publicationDate,title,abstract,id
2022-08-09,Fractalized magnon transport on the quasicrystal with enhanced stability,"Magnonics has been receiving significant attention in magnetism and
spintronics because of its premise for devices using spin current carried by
magnons, quanta of spin-wave excitations of a macroscopically ordered magnetic
media. Although magnonics has clear energy-wise advantage over conventional
electronics due to the absence of the Joule heating, the inherent magnon-magnon
interactions give rise to finite lifetime of the magnons which has been
hampering the efficient realizations of magnonic devices. To promote magnonics,
it is imperative to identify the delocalized magnon modes that are minimally
affected by magnon-magnon interactions and thus possess a long lifetime and use
them to achieve efficient magnon transport. Here, we suggest that quasicrystals
may offer the solution to this problem via the critical magnon modes that are
neither extended nor localized. We find that the critical magnon exhibits
fractal characteristics that are absent in conventional magnon modes in regular
solids such as a unique power-law scaling and a self-similar distribution of
distances showing perfect magnon transmission. Moreover, the critical magnons
have longer lifetimes compared to the extended ones in a periodic system, by
suppressing the magnon-magnon interaction decay rate. Such enhancement of the
magnon stability originates from the presence of the quasi-periodicity and
intermediate localization behavior of the critical magnons. Thus, we offer the
utility of quasicrystals and their critical spin wave functions in magnonics as
unique fractal transport characteristics and enhanced stability.",2208.04967v1
2023-11-30,Nanoscaled magnon transistor based on stimulated three-magnon splitting,"Magnonics is a rapidly growing field, attracting much attention for its
potential applications in data transport and processing. Many individual
magnonic devices have been proposed and realized in laboratories. However, an
integrated magnonic circuit with several separate magnonic elements has yet not
been reported due to the lack of a magnonic amplifier to compensate for
transport and processing losses. The magnon transistor reported in [Nat.
Commun. 5, 4700, (2014)] could only achieve a gain of 1.8, which is
insufficient in many practical cases. Here, we use the stimulated three-magnon
splitting phenomenon to numerically propose a concept of magnon transistor in
which the energy of the gate magnons at 14.6 GHz is directly pumped into the
energy of the source magnons at 4.2 GHz, thus achieving the gain of 9. The
structure is based on the 100 nm wide YIG nano-waveguides, a directional
coupler is used to mix the source and gate magnons, and a dual-band magnonic
crystal is used to filter out the gate and idler magnons at 10.4 GHz frequency.
The magnon transistor preserves the phase of the signal and the design allows
integration into a magnon circuit.",2311.18479v1
2023-02-03,Sub-millimeter propagation of antiferromagnetic magnons via magnon-photon coupling,"For the realization of magnon-based current-free technologies, referred to as
magnonics, all-optical control of magnons is an important technique for
fundamental research and application. Magnon-polariton is a coupled state of
magnon and photon in a magnetic medium, which is expected to exhibit a
magnon-like controllability and a photon-like high-speed propagation. Recent
studies have observed magnon-polaritons as modulation of the incident terahertz
wave; however, the influence of magnon-photon coupling on the magnon
propagation property has not been explored. This study aimed to observe the
spatiotemporal dynamics of coherent magnon-polariton through time-resolved
imaging measurements. $\mathrm{BiFeO_3}$ was chosen as the sample, because it
is expected to exhibit strong coupling between the magnon and photon. The
observed dynamics suggested that antiferromagnetic magnons can propagate over
long distances up to hundreds of micrometers through strong coupling with the
photons. The results shed light on the understanding of the optical control of
the magnonic systems thereby paving the way for terahertz opto-magnonics.",2302.01821v1
2017-12-13,Anomalous magnon Nernst effect of topological magnonic materials,"The magnon transport driven by thermal gradient in a perpendicularly
magnetized honeycomb lattice is studied. The system with the nearest-neighbor
pseudodipolar interaction and the next-nearest-neighbor Dzyaloshinskii-Moriya
interaction (DMI) has various topologically nontrivial phases. When an in-plane
thermal gradient is applied, a transverse in-plane magnon current is generated.
This phenomenon is termed as the anomalous magnon Nernst effect that closely
resembles the anomalous Nernst effect for an electronic system. The anomalous
magnon Nernst coefficient and its sign are determined by the magnon Berry
curvatures distribution in the momentum space and magnon populations in the
magnon bands. We predict a temperature-induced sign reversal in anomalous
magnon Nernst effect under certain conditions.",1712.05027v1
2023-08-28,Exploring wavefunction hybridization of magnon-magnon hybrid state,"We investigate magnon magnon hybrid states using a non Hermitian two band
Hamiltonian and the concept of wavefunction hybridization. By comparing our
model with micromagnetic simulations conducted on a synthetic antiferromagnet
with strong magnon magnon coupling, we successfully reproduce not only the
resonance frequencies and linewidths but also the phases and amplitudes of the
magnon wavefunction. The hybridization effect influences the dissipation rate,
leading to the crossing of linewidths. Additionally, we quantify the magnon
hybridization within a magnonic Bloch sphere, which enhances the ability to
manipulate hybrid magnons for coherent information processing.",2308.14463v1
2015-02-13,Magnon transport through microwave pumping,"We present a microscopic theory of magnon transport in ferromagnetic
insulators (FIs). Using magnon injection through microwave pumping, we propose
a way to generate magnon dc currents and show how to enhance their amplitudes
in hybrid ferromagnetic insulating junctions. To this end focusing on a single
FI, we first revisit microwave pumping at finite (room) temperature from the
microscopic viewpoint of magnon injection. Next, we apply it to two kinds of
hybrid ferromagnetic insulating junctions. The first is the junction between a
quasi-equilibrium magnon condensate and magnons being pumped by microwave,
while the second is the junction between such pumped magnons and noncondensed
magnons. We show that quasi-equilibrium magnon condensates generate ac and dc
magnon currents, while noncondensed magnons produce essentially a dc magnon
current. The ferromagnetic resonance (FMR) drastically increases the density of
the pumped magnons and enhances such magnon currents. Lastly, using microwave
pumping in a single FI, we discuss the possibility that a magnon current
through an Aharonov-Casher phase flows persistently even at finite temperature.
We show that such a magnon current arises even at finite temperature in the
presence of magnon-magnon interactions. Due to FMR, its amplitude becomes much
larger than the condensed magnon current.",1502.03865v2
2021-07-07,Skyrmion Based Magnonic Crystals,"Magnonics is now an attractive field which focuses on the dynamic
characteristics of magnons, a kind of quasiparticles in magnetic media, and
attempt to apply magnons for functional devices. In order to construct magnon
based devices, it is necessary to fabricate materials with specific and tunable
magnon bands and band gaps. Skyrmion based magnonic crystals is one of the most
suitable materials which possess periodical skyrmion structure and show
applicative magnon bands and band gaps. In this review, we provide an overview
over recent theoretical and experimental research on skyrmion based magnonic
crystals. We will firstly provide an introduction of magnonic crystals and
magnetic skyrmion. And then we will show the theoretical and experimental
progress on skyrmion based magnonic crystals and their magnon band
characteristics. At the end, we will give an outlook and perspectives of new
fascinating fields on topological nontrivial magnon modes, as well as hybrid
and quantum magnonic phenomena of skyrmion based magnonic crystals.",2107.03148v1
2020-08-26,One- and two-magnon excitations in antiferromagnet PbFeBO4,"The linear spin-wave theory study of PbFeBO4 spin dynamics is presented. It
is shown that the modes observed in Raman scattering experiments below Neel
temperature are optical magnon and two-magnon excitations. Based on the magnon
energy, two-magnon band lineshape, and Weiss temperature, the consistent set of
the exchange coupling constants up the third neighbor is derived and compared
with the results of ab initio calculations. The small deviation of the observed
two-magnon band from the one-magnon density of states suggests a surprisingly
negligible role of magnon-magnon interactions.",2008.11530v2
2024-04-04,Interaction-induced nonlinear magnon transport in noncentrosymmetric ferromagnets,"We study the effect of the magnon-magnon interaction on the nonlinear magnon
transport. The magnon-magnon interaction induces nonreciprocal magnon decay
when the time-reversal symmetry is broken, and leads to nonlinear thermal
responses of magnons. We construct a theoretical framework to study the
nonlinear thermal responses due to the nonreciprocal magnon decay by using the
imaginary Dyson equation and quantum kinetic theory, which is then applied to a
model of honeycomb ferromagnets with Dzyaloshinskii-Moriya interactions. An
order estimate shows that the nonlinear thermal response from the present
mechanism is feasible for experimental measurement.",2404.03260v1
2018-09-30,Magnon Valves Based on YIG/NiO/YIG All-Insulating Magnon Junctions,"As an alternative angular momentum carrier, magnons or spin waves can be
utilized to encode information and breed magnon-based circuits with ultralow
power consumption and non-Boolean data processing capability. In order to
construct such a circuit, it is indispensable to design some electronic
components with both long magnon decay and coherence length and effective
control over magnon transport. Here we show that an all-insulating magnon
junctions composed by a magnetic insulator (MI1)/antiferromagnetic insulator
(AFI)/magnetic insulator (MI2) sandwich (Y3Fe5O12/NiO/Y3Fe5O12) can completely
turn a thermogradient-induced magnon current on or off as the two Y3Fe5O12
layers are aligned parallel or anti-parallel. The magnon decay length in NiO is
about 3.5~4.5 nm between 100 K and 200 K for thermally activated magnons. The
insulating magnon valve (magnon junction), as a basic building block, possibly
shed light on the naissance of efficient magnon-based circuits, including
non-Boolean logic, memory, diode, transistors, magnon waveguide and switches
with sizable on-off ratios.",1810.00380v1
2020-09-11,Magnon-magnon interaction and magnon relaxation time in ferromagnetic Cr2Ge2Te6 monolayer,"Despite the intense amount of attention and huge potential of two-dimensional
(2D) magnets for applications in novel magnetic, magneto-optical,
magneto-thermal and magneto-electronic devices, there has yet to be a robust
strategy developed to systematically understand magnon-magnon (MMI)
interactions at finite temperature. In this paper, we present a
first-principles theoretical method to introduce the finite temperature
magnon-magnon interaction into Heisenberg Hamiltonian through a nonlinear
correction energy. The Wick theorem is used to decouple the four-magnon
operators to two-magnon order. We demonstrate the capabilities of this method
by studying the strength of MMI in Cr2Ge2Te6 (CGT) monolayer. The spin wave
spectrum at finite temperature and the time-dependent spin autocorrelation
function are explored. It is found that the magnon relaxation time due to
magnon-magnon scattering increases with temperature because of the reduction in
magnon energy, while decreases with wavevector and external magnetic field. Our
results provide a new insight to understand the magnon damping and energy
dissipation in two-dimensional ferromagnetic materials.",2009.05225v1
2022-08-27,Magnon drag induced by magnon-magnon interactions characteristic of noncollinear magnets,"A noncollinear magnet consists of the magnetic moments forming a noncollinear
spin structure. Because of this structure, the Hamiltonian of magnons acquires
the cubic terms. Although the cubic terms are the magnon-magnon interactions
characteristic of noncollinear magnets, their effects on magnon transport have
not been clarified yet. Here we show that in a canted antiferromagnet the cubic
terms cause a magnon drag that magnons drag magnon spin current and heat
current, which can be used to enhance these currents by tuning a magnetic
field. For a strong magnetic field, we find that the cubic terms induce
low-temperature peaks of a spin-Seebeck coefficient, a magnon conductivity, and
a magnon thermal conductivity, and that each value is one order of magnitude
larger than the noninteracting value. This enhancement is mainly due to the
magnetic field dependence of the coupling constant of the cubic terms through
the magnetic-field dependent canting angle. Our magnon drag offers a way for
controlling the magnon currents of noncollinear magnets via the many-body
effect.",2208.12939v1
2023-03-28,Tutorial: Nonlinear magnonics,"Nonlinear magnonics studies the nonlinear interaction between magnons and
other physical platforms (phonon, photon, qubit, spin texture) to generate
novel magnon states for information processing. In this tutorial, we first
introduce the nonlinear interactions of magnons in pure magnetic systems and
hybrid magnon-phonon and magnon-photon systems. Then we show how these
nonlinear interactions can generate exotic magnonic phenomena. In the classical
regime, we will cover the parametric excitation of magnons, bistability and
multistability, and the magnonic frequency comb. In the quantum regime, we will
discuss the single magnon state, Schr\""{o}dinger cat state and the entanglement
and quantum steering among magnons, photons and phonons. The applications of
the hybrid magnonics systems in quantum transducer and sensing will also be
presented. Finally, we outlook the future development direction of nonlinear
magnonics.",2303.16313v1
2023-04-04,Full quantum theory for magnon transport in two-sublattice magnetic insulators and magnon junctions,"Magnon, as elementary excitation in magnetic systems, can carry and transfer
angular momentum. Due to the absence of Joule heat during magnon transport,
researches on magnon transport have gained considerable interests over the past
decade. Recently, a full quantum theory has been employed to investigate magnon
transport in ferromagnetic insulators (FMIs). However, the most commonly used
magnetic insulating material in experiments, yttrium iron garnet (YIG), is a
ferrimagnetic insulator (FIMI). Therefore, a full quantum theory for magnon
transport in FIMI needs to be established. Here, we propose a Green's function
formalism to compute the magnon bulk and interface current in both FIMIs and
antiferromagnetic insulators (AFMIs). We investigate the spatial distribution
and temperature dependence of magnon current in FIMIs and AFMIs generated by
temperature or spin chemical potential step. In AFMIs, magnon currents
generated by temperature step in the two sublattices cancel each other out.
Subsequently, we numerically simulate the magnon junction effect using the
Green's function formalism, and result shows near 100\% magnon junction ratio.
This study demonstrates the potential for investigating magnon transport in
specific magnonic devices using a full quantum theory.",2304.01930v1
2020-05-27,Magnon antibunching in a nanomagnet,"We investigate the correlations of magnons inside a nanomagnet and identify a
regime of parameters where the magnons become antibunched, i.e., where there is
a large probability for occupation of the single-magnon state. This antibunched
state is very different from magnons at thermal equilibrium and
microwave-driven coherent magnons. We further obtain the steady state
analytically and describe the magnon dynamics numerically, and ascertain the
stability of such antibunched magnons over a large window of magnetic
anisotropy, damping and temperature. This means that the antibunched magnon
state is feasible in a wide class of low-damping magnetic nanoparticles. To
detect this quantum effect, we propose to transfer the quantum information of
magnons to photons by magnon-photon coupling and then measure the correlations
of photons to retrieve the magnon correlations. Our findings may provide a
promising platform to study quantum-classical transitions and for designing a
single magnon source.",2005.13637v1
2023-03-21,Intrinsic Magnon Orbital Hall Effect in Honeycomb Antiferromagnets,"We theoretically investigate the transport of magnon orbitals in a honeycomb
antiferromagnet. We find that the magnon orbital Berry curvature is finite even
without spin-orbit coupling and thus the resultant magnon orbital Hall effect
is an intrinsic property of the honeycomb antiferromagnet rooted only in the
exchange interaction and the lattice structure. Due to the intrinsic nature of
the magnon orbital Hall effect, the magnon orbital Nernst conductivity is
estimated to be orders of magnitude larger than the predicted values of the
magnon spin Nernst conductivity that requires finite spin-orbit coupling. For
the experimental detection of the predicted magnon orbital Hall effect, we
invoke the magnetoelectric effect that couples the magnon orbital and the
electric polarization, which allows us to detect the magnon orbital
accumulation through the local voltage measurement. Our results pave a way for
a deeper understanding of the topological transport of the magnon orbitals and
also its utilization for low-power magnon-based orbitronics, namely magnon
orbitronics.",2303.11687v1
2023-04-11,Sizable suppression of magnon Hall effect by magnon damping in Cr$_2$Ge$_2$Te$_6$,"Two-dimensional (2D) Heisenberg honeycomb ferromagnets are expected to have
interesting topological magnon effects as their magnon dispersion can have
Dirac points. The Dirac points are gapped with finite second nearest neighbor
Dzyaloshinskii-Moriya interaction, providing nontrivial Berry curvature with
finite magnon Hall effect. Yet, it is unknown how the topological properties
are affected by magnon damping. We report the thermal Hall effect in
Cr$_2$Ge$_2$Te$_6$, an insulating 2D honeycomb ferromagnet with a large Dirac
magnon gap and significant magnon damping. Interestingly, the thermal Hall
conductivity in Cr$_2$Ge$_2$Te$_6$ shows the coexisting phonon and magnon
contributions. Using an empirical two-component model, we successfully estimate
the magnon contribution separate from the phonon part, revealing that the
magnon Hall conductivity was 20 times smaller than the theoretical calculation.
Finally, we suggest that such considerable suppression in the magnon Hall
conductivity is due to the magnon damping effect in Cr$_2$Ge$_2$Te$_6$.",2304.04922v1
2023-07-03,Cavity-Induced Strong Magnon-Magnon Coupling in Altermagnets,"Long-distance strong coupling between short-wavelength magnons remains an
outstanding challenge in quantum magnonics, an emerging interdiscipline between
magnonics and quantum information science. Recently, altermagnets are
identified as the third elementary class of magnets that break the
time-reversal symmetry without magnetization and thus combine characteristics
of conventional collinear ferromagnets and antiferromagnets. In this work, we
show that cavity photons can mediate the long-distance strong coupling of
exchange magnons with opposite chiralities in altermagnets, manifesting as an
anticrossing of the magnon-polariton spectrum in the extremely dispersive
regime. The predicted effective magnon-magnon coupling strongly depends on the
magnon propagation direction, and is thus highly anisotropic. Our findings are
intimately connected to the intrinsic nature of altermagnetic magnons, i.e.,
chirality-splitting-induced crossing of exchange magnons, which has no
counterpart in conventional ferromagnets or antiferromagnets, and may open a
new path way for magnon-based quantum information processing in altermagnets.",2307.00909v1
2016-05-15,Propagation of Thermally Induced Magnonic Spin Currents,"The propagation of magnons in temperature gradients is investigated within
the framework of an atomistic spin model with the stochastic
Landau-Lifshitz-Gilbert equation as underlying equation of motion. We analyze
the magnon accumulation, the magnon temperature profile as well as the
propagation length of the excited magnons. The frequency distribution of the
generated magnons is investigated in order to derive an expression for the
influence of the anisotropy and the damping parameter on the magnon propagation
length. For soft ferromagnetic insulators with low damping a propagation length
in the range of some $\mu$m can be expected for exchange driven magnons.",1605.04543v1
2022-01-24,Efficient Gating of Magnons by Proximity Superconductors,"Electrostatic gating confines and controls the transport of electrons in
integrated circuits. Magnons, the quanta of spin waves of the magnetic order,
are promising alternative information carriers, but difficult to gate. Here we
report that superconducting strips on top of thin magnetic films can totally
reflect magnons by its diamagnetic response to the magnon stray fields. The
induced large frequency shifts unidirectionally blocks the magnons propagating
normal to the magnetization. Two superconducting gates parallel to the
magnetization create a magnonic cavity. The option to gate coherent magnons
adds functionalities to magnonic devices, such as reprogrammable logical
devices and increased couplings to other degrees of freedom.",2201.09532v2
2021-08-18,Thermal squeezing and nonlinear spectral shift of magnons in antiferromagnetic insulators,"We investigate the effect of magnon-magnon interactions on the dispersion and
polarization of magnons in collinear antiferromagnetic (AF) insulators at
finite temperatures. In two-sublattice AF systems with either uniaxial or
biaxial magnetocrystalline anisotropies, we implement a self-consistent
Hartree-Fock mean-field approximation to explore the nonlinear thermal
interactions. The resulting nonlinear magnon interactions separate into
two-magnon intra- and interband scattering processes. Furthermore, we compute
the temperature dependence of the magnon bandgap and AF resonance modes due to
nonlinear magnon interactions for square and hexagonal lattices. In addition,
we study the effect of magnon interactions on the polarization of magnon modes.
We find that although the noninteracting eigenmodes in the uniaxial case are
circularly polarized, but in the presence of nonlinear thermal interactions the
local U(1) symmetry of the Hamiltonian is broken. The attractive nonlinear
interactions squeeze the low-energy magnon modes and make them elliptical. In
the biaxial case, on the other hand, the bare eigenmodes of low energy magnons
are elliptically polarized but thermal nonlinear interactions squeeze them
further. Direct measurements of the predicted temperature-dependent AF
resonance modes and their polarization can be used as a tool to probe the
nonlinear interactions. Our findings establish a framework for exploring the
effect of thermal magnon interactions in technologically important magnetic
systems, such as magnetic stability of recently discovered two-dimensional
magnetic materials, coherent transport of magnons, Bose-Einstein condensation
of magnons, and magnonic topological insulators.",2108.08374v4
2022-09-07,Master equation approach to magnon relaxation and dephasing,"There has been a recent upsurge of interest in the quantum properties of
magnons for quantum information processing. An important issue is to examine
the stability of quantum states of magnons against various relaxation and
dephasing channels. Since the interaction of magnons in magnetic systems may
fall in the ultra-strong and even deep-strong coupling regimes, the relaxation
process of magnon states is quite different from the more common quantum
optical systems. Here we study the relaxation and dephasing of magnons based on
the Lindblad formalism and derive a generalized master equation that describes
the quantum dynamics of magnons. Employing this master equation, we identify
two distinct dissipation channels for squeezed magnons, i.e., the local
dissipation and collective dissipation, which play a role for both ferromagnets
and antiferromagnets. The local dissipation is caused by the independent
exchange of angular momentum between the magnonic system and the environment,
while the collective dissipation is dressed by the parametric interactions of
magnons and it enhances the quantumness and thermal stability of squeezed
magnons. Further, we show how this formalism can be applied to study the pure
dephasing of magnons caused by four-magnon scattering and magnon-phonon
interactions. Our results provide the theoretical tools to study the
decoherence of magnons within a full quantum-mechanical framework and further
benefit the use of quantum states of magnons for information processing.",2209.02961v1
2021-09-27,Numerical model for 32-bit magnonic ripple carry adder,"In CMOS-based electronics, the most straightforward way to implement a
summation operation is to use the ripple carry adder (RCA). Magnonics, the
field of science concerned with data processing by spin-waves and their quanta
magnons, recently proposed a magnonic half-adder that can be considered as the
simplest magnonic integrated circuit. Here, we develop a computation model for
the magnonic basic blocks to enable the design and simulation of magnonic gates
and magnonic circuits of arbitrary complexity and demonstrate its functionality
on the example of a 32-bit integrated RCA. It is shown that the RCA requires
the utilization of additional regenerators based on magnonic directional
couplers with embedded amplifiers to normalize the magnon signals in-between
the half-adders. The benchmarking of large-scale magnonic integrated circuits
is performed. The energy consumption of 30 nm-based magnonic 32-bit adder can
be as low as 961aJ per operation with taking into account all required
amplifiers.",2109.12973v1
2023-05-15,Quantum interference induced magnon blockade and antibunching in a hybrid quantum system,"In this work, we study the phenomena of quantum interference assisted magnon
blockade and magnon antibunching in a weakly interacting hybrid
ferromagnet-superconductor system. The magnon excitations in two yttrium iron
garnet spheres are indirectly coupled to a superconducting qubit through
microwave cavity modes of two mutually perpendicular cavities. We find that
when one of the magnon mode is driven by a weak optical field, the destructive
interference between more than two distinct transition pathways restricts
simultaneous excitation of two magnons. We analyze the magnon correlations in
the driven magnon mode for the case of zero detunings as well as finite
detunings of the magnon modes and the qubit. We show that the magnon
antibunching can be tuned by changing the magnon-qubit coupling strength ratio
and the driving detuning. Our work proposes a possible scheme which have
significant role in the construction of single magnon generating devices.",2305.08444v1
2023-05-24,Topological Phases in Magnonics,"Magnonics or magnon spintronics is an emerging field focusing on generating,
detecting, and manipulating magnons. As charge-neutral quasi-particles, magnons
are promising information carriers because of their low energy dissipation and
long coherence length. In the past decade, topological phases in magnonics have
attracted intensive attention due to their fundamental importance in
condensed-matter physics and potential applications of spintronic devices. In
this review, we mainly focus on recent progress in topological magnonics, such
as the Hall effect of magnons, magnon Chern insulators, topological magnon
semimetals, etc. In addition, the evidence supporting topological phases in
magnonics and candidate materials are also discussed and summarized. The aim of
this review is to provide readers with a comprehensive and systematic
understanding of the recent developments in topological magnonics.",2305.14861v2
2023-08-11,Kerr nonlinearity induced strong spin-magnon coupling,"One pillar of quantum magnonics is the exploration of the utilization of the
mediation role of magnons in different platforms to develop quantum
technologies. The efficient coupling between magnons and various quantum
entities is a prerequisite. Here, we propose a scheme to enhance the
spin-magnon coupling by the magnonic Kerr nonlinearity in a YIG sphere. We find
that the Kerr-enhanced spin-magnon coupling invalidates the widely used
single-Kittel-mode approximation to magnons. It is revealed that the spin
decoherence induced by the multimode magnons in the strong-coupling regime
becomes not severe, but suppressed, manifesting as either population trapping
or persistent Rabi-like oscillation. This anomalous effect is because the spin
changes to be so hybridized with the magnons that one or two bound states are
formed between them. Enriching the spin-magnon coupling physics, the result
supplies a guideline to control the spin-magnon interface.",2308.05927v2
2024-03-11,Handedness manipulation of propagating antiferromagnetic magnons,"Antiferromagnetic magnons possess a distinctive feature absent in their
ferromagnetic counterparts: the presence of two distinct handedness modes, the
right-handed (RH) and left-handed (LH) precession modes. The magnon handedness
determines the sign of spin polarization carried by the propagating magnon,
which is indispensable for harnessing the diverse functionalities. However, the
control of coherently propagating magnon handedness in antiferromagnets has
remained elusive so far. Here we demonstrate the manipulation and electrical
readout of propagating magnon handedness in perpendicularly magnetized
synthetic antiferromagnets (SAF). We find that the antiferromagnetic magnon
handedness can be directly identified by measuring the inverse spin Hall effect
(ISHE) voltage, which arises from the spin pumping effect caused by the
propagating antiferromagnetic magnons in the SAF structure. The RH and LH modes
of the magnon can be distinguishable particularly when the SAF structure is
sandwiched by heavy metals with the same sign of spin Hall angle. Moreover, we
succeed in controlling the handedness of propagating antiferromagnetic magnons
by tuning the excitation microwave frequency. This work unveils promising
avenues for harnessing magnon unique properties in antiferromagnet-based
magnonic applications.",2403.06549v1
2021-03-08,Anisotropic magnon-magnon coupling in synthetic antiferromagnets,"The magnon-magnon coupling in synthetic antiferromagnets advances it as
hybrid magnonic systems to explore the quantum information technologies. To
induce the magnon-magnon coupling, the parity symmetry between two
magnetization needs to be broken. Here we experimentally demonstrate a
convenient method to break the parity symmetry by the asymmetric thickness of
two magnetic layers and thus introduce a magnon-magnon coupling in Ir-based
synthetic antiferromagnets CoFeB(10 nm)/Ir(tIr=0.6 nm, 1.2 nm)/CoFeB(13 nm).
Remarkably, we find that the weakly uniaxial anisotropy field (~ 20 Oe) makes
the magnon-magnon coupling anisotropic. The coupling strength presented by a
characteristic anticrossing gap varies in the range between 0.54 GHz and 0.90
GHz for tIr =0.6 nm, and between nearly zero to 1.4 GHz for tIr = 1.2 nm,
respectively. Our results demonstrate a feasible way to induce the
magnon-magnon coupling by an asymmetric structure and tune the coupling
strength by varying the direction of in-plane magnetic field. The magnon-magnon
coupling in this highly tunable material system could open exciting
perspectives for exploring quantum-mechanical coupling phenomena.",2103.04512v2
2023-01-18,Magnon-magnon interactions induced by spin pumping-driven symmetry breaking in synthetic antiferromagnets,"The richness in both the dispersion and energy of antiferromagnetic magnons
has spurred the magnetism community to consider antiferromagnets for future
spintronic/magnonic applications. However, the excitation and control of
antiferromagnetic magnons remains challenging, especially when compared to
ferromagnetic counterparts. A middle ground is found with synthetic
antiferromagnet metamaterials, where acoustic and optical magnons exist at GHz
frequencies. In these materials, the magnon energy spectrum can be tuned by
static symmetry-breaking external fields or dipolar interactions hybridizing
optical and acoustic magnon branches. Here, we experimentally measure the
magnon energy spectrum of synthetic antiferromagnetic tetralayers, and discover
avoided energy level crossings in the energy spectrum that are unexplained by
the antiferromagnetic interlayer coupling. We explain our experimental results
using a phenomenological model incorporating both fieldlike and dampinglike
torques generated by spin pumping in noncollinear magnetic multilayers
separated by normal-metal spacers. We show that an asymmetry in the fieldlike
torques acting on different magnetic layers can lift the spectral degeneracies
of acoustic and optical magnon branches and yield symmetry-breaking induced
magnon-magnon interactions. Our work extends the phenomenology of spin pumping
to noncollinear magnetization configurations and significantly expands ways of
engineering magnon-magnon interactions within antiferromagnets and quantum
hybrid magnonic materials.",2301.07311v1
1999-07-23,Magnon Broadening Effects in Double Layered Manganite La_1.2 Sr_1.8 Mn_2 O_7,"Magnon linewidth of La_1.2 Sr_1.8 Mn_2 O_7 near the Brillouin zone boundary
is investigated from both theoretical and experimental points of view. Abrupt
magnon broadening is ascribed to a strong magnon-phonon coupling. Magnon
broadening observed in cubic perovskite manganites is also discussed.",9907362v1
2018-02-12,Photoinduced topologically trivial magnons with finite thermal Hall effect,"In two-dimensional (2D) insulating magnets, the thermal Hall effect of
magnons is believed to be a consequence of topological magnon insulator with
separated magnon bands and a well-defined Chern number. Due to broken
time-reversal symmetry the thermal Hall effect vanishes in Dirac magnons. In
this paper, we show that periodically driven semi-Dirac magnon in 2D insulating
honeycomb ferromagnet results in a photoinduced Dirac magnon at the topological
phase transition between a photoinduced topological and trivial magnon
insulator. Remarkably, the photoinduced Dirac magnon and the photoinduced
trivial magnon insulator possess a nonzero Berry curvature and exhibit a finite
thermal Hall effect. These intriguing properties of periodically driven 2D
insulating magnets originate from the bosonic nature of magnons. Hence, they
are not expected to exist in 2D electronic Floquet systems.",1802.04268v2
2016-10-04,Resolving magnon number states in quantum magnonics,"Collective excitation modes in solid state systems play a central role in
circuit quantum electrodynamics, cavity optomechanics, and quantum magnonics.
In the latter, quanta of collective excitation modes in a ferromagnet, called
magnons, interact with qubits to provide the nonlinearity necessary to access
quantum phenomena in magnonics. A key ingredient for future quantum magnonics
systems is the ability to probe magnon states. Here we observe individual
magnons in a millimeter-sized ferromagnet coherently coupled to a
superconducting qubit. Specifically, we resolve magnon number states in
spectroscopic measurements of a transmon qubit with the hybrid system in the
strong dispersive regime. This enables us to detect a change in the magnetic
dipole of the ferromagnet equivalent to a single spin flipped among more than
$10^{19}$ spins. The strong dispersive regime of quantum magnonics opens up the
possibility of encoding superconducting qubits into non-classical magnon
states, potentially providing a coherent interface between a superconducting
quantum processor and optical photons.",1610.00839v1
2018-10-01,Stabilizing Mechanism for Bose-Einstein Condensation of Interacting Magnons in Ferrimagnets and Ferromagnets,"We propose a stabilizing mechanism for the Bose-Einstein condensation (BEC)
of interacting magnons in ferrimagnets and ferromagnets. By studying the
effects of the magnon-magnon interaction on the stability of the magnon BEC in
a ferrimagnet and two ferromagnets, we show that the magnon BEC remains stable
even in the presence of the magnon-magnon interaction in the ferrimagnet and
ferromagnet with a sublattice structure, whereas it becomes unstable in the
ferromagnet without a sublattice structure. This indicates that the existence
of a sublattice structure is the key to stabilizing the BEC of interacting
magnons, and the difference between the spin alignments of a ferrimagnet and a
ferromagnet is irrelevant. Our result can resolve a contradiction between
experiment and theory in the magnon BEC of yttrium iron garnet. Our theoretical
framework may provide a starting point for understanding the physics of the
magnon BEC including the interaction effects.",1810.00584v2
2019-10-09,Magnon Blockade in a Hybrid Ferromagnet-Superconductor Quantum System,"The implementation of a single magnon level quantum manipulation is one of
the fundamental targets in quantum magnetism with a significant practical
relevance for precision metrology, quantum information processing, and quantum
simulation. Here, we demonstrate theoretically the feasibility of using a
hybrid ferromagnet-superconductor quantum system to prepare a single magnon
source based on magnon blockade effects. By numerically solving the quantum
master equation, we show that the second-order correlation function of the
magnon mode depends crucially on the relation between the qubit-magnon coupling
strength and the driving detuning, and simultaneously signatures of the magnon
blockade appear only under quite stringent conditions of a cryogenic
temperature. In addition to providing perception into the quantum phenomena of
magnon, the study of magnon blockade effects will help to develop novel
technologies for exploring the undiscovered magnon traits at the quantum level
and may find applications in designing single magnon emitters.",1910.03738v1
2020-01-20,Effects of magnon-magnon interactions in a noncollinear magnet induced by combination of a symmetric and an antisymmetric exchange interaction,"We study magnon-magnon interactions and their effects in a spiral magnet
induced by combination of an antiferromagnetic Heisenberg interaction and a
Dzyaloshinsky-Moriya interaction. We show that the main effect of magnon-magnon
interactions on low-energy magnons is to renormalize the coefficient of energy
dispersion. This could explain why some experiments are consistent with the
noninteracting theory. We also show that although the magnon-magnon
interactions induce the pair amplitude for low-energy magnons, its effect on
the excitation energy is negligible. This suggests that for magnons the finite
pair amplitude does not necessarily accompany the pair condensation.",2001.06981v2
2020-04-23,Strong coupling between magnons confined in a single magnonic cavity,"Strong coupling between magnons and cavity photons was studied extensively
for quantum electrodynamics in the past few years. Recently, the strong
magnon-magnon coupling between adjacent layers in magnetic multilayers has been
reported. However, the strongly coupled magnons confined in a single nanomagnet
remains to be revealed. Here, we report the interaction between different
magnon modes in a single magnonic cavity. The intermodel coupling between edge
and center magnon modes in the strong coupling regime was approached with a
maximum coupling strength of 0.494 GHz and cooperativity of 60.1 with a damping
of 1X10-3. Furthermore, it is found that the coupling strength is highly
dependent on the geometric parameters of the magnonic cavity. Our findings
could greatly enrich the still evolving field of quantum magnonics.",2004.11058v1
2020-08-15,Magnon laser based on Brillouin light scattering,"An analogous laser action of magnons would be a subject of interest and is
crucial for the study of nonlinear magnons spintronics. Here, we demonstrate
the magnon laser behavior based on Brillouin light scattering in a
ferrimagnetic insulator sphere which supports optical whispering gallery modes
and magnon resonances. We show that the excited magnon plays what has
traditionally been the role of the Stokes wave and is coherently amplified
during the Brillouin scattering process, making magnon laser possible.
Furthermore, the stimulating excited magnon number increasing exponentially
with the input light power can be manipulated by adjusting the external
magnetic field. In addition to providing insight into magneto-optical
interaction, the study of magnon laser action will help to develop novel
technologies for handling spin-wave excitations and could affect scientific
fields beyond magnonics. Potential applications range from preparing coherent
magnon sources to operating onchip functional magnetic devices.",2008.06628v2
2020-10-31,Facet-dependent magnon-polarons in epitaxial ferrimagnetic Fe3O4 thin films,"Magnon-polarons are coherently mixed quasiparticles that originate from the
strong magnetoelastic coupling of lattice vibrations and spin waves in
magnetic-ordered materials. Recently, magnon-polarons have attracted a lot of
attention since they provide a powerful tool to manipulate magnons, which is
essential for magnon-based spintronic devices. In this work, we report the
experimental observation of facet-dependent magnon-polarons in epitaxial
ferrimagnetic Fe3O4 thin films via spin Seebeck effect measurement. The
critical magnetic fields for the magnon-polarons in the (110)- and
(100)-oriented Fe3O4 films are 1.5 T and 1.8 T, respectively, which arises from
the different phonon velocities along the [110] and [100] directions. As the
temperature decreases, the magnon-polarons-enhanced spin Seebeck voltage
decreases in both (110)- and (100)-oriented Fe3O4 films, which could be
attributed to the enhanced magnon-polarons scattering at elevated temperatures.
This work demonstrates the crystal structure engineering in epitaxial magnetic
films as a promising route to manipulate the magnon-polarons for future magnon
spintronic applications.",2011.00195v1
2021-06-12,Magnon-magnon entanglement and its detection in a microwave cavity,"Quantum magnonics is an emerging research field, with great potential for
applications in magnon based hybrid systems and quantum information processing.
Quantum correlation, such as entanglement, is a central resource in many
quantum information protocols that naturally comes about in any study toward
quantum technologies. This applies also to quantum magnonics. Here, we
investigate antiferromagnets in which sublattices with ferromagnetic
interactions can have two different magnon modes, and we show how this may lead
to experimentally detectable bipartite continuous variable magnon-magnon
entanglement. The entanglement can be fully characterized via a single
squeezing parameter, or, equivalently, entanglement parameter. The clear
relation between the entanglement parameter and the Einstein, Podolsky, and
Rosen (EPR) function of the ground state opens up for experimental observation
of magnon-magnon continuous variable entanglement and EPR non-locality. We
propose a practical experimental realization to detect the EPR function of the
ground state, in a setting that relies on magnon-photon interaction in a
microwave cavity.",2106.06862v1
2021-12-04,Dissipation-induced nonreciprocal magnon blockade in a magnon-based hybrid system,"We propose an experimentally realizable nonreciprocal magnonic device at the
single-magnon level by exploiting magnon blockade in a magnon-based hybrid
system. The coherent qubit-magnon coupling, mediated by virtual photons in a
microwave cavity, leads to the energy-level anharmonicity of the composite
modes. In contrast, the corresponding dissipative counterpart, induced by
traveling microwaves in a waveguide, yields inhomogeneous broadenings of the
energy levels. As a result, the cooperative effects of these two kinds of
interactions give rise to the emergence of the direction-dependent magnon
blockade. We show that this can be demonstrated by studying the equal-time
second-order correlation function of the magnon mode. Our study opens an avenue
to engineer nonreciprocal magnonic devices in the quantum regime involving only
a small number of magnons.",2112.02351v1
2022-09-09,Parity-Time Symmetry-Enhanced Simultaneous Magnon and Photon Blockade in Cavity Magnonic System,"The main challenge in the recent demonstration of conventional magnon
blockade is to increase the nonlinearity of the system especially in comparison
with the dissipation channels. One can consider the Kerr nonlinearity through
which magnon blockade in a cavity magnonic system is possible provided that the
Kerr nonlinearity is much stronger than the cavity and magnonic mode
dissipation rates. In the present contribution, we consider a PT-symmetric
cavity magnonic system and study the effect of PT-symmetric phase on the magnon
statistics and hence magnon blockade. We show that the PT-symmetric phase,
which is achievable by properly selecting the system parameters, can relax the
requirement of large Kerr nonlinearity such that a perfect magnon blockade can
be easily obtained even under a small value of Kerr nonlinearity. Surprisingly,
although there is no photonic Kerr nonlinearity in the scheme, photon blockade
can also occur simultaneously with magnon blockade. This result is arising from
the PT-symmetric phase which can generate an effective photonic Kerr
nonlinearity.",2209.04228v1
2023-01-22,Magnon bundle in a strongly dissipative magnet,"Hybrid quantum systems based on magnetic platforms have witnessed the birth
and fast development of quantum spintronics. Until now, most of the studies
rely on magnetic excitations in low-damping magnetic insulators, particularly
yttrium iron garnet, while a large class of magnetic systems is ruled out in
this interdisciplinary field. Here we propose the generation of a magnon bundle
in a hybrid magnet-qubit system, where two or more magnons are emitted
simultaneously. By tuning the driving frequency of qubit to match the detuning
between magnon and qubit mode, one can effectively generate a magnon bundle via
super-Rabi oscillations. In contrast with general wisdom, magnetic dissipation
plays an enabling role in generating the magnon bundle, where the relaxation
time of magnons determines the typical time delay between two successive
magnons. The maximal damping that allows an antibunched magnon bundle can reach
the order of 0.1, which may break the monopoly of low-dissipation magnetic
insulators in quantum spintronics and enables a large class of magnetic
materials for quantum manipulation. Further, our finding may provide a scalable
and generic platform to study multi-magnon physics and benefit the design of
magnonic networks for quantum information processing.",2301.09095v1
2023-10-13,Magnon spin capacitor,"In this work we show that a magnon spin capacitor can be realized at a
junction between two exchange coupled ferromagnets. In this junction, the
buildup of magnon spin over the junction is coupled to the difference in magnon
chemical potential, realizing the magnon spin analogue of an electrical
capacitor. The relation between magnon spin and magnon chemical potential
difference directly follows from considering the magnon density-density
interaction between the two ferromagnets. We analyse the junction in detail by
considering spin injection and detection from normal metal leads, the tunneling
current across the junction and magnon decay within the ferromagnet, showing
that such a structure realizes a magnon spin capacitor in series with a spin
resistor. Choosing yttrium iron garnet as the ferromagnet, we numerically
calculate the capacitance, which ranges from picofarad to microfarad, depending
on the area of the junction. We therefore conclude that the magnon spin
capacitor could directly be of use in applications.",2310.09064v3
2024-03-27,Extreme Terahertz Magnon Multiplication Induced by Resonant Magnetic Pulse Pairs,"Nonlinear interactions of spin-waves and their quanta, magnons, have emerged
as prominent candidates for interference-based technology, ranging from quantum
transduction to antiferromagnetic spintronics. Yet magnon multiplication in the
terahertz (THz) spectral region represents a major challenge. Intense, resonant
magnetic fields from THz pulse-pairs with controllable phases and amplitudes
enable high order THz magnon multiplication, distinct from non-resonant
nonlinearities such as the high harmonic generation by below-band gap electric
fields. Here, we demonstrate exceptionally high-order THz nonlinear magnonics.
It manifests as 7$^\text{th}$-order spin-wave-mixing and 6$^\text{th}$ harmonic
magnon generation in an antiferromagnetic orthoferrite. We use THz
multi-dimensional coherent spectroscopy to achieve high-sensitivity detection
of nonlinear magnon interactions up to six-magnon quanta in strongly-driven
many-magnon correlated states. The high-order magnon multiplication, supported
by classical and quantum spin simulations, elucidates the significance of
four-fold magnetic anisotropy and Dzyaloshinskii-Moriya symmetry breaking.
Moreover, our results shed light on the potential quantum fluctuation
properties inherent in nonlinear magnons.",2403.18168v1
1999-05-11,Magnon Broadening Effect by Magnon-Phonon Interaction in Colossal Magnetoresistance Manganites,"In order to study the magnetic excitation behaviors in colossal
magnetoresistance manganites, a magnon-phonon interacting system is
investigated. Sudden broadening of magnon linewidth is obtained when a magnon
branch crosses over an optical phonon branch. Onset of the broadening is
approximately determined by the magnon density of states. Anomalous magnon
damping at the brillouine zone boundary observed in low Curie temperature
manganites is explained.",9905133v2
2005-11-11,Spin wave spectrum of a magnonic crystal with an isolated defect,"Real magnonic crystals - periodic magnetic media for spin wave (magnon)
propagation - may contain some defects. We report theoretical spin wave spectra
of a one dimensional magnonic crystal with an isolated defect. The latter is
modeled by insertion of an additional layer with thickness and magnetic
anisotropy values different from those of the magnonic crystal constituent
layers. The defect layer leads to appearance of several localized defect modes
within the magnonic band gaps. The frequency and the number of the defect modes
may be controlled by varying parameters of the constituent layers of the
magnonic crystal.",0511295v1
2007-11-10,Stability of Bose Einstein condensates of hot magnons in YIG,"We investigate the stability of the recently discovered room temperature
Bose-Einstein condensate (BEC) of magnons in Ytrrium Iron Garnet (YIG) films.
We show that magnon-magnon interactions depend strongly on the external field
orientation, and that the BEC in current experiments is actually metastable -
it only survives because of finite size effects, and because the BEC density is
very low. On the other hand a strong field applied perpendicular to the sample
plane leads to a repulsive magnon-magnon interaction; we predict that a
high-density magnon BEC can then be formed in this perpendicular field
geometry.",0711.1574v1
2008-03-19,Magnetic field-induced one-magnon Raman scattering in the magnon Bose-Einstein condensation phase of TlCuCl$_{3}$,"We report the observation of the $A_{\rm g}$-symmetric one-magnon Raman peak
in the magnon Bose-Einstein condensation phase of TlCuCl$_{3}$. Its Raman shift
traces the one-magnon energy at the magnetic $\Gamma$ point, and its intensity
is proportional to the squared transverse magnetization. The appearance of the
one-magnon Raman scattering originates from the exchange magnon Raman process
and reflects the change of the magnetic-state symmetry. Using the bond-operator
representation, we theoretically clarify the Raman selection rules, being
consistent with the experimental results.",0803.2815v1
2021-06-23,Stimulated-Raman-Adiabatic-Passage mechanism in a magnonic environment,"We discuss the realization of a magnonic version of the
STImulated-Raman-Adiabatic-Passage (m-STIRAP) mechanism using micromagnetic
simulations. We consider the propagation of magnons in curved magnonic
directional couplers. Our results demonstrate that quantum-classical analogy
phenomena are accessible in magnonics. Specifically, the inherent advantages of
the STIRAP mechanism, associated with dark states, can now be utilized in
magnonics. Applications of this effect for future magnonic device
functionalities and designs are discussed.",2106.12318v1
2022-08-01,Calculation of magnon drag force induced by an electric current in ferromagnetic metals,"Magnon drag effect induced by an applied electric field in ferromagnetic
metals is theoretically studied by a microscopic calculation of the force on
magnons arising from magnon emission/absorption and scattering due to driven
electrons. It is shown that magnon scattering contribution dominates over the
emission/absorption one in a wide temperature regime in good metals with long
elastic lifetime $\taue$, as the latter has a relative suppression factor of
$(\Deltasd\taue)^{-2}$ due to the electron spin flip by the magnon, where
$\Deltasd$ is the $sd$ exchange interaction energy. Spin-transfer efficiency is
discussed including the magnon drag effect.",2208.00622v1
2004-12-14,Current-induced magnetization changes in a spin valve due to incoherent emission of non-equilibrium magnons,"We describe spin transfer in a ferromagnet/normal metal/ferromagnet
spin-valve point contact. Spin is transferred from the spin-polarized device
current to the magnetization of the free layer by the mechanism of incoherent
magnon emission by electrons. Our approach is based on the rate equation for
the magnon occupation, using Fermi's golden rule for magnon emission and
absorption and the non-equilibrium electron distribution for a biased spin
valve. The magnon emission reduces the magnetization of the free layer. For
anti-parallel alignment of the magnetizations of the layers and at a critical
bias a magnon avalanche occurs, characterized by a diverging effective magnon
temperature. This critical behavior can result in magnetization reversal and
consequently to suppression of magnon emission. However, magnon-magnon
scattering can lead to saturation of the magnon concentration at a large but
finite value. The further behavior depends on the parameters of the system. In
particular, gradual evolution of the magnon concentration followed by a
magnetization reversal is possible. Another scenario corresponds to a step-like
increase of the magnon concentration followed by a slow decrease. In the latter
case a spike in the differential resistance is expected due to a contribution
of electron-magnon scattering. A comparison of the obtained results to existing
experimental data and theoretical approches is given.",0412348v1
2023-12-26,Direct observation of topological magnon polarons in a multiferroic material,"Magnon polarons are novel elementary excitations possessing hybrid magnonic
and phononic signatures, and are responsible for many exotic spintronic and
magnonic phenomena. Despite long-term sustained experimental efforts in chasing
for magnon polarons, direct spectroscopic evidence of their existence is hardly
observed. Here, we report the direct observation of magnon polarons using
neutron spectroscopy on a multiferroic Fe$_{2}$Mo$_{3}$O$_{8}$ possessing
strong magnon-phonon coupling. Specifically, below the magnetic ordering
temperature, a gap opens at the nominal intersection of the original magnon and
phonon bands, leading to two separated magnon-polaron bands. Each of the bands
undergoes mixing, interconverting and reversing between its magnonic and
phononic components. We attribute the formation of magnon polarons to the
strong magnon-phonon coupling induced by Dzyaloshinskii-Moriya interaction.
Intriguingly, we find that the band-inverted magnon polarons are topologically
nontrivial. These results uncover exotic elementary excitations arising from
the magnon-phonon coupling, and offer a new route to topological states by
considering hybridizations between different types of fundamental excitations.",2312.15943v1
2014-07-12,Role of dimensionality in spontaneous magnon decay: easy-plane ferromagnet,"We calculate magnon lifetime in an easy-plane ferromagnet on a tetragonal
lattice in transverse magnetic field. At zero temperature magnons are unstable
with respect to spontaneous decay into two other magnons. Varying ratio of
intrachain to interchain exchanges in this model we consider the effect of
dimensionality on spontaneous magnon decay. The strongest magnon damping is
found in the quasi-one-dimensional case for momenta near the Brillouin zone
boundary. The sign of a weak interchain coupling has a little effect on the
magnon decay rate. The obtained theoretical results suggest possibility of
experimental observation of spontaneous magnon decay in a quasi-one-dimensional
ferromagnet CsNiF$_3$. We also find an interesting enhancement of the magnon
decay rate for a three-dimensional ferromagnet. The effect is present only for
the nearest-neighbor model and is related to effective dimensionality reduction
in the two-magnon continuum.",1407.3402v1
2016-06-09,Integer Quantum Magnon Hall Plateau-Plateau Transition in a Spin Ice Model,"Low-energy magnon bands in a two-dimensional spin ice model become integer
quantum magnon Hall bands. By calculating the localization length and the
two-terminal conductance of magnon transport, we show that the magnon bands
with disorders undergo a quantum phase transition from an integer quantum
magnon Hall regime to a conventional magnon localized regime. Finite size
scaling analysis as well as a critical conductance distribution shows that the
quantum critical point belongs to the same universality class as that in the
quantum Hall transition. We characterize thermal magnon Hall conductivity in
disordered quantum magnon Hall system in terms of robust chiral edge magnon
transport.",1606.02839v2
2016-05-13,Nonlocal Drag of Magnons in a Ferromagnetic Bilayer,"Quantized spin waves, or magnons, in a magnetic insulator are assumed to
interact weakly with the surroundings, and to flow with little dissipation or
drag, producing exceptionally long diffusion lengths and relaxation times. In
analogy to Coulomb drag in bilayer two dimensional electron gases, in which the
contribution of the Coulomb interaction to the electric resistivity is studied
by measuring the interlayer resistivity (transresistivity), we predict a
nonlocal drag of magnons in a ferromagnetic bilayer structure based on
semiclassical Boltzmann equations. Nonlocal magnon drag depends on magnetic
dipolar interactions between the layers and manifests in the magnon current
transresistivity and the magnon thermal transresistivity, whereby a magnon
current in one layer induces a chemical potential gradient and/or a temperature
gradient in the other layer. The largest drag effect occurs when the magnon
current flows parallel to the magnetization, however for oblique magnon
currents a large transverse current of magnons emerges. We examine the effect
for practical parameters, and find that the predicted induced temperature
gradient is readily observable.",1605.04336v1
2020-12-22,Dirac magnons pairing via pumping,"We study pumping of magnons to the Dirac points of magnon's Brillouin zone of
a ferromagnet on a honeycomb lattice. In particular, we consider second-order
Suhl process, when due to interaction between magnons, a pair of magnons is
created due to absorption of two electromagnetic wave quanta. We introduce a
bosonic analog of the Cooper ladder for the magnon pair, which is shown to
enhance the pairing of magnons at the Dirac points. As a result of pairing of
the Dirac magnons, the system becomes unstable towards formation of a magnetic
state with zero or reduced magnetization - the Dirac magnon paired state. In
this case the resonant frequency of the pump equals to that of energy of the
Dirac points. Our estimates suggest that the Dirac magnon paired state can be
found in the CrBr$_{3}$ or CrCl$_{3}$ ferromagnet below in vicinity of the
Curie temperature.",2012.12051v4
2013-06-24,Magnon Energy Renormalization and Low-Temperature Thermodynamics of O(3) Heisenberg Ferromagnets,"We present the perturbation theory for lattice magnon fields of
$D$-dimensional O(3) Heisenberg ferromagnet. The effective Hamiltonian for the
lattice magnon fields is obtained starting from the effective Lagrangian, with
two dominant contributions that describe magnon-magnon interactions identified
as a usual gradient term for the unit vector field and a part originating in
the Wess-Zumino-Witten term of effective Lagrangian. Feynman diagrams for
lattice scalar fields with derivative couplings are introduced, on basis of
which we investigate the influence of magnon-magnon interactions on magnon
self-energy and ferromagnet free energy. We also comment appearance of spurious
terms in the low-temperature series for the free energy by examining
magnon-magnon interactions and internal symmetry of the effective Hamiltonian
(Lagrangian).",1306.5584v1
2020-01-30,Magnon-phonon interactions in magnon spintronics,"Nowadays, the interaction between phonon and magnon subsystems of a magnetic
medium is a hot topic of research. The complexity of phonon and magnon spectra,
the existence of both bulk and surface modes, the quantization effects, and the
dependence of magnon properties on applied magnetic field, make this field very
complex and intriguing. Moreover, the recent advances in the fields of
spin-caloritronics and magnon spintronics as well as the observation of the
spin Seebeck effect (SSE) in magnetic insulators points on the crucial role of
magnons in spin-caloric transport processes. In this review, we collect the
variety of different studies in which magnon-phonon interaction play important
role. The scope of the paper covers the wide range of phenomena starting from
the interaction of the coherent magnons with surface acoustic wave and
finishing with the formation of magnon supercurrents in the thermal gradients.",2001.11447v1
2021-03-11,Hybrid topological magnon-phonon modes in honeycomb and kagome lattices,"Magnons and phonons are two fundamental neutral excitations of magnetically
ordered materials which can significantly dominate the low-energy thermal
properties. In this work we study the interplay of magnons and phonons in
honeycomb and Kagome lattices. When the mirror reflection with respect to the
magnetic ordering direction is broken, the symmetry-allowed in-plane
Dzyaloshinskii-Moriya (DM) interaction will couple the magnons to the phonons
and the magnon-polaron states are formed. Besides, both lattice structures also
allow for an out-of-plane DM interaction rendering the uncoupled magnons to be
topological. Our aim is to study the interplay of such topological magnons with
phonons. We show that the hybridization between magnons and phonons can
significantly redistribute the Berry curvature among the bands. Especially, we
found that the topological magnon band becomes trivial while the hybridized
states at lower energy acquire Berry curvature strongly peaked near the avoided
crossings. As such the thermal Hall conductivity of topological magnons shows
significant changes due to coupling to the phonons.",2103.06959v2
2021-06-24,Control of nonlocal magnon spin transport via magnon drift currents,"Spin transport via magnon diffusion in magnetic insulators is important for a
broad range of spin-based phenomena and devices. However, the absence of the
magnon equivalent of an electric force is a bottleneck. In this work, we
demonstrate the controlled generation of magnon drift currents in yttrium iron
garnet/platinum heterostructures. By performing electrical injection and
detection of incoherent magnons, we find magnon drift currents that stem from
the interfacial Dzyaloshinskii-Moriya interaction. We can further control the
magnon drift by the orientation of the magnetic field. The drift current
changes the magnon propagation length by up to $\pm$ 6 % relative to diffusion.
We generalize the magnonic spin transport theory to include a finite drift
velocity resulting from any inversion asymmetric interaction, and obtain
results consistent with our experiments.",2106.12946v1
2021-09-02,Enhanced magnon spin current using the bosonic Klein paradox,"Efficient manipulation of magnons for information processing is a central
topic in spintronics and magnonics. An outstanding challenge for long-distance
spin transport with minimal dissipation is to overcome the relaxation of
magnons and to amplify the spin current they carry. Here, we propose to amplify
magnon currents based on the realization of the bosonic Klein paradox in
magnetic nanostructures. This paradox involves the magnon's antiparticle, the
antimagnon, of which the existence is usually precluded by magnetic
instabilities as it is an excitation at negative energy. We show that, by
appropriately tuning the effective dissipation through spin-orbit torques, both
magnons and antimagnons are dynamically stabilized. As a result, we find that
the reflection coefficient of incident magnons at an interface between two
coupled magnets can become larger than one, thereby amplifying the reflected
magnon current. Our findings can lead to magnon amplifier devices for
spintronic applications. Furthermore, our findings yield a solid-state platform
to study the relativistic behavior of bosonic particles, which is an
outstanding challenge with fundamental particles.",2109.00865v3
2022-01-17,Pure dephasing of magnonic quantum states,"For a wide range of nonclassical magnonic states that have been proposed and
demonstrated recently, a new time scale besides the magnon lifetime - the
magnon dephasing time - becomes important, but this time scale is rarely
studied. Considering exchange interaction and spin-phonon coupling, we evaluate
the pure magnon dephasing time and find it to be smaller than the magnon
lifetime at temperatures of a few Kelvins. By examining a magnonic cat state as
an example, we show how pure dephasing of magnons destroys and limits the
survival of quantum superpositions. Thus it will be critical to perform quantum
operations within the pure dephasing time. We further derive the master
equation for the density matrix describing such magnonic quantum states taking
into account the role of pure dephasing, whose methodology can be generalized
to include additional dephasing channels that experiments are likely to
encounter in the future. Our findings enable one to design and manipulate
robust quantum states of magnons for information processing.",2201.06637v1
2022-03-07,Magnetoelectric Cavity Magnonics in Skyrmion Crystals,"We present a theory of magnetoelectric magnon-photon coupling in cavities
hosting noncentrosymmetric magnets. Analogously to nonreciprocal phenomena in
multiferroics, the magnetoelectric coupling is time-reversal and inversion
asymmetric. This asymmetry establishes a means for exceptional tunability of
magnon-photon coupling, which can be switched on and off by reversing the
magnetization direction. Taking the multiferroic skyrmion-host Cu$_2$OSeO$_3$
with ultralow magnetic damping as an example, we reveal the electrical activity
of skyrmion eigenmodes and propose it for magnon-photon splitting of
``magnetically dark'' elliptic modes. Furthermore, we predict a cavity-induced
magnon-magnon coupling between magnetoelectrically active skyrmion excitations.
We discuss applications in quantum information processing by proposing
protocols for all-electrical magnon-mediated photon quantum gates, and a
photon-mediated SPLIT operation of magnons. Our study highlights
magnetoelectric cavity magnonics as a novel platform for realizing
quantum-hybrid systems and the coherent transduction between photons and
magnons in topological magnetic textures.",2203.03241v2
2022-04-25,Oscillations and confluence in three-magnon scattering of ferromagnetic resonance,"We have performed a time-resolved and phase-sensitive investigation of
three-magnon scattering of ferromagnetic resonance (FMR) over several orders of
magnitude in excitation power. We observe a regime that hosts transient
oscillations of the FMR magnon population, despite higher-order magnon
interactions at large powers. Also at high powers, the scattering generates
$180^\circ$ phase shifts of the FMR magnons. These phase shifts correspond to
reversals in the three-magnon scattering direction, between splitting and
confluence. These scattering reversals are most directly observed after
removing the microwave excitation, generating coherent oscillations of the FMR
magnon population much larger than its steady-state value during the
excitation. Our model is in strong agreement with these findings. These
findings reveal the transient behavior of this three-magnon scattering process,
and the nontrivial interplay between three-magnon scattering and the magnons'
phases.",2204.11969v4
2022-05-06,Interband magnon drag in ferrimagnetic insulators,"We propose a new drag phenomenon, an interband magnon drag, and report on
interaction effects and multiband effects in magnon transport of ferrimagnetic
insulators. We study a spin-Seebeck coefficient $S_{\textrm{m}}$, a magnon
conductivity $\sigma_{\textrm{m}}$, and a magnon thermal conductivity
$\kappa_{\textrm{m}}$ of interacting magnons for a minimal model of
ferrimagnetic insulators using a $1/S$ expansion of the Holstein-Primakoff
method, the linear-response theory, and a method of Green's functions. We show
that the interband magnon drag enhances $\sigma_{\textrm{m}}$ and reduces
$\kappa_{\textrm{m}}$, whereas its total effects on $S_{\textrm{m}}$ are small.
This drag results from the interband momentum transfer induced by the
magnon-magnon interactions. We also show that the higher-energy band magnons
contribute to $S_{\textrm{m}}$, $\sigma_{\textrm{m}}$, and
$\kappa_{\textrm{m}}$ even for temperatures smaller than the energy difference
between the two bands.",2205.03058v1
2022-09-02,Tunable phonon-driven magnon-magnon entanglement at room temperature,"We report the existence of entangled steady-states in bipartite quantum
magnonic systems at elevated temperatures. We consider dissipative dynamics of
two magnon modes in a bipartite antiferromagnet, subjected to interaction with
a phonon mode and an external rotating magnetic field. To quantify the
bipartite magnon-magnon entanglement, we use entanglement negativity and
compute its dependence on temperature and magnetic field. We provide evidence
that the coupling between magnon and phonon modes is necessary for the
entanglement, and that, for any given phonon frequency and magnon-phonon
coupling rate, there are always ranges of the magnetic field amplitudes and
frequencies for which magnon-magnon entanglement persists at room temperature.",2209.01032v2
2023-01-27,Tunable Strong Magnon-Magnon Coupling in Two-Dimensional Array of Diamond Shaped Ferromagnetic Nanodots,"Hybrid magnonics involving coupling between magnons and different quantum
particles have been extensively studied during past few years for varied
interests including quantum electrodynamics. In such systems, magnons in
magnetic materials with high spin density are utilized where the coupling
strength is collectively enhanced by the square root of the number of spins to
overcome the weaker coupling between individual spins and the microwave field.
However, achievement of strong magnon-magnon coupling in a confined nanomagnets
would be essential for on-chip integration of such hybrid systems. Here,
through intensive study of interaction between different magnon modes in a
Ni80Fe20 (Py) nanodot array, we demonstrate that the intermodal coupling can
approach the strong coupling regime with coupling strength up to 0.82 GHz and
cooperativity of 2.51. Micromagnetic simulations reveal that the intermodal
coupling is mediated by the exchange field inside each nanodot. The coupling
strength could be continuously tuned by varying the bias field strength and
orientation, opening routes for external control over hybrid magnonic systems.
These findings could greatly enrich the rapidly evolving field of quantum
magnonics.",2301.11583v1
2023-02-21,Adiabatic magnon spectra with and without constraining field: Benchmark against an exact magnon spectrum,"The spectrum of magnon excitations in magnetic materials can be obtained
exactly from the transverse dynamic magnetic susceptibility, which is however
in practice numerically expensive. Many ab initio approaches therefore consider
instead the adiabatic magnon spectrum, which assumes a separation of time
scales of magnons and electronic excitations. There exist two alternative
implementations for adiabatic magnon spectra: one based on the magnetic force
theorem (MFT) and the other with a constraining field that enforces static
non-collinear spin configurations. We benchmark both implementations against
the exact magnon spectrum of an exactly solvable mean-field model. While both
adiabatic methods are equally valid in the low magnon energy and strong Stoner
coupling limits, we find that the constraining field method performs better
than the MFT in both the cases of strong Stoner coupling and high magnon
energies,while the MFT performs better for combined weak coupling and low
magnon energies.",2302.10958v1
2023-05-05,Nonreciprocal entanglement in cavity-magnon optomechanics,"Cavity optomechanics, a promising platform to investigate macroscopic quantum
effects, has been widely used to study nonreciprocal entanglement with Sagnec
effect. Here we propose an alternative way to realize nonreciprocal
entanglemment among magnons, photons, and phonons in a hybrid cavity-magnon
optomechanics, where magnon Kerr effect is used. We show that the Kerr effect
gives rise to a magnon frequency shift and an additional two-magnon effect.
Both of them can be tuned from positive to negative via tuning the magectic
field direction, leading to nonreciprocity. By tuning system parameters such as
magnon frequency detuning or the coefficient of the two-magnon effect,
bipartite and tripartite entanglements can be nonreciprocally enhanced. By
further studying the defined bidirectional contrast ratio, we find that
nonreciprocity in our system can be switch on and off, and can be engineered by
the bath temperature. Our proposal not only provides a potential path to
demonstrate nonreciprocal entanglement with the magnon Kerr effect, but also
opens a direction to engineer and design diverse nonreciprocal devices in
hybrid cavity-magnon optomechanics with nonlinear effects.",2305.03325v3
2023-09-08,Programmable Real-Time Magnon Interference in Two Remotely Coupled Magnonic Resonators,"Magnon interference is a signature of coherent magnon interactions for
coherent information processing. In this work, we demonstrate programmable
real-time magnon interference, with examples of nearly perfect constructive and
destructive interference, between two remotely coupled yttrium iron garnet
spheres mediated by a coplanar superconducting resonator. Exciting one of the
coupled resonators by injecting single- and double-microwave pulse leads to the
coherent energy exchange between the remote magnonic resonators and allows us
to realize a programmable magnon interference that can define an arbitrary
state of coupled magnon oscillation. The demonstration of time-domain coherent
control of remotely coupled magnon dynamics offers new avenues for advancing
coherent information processing with circuit-integrated hybrid magnonic
networks.",2309.04289v1
2024-01-19,Hybridized magnonic materials for THz frequency applications,"The capability of magnons to hybridize and strongly couple with diverse
excitations offers a promising avenue for realizing and controlling emergent
properties that hold significant potential for applications in devices,
circuits, and information processing. In this letter, we present recent
theoretical and experimental developments in magnon-based hybrid systems,
focusing on the combination of magnon excitation in an antiferromagnet with
other excitations, namely plasmons in a topological insulator, phonons in a 2D
AFM, and photons. The existence of THz frequency magnons, plasmons, and phonons
makes magnon-based hybrid systems particularly appealing for
high-operating-speed devices. In this context, we explore several directions to
advance magnon hybrid systems, including strong coupling between a surface
plasmon and magnon polariton in a TI/AFM bilayer, a giant spin Nernst effect
induced by magnon phonon coupling in 2D AFMs, and control of magnon-photon
coupling using spin torque.",2401.11010v2
2024-03-01,Minority magnons and mode branching in monolayer Fe$_3$GeTe$_2$,"In this letter, we predict the existence of minority magnons in a monolayer
of Fe$_3$GeTe$_2$ using first principles calculations. Minority magnons
constitute a new type of collective magnetic excitations which increase the
magnetic moment rather than lower it, contrary to ordinary (majority) magnons.
The presence of such quasi-particles is made possible by the nontrivial
ferromagnetic band structure of Fe$_3$GeTe$_2$ originating from its
nonequivalent Fe sublattices. The result is a strong peak in the dynamic
spin-raising susceptibility $\chi^{-+}(\omega)$ in the long wavelength limit,
which is the hallmark of minority magnon physics. We calculate the
susceptibility using time-dependent density functional theory and perform a
detailed mode analysis, which allows us to separate and investigate the
individual magnon modes as well as the Stoner excitations that constitute the
many-body spectrum. For minority as well as majority magnons, the analysis
reveals a plethora of magnetic excitations, which in addition to the standard
main magnon branches include both satellite, valley and spin-inversion magnons,
originating from the itinerancy of the ferromagnetic order. The physics
underlying this analysis is in no way restricted to Fe$_3$GeTe$_2$, and
minority magnons are expected to be observable in many complex ferromagnetic
materials.",2403.00525v1
2019-12-01,Differences in the magnon diffusion length for electrically and thermally driven magnon currents in Y$_3$Fe$_5$O$_{12}$,"Recent demonstration of efficient transport and manipulation of spin
information by magnon currents have opened exciting prospects for processing
information in devices. Magnon currents can be driven both electrically and
thermally, even in magnetic insulators, by applying charge currents in an
adjacent metal layer. Earlier reports in thin yttrium iron garnet (YIG) films
suggested that the diffusion length of magnons is independent on the biasing
method, but different values were obtained in thicker films. Here, we study the
magnon diffusion length for electrically and thermally driven magnon currents
in the linear regime in a 2-$\mu$m-thick YIG film as a function of temperature
and magnetic field. Our results show a decrease of the magnon diffusion length
with magnetic field for both biasing methods and at all temperatures from 5 to
300 K, indicating that sub-thermal magnons dominate the long-range transport.
Moreover, we demonstrate that the value of the magnon diffusion length depends
on the driving mechanism, suggesting that different non-equilibrium magnon
distributions are biased for each method. Finally, we demonstrate that the
magnon diffusion length for thermally driven magnon currents is independent of
the YIG thickness and material growth conditions, confirming that this quantity
is an intrinsic parameter of YIG.",1912.00490v2
2018-01-10,Floquet Weyl Magnons,"In three-dimensional (3D) quantum magnets, magnonic Weyl points (WPs)
featuring linear band crossing of two non-degenerate magnon branches can emerge
in certain lattice geometry when time-reversal symmetry is broken
macroscopically. Unfortunately, there are very limited 3D quantum magnets that
host magnonic WPs, and they are yet to be observed experimentally because the
intrinsic perturbative interactions that break time-reversal symmetry
macroscopically can be very negligible. Here, we present an alternative means
via photo-irradiation, in which magnonic WPs can emerge in 3D quantum magnets
without relying on intrinsic perturbative interactions to break time-reversal
symmetry. By utilizing the magnonic Floquet-Bloch theory, we put forward the
general theory of magnonic Floquet WPs in 3D quantum magnets. We show that
periodically driven 3D magnonic Dirac nodal-line (DNL) and 3D magnonic gapped
trivial insulators can generate 3D magnonic Floquet WPs, which can be tuned by
the incident circularly-polarized light. We demonstrate the existence of
magnonic Floquet WPs by combining the study of the magnon dispersions, Berry
curvatures, and the anomalous thermal Hall effect. The general theoretical
formalism can be applied to different magnetic insulators, and thus extending
the concept of magnonic WPs to a broader class of 3D magnetically ordered
systems.",1801.03499v6
2023-01-13,Voltage-Controlled Magnon Transistor via Tunning Interfacial Exchange Coupling,"Magnon transistors that can effectively regulate magnon transport by an
electric field are desired for magnonics which aims to provide a Joule-heating
free alternative to the conventional electronics owing to the electric
neutrality of magnons (the key carriers of spin-angular momenta in the
magnonics). However, also due to their electric neutrality, magnons have no
access to directly interact with an electric field and it is thus difficult to
manipulate magnon transport by voltages straightforwardly. Here, we
demonstrated a gate voltage ($V_{\rm g}$) applied on a nonmagnetic
metal/magnetic insulator (NM/MI) interface that bended the energy band of the
MI and then modulated the possibility for conduction electrons in the NM to
tunnel into the MI can consequently enhance or weaken the spin-magnon
conversion efficiency at the interface. A voltage-controlled magnon transistor
based on the magnon-mediated electric current drag (MECD) effect in a
Pt/Y$_{\rm 3}$Fe$_{\rm 5}$O$_{\rm 12}$ (YIG)/Pt sandwich was then
experimentally realized with $V_{\rm g}$ modulating the magnitude of the MECD
signal. The obtained efficiency (the change ratio between the MECD voltage at
$\pm V_{\rm g}$) reached 10%/(MV/cm) at 300 K. This prototype of magnon
transistor offers an effective scheme to control magnon transport by a gate
voltage.",2301.05592v1
2023-05-14,Cavity magnonics with easy-axis ferromagnet: Critically enhanced magnon squeezing and light-matter interaction,"Generating and probing the magnon squeezing is an important challenge in the
field of quantum magnonics. In this work, we propose a cavity magnonics setup
with an easy-axis ferromagnet to address this challenge. To this end, we first
establish a mechanism for the generation of magnon squeezing in the easy-axis
ferromagnet and show that the magnon squeezing can be critically enhanced by
tuning an external magnetic field near the Ising phase transition point. When
the magnet is coupled to the cavity field, the effective cavity-magnon
interaction becomes proportional to the magnon squeezing, allowing one to
enhance the cavity-magnon coupling strength using a static field. We
demonstrate that the magnon squeezing can be probed by measuring the frequency
shift of the cavity field. Moreover, a magnonic superradiant phase transition
can be observed in our setup by tuning the static magnetic field, overcoming
the challenge that the magnetic interaction between the cavity and the magnet
is typically too weak to drive the superradiant transition. Our work paves the
way to develop unique capabilities of cavity magnonics that goes beyond the
conventional cavity QED physics by harnessing the intrinsic property of a
magnet.",2305.08119v2
2023-08-06,Breakdown of Chiral Edge Modes in Topological Magnon Insulators,"Topological magnon insulators (TMI) are ordered magnets supporting chiral
edge magnon excitations. These edge states are envisioned to serve as
topologically protected information channels in low-loss magnonic devices. The
standard description of TMI is based on linear spin-wave theory (LSWT), which
approximates magnons as free non-interacting particles. However, magnon
excitations of TMI are genuinely interacting even at zero temperature, calling
into question descriptions based on LSWT alone. Here we perform a detailed
non-linear spin-wave analysis to investigate the stability of chiral edge
magnons. We identify three general breakdown mechanisms: (1) The edge magnon
couples to itself, generating a finite lifetime that can be large enough to
lead to a spectral annihilation of the chiral state; (2) The edge magnon
hybridizes with the extended bulk magnons and, as a consequence, delocalizes
away from the edge; (3) Due to a bulk-magnon mediated edge-to-edge coupling,
the chiral magnons at opposite edges hybridize. We argue that, in general,
these breakdown mechanisms may invalidate predictions based on LSWT and violate
the notion of topological protection. We discuss strategies how the breakdown
of chiral edge magnons can be avoided, e.g. via the application of large
magnetic fields. Our results highlight a challenge for the realization of
chiral edge states in TMI and in other bosonic topological systems without
particle number conservation.",2308.03168v1
2024-02-29,Magnon spectrum of altermagnets: Time-dependent matrix product states vs. linearized Holstein-Primakoff calculations unravelling spontaneous magnon decay,"The energy-momentum dispersion of magnons, viewed as noninteracting and
infinitely long-lived quasiparticles describing collective low-energy
excitations of magnetic materials, is often presented as sharp bands obtained
from the effective quantum spin Hamiltonian, after being simplified via
linearized Holstein-Primakoff (HP) transformations. However, magnons are prone
to many-body interactions with other quasiparticles which can lead to their
spontaneous decay. The magnon-magnon interactions could affect newly classified
altermagnets. On the other hand, sharp bands of noninteracting chiral magnons
in RuO2, as the canonical example of altermagnets, have been very recently
predicted. Here, we employ nonperturbative numerically (quasi)exact quantum
many-body calculations, via time-dependent matrix product states (TDMPS), to
obtain magnon spectral function of RuO2. These calculations produce a broadened
magnon dispersion, which overlaps with linearized HP theory sharp bands only at
edges/center of the Brillouin zone. Substantially deviating otherwise.
Artificially making exchange interaction within two sublattices of RuO2 closer
in value forces these two spectra to overlap, thereby explaining the origin of
the failure of linearized HP theory. Such features translate into the
difference between their respective density of states, which we also compute
and which could be tested by Raman scattering experiments. Finally, we employ
popular Landau-Lifshitz-Gilbert (LLG) equation-based classical atomistic spin
dynamics (ASD) simulations to obtain dynamical structure factor and extract
magnon spectrum from it at finite temperature. Despite including magnon-magnon
interactions via nonlinearity of LLG equation, ASD simulations cannot fully
match the TDMPS-computed magnon spectrum due to nonclassical effects harbored
by altermagnets.",2402.19433v1
1999-05-24,Soliton-Magnon Scattering in Two-Dimensional Isotropic Ferromagnets,"It is studied the scattering of magnons by the 2d topological
Belavin-Polyakov soliton in isotropic ferromagnet. Analytical solutions of the
scattering problem are constructed: (i) exactly for any magnon wave vectors for
the partial wave with the azimuthal number m=1 (translational mode), and (ii)
in the long- and short-wave limits for the rest modes. The magnon mode
frequencies are found for the finite size magnets. An effective equation of the
soliton motion is constructed. The magnon density of states, connected with the
soliton-magnon interaction, is found in a long-wave approximation.",9905348v1
2006-08-07,On the Scattering of Magnon Boundstates,"We study the scattering of magnon boundstates in the spin-chain description
of planar N=4 SUSY Yang-Mills. Starting from the conjectured exact S-matrix for
magnons in the SU(2) sector, we calculate the corresponding S-matrix for
boundstates with an arbitrary number of constituent magnons. The resulting
expression has an interesting analytic structure with both simple and double
poles. We also calculate the semiclassical S-matrix for the scattering of the
corresponding excitations on the string worldsheet known as Dyonic Giant
Magnons. We find precise agreement with the magnon boundstate S-matrix in the
limit of large 't Hooft coupling.",0608047v1
2007-12-25,One-magnon (electromagnon) light scattering in BiFeO3 single crystals,"We observed Raman scattering from magnon in frequency range from 10 to 65
cm-1 in BiFeO3 single crystals at cryogenic temperatures; the temperature
dependence of the magnon frequency at 18.2 cm-1 approximates an S=5/2 Brillouin
function up to the temperature (280 K) at which the magnon becomes overdamped.
The diverging cross-section and the frequency-shift at 140K and 200 K implies a
magnon-reorientation transition as in orthoferrites. Magnons in polar materials
such as BiFeO3 are often termed electromagnons meaning that they possess an
electric dipole moment due to magnetoelectric coupling.",0712.4040v2
2008-09-30,Giant Magnon and Spike Solutions with Two Spins in AdS4xCP3,"In the string theory in AdS_4 \times CP^3 we construct the giant magnon and
spike solutions with two spins in two kinds of subspaces of R_t \times CP^3 and
derive the dispersion relations for them. For the single giant magnon solution
in one subspace we show that its dispersion relation is associated with that of
the big one-spin giant magnon solution in the RP^2 subspace. For the single
giant magnon solution in the other complementary subspace its dispersion
relation is similar to that of the one-spin giant magnon solution living in the
S^2 subspace but has one additional spin dependence.",0809.5106v1
2008-10-29,Finite size giant magnons and interactions,"Magnon interactions give important contributions to the wrapping interactions
of the N=4 spin-chain. Similar effects are expected for the finite size
corrections to the giant magnon energy in AdS_5 x S^5. In this paper I review
the finite gap description of giant magnons and the leading order calculation
of the finite size corrections to the giant magnon dispersion relation for
multi-magnon states.",0810.5236v2
2010-02-19,Strong Three-magnon Scattering in Cuprates by Resonant X-rays,"We show that Resonant Inelastic X-ray scattering (RIXS) is sensitive to
three-magnon excitations in cuprates. Even if it requires three electrons to
simultaneously flip their spin, the RIXS tri-magnon scattering amplitude is not
small. At the Cu $L$-edge its intensity is generally larger than the bi-magnon
one and at low transferred momentum even larger than the single-magnon
intensity. At the copper $M$-edge the situation is yet more extreme: in this
case three-magnon scattering is dominating over all other magnetic channels.",1002.3773v1
2011-01-11,Magnon Pumping by a Time-Dependent Transverse Magnetic Field in Ferromagnetic Insulators,"The magnon pumping effect in ferromagnetic insulators under an external
time-dependent transverse magnetic field is theoretically studied. Generation
of a magnon current is discussed by calculating the magnon source term in the
spin continuity equation. This term represents the non-conservation of magnons
arising from an applied transverse magnetic field. The magnon source term has a
resonance structure as a function of the angular frequency of the transverse
field, and this fact is useful to enhance the pumping effect.",1101.2137v2
2012-02-14,Magnon qubit on double Bose-Einstein condensate,"We have proposed a magnon qubit based on coupled configuration of
Bose-Einstein condensates (BEC) in two ferromagnetic samples placed closely to
each other. We have evaluated the magnon BEC qubit realization in the double
BEC scheme where we found a quantum synchronism condition providing an
effective Hamiltonian of magnon qubit. It has the form of well-known
superconducting Josephson qubit. The possibilities for coherent magnon BEC
qubit rotation are analyzed. Implementations of the magnon BEC qubit are
considered for small samples and thin ferromagnetic films. Advantages of the
proposed macroscopic qubit realization are discussed.",1202.3057v1
2015-02-14,Energy shift of magnons in a ferromagnetic spinor-dipolar Bose-Einstein condensate,"Motivated by the recent experiment performed by the Berkeley group [G. E.
Marti {\it et al.}, Phys. Rev. Lett. {\bf 113}, 155302 (2014)], we consider the
dynamics of magnons in a spin-1 spinor-dipolar Bose-Einstein condensate, using
mean-field theory. We show that the effective mass of a magnon is increased by
the magnetic dipole-dipole interaction, as observed in the experiment. The
magnon mass is also decreased by changing the direction of the magnetic field.
The increase and decrease in the magnon mass manifest themselves in the
acceleration of the magnons.",1502.04161v2
2015-04-28,Roton minimum as fingerprint of magnon-Higgs scattering in ordered quantum antiferromagnets,"A quantitative description of magnons in long-range ordered quantum
antiferromagnets is presented which is consistent from low to high energies. It
is illustrated for the generic $S=1/2$ Heisenberg model on the square lattice.
The approach is based on a continuous similarity transformation in momentum
space using the scaling dimension as truncation criterion. Evidence is found
for significant magnon-magnon attraction inducing a Higgs resonance. The
high-energy roton minimum in the magnon dispersion appears to be induced by
strong magnon-Higgs scattering.",1504.07371v2
2008-07-01,Finite size Giant Magnons in the string dual of N=6 superconformal Chern-Simons theory,"We find the exact solution for a finite size Giant Magnon in the $SU(2)\times
SU(2)$ sector of the string dual of the $\mathcal{N}=6$ superconformal
Chern-Simons theory recently constructed by Aharony, Bergman, Jafferis and
Maldacena. The finite size Giant Magnon solution consists of two magnons, one
in each $SU(2)$. In the infinite size limit this solution corresponds to the
Giant Magnon solution of arXiv:0806.4959. The magnon dispersion relation
exhibits finite-size exponential corrections with respect to the infinite size
limit solution.",0807.0205v2
2017-06-24,Magnon cotunneling through a quantum dot,"I consider a single-level quantum dot coupled to two reservoirs of spin waves
(magnons). Such systems have been studied recently from the point of view of
possible coupling between electronic and magnonic spin currents. However,
usually weakly coupled systems were investigated. When coupling between the dot
and reservoirs is not weak, then higher order processes play a role and have to
be included. Here I consider cotunneling of magnons through a spin-occupied
quantum dot, which can be understood as a magnon (spin) leakage current in
analogy to leakage currents in charge-based electronics. Particular emphasis
has been put on investigating the effect of magnetic field and temperature
difference between the magnonic reservoirs.",1706.07914v1
2015-08-21,Quantum magnonics: magnon meets superconducting qubit,"The techniques of microwave quantum optics are applied to collective spin
excitations in a macroscopic sphere of ferromagnetic insulator. We demonstrate,
in the single-magnon limit, strong coupling between a magnetostatic mode in the
sphere and a microwave cavity mode. Moreover, we introduce a superconducting
qubit in the cavity and couple the qubit with the magnon excitation via the
virtual photon excitation. We observe the magnon-vacuum-induced Rabi splitting.
The hybrid quantum system enables generation and characterization of
non-classical quantum states of magnons.",1508.05290v1
2015-08-27,Guided magnonic Michelson interferometer,"Magnonics is an emerging field with potential applications in classical and
quantum information processing. Freely propagating magnons in two-dimensional
media suffer from dispersion, which limits their effective range and fidelity.
We show the design of controllable magnonic circuitry, that utilise surface
current carrying wires to create magnonic waveguides. We also show the design
of a magnonic directional coupler and controllable Michelson interferometer to
demonstrate its utility for information processing tasks.",1508.06722v2
2020-09-14,Antiferromagnetic Dispersion Relations and Nature of Magnon Pressure,"We derive higher-order corrections in the magnon dispersion relations for
two- and three-dimensional antiferromagnets exposed to magnetic and staggered
fields that are mutually aligned. ""Dressing"" the magnons is the prerequisite to
separate the low-temperature representation of the pressure into a piece due to
noninteracting magnons and a piece that corresponds to the magnon-magnon
interaction. Both in two and three spatial dimensions, the interaction in the
pressure turns out to be attractive. While concrete figures refer to the
spin-$\frac{1}{2}$ square-lattice and the spin-$\frac{1}{2}$ simple cubic
lattice antiferromagnet, our results are valid for arbitrary bipartite
geometry.",2009.06584v1
2015-12-15,Magnon Dirac materials,"We demonstrate how a Dirac-like magnon spectrum is generated for localized
magnetic moments forming a two-dimensional honeycomb lattice. The Dirac
crossing point is proven to be robust against magnon-magnon interactions, as
these only shift the spectrum. Local defects induce impurity resonances near
the Dirac point, as well as magnon Friedel oscillations. The energy of the
Dirac point is controlled by the exchange coupling, and thus a two-dimensional
array of magnetic dots is an experimentally feasible realization of Dirac
magnons with tunable dispersion.",1512.04902v2
2020-08-31,Magnon-squeezing as a niche of quantum magnonics,"The spin excitations of ordered magnets - magnons - mediate transport in
magnetic insulators. Their bosonic nature makes them qualitatively distinct
from electrons. These features include quantum properties traditionally
realized with photons. In this perspective, we present an intuitive discussion
of one such phenomenon. Equilibrium magnon-squeezing manifests unique
advantageous with magnons as compared to photons, including properties such as
entanglement. Building upon the recent progress in the fields of spintronics
and quantum optics, we outline challenges and opportunities in this emerging
field of quantum magnonics.",2008.13536v1
2022-07-26,Rectification of the spin Seebeck current in noncollinear antiferromagnets,"In the absence of an external magnetic field and a spin-polarized charge
current, an antiferromagnetic system supports two degenerate magnon modes. An
applied thermal bias activates the magnetic dynamics, leading to a magnon flow
from the hot to the cold edge (magnonic spin Seebeck current). Both degenerate
bands contribute to the magnon current but the orientations of the magnetic
moments underlying the magnons are opposite in different bands. Therefore,
while the magnon current is nonzero, the net spin current is zero.",2207.12890v1
2022-08-30,Breaking down the magnonic Wiedemann-Franz law in the hydrodynamic regime,"Recent experiments have shown an indication of a hydrodynamic magnon behavior
in ultrapure ferromagnetic insulators; however, its direct observation is still
lacking. Here, we derive a set of coupled hydrodynamic equations and study the
thermal and spin conductivities for such a magnon fluid. We reveal the drastic
breakdown of the magnonic Wiedemann-Franz law as a hallmark of the
hydrodynamics regime, which will become key evidence for the experimental
realization of an emergent hydrodynamic magnon behavior. Therefore, our results
pave the way towards the direct observation of magnon fluids.",2208.14458v3
2023-05-11,Correlation-enhanced interaction of a Bose-Einstein condensate with parametric magnon pairs and virtual magnons,"Nonlinear interactions are crucial in science and engineering. Here, we
investigate wave interactions in a highly nonlinear magnetic system driven by
parametric pumping leading to Bose--Einstein condensation of spin-wave quanta
-- magnons. Using Brillouin light scattering spectroscopy in yttrium-iron
garnet films, we found and identified a set of nonlinear processes resulting in
off-resonant spin-wave excitations -- virtual magnons. In particular, we
discovered a dynamically-strong, correlation-enhanced four-wave interaction
process of the magnon condensate with pairs of parametric magnons having
opposite wavevectors and fully correlated phases.",2305.06896v1
2023-06-08,Tunable magnon topology in monolayer CrI$_\mathbf{3}$ under external stimuli,"Two-dimensional (2D) honeycomb ferromagnets, such as monolayer
chromium-trihalides, are predicted to behave as topological magnon insulators -
characterized by an insulating bulk and topologically protected edge states,
giving rise to a thermal magnon Hall effect. Here we report the behavior of the
topological magnons in monolayer CrI$_3$ under external stimuli, including
biaxial and uniaxial strain, electric gating, as well as in-plane and
out-of-plane magnetic field, revealing that one can thereby tailor the magnetic
states as well as the size and the topology of the magnonic bandgap. These
findings broaden the perspective of using 2D magnetic materials to design
topological magnonic devices.",2306.05104v1
2023-08-11,Achieving Strong Magnon Blockade through Magnon Squeezing in a Cavity Magnetomechanical System,"We propose a scheme to achieve magnon (photon) blockade by using magnon
squeezing within a cavity magnomechanical system under weak pump driving. Under
ideal conditions, we observe a substantial magnon blockade effect, as well as
simultaneous photon blockade. Moreover, both numerical and analytical results
match perfectly, providing robust evidence of consistency. In addition to
calculating optimal parametric gain and detuning values, we can improve the
second-order correlation function. The proposed scheme will be a pioneering
approach towards magnon (photon) blockade in experimental cavity
magnomechanical systems.",2308.06367v1
2023-12-19,Electrical Activity of Topological Chiral Edge Magnons,"Topological magnon insulators support chiral edge excitations, whose lack of
electric charge makes them notoriously difficult to detect experimentally. We
show that relativistic magnetoelectric coupling universally renders chiral edge
magnons electrically active, thereby facilitating electrical probes of magnon
topology. Considering a two-dimensional out-of-plane magnetized topological
magnon insulator, we predict a fluctuation-activated electric polarization
perpendicular to the sample edges. Furthermore, the chiral topological
electromagnons give rise to a unique in-gap signal in electrical absorption
experiments. These results suggest THz spectroscopy as a promising probe for
topological magnons.",2312.12316v1
2014-08-21,Magnonic band structure of domain wall magnonic crystals,"Magnonic crystals are prototype magnetic metamaterials designed for the
control of spin wave propagation. Conventional magnonic crystals are composed
of single domain elements. If magnetization textures, such as domain walls,
vortices and skyrmions, are included in the building blocks of magnonic
crystals, additional degrees of freedom over the control of the magnonic band
structure can be achieved. We theoretically investigate the influence of domain
walls on the spin wave propagation and the corresponding magnonic band
structure. It is found that the rotation of magnetization inside a domain wall
introduces a geometric vector potential for the spin wave excitation. The
corresponding Berry phase has quantized value $4 n_w \pi$, where $n_w$ is the
winding number of the domain wall. Due to the topological vector potential, the
magnonic band structure of magnonic crystals with domain walls as comprising
elements differs significantly from an identical magnonic crystal composed of
only magnetic domains. This difference can be utilized to realize dynamic
reconfiguration of magnonic band structure by a sole nucleation or annihilation
of domain walls in magnonic crystals.",1408.4889v3
2018-10-02,The Discrete Noise of Magnons,"Magnonics is a rapidly developing subfield of spintronics, which deals with
devices and circuits that utilize spin currents carried by magnons - quanta of
spin waves. Magnon current, i.e. spin waves, can be used for information
processing, sensing, and other applications. A possibility of using the
amplitude and phase of magnons for sending signals via electrical insulators
creates conditions for avoiding Ohmic losses, and achieving ultra-low power
dissipation. Most of the envisioned magnonic logic devices are based on spin
wave interference, where the minimum energy per operation is limited by the
noise level. The sensitivity and selectivity of magnonic sensors is also
limited by the low frequency noise. However, the fundamental question ""do
magnons make noise?"" has not been answered yet. It is not known how noisy
magnonic devices are compared to their electronic counterparts. Here we show
that the low-frequency noise of magnonic devices is dominated by the random
telegraph signal noise rather than 1/f noise - a striking contrast to
electronic devices (f is a frequency). We found that the noise level of surface
magnons depends strongly on the power level, increasing sharply at the on-set
of nonlinear dissipation. The presence of the random telegraph signal noise
indicates that the current fluctuations involve random discrete macro events.
We anticipate that our results will help in developing the next generation of
magnonic devices for information processing and sensing.",1810.01500v1
2021-12-07,Influence of low-energy magnons on magnon Hanle experiments in easy-plane antiferromagnets,"Antiferromagnetic materials host pairs of spin-up and spin-down magnons which
can be described in terms of a magnonic pseudospin. The close analogy between
this magnonic pseudospin systems and that of electronic charge carriers led to
the prediction of fascinating phenomena in antiferromagnets. Recently, the
associated dynamics of antiferromagnetic pseudospin has been experimentally
demonstrated and, in particular, the first observation of the magnon Hanle
effect has been reported. We here expand the magnonic spin transport
description by explicitly taking into account contributions of finite-spin
low-energy magnons. In our experiments we realize the spin injection and
detection process by two Platinum strips and investigate the influence of the
Pt-strips on the generation and diffusive transport of magnons in films of the
antiferromagnetic insulator hematite. For both a 15 nm and a 100 nm thick film,
we find a distinct signal caused by the magnon Hanle effect. However, the
magnonic spin signal exhibits clear differences in both films. In contrast to
the thin film, for the thicker one, we observe an oscillating behavior in the
high magnetic field range as well as an additional offset signal in the low
magnetic field regime. We attribute this offset signal to the presence of
finite-spin low-energy magnons.",2112.03820v1
2022-09-01,Magnon dynamics in parity-time-symmetric dipolarly coupled waveguides and magnonic crystals,"We consider the magnonic properties of two dipolarly coupled magnetic
stripes, both deposited on a normal conductive substrate with strong spin-orbit
coupling. A charge current in the substrate acts on the adjacent magnets with
spin-orbit torques, which result in magnonic damping or antidamping of the spin
waves, and hence a gain-loss coupling of the two magnetic stripes. The whole
setup is demonstrated to exhibit features typical for parity-time (PT)
symmetric systems. Phenomena are demonstrated that can be functionalized in
magnonic devices, including reconfigurable magnonic diodes and logic devices.
Alternative stripes designs and PT-symmetric, periodic, coupled magnonic
textures are studied. Analytical and full numerical analysis identify the
conditions for the appearance of exceptional points (EPs), where magnonic gain
and loss are balanced and evidence nonreciprocal magnon propagation and
enhanced magnon excitation around EPs. Furthermore, the dipolar coupling is
shown to bring in a wave vector-dependent PT-symmetric behavior. Proposing and
simulating a PT-symmetric magnonic crystal, we show how EPs and hence
associated phenomena can be steered to a particular wave vector in a gaped
spectrum via material design. The phenomena offer additional tools for
magnonic-based communication and computational devices.",2209.00180v1
2022-09-05,Nonlocal detection of interlayer three-magnon coupling,"A leading nonlinear effect in magnonics is the interaction that splits a
high-frequency magnon into two low-frequency ones with conserved linear
momentum. Here, we report experimental observation of nonlocal three-magnon
scattering between spatially separated magnetic systems, viz. a CoFeB nanowire
and an yttrium iron garnet (YIG) thin film. Above a certain threshold power of
an applied microwave field, a CoFeB Kittel magnon splits into a pair of
counter-propagating YIG magnons that induce voltage signals in Pt electrodes on
each side, in excellent agreement with model calculations based on the
interlayer dipolar interaction. The excited YIG magnon pairs reside mainly in
the first excited (n=1) perpdendicular standing spin-wave mode. With increasing
power, the n=1 magnons successively scatter into nodeless (n=0) magnons through
a four-magnon process. Our results help to assess non-local scattering
processes in magnonic circuits that may enable quantum entanglement between
distant magnons for quantum information applications.",2209.01875v1
2023-03-24,Magnon blockade in magnon-qubit systems,"A hybrid system established by the direct interaction between a magnon mode
and a superconducting transmon qubit is used to realize a high-degree blockade
for magnon. It is a fundamental way toward quantum manipulation at the level of
a single magnon and preparation of single magnon sources. Through weakly
driving the magnon and probing the qubit, our magnon-blockade proposal can be
optimized when the transversal coupling strength between the magnon and qubit
is equivalent to the detuning of the qubit and the probing field or that of the
magnon and the driving field. Under this condition, the equal-time second-order
correlation function $g^{(2)}(0)$ can be analytically minimized when the
probing intensity is about three times the driving intensity. Moreover, the
magnon blockade could be further enhanced by proper driving intensity and
system decay rate, whose magnitudes outrange the current systems of cavity QED
and cavity optomechanics. In particular, the correlation function achieves
$g^{(2)}(0)\sim10^{-7}$, about two orders lower than that for the photon
blockade in cavity optomechanics. Also, we discuss the effects on $g^{(2)}(0)$
from thermal noise and the extra longitudinal interaction between the magnon
and qubit. Our optimized conditions for blockade are found to persist in these
nonideal situations.",2303.13823v2
2023-04-21,Magnon squeezing by two-tone driving of a qubit in cavity-magnon-qubit systems,"We propose a scheme for preparing magnon squeezed states in a hybrid
cavity-magnon-qubit system. The system consists of a microwave cavity that
simultaneously couples to a magnon mode of a macroscopic yttrium-iron-garnet
(YIG) sphere via the magnetic-dipole interaction and to a transmon-type
superconducting qubit via the electric-dipole interaction. By far detuning from
the magnon-qubit system, the microwave cavity is adiabatically eliminated. The
magnon mode and the qubit then get effectively coupled via the mediation of
virtual photons of the microwave cavity. We show that by driving the qubit with
two microwave fields and by appropriately choosing the drive frequencies and
strengths, magnonic parametric amplification can be realized, which leads to
magnon quadrature squeezing with the noise below vacuum fluctuation. We provide
optimal conditions for achieving magnon squeezing, and moderate squeezing can
be obtained using currently available parameters. The generated squeezed states
are of a magnon mode involving more than $10^{18}$ spins and thus macroscopic
quantum states. The work may find promising applications in quantum information
processing and high-precision measurements based on magnons and in the study of
macroscopic quantum states.",2304.10760v4
2024-03-20,All-magnonic repeater based on bistability,"Bistability, a universal phenomenon found in diverse fields such as biology,
chemistry, and physics, describes a scenario in which a system has two stable
equilibrium states and resets to one of the two states. The ability to switch
between these two states is the basis for a wide range of applications,
particularly in memory and logic operations. Here, we present a universal
approach to achieve bistable switching in magnonics, the field processing data
using spin waves. As an exemplary application, we use magnonic bistability to
experimentally demonstrate the still missing magnonic repeater. A pronounced
bistable window is observed in a 1um wide magnonic conduit under an external rf
drive characterized by two magnonic stable states defined as low and high
spin-wave amplitudes. The switching between these two states is realized by
another propagating spin wave sent into the rf driven region. This magnonic
bistable switching is used to design the magnonic repeater, which receives the
original decayed and distorted spin wave and regenerates a new spin wave with
amplified amplitude and normalized phase. Our magnonic repeater is proposed to
be installed at the inputs of each magnonic logic gate to overcome the
spin-wave amplitude degradation and phase distortion during previous
propagation and achieve integrated magnonic circuits or magnonic neuromorphic
networks.",2403.13276v1
1999-04-17,The sublattice magnetizations of the spin-s quantum Heisenberg antiferromagnets,"We study the spin-$s$ quantum Heisenberg antiferromagnetic models in a magnon
theory free of any unphysical magnon state. Because the unphysical magnon
states are completely removed in the magnon Hamiltonians and during the
approximation process, we derive spin-s (s>1/2) reliable Neel temperature $T_N$
and reasonable sublattice magnetization unified for $T\leq T_N$.",9904243v1
1999-07-27,The Three-Magnon Contribution to the Spin Correlation Function in Integer-Spin Antiferromagnetic Chains,"The exact form factor for the O(3) non-linear sigma model is used to predict
the three-magnon contribution to the spin correlation function, S(q,w), near
wavevector q=pi in an integer spin, one-dimensional antiferromagnet. The
three-magnon contribution is extrememly broad and extremely weak; the
integrated intensity is <2% of the single-magnon contribution.",9907431v1
2008-03-02,Raman scattering in a Heisenberg {\boldmath $S=1/2$} antiferromagnet on the triangular lattice,"We investigate two-magnon Raman scattering from the $S=1/2$ Heisenberg
antiferromagnet on the triangular lattice, considering both the effect of
renormalization of the one-magnon spectrum by 1/S corrections and final-state
magnon-magnon interactions. The bare Raman intensity displays two peaks related
to one-magnon van-Hove singularities. We find that 1/S self-energy corrections
to the one-magnon spectrum strongly modify this intensity profile. The central
Raman-peak is significantly enhanced due to plateaus in the magnon dispersion,
the high frequency peak is suppressed due to magnon damping, and the overall
spectral support narrows considerably. Additionally we investigate final-state
interactions by solving the Bethe-Salpeter equation to $O(1/S)$. In contrast to
collinear antiferromagnets, the non-collinear nature of the magnetic ground
state leads to an irreducible magnon scattering which is retarded and
non-separable already to lowest order. We show that final-state interactions
lead to a rather broad Raman-continuum centered around approximately twice the
'roton'-energy. We also discuss the dependence on the scattering geometry.",0803.0130v1
2015-02-27,Stimulated Thermalization of a Parametrically Driven Magnon Gas as a Prerequisite for Bose-Einstein Magnon Condensation,"Thermalization of a parametrically driven magnon gas leading to the formation
of a Bose-Einstein condensate at the bottom of a spin-wave spectrum was studied
by time- and wavevector-resolved Brillouin light scattering spectroscopy. It
has been found that the condensation is preceded by the conversion of initially
pumped magnons into a second group of frequency degenerated magnons, which
appear due to parametrically stimulated scattering of the initial magnons to a
short-wavelength spectral region. In contrast to the first magnon group, which
wavevectors are orthogonal to the wavevectors of the magnons at the lowest
energy states, the secondary magnons can effectively scatter to the bottom of
the spectrum and condense there.",1502.07836v1
2016-01-08,Microscopic origin of subthermal magnons and the spin Seebeck effect,"Recent experimental evidence points to low-energy magnons as the primary
contributors to the spin Seebeck effect. This spectral dependence is puzzling
since it is not observed on other thermocurrents in the same material. Here, we
argue that the physical origin of this behavior is the magnon-magnon scattering
mediated by phonons, in a process which conserves the number of magnons. To
assess the importance and features of this kind of scattering, we derive the
effective magnon-phonon interaction from a microscopic model, including band
energy, a screened electron-electron interaction and the electron-phonon
interaction. Unlike higher order magnon-only scattering, we find that the
coupling with phonons induce a scattering which is very small for low-energy
(or subthermal) magnons but increases sharply above a certain energy --
rendering magnons above this energy poor spin-current transporters.",1601.02041v2
2017-02-20,Temperature Dependence of Magnetic Excitations: Terahertz Magnons above the Curie Temperature,"When an ordered spin system of a given dimensionality undergoes a second
order phase transition the dependence of the order parameter i.e. magnetization
on temperature can be well-described by thermal excitations of elementary
collective spin excitations (magnons). However, the behavior of magnons
themselves, as a function of temperature and across the transition temperature
TC, is an unknown issue. Utilizing spin-polarized high resolution electron
energy loss spectroscopy we monitor the high-energy (terahertz) magnons,
excited in an ultrathin ferromagnet, as a function of temperature. We show that
the magnons' energy and lifetime decrease with temperature. The
temperature-induced renormalization of the magnons' energy and lifetime depends
on the wave vector. We provide quantitative results on the temperature-induced
damping and discuss the possible mechanism e.g., multi-magnon scattering. A
careful investigation of physical quantities determining the magnons'
propagation indicates that terahertz magnons sustain their propagating
character even at temperatures far above TC.",1702.06104v1
2017-02-28,Linking Magnon-Cavity Strong Coupling to Magnon-Polaritons through Effective Permeability,"Strong coupling in cavity-magnon systems has shown great potential for use in
spintronics and information processing technologies due to the low damping
rates and long coherence times. Although such systems are conceptually similar
to those coupled by magnon-polaritons (MPs), the link between magnon-cavity
coupling and MPs has not been explicitly defined. In this work we establish
such a connection by studying the frequency-wavevector dispersion of a strongly
coupled magnon-cavity system, using a height-adjustable microwave cavity, and
by modelling the observed behaviour through the system's effective
permeability. A polariton gap between the upper and lower coupled modes of the
magnon-cavity system is defined, and is seen to be dependent on the system's
effective filling factor. This gap is equal to the MP polariton gap in the
limit where filling factor = 1, corresponding to the removal of the microwave
cavity. Thus, our work clarifies the connection between magnon-cavity and MP
coupling, improving our understanding of magnon-photon interactions in coupled
systems.",1703.00074v1
2010-05-14,Finite-Size Dyonic Giant Magnons in TsT-transformed $AdS_5\times S^5$,"We investigate dyonic giant magnons propagating on $\gamma$-deformed
$AdS_5\times S^5$ by Neumann-Rosochatius reduction method with a twisted
boundary condition. We compute finite-size effect of the dispersion relations
of dyonic giant magnons which generalizes the previously known case of the
giant magnons with one angular momentum found by Bykov and Frolov.",1005.2508v1
2017-01-24,Pumping of magnons in a Dzyaloshinskii-Moriya ferromagnet,"We formulate a microscopic linear response theory of nonequilibrium magnonic
torques and magnon pumping applicable to multiple-magnonic-band uniform
ferromagnets with Dzyaloshinskii-Moriya interactions. From the linear response
theory, we identify the extrinsic and intrinsic contributions where the latter
is expressed via the Berry curvature of magnonic bands. We observe that in the
presence of a time-dependent magnetization Dzyaloshinskii-Moriya interactions
can act as fictitious electric fields acting on magnons. We study various
current responses to this fictitious field and analyze the role of Berry
curvature. After identifying the magnon-mediated contribution to the
equilibrium Dzyaloshinskii-Moriya interaction, we also establish the Onsager
reciprocity between the magnon-mediated torques and heat pumping. We apply our
theory to the magnonic heat pumping and torque responses in honeycomb and
kagome lattice ferromagnets.",1701.06990v3
2017-01-31,Lateral transport properties of thermally excited magnons in yttrium iron garnet films,"Spin information carried by magnons is attractive for computing technology
and the development of magnon-based computing circuits is of great interest.
However, magnon transport in insulators has been challenging, different from
the clear physical picture for spin transport in conductors. Here we
investigate the lateral transport properties of thermally excited magnons in
yttrium iron garnet (YIG), a model magnetic insulator. Polarity reversals of
detected spins in non-local geometry devices have been experimentally observed
and are strongly dependent on temperature, YIG film thickness, and
injector-detector separation distance. A competing two-channel transport model
for thermally excited magnons is proposed, which is qualitatively consistent
with the spin signal behavior. In addition to the fundamental significance for
thermal magnon transport, our work furthers the development of magnonics by
creating an easily accessible magnon source with controllable transport",1701.08932v1
2018-05-23,Renormalization of spin excitations in hexagonal HoMnO3 by magnon-phonon coupling,"Hexagonal HoMnO3, a two-dimensional Heisenberg antiferromagnet, has been
studied via inelastic neutron scattering. A simple Heisenberg model with a
single-ion anisotropy describes most features of the spin-wave dispersion
curves. However, there is shown to be a renormalization of the magnon energies
located at around 11 meV. Since both the magnon-magnon interaction and
magnon-phonon coupling can affect the renormalization in a noncollinear magnet,
we have accounted for both of these couplings by using a Heisenberg XXZ model
with 1=S expansions [1] and the Einstein site phonon model [13], respectively.
This quantitative analysis leads to the conclusion that the renormalization
effect primarily originates from the magnon-phonon coupling, while the
spontaneous magnon decay due to the magnon-magnon interaction is suppressed by
strong two-ion anisotropy.",1805.08919v1
2014-06-26,Josephson and Persistent Spin Currents in Bose-Einstein Condensates of Magnons,"Using the Aharonov-Casher (A-C) phase, we present a microscopic theory of the
Josephson and persistent spin currents in quasi-equilibrium Bose-Einstein
condensates (BECs) of magnons in ferromagnetic insulators. Starting from a
microscopic spin model that we map onto a Gross-Pitaevskii Hamiltonian, we
derive a two-state model for the Josephson junction between the weakly coupled
magnon-BECs. We then show how to obtain the alternating-current (ac) Josephson
effect with magnons as well as macroscopic quantum self-trapping in a
magnon-BEC. We next propose how to control the direct-current (dc) Josephson
effect electrically using the A-C phase, which is the geometric phase acquired
by magnons moving in an electric field. Finally, we introduce a magnon-BEC ring
and show that persistent magnon-BEC currents flow due to the A-C phase.
Focusing on the feature that the persistent magnon-BEC current is a steady flow
of magnetic dipoles that produces an electric field, we propose a method to
directly measure it experimentally.",1406.7004v2
2020-12-14,Effect of magnon decays on parametrically pumped magnons,"We investigate the influence of magnon decays on the non-equilibrium dynamics
of parametrically excited magnons in the magnetic insulator yttrium-iron garnet
(YIG). Our investigations are motivated by a recent experiment by Noack et al.
[Phys. Status Solidi B 256, 1900121 (2019)] where an enhancement of the spin
pumping effect in YIG was observed near the magnetic field strength where
magnon decays via confluence of magnons becomes kinematically possible. To
explain the experimental findings, we have derived and solved kinetic equations
for the non-equilibrium magnon distribution. The effect of magnon decays is
taken into account microscopically via collision integrals derived from
interaction vertices involving three powers of magnon operators. Our results
agree quantitatively with the experimental data.",2012.07870v2
2019-02-13,Magnon transport in quasi-two-dimensional van der Waals antiferromagnets,"The recent emergence of 2D van der Waals magnets down to atomic layer
thickness provides an exciting platform for exploring quantum magnetism and
spintronics applications. The van der Waals nature stabilizes the long-range
ferromagnetic order as a result of magnetic anisotropy. Furthermore, giant
tunneling magnetoresistance and electrical control of magnetism have been
reported. However, the potential of 2D van der Waals magnets for magnonics,
magnon-based spintronics, has not been explored yet. Here, we report the
experimental observation of long-distance magnon transport in
quasi-twodimensional van der Waals antiferromagnet MnPS3, which demonstrates
the 2D magnets as promising material candidates for magnonics. As the 2D MnPS3
thickness decreases, a shorter magnon diffusion length is observed, which could
be attributed to the surface-impurity-induced magnon scattering. Our results
could pave the way for exploring quantum magnonics phenomena and designing
future magnonics devices based on 2D van der Waals magnets.",1902.04719v1
2019-03-01,Entangling two magnon modes via magnetostrictive interaction,"We present a scheme to entangle two magnon modes in a cavity magnomechanical
system. The two magnon modes are embodied by collective motions of a large
number of spins in two macroscopic ferrimagnets, and couple to a single
microwave cavity mode via magnetic dipole interaction. We show that by
activating the nonlinear magnetostrictive interaction in one ferrimagnet,
realized by driving the magnon mode with a strong red-detuned microwave field,
the two magnon modes can be prepared in an entangled state. The entanglement is
achieved by exploiting the nonlinear magnon-phonon coupling and the linear
magnon-cavity coupling, and is in the steady state and robust against
temperature. The entangled magnon modes in two massive ferrimagnets represent
genuinely macroscopic quantum states, and may find applications in the study of
macroscopic quantum mechanics and quantum information processing based on
magnonics.",1903.00221v2
2017-10-31,Magnonic triply-degenerate nodal points,"We generalize the concept of triply-degenerate nodal points to non-collinear
antiferromagnets. Here, we introduce this concept to insulating quantum
antiferromagnets on the decorated honeycomb lattice, with spin-$1$ bosonic
quasiparticle excitations known as magnons. We demonstrate the existence of
magnonic surface states with constant energy contours that form pairs of
magnonic arcs connecting the surface projection of the magnonic triple nodal
points. The quasiparticle excitations near the triple nodal points represent
three-component bosons beyond that of magnonic Dirac, Weyl, and nodal-line
cases. They can be regarded as a direct reflection of the intrinsic spin
carried by magnons. Furthermore, we show that the magnonic triple nodal points
can split into magnonic Weyl points, as the system transits from a
non-collinear spin structure to a noncoplanar one with a nonzero scalar spin
chirality. Our results not only apply to insulating antiferromagnets, but also
provide a platform to seek for triple nodal points in metallic
antiferromagnets.",1711.00007v4
2017-11-11,Observation of the exceptional point in cavity magnon-polaritons,"Magnon-polaritons are hybrid light-matter quasiparticles originating from the
strong coupling between magnons and photons. They have emerged as a potential
candidate for implementing quantum transducers and memories. Owing to the
dampings of both photons and magnons, the polaritons have limited lifetimes.
However, stationary magnon-polariton states can be reached by a dynamical
balance between pumping and losses, so the intrinsical nonequilibrium system
may be described by a non-Hermitian Hamiltonian. Here we design a tunable
cavity quantum electrodynamics system with a small ferromagnetic sphere in a
microwave cavity and engineer the dissipations of photons and magnons to create
cavity magnon-polaritons which have non-Hermitian spectral degeneracies. By
tuning the magnon-photon coupling strength, we observe the polaritonic coherent
perfect absorption and demonstrate the phase transition at the exceptional
point. Our experiment offers a novel macroscopic quantum platform to explore
the non-Hermitian physics of the cavity magnon-polaritons.",1711.04176v1
2018-10-27,Microwave control of thermal magnon spin transport,"We observe that an rf microwave field strongly influences the transport of
incoherent thermal magnons in yttrium iron garnet. Ferromagnetic resonance in
the nonlinear regime suppresses thermal magnon transport by 95%. The transport
is also modulated at non-resonant conditions in two cases, both related to the
magnon band minimum. Firstly, a strong enhancement of the nonlocal signal
appears at a static magnetic field below the resonance condition. This increase
only occurs at one field polarity and can be as large as 800%. We attribute
this effect to magnon kinetic processes, which give rise to band-minimum
magnons and high-energy chiral surface modes. Secondly, the signal increases at
a static field above the resonance condition, where the rf frequency coincides
with the magnon band minimum. Our study gives insight into the interplay
between coherent and incoherent spin dynamics: The rf field modifies the
occupation of relevant magnon states and, via kinetic processes, the magnon
spin transport.",1810.11667v1
2019-05-11,Topological magnon insulator and quantized pumps from strongly-interacting bosons in optical superlattices,"We propose a scheme realizing topological insulators and quantized pumps for
magnon excitations, based on strongly-interacting two-component ultracold atoms
trapped in optical superlattices. Specifically, we show how to engineer the
Su-Schrieffer-Heeger model for magnons using state-independent superlattices,
and the Rice-Mele model using state-dependent superlattices. We describe
realistic experimental protocols to detect the topological signatures of magnon
excitations in these two models. In particular, we show that the
non-equilibrium dynamics of a single magnon can be exploited to directly detect
topological winding numbers and phase transitions. We also describe how
topological (quantized) pumps can be realized with magnons, and study how this
phenomenon depends on the initial magnon state preparation. Our study opens a
new avenue for exploring magnonic topological phases of matter and their
potential applications in the context of topological magnon transport.",1905.04549v1
2021-02-22,"The role of density-dependent magnon hopping and magnon-magnon repulsion in ferrimagnetic spin-(1/2, $S$) chains in a magnetic field","We compare the ground-state features of alternating ferrimagnetic chains
$(1/2, S)$ with $S=1,3/2,2,5/2$ in a magnetic field and the corresponding
Holstein-Primakoff bosonic models up to order $\sqrt{s/S}$, with $s=1/2$,
considering the fully polarized magnetization as the boson vacuum. {The
single-particle Hamiltonian is a Rice-Mele model with uniform hopping and
modified boundaries, while the interactions have a correlated
(density-dependent) hopping term and magnon-magnon repulsion.} The
magnon-magnon repulsion increases the many-magnon energy and the
density-dependent hopping decreases the kinetic energy. We use density matrix
renormalization group calculations to investigate the effects of these two
interaction terms in the bosonic model{, and display the quantitative agreement
between the results from the spin model and the full bosonic approximation. In
particular, we verify the good accordance in the behavior of the edge states,
associated with the ferrimagnetic plateau, from the spin and from the bosonic
models. Furthermore, we show that the boundary magnon density strongly depends
on the interactions and particle statistics.",2102.11143v1
2016-09-20,Magnon nodal-line semimetals and drumhead surface states in anisotropic pyrochlore ferromagnets,"We introduce a new type of topological magnon matter: the magnonic pendant to
electronic nodal-line semimetals. Magnon spectra of anisotropic pyrochlore
ferromagnets feature twofold degeneracies of magnon bands along a closed loop
in reciprocal space. These magnon nodal lines are topologically protected by
the coexistence of inversion and time-reversal symmetry; they require the
absence of spin-orbit interaction (no Dzyaloshinskii-Moriya interaction). We
calculate the topological invariants of the nodal lines and show that details
of the associated magnon drumhead surface states depend strongly on the
termination of the surface. Magnon nodal-line semimetals complete the family of
topological magnons in three-dimensional ferromagnetic materials.",1609.06131v1
2016-09-26,Magnon Kerr effect in a strongly coupled cavity-magnon system,"We experimentally demonstrate magnon Kerr effect in a cavity-magnon system,
where magnons in a small yttrium iron garnet (YIG) sphere are strongly but
dispersively coupled to the photons in a three-dimensional cavity. When the YIG
sphere is pumped to generate considerable magnons, the Kerr effect yields a
perceptible shift of the cavity's central frequency and more appreciable shifts
of the magnon modes. We derive an analytical relation between the magnon
frequency shift and the drive power for the uniformly magnetized YIG sphere and
find that it agrees very well with the experimental results of the Kittel mode.
Our study paves the way to explore nonlinear effects in the cavity-magnon
system.",1609.07891v2
2019-07-17,Laser control of magnonic topological phases in antiferromagnets,"We study the laser control of magnon topological phases induced by the
Aharonov-Casher effect in insulating antiferromagnets (AFs). Since the laser
electric field can be considered as a time-periodic perturbation, we apply the
Floquet theory and perform the inverse frequency expansion by focusing on the
high frequency region. Using the obtained effective Floquet Hamiltonian, we
study nonequilibrium magnon dynamics away from the adiabatic limit and its
effect on topological phenomena. We show that a linearly polarized laser can
generate helical edge magnon states and induce the magnonic spin Nernst effect,
whereas a circularly polarized laser can generate chiral edge magnon states and
induce the magnonic thermal Hall effect. In particular, in the latter, we find
that the direction of the magnon chiral edge modes and the resulting thermal
Hall effect can be controlled by the chirality of the circularly polarized
laser through the change from the left-circular to the right-circular
polarization. Our results thus provide a handle to control and design magnon
topological properties in the insulating AF.",1907.07636v1
2021-01-03,Magnon-polaron formation in XXZ quantum Heisenberg chains,"We study the formation of magnon-polaron excitations and the consequences of
different time scales between the magnon and lattice dynamics. The spin-spin
interactions along the 1D lattice are ruled by a Heisenberg Hamiltonian in the
anisotropic form XXZ, in which each spin exhibits a vibrational degree of
freedom around its equilibrium position. By considering a magnetoelastic
coupling as a linear function of the relative displacement between
nearest-neighbor spins, results provide an original framework for achieving a
hybridized state of magnon-polaron. Such state is characterized by high
cooperation between the underlying excitations, where the traveling or
stationary formation of magnon-polaron depends on the effective magnetoelastic
coupling. A systematic investigation reveals the critical amount of the
magnon-lattice interaction ($\chi_c$) necessary to emergence of the stationary
magnon-polaron quasi-particle. Different characteristic time scales of the
magnon and the vibrational dynamics unveiled the threshold between the two
regimes, as well as a limiting value of critical magnetoelastic interaction,
above which the magnon velocity no longer interferes at the critical
magnetoelastic coupling capable of inducing the stationary regime.",2101.00673v1
2021-05-16,Voltage-Controlled Reconfigurable Magnonic Crystal at the Submicron Scale,"Multiferroics offer an elegant means to implement voltage-control and on the
fly reconfigurability in microscopic, nanoscaled systems based on ferromagnetic
materials. These properties are particularly interesting for the field of
magnonics, where spin waves are used to perform advanced logical or analogue
functions. Recently, the emergence of nano-magnonics {\color{black} is expected
to} eventually lead to the large-scale integration of magnonic devices.
However, a compact voltage-controlled, on demand reconfigurable magnonic system
has yet to be shown. Here, we introduce the combination of multiferroics with
ferromagnets in a fully epitaxial heterostructure to achieve such
voltage-controlled and reconfigurable magnonic systems. Imprinting a remnant
electrical polarization in thin multiferroic $\mathrm{BiFeO_3}$ with a
periodicity of $500\,\mathrm{nm}$ yields a modulation of the effective magnetic
field in the micron-scale, ferromagnetic $\mathrm{La_{2/3}Sr_{1/3}MnO_3}$
magnonic waveguide. We evidence the magneto-electrical coupling by
characterizing the spin wave propagation spectrum in this artificial, voltage
induced, magnonic crystal and demonstrate the occurrence of a robust magnonic
bandgap with $>20 \,\mathrm{dB}$ rejection.",2105.07418v1
2022-01-06,A Perspective on Magnon Spin Nernst Effect in Antiferromagnets,"Magnon excitations in antiferromagnetic materials and their physical
implications enable novel device concepts not available in ferromagnets,
emerging as a new area of active research. A unique characteristic of
antiferromagnetic magnons is the coexistence of opposite spin polarization,
which mimics the electron spin in a variety of transport phenomena. Among them,
the most prominent spin-contrasting phenomenon is the magnon spin Nernst effect
(SNE), which refers to the generation of transverse pure magnon spin current
through a longitudinal temperature gradient. We introduce selected recent
progress in the study of magnon SNE in collinear antiferromagnets with a focus
on its underlying physical mechanism entailing profound topological features of
the magnon band structures. By reviewing how the magnon SNE has inspired and
enriched the exploration of topological magnons, we offer our perspectives on
this emerging frontier that holds potential in future spintronic
nano-technology.",2201.01907v2
2022-02-24,Magnon boundary states tailored by longitudinal spin-spin interactions and topology,"Since longitudinal spin-spin interaction is ubiquitous in magnetic materials,
it is very interesting to explore the interplay between topology and
longitudinal spin-spin interaction. Here, we examine the role of longitudinal
spin-spin interaction on topological magnon excitations. Remarkably, even for
single-magnon excitations, we discover topological edge states and defect edge
states of magnon excitations in a dimerized Heisenberg XXZ chain and their
topological properties can be distinguished via adiabatic quantum transport. We
uncover topological phase transitions induced by longitudinal spin-spin
interactions whose boundary is analytically obtained via the transfer matrix
method. For multi-magnon excitations, even-magnon bound states are found to be
always topologically trivial, but odd-magnon bound states may be topologically
nontrivial due to the interplay between the transverse dimerization and the
longitudinal spin-spin interaction. For two-dimensional spin systems, the
longitudinal spin-spin interaction contributes to the coexistence of defect
corner states, second-order topological corner states and first-order
topological edge states. Our work opens an avenue for exploring topological
magnon excitations and has potential applications in topological magnon
devices.",2202.13780v2
2022-03-25,Topological state transfers in cavity-magnon system,"We propose an experimentally feasible scheme for realizing quantum state
transfer via the topological edge states in a one-dimensional cavity-magnon
lattice. We find that the cavity-magnon system can be mapped analytically into
the generalized Su-Schrieffer-Heeger model with tunable cavity-magnon coupling.
It can be shown that the edge state can be served as a quantum channel to
realize the photonic and magnonic state transfers by adjusting the
cavity-cavity coupling strength. Further, our scheme can realize the quantum
state transfer between photonic state and magnonic state by changing the
amplitude of the intracell hopping. With a numerical simulation, we
quantitatively show that the photonic, magnonic and magnon-to-photon state
transfers can be achieved with high fidelity in the cavity-magnon lattice.
Spectacularly, the three different types of quantum state transfer schemes can
be even transformed to each other in a controllable fashion. This system
provides a novel way of realizing quantum state transfer and can be implemented
in quantum computing platforms.",2203.13727v1
2022-06-16,Confinement and oscillating deconfinement crossover of two-magnon excitations in quantum spin chains quantified by spin entanglement entropy,"We introduce the spin entanglement entropy (EE) to characterize spin
excitations. The scalings of EEs are elaborated as $\ln N$, $\ln N+D_k$ and
$2\ln N+D$ for the single-magnon states, two-magnon bound states and two-magnon
continuum, respectively, based on the Bethe ansatz solutions of the
ferromagnetic spin chain with $N$ sites. The $\ln N$ divergence for the bound
states reveals the two magnons emerge as a new quasiparticle. More importantly,
the nonzero intercepts ($D_k,D$) embody the many-body effects of two-magnon
states. In particular, an exact relation between the intercept of EEs and an
observable quantity, the two-magnon distance, is established for the bound
states. In such a case, we quantify the two-magnon confinement in a spin system
by the increasing entanglement with the distance as a particle physics
analogue. Moreover, the EEs can also be used to study the evolution of the
excitations. When the bound states are immersed in the continuum in the
alternating chain, they undergo an oscillating confinement-deconfinement
crossover, shown by the oscillations of the EEs and two-magnon distance with
the chain length.",2206.08018v1
2022-09-07,Acoustic attenuation in magnetic insulator films: effects of magnon polaron formation,"A magnon and a phonon are the quanta of spin wave and lattice wave,
respectively, and they can hybridize into a magnon polaron when their
frequencies and wavenumbers match close enough the values at the exceptional
point. Guided by an analytically calculated magnon polaron dispersion,
dynamical phase-field simulations are performed to investigate the effects of
magnon polaron formation on the attenuation of a bulk acoustic wave in a
magnetic insulator film. It is shown that a stronger magnon-phonon coupling
leads to a larger attenuation. The simulations also demonstrate the existence
of a minimum magnon-phonon interaction time required for the magnon polaron
formation, which is found to decrease with the magnetoelastic coupling
coefficient but increase with the magnetic damping coefficient. These results
deepen the understanding of the mechanisms of acoustic attenuation in magnetic
crystals and provide insights into the design of new-concept spin interconnects
that operate based on acoustically driven magnon propagation.",2209.03481v2
2022-09-27,Radiation-free and non-Hermitian topology inertial defect states of on-chip magnons,"Radiative damping is a strong dissipation source for the quantum emitters
hybridized with propagating photons, electrons, or phonons, which is not easily
avoidable for on-chip magnonic emitters as well that can radiate via the
surface acoustic waves of the substrate. Here we demonstrate in an array of
on-chip nano-magnets coupled in a long range via exchanging the surface
acoustic waves that a point defect in the array, which can be introduced by the
local magnon frequency shift by a local biased magnetic field or the absence of
a magnetic wire, strongly localizes the magnons, in contrast to the well
spreading Bloch-like collective magnon modes in such an array setting. The
radiation of the magnon defect states is exponentially suppressed by the
distance of the defect to the array edges. Moreover, this defect state is
strikingly inertial to the non-Hermitian topology that localizes all the
extended states at one boundary. Such configuration robust magnon defect states
towards radiation-free limit may be suitable for high-fidelity magnon quantum
information storage in the future on-chip magnonic devices.",2209.13386v1
2022-10-29,Antimagnonics,"Magnons are the quanta of collective spin excitations in magnetically-ordered
systems and manipulation of magnons for computing and information processing
has witnessed the development of ``magnonics"". A magnon corresponds to an
excitation of the magnetic system from its ground state and the creation of a
magnon thus increases the total energy of the system. In this perspective, we
introduce the antiparticle of a magnon, dubbed the antimagnon, as an excitation
that lowers the magnetic energy. We investigate the stability and thermal
occupation of antimagnons and verify our theory by micromagnetic simulations.
Furthermore, we show how the concept of antimagnons yields a unified picture to
understand the magnonic analog of the Klein effect, magnonic black-hole
horizons, and magnonic black-hole lasing. Our work may stimulate fundamental
interest in antimagnons, as well as their applications to spintronic devices.",2210.16698v1
2022-11-01,Ultrastrong magnon-photon coupling and entanglement in superconductor/ferromagnet nanostructures,"Ultrastrong light-matter coupling opens exciting possibilities to generate
squeezed quantum states and entanglement. Here we propose a way to achieve this
regime in superconducting hybrid nanostructures with ferromagnetic interlayers.
Strong confinement of electromagnetic field between superconducting plates is
found to result in the existence of magnon-polariton modes with ultrastrong
magnon-photon coupling, ultra-high cooperativity and very large group
velocities. These modes provide a numerically accurate explanation of recent
experiments and have intriguing quantum properties. The magnon-polariton
quantum vacuum consists of the squeezed magnon and photon states with the
degree of squeezing controlled in wide limits by the external magnetic field.
The ground state population of virtual photons and magnons is shown to be very
large which can be used for generating correlated magnon and photon pairs.
Excited states of magnon-polaritons contain bipartite entanglement between
magnons and photons. This property can be used for transferring entanglement
between different types of quantum systems.",2211.00462v3
2023-01-09,Nonlinear Topological Magnon Spin Hall Effect,"When a magnon passes through two-dimensional magnetic textures, it will
experience a fictitious magnetic field originating from the $3\times 3$
skew-symmetric gauge fields. To date, only one of the three independent
components of the gauge fields has been found to play a role in generating the
fictitious magnetic field while the rest two are perfectly hidden. In this
work, we show that they are concealed in the nonlinear magnon transport in
magnetic textures. Without loss of generality, we theoretically study the
nonlinear magnon-skyrmion interaction in antiferromagnets. By analyzing the
scattering features of three-magnon processes between the circularly-polarized
incident magnon and breathing skyrmion, we predict a giant Hall angle of both
the confluence and splitting modes. Furthermore, we find that the Hall angle
reverses its sign when one switches the handedness of the incident magnons. We
dub it nonlinear topological magnon spin Hall effect. Our findings are deeply
rooted in the bosonic nature of magnons that the particle number is not
conserved, which has no counterpart in low-energy fermionic systems, and may
open the door for probing gauge fields by nonlinear means.",2301.03211v1
2023-06-24,Magnon confinement in an all-on-chip YIG cavity resonator using hybrid YIG/Py magnon barriers,"Confining magnons in cavities can introduce new functionalities to magnonic
devices, enabling future magnonic structures to emulate established photonic
and electronic components. As a proof-of-concept, we report magnon confinement
in a lithographically defined all-on-chip YIG cavity created between two
YIG/Permalloy bilayers. We take advantage of the modified magnetic properties
of covered/uncovered YIG film to define on-chip distinct regions with
boundaries capable of confining magnons. We confirm this by measuring multiple
spin pumping voltage peaks in a 400 nm wide platinum strip placed along the
center of the cavity. These peaks coincide with multiple spin-wave resonance
modes calculated for a YIG slab with the corresponding geometry. The
fabrication of micrometer-sized YIG cavities following this technique
represents a new approach to control coherent magnons, while the spin pumping
voltage in a nanometer-sized Pt strip demonstrates to be a non-invasive local
detector of the magnon resonance intensity.",2306.14029v2
2023-07-07,Interacting topological magnons in a checkerboard ferromagnet,"This work is devoted to studying the magnon-magnon interaction effect in a
two-dimensional checkerboard ferromagnet with the Dzyaloshinskii-Moriya
interaction. By means of the first-order Green function formalism, the
influence of magnon-magnon interaction on the magnon band topology is analyzed.
In order to verify that the gap-closing phenomenon is a signature for the
topological phase transitions of the checkerboard ferromagnet, we display that
the Chern numbers of renormalized magnon bands are distinct above and below the
critical temperature. Our results show that the checkerboard ferromagnet
possesses two topological phases and its topological phase can be controlled
either by the temperature or applied magnetic field due to magnon-magnon
interactions. Interestingly, we find that the topological phase transition
occurs twice with the increase of the temperature, which is different from the
result of the honeycomb ferromagnet.",2307.03361v1
2023-09-17,Highly-tunable and strong nonreciprocity in coupled nonlinear cavity magnonics,"Nonreciprocity, which violates Lorentz reciprocity, plays a pivotal role in
quantum information processing and networks. Nevertheless, achieving a desired
and highly-tunable level of nonreciprocity has proven to be a formidable
challenge. Here, we propose a coupled nonlinear cavity magnonic system,
consisting of two cavities, a second-order nonlinear element, and a
yttrium-iron-garnet sphere for supporting Kerr magnons, to realize this
sought-after highly-tunable nonreciprocity. We first derive the critical
condition for switching between reciprocity and nonreciprocity with undriven
magnons, then we numerically demonstrate that a strong magnonic nonreciprocity
can be obtained by breaking the critical condition. When magnons are driven, we
show that a strong magnonic nonreciprocity can also be attained within the
critical condition. Compared to previous study, the introduced nonlinear
element not only relaxes the critical condition to both the weak and strong
regimes, but also offers an alternative path to adjust the magnonic
nonreciprocity. Our work provides a promising avenue to realize highly-tunable
nonreciprocal devices with Kerr magnons.",2309.09245v2
2023-10-12,Electrical noise spectroscopy of magnons in a quantum Hall ferromagnet,"Collective spin-wave excitations-magnons-in a quantum Hall ferromagnet are
promising quasi-particles for next-generation spintronics devices, including
platforms for information transfer. Detection of these charge-neutral
excitations relies on the conversion of magnons into electrical signals in the
form of excess electrons and holes, but if these signals are equal the magnon
detection remains elusive. In this work, we overcome this shortcoming by
measuring the electrical noise generated by magnons. We use the symmetry-broken
quantum Hall ferromagnet of the zeroth Landau level in graphene to launch
magnons. Absorption of these magnons creates excess noise above the Zeeman
energy and remains finite even when the average electrical signal is zero.
Moreover, we formulate a theoretical model in which the noise is generated by
equilibration (partial or full, depending on the bias voltage) between edge
channels and propagating magnons. Our model, which agrees with experimental
observations, also allows us to pinpoint the regime of ballistic magnon
transport in our device.",2310.08703v1
2024-01-10,Electrical Non-Hermitian Control of Topological Magnon Spin Transport,"Magnonic topological phases realize chiral edge spin waves that are protected
against backscattering, potentially enabling highly efficient spin transport.
Here we show that the spin transport through these magnonic chiral edge states
can be electrically manipulated by non-Hermitian control. We consider the
paradigmatic magnon Haldane model and show that it is transformed into an
effective non-Hermitian magnon Chern insulator by including a
sublattice-dependent spin-orbit torque. In linear spin-wave theory, this
electrically induced torque causes a lasing of the chiral edge magnons along
certain edge directions, leading to an enhancement of the spin-wave amplitude.
This prediction is confirmed by numerical simulations based on the
Landau-Lifshitz-Gilbert equation. For a spin-wave transport setup, in which
magnons are excited by a microwave field and detected with a normal metal
conductor, we find that the magnon amplification is remarkably robust against
disorder, establishing non-Hermitian control as a promising avenue for
topological magnonics.",2401.04967v2
2024-02-22,Impact of Magnon Interactions on Transport in Honeycomb Antiferromagnets,"The thermal transport of magnons has attracted substantial attention as an
energy-efficient alternative to the transport of electrons. Most theoretical
studies so far have been carried out within the frame of the linear spin-wave
theory, which dramatically fails upon increasing the temperature and in the
presence of competing interactions. In this work, we consider the impact of
three- and four-magnon interactions in a honeycomb antiferromagnet, where such
interactions are remarkably strong even at zero temperature. Using a
combination of quantum field theory and mean-field theory, we compute the band
structure of the interacting magnons and investigate the spin Nernst effect. We
find that in the presence of in-plane Dzyaloshinskii-Moriya Interaction, the
three-magnon interaction induces a non-reciprocal band splitting, even at zero
temperature, leading to an enhancement of the spin Nernst conductivity. In
contrast, the four-magnon interaction renormalizes the magnon spectrum at high
temperatures, leading to a reduction of the overall magnon spin Nernst effect.
These results suggest that interactions can massively influence the transport
properties of magnons in antiferromagnets, even at zero temperature, and should
be taken into account for predictive modeling.",2402.14572v1
2024-02-25,Enhancement of Entanglement via Josephson Parametric Amplifier in a Dual Cavity-Magnon System,"In the two microwave (MW) cross-shaped cavity magnon system, we describe a
method to produce multipartite entanglement and quantum steering. To achieve
squeezed states of the magnons, a Josephson parametric amplifier (JPA) creates
a squeezed vacuum field that drives the two cavities. We theoretically
demonstrate that the cavity-cavity entanglement can be generated at the
resonance point, however, increasing the cavity and magnon decay rates generate
the cavity-magnon entanglement. By changing the squeezing parameter and
increasing the decay rates, we can transfer the cavity-cavity entanglement to
cavity-magnon entanglement. Furthermore, the cavity-cavity entanglement
(survive up to 2.8K) not only found to be much stronger but also more robust as
compared to cavity-magnon entanglement (survive up to 0.4K). More importantly,
the genuine photon-magnon-photon tripartite entanglement could be achieved,
which is robust against the thermal fluctuations and depends strongly on
squeezing parameter. Furthermore, for current dual cavity-magnon system,
two-way quantum steering is found when the optomagnonical couplings are equal.
The current study offers a straightforward and practical method for achieving
multipartite quantum correlations.",2402.16042v1
2018-02-13,Electric control of emergent magnonic spin current and dynamic multiferroicity in magnetic insulators at finite temperatures,"Conversion of thermal energy into magnonic spin currents and/or effective
electric polarization promises new device functionalities. A versatile approach
is presented here for generating and controlling open circuit magnonic spin
currents and an effective multiferroicity at a uniform temperature with the aid
of spatially inhomogeneous, external, static electric fields. This field
applied to a ferromagnetic insulator with a Dzyaloshinskii-Moriya type coupling
changes locally the magnon dispersion and modifies the density of thermally
excited magnons in a region of the scale of the field inhomogeneity. The
resulting gradient in the magnon density can be viewed as a gradient in the
effective magnon temperature. This effective thermal gradient together with
local magnon dispersion result in an open-circuit, electric field controlled
magnonic spin current. In fact, for a moderate variation in the external
electric field the predicted magnonic spin current is on the scale of the spin
(Seebeck) current generated by a comparable external temperature gradient.
Analytical methods supported by full-fledge numerics confirm that both, a
finite temperature and an inhomogeneous electric field are necessary for this
emergent non-equilibrium phenomena. The proposal can be integrated in magnonic
and multiferroic circuits, for instance to convert heat into electrically
controlled pure spin current using for example nanopatterning, without the need
to generate large thermal gradients on the nanoscale.",1802.04614v1
2017-09-30,Tuning the diffusion of magnon in Y3Fe5O12 by light excitation,"Deliberate control of magnon transportation will lead to an energy-efficient
technology for information transmission and processing. Y3Fe5O12(YIG),
exhibiting extremely large magnon diffusion length due to the low magnetic
damping constant, has been intensively investigated for decades. While most of
the previous works focused on the determination of magnon diffusion length by
various techniques, herein we demonstrated how to tune magnon diffusion by
light excitation. We found that the diffusion length of thermal magnons is
strongly dependent on light wavelength when the magnon is generated by exposing
YIG directly to laser beam. The diffusion length, determined by a nonlocal
geometry at room temperature, is ~30 um for the magnons produced by visible
light (400-650 nm), and ~136-156 um for the laser between 808 nm and 980 nm.
The diffusion distance is much longer than the reported value. In addition to
thermal gradient, we found that light illumination affected the electron
configuration of the Fe3+ ion in YIG. Long wavelength laser triggers a high
spin to low spin state transition of the Fe3+ ions in FeO6 octahedron. This in
turn causes a substantial softening of the magnon thus a dramatic increase in
diffusion distance. The present work paves the way towards an efficient tuning
of magnon transport behavior which is crucially important for magnon
spintronics.",1710.00222v2
2017-07-24,Magnonic topological insulators in antiferromagnets,"Extending the notion of symmetry protected topological phases to insulating
antiferromagnets (AFs) described in terms of opposite magnetic dipole moments
associated with the magnetic N$\acute{{\rm{e}}} $el order, we establish a
bosonic counterpart of topological insulators in semiconductors. Making use of
the Aharonov-Casher effect, induced by electric field gradients, we propose a
magnonic analog of the quantum spin Hall effect (magnonic QSHE) for edge states
that carry helical magnons. We show that such up and down magnons form the same
Landau levels and perform cyclotron motion with the same frequency but
propagate in opposite direction. The insulating AF becomes characterized by a
topological ${\mathbb{Z}}_{2}$ number consisting of the Chern integer
associated with each helical magnon edge state. Focusing on the topological
Hall phase for magnons, we study bulk magnon effects such as magnonic spin,
thermal, Nernst, and Ettinghausen effects, as well as the thermomagnetic
properties of helical magnon transport both in topologically trivial and
nontrivial bulk AFs and establish the magnonic Wiedemann-Franz law. We show
that our predictions are within experimental reach with current device and
measurement techniques.",1707.07427v2
2018-01-15,Photoinduced Topological Phase Transitions in Topological Magnon Insulators,"Topological magnon insulators are the bosonic analogs of electronic
topological insulators. They are manifested in magnetic materials with
topologically nontrivial magnon bands as realized experimentally in a
quasi-two-dimensional (quasi-2D) kagome ferromagnet Cu(1-3, bdc), and they also
possess protected magnon edge modes. These topological magnetic materials can
transport heat as well as spin currents, hence they can be useful for
spintronic applications. Moreover, as magnons are charge-neutral spin-${\bf 1}$
bosonic quasiparticles with a magnetic dipole moment, topological magnon
materials can also interact with electromagnetic fields through the
Aharonov-Casher effect. In this report, we study photoinduced topological phase
transitions in intrinsic topological magnon insulators in the kagom\'e
ferromagnets. Using magnonic Floquet-Bloch theory, we show that by varying the
light intensity, periodically driven intrinsic topological magnetic materials
can be manipulated into different topological phases with different sign of the
Berry curvatures and the thermal Hall conductivity. We further show that, under
certain conditions, periodically driven gapped topological magnon insulators
can also be tuned to synthetic gapless topological magnon semimetals with
Dirac-Weyl magnon cones. We envision that this work will pave the way for
interesting new potential practical applications in topological magnetic
materials",1801.04932v3
2020-07-11,Magnon-bipolar carrier drag thermopower in antiferromagnetic/ferromagnetic semiconductors: theoretical formulation and experimental evidence,"Quantized spin-wave known as magnon, a bosonic quasiparticle, can drag
electrons or holes via sd exchange interaction and boost the thermopower over
the conventional diffusive thermopower. P-type magnon-drag thermopower has been
observed in both ferromagnetic and antiferromagnetic metallic and degenerate
semiconductors. However, it has been less reported for intrinsic or n-type
magnetic semiconductors; therefore, the impact of magnon-bipolar carrier drag
on thermopower has remained unexplored. Here, a theoretical model for
magnon-bipolar carrier drag thermopower is derived based on the magnon-carrier
interaction lifetimes. The model predicts that the bipolar carrier drag
thermopower becomes independent of both the carrier and magnon relaxation
times. A proof of concept experiment is presented that confirms this
prediction. We also report the observation of magnon-carrier drag thermopower
in n-type and intrinsic ferromagnetic semiconductors experimentally. The p-type
antiferromagnetic MnTe is doped with different amounts of Cr to produce
non-degenerate and n-type semiconductors of various carrier concentrations. Cr
dopants have a donor nature and create ferromagnetic-antiferromagnetic clusters
due to the Cr3+ oxidation state. Heat capacity measurements confirm the
presence of magnons in Cr-doped MnTe. It is shown that the magnon-drag
thermopower is significantly reduced for 3%-5% Cr-doped samples due to bipolar
drag effects and becomes negative for 14% and 20% Cr-doped MnTe due to dominant
magnon-electron drag thermopower.",2007.05682v1
2021-03-01,Magnon driven skyrmion dynamics in antiferromagnets: The effect of magnon polarization,"The controllable magnetic skyrmion motion represents a highly concerned issue
in preparing advanced skyrmion-based spintronic devices. Specifically,
magnon-driven skyrmion motion can be easily accessible in both metallic and
insulating magnets, and thus is highly preferred over electric current control
further for the ultra-low energy consumption. In this work, we investigate
extensively the dynamics of skyrmion motion driven by magnon in an
antiferromagnet using the collective coordinate theory, focusing on the effect
of magnon polarization. It is revealed that the skyrmion Hall motion driven by
circularly polarized magnon becomes inevitable generally, consistent with
earlier report. Furthermore, the elastic scattering theory and numerical
results unveil the strong inter-dependence between the linearly polarized
magnon and skyrmion motion, suggesting the complicated dependence of the
skyrmion motion on the polarization nature of driving magnon. On the reversal,
the scattering from the moving skyrmion may lead to decomposition of the
linearly polarized magnon into two elliptically polarized magnon bands.
Consequently, a net transverse force acting on skyrmion is generated owing to
the broken mirror symmetry, which in turn drives a skyrmion Hall motion. The
Hall motion can be completely suppressed only in some specific condition where
the mirror symmetry is preserved. The present work unveils non-trivial
skyrmion-magnon scattering behavior in antiferromagnets, advancing the
antiferromagnetic spintronics and benefiting to high-performance devices.",2103.00898v1
2022-01-23,Squeezed driving induced entanglement and squeezing among cavity modes and magnon mode in a magnon-cavity QED system,"We propose a scheme to generate entanglement between two cavity modes and
squeeze magnon mode in a magnon-cavity QED system, where the two microwave
cavity modes are coupled with a massive yttrium iron garnet (YIG) sphere
through magnetic dipole interaction. The nonlinearity used in our system
originates from a squeezed driving via parametric down-conversion process,
which is the reason to cause entanglement and squeezing. By using the mean
field approximation and employing experimentally feasible parameters, we
demonstrate that the system shows zero entanglement and squeezing without
squeezed driving. Meanwhile, our QED system denotes that the entanglement
between squeezed cavity mode and magnon mode can be transferred to the other
cavity mode and magnon mode via magnon-cavity coupling interaction, and then
the two cavity modes get entangled. A genuinely tripartite entangled state is
formed. We also show that magnon mode can be prepared in a squeezed state via
magnon-cavity beam-splitter interaction, which is as a result of the squeezed
field. Moreover, we show that it is a good way to enhance entanglement and
squeezing by increasing the nonlinear gain coefficient of squeezed driving. Our
results denote that magnon-cavity QED system is a powerful platform for
studying macroscopic quantum phenomena, which illustrates a new method to
photon-photon entanglement and magnon squeezing.",2201.09154v1
2022-07-04,Long-distance coupling of spin qubits via topological magnons,"We consider two distant spin qubits in quantum dots, both coupled to a
two-dimensional topological ferromagnet hosting chiral magnon edge states at
the boundary. The chiral magnon is used to mediate entanglement between the
spin qubits, realizing a fundamental building block of scalable quantum
computing architectures: a long-distance two-qubit gate. Previous proposals for
long-distance coupling with magnons involved off-resonant coupling, where the
detuning of the spin-qubit frequency from the magnonic band edge provides
protection against spontaneous relaxation. The topological magnon mode, on the
other hand, lies in-between two magnonic bands far away from any bulk magnon
resonances, facilitating strong and highly tuneable coupling between the two
spin qubits. Even though the coupling between the qubit and the chiral magnon
is resonant for a wide range of qubit splittings, we find that the
magnon-induced qubit relaxation is vastly suppressed if the coupling between
the qubit and the ferromagnet is antiferromagnetic. A fast and high-fidelity
long-distance coupling protocol is presented capable of achieving spin-qubit
entanglement over micrometer distances with $1\,$MHz gate speed and up to
$99.9\%$ fidelities. The resulting spin-qubit entanglement may be used as a
probe for the long-sought detection of topological edge magnons.",2207.01264v1
2022-09-27,Tunable Exciton-Hybridized Magnon Interactions in a Layered Semiconductor,"The interaction between distinct excitations in solids is of both fundamental
interest and technological importance. One example of such interactions is
coupling between an exciton, a Coulomb bound electron-hole pair, and a magnon,
a collective spin excitation. The recent emergence of van der Waals magnetic
semiconductors provides a powerful platform for exploring these exciton-magnon
interactions and their fundamental properties, such as strong correlation, as
well as their photo-spintronic and quantum transduction applications. Here we
demonstrate precise control of coherent exciton-magnon interactions in the
layered magnetic semiconductor CrSBr. We show that by controlling the direction
of applied magnetic fields relative to the crystal axes, and thus the
rotational symmetry of the magnetic system, we can tune not only the exciton
coupling to the bright magnon, but also to an optically dark mode via magnon
hybridization. The exciton-magnon coupling and associated magnon dispersion
curves can be further modulated by applying a uniaxial strain. At the critical
strain, a dispersionless dark magnon band emerges. Our results demonstrate
unprecedented control of the opto-mechanical-magnonic coupling, and a step
towards the predictable and controllable implementation of hybrid quantum
magnonics.",2209.13744v1
2023-03-04,Controllable magnon-induced transparency in a ferromagnetic material via cross- and self-Kerr effects,"Nonlinear interactions between optical fields and magnetic modes in cavity
magnonics constitute a rich source of various nontrivial effects in optics and
quantum information processing. In cavity magnonics, the nonlinear cross-Kerr
effect, which shifts the cavity's central frequency when a magnetic material is
pumped, causes the system to exhibit both Kittle and magnetostatic modes. Here,
we propose a new scheme for the investigation of probe fields transmission
profiles in cavity magnonic systems composed of a microwave cavity and a
ferromagnetic material (Yttrium iron garnet sphere). We report single-to-double
magnon-induced transparency (MIT) dips and a sharp magnon-induced absorption
(MIA) peak, and demonstrate how nonlinear cross- and self-Kerr interactions can
significantly enhance or suppress these phenomena. It is observed that the
splitting of the MIT window occurs when we incorporate magnon-magnon modes
coupling, which helps introducing a new degree of freedom to light-matter
interaction problems. Moreover, we investigate the propagation of group delay
in the vicinity of transparency and demonstrate how a sharp dip allows the
realization of slow light for a longer period of time. We found that both the
cavity-Kittle and magnon-magnon modes coupling parameters influence the
propagation of group delay, which demonstrates how subluminal-to-superluminal
(and vice versa) propagation phenomena may occur and transform. These findings
could pave the way for future research into nonlinear effects with novel
applications in cavity magnonics devices, which might be exploited for several
applications such as quantum computing devices and quantum memories.",2303.02332v1
2023-05-25,Type-II Dirac points and Dirac nodal loops on the magnons of square-hexagon-octagon lattice,"We study topological magnons on an anisotropic square-hexagon-octagon (SHO)
lattice which has been found by a two-dimensional Biphenylene network (BPN). We
propose the concepts of type-II Dirac magnonic states where new schemes to
achieve topological magnons are unfolded without requiring the
Dzyaloshinsky-Moriya interactions (DMIs). In the ferromagnetic states, the
topological distinctions at the type-II Dirac points along with one-dimensional
(1D) closed lines of Dirac magnon nodes are characterized by the $\mathbb{Z}_2$
invariant. We find pair annihilation of the Dirac magnons and use the Wilson
loop method to depict the topological protection of the band-degeneracy. The
Green's function approach is used to calculte chiral edge modes and magnon
density of states (DOS). We introduce the DMIs to gap the type-II Dirac magnon
points and demonstrate the Dirac nodal loops (DNLs) are robust against the DMIs
within a certain parameter range. The topological phase diagram of magnon bands
is given via calculating the Berry curvature and Chern number. We find that the
anomalous thermal Hall conductivity gives connection to the magnon edge
current. Furthermore, we derive the differential gyromagnetic ratio to exhibit
the Einstein-de Haas effect (EdH) of magnons with topological features.",2305.16419v2
2023-06-07,Non-Hermitian Topological Magnonics,"Dissipation in mechanics, optics, acoustics, and electronic circuits is
nowadays recognized to be not always detrimental but can be exploited to
achieve non-Hermitian topological phases or properties with functionalities for
potential device applications. As elementary excitations of ordered magnetic
moments that exist in various magnetic materials, magnons are the information
carriers in magnonic devices with low-energy consumption for reprogrammable
logic, non-reciprocal communication, and non-volatile memory functionalities.
Non-Hermitian topological magnonics deals with the engineering of dissipation
and/or gain for non-Hermitian topological phases or properties in magnets that
are not achievable in the conventional Hermitian scenario, with associated
functionalities cross-fertilized with their electronic, acoustic, optic, and
mechanic counterparts, such as giant enhancement of magnonic frequency combs,
magnon amplification, (quantum) sensing of the magnetic field with
unprecedented sensitivity, magnon accumulation, and perfect absorption of
microwaves. In this review article, we address the unified approach in
constructing magnonic non-Hermitian Hamiltonian, introduce the basic
non-Hermitian topological physics, and provide a comprehensive overview of the
recent theoretical and experimental progress towards achieving distinct
non-Hermitian topological phases or properties in magnonic devices, including
exceptional points, exceptional nodal phases, non-Hermitian magnonic SSH model,
and non-Hermitian skin effect. We emphasize the non-Hermitian Hamiltonian
approach based on the Lindbladian or self-energy of the magnonic subsystem but
address the physics beyond it as well, such as the crucial quantum jump effect
in the quantum regime and non-Markovian dynamics. We provide a perspective for
future opportunities and challenges before concluding this article.",2306.04348v2
2023-09-26,Multiferroic Magnon Spin-Torque Based Reconfigurable Logic-In-Memory,"Magnons, bosonic quasiparticles carrying angular momentum, can flow through
insulators for information transmission with minimal power dissipation.
However, it remains challenging to develop a magnon-based logic due to the lack
of efficient electrical manipulation of magnon transport. Here we present a
magnon logic-in-memory device in a spin-source/multiferroic/ferromagnet
structure, where multiferroic magnon modes can be electrically excited and
controlled. In this device, magnon information is encoded to ferromagnetic bits
by the magnon-mediated spin torque. We show that the ferroelectric polarization
can electrically modulate the magnon spin-torque by controlling the
non-collinear antiferromagnetic structure in multiferroic bismuth ferrite thin
films with coupled antiferromagnetic and ferroelectric orders. By manipulating
the two coupled non-volatile state variables (ferroelectric polarization and
magnetization), we further demonstrate reconfigurable logic-in-memory
operations in a single device. Our findings highlight the potential of
multiferroics for controlling magnon information transport and offer a pathway
towards room-temperature voltage-controlled, low-power, scalable magnonics for
in-memory computing.",2309.14614v1
2023-11-26,Electric Field Switching of Magnon Spin Current in a Compensated Ferrimagnet,"Manipulation of directional magnon propagation, known as magnon spin current,
is essential for developing magnonic memory and logic devices featuring
nonvolatile functionalities and ultralow power consumption. Magnon spin current
can usually be modulated by magnetic field or current-induced spin torques.
However, these approaches may lead to energy dissipation caused by Joule
heating. Electric-field switching of magnon spin current without charge current
is highly desired but very challenging to realize. By integrating magnonic and
piezoelectric materials, we demonstrate manipulation of the magnon spin current
generated by the spin Seebeck effect in the ferrimagnetic insulator Gd3Fe5O12
(GdIG) film on a piezoelectric substrate. We observe reversible electric-field
switching of magnon polarization without applied charge current. Through
strain-mediated magnetoelectric coupling, the electric field induces the
magnetic compensation transition between two magnetic states of the GdIG,
resulting in its magnetization reversal and the simultaneous switching of
magnon spin current. Our work establishes a prototype material platform that
pave the way for developing magnon logic devices characterized by all electric
field reading and writing and reveals the underlying physics principles of
their functions.",2311.15183v1
2024-01-03,High-degree magnon blockade with large single-excitation probability,"Optimized blockade is an efficient tool in generating a single-magnon state,
that is fundamental to manipulate the magnonic systems at the quantum level. In
this study, we consider a hybrid system in which a qubit is strongly coupled to
$N$ magnons via the exchange interaction. The qubit and the magnon modes are
subject to the probing field and driving fields, respectively. It is
interesting to find the scalable conditions in minimizing the equal-time
second-order correlation function $g^{(2)}(0)$ for each magnon with respect to
$N$. In particular, the simultaneous blockade is optimized when (i) the
detuning between the qubit (magnon) and the probing (driving field) field is
$\sqrt{N}$ times the magnon-qubit coupling strength, (ii) the probing intensity
is $3\sqrt{N}$ times the driving intensity, and (iii) the relative phase
between probing and driving fields is $2/(3\sqrt{N}$) times the ratio of the
system decay rate to the magnon-qubit coupling strength. More than a
high-degree blockade, we can generate a significant population on the
single-magnon state. With experimental-relevant driving intensity and decay
rate, the correlation function can achieve about $g^{(2)}(0)\sim10^{-7}$ in
company with a large single-magnon population $P_1\sim0.24$ when $N=1$ and
$g^{(2)}(0)\sim10^{-7}$ with $P_1\sim0.12$ when $N=2$.",2401.01590v1
2024-03-13,Thermal Hall effect incorporating magnon damping in localized spin systems,"We propose a theory for thermal Hall transport mediated by magnons to address
the impact of their damping resulting from magnon-magnon interactions in
insulating magnets. This phenomenon is anticipated to be particularly
significant in systems characterized by strong quantum fluctuations,
exemplified by spin-1/2 systems. Employing a nonlinear flavor-wave theory, we
analyze a general model for localized electron systems and develop a
formulation for thermal conductivity based on a perturbation theory, utilizing
bosonic Green's functions with a nonzero self-energy. We derive the expression
of the thermal Hall conductivity incorporating magnon damping. To demonstrate
the applicability of the obtained representation, we adopt it to two $S=1/2$
quantum spin models on a honeycomb lattice. In calculations for these systems,
we make use of the self-consistent imaginary Dyson equation approach at finite
temperatures for evaluating the magnon damping rate. In both systems, the
thermal Hall conductivity is diminished due to the introduction of magnon
damping over a wide temperature range. This effect arises due to the smearing
of magnon spectra with nonzero Berry curvatures. We also discuss the relation
to the damping of chiral edge modes of magnons. Our formulation can be applied
to various localized electron systems as we begin with a general Hamiltonian
for these systems. Our findings shed light on a new aspect of topological
magnonics emergent from many-body effects and will stimulate further
investigations on the impact of magnon damping on topological phenomena.",2403.08478v1
1996-05-09,Two-magnon Raman scattering in a spin density wave antiferromagnet,"We present the results for a model calculation of resonant two-magnon Raman
scattering in a spin density wave (SDW) antiferromagnet. The resonant
enhancement of the two-magnon intensity is obtained from a microscopic analysis
of the photon-magnon coupling vertex. By combining magnon-magnon interactions
with `triple resonance` phenomena in the vertex function the resulting
intensity line shape is found to closely resemble the measured two-magnon Raman
signal in antiferromagnetic cuprates. Both, resonant and non-resonant Raman
scattering are discussed for the SDW antiferromagnet and a comparison is made
to the conventional Loudon-Fleury theory of two-magnon light scattering.",9605056v1
2012-09-04,Microscopic Theory of Magnon-Drag Thermoelectric Transport in Ferromagnetic Metals,"A theoretical study of the magnon-drag Peltier and Seebeck effects in
ferromagnetic metals is presented. A magnon heat current is described
perturbatively from the microscopic viewpoint with respect to electron--magnon
interactions and the electric field. Then, the magnon-drag Peltier coefficient
$\Pi_\MAG$ is obtained as the ratio between the magnon heat current and the
electric charge current. We show that $\Pi_\MAG=C_\MAG T^{5/2}$ at a low
temperature $T$; that the coefficient $C_\MAG$ is proportional to the spin
polarization $P$ of the electric conductivity; and that $P>0$ for $C_\MAG<0$,
but $P<0$ for $C_\MAG>0$. From experimental results for magnon-drag Peltier
effects, we estimate that the strength of the electron--magnon interaction is
about 0.3 eV$\cdot\AA^{3/2}$ for permalloy.",1209.0685v2
2013-02-24,Electron spin resonance in a dilute magnon gas as a probe of magnon scattering resonances,"We study the electron spin resonance in a dilute magnon gas that is realized
in a ferromagnetic spin system at low temperature. A quantum cluster expansion
is developed to show that the frequency shift of the single-magnon peak changes
its sign and the linewidth reaches its maximum across a scattering resonance
between magnons. Such characteristic behaviors are universal and can be used to
experimentally locate the two-magnon resonance when an external parameter such
as pressure is varied. Future achievement of the two-magnon resonance may have
an impact comparable to the Feshbach resonance in ultracold atoms and will open
up a rich variety of strongly correlated physics such as the recently proposed
Efimov effect in quantum magnets. We also suggest how the emergence of an
Efimov state of three magnons and its binding energy may be observed with the
electron spin resonance.",1302.5908v2
2013-07-05,Theory of asymmetric and negative differential magnon tunneling under temperature bias: Towards a spin Seebeck diode and transistor,"We study the nonequilibrium transport for the asymmetric and negative
differential magnon tunneling driven by temperature bias. We demonstrate that
the many-body magnon interaction that makes the magnonic spectrum
temperature-dependent is the crucial factor for the emergence of rectification
and negative differential spin Seebeck effects in magnon tunneling junctions.
When magnonic junctions have temperature-dependent density of states, reversing
the temperature bias is able to give asymmetric spin currents and increasing
temperature bias could give an anomalously decreasing magnonic spin current. We
show that these properties are relevant for building spin Seebeck diodes and
transistors, which could play important roles in controlling information and
energy in magnonics and spin caloritronics.",1307.1726v3
2014-02-26,Magneto-elastic modes and lifetime of magnons in thin yttrium-iron garnet films,"We calculate the effects of the spin-lattice coupling on the magnon spectrum
of thin ferromagnetic films consisting of the magnetic insulator yttrium-iron
garnet. The magnon-phonon hybridisation generates a characteristic minimum in
the spin dynamic structure factor which quantitatively agrees with recent
Brillouin light scattering experiments. We also show that at room temperature
the phonon contribution to the magnon damping exhibits a rather complicated
momentum dependence: In the exchange regime the magnon damping is dominated by
Cherenkov type scattering processes, while in the long-wavelength dipolar
regime these processes are subdominant and the magnon damping is two orders of
magnitude smaller. We supplement our calculations by actual measurements of the
magnon relaxation in the dipolar regime. Our theory provides a simple
explanation of a recent experiment probing the different temperatures of the
magnon and phonon gases in yttrium-iron garnet.",1402.6575v2
2014-08-06,Observation of Magnonic Band Gaps in Magnonic Crystals with Nopnreciprocal Dispersion Relation,"An effect of metallization of the magnonic crystal surface on the band gaps
formation in the spectra of the surface spin wave (SSW) is studied both
theoretically and experimentally. The structures under consideration are
one-dimensional magnonic crystals based on yttrium iron garnet with an array of
etched grooves with metal screen on the top of the corrugated surface and
without it. Due to nonreciprocity of propagation of the SSW the shift of band
gap to higher frequency and from the border of the Brillouin zone in presence
of conducting overlayer was measured in transmission line experiment. Results
of numerical calculations and model analysis are in agreement with experimental
data and give further insight into origin of the band gap and properties of the
nonreciprocal SSW in metallized magnonic crystals. This gives positive answer
to the outstanding question about possibility of detection of magnonic band
gaps in the spectra of the spin waves with nonreciprocal dispersion in magnonic
crystals and creates potential for new applications and improvements of already
existing prototype magnonic devices.",1408.1221v2
2015-03-02,Supercurrent in a room temperature Bose-Einstein magnon condensate,"We report evidence for the existence of a supercurrent of magnons in a magnon
Bose-Einstein condensate prepared in a room temperature yttrium-iron-garnet
magnetic film and subject to a thermal gradient. The magnon condensate is
formed in a parametrically populated magnon gas, and its temporal evolution is
studied by time-, frequency- and wavector-resolved Brillouin light scattering
spectroscopy. It has been found that local heating in the focal point of a
probing laser beam enhances the temporal decrease in the density of the freely
evolving magnon condensate after the termination of the pumping pulse, but it
does not alter the relaxation dynamics of the gaseous magnon phase. This
phenomenon is understood as the appearance of a magnon supercurrent within the
condensate due to a temperature- and, consequently, magnetisation-gradient
induced phase gradient in the condensate wave function.",1503.00482v2
2016-06-27,Stability of a skyrmion and interaction of magnons,"The stability of a single Belavin-Polyakov (BP) skyrmion in isotropic
Heisenberg ferromagnet is studied. Such skyrmion is higher in energy than the
uniform ferromagnetic state and is thus metastable. Starting from the lattice
model in two spatial dimensions and using Maleyev-Dyson representation for spin
operators, we examine the effects of magnon-magnon interaction for two
quantities at $T=0$. First we discuss the self-energy corrections to magnon
energy. Second we analyze the two-particle Green's function and possible bound
states of two magnons. The simplicity of the model makes possible full analytic
treatment of all relevant processes. We found that the magnons remain
well-defined quasiparticles with a finite lifetime. The bound states of two
magnons are suppressed near the skyrmion, although they are not excluded far
away from it. A resonance for the magnons of dilational mode in the vicinity of
BP skyrmion is also found, which leads to a redistribution of the spectral
weight. We conclude that the BP skyrmion as the classical topological object is
not destroyed by quantum fluctuations.",1606.08308v2
2016-07-06,Temperature dependence of the magnon spin diffusion length and magnon spin conductivity in the magnetic insulator yttrium iron garnet,"We present a systematic study of the temperature dependence of diffusive
magnon spin transport, using a non-local device geometry. In our measurements,
we detect spin signals arising from electrical and thermal magnon generation,
and we directly extract the magnon spin diffusion length $\lambda_m$ for
temperatures from 2 to 293 K. Values of $\lambda_m$ obtained from electrical
and thermal generation agree within the experimental error, with
$\lambda_m=9.6\pm0.9$ $\mu$m at room temperature to a minimum of
$\lambda_m=5.5\pm0.7$ $\mu$m at 30 K. Using a 2D finite element model to fit
the data obtained for electrical magnon generation we extract the magnon spin
conductivity $\sigma_m$ as a function of temperature, which is reduced from
$\sigma_m=5.1\pm0.2\times10^5$ S/m at room temperature to
$\sigma_m=0.7\pm0.4\times10^5$ S/m at 5 K. Finally, we observe an enhancement
of the signal originating from thermally generated magnons for low
temperatures, where a maximum is observed around $T=7$ K. An explanation for
this low temperature enhancement is however still missing and requires
additional investigations.",1607.01506v1
2018-11-21,Squeezed states of magnons and phonons in cavity magnomechanics,"We show how to create quantum squeezed states of magnons and phonons in a
cavity magnomechanical system. The magnons are embodied by a collective motion
of a large number of spins in a macroscopic ferrimagnet, and couple to cavity
microwave photons and phonons (vibrational modes of the ferrimagnet) via the
magnetic dipole interaction and magnetostrictive interaction, respectively. The
cavity is driven by a weak squeezed vacuum field generated by a flux-driven
Josephson parametric amplifier, which is essential to get squeezed states of
the magnons and phonons. We show that the magnons can be prepared in a squeezed
state via the cavity-magnon beamsplitter interaction, and by further driving
the magnon mode with a strong red-detuned microwave field, the phonons are
squeezed. We show optimal parameter regimes for obtaining large squeezing of
the magnons and phonons, which are robust against temperature and could be
realized with experimentally reachable parameters.",1811.09668v4
2018-11-25,Spin current cross-correlations as a probe of magnon coherence,"Motivated by the important role of the normalized second order coherence
function, often called $g^{(2)}$, in the field of quantum optics, we propose a
method to determine magnon coherence in solid-state devices. Namely, we show
that the cross-correlations of pure spin-currents injected by a ferromagnet
into two metal leads, normalized by their dc value, replicate the behavior of
$g^{(2)}$ when magnons are driven far from equilibrium. We consider two
scenarios: driving by ferromagnetic resonance, which leads to the coherent
occupation of a single mode, and driving by heating of the magnons, which leads
to an excess of incoherent magnons. We find an enhanced normalized
cross-correlation in the latter case, thereby demonstrating bunching of
nonequilibrium thermal magnons due to their bosonic statistics. Our results
contribute to the burgeoning field of quantum magnonics, which seeks to explore
and exploit the quantum nature of magnons.",1811.10001v1
2017-06-06,Spin Seebeck effect and magnon-magnon drag in Pt/YIG/Pt structures,"The formation of the two: injected (""coherent"") and ""thermally"" excited,
different in energies magnon subsystems and the influence of its interaction
with phonons and between on drag effect under spin Seebeck effect conditions in
the magnetic insulator part of the metal/ferromagnetic insulator/metal
structure is studied. An approximation of the effective parameters, when each
of the interacting subsystems (""injected"", ""thermal"" magnons, and phonons) is
characterized by its own effective temperature and drift velocities have been
considered. The analysis of the macroscopic momentum balance equations of the
systems of interest conducted for different ratios of the drift velocities of
the magnon and phonon currents show that the ""injected"" magnons relaxation on
the ""thermal"" ones is possible to be dominant over its relaxation on phonons.
This interaction will be the defining in the forming of the temperature
dependence of the spin-wave current under spin Seebeck effect conditions, and
inelastic part of the magnon-magnon interaction is the dominant spin relaxation
mechanism.",1706.02154v1
2017-10-07,Magnon-phonon relaxation in yttrium iron garnet from first principles,"We combine the theoretical method of calculating spin wave excitation with
the finite-temperature modeling and calculate the magnon-phonon relaxation time
in the technologically important material Yttrium iron garnet (YIG) from first
principles. The finite lifetime of magnon excitation is found to arise from the
fluctuation of the exchange interaction of magnetic atoms in YIG. At room
temperature, the magnon spectra have significant broadening that is used to
extract the magnon-phonon relaxation time quantitatively. The latter is a
phenomenological parameter of great importance in YIG-based spintronics
research. We find that the magnon-phonon relaxation time for the optical magnon
is a constant while that for the acoustic magnon is proportional to $1/k^2$ in
the long-wavelength regime.",1710.02647v1
2017-12-22,Chiral topological insulator of magnons,"We propose a magnon realization of 3D topological insulator in the AIII
(chiral symmetry) topological class. The topological magnon gap opens due to
the presence of Dzyaloshinskii-Moriya interactions. The existence of the
topological invariant is established by calculating the bulk winding number of
the system. Within our model, the surface magnon Dirac cone is protected by the
sublattice chiral symmetry. By analyzing the magnon surface modes, we confirm
that the backscattering is prohibited. By weakly breaking the chiral symmetry,
we observe the magnon Hall response on the surface due to opening of the gap.
Finally, we show that by changing certain parameters the system can be tuned
between the chiral topological insulator (mcTI), three dimensional magnon
anomalous Hall (3D-mAH), and Weyl magnon phases.",1712.08612v3
2018-09-18,Many-body theory of spin-current driven instabilities in magnetic insulators,"We consider a magnetic insulator in contact with a normal metal. We derive a
self-consistent Keldysh effective action for the magnon gas that contains the
effects of magnon-magnon interactions and contact with the metal to lowest
order. Self-consistent expressions for the dispersion relation, temperature and
chemical potential for magnons are derived. Based on this effective action, we
study instabilities of the magnon gas that arise due to spin-current flowing
across the interface between the normal metal and the magnetic insulator. We
find that the stability phase diagram is modified by an interference between
magnon-magnon interactions and interfacial magnon-electron coupling. These
effects persist at low temperatures and for thin magnetic insulators.",1809.06738v1
2020-12-08,Observation of 4- and 6-magnon bound-states in the spin-anisotropic frustrated antiferromagnet FeI$_2$,"Spin-waves e.g. magnons are the conventional elementary excitations of
ordered magnets. However, other possibilities exist. For instance, magnon
bound-states can arise due to attractive magnon-magnon interactions and
drastically impact the static and dynamic properties of materials. Here, we
demonstrate a zoo of distinct multi-magnon quasiparticles in the frustrated
spin-1 triangular antiferromagnet FeI$_2$ using time-domain terahertz
spectroscopy. The energy-magnetic field excitation spectrum contains signatures
of one-, two-, four- and six-magnon bound-states, which we analyze using an
exact diagonalization approach for a dilute gas of interacting magnons. The
two-magnon single-ion bound states occur due to strong anisotropy and the
preponderance of even higher order excitations arises from the tendency of the
single-ion bound states to themselves form bound states due to their very flat
dispersion. This menagerie of tunable interacting quasiparticles provides a
unique platform in a condensed matter setting that is reminiscent of the
few-body quantum phenomena central to cold-atom, nuclear, and particle physics
experiments.",2012.04205v1
2020-12-08,Inverse-design magnonic devices,"The field of magnonics offers a new type of low-power information processing,
in which magnons, the quanta of spin waves, carry and process data instead of
electrons. Many magnonic devices were demonstrated recently, but the
development of each of them requires specialized investigations and, usually,
one device design is suitable for one function only. Here, we introduce the
method of inverse-design magnonics, in which any functionality can be specified
first, and a feedback-based computational algorithm is used to obtain the
device design. Our proof-of-concept prototype is based on a rectangular
ferromagnetic area which can be patterned using square shaped voids. To
demonstrate the universality of this approach, we explore linear, nonlinear and
nonreciprocal magnonic functionalities and use the same algorithm to create a
magnonic (de-)multiplexer, a nonlinear switch and a circulator. Thus,
inverse-design magnonics can be used to develop highly efficient rf
applications as well as Boolean and neuromorphic computing building blocks.",2012.04544v1
2018-12-03,Microscopic theory of magnon-drag electron flow in ferromagnetic metals,"A temperature gradient applied to a ferromagnetic metal induces not only
independent flows of electrons and magnons but also drag currents because of
their mutual interaction. In this paper, we present a microscopic study of the
electron flow induced by the drag due to magnons. The analysis is based on the
$s$-$d$ model, which describes conduction electrons and magnons coupled via the
$s$-$d$ exchange interaction. Magnetic impurities are introduced in the
electron subsystem as a source of spin relaxation. The obtained magnon-drag
electron current is proportional to the entropy of magnons and to $\alpha -
\beta$ (more precisely, to $1 - \beta/\alpha$), where $\alpha$ is the Gilbert
damping constant and $\beta$ is the dissipative spin-transfer torque parameter.
This result almost coincides with the previous phenomenological result based on
the magnonic spin-motive forces, and consists of spin-transfer and
momentum-transfer contributions, but with a slight disagreement in the former.
The result is interpreted in terms of the nonequilibrium spin chemical
potential generated by nonequilibrium magnons.",1812.00720v1
2018-12-05,Controlling stability of Bose-Einstein condensation of interacting magnons in an antiferromagnet by an external magnetic field,"We propose a mechanism for destabilizing or stabilizing Bose-Einstein
condensation (BEC) of interacting magnons in an antiferromagnet. We study how
the interaction between magnons affects the magnon BEC in a two-sublattice
antiferromagnet without and with an external magnetic field. We show that in
the absence of the magnetic field the magnon BEC is destabilized by the
attractive interband interaction, which is larger than the repulsive intraband
interaction. We also show that in the presence of the magnetic field the magnon
BEC is stabilized only if the magnetic field is large enough to make the
intraband interaction larger than the interband interaction. Our results
provide the first step for understanding the interaction effects on the magnon
BEC in antiferromagnets and may open interaction physics of multicomponent BEC
of magnons in magnets.",1812.01787v2
2018-12-20,Magnon-polarons in cubic collinear Antiferromagnets,"We present a theoretical study of excitations formed by hybridization between
magnons and phonons - magnon-polarons - in antiferromagnets. We first outline a
general approach to determining which magnon and phonon modes can and cannot
hybridize in a system thereby addressing the qualitative questions concerning
magnon-polaron formation. As a specific and experimentally relevant case, we
study Nickel Oxide quantitatively and find perfect agreement with the
qualitative analysis, thereby highlighting the strength of the former. We find
that there are two distinct features of antiferromagnetic magnon-polarons which
differ from the ferromagnetic ones. First, hybridization between magnons and
the longitudinal phonon modes is expected in many cubic antiferromagnetic
structures. Second, we find that the very existence of certain hybridizations
can be controlled via an external magnetic field, an effect which comes in
addition to the ability to move the magnon modes relative to the phonons modes.",1812.09239v1
2019-03-09,Theory of the magnon Kerr effect in cavity magnonics,"We develop a theory for the magnon Kerr effect in a cavity magnonics system,
consisting of magnons in a small yttrium iron garnet (YIG) sphere strongly
coupled to cavity photons, and use it to study the bistability in this hybrid
system. To have a complete picture of the bistability phenomenon, we analyze
two different cases in driving the cavity magnonics system, i.e., directly
pumping the YIG sphere and the cavity, respectively. In both cases, the magnon
frequency shifts due to the Kerr effect exhibit a similar bistable behavior but
the corresponding critical powers are different. Moreover, we show how the
bistability of the system can be demonstrated using the transmission spectrum
of the cavity. Our results are valid in a wide parameter regime and generalize
the theory of bistability in a cavity magnonics system.",1903.03754v1
2015-08-09,Magnon-Magnon Interactions in O(3) Ferromagnets and Equations of Motion for Spin Operators,"The method of equations of motion for spin operators in the case of O(3)
Heisenberg ferromagnet is systematically analyzed starting from the effective
Lagrangian. It is shown that the random phase approximation and the Callen
approximation can be understood in terms of perturbation theory for type B
magnons. Also, the second order approximation of Kondo and Yamaji for one
dimensional ferromagnet is reduced to the perturbation theory for type A
magnons. An emphasis is put on the physical picture, i.e. on magnon-magnon
interactions and symmetries of the Heisenberg model. Calculations demonstrate
that all three approximations differ in manner in which the magnon-magnon
interactions arising from the Wess-Zumino term are treated, from where specific
features and limitations of each of them can be deduced.",1508.01974v1
2020-06-17,Magnon Polarons induced by a magnetic field gradient,"In this work, we report the theoretical possibility of generating magnon
polaron excitations through a space-varying magnetic field. The spatial
dependence of the magnetic field in the Zeeman interaction gives rise to a
magnon-phonon coupling when a magnetic field gradient is applied, and such a
coupling depends directly on the strength of the gradient. It is also predicted
that the direction of the magnetic field gradient allows control over which
phonon polarization couples to the magnons in the material. Here we develop the
calculations of the magnon-phonon coupling for an arbitrary (anti)ferromagnet,
which are later used to numerically study its consequences. These results are
compared to the ones obtained with the phenomenological magnetoelastic coupling
in YIG, where we show that the magnon polaron bandgap seen in YIG can be also
obtained with a magnetic field gradient of $\sim 0.1$T/m which can be achieved
with the current experimental techniques. Our results propose a new way of
controlling the magnetoelastic coupling in an arbitrary material and open a new
route to exploit the magnon-phonon interaction in magnonic and spintronic
devices.",2006.09839v1
2020-09-26,Bose condensation and spin superfluidity of magnons in a perpendicularly magnetized film of yttrium iron garnet,"The formation of a Bose condensate of magnons in a perpendicularly magnetized
film of yttrium iron garnet under radio-frequency pumping in a strip line is
studied experimentally. The characteristics of nonlinear magnetic resonance and
the spatial distribution of the Bose condensate of magnons in the magnetic
field gradient are investigated. In these experiments, the Bosonic system of
magnons behaves similarly to the Bose condensate of magnons in the
antiferromagnetic superfluid 3He-B, which was studied in detail earlier.
Magnonic BEC forms a coherently precessing state with the properties of
magnonic superfluidity. Its stability is determined by the repulsive potential
between excited magnons, which compensates for the inhomogeneity of the
magnetic field.",2009.12595v1
2021-02-04,Magnonic frequency comb through nonlinear magnon-skyrmion scattering,"An optical frequency comb consists of a set of discrete and equally spaced
frequencies and has found wide applications in the synthesis over broad
spectral frequencies of electromagnetic wave and precise optical frequency
metrology. Despite the analogies between magnons and photons in many aspects,
the analogue of optical frequency comb in magnonic system has not been
reported. Here, we theoretically study the magnon-skyrmion interaction and find
that magnonic frequency comb (MFC) can be generated above a threshold of
driving amplitude, where the nonlinear scattering process involving three
magnons prevails. The mode-spacing of the MFC is equal to the breathing-mode
frequency of skyrmion and is thus tunable by either electric or magnetic means.
The theoretical prediction is verified by micromagnetic simulations and the
essential physics can be generalized to a large class of magnetic solitons. Our
findings open a new pathway to observe the frequency comb structure in magnonic
devices, that may inspire the study of fundamental nonlinear physics in
spintronic platform in the future.",2102.02571v3
2019-07-10,Mechanism for a Chemical Potential of Nonequilibrium Magnons in Parametric Parallel Pumping,"We demonstrate how a magnon chemical potential is generated in parametric
parallel pumping. We study how a time-periodic magnetic field of this pumping
affects magnon properties of a ferrimagnet in a nonequilibrium steady state. We
show that the magnon distribution function of our nonequilibrium steady state
becomes the Bose distribution function with $\mu=\omega_{\textrm{p}}/2$, where
$\mu$ is the magnon chemical potential and $\omega_{\textrm{p}}$ is the pumping
frequency. This result is distinct from the absence of the magnon chemical
potential in the standard theory and can qualitatively explain its generation
in experiments. We believe our result is a first theoretical demonstration of
the generation of the magnon chemical potential in the parametric parallel
pumping, providing an important step towards a thorough understanding of
properties of nonequilibrium magnons.",1907.04552v1
2019-10-23,Electrical manipulation of spin pumping signal through nonlocal thermal magnon transport,"We study the magnon transport in the nonlocal configuration composed of two
Pt strips on top of yttrium iron garnet, with and without the presence of RF
microwave generated by an on-chip antenna. We find that the spin-Hall induced
thermal magnon heating/cooling, the Oersted field as well as the Joule heating
generated by the a.c. current in the Pt injector can significantly influence
the spin-pumping signal measured by the Pt detector in the presence of RF
microwave, forcing the spin-pumping voltage to show up in the first and second
harmonic signals in the nonlocal magnon transport measurement. These results
indicate that nonlocal magnon transport configuration can serve as a structure
to electrically detect and manipulate the spin-pumping signal. Furthermore,
certain caution is needed when studying the interplay between incoherent magnon
and coherent magnon spin transport in the nonlocal transport configuration,
since the change in microwave-induced spin-pumping voltage can overwhelm the
incoherent magnon transport signals.",1910.10326v2
2019-10-23,Magnon crystallization in the kagome lattice antiferromagnet,"We present numerical evidence for the crystallization of magnons below the
saturation field at non-zero temperatures for the highly frustrated spin-half
kagome Heisenberg antiferromagnet. This phenomenon can be traced back to the
existence of independent localized magnons or equivalently flat-band
multi-magnon states. We present a loop-gas description of these localized
magnons and a phase diagram of this transition, thus providing information for
which magnetic fields and temperatures magnon crystallization can be observed
experimentally. The emergence of a finite-temperature continuous transition to
a magnon-crystal is expected to be generic for spin models in dimension $D>1$
where flat-band multi-magnon ground states break translational symmetry.",1910.10448v3
2020-03-25,Two-photon driven magnon-pair resonance as a signature of spin-nematic order,"We theoretically study the nonlinear magnetic resonance driven by intense
laser or electromagnetic wave in a fully polarized frustrated magnet near a
less-visible spin-nematic ordered phase. In general, both magnons and magnon
pairs (two-magnon bound state) appear as the low-energy excitation in the
saturated state of spin-nematic magnets. Their excitation energies are usually
in terahertz (THz) or gigahertz range. Magnon pairs with angular momentum
2$\hbar$ can be excited by the simultaneous absorption of two photons, and such
multi-photon processes occur if the applied THz laser is strong enough. We
compute laser-driven magnetic dynamics of a frustrated four-spin system with
both magnon ($\hbar$) and magnon-pair (2$\hbar$) like excitations which is
analogous to a macroscopic frustrated magnet with a spin nematic phase. We
estimate the required strength of magnetic field of laser for the realization
of two photon absorption, taking into account dissipation effects with the
Lindblad equation. We show that intense THz laser with ac magnetic field of
0.1-1.0 Tesla is enough to observe magnon-pair resonance.",2003.11240v2
2020-04-13,Revealing ultra-strong magnon-photon coupling in a polar antiferromagnet Fe2Mo3O8 by time domain terahertz spectroscopy,"Strong coupling between magnon and electromagnetic wave can lead to the
formation of a coupled spinphoton quasiparticle named as magnon-polariton. The
phenomenon is well studied for ferromagnetic systems inside microwave cavities
in recent years. However, formation of magnon-polariton is rarely seen for an
antiferromagnet (AFM) because the strong coupling condition is not easily
fulfilled. Here we present time-domain terahertz measurement on a multiferroic
polar antiferromagnet Fe2Mo3O8. We find clearly beating between two modes at
frequencies above and below the electric-active magnon frequency below TN,
which we assign to the formation of AFM magnon-polariton. An ultra-strong
spin-photon coupling effect is derived based on the energy level splitting.
However, the AFM magnon-polariton is absent in the frequency domain
measurement. Our work reveals that the coherent magnon formation driven by the
ultrashort THz pulse provides a new way to detect polariton mode splitting.",2004.05823v2
2020-05-25,Nonlinear losses in magnon transport due to four-magnon scattering,"We report on the impact of nonlinear four-magnon scattering on magnon
transport in microstructured Co25Fe75 waveguides with low magnetic damping. We
determine the magnon propagation length with microfocused Brillouin light
scattering over a broad range of excitation powers and detect a decrease of the
attenuation length at high powers. This is consistent with the onset of
nonlinear four-magnon scattering. Hence, it is critical to stay in the linear
regime, when deriving damping parameters from the magnon propagation length.
Otherwise, the intrinsic nonlinearity of magnetization dynamics may lead to a
misinterpretation of magnon propagation lengths and, thus, to incorrect values
of the magnetic damping of the system.",2005.12113v2
2020-07-17,A nonlinear magnonic nano-ring resonator,"The field of magnonics, which aims at using spin waves as carriers in data
processing devices, has attracted increasing interest in recent years. We
present and study micromagnetically a nonlinear nanoscale magnonic ring
resonator device for enabling implementations of magnonic logic gates and
neuromorphic magnonic circuits. In the linear regime, this device efficiently
suppresses spin-wave transmission using the phenomenon of critical resonant
coupling, thus exhibiting the behavior of a notch filter. By increasing the
spin-wave input power, the resonance frequency is shifted leading to
transmission curves, depending on the frequency, reminiscent of the activation
functions of neurons or showing the characteristics of a power limiter. An
analytical theory is developed to describe the transmission curve of magnonic
ring resonators in the linear and nonlinear regimes and validated by a
comprehensive micromagnetic study. The proposed magnonic ring resonator
provides a multi-functional nonlinear building block for unconventional
magnonic circuits.",2007.09205v2
2020-07-27,Probing thermal magnon current mediated by coherent magnon via nitrogen-vacancy centers in diamond,"Currently, thermally excited magnons are being intensively investigated owing
to their potential in computing devices and thermoelectric conversion
technologies. We report the detection of thermal magnon current propagating in
a magnetic insulator yttrium iron garnet under a temperature gradient using a
quantum sensor: electron spins associated with nitrogen-vacancy (NV) centers in
diamond. Thermal magnon current was observed as modified Rabi oscillation
frequencies of NV spins hosted in a beam-shaped bulk diamond that resonantly
coupled with coherent magnon propagating over a long distance. Additionally,
using a nanodiamond, alteration in NV spin relaxation rates depending on the
applied temperature gradient were observed under a non-resonant NV excitation
condition. The demonstration of probing thermal magnon current mediated by
coherent magnon via NV spin states serves as a basis for creating a device
platform hybridizing spin caloritronics and spin qubits.",2007.13433v6
2020-08-02,Observation of Antiferromagnetic Magnon Pseudospin Dynamics and the Hanle effect,"We report on experiments demonstrating coherent control of magnon spin
transport and pseudospin dynamics in a thin film of the antiferromagnetic
insulator hematite utilizing two Pt strips for all-electrical magnon injection
and detection. The measured magnon spin signal at the detector reveals an
oscillation of its polarity as a function of the externally applied magnetic
field. We quantitatively explain our experiments in terms of diffusive magnon
transport and a coherent precession of the magnon pseudospin caused by the
easy-plane anisotropy and the Dzyaloshinskii-Moriya interaction. This
experimental observation can be viewed as the magnonic analogue of the
electronic Hanle effect and the Datta-Das transistor, unlocking the high
potential of antiferromagnetic magnonics towards the realization of rich
electronics-inspired phenomena.",2008.00440v2
2020-10-18,Observation of magnon-polarons in a uniaxial antiferromagnetic insulator,"Magnon-polarons, a type of hybridized excitations between magnons and
phonons, were first reported in yttrium iron garnet as anomalies in the spin
Seebeck effect responses. Here we report an observation of antiferromagnetic
(AFM) magnon-polarons in a uniaxial AFM insulator Cr2O3. Despite the relatively
higher energy of magnon than that of the acoustic phonons, near the spin-flop
transition of ~ 6 T, the left-handed magnon spectrum shifts downward to
hybridize with the acoustic phonons to form AFM magnon-polarons, which can also
be probed by the spin Seebeck effect. The spin Seebeck signal is founded to be
enhanced due to the magnon-polarons at low temperatures.",2010.08926v1
2020-10-20,Valley Modulation and Single-Edge Transport of Magnons in Staggered Kagome Ferromagnets,"Owing to its charge-free property, magnon is highly promising to achieve
dissipationless transport without Joule heating and thus potentially applicable
to energy-efficient devices. Moreover, a kagome lattice, as stacking layers of
many magnon ferromagnets, also exhibits valley structure in quasiparticle
spectra, which are likely to add a new dimension to magnon excitation. Here, we
investigate valley magnon and associated valley modulation in a kagome lattice,
with staggered exchange interaction and Dzyaloshinskii-Moriya interaction. The
staggered exchange interaction breaks spatial inversion symmetry, leading to
gapped degenerate valleys at $\pm K$ and consequent valley magnon Hall effect.
When the Dzyaloshinskii-Moriya interaction is further included, the valley
degeneracy is lifted. As a result, net magnon anomalous Hall effect and
topological phase transition are realized. More interestingly, by tuning valley
splitting and excitation frequency, heat currents in the kagome strip can be
localized at one edge to achieve single-edge transport. Besides, for the kagmon
lattice, the edge heat currents include local circulating contribution within
triangular fine structure, together with currents flowing parallel to the
edges. These findings give full play to spin and valley degrees of freedom and
enrich energy-efficient magnonic device paradigms.",2010.09945v1
2020-11-17,Local and nonlocal spin Seebeck effect in lateral Pt-$\mathrm{Cr_2O_3}$-Pt devices at low temperatures,"We have studied thermally driven magnon spin transport (spin Seebeck effect,
SSE) in heterostructures of antiferromagnetic $\alpha$-$\mathrm{Cr_2O_3}$ and
Pt at low temperatures. Monitoring the amplitude of the local and nonlocal SSE
signals as a function of temperature, we found that both decrease with
increasing temperature and disappear above 100 K and 20 K, respectively.
Additionally, both SSE signals show a tendency to saturate at low temperatures.
The nonlocal SSE signal decays exponentially for intermediate injector-detector
separation, consistent with magnon spin current transport in the relaxation
regime. We estimate the magnon relaxation length of our
$\alpha$-$\mathrm{Cr_2O_3}$ films to be around 500 nm at 3 K. This short magnon
relaxation length along with the strong temperature dependence of the SSE
signal indicates that temperature-dependent inelastic magnon scattering
processes play an important role in the intermediate range magnon transport.
Our observation is relevant to low-dissipation antiferromagnetic magnon memory
and logic devices involving thermal magnon generation and transport.",2011.08885v2
2021-06-17,Cavity Magnonics,"Cavity magnonics deals with the interaction of magnons - elementary
excitations in magnetic materials - and confined electromagnetic fields. We
introduce the basic physics and review the experimental and theoretical
progress of this young field that is gearing up for integration in future
quantum technologies. Much of its appeal is derived from the strong
magnon-photon coupling and the easily-reached nonlinear regime in microwave
cavities. The interaction of magnons with light as detected by Brillouin light
scattering is enhanced in magnetic optical resonators, which can be employed to
manipulate magnon distributions. The cavity photon-mediated coupling of a
magnon mode to a superconducting qubit enables measurements in the single
magnon limit.",2106.09312v1
2021-07-23,Dzyaloshinskii-Moriya Induced Topological Magnon-Phonon Hybridization in 2D Antiferromagnetic Insulators with Tunable Chern Numbers,"We theoretically study magnon-phonon hybrid excitations (magnon-polarons) in
two-dimensional antiferromagnets on a honeycomb lattice. With an in-plane
Dzyaloshinskii-Moriya interaction (DMI) allowed from mirror symmetry breaking
from phonons, we find non-trivial Berry curvature around the anti-crossing
rings among magnon and both optical and acoustic phonon bands, which gives rise
to finite Chern numbers. We show that the Chern numbers of the magnon-polaron
bands can be manipulated by changing the magnetic field direction or strength.
We evaluate the thermal Hall conductivity reflecting the non-trivial Berry
curvatures of magnon-polarons and propose a valley Hall effect resulting from
spin-induced chiral phonons as a possible experimental signature. Our study
complements prior work on magnon-phonon hybridized systems without optical
phonons and suggests possible applications in spin caloritronics with
topological magnons and chiral phonons.",2107.11484v1
2021-11-24,A generalized model of magnon kinetics and subgap magnetic noise,"Magnetic noise spectroscopy provides a noninvasive probe of spin dynamics in
magnetic materials. We consider two-dimensional magnetically ordered insulators
with magnon excitations, especially those supporting long-distance magnon
transport, where nitrogen-vacancy (NV) centers enable the access to (nearly)
ballistic transport regime of magnons. We develop a generalized theory to
describe the magnon transport across a wide range of length scales. The
longitudinal dynamic spin susceptibility is derived from the Boltzmann equation
and extended to a Lindhard form, which is modified by both the spin-conserving
magnon collisions and spin relaxation. Our result is consistent with the
diffusive (ballistic) model for the length scale much larger (smaller) than the
magnon mean free path, and provides a description for the intermediate regime.
We also give a prediction for the NV transition rate in different magnon
transport regimes.",2111.12613v2
2021-12-01,Strong Long-Range Spin-Spin Coupling via a Kerr Magnon Interface,"Strong long-range coupling between distant spins is crucial for spin-based
quantum information processing. However, achieving such a strong spin-spin
coupling remains challenging. Here we propose to realize a strong coupling
between two distant spins via the Kerr effect of magnons in a
yttrium-iron-garnet nanosphere. By applying a microwave field on this
nanosphere, the Kerr effect of magnons can induce the magnon squeezing, so that
the coupling between the spin and the squeezed magnons can be exponentially
enhanced. This in turn allows the spin-magnon distance to increase from nano-
to micrometer scale. By considering the virtual excitation of the squeezed
magnons in the dispersive regime, strong spin-spin coupling mediated by the
squeezed magnons can be achieved, and a remote quantum-state transfer, as well
as the nonlocal two-qubit ISWAP gate with high fidelity becomes implementable.
Our approach offers a feasible scheme to perform quantum information processing
among distant spins.",2112.00452v3
2021-12-21,Giant dynamical electron-magnon coupling in metal-metal-ferromagnetic insulator heterostructure,"Magnon-mediated spin transport across nonmagnetic metal (NM) and
ferromagnetic insulator (FI) interface depends critically on electron-magnon
coupling. We propose a novel route to enhance electron-magnon coupling
dynamically from transport viewpoint. Using non-equilibrium Green's function a
theoretical formalism for magnon-mediated spin current is developed. In the
language of transport, the effective electron-magnon coupling at NM/FI
interface is determined by self-energy of FI lead, which is proportional to
density of states (DOS) at NM/FI interface due to nonlinear process of
electron-magnon conversion. By modifying interfacial DOS, the spin conductance
of 2D and 3D NM/FI systems can be increased by almost three orders of
magnitude, setting up a new platform of manipulating dynamical electron-magnon
coupling.",2112.11021v1
2021-12-22,Topological magnon modes on honeycomb lattice with coupling textures,"Topological magnon modes are expected to be useful for novel applications
such as robust information propagation, since they are immune to backscattering
and robust against disorder. Although there are several of theoretical
proposals for topological magnon modes and growing experimental efforts for
realizing them by now, it is still desirable to add complementary insights on
this important phenomenon. Here, we propose a new scheme to achieve topological
magnon where only nearest-neighbour exchange couplings on honeycomb lattice are
necessary. In both ferromagnets and antiferromagnets, tuning exchange couplings
between and inside hexagonal unit cells induces a topological state accompanied
by a band inversion between p-orbital and d-orbital like magnon modes.
Topological magnon modes appear at the interface between a topological domain
and a trivial domain with magnon currents, which counterpropagate depending on
pseudospins originated from orbital angular momenta of magnon modes. This
mimics the spin-momentum locking phenomenon in the quantum spin Hall effect.",2112.11652v1
2021-12-29,Excitation and detection of coherent sub-terahertz magnons in ferromagnetic and antiferromagnetic heterostructures,"Excitation of coherent high-frequency magnons (quanta of spin waves) is
critical to the development of high-speed magnonic devices. Here we
computationally demonstrate the excitation of coherent sub-terahertz (THz)
magnons in ferromagnetic (FM) and antiferromagnetic (AFM) thin films by a
photoinduced picosecond acoustic pulse. Analytical calculations are also
performed to reveal the magnon excitation mechanism. Through spin pumping and
spin-charge conversion, these magnons can inject sub-THz charge current into an
adjacent heavy-metal film which in turn emits electromagnetic (EM) waves. Using
a dynamical phase-field model that considers the coupled dynamics of acoustic
waves, spin waves, and EM waves, we show that the emitted EM wave retains the
spectral information of all the sub-THz magnon modes and has a sufficiently
large amplitude for near-field detection. These predictions indicate that the
excitation and detection of sub-THz magnons can be realized in rationally
designed FM or AFM thin-film heterostructures via ultrafast optical-pump
THz-emission-probe spectroscopy.",2112.14749v2
2022-05-29,Magnon squeezing enhanced entanglement in a cavity magnomechanical system,"We investigate the generation of the entanglement in a cavity magnomechanical
system, which consists of three modes: a magnon mode, a microwave cavity mode
and a mechanical vibration mode, the couplings of the magnon-photon and the
magnon-phonon are achieved by the magnetic dipole interaction and the
magnetostrictive interaction, respectively. By introducing a squeezing of the
magnon mode, the magnon-photon and the magnon-phonon entanglements are
significantly enhanced compared with the case without inserting the magnon
squeezing. We find that an optimal parameter of the squeezing exists, which
yields the maximum entanglement. This study provides a new idea for exploring
the properties of quantum entanglement in the the cavity magnomechanical
systems, and may have some potential applications in the quantum state
engineering.",2205.14569v2
2022-10-21,Tuning bulk topological magnon properties with light-induced magnons,"Although theoretical modelling and inelastic neutron scattering measurements
have indicated the presence of topological magnon bands in multiple quantum
magnets, experiments remain unable to detect signal of magnon thermal Hall
effect in the quantum magnets, which is a consequence of magnons condensation
at the bottom of the bands following Bose Einstein statistics as well as the
concentration of Berry curvature at the higher energies. In a recent work, Malz
et al.[Nature Communications 10, 3937 (2019)] have shown that topological
magnons in edge states in a finite sample can be amplified using tailored
electromagnetic fields. We extend their approach by showing that a uniform
electromagnetic field can selectively amplify magnons with finite Berry
curvature by breaking inversion symmetry of a lattice. Using this approach, we
demonstrate the generation of bulk topological magnons in a Heisenberg
ferromagnet on the breathing kagome lattice and the consequent amplification of
thermal Hall effect.",2210.12087v2
2022-10-31,Nonlinear spin current of photoexcited magnons in collinear antiferromagnets,"We study the nonlinear magnon spin current induced by an ac electric field
under light irradiation in collinear antiferromagnets with broken inversion
symmetry. For linearly polarized light, we find that a dc spin current appears
through ``the magnon spin shift current"" mechanism, which is driven by a spin
polarization generation in the two magnon creation process and has a close
relationship to the geometry of magnon bands through Berry connection. For
circularly polarized light, a dc spin current appears through ``the spin
injection current"" mechanism, which is proportional to the relaxation time of
magnons and can be large when the magnon lifetime is long. We demonstrate
generation of the magnon spin shift and injection currents, based on a few toy
models and a realistic model for a multiferroic material M$_2$Mo$_3$O$_8$.",2210.17099v3
2022-12-12,Magnon-Plasmon Hybridization Mediated by Spin-Orbit Interaction in Magnetic Materials,"We propose a mechanism for magnon-plasmon coupling and hybridization in
ferromagnetic (FM) and antiferromagnetic (AFM) systems. The electric field
associated with plasmon oscillations creates a non-equilibrium spin density via
the inverse spin galvanic effect. This plasmon-induced spin density couples to
magnons by an exchange interaction. The strength of magnon-plasmon coupling
depends on the magneto-electric susceptibility of the system and the wavevector
at which the level repulsion is happened. This wavevector may be tuned by an
applied magnetic field. In AFM systems, the degeneracy of two chiral magnons is
broken in the presence of a magnetic field, and we find two separate hybrid
modes for left-handed and right-handed AFM magnons. Furthermore, we show that
magnon-plasmon coupling in AFM systems is enhanced because of strong
intra-sublattice spin dynamics. We argue that the recently discovered
two-dimensional magnetic systems are ideal platforms to investigate proposed
magnon-plasmon hybrid modes.",2212.06105v1
2023-02-14,Switchable Superradiant Phase Transition with Kerr Magnons,"The superradiant phase transition (SPT) has been widely studied in cavity
quantum electrodynamics (CQED). However, this SPT is still subject of ongoing
debates due to the no-go theorem induced by the so-called ${\bf A}^2$ term
(AT). We propose a hybrid quantum system, consisting of a single-mode cavity
simultaneously coupled to both a two-level system and yttrium-iron-garnet
sphere supporting magnons with Kerr nonlinearity, to restore the SPT against
the AT. The Kerr magnons here can effectively introduce an additional strong
and tunable AT to counteract the intrinsic AT, via adiabatically eliminating
the degrees of freedom of the magnons. We show that the Kerr magnons induced
SPT can exist in both cases of ignoring and including the intrinsic AT. Without
the intrinsic AT, the critical coupling strength can be dramatically reduced by
introducing the Kerr magnons, which greatly relaxes the experimental conditions
for observing the SPT. With the intrinsic AT, the forbidden SPT can be
recovered with the Kerr magnons in a reversed way. Our work paves a potential
way to manipulate the SPT against the AT in hybrid systems combining CQED and
nonlinear magnonics.",2302.07163v2
2023-02-17,Entangling ferrimagnetic magnons with an atomic ensemble via opto-magnomechanics,"We show how to prepare macroscopic entanglement between an atomic ensemble
and a large number of magnons in a ferrimagnetic YIG crystal. Specifically, we
adopt an opto-magnomechanical configuration where the magnetostriction-induced
magnomechanical displacement couples to an optical cavity via radiation
pressure, and the latter further couples to an ensemble of two-level atoms that
are placed inside the cavity. We show that by properly driving the cavity and
magnon modes, optomechanical entanglement is created which is further
distributed to the atomic and magnonic systems, yielding stationary
entanglement between atoms and magnons. The atom-magnon entanglement is a
result of the combined effect of opto- and magnomechanical cooling and
optomechanical parametric down-conversion interactions. A competition mechanism
between two mechanical cooling channels is revealed. We further show that
genuine tripartite entanglement of three massive subsystems, i.e., atoms,
magnons and phonons, can also be achieved in the same system. Our results
indicate that the hybrid opto-magnomechanical system may become a promising
system for preparing macroscopic quantum states involving magnons, photons,
phonons and atoms.",2302.08684v2
2023-03-20,Meter-scale strong coupling between magnons and photons,"We experimentally realize a meter-scale strong coupling effect between
magnons and photons at room temperature, with a coherent coupling of 20 m and a
dissipative coupling of 7.6 m. To this end, we integrate a saturable gain into
a microwave cavity and then couple this active cavity to a magnon mode via a
long coaxial cable. The gain compensates for the cavity dissipation, but
preserves the cavity radiation that mediates the indirect photon-magnon
coupling. It thus enables the long-range strong photon-magnon coupling. With
full access to traveling waves, we demonstrate a remote control of
photon-magnon coupling by modulating the phase and amplitude of traveling
waves, rather than reconfiguring subsystems themselves. Our method for
realizing long-range strong coupling in cavity magnonics provides a general
idea for other physical systems. Our experimental achievements may promote the
construction of information networks based on cavity magnonics.",2303.10925v3
2023-04-20,Magnon peak lineshape in the transverse dynamical structure factor of a magnetically polarized easy-axis $XXZ$ chain at low temperatures,"The ferromagnetically polarized gapped XXZ spin chain is studied at low
temperatures. Utilizing only the one- and two-magnon spectrums and focusing on
the magnon-creation contribution to the transverse dynamical susceptibility, we
represent the latter in the form of the Dyson equation. Then, following the
well known correspondence between the imaginary part of magnetic susceptibility
and dynamical structure factor, we get the low-temperature formula for the
magnon-peak lineshape. The suggested approach is effective only if the
processes related to magnon creations and to transitions from magnons to
coupled magnon pairs are energetically separated. As it is shown in the paper,
such separation is inherent in the easy-axis chains with rather strong
anisotropy. We present several plots and discuss their lineshapes. As the
supplemental result we obtain integral representations for the
temperature-dependent magnon resonance shift and the parameter which is usually
associated with the decay rate. The low-temperature behavior of the resonance
shift is studied in details. All calculations are performed up to {\it
controllable} error $o({\rm e}^{-\beta E_{gap}})$.",2304.10645v2
2023-06-09,Magnonic frequency comb in the magnomechanical resonator,"An optical frequency comb is a spectrum of optical radiation which consists
of evenly spaced and phase-coherent narrow spectral lines and is initially
invented in laser for frequency metrology purposes. A direct analogue of
frequency combs in the magnonic systems has not been demonstrated to date. In
our experiment, we generate a new magnonic frequency comb in the resonator with
giant mechanical oscillation through the magnomechanical interaction. We
observe the magnonic frequency comb contains up to 20 comb lines, which are
separated to the mechanical frequency of the 10.08 MHz. The thermal effect
based on the strong pump power induces the cyclic oscillation of the magnon
frequency shift, which leads to a periodic oscillation of the magnonic
frequency comb. Moreover, we demonstrate the stabilization and control of the
frequency spacing of the magnonic frequency comb via injection locking. Our
work lays the groundwork of magnonic frequency combs for sensing and metrology.",2306.07985v1
2023-12-12,Interacting Floquet topological magnons in laser-irradiated Heisenberg honeycomb ferromagnets,"When a Heisenberg honeycomb ferromagnet is irradiated by high frequency
circularly polarized light, the underlying uncharged magnons acquire a time
dependent Aharonov Casher phase, which makes it a Floquet topological magnon
insulator. In this context, we investigate the many body interaction effects of
Floquet magnons in laser irradiated Heisenberg honeycomb ferromagnets with
ocontaining Dzyaloshinskii Moriya interaction under the application of
circularly polarized off resonant light. We demonstrate that the quantum
ferromagnet systems periodically laser driven exhibits temperature driven
topological phase transitions due to Floquet magnon magnon interactions. The
thermal Hall effect of Floquet magnons serves as a prominent signature for
detecting these many body effects near the critical point, enabling
experimental investigation into this phenomenon. Our study complements the lack
of previous theoretical works that the topological phase transition of the
Floquet magnon under the linear spin wave approximation is only tunable by the
light field. Our study presents a novel approach for constructing Floquet
topological phases in periodically driven quantum magnet systems that goes
beyond the limitations of the linear spin wave theory. We provide numerical
results based on the well known van der Waals quantum magnet CrX3 (X=F, Cl, Br,
and I), calling for experimental implementation.",2312.06929v1
2023-12-14,Magnetization Reversal of 50-nm-wide Ni81Fe19 Nanostripes by Ultrashort Magnons in Yttrium Iron Garnet for Memory-Enhanced Magnonic Circuits,"Spin waves (magnons) can enable wave-based neuromorphic computing by which
one aims at overcoming limitations inherent to conventional electronics and the
von Neumann architecture. In this study, we explore the storage of magnon
signals and the magnetization switching of periodic and aperiodic arrays of
Ni81Fe19 (Py) nanostripes with widths (w) between 50 nm and 200 nm. Spin waves
excited with low microwave power in yttrium iron garnet induce the reversal of
the nanostripes of different w in a small opposing field. Exploiting
microwave-to-magnon transducers for magnon modes with ultrashort wavelengths,
we demonstrate the reversal of 50-nm-wide Py nanostripes by magnons with
wavelength ~ 100 nm after they have propagated over 25 micrometer in YIG. The
findings are important for designing a magnon-based in-memory computing device.",2312.09177v1
2024-03-22,Tunable ultrastrong magnon magnon coupling and spin Hall magnetoresistance in a van der Waals antiferromagnet,"Antiferromagnetic (AFM) magnons and the manipulation of magnetisation via
spin currents in van der Waals (vdW) materials offer substantial potential for
applications in magnonics and spintronics. In this study, we demonstrate
ultrastrong magnon-magnon coupling in the GHz regime within a vdW AFM,
achieving a remarkable maximum normalised coupling strength of 0.47. Our
investigation unveils the tunability of coupling strength through
temperature-dependent magnetic anisotropies. Additionally, we show the prospect
for further enhancement to the deep-strong magnon-magnon coupling regime by
manipulating the anisotropy. Furthermore, we report spin Hall magnetoresistance
in a vdW AFM-based device and study the field and temperature dependence. These
findings highlight the transformative potential of vdW AFMs in advancing the
field of spin-based technologies.",2403.15558v1
2016-02-22,A first theoretical realization of honeycomb topological magnon insulator,"It has been recently shown that in the Heisenberg (anti)ferromagnet on the
honeycomb lattice, the magnons (spin wave quasipacticles) realize a massless
two-dimensional (2D) Dirac-like Hamiltonian. It was shown that the Dirac magnon
Hamiltonian preserves time-reversal symmetry defined with the sublattice pseudo
spins and the Dirac points are robust against magnon-magnon interactions. The
Dirac points also occur at nonzero energy. In this paper, we propose a simple
realization of nontrivial topology (magnon edge states) in this system. We show
that the Dirac points are gapped when the inversion symmetry of the lattice is
broken by introducing a next-nearest neighbour Dzyaloshinskii-Moriya (DM)
interaction. Thus, the system realizes magnon edge states similar to Haldane
model for quantum anomalous Hall effect in electronic systems. However, in
contrast to electronic spin current where dissipation can be very large due to
Ohmic heating, noninteracting topological magnons can propagate for long time
without dissipation as magnons are uncharged particles. We observe the same
magnon edge states for the XY model on the honeycomb lattice. Remarkably, in
this case the model maps to interacting hardcore bosons on the honeycomb
lattice. Quantum magnetic systems with nontrivial magnon edge states are called
topological magnon insulators. They have been studied theoretically on the
kagome lattice and recently observed experimentally on the kagome magnet
Cu(1-3, bdc) with three magnon bulk bands. Our results for the honeycomb
lattice suggests an experimental procedure to search for honeycomb topological
magnon insulators within a class of 2D quantum magnets and ultracold atoms
trapped in honeycomb optical lattices.",1602.06772v6
1997-10-24,Infinite coupling magnon theory of quantum Heisenberg magnetic models of spin s,"An infinity magnon coupling term is introduced into the Holstein-Primakoff
transformed forms of the Heisenberg ferromagnetic and antiferromagnetic models
of any spin $s$ to rigorously remove the unphysical magnon states. This term
makes the series expansion of the square root of the magnon operators become a
finite series of magnon operator products. Under a simple Hubbard-like
approximation our infinite coupling theory yields much better result than the
existing spin wave theories, especially near transition temperatures.",9710261v1
1999-04-26,Magnon Damping by magnon-phonon coupling in Manganese Perovskites,"Inelastic neutron scattering was used to systematically investigate the
spin-wave excitations (magnons) in ferromagnetic manganese perovskites. In
spite of the large differences in the Curie temperatures ($T_C$s) of different
manganites, their low-temperature spin waves were found to have very similar
dispersions with the zone boundary magnon softening. From the wavevector
dependence of the magnon lifetime effects and its correlation with the
dispersions of the optical phonon modes, we argue that a strong magneto-elastic
coupling is responsible for the observed low temperature anomalous spin
dynamical behavior of the manganites.",9904372v1
2003-09-09,Magnon-Hole Scattering and Charge Order in $Sr_{14-x}Ca_xCu_{24}O_{41}$,"The magnon thermal conductivity $\kappa_{\mathrm{mag}}$ of the hole doped
spin ladders in $\rm Sr_{14-x}Ca_xCu_{24}O_{41}$ has been investigated at low
doping levels $x$. The analysis of $\kappa_{\mathrm{mag}}$ reveals a strong
doping and temperature dependence of the magnon mean free path
$l_{\mathrm{mag}}$ which is a local probe for the interaction of magnons with
the doped holes in the ladders. In particular, this novel approach to studying
charge degrees of freedom via spin excitations shows that charge ordering of
the holes in the ladders leads to a freezing out of magnon-hole scattering
processes.",0309232v2
2005-11-14,Magnon decay in gapped quantum spin systems,"In the O(3) sigma-model description of gapped spin systems, S=1 magnons can
only decay into three lower energy magnons. We argue that the symmetry of the
quantum spin Hamiltonian often allows decay into two magnons, and compute this
decay rate in model systems. Two magnon decay is present in Haldane gap S=1
spin chains, even though it cannot be induced by any allowed term written in
powers and gradients of the sigma-model field. We compare our results with
recent measurements of Stone et al. (cond-mat/0511266) on a two-dimensional
spin system.",0511353v1
2008-10-07,Finite size giant magnons in the SU(2) x SU(2) sector of AdS_4 x CP^3,"We use the algebraic curve and Luscher's mu-term to calculate the leading
order finite size corrections to the dispersion relation of giant magnons in
the SU(2) x SU(2) sector of AdS_4 x CP^3. We consider a single magnon as well
as one magnon in each SU(2). In addition the algebraic curve computation is
generalized to give the leading order correction for an arbitrary multi-magnon
state in the SU(2) x SU(2) sector.",0810.1246v3
2008-10-22,Probing of a parametrically pumped magnon gas with a non-resonant packet of traveling spin waves,"The magnon gas created by spatially localized longitudinal parametric pumping
in an yttrium-iron-garnet film is probed by a traveling packet of spin waves
non-resonant with the pumping field. The analysis of the influence of the
magnon gas on the amplitude and phase of the propagating spin waves allows to
determine characteristic properties of the parametrically pumped magnon gas. A
simple theoretical model is proposed from which the magnon density in the
pumping region is calculated.",0810.4033v1
2008-12-04,Bose-Einstein condensation of magnons under incoherent pumping,"Bose-Einstein condensation in a gas of magnons pumped by an incoherent
pumping source is experimentally studied at room temperature. We demonstrate
that the condensation can be achieved in a gas of bosons under conditions of
incoherent pumping. Moreover, we show the critical transition point is almost
independent of the frequency spectrum of the pumping source and is solely
determined by the density of magnons. The electromagnetic power radiated by the
magnon condensate was found to scale quadratically with the pumping power,
which is in accordance with the theory of Bose-Einstein condensation in magnon
gases.",0812.0953v1
2008-12-15,Unbounded Multi-Magnon and Spike,"We generalize the one magnon solution in R X S^2 to unbounded M magnon and
find the corresponding solitonic string configuration in the string sigma
model. This configuration gives rise to the expected dispersion relation
obtained from the spin chain model in the large 't Hooft coupling limit. After
considering (M,M) multi-magnon or spike on R X S^2 X S^2 as a subspace of
AdS(5)XS^5 or AdS(4)XCP^3, we investigate the dispersion relation and the
finite size effect for (M,M) multi-magnon or spike.",0812.2727v2
2009-01-28,Relativistic ferromagnetic magnon at the zigzag edge of graphene,"We study the spin-wave excitations near the zigzag edge of graphene. It is
rather interesting that we obtain a single branch of relativistic ferromagnetic
magnon due to the presence of the open boundary. Note that magnons in
antiferomagnets appear in pairs, while the single brach magnon in ferromagnets
does not have relativistic dispersion. Thus, the magnon near the zigzag edge of
graphene is a hybrid of both, signaling its intrinsic property as a boundary
excitation that must be embedded in a higher dimensional bulk system.",0901.4567v1
2010-12-18,Raman scattering due to a one-magnon excitation process in ${\rm MnV_2O_4}$,"Unconventional peak structure in the Raman spectra due to magnon excitation
at low temperature is observed in spinel magnet ${\rm MnV_2O_4}$, where a
noncollinear spin state is realized by geometrical frustration. We propose a
new mechanism to induce such a Raman scattering process due to a one-magnon
excitation of the noncollinear spin state. Novel features of the scattering
such as selection rules and peak position observed experimentally in ${\rm
MnV_2O_4}$ can be explained quite naturally by considering the present
one-magnon process. We also discuss that such one-magnon process may exist in
various materials with noncollinear spin structures.",1012.4073v1
2011-09-01,Light-matter interaction in antiferromagnets: the exchange-induced magnetic dipole mechanism,"We propose a novel mechanism for exchange-induced exciton-magnon absorptions
via hopping between two antiferromagnetically coupled sites and a simultaneous
magnetic dipole transition to an excited orbital state. The obtained selection
rules correspond to ones for magnetic dipole transitions and are in agreement
with the exciton-magnon transitions observed in the quasi one-dimensional
Heisenberg antiferromagnet KCuF3. The calculated magnon density of states in
combination with a structure dependent factor identifies the observed optical
magnon sideband to originate from transverse magnon modes.",1109.0295v2
2011-09-12,Spin-transfer mechanism for magnon-drag thermopower,"We point out a relation between the dissipative spin-transfer-torque
parameter $\beta$ and the contribution of magnon drag to the thermoelectric
power in conducting ferromagnets. Using this result we estimate $\beta$ in iron
at low temperatures, where magnon drag is believed to be the dominant
contribution to the thermopower. Our results may be used to determine $\beta$
from magnon-drag-thermopower experiments, or, conversely, to infer the strength
of magnon drag via experiments on spin transfer.",1109.2414v1
2013-02-22,s-f(d) exchange mechanism of magnon generation by slow spinpolarons,"It is shown that in a single-axis antiferromagnetic semiconductor placed in a
strong magnetic field, dispersionless magnons start emitting at any arbitrarily
small velocity of an electron occurring in a spinpolaron state. If magnons are
dispersed they are generated when the spinpolaron velocity exceeds the minimum
phase velocity of magnons. The maximum power of magnon generation caused by the
drift of spinpolarons is estimated.",1302.5698v1
2013-04-07,Phenomenological model of anomalous magnon softening and damping in half-metallic manganites,"To describe anomalous zone-boundary softening and damping of magnons in
manganites we present a phenomenological two-fluid model containing
ferromagnetic Fermi-liquid and non-Fermi-liquid components. The Fermi-liquid
component accounts for softening of zone-boundary magnons and for the Landau
damping of magnons in the Stoner continuum arising at low frequencies due to
zero-point effects. Coupling of the Fermi-liquid and non-Fermi-liquid fluids
yields conventional long wavelength magnons damped due to their coupling with
longitudinal spin fluctuations.",1304.1983v1
2013-05-18,Josephson Effects in a Bose-Einstein Condensate of Magnons,"A phenomenological theory is developed, that accounts for the collective
dynamics of a Bose-Einstein condensate of magnons. In terms of such description
we discuss the nature of spontaneous macroscopic interference between magnon
clouds, highlighting the close relation between such effects and the well known
Josephson effects. Using those ideas we present a detailed calculation of the
Josephson oscillations between two magnon clouds, spatially separated in a
magnonic Josephson junction.",1305.4285v2
2015-05-08,Thermal vector potential theory of magnon-driven magnetization dynamics,"Thermal vector potential formulation is applied to study thermal dynamics of
magnetic structures in insulating ferromagnets. By separating variables of the
magnetic structure and magnons, the equation of motion for the structure
including spin-transfer effect due to thermal magnons is derived in the case of
a domain wall and a vortex. The magnon current is evaluated based on a linear
response theory with respect to the thermal vector potential representing the
temperature gradient. It is shown that the velocity of a domain wall when
driven by thermal magnon has a strong temperature dependence unlike the case of
an electrically-driven domain wall in metals.",1505.01908v1
2018-05-25,Theory of Cross-correlated Electron-Magnon Transport Phenomena: Case of Magnetic Topological Insulator,"We study transport phenomena cross-correlated among the heat and electric
currents of magnons and Dirac electrons on the surface of ferromagnetic
topological insulators. For a perpendicular magnetization, we calculate magnon-
(electron-) drag anomalous Nernst/Seebeck (anomalous Ettingshausen/Peltier)
effects and magnon-/electron-drag thermal Hall effects. The magnon-drag
thermoelectric effects are interpreted to be caused by magnon-induced
electromotive force. When the magnetization has in-plane components, there
arise thermal/thermoelectric analogs of anisotropic magnetoresistance (AMR). In
the insulating state, the thermal AMR is realized as a magnonic analog of AMR.",1805.10323v2
2017-06-13,Signature of magnon Nernst effect in an antiferromagnetic insulator,"A magnon Nernst effect, an antiferromagnetic analogue of the magnon Hall
effect in ferromagnetic insulators, has been studied experimentally for a
layered antiferromagnetic insulator MnPS3 in contact with two Pt strips.
Thermoelectric voltage in the Pt strips grown on MnPS3 single crystals exhibits
non-monotonic temperature dependence at low temperatures, which cannot be
explained by electronic origins in Pt but can be ascribed to the inverse spin
Hall voltage induced by a magnon Nernst effect. Control of antiferromagnetic
domains in the MnPS3 crystal by magnetoelectric cooling is found to modulate
the low-temperature thermoelectric voltage in Pt, which corroborates the
emergence of the magnon Nernst effect in Pt|MnPS3 hybrid structures.",1706.03978v1
2018-06-04,An analog magnon adder for all-magnonic neurons,"Spin-waves are excellent data carriers with a perspective use in neuronal
networks: Their lifetime gives the spin-wave system an intrinsic memory, they
feature strong nonlinearity, and they can be guided and steered through
extended magnonic networks. In this work, we present a magnon adder that
integrates over incoming spin-wave pulses in an analog fashion. Such an adder
is a linear prequel to a magnonic neuron, which would integrate over the
incoming pulses until a certain nonlinearity is reached. In this work, the
adder is realized by a resonator in combination with a parametric amplifier
which is just compensating the resonator losses.",1806.01389v1
2018-06-18,Relation between unidirectional spin Hall magnetoresistance and spin current-driven magnon generation,"We perform electronic measurements of unidirectional spin Hall
magnetoresistance (USMR) in a Permalloy/Pt bilayer, in conjunction with
magneto-optical Brillouin light spectroscopy of spin current-driven magnon
population. We show that the current dependence of USMR closely follows the
dipolar magnon density, and that both dependencies exhibit the same scaling
over a large temperature range of 80-400 K. These findings demonstrate a close
relationship between spin current-driven magnon generation and USMR, and
indicate that the latter is likely dominated by the dipolar magnons.",1806.06581v1
2018-12-28,Topological Magnons and Edge States in Antiferromagnetic Skyrmion Crystals,"Antiferromagnetic skyrmion crystals are magnetic phases predicted to exist in
antiferromagnets with Dzyaloshinskii-Moriya interactions. Their spatially
periodic noncollinear magnetic texture gives rise to topological bulk magnon
bands characterized by nonzero Chern numbers. We find topologically-protected
chiral magnonic edge states over a wide range of magnetic fields and
Dzyaloshinskii-Moriya interaction values. Moreover, and of particular
importance for experimental realizations, edge states appear at the lowest
possible energies, namely, within the first bulk magnon gap. Thus,
antiferromagnetic skyrmion crystals show great promise as novel platforms for
topological magnonics.",1812.11125v1
2019-02-11,Magnon excitations in $Cs_2CuAl_4O_8$ - a bond alternating S=1/2 spin chain with next nearest neighbor coupling,"A recent density functional theory (DFT) based analysis, complemented with
Quantum Monte Carlo calculations revealed a highly spin-frustrating nature of
the one-dimensional spin-$\frac{1}{2}$ compound $Cs_2CuAl_4O_8$ that comprises
of unique bond alternations and relatively strong next nearest neighbor
interactions. This article gives a brief account on possible magnon excitations
that can appear in the ground state of such systems. We find that the spin
waves obtained on top of coplanar helical reference states show multiple magnon
modes (both acoustic and optical). However, those magnon modes turn out to be
stable only in the absence of bond alternations.",1902.04008v1
2019-11-12,Coherent pumping of high momentum magnons by light,"We propose to excite a large number of coherent magnons with high momentum in
optical cavities. This is achieved by two counterpropagating optical modes that
are detuned by the frequency of a selected magnon, similar to stimulated Raman
scattering. In sub-mm size yttrium iron garnet spheres, a mW laser input power
generates 10^6-10^8 coherent magnons. The large magnon population enhances
Brillouin light scattering, a probe suitable to access their quantum
properties.",1911.04728v1
2019-12-25,Magnon polarons in the spin Peltier effect,"We report the observation of anomalous peak structures induced by hybridized
magnon-phonon excitation (magnon polarons) in the magnetic field dependence of
the spin Peltier effect (SPE) in a Lu$_{2}$Bi$_{1}$Fe$_{4}$Ga$_{1}$O$_{12}$
(BiGa:LuIG) with Pt contact. The SPE peaks coincide with magnetic fields tuned
to the threshold of magnon-polaron formation, consistent with the previous
observation in the spin Seebeck effect. The enhancement of SPE is attributed to
the lifetime increase in spin current caused by magnon-phonon hybridization in
BiGa:LuIG.",1912.11573v1
2019-06-03,Magnon-phonon interactions in magnetic insulators,"We address the theory of magnon-phonon interactions and compute the
corresponding quasi-particle and transport lifetimes in magnetic insulators
with focus on yttrium iron garnet at intermediate temperatures from anisotropy-
and exchange-mediated magnon-phonon interactions, the latter being derived from
the volume dependence of the Curie temperature. We find in general weak effects
of phonon scattering on magnon transport and the Gilbert damping of the
macrospin Kittel mode. The magnon transport lifetime differs from the
quasi-particle lifetime at shorter wavelengths.",1906.01042v1
2019-10-02,Direct Observation of Magnon Modes in Kagome Artificial Spin Ice with Topological Defects,"We investigate spin dynamics of artificial spin ice (ASI) where topological
defects confine magnon modes in Ni$_{81}$Fe$_{19}$ nanomagnets arranged on an
interconnected kagome lattice. Brillouin light scattering microscopy performed
on magnetically disordered states exhibit a series of magnon resonances which
depend on topological defect configurations detected by magnetic force
microscopy. Nanomagnets on a Dirac string and between a monopole-antimonopole
pair show pronounced modifications in magnon frequencies both in experiments
and simulations. Our work is key for the creation and annihilation of Dirac
strings via microwave assisted switching and reprogrammable magnonics based on
ASIs.",1910.00874v1
2020-10-06,A search for non-reciprocal magnons in MnPS$_3$,"Recent articles have suggested that the quasi-two dimensional antiferromagnet
MnPS$_3$ may have non-reciprocal magnons, whereby magnons in a Brillouin zone
corner at +q have different energies than those at $-$q. The magnons along the
Brillouin zone boundaries were measured using neutron three-axis spectrometry,
paying careful attention to the resolution function, to determine whether such
non-reciprocity was present. The data show that, within the resolution, there
are no significant differences between the magnons in opposite Brillouin zone
corners.",2010.02898v1
2021-06-03,Magnon spin transport around the compensation magnetic field in easy-plane antiferromagnetic insulators,"In this work, we theoretically study the magnon spin transport in easy-plane
antiferromagnetic insulators in the presence of an in-plane magnetic field. By
exactly calculating the magnon spectrum, we find the band splitting due to the
magnetic anisotropy can be fully compensated by the external field at a
particular strength, which makes its dynamics nearly equivalent to an easy-axis
antiferromagnet. As a result, the intrinsic magnon spin Hall effect due to the
dipole-dipole interaction, previously predicted in easy-axis antiferromagnets
is activated in easy-plane antiferromagnets. The compensation feature also
allows the field control of magnon spin lifetime and hence the spin diffusion
length. The compensation feature is robust against the biaxial anisotropy.",2106.02178v1
2021-09-19,Theory of the Magnon Parametron,"The 'magnon parametron' is a ferromagnetic particle that is parametrically
excited by microwaves in a cavity. Above a certain threshold of the microwave
power, a bistable steady state emerges that forms an effective Ising spin. We
calculate the dynamics of the magnon parametron as a function of microwave
power, applied magnetic field and temperature for the interacting magnon
system, taking into account thermal and quantum fluctuations. We predict three
dynamical phases, viz. a stable Ising spin, telegraph noise of thermally
activated switching, and an intermediate regime that at lower temperatures is
quantum correlated with significant distillible magnon entanglement. These
three regimes of operation are attractive for alternative computing schemes.",2109.09117v1
2021-09-20,Nonlinear magnon spin Nernst effect in antiferromagnets and strain-tunable pure spin current,"In this Letter, we study the spin Nernst effect (SNE) of magnons in the
nonlinear response regime. We derive the formula for the nonlinear magnon spin
Nernst current by solving the Boltzmann equation and find out that it is
described by an extended Berry curvature dipole of magnons. The nonlinear
magnon SNE is expected to occur in various N\'eel antiferromagnets without
Dzyaloshinskii-Moriya interaction. In particular, the nonlinear spin Nernst
current in the honeycomb and diamond lattice antiferromagnets can be controlled
by strain/pressure.",2109.09464v4
2022-06-09,Magnon-phonon interaction induced electromagnetic wave radiation in the strong coupling region,"We theoretically study the electromagnetic wave radiation of magnons driven
by acoustic phonons in systems with strong magnon-phonon interaction. We
evaluate the field dependence of radiation intensity spectra which exhibits the
avoided crossing, a characteristic of strongly coupled systems. At the
crossover where the magnon and phonon eigenstates are hybridized, we
demonstrate the existence of two resonant radiation frequencies with circular
polarization and the enhancement of antenna radiation efficiency by over 100
times. Our results open up possibilities of developing ultra-compact antennas
by using the hybridized magnon-phonon mode.",2206.04454v1
2023-01-24,Direct observation of magnon BEC in an out-of-plane magnetized yttrium iron garnet film,"Bose-Einstain condensation occurs at an appropriate density of bosonic
particles, depending on their mass and temperature. We were able to
experimentally observe the transition from the spin wave regime to the magnon
Bose-Einstein condensed state (mBEC) with increasing magnon density by a
microwave pumping. We used optical methods to register the spatial distribution
of the magnon density and phase. For the first time, a coherent state of
stationary magnons was demonstrated far from the region of their excitation.",2301.10725v1
2023-06-28,Time-dependent Schwinger boson mean-field theory of supermagnonic propagation in 2D antiferromagnets,"Understanding the speed limits for the propagation of magnons is of key
importance for the development of ultrafast spintronics and magnonics.
Recently, it was predicted that in 2D antiferromagnets, spin correlations can
propagate faster than the highest magnon velocity. Here we gain deeper
understanding of this supermagnonic effect based on time-dependent Schwinger
boson mean-field theory. We find that the supermagnonic effect is determined by
the competition between propagating magnons and a localized quasi-bound state,
which is tunable by lattice coordination and quantum spin value $S$, suggesting
a new scenario to enhance magnon propagation.",2306.16382v1
2024-03-21,Magnonic Superconductivity,"We uncover a new superconducting state with partial spin polarization induced
by a magnetic field. This state, which we call ""magnonic superconductor"", lacks
a conventional pairing order parameter, but is characterized instead by a
composite order parameter that represents the binding of electron pairs and
magnons. We rigorously demonstrate the existence of magnonic superconductivity
with high transition temperature in a triangular lattice Hubbard model with
repulsive interaction. We further show that magnonic Cooper pairs can attract
to form higher-charge bound states, which can give rise to charge-$6e$
superconductivity.",2403.14756v1
2012-03-26,Magnon-drag thermopile,"Thermoelectric effects in spintronics are gathering increasing attention as a
means of managing heat in nanoscale structures and of controlling spin
information by using heat flow. Thermal magnons (spin-wave quanta) are expected
to play a major role, however, little is known about the underlying physical
mechanisms involved. The reason is the lack of information about magnon
interactions and of reliable methods to obtain it, in particular for electrical
conductors because of the intricate influence of electrons. Here, we
demonstrate a conceptually new device that allows us to gather information on
magnon-electron scattering and magnon-drag effects. The device resembles a
thermopile formed by a large number of pairs of ferromagnetic wires placed
between a hot and a cold source and connected thermally in parallel and
electrically in series. By controlling the relative orientation of the
magnetization in pairs of wires, the magnon-drag can be studied independently
of the electron and phonon-drag thermoelectric effects. Measurements as a
function of temperature reveal the effect on magnon drag following a variation
of magnon and phonon populations. This information is crucial to understand the
physics of electron-magnon interactions, magnon dynamics and thermal spin
transport.",1203.5628v1
2017-03-11,Magnonic crystals - prospective structures for shaping spin waves in nanoscale,"We have investigated theoretically band structure of spin waves in magnonic
crystals with periodicity in one-(1D), two- (2D) and three-dimensions (3D). We
have solved Landau-Lifshitz equation with the use of plane wave method, finite
element method in frequency domain and micromagnetic simulations in time domain
to find the dynamics of spin waves and spectrum of their eigenmodes. The spin
wave spectra were calculated in linear approximation. In this paper we show
usefulness of these methods in calculations of various types of spin waves. We
demonstrate the surface character of the Damon-Eshbach spin wave in 1D magnonic
crystals and change of its surface localization with the band number and
wavenumber in the first Brillouin zone. The surface property of the spin wave
excitation is further exploited by covering plate of the magnonic crystal with
conductor. The band structure in 2D magnonic crystals is complex due to
additional spatial inhomogeneity introduced by the demagnetizing field. This
modifies spin wave dispersion, makes the band structure of magnonic crystals
strongly dependent on shape of the inclusions and type of the lattice. The
inhomogeneity of the internal magnetic field becomes unimportant for magnonic
crystals with small lattice constant, where exchange interactions dominate. For
3D magnonic crystals, characterized by small lattice constant, wide magnonic
band gap is found. We show that the spatial distribution of different materials
in magnonic crystals can be explored for tailored effective damping of spin
waves.",1703.04012v1
2019-09-09,Klein tunneling of Weyl magnons,"Similar to Weyl semimetals, in magnetic materials, magnon bands can host Weyl
points, around which the bosonic excitations are called Weyl magnons. Here, we
investigate the Klein tunneling of Weyl magnons, during the process of which
Weyl magnons tunnel through a high potential barrier. Specifically, we study
the magnetization current carried by Weyl magnons in a quasi-one-dimensional
magnetic wire, in the middle of which a gate magnetic field is applied to
generate a potential barrier. The transmission probability is calculated and
the Landauer-B\""uttiker formalism is used to find the magnetization current.
Various types of Weyl magnons are considered, including isotropic, tilted, and
double Weyl magnons. Unlike in Weyl semimetals where fermionic statistics is in
charge and the current oscillates with the gate field as a result of
Fabry-P\'erot resonances, here Bose distribution smears out the oscillations.
We find that the tilting of the Weyl cone causes the decrease of the
magnetization current from Klein tunneling, while for double Weyl magnons,
Klein tunneling is absent in the direction of quadratic dispersion, but is
enhanced in the direction of linear dispersion. Our results show that the
behaviour of the current-voltage characteristics of magnons is rather different
from that of electrons due to different statistics, although the
single-particle properties, such as the transmission probabilities, are the
same for both Weyl bosons and Weyl fermions.",1909.04109v1
2019-02-18,Coherent control of magnon radiative damping with local photon states,"The collective excitation of ordered spins, known as spin waves or magnons,
can in principle radiate by emitting travelling photons to an open system when
decaying to the ground state. However, in contrast to the electric dipoles,
magnetic dipoles contributed by magnons are more isolated from electromagnetic
environment with negligible radiation in the vacuum, limiting their application
in coherent communication by photons. Recently, strong interaction between
cavity standing-wave photons and magnons has been reported, indicating the
possible manipulation of magnon radiation via tailoring photon states. Here,
with loading an yttrium iron garnet sphere in a one-dimensional circular
waveguide cavity in the presence of both travelling and standing photon modes,
we demonstrate an efficient photon emissions from magnon and a significant
magnon radiative damping with radiation rate found to be proportional to the
local density of states (LDOS) of photon. By modulating the LDOS including its
magnitude and/or polarization, we can flexibly tune the photon emission and
magnon radiative damping on demand. Our findings provide a general way in
manipulating photon emission from magnon radiation for harnessing energy and
angular momentum generation, transfer and storage modulated by magnon in the
cavity and waveguide electrodynamics.",1902.06795v2
2019-03-06,Enhancement of antiferromagnetic magnon-magnon entanglement by cavity cooling,"Magnon-photon coupling has been experimentally realized inside a cavity and
the emerging field known as cavity spintronics has attracted significant
attention for its potential docking with quantum information science. However,
one seldom knows whether this coupling implies an entanglement state among
magnons and photons or not, which is crucial for its usage in quantum
information. Here we study the entanglement properties among magnons and
photons in an antiferromagnet-light system and find that the entanglement
between magnon and photon is nearly zero while the magnon-magnon entanglement
is very strong and it can be even further enhanced through the coupling with
the cavity photons. The maximum enhancement occurs when the antiferromagnet
reaches resonant with the light. The essential physics can be well understood
within the picture of cavity induced cooling of magnon-magnon state near its
joint vacuum with stronger entanglement. Our results are significant to extend
the cavity spintronics to quantum manipulation and further provide an alternate
to manipulate the deep strong correlations of continuous variable entanglement
with a generic stable condition and easy tunability.",1903.02484v2
2018-10-05,Magnon contribution to unidirectional spin Hall magnetoresistance,"We develop a model for the magnonic contribution to the unidirectional spin
Hall magnetoresistance (USMR) of heavy metal/ferromagnetic insulator bilayer
films. We show that diffusive transport of Holstein-Primakoff magnons leads to
an accumulation of spin near the bilayer interface, giving rise to a
magnoresistance which is not invariant under inversion of the current
direction. Unlike the electronic contribution described by Zhang and Vignale
[Phys. Rev. B 94, 140411 (2016)], which requires an electrically conductive
ferromagnet, the magnonic contribution can occur in ferromagnetic insulators
such as yttrium iron garnet. We show that the magnonic USMR is, to leading
order, cubic in the spin Hall angle of the heavy metal, as opposed to the
linear relation found for the electronic contribution. We estimate that the
maximal magnonic USMR in Pt|YIG bilayers is on the order of $10^{-8}$, but may
reach values of up to $10^{-5}$ if the magnon gap is suppressed, and can thus
become comparable to the electronic contribution in, e.g., Pt|Co. We show that
the magnonic USMR at a finite magnon gap may be enhanced by an order of
magnitude if the magnon diffusion length is decreased to a specific optimal
value that depends on various system parameters.",1810.02610v2
2020-06-29,"Hybrid magnonics: physics, circuits and applications for coherent information processing","Hybrid dynamic systems have recently gained interests with respect to both
fundamental physics and device applications, particularly with their potential
for coherent information processing. In this perspective, we will focus on the
recent rapid developments of magnon-based hybrid systems, which seek to combine
magnonic excitations with diverse excitations for transformative applications
in devices, circuits and information processing. Key to their promising
potentials is that magnons are highly tunable excitations and can be easily
engineered to couple with various dynamic media and platforms. The capability
of reaching strong coupling with many different excitations has positioned
magnons well for studying solid-state coherent dynamics and exploiting unique
functionality. In addition, with their gigahertz frequency bandwidth and the
ease of fabrication and miniaturization, magnonic devices and systems can be
conveniently integrated into microwave circuits for mimicking a broad range of
device concepts that have been applied in microwave electronics, photonics and
quantum information. We will discuss a few potential directions for advancing
magnon hybrid systems, including on-chip geometry, novel coherent magnonic
functionality, and coherent transduction between different platforms. As future
outlook, we will discuss the opportunities and challenges of magnonic hybrid
systems for their applications in quantum information and magnonic logic.",2006.16158v1
2019-10-11,Chiral Magnonic Edge States in Ferromagnetic Skyrmion Crystals Controlled by Magnetic Fields,"Achieving control over magnon spin currents in insulating magnets - where
dissipation due to Joule heating is highly suppressed - is an active area of
research that could lead to energy-efficient spintronics applications. However,
magnon spin currents supported by conventional systems with uniform magnetic
order have proven hard to control. An alternative approach that relies on
topologically protected magnonic edge states of spatially periodic magnetic
textures has recently emerged. A prime example of such textures is the
ferromagnetic skyrmion crystal which hosts chiral edge states providing a
platform for magnon spin currents. Here, we show, for the first time, an
external magnetic field can drive a topological phase transition in the spin
wave spectrum of a ferromagnetic skyrmion crystal. The topological phase
transition is signaled by the closing of a low-energy bulk magnon gap at a
critical field. In the topological phase, below the critical field, two
topologically protected chiral magnonic edge states lie within this gap, but
they unravel in the trivial phase, above the critical field. Remarkably, the
topological phase transition involves an inversion of two magnon bands that at
the $\Gamma$ point correspond to the breathing and anticlockwise modes of the
skyrmions in the crystal. Our findings suggest that an external magnetic field
could be used as a knob to switch on and off magnon spin currents carried by
topologically protected chiral magnonic edge states.",1910.05214v1
2021-08-25,Quantum network with magnonic and mechanical nodes,"A quantum network consisting of magnonic and mechanical nodes connected by
light is proposed. Recent years have witnessed a significant development in
cavity magnonics based on collective spin excitations in ferrimagnetic
crystals, such as yttrium iron garnet (YIG). Magnonic systems are considered to
be a promising building block for a future quantum network. However, a major
limitation of the system is that the coherence time of the magnon excitations
is limited by their intrinsic loss (typically in the order of 1 $\mu$s for
YIG). Here, we show that by coupling the magnonic system to a mechanical system
using optical pulses, an arbitrary magnonic state (either classical or quantum)
can be transferred to and stored in a distant long-lived mechanical resonator.
The fidelity depends on the pulse parameters and the transmission loss. We
further show that the magnonic and mechanical nodes can be prepared in a
macroscopic entangled state. These demonstrate the quantum state transfer and
entanglement distribution in such a novel quantum network of magnonic and
mechanical nodes. Our work shows the possibility to connect two separate fields
of optomagnonics and optomechanics, and to build a long-distance quantum
network based on magnonic and mechanical systems.",2108.11156v3
2021-11-11,Evidence for spin current driven Bose-Einstein condensation of magnons,"The quanta of magnetic excitations - magnons - are known for their unique
ability to undergo Bose-Einstein condensation at room temperature. This
fascinating phenomenon reveals itself as a spontaneous formation of a
macroscopic coherent state under the influence of incoherent stimuli. Spin
currents have been predicted to offer electronic control of magnon
Bose-Einstein condensates, but this phenomenon has not been experimentally
evidenced up to now. Here we experimentally show that current-driven
Bose-Einstein condensation can be achieved in nanometer-thick films of magnetic
insulators with tailored dynamic magnetic nonlinearities and minimized
magnon-magnon interactions. We demonstrate that, above a certain threshold,
magnons injected by the spin current overpopulate the lowest-energy level
forming a highly coherent spatially extended state. By accessing magnons with
essentially different energies, we quantify the chemical potential of the
driven magnon gas and show that, at the critical current, it reaches the energy
of the lowest magnon level. Our results pave the way for implementation of
integrated microscopic quantum magnonic and spintronic devices.",2111.06356v1
2022-01-14,Cavity mediated level attraction and repulsion between magnons,"We characterize some of the distinctive hallmarks of magnon-magnon
interaction mediated by the intracavity field of a microwave cavity, along with
their testable ramifications. In general, we foreground two widely dissimilar
parameter domains that bring forth the contrasting possibilities of level
splitting and level crossing. The former is observed in the regime of strong
magnon-photon couplings, particularly when the three modes bear comparable
relaxation rates. This character is marked by the appearance of three
distinguishable and non-converging polariton branches in the spectral response
to a cavity drive. However, when the bare modes are resonant and the couplings
perfectly symmetrical, one of the spectral peaks gets wiped out. This anomalous
extinction of polaritonic response can be traced down to the existence of a
conspicuous dark mode alongside two frequency-shifted bright modes. In an
alternate parameter regime, where the magnon modes are weakly coupled to the
cavity, features of level attraction unfold, subject to a large relaxation rate
for the cavity mode. Concurrently, for antisymmetric detunings to the magnon
modes, a transmission window springs into existence, exhibiting transparency in
the limit of negligible dissipation from the magnons. The emergence of level
attraction can be reconciled with a theoretical model that embodies the
dynamics of the magnon-magnon subsystem when the cavity field decays rapidly
into its steady state. In this limit, we identify a purely dissipative coupling
between the magnon modes.",2201.05685v1
2022-06-13,An efficient material search for room temperature topological magnons,"Topologically protected magnon surface states are highly desirable as an
ideal platform to engineer low-dissipation spintronics devices. However,
theoretical prediction of topological magnons in strongly correlated materials
proves to be challenging because the ab initio density functional theory
calculations fail to reliably predict magnetic interactions in correlated
materials. Here, we present a symmetry-based approach, which predicts
topological magnons in magnetically ordered crystals, upon applying external
perturbations such as magnetic/electric fields and/or mechanical strains. We
apply this approach to carry out an efficient search for magnetic materials in
the Bilbao Crystallographic Server, where, among 198 compounds with an over
300-K transition temperature, we identify 12 magnetic insulators that support
room-temperature topological magnons. They feature Weyl magnons with surface
magnon arcs and magnon axion insulators with either chiral surface or hinge
magnon modes, offering a route to realize energy-efficient devices based on
protected surface magnons.",2206.06248v2
2022-06-23,High pressure tuning of magnon-polarons in the layered antiferromagnet FePS$_3$,"Magnetic layered materials have emerged recently as promising systems to
introduce magnetism in structures based on two-dimensional (2D) materials and
to investigate exotic magnetic ground states in the 2D limit. In this work, we
apply high hydrostatic pressures up to P = 8.7 GPa to the bulk layered
antiferromagnet FePS$_3$ to tune the collective lattice excitations (phonons)
in resonance with magnetic excitations (magnons). Close to P = 4 GPa, the
magnon-phonon resonance is achieved and the strong coupling between these
collective modes leads to the formation of new quasi-particles, the
magnon-polarons, evidenced in our low temperature Raman scattering experiments
by a particular avoided crossing behavior between the phonon and the doubly
degenerate antiferromagnetic magnon. At the pressure-induced magnon-phonon
resonance, three distinct coupled modes emerge. As it is mainly defined by
intralayer properties, we show that the energy of the magnon is nearly pressure
independent. We additionally apply high magnetic fields up to B = 30 T to fully
identify and characterize the magnon excitations, and to explore the different
magnon-polaron regimes for which the phonon has an energy lower-, equal to-, or
higher- than the magnon energy. The description of our experimental data
requires introducing a phonon-phonon coupling not taken into account in actual
calculations.",2206.11963v1
2022-08-29,Confinement of Bose-Einstein magnon condensates in adjustable complex magnetization landscapes,"Coherent wave states such as Bose-Einstein condensates (BECs), which
spontaneously form in an overpopulated magnon gas even at room temperature,
have considerable potential for wave-based computing and information processing
at microwave frequencies. The ability to control the transport properties of
magnon BECs plays an essential role for their practical use. Here, we
demonstrate spatio-temporal control of the BEC density distribution through the
excitation of magnon supercurrents in an inhomogeneously magnetized yttrium
iron garnet film. The BEC is created by microwave parametric pumping and probed
by Brillouin light scattering spectroscopy. The desired magnetization profile
is prepared by heating the film with optical patterns projected onto its
surface using a phase-based wavefront modulation technique. Specifically, we
observe a pronounced spatially localized magnon accumulation caused by magnon
supercurrents flowing toward each other originating in two heated regions. This
accumulation effect increases the BEC lifetime due to the constant influx of
condensed magnons into the confinement region. The shown approach to manipulate
coherent waves provides an opportunity to extend the lifetime of freely
evolving magnon BECs, create dynamic magnon textures, and study the interaction
of magnon condensates formed in different regions of the sample.",2208.13507v1
2022-09-08,Perspective: non-Hermitian physics in magnetic systems,"Non-Hermitian Hamiltonians provide an alternative perspective on the dynamics
of quantum and classical systems coupled non-conservatively to an environment.
Once primarily an interest of mathematical physicists, the theory of
non-Hermitian Hamiltonians has solidified and expanded to describe various
physically observable phenomena in optical, photonic, and condensed matter
systems. Self-consistent descriptions of quantum mechanics based on
non-Hermitian Hamiltonians have been developed and continue to be refined. In
particular, non-Hermitian frameworks to describe magnonic and hybrid magnonic
systems have gained popularity and utility in recent years, with new insights
into the magnon topology, transport properties, and phase transitions coming
into view. Magnonic systems are in many ways a natural platform in which to
realize non-Hermitian physics because they are always coupled to a surrounding
environment and exhibit lossy dynamics. In this perspective we review recent
progress in non-Hermitian frameworks to describe magnonic and hybrid magnonic
systems, such as cavity magnonic systems and magnon-qubit coupling schemes. We
discuss progress in understanding the dynamics of inherently lossy magnetic
systems as well as systems with gain induced by externally applied spin
currents. We enumerate phenomena observed in both purely magnonic and hybrid
magnonic systems which can be understood through the lens of non-Hermitian
physics, such as PT and Anti-PT-symmetry breaking, dynamical magnetic phase
transitions, non-Hermitian skin effect, and the realization of exceptional
points and surfaces. Finally, we comment on some open problems in the field and
discuss areas for further exploration.",2209.03946v2
2022-11-15,Modification of three-magnon splitting in a flexed magnetic vortex,"We present an experimental and numerical study of three-magnon splitting in a
micrometer-sized magnetic disk with the vortex state strongly deformed by
static in-plane magnetic fields. Excited with a large enough power at frequency
$f_\mathrm{RF}$, the primary radial magnon modes of a cylindrical magnetic
vortex can decay into secondary azimuthal modes via spontaneous three-magnon
splitting. This nonlinear process exhibits selection rules leading to
well-defined and distinct frequencies $f_\mathrm{RF}/2\pm \Delta f$ of the
secondary modes. Here, we demonstrate that three-magnon splitting in vortices
can be significantly modified by deforming the magnetic vortex with in-plane
magnetic fields, leading to a much richer three-magnon response. We find that,
with increasing field, an additional class of secondary modes is excited which
are localized to the highly-flexed regions adjacent to the displaced vortex
core. While these modes satisfy the same selection rules of three-magnon
splitting, they exhibit a much lower three-magnon threshold power compared to
regular secondary modes of a centered vortex. The applied static magnetic
fields are small ($\simeq$ 10 mT), providing an effective parameter to control
the nonlinear spectral response of confined vortices. Our work expands the
understanding of nonlinear magnon dynamics in vortices and advertises these for
potential neuromorphic applications based on magnons.",2211.08226v2
2023-07-12,Quantum information diode based on a magnonic crystal,"Exploiting the effect of nonreciprocal magnons in a system with no inversion
symmetry, we propose a concept of a quantum information diode, {\it i.e.}, a
device rectifying the amount of quantum information transmitted in the opposite
directions. We control the asymmetric left and right quantum information
currents through an applied external electric field and quantify it through the
left and right out-of-time-ordered correlation (OTOC). To enhance the
efficiency of the quantum information diode, we utilize a magnonic crystal. We
excite magnons of different frequencies and let them propagate in opposite
directions. Nonreciprocal magnons propagating in opposite directions have
different dispersion relations. Magnons propagating in one direction match
resonant conditions and scatter on gate magnons. Therefore, magnon flux in one
direction is damped in the magnonic crystal leading to an asymmetric transport
of quantum information in the quantum information diode. A quantum information
diode can be fabricated from an yttrium iron garnet (YIG) film. This is an
experimentally feasible concept and implies certain conditions: low temperature
and small deviation from the equilibrium to exclude effects of phonons and
magnon interactions. We show that rectification of the flaw of quantum
information can be controlled efficiently by an external electric field and
magnetoelectric effects.",2307.06047v1
2023-07-26,Tunable Magnon-Photon Coupling by Magnon Band Gap in a Layered Hybrid Perovskite Antiferromagnet,"Tunability of coherent coupling between fundamental excitations is an
important prerequisite for expanding their functionality in hybrid quantum
systems. In hybrid magnonics, the dipolar interaction between magnon and photon
usually persists and cannot be switched off. Here, we demonstrate this
capability by coupling a superconducting resonator to a layered hybrid
perovskite antiferromagnet, which exhibits a magnon band gap due to its
intrinsic Dzyaloshinskii-Moriya interaction. The pronounced temperature
sensitivity of the magnon band gap location allows us to set the photon mode
within the gap and to disable magnon-photon hybridization. When the resonator
mode falls into the magnon band gap, the resonator damping rate increases due
to the nonzero coupling to the detuned magnon mode. This phenomena can be used
to quantify the magnon band gap using an analytical model. Our work brings new
opportunities in controlling coherent information processing with quantum
properties in complex magnetic materials.",2307.14447v1
2023-09-25,Ultrafast Demagnetization through Femtosecond Generation of Non-thermal Magnons,"Ultrafast laser excitation of ferromagnetic metals gives rise to correlated,
highly non-equilibrium dynamics of electrons, spins and lattice, which are,
however, poorly described by the widely-used three-temperature model (3TM).
Here, we develop a fully ab-initio parameterized out-of-equilibrium theory
based on a quantum kinetic approach--termed (N+2) temperature model--that
describes magnon occupation dynamics due to electron-magnon scattering. We
apply this model to perform quantitative simulations on the ultrafast,
laser-induced generation of magnons in iron and demonstrate that on these
timescales the magnon distribution is non-thermal: predominantly high-energy
magnons are created, while the magnon occupation close to the center of the
Brillouin zone even decreases, due to a repopulation towards higher energy
states via a so-far-overlooked scattering term. We demonstrate that the simple
relation between magnetization and temperature computed at equilibrium does not
hold in the ultrafast regime and that the 3TM greatly overestimates the
demagnetization. The ensuing Gilbert damping becomes strongly magnon wavevector
dependent and requires a description beyond the conventional
Landau-Lifshitz-Gilbert spin dynamics. Our ab-initio-parameterized calculations
show that ultrafast generation of non-thermal magnons provides a sizable
demagnetization within 200fs in excellent comparison with experimentally
observed laser-induced demagnetizations. Our investigation emphasizes the
importance of non-thermal magnon excitations for the ultrafast demagnetization
process.",2309.14167v3
2023-12-15,Magnon Bose-Einstein condensates: from time crystals and quantum chromodynamics to vortex sensing and cosmology,"Under suitable experimental conditions collective spin-wave excitations,
magnons, form a Bose-Einstein condensate (BEC) where the spins precess with a
globally coherent phase. Bose-Einstein condensation of magnons has been
reported in a few systems, including superfluid phases of $^3$He, solid state
systems such as Yttrium-iron-garnet (YIG) films, and cold atomic gases. Among
these systems, the superfluid phases of $^3$He provide a nearly ideal test
bench for coherent magnon physics owing to experimentally proven spin
superfluidity, the long lifetime of the magnon condensate, and the versatility
of the accessible phenomena. We first briefly recap the properties of the
different magnon BEC systems, with focus on superfluid $^3$He. The main body of
this review summarizes recent advances in application of magnon BEC as a
laboratory to study basic physical phenomena connecting to diverse areas from
particle physics and cosmology to new phases of condensed matter. This line of
research complements the ongoing efforts to utilize magnon BECs as probes and
components for potentially room-temperature quantum devices. In conclusion, we
provide a roadmap for future directions in the field of applications of magnon
BEC to fundamental research.",2312.10119v1
2024-03-03,Magnonic $\varphi$ Josephson Junctions and Synchronized Precession,"There has been a growing interest in non-Hermitian physics. One of its main
goals is to engineer dissipation and to explore ensuing functionality. In
magnonics, the effect of dissipation due to local damping on magnon transport
has been explored. However, the effects of non-local damping on the magnonic
analog of the Josephson effect remain missing, despite that non-local damping
is inevitable and has been playing a central role in magnonics. Here, we
uncover theoretically that a surprisingly rich dynamics can emerge in magnetic
junctions due to intrinsic non-local damping, using analytical and numerical
methods. In particular, under microwave pumping, we show that coherent spin
precession in the right and left insulating ferromagnet (FM) of the junction
becomes synchronized by non-local damping and thereby a magnonic analog of the
$\varphi$ Josephson junction emerges, where $\varphi$ stands here for the
relative precession phase of right and left FM in the stationary limit.
Remarkably, $\varphi$ decreases monotonically from $ \pi$ to $\pi/2$ as the
magnon-magnon interaction, arising from spin anisotropies, increases. Moreover,
we also find a magnonic diode effect giving rise to rectification of magnon
currents. Our predictions are readily testable with current device and
measurement technologies at room temperatures.",2403.01625v1
2019-09-04,Magnon polaron formed by selectively coupled coherent magnon and phonon modes of a surface patterned ferromagnet,"Strong coupling between two quanta of different excitations leads to the
formation of a hybridized state which paves a way for exploiting new degrees of
freedom to control phenomena with high efficiency and precision. A magnon
polaron is the hybridized state of a phonon and a magnon, the elementary quanta
of lattice vibrations and spin waves in a magnetically-ordered material. A
magnon polaron can be formed at the intersection of the magnon and phonon
dispersions, where their frequencies coincide. The observation of magnon
polarons in the time domain has remained extremely challenging because the weak
interaction of magnons and phonons and their short lifetimes jeopardize the
strong coupling required for the formation of a hybridized state. Here, we
overcome these limitations by spatial matching of magnons and phonons in a
metallic ferromagnet with a nanoscale periodic surface pattern. The spatial
overlap of the selected phonon and magnon modes formed in the periodic
ferromagnetic structure results in a high coupling strength which, in
combination with their long lifetimes allows us to find clear evidence of an
optically excited magnon polaron. We show that the symmetries of the localized
magnon and phonon states play a crucial role in the magnon polaron formation
and its manifestation in the optically excited magnetic transients.",1909.01886v4
2022-10-15,Non-local magnon transconductance in extended magnetic insulating films.\\ Part I: spin diode effect,"This review provides a comprehensive study of the nonlinear transport
properties of magnons, which are electrically emitted or absorbed inside
extended YIG films by spin transfer effects via a YIG$\vert$Pt interface. Our
purpose is to experimentally elucidate the pertinent picture behind the
asymmetric electrical variation of the magnon transconductance analogous to an
electric diode. The feature is rooted in the variation of the density of
low-lying spin excitations via an electrical shift of the magnon chemical
potential. As the intensity of the spin transfer increases in the forward
direction (regime of magnon emission), the transport properties of low-energy
magnon go through 3 distinct regimes: \textit{i)} at low currents, where the
spin current is a linear function of the electrical current, the spin transport
is ballistic and set by the film thickness; \textit{ii)} for amplitudes of the
order of the damping compensation threshold, it switches to a highly correlated
regime limited by magnon-magnon relaxation process and marked by a saturation
of the magnon transconductance. Here the main bias, that controls the magnon
density, are thermal fluctuations beneath the emitter. \textit{iii)} As the
temperature under the emitter approaches the Curie temperature, scattering with
high-energy magnons dominates, leading to diffusive transport. We note that
such sequence of transport regimes bears analogy with electron hydrodynamic
transport in ultra-pure media predicted by Radii Gurzhi. This study restricted
to low energy part of the magnon manifold complements part II of this
review\cite{kohno_2F}, which concentrates instead on the whole spectrum of
propagating magnons.",2210.08304v2
1998-02-04,Magnetic impurities in Mott-Hubbard antiferromagnets,"A formalism is developed to treat magnetic impurities in a Mott-Hubbard
antiferromagnetic insulator within a representation involving multiple orbitals
per site. Impurity scattering of magnons is found to be strong when the number
of orbitals N' on impurity sites is different from the number N on host sites.
The impurity-scattering-induced softening of magnon modes leads to enhancement
in thermal excitation of magnons, and hence to a lowering of the Neel
temperature in layered or three dimensional systems. Weak impurity scattering
of magnons is obtained in the case N'=N, where the impurity is represented in
terms of modified hopping strength, and a momentum-independent, multiplicative
renormalization of magnon energies is obtained. Split-off magnon modes are
obtained when the impurity-host coupling is stronger, and implications are
discussed for two-magnon Raman scattering. The mapping between antiferromagnets
and superconductors is utilized to contrast formation of impurity-induced
states.",9802052v1
2005-01-11,Ballistic magnon transport and phonon scattering in the antiferromagnet Nd$_2$CuO$_4$,"The thermal conductivity of the antiferromagnet Nd$_2$CuO$_4$ was measured
down to 50 mK. Using the spin-flop transition to switch on and off the acoustic
Nd magnons, we can reliably separate the magnon and phonon contributions to
heat transport. We find that magnons travel ballistically below 0.5 K, with a
thermal conductivity growing as $T^3$, from which we extract their velocity. We
show that the rate of scattering of acoustic magnons by phonons grows as $T^3$,
and the scattering of phonons by magnons peaks at twice the average Nd magnon
frequency.",0501220v2
2008-04-21,Giant magnons in the D1-D5 system,"We study giant magnons in the the D1-D5 system from both the boundary CFT and
as classical solutions of the string sigma model in $AdS_3\times S^3\times
T^4$. Re-examining earlier studies of the symmetric product conformal field
theory we argue that giant magnons in the symmetric product are BPS states in a
centrally extended $SU(1|1)\times SU(1|1)$ superalgebra with two more
additional central charges. The magnons carry these additional central charges
locally but globally they vanish. Using a spin chain description of these
magnons and the extended superalgebra we show that these magnons obey a
dispersion relation which is periodic in momentum. We then identify these
states on the string theory side and show that here too they are BPS in the
same centrally extended algebra and obey the same dispersion relation which is
periodic in momentum. This dispersion relation arises as the BPS condition for
the extended algebra and is similar to that of magnons in ${\cal N}=4$
Yang-Mills",0804.3267v3
2013-04-26,Strong magnon softening in tetragonal FeCo compounds,"Magnons play an important role in fast precessional magnetization reversal
processes serving as a heat bath for dissipation of the Zeeman energy and thus
being responsible for the relaxation of magnetization. Employing \emph{ab
initio} many-body perturbation theory we studied the magnon spectra of the
tetragonal FeCo compounds considering three different experimental $c/a$
ratios, $c/a=$1.13, 1.18, and 1.24 corresponding to FeCo grown on Pd, Ir, and
Rh, respectively. We find that for all three cases the short-wave-length
magnons are strongly damped and tetragonal distortion gives rise to a
significant magnon softening. The magnon stiffness constant $D$ decreases
almost by a factor of two from FeCo/Pd to FeCo/Rh. The combination of soft
magnons together with the giant magnetic anisotropy energy suggests FeCo/Rh to
be a promising material for perpendicular magnetic recording applications.",1304.7091v1
2014-11-26,Pattern Recognition with Magnonic Holographic Memory Device,"In this work, we present experimental data demonstrating the possibility of
using magnonic holographic devices for pattern recognition. The prototype
eight-terminal device consists of a magnetic matrix with micro-antennas placed
on the periphery of the matrix to excite and detect spin waves. The principle
of operation is based on the effect of spin wave interference, which is similar
to the operation of optical holographic devices. Input information is encoded
in the phases of the spin waves generated on the several edges of the magnonic
matrix, while the output corresponds to the amplitude of the inductive voltage
produced by the interfering spin waves on the other side of the matrix. The
level of the output voltage depends on the combination of the input phases as
well as on the internal structure of the magnonic matrix. Experimental data
collected for several magnonic matrixes show the unique output signatures in
which maxima and minima correspond to specific input phase patterns.
Potentially, magnonic holographic devices may provide a higher storage density
compare to the optical counterparts due to a shorter wavelength and
compatibility with conventional electronic devices. The challenges and
shortcoming of the magnonic holographic devices are also discussed.",1411.7082v1
2014-12-07,Magnonic band gaps in YIG based magnonic crystals: array of grooves versus array of metallic stripes,"The magnonic band gaps of the two types of planar one-dimensional magnonic
crystals comprised of the periodic array of the metallic stripes on yttrium
iron garnet (YIG) film and YIG film with an array of grooves was analyzed
experimentally and theoretically. In such periodic magnetic structures the
propagating magnetostatic surface spin waves were excited and detected by
microstripe transducers with vector network analyzer and by Brillouin light
scattering spectroscopy. Properties of the magnonic band gaps were explained
with the help of the finite element calculations. The important influence of
the nonreciprocal properties of the spin wave dispersion induced by metallic
stripes on the magnonic band gap width and its dependence on the external
magnetic field has been shown. The usefulness of both types of the magnonic
crystals for potential applications and possibility for miniaturization are
discussed.",1412.2367v2
2015-01-26,Dynamics of magnon fluid in Dzyaloshinskii-Moriya magnet and its manifestation in magnon-Skyrmion scattering,"We construct Holstein-Primakoff Hamiltonian for magnons in arbitrary slowly
varying spin background, for a microscopic spin Hamiltonian consisting of
ferromagnetic spin exchange,Dzyaloshinskii-Moriya exchange, and the Zeeman
term. The Gross-Pitaevskii-type equation for magnon dynamics contains several
background gauge fields pertaining to local spin chirality, inhomogeneous
potential, and anomalous scattering that violates the boson number
conservation. Non-trivial corrections to previous formulas derived in the
literature are given. Subsequent mapping to hydrodynamic fields yields the
continuity equation and the Euler equation of the magnon fluid dynamics. Magnon
wave scattering off a localized Skyrmion is examined numerically based on our
Gross-Pitaevskii formulation. Dependence of the effective flux experienced by
the impinging magnon on the Skyrmion radius is pointed out, and compared with
analysis of the same problem using the Landau-Lifshitz-Gilbert equation.",1501.06467v1
2015-04-03,The effect of the magnon dispersion on the longitudinal spin Seebeck effect in yttrium iron garnets (YIG),"We study the temperature dependence of the longitudinal spin-Seebeck effect
(LSSE) in a yttrium iron garnet Y3Fe5O12 (YIG) / Pt system for samples of
different thicknesses. In this system, the thermal spin torque is
magnon-driven. The LSSE signal peaks at a specific temperature that depends on
the YIG sample thickness. We also observe freeze-out of the LSSE signal at high
magnetic fields, which we attribute to the opening of an energy gap in the
magnon dispersion. We observe partial freeze-out of the LSSE signal even at
room temperature, where kBT is much larger than the gap. This suggests that a
subset of the magnon population with an energy below kB x TC (TC about 40 K)
contribute disproportionately to the LSSE; at temperatures below TC, we label
these magnons subthermal magnons. The T-dependence of the LSSE at temperatures
below the maximum is interpreted in terms of a new empirical model that
ascribes most of the temperature dependence to that of the thermally driven
magnon flux.",1504.00895v1
2015-11-11,Cavity magnomechanics,"A dielectric body couples with electromagnetic fields through radiation
pressure and electrostrictive forces, which mediate phonon-photon coupling in
cavity optomechanics. In a magnetic medium, according to Korteweg-Helmholtz
formula, magnetostrictive forces should arise and lead to phonon-magnon
interaction. Here we report such a coupled phonon-magnon system based on
ferrimagnetic spheres, which we term as cavity magnomechanics, by analogy to
cavity optomechanics. Coherent phonon-magnon interactions, including
electromagnetically induced transparency and absorption, are demonstrated.
Excitingly, due to strong hybridization of magnon and microwave photon modes
and their high tunability, our platform exhibits new features including
parametric amplification of magnons and phonons, triply resonant
photon-magnon-phonon coupling and phonon lasing. Our work demonstrates the
fundamental principle of cavity magnomechanics and its application as a new
information transduction platform based on coherent coupling between photons,
phonons and magnons.",1511.03680v2
2015-11-16,Condensing magnons in a degenerate ferromagnetic spinor Bose gas,"We observe the condensation of magnon excitations within an $F=1$ $^{87}$Rb
spinor Bose-Einstein condensed gas. Magnons are pumped into a longitudinally
spin-polarized gas, allowed to equilibrate to a non-degenerate distribution,
and then cooled evaporatively at near-constant net longitudinal magnetization
whereupon they condense. We find magnon condensation to be described
quantitatively as the condensation of free particles in an effective potential
that is uniform within the ferromagnetic condensate volume, evidenced by the
number and distribution of magnons at the condensation transition. Transverse
magnetization images reveal directly the spontaneous, inhomogeneous symmetry
breaking by the magnon quasi-condensate, including signatures of Mermin-Ho spin
textures that appear as phase singularities in the magnon condensate
wavefunction.",1511.05193v1
2016-04-07,Magnon-mediated spin current noise in ferromagnet$|$non-magnetic conductor hybrids,"The quantum excitations of the collective magnetization dynamics in a
ferromagnet (F) - magnons - enable spin transport without an associated charge
current. This pure spin current can be transferred to electrons in an adjacent
non-magnetic conductor (N). We evaluate the finite temperature noise of the
magnon-mediated spin current injected into N by an adjacent F driven by a
coherent microwave field. We find that the dipolar interaction leads to
squeezing of the magnon modes giving them wavevector dependent non-integral
spin, which directly manifests itself in the shot noise. For temperatures
higher than the magnon gap, the thermal noise is dominated by large wavevector
magnons which exhibit negligible squeezing. The noise spectrum is white up to
the frequency corresponding to the maximum of the temperature or the magnon
gap. At larger frequencies, the noise is dominated by vacuum fluctuations. The
shot noise is found to be much larger than its thermal counterpart over a broad
temperature range, making the former easier to be measured experimentally.",1604.02079v2
2016-04-13,Magnon spin transport driven by the magnon chemical potential in a magnetic insulator,"We develop a linear-response transport theory of diffusive spin and heat
transport by magnons in magnetic insulators with metallic contacts. The magnons
are described by a position dependent temperature and chemical potential that
are governed by diffusion equations with characteristic relaxation lengths.
Proceeding from a linearized Boltzmann equation, we derive expressions for
length scales and transport coefficients. For yttrium iron garnet (YIG) at room
temperature we find that long-range transport is dominated by the magnon
chemical potential. We compare the model's results with recent experiments on
YIG with Pt contacts [L.J. Cornelissen, et al., Nat. Phys. 11, 1022 (2015)] and
extract a magnon spin conductivity of $\sigma_{m}=5\times10^{5}$ S/m. Our
results for the spin Seebeck coefficient in YIG agree with published
experiments. We conclude that the magnon chemical potential is an essential
ingredient for energy and spin transport in magnetic insulators.",1604.03706v1
2016-04-28,Chirality-driven Hall Transport Phenomena of Spins,"Experimental and theoretical aspects of Hall-type transport of spins in
magnetic insulators are reviewed, with emphasis on their spin chirality origin.
A general formalism for linear response theory of thermal Hall transport in the
spin model is developed, which can be applied to both the magnon and the
paramagnetic, spin-liquid regimes. Recent experiments on magnon-mediated
thermal Hall transport in the two-dimensional kagome, and three-dimensional
pyrochlore ferromagnetic insulators are reviewed in light of the multi-band
magnon theory of Hall transport, and compared to the more mysterious thermal
Hall transport found in the putative quantum spin ice material. As realizations
of spin-chirality driven magnon transport in the real space, we review the
general theory of emergent gauge fields governing the magnon dynamics in the
textured magnet, and discuss its application to the magnon-Skyrmion scattering
problem. Topological magnon Hall effect driven by the Skyrmion texture is
discussed.",1604.08290v2
2017-03-22,Dirac Magnon Nodal Loops in Quasi-2D Quantum Magnets,"In this report, we propose a new concept of one-dimensional (1D) closed lines
of Dirac magnon nodes in two-dimensional (2D) momentum space of quasi-2D
quantum magnetic systems. They are termed ""2D Dirac magnon nodal-line loops"".
We utilize the bilayer honeycomb ferromagnets with intralayer coupling $J$ and
interlayer coupling $J_L$, which is realizable in the honeycomb chromium
compounds CrX$_3$ (X $\equiv$ Br, Cl, and I). However, our results can also
exist in other layered quasi-2D quantum magnetic systems. Here, we show that
the magnon bands of the bilayer honeycomb ferromagnets overlap for $J_L\neq 0$
and form 1D closed lines of Dirac magnon nodes in 2D momentum space. The 2D
Dirac magnon nodal-line loops are topologically protected by inversion and
time-reversal symmetry. Furthermore, we show that they are robust against weak
Dzyaloshinskii-Moriya interaction $ \Delta_{DM}< J_L$ and possess chiral magnon
edge modes.}",1703.07783v5
2017-08-09,Weyl magnons in pyrochlore antiferromagnets with all-in-all-out orders,"We investigate novel topological magnon band crossings of pyrochlore
antiferromagnets with all-in-all-out (AIAO) magnetic order. By general symmetry
analysis and spin-wave theory, we show that pyrochlore materials with AIAO
orders can host Weyl magnons under external magnetic fields or uniaxial
strains. Under a small magnetic field, the magnon bands of the pyrochlore with
AIAO background can feature two opposite-charged Weyl points, which is the
minimal number of Weyl points realizable in quantum materials and has not be
experimentally observed so far. We further show that breathing pyrochlores with
AIAO orders can exhibit Weyl magnons upon uniaxial strains. These findings
apply to any pyrochlore material supporting AIAO orders, irrespective of the
forms of interactions. Specifically, we show that the Weyl magnons are robust
against direct (positive) Dzyaloshinskii-Moriya interactions. Because of the
ubiquitous AIAO orders in pyrochlore magnets including R$_2$Ir$_2$O$_7$, and
experimentally achievable external strain and magnetic field, our predictions
provide promising arena to witness the Weyl magnons in quantum magnets.",1708.02948v2
2018-04-12,A shortcut to gradient-corrected magnon dispersion: exchange-only case,"Ab initio calculations of the magnon dispersion in ferromagnetic materials
typically rely on the adiabatic local density approximation (ALDA) in which the
effective exchange-correlation field is everywhere parallel to the
magnetization. These calculations, however, tend to overestimate the ""magnon
stiffness"", defined as the curvature of the magnon frequency vs. wave vector
relation evaluated at zero wave vector. Here we suggest a simple procedure to
improve the magnon dispersion by taking into account gradient corrections to
the ALDA at the exchange-only level. We find that this gradient correction
always reduces the magnon stiffness. The surprisingly large size of these
corrections ($\sim 30\%$) greatly improves the agreement between the calculated
and the observed magnon stiffness for cobalt and nickel, which are known to be
overestimated within the ALDA.",1804.04556v2
2013-08-05,Magnonic momentum transfer force on domain walls confined in space,"Momentum transfer from incoming magnons to a Bloch domain wall is calculated
using one dimensional continuum micromagnetic analysis. Due to the confinement
of the wall in space, the dispersion relation of magnons is different from that
of a single domain. This mismatch of dispersion relations can result in
reflection of magnons upon incidence on the domain wall, whose direct
consequence is a transfer of momentum between magnons and the domain wall. The
corresponding counteraction force exerted on the wall can be used for the
control of domain wall motion through magnonic linear momentum transfer, in
analogy with the spin transfer torque induced by magnonic angular momentum
transfer.",1308.0965v1
2013-09-10,Theory of magnon-skyrmion scattering in chiral magnets,"We study theoretically the dynamics of magnons in the presence of a single
skyrmion in chiral magnets featuring Dzyaloshinskii-Moriya interaction. We show
by micromagnetic simulations that the scattering process of magnons by a
skyrmion can be clearly defined although both originate in the common spins. We
find that (i) the magnons are deflected by a skyrmion, with the angle strongly
dependent on the magnon wavenumber due to the effective magnetic field of the
topological texture, and (ii) the skyrmion motion is driven by magnon
scattering through exchange of the momenta between the magnons and a skyrmion:
the total momentum is conserved. This demonstrates that the skyrmion has a
well-defined, though highly non-Newtonian, momentum.",1309.2361v1
2017-06-02,Experimental investigation of the temperature-dependent magnon density and its influence on studies of spin-transfer-torque-driven systems,"We present the temperature dependence of the thermal magnon density in a thin
ferromagnetic layer. By employing Brillouin light scattering and varying the
temperature, an increase of the magnon density accompanied by a lowering of the
spin-wave frequency is observed with increasing temperature. The magnon density
follows the temperature according to the Bose-Einstein distribution function
which leads to an approximately linear dependency. In addition, the influence
of this effect in spin-transfer-torque-driven systems is presented. In
particular, the increase in the magnon density with temperature sets the limit
for a suppression of magnons in charge current-driven systems. Hence, the
maximum possible suppression of thermal magnons occurs at a finite current.",1706.00619v1
2018-09-04,Level Attraction Due to Dissipative Magnon-Photon Coupling,"We report dissipative magnon-photon coupling caused by cavity Lenz effect,
where the magnons in a magnet induce a rf current in the cavity, leading to a
cavity back action that impedes the magnetization dynamics. This effect is
revealed in our experiment as level attraction with a coalescence of hybridized
magnon-photon modes, which is distinctly different from level repulsion with
mode anticrossing caused by coherent magnon-photon coupling. We develop a
method to control the in- terpolation of coherent and dissipative magnon-photon
coupling, and observe a matching condition where the two effects cancel. Our
work sheds light on the so-far hidden side of magnon-photon coupling, opening a
new avenue for controlling and utilizing light-matter interactions.",1809.01233v1
2020-12-03,Parameter estimation and quantum entanglement in PT symmetrical cavity magnonics system,"We investigate the parameter estimation in a magnon-cavity-magnon coupled
system. PT symmetrical two magnons system can be formed in the gain magnetic
materials by the adiabatic elimination of the cavity field mode. We show that
the optimal estimation will not appear at the exceptional point due to that the
quantum fluctuations are the strongest at the exceptional point. Moreover, we
demonstrate that the measurements at the exceptional point tend to be optimal
with the increase of prepared time. And the direct photon detection is the
optimal measurement for the initial state in the vacuum input state. For the
open PT symmetrical two magnons system, the quantum fluctuations will greatly
reduce the degree of entanglement. Finally, we show that a higher estimated
magnetic sensitivity can be obtained by measuring the frequency of one magnon
in the PT symmetrical two magnons system.",2012.01890v1
2017-05-12,Floquet Topological Magnons,"We introduce the concept of Floquet topological magnons --- a mechanism by
which a synthetic tunable Dzyaloshinskii-Moriya interaction (DMI) can be
generated in quantum magnets using circularly polarized electric (laser) field.
The resulting effect is that Dirac magnons and nodal magnons in two-dimensional
(2D) and three-dimensional (3D) quantum magnets can be tuned to magnon Chern
insulators and Weyl magnons respectively under circularly polarized laser
field. The Floquet formalism also yields a tunable intrinsic DMI in insulating
quantum magnets without an inversion center. We demonstrate that the Floquet
topological magnons possess a finite thermal Hall conductivity tunable by the
laser field.",1705.04694v5
2018-07-05,Bridging magnonics and spin-orbitronics,"The emerging field of nano-magnonics utilizes high-frequency waves of
magnetization - the spin waves - for the transmission and processing of
information on the nanoscale. The advent of spin-transfer torque has spurred
significant advances in nano-magnonics, by enabling highly efficient local
spin-wave generation in magnonic nanodevices. Furthermore, the recent emergence
of spin-orbitronics, which utilizes spin-orbit interaction as the source of
spin torque, has provided a unique ability to exert spin torque over spatially
extended areas of magnonic structures, enabling enhanced spin-wave
transmission. Here, we experimentally demonstrate that these advances can be
efficiently combined. We utilize the same spin-orbit torque mechanism for the
generation of propagating spin waves, and for the long-range enhancement of
their propagation, in a single integrated nano-magnonic device. The
demonstrated system exhibits a controllable directional asymmetry of spin wave
emission, which is highly beneficial for applications in non-reciprocal
magnonic logic and neuromorphic computing.",1807.02050v1
2018-07-24,Enhanced magnon spin transport in NiFe$_2$O$_4$ thin films on a lattice-matched substrate,"We investigate magnon spin transport in epitaxial nickel ferrite
(NiFe$_2$O$_4$, NFO) films grown on magnesium gallate spinel (MgGa$_2$O$_4$,
MGO) substrates, which have a lattice mismatch with NFO as small as 0.78%,
resulting in the reduction of antiphase boundary defects and thus in improved
magnetic properties in the NFO films. In the nonlocal transport experiments,
enhanced signals are observed for both electrically and thermally excited
magnons, and the magnon relaxation length ($\lambda_m$) of NFO is found to be
around 2.5 $\mu$m at room temperature. Moreover, at both room and low
temperatures, we present distinct features from the nonlocal spin Seebeck
signals which arise from magnon polaron formation. Our results demonstrate
excellent magnon transport properties (magnon spin conductivity, $\lambda_m$
and spin mixing conductance at the interface between Pt) of NFO films grown on
a lattice-matched substrate that are comparable with those of yttrium iron
garnet.",1807.09013v1
2018-12-21,Magnon heralding in cavity optomagnonics,"In the emerging field of cavity optomagnonics, photons are coupled coherently
to magnons in solid-state systems. These new systems are promising for
implementing hybrid quantum technologies. Being able to prepare Fock states in
such platforms is an essential step towards the implementation of quantum
information schemes. We propose a magnon-heralding protocol to generate a
magnon Fock state by detecting an optical cavity photon. Due to the
peculiarities of the optomagnonic coupling, the protocol involves two distinct
cavity photon modes. Solving the quantum Langevin equations of the coupled
system, we show that the temporal scale of the heralding is governed by the
magnon-photon cooperativity and derive the requirements for generating high
fidelity magnon Fock states. We show that the nonclassical character of the
heralded state, which is imprinted in the autocorrelation of an optical ""read""
mode, is only limited by the magnon lifetime for small enough temperatures. We
address the detrimental effects of nonvacuum initial states, showing that high
fidelity Fock states can be achieved by actively cooling the system prior to
the protocol.",1812.09165v3
2019-01-08,Topological magnon amplification,"Topology is quickly becoming a cornerstone in our understanding of electronic
systems. Like their electronic counterparts, bosonic systems can exhibit a
topological band structure, but in real materials it is difficult to ascertain
their topological nature, as their ground state is a simple condensate or the
vacuum, and one has to rely instead on excited states, for example a
characteristic thermal Hall response. Here we propose driving a topological
magnon insulator with an electromagnetic field and show that this causes edge
mode instabilities and a large non-equilibrium steady-state magnon edge
current. Building on this, we discuss several experimental signatures that
unambiguously establish the presence of topological magnon edge modes.
Furthermore, our amplification mechanism can be employed to power a topological
travelling-wave magnon amplifier and topological magnon laser, with
applications in magnon spintronics. This work thus represents a step toward
functional topological magnetic materials.",1901.02282v2
2019-02-02,Hybrid plasmon-magnon polaritons in graphene-antiferromagnet heterostructures,"We consider a hybrid structure formed by graphene and an insulating
antiferromagnet, separated by a dielectric of thickness up to $d\simeq 500
\,nm$. When uncoupled, both graphene and the antiferromagnetic surface host
their own polariton modes coupling the electromagnetic field with plasmons in
the case of graphene, and with magnons in the case of the antiferromagnet. We
show that the hybrid structure can host two new types of hybrid polariton
modes. First, a surface magnon-plasmon polariton whose dispersion is radically
changed by the carrier density of the graphene layer, including a change of
sign in the group velocity. Second, a surface plasmon-magnon polariton formed
as a linear superposition of graphene surface plasmon and the antiferromagnetic
bare magnon. This polariton has a dispersion with two branches, formed by the
anticrossing between the dispersive surface plasmon and the magnon. We discuss
the potential these new modes have for combining photons, magnons, and plasmons
to reach new functionalities.",1902.00708v1
2019-02-07,Integrated magnonic half-adder,"Spin waves and their quanta magnons open up a promising branch of high-speed
and low-power information processing. Several important milestones were
achieved recently in the realization of separate magnonic data processing units
including logic gates, a magnon transistor and units for non-Boolean computing.
Nevertheless, the realization of an integrated magnonic circuit consisting of
at least two logic gates and suitable for further integration is still an
unresolved challenge. Here we demonstrate such an integrated circuit
numerically on the example of a magnonic half-adder. Its key element is a
nonlinear directional coupler serving as combined XOR and AND logic gate that
utilizes the dependence of the spin wave dispersion on its amplitude. The
circuit constitutes of only three planar nano-waveguides and processes all
information within the magnon domain. Benchmarking of the proposed device is
performed showing the potential for sub-aJ energy consumption per operation.",1902.02855v2
2019-03-07,Nonlocal Spin Transport as a Probe of Viscous Magnon Fluids,"Magnons in ferromagnets behave as a viscous fluid over a length scale, the
momentum-relaxation length, below which momentum-conserving scattering
processes dominate. We show theoretically that in this hydrodynamic regime
viscous effects lead to a sign change in the magnon chemical potential, which
can be detected as a sign change in the nonlocal resistance measured in spin
transport experiments. This sign change is observable when the
injector-detector distance becomes comparable to the momentum-relaxation
length. Taking into account momentum- and spin-relaxation processes, we
consider the quasiconservation laws for momentum and spin in a magnon fluid.
The resulting equations are solved for nonlocal spin transport devices in which
spin is injected and detected via metallic leads. Because of the finite
viscosity we also find a backflow of magnons close to the injector lead. Our
work shows that nonlocal magnon spin transport devices are an attractive
platform to develop and study magnon-fluid dynamics.",1903.02790v2
2019-03-11,Exchange-enhanced Ultrastrong Magnon-Magnon Coupling in a Compensated Ferrimagnet,"The ultrastrong coupling of (quasi-)particles has gained considerable
attention due to its application potential and richness of the underlying
physics. Coupling phenomena arising due to electromagnetic interactions are
well explored. In magnetically ordered systems, the quantum-mechanical
exchange-interaction should furthermore enable a fundamentally different
coupling mechanism. Here, we report the observation of ultrastrong intralayer
exchange-enhanced magnon-magnon coupling in a compensated ferrimagnet. We
experimentally study the spin dynamics in a gadolinium iron garnet single
crystal using broadband ferromagnetic resonance. Close to the ferrimagnetic
compensation temperature, we observe ultrastrong coupling of clockwise and
anticlockwise magnon modes. The magnon-magnon coupling strength reaches more
than 30% of the mode frequency and can be tuned by varying the direction of the
external magnetic field. We theoretically explain the observed phenomenon in
terms of an exchange-enhanced mode-coupling mediated by a weak cubic
anisotropy.",1903.04330v2
2019-08-25,Itinerant topological magnons in Haldane Hubbard model with a nearly-flat electron band,"We elaborate the first theoretical realization of two dimensional itinerant
topological magnons, based on the quarter filled Haldane-Hubbard model with a
nearly-flat electron band. By using the exact diagonalization method with a
projection onto this band, we obtain the spin wave excitations over the
itinerant ferromagnetic ground state. In the flatband limit, the excitation
exhibits similar dispersion to the free electron band with Dirac magnons. The
nonflatness of the electron band opens a topological gap at Dirac points and
leads to an acoustic magnon band with a nonzero Chern number. We further show
that tuning the sublattice Hubbard interactions or the next-nearest-neighbor
hopping can induce a topological transition characterized by the gap closing
and reopening, and the existence of the in-gap magnons on magnetic domain
walls. We find an exact set of bases for magnons in the flatband limit
constructed from sublattice particle-hole vectors and derive an effective model
to explore the origin of the topological magnon which is attributed to the
``mass inversion mechanism''.",1908.09255v1
2016-10-13,Spin wave surface states in one-dimensional planar magnonic crystals,"We have investigated surface spin wave states in one-dimensional planar
bi-component magnonic crystals, localized on the surfaces resulting from the
breaking of the periodic structure. The two systems have been considered: the
magnonic crystal with periodic changes of the anisotropy field in exchange
regime and the magnonic crystal composed of Fe and Ni stripes in dipolar regime
with exchange interactions included. We chose the symmetric unit cell for both
systems to implement the symmetry related criteria for existence of the surface
states. We investigated also the surface states induced by the presence of
perturbation of the surface areas of the magnonic crystals. We showed, that the
system with modulated anisotropy is a direct analog of the electronic crystal.
Therefore, the surface states in both systems have the same properties. For
surface states existing in magnonic crystals in dipolar regime we demonstrated
that spin waves preserve distinct differences to the electronic crystals, which
are due to long-range dynamic dipolar interactions. We found that tuning of the
strength of magnetization pinning resulting from the surface anisotropy or
dipolar effect is vitally important for existence of the surface states in
magnonic crystals.",1610.04083v1
2017-11-13,Thermodynamic Entanglement of Magnonic Condensates,"Over the last decade, significant progresses have been achieved to create
Bose-Einstein condensates (BEC) of magnetic excitations, i.e., magnons, at the
room temperature, which is a novel quantum many-body system with a strong
spin-spin correlation, and contains potential applications in magnonic
spintronics. For quantum information science, the magnonic condensates can
become an attractive source of quantum entanglement, which plays a central role
in most of the quantum information processing tasks. Here we theoretically
study the entanglement properties of a magnon gas above and below the
condensation temperature. We show that the thermodynamic entanglement of the
magnons is a manifestation of the off-diagonal long-range order; the
entanglement of the condensate does not vanish, even if the spins are separated
by an infinitely large distance, which is fundamentally distinct from the
normal magnetic ordering below the Curie temperature. In addition, the phase
transition point occurs when the derivative of the entanglement changes
abruptly. Furthermore, the spin-spin entanglement can be experimentally
accessed with the current technology. These results provide a theoretical
foundation for a future experimental investigation of the magnon BEC in terms
of quantum entanglement.",1711.04394v1
2018-10-17,Conventional magnon BEC in YIG film,"The conventional magnon Bose-Einstein condensation (BEC of magnons with k =
0) is a coherent state of excited magnons described by a common wave function.
It was observed first in antiferromagnetic superfluid states of 3He. Here we
report on the discovery of a very similar magnon BEC in ferrimagnetic film at
room temperature. The experiments were performed in Yttrium Iron Garnet (YIG)
films at a magnetic field oriented perpendicular to the film. The high-density
quasiequilibrium state of excited magnon was formed by methods of pulse and/or
Continuous Waves (CW) magnetic resonance. We have observed a Long Lived
Induction Decay Signals (LLIDS), well known as a signature of spin
superfluidity. We demonstrate that the BEC state may maintain permanently by
continuous replenishment of magnons with a small radiofrequency (RF) field. Our
finding opens the way for development of potential supermagnonic applications
at an ambient conditions.",1810.08051v2
2020-06-22,Imprinting and driving electronic orbital magnetism using magnons,"Magnons, as the most elementary excitations of magnetic materials, have
recently emerged as a prominent tool in electrical and thermal manipulation and
transport of spin, and magnonics as a field is considered as one of the pillars
of modern spintronics. On the other hand, orbitronics, which exploits the
orbital degree of freedom of electrons rather than their spin, emerges as a
powerful platform in efficient design of currents and redistribution of angular
momentum in structurally complex materials. Here, we uncover a way to bridge
the worlds of magnonics and electronic orbital magnetism, which originates in
the fundamental coupling of scalar spin chirality, inherent to magnons, to the
orbital degree of freedom in solids. We show that this can result in efficient
generation and transport of electronic orbital angular momentum by magnons,
thus opening the road to combining the functionalities of magnonics and
orbitronics to their mutual benefit in the realm of spintronics applications.",2006.13033v2
2020-06-29,Bosonic Bott Index and Disorder-Induced Topological Transitions of Magnons,"We investigate the role of disorder on the various topological magnonic
phases present in deformed honeycomb ferromagnets. To this end, we introduce a
bosonic Bott index to characterize the topology of magnon spectra in finite,
disordered systems. The consistency between the Bott index and Chern number is
numerically established in the clean limit. We demonstrate that topologically
protected magnon edge states are robust to moderate disorder and, as
anticipated, localized in the strong regime. We predict a disorder-driven
topological phase transition, a magnonic analog of the ""topological Anderson
insulator"" in electronic systems, where the disorder is responsible for the
emergence of the nontrivial topology. Combining the results for the Bott index
and transport properties, we show that bulk-boundary correspondence holds for
disordered topological magnons. Our results open the door for research on
topological magnonics as well as other bosonic excitations in finite and
disordered systems.",2006.16310v4
2020-09-02,Non-local spin Seebeck effect in the bulk easy-plane antiferromagnet NiO,"We report the observation of magnon spin currents generated by the Spin
Seebeck effect (SSE) in a bulk single crystal of the easy-plane antiferromagnet
NiO. A magnetic field induces a non-degeneracy and thereby an imbalance in the
population of magnon modes with opposite spin. A temperature gradient then
gives rise to a non-zero magnon spin current. This SSE is measured both in a
local and a non-local geometry at 5$\,$K in bulk NiO. The magnetic field
dependence of the obtained signal is modelled by magnetic field splitting of
the low energy magnon modes, affecting the spin Seebeck coefficient. The
relevant magnon modes at this temperature are linked to cubic anisotropy and
magnetic dipole-dipole interactions. The non-local signal deviates from the
expected quadratic Joule heating by saturating at a current from around
75$\,\mu A$ in the injector. The magnon chemical potential does not decay
exponentially with distance and inhomogeneities may be the result of local
magnon accumulations.",2009.01160v1
2020-09-04,Magnonic Su-Schrieffer-Heeger Model in Honeycomb Ferromagnets,"Topological electronics has extended its richness to non-electronic systems
where phonons and magnons can play the role of electrons. In particular,
topological phases of magnons can be enabled by the Dzyaloshinskii-Moriya
interaction (DMI) which acts as an effective spin-orbit coupling. We show that
besides DMI, an alternating arrangement of Heisenberg exchange interactions
critically determines the magnon band topology, realizing a magnonic analog of
the Su-Schrieffer-Heeger model. On a honeycomb ferromagnet with perpendicular
anisotropy, we calculate the topological phase diagram, the chiral edge states,
and the associated magnon Hall effect by allowing the relative strength of
exchange interactions on different links to be tunable. Including weak
phonon-magnon hybridization does not change the result. Candidate materials are
discussed.",2009.02291v1
2015-12-04,Magnetic field dependence of the magnon spin diffusion length in the magnetic insulator yttrium iron garnet,"We investigated the effect of an external magnetic field on the diffusive
spin transport by magnons in the magnetic insulator yttrium iron garnet (YIG),
using a non-local magnon transport measurement geometry. We observed a decrease
in magnon spin diffusion length $\lambda_m$ for increasing field strengths,
where $\lambda_m$ is reduced from 9.6$\pm1.2$ $\mu$m at 10 mT to 4.2$\pm0.6$
$\mu$m at 3.5 T at room temperature. In addition, we find that there must be at
least one additional transport parameter that depends on the external magnetic
field. Our results do not allow us to unambiguously determine whether this is
the magnon equilibrium density or the magnon diffusion constant. These results
are significant for experiments in the more conventional longitudinal spin
Seebeck geometry, since the magnon spin diffusion length sets the length scale
for the spin Seebeck effect as well and is relevant for its understanding.",1512.01410v1
2017-04-18,Giant Magnons of String Theory in the Lambda Background,"The analogues of giant magnon configurations are studied on the string world
sheet in the lambda background. This is a discrete deformation of the
AdS(5)xS(5) background that preserves the integrability of the world sheet
theory. Giant magnon solutions are generated using the dressing method and
their dispersion relation is found. This reduces to the usual dyonic giant
magnon dispersion relation in the appropriate limit and becomes relativistic in
another limit where the lambda model becomes the generalized sine-Gordon theory
of the Pohlmeyer reduction. The scattering of giant magnons is then shown in
the semi-classical limit to be described by the quantum S-matrix that is a
quantum group deformation of the conventional giant magnon S-matrix. It is
further shown that in the small g limit, a sector of the S-matrix is related to
the XXZ spin chain whose spectrum matches the spectrum of magnon bound states.",1704.05437v2
2019-10-14,Direct evidence of spatial stability of Bose-Einstein condensate of magnons,"Bose-Einstein condensation of quasi-equilibrium magnons is one of few
macroscopic quantum phenomena observed at room temperature. Since its
discovery, it became an object of intense research, which led to the
observation of many exciting phenomena such as quantized vortices, second
sound, and Bogolyubov waves. However, for a long time it remained unclear, what
physical mechanisms can be responsible for the spatial stability of the magnon
condensate. Indeed, since magnons are believed to exhibit attractive
interaction, it is generally expected that the magnon condensate should be
unstable with respect to the real-space collapse, which contradicts all the
experimental findings. Here, we provide direct experimental evidence that
magnons in a condensate exhibit repulsive interaction resulting in the
condensate stabilization and propose a mechanism, which is responsible for the
interaction inversion. Our experimental conclusions are additionally supported
by the theoretical model based on the Gross-Pitaevskii equation. Our findings
solve a long-standing problem and provide a new insight into the physics of
magnon Bose-Einstein condensates.",1910.06013v1
2019-10-21,Generation of exchange magnons in thin ferromagnetic films by ultrashort acoustic pulses,"We investigate generation of exchange magnons by ultrashort, picosecond
acoustic pulses propagating through ferromagnetic thin films. Using the
Landau-Lifshitz-Gilbert equations we derive the dispersion relation for
exchange magnons for an external magnetic field tilted with respect to the film
normal. Decomposing the solution in a series of standing spin wave modes, we
derive a system of ordinary differential equations and driven harmonic
oscillator equations describing the dynamics of individual magnon mode. The
external magnetoelastic driving force is given by the time-dependent spatial
Fourier components of acoustic strain pulses inside the layer. Dependencies of
the magnon excitation efficiencies on the duration of the acoustic pulses and
the external magnetic field highlight the role of acoustic bandwidth and
phonon-magnon phase matching. Our simulations for ferromagnetic nickel evidence
the possibility of ultrafast magneto-acoustic excitation of exchange magnons
within the bandwidth of acoustic pulses in thin samples under conditions
readily obtained in femtosecond pump-probe experiments.",1910.09481v1
2020-02-01,Coplanar cavity for strong coupling between photons and magnons in van der Waals antiferromagnet,"We investigate the performance of niobium nitride superconducting coplanar
waveguide resonators towards hybrid quantum devices with magnon-photon
coupling. We find internal quality factors ~ 20000 at 20 mK base temperature,
in zero magnetic field. We find that by reducing film thickness below 100 nm
internal quality factor greater than 1000 can be maintained up to parallel
magnetic field of ~ 1 T and perpendicular magnetic field of ~ 100 mT. We
further demonstrate strong coupling of microwave photons in these resonators,
with magnons in chromium trichloride, a van der Waals antiferromagnet, which
shows that these cavities serve as a good platform for studying magnon-photon
coupling in 2D magnonics based hybrid quantum systems. We demonstrate strong
magnon-photon coupling for both optical and acoustic magnon modes of an
antiferromagnet.",2002.00197v2
2020-02-03,Thermal relaxation of magnons and phonons near resonance points in magnetic insulators,"We theoretically investigate the energy relaxation rate of magnons and
phonons near the resonance points to clarify the underlying mechanism of heat
transport in ferromagnetic materials. We find that the simple two-temperature
model is valid for the one-phonon/one-magnon process, as the rate of energy
exchange between magnons and phonons is proportional to the temperature
difference between them, and it is independent of temperature in the high
temperature limit. We found that the magnon-phonon relaxation time due to the
one-phonon/one-magnon interaction could be reduced to 1.48 $\mu s$ at the
resonance point by applying an external magnetic field. It means that the
resonance effect plays a significant role in enhancing the total magnon-phonon
energy exchange rate, apart from the higher order interaction processes.",2002.00547v1
2020-02-18,Ultrafast Light-driven Simultaneous Excitation of Coherent Terahertz Magnons and Phonons in Multiferroic BiFeO$_3$,"The ultrafast switching of magnetization in multiferroic materials by a
femtosecond laser could provide various advantages in photonics and magnonics.
An efficient approach to control the light matter interaction is the modulation
of ultrafast coherent magnons and phonons in the high frequency range.
Spontaneous Raman and infrared spectra reveal the excitation of magnons and
optical phonons in multiferroic BiFeO3 in the sub few terahertz range. However,
coherent control of such quasiparticles has not been achieved yet. In this
study, we demonstrate that linearly polarized laser pulses simultaneously
excite coherent magnons out of plane and in plane cyclone modes and optical
phonon E mode in BiFeO3. Experimental results in conjugation with
phenomenological theory, by considering three uniformly distributed magnetic
domains reveal that impulsive stimulated Raman scattering is responsible for
the generation of coherent magnons and phonons in BiFeO3. The observation of
these terahertz magnon and optical phonon modes paves the way for the
development of ultrafast magneto electro optical devices.",2002.07449v2
2020-02-22,Bose-Einstein Condensation of Nonequilibrium Magnons in Confined Systems,"We study the formation of a room temperature magnon Bose-Einstein condensate
(BEC) in nanoscopic systems and demonstrate that its lifetime is influenced by
the spatial confinement. We predict how dipolar interactions and nonlinear
magnon scattering assist in the generation of a metastable magnon BEC in
energy-quantized nanoscopic devices. We verify our prediction by a full
numerical simulation of the Landau-Lifshitz-Gilbert equation and demonstrate
the generation of magnon BEC in confined insulating magnets of yttrium iron
garnet. We directly map out the nonlinear magnon scattering processes behind
this phase transition to show how fast quantized thermalization channels allow
the BEC formation in confined structures. Based on our results, we discuss a
new mechanism to manipulate the BEC lifetime in nanoscaled systems. Our study
greatly extends the freedom to study the dynamics of magnon BEC in realistic
systems and to design integrated circuits for BEC-based applications at room
temperature.",2002.09777v2
2020-02-23,"Generating lightwave-photon-and-magnon entanglement with a mechanical oscillator as a ""cold reservoir""","We present a scheme to implement a steady lightwave-photon-and-magnon
entanglement in a hybrid photon-magnon system by adiabatically eliminating the
auxiliary microwave cavity and effectively laser cooling a delocalized
Bogoliubov mode. The system consists of magnons, lightwave and microwave
photons, and phonons. The magnons are embodied by a collective motion of a
large number of spins in a macroscopic ferrimagnet. To achieve an entangling
interaction between magnons and lightwave photons, we drive optical cavity and
magnon at the red and blue sideband associated with the mechanical resonator.
In particular, optimizing the relative ratio of effect couplings, rather than
simply increasing their magnitudes, is essential for achieving strong
entanglement. Unlike typical dissipative entanglement schemes, our results
cannot be described by treating the effects of the entangling reservoir via a
Linblad master equation.",2002.10877v2
2020-03-10,Effect of magnons on the temperature dependence and anisotropy of spin-orbit torque,"We investigate the influence of magnons on the temperature-dependence and the
anisotropy of the spin-orbit torque (SOT). For this purpose we use 3rd order
perturbation theory in the framework of the Keldysh formalism in order to
derive suitable equations to compute the magnonic SOT. We find several
contributions to the magnonic SOT, which depend differently on the spin-wave
stiffness $\mathcal{A}$ and on the temperature $T$, with the dominating
contribution scaling like $T^{2}/\mathcal{A}^{2}$. Based on this formalism we
compute the magnonic SOT in the ferromagnetic Rashba model. For large Rashba
parameters the magnonic SOT is strongly anisotropic and for small quasiparticle
broadening it may become larger than the non-magnonic SOT.",2003.05028v3
2020-05-26,Nonlocal stimulation of three-magnon splitting in a magnetic vortex,"We present a combined numerical, theoretical and experimental study on
stimulated three-magnon splitting in a magnetic disk in the vortex equilibrium
state. Our micromagnetic simulations and Brillouin-light-scattering results
confirm that three-magnon splitting can be triggered even below threshold by
exciting one of the secondary modes by magnons propagating in a waveguide next
to the disk. The experiments show that stimulation is possible over an extended
range of excitation powers and a wide range of frequencies around the
eigenfrequencies of the secondary modes. Rate-equation calculations predict an
instantaneous response to stimulation and the possibility to prematurely
trigger three-magnon splitting even above threshold in a sustainable manner.
These predictions are confirmed experimentally using time-resolved
Brillouin-light-scattering measurements and are in a good qualitative agreement
with the theoretical results. We believe that the controllable mechanism of
stimulated three-magnon splitting could provide a possibility to utilize
magnon-based nonlinear networks as hardware for reservoir or neuromorphic
computing.",2005.12663v1
2020-07-20,Stacking Domain Wall Magnons in Twisted van der Waals Magnets,"Using bilayer CrI$_3$ as an example, we demonstrate that stacking domain
walls in van der Waals magnets can host one dimensional (1D) magnon channels,
which have lower energies than bulk magnons. Interestingly, some magnon
channels are hidden in magnetically homogeneous background and can only be
inferred with the knowledge of stacking domain walls. Compared to 1D magnons
confined in magnetic domain walls, 1D magnons in stacking domain walls are more
stable against external perturbations. We show that the relaxed moir\'e
superlattices of small-angle twisted bilayer CrI$_3$ is a natural realization
of stacking domain walls and host interconnected moir\'e magnon network. Our
work reveals the importance of stacking domain walls in understanding magnetic
properties of van der Waals magnets, and extends the scope of stacking
engineering to magnetic dynamics.",2007.10398v2
2020-08-04,Self-hybridization and tunable magnon-magnon coupling in van der Waals synthetic magnets,"Van der Waals magnets are uniquely positioned at the intersection between
two-dimensional materials, antiferromagnetic spintronics, and magnonics. The
interlayer exchange interaction in these materials enables antiferromagnetic
resonances to be accessed at GHz frequencies. Consequently, these layered
antiferromagnets are intriguing materials out of which quantum hybrid magnonic
devices can be fashioned. Here, we use both a modified macrospin model and
micromagnetic simulations to demonstrate a comprehensive antiferromagnetic
resonance spectra in van der Waals magnets near the ultrathin (monolayer)
limit. The number of optical and acoustic magnon modes, as well as the mode
frequencies, are found to be exquisitely sensitive to the number of layers. We
discover a self-hybridization effect where pairs of either optical or acoustic
magnons are found to interact and self-hybridize through the dynamic exchange
interaction. This leads to characteristic avoided energy level crossings in the
energy spectra. Through simulations, we show that by electrically controlling
the damping of surface layers within heterostructures both the strength and
number of avoided energy level crossings in the magnon spectra can be
controlled.",2008.01298v1
2020-08-14,"Magnonic bending, phase shifting and interferometry in a 2D reconfigurable nanodisk crystal","Strongly-interacting nanomagnetic systems are pivotal across next-generation
technologies including reconfigurable magnonics and neuromorphic computation.
Controlling magnetisation state and local coupling between neighbouring
nanoelements allows vast reconfigurable functionality and a host of associated
functionalities. However, existing designs typically suffer from an inability
to tailor inter-element coupling post-fabrication and nanoelements restricted
to a pair of Ising-like magnetisation states. Here, we propose a new class of
reconfigurable magnonic crystal incorporating nanodisks as the functional
element. Magnetic nanodisks are crucially bistable in macrospin and vortex
states, allowing inter-element coupling to be selectively activated (macrospin)
or deactivated (vortex). Through microstate engineering, we leverage the
distinct coupling behaviours and magnonic band structures of bistable nanodisks
to achieve reprogrammable magnonic waveguiding, bending, gating and
phase-shifting across a 2D network. The potential of nanodisk-based magnonics
for wave-based computation is demonstrated via an all-magnon interferometer
exhibiting XNOR logic functionality. Local microstate control is achieved here
via topological magnetic writing using a magnetic force microscope tip.",2008.06451v2
2020-10-09,Einstein-Podolsky-Rosen entanglement and asymmetric steering between distant macroscopic mechanical and magnonic systems,"We propose a deterministic scheme for establishing hybrid
Einstein-Podolsky-Rosen (EPR) entanglement channel between a macroscopic
mechanical oscillator and a magnon mode in a distant yttrium-iron-garnet (YIG)
sphere across about ten gigahertz of frequency difference. The system consists
of a driven electromechanical cavity which is unidirectionally coupled to a
distant electromagnonical cavity inside which a YIG sphere is placed. We find
that far beyond the sideband-resolved regime in the electromechanical
subsystem, stationary phonon-magnon EPR entanglement can be achieved. This is
realized by utilizing the output field of the electromechanical cavity being an
intermediary which distributes the electromechanical entanglement to the
magnons, thus establishing a remote phonon-magnon entanglement. The EPR
entanglement is strong enough such that phonon-magnon quantum steering can be
attainable in an asymmetric manner. This long-distance macroscopic hybrid EPR
entanglement and steering enable potential applications not only in fundamental
tests of quantum mechanics at the macro scale, but also in quantum networking
and one-sided device-independent quantum cryptography based on magnonics and
electromechanics.",2010.04357v1
2020-10-16,Insights on magnon topology and valley-polarization in 2D bilayer quantum magnets,"The rich and unconventional physics in layered 2D magnets can open new
avenues for topological magnonics and magnon valleytronics. In particular,
two-dimensional (2D) bilayer quantum magnets are gaining increasing attention
due to their intriguing stacking-dependent magnetism, controllable ground
states, and topological excitations induced by magnetic spin-orbit couplings
(SOCs). Despite the substantial research on these materials, their topological
features remain widely unexplored to date. The present study comprehensively
investigates the magnon topology and magnon valley-polarization in honeycomb
bilayers with collinear magnetic order. We elucidate the separate and combined
effects of the SOC, magnetic ground-states, stacking order, and inversion
symmetry breaking on the topological phases, magnon valley transport, and the
Hall and Nernst effects. The comprehensive analysis suggests clues to determine
the SOC's nature and predicts unconventional Hall and Nernst conductivities in
topologically trivial phases. We further report on novel bandgap closures in
layered antiferromagnets and detail their topological implications. We believe
the present study provides important insights into the fundamental physics and
technological potentials of topological 2D magnons.",2010.08621v4
2021-01-19,Evolution of room-temperature magnon gas toward coherent Bose-Einstein condensate,"The appearance of spontaneous coherence is a fundamental feature of a
Bose-Einstein condensate and an essential requirement for possible applications
of the condensates for data processing and quantum computing. In the case of a
magnon condensate in a magnetic crystal, such computing can be performed even
at room temperature. So far, the process of coherence formation in a magnon
condensate was inaccessible. We study the evolution of magnon radiation spectra
by direct detection of microwave radiation emitted by magnons in a
parametrically driven yttrium iron garnet crystal. By using specially shaped
bulk samples, we show that the parametrically overpopulated magnon gas evolves
to a state, whose coherence is only limited by the natural magnon relaxation
into the crystal lattice.",2101.07890v1
2021-03-02,Hinge Magnons from Non-collinear Magnetic Order in Honeycomb Antiferromagnet,"We propose that non-collinear magnetic order in quantum magnets can harbor a
novel higher-order topological magnon phase with non-Hermitian topology and
hinge magnon modes. We consider a three-dimensional system of interacting local
moments on stacked-layers of honeycomb lattice. It initially favors a collinear
magnetic order along an in-plane direction, which turns into a non-collinear
order upon applying an external magnetic field perpendicular to the easy axis.
We exploit the non-Hermitian nature of the magnon Hamiltonian to show that this
field-induced transition corresponds to the transformation from a topological
magnon insulator to a higher-order topological magnon state with a
one-dimensional hinge mode. As a concrete example, we discuss the
recently-discovered monoclinic phase of the thin chromium trihalides, which we
propose as the first promising material candidate of the higher-order
topological magnon phase.",2103.01919v1
2021-05-05,Eliashberg study of superconductivity induced by interfacial coupling to antiferromagnets,"We perform Eliashberg calculations for magnon-mediated superconductivity in a
normal metal, where the electron-magnon interaction arises from interfacial
coupling to antiferromagnetic insulators. In agreement with previous studies,
we find $p$-wave pairing for large doping when the antiferromagnetic interfaces
are uncompensated, and $d$-wave pairing close to half-filling when the
antiferromagnetic interfaces are compensated. However, for the $p$-wave phase,
we find a considerable reduction in the critical temperature compared to
previous weak-coupling results, as the effective frequency cutoff on the magnon
propagator in this case is found to be much smaller than the cutoff on the
magnon spectrum. The $d$-wave phase, on the other hand, relies less on
long-wavelength magnons, leading to a larger effective cutoff on the magnon
propagator. Combined with a large density of states close to half-filling, this
might allow the $d$-wave phase to survive up to higher critical temperatures.
Based on our findings, we provide new insight into how to realize interfacially
induced magnon-mediated superconductivity in experiments.",2105.02235v2
2021-07-05,Unified Treatment of Magnons and Excitons in Monolayer CrI$_3$ from Many-Body Perturbation Theory,"We present first principles calculations of the two-particle excitation
spectrum of CrI$_3$ using many-body perturbation theory including spin-orbit
coupling. Specifically, we solve the Bethe-Salpeter equation, which is
equivalent to summing up all ladder diagrams with static screening and it is
shown that excitons as well as magnons can be extracted seamlessly from the
calculations. The resulting optical absorption spectrum as well as the magnon
dispersion agree very well with recent measurements and we extract the
amplitude for optical excitation of magnons resulting from spin-orbit
interactions. Importantly, the results do not rely on any assumptions on the
microscopic magnetic interactions such as Dzyaloshinskii-Moriya (DM), Kitaev or
biquadratic interactions and we obtain a model independent estimate of the gap
between acoustic and optical magnons of 0.3 meV. In addition, we resolve the
magnon wavefunction in terms of band transitions and show that the magnon
carries a spin that is significantly smaller than $\hbar$. This highlights the
importance of terms that do not commute with $S^z$ in any Heisenberg model
description.",2107.02284v1
2021-08-24,Magnon-polarons in van der Waals antiferromagnet FePS3,"The hybridization of magnons (spin waves) with phonons, if sufficiently
strong and comprising long wavelength excitations, may offer a new playground
when manipulating the magnetically ordered systems with light. Applying a
magnetic field to a quasi-2D antiferromagnet, FePS3, we tune the magnon-gap
excitation towards coincidence with the initially lower-in-energy phonon modes.
Hybrid magnon-phonon modes, the magnon polarons are unveiled with demonstration
of a pronounced avoided crossing between the otherwise bare magnon and phonon
excitations. The magnon polarons in FePS3 are primary traced with Raman
scattering experiments, but, as we show, they also couple directly to terahertz
photons, what evokes their further explorations in the domain of
antiferromagnetic optospintronics.",2108.10945v3
2021-09-16,Strong Coupling between Propagating Magnons and High-order Gyrotropic Skyrmion Modes,"Inspired by the recent achievements of the strong magnons- and spin
textures-photons coupling via dipolar interaction, the coupling between magnons
and the local resonances of spin textures through direct exchange interaction
is expected but not realized yet. In this work, we demonstrated the coherent
coupling between propagating magnons and local skyrmion resonances. Besides the
Rabbi coupling gap (RCG) in the frequency field dispersion, a magnonic analog
of polariton gap, polaragnonic band gap (PBG), is also observed in the
frequency-wavenumber dispersion. The realization of coupling requires the
gyrotropic skyrmion modes to satisfy not only their quantum number larger than
one but also their chirality opposite to that of magnons. The observed PBG and
RCG can be controlled to exist within different Brillouin zones (BZs) as well
as at BZ boundaries. The coupling strength can approach the strong regime by
selecting the wavenumber of propagating magnons. Our findings could provide a
pure magnonic platform for investigating quantum optics phenomena in quantum
information technology.",2109.07671v1
2021-10-06,Optomagnonics in dispersive media: magnon-photon coupling enhancement at the epsilon-near-zero frequency,"Reaching strong light-matter coupling in solid-state systems has been long
pursued for the implementation of scalable quantum devices. Here, we put
forward the concept of a platform capable of achieving strong coupling between
magnetic excitations (magnons) and optics based in an epsilon-near-zero medium,
that's it, a medium in which the permittivity is close to zero. We adopt a
phenomenological approach to quantize the electromagnetic field inside a
dispersive magnetic medium and obtain a Hamiltonian describing the interaction
between photons and magnons and the frequency-dependent coupling. We predict
that, in the epsilon-near-zero regime, the single-magnon photon optomagnonic
coupling can be comparable to the uniform magnon's frequency for small magnetic
volumes. For state-of-the-art illustrative values, this would correspond to
achieving the single-magnon strong coupling regime, where the coupling rate is
larger than all the decay rates. Finally, we show that the non-linear energy
spectrum intrinsic to this coupling regime regime can be probed via the
characteristic multiple magnon sidebands in the photon power spectrum.",2110.02984v2
2021-10-15,Functionalized high-speed magnon-polaritons resulting from the magnetic antenna effect,"Magnon-polaritons (MPs) refer to a light--magnon coupled state and can
potentially act as information carriers, possibly enabling charge-free
computation. However, the light--magnon coupling is inherently weak. To achieve
sufficiently strong coupling, a large ferromagnet or coupling with a microwave
cavity is necessary. Herein, we theoretically propose a fundamental platform
for magnonic and magnon--optical information storage devices and discuss the
transport properties of MP's. The proposed multi-layered structure overcomes
the aforementioned issues. Owing to the waveguide modes, magnons placed in a
nanometer-thin layer are strongly coupled with light, exhibiting rich
functionalities of thick-layer MPs via the `magnetic antenna effect'. Thus, the
thin-layer MPs are faster, and the direction is switchable. The results of this
study will enable the integration of ferromagnetic micro and nanostructures for
MP-based information devices without any restrictions due to cavities.",2110.08147v1
2021-10-20,Gapless Dirac magnons in CrCl$_{3}$,"Bosonic Dirac materials are testbeds for dissipationless spin-based
electronics. In the quasi two-dimensional honeycomb lattice of CrX$_{3}$ (X=Cl,
Br, I), Dirac magnons have been predicted at the crossing of acoustical and
optical spin waves, analogous to Dirac fermions in graphene. Here we show that,
distinct from CrBr$_{3}$ and CrI$_{3}$, gapless Dirac magnons are present in
bulk CrCl$_{3}$, with inelastic neutron scattering intensity at low
temperatures approaching zero at the Dirac $K$ point. Upon warming,
magnon-magnon interactions induce strong renormalization and decreased
lifetimes, with a ~25% softening of the upper magnon branch intensity from 5 to
50 K, though magnon features persist well above T$_{N}$. Moreover, an unusual
negative thermal expansion (NTE) of the $a$-axis lattice constant and anomalous
phonon behavior are observed below 50 K, indicating magnetoelastic and
spin-phonon coupling arising from an increase in the in-plane spin correlations
that begins tens of Kelvin above T$_{N}$.",2110.10771v1
2021-11-01,Magnon-driven dynamics of frustrated skyrmion in synthetic antiferromagnets: Effect of skyrmion precession,"A theoretical study on the interplay of frustrated skyrmion and magnons is
useful for revealing new physics and future experiments design. In this work,
we investigated the magnon-driven dynamics of frustrated skyrmion in synthetic
antiferromagnets, focusing on the effect of skyrmion precession. It is
theoretically revealed that the scattering cross section of the injected
magnons depends on the skyrmion precession, which in turn effectively modulates
the skyrmion Hall motion. Specifically, the Hall angle decreases as the
precession speed increases, which is also verified by the atomistic
micromagnetic simulations. Moreover, the precession speed and the Hall angle of
the frustrated skyrmion depending on the magnon intensity and damping constant
are simulated, demonstrating the effective suppression of the Hall motion by
the skyrmion precession. This work provides a comprehensive understanding of
the magnon-skyrmion scattering in frustrated magnets, benefiting future
spintronic and magnonic applications.",2111.00738v1
2021-11-16,Optical sensing of magnons via the magnetoelastic displacement,"We show how to measure a steady-state magnon population in a magnetostatic
mode of a ferromagnet or ferrimagnet, such as yttrium iron garnet. We adopt an
optomechanical approach and utilize the magnetoelasticity of the ferromagnet.
The magnetostrictive force dispersively couples magnons to the deformation
displacement of the ferromagnet, which is proportional to the magnon
population. By further coupling the mechanical displacement to an optical
cavity that is resonantly driven by a weak laser, the magnetostrictively
induced displacement can be sensed by measuring the phase quadrature of the
optical field. The phase shows an excellent linear dependence on the magnon
population for a not very large population, and can thus be used as a
`magnometer' to measure the magnon population. We further study the effect of
thermal noises, and find a high signal-to-noise ratio even at room temperature.
At cryogenic temperatures, the resolution of magnon excitation numbers is
essentially limited by the vacuum fluctuations of the phase, which can be
significantly improved by using a squeezed light.",2111.08376v2
2021-12-30,Giant magnon spin conductivity approaching the two-dimensional transport regime in ultrathin yttrium iron garnet films,"Conductivities are key material parameters that govern various types of
transport (electronic charge, spin, heat etc.) driven by thermodynamic forces.
Magnons, the elementary excitations of the magnetic order, flow under the
gradient of a magnon chemical potential in proportion to a magnon (spin)
conductivity $\sigma_{m}$. The magnetic insulator yttrium iron garnet (YIG) is
the material of choice for efficient magnon spin transport. Here we report an
unexpected giant $\sigma_{m}$ in record-thin YIG films with thicknesses down to
3.7 nm when the number of occupied two-dimensional (2D) subbands is reduced
from a large number to a few, which corresponds to a transition from 3D to 2D
magnon transport. We extract a 2D spin conductivity ($\approx1$ S) at room
temperature, comparable to the (electronic) spin conductivity of the
high-mobility two-dimensional electron gas in GaAs quantum wells at millikelvin
temperatures. Such high conductivities offer unique opportunities to develop
low-dissipation magnon-based spintronic devices.",2112.15165v3
2022-01-15,Physical Nature of Magnon Spin Seebeck Effect in Ferrimagnetic Insulators,"The spin Seebeck effect (SSE) in ferrimagnetic insulators (FMI) provides a
simple method of using heat to manipulate magnons, which could be used as
carriers of information and energy conversion. However, a theory that can
quantitively interpret experimental results is still lacking. In this paper, we
develop a transport theory of magnons in FMI at low temperatures by combining
the macroscopic Boltzmann equation with microscopic quantum scattering theory.
It is found that the scattering of magnons is dominated by phonons rather than
magnons, and the relaxation time of magnon is inversely proportional to the
cube of temperature. At extremely low temperature region, the magnon enters the
ballistic transport process. In addition, we also derive the linear spatial
distribution of the transverse SSE signal with sample position. All the
theoretical results are in excellent agreement with the experimental data.",2201.05806v1
2022-03-08,Topological phase transition in magnon bands in a honeycomb ferromagnet driven by sublattice symmetry breaking,"Ferromagnetic honeycomb systems are known to exhibit a magnonic topological
phase under the existence of the next-nearest neighbor Dzyaloshinskii-Moriya
interaction (DMI). Motivated by the recent progress in the sublattice-specific
control of magnetic anisotropy, we study the topological phase of magnon bands
of honeycomb ferromagnetic monolayer and bilayer with the sublattice symmetry
breaking due to the different anisotropy energy in the presence of the DMI. We
show that there is a topological phase transition between the topological
magnon insulator and the topologically trivial magnon phase driven by the
change of the relative size of the DMI and the anisotropy differences between
the sublattices. The magnon thermal Hall conductivity is proposed as an
experimental probe of the magnon topology.",2203.03845v1
2022-03-30,Hybrid magnonics for short-wavelength spin waves facilitated by a magnetic heterostructure,"Recent research on hybrid magnonics has been restricted by the long magnon
wavelengths of the ferromagnetic resonance modes. We present an experiment on
the hybridization of 250-nm wavelength magnons with microwave photons in a
multimode magnonic system consists of a planar cavity and a magnetic bilayer.
The coupling between magnon modes in the two magnetic layers, i.e., the uniform
mode in Permalloy (Py) and the perpendicular standing spin waves (PSSWs) in
YIG, serves as an effective means for exciting short-wavelength PSSWs, which is
further hybridized with the photon mode of the microwave resonator. The
demonstrated magnon-photon coupling approaches the superstrong coupling regime,
and can even be achieved near zero bias field.",2203.16310v1
2022-04-04,Twisted Magnon Frequency Comb and Penrose Superradiance,"Quantization effects of the nonlinear magnon-vortex interaction in
ferromagnetic nanodisks are studied. We show that the circular geometry twists
the spin-wave fields with spiral phase dislocations carrying quantized orbital
angular momentum (OAM). Meanwhile, the confluence and splitting scattering of
twisted magnons off the gyrating vortex core (VC) generates a frequency comb
consisting of discrete and equally spaced spectral lines, dubbed as twisted
magnon frequency comb (tMFC). It is found that the mode spacing of the tMFC is
equal to the gyration frequency of the VC and the OAM quantum numbers between
adjacent spectral lines differ by one. By applying a magnetic field
perpendicular to the plane of a thick nanodisk, we observe a magnonic Penrose
superradiance inside the cone vortex state, which mimics the amplification of
waves scattered from a rotating black hole. It is demonstrated that the
higher-order modes of tMFC are significantly amplified while the lower-order
ones are trapped within the VC gyrating orbit which manifests as the
ergoregion. These results suggest a promising way to generate twisted magnons
with large OAM and to drastically improve the flatness of the magnon comb.",2204.01582v2
2022-06-12,Stationary optomagnonic entanglement and magnon-to-optics quantum state transfer via opto-magnomechanics,"We show how to prepare a steady-state entangled state between magnons and
optical photons in an opto-magnomechanical configuration, where a mechanical
vibration mode couples to a magnon mode in a ferrimagnet by the dispersive
magnetostrictive interaction, and to an optical cavity by the radiation
pressure. We find that, by appropriately driving the magnon mode and the cavity
to simultaneously activate the magnomechanical Stokes and the optomechanical
anti-Stokes scattering, a stationary optomagnonic entangled state can be
created. We further show that, by activating the magnomechanical state-swap
interaction and subsequently sending a weak red-detuned optical pulse to drive
the cavity, the magnonic state can be read out in the cavity output field of
the pulse via the mechanical transduction. The demonstrated entanglement and
state-readout protocols in such a novel opto-magnomechanical configuration
allow us to optically control, prepare, and read out quantum states of
collective spin excitations in solids, and provide promising opportunities for
the study of quantum magnonics, macroscopic quantum states, and magnonic
quantum information processing.",2206.05688v3
2022-07-08,Interacting topological Dirac magnons,"In this work, we study the magnon-magnon interaction effect in typical
honeycomb ferromagnets consisting of van der Waals-bonded stacks of honeycomb
layers, e.g., chromium trihalides CrX3 (X = F, Cl, Br, and I), that display two
spin-wave modes (Dirac magnon). Using Green's function formalism with the
presence of the Dzyaloshinskii-Moriya interaction, we obtain a spinor Dyson
equation up to the second-order approximation by the cluster expansion method.
Numerical calculations show prominent renormalizations of the single-particle
spectrum. Furthermore, we propose a tunable renormalization effect using a
parametric magnon amplification scheme. By amplifying the magnon population at
different k points, the enabled renormalization effect not only reshapes the
band structure but also modifies the Berry curvature distribution. Our work
demonstrates the interplay between band geometry, interactions, and the
external light field in the bosonic system and can potentially lead to new
insights into the properties of magnon-based spintronic devices.",2207.03964v2
2022-07-11,Direct optical probe of magnon topology in two-dimensional quantum magnets,"Controlling edge states of topological magnon insulators is a promising route
to stable spintronics devices. However, to experimentally ascertain the
topology of magnon bands is a challenging task. Here we derive a fundamental
relation between the light-matter coupling and the quantum geometry of magnon
states. This allows to establish the two-magnon Raman circular dichroism as an
optical probe of magnon topology in honeycomb magnets, in particular of the
Chern number and the topological gap. Our results pave the way for interfacing
light and topological magnons in functional quantum devices.",2207.04745v1
2022-07-20,Exact Results for the Orbital Angular Momentum of Magnons on Honeycomb Lattices,"We obtain exact results for the orbital angular momentum (OAM) of magnons at
the high symmetry points of ferromagnetic (FM) and antiferromagnetic (AF)
honeycomb lattices in the presence of Dzyallonshinskii-Moriya (DM)
interactions. For the FM honeycomb lattice in the absence of DM interactions,
the values of the OAM at the corners of the Brillouin zone (BZ) (${\bf
k}_1^*=(0,2\sqrt{3}/9)2\pi/a $, ${\bf k}_2^*=(1/3,\sqrt{3}/9)2\pi/a ,\ldots $)
are alternately $\pm 3\hbar /16$ for both magnon bands. The presence of DM
interactions dramatically changes those values by breaking the degeneracy of
the two magnon bands. The OAM values are alternately $3\hbar /8$ and 0 for the
lower magnon band and $-3\hbar /8$ and 0 for the upper magnon band. For the AF
honeycomb lattice, the values of the OAM at the corners of the BZ are $\mp
(3\hbar /16)\kappa $ on one of the degenerate magnon bands and $\pm (3\hbar /8)
(1+\kappa /2)$ on the other, where $\kappa $ measures the anisotropy and the
result is independent of the DM interaction.",2207.09883v2
2022-08-01,Time-dependent strain-tuning topological magnon phase transition,"Collinear magnets in honeycomb lattices under the action of time-dependent
strains are investigated. Given the limits of high-frequency periodically
varying deformations, we derive an effective Floquet theory for spin system
that results in the emergence of a spin chirality. We find that the coupling
between magnons and spin chirality depends on the details of the strain such as
the spatial dependence and applied direction. Magnonic fluctuations about the
ferromagnetic state are determined, and it is found that spatially homogeneous
strains drive the magnon system into topologically protected phases. In
particular, we show that certain uniform strain fields play the role of an
out-of-plane next-neighbor Dzyaloshinskii-Moriya interaction. Furthermore, we
explore the application of nonuniform strains, which lead to a confinement of
magnon states that for uniaxial strains, propagates along the direction that
preserves translational symmetry. Our work demonstrates a direct and novel way
in which to manipulate the magnon spectrum based on time-dependent strain
engineering that is relevant for exploring topological transitions in quantum
magnonics.",2208.01170v1
2022-09-21,Temperature-Induced Magnonic Chern Insulator in Collinear Antiferromagnets,"Thermal fluctuation in magnets will bring temperature-dependent self-energy
corrections to the magnons, however, their effects on the topological orders of
magnons is not well explored. Here we demonstrate that such corrections can
induce a Chern insulating phase in two-dimensional collinear antiferromagnets
with sublattice asymmetries by increasing temperature. We present the phase
diagram of the system and show that the trivial magnon bands at zero
temperature exhibit Chern insulating phase above a critical temperature before
the paramagnetic phase transition. The self-energy corrections close and reopen
the bandgap at {\Gamma} or K points, accompanied by a magnon chirality switch
and nontrivial Berry curvature transition. The thermal Hall effect of magnons
or detecting the magnon polarization can give experimentally prominent
signatures of topological transitions. We include the numerical results based
on van der Waals magnet MnPS3, calling for experimental implementation. Our
work presents a new paradigm for constructing topological phase that is beyond
the linear spin wave theory.",2209.10184v3
2022-10-04,The spin-flop transition in the quasi two dimensional antiferromagnet MnPS3 detected via thermally generated magnon transport,"We present the detection of the spin-flop transition in the antiferromagnetic
van der Waals material MnPS3 via thermally generated nonlocal magnon transport
using permalloy detector strips. The inverse anomalous spin Hall effect has the
unique power to detect an out-of-plane spin accumulation which enables us to
detect magnons with an out-of-plane spin polarization; in contrast to strips of
high spin-orbit material such as Pt which only possess the spin Hall effect and
are only sensitive to an in-plane spin polarization of the spin accumulation.
We show that nonlocal magnon transport is able to measure the spin-flop
transition in the absence of other spurious effects. Our measurements show the
detection of magnons generated by the spin Seebeck effect before and after the
spin-flop transition where the signal reversal of the magnon spin accumulation
agrees with the OOP spin polarization carried by magnon modes before and after
the SF transition.",2210.01418v1
2022-10-07,Composition-tunable magnon-polaron anomalies in spin Seebeck effects in epitaxial Bi$_x$Y$_{3-x}$Fe$_{5}$O$_{12}$ films,"We have investigated hybridized magnon-phonon excitation (magnon polarons) in
spin Seebeck effects (SSEs) in Bi$_x$Y$_{3-x}$Fe$_{5}$O$_{12}$
(Bi$_x$Y$_{3-x}$IG; $x=0$, $0.5$, and $0.9$) films with Pt contact. We observed
sharp peak structures in the magnetic field $H$ dependence of the longitudinal
SSE (LSSE) voltages, which appear when the phonon dispersions are tangential to
the magnon dispersion curve in Bi$_x$Y$_{3-x}$IG. By increasing the Bi amount
$x$, the peak fields in the LSSE shift toward lower $H$ values due to the
reduction of the sound velocities in Bi$_x$Y$_{3-x}$IG. We also measured the
SSE in a nonlocal configuration and found that magnon-polaron anomalies appear
with different signs and intensities. Our result shows composition-tunability
of magnon-polaron anomalies and provides a clue to further unravel the physics
of magnon-polaron SSEs.",2210.03346v1
2022-10-27,Topological magnons in one-dimensional ferromagnetic Su-Schrieffer-Heeger model with anisotropic interaction,"Topological magnons in a one-dimensional (1D) ferromagnetic (FM)
Su-Schrieffer-Heeger (SSH) model with anisotropic exchange interactions are
investigated. Apart from the inter-cellular isotropic Heisenberg interaction,
the intercellular anisotropic exchange interactions, i.e. Dzyaloshinskii-Moriya
interaction (DMI) and pseudo-dipolar interaction (PDI), also can induce the
emergence of the non-trivial phase with two degenerate in-gap edge states
separately localized at the two ends of the 1D chain, while the intracellular
interactions instead unfavors the topological phase. The interplay among them
has synergistic effects on the topological phase transition, very different
from that in the two-dimensional (2D) ferromagnet. These results demonstrate
that the 1D magnons possess rich topological phase diagrams distinctly
different from the electronic version of the SSH model and even the 2D magnons.
Due to the lower dimensional structural characteristics of this 1D topological
magnonic system, the magnonic crystals can be constructed from bottom to top,
which has important potential applications in the design of novel magnonic
devices.",2210.15530v1
2022-10-31,Magnon dynamics in a Skyrmion-textured domain wall of antiferromagnets,"We theoretically investigate the interaction between magnons and a
Skyrmion-textured domain wall in a two-dimensional antiferromagnet and
elucidate the resultant properties of magnon transport. Using supersymmetric
quantum mechanics, we solve the scattering problem of magnons on top of the
domain wall and obtain the exact solutions of propagating and bound magnon
modes. Then, we find their properties of reflection and refraction in the
Skyrmion-textured domain wall, where magnons experience an emergent magnetic
field due to its non-trivial spin texture-induced effective gauge field. Based
on the obtained scattering properties of magnons and the domain wall, we show
that the thermal transport decreases as the domain wall's chirality increases.
Our results suggest that the thermal transport of an antiferromagnet is tunable
by modulating the Skyrmion charge density of the domain wall, which might be
useful for realizing electrically tunable spin caloritronic devices.",2211.00030v1
2022-11-10,Continuous similarity transformation for critical phenomena: easy-axis antiferromagnetic XXZ model,"We apply continuous similarity transformations (CSTs) to the easy-axis
antiferromagnetic XXZ-model on the square lattice. The CST flow equations are
truncated in momentum space by the scaling dimension $d$ so that all
contributions with $d\le 2$ are taken into account. The resulting quartic
magnon-conserving effective Hamiltonian is analyzed in the zero-, one-, and
two-magnon sector. In this way, a quantitative description of the ground-state
energy, the one-magnon dispersion and its gap as well as of two-magnon bound
states is gained for anisotropies ranging from the gapped Ising model to the
gapless Heisenberg model. We discuss the critical properties of the gap closing
as well as the evolution of the one-magnon roton mininum. The excitation
energies of two-magnon bound states are calculated and their decay into the
two-magnon continuum is determined via the inverse participation ratio.",2211.05689v1
2022-11-16,Strongly coupled magnon-plasmon polaritons in graphene- 2D ferromagnet heterostructures,"Magnons and plasmons are two very different types of collective modes, acting
on the spin and charge degrees of freedom, respectively. At first sight, the
formation of hybrid plasmon-magnon polaritons in heterostructures of plasmonic
and magnetic systems would face two challenges, the small mutual interaction,
via Zeeman coupling of the electromagnetic field of the plasmon with the spins,
and the energy mismatch, as in most systems plasmons have energies in the eV
range, orders of magnitude larger than magnons. Here we show that graphene
plasmons form polaritons with the magnons of two-dimensional ferrromagnetic
insulators, placed up to to half a micron apart, with Rabi couplings in the
range of 100 GHz (dramatically larger than cavity QED magnonics). This strong
coupling is facilitated both by the small energy of graphene plasmons and the
cooperative super-radiant nature of the plasmon-magnon coupling afforded by
phase matching. We show that the Rabi coupling can be modulated both
electrically and mechanically and we propose a attenuated total internal
reflection experiment to implement ferromagnetic resonance experiments on 2D
ferromagnets driven by plasmon excitation.",2211.08949v1
2022-12-02,Coherent magnon-induced domain wall motion in a magnetic insulator channel,"Advancing the development of spin-wave devices requires high-quality
low-damping magnetic materials where magnon spin currents can propagate
efficiently and interact effectively with local magnetic textures. We show that
magnetic domain walls (DW) can modulate spin-wave transport in perpendicularly
magnetized channels of Bi-doped yttrium-iron-garnet (BiYIG). Conversely, we
demonstrate that the magnon spin current can drive DW motion in the BiYIG
channel device by means of magnon spin-transfer torque. The DW can be reliably
moved over 15 um distances at zero applied magnetic field by a magnon spin
current excited by an RF pulse as short as 1 ns. The required energy for
driving DW motion is orders of magnitude smaller than those reported for
metallic systems. These results facilitate low-switching-energy magnonic
devices and circuits where magnetic domains can be efficiently reconfigured by
magnon spin currents flowing within magnetic channels.",2212.01408v1
2023-01-29,Three-wave mixing of anharmonically coupled magnons,"Magnons are quantized collective spin-wave excitations in magnetically
ordered systems. Revealing their interactions among these collective modes is
crucial for the understanding of fundamental many-body effects in such systems
and the development of high-speed information transport and processing devices
based on them. Nevertheless, identifying couplings between individual magnon
modes remains a long-standing challenge. Here, we observe unambiguous
spectroscopic fingerprints of anharmonic coupling between distinct magnon modes
in an antiferromagnet, as evidenced by coherent photon emission at the sum and
difference frequencies of the two modes. This discovery is enabled by driving
two magnon modes coherently with a pair of tailored terahertz fields and then
disentangling a mixture of nonlinear responses with different origins,
symmetries, and field dependences in a two-dimensional frequency-frequency
correlation map. Our approach provides a new platform for generating nonlinear
magnon-magnon mixings and establishes a systematic means of unveiling intricate
couplings among distinct low-energy collective modes.",2301.12555v1
2023-04-26,Critical Cavity-Magnon Polariton Mediated Strong Long-Distance Spin-Spin Coupling,"Strong long-distance spin-spin coupling is desperately demanded for
solid-state quantum information processing, but it is still challenged. Here,
we propose a hybrid quantum system, consisting of a coplanar waveguide (CPW)
resonator weakly coupled to a single nitrogen-vacancy spin in diamond and a
yttrium-iron-garnet (YIG) nanosphere holding Kerr magnons, to realize strong
long-distance spin-spin coupling. With a strong driving field on magnons, the
Kerr effect can squeeze magnons, and thus exponentially enhance the coupling
between the CPW resonator and the squeezed magnons, which produces two
cavity-magnon polaritons, i.e., the high-frequency polariton (HP) and
low-frequency polariton (LP). When the enhanced cavity-magnon coupling
approaches to the critical value, the spin is fully decoupled from the HP,
while the coupling between the spin and the LP is significantly improved. In
the dispersive regime, a strong spin-spin coupling is achieved with accessible
parameters, and the coupling distance can be up to $\sim$cm. Our proposal
provides a promising way to manipulate remote solid spins and perform quantum
information processing in weakly coupled hybrid systems.",2304.13553v2
2023-06-02,Chiral Anomaly Beyond Fermionic Paradigm,"Two-dimensional magnets have manifested themselves as promising candidates
for quantum devices. We here report that the edge and strain effects during the
device fabrication with two-dimensional honeycomb ferromagnets such as CrX$_3$
(X=Cl, I, Br) and CrXTe$_3$ (X=Si, Ge) can be characterized by a
(1+1)-dimensional magnon chiral anomaly beyond the fermionic paradigm. In the
presence of zigzag edges, a pair of chiral bulk-edge magnon bands appear and
cause an imbalance of left- and right-chirality magnons when subjected to
nonuniform temperature or magnetic fields. In the presence of a uniaxial
strain, the bulk Dirac magnons are broken into chiral magnon pseudo-Landau
levels, resulting in a magnon chiral anomaly observable through a negative
strain-resistivity of the magnetic dipole and heat. Our work demonstrates a
chiral anomaly with (quasi)particles obeying non-fermionic statistics and will
be instructive in understanding anomalous magnon transport.",2306.01446v2
2023-06-26,Exciton-magnon splitting in van der Waals antiferromagnet MnPS$_3$ unveiled by second-harmonic generation,"Ultrafast and coherent generation of magnons is of great significance for
high-speed antiferromagnetic spintronics. One possible route is by
exciton-magnon pairwise optical excitation. To date, such exciton-magnon
transitions have been studied mostly by linear optical means in a limited
number of conventional three-dimensional antiferromagnets. Here we investigate
this correlated transition in van der Waals antiferromagnet MnPS$_3$ by using
resonant second-harmonic generation spectroscopy, a nonlinear optical probe
sensitive to the symmetry of electronic and magnetic excitations. Two
exciton-magnon peaks are observed, in line with the exciton-induced splitting
of magnon density of states predicted by a Koster-Slater type theory,
indicating significant exciton-magnon interactions. In addition, a large linear
magnetoelectric effect of excitons is observed. These findings provide renewed
understandings on the correlation effects in two-dimensional magnets with
enhanced quasiparticle scattering cross-sections, and point to potential
coherent control among charge, spin and orbital degrees of freedom in
two-dimensional magnets by optical means.",2306.14642v1
2023-08-02,Topological magnon-photon interaction for cavity magnonics,"The study of cavity magnonics and topological insulators has made significant
advances over the past decade, however the possibility of combining the two
fields is still unexplored. Here, we explore such connection by investigating
hybrid cavity systems that incorporate both a ferromagnet and a topological
insulator. We find that electrons in the topological surface state efficiently
mediate the effective electric dipole coupling between the spin of the
ferromagnet and the electric field of the cavity, in contrast with the
conventional cavity magnonics theory based on magnetic dipole coupling. We
refer to this coupling as topological magnon-photon interaction, estimating it
one order of magnitude stronger than the conventional magnon-photon coupling,
and showing that its sign can be manipulated. We discuss the potential of our
proposed device to allow for scaling down and controlling the cavity system
using electronics. Our results provide solid ground for exploring the
functionalities enabled by merging cavity magnonics with topological
insulators.",2308.01349v1
2023-08-21,Magnon-Phonon coupling in Fe$_3$GeTe$_2$,"We study the dynamic coupling of magnons and phonons in single crystals of
Fe3GeTe2 (FGT) using inelastic scanning tunneling spectroscopy (ISTS) with an
ultra-low temperature scanning tunneling microscope. Inelastic scattering of
hot carriers off phonons or magnons has been widely studied using ISTS, and we
use it to demonstrate strong magnon-phonon coupling in FGT. We show a strong
interaction between magnons and acoustic phonons which leads to formation of
van Hove singularities originating in avoided level crossings and hybridization
between the magnonic and phononic bands in this material. We identify these
additional hybridization points in experiments and compare their energy with
density functional theory calculations. Our findings provide a platform for
designing the properties of dynamic magnon-phonon coupling in two-dimensional
materials.",2308.10774v1
2023-10-29,Magnomechanically controlled Goos-Hänchen shift in cavity QED,"Phenomena involving interactions among magnons, phonons, and photons in
cavity magnomechanical systems have attracted considerable attention recently,
owing to their potential applications in the microwave frequency range. One
such important effect is the response of a probe field to such tripartite
interaction between photon-magnon-phonon. In this paper, we study
Goos-H\""{a}nchen shift (GHS) of a reflected probe field in a cavity
magnomechanical system. We consider a YIG sphere positioned within a microwave
cavity. A microwave control field directly drives the magnon mode in YIG
sphere, whereas the cavity is driven via a weak probe field. Our results show
that the GHS can be coherently controlled through magnon-phonon coupling via
the control field. For instance, GHS can be tuned from positive to negative by
tuning the magnon-phonon coupling. Similarly, the effective cavity detuning is
another important controlling parameter for GHS. Furthermore, we observe that
the enhancement of GHS occurs when magnon-phonon coupling is weak at resonance,
and when the magnon-photon coupling is approximately equal to the loss of
microwave photons. Our findings may have potential significance in applications
related to microwave switching and sensing.",2310.19111v1
2023-11-24,Local temperature control of magnon frequency and direction of supercurrents in a magnon Bose-Einstein condensate,"The creation of temperature variations in magnetization, and hence in the
frequencies of the magnon spectrum in laser-heated regions of magnetic films,
is an important method for studying Bose-Einstein condensation of magnons,
magnon supercurrents, Bogoliubov waves, and similar phenomena. In our study, we
demonstrate analytically, numerically, and experimentally that, in addition to
the magnetization variations, it is necessary to consider the connected
variations of the demagnetizing field. In case of a heat induced local minimum
of the saturation magnetization, the combination of these two effects results
in a local increase in the minimum frequency value of the magnon dispersion at
which the Bose-Einstein condensate emerges. As a result, a magnon supercurrent
directed away from the hot region is formed.",2311.14476v1
2023-12-09,Nonreciprocal Photon-Phonon Entanglement in Kerr-Modified Spinning Cavity Magnomechanics,"Cavity magnomechanics has shown great potential in studying macroscopic
quantum effects, especially for quantum entanglement, which is a key resource
for quantum information science. Here we propose to realize magnons mediated
nonreciprocal photon-phonon entanglement with both the magnon Kerr and Sagnac
effects in cavity magnomechanics. We find that the mean magnon number can
selectively exhibit nonreciprocal linear or nonlinear (bistable) behavior with
the strength of the strong driving field on the cavity. Assisted by this
driving field, the magnon-phonon coupling is greatly enhanced, leading to the
nonreciprocal photon-phonon entanglement via the swapping interaction between
the magnons and photons. This nonreciprocal entanglement can be significantly
enhanced with the magnon Kerr and Sagnac effects. Given the available
parameters, the nonreciprocal photon-phonon entanglement can be preserved at
$\sim3$ K, showing remarkable resilience against the bath temperature. The
result reveals that our work holds promise in developing various nonreciprocal
devices with both the magnon Kerr and Sagnac effects in cavity magnomechanics.",2312.05561v1
2024-01-01,Magnon Damping Minimum and Logarithmic Scaling in a Kondo-Heisenberg Model,"Recently, an anomalous temperature evolution of spin wave excitations has
been observed in a van der Waals metallic ferromagnet Fe$_3$GeTe$_2$ (FGT) [S.
Bao, et al., Phys. Rev. X 12, 011022 (2022)], whose theoretical understanding
yet remains elusive. Here we study the spin dynamics of a ferromagnetic
Kondo-Heisenberg lattice model at finite temperature, and propose a mechanism
of magnon damping that explains the intriguing experimental results. In
particular, we find the magnon damping rate $\gamma(T)$ firstly decreases as
temperature lowers, due to the reduced magnon-magnon scatterings. It then
reaches a minimum at $T_{\rm d}^*$, and rises up again following a logarithmic
scaling $\gamma(T) \sim \ln{(T_0/T)}$ (with $T_0$ a constant) for $T < T_{\rm
d}^*$, which can be attributed to electron-magnon scatterings of spin-flip
type. Moreover, we obtain the phase diagram containing the ferromagnetic and
Kondo insulator phases by varying the Kondo coupling, which may be relevant for
experiments on pressured FGT. The presence of a magnon damping minimum and
logarithmic scaling at low temperature indicates the emergence of the Kondo
effect reflected in the collective excitations of local moments in a Kondo
lattice system.",2401.00758v1
2024-01-02,Dissipative coupling induced UWB magnonic frequency combs generation,"Magnonic frequency combs have recently attracted particular attention due to
their potential impact on spin-wave science. Here, we demonstrate theoretically
the generation of ultra-wideband (UWB) magnonic frequency combs induced by
dissipative coupling in an open cavity magnomechanical system. A broadband comb
with gigahertz repetition rates is obtained in the magnonic spectrum and a
typical non-perturbation frequency-comb structure is also observed. The total
width of the magnonic comb in the robust plateau region can be up to 400 comb
lines, which is much broader and flatter than the reported in the previous
works. Furthermore, when the dissipative coupling strength is further
increased, the chaotic motion is predicted in the magnonic spectrum. Our
results provide an in-depth understanding of nonlinear magnomechanic dynamics
in open quantum systems and fundamentally broadens the research range of magnon
in new spectral regimes.",2401.01260v1
2024-01-04,Nonreciprocal entanglement in cavity magnomechanics exploiting chiral cavity-magnon coupling,"We show how to achieve nonreciprocal quantum entanglement in a cavity
magnomechanical system by exploiting the chiral cavity-magnon coupling. The
system consists of a magnon mode, a mechanical vibration mode, and two
degenerate counter-propagating microwave cavity modes in a torus-shaped cavity.
We show that nonreciprocal stationary microwave-magnon and -phonon bipartite
entanglements and photon-magnon-phonon tripartite entanglement can be achieved
by respectively driving different circulating cavity modes that hold a chiral
coupling to the magnon mode. The nonreciprocal entanglements are shown to be
robust against various experimental imperfections. The work may find promising
applications of the cavity magnomechanical systems in nonreciprocal
electromechanical quantum teleportation and chiral magnonic quantum networks.",2401.02280v2
2024-01-25,Magnon dispersion and spin transport in CrCl$_3$ bilayers under different strain-induced magnetic states,"Atomically-thin van der Waals magnetic materials offer exceptional
opportunities to mechanically and electrically manipulate magnetic states and
spin textures. The possibility of efficient spin transport in these materials
makes them promising for the development of novel nanospintronics technology.
Using atomistic spin dynamics simulations, we investigate magnetic ground
state, magnon dispersion, critical temperature, and magnon spin transport in
CrCl$_3$ bilayers in the absence and presence of compressive and tensile
strains. We show that in the presence of mechanical strain, the magnon band gap
at the $\Gamma$ point and the critical temperature of the bilayer are
increased. Furthermore, our simulations show that the magnon diffusion length
is reduced in the presence of strain. Moreover, by exciting magnons through the
spin Seebeck effect and spin Hall-induced torque, we illustrate distinctions
between magnon spin transport in the antiferromagnetic state, under compressive
strains, and ferromagnetic states, under tensile strains or in the unstrained
case.",2401.13984v1
2024-02-21,Many-body effects on superconductivity mediated by double-magnon processes in altermagnets,"Altermagnets exhibit a large electron spin splitting which can be understood
as a result of strong coupling between itinerant electrons and localized spins.
We consider superconductivity due to electron-magnon scattering, using
strong-coupling Eliashberg theory to capture many-body effects that are not
covered by a weak-coupling approach. The characteristic band structure of
altermagnets puts significant constraints on the spin structure of electron
scattering on the Fermi surface. We emphasize the role of spin-preserving,
double-magnon scattering processes compared to conventional spin-flip processes
involving a single magnon. Then, we derive the Eliashberg equations for a
situation where double-magnon scattering mediates spin-polarized Cooper pairs,
while both double-magnon and single-magnon scatterings contribute to many-body
effects. These many-body effects impact superconducting properties in a way
that differs significantly from systems where conventional spin-flip processes
mediate superconductivity. To highlight the role of $d$-wave magnetism on
superconductivity in altermagnets, we compare our results to those found in
ferromagnetic half-metals and conventional antiferromagnetic metals.",2402.14061v1
2024-03-11,Impact of spin torques and spin pumping phenomena on magnon-plasmon polaritons in antiferromagnetic insulator-semiconductor heterostructures,"We investigate the impact of spin torque and spin pumping on the surface
magnon polariton dispersion in a antiferromagnetic insulator-semiconductor
heterostructure. In the bilayer system, the surface magnon polaritons
conventionally couple to the plasma-oscillations in the semiconductor via
electromagnetic fields. Additionally, magnons in the antiferromagnetic
insulator layer may interact with the semiconductor layer via spin torques and
their reciprocal phenomena of spin pumping. Due to the spin-to-charge
conversion from the spin Hall and inverse spin Hall effects in the
semiconductor layer with a strong spin-orbit coupling, this can couple the
magnons to the plasmons in the semiconductor layer. Our research reveals that
modifications in the mode frequency and the hybridization gap induced by these
phenomena depend on the thickness of the antiferromagnetic layer. In thick
layers, the spin-pumping contribution to the frequency shift and damping is
inversely proportional to the wavelength, while in thin layers it is inversely
proportional to the thickness. Furthermore, hybridization of the surface magnon
polariton and dispersive magnons in the antiferromagnet is shown to depend on
both the thickness and wavelength of the modes.",2403.06934v1
2014-10-14,Coherent coupling between ferromagnetic magnon and superconducting qubit,"Rigidity of an ordered phase in condensed matter results in collective
excitation modes spatially extending in macroscopic dimensions. Magnon is a
quantum of an elementary excitation in the ordered spin system, such as
ferromagnet. Being low dissipative, dynamics of magnons in ferromagnetic
insulators has been extensively studied and widely applied for decades in the
contexts of ferromagnetic resonance, and more recently of Bose-Einstein
condensation as well as spintronics. Moreover, towards hybrid systems for
quantum memories and transducers, coupling of magnons and microwave photons in
a resonator have been investigated. However, quantum-state manipulation at the
single-magnon level has remained elusive because of the lack of anharmonic
element in the system. Here we demonstrate coherent coupling between a magnon
excitation in a millimetre-sized ferromagnetic sphere and a superconducting
qubit, where the interaction is mediated by the virtual photon excitation in a
microwave cavity. We obtain the coupling strength far exceeding the damping
rates, thus bringing the hybrid system into the strong coupling regime.
Furthermore, we find a tunable magnon-qubit coupling scheme utilising a
parametric drive with a microwave. Our approach provides a versatile tool for
quantum control and measurement of the magnon excitations and thus opens a new
discipline of quantum magnonics.",1410.3781v1
2015-06-23,Breakdown of long-wavelength magnons in cubic antiferromagnets with dipolar forces at small temperature,"Using $1/S$ expansion, we discuss the magnon spectrum of Heisenberg
antiferromagnet (AF) on a simple cubic lattice with small dipolar interaction
at small temperature $T\ll T_N$, where $T_N$ is the Neel temperature. Similar
to 3D and 2D ferromagnets, quantum and thermal fluctuations renormalize greatly
the bare gapless spectrum leading to a gap $\Delta\sim \omega_0$, where
$\omega_0$ is the characteristic dipolar energy. This gap is accompanied by
anisotropic corrections to the free energy which make the cube edges easy
directions for the staggered magnetization (dipolar anisotropy). In accordance
with previous results, we find that dipolar forces split the magnon spectrum
into two branches. This splitting makes possible two types of processes which
lead to a considerable enhance of the damping compared to the Heisenberg AF: a
magnon decay into two other magnons and a confluence of two magnons. It is
found that magnons are well defined quasiparticles in quantum AF. We
demonstrate however that a small fraction of long-wavelength magnons can be
overdamped in AFs with $S\gg1$ and in quantum AFs with a single-ion anisotropy
competing with the dipolar anisotropy. Particular materials are pointed out
which can be suitable for experimental observation of this long-wavelength
magnons breakdown that contradicts expectation of the quasiparticle concept.",1506.06989v2
2017-02-08,Magnon Spin-Momentum Locking: Various Spin Vortices and Dirac Magnons in Noncollinear Antiferromagnets,"We generalize the concept of the spin-momentum locking to magnonic systems
and derive the formula to calculate the spin expectation value for one-magnon
states of general two-body spin Hamiltonians. We give no-go conditions for
magnon spin to be independent of momentum. As examples of the magnon
spin-momentum locking, we analyze a one-dimensional antiferromagnet with the
N\'eel order and two-dimensional kagome lattice antiferromagnets with the
120$^\circ$ structure. We find that the magnon spin depends on its momentum
even when the Hamiltonian has the $z$-axis spin rotational symmetry, which can
be explained in the context of a singular band point or a $U(1)$ symmetry
breaking. A spin vortex in momentum space generated in a kagome lattice
antiferromagnet has the winding number $Q=-2$, while the typical one observed
in topological insulator surface states is characterized by $Q=+1$. A magnonic
analogue of the surface states, the Dirac magnon with $Q=+1$, is found in
another kagome lattice antiferromagnet. We also derive the sum rule for $Q$ by
using the Poincar\'e-Hopf index theorem.",1702.02575v4
2018-11-08,Magnon dressing by orbital excitations in ferromagnetic planes of K$_2$CuF$_4$ and LaMnO$_3$,"We show that even when spins and orbitals disentangle in the ground state,
spin excitations are renormalized by the local tuning of $e_g$ orbitals in
ferromagnetic planes of K$_2$CuF$_4$ and LaMnO$_3$. As a result, dressed spin
excitations (magnons) obtained within the electronic model propagate as
quasiparticles and their energy renormalization depends on momentum ${\vec k}$.
Therefore magnons in spin-orbital systems go beyond the paradigm of the
effective Heisenberg model with nearest neighbor spin exchange derived from the
ground state --- spin-orbital entanglement in excited states predicts large
magnon softening at the Brillouin zone boundary, and in case of LaMnO$_3$ the
magnon energy at the $M=(\pi,\pi)$ point may be reduced by $\sim 45$\%. In
contrast, simultaneously the stiffness constant near the Goldstone mode is
almost unaffected. We elucidate physics behind magnon renormalization in
spin-orbital systems and explain why long wavelength magnons are unrenormalized
while simultaneously energies of short wavelength magnons are reduced by
orbital fluctuations. In fact, the ${\vec k}$-dependence of the magnon energy
is modified mainly by dispersion which originates from spin exchange between
second neighbors along the cubic axes $a$ and $b$.",1811.03372v1
2014-05-07,Magnon-skyrmion scattering in chiral magnets,"Chiral magnets support topological skyrmion textures due to the
Dzyaloshinskii-Moriya spin-orbit interaction. In the presence of a sufficiently
large applied magnetic field, such skyrmions are large amplitude excitations of
the field-polarized magnetic state. We investigate analytically the interaction
between such a skyrmion excitation and its small amplitude fluctuations, i.e.,
the magnons in a clean two-dimensional chiral magnet. The magnon spectrum is
found to include two magnon-skyrmion bound states corresponding to a breathing
mode and, for intermediate fields, a quadrupolar mode, which will give rise to
subgap magnetic and electric resonances. Due to the skyrmion topology, the
magnons scatter from a Aharonov-Bohm flux density that leads to skew and
rainbow scattering, characterized by an asymmetric differential cross section
with, in general, multiple peaks. As a consequence of the skew scattering, a
finite density of skyrmions will generate a topological magnon Hall effect.
Using the conservation law for the energy-momentum tensor, we demonstrate that
the magnons also transfer momentum to the skyrmion. As a consequence, a magnon
current leads to magnon pressure reflected in a momentum-transfer force in the
Thiele equation of motion for the skyrmion. This force is reactive and governed
by the scattering cross sections of the skyrmion; it causes not only a finite
skyrmion velocity but also a large skyrmion Hall effect. Our results provide,
in particular, the basis for a theory of skyrmion caloritronics for a dilute
skyrmion gas in clean insulating chiral magnets.",1405.1568v3
2018-09-24,Magnonic Floquet Hofstadter Butterfly,"We introduce the magnonic Floquet Hofstadter butterfly in the two-dimensional
insulating honeycomb ferromagnet. We show that when the insulating honeycomb
ferromagnet is irradiated by an oscillating space- and time-dependent electric
field, the hopping magnetic dipole moment (i.e. magnon quasiparticles)
accumulate the Aharonov-Casher phase. In the case of only space-dependent
electric field, we realize the magnonic Hofstadter spectrum with similar
fractal structure as graphene subject to a perpendicular magnetic field, but
with no spin degeneracy due to broken time-reversal symmetry by the
ferromagnetic order. In addition, the magnonic Dirac points and Landau levels
occur at finite energy as expected in a bosonic system. Remarkably, this
discrepancy does not affect the topological invariant of the system.
Consequently, the magnonic Chern number assumes odd values and the magnon Hall
conductance gets quantized by odd integers. In the case of both space- and
time-dependent electric field, the theoretical framework is studied by the
Floquet formalism. We show that the magnonic Floquet Hofstadter spectrum
emerges entirely from the oscillating space- and time-dependent electric field,
which is in stark contrast to electronic Floquet Hofstadter spectrum, where
irradiation by circularly polarized light and a perpendicular magnetic field
are applied independently. We study the deformation of the fractal structure at
different laser frequencies and amplitudes, and analyze the topological phase
transitions associated with gap openings in the magnonic Floquet Hofstadter
butterfly.",1809.09102v1
2017-09-22,Topological thermal Hall effect due to Weyl magnons,"We present the first theoretical evidence of zero magnetic field topological
(anomalous) thermal Hall effect due to Weyl magnons. Here, we consider Weyl
magnons in stacked noncoplanar frustrated kagom\'e antiferromagnets recently
proposed by Owerre, [arXiv:1708.04240]. The Weyl magnons in this system result
from macroscopically broken time-reversal symmetry by the scalar spin chirality
of noncoplanar chiral spin textures. Most importantly, they come from the
lowest excitation, therefore they can be easily observed experimentally at low
temperatures due to the population effect. Similar to electronic Weyl nodes
close to the Fermi energy, Weyl magnon nodes in the lowest excitation are the
most important. Indeed, we show that the topological (anomalous) thermal Hall
effect in this system arises from nonvanishing Berry curvature due to Weyl
magnon nodes in the lowest excitation, and it depends on their distribution
(distance) in momentum space. The present result paves the way to directly
probe low excitation Weyl magnons and macroscopically broken time-reversal
symmetry in three-dimensional frustrated magnets with the anomalous thermal
Hall effect.",1709.07879v2
2016-10-27,Spin Currents and Magnon Dynamics in Insulating Magnets,"Nambu-Goldstone theorem provides gapless modes to both relativistic and
nonrelativistic systems. The Nambu-Goldstone bosons in insulating magnets are
called magnons or spin-waves and play a key role in magnetization transport. We
review here our past works on magnetization transport in insulating magnets and
also add new insights, with a particular focus on magnon transport. We
summarize in detail the magnon counterparts of electron transport, such as the
Wiedemann-Franz law, the Onsager reciprocal relation between the Seebeck and
Peltier coefficients, the Hall effects, the superconducting state, the
Josephson effects, and the persistent quantized current in a ring to list a
few. Focusing on the electromagnetism of moving magnons, i.e., magnetic
dipoles, we theoretically propose a way to directly measure magnon currents. As
a consequence of the Mermin-Wagner-Hohenberg theorem, spin transport is
drastically altered in one-dimensional antiferromagnetic (AF) spin-1/2 chains;
where the N\'eel order is destroyed by quantum fluctuations and a quasiparticle
magnon-like picture breaks down. Instead, the low-energy collective excitations
of the AF spin chain are described by a Tomonaga-Luttinger liquid (TLL) which
provides the spin transport properties in such antiferromagnets some universal
features at low enough temperature. Finally, we enumerate open issues and
provide a platform to discuss the future directions of magnonics.",1610.08901v2
2020-06-18,Non-Hermiticity and topological invariants of magnon Bogoliubov-de Gennes systems,"Since the theoretical prediction and experimental observation of the thermal
Hall effect of magnons, a variety of novel phenomena that may occur in magnonic
systems have been proposed. In this paper, we review the recent advances in the
study of topological phases of magnon Bogoliubov-de Gennes (BdG) systems. After
giving an overview of the previous works on electronic topological insulators
and the thermal Hall effect of magnons, we provide the necessary background for
bosonic BdG systems, with a particular emphasis on their non-Hermiticity
arising from the diagonalization of the BdG Hamiltonian. After that, we
introduce the definitions of $ \mathbb{Z}_2 $ topological invariants for
bosonic systems with pseudo-time-reversal symmetry, which ensures the existence
of bosonic counterparts of ""Kramers pairs"". Because of the intrinsic
non-Hermiticity of the bosonic BdG systems, these topological invariants have
to be defined in terms of the bosonic Berry connection and curvature. We then
introduce theoretical models that can be thought of as magnonic analogues of
two- and three-dimensional topological insulators in class AII. We demonstrate
analytically and numerically that the $ \mathbb{Z}_2 $ topological invariants
precisely characterize the presence of gapless edge/surface states. We also
predict that bilayer CrI$_3$ with a particular stacking would be an ideal
candidate for realization of a two-dimensional magnon system characterized by a
nontrivial $ \mathbb{Z}_2 $ topological invariant. For three-dimensional
topological magnon systems, the thermal Hall effect of magnons is expected to
occur when a magnetic field is applied to the surface.",2006.10391v3
2018-01-20,Magnon Valve Effect Between Two Magnetic Insulators,"The key physics of the spin valve involves spin-polarized conduction
electrons propagating between two magnetic layers such that the device
conductance is controlled by the relative magnetization orientation of two
magnetic layers. Here, we report the effect of a magnon valve which is made of
two ferromagnetic insulators (YIG) separated by a nonmagnetic spacer layer
(Au). When a thermal gradient is applied perpendicular to the layers, the
inverse spin Hall voltage output detected by a Pt bar placed on top of the
magnon valve depends on the relative orientation of the magnetization of two
YIG layers, indicating the magnon current induced by spin Seebeck effect at one
layer affects the magnon current in the other layer separated by Au. We
interpret the magnon valve effect by the angular momentum conversion and
propagation between magnons in two YIG layers and conduction electrons in the
Au layer. The temperature dependence of magnon valve ratio shows approximately
a power law, supporting the above magnon-electron spin conversion mechanism.
This work opens a new class of valve structures beyond the conventional spin
valves.",1801.06617v2
2021-02-26,Thermodynamics of free and bound magnons in graphene,"Symmetry-broken electronic phases support neutral collective excitations. For
example, monolayer graphene in the quantum Hall regime hosts a nearly ideal
ferromagnetic phase at filling factor $\nu=1$ that spontaneously breaks spin
rotation symmetry. This ferromagnet has been shown to support spin-wave
excitations known as magnons which can be generated and detected electrically.
While long-distance magnon propagation has been demonstrated via transport
measurements, important thermodynamic properties of such magnon
populations--including the magnon chemical potential and density--have thus far
proven out of reach of experiments. Here, we present local measurements of the
electron compressibility under the influence of magnons, which reveal a
reduction of the $\nu=1$ gap by up to 20%. Combining these measurements with
estimates of the temperature, our analysis reveals that the injected magnons
bind to electrons and holes to form skyrmions, and it enables extraction of the
free magnon density, magnon chemical potential, and average skyrmion spin. Our
methods furnish a novel means of probing the thermodynamic properties of
charge-neutral excitations that is applicable to other symmetry-broken
electronic phases.",2103.00015v1
2021-04-01,Topological Phase Transitions of Dirac Magnons in Honeycomb Ferromagnets,"The study of the magnonic thermal Hall effect in magnets with
Dzyaloshinskii-Moriya interaction (DMI) has recently drawn attention because of
the underlying topology. Topological phase transitions may arise when there
exist two or more distinct topological phases, and they are often revealed by a
gap-closing phenomenon. In this work, we consider the magnons in honeycomb
ferromagnets described by a Heisenberg Hamiltonian containing both an
out-of-plane DMI and a Zeeman interaction. We demonstrate that the magnonic
system exhibits temperature (or magnetic field) driven topological phase
transitions due to magnon-magnon interactions. Specifically, when the
temperature increases, the magnonic energy gap at Dirac points closes and
reopens at a critical temperature, $T_c$. By showing that the Chern numbers of
the magnonic bands are distinct above and below $T_c$, we confirm that the
gap-closing phenomenon is indeed a signature for the topological phase
transitions. Furthermore, our analysis indicates that the thermal Hall
conductivity in the magnonic system exhibits a sign reversal at $T_c$, which
can serve as an experimental probe of its topological nature. Our theory
predicts that in $\rm{CrI_3}$ such a phenomenon exists and is experimentally
accessible.",2104.00266v2
2021-06-28,Stabilization of a nonlinear bullet coexisting with a Bose-Einstein condensate in a rapidly cooled magnonic system driven by a spin-orbit torque,"We have recently shown that injection of magnons into a magnetic dielectric
via the spin-orbit torque (SOT) effect in the adjacent layer of a heavy metal
subjected to the action of short (0.1 $\mu$s) current pulses allows for control
of a magnon Bose-Einstein Condensate (BEC). Here, the BEC was formed in the
process of rapid cooling (RC), when the electric current heating the sample is
abruptly terminated. In the present study, we show that the application of a
longer (1.0 $\mu$s) electric current pulse triggers the formation of a
nonlinear localized magnonic bullet below the linear magnon spectrum. After
pulse termination, the magnon BEC, as before, is formed at the bottom of the
linear spectrum, but the nonlinear bullet continues to exist, stabilized for
additional 30 ns by the same process of RC-induced magnon condensation. Our
results suggest that a stimulated condensation of excess magnons to all highly
populated magnonic states occurs.",2106.14710v1
2022-01-27,Magnon transport and thermoelectric effects in ultrathin Tm3Fe5O12/Pt nonlocal devices,"The possibility of electrically exciting and detecting magnon currents in
magnetic insulators has opened exciting perspectives for transporting spin
information in electronic devices. However, the role of the magnetic field and
the nonlocal thermal gradients on the magnon transport remain unclear. Here, by
performing nonlocal harmonic voltage measurements, we investigate magnon
transport in perpendicularly magnetized ultrathin Tm3Fe5O12 (TmIG) films
coupled to Pt electrodes. We show that the first harmonic nonlocal voltage
captures spin-driven magnon transport in TmIG, as expected, and the second
harmonic is dominated by thermoelectric voltages driven by current-induced
thermal gradients at the detector. The magnon diffusion length in TmIG is found
to be on the order of 0.3 {\mu}m at 0.5 T and gradually decays to 0.2 {\mu}m at
0.8 T, which we attribute to the suppression of the magnon relaxation time due
to the increase of the Gilbert damping with field. By performing current,
magnetic field, and distance dependent nonlocal and local measurements we
demonstrate that the second harmonic nonlocal voltage exhibits five
thermoelectric contributions, which originate from the nonlocal spin Seebeck
effect and the ordinary, planar, spin, and anomalous Nernst effects. Our work
provides a guide on how to disentangle magnon signals from diverse
thermoelectric voltages of spin and magnetic origin in nonlocal magnon devices,
and establish the scaling laws of the thermoelectric voltages in
metal/insulator bilayers.",2201.11353v1
2022-01-31,Exciton-Coupled Coherent Magnons in a 2D Semiconductor,"Two-dimensional (2D) magnetic semiconductors feature both tightly-bound
excitons with large oscillator strength and potentially long-lived coherent
magnons due to the presence of bandgap and spatial confinement. While magnons
and excitons are energetically mismatched by orders of magnitude, their
coupling can lead to efficient optical access to spin information. Here we
report strong magnon-exciton coupling in the 2D van der Waals (vdW)
antiferromagnetic (AFM) semiconductor CrSBr. Coherent magnons launched by
above-gap excitation modulate the interlayer hybridization, which leads to
dynamic modulation of excitonic energies. Time-resolved exciton sensing reveals
magnons that can coherently travel beyond 7 micrometer, with coherence time
above 5 ns. We observe this exciton-coupled coherent magnons in both even and
odd number of layers, with and without compensated magnetization, down to the
bilayer limit. Given the versatility of vdW heterostructures, these coherent 2D
magnons may be basis for optically accessible magnonics and quantum
interconnects.",2201.13197v2
2022-03-09,Magnons in antiferromagnetic bcc-Cr and Cr$_2$O$_3$ from time-dependent density functional theory,"We apply time-dependent density functional theory to calculate the transverse
magnetic susceptibility of bcc-Cr and Cr$_2$O$_3$, which constitute
prototypical examples of antiferromagnets with itinerant and localized magnetic
moments respectively. The exchange-correlation kernel is rescaled in order to
enforce the Goldstone condition and the magnon dispersion relations are
extracted based on a symmetry analysis relying on the generalized Onsager
relation. Doing so, our calculations yield the characteristic linear magnon
dispersion of antiferromagnets in the long wavelength limit. In the case of
Cr$_2$O$_3$, we find that the adiabatic local density approximation yields a
good qualitative agreement with the measured dispersion, but overestimates the
magnon velocity and bandwidth by a factor of two. Including a Hubbard
correction improves the magnon velocity, but at the expense of the overall
qualitative agreement with the experimental magnon dispersion. For bcc-Cr we
find a sharp acoustic magnon mode at low energies with a velocity in agreement
with previously reported values. At higher energies, the acoustic magnon mode
becomes subject to strong Landau damping and rapidly vanishes once it enters
the Stoner continuum. In addition to the acoustic magnon mode, we also observe
an additional collective mode along the $\Gamma\rightarrow\mathrm{R}$ direction
with an energy of $\sim$ 1 eV, which is located inside the Stoner continuum,
but appears to elude the effect of Landau damping.",2203.04796v2
2022-08-27,Optomechanical-interface-induced strong spin-magnon coupling,"Strong long-distance spin-magnon coupling is essential for solid-state
quantum information processing and single qubit manipulation. Here, we propose
an approach to realize strong spin-magnon coupling in a hybrid optomechanical
cavity-spin-magnon system, where the optomechanical system, consisting of two
cavities coupled to a common high-frequency mechanical resonator, acts as
quantum interface. By eliminating the mechanical mode, a position-position
coupling and two-mode squeezing of two cavities are induced. In the squeezing
presentation, the spin-photon, magnon-photon and photon-photon coupling
strengths are exponentially amplified, thus lower- and upper-branch polaritons
(LBP and UBP) are generated by strongly coupled squeezed modes of two cavities.
Utilizing the critical property of the LBP, the coupling between the spin qubit
(magnon) and LBP is greatly enhanced, while the coupling between the spin qubit
(magnon) and UBP is fully suppressed. In the dispersive regime, strong and
tunable spin-magnon coupling is induced by the virtual LBP, allowing quantum
state exchange between them. Our proposal provides a promising platform to
construct magnon-based hybrid systems and realize solid-state quantum
information processing with optomechanical interfaces.",2208.12988v3
2022-09-20,Electronic Manipulation of Magnon Topology by Chirality Injection from Boundaries,"Magnon bands are known to exhibit nontrivial topology in ordered magnets
under suitable conditions, engendering topological phases referred to as
magnonic topological insulators. Conventional methods to drive a magnonic
topological phase transition are bulk magnetic or thermal operations such as
changing the direction of an external magnetic field or varying the temperature
of the system, which are undesired in device applications of magnon topology.
In this work, we lift the limitation of the magnon topology control on the bulk
non-electronic manipulation by proposing a scheme to manipulate magnonic
topological phases by electronic boundary operations of spin chirality
injection. More specifically, we consider a ferromagnetic honeycomb lattice and
show that a finite spin chirality injected from the boundary of the system via
the spin Hall effects introduces a tunable sublattice-symmetry-breaking mass
term to the bosonic counterpart of the Haldane model for the Chern insulators
and thereby allows us to electronically manipulate the bulk topology of magnons
from the boundary. The ""shoulder"" in the thermal Hall conductivity profile is
proposed as an experimental probe of the chirality-induced topological phase
transition. The scheme for the boundary manipulation of the magnon topology is
shown to work for a honeycomb antiferromagnet as well. We envisage that the
interfacial chirality injection may offer a nonintrusive electronic means to
tune the static and the dynamical bulk properties of general magnetic systems.",2209.09435v1
2022-11-16,Detection sensitivity enhancement of magnon Kerr nonlinearity in cavity magnonics induced by coherent perfect absorption,"We show how to enhance the detection sensitivity of magnon Kerr nonlinearity
(MKN) in cavity magnonics. The considered cavity-magnon system consists of a
three-dimensional microwave cavity containing two yttrium iron garnet (YIG)
spheres, where the two magnon modes (one has the MKN, while the other is
linear) in YIG spheres are simultaneously coupled to microwave photons. To
obtain the effective gain of the cavity mode, we feed two input fields into the
cavity. By choosing appropriate parameters, the coherent perfect absorption of
the two input fields occurs, and the cavity-magnon system can be described by
an effective non-Hermitian Hamiltonian. Under the pseudo-Hermitian conditions,
the effective Hamiltonian can host the third-order exceptional point (EP3),
where the three eigenvalues of the Hamiltonian coalesce into one. When the
magnon frequency shift $\Delta_K$ induced by the MKN is much smaller than the
linewidths $\Gamma$ of the peaks in the transmission spectrum of the cavity
(i.e., $\Delta_K\ll \Gamma$), the magnon frequency shift can be amplified by
the EP3, which can be probed via the output spectrum of the cavity. The scheme
we present provides an alternative approach to measure the MKN in the region
$\Delta_K\ll \Gamma$ and has potential applications in designing low-power
nonlinear devices based on the MKN.",2211.08922v2
2022-12-08,Strong photon-magnon coupling using a lithographically defined organic ferrimagnet,"We demonstrate a hybrid quantum system composed of superconducting resonator
photons and magnons hosted by the organic-based ferrimagnet vanadium
tetracyanoethylene (V[TCNE]$_x$). Our work is motivated by the challenge of
scalably integrating an arbitrarily-shaped, low-damping magnetic system with
planar superconducting circuits, thus enabling a host of quantum magnonic
circuit designs that were previously inaccessible. For example, by leveraging
the inherent properties of magnons, one can enable nonreciprocal
magnon-mediated quantum devices that use magnon propagation rather than
electrical current. We take advantage of the properties of V[TCNE]$_x$, which
has ultra-low intrinsic damping, can be grown at low processing temperatures on
arbitrary substrates, and can be patterned via electron beam lithography. We
demonstrate the scalable, lithographically integrated fabrication of hybrid
quantum magnonic devices consisting of a thin-film superconducting resonator
coupled to a low-damping, thin-film V[TCNE]$_x$ microstructure. Our devices
operate in the strong coupling regime, with a cooperativity as high as 1181(44)
at T$\sim$0.4 K, suitable for scalable quantum circuit integration. This work
paves the way for the exploration of high-cooperativity hybrid magnonic quantum
devices in which magnonic circuits can be designed and fabricated as easily as
electrical wires.",2212.04423v1
2023-02-21,Chirality selective magnon-phonon hybridization and magnon-induced chiral phonons in a layered zigzag antiferromagnet,"Two-dimensional (2D) magnetic systems possess versatile magnetic order and
can host tunable magnons carrying spin angular momenta. Recent advances show
angular momentum can also be carried by lattice vibrations in the form of
chiral phonons. However, the interplay between magnons and chiral phonons as
well as the details of chiral phonon formation in a magnetic system are yet to
be explored. Here, we report the observation of magnon-induced chiral phonons
and chirality selective magnon-phonon hybridization in a layered zigzag
antiferromagnet (AFM) FePSe$_3$. With a combination of magneto-infrared and
magneto-Raman spectroscopy, we observe chiral magnon polarons (chiMP), the new
hybridized quasiparticles, at zero magnetic field. The hybridization gap
reaches 0.25~meV and survives down to the quadrilayer limit. Via first
principle calculations, we uncover a coherent coupling between AFM magnons and
chiral phonons with parallel angular momenta, which arises from the underlying
phonon and space group symmetries. This coupling lifts the chiral phonon
degeneracy and gives rise to an unusual Raman circular polarization of the
chiMP branches. The observation of coherent chiral spin-lattice excitations at
zero magnetic field paves the way for angular momentum-based hybrid phononic
and magnonic devices.",2302.10656v2
2023-09-06,Strong magnon-magnon coupling in an ultralow damping all-magnetic-insulator heterostructure,"Magnetic insulators such as yttrium iron garnets (YIGs) are of paramount
importance for spin-wave or magnonic devices as their ultralow damping enables
ultralow power dissipation that is free of Joule heating, exotic magnon quantum
state, and coherent coupling to other wave excitations. Magnetic insulator
heterostructures bestow superior structural and magnetic properties and house
immense design space thanks to the strong and engineerable exchange interaction
between individual layers. To fully unleash their potential, realizing low
damping and strong exchange coupling simultaneously is critical, which often
requires high quality interface. Here, we show that such a demand is realized
in an all-insulator thulium iron garnet (TmIG)/YIG bilayer system. The ultralow
dissipation rates in both YIG and TmIG, along with their significant spin-spin
interaction at the interface, enable strong and coherent magnon-magnon coupling
with a benchmarking cooperativity value larger than the conventional
ferromagnetic metal-based heterostructures. The coupling strength can be tuned
by varying the magnetic insulator layer thickness and magnon modes, which is
consistent with analytical calculations and micromagnetic simulations. Our
results demonstrate TmIG/YIG as a novel platform for investigating hybrid
magnonic phenomena and open opportunities in magnon devices comprising
all-insulator heterostructures.",2309.03116v1
2023-10-31,Dynamics of nonequilibrium magnons in gapped Heisenberg antiferromagnets,"Nonequilibrium dynamics in spin systems is a topic currently under intense
investigation as it provides fundamental insights into thermalization,
universality, and exotic transport phenomena. While most of the studies have
been focused on ideal closed quantum many-body systems such as ultracold atomic
quantum gases and one-dimensional spin chains, driven-dissipative Bose gases in
steady states away from equilibrium in classical systems also lead to
intriguing nonequilibrium physics. In this work, we theoretically investigate
out-of-equilibrium dynamics of magnons in a gapped Heisenberg quantum
antiferromagnet based on Boltzmann transport theory. We show that, by treating
scattering terms beyond the relaxation time approximation in the Boltzmann
transport equation, energy and particle number conservation mandate that
nonequilibrium magnons cannot relax to equilibrium, but decay to other
nonequilibrium stationary states, partially containing information about the
initial states. The only decay channel for these stationary states back to
equilibrium is through the non-conserving interactions such as boundary or
magnon-phonon scattering. At low temperatures, these non-conserving
interactions are much slower processes than intrinsic magnon-magnon interaction
in a gapped spin system. Using magnon-phonon interaction as a quintessential
type of non-conserving interaction, we then propose that nonequilibrium steady
states of magnons can be maintained and tailored using periodic driving at
frequencies faster than relaxation due to phonon interactions. These findings
reveal a class of classical material systems that are suitable platforms to
study nonequilibrium statistical physics and macroscopic phenomena such as
classical Bose-Einstein condensation of quasiparticles and magnon supercurrents
that are relevant for spintronic applications.",2310.20617v1
2023-12-25,Controllable magnon frequency comb in synthetic ferrimagnets,"Magnon frequency comb provides opportunities for exploring magnon nonlinear
effects and measuring the transmission magnon frequency in magnets, whose
controllability becomes vital for modulating the operating frequency and
improving the measurement accuracy. Nevertheless, such controllable frequency
comb remains to be explored. In this work, we investigate theoretically and
numerically the skyrmion-induced magnon frequency comb effect generated by
interaction between the magnon excitation mode and skyrmion breathing mode in
synthetic ferrimagnets. It is revealed that both the skyrmion breathing mode
and the magnon frequency gap closely depend on the net angular momentum
{\delta}s, emphasizing the pivotal role of {\delta}s as an effective control
parameter in governing the comb teeth. With the increase of {\delta}s, the
skyrmion size decreases, which results in the enlargement of the breathing
frequency and the distance between the comb teeth. Moreover, the dependences of
the magnon frequency gap on {\delta}s and the inter-layer coupling allow one to
modulate the comb lowest coherent frequency via structural control.
Consequently, the coherent modes generated by the comb may range from gigahertz
to terahertz frequencies, serving as a bridge between microwave and terahertz
waves. Thus, this work represents a substantial advance in understanding the
magnon frequency comb effect in ferrimagnets.",2312.15584v2
1999-04-22,Orbital Dynamics: The Origin of Anomalous Magnon Softening in Ferromagnetic Manganites,"We study the renormalization of magnons by charge and coupled orbital-lattice
fluctuations in colossal magnetoresistance compounds. The model considered is
an orbitally degenerate double-exchange system coupled to Jahn-Teller active
phonons. The modulation of ferromagnetic bonds by low-energy orbital
fluctuations is identified as the main origin of the unusual softening of the
zone-boundary magnons observed experimentally in manganites.",9904316v2
2006-03-14,Comment on ``Magnon wave forms in the presence of a soliton in two--dimensional antiferromagnets with a staggered field'',"Very recently Fonseca and Pires [Phys. Rev. B 73, 012403(2006)] have studied
the soliton--magnon scattering for the isotropic antiferromagnet and calculated
``exact'' phase shifts, which were compared with the ones obtained by the Born
approximation. In this Comment we correct both the soliton and magnon solutions
and point out the way how to study correctly the scattering problem.",0603395v1
1999-03-01,Mathematical Structure of Magnons in Quantum Ferromagnets,"We provide the mathematical structure and a simple, transparent and rigorous
derivation of the magnons as elementary quasi-particle excitations at low
temperatures and in the infinite spin limit for a large class of Heisenberg
ferromagnets. The magnon canonical variables are obtained as fluctuation
operators in the infinite spin limit. Their quantum character is governed by
the size of the magnetization.",9903004v1
2010-02-14,The Force Between Giant Magnons,"We compute the force and torque between well-separated, slowly-moving Giant
Magnons with arbitrary orientations on S^5. We propose an effective Hamiltonian
for Giant Magnons in this regime.",1002.2787v1
2012-07-24,Magnon mechanism of Josephson coupling in SFS structures,"It is shown that Josephson coupling in SFS junction due to electron-magnon
interaction remains at a distance, when the usual proximity effect decreases
exponentially. We obtain expression for the Josephson energy, which contain the
parameters of the magnon spectrum and allow to estimate the value of the
maximum superconducting current.",1207.5828v1
2015-06-04,Three magnons in an isotropic $S=1$ ferromagnetic chain as an exactly solvable non-integrable system,"It is shown that a generalization of Bethe Ansatz based on an utilization of
{\it degenerative discrete-diffractive} wave functions solves the three-magnon
problem for the $S=1$ isotropic ferromagnetic infinite chain. The four-magnon
problem is briefly discussed.",1506.01554v2
2018-04-16,Phase modulation and amplitude modulation interconversion for magnonic circuits,"Circuit elements within magnonic computers generally operate using signals
that are either amplitude or phase modulated (AM or PM). We propose a simple
all-magnon circuit element capable of converting between these two types of
encoding. We highlight the potential of our technique to augment existing
devices and propose a novel schematic for an 'equality' gate and XNOR gate.",1804.05575v1
2008-07-23,Issues on magnon reflection,"Two questions related to reflections of magnons in AdS/CFT are discussed:
namely the problem of explaining the (physical) poles of the reflection
amplitudes using Landau type diagrams and the generalization of the
Ghoshal-Zamolodchikov boundary state formalism to magnon reflections.",0807.3646v1
2021-10-02,Electrically Switchable van der Waals Magnon Valves,"Van der Waals magnets have emerged as a fertile ground for the exploration of
highly tunable spin physics and spin-related technology. Two-dimensional (2D)
magnons in van der Waals magnets are collective excitation of spins under
strong confinement. Although considerable progress has been made in
understanding 2D magnons, a crucial magnon device called the van der Waals
magnon valve, in which the magnon signal can be completely and repeatedly
turned on and off electrically, has yet to be realized. Here we demonstrate
such magnon valves based on van der Waals antiferromagnetic insulator MnPS3. By
applying DC electric current through the gate electrode, we show that the
second harmonic thermal magnon (SHM) signal can be tuned from positive to
negative. The guaranteed zero crossing during this tuning demonstrates a
complete blocking of SHM transmission, arising from the nonlinear gate
dependence of the non-equilibrium magnon density in the 2D spin channel. Using
the switchable magnon valves we demonstrate a magnon-based inverter. These
results illustrate the potential of van der Waals anti-ferromagnets for
studying highly tunable spin-wave physics and for application in magnon-base
circuitry in future information technology.",2110.00700v1
2022-10-15,Non-local magnon transconductance in extended magnetic insulating films.\\Part II: two-fluid behavior,"This review presents a comprehensive study of the spatial dispersion of
propagating magnons electrically emitted in extended yttrium-iron garnet (YIG)
films by the spin transfer effects across a YIG$\vert$Pt interface. Our goal is
to provide a generic framework to describe the magnon transconductance inside
magnetic films. We experimentally elucidate the relevant spectral contributions
by studying the lateral decay of the magnon signal. While most of the injected
magnons do not reach the collector, the propagating magnons can be split into
two-fluids: \textit{i)} a large fraction of high-energy magnons carrying energy
of about $k_B T_0$, where $T_0$ is the lattice temperature, with a
characteristic decay length in the sub-micrometer range, and \textit{ii)} a
small fraction of low-energy magnons, which are particles carrying energy of
about $\hbar \omega_K$, where $\omega_K/(2 \pi)$ is the Kittel frequency, with
a characteristic decay length in the micrometer range. Taking advantage of
their different physical properties, the low-energy magnons can become the
dominant fluid \textit{i)} at large spin transfer rates for the bias causing
the emission of magnons, \textit{ii)} at large distance from the emitter,
\textit{iii)} at small film thickness, or \textit{iv)} for reduced band
mismatch between the YIG below the emitter and the bulk due to variation of the
magnon concentration. This broader picture complements part I \cite{kohno_SD},
which focuses solely on the nonlinear transport properties of low-energy
magnons.",2210.08283v2
2023-04-21,Soft Magnons in Anisotropic Ferromagnets,"We discuss spin-wave transport in anisotropic ferromagnets with an emphasis
on the zeroes of the band edges as a function of a magnetic field. An
associated divergence of the magnon spin should be observable by enhanced
magnon conductivities in non-local experiments, especially in two-dimensional
ferromagnets.",2304.10709v1
1999-09-28,"Finite size spectrum, magnon interactions and magnetization of S=1 Heisenberg spin chains","We report our density matrix renormalization-group and analytical work on S=1
antiferromagnetic Heisenberg spin chains. We study the finite size behavior
within the framework of the non-linear sigma model. We study the effect of
magnon-magnon interactions on the finite size spectrum and on the magnetization
curve close to the critical magnetic field, determine the magnon scattering
length and compare it to the prediction from the non-linear $\sigma$ model.",9909388v3
1999-12-14,Relation between the superconducting gap energy and the two-magnon Raman peak energy in Bi2Sr2Ca{1-x}YxCu2O{8+δ},"The relation between the electronic excitation and the magnetic excitation
for the superconductivity in Bi2Sr2Ca{1-x}YxCu2O{8+\delta} was investigated by
wide-energy Raman spectroscopy. In the underdoping region the B1g scattering
intensity is depleted below the two-magnon peak energy due to the ""hot spots""
effects. The depleted region decreases according to the decrease of the
two-magnon peak energy, as the carrier concentration ncreases. This two-magnon
peak energy also determines the B1g superconducting gap energy as $2\Delta
\approx \alpha \hbar \omega_{\rm Two-Magnon} \approx J_{\rm effective}$
$(\alpha=0.34-0.41)$ from under to overdoping hole concentration.",9912232v1
2001-12-20,Zone Edge Softening and Relaxation in the Double Exchange Model,"The $J\to \infty$ double exchange model is formulated in terms of three
auxiliary particles. A slow true bosonic magnon propagates by admixture with a
fast fermionic pseudo-magnon. This process involves the absorption of a
conduction electron which, for this half-metal, carries only charge degrees of
freedom. The magnon dispersion becomes much weaker and the relaxation rate
increases rapidly upon approaching the zone boundary. That the magnons relax
for all wave vector values implies the existence of a low energy spin
continuum.",0112399v1
2003-01-10,Magnon Heat Transport in doped $\rm La_2CuO_4$,"We present results of the thermal conductivity of $\rm La_2CuO_4$ and $\rm
La_{1.8}Eu_{0.2}CuO_4$ single-crystals which represent model systems for the
two-dimensional spin-1/2 Heisenberg antiferromagnet on a square lattice. We
find large anisotropies of the thermal conductivity, which are explained in
terms of two-dimensional heat conduction by magnons within the CuO$_2$ planes.
Non-magnetic Zn substituted for Cu gradually suppresses this magnon thermal
conductivity $\kappa_{\mathrm{mag}}$. A semiclassical analysis of
$\kappa_{\mathrm{mag}}$ is shown to yield a magnon mean free path which scales
linearly with the reciprocal concentration of Zn-ions.",0301164v2
2005-11-15,Magnon-mediated Binding between Holes in an Antiferromagnet,"The long-range forces between holes in an antiferromagnet are due to magnon
exchange. The one-magnon exchange potential between two holes is proportional
to cos(2\phi)/r^2 where r is the distance vector of the holes and \phi is the
angle between r and an axis of the square crystal lattice. One-magnon exchange
leads to bound states of holes with antiparallel spins resembling d-wave
symmetry. The role of these bound states as potential candidates for the
preformed Cooper pairs of high-temperature superconductivity is discussed
qualitatively.",0511367v2
2006-06-30,A variational coupled-cluster study of magnon-density-wave excitations in quantum antiferromagnets,"We extend recently proposed variational coupled-cluster method to describe
excitation states of quantum antiferromagnetic bipartite lattices. We reproduce
the spin-wave excitations (i.e., magnons with spin $\pm 1$). In addition, we
obtain a new, spin-zero excitation (magnon-density waves) which has been
missing in all existing spin-wave theories. Within our approximation, this
magnon-density-wave excitation has a nonzero energy gap in a cubic lattice and
is gapless in a square lattice, similar to those charge-density-wave
excitations (plasmons) in quantum electron gases.",0606813v2
2006-10-03,Three-Spin Giant Magnons in AdS_5xS^5,"From the Polyakov string action using a conformal gauge we construct a
three-spin giant magnon solution describing a long open string in AdS_5 \times
S^5 which rotates both in two angular directions of S^5 and in one angular
direction of AdS_5. Through the Virasoro constraints the string motion in AdS_5
takes an effect from the string configuration in S^5. The dispersion relation
of the soliton solution is obtained as a superposition of two bound states of
magnons. We show that there is a correspondence between a special giant magnon
in AdS_2 and the sinh-Gordon soliton.",0610037v1
2006-11-02,Dressing the Giant Magnon II,"We extend our earlier work by demonstrating how to construct classical string
solutions describing arbitrary superpositions of scattering and bound states of
dyonic giant magnons on S^5 using the dressing method for the SU(4)/Sp(2) coset
model. We present a particular scattering solution which generalizes solutions
found in hep-th/0607009 and hep-th/0607044 to the case of arbitrary magnon
momenta. We compute the classical time delay for the scattering of two dyonic
magnons carrying angular momenta with arbitrary relative orientation on the
S^5.",0611033v1
2007-03-27,Giant Magnon in NS5-brane Background,"We study the giant magnon solutions in the near horizon geometry of the
Neveu-Schwarz (NS) 5-brane background. In conformal gauge, we find magnon
dispersion relation in the large angular momentum (J) limit. We further show
that the giant magnon poses uniform distribution of the angular momentum along
the string world-sheet as in case of AdS_5\times S^5 spacetime.",0703244v2
1995-01-19,Phonon Assisted Multimagnon Optical Absorption and Long Lived Two-Magnon States in Undoped Lamellar Copper Oxides,"We calculate the effective charge for multimagnon infrared (IR) absorption
assisted by phonons in the parent insulating compounds of cuprate
superconductors and the spectra for two-magnon absorption using interacting
spin-wave theory. Recent measured bands in the mid IR [Perkins et al. Phys.
Rev. Lett. {\bf 71} 1621 (1993)] are interpreted as involving one phonon plus a
two-magnon virtual bound state, and one phonon plus higher multimagnon
absorption processes. The virtual bound state consists of a narrow resonance
occurring when the magnon pair has total momentum close to $(\pi,0)$.",9501001v1
2007-06-10,Magnon mode truncation in a rung-dimerized asymmetric spin ladder,"An exactly solvable effective model is suggested for an asymmetric spin
ladder with dimerized rungs. Magnon mode truncation originated from magnon
decay (recently observed in the 1D compound ${\rm IPA-CuCl}_3$) is naturally
described within this model. Using Bethe Ansatze we described a one-magnon
sector and obtained relations between interaction constants of the model and
experimentally observable quantities such as the gap and truncation energies,
spin velocity and the truncation wave vector. It is also shown that structure
factor turns to zero at the truncation point.",0706.1370v2
2008-02-25,Quantum Giant Magnons,"The giant magnons are classical solitons of the O(N) sigma-model, which play
an important role in the AdS/CFT correspondence. We study quantum giant magnons
first at large N and then exactly using Bethe Ansatz, where giant magnons can
be interpreted as holes in the Fermi sea. We also identify a solvable limit of
Bethe Ansatz in which it describes a weakly-interacting Bose gas at zero
temperature. The examples include the O(N) model at large N, weakly interacting
non-linear Schrodinger model, and nearly isotropic XXZ spin chain in the
magnetic field.",0802.3681v2
2008-11-25,Theory of electric field induced one-magnon resonance in cycloidal spin magnets,"We propose a new mechanism to induce a novel one-magnon excitation by the
electric component of light in cycloidal spin states, i.e. so called
electromagnon process. We calculated optical spectra in the cycloidal spin
structures as observed in multiferroic perovskite manganites RMnO3 where novel
magnetic excitations induced by oscillating electric fields are observed. When
symmetric spin-dependent electric polarizations are introduced, we have light
absorptions at terahertz frequencies with one- and two-magnon excitations
driven by the electric component of light. Our results show that some parts of
optical spectra observed experimentally at terahertz frequencies are one-magnon
excitation absorptions.",0811.4082v1
2009-01-20,Role of Bound Magnon in Magnetic Domain Wall Motion,"We report on a quantum description of the domain wall (DW) motion under a
spin current. A bound magnon, which is the zero mode of DW, is found to play a
dominant role in DW dynamics. The bound magnon acquires its inertia by the hard
axis anisotropy and is a free particle even under the spin current. The full
transfer of spin angular momentum from the spin current to DW via the bound
magnon leads to the DW motion with the adiabatic velocity, decoupling of spin
waves from DW, and no Doppler shift in spin waves.",0901.3066v1
2011-06-22,All-magnonic spin-transfer torque and domain wall propagation,"The spin wave transportation through a transverse magnetic domain wall (DW)
in a magnetic nanowire is studied. It is found that spin wave passes through a
DW without reflection. A magnon, the quantum of the spin wave, carries opposite
spins on the two sides of the DW. As a result, there is a spin angular momentum
transfer from the propagating magnons to the DW. This magnonic spin-transfer
torque can efficiently drive a DW to propagate in the opposite direction to
that of the spin wave.",1106.4382v2
2013-02-20,"Lifetimes of antiferromagnetic magnons in two and three dimensions: experiment, theory, and numerics","A high-resolution neutron spectroscopic technique is used to measure
momentum-resolved magnon lifetimes in the prototypical two- and
three-dimensional antiferromagnets Rb2MnF4 and MnF2, over the full Brillouin
zone and a wide range of temperatures. We rederived theories of the lifetime
resulting from magnon-magnon scattering, thereby broadening their applicability
beyond asymptotically small regions of wavevector and temperature.
Corresponding computations, combined with a small contribution reflecting
collisions with domain boundaries, yield excellent quantitative agreement with
the data.",1302.4867v1
2013-02-26,Magnon-mediated thermal transport in antiferromagnets: the link to momentum-resolved magnon lifetime data,"Transport currents in solids decay through collisions of quasiparticles with
each other and with defects or boundaries. Since information about collisional
lifetimes is difficult to obtain, most calculations of transport properties
rely on parameters that are not known independently. Here, we use magnon
lifetime data for the two-dimensional antiferromagnet Rb2MnF4 to calculate the
magnon-mediated thermal conductivity without any adjustable parameters, thereby
quantifying the influence of scattering from domain boundaries on transport.
Related strategies have the potential to enhance our understanding of thermal
transport by electronic and phononic quasiparticles greatly.",1302.6476v1
2015-02-05,Large-Spin Expansions of Giant Magnons,"This is a talk delivered at the Workshop on Quantum Fields and Strings of the
2014 Corfu Summer Institute. We discuss how giant magnons emerge in the context
of the AdS5/CFT4 correspondence as the gravity duals of N = 4 super Yang-Mills
magnon excitations. Then we present a new analytic expression for the
dispersion relation of classical finite-size giant magnons with Lambert's
W-function.",1502.01630v2
2015-03-13,Upper bound of one-magnon excitation and lower bound of effective mass for ferromagnetic spinor Bose and Fermi gases,"Using a variational method, we derive an exact upper bound for one-magnon
excitation energy in ferromagnetic spinor gases, which limits the quantum
corrections to the effective mass of a magnon to be positive. We also derive an
upper bound for one-magnon excitation energy in lattice systems. The results
hold for both Bose and Fermi systems in $d$ dimensions as long as the
interaction is local and invariant under spin rotation.",1503.03951v3
2015-09-01,Magnon instability driven by heat current in magnetic bilayers,"We theoretically demonstrate that, in a ferromagnet/paramagnet bilayer, a
magnon instability accompanied by a gigahertz microwave emission can be driven
simply by means of a temperature bias. Employing many-body theory for
investigating the effects of a phonon heat current on the magnon lifetime, we
show that the magnon instability occurs upon the suppression of the Umklapp
scattering at low temperatures, leading to microwave emission. The present
finding provides crucial information about the interplay of spin current and
heat current.",1509.00132v2
2015-09-19,Spin torque and Nernst effects in Dzyaloshinskii-Moriya ferromagnets,"We predict that a temperature gradient can induce a magnon-mediated intrinsic
torque and a transverse spin current in systems with non-trivial magnon Berry
curvature. With the help of a microscopic linear response theory of
nonequilibrium magnon-mediated torques and spin currents we identify the
interband and intraband components that manifest in ferromagnets with
Dzyaloshinskii-Moriya interactions and magnetic textures. To illustrate and
assess the importance of such effects, we apply our theory to the
magnon-mediated spin Nernst and torque responses in a kagome lattice
ferromagnet.",1509.05847v3
2016-06-09,Magnon spin Nernst effect in antiferromagnets,"We predict that a temperature gradient can induce a magnon-mediated spin Hall
response in an antiferromagnet with non-trivial magnon Berry curvature. We
develop a linear response theory which gives a general condition for a Hall
current to be well defined, even when the thermal Hall response is forbidden by
symmetry. We apply our theory to a honeycomb lattice antiferromagnet and
discuss a role of magnon edge states in a finite geometry.",1606.03088v3
2016-06-29,Damped Topological Magnons in the Kagomé-Lattice Ferromagnets,"We demonstrate that interactions can substantially undermine the
free-particle description of magnons in ferromagnets on geometrically
frustrated lattices. The anharmonic coupling, facilitated by the
Dzyaloshinskii-Moriya interaction, and a highly-degenerate two-magnon continuum
yield a strong, non-perturbative damping of the high-energy magnon modes. We
provide a detailed account of the effect for the $S=1/2$ ferromagnet on the
kagom\'e lattice and propose further experiments.",1606.09249v3
2017-02-02,Magnon Condensation and Spin Superfluidity,"We consider the phenomenon of Bose-Einstein condensation of quasi-equilibrium
magnons which leads to a spin superfluidity, the coherent quantum transfer of
magnetization in magnetic materials. These phenomena are beyond the classical
Landau-Lifshitz-Gilbert paradigm. The critical conditions for excited magnon
density for ferro- and antiferromagnets, bulk and thin films are estimated and
discussed. The BEC should occur in the antiferromagnetic hematite at much lower
excited magnon density compared to the ferromagnetic YIG.",1702.00846v2
2013-08-29,Exchange parameters and adiabatic magnon energies from spin-spiral calculations,"We present a method of extracting the exchange parameters of the classical
Heisenberg model from first-principles calculations of spin-spiral total
energies based on density functional theory. The exchange parameters of the
transition-metal monoxides MnO and NiO are calculated and used to estimate
magnetic properties such as transition temperatures and magnon energies.
Furthermore we show how to relate the magnon energies directly to differences
in spin-spiral total energies for systems containing an arbitrary number of
magnetic sublattices. This provides a comparison between magnon energies using
a finite number of exchange parameters and the infinite limit.",1308.6466v1
2014-05-02,Dynamic phase diagram of dc-pumped magnon condensates,"We study the effects of nonlinear dynamics and damping by phonons on a system
of interacting electronically pumped magnons in a ferromagnet. The nonlinear
effects are crucial for constructing the dynamic phase diagram, which describes
how ""swasing"" and Bose-Einstein condensation emerge out of the
quasiequilibrated thermal cloud of magnons. We analyze the system in the
presence of magnon damping and interactions, demonstrating the continuous onset
of stable condensates as well as hysteretic transitions.",1405.0522v1
2017-12-21,Topological magnons with nodal-line and triple-point degeneracies: Implications for thermal Hall effect in pyrochlore iridates,"We analyze the magnon excitations in pyrochlore iridates with all-in-all-out
(AIAO) antiferromagnetic order, focusing on their topological features. We
identify the magnetic point group symmetries that protect the nodal-line band
crossings and triple-point degeneracies that dominate the Berry curvature. We
find three distinct regimes of magnon band topology, as a function of the ratio
of Dzyaloshinskii-Moriya (DM) interaction to the antiferromagnetic exchange. We
show how the thermal Hall response provides a unique probe of the topological
magnon band structure in AIAO systems.",1712.08170v2
2017-12-27,Finite temperature magnon spectra in yttrium iron garnet from mean field approach in tight-binding model,"We study magnon spectra at finite temperature in yttrium iron garnet from
tight-binding model with nearest neighboring exchange interaction. The spin
reduction due to thermal magnon excitation are taken into account via the mean
field approximation to the local spin and found to be different at two sets of
iron atoms. The resulting temperature dependence of the spin wave gap shows
good agreement with experiment. We find only two magnon modes are relevant to
ferromagnetic resonance.",1712.09512v1
2019-09-03,Magnon Accumulation in Chirally Coupled Magnets,"We report strong chiral coupling between magnons and photons in microwave
waveguides that contain chains of small magnets on special lines. Large magnon
accumulations at one edge of the chain emerge when exciting the magnets by a
phased antenna array. This mechanism holds the promise of new functionalities
in non-linear and quantum magnonics.",1909.00953v1
2020-12-03,Dirac surface states in magnonic analogs of topological crystalline insulators,"We propose magnonic analogs of topological crystalline insulators which
possess Dirac surface states protected by the combined symmetry of
time-reversal and half translation. Constructing models of the topological
magnon systems, we demonstrate that the energy current flows through the
systems in response to an electric field, owing to the Dirac surface states
with the spin-momentum locking. We also propose a realization of the magnonic
analogs of topological crystalline insulators in a magnetic compound CrI$_{3}$
with a monoclinic structure.",2012.02034v2
2000-06-01,High-energy magnon dispersion and multi-magnon continuum in the two-dimensional Heisenberg antiferromagnet,"We use quantum Monte Carlo simulations and numerical analytic continuation to
study high-energy spin excitations in the two-dimensional S=1/2 Heisenberg
antiferromagnet at low temperature. We present results for both the transverse
and longitudinal dynamic spin structure factor S(q,w) at q=(pi,0) and
(pi/2,pi/2). Linear spin-wave theory predicts no dispersion on the line
connecting these momenta. Our calculations show that in fact the magnon energy
at (pi,0) is 10% lower than at (pi/2,pi/2). We also discuss the transverse and
longitudinal multi-magnon continua and their relevance to neutron scattering
experiments.",0006021v1
2017-05-29,Chemical potential of quasi-equilibrium magnon gas driven by pure spin current,"We show experimentally that the spin current generated by the spin Hall
effect drives the magnon gas in a ferromagnet into a quasi-equilibrium state
that can be described by the Bose-Einstein statistics. The magnon population
function is characterized either by an increased effective chemical potential
or by a reduced effective temperature, depending on the spin current
polarization. In the former case, the chemical potential can closely approach,
at large driving currents, the lowest-energy magnon state, indicating the
possibility of spin current-driven Bose-Einstein condensation.",1705.10047v1
2016-10-07,Magnonic black holes,"We show that the interaction between spin-polarized current and magnetization
dynamics can be used to implement black-hole and white-hole horizons for
magnons - the quanta of oscillations in the magnetization direction in magnets.
We consider three different systems: easy-plane ferromagnetic metals, isotropic
antiferromagnetic metals, and easy-plane magnetic insulators. Based on
available experimental data, we estimate that the Hawking temperature can be as
large as 1 K. We comment on the implications of magnonic horizons for spin-wave
scattering and transport experiments, and for magnon entanglement.",1610.02313v1
2020-06-22,Magnon Landau Levels and Spin Responses in Antiferromagnets,"We study gauge fields produced by gradients of the Dzyaloshinskii-Moriya
interaction and propose a model of AFM topological insulator of magnons. In the
long wavelength limit, the Landau levels induced by the inhomogeneous
Dzyaloshinskii-Moriya interaction exhibit relativistic physics described by the
Klein-Gordon equation. The spin Nernst response due to the formation of
magnonic Landau levels is compared to similar topological responses in skyrmion
and vortex-antivortex crystal phases of AFM insulators. Our studies show that
AFM insulators exhibit rich physics associated with topological magnon
excitations.",2006.12423v2
2020-02-02,Topological Valley Transport of Gapped Dirac Magnons in Bilayer Ferromagnetic Insulators,"Bilayer Heisenberg ferromagnetic insulators hold degenerate terahertz Dirac
magnon modes associated with two opposite valleys of the hexagonal Brillouin
zone. We show that this energy degeneracy can be removed by breaking of the
inversion symmetry (I), leading to a topological magnon valley current. We show
furthermore that this current leads to valley Seebeck effect for magnons and is
thereby detectable. We perform calculations in the specific example of bilayer
CrBr3, where I can be broken by electrostatic doping.",2002.00446v2
2020-04-03,Twist-induced magnon Landau levels in honeycomb magnets,"Lattice deformation resulting from elastic strain is known to spatially
modulate the wave function overlap of the atoms on the lattice and can
drastically alter the properties of the quasiparticles. Here we elaborate that
a twist lattice deformation in two-dimensional honeycomb quantum magnet
nanoribbons is equivalent to an elastic gauge field giving rise to magnon
Landau quantization. When the ground state is ferromagnetic, dispersive
Dirac-Landau levels are induced in the center of magnon bands, while for
antiferromagnetic nanoribbons, the twist results in dispersive equidistant
Landau levels at the top of magnon bands. The dispersions for both types of
Landau levels are derived in the framework of the band theory.",2004.01371v1
2020-05-28,Nonreciprocal Magnon by Symmetric Anisotropic Exchange Interaction in Honeycomb Antiferromagnet,"We investigate a microscopic origin of nonreciprocal magnon that is distinct
from the Dzyaloshinskii-Moriya interaction in a honeycomb antiferromagnet. The
key ingredient is a symmetric anisotropic exchange interaction depending on the
bond direction, which results in a valley-type nonreciprocal magnon excitations
under the staggered antiferromagnetic ordering. Furthermore, we find this type
of nonreciprocal magnon exhibits a peculiar magnetic-field response; the
nonreciprocal direction can be manipulated by the in-plane rotating magnetic
field. The obtained results can be accounted for the emergence of the magnetic
toroidal multipoles.",2005.13729v1
2020-07-22,Nanomagnonic cavities for strong spin-magnon coupling,"We present a theoretical approach to use ferro- or ferrimagnetic
nanoparticles as microwave nanomagnonic cavities to concentrate microwave
magnetic fields into deeply subwavelength volumes $\sim 10^{-13}$ mm$^3$. We
show that the field in such nanocavities can efficiently couple to isolated
spin emitters (spin qubits) positioned close to the nanoparticle surface
reaching the single magnon-spin strong-coupling regime and mediate efficient
long-range quantum state transfer between isolated spin emitters. Nanomagnonic
cavities thus pave the way towards magnon-based quantum networks and
magnon-mediated quantum gates.",2007.11595v1
2020-08-16,Magnon dispersion in bilayers of two-dimensional ferromagnets,"We determine magnon spectra of an atomic bilayer magnet with ferromagnetic
intra- and both ferro- and anti- ferromagnetic interlayer coupling. Analytic
expressions for the full magnon band of the latter case reveal that both
exchange interactions govern the fundamental magnon gap. The inter and
intralayer magnetic ordering are not independent: the intralayer ferromagnetism
stabilizes antiferromagnetic inter-layer order. The topology of these
exchange-anisotropy spin models without spin-orbit interaction turns out to be
trivial.",2008.06875v2
2021-06-24,Magnonics vs. Ferronics,"Magnons are the elementary excitations of the magnetic order that carry spin,
momentum, and energy. Here we compare the magnon with the ferron, i.e. the
elementary excitation of the electric dipolar order that transports
polarization and heat in ferroelectrics.",2106.12775v1
2021-09-10,Numerical Simulation of the Coupling between Split-Ring Resonators and Antiferromagnetic Magnons,"We report on the results of simulations of the terahertz response of a split
ring resonator (SRR) metamaterial coupled to a hypothetical antiferromagnetic
material characterized by a magnon resonance. The simulations were done using
finite difference time domain (FDTD) techniques. By adjusting the magnon
frequency we find a hybridization of the resonant normal modes of the SRR and
the magnon manifested as an avoided crossing. By varying the physical
separation between the metamaterial and the antiferromagnet with a dielectric
spacer, we evaluated the coupling strength between the two.",2109.05086v1
2021-12-13,Magnon frequency renormalization by the electronic geometrical spin torque in itinerant magnets,"We investigate non-adiabatic effects on the magnon frequency in an
interacting system of localized spins and itinerant electrons. Including the
lowest order corrections to the adiabatic dynamics in an analytically solvable
model, applicable to simple ferromagnets like Fe, Co and Ni, we find that the
magnon frequency is renormalized by a geometrical torque arising from the
electronic spin Berry curvature. Comparison to exact numerical simulations
reveals that our analytical solution captures essential low-energy features,
and provides a mechanism for the magnon frequency hardening observed in recent
first principles calculations for Fe, provided the geometrical torque is taken
into account.",2112.06547v1
2023-01-06,Stacking-dependent topological magnons in bilayer CrI$_3$,"Motivated by the potential of atomically-thin magnets towards tunable
high-frequency magnonics, we detail the spin-wave dispersion of bilayer
CrI$_3$. We demonstrate that the magnonic behavior of the bilayer strongly
depends on its stacking configuration and the interlayer magnetic ordering,
where a topological bandgap opens in the dispersion caused by the
Dzyaloshinskii-Moriya and Kitaev interactions, classifying bilayer CrI$_3$ as a
topological magnon insulator. We further reveal that both size and topology of
the bandgap in a CrI$_3$ bilayer with an antiferromagnetic interlayer ordering
are tunable by an external magnetic field.",2301.02502v2
2023-01-19,Interfacial Magnon-Mediated Superconductivity in Twisted Bilayer Graphene,"The interfacial coupling between electrons and magnons in adjacent layers can
mediate an attractive electron-electron interaction and induce
superconductivity. We consider magic-angle twisted bilayer graphene sandwiched
between two ferromagnetic insulators to optimize this effect. As a result,
magnons induce an interlayer superconducting state characterized by $p$-wave
symmetry. We investigate two candidate ferromagnets. The van der Waals
ferromagnet CrI$_3$ stands out because it allows compression to tune the
superconducting state with an exponential sensitivity. This control adds a new
dimension to the tunability of twisted bilayer graphene. Our results open a new
path for exploring magnon-induced superconductivity.",2301.07909v1
2023-06-14,Chirality Enables Thermal Magnon Transistors,"We report a theory of thermal spin pumping into proximity magnets under a
transverse-bias-driven heat flow of magnons in magnetic films when the dipolar
coupling to the magnetic gate is tuned to be ""chiral"". While there is no
rectification of the magnon current in the film, we predict that chirality
diverts a large percentage (50$\%$ for perfect chirality) of it into the gate.
This transverse thermal spin pumping effect can be controlled by rotating the
film magnetization and may help manage the heat flow in future magnonic
circuits.",2306.08371v3
2023-11-17,Magnon topological transition in skyrmion crystal,"We study the magnon spectrum in skyrmion crystal formed in thin ferromagnetic
films with Dzyalosinskii-Moria interaction in presence of magnetic field.
Focusing on two low-lying observable magnon modes and employing stereographic
projection method, we develop a theory demonstrating a topological transition
in the spectrum. Upon the increase of magnetic field, the gap between two
magnon bands closes, with the ensuing change in the topological character of
both bands. This phenomenon of gap closing, if confirmed in magnetic resonance
experiments, may deserve further investigation by thermal Hall conductivity
experiments.",2311.10622v1
2023-12-05,Interplay between magnetism and superconductivity in a hybrid magnon-photon bilayer system,"Spin waves in magnetic films are affected by the vicinity to a
superconductor. Here we studied a bilayer stack made of an insulating Yttrium
Iron Garnet (YIG) film and a high-$T_c$ YBCO superconducting planar resonator.
We investigated the hybridization of magnon and photon modes reporting the
temperature evolution of microwave transmission spectra. Data analysis, based
on the description of magnon modes and on the Hopfield model, shows that the
magnon-photon coupling strength and the mode frequency shift can be ultimately
related to the temperature dependence of the penetration depth of YBCO.",2312.02785v2
2024-01-22,Damping-Enhanced Magnon Transmission,"The inevitable Gilbert damping in magnetization dynamics is usually regarded
as detrimental to spin transport. Here we demonstrate in a
ferromagnetic-insulator--normal-metal heterostructure that the strong momentum
dependence and chirality of the eddy-current-induced damping causes also
beneficial scattering properties. Here we show that a potential barrier that
reflects magnon wave packets becomes transparent in the presence of a metallic
cap layer, but only in one direction. We formulate the unidirectional
transmission in terms of a generalized group velocity with an imaginary
component and the magnon skin effect. This trick to turn presumably harmful
dissipation into useful functionalities should be useful for future quantum
magnonic devices.",2401.12022v1
2021-11-28,Quantum magnonics: when magnon spintronics meets quantum information science,"Spintronics and quantum information science are two promising candidates for
innovating information processing technologies. The combination of these two
fields enables us to build solid-state platforms for studying quantum phenomena
and for realizing multi-functional quantum tasks. For a long time, however, the
intersection of these two fields was limited. This situation has changed
significantly over the last few years because of the remarkable progress in
coding and processing information using magnons. On the other hand, significant
advances in understanding the entanglement of quasi-particles and in designing
high-quality qubits and photonic cavities for quantum information processing
provide physical platforms to integrate magnons with quantum systems. From
these endeavours, the highly interdisciplinary field of quantum magnonics
emerges, which combines spintronics, quantum optics and quantum information
science.Here, we give an overview of the recent developments concerning the
quantum states of magnons and their hybridization with mature quantum
platforms. First, we review the basic concepts of magnons and quantum
entanglement and discuss the generation and manipulation of quantum states of
magnons, such as single-magnon states, squeezed states and quantum many-body
states including Bose-Einstein condensation and the resulting spin
superfluidity. We discuss how magnonic systems can be integrated and entangled
with quantum platforms including cavity photons, superconducting qubits,
nitrogen-vacancy centers, and phonons for coherent information transfer and
collaborative information processing. The implications of these hybrid quantum
systems for non-Hermitian physics and parity-time symmetry are highlighted,
together with applications in quantum memories and high-precision measurements.
Finally, we present an outlook on the opportunities in quantum magnonics.",2111.14241v2
2007-01-17,Two-magnon excitations in resonant inelastic x-ray scattering from quantum Heisenberg antiferromagnets,"We study two-magnon spectra in resonant inelastic x-ray scattering (RIXS)
from the Heisenberg antiferromanets by extending the formula of Nomura
andIgarashi (Phys. Rev. B 71, 035110 (2005)). The core-hole potential in the
intermediate state of RIXS gives rise to a change in the exchange coupling
between 3d electrons, leading to an effective interaction between the core hole
and spins of 3d electrons. We derive a formula suitable to calculate the
two-magnon RIXS intensities, replacing the bare core-hole potential responsible
to charge excitations by this effective interaction creating two magnons in our
previous formula. It consists of two factors, one of which determines the
incident-photon-energy dependence and another is the two-magnon correlation
function. We evaluate the former factor for La_{2}CuO_{4} in terms of the
density of states of the 4p states obtained by the band calculation. We also
calculate the two-magnon correlation function as a function of energy loss
\omega and momentum transfer \textbf{q} of the Heisenberg model on a square
lattice,by summing up the ladder diagrams after transforming the magnon-magnon
interaction into a separable form. The calculated spectra form a broad peak
around \omega=3J for S=1/2 on the magnetic Brillouin zone boundary and vanish
at \textbf{q}=(0,0) and (\pi,\pi). Such momentum dependence of the RIXS spectra
could provide an excellent opportunity to study the dynamics in the Heisenberg
model.",0701399v1
2015-10-18,Magnetization dynamics in Fe$_x$Co$_{1-x}$ in presence of chemical disorder,"In this paper, we present a theoretical formulation of magnetization dynamics
in disordered binary alloys based on Kubo linear response theory interfaced
with the combination of seamlessly three approaches; density functional based
tight-binding linear muffin-tin orbitals, generalized recursion and Augmented
space formalism. We apply this method to study the magnetization dynamics in
chemically disordered Fe$_x$Co$_{1-x}$ ($x$ = 0.2, 0.5, 0.8) alloys. We
reported that the magnon energies decrease with an increase in Co
concentration. Significant magnon softening has been observed in
Fe$_{20}$Co$_{80}$ at the Brillouin zone boundary. The magnon-electron
scattering increases with increasing Co content which in turn modifies the
hybridization between the Fe and Co atoms. This reduces the exchange energy
between the atoms and soften down the magnon energy. The lowest magnon lifetime
in found in Fe$_{50}$Co$_{50}$, where disorder is maximum. This clearly
indicates that the damping of magnon energies in Fe$_x$Co$_{1-x}$ is governed
by the hybridization between Fe and Co whereas the magnon lifetime is
controlled by disorder configuration. Our atomistic spin dynamics simulations
show a reasonable agreement with our theoretical approach in magnon dispersion
for different alloy compositions.",1510.05200v4
2016-01-08,Magnons and Phonons Optically Driven Out of Local Equilibrium in a Magnetic Insulator,"Magnons are the energy quanta of fundamental spin excitations, namely spin
waves, and they can make a considerable contribution to energy transport in
some magnetic materials in a similar manner as lattice vibration waves or
phonons. The coupling and possible non-equilibrium between magnons and other
energy carriers have been used to explain several recently discovered thermally
driven spin transport and energy conversion phenomena. Here, we report
experiments in which local non-equilibrium between magnons and phonons in a
single crystalline bulk magnetic insulator, Y3Fe5O12 (yttrium iron garnet, or
YIG), has been created optically within a focused laser spot and probed
directly with the use of micro-Brillouin light scattering (BLS). By analyzing
the experimental results with a thermally induced magnon diffusion model, we
obtain the magnon diffusion length of thermal magnons. By explicitly
establishing non-equilibrium between magnons and phonons, our studies represent
an important step toward a quantitative understanding of various spin-heat
coupling phenomena.",1601.01982v3
2017-01-09,Spin-Wave Modes in Transition from a Thin Film to a Full Magnonic Crystal,"Spin-wave modes are studied under the gradual transition from a flat thin
film to a 'full' (one-dimensional) magnonic crystal. For this purpose, the
surface of a pre-patterned 36.8 nm thin permalloy film was sequentially ion
milled resulting in magnonic hybrid structures, referred to as
surface-modulated magnonic crystals, with increasing modulation depth. After
each etching step, ferromagnetic resonance measurements were performed yielding
the spin-wave resonance modes in backward-volume and Damon-Eshbach geometry.
The spin-wave spectra of these hybrid systems reveal an even larger variety of
spin-wave states compared to the 'full' magnonic crystal. The measurements are
corroborated by quasi-analytical theory and micromagnetic simulations in order
to study the changing spin-wave mode character employing spin-wave mode
profiles. In backward-volume geometry, a gradual transition from the uniform
mode in the film limit to a fundamental mode in the thin part of the magnonic
crystal was observed. Equivalently, the first and the second film modes are
transform into a center and an edge mode of the thick part of the magnonic
crystal. Simple transition rules from the $n^{\mathrm{th}}$ film mode to the
$m^{\mathrm{th}}$ mode in the 'full' magnonic crystal are formulated unraveling
the complex mode structure particularly in the backward-volume geometry. An
analogous analysis was performed in the Damon-Eshbach geometry.",1702.05675v3
2017-01-30,Effects of Grain Boundaries and Defects on Anisotropic Magnon Transport in Textured Sr14Cu24O41,"The strong spin-spin exchange interaction in some low-dimensional magnetic
materials can give rise to a high group velocity and thermal conductivity
contribution from magnons. One example is the incommensurate layered compounds
(Sr,Ca,La)14Cu24O41. The effects of grain boundaries and defects on
quasi-one-dimensional magnon transport in these compounds are not well
understood. Here we report the microstructures and anisotropic thermal
transport properties of textured Sr14Cu24O41, which are prepared by solid-state
reaction followed by spark plasma sintering. Transmission electron microscopy
clearly reveals nano-layered grains and the presence of dislocations and planar
defects. The thermal conductivity contribution and mean free paths of magnons
in the textured samples are evaluated with the use of a kinetic model for
one-dimensional magnon transport, and found to be suppressed significantly as
compared to single crystals at low temperatures. The experimental results can
be explained by a one-dimensional magnon-defect scattering model, provided that
the magnon-grain boundary scattering mean free path in the anisotropic magnetic
structure is smaller than the average length of these nano-layers along the c
axis. The finding suggests low transmission coefficients for magnons across
grain boundaries.",1701.08519v1
2016-05-07,Weak coupling of pseudoacoustic phonons and magnon dynamics in incommensurate spin ladder compound Sr14Cu24O41,"Intriguing lattice dynamics has been predicted for aperiodic crystals that
contain incommensurate substructures. Here we report inelastic neutron
scattering measurements of phonon and magnon dispersions in Sr14Cu24O41, which
contains incommensurate one-dimensional (1D) chain and two-dimensional (2D)
ladder substructures. Two distinct pseudoacoustic phonon modes, corresponding
to the sliding motion of one sublattice against the other, are observed for
atomic motions polarized along the incommensurate axis. In the long wavelength
limit, it is found that the sliding mode shows a remarkably small energy gap of
1.7-1.9 meV, indicating very weak interactions between the two incommensurate
sublattices. The measurements also reveal a gapped and steep linear magnon
dispersion of the ladder sublattice. The high group velocity of this magnon
branch and weak coupling with acoustic and pseudoacoustic phonons can explain
the large magnon thermal conductivity in Sr14Cu24O41 crystals. In addition, the
magnon specific heat is determined from the measured total specific heat and
phonon density of states, and exhibits a Schottky anomaly due to gapped magnon
modes of the spin chains. These findings offer new insights into the phonon and
magnon dynamics and thermal transport properties of incommensurate magnetic
crystals that contain low-dimensional substructures.",1605.02133v3
2016-08-29,Field induced spontaneous quasiparticle decay and renormalization of quasiparticle dispersion in a quantum antiferromagnet,"The notion of a quasiparticle, such as a phonon, a roton, or a magnon, is
used in modern condensed matter physics to describe an elementary collective
excitation. The intrinsic zero-temperature magnon damping in quantum spin
systems can be driven by the interaction of the one-magnon states and
multi-magnon continuum. However, detailed experimental studies on this quantum
many-body effect induced by an applied magnetic field are rare. Here we present
a high-resolution neutron scattering study in high fields on an S=1/2
antiferromagnet C9H18N2CuBr4. Compared with the non-interacting linear
spin-wave theory, our results demonstrate a variety of phenomena including
field-induced renormalization of one-magnon dispersion, spontaneous magnon
decay observed via intrinsic linewidth broadening, unusual non-Lorentzian
two-peak structure in the excitation spectra, and a dramatic shift of spectral
weight from one-magnon state to the two-magnon continuum.",1608.08172v3
2018-12-19,Decay rates and energies of free magnons and bound states in dissipative XXZ chains,"Chains of coupled two-level atoms behave as 1D quantum spin systems,
exhibiting free magnons and magnon bound states. While these excitations are
well studied for closed systems, little consideration has been given to how
they are altered by the presence of an environment. This will be especially
important in systems that exhibit nonlocal dissipation, e.g. systems in which
the magnons decay due to optical emission. In this work, we consider free
magnon excitations and two-magnon bound states in an XXZ chain with nonlocal
dissipation. We prove that whilst the energy of the bound state can lie outside
the two-magnon continuum of energies, the decay rate of the bound state has to
always lie within the two-magnon continuum of decay rates. We then derive
analytically the bound state solutions for a system with nearest-neighbour and
next-nearest-neighbour XY interaction and nonlocal dissipation, finding that
the inclusion of nonlocal dissipation allows more freedom in engineering the
energy and decay rate dispersions for the bound states. Finally, we numerically
study a model of an experimental set-up that should allow the realisation of
dissipative bound states by using Rydberg-dressed atoms coupled to a photonic
crystal waveguide (PCW). We demonstrate that this model can exhibit many key
features of our simpler models.",1812.07893v1
2019-04-05,"Cluster-based Haldane phases, bound magnon crystals and quantum spin liquids of a mixed spin-1 and spin-1/2 Heisenberg octahedral chain","The mixed spin-1 and spin-1/2 Heisenberg octahedral chain with regularly
alternating monomeric spin-1 sites and square-plaquette spin-1/2 sites is
investigated using variational technique, localized-magnon approach, exact
diagonalization (ED) and density-matrix renormalization group (DMRG) method.
The investigated model has in a magnetic field an extraordinarily rich
ground-state phase diagram, which includes the uniform and cluster-based
Haldane phases, two ferrimagnetic phases of Lieb-Mattis type, two quantum spin
liquids and two bound magnon crystals in addition to the fully polarized
ferromagnetic phase. The lowest-energy eigenstates in a highly-frustrated
parameter region belong to flat bands and hence, low-temperature thermodynamics
above the bound magnon-crystal ground states can be satisfactorily described
within the localized-magnon approach. The variational method provides an exact
evidence for the magnon-crystal phase with a character of the monomer-tetramer
ground state at zero field, while another magnon-crystal phase with a single
bound magnon at each square plaquette is found in a high-field region. A
diversity of quantum ground states gives rise to manifold zero-temperature
magnetization curves, which may involve up to four wide intermediate plateaus
at zero, one-sixth, one-third and two-thirds of the saturation magnetization,
two quantum spin-liquid regions and two tiny plateaus at one-ninth and
one-twelfth of the saturation magnetization corresponding to the fragmentized
cluster-based Haldane phases.",1904.02889v2
2019-04-29,Experimental Realization of a Passive GHz Frequency-Division Demultiplexer for Magnonic Logic Networks,"The emerging field of magnonics employs spin waves and their quanta, magnons,
to implement wave-based computing on the micro- and nanoscale. Multi-frequency
magnon networks would allow for parallel data processing within single logic
elements whereas this is not the case with conventional transistor-based
electronic logic. However, a lack of experimentally proven solutions to
efficiently combine and separate magnons of different frequencies has impeded
the intensive use of this concept. In this Letter, the experimental realization
of a spin-wave demultiplexer enabling frequency-dependent separation of
magnonic signals in the GHz range is demonstrated. The device is based on
two-dimensional magnon transport in the form of spin-wave beams in unpatterned
magnetic films. The intrinsic frequency-dependence of the beam direction is
exploited to realize a passive functioning obviating an external control and
additional power consumption. This approach paves the way to magnonic
multiplexing circuits enabling simultaneous information transport and
processing.",1904.12744v2
2017-11-08,Discovery of coexisting Dirac and triply degenerate magnons in a three-dimensional antiferromagnet,"Topological magnons are emergent quantum spin excitations featured by magnon
bands crossing linearly at the points dubbed nodes, analogous to fermions in
topological electronic systems. Experimental realization of topological magnons
in three dimensions has not been reported so far. Here, by measuring spin
excitations (magnons) of a three-dimensional antiferromagnet Cu$_{3}$TeO$_{6}$
with inelastic neutron scattering, we provide direct spectroscopic evidence for
the coexistence of symmetry-protected Dirac and triply degenerate nodes, the
latter involving three-component magnons beyond the Dirac-Weyl framework. Our
theoretical calculations show that the observed topological magnon band
structure can be well described by the linear-spin-wave theory based on a
Hamiltonian dominated by the nearest-neighbour exchange interaction $J_1$. As
such, we showcase Cu$_{3}$TeO$_{6}$ as an example system where Dirac and triply
degenerate magnonic nodal excitations coexist, demonstrate an exotic
topological state of matter, and provide a fresh ground to explore the
topological properties in quantum materials.",1711.02960v2
2017-11-27,Cavity Magnon Polaritons with Lithium Ferrite and 3D Microwave Resonators at milli-Kelvin Temperatures,"Single crystal Lithium Ferrite (LiFe) spheres of sub-mm dimension are
examined at mK temperatures, microwave frequencies and variable DC magnetic
field, for use in hybrid quantum systems and condensed matter and fundamental
physics experiments. Strong coupling regimes of the photon-magnon interaction
(cavity magnon polariton quasi-particles) were observed with coupling strength
of up to 250 MHz at 9.5 GHz (2.6\%) with magnon linewidths of order 4 MHz (with
potential improvement to sub-MHz values). We show that the photon-magnon
coupling can be significantly improved and exceed that of the widely used
Yttrium Iron Garnet crystal, due to the small unit cell of LiFe, allowing twice
more spins per unit volume. Magnon mode softening was observed at low DC fields
and combined with the normal Zeeman effect creates magnon spin wave modes that
are insensitive to first order order magnetic field fluctuations. This effect
is observed in the Kittel mode at 5.5 GHz (and another higher order mode at 6.5
GHz) with a DC magnetic field close to 0.19 Tesla. We show that if the cavity
is tuned close to this frequency, the magnon polariton particles exhibit an
enhanced range of strong coupling and insensitivity to magnetic field
fluctuations with both first order and second order insensitivity to magnetic
field as a function of frequency (double magic point clock transition), which
could potentially be exploited in cavity QED experiments.",1711.09980v2
2019-05-29,A magnonic directional coupler for integrated magnonic half-adders,"Magnons, the quanta of spin waves, could be used to encode information in
beyond-Moore computing applications, and magnonic device components, including
logic gates, transistors, and units for non-Boolean computing, have already
been developed. Magnonic directional couplers, which can function as circuit
building blocks, have also been explored, but have been impractical because of
their millimetre dimensions and multi-mode spectra. Here, we report a magnonic
directional coupler based on yttrium iron garnet single-mode waveguides of 350
nm width. We use the amplitude of a spin-wave to encode information and to
guide it to one of the two outputs of the coupler depending on the signal
magnitude, frequency, and the applied magnetic field. Using micromagnetic
simulations, we also propose an integrated magnonic half-adder that consists of
two directional couplers and processes all information within the magnon domain
with aJ energy consumption.",1905.12353v3
2020-06-12,Evidence for magnon-phonon coupling in the topological magnet Cu$_3$TeO$_6$,"We perform thermodynamic and inelastic neutron scattering (INS) measurements
to study the lattice dynamics (phonons) of a cubic collinear antiferromagnet
Cu$_3$TeO$_6$ which hosts topological spin excitations (magnons). While the
specific heat and thermal conductivity results show that the thermal transport
is dominated by phonons, the deviation of the thermal conductivity from a pure
phononic model indicates that there is a strong coupling between magnons and
phonons. In the INS measurements, we find a mode in the excitation spectra at
4.5 K, which exhibits a slight downward dispersion around the Brillouin zone
center. This mode disappears above the N\'{e}el temperature, and thus cannot be
a phonon. Furthermore, the dispersion is distinct from that of a magnon.
Instead, it can be explained by the magnon-polaron mode, which is new
collective excitations resulting from the hybridization between magnons and
phonons. We consider the suppression of the thermal conductivity and emergence
of the magnon-polaron mode to be evidence for magnon-phonon coupling in
Cu$_3$TeO$_6$.",2006.07012v2
2021-02-03,Unveiling excitonic properties of magnons in a quantum Hall ferromagnet,"Magnons enable transferring a magnetic moment or spin over macroscopic
distance. In quantum Hall ferromagnet, it has been predicted in the early 90s
that spin and charges are entangled, meaning that any change of the spin
texture modifies the charge distribution. As a direct consequence of this
entanglement, magnons carry an electric dipole moment. Here we report the first
evidence of the existence of this electric dipole moment in a graphene quantum
Hall ferromagnet using a Mach-Zehnder interferometer as a quantum sensor. By
propagating towards the interferometer through an insulating bulk, the magnon
electric dipole moment modifies the Aharonov-Bohm flux through the
interferometer, changing both its phase and its visibility. In particular, we
relate the phase shift to the sign of this electric dipole moment, and the
exponential loss of visibility to the flux of emitted magnons. Finally, we
probe the emission energy threshold of the magnons close to filling factor v=1.
Approaching v=0, we observe that the emission energy threshold diminishes
towards zero, which might be linked to the existence of gapless mode in the
canted-antiferromagnetic (CAF) phase at v=0. The detection and manipulation of
magnons based on their electric dipole open the field for a new type of
coherent magnon quantum circuits that will be electrostatically controlled.",2102.02068v1
2016-11-17,Parallel pumping for magnon spintronics: Amplification and manipulation of magnon spin currents on the micron-scale,"Magnonics and magnon spintronics aim at the utilization of spin waves and
magnons, their quanta, for the construction of wave-based logic networks via
the generation of pure all-magnon spin currents and their interfacing with
electrical charge transport. The promise of efficient parallel data processing
and low power consumption renders this field one of the most promising research
areas in spintronics. In this context, the process of parallel parametric
amplification, i.e., the conversion of microwave photons into magnons at one
half of the microwave frequency, has proven to be a versatile tool. Its
beneficial and unique properties, such as frequency and mode-selectivity, the
possibility to excite spin waves in a wide wavevector range and the creation of
phase-correlated wave pairs, render it one of the key methods of spin-wave
generation and amplification.
  The application of parallel parametric amplification to micro- and
nanostructures is an important step towards the realization of magnonic
networks. This is motivated not only by the fact that amplifiers are an
important tool for the construction of any extended logic network but also by
the unique properties of parallel parametric amplification, such as a
phase-dependent amplification. Recently, the successful application of parallel
parametric amplification to metallic microstructures has been reported. It has
been demonstrated that parametric amplification provides an excellent tool to
generate and to amplify spin waves in these systems. In particular, the
amplification greatly benefits from the discreteness of the spin-wave spectra.
This opens up new, interesting routes of spin-wave amplification and
manipulation. In this Review, we give an overview over the recent developments
and achievements in this field.",1611.05893v1
2018-01-10,Topological magnon nodal-lines and absence of magnon spin Nernst effect in layered collinear antiferromagnets,"We propose the existence of a symmetry-protected topological Dirac nodal line
(DNL) magnonic phase in layered honeycomb collinear antiferromagnets even in
the presence of spin-orbit Dzyaloshinskii-Moriya interaction. We show that the
magnon spin Nernst effect, predicted to occur in strictly two-dimensional (2D)
honeycomb collinear antiferromagnets cancels out in the layered honeycomb
collinear antiferromagnets. In other words, the magnon spin Nernst effect in
each 2D antiferromagnetic layer cancels out the succeeding layer. Hence, the
Berry curvature vanish in the entire Brillouin zone due to the combination of
time-reversal and space-inversion ($\mathcal{PT}$) symmetry. However, upon
symmetry breaking by an external magnetic field, we show that a non-vanishing
Berry curvature and Chern number protected topological magnon bands are induced
in the non-collinear spin structure. This leads to an experimentally accessible
magnon thermal Hall effect in the $\mathcal{PT}$ symmetry-broken topological
DNL magnonic phase of layered honeycomb antiferromagnets. We propose that the
current predicted results can be experimentally investigated in the layered
honeycomb antiferromagnets CaMn$_2$Sb$_2$, BaNi$_2$V$_2$O$_8$, and
Bi$_3$Mn$_4$O$_{12}$(NO$_3$).",1801.03498v6
2019-06-13,Impact of electromagnetic fields and heat on spin transport signals in Y$_{3}$Fe$_{5}$O$_{12}$,"Exploring new strategies to perform magnon logic is a key requirement for the
further development of magnon-based spintronics. In this work, we realize a
three-terminal magnon transport device to study the possibility of manipulating
magnonic spin information transfer in a magnetic insulator via localized
magnetic fields and heat generation. The device comprises two parallel Pt wires
as well as a Cu center wire that are deposited on the ferrimagnetic insulator
Y$_{3}$Fe$_{5}$O$_{12}$. While the Pt wires act as spin current injector and
detector, the Cu wire is used to create local magnetostatic fields and
additional heat, which impact both the magnetic configuration and the magnons
within the Y$_{3}$Fe$_{5}$O$_{12}$ below. We show that these factors can create
a non-local signal that shows similar features as compared to an electrically
induced magnon flow. Furthermore, a modulation of the spin transport signal
between the Pt wires is observed, which can be partly explained by thermally
excited spin currents of different polarization. Our results indicate a
potential way towards the manipulation of non-local magnon signals, which could
be useful for magnon logic.",1906.05631v2
2019-07-01,Spin transport in an electrically-driven magnon gas near Bose-Einstein condensation: Hartree-Fock-Keldysh theory,"An easy-plane ferromagnetic insulator in a uniform external magnetic field
and in contact with a phonon bath and a normal metal bath is studied
theoretically in the presence of dc spin current injection via the spin Hall
effect in the metal. The Keldysh path integral formalism is used to model the
magnon gas driven into a nonequilibrium steady state by mismatched bath
temperatures and/or electrical injection, and we analyze the magnon system in
the normal (uncondensed) state, but close to the instability to Bose-Einstein
condensation (BEC), within the self-consistent Hartree-Fock approximation. We
find that the steady state magnon distribution function generally has a
non-thermal form that cannot be described by a single effective chemical
potential and effective temperature. We also show that the BEC instability in
the electrically-driven magnon system is signaled by a sign change in the
imaginary part of the poles for long-wavelength magnon modes and by the
divergence of the nonequilibrium magnon distribution function. In the presence
of two bath temperatures, we find that the correlation length of the superfluid
order parameter fluctuations exhibits nontrivial finite temperature crossover
behaviors that are richer than the standard crossover behaviors obtained for
the vacuum-superfluid transition in an equilibrium dilute Bose gas. We study
the consequences of these thermal crossovers on the magnon spin conductivity
and obtain an inverse square-root divergence in the spin conductivity in the
vicinity of the electrically-induced BEC instability. A spintronics device
capable of testing our spin transport predictions is discussed.",1907.01043v2
2019-10-16,"Thermal Hall effect, spin Nernst effect, and spin density induced by thermal gradient in collinear ferrimagnets from magnon-phonon interaction","We theoretically study the intrinsic thermal Hall and spin Nernst effect in
collinear ferrimagnets on a honeycomb lattice with broken inversion symmetry.
The broken inversion symmetry allows in-plane Dzyaloshinskii-Moriya interaction
between the nearest neighbors, which does not affect the magnon spectrum in the
linear spin wave theory. However, the Dzyaloshinskii-Moriya interaction can
induce large Berry curvature in the magnetoelastic excitation spectrum through
the magnon-phonon interaction to produce thermal Hall current. Furthermore, we
find that the magnetoelastic excitations transport spin, which is inherited
from the magnon bands. Therefore, the thermal Hall current is accompanied by
spin Nernst current. Because the magnon-phonon interaction does not conserve
the spin, we also study the spin density induced by thermal gradient in the
presence of magnon-phonon interaction. We find that the intrinsic part of the
spin density shows no asymmetric spin accumulation near the boundary of the
system having a stripe geometry. However, because of the magnon-phonon
interaction, we find nonzero total spin density in the system having armchair
edges. The extrinsic part of the spin density, on the other hand, shows
asymmetric spin accumulation near the boundary for both armchair and zigzag
edges because of the magnon-phonon interaction. In addition, we find nonzero
total spin density in the system having zigzag edge.",1910.07206v2
2020-01-31,Magnon valley Hall effect in CrI3-based vdW heterostructures,"Magnonic excitations in the two-dimensional (2D) van der Waals (vdW)
ferromagnet CrI3 are studied. We find that bulk magnons exhibit a non-trivial
topological band structure without the need for Dzyaloshinskii-Moriya (DM)
interaction. This is shown in vdW heterostructures, consisting of single-layer
CrI3 on top of different 2D materials as MoTe2, HfS2 and WSe2. We find
numerically that the proposed substrates modify substantially the out-of-plane
magnetic anisotropy on each sublattice of the CrI3 subsystem. The induced
staggered anisotropy, combined with a proper band inversion, leads to the
opening of a topological gap of the magnon spectrum. Since the gap is opened
non-symmetrically at the K+ and K- points of the Brillouin zone, an imbalance
in the magnon population between these two valleys can be created under a
driving force. This phenomenon is in close analogy to the so-called valley Hall
effect (VHE), and thus termed as magnon valley Hall effect (MVHE). In linear
response to a temperature gradient we quantify this effect by the evaluation of
the temperature-dependence of the magnon thermal Hall effect. These findings
open a different avenue by adding the valley degrees of freedom besides the
spin, in the study of magnons.",2001.11934v1
2020-02-19,Manipulation of coupling and magnon transport in magnetic metal-insulator hybrid structures,"Ferromagnetic metals and insulators are widely used for generation, control
and detection of magnon spin signals. Most magnonic structures are based
primarily on either magnetic insulators or ferromagnetic metals, while
heterostructures integrating both of them are less explored. Here, by
introducing a Pt/yttrium iron garnet (YIG)/permalloy (Py) hybrid structure
grown on Si substrate, we studied the magnetic coupling and magnon transmission
across the interface of the two magnetic layers. We found that within this
structure, Py and YIG exhibit an antiferromagnetic coupling field as strong as
150 mT, as evidenced by both the vibrating-sample magnetometry and polarized
neutron reflectometry measurements. By controlling individual layer thicknesses
and external fields, we realize parallel and antiparallel magnetization
configurations, which are further utilized to control the magnon current
transmission. We show that a magnon spin valve with an ON/OFF ratio of ~130%
can be realized out of this multilayer structure at room temperature through
both spin pumping and spin Seebeck effect experiments. Thanks to the efficient
control of magnon current and the compatibility with Si technology, the
Pt/YIG/Py hybrid structure could potentially find applications in magnon-based
logic and memory devices.",2002.08266v1
2020-11-11,Magnon-assisted photon-phonon conversion in the presence of the structured environments,"Quantum conversion or interface is one of the most prominent protocols in
quantum information processing and quantum state engineering. We propose a
photon-phonon conversion protocol in a hybrid magnomechanical system comprising
a microwave optical mode, a driven magnon mode and a mechanical-vibrating mode.
The microwave photons in the optical cavity are coupled to the magnons by the
magnetic-dipole interaction, and the latter are coupled to the mechanical
phonons by the magnetostrictive interaction. With strong photon-magnon
interaction and strong driving on magnon, an effective Hamiltonian is
constructed to describe the conversion between photons and phonons nearby their
resonant point. The cavity-magnon system can then play the role of a quantum
memory. Moreover, the faithfulness of the photon-phonon conversion is estimated
in terms of fidelities for state evolution and state-independent transfer. The
former is discussed in the Lindblad master equation taking account the leakages
of photon, phonon and magnon into consideration. The latter is derived by the
Heisenberg-Langevin equation considering the non-Markovian noise from the
structured environments for both optical and mechanical modes. The
state-evolution fidelity is found to be robust to the weak leakage. The
transfer fidelity can be maintained by the Ohmic and sub-Ohmic environments of
the photons and is insensitive to the $1/f$ noise of the phonons. Our work thus
provides an interesting application for the magnon system as a photon-phonon
converter in the microwave regime.",2011.05642v2
2022-06-10,Observing high-k magnons with Mie-resonance-enhanced Brillouin light scattering,"Magnonics is a prospective beyond CMOS technology which uses magnons, the
quanta of spin waves, for low-power information processing. Many magnonic
concepts and devices were recently demonstrated at macro- and microscale, and
now these concepts need to be realized at nanoscale. Brillouin light scattering
spectroscopy and microscopy (BLS) has become a standard technique for spin wave
visualization and characterization, and enabled many pioneering magnonic
experiments. However, due to its fundamental limit in maximum detectable magnon
momentum, the conventional BLS cannot be used to detect nanoscale spin waves.
Here we show that optically induced Mie resonances in dielectric nanoparticles
can be used to extend the range of accessible spin wave wavevectors beyond the
BLS fundamental limit. The method is universal and can be used in many magnonic
experiments dealing with thermally excited as well as coherently excited
high-momentum, short-wavelength spin waves. This discovery significantly
extends the usability and relevance of the BLS technique for nanoscale magnonic
research.",2206.05178v3
2023-01-18,Charged magnons on the surface of a topological insulator,"We study a system of two-dimensional Dirac electrons (as is realized on the
surface of a 3D topological insulator) coupled to an array of localized spins.
The spins are coupled ferromagnetically to each other, forming an ordered
ground state with low-energy spin-wave excitations (magnons). The Dirac
electrons couple to the spins through a spin-dependent effective Zeeman field.
The out-of-plane effective Zeeman field therefore serves as a Dirac mass that
gaps the electronic spectrum. Once a spin is flipped, it creates a surrounding
domain in which the sign of the Dirac mass is opposite to that of the rest of
the sample. Therefore, an electronic bound state appears on the domain wall, as
predicted by Jackiw and Rebbi. However, in a quantum magnet, a localized spin
flip does not produce an eigenstate. Instead, the eigenstates correspond to
delocalized spin waves (magnons). As in the case of the single flipped spin,
the delocalized magnon also binds an in-gap electronic state. We name this
excitation a `Jackiw-Rebbi-Magnon' (JRM) and study its signature in the dynamic
spin susceptibility. When the sample is tunnel-coupled to an electronic
reservoir, a magnon produced in a system without any electrons hybridizes with
a JRM (which binds a single electron), producing magnon-JRM polaritons. For
such a system, we identify a quantum phase transition when the magnon-JRM
polariton energy falls below that of the fully polarized ferromagnetic ground
state.",2301.07754v2
2023-04-15,Temperature-anisotropy conjugate magnon squeezing in antiferromagnets,"Quantum squeezing is an essential asset in the field of quantum science and
technology. In this study, we investigate the impact of temperature and
anisotropy on squeezing of quantum fluctuations in two-mode magnon states
within uniaxial antiferromagnetic materials. Through our analysis, we discover
that the inherent nonlinearity in these bipartite magnon systems gives rise to
a conjugate magnon squeezing effect across all energy eigenbasis states, driven
by temperature and anisotropy. We show that temperature induces amplitude
squeezing, whereas anisotropy leads to phase squeezing. In addition, we observe
that the two-mode squeezing characteristic of magnon eigenenergy states is
associated with amplitude squeezing. This highlights the constructive impact of
temperature and the destructive impact of anisotropy on two-mode magnon
squeezing. Nonetheless, our analysis shows that the destructive effect of
anisotropy is bounded. We demonstrate this by showing that, at a given
temperature, the squeezing of the momentum (phase) quadrature (or equivalently,
the stretching of the position (amplitude) quadrature) approaches a constant
function of anisotropy after a finite value of anisotropy. Moreover, our study
demonstrates that higher magnon squeeze factors can be achieved at higher
temperatures, smaller levels of anisotropy, and closer to the Brillouin zone
center. All these characteristics are specific to low-energy magnons in the
uniaxial antiferromagnetic materials that we examine here.",2304.07602v2
2023-04-28,Interplay of electron-magnon scattering and spin-orbit induced electronic spin-flip fcattering in a two-band Stoner model,"Magnons are one of the carriers of angular momentum that are involved in the
ultrafast magnetization dynamics in ferromagnets, but their contribution to the
electronic dynamics and their interplay with other scattering process that
occur during ultrafast demagnetization has not yet been studied in the
framework of a microscopic dynamical model. The present paper presents such an
investigation of electronic scattering dynamics in itinerant ferromagnets at
the level of Boltzmann scattering integrals for the magnon distributions and
spin-dependent electron distributions. In addition to electron-magnon
scattering, we include spin-conserving and effective Elliott-Yafet like
spin-flip electron-electron scattering processes and the influence of phonons.
In our model system, the creation or annihilation of magnons leads to
transitions between two spin-split electronic bands with energy and momentum
conservation. Due to the presence of spin-orbit coupling, Coulomb scattering
transitions between these bands are also possible, and we describe them on an
equal footing in terms of Boltzmann scattering integrals. For an instantaneous
carrier excitation process we analyze the influence of both interaction
processes on the magnon and spin-dependent electron dynamics, and show that
their interplay gives rise to an efficient creation of magnons at higher
energies and wave vectors accompanied by only a small increase of the
electronic spin polarization. These results present a microscopic dynamical
scenario that shows how non-equilibrium magnons may dominate the magnetic
response of a ferromagnet on ultrafast timescales.",2304.14978v1
2023-06-28,Dipolar Ultrastrong Magnon-Magnon Coupling in a 3D Multilayered Artificial Spin-Vortex Ice,"Strongly-interacting nanomagnetic arrays are ideal systems for exploring
reconfigurable magnonics. They provide huge microstate spaces and integrated
solutions for storage and neuromorphic computing alongside GHz functionality.
These systems may be broadly assessed by their range of reliably accessible
states and the strength of magnon coupling phenomena and nonlinearities.
  Increasingly, nanomagnetic systems are expanding into three-dimensional
architectures. This has enhanced the range of available magnetic microstates
and functional behaviours, but engineering control over 3D states and dynamics
remains challenging.
  Here, we introduce a 3D magnonic metamaterial composed from multilayered
artificial spin ice nanoarrays. Comprising two magnetic layers separated by a
non-magnetic spacer, each nanoisland may assume four macrospin or vortex states
per magnetic layer. This creates a system with a rich $16^N$ microstate space
and intense static and dynamic dipolar magnetic coupling.
  The system exhibits a broad range of emergent phenomena driven by the strong
inter-layer dipolar interaction, including ultrastrong magnon-magnon coupling
with normalised coupling rates of $\frac{\Delta \omega}{\gamma} = 0.57$, GHz
mode shifts in zero applied field and chirality-selective magneto-toroidal
microstate programming and corresponding magnonic spectral control.",2306.16159v2
2023-11-10,Perspective on Nanoscaled Magnonic Networks,"With the rapid development of artificial intelligence in recent years,
mankind is facing an unprecedented demand for data processing. Today, almost
all data processing is performed using electrons in conventional complementary
metal-oxide-semiconductor (CMOS) circuits. Over the past few decades,
scientists have been searching for faster and more efficient ways to process
data. Now, magnons, the quanta of spin waves, show the potential for higher
efficiency and lower energy consumption in solving some specific problems.
While magnonics remains predominantly in the realm of academia, significant
efforts are being made to explore the scientific and technological challenges
of the field. Numerous proof-of-concept prototypes have already been
successfully developed and tested in laboratories. In this article, we review
the developed magnonic devices and discuss the current challenges in realizing
magnonic circuits based on these building blocks. We look at the application of
spin waves in neuromorphic networks, stochastic and reservoir computing and
discuss the advantages over conventional electronics in these areas. We then
introduce a new powerful tool, inverse design magnonics, which has the
potential to revolutionize the field by enabling the precise design and
optimization of magnonic devices in a short time. Finally, we provide a
theoretical prediction of energy consumption and propose benchmarks for
universal magnonic circuits.",2311.06129v1
2023-11-16,Quantum tomography of magnons using Brillouin light scattering,"Quantum magnonics, an emerging field focusing on the study of magnons for
quantum applications, requires precise measurement methods capable of resolving
single magnons. Existing techniques introduce additional dissipation channels
and are not apt for magnets in free space. Brillouin light scattering (BLS) is
a well-established technique for probing the magnetization known for its high
sensitivity and temporal resolution. The coupling between magnons and photons
is controlled by a laser input, so it can be switched off when a measurement is
not needed. In this article, we theoretically investigate the efficacy of BLS
for quantum tomography of magnons. We model a finite optomagnonic waveguide,
including the optical noise added by the dielectric, to calculate the
signal-to-noise ratio (SNR). We find that the SNR is typically low due to a
small magneto-optical coupling; nevertheless, it can be significantly enhanced
by injecting squeezed vacuum into the waveguide. We reconstruct the density
matrix of the magnons from the statistics of the output photons using a maximum
likelihood estimate. The classical component of a magnon state, defined as the
regions of positive Wigner function, can be reconstructed with a high accuracy
while the non-classical component necessitates either a higher SNR or a larger
dataset. The latter requires more compact data structures and advanced
algorithms for post-processing. The SNR is limited partially by the input laser
power that can be increased by designing the optomagnonic cavity with a heat
sink.",2311.10152v2
2023-12-15,Topological magnon gap engineering in van der Waals CrI$_3$ ferromagnets,"The microscopic origin of the topological magnon band gap in CrI$_3$
ferromagnets has been a subject of controversy for years since two main models
with distinct characteristics, i.e., Dzyaloshinskii-Moriya (DM) and Kitaev,
provided possible explanations with different outcome implications. Here we
investigate the angular magnetic field dependence of the magnon gap of CrI$_3$
by elucidating what main contributions play a major role in its generation. We
implement stochastic atomistic spin dynamics simulations to compare the impact
of these two spin interactions on the magnon spectra. We observe three distinct
magnetic field dependencies between these two gap opening mechanisms. First, we
demonstrate that the Kitaev-induced magnon gap is influenced by both the
direction and amplitude of the applied magnetic field, while the DM-induced gap
is solely affected by the magnetic field direction. Second, the position of the
Dirac cones within the Kitaev-induced magnon gap shifts in response to changes
in the magnetic field direction, whereas they remain unaffected by the magnetic
field direction in the DM-induced gap scenario. Third, we find a
direct-indirect magnon band-gap transition in the Kitaev model by varying the
applied magnetic field direction. These differences may distinguish the origin
of topological magnon gaps in CrI$_3$ and other van der Waals magnetic layers.
Our findings pave the way for exploration and engineering topological gaps in
van der Waals materials.",2312.09903v1
2023-12-22,Magnon-assisted magnetization reversal of Ni81Fe19 nanostripes on Y3Fe5O12 with different interfaces,"Magnetic bit writing by short-wave magnons without conversion to the
electrical domain is expected to be a game-changer for in-memory computing
architectures. Recently, the reversal of nanomagnets by propagating magnons was
demonstrated. However, experiments have not yet explored different wavelengths
and the nonlinear excitation regime of magnons required for computational
tasks. We report on the magnetization reversal of individual 20-nm-thick
Ni81Fe19 (Py) nanostripes integrated onto 113-nm-thick yttrium iron garnet
(YIG). We suppress direct interlayer exchange coupling by an intermediate layer
such as Cu and SiO2. Exciting magnons in YIG with wavelengths {\lambda} down to
148 nm we observe the reversal of the integrated ferromagnets in a small
opposing field of 14 mT. Magnons with a small wavelength of {\lambda} = 195 nm,
i.e., twice the width of the Py nanostripes, induced the reversal at an
unprecedentedly small spin precessional power of about 1 nW after propagating
over 15 {\mu}m in YIG. Considerations based on dynamic dipolar coupling explain
the observed wavelength dependence of magnon-induced reversal efficiency. For
an increased power the stripes reversed in an opposing field of only about 1
mT. Our findings are important for the practical implementation of nonvolatile
storage of broadband magnon signals in YIG by means of bistable nanomagnets
without the need of an appreciable global magnetic field.",2312.15107v1
2024-01-19,Exploration of magnon-magnon coupling in an antidot lattice: The role of non-uniform magnetization texture,"We numerically study the spin wave dynamics in an antidot lattice based on a
Co/Pd multilayer structure with reduced perpendicular magnetic anisotropy at
the edges of the antidots. This structure forms a magnonic crystal with a
periodic antidot pattern and a periodic magnetization configuration consisting
of out-of-plane magnetized bulk and in-plane magnetized rims. Our results show
the different behavior of spin waves in the bulk and in the rims under varying
out-of-plane external magnetic field strength, revealing complex spin-wave
spectra and hybridizations between the modes of these two subsystems. A
particularly strong magnon-magnon coupling, due to exchange interactions, is
found between the fundamental bulk spin-wave mode and the second-order radial
rim modes. However, the dynamical coupling between the spin-wave modes at low
frequencies, involving the first-order radial rim modes, is masked by the
changes in the static magnetization at the bulk-rim interface with magnetic
field changes. The study expands the horizons of magnonic-crystal research by
combining periodic structural patterning and non-collinear magnetization
texture to achieve strong magnon-magnon coupling, highlighting the significant
role of exchange interactions in the coupling.",2401.10540v2
2024-02-14,Dynamic magnetic phase transition induced by parametric magnon pumping,"Uncovering pathways to optically drive magnetic order-disorder transitions on
ultrashort timescales can lead to the realization of novel out-of-equilibrium
quantum phenomena. A long-sought pathway is to directly excite a highly
non-thermal energy-momentum distribution of magnons, bypassing both charge and
lattice degrees of freedom. However, this remains elusive owing to the weak
coupling and large momentum mismatch between photons and magnons. Here we
demonstrate strong parametric excitation of magnons across the entire Brillouin
zone of the antiferromagnetic insulator Sr$_2$Cu$_3$O$_4$Cl$_2$ by periodically
modulating the superexchange interaction with the electric field of light. The
excitation efficiency is greatly enhanced by tuning to the van Hove singularity
in the magnon spectrum, sufficient to transiently collapse the
antiferromagnetic state using a pulsed laser field of 10$^9$ V/m. The order
parameter recovery timescale increases by over 1000 times as a function of
excitation density, reflecting a crossover from high- to low-energy magnon
dominated decay dynamics. This electric-field induced parametric magnon pumping
mechanism is applicable to a broad range of magnetic insulators and opens up
the possibility of dynamically engineering magnon distributions by design.",2402.09521v1
2024-03-02,Signatures of magnon hydrodynamics in an atomically-thin ferromagnet,"Strong interactions between particles can lead to emergent collective
excitations. These phenomena have been extensively established in electronic
systems, but are also expected to occur for gases of neutral particles like
spin waves, also known as magnons, in a ferromagnet. In a hydrodynamic regime
where magnons are strongly interacting, they can form a slow collective density
mode -- in analogy to sound waves in water -- with characteristic low-frequency
signatures. While such a mode has been predicted in theory, its signatures have
yet to be observed experimentally. In this work, we isolate atomically-thin
sheets of ferromagnetic CrCl$_3$ where magnon interactions are strong, and
develop a technique to measure its collective magnon dynamics via the quantum
coherence of nearby Nitrogen-Vacancy (NV) centers in diamond. We find that the
thermal magnetic fluctuations generated by CrCl$_3$ exhibit an anomalous
temperature-dependence, whereby fluctuations increase upon decreasing
temperature. Our analysis reveals that this anomalous trend is a consequence of
the damping rate of a low-energy magnon sound mode which sharpens as magnon
interactions increase with increasing temperature, providing a first glimpse of
the magnon hydrodynamic regime.",2403.01057v1
1996-12-11,Magnon-magnon interactions in the Spin-Peierls compound CuGeO_3,"In a magnetic substance the gap in the Raman spectrum, Delta_R, is
approximatively twice the value of the neutron scattering gap, Delta_S, if the
the magnetic excitations (magnons) are only weakly interacting.
  But for CuGeO_3 the experimentally observed ratio Delta_R/Delta_S is
approximatively 1.49-1.78, indicating attractive magnon-magnon interactions in
the quasi-1D Spin-Peierls compound CuGe_3.
  We present numerical estimates for Delta_R/Delta_S from exact diagonalization
studies for finite chains and find agreement with experiment for intermediate
values of the frustration parameter alpha.
  An analysis of the numerical Raman intensity leads us to postulate a
continuum of two-magnon bound states in the Spin-Peierls phase. We discuss in
detail the numerical method used, the dependence of the results on the model
parameters and a novel matrix-element effect due to the dimerization of the
Raman-operator in the Spin-Peierls phase.",9612101v1
1998-10-07,From spinons to magnons in explicit and spontaneously dimerized antiferromagnetic chains,"We reconsider the excitation spectra of a dimerized and frustrated
antiferromagnetic Heisenberg chain. This model is taken as the simpler example
of compiting spontaneous and explicit dimerization relevant for Spin-Peierls
compounds. The bosonized theory is a two frequency Sine-Gordon field theory. We
analize the excitation spectrum by semiclassical methods. The elementary
triplet excitation corresponds to an extended magnon whose radius diverge for
vanishing dimerization. The internal oscilations of the magnon give rise to a
series of excited state until another magnon is emited and a two magnon
continuum is reached. We discuss, for weak dimerization, in which way the
magnon forms as a result of a spinon-spinon interaction potential.",9810073v1
2003-02-24,Magnon-assisted transport and thermopower in ferromagnet-normal metal tunnel junctions,"We develop a theoretical model of magnon-assisted transport in a mesoscopic
tunnel junction between a ferromagnetic metal and a normal (non-magnetic)
metal. The current response to a bias voltage is dominated by the contribution
of elastic processes rather than magnon-assisted processes and the degree of
spin polarization of the current, parameterized by a function $P (\Pi_{\uparrow
(\downarrow)},\Pi_{N})$, $0 \leq P \leq 1$, depends on the relative sizes of
the majority $\Pi_{\uparrow}$ and minority $\Pi_{\downarrow}$ band Fermi
surface in the ferromagnet and of the Fermi surface of the normal metal
$\Pi_{N}$. On the other hand, magnon-assisted tunneling gives the dominant
contribution to the current response to a temperature difference across the
junction. The resulting thermopower is large, $S \sim - (k_B/e)
(k_BT/\omega_{D})^{3/2} P (\Pi_{\uparrow (\downarrow)},\Pi_{N})$, where the
temperature dependent factor $(k_{B}T/\omega_{D})^{3/2}$ reflects the
fractional change in the net magnetization of the ferromagnet due to thermal
magnons at temperature $T$ (Bloch's $T^{3/2}$ law) and $\omega_{D}$ is the
magnon Debye energy.",0302485v1
2003-06-24,Sound attenuation on the Bose-Einstein condensation of magnons in TlCuCl_3,"We investigated experimentally and theoretically sound attenuation in the
quantum spin system TlCuCl_3 in magnetic fields at low temperatures. Near the
point of Bose-Einstein condensation (BEC) of magnons a sharp peak in the sound
attenuation is observed. The peak demonstrates a hysteresis as function of the
magnetic field pointing to a first-order contribution to the transition. The
sound damping has a Drude-like form arising as a result of hardcore
magnon-magnon collisions. The strength of the coupling between lattice and
magnons is estimated from the experimental data. The puzzling relationship
between the transition temperature and the concentration of magnons is
explained by their ""relativistic"" dispersion.",0306623v1
2006-10-28,Exact one- and two-particle excitation spectra of acute-angle helimagnets above their saturation magnetic field,"The two-magnon problem for the frustrated XXZ spin-1/2 Heisenberg Hamiltonian
and external magnetic fields exceeding the saturation field Bs is considered.
We show that the problem can be exactly mapped onto an effective tight-binding
impurity problem. It allows to obtain explicit exact expressions for the
two-magnon Green's functions for arbitrary dimension and number of
interactions. We apply this theory to a quasi-one dimensional helimagnet with
ferromagnetic nearest neighbor J1 < 0 and antiferromagnetic next-nearest
neighbor J2 > 0 interactions. An outstanding feature of the excitation spectrum
is the existence of two-magnon bound states. This leads to deviations of the
saturation field Bs from its classical value Bs(classical) which coincides with
the one-magnon instability. For the refined frustration ratio |J2/J1|> 0.374661
the minimum of the two-magnon spectrum occurs at the boundary of the Brillouin
zone. Based on the two-magnon approach, we propose general analytic expressions
for the saturation field Bs, confirming known previous results for
one-dimensional isotropic systems, but explore also the role of interchain and
long-ranged intrachain interactions as well as of the exchange anisotropy.",0610801v1
2007-11-14,Magnetic-Field Induced Bose-Einstein Condensation of Magnons and Critical Behavior in Interacting Spin Dimer System TlCuCl$_3$,"Magnetization measurements were performed to investigate the critical
behavior of the field-induced magnetic ordering in gapped spin system
TlCuCl$_3$. The critical density of the magnons was obtained as a function of
temperature and the magnon-magnon interaction constant was evaluated. The
experimental phase boundary for $T < 5$ K agrees almost perfectly with the
magnon BEC theory based on the Hartree-Fock approximation with realistic
dispersion relations. The phase boundary can be described by the power law
$[H_{N}(T)-H_{c}] propto T^{phi}$. With decreasing fitting temperature range,
the critical exponent ${phi}$ decreases and converges at $phi_{BEC} =3/2$
predicted by the magnon BEC theory.",0711.2110v1
2008-01-14,Finite size effects for giant magnons on physical strings,"Using finite gap methods, we find the leading order finite size corrections
for an arbitrary number of giant magnons on physical strings, where the sum of
the momenta is a multiple of 2\pi. Our results are valid for the
Hofman-Maldacena fundamental giant magnons as well as their dyonic
generalizations. The energy corrections turn out to be surprisingly simple,
especially if all the magnons are fundamental, and at leading order are
independent of the magnon flavors. We also show how to use the Bethe ansatz to
find finite size corrections for dyonic giant magnons with large R-charges.",0801.2064v2
2008-10-27,N-body Dynamics of Giant Magnons in RxS2,"We pursue the question of multi-magnon dynamics, focusing on the simplest
case of magnons moving on RxS2 and working at the semiclassical level. Through
a Pohlmeyer reduction, the problem reduces to another well known integrable
field theory, the sine-Gordon model, which can be exactly described through an
N-body model of Calogero type. The two theories coincide at the level of
equations of motion, but physical quantities like the energies (of magnons and
solitons) and the associated phase shifts are different. We start from the
equivalence of the two systems at the level of equations of motion and require
that the new (string theory) model reproduces the correct magnon energies and
the phase shift, both of which differ from the soliton case. From the
comparison of energies we suggest a Hamiltonian, and from requiring the correct
phase shift we are led to a nontrivial Poisson structure representing the
magnons.",0810.4548v1
2009-02-09,Wrapping in maximally supersymmetric and marginally deformed N=4 Yang-Mills,"In this note we give evidence for an equality of the spectra, including
wrapping, of the SU(2)-sector spin chain for real deformations beta and
beta+1/L, in marginally beta-deformed N=4 Yang-Mills, which appears after
relaxing the cyclicity constraint. Evidence for the equality is given by
evaluating the first wrapping correction to the energy of the undeformed magnon
of momentum pi, and the beta=1/2, physical magnon, for several spin chain
lengths L. We also show that the term of maximal transcendentality coincides
for both magnons to all L. As a by-product we provide an expression for the
first wrapping correction to the beta = 1/2 single-magnon operator dimension,
valid for all even L. We then apply the symmetry to the magnon dispersion
relation of N=4, obtaining its first wrapping correction for a discrete set of
magnon momenta.",0902.1427v3
2009-03-19,Dyonic Giant Magnons in CP^3: Strings and Curves at Finite J,"This paper studies giant magnons in AdS_4 x CP^3 using both the string
sigma-model and the algebraic curve. We complete the dictionary of solutions by
finding the dyonic generalisation of the CP^1 string solution, which matches
the `small' giant magnon in the algebraic curve, and by pointing out that the
solution recently constructed by the dressing method is the `big' giant magnon.
We then use the curve to compute finite-J corrections to all cases, which for
the non-dyonic cases always match the AFZ result. For the dyonic RP^3 magnon we
recover the S^5 answer, but for the `small' and `big' giant magnons we obtain
new corrections.",0903.3365v2
2010-02-21,Vacuum entanglement governs the bosonic character of magnons,"It is well known that magnons, elementary excitations in a magnetic material,
behave as bosons when their density is low. We study how the bosonic character
of magnons is governed by the amount of a multipartite entanglement in the
vacuum state on which magnons are excited. We show that if the multipartite
entanglement is strong, magnons cease to be bosons. We also consider some
examples, such as ground states of the Heisenberg ferromagnet and the
transverse Ising model, the condensation of magnons, the one-way quantum
computer, and Kitaev's toric code. Our result provides insights into the
quantum statistics of elementary excitations in these models, and into the
reason why a non-local transformation, such as the Jordan-Wigner
transformation, is necessary for some many-body systems.",1002.4010v2
2011-03-07,Theoretical prediction of rotating magnon wavepacket in ferromagnets,"We theoretically show that the magnon wavepacket has a rotational motion in
two ways; a self-rotation and a motion along the boundary of the sample (edge
current). They are similar to cyclotron motion of electrons, but unlike
electrons the magnons have no charge and the rotation is not due to Lorenz
force. These rotational motions are caused by the Berry phase in momentum space
from magnon band structure. Furthermore, these rotational motions of the magnon
give an additional correction term to the magnon Hall effect. We also discuss
the Berry curvature effect in the classical limit of long-wavelength
magnetostatic spin waves having macroscopic coherence length.",1103.1221v3
2011-03-11,Spin pumping by parametrically excited exchange magnons,"We experimentally show that exchange magnons can be detected using a
combination of spin pumping and inverse spin-Hall effect (iSHE) proving its
wavelength integrating capability down to the sub-micrometer scale. The magnons
were injected in a ferrimagnetic yttrium iron garnet film by parametric pumping
and the iSHE-induced voltage was detected in an attached Pt layer. The role of
the density, wavelength, and spatial localization of the magnons for the spin
pumping efficiency is revealed. This study opens the field of the magnon-based
information processing to magnons with nano-scale wavelengths.",1103.2229v2
2012-09-15,Magnon-phonon coupling unmasked: a direct measurement of magnon temperature,"Thermoelectric phenomena in magnetic materials present tantalizing
possibilities for manipulating spin-information using heat in future 'spin
caloritronic' devices. Key to unraveling their underlying physics is to
understand spin-lattice interactions, i.e. the coupling between magnons (the
quanta of magnetization excitations) and phonons (the quanta of lattice
vibrations). Here, we present the first measurements of the spatial
distribution of magnon temperature in a magnetic system subject to a lateral
thermal (i.e. phonon temperature) gradient and demonstrate that, contrary to
currently accepted theory, the magnon and phonon temperatures do not differ.
This result has profound implications. In particular, it re-opens the question
of how the spin Seebeck effect-which allows spin currents to be produced from
thermal gradients, and is arguably the most intriguing and technologically
relevant thermoelectric phenomenon of all-can exist, and which physics
underpins it. Specifically, it reveals that if the general framework of the
current theory of the effect holds, we must adopt a new concept of spectrally
non-uniform magnon temperature.",1209.3405v2
2012-11-20,Bond Randomness Induced Magnon Decoherence in a Spin-1/2 Ladder Compound,"We have used a combination of neutron resonant spin-echo and triple-axis
spectroscopies to determine the energy and linewidth of the magnon resonance in
IPA-Cu(Cl$_{0.95}$Br$_{0.05}$)$_3$, a model spin-1/2 ladder antiferromagnet
where Br substitution induces bond randomness. We find that the bond defects
induce a blueshift, $\delta \Delta$, and broadening, $\delta \Gamma$, of the
magnon gap excitation compared to the pure compound. At temperatures exceeding
the energy scale of the inter-ladder exchange interactions, $\delta \Delta$ and
$\delta \Gamma$ are temperature independent within the experimental error, in
agreement with Matthiessen's rule according to which magnon-defect scattering
yields a temperature independent contribution to the magnon mean free path.
Upon cooling, $\delta \Delta$ and $\delta \Gamma$ become temperature dependent
and saturate at values lower than those observed at higher temperature,
consistent with the crossover from one-dimensional to two-dimensional spin
correlations with decreasing temperature previously observed in pure
IPA-CuCl$_3$. These results indicate limitations in the applicability of
Matthiessen's rule for magnon scattering in low-dimensional magnets.",1211.4741v1
2016-02-09,Topological magnon bound-states in periodically modulated Heisenberg XXZ chains,"Strongly interacting topological states in multi-particle quantum systems
pose great challenges to both theory and experiment. Recently, bound states of
elementary spin waves (magnons) in quantum magnets have been experimentally
observed in quantum Heisenberg chains comprising ultracold Bose atoms in
optical lattices. Here, we explore a strongly interacting topological state
called topological magnon bound-state in the quantum Heisenberg chain under
cotranslational symmetry. We find that the cotranslational symmetry is the key
to the definition of a topological invariant for multi-particle quantum states,
which enables us to characterize the topological features of multi-magnon
excitations. We calculate energy spectra, density distributions, correlations
and Chern numbers of the two-magnon bound-states and show the existence of
topological protected edge bound-states. Our study not only opens a new
prospect to pursue topological magnon bound-states, but also gives insights
into the characterization and understanding of strongly interacting topological
states.",1602.03217v2
2016-02-13,Weyl magnons in breathing pyrochlore antiferromagnets,"Frustrated quantum magnets not only provide exotic ground states and unusual
magnetic structures, but also support unconventional excitations in many cases.
Using a physically relevant spin model for a breathing pyrochlore lattice, we
discuss the presence of topological linear band crossings of magnons in
antiferromagnets. These are the analogs of Weyl fermions in electronic systems,
which we dub Weyl magnons. The bulk Weyl magnon implies the presence of chiral
magnon surface states forming arcs at finite energy. We argue that such
antiferromagnets present a unique example in which Weyl points can be
manipulated in situ in the laboratory by applied fields. We discuss their
appearance specifically in the breathing pyrochlore lattice, and give some
general discussion of conditions to find Weyl magnons and how they may be
probed experimentally. Our work may inspire a re-examination of the magnetic
excitations in many magnetically ordered systems.",1602.04288v2
2016-06-24,Tunable magnon Weyl points in ferromagnetic pyrochlores,"The dispersion relations of magnons in ferromagnetic pyrochlores with
Dzyaloshinskii-Moriya interaction is shown to possess Weyl points, i.\,e.,
pairs of topological nontrivial crossings of two magnon branches with opposite
topological charge. As a consequence of their topological nature, their
projections onto a surface are connected by magnon arcs, thereby resembling
closely Fermi arcs of electronic Weyl semimetals. On top of this, the positions
of the Weyl points in reciprocal space can be tuned widely by an external
magnetic field: rotated within the surface plane, the Weyl points and magnon
arcs are rotated as well; tilting the magnetic field out-of-plane shifts the
Weyl points toward the center $\bar{\Gamma}$ of the surface Brillouin zone. The
theory is valid for the class of ferromagnetic pyrochlores, i.\,e.,
three-dimensional extensions of topological magnon insulators on kagome
lattices. In this Letter, we focus on the $(111)$ surface, identify candidates
of established ferromagnetic pyrochlores which apply to the considered spin
model, and suggest experiments for the detection of the topological features.",1606.07612v1
2017-02-14,Inelastic Light Scattering Spectroscopy of Magnons and Phonons in Nickel Oxide: Effects of Temperature,"We report results of an investigation of the temperature dependence of the
magnon and phonon frequencies in NiO. A combination of Brillouin - Mandelstam
and Raman spectroscopies allowed us to elucidate the evolution of the phonon
and magnon spectral signatures from the Brillouin zone center (GHz range) to
the second-order peaks from the zone boundary (THz range). The
temperature-dependent behavior of the magnon and phonon bands in the NiO
spectrum indicates the presence of antiferromagnetic (AF) order fluctuation or
a persistent AF state at temperatures above the Neel temperature (T=523 K).
Tuning the intensity of the excitation laser provides a method for
disentangling the features of magnons from acoustic phonons without the
application of a magnetic field. Our results are useful for interpretation of
the inelastic-light scattering spectrum of NiO, and add to the knowledge of its
magnon properties important for THz spintronic devices.",1702.04366v1
2017-02-22,Magnonic crystals for data processing,"Magnons - the quanta of spin waves - propagating in magnetic materials with
wavelengths at the nanometer-scale and carrying information in the form of an
angular momentum, can be used as data carriers in next-generation, nano-sized
low-loss information processing systems. In this respect, artificial magnetic
materials with properties periodically varied in space, known as magnonic
crystals, are especially promising for controlling and manipulating the magnon
currents. In this article, different approaches for the realization of static,
reconfigurable, and dynamic magnonic crystals are presented along with a
variety of novel wave phenomena discovered in these crystals. Special attention
is devoted to the utilization of magnonic crystals for processing of analog and
digital information.",1702.06701v1
2017-02-26,Magnon-photon coupling in antiferromagnets,"Magnon-photon coupling in antiferromagnets has many attractive features that
do not exist in ferro- or ferrimagnets. We show quantum-mechanically that, in
the absence of an external field, one of the two degenerated spin wave bands
couples with photons while the other does not. The photon mode anticrosses with
the coupled spin waves when their frequencies are close to each other. Similar
to its ferromagnetic counterpart, the magnon-photon coupling strength is
proportional to the square root of number of spins $\sqrt{N}$ in
antiferromagnets. An external field removes the spin wave degeneracy and both
spin wave bands couple to the photons, resulting in two anticrossings between
the magnons and photons. Two transmission peaks were observed near the
anticrossing frequency. The maximum damping that allows clear discrimination of
the two transmission peaks is proportional to $\sqrt{N}$ and it's well below
the damping of antiferromagnetic insulators. Therefore the strong magnon-photon
coupling can be realized in antiferromagnets and the coherent information
transfer between the photons and magnons is possible.",1702.07977v1
2017-03-09,Dynamics and inertia of a skyrmion in chiral magnets and interfaces: A linear response approach based on magnon excitations,"Taking all the magnon modes into account, we derive the skyrmion dynamics in
response to a weak external drive. A skyrmion has rotational symmetry and the
magnon modes can be characterized by an angular momentum. For a weak distortion
of a skyrmion, only the magnon modes with an angular momentum $|m|=1$ govern
the dynamics of skyrmion topological center. The skyrmion inertia is determined
by the magnon modes in the continuum spectrum. For a skyrmion driven by a
magnetic field gradient or by a spin transfer torque generated by a current,
the dynamical response is practically instantaneous. This justifies the rigid
skyrmion approximation used in Thiele's collective coordinate approach. For a
skyrmion driven by a spin Hall torque, the torque couples to the skyrmion
motion through the magnons in the continuum and damping, therefore the skyrmion
dynamics shows sizable inertia in this case. The trajectory of a skyrmion is an
ellipse for an ac drive of spin Hall torque.",1703.03099v2
2018-02-15,Electron drag in ferromagnetic structures separated by an insulating interface,"We consider electron drag in a system of two ferromagnetic layers separated
by an insulating interface. The source of it is expected to be magnon-electron
interactions. Namely, we assume that the external voltage is applied to the
""active"" layer stimulating electric current through this layer. In its turn,
the scattering of the current-carrying electrons by magnons leads to a magnon
drag current within this layer. The 3-magnons interactions between magnons in
the two layers (being of non-local nature) lead to magnon drag within the
""passive"" layer which, correspondingly, produce electron drag current via
processes of magnon-electron scattering. We estimate the drag current and
compare it to the phonon-induced one.",1802.05485v1
2018-02-21,Magnonic Spin-Transfer Torque in Ferromagnet/Antiferromagnet/Ferromagnet Trilayer,"In an antiferromagnet (AF) with uniaxial anisotropy, spin-up and spin-down
magnons coexist and form an intrinsic degree of freedom resembling electrons.
When polarized by an adjacent ferromagnet (F), a magnonic pure spin current can
be thermally generated in an AF. We explore thermal magnon transport in an
insulating F/AF/F trilayer where propagating magnons inside the AF spacer can
transfer angular momenta between the two Fs. We find that a sufficiently large
temperature gradient can switch the downstream F via magnonic spin-transfer
torque if it is initially antiparallel with the upstream F. A reciprocal
switching is achievable by reversing the temperature gradient. Using typical
material parameters, we estimate the threshold to be less than 1 K/nm at room
temperature, which can be reduced by raising temperature and enhancing the
interfacial exchange coupling.",1802.07709v2
2018-04-04,Anomalous nonlinearity of the magnonic edge mode,"Nonlinearity of magneto-dynamics is typically described by a single constant,
$\mathcal{N}$, with positive and negative values indicating repulsion and
attraction of magnons, respectively. In thin magnetic films with easy-plane
magnetic anisotropy, magnon attraction is typically achieved for an in-plane
magnetization. At sufficient stimulus, e.g. via application of spin transfer
torque, the attraction can give rise to self-localized magnetic solitons, such
as spin wave bullets, which shrink as their amplitude increases. In contrast,
for an oblique magnetization above a certain critical angle, the repulsion of
magnons only allows for propagating modes, which expand when pumped more
strongly. Here we demonstrate, both analytically and using micromagnetic
simulations, that such a dichotomic description is inadequate for magnonic edge
modes, which naturally appear in confined magnetic systems. In particular, we
demonstrate that the confinement potential of such modes is nonlinear in nature
and its contribution makes $\mathcal{N}$ non-monotonically dependent on their
amplitude. As a prominent example, edge modes show compression and expansion
for negative and positive $\mathcal{N}$, yet remain localized. In striking
contrast to the extended geometries, edge magnons might also repeal even for an
in-plane magnetization.",1804.01585v1
2018-11-22,Topological magnon bands for magnonics,"Topological excitations in magnetically ordered systems have attracted much
attention lately. We report on topological magnon bands in ferromagnetic
Shastry-Sutherland lattices whose edge modes can be put to use in magnonic
devices. The synergy of Dzyaloshinskii-Moriya interactions and geometrical
frustration is responsible for the topologically non-trivial character. Using
exact spin wave theory we determine the finite Chern numbers of the magnon
bands which give rise to chiral edge states. The quadratic band crossing point
vanishes due the present anisotropies and the system enters a topological
phase. We calculate the thermal Hall conductivity as an experimental signature
of the topological phase. Different promising compounds are discussed as
possible physical realizations of ferromagnetic Shastry-Sutherland lattices
hosting the required antisymmetric Dzyaloshinskii-Moriya interactions. Routes
to applications in magnonics are pointed out.",1811.08981v2
2016-05-28,"Magnon Waves on Chains of YIG particles: Dispersion Relations, Faraday Rotation, and Power Transmission","We calculate the dispersion relations for magnon waves on a periodic chain of
spherical or cylindrical Yttrium Iron Garnet (YIG) particles. We use the
quasistatic approximation, appropriate when $kd \ll 1$, where $k$ is the wave
number and $d$ the interparticle spacing. In this regime, because of the
magnetic dipole-dipole interaction between the localized magnetic excitations
on neighboring particles, dispersive magnon waves can propagate along the
chain. The waves are analogous to plasmonic waves generated by electric
dipole-dipole interactions between plasmons on neighboring metallic particles.
The magnon waves can be longitudinal ($L$), transverse ($T$), or elliptically
polarized. We find that a linearly polarized magnon wave undergoes a Faraday
rotation as it propagates along the chain. The amount of Faraday rotation can
be tuned by varying the off-diagonal component of the permeability tensor. We
also discuss the possibility of wireless power transmission along the chain
using these coupled magnon waves.",1605.08931v1
2016-08-03,Influence of yttrium iron garnet thickness and heater opacity on the nonlocal transport of electrically and thermally excited magnons,"We studied the nonlocal transport behavior of both electrically and thermally
excited magnons in yttrium iron garnet (YIG) as a function of its thickness.
For electrically injected magnons, the nonlocal signals decrease monotonically
as the YIG thickness increases. For the nonlocal behavior of the thermally
generated magnons, or the nonlocal spin Seebeck effect (SSE), we observed a
sign reversal which occurs at a certain heater-detector distance, and it is
influenced by both the opacity of the YIG/heater interface and the YIG
thickness. Our nonlocal SSE results can be qualitatively explained by the
bulk-driven SSE mechanism together with the magnon diffusion model. Using a
two-dimensional finite element model (2D-FEM), we estimated the bulk spin
Seebeck coefficient of YIG at room temperature. The quantitative disagreement
between the experimental and modeled results indicates more complex processes
going on in addition to magnon diffusion and relaxation, especially close to
the contacts.",1608.01178v1
2016-08-08,Reconstruction of an effective magnon mean free path distribution from spin Seebeck measurements in thin films,"A thorough understanding of the mean-free-path (MFP) distribution of the
energy carriers is crucial to engineer and tune the transport properties of
materials. In this context, a significant body of work has investigated the
phonon and electron MFP distribution, however, similar studies of the magnon
MFP distribution have not been carried out so far. In this work, we used
thickness-dependence measurements of the longitudinal spin Seebeck (LSSE)
effect of yttrium iron garnet films to reconstruct the cumulative distribution
of a SSE related effective magnon MFP. By using the experimental data reported
by Guo et al. [Phys. Rev. X 6, 031012 (2016)], we adapted the phonon MFP
reconstruction algorithm proposed by A.J. Minnich, [Phys. Rev. Lett. 109,
205901 (2012)] and apply it to magnons. The reconstruction showed that magnons
with different MFP contribute in different manner to the total LSSE and the
effective magnon MFP distribution spreads far beyond their typical averaged
values.",1608.02474v3
2017-10-18,Strong Coupling of 3D Cavity Photons to Travelling Magnons At Low Temperatures,"We demonstrate strong coupling between travelling magnons in an Yttrium Iron
Garnet film and 3D microwave cavity photons at milli-Kelvin temperatures. The
coupling strength of $350$MHz or $7.3$\% of resonance frequency is observed.
The magnonic subsystem is represented by the Damon-Eshbach magnetostatic
surface wave with a distribution of wave numbers giving the linewidth of 15MHz.
The ways to improve this parameter are discussed. The energy gap in the
spectrum given by the Zeeman energy and the shape-anisotropy energy in the film
geometry give rise to a significant asymmetry of the double peak structure of
the photon-magnon avoided level crossing. A structure of two parallel YIG films
is investigated using the same re-entrant magnetostatic surface wave transducer
revealing a higher order magnon modes existing in both films. Combination of a
multi-post re-entrant cavity and multiple films is a potential base for
engineering both magnon and photon spectra.",1710.06601v1
2017-10-27,Bulk and edge spin transport in topological magnon insulators,"We investigate the spin transport properties of a topological magnon
insulator, a magnetic insulator characterized by topologically nontrivial bulk
magnon bands and protected magnon edge modes located in the bulk band gaps.
Employing the Landau-Lifshitz-Gilbert phenomenology, we calculate the spin
current driven through a normal metal$|$topological magnon insulator$|$normal
metal heterostructure by a spin accumulation imbalance between the metals, with
and without random lattice defects. We show that bulk and edge transport are
characterized by different length scales. This results in a characteristic
system size where the magnon transport crosses over from being bulk-dominated
for small systems to edge-dominated for larger systems. These findings are
generic and relevant for topological transport in systems of nonconserved
bosons.",1710.09998v2
2017-12-25,"Magnon and Phonon Dispersion, Lifetime and Thermal Conductivity of Iron from Spin-Lattice Dynamics Simulations","In recent years, the fundamental physics of spin-thermal (i.e.,
magnon-phonon) interaction has attracted significant experimental and
theoretical interests given its potential paradigm-shifting impacts in areas
like spin-thermoelectrics, spin-caloritronics and spintronics. Modelling
studies of the transport of magnons and phonons in magnetic crystals are very
rare. In this paper, we use spin-lattice dynamics (SLD) simulations to model
ferromagnetic crystalline iron, where the spin and lattice systems are coupled
through the atomic position-dependent exchange function, and thus the
interaction between magnon and phonon is naturally considered. We then present
a method combining SLD simulations with spectral energy analysis to calculate
the magnon and phonon harmonic (e.g., dispersion, specific heat, group
velocity) and anharmonic (e.g., scattering rate) properties, based on which
their thermal conductivity values are calculated. This work represents an
example of using SLD simulations to understand the transport properties
involving coupled magnon and phonon dynamics.",1712.09002v1
2018-09-13,Hybrid Architecture for Engineering Magnonic Quantum Networks,"We show theoretically that a network of superconducting loops and magnetic
particles can be used to implement magnonic crystals with tunable magnonic band
structures. In our approach, the loops mediate interactions between the
particles and allow magnetic excitations to tunnel over long distances. As a
result, different arrangements of loops and particles allow one to engineer the
band structure for the magnonic excitations. Furthermore, we show how magnons
in such crystals can serve as a quantum bus for long-distance magnetic coupling
of spin qubits. The qubits are coupled to the magnets in the network by their
local magnetic-dipole interaction and provide an integrated way to measure the
state of the magnonic quantum network.",1809.04901v2
2019-09-09,Modulation of magnon spin transport in a magnetic gate transistor,"We demonstrate a modulation of up to 18% in the magnon spin transport in a
magnetic insulator (Y$_{3}$Fe$_{5}$O$_{12}$, YIG) using a common ferromagnetic
metal (permalloy, Py) as a magnetic control gate. A Py electrode, placed
between two Pt injector and detector electrodes, acts as a magnetic gate in our
prototypical magnon transistor device. By manipulating the magnetization
direction of Py with respect to that of YIG, the transmission of magnons
through the Py|YIG interface can be controlled, resulting in a modulation of
the non-equilibrium magnon density in the YIG channel between the Pt injector
and detector electrodes. This study opens up the possibility of using the
magnetic gating effect for magnon-based spin logic applications.",1909.03775v1
2020-12-26,The interplay of Dzyaloshinskii-Moriya and Kitaev interactions for magnonic properties of Heisenberg-Kitaev honeycomb ferromagnets,"The properties of Kitaev materials are attracting ever increasing attention
owing to their exotic properties. In realistic two-dimensional materials,
Kitaev interaction is often accompanied by the Dzyloshinskii-Moriya
interaction, which poses a challenge of distinguishing their magnitude
separately. In this work, we demonstrate that it can be done by accessing
magnonic transport properties. By studying honeycomb ferromagnets exhibiting
Dzyaloshinskii-Moriya and Kitaev interactions simultaneously, we reveal
non-trivial magnonic topological properties accompanied by intricate magnonic
transport characteristics as given by thermal Hall and magnon Nernst effects.
We also investigate the effect of a magnetic field, showing that it does not
only break the symmetry of the system but also brings drastic modifications to
magnonic topological transport properties, which serve as hallmarks of the
relative strength of anisotropic exchange interactions. Based on our findings,
we suggest strategies to estimate the importance of Kitaev interactions in real
materials.",2012.13729v2
2012-05-20,Brillouin light scattering spectroscopy of parametrically excited dipole-exchange magnons,"The spectral distribution of parametrically excited dipole-exchange magnons
in an in-plane magnetized epitaxial film of yttrium-iron garnet was studied by
means of frequency- and wavevector-resolved Brillouin light scattering
spectroscopy. The experiment was performed in a parallel pumping geometry where
an exciting microwave magnetic field was parallel to the magnetizing field. It
was found that for both dipolar and exchange spectral areas parallel pumping
excites the lowest volume magnon modes propagating in the film plane
perpendicularly to the magnetization direction. In order to interpret the
experimental observations, we used a microscopic Heisenberg model that includes
exchange as well as dipole-dipole interactions to calculate the magnon spectrum
and construct the eigenstates. As proven in our calculations, the observed
magnons are characterized by having the highest possible ellipticity of
precession which suggests the lowest threshold of parametric generation.
Applying different pumping powers we observe modifications in the magnon
spectrum that are described theoretically by a softening of the spin stiffness.",1205.4396v1
2017-05-08,Non-local magnon transport in the compensated ferrimagnet GdIG,"We study the diffusive transport of magnons through the compensated
ferrimagnetic insulator Gd3Fe5O12 (GdIG). The magnons are injected and detected
electrically in a non-local measurement configuration via two parallel Pt
strips deposited on top of the ferrimagnet. GdIG exhibits a rich magnon
spectrum, with several thermally populated magnon bands at room temperature. We
observe a strong temperature and field dependence of the non-local voltage in
the detector strip. Just below the magnetization compensation temperature we
find that the increasing magnetic field causes an unexpected enhancement of the
non-local signal. A comparison with GdIG spin wave spectra obtained from
atomistic modeling indicates that the thermal magnon population is important
for understanding the non-local voltage signal.",1705.02871v1
2017-05-11,Chiral anomaly of Weyl magnons in stacked honeycomb ferromagnets,"Chiral anomaly of Weyl magnons (WMs), featured by nontrivial band crossings
at paired Weyl nodes (WNs) of opposite chirality, is investigated. It is shown
that WMs can be realized in stacked honeycomb ferromagnets. Using the
Aharonov-Casher effect that is about the interaction between magnetic moments
and electric fields, the magnon motion in honeycomb layers can be quantized
into magnonic Landau levels (MLLs). The zeroth MLL is chiral so that
unidirectional WMs propagate in the perpendicular (to the layer) direction for
a given WN under a magnetic field gradient from one WN to the other and change
their chiralities, resulting in the magnonic chiral anomaly (MCA). A net magnon
current carrying spin and heat through the zeroth MLL depends linearly on the
magnetic field gradient and the electric field gradient in the ballistic
transport.",1705.04164v1
2017-05-17,Ballistic magnon heat conduction and possible Poiseuille flow in the helimagnetic insulator Cu$_2$OSeO$_3$,"We report on the observation of magnon thermal conductivity $\kappa_m\sim$ 70
W/mK near 5 K in the helimagnetic insulator Cu$_2$OSeO$_3$, exceeding that
measured in any other ferromagnet by almost two orders of magnitude. Ballistic,
boundary-limited transport for both magnons and phonons is established below 1
K, and Poiseuille flow of magnons is proposed to explain a magnon mean-free
path substantially exceeding the specimen width for the least defective
specimens in the range 2 K $<T<$ 10 K. These observations establish
Cu$_2$OSeO$_3$ as a model system for studying long-wavelength magnon dynamics.",1705.06328v2
2018-03-30,From Kinetic Instability to Bose-Einstein Condensation and Magnon Supercurrents,"Evolution of an overpopulated gas of magnons to a Bose-Einstein condensate
and excitation of a magnon supercurrent, propelled by a phase gradient in the
condensate wave function, can be observed at room-temperature by means of the
Brillouin light scattering spectroscopy in an yttrium iron garnet material. We
study these phenomena in a wide range of external magnetic fields in order to
understand their properties when externally pumped magnons are transferred
towards the condensed state via two distinct channels: A multistage
Kolmogorov-Zakharov cascade of the weak-wave turbulence or a one-step
kinetic-instability process. Our main result is that opening the kinetic
instability channel leads to the formation of a much denser magnon condensate
and to a stronger magnon supercurrent compared to the cascade mechanism alone.",1803.11548v1
2018-06-14,Nonlinear relaxation between magnons and phonons in insulating ferromagnets,"Nonlinear relaxation between spin waves (magnons) and the crystal lattice
(phonons) in an insulating ferromagnet is investigated theoretically. Magnons
and phonons are described by the equilibrium Bose-Einstein distributions with
different temperatures. The nonlinear heat current from magnons to phonons is
calculated microscopically in terms of the Cherenkov radiation of phonons by
magnons. The results are discussed in comparison with the well-known
theoretical results on the nonlinear electron-phonon relaxation in metals
[Kaganov, Lifshitz, Tanatarov, J. Exp. Theor. Phys. 31, 232 (1956)]. The
elaborated theoretical description is relevant for spin-pumping experiments and
thermoelectric devices in which the magnon temperature is essentially higher
than the phonon one.",1806.05501v1
2018-06-18,Magnonic Noise and Wiedemann-Franz Law,"We theoretically establish mutual relations among magnetic momentum, heat,
and fluctuations of propagating magnons in a ferromagnetic insulating junction
in terms of noise and the bosonic Wiedemann-Franz (WF) law. Using the
Schwinger-Keldysh formalism, we calculate all transport coefficients of a noise
spectrum for both magnonic spin and heat currents, and establish Onsager
relations between the thermomagnetic currents and the zero-frequency noise.
Making use of the magnonic WF law and the Seebeck coefficient in the
low-temperature limit, we theoretically discover universal relations, i.e.
being independent of material parameters, for both the nonequilibrium and
equilibrium noise, and show that each noise is described solely in terms of
thermal conductance. Finally, we introduce a magnonic spin-analog of the Fano
factor, noise-to-current ratio, and demonstrate that the magnonic spin-Fano
factor reduces to a universal value in the low-temperature limit and it remains
valid even beyond a linear response regime.",1806.06965v1
2018-07-18,Magnon-photon-phonon entanglement in cavity magnomechanics,"We show how to generate tripartite entanglement in a cavity magnomechanical
system which consists of magnons, cavity microwave photons, and phonons. The
magnons are embodied by a collective motion of a large number of spins in a
macroscopic ferrimagnet, and are driven directly by an electromagnetic field.
The cavity photons and magnons are coupled via magnetic dipole interaction, and
the magnons and phonons are coupled via magnetostrictive (radiation
pressure-like) interaction. We show optimal parameter regimes for achieving the
tripartite entanglement where magnons, cavity photons, and phonons are
entangled with each other, and we further prove that the steady state of the
system is a genuinely tripartite entangled state. The entanglement is robust
against temperature. Our results indicate that cavity magnomechanical systems
could provide a promising platform for the study of macroscopic quantum
phenomena.",1807.07158v3
2019-01-04,Atomistic Simulation of Phonon and Magnon Thermal Transport across the Ferro-Paramagnetic Transition,"A temperature-dependent approach involving Green-Kubo equilibrium atomic and
spin dynamics (GKEASD) is reported to assess phonon and magnon thermal
transport processes accounting for phonon-magnon interactions. Using
body-center cubic (BCC) iron as a case study, GKEASD successfully reproduces
its characteristic temperature-dependent spiral and lattice thermal
conductivities. The non-electronic thermal conductivity, i.e., the sum of
phonon and magnon thermal conductivities, calculated using GKEASD for BCC Fe
agrees well with experimental measurements. Spectral energy analysis reveals
that high-frequency phonon-magnon scattering rates are one order of magnitude
larger than those at low frequencies due to energy scattering conservation
rules and high densities of states. Higher temperatures further accentuate this
phenomenon. This new framework fills existing gaps in simulating thermal
transport across the ferro- to para-magnetic transition. Future application of
this methodology to phonon- and magnon-dominant insulators and semiconductors
will enhance understanding of emerging thermoelectric, spin caloritronic and
superconducting materials.",1901.00966v4
2019-01-19,Topological Magnetoelastic Excitations in Non-Collinear Antiferromagnets,"We study the topological property of the magnetoelastic excitation in
non-collinear antiferromagnets. As a toy model, we consider the magnon-phonon
coupling in a triangular antiferromagnet with a $120^\circ$ N\`eel order. We
find that in the presence of out-of-plane external magnetic field, the
magnon-polaron bands, which arise from hybridization of magnons and phonons,
can carry Chern number, even though the individual magnon and phonon bands are
topologically trivial. Large Berry curvature is induced from the anti-crossing
regions between the magnon and phonon bands, which renormalizes the thermal
Hall conductivity of phonon bands. To compute the Berry curvature and Chern
number of magnon-polarons, we give a simple algorithm to diagonalize
magnetoelastic Hamiltonian without diagonalizing the phonon Hamiltonian, by
mapping the problem to the diagonalization of bosonic Bogoliubov-de-Gennes
(BdG) Hamiltonian. This is necessary because the contribution to the Berry
curvature from phonon cannot be properly captured if we compute the Berry
curvature from magnetoelastic Hamiltonian whose phonon sector has been already
diagonalized.",1901.06500v2
2019-02-01,Bloch oscillations of multi-magnon excitations in a Heisenberg XXZ chain,"The studies of multi-magnon excitations will extend our understandings of
quantum magnetism and strongly correlated matters. Here, by using the
time-evolving block decimation algorithm, we investigate the Bloch oscillations
of two-magnon excitations under a gradient magnetic field. Through analyzing
the symmetry of the Hamiltonian, we derive a rigorous and universal relation
between ferromagnetic and anti-ferromagnetic systems. Under strong
interactions, in addition to free-magnon Bloch oscillations, there appear
fractional bounded-magnon Bloch oscillations which can be understood by an
effective single-particle model. To extract the frequencies of Bloch
oscillations and determine the gradient of magnetic field, we respectively
calculate the fidelity in the time domain and the sub-standard deviation in the
frequency domain. Our study not only explore the interaction-induced Bloch
oscillations of multi-magnon excitations, but also provides an alternative
approach to determine the gradient of magnetic field via ultracold atoms in
optical lattices.",1902.00273v2
2019-02-20,Photoinduced Floquet topological magnons in Kitaev magnets,"We study periodically driven pure Kitaev model and ferromagnetic phase of the
Kitaev-Heisenberg model on the honeycomb lattice by off-resonant linearly and
circularly-polarized lights at zero magnetic field. Using a combination of
linear spin wave and Floquet theories, we show that the effective
time-independent Hamiltonians in the off-resonant regime map onto the
corresponding anisotropic static spin model, plus a tunable photoinduced
magnetic field along the $[111]$ direction, which precipitates Floquet
topological magnons and chiral magnon edge modes. They are tunable by the light
amplitude and polarization. Similarly, we show that the thermal Hall effect
induced by the Berry curvature of the Floquet topological magnons can also be
tuned by the laser field. Our results pave the way for ultrafast manipulation
of topological magnons in irradiated Kitaev magnets, and could play a pivotal
role in the investigation of ultrafast magnon spin current generation in Kitaev
materials.",1902.07716v2
2019-02-22,Thermal induced monochromatic microwave generation in magnon-polariton,"We propose thermal induced generation of monochromatic microwave radiation in
magnon-polariton. Mechanism of thermal to microwave energy transformation is
based on intrinsic energy loss compensation of coupled magnon and microwave
cavity oscillators by thermal induced ""negative damping"". A singularity at an
exceptional point is achieved when at the critical value of ""negative damping""
the damping of the system is fully compensated. At the exceptional point, the
input energy is equally distributed between the magnon and photon subsystems of
the magnon-polariton. The efficiency of transformation of thermal energy into
useful microwave radiation is estimated to be as large as 17 percent due to
magnon-photon coupling mediated direct conversation of spin current into
microwave photons.",1902.08383v1
2019-04-02,Universal $1/3$-suppression of magnonic shot noise in diffusive insulating magnets,"Extending a Boltzmann-Langevin theory to magnons, we show a universality of
current-noise suppression in diffusive systems against the difference of
quantum-statistical properties of bosons and fermions. To this end, starting
from a quantum kinetic equation for magnons subjected to thermal gradient in
dilute impurities, we derive a bosonic counterpart of the semiclassical
Boltzmann-Langevin equation for electrons and evaluate a magnonic current-noise
in the diffusive insulating magnet. We theoretically discover that compared
with a Poissonian shot noise of magnons in an insulating ferromagnetic
junction, the magnonic shot noise is suppressed in the diffusive insulating
bulk magnet and noise-to-current ratio (Fano factor) at low temperatures
exhibits a universal behavior, i.e., the same suppression $1/3$ as the one for
electron transport in diffusive conductors, despite the difference of
quantum-statistical properties. Finally, we show that our predictions are
within experimental reach with current device and measurement technologies.",1904.01151v2
2019-04-09,Antiferromagnetic magnons as highly squeezed Fock states underlying quantum correlations,"Employing the concept of two-mode squeezed states from quantum optics, we
demonstrate a revealing physical picture for the antiferromagnetic ground state
and excitations. Superimposed on a N{\'e}el ordered configuration, a spin-flip
restricted to one of the sublattices is called a sublattice-magnon. We show
that an antiferromagnetic spin-up magnon is comprised by a quantum
superposition of states with $n+1$ spin-up and $n$ spin-down
sublattice-magnons, and is thus an enormous excitation despite its unit net
spin. Consequently, its large sublattice-spin can amplify its coupling to other
excitations. Employing von Neumann entropy as a measure, we show that the
antiferromagnetic eigenmodes manifest a high degree of entanglement between the
two sublattices, thereby establishing antiferromagnets as reservoirs for strong
quantum correlations. Based on these novel insights, we outline strategies for
exploiting the strong quantum character of antiferromagetic (squeezed-)magnons
and give an intuitive explanation for recent experimental and theoretical
findings in antiferromagnetic magnon spintronics.",1904.04553v3
2019-08-28,Spin functional renormalization group for quantum Heisenberg ferromagnets: Magnetization and magnon damping in two dimensions,"We use the spin functional renormalization group recently developed by two of
us [J. Krieg and P. Kopietz, Phys. Rev. B $\bf{99}$, 060403(R) (2019)] to
calculate the magnetization $M ( H , T )$ and the damping of magnons due to
classical longitudinal fluctuations of quantum Heisenberg ferromagnets. In
order to guarantee that for vanishing magnetic field $H \rightarrow 0$ the
magnon spectrum is gapless when the spin rotational invariance is spontaneously
broken, we use a Ward identity to express the magnon self-energy in terms of
the magnetization. In two dimensions our approach correctly predicts the
absence of long-range magnetic order for $H=0$ at finite temperature $T$. The
magnon spectrum then exhibits a gap from which we obtain the transverse
correlation length. We also calculate the wave-function renormalization factor
of the magnons. As a mathematical by-product, we derive a recursive form of the
generalized Wick theorem for spin operators in frequency space which
facilitates the calculation of arbitrary time-ordered connected correlation
functions of an isolated spin in a magnetic field.",1908.10753v2
2019-11-16,Magnon magic angles and tunable Hall conductivity in 2D twisted ferromagnetic bilayers,"Twistronics is currently one of the most active research fields in condensed
matter physics, following the discovery of correlated insulating and
superconducting phases in twisted bilayer graphene (tBLG). Here, we present a
magnonic analogue of tBLG. We study magnons in twisted ferromagnetic bilayers
(tFBL), including exchange and Dzyaloshinskii-Moriya interactions (DMI). For
negligible DMI, tFBL presents discrete magnon magic angles and flat moir\'e
minibands analogous to tBLG. The DMI, however, changes the picture and renders
the system much more exotic. The DMI in tFBL induces a rich topological magnon
band structure for any twist angle. The twist angle turns to a control knob for
the magnon Hall and Nernst conductivities. Gapped flat bands appear in a
continuum of magic angles in tFBL with DMI. In the lower limit of the
continuum, the band structure reconstructs to form bundles of topological flat
bands. The luxury of twist-angle control over band gaps, topological
properties, number of flat bands, Hall and Nernst conductivities renders tFBL a
novel device from fundamental and applied perspectives.",1911.07009v5
2015-08-04,Low-energy singlet excitations in spin- 1/2 Heisenberg antiferromagnet on square lattice,"We present an approach based on a dimer expansion which describes low-energy
singlet excitations (singlons) in spin-$\frac12$ Heisenberg antiferromagnet on
simple square lattice. An operator (""effective Hamiltonian"") is constructed
whose eigenvalues give the singlon spectrum. The ""effective Hamiltonian"" looks
like a Hamiltonian of a spin-$\frac12$ magnet in strong external magnetic field
and it has a gapped spectrum. It is found that singlet states lie above triplet
ones (magnons) in the whole Brillouin zone except in the vicinity of the point
$(\pi,0)$, where their energies are slightly smaller. Based on this finding, we
suggest that a magnon decay is possible near $(\pi,0)$ into another magnon and
a singlon which may contribute to the dip of the magnon spectrum near $(\pi,0)$
and reduce the magnon lifetime. It is pointed out that the singlon-magnon
continuum may contribute to the continuum of excitations observed recently near
$(\pi,0)$.",1508.00744v3
2016-12-25,Magnon Planar Hall Effect and Anisotropic Magnetoresistance in a Magnetic Insulator,"Electrical resistivities can be different for charge currents travelling
parallel or perpendicular to the magnetization in magnetically ordered
conductors or semiconductors, resulting in the well-known planar Hall effect
and anisotropic magnetoresistance. Here, we study the analogous anisotropic
magnetotransport behavior for magnons in a magnetic insulator
Y$_{3}$Fe$_{5}$O$_{12}$. Electrical and thermal magnon injection, and
electrical detection methods are used at room temperature with transverse and
longitudinal geometries to measure the magnon planar Hall effect and
anisotropic magnetoresistance, respectively. We observe that the relative
difference between magnon current conductivities parallel and perpendicular to
the magnetization, with respect to the average magnon conductivity, i.e.
$|(\sigma_{\parallel}^{\textrm{m}}-\sigma_{\perp}^{\textrm{m}})/\sigma_{0}^{\textrm{m}}|$
, is approximately 5% with the majority of the measured devices showing
$\sigma_{\perp}^{\textrm{m}}>\sigma_{\parallel}^{\textrm{m}}$.",1612.08233v1
2017-07-20,Bistability of Cavity Magnon Polaritons,"We report the first observation of the magnon-polariton bistability in a
cavity magnonics system consisting of cavity photons strongly interacting with
the magnons in a small yttrium iron garnet (YIG) sphere. The bistable behaviors
are emerged as sharp frequency switchings of the cavity magnon-polaritons
(CMPs) and related to the transition between states with large and small number
of polaritons. In our experiment, we align, respectively, the [100] and [110]
crystallographic axes of the YIG sphere parallel to the static magnetic field
and find very different bistable behaviors (e.g., clockwise and
counter-clockwise hysteresis loops) in these two cases. The experimental
results are well fitted and explained as being due to the Kerr nonlinearity
with either positive or negative coefficient. Moreover, when the magnetic field
is tuned away from the anticrossing point of CMPs, we observe simultaneous
bistability of both magnons and cavity photons by applying a drive field on the
lower branch.",1707.06509v2
2017-11-02,Measurement of a Magnonic Crystal at Millikelvin Temperatures,"Hybrid systems combining magnons and superconducting quantum circuits have
attracted increasing interest in recent years. Magnonic crystals (MCs) are one
of the building blocks of room-temperature magnonics and can be used to create
devices with an engineered band structure. These devices, exhibiting tunable
frequency selectivity and the ability to store travelling excitations in the
microwave regime, may form the basis of a set of new tools to be used in the
context of quantum information processing. In order to ascertain the
feasibility of such plans, MCs must be demonstrated to work at the low
temperatures required for microwave-frequency quantum experiments. We report
the first measurements of the transmission of microwave signals through an MC
at 20 mK and observe a magnonic bandgap in both continuous-wave and pulsed
excitation experiments. The spin-wave damping at low temperatures in our
yttrium iron garnet MC is higher than expected, indicating that further work is
necessary before the full potential of quantum experiments using magnonic
crystals can be realised.",1711.00958v2
2017-11-23,Elastic Gauge Fields and Hall Viscosity of Dirac Magnons,"We analyze the coupling of elastic lattice deformations to the magnon degrees
of freedom of magnon Dirac materials. For a Honeycomb ferromagnet we find that,
as it happens in the case of graphene, elastic gauge fields appear coupled to
the magnon pseudospinors. For deformations that induce constant pseudomagnetic
fields, the spectrum around the Dirac nodes splits into pseudoLandau levels. We
show that when a Dzyaloshinskii-Moriya interaction is considered, a topological
gap opens in the system and a Chern-Simons effective action for the elastic
degrees of freedom is generated. Such a term encodes a phonon Hall viscosity
response, entirely generated by fluctuations of magnons living in the vicinity
of the Dirac points. The magnon Hall viscosity vanishes at zero temperature,
and grows as temperature is raised and the states around the Dirac points are
increasingly populated.",1711.08653v2
2018-10-09,Phase-mediated magnon chaos-order transition in cavity optomagnonics,"Magnon as a quantized spin wave has attracted extensive attentions in various
fields of physics, such as magnon spintronics, microwave photonics, and cavity
quantum electrodynamics. Here, we explore theoretically magnon chaos-order
transition in cavity optomagnonics, which is still remains largely unexplored
in this emerging field. We find that the evolution of the magnon experiences
the transition from order to period-doubling bifurcation and finally enter
chaos by adjusting the microwave driving power. Different from the normal
chaos, the magnon chaos-order transition proposed here is phase-mediated.
Beyond their fundamental scientific significance, our results will contribute
to the comprehending of nonlinear phenomena and chaos in optomagnonical system,
and may find applications in chaos-based secure communication.",1810.04330v2
2018-10-29,Hydrodynamic sound modes and Galilean symmetry breaking in a magnon fluid,"The non-interacting magnon gas description in ferromagnets breaks down at
finite magnon density where momentum-conserving collisions between magnons
become important. Observation of the collision-dominated regime, however, has
been hampered by the lack of probes to access the energy and lengthscales
characteristic of this regime. Here we identify a key signature of the
collision-dominated hydrodynamic regime---a magnon sound mode---which governs
dynamics at low frequencies and can be detected with recently-introduced spin
qubit magnetometers. The magnon sound mode is an excitation of the longitudinal
spin component with frequencies below the spin wave continuum in gapped
ferromagnets. We also show that, in the presence of exchange interactions with
SU(2) symmetry, the ferromagnet hosts an usual hydrodynamic regime that lacks
Galilean symmetry at all energy and lengthscales. The hydrodynamic sound mode,
if detected, can lead to a new platform to explore hydrodynamic behavior in
quantum materials.",1810.12333v2
2019-05-12,Optical heterodyne imaging of magnetostatic modes in one-dimensional magnonic crystals,"We demonstrate a real-space imaging of a heterodyne signal of light that is
produced as a result of the Brillouin light scattering by coherently driven
magnons in magnetostatic modes. With this imaging technique, we characterize
surface magnetostatic modes (Damon-Eshbach modes) in a one-dimensional magnonic
crystal, which is formed by patterned aluminum strips deposited on the
ferromagnetic film. The modified band structures of the magnonic crystal are
deduced from the Fourier transforms of the real-space images. The heterodyne
imaging provides a simple and powerful method to probe magnons in structured
ferromagnetic films, paving a way to investigate more complex phenomena, such
as Anderson localization and topological transport with magnons.",1905.04683v4
2020-06-05,Controlling the nonlinear relaxation of quantized propagating magnons in nanodevices,"Relaxation of linear magnetization dynamics is well described by the viscous
Gilbert damping processes. However, for strong excitations, nonlinear damping
processes such as the decay via magnon-magnon interactions emerge and trigger
additional relaxation channels. Here, we use space- and time-resolved
microfocused Brillouin light scattering spectroscopy and micromagnetic
simulations to investigate the nonlinear relaxation of strongly driven
propagating spin waves in yttrium iron garnet nanoconduits. We show that the
nonlinear magnon relaxation in this highly quantized system possesses
intermodal features, i.e., magnons scatter to higher-order quantized modes
through a cascade of scattering events. We further show how to control such
intermodal dissipation processes by quantization of the magnon band in
single-mode devices, where this phenomenon approaches its fundamental limit.
Our study extends the knowledge about nonlinear propagating spin waves in
nanostructures which is essential for the construction of advanced spin-wave
elements as well as the realization of Bose-Einstein condensates in scaled
systems.",2006.03400v2
2020-06-05,Hierarchy of magnon entanglement in antiferromagnets,"Continuous variable entanglement between magnon modes in Heisenberg
antiferromagnet with Dzyaloshinskii-Moryia (DM) interaction is examined.
Different bosonic modes are identified, which allows to establish a hierarchy
of magnon entanglement in the ground state. We argue that entanglement between
magnon modes is determined by a simple lattice specific factor, together with
the ratio of the strengths of the DM and Heisenberg exchange interactions, and
that magnon entanglement can be detected by means of quantum homodyne
techniques. As an illustration of the relevance of our findings for possible
entanglement experiments in the solid state, a typical antiferromagnet with the
perovskite crystal structure is considered, and it is shown that long wave
length magnon modes have the highest degree of entanglement.",2006.03479v1
2020-06-14,Hypersonic heat-induced flows of magnons induced by femtosecond laser pulses,"In this work, we present evidence for the existence of a magnonic current on
the sub-picosecond time-scale in a ferrimagnetic bilayer and its effect on
ultrafast spin dynamics. The ferrimagnet, GdFeCo, is a material known to
undergo ultrafast switching within 1-2ps after excitation with femtosecond
laser pulses. Here, we show that the strong thermal gradients induced by
applying femtosecond laser pulses and the presence of chemical inhomogeneities
lead to local imbalances in the effective temperatures of the spins that
produces a rapid transfer of spin angular momentum, which we interpret as an
ultrafast spin Seebeck effect. We have quantified the typical magnon
propagation in such a system. The results show ballistic magnon propagation
with 30nm/ps velocities. The characteristic time scale of such magnon
propagation indicates that this magnon transport can play an important role in
switching, a crucial piece of understanding towards realising next generation
data processing devices that operate at much higher frequencies.",2006.07935v1
2013-06-12,Magnon self energy in the correlated ferromagnetic Kondo lattice model: spin-charge coupling effects on magnon excitations in manganites,"Magnon self energy due to spin-charge coupling is calculated for the
correlated ferromagnetic Kondo lattice model using a diagrammatic expansion
scheme. Systematically incorporating correlation effects in the form of
self-energy and vertex corrections, the expansion scheme explicitly preserves
the continuous spin rotation symmetry and hence the Goldstone mode. Due to a
near cancellation of the correlation-induced quantum correction terms at
intermediate coupling and optimal band filling relevant for ferromagnetic
manganites, the renormalized magnon energies for the correlated FKLM are nearly
independent of correlation term. Even at higher band fillings, despite
exhibiting overall non-Heisenberg behavior, magnon dispersion in the \Gamma-X
direction retains nearly Heisenberg form. Therefore, the experimentally
observed doping dependent zone-boundary magnon softening must be ascribed to
spin-orbital coupling effects.",1306.2769v1
2016-09-06,Magnonic Weyl semimetal in pyrochlore ferromagnets,"Topological states of matter have been a subject of intensive studies in
recent years because of their exotic properties such as the topologically
protected edge and surface states. The initial studies were exclusively for
electron systems. It is now known that topological states can also exist for
other particles. Indeed, topologically protected edge states have already been
found for phonons and photons. In spite of active searching for topological
states in many fields, the studies in magnetism are relatively rare although
topological states are apparently important and useful in magnonics. Here we
show that the pyrochlore ferromagnets with the Dzyaloshinskii-Moriya
interaction are intrinsic magnonic Weyl semimetals. Similar to the electronic
Weyl semimetals, the magnon bands in a magnonic Weyl semimetal are nontrivially
crossing in pairs at special points (called Weyl nodes) in momentum space. The
equal energy contour around the Weyl nodes gives rise to the Fermi arcs on
sample surfaces due to the topologically protected surface states between each
pair of Weyl nodes. Additional Weyl nodes and Fermi arcs can be generated in
lower energy magnon bands when an anisotropic exchange interaction is
introduced.",1609.01500v1
2016-11-29,Magnonic quantum Hall effect and Wiedemann-Franz law,"We present a quantum Hall effect of magnons in two-dimensional clean
insulating magnets at finite temperature. Through the Aharonov-Casher effect, a
magnon moving in an electric field acquires a geometric phase and forms Landau
levels in an electric field gradient of sawtooth form. At low temperatures, the
lowest energy band being almost flat carries a Chern number associated with a
Berry curvature. Appropriately defining the thermal conductance for bosons, we
find that the magnon Hall conductances get quantized and show a universal
thermomagnetic behavior, i.e., are independent of materials, and obey a
Wiedemann-Franz law for magnon transport. We consider magnons with quadratic
and linear (Dirac-like) dispersions. Finally, we show that our predictions are
within experimental reach for ferromagnets and skyrmion lattices with current
device and measurement techniques.",1611.09752v3
2018-01-16,Topological Magnons in a One-dimensional Itinerant Flatband Ferromagnet,"Different from previous scenarios that topological magnons emerge in local
spin models, we propose an alternative that itinerant electron magnets can host
topological magnons. A one-dimensional Tasaki model with a flat band is
considered as the prototype. This model can be viewed as a quarter filled
periodic Anderson model with impurities located in between and hybridizing with
the nearest-neighbor conducting electrons, together with a Hubbard repulsion
for these electrons. By increasing the Hubbard interaction, the gap between the
acoustic and optical magnons closes and reopens while the Berry phase of the
acoustic band changes from 0 to $\pi$, leading to the occurrence of a
topological transition. After this transition, there always exist in-gap edge
magnonic modes which is consistent with the bulk-edge correspondence. The
Hubbard interaction driven transition reveals a new mechanism to realize
non-trivial magnon bands.",1801.05255v2
2018-08-20,Tunable Magnonic Thermal Hall Effect in Skyrmion Crystal Phases of Ferrimagnets,"We theoretically study the thermal Hall effect by magnons in skyrmion crystal
phases of ferrimagnets in the vicinity of the angular momentum compensation
point (CP). To this end, we start by deriving the equation of motion for
magnons in the background of an arbitrary equilibrium spin texture, which gives
rise to the fictitious electromagnetic field for magnons. As the net spin
density varies, the resultant equation of motion interpolates between the
relativistic Klein-Gordon equation at CP and the nonrelativistic
Schr{\""o}dinger-like equation away from it. In skyrmion crystal phases, the
right- and the left-circularly polarized magnons with respect to the order
parameter are shown to form the Landau levels separately within the uniform
skyrmion-density approximation. For an experimental proposal, we predict that
the magnonic thermal Hall conductivity changes its sign when the ferrimagnet is
tuned across CP, providing a way to control heat flux in spin-caloritronic
devices on the one hand and a feasible way to detect CP of ferrimagnets on the
other hand.",1808.06690v1
2019-06-18,Reversible tuning of omnidirectional band gaps in two-dimensional magnonic crystals by magnetic field and in-plane squeezing,"By means of the plane wave method, we study nonuniform, i.e., mode- and
k-dependent, effects in the spin-wave spectrum of a two-dimensional bicomponent
magnonic crystal. We use the crystal based on a hexagonal lattice squeezed in
the direction of the external magnetic field wherein the squeezing applies to
the lattice and the shape of inclusions. The squeezing changes both the
demagnetizing field and the spatial confinement of the excitation, which may
lead to the occurrence of an omnidirectional magnonic band gaps. In particular,
we study the role played by propagational effects, which allows us to explain
the k-dependent softening of modes. The effects we found enabled us not only to
design the width and position of magnonic band gaps, but also to plan their
response to a change in the external magnetic field magnitude. This allows the
reversible tuning of magnonic band gaps, and it shows that the studied
structures are promising candidates for designing magnonic devices that are
tunable during operation.",1906.07469v3
2019-06-19,Introducing coherent time control to cavity-magnon-polariton modes,"By connecting light to magnetism, cavity-magnon-polaritons (CMPs) can build
links from quantum computation to spintronics. As a consequence, CMP-based
information processing devices have thrived over the last five years, but
almost exclusively been investigated with single-tone spectroscopy. However,
universal computing applications will require a dynamic control of the CMP on
demand and within nanoseconds. In this work, we perform fast manipulations of
the different CMP modes with independent but coherent pulses to the cavity and
magnon system. We change the state of the CMP from the energy exchanging beat
mode to its normal modes and further demonstrate two fundamental examples of
coherent manipulation: First, a dynamic control over the appearance of
magnon-Rabi oscillations, i.e., energy exchange, and second, a complete energy
extraction by applying an anti-phase drive to the magnon. Our results show a
promising approach to control different building blocks for a quantum internet
and pave the way for further magnon-based quantum computing research.",1906.08103v2
2019-06-24,Macroscopic entanglement of two magnon modes via quantum correlated microwave fields,"We present a scheme to entangle two magnon modes in two macroscopic
yttrium-iron-garnet spheres. The two spheres are placed inside two microwave
cavities, which are driven by a two-mode squeezed microwave field. By using the
linear state-swap interaction between the cavity and the magnon mode in each
cavity, the quantum correlation of the two driving fields is with high
efficiency transferred to the two magnon modes. Considerable entanglement could
be achieved under experimentally achievable conditions $g \gg \kappa_a \gg
\kappa_m$, where $g$ is the cavity-magnon coupling rate and $\kappa_a$,
$\kappa_m$ are the decay rates of the cavity and magnon modes, respectively.
The entanglement is in the steady state and robust against temperature,
surviving up to hundreds of milliKelvin with experimentally accessible two-mode
squeezed source.",1906.09921v1
2019-06-28,Control of magnons via ultrafast magnetization modulation,"We demonstrate optical control of magnons using femtosecond laser pulses by
performing ab-initio real-time time-dependent density functional theory (TDDFT)
simulations. We predict that the spin-wave dynamics in Fe$_{50}$Ni$_{50}$ can
be manipulated by tailoring the applied laser pulse via three distinct
mechanisms: (1) element selective destruction of magnon modes depending on the
laser intensity, (2) delay dependent freezing of the magnon mode into a
transient non-collinear state (where delay is in the pulse peak with respect to
the start of simulations), and (3) optically induced inter sublattice transfer
(OISTR) driven renormalization of the optical magnon frequency. Harnessing such
processes would significantly speed up magnonic devices.",1906.12270v2
2019-07-04,Topological Magnon-Phonon Hybrid Excitations in Two-Dimensional Ferromagnets with Tunable Chern Numbers,"We theoretically investigate magnon-phonon hybrid excitations in
two-dimensional ferromagnets. The bulk bands of hybrid excitations, which are
referred to as magnon-polarons, are analytically shown to be topologically
nontrivial, possessing finite Chern numbers. We also show that the Chern
numbers of magnon-polaron bands and the number of band-crossing lines can be
manipulated by an external magnetic field. For experiments, we propose to use
the thermal Hall conductivity as a probe of the finite Berry curvatures of
magnon-polarons. Our results show that a simple ferromagnet on a square lattice
supports topologically nontrivial magnon-polarons, generalizing topological
excitations in conventional magnetic systems.",1907.02224v1
2019-07-20,Magnon Bose-Einstein condensate and supercurrents over a wide temperature range,"Magnon Bose-Einstein Condensates (BECs) and supercurrents are coherent
quantum phenomena, which appear on a macroscopic scale in parametrically
populated solid state spinsystems. One of the most fascinating and attractive
features of these processes is the possibility of magnon condensation and
supercurrent excitation even at room temperature. At the same time, valuable
information about a magnon BEC state, such as its lifetime, its formation
threshold, and coherency, is provided by experiments at various temperatures.
Here, we use Brillouin Light Scattering (BLS) spectroscopy for the
investigation of the magnon BEC dynamics in a single-crystal film of yttrium
iron garnet in a wide temperature range from 30 K to 380 K. By comparing the
BLS results with previous microwave measurements, we re-vealed the direct
relation between the damping of the condensed and the parametrically injected
magnons. The enhanced supercurrent dynamics was detected at 180 K near the
minimum of BEC damping.",1907.08805v1
2019-10-08,Orbital Nernst Effect of Magnons,"In the past, magnons have been shown to mediate thermal transport of spin in
various systems. Here, we reveal that the fundamental coupling of scalar spin
chirality, inherent to magnons, to the electronic degrees of freedom in the
system can result in the generation of sizeable orbital magnetization and
thermal transport of orbital angular momentum. We demonstrate the emergence of
the latter phenomenon of orbital Nernst effect by referring to the spin-wave
Hamiltonian of kagome ferromagnets, predicting that in a wide range of systems
the transverse current of orbital angular momentum carried by magnons in
response to an applied temperature gradient can overshadow the accompanying
spin current. We suggest that the discovered effect fundamentally correlates
with the topological Hall effect of fluctuating magnets, and it can be utilized
in magnonic devices for generating magnonic orbital torques.",1910.03317v2
2019-10-25,Twisted magnon as a magnetic tweezer,"Wave fields with spiral phase dislocations carrying orbital angular momentum
(OAM) have been realized in many branches of physics, such as for photons,
sound waves, electron beams, and neutrons. However, the OAM states of magnons
(spin waves)$-$the building block of modern magnetism$-$and particularly their
implications have yet to be addressed. Here, we theoretically investigate the
twisted spin-wave generation and propagation in magnetic nanocylinders. The OAM
nature of magnons is uncovered by showing that the spin-wave eigenmode is also
the eigenstate of the OAM operator in the confined geometry. Inspired by
optical tweezers, we predict an exotic ""magnetic tweezer"" effect by showing
skyrmion gyrations under twisted magnons in exchange coupled
nanocylinder$|$nanodisk heterostructure, as a practical demonstration of
magnonic OAM to manipulate topological spin defects. Our study paves the way
for the emerging magnetic manipulations by harnessing the OAM degree of freedom
of magnons.",1910.11528v2
2020-01-03,Dissipative couplings in cavity magnonics,"Cavity Magnonics is an emerging field that studies the strong coupling
between cavity photons and collective spin excitations such as magnons. This
rapidly developing field connects some of the most exciting branches of modern
physics, such as quantum information and quantum optics, with one of the oldest
science on the earth, the magnetism. The past few years have seen a steady
stream of exciting experiments that demonstrate novel magnon-based transducers
and memories. Most of such cavity magnonic devices rely on coherent coupling
that stems from direct dipole-dipole interaction. Recently, a distinct
dissipative magnon-photon coupling was discovered. In contrast to coherent
coupling that leads to level repulsion between hybridized modes, dissipative
coupling results in level attraction. It opens an avenue for engineering and
harnessing losses in hybrid systems. This article gives a brief review of this
new frontier. Experimental observations of level attraction are reviewed.
Different microscopic mechanisms are compared. Based on such experimental and
theoretical reviews, we present an outlook for developing open cavity systems
by engineering and harnessing dissipative couplings.",2001.00978v2
2020-01-21,High order magnon bound states in the quasi-one-dimensional antiferromagnet $α$-NaMnO$_2$,"Here we report on the formation of two and three magnon bound states in the
quasi-one-dimensional antiferromagnet $\alpha$-NaMnO$_2$, where the single-ion,
uniaxial anisotropy inherent to the Mn$^{3+}$ ions in this material provides a
binding mechanism capable of stabilizing higher order magnon bound states.
While such states have long remained elusive in studies of antiferromagnetic
chains, neutron scattering data presented here demonstrate that higher order
$n>2$ composite magnons exist, and, specifically, that a weak three-magnon
bound state is detected below the antiferromagnetic ordering transition of
NaMnO$_2$. We corroborate our findings with exact numerical simulations of a
one-dimensional Heisenberg chain with easy-axis anisotropy using matrix-product
state techniques, finding a good quantitative agreement with the experiment.
These results establish $\alpha$-NaMnO$_2$ as a unique platform for exploring
the dynamics of composite magnon states inherent to a classical
antiferromagnetic spin chain with Ising-like single ion anisotropy.",2001.07300v1
2020-01-31,Topological magnons in CrI$_3$ monolayers: an itinerant fermion description,"Magnons dominate the magnetic response of the recently discovered insulating
ferromagnetic two dimensional crystals such as CrI$_3$. Because of the
arrangement of the Cr spins in a honeycomb lattice, magnons in CrI$_3$ bear a
strong resemblance with electronic quasiparticles in graphene. Neutron
scattering experiments carried out in bulk CrI$_3$ show the existence of a gap
at the Dirac points, that has been conjectured to have a topological nature.
Here we propose a theory for magnons in ferromagnetic CrI$_3$ monolayers based
on an itinerant fermion picture, with a Hamiltonian derived from first
principles. We obtain the magnon dispersion for 2D CrI$_3$ with a gap at the
Dirac points with the same Berry curvature in both valleys. For CrI$_3$
ribbons, we find chiral in-gap edge states. Analysis of the magnon wave
functions in momentum space further confirms their topological nature.
Importantly, our approach does not require to define a spin Hamiltonian, and
can be applied to both insulating and conducting 2D materials with any type of
magnetic order.",2002.00077v1
2020-02-26,Driving a magnetized domain wall in an antiferromagnet by magnons,"We theoretically study the interaction of magnons, quanta of spin waves, and
a domain wall in a one-dimensional easy-axis antiferromagnet in the presence of
an external magnetic field applied along the easy axis. To this end, we begin
by obtaining the exact solution for spin waves in the background of a domain
wall magnetized by an external field. The finite magnetization inside the
domain wall is shown to give rise to reflection of magnons scattering off the
domain wall, deviating from the well-known result of reflection-free magnons in
the absence of a magnetic field. For practical applications of the predicted
reflection of magnons, we show that the magnon reflection contributes to the
thermally-driven domain-wall motion. Our work leads us to envision that
inducing a finite magnetization in antiferromagnetic solitons such as vortices
and skyrmions can be used to engender phenomena that do not occur in the
absence of magnetization.",2002.11777v1
2020-03-12,Collisionless kinetic theory for parametrically pumped magnons,"We discuss collisionless kinetic equations describing the non-equilibrium
dynamics of magnons in a ferromagnet exposed to an oscillating microwave field.
Previously, this problem has been treated within the so-called ""S-theory"" where
the collision integral in the kinetic equation for the magnon distribution is
either neglected or taken into account phenomenologically via an effective
relaxation time. However, the possibility of magnon condensation has not been
included in S-theory. Moreover, the momentum integrations appearing in the
magnon self-energies are usually decoupled by retaining only the term where the
loop momentum is equal to the external momentum. In this work we critically
examine the accuracy of these approximations and develop the proper extensions
of S-theory. We show that these extensions can significantly modify the time
evolution of the magnon distribution.",2003.05949v2
2020-03-28,Valley-polarized domain wall magnons in 2D ferromagnetic bilayers,"Valleytronics is a pioneering technological field relying on the valley
degree of freedom to achieve novel electronic functionalities. Topological
valley-polarized electrons confined to domain walls in bilayer graphene were
extensively studied in view of their potentials in valleytronics. Here, we
study the magnonic version of domain wall excitations in 2D honeycomb
ferromagnetic bilayers (FBL) with collinear order. In particular, we explore
the implications of Dzyaloshinskii-Moriya interaction (DMI) and electrostatic
doping (ED) on the existence and characteristics of 1D magnons confined to
layer stacking domain walls in FBL. The coexistence of DMI and ED is found to
enrich the topology in FBL, yet the corresponding domain wall magnons do not
carry a well-defined valley index. On the other hand, we show that layer
stacking domain walls in DMI-free FBL constitute 1D channels for ballistic
transport of topological valley-polarized magnons. Our theoretical results
raise hope towards magnon valleytronic devices based on atomically thin
topological magnetic materials.",2003.12791v3
2020-04-08,A formalism for magnon gravitational wave detectors,"In order to detect high frequency gravitational waves, we need a new
detection method. In this paper, we develop a formalism for a gravitational
wave detector using magnons in a cavity. Using Fermi normal coordinates and
taking the non-relativistic limit, we obtain a Hamiltonian for magnons in
gravitational wave backgrounds. Given the Hamiltonian, we show how to use the
magnons for detecting high frequency gravitational waves. Furthermore, as a
demonstration of the magnon gravitational wave detector, we give upper limits
on GHz gravitational waves by utilizing known results of magnon experiments for
an axion dark matter search.",2004.04646v3
2020-05-08,Electrical detection of magnon-photon interaction via auxiliary spin wave mode,"We report on the electrical detection of a hybrid magnon-photon system, which
is comprised of a magnetic sample coupled to a planar cavity. While the uniform
Kittel mode has the largest coupling strength among all the magnon modes, it
only generates a modest voltage signal by means of inverse spin-Hall effect. We
have found that the generated voltage can be significantly enhanced by
introducing a higher order magnon mode, which possesses a much higher spin
pumping efficiency and furthermore, it is nearly degenerated with the Kittel
mode. The experimental results can be explained by our theoretical model, and
suggest that the use of an auxiliary magnon mode can realize the configuration
of a magnon-photon system with both strong coupling and large spin current.",2005.04161v1
2020-05-12,Magnonic Quadrupole Topological Insulator in Antiskyrmion Crystals,"When the crystalline symmetries that protect a higher-order topological phase
are not preserved at the boundaries of the sample, gapless hinge modes or
in-gap corner states cannot be stabilized. Therefore, careful engineering of
the sample termination is required. Similarly, magnetic textures, whose quantum
fluctuations determine the supported magnonic excitations, tend to relax to new
configurations that may also break crystalline symmetries when boundaries are
introduced. Here we uncover that antiskyrmion crystals provide an
experimentally accessible platform to realize a magnonic topological quadrupole
insulator, whose hallmark signature are robust magnonic corner states.
Furthermore, we show that tuning an applied magnetic field can trigger the
self-assembly of antiskyrmions carrying a fractional topological charge along
the sample edges. Crucially, these fractional antiskyrmions restore the
symmetries needed to enforce the emergence of the magnonic corner states. Using
the machinery of nested Wilson loops, adapted to magnonic systems supported by
noncollinear magnetic textures, we demonstrate the quantization of the bulk
quadrupole moment, edge dipole moments, and corner charges.",2005.05884v2
2020-07-29,Magnon blockade in a PT-symmetric-like cavity magnomechanical system,"We investigate the magnon blockade effect in a parity-time (PT)
symmetric-like three-mode cavity magnomechanical system involving the
magnon-photon and magnon-phonon interactions. In the broken and unbroken
PT-symmetric regions, we respectively calculate the second-order correlation
function analytically and numerically and further determine the optimal value
of detuning. By adjusting different system parameters, we study the different
blockade mechanisms and find that the perfect magnon blockade effect can be
observed under the weak parameter mechanism. Our work paves a way to achieve
the magnon blockade in experiment.",2007.14645v1
2020-08-04,Quantitative comparison of magnon transport experiments in three-terminal YIG/Pt nanostructures acquired via dc and ac detection techniques,"All-electrical generation and detection of pure spin currents is a promising
way towards controlling the diffusive magnon transport in magnetically ordered
insulators. We quantitatively compare two measurement schemes, which allow to
measure the magnon spin transport in a three-terminal device based on a yttrium
iron garnet thin film. We demonstrate that the dc charge current method based
on the current reversal technique and the ac charge current method utilizing
first and second harmonic lock-in detection can both efficiently distinguish
between electrically and thermally injected magnons. In addition, both
measurement schemes allow to investigate the modulation of magnon transport
induced by an additional dc charge current applied to the center modulator
strip. However, while at low modulator charge current both schemes yield
identical results, we find clear differences above a certain threshold current.
This difference originates from nonlinear effects of the modulator current on
the magnon conductance.",2008.01416v1
2020-08-26,Direct Imaging of Resonant Phonon-Magnon Coupling,"Direct detection of phonons is important for the investigation of information
interconversion between the resonantly coupled magnons and phonons. Here we
report resonant coupling of magnons and phonons, which can be directly
visualized by using micro focused Brillouin light scattering in Ni/LiNbO3
hybrid heterostructures. The patterns of surface acoustic wave phonons,
originating from the interference between the original wave {\psi}0(A_0,k) and
reflected wave {\psi}1(A_1,-k), can be modulated by magnetic field due to the
magnon-phonon coupling. By analyzing the information of phonons obtained from
Brillouin spectroscopy, the properties of the magnon system (Ni film), e.g.,
ferromagnetic resonance field and resonance linewidth can be determined. The
results provide spatially resolved information about phonon manipulation and
detection in a coupled magnon-phonon system.",2008.11391v2
2020-08-26,Magnon bands in twisted bilayer honeycomb quantum magnets,"We study the magnon bands of twisted bilayer honeycomb quantum magnets using
linear spin wave theory. Although the interlayer coupling can be ferromagnetic
or antiferromagnetic, we keep the intralayer one ferromagnetic to avoid
possible frustration. For the interlayer ferromagnetic case, we find the magnon
bands have similar features with the corresponding electronic energy spectrums.
Although the linear dispersions near the Dirac points are preserved in the
magnon bands of twisted bilayer magnets, their slopes are graduately reduced
with the decrease of the twist angles. On the other hand, the interlayer
antiferromagnetic couplings generate quite different magnon spectra. The two
single-layered magnon spectra are usually undecoupled due to the opposite
orientations of the spins in the two layers. We also develop a low-energy
continuous theory for very small twist angles, which has been verified to fit
well with the exact tight-binding calculations. Our results may be
experimentally observed due to the rapid progress in two-dimensional magnetic
materials.",2008.11640v1
2020-11-30,Spin-Wave Doppler Shift by Magnon Drag in Magnetic Insulators,"The Doppler shift of the quasiparticle dispersion by charge currents is
responsible for the critical supercurrents in superconductors and instabilities
of the magnetic ground state of metallic ferromagnets. Here we predict an
analogous effect in thin films of magnetic insulators in which microwaves
emitted by a proximity stripline generate coherent chiral spin currents that
cause a Doppler shift in the magnon dispersion. The spin-wave instability is
suppressed by magnon-magnon interactions that limit spin currents to values
close to but below the threshold for the instability. The spin current
limitations by the backaction of magnon currents on the magnetic order should
be considered as design parameters in magnonic devices.",2011.15008v2
2021-03-16,All-electrical Magnon Transport Experiments in Magnetically Ordered Insulators,"Angular momentum transport is one of the cornerstones of spintronics. Spin
angular momentum is not only transported by mobile charge carriers, but also by
the quantized excitations of the magnetic lattice in magnetically ordered
systems. In this regard, magnetically ordered insulators provide a platform for
magnon spin transport experiments without additional contributions from spin
currents carried by mobile electrons. In combination with charge-to-spin
current conversion processes in conductors with finite spin-orbit coupling it
is possible to realize all-electrical magnon transport schemes in thin film
heterostructures. This review provides an insight into such experiments and
recent breakthroughs achieved. Special attention is given to charge current
based manipulation via an adjacent normal metal of magnon transport in
magnetically ordered insulators in terms of spin-transfer torque. Moreover, the
influence of two magnon modes with opposite spin in antiferromagnetic
insulators on all-electrical magnon transport experiments is discussed.",2103.08996v2
2021-04-22,All-Optical Generation of Antiferromagnetic Magnon Currents via the Magnon Circular Photogalvanic Effect,"We introduce the magnon circular photogalvanic effect enabled by stimulated
Raman scattering. This provides an all-optical pathway to the generation of
directed magnon currents with circularly polarized light in honeycomb
antiferromagnetic insulators. The effect is the leading order contribution to
magnon photocurrent generation via optical fields. Control of the magnon
current by the polarization and angle of incidence of the laser is
demonstrated. Experimental detection by sizeable inverse spin Hall voltages in
platinum contacts is proposed.",2104.10914v2
2021-04-26,Cavity magnomechanical storage and retrieval of quantum states,"We show how a quantum state in a microwave cavity mode can be transferred to
and stored in a phononic mode via an intermediate magnon mode in a
magnomechanical system. For this we consider a ferrimagnetic yttrium iron
garnet (YIG) sphere inserted in a microwave cavity, where the microwave and
magnon modes are coupled via a magnetic-dipole interaction and the magnon and
phonon modes in the YIG sphere are coupled via magnetostrictive forces. By
modulating the cavity and magnon detunings and the driving of the magnon mode
in time, a Stimulated Raman Adiabatic Passage (STIRAP)-like coherent transfer
becomes possible between the cavity mode and the phonon mode. The phononic mode
can be used to store the photonic quantum state for long periods as it
possesses lower damping than the photonic and magnon modes. Thus our proposed
scheme offers a possibility of using magnomechanical systems as quantum memory
for photonic quantum information.",2104.12323v1
2021-05-10,Magnon diffuse scattering in scanning transmission electron microscopy,"We present a theory and a simulation of thermal diffuse scattering due to the
excitation of magnons in scanning transmission electron microscopy. The
calculations indicate that magnons can present atomic contrast when detected by
electron energy-loss spectroscopy using atomic-size electron beams. The results
presented here indicate that the intensity of the magnon diffuse scattering in
bcc iron at 300~K is 4 orders of magnitude weaker than the intensity of thermal
diffuse scattering arising from atomic vibrations. In an energy range where the
phonon and magnon dispersions do not overlap, a monochromated scanning
transmission electron microscope equipped with direct electron detectors for
spectroscopy is expected to resolve the magnon spectral signatures.",2105.04467v1
2021-05-16,Anatomy of inertial magnons in ferromagnets,"We analyze dispersion relations of magnons in ferromagnetic nanostructures
with uniaxial anisotropy taking into account inertial terms, i.e. magnetic
nutation. Inertial effects are parametrized by damping-independent parameter
$\beta$, which allows for an unambiguous discrimination of inertial effects
from Gilbert damping parameter $\alpha$. The analysis of magnon dispersion
relation shows its two branches are modified by the inertial effect, albeit in
different ways. The upper nutation branch starts at $\omega=1/ \beta$, the
lower branch coincides with FMR in the long-wavelength limit and deviates from
the zero-inertia parabolic dependence $\simeq\omega_{FMR}+Dk^2$ of the exchange
magnon. Taking a realistic experimental geometry of magnetic thin films,
nanowires and nanodiscs, magnon eigenfrequencies, eigenvectors and $Q$-factors
are found to depend on the shape anisotropy. The possibility of phase-matched
magneto-elastic excitation of nutation magnons is discussed and the condition
was found to depend on $\beta$, exchange stiffness $D$ and the acoustic
velocity.",2105.07376v1
2021-06-30,Confined Magnons,"Magnetic structures are known to possess magnon excitations confined to their
surfaces and interfaces, but these spatially localized modes are often not
resolved in spectroscopy experiments. We develop a theory to calculate the
confined magnon spectra and its associated spin scattering function, which is
the physical observable in neutron and electron scattering, and a proxy for
photon spectroscopy based on X-ray, Raman and THz sources. We show that extra
anisotropy at the surface or interface plays a key role in magnon confinement.
We obtain analytical expressions for the confinement length scale, and show
that it is qualitatively similar for ferromagnets and antiferromagnets in
dimension d>=2. For d=1 we find remarkable differences between ferromagnetic
and antiferromagnetic models. The theory indicates the presence of several
confined magnon resonances in addition to the usual magnons thought to explain
the excitations of magnetic nanostructures. Detecting these modes may elucidate
the impact of the interface on spin anisotropy and magnetic order.",2106.15771v2
2021-07-27,Nonreciprocal magnon excitations by the Dzyaloshinskii-Moriya interaction on the basis of bond magnetic toroidal multipoles,"The microscopic conditions for the emergence of nonreciprocal magnon
excitations in noncentrosymmetric magnets are theoretically investigated. We
show that asymmetric magnon excitations appear when a bond magnetic toroidal
dipole becomes active, which is defined as a parallel component between the
Dzyaloshinskii-Moriya vector and the averaged spin moments at the ends of the
bonds. Depending on magnetic structures accompanying the bond magnetic toroidal
dipoles, the higher-rank magnetic toroidal multipoles can also be activated in
a magnetic cluster, which describes various angle dependences of asymmetric
magnon excitations. We demonstrate a variety of asymmetric magnon excitations
for two magnetic systems in the noncentrosymmetric lattice structures; a
one-dimensional breathing chain under a uniform crystalline electric field and
a three-dimensional layered breathing kagome structure. We show that a bottom
shift of magnon bands occurs by the magnetic toroidal dipole and a valley
splitting occurs by the magnetic toroidal octupole under magnetic orderings.",2107.12743v1
2021-08-11,Remote generation of magnon Schrödinger cat state via magnon-photon entanglement,"Magnon cat state represents a macroscopic quantum superposition of collective
magnetic excitations of large number spins that not only provides fundamental
tests of macroscopic quantum effects but also finds applications in quantum
metrology and quantum computation. In particular, remote generation and
manipulation of Schr\""{o}dinger cat states are particularly interesting for the
development of long-distance and large-scale quantum information processing.
Here, we propose an approach to remotely prepare magnon even/odd cat states by
performing local non-Gaussian operations on the optical mode that is entangled
with magnon mode through pulsed optomagnonic interaction. By evaluating key
properties of the resulting cat states, we show that for experimentally
feasible parameters they are generated with both high fidelity and
nonclassicality, and with a size large enough to be useful for quantum
technologies. Furthermore, the effects of experimental imperfections such as
the error of projective measurements and dark count when performing
single-photon operations have been discussed, where the lifetime of the created
magnon cat states is expected to be $t\sim1\,\mu$s.",2108.05095v1
2021-10-13,Generation of Bell and GHZ states from a hybrid qubit-photon-magnon system,"We propose a level-resolved protocol in a hybrid architecture for connecting
a superconducting qubit and a magnon mode contained within a microwave cavity
(resonator) to generate the local and global entangled states, enabling a wide
range of applications in quantum communication, quantum metrology, and quantum
information processing. Exploiting the high-degree of controllability in such a
hybrid qubit-photon-magnon system, we derive effective Hamiltonians at the
second- or the third-order resonant points by virtue of the strong
counter-rotating interactions between the resonator and the qubit and between
the resonator and the magnon. Consequently, we can efficiently generate the
Bell states of the photon-magnon and the qubit-magnon subsystems and the
Greenberger-Horne-Zeilinger state of the whole hybrid system. We also check the
robustness of our protocol against the environmental noise by the Lindblad
master equation. Our work makes this hybrid platform of high-degree of
controllability a high-fidelity candidate for the realization of the
maximally-entangled multiple states.",2110.06531v1
2021-11-23,Modeling ultrafast demagnetization and spin transport: the interplay of spin-polarized electrons and thermal magnons,"We theoretically investigate laser-induced spin transport in metallic
magnetic heterostructures using an effective spin transport description that
treats itinerant electrons and thermal magnons on an equal footing.
Electron-magnon scattering is included and taken as the driving force for
ultrafast demagnetization. We assume that in the low-fluence limit the magnon
system remains in a quasi-equilibrium, allowing a transient nonzero magnon
chemical potential. In combination with the diffusive transport equations for
the itinerant electrons, the description is used to chart the full spin
dynamics within the heterostructure. In agreement with recent experiments, we
find that in case the spin-current-receiving material includes an efficient
spin dissipation channel, the interfacial spin current becomes directly
proportional to the temporal derivative of the magnetization. Based on an
analytical calculation, we discuss that other relations between the spin
current and magnetization may arise in case the spin-current-receiving material
displays inefficient spin-flip scattering. Finally, we discuss the role of
(interfacial) magnon transport and show that, a priori, it cannot be neglected.
However, its significance strongly depends on the system parameters.",2111.11961v1
2022-01-02,Tunable Magnonic Chern Bands and Chiral Spin Currents in Magnetic Multilayers,"Realization of novel topological phases in magnonic band structures
represents a new opportunity for the development of spintronics and magnonics
with low power consumption. In this work, we show that in antiparallelly
aligned magnetic multilayers, the long-range, chiral dipolar interaction
generates bulk bands with non-zero Chern integers and magnonic surface states
carrying chiral spin currents. The surface states are strictly localized and
can be easily toggled between non-trivial and trivial phases through an
external magnetic field. The realization of chiral surface spin currents in
this dipolarly coupled heterostructure represents a magnonic implementation of
the coupled wire model that has been extensively explored in electronic
systems. Our work presents an easy-to-implement system for realizing
topological magnonic surface states and low-dissipation spin current transport
in a tunable manner.",2201.00312v2
2022-01-11,Parametric Excitation and Instabilities of Spin Waves driven by Surface Acoustic Waves,"The parametric excitation of spin waves by coherent surface acoustic waves is
demonstrated experimentally in metallic magnetic thin film structures. The
involved magnon modes are analyzed with micro-focused Brillouin light
scattering spectroscopy and complementary micromagnetic simulations combined
with analytical modelling are used to determine the origin of the spin-wave
instabilities. Depending on the experimental conditions, we observe spin-wave
instabilities originating from different phonon-magnon and magnon-magnon
scattering processes. Our results demonstrate that an efficient excitation of
high amplitude, strongly nonlinear magnons in metallic ferromagnets is possible
by surface acoustic waves, which opens novel ways to create micro-scaled
nonlinear magnonic systems for logic and data processing that can profit from
the high excitation efficiency of phonons using piezoelectricity.",2201.04033v2
2022-02-11,"Spin stiffness, spectral weight, and Landau damping of magnons in metallic spiral magnets","We analyze the properties of magnons in metallic electron systems with spiral
magnetic order. Our analysis is based on the random phase approximation for the
susceptibilities of tight binding electrons with a local Hubbard interaction in
two or three dimensions. We identify three magnon branches from poles in the
susceptibilities, one associated with in-plane, the other two associated with
out-of-plane fluctuations of the spiral order parameter. We derive general
expressions for the spin stiffnesses and the spectral weights of the magnon
modes, from which also the magnon velocities can be obtained. Moreover, we
determine the size of the decay rates of the magnons due to Landau damping.
While the decay rate of the in-plane mode is of the order of its excitation
energy, the decay rate of the out-of-plane mode is smaller so that these modes
are asymptotically stable excitations even in the presence of Landau damping.",2202.05660v1
2022-03-13,"Identifying, and constructing, complex magnon band topology","Magnetically ordered materials tend to support bands of coherent propagating
spin wave, or magnon, excitations. Topologically protected surface states of
magnons offer a new path towards coherent spin transport for spintronics
applications. In this work we explore the variety of topological magnon band
structures and provide insight into how to efficiently identify topological
magnon bands in materials. We do this by adapting the topological quantum
chemistry approach that has used constraints imposed by time reversal and
crystalline symmetries to enumerate a large class of topological electronic
bands. We show how to identify physically relevant models of gapped magnon band
topology by using so-called decomposable elementary band representations, and
in turn discuss how to use symmetry data to infer the presence of exotic
symmetry enforced nodal topology.",2203.06678v1
2022-03-23,Topological Hybrids of Magnons and Magnon Bound Pairs,"We consider quantum condensed matter systems without particle-number
conservation. Since the particle number is not a good quantum number, states
belonging to different particle-number sectors can hybridize, which causes
topological anticrossings in the spectrum. The resulting spectral gaps support
chiral edge excitations whose wavefunction is a superposition of states in the
two hybridized sectors. This situation is realized in fully saturated
spin-anisotropic quantum magnets without spin conservation, in which single
magnons hybridize with magnon bound pairs, i.e., two-magnon bound states. The
resulting chiral edge excitations are exotic composites that carry mixed
spin-multipolar character, inheriting spin-dipolar and spin-quadrupolar
character from their single-particleness and two-particleness, respectively. In
contrast to established topological magnons, the topological effects discussed
here are of genuine quantum mechanical origin and vanish in the classical
limit. We discuss implications for both intrinsic anomalous Hall-type transport
and beyond-spintronics computation paradigms. We conclude that fully polarized
quantum magnets are a promising platform for topology caused by hybridizations
between particle-number sectors, complementing the field of ultracold atoms
working with a conserved number of particles.",2203.12374v1
2022-03-31,Thermal Hall responses in frustrated honeycomb spin systems,"We study geometrical responses of magnons driven by a temperature gradient in
frustrated spin systems. While Dzyaloshinskii-Moriya (DM) interactions are
usually incorporated to obtain geometrically nontrivial magnon bands, here we
investigate thermal Hall responses of magnons that do no rely on the DM
interactions. Specifically, we focus on frustrated spin systems with sublattice
degrees of freedom and show that a nonzero Berry curvature requires breaking of
an effective $PT$ symmetry. According to this symmetry consideration, we study
the $J_1$-$J_2$-$J_2^\prime$ Heisenberg models on a honeycomb lattice as a
representative example, and demonstrate that magnons in the spiral phase
support the thermal Hall effect once we introduce a magnetic field and
asymmetry between the two sublattices. We also show that driving the magnons by
a temperature gradient induces spin current generation (i.e., magnon spin
Nernst effect) in the $J_1$-$J_2$-$J_2^\prime$ Heisenberg models.",2203.16853v1
2022-04-04,Generation of twisted magnons via spin-to-orbital angular momentum conversion,"Twisted magnons (TMs) carrying orbital angular momentum (OAM) have attracted
much attention from the magnonic community. The fabrication of such novel
magnon state however is still challenging. Here we present a simple method to
generate TMs with arbitrary radial and azimuthal quantum numbers through the
spin-to-orbital angular momentum conversion. The conversion rate from
plane-wave magnons to twisted ones is shown to be insensitive to the quantum
index. The spectrum of TMs in thin nanodisks is solved analytically, showing a
good agreement with micromagnetic simulations. Moreover, we numerically study
the propagation of TMs in magnetic nanodisk arrays and obtain the quantitative
dependence of the decay length on quantum indexes. Our results are helpful for
realizing TMs with large OAMs that are indispensable for future high-capacity
magnonic communications and computings.",2204.01629v1
2022-04-20,Magnon drag in a metal-insulating antiferromagnet bilayer,"We study a bilayer structure consisting of an antiferromagnetic insulator and
a normal metal. An electron current is driven in the normal metal with
direction parallel to the interface between the materials. Due to interfacial
exchange coupling between the localized spins in the antiferromagnet and the
itinerant electrons in the normal metal, a magnon current can then be induced
in the antiferromagnet. Using an uncompensated antiferromagnetic interface,
creating an asymmetry in the interfacial coupling to the two degenerate magnon
modes, we find that it is possible to generate a magnon spin-current. The
magnon spin-current can be enhanced by increasing the temperature or by
spin-splitting the magnon modes.",2204.09686v2
2022-04-29,All-magnonic Stern-Gerlach effect in antiferromagnets,"The Stern-Gerlach (SG) effect is well known as the spin-dependent splitting
of a beam of atoms carrying magnetic moments by a magnetic-field gradient,
leading to the concept of electron spin. Antiferromagnets can accommodate two
magnon modes with opposite spin polarizations, which is equivalent to the spin
property of electrons. Here, we propose the existence of an all-magnonic SG
effect in antiferromagnetic magnonic system, where a linearly polarized
spin-wave beam is deflected by a straight Dzyaloshinskii-Moriya interaction
(DMI) interface into two opposite polarized spin-wave beams propagating in two
discrete directions. Moreover, we observe bi-focusing of antiferromagnetic spin
waves induced by a curved DMI interface, which can also spatially separate
thermal magnons with opposite polarizations. Our findings provide a unique
perspective to understand the rich phenomena associated with antiferromagnetic
magnon spin and would be helpful for polarization-dependent application of
antiferromagnetic spintronic devices.",2204.13942v1
2022-05-04,Magnon Nernst Effect in Magnon Spin Hall Systems,"Magnon spin Hall systems could hardly show experimentally observable particle
and thermal transport phenomena intrinsically due to the spin cancellation.
Here we demonstrated that the magnon spin Hall systems can exhibit magnon
Nernst effect and thermal Hall effect under external magnetic field by
considering two typical systems, i.e. the antiferromagnetically (AFM) coupled
bilayer honeycomb ferromagnets and monolayer collinear honeycomb
antiferromagnet. The both systems experience magnetic phase transitions from
AFM phase to a field-polarized phase via a spin-flop (SF) phase or directly. In
both systems, there exist magnon Nerst effect and also thermal Hall effect
under a longitudinal temperature gradient, which can be regarded as the
indicator of the magnetic phase transitions, with the Hall conductivity
dependence on magnetic field consistent with the order of magnetic phase
transition.",2205.01891v1
2022-06-02,Bistability in dissipatively coupled cavity magnonics,"Dissipative coupling of resonators arising from their cooperative dampings to
a common reservoir induces intriguingly new physics such as energy level
attraction. In this study, we report the nonlinear properties in a
dissipatively coupled cavity magnonic system. A magnetic material YIG (yttrium
iron garnet) is placed at the magnetic field node of a Fabry-Perot-like
microwave cavity such that the magnons and cavity photons are dissipatively
coupled. Under high power excitation, a nonlinear effect is observed in the
transmission spectra, showing bistable behaviors. The observed bistabilities
are manifested as clockwise, counterclockwise, and butterfly-like hysteresis
loops with different frequency detuning. The experimental results are well
explained as a Duffing oscillator dissipatively coupled with a harmonic one and
the required trigger condition for bistability could be determined
quantitatively by the coupled oscillator model. Our results demonstrate that
the magnon damping has been suppressed by the dissipative interaction, which
thereby reduces the threshold for conventional magnon Kerr bistability. This
work sheds light upon potential applications in developing low power
nonlinearity devices, enhanced anharmonicity sensors and for exploring the
non-Hermitian physics of cavity magnonics in the nonlinear regime.",2206.01231v1
2022-06-30,Second order nonlinearity induced multipartite entanglement in a hybrid magnon cavity QED system,"We present a proposal to produce bipartite and tripartite entanglement in a
hybrid magnon-cavity QED system. Two macroscopic yttrium iron garnet (YIG)
spheres are coupled to a single-mode microwave cavity via magnetic dipole
interaction, while the cavity photons are generated via the two photon process
induced by a pump field. Using the mean field theory, we show that the second
order nonlinearity can result in strong bipartite entanglement between cavity
photons and magnonic modes under two conditions, i.e., $\delta_c
\delta_{m}=2g^2$ and $\delta_c=-\delta_{m}$. For the later one, we also show
the possibility for producing the bipartite entanglement between two magnonic
modes and tripartite entanglement among the cavity photons and two magnonic
modes. Combining these two conditions, we further derive a third condition,
i.e., $\delta_m^2-\phi^2+2g^2=0$, where the tripartite entanglement can be
achieved when two magnonic modes have different resonant frequencies.",2206.15326v1
2022-08-05,Floquet generation of magnonic NOON state,"We propose a concise and deterministic protocol to generate NOON states in a
hybrid system consisting of a superconducting qubit, a circuit resonator mode,
and two magnonic modes, based on Floquet engineering. In particular, we
construct a time-reversal-symmetry broken Hamiltonian for chiral state
propagation of the three continuous-variable modes depending on qubit state, by
the time modulation over qubit-resonator interaction and magnon frequency. Then
an arbitrary magnonic NOON state can be generated by a typical
preparing-and-measurement procedure. We analyze the robustness of our protocol
against the systematic errors in the qubit-magnon coupling strength, the
Floquet-driving intensity, the frequency mismatch of the magnons, and the
counter-rotating interactions. We can obtain a high-fidelity NOON state in the
presence of the quantum dissipation on all components.",2208.02976v2
2022-08-05,Novel magnetic excitations beyond the single- and double-magnons,"Conventional wisdom suggests that one photon that carries one unit of angular
momentum can change the spin angular momentum of a magnetic system with one
unit (delta Ms = +-1) at most. This would imply that a two-photon scattering
process can manipulate the spin angular momentum of the magnetic system with a
maximum of two units. Here we examine the fundamental limit of the
photon-driven transport of angular momentum by studying the magnon spectrum of
{\alpha}-Fe2O3 using resonant inelastic x-ray scattering. We discovered a
cascade of higher-rank magnons carrying double, triple, quadruple, and
quintuple the spin angular momentum of a single-magnon. Guided by theoretical
calculations, we reveal how a two-photons scattering process can create exotic
higher-rank magnons and the relevance of these quasiparticles for magnon-based
applications.",2208.03198v1
2022-08-08,Temperature and Field Dependence of Ferromagnetic Magnon in Monolayer Honeycomb Spin Lattice,"Temperature and field dependence of collective spin excitations or magnon in
monolayer honeycomb spin lattices is investigated using an anisotropic exchange
XZ-Heisenberg model in an external field. Magnetic phase transition in the
presence of the transverse field is the spin reorientation (SR) transition with
magnon intensity existing above the SR temperature. The transverse field either
decreases or sustains the spin-wave intensity in the temperature region below
or above the SR temperature, respectively. The gap of the zero-momentum
low-energy magnon branch closes at the SR transverse field, which is the
critical quantum phase transition field at zero temperature. The application of
the model to a two-dimensional CrI$_3$ explains the existence of the
zero-momentum magnon mode above the Curie temperature and shows the suitable
values of the exchange parameters compared with the DFT calculations. The
estimated magnon velocity near the Dirac point in this material is about 1.74
km/s.",2208.03913v1
2022-09-01,\textit{Ab initio} study on spin fluctuations of itinerant kagome magnet FeSn,"Kagome antiferromagnetic metal FeSn has become an attracting platform for the
exploration of novel electronic states, such as topological Dirac states and
the formation of flat bands by localized electrons. Apart from the electronic
properties, Dirac magnons and flat magnon bands have also been proposed by
applying simplified Heisenberg models to kagome magnetic systems.Inelastic
neutron scattering studies on FeSn found well defined magnon dispersions at low
energies,but magnons at high energies are strongly dampled, which can not be
explained by localized spin models. In this paper, we utilize both linear spin
wave theory and time-dependent density functional perturbation theory to
investigate spin fluctuations of FeSn. Through the comparison of calculated
spin wave spectra and Stoner continuum, we explicitly show that the damping of
magnons at high energies are due to the Landau damping, and the appearance of
high energy optical-magnon like branches at the M and K point are resulted by
relatively low Stoner excitation intensity at those regions.",2209.00187v1
2022-09-07,Nonequilibrium magnons from hot electrons in antiferromagnetic systems,"We describe a \emph{nonthermal} magnon activation mechanism in
antiferromagnetic (AFM) systems via locally equilibrated
\emph{spin-unpolarized} hot electrons excited by an ultrafast intense laser
pulse. We employ a quantum kinetic equation that takes into account a direct
electron-magnon scattering channel in either bulk AFM metal or at the interface
of the AFM/normal-metal heterostructure. The mechanism is responsible for the
nonequilibrium population of AFM magnon modes on a subnanosecond timescale,
which are formed shortly after the local thermalization of hot electrons by
Coulomb interactions. Nonequilibrium magnon populations can be additionally
manipulated by applying an external magnetic field. Our work paves the way
toward spin dynamics control in AFM systems via the ultrafast manipulation of
out-of-equilibrium magnon excitations.",2209.03469v2
2022-12-11,Manifestation of the coupling phase in microwave cavity magnonics,"The interaction between microwave photons and magnons is well understood and
originates from the Zeeman coupling between spins and a magnetic field.
Interestingly, the magnon/photon interaction is accompanied by a phase factor
which can usually be neglected. However, under the rotating wave approximation,
if two magnon modes simultaneously couple with two cavity resonances, this
phase cannot be ignored as it changes the physics of the system. We consider
two such systems, each differing by the sign of one of the magnon/photon
coupling strengths. This simple difference, originating from the various
coupling phases in the system, is shown to preserve, or destroy, two potential
applications of hybrid photon/magnon systems, namely dark mode memories and
cavity-mediated coupling. The observable consequences of the coupling phase in
this system is akin to the manifestation of a discrete Pancharatnam-Berry
phase, which may be useful for quantum information processing.",2212.05389v1
2023-01-23,Magnon-bandgap controllable artificial domain wall waveguide,"In this paper, a magnon-bandgap controllable artificial domain wall waveguide
is proposed by means of micromagnetic simulation. By the investigation of the
propagation behavior and dispersion relationship of spin waves in artificial
domain wall waveguides, it is found that the nonreciprocal propagation of spin
waves in the artificial domain walls are mainly affected by the local effective
exchange field, and the magnon bandgap can be controlled by changing the
maximum value of the effective exchange field. In addition, it is observed that
the artificial domain wall waveguides are structurally more stable than the
natural domain wall waveguides under the same spin wave injection conditions,
and the magnon bandgap of the artificial domain wall waveguides can be adjusted
by its width and magnetic anisotropy parameters. The bandgap controllable
artificial domain wall scheme is beneficial to the miniaturization and
integration of magnon devices and can be applied to future magnonic technology
as a novel frequency filter.",2301.09291v1
2023-02-25,Transmon probe for quantum characteristics of magnons in antiferromagnets,"The detection of magnons and their quantum properties, especially in
antiferromagnetic (AFM) materials, is a substantial step to realize many
ambitious advances in the study of nanomagnetism and the development of energy
efficient quantum technologies. The recent development of hybrid systems based
on superconducting circuits provides the possibility of engineering quantum
sensors that exploit different degrees of freedom. Here, we examine the
magnon-photon-transmon hybridisation based on bipartite AFM materials, which
gives rise to an effective coupling between a transmon qubit and magnons in a
bipartite AFM. We demonstrate how magnetically invisible magnon modes, their
chiralities and quantum properties such as nonlocality and two-mode magnon
entanglement in bipartite AFMs can be characterized through the Rabi frequency
of the superconducting transmon qubit.",2302.13167v1
2023-04-27,Conversion of chiral phonons into magnons in ferromagnets and antiferromagnets,"Chiral phonons with atomic rotations converted into electron spins result in
a change of spin magnetizations in crystals. In this paper, we investigate a
new conversion of chiral phonons into magnons both in ferromagnets and
antiferromagnets by spin models with exchange and Dzyaloshinskii-Moriya
interactions. The atomic rotations in chiral phonons are treated as slow
phonons, which modulate spin-spin interaction and induce time-dependent magnon
excitations due to geometric effect within adiabatic approximation. We
demonstrate that a non-trivial change of the number of magnons requires
breaking of the spin-rotation symmetry around the spin quantization axis. As a
result, the clockwise and counterclockwise chiral phonons induce a change of
the magnon number with opposite signs, which corresponds to an increasing or
decreasing spin magnetizaton due to the chiral nature of the atomic rotations.
In particular, in antiferromagnets, the modulation of magnons due to chiral
phonons generates a non-zero net magnetization by the proposed effect, which is
expect to be observed in experiments.",2304.14000v3
2023-05-17,Spin-wave dynamics controlled by tunable ac magnonic crystal,"The magnonic crystal, which has a spatial modulation wave vector $q$, couples
the spin wave with wave vector $k$ to the one with wave vector $k-q$. For a
conventional magnonic crystal with direct current (dc) supply, the spin waves
around $q/2$ are resonantly coupled to the waves near $-q/2$, and a band gap is
opened at $k=\pm q/2$. If instead of the dc current the magnonic crystal is
supplied with an alternating current (ac), then the band gap is \emph{shifted}
to $k$ satisfying $|\omega_{s}(k)-\omega_{s}(k-q)|=\omega_{ac}$; here
$\omega_{s}(k)$ is the dispersion of the spin wave, while $\omega_{ac}$ is the
frequency of the ac modulation. The resulting gap in the case of the ac
magnonic crystal is the half of the one caused by the dc with the same
amplitude of modulation. The time evolution of the resonantly coupled spin
waves controlled by properly suited ac pulses can be well interpreted as the
motion on a Bloch sphere. The tunability of the ac magnonic crystal broadens
the perspective of spin-wave computing.",2305.09876v1
2023-05-18,Two-tone modulated cavity electromagnonics,"Cavity electromagnonics has increasingly emerged as a new platform for the
fundamental study of quantum mechanics and quantum technologies. Since the
coupling between the microwave field and magnon Kittle modes in current
experiments is much weaker than their resonant frequencies, the anti-rotating
terms in magnon-microwave-photon interaction can be neglected and only the
beam-splitter-like part takes effect. In this situation, the direct generation
of magnonic nonclassical states is impossible, unless other subsystems e.g.
phonons, squeezed photons or superconducting qubits are incorporated. In this
paper, we consider two-tone modulated cavity electromagnonics to keep the
nontrivial anti-rotating terms and obtain tunable phase factors, resulting in
an effective Hamiltonian exactly the same as that of generic linearized cavity
optomechanics. This can therefore be exploited to directly prepare macroscopic
magnonic quantum states, as detailedly exemplified by the generation of steady
and strongly squeezed and entangled states, realize ultra-sensitive
magnon-based sensing by engineering backaction-evading interaction of magnons
and photons, and develop spintronics-related quantum information processing
devices.",2305.10653v1
2023-05-22,Switching of magnon parametric oscillation by magnetic field direction,"Parametric oscillation occurs when the resonance frequency of an oscillator
is periodically modulated. Owing to time-reversal symmetry breaking in magnets,
nonreciprocal magnons can be parametrically excited when spatial-inversion
symmetry breaking is provided. This means that magnons with opposite
propagation directions have different amplitudes. Here we demonstrate switching
on and off the magnon parametric oscillation by reversing the external field
direction applied to a Y$_3$Fe$_5$O$_{12}$ micro-structured film. The result
originates from the nonreciprocity of surface mode magnons, leading to
field-direction dependence of the magnon accumulation under a nonuniform
microwave pumping. Our numerical calculation well reproduces the experimental
result.",2305.12787v2
2023-06-04,Topological phase transitions in a honeycomb ferromagnet with unequal Dzyaloshinskii-Moriya interactions,"This theoretical research is devoted to study topological phase transitions
in a two-dimensional honeycomb ferromagnetic lattice with unequal
Dzyaloshinskii-Moriya interactions for the two sublattices. With the help of a
first-order Green function formalism, we analyze the influence of magnon-magnon
interaction on the magnon band topology. It is found that the existence of the
antichiral Dzyaloshinskii-Moriya interaction can led to a tilting of the
renormalized magnon bands near the Dirac momenta. Then, the renormalized magnon
band gaps at Dirac points have different widths. Through changing the
temperature, we can observe the renormalized magnon band gap closing-reopening
phenomenon, which corresponds to the topological phase transition. Our results
show that the critical temperature of the topological phase transition is
related to the strength of the antichiral Dzyaloshinskii-Moriya interaction.",2306.02505v2
2023-07-04,Magnonic Hong-Ou-Mandel Effect,"We carried out numerical simulations of propagation of spin waves (magnons in
quantum language) in a yttrium-iron garnet film. The numerical model is based
on an original formalism. We demonstrated that a potential barrier for magnons,
created by an Oersted field of a dc current flowing through a wire sitting on
top of the film, is able to act as an electrically controlled partly
transparent mirror for the magnons. We found that the mirror transparency can
be set to 50% by properly adjusting the current strength, thus creating a
semi-transparent mirror. A strong Hong-Ou-Mandel Effect for single magnons is
expected in this configuration. The effect must be seen as two single magnons,
launched simultaneously into the film from two transducers located from the
opposite sides of the mirror, creating a two-microwave-photon state at the
output port of one of the transducers. The probability of seeing those
two-photon states at the output port of either transducer must be the same for
both transducers.",2307.01547v1
2023-07-07,Valley-Selective Phonon-Magnon Scattering in Magnetoelastic Superlattices,"Phonons and magnons are engineered by periodic potential landscapes in
phononic and magnonic crystals, and their combined studies may enable valley
phonon transport tunable by the magnetic field. Through nonreciprocal surface
acoustic wave transmission, we demonstrate valley-selective phonon-magnon
scattering in magnetoelastic superlattices. The lattice symmetry and the
out-of-plane magnetization component control the sign of nonreciprocity. The
phonons in the valleys play a crucial role in generating nonreciprocal
transmission by inducing circularly polarized strains that couple with the
magnons. The transmission spectra show a nonreciprocity peak near a
transmission gap, matching the phononic band structure. Our results open the
way for manipulating valley phonon transport through periodically varying
magnon-phonon coupling.",2307.03367v2
2023-07-19,Enhanced bipartite entanglement and Gaussian quantum steering of squeezed magnon modes,"We theoretically investigate a scheme to entangle two squeezed magnon modes
in a double cavitymagnon system, where both cavities are driven by a two-mode
squeezed vacuum microwave field. Each cavity contains an optical parametric
amplifier as well as a macroscopic yttrium iron garnet (YIG) sphere placed near
the maximum bias magnetic fields such that this leads to the excitation of the
relevant magnon mode and its coupling with the corresponding cavity mode. We
have obtained optimal parameter regimes for achieving the strong magnon-magnon
entanglement and also studied the effectiveness of this scheme towards the
mismatch of both the cavity-magnon couplings and decay parameters. We have also
explored the entanglement transfer efficiency including Gaussian quantum
steering in our proposed system",2307.09846v1
2023-07-23,Spin Space Group Theory and Unconventional Magnons in Collinear Magnets,"Topological magnons have received substantial interest for their potential in
both fundamental research and device applications due to their exotic uncharged
yet topologically protected boundary modes. However, their understanding has
been impeded by the lack of fundamental symmetry descriptions of magnetic
materials, of which the spin Hamiltonians are essentially determined by the
isotropic Heisenberg interaction. The corresponding magnon band structures
allows for more symmetry operations with separated spin and spatial operations,
forming spin space groups (SSGs), than the conventional magnetic space groups.
Here we developed spin space group (SSG) theory to describe collinear magnetic
configurations, identifying all the 1421 collinear SSGs and categorizing them
into four types, constructing band representations for these SSGs, and
providing a full tabulation of SSGs with exotic nodal topology. Our
representation theory perfectly explains the band degeneracies of previous
experiments and identifies new magnons beyond magnetic space groups with
topological charges, including duodecuple point, octuple nodal line and
charge-4 octuple point. With an efficient algorithm that diagnoses topological
magnons in collinear magnets, our work offers new pathways to exploring exotic
phenomena of magnonic systems, with the potential to advance the
next-generation spintronic devices.",2307.12366v1
2023-07-24,"Correlations, disorder, and multi-magnon processes in terahertz spin dynamics of magnetic nanostructures: A first-principles investigation","Understanding the profound impact of correlation effects and crystal
imperfections is essential for an accurate description of solids. Here we study
the role of correlation, disorder, and multi-magnon processes in THz magnons.
Our findings reveal that a significant part of the electron self-energy, which
goes beyond the adiabatic local spin density approximation, arises from the
interaction between electrons and a virtual magnon gas. This interaction leads
to a substantial modification of the exchange splitting and a renormalization
of magnon energies, in agreement with the experimental data. We establish a
quantitative hierarchy of magnon relaxation processes based on first
principles.",2307.12649v3
2023-08-20,Terahertz cavity magnon polaritons,"Hybrid light-matter coupled states, or polaritons, in magnetic materials have
attracted significant attention due to their potential for enabling novel
applications in spintronics and quantum information processing. However, most
studies to date have been carried out for ferromagnetic materials with magnon
excitations at gigahertz frequencies. Here, we have investigated strong
resonant photon-magnon coupling at frequencies above 1 terahertz for the first
time in a prototypical room-temperature antiferromagnetic insulator, NiO,
inside a Fabry-P\'erot cavity. The cavity was formed by the crystal itself when
it was thinned down to an optimized thickness. By using terahertz time-domain
spectroscopy in high magnetic fields up to 25 T, we swept the magnon frequency
through Fabry-P\'erot cavity modes and observed photon-magnon anticrossing
behavior, demonstrating clear vacuum Rabi splittings exceeding the polariton
linewidths. These results show that NiO is a promising platform for exploring
antiferromagnetic spintronics and cavity magnonics in the terahertz frequency
range.",2308.10159v1
2023-08-25,Macroscopic distant magnon modes entanglement via a squeezed reservoir,"The generation of robust entanglement in quantum system arrays is a crucial
aspect of the realization of efficient quantum information processing.
Recently, the field of quantum magnonics has garnered significant attention as
a promising platform for advancing in this direction. In our proposed scheme,
we utilize a one-dimensional array of coupled cavities, with each cavity
housing a single yttrium iron garnet (YIG) sphere coupled to the cavity mode
through magnetic dipole interaction. To induce entanglement between YIGs, we
employ a local squeezed reservoir, which provides the necessary nonlinearity
for entanglement generation. Our results demonstrate the successful generation
of bipartite and tripartite entanglement between distant magnon modes, all
achieved through a single quantum reservoir. Furthermore, the steady-state
entanglement between magnon modes is robust against magnon dissipation rates
and environment temperature. Our results may lead to applications of
cavity-magnon arrays in quantum information processing and quantum
communication systems.",2308.13586v3
2023-09-18,Terahertz magnon frequency comb,"Magnon frequency comb (MFC), the spin-wave spectra composing of equidistant
coherent peaks, is attracting much attention in magnonics. A terahertz (THz)
MFC, combining the advantages of the THz and MFC technologies, is highly
desired because it would significantly advance the MFC applications in
ultrafast magnonic metrology, sensing, and communications. Here, we show that
the THz MFC can be generated by nonlinear interactions between spin waves and
skyrmions in antiferromagnets [Z. Jin \emph{et al}.,
\href{https://doi.org/10.48550/arXiv.2301.03211}{arXiv:2301.03211}]. It is
found that the strength of the three-wave mixing between propagating magnons
and breathing skyrmions follows a linear dependence on the driving frequency
and the MFC signal can be observed over a broad driving frequency range. Our
results extend the working frequency of MFC to the THz regime, which would have
potential applications in ultrafast spintronic devices and promote the
development of nonlinear magnonics in antiferromagnets.",2309.09475v1
2023-09-28,Engineering Entangled Coherent States of Magnons and Phonons via a Transmon Qubit,"We propose a scheme for generating and controlling entangled coherent states
(ECS) of magnons, i.e. the quanta of the collective spin excitations in
magnetic systems, or phonons in mechanical resonators. The proposed hybrid
circuit architecture comprises a superconducting transmon qubit coupled to a
pair of magnonic Yttrium Iron Garnet (YIG) spherical resonators or mechanical
beam resonators via flux-mediated interactions. Specifically, the coupling
results from the magnetic/mechanical quantum fluctuations modulating the qubit
inductor, formed by a superconducting quantum interference device (SQUID). We
show that the resulting radiation-pressure interaction of the qubit with each
mode, can be employed to generate maximally-entangled states of magnons or
phonons. In addition, we numerically demonstrate a protocol for the preparation
of magnonic and mechanical Bell states with high fidelity including realistic
dissipation mechanisms. Furthermore, we have devised a scheme for reading out
the prepared states using standard qubit control and resonator field
displacements. Our work demonstrates an alternative platform for quantum
information using ECS in hybrid magnonic and mechanical quantum networks.",2309.16514v1
2023-10-09,Estimation theory of photon-magnon coupling strength in a driven-dissipative double-cavity-magnon system,"Cavity-magnon systems are emerging as a fruitful architecture for the
integration of quantum technologies and spintronic technologies, where magnons
are coupled to microwave photons via the magnetic-dipole interaction.
Controllable the photon-magnon (P-M) couplings provide a powerful means of
accessing and manipulating quantum states in such hybrid systems. Thus
determining the relevant P-M couplings is a fundamental task. Here we address
the quantum estimation problem for the P-M coupling strength in a
double-cavity-magnon system with drive and dissipation. The effects of various
physical factors on the estimation precision are investigated and the
underlying physical mechanisms are discussed in detail. Considering that in
practical experiments it is almost infeasible to perform measurements on the
global quantum state of this composite system, we identify the optimal
subsystem for performing measurements and estimations. Further, we evaluate the
performance of different Gaussian measurements, indicating that optimal
Gaussian measurement almost saturates the ultimate theoretical bound on the
estimation precision given by the quantum Fisher information.",2310.05412v1
2023-10-26,Effects of Finite Material Size On Axion-magnon Conversion,"Magnetic materials are particularly favorable targets for detecting axions
interacting with electrons because the collective excitation of electron spins,
the magnon, can be excited through the axion-magnon conversion process. It is
often assumed that only the zero-momentum uniformly precessing magnetostatic
(Kittel) mode of the magnon is excited. This is justified if the de Broglie
wavelength of the axion is much longer than the size of the target magnetic
material. However, if the de Broglie wavelength is shorter, finite-momentum
magnon modes can also be excited. We systematically analyze the target material
size dependence of the axion-magnon conversion rate. We discuss the importance
of these effects in the detection of relativistic axions as well as in the
detection of axion dark matter of relatively heavy mass with large material
size.",2310.17704v2
2023-11-10,Nonreciprocal Spin Waves in Nanoscale Domain Walls Detected by Scanning X-ray Microscopy in Perpendicular Magnetic Anisotropic Fe/Gd Multilayers,"Spin wave nonreciprocity in domain walls (DWs) allows for unidirectional
signal processing in reconfigurable magnonic circuits. Using scanning
transmission x-ray microscopy (STXM), we examined coherently-excited magnons
propagating in Bloch-like DWs in amorphous Fe/Gd multilayers with perpendicular
magnetic anisotropy (PMA). Near 1 GHz we detected magnons with short
wavelengths down to $\lambda = 281$ nm in DWs whose minimum width amounted to
$\delta_{\rm DW} = 52$ nm. Consistent with micromagnetic simulations, the STXM
data reveal their nonreciprocal magnon band structures. We identified Bloch
points which disrupted the phase evolution of magnons and induced different
$\lambda$ adjacent to the topological defects. Our observations provide direct
evidence of nonreciprocal spin waves within Bloch-like DWs, serving as
programmable waveguides in magnonic devices with directed information flow.",2311.06186v1
2023-12-12,Topological magnon-polaron transport in a bilayer van der Waals magnet,"The stacking of intrinsically magnetic van der Waals materials provides a
fertile platform to explore tunable transport effects of magnons, presenting
significant prospects for spintronic applications. The possibility of having
topologically nontrivial magnons in these systems can further expand the scope
of exploration. In this work, we consider a bilayer system with intralayer
ferromagnetic exchange and a weak interlayer antiferromagnetic exchange, and
study the topological magnon-polaron excitations induced by magnetoelastic
couplings. Under an applied magnetic field, the system features a metamagnetic
transition, where the magnetic ground state changes from antiparallel layers to
parallel. We show that the metamagnetic transition is accompanied by a
transition of the topological structure of the magnon polarons, which results
in discernible changes in the topology induced transport effects. The
magnetic-field dependence of the thermal Hall conductivity and spin Nernst
coefficient is analyzed with linear response theories.",2312.07463v2
2023-12-19,Macroscopic entanglement between ferrimagnetic magnons and atoms via crossed optical cavity,"We consider a two-dimensional opto-magnomechanical (OMM) system including two
optical cavity modes, a magnon mode, a phonon mode, and a collection of
two-level atoms. In this study, we demonstrate the methodology for generating
stationary entanglement between two-level atoms and magnons, which are
implemented using two optical cavities inside the setup. Additionally, we
investigate the efficiency of transforming entanglement from atom-phonon
entanglement to atom-magnon entanglement. The magnons are stimulated by both a
bias magnetic field and a microwave magnetic field, and they interact with
phonons through the mechanism of magnetostrictive interaction. This interaction
generates magnomechanical displacement, which couples to an optical cavity via
radiation pressure. We demonstrate that by carefully selecting the frequency
detuning of an optical cavity, it is possible to achieve an increase in
bipartite entanglements. Furthermore, this improvement is found to be resistant
to changes in temperature. The entanglement between atoms and magnons plays a
crucial role in the construction of hybrid quantum networks. Our modeling
approach exhibits potential applications in the field of magneto-optical trap
systems as well.",2312.11864v1
2024-01-30,Entropy production rate and correlations of cavity magnomechanical system,"We present the irreversibility generated by a stationary cavity
magnomechanical system composed of a yttrium iron garnet (YIG) sphere with a
diameter of a few hundred micrometers inside a microwave cavity. In this
system, the magnons, i.e., collective spin excitations in the sphere, are
coupled to the cavity photon mode via magnetic dipole interaction and to the
phonon mode via magnetostrictive force (optomechanical-like). We employ the
quantum phase space formulation of the entropy change to evaluate the
steady-state entropy production rate and associated quantum correlation in the
system. We find that the behavior of the entropy flow between the cavity photon
mode and the phonon mode is determined by the magnon-photon coupling and the
cavity photon dissipation rate. Interestingly, the entropy production rate can
increase/decrease depending on the strength of the magnon-photon coupling and
the detuning parameters. We further show that the amount of correlations
between the magnon and phonon modes is linked to the irreversibility generated
in the system for small magnon-photon coupling. Our results demonstrate the
possibility of exploring irreversibility in driven magnon-based hybrid quantum
systems and open a promising route for quantum thermal applications.",2401.16857v1
2024-02-14,Slow-Wave Hybrid Magnonics,"Cavity magnonics is an emerging research area focusing on the coupling
between magnons and photons. Despite its great potential for coherent
information processing, it has been long restricted by the narrow interaction
bandwidth. In this work, we theoretically propose and experimentally
demonstrate a novel approach to achieve broadband photon-magnon coupling by
adopting slow waves on engineered microwave waveguides. To the best of our
knowledge, this is the first time that slow wave is combined with hybrid
magnonics. Its unique properties promise great potentials for both fundamental
research and practical applications, for instance, by deepening our
understanding of the light-matter interaction in the slow wave regime and
providing high-efficiency spin wave transducers. The device concept can be
extended to other systems such as optomagnonics and magnomechanics, opening up
new directions for hybrid magnonics.",2402.08872v1
2024-02-20,Quantum Sensing of Antiferromagnetic Magnon Two-Mode Squeezed Vacuum,"N\'eel ordered antiferromagnets exhibit two-mode squeezing such that their
ground state is a nonclassical superposition of magnon Fock states. Here we
theoretically demonstrate that antiferromagnets can couple to spin qubits via
direct dispersive interaction stemming from, e.g., interfacial exchange. We
demonstrate that this kind of coupling induces a magnon number dependent level
splitting of the excited state resulting in multiple system excitation
energies. This series of level splittings manifests itself as nontrivial
excitation peaks in qubit spectroscopy thereby revealing the underlying
nonclassical magnon composition of the antiferromagnetic quantum state. By
appropriately choosing the drive or excitation energy, the magnonic state can
be controlled via the qubit, suggesting that Fock states of magnon pairs can be
generated deterministically. This enables achieving states useful for quantum
computing and quantum information science protocols.",2402.13203v1
2024-02-23,Magnons and fundamental magnetic interactions in a ferromagnetic monolayer: The case of Ni monolayer,"The experimental investigations of the magnetic interactions in an atomically
thin magnetic layer are essential to understand the physics of low-dimensional
magnets. The full spectrum of collective magnetic excitations (magnons) would
provide an access to these fundamental interactions on the atomic scale. Here
in order to be able to excite the magnons by means of spin-polarized electrons
we couple a Ni monolayer to one and two atomic layers of Co and probe the full
experimental magnon dispersion relation up to the Brillouin zone boundary.
Comparing to the results of ab initio calculations we quantify the complex
pattern of the magnetic exchange interaction in the Ni monolayer. We show that
although the magnons in this system are rather stiff, the Heisenberg exchange
coupling between the Ni spins is weak. We unravel the origin of the observed
large magnon stiffness constant being a consequence of the small spin density
of the Ni atoms.",2402.15251v1
2024-03-12,Antiferromagnetic magnons on a Möbius strip: topology-induced symmetry breaking,"A M\""obius strip comprising of two antiferromagnetically coupled spin chains
showcase exotic magnon excitations with non-trivial physical behavior, which is
solely attributed to the real-space boundary condition even in the absence of
local curvature effects. Counterintuitively, magnons on a M\""obius strip
feature linear polarization of the N\'eel vector devoid of chirality even when
the spin Hamiltonian preserves local rotational symmetry around the anisotropic
easy axis. These linearly-polarized magnons form two non-degenerate branches
that can neither be smoothly connected to nor be decomposed by the
circularly-polarized magnons of opposite chirality commonly found in
antiferromagnets. Only one branch supports standing-wave formation on the
M\""obius strip while the other does not, owing to its spectral shift incurred
by the boundary condition. By unraveling a hitherto unknown mechanism of
topology-induced symmetry breaking, our findings highlight the profound impact
of real-space topology on the physical nature of not only magnons but also
other bosonic quasiparticles.",2403.07846v2
2024-03-13,Thermal Hall effect in a van der Waals ferromagnet CrI3,"CrI3 is a prototypical van der Waals ferromagnet with a magnetic honeycomb
lattice. Previous inelastic neutron scattering studies have suggested
topological nature of its magnetic excitations with a magnon gap at the Dirac
points, which are anticipated to give rise to magnon thermal Hall effect. Here
we report thermal transport properties of CrI3 and show that the long-sought
thermal Hall signal anticipated for topological magnons is fairly small. In
contrast, we find that CrI3 exhibits an appreciable anomalous thermal Hall
signal at lower temperature which may arise from magnon-phonon hybridization or
magnon-phonon scattering. These findings are anticipated to stimulate further
neutron scattering studies on CrI3 single crystal, which can shed light not
only on the intrinsic nature of magnetic excitations but also on the
magnon-phonon interaction.",2403.08180v1
2024-03-17,Electrical reversal of the sign for magnon thermal Hall coefficient in van der Waals bilayer antiferromagnet,"With spin-layer locking, the manipulation of spin degree of freedom via
perpendicular electric field can be realized in a typical antiferromagnetically
coupled bilayer. In analogy to the electric control of the anomalous layer Hall
effect of electron within such bilayer system, we propose here its magnon
counterpart i.e., thermal Hall effect controlled by a perpendicular electric
field. Unlike electrons, magnon is charged neutral and its transport in solids
can be driven by a thermal gradient. It also exhibits Hall response due to the
intrinsic Berry curvature of magnon, analogous to the achievement in electron
system. Taking bilayer 2H-VSe2 with both H-type stacking interlayer
antiferromagnetic coupling as a platform, we perform first-principles
calculations towards the magnetic exchange coupling parameters under applied
electric field perpendicular to the plane. Based on linear spin wave
approximation, we then fit the magnon band structures accordingly and calculate
the corresponding Berry curvature. The thermal Hall coefficient dependence on
the temperature under thermal gradient can be calculated correspondingly in
linear response regime. It is shown that electric field reversal is able to
reverse the sign of the coefficient. These findings provide a platform for the
realization of all-electric magnon spintronics.",2403.11119v1
2002-09-23,Giant magnetothermopower of magnon-assisted transport in ferromagnetic tunnel junctions,"We present a theoretical description of the thermopower due to
magnon-assisted tunneling in a mesoscopic tunnel junction between two
ferromagnetic metals. The thermopower is generated in the course of thermal
equilibration between two baths of magnons, mediated by electrons. For a
junction between two ferromagnets with antiparallel polarizations, the ability
of magnon-assisted tunneling to create thermopower $S_{AP}$ depends on the
difference between the size $\Pi_{\uparrow, \downarrow}$ of the majority and
minority band Fermi surfaces and it is proportional to a temperature dependent
factor $(k_{B}T/\omega_{D})^{3/2}$ where $\omega_{D}$ is the magnon Debye
energy. The latter factor reflects the fractional change in the net
magnetization of the reservoirs due to thermal magnons at temperature $T$
(Bloch's $T^{3/2}$ law). In contrast, the contribution of magnon-assisted
tunneling to the thermopower $S_P$ of a junction with parallel polarizations is
negligible. As the relative polarizations of ferromagnetic layers can be
manipulated by an external magnetic field, a large difference $\Delta S =
S_{AP} - S_P \approx S_{AP} \sim - (k_B/e) f (\Pi_{\uparrow},\Pi_{\downarrow})
(k_BT/\omega_{D})^{3/2}$ results in a magnetothermopower effect. This
magnetothermopower effect becomes giant in the extreme case of a junction
between two half-metallic ferromagnets, $\Delta S \sim - k_B/e$.",0209520v1
2003-10-15,Pions versus Magnons: From QCD to Antiferromagnets and Quantum Hall Ferromagnets,"The low-energy dynamics of pions and magnons -- the Goldstone bosons of the
strong interactions and of magnetism -- are analogous in many ways. The
electroweak interactions of pions result from gauging an SU(2)_L x U(1)_Y
symmetry which then breaks to the U(1)_{em} gauge symmetry of electromagnetism.
The electromagnetic interactions of magnons arise from gauging not only
U(1)_{em} but also the SU(2)_s spin rotational symmetry, with the
electromagnetic fields E and B appearing as non-Abelian vector potentials.
Pions couple to electromagnetism through a Goldstone-Wilczek current that
represents the baryon number of Skyrmions and gives rise to the decay \pi^0 to
\gamma \gamma. Similarly, magnons may couple to an analogue of the
Goldstone-Wilczek current for baby-Skyrmions which induces a magnon-two-photon
vertex. The corresponding analogue of photon-axion conversion is photon-magnon
conversion in an external magnetic field. The baryon number violating decay of
Skyrmions can be catalyzed by a magnetic monopole via the Callan-Rubakov
effect. Similarly, baby-Skyrmion decay can be catalyzed by a charged wire. For
more than two flavors, the Wess-Zumino-Witten term enters the low-energy pion
theory with a quantized prefactor N_c -- the number of quark colors. The magnon
analogue of this prefactor is the anyon statistics angle \theta which need not
be quantized.",0310353v1
2006-07-24,Giant Magnons under NS-NS and Melvin Fields,"The giant magnon is a rotating spiky string configuration which has the same
dispersion relation between the energy and angular momentum as that of a spin
magnon. In this paper we investigate the effects of the NS-NS and Melvin fields
on the giant magnon. We first analyze the energy and angular momenta of the
two-spin spiky D-string moving on the $AdS_3\times S^1$ with the NS-NS field.
Due to the infinite boundary of the AdS spacetime the D-string solution will
extend to infinity and it appears the divergences. After adding the counter
terms we obtain the dispersion relation of the corresponding giant magnon. The
result shows that there will appear a prefactor before the angular momentum, in
addition to some corrections in the sine function. We also see that the spiky
profile of a rotating D-string plays an important role in mapping it to a spin
magnon. We next investigate the energy and angular momentum of the one-spin
spiky fundamental string moving on the $R \times S^2$ with the electric or
magnetic Melvin field. The dispersion relation of the corresponding deformed
giant magnon is also obtained. We discuss some properties of the correction
terms and their relations to the spin chain with deformations.",0607161v4
2008-04-10,Quantum Heisenberg antiferromagnets in a uniform magnetic field: nonanalytic magnetic field dependence of the magnon spectrum,"We reexamine the 1/S-correction to the self-energy of the gapless magnon of a
D-dimensional quantum Heisenberg antiferromagnet in a uniform magnetic field h
using a hybrid approach between 1/S-expansion and non-linear sigma model, where
the Holstein-Primakoff bosons are expressed in terms of Hermitian field
operators representing the uniform and the staggered components of the
spin-operators [N. Hasselmann and P. Kopietz, Europhys. Lett. {\bf{74}}, 1067
(2006)]. By integrating over the field associated with the uniform
spin-fluctuations we obtain the effective action for the staggered
spin-fluctuations on the lattice, which contains fluctuations on all length
scales and does not have the cutoff ambiguities of the non-linear sigma model.
We show that in dimensions D <= 3 the magnetic field dependence of the
spin-wave velocity c(h) is non-analytic in h^2, with c(h) - c(0) proportional
to h^2 \ln | h | in D=3, and c(h) - c(0) proportional to | h | in D=2. The
frequency dependent magnon self-energy is found to exhibit an even more
singular magnetic field dependence, implying a strong momentum dependence of
the quasi-particle residue of the gapless magnon. We also discuss the problem
of spontaneous magnon decay and show that in D > 1 dimensions the damping of
magnons with momentum k is proportional to |k|^{2D -1} if spontaneous magnon
decay is kinematically allowed.",0804.1704v2
2013-05-28,Microscopic observation of magnon bound states and their dynamics,"More than eighty years ago, H. Bethe pointed out the existence of bound
states of elementary spin waves in one-dimensional quantum magnets. To date,
identifying signatures of such magnon bound states has remained a subject of
intense theoretical research while their detection has proved challenging for
experiments. Ultracold atoms offer an ideal setting to reveal such bound states
by tracking the spin dynamics after a local quantum quench with single-spin and
single-site resolution. Here we report on the direct observation of two-magnon
bound states using in-situ correlation measurements in a one-dimensional
Heisenberg spin chain realized with ultracold bosonic atoms in an optical
lattice. We observe the quantum walk of free and bound magnon states through
time-resolved measurements of the two spin impurities. The increased effective
mass of the compound magnon state results in slower spin dynamics as compared
to single magnon excitations. In our measurements, we also determine the decay
time of bound magnons, which is most likely limited by scattering on thermal
fluctuations in the system. Our results open a new pathway for studying
fundamental properties of quantum magnets and, more generally, properties of
interacting impurities in quantum many-body systems.",1305.6598v2
2016-06-15,Topological Magnon Bands in Ferromagnetic Star Lattice,"The experimental observation of topological magnon bands and thermal Hall
effect in a kagom\'e lattice ferromagnet Cu(1-3, bdc) has inspired the search
for topological magnon effects in various insulating ferromagnets that lack an
inversion center allowing a Dzyaloshinskii-Moriya (DM) spin-orbit interaction.
The star lattice (also known as the decorated honeycomb lattice) ferromagnets
is an ideal candidate for this purpose because it is a variant of the kagom\'e
lattice with additional links that connect the up-pointing and down-pointing
triangles. This gives rise to twice the unit cell of the kagom\'e lattice,
hence a more interesting topological magnon effects. In particular, the
triangular bridges on the star lattice can be coupled either ferromagnetically
or antiferromagnetically which is not possible on the kagome lattice
ferromagnets. Here, we study DM-induced topological magnon bands, chiral edge
modes, and thermal magnon Hall effect on the star lattice ferromagnet in
different parameter regimes. The star lattice can also be visualized as the
parent material from which topological magnon bands can be realized for the
kagom\'e and honeycomb lattices in some limiting cases.",1606.04904v8
2017-03-11,Spin-wave dynamics in Permalloy/Cobalt magnonic crystals in the presence of a non-magnetic spacer,"In this paper, we theoretically study the influence of a non-magnetic spacer
between ferromagnetic dots and ferromagnetic matrix on the frequency dispersion
of the spin wave excitations in two-dimensional bi-component magnonic crystals.
By means of the dynamical matrix method we investigate structures inhomogeneous
across the thickness represented by square arrays of Cobalt or Permalloy dots
in a Permalloy matrix. We show that the introduction of a non-magnetic spacer
significantly modifies the total internal magnetic field especially at the
edges of the grooves and dots. This permits the manipulation of the magnonic
band structure of spin waves localized either at the edges of the dots or in
matrix material at the edges of grooves. According to the micromagnetic
simulations two types of end modes were found. The corresponding frequencies
are significantly influenced by the end modes localization region. We also show
that, with the use of a single ferromagnetic material, it is possible to design
a magnonic crystal preserving properties of bi-component magnonic crystals and
magnonic antidot lattices. Finally, the influence of the non-magnetic spacers
on the technologically relevant parameters like group velocity and magnonic
band width are discussed.",1703.03978v1
2012-04-26,Thermal spin pumping mediated by magnon in the semiclassical regime,"We microscopically analyze thermal spin pumping mediated by magnons, at the
interface between a ferromagnetic insulator and a non-magnetic metal, in the
semiclassical regime. The generation of a spin current is discussed by
calculating the thermal spin transfer torque, which breaks the spin
conservation law for conduction electrons and operates the coherent magnon
state. Inhomogeneous thermal fluctuations between conduction electrons and
magnons induce a net spin current, which is pumped into the adjacent
non-magnetic metal. The pumped spin current is proportional to the temperature
difference. When the effective temperature of magnons is lower than that of
conduction electrons, localized spins lose spin angular momentum by emitting
magnons and conduction electrons flip from down to up by absorbing all the
emitted momentum, and vice versa. Magnons at the zero mode cannot contribute to
thermal spin pumping because they are eliminated by the spin-flip condition.
Consequently thermal spin pumping does not cost any kinds of applied magnetic
fields. We have discussed the distinction from the theory proposed by Xiao et
al. [Phys. Rev. B, 81 (2010) 214418], Adachi et al. [Phys. Rev. B, 83 (2011)
094410], and Bender et al. [arXiv:1111.2382].",1204.5811v2
2014-06-23,Propulsion of a domain wall in an antiferromagnet by magnons,"We analyze the dynamics of a domain wall in an easy-axis antiferromagnet
driven by circularly polarized magnons. Magnons pass through a stationary
domain wall without reflection and thus exert no force on it. However, they
reverse their spin upon transmission, thereby transferring two quanta of
angular momentum to the domain wall and causing it to precess. A precessing
domain wall partially reflects magnons back to the source. The reflection of
spin waves creates a previously identified reactive force. We point out a
second mechanism of propulsion, which we term redshift: magnons passing through
a precessing domain wall lower their frequency by twice the angular velocity of
the domain wall; the concomitant reduction of magnons' linear momentum
indicates momentum transfer to the domain wall. We solve the equations of
motion for spin waves in the background of a uniformly precessing domain wall
with the aid of supersymmetric quantum mechanics and compute the net force and
torque applied by magnons to the domain wall. Redshift is the dominant
mechanism of propulsion at low spin-wave intensities; reflection dominates at
higher intensities. We derive a set of coupled algebraic equations to determine
the linear velocity and angular frequency of the domain wall in a steady state.
The theory agrees well with numerical micromagnetic simulations.",1406.6051v1
2017-06-14,Nonlocal magnon-polaron transport in yttrium iron garnet,"The spin Seebeck effect (SSE) is observed in magnetic insulator|heavy metal
bilayers as an inverse spin Hall effect voltage under a temperature gradient.
The SSE can be detected nonlocally as well, viz. in terms of the voltage in a
second metallic contact (detector) on the magnetic film, spatially separated
from the first contact that is used to apply the temperature bias (injector).
Magnon-polarons are hybridized lattice and spin waves in magnetic materials,
generated by the magnetoelastic interaction. Kikkawa et al. [Phys. Rev. Lett.
\textbf{117}, 207203 (2016)] interpreted a resonant enhancement of the local
SSE in yttrium iron garnet (YIG) as a function of the magnetic field in terms
of magnon-polaron formation. Here we report the observation of magnon-polarons
in \emph{nonlocal} magnon spin injection/detection devices for various
injector-detector spacings and sample temperatures. Unexpectedly, we find that
the magnon-polaron resonances can suppress rather than enhance the nonlocal
SSE. Using finite element modelling we explain our observations as a
competition between the SSE and spin diffusion in YIG. These results give
unprecedented insights into the magnon-phonon interaction in a key magnetic
material.",1706.04373v1
2019-09-12,Spin Transport in Thick Insulating Antiferromagnetic Films,"Spin transport of magnonic excitations in uniaxial insulating
antiferromagnets (AFs) is investigated. In linear response to spin biasing and
a temperature gradient, the spin transport properties of
normal-metal--insulating antiferromagnet--normal-metal heterostructures are
calculated. We focus on the thick-film regime, where the AF is thicker than the
magnon equilibration length. This regime allows the use of a drift-diffusion
approach, which is opposed to the thin-film limit considered by Bender {\it et
al.} 2017, where a stochastic approach is justified. We obtain the temperature-
and thickness-dependence of the structural spin Seebeck coefficient
$\mathcal{S}$ and magnon conductance $\mathcal{G}$. In their evaluation we
incorporate effects from field- and temperature-dependent spin conserving
inter-magnon scattering processes. Furthermore, the interfacial spin transport
is studied by evaluating the contact magnon conductances in a microscopic model
that accounts for the sub-lattice symmetry breaking at the interface. We find
that while inter-magnon scattering does slightly suppress the spin Seebeck
effect, transport is generally unaffected, with the relevant spin decay length
being determined by non-magnon-conserving processes such as Gilbert damping. In
addition, we find that while the structural spin conductance may be enhanced
near the spin flip transition, it does not diverge due to spin impedance at the
normal metal|magnet interfaces.",1909.05881v2
2020-12-23,Topological Weyl magnons and thermal Hall effect in layered honeycomb ferromagnets,"In this work, we study the topological properties and magnon Hall effect of a
three-dimensional ferromagnet in the ABC stacking honeycomb lattice, motivated
by the recent inelastic neutron scattering study of CrI$_3$. We show that the
magnon band structure and Chern numbers of the magnon branches are
significantly affected by the interlayer coupling $J_c$, which moreover has a
qualitatively different effect in the ABC stacking compared to the AA stacking
adopted by other authors. The nontrivial Chern number of the lowest magnon band
is stabilized by the next-nearest-neighbour Dzyaloshinsky-Moriya interaction in
each honeycomb layer, resulting in the hopping term similar to that in the
electronic Haldane model for graphene. However, we also find several gapless
Weyl points, separating the non-equivalent Chern insulating phases, tuned by
the ratio of the interlayer coupling $J_c$ and the third-neighbour Heisenberg
interaction $J_3$. We further show that the topological character of magnon
bands results in non-zero thermal Hall conductivity, whose sign and magnitude
depend on $J_c$ and the intra-layer couplings. Since the interlayer coupling
strength $J_c$ can be easily tuned by applying pressure to the quasi-2D
material such as CrI$_3$, this provides a potential route to tuning the magnon
thermal Hall effect in an experiment.",2012.12924v2
2018-03-30,Nonlocal magnon spin transport in yttrium iron garnet with tantalum and platinum spin injection/detection electrodes,"We study the magnon spin transport in the magnetic insulator yttrium iron
garnet (YIG) in a nonlocal experiment and compare the magnon spin excitation
and detection for the heavy metal paramagnetic electrodes platinum (Pt|YIG|Pt)
and tantalum (Ta|YIG|Ta). The electrical injection and detection processes rely
on the (inverse) spin Hall effect in the heavy metals and the conversion
between the electron spin and magnon spin at the heavy metal|YIG interface. Pt
and Ta possess opposite signs of the spin Hall angle. Furthermore, their
heterostructures with YIG have different interface properties, i.e. spin mixing
conductances. By varying the distance between injector and detector, the magnon
spin transport is studied. Using a circuit model based on the
diffusion-relaxation transport theory, a similar magnon relaxation length of ~
10 \mu m was extracted from both Pt and Ta devices. By changing the injector
and detector material from Pt to Ta, the influence of interface properties on
the magnon spin transport has been observed. For Ta devices on YIG the spin
mixing conductance is reduced compared with Pt devices, which is quantitatively
consistent when comparing the dependence of the nonlocal signal on the
injector-detector distance with the prediction from the circuit model.",1803.11382v1
2018-06-11,Excitation of whispering gallery magnons in a magnetic vortex,"One of the most fascinating topics in current quantum physics are hybridised
systems, in which different quantum resonators are strongly coupled. Prominent
examples are circular resonators with high quality factors that allow the
coupling of optical whispering gallery modes to microwave cavities or magnon
resonances in optomagnonics. Whispering gallery modes play a special role in
this endeavour because of their high quality factor and strong localisation,
which ultimately increases the overlap of the wavefunctions of quantum
particles in hybridised systems. The hybridisation with magnons, the collective
quantum excitations of the electron spins in a magnetically ordered material,
is of particular interest because magnons can take over two functionalities:
due to their collective nature they are robust and can serve as a quantum
memory and, moreover, they can act as a wavelength converter between microwave
and THz photons. However, the observation of whispering gallery magnons has not
yet been achieved due to the lack of efficient excitation schemes for magnons
with large wave vectors in a circular geometry. To tackle this problem, we
studied nonlinear 3-magnon scattering as a means to generate whispering gallery
magnons. This Letter discusses the basics of this nonlinear mechanism in a
confined, circular geometry from experimental and theoretical point of view.",1806.03910v1
2018-12-03,Symmetry and localization properties of defect modes in magnonic superlattices,"Symmetry and localization properties of defect modes of a one-dimensional
bi-component magnonic superlattice are theoretically studied. The magnonic
superlattice can be seen as a periodic array of nanostripes, where stripes with
different width, termed as defect stripes, are periodically introduced. By
controlling the geometry of the defect stripes, a transition from dispersive to
practically flat spin-wave defect modes can be observed inside the magnonic
band gaps. It is shown that the spin-wave profile of the defect modes can be
either symmetric or antisymmetric, depending on the geometry of the defect. Due
to the localized character of the defect modes, a particular magnonic
superlattice is proposed, wherein the excitation of either symmetric or
antisymmetric flat magnonic modes is enabled at the same time. Also, it is
demonstrated that the relative frequency position of the antisymmetric mode
inside the band gap does not significantly change with the application of an
external field, while the symmetric modes move to the edges of the frequency
band gaps. The results are complemented by numerical simulations, where an
excellent agreement is observed between both methods. The proposed theory
allows exploring different ways to control the dynamic properties of the defect
modes in metamaterial magnonic superlattices, which can be useful for
applications on multifunctional microwave devices operating over a broad
frequency range.",1812.00897v1
2019-01-24,Fundamentals of magnon-based computing,"A disturbance in the local magnetic order of a solid body can propagate
across a material just like a wave. This wave is named spin wave, and its
quanta are known as magnons. Recently, physicists proposed the usage of magnons
to carry and process information instead of electrons as it is the case of
electronics. This technology opens access to a new generation of computers in
which data are processed without motion of any real particles like electrons.
This leads to a sizable decrease in the accompanying heating losses and,
consequently, to lower energy consumption, which is crucial due to the ever
increasing demand for computing devices. Moreover, unique properties of spin
waves allow for the utilisation of unconventional computing concepts, giving
the vision of a significantly faster and more powerful next-generation of
information processing systems.
  The current review addresses a selection of fundamental topics that form the
basis of the magnon-based computing and are of primary importance for the
further development of this concept. First, the transport of spin-wave-carried
information in one and two dimensions that is required for the realization of
logic elements and integrated magnon circuits is covered. Second, the
convertors between spin waves and electron (charge and spin) currents are
discussed. These convertors are necessary for the compatibility of magnonic
devices with modern CMOS technology. The paper starts with basics on spin waves
and the related methodology. In addition, the general ideas behind magnon-based
computing are presented. The review finishes with conclusions and an outlook on
the perspective use of spin waves.",1901.08934v1
2019-03-10,Magnons at low excitations: Observation of incoherent coupling to a bath of two-level-systems,"Collective magnetic excitation modes, magnons, can be coherently coupled to
microwave photons in the single excitation limit. This allows for access to
quantum properties of magnons and opens up a range of applications in quantum
information processing, with the intrinsic magnon linewidth representing the
coherence time of a quantum resonator. Our measurement system consists of a
yttrium iron garnet (YIG) sphere and a three-dimensional (3D) microwave cavity
at temperatures and excitation powers typical for superconducting quantum
circuit experiments. We perform spectroscopic measurements to determine the
limiting factor of magnon coherence at these experimental conditions. Using the
input-output formalism, we extract the magnon linewidth $\kappa_\mathrm{m}$. We
attribute the limitations of the coherence time at lowest temperatures and
excitation powers to incoherent losses into a bath of near-resonance two-level
systems (TLSs), a generic loss mechanism known from superconducting circuits
under these experimental conditions. We find that the TLSs saturate when
increasing the excitation power from quantum excitation to multi-photon
excitation and their contribution to the linewidth vanishes. At higher
temperatures, the TLSs saturate thermally and the magnon linewidth decreases as
well.",1903.03981v3
2019-11-03,Ferromagnetic dynamics detected via one- and two-magnon NV relaxometry,"The NV center in diamond has proven to be a powerful tool for locally
characterizing the magnetic response of microwave excited ferromagnets. To
date, this has been limited by the requirement that the FMR excitation
frequency be less than the NV spin resonance frequency. Here we report NV
relaxometry based on a two-magnon Raman-like process, enabling detection of FMR
at frequencies higher than the NV frequency. For high microwave drive powers,
we observe an unexpected field-shift of the NV response relative to a
simultaneous microwave absorption signal from a low damping ferrite film. We
show that the field-shifted NV response is due to a second order Suhl
instability. The instability creates a large population of non-equilibrium
magnons which relax the NV spin, even when the uniform mode FMR frequency
exceeds that of the NV spin resonance frequency, hence ruling out the
possibility that the NV is relaxed by a single NV-resonant magnon. We argue
that at high frequencies the NV response is due to a two-magnon relaxation
process in which the difference frequency of two magnons matches the NV
frequency, and at low frequencies we evaluate the lineshape of the one-magnon
NV relaxometry response using spinwave instability theory.",1911.00829v1
2019-12-05,Steering magnonic dynamics and permeability at exceptional points in a parity-time symmetric waveguide,"Tuning the low-energy magnetic dynamics is a key element in designing novel
magnetic metamaterials, spintronic devices and magnonic logic circuits. This
study uncovers a new, highly effective way of controlling the magnetic
permeability via shaping the magnonic properties in coupled magnetic waveguides
separated by current carrying spacer with strong spin-orbit coupling. The
spin-orbit torques exerted on the waveguides leads to an externally tunable
enhancement of magnetic damping in one waveguide and a decreased damping in the
other, constituting so a magnetic parity-time (PT) symmetric system with
emergent magnetic properties at the verge of the exceptional point where
magnetic gains/losses are balanced. In addition to controlling the magnetic
permeability, phenomena inherent to PT-symmetric systems are identified,
including the control on magnon power oscillations, nonreciprocal magnon
propagation, magnon trapping and enhancement as well as the increased
sensitivity to magnetic perturbation and abrupt spin reversal. These
predictions are demonstrated analytically and confirmed by full numerical
simulations under experimentally feasible conditions. The position of the
exceptional points and the strength of the spontaneous PT symmetry breaking can
be tuned by external electric and/or magnetic fields. The roles of the
intrinsic magnetic damping, and the possibility of an electric control via
Dzyaloshinskii-Moriya interaction are exposed and utilized for mode dispersion
shaping and magnon amplification and trapping. The results point to a new route
to designing optomagnonic waveguides, traps, sensors, and circuits.",1912.02500v1
2016-03-11,"Magnon-drag thermopower and Nernst coefficient in Fe, Co, and Ni","Magnon-drag is shown to dominate the thermopower of elemental Fe from 2 to 80
K and of elemental Co from 150 to 600 K; it is also shown to contribute to the
thermopower of elemental Ni from 50 to 500 K. Two theoretical models are
presented for magnon-drag thermopower. One is a hydrodynamic theory based
purely on non-relativistic, Galilean, spin-preserving electron-magnon
scattering. The second is based on spin-motive forces, where the thermopower
results from the electric current pumped by the dynamic magnetization
associated with a magnon heat flux. In spite of their very different
microscopic origins, the two give similar predictions for pure metals at low
temperature, allowing us to semi-quantitatively explain the observed
thermopower of elemental Fe and Co without adjustable parameters. We also find
that magnon-drag may contribute to the thermopower of Ni. A spin-mixing model
is presented that describes the magnon-drag contribution to the Anomalous
Nernst Effect in Fe, again enabling a semi-quantitative match to the
experimental data without fitting parameters. Our work suggests that particle
non-conserving processes may play an important role in other types of drag
phenomena, and also gives a predicative theory for improving metals as
thermoelectric materials.",1603.03736v3
2017-07-27,Electron spin resonance for the detection of long-range spin nematic order,"In this paper we propose that electron spin resonance (ESR) measurements
enable us to detect the long-range spin nematic order. We show that the
frequency of the paramagnetic resonance peak in the ESR spectrum is shifted by
the ferroquadrupolar order parameter together with other quantities. The
ferroquadrupolar order parameter is extractable from the angular dependence of
the frequency shift. In contrast, the antiferroquadrupolar order parameter is
usually invisible in the frequency shift. Instead, the long-range
antiferroquadrupolar order yields a characteristic resonance peak in the ESR
spectrum, which we call a magnon-pair resonance peak. This resonance
corresponds to the excitation of the bound magnon pair at the wave vector $\bm
k={\bm 0}$. Reflecting the condensation of bound magnon pairs, the field
dependence of the magnon-pair resonance frequency shows a singular upturn at
the saturation field. Moreover, the intensity of the magnon-pair resonance peak
shows a characteristic angular dependence and it vanishes when the magnetic
field is parallel to one of the axes that diagonalize the weak anisotropic
interactions. We confirm these general properties of the magnon-pair resonance
peak in the spin nematic phase by studying an $S=1$ bilinear-biquadratic model
on the square lattice in the linear flavor-wave approximation. In addition, we
argue applications to the $S=1/2$ frustrated ferromagnets and also the $S=1/2$
orthogonal dimer spin system SrCu$_2$(BO$_3$)$_2$, both of which are candidate
materials of spin nematics. Our theory for the antiferroquadrupolar ordered
phase is consistent with many features of the magnon-pair resonance peak
experimentally observed in the low-magnetization regime of
SrCu$_2$(BO$_3$)$_2$.",1707.08784v2
2017-11-20,Temperature dependent relaxation of dipole-exchange magnons in yttrium iron garnet films,"Low energy consumption enabled by charge-free information transport, which is
free from ohmic heating, and the ability to process phase-encoded data by
nanometer-sized interference devices at GHz and THz frequencies are just a few
benefits of spin-wave-based technologies. Moreover, when approaching cryogenic
temperatures, quantum phenomena in spin-wave systems pave the path towards
quantum information processing. In view of these applications, the lifetime of
magnons$-$spin-wave quanta$-$is of high relevance for the fields of magnonics,
magnon spintronics and quantum computing. Here, the relaxation behavior of
parametrically excited magnons having wavenumbers from zero up to $6\cdot 10^5
\mathrm{rad~cm}^{-1}$ was experimentally investigated in the temperature range
from 20 K to 340 K in single crystal yttrium iron garnet (YIG) films
epitaxially grown on gallium gadolinium garnet (GGG) substrates as well as in a
bulk YIG crystal$-$the magnonic materials featuring the lowest magnetic damping
known so far. As opposed to the bulk YIG crystal in YIG films we have found a
significant increase in the magnon relaxation rate below 150 K$-$up to 10.5
times the reference value at 340 K$-$in the entire range of probed wavenumbers.
This increase is associated with rare-earth impurities contaminating the YIG
samples with a slight contribution caused by coupling of spin waves to the spin
system of the paramagnetic GGG substrate at the lowest temperatures.",1711.07517v1
2018-10-10,Magnonic Floquet Quantum Spin Hall Insulator in Bilayer Collinear Antiferromagnets,"We study irradiated two-dimensional insulating bilayer honeycomb ferromagnets
and antiferromagnets coupled antiferromagnetically with a zero net
magnetization. The former is realized in the recently synthesized bilayer
honeycomb chromium triiodide CrI$_{\bf 3}$. In both systems, we show that
circularly-polarized electric field breaks time-reversal symmetry and induces a
dynamical Dzyaloshinskii-Moriya interaction in each honeycomb layer. However,
the resulting bilayer antiferromagnetic system still preserves a combination of
time-reversal and space-inversion ($\mathcal{PT}$) symmetry. We show that the
magnon topology of the bilayer antiferromagnetic system is characterized by a
$\pmb{\mathbb{Z}_2}$ Floquet topological invariant. Therefore, the system
realizes a magnonic Floquet quantum spin Hall insulator with spin filtered
magnon edge states. This leads to a non-vanishing Floquet magnon spin Nernst
effect, whereas the Floquet magnon thermal Hall effect vanishes due to
$\mathcal{PT}$ symmetry. We study the rich $\pmb{\mathbb{Z}_2}$ Floquet
topological magnon phase diagram of the system as a function of the light
amplitudes and polarizations. We further discuss the great impact of the
results on future experimental realizations.",1810.04677v4
2018-10-11,"Magnons in a Quasicrystal: Propagation, Localization and Extinction of Spin Waves in Fibonacci Structures","Magnonic quasicrystals exceed the possibilities of spin wave (SW)
manipulation offered by regular magnonic crystals, because of their more
complex SW spectra with fractal characteristics. Here, we report the direct
x-ray microscopic observation of propagating SWs in a magnonic quasicrystal,
consisting of dipolarly coupled permalloy nanowires arranged in a
one-dimensional Fibonacci sequence. SWs from the first and second band as well
as evanescent waves from the band gap between them are imaged. Moreover,
additional mini-band gaps in the spectrum are demonstrated, directly indicating
an influence of the quasiperiodicity of the system. The experimental results
are interpreted using numerical calculations and we deduce a simple model to
estimate the frequency position of the magnonic gaps in quasiperiodic
structures. The demonstrated features of SW spectra in one-dimensional magnonic
quasicrystals allows utilizing this class of metamaterials for magnonics and
makes them an ideal basis for future applications.",1810.04931v2
2020-01-20,Direct observation of 2D magnons in atomically thin CrI$_3$,"Exfoliated chromium triiodide (CrI$_3$) is a layered van der Waals (vdW)
magnetic insulator that consists of ferromagnetic layers coupled through
antiferromagnetic interlayer exchange. The resulting permutations of magnetic
configurations combined with the underlying crystal symmetry produces tunable
magneto-optical phenomena that is unique to the two-dimensional (2D) limit.
Here, we report the direct observation of 2D magnons through magneto-Raman
spectroscopy with optical selection rules that are strictly determined by the
honeycomb lattice and magnetic states of atomically thin CrI$_3$. In
monolayers, we observe an acoustic magnon mode of ~0.3 meV with
cross-circularly polarized selection rules locked to the magnetization
direction. These unique selection rules arise from the discrete conservation of
angular momentum of photons and magnons dictated by threefold rotational
symmetry in a rotational analogue to Umklapp scattering. In bilayers, by tuning
between the layered antiferromagnetic and ferromagnetic-like states, we observe
the switching of two magnon modes. The bilayer structure also enables Raman
activity from the optical magnon mode at ~17 meV (~4.2 THz) that is otherwise
Raman-silent in the monolayer. From these measurements, we quantitatively
extract the spin wave gap, magnetic anisotropy, intralayer and interlayer
exchange constants, and establish 2D magnets as a new system for exploring
magnon physics.",2001.07025v1
2020-03-27,Spatial separation of degenerate components of magnon Bose-Einstein condensate by using a local acceleration potential,"Bose-Einstein condensation (BEC) of magnons is one of few macroscopic quantum
phenomena observable at room temperature. Due to competition of the exchange
and the magnetic dipole interactions the minimum-energy magnon state is doubly
degenerate and corresponds to two antiparallel non-zero wavevectors.
Correspondingly, magnon BEC differs essentially from other condensates, since
it takes place simultaneously at +/-k_min. The degeneracy of BEC and
interaction between its two components have significant impact on the
condensate properties. Phase locking of the two condensates causes formation of
a standing wave of the condensate density and quantized vortices. Additionally,
interaction between the two components is believed to be important for
stabilization of the condensate with respect to the real-space collapse. Thus,
the possibility to create a non-degenerate, single-component condensate is
decisive for understanding of underlying physics of magnon BEC. Here, we
experimentally demonstrate an approach, which allows one to accomplish this
challenging task. We show that this can be achieved by using a separation of
the two components of the degenerate condensate in the real space by applying a
local pulsed magnetic field, which causes their motion in the opposite
directions. Thus, after a certain delay, the two clouds corresponding to
different components become well separated in the real space. We find that the
motion of the clouds can be described well based on the peculiarities of the
magnon dispersion characteristics. Additionally, we show that, during the
motion, the condensate cloud harvests non-condensed magnons, which results in a
partial compensation of the condensate depletion.",2003.12436v1
2020-08-03,Symmetry-resolved two-magnon excitations in a strong spin-orbit-coupled bilayer antiferromagnet,"We used a combination of polarized Raman spectroscopy and spin wave
calculations to study magnetic excitations in the strong spin-orbit-coupled
(SOC) bilayer perovskite antiferromagnet $Sr_3Ir_2O_7$. We observed two broad
Raman features at ~ 800 $cm^{-1}$ and ~ 1400 $cm^{-1}$ arising from magnetic
excitations. Unconventionally, the ~ 800 $cm^{-1}$ feature is fully symmetric
($A_{1g}$) with respect to the underlying tetragonal ($D_{4h}$) crystal lattice
which, together with its broad line shape, definitively rules out the
possibility of a single magnon excitation as its origin. In contrast, the ~
1400 $cm^{-1}$ feature shows up in both the $A_{1g}$ and $B_{2g}$ channels.
From spin wave and two-magnon scattering cross-section calculations of a
tetragonal bilayer antiferromagnet, we identified the ~ 800 $cm^{-1}$ (~ 1400
$cm^{-1}$) feature as two-magnon excitations with pairs of magnons from the
zone-center $\Gamma$ point (zone-boundary van Hove singularity X point). We
further found that this zone-center two-magnon scattering is unique to bilayer
perovskite magnets which host an optical branch in addition to the acoustic
branch, as compared to their single layer counterparts. This zone-center
two-magnon mode is distinct in symmetry from the time-reversal symmetry broken
spin wave gap and phase mode proposed to explain the ~ 92 meV (742 $cm^{-1}$)
gap in RIXS magnetic excitation spectra of $Sr_3Ir_2O_7$.",2008.01052v1
2020-10-26,Ultra-strong photon-to-magnon coupling in multilayered heterostructures involving superconducting coherence via ferromagnetic layers,"The critical step for future quantum industry demands realization of
efficient information exchange between different-platform hybrid systems,
including photonic and magnonic systems, that can harvest advantages of
distinct platforms. The major restraining factor for the progress in certain
hybrid systems is the fundamentally weak coupling parameter between the
elemental particles. This restriction impedes the entire field of hybrid
magnonics by making realization of scalable on-chip hybrid magnonic systems
unattainable. In this work, we propose a general flexible approach for
realization of on-chip hybrid magnonic systems with unprecedentedly strong
coupling parameters. The approach is based on multilayered micro-structures
containing superconducting, insulating and ferromagnetic layers with modified
both photon phase velocities and magnon eigen-frequencies. Phenomenologically,
the enhanced coupling strength is provided by the radically reduced photon mode
volume. The microscopic mechanism of the phonon-to-magnon coupling in studied
systems evidences formation of the long-range superconducting coherence via
thick strong ferromagnetic layers. This coherence is manifested by coherent
superconducting screening of microwave fields by the
superconductor/ferromagnet/superconductor three-layers in presence of
magnetization precession. This discovery offers new opportunities in microwave
superconducting spintronics for quantum technologies.",2010.13553v1
2020-11-12,Interaction-stabilized topological magnon insulator in ferromagnets,"Condensed matter systems admit topological collective excitations above a
trivial ground state, an example being Chern insulators formed by Dirac bosons
with a gap at finite energies. However, in contrast to electrons, there is no
particle-number conservation law for collective excitations. This gives rise to
particle number-nonconserving many-body interactions whose influence on
single-particle topology is an open issue of fundamental interest in the field
of topological quantum materials. Taking magnons in ferromagnets as an example,
we uncover topological magnon insulators that are stabilized by interactions
through opening Chern-insulating gaps in the magnon spectrum. This can be
traced back to the fact that the particle-number nonconserving interactions
break the effective time-reversal symmetry of the harmonic theory. Hence,
magnon-magnon interactions are a source of topology that can introduce chiral
edge states, whose chirality depends on the magnetization direction.
Importantly, interactions do not necessarily cause detrimental damping but can
give rise to topological magnons with exceptionally long lifetimes. We identify
two mechanisms of interaction-induced topological phase transitions---one
driven by an external field, the other by temperature---and show that they
cause unconventional sign reversals of transverse transport signals, in
particular of the thermal Hall conductivity. We identify candidate materials
where this many-body mechanism is expected to occur, such as the metal-organic
kagome-lattice magnet Cu(1,3-benzenedicarboxylate), the van der Waals
honeycomb-lattice magnet CrI$_3$, and the multiferroic kamiokite
(Fe$_2$Mo$_3$O$_8$). Our results demonstrate that interactions can play an
important role in generating nontrivial topology.",2011.06543v1
2021-05-12,Coherent spin-wave transport in an antiferromagnet,"Magnonics is a research field complementary to spintronics, in which the
quanta of spin waves (magnons) replace electrons as information carriers,
promising less energy dissipation. The development of ultrafast nanoscale
magnonic logic circuits calls for new tools and materials to generate coherent
spin waves with frequencies as high, and wavelengths as short, as possible.
Antiferromagnets can host spin waves at THz frequencies and are therefore seen
as a future platform for the fastest and the least dissipative transfer of
information. However, the generation of short-wavelength coherent propagating
magnons in antiferromagnets has so far remained elusive. Here we report the
efficient emission and detection of a nanometer-scale wavepacket of coherent
propagating magnons in antiferromagnetic DyFeO3 using ultrashort pulses of
light. The subwavelength nanoscale confinement of the laser field due to large
absorption creates a strongly non-uniform spin excitation profile, thereby
enabling the propagation of a broadband continuum of coherent THz spin waves.
The wavepacket features magnons with detected wavelengths down to 125 nm and
supersonic velocities up to 13 km/s that propagate over macroscopic distances.
The long-sought source of coherent short-wavelength spin carriers demonstrated
here opens up new prospects for THz antiferromagnetic magnonics and coherence
mediated logic devices at THz frequencies.",2105.05886v1
2021-08-26,Coherent strong-coupling of terahertz magnons and phonons in a Van der Waals antiferromagnetic insulator,"Emergent cooperative motions of individual degrees of freedom, i.e.
collective excitations, govern the low-energy response of system ground states
under external stimulations and play essential roles for understanding
many-body phenomena in low-dimensional materials. The hybridization of distinct
collective modes provides a route towards coherent manipulation of coupled
degrees of freedom and quantum phases. In magnets, strong coupling between
collective spin and lattice excitations, i.e., magnons and phonons, can lead to
coherent quasi-particle magnon polarons. Here, we report the direct observation
of a series of terahertz magnon polarons in a layered zigzag antiferromagnet
FePS3 via far-infrared (FIR) transmission measurements. The characteristic
avoided-crossing behavior is clearly seen as the magnon-phonon detuning is
continuously changed via Zeeman shift of the magnon mode. The coupling strength
g is giant, achieving 120 GHz (0.5 meV), the largest value reported so far.
Such a strong coupling leads to a large ratio of g to the resonance frequency
(g/{\omega}) of 4.5%, and a value of 29 in cooperativity
(g^2/{\gamma}_{ph}{\gamma}_{mag}). Experimental results are well reproduced by
first-principle calculations, where the strong coupling is identified to arise
from phonon-modulated anisotropic magnetic interactions due to spin-orbit
coupling. These findings establish FePS3 as an ideal testbed for exploring
hybridization-induced topological magnonics in two dimensions and the coherent
control of spin and lattice degrees of freedom in the terahertz regime.",2108.11619v1
2021-11-01,Magnon-phonon coupling from a crossing symmetric screened interaction,"The magnon-phonon coupling has received growing attention in recent years due
to its central role in spin caloritronics and the emerging field of acoustic
spintronics. At resonance, this magnetoelastic interaction drives the formation
of magnon polarons, which underpin exotic phenomena such as magnonic heat
currents and phononic spin, but has with a few recent exceptions only been
investigated using mesoscopic spin-lattice models. Motivated to integrate the
magnon-phonon coupling into first-principle many-body electronic structure
theory, we set up to derive the non-relativistic exchange-contribution, which
is more subtle than the spin-orbit contribution, using Schwinger's method of
functional derivatives. To avoid having to solve the famous Hedin-Baym
equations self-consistently, the phonons are treated as a perturbation to the
electronic structure. A formalism is developed around the idea of imposing
crossing symmetry on the interaction, in order to treat charge and spin on
equal footing. By an iterative scheme, we find that the spin-flip component of
the ${\mathit collective}$ four-point interaction, $\mathcal{V}$, which is used
to calculate the magnon spectrum, contains a first-order ""screened T matrix""
part and an arguably more important second-order part, which in the limit of
local spins describes the same processes of phonon emission and absorption as
obtained from phenomenological magnetoelastic models. Here, the ""order"" refers
to the ${\mathit screened}$ ${\mathit collective}$ four-point interaction,
$\mathcal{W}$ - the crossing-symmetric analog of Hedin's $W$.
Proof-of-principle model calculations are performed at varying temperatures for
the isotropic magnon spectrum in three dimensions in the presence of a flat
optical phonon branch.",2111.00908v1
2022-02-25,Direct probing of strong magnon-photon coupling in a planar geometry,"We demonstrate direct probing of strong magnon-photon coupling using
Brillouin light scattering spectroscopy in a planar geometry. The magnonic
hybrid system comprises a split-ring resonator loaded with epitaxial yttrium
iron garnet thin films of 200 nm and 2.46 $\mu$m thickness. The Brillouin light
scattering measurements are combined with microwave spectroscopy measurements
where both biasing magnetic field and microwave excitation frequency are
varied. The cooperativity for the 200 nm-thick YIG films is 4.5, and larger
cooperativity of 137.4 is found for the 2.46 $\mu$m-thick YIG film. We show
that Brillouin light scattering is advantageous for probing the magnonic
character of magnon-photon polaritons, while microwave absorption is more
sensitive to the photonic character of the hybrid excitation. A miniaturized,
planar device design is imperative for the potential integration of magnonic
hybrid systems in future coherent information technologies, and our results are
a first stepping stone in this regard. Furthermore, successfully detecting the
magnonic hybrid excitation by Brillouin light scattering is an essential step
for the up-conversion of quantum signals from the optical to the microwave
regime in hybrid quantum systems.",2202.12696v1
2022-04-07,Gaps in Topological Magnon Spectra: Intrinsic vs. Extrinsic Effects,"For topological magnon spectra, determining and explaining the presence of a
gap at a magnon crossing point is a key to characterize the topological
properties of the system. An inelastic neutron scattering study of a single
crystal is a powerful experimental technique that is widely employed to probe
the magnetic excitation spectra of topological materials. Here, we show that
when the scattering intensity rapidly disperses in the vicinity of a crossing
point, such as a Dirac point, the apparent topological gap size is extremely
sensitive to experimental conditions including sample mosaic, resolution, and
momentum integration range. We demonstrate these effects using comprehensive
neutron-scattering measurements of CrCl$_3$. Our measurements confirm the
gapless nature of the Dirac magnon in CrCl$_3$, but also reveal an artificial,
i.e. extrinsic, magnon gap unless the momentum integration range is carefully
controlled. Our study provides an explanation of the apparent discrepancies
between spectroscopic and first-principles estimates of Dirac magnon gap sizes,
and provides guidelines for accurate spectroscopic measurement of topological
magnon gaps.",2204.03720v1
2022-08-18,Nonlinear magnon control of atomic spin defects in scalable quantum devices,"Ongoing efforts in quantum engineering have recently focused on integrating
magnonics into hybrid quantum architectures for novel functionalities. While
hybrid magnon-quantum spin systems have been demonstrated with nitrogen-vacancy
(NV) centers in diamond, they have remained elusive on the technologically
promising silicon carbide (SiC) platform mainly due to difficulties in finding
a resonance overlap between the magnonic system and the spin centers. Here we
circumvent this challenge by harnessing nonlinear magnon scattering processes
in a magnetic vortex to access magnon modes that overlap in frequency with
silicon-vacancy ($\textrm{V}_{\mathrm{Si}}$) spin transitions in SiC. Our
results offer a route to develop hybrid systems that benefit from marrying the
rich nonlinear dynamics of magnons with the advantageous properties of SiC for
scalable quantum technologies.",2208.09036v1
2022-11-24,Chiral magnons in altermagnetic RuO2,"Magnons in ferromagnets have one chirality, and typically are in the GHz
range and have a quadratic dispersion near the zero wavevector. In contrast,
magnons in antiferromagnets are commonly considered to have bands with both
chiralities that are degenerate across the entire Brillouin zone, and to be in
the THz range and to have a linear dispersion near the center of the Brillouin
zone. Here we theoretically demonstrate a new class of magnons on a
prototypical $d$-wave altermagnet RuO$_2$ with the compensated antiparallel
magnetic order in the ground state. Based on density-functional-theory
calculations we observe that the THz-range magnon bands in RuO$_2$ have an
alternating chirality splitting, similar to the alternating spin splitting of
the electronic bands, and a linear magnon dispersion near the zero wavevector.
We also show that, overall, the Landau damping of this metallic altermagnet is
suppressed due to the spin-split electronic structure, as compared to an
artificial antiferromagnetic phase of the same RuO$_2$ crystal with
spin-degenerate electronic bands and chirality-degenerate magnon bands.",2211.13806v1
2023-02-10,Laser-induced magnonic band gap formation and control in YIG/GaAs heterostructure,"We demonstrate the laser-induced control over spin-wave (SW) transport in the
magnonic crystal (MC) waveguide formed from the semiconductor slab placed on
the ferrite film. We considered bilayer MC with periodical grooves performed on
the top of the n-type gallium arsenide slab side that oriented to the yttrium
iron garnet film. To observe the appearance of magnonic gap induced by laser
radiation, the fabricated structure was studied by the use of microwave
spectroscopy and Brillouin light-scattering. We perform detailed numerical
studies of this structure. We showed that the optical control of the magnonic
gaps (frequency width and position) is related to the variation of the charge
carriers' concentration in GaAs. We attribute these to nonreciprocity of SW
transport in the layered structure. Nonreciprocity was induced by the laser
exposure of the GaAs slab due to SWs' induced electromagnetic field screening
by the optically-generated charge carriers. We showed that SW dispersion,
nonreciprocity, and magnonic band gap position and width in the
ferrite-semiconductor magnonic crystal can be modified in a controlled manner
by laser radiation. Our results show the possibility of the integration of
magnonics and semiconductor electronics on the base of YIG/GaAs structures.",2302.05310v1
2023-02-13,Raman scattering study of multimagnon (bi- and tri-magnon) excitations and rotonlike points in the distorted triangular lattice antiferromagnet,"We investigate the experimental signatures of Raman spectroscopy of bi- and
tri-magnon excitations in the distorted triangular lattice antiferromagnets
alpha-LCr2O4 (L=Sr, Ca). We utilize spin wave theory to analyze the nearly 120
degree spin-3/2 spiral ordered antiferromagnetic ground state to compute the
single-magnon density of states, single-magnon dispersion, and bimagnon and
trimagnon Raman spectra (polarized and unpolarized). It is found that Raman
scattering is capable of capturing the effect of the rotonlike M and M' points
on the bimagnon Raman spectrum. Our calculation confirms the connection between
single-magnon rotonlike excitation energy and bimagnon Raman excitation
spectrum observed experimentally. The roton energy minimum in momentum space is
half of the energy of a bimagnon excitation signal. The experimental magnetic
Raman scattering result displays two peaks which have a Raman shift of 15 meV
and 40 meV, respectively. Theoretical modeling and analysis of the experimental
spectrum of alpha-SrCr2O4 within our distorted Heisenberg Hamiltonian lattice
suggests that the low-energy peak at 15 meV is associated with the bimagnon
excitation, whereas the high-energy peak around 40 meV is primarily a trimagnon
excitation. Based on our fitting procedure we propose a new set of magnetic
interaction parameters for alpha-SrCr2O4. These parameters reproduce not only
the experimental Raman spectrum, but also the inelastic neutron scattering
response (including capturing high energy magnon branches). We also compute the
unpolarized bimagnon and trimagnon Raman spectra for alpha-CaCr2O4.
Furhtermore, we found that the polarization sensitivity of Raman spectrum can
be utilized to distinguish the bi- and tri-magnon excitation channels.",2302.06642v1
2023-03-08,True amplification of spin waves in magnonic nano-waveguides,"Magnonic nano-devices exploit magnons -- quanta of spin waves -- to transmit
and process information within a single integrated platform that has the
potential to outperform traditional semiconductor-based electronics for low
power applications. The main missing cornerstone of this information
nanotechnology is an efficient scheme for the direct amplification of
propagating spin waves. The recent discovery of spin-orbit torque provided an
elegant mechanism for propagation losses compensation. While partial
compensation of the spin-wave damping has allowed for spin-wave signal
modulation, true amplification - the exponential increase in the spin-wave
intensity during propagation - has so far remained elusive. Here we evidence
the operating conditions to achieve unambiguous amplification using clocked
nanoseconds-long spin-orbit torque pulses in sub-micrometer wide magnonic
waveguides, where the effective magnetization has been engineered to be close
to zero to suppress the detrimental magnon-magnon scattering. As a result, we
achieve an exponential increase in the intensity of propagating spin waves up
to 500 % at a propagation distance of several micrometers. These results pave
the way towards the implementation of energy efficient, cascadable magnonic
architectures for wave-based information processing and complex on-chip
computation.",2303.04695v2
2023-04-03,Observation of spin-wave moiré edge and cavity modes in twisted magnetic lattices,"We report the experimental observation of the spin-wave moir\'e edge and
cavity modes using Brillouin light scattering spectro-microscopy in a
nanostructured magnetic moir\'e lattice consisting of two twisted triangle
antidot lattices based on an yttrium iron garnet thin film. Spin-wave moir\'e
edge modes are detected at an optimal twist angle and with a selective
excitation frequency. At a given twist angle, the magnetic field acts as an
additional degree of freedom for tuning the chiral behavior of the magnon edge
modes. Micromagnetic simulations indicate that the edge modes emerge within the
original magnonic band gap and at the intersection between a mini-flatband and
a propagation magnon branch. Our theoretical estimate for the Berry curvature
of the magnon-magnon coupling suggests a non-trivial topology for the chiral
edge modes and confirms the key role played by the dipolar interaction. Our
findings shed light on the topological nature of the magnon edge mode for
emergent moir\'e magnonics.",2304.01001v1
2023-04-19,Magnon-magnon interaction in monolayer MnBi$_2$Te$_4$,"MnBi$_2$Te$_4$, the first confirmed intrinsic antiferromagnetic topological
insulator, has garnered increasing attention in recent years. Here we
investigate the energy correction and lifetime of magnons in MnBi$_2$Te$_4$
caused by magnon-magnon interaction. First, a calculation based on the density
functional theory was performed to get the parameters of the magnetic
Hamiltonian of MnBi$_2$Te$_4$. Subsequently, the perturbation method of
many-body Green's function was employed and the first-order self-energy
[$\Sigma^{(1)}(\bm k)$] and second-order self-energy [$\Sigma^{(2)}(\bm
k,\varepsilon_{\bm k})$] of magnon were obtained. Numerical computations reveal
that the corrections from both $\Sigma^{(1)}(\bm k)$ and $\Sigma^{(2)}(\bm
k,\varepsilon_{\bm k})$ strongly rely on momentum and temperature, with the
energy renormalization near the Brillouin zone (BZ) boundary being
significantly more pronounced than that near the BZ center. Furthermore, our
findings indicate the occurrence of dip structures in the renormalized magnon
spectrum near the $\rm K$ and $\rm M$ points. These dip structures are
determined to be attributed to the influence of $\Sigma^{(2)}(\bm
k,\varepsilon_{\bm k})$.",2304.09637v3
2023-07-20,The challenges of measuring spin Seebeck noise,"Just as electronic shot noise in driven conductors results from the
granularity of charge and the statistical variation in the arrival times of
charge carriers, there are predictions for fundamental noise in magnon currents
due to angular momentum being carried by discrete excitations. The inverse spin
Hall effect as a transduction mechanism to convert spin current into charge
current raises the prospect of experimental investigations of such magnon shot
noise. Spin Seebeck effect measurements have demonstrated the electrical
detection of thermally driven magnon currents and have been suggested as an
avenue for accessing spin current fluctuations. Using spin Seebeck structures
made from yttrium iron garnet on gadolinium gallium garnet, we demonstrate the
technical challenges inherent in such noise measurements. While there is a
small increase in voltage noise in the inverse spin Hall detector at low
temperatures associated with adding a magnetic field, the dependence on field
orientation implies that this is not due to magnon shot noise. We describe
theoretical predictions for the expected magnitude of magnon shot noise,
highlighting ambiguities that exist. Further, we show that magnon shot noise
detection through the standard inverse spin Hall approach is likely impossible
due to geometric factors. Implications for future attempts to measure magnon
shot noise are discussed.",2307.11218v2
2023-08-02,Impulsive Fermi magnon-phonon resonance in antiferromagnetic $CoF_{2}$,"Understanding spin-lattice interactions in antiferromagnets is one of the
most fundamental issues at the core of the recently emerging and booming fields
of antiferromagnetic spintronics and magnonics. Recently, coherent nonlinear
spin-lattice coupling was discovered in an antiferromagnet which opened the
possibility to control the nonlinear coupling strength and thus showing a novel
pathway to coherently control magnon-phonon dynamics. Here, utilizing intense
narrow band terahertz (THz) pulses and tunable magnetic fields up to 7 T, we
experimentally realize the conditions of the Fermi magnon-phonon resonance in
antiferromagnetic $CoF_{2}$. These conditions imply that both the spin and the
lattice anharmonicities harvest energy transfer between the subsystems, if the
magnon eigenfrequency $f_{m}$ is twice lower than the frequency of the phonon
$2f_{m}=f_{ph}$. Performing THz pump-infrared probe spectroscopy in conjunction
with simulations, we explore the coupled magnon-phonon dynamics in the vicinity
of the Fermi-resonance and reveal the corresponding fingerprints of an
impulsive THz-induced response. This study focuses on the role of nonlinearity
in spin-lattice interactions, providing insights into the control of coherent
magnon-phonon energy exchange.",2308.01052v1
2023-12-12,Spatiotemporal visualization of a surface acoustic wave coupled to magnons across a submillimeter-long sample by pulsed laser interferometry,"Surface acoustic waves (SAWs) coupled to magnons have attracted much
attention because they allow for the long-range transport of magnetic
information which cannot be achieved by magnon alone. We employed pulsed laser
interferometry to visualize the entire spatiotemporal dynamics of a SAW that
travels on a nickel (Ni) thin film and is coupled to magnons. It was possible
to trace the coupling-induced amplitude reduction and phase shift that occurs
as the SAW propagates over a distance of 0.4 mm. The observed changes are
consistent with results obtained from conventional radio-frequency transmission
measurements, which probe the total SAW absorption due to magnon--phonon
coupling. This result verifies that our method can accurately capture the
spatiotemporal dynamics of a SAW coupled to magnons across the entire length of
the sample. Additionally, we validated our time-resolved profiles by comparing
them with theoretical results that take the echo wave due to reflection into
account. The impact of the echo wave is significant even when it has propagated
over a distance on the order of millimeters. Our imaging results highlight the
visualization of the long-range propagation of the SAW coupled to magnons and
offer more information about the surface vibration profiles in such devices.",2312.07029v1
2023-12-12,Confined magnon dispersion in ferromagnetic and antiferromagnetic thin films in a second quantization approach: the case of Fe and NiO,"We present a methodology based on the calculation of the inelastic scattering
from magnons via the spin scattering function in confined geometries such as
thin films using a second quantization formalism, for both ferromagnetic and
antiferromagnetic materials. The case studies are chosen with an aim to
demonstrate the effects of film thickness and crystal orientation on magnon
modes, using bcc Fe(100) and NiO with (100) and (111) crystallographic
orientations as prototypical systems. Due to the quantization of the
quasi-momentum we observe a granularity in the inelastic spectra in the
reciprocal space path reflecting the orientation of the thin film. This
approach also allows to capture softer modes that appear due to the partial
interaction of magnetic moments close to the surface in a thin film geometry,
in addition to bulk modes. The softer modes are also affected by
crystallographic orientations as illustrated by the different surface-related
peaks of NiO magnon density of states at approximately ~ 65 meV for (100) and ~
42 meV for (111). Additionally, we explore the role of anisotropy on magnon
modes, revealing that introducing anisotropy to both Fe and NiO films increases
the overall hardness of the magnon modes. The introduction of a surface
anisotropy produces a shift of the surface-related magnon DOS peak to higher
energies with increased surface anisotropy, and in some cases leading to
surface confined mode.",2312.07715v1
2024-03-13,Hybrid magnon-phonon cavity for large-amplitude terahertz spin-wave excitation,"Terahertz (THz) spin waves or their quanta, magnons, can be efficiently
excited by acoustic phonons because these excitations have similar wavevectors
in the THz regime. THz acoustic phonons can be produced using photoacoustic
phenomena but typically have a low population and thus a relatively low
displacement amplitude. The magnetization amplitude and population of the
acoustically excited THz magnons are thus usually small. Using analytical
calculations and dynamical phase-field simulations, we show that a freestanding
metal/magnetic-insulator (MI)/dielectric multilayer can be designed to produce
large-amplitude THz spin wave via cavity-enhanced magnon-phonon interaction.
The amplitude of the acoustically excited THz spin wave in the freestanding
multilayer is predicted to be more than ten times larger than in a
substrate-supported multilayer. Acoustically excited nonlinear magnon-magnon
interaction is demonstrated in the freestanding multilayer. The simulations
also indicate that the magnon modes can be detected by probing the charge
current in the metal layer generated via spin-charge conversion across the
MI/metal interface and the resulting THz radiation. Applications of the
freestanding multilayer in THz optoelectronic transduction are computationally
demonstrated.",2403.08186v1
2017-08-06,Thermally Driven Long Range Magnon Spin Currents in Yttrium Iron Garnet due to Intrinsic Spin Seebeck Effect,"The longitudinal spin Seebeck effect refers to the generation of a spin
current when heat flows across a normal metal/magnetic insulator interface.
Until recently, most explanations of the spin Seebeck effect use the
interfacial temperature difference as the conversion mechanism between heat and
spin fluxes. However, recent theoretical and experimental works claim that a
magnon spin current is generated in the bulk of a magnetic insulator even in
the absence of an interface. This is the so-called intrinsic spin Seebeck
effect. Here, by utilizing a non-local spin Seebeck geometry, we provide
additional evidence that the total magnon spin current in the ferrimagnetic
insulator yttrium iron garnet (YIG) actually contains two distinct terms: one
proportional to the gradient in the magnon chemical potential (pure magnon spin
diffusion), and a second proportional to the gradient in magnon temperature
($\nabla T_m$). We observe two characteristic decay lengths for magnon spin
currents in YIG with distinct temperature dependences: a temperature
independent decay length of ~ 10 ${\mu}$m consistent with earlier measurements
of pure ($\nabla T_m = 0$) magnon spin diffusion, and a longer decay length
ranging from about 20 ${\mu}$m around 250 K and exceeding 80 ${\mu}$m at 10 K.
The coupled spin-heat transport processes are modeled using a finite element
method revealing that the longer range magnon spin current is attributable to
the intrinsic spin Seebeck effect ($\nabla T_m \neq 0$), whose length scale
increases at lower temperatures in agreement with our experimental data.",1708.01941v3
2018-02-27,Strain-Induced Topological Magnon Phase Transitions: Applications to Kagome-Lattice Ferromagnets,"A common feature of topological insulators is that they are characterized by
topologically invariant quantity such as the Chern number and the
$\mathbb{Z}_2$ index. This quantity distinguishes a nontrivial topological
system from a trivial one. A topological phase transition may occur when there
are two topologically distinct phases, and it is usually defined by a gap
closing point where the topologically invariant quantity is ill-defined. In
this paper, we show that the magnon bands in the strained (distorted)
kagome-lattice ferromagnets realize an example of a topological magnon phase
transition in the realistic parameter regime of the system. When spin-orbit
coupling (SOC) is neglected (i.e. no Dzyaloshinskii-Moriya interaction), we
show that all three magnon branches are dispersive with no flat band, and there
exists a critical point where tilted Dirac and semi-Dirac point coexist in the
magnon spectra. The critical point separates two gapless magnon phases as
opposed to the usual phase transition. Upon the inclusion of SOC, we realize a
topological magnon phase transition point at the critical strain
$\delta_c=\frac{1}{2}\big[ 1-(D/J)^2\big]$, where $D$ and $J$ denote the
perturbative SOC and the Heisenberg spin exchange interaction respectively. It
separates two distinct topological magnon phases with different Chern numbers
for $\delta<\delta_c$ and for $\delta>\delta_c$. The associated anomalous
thermal Hall conductivity develops an abrupt change at $\delta_c$, due to the
divergence of the Berry curvature in momentum space. The proposed topological
magnon phase transition is experimentally feasible by applying external
perturbations such as uniaxial strain or pressure.",1802.10095v3
2021-02-05,Quantum engineering with hybrid magnonics systems and materials,"Quantum technology has made tremendous strides over the past two decades with
remarkable advances in materials engineering, circuit design and dynamic
operation. In particular, the integration of different quantum modules has
benefited from hybrid quantum systems, which provide an important pathway for
harnessing the different natural advantages of complementary quantum systems
and for engineering new functionalities. This review focuses on the current
frontiers with respect to utilizing magnetic excitatons or magnons for novel
quantum functionality. Magnons are the fundamental excitations of magnetically
ordered solid-state materials and provide great tunability and flexibility for
interacting with various quantum modules for integration in diverse quantum
systems. The concomitant rich variety of physics and material selections enable
exploration of novel quantum phenomena in materials science and engineering. In
addition, the relative ease of generating strong coupling and forming hybrid
dynamic systems with other excitations makes hybrid magnonics a unique platform
for quantum engineering. We start our discussion with circuit-based hybrid
magnonic systems, which are coupled with microwave photons and acoustic
phonons. Subsequently, we are focusing on the recent progress of magnon-magnon
coupling within confined magnetic systems. Next we highlight new opportunities
for understanding the interactions between magnons and nitrogen-vacancy centers
for quantum sensing and implementing quantum interconnects. Lastly, we focus on
the spin excitations and magnon spectra of novel quantum materials investigated
with advanced optical characterization.",2102.03222v1
2023-06-01,Impact of magnetic anisotropy on the magnon Hanle effect in $α$-Fe$_2$O$_3$,"In easy-plane antiferromagnets, the nature of the elementary excitations of
the spin system is captured by the precession of the magnon pseudospin around
its equilibrium pseudofield, manifesting itself in the magnon Hanle effect.
Here, we investigate the impact of growth-induced changes in the magnetic
anisotropy on this effect in the antiferromagnetic insulator
$\alpha$-Fe$_2$O$_3$ (hematite). To this end, we compare the structural,
magnetic, and magnon-based spin transport properties of $\alpha$-Fe$_2$O$_3$
films with different thicknesses grown by pulsed laser deposition in molecular
and atomic oxygen atmospheres. While in films grown with molecular oxygen a
spin-reorientation transition (Morin transition) is absent down to $10\,$K, we
observe a Morin transition for those grown by atomic-oxygen-assisted
deposition, indicating a change in magnetic anisotropy. Interestingly, even for
a $19\,$nm thin $\alpha$-Fe$_2$O$_3$ film grown with atomic oxygen we still
detect a Morin transition at $125\,$K. We characterize the magnon Hanle effect
in these $\alpha$-Fe$_2$O$_3$ films via all-electrical magnon transport
measurements. The films grown with atomic oxygen show a markedly different
magnon spin signal from those grown in molecular oxygen atmospheres. Most
importantly, the maximum magnon Hanle signal is significantly enhanced and the
Hanle peak is shifted to lower magnetic field values for films grown with
atomic oxygen. These observations suggest a change of magnetic anisotropy for
$\alpha$-Fe$_2$O$_3$ films fabricated by atomic-oxygen-assisted deposition
resulting in an increased oxygen content in these films. Our findings provide
new insights into the possibility to fine-tune the magnetic anisotropy in
$\alpha$-Fe$_2$O$_3$ and thereby to engineer the magnon Hanle effect.",2306.00375v1
2023-10-30,Dynamical renormalisation of a spin Hamiltonian via high-order nonlinear magnonics,"The macroscopic magnetic order in the ground state of solids is determined by
the spin-dependent Hamiltonian of the system. In the absence of external
magnetic fields, this Hamiltonian contains the exchange interaction, which is
of electrostatic origin, and the spin-orbit coupling, whose magnitude depends
on the atomic charge. Spin-wave theory provides a representation of the entire
spectrum of collective magnetic excitations, called magnons, assuming the
interactions to be constant and the number of magnons in the system negligible.
However, the electric field component of light is able to perturb electrostatic
interactions, charge distributions and, at the same time, can create a magnon
population. A fundamental open question therefore concerns the possibility to
optically renormalise the spin Hamiltonian. Here, we test this hypothesis by
using femtosecond laser pulses to resonantly pump electric-dipole-active pairs
of high-energy magnons near the edges of the Brillouin zone. The transient spin
dynamics reveals the activation and a surprising amplification of coherent
low-energy zone-centre magnons, which are not directly driven. Strikingly, the
spectrum of these low-energy magnons differs from the one observed in thermal
equilibrium, the latter being consistent with spin-wave theory. The light-spin
interaction thus results in a room-temperature renormalisation of the magnetic
Hamiltonian, with an estimated modification of the magnetic interactions by 10%
of their ground-state values. We rationalise the observation in terms of a
novel resonant scattering mechanism, in which zone-edge magnons couple
nonlinearly to the zone-centre modes. In a quantum mechanical model, we
analytically derive the corrections to the spectrum due to the photo-induced
magnon population, which are consistent with our experiments. Our results
present a milestone for an all-optical engineering of Hamiltonians.",2310.19667v1
1997-02-18,The NMR relaxation rate of O in undoped Sr_2CuO_2Cl_2: Probing two-dimensional magnons at short distances,"We calculate the nuclear relaxation rate of oxygen in the undoped quasi
two-dimensional quantum Heisenberg antiferromagnet Sr_2CuO_2Cl_2 above the Neel
temperature. The calculation is performed at two-loop order with the help of
the Dyson-Maleev formulation of the spin-wave expansion, taking all scattering
processes involving two and three magnons into account. At low temperatures T
we find 1 / T_1 = c_1 T^3 + c_2 T^4 + O (T^5), and give explicit expressions
for the coefficients c_1 (two-magnon scattering) and c_2 (three magnon
scattering). We compare our result with a recent experiment by Thurber et al.
and show that this experiment directly probes the existence of short-wavelength
magnons in a two-dimensional antiferromagnet.",9702160v2
1997-11-11,Accurate mapping of quantum Heisenberg magnetic models of spin $s$ on strong-coupling magnon systems,"An infinite-$U$ term is introduced into the Holstein-Primakoff-transformed
magnon hamiltonian of quantum Heisenberg magnetic models of spin $s$. This term
removes the unphysical spin wave states on every site and truncates
automatically the expansion in powers of the magnon occupation operator. The
resultant strong-coupling magnon hamiltonians are accurately equivalent to the
original spin hamiltonians. The on-site $U$ levels and their implications are
studied. Within a simple decoupling approximation for our strong-coupling
magnon models we can easily reproduce the results for the (sublattice)
magnetizations obtained previously for the original spin model. But our bosonic
hamiltonians without any unphysical states allow for substantially improved
values for the spectral weight in the ground state and for lower ground-state
energies than those obtained within previous approximations.",9711086v1
1998-05-14,Magnon modes and magnon-vortex scattering in two-dimensional easy-plane ferromagnets,"We calculate the magnon modes in the presence of a vortex in a circular
system, combining analytical calculations in the continuum limit with a
numerical diagonalization of the discrete system. The magnon modes are
expressed by the S-matrix for magnon-vortex scattering, as a function of the
parameters and the size of the system and for different boundary conditions.
Certain quasi-local translational modes are identified with the frequencies
which appear in the trajectory X(t) of the vortex center in recent Molecular
Dynamics simulations of the full many-spin model. Using these quasi-local modes
we calculate the two parameters of a 3rd-order quation of motion for X(t). This
equation was recently derived by a collective variable theory and describes
very well the trajectories observed in the simulations. Both parameters, the
vortex mass and a factor on the third time derivative of X(t), depend strongly
on the boundary conditions.",9805177v1
1998-09-18,Hopping-disorder-induced effects upon the two-Magnon Raman Scattering spectrum in an Antiferromagnet,"Two-magnon Raman scattering intensity is obtained for two-dimensional
spin-1/2 antiferromagnets in presence of hopping disorder. A consistent mode
assignment scheme is prescribed which essentially establishes a correspondence
between the hopping-disordered and the pure system. It is seen that a minute
amount of disorder in hopping leads to a good agreement of the Raman lineshape
experimentally obtained for the copper-oxide insulators such as ${\rm
{La_{2}CuO_{4}}}$. Also it is observed that a considerable asymmetry with
respect to the two-magnon peak appears and the spectral intensity persisting
much beyond $4J$ (where the joint magnon density of states peaks). This has
been argued earlier by us [S. Basu and A. Singh, Phys. Rev. B, {\bf {53}}, 6406
(1996)] to be arising due to highly asymmetric magnon-energy renormalization
because of a cooperative effect arising from local correlation in hopping
disorder.",9809251v1
2001-05-21,"Magnon Heat Transport in (Sr,La)_14Cu_24O_41","We have measured the thermal heat conductivity kappa of the compounds
Sr_14Cu_24O_41 and Ca_9La_5Cu_24O_41 containing doped and undoped spin ladders,
respectively. We find a huge anisotropy of both, the size and the temperature
dependence of kappa which we interpret in terms of a very large heat
conductivity due to the magnetic excitations of the one-dimensional spin
ladders. This magnon heat conductivity decreases with increasing hole doping of
the ladders. The magnon heat transport is analyzed theoretically using a simple
kinetic model. From this analysis we determine the spin gap and the temperature
dependent mean free path of the magnons which ranges by several thousand
angstroms at low temperature. The relevance of several scattering channels for
the magnon transport is discussed.",0105407v1
2001-06-08,Subgap transport in ferromagnet-superconductor junctions due to magnon-assisted Andreev reflection,"We propose a new process of magnon-assisted Andreev reflection at a
ferromagnetic metal - superconductor interface, which consists of the
simultaneous injection of a Cooper pair from the superconductor and the
emission of a magnon inside the ferromagnet. At low temperature this process
represents an additional channel for sub-gap transport across an FS interface,
which lifts restrictions on the current resulting from the necessity to match
spin-polarized current in the ferromagnet with spin-less current in the
superconductor. For a junction between a superconductor and a ferromagnet with
an arbitrary degree of polarization, the inelastic magnon-assisted Andreev
reflection process would manifest itself as a nonlinear addition to the $I(V)$
characteristics which is asymmetric with respect to the sign of the bias
voltage and is related to the density of states of magnons in the ferromagnet.
Expressions for the subgap $I(V)$ characteristics are given for arbitrary
interfacial quality whilst the limiting cases of uniformly transparent and
disordered interfaces are discussed in detail.",0106161v1
2001-12-14,Magnon delocalization in ferromagnetic chains with long-range correlated disorder,"We study one-magnon excitations in a random ferromagnetic Heisenberg chain
with long-range correlations in the coupling constant distribution. By
employing an exact diagonalization procedure, we compute the localization
length of all one-magnon states within the band of allowed energies $E$. The
random distribution of coupling constants was assumed to have a power spectrum
decaying as $S(k)\propto 1/k^{\alpha}$. We found that for $\alpha < 1$,
one-magnon excitations remain exponentially localized with the localization
length $\xi$ diverging as 1/E. For $\alpha = 1$ a faster divergence of $\xi$ is
obtained. For any $\alpha > 1$, a phase of delocalized magnons emerges at the
bottom of the band. We characterize the scaling behavior of the localization
length on all regimes and relate it with the scaling properties of the
long-range correlated exchange coupling distribution.",0112274v1
2003-09-08,Dynamical structure factors of S=1 bond-alternating Heisenberg chains,"We calculate the dynamical structural factor of the S=1 bond-alternating
Heisenberg chain. In the Haldane phase, the lowest excited states form the
lower edge of the multimagnon continuum in $0 \leq q \leq q_c$ and the
one-magnon mode in $q_c \leq q \leq \pi$. As the system approaches the gapless
point, $q_c$ shifts towards $q=\pi$ and the largest integrated intensity of the
one-magnon mode is decreased. In the singlet-dimer phase, the one-magnon mode
appears in $0 \leq q \leq q_c$. As the bond-alternation becomes strong, $q_c$
shifts towards $q=\pi$. In the antiferromagnetic-ferromagnetic bond-alternation
region with a strong ferromagnetic coupling, the lowest excited states form the
lower edge of the multimagnon continuum in $0 \leq q \leq 0.2\pi$ and $0.8\pi
\leq q \leq \pi$, and the one-magnon mode appears in $0.2\pi<q<0.8\pi$. The
largest integrated intensity of the one-magnon mode is 93%, which is slightly
smaller than that in the S=1 Haldane-gap system. We further discuss the
dynamical structural factor in connection with the inelastic neutron-scattering
experiments.",0309178v1
2003-09-19,Exact eigenstates and macroscopic magnetization jumps in strongly frustrated spin lattices,"For a class of frustrated spin lattices including e.g. the 1D sawtooth chain,
the 2D kagom\'e and checkerboard, as well as the 3D pyrochlore lattices we
construct exact product eigenstates consisting of several independent,
localized one-magnon states in a ferromagnetic background. Important
geometrical elements of the relevant lattices are triangles being attached to
polygons or lines. Then the magnons can be trapped on these polygons/lines. If
the concentration of localized magnons is small they can be distributed
randomly over the lattice. Increasing the number of localized magnons their
distribution over the lattice becomes more and more regular and finally the
magnons condensate in a crystal-like state. The physical relevance of these
eigenstates emerges in high magnetic fields where they become groundstates of
the system. As a result a macroscopic magnetization jump appears in the
zero-temperature magnetization curve just below the saturation field. The
height of the jump decreases with increasing spin quantum number and vanishes
in the classical limit. Thus it is a true macroscopic quantum effect.",0309455v1
2003-09-25,Magnon Localization in Mattis Glass,"We study the spectral and transport properties of magnons in a model of a
disordered magnet called Mattis glass, at vanishing average magnetization. We
find that in two dimensional space, the magnons are localized with the
localization length which diverges as a power of frequency at small
frequencies. In three dimensional space, the long wavelength magnons are
delocalized. In the delocalized regime in 3d (and also in 2d in a box whose
size is smaller than the relevant localization length scale) the magnons move
diffusively. The diffusion constant diverges at small frequencies. However, the
divergence is slow enough so that the thermal conductivity of a Mattis glass is
finite, and we evaluate it in this paper. This situation can be contrasted with
that of phonons in structural glasses whose contribution to thermal
conductivity is known to diverge (when inelastic scattering is neglected).",0309594v1
2005-03-01,Berry phase of magnons in textured ferromagnets,"We study the energy spectrum of magnons in a ferromagnet with topologically
nontrivial magnetization profile. In the case of inhomogeneous magnetization
corresponding to a metastable state of ferromagnet, the spin-wave equation of
motion acquires a gauge potential leading to a Berry phase for the magnons
propagating along a closed contour. The effect of magnetic anisotropy is
crucial for the Berry phase: we show that the anisotropy suppresses its
magnitude, which makes the Berry phase observable in some cases, similar to the
Aharonov-Bohm effect for electrons. For example, it can be observed in the
interference of spin waves propagating in mesoscopic rings. We discuss the
effect of domain walls on the interference in ferromagnetic rings, and propose
some experiments with a certain geometry of magnetization. We also show that
the nonvanishing average topological field acts on the magnons like a uniform
magnetic field on electrons. It leads to the quantization of the magnon
spectrum in the topological field.",0503013v1
2005-03-10,Localized-magnon states in strongly frustrated quantum spin lattices,"Recent developments concerning localized-magnon eigenstates in strongly
frustrated spin lattices and their effect on the low-temperature physics of
these systems in high magnetic fields are reviewed. After illustrating the
construction and the properties of localized-magnon states we describe the
plateau and the jump in the magnetization process caused by these states.
Considering appropriate lattice deformations fitting to the localized magnons
we discuss a spin-Peierls instability in high magnetic fields related to these
states. Last but not least we consider the degeneracy of the localized-magnon
eigenstates and the related thermodynamics in high magnetic fields. In
particular, we discuss the low-temperature maximum in the isothermal entropy
versus field curve and the resulting enhanced magnetocaloric effect, which
allows efficient magnetic cooling from quite large temperatures down to very
low ones.",0503243v2
2005-09-21,Bose-Einstein condensation of magnons in Cs$_{2}$CuCl$_{4}$: a dilute gas limit near the saturation magnetic field,"Based on a realistic spin Hamiltonian for a frustrated quasi-two dimensional
spin-1/2 antiferromagnet Cs$_{2}$CuCl$_{4}$, a three-dimensional spin ordering
in the applied magnetic field $B$ near the saturation value $B_{c}$ is studied
within the magnon Bose-Einstein condensation (BEC) scenario. With the use of a
hard-core boson formulation of the spin model, a strongly anysotropic magnon
dispersion in Cs$_{2}$CuCl$_{4}$ is calculated. In the dilute magnon limit near
$B_{c}$, the hard-core boson constraint is resulted in an effective magnon
interaction which is treated in the Hartree-Fock approximation. The critical
temperature $T_{c}$ is calculated as a function of a magnetic field $B$ and
compared with the phase boundary $T_{c}(B) $ experimentally determined in
Cs$_{2}$CuCl$_{4}$ [Phys. Rev. Lett. \textbf{95}, 127202 (2005)].",0509552v1
2005-12-12,Hardcore Magnons in the S=1/2 Heisenberg Model on the Square Lattice,"We propose a versatile approach to treat commonly arising constraints. It is
illustrated for interacting magnons of the Heisenberg antiferromagnet on a
square lattice. For systems of $L\times L$ sites a non-perturbative continuous
unitary transformation (CUT) is used to derive an effective Hamiltonian
conserving the number of magnons. They are bosonic particles with a hardcore
constraint which is captured by a local, repulsive interaction U. The limits
$U\to\infty$ and $L\to\infty$ are achieved by extrapolation. The residual spin
gap $\Delta_1$ is smaller than 0.01J reflecting the gapless nature of the
magnons. The one-magnon dispersion displays all known characteristics.",0512244v2
2006-04-28,Linear independence of localized magnon states,"At the magnetic saturation field, certain frustrated lattices have a class of
states known as ""localized multi-magnon states"" as exact ground states. The
number of these states scales exponentially with the number $N$ of spins and
hence they have a finite entropy also in the thermodynamic limit $N\to \infty$
provided they are sufficiently linearly independent. In this article we present
rigorous results concerning the linear dependence or independence of localized
magnon states and investigate special examples. For large classes of spin
lattices including what we called the orthogonal type and the isolated type as
well as the kagom\'{e}, the checkerboard and the star lattice we have proven
linear independence of all localized multi-magnon states. On the other hand the
pyrochlore lattice provides an example of a spin lattice having localized
multi-magnon states with considerable linear dependence.",0604649v1
2006-04-24,Magnon Bound States and the AdS/CFT Correspondence,"We study the spectrum of asymptotic states in the spin-chain description of
planar N=4 SUSY Yang-Mills. In addition to elementary magnons, the asymptotic
spectrum includes an infinite tower of multi-magnon bound states with exact
dispersion relation, Delta-J_{1} = sqrt{Q^{2}+(lambda/pi^2)sin^2(p/2)}, where
the positive integer Q is the number of constituent magnons. These states
account precisely for the known poles in the exact S-matrix. Like the
elementary magnon, they transform in small representations of supersymmetry and
are present for all values of the 't Hooft coupling. At strong coupling we
identify the dual states in semiclassical string theory.",0604175v2
2006-07-04,Multispin Giant Magnons,"We investigate giant magnons from classical rotating strings in two different
backgrounds. First we generalize the solution of Hofman and Maldacena and
investigate new magnon excitations of a spin chain which are dual to a string
on $R\times S^5$ with two non-vanishing angular momenta. Alowing string
dynamics along the third angle in the five sphere, we find a dispersion
relation that reproduces the Hofman and Maldacena and the one found by Dorey
for the two spin case. In the second part of the paper we generalize the two
""spin"" giant magnon to the case of $\b$-deformed $\axs$ background. We find
agreement between the dispersion relation of the rotating string and the
proposed dispersion relation of the magnon bound state on the spin chain.",0607018v3
2006-08-03,The magnon kinematics of the AdS/CFT correspondence,"The planar dilatation operator of N=4 supersymmetric Yang-Mills is the
hamiltonian of an integrable spin chain whose length is allowed to fluctuate.
We will identify the dynamics of length fluctuations of planar N=4 Yang-Mills
with the existence of an abelian Hopf algebra Z symmetry with non-trivial
co-multiplication and antipode. The intertwiner conditions for this Hopf
algebra will restrict the allowed magnon irreps to those leading to the magnon
dispersion relation. We will discuss magnon kinematics and crossing symmetry on
the spectrum of Z. We also consider general features of the underlying Hopf
algebra with Z as central Hopf subalgebra, and discuss the giant magnon
semiclassical regime.",0608029v2
2006-10-02,Fat Magnon,"We consider a D-brane type state which shares the characteristic of the
recently found giant magnon of Hofman and Maldacena. More specifically we find
a bound state of giant graviton (D3-brane) and giant magnon (F-string), which
has exactly the same anomalous dimension as that of the giant magnon. It is
described by the D3-brane with electric flux which is topologically a $S^3$
elongated by the electric flux. The angular momentum and energy are infinite,
but split sensibly into two parts -- the infinite part precisely the same as
that of the giant magnon and the finite part which can be identified as the
contribution from the giant graviton. We discuss that the corresponding dual
gauge theory operator is not a simple chain type but rather admixture of the
(sub-)determinant and chain types.",0610027v4
2008-04-04,"Spin-charge coupling in a band ferromagnet: magnon-energy reduction, anomalous softening, and damping","The effects of correlation-induced coupling between spin and charge
fluctuations on spin-wave excitations in a band ferromagnet are investigated by
including self-energy and vertex corrections within a systematic
inverse-degeneracy expansion scheme which explicitly preserves the Goldstone
mode. Arising from the scattering of a magnon into intermediate spin-excitation
states (including both magnon and Stoner excitations) accompanied with charge
fluctuations in the majority spin band, this spin-charge coupling results not
only in a substantial reduction of magnon energies but also in anomalous
softening and significant magnon damping for zone-boundary modes lying within
the Stoner gap. Our results are in good qualitative agreement with recent
spin-wave excitation measurements in colossal magneto-resistive manganites and
ferromagnetic ultrathin films of transition metals.",0804.0680v1
2008-11-17,"Giant Magnons in AdS4 x CP3: Embeddings, Charges and a Hamiltonian","This paper studies giant magnons in CP3, which in all known cases are old
solutions from S5 placed into two- and three-dimensional subspaces of CP3,
namely CP1, RP2 and RP3. We clarify some points about these subspaces, and
other potentially interesting three- and four-dimensional subspaces. After
confirming that E-(J1-J4)/2 is a Hamiltonian for small fluctuations of the
relevant 'vacuum' point particle solution, we use it to calculate the
dispersion relation of each of the inequivalent giant magnons. We comment on
the embedding of finite-J solutions, and use these to compare string solutions
to giant magnons in the algebraic curve.",0811.2423v2
2009-02-18,Theory of coherence in Bose-Einstein condensation phenomena in a microwave driven interacting magnon gas,"Strong experimental evidences of the formation of quasi-equilibrium
Bose-Einstein condensation (BEC) of magnons at room temperature in a film of
yttrium iron garnet (YIG) excited by microwave radiation have been recently
reported. Here we present a theory for the magnon gas driven by a microwave
field far out of equilibrium showing that the nonlinear magnetic interactions
create cooperative mechanisms for the onset of a phase transition leading to
the spontaneous generation of quantum coherence and magnetic dynamic order in a
macroscopic scale. The theory provides rigorous support for the formation of a
BEC of magnons in a YIG film magnetized in the plane. We show that the system
develops coherence only when the microwave driving power exceeds a threshold
value and that the theoretical result for the intensity of the Brillouin light
scattering from the BEC as a function of power agrees with the experimental
data. The theory also explains quantitatively experimental measurements of
microwave emission from the uniform mode generated by the confluence of BEC
magnon pairs in a YIG film when the driving power exceeds a critical value.",0902.3138v1
2009-02-18,Dressed Giant Magnons on CP^3,"A new example of AdS/CFT duality relating IIA string theory on AdS_4 x CP^3
to N=6 superconformal Chern-Simons theory has recently been provided by ABJM.
By now a number of papers have considered particular giant magnon classical
string solutions in the CP^3 background, corresponding to excitations in the
spin chain picture of the dual field theory. In this paper we apply the CP^3 =
SU(4)/S(U(3) x U(1)) dressing method to the problem of constructing general
classical string solutions describing various configurations of giant magnons.
As a particular application we present a new giant magnon solution on CP^3.
Interestingly the dressed solution carries only a single SO(6) charge, in
contrast with the dyonic magnons found in previous applications of the dressing
method.",0902.3179v3
2009-03-11,Low-Temperature Heat Transport in the Low-Dimensional Quantum Magnet NiCl_{2}-4SC(NH_{2})_{2},"We report a study of the low-temperature thermal conductivity of
NiCl_{2}-4SC(NH_{2})_{2}, which is a spin-1 chain system exhibiting the magnon
Bose-Einstein condensation (BEC) in magnetic field. It is found that the low-T
thermal conductivity along the spin-chain direction shows strong anomalies at
the lower and upper critical fields of the magnon BEC state. In this state,
magnons act mainly as phonon scatterers at relatively high temperature, but
change their role to heat carriers upon temperature approaching zero. The
result demonstrates a large thermal conductivity in the magnon BEC state and
points to a direct analog between the magnon BEC and the conventional one.",0903.1912v2
2010-03-22,Magnon pairing in quantum spin nematic,"Competing ferro- and antiferromagnetic exchange interactions may lead to the
formation of bound magnon pairs in the high-field phase of a frustrated quantum
magnet. With decreasing field, magnon pairs undergo a Bose-condensation prior
to the onset of a conventional one-magnon instability. We develop an analytical
approach to study the zero-temperature properties of the magnon-pair
condensate, which is a bosonic analog of the BCS superconductors.
Representation of the condensate wave-function in terms of the coherent bosonic
states reveals the spin-nematic symmetry of the ground-state and allows one to
calculate various static properties. Sharp quasiparticle excitations are found
in the nematic state with a small finite gap. We also predict the existence of
a long-range ordered spin-nematic phase in the frustrated chain material
LiCuVO4 at high fields.",1003.4096v3
2010-07-12,Variable damping and coherence in a high-density magnon gas,"We report on the fast relaxation behavior of a high-density magnon gas
created by a parametric amplification process. The magnon gas is probed using
the technique of spin-wave packet recovery by parallel parametric pumping.
Experimental results show a damping behavior which is in disagreement with both
the standard model of exponential decay and with earlier observations of
non-linear damping. In particular, the inherent magnon damping is found to
depend upon the presence of the parametric pumping field. A phenomenological
model which accounts for the dephasing of the earlier injected magnons is in
good agreement with the experimental data.",1007.1895v3
2011-03-09,Magnetization and magnon excitation energies of the magnetic semiconductors EuTe and EuO on the basis of the renormalized spin wave theory,"We present analytic expressions for the temperature dependent magnetization
and magnon dispersion relation of the antiferromagnetic (AFM) EuTe and the
ferromagnetic (FM) EuO. These bulk semiconductors represent concentrated spin
systems for which the interaction between magnons has to be taken into account.
We do this using the renormalized spin wave theory. A higher order Green's
function according to Tjablikov is used for their description. As a result, we
obtain a modified Bloch- T3/2 law, for low temperatures and high magnetic
fields. A full analytic expression is given for the sublattice magnetization of
the AFM EuTe (B0=0), with a N\'eel temperature of TN = 9.81 K. The
magnetization curves {\sigma}N(T) of EuO (for 0 \leq T \leq 0.68TC) and
{\sigma}R(T) (for 0.82TC < T < TC) agree well with experiment. The spin wave
excitation energy perfectly matches experimental data from inelastic neutron
scattering. In the case of EuTe the magnon excitation energies exhibit a
characteristic maximum. For q = 0, an energy gap Eg occurs in the spin wave
spectrum, a consequence of the AFM exchange interaction J2(T). Such energy gaps
exist in the spectrum of magnon excitation energies only in systems with AFM
interactions.",1103.1831v1
2011-03-16,The Semi-Classical Spectrum of Solitons and Giant Magnons,"In this note, we summarize recent progress in constructing and then
semi-classically quantizing solitons, or non-abelian Q-balls, in the symmetric
space sine-Gordon theories. We then consider the images of these solitons in
the related constrained sigma model, which are the dyonic giant magnons on the
string theory world-sheet. Focussing on the case of the symmetric space S^5, we
perform a semi-classical quantization of the solitons and magnons and show that
both lead to Chern-Simons quantum mechanics on the internal moduli space which
is a real Grassmannian SO(4)/SO(2)xSO(2) but---importantly---with a different
coupling constant. Quantizing this system shows that both the Q-balls and
magnons come in a tower of states transforming in symmetric representations of
the SO(4) symmetry group; however, the former come in a finite tower whereas
the latter come in the well-known infinite tower of dyonic giant magnons.",1103.3148v2
2011-05-27,"Magnonic spin-transfer torque MRAM with low power, high speed, and error-free switching","A new class of spin-transfer torque magnetic random access memory (STT-MRAM)
is discussed, in which writing is achieved using thermally initiated magnonic
current pulses as an alternative to conventional electric current pulses. The
magnonic pulses are used to destabilize the magnetic free layer from its
initial direction, and are followed immediately by a bipolar electric current
exerting conventional spin-transfer torque on the free layer. The combination
of thermal and electric currents greatly reduces switching errors, and
simultaneously reduces the electric switching current density by more than an
order of magnitude as compared to conventional STT-MRAM. The energy efficiency
of several possible electro-thermal circuit designs have been analyzed
numerically. As compared to STT-MRAM with perpendicular magnetic anisotropy,
magnonic STT-MRAM reduces the overall switching energy by almost 80%.
Furthermore, the lower electric current density allows the use of thicker
tunnel barriers, which should result in higher tunneling magneto-resistance and
improved tunnel barrier reliability. The combination of lower power, improved
reliability, higher integration density, and larger read margin make magnonic
STT-MRAM a promising choice for future non-volatile storage.",1105.5473v1
2011-07-06,Edge magnetism of Heisenberg model on honeycomb lattice,"In our previous study, a single-branch of ferromagnetic magnon with linear
dispersion is shown to exist near the (uncompensated) zigzag edge for
Heisenberg model on honeycomb lattice. Here we develop a field-theory
description for the edge magnon and find its dynamics is captured by the
one-dimensional relativistic Klein-Gordon equation. It is intriguing that the
boundary field theory for the edge magnon is tied up with its bulk counterpart,
described by the two-dimensional Klein-Gordon equation. Furthermore, we also
reveal how the parity symmetry relates evanescent modes on opposite edges in a
honeycomb nanoribbon. By employing alternative methods, including Schwinger
bosons and density-matrix renormalization group, we also demonstrate that the
relativistic edge magnon is robust even when the Neel order in the bulk is
destroyed by quantum fluctuations. The edge magnon is a direct consequence of
uncompensated edge and may be verified in realistic materials by experimental
probes.",1107.1102v2
2012-03-07,Magnonic band gap design by the edge modulation of micro-sized waveguides,"The potential to control the number of the spin-wave band gaps of a magnonic
crystal (MC) by variation of its geometry is investigated by numerical
simulations. The magnonic crystal is represented by a micro-sized planar
ferromagnetic waveguide with periodically variable width. By choosing a
step-like or sinusoidal variation of the width, the magnonic crystal reveals
multiple or single band gaps, respectively. This allows for additional degrees
of freedom in the design of MC based microwave filters and phase shifters with
desired characteristics. The MCs band gaps have been studied in the space and
frequency domains exploring the spin-wave spectrum dependence on the probing
position inside the magnonic crystal.",1203.1469v1
2012-06-28,Thermalization of magnons in yttrium-iron garnet: nonequilibrium functional renormalization group approach,"Using a nonequilibrium functional renormalization group (FRG) approach we
calculate the time evolution of the momentum distribution of a magnon gas in
contact with a thermal phonon bath. As a cutoff for the FRG procedure we use a
hybridization parameter {\Lambda} giving rise to an artificial damping of the
phonons. Within our truncation of the FRG flow equations the time evolution of
the magnon distribution is obtained from a rate equation involving
cutoff-dependent nonequilibrium self-energies, which in turn satisfy FRG flow
equations depending on cutoff-dependent transition rates. Our approach goes
beyond the Born collision approximation and takes the feedback of the magnons
on the phonons into account. We use our method to calculate the thermalization
of a quasi two-dimensional magnon gas in the magnetic insulator yttrium-iron
garnet after a highly excited initial state has been generated by an external
microwave field. We obtain good agreement with recent experiments.",1206.6689v3
2012-08-03,Field-dependence of magnon decay in yttrium iron garnet thin films,"We discuss threshold field-dependence of the decay rate of the uniform magnon
mode in yttrium iron garnet (YIG) thin films. We demonstrate that decays must
cease to exist in YIG films of thickness less than 1 \mu m, the lengthscale
defined by the exchange length. We show that due to the symmetry of the
three-magnon coupling the decay rate is linear in \Delta H=(Hc-H) in the
vicinity of the threshold field Hc instead of the step-like \Gamma
\Theta(\Delta H) expected from the two-dimensional character of magnon
excitations in such films. For thicker films, the decay rate should exhibit
multiple steps due to thresholds for decays into a sequence of the
two-dimensional magnon bands. For yet thicker films, such thresholds merge and
crossover to the three-dimensional single-mode behavior: \Gamma |\Delta
H|^{3/2}.",1208.0831v1
2012-10-12,Topological Magnon Insulator in Insulating Ferromagnet,"In the ferromagnetic insulator with Dzyaloshinskii-Moriya interaction, we
theoretically predict and numerically verify a topological magnon insulator,
where the charge-free magnon is topologically protected to transport along the
edge while it is insulating in the bulk. Within the bulk band gaps, edge states
form a connected loop as a $4\pi$- or $8\pi$-period M\""{o}bius strip in the
wave vector space. As a consequence, the chiral energy current traveling along
the corresponding edge is topologically protected from defects or disorders.
Using the nonequilibrium Green's function method, we demonstrate that the
energy current carried by magnons with energy in the bulk gap localizes at
edges and prefers to travel along one edge in only one direction at
nonequilibrium steady state. Our prediction about topological magnon insulator
could be observed at a wide energy range in the thin film of the insulating
ferromagnet, such as Lu$_2$V$_2$O$_7$.",1210.3487v2
2013-02-07,Dynamics of a Bose-Einstein Condensate of Excited Magnons,"The emergence of a non-equilibrium Bose-Einstein-like condensation of magnons
in rf-pumped magnetic thin films has recently been experimentally observed. We
present here a complete theoretical description of the non-equilibrium
processes involved. It it demonstrated that the phenomenon is another example
of the presence of a Bose-Einstein-like condensation in non-equilibrium
many-boson systems embedded in a thermal bath, better referred-to as
Fr\""{o}hlich-Bose-Einstein condensation. The complex behavior emerges after a
threshold of the exciting intensity is attained. It is inhibited at higher
intensities when the magnon-magnon interaction drives the magnons to internal
thermalization. The observed behavior of the relaxation to equilibrium after
the end of the pumping pulse is also accounted for and the different processes
fully described.",1302.1765v1
2013-12-05,Signature of Electron-magnon Umklapp Scattering in L10 FePt probed by Thermoelectric Measurements,"We report unconventional thermoelectric power (Seebeck coefficient, S) in L10
structured FePt films. The temperature dependence of S can be well fitted by a
phenomenological expression consisting of electron diffusion and magnon-drag
contributions. Interestingly, the magnon drag coefficient carries an opposite
sign to that of electron diffusion, revealing a dominant contribution from the
elusive electron-magnon Umklapp scattering. DFT calculations identify several
bands crossing the Brillouin zone boundaries, facilitating the Umklapp process.
The large spin-orbit coupling in FePt results in strong mixing of majority and
minority spins among some of those bands, greatly enhancing the electron-magnon
scattering.",1312.1634v4
2014-01-13,Microkelvin thermometry with Bose-Einstein condensates of magnons and applications to studies of the AB interface in superfluid $^3$He,"Coherent precession of trapped Bose-Einstein condensates of magnons is a
sensitive probe for magnetic relaxation processes in superfluid 3He-B down to
the lowest achievable temperatures. We use the dependence of the relaxation
rate on the density of thermal quasiparticles to implement thermometry in 3He-B
at temperatures below 300 $\mu$K. Unlike popular vibrating wire or quartz
tuning fork based thermometers, magnon condensates allow for contactless
temperature measurement and make possible an independent in situ determination
of the residual zero-temperature relaxation provided by the radiation damping.
We use this magnon-condensate-based thermometry to study the thermal impedance
of the interface between A and B phases of superfluid 3He. The magnon
condensate is also a sensitive probe of the orbital order-parameter texture.
This has allowed us to observe for the first time the non-thermal signature of
the annihilation of two AB interfaces.",1401.2953v2
2014-01-16,A thermodynamic theory for thermal-gradient-driven domain wall motion,"Spin waves (or magnons) interact with magnetic domain walls (DWs) in a
complicated way that a DW can propagate either along or against magnon flow.
However, thermally activated magnons always drive a DW to the hotter region of
a nanowire of magnetic insulators under a temperature gradient. We
theoretically illustrate why it is surely so by showing that DW entropy is
always larger than that of a domain as long as material parameters do not
depend on spin textures. Equivalently, the total free energy of the wire can be
lowered when the DW moves to the hotter region. The larger DW entropy is
related to the increase of magnon density of states at low energy originated
from the gapless magnon bound states.",1401.4021v2
2014-04-22,Coherent magnon optics in a ferromagnetic spinor Bose-Einstein condensate,"We measure the mass, gap, and magnetic moment of a magnon in the
ferromagnetic $F=1$ spinor Bose-Einstein condensate of $^{87}$Rb. We find an
unusually heavy magnon mass of $1.038(2)_\mathrm{stat}(8)_\mathrm{sys}$ times
the atomic mass, as determined by interfering standing and running coherent
magnon waves within the dense and trapped condensed gas. This measurement is
shifted significantly from theoretical estimates. The magnon energy gap of
$h\times 2.5(1)_\mathrm{stat}(2)_\mathrm{sys}\;\mathrm{Hz}$ and the effective
magnetic moment of $-1.04(2)_\mathrm{stat}(8)\,\mu_\textrm{bare}$ times the
atomic magnetic moment are consistent with mean-field predictions. The nonzero
energy gap arises from magnetic dipole-dipole interactions.",1404.5631v1
2014-12-25,Normal-mode splitting in the coupled system of hybridized nuclear magnons and microwave photons,"In the weak ferromagnetic MnCO$_3$ system, a low-frequency collective spin
excitation (magnon) is the hybridized oscillation of nuclear and electron spins
coupled through the hyperfine interaction. By using a split-ring resonator, we
performed transmission spectroscopy measurements of MnCO$_3$ system and
observed, for the first time, avoiding crossing between the hybridized
nuclear-electron magnon mode and the resonator mode in the NMR-frequency range.
The splitting strength is quite large due to the large spin density of
$^{55}$Mn, and the cooperativity value $C=0.2$ (magnon-photon coupling
parameter) is close to the conditions of strong coupling. The results reveal a
new class of spin systems, in which the coupling between nuclear spins and
photons is mediated by electron spins via the hyperfine interaction, and in
which the similar normal-mode splitting of the hybridized nuclear magnon mode
and the resonator mode can be observed.",1412.7799v3
2015-02-13,Bose-Einstein condensation of magnons in atomic hydrogen gas,"We report on experimental observation of BEC-like behaviour of quantized
electron spin waves (magnons) in a dense gas of spin polarized atomic hydrogen.
The magnons are trapped and controlled with inhomogeneous magnetic fields, and
described by a Schr\""odinger-like wave equation, in analogy to the BEC
experiments with neutral atoms. We have observed the appearance of a sharp
feature in the ESR spectrum displaced from the normal spin wave spectrum. We
believe that this observation corresponds to a sudden growth of the ground
state population of the magnons and emergence of their spontaneous coherence
for hydrogen gas densities exceeding a critical value, dependent on the
trapping potential. We interpret the results as a BEC of non-equilibrium
magnons which were formed by applying the rf power.",1502.03963v1
2015-02-27,Collective dynamical skyrmions excitations in magnonic crystal,"We investigate theoretically skyrmion magnonic crystal, i.e., the dynamics of
the magnetization in a chain of the ferromagnetic nanodots being in skyrmion
magnetic configuration. We show that collective excitations are possible to be
observed in the structure. We present the dispersion relation of the coupled
skyrmions. It exhibit a periodical property in dependence on wave vector,
characteristic feature of the band structure in magnonic crystals. Spatial
analysis of the magnetization amplitude associated with the magnonic bands
confirms type of the excited modes, as breathing and clockwise gyrotropic
dynamical skyrmions. These high and low frequency excitations, propagate with
negative and positive group velocity, respectively, and can be explored for
study fundamental properties and technological applications in spintronics and
magnonics.",1502.08024v1
2015-03-19,Critical suppression of spin Seebeck effect by magnetic fields,"The longitudinal spin Seebeck effect (LSSE) in Pt/Y$_3$Fe$_5$O$_{12}$ (YIG)
junction systems has been investigated at various magnetic fields and
temperatures. We found that the LSSE voltage in a Pt/YIG-slab system is
suppressed by applying high magnetic fields and this suppression is critically
enhanced at low temperatures. The field-induced suppression of the LSSE in the
Pt/YIG-slab system is too large at around room temperature to be explained
simply by considering the effect of the Zeeman gap in magnon excitation. This
result requires us to introduce magnon-frequency-dependent mechanism into the
scenario of LSSE; low-frequency magnons dominantly contribute to the LSSE. The
magnetic field dependence of the LSSE voltage was observed to change by
changing the thickness of YIG, suggesting that the thermo-spin conversion by
the low-frequency magnons is suppressed in thin YIG films due to the long
characteristic lengths of such magnons.",1503.05764v2
2015-04-08,Scattering of high-energy magnons off a magnetic skyrmion,"We discuss the scattering of high-energy magnons off a single magnetic
skyrmion within the field-polarized ground state of a two-dimensional chiral
magnet. For wavevectors larger than the inverse skyrmion radius, $k r_s \gg 1$,
the magnon scattering is dominated by an emerging magnetic field whose flux
density is essentially determined by the topological charge density of the
skyrmion texture. This leads to skew and rainbow scattering characterized by an
asymmetric and oscillating differential cross section. We demonstrate that the
transversal momentum transfer to the skyrmion is universal due to the
quantization of the total emerging flux while the longitudinal momentum
transfer is negligible in the high-energy limit. This results in a
magnon-driven skyrmion motion approximately antiparallel to the incoming magnon
current and a universal relation between current and skyrmion-velocity.",1504.02108v2
2015-05-08,Nonreciprocal magnon propagation in a noncentrosymmetric ferromagnet,"Relativistic spin-orbit interaction drastically modifies electronic band and
endows emergent functionalities. One of the example is the Rashba effect. In
noncentrosymmetric systems such as interface and polar materials, the
electronic band is spin-splitted depending on the momentum direction owing to
the spin-orbit interaction, which is useful for the electric manipulation of
spin current. Similar relativistic band-modification is also emergent for spin
wave (magnon) in magnetic materials. The asymmetric magnon band dispersion
induced by the Dzyaloshinskii-Moriya interaction, which is antisymmetric
exchange interaction originating from the spin-orbit interaction, is
theoretically expected, and experimentally observed recently in
noncentrosymmetric ferromagnets. Here, we demonstrate that the nonreciprocal
microwave response can be induced by the asymmetric magnon band in a
noncentrosymmetric ferrimagnet LiFe$_5$O$_8$. This result may pave a new path
to designing magnonic device based on the relativistic band engineering.",1505.01942v1
2015-05-23,Long distance transport of magnon spin information in a magnetic insulator at room temperature,"The transport of spin information has been studied in various materials, such
as metals, semiconductors and graphene. In these materials, spin is transported
by diffusion of conduction electrons. Here we study the diffusion and
relaxation of spin in a magnetic insulator, where the large bandgap prohibits
the motion of electrons. Spin can still be transported, however, through the
diffusion of non-equilibrium magnons, the quanta of spin wave excitations in
magnetically ordered materials. Here we show experimentally that these magnons
can be excited and detected fully electrically in linear response, and can
transport spin angular momentum through the magnetic insulator yttrium iron
garnet (YIG) over distances as large as 40 micrometer. We identify two
transport regimes: the diffusion limited regime for distances shorter than the
magnon relaxation length, and the relaxation limited regime for larger
distances. With a model similar to the diffusion-relaxation model for electron
spin transport in (semi)conducting materials, we extract the magnon relaxation
length lambda = 9.4 micrometer in a 200 nm thin YIG film at room temperature.",1505.06325v1
2016-04-24,Optomagnonics in Magnetic Solids,"Coherent conversion of photons to magnons, and back, provides a natural
mechanism for rapid control of interactions between stationary spins with long
coherence times and high-speed photons. Despite the large frequency difference
between optical photons and magnons, coherent conversion can be achieved
through a three-particle interaction between one magnon and two photons whose
frequency difference is resonant with the magnon frequency, as in optomechanics
with two photons and a phonon. The large spin density of a transparent
ferromagnetic insulator (such as the ferrite yttrium iron garnet) in an optical
cavity provides an intrinsic photon-magnon coupling strength that we calculate
to exceed reported optomechanical couplings. A large cavity photon number and
properly selected cavity detuning produce a predicted effective coupling
strength sufficient for observing electromagnetically induced transparency and
the Purcell effect, and even to reach the ultra-strong coupling regime.",1604.07052v2
2016-06-09,Spin dynamical phase and anti-resonance in a strongly coupled magnon-photon system,"We experimentally studied a strongly coupled magnon-photon system via
microwave transmission measurements. An anti-resonance, i.e. the suppression of
the microwave transmission, is observed, indicating a relative phase change
between the magnon response and the driving microwave field. We show that this
anti-resonance feature can be used to interpret the phase evolution of the
coupled magnon-microwave system and apply this technique to reveal the phase
evolution of magnon dark modes. Our work provides a standard procedure for the
phase analysis of strongly coupled systems, enabling the phase characterization
of each subsystem, and can be generally applied to other strongly coupled
systems.",1606.03056v1
2017-03-13,Quasiparticle Breakdown and Spin Hamiltonian of the Frustrated Quantum Pyrochlore Yb$_2$Ti$_2$O$_7$ in Magnetic Field,"The frustrated pyrochlore magnet Yb$_2$Ti$_2$O$_7$ has the remarkable
property that it orders magnetically, but has no propagating magnons over wide
regions of the Brillouin zone. Here we use inelastic neutron scattering to
follow how the spectrum evolves in cubic-axis magnetic fields. At high fields
we observe in addition to dispersive magnons also a two-magnon continuum, which
grows in intensity upon reducing the field and overlaps with the one-magnon
states at intermediate fields leading to strong renormalization of the
dispersion relations, and magnon decays. Using heat capacity measurements we
find that the low and high field regions are smoothly connected with no sharp
phase transition, with the spin gap increasing monotonically in field. Through
fits to an extensive data set we re-evaluate the spin Hamiltonian finding
dominant quantum exchange terms, which we propose are responsible for the
anomalously strong fluctuations and quasiparticle breakdown effects observed at
low fields.",1703.04506v1
2018-02-22,Interlayer Couplings Mediated by Antiferromagnetic Magnons,"Collinear antiferromagnets (AFs) support two degenerate magnon excitations
carrying opposite spin polarizations, by which magnons can function as
electrons in various spin-related phenomena. In an insulating
ferromagnet(F)/AF/F trilayer, we explore the magnon-mediated interlayer
coupling by calculating the magnon thermal energy in the AF as a function of
the orientations of the Fs. The effect manifests as an interlayer exchange
interaction and a perpendicular magnetic anisotropy; they both depend on
temperature and the AF thickness. In particular, the exchange interaction turns
out to be antiferromagnetic at low temperatures and ferromagnetic at high
temperatures, whose magnitude can be $10-100$ $\mu$eV for nanoscale
separations, allowing experimental verification.",1802.07867v2
2018-02-28,The effects of thermal and correlated noise on magnons in a quantum ferromagnet,"The dynamics and thermal equilibrium of spin waves (magnons) in a quantum
ferromagnet as well as the macroscopic magnetisation are investigated. Thermal
noise due to an interaction with lattice phonons and the effects of spatial
correlations in the noise are considered. We first present a Markovian master
equation approach with analytical solutions for any homogeneous spatial
correlation function of the noise. We find that spatially correlated noise
increases the decay rate of magnons with low wave vectors to their thermal
equilibrium, which also leads to a faster decay of the ferromagnet's
magnetisation to its steady-state value. For long correlation lengths and
higher temperature we find that additionally there is a component of the
magnetisation which decays very slowly, due to a reduced decay rate of fast
magnons. This effect could be useful for fast and noise-protected quantum or
classical information transfer and magnonics. We further compare ferromagnetic
and antiferromagnetic behaviour in noisy environments and find qualitatively
similar behaviour in Ohmic but fundamentally different behaviour in super-Ohmic
environments.",1802.10258v2
2018-04-25,Magnon thermal Hall effect in kagome antiferromagnets with Dzyaloshinskii-Moriya interactions,"We theoretically study magnetic and topological properties of
antiferromagnetic kagome spin systems in the presence of both in- and
out-of-plane Dzyaloshinskii-Moriya interactions. In materials such as the iron
jarosites, the in-plane interactions stabilize a canted noncollinear ""umbrella""
magnetic configuration with finite scalar spin chirality. We derive expressions
for the canting angle, and use the resulting order as a starting point for a
spin-wave analysis. We find topological magnon bands, characterized by non-zero
Chern numbers. We calculate the magnon thermal Hall conductivity, and propose
the iron jarosites as a promising candidate system for observing the magnon
thermal Hall effect in a noncollinear spin configuration. We also show that the
thermal conductivity can be tuned by varying an applied magnetic field, or the
in-plane Dzyaloshinskii-Moriya strength. In contrast with previous studies of
topological magnon bands, the effect is found to be large even in the limit of
small canting.",1804.09783v2
2018-11-01,Non-Hermitian magnon-photon interference in an atomic ensemble,"The beam-splitter (BS) is one of the most common and important components in
modern optics, and lossless BS which features unitary transformation induces
Hermitian evolution of light. However, the practical BS based on the conversion
between different degree of freedoms are naturally non-Hermitian, as a result
of essentially open quantum dynamics. In this work, we experimentally
demonstrate a non-Hermitian BS for the interference between traveling photonic
and localized magnonic modes. The non-Hermitian magnon-photon BS is achieved by
the coherent and incoherent interaction mediated by the excited levels of
atoms, which is reconfigurable by adjusting the detuning of excitation.
Unconventional correlated interference pattern is observed at the photon and
magnon output ports. Our work is potential for extending to single-quantum
level to realize interference between a single photon and magnon, which
provides an efficient and simple platform for future tests of non-Hermitian
quantum physics.",1811.00307v1
2018-11-05,Topological transitions of spin-excitations in insulating chiral antiferromagnets,"We present a comprehensive study of strain-induced topological magnon phase
transitions in insulating three-dimensional (3D) topological chiral
antiferromagnets on the kagome-lattice. We show that by applying (100) uniaxial
strain, 3D insulating antiferromagnetic Weyl magnons (WMs) manifest as an
intermediate phase between a strain-induced 3D magnon Chern insulator (MCI)
with integer Chern numbers and a 3D trivial magnon insulator with zero Chern
number. In addition, we show that strain suppresses the topological thermal
Hall conductivity of magnons in these systems. Due to the similarity between 3D
insulating and metallic kagome chiral antiferromagnets, we envision that the
current results could also manifest in the 3D antiferromagnetic topological
Weyl semimetals Mn$_3$Sn\slash Ge.",1811.01946v6
2010-05-06,"Giant Magnons in Symmetric Spaces: Explicit N-soliton solutions for CP^n, SU(n) and S^n","Giant magnons are one of the main manifestations of integrability on the
string theory side of the AdS/CFT correspondence. Motivated by the recent
advances in their study, especially in the context of the string theory dual of
ABJM theory, we present and prove explicit N-soliton solutions for the relevant
CP^n, SU(n) and S^n sigma models. The proof is based on solving the dressing
method recursion with the help of determinant operations, and our solutions
hold for any choice of vacuum and soliton parameters. We further specialize our
results for the choices that lead to giant magnons, and as an application, we
calculate the classical time delay due to the scattering of an arbitrary number
of CP^2 elementary dyonic magnons. The determinant expressions for our
N-soliton solutions could possibly be used for the derivation of an effective
particle description of magnon scattering.",1005.1066v2
2017-01-08,Nonlocal magnon spin transport in NiFe$_2$O$_4$ thin films,"We report magnon spin transport in nickel ferrite (NiFe$_2$O$_4$, NFO)/
platinum (Pt) bilayer systems at room temperature. A nonlocal geometry is
employed, where the magnons are excited by the spin Hall effect or by the Joule
heating induced spin Seebeck effect at the Pt injector, and detected at a
certain distance away by the inverse spin Hall effect at the Pt detector. The
dependence of the nonlocal magnon spin signals as a function of the magnetic
field is closely related to the NFO magnetization behavior. In contrast, we
observe that the magnetoresistance measured locally at the Pt injector does not
show a clear relation with the average NFO magnetization. We estimate the
magnon spin relaxation length to be 3.1 $\pm$ 0.2 $\mu$m in the investigated
NFO samples.",1701.02041v2
2017-01-18,Conversion of electronic to magnonic spin current at heavy-metal magnetic-insulator interface,"Electronic spin current is convertible to magnonic spin current via the
creation or annihilation of thermal magnons at the interface of a magnetic
insulator and a metal with a strong spin-orbital coupling. So far this
phenomenon was evidenced in the linear regime. Based on analytical and
fulledged numerical results for the non-linear regime we demonstrate that the
generated thermal magnons or magnonic spin current in the insulator is
asymmetric with respect to the charge current direction in the metal and
exhibits a nonlinear dependence on the charge current density, which is
explained by the tuning effect of the spin Hall torque and the magnetization
damping. The results are also discussed in light of and are in line with recent
experiments pointing to a new way of non-linear manipulation of spin with
electrical means.",1701.05148v1
2017-01-20,Two-magnon excitations in resonant inelastic x-ray scattering studied by spin-density-wave formalism,"We study two-magnon excitations in resonant inelastic x-ray scattering (RIXS)
at the transition-metal $K$-edge. Instead of working with effective Heisenberg
spin models, we work with a Hubbard-type model ($d$-$p$ model) for a typical
insulating cuprate La$_2$CuO$_4$. For the antiferromagnetic ground state within
the spin-density-wave (SDW) mean-field formalism, we calculate the dynamical
correlation function within the random-phase approximation (RPA), and then
obtain two-magnon excitation spectra by calculating the convolution of it.
Coupling between the $K$-shell hole and the magnons in the intermediate state
is calculated by means of diagrammatic perturbation expansion in the Coulomb
interaction. Calculated momentum dependence of RIXS spectra agrees well with
that of experiments. A notable difference from previous calculations based on
the Heisenberg spin models is that RIXS spectra have a large two-magnon weight
near the zone center, which may be confirmed by further careful high-resolution
experiments.",1701.05684v2
2017-01-28,Magnetic Domain Walls as Hosts of Spin Superfluids and Generators of Skyrmions,"A domain wall in a magnet with easy-axis anisotropy is shown to harbor spin
superfluid associated with its spontaneous breaking of the U(1) spin-rotational
symmetry. The spin superfluid is shown to have several topological properties,
which are absent in conventional superfluids. First, the associated phase slips
create and destroy skyrmions to obey the conservation of the total skyrmion
charge, which allows us to use a domain wall as a generator and detector of
skyrmions. Secondly, the domain wall engenders the emergent magnetic flux for
magnons along its length, which are proportional to the spin supercurrent
flowing through it, and thereby provides a way to manipulate magnons. Thirdly,
the spin supercurrent can be driven by the magnon current traveling across it
owing to the spin transfer between the domain wall and magnons, leading to the
magnonic manipulation of the spin superfluid. The theory for superfluid spin
transport within the domain wall is confirmed by numerical simulations.",1701.08273v2
2018-05-30,Magnonic band gap and mode hybridization in continuous Permalloy film induced by vertical coupling with an array of Permalloy ellipses,"We investigate magnonic band structure in thin homogeneous permalloy film
decorated with periodic array of elliptically shaped permalloy dots and
separated by non-magnetic Pt spacer. We demonstrated experimentally formation
of the magnonic band structure for Damon-Eshbach wave propagating in permalloy
film with the band gap opened at the Brillouin zone border and band splitting
at smaller wavenumbers, due to the Bragg interference and interaction of
propagating wave of the continuous film with a standing resonant mode of the
nano-ellipses, respectively. The shape anisotropy of the permalloy nanodots
allows to control the spin wave dynamics through the switch between two states
of the magnetization with respect to the underneath film magnetization, thus
enabling magnonic band structure reprogrammability. With numerical analysis we
show, that predominant role in formation of the magnonic band structure is
played by a vertical dynamic coupling between propagating wave in the film and
magnetization oscillations in the nanodots.",1805.12178v2
2008-07-07,Finite-Size Effects for Multi-Magnon States,"We propose the generalized Luscher formula for multi-magnon states by which
one can compute the finite-size correction to the energy of multi giant magnons
at classical and one-loop levels. It is shown that the F-term of our formula is
consistent with the exact finite-size spectrum of the sinh-Gordon model, and
the mu-term agrees with the finite-size energy of magnon boundstate of the
asymptotic Bethe Ansatz in the su(2) sector at strong coupling. In an appendix,
we evaluate our formula at weak coupling under some approximations, and find
that the transcendental terms arise from a sum over an infinite tower of BPS
boundstates.",0807.0643v4
2008-07-15,Magnons and BFKL,"We extract from the double logarithmic contributions to DGLAP anomalous
dimensions for twist-two operators up to three-loops the magnon dispersion
relation for planar N=4 supersymmetric Yang-Mills. Perturbatively the magnon
dispersion relation agrees with the expansion of the anomalous dimension for
spin-one as well as with the non-collinear double logarithmic contributions to
the BFKL anomalous dimensions analytically extended to negative spin. The
all-loop expression for the magnon dispersion relation is determined by the
double logarithmic resummation of the corresponding Bethe-Salpeter equation. A
potential map relating the spin chain magnon to BFKL eigenfunctions in the
double logarithm approximation is suggested.",0807.2339v2
2009-12-16,Parametric pumping and kinetics of magnons in dipolar ferromagnets,"The time evolution of magnons subject to a time-dependent microwave field is
usually described within the so-called ""S-theory"", where kinetic equations for
the distribution function are obtained within the time-dependent Hartree-Fock
approximation. To explain the recent observation of ""Bose-Einstein condensation
of magnons"" in an external microwave field [Demokritov et al., Nature 443, 430
(2006)], we extend the ""S-theory"" to include the Gross-Pitaevskii equation for
the time-dependent expectation values of the magnon creation and annihilation
operators. We explicitly solve the resulting coupled equations within a simple
approximation where only a single condensed mode is retained. We also
re-examine the usual derivation of an effective boson model from a realistic
spin model for yttrium-iron garnet films and argue that in the parallel pumping
geometry (where both the static and the time-dependent magnetic field are
parallel to the macroscopic magnetization) the time-dependent Zeemann energy
cannot give rise to magnon condensation.",0912.3254v2
2009-12-23,The Ginzburg-Landau model of Bose-Einstein condensation of magnons,"We introduce a system of phenomenological equations for Bose-Einstein
condensates of magnons in the one-dimensional setting. The nonlinearly coupled
equations, written for amplitudes of the right-and left-traveling waves,
combine basic features of the Gross-Pitaevskii and complex Ginzburg-Landau
models. They include localized source terms, to represent the microwave
magnon-pumping field. With the source represented by the $\delta $-functions,
we find analytical solutions for symmetric localized states of the magnon
condensates. We also predict the existence of asymmetric states with unequal
amplitudes of the two components. Numerical simulations demonstrate that all
analytically found solutions are stable. With the $\delta $-function terms
replaced by broader sources, the simulations reveal a transition from the
single-peak stationary symmetric states to multi-peak ones, generated by the
modulational instability of extended nonlinear-wave patterns. In the
simulations, symmetric initial conditions always converge to symmetric
stationary patterns. On the other hand, asymmetric inputs may generate
nonstationary asymmetric localized solutions, in the form of traveling or
standing waves. Comparison with experimental results demonstrates that the
phenomenological equations provide for a reasonably good model for the
description of the spatiotemporal dynamics of magnon condensates.",0912.4663v1
2012-04-16,Topological chiral magnonic edge mode in a magnonic crystal,"Topological phases have been explored in various fields in physics such as
spintronics, photonics, liquid helium, correlated electron system and
cold-atomic system. This leads to the recent foundation of emerging materials
such as topological band insulators, topological photonic crystals and
topological superconductors/superfluid. In this paper, we propose a topological
magnonic crystal which provides protected chiral edge modes for magnetostatic
spin waves. Based on a linearized Landau-Lifshitz equation, we show that a
magnonic crystal with the dipolar interaction acquires spin-wave volume-mode
band with non-zero Chern integer. We argue that such magnonic systems are
accompanied by the same integer numbers of chiral spin-wave edge modes within a
band gap for the volume-mode bands. In these edge modes, the spin wave
propagates in a unidirectional manner without being scattered backward, which
implements novel fault-tolerant spintronic devices.",1204.3349v3
2016-08-01,Noncollinear Antiferromagnetic Haldane Magnon Insulator,"In this Letter we present a comprehensive study of topological magnon bands
and thermal Hall effect in non-collinear antiferromagnetic systems on the
honeycomb lattice with an intrinsic Dzyaloshinskii-Moriya interaction. We
theoretically show that the system possesses topological magnon bands with
Chern number protected edge modes accompanied by a nonzero thermal magnon Hall
effect. These features result from non-collinearity of the magnetic moments due
to an applied out-of-plane magnetic field. Our results provide an experimental
clue towards the realization of topological magnon transports in honeycomb
antiferromagnetic compounds such as XPS$_3$ (X=Mn,Fe) and
$\alpha$-Cu$_2$V$_2$O$_7$.",1608.00545v12
2016-08-05,On supercurrents in Bose-Einstein magnon condensates in YIG ferrimagnet,"Recently E. Sonin commented [1] on our preprint ""Supercurrent in a room
temperature Bose-Einstein magnon condensate"" [2,3], arguing that our ""claim of
detection of spin supercurrent is premature and has not been sufficiently
supported by presented experimental results and their theoretical
interpretation."" We consider the appearance of this Comment as a sign of
significant interest into the problem of supercurrents in Bose-Einstein magnon
condensates. Here, we explicitly address E. Sonin's comments and show that our
interpretation of our experimental results as a detection of a magnon
supercurrent is fully supported not only by the experimental results
themselves, but also by independent theoretical analysis [4].",1608.01813v2
2016-08-30,Magnon Hall Effect without Dzyaloshinskii-Moriya Interaction,"Topological magnon bands and magnon Hall effect in insulating collinear
ferromagnets are induced by the Dzyaloshinskii-Moriya interaction (DMI) even at
zero magnetic field. In the geometrically frustrated star lattice, a
coplanar/noncollinear $\mathbf q=0$ magnetic ordering may be present due to
spin frustration. This magnetic structure, however, does not exhibit
topological magnon effects even with DMI in contrast to collinear ferromagnets.
We show that a magnetic field applied perpendicular to the star plane induces a
non-coplanar spin configuration with nonzero spin scalar chirality, which
provides topological effects without the need of DMI. The non-coplanar spin
texture originates from the topology of the spin configurations and does not
need the presence of DMI or magnetic ordering, which suggests that this
phenomenon may be present in the chiral spin liquid phases of frustrated
magnetic systems. We propose that these anomalous topological magnon effects
can be accessible in Polymeric Iron (III) Acetate --- a star-lattice
antiferromagnet with both spin frustration and long-range magnetic ordering.",1608.08605v6
2017-06-13,Light scattering by magnons in whispering gallery mode cavities,"Brillouin light scattering is an established technique to study magnons, the
elementary excitations of a magnet. Its efficiency can be enhanced by cavities
that concentrate the light intensity. Here, we theoretically study inelastic
scattering of photons by a magnetic sphere that supports optical whispering
gallery modes in a plane normal to the magnetization. Magnons with low angular
momenta scatter the light in the forward direction with a pronounced asymmetry
in the Stokes and the anti-Stokes scattering strength, consistent with earlier
studies. Magnons with large angular momenta constitute Damon Eschbach modes are
shown to inelastically reflect light. The reflection spectrum contains either a
Stokes or anti-Stokes peak, depending on the direction of the magnetization, a
selection rule that can be explained by the chirality of the Damon Eshbach
magnons. The controllable energy transfer can be used to manage the
thermodynamics of the magnet by light.",1706.04106v1
2017-10-10,Weak localization of magnons in a disordered two-dimensional antiferromagnet,"We propose the weak localization of magnons in a disordered two-dimensional
antiferromagnet. We derive the longitudinal thermal conductivity $\kappa_{xx}$
for magnons of a disordered Heisenberg antiferromagnet in the linear-response
theory with the linear-spin-wave approximation. We show that the back
scattering of magnons is enhanced critically by the particle-particle-type
multiple impurity scattering. This back scattering causes a logarithmic
suppression of $\kappa_{xx}$ with the length scale in two dimensions. We also
argue a possible effect of inelastic scattering on the temperature dependence
of $\kappa_{xx}$. This weak localization is useful to control turning the
magnon thermal current on and off.",1710.03445v2
2017-10-10,Negative magneto-thermal-resistance in a disordered two-dimensional antiferromagnet,"We demonstrate that a weak external magnetic field can induce negative
magneto-thermal-resistance for magnons in a disordered two-dimensional
antiferromagnet. We study the main effect of a weak external magnetic field on
the longitudinal thermal conductivity, $\kappa_{xx}$, for a disordered
antiferromagnet using the weak-localization theory for magnons. We show that
the weak-localization correction term of $\kappa_{xx}$ positively increases
with increasing the magnetic field parallel to the ordered spins. Since this
increase corresponds to a decrease of the thermal resistivity, this phenomenon
is negative magneto-thermal-resistance for magnons. This negative
magneto-thermal-resistance and the weak localization of magnons will be used to
control the magnon thermal current in antiferromagnetic spintronics devices. We
also discuss several implications for further experimental and theoretical
studies for disordered magnets.",1710.03446v2
2017-12-22,Neutron Scattering Signatures of Magnon Weyl Points,"We study the inelastic neutron scattering cross section in the vicinity of
touching points in magnon bands. Among the possible touching points are magnon
Weyl points in three dimensional ordered magnets with significant spin-orbit
coupling that are characterized by a linear dispersion in their vicinity. A
Weyl point is topologically protected by its net chirality and here we show
that this leads to a characteristic form for the dynamical structure factor. To
address this question, we show that scattering intensities in the vicinity of
arbitrary magnon two-band touching points are identical to expectation values
of the pseudospin polarization along some direction $\hat{\boldsymbol{n}}$ in
momentum space fixed by the magnetic Hamiltonian. This approach applied to Weyl
points shows that they are singular points in the intensity of the form
$\hat{\boldsymbol{n}}\cdot \delta\hat{\boldsymbol{k}}$ regardless of the
magnetic ground state. We make specific predictions for the experimental
signatures of such intensity singularities in several spin models hosting Weyl
magnons applicable to candidate materials.",1712.08535v1
2017-12-28,Topological magnon bands in the zigzag and stripy phases of antiferromagnetic honeycomb lattice,"We investigated the topological property of magnon bands in the collinear
magnetic orders of zigzag and stripy phases for the antiferromagnetic honeycomb
lattice and identified Berry curvature and symmetry constraints on the magnon
band structure. Different symmetries of both zigzag and stripy phases lead to
different topological properties, in particular, the magnon bands of the stripy
phase being disentangled with a finite Dzyaloshinskii-Moriya (DM) term with
non-zero spin Chern number. This is corroborated by calculating the spin Nernst
effect. Our study establishes the existence of the non-trivial magnon band
topology for all observed collinear antiferromagnetic honeycomb lattice in the
presence of the DM term.",1712.09801v1
2018-09-17,Unraveling the influence of electronic and magnonic spin current injection near the magnetic ordering transition of IrMn metallic antiferromagnets,"Although spin injection at room temperature in an IrMn metallic
antiferromagnet strongly depends on the transport regime, and is more efficient
in the case of magnonic transport, in this article, we present experimental
data demonstrating that the enhanced efficiency of spin injection caused by
spin fluctuations near the ordering temperature can be as efficient for the
electronic and magnonic transport regimes. By selecting representative
interacting environments, we also demonstrated that the amplification of spin
injection near the ordering temperature of the IrMn antiferromagnet is
independent of exchange coupling with an adjacent NiFe ferromagnet. In
addition, our findings confirm that the spin current carried by magnons
penetrates deeper than that transported by conduction electrons in IrMn.
Finally, our data indicates that the value of the ordering temperature for the
IrMn antiferromagnet is not significantly affected by either the electronic or
magnonic nature of the spin current probe, or by exchange coupling.",1809.06174v1
2020-12-02,Electric field control of interaction between magnons and quantum spin defects,"Hybrid systems coupling quantum spin defects (QSD) and magnons can enable
unique spintronic device functionalities and probes for magnetism. Here, we add
electric field control of magnon-QSD coupling to such systems by integrating
ferromagnet-ferroelectric multiferroic with nitrogen-vacancy (NV) center spins.
Combining quantum relaxometry with ferromagnetic resonance measurements and
analytical modeling, we reveal that the observed electric-field tuning results
from ferroelectric polarization control of the magnon-generated fields at the
NV. Exploiting the demonstrated control, we also propose magnon-enhanced hybrid
electric field sensors with improved sensitivity.",2012.01497v2
2020-12-29,Twist-induced Near-field Thermal Switch Using Nonreciprocal Surface Magnon-Polaritons,"We explore that two ferromagnetic insulator slabs host a strong twist-induced
near-field radiative heat transfer in the presence of twisted magnetic fields.
Using the formalism of fluctuational electrodynamics, we find the existence of
large twist-induced thermal switch ratio in large damping condition and
nonmonotonic twist manipulation for heat transfer in small damping condition,
associated with the different twist-induced effects of nonreciprocal elliptic
surface magnon-polaritons, hyperbolic surface magnon-polaritons, and
twist-non-resonant surface magnon-polaritons. Moreover, the near-field
radiative heat transfer can be significantly enhanced by the twist-non-resonant
surface magnon-polaritons in the ultra-small damping condition. Such
twist-induced effect is applicable for other kinds of anisotropic slabs with
timereversal symmetry breaking. Our findings provide a way to twisted and
magnetic control in nanoscale thermal management and improve it with
twistronics concepts.",2012.14733v1
2012-05-23,Spontaneous Magnon Decays,"A theoretical overview of the phenomenon of spontaneous magnon decays in
quantum antiferromagnets is presented. The intrinsic zero-temperature damping
of magnons in quantum spin systems is a fascinating many-body effect, which has
recently attracted significant attention in view of its possible observation in
neutron-scattering experiments. An introduction to the theory of magnon
interactions and a discussion of necessary symmetry and kinematic conditions
for spontaneous decays are provided. Various parallels with the decays of
anharmonic phonons and excitations in superfluid 4He are extensively used.
Three principal cases of spontaneous magnon decays are considered:
field-induced decays in Heisenberg antiferromagnets, zero-field decays in
spiral antiferromagnets, and triplon decays in quantum-disordered magnets.
Analytical results are compared with available numerical data and prospective
materials for experimental observation of the decay-related effects are briefly
discussed.",1205.5278v3
2012-05-24,Giant Magnon Gap in Bilayer Iridate Sr3Ir2O7: Enhanced Pseudo-dipolar Interactions Near the Mott Transition,"Using resonant inelastic x-ray scattering, we observe in the bilayer iridate
Sr3Ir2O7, a spin-orbit coupling driven magnetic insulator with a small charge
gap, a magnon gap of ~92 meV for both acoustic and optical branches. This
exceptionally large magnon gap exceeds the total magnon bandwidth of ~70 meV
and implies a marked departure from the Heisenberg model, in stark contrast to
the case of the single-layer iridate Sr2IrO4. Analyzing the origin of these
observations, we find that the giant magnon gap results from bond-directional
pseudo-dipolar interactions that are strongly enhanced near the metal-insulator
transition boundary. This suggests that novel magnetism, such as that inspired
by the Kitaev model built on the pseudo-dipolar interactions, may emerge in
small charge-gap iridates.",1205.5337v1
2017-05-05,Nano-patterned magnonic crystals based on ultrathin YIG films,"We demonstrate a microscopic magnonic-crystal waveguide produced by
nano-patterning of a 20 nm thick film of Yttrium Iron Garnet. By using the
phase-resolved micro-focus Brillouin light scattering spectroscopy, we map the
intensity and the phase of spin waves propagating in such a periodic magnetic
structure. Based on these maps, we obtain the dispersion and the attenuation
characteristics of spin waves providing detailed information about the physics
of spin-wave propagation in the magnonic crystal. We show that, in contrast to
the simplified physical picture, the maximum attenuation of spin waves is
achieved close to the edge of the magnonic band gap, which is associated with
non-trivial reflection characteristics of spin waves in non-uniform field
potentials.",1705.02267v1
2017-05-24,Topological Magnon Bands and Unconventional Thermal Hall Effect on the Frustrated Honeycomb and Bilayer Triangular Lattice,"In the conventional ferromagnetic systems, topological magnon bands and
thermal Hall effect are due to the Dzyaloshinskii-Moriya interaction (DMI). In
principle, however, the DMI is either negligible or it is not allowed by
symmetry in some quantum magnets. Therefore, we expect that topological magnon
features will not be present in those systems. In addition, quantum magnets on
the triangular-lattice are not expected to possess topological features as the
DMI or spin-chirality cancels out due to equal and opposite contributions from
adjacent triangles. Here, however, we predict that the isomorphic frustrated
honeycomb-lattice and bilayer triangular-lattice antiferromagnetic system will
exhibit topological magnon bands and topological thermal Hall effect in the
absence of an intrinsic DMI. These unconventional topological magnon features
are present as a result of magnetic-field-induced non-coplanar spin
configurations with nonzero scalar spin chirality. The relevance of the results
to realistic bilayer triangular antiferromagnetic materials are discussed.",1705.08892v5
2017-09-12,Direct observation of magnon-phonon coupling in yttrium iron garnet,"The magnetic insulator yttrium iron garnet (YIG) with a ferrimagnetic
transition temperature of $\sim$560 K has been widely used in microwave and
spintronic devices. Anomalous features in the spin Seeback effect (SSE)
voltages have been observed in Pt/YIG and attributed to the magnon-phonon
coupling. Here we use inelastic neutron scattering to map out low-energy spin
waves and acoustic phonons of YIG at 100 K as a function of increasing magnetic
field. By comparing the zero and 9.1 T data, we find that instead of splitting
and opening up gaps at the spin wave and acoustic phonon dispersion
intersecting points, magnon-phonon coupling in YIG enhances the hybridized
scattering intensity. These results are different from expectations of
conventional spin-lattice coupling, calling for new paradigms to understand the
scattering process of magnon-phonon interactions and the resulting
magnon-polarons.",1709.03940v1
2018-07-13,Magnon band topology in spin-orbital coupled magnets: classification and application to $α$-RuCl$_3$,"In spite of flourishing studies on the topology of spin waves, a generic
framework to classify and compute magnon band topology in non-collinear magnets
is still missing. In this work we provide such a theory framework, by mapping
an arbitrary linear spin wave into a local free-fermion Hamiltonian with
exactly the same spectrum, symmetry implementation and band topology, which
allows for a full classification and calculation on any topological properties
of magnon bands. We apply this fermionization approach to honeycomb Kitaev
magnet $\alpha$-RuCl$_3$, and show the existence of topologically protected
magnon band crossings, and field-induced magnon Chern bands under small
magnetic fields.",1807.05232v2
2018-07-25,Strong microwave photon-magnon coupling in multiresonant dielectric antennas (Perspective),"Achieving quantum-level control over electromagnetic waves, magnetisation
dynamics, vibrations and heat is invaluable for many practical application and
possible by exploiting the strong radiation-matter coupling. Most of the modern
strong microwave photon-magnon coupling developments rely on the integration of
metal-based microwave resonators with a magnetic material. However, it has
recently been realised that all-dielectric resonators made of or containing
magneto-insulating materials can operate as a standalone strongly-coupled
system characterised by low dissipation losses and strong local microwave field
enhancement. Here, after a brief overview of recent developments in the field,
I discuss examples of such dielectric resonant systems and demonstrate their
ability to operate as multiresonant antennas for light, microwaves, magnons,
sound, vibrations and heat. This multiphysics behaviour opens up novel
opportunities for the realisation of multiresonant coupling such as, for
example, photon-magnon-phonon coupling. I also propose several novel systems in
which strong photon-magnon coupling in dielectric antennas and similar
structures is expected to extend the capability of existing devices or may
provide an entirely new functionality. Examples of such systems include novel
magnetofluidic devices, high-power microwave power generators, and hybrid
devices exploiting the unique properties of electrical solitons.",1807.09827v1
2018-12-26,Magnonic Band Structure Established by Chiral Spin-Density Waves in Thin Film Ferromagnets,"Recent theoretical studies have demonstrated the possibility to excite and
sustain noncollinear magnetization states in ferromagnetic nanowires. The
resulting state is referred to as a spin-density wave (SDW). SDWs can be
interpreted as hydrodynamic states with a constant fluid density and fluid
velocity in systems with easy-plane anisotropy. Here, we consider the effect of
the nonlocal dipole field arising from the finite thickness of magnetic thin
films on the spatial profile of the SDW and on the associated magnon
dispersion. Utilizing a hydrodynamic formulation of the Larmor torque equation,
it is found that the nonlocal dipole field modulates the fluid velocity. Such a
modulation induces a magnonic band structure unlike the typical dispersion
relation for magnons on uniform magnetization. The analytical results are
validated by micromagnetic simulations. Band gaps on the order of GHz are
numerically observed to depend on the SDW fluid velocity and film thickness for
realistic material parameters. The presented results suggest that SDWs can find
applications as reconfigurable magnonic crystals.",1812.10571v1
2019-01-12,Kelvin modes of a skyrmion line in chiral magnets and the associated magnon transport,"Magnetic skyrmions in bulk crystals are line-like topologically protected
spin textures. They allow for the propagation of magnons along the skyrmion
line but are localized inside the skyrmion line. Analogous to the vortex line,
these propagating modes are the Kelvin modes of a skyrmion line. In crystals
without an inversion center, it is known that the magnon dispersion in the
ferromagnetic state is asymmetric in the wavevector. It is natural to expect
that the dispersion of the Kelvin modes is also asymmetric with respect to the
wavevector. We study the Kelvin modes of a skyrmion line in the ferromagnetic
background. In contrast, we find that the lowest Kelvin mode is symmetric in
the wavevector in the low energy region despite the inversion symmetry
breaking. Other Kelvin modes below the magnon continuum are asymmetric, and
most of them have a positive group velocity. Our results suggest that a
skyrmion line can function as a one-way waveguide for magnons.",1901.03812v2
2019-01-17,Topological pumping of a single magnon in a one-dimensional spin-dependent optical superlattice,"Topological pumping of ultracold atomic gases has recently been demonstrated
in two experiments (Nat. Phys. 12, 296; 12, 350 (2016)). Here we study the
topological pumping of a single magnon in a dynamically controlled
spin-dependent optical superlattice. When the interaction between atoms is
strong, this system supports a dynamical version of topological magnon
insulator phase. By initially putting a single magnon in the superlattice and
slowly varying the dynamical controlled parameter over one period, the shift of
the magnon density center is quantized and equal to the topological Chern
number. Moreover, we also find that the direction of this quantized shift is
entanglement-dependent. Our result provides a route for realizing topological
pumping of quasiparticles in strongly correlated ultracold atomic system and
for studying the interplay between topological pumping and quantum
entanglement.",1901.05614v1
2019-02-08,Hybrid quantum systems based on magnonics,"Engineered quantum systems enabling novel capabilities for communication,
computation, and sensing have blossomed in the last decade. Architectures
benefiting from combining distinct and complementary physical quantum systems
have emerged as promising platforms for developing quantum technologies. A new
class of hybrid quantum systems based on collective spin excitations in
ferromagnetic materials has led to the diverse set of experimental platforms
which are outlined in this review article. The coherent interaction between
microwave cavity modes and collective spin-wave modes is presented as the
backbone of the development of more complex hybrid quantum systems. Indeed,
quanta of excitation of the spin-wave modes, called magnons, can also interact
coherently with optical photons, phonons, and superconducting qubits in the
fields of cavity optomagnonics, cavity magnomechanics, and quantum magnonics,
respectively. Notably, quantum magnonics provides a promising platform for
performing quantum optics experiments in magnetically-ordered solid-state
systems. Applications of hybrid quantum systems based on magnonics for quantum
information processing and quantum sensing are also outlined briefly.",1902.03024v2
2019-02-11,Interplay of electronic and spin degrees in ferromagnetic SrRuO$_3$: anomalous softening of magnon gap and stiffness,"The magnon dispersion of ferromagnetic SrRuO$_3$ was studied by inelastic
neutron scattering experiments on single crystals as function of temperature.
Even at low temperature the magnon modes exhibit substantial broadening
pointing to strong interaction with charge carriers. We find an anomalous
temperature dependence of both the magnon gap and the magnon stiffness, which
soften upon cooling in the ferromagnetic phase. Both effects trace the
temperature dependence of the anomalous Hall effect. We argue that these
results show that Weyl points and the anomalous Hall effect can directly
influence the spin dynamics in metallic ferromagnets.",1902.04036v1
2019-03-01,Indirect observation of phase conjugate magnons from non-degenerate four-wave mixing,"A phase conjugate mirror utilising four-wave mixing in a magnetic system is
experimentally realised for the first time. Indirect evidence of
continuous-wave phase conjugation has been observed experimentally and is
supported by simulations. The experiment utilizes a pump-probe method to excite
a four-wave mixing process. Two antennae are used to pump a region of a
thin-film yttrium iron garnet waveguide with magnons of frequency $f_{1}$ to
create a spatio-temporally periodic potential. As the probe magnons of
$f_{\mathrm{p}}$ impinge on the pumped region, a signal with frequency
$f_{\mathrm{c}} = 2f_{1}-f_{\mathrm{p}}$ is observed. The amplitude of the
nonlinear signal was highly dependent on the applied magnetic field $H$. Width
modes of the probe magnons and standing wave modes of the pump magnons were
shown to affect the amplitude of the signal at $f_{\mathrm{c}}$. Experimental
data is compared with simulations and theory to suggest that $f_{\mathrm{c}}$
is a phase conjugate of $f_{\mathrm{p}}$.",1903.00400v1
2019-03-06,Symmetry-protected topological magnons in three dimensional Kitaev materials,"Topological phases in magnetic materials offer novel tunability of
topological properties via varying the underlying magnetism. We show that three
dimensional Kitaev materials can provide a great opportunity for controlling
symmetry-protected topological nodal magnons. These materials are originally
considered as strong candidates for the Kitaev quantum spin liquid due to the
bond-dependent frustrating spin exchange interactions. As a concrete example,
we consider the symmetry and topology of the magnons in the canted zig-zag
ordered state in the hyperhoneycomb $\beta\text{-}\mathrm{Li_2IrO_3}$, which
can be obtained by applying a magnetic field in the counter-rotating spiral
state at zero field. It is shown that the magnetic glide symmetries and the
non-Hermitian nature of the bosonic magnons lead to unique topological
protection that is different from the case of the fermionic counterparts. We
investigate how such topological magnons can be controlled by changing the
symmetry of the underlying spin exchange interactions.",1903.02559v1
2019-03-08,Magnon quantum anomalies in Weyl ferromagnets,"When subjected to parallel electric field $\mathbf E$ and magnetic field
$\mathbf B$, Weyl semimetals exhibit the exotic transport property known as the
chiral anomaly due to the pumping of electrons between Weyl cones of opposite
chiralities. When one or both electromagnetic (EM) fields are replaced by
strain-induced chiral pseudo-electromagnetic (pseudo-EM) fields, other types of
quantum anomalies occur. In the present paper, we will show that such quantum
anomalies can be reproduced in a completely different system -- a Weyl
ferromagnet whose magnonic structure remarkably encodes Weyl physics. By
analytical and numerical calculations, we will show that the magnon bands can
be Landau-quantized by either an inhomogeneous electric field $\mathbf E$ or a
chiral pseudo-electric field $\mathbf e$ induced by a torsional strain, and
magnons can be pumped along bands by either an inhomogeneous magnetic field
$\mathbf B$ or a chiral pseudo-magnetic field $\mathbf b$ due to a dynamic
uniaxial strain. We list the magnon quantum anomalies and the associated
anomalous spin and heat currents in the Weyl ferromagnet, and show they are
distinct from their Weyl semimetal counterparts.",1903.03664v1
2019-03-12,Probing GHz Gravitational Waves with Graviton-magnon Resonance,"A novel method for extending frequency frontier in gravitational wave
observations is proposed. It is shown that gravitational waves can excite a
magnon. Thus, gravitational waves can be probed by a graviton-magnon detector
which measures resonance fluorescence of magnons. Searching for gravitational
waves with a wave length $\lambda$ by using a ferromagnetic sample with a
dimension $l$, the sensitivity of the graviton-magnon detector reaches spectral
densities, around $5.4 \times 10^{-22} \times (\frac{l}{\lambda /2\pi})^{-2} \
[{\rm Hz}^{-1/2}]$ at 14 GHz and $8.6 \times 10^{-21} \times (\frac{l}{\lambda
/2\pi})^{-2} \ [{\rm Hz}^{-1/2}]$ at 8.2 GHz, respectively.",1903.04843v4
2019-03-26,Asymmetric quantum shot noise in magnon transport,"We study a frequency-dependent noise-to-current ratio for asymmetric,
symmetric, and noncommutative current-noise in a ferromagnetic insulating
junction, and extract quantum-mechanical properties of magnon transport at low
temperatures. We demonstrate that the noncommutative noise, vanished in the
dc-limit (i.e., a classical regime), increases monotonically as a function of
frequency, and show that the noncommutative noise associated directly with
quantum fluctuations of magnon currents breaks through the classical upper
limit determined by the symmetric noise and realizes asymmetric quantum shot
noise. Finally, we show that our theoretical predictions are within
experimental reach with current device and measurement scheme while exploiting
low temperatures. Our work provides a platform toward experimental access to
quantum fluctuations of magnon currents.",1903.10639v1
2019-04-03,Non-Hermitian topology of spontaneous magnon decay,"Spontaneous magnon decay is a generic feature of the magnetic excitations of
anisotropic magnets and isotropic magnets with non-collinear order. In this
paper, we argue that the effect of interactions on one-magnon states can, under
many circumstances, be treated in terms of an effective, energy independent,
non-Hermitian Hamiltonian for the magnons. In the vicinity of Dirac or Weyl
touching points, we show that the spectral function has a characteristic
anisotropy arising from topologically protected exceptional points or lines in
the non-Hermitian spectrum. Such features can, in principle, be detected using
inelastic neutron scattering or other spectroscopic probes. We illustrate this
physics through a concrete example: a honeycomb ferromagnet with
Dzyaloshinskii-Moriya exchange. We perform interacting spin wave calculations
of the structure factor and spectral function of this model, showing good
agreement with results from a simple effective non-Hermitian model for the
splitting of the Dirac point. Finally, we argue that the zoo of known
topological protected magnon band structures may serve as a nearly ideal
platform for realizing and exploring non-Hermitian physics in solid-state
systems.",1904.02160v1
2019-04-11,Thermalization of hot electrons via interfacial electron-magnon interaction,"Recent work on layered structures of superconductors (S) or normal metals (N)
in contact with ferromagnetic insulators (FI) has shown how the properties of
the previous can be strongly affected by the magnetic proximity effect due to
the static FI magnetization. Here we show that such structures can also exhibit
a new electron thermalization mechanism due to the coupling of electrons with
the dynamic magnetization, i.e., magnons in FI. We here study the heat flow
between the two systems and find that in thin films the heat conductance due to
the interfacial electron-magnon collisions can dominate over the well-known
electron-phonon coupling below a certain characteristic temperature that can be
straightforwardly reached with present-day experiments. We also study the role
of the magnon band gap and the induced spin-splitting field induced in S on the
resulting heat conductance and show that heat balance experiments can reveal
information about such quantities in a way quite different from typical magnon
spectroscopy experiments.",1904.05696v2
2019-04-12,Magnetic excitations in hole-doped Sr2IrO4: A comparison with electron-doped cuprates,"We have studied the evolution of magnetic and orbital excitations as a
function of hole-doping in single crystal samples of Sr2Ir(1-x)Rh(x)O4 (0.07 <
x < 0.42) using high resolution Ir L3-edge resonant inelastic x-ray scattering
(RIXS). Within the antiferromagnetically ordered region of the phase diagram (x
< 0.17) we observe highly dispersive magnon and spin-orbit exciton modes.
Interestingly, both the magnon gap energy and the magnon bandwidth appear to
increase as a function of doping, resulting in a hardening of the magnon mode
with increasing hole doping. As a result, the observed spin dynamics of
hole-doped iridates more closely resemble those of the electron-doped, rather
than hole-doped, cuprates. Within the paramagnetic region of the phase diagram
(0.17 < x < 0.42) the low-lying magnon mode disappears, and we find no evidence
of spin fluctuations in this regime. In addition, we observe that the orbital
excitations become essentially dispersionless in the paramagnetic phase,
indicating that magnetic order plays a crucial role in the propagation of the
spin-orbit exciton.",1904.06402v1
2019-11-27,Ellipticity and Dissipation Effects in Magnon Spin Valves,"We consider alignment-dependent spin and heat transport across a magnon spin
valve in the tunneling regime, i.e., a junction consisting of two weakly
coupled ferromagnetic insulators. We determine the difference in spin and heat
conductance between the parallel and antiparallel configuration of the
magnetization direction. The dependence of these conductances on both the
Gilbert damping and ellipticity is studied. We find that both magnon
ellipticity and dissipation open channels for magnons to tunnel through in the
antiparallel configuration. Our results highlight an important difference
between electronic and magnon spin transport in spin-valve structures and may
be important for the development of devices based on magnetic insulators.",1911.12017v2
2019-12-31,Probing magnon-magnon coupling in exchange coupled Y$_3$Fe$_5$O$_{12}$/Permalloy bilayers with magneto-optical effects,"We demonstrate the magnetically-induced transparency (MIT) effect in
Y$_3$Fe$_5$O$_{12}$(YIG)/Permalloy(Py) coupled bilayers. The measurement is
achieved via a heterodyne detection of the coupled magnetization dynamics using
a single wavelength that probes the magneto-optical Kerr and Faraday effects of
Py and YIG, respectively. Clear features of the MIT effect are evident from the
deeply modulated ferromagnetic resonance of Py due to the
perpendicular-standing-spin-wave of YIG. We develop a phenomenological model
that nicely reproduces the experimental results including the induced amplitude
and phase evolution caused by the magnon-magnon coupling. Our work offers a new
route towards studying phase-resolved spin dynamics and hybrid magnonic
systems.",1912.13407v2
2015-08-23,Asymmetric Magnon Excitation by Spontaneous Toroidal Ordering,"Effects of spontaneous toroidal ordering on magnetic excitation are
theoretically investigated for a localized spin model that includes a staggered
Dzyaloshinsky-Moriya interaction and anisotropic exchange interactions, which
arise from the antisymmetric spin-orbit coupling and the multi-orbital
correlation effect. We show that the model exhibits a Neel-type
antiferromagnetic order, which simultaneously accompanies a ferroic toroidal
order. We find that the occurrence of toroidal order modulates the magnon
dispersion in an asymmetric way with respect to the wave number: a toroidal
dipole order on the zigzag chain leads to a band-bottom shift, while a toroidal
octupole order on the honeycomb lattice gives rise to a valley splitting. These
asymmetric magnon excitations could be a source of unusual magnetic responses,
such as nonreciprocal magnon transport. A variety of modulations are discussed
while changing the lattice and magnetic symmetries. Implications of candidate
materials for asymmetric magnon excitations are presented.",1508.05627v2
2016-03-02,"Magnetic transitions in the topological magnon insulator Cu(1,3-bdc)","Topological magnon insulators are a new class of magnetic materials that
possess topologically nontrivial magnon bands. As a result, magnons in these
materials display properties analogous to those of electrons in topological
insulators. Here, we present magnetization, specific heat, and neutron
scattering measurements of the ferromagnetic kagome magnet Cu(1,3-bdc). Our
measurements provide a detailed description of the magnetic structure and
interactions in this material, and confirm that it is an ideal prototype for
topological magnon physics in a system with a simple spin Hamiltonian.",1603.00901v2
2016-10-12,Microscopic derivation of magnon spin current in a topological insulator/ferromagnet heterostructure,"We investigate a spin-electricity conversion effect in a topological
insulator/ferromagnet heterostructure. In the spin-momentum-locked surface
state, an electric current generates nonequilibrium spin accumulation, which
causes a spin-orbit torque that acts on the ferromagnet. When spins in the
ferromagnet are completely parallel to the accumulated spin, this spin-orbit
torque is zero. In the presence of spin excitations, however, a coupling
between magnons and electrons enables us to obtain a nonvanishing torque. In
this paper, we consider a model of the heterostructure in which a
three-dimensional magnon gas is coupled with a two-dimensional massless Dirac
electron system at the interface. We calculate the torque induced by an
electric field, which can be interpreted as a magnon spin current, up to the
lowest order of the electron-magnon interaction. We derive the expressions for
high and low temperatures and estimate the order of magnitude of the induced
spin current for realistic materials at room temperature.",1610.03636v3
2016-12-18,Bottleneck accumulation of hybrid magneto-elastic bosons,"It is known that an ensemble of magnons, quanta of spin waves, can be
prepared as a Bose gas of weakly interacting quasiparticles with conservation
of the particle number. Furthermore, the thermalization of the overpopulated
magnon gas can lead to the formation of a Bose-Einstein condensate at the
bottom of a spin-wave spectrum. However, magnon-phonon scattering processes can
significantly modify this scenario and new quasiparticles are formed -
magneto-elastic bosons. Our observations of a parametrically populated magnon
gas in a single-crystal film of Yttrium Iron Garnet by means of
wavevector-resolved Brillouin light scattering spectroscopy including
magneto-elastic coupling resulted in the discovery of a novel condensation
phenomenon: A spontaneous accumulation of hybrid magneto-elastic bosonic
quasiparticles at the intersection of the lowest magnon mode and a transversal
acoustic wave.",1612.05925v1
2017-07-03,Tunneling between chiral magnets: Spin current generation without external fields,"Magnons can generate a spin current, and the standard generating mechanism
requires at least one external field. Since this mechanism is often applied to
a multilayer system including a magnet and a paramagnetic metal, the system can
possess not only the charge current induced by the spin current but also the
charge current induced by the external field. The latter is an unnecessary
accompaniment. Here we show that the tunneling of a magnon pair between chiral
magnets can generate a spin current even without external fields. This
phenomenon originates from a phase difference between magnon pairs of separate,
weakly coupled chiral magnets, and is essentially different from the mechanism
using the angle degree of freedom of the magnon Bose-Einstein condensates. The
pair's tunneling is possible in chiral magnets due to lack of the
Goldstone-type gapless excitations. This phenomenon opens the door to
spintronics not requiring any external field and using the magnon pair
tunneling.",1707.00382v1
2018-10-19,Magnon properties of random alloys,"We study magnon properties in terms of spin stiffness, Curie temperatures and
magnon spectrum of Fe-Ni, Co-Ni and Fe-Co random alloys using a combination of
electronic structure calculations and atomistic spin dynamics simulations.
Influence of the disorder are studied in detail by use of large supercells with
random atomic arrangement. It is found that disorder affects the magnon
spectrum in vastly different ways depending on the system. Specifically, it is
more pronounced in Fe-Ni alloys compared to Fe-Co alloys. In particular, the
magnon spectrum at room temperature in Permalloy (Fe$_{20}$Ni$_{80}$) is found
to be rather diffuse in a large energy interval while in Fe$_{75}$Co$_{25}$ it
forms sharp branches. Fe-Co alloys are very interesting from a technological
point of view due to the combination of large Curie temperatures and very low
calculated Gilbert damping of $\sim$0.0007 at room temperature for Co
concentrations around 20--30\%.",1810.08487v1
2019-05-10,Helicity-Changing Brillouin Light Scattering by Magnons in a Ferromagnetic Crystal,"Brillouin light scattering in ferromagnetic materials usually involves one
magnon and two photons and their total angular momentum is conserved. Here, we
experimentally demonstrate the presence of a helicity-changing two-magnon
Brillouin light scattering in a ferromagetic crystal, which can be viewed as a
four-wave mixing process involving two magnons and two photons. Moreover, we
observe an unconventional helicity-changing one-magnon Brillouin light
scattering, which apparently infringes the conservation law of the angular
momentum. We show that the crystal angular momentum intervenes to compensate
the missing angular momentum in the latter scattering process.",1905.04018v5
2019-05-27,Steady Bell state generation via magnon-photon coupling,"We show that parity-time ($\mathcal{PT}$) symmetry can be spontaneously
broken in the recently reported energy level attraction of magnons and cavity
photons. In the $\mathcal{PT}$-broken phase, magnon and photon form a
high-fidelity Bell state with maximum entanglement. This entanglement is steady
and robust against the perturbation of environment, in contrast to the general
wisdom that expects instability of the hybridized state when the symmetry is
broken. This anomaly is further understood by the compete of non-Hermitian
evolution and particle number conservation of the hybridized system. As a
comparison, neither $\mathcal{PT}$-symmetry broken nor steady magnon-photon
entanglement is observed inside the normal level repulsion case. Our results
may open a novel window to utilize magnon-photon entanglement as a resource for
quantum technologies.",1905.11117v1
2020-06-25,Magnon-exciton proximity coupling at a van der Waals heterointerface,"Spin and photonic systems are at the heart of modern information devices and
emerging quantum technologies. An interplay between electron-hole pairs
(excitons) in semiconductors and collective spin excitations (magnons) in
magnetic crystals would bridge these heterogeneous systems, leveraging their
individual assets in novel interconnected devices. Here, we report the
magnon-exciton coupling at the interface between a magnetic thin film and an
atomically-thin semiconductor. Our approach allies the long-lived magnons
hosted in a film of yttrium iron garnet (YIG) to strongly-bound excitons in a
flake of a transition metal dichalcogenide, MoSe$_2$. The magnons induce on the
excitons a dynamical valley Zeeman effect ruled by interfacial exchange
interactions. This nascent class of hybrid system suggests new opportunities
for information transduction between microwave and optical regions.",2006.14257v2
2020-09-17,Investigation of entanglement measures across the magnetization process of a highly frustrated spin-1/2 Heisenberg octahedral chain as a new paradigm of the localized-magnon approach,"The bipartite entanglement across the magnetization process of a highly
frustrated spin-1/2 Heisenberg octahedral chain is examined within the concept
of localized magnons, which enables a simple calculation of the concurrence
measuring a strength of the pairwise entanglement between nearest-neighbor and
next-nearest-neighbor spins from square plaquettes. A full exact
diagonalization of the finite-size Heisenberg octahedral chain with up to 4
unit cells (20 spins) evidences an extraordinary high precision of the
localized-magnon theory in predicting measures of the bipartite entanglement at
sufficiently low temperatures. While the monomer-tetramer phase emergent at low
enough magnetic fields exhibits presence (absence) of the bipartite
entanglement between the nearest-neighbor (next-nearest-neighbor) spins, the
magnon-crystal phase emergent below the saturation field contrarily displays
identical bipartite entanglement between the nearest-neighbor and
next-nearest-neighbor spins. The presented results verify a new paradigm of the
localized-magnon approach concerned with a simple calculation of entanglement
measures.",2009.08504v1
2020-09-21,"Simultaneous blockade of a photon phonon, and magnon induced by a two-level atom","The hybrid microwave optomechanical-magnetic system has recently emerged as a
promising candidate for coherent information processing because of the
ultrastrong microwave photon-magnon coupling and the longlife of the magnon and
phonon. As a quantum information processing device, the realization of a single
excitation holds special meaning for the hybrid system. In this paper, we
introduce a single two-level atom into the optomechanical-magnetic system and
show that an unconventional blockade due to destructive interference cannot
offer a blockade of both the photon and magnon. Meanwhile, under the condition
of single excitation resonance, the blockade of the photon, phonon, and magnon
can be achieved simultaneously even in a weak optomechanical region, but the
phonon blockade still requires the cryogenic temperature condition.",2009.09871v1
2020-09-22,Magnon Thermal Edelstein Effect Detected by Inverse Spin Hall Effect,"In an easy-plane antiferromagnet with the Dzyaloshinskii-Moriya interaction
(DMI), magnons are subject to an effective spin-momentum locking. An in-plane
temperature gradient can generate interfacial accumulation of magnons with a
specified polarization, realizing the magnon thermal Edelstein effect. We
theoretically investigate the injection and detection of this thermally-driven
spin polarization in an adjacent heavy metal with strong spin Hall effect. We
find that the inverse spin Hall voltage depends monotonically on both
temperature and the DMI but non-monotonically on the hard-axis anisotropy.
Counterintuitively, the magnon thermal Edelstein effect is an even function of
a magnetic field applied along the N\'eel vector.",2009.10703v2
2021-02-12,Topological Thermal Hall Effect of Magnons in Magnetic Skyrmion Lattice,"Topological transports of fermions are governed by the Chern numbers of the
energy bands lying below the Fermi energy. For bosons, e.g. phonons and magnons
in a crystal, topological transport is dominated by the Chern number of the
lowest energy band when the band gap is comparable to the thermal energy. Here,
we demonstrate the presence of topological transport by bosonic magnons in a
lattice of magnetic skyrmions - topological defects formed by a vortex-like
texture of spins. We find a distinct thermal Hall signal in the magnetic
skyrmion phase of an insulating polar magnet GaV4Se8, identified as the
topological thermal Hall effect of magnons governed by the Chern number of the
lowest energy band of the magnons in a triangular lattice of magnetic
skyrmions. Our findings lay a foundation for studying topological phenomena of
other bosonic excitations through thermal Hall probe.",2102.06430v3
2015-07-10,Magnon dark modes and gradient memory,"Extensive efforts have been expended in developing hybrid quantum systems to
overcome the short coherence time of superconducting circuits by introducing
the naturally long-lived spin degree of freedom. Among all the possible
materials, single-crystal yttrium iron garnet has shown up very recently as a
promising candidate for hybrid systems, and various highly coherent
interactions, including strong and even ultra-strong coupling, have been
demonstrated. One distinct advantage of these systems is that the spins are in
the form of well-defined magnon modes, which allows flexible and precise
tuning. Here we demonstrate that by dissipation engineering, a non-Markovian
interaction dynamics between the magnon and the microwave cavity photon can be
achieved. Such a process enables us to build a magnon gradient memory to store
information in the magnon dark modes, which decouple from the microwave cavity
and thus preserve a long life-time. Our findings provide a promising approach
for developing long-lifetime, multimode quantum memories.",1507.02791v1
2015-12-02,Bose-Einstein Condensation of Magnons Pumped by the Bulk Spin Seebeck Effect,"We propose inducing Bose-Einstein condensation of magnons in a magnetic
insulator by a heat flow oriented toward its boundary. At a critical heat flux,
the oversaturated thermal gas of magnons accumulated at the boundary
precipitates the condensate, which then grows gradually as the thermal bias is
dialed up further. The thermal magnons thus pumped by the magnonic bulk (spin)
Seebeck effect must generally overcome both the local Gilbert damping
associated with the coherent magnetic dynamics as well as the radiative
spin-wave losses toward the magnetic bulk, in order to achieve the threshold of
condensation. We quantitatively estimate the requisite bias in the case of the
ferrimagnetic yttrium iron garnet, discuss different physical regimes of
condensation, and contrast it with the competing (so-called Doppler-shift) bulk
instability.",1512.00557v1
2016-09-03,Magnon-phonon coupling and two-magnon continuum in two-dimensional triangular antiferromagnet CuCrO2,"CuCrO2 is a manifestation of a two-dimensional triangular antiferromagnet
which exhibits an incommensurate noncollinear magnetic structure similar to a
classical 120 degree ordering. Using the inelastic neutron scattering
technique, direct evidence of a magnon-phonon coupling in CuCrO2 is revealed
via the mixed magnon-phonon character of the excitation mode at 12.5 meV as
well as a minimum at the zone boundary. A simple model Hamiltonian that
incorporates an exchange striction type magnon-phonon coupling reproduces the
observed features accurately. Also, continuum excitations originating from the
interaction between quasiparticles are observed with strong intensity at the
zone boundary. These features of the magnetic excitations are key to an
understanding of the emergent excitations in noncollinear antiferromagnetic
compounds.",1609.00785v1
2016-09-13,Formation of new phase inclusions in the system of quasi-equilibrium magnons of high density,"The paper studies the spatial variation of the magnetization in a
nonconducting magnetic sample with an excess number of magnons in comparison to
the equilibrium. The phenomenon is considered using the Landau-Lifshits
equation with additional terms describing the longitudinal relaxation of the
magnetization, the magnon diffusion and the magnon creation by external
pumping. The free energy of the system is presented in the mean field
approximation. It is shown that, if the pumping exceeds some critical value,
regions of the new phase arise where the magnetic moments are oriented opposite
to the magnetization of the magnetic sample. The phenomenon is similar to the
appearance of droplets of the condensed phase in the supersaturated vapor. The
appearance of the new phase either in the form of a single domain or a
periodical lattice is demonstrated. The studied process is a competitor to the
process of the Bose-Einstein condensation of magnons.",1609.04768v1
2018-01-19,Nonabelian magnonics in antiferromagnets,"We present a semiclassical formalism for antiferromagnetic (AFM) magnonics
which promotes the central ingredient of spin wave chirality, encoded in a
quantity called magnonic isospin, to a first-class citizen of the theory. We
use this formalism to unify results of interest from the field under a single
chirality-centric formulation. Our main result is that the isospin is governed
by unitary time evolution, through a Hamiltonian projected down from the full
spin wave dynamics. Because isospin is SU(2)-valued, its dynamics on the Bloch
sphere are precisely rotations - which, in general, do not commute.
Consequently, the induced group of operations on AFM spin waves is nonabelian.
This is a paradigmatic departure from ferromagnetic magnonics, which operates
purely within the abelian group generated by spin wave phase and amplitude. Our
investigation of this nonabelian magnonics in AFM insulators focuses on
studying several simple gate operations, and offering in broad strokes a
program of study for interesting new logic families in antiferromagnetic spin
wave systems",1801.06535v3
2019-06-10,Low-energy dynamics of the Affleck-Kennedy-Lieb-Tasaki model in the one- and two-triplon basis,"The elementary excitation in the antiferromagnetic spin-1 model known as the
Affleck-Kennedy-Lieb-Tasaki (AKLT) Hamiltonian has been described alternatively
as magnons or kink-like solitons (triplons). The latter, which we call the
triplon throughout this paper, has been proven equivalent descriptions of the
same magnon excitation and not an independent branch of excited states. On the
other hand, no careful examination of multi-magnon and multi-triplon
equivalence was made in the past. In this paper we prove that two-magnon and
two-triplon states are also identical descriptions of the same excited states,
and furthermore that their energies break down as the sum of one-triplon
energies exactly for the AKLT Hamiltonian. The statement holds despite the fact
that the model is non-integrable. Such magnon/triplon dichotomy is conjectured
to hold for arbitrary n-magnon and n-triplon states. The one- and two-triplon
states form orthogonal sets that can be used to span the low-energy Hilbert
space. We construct an effective version of the AKLT Hamiltonian within such
subspace, and work out the correction to the one-triplon energy gap that finds
excellent agreement with the known exact value.",1906.03758v1
2019-07-02,Bias-free reconfigurable magnonic phase shifter based on a spin-current controlled ferromagnetic resonator,"Controllable phase modulation plays a pivotal role in the researches of
magnonic logic gates. Here we propose a reconfigurable spin-current controlled
magnonic phase shifter based on a ferromagnetic resonator. The proposed phase
shifter requires no magnetic bias field during operation. The device is
directly configured over the waveguide while keeping the original structure of
the waveguide unaffected. Numerical micromagnetic simulations show that the
phase shifter could yield either a {\pi}-phase or no shift depending on the
magnetization status of the resonator, which can be controlled by a current
pulse. Moreover, the phase-shifting operation could be affected by spin
current. At different input current density, the device could be either used as
a dynamic controlled phase shifter or a spin-wave valve. Finally, a XNOR
magnonic logic gate is demonstrated using the proposed phase shifter. Our work
can be a beneficial step to enhance the functionality and compatibility of the
magnonic logic circuits.",1907.01303v1
2019-10-24,Resources of nonlinear cavity magnonics for quantum information,"We theoretically explore nonlinearities of ferromagnets in microwave cavities
in the classical and quantum regimes, and assess the resources for quantum
information, i.e. fluctuation squeezing and bipartite entanglement. The
(semi-)classical analysis of the anharmonic oscillator (Duffing) model for the
Kittel mode when including all other magnon modes, reveals chaotic and
limit-cycle phases that do not survive in quantum calculations. However,
magnons with nonzero wavenumbers that are driven by the Suhl instability of the
Kittel mode, form a genuine limit cycle. We subsequently compute bounds for the
distillable entanglement, as well as entanglement of formation for the
bipartite configurations of the mixed magnon modes. The distillable
entanglement of bipartite states accessible from a covariance matrix vanishes,
but can be recovered by injection locking. The predicted magnon entanglement
can be experimentally tested with yttrium iron garnet samples under realistic
conditions.",1910.11130v1
2020-01-20,The topological magnon bands in the Flux state in Sashtry-Sutherland lattice,"We investigate low energy magnon excitations above the non-collinear flux
state and non-coplanar canted flux state in a Heisenberg anti-ferromagnet with
Dzyaloshinskii-Moriya interaction~(DMI) on a Sashtry-Sutherland lattice.While
previous studies have shown the presence of topological magnetic excitation in
the dimer and ferromagnetic phases on the Shastry-Sutherland lattice, our
results establish the non-trivial topology of magnons in the anti-ferromagnetic
flux and canted flux states. Our results uncover the existence of a multitude
of topological phase transitions in the magnon sector -- evidenced by the
changing Chern numbers of the single magnon bands -- as the Hamiltonian
parameters are varied, even when the ground state remains unchanged. The
thermal Hall conductivity is calculated and its derivative is shown to exhibit
a logarithmic divergence at the phase transitions, independent of the type of
band touching involved. This may provide a useful means to identify the energy
at which the transition occurs. Finally, we propose the way to realize the
studied model in a practical material.",2001.07008v1
2020-02-28,Spin Hall magnetoresistance in Pt/YIG bilayers via varying magnon excitation,"Spin Hall magnetoresistance (SMR) and magnon excitation magnetoresistance
(MMR) that all generate via the spin Hall effect and inverse spin Hall effect
in a nonmagnetic material are always related to each other. However, the
influence of magnon excitation for SMR is often overlooked due to the
negligible MMR. Here, we investigate the SMR in Pt/Y3Fe5O12 (YIG) bilayers from
5 to 300K, in which the YIG are treated after Ar+-ion milling. The SMR in the
treated device is smaller than in the non-treated. According to theoretical
simulation, we attribute this phenomenon to the reduction of the interfacial
spin-mixing conductance at the treated Pt/YIG interface induced by the magnon
suppression. Our experimental results point out that the SMR and the MMR are
inter-connected, and the former could be modulated via magnon excitation. Our
findings provide a new approach for separating and clarifying the underlying
mechanisms.",2002.12550v1
2020-02-29,Nonreciprocal Emergence of Hybridized Magnons in magnetic thin Films,"We investigate the transfer and control of nonreciprocity through magnons
themselves in permalloy thin films deposited on surface oxide silicon
substrate. Evidences of nonreciprocal emergence of hybridized dipole exchange
magnons (spin waves) at two permalloy surfaces are provided by studying magnon
transmission and asymmetry, via Brillouin light scattering measurements. The
dipole dominated spin wave and exchange dominated spin wave are found to be
localized near the top and bottom surfaces, respectively, and traveling along
opposite directions. The nonreciprocity and the localization are intertwined
and ca n be tuned by an in plane magnetic field. The effects are well explained
by the magnetostatic theory and can be quantitatively reproduced by the
micromagnetic simulations. Our findings provide a simple and flexible approach
to nonreciprocal all magnon logi c devices with highly compatible with silicon
based integrated circuit technology.",2003.00230v4
2020-03-12,"Tunable multiwindow magnomechanically induced transparency, Fano resonances, and slow-to-fast light conversion","We investigate the absorption and transmission properties of a weak probe
field under the influence of a strong control field in a hybrid cavity
magnomechanical system in the microwave regime. This hybrid system consists of
two ferromagnetic material yttrium iron garnet (YIG) spheres strongly coupled
to a single cavity mode. In addition to two magnon-induced transparency (MIT)
that arise due to strong photon-magnon interactions, we observe a
magnomechanically induced transparency (MMIT) due to the presence of nonlinear
phonon-magnon interaction. In addition, we discuss the emergence and tunability
of the multiple Fano resonances in our system. We find that due to strong
photon-magnon coupling the group delay of the probe field can be enhanced
significantly. The subluminal or superluminal propagation depends on the
frequency of the magnons, which can be easily tuned by an external bias
magnetic field. Besides, the group delay of the transmitted field can also be
controlled with the control field power.",2003.13760v4
2020-04-07,Optomagnonic Barnett effect,"Combining the technologies of quantum optics and magnonics, we find that the
circularly polarized laser can dynamically realize the quasiequilibrium magnon
Bose-Einstein condensates (BEC). The Zeeman coupling between the laser and
spins generates the optical Barnett field, and its direction is controllable by
switching the laser chirality. We show that the optical Barnett field develops
the total magnetization in insulating ferrimagnets with reversing the local
magnetization, which leads to the quasiequilibrium magnon BEC. This
laser-induced magnon BEC transition through optical Barnett effect, dubbed the
optomagnonic Barnett effect, provides an access to coherent magnons in the high
frequency regime of the order of terahertz. We also propose a realistic
experimental setup to observe the optomagnonic Barnett effect using current
device and measurement technologies as well as the laser chirping. The
optomagnonic Barnett effect is a key ingredient for the application to
ultrafast spin transport.",2004.03353v2
2020-04-15,Topological magnon bands in a room temperature Kagome magnet,"Topological magnon is a vibrant research field gaining more and more
attention in the past few years. Among many theoretical proposals and limited
experimental studies, ferromagnetic Kagome lattice emerges as one of the most
elucidating systems. Here we report neutron scattering studies of YMn6Sn6, a
metallic system consisting of ferromagnetic Kagome planes. This system
undergoes a commensurate-to-incommensurate antiferromagnetic phase transition
upon cooling with the incommensurability along the out-of-plane direction. We
observe magnon band gap opening at the symmetry-protected K points and ascribe
this feature to the antisymmetric Dzyaloshinskii-Moriya (DM) interactions. Our
observation supports the existence of topological Dirac magnons in both the
commensurate collinear and incommensurate coplanar magnetic orders, which is
further corroborated by symmetry analysis. This finding places YMn6Sn6 as a
promising candidate for room-temperature magnon spintronics applications.",2004.07121v1
2020-04-23,Nanoscale Detection of Magnon Excitations with Variable Wavevectors Through a Quantum Spin Sensor,"We report the optical detection of magnons with a broad range of wavevectors
in magnetic insulator Y3Fe5O12 thin films by proximate nitrogen-vacancy (NV)
single-spin sensors. Through multi-magnon scattering processes, the excited
magnons generate fluctuating magnetic fields at the NV electron spin resonance
frequencies, which accelerate the relaxation of NV spins. By measuring the
variation of the emitted spin-dependent photoluminescence of the NV centers,
magnons with variable wavevectors up to ~ 5 x 10^7 m-1 can be optically
accessed, providing an alternative perspective to reveal the underlying spin
behaviors in magnetic systems. Our results highlight the significant
opportunities offered by NV single-spin quantum sensors in exploring nanoscale
spin dynamics of emergent spintronic materials.",2004.11067v1
2020-05-03,Berry curvature for coupled waves of magnons and electromagnetic waves,"In this paper, we introduce Berry curvature, topological Chern number and
topological chiral edge mode, that emerge from a hybridization between magnon
and electromagnetic wave in a ferromagnet insulator. By focusing on the energy
conservation, we first reformulate the Landau-Lifshitz-Maxwell equation into a
Hermitian eigenvalue equation. From the eigenvalue equation, we define the
Berry curvature of the magnon-photon coupled waves. We show that the Berry
curvature thus introduced shows a prominent peak around a hybridization point
between magnon mode and photon mode, and a massive hybrid mode takes a non-zero
Chern number ($\pm 1$) due to the magnon-photon coupling. In accordance with
the non-zero Chern number, the topological edge modes emerge inside the
hybridization gap at a domain wall between two ferromagnetic insulators with
opposite magnetizations.",2005.01105v1
2020-05-10,Stationary entanglement between light and microwave via ferromagnetic magnons,"We show how to generate stationary entanglement between light and microwave
in a hybrid opto-electro-magnonical system which mainly consists of a microwave
cavity, a yttrium iron garnet (YIG) sphere and a nanofiber. The optical modes
in nanofiber can evanescently coupled to whispering gallery modes, that are
able to interact with magnon mode via spin-orbit interaction, in YIG sphere,
while the microwave cavity photons and magnons are coupled through magnetic
dipole interaction simultaneously. Under reasonable parameter regimes, pretty
amount of entanglement can be generated, and it also shows persistence against
temperature. Our work is expected to provide a new perspective for building
more advanced and comprehensive quantum networks along with magnons for
fast-developing quantum technology and for studying the macroscopic quantum
phenomena.",2005.04581v3
2020-05-20,Detectability of Axion Dark Matter with Phonon Polaritons and Magnons,"Collective excitations in condensed matter systems, such as phonons and
magnons, have recently been proposed as novel detection channels for light dark
matter. We show that excitation of i) optical phonon polaritons in polar
materials in an ${\mathcal O}$(1 T) magnetic field (via the axion-photon
coupling), and ii) gapped magnons in magnetically ordered materials (via the
axion wind coupling to the electron spin), can cover the difficult-to-reach
${\mathcal O}$(1-100) meV mass window of QCD axion dark matter with less than a
kilogram-year exposure. Finding materials with a large number of optical phonon
or magnon modes that can couple to the axion field is crucial, suggesting a
program to search for a range of materials with different resonant energies and
excitation selection rules; we outline the rules and discuss a few candidate
targets, leaving a more exhaustive search for future work. Ongoing development
of single photon, phonon and magnon detectors will provide the key for
experimentally realizing the ideas presented here.",2005.10256v1
2020-05-28,Moiré magnons in twisted bilayer magnets with collinear order,"We explore the moir\'e magnon bands in twisted bilayer magnets with
next-nearest neighboring Dzyaloshinskii-Moriya interactions, assuming that the
out-of-plane collinear magnetic order is preserved under weak interlayer
coupling. By calculating the magnonic band structures and the topological Chern
numbers for four representative cases, we find that (i) the valley moir\'e
bands are extremely flat over a wide range of continuous twist angles; (ii) the
topological Chern numbers of the lowest few flat bands vary significantly with
the twist angle; and (iii) the lowest few topological flat bands in bilayer
antiferromagnets entail nontrivial thermal spin transport in the transverse
direction; These properties make twisted bilayer magnets an ideal platform to
study the magnonic counterparts of moir\'e electrons, where the statistical
distinction between magnons and electrons leads to fundamentally new physical
behavior.",2005.14096v2
2020-07-22,Theoretical realization of rich magnon topology by symmetry-breaking in honeycomb bilayer ferromagnets,"We reveal the rich magnon topology in honeycomb bilayer ferromagnets (HBF)
induced by the combined effect of interlayer exchange, Dzyaloshinskii-Moriya
interaction (DMI), and electrostatic doping (ED). In particular, we present a
systematic study of the Hamiltonian non-adiabatic evolution in the HBF
parametric space, spanned by the symmetry-breaking terms (DMI and ED) and
interlayer exchange. We determine the band closure manifolds which are found to
divide the parametric space into six distinct regions, matched with five
distinct topological phases and one topologically trivial phase. The
characteristic Chern numbers and thermal Hall conductivities are calculated for
the topological phases. Edge spectra, dictated by the bulk-edge correspondence,
are also analyzed in the nanoribbon version of the model. Both bulk and edge
spectra are found to be nonreciprocal as a consequence of ED and edge magnons
are observed to counter propagate on opposite edges. The predicted results
offer new insights on the manipulation of magnonic Chern numbers and magnon
topological transport via experimentally tunable parameters.",2007.11692v2
2020-07-27,Magnonic Superradiant Phase Transition,"We show that the low-temperature phase transition in ErFeO3 that occurs at a
critical temperature of ~ 4 K can be described as a magnonic version of the
superradiant phase transition (SRPT). The role of photons in the
quantum-optical SRPT is played by Fe magnons, while that of two-level atoms is
played by Er spins. Our spin model, which is reduced to an extended Dicke
model, takes into account the short-range, direct exchange interactions between
Er spins in addition to the long-range Er-Er interactions mediated by Fe
magnons. By using realistic parameters determined by recent terahertz
magnetospectroscopy and magnetization experiments, we demonstrate that it is
the cooperative, ultrastrong coupling between Er spins and Fe magnons that
causes the phase transition. This work thus proves ErFeO3 to be a unique system
that exhibits a SRPT in thermal equilibrium, in contrast to previous
observations of laser-driven non-equilibrium SRPTs.",2007.13263v1
2020-07-30,Electron-magnon spin conversion and magnonic spin pumping in antiferromagnet/heavy metal heterostructure,"We study the exchange between electron and magnon spins at the interface of
an antiferromagnet and a heavy metal at finite temperatures. The underlying
physical mechanism is based on spin torque associated with the
creation/annihilation of thermal magnons with right-hand and left-hand
polarization. The creation/annihilation process depends strongly on the
relative orientation between the polarization of the electron and the magnon
spins. For a sufficiently strong spin transfer torque (STT), the conversion
process becomes nonlinear, generating a nonzero net spin pumping current in the
AFM that can detected in the neighboring metal layer. Applying an external
magnetic field renders possible the manipulation of the STT driving thermal
spin pumping. Our theoretical results are experimentally feasible and are of a
direct relevance to antiferromagnet-based spintronic devices.",2007.15512v1
2020-08-07,Cavity magnon polariton based precision magnetometry,"A photon-magnon hybrid system can be realised by coupling the electron spin
resonance of a magnetic material to a microwave cavity mode. The quasiparticles
associated with the system dynamics are the cavity magnon polaritons, which
arise from the mixing of strongly coupled magnons and photons. We illustrate
how these particles can be used to probe the magnetisation of a sample with a
remarkable sensitivity, devising suitable spin-magnetometers which ultimately
can be used to directly assess oscillating magnetic fields. Specifically, the
capability of cavity magnon polaritons of converting magnetic excitations to
electromagnetic ones, allows for translating to magnetism the quantum-limited
sensitivity reached by state-of-the-art electronics. Here we employ hybrid
systems composed of microwave cavities and ferrimagnetic spheres, to
experimentally implement two types of novel spin-magnetometers.",2008.03062v2
2020-08-08,Two-magnon frequency-pulling effect in ferromagnetic resonance,"We report the experimental observation in thin films of the hybridization of
the uniform ferromagnetic resonance mode with nonuniform magnons as a result of
the two-magnon scattering mechanism, leading to a frequency-pulling effect on
the ferromagnetic resonance. This effect, when not properly accounted for,
leads to a discrepancy in the dependence of the ferromagnetic resonance field
on frequency for different field orientations. The frequency-pulling effect is
the complement of the broadening of the ferromagnetic resonance lineshape by
two-magnon scattering and can be calculated using the same parameters. By
accounting for the two-magnon frequency shifts through these means, consistency
is achieved in fitting data from in-plane and perpendicular-to-plane resonance
conditions.",2008.03423v2
2020-08-17,Scattering of magnons at graphene quantum-Hall-magnet junctions,"Motivated by recent non-local transport studies of quantum-Hall-magnet (QHM)
states formed in monolayer graphene's $N=0$ Landau level, we study the
scattering of QHM magnons by gate-controlled junctions between states with
different integer filling factors $\nu$. For the $\nu=1|-1|1$ geometry we find
magnons are weakly scattered by electric potential variation in the junction
region, and that the scattering is chiral when the junction lacks a mirror
symmetry. For the $\nu=1|0|1$ geometry, %in which the scattering region
contains a $\nu=0$ canted antiferromagnet, we find that kinematic constraints
completely block magnon transmission if the incident angle exceeds a critical
value. Our results explain the suppressed non-local-voltage signals observed in
the $\nu=1|0|1$ case. We use our theory to propose that valley-waves generated
at $\nu=-1|1$ junctions and magnons can be used in combination to probe the
spin/valley flavor structure of QHM states at integer and fractional filling
factors.",2008.07583v1
2020-10-08,Chiral Hinge Magnons in Second-Order Topological Magnon Insulators,"When interacting spins in condensed matter order ferromagnetically, their
ground state wave function is topologically trivial. Nonetheless, in two
dimensions, the ferromagnetic state can support spin excitations with
nontrivial topology, an exotic state known as topological magnon insulator
(TMI). Here, we theoretically unveil and numerically confirm a novel
ferromagnetic state in three dimensions dubbed second-order TMI, whose
hallmarks are excitations at its hinges, where facets intersect. Since
ferromagnetism naturally comes with broken time-reversal symmetry, the hinge
magnons are chiral, rendering backscattering impossible. Hence, they trace out
a three-dimensional path about the sample unimpeded by defects and are
topologically protected by the spectral gap. They are remarkably robust against
disorder and simultaneously highly tunable by atomic-level engineering of the
sample termination. Our findings empower magnonics with the tools of
higher-order topology, a promising route to combine low-energy information
transfer free of Joule heating with three-dimensional vertical integration.",2010.04142v1
2020-10-19,Magnetic texture based magnonics,"The spontaneous magnetic orders arising in ferro-, ferri- and
antiferromagnets stem from various magnetic interactions. Depending on the
interplay and competition among the Heisenberg exchange interaction,
Dzyaloshinskii-Moriya exchange interaction, magnetic dipolar interaction and
crystal anisotropies, a great variety of magnetic textures may be stabilized,
such as magnetic domain walls, vortices, Skyrmions and spiral helical
structures. While each of these spin textures responds to external forces in a
specific manner with characteristic resonance frequencies, they also interact
with magnons, the fundamental collective excitation of the magnetic order,
which can propagate in magnetic materials free of charge transport and
therefore with low energy dissipation. Recent theories and experiments found
that the interplay between spin waves and magnetic textures is particularly
interesting and rich in physics. In this review, we introduce and discuss the
theoretical framework of magnons living on a magnetic texture background, as
well as recent experimental progress in the manipulation of magnons via
magnetic textures. The flexibility and reconfigurability of magnetic textures
are discussed regarding the potential for applications in information
processing schemes based on magnons.",2010.09180v1
2020-10-28,Floquet Cavity Electromagnonics,"Hybrid magnonics has recently attracted intensive attentions as a promising
platform for coherent information processing. In spite of its rapid
development, on-demand control over the interaction of magnons with other
information carriers, in particular microwave photons in electromagnonic
systems, has been long missing, significantly limiting the broad applications
of hybrid magnonics. Here, we show that by introducing Floquet engineering into
cavity electromagnonics, coherent control on the magnon-microwave photon
coupling can be realized. Leveraging the periodic temporal modulation from a
Floquet drive, our first-of-its-kind Floquet cavity electromagnonic system can
manipulate the interaction between hybridized cavity electromagnonic modes on
demand. Moreover, we demonstrate a new coupling regime in such systems: the
Floquet ultrastrong coupling, where the Floquet splitting is comparable with or
even larger than the level spacing of the two interacting modes, resulting in
the breakdown of the rotating wave approximation. Our findings open up new
directions for magnon-based coherent signal processing.",2010.14727v1
2020-11-25,Antiferromagnetic magnon pseudospin: Dynamics and diffusive transport,"We formulate a theoretical description of antiferromagnetic magnons and their
transport in terms of an associated pseudospin. The need and strength of this
formulation emerges from the antiferromagnetic eigenmodes being formed from
superpositions of spin-up and -down magnons, depending on the material
anisotropies. Consequently, a description analogous to that of spin-1/2
electrons is demonstrated while accounting for the bosonic nature of the
antiferromagnetic eigenmodes. Introducing the concepts of a pseudospin chemical
potential together with a pseudofield and relating magnon spin to pseudospin
allows a consistent description of diffusive spin transport in
antiferromagnetic insulators with any given anisotropies and interactions.
Employing the formalism developed, we elucidate the general features of recent
non-local spin transport experiments in antiferromagnetic insulators hosting
magnons with different polarisations. The pseudospin formalism developed herein
is valid for any pair of coupled bosons and is likely to be useful in other
systems comprising interacting bosonic modes.",2011.12927v1
2021-01-22,Opportunities for long-range magnon-mediated entanglement of spin qubits via on- and off-resonant coupling,"The ability to manipulate entanglement between multiple spatially-separated
qubits is essential for quantum information processing. Although
nitrogen-vacancy (NV) centers in diamond provide a promising qubit platform,
developing scalable two-qubit gates remains a well-known challenge. To this
end, magnon-mediated entanglement proposals have attracted attention due to
their long-range spin-coherent propagation. Optimal device geometries and gate
protocols of such schemes, however, have yet to be determined. Here we predict
strong long-distance ($>\mu$m) NV-NV coupling via magnon modes with
cooperativities exceeding unity in ferromagnetic bar and waveguide structures.
Moreover, we explore and compare on-resonant transduction and off-resonant
virtual-magnon exchange protocols, and discuss their suitability for generating
or manipulating entangled states at low temperatures ($T\lesssim 150$ mK) under
realistic experimental conditions. This work will guide future experiments that
aim to entangle spin qubits in solids with magnon excitations.",2101.09220v2
2021-03-09,Strong Magnon-Magnon Coupling in Synthetic Antiferromagnets,"Synthetic antiferromagnet, comprised of two ferromagnetic layers separated by
a non-magnetic layer, possesses two uniform precession resonance modes:
in-phase acoustic mode and out-of-phase optic mode. In this work, we
theoretically and numerically demonstrated the strong coupling between acoustic
and optic magnon modes. The strong coupling is attributed to the symmetry
breaking of the system, which can be realized by tilting the bias field or
constructing an asymmetrical synthetic antiferromagnet. It is found that the
coupling strength can be highly adjusted by tuning the tilting angle of bias
field, the magnitude of antiferromagnetic interlayer exchange coupling, and the
thicknesses of ferromagnetic layers. Furthermore, the coupling between acoustic
and optic magnon modes can even reach the ultrastrong coupling regime. Our
findings show high promise for investigating quantum phenomenon with a magnonic
platform.",2103.05153v1
2021-03-19,Remote magnon entanglement between two massive ferrimagnetic spheres via cavity optomagnonics,"Recent studies show that hybrid quantum systems based on magnonics provide a
new and promising platform for generating macroscopic quantum states involving
a large number of spins. Here we show how to entangle two magnon modes in two
massive yttrium-iron-garnet (YIG) spheres using cavity optomagnonics, where
magnons couple to high-quality optical whispering gallery modes supported by
the YIG sphere. The spheres can be as large as 1 mm in diameter and each sphere
contains more than $10^{18}$ spins. The proposal is based on the asymmetry of
the Stokes and anti-Stokes sidebands generated by the magnon-induced Brillouin
light scattering in cavity optomagnonics. This allows one to utilize the Stokes
and anti-Stokes scattering process, respectively, for generating and verifying
the entanglement. Our work indicates that cavity optomagnonics could be a
promising system for preparing macroscopic quantum states.",2103.10595v2
2021-03-20,Ultrafast amplification and non-linear magneto-elastic coupling of coherent magnon modes in an antiferromagnet,"We investigate the role of domain walls in the ultrafast magnon dynamics of
an antiferromagnetic NiO single crystal in a pump-probe experiment with
variable pump photon energy. Analysing the amplitude of the energy-dependent
photo-induced ultrafast spin dynamics, we detect a yet unreported coupling
between the material's characteristic THz- and a GHz-magnon modes. We explain
this unexpected coupling between two orthogonal eigenstates of the
corresponding Hamiltonian by modelling the magneto-elastic interaction between
spins in different domains. We find that such interaction, in the non-linear
regime, couples the two different magnon modes via the domain walls and it can
be optically exploited via the exciton-magnon resonance.",2103.11203v2
2021-03-26,Switching magnon chirality in artificial antiferromagnet,"Magnons in antiferromagnets can support both right-handed and left-handed
chiralities, which shed a light on the chirality-based spintronics. Here we
demonstrate the switching and reading of magnon chirality in an artificial
antiferromagnet. The coexisting antiferromagnetic and ferromagnetic
characteristic resonance modes are discovered, which permits a high tunability
in the modulation of magnon chirality. The reading of the chirality is
accomplished via the chirality-dependent spin pumping as well as spin
rectification effect. Our result illustrates an ideal antiferromagnetic
platform for handling magnon chirality and paves the way for chirality-based
spintronics.",2103.14483v2
2021-04-07,Strain engineering of topological magnons in chromium trihalides from first-principles,"Recent experiments evidence the direct observation of spin waves in chromium
trihalides and a gap at the Dirac points of the magnon dispersion in bulk
CrI$_3$. However, the topological origin of this feature remains unclear and
its emergence at the 2D limit has not yet been proven experimentally. Herein,
we perform a fully self-consistent ab initio analysis that supports the
presence of topological magnons in chromium trihalides monolayers. Our results
confirm the existence of a gap around the K high-symmetry point in the linear
magnon dispersion of CrI$_3$, which originates as a direct consequence of
intralayer Dzyaloshinskii-Moriya (DM) interaction. In addition, our orbital
resolved analysis reveals the microscopic mechanisms that can be exploited
using strain engineering to increase the strength of the DM interaction and
thus control the gap size in CrI$_3$. This paves the way to the further
development of this family of materials as building-blocks for topological
magnonics at the limit of miniaturization.",2104.03023v2
2021-05-18,Parity-symmetry-breaking quantum phase transition via parametric drive in a cavity magnonic system,"We study the parity-symmetry-breaking quantum phase transition (QPT) in a
cavity magnonic system driven by a parametric field, where the magnons in a
ferrimagnetic yttrium-iron-garnet sphere strongly couple to a microwave cavity.
With appropriate parameters, this cavity magnonic system can exhibit a rich
phase diagram, including the parity-symmetric phase, parity-symmetry-broken
phase, and bistable phase. When increasing the drive strength beyond a critical
threshold, the cavity magnonic system undergoes either a first- or second-order
nonequilibrium QPT from the parity-symmetric phase with microscopic excitations
to the parity-symmetry-broken phase with macroscopic excitations, depending on
the parameters of the system. Our work provides an alternate way to engineer
the QPT in a hybrid quantum system containing the spin ensemble in a ferri- or
ferromagnetic material with strong exchange interactions.",2105.08371v3
2021-05-24,Phase-resolved electrical detection of coherently coupled magnonic devices,"We demonstrate the electrical detection of magnon-magnon hybrid dynamics in
yttrium iron garnet/permalloy (YIG/Py) thin film bilayer devices. Direct
microwave current injection through the conductive Py layer excites the hybrid
dynamics consisting of the uniform mode of Py and the first standing spin wave
($n=1$) mode of YIG, which are coupled via interfacial exchange. Both the two
hybrid modes, with Py or YIG dominated excitations, can be detected via the
spin rectification signals from the conductive Py layer, providing phase
resolution of the coupled dynamics. The phase characterization is also applied
to a nonlocally excited Py device, revealing the additional phase shift due to
the perpendicular Oersted field. Our results provide a device platform for
exploring hybrid magnonic dynamics and probing their phases, which are crucial
for implementing coherent information processing with magnon excitations",2105.11057v1
2021-06-10,Photoinduced Floquet topological magnons in a ferromagnetic checkerboard lattice,"This theoretical work is devoted to investigating laser-irradiated Floquet
topological magnon insulators on a two-dimensional checkerboard ferromagnet and
corresponding topological phase transitions. It is shown that the checkerboard
Floquet topological magnon insulator is able to be transformed from a
topological magnon insulator into another one possessing various Berry
curvatures and Chern numbers by changing the light intensity. Especially, we
also show that both Tamm-like and topologically protected Floquet magnon edge
states can exist when a nontrivial gap opens. In addition, our results display
that the sign of the Floquet thermal Hall conductivity is also tunable via
varying the light intensity of the laser field.",2106.05936v3
2021-07-20,Intrinsic Magnon Nernst Effects in Pyrochlore Iridate Thin Films,"We theoretically study the magnon spin thermal transport using a strong
coupling approach in pyrochlore iridate trilayer thin films grown along the
[111] direction. As a result of the Dzyaloshinskii-Moriya interaction (DMI),
the spin configuration of the ground state is an all-in/all-out ordering on
neighboring tetrahedra of the pyrochlore lattice. In such a state, the system
has an inversion symmetry and a Nernst-type thermal spin current response is
well defined. We calculate the temperature dependence of the magnon Nernst
response with respect to the magnon band topology controlled by the spin-orbit
coupling parameters and observe topologically protected chiral edge modes over
a range of parameters. Our study complements prior work on the magnon thermal
Hall effect in thin-film pyrochlore iridates and suggests that the [111] grown
thin-film pyrochlore iridates are a promising candidate for thermal spin
transport and spin caloritronic devices.",2107.09613v1
2021-07-22,Driven-dissipative Quantum Dynamics in Cavity Magnon-Polariton System,"The dynamics of arbitrary-order quantum correlations in a cavity
magnon-polariton system are investigated based on the quantum master equation
in the coherent state representation. The phenomena of Rabi-like oscillation
and level repulsion of the average cavity-photon number agree remarkably well
with existing experimental observations. The competing nature of coherent and
incoherent components in these two cases is further revealed by the
second-order quantum coherence of the cavity photons and magnons, which can be
systematically tuned by the driving microwave and thermal bath. Our results
demonstrate the rich higher-order quantum dynamics induced by magnetic
light-matter interaction, and serve as an indispensable step toward exploring
nonclassical states for cavity photons and magnons in quantum cavity magnonics.",2107.10444v2
2021-08-09,Conductivity Enhancement in a Diffusive Fermi Liquid due to Bose-Einstein Condensation of Magnons,"We theoretically study the conductivity of a disordered 2D metal when it is
coupled to ferromagnetic magnons with a quadratic spectrum and a gap $\Delta$.
In the diffusive limit, a combination of disorder and magnon-mediated electron
interaction leads to a sharp metallic correction to the Drude conductivity as
the magnons approach criticality, i.e., $\Delta \to 0$. The correction is
non-singular and is distinctively weaker than, for example, the logsquared
correction obtained when disordered electrons couple to diffusive spin
fluctuations near a Hertz-Millis transition. The possibility of verifying this
prediction in an S = 1/2 easy-plane ferromagnetic insulator K$_2$CuF$_4$ under
an external magnetic field is proposed. Our results show that the onset of a
magnon BEC in an insulator can be detected via electrical transport
measurements on the proximate metal.",2108.04277v4
2021-09-09,Magnonic spin Joule heating and rectification effects,"Nonlinear devices, such as transistors, enable contemporary computing
technologies. We theoretically investigate nonlinear effects, bearing a high
fundamental scientific and technical relevance, in magnonics with emphasis on
superconductor-ferromagnet hybrids. Accounting for finite magnon chemical
potential, we theoretically demonstrate magnonic spin-Joule heating, the spin
analogue of conventional electronic Joule heating. Besides suggesting a key
contribution to magnonic heat transport in a broad range of devices, it
provides insights into the thermal physics of non-conserved bosonic
excitations. Considering a spin-split superconductor self-consistently, we
demonstrate its interface with a ferromagnetic insulator to harbor large
tunability of spin and thermal conductances. We further demonstrate hysteretic
rectification I-V characteristics in this hybrid, where the hysteresis results
from the superconducting state bistability.",2109.04329v2
2021-11-02,Chiral Magnonic Resonators: Rediscovering the Basic Magnetic Chirality in Magnonics,"The outlook for producing useful practical devices within the paradigm of
magnonics rests on our ability to emit, control and detect coherent exchange
spin waves on the nanoscale. Here, we argue that all these key functionalities
can be delivered by chiral magnonic resonators - soft magnetic elements
chirally coupled, via magneto-dipole interaction, to magnonic media nearby.
Starting from the basic principles of chiral coupling, we outline how they
could be used to construct devices and explore underpinning physics, ranging
from basic logic gates to field programmable gate arrays, in-memory computing,
and artificial neural networks, and extending from one- to two- and
three-dimensional architectures.",2111.01508v1
2021-11-29,Tailoring High-Frequency Magnonics in Monolayer Chromium Trihalides,"Monolayer chromium trihalides, the archetypal two dimensional (2D) magnetic
materials, are readily suggested as a promising platform for high frequency
magnonics. Here we detail the spin wave properties of monolayer CrBr$_3$ and
CrI$_3$, using spin dynamics simulations parametrized from the first
principles. We reveal that spin wave dispersion can be tuned in a broad range
of frequencies by strain, paving the way towards flexo magnonic applications.
We further show that ever present halide vacancies in these monolayers host
sufficiently strong Dzyaloshinskii Moriya interaction to scatter spin waves,
which promotes design of spin-wave guides by defect engineering. Finally we
discuss the spectra of spin-waves propagating across a moir\'e periodic
modulation of magnetic parameters in a van der Waals heterobilayer, and show
that the nanoscale moir\'e periodicities in such samples are ideal for
realization of a magnonic crystal in the terahertz frequency range. Recalling
the additional tunability of magnetic 2D materials by electronic gating, our
results situate these systems among the front-runners for prospective high
frequency magnonic applications.",2111.14305v1
2021-12-08,Role of Magnon-Magnon Scattering in Magnon Polaron Spin Seebeck Effect,"The spin Seebeck effect (SSE) signal of magnon polarons in bulk-Y3Fe5O12
(YIG)/Pt heterostructures is found to drastically change as a function of
temperature. It appears as a dip in the total SSE signal at low temperatures,
but as the temperature increases, the dip gradually decreases before turning to
a peak. We attribute the observed dip-to-peak transition to the rapid rise of
the four-magnon scattering rate. Our analysis provides important insights into
the microscopic origin of the hybridized excitations and the overall
temperature dependence of the SSE anomalies.",2112.04621v1
2021-12-09,Tuneable electron-magnon coupling of ferromagnetic surface states in PdCoO$_2$,"Controlling spin wave excitations in magnetic materials underpins the
burgeoning field of magnonics. Yet, little is known about how magnons interact
with the conduction electrons of itinerant magnets, or how this interplay can
be controlled. Via a surface-sensitive spectroscopic approach, we demonstrate a
strong and highly-tuneable electron-magnon coupling at the Pd-terminated
surface of the delafossite oxide PdCoO$_2$, where a polar surface charge
mediates a Stoner transition to itinerant surface ferromagnetism. We show how
the coupling can be enhanced 7-fold with increasing surface disorder, and
concomitant charge carrier doping, becoming sufficiently strong to drive the
system into a polaronic regime, accompanied by a significant quasiparticle mass
enhancement. Our study thus sheds new light on electron-magnon interactions in
solid-state materials, and the ways in which these can be controlled.",2112.04869v1
2021-12-20,Theory of drift-enabled control in nonlocal magnon transport,"Electrically injected and detected nonlocal magnon transport has emerged as a
versatile method for transporting spin as well as probing the spin excitations
in a magnetic insulator. We examine the role of drift currents in this
phenomenon as a method for controlling the magnon propagation length.
Formulating a phenomenological description, we identify the essential
requirements for existence of magnon drift. Guided by this insight, we examine
magnetic field gradient, asymmetric contribution to dispersion, and temperature
gradient as three representative mechanisms underlying a finite magnon drift
velocity, finding temperature gradient to be particularly effective.",2112.10819v2
2021-12-27,Observation of magnon cross-Kerr effect in cavity magnonics,"When there is a certain amount of field inhomogeneity, the biased
ferrimagnetic crystal will exhibit the higher-order magnetostatic (HMS) mode in
addition to the uniform-precession Kittel mode. In cavity magnonics, we show
both experimentally and theoretically the cross-Kerr-type interaction between
the Kittel mode and HMS mode. When the Kittel mode is driven to generate a
certain number of excitations, the HMS mode displays a corresponding frequency
shift and vice versa. The cross-Kerr effect is caused by an exchange
interaction between these two spin-wave modes. Utilizing the cross-Kerr effect,
we realize and integrate a multi-mode cavity magnonic system with only one
yttrium iron garnet (YIG) sphere. Our results will bring new methods to
magnetization dynamics studies and pave a way for novel cavity magnonic devices
by including the magnetostatic mode-mode interaction as an operational degree
of freedom.",2112.13807v1
2022-01-16,Coherent coupling of two remote magnonic resonators mediated by superconducting circuits,"We demonstrate microwave-mediated distant magnon-magnon coupling on a
superconducting circuit platform, incorporating chip-mounted single-crystal
Y$_3$Fe$_5$O$_{12}$ (YIG) spheres. Coherent level repulsion and dissipative
level attraction between the magnon modes of the two YIG spheres are
demonstrated. The former is mediated by cavity photons of a superconducting
resonator, and the latter is mediated by propagating photons of a coplanar
waveguide. Our results open new avenues towards exploring integrated hybrid
magnonic networks for coherent information processing on a quantum-compatible
superconducting platform.",2201.05937v1
2022-01-18,Magnon Spectrum of the Amorphous Ferromagnet Co$_4$P from Atomistic Spin Dynamics,"An anomaly in the magnon dispersion of the amorphous ferromagnet Co$_{4}$P,
often referred to as a `roton-like' excitation, attracted much attention half a
century ago. With the current interest in heat and spin currents in amorphous
magnets, we apply modern simulation methods, combining reverse Monte Carlo to
build the atomic structure and the stochastic Landau-Lifshitz equation for spin
dynamics, to re-investigate the magnetic excitation spectrum. We find two
magnon valleys, one at the origin and another at a finite wavenumber close to
the observations, but without a magnon gap. We conclude that the second dip is
due to Umklapp scattering caused by residual long-range order, which may be an
alternative explanation of the putative roton excitation. Our study paves the
way to study magnon transport in amorphous magnets and related spintronic
applications.",2201.06986v1
2022-01-26,Nonlinear magnon polaritons,"We experimentally and theoretically demonstrate that nonlinear spin-wave
interactions suppress the hybrid magnon-photon quasiparticle or ""magnon
polariton"" in microwave spectra of an yttrium iron garnet film detected by an
on-chip split-ring resonator. We observe a strong coupling between the Kittel
and microwave cavity modes in terms of an avoided crossing as a function of
magnetic fields at low microwave input powers, but a complete closing of the
gap at high powers. The experimental results are well explained by a
theoretical model including the three-magnon decay of the Kittel magnon into
spin waves. The gap closure originates from the saturation of the ferromagnetic
resonance above the Suhl instability threshold by a coherent back reaction from
the spin waves.",2201.10889v2
2022-01-26,Spin Wave Electromagnetic Nano-Antenna Enabled by Tripartite Phonon-Magnon-Photon Coupling,"We investigate tripartite coupling between phonons, magnons and photons in a
periodic array of elliptical magnetostrictive nanomagnets delineated on a
piezoelectric substrate to form a two-dimensional two-phase multiferroic
crystal. A surface acoustic wave (phonons) of 5 - 35 GHz frequency launched
into the substrate causes the magnetizations of the nanomagnets to precess at
the frequency of the wave, giving rise to spin waves (magnons). The spin waves,
in turn, radiate electromagnetic waves (photons) into the surrounding space at
the surface acoustic wave frequency. Here, the phonons couple into magnons,
which then couple into photons. This tripartite phonon-magnon-photon coupling
is exploited to implement an extreme sub-wavelength electromagnetic antenna
whose measured radiation efficiency and antenna gain exceed the theoretical
limits for traditional antennas by more than two orders of magnitude at some
frequencies. Micro-magnetic simulations are in excellent agreement with
experimental observations and provide insight into the spin wave modes that
couple into radiating electromagnetic modes to implement the antenna.",2201.11110v1
2022-02-09,Electric field control of magnons in magnetic thin films: ab initio predictions for 2D metallic heterostructures,"We explore possibilities for control of magnons in two-dimensional
heterostructures by an external electric field acting across a dielectric
barrier. By performing ab-initio calculations for a Fe monolayer and a Fe
bilayer, both suspended in vacuum and deposited on Cu(001), we demonstrate that
external electric field can significantly modify magnon lifetimes and that
these changes can be related to field-induced changes in the layer-resolved
Bloch spectral functions. For systems with more magnon dispersion branches, the
gap between high- and low-energy eigenmodes varies with the external field.
These effects are strongly influenced by the substrate. Considerable
variability in how the magnon spectra are sensitive to the external electric
field can be expected, depending on the substrate and on the thickness of the
magnetic layer.",2202.04525v1
2022-03-10,Magnon-Cooparons in magnet-superconductor hybrids,"Generation and detection of spinful Cooper pairs in conventional
superconductors has been intensely pursued by designing increasingly complex
magnet-superconductor hybrids. Here, we demonstrate theoretically that magnons
with nonzero wavenumbers universally induce a cloud of spinful triplet Cooper
pairs around them in an adjacent conventional superconductor. The resulting
composite quasiparticle, termed magnon-cooparon, consists of a spin flip in the
magnet screened by a cloud of the spinful superfluid condensate. Thus, it
inherits a large effective mass, which can be measured experimentally.
Furthermore, we demonstrate that two magnetic wires deposited on a
superconductor serve as a controllable magnonic directional coupler mediated by
the nonlocal and composite nature of magnon-cooparons. Our analysis predicts a
quasiparticle that enables generation, control, and use of spinful triplet
Cooper pairs in the simplest magnet-superconductor heterostructures.",2203.05336v2
2022-03-22,Analog quantum control of magnonic cat states on-a-chip by a superconducting qubit,"We propose to directly and quantum-coherently couple a superconducting
transmon qubit to magnons - the quanta of the collective spin excitations, in a
nearby magnetic particle. The magnet's stray field couples to the qubit via a
superconducting quantum interference device (SQUID). We predict a resonant
qubit-magnon exchange and a nonlinear radiation-pressure interaction that are
both stronger than dissipation rates and tunable by an external flux bias. We
additionally demonstrate a quantum control scheme that generates qubit-magnon
entanglement and magnonic Schr\""{o}dinger cat states with high fidelity.",2203.11893v2
2022-03-30,Excitation and transport of bound magnon clusters in frustrated ferromagnetic chain,"We investigate magnetic excitations and spin transport properties of a
frustrated spin chain with ferromagnetic nearest-neighbor and antiferromagnetic
next-nearest-neighbor exchange couplings in a magnetic field by density-matrix
renormalization group methods. In the field-induced quadrupole and octupole
regimes, the low-energy excitation is governed by two- and three-magnon bound
states, respectively, so that bound magnon clusters would contribute to the
spin transport. We show that spin current correlations decrease when the system
goes from the quadrupole regime into the octupole regime. This indicates that
the spin transport is suppressed in the octupole regime, although bound
three-magnon clusters carry a larger amount of angular momentum than bound
two-magnon clusters.",2203.16321v1
2022-04-24,Thermal Evolution of Dirac Magnons in the Honeycomb Ferromagnet CrBr$_3$,"CrBr$_3$ is an excellent realization of the two-dimensional honeycomb
ferromagnet, which offers a bosonic equivalent of graphene with Dirac magnons
and topological character. We perform inelastic neutron scattering (INS)
measurements using state-of-the-art instrumentation to update 50-year-old data,
thereby enabling a definitive comparison both with recent experimental claims
of a significant gap at the Dirac point and with theoretical predictions for
thermal magnon renormalization. We demonstrate that CrBr$_3$ has next-neighbor
$J_2$ and $J_3$ interactions approximately 5\% of $J_1$, an ideal Dirac magnon
dispersion at the K point, and the associated signature of isospin winding. The
magnon lifetime and the thermal band renormalization show the universal $T^2$
evolution expected from an interacting spin-wave treatment, but the measured
dispersion lacks the predicted van Hove features, highlighting the need for a
deeper theoretical analysis.",2204.11355v1
2022-05-05,Spin-wave localization on phasonic defects in one-dimensional magnonic quasicrystal,"We report on the evolution of the spin-wave spectrum under structural
disorder introduced intentionally into one-dimensional magnonic quasicrystal.
We study theoretically a system composed of ferromagnetic strips arranged in a
Fibonacci sequence. We considered several stages of disorder in the form of
phasonic defects, where different rearrangements of strips are introduced. By
transition from the quasiperiodic order towards disorder, we show a gradual
degradation of spin-waves fractal spectra and closing of the frequency gaps. In
particular, the phasonic defects lead to the disappearance of the van Hove
singularities at the frequency gap edges by moving modes into the frequency
gaps and appearing new modes inside the frequency gaps. These modes disperse
and eventually can close the gap, with increasing disorder levels. The work
reveals how the the presence of disorder modifies the intrinsic spin wave
localization existing in undefected magnonic quasicrystals. The paper
contributes to the knowledge of magnonic Fibonacci quasicrystals and opens the
way to study of the phasonic defects in two-dimensional magnonic quasicrystals.",2205.02702v2
2022-05-09,Direct and alternating magnon spin currents across a junction interface irradiated by linearly polarized laser,"The developments in the field of quantum optics raise expectations that
laser-matter coupling is a promising building block for magnonics. Here, we
propose a method for the generation of direct and alternating spin currents of
magnons across the junction interface irradiated by linearly polarized laser.
In a junction of ferromagnetic insulators with a large electronic gap, the spin
angular momentum is exchanged during the tunneling process of magnons across
the junction interface. The advanced technology in the field of plasmonics and
metamaterials realizes that spins irradiated by the laser field interact only
with the magnetic component of the laser through the Zeeman coupling. Using an
analytic perturbation theory, we provide a general formula for magnon transport
induced by the inversion symmetry breaking across the junction interface. Then,
we show that those spin currents are enhanced by the ferromagnetic resonance,
and the period of the ac spin current is one-half of that of the laser magnetic
field. Finally, we estimate the magnitude of the spin current, and find that it
will be within experimental reach.",2205.04572v1
2022-06-18,Magnon straintronics in the 2D van der Waals ferromagnet CrSBr from first-principles,"The recent isolation of two-dimensional (2D) magnets offers tantalizing
opportunities for spintronics and magnonics at the limit of miniaturization.
One of the key advantages of atomically-thin materials is their outstanding
deformation capacity, which provides an exciting avenue to control their
properties by strain engineering. Herein, we investigate the magnetic
properties, magnon dispersion and spin dynamics of the air-stable 2D magnetic
semiconductor CrSBr ($T_C$ = 146 K) under mechanical strain using
first-principles calculations. Our results provide a deep microscopic analysis
of the competing interactions that stabilize the long-range ferromagnetic order
in the monolayer. We showcase that the magnon dynamics of CrSBr can be modified
selectively along the two main crystallographic directions as a function of
applied strain, probing the potential of this quasi-1D electronic system for
magnon straintronics applications. Moreover, we predict a strain-driven
enhancement of $T_C$ considering environmental screening by ~30%, allowing the
propagation of spin waves at higher temperatures.",2206.09277v1
2022-06-20,Chiral current in Floquet cavity-magnonics,"Floquet engineering can induce complex collective behaviour and interesting
synthetic gauge-field in quantum systems through temporal modulation of system
parameters by periodic drives. Using a Floquet drive on frequencies of the
magnon modes, we realize a chiral state-transfer in a cavity-magnonic system.
The time-reversal symmetry is broken in such a promising platform for coherent
information processing. In particular, the photon mode is adiabatically
eliminated in the large-detuning regime and the magnon modes under conditional
longitudinal drives can be indirectly coupled to each other with a
phase-modulated interaction. The effective Hamiltonian is then used to generate
chiral currents in a circular loop, whose dynamics is evaluated to measure the
symmetry of the system Hamiltonian. Beyond the dynamics limited in the manifold
with a fixed number of excitations, our protocol applies to the
continuous-variable systems with arbitrary states. Also it is found to be
robust against the systematic errors in the photon-magnon coupling strength and
Kerr nonlinearity.",2206.09540v2
2022-06-23,Anisotropic magnon damping by zero-temperature quantum fluctuations in ferromagnetic CrGeTe$_3$,"Spin and lattice are two fundamental degrees of freedom in a solid, and their
fluctuations about the equilibrium values in a magnetic ordered crystalline
lattice form quasiparticles termed magnons (spin waves) and phonons (lattice
waves), respectively. In most materials with strong spin-lattice coupling
(SLC), the interaction of spin and lattice induces energy gaps in the spin wave
dispersion at the nominal intersections of magnon and phonon modes. Here we use
neutron scattering to show that in the two-dimensional (2D) van der Waals
honeycomb lattice ferromagnetic CrGeTe3, spin waves propagating within the 2D
plane exhibit an anomalous dispersion, damping, and break-down of quasiparticle
conservation, while magnons along the c axis behave as expected for a local
moment ferromagnet. These results indicate the presence of dynamical SLC
arising from the zero-temperature quantum fluctuations in CrGeTe3, suggesting
that the observed in-plane spin waves are mixed spin and lattice quasiparticles
fundamentally different from pure magnons and phonons.",2206.11962v1
2022-07-07,Exciton-assisted low-energy magnetic excitations in a photoexcited Mott insulator on a square lattice,"The photoexcitation of a Mott insulator on a square lattice weakens the
intensity of both single- and two-magnon excitations as observed in
time-resolved resonant-inelastic X-ray scattering and time-resolved Raman
scattering, respectively. However, the spectral changes in the low-energy
regions below the magnons have not yet been clearly understood. To uncover the
nature of the photoinduced low-energy magnetic excitations of the Mott
insulator, we numerically investigate the transient magnetic dynamics in a
photoexcited half-filled Hubbard model on a square lattice. After turning off a
pump pulse tuned for an absorption edge, new magnetic signals clearly emerge
well below the magnon energy in both single- and two-magnon excitations. We
find that the low-energy excitations are predominantly created via excitonic
states at the absorption edge. These exciton-assisted magnetic excitations may
provide a possible explanation for the low-energy spectral weight in a recent
time-resolved two-magnon Raman scattering experiment for insulating
YBa$_2$Cu$_3$O$_{6.1}$.",2207.03141v2
2022-08-19,Phononic-crystal cavity magnomechanics,"Establishing a way to control magnetic dynamics and elementary excitations
(magnons) is crucial to fundamental physics and the search for novel phenomena
and functions in magnetic solid-state systems. Electromagnetic waves have been
developed as means of driving and sensing in magnonic and spintronics devices
used in magnetic spectroscopy, non-volatile memory, and information processors.
However, their millimeter-scale wavelengths and undesired cross-talk have
limited operation efficiency and made individual control of densely integrated
magnetic systems difficult. Here, we utilize acoustic waves (phonons) to
control magnetic dynamics in a miniaturized phononic crystal micro-cavity and
waveguide architecture. We demonstrate acoustic pumping of localized
ferromagnetic magnons, where their back-action allows dynamic and
mode-dependent modulation of phononic cavity resonances. The phononic crystal
platform enables spatial driving, control and read-out of tiny magnetic states
and provides a means of tuning acoustic vibrations with magnons. This
alternative technology enhances the usefulness of magnons and phonons for
advanced sensing, communications and computation architectures that perform
transduction, processing, and storage of classical and quantum information.",2208.09105v1
2022-08-23,Visualization of moiré magnons in monolayer ferromagnet,"Two-dimensional magnetic materials provide an ideal platform to explore
collective many-body excitations associated with spin fluctuations. In
particular, it should be feasible to explore, manipulate and ultimately design
magnonic excitations in two-dimensional van der Waals magnets in a controllable
way. Here we demonstrate the emergence of moir\'e magnon excitations, stemming
from the interplay of spin-excitations in monolayer CrBr$_3$ and the moir\'e
pattern stemming from the lattice mismatch with the underlying substrate. The
existence of moir\'e magnons is further confirmed via inelastic quasiparticle
interference, showing the appearance of a dispersion pattern correlated with
the moir\'e length scale. Our results provide a direct visualization in
real-space of the dispersion of moir\'e magnons, demonstrating the versatility
of moir\'e patterns in creating emerging many-body excitations.",2208.10991v2
2022-08-24,Stimulated amplification of propagating spin waves,"Spin-wave amplification techniques are key to the realization of magnon-based
computing concepts. We introduce a novel mechanism to amplify spin waves in
magnonic nanostructures. Using the technique of rapid cooling, we create a
non-equilibrium state in excess of high-energy magnons and demonstrate the
stimulated amplification of an externally seeded, propagating spin wave. Using
an extended kinetic model, we qualitatively show that the amplification is
mediated by an effective energy flux of high energy magnons into the low energy
propagating mode, driven by a non-equilibrium magnon distribution.",2208.11455v2
2022-09-08,Dominance of Electron-Magnon Scattering in Itinerant Ferromagnet Fe3GeTe2,"Fe3GeTe2 is a 2-dimensional van der Waals material exhibiting itinerant
ferromagnetism upto 230 K. Here, we study aspects of scattering mechanism in
Fe3Ge2Te2 single crystals via resistivity, magneto-transport and Hall effect
measurements. The quadratic temperature dependence of electrical resistivity
below the Curie temperature hints towards the dominance of electron-magnon
scattering. A non-saturating positive magnetoresistance (MR) is observed at low
temperatures when the magnetic field is applied parallel to the sample plane.
The linear negative MR at high fields for T < TC corroborates to the
suppression in magnon population due to the damping of spin waves. In the high
temperature regime T > TC,MR can be described by the scattering from spin
fluctuations using the model described by Khosla and Fischer. Isothermal Hall
resistivity curves unveil the presence of anomalous Hall resistivity.
Correlation between MR and side jump mechanism further reveals that the
electron-magnon scattering is responsible for the side jump contribution to the
anomalous Hall effect. Our results provide a clear understanding of the role of
electron-magnon scattering on anomalous Hall effect that rules out its origin
to be the topological band structure.",2209.03555v1
2022-10-12,Quantum amplification of spin currents in cavity magnonics by a parametric drive induced long-lived mode,"Cavity-mediated magnon-magnon coupling can lead to a transfer of spin-wave
excitations between two spatially separated magnetic samples. We enunciate how
the application of a two-photon parametric drive to the cavity can lead to
stark amplification in this transfer efficiency. The recurrent multiphoton
absorption by the cavity opens up an infinite ladder of accessible energy
levels, which can induce higher-order transitions within the magnon Fock space.
This is reflected in a heightened spin-current response from one of the
magnetic samples when the neighboring sample is coherently pumped. The
enhancement induced by the parametric drive can be considerably high within the
stable dynamical region. Specifically, near the periphery of the stability
boundary, the spin current is amplified by several orders of magnitude. Such
striking enhancement factors are attributed to the emergence of parametrically
induced strong coherences precipitated by a long-lived mode. While
contextualized in magnonics, the generality of the principle would allow
applications to energy transfer between systems contained in parametric
cavities.",2210.05898v1
2022-10-13,Nonlocal drag by topological surface magnons in a pyrochlore ferromagnet,"The nontrivial topology of quasiparticle wavefunctions can manifest
themselves in the form of observable surface states. This is now well
established in electronic systems, with Dirac and Weyl semimetals bringing to
fore the exotic nature of these topologically protected entities. Magnons -
which refer to collective excitations of localized spins - offer another sector
where many of these concepts could be realized. Here, we report magneto-thermal
measurements on a pyrochlore ferromagnet which is theoretically predicted to
host such topological magnons. It is demonstrated that the thermoelectric
potential across a metal layer deposited on single crystalline specimens of
Y$_2$V$_2$O$_7$ can be used to measure the magnon Hall effect. Moreover, a
direct manifestation of topologically protected magnon surface states is
observed - via the interfacial drag which these surface spin currents impose on
the conduction electrons of the adjacent metallic layer.",2210.07058v1
2022-11-24,Influence of non-local damping on magnon properties of ferromagnets,"We study the influence of non-local damping on magnon properties of Fe, Co,
Ni and Fe$_{1-x}$Co$_{x}$ ($x=30\%,50\%$) alloys. The Gilbert damping parameter
is typically considered as a local scalar both in experiment and in theoretical
modelling. However, recent works have revealed that Gilbert damping is a
non-local quantity that allows for energy dissipation between atomic sites.
With the Gilbert damping parameters calculated from a state-of-the-art
real-space electronic structure method, magnon lifetimes are evaluated from
spin dynamics and linear response, where a good agreement is found between
these two methods. It is found that non-local damping affects the magnon
lifetimes in different ways depending on the system. Specifically, we find that
in Fe, Co, and Ni the non-local damping decreases the magnon lifetimes, while
in $\rm Fe_{70}Co_{30}$ and Fe$_{50}$Co$_{50}$ an opposite, non-local damping
effect is observed, and our data show that it is much stronger in the former.",2211.13486v1
2022-11-28,Spurious Symmetry Enhancement and Interaction-Induced Topology in Magnons,"Linear spin wave theory (LSWT) is the standard technique to compute the
spectra of magnetic excitations in quantum materials. In this paper, we show
that LSWT, even under ordinary circumstances, may fail to implement the
symmetries of the underlying ordered magnetic Hamiltonian leading to spurious
degeneracies. In common with pseudo-Goldstone modes in cases of quantum
order-by-disorder these degeneracies tend to be lifted by magnon-magnon
interactions. We show how, instead, the correct symmetries may be restored at
the level of LSWT. In the process we give examples, supported by
nonperturbative matrix product based time evolution calculations, where
symmetries dictate that there should be a topological magnon gap but where LSWT
fails to open up this gap. We also comment on possible spin split magnons in
MnF$_2$ and similar rutiles by analogy to recently proposed altermagnets.",2211.15157v1
2022-11-29,Ground states and magnonics in orthogonally-coupled symmetric all-antiferromagnetic junctions,"In this work, the rich ground-state structure of orthogonally-coupled
symmetric all-antiferromagnetic junctions with easy-plane anisotropy is
reported. Spin reorientation process rather than the traditional spin flop (SF)
occurs, resulting in a novel phase in which N\'{e}el vectors preserve the
mirror-reflection symmetry (termed as ``MRS phase""). The phase transitions
between SF and MRS phases can be either the first- or second-order. After
disturbed by external stimuli, magnons with different parities emerge. For
in-plane dc fields, no couplings between magnons occur. When dc fields become
oblique, coherent couplings between magnons with opposite parity emerge,
leading to anticrossings in resonance frequencies. However, self-hybridization
among magnons with the same parity never happens. More interestingly, spin
waves based on MRS phase are linearly polarized and their polarization
directions can be fine controlled.",2211.16063v1
2022-12-04,Renormalization of antiferromagnetic magnons by superconducting condensate and quasiparticles,"The ability to modify and tune the spin-wave dispersion is one of the most
important requirements for engineering of magnonic networks. In this study we
demonstrate the promise of synthetic thin-film hybrids composed of an
antiferromagnetic insulator (AF) and a normal (N) or superconducting (S) metal
for tuning and modifying the spin-wave dispersion in antiferromagnetic
insulators. The key ingredient is the uncompensated magnetic moment at the
AF/S(N) interface, which induces an effective exchange field in the adjacent
metal via the interface exchange interaction. The exchange field spin polarizes
quasiparticles in the metal and induces spinful triplet Cooper pairs screening
the magnon. The quasiparticle and Cooper pair polarization renormalizes the
magnon dispersion. The renormalization results in the splitting of the
otherwise degenerate AF magnon modes with no need to apply a magnetic field. It
is also proposed that measurements of the renormalized dispersion relations can
provide the amplitude of the effective exchange field induced by the AF in the
adjacent metal.",2212.01831v2
2022-12-07,"Observation of magnon bound states in the long-range, anisotropic Heisenberg model","Over the recent years coherent, time-periodic modulation has been established
as a versatile tool for realizing novel Hamiltonians. Using this approach,
known as Floquet engineering, we experimentally realize a long-ranged,
anisotropic Heisenberg model with tunable interactions in a trapped ion quantum
simulator. We demonstrate that the spectrum of the model contains not only
single magnon excitations but also composite magnon bound states. For the
long-range interactions with the experimentally realized power-law exponent,
the group velocity of magnons is unbounded. Nonetheless, for sufficiently
strong interactions we observe bound states of these unconventional magnons
which possess a non-diverging group velocity. By measuring the configurational
mutual information between two disjoint intervals, we demonstrate the
implications of the bound state formation on the entanglement dynamics of the
system. Our observations provide key insights into the peculiar role of
composite excitations in the non-equilibrium dynamics of quantum many-body
systems.",2212.03899v1
2022-12-12,Slow inter-minima relaxation and its consequence for BEC of magnons,"Two recent articles of the Munster University experimental team led by S.O.
Demokritov displayed several important facts related to the Bose-Einstein
condensation of magnons (BECM) under permanent pumping first discovered in
2006. They contradicted existing theories of this phenomenon, which predict the
attractive interaction between magnons and strong spontaneous violation of the
reflection symmetry. In this article, we show that these theories implicitly
assumed all relaxation processes to be fast compared with the lifetime of the
magnons, whereas one of them -- relaxation between two minima of energy -- is
slow. We classify processes responsible for the inter-minima relaxation and
present their analytic theory. We analyze how the slow inter-minima relaxation
modifies the anticipated properties of a ferromagnet with the magnon
condensate.",2212.06085v2
2023-02-13,Zero-frequency chiral magnonic edge states protected by non-equilibrium topology,"Topological bosonic excitations must, in contrast to their fermionic
counterparts, appear at finite energies. This is a key challenge for magnons,
as it prevents straightforward excitation and detection of
topologically-protected magnonic edge states and their use in magnonic devices.
In this work, we show that in a non-equilibrium state, in which the
magnetization is pointing against the external magnetic field, the
topologically-protected chiral edge states in a magnon Chern insulator can be
lowered to zero frequency, making them directly accessible by existing
experimental techniques. We discuss the spin-orbit torque required to stabilize
this non-equilibrium state, and show explicitly using numerical
Landau-Lifshitz-Gilbert simulations that the edge states can be excited with a
microwave field. Finally, we consider a propagating spin wave spectroscopy
experiment, and demonstrate that the edge states can be directly detected.",2302.06597v3
2023-02-16,Coherent feedback control of quantum correlations in cavity magnomechanical system with magnon squeezing,"We address a scheme to enhance the quantum correlations in cavity
opto-magnomechanical system by using the coherent feedback loop in the presence
of magnon squeezing. The proposed coherent feedback-control allows a
significant enhancement of the entanglement of three bipartite subsystems,
i.e., photon-phonon, photon-magnon and phonon-magnon. We also study the
Einstein-Podolsky-Rosen steering and one-way steering in the presence of
thermal effects without imposing additional conditions of asymmetric losses or
noises in the subsystems. Furthermore, we investigate the sensitiveness of the
scheme to the magnon squeezing, and its performance in non-ideal situations in
which losses and noises are taken into account.",2302.08567v2
2023-03-02,Unidirectional Microwave Transduction with Chirality Selected Short-Wavelength Magnon Excitations,"Nonreciprocal magnon propagation has recently become a highly potential
approach of developing chip-embedded microwave isolators for advanced
information processing. However, it is challenging to achieve large
nonreciprocity in miniaturized magnetic thin-film devices because of the
difficulty of distinguishing propagating surface spin waves along the opposite
directions when the film thickness is small. In this work, we experimentally
realize unidirectional microwave transduction with sub-micron-wavelength
propagating magnons in a yttrium iron garnet (YIG) thin film delay line. We
achieve a non-decaying isolation of 30 dB with a broad field-tunable band-pass
frequency range up to 14 GHz. The large isolation is due to the selection of
chiral magnetostatic surface spin waves with the Oersted field generated from
the coplanar waveguide antenna. Increasing the geometry ratio between the
antenna width and YIG thickness drastically reduces the nonreciprocity and
introduces additional magnon transmission bands. Our results pave the way for
on-chip microwave isolation and tunable delay line with short-wavelength
magnonic excitations.",2303.00936v1
2023-03-13,Anti-helical edge magnons in patterned antiferromagnetic thin film,"Helical edge states in topological insulators give counterpropagating spin
current on the two parallel edges. We here propose anti-helical edge states of
magnons in patterned antiferromagnetic thin films, which host copropagating
spin current on the two parallel edges, where the two magnon modes with
opposite chirality act like the spin. The embedded heavy metal dot array in the
thin film induces interfacial Dzyaloshinskii-Moriya interactions (iDMIs),
drives the magnon bands into nontrivial topological phases, characterized by
spin Chern number. The resulting helical edge modes lead to spin current with
the direction dependent on the sign of iDMI parameter. In a strip geometry, we
combine two subsystems with two embedded metal dot arrays, which give opposite
iDMI parameters. Anti-helical edge states emerge, compensated by the
counterpropagating bulk confined states. Helical and anti-helical edge states
are verified by the micromagnetic simulations. Our work is quite helpful in the
field of magnon spintronics based on antiferromagnets.",2303.06830v1
2023-03-27,Entangling cavity-magnon polaritons by interacting with phonons,"We show how to entangle two cavity-magnon polaritons (CMPs) formed by two
strongly coupled microwave cavity and magnon modes. This is realized by
introducing vibration phonons, via magnetostriction, into the system that are
dispersively coupled to the magnon mode. Stationary entanglement between two
CMPs can be achieved when they are respectively resonant with the two sidebands
of the drive field scattered by the phonons, and when the proportions of the
cavity and magnon modes in the two polaritons are appropriately chosen. The
entangled CMPs can improve the detection sensitivity in the dark matter search
experiments using CMPs, and can also lead to the emission of
frequency-entangled microwave photons.",2303.15217v5
2023-04-18,Linear scaling relationship of Néel temperature and dominant magnons in pyrochlore ruthenates,"We present a systematic Raman spectroscopy study on a series of pyrochlore
ruthenates, a system which is not yet clearly settled on its magnetic origin
and structure. Apart from the Raman-active phonon modes, new peaks that appear
in the energy range of 15 - 35 meV below the N\'{e}el temperature are assigned
as one-magnon modes. The temperature evolution of one-magnon modes displays no
significant thermal dependence in mode frequencies while the intensities
decrease monotonically. Remarkably, one-magnons from all compounds show similar
characteristics with a single dominant peak at lower energy and weaker side
peaks at a couple of meV higher energy. Most importantly, we uncover a striking
proportionality between the dominant magnon mode energies and the N\'{e}el
temperature values. Our results suggest the Ru ions may have similar or the
same magnetic phase in all pyrochlore ruthenates of our study. We have thus
found an avenue for directly tuning the magnetic exchange interaction by the
selection of the $A$-site ion.",2304.08815v2
2023-04-19,Cavity magnomechanical coupling with coupled magnon modes in a synthetic antiferromagnet,"On-chip cavity magnomechanics is an emerging field exploring acoustic and
magnonic functionalities of various ferromagnetic materials and structures
using strongly confined phonons. It is expected that such cavity magnomechanics
can be extended to multilayer ferromagnets, especially synthetic
antiferromagnets (SAFs) that exhibit zero net magnetization through interlayer
exchange coupling. However, the conventional theoretical framework for a single
ferromagnet cannot be used directly because of the antiferromagnetic
magnetization dynamics associated with the interlayer exchange coupling. In
this paper, we theoretically investigate phonon-magnon coupling with a
three-layer SAF. Our formulation of the phonon-magnon coupling constants
reveals that the acoustic (optical) magnon mode dominantly couples to the
cavity phonon when the magnetization angles in the two ferromagnetic layers are
antiparallel (orthogonal). Moreover, numerical calculations including the
effects of dipole-dipole interactions and in-plane uniaxial magnetic anisotropy
allow us to predict phonon frequency shifts and linewidth broadening that can
be detected in experiments. These theoretical insights would greatly help us to
make a strategy for bringing the system into the strong coupling regime and to
devise novel control protocols in analogy to cavity quantum electrodynamics and
cavity optomechanics.",2304.09458v2
2023-05-01,Theoretical analysis of multi-magnon excitations in resonant inelastic x-ray scattering spectra of two-dimensional antiferromagnets,"Resonant inelastic x-ray spectroscopy (RIXS) has emerged as an important tool
to explore magnetism in two-dimensional (2D) antiferromagnet realized in
strongly correlated materials. Here we consider the Heisenberg model with
nearest and next nearest neighbor hopping relevant to the study of magnetic
excitations of the cuprate family. We compute the RIXS cross-section within the
ultra-short core-hole lifetime (UCL) expansion of the Kramers-Heisenberg
scattering amplitude that allows perturbative solution within linear spin wave
theory (LSWT). We report detailed results for both spin-conserving and
non-conserving channels. Apart from the widely discussed single magnon and
bimagnon contributions, we show that three-magnon contributions in the spin
non-conserving channel are useful to explain certain features of the RIXS data
for two-dimensional cuprates. We confirm the qualitative correctness of the
LSWT conclusions for the three-magnon excitation with exact diagonalization.
Our work puts constraints on the dispersion of the three-magnon in the
Brillouin zone, opening new avenues for realizing higher modes of
quasiparticles using RIXS.",2305.00802v1
2023-05-01,Coherent and incoherent magnons induced by strong ultrafast demagnetization in thin permalloy films,"Understanding spin dynamics on femto- and picosecond timescales offers new
opportunities for faster and more efficient spintronic devices. Here, we
experimentally investigate the coherent spin dynamics after ultrashort laser
excitation by time-resolved magneto optical Kerr effect (TR-MOKE) in thin
Ni80Fe20 films. We provide a detailed study of the magnetic field and pump
fluence dependence of the coherent precessional dynamics. We show that the
coherent precession lifetime increases with the applied external magnetic field
which cannot be understood by viscous Gilbert damping of the coherent magnons.
Instead, it can be explained by nonlinear magnon interactions and by the change
in the fraction of incoherent magnons. This interpretation is in agreement with
the observed trends of the coherent magnon amplitude and lifetime as a function
of the exciting laser fluence. Our results provide a new insight into the
magnetization relaxation processes in ferromagnetic thin films, which is of
great importance for further spintronic applications.",2305.00814v2
2023-05-19,Quantum transduction of superconducting qubit in electro-optomechanical and electro-optomagnonical system,"We study the quantum transduction of a superconducting qubit to an optical
photon in electro-optomechanical and electro-optomagnonical systems. The
electro-optomechanical system comprises a flux-tunable transmon qubit coupled
to a suspended mechanical beam, which then couples to an optical cavity.
Similarly, in an electro-optomagnonical system, a flux-tunable transmon qubit
is coupled to an optical whispering gallery mode via a magnon excitation in a
YIG ferromagnetic sphere. In both systems, the transduction process is done in
sequence. In the first sequence, the qubit states are encoded in coherent
excitations of phonon/magnon modes through the phonon/magnon-qubit interaction,
which is non-demolition in the qubit part. We then measure the phonon/magnon
excitations, which reveal the qubit states, by counting the average number of
photons in the optical cavities. The measurement of the phonon/magnon
excitations can be performed at a regular intervals of time.",2305.11629v1
2023-05-19,Antiferromagnetic-ferromagnetic homostructures with Dirac magnons in van der Waals magnet CrI$_3$,"Van der Waals (vdW) Dirac magnon system CrI$_3$, a potential host of
topological edge magnons, orders ferromagnetically (FM) (T$_C=61$ K) in the
bulk, but antiferromagnetic (AFM) order has been observed in nanometer thick
flakes, attributed to monoclinic (M) type stacking. We report neutron
scattering measurements on a powder sample where the usual transition to the
rhombohedral (R) phase was inhibited for a majority of the structure. Elastic
measurements (and the opening of a hysteresis in magnetization data on a
pressed pellet) showed that an AFM transition is clearly present below $\sim$50
K, coexisting with the R-type FM order. Inelastic measurements showed a
decrease in magnon energy compared to the R phase, consistent with a smaller
interlayer magnetic coupling in M-type stacking. A gap remains at the Dirac
point, suggesting that the same nontrivial magnon topology reported for the R
phase may be present in the M phase as well.",2305.11750v1
2023-06-08,Resolving nonclassical magnon composition of a magnetic ground state via a qubit,"Recently gained insights into equilibrium squeezing and entanglement harbored
by magnets point towards exciting opportunities for quantum science and
technology, while concrete protocols for exploiting these are needed. Here, we
theoretically demonstrate that a direct dispersive coupling between a qubit and
a noneigenmode magnon enables detecting the magnonic number states' quantum
superposition that forms the ground state of the actual eigenmode -
squeezed-magnon - via qubit excitation spectroscopy. Furthermore, this unique
coupling is found to enable control over the equilibrium magnon squeezing and a
deterministic generation of squeezed even Fock states via the qubit state and
its excitation. Our work demonstrates direct dispersive coupling to
noneigenmodes, realizable in spin systems, as a general pathway to exploiting
the equilibrium squeezing and related quantum properties thereby motivating a
search for similar realizations in other platforms.",2306.05065v2
2023-07-10,Realization of an extremely anisotropic Heisenberg magnet in Rydberg atom arrays,"Strong mutual interactions correlate elementary excitations of quantum matter
and plays a key role in a range of emergent phenomena, from binding and
condensation to quantum thermalization and many-body localization. Here, we
employ a Rydberg quantum simulator to experimentally demonstrate strongly
correlated spin transport in anisotropic Heisenberg magnets, where the
magnon-magnon interaction can be tuned two orders of magnitude larger than the
magnon hopping strength. In our approach, the motion of magnons is controlled
by an induced spin-exchange interaction through Rydberg dressing, which enables
coherent transport of a single Rydberg excitation across a chain of
ground-state atoms. As the most prominent signature of a giant anisotropy, we
show that nearby Rydberg excitations form distinct types of magnon bound
states, where a tightly bound pair exhibits frozen dynamics in a fragmented
Hilbert space, while a loosely bound pair propagates and establishes
correlations beyond a single lattice site. Our scheme complements studies using
resonant dipole-dipole interactions between Rydberg states, and opens the door
to exploring quantum thermodynamics with ultrastrong interactions and kinetic
constraints.",2307.04342v1
2023-07-20,Magnon Spin Photogalvanic Effect in Collinear Ferromagnets,"We propose a spin photogalvanic effect of magnons with broken inversion
symmetry. The dc spin photocurrent is generated via the Aharonov-Casher effect,
which includes the Drude, Berry curvature dipole, shift, injection, and
rectification components with distinct quantum geometric origin. Based on a
symmetry classification, we uncover that there exist linearly polarized (LP)
magnon spin photocurrent responses in the breathing kagome-lattice ferromagnet
with Dzyaloshinskii-Moriya interaction, and the circularly polarized (CP)
responses due to the symmetry breaking by applying a uniaxial strain. We
address that the topological phase transitions can be characterized by the spin
photocurrents. This study presents a deeper insight into the nonlinear
responses of light-magnon interactions, and suggests a possible way to generate
and control the magnon spin current in real materials.",2307.10882v1
2023-08-30,Chiral cavity-magnonic system for the unidirectional emission of a tunable squeezed microwave field,"Unidirectional photon emission is crucial for constructing quantum networks
and realizing scalable quantum information processing. In the present work an
efficient scheme is developed for the unidirectional emission of a tunable
squeezed microwave field. Our scheme is based on a chiral cavity magnonic
system, where a magnon mode in a single-crystalline yttrium iron garnet (YIG)
sphere is selectively coupled to one of the two degenerate rotating microwave
modes in a torus-shaped cavity with the same chirality. With the YIG sphere
driven by a two-color Floquet field to induce sidebands in the magnon-photon
coupling, we show that the unidirectional emission of a tunable squeezed
microwave field can be generated via the assistance of the dissipative magnon
mode and a waveguide. Moreover, the direction of the proposed one-way emitter
can be controlled on demand by reversing the biased magnetic field. Our work
opens up an avenue to create and manipulate one-way nonclassical microwave
radiation field and could find potential quantum technological applications.",2308.15826v1
2023-09-12,Anisotropy-assisted magnon condensation in ferromagnetic thin films,"We theoretically demonstrate that adding an easy-axis magnetic anisotropy
facilitates magnon condensation in thin yttrium iron garnet (YIG) films.
Dipolar interactions in a quasi-equilibrium state stabilize room-temperature
magnon condensation in YIG. Even though the out-of-plane easy-axis anisotropy
generally competes with the dipolar interactions, we show that adding such
magnetic anisotropy may even assist the generation of the magnon condensate
electrically via the spin transfer torque mechanism. We use analytical
calculations and micromagnetic simulations to illustrate this effect. Our
results may explain the recent experiment on Bi-doped YIG and open a pathway
toward applying current-driven magnon condensation in quantum spintronics.",2309.05982v3
2023-11-09,Magnon-phonon coupling of synthetic antiferromagnets in a surface acoustic wave cavity resonator,"We use a surface acoustic wave (SAW) cavity resonator to study the coupling
of acoustic magnons in a synthetic antiferromagnet (SAF) and the phonons
carried by SAWs. The SAF is composed of a CoFeB/Ru/CoFeB trilayer and the
scattering matrix of the SAW resonator is studied to assess the coupling. We
find that the spectral linewidth of the SAW resonator is modulated when the
frequency of the excited magnons approaches the SAW resonance frequency.
Moreover, the linewidth modulation varies with the magnitude and orientation of
the external magnetic field. Such change in the spectral linewidth can be well
reproduced using macrospin-like model calculations. From the model analyses, we
estimate the magnon-phonon coupling strength to be $\sim$15.6 MHz at a SAW
resonance frequency of 1.8 GHz: the corresponding magnomechanical cooperativity
is $\sim$0.66. As the spectral shape hardly changes in a CoFeB single layer
reference sample under the same experimental condition, these results show that
SAF provides an ideal platform to study magnon-phonon coupling in a SAW cavity
resonator.",2311.05275v1
2024-02-02,Entanglement enhancement of two different magnon modes via nonlinear effect in cavity magnomechanics,"We present a scheme to enhance two different magnon modes entanglement in
cavity magnomechanics via nonlinear effect. The scheme demonstrated that
nonlinear effects enhance entanglement of the two magnon modes. Moreover, the
entanglement of the two magnon modes is also significantly enhanced by
microwave parametric amplification (PA) and magnon self-Kerr nonlinearity. Not
only dose nonlinear effect enhances the strength of entanglement, but it also
increases the robustness of entanglement against temperature. Our proposed
scheme plays an important role in the research of fundamental theories of
quantum physics and quantum information processing theory.",2402.01150v1
2024-02-05,Controlling magnon-photon coupling in a planar geometry,"The tunability of magnons enables their interaction with various other
quantum excitations, including photons, paving the route for novel hybrid
quantum systems. Here, we study magnon-photon coupling using a high-quality
factor split-ring resonator and single-crystal yttrium iron garnet (YIG)
spheres at room temperature. We investigate the dependence of the coupling
strength on the size of the sphere and find that the coupling is stronger for
spheres with a larger diameter as predicted by theory. Furthermore, we
demonstrate strong magnon-photon coupling by varying the position of the YIG
sphere within the resonator. Our experimental results reveal the expected
correlation between the coupling strength and the rf magnetic field. These
findings demonstrate the control of coherent magnon-photon coupling through the
theoretically predicted square-root dependence on the spin density in the
ferromagnetic medium and the magnetic dipolar interaction in a planar
resonator.",2402.03071v1
2024-03-05,Magnon kinetic theory of the antiferromagnetic Hanle effect,"Motivated by the recently discovered magnonic Hanle effect in an insulating
antiferromagnet [Wimmer et al., Phys. Rev. Lett. 125, 247204 (2020)], we
develop a spin transport theory based on low-energy waves of antiferromagnetic
N\'eel order. These waves have two polarizations, which we describe in analogy
to optics using the Stokes vector on the Poincar\'e sphere. We find that the
polarization, which encodes the magnon spin angular momentum, changes
periodically with a frequency that is nonlinear in the magnetic field. This
explains the observed asymmetry in the Hanle signal as a function of the
magnetic field, along with other salient experimental features. By providing an
energy-resolved description of the spin injection, our theory combines the
kinetic transport of magnons with the coherent dynamics of their polarization
in an intuitive way. This opens a general perspective on a coherent control of
magnonic spin density in collinear antiferromagnets.",2403.03358v1
2024-03-11,Magnon bands and transverse transport in a proposed two-dimensional $Cu_2F_5$ ferrimagnet,"The copper fluoride $Cu_2F_5$ is a proposed stable compound that can be seen
as a layered magnetic lattice of $S=1$ and $S=1/2$ sites, corresponding to
copper ions. Intending to cast light on the transport properties of
ferrimagnetic magnons, we use the linear spin wave approach to study the magnon
band structure of the 2D lattice in a ferrimagnetic off-plane order, as well as
the transverse transport of magnons in the crystal bulk. That transverse
(Hall-like) transport can be induced by a magnetic field or temperature
gradient, and within the linear response theory is generated by the Berry
curvature of the eigenstates. As in most of the cases for magnons, the Berry
curvature here is related to Dzyaloshinskii-Moriya interactions between
next-near-neighbors. The band structure of the system is non-degenerate and the
transport coefficients are non-null. We also determine the condition for two
transport coefficients to change sign in response to temperature.",2403.07169v1
2024-04-02,Understanding spin currents from magnon dispersion and polarization: Spin-Seebeck effect and neutron scattering study on Tb3Fe5O12,"Magnon spin currents in the ferrimagnetic garnet Tb3Fe5O12 with 4f electrons
were examined through the spin-Seebeck effect and neutron scattering
measurements. The compound shows a magnetic compensation, where the
spin-Seebeck signal reverses above and below Tcomp = 249.5(4) K. Unpolarized
neutron scattering unveils two major magnon branches with finite energy gaps,
which are well-explained in the framework of spin-wave theory. Their
temperature dependencies and the direction of the precession motion of magnetic
moments, i.e. magnon polarization, defined using polarized neutrons, explain
the reversal at Tcomp and decay of the spin-Seebeck signals at low
temperatures. We illustrate an example that momentum- and energy-resolved
microscopic information is a prerequisite to understanding the magnon spin
current.",2404.01603v1
1995-11-15,Resonant Two-Magnon Raman Scattering in Cuprate Antiferromagnetic Insulators,"We present results of low-temperature two-magnon resonance Raman excitation
profile measurements for single layer Sr_2CuO_2Cl_2 and bilayer YBa_2Cu_3O_{6 +
\delta} antiferromagnets over the excitation region from 1.65 to 3.05 eV. These
data reveal composite structure of the two-magnon line shape and strong
nonmonotic dependence of the scattering intensity on excitation energy. We
analyze these data using the triple resonance theory of Chubukov and Frenkel
(Phys. Rev. Lett., 74, 3057 (1995)) and deduce information about magnetic
interaction and band parameters in these materials.",9511080v1
1997-10-15,Softening and Broadening of the Zone Boundary Magnons in Pr0.63Sr0.37MnO3,"We have studied the spin dynamics in Pr$_{0.63}$Sr$_{0.37}$MnO$_3$ above and
below the Curie temperature $T_C=301$ K. Three distinct new features have been
observed: a softening of the magnon dispersion at the zone boundary for
$T<T_C$, significant broadening of the zone boundary magnons as $T\to T_C$, and
no evidence for residual spin-wave like excitations just above $T_C$. The
results are inconsistent with double exchange models that have been
successfully applied to higher $T_C$ samples, indicating an evolution of the
spin system with decreasing $T_C$.",9710157v1
1998-06-06,Magnons and skyrmions in fractional Hall ferromagnets,"Recent experiments have established a qualitative difference between the
magnetization temperature-dependences $M(T)$ of quantum Hall ferromagnets at
integer and fractional filling factors. We explain this difference in terms of
the relative energies of collective magnon and particle-hole excitations in the
two cases. Analytic calculations for hard-core model systems are used to
demonstrate that, in the fractional case, interactions suppress the
magnetization at finite temperatures and that particle-hole excitations rather
than long-wavelength magnons control $M(T)$ at low $T$.",9806087v1
1998-07-13,Magnetic Properties of (VO)_2P_2O_7 from Frustrated Interchain Coupling,"Neutron-scattering experiments on (VO)_2P_2O_7 reveal both a gapped magnon
dispersion and an unexpected, low-lying second mode. The proximity and
intensity of these modes suggest a frustrated coupling between the alternating
spin chains. We deduce the minimal model containing such a frustration, and
show that it gives an excellent account of the magnon dispersion, static
susceptibility and electron spin resonance absorption. We consider two-magnon
states which bind due to frustration, and demonstrate that these may provide a
consistent explanation for the second mode.",9807186v2
1998-08-10,Instability of antiferromagnetic magnons in strong fields,"We predict that spin-waves in an ordered quantum antiferromagnet (AFM) in a
strong magnetic field become unstable with respect to spontaneous two-magnon
decays. At zero temperature, the instability occurs between the threshold field
$H^*$ and the saturation field $H_c$. As an example, we investigate the
high-field dynamics of a Heisenberg antiferromagnet on a square lattice and
show that the single-magnon branch of the spectrum disappears in the most part
of the Brillouin zone.",9808100v2
1998-08-31,Magnon Wave-function and Impurity Effects in S=1 Antiferromagnetic Chains: A Large-n Approach,"A large-n approximation to the S=1 antiferromagnetic chain, using the
symmetric tensor representation and its conjugate, is developed to order 1/n in
order to calculate the magnon wave-function and to study the effect of
modifying the exchange coupling from J to J' on a single link. It is shown that
a magnon boundstate exists below the Haldane gap for arbitrarily small negative
J'-J but only above a certain critical value of J'-J for positive values. In
the former case the binding energy vanishes as the square of (J-J').",9808345v1
1999-05-04,Quantum spin ladder with dynamic phonons. Bound states of magnons and phonons,"We consider the two-chain Heisenberg ladder with antiferromagnetic
interactions. Our approach is based on the description of spin excitations as
triplets above a strong-coupling singlet ground state. It is shown that
interaction with dynamic phonons drastically changes the dispersion of the
elementary triplet excitation (magnon) and the spin structure factor. It is
also shown that a new triplet excitation appears in the spectrum: the bound
state of a magnon and a phonon.",9905030v1
2000-01-31,Magnon dispersions in quantum Heisenberg ferrimagnetic chains at zero temperature,"Within the Dyson-Maleev boson formalism, we study the zero-temperature magnon
dispersions in a family of one-dimensional quantum Heisenberg ferrimagnets
composed of two different spins $(S_1,S_2)$ in the elementary cell. It is shown
that the spin-wave theory can produce precise quantitative results for the
low-energy excitations. The spin-stiffness constant $\rho_s$ and the optical
magnon gap $\Delta$ of different $(S_1,S_2)$ ferrimagnetic systems are
calculated, respectively, to second and third order in the quasiparticle
interaction. The spin-wave results are compared with available numerical
estimates.",0001441v1
2000-10-26,Magnon-assisted Andreev reflection in a ferromagnet-superconductor junction,"We study subgap transport in a ferromagnet-superconductor junction at low
temperature due to Andreev reflection. The mismatch of spin polarized current
in the ferromagnet and spinless current in the superconductor results in an
additional contact resistance which can be reduced by magnon emission in the
ferromagnet. Using the s-f model and focusing on half-metallic ferromagnets, we
calculate the corresponding nonlinear contribution to the current which is
asymmetric with respect to the sign of the bias voltage and is related to the
density of states of magnons in the ferromagnet.",0010421v1
2001-08-01,Order from disorder: Quantum spin gap in magnon spectra of LaTiO_3,"A theory of the anisotropic superexchange and low energy spin excitations in
a Mott insulator with t_{2g} orbital degeneracy is presented. We observe that
the spin-orbit coupling induces frustrating Ising-like anisotropy terms in the
spin Hamiltonian, which invalidate noninteracting spin wave theory. The
frustration of classical states is resolved by an order from disorder
mechanism, which selects a particular direction of the staggered moment and
generates a quantum spin gap. The theory explains well the observed magnon gaps
in LaTiO_3. As a test case, a specific prediction is made on the splitting of
magnon branches at certain momentum directions.",0108016v2
2001-12-13,Magnon softening and damping in the ferromagnetic manganites due to orbital correlations,"We present a theory for spin excitations in ferromagnetic metallic manganites
and demonstrate that orbital fluctuations have strong effects on the magnon
dynamics in the case these compounds are close to a transition to an orbital
ordered state. In particular we show that the scattering of the spin
excitations by low-lying orbital modes with cubic symmetry causes both the
magnon softening and damping observed experimentally.",0112252v2
2002-02-26,Magnetothermopower and magnon-assisted transport in ferromagnetic tunnel junctions,"We present a model of the thermopower in a mesoscopic tunnel junction between
two ferromagnetic metals based upon magnon-assisted tunneling processes. In our
model, the thermopower is generated in the course of thermal equilibration
between two baths of magnons, mediated by electrons. We predict a particularly
large thermopower effect in the case of a junction between two half-metallic
ferromagnets with antiparallel polarizations, $S_{AP} \sim - (k_B/e)$, in
contrast to $S_{P} \approx 0$ for a parallel configuration.",0202475v1
2002-04-17,Inelastic neutron scattering signal from deconfined spinons in a fractionalized antiferromagnet,"We calculate the contribution of deconfined spinons to inelastic neutron
scattering (INS) in the fractionalized antiferromagnet (AF*), introduced
elsewhere. We find that the presence of free spin-1/2 charge-less excitations
leads to a continuum INS signal above the Neel gap. This signal is found above
and in addition to the usual spin-1 magnon signal, which to lowest order is the
same as in the more conventional confined antiferromagnet. We calculate the
relative weights of these two signals and find that the spinons contribute to
the longitudinal response, where the magnon signal is absent to lowest order.
Possible higher-order effects of interactions between magnons and spinons in
the AF* phase are also discussed.",0204382v1
2002-05-02,Magnon Dispersion in the Field-Induced Magnetically Ordered Phase of TlCuCl3,"The magnetic properties of the interacting dimer system TlCuCl3 are
investigated within a bond-operator formulation. The observed field-induced
staggered magnetic order perpendicular to the field is described as a Bose
condensation of magnons which are linear combinations of dimer singlet and
triplet modes. This technique accounts for the magnetization curve and for the
field dependence of the magnon dispersion curves observed by high-field neutron
scattering measurements.",0205052v2
2002-05-06,High-energy magnon dispersion in the half-filled Hubbard model: A comparison with La$_2$CuO$_4$,"We use quantum Monte Carlo methods and single-mode approximation to study the
magnon dispersion in the 2D half-filled Hubbard and phonon-coupled Heisenberg
models. We find that in the Hubbard model with $U/t< 8$, high-energy magnon
dispersion is similar to those observed in inelastic neutron scattering
experiments in ${La}_2{CuO}_4$. On the other hand, our studies of a 2D
Heisenberg model coupled to dynamic optical bond phonons, fails to reproduce
the experimental dispersion. These results can be interpreted as evidence for
intermediate $U/t$ and charge fluctuations in the cuprate materials.",0205107v1
2003-10-07,One dimensional gapless magnons in a single anisotropic ferromagnetic nanolayer,"Gapless magnons in a plane ferromagnet with normal axis anisotropy are shown
to exist besides the usual gapped modes that affect spin dependent transport
properties only above a finite temperature. These magnons are one dimensional
objects, in the sense that they are localized inside the domain walls that form
in the film. They may play an essential role in the spin dependent scattering
processes even down to very low temperatures.",0310159v1
2004-04-10,Nuclear Spin-Lattice Relaxation in One-Dimensional Heisenberg Ferrimagnets: Three-Magnon versus Raman Processes,"Nuclear spin-lattice relaxation in one-dimensional Heisenberg ferrimagnets is
studied by means of a modified spin-wave theory. We consider the second-order
process, where a nuclear spin flip induces virtual spin waves which are then
scattered thermally via the four-magnon exchange interaction, as well as the
first-order process, where a nuclear spin directly interacts with spin waves
via the hyperfine interaction. We point out a possibility of the three-magnon
relaxation process predominating over the Raman one and suggest model
experiments.",0404243v2
2004-10-08,Quasiparticle excitations in frustrated antiferromagnets,"We have computed the quasiparticle wave function corresponding to a hole
injected in a triangular antiferromagnet. We have taken into account
multi-magnon contributions within the self consistent Born approximation. We
have found qualitative differences, under sign reversal of the integral
transfer t, regarding the multi-magnon components and the own existence of the
quasiparticle excitations. Such differences are due to the subtle interplay
between magnon-assisted and free hopping mechanisms. We conclude that the
conventional quasiparticle picture can be broken by geometrical frustration
without invoking spin liquid phases.",0410209v1
2005-04-26,Two-magnon excitations observed by neutron scattering in the two-dimensional spin-5/2 Heisenberg antiferromagnet Rb2MnF4,"The low-temperature magnetic excitations of the two-dimensional spin-5/2
square-lattice Heisenberg antiferromagnet Rb2MnF4 have been probed using pulsed
inelastic neutron scattering. In addition to dominant sharp peaks identified
with one-magnon excitations, a relatively weak continuum scattering is also
observed at higher energies. This is attributed to neutron scattering by pairs
of magnons and the observed intensities are consistent with predictions of spin
wave theory.",0504684v1
2005-08-17,Magnonic Crystal Theory of the Spin-Wave Frequency Gap in Low-Doped $La_{1-x}Ca_{x}MnO_{3}$ Manganites,"A theory of three-dimensional (3D) hypothetical magnonic crystal (conceived
as the magnetic counterpart of the well-known photonic crystal) is developed
and applied to explain the existence of a spin-wave frequency gap recently
revealed in low-doped manganites $La_{1-x}Ca_{x}MnO_{3}$ by neutron scattering.
A successful confrontation with the experimental results allows us to formulate
a working hypothesis that certain manganites could be regarded as 3D magnonic
crystals existing in nature.",0508389v1
2005-08-17,Magnetic Composites: Magnonic Excitations vs. Three-Dimensional Structural Periodicity,"This study deals with the spin wave spectrum in magnetic macrostructure
(composed of two ferromagnetic materials) showing a 3D periodicity: spherical
ferromagnetic grains disposed in the nodes of a simple cubic crystal lattice
are embedded in a matrix with different ferromagnetic properties. It is shown
that the \textit{magnonic spectrum} of this composite structure exhibits
frequency regions \textit{forbidden} for magnon propagation, and the energy
gaps are found to be sensitive to the exchange contrast between the constituent
materials as well as to the magnetization contrast. The widths of the
respective magnonic gaps are studied as functions of parameters characterizing
the magnetic structure.",0508390v1
2005-12-24,Modified spin-wave theory of nuclear magnetic relaxation in one-dimensional quantum ferrimagnets: Three-magnon versus Raman processes,"Nuclear spin-lattice relaxation in one-dimensional Heisenberg ferrimagnets is
studied by means of a modified spin-wave theory. Calculating beyond the
first-order mechanism, where a nuclear spin directly interacts with spin waves
through the hyperfine coupling, we demonstrate that the
exchange-scattering-enhanced three-magnon nuclear relaxation may generally
predominate over the Raman one with increasing temperature and decreasing
field. Recent proton spin-lattice relaxation-time (T_1_) measurements on the
ferrimagnetic chain compound NiCu(C_7_H_6_N_2_O_6_)(H_2_O)_3_2H_2_O suggest
that the major contribution to 1/T_1_ be made by the three-magnon scattering.",0512629v1
2006-06-07,Ferromagnetic relaxation by magnon-induced currents,"A theory for calculating spin wave relaxation times based on the
magnon-electron interaction is developed. The theory incorporates a thin film
geometry and is valid for a large range of magnon frequencies and wave vectors.
For high conductivity metals such as permalloy, the wave vector dependent
damping constant approaches values as high as 0.2, showing the large magnitude
of the effect, and can dominate experimentally observed relaxation.",0606197v1
2006-06-30,Spin-Wave Lifetimes Throughout the Brillouin Zone,"We use a neutron spin-echo method with $\mu$eV resolution to determine the
lifetimes of spin waves in the prototypical antiferromagnet MnF$_2$ over the
entire Brillouin zone. A theory based on the interaction of magnons with
longitudinal spin fluctuations provides an excellent, parameter-free
description of the data, except at the lowest momenta and temperatures. This is
surprising, given the prominence of alternative theories based on magnon-magnon
interactions in the literature. The results and technique open up a new avenue
for the investigation of fundamental concepts in magnetism. The technique also
allows measurement of the lifetimes of other elementary excitations (such as
lattice vibrations) throughout the Brillouin zone.",0606814v1
2006-10-25,Domain-Wall Waves (2D Magnons) in Superconducting Ferromagnets,"Propagation of the magnetization waves along domain walls (2D magnons) in a
superconducting ferromagnet has been studied theoretically. The magnetostatic
fields (long-range dipole-dipole interaction) have a crucial effect on the
spectrum of 2D magnons. But this effect is essentially affected by the
superconducting Meissner currents, which screen the magnetostatic fields and
modify the long-wavelength spectrum from square-root to linear. The excitation
of the domain wall waves by an electromagnetic wave incident on a
superconducting-ferromagnet sample has been considered. This suggests using
measurements of the surface impedance for studying the domain wall waves, and
eventually for effective probing of superconductivity-ferromagnetism
coexistence.",0610699v1
2007-01-12,Spectroscopy of the parametric magnons excited by 4-wave process,"Using a Magnetic Resonace Force Microscope, we have performed ferromagnetic
resonance (FMR) spectroscopy on parametric magnons created by 4-wave process.
This is achieved by measuring the differential response to a small source
modulation superimposed to a constant excitation power that drives the dynamics
in the saturation regime of the transverse component. By sweeping the applied
field, we observe abrupt readjustement of the total number of magnons each time
the excitation coincides with a parametric mode. This gives rise to
ultra-narrow peaks whose linewith is lower than $5~10^{-6}$ of the applied
field.",0701287v1
2006-05-16,Dyonic Giant Magnons,"We study the classical spectrum of string theory on AdS_5 X S^5 in the
Hofman-Maldacena limit. We find a family of classical solutions corresponding
to Giant Magnons with two independent angular momenta on S^5. These solutions
are related via Pohlmeyer's reduction procedure to the charged solitons of the
Complex sine-Gordon equation. The corresponding string states are dual to BPS
boundstates of many magnons in the spin-chain description of planar N=4 SUSY
Yang-Mills. The exact dispersion relation for these states is obtained from a
purely classical calculation in string theory.",0605155v2
2007-03-20,Giant Magnons and Singular Curves,"We obtain the giant magnon of Hofman-Maldacena and its dyonic generalisation
on R x S^3 < AdS_5 x S^5 from the general elliptic finite-gap solution by
degenerating its elliptic spectral curve into a singular curve. This alternate
description of giant magnons as finite-gap solutions associated to singular
curves is related through a symplectic transformation to their already
established description in terms of condensate cuts developed in
hep-th/0606145.",0703180v2
1996-12-20,On the two-magnon bound states for the quantum Heisenberg chain with variable range exchange,"The spectrum of finite-difference two-magnon operator is investigated for
quantum S=1/2 chain with variable range exchange of the form $h(j-k)\propto
\sinh^{-2}a(j-k)$. It is found that usual bound state appears for some values
of the total pseudomomentum of two magnons as for the Heisenberg chain with
nearest-neighbor spin interaction. Besides this state, a new type of bound
state with oscillating wave function appears at larger values of the total
pseudomomentum.",9612008v1
2007-04-18,Semiclassical Quantization of the Giant Magnon,"Solitons in field theory provide a window into regimes not directly
accessible by the fundamental perturbative degrees of freedom. Motivated by
interest in the worldsheet S-matrix of string theory in AdS_5 x S^5 in the
limit of infinite worldsheet volume we consider the semiclassical quantization
of a particular soliton of this theory: the Hofman-Maldacena `giant magnon'
spinning string. We obtain explicit formulas for the complete spectrum of
bosonic and fermionic fluctuations around the giant magnon. As an application
of these results we confirm that the one-loop correction to the classical
energy vanishes as expected.",0704.2389v2
2007-08-06,Multi-magnon bound states in the frustrated ferromagnetic 1D chain,"We study a one-dimensional Heisenberg chain with competing ferromagnetic
nearest-neighbor and antiferromagnetic next-nearest neighbor interactions in
magnetic field. Starting from the fully polarized high-field state, we
calculate the dispersions of the lowest-lying $n$-magnon excitations and the
saturation field (n=2,3,4). We show that the lowest-lying excitations are
always bound multi-magnon states with a total momentum of $\pi$ except for a
small parameter range. We argue that bose condensation of the bound $n$ magnons
leads to novel Tomonaga-Luttinger liquids with multi-polar correlations;
nematic and triatic ordered liquids correspond to n=2 and n=3.",0708.0701v1
2007-10-18,Spin Vortex in Magnon BEC of Superfluid 3He-B,"The phenomenon of the spontaneous phase-coherent precession of magnetization
in superfluid 3He and the related effects of spin superfluidity are based on
the true Bose-Einstein condensation of magnons. Several different magnon BEC
states have been observed: homogeneously precessing domain (HPD); BEC
condensation in the spin-orbit potential trap (Q-balls); coherent precession
with fractional magnetization; and two modes of the coherent precession in
squeezed aerogel. The spin superfluidity effects, like spin Josephson
phenomena, spin current vortices, spin phase slippage, long distance
magnetization transport by spin supercurrents have been observed.",0710.3448v1
2008-01-19,Electrical control of magnon propagation in multiferroic BiFeO3 films,"The spin wave spectra of multiferroic BiFeO3 films is calculated using a
phenomenological Landau theory that includes magnetostatic effects. The lowest
frequency magnon dispersion is shown to be quite sensitive to the angle between
spin wave propagation vector and the Neel moment. Since electrical switching of
the Neel moment has recently been demonstrated in this material, the
sensitivity of the magnon dispersion permits direct electrical switching of
spin wave propagation. This effect can be used to construct spin wave logical
gates without current pulses, potentially allowing reduced power dissipation
per logical operation.",0801.3012v1
2008-04-23,Phonons in magnon superfluid and symmetry breaking field,"Recent experiments [1],[2] which measured the spectrum of the Goldstone
collective mode of coherently precessing state in 3He-B are discussed using the
presentation of the coherent spin precession in terms of the Bose-Einstein
condensation of magnons. The mass in the spectrum of the Goldstone boson --
phonon in the superfluid magnon liquid -- is induced by the symmetry breaking
field, which is played by the RF magnetic field",0804.3709v3
2008-06-16,Exact solutions for N-magnon scattering,"Giant magnon solutions play an important role in various aspects of the
AdS/CFT correspondence. We apply the dressing method to construct an explicit
formula for scattering states of an arbitrary number N of magnons on R x S^3.
The solution can be written in Hirota form and in terms of determinants of N x
N matrices. Such a representation may prove useful for the construction of an
effective particle Hamiltonian describing magnon dynamics.",0806.2466v1
2008-08-04,Dynamics of a one-dimensional spinor Bose liquid: a phenomenological approach,"The ground state of a spinor Bose liquid is ferromagnetic, while the softest
excitation above the ground state is the magnon mode. The dispersion relation
of the magnon in a one-dimensional liquid is periodic in the wavenumber q with
the period 2\pi n, determined by the density n of the liquid. Dynamic
correlation functions, such as e.g. spin-spin correlation function, exhibit
power-law singularities at the magnon spectrum, $\omega\to\omega_m(q,n)$.
Without using any specific model of the inter-particle interactions, we relate
the corresponding exponents to independently measurable quantities
$\partial\omega_m/\partial q$ and $\partial\omega_m/\partial n$.",0808.0479v2
2008-09-18,Giant Magnons and Spiky Strings on S^3 with B-field,"We study solutions for a rotating string on S^3 with a background NS-NS
B-field and show the existence of spiky string and giant magnon as two limiting
solutions. We make a connection to the sine-Gordon model via the Polyakov
worldsheet action and study the effect of B-field. In particular, we find the
magnon solution can be mapped to the excitation of a fractional spin chain. We
conjecture a B-deformed SYM to be the gauge theory dual to this background.",0809.3269v2
2008-12-08,Multi-magnon bound states in easy-axis ferromagnetic zigzag spin chain,"The frustrated spin-1/2 chain with weakly anisotropic ferromagnetic
nearest-neighbor and antiferromagnetic next-nearest-neighbor exchanges is
studied. We focus on the excitation spectrum and the low-temperature
thermodynamics in the ferromagnetic region of the ground state phase diagram.
It is shown that the excitation spectrum of the model is characterized by the
existence of the multi-magnon bound states. These excitations determine the
low-temperature magnetic susceptibility. The energy of the bound magnon
complexes is found using the scaling estimates of the perturbation theory and
numerical calculations. The relation of the considered model to the
edge-sharing cuprate $Li_2CuO_4$ is discussed.",0812.1425v1
2009-02-28,Magnon BEC in superfluid 3He-A,"The new mode of magnetization precession in superfluid 3He-A in a squeezed
aerogel has been recently reported. We consider this mode in terms of the
Bose-Einstein condensation (BEC) of magnons. The difference between magnon BEC
states in 3He-A and in 3He-B is discussed.",0903.0016v2
2009-11-04,Theory of Resonant Inelastic X-ray Scattering by Collective Magnetic Excitations,"I present a tractable theory for the Resonant Inelastic X-ray Scattering
(RIXS) spectral function of magnons. The low-energy transition operator is
written as a product of local spin operators times fundamental x-ray absorption
spectra. This leads to simple selection rules for the magnetic cross section.
The scattering cross section linear (quadratic) in spin operators is
proportional to the magnetic circular (linear) dichroic absorption. RIXS is a
novel tool to measure magnetic quasi particles (magnons) and the incoherent
spectral weight, as well as multiple magnons up to very high energy losses, in
small samples, thin films and multilayers, complementary to Neutron scattering.",0911.0706v2
2009-11-16,Electron-Magnon Scattering in Anomalous Hall Effect,"We study the role played by electron-magnon scattering in the anomalous Hall
effect. We find that it has important contributions distinct from other
scattering processes like impurities scattering and phonon scattering. As a
demonstration, we calculate the Hall conductivity for a two dimensional Dirac
model. The result indicates that as system control parameter varies, the
competition between magnon scattering and other types of scattering changes the
Hall conductivity drastically. In particular, the side jump contribution could
acquire a strong temperature dependence.",0911.3134v1
2010-04-08,Spin-charge and spin-orbital coupling effects on spin dynamics in ferromagnetic manganites,"Correlation-induced spin-charge and spin-orbital coupling effects on spin
dynamics in ferromagnetic manganites are calculated with realistic parameters
in order to provide a quantitative comparison with experimental results for
spin stiffness, magnon dispersion, magnon damping, anomalous zone-boundary
magnon softening, and Curie temperature. The role of orbital degeneracy,
orbital ordering, and orbital correlations on spin dynamics in different doping
regimes is highlighted.",1004.1352v2
2010-07-09,Boundary Giant Magnons and Giant Gravitons,"We construct the full set of boundary giant magnons on $\mathbb{R}\times
S^{2}$ attached to the maximal $Z=0$ giant graviton by mapping from the general
solution to static sine-Gordon theory on the interval and compute the values of
$\Delta-J$ at finite $J$, including the leading order corrections when $J$ is
large. We then consider the Born-Infeld theory of the giant graviton itself to
construct BIon spike solutions that correspond to the world volume description
of the boundary giant magnons at finite $J$.",1007.1674v2
2010-11-05,Giant magnons and non-maximal giant gravitons,"We produce the open strings on $\mathbb{R}\times S^{2}$ that correspond to
the solutions of integrable boundary sine-Gordon theory by making use of the
$N$-magnon solutions provided in \cite{KPV} together with explicit moduli.
Relating the two boundary parameters in a special way we describe the
scattering of giant magnons with non-maximal $Y=0$ giant gravitons and
calculate the leading contribution to the associated magnon scattering phase.",1011.1440v1
2011-05-16,Three-point Correlation functions of Giant magnons with finite size,"We compute holographic three-point correlation functions or structure
constants of a zero-momentum dilaton operator and two (dyonic) giant magnon
string states with a finite-size length in the semiclassical approximation. We
show that the semiclassical structure constants match exactly with the
three-point functions between two su(2) magnon single trace operators with
finite size and the Lagrangian in the large 't Hooft coupling constant limit. A
special limit J>>sqrt(lambda) of our result is compared with the relevant
result based on the L\""uscher corrections.",1105.3084v2
2011-06-10,Rotational motion of magnon and thermal Hall effect,"Due to the Berry curvature in momentum space, the magnon wavepacket undergoes
two types of orbital motions in analogy with the electron system: the
self-rotation motion and a motion along the boundary of the sample (edge
current). The magnon edge current causes the thermal Hall effect, and these
orbital motions give corrections to the thermal transport coefficients. We also
apply our theory to the magnetostatic spin wave in a thin-film ferromagnet, and
derive expression for the Berry curvature.",1106.1987v1
2012-08-27,Spin-wave modes and band structure of rectangular CoFeB antidot lattices,"We present an investigation of rectangular antidot lattices in a CoFeB film.
Magnonic band structures are numerically calculated, and band gaps are
predicted which shift in frequency by 0.9 GHz when rotating the external field
from the long to the short axis of the unit cell. We demonstrate by
time-resolved experiments that magnonic dipolar surface modes are split in
frequency by 0.6 GHz which agrees well with the theoretical prediction. These
findings provide the basis for directional spin-wave filtering with magnonic
devices.",1208.5339v1
2012-08-28,Magnon Mediated Electric Current Drag Across a Ferromagnetic Insulator Layer,"In a semiconductor hererostructure, the Coulomb interaction is responsible
for the electric current drag between two 2D electron gases across an electron
impenetrable insulator. For two metallic layers separated by a ferromagnetic
insulator (FI) layer, the electric current drag can be mediated by a
nonequilibrium magnon current of the FI. We determine the drag current by using
the semiclassical Boltzmann approach with proper boundary conditions of
electrons and magnons at the metal-FI interface.",1208.5812v1
2012-10-09,Spin convertance at magnetic interfaces,"The exchange interaction between the conduction electrons and magnetic
moments at magnetic interface leads to mutual conversion between electron spin
current and magnon current. We introduce a concept of spin convertance which
quantitatively measures the magnon current induced by electron spin
accumulation and the spin current created by magnon accumulation at the
interface. We predict the phenomena on charge and spin drag across a magnetic
insulator spacer for several layered structures.",1210.2735v3
2012-10-22,Excitations in a quantum spin liquid with random bonds,"We present results of inelastic neutron scattering study on two bond
disordered quasi twodimensional quantum magnets
(C$_4$H$_12$N$_2$)Cu$_2$(Cl$_{1-x}$Br$_x$)$_6$ with x=0.035 and 0.075. We
observe the increase of spin gap, reduction of magnon bandwidth and a decrease
of magnon lifetimes compared to x=0 sample. Additional magnon damping is
observed at higher energies away from zone center which is found to follow the
density of single particle states.",1210.5899v1
2012-11-14,Ultrafast charge recombination in photoexcited Mott-Hubbard insulator,"We present a calculation of the recombination rate of the excited
holon-doublon pairs based on the two-dimensional model relevant for undoped
cuprates which shows that fast processes, observed in pump-probe experiments on
Mott-Hubbard insulators in picosecond range, can be explained even
quantitatively with the multi-magnon emission. The precondition is the
existence of the Mott-Hubbard bound exciton of the s-type. We find that its
decay is exponentially dependent on the Mott-Hubbard gap and on the magnon
energy, with a small prefactor which can be traced back to strong correlations
and consequently large exciton-magnon coupling.",1211.3236v1
2013-02-19,The temperature dependence of quantum spin pumping generated using electron spin resonance with three-magnon splittings,"On the basis of the Schwinger-Keldysh formalism, we have closely investigated
the temperature dependence of quantum spin pumping by electron spin resonance.
We have clarified that three-magnon splittings excite non-zero modes of magnons
and characterize the temperature dependence of quantum spin pumping. Our
theoretical result qualitatively agrees with the experiment by Czeschka et al.
that the mixing conductance is little influenced by temperature [F. D. Czeschka
et al., Phys. Rev. Lett., 107, 046601 (2011)].",1302.4777v1
2013-02-24,Bose-Einstein condensation of magnons in spin pumping systems,"We clarify the condition for the occurrence of magnon Bose-Einstein
condensation (BEC) in spin pumping systems without using external pumping
magnetic fields. The Goldstone model is generalized and the stability of the
vacuum is closely investigated. By applying the generalized Goldstone model to
spin pumping systems, the condition for the experimental realization of the
stable magnon BEC in spin pumping systems is theoretically proposed.",1302.5858v1
2014-07-31,Universal critical behavior of the two-magnon-bound-state mass gap for the (2+1)-dimensional Ising model,"The two-magnon-bound-state mass gap m_2 for the two-dimensional quantum Ising
model was investigated by means of the numerical diagonalization method; the
low-lying spectrum is directly accessible via the numerical diagonalization
method. It has been claimed that the ratio m_2/m_1 (m_1: one-magnon mass gap)
is a universal constant in the vicinity of the critical point. Aiming to
suppress corrections to scaling (lattice artifact), we consider the spin-S=1
Ising model with finely-adjusted extended interactions. The simulation result
for the finite-size cluster with N \le 20 spins indicates the mass-gap ratio
m_2/m_1=1.84(1).",1407.8243v1
2014-09-25,Interfacial Spin and Heat Transfer between Metals and Magnetic Insulators,"We study the role of thermal magnons in the spin and heat transport across a
normal-metal/insulating-ferromagnet interface, which is beyond an elastic
electronic spin transfer. Using an interfacial exchange Hamiltonian, which
couples spins of itinerant and localized orbitals, we calculate spin and energy
currents for an arbitrary interfacial temperature difference and misalignment
of spin accumulation in the normal metal relative to the ferromagnetic order.
The magnonic contribution to spin current leads to a temperature-dependent
torque on the magnetic order parameter; reciprocally, the coherent precession
of the magnetization pumps spin current into the normal metal, the magnitude of
which is affected by the presence of thermal magnons.",1409.7128v1
2014-12-19,Spin waves in one-dimensional bi-component quasicrystals,"We studied finite Fibonacci sequence of Co and Py stripes aligned
side-by-side and in direct contact with each other. Calculations based on
continuous model including exchange and dipole interactions were performed for
structures feasible for fabrication and characterization of main properties of
magnonic quasicrystals. We have shown the fractal structure of the magnonic
spectrum with a number of magnonics gaps of different width. Also localization
of spin waves in quasicrystals and existence of surface spin waves in finite
quaiscrystal structure is demonstrated.",1412.6301v4
2015-01-13,Spin effects induced by thermal perturbation in a normal metal/magnetic insulator system,"Using one of the methods of quantum nonequilibrium statistical physics we
have investigated the spin transport transverse to the normal
metal/ferromagnetic insulator interface in hybrid nanostructures. An
approximation of the effective parameters, when each of the interacting
subsystems (electron spin, magnon, and phonon) is characterized by its own
effective temperature have been considered. The generalized Bloch equations
which describe the spin-wave current propagation in the dielectric have been
derived. Finally, two sides of the spin transport ""coin"" have been revealed:
the diffusive nature of the magnon motion and magnon relaxation processes,
responsible for the spin pumping and the spin-torque effect.",1501.02983v1
2015-05-06,Spintronics and Magnon Bose-Einstein Condensation,"Spintronics is the science and technology of electric control over spin
currents in solid-state-based devices. Recent advances have demonstrated a
coupling between electronic spin currents in non-magnetic metals and magnons in
magnetic insulators. The coupling is due to spin transfer and spin pumping at
interfaces between the normal metals and magnetic insulators. In this Chapter,
we review these developments and the prospects they raise for electric control
of quasi-equilibrium magnon Bose-Einstein condensates and spin superfluidity.",1505.01329v1
2016-01-16,Measurements of anisotropic mass of magnons confined in a harmonic trap in superfluid $^3$He-B,"We can pump magnons to a nearly harmonic magneto-textural trap in superfluid
$^3$He-B. Using the NMR spectroscopy of levels in the trap we have measured the
anisotropic magnon mass and related values of the spin-wave velocities. Based
on our measurements we provide values of the Fermi-liquid parameter $F^a_1$.",1601.04192v1
2016-02-22,Exchange magnon induced resistance asymmetry in permalloy spin-Hall oscillators,"We investigate magnetization dynamics in a spin-Hall oscillator using a
direct current measurement as well as conventional microwave spectrum analysis.
When the current applies an anti-damping spin-transfer torque, we observe a
change in resistance which we ascribe to the excitation of incoherent exchange
magnons. A simple model is developed based on the reduction of the effective
saturation magnetization, quantitatively explaining the data. The observed
phenomena highlight the importance of exchange magnons on the operation of
spin-Hall oscillators.",1602.06710v1
2017-02-14,Cavity mediated effective interaction between phonon and magnon,"Optomagnonics and optomechanics have various applications ranging from
tunable light sources to optical manipulation for quantum information science.
Here, we propose a hybrid system with the interaction between phonon and magnon
which could be tuned by the electromagnetic field based on the radiation
pressure and magneto-optical effect. The self energy of the magnon and phonon
induced by the electromagnetic field and the influence of the thermal noise are
studied. Moreover, the topological features of the hybrid system is illustrated
with resort to the dynamical encircling with the exceptional points, and the
chiral characteristics under these encirclements are found.",1702.04030v1
2018-11-25,Self-localization of magnons and magnetoroton in a binary Bose-Einstein condensate,"We consider a two-component Bose-condensed mixture characterized by positive
s-wave scattering lengths. We assume equal densities and intra-species
interactions. By doing the Bogoliubov transformation of an effective
Hamiltonian we obtain the lower energy magnon dispersion incorporating the
superfluid entrainment between the components. We argue that p-wave pairing of
distinct bosons should be accompanied by self-localization of magnons and
formation of a magnetoroton. We demonstrate the effect on a model system of
particles interacting via step potentials.",1811.10099v2
2014-05-20,Giant Magnon on Deformed AdS(3)xS(3),"We study giant magnon solutions for strings moving on a deformed AdS(3)xS(3)
background. We impose a conformal gauge on the Polyakov action and proceed with
solving the Virasoro constraints. The expressions of the conserved charge J and
the energy of a single magnon excitation are then computed. Then we determine
the dispersion relation in the infinite J limit configuration and we find that
it reduces to celebrated Hofman-Maldacena dispersion relation when the
deformation parameter goes to zero.",1405.5017v2
2014-06-03,Finite-size giant magnons on η-deformed AdS_5 x S^5,"We consider strings moving in the R_t x S^3_\eta subspace of the
\eta-deformed AdS_5 x S^5 and obtain a class of solutions depending on several
parameters. They are characterized by the string energy and two angular
momenta. Finite-size dyonic giant magnon belongs to this class of solutions.
Further on, we restrict ourselves to the case of giant magnon with one nonzero
angular momentum, and obtain the leading finite-size correction to the
dispersion relation.",1406.0628v1
2016-05-05,Theory of magnon motive force in chiral ferromagnets,"We predict that magnon motive force can lead to temperature dependent,
nonlinear chiral damping in both conducting and insulating ferromagnets. We
estimate that this damping can significantly influence the motion of skyrmions
and domain walls at finite temperatures. We also find that in systems with low
Gilbert damping moving chiral magnetic textures and resulting magnon motive
forces can induce large spin and energy currents in the transverse direction.",1605.01694v2
2016-07-30,Magnon-bound-state hierarchy for the two-dimensional transverse-field Ising model in the ordered phase,"In the ordered phase for an Ising ferromagnet, the magnons are attractive to
form a series of bound states with the mass gaps, $m_2<m_3 < \dots$. Each ratio
$m_{2,3,\dots}/m_1$ ($m_1$: the single-magnon mass) is expected to be a
universal constant in the vicinity of the critical point. In this paper, we
devote ourselves to the $(2+1)$-dimensional counterpart, for which the
universal hierarchical character remains unclear. We employed the exact
diagonalization method, which enables us to calculate the dynamical
susceptibility via the continued-fraction expansion. Thereby, we observe a
variety of signals including $m_{2,3,4}$, and the spectrum is analyzed with the
finite-size-scaling method to estimate the universal mass-gap ratios.",1608.00083v1
2018-03-09,Non-local coupling between antiferromagnets and ferromagnets in cavities,"Microwaves couple to magnetic moments in both ferromagnets and
antiferromagnets. Although the magnons in ferromagnets and antiferromagnets
radically differ, they can become entangled via strong coupling to the same
microwave mode in a cavity. The equilibrium configuration of the magnetic
moments crucially governs the coupling between the different magnons because
the antiferromagnetic and ferromagnetic magnons have the opposite spins when
their dispersion relations cross. We derive analytical expressions for the
coupling strengths and find that the coupling between antiferromagnets and
ferromagnets is comparable to the coupling between two ferromagnets. Our
findings reveal a robust link between cavity spintronics with ferromagnets and
antiferromagnets.",1803.03486v1
2018-07-26,Emission of coherent propagating magnons by insulator-based spin-orbit torque oscillators,"We experimentally demonstrate generation of coherent propagating magnons in
ultra-thin magnetic-insulator films by spin-orbit torque induced by dc electric
current. We show that this challenging task can be accomplished by utilizing
magnetic-insulator films with large perpendicular magnetic anisotropy. We
demonstrate simple and flexible spin-orbit torque devices, which can be used as
highly efficient nanoscale sources of coherent propagating magnons for
insulator-based spintronic applications.",1807.09976v1
2019-01-29,Evaluation protocol for revealing magnonic contrast in STXM-FMR measurements,"We present a statistically motivated method to extract magnonic contrast from
STXM-FMR measurement with microwave frequencies of the order of \unit[10]{GHz}.
With this method it is possible to generate phase and amplitude profiles with a
spatial resolution of about \unit[30]{nm} given by the STXM resolution,
furthermore this method allows fo a rigoros transformation to reciprocal
\vec{k}-space, revealing \vec{k}-dependent magnon properties.",1901.10595v1
2019-02-24,Half-Metallic Ferromagnets and Spin Gapless Semiconductors,"A brief review of experimental and theoretical studies of half-metallic
ferromagnets (HMF) and spin gapless semiconductors (SGS) is given. The data on
resistivity and magnetoresistivity are presented. An important role of
non-quasiparticle states owing to electron-magnon scattering in transport
properties is discussed. The problem of low-temperature resistivity in HMF is
treated in terms of one-magnon and two-magnon scattering processes.",1902.08972v1
2019-03-18,Theory of spin transport through antiferromagnetic insulator,"A theoretical formulation for spin transport through an antiferromagnetic
(AF) insulator is presented in the case driven/detected by direct/inverse spin
Hall effect in two heavy metal contacts. The spin signal is shown to be
transferred by the ferromagnetic correlation function of the antiferromagnet,
which is calculated based on a magnon representation. To cover high temperature
regimes, we include an auxiliary field representing short AF correlations and a
temperature-dependent damping due to magnon scattering. The diffusion length
for spin is long close to the degeneracy of the two AF magnons, and has a
maximum as function of temperature near the N\'eel transition.",1903.07223v1
2019-03-30,Superconductivity at metal-antiferromagnetic insulator interfaces,"Magnons in antiferromagnetic insulators couple strongly to conduction
electrons in adjacent metals. We show that this interfacial tie can lead to
superconductivity in a tri-layer consisting of a metal sandwiched between two
antiferromagnetic insulators. The critical temperature is closely related to
the magnon gap, which can be in the THz range. We estimate the critical
temperature in MnF$_2$-Au-MnF$_2$ to be on the order of 1 K. The Umklapp
scattering at metal-antiferromagnet interfaces leads to a d-wave
superconductive pairing, in contrast to the p-wave superconductivity mediated
by magnons in ferromagnets.",1904.00233v1
2015-08-19,Antiferromagnetic Magnonic Crystals,"We describe the features of magnonic crystals based upon antiferromagnetic
elements. Our main results are that with a periodic modulation of either
magnetic fields or system characteristics, such as the anisotropy, it is
possible to tailor the spin wave spectra of antiferromagnetic systems into a
band-like organization that displays a segregation of allowed and forbidden
bands. The main features of the band structure, such as bandwidths and
bandgaps, can be readily manipulated. Our results provide a natural link
between two steadily growing fields of spintronics: antiferromagnetic
spintronics and magnonics.",1508.04497v1
2017-07-12,Superconductivity Induced by Interfacial Coupling to Magnons,"We consider a thin normal metal sandwiched between two ferromagnetic
insulators. At the interfaces, the exchange coupling causes electrons within
the metal to interact with magnons in the insulators. This electron-magnon
interaction induces electron-electron interactions, which, in turn, can result
in p-wave superconductivity. In the weak-coupling limit, we solve the gap
equation numerically and estimate the critical temperature. In YIG-Au-YIG
trilayers, superconductivity sets in at temperatures somewhere in the interval
between 1 and 10 K. EuO-Au-EuO trilayers require a lower temperature, in the
range from 0.01 to 1 K.",1707.03754v1
2017-11-25,Giant magnon-like solution in Sch_5 x S^5,"In this paper we have found a classical giant magnon-like solution with both
infinite and finite angular momentum moving in Sch_5 x S^5 with B-field, which
is believed to be dual to dipole-deformed N=4 super Yang-Mills theory. This
string state propagates as a point particle in non-trivial subspace of the
Sch_5 space but shows a giant magnon-like property in the S^2 subspace. We
derive the energy-momentum dispersion relations and their finite-size
correction for the case of finite but large angular momentum.",1711.09252v3
2019-05-19,Curvature induced magnonic crystal in nanowires,"A new type of magnonic crystals, curvature induced ones, is realized in
ferromagnetic nanowires with periodically deformed shape. A magnon band
structure of such crystal is fully determined by its curvature: the developed
theory is well confirmed by simulations. An application to nanoscale spintronic
devises with the geometrically tunable parameters is proposed, namely, to
filter elements.",1905.07795v4
2020-06-22,Semiclassical quantization of the mixed-flux AdS$_3$ giant magnon,"We obtain explicit formulas for the eight bosonic and eight fermionic
fluctuations around the mixed-flux generalization of the Hofman-Maldacena giant
magnon on AdS$_3 \times$S$^3 \times$T$^4$ and AdS$_3 \times$S$^3 \times$S$^3
\times$S$^1$. As a check of our results, we confirm that the semiclassical
quantization of these fluctuations leads to a vanishing one-loop correction to
the magnon energy, as expected from symmetry based arguments.",2006.12080v2
2015-07-24,On-chip artificial magnon-polariton device for voltage control of electromagnetically induced transparency,"We demonstrate an on-chip device utilizing the concept of artificial cavity
magnon-polariton (CMP) coupling between the microwave cavity mode and the
dynamics of the artificial magnetism in a split ring resonator. This on-chip
device allows the easy tuning of the artificial CMP gap by using a DC voltage
signal, which enables tuneable electrodynamically induced transparency. The
high tunability of the artificial magnon-polariton system not only enables the
study of the characteristic phenomena associated with distinct coupling
regimes, but also may open up avenues for designing novel microwave devices and
ultra-sensitive sensors.",1507.06940v1
2016-11-28,Berry curvature in magnon-phonon hybrid systems,"We theoretically study the Berry curvature of the magnon induced by the
hybridization with the acoustic phonons via the spin-orbit and dipolar
interactions. We first discuss the magnon-phonon hybridization via the dipolar
interaction, and show that the dispersions have gapless points in momentum
space, some of which form a loop. Next, when both spin-orbit and dipolar
interactions are considered, we show anisotropic texture of the Berry curvature
and its divergence with and without gap-closing. Realistic evaluation of the
consequent anomalous velocity is given for yttrium iron garnet.",1611.08993v1
2017-04-17,Weyl and nodal ring magnons in three-dimensional honeycomb lattices,"We study the topological properties of magnon excitations in a wide class of
three dimensional (3D) honeycomb lattices with ferromagnetic ground states. It
is found that they host nodal ring magnon excitations. These rings locate on
the same plane in the momentum space. The rings can be gapped by
Dzyaloshinskii-Moriya (DM) interactions to form two Weyl points with opposite
charges. We explicitly discuss these physics in the simplest 3D honeycomb
lattice, the hyperhoneycomb lattice and show drumhead and arc surface states in
the nodal ring and Weyl phases, respectively, due to the bulk-boundary
correspondence.",1704.04898v1
2020-01-04,Nonrelativistic giant magnons from Newton Cartan strings,"We show nonrelativistic (NR) giant magnon dispersion relations by probing the
torsional Newton Cartan (TNC) geometry with (semi)classical nonrelativistic
rigidly rotating strings. We construct NR sigma models over $ R \times S^2 $
and consider two specific limiting cases those are of particular interest. Both
of these limiting conditions give rise to what we identify as the \emph{small}
momentum limit of the giant magnon dispersion relation in the dual SMT at
\emph{strong} coupling. We further generalize our results in the presence of
background NS-NS fluxes. Our analysis reveals that unlike its relativistic
counterpart, the NR string theory lacks of single spike solutions.",2001.01061v2
2020-04-30,Magnon Current Generation by Dynamical Distortion,"The interaction between spin and nanomechanical degrees of freedom attracts
interest from the viewpoint of basic science and device applications. We study
the magnon current induced by the torsional oscillation of ferromagnetic
nanomechanical cantilever. We find that a finite Dzyaloshinskii-Moriya (DM)
interaction emerges by the torsional oscillation, which is described by the
spin gauge field, and the DM interaction leads to the detectably-large magnon
current with frequency same as that of the torsional oscillation. Our theory
paves the way for studying torsional spin-nanomechanical phenomena by using the
spin gauge field.",2004.14707v1
2020-10-11,Multispin magnons from Spin-Matrix strings on $ AdS_5 \times S^5 $,"The present Letter derives multispin nonrelativistic magnon spectrum
considering various near BPS corners within $ SU(1,2|3) $ Spin-Matrix theory
(SMT) limit of strings on $ AdS_5 \times S^5 $. In particular, we focus on some
typical rotating string solutions those correspond to two spin as well as three
spin configurations in the bulk and identify the associated nonrelativistic
magnon like excitations in these models.",2010.05179v2
2020-10-21,Thermal gating of magnon exchange in magnetic multilayers with antiferromagnetic spacers,"We observe a strong thermally-controlled magnon-mediated interlayer coupling
of two ferromagnetic layers via an antiferromagnetic spacer in spin-valve type
trilayers. The effect manifests itself as a field-induced coherent switching of
the two ferromagnets, which can be controlled by varying temperature and the
spacer thickness. We explain the observed behavior as due to a strong
hybridization of the ferro- and antiferro-magnetic magnon modes in the trilayer
at temperatures just below the N\'eel temperature of the antiferromagnetic
spacer.",2010.11134v1
2021-04-30,Micromagnetic modeling of magnon coherent states in a nonuniform magnetic field,"The study of the dynamics of magnetically ordered states in strong excitation
through micromagnetic modeling has become relevant due to the observation of
magnon Bose condensation. In particular, the question has arisen about the
possibility of describing the coherent quantum state by the quasi-classical
Landau-Lifshitz-Gilbert equations. We performed micromagnetic simulations of
magnetization precession with a high angle of deviation in an out-of-plane
nonuniform dc field. Our results confirm the formation of coherent magnon state
under conditions of high excitation. This coherent state extends over long
distances and described by a spatially inhomogeneous amplitude and a
homogeneous precession phase.",2104.14804v1
2022-06-19,Low-temperature asymptotics for transverse autocorrelator of the magnetically polarized Ising chain studied by ordinary and nested Dyson equations,"Suggesting two versions for the Plakida-Tserkovnikov algorithm breakdown in
the low-temperature regime, we derive ordinary and nested Dyson equations for
the transverse autocorrelator of the magnetically polarized Ising chain. Using
them we get the corresponding low-temperature asymptotics for the
autocorrelator. We show that the ordinary Dyson equation results in a correct
$o({\rm e}^{-\beta E_{gap}})$ account of the magnon creation process, while the
nested Dyson equation additionally gives the correct $o({\rm e}^{-\beta
E_{gap}})$ contribution associated with transitions from magnons to bound
two-magnon states. The obtained result may be useful for the extension of the
suggested approach on the polarized $XXZ$-chain.",2206.09301v1
2023-01-14,Quantum entanglement generation on magnons assisted with microwave cavities coupled to a superconducting qubit,"We present protocols to generate quantum entanglement on nonlocal magnons in
hybrid systems composed of yttrium iron garnet (YIG) spheres, microwave
cavities and a superconducting (SC) qubit. In the schemes, the YIGs are coupled
to respective microwave cavities in resonant way, and the SC qubit is placed at
the center of the cavities, which interacts with the cavities simultaneously.
By exchanging the virtual photon, the cavities can indirectly interact in the
far-detuning regime. Detailed protocols are presented to establish entanglement
for two, three and arbitrary $N$ magnons with reasonable fidelities.",2301.05820v1
2023-02-08,Dielectric relaxation by quantum critical magnons,"We report the experimental observation of dielectric relaxation by quantum
critical magnons. Complex capacitance measurements reveal a dissipative feature
with a temperature-dependent amplitude due to low-energy lattice excitations
and an activation behavior of the relaxation time. The activation energy
softens close to a field-tuned magnetic quantum critical point at $H=H_c$ and
follows single-magnon energy for $H>H_c$, showing its magnetic origin. Our
study demonstrates the electrical activity of coupled low-energy spin and
lattice excitations, an example of quantum multiferroic behavior.",2302.04234v1
2023-02-17,Control of magnon-photon coupling by spin torque,"We demonstrate the influence of damping and field-like torques in the
magnon-photon coupling process by classically integrating the generalized
Landau-Lifshitz-Gilbert equation with RLC equation in which a phase correlation
between dynamic magnetization and microwave current through combined Amp\`ere
and Faraday effects are considered. We show that the gap between two hybridized
modes can be controlled in samples with damping parameter in the order of
$10^{-3}$ by changing the direction of the dc current density $J$ if a certain
threshold is reached. Our results suggest that an experimental realization of
the proposed magnon-photon coupling control mechanism is feasible in yttrium
iron garnet/Pt hybrid structures.",2302.08910v1
2023-03-15,Non-ergodic one-magnon magnetization dynamics of the antiferromagnetic delta chain,"We investigate the one-magnon dynamics of the antiferromagnetic delta chain
as a paradigmatic example of tunable equilibration. Depending on the ratio of
nearest and next-nearest exchange interactions the spin system exhibits a flat
band in one-magnon space - in this case equilibration happens only partially,
whereas it appears to be complete with dispersive bands as generally expected
for generic Hamiltonians. We provide analytical as well as numerical insight
into the phenomenon.",2303.08638v1
2023-04-13,Magnon squeezing in conical spin spirals,"We investigate squeezing of magnons in a conical spin spiral configuration.
We find that while the energy of magnons propagating along the $\boldsymbol{k}$
and the $-\boldsymbol{k}$ directions can be different due to the non-reciprocal
dispersion, these two modes are connected by the squeezing, hence can be
described by the same squeezing parameter. The squeezing parameter diverges at
the center of the Brillouin zone due to the translational Goldstone mode of the
system, but the squeezing also vanishes for certain wave vectors. We discuss
possible ways of detecting the squeezing.",2304.06338v2
2023-10-01,Exact Entanglement Propagation Dynamics in Integrable Heisenberg Chains,"The exact single-magnon entanglement evolution in Heisenberg chains is
obtained using the Quantum Correlation Transfer Function (QCTF) formulation.
The individual spins' entanglement is given by a hypergeometric function, and
its transient behavior is described via a Bessel function of the first kind.
The presented characterization through the lens of QCTF allowed for calculating
the ballistic single-magnon entanglement edge velocity in Heisenberg chains,
which has not been achieved before. Our results can be extended to the
multi-magnon regime, therefore opening up the means to explain equilibration
dynamics and thermodynamics in Heisenberg chains.",2310.00717v2
2023-12-07,Thermomagnetic Anomalies by Magnonic Criticality in Ultracold Atomic Transport,"We investigate thermomagnetic transport in an ultracold atomic system with
two ferromagnets linked via a magnetic quantum point contact. Using
nonequilibrium Green's function approach, we show a divergence in spin
conductance and a slowing down of spin relaxation that manifest in the weak
effective-Zeeman-field limit. These anomalous spin dynamics result from the
magnonic critical point at which magnons become gapless due to spontaneous
magnetization. Our findings unveil untapped dynamics in ultracold atomic
systems, opening new avenues in thermomagnetism.",2312.04280v1
2024-01-02,Efficient Magnon Injection and Detection via the Orbital Rashba Edelstein Effect,"Orbital currents and accumulation provide a new avenue to boost spintronic
effects in nanodevices. Here we use interconversion effects between charge
current and orbital angular momentum to demonstrate a dramatic increase in the
magnon spin injection and detection efficiencies in nanodevices consisting of a
magnetic insulator contacted by Pt/CuOx electrodes. Moreover, we note distinct
variations in efficiency for magnon spin injection and detection, indicating a
disparity in the direct and inverse orbital Rashba Edelstein effect
efficiencies.",2401.01090v1
2024-03-13,Are Magnons just the van der Waals interaction in Disguise?,"The MBD model of the van der Waals interaction is extended to also consider
magnetic interactions, and it is demonstrated how this can be made to reproduce
the Heisenberg Hamiltonian. It is found that this leads to a weak coupling
between the charge dipole waves that are the basis for the electric-only van
der Waals interaction and the spin-dipole waves (magnons) of the Heisenberg
model. By applying the same level of theory to both simultaneously we
demonstrate that magnons and charge-dipole waves may both be considered to be a
basis for the fluctuating part of the many-body electron density.",2403.08676v1
2024-04-08,Lattice-Assisted Molecular Excitations in Frustrated Pyrochlores,"Resonance-like molecular excitations have been discovered in a series of
pyrochlore antiferromagnets, yet their origins and relationships with the
coexisting magnon excitations remain a puzzle. Here, by incorporating the
spin-lattice coupling and the consequent four-spin interactions through the
site-phonon model, we accomplish a unified description of the molecular and
magnon excitations, revealing the molecular modes as statistical approximations
of the dispersive magnon excitations. Our work demonstrates a general approach
to understand exotic spin dynamics in magnetoelastic systems.",2404.05523v1
2024-04-09,Magnon transmission across $ν=1|-1|1$ mono-layer graphene junction as a probe of electronic structure,"We study magnon transmission across gate-controlled junctions in the $n=0$
manifold of Landau levels in monolayer graphene, allowing for both spin and
valley Zeeman fields. We show that by tuning the external perpendicular
magnetic field, the phase in the intermediate region of a $1|\nu_m|1$ sandwich
geometry can be changed, due to which the magnon transmission can be switched
between fully transmitting and fully blocked. Our analysis, along with the
experimental measurements, can be used to determine the anisotropic couplings
in the sample.",2404.06355v1
1995-03-02,Resonant two-magnon Raman scattering in parent compounds of high-T$_c$ superconductors.,"We propose a theory of two-magnon Raman scattering from the insulating parent
compounds of high-T$_c$ superconductors, which contains information not only on
magnetism, but also on the electronic properties in these materials. We use
spin density wave formalism for the Hubbard model, and study diagrammatically
the profile of the two-magnon scattering and its intensity dependence on the
incoming photon frequency $\omega_i$ both for $\omega_i \ll U$ and in the
resonant regime, in which the energy of the incident photon is close to the gap
between conduction and valence bands. In the nonresonant case, we identify the
diagrams which contribute to the conventional Loudon-Fleury Hamiltonian. In the
resonant regime, where most of the experiments have been done, we find that the
dominant contribution to Raman intensity comes from a different diagram, one
which allows for a simultaneous vanishing of all three of its denominators
(i.e., a triple resonance). We study this diagram in detail and show that the
triple resonance, combined with the spin-density-wave dispersion relation for
the carriers, explains the unusual features found in the two-magnon profile and
in the two-magnon peak intensity dependence on the incoming photon frequency.
In particular, our theory predicts a maximum of the two-magnon peak intensity
right at the upper edge of the features in the optical data, which has been one
of the key experimental puzzles.",9503010v1
2007-12-26,Reflecting Magnon Bound States,"In N=4 super Yang-Mills spin chain, we compute reflection amplitudes of
magnon bound-state off giant graviton. We first compute the reflection
amplitude off Y=0 brane boundary and compare it with the scattering amplitude
between two magnon bound-states in the bulk. We find that analytic structure of
the two amplitudes are intimately related each other: the boundary reflection
amplitude is a square-root of the bulk scattering amplitude. Using such
relation as a guide and taking known results at weak and strong coupling limits
as inputs, we find the reflection amplitude of an elementary magnon off Z=0
giant graviton boundary. The reflection phase factor is shown to solve crossing
and unitarity relations. We then compute the reflection amplitude of magnon
bound-state off the Z=0 brane boundary and observe that its analytic structures
are again intimately related to the bulk scattering and the Y=0 boundary
reflection amplitudes. We also take dyonic giant magnon limit of these
reflection amplitudes and confirm that their phase-shifts agree completely with
string worldsheet computations based on complex sine-Gordon soliton scattering.",0712.4144v3
2012-10-15,"Exciton-magnon transitions in the frustrated chromium antiferromagnets CuCrO2, alpha-CaCr2O4, CdCr2O4, and ZnCr2O4","We report on optical transmission spectroscopy of the Cr-based frustrated
triangular antiferromagnets CuCrO2 and alpha-CaCr2O4, and the spinels CdCr2O4
and ZnCr2O4 in the near-infrared to visible-light frequency range. We explore
the possibility to search for spin correlations far above the magnetic ordering
temperature and for anomalies in the magnon lifetime in the magnetically
ordered state by probing exciton-magnon sidebands of the spin-forbidden
crystal-field transitions of the Cr3+ ions (spin S = 3/2). In CuCrO2 and
alpha-CaCr2O4 the appearance of fine structures below T_N is assigned to magnon
sidebands by comparison with neutron scattering results. The temperature
dependence of the line width of the most intense sidebands in both compounds
can be described by an Arrhenius law. For CuCrO2 the sideband associated with
the 4A2 -> 2T2 transition can be observed even above T_N. Its line width does
not show a kink at the magnetic ordering temperature and can alternatively be
described by a Z2 vortex scenario proposed previously for similar materials.
The exciton-magnon features in alpha-CaCr2O4 are more complex due to the
orthorhombic distortion. While for CdCr2O4 magnon sidebands are identified
below T_N and one sideband excitation is found to persist across the magnetic
ordering transition, only a weak fine structure related to magnetic ordering
has been observed in ZnCr2O4.",1210.3938v2
2014-08-13,High Cooperativity Cavity QED with Magnons at Microwave Frequencies,"Using a sub-millimetre sized YIG (Yttrium Iron Garnet) sphere mounted in a
magnetic field-focusing cavity, we demonstrate an ultra-high cooperativity of
$10^5$ between magnon and photon modes at millikelvin temperatures and
microwave frequencies. The cavity is designed to act as a magnetic dipole by
using a novel multiple-post approach, effectively focusing the cavity magnetic
field within the YIG crystal with a filling factor of 3%. Coupling strength
(normal-mode splitting) of 2 GHz, (equivalent to 76 cavity linewidths or $0.3$
Hz per spin), is achieved for a bright cavity mode that constitutes about 10%
of the photon energy and shows that ultra-strong coupling is possible in spin
systems at microwave frequencies. With straight forward optimisations we
demonstrate that with that this system has the potential to reach
cooperativities of $10^7$, corresponding to a normal mode splitting of 5.2 GHz
and a coupling per spin approaching 1 Hz. We also observe a three-mode strong
coupling regime between a dark cavity mode and a magnon mode doublet pair,
where the photon-magnon and magnon-magnon couplings (normal-mode splittings)
are 143 MHz and 12.5 MHz respectively, with HWHM bandwidth of about 0.5 MHz.",1408.2905v3
2014-10-14,Low frequency noise peak near magnon emission energy in magnetic tunnel junctions,"We report on the low frequency (LF) noise measurements in magnetic tunnel
junctions (MTJs) below 4 K and at low bias, where the transport is strongly
affected by scattering with magnons emitted by hot tunnelling electrons, as
thermal activation of magnons from the environment is suppressed. For both
CoFeB/MgO/CoFeB and CoFeB/AlO$_{x}$/CoFeB MTJs, enhanced LF noise is observed
at bias voltage around magnon emission energy, forming a peak in the bias
dependence of noise power spectra density, independent of magnetic
configurations. The noise peak is much higher and broader for unannealed
AlO$_{x}$-based MTJ, and besides Lorentzian shape noise spectra in the
frequency domain, random telegraph noise (RTN) is visible in the time traces.
During repeated measurements the noise peak reduces and the RTN becomes
difficult to resolve, suggesting defects being annealed. The Lorentzian shape
noise spectra can be fitted with bias-dependent activation of RTN, with the
attempt frequency in the MHz range, consistent with magnon dynamics. These
findings suggest magnon-assisted activation of defects as the origin of the
enhanced LF noise.",1410.3586v2
2015-02-06,Higgs Mode and Magnon Interactions in 2D Quantum Antiferromagnets from Raman Scattering,"We present a theory for Raman scattering on 2D quantum antiferromagnets. The
microscopic Fleury-Loudon Hamiltonian is expressed in terms of an effective
$O(3)$ - model. Well within the N\'eel ordered phase, the Raman spectrum
contains a two-magnon and a two-Higgs contribution, which are calculated
diagramatically. The vertex functions for both the Higgs and magnon
contributions are determined from a numerical solution of the corresponding
Bethe-Salpeter equation. Due to the momentum dependence of the Raman vertex in
the relevant $B_{1g}+E_{2g}$ symmetry, the contribution from the Higgs mode is
strongly suppressed. Except for intermediate values of the Higgs mass, it does
not show up as separate peak in the spectrum but gives rise to a broad
continuum above the dominant contribution from two-magnon excitations. The
latter give rise to a broad, asymmetric peak at $\omega\simeq 2.44\, J$, which
is a result of magnon-magnon interactions mediated by the Higgs mode. The full
Raman spectrum is determined completely by the antiferromagnetic exchange
coupling $J$ and a dimensionless Higgs mass. Experimental Raman spectra of
undoped cuprates turn out to be in very good agreement with the theory only
with inclusion of the Higgs contribution. They thus provide a clear signature
of the presence of a Higgs mode in spin one-half 2D quantum antiferromagnets.",1502.01857v4
2015-03-25,Rigorous numerical study of strong microwave photon-magnon coupling in all-dielectric magnetic multilayers,"We demonstrate theoretically a strong local enhancement of the intensity of
the in-plane microwave magnetic field in multilayered structures made from a
magneto-insulating yttrium iron garnet (YIG) layer sandwiched between two
non-magnetic layers with a high dielectric constant matching that of YIG. The
enhancement is predicted for the excitation regime when the microwave magnetic
field is induced inside the multilayer by the transducer of a stripline
Broadband Ferromagnetic Resonance (BFMR) setup. By means of a rigorous
numerical solution of the Landau-Lifshitz-Gilbert equation consistently with
the Maxwell's equations, we investigate the magnetisation dynamics in the
multilayer. We reveal a strong photon-magnon coupling, which manifests itself
as anti-crossing of the ferromagnetic resonance (FMR) magnon mode supported by
the YIG layer and the electromagnetic resonance mode supported by the whole
multilayered structure. The frequency of the magnon mode depends on the
external static magnetic field, which in our case is applied tangentially to
the multilayer in the direction perpendicular to the microwave magnetic field
induced by the stripline of the BFMR setup. The frequency of the
electromagnetic mode is independent of the static magnetic field. Consequently,
the predicted photon-magnon coupling is sensitive to the applied magnetic field
and thus can be used in magnetically tuneable metamaterials based on
simultaneously negative permittivity and permeability achievable thanks to the
YIG layer. We also suggest that the predicted photon-magnon coupling may find
applications in microwave quantum information systems.",1503.07282v1
2016-04-18,Magnon Hall effect in AB-stacked bilayer honeycomb quantum magnets,"Motivated by the fact that many bilayer quantum magnets occur in nature, we
generalize the study of thermal Hall transports of spin excitations to bilayer
magnetic systems. It is shown that bilayer magnetic systems can be coupled
either ferromagnetically or antiferromagnetically. We study both scenarios on
the honeycomb lattice and show that the system realizes topologically
nontrivial magnon bands induced by alternating next-nearest-neighbour
Dzyaloshinsky-Moriya interaction (DMI). As a result, the bilayer system
realizes both magnon Hall effect and magnon spin Nernst effect. We show that
antiferromagnetically coupled layers differ from ferromagnetically coupled
layers by a sign change in the conductivities as the magnetic field is
reversed. Furthermore, Chern number protected magnon edge states are observed
and propagate in the same direction on the top and bottom layers in
ferromagnetically coupled layers, whereas the magnon edge states propagate in
opposite directions for antiferromagnetically coupled layers.",1604.05292v9
2017-08-14,Weyl magnons in noncoplanar stacked kagome antiferromagnets,"Weyl nodes have been experimentally realized in photonic, electronic, and
phononic crystals. However, magnonic Weyl nodes are yet to be seen
experimentally. In this paper, we propose Weyl magnon nodes in noncoplanar
stacked frustrated kagome antiferromagnets, naturally available in various real
materials. Most crucially, the Weyl nodes in the current system occur at the
lowest excitation and possess a topological (anomalous) thermal Hall effect,
therefore they are experimentally accessible at low temperatures due to the
population effect of bosonic quasiparticles. In stark contrast to other
magnetic systems, the current Weyl nodes do not rely on time-reversal symmetry
breaking by the magnetic order. Rather, they result from explicit
macroscopically broken time reversal symmetry by the scalar spin chirality of
noncoplanar spin textures, and can be generalized to chiral spin liquid states.
Moreover, the scalar spin chirality gives a real space Berry curvature which is
not available in previously studied magnetic Weyl systems. We show the
existence of magnon arc surface states connecting projected Weyl magnon nodes
on the surface Brillouin zone. We also uncover the first realization of
triply-degenerate nodal magnon point in the non-collinear regime with zero
scalar spin chirality.",1708.04240v6
2018-04-19,Effect of magnons on interfacial phonon drag in YIG/metal systems,"We examine substrate-to-film interfacial phonon drag on typical spin Seebeck
heterostructures, in particular studying the effect of ferromagnetic magnons on
the phonon-electron drag dynamics at the interface. We investigate with high
precision the effect of magnons in the Pt|YIG heterostructure by designing a
magnon drag thermocouple; a hybrid sample with both a Pt|YIG film and Pt|GGG
interface accessible isothermally via a 6 nm Pt film patterned in a rectangular
U shape with one arm on the 250 nm YIG film and the other on GGG. We measure
the voltage between the isothermal ends of the U, while applying a temperature
gradient parallel to the arms and perpendicular to the bottom connection. With
a uniform applied temperature gradient, the Pt acts as a differential
thermocouple. We conduct temperature-dependent longitudinal thermopower
measurements on this sample. Results show that the YIG interface actually
decreases the thermopower of the film, implying that magnons impede phonon
drag. We repeat the experiment using metals with low spin Hall angles, Ag and
Al, in place of Pt. We find that the phonon drag peak in thermopower is killed
in samples where the metallic interface is with YIG. We also investigate
magneto-thermopower and YIG film thickness dependence. These measurements
confirm our findings that magnons impede the phonon-electron drag interaction
at the metallic interface in these heterostructures.",1804.07023v1
2017-12-18,Magnon-induced non-Markovian friction of a domain wall in a ferromagnet,"Motivated by the recent study on the quasiparticle-induced friction of
solitons in superfluids, we theoretically study magnon-induced intrinsic
friction of a domain wall in a one-dimensional ferromagnet. To this end, we
start by obtaining the hitherto overlooked dissipative interaction of a domain
wall and its quantum magnon bath to linear order in the domain-wall velocity
and to quadratic order in magnon fields. An exact expression for the pertinent
scattering matrix is obtained with the aid of supersymmetric quantum mechanics.
We then derive the magnon-induced frictional force on a domain wall in two
different frameworks: time-dependent perturbation theory in quantum mechanics
and the Keldysh formalism, which yield identical results. The latter, in
particular, allows us to verify the fluctuation-dissipation theorem explicitly
by providing both the frictional force and the correlator of the associated
stochastic Langevin force. The potential for magnons induced by a domain wall
is reflectionless, and thus the resultant frictional force is non-Markovian
similarly to the case of solitons in superfluids. They share an intriguing
connection to the Abraham-Lorentz force that is well-known for its causality
paradox. The dynamical responses of a domain wall are studied under a few
simple circumstances, where the non-Markovian nature of the frictional force
can be probed experimentally. Our work, in conjunction with the previous study
on solitons in superfluids, shows that the macroscopic frictional force on
solitons can serve as an effective probe of the microscopic degrees of freedom
of the system.",1712.06578v1
2018-09-19,"From ""Weak"" to ""Strong"" Hole Confinement in a Mott Insulator","We study the problem of a single hole in an Ising antiferromagnet and, using
the magnon expansion and analytical methods, determine the expansion
coefficients of its wave function in the magnon basis. In the 1D case, the hole
is ""weakly"" confined in a potential well and the magnon coefficients decay
exponentially in the absence of a string potential. This behavior is in sharp
contrast to the 2D square plane where the hole is ""strongly"" confined by a
string potential and the magnon coefficients decay superexponentially. The
latter is identified here to be a fingerprint of the strings in doped
antiferromagnets that can be recognized in the numerical or cold atom
simulations of the 2D doped Hubbard model. Finally, we attribute the
differences between the 1D and 2D cases to the magnon-magnon interactions being
crucially important in a 1D spin system.",1809.07120v6
2019-11-28,Magnon damping in the zigzag phase of the Kitaev-Heisenberg-$Γ$ model on a honeycomb lattice,"We calculate magnon dispersions and damping in the Kitaev-Heisenberg model
with an off-diagonal exchange $\Gamma$ and isotropic third-nearest-neighbor
interaction $J_3$ on a honeycomb lattice. This model is relevant to a
description of the magnetic properties of iridium oxides $\alpha$-Li$_2$IrO$_3$
and Na$_2$IrO$_3$, and Ru-based materials such as $\alpha$-RuCl$_3$. We use an
unconventional parametrization of the spin-wave expansion, in which each
Holstein-Primakoff boson is represented by two conjugate hermitian operators.
This approach gives us an advantage over the conventional one in identifying
parameter regimes where calculations can be performed analytically. Focusing on
the parameter regime with the zigzag spin pattern in the ground state that is
consistent with experiments, we demonstrate that one such region is $\Gamma =
K>0$, where $K$ is the Kitaev coupling. Within our approach we are able to
obtain explicit analytical expressions for magnon energies and eigenstates and
go beyond the standard linear spin-wave theory approximation by calculating
magnon damping and demonstrating its role in the dynamical structure factor. We
show that the magnon damping effects in both Born and self-consistent
approximations are very significant, underscoring the importance of non-linear
magnon coupling in interpreting broad features in the neutron-scattering
spectra.",1911.12829v2
2019-12-22,First-principles study of magnon-phonon interactions in gadolinium iron garnet,"We obtained the spin-wave spectrum based on a first-principles method of
exchange constants, calculated the phonon spectrum by the first-principles
phonon calculation method, and extracted the broadening of the magnon spectrum,
$\Delta \omega$, induced by magnon-phonon interactions in gadolinium iron
garnet (GdIG). Using the obtained exchange constants, we reproduce the
experimental Curie temperature and the compensation temperature from spin
models using Metropolis Monte Carlo (MC) simulations. In the lower-frequency
regime, the fitted positions of the magnon-phonon dispersion crossing points
are consistent with the inelastic neutron scattering experiment. We found that
the $\Delta \omega$ and magnon wave vector $k$ have a similar relationship in
YIG. The broadening of the acoustic spin-wave branch is proportional to
$k^{2}$, while that of the YIG-like acoustic branch and the optical branch are
a constant. At a specific $k$, the magnon-phonon thermalization time of
$\tau_{mp}$ are approximately $10^{-9}$~s, $10^{-13}$~s, and $10^{-14}$~s for
acoustic branch, YIG-like acoustic branch, and optical branch, respectively.
This research provides specific and effective information for developing a
clear understanding of the spin-wave mediated spin Seebeck effect and
complements the lack of lattice dynamics calculations of GdIG.",1912.10432v1
2019-05-12,Elementary excitations in the ordered phase of spin-1/2 J1-J2 model on square lattice,"We use recently proposed four-spin bond-operator technique (BOT) to discuss
spectral properties of frustrated spin-$\frac12$ $J_1$--$J_2$ Heisenberg
antiferromagnet on square lattice at $J_2<0.4J_1$ (i.e., in the N\'eel ordered
phase). This formalism is convenient for the consideration of low-lying
excitations which appear in conventional approaches as multi-magnon bound
states (e.g., the Higgs excitation) because separate bosons describe them in
BOT. At $J_2=0$, the obtained magnon spectrum describes accurately available
experimental data. However, calculated one-magnon spectral weights and the
transverse dynamical structure factor (DSF) do not reproduce experimental
findings quantitatively around the momentum ${\bf k}=(\pi,0)$. Then, we do not
support the conjecture that the continuum of excitations observed
experimentally and numerically near ${\bf k}=(\pi,0)$ is of the Higgs-magnon
origin. Upon $J_2$ increasing, one-magnon spectral weights decrease and spectra
of high-energy spin-0 and spin-1 excitations move down. One of spin-0
quasiparticles becomes long-lived and its spectrum merges with the magnon
spectrum in the most part of the Brillouin zone at $J_2\approx0.3J_1$. We
predict that the Higgs excitation and another spin-0 quasiparticle become
long-lived around ${\bf k}=(\pi/2,\pi/2)$ at $J_2\agt0.3J_1$ and produce sharp
anomalies in the longitudinal DSF.",1905.04710v2
2020-09-10,Spin waves in alloys at finite temperatures: application for FeCo magnonic crystal,"We study theoretically the influence of the temperature and disorder on the
spin wave spectrum of the magnonic crystal Fe$_{1-c}$Co$_{c}$. Our formalism is
based on the analysis of a Heisenberg Hamiltonian by means of the wave vector
and frequency dependent transverse magnetic susceptibility. The exchange
integrals entering the model are obtained from the \emph{ab initio} magnetic
force theorem. The coherent potential approximation is employed to treat the
disorder and random phase approximation in order to account for the softening
of the magnon spectrum at finite temperatures. The alloy turns out to exhibit
many advantageous properties for spintronic applications. Apart from high Curie
temperature, its magnonic bandgap remains stable at elevated temperatures and
is largely unaffected by the disorder. We pay particular attention to the
attenuation of magnons introduced by the alloying. The damping turns out to be
a non-monotonic function of the impurity concentration due to the non-trivial
evolution of the value of exchange integrals with the Co concentration. The
disorder induced damping of magnons is estimated to be much smaller than their
Landau damping.",2009.04712v6
2021-02-26,Control of the Bose-Einstein Condensation of Magnons by the Spin-Hall Effect,"Previously, it has been shown that rapid cooling of yttrium-iron-garnet
(YIG)/platinum (Pt) nano structures, preheated by an electric current sent
through the Pt layer, leads to overpopulation of a magnon gas and to subsequent
formation of a Bose-Einstein condensate (BEC) of magnons. The spin Hall effect
(SHE), which creates a spin-polarized current in the Pt layer, can inject or
annihilate magnons depending on the electric current and applied field
orientations. Here we demonstrate that the injection or annihilation of magnons
via the SHE can prevent or promote the formation of a rapid cooling induced
magnon BEC. Depending on the current polarity, a change in the BEC threshold of
-8% and +6% was detected. These findings demonstrate a new method to control
macroscopic quantum states, paving the way for their application in spintronic
devices.",2102.13481v2
2015-07-14,Wiedemann-Franz Law for Magnon Transport,"One of the main goals of spintronics is to improve transport of information
carriers and to achieve new functionalities with ultra-low dissipation. A most
promising strategy for this holy grail is to use pure magnon currents created
and transported in insulating magnets, in the complete absence of any
conducting metallic elements. Here we propose a realistic solution to this
fundamental challenge by analyzing magnon and heat transport in insulating
ferromagnetic junctions. We calculate all transport coefficients for magnon
transport and establish Onsager relations between them. We theoretically
discover that magnon transport in junctions has a universal behavior, i.e. is
independent of material parameters, and establish a magnon analog of the
celebrated Wiedemann-Franz law which governs charge transport at low
temperatures. We calculate the Seebeck and Peltier coefficients which are
crucial quantities for spin caloritronics and demonstrate that they assume
universal values in the low temperature limit. Finally, we show that our
predictions are within experimental reach with current device and measurement
technologies.",1507.03807v5
2018-08-22,Role of magnons and the size effect in heat transport through an insulating ferromagnet/insulator interface,"While recent experiments on the spin Seebeck effect have revealed the
decisive role of the magnon contribution to the heat current $Q$ in hybrid
systems containing thin ferromagnetic layers, the available acoustic mismatch
theory does not account for their magnetic properties. Here, we analyze
theoretically the heat transfer through an insulating ferromagnet (F)
sandwiched between two insulators (I). Depending on the relation between the F
thickness, $d$, and the mean free path of phonons generated by magnons,
$l_{ls}$, we reveal two qualitatively different regimes in the nonlinear heat
transport through the F/I interfaces. Namely, in thick F layers the regime of
conventional ""Joule"" heating with $Q \propto T_s^4$ is realized, in which the
detailed structure of the F/I interfaces is inessential. Here $T_s$ is the
magnon temperature. By contrast, in thin F layers with $d\ll l_{ls}$, most of
phonons emitted by magnons can leave F without being absorbed in its interior,
giving rise to the \emph{magnon overheating} regime with $Q \propto T_s^m$ and
$m\gtrsim7$. Conditions for the examination of both regimes and the
determination of $T_s$ from experiments are discussed. The reported results are
relevant for the theoretical analysis of the spin Seebeck effect and the
development of magnon-based spin caloritronic devices.",1808.07294v1
2019-07-17,Neutron Scattering Study of Magnetic Anisotropy in a Tetragonal Antiferromagnet Bi$_2$CuO$_4$,"We present a comprehensive study of magnon excitations in the tetragonal
easy-plane anti-ferromagnet Bi$_2$CuO$_4$ using inelastic neutron scattering
and spin wave analyses. The nature of low energy magnons, and hence the
anisotropy in this material, has been controversial. We show unambiguously that
the low energy magnon spectrum consists of a gapped and a gapless mode, which
we attribute to out-of-plane and in-plane spin fluctuations, respectively. We
modelled the observed magnon spectrum using linear spin wave analysis of a
minimal anisotropic spin model motivated by the lattice symmetry. By studying
the magnetic field dependence of the (1, 0, 0) Bragg peak intensity and the
in-plane magnon intensity, we observed a spin-flop transition in the $ab$ plane
at $\sim0.4$~T which directly indicates the existence of a small in-plane
anisotropy that is classically forbidden. It is only by taking into account
magnon zero-point fluctuations beyond the linear spin wave approximation, we
could explain this in-plane anisotropy and its magnitude, the latter of which
is deduced from critical field of the spin-flop transition. The microscopic
origins of the observed anisotropic interactions are also discussed. We found
that our data is inconsistent with a large Dzyaloshinskii-Moriya interaction,
which suggests a potential departure of Bi$_2$CuO$_4$ from the conventional
theories of magnetic anisotropy for other cuprates.",1907.07784v2
2020-03-09,Predicted strong coupling of solid-state spins via a single magnon mode,"We propose an approach to realize a hybrid quantum system composed of a
diamond nitrogen-vacancy (NV) center spin coupled to a magnon mode of the
low-damping, low-moment organic ferrimagnet vanadium tetracyanoethylene. We
derive an analytical expression for the spin-magnon cooperativity as a function
of NV position under a micron-scale perpendicularly magnetized disk, and show
that, surprisingly, the cooperativity will be higher using this magnetic
material than in more conventional materials with larger magnetic moments, due
to in part to the reduced demagnetization field. For reasonable experimental
parameters, we predict that the spin-magnon-mode coupling strength is $g\sim
10$ kHz. For isotopically pure $^{12}$C diamond we predict strong coupling of
an NV spin to the unoccupied magnon mode, with cooperativity $\mathcal C=6$ for
a wide range of NV spin locations within the diamond, well within the spatial
precision of NV center implantation. Thus our proposal describes a practical
pathway for single-spin-state-to-single-magnon-occupancy transduction and for
entangling NV centers over micron length scales.",2003.04341v2
2020-04-20,Giant transition-state enhancement of quasiparticle spin-Hall effect in an exchange-spin-split superconductor detected by non-local magnon spin-transport,"Although recent experiments and theories have shown a variety of exotic
transport properties of non-equilibrium quasiparticles (QPs) in superconductor
(SC)-based devices with either Zeeman or exchange spin-splitting, how QP
interplays with magnon spin currents remains elusive. Here, using non-local
magnon spin-transport devices where a singlet SC (Nb) on top of a ferrimagnetic
insulator (Y3Fe5O12) serves as a magnon spin detector, we demonstrate that the
conversion efficiency of magnon spin to QP charge via inverse spin-Hall effect
(iSHE) in such an exchange-spin-split SC can be greatly enhanced by up to 3
orders of magnitude compared with that in the normal state, particularly when
its interface superconducting gap matches the magnon spin accumulation. Through
systematic measurements with varying the current density and SC thickness, we
identify that superconducting coherence peaks and exchange spin-splitting of
the QP density-of-states, yielding a larger spin excitation while retaining a
modest QP charge-imbalance relaxation, are responsible for the giant QP iSHE.
The latter exchange-field-modified QP relaxation is experimentally proved by
spatially resolved measurements with varying the separation of electrical
contacts on the spin-split Nb.",2004.09467v2
2020-07-03,Light-induced topological magnons in two-dimensional van der Waals magnets,"Driving a two-dimensional Mott insulator with circularly polarized light
breaks time-reversal and inversion symmetry, which induces an optically-tunable
synthetic scalar spin chirality interaction in the effective low-energy spin
Hamiltonian. Here, we show that this mechanism can stabilize topological magnon
excitations in honeycomb ferromagnets and in optical lattices. We find that the
irradiated quantum magnet is described by a Haldane model for magnons that
hosts topologically-protected edge modes. We study the evolution of the magnon
spectrum in the Floquet regime and via time propagation of the magnon
Hamiltonian for a slowly varying pulse envelope. Compared to similar but
conceptually distinct driving schemes based on the Aharanov-Casher effect, the
dimensionless light-matter coupling parameter $\lambda = eEa/\hbar\omega$ at
fixed electric field strength is enhanced by a factor $\sim 10^5$. This
increase of the coupling parameter allows to induce a topological gap of the
order of $\Delta \approx 2$ meV with realistic laser pulses, bringing an
experimental realization of light-induced topological magnon edge states within
reach.",2007.01714v4
2020-08-24,Loop-gas description of the localized-magnon states on the kagome lattice with open boundary conditions,"The high-field regime of the spin-s XXZ antiferromagnet on the kagome lattice
gives rise to macroscopically degenerate ground states thanks to a completely
flat lowest single-magnon band. The corresponding excitations can be localized
on loops in real space and have been coined ""localized magnons"". Thus, the
description of the many-body ground states amounts to characterizing the
allowed classical loop configurations and eliminating the quantum mechanical
linear relations between them. Here, we investigate this loop-gas description
on finite kagome lattices with open boundary conditions and compare the results
with exact diagonalization for the spin-1/2 XY model on the same lattice. We
find that the loop gas provides an exact account of the degenerate ground-state
manifold while a hard-hexagon description misses contributions from nested loop
configurations. The densest packing of the loops corresponds to a magnon
crystal that according to the zero-temperature magnetization curve is a stable
ground state of the spin-1/2 XY model in a window of magnetic fields of about
4% of the saturation field just below this saturation field. We also present
numerical results for the specific heat obtained by the related methods of
thermal pure quantum (TPQ) states and the finite-temperature Lanczos method
(FTLM). For a field in the stability range of the magnon crystal, one finds a
low-temperature maximum of the specific heat that corresponds to a
finite-temperature phase transition into the magnon crystal at low
temperatures.",2008.10614v2
2020-11-19,Topological insulators and semimetals in classical magnetic systems,"Pursuing topological phases in natural and artificial materials is one of the
central topics in modern physical science and engineering. In classical
magnetic systems, spin waves (or magnons) and magnetic solitons (such as domain
wall, vortex, skyrmion, etc) represent two important excitations. Recently, the
topological insulator and semimetal states in magnon- and soliton-based
crystals (or metamaterials) have attracted growing attention owing to their
interesting dynamics and promising applications for designing robust spintronic
devices. Here, we give an overview of current progress of topological phases in
structured classical magnetism. We first provide a brief introduction to spin
wave, and discuss its topological properties including magnon Hall effects,
topological magnon insulators, and Dirac (Weyl) magnon semimetals. Appealing
proposal of topological magnonic devices is also highlighted. We then review
the collective-coordinate approach for describing the dynamics of magnetic
soliton lattice. Pedagogical topological models such as the
Su-Schrieffer-Heeger model and the Haldane model and their manifestation in
magnetic soliton crystals are elaborated. Then we focus on the topological
properties of magnetic solitons, by theoretically analyzing the first-order
topological insulating phases in low dimensional systems and higher-order
topological states in breathing crystals. Finally, we discuss the experimental
realization and detection of the edge states in both the magnonic and solitonic
crystals. We remark the challenges and future prospects before concluding this
article.",2011.09751v1
2021-01-21,Magnon-mediated interlayer coupling in an all-antiferromagnetic junction,"The interlayer coupling mediated by fermions in ferromagnets brings about
parallel and anti-parallel magnetization orientations of two magnetic layers,
resulting in the giant magnetoresistance, which forms the foundation in
spintronics and accelerates the development of information technology. However,
the interlayer coupling mediated by another kind of quasi-particle, boson, is
still lacking. Here we demonstrate such a static interlayer coupling at room
temperature in an antiferromagnetic junction Fe2O3/Cr2O3/Fe2O3, where the two
antiferromagnetic Fe2O3 layers are functional materials and the
antiferromagnetic Cr2O3 layer serves as a spacer. The N\'eel vectors in the top
and bottom Fe2O3 are strongly orthogonally coupled, which is bridged by a
typical bosonic excitation (magnon) in the Cr2O3 spacer. Such an orthogonally
coupling exceeds the category of traditional collinear interlayer coupling via
fermions in ground state, reflecting the fluctuating nature of the magnons, as
supported by our magnon quantum well model. Besides the fundamental
significance on the quasi-particle-mediated interaction, the strong coupling in
an antiferromagnetic magnon junction makes it a realistic candidate for
practical antiferromagnetic spintronics and magnonics with ultrahigh-density
integration.",2101.08665v1
2021-07-06,Topological magnon insulator spin excitations in the two-dimensional ferromagnet CrBr$_3$,"Topological magnons are bosonic analogues of topological fermions in
electronic systems. They have been studied extensively by theory but rarely
realized by experiment. Here, by performing inelastic neutron scattering
measurements on single crystals of a two-dimensional ferromagnet CrBr$_3$,
which was classified as Dirac magnon semimetal featured by the linear bands
crossing at the Dirac points, we fully map out the magnetic excitation spectra,
and reveal that there is an apparent gap of $\sim$3.5~meV between the acoustic
and optical branches of the magnons at the K point. By collaborative efforts
between experiment and theoretical calculations using a five-orbital Hubbard
model obtained from first-principles calculations to derive the exchange
parameters, we find that a Hamiltonian with Heisenberg exchange interactions,
next-nearest-neighbor Dzyaloshinskii-Moriya (DM) interaction, and single-ion
anisotropy is more appropriate to describe the system. Calculations using the
model show that the lower and upper magnon bands separated by the gap exhibit
Chern numbers of $\pm1$. These results indicate that CrBr$_3$ is a topological
magnon insulator, where the nontrivial gap is a result of the DM interaction.",2107.02486v1
2021-08-01,Directional excitation of a high-density magnon gas using coherently driven spin waves,"Controlling magnon densities in magnetic materials enables driving spin
transport in magnonic devices. We demonstrate the creation of large,
out-of-equilibrium magnon densities in a thin-film magnetic insulator via
microwave excitation of coherent spin waves and subsequent multi-magnon
scattering. We image both the coherent spin waves and the resulting incoherent
magnon gas using scanning-probe magnetometry based on electron spins in
diamond. We find that the gas extends unidirectionally over hundreds of
micrometers from the excitation stripline. Surprisingly, the gas density far
exceeds that expected for a boson system following a Bose-Einstein distribution
with a maximum value of the chemical potential. We characterize the momentum
distribution of the gas by measuring the nanoscale spatial decay of the
magnetic stray fields. Our results show that driving coherent spin waves leads
to a strong out-of-equilibrium occupation of the spin-wave band, opening new
possibilities for controlling spin transport and magnetic dynamics in target
directions.",2108.00467v1
2021-12-09,From the spin eigenmodes of isolated Néel skyrmions to the magnonic bands of a skyrmionic crystal: a micromagnetic study as a function of the strength of both the interfacial Dzyaloshinskii-Moriya and the exchange constants,"The presence of interfacial Dzyaloshinskii-Moriya interaction (DMI) may lead
to the appearance of N\'eel skyrmions in ferromagnetic films. These
topologically protected structures, whose diameter is as small as a few
nanometers, can be nowadays stabilized at room temperature and have been
proposed for the realization of artificial magnonic crystals and new spintronic
devices, such as racetrack memories. In this perspective, it is of utmost
importance to analyze their dynamical properties in the GHz range, i.e. in the
operation range of current communication devices. Here we exploited the
software MuMax3 to calculate the dynamics of N\'eel skyrmions in the range
between 1 and 30 GHz, considering first the eigenmodes of an isolated skyrmion,
then the case of two interacting skyrmions and finally a linear chain,
representing a one-dimensional magnonic crystal, whose magnonic band structure
has been calculated as a function of the strength of both the DMI- and the
exchange-constants, namely D and A. The magnonic bands can be interpreted as
derived from the eigenmodes of isolated skyrmions, even if hybridization and
anti-crossing phenomena occur for specific ranges of values of D and A.
Therefore, varying the latter parameters, for instance by a proper choice of
the materials and thicknesses, may enable one to fine-tune the permitted and
forbidden frequency interval of the corresponding magnonic crystal.",2112.04967v2
2022-03-21,Magnon squeezing enhanced ground-state cooling in cavity magnomechanics,"Cavity magnomechanics has recently become a new platform for studying
macroscopic quantum phenomena. The magnetostriction induced vibration mode of a
large-size ferromagnet or ferrimagnet reaching its ground state represents a
genuine macroscopic quantum state. Here we study the ground-state cooling of
the mechanical vibration mode in a cavity magnomechanical system, and focus on
the role of magnon squeezing in improving the cooling efficiency. The magnon
squeezing is obtained by exploiting the magnon self-Kerr nonlinearity. We find
that the magnon squeezing can significantly and even completely suppress the
magnomechanical Stokes scattering. It thus becomes particularly useful in
realizing ground-state cooling in the unresolved-sideband regime, where the
conventional sideband cooling protocols become inefficient. We also find that
the coupling to the microwave cavity plays only an adverse effect in mechanical
cooling. This makes essentially the two-mode magnomechanical system (without
involving the microwave cavity) a preferred system for cooling the mechanical
motion, in which the magnon mode is established by a uniform bias magnetic
field and a microwave drive field.",2203.10767v4
2022-03-30,Tunable magnetically induced transparency spectra in magnon-magnon coupled Y3Fe5O12/permalloy bilayers,"Hybrid magnonic systems host a variety of characteristic quantum phenomena
such as the magnetically-induced transparency (MIT) and Purcell effect, which
are considered useful for future coherent quantum information processing. In
this work, we experimentally demonstrate a tunable MIT effect in the
Y3Fe5O12(YIG)/Permalloy(Py) magnon-magnon coupled system via changing the
magnetic field orientations. By probing the magneto-optic effects of Py and
YIG, we identify clear features of MIT spectra induced by the mode
hybridization between the uniform mode of Py and the perpendicular standing
spin-wave modes of YIG. By changing the external magnetic field orientations,
we observe a tunable coupling strength between the YIG's spin-wave modes and
the Py's uniform mode, upon the application of an out-of-plane magnetic field.
This observation is theoretically interpreted by a geometrical consideration of
the Py and YIG magnetization under the oblique magnetic field even at a
constant interfacial exchange coupling. Our findings show high promise for
investigating tunable coherent phenomena with hybrid magnonic platforms.",2203.16303v1
2022-04-29,Entangling mechanical vibrations of two massive ferrimagnets by fully exploiting the nonlinearity of magnetostriction,"Quantum entanglement in the motion of macroscopic objects is of significance
to both fundamental studies and quantum technologies. Here we show how to
entangle the mechanical vibration modes of two massive ferrimagnets that are
placed in the same microwave cavity. Each ferrimagnet supports a magnon mode
and a low-frequency vibration mode coupled by the magnetostrictive force. The
two magnon modes are, respectively, coupled to the microwave cavity by the
magnetic dipole interaction. We first generate a stationary nonlocal entangled
state between the vibration mode of the ferrimagnet-1 and the magnon mode of
the ferrimagnet-2. This is realized by continuously driving the ferrimagnet-1
with a strong red-detuned microwave field and the entanglement is achieved by
exploiting the magnomechanical parametric down-conversion and the cavity-magnon
state-swap interaction. We then switch off the pump on the ferrimagnet-1 and,
simultaneously, turn on a red-detuned pulsed drive on the ferrimagnet-2. The
latter drive is used to activate the magnomechanical beamsplitter interaction,
which swaps the magnonic and mechanical states of the ferrimagnet-2.
Consequently, the previously generated phonon-magnon entanglement is
transferred to the mechanical modes of two ferrimagnets. The work provides a
scheme to prepare entangled states of mechanical motion of two massive objects,
which may find applications in various studies exploiting macroscopic entangled
states.",2204.14010v4
2022-05-27,Magnonic Casimir Effect in Ferrimagnets,"Quantum fluctuations are the key concepts of quantum mechanics. Quantum
fluctuations of quantum fields induce a zero-point energy shift under spatial
boundary conditions. This quantum phenomenon, called the Casimir effect, has
been attracting much attention beyond the hierarchy of energy scales, ranging
from elementary particle physics to condensed matter physics together with
photonics. However, the application of the Casimir effect to spintronics has
not yet been investigated enough, particularly to ferrimagnetic thin films,
although yttrium iron garnet (YIG) is one of the best platforms for
spintronics. Here we fill this gap. Using the lattice field theory, we
investigate the Casimir effect induced by quantum fields for magnons in
insulating magnets and find that the magnonic Casimir effect can arise not only
in antiferromagnets but also in ferrimagnets including YIG thin films. Our
result suggests that YIG, the key ingredient of magnon-based spintronics, can
serve also as a promising platform for manipulating and utilizing Casimir
effects, called Casimir engineering. Microfabrication technology can control
the thickness of thin films and realize the manipulation of the magnonic
Casimir effect. Thus, we pave the way for magnonic Casimir engineering.",2205.13802v3
2022-07-14,Terahertz field-driven magnon upconversion in an antiferromagnet,"Tailored light excitation and nonlinear control of lattice vibrations have
emerged as powerful strategies to manipulate the properties of quantum
materials out of equilibrium. Generalizing the use of coherent phonon-phonon
interactions to nonlinear couplings among other types of collective modes would
open unprecedented opportunities in the design of novel dynamic functionalities
in solids. For example, the collective excitations of magnetic order -- magnons
-- can carry information with little energy dissipation, and their coherent and
nonlinear control would provide an attractive route to achieve
collective-mode-based information processing and storage in forthcoming
spintronics and magnonics. Here, we discover that intense terahertz (THz)
fields can initiate processes of magnon upconversion mediated by an
intermediate magnetic resonance. By using a suite of advanced spectroscopic
tools, including a newly demonstrated two-dimensional (2D) THz polarimetry
technique enabled by single-shot detection, we unveil the unidirectional nature
of coupling between distinct magnon modes of a canted antiferromagnet.
Calculations of spin dynamics further suggest that this coupling is a universal
feature of antiferromagnets with canted magnetic moments. These results
demonstrate a route to inducing desirable energy transfer pathways and
THz-induced coupling between coherent magnons in solids and pave the way for a
new era in the development of ultrafast control of magnetism.",2207.07103v2
2022-09-18,Theory of magnon-polaritons in quantum Ising materials,"We present a theory of magnon-polaritons in quantum Ising materials, and
develop a formalism describing the coupling between light and matter as an
Ising system is tuned through its quantum critical point. The theory is applied
to Ising materials having multilevel single-site Hamiltonians, in which
multiple magnon modes are present, such as the insulating Ising magnet
LiHoF$_4$ . We find that the magnon-photon coupling strengths may be tuned by
the applied transverse field, with the coupling between the soft mode present
in the quantum Ising material and a photonic resonator mode diverging at the
quantum critical point of the material. A fixed system of spins will not
exhibit the diamagnetic response expected when light is coupled to mobile spins
or atoms. Without the diamagnetic response, one expects a divergent
magnon-photon coupling strength to lead to a superradiant quantum phase
transition. However, this neglects the effects of damping and decoherence
present in any real system. We show that damping and decoherence may block the
superradiant quantum phase transition, and lead to weak coupling between the
soft magnon mode and the resonator mode. The results of the theory are applied
to experimental data on the model system LiHoF$_4$ in a microwave resonator.",2209.08674v1
2022-09-19,Observation of nonreciprocal magnon Hanle effect,"The precession of magnon pseudospin about the equilibrium pseudofield, the
latter capturing the nature of magnonic eigen-excitations in an
antiferromagnet, gives rise to the magnon Hanle effect. Its realization via
electrically injected and detected spin transport in an antiferromagnetic
insulator demonstrates its high potential for devices and as a convenient probe
for magnon eigenmodes and the underlying spin interactions in the
antiferromagnet. Here, we observe a nonreciprocity in the Hanle signal measured
in hematite using two spatially separated platinum electrodes as spin
injector/detector. Interchanging their roles was found to alter the detected
magnon spin signal. The recorded difference depends on the applied magnetic
field and reverses sign when the signal passes its nominal maximum at the
so-called compensation field. We explain these observations in terms of a spin
transport direction-dependent pseudofield. The latter leads to a
nonreciprocity, which is found to be controllable via the applied magnetic
field. The observed nonreciprocal response in the readily available hematite
films opens interesting opportunities for realizing exotic physics predicted so
far only for antiferromagnets with special crystal structures.",2209.09040v1
2022-09-29,Spin Dynamics in Patterned Magnetic Multilayers with Perpendicular Magnetic Anisotropy,"The magnetization dynamics in nanostructures has been extensively studied in
the last decades, and nanomagnetism has evolved significantly over that time,
discovering new effects, developing numerous applications, and identifying
promising new directions. This includes magnonics, an emerging research field
oriented on the study of spin-wave dynamics and their applications. In this
context, thin ferromagnetic films with perpendicular magnetic anisotropy (PMA)
offer interesting opportunities to study spin waves, in particular, due to
out-of-plane magnetization in remanence or at relatively weak external magnetic
fields. This is the only magnetization configuration offering isotropic
in-plane spin-wave propagation within the sample plane, the forward volume
magnetostatic spin-wave geometry. The isotropic dispersion relation is highly
important in designing signal-processing devices, offering superior prospects
for direct replicating various concepts from photonics into magnonics.
Analogous to photonic or phononic crystals, which are the building blocks of
optoelectronics and phononics, magnonic crystals are considered as key
components in magnonics applications. Arrays of nanodots and structured
ferromagnetic thin films with a periodic array of holes, popularly known as
antidot lattices based on PMA multilayers have been recently studied. Novel
magnonic properties related to propagating spin-wave modes, exploitation of the
band gaps, and confined modes, were demonstrated. Also, the existence of
nontrivial magnonic band topologies has been shown. Moreover, the combination
of PMA and Dzyaloshinskii-Moriya interaction leads to the formation of chiral
magnetization states, including N\'eel domain walls, skyrmions, and skyrmionium
states.",2209.14824v2
2022-11-30,Topological magnons driven by the Dzyaloshinskii-Moriya interaction in the centrosymmetric ferromagnet Mn$_5$Ge$_3$,"The phase of the quantum-mechanical wave function can encode a topological
structure with wide-ranging physical consequences, such as anomalous transport
effects and the existence of edge states robust against perturbations. While
this has been exhaustively demonstrated for electrons, properties associated
with the elementary quasiparticles in magnetic materials are still
underexplored. Here, we show theoretically and via inelastic neutron scattering
experiments that the bulk ferromagnet Mn$_5$Ge$_3$ hosts gapped topological
Dirac magnons. Although inversion symmetry prohibits a net
Dzyaloshinskii-Moriya interaction in the unit cell, it is locally allowed and
is responsible for the gap opening in the magnon spectrum. This gap is
predicted and experimentally verified to close by rotating the magnetization
away from the $c$-axis with an applied magnetic field. Hence, Mn$_5$Ge$_3$
realizes a gapped Dirac magnon material in three dimensions. Its tunability by
chemical doping or by thin film nanostructuring defines an exciting new
platform to explore and design topological magnons. More generally, our
experimental route to verify and control the topological character of the
magnons is applicable to bulk centrosymmetric hexagonal materials, which calls
for systematic investigation.",2211.16925v2
2023-03-07,Magnon currents excited by the spin Seebeck effect in ferromagnetic EuS thin films,"A magnetic insulator is an ideal platform to propagate spin information by
exploiting magnon currents. However, until now, most studies have focused on
Y$_3$Fe$_5$O$_{12}$ (YIG) and a few other ferri- and antiferromagnetic
insulators, but not on pure ferromagnets. In this study, we demonstrate for the
first time that magnon currents can propagate in ferromagnetic insulating thin
films of EuS. By performing both local and non-local transport measurements in
18-nm-thick films of EuS using Pt electrodes, we detect magnon currents arising
from thermal generation by the spin Seebeck effect. By comparing the dependence
of the local and non-local signals with the temperature (< 30 K) and magnetic
field (< 9 T), we confirm the magnon transport origin of the non-local signal.
Finally, we extract the magnon diffusion length in the EuS film (~140 nm), a
short value in good correspondence with the large Gilbert damping measured in
the same film.",2303.03833v2
2023-07-21,Observation of strong coupling between a mechanical oscillator and a cavity-magnon polariton,"Cavity magnomechanics (CMM) is an emerging field and has received much
attention in the past decade. It deals with coherent couplings among microwave
cavity photons, magnons and vibration phonons. So far, all previous CMM
experiments have been operated in the weak-coupling regime. This considerably
limits prospective various applications of the system. Here, we demonstrate the
CMM system in the strong-coupling regime and observe the associated normal-mode
splitting. In this regime, the mechanical oscillator is strongly coupled to a
cavity-magnon polariton that is formed by strongly coupled cavity photons and
magnons, and the polariton-mechanics cooperativity reaches $4\times10^3$, which
is improved by three orders of magnitude than previous CMM experiments. The
system is then in the triple-strong-coupling regime and the normal modes of the
system are the hybridization of microwave photons, magnons and phonons. This is
achieved by significantly reducing the decay rate of the polariton mode using
coherent perfect absorption and the decay rate is reduced by four orders of
magnitude. The work paves the way towards coherent control and measurement of
the quantum states of phonons, photons and magnons, and provides a new platform
for the study of rich strong-coupling effects in multipartite hybrid systems.",2307.11328v2
2023-07-22,Quantum parametric amplifiation of phonon-mediated magnon-spin interaction,"The recently developed hybrid magnonics provides new opportunities for
advances in both the study of magnetism and the development of quantum
information processing. However, engineering coherent quantum state transfer
between magnons and specific information carriers, in particular, mechanical
oscillators and solid-state spins, remains challenging due to the intrinsically
weak interactions and the frequency mismatch between diffrent components. Here,
we show how to strongly couple the magnon modes in a nanomagnet to the
quantized mechanical motion (phonons) of a micromechanical cantilever in a
hybrid tripartite system. The coherent and enhanced magnon-phonon coupling is
engineered by introducing the quantum parametric amplifiation of the mechanical
motion. With experimentally feasible parameters, we show that the mechanical
parametric drive can be adjusted to drive the system into the strong-coupling
regime and even the ultrastrong-coupling regime. Furthermore, we show the
coherent state transfer between the nanomagnet and a nitrogen-vacancy center in
the dispersive-coupling regime, with the magnon-spin interaction mediated by
the virtually-excited squeezed phonons. The amplifid mechanical noise can
hardly interrupt the coherent dynamics of the system even for low mechanical
quality factors, which removes the requirement of applying additional
engineered-reservoir techniques. Our work opens up prospects for developing
novel quantum transducers, quantum memories and high-precision measurements.",2307.11961v1
2023-08-22,Magnon-mediated qubit coupling determined via dissipation measurements,"Controlled interaction between localized and delocalized solid-state spin
systems offers a compelling platform for on-chip quantum information processing
with quantum spintronics. Hybrid quantum systems (HQSs) of localized
nitrogen-vacancy (NV) centers in diamond and delocalized magnon modes in
ferrimagnets-systems with naturally commensurate energies-have recently
attracted significant attention, especially for interconnecting isolated spin
qubits at length-scales far beyond those set by the dipolar coupling. However,
despite extensive theoretical efforts, there is a lack of experimental
characterization of the magnon-mediated interaction between NV centers, which
is necessary to develop such hybrid quantum architectures. Here, we
experimentally determine the magnon-mediated NV-NV coupling from the
magnon-induced self-energy of NV centers. Our results are quantitatively
consistent with a model in which the NV center is coupled to magnons by dipolar
interactions. This work provides a versatile tool to characterize HQSs in the
absence of strong coupling, informing future efforts to engineer entangled
solid-state systems.",2308.11710v1
2023-10-29,Higher-order topological corner and bond-localized modes in magnonic insulators,"We theoretically investigate a two-dimensional decorated honeycomb lattice
framework to realize a second-order topological magnon insulator (SOTMI) phase
featuring distinct corner-localized modes. Our study emphasizes the pivotal
role of spin-magnon mapping in characterizing bosonic topological properties,
which exhibit differences from their fermionic counterparts. We employ a
symmetry indicator topological invariant to identify and characterize this
SOTMI phase, particularly for systems respecting time-reversal and ${\sf{C}}_6$
rotational symmetry. Using a spin model defined on a honeycomb lattice
geometry, we demonstrate that introducing ``\textit{kekul\'e}'' type
distortions yields a topological phase. In contrast, ``\textit{anti-kekul\'e}''
distortions result in a non-topological magnonic phase. The presence of
kekul\'e distortions manifests in two distinct topologically protected bosonic
corner modes - an \textit{intrinsic} and a \textit{pseudo}, based on the
specific edge terminations. On the other hand, anti-kekul\'e distortions give
rise to \SW{\textit{Tamm/Shockley}} type bond-localized boundary modes, which
are non-topological and reliant on particular edge termination. We further
investigate the effects of random out-of-plane exchange anisotropy disorder on
the robustness of these bosonic corner modes. The distinction between SOTMIs
and their fermionic counterparts arises due to the system-specific magnonic
onsite energies, a crucial feature often overlooked in prior literature. Our
study unveils exciting prospects for engineering higher-order topological
phases in magnon systems and enhances our understanding of their unique
behavior within decorated honeycomb lattices.",2310.19010v2
2023-10-30,Cavity magnomechanics: from classical to quantum,"Hybrid quantum systems based on magnons in magnetic materials have made
significant progress in the past decade. They are built based on the couplings
of magnons with microwave photons, optical photons, vibration phonons, and
superconducting qubits. In particular, the interactions among magnons,
microwave cavity photons, and vibration phonons form the system of cavity
magnomechanics (CMM), which lies in the interdisciplinary field of cavity QED,
magnonics, quantum optics, and quantum information. Here, we review the
experimental and theoretical progress of this emerging field. We first
introduce the underlying theories of the magnomechanical coupling, and then
some representative classical phenomena that have been experimentally observed,
including magnomechanically induced transparency, magnomechanical dynamical
backaction, magnon-phonon cross-Kerr nonlinearity, etc. We also discuss a
number of theoretical proposals, which show the potential of the CMM system for
preparing different kinds of quantum states of magnons, phonons, and photons,
and hybrid systems combining magnomechanics and optomechanics and relevant
quantum protocols based on them. Finally, we summarize this review and provide
an outlook for the future research directions in this field.",2310.19237v3
2023-11-09,Electrically induced angular momentum flow between separated ferromagnets,"Converting angular momentum between different degrees of freedom within a
magnetic material results from a dynamic interplay between electrons, magnons
and phonons. This interplay is pivotal to implementing spintronic device
concepts that rely on spin angular momentum transport. We establish a new
concept for long-range angular momentum transport that further allows to
address and isolate the magnonic contribution to angular momentum transport in
a nanostructured metallic ferromagnet. To this end, we electrically excite and
detect spin transport between two parallel and electrically insulated
ferromagnetic metal strips on top of a diamagnetic substrate. Charge-to-spin
current conversion within the ferromagnetic strip generates electronic spin
angular momentum that is transferred to magnons via electron-magnon coupling.
We observe a finite angular momentum flow to the second ferromagnetic strip
across a diamagnetic substrate over micron distances, which is electrically
detected in the second strip by the inverse charge-to-spin current conversion
process. We discuss phononic and dipolar interactions as the likely cause to
transfer angular momentum between the two strips. Moreover, our work provides
the experimental basis to separate the electronic and magnonic spin transport
and thereby paves the way towards magnonic device concepts that do not rely on
magnetic insulators.",2311.05290v1
2023-11-22,Controlling selection rules for magnon scattering in nanomagnets by spatial symmetry breaking,"Nanomagnets are the building blocks of many existing and emergent spintronic
technologies. Magnetization dynamics of nanomagnets is often dominated by
nonlinear processes, which have been recently shown to have many surprising
features and far-reaching implications for applications. Here we develop a
theoretical framework uncovering the selection rules for multimagnon processes
and discuss their underlying mechanisms. For its technological relevance, we
focus on the degenerate three-magnon process in thin elliptical nanodisks to
illustrate our findings. We parameterize the selection rules through a set of
magnon interaction coefficients which we calculate using micromagnetic
simulations. We postulate the selection rules and investigate how they are
altered by perturbations, that break the symmetry of static magnetization
configuration and spatial spin-wave profiles and that can be realized by
applying off-symmetry-axis or nonuniform magnetic fields. Our work provides the
phenomenological understanding into the mechanics of magnon interaction as well
as the formalism for determining the interaction coefficients from simulations
and experimental data. Our results serve as a guide to analyze magnon processes
inherently present in spin-torque devices for boosting their performance or to
engineer a specific nonlinear response of a nanomagnet used in neuromorphic or
quantum magnonic application.",2311.13726v1
2023-12-31,"Magnon, doublon and quarton excitations in 2D trimerized Heisenberg models","We investigate the magnetic excitations of the trimerized Heisenberg models
with intra-trimer interaction $J_1$ and inter-trimer interaction $J_2$ on four
different two-dimensional lattices using a combination of stochastic series
expansion quantum Monte Carlo (SSE QMC) and stochastic analytic continuation
methods (SAC), complemented by cluster perturbation theory (CPT). These models
exhibit quasi-particle-like excitations when $g=J_2/J_1$ is small,
characterized by low-energy magnons, intermediate-energy doublons, and
high-energy quartons. The low-energy magnons are associated with the magnetic
ground states. They can be described by the linear spin wave theory (LSWT) of
the effective block spin model and the original spin model. Doublons and
quartons emerge from the corresponding internal excitations of the trimers with
distinct energy levels, which can be effectively analyzed using perturbation
theory when the ratio of exchange interactions $g$ is small. In this small $g$
regime, we observe a clear separation between the magnon and higher-energy
spectra. However, as $g$ increases, these three spectra gradually merge into
the magnon modes or continua. Nevertheless, the LSWT fails to provide
quantitative descriptions of the higher-energy excitation bands due to
significant quantum fluctuations. Notably, in the Collinear II and trimerized
hexagon lattice, a broad continuum emerges above the single-magnon spectrum,
originating from the quasi-1D physics due to the dilute connections between
chains. Our numerical analysis of these 2D trimers yields valuable theoretical
predictions and explanations for the inelastic neutron scattering (INS) spectra
of 2D magnetic materials featuring trimerized lattices.",2401.00376v1
2024-01-04,Nonreciprocal photon blockade with Kerr magnons,"Nonreciprocal devices, allowing to manipulate one-way signals, are crucial to
quantum information processing and quantum network. Here we propose a nonlinear
cavity-magnon system, consisting of a microwave cavity coupled to one or two
yttrium-iron-garnet (YIG) spheres supporting magnons with Kerr nonlinearity, to
investigate nonreciprocal photon blockade. The nonreciprocity originates from
the direction-dependent Kerr effect, distinctly different from previous
proposals with spinning cavities and dissipative couplings. For a single sphere
case, nonreciprocal photon blockade can be realized by manipulating the
nonreciprocal destructive interference between two active paths, via vary the
Kerr coefficient from positive to negative, or vice versa. By optimizing the
system parameters, the perfect and well tuned nonreciprocal photon blockade can
be predicted. For the case of two spheres with opposite Kerr effects, only
reciprocal photon blockade can be observed when two cavity-magnon coupling
strengths Kerr strengths are symmetric. However, when coupling strengths or
Kerr strengths become asymmetric, nonreciprocal photon blockade appears. This
implies that two-sphere nonlinear cavity-magnon systems can be used to switch
the transition between reciprocal and nonreciprocal photon blockades. Our study
offers a potential platform for investigating nonreciprocal photon blockade
effect in nonlinear cavity magnonics.",2401.02251v1
2024-02-08,Assessing the atomic moment picture of spin dynamics: the perspective of \textit{ab initio} magnon wavefunction,"Our understanding of collective spin fluctuation in materials relies largely
on Heisenberg-type spin Hamiltonians. Implicit in these spin models is the
atomic moment picture that in transverse spin dynamics the magnetization around
an atom undergoes precessional motion as a rigid moment, which has been
challenged by emerging theoretical and experimental advances. To assess the
validity of the atomic moment picture in spin dynamics, however, necessitates
magnon wavefunctions from \textit{ab initio} methods without \textit{a priori}
spin models. To this end, we develop an efficient model-free {\it ab initio}
method for computing magnon spectrum and wavefunctions. Niu-Kleinman's
adiabatic spin-wave dynamics is reformulated using linear perturbation theory
into a generalized eigenvalue problem, which can be solved to produce magnon
spectrum and wavefunctions without assuming atomic moments. We have implemented
this method in the framework of density functional perturbation theory (DFPT).
A dynamical extension of Niu-Kleinman equation of motion is proposed to improve
inaccurate predicted magnon energies due to imperfect adiabaticity at higher
energies. Based on so-obtained {\it ab initio} magnon wavefunctions, we find
the atomic moment picture to be valid in typical ferromagnets and
antiferromagnets, but fails in the molecular orbital crystal Na$_2$IrO$_3$. Our
results suggest that the usual spin Hamiltonian approach should be taken with a
grain of salt, and possible experimental ramification on the issue is
discussed.",2402.05473v2
2024-02-22,Combinatorial split-ring and spiral meta-resonator for efficient magnon-photon coupling,"Developing hybrid materials and structures for electromagnetic wave
engineering has been a promising route towards novel functionalities and
tunabilities in many modern applications and perspectives in new quantum
technologies. Despite its established success in engineering optical light and
terahertz waves, the implementation of meta-resonators operating at the
microwave band is still emerging, especially those that allow for on-chip
integration and size miniaturization, which has turned out crucial to
developing hybrid quantum systems at the microwave band. In this work, we
present a microwave meta-resonator consisting of split-ring and and spiral
resonators, and implement it to the investigation of photon-magnon coupling for
hybrid magnonic applications. We observe broadened bandwidth to the split ring
modes augmented by the additional spiral resonator, and, by coupling the modes
to a magnetic sample, the resultant photon-magnon coupling can be significantly
enhanced to more than ten-fold. Our work suggests that combinatorial, hybrid
microwave resonators may be a promising approach towards future development and
implementation of photon-magnon coupling in hybrid magnonic systems.",2402.15535v2
2024-03-07,Nonequilibrium magnonic thermal transport engineering,"Thermal conductivity, a fundamental parameter characterizing thermal
transport in solids, is typically determined by electron and phonon transport.
Although other transport properties including electrical conductivity and
thermoelectric conversion coefficients have material-specific values, it is
known that thermal conductivity can be modulated artificially via phonon
engineering techniques. Here, we demonstrate another way of artificially
modulating the heat conduction in solids: magnonic thermal transport
engineering. The time-domain thermoreflectance measurements using ferromagnetic
metal/insulator junction systems reveal that the thermal conductivity of the
ferromagnetic metals and interfacial thermal conductance vary significantly
depending on the spatial distribution of nonequilibrium spin currents.
Systematic measurements of the thermal transport properties with changing the
boundary conditions for spin currents show that the observed thermal transport
modulation stems from magnon origin. This observation unveils that magnons
significantly contribute to the heat conduction even in ferromagnetic metals at
room temperature, upsetting the conventional wisdom that the thermal
conductivity mediated by magnons is very small in metals except at low
temperatures. The magnonic thermal transport engineering offers a new principle
and method for active thermal management.",2403.04166v1
1997-10-06,Multi-Magnon Scattering in the Ferromagnetic XXX-Model with Inhomogeneities,"We determine the transition amplitude for multi-magnon scattering induced
through an inhomogeneous distribution of the coupling constant in the
ferromagnetic XXX-model. The two and three particle amplitudes are explicitely
calculated at small momenta. This suggests a rather plausible conjecture also
for a formula of the general n-particle amplitude.",9710055v1
1998-09-10,Introduction to the Bethe ansatz I,"The Bethe ansatz for the one-dimensional s=1/2 Heisenberg ferromagnet is
introduced at an elementary level. The presentation follows Bethe's original
work very closely. A detailed description and a complete classification of all
two-magnon scattering states and two-magnon bound states are given for finite
and infinite chains. The paper is designed as a tutorial for beginning graduate
students. It includes 10 problems for further study.",9809162v1
1999-10-21,Optical absorption of spin ladders,"We present a theory of phonon-assisted optical two-magnon absorption in
two-leg spin-ladders. Based on the strong intra-rung-coupling limit we show
that collective excitations of total spin S=0, 1 and 2 exist outside of the
two-magnon continuum. It is demonstrated that the singlet collective state has
a clear signature in the optical spectrum.",9910322v1
2004-12-15,Nuclear Magnetic Relaxation in the Ferrimagnetic Chain Compound NiCu(C_7_H_6_N_2_O_6_)(H_2_O)_3_2H_2_O: Three-Magnon Scattering?,"Recent proton spin-lattice relaxation-time (T_1_) measurements on the
ferrimagnetic chain compound NiCu(C_7_H_6_N_2_O_6_)(H_2_O)_3_2H_2_O are
explained by an elaborately modified spin-wave theory. We give a strong
evidence of the major contribution to 1/T_1_ being made by the three-magnon
scattering rather than the Raman one.",0412389v1
1998-07-14,A New Look at the Ashtekar-Magnon Energy Condition,"In 1975, Ashtekar and Magnon showed that an energy condition selects a unique
quantization procedure for certain observers in general, curved spacetimes. We
generalize this result in two important ways, by eliminating the need to assume
a particular form for the (quantum) Hamiltonian, and by considering the
surprisingly nontrivial extension to nonminimal coupling.",9807028v1
1998-06-26,Cherenkov radiation of magnon and phonon by the slow magnetic monopole,"The Cherenkov radiation of magnons at passage of the heavy slow magnetic
monopole through an ordered magnetic matter is considered. Also the Cherenkov
radiation of phonons at monopole movement in medium is discussed.",9806495v1
2006-07-03,Dressing the Giant Magnon,"We apply the dressing method to construct new classical string solutions
describing various scattering and bound states of magnons. These solutions
carry one, two or three SO(6) charges and correspond to multi-soliton
configurations in the generalized sine-Gordon models.",0607009v3
2006-12-16,A note on two-spin magnon-like energy-charge relations from M-theory viewpoint,"We show that for each M-theory background, having subspaces with metrics of
given type, there exist M2-brane configurations, which in appropriate limit
lead to two-spin magnon-like energy-charge relations, established for strings
on AdS_5 x S^5, its beta-deformation, and for membrane in AdS_4 x S^7.",0612175v2
2008-03-27,Finite Size Giant Magnon,"The quantization of the giant magnon away from the infinite size limit is
discussed. We argue that this quantization inevitably leads to string theory on
a Z_M-orbifold of S^5. This is shown explicitly and examined in detail in the
near plane-wave limit.",0803.4028v2
2009-01-29,"Spin-waves in triangular lattice antiferromagnet: decays, spectrum renormalization, and singularities","We present a comprehensive study of the dynamical properties of the quantum
Heisenberg antiferromagnet on a triangular lattice within the framework of
spin-wave theory. The distinct features of spin-wave excitations in the
triangular-lattice antiferromagnet are (i) finite lifetime at zero temperature
due to spontaneous two-magnon decays, (ii) strong renormalization of magnon
energies with respect to the harmonic result, and (iii) logarithmic
singularities in the decay rate Gamma_k. Quantum corrections to the magnon
spectrum are obtained using both the on-shell and off-shell solutions of the
Dyson equation with the lowest-order magnon self-energy. At low-energies magnon
excitations remain well-defined albeit with the anomalous decay rate Gamma_k ~
k^2 at k->0 and Gamma_k ~ |k-Q_AF|^7/2 at k->Q_AF. At high energies, magnons
are heavily damped with the decay rate reaching (2 Gamma_k/E_k) ~ 0.3 for the
case S=1/2. The on-shell solution shows logarithmic singularities in Gamma_k
with the concomitant jump-like discontinuities in E_k along certain contours in
the momentum space. Such singularities are even more prominent in the magnon
spectral function A(k,w). Although the off-shell solution removes such
log-singularities, the decay rates remain strongly enhanced. We also discuss
the role of higher-order corrections and show that such singularities may lead
to complete disappearance of the spectrum in the vicinity of certain k-points.
The kinematic conditions for two-magnon decays are analyzed for various
generalizations of the triangular-lattice antiferromagnet as well as for the
XXZ model on a kagome lattice. Our results suggest that decays and
singularities in the spin-wave spectra must be ubiquitous in all these systems.",0901.4803v3
2009-04-27,The Fate of the Two-Magnon Bound State in the Heisenberg Antiferromagnet,"The energy spectrum of the two-magnon bound states in the Heisenberg-Ising
antiferromagnet on the square lattice are calculated using series expansion
methods. The results confirm an earlier spin-wave prediction of Oguchi and
Ishikawa, that the bound states vanish into the continuum before the isotropic
Heisenberg limit is reached.",0904.4085v1
2010-12-09,Twisted Magnons,"We study spin chains for superconformal quiver gauge theories in the moduli
space of N=2 orbifolds. Independent of integrability, which is generally
broken, we use the centrally extended SU(2|2) symmetry of the magnons to fix
their dispersion relations and two-body S-matrices, as functions of the exactly
marginal couplings.",1012.2097v2
2011-05-17,Finite size effect on the magnon's correlation functions,"We calculate the finite size correction on the three-point correlation
function between two giant magnons and one marginal operator. We also check
that the structure constant in the string set-up is exactly the same as one of
the RG analysis in the gauge theory.",1105.3279v2
2011-06-18,Finite-size Giant Magnons on AdS_4 x CP^3_γ,"We investigate finite-size giant magnons propagating on \gamma-deformed AdS_4
x CP^3_{\gamma} type IIA string theory background, dual to one parameter
deformation of the N=6 super Chern-Simoms-matter theory. Analyzing the
finite-size effect on the dispersion relation, we find that it is modified
compared to the undeformed case, acquiring \gamma dependence.",1106.3686v1
2012-08-03,"On finite size corrections to the dispersion relations of giant magnon and single spike on γ-deformed T^{1,1}","In this paper we consider the finite size effects for the strings in \beta
-deformed AdS_{5}\times T^{1,1} background. We analyze the finite size
corrections for the cases of giant magnon and single spike string solution. The
finite size corrections for the undeformed case are straightforwardly obtained
sending the deformation parameter to zero.",1208.0698v1
2012-08-30,Free from spurious solutions integral equation for three-magnon bound states in 1D XXZ ferromagnet,"A new integral equation for three-magnon bound states in XXZ spin chain is
suggested. Unlike the one presented by C. K. Majumdar about 40 years ago this
equation does not have spurious solutions. Such an advantage is a result of
decomposition of the wave function in the Bloch basis rather than in the basis
of flat waves.",1208.6177v1
2016-01-19,Classification of magnons in Rotated Ferromagnetic Heisenberg model and their competing responses in transverse fields,"Competing orders is a general concept to describe various quantum phases and
transitions in various materials. One efficient way to investigate competing
orders is to first classify different class of excitations in a given quantum
phase, then study their competing responses under various external probes. This
strategy may not only lead to deep understanding of the quantum phase itself,
but also its deep connections to various other quantum phases nearby. We
implement this approach by studying the Rotated Ferromagnetic Heisenberg model
(RFHM) in two different transverse fields $h_x$ and $h_z$ which can be
intuitively visualized as studying spin-orbit couplings (SOC) effects in 2d
Ising or anisotropic XY model in a transverse field. At a special SOC class, it
was known that the RFHM at a zero field owns an exact ground state called Y-x
state. It supports non only the commensurate C-C$_0$ and C-C$_{\pi} $ magnons,
but also the in-commensurate C-IC magnons. These magnons are non-relativistic,
not contained in the exact ground state, so need to be thermally excited. Their
dramatic response under the longitudinal $ h_y $ field was recently worked out
by the authors. Here we find they respond very differently under the two
transverse fields. Any $h_x$ ($h_z$) changes the collinear Y-x state to a
canted co-planar YX-x (YZ-x) state which suffers quantum fluctuations. The
C-C$_0$, C-C$_{\pi} $ and C-IC magnons sneak into the quantum ground state,
become relativistic and play leading roles even at $ T=0 $. We map out the
boundaries among the C-C$_0$, C-C$_{\pi} $ and C-IC magnons, especially the
detailed evolution of the C-IC magnons inside the canted phases. As $h_x$
($h_z$) increases further, the C-C$_0$ magnons always win the competition and
emerge as the seeds to drive a transition from the YX-x (YZ-x) to the X-FM (
Z-FM ) which is shown to be in the 3d Ising universality class.",1601.05067v1
2018-11-03,Tunable Spin Seebeck Diode with Magnonic Spin Tunneling Junction,"We theoretically investigate the spin--wave spin current induced by the spin
Seebeck effect in magnonic spin tunneling junctions (MSTJs) for arbitrary
magnetization directions. We show that the MSTJ functions as a \textit{tunable}
spin Seebeck diode in which the tunneling spin current can be turned on and off
with high efficiency by controlling the magnetization direction.",1811.01141v1
2008-07-11,Numerical evidence for unstable magnons at high fields in the Heisenberg antiferromagnet on the square lattice,"We find evidence for decaying magnons at strong magnetic field in the square
lattice spin-1/2 Heisenberg antiferromagnet. The results are obtained using
Quantum Monte Carlo simulations combined with a Bayesian inference technique to
obtain dynamics and are consistent with predictions from spin wave theory.",0807.1837v2
2016-08-22,"Thermally induced magnonic spin current, thermomagnonic torques and domain wall dynamics in the presence of Dzyaloshinskii-Moriya interaction","Thermally activated domain wall (DW) motion in magnetic insulators has been
considered theoretically, with a particular focus on the role of
Dzyaloshinskii-Moriya Interaction (DMI) and thermomagnonic torques. The
thermally assisted DW motion is a consequence of the magnonic spin current due
to the applied thermal bias. In addition to the exchange magnonic spin current
and the exchange adiabatic and the entropic spin transfer torques, we also
consider the DMI-induced magnonic spin current, thermomagnonic DMI field-like
torque and the DMI entropic torque. Analytical estimations are supported by
numerical calculations. We found that the DMI has a substantial influence on
the size and the geometry of DWs, and that the DWs become oriented parallel to
the long axis of the nanostrip. Increasing the temperature smoothes the DWs.
Moreover, the thermallyinduced magnonic current generates a torque on the DWs,
which is responsible for their motion. From our analysis it follows that for a
large enough DMI the influence of DMI-induced field-like torque is much
stronger than that of the DMI and the exchange entropic torques. By
manipulating the strength of the DMI constant, one can control the speed of the
DW motion, and the direction of the DW motion can be switched, as well. We also
found that DMI not only contributes to the total magnonic current, but also it
modifies the exchange magnonic spin current, and this modification depends on
the orientation of the steady state magnetization. The observed phenomenon can
be utilized in spin caloritronics devices, for example in the DMI based thermal
diodes. By switching the magnetization direction, one can rectify the total
magnonic spin current.",1608.06179v1
2017-10-15,Long-wave magnons in a ferromagnetic film,"An asymptotically exact theory of spectrum and transverse distribution of
magnetization in long-wave magnons is presented. It is based on exact
analytical solution of linearized Landau-Lifshitz equation in a film. The
quantization of the transverse wave parameters and wave vectors is studied.",1710.05362v1
2018-03-02,Long lifetime of thermally-excited magnons in bulk yttrium iron garnet,"Spin currents are generated within the bulk of magnetic materials due to heat
flow, an effect called intrinsic spin-Seebeck. This bulk bosonic spin current
consists of a diffusing thermal magnon cloud, parametrized by the magnon
chemical potential ($\mu_{m}$), with a diffusion length of several microns in
yttrium iron garnet (YIG). Transient opto-thermal measurements of the
spin-Seebeck effect (SSE) as a function of temperature reveal the time
evolution of $\mu_{m}$ due to intrinsic SSE in YIG. The interface SSE develops
at times < 2 ns while the intrinsic SSE signal continues to evolve at times >
500 $\mu$s, dominating the temperature dependence of SSE in bulk YIG.
Time-dependent SSE data are fit to a multi-temperature model of coupled
spin/heat transport using finite element method (FEM), where the magnon spin
lifetime ($\tau$) and magnon-phonon thermalization time ($\tau_{mp}$) are used
as fit parameters. From 300 K to 4 K, $\tau_{mp}$ varies from 1 to 10 ns,
whereas $\tau$ varies from 2 to 60 $\mu$s with the spin lifetime peaking at 90
K. At low temperature, a reduction in $\tau$ is observed consistent with
impurity relaxation reported in ferromagnetic resonance measurements. These
results demonstrate that the thermal magnon cloud in YIG contains extremely low
frequency magnons (~10 GHz) providing spectral insight to the microscopic
scattering processes involved in magnon spin/heat diffusion.",1803.01054v3
2015-08-11,Transport between metals and magnetic insulators,"We derive the Onsager response matrix of fluctuation-mediated spin-collinear
transport through a ferromagnetic insulator and normal metal interface driven
by a temperature difference, spin accumulation, or magnetic field. We predict
magnon-squeezing spin currents, magnetic field-induced cooling (magnon Peltier
effect), temperature induced magnetization (thermal magnetic field) as well as
universal spin Seebeck/Peltier coefficients.",1508.02486v2
2016-09-26,Bethe Ansatz approach to the three-magnon problem in an infinite $XX$ spin chain with Izing next neighboring ferromagnetic interaction,"Using the traditional and degenerative discrete-diffractive versions of Bethe
Ansatz we construct a rude set of three-magnon states in the infinite $XX$ spin
chain with Izing next neighboring ferromagnetic term.",1609.07941v1
2019-10-02,Magnonics and Supermagnonics,"The magnetic community continues to discuss the possibility to observe the
magnetic superfluidity, despite the fact that it has been discovered long time
ago. It was observed in antiferromagnetic states of superfluid $^3$He in 1984.
In this article we reminds the main principles of spin superfluidity and
related Bose-Einstain magnon condensation. We discuss applications of this
phenomenon in supermagnonic devises.",1910.02841v1
2022-06-10,Theory of Moire Magnets and Topological Magnons: Applications to Twisted Bilayer CrI3,"We develop a comprehensive theory of twisted bilayer magnetism. Starting from
the first-principles calculations of two-dimensional honeycomb magnet CrI3, we
construct the generic spin models that represent a broad class of twisted
bilayer magnetic systems. Using Monte-Carlo method, we discover a variety of
non-collinear magnetic orders and topological magnons that have been overlooked
in the previous theoretical and experimental studies. As a function of the
twist angle, the collinear magnetic order undergoes the phase transitions to
the non-collinear order and the magnetic domain phase. In the magnetic domain
phase, we find that the spatially varying interlayer coupling produces the
magnetic skyrmions even in the absence of the Dzyaloshinskii-Moriya
interactions. In addition, we describe the critical phenomena of the magnetic
phase transitions by constructing the field theoretical model of the moire
magnet. Our continuum model well-explains the nature of the phase transitions
observed in the numerical simulations. Finally, we classify the topological
properties of the magnon excitations. The magnons in each phases are
characterized by the distinct mass gaps with different physical origins. In the
collinear ferromagnetic order, the higher-order topological magnonic insulator
phase occurs. It serves as a unique example of the higher-order topological
phase in magnonic system, since it does not require non-collinear order or
asymmetric form of the interactions. In the magnetic domain phases, the magnons
are localized along the domain wall and form one-dimensional topological edge
mode. As the closed domain walls deform to a open network, the confined edge
mode extends to form a network model of the topological magnons.",2206.05264v1
2023-11-28,Temperature dependence of the magnon-phonon interaction in high overtone bulk acoustic resonator-ferromagnetic thin film hybrids,"Tailored magnon-phonon hybrid systems, where high overtone bulk acoustic
resonators couple resonantly to the magnonic mode of a ferromagnetic thin film,
are considered optimal for the creation of acoustic phonons with a defined
circular polarization. This class of devices is therefore ideal for the
investigation of phonon propagation properties and assessing their capacity to
transport angular momentum in the classical and potentially even in the quantum
regime. Here, we study the coupling between the magnons in a ferromagnetic
\ch{Co25Fe75} thin film and the transverse acoustic phonons in a bulk acoustic
wave resonators formed by the sapphire substrate onto which the film is
deposited. Using broadband ferromagnetic resonance experiments as a function of
temperature, we investigate the strength of the coherent magnon-phonon
interaction and the individual damping rates of the magnons and phonons
participating in the process. This demonstrates that this coupled magnon-phonon
system can reach a cooperativity $C\approx 1$ at cryogenic temperatures. Our
experiments also showcase the potential of strongly coupled magnon-phonon
systems for strain sensing applications.",2311.16725v1
1994-09-27,Resonant two-magnon Raman scattering in antiferromagnetic insulators,"We propose a theory of two-magnon {\it resonant\/} Raman scattering from
antiferromagnetic insulators, which contains information both on the magnetism
and the carrier properties in the lighly doped phases. We argue that the
conventional theory does not work in the resonant regime, in which the energy
of the incident photon is close to the gap between the conduction and valence
bands. We identify the diagram which gives the dominant contribution to Raman
intensity in this regime and show that it can explain the unusual features in
the two-magnon profile and in the two-magnon peak intensity dependence on the
incoming photon frequency.",9409114v1
1995-11-01,Theory of Phonon-Assisted Multimagnon Optical Absorption and Bimagnon States in Quantum Antiferromagnets,"We calculate the effective charge for multimagnon infrared (IR) absorption
assisted by phonons in a perovskite like antiferromagnet and we compute the
spectra for two magnon absorption using interacting spin-wave theory. The full
set of equations for the interacting two magnon problem is presented in the
random phase approximation for arbitrary total momentum of the magnon pair. The
spin wave theory results fit very well the primary peak of recent measured
bands in the parent insulating compounds of cuprate superconductors. The line
shape is explained as being due to the absorption of one phonon plus a new
quasiparticle excitation of the Heisenberg Hamiltonian that consists off a long
lived virtual bound state of two magnons (bimagnon). The bimagnon states have
well defined energy and momentum in a substantial portion of the Brillouin
zone. The higher energy bands are explained as one phonon plus higher
multimagnon absorption processes. Other possible experiments for observing
bimagnons are proposed. In addition we predict the line shape for the spin one
system La$_2$NiO$_4$.",9511005v1
1998-09-10,Magnon splitting induced by charge ordering in NaV_2O_5,"We consider the effects of charge ordering in NaV_2O_5 (below T_SP) on the
exchange constants and on the magnon dispersion. We show that the
experimentally observed splitting of the magnon branches along the a direction
is induced by charge ordering. We find that one can distinguish between the
proposed 'zig-zag' and 'in-line' patterns of charge ordering. Only the zig-zag
ordering is consistent with the experimental results regarding (i) the unusual
intensity modulation observed in magnetic neutron scattering, (ii) the
reduction in the intra-ladder exchange constant below T_SP, and (iii) the
magnon dispersion along a. We estimate the inter-ladder exchange constant to be
1.01meV=11.7K for T>T_SP.",9809154v2
1998-12-02,Observation of magnetic order in La_{1.9}Sr_{0.1}CuO_{4} from two-magnon Raman scattering,"We reported two-magnon Raman scattering from La_{1.9}Sr_{0.1}CuO_4, which has
a suppressed Tc=12 K, as the temperature is lowered below 37 K and an ordered
spin phase is formed. The two-magnon Raman intensity increases with decreasing
temperature. The magnetic scattering in La_{1.9}Sr_{0.1}CuO_4 is totally
different from that reported in the parent compound La_2CuO_4. We analyze the
line shape of the two-magnon scattering within the traditional Loudon-Fleury
theory and find the superexchange constant J=1052 cm^{-1}. The calculation of
the frequency moment suggests that the quantum fluctuations are very weak in
the system. The room temperature Raman scattering from La_2CuO_4 is also
measured. Strong features appear in the one-phonon spectrum at the frequencies
of the longitudinal optical (LO) infrared modes which we suggest become Raman
active through a Fr\""{o}hlich-interaction.",9812027v1
1999-04-27,Spin-wave contributions to nuclear magnetic relaxation in magnetic metals,"The longitudinal and transverse nuclear magnetic relaxation rates $1/T_1(T)$
and $1/T_2(T)$ are calculated for three- and two-dimensional (3D and 2D)
metallic ferro- and antiferromagnets (FM and AFM) with localized magnetic
moments in the spin-wave temperature region. The contribution of the one-magnon
decay processes is strongly enhanced in comparison with the standard $T$-linear
Korringa term, especially for the FM case. For the 3D AFM case this
contribution diverges logarithmically, the divergence being cut at the magnon
gap $\omega_0$ due to magnetic anisotropy, and for the 2D AFM case as
$\omega_0^{-1}$. The electron-magnon scattering processes yield $T^2\ln
(T/\omega_0)$ and $T^2/\omega_0^{1/2}$-terms in $1/T_1$ for the 3D AFM and 2D
FM cases, respectively. The two-magnon (``Raman'') contributions are
investigated and demonstrated to be large in the 2D FM case. Peculiarities of
the isotropic 2D limit (where the correlation length is very large) are
analyzed.",9904384v3
2000-01-12,Raman Scattering from Magnetic Excitations in the Spin Ladder Compounds CaV2O5 and MgV2O5,"We present the Raman-scattering spectra of CaV2O5 and MgV2O5. The magnetic
contribution in the Raman spectra of CaV2O5 is found in the form of a strong
asymmetric line, centered at 2Delta, with a tail on the high-energy side. Our
analysis of its spectral shape shows that the magnetic ordering in CaV2O5 can
be described using a S=1/2 two-leg ladder Heisenberg antiferromagnetic model
with Jparallel/Jperp = 0.1, and a small interladder exchange. The spin gap and
exchange constant are estimated to be Delta = 400 cm-1 (570 K) and Jperp = 640
K. No magnon bound states are found. In contrast to CaV2O5 the existence of the
spin-gap is not confirmed in MgV2O5, since we found no feature in the spectra
which could be associated with the onset of the two-magnon continuum. Instead,
we observe two-magnon excitation at 340 cm-1, presumably related to the top of
the two-magnon brunch.",0001168v2
2000-04-21,Thermal conductivity of Mg-doped CuGeO_3 at very low temperatures: Heat conduction by antiferromagnetic magnons,"Thermal conductivity \kappa is measured at very low temperatures down to 0.28
K for pure and Mg-doped CuGeO_3 single crystals. The doped samples carry larger
amount of heat than the pure sample at the lowest temperature. This is because
antiferromagnetic magnons appear in the doped samples and are responsible for
the additional heat conductivity, while \kappa of the pure sample represents
phonon conductivity at such low temperatures. The maximum energy of the magnon
is estimated to be much lower than the spin-Peierls-gap energy. The result
presents the first example that \kappa at very low temperatures probes the
magnon transport in disorder-induced antiferromagnetic phase of spin-gap
systems.",0004367v1
2000-05-12,Two-magnon states in Cu(NO3)2*2.5D2O using inelastic neutron scattering,"We report measurements of the two-magnon states in a dimerized
antiferromagnetic chain material, copper nitrate
(\QTR{group}{Cu(NO$_{3}$)$_{2}\cdot 2.5$D$_{2}$O}). Using inelastic neutron
scattering, we have studied the one- and two-magnon excitation spectra in a
large single crystal of this material. We compare this new data with
perturbative expansions of the alternating Heisenberg chain and find good
agreement with these calculations. The data may also show evidence for the
recently proposed S=1 two-magnon bound state (Phys. Rev. B54, R9624 (1996)).",0005222v1
2000-07-11,Spin-1 Antiferromagnetic Heisenberg Chains in an External Staggered Field,"We present in this paper a nonlinear sigma-model analysis of a spin-1
antiferromagnetic Heisenberg chain in an external commensurate staggered
magnetic field. After rediscussing briefly and extending previous results for
the staggered magnetization curve, the core of the paper is a novel
calculation, at the tree level, of the Green functions of the model. We obtain
precise results for the elementary excitation spectrum and in particular for
the spin gaps in the transverse and longitudinal channels. It is shown that,
while the spectral weight in the transverse channel is exhausted by a single
magnon pole, in the longitudinal one, besides a magnon pole a two-magnon
continuum appears as well whose weight is a stedily increasing function of the
applied field, while the weight of the magnon decreases correspondingly. The
balance between the two is governed by a sum rule that is derived and
discussed. A detailed comparison with the present experimental and numerical
(DMRG) status of the art as well as with previous analytical approaches is also
made.",0007188v1
2000-11-29,Magnon dispersion and thermodynamics in CsNiF_3,"We present an accurate transfer matrix renormalization group calculation of
the thermodynamics in a quantum spin-1 planar ferromagnetic chain. We also
calculate the field dependence of the magnon gap and confirm the accuracy of
the magnon dispersion derived earlier through an 1/n expansion. We are thus
able to examine the validity of a number of previous calculations and further
analyze a wide range of experiments on CsNiF_3 concerning the magnon
dispersion, magnetization, susceptibility, and specific heat. Although it is
not possible to account for all data with a single set of parameters, the
overall qualitative agreement is good and the remaining discrepancies may
reflect departure from ideal quasi-one-dimensional model behavior. Finally, we
present some indirect evidence to the effect that the popular interpretation of
the excess specific heat in terms of sine-Gordon solitons may not be
appropriate.",0011496v1
2002-07-02,"Magnon Exchange Mechanism of Superconductivity: ZrZn_2, URhGe","The magnon exchange mechanism of superconductivity was developed to explain
in a natural way the fact that the superconductivity in $UGe_2$, $ZrZn_2$ and
$URhGe$ is confined to the ferromagnetic phase.The order parameter is a spin
anti-parallel component of a spin-1 triplet with zero spin projection. The
transverse spin fluctuations are pair forming and the longitudinal ones are
pair breaking. In the present paper, a superconducting solution, based on the
magnon exchange mechanism, is obtained which closely matches the experiments
with $ZrZn_2$ and $URhGe$. The onset of superconductivity leads to the
appearance of complicated Fermi surfaces in the spin up and spin down momentum
distribution functions. Each of them consist of two pieces, but they are
simple-connected and can be made very small by varying the microscopic
parameters. As a result, it is obtained that the specific heat depends on the
temperature linearly, at low temperature, and the coefficient $\gamma=\frac
{C}{T}$ is smaller in the superconducting phase than in the ferromagnetic one.
The absence of a quantum transition from ferromagnetism to ferromagnetic
superconductivity in a weak ferromagnets $ZrZn_2$ and $URhGe$ is explained
accounting for the contribution of magnon self-interaction to the spin
fluctuations' parameters. It is shown that in the presence of an external
magnetic field the system undergoes a first order quantum phase transition.",0207078v1
2002-07-29,A neutron scattering study of two-magnon states in the quantum magnet copper nitrate,"We report measurements of the two-magnon states in a dimerized
antiferromagnetic chain material, copper nitrate (Cu(NO3)2*2.5D2O). Using
inelastic neutron scattering we have measured the one and two magnon excitation
spectra in a large single crystal. The data are in excellent agreement with a
perturbative expansion of the alternating Heisenberg Hamiltonian from the
strongly dimerized limit. The expansion predicts a two-magnon bound state for q
~ (2n+1)pi*d which is consistent with the neutron scattering data.",0207678v1
2002-12-01,Theoretical analysis of magnetic Raman scattering in La2CuO4: two-magnon intensity with the inclusion of ring exchange,"We evaluate the Raman light scattering intensity for the square lattice
Heisenberg antiferromagnet with plaquette ring exchange J_\Box. With the
exchange couplings as fixed before from an accurate fit to the spin-wave
dispersion in La2CuO4, leading in particular to J_\Box=0.24J, we demonstrate in
a parameter free calculation that the inclusion of the plaquette exchange
contribution to the dispersion and the magnon-magnon interaction vertex gives a
peak position in $B_{1g}$ scattering geometry E_max = 2.71J which is in
excellent agreement with the experimental data. Yet, the intrinsic width and
the lineshape of the two-magnon remain beyond a descriptions in terms of a
spin-only Hamiltonian.",0212014v1
2003-09-27,Amplitudes for magnon scattering by vortices in two-dimensional weakly easy-plane ferromagnets,"We study magnon modes in the presence of a vortex in a circular easy-plane
ferromagnet. The problem of vortex-magnon scattering is investigated for
partial modes with different values of the azimuthal quantum number m over a
wide range of wave numbers. The analysis was done by combining analytical and
numerical calculations in the continuum limit with numerical diagonalization of
adequately large discrete systems. The general laws governing vortex-magnon
interactions are established. We give simple physical explanations of the
scattering results: the splitting of doublets for the modes with opposite signs
of $m$, which takes place for the long wavelength limit, is an analogue of the
Zeeman splitting in the effective magnetic field of the vortex. A singular
behavior for the scattering amplitude, $\sigma_m\propto k$, takes place as $k$
diverges; it corresponds to the generalized Levinson theorem and can be
explained by the singular behavior of the effective magnetic field at the
origin.",0309640v1
2004-01-23,Nonmonotonic inelastic tunneling spectra due to surface spin excitations in ferromagnetic junctions,"The paper addresses inelastic spin-flip tunneling accompanied by surface spin
excitations (magnons) in ferromagnetic junctions. The inelastic tunneling
current is proportional to the magnon density of states which is
energy-independent for the surface waves and, for this reason, cannot account
for the bias-voltage dependence of the observed inelastic tunneling spectra.
This paper shows that the bias-voltage dependence of the tunneling spectra can
arise from the tunneling matrix elements of the electron-magnon interaction.
These matrix elements are derived from the Coulomb exchange interaction using
the itinerant-electron model of magnon-assisted tunneling. The results for the
inelastic tunneling spectra, based on the nonequilibrium Green's function
calculations, are presented for both parallel and antiparallel magnetizations
in the ferromagnetic leads.",0401460v6
2005-07-26,One-magnon states and electron spin resonance in spin-ladders with singlet-rung ground statein a staggered magnetic field,"One-magnon problem for spin-ladders with exact singlet-rung ground state in
arbitrary oriented constant and staggered magnetic fields is studied. For two
special configurations when the both fields are parallel or perpendicular to
each other the exact formulas for eigenstates and eigenvectors are presented.
In the parallel case the one-magnon contribution to the ESR absorption line
shape is obtained. The latter is non-Lorentzian, has finite range and a gap
around resonance. For low fields both the range of the spectrum and the width
of the gap are controlled by the width of the one-magnon zone in zero magnetic
field. Some other interactions usually contributing to the ESR line shape are
also briefly discussed.",0507619v1
2006-01-26,Spin dynamics characterization in magnetic dots,"The spin structure in a magnetic dot, which is an example of a quantum
few-body system, is studied as a function of exchange coupling strength and dot
size with in the semiclassical approximation on a discrete lattice. As the
exchange coupli ng is decreased or the size is increased, the ground state
undergoes a phase cha nge from a single domain ferromagnet to a spin vortex.
The line separating these two phases has been calculated numerically for small
system sizes. %, and analytically for larger dots. The dipolar interaction has
been fully included in our calculations. Magnon frequencies in such a dot have
also been calculated in both phases by the linearized equation of motion
method. These results have also been reproduced f rom the Fourier transform of
the spin autocorrelation function. From the magnon Density Of States (DOS), it
is possible to identify the magnetic phase of the dot. Furthermore, the magnon
modes have been characterized for both the ferromagnetic and the vortex phase,
and the magnon instability mechanism leading to the vortex-ferro transition has
also been identified. The results can also be used to compute finite
temperature magnetization or vort icity of magnetic dots.",0601624v1
2006-06-26,From the Hubbard Model to a Systematic Low-Energy Effective Field Theory for Magnons and Holes in an Antiferromagnet,"The low-energy physics of antiferromagnets is governed by their Goldstone
bosons -- the magnons -- and it is described by a low-energy effective field
theory. In analogy to baryon chiral perturbation theory, we construct the
effective field theory for magnons and holes in an antiferromagnet. It is a
systematic low-energy expansion based on symmetry considerations and on the
fact that the holes are located in pockets centered at k=(pi/2a,\pm pi/2a).
Even though the symmetries are extracted from the Hubbard model, the effective
theory is universal and makes model-independent predictions about the dynamical
mechanisms in the antiferromagnetic phase. The low-energy effective theory has
been used to investigate one-magnon exchange which leads to a d-wave-shaped
bound state of holes.",0606662v1
2006-06-22,"Magnons, Classical Strings and beta-Deformations","Motivated by the recent work of Hofman and Maldacena we construct a classical
string solution on the beta-deformed AdS_5 \times \tilde{S}^5 background. This
string solution is identified with a magnon state of the integrable spin chain
description of the N=1 supersymmetric beta-deformed gauge theory. The string
solution carries two angular momenta, an infinite J_1 and a finite J_2 which
classically can take arbitrary values. It corresponds to the magnon of charge
J_2 propagating on an infinite spin chain. We derive an exact dispersion
relation for this magnon from string theory.",0606220v2
2006-07-06,Spiky strings and giant magnons on S5,"Recently, classical solutions for strings moving in AdS5 x S5 have played an
important role in understanding the AdS/CFT correspondence. A large set of them
were shown to follow from an ansatz that reduces the solution of the string
equations of motion to the study of a well-known integrable 1-d system known as
the Neumann-Rosochatius (NR) system. However, other simple solutions such as
spiky strings or giant magnons in S5 were not included in the NR ansatz. We
show that, when considered in the conformal gauge, these solutions can be also
accomodated by a version of the NR-system. This allows us to describe in detail
a giant magnon solution with two additional angular momenta and show that it
can be interpreted as a superposition of two magnons moving with the same
speed. In addition, we consider the spin chain side and describe the
corresponding state as that of two bound states in the infinite SU(3) spin
chain. We construct the Bethe ansatz wave function for such bound state.",0607044v3
2007-03-11,On the Singularities of the Magnon S-matrix,"We investigate the analytic structure of the magnon S-matrix in the
spin-chain description of planar ${\cal N}=4$ SUSY Yang-Mills/$AdS_{5}\times
S^{5}$ strings. Semiclassical analysis suggests that the exact S-matrix must
have a large family of poles near the real axis in momentum space. In this
article we show that these are double poles corresponding to the exchange of
pairs of BPS magnons. Their locations in the complex plane are uniquely fixed
by the known dispersion relation for the BPS particles. The locations precisely
agree with the recent conjecture for the $S$ matrix by Beisert, Hernandez,
Lopez, Eden and Staudacher (hep-th/0609044 and hep-th/0610251). These poles do
not signal the presence of new bound states. In fact, a certain non-BPS
localized classical solution, which was thought to give rise to new bound
states, can actually decay into a pair of BPS magnons.",0703104v2
2007-05-09,Dynamics of quantum spin liquid and spin solid phases in IPA-CuCl3 under field,"Inelastic and elastic neutron scattering is used to study spin correlations
in the quasi-one dimensional quantum antiferromagnet IPA-CuCl3 in strong
applied magnetic fields. A condensation of magnons and commensurate transverse
long-range ordering is observe at a critical filed $H_c=9.5$ T. The field
dependencies of the energies and polarizations of all magnon branches are
investigated both below and above the transition point. Their dispersion is
measured across the entire 1D Brillouin zone in magnetic fields up to 14 T. The
critical wave vector of magnon spectrum truncation [Masuda et al., Phys. Rev.
Lett. 96, 047210 (2006)] is found to shift from h_c 0.35 at H>H_c. A drastic
reduction of magnon bandwidths in the ordered phase [Garlea et al., Phys. Rev.
Lett. 98, 167202 (2007)] is observed and studied in detail. New features of the
spectrum, presumably related to this bandwidth collapse, are observed just
above the transition field.",0705.1360v1
2007-08-02,On the multi-spin magnon and spike solutions from membranes,"Recently important classes of solitonic string solutions were obtained -
giant magnons and single spikes. In previous study we showed the existence of
giant magnon-like membrane solutions and studied their properties. In this
paper we investigate classical rotating membranes representing analog of a
specific class of string spiky solutions. Using the reduction to the
Neumann-Rosochatius integrable system we find analog of the string single spike
solutions. In contrast to the magnon-like solutions, this case is characterized
with finite difference of energy and ``winding number'' and finite spins as
well.",0708.0325v3
2007-08-05,Bose-Einstein condensation of magnons in superfluid 3He,"The possibility of Bose-Einstein condensation of excitations has been
discussed for a long time. The phenomenon of the phase-coherent precession of
magnetization in superfluid 3He and the related effects of spin superfluidity
are based on the true Bose-Einstein condensation of magnons. Several different
states of coherent precession has been observed in 3He-B: homogeneously
precessing domain (HPD); persistent signal formed by Q-balls at very low
temperatures; coherent precession with fractional magnetization; and two new
modes of the coherent precession in compressed aerogel. In compressed aerogel
the coherent precession has been also found in 3He-A. Here we demonstrate that
all these cases are examples of a Bose-Einstein condensation of magnons, with
the magnon interaction term in the Gross-Pitaevskii equation being provided by
different types of spin-orbit coupling in the background of the coherent
precession.",0708.0663v1
2007-08-21,Magnons in Ferromagnetic Metallic Manganites,"Ferromagnetic (FM) manganites, a group of likely half-metallic oxides, are of
special interest not only because they are a testing ground of the classical
doubleexchange interaction mechanism for the colossal magnetoresistance, but
also because they exhibit an extraordinary arena of emergent phenomena. These
emergent phenomena are related to the complexity associated with strong
interplay between charge, spin, orbital, and lattice. In this review, we focus
on the use of inelastic neutron scattering to study the spin dynamics, mainly
the magnon excitations in this class of FM metallic materials. In particular,
we discussed the unusual magnon softening and damping near the Brillouin zone
boundary in relatively narrow band compounds with strong Jahn-Teller lattice
distortion and charge/orbital correlations. The anomalous behaviors of magnons
in these compounds indicate the likelihood of cooperative excitations involving
spin, lattice, as well as orbital degrees of freedom.",0708.2891v1
2007-10-15,Fermionic representation for the ferromagnetic Kondo lattice model -- diagrammatic study of spin-charge coupling effects on magnon excitations,"A purely fermionic representation is introduced for the ferromagnetic Kondo
lattice model which allows conventional diagrammatic tools to be employed to
study correlation effects. Quantum 1/S corrections to magnon excitations are
investigated using a systematic inverse-degeneracy expansion scheme which
incorporates correlation effects in the form of self-energy and vertex
corrections, while explicitly preserving the continuous spin-rotation symmetry.
Magnon self-energy is studied in the full range of interaction strength, and
shown to result in strong magnon damping and anomalous softening for zone
boundary modes, which accounts for several zone-boundary anomalies observed in
recent spin-wave measurements of ferromagnetic manganites.",0710.2828v1
2008-03-17,Interacting finite-size magnons,"We explicitly construct a large class of finite-volume two-magnon string
solutions moving on R x S^2. In particular, by making use of the relationship
between the O(3) sigma model and sine-Gordon theory we are able to find
solutions corresponding to the periodic analogues of magnon scattering and
breather-like solutions. After semi-classically quantizing these solutions we
invert the implicit expressions for the excitation energies in certain limits
and find the corrections for the multi-magnon states. For the breather-like
solutions we express the energies directly in terms of the action variable
whereas for the scattering solution we express the result as a combination of
corrections to the dispersion relation and to the scattering phase.",0803.2324v2
2008-05-08,Giant magnons in TsT-transformed AdS_5 x S^5,"We consider giant magnons propagating in a \gamma-deformed AdS_5 x S^5
background obtained from AdS_5 x S^5 by means of a chain of TsT
transformations. We point out that in the light-cone gauge and in the infinite
J limit the deformed and undeformed string models share the same magnon
dispersion relation, the \su(2|2)\oplus \su(2|2)-invariant world-sheet S-matrix
and the dressing factor. The \gamma-dependence in the limit is only due to
different level-matching conditions. We consider the reduction of the deformed
model to R x S^3 and determine the leading \gamma-dependence of the dispersion
relation for a finite J giant magnon.",0805.1070v2
2008-12-24,Design and optimization of one-dimensional ferrite-film based magnonic crystals,"One-dimensional magnonic crystals have been implemented as gratings of
shallow grooves chemically etched into the surface of yttrium-iron garnet
films. Scattering of backward volume magnetostatic spin waves from such
structures is investigated experimentally and theoretically. Well-defined
rejection frequency bands are observed in transmission characteristics of the
magnonic crystals. The loss inserted by the gratings and the rejections bands
bandwidths are studied as a function of the film thickness, the groove depth,
the number of grooves, and the groove width. The experimental data are well
described by a theoretical model based on the analogy of a spin-wave
film-waveguide with a microwave transmission line. Our study shows that
magnonic crystals with required operational characteristics can be engineered
by adjusting these geometrical parameters.",0812.4553v1
2009-04-02,S-duality and the giant magnon dispersion relation,"We use S-duality and planarity to propose an argument for the
non-renormalization of the dispersion relation of giant magnon solutions in
type IIB string theory on AdS_5 x S^5. We compute the spectrum of giant magnons
for (p,q)-strings from field theory at strong coupling by using the central
charge properties of electrically and magnetically charged supersymmetric
states in the Coulomb branch of N=4 super Yang-Mills. We argue that the
coupling dependence of the giant magnon dispersion relation conjectured in the
literature using integrability assumptions is in fact the only functional
dependence compatible with S-duality.",0904.0444v3
2009-04-09,Evaluating the locality of intrinsic precession damping in transition metals,"The Landau-Lifshitz-Gilbert damping parameter is typically assumed to be a
local quantity, independent of magnetic configuration. To test the validity of
this assumption we calculate the precession damping rate of small amplitude
non-uniform mode magnons in iron, cobalt, and nickel. At scattering rates
expected near and above room temperature, little change in the damping rate is
found as the magnon wavelength is decreased from infinity to a length shorter
than features probed in recent experiments. This result indicates that
non-local effects due to the presence of weakly non-uniform modes, expected in
real devices, should not appreciably affect the dynamic response of the element
at typical operating temperatures. Conversely, at scattering rates expected in
very pure samples around cryogenic temperatures, non-local effects result in an
order of magnitude decrease in damping rates for magnons with wavelengths
commensurate with domain wall widths. While this low temperature result is
likely of little practical importance, it provides an experimentally testable
prediction of the non-local contribution of the spin-orbit torque-correlation
model of precession damping. None of these results exhibit strong dependence on
the magnon propagation direction.",0904.1455v1
2009-04-24,Magnon BEC and Spin Superfluidity: a 3He primer,"Bose-Einstein condensation (BEC) is a quantum phenomenon of formation of the
collective quantum state, in which the macroscopic number of particles occupies
the lowest energy state and thus is governed by a single wave function. Here we
highlight the BEC in a magnetic subsystem -- the BEC of magnons, elementary
magnetic excitations. Magnon BEC is manifested as the spontaneously emerging
state of the precessing spins, in which all spins precess with the same
frequency and phase even in the inhomogeneous magnetic field. We consider this
phenomenon on example of spin precession in superfluid phases of $^3$He. The
magnon BEC in these phases has all the properties of spin superfluidity. The
states of the phase-coherent precession belong to the class of the coherent
quantum states, which manifest themselves by superfluidity, superconductivity,
quantum Hall effect, Josephson effect and many other macroscopic quantum
phenomena.",0904.3889v1
2009-05-15,A New and Elementary CP^n Dyonic Magnon,"We show that the dressing transformation method produces a new type of dyonic
CP^n magnon in terms of which all the other known solutions are either
composites or arise as special limits. In particular, this includes the
embedding of Dorey's dyonic magnon via an RP^3 subspace of CP^n. We also show
how to generate Dorey's dyonic magnon directly in the S^n sigma model via the
dressing method without resorting to the isomorphism with the SU(2) principle
chiral model when n=3. The new dyon is shown to be either a charged dyon or
topological kink of the related symmetric-space sine-Gordon theories associated
to CP^n and in this sense is a direct generalization of the soliton of the
complex sine-Gordon theory.",0905.2534v2
2009-10-28,"Orbital fluctuations, spin-orbital coupling, and anomalous magnon softening in an orbitally degenerate ferromagnet","The correlated motion of electrons in the presence of strong orbital
fluctuations and correlations is investigated with respect to magnetic
couplings and excitations in an orbitally degenerate ferromagnet. Introduction
of the orbital degree of freedom results in a class of diagrams representing
spin-orbital coupling which become particularly important near the orbital
ordering instability. Low-energy staggered orbital fluctuation modes,
particularly with momentum near (\pi/2,\pi/2,0) (corresponding to CE-type
orbital correlations), are shown to generically yield strong intrinsically
non-Heisenberg (1-\cos q)^2 magnon self energy correction, resulting in no spin
stiffness reduction, but strongly suppressed zone-boundary magnon energies in
the Gamma-X direction. The zone-boundary magnon softening is found to be
strongly enhanced with increasing hole doping and for narrow-band materials,
which provides insight into the origin of zone-boundary anomalies observed in
ferromagnetic manganites.",0910.5321v2
2009-11-09,Giant Spinons,"We study the spectrum around the ""antiferromagnetic"" states of the planar
AdS_5/CFT_4 duality. In contrast to the familiar large-spin limit J \to \infty
where each magnon momentum scales as p \sim 1/J << 1, we consider a novel
""large-winding"" limit in which the total momentum becomes infinitely large,
\sum_j p_j \to \infty. Upon taking the limit we identify ""spinon"" excitations
of both gauge and string theories. In particular, a (classical) string spinon
turns out to be an infinite set of spiky strings, which are closely related to
well-known infinite-spin strings: giant magnons. Furthermore, we show that the
curious agreement of scattering phase-shifts of two spikes and that of two
giant magnons can be accounted for by regarding the spinon scattering as
factorised scatterings of infinitely many magnons.",0911.1528v2
2010-02-08,Self-localization of magnon Bose-Einstein condensates in the ground state and on excited levels: from harmonic to box-like trapping potential,"Long-lived coherent spin precession of 3He-B at low temperatures around 0.2
Tc is a manifestation of Bose-Einstein condensation of spin-wave excitations or
magnons in a magnetic trap which is formed by the order-parameter texture and
can be manipulated experimentally. When the number of magnons increases, the
orbital texture reorients under the influence of the spin-orbit interaction and
the profile of the trap gradually changes from harmonic to a square well, with
walls almost impenetrable to magnons. This is the first experimental example of
Bose condensation in a box. By selective rf pumping the trap can be populated
with a ground-state condensate or one at any of the excited energy levels. In
the latter case the ground state is simultaneously populated by relaxation from
the exited level, forming a system of two coexisting condensates.",1002.1674v3
2011-02-01,Low Temperature Dynamics of Magnons in a Spin-1/2 Ladder Compound,"We have used a combination of neutron resonant spin-echo and triple-axis
spectroscopies to determine the energy, fine structure, and linewidth of the
magnon resonance in the model spin-1/2 ladder antiferromagnet IPA-CuCl_3 at
temperatures T << Delta_0 /k_B, where Delta_0 is the spin gap at T=0. In this
low-temperature regime we find that the results deviate substantially from the
predictions of the non-linear sigma model proposed as a description of magnon
excitations in one-dimensional quantum magnets and attribute these deviations
to real-space and spin-space anisotropies in the spin Hamiltonian as well as
scattering of magnon excitations from a dilute density of impurities. These
effects are generic to experimental realizations of one-dimensional quantum
magnets.",1102.0214v1
2011-04-08,Low-temperature heat transport of Nd_2CuO_4: Roles of Nd magnons and spin-structure transitions,"We report the magnetic-field dependence of thermal conductivity (\kappa) of
an insulating cuprate Nd_2CuO_4 at very low temperatures down to 0.3 K. It is
found that apart from the paramagnetic moments scattering on phonons, the
Nd^{3+} magnons can act as either heat carriers or phonon scatterers, which
strongly depends on the long-range antiferromagnetic transition and the
field-induced transitions of spin structure. In particular, the Nd^{3+} magnons
can effectively transport heat in the spin-flopped state of the Nd^{3+}
sublattice. However, both the magnon transport and the magnetic scattering are
quenched at very high fields. The spin re-orientations under the in-plane field
can be conjectured from the detailed field dependence of \kappa.",1104.1514v1
2011-06-14,Two-magnon Raman scattering in A$_{0.8}$Fe$_{1.6}$Se$_2$ systems: competition between superconductivity and antiferromagnetic order,"We have performed Raman-scattering measurements on high-quality single
crystals of A$_{0.8}$Fe$_{1.6}$Se$_2$ superconductors of several compositions.
We find a broad, asymmetric peak around 1600 cm$^{-1}$ (200 meV), which we
identify as a two-magnon process involving optical magnons. The intensity of
the two-magnon peak falls sharply on entering the superconducting phase. This
effect, which is entirely absent in the non-superconducting system
KFe$_{1.5}$Se$_2$, requires a strong mutual exclusion between
antiferromagnetism and superconductivity arising from proximity effects within
regions of microscale phase separation.",1106.2706v2
2011-08-11,Thermoelectric Spin-Transfer Torque MRAM with Sub-Nanosecond Bi-Directional Writing using Magnonic Current,"A new genre of Spin-Transfer Torque (STT) MRAM is proposed, in which
bi-directional writing is achieved using thermoelectrically controlled magnonic
current as an alternative to conventional electric current. The device uses a
magnetic tunnel junction (MTJ), which is adjacent to a non-magnetic metallic
and a ferrite film. This film stack is heated or cooled by a Peltier element
which creates a bi-directional magnonic pulse in the ferrite film. Conversion
of magnons to spin current occurs at the ferrite-metal interface, and the
resulting spin-transfer torque is used to achieve sub-nanosecond precessional
switching of the ferromagnetic free layer in the MTJ. Compared to electric
current driven STT-MRAM with perpendicular magnetic anisotropy (PMA),
thermoelectric STT-MRAM reduces the overall magnetization switching energy by
more than 40% for nano-second switching, combined with a write error rate (WER)
of less than 10-9 and a lifetime of 10 years or higher. The combination of
higher thermal activation energy, sub-nanosecond read/write speed, improved
tunneling magneto-resistance (TMR) and tunnel barrier reliability make
thermoelectric STT-MRAM a promising choice for future non-volatile memory
applications.",1108.2386v1
2011-12-24,Frustrated spin chains in strong magnetic field: dilute two-component Bose gas regime,"We study the ground state of frustrated spin-S chains in a strong magnetic
field in the immediate vicinity of saturation. In strongly frustrated chains,
the magnon dispersion has two degenerate minima at inequivalent momenta $\pm
Q$, and just below the saturation field the system can be effectively
represented as a dilute one-dimensional lattice gas of two species of bosons
that correspond to magnons with momenta around $\pm Q$. We present a theory of
effective interactions in such a dilute magnon gas that allows us to make
quantitative predictions for arbitrary values of the spin. With the help of
this method, we are able to establish the magnetic phase diagram of frustrated
chains close to saturation and study phase transitions between several
nontrivial states, including a two-component Luttinger liquid, a vector chiral
phase, and phases with bound magnons. We study those phase transitions
numerically and find a good agreement with our analytical predictions.",1112.5720v2
2012-02-26,"Magnons and crystal field transitions in hexagonal REMnO3 (RE = Er, Tm, Yb, Lu) single crystals","Far-infrared optical transmission spectra of the antiferromagnetic
resonances, or magnons, and crystal field transitions have been studied in
hexagonal REMnO3 (RE = Er, Tm, Yb, Lu) single crystals. The magnon and CF
frequencies, their oscillator strengths, and effective g-factors have been
measured using external magnetic fields up to 9 T in the temperature range
between 1.5 K and 100 K. The magnon frequency increases systematically with a
decrease of the RE ion radius. The magnetic ordering of RE ions (RE=Er, Tm, Yb)
was observed at low temperatures T<3.5 K and in strong external magnetic
fields. The observed effects are analyzed taking into account main magnetic
interactions in the system including exchange of the Mn3+ spins with RE3+
paramagnetic moments.",1202.5803v1
2012-06-20,Lifetime of Gapped Excitations in a Collinear Quantum Antiferromagnet,"We demonstrate that local modulations of magnetic couplings have a profound
effect on the temperature dependence of the relaxation rate of optical magnons
in a wide class of antiferromagnets in which gapped excitations coexist with
acoustic spin waves. In a two-dimensional collinear antiferromagnet with an
easy-plane anisotropy, the disorder-induced relaxation rate of the gapped mode,
Gamma_imp=Gamma_0+A(TlnT)^2, greatly exceeds the magnon-magnon damping,
Gamma_m-m=BT^5, negligible at low temperatures. We measure the lifetime of
gapped magnons in a prototype XY antiferromagnet BaNi2(PO4)2 using a
high-resolution neutron-resonance spin-echo technique and find experimental
data in close accord with the theoretical prediction. Similarly strong effects
of disorder in the three-dimensional case and in noncollinear antiferromagnets
are discussed.",1206.4690v2
2012-08-30,Efimov effect in quantum magnets,"Physics is said to be universal when it emerges regardless of the underlying
microscopic details. A prominent example is the Efimov effect, which predicts
the emergence of an infinite tower of three-body bound states obeying discrete
scale invariance when the particles interact resonantly. Because of its
universality and peculiarity, the Efimov effect has been the subject of
extensive research in chemical, atomic, nuclear and particle physics for
decades. Here we employ an anisotropic Heisenberg model to show that collective
excitations in quantum magnets (magnons) also exhibit the Efimov effect. We
locate anisotropy-induced two-magnon resonances, compute binding energies of
three magnons and find that they fit into the universal scaling law. We propose
several approaches to experimentally realize the Efimov effect in quantum
magnets, where the emergent Efimov states of magnons can be observed with
commonly used spectroscopic measurements. Our study thus opens up new avenues
for universal few-body physics in condensed matter systems.",1208.6214v2
2012-09-25,Ultrafast magnon-transistor at room temperature,"We study sequential tunneling of magnetic excitations in nonitinerant systems
(either magnons or spinons) through triangular molecular magnets. It is known
that the quantum state of such molecular magnets can be controlled by
application of an electric- or a magnetic field. Here, we use this fact to
control the flow of a spin current through the molecular magnet by electric- or
magnetic means. This allows us to design a system that behaves as a
magnon-transistor. We show how to combine three magnon-transistors to form a
NAND-gate, and give several possible realizations of the latter, one of which
could function at room temperature using transistors with a 11 ns switching
time.",1209.5594v2
2013-02-25,Phase Diagram for Magnon Condensate in Yttrium Iron Garnet Film,"Recently, magnons, which are quasiparticles describing the collective motion
of spins, were found to undergo Bose-Einstein condensation (BEC) at room
temperature in films of Yttrium Iron Garnet (YIG). Unlike other quasiparticle
BEC systems, this system has a spectrum with two degenerate minima, which makes
it possible for the system to have two condensates in momentum space. Recent
Brillouin Light scattering studies for a microwave-pumped YIG film of thickness
d=5 $\mu$m and field H=1 kOe find a low-contrast interference pattern at the
characteristic wavevector $Q$ of the magnon energy minimum. In this report, we
show that this modulation pattern can be quantitatively explained as due to
non-symmetric but coherent Bose-Einstein condensation of magnons into the two
energy minima. Our theory predicts a transition from a high-contrast symmetric
phase to a low-contrast non-symmetric phase on varying the $d$ and $H$, and a
new type of collective oscillations.",1302.6128v1
2013-02-27,Specific heat and thermal conductivity of ferromagnetic magnons in Yttrium Iron Garnet,"The specific heat and thermal conductivity of the insulating ferrimagnet
Y$_3$Fe$_5$O$_{12}$ (Yttrium Iron Garnet, YIG) single crystal were measured
down to 50 mK. The ferromagnetic magnon specific heat $C$$_m$ shows a
characteristic $T^{1.5}$ dependence down to 0.77 K. Below 0.77 K, a downward
deviation is observed, which is attributed to the magnetic dipole-dipole
interaction with typical magnitude of 10$^{-4}$ eV. The ferromagnetic magnon
thermal conductivity $\kappa_m$ does not show the characteristic $T^2$
dependence below 0.8 K. To fit the $\kappa_m$ data, both magnetic defect
scattering effect and dipole-dipole interaction are taken into account. These
results complete our understanding of the thermodynamic and thermal transport
properties of the low-lying ferromagnetic magnons.",1302.6739v1
2013-04-26,A micro-structured ion-implanted magnonic crystal,"We investigate spin-wave propagation in a microstructured magnonic-crystal
waveguide fabricated by localized ion implantation. The irradiation caused a
periodic variation in the saturation magnetization along the waveguide. As a
consequence, the spin-wave transmission spectrum exhibits a set of frequency
bands, where spin-wave propagation is suppressed. A weak modification of the
saturation magnetization by 7% is sufficient to decrease the spin-wave
transmission in the band gaps by a factor of 10. These results evidence the
applicability of localized ion implantation for the fabrication of efficient
micron- and nano-sized magnonic crystals for magnon spintronic applications.",1304.7122v1
2013-05-21,Effect of Hole Shape on Spin-Wave Band Structure in One-Dimensional Magnonic Antidot Waveguide,"We present the possibility of tuning the spin-wave band structure,
particularly the bandgaps in a nanoscale magnonic antidot waveguide by varying
the shape of the antidots. The effects of changing the shape of the antidots on
the spin-wave dispersion relation in a waveguide have been carefully monitored.
We interpret the observed variations by analysing the equilibrium magnetic
configuration and the magnonic power and phase distribution profiles during
spin-wave dynamics. The inhomogeneity in the exchange fields at the antidot
boundaries within the waveguide is found to play a crucial role in controlling
the band structure at the discussed length scales. The observations recorded
here will be important for future developments of magnetic antidot based
magnonic crystals and waveguides.",1305.4812v2
2013-11-26,Magnon breakdown in a two dimensional triangular lattice Heisenberg antiferromagnet of multiferroic LuMnO$_3$,"The breakdown of magnons, the quasiparticles of magnetic systems, has rarely
been seen. By using an inelastic neutron scattering technique we report the
observation of spontaneous magnon decay in multiferroic LuMnO$_3$, a simple
two-dimensional Heisenberg triangular lattice antiferromagnet, with large spin,
S = 2. The origin of this rare phenomenon lies in the non-vanishing cubic
interaction between magnons in the spin Hamiltonian arising from the
noncollinear 120$^o$ spin structure. We observed all three key features of the
nonlinear effects as theoretically predicted: a roton-like minimum, a flat
mode, and a linewidth broadening, in our inelastic neutron scattering
measurements of single crystal LuMnO$_3$. Our results show that quasiparticles
in a system hitherto thought of as ""classical"" can indeed break down.",1311.6652v1
2014-01-05,Magnon-Mediated Dzyaloshinskii-Moriya Torque in Homogeneous Ferromagnets,"In thin magnetic layers with structural inversion asymmetry and spin-orbit
coupling, a Dzyaloshinskii-Moriya interaction arises at the interface. When a
spin wave current ${\bf j}_m$ flows in a system with a homogeneous
magnetization {\bf m}, this interaction produces an effective field-like torque
on the form ${\bf T}_{\rm FL}\propto{\bf m}\times({\bf z}\times{\bf j}_m)$ as
well as a damping-like torque, ${\bf T}_{\rm DL}\propto{\bf m}\times[({\bf
z}\times{\bf j}_m)\times{\bf m}]$ in the presence of spin-wave relaxation
(${\bf z}$ is normal to the interface). These torques mediated by the magnon
flow can reorient the time-averaged magnetization direction and display a
number of similarities with the torques arising from the electron flow in a
magnetic two dimensional electron gas with Rashba spin-orbit coupling. This
magnon-mediated spin-orbit torque can be efficient in the case of magnons
driven by a thermal gradient.",1401.0883v2
2014-09-30,Direct observation of the Dzyaloshinskii-Moriya interaction in a Pt/Co/Ni film,"The interfacial Dzyaloshinskii-Moriya interaction (DMI) in an in-plane
anisotropic Pt(4nm)/Co(1.6nm)/Ni(1.6nm) film has been directly observed by
Brillouin spectroscopy. It is manifested in the asymmetry of the measured
magnon dispersion relation, from which the DMI constant has been evaluated.
Linewidth measurements reveal that the lifetime of the magnons is asymmetric
with respect to their counter-propagating directions. The lifetime asymmetry is
dependent on the magnon frequency, being more pronounced the higher the
frequency. Analytical calculations of the magnon dispersion relation and
linewidth agree well with experiments.",1409.8570v2
2014-11-14,Lifetimes of Magnons in Two-Dimensional Diluted Ferromagnetic Systems,"Spin dynamics in low dimensional magnetic systems has been of fundamental
importance for a long time and has currently received an impetus owing to the
emerging field of nanoelectronics. Knowledge of the spin wave lifetimes, in
particular, can be favorable for future potential applications. We investigate
the low-temperature spin wave excitations in two-dimensional disordered
ferromagnetic systems, with a particular focus on the long wavelength magnon
lifetimes. A semi-analytical Green's functions based approach is used to
determine the dynamical spectral functions, for different magnetic impurity
concentrations, from which the intrinsic linewidth is extracted. We obtain an
unambiguous $q^4$ scaling of the magnon linewidth which is ascribed to the
disorder induced damping of the spin waves, thereby settling a longstanding
unresolved issue on the wave-vector dependence. Our findings are also in good
agreement with previous theoretical studies on Heisenberg ferromagnets.
Additionally, we demonstrate the futility of using the low moments associated
with the spectral densities to evaluate the magnon dispersions and lifetimes.",1411.3876v2
2014-12-15,String theory in AdS_3 x S^3 x T^4 with mixed flux: semiclassical and 1-loop phase in the S-matrix,"We present a semiclassical derivation of the tree-level and 1-loop dressing
phases in the massive sector of string theory on AdS_3 x S^3 x T^4 supplemented
by R-R and NS-NS 3-form fluxes. In analogy with the AdS_5 x S^5 case, we use
the dressing method to obtain scattering solutions for dyonic giant magnons
which allows us to determine the semiclassical bound-state S-matrix and its
1-loop correction. We also find that the 1-loop correction to the dyonic giant
magnon energy vanishes. Looking at the relation between the bound-state picture
and elementary magnons in terms of the fusion procedure we deduce the
elementary dressing phases. In both the semiclassical and 1-loop cases we find
agreement with recent proposals from finite-gap equations and unitarity cut
methods. Further, we find consistency with the finite-gap picture by
determining the resolvent for the dyonic giant magnon from the semiclassical
bosonic scattering data.",1412.4764v3
2015-01-14,A spin-wave logic gate based on a width-modulated dynamic magnonic crystal,"An electric current controlled spin-wave logic gate based on a
width-modulated dynamic magnonic crystal is realized. The device utilizes a
spin-wave waveguide fabricated from a single-crystal Yttrium Iron Garnet film
and two conducting wires attached to the film surface. Application of electric
currents to the wires provides a means for dynamic control of the effective
geometry of the waveguide and results in a suppression of the magnonic band
gap. The performance of the magnonic crystal as an AND logic gate is
demonstrated.",1501.03486v2
2015-01-17,Kondo dynamics in one-dimensional doped ferromagnetic insulators,"Some well-established examples of itinerant-electron ferromagnetism in one
dimension occur in a Mott-insulating phase. We examine the consequences of
doping a ferromagnetic insulator and cou- pling magnons to gapless charge
fluctuations. Using a bosonization scheme for strongly interacting electrons,
we derive an effective field theory for the magnon-holon interaction. When the
magnon momentum matches the Fermi momentum of the holons, the backscattering of
the magnon at low energies gives rise to a Kondo effect of a pseudospin defined
from the chirality degree of freedom (right- or left-moving particles). The
crossover between weak-coupling and strong-coupling fixed points of the
effective mobile-impurity model is then investigated using a numerical
renormalization group approach.",1501.04220v1
2015-02-03,Probing single magnon excitations in Sr$_2$IrO$_4$ using O $K$-edge resonant inelastic X-ray scattering,"Resonant inelastic X-ray scattering (RIXS) at the $L$-edge of transition
metal elements is now commonly used to probe single magnon excitations. Here we
show that single magnon excitations can also be measured with RIXS at the
$K$-edge of the surrounding ligand atoms when the center heavy metal elements
have strong spin-orbit coupling. This is demonstrated with oxygen $K$-edge RIXS
experiments on the perovskite Sr$_2$IrO$_4$, where low energy peaks from single
magnon excitations were observed. This new application of RIXS has excellent
potential to be applied to a wide range of magnetic systems based on heavy
elements, for which the $L$-edge RIXS energy resolutions in the hard X-ray
region is usually poor.",1502.00752v1
2015-02-22,Excitation and detection of propagating spin waves at the single magnon level,"Ferro- and ferrimagnets play host to small-signal, microwave-frequency
magnetic excitations called spin waves, the quanta of which are known as
magnons. Over the last decade, the field of spin-wave dynamics has contributed
much to our understanding of fundamental magnetism. To date, experiments have
focussed overwhelmingly on the study of room-temperature systems within
classical limits. Here we demonstrate, for the first time, the excitation and
detection of propagating spin waves at the single magnon level. Our results
allow us to project that coupling of propagating spin-wave excitations to
quantum circuits is achievable, enabling fundamental quantum-level studies of
magnon systems and potentially opening doors to novel hybrid quantum
measurement and information processing devices.",1502.06263v1
2015-03-25,Effect of broken symmetry on resonant inelastic x-ray scattering from undoped cuprates,"We study the magnetic excitation spectra of resonant inelastic x-ray
scattering (RIXS) at the $L$-edge from undoped cuprates beyond the fast
collision approximation. We analyse the effect of the symmetry breaking ground
state on the RIXS process of the Heisenberg model by using a projection
procedure. We derive the expressions of the scattering amplitude in both
one-magnon and two-magnon excitation channels. Each of them consists of the
isotropic and anisotropic contributions. The latter is a new finding and
attributed to the long range order of the ground state. The presence of
anisotropic terms is supported by numerical calculations on a two-dimensional
spin cluster. We express the RIXS spectra in the form of spin-correlation
functions with the coefficients evaluated on the cluster, and calculate the
function in a two dimensional system within the $1/S$ expansion. Due to the
anisotropic terms, the spectral intensities are considerably enhanced around
momentum transfer $\textbf{q}=0$ in both one-magnon and two-magnon excitation
channels. This finding may be experimentally confirmed by examining carefully
the $\textbf{q}$-dependence of the spectra.",1503.07264v1
2015-04-06,Spin Pumping in Electrodynamically Coupled Magnon-Photon Systems,"We use electrical detection, in combination with microwave transmission, to
investigate both resonant and non-resonant magnon-photon coupling at room
temperature. Spin pumping in a dynamically coupled magnon-photon system is
found to be distinctly different from previous experiments. Characteristic
coupling features such as modes anti-crossing, line width evolution, peculiar
line shape, and resonance broadening are systematically measured and
consistently analyzed by a theoretical model set on the foundation of classical
electrodynamic coupling. Our experimental and theoretical approach pave the way
for pursuing microwave coherent manipulation of pure spin current via the
combination of spin pumping and magnon-photon coupling.",1504.01335v1
2015-05-05,Spin pumping from a ferromagnet into a hopping insulator: the role of resonant absorption of magnons,"Motivated by recent experiments on spin pumping from a ferromagnet into
organic materials in which the charge transport is due to hopping, we study
theoretically the generation and propagation of spin current in a hopping
insulator. Unlike metals, the spin polarization at the boundary with
ferromagnet is created as a result of magnon absorption within pairs of
localized states and it spreads following the current-currying resistor network
(although the charge current is absent). We consider a classic resonant
mechanism of the ac absorption in insulators and adapt it to the absorption of
magnons. A strong enhancement of pumping efficiency is predicted when the
Zeeman splitting of the localized states in external magnetic field is equal to
the frequency of ferromagnetic resonance. Under this condition the absorption
of a magnon takes place within individual sites.",1505.01211v1
2015-06-17,Anomalous Dimensions of Heavy Operators from Magnon Energies,"We study spin chains with boundaries that are dual to open strings suspended
between systems of giant gravitons and dual giant gravitons. The anomalous
dimensions computed in the gauge theory are in complete quantitative agreement
with energies computed in the dual string theory. The comparison makes use of a
description in terms of magnons, generalizing results for a single maximal
giant graviton. The symmetries of the problem determine the structure of the
magnon boundary reflection/scattering matrix up to a phase. We compute a
reflection/scattering matrix element at weak coupling and verify that it is
consistent with the answer determined by symmetry. We find the
reflection/scattering matrix does not satisfy the boundary Yang-Baxter equation
so that the boundary condition on the open spin chain spoils integrability. We
also explain the interpretation of the double coset ansatz in the magnon
language.",1506.05224v2
2015-06-17,Magnetic field control of the spin Seebeck effect,"The origin of the suppression of the longitudinal spin Seebeck effect by
applied magnetic fields is studied. We perform numerical simulations of the
stochastic Landau-Lifshitz-Gilbert equation of motion for an atomistic spin
model and calculate the magnon accumulation in linear temperature gradients for
different strengths of applied magnetic fields and different length scales of
the temperature gradient. We observe a decrease of the magnon accumulation with
increasing magnetic field and we reveal that the origin of this effect is a
field dependent change of the frequency distribution of the propagating
magnons. With increasing field the magnonic spin currents are reduced due to a
suppression of parts of the frequency spectrum. By comparison with measurements
of the magnetic field dependent longitudinal spin Seebeck effect in YIG thin
films with various thicknesses, we find that our model describes the
experimental data very well, demonstrating the importance of this effect for
experimental systems.",1506.05290v1
2015-10-12,Spin wave approach to the two-magnon Raman scattering in an J1x-J1y-J2-Jc antiferromagnetic Heisenberg model,"We study the two-magnon non-resonant Raman scattering in the (pi,pi) and
(pi,0) ordered antiferromagnetic phases of a J1x-J1y-J2-Jc Heisenberg model on
the tetragonal lattice within the framework of the spin-wave theory. We discuss
the effects of various tuning factors to the two-magnon Raman spectra. We find
that both the magnetic frustration J2/J1 and the interlayer exchange coupling
Jc may significantly affect the spectra in both the B1g and A1g' channels in
the (pi,pi) Neel ordered phase. Moreover, we find a splitting of the two-magnon
peak in the (pi,0) antiferromagnetic phase. We further discuss the implications
of our results to the BaMnBi2 and iron pnictide systems.",1510.03359v1
2016-05-31,Electric field control of magnon-induced magnetization dynamics in multiferroics,"We consider theoretically the effect of an inhomogeneous magnetoelectric
coupling on the magnon-induced dynamics of a ferromagnet. The magnon-mediated
magnetoelectric torque affects both the homogeneous magnetization and
magnon-driven domain wall motion. In the domains, we predict a reorientation of
the magnetization, controllable by the applied electric field, which is almost
an order of magnitude larger than that observed in other physical systems via
the same mechanism. The applied electric field can also be used to tune the
domain wall speed and direction of motion in a linear fashion, producing domain
wall velocities several times the zero field velocity. These results show that
multiferroic systems offer a promising arena to achieve low-dissipation
magnetization rotation and domain wall motion by exciting spin-waves.",1606.00007v2
2016-06-23,Electromagnon dispersion probed by inelastic x-ray scattering,"Inelastic x-ray scattering with meV energy resolution (IXS) is an ideal tool
to measure collective excitations in solids and liquids. In non-resonant
scattering condition, the cross section is strongly dominated by lattice
vibrations (phonons). However, it is possible to probe additional degrees of
freedom such as magnetic fluctuations that are strongly coupled to the phonons.
The IXS spectrum of the coupled system will contain not only the phonon
dispersion (majority component) but also the so far undetected magnetic
correlation function (minority component). Here we report the discovery of
strong magnon-phonon coupling in LiCrO$_2$ that enables the measurement of
magnetic correlations throughout the Brillouin-zone via IXS. We found
electromagnon excitations and electric dipole active two-magnon excitations in
the magnetically ordered phase and paraelectromagnons in the paramagnetic phase
of LiCrO$_2$. We predict that the numerous group of (frustrated) magnets with
dominant direct exchange and non-collinear magnetism shows similarly strong
coupling and surprisingly large and measurable IXS cross section for magnons
and multi-magnon processes.",1606.07321v1
2016-07-27,Voltage-Controlled Nano-Scale Reconfigurable Magnonic Crystal,"A nano-scale reconfigurable magnonic crystal is designed using
voltage-controlled perpendicular magnetic anisotropy (PMA) in
ferromagnetic-dielectric hetero-structures. A periodic array of gate metallic
stripes is placed on top of a MgO/Co structure in order to apply a periodic
electric field and to modify the PMA in Co. It is numerically demonstrated that
the application of the voltage to the gate stripes modifies the spin-wave
propagation and leads to the formation of band gaps in the spin-wave spectrum.
The band gaps are dynamically controllable, i.e. it is possible to switch band
gaps on and off within a few nanoseconds. The width and the center frequency of
the band gaps is defined by the applied voltage. At last, it is shown that the
application of the voltage to selected, rather than to all gate stripes allows
for a pre-defined modification of the band gap spectra. The proposed
voltage-controlled reconfigurable magnonic crystal opens a new way to low power
consumption magnonic applications.",1607.07975v1
2017-02-08,Magnon-polaron transport in magnetic insulators,"We theoretically study the effects of strong magnetoelastic coupling on the
transport properties of magnetic insulators. We develop a Boltzmann transport
theory for the mixed magnon-phonon modes (magnon polarons) and determine
transport coefficients and spin diffusion length. Magnon-polaron formation
causes anomalous features in the magnetic field and temperature dependence of
the spin Seebeck effect when the disorder scattering in the magnetic and
elastic subsystems is sufficiently different. Experimental data by Kikkawa et
al. [PRL 117, 207203 (2016)] on yttrium iron garnet films can be explained by
an acoustic quality that is much better than the magnetic quality of the
material. We predict similar anomalous features in the spin and heat
conductivity and non-local spin transport experiments.",1702.02270v1
2017-01-30,Ultrafast optical excitation of coherent magnons in antiferromagnetic NiO,"In experiment and theory, we resolve the mechanism of ultrafast optical
magnon excitation in antiferromagnetic NiO. We employ time-resolved optical
two-color pump-probe measurements to study the coherent non-thermal spin
dynamics. Optical pumping and probing with linearly and circularly polarized
light along the optic axis of the NiO crystal scrutinizes the mechanism behind
the ultrafast optical magnon excitation. A phenomenological symmetry-based
theory links these experimental results to expressions for the optically
induced magnetization via the inverse Faraday effect and the inverse
Cotton-Mouton effect. We obtain striking agreement between experiment and
theory that, furthermore, allows us to extract information about the spin
domain distribution. We also find that in NiO the energy transfer into the
magnon mode via the inverse Cotton-Mouton effect is about three orders of
magnitude more efficient than via the inverse Faraday effect.",1702.05666v1
2017-03-18,Entanglement of condensed magnons via momentum-space fragmentation,"A scheme is presented for engineering momentum-space entanglement of
fragmented magnon condensates. We consider easy plane frustrated
antiferromagnets in which the magnon dispersion has degenerate minima that
represent umbrella chiral spin textures. With an applied magnetic field, we
tune the Hamiltonian near a quantum critical point that is is signaled by a
singularity in the entanglement entropy. The ground state develops
momentum-space entanglement of the chiral spin textures. The size of the
entangled superposition is accessible experimentally through the magnetic
structure factor. Our model is motivated by equilibrium magnon condensates in
frustrated antiferromagnets such as CsCuCl3, and it can also be simulated in
spin-orbit coupled Mott insulators in atomic optical lattices and circuit
quantum electrodynamics.",1703.06232v1
2017-03-23,Magnon activation by hot electrons via non-quasiparticle states,"We consider the situation when a femtosecond laser pulse creates a hot
electron state in half-metallic ferromagnet (e. g. ferromagnetic semiconductor)
on a picosecond timescale but do not act directly on localized spin system. We
show that the energy and magnetic moment transfer from hot itinerant electrons
to localized spins is facilitated by the so-called non-quasiparticle states,
which are the scattering states of a magnon and spin-majority electron. The
magnon distribution is described by a quantum kinetic equation that we derive
using the Keldysh diagram technique. In a typical ferromagnetic semiconductor
such as EuO magnons remain essentially in non-equilibrium on a scale of the
order of microsecond after the laser pulse.",1703.08106v1
2017-03-24,Dirac and nodal line magnons in three-dimensional antiferromagnets,"We study the topological properties of magnon excitations in
three-dimensional antiferromagnets, where the ground state configuration is
invariant under time-reversal followed by space-inversion ($PT$-symmetry). We
prove that Dirac points and nodal lines, the former being the limiting case of
the latter, are the generic forms of symmetry-protected band crossings between
magnon branches. As a concrete example, we study a Heisenberg spin model for a
""spin-web"" compound, Cu$_3$TeO$_6$, and show the presence of the magnon Dirac
points assuming a collinear magnetic structure. Upon turning on
symmetry-allowed Dzyaloshinsky-Moriya interactions, which introduce a small
non-collinearity in the ground state configuration, we find that the Dirac
points expand into nodal lines with nontrivial $Z_2$-topological charge, a new
type of nodal lines unpredicted in any materials so far.",1703.08545v3
2018-02-03,Magnon scattering in the transport coefficients of CoFe alloys,"Resistivity $\rho$, thermopower ${\cal S}$, and thermal conductivity $\kappa$
were measured simultaneously on a set of CoFe alloy films. Variation of the
Co-content $x_\mathrm{Co}$ allows for a systematic tuning of the Fermi level
through the band structure, and the study of the interplay between electronic
and magnetic contributions to the transport coefficients. While band structure
and magnon effects in $\rho$ and $\kappa$ are rather weak, they turn out to be
very significant in ${\cal S}$. The evolution of Mott and magnon drag
contributions to ${\cal S}$ is traced between the two limiting cases of pure Fe
and pure Co. In addition, we find an interesting sign change of the magnon
drag.",1802.01038v1
2018-02-12,Topological Magnons in Kitaev Magnets at High Fields,"We study the Kitaev-Heisenberg-$\Gamma$-$\Gamma'$ model that describes the
magnetism in strong spin-orbit coupled honeycomb lattice Mott insulators. In
strong $[111]$ magnetic fields that bring the system into the fully polarized
paramagnetic phase, we find that the spin wave bands carry nontrivial Chern
numbers over large regions of the phase diagram implying the presence of chiral
magnon edge states. In contrast to other topological magnon systems, the
topological nontriviality of these systems results from the presence of magnon
number non-conserving terms in the Hamiltonian. Since the effects of
interactions are suppressed by $J/h$, the validity of the single particle
picture is tunable making paramagnetic phases particularly suitable for the
exploration of this physics. Using time dependent DMRG and interacting spin
wave theory, we demonstrate the presence of the chiral edge mode and its
evolution with field.",1802.04283v1
2018-04-06,Magnons versus electrons in thermal spin transport through metallic interfaces,"We develop a theory for spin transport in magnetic metals that treats the
contribution of magnons and electrons on equal footing. As an application we
consider thermally-driven spin injection across an interface between a magnetic
metal and a normal metal, i.e., the spin-dependent Seebeck effect. We show that
the ratio between magnonic and electronic contribution scales as
$\sqrt{T/T_C}T_F/T_C$, with the Fermi temperature $T_F$ and the Curie
temperature $T_C$. Since, typically, $T_C \ll T_F$, the magnonic contribution
may dominate the thermal spin injection, even though the interface is more
transparent for electronic spin current.",1804.02172v1
2018-04-06,Thermoelastic enhancement of the magnonic spin Seebeck effect in thin films and bulk samples,"A non-uniform temperature profile may generate a pure spin current in
magnetic films, as observed for instance in the spin Seebeck effect. In
addition, thermally induced elastic deformations may set in that could affect
the spin current. A self-consistent theory of the magnonic spin Seebeck effect
including thermally activated magneto-elastic effects is presented and
analytical expressions for the thermally activated deformation tensor and
dispersion relations for coupled magneto-elastic modes are obtained. We derived
analytical results for bulk (3D) systems and thin magnetic (2D) films.
  We observed that the displacement vector and the deformation tensor in bulk
systems decay asymptotically as $u\sim1/R^{2}$ and $\varepsilon\sim1/R^{3}$,
respectively, while the decays in thin magnetic films proceed slower following
$u\sim1/R$ and $\varepsilon\sim1/R^{2}$. The dispersion relations evidence a
strong anisotropy in the magnetic excitations. We observed that a thermoelastic
steady state deformation may lead to both an enchantment or a reduction of the
gap in the magnonic spectrum. The reduction of the gap increases the number of
magnons contributing to the spin Seebeck effect and offers new possibilities
for the thermoelastic control of the Spin Seebeck effect.",1804.02185v1
2018-04-08,Optical cooling of magnons,"Inelastic scattering of light by spin waves generates an energy flow between
the light and magnetization fields, a process that can be enhanced and
controlled by concentrating the light in magneto-optical resonators. Here, we
model the cooling of a sphere made of a magnetic insulator, such as yttrium
iron garnet (YIG), using a monochromatic laser source. When the magnon
lifetimes are much larger than the optical ones, we can treat the latter as a
Markovian bath for magnons. The steady-state magnons are canonically
distributed with a temperature that is controlled by the light intensity. We
predict that such a cooling process can significantly reduce the temperature of
the magnetic order within current technology.",1804.02683v1
2018-11-14,Microscopic Theory of Ultrafast Out-of-Equilibrium Dynamics in Magnetic Insulators. Unraveling the Magnon-Phonon Coupling,"The interaction between lattice and spins is at the heart of an extremely
intriguing ultrafast dynamics in magnetic materials. In this work we formulate
a general non-equilibrium theory that disentangles the complex interplay
between them in a THz laser-excited antiferromagnetic insulator. The theory
provides a quantitative description of the transient energy flow between the
spin and lattice sub-systems, subject to magnon-phonon and phonon-phonon
scatterings, giving rise to finite life-times of the quasiparticles and to the
equilibration time of the system. We predict a novel kind of scattering process
where two magnons of opposite polarizations decay into a phonon, previously
omitted in the literature. The theory is combined with first-principle
calculations and then applied to simulate a realistic dynamics of NiO. The main
relaxation channels and hot spots in the reciprocal space, giving the strongest
contribution to the energy transfer between phonons and magnons are identified.
The diverse interaction strengths lead to distinct coupled dynamics of the
lattice and spin systems and subsequently to different equilibration
timescales.",1811.05841v1
2013-08-09,Relativistic and thermal effects on the magnon spectrum of a ferromagnetic monolayer,"A spin model including magnetic anisotropy terms and Dzyaloshinsky-Moriya
interactions is studied for the case of a ferromagnetic monolayer with C2v
symmetry like Fe/W(110). Using the quasiclassical stochastic
Landau-Lifshitz-Gilbert equations, the magnon spectrum of the system is derived
using linear response theory. The Dzyaloshinsky-Moriya interaction leads to
asymmetry in the spectrum, while the anisotropy terms induce a gap. It is shown
that in the presence of lattice defects, both the Dzyaloshinsky-Moriya
interactions and the two-site anisotropy lead to a softening of the magnon
energies. Two methods are developed to investigate the magnon spectrum at
finite temperatures. The theoretical results are compared to atomistic spin
dynamics simulations and a good agreement is found between them.",1308.2082v2
2014-03-02,Generalized two-temperature model for coupled phonon-magnon diffusion,"We generalize the two-temperature model [Sanders and Walton, Phys. Rev. B,
15, 1489 (1977)] for coupled phonon-magnon diffusion to include the effect of
the concurrent magnetization flow. Working within the framework of Boltzmann
transport equation, we derive the constitutive equations for coupled
phonon-magnon transport driven by gradients of both temperature and external
magnetic fields, and the corresponding conservation laws. Our equations reduce
to the original Sanders-Walton two-temperature model under a uniform external
field, but predict a new magnon cooling effect driven by a non-uniform magnetic
field in a homogeneous single-domain ferromagnet. We estimate the magnitude of
the cooling effect in yttrium iron garnet, and show it is within current
experimental reach. With properly optimized materials, the predicted cooling
effect can potentially supplement the conventional magnetocaloric effect in
cryogenic applications in the future.",1403.0279v1
2018-05-02,RIXS studies of magnons and bimagnons in the lightly doped cuprate La2-xSrxCuO4,"We investigated the doping dependence of magnetic excitations in the lightly
doped cuprate La2-xSrxCuO4 via combined studies of resonant inelastic x-ray
scattering (RIXS) at the Cu L3-edge and theoretical calculations. With
increasing doping, the magnon dispersion is found to be essentially unchanged,
but the spectral width broadens and the spectral weight varies differently at
different momenta. Near the Brillouin zone center, we directly observe bimagnon
excitations which possess the same energy scale and doping dependence as
previously observed by Raman spectroscopy. They disperse weakly in
energy-momentum space, and are consistent with a bimagnon dispersion that is
renormalized by the magnon-magnon interaction at the zone center.",1805.00701v1
2018-05-10,Designing Rashba-Dresselhaus effect in magnetic insulators,"One of the major strategies to control magnetism in spintronics is to utilize
the coupling between electron spin and its orbital motion. The Rashba and
Dresselhaus spin-orbit couplings induce magnetic textures of band electrons
called spin momentum locking, which produces a spin torque by the injection of
electric current. However, joule heating had been a bottleneck for device
applications. Here, we propose a theory to generate further rich spin textures
in insulating antiferromagnets with broken spatial inversion symmetry (SIS),
which is easily controlled by a small magnetic field. In antiferromagnets, the
ordered moments host two species of magnons that serve as internal degrees of
freedom in analogy with electron spins. The Dzyaloshinskii-Moriya interaction
introduced by the SIS breaking couples the two-magnon-degrees of freedom with
the magnon momentum. We present a systematic way to design such texture and to
detect it via magnonic spin current for the realization of antiferromagnetic
memory.",1805.03925v2
2012-01-10,Quantum spin pumping mediated by magnon,"We theoretically propose quantum spin pumping mediated by magnons, under a
time-dependent transverse magnetic field, at the interface between a
ferromagnetic insulator and a non-magnetic metal. The generation of a spin
current under a thermal equilibrium condition is discussed by calculating the
spin transfer torque, which breaks the spin conservation law for conduction
electrons and operates the coherent magnon state. Localized spins lose spin
angular momentum by emitting magnons and conduction electrons flip from down to
up by absorbing the momentum. The spin transfer torque has a resonance
structure as a function of the angular frequency of the applied transverse
field. This fact is useful to enhance the spin pumping effect induced by
quantum fluctuations. We also discuss the distinction between our quantum spin
pumping theory and the one proposed by Tserkovnyak et al.",1201.1947v3
2012-04-16,Bose analogs of MIT bag model of hadrons in coherent precession,"Recently it was demonstrated that magnon condensation in the trap exhibits
the phenomenon of self-localization. When the number of magnons in the textural
trap increases, they drastically modify the profile of the gap and highly
increase its size. The trap gradually transforms from the initial harmonic one
to the box with walls almost impenetrable for magnons. The resulting
texture-free ""cavity"" filled by the magnon condensate wave function becomes the
bosonic analog of the MIT bag, in which hadron is seen as a cavity surrounded
by the QCD vacuum, in which the free quarks are confined in the ground or
excited state. Here we consider the bosonic analog of the MIT bag with quarks
on the ground and excited levels.",1204.3423v2
2013-09-12,One-Dimensional Dispersive Magnon Excitation in the Frustrated Spin-2 Chain System Ca3Co2O6,"Using inelastic neutron scattering, we have observed a quasi-one-dimensional
dispersive magnetic excitation in the frustrated triangular-lattice spin-2
chain oxide Ca3Co2O6. At the lowest temperature (T = 1.5 K), this magnon is
characterized by a large zone-center spin gap of ~27 meV, which we attribute to
the large single-ion anisotropy, and disperses along the chain direction with a
bandwidth of ~3.5 meV. In the directions orthogonal to the chains, no
measurable dispersion was found. With increasing temperature, the magnon
dispersion shifts towards lower energies, yet persists up to at least 150 K,
indicating that the ferromagnetic intrachain correlations survive up to 6 times
higher temperatures than the long-range interchain antiferromagnetic order. The
magnon dispersion can be well described within the predictions of linear
spin-wave theory for a system of weakly coupled ferromagnetic chains with large
single-ion anisotropy, enabling the direct quantitative determination of the
magnetic exchange and anisotropy parameters.",1309.3222v2
2013-09-19,Exact spectral function for hole-magnon coupling in the ferromagnetic CuO$_3$-like chain,"We present the exact spectral function for a single oxygen hole with spin
opposite to ferromagnetic order within a one-dimensional CuO$_{3}$-like spin
chain. We find that local Kondo-like exchange interaction generates five
different states in the strong coupling regime. It stabilizes a spin polaron
which is a bound state of a moving charge dressed by magnon excitations, with
essentially the same dispersion as predicted by mean field theory. We then
examine in detail the evolution of the spectral function for increasing
strength of the hole-magnon interaction. We also demonstrate that the $s$ and
$p$ symmetry of orbital states in the conduction band are essentially
equivalent to each other and find that the simplified models do not suffice to
reproduce subtle aspects of hole-magnon coupling in the charge-transfer model.",1309.5054v1
2014-06-03,Large-Spin and Large-Winding Expansions of Giant Magnons and Single Spikes,"We generalize the method of our recent paper on the large-spin expansions of
Gubser-Klebanov-Polyakov (GKP) strings to the large-spin and large-winding
expansions of finite-size giant magnons and finite-size single spikes. By
expressing the energies of long open strings in RxS2 in terms of Lambert's
W-function, we compute the leading, subleading and next-to-subleading series of
classical exponential corrections to the dispersion relations of
Hofman-Maldacena giant magnons and infinite-winding single spikes. We also
compute the corresponding expansions in the doubled regions of giant magnons
and single spikes that are respectively obtained when their angular and linear
velocities become smaller or greater than unity.",1406.0796v4
2014-06-30,Magnons and continua in a magnetized and dimerized spin-1/2 chain,"We examine the magnetic field dependent excitations of the dimerized spin-1/2
chain, copper nitrate, with antiferromagnetic intra-dimer exchange $J_1=0.44$
meV and exchange alternation $\alpha=J_2/J_1=0.24$. Magnetic excitations in
three distinct regimes of magnetization are probed through inelastic neutron
scattering at low temperatures. At low and high fields there are three and two
long-lived magnon-like modes, respectively. The number of modes and the
anti-phase relationship between the wave-vector dependent energy and intensity
of magnon scattering reflect the distinct ground states: A singlet ground state
at low fields $\mu_0H < \mu_0H_{c1} = 2.8$~T and an $S_z=1/2$ product state at
high fields $\mu_0H > \mu_0H_{c2} = 4.2$~T. In the intermediate field regime, a
continuum of scattering for $\hbar\omega\approx J_1$ is indicative of a
strongly correlated gapless quantum state without coherent magnons.",1406.7596v1
2016-05-21,Landau-Lifshitz theory of the magnon-drag thermopower,"Metallic ferromagnets subjected to a temperature gradient exhibit a magnonic
drag of the electric current. We address this problem by solving a stochastic
Landau-Lifshitz equation to calculate the magnon-drag thermopower. The
long-wavelength magnetic dynamics result in two contributions to the
electromotive force acting on electrons: (1) An adiabatic Berry-phase force
related to the solid angle subtended by the magnetic precession and (2) a
dissipative correction thereof, which is rooted microscopically in the
spin-dephasing scattering. The first contribution results in a net force
pushing the electrons towards the hot side, while the second contribution drags
electrons towards the cold side, i.e., in the direction of the magnonic drift.
The ratio between the two forces is proportional to the ratio between the
Gilbert damping coefficient $\alpha$ and the coefficient $\beta$ parametrizing
the dissipative contribution to the electromotive force.",1605.06578v1
2016-05-27,Combined Brillouin light scattering and microwave absorption study of magnon-photon coupling in a split-ring resonator/YIG film system,"Microfocused Brillouin light scattering (BLS) and microwave absorption (MA)
are used to study magnon-photon coupling in a system consisting of a split-ring
microwave resonator and a yttrium iron garnet (YIG) film. The split-ring
resonantor is defined by optical lithography and loaded with a 1 $\mu$m-thick
YIG film grown by liquid phase epitaxy. BLS and MA spectra of the hybrid system
are simultaneously recorded as a function of the applied magnetic field
magnitude and microwave excitation frequency. Strong coupling of the magnon and
photon modes is found with a coupling strength of $g_\text{eff}/2 \pi = 63$
MHz. The combined BLS and MA data allows to study the continuous transition of
the hybridized modes from a purely magnonic to a purely photonic mode by
varying the applied magnetic field and microwave frequency. Furthermore, the
BLS data represent an up-conversion of the microwave frequency coupling to
optical frequencies.",1605.08531v1
2017-06-10,Spin wave nonreciprocity and magnonic band structure in thin permalloy film induced by dynamical coupling with an array of Ni stripes,"An efficient way for the control of spin wave propagation in a magnetic
medium is the use of periodic patterns known as magnonic crystals (MCs).
However, the fabrication of MCs especially bi-components, with periodicity on
nanoscale, is a challenging task due to the requirement for sharp interfaces.
An alternative method to circumvent this problem is to use homogeneous
ferromagnetic film with modified periodically surrounding. The structure is
also suitable for exploiting nonreciprocal properties of the surface spin
waves. In this work, we demonstrate that the magnonic band structure forms in
thin permalloy film due to dynamical magnetostatic coupling with Ni stripes
near its surface. We show, that the band gap width can be systematically tuned
by the changing interlayer thickness between film and stripes. We show also the
effect of nonreciprocity, which is seen at the band gap edge shifted from the
Brillouin zone boundary and also in nonreciprocal interaction of propagating
spin waves in Py film with the standing spin waves in Ni stripes. Our findings
open possibility for further investigation and exploitation of the
nonreciprocity and band structure in magnonic devices.",1706.03202v1
2017-06-15,Squeezed Dirac and Topological Magnons in a Bosonic Honeycomb Optical Lattice,"Quantum information storage using charge-neutral quasiparticles are expected
to play a crucial role in the future of quantum computers. In this regard,
magnons or collective spin-wave excitations in solid-state materials are
promising candidates in the future of quantum computing. Here, we study the
quantum squeezing of Dirac and topological magnons in a bosonic honeycomb
optical lattice with spin-orbit interaction by utilizing the mapping to quantum
spin-$1/2$ XYZ Heisenberg model on the honeycomb lattice with discrete Z$_2$
symmetry and a Dzyaloshinskii-Moriya interaction. We show that the squeezed
magnons can be controlled by the Z$_2$ anisotropy and demonstrate how the noise
in the system is periodically modified in the ferromagnetic and
antiferromagnetic phases of the model. Our results also apply to solid-state
honeycomb (anti)ferromagnetic insulators.",1706.04974v2
2017-06-19,Magnon transport through a quantum dot: Conversion to electronic spin and charge currents,"We consider a single-level quantum dot coupled to magnetic insulators
(magnonic reservoirs) and magnetic metals (electronic reservoirs). The whole
system is in an external magnetic field. In a general case, the system includes
two magnonic and two electronic reservoirs, but we also present results for
some specific situations, where only two or three reservoirs are effectively
connected to the dot. The main objective is the analysis of the conversion of
magnon current to electronic spin and charge currents, and {\it vice versa}. We
consider the limiting case of large Coulomb energy in the dot (Coulomb
blockade), as well as the case when the Coulomb energy is finite and double
occupancy is allowed.",1706.05991v1
2017-10-05,Spin Hall and Nernst effects of Weyl magnons,"In this paper, we present a simple model of a three-dimensional insulating
magnetic structure which represents a magnonic analog of the layered electronic
system described in [Phys. Rev. Lett. {\bf 107}, 127205 (2011)]. In particular,
our model realizes Weyl magnons as well as surface states with a Dirac
spectrum. In this model, the Dzyaloshinskii-Moriya interaction is responsible
for the separation of opposite Weyl points in momentum space. We calculate the
intrinsic (due to the Berry curvature) transport properties of Weyl and
so-called anomalous Hall effect (AHE) magnons. The results are compared with
fermionic analogs.",1710.02115v2
2017-10-17,One dimensional Cherenkov processes in ferromagnetic insulator,"One dimensional Cherenkov processes in ferromagnetic isolators are studied
with perturbation theory under the constraint condition of conservation of
energy and momentum. It is shown that the magnon-phonon interaction channels
are limited and wave number dependent, which result in respectively $1/k^{2}$
and $1/k^{4}$ dependence of the lifetime and the relaxation time of long
wavelength magnons. The reciprocal of relaxation time between magnons and
phonons, $1/\tau_{mp}$, is found to be a linearly increasing function of the
temperature as $T>$ 70 K. Based on the Sanders-Walton model, we further show
that when a thermal (phonon) gradient is applied along the system, the
temperature difference between the phonon bath and the magnons with wave-vector
$k$ becomes more pronounced as $k$ decreasing.",1710.06155v2
2018-09-04,Topological Magnon Insulator with a Kekule Bond Modulation,"We examine the combined effects of a Kekule coupling texture (KC) and a
Dzyaloshinskii-Moriya interaction (DMI) in a two-dimensional ferromagnetic
honeycomb lattice. By analyzing the gap closing conditions and the inversions
of the bulk bands, we identify the parameter range in which the system behaves
as a trivial or a nontrivial topological magnon insulator. We find four
topological phases in terms of the KC parameter and the DMI strength. We
present the bulk-edge correspondence for the magnons in a honeycomb lattice
with an armchair or a zigzag boundary. Furthermore, we find Tamm-like edge
states due to the intrinsic on-site interactions along the boundary sites. Our
results may have significant implications to magnon transport properties in the
2D magnets at low temperatures.",1809.01073v2
2019-08-30,Amplitude and Phase Noise of Magnons,"The low-frequency amplitude and phase noise spectra of magnetization waves,
i.e. magnons, was measured in the yttrium iron garnet (YIG) waveguides. This
type of noise, which originates from the fluctuations of the physical
properties of the YIG crystals, has to be taken into account in the design of
YIG-based RF generators and magnonic devices for data processing, sensing and
imaging applications. It was found that the amplitude noise level of magnons
depends strongly on the power level, increasing sharply at the on-set of
nonlinear dissipation. The noise spectra of both the amplitude and phase noise
have the Lorentzian shape with the characteristic frequencies below 100 Hz.",1909.00085v1
2019-09-04,Chiral coupling of magnons in waveguides,"We theoretically investigate the collective excitation of multiple
(sub)millimeter-sized ferromagnets mediated by waveguide photons. By the
position of the magnets in the waveguide, the magnon-photon coupling can be
tuned to be chiral, i.e., magnons only couple with photons propagating in one
direction, leading to asymmetric transfer of angular momentum and energy
between the magnets. A large imbalance in the magnon number distribution over
the magnets can be achieved with a long chain of magnets, which concentrate at
one edge. The chain also supports standing waves with low radiation efficiency
that is inert to the chirality.",1909.01817v1
2020-12-01,Magnon hybridization in ferrimagnetic heterostructures,"We study magnon hybridization in a ferrimagnetic heterostructure consisting
of ultrathin gadolinium iron garnet and yttrium iron garnet layers and show the
localized and extended spatial profiles of the magnon modes with different
polarizations. These modes are expected to have distinct thermal excitation
properties in the presence of a temperature gradient across the
heterostructure. From a quantitative analysis of their consequences on
longitudinal spin Seebeck effect, we predict an observable shift of the
sign-changing temperature with respect to the one previously observed in
gadolinium iron garnet. Moreover, the sign-changing point of spin Seebeck
signal is found to be tunable by YIG thickness. Our results suggest the
necessity of taking into account the temperature difference between the magnon
modes in ferrimagnetic heterostructures.",2012.00576v1
2020-12-01,Design of an optomagnonic crystal: towards optimal magnon-photon mode matching at the microscale,"We put forward the concept of an optomagnonic crystal: a periodically
patterned structure at the microscale based on a magnetic dielectric, which can
co-localize magnon and photon modes. The co-localization in small volumes can
result in large values of the photon-magnon coupling at the single quanta
level, which opens perspectives for quantum information processing and quantum
conversion schemes with these systems. We study theoretically a simple geometry
consisting of a one-dimensional array of holes with an abrupt defect,
considering the ferrimagnet Yttrium Iron Garnet (YIG) as the basis material. We
show that both magnon and photon modes can be localized at the defect, and use
symmetry arguments to select an optimal pair of modes in order to maximize the
coupling. We show that an optomagnonic coupling in the kHz range is achievable
in this geometry, and discuss possible optimization routes in order to improve
both coupling strengths and optical losses.",2012.00760v2
2020-12-22,Crystalline symmetry-dependent magnon formation in itinerant ferromagnet SrRuO3,"SrRuO3 (SRO) is an itinerant ferromagnet with strong coupling between the
charge, spin, and lattice degrees of freedom. This strong coupling suggests
that the electronic and magnetic behaviors of SRO are highly susceptible to
changes in the lattice distortion. Here we show how the spin interaction and
resultant magnon formation change with the modification in the crystallographic
orientation. We fabricated SRO epitaxial thin films with (100), (110), and
(111) surface orientations, to systematically modulate the spin interaction and
spin dimensionality. The reduced spin dimensionality and enhanced exchange
interaction in the (111)-oriented SRO thin film significantly suppresses magnon
formation. Our study comprehensively demonstrates the facile tunability of
magnon formation and spin interaction in correlated oxide thin films.",2012.11865v1
2020-12-29,Nonreciprocal Multi-mode and Indirect Couplings in Cavity Magnonics,"We investigate the magnon-photon couplings by employing a small magnet within
an irregular resonant cavity, which leads to a desirable nonreciprocity with a
big isolation ratio. Moreover, the higher-order couplings between the spin wave
modes with the polarized photon modes also exhibit the nonreciprocity. These
couplings between polarized photon and spin waves could be regarded as an
indirect multi-modes coupling between the ferromagnetic resonance (FMR) mode
and spin wave mode magnons mediated by the cavity mode photons. We also derive
a coupling matrix to predict the characteristics of this kind of indirect
coupling. The existence of the indirect couplings broaden the field range of
the nonreciprocity of the system. The achieved nonreciprocal multi-mode
magnon-photon couplings in a single system offer a feasible method to improve
the signal transmission quality.",2012.14765v2
2000-06-06,Spectral function of transverse spin fluctuations in an antiferromagnet,"The spectral function of transverse spin fluctuations, including the
contributions from both the single-particle and the collective (magnon)
excitations in an antiferromagnet, is evaluated for the Hubbard model with NN
and NNN hoppings in the full U-range from weak coupling to strong coupling. For
the NN hopping model, the magnon excitations are dominant for U > 2.5 (d=2), so
that an effective spin description of the AF state holds down to a surprisingly
low U value. In the weak coupling limit the spectral function is suppressed at
low energies, as if due to an effective gap. NNN hopping t' leads to magnon
softening and also a significant increase in the low-energy spectral function
due to single-particle excitations. Evolution of the magnon spectrum with t' is
studied in the strong coupling limit, and the quantum spin- fluctuation
correction to sublattice magnetization in d=2 and the Neel temperature in d=3
are also evaluated.",0006079v1
2017-04-29,Magnon spectrum in two- and three- dimensional skyrmion crystals,"We study the low-energy magnon spectrum of the skyrmion crystal (SkX) ground
state, appearing in two-dimensional ferromagnet with Dzyaloshinskii-Moriya
interaction and magnetic field. We approximate SkX hexagonal superlattice by a
set of overlapping disks, and find the lattice period by minimizing the
classical energy density. The determined spectrum of magnons on the disc of
optimal radius is stable and only two lowest energy levels can be considered as
localized. The subsequent hybridization of these levels in the SkX lattice
leads to tight-binding spectrum. The localized character of the lowest magnon
states is lost at small and at high fields, which is interpreted as melting of
SkX. The classical energy of SkX is slightly above the energy of a single
conical spiral, and a consideration of quantum corrections can favor the
skyrmion ground state. Extending our analysis to three-dimensional case, we
argue that these quantum corrections become more important at finite
temperatures, when the average spin value is decreased.",1705.00196v1
2017-05-08,Helimagnon resonances in an intrinsic chiral magnonic crystal,"We experimentally study magnetic resonances in the helical and conical
magnetic phases of the chiral magnetic insulator Cu$_2$OSeO$_3$ at the
temperature $T$=5 K. Using a broadband microwave spectroscopy technique based
on vector network analysis, we identify three distinct sets of helimagnon
resonances in the frequency range 2 GHz $\leq f \leq$ 20 GHz. The extracted
resonance frequencies are in accordance with calculations of the helimagnon
bandstructure found in an intrinsic chiral magnonic crystal. The periodic
modulation of the equilibrium spin direction that leads to the formation of the
magnonic crystal is a direct consequence of the chiral magnetic ordering caused
by the Dzyaloshinskii-Moriya interaction. The opening of magnon band-gaps
allows for excitation of helimagnons with wave vectors that are multiples of
the spiral wave vector.",1705.02874v2
2017-05-18,The Final Chapter In The Saga Of YIG,"The magnetic insulator Yttrium Iron Garnet can be grown with exceptional
quality, has a ferrimagnetic transition temperature of nearly 600 K, and is
used in microwave and spintronic devices that can operate at room temperature.
The most accurate prior measurements of the magnon spectrum date back nearly 40
years, but cover only 3 of the lowest energy modes out of 20 distinct magnon
branches. Here we have used time-of-flight inelastic neutron scattering to
measure the full magnon spectrum throughout the Brillouin zone. We find that
the existing model of the excitation spectrum, well known from an earlier work
titled ""The Saga of YIG"", fails to describe the optical magnon modes. Using a
very general spin Hamiltonian, we show that the magnetic interactions are both
longer-ranged and more complex than was previously understood. The results
provide the basis for accurate microscopic models of the finite temperature
magnetic properties of Yttrium Iron Garnet, necessary for next-generation
electronic devices.",1705.06594v1
2018-03-06,Combined Molecular and Spin Dynamics Simulation of BCC Iron with Vacancy Defects,"Utilizing an atomistic computational model which handles both translational
and spin degrees of freedom, combined molecular and spin dynamics simulations
have been performed to investigate the effect of vacancy defects on spin wave
excitations in ferromagnetic iron. Fourier transforms of space and
time-displaced correlation functions yield the dynamic structure factor,
providing characteristic frequencies and lifetimes of the spin wave modes.
Comparison of the system with a 5% vacancy concentration with pure lattice data
shows a decrease in frequency as well as a decrease in lifetime for all
transverse spin wave excitations observed. Additionally, a rugged spin wave
line shape for low-q spin waves indicates the presence of multiple localized
excitations near defect sites resulting in reduced excitation lifetimes due to
increased magnon-magnon scattering. We observe further evidence of increased
magnon-magnon scattering as the peaks in the longitudinal spin wave spectrum
become less distinct.",1803.02468v1
2018-03-21,Spin caloric effects in antiferromagnets assisted by an external spin current,"Searching for novel spin caloric effects in antiferromagnets we study the
properties of thermally activated magnons in the presence of an external spin
current and temperature gradient. We predict the spin Peltier effect --
generation of a heat flux by spin accumulation -- in an antiferromagnetic
insulator with cubic or uniaxial magnetic symmetry. This effect is related with
spin-current induced splitting of the relaxation times of the magnons with
opposite spin direction. We show that the Peltier effect can trigger
antiferromagnetic domain wall motion with a force whose value grows with the
temperature of a sample. At a temperature, larger than the energy of the
low-frequency magnons, this force is much larger than the force caused by
direct spin transfer between the spin current and the domain wall. We also
demonstrate that the external spin current can induce the magnon spin Seebeck
effect. The corresponding Seebeck coefficient is controlled by the current
density. These spin-current assisted caloric effects open new ways for the
manipulation of the magnetic states in antiferromagnets.",1803.07949v1
2018-06-21,Magnon-induced high-order sideband generation,"Magnon Kerr nonlinearity plays crucial roles in the study of cavity
optomagnonics system and may bring many novel physical phenomena and important
applications. In this work, we report the investigation of high- order sideband
generation induced by magnon Kerr nonlinearity in a cavity-magnon system, which
is critically lacking in this emerging research field. We uncover that the
driving field plays a significant role in controlling the generation and
amplification of the higher-order sidebands and the sideband spacing can be
adjusted by regulating the beat frequency between the pump laser and the probe
laser, which is extremely eventful for the spacing modulation of the sideband
spectrum. Based on the recent experimental progress, our results deepen our
cognition into optomagnonics nonlinearity and may find interesting applications
in optical frequency metrology and optical communications.",1806.08289v2
2018-06-27,Boltzmann approach to the longitudinal spin Seebeck effect,"We develop a Boltzmann transport theory of coupled magnon-phonon transport in
ferromagnetic insulators. The explicit treatment of the magnon-phonon coupling
within the Boltzmann approach allows us to calculate the low-temperature
magnetic-field dependence of the spin-Seebeck voltage. Within the Boltzmann
theory we find that this magnetic field dependence shows similar features as
found by Flebus et al. [Phys. Rev. B 95, 144420 (2017)] for a strongly coupled
magnon phonon system that forms magnon-polarons, and consistent with
experimental findings in yttrium iron garnet by Kikkawa et al. [Phys. Rev.
Lett. 117, 207203 (2016)]. In addition to the anomalous magnetic-field
dependence of the spin Seebeck effect, we also predict a dependence on the
system size.",1806.10445v1
2018-12-10,Topologically nontrivial magnonic solitons,"The intrinsic non-linearities of the spin dynamics in condensed matter
systems give rise to a rich phenomenology that can be strongly affected by
topology. Here we study formation of magnonic solitons in the topologically
nontrivial bandgap of a spin lattice realization of the Haldane model, in both
static and dynamic (Floquet) regimes. We consider nonlinearities caused by
magnetic crystalline anisotropy and magnon-magnon interactions. We find soliton
formation power thresholds as a function of anisotropy coefficient and
interaction strength. We predict different classes of topological solitons for
the same topological class of the underlying lattice and explain it in terms of
a transition from a topologically nontrivial mass to a trivial one. Our
findings imply that a soliton can phase-separate, containing boundaries between
topologically trivial and non-trivial phases, which is associated with a
vanishing spin wave gap.",1812.03738v2
2018-12-11,Uniform Perpendicular and Parallel Staggered Susceptibility of Antiferromagnetic Films,"We investigate the thermodynamic behavior of antiferromagnetic films in
external magnetic fields oriented perpendicular to the staggered magnetization.
Within the systematic effective Lagrangian framework we first calculate the
two-point function and the dispersion relation for the two types of magnons up
to one-loop order. This allows us to split the two-loop free energy density
into a piece that originates from noninteracting dressed magnons, and a second
piece that corresponds to the genuine magnon-magnon interaction. We then
discuss the low-temperature series for various thermodynamic quantities,
including the parallel staggered and uniform perpendicular susceptibilities,
and analyze the role of the spin-wave interaction at finite temperature.",1812.04392v2
2019-01-04,Spin phases of the helimagnetic insulator Cu$_2$OSeO$_3$ probed by magnon heat conduction,"We report studies of thermal conductivity as functions of magnetic field and
temperature in the helimagnetic insulator Cu$_2$OSeO$_3$ that reveal novel
features of the spin-phase transitions as probed by magnon heat conduction. The
tilted conical spiral and low-temperature skyrmion phases, recently identified
in small-angle neutron scattering studies, are clearly identified by sharp
signatures in the magnon thermal conductivity. Magnon scattering associated
with the presence of domain boundaries in the tilted conical phase and regions
of skyrmion and conical-phase coexistence are identified.",1901.01242v1
2019-01-17,Magnon Condensation in a Dense Nitrogen-Vacancy Spin Ensemble,"The feasibility of creating a Bose-Einstein condensate of magnons using a
dense ensemble of nitrogen-vacancy spin defects in diamond is investigated.
Through assessing a density-dependent spin exchange interaction strength and
the magnetic phase transition temperature ($T_c$) using the
Sherrington-Kirkpatrick model, the minimum temperature-dependent concentration
for magnetic self-ordering is estimated. For a randomly dispersed spin
ensemble, the calculated average exchange constant exceeds the average dipole
interaction strengths for concentrations approximately greater than 70 ppm,
while $T_c$ is estimated to exceed 10 mK beyond 90 ppm, reaching 300 K at a
concentration of approximately 450 ppm. On this basis, the existence of
dipole-exchange spin waves and their plane-wave dispersion is postulated and
estimated using a semiclassical magnetostatic description. This is discussed
along with a $T_c$-based estimate of the four-magnon scattering rate, which
indicates magnons and their condensation may be detectable in thin films for
concentrations greater than 90 ppm.",1901.05818v1
2019-01-18,Magnon-Fluxon interaction in a ferromagnet/superconductor heterostructure,"Ferromagnetism and superconductivity are most fundamental phenomena in
condensed matter physics. Entailing opposite spin orders, they share an
important conceptual similarity: Disturbances in magnetic ordering in magnetic
materials can propagate in the form of spin waves (magnons) while magnetic
fields penetrate superconductors as a lattice of magnetic flux quanta
(fluxons). Despite a rich choice of wave and quantum phenomena predicted,
magnon-fluxon coupling has not been observed experimentally so far. Here, we
clearly evidence the interaction of spin waves with a flux lattice in
ferromagnet/superconductor Py/Nb bilayers. We demonstrate that, in this system,
the magnon frequency spectrum exhibits a Bloch-like band structure which can be
tuned by the biasing magnetic field. Furthermore, we observe Doppler shifts in
the frequency spectra of spin waves scattered on a flux lattice moving under
the action of a transport current in the superconductor.",1901.06156v1
2019-02-15,Gigahertz frequency antiferromagnetic resonance and strong magnon-magnon coupling in the layered crystal CrCl3,"We report broadband microwave absorption spectroscopy of the layered
antiferromagnet CrCl3. We observe a rich structure of resonances arising from
quasi-two-dimensional antiferromagnetic dynamics. Due to the weak interlayer
magnetic coupling in this material, we are able to observe both optical and
acoustic branches of antiferromagnetic resonance in the GHz frequency range and
a symmetry-protected crossing between them. By breaking rotational symmetry, we
further show that strong magnon-magnon coupling with large tunable gaps can be
induced between the two resonant modes.",1902.05669v2
2019-02-26,Strong magnon-photon coupling in ferromagnet-superconducting resonator thin-film devices,"We demonstrate strong magnon-photon coupling of a thin-film permalloy device
fabricated on a coplanar superconducting resonator. A coupling strength of
0.152 GHz and a cooperativity of 68 are found for a 30-nm-thick permalloy
stripe. The coupling strength is tunable by rotating the biasing magnetic field
or changing the volume of permalloy. We also observe an enhancement of
magnon-photon coupling in the nonlinear regime of the superconducting
resonator, which is mediated by the nucleation of dynamic flux vortices. Our
results demonstrate a critical step towards future integrated hybrid systems
for quantum magnonics and on-chip coherent information transfer.",1902.09715v1
2019-03-06,Microwave magnon damping in YIG films at millikelvin temperatures,"Magnon systems used in quantum devices require low damping if coherence is to
be maintained. The ferrimagnetic electrical insulator yttrium iron garnet (YIG)
has low magnon damping at room temperature and is a strong candidate to host
microwave magnon excitations in future quantum devices. Monocrystalline YIG
films are typically grown on gadolinium gallium garnet (GGG) substrates. In
this work, comparative experiments made on YIG waveguides with and without GGG
substrates indicate that the material plays a significant role in increasing
the damping at low temperatures. Measurements reveal that damping due to
temperature-peak processes is dominant above 1 K. Damping behaviour that we
show can be attributed to coupling to two-level fluctuators (TLFs) is observed
below 1 K. Upon saturating the TLFs in the substrate-free YIG at 20 mK,
linewidths of 1.4 MHz are achievable: lower than those measured at room
temperature.",1903.02527v3
2019-03-11,Spin waves in thin films and magnonic crystals with Dzyaloshinskii-Moriya interactions,"The influence of the Dzyaloshinskii-Moriya interaction in ultra-thin
ferromagnetic films and chiral magnonic crystals on the behavior of spin waves
is reviewed. During the last decade, it has been shown, both theoretically and
experimentally, that this anisotropic exchange interaction produces
non-reciprocal features on the spin-wave spectrum of a magnetic system, a
phenomenon that occurs both for bulk and interfacial Dzyaloshinskii-Moriya
coupling. More recently, the concept of a chiral magnonic crystal has been
introduced, where the interfacial Dzyaloshinskii-Moriya interaction is
periodic. The effect of this periodicity include additional features such as
flat bands, indirect gaps, and an unusual spin-wave evolution.",1903.04288v2
2019-03-12,Magnon Landau levels and emergent supersymmetry in strained antiferromagnets,"Inhomogeneous strain applied to lattice systems can induce artificial gauge
fields for particles moving on this lattice. Here we demonstrate how to
engineer a novel state of matter, namely an antiferromagnet with a Landau-level
excitation spectrum of magnons. We consider a honeycomb-lattice Heisenberg
model and show that triaxial strain leads to equally spaced pseudo-Landau
levels at the upper end of the magnon spectrum, with degeneracies
characteristic of emergent supersymmetry. We also present a particular strain
protocol which induces perfectly quantized magnon Landau levels over the whole
bandwidth. We discuss experimental realizations and generalizations.",1903.05097v2
2019-03-18,A series of magnon crystals appearing under ultrahigh magnetic fields in a kagomé antiferromagnet,"Search for a new quantum state of matter emerging in a crystal is one of
recent trends in condensed matter physics. For magnetic materials, geometrical
frustration and high magnetic field are two key ingredients to realize it: a
conventional magnetic order is possibly destroyed by competing interactions
(frustration) and is replaced by an exotic state that is characterized in terms
of quasiparticles, that are magnons, and the magnetic field can control the
density and chemical potential of the magnons. Here we show that a synthetic
copper mineral, Cd-kapellasite, comprising a kagome lattice made of
corner-sharing triangles of Cu2+ ions carrying spin-1/2 exhibits an
unprecedented series of fractional magnetization plateaux in ultrahigh magnetic
fields up to 160 T, which may be interpreted as crystallizations of emergent
magnons localized on the hexagon of the kagome lattice. Our observation reveals
a novel type of particle physics realized in a highly frustrated magnet.",1903.07283v1
2019-03-18,Thermal Hall Effect Induced by Magnon-Phonon Interactions,"We propose a new mechanism for the thermal Hall effect in exchange spin-wave
systems, which is induced by the magnon-phonon interaction. Using symmetry
arguments, we first show that this effect is quite general, and exists whenever
the mirror symmetry in the direction of the magnetization is broken. We then
demonstrate our result in a collinear ferromagnet on a square lattice, with
perpendicular easy-axis anisotropy and Dzyaloshinskii-Moriya interaction from
mirror symmetry breaking. We show that the thermal Hall conductivity is
controlled by the resonant contribution from the anti-crossing points between
the magnon and phonon branches, and estimate its size to be comparable to that
of the magnon mediated thermal Hall effect.",1903.07702v2
2019-04-25,Microscopic origin of level attraction for a coupled magnon-photon system in a microwave cavity,"We discuss various microscopic mechanisms for level attraction in a
hybridized magnon-photon system of a ferromagnet in a microwave cavity. The
discussion is based upon the electromagnetic theory of continuous media where
the effects of the internal magnetization dynamics of the ferromagnet are
described using dynamical response functions. This approach is in agreement
with quantized multi-oscillator models of coupled photon-magnon dynamics. We
demonstrate that to provide the attractive interaction between the modes, the
effective response functions should be diamagnetic. Magneto-optical coupling is
found to be one mechanism for the effective diamagnetic response, which is
proportional to photon number. A dual mechanism based on the Aharonov-Casher
effect is also highlighted, which is instead dependent on magnon number.",1904.11570v2
2019-08-01,Universal prethermal dynamics in Heisenberg ferromagnets,"We study the universal far from equilibrium dynamics of magnons in Heisenberg
ferromagnets. We show that such systems exhibit universal scaling in momentum
and time of the quasiparticle distribution function, with the universal
exponents distinct from those recently observed in Bose-Einstein condensates.
This new universality class originates from the SU(2) symmetry of the
Hamiltonian, which leads to a strong momentum-dependent magnon-magnon
scattering amplitude. We compute the universal exponents using the Boltzmann
kinetic equation and incoherent initial conditions that can be realized with
microwave pumping of magnons. We compare our numerical results with analytic
estimates of the scaling exponents and demonstrate the robustness of the
scaling to variations in the initial conditions. Our predictions can be tested
in quench experiments of spin systems in optical lattices and pump-probe
experiments in ferromagnetic insulators such as yttrium iron garnet.",1908.00554v2
2019-08-09,Observation of anti-PT symmetry phase transition in the magnon-cavity-magnon coupled system,"As the counterpart of PT symmetry, abundant phenomena and potential
applications of anti-PT symmetry have been predicted or demonstrated
theoretically. However, experimental realization of the coupling required in
the anti-PT symmetry is difficult. Here, by coupling two YIG spheres to a
microwave cavity, the large cavity dissipation rate makes the magnons coupled
dissipatively with each other, thereby obeying a two-dimensional anti-PT
Hamiltonian. In terms of the magnon-readout method, a new method adopted here,
we demonstrate the validity of our method in constructing an anti-PT system and
present the counterintuitive level attraction process. Our work provides a new
platform to explore the anti-PT symmetry properties and paves the way to study
multi-magnoncavity-polariton systems.",1908.03358v1
2019-08-21,Nonreciprocity and Unidirectional Invisibility in Cavity Magnonics,"We reveal the cooperative effect of coherent and dissipative magnon-photon
couplings in an open cavity magnonic system, which leads to nonreciprocity with
a considerably large isolation ratio and flexible controllability. Furthermore,
we discover unidirectional invisibility for microwave propagation, which
appears at the zero-damping condition for hybrid magnon-photon modes. A simple
model is developed to capture the generic physics of the interference between
coherent and dissipative couplings, which accurately reproduces the
observations over a broad range of parameters. This general scheme could
inspire methods to achieve nonreciprocity in other systems.",1908.07907v1
2019-08-28,Two-Magnon Bound States in the Kitaev Model in a $[111]$-Field,"It is now well established that the Kitaev honeycomb model in a magnetic
field along the $[111]$-direction harbors an intermediate gapless quantum spin
liquid (QSL) phase sandwiched between a gapped non-abelian QSL at low fields
$H< H_{c1}$ and a partially polarized phase at high fields $H> H_{c2}$. Here,
we analyze the low field and high field phases and phase transitions in terms
of single- and two-magnon excitations using exact diagonalization (ED) and
density matrix renormalization group (DMRG) methods. We find that the energy to
create a bound state of two-magnons $\Delta_p$ becomes lower than the energy to
create a single spin flip $\Delta_s$ near $H_{c2}$. In the entire Kitaev spin
liquid $\Delta_p<\Delta_s$ and both gaps vanish at $H_{c2}$. We make testable
predictions for magnon pairing that could be observable in Raman scattering
measurements on Kitaev QSL candidate materials.",1908.10877v1
2019-11-05,Exceptional points in dissipatively coupled spin dynamics,"We theoretically investigate dynamics of classical spins exchange-coupled
through an isotropic medium. The coupling is treated at the adiabatic level of
the medium's response, which mediates a first-order in frequency dissipative
interaction along with an instantaneous Heisenberg exchange. The resultant
damped spin precession yields exceptional points (EPs) in the coupled spin
dynamics, which should be experimentally accessible with the existing magnetic
heterostructures. In particular, we show that an EP is naturally approached in
an antiferromagnetic dimer by controlling local damping, while the same is
achieved by tuning the dissipative coupling between spins in the ferromagnetic
case. Extending our treatment to one-dimensional spin chains, we show how EPs
can emerge within the magnonic Brillouin zone by tuning the dissipative
properties. The critical point, at which an EP pair emerges out of the
Brillouin zone center, realizes a gapless Weyl point in the magnon spectrum.
Tuning damping beyond this critical point produces synchronization (level
attraction) of magnon modes over a finite range of momenta, both in ferro- and
antiferromagnetic cases. We thus establish that damped magnons can generically
yield singular points in their band structure, close to which their kinematic
properties, such as group velocity, become extremely sensitive to the control
parameters.",1911.01619v2
2019-11-13,Tailoring spin wave channels in a reconfigurable artificial spin ice,"Artificial spin ices are ensembles of geometrically-arranged, interacting
nanomagnets which have shown promising potential for the realization of
reconfigurable magnonic crystals. Such systems allow for the manipulation of
spin waves on the nanoscale and their potential use as information carriers.
However, there are presently two general obstacles to the realization of
artificial spin ice-based magnonic crystals: the magnetic state of artificial
spin ices is difficult to reconfigure and the magnetostatic interactions
between the nanoislands are often weak, preventing mode coupling. We
demonstrate, using micromagnetic modeling, that coupling a reconfigurable
artificial spin ice geometry made of weakly interacting nanomagnets to a soft
magnetic underlayer creates a complex system exhibiting dynamically coupled
modes. These give rise to spin wave channels in the underlayer at well-defined
frequencies, based on the artificial spin ice magnetic state, which can be
reconfigured. These findings open the door to the realization of reconfigurable
magnonic crystals with potential applications for data transport and processing
in magnonic-based logic architectures.",1911.05354v4
2019-11-26,Ultrastrong coupling between a microwave resonator and antiferromagnetic resonances of rare earth ion spins,"Quantum magnonics is a new and active research field, leveraging the strong
collective coupling between microwaves and magnetically ordered spin systems.
To date work in quantum magnonics has focused on transition metals and almost
entirely on ferromagnetic resonances in yttrium iron garnet (YIG).
Antiferromagnetic systems have gained interest as they produce no stray field,
and are therefore robust to magnetic perturbations and have narrow, shape
independent resonant linewidths. Here we show the first experimental evidence
of ultrastrong-coupling between a microwave cavity and collective
antiferromagnetic resonances (magnons) in a rare earth crystal. The combination
of the unique optical and spin properties of the rare earths and collective
antiferromagnetic order paves the way for novel quantum magnonic applications.",1911.11311v1
2019-11-27,Observation of the Magnon Polarization,"We measure the mode-resolved direction of the precessional motion of the
magnetic order, i.e., magnon polarization, via the chiral term of inelastic
polarized neutron scattering spectra. The magnon polarisation is important in
spintronics, affecting thermodynamic properties such as the magnitude and sign
of the spin Seebeck effect. The observation of both signs of magnon
polarization in Y3Fe5O12 also gives direct proof of its ferrimagnetic nature.
The experiments agree very well with atomistic simulations of the scattering
cross section.",1911.11968v1
2019-12-23,Ultrafast dynamics of entanglement in Heisenberg antiferromagnets,"We investigate entanglement dynamics in the antiferromagnetic Heisenberg
model in two dimensions following a spatially anisotropic quench of the
exchange interactions. Opposed to established results in one dimension, the
magnon quasiparticles show an initial growth of entanglement dynamics that does
not depend on the system size and is governed by the oscillation period of the
exchange interaction. We ascribe this to the dominance of the intrinsic
entanglement of short wavelength non-propagating magnon-pairs, which also leads
to a competition between area-law and volume-law contribution in the
entanglement dynamics. Furthermore, by adopting the neural-network quantum
states, we provide numerical evidence that this behavior survives even in the
presence of strong magnon-magnon interactions, suggesting new avenues for
manipulating entanglement dynamics in quantum materials.",1912.10845v3
2016-03-03,Thermal spin current and spin accumulation at ferromagnetic insulator/nonmagnetic metal interface,"Spin current injection and spin accumulation near a ferromagnetic insulator
(FI)/nonmagnetic metal (NM) bilayer film under a thermal gradient is
investigated theoretically. Using the Fermi golden rule and the Boltzmann
equations, we find that FI and NM can exchange spins via interfacial
electron-magnon scattering because of the imbalance between magnon emission and
absorption caused by either non-equilibrium distribution of magnons or
non-equilibrium between magnons and electrons. A temperature gradient in FI
and/or a temperature difference across the FI/NM interface generates a spin
current which carries angular momenta parallel to the magnetization of FI from
the hotter side to the colder one. Interestingly, the spin current induced by a
temperature gradient in NM is negligibly small due to the nonmagnetic nature of
the non-equilibrium electron distributions. The results agree well with all
existing experiments.",1603.00942v1
2016-03-03,Cloaking the magnons,"We propose two approaches to cloak the spin waves (magnons) by investigating
magnetization dynamics. One approach is based on a spatially inhomogeneous
anisotropic magnetic moment tensor. The other mechanism is using a spatially
inhomogeneous anisotropic gyromagnetic factor tensor and an inhomogeneous
external magnetic field. For both approaches, the damping tensor is also
inhomogeneous and anisotropic. The magnetic characteristic functions of the
magnetic materials have been theoretically derived for both mechanisms. A
non-magnetic core, which prevents magnons from entering and consequently
distorts the spin wave propagation, can be cloaked by a structured magnetic
shell to redirect the spin wave around the core using the above design
mechanisms. We discuss the feasibility of the proposed mechanisms in an
ensemble of quantum dot molecules and magnetic semiconductors. The proposed
approaches shed light on transformation magnonics, and can be utilized for
future spin-wave lenses, concentrators, low back-scattering waveguides, and
ultimately quantum computing.",1603.01000v1
2016-10-14,Edge Magnon Excitation in Spin Dimer Systems,"Magnetic excitation in a spin dimer system on a bilayer honeycomb lattice is
investigated in the presence of a zigzag edge, where disordered and ordered
phases can be controlled by a quantum phase transition. In analogy with the
case of graphene with a zigzag edge, a flat edge magnon mode appears in the
disordered phase. In an ordered phase, a finite magnetic moment generates a
mean-field potential to the magnon. Since the potential is nonuniform on the
edge and bulk sites, it affects the excitation, and the dispersion of the edge
mode deviates from the flat shape. We investigate how the edge magnon mode
evolves when the phase changes through the quantum phase transition and discuss
the similarities to ordered spin systems on a monolayer honeycomb lattice.",1610.04333v1
2016-12-01,Electrically Driven Bose-Einstein Condensation of Magnons in Antiferromagnets,"We explore routes to realize electrically driven Bose-Einstein condensation
of magnons in insulating antiferromagnets. Even in insulating antiferromagnets,
the localized spins can strongly couple to itinerant spins in adjacent metals
via spin-transfer torque and spin pumping. We describe the formation of
steady-state magnon condensates controlled by a spin accumulation polarized
along the staggered field in an adjacent normal metal. Two types of magnons
exist in antiferromagnets, which carry opposite magnetic moments. Consequently,
and in contrast to ferromagnets, Bose-Einstein condensation can occur for
either sign of the spin accumulation. This condensation may occur even at room
temperature when the interaction with the normal metal is fast compared to the
relaxation processes within the antiferromagnet. In antiferromagnets, the
operating frequencies of the condensate are orders of magnitude faster than in
ferromagnets.",1612.00366v1
2016-12-14,Finite-size Effect for Dyonic Giant Magnons in $CP^3_β$,"We studied the finite-size giant magnons in
$\text{AdS}_4\times\text{CP}^3_{\beta}$ background using the classical spectral
curve constructed in this paper. We computed the finite-size corrections to the
dispersion relations for the $RP^3$ giant magnons using our twisted algebraic
curve based on the method proposed in arXiv:0810.1246, in which the authors
computed the finite-size corrections of giant magnons in
$\text{AdS}_4\times\text{CP}^3$ by introducing a finite-size resolvent
$G_{\text{finite}}(x)$. We obtained exactly the same result as in
arXiv:1106.3686, where a totally different approach was used.",1612.04613v5
2016-12-21,Topologically non-trivial magnon bands in artificial square spin ices subject to Dzyaloshinskii-Moriya interaction,"Systems that exhibit topologically protected edge states are interesting both
from a fundamental point of view as well as for potential applications, the
latter because of the absence of back-scattering and robustness to
perturbations. It is desirable to be able to control and manipulate such edge
states. Here, we show that artificial square ices can incorporate both
features: an interfacial Dzyaloshinksii-Moriya gives rise to topologically
non-trivial magnon bands, and the equilibrium state of the spin ice is
reconfigurable with different configurations having different magnon
dispersions and topology. The topology is found to develop as odd-symmetry bulk
and edge magnon bands approach each other, so that constructive band inversion
occurs in reciprocal space. Our results show that topologically protected bands
are supported in square spin ices.",1612.07203v5
2017-07-12,THz-frequency cavity magnon-phonon-polaritons in the strong coupling regime,"We demonstrate the strong coupling of both magnons and phonons to terahertz
(THz) frequency electromagnetic (EM) waves confined to a photonic crystal (PhC)
cavity. Our cavity consists of a two-dimensional array of air-holes cut into a
hybrid slab of ferroelectric lithium niobate (LiNbO$_3$) and erbium
orthoferrite (ErFeO$_3$), a canted antiferromagnetic crystal. The phonons in
LiNbO$_3$ and the magnons in ErFeO$_3$ are strongly coupled to the electric and
magnetic field components of the confined EM wave, respectively. This leads to
the formation of new cavity magnon-phonon-polariton modes, which we
experimentally observe as a normal-mode splitting in the frequency spectrum and
an avoided crossing in the temperature-frequency plot. The cavity also has a
mode volume of $V=3.4\times10^{-3}\lambda^3\simeq0.5(\lambda/n)^3$ $\mu$m$^3$
and can achieve a Q-factor as high as 1000. These factors facilitate the
pursuit of the fields of THz cavity spintronics and quantum electrodynamics.",1707.03503v1
2018-10-09,Non-uniform spin wave softening in 2D magnonic crystals as a tool for opening omnidirectional magnonic band gaps,"By means of the plane wave method we study spin wave dynamics in
two-dimensional bi-component magnonic crystals based on a squeezed hexagonal
lattice and consist of a permalloy thin film with cobalt inclusions. We explore
the dependence of a spin wave frequency on the external magnetic field,
especially in weak fields where the mode softening takes place. For considered
structures, the mode softening proves to be highly non-uniform on both the mode
number and the wave vector. We found this effect to be responsible for the
omnidirectional band gap opening. Moreover, we show that the enhancement of the
demagnetizing field caused by the squeezing of the structure is of crucial
importance for the non-uniform mode softening. This allows us to employ this
mechanism to design magnonic gaps with different sensitivity for the tiny
change of the external field. The effects we have found should be useful in
designing and optimization of spin wave filters highly tunable by a small
external magnetic field.",1810.04005v3
2020-06-11,Nonlocal magnon entanglement generation in coupled hybrid cavity systems,"We investigate dynamical generation of macroscopic nonlocal entanglements
between two remote massive magnon-superconducting-circuit hybrid systems. Two
fiber-coupled microwave cavities are employed to serve as an interaction
channel connecting two sets of macroscopic hybrid units each containing a
magnon (hosted by a Yttrium-Iron-Garnet sphere) and a superconducting-circuit
qubit. Surprisingly, it is found that stronger coupling does not necessarily
mean faster entanglement generation. The proposed hybrid system allows the
existence of an optimal fiber coupling strength that requests the shortest
amount of time to generate a systematic maximal entanglement. Our theoretical
results are shown to be within the scope of specific parameters that can be
achieved with current technology. The noise effects on the implementation of
systems are also treated in a general environment suggesting the robustness of
entanglement generation. Our discrete-variable qubit-like entanglement theory
of magnons may lead to direct applications in various quantum information
tasks.",2006.06132v3
2020-06-19,Coexistence and interaction of spinons and magnons in an antiferromagnet with alternating antiferromagnetic and ferromagnetic quantum spin chains,"In conventional quasi-one-dimensional antiferromagnets with quantum spins,
magnetic excitations are carried by either magnons or spinons in different
energy regimes: they do not coexist independently, nor could they interact with
each other. In this Letter, by combining inelastic neutron scattering, quantum
Monte Carlo simulations and Random Phase Approximation calculations, we report
the discovery and discuss the physics of the coexistence of magnons and spinons
and their interactions in Botallackite-Cu2(OH)3Br. This is a unique quantum
antiferromagnet consisting of alternating ferromagnetic and antiferromagnetic
Spin-1/2 chains with weak inter-chain couplings. Our study presents a new
paradigm where one can study the interaction between two different types of
magnetic quasiparticles, magnons and spinons.",2006.10922v2
2020-06-19,Orbital Magnetic Moment of Magnons,"In experiments and applications usually the spin magnetic moment of magnons
is considered. In this Paper we identify an additional degree of freedom of
magnons: an \emph{orbital} magnetic moment brought about by spin-orbit
coupling.Our microscopic theory uncovers that spin magnetization
$\vec{M}^\mathrm{S}$ and orbital magnetization $\vec{M}^\mathrm{O}$ are
independent quantities. They are not necessarily collinear; thus, even when the
total spin moment is compensated due to antiferromagnetism ($\vec{M}^\mathrm{S}
= \vec{0}$), $\vec{M}^\mathrm{O}$ may be nonzero. This scenario of orbital weak
ferromagnetism is realized in paradigmatic kagome antiferromagnets with
Dzyaloshinskii-Moriya interaction. We demonstrate that magnets exhibiting a
magnonic orbital moment are omnipresent and propose transport experiments for
probing it.",2006.11107v1
2020-06-20,Tunable magnon-magnon coupling in synthetic antiferromagnets,"In this work, we study magnon-magnon coupling in synthetic antiferromagnets
(SyAFs) using microwave spectroscopy at room temperature. Two distinct
spin-wave modes are clearly observed and are hybridised at degeneracy points.
We provide a phenomenological model that captures the coupling phenomena and
experimentally demonstrate that the coupling strength is controlled by the
out-of-plane tilt angle as well as the interlayer exchange field. We
numerically show that a spin-current mediated damping in SyAFs plays a role in
influencing the coupling strength.",2006.11633v2
2020-09-04,Magnon-induced Giant Anomalous Nernst Effect in Single Crystal MnBi,"Thermoelectric modules are a promising approach to energy harvesting and
efficient cooling. In addition to the longitudinal Seebeck effect, recently
transverse devices utilizing the anomalous Nernst effect (ANE) have attracted
interest. For high conversion efficiency, it is required that the material
should have a large ANE thermoelectric power and low electrical resistance, the
product of which is the ANE conductivity. ANE is usually explained in terms of
intrinsic contributions from Berry curvature. Our observations suggest that
extrinsic contributions also matter. Studying single-crystal MnBi, we find a
very high ANE thermopower (~10 $\mu$V/K) under 0.6 T at 80 K, and a transverse
thermoelectric conductivity of over 40 A/Km. With insight from theoretical
calculations, we attribute this large ANE predominantly to a new advective
magnon contribution arising from magnon-electron spin-angular momentum
transfer. We propose that introducing large spin-orbit coupling into
ferromagnetic materials may enhance the ANE through the extrinsic contribution
of magnons.",2009.02211v3
2020-09-08,Unconventional Singularity in Anti-Parity-Time Symmetric Cavity Magnonics,"By engineering an anti-parity-time (anti-PT) symmetric cavity magnonics
system with precise eigenspace controllability, we observe two different
singularities in the same system. One type of singularity, the exceptional
point (EP), is produced by tuning the magnon damping. Between two EPs, the
maximal coherent superposition of photon and magnon states is robustly
sustained by the preserved anti-PT symmetry. The other type of singularity,
arising from the dissipative coupling of two anti-resonances, is an
unconventional bound state in the continuum (BIC). At the settings of BICs, the
coupled system exhibits infinite discontinuities in the group delay. We find
that both singularities co-exist at the equator of the Bloch sphere, which
reveals a unique hybrid state that simultaneously exhibits the maximal coherent
superposition and slow light capability.",2009.03950v1
2020-09-28,Spin-orbit coupling and linear crossings of dipolar magnons in van der Waals antiferromagnets,"A magnon spin-orbit coupling, induced by the dipole-dipole interaction, is
derived in monoclinic-stacked bilayer honeycomb spin lattice with perpendicular
magnetic anisotropy and antiferromagnetic interlayer coupling. Linear crossings
are predicted in the magnon spectrum around the band minimum in G valley, as
well as in the high frequency range around the zone boundary. The linear
crossings in K and K' valleys, which connect the acoustic and optical bands,
can be gapped when the intralayer dipole-dipole or Kitaev interactions exceed
the interlayer dipole-dipole interaction, resulting in a phase transition from
semimetal to insulator. Our results are useful for analyzing the magnon spin
dynamics and transport properties in van der Waals antiferromagnet.",2009.13660v1
2021-01-31,Vortex-mediated relaxation of magnon BEC into light Higgs quasiparticles,"A magnon Bose-Einstein condensate in superfluid $^3$He is a fine instrument
for studying the surrounding macroscopic quantum system. At zero temperature,
the BEC is subject to a few, distinct forms of decay into other collective
excitations, owing to momentum and energy conservation in a quantum vacuum. We
study the vortex-Higgs mechanism: the vortices relax the requirement for
momentum conservation, allowing the optical magnons of the BEC to transform
into light Higgs quasiparticles. This observation expands the spectrum of
possible interactions between magnetic quasiparticles in $^3$He-B, opens
pathways for hunting down elusive phenomena such as the Kelvin wave cascade or
bound Majorana fermions, and lays groundwork for building magnon-based quantum
devices.",2102.00419v2
2021-02-15,Magnetodynamic properties of dipole-coupled 1D magnonic crystals,"Magnonic crystals are magnetic metamaterials, that provide a promising way to
manipulate magnetodynamic properties by controlling the geometry of the
patterned structures. Here, we study the magnetodynamic properties of 1D
magnonic crystals consisting of parallel NiFe strips with different strip
widths and separations. The strips couple via dipole-dipole interactions. As an
alternative to experiments and/or micromagnetic simulations, we investigate the
accuracy of a simple macrospin model. For the case of simple strips, a model
with a single free parameter to account for an overestimation of the
out-of-plane demagnetization of the magnonic lattice is described. By adjusting
this parameter a good fit with experimental as well as micromagnetic results is
obtained. Moreover, the Gilbert damping is found independent of the lattice
constant however the inhomogeneous linewidth broadening found to increase with
decreasing stripe separation.",2102.07712v2
2013-06-07,Theory of carrier-mediated magnonic superlattices,"We present a minimal one-dimensional model of collective spin excitations in
itinerant ferromagnetic superlattices within the regime of parabolic
spin-carrier dispersion. We discuss the cases of weakly and strongly modulated
magnetic profiles finding evidences of antiferromagnetic correlations for
long-wave magnons (especially significant in layered systems), with an insight
into the ground state properties. In addition, the presence of local minima in
the magnonic dispersion suggests the possibility of (thermal) excitation of
spin waves with a relatively well controlled wave length. Some of these
features could be experimentally tested in DMS superlattices based on thin
doped magnetic layers, acting as natural interfaces between (spin)electronic
and magnonic degrees of freedom.",1306.1658v2
2015-07-07,Rayleigh-Jeans condensation of pumped magnons in thin film ferromagnets,"We show that the formation of a magnon condensate in thin ferromagnetic films
can be explained within the framework of a classical stochastic non-Markovian
Landau-Lifshitz-Gilbert equation where the properties of the random magnetic
field and the dissipation are determined by the underlying phonon dynamics. We
have numerically solved this equation for a tangentially magnetized
yttrium-iron garnet film in the presence of a parallel parametric pumping
field. We obtain a complete description of all stages of the nonequilibrium
time evolution of the magnon gas which is in excellent agreement with
experiments. Our calculation proves that the experimentally observed
condensation of magnons in yttrium-iron garnet at room temperature is a purely
classical phenomenon which should be called Rayleigh-Jeans rather than
Bose-Einstein condensation.",1507.01717v2
2016-09-30,Magnon dispersion shift in the induced-ferromagnetic phase of the noncentrosymmetric MnSi,"Small angle neutron inelastic scattering measurement has been performed to
study the magnon dispersion relation in the field-induced-ferromagnetic phase
of the noncentrosymmetric binary compound MnSi. For the magnons propagating
parallel or anti-parallel to the external magnetic field, we experimentally
confirmed that the dispersion relation is asymmetrically shifted along the
magnetic field direction. This magnon dispersion shift is attributed to the
relativistic Dzyaloshinskii-Moriya interaction, which is finite in
noncentrosymmetric magnets, such as MnSi. The shift direction is found to be
switchable by reversing the external magnetic field direction.",1609.09583v1
2018-08-16,Chiral Phonon Transport Induced by Topological Magnons,"The plethora of recent discoveries in the field of topological electronic
insulators has inspired a search for boson systems with similar properties.
There are predictions that ferromagnets on a two-dimensional honeycomb lattice
may host chiral edge magnons. In such systems, we theoretically study how
magnons and phonons couple. We find topological magneto-polarons around the
avoided crossings between phonons and topological magnons. Exploiting this
feature along with our finding of Rayleigh edge phonons in armchair ribbons, we
demonstrate the existence of chiral edge modes with a phononic character. We
predict that these modes mediate a chirality in the coherent phonon response
and suggest to measure this effect via elastic transducers. These findings
reveal a possible approach towards heat management in future devices.",1808.05493v2
2018-08-28,${\mathbb Z}_2$ Topological Invariant for Magnon Spin Hall Systems,"We propose a definition of a ${\mathbb Z}_2$ topological invariant for magnon
spin Hall systems which are the bosonic analog of two-dimensional topological
insulators in class AII. The existence of ""Kramers pairs"" in these systems is
guaranteed by pseudo-time-reversal symmetry which is the same as time-reversal
symmetry up to some unitary transformation. The ${\mathbb Z}_2$ index of each
Kramers pair of bands is expressed in terms of the bosonic counterparts of the
Berry connection and curvature. We construct explicit examples of magnon spin
Hall systems and demonstrate that our ${\mathbb Z}_2$ index precisely
characterizes the presence or absence of helical edge states. The proposed
${\mathbb Z}_2$ index and the formalism developed can be applied not only to
magnonic systems but also to other non-interacting bosonic systems.",1808.09149v4
2019-05-31,Experimental observation of exceptional surface in synthetic dimensions with magnon polaritons,"Exceptional points (EPs) are singularities of energy levels in non-Hermitian
systems. In this Letter, we demonstrate the surface of EPs on a magnon
polariton platform composed of coupled magnons and microwave photons. Our
experiments show that EPs form a three-dimensional exceptional surface (ES)
when the system is tuned in a four-dimensional synthetic space. We demonstrated
that there exists an exceptional saddle point (ESP) in the ES which originates
from the unique couplings between magnons and microwave photons. Such an ESP
exhibits unique anisotropic behaviors in both the real and imaginary part of
the eigenfrequencies. To the best of our knowledge, this is the first
experimental observation of ES, opening up new opportunities for
high-dimensional control of non-Hermitian systems.",1906.00044v1
2019-06-14,Short-range antiferromagnetic correlation effect on conduction electrons in two-dimensional strongly correlated electron systems,"We investigate magnetic polarons in two-dimensional strongly correlated
electron systems, where conduction electrons interact with
antiferromagnetically interacting localized spins. Starting from a basic model,
we derive a simplified model with the help of spin Green's function and a
perturbation analysis. A strong coupling analysis is applied to the model,
where the sum of the scattering wave vectors is approximated to be $(\pi,\pi)$
or zero, using the equation of motion for the conduction electron Green's
function, and we discuss the pseudogap like behavior associated with the
suppression of the quasiparticle weights and the transition from the large
magnetic polaron to the small magnetic polaron. In the antiferromagnetic
long-range ordered state, the spectral weight of the conduction electrons has a
form of broad humps due to Franck-Condon broadening associated with the
multi-magnon scattering. The band folding feature due to the $(\pi,\pi)$
scattering disappears as we increase the number of the magnons involved in the
multi-magnon scattering. It is crucial to include long-range antiferromagnetic
correlations as well as dumping of magnons.",1906.06031v2
2019-06-28,Magnon gravitomagnetoelectric effect in noncentrosymmetric antiferromagnetic insulators,"We study the magnon contribution to the gravitomagnetoelectric (gravito-ME)
effect, in which the magnetization is induced by a temperature gradient, in
noncentrosymmetric antiferromagnetic insulators. This phenomenon is totally
different from the ME effect, because the temperature gradient is coupled to
magnons but an electric field is not. We derive a general formula of the
gravito-ME susceptibility in terms of magnon wave functions and find that a
difference in $g$ factors of magnetic ions is crucial. We also apply our
formula to a specific model. Although the obtained gravito-ME susceptibility is
small, we discuss several ways to enhance this phenomenon.",1906.12031v3
2019-06-28,Microscopic mechanism of level attraction,"The emerging level attraction from dissipative light-matter coupling
converges the typical Rabi-splitting feature from coherent coupling and
exhibits potentials in topological information processing. However, the
underlying microscopic quantum mechanism of dissipative coupling still remains
unclear, which brings difficulties in quantifying and manipulating
coherence-dissipation competition and thereby the flexible control of level
attraction. Here, by coupling magnon to a cavity supporting both standing and
travelling waves, we identify the travelling-wave state to be responsible for
magnon-photon dissipative coupling. By characterizing radiative broadening of
magnon linewidth, we quantify the coherent and dissipative coupling strengths
and their competition. The effective magnon-photon coupling strength, as a net
result of competition, is analytically presented in quantum theory to show good
agreement with measurements. In this manner, we extend the control dimension of
level attraction by tuning field torque on magnetization or global cavity
geometry. Our finding opens new routines to engineer coupled harmonic
oscillator system.",1906.12142v2
2019-07-01,Magnon decay theory of Gilbert damping in metallic antiferromagnets,"Gilbert damping is a key property governing magnetization dynamics in ordered
magnets. We present a theoretical study of intrinsic Gilbert damping induced by
magnon decay in antiferromagnetic metals through $s$-$d$ exchange interaction.
Our theory delineates the qualitative features of damping in metallic
antiferromagnets owing to their bipartite nature, in addition to providing
analytic expressions for the damping parameters. Magnon-induced intraband
electron scattering is found to predominantly cause magnetization damping,
whereas the N\'eel field is found to be damped via disorder. Depending on the
conduction electron band structure, we predict that magnon-induced interband
electron scattering around band crossings may be exploited to engineer a strong
N\'eel field damping.",1907.01045v1
2019-07-15,Probing magnon dynamics and interactions in a ferromagnetic spin-1 chain,"NiNb$_{2}$O$_{6}$ is an almost ideal realization of a 1D spin-1 ferromagnetic
Heisenberg chain compound with weak unidirectional anisotropy. Using
time-domain THz spectroscopy, we measure the low-energy electrodynamic response
of NiNb$_{2}$O$_{6}$ as a function of temperature and external magnetic field.
At low temperatures, we find a magnon-like spin-excitation, which corresponds
to the lowest energy excitation at $q\sim0$. At higher temperatures, we
unexpectedly observe a temperature-dependent renormalization of the
spin-excitation energy, which has a strong dependence on field direction. Using
theoretical arguments, exact diagonalizations and finite temperature dynamical
Lanczos calculations, we construct a picture of magnon-magnon interactions that
naturally explains the observed renormalization. This unique scenario is a
consequence of the spin-1 nature and has no analog in the more widely studied
spin-1/2 systems.",1907.06542v1
2019-07-15,Cavity mediated dissipative coupling of distant magnetic moments: theory and experiment,"We investigate long-range coherent and dissipative coupling between two
spatially separated magnets while both are coupled to a microwave cavity. A
careful examination of the system shows that the indirect interaction between
two magnon modes is dependent on their individual mechanisms of direct coupling
to the cavity. If both magnon modes share the same form of coupling to the
cavity (either coherent or dissipative), then the indirect coupling between
them will produce level repulsion. Conversely, if the magnon modes have
different forms of coupling to the cavity (one coherent and one dissipative),
then their indirect coupling will produce level attraction. We further
demonstrate the cavity-mediate nature of the indirect interaction through
investigating the dependence of the indirect coupling strength on the frequency
detuning between the magnon and cavity modes. Our work theoretically and
experimentally explores indirect cavity mediate interactions in systems
exhibiting both coherent and dissipative coupling, which opens a new avenue for
controlling and utilizing light-matter interactions.",1907.06783v1
2019-07-24,Intrinsic spin Nernst effect of magnons in a noncollinear antiferromagnet,"We investigate the intrinsic magnon spin current in a noncollinear
antiferromagnetic insulator. We introduce a definition of the magnon spin
current in a noncollinear antiferromagnet and find that it is in general
non-conserved, but for certain symmetries and spin polarizations the averaged
effect of non-conserving terms can vanish. We formulate a general linear
response theory for magnons in noncollinear antiferromagnets subject to a
temperature gradient and analyze the effect of symmetries on the response
tensor. We apply this theory to single-layer potassium iron jarosite
KFe$_3$(OH)$_6$(SO$_4$)$_2$ and predict a measurable spin current response.",1907.10567v3
2019-10-11,Hannay Angles in Magnetic Dynamics,"We consider, within the framework developed by Hannay for classical
integrable systems [Journal of Physics A: Mathematical and General {\bf 18},
221 (1985)], the geometric phases that occur in semi-classical magnetic
dynamics. Such geometric phases are generically referred to as Hannay angles,
and, in the context of magnetic dynamics, may arise as a result of both
adiabatically-varying ellipticity and axis of magnetization precession. We
elucidate both effects and their interplay for single-domain magnetic dynamics
within a simple model with time-dependent anisotropies and external field.
Subsequently, we consider spin waves and rederive, from our classical approach,
some known results on what is commonly referred to as the magnon Berry phase.
As an aside, these results are used to give an interpretation for geometric
phases that occur in superfluids. Finally, we develop a Green's function
formalism for elliptical magnons. Within this formalism, we consider magnon
transport in a mesoscopic ring and show how it is influenced by interference
effects that are tuned by the Hannay angle that results from a varying
ellipticity. Our results may inform the field of magnonics that seeks to
utilize spin waves in applications.",1910.05099v2
2019-10-21,Entanglement-based single-shot detection of a single magnon with a superconducting qubit,"The recent development of hybrid systems based on superconducting circuits
has opened up the possibility of engineering sensors of quanta of different
degrees of freedom. Quantum magnonics, which aims to control and read out
quanta of collective spin excitations in magnetically-ordered systems,
furthermore provides unique opportunities for advances in both the study of
magnetism and the development of quantum technologies. Using a superconducting
qubit as a quantum sensor, we report the detection of a single magnon in a
millimeter-sized ferromagnetic crystal with a quantum efficiency of up
to~$0.71$. The detection is based on the entanglement between a magnetostatic
mode and the qubit, followed by a single-shot measurement of the qubit state.
This proof-of-principle experiment establishes the single-photon detector
counterpart for magnonics.",1910.09096v1
2019-10-31,Coherent spin pumping in a strongly coupled magnon-magnon hybrid system,"We experimentally identify coherent spin pumping in the magnon-magnon hybrid
modes of permalloy/yttrium iron garnet (Py/YIG) bilayers. Using broadband
ferromagnetic resonance, an ""avoided crossing"" is observed between the uniform
mode of Py and the spin wave mode of YIG due to the fieldlike interfacial
exchange coupling. We also identify additional linewidth suppression and
enhancement for the in-phase and out-of-phase hybrid modes, respectively,
\textcolor{black}{which can be interpreted as concerted dampinglike torque from
spin pumping}. Our analysis predicts inverse proportionality of both fieldlike
and dampinglike torques to the square root of the Py thickness, which
quantitatively agrees with experiments.",1910.14470v2
2020-01-11,Spin Seebeck effect and magnon diffusion length in $\rm{\mathbf{Fe}}_{\mathbf{3}}\rm{\mathbf{O}}_{\mathbf{4}}$,"The determination of the magnon diffusion length (MDL) is important for
increasing the efficiency of spin Seebeck effect (SSE) based devices utilising
non-metallic magnets. We extract the MDL at $50$ and $300\,\rm{K}$ in an
$\rm{Fe}_{3}\rm{O}_{4}$ single crystal from the magnon dispersion obtained
using inelastic neutron scattering (INS) and find them to be equal within
error. We then measure the heat flux normalised SSE responses and in-plane
magnetization of $\rm{Fe}_{3}\rm{O}_{4}$ thin films and normalise by the static
magnetization contribution to the SSE before determining the MDLs from a fit of
the thickness dependence. We find that the MDLs determined in this way are
smaller than that measured from INS which maybe due to differences in magnon
propagation between bulk and thin film $\rm{Fe}_{3}\rm{O}_{4}$.",2001.03738v1
2020-02-12,Topological magnonics in the two-dimensional van der Waals magnet CrI3,"In this article, we calculate the magnon spectrum of Kitaev-Heisenberg
magnets. This model has been recently proposed as a spin Hamiltonian to model
$\text{CrI}_3$ and other two-dimensional magnets. It is a minimal spin
Hamiltonian that includes a contribution stemming from a Heisenberg, isotropic
exchange, and a contribution arising from a Kitaev interaction, anisotropic and
frustrated exchange. Our calculations reveal the topological nature of the
magnons and a gap that opens at the $K$ and $K^\prime$ points. These
topological properties give rise to effects such as thermal Hall effect. In
addition to the bulk properties, we calculate the magnon spectrum of
nanoribbons illustrating the corresponding edge states.",2002.05266v1
2020-04-01,Gradual pressure-induced enhancement of magnon excitations in CeCoSi,"CeCoSi is an intermetallic antiferromagnet with a very unusual
temperature-pressure phase diagram: at ambient pressure it orders below
$T_{\mathrm{N}} = 8.8$ K, while application of hydrostatic pressure induces a
new magnetically ordered phase with exceptionally high transition temperature
of $\sim40$ K at 1.5 GPa. We studied the magnetic properties and the
pressure-induced magnetic phase of CeCoSi by means of elastic and inelastic
neutron scattering (INS) and heat capacity measurements. At ambient pressure
CeCoSi orders into a simple commensurate AFM structure with a reduced ordered
moment of only $m_{\mathrm{Ce}} = 0.37(6)$ $\mu_{\mathrm{B}}$. Specific heat
and low-energy INS indicate a significant gap in the low-energy magnon
excitation spectrum in the antiferromagnetic phase, with the CEF excitations
located above 10 meV. Hydrostatic pressure gradually shifts the energy of the
magnon band towards higher energies, and the temperature dependence of the
magnons measured at 1.5 GPa is consistent with the phase diagram. Moreover, the
CEF excitations are also drastically modified under pressure.",2004.00442v2
2020-04-29,Non-reciprocal magnons in a two dimensional crystal with off-plane magnetization,"Non reciprocal spin waves have a chiral asymmetry so that their energy is
different for two opposite wave vectors. They are found in atomically thin
ferromagnetic overlayers with in plane magnetization and are linked to the
anti-symmetric Dzyaloshinskii-Moriya surface exchange. We use an itinerant
fermion theory based on first principles calculations to predict that
non-reciprocal magnons can occur in Fe$_3$GeTe$_2$, the first stand alone
metallic two dimensional crystal with off-plane magnetization. We find that
both the energy and lifetime of magnons are non-reciprocal and we predict that
acoustic magnons can have lifetimes up to hundreds of picoseconds, orders of
magnitude larger than in other conducting magnets.",2004.14179v2
2020-05-19,Dissipation-based Quantum Sensing of Magnons with a Superconducting Qubit,"Hybrid quantum devices expand the tools and techniques available for quantum
sensing in various fields. Here, we experimentally demonstrate quantum sensing
of the steady-state magnon population in a magnetostatic mode of a
ferrimagnetic crystal. Dispersively coupling the magnetostatic mode to a
superconducting qubit allows the detection of magnons using Ramsey
interferometry with a sensitivity on the order of $10^{-3}$
$\text{magnons}/\sqrt{\text{Hz}}$. The protocol is based on dissipation as
dephasing via fluctuations in the magnetostatic mode reduces the qubit
coherence proportionally to the number of magnons.",2005.09250v1
2020-05-21,Non-Hermitian coherent coupling of nanomagnets by exchange spin waves,"Non-Hermitian physics has recently attracted much attention in optics and
photonics. Less explored is non-Hermitian magnonics that provides opportunities
to take advantage of the inevitable dissipation of magnons or spin waves in
magnetic systems. Here we demonstrate non-Hermitian coherent coupling of two
distant nanomagnets by fast spin waves with sub-50 nm wavelengths. Magnons in
two nanomagnets are unidirectionally phase-locked with phase shifts controlled
by magnon spin torque and spin-wave propagation. Our results are attractive for
analog neuromorphic computing that requires unidirectional information
transmission.",2005.10452v1
2020-07-06,Possible odd-frequency Amperean magnon-mediated superconductivity in topological insulator -- ferromagnetic insulator bilayer,"We study the magnon-mediated pairing between fermions on the surface of a
topological insulator (TI) coupled to a ferromagnetic insulator with a tilted
mean field magnetization. Tilting the magnetization towards the interfacial
plane reduces the magnetic band gap and leads to a shift in the effective TI
dispersions. We derive and solve the self-consistency equation for the
superconducting gap in two different situations, where we neglect or include
the frequency dependence of the magnon propagator. Neglecting the frequency
dependence results in p-wave Amperean solutions. We also find that tilting the
magnetization into the interface plane favors Cooper pairs with center of mass
momenta parallel to the magnetization vector, increasing $T_c$ compared to the
out-of-plane case. Including the frequency dependence of the magnon propagator,
and solving for a low number of Matsubara frequencies, we find that the
eigenvectors of the Amperean solutions at the critical temperature are
dominantly odd in frequency and even in momentum, thus opening the possibility
for odd-frequency Amperean pairing.",2007.02953v2
2020-07-17,Entangling the vibrational modes of two massive ferromagnetic spheres using cavity magnomechanics,"We present a scheme to entangle the vibrational phonon modes of two massive
ferromagnetic spheres in a dual-cavity magnomechanical system. In each cavity,
a microwave cavity mode couples to a magnon mode (spin wave) via the magnetic
dipole interaction, and the latter further couples to a deformation phonon mode
of the ferromagnetic sphere via a nonlinear magnetostrictive interaction. We
show that by directly driving the magnon mode with a red-detuned microwave
field to activate the magnomechanical anti-Stokes process a
cavity-magnon-phonon state-swap interaction can be realized. Therefore, if the
two cavities are further driven by a two-mode squeezed vacuum field, the
quantum correlation of the driving fields is successively transferred to the
two magnon modes and subsequently to the two phonon modes, i.e., the two
ferromagnetic spheres become remotely entangled. Our work demonstrates that
cavity magnomechanical systems allow to prepare quantum entangled states at a
more massive scale than currently possible with other schemes.",2007.09083v3
2020-07-28,Pair emission from a relativistic domain wall in antiferromagnets,"Magnon emission and excitation by a relativistic domain wall at a constant
velocity in antiferromagnet is theoretically studied. A pair emission due to a
quadratic magnon coupling is shown to be dominant. The emission corresponds in
the comoving frame to a vacuum polarization induced by a zero-energy
instability of the Lorentz-boosted anomalous response function. The emission
rate is sensitive to the magnon dispersion and wall profile, and is
significantly enhanced for a thin wall with velocity close to the effective
light velocity. The Ohmic damping constant due to magnon excitation at low
velocity is calculated.",2007.13939v1
2020-08-12,Topological properties of multilayer magnon insulators,"Two-dimensional magnetic insulators can be promising hosts for topological
magnons. In this study, we show that ABC-stacked honeycomb lattice multilayers
with alternating Dzyaloshinskii-Moriya interaction (DMI) reveal a rich
topological magnon phase diagram. Based on our bandstructure and Berry
curvature calculations, we demonstrate jumps in the thermal Hall behavior that
corroborate with topological phase transitions triggered by adjusting the DMI
and interlayer coupling. We connect the phase diagram of generic multilayers to
a bilayer and a trilayer system. We find an even-odd effect amongst the
multilayers where the even layers show no jump in thermal Hall conductivity,
but the odd layers do. We also observe the presence of topological proximity
effect in our trilayer. Our results offer new schemes to manipulate Chern
numbers and their measurable effects in topological magnonic systems.",2008.05487v2
2020-11-03,Symmetry breaking induced magnon-magnon coupling in synthetic antiferromagnets,"We propose a general theory of microwave absorption spectroscopy for
symmetry-breaking synthetic antiferromagnets (SAFs). Generally, inhomogeneity
or different thickness of the two ferromagnetic sublayers of a SAF results in
the intrinsic symmetry breaking, while out-of-plane components of dc magnetic
fields lead to the extrinsic one. The broken symmetry of SAFs excludes the
original symmetry-protected crossing between pure in-phase and out-of-phase
resonance modes with opposite parity. Alternatively, new frequency branches
become hybridization of original bare modes in terms of
symmetry-breaking-induced magnon-magnon coupling, which results in an indirect
gap in ferromagnetic resonance frequencies. Also, the dependence of gap width
on the degree of symmetry breaking for several typical cases are presented and
compared with existing experiments. Our theory provides a simple but physical
understanding on the rich structure of ferromagnetic resonance spectra for
asymmetric SAFs.",2011.01583v1
2020-11-12,"Experimental parameters, combined dynamics, and nonlinearity of a Magnonic-Opto-Electronic Oscillator (MOEO)","We report the construction and characterization of a comprehensive
magnonic-opto-electronic oscillator (MOEO) system based on 1550-nm photonics
and yttirum iron garnet (YIG) magnonics. The system exhibits a rich and
synergistic parameter space because of the ability to control individual
photonic, electronic, and magnonic components. Taking advantage of the spin
wave dispersion of YIG, the frequency self-generation as well as the related
nonlinear processes become sensitive to the external magnetic field. Besides
being known as a narrowband filter and a delay element, the YIG delayline
possesses spin wave modes that can be controlled to mix with the optoelectronic
modes to generate higher-order harmonic beating modes. With the high
sensitivity and external tunability, the MOEO system may find usefulness in
sensing applications in magnetism and spintronics beyond optoelectronics and
photonics.",2011.06155v1
2020-11-16,Electrically induced strong modulation of magnons transport in ultrathin magnetic insulator films,"Magnon transport through a magnetic insulator can be controlled by
current-biased heavy-metal gates that modulate the magnon conductivity via the
magnon density. Here, we report nonlinear modulation effects in 10$\,$nm thick
yttrium iron garnet (YIG) films. The modulation efficiency is larger than
40\%/mA. The spin transport signal at high DC current density (2.2$\times
10^{11}\,$A/m$^{2}$) saturates for a 400$\,$nm wide Pt gate, which indicates
that even at high current levels a magnetic instability cannot be reached in
spite of the high magnetic quality of the films.",2011.07800v1
2020-11-17,Sum-frequency excitation of coherent magnons,"Coherent excitation of magnons is conventionally achieved through Raman
scattering processes, in which the difference-frequency components of the
driving field are resonant with the magnon energy. Here, we describe mechanisms
by which the sum-frequency components of the driving field can be used to
coherently excite magnons through two-particle absorption processes. We use the
Landau-Lifshitz-Gilbert formalism to compare the spin-precession amplitudes
that different types of impulsive stimulated and ionic Raman scattering
processes and their sum-frequency counterparts induce in an antiferromagnetic
model system. We show that sum-frequency mechanisms enabled by linearly
polarized driving fields yield excitation efficiencies comparable or larger
than established Raman techniques, while elliptical polarizations produce only
weak and circularly polarizations no sum-frequency components at all. The
mechanisms presented here complete the map for dynamical spin control by the
means of Raman-type processes.",2011.08730v1
2020-11-25,Magnon-phonon hybridization in quasi-2D antiferromagnet MnPSe$_3$,"Magnetic excitations in van der Waals (vdW) materials, especially in the
two-dimensional (2D) limit, are an exciting research topic from both the
fundamental and applied perspectives. Using temperature-dependent,
magneto-Raman spectroscopy, we identify the hybridization of two-magnon
excitations with two separate E$_\mathrm{g}$ phonons in MnPSe$_3$, a magnetic
vdW material that could potentially host 2D antiferromagnetism. Results from
first principles calculations of the phonon and magnon spectra further support
our identification. The Raman spectra's rich temperature dependence through the
magnetic transition displays an avoided-crossing behavior in the phonons'
frequency and a concurrent decrease in their lifetimes. We construct a model
based on the interaction between a discrete level and a continuum that
reproduces these observations. The strong magnon-phonon hybridization reported
here highlights the need to understand its effects on spin transport
experiments in magnetic vdW materials.",2011.12557v2
2021-01-26,Supermagnonic propagation in two-dimensional antiferromagnets,"We investigate the propagation of magnons after ultrashort perturbations of
the exchange interaction in the prototype two-dimensional Heisenberg
antiferromagnet. Using the recently proposed neural quantum states, we predict
highly anisotropic spreading in space constrained by the symmetry of the
perturbation. Interestingly, the propagation speed at the shortest length and
time scale is up to 40% higher than the highest magnon velocity. We argue that
the enhancement stems from extraordinary strong magnon-magnon interactions,
suggesting new avenues for manipulating information transfer on ultrashort
length and time scales.",2101.10945v2
2021-03-09,"Dynamic transverse magnetic susceptibility in the projector augmented-wave method. Application to Fe, Ni, and Co","We present a first principles implementation of the dynamic transverse
magnetic susceptibility in the framework of linear response time-dependent
density functional theory. The dynamic susceptibility allows one to obtain the
magnon dispersion as well as magnon lifetimes for a particular material, which
strongly facilitates the interpretation of inelastic neutron scattering
experiments as well as other spectroscopic techniques. We apply the method to
Fe, Ni, and Co and perform a thorough convergence analysis with respect the
basis set size, $k$-point sampling, spectral smearing and unoccupied bands. In
particular, it is shown that while the gap error (acoustic magnon energy at
$\mathbf{q}=\mathbf{0}$) is highly challenging to converge, the spin-wave
stiffness and the dispersion relation itself are much less sensitive to
convergence parameters. Our final results agrees well with experimentally
extracted magnon dispersion relations except for Ni, where it is well-known
that the exchange splitting energy is poorly represented in the local density
approximation. We also find good agreement with previous first principles
calculations and explain how differences in the calculated dispersion relations
can arise from subtle differences in computational approaches.",2103.05409v2
2021-03-11,Proposal for a Bell test in cavity optomagnonics,"We present a proposal to test Bell inequality in the emerging field of cavity
optomagnonics, where a sphere of ferromagnetic crystal supports two optical
whispering gallery modes and one magnon mode. The two optical modes are driven
by two laser pulses, respectively. Entanglement between magnon mode and one of
the two optical modes will be generated by the first pulse, and the state of
magnon mode is subsequently mapped into another optical mode via the second
pulse. Hence correlated photon-photon pairs is created out of the cavity. A
Bell test can be implemented using these pairs, which enables us to test local
hidden-variable models at macroscopic optomagnonical system. Our results show
that a significant violation of Bell inequality can be obtained in the
experimentally relevant weak-coupling regime. The violation of Bell inequality
not only verifies the entanglement between magnons and photons, but also
implies that cavity optomagnonics is a promising platform for quantum
information processing tasks.",2103.06429v2
2021-03-12,Double accumulation and anisotropic transport of magneto-elastic bosons in yttrium iron garnet films,"Interaction between quasiparticles of a different nature, such as magnons and
phonons in a magnetic medium, leads to the mixing of their properties and the
formation of hybrid states in the areas of intersection of individual spectral
branches. We recently reported the discovery of a new phenomenon mediated by
the magnon-phonon interaction: the spontaneous bottleneck accumulation of
magneto-elastic bosons under electromagnetic pumping of pure magnons into a
ferrimagnetic yttrium iron garnet film. Here, by studying the transport
properties of the accumulated magneto-elastic bosons, we reveal that such
accumulation occurs in two frequency-distant groups of quasiparticles:
quasi-phonons and quasi-magnons. They propagate with different speeds in
different directions relative to the magnetization field. The theoretical model
we propose qualitatively describes the double accumulation effect, and the
analysis of the two-dimensional spectrum of quasiparticles in the hybridization
region allows us to determine the wavevectors and frequencies of each of the
groups.",2103.07338v1
2021-03-12,Superconductivity provides a giant enhancement to the spin battery effect,"We develop a theory of the spin battery effect in
superconductor/ferromagnetic insulator (SC/FI) systems taking into account the
magnetic proximity effect. We demonstrate that the spin-energy mixing enabled
by the superconductivity leads to the enhancement of spin accumulation by
several orders of magnitude relative to the normal state. This finding can
explain the recently observed giant inverse spin Hall effect generated by
thermal magnons in the SC/FI system. We suggest a non-local electrical
detection scheme which can directly probe the spin accumulation driven by the
magnetization dynamics. We predict a giant Seebeck effect converting the magnon
temperature bias into the non-local voltage signal. We also show how this can
be used to enhance the sensitivity of magnon detection even up to the
single-magnon level.",2103.07412v1
2021-04-20,Magnetic dipolar modes in magnon-polariton condensates,"For dipole-carrying excitations observed in a high-quality resonator,
strong-coupling modes can appear as composite bosons with the spontaneous
formation of quantized vortices in the condensed phase of a polariton fluid. In
exciton-polaritons, in particular, it leads to sustained trapping of the
emitted photon. In this paper, we show that magnon-polaritons can be realized
due to magnon condensation caused by magnetic dipole-dipole interaction. In a
quasi-2D ferrite disk placed in a microwave cavity, one observes quantum
confinement effects of magnetic-dipolar-mode (MDM) oscillations. These modes,
characterized by energy eigenstates with rotational superflows and quantized
vortices, are exhibited as spinor condensates. Along with the condensation of
MDM magnons in the quasi-2D disk of the magnetic insulator, electric dipole
condensation is also observed. At the MDM resonances, transfer between angular
momenta in the magnetic insulator and in the vacuum cavity, demonstrates
generation of vortex flows with fixed handedness. This indicates unique
topological properties of polariton wavefronts. One observes curved wavefronts
and effects of supperradiance in microwave structures. In an environment of
scattering states of microwave waveguide, EM waves can carry the topological
phases of MDM resonances.",2104.10048v1
2021-05-04,Non-local magnon transport in a magnetic domain wall wave guide,"Magnetic domain walls function as a wave guide for low energy magnons. In
this paper we develop the theory for the non-local transport of these bound
magnons through a ferromagnetic insulator that are injected and detected
electrically in adjacent normal metal leads by spin-flip scatting processes and
the (inverse) spin-Hall effect. Our set-up requires a twofold degeneracy of the
magnetic ground state, which we realize by an easy axis and hard axis
anisotropy, in the ferromagnetic insulator. This is readily provided by a broad
range of materials. The domain wall is a a topologically protected feature of
the system and we obtain the non-local spin transport through it. Thereby we
provide a framework for reconfigureable magnonic devices.",2105.01694v1
2021-05-18,Magnon-mediated quantum battery under systematic errors,"Quantum battery is one of the most prominent micro-devices in the
rapid-developing quantum thermodynamics. We propose a quantum charging protocol
in which both battery and charger are consisted of a many-spin system. The
battery and charger are connected via the Kittle mode of the magnon system,
that serves as a charging wire and then enables a long-range charging protocol.
Counter-intuitively, we find that the spin-spin couplings insider both battery
and charger can be used to promote the charging performance in comparison to
the noninteracting condition. And a remarkable promotion is realized at a
""sweet"" spot in terms of the average coupling strength. Moreover, we apply the
quantum-state-diffusion equation to test the robustness of our magnon-mediated
charging protocol to the systematic errors about the magnon frequency.",2105.08387v2
2021-05-21,Coherent and Dissipative Cavity Magnonics,"Strong interactions between magnetic materials and electrodynamic cavities
mix together spin and photon properties, producing unique hybridized behaviour.
The study of such coupled spin-photon systems, known as cavity magnonics, is
motivated by the flexibility and controllability of these hybridized states for
spintronic and quantum information technologies. In this tutorial we examine
and compare both coherent and dissipative interactions in cavity magnonics. We
begin with a familiar case study, the coupled harmonic oscillator, which
provides insight into the unique characteristics of coherent and dissipative
coupling. We then examine several canonical cavity magnonic systems,
highlighting the requirements for different coupling mechanisms, and conclude
with recent applications of spin-photon hybridization, for example, the
development of quantum transducers, memory architectures, isolators and
enhanced sensing.",2105.10491v2
2021-05-24,Advances in coherent coupling between magnons and acoustic phonons,"The interaction between magnetic and acoustic excitations have recently
inspired many interdisciplinary studies ranging from fundamental physics to
circuit implementation. Specifically, the exploration of their coherent
interconversion enabled via the magnetoelastic coupling opens a new playground
combining straintronics and spintronics, and provides a unique platform for
building up on-chip coherent information processing networks with miniaturized
magnonic and acoustic devices. In this Perspective, we will focus on the recent
progress of magnon-phonon coupled dynamic systems, including materials,
circuits, imaging and new physics. In particular, we highlight the unique
features such as nonreciprocal acoustic wave propagation and strong coupling
between magnons and phonons in magnetic thin-film systems, which provides a
unique platform for their coherent manipulation and transduction. We will also
review the frontier of surface acoustic wave resonators in coherent quantum
transduction and discuss how the novel acoustic circuit design can be applied
in microwave spintronics.",2105.11051v1
2021-05-28,Exceptional sign changes of the non-local spin Seebeck effect in antiferromagnetic hematite,"Low power spintronic devices based on the propagation of pure magnonic spin
currents in antiferromagnetic insulator materials offer several distinct
advantages over ferromagnetic components including higher frequency magnons and
a stability against disturbing external magnetic fields. In this work, we make
use of the insulating antiferromagnetic phase of iron oxide, the mineral
hematite $\alpha$-Fe$_2$O$_3$ to investigate the long distance transport of
thermally generated magnonic spin currents. We report on the excitation of
magnons generated by the spin Seebeck effect, transported both parallel and
perpendicular to the antiferromagnetic easy-axis under an applied magnetic
field. Making use of an atomistic hematite toy model, we calculate the
transport characteristics from the deviation of the antiferromagnetic ordering
from equilibrium under an applied field. We resolve the role of the magnetic
order parameters in the transport, and experimentally we find significant
thermal spin transport without the need for a net magnetization.",2105.13653v1
2021-06-02,Topological Magnons: A Review,"At sufficiently low temperatures magnetic materials often enter a correlated
phase hosting collective, coherent magnetic excitations such as magnons or
triplons. Drawing on the enormous progress on topological materials of the last
few years, recent research has led to new insights into the geometry and
topology of these magnetic excitations. Berry phases associated to magnetic
dynamics can lead to observable consequences in heat and spin transport while
analogues of topological insulators and semimetals can arise within magnon band
structures from natural magnetic couplings. Magnetic excitations offer a
platform to explore the interplay of magnetic symmetries and topology, to drive
topological transitions using magnetic fields. examine the effects of
interactions on topological bands and to generate topologically protected spin
currents at interfaces. In this review, we survey progress on all these topics,
highlighting aspects of topological matter that are unique to magnon systems
and the avenues yet to be fully investigated.",2106.01430v1
2021-06-03,Mass-energy equivalence for terahertz magnon excitation in antiferromagnetic domain walls,"The theory of special relativity is one of the most significant achievements
in modern physics, with several important predictions such as time dilation,
size contraction for a moving object and mass-energy equivalence. Recent
studies have demonstrated size contraction for an antiferromagnetic (AFM)
domain wall (DW). Here, we show the mass-energy equivalence by numerically
investigating the excitation of terahertz (THz) magnons from a moving AFM DW
under the magnetic anisotropy energy gradient. The energy of magnons comes from
the loss of DW mass, accompanied with a DW width broadening, overcoming the
Lorentz contraction effect. Our results pave the way to study relativistic
physics in AFM textures and to efficiently generate THz magnons by electric
means.",2106.01631v2
2021-06-15,Nonreciprocal high-order sidebands induced by magnon Kerr nonlinearity,"We propose an effective approach for creating robust nonreciprocity of
high-order sidebands, including the first-, second- and third-order sidebands,
at microwave frequencies. This approach relies on magnon Kerr nonlinearity in a
cavity magnonics system composed of two microwave cavities and one yttrium iron
garnet (YIG) sphere. By manipulating the driving power applied on YIG and the
frequency detuning between the magnon mode in YIG and the driving field, the
effective Kerr nonlinearity can be strengthened, thereby inducing strong
transmission non-reciprocity. More interestingly, we find the higher the
sideband order, the stronger the transmission nonreciprocity marked by the
higher isolation ratio in the optimal detuning regime. Such a series of
equally-spaced high-order sidebands have potential applications in frequency
comb-like precision measurement, besides structuring high-performance on-chip
nonreciprocal devices.",2106.09542v1
2021-07-08,Nonreciprocal magnon fluxonics upon ferromagnet/superconductor hybrids,"Ferromagnet/superconductor heterostructures allow for the combination of
unique physical phenomena offered by the both fields of magnetism and
superconductivity. It was shown recently that spin waves can be efficiently
scattered in such structures by a lattice of static or moving magnetic flux
quanta (Abrikosov vortices), resulting in bandgaps in the spin-wave spectra.
Here, we realize a nonreciprocal motion of a vortex lattice in nanoengineered
symmetric and asymmetric pinning landscapes and investigate the non-reciprocal
scattering of magnons on fluxons. We demonstrate that the magnon bandgap
frequencies can be tuned by the application of a low-dissipative transport
current and by its polarity reversal. Furthermore, we exploit the rectifying
(vortex diode or ratchet) effect by the application of a 100 MHz-frequency ac
current to deliberately realize bandgap up- or downshifts during one ac
halfwave while keeping the bandgap frequency constant during the other ac
halfwave. The investigated phenomena allow for the realization of
energy-efficient hybrid magnonic devices, such as microwave filters with an
ultra-high bandgap tunability of 10 GHz/mA and a fast modulation of the
transmission characteristics on the 10 ns time scale.",2107.04095v1
2021-07-23,Green's function formalism for nonlocal elliptical magnon transport,"We develop a non-equilibrium Green's function formalism to study magnonic
spin transport through a strongly anisotropic ferromagnetic insulator contacted
by metallic leads. We model the ferromagnetic insulator as a finite-sized
one-dimensional spin chain, with metallic contacts at the first and last sites
that inject and detect spin in the form of magnons. In the presence of
anisotropy, these ferromagnetic magnons become elliptically polarized, and spin
conservation is broken. We show that this gives rise to a novel parasitic spin
conductance, which becomes dominant at high anisotropy. Moreover, the spin
state of the ferromagnet becomes squeezed in the high-anisotropy regime. We
show that the squeezing may be globally reduced by the application of a local
spin bias.",2107.11072v2
2021-08-04,Theory of Huge Thermoelectric Effect Based on Magnon Drag Mechanism: Application to Thin-Film Heusler Alloy,"To understand the unexpectedly high thermoelectric performance observed in
the thin-film Heusler alloy Fe$_2$V$_{0.8}$W$_{0.2}$Al, we study the magnon
drag effect, generated by the tungsten based impurity band, as a possible
source of this enhancement, in analogy to the phonon drag observed in FeSb$_2$.
Assuming that the thin-film Heusler alloy has a conduction band integrating
with the impurity band, originated by the tungsten substitution, we derive the
electrical conductivity $L_{11}$ based on the self-consistent t-matrix
approximation and the thermoelectric conductivity $L_{12}$ due to magnon drag,
based on the linear response theory, and estimate the temperature dependent
electrical resistivity, Seebeck coefficient and power factor. Finally, we
compare the theoretical results with the experimental results of the thin-film
Heusler alloy to show that the origin of the exceptional thermoelectric
properties is likely to be due to the magnon drag related with the
tungsten-based impurity band.",2108.01880v1
2021-08-04,Spin interactions and topological magnonics in chromium trihalide CrClBrI,"The discovery of spontaneous magnetism in van der Waal (vdW) magnetic
monolayers has opened up an unprecedented platform for investigating magnetism
in purely two-dimensional systems. Recently, it has been shown that the
magnetic properties of vdW magnets can be easily tuned by adjusting the
relative composition of halides. Motivated by these experimental advances, here
we derive a model for a trihalide CrClBrI monolayer from symmetry principles
and we find that, in contrast to its single-halide counterparts, it can display
highly anisotropic nearest- and next-to-nearest neighbor Dzyaloshinskii-Moriya
and Heisenberg interactions. Depending on the parameters, the DM interactions
are responsible for the formation of exotic chiral spin states, such as
skyrmions and spin cycloids, as shown by our Monte Carlo simulations. Focusing
on a ground state with a two-sublattice unit cell, we find spin-wave bands with
nonvanishing Chern numbers. The resulting magnon edge states yield a magnon
thermal Hall conductivity that changes sign as function of temperature and
magnetic field, suggesting chromium trihalides as a candidate for testing
topological magnon transport in two-dimensional noncollinear spin systems.",2108.02165v4
2021-08-06,Magnon transport in $\mathrm{\mathbf{Y_3Fe_5O_{12}}}$/Pt nanostructures with reduced effective magnetization,"For applications making use of magnonic spin currents damping effects, which
decrease the spin conductivity, have to be minimized. We here investigate the
magnon transport in an yttrium iron garnet thin film with strongly reduced
effective magnetization. We show that in a three-terminal device the effective
magnon conductivity can be increased by a factor of up to six by a current
applied to a modulator electrode, which generates damping compensation above a
threshold current. Moreover, we find a linear dependence of this threshold
current on the applied magnetic field. We can explain this behavior by the
reduced effective magnetization and the associated nearly circular
magnetization precession.",2108.03263v1
2021-08-18,Magnetoelastic coupling enabled tunability of magnon spin current generation in 2D antiferromagnets,"We theoretically investigate the magnetoelastic coupling (MEC) and its effect
on magnon transport in two-dimensional antiferromagnets with a honeycomb
lattice. MEC coeffcient along with magnetic exchange parameters and spring
constants are computed for monolayers of transition metal trichalcogenides with
N\'eel order ($\text{MnPS}_3$ and $\text{VPS}_3$) and zigzag order
($\text{CrSiTe}_3$, $\text{NiPS}_3$ and $\text{NiPSe}_3$) by $ab$ $initio$
calculations. Using these parameters, we predict that the spin-Nernst
coefficient is significantly enhanced due to magnetoelastic coupling. Our study
shows that although Dzyaloshinskii-Moriya interaction can produce spin Nernst
effect in these materials, other mechanisms such as magnon-phonon coupling
should be taken into account. We also demonstrate that the magnetic anisotropy
is an important factor for control of magnon-phonon hybridization and
enhancement of the Berry curvature and thus the spin-Nernst coefficient. Our
results pave the way towards gate tunable spin current generation in 2D magnets
by SNE via electric field modulation of MEC and anisotropy.",2108.07999v1
2021-08-19,Perturbations of giant magnons and single spikes in $\mathbb R \times S^2$,"Perturbations of giant magnons and single spikes in a $2+1$ dimensional
$\mathbb R \times S^2$ background spacetime are analysed. Using the form of the
giant magnon solution in the Jevicki-Jin gauge,the well-known Jacobi equation
for small normal deformations of an embedded time-like surface are written
down. Surprisingly, this equation reduces to a simple wave equation in a
Minkowski background. The finiteness of perturbations and the ensuing stability
of such giant magnons under small deformations are then discussed. It turns out
that only the zero mode has finite deformations and is stable. Thereafter, we
move on to explore the single spike solution in the Jevicki-Jin gauge. We
obtain and solve the perturbation equation numerically and address stability
issues.",2108.08622v1
2021-09-01,Thermal Hall effect of magnons in collinear antiferromagnetic insulators: signatures of magnetic and topological phase transitions,"We demonstrate theoretically that the thermal Hall effect of magnons in
collinear antiferromagnetic insulators is an indicator of magnetic and
topological phase transitions in the magnon spectrum. The transversal heat
current of magnons caused by a thermal gradient is calculated for an
antiferromagnet on a honeycomb lattice. An applied magnetic field drives the
system from the antiferromagnetic phase via a spin-flop phase into the
field-polarized phase. Besides these magnetic phase transitions we find
topological phase transitions within the spin-flop phase. Both types of
transitions manifest themselves in prominent and distinguishing features in the
thermal conductivities; depending on the temperature, the conductivity changes
by several orders of magnitude, providing a tool to discern experimentally the
two types of phase transitions. We include numerical results for the van der
Waals magnet MnPS$_3$.",2109.00278v1
2021-09-14,Magnon Condensation in Dimerized Antiferromagnets with Spin-Orbit Coupling,"Bose-Einstein condensation (BEC) of triplet excitations triggered by a
magnetic field, sometimes called magnon BEC, in dimerized antiferromagnets
gives rise to a long-range antiferromagnetic order in the plane perpendicular
to the applied magnetic field. To explore the effects of spin-orbit coupling on
magnon condensation, we study a spin model on a distorted honeycomb lattice
with dimerized Heisenberg exchange ($J$ terms) and uniform off-diagonal
exchange ($\Gamma'$ terms) interactions. Via variational Monte Carlo
calculations and spin-wave theory, we find that an out-of-plane magnetic field
can induce different types of long-range magnetic orders, no matter if the
ground state is a non-magnetic dimerized state or an ordered N\'{e}el state.
Furthermore, the critical properties of field-driven phase transitions in the
presence of spin-orbit couplings, as illustrated from spin-wave spectrum and
interpreted by effective field theory, can be different from the conventional
magnon BEC. Our study is helpful to understand the rich phases of spin-orbit
coupled antiferromagnets induced by magnetic fields.",2109.06518v2
2021-09-30,Interaction of magnons with plasmons in a system of antiferromagnetic insulators coupled to a superconductor microwave cavity through the interfacial exchange interaction,"A superconductor can interact with magnets through the magnetic field which
is induced by the supercurrent. However, this interaction is rather weak and
requires relatively large magnets to reach a sizable hybridization of microwave
electromagnetic modes with magnons. The size of the device can be drastically
reduced by means of the large interfacial exchange interaction between spins in
a magnet and a superconductor. In the spin-orbit coupled superconductor this
interaction can be transmitted to the s-wave condensate through a mixing of
triplet and singlet Cooper pairs. In this paper we theoretically consider thin
antiferromagnetic films, which make a spin compensated contact to a
superconducting wire, which forms a cavity for Mooij-Sch\""{o}n plasma waves.
The effective Lagrangian is derived where the interaction of these waves with
magnons is taken into account. Frequencies of hybrid magnon-plasmon modes are
calculated. A system of two micromagnets placed far apart are also considered
and their mutual coupling through the cavity mode is calculated.",2109.14973v1
2021-09-30,Optomagnonic Josephson effect in antiferromagnets,"Combining advanced technologies of optics and antiferromagnetic spintronics,
we present a method to realize ultrafast spin transport. The optical Barnett
effect provokes quasiequilibrium Bose-Einstein condensates (BECs) of magnons
associated with the fully spin-polarized state in insulating antiferromagnets
(AFs). This optomagnonic Barnett effect enables us to exploit coherent magnons
of high frequency over the conventional ones of (sub-) terahertz associated
with the N\'eel magnetic order. We show that the macroscopic coherence of those
optical magnon BECs induces a spin current across the junction interface of
weakly coupled two insulating AFs, and this optomagnonic Josephson effect
realizes ultrafast spin transport. The period of the optomagnonic Josephson
oscillation is much shorter than the conventional one of the order of
picoseconds. Thus we propose a way to realize ultrafast spin transport in AFs
by means of the macroscopic coherence of optical magnon BECs.",2109.15202v1
2021-10-21,Multi-magnon quantum many-body scars from tensor operators,"We construct a family of three-body spin-1/2 Hamiltonians with a
super-extensive set of infinitely long-lived multi-magnon states. A magnon in
each such state carries either quasi-momentum zero or fixed $p_0\neq$ 0, and
energy $\Omega$ . These multi-magnon states provide an archetypal example of
quantum many-body scars: they are eigenstates at finite energy density that
violate the eigenstate thermalization hypothesis, and lead to persistent
oscillations in local observables in certain quench experiments. On the
technical side, we demonstrate the systematic derivation of scarred
Hamiltonians that satisfy a restricted spectrum generating algebra using an
operator basis built out of irreducible tensor operators. This operator basis
can be constructed for any spin, spatial dimension or continuous non-Abelian
symmetry that generates the scarred subspace.",2110.11448v3
2021-10-27,Interface modes in planar one-dimensional magnonic crystals,"We present the concept of Zak phase for spin waves in planar magnonic
crystals and discuss the conditions for the existence of interface modes
localized on the boundary between two magnonic crystals with centrosymmetric
unit cells. Using the symmetry criterion and the calculated logarithmic
derivative of the Bloch function we study the interface modes to demonstrate
the bulk-edge correspondence. Our theoretical results are verified numerically
for structures in the exchange and exchange-dipolar regimes and extended to the
case in which one of the magnonic crystals has an arbitrary unit cell.
Moreover, we show that by shifting the unit cell the interface modes can be
caused to traverse the bandgap edges.",2110.14726v4
2021-11-01,Long-Time Memory and Ternary Logic Gate Using a Multistable Cavity Magnonic System,"Multistability is an extraordinary nonlinear property of dynamical systems
and can be explored to implement memory and switches. Here we experimentally
realize the tristability in a three-mode cavity magnonic system with Kerr
nonlinearity. The three stable states in the tristable region correspond to the
stable solutions of the frequency shift of the cavity magnon polariton under
specific driving conditions. We find that the system staying in which stable
state depends on the history experienced by the system, and this state can be
harnessed to store the history information. In our experiment, the memory time
can reach as long as 5.11 s. Moreover, we demonstrate the ternary logic gate
with good on-off characteristics using this multistable hybrid system. Our new
findings pave a way towards cavity magnonics-based information storage and
processing.",2111.01558v1
2021-11-10,Light propagation and magnon-photon coupling in optically dispersive magnetic media,"Achieving strong coupling between light and matter excitations in hybrid
systems is a benchmark for the implementation of quantum technologies. We
recently proposed [arXiv:2110.02984] that strong single-particle coupling
between magnons and light can be realized in a magnetized epsilon-near-zero
(ENZ) medium, in which magneto-optical effects are enhanced. Here we present a
detailed derivation of the magnon-photon coupling Hamiltonian in dispersive
media both for degenerate and non-degenerate optical modes, and show the
enhancement of the coupling near the ENZ frequency. Moreover, we show that the
coupling of magnons to plane-wave non-degenerate Voigt modes vanishes at
specific frequencies due to polarization selection rules tuned by dispersion.
Finally, we present specific results using a Lorentz dispersion model. Our
results pave the way for the design of dispersive optomagnonic systems,
providing a general theoretical framework for describing engineering ENZ-based
optomagnonic systems.",2111.05851v1
2021-11-11,Anomalous Thermal Hall Effect in an Insulating van der Waals Magnet VI3,"Two-dimensional (2D) van der Waals (vdW) magnets have been a fertile
playground for the discovery and exploration of physical phenomena and new
physics. In this Letter, we report the observation of an anomalous thermal Hall
effect (THE) with \k{appa}_xy ~ 1x10^(-2) W K^(-1) m^(-1) in an insulating van
der Waals ferromagnet VI3. The thermal Hall signal persists in the absence of
an external magnetic field and flips sign upon the switching of the
magnetization. In combination with theoretical calculations, we show that VI3
exhibits a dual nature of the THE, i.e., dominated by topological magnons
hosted by the ferromagnetic honeycomb lattice at higher temperatures and by
phonons induced by the magnon-phonon coupling at lower temperatures. Our
results not only position VI3 as the first ferromagnetic system to investigate
both anomalous magnon and phonon THEs, but also render it as a potential
platform for spintronics/magnonics applications.",2111.06039v1
2021-11-12,Classical analog of qubit logic based on a magnon Bose-Einstein condensate,"We present a classical version of several quantum bit (qubit) functionalities
using a two-component magnon Bose-Einstein condensate formed at opposite
wavevectors in a room-temperature yttrium-iron-garnet ferrimagnetic film. The
macroscopic wavefunctions of these two condensates serve as orthonormal basis
states that form a system being a classical counterpart of a single qubit.
Solving the Gross-Pitaevskii equation and employing micromagnetic numerical
simulations, we first show how to initialize the system in one of the basis
states: the application of wavevector-selective parallel parametric pumping
allows us to form only a single condensate in one of the two lowest energy
states of the magnon gas. Next, by translating the concept of Rabi-oscillations
into the wavevector domain, we demonstrate how to manipulate the magnon-BEC
system along the polar axis in the Bloch sphere representation. We also discuss
the manipulation regarding the azimuthal angle.",2111.06798v2
2021-12-06,Accumulation of magnetoelastic bosons in yttrium iron garnet: kinetic theory and wave vector resolved Brillouin light scattering,"We derive and solve quantum kinetic equations describing the accumulation of
magnetoelastic bosons in an overpopulated magnon gas realized in a thin film of
the magnetic insulator yttrium iron garnet. We show that in the presence of a
magnon condensate, there is a non-equilibrium steady state in which incoherent
magnetoelastic bosons accumulate in a narrow region in momentum space for
energies slightly below the bottom of the magnon spectrum. The results of our
calculations agree quite well with Brillouin light scattering measurements of
the stationary non-equilibrium state of magnons and magnetoelastic bosons in
yttrium iron garnet.",2112.03310v2
2021-12-14,Essential model parameters for nonreciprocal magnons in multisublattice systems,"We theoretically investigate the microscopic conditions for emergent
nonreciprocal magnons toward unified understanding on the basis of a
microscopic model analysis. We show that the products of the Bogoliubov
Hamiltonian obtained within the linear spin wave approximation is enough to
obtain the momentum-space functional form and the key ingredients in the
nonreciprocal magnon dispersions in an analytical way even without solving the
eigenvalue problems. We find that the odd order of an effective antisymmetric
Dzyaloshinskii-Moriya interaction and/or the even order of an effective
symmetric anisotropic interaction in the spin rotated frame can be a source of
the antisymmetric dispersions. We present possible kinetic paths of magnons
contributing to the antisymmetric dispersions in the one- to four-sublattice
systems with the general exchange interactions. We also test the formula for
both ferromagnetic and antiferromagnetic orderings in the absence of spatial
inversion symmetry.",2112.07071v1
2022-01-25,Protocol for generating an arbitrary quantum state of the magnetization in cavity magnonics,"We propose and numerically evaluate a protocol to generate an arbitrary
quantum state of the magnetization in a magnet. The protocol involves
repeatedly exciting a frequency-tunable superconducting transmon and
transferring the excitations to the magnet via a microwave cavity. To avoid
decay, the protocol must be much shorter than magnon lifetime. Speeding up the
protocol by simply shortening the pulses leads to non-resonant leakage of
excitations to higher levels of the transmon accompanied by higher decoherence.
We discuss how to correct for such leakages by applying counter pulses to
de-excite these higher levels. In our protocol, states with a maximum magnon
occupation of up to $\sim9$ and average magnon number up to $\sim4$ can be
generated with fidelity $>0.75$.",2201.10170v1
2022-02-06,Coexistence of coupling-induced transparency and absorption of transmission signals in magnon-mediated photon-photon coupling,"Coexistence of coupling-induced transparency (CIT) and absorption (CIA) of
signals in magnon-mediated photon-photon coupling was experimentally determined
in a planar hybrid structure consisting of a yttrium iron garnet (YIG) film and
three concentric inverted-split-ring resonators (ISRRs). The experimental
observation of simultaneous CIT and CIA phenomena was ascribed to
magnon-mediated photon-photon coupling between the individually decoupled
ISRRs. In order to capture the generic physics of the observed interactions, we
constructed an appropriate analytical model based on the balance between the
coherent and dissipative multiple-paths interactions, which model precisely
reproduced both the CIT and CIA experimentally observed from a single hybrid
system. This work, promisingly, can provide guidance for design of efficient,
flexible, and well-controllable photon-magnonic devices that are highly in
demand for applications to quantum technologies currently under development.",2202.02667v1
2022-02-17,Higher-order Topological Phases of Magnons in van der Waals Honeycomb Ferromagnets,"We theoretically propose a second-order topological magnon insulator by
stacking the van der Waals honeycomb ferromagnets with antiferromagnetic
interlayer coupling. The system exhibits Z$_{2}$ topological phase, protected
by pseudo-time-reversal symmetry (PTRS). An easy-plane anisotropy term breaks
PTRS and destroys the topological phase. Nevertheless, it respects a magnetic
two-fold rotational symmetry which protects a second-order topological phase
with corner modes in bilayer and hinge modes along stacking direction.
Moreover, an introduced staggered interlayer coupling establishes a
Z$_{2}$$\times$Z topology, giving rise to gapped topological surface modes
carrying non-zero Chern numbers. Consequently, chiral hinge modes propagate
along the horizontal hinges in a cuboid geometry and are robust against
disorders. Our work bridges the higher-order topology and magnons in van der
Waals platforms, and could be used for constructing topological magnonic
devices.",2202.08424v1
2022-03-11,Laser induced enhanced coupling between photons and squeezed magnons in antiferromagnets,"In this paper we consider a honeycomb antiferromagnet subject to an external
laser field. Obtaining a time-independent effective Hamiltonian, we find that
the external laser renormalizes the exchange interaction between the in-plane
components of the spin-operators, and induces a synthetic Dzyaloshinskii-Moria
interaction (DMI) between second neighbors. The former allows the control of
the magnon dispersion's bandwidth and the latter breaks time-reversal symmetry
inducing non-reciprocity in momentum space. The eigen-excitations of the system
correspond to squeezed magnons whose squeezing parameters depend on the
properties of the laser. When studying how these spin excitations couple with
cavity photons, we obtain a coupling strength which can be enhanced by an order
of magnitude via careful tuning of the laser's intensity, when compared to the
case where the laser is absent. The transmission plots through the cavity are
presented, allowing the mapping of the magnons' dispersion relation.",2203.05809v1
2022-03-15,Topological magnon band structure of emergent Landau levels in a skyrmion lattice,"The motion of a spin excitation across topologically non-trivial magnetic
order exhibits a deflection that is analogous to the effect of the Lorentz
force on an electrically charged particle in an orbital magnetic field. We used
polarized inelastic neutron scattering to investigate the propagation of
magnons (i.e., bosonic collective spin excitations) in a lattice of skyrmion
tubes in manganese silicide. For wave vectors perpendicular to the skyrmion
tubes, the magnon spectra are consistent with the formation of finely spaced
emergent Landau levels that are characteristic of the fictitious magnetic field
used to account for the nontrivial topological winding of the skyrmion lattice.
This provides evidence of a topological magnon band structure in reciprocal
space, which is borne out of the nontrivial real-space topology of a magnetic
order.",2203.08159v1
2022-03-21,Antiferromagnetic cavity magnon polaritons in collinear and canted phases of hematite,"Cavity spintronics explores light matter interactions at the interface
between spintronic and quantum phenomena. Until now, studies have focused on
the hybridization between ferromagnets and cavity photons.In this article, we
realize antiferromagnetic cavity-magnon polaritons. The collective spin motion
in single hematite crystals ({\alpha}-Fe2O3) hybridizes with 18 - 45 GHz
microwave cavity photons with required specific symmetries. We show
theoretically and experimentally that the photon-magnon coupling in the
collinear phase is mediated by the dynamical Neel vector and the weak magnetic
moment in the canted phase by measuring across the Morin transition. The
coupling strength g is shown to scale with the anisotropy field in the
collinear phase and with the Dzyaloshinskii-Moriya field in the canted phase.
We achieve a strong coupling regime both in canted (C > 25 at 300 K) and
noncolinear phases (C > 4 at 150 K) and thus coherent information exchange with
antiferromagnets These results evidence a generic strategy to achieve
cavity-magnon polaritons in antiferromagnets for different symmetries, opening
the field of cavity spintronics to antiferromagnetic materials.",2203.10924v1
2022-03-29,Orbital Angular Momentum of Magnons in Collinear Magnets,"We study the orbital angular momentum of magnons for collinear ferromagnet
(FM) and antiferromagnetic (AF) systems with nontrivial networks of exchange
interactions. The orbital angular momentum of magnons for AF and FM zig-zag and
honeycomb lattices becomes nonzero when the lattice contains two inequivalent
sites and is largest at the avoided-crossing points or extremum of the
frequency bands. Hence, the arrangement of exchange interactions may play a
more important role at producing the orbital angular momentum of magnons than
the spin-orbit coupling energy and the resulting non-collinear arrangement of
spins.",2203.15677v3
2022-04-06,Magnon mediated spin entanglement in the strong coupling regime,"We present that two spin defects (SDs) can be entangled through a magnon
polariton mode, within the strong coupling regime. The magnonic modes are
provided by an antiferromagnetic (AF) MnF$_{2}$ layer and their dispersion is
characterized by the layer's thickness. The macroscopic quantum electrodynamics
theory is used to describe the light-matter interactions, where the Green's
functions are its core element. The individual SD relaxes by exciting the
magnon polariton modes, exhibiting high enhancement values of the Purcell
factor. When two SDs are considered, an oscillatory population exchange is
observed between them, a sign of strong light-matter coupling, where the
concurrence value is used to quantify the level of entanglement. The thinner AF
layers can potentially be used to promote interactions between multiple spins
through long range coupling, this is a desired feature to fabricate high demand
applications in the fields of quantum measurement and computation.",2204.02533v1
2022-04-26,Spin waves and magnetic exchange Hamiltonian in CrSBr,"CrSBr is an air-stable 2D van der Waals semiconducting magnet with great
technological promise, but its atomic-scale magnetic interactions -- crucial
information for high-frequency switching -- are poorly understood. We present
an experimental study to determine the CrSBr magnetic exchange Hamiltonian and
bulk magnon spectrum. We confirm the $A$-type antiferromagnetic order using
single crystal neutron diffraction. We also measure the magnon dispersions
using inelastic neutron scattering and rigorously fit the excitation modes to a
spin wave model. The magnon spectrum is well described by an intra-plane
ferromagnetic Heisenberg exchange model with seven nearest in-plane exchanges.
This fitted exchange Hamiltonian enables theoretical predictions of CrSBr
behavior: as one example, we use the fitted Hamiltonian to predict the presence
of chiral magnon edge modes with a spin-orbit enhanced CrSBr heterostructure.",2204.12461v2
2022-04-27,Beating Fabry-Pérot interference pattern in a magnonic scattering junction in the graphene quantum Hall ferromagnet,"At filling factor $\nu=0,\pm1$, the ground state of graphene is a particular
SU(4) ferromagnet which hosts a rich phase diagram along with several spin,
pseudospin or ""entanglement"" magnon modes. Motivated by recent experiments, we
study a $\nu=-1|0|-1$ Fabry-P\'erot magnonic junction. If the ground state at
$\nu=0$ is spin polarized, there exist two spin modes which interfere and
create a beating pattern, while pseudospin modes are reflected. The same
scenario occurs for pseudospin magnon if the $\nu=0$ ground state is pseudospin
polarized. The observation of such an interference pattern would provide
information on the low-energy anisotropies and thus on the ground state.",2204.12795v2
2022-04-29,Electrically Active Domain Wall Magnons in Layered van der Waals Antiferromagnets,"We study theoretically domain wall (DW) magnons-elementary collective
excitations of magnetic DWs -- in easy-axis layered van der Waals (vdW)
antiferromagnets, where they behave as normal modes of coupled spin
superfluids. We uncover that, due to spin-charge coupling in vdW magnets, such
DW magnons can be activated by voltage-induced torques, thereby providing a
path for their low-dissipation and nanoscale excitation. Moreover, the
electrical activation and the number of DW magnons at a frequency can be
controlled by applying symmetry-breaking static magnetic field, adding
tunability of signal transmission by them. Our results highlight that domain
walls in vdW magnets provide a promising platform to route coherent spin
information for a broad range of explorations in spintronics and magnetism.",2204.13909v1
2022-05-12,Violation of the magnonic Wiedemann-Franz law in the strong nonlinear regime,"The celebrated Wiedemann-Franz (WF) law which governs the relation between
charge and heat transport traces back to the experimental discovery in 1853 by
Wiedemann and Franz. Despite the fundamental difference of the
quantum-statistical properties between fermions and bosons, the linear-in-$T$
behavior of the WF law at low temperatures has recently been found to be the
universal property by the discovery of the WF law for magnon transport.
However, the WF law is for the linear response, and whether or not the
universal law is valid even in the nonlinear regime of Bose systems remains an
open issue. Here we provide a solution to this fundamental challenge. We show
that the ratio of the thermal to spin transport coefficient of magnons in
topologically trivial insulating magnets exhibits a different behavior from the
linear response and the universal law breaks down in the strong nonlinear
regime. This finding is within experimental reach with current device and
measurement technologies. Our discovery is the key ingredient in magnon-based
spintronics, in the evaluation of the figure of merit for thermomagnetic
conversion elements of spintronics devices.",2205.06366v1
2022-06-16,Defect-induced edge ferromagnetism and fractional spin excitations of the SU(4) $π$-flux Hubbard model on honeycomb lattice,"Recently, a SU(4) $\pi$-flux Hubbard model on the honeycomb lattice has been
proposed to study the spin-orbit excitations of $\alpha$-ZrCl$_3$
[Phys.~Rev.~Lett. 121.097201~(2017)]. Based on this model with a zigzag edge,
we show the edge defects can induce edge flat bands that result in a SU(4) edge
ferromagnetism. We develop an effective one-dimensional interaction Hamiltonian
to study the corresponding SU(4) spin excitations. Remarkably, SU(4) spin
excitations of the edge ferromagnet appear as a continuum covering the entire
energy region rather than usual magnons. Through further entanglement entropy
analysis, we suggest that the continuum consists of fractionalized spin
excitations from the disappeared magnons, except for that from the
particle-hole Stoner excitations. Moreover, in ribbon systems with finite
widths, the disappeared magnons can be restored in the gap formed by the
finite-size effect and the optical branch of the restored magnons are found to
be topological nontrivial.",2206.08036v1
2022-06-24,Ground state selection by magnon interactions in the fcc antiferromagnet,"We study the nearest-neighbor Heisenberg antiferromagnet on a face-centered
cubic lattice with arbitrary spin S. The model exhibits degenerate classical
ground states including two collinear structures AF1 and AF3 described by
different propagation vectors that are prime candidates for the quantum ground
state. We compute the energy for each of the two states as a function of S
using the spin-wave theory that includes magnon-magnon interaction in a
self-consistent way and the numerical coupled cluster method. Our results
unambiguously demonstrate that quantum fluctuations stabilize the AF1 state for
realistic values of spin. Transition to the harmonic spin-wave result, which
predicts the AF3 state, takes place only for S > 10. We also study quantum
renormalization of the magnon spectra for both states as a function of spin.",2206.12102v1
2022-06-29,Mechanical Bistability in Kerr-modified Cavity Magnomechanics,"Bistable mechanical vibration is observed in a cavity magnomechanical system,
which consists of a microwave cavity mode, a magnon mode, and a mechanical
vibration mode of a ferrimagnetic yttrium-iron-garnet (YIG) sphere. The
bistability manifests itself in both the mechanical frequency and linewidth
under a strong microwave drive field, which simultaneously activates three
different kinds of nonlinearities, namely, magnetostriction, magnon self-Kerr,
and magnon-phonon cross-Kerr nonlinearities. The magnon-phonon cross-Kerr
nonlinearity is first predicted and measured in magnomechanics. The system
enters a regime where Kerr-type nonlinearities strongly modify the conventional
cavity magnomechanics that possesses only a radiation-pressure-like
magnomechanical coupling. Three different kinds of nonlinearities are
identified and distinguished in the experiment. Our work demonstrates a new
mechanism for achieving mechanical bistability by combining magnetostriction
and Kerr-type nonlinearities, and indicates that such Kerr-modified cavity
magnomechanics provides a unique platform for studying many distinct
nonlinearities in a single experiment.",2206.14588v2
2022-06-29,Spin-Wave Optics in YIG by Ion-Beam Irradiation,"We demonstrate direct focused ion beam (FIB) writing as an enabling
technology for realizing spin-wave-optics devices. It is shown that ion-beam
irradiation changes the characteristics of YIG films on a submicron scale in a
highly controlled way, allowing to engineer the magnonic index of refraction
adapted to desired applications. This technique does not physically remove
material, and allows rapid fabrication of high-quality architectures of
modified magnetization in magnonic media with minimal edge damage (compared to
more common techniques such as etching or milling). By experimentally showing
magnonic versions of a number of optical devices (lenses, gratings,
Fourier-domain processors) we envision this technology as the gateway to
building magnonic computing devices that rival their optical counterparts in
their complexity and computational power.",2206.14696v1
2022-07-06,Topological magnons on the triangular kagome lattice,"We present the topology of magnons on the triangular kagome lattice (TKL) by
calculating its Berry curvature, Chern number and edge states. In addition to
the ferromagnetic state, the TKL hosts ferrimagnetic ground state as its two
sublattices can couple with each other either ferromagnetically or
antiferromagnetically. Using Holstein-Primakoff (HP) boson theory and Green's
function approach, we find that the TKL has a rich topological band structure
with added high Chern numbers compared with the kagome and honeycomb lattices.
The magnon edge current allows a convenient calculation of thermal Hall
coefficients and the orbital angular momentum gives correlation to the
Einstein-de Haas effect. We apply the calculations to the TKL and derive the
topological gyromagnetic ratio showing a nonzero Einstein-de Haas effect in the
zero temperature limit. Our results render the TKL as a potential platform for
quantum magnonics applications including high-precision mechanical sensors and
information transmission.",2207.02886v2
2022-07-26,Anderson-Higgs mass of magnons in superconductor/ferromagnet/superconductor systems,"Anderson-Higgs mechanism of mass generation is a generic concept in
high-energy and condensed matter physics. It shows up through the Meissner
effect providing the expulsion of static and low-frequency magnetic fields from
superconductors. However, it does not affect propagating electromagnetic waves
with a spectrum gap determined by the plasma frequency, which is too large to
be sensitive to the superconducting transition. Here we demonstrate the
spectroscopic manifestation of the Anderson-Higgs mass, showing that it
determines the spectrum gap of magnons in
superconductor/ferromagnet/superconductor multilayers. Moreover, we show that
this effect has been observed in recent experiments as a spontaneous
ferromagnetic resonance frequency shift in such systems. Our theory explains
many unusual experimental features and suggests effective controls over the
magnon spectrum with tunable spectral gap and group-velocity reversal. These
findings pave the way to a wide range of advanced functionalities for possible
applications in magnonics.",2207.13201v2
2022-08-22,Magnetically Tuned Continuous Transition from Weak to Strong Coupling in Terahertz Magnon Polaritons,"Depending on the relative rates of coupling and dissipation, a light-matter
coupled system is either in the weak- or strong-coupling regime. Here, we
present a unique system where the coupling rate continuously increases with an
externally applied magnetic field while the dissipation rate remains constant,
allowing us to monitor a weak-to-strong coupling transition as a function of
magnetic field. We observed a Rabi splitting of a terahertz magnon mode in
yttrium orthoferrite above a threshold magnetic field of ~14 T. Based on a
microscopic theoretical model, we show that with increasing magnetic field the
magnons transition into magnon polaritons through an exceptional point, which
will open up new opportunities for in situ control of non-Hermitian systems.",2208.10030v1
2022-08-23,Reversal of nanomagnets by propagating magnons in ferrimagnetic yttrium iron garnet enabling nonvolatile magnon memory,"Despite the unprecedented downscaling of CMOS integrated circuits,
memory-intensive machine learning and artificial intelligence applications are
limited by data conversion between memory and processor. There is a challenging
quest for novel approaches to overcome this so-called von Neumann bottleneck.
Magnons are the quanta of spin waves and transport angular momenta through
magnets. They enable power-efficient computation without charge flow and would
solve the conversion problem if spin wave amplitudes could be stored directly
in a magnetic memory cell. Here, we report the reversal of ferromagnetic
nanostripes by spin waves which propagate through an underlying spin-wave bus
made from yttrium iron garnet. Thereby, the charge-free angular momentum flow
is stored after transmission over a macroscopic distance. We show that spin
waves can reverse large arrays of ferromagnetic stripes at a strikingly small
power level of nW. Combined with the already existing wave logic, our discovery
is path-breaking for the new era of magnonics-based in-memory computation and
beyond von Neumann computer architectures.",2208.10923v1
2022-08-23,Field-induced spin nematic Tomonaga-Luttinger liquid of the $S=1/2$ spin ladder system with the anisotropic ferromagnetic rung interaction,"The $S=1/2$ quantum spin ladder system with the anisotropic ferromagnetic
exchange interaction on the rung under magnetic field is investigated using the
numerical diagonalization and the density matrix renormalization group (DMRG)
analyses. It is found that the nematic-spin-correlation-dominant
Tomonaga-Luttinger liquid (TLL) appears in some high magnetic field. It is
included in the TLL phase where the two-magnon bound state is realized. For
some suitable parameters, after the field-induced phase transition from this
two-magnon-bound TLL phase to the single-magnon TLL one, the re-entrant
transition to the two-magnon-bound TLL phase occurs, which is confirmed by the
magnetization curves by the DMRG. Several phase diagrams on the plane of the
coupling anisotropy versus the magnetization and the magnetic field are
presented. The present result is a proposal of the candidate system which
exhibits the spin nematic phase without the biquadratic interaction or the
frustration.",2208.10994v1
2022-09-14,Generation of Spin-Wave Pulses by Inverse Design,"The development of fast magnonic information processing nanodevices requires
operating with short spin-wave pulses, but, the shorter the pulses, the more
affected they are by information loss due to broadening and dispersion. The
capability of engineering spin-wave pulses and controlling their propagation
could solve this problem. Here, we provide a method to generate linear
spin-wave pulses with a desired spatial-temporal profile in magnonic waveguides
based on inverse design. As relevant examples, we theoretically predict that
both rectangular and self-compressing spin-wave pulses can be generated in
state-of-the-art waveguides with fidelities >96% using narrow stripline
antennas. The method requires minimal computational overhead and is universal,
i.e., it applies to arbitrary targeted pulse shapes, type of waves (exchange or
dipolar), waveguide materials, and waveguide geometries. It can also be
extended to more complex magnonic structures. Our results could lead to the
utilization of large-scale magnonic circuits for classical and quantum
information processing.",2209.06608v1
2022-10-14,Transversal transport of magnons in a modified Lieb lattice,"We studied a two-band magnon insulating model whose geometry is that of a
modified Lieb lattice in which one of the sites was removed. Anisotropic
ferromagnetic exchange interactions exist between the three nearest neighbors,
and the anisotropy opens a gap in the magnon energy band structure. A
non-vanishing Berry curvature is induced by a Dzyaloshinskii-Moriya interaction
(DMI). The topology of the bands is trivial (in the sense of a null Chern
number), but the finite Berry curvature induces Hall-like transport effects
whose coefficients were calculated. Their dependence on temperature was studied
and shows a resemblance with other magnon insulating systems found in the
literature. The dependence on exchange couplings, DMI parameter, and external
magnetic field was also investigated.",2210.07757v2
2022-10-21,Easy-plane antiferromagnet in tilted field: gap in magnon spectrum and susceptibility,"Motivated by recent experimental data on dichloro-tetrakis thiourea-nickel
(DTN) [T.A. Soldatov $\textit{et al}$, Phys. Rev. B ${\bf 101}$, 104410
(2020)], a model of antiferromagnet on a tetragonal lattice with single-ion
easy-plane anisotropy in the tilted external magnetic field is considered.
Using the smallness of the in-plane field component, we analytically address
field dependence of the energy gap in ``acoustic'' magnon mode and transverse
uniform magnetic susceptibility in the ordered phase. It is shown that the
former is non-monotonic due to quantum fluctuations, which was indeed observed
experimentally. The latter is essentially dependent on the ``optical'' magnon
rate of decay on two magnons. At magnetic fields close to the one which
corresponds to the center of the ordered phase, it leads to experimentally
observed dynamical diamagnetism phenomenon.",2210.12058v1
2022-10-27,Quantum-state engineering in cavity magnomechanics formed by two-dimensional magnetic materials,"Cavity magnomechanics has become an ideal platform to explore macroscopic
quantum effects. Bringing together magnons, phonons, and photons in a system,
it opens many opportunities for quantum technologies. It was conventionally
realized by an yttrium iron garnet, which exhibits a parametric magnon-phonon
coupling $\hat{m}^\dag\hat{m}(\hat{b}^\dag+\hat{b})$, with $\hat{m}$ and
$\hat{b}$ being the magnon and phonon modes. Inspired by the recent realization
of two-dimensional (2D) magnets, we propose a cavity magnomechanical system
using a 2D magnetic material with both optical and magnetic drivings. It
features the coexisting photon-phonon radiation-pressure coupling and quadratic
magnon-phonon coupling $\hat{m}^\dag\hat{m}(\hat{b}^\dag+\hat{b})^2$ induced by
the magnetostrictive interaction. A stable squeezing of the phonon and bi- and
tri-partite entanglements among the three modes are generated in the regimes
with a suppressed phonon number. Compared with previous schemes, ours does not
require any extra nonlinear interaction and reservoir engineering and is robust
against the thermal fluctuation. Enriching the realization of cavity
magnomechanics, our system exhibits its superiority in quantum-state
engineering due to the versatile interactions enabled by its 2D feature.",2210.15519v2
2022-11-04,Pattern recognition with a magnon-scattering reservoir,"Magnons are elementary excitations in magnetic materials and undergo
nonlinear multimode scattering processes at large input powers. In experiments
and simulations, we show that the interaction between magnon modes of a
confined magnetic vortex can be harnessed for pattern recognition. We study the
magnetic response to signals comprising sine wave pulses with frequencies
corresponding to radial mode excitations. Three-magnon scattering results in
the excitation of different azimuthal modes, whose amplitudes depend strongly
on the input sequences. We show that recognition rates above 95\% can be
attained for four-symbol sequences using the scattered modes, with strong
performance maintained with the presence of amplitude noise in the inputs.",2211.02328v1
2022-11-07,Interaction of gapless spin waves and a domain wall in an easy-cone ferromagnet,"We theoretically study the interaction of spin waves and a domain wall in a
quasi-one-dimensional easy-cone ferromagnet. The gapless spin waves on top of a
domain wall are found to exhibit finite reflection in contrast to the
well-known perfect transmission of gapful spin waves in easy-axis magnets.
Based on the obtained scattering properties, we study the thermal-magnon-driven
dynamics of a domain wall subjected to a thermal bias within the
Landau-B\""uttiker formalism, where transmitted magnons are shown to exert the
magnonic torque on the domain wall and thereby drive it with the velocity
linear to the applied thermal bias. The peculiar gapless nature of spin waves
in easy-cone magnets enables the thermally-driven domain-wall motion even at
low temperatures, differing from the easy-axis case where the domain-wall
velocity is exponentially suppressed at low temperatures. Our work suggests
that easy-cone magnets can serve as a useful platform to study the interaction
of gapless spin waves and nonlinear excitations and thereby realize
low-temperature magnon-related phenomena.",2211.03331v1
2022-11-10,Chiral coupling between a ferromagnetic magnon and a superconducting qubit,"Chiral coupling at the single-quantum level promises to be a remarkable
potential for quantum information processing. Here we propose to achieve a
chiral interaction between a magnon mode in a ferromagnetic sphere and a
superconducting qubit mediated by a one-dimensional coupled-cavity array. When
the qubit is coupled to two lattice sites of the array and each one is encoded
with a tunable phase, we can acquire a directional qubit-magnon interaction via
the quantum interference effect. This work opens up a new route to construct
chiral devices, which are expected to become a building block in quantum
magnonic networks.",2211.05285v2
2022-12-03,Strong On-Chip Microwave Photon-Magnon Coupling Using Ultra-low Damping Epitaxial Y3Fe5O12 Films at 2 Kelvin,"Y3Fe5O12 is arguably the best magnetic material for magnonic quantum
information science (QIS) because of its extremely low damping. We report
ultralow damping at 2 K in epitaxial Y3Fe5O12 thin films grown on a diamagnetic
Y3Sc2Ga3O12 substrate that contains no rare-earth elements. Using these
ultralow damping YIG films, we demonstrate for the first time strong coupling
between magnons in patterned YIG thin films and microwave photons in a
superconducting Nb resonator. This result paves the road towards scalable
hybrid quantum systems that integrate superconducting microwave resonators, YIG
film magnon conduits, and superconducting qubits into on-chip QIS devices.",2212.01708v1
2023-01-12,Towards Magnonic Logic and Neuromorphic Computing: Controlling Spin-Waves by Spin-Polarized Current,"Spin-waves (magnons) are among the prime candidates for building fast yet
energy-efficient platforms for information transport and computing. We here
demonstrate theoretically and in state-of-the-art micromagnetic simulation the
effects that strategically-injected spin-polarized current can have on
controlling magnonic transport. We reveal analytically that the Zhang-Li
spin-transfer-torque induced by applied current is analogous to the
Dzyaloshinskii-Moriya interaction for scattering the magnons in the linear
regime, to then provide a generalized Snell's law that describes the spin-wave
propagation across regions with different current densities. We validate the
latter in numerical simulations of realistic systems, and exemplify how these
findings may help advance the design of spin-wave logic and neuromorphic
computing devices.",2301.04922v1
2023-02-17,Control of the magnon-polariton hybridization with a microwave pump,"Pump-induced magnon modes (PIMs) are recently discovered elementary
excitations in ferrimagnets that offer significant tunability to spin dynamics.
Here, we investigate the coupling between a PIM and cavity magnon polaritons
(CMPs) by driving a cavity magnonic system away from equilibrium with a
microwave pump. In our experiment, the Walker mode simultaneously couples with
the PIM and cavity photons and thus combines two strongly coherent coupling
processes in a single cavity structure. Such a PIM-CMP hybridization system
acquires complementary properties from both the PIM and CMPs, allowing it to be
freely manipulated by the magnetic field, the pump power and the pump
frequency. These coherent manipulations exhibit unique behaviors beyond the
intrinsic properties limited by the material nature and electromagnetic
boundary conditions, thereby creating opportunities for extending the control
of hybrid devices.",2302.08665v2
2023-02-23,Giant supermagnonic Bloch point velocities by jet propulsion effect in cylindrical ferromagnetic nanowires,"Achieving high velocities of magnetic domain walls is a crucial factor for
their use as information carriers in modern nanoelectronic applications. In
nanomagnetism and spintronics, these velocities are often limited either by
internal domain wall instabilities, known as the Walker breakdown phenomenon,
or by spin wave emission, known as the magnonic regime. In the rigid domain
wall model, the maximum magnon velocity acts as an effective ""speed of light"",
providing a relativistic analogy for the domain wall speed limitation.
Cylindrical magnetic nanowires are an example of systems with the absence of
the Walker breakdown phenomenon. Here we demonstrate that in cylindrical
nanowires with high magnetization such as Iron, also the magnonic limit could
be outstandingly surpassed. Our numerical modelling shows the Bloch point
domain wall velocities as high as 14 km/s, well above the magnonic limit
estimated in the interval 1.7-2.0 km/s. The key ingredient is the conical shape
of the domain wall which elongates and breaks during the dynamics, leading to
domain wall acceleration due to the jet propulsion effect. This effect will be
very important for three-dimensional spintronic networks based on cylindrical
magnetic nanowires.",2302.12329v1
2023-03-20,Secondary Excitation of Spin-Waves: How Electromagnetic Cross-Talk Impacts on Magnonic Devices,"This work examines the impact of electromagnetic cross-talk in magnonic
devices when using inductive spin-wave (SW) transducers. We present detailed
electrical SW spectroscopy measurements showing the signal contributions to be
considered in magnonic device design. We further provide a rule of thumb
estimation for the cross-talk that is responsible for the secondary SW
excitation at the output transducer. Simulations and calibrated electrical
characterizations underpin this method. Additionally, we visualize the
secondary SW excitation via time-resolved MOKE imaging in the forward-volume
configuration in a 100nm Yttrium-Iron-Garnet (YIG) system. Our work is a step
towards fast yet robust joint electromagentic-micromagnetic magnonic device
design.",2303.11303v2
2023-03-27,Refined Spin Wave Model and Multi-magnon Bound States in $Li_{2}CuO_{2}$,"Here we report a study of the spin dynamics in the ferromagnetic chain
compound $Li_{2}CuO_{2}$. Inelastic neutron scattering measurements allow for
the spin Hamiltonian to be determined using a $J_{1}-J_{2}$ $XXZ$-Heisenberg
spin chain model with weak interchain interactions. The primary exchange
parameters determined from our data are qualitatively consistent with those of
Lorenz $et$ $al.$ [Europhys. Lett. 88, 37002 (2009)], and our data allow for
the resolution of additional interchain exchange interactions. We also observe
the formation of two- and, potentially, three-magnon bound states. The
two-magnon bound state exists only in the magnetically ordered phase of this
material, consistent with stabilization by the weak, Ising-like exchange
anisotropy of the nearest-neighbor intrachain interaction. In contrast, the
potential three-magnon state persists in a finite temperature regime above
$T_{N}$, indicating an unconventional character. Our results establish
$Li_{2}CuO_{2}$ as an experimental platform for the study of exchange
anisotropy-stabilized bound states in a ferromagnetic chain.",2303.15639v2
2023-03-28,Topological phase diagrams of in-plane field polarized Kitaev magnets,"While the existence of a magnetic field induced quantum spin liquid in Kitaev
magnets remains under debate, its topological properties often extend to
proximal phases where they can lead to unusual behaviors of both fundamental
and applied interests. Subjecting a generic nearest neighbor spin model of
Kitaev magnets to a sufficiently strong in-plane magnetic field, we study the
resulting polarized phase and the associated magnon excitations. In contrast to
the case of an out-of-plane magnetic field where the magnon band topology is
enforced by a three-fold symmetry, we find that it is possible for
topologically trivial and nontrivial parameter regimes to coexist under
in-plane magnetic fields. We map out the topological phase diagrams of the
magnon bands, revealing a rich pattern of variation of the Chern number over
the parameter space and the field angle. We further compute the magnon thermal
Hall conductivity as a weighted summation of Berry curvatures, and discuss
experimental implications of our results to planar thermal Hall effects in
Kitaev magnets.",2303.16222v1
2023-04-01,Controllable Fano-type optical response and four-wave mixing via magnetoelastic coupling in a opto-magnomechanical system,"We analytically investigate the Fano-type optical response and four-wave
mixing (FWM) process by exploiting the magnetoelasticity of a ferromagnetic
material. The deformation of the ferromagnetic material plays the role of
mechanical displacement, which is simultaneously coupled to both optical and
magnon modes. We report that the magnetostrictively induced displacement
demonstrates Fano profiles, in the output field, which is well-tuned by
adjusting the system parameters, like effective magnomechanical coupling,
magnon detuning, and cavity detuning. It is found that the magnetoelastic
interaction also gives rise to the FWM phenomenon. The number of the FWM
signals mainly depends upon the effective magnomechanical coupling and the
magnon detuning. Moreover, the FWM spectrum exhibits suppressive behavior upon
increasing (decreasing) the magnon (cavity) decay rate. The present scheme will
open new perspectives in highly sensitive detection and quantum information
processing.",2304.00237v1
2023-04-17,Geometric similarities and topological phases in surface magnon polaritons,"Highly spatially-squeezed polaritons, with propagation momentum significantly
larger than free-space modes at the same frequency, enable varied and extreme
control over light-matter interaction. Compared to other polaritons, surface
magnon polaritons, the magnetic counterpart of surface phonon polaritons, have
received relatively little attention. Here, we investigate the dispersion and
properties of surface-magnon polaritons, highlighting the impact of geometric
similarities and applying them to various surface-magnon polariton devices in
both conventional and topological settings. Our theory predicts a method for
strongly localizing and significantly enhancing magnetic fields in the
microwave range and developing compact and lossless connectors capable of
interconnecting waveguides with vastly different input and output impedances.
Our work opens new avenues for manipulating magnetic fields in the microwave
regime and for exploring topological phases in polariton platforms.",2304.09763v1
2023-05-09,Magnon dispersion in ferromagnetic SrRuO$_3$,"The magnetic excitations in ferromagnetic SrRuO$_3$ were studied by inelastic
neutron scattering combining experiments on triple-axis and time-of-flight
spectrometers with and without polarization analysis. A quadratic spin-wave
dispersion with an anisotropy gap describes the low-energy low-temperature
response. The magnon dispersion extends to at least 35 meV and there is no
direct evidence for a continuum of Stoner excitations below this energy.
However, the magnon response is weakened at higher energy. In addition to the
anomalous softening of the spin-wave stiffness and of the gap, which is induced
by the topology of the Bloch states, the magnon excitations are broadened in
energy and this effect increases upon heating.",2305.05497v1
2023-05-10,Proposal for optomagnonic teleportation and entanglement swapping,"A protocol for realizing discrete-variable quantum teleportation in an
optomagnonic system is provided. Using optical pulses, an arbitrary photonic
qubit state encoded in orthogonal polarizations is transferred onto the joint
state of a pair of magnonic oscillators in two macroscopic yttrium-iron-garnet
(YIG) spheres that are placed in an optical interferometer. We further show
that optomagnonic entanglement swapping can be realized in an extended
dual-interferometer configuration with a joint Bell-state detection.
Consequently, magnon Bell states are prepared. We analyze the effect of the
residual thermal occupation of the magnon modes on the fidelity in both the
teleportation and entanglement swapping protocols.",2305.05889v1
2023-06-03,Giant Enhancement of Magnonic Frequency Combs by Exceptional Points,"With their incomparable time-frequency accuracy, frequency combs have
significantly advanced precision spectroscopy, ultra-sensitive detection, and
atomic clocks. Traditional methods to create photonic, phononic, and magnonic
frequency combs hinge on material nonlinearities which are often weak,
necessitating high power densities to surpass their initiation thresholds,
which subsequently limits their applications. Here, we introduce a novel
nonlinear process to efficiently generate magnonic frequency combs (MFCs) by
exploiting exceptional points (EPs) in a coupled system comprising a
pump-induced magnon mode and a Kittel mode. Even without any cavity, our method
greatly improves the efficiency of nonlinear frequency conversion and achieves
optimal MFCs at low pump power. Additionally, our novel nonlinear process
enables excellent tunability of EPs using the polarization and power of the
pump, simplifying MFC generation and manipulation. Our work establishes a
synergistic relationship between non-Hermitian physics and MFCs, which is
advantages for coherent/quantum information processing and ultra-sensitive
detection.",2306.02120v1
2023-06-04,Topological magnets and magnons in twisted bilayer MoTe$_2$ and WSe$_2$,"Twisted homobilayer transition metal dichalcogenide (TMD) offers a versatile
platform for exploring band topology, interaction-driven phases, and magnetic
orders. We study the interaction-driven phases in twisted TMD homobilayers and
their low-energy collective excitations, focusing on the effect of band
topology on magnetism and its thermal stability. From Hartree-Fock theory of
the continuum model, we identify several magnetic and topological phases. By
tuning the displacement field, we find two phase transitions involving a change
in topology and magnetism respectively. We analyze the magnon spectrum,
revealing the crucial role of band topology in stabilizing 2D ferromagnetism by
amplifying easy-axis magnetic anisotropy, resulting in a large magnon gap of up
to 7 meV. As the magnon gap is directly tied to the stability of the magnetic
phase to thermal fluctuations, our findings have several important experimental
implications.",2306.02501v2
2023-06-26,Repulsively bound magnon excitations of a spin-1/2 XXZ chain in a staggered transverse field,"We study the excitation spectrum of the one-dimensional spin-1/2 XXZ chain
with antiferromagnetic Ising anisotropy across a magnetic quantum phase
transition induced by the application of a site-dependent transverse magnetic
field. Motivated by the chain antiferromagnet BaCo$_2$V$_2$O$_8$, we consider a
situation where the transverse magnetic field has a strong uniform component
and a weaker staggered part. To determine the nature of the excitations giving
rise to the spin dynamical structure factor, we use a combination of analytical
approaches and the numerically exact time-dependent matrix product state
method. We identify below the quantum phase transition high-energy many-body
two-magnon and three-magnon repulsively bound states which are clearly visible
due to the staggered component of the magnetic field. At high magnetic fields
and low temperature, single magnons dominate the dynamics. Our theory results
are in very good agreement with terahertz spectroscopy experimental results
presented in [Wang et al., in preparation].",2306.14742v2
2023-06-30,Spin Swapping for an Exchange Magnon Spin Current,"We propose the spin swapping effect for an exchange magnon spin current in a
perpendicularly magnetized ferromagnetic medium with in plane anisotropy on the
surface. The excitation of a magnon current flowing along an in-plane direction
with an out-of-plane spin polarization leads to the generation of a secondary
exchange spin current propagating along the out-of-plane direction,
characterized by an in-plane spin polarization. The resulting exchange magnon
spin current can induce an inverse spin Hall voltage of micro-volts. The
exchange coupling at the interface between regions with different magnetic
anisotropies plays a crucial role in generating the spin swapping effect. This
is in contrast to the recently reported spin swapping for an exchange spin
current in a canted antiferromagnet due to the Dzyaloshiskii-Moriya
interaction.",2306.17383v1
2023-07-06,Realization of the unidirectional amplification in a cavity magnonic system,"We experimentally demonstrate the nonreciprocal microwave amplification using
a cavity magnonic system, consisting of a passive cavity (i.e., the split-ring
resonator), an active feedback circuit integrated with an amplifier, and a
ferromagnetic spin ensemble (i.e., a yttrium-iron-garnet sphere). Combining the
amplification provided by the active circuit and the nonreciprocity supported
by the cavity magnonics, we implement a nonreciprocal amplifier with the
functions of both unidirectional amplification and reverse isolation. The
microwave signal is amplified by 11.5 dB in the forward propagating direction
and attenuated in the reverse direction by -34.7 dB, giving an isolation ratio
of 46.2 dB. Such a unidirectional amplifier can be readily employed in quantum
technologies, where the device can simultaneously amplify the weak signal
output by the quantum system and isolate the sensitive quantum system from the
backscattered external noise. Also, it is promising to explore more functions
and applications using a cavity magnonic system with real gain.",2307.02826v2
2023-07-24,In-plane magnetocrystalline anisotropy in the van der Waals antiferromagnet FePSe$_3$ probed by magneto-Raman scattering,"Magnon gap excitations selectively coupled to phonon modes have been studied
in FePSe$_3$ layered antiferromagnet with magneto-Raman scattering experiments
performed at different temperatures. The bare magnon excitation in this
material has been found to be split (by $\approx~1.2$ cm$^{-1}$) into two
components each being selectively coupled to one of the two degenerated, nearby
phonon modes. Lifting the degeneracy of the fundamental magnon mode points out
toward the biaxial character of the FePS$_3$ antiferromagnet, with an
additional in-plane anisotropy complementing much stronger, out-of-plane
anisotropy. Moreover, the tunability, with temperature, of the phonon- versus
the magnon-like character of the observed coupled modes has been demonstrated.",2307.12692v1
2023-08-04,Strong squeezing of microwave output fields via reservoir-engineered cavity magnomechanics,"We show how to achieve strong squeezing of a microwave output field by
reservoir engineering a cavity magnomechanical system, consisting of a
microwave cavity, a magnon mode, and a mechanical vibration mode. The magnon
mode is simultaneously driven by two microwave fields at the blue and red
sidebands associated with the vibration mode. The two-tone drive induces a
squeezed magnonic reservoir for the intracavity field, leading to a squeezed
cavity mode due to the cavity-magnon state swapping, which further yields a
squeezed cavity output field. The squeezing of the output field is stationary
and substantial using currently available parameters in cavity magnomechanics.
The work indicates the potential of the cavity magnomechanical system in
preparing squeezed microwave fields, and may find promising applications in
quantum information science and quantum metrology.",2308.02222v3
2023-08-15,Tunable topological magnon-polaron states and anomalous Hall phenomena in two-dimensional ferromagnetic insulators,"We study magnon-polaron hybrid states, mediated by Dzyaloshinskii-Moriya and
magnetoelastic interactions, in a two-dimensional ferromagnetic insulator. The
magnetic system consists of both in-plane and flexural acoustic and optical
phonon bands, as well as acoustic and optical magnon bands. Through
manipulation of the ground-state magnetization direction using a magnetic
field, we demonstrate the tunability of Chern numbers and (spin) Berry
curvatures of magnon-polaron hybrid bands. This adjustment subsequently
modifies two anomalous Hall responses of the system, namely, thermal Hall and
spin Nernset signals. Notably, we find that by changing the magnetic field
direction in particular directions, it is possible to completely suppress the
thermal Hall signal while maintaining a finite spin Nernst signal. Our finding
reveals the intricate interplay between topological and quantum geometrical
phenomena and magnetic ordering, offering compelling avenues for on-demand
control over emergent quantum states in condensed matter systems.",2308.08048v2
2023-08-18,Spin-wave dispersion and magnon chirality in multiferroic TbMnO3,"Inelastic neutron scattering experiments combining time-of-flight and
polarized techniques yield a comprehensive picture of the magnon dispersion in
multiferroic TbMnO3 including the dynamic chirality. Taking into account only
Mn3+ moments, spin-wave calculations including nearest-neighbor interactions,
frustrating next-nearest neighbor exchange as well as single-ion anisotropy and
antisymmetric terms describe the energy dispersion and the distribution of
neutron scattering intensity in the multiferroic state very well. Polarized
neutron scattering reveals strong dynamic chirality of both signs that may be
controlled by external electric fields in the multiferroic phase. Also above
the onset of long-range multiferroic order in zero electric field, a small
inelastic chiral component can be inverted by an electric field. The
microscopic spin-wave calculations fully explain also the dynamic chirality of
magnetic excitations, which is imprinted by the static chirality of the
multiferroic phase. The ordering of Tb3+ moments at lower temperature reduces
the broadening of magnons but also renders the magnon dispersion more complex.",2308.09407v1
2023-08-28,Spin wave mode conversion in an in-plane magnetized microscale T-shaped YIG magnonic splitter,"As one of the fundamental magnonic devices, a magnonic splitter device has
been proposed and spin wave propagation in this device has been studied
numerically and experimentally. In the present work, we fabricated a T-shaped
magnonic splitter with 6 $\mu$m-wide three arms using a 100 nm-thick yttrium
iron garnet film and, using time-resolved magneto-optic Kerr microscopy,
observed that spin waves split into both, the vertical and the horizontal
direction at the junction. Analyzing the results, we found that spin wave modes
are converted into another during the splitting process and the splitting
efficiency is dominantly dependent on the 1st order of incoming spin waves.",2308.14327v1
2023-09-13,Magnon gap excitations in van der Waals antiferromagnet MnPSe$_3$,"Magneto-spectroscopy methods have been employed to study the zero-wavevector
magnon excitations in MnPSe$_3$. Experiments carried out as a function of
temperature and the applied magnetic field show that two low-energy magnon
branches of MnPSe$_3$ in its antiferromagnetic phase are gapped. The
observation of two low-energy magnon gaps (at 14 and 0.7 cm$^{-1}$) implies
that MnPSe$_3$ is a biaxial antiferromagnet. A relatively strong out-of-plane
anisotropy imposes the spin alignment to be in-plane whereas the spin
directionality within the plane is governed by a factor of 2.5 $\times$
10$^{-3}$ weaker in-plane anisotropy.",2309.06866v1
2023-09-13,Topological superconductivity mediated by magnons of helical magnetic states,"We recently showed that spin fluctuations of noncoplanar magnetic states can
induce topological superconductivity in an adjacent normal metal [M{\ae}land et
al., Phys. Rev. Lett. 130, 156002 (2023)]. The noncolinear nature of the spins
was found to be essential for this result, while the necessity of noncoplanar
spins was unclear. In this paper we show that magnons in coplanar, noncolinear
magnetic states can mediate topological superconductivity in a normal metal.
Two models of the Dzyaloshinskii-Moriya interaction are studied to illustrate
the need for a sufficiently complicated Hamiltonian describing the magnetic
insulator. The Hamiltonian, in particular the specific form of the
Dzyaloshinskii-Moriya interaction, affects the magnons and by extension the
effective electron-electron interaction in the normal metal. Symmetry arguments
are applied to complement this discussion. We solve a linearized gap equation
in the case of weak-coupling superconductivity. The result is a
time-reversal-symmetric topological superconductor, as confirmed by calculating
the topological invariant. In analogy with magnon-mediated superconductivity
from antiferromagnets, Umklapp scattering enhances the critical temperature of
superconductivity for certain Fermi momenta.",2309.07211v2
2023-09-26,Kinetic Ferromagnetism and Topological Magnons of the Hole-Doped Kitaev Spin Liquid,"We study the effect of hole doping on the Kitaev spin liquid (KSL) and find
that for ferromagnetic (FM) Kitaev exchange $K$ the system is very susceptible
to the formation of a FM spin polarization. Through density matrix
renormalization group (DMRG) simulations on finite systems, we uncover that the
introduction of a single hole with a hopping strength of just $t\sim{}0.28K$ is
enough to disrupt fractionalization and polarize the spins in the [001]
direction due to an order-by-disorder mechanism. Taking into account a material
relevant FM anisotropic spin exchange $\varGamma$ drives the polarization
towards the [111] direction via a reorientation transition into a topological
FM state with chiral magnon excitations. We develop a parton mean-field theory
incorporating fermionic holons and bosonic spinons/magnons, which accounts for
the doping induced FM phases and topological magnon excitations. We discuss
experimental signatures and implications for Kitaev candidate materials.",2309.15153v1
2023-10-23,Spin pumping in an altermagnet/normal metal bilayer,"Altermagnetism is a subclass of antiferromagnetism that features
spin-polarized electron bands of a non-relativistic origin despite the absence
of a net magnetiation in the material. We here theoretically study spin pumping
from an altermagnetic insulator into a normal metal. The symmetry properties of
the lattice and spin order of the altermagnet alters the magnon dispersion
compared to a conventional square lattice antiferromagnet. Nevertheless, the
pumped spin current turns out to be equal to the current which is pumped from a
conventional antiferromagnet. This occurs so long that the magnetic field which
sets the altermagnetic spins into precessional motion is spatially homogeneous.
These results show that while spin pumping is possible using altermagnets, the
altermagnetic spin order does not readily leave unique fingerprints in the
pumped spin current. Our model provides a suitable starting point for
investigating more complex models where finite momentum magnons contribute to
spin pumping due to magnon-magnon interactions or where the magnetic field
inducing spin pumping is spatially inhomogeneous.",2310.15220v1
2023-11-15,Broad-Wavevector Spin Pumping of Flat-Band Magnons,"We report the experimental observation of large spin pumping signals in
YIG/Pt system driven by broad-wavevector spin-wave spin current. 280 nm-wide
microwave inductive antennas offer broad-wavevector excitation which, in
combination with quasi-flatband of YIG, allows a large number of magnons to
participate in spin pumping at a given frequency. Through comparison with
ferromagnetic resonance spin pumping, we attribute the enhancement of the spin
current to the multichromatic magnons. The high efficiency of spin current
generation enables us to uncover nontrivial propagating properties in ultra-low
power regions. Additionally, our study achieves the spatially separated
detection of magnons, allowing the direct extraction of the decay length. The
synergistic combination of the capability of broad-wavevector excitation,
enhanced voltage signals, and nonlocal detection provides a new avenue for the
electrical exploration of spin waves dynamics.",2311.09098v1
2023-11-23,Exciting high-frequency short-wavelength spin waves using high harmonics of a magnonic cavity mode,"Confined spin-wave modes are a promising object for studying nonlinear
effects and future quantum technologies. Here, using micromagnetic simulations,
we use a microwave magnetic field from a coplanar waveguide (CPW) to pump a
standing spin-wave confined in the cavity of magnonic crystal. We find that the
frequency of the fundamental cavity mode is equal to the ferromagnetic
resonance frequency of the plane film and overlaps with the magnonic bandgap,
allowing high magnetic field tunability. Multi-frequency harmonics of the
cavity mode are generated once the microwave amplitude surpasses a certain
threshold. Specifically, the second and third harmonics at 0.5 T equate to 48.6
and 72.9 GHz with wavelengths of 44 and 22 nm respectively, which propagate
into the crystal. This effect reaches saturation when the CPW covers the entire
cavity, making the system feasible for realization. These processes show
potential for the advancement of magnonics at high-frequencies and very
short-wavelengths.",2311.14143v1
2023-11-27,Zero-field spin wave turns,"Spin-wave computing, a potential successor to CMOS-based technologies, relies
on the efficient manipulation of spin waves for information processing. While
basic logic devices like magnon transistors, gates, and adders have been
experimentally demonstrated, the challenge for complex magnonic circuits lies
in steering spin waves through sharp turns. In this study we demonstrate with
micromagnetic simulations and Brillouin light scattering microscopy
experiments, that dipolar spin waves can propagate through 90-degree turns
without distortion. The key lies in carefully designed in-plane magnetization
landscapes, addressing challenges posed by anisotropic dispersion. The
experimental realization of the required magnetization landscape is enabled by
spatial manipulation of the uniaxial anisotropy using corrugated magnonic
waveguides. The findings presented in this work should be considered in any
magnonic circuit design dealing with anisotropic dispersion and spin wave
turns.",2311.16335v1
2023-11-28,Coherent phonon-magnon interactions detected by micro-focused Brillouin light scattering spectroscopy,"We investigated the interaction of surface acoustic waves and spin waves with
spatial resolution by micro-focused Brillouin light scattering spectroscopy in
a Co$_{40}$Fe$_{40}$B$_{20}$ ferromagnetic layer on a LiNbO$_{3}$-piezoelectric
substrate. We experimentally demonstrate that the magnetoelastic excitation of
magnons by phonons is coherent by studying the interfering BLS-signals of the
phonons and magnons during their conversion process.We find a pronounced
spatial dependence of the phonon annihilation and magnon excitation which we
map as a function of the magnetic field. The coupling efficiency of the surface
acoustic waves (SAWs) and the spin waves (SWs) is characterized by a magnetic
field dependent decay of the SAWs amplitude.",2311.16688v1
2023-12-14,"Tripartite Phonon-Magnon-Plasmon Coupling, Parametric Amplification, and Formation of a Phonon-Magnon-Plasmon Polariton in a Two-Dimensional Periodic Array of Magnetostrictive/Plasmonic Bilayered Nanodots","Coupling between spin waves (SWs) and other types of waves in nanostructured
magnetic media has garnered increased attention in recent years because of the
rich physics and the potential to produce disruptive technologies. Among this
family of intriguing phenomena, we recently reported a new one: coupling
between SWs and hybridized phonon-plasmon waves, resulting in tripartite
coupling of magnons, phonons, and plasmons. Here, this acousto-plasmo-magnonic
phenomenon is studied in a two-dimensional periodic array of bilayered [Co/Al]
nanodots on a silicon substrate, where the Co is a magnetostrictive constituent
responsive to magneto-elastic coupling and the Al acts as a source of surface
plasmons. Time-resolved magneto-optical-Kerr-effect microscopy revealed
parametric amplification and strong coupling between two spin wave modes
mediated by a hybrid phonon-plasmon wave. The strong coupling forms a new
quasi-particle: the phonon-plasmonmagnon polariton.",2312.09343v1
2023-12-17,Cryogenic hybrid magnonic circuits based on spalled YIG thin films,"Yttrium iron garnet (YIG) magnonics has sparked extensive research interests
toward harnessing magnons (quasiparticles of collective spin excitation) for
signal processing. In particular, YIG magnonics-based hybrid systems exhibit
great potentials for quantum information science because of their wide
frequency tunability and excellent compatibility with other platforms. However,
the broad application and scalability of thin-film YIG devices in the quantum
regime has been severely limited due to the substantial microwave loss in the
host substrate for YIG, gadolinium gallium garnet (GGG), at cryogenic
temperatures. In this study, we demonstrate that substrate-free YIG thin films
can be obtained by introducing the controlled spalling and layer transfer
technology to YIG/GGG samples. Our approach is validated by measuring a hybrid
device consisting of a superconducting resonator and a spalled YIG film, which
gives a strong coupling feature indicating the good coherence of our system.
This advancement paves the way for enhanced on-chip integration and the
scalability of YIG-based quantum devices.",2312.10660v2
2024-01-06,Cavity magnonics with domain walls in insulating ferromagnetic wires,"Magnetic domain walls (DWs) are topological defects that exhibit robust
low-energy modes that can be harnessed for classical and neuromorphic
computing. However, the quantum nature of these modes has been elusive thus
far. Using the language of cavity optomechanics, we show how to exploit a
geometric Berry-phase interaction between the localized DWs and the extended
magnons in short ferromagnetic insulating wires to efficiently cool the DW to
its quantum ground state or to prepare nonclassical states exhibiting a
negative Wigner function that can be extracted from the power spectrum of the
emitted magnons. Moreover, we demonstrate that magnons can mediate long-range
entangling interactions between qubits stored in distant DWs, which could
facilitate the implementation of a universal set of quantum gates. Our proposal
relies only on the intrinsic degrees of freedom of the ferromagnet, and can be
naturally extended to explore the quantum dynamics of DWs in ferrimagnets and
antiferromagnets, as well as quantum vortices or skyrmions confined in
insulating magnetic nanodisks.",2401.03164v1
2024-01-09,Signatures of novel magnon-phonon coupling in frustrated double perovskite square lattices,"Low-dimensional frustrated magnetic square networks feature a variety of
unconventional phases with novel emergent excitations. Often these excitations
are intertwined and manifest into intriguing phenomena, an area that has
remained largely unexplored in square-lattice systems, especially, double
perovskites (A2BB'O6). In this study, we explore these interactions between the
fundamental excitations such as phonons and magnons in square-lattice
Sr2CuTeO6, Sr2CuWO6, and Ba2CuWO6 isostructural double perovskites that exhibit
both short-ranged (TS) as well as long-ranged Neel antiferromagnetic (TN)
transitions. Our Raman measurements at variable temperatures reveal an
intriguing broad peak (identified as 2-magnon (2M)) surviving beyond TS for
W-based compositions contrary to the Te-based system, suggesting a key role of
diamagnetic B'-site cation on their magnetism. The thermal response of 2M
intriguingly shows signatures of correlation with phonons and control over
their anharmonicity, depicting magnon-phonon interaction. Further, a few
phonons exhibit anomalies across the magnetic transitions implying the presence
of spin-phonon coupling. In particular, the phonon modes at ~ 194 cm-1 of
Sr2CuTeO6 and ~ 168 cm-1 of Sr2CuWO6, that show a strong correlation with the
2M, exhibit the strongest spin-phonon coupling suggesting their roles in
mediating magnon-phonon interactions in these systems.",2401.04624v1
2024-01-11,Room-temperature Magnetic Thermal Switching by Suppressing Phonon-Magnon Scattering,"Thermal switching materials, whose thermal conductivity can be controlled
externally, show great potential in contemporary thermal management.
Manipulating thermal transport properties through magnetic fields has been
accomplished in materials that exhibit a high magnetoresistance. However, it is
generally understood that the lattice thermal conductivity attributed to
phonons is not significantly impacted by the magnetic fields. In this study, we
experimentally demonstrate the significant impact of phonon-magnon scattering
on the thermal conductivity of the rare-earth metal gadolinium near room
temperature, which can be controlled by a magnetic field to realize thermal
switching. Using first-principles lattice dynamics and spin-lattice dynamics
simulations, we attribute the observed change in phononic thermal conductivity
to field-suppressed phonon-magnon scattering. This research suggests that
phonon-magnon scattering in ferromagnetic materials is crucial for determining
their thermal conductivity, opening the door to innovative
magnetic-field-controlled thermal switching materials.",2401.05607v1
2024-01-20,Finite-Temperature Hole-Magnon Dynamics in an Antiferromagnet,"Employing the numerically accurate multiple Davydov Ansatz in combination
with the thermo-field dynamics approach, we delve into interplay of the
finite-temperature dynamics of holes and magnons in an antiferromagnet, which
allows for scrutinizing previous predictions from self-consistent Born
approximation while offering, for the first time, accurate finite-temperature
computation of detailed magnon dynamics as a response and a facilitator to the
hole motion. The study also uncovers pronounced temperature dependence of the
magnon and hole populations, pointing to the feasibility of potential thermal
manipulation and control of hole dynamics. Our methodology can be applied not
only to the calculation of steady-state angular-resolved photoemission spectra,
but also to the simulation of femtosecond terahertz pump-probe and other
nonlinear signals for the characterization of antiferromagnetic materials.",2401.11184v1
2024-01-31,Keldysh Theory of Thermal Transport in Multiband Hamiltonians,"We establish a comprehensive theoretical framework for systems subjected to a
static uniform temperature gradient, employing the non-equilibrium
Keldysh-Dyson formalism. This framework interprets the statistical force due to
the temperature gradient as a mechanical force, utilizing both Luttinger's
scalar and Moreno-Coleman-Tatara's vector potentials, which collectively
emulate the gauge invariance stemming from the conservation of energy. Our
approach has the ability to treat heat current and heat magnetization on an
equal footing, thereby extending and generalizing previous formalisms. The
derived result for the thermal conductivity is applied to investigate the
thermal characteristics of Weyl magnons in a stacked honeycomb ferromagnet
featuring a trivial insulator phase, a magnon Chern insulator phase, and three
Weyl magnon phases. Against the expectation from the Berry curvature, the
magnon Chern insulator phase exhibits the highest transverse thermal
conductivity.",2401.17652v1
2024-02-10,Magnons and spikes for $\mathcal{N}=2$ linear quivers and their non-Abelian T-duals,"We compute the spectra associated with various semiclassical string states
that propagate over $\mathcal{N}=2$ Gaiotto-Maldacena backgrounds. As an
interesting special case, for the Abelian T- dual solution, we discover giant
magnon and single spike configurations while imposing appropriate boundary
conditions. However, for Sfetsos-Thompson backgrounds, one has to adopt a
different string embedding which reveals ``modified'' dispersion relations both
for magnons and spikes. These results boil down into the standard dispersion
relations in the limit when the rank of the associated $SU(N_c)$ color gauge
group becomes large enough. We further generalize our analysis in the presence
of flavor D6 branes. Our analysis reveals a new set of dispersion relations,
which shows that both magnons and spikes are ceased to exist in the presence of
flavor excitations.",2402.06897v1
2024-03-18,SU(3) gauge field of magnons in antiferromagnetic skyrmion crystals,"Quasiparticle excitations in material solids often experience a fictitious
gauge field, which can be a potential source of intriguing transport phenomena.
Here, we show that low-energy excitations in insulating antiferromagnetic
skyrmion crystals on the triangular lattice are effectively described by
magnons with an SU(3) gauge field. The three-sublattice structure in the
antiferromagnetic skyrmion crystals is inherited as three internal degrees of
freedom for the magnons, which are coupled with their kinetic motion via the
SU(3) gauge field that arises from the topologically nontrivial spin texture in
real space. We also demonstrate that the non-commutativity of the SU(3) gauge
field breaks an effective time-reversal symmetry and contributes to a magnon
thermal Hall effect.",2403.11655v2
2024-04-04,Spin dynamics and possible topological magnons in non-stoichiometric pyrochlore iridate Tb$_2$Ir$_2$O$_7$ studied by RIXS,"We report a resonant inelastic X-ray scattering study on a single crystal of
a non-stoichiometric pyrochlore iridate Tb$_{2+x}$Ir$_{2-x}$O$_{7-y}$ ($x
\simeq 0.25$) that magnetically orders at $T_{\rm{N}}\simeq 50$ K. We find that
the strength of the spin-orbit coupling and the trigonal distortion of the
IrO$_6$ octahedra are comparable with the ones obtained in other pyrochlore
iridates. We observe a propagating gapped magnon mode at low energy, and model
it using a Hamiltonian consisting of a Heisenberg exchange [$J = 16.2(9)$ meV]
and Dzyaloshinskii-Moriya interactions [$D = 5.2(3)$ meV], which shows the
robustness of interactions despite Tb-stuffing at the Ir-site. Strikingly, the
ratio $D/J = 0.32(3)$ supports possible non-trivial topological magnon band
crossing. This material may thus host coexisting fermionic and bosonic
topology, with potential for manipulating electronic and magnonic topological
bands thanks to the $d-f$ interaction.",2404.03829v1
2013-10-31,Signatures of electron-magnon interaction in charge and spin currents in magnetic tunnel junctions: A nonequilibrium many-body perturbation theory approach,"We develop a numerically exact scheme for resumming certain classes of
Feynman diagrams in the self-consistent perturbation expansion for the electron
and magnon self-energies in the nonequilibrium Green function formalism applied
to a coupled electron-magnon (\mbox{e-m}) system which is driven out of
equilibrium by the applied finite bias voltage. Our scheme operates with the
electronic and magnonic GFs and the corresponding self-energies viewed as
matrices in the Keldysh space, rather than conventionally extracting their
retarded and lesser components. This is employed to understand the effect of
inelastic \mbox{e-m} scattering on charge and spin current vs. bias voltage
$V_b$ in F/I/F magnetic tunnel junctions (MTJs), which are modeled on a
one-dimensional (1D) tight-binding lattice for the electronic subsystem and 1D
Heisenberg model for the magnonic subsystem. For this purpose, we evaluate Fock
diagram for the electronic self-energy and the electron-hole polarization
bubble diagram for the magnonic self-energy. The respective electronic and
magnonic GF lines within these diagrams are the fully interacting ones, thereby
requiring to solve the ensuing coupled system of nonlinear integral equations
self-consistently. Despite using the 1D model and treating \mbox{e-m}
interaction in many-body fashion only within a small active region consisting
of few lattice sites around the F/I interface, our analysis captures essential
features of the so-called zero-bias anomaly observed in both MgO- and
AlO$_x$-based realistic 3D MTJs where the second derivative $d^2 I/dV_b^2$
(i.e., inelastic electron tunneling spectrum) of charge current exhibits sharp
peaks of opposite sign on either side of the zero bias voltage.",1310.8551v4
2017-08-10,Nearly deconfined spinon excitations in the square-lattice spin-1/2 Heisenberg antiferromagnet,"We study the dynamic spin structure factor of the spin-$1/2$ square-lattice
Heisenberg antiferromagnet and of the $J$-$Q$ model (with 4-spin interactions
$Q$ and Heisenberg exchange $J$). Using an improved method for stochastic
analytic continuation of imaginary-time correlation functions computed with QMC
simulations, we can treat the sharp ($\delta$-function) contribution from
spinwaves (magnons) and a continuum at higher energy. The results for the
Heisenberg model agree with neutron scattering experiments on
Cu(DCOO)$_2$$\cdot$4D$_2$O, where a broad spectral-weight continuum at
$q=(\pi,0)$ was interpreted as deconfined spinons. Our results at $(\pi,0)$
show a similar reduction of the magnon weight and a large continuum, while the
continuum is much smaller at $q=(\pi/2,\pi/2)$ (as also seen experimentally).
Turning on $Q$, we observe a rapid reduction of the $(\pi,0)$ magnon weight to
zero, well before the deconfined quantum phase transition into a spontaneously
dimerized state. We re-interpret the picture of deconfined spinons at $(\pi,0)$
in the experiments as nearly deconfined spinons---a precursor to deconfined
quantum criticality. To further elucidate the picture of a fragile
$(\pi,0)$-magnon in the Heisenberg model and its depletion in the $J$-$Q$
model, we introduce an effective model in which a magnon can split into two
spinons that do not separate but fluctuate in and out of the magnon space (in
analogy with the resonance between a photon and a particle-hole pair in the
exciton-polariton problem). The model reproduces the $(\pi,0)$ and
$(\pi/2,\pi/2)$ features of the Heisenberg model. It can also account for the
rapid loss of the $(\pi,0)$ magnon with increasing $Q$ and a remarkable
persistence of a large magnon pole at $q=(\pi/2,\pi/2)$ even at the deconfined
critical point.",1708.03232v3
2017-11-21,Eavesdropping on spin waves inside the domain-wall nanochannel via three-magnon processes,"One recent breakthrough in the field of magnonics is the experimental
realization of reconfigurable spin-wave nanochannels formed by magnetic domain
wall with a width of $10-100$ nm [Wagner \emph{et al}., Nat. Nano. \textbf{11},
432 (2016)]. This remarkable progress enables an energy-efficient spin-wave
propagation with a well-defined wave vector along its propagating path inside
the wall. In the mentioned experiment, a micro-focus Brillouin light scattering
spectroscopy was taken in a line-scans manner to measure the frequency of the
bounded spin wave. Due to their localization nature, the confined spin waves
can hardly be detected from outside the wall channel, which guarantees the
information security to some extent. In this work, we theoretically propose a
scheme to detect/eavesdrop on the spin waves inside the domain-wall nanochannel
via nonlinear three-magnon processes. We send a spin wave in one magnetic
domain to interact with the bounded mode in the wall. Two kinds of three-magnon
processes, i.e., confluence and splitting, are expected to occur. The
confluence process is conventional. We predict a stimulated three-magnon
splitting (or ""magnon laser"") effect: the presence of a bound magnon
propagating along the domain wall channel assists the splitting of the incident
wave into two modes, one of which is identical to the bound mode in the
channel. Micromagnetic simulations confirm our theoretical analysis. These
results demonstrate that one is able to uniquely infer the spectrum of the
spin-wave in the domain-wall nanochannel once we know both the injection and
the transmitted waves.",1711.07615v2
2020-09-21,Thermal Hall effect from two-dimensional Schwinger-boson gas with Rashba spin-orbit interaction: application to ferromagnets with in-plane Dzyaloshinskii-Moriya interaction,"Recently, uncovering the sources of thermal Hall effect in insulators has
become an important issue. In the case of ferromagnetic insulators, it is well
known that Dzyaloshinskii-Moriya (DM) interaction can induce magnon thermal
Hall effect. Specifically, the DM vector parallel to the magnetization
direction induces complex magnon hopping amplitudes, so that magnons act as if
they feel Lorentz force. However, the DM vector which is orthogonal to the
magnetization direction has hitherto been neglected as a possible source of
magnon thermal Hall effect. This is because they play no role in the linear
spin wave theory, an often invoked approximation when computing the magnon
thermal Hall effect. Here, we challenge this expectation by presenting the
self-consistent Schwinger-boson mean field study of two-dimensional magnets
with ferromagnetic Heisenberg interaction and in-plane DM interaction. We find
that the relevant Schwinger-boson mean field Hamiltonian takes the form of
two-dimensional electron gas with Rashba spin-orbit interaction, which is known
to show anomalous Hall effect, spin Hall effect, and Rashba-Edelstein effect,
whose thermal counterparts also appear in our system. Importantly, the thermal
Hall effect can be induced when out-of-plane magnetic field is applied, and
persists even when the magnetic field is large, so that the spins are
significantly polarized, and the linear spin wave theory is expected to be a
reasonable approximation. Since the linear spin wave theory predicts vanishing
thermal Hall effect, our result implies that linear spin wave is not a
sufficient approximation, and that magnon-magnon interaction must be taken into
account to predict the correct thermal Hall conductivity.",2009.09621v1
2020-07-30,Magnon and Spin Transition Contribution in Heat Capacity of Ferromagnetic Cr-doped MnTe: Experimental Evidence for a Paramagnetic Spin-Caloritronic Effect,"We present experimental evidence for the simultaneous existence of the
magnons and spin-state transition contributions to the heat capacity in
ferromagnetic (FM) Cr-doped MnTe (Tc~280K), where the magnon heat capacity is
attributed to the observed magnon-bipolar carrier-drag thermopower. The
pristine antiferromagnetic (AFM) MnTe shows only a magnon-induced peak in the
heat capacity near the Neel temperature, TN~307K. However, Cr-doped MnTe shows
a magnon-contributed heat capacity peak at ~293K with an additional peak in the
deep paramagnetic domain near 780K. Temperature-dependent magnetic
susceptibility reveals that Cr-doping initially creates low-spin (LS) states
Mn2+ ions into MnTe near and below TN due to a higher crystal field induced by
Cr ions. Above 400K, LS Mn2+ ions start converting into high-spin (HS) Mn2+
ions. The LS-to-HS transition of Mn2+ leads to an excess entropy and hence
excess heat capacity contribution in the system. Temperature-dependent X-ray
diffraction (XRD) and magnetic field-dependent susceptibility (M-H) confirmed
no presence of any structural changes and magnetic polaron, respectively. Both
XRD and M-H ensure that the peak of the heat capacity in the paramagnetic
domain is originated solely by the spin-state transition. The heat capacity
versus temperature was calculated to explain the contribution of each
component, including the ones due to the phonons, magnons, spin-transition,
Schottky anomaly, and lattice dilation. With the recent advances in
spin-caloritronics extending the spin-based effects from magnetic to
paramagnetic materials, the data from the heat capacity can play a crucial role
to probe the presence of different phenomena such as paramagnon-carrier-drag
and spin-entropy thermopowers.",2007.15232v1
2022-10-07,Magnon bands in pyrochlore slabs with Heisenberg exchange and anisotropies,"The pyrochlore lattice is a versatile venue to probe the properties of
magnetically ordered states induced or perturbed by anisotropic terms like the
Dzyaloshinskii-Moriya interactions or single-ion anisotropy. Several such
ordered states have been investigated recently as precursors of topological
magnons and the associated surface states. In parallel, there has been recent
progress in growing thin films of magnetic materials with this lattice
structure along high symmetry directions of the lattice. In both cases, an
account of the magnetic excitations of relevant Hamiltonians for finite slabs
is a necessary step in the analysis of the physics of these systems. We study
here magnon bands in the slab geometry for a class of spin models on the
pyrochlore lattice with Heisenberg exchange, Dzyaloshinskii-Moriya interaction
and spin-ice anisotropy. For a range of model parameters, for both
ferromagnetic and antiferromagnetic exchange, we compute the classical ground
states for different slab orientations and determine the spin wave excitations
above them. We analyze the ferromagnetic splay phase, the all-in-all-out phase
and a coplanar phase and evaluate magnon dispersions for slabs oriented
perpendicular to the $[111]$, $[100]$ and $[110]$ directions. For all the
phases considered, depending on the slab orientation, magnon band structures
can be non-reciprocal and we highlight the differences in the three
orientations from this point-of-view. Finally, we present details of the
surface localized magnons for all the three slab orientations in the phases we
study. For the ferromagnetic splay phase and the all-in-all-out phase we
analyze surface states associated with point degeneracies or nodal lines in the
bulk spectrum by computing the magnonic Berry curvature and Weyl charges or
Chern numbers associated with it.",2210.03548v3
2023-08-18,"Magnon Diffusion Length and Longitudinal Spin Seebeck Effect in Vanadium Tetracyanoethylene (V[TCNE]$_x$, $x \sim 2$)","Spintronic, spin caloritronic, and magnonic phenomena arise from complex
interactions between charge, spin, and structural degrees of freedom that are
challenging to model and even more difficult to predict. This situation is
compounded by the relative scarcity of magnetically-ordered materials with
relevant functionality, leaving the field strongly constrained to work with a
handful of well-studied systems that do not encompass the full phase space of
phenomenology predicted by fundamental theory. Here we present an important
advance in this coupled theory-experiment challenge, wherein we extend existing
theories of the spin Seebeck effect (SSE) to explicitly include the
temperature-dependence of magnon non-conserving processes. This expanded theory
quantitatively describes the low-temperature behavior of SSE signals previously
measured in the mainstay material yttrium iron garnet (YIG) and predicts a new
regime for magnonic and spintronic materials that have low saturation
magnetization, $M_S$, and ultra-low damping. Finally, we validate this
prediction by directly observing the spin Seebeck resistance (SSR) in the
molecule-based ferrimagnetic semiconductor vanadium tetracyanoethylene
(V[TCNE]$_x$, $x \sim 2$). These results validate the expanded theory, yielding
SSR signals comparable in magnitude to YIG and extracted magnon diffusion
length ($\lambda_m>1$ $\mu$ m) and magnon lifetime for V[TCNE]$_x$
($\tau_{th}\approx 1-10$ $\mu$ s) exceeding YIG ($\tau_{th}\sim 10$ ns).
Surprisingly, these properties persist to room temperature despite relatively
low spin wave stiffness (exchange). This identification of a new regime for
highly efficient SSE-active materials opens the door to a new class of magnetic
materials for spintronic and magnonic applications.",2308.09752v1
2023-10-08,Floquet-engineering the exceptional points in parity-time-symmetric magnonics,"Magnons serve as a testing ground for fundamental aspects of Hermitian and
non-Hermitian wave mechanics and are of high relevance for information
technology. This study presents setups for realizing spatio-temporally driven
parity-time (PT) symmetric magnonics based on coupled magnetic waveguides and
magnonic crystals. A charge current in a metal layer with strong spin-orbit
coupling sandwiched between two insulating magnetic waveguides leads to gain or
loss in the magnon amplitude depending on the directions of the magnetization
and the charge currents. When gain in one waveguide is balanced by loss in the
other waveguide a PT-symmetric system hosting non-Hermitian degeneracies (or
exceptional points (EPs)) is realized. For AC current multiple EPs appear for a
certain gain/loss strength and mark the boundaries between the preserved
PT-symmetry and the broken PT-symmetry phases. The number of islands of broken
PT-symmetry phases and their extensions is tunable by the frequency and the
strength of the spacer current. At EP and beyond, the induced and amplified
magnetization oscillations are strong and self-sustained. In particular, these
magnetization auto-oscillations in broken PT-symmetry phase occur at low
current densities and do not require further adjustments such as tilt angle
between electric polarization and equilibrium magnetization direction in
spin-torque oscillators, pointing to a new design of these oscillators and
their utilization in computing and sensoric. It is also shown how the periodic
gain/loss mechanism allows for the generation of high-frequency spin waves with
low-frequency currents. For spatially-periodic gain/loss acting on a magnonic
crystal, magnon modes approaching each other at the Brillouin-zone boundaries
are highly susceptible to PT-symmetry, allowing for a wave-vector-resolved
experimental realization at very low currents.",2310.09300v1
2023-10-30,Schwinger mechanism of magnon-antimagnon pair production on magnetic field inhomogeneities and the bosonic Klein effect,"Effective field theory of low-energy exitations-magnons that describes
antiferromagnets is mapped into scalar electrodynamics of a charged scalar
field interacting with an external electromagnetic potential. In the presence
of a constant inhomogeneous external magnetic field the latter problem is
technically reduced to the problem of charged-particle creation from the vacuum
by an electric potential step (x-step). Magnetic moment plays here the role of
the electric charge, and magnons and antimagnons differ from each other in the
sign of the magnetic moment. In the framework of such a consideration, it is
important to take into account the vacuum instability (the Schwinger effect)
under the magnon-antimagnon production on magnetic field inhomogeneities (an
analog of pair creation from the vacuum by electric-like fields). We
demonstrate how to use the strong field QED with x-steps developed by the
authors (SPG and DMG) to study the magnon-antimagnon pair production on
magnetic field inhomogeneities. Characteristics of the vacuum instability
obtained for some magnetic steps that allows exact solving the Klein-Gordon
equation are presented. In particular, we consider examples of magnetic steps
with very sharp field derivatives that correspond to a regularization of the
Klein step. In the case of smooth-gradient steps, we describe an universal
behavior of the flux density of created magnon pairs. We also note that since
the low-energy magnons are bosons with small effective mass, then for the first
time maybe the opportunity will arise to observe the Schwinger effect in the
case of the Bose statistics, in particular, the bosonic Klein effect in
laboratory conditions. Moreover, it turns out that in the case of the Bose
statistics appears a new mechanism for amplifying the effect of the pair
creation, which we call statistically-assisted Schwinger effect.",2310.20035v1
2024-01-25,Few-magnon excitations in a frustrated spin-$S$ ferromagnetic chain with single-ion anisotropy,"We study few-magnon excitations in a finite-size spin-$S$ chain with
ferromagnetic nearest-neighbor (NN) interaction $J>0$ and antiferromagnetic
next-nearest-neighbor (NNN) interaction $J'<0$, in the presence of the
single-ion (SI) anisotropy $D$. We first reveal the condition for the emergence
of zero-excitation-energy states. In the isotropic case with $\Delta=\Delta'=1$
($\Delta$ and $\Delta'$ are the corresponding anisotropy parameters), a
threshold of $J/|J'|$ above which the ground state is ferromagnetic is
determined by exact diagonalization for short chains up to $12$ sites. Using a
set of exact two-magnon Bloch states, we then map the two-magnon problem to a
single-particle one on an effective open chain with both NN and NNN hoppings.
The whole two-magnon excitation spectrum is calculated for large systems and
the commensurate-incommensurate transition in the lowest-lying mode is found to
exhibit different behaviors between $S=1/2$ and higher spins due to the
interplay of the SI anisotropy and the NNN interaction. For the commensurate
momentum $k=-\pi$, the effective lattice is decoupled into two NN open chains
that can be exactly solved via a plane-wave ansatz. Based on this, we
analytically identify in the $\Delta'-D/|J'|$ plane the regions supporting the
SI or NNN exchange two-magnon bound states near the edge of the band. In
particular, we prove that there always exists a lower-lying NN exchange
two-magnon bound state near the band edge for arbitrary $S\geq 1/2$. Finally,
we numerically calculate the $n$-magnon spectra for $S=1/2$ with $n\leq5$ by
using a spin-operator matrix element method. The corresponding $n$-magnon
commensurate instability regions are determined for finite chains and
consistent results with prior literature are observed.",2401.14101v2
1994-05-14,Dynamical T=0 correlations of the S=1/2 1D Heisenberg Anti-Ferromagnet with 1/r^2 exchange in a magnetic field,"We present a new selection rule for matrix elements of local spin operators
in the $S=1/2$ ``Haldane-Shastry'' model. Based on this rule we extend a recent
exact calculation \cite{H93} of the ground-state dynamical spin correlation
function $S^{ab}(n,t)$ = $\langle 0 | S^a(n,t)S^b(0,0)| 0 \rangle$ and its
Fourier-transform $S^{ab}(Q,E)$ of this model to a finite magnetic field. In
zero field, only {\it two-spinon} excitations contribute to the spectral
function; in the (positively) partially-spin-polarized case, there are two
types of elementary excitations: {\it spinons} ($\Delta S^z = \pm 1/2$) and
{\it magnons} ($\Delta S^z = -1 $). The magnons are divided into left- or
right-moving branches. The only classes of excited states contributing to the
spectral functions are: (I) two spinons, (II) two spinons + one magnon, (IIIa)
two spinons + two magnons (moving in opposite directions), (IIIb) one magnon.
The contributions to the various correlations are: $S^{-+}$: (I); $S^{zz}$:
(I)+(II); $S^{+-}$: (I)+(II)+(III). In the zero-field limit there are no
magnons, while in the fully-polarized case, there are no spinons. We discuss
the relation of the spectral functions to correlations of the
Calogero-Sutherland model at coupling $\lambda = 2$.",9405043v1
2006-04-19,Giant Magnons,"Studies of ${\cal N}=4$ super Yang Mills operators with large R-charge have
shown that, in the planar limit, the problem of computing their dimensions can
be viewed as a certain spin chain. These spin chains have fundamental
``magnon'' excitations which obey a dispersion relation that is periodic in the
momentum of the magnons. This result for the dispersion relation was also shown
to hold at arbitrary 't Hooft coupling. Here we identify these magnons on the
string theory side and we show how to reconcile a periodic dispersion relation
with the continuum worldsheet description. The crucial idea is that the
momentum is interpreted in the string theory side as a certain geometrical
angle. We use these results to compute the energy of a spinning string. We also
show that the symmetries that determine the dispersion relation and that
constrain the S-matrix are the same in the gauge theory and the string theory.
We compute the overall S-matrix at large 't Hooft coupling using the string
description and we find that it agrees with an earlier conjecture. We also find
an infinite number of two magnon bound states at strong coupling, while at weak
coupling this number is finite.",0604135v2
2006-06-14,Finite-size Effects from Giant Magnons,"In order to analyze finite-size effects for the gauge-fixed string sigma
model on AdS_5 x S^5, we construct one-soliton solutions carrying finite
angular momentum J. In the infinite J limit the solutions reduce to the
recently constructed one-magnon configuration of Hofman and Maldacena. The
solutions do not satisfy the level-matching condition and hence exhibit a
dependence on the gauge choice, which however disappears as the size J is taken
to infinity. Interestingly, the solutions do not conserve all the global
charges of the psu(2,2|4) algebra of the sigma model, implying that the
symmetry algebra of the gauge-fixed string sigma model is different from
psu(2,2|4) for finite J, once one gives up the level-matching condition. The
magnon dispersion relation exhibits exponential corrections with respect to the
infinite J solution. We also find a generalisation of our one-magnon
configuration to a solution carrying two charges on the sphere. We comment on
the possible implications of our findings for the existence of the Bethe ansatz
describing the spectrum of strings carrying finite charges.",0606126v2
2009-06-25,Intermediate-statistics spin waves,"In this paper, we show that spin waves, the elementary excitation of the
Heisenberg magnetic system, obey a kind of intermediate statistics with a
finite maximum occupation number n. We construct an operator realization for
the intermediate statistics obeyed by magnons, the quantized spin waves, and
then construct a corresponding intermediate-statistics realization for the
angular momentum algebra in terms of the creation and annihilation operators of
the magnons. In other words, instead of the Holstein-Primakoff representation,
a bosonic representation subject to a constraint on the occupation number, we
present an intermediate-statistics representation with no constraints. In this
realization, the maximum occupation number is naturally embodied in the
commutation relation of creation and annihilation operators, while the
Holstein-Primakoff representation is a bosonic operator relation with an
additional putting-in-by-hand restriction on the occupation number. We deduce
the intermediate-statistics distribution function for magnons. On the basis of
these results, we calculate the dispersion relations for ferromagnetic and
antiferromagnetic spin waves. The relations between the intermediate statistics
that magnons obey and the other two important kinds of intermediate statistics,
Haldane-Wu statistics and the fractional statistics of anyons, are discussed.
We also compare the spectrum of the intermediate-statistics spin wave with the
exact solution of the one-dimensional s = 1/2 Heisenberg model, which is
obtained by the Bethe ansatz method. For ferromagnets, we take the
contributions from the interaction between magnons (the quartic contribution),
the next-to-nearest neighbor interaction, and the dipolar interaction into
account for comparison with the experiment.",0906.4818v1
2010-03-25,Spin superfluidity and magnon BEC,"The spin superfluidity -- superfluidity in the magnetic subsystem of a
condensed matter - is manifested as the spontaneous phase-coherent precession
of spins first discovered in 1984 in 3He-B. This superfluid current of spins -
spin supercurrent - is one more representative of superfluid currents known or
discussed in other systems, such as the superfluid current of mass and atoms in
superfluid 4He; superfluid current of electric charge in superconductors;
superfluid current of hypercharge in Standard Model of particle physics;
superfluid baryonic current and current of chiral charge in quark matter; etc.
Spin superfluidity can be described in terms of the Bose condensation of spin
waves - magnons. We discuss different phases of magnon superfluidity, including
those in magnetic trap; and signatures of magnons superfluidity: (i) spin
supercurrent, which transports the magnetization on a macroscopic distance more
than 1 cm long; (ii) spin current Josephson effect which shows interference
between two condensates; (iii) spin current vortex - a topological defect which
is an analog of a quantized vortex in superfluids, of an Abrikosov vortex in
superconductors, and cosmic strings in relativistic theories; (iv) Goldstone
modes related to the broken U(1) symmetry - phonons in the spin-superfluid
magnon gas; etc. We also touch the topic of spin supercurrent in general
including spin Hall and intrinsic quantum spin Hall effects.",1003.4889v3
2013-12-11,"Magnonic Band Structure, Complete Bandgap and Collective Spin Wave Excitation in Nanoscale Two--Dimensional Magnonic Crystals","We present the observation of a complete bandgap and collective spin wave
excitation in two-dimensional magnonic crystals comprised of arrays of
nanoscale antidots and nanodots, respectively. Considering that the frequencies
dealt with here fall in the microwave band, these findings can be used for the
development of suitable magnonic metamaterials and spin wave based signal
processing. We also present the application of a numerical procedure, to
compute the dispersion relations of spin waves for any high symmetry direction
in the first Brillouin zone. The results obtained from this procedure has been
reproduced and verified by the well established plane wave method for an
antidot lattice, when magnetization dynamics at antidot boundaries is pinned.
The micromagnetic simulation based method can also be used to obtain
iso--frequency countours of spin waves. Iso--frequency contours are analougous
of the Fermi surfaces and hence, they have the potential to radicalize our
understanding of spin wave dynamics. The physical origin of bands, partial and
full magnonic bandgaps has been explained by plotting the spatial distribution
of spin wave energy spectral density. Although, unfettered by rigid assumptions
and approximations, which afflict most analytical methods used in the study of
spin wave dynamics, micromagnetic simulations tend to be computationally
demanding. Thus, the observation of collective spin wave excitation in the case
of nanodot arrays, which can obviate the need to perform simulations may also
prove to be valuable.",1312.3044v1
2013-12-20,Magnon condensation with finite degeneracy on the triangular lattice,"We study the spin 1/2 triangular-lattice $J_1$-$J_2$-$J_3$ antiferromagnet
close to the saturation field using the dilute Bose gas theory, where the
magnetic structure is determined by the condensation of magnons. We focus on
the case of ferromagnetic $J_1$ and antiferromagnetic $J_2,J_3$, that is
particularly rich because frustration effects allow the single-magnon energy
dispersion to have six-fold degenerate minima at incommensurate momenta. Our
calculation also includes an interlayer coupling $J_0$, which covers both
antiferromagnetic and ferromagnetic cases including negligibly small regime
(two-dimensional case). Besides the spiral and fan phases, we find a new
double-$q$ phase (superposition of two modes), dubbed ""${\bf Q}_0$-${\bf Q}_1$""
(or simply ""01"") phase, that enjoys a new type of multiferroic character.
Certain phase boundaries have a singular $J_0$ dependence for $J_0\to 0$,
implying that even a very small interlayer coupling drastically changes the
ground state. A mechanism for this singularity is presented. Moreover, in some
regions of the parameter space, we show that a dilute gas of magnons can not be
stable, and phase separation (corresponding to a magnetization jump) is
expected. In the $J_1$-$J_2$ model ($J_3=0$), formation of two-magnon bound
states is observed, which can lead to a quadrupolar (spin-nematic) ordered
phase. Exact diagonalization analysis is also applied to the search of bound
states.",1312.5935v2
2014-11-12,Magnonic Holographic Memory: from Proposal to Device,"In this work, we present recent developments in magnonic holographic memory
devices exploiting spin waves for information transfer. The devices comprise a
magnetic matrix and spin wave generating/detecting elements placed on the edges
of the waveguides. The matrix consists of a grid of magnetic waveguides
connected via cross junctions. Magnetic memory elements are incorporated within
the junction while the read-in and read-out is accomplished by the spin waves
propagating through the waveguides. We present experimental data on spin wave
propagation through NiFe and YIG magnetic crosses. The obtained experimental
data show prominent spin wave signal modulation (up to 20 dB for NiFe and 35 dB
for YIG) by the external magnetic field, where both the strength and the
direction of the magnetic field define the transport between the cross arms. We
also present experimental data on the 2-bit magnonic holographic memory built
on the double cross YIG structure with micro-magnets placed on the top of each
cross. It appears possible to recognize the state of each magnet via the
interference pattern produced by the spin waves with all experiments done at
room temperature. Magnonic holographic devices aim to combine the advantages of
magnetic data storage with wave-based information transfer. We present
estimates on the spin wave holographic devices performance, including power
consumption and functional throughput. According to the estimates, magnonic
holographic devices may provide data processing rates higher than 10^18
bits/cm2/s while consuming 0.15uW. Technological challenges and fundamental
physical limits of this approach are also discussed.",1411.3388v1
2015-01-27,"Magnon spectra and strong spin-lattice coupling in magnetically frustrated MnB2O4 (B = Mn,V): Inelastic light scattering studies","The ferrimagnetic spinels MnB2O4 (B = Mn,V) exhibit a similar series of
closely spaced magnetic and structural phase transitions at low temperatures,
reflecting both magnetic frustration and a strong coupling between the spin and
lattice degrees of freedom. Careful studies of excitations in MnB2O4 (B =
Mn,V), and the evolution of these excitations with temperature, are important
for obtaining a microscopic description of the role that magnetic excitations
and spin-lattice coupling play in the low temperature phase transitions of
these materials. We report an inelastic light (Raman) scattering study of the
temperature and magnetic field dependences of one- and two-magnon excitations
in MnV2O4 and Mn3O4. We observe a pair of q=0 one-magnon modes at 74 cm^{-1}
and 81 cm^{-1} in MnV2O4, which is in contrast with the single 80 cm^{-1} q=0
magnon that has been reported for MnV2O4 based on previous neutron scattering
measurements and spin wave calculations. Additionally, we find that the
two-magnon energy of MnV2O4 decreases (""softens"") with decreasing temperature
below T_{N}, which we attribute to strong coupling between magnetic and
vibrational excitations near the zone boundary.",1501.06897v1
2015-10-14,Field-induced dynamical properties of the $XXZ$ model on a honeycomb lattice,"We present a comprehensive $1/S$ study of the field-induced dynamical
properties of the nearest-neighbor $XXZ$ antiferromagnet on a honeycomb lattice
using the formalism of the nonlinear spin-wave theory developed for this model.
The external magnetic field controls spin frustration in the system and induces
non-collinearity of the spin structure, which is essential for the two-magnon
decay processes. Our results include an intriguing field-evolution of the
regions of the Brillouin zone wherein decays of spin excitations are prominent,
a detailed classification of the decay channels involving magnons from both
excitation branches, and a thorough analysis of the singularities in the magnon
spectra due to coupling to the two magnon continuum, all of which are
illustrated for several field and anisotropy values. We highlight a number of
features related to either the non-Bravais nature of the lattice, or the
existence of the Dirac-like points in the spectrum. In addition, the asymptotic
behavior of the decay rates near high-symmetry points is analyzed in detail.
The inelastic neutron-scattering spin-spin structure factor is obtained in the
leading $1/S$ order and is shown to exhibit qualitatively distinct fingerprints
of the decay-induced magnon dynamics such as quasiparticle peaks broadened by
decays and strong spectral weight redistribution.",1510.04275v2
2017-02-16,Nonreciprocal magnons and symmetry-breaking in the noncentrosymmetric antiferromagnet,"Magnons, the spin-wave quanta, are disturbances that embody a wave
propagating through a background medium formed by ordered magnetic moments. In
an isotropic Heisenberg system, these disturbances vary in a continuous manner
around an ordered spin structure, thus requiring infinitesimal energy as a
wavevector approaches a magnetic zone centre. However, competing anisotropic
interactions arising from broken symmetry can favour a distinct static and
dynamic spin state causing a shift of the minimum point of the magnon
dispersion to a nonreciprocal wavevector. Here we report the first direct
evidence of these nonreciprocal magnons in an antiferromagnet. In the
antiferromagnet we investigated, namely, noncentrosymmetric
$\alpha$-Cu$_2$V$_2$O$_7$, they are caused by the incompatibility between
anisotropic exchange and antisymmetric Dzyaloshinskii-Moriya interactions
resulting in competing collinear and helical spin structures, respectively. The
nonreciprocity introduces the difference in the phase velocity of the
counter-rotating modes, causing the opposite spontaneous magnonic Faraday
rotation of the left- and right-propagating spin-waves. The breaking of spatial
inversion and time reversal symmetry is revealed as a magnetic-field-induced
asymmetric energy shift, which provides a test for the detailed balance
relation.",1702.04889v1
2017-02-17,Spin conductance of YIG thin films driven from thermal to subthermal magnons regime by large spin-orbit torque,"We report a study on spin conductance in ultra-thin films of Yttrium Iron
Garnet (YIG), where spin transport is provided by propagating spin waves, that
are generated and detected by direct and inverse spin Hall effects in two Pt
wires deposited on top. While at low current the spin conductance is dominated
by transport of thermal magnons, at high current, the spin conductance is
dominated by low-damping non-equilibrium magnons thermalized near the spectral
bottom by magnon-magnon interaction, with consequent a sensitivity to the
applied magnetic field and a longer decay length. This picture is supported by
microfocus Brillouin Light Scattering spectroscopy.",1702.05226v3
2017-08-18,Magnon Dispersion and Specific Heat of Chiral Magnets on the Pyrochlore Lattice,"Chiral magnets are magnetically ordered insulators having spin scalar
chirality, and magnons of chiral magnets have been poorly understood. We study
the magnon dispersion and specific heat for four chiral magnets with Q=0 on the
pyrochlore lattice. This study is based on the linear-spin-wave approximation
for the S=1/2 effective Hamiltonian consisting of two kinds of Heisenberg
interaction and two kinds of Dzyaloshinsky-Moriya interaction. We show that the
three-in-one-out type chiral magnets possess an optical branch of the magnon
dispersion near q=0, in addition to three quasiacoustic branches. This differs
from the all-in/all-out type chiral magnets, which possess four quasiacoustic
branches. We also show that all four chiral magnets have a gapped magnon energy
at q=0, indicating the absence of the Goldstone type gapless excitation. These
results are useful for experimentally identifying the three-in-one-out or
all-in/all-out type chiral order. Then, we show that there is no qualitative
difference in the specific heat among the four magnets. This indicates that the
specific heat is not useful for distinguishing the kinds of chiral orders. We
finally compare our results with experiments and provide a proposal for the
three-in-one-out type chiral magnets.",1708.05532v1
2018-02-26,Spin Seebeck effect and ballistic transport of quasi-acoustic magnons in room-temperature yttrium iron garnet films,"We studied the transient behavior of the spin current generated by the
longitudinal spin Seebeck effect (LSSE) in a set of platinum-coated yttrium
iron garnet (YIG) films of different thicknesses. The LSSE was induced by means
of pulsed microwave heating of the Pt layer and the spin currents were measured
electrically using the inverse spin Hall effect in the same layer. We
demonstrate that the time evolution of the LSSE is determined by the evolution
of the thermal gradient triggering the flux of thermal magnons in the vicinity
of the YIG/Pt interface. These magnons move ballistically within the YIG film
with a constant group velocity, while their number decays exponentially within
an effective propagation length. The ballistic flight of the magnons with
energies above 20K is a result of their almost linear dispersion law, similar
to that of acoustic phonons. By fitting the time-dependent LSSE signal for
different film thicknesses varying by almost an order of magnitude, we found
that the effective propagation length is practically independent of the YIG
film thickness. We consider this fact as strong support of a ballistic
transport scenario - the ballistic propagation of quasi-acoustic magnons in
room temperature YIG.",1802.09593v1
2017-01-18,Excitation of magnon accumulation by laser clocking as a source of long-range spin waves in transparent magnetic films,"Optical tools are of great promise for generation of spin waves due to the
possibility to manipulate on ultrashort time scales and to provide local
excitation. However, a single laser pulse can inject spin waves only with a
broad frequency spectrum, resulting in a short propagation distance and low
amplitude. Here we excite a magnetic garnet film by a train of fs-laser pulses
with 1 GHz repetition rate so that pulse separation is smaller than decay time
of the magnetic modes which allows to achieve collective photonic impact on
magnetization. It establishes a quasi-stationary source of SWs, namely a
coherent magnon accumulation (""magnon cloud""). This approach has several
appealing features: (i) the source is tunable; (ii) the SW amplitude can be
significantly enhanced; (iii) the spectrum of the generated SWs is quite narrow
that provides longer propagation distance; (iv) the periodic pumping results in
almost constant in time SW amplitude up to 100 um away from the source; and (v)
the SW emission shows a pronounced directionality. These results expand the
capabilities of ultrafast coherent optical control of magnetization and pave a
way for applications in data processing, including the quantum regime. The
quasi-stationary magnon accumulation might be also of interest for the problem
of magnon Bose-Einstein condensate.",1701.05109v1
2011-11-11,Real and Virtual Bound States in Lüscher Corrections for CP3 Magnons,"We study classical and quantum finite-size corrections to giant magnons in
AdS_4 x CP^3 using generalised L\""uscher formulae. L\""uscher F-terms are
organised in powers of the exponential suppression factor exp(-Delta/2h)^m, and
we calculate all terms in this series, matching one-loop algebraic curve
results from our previous paper arXiv:1006.2174. Starting with the second term,
the structure of these terms is different to those in AdS_5 x S^5 thanks to the
appearance of heavy modes in the loop, which can here be interpreted as
two-particle bound states in the mirror theory. By contrast, physical bound
states can represent dyonic giant magnons, and we also calculate F-terms for
these solutions. L\""uscher mu-terms, suppressed by exp(-Delta/E), instead give
at leading order the classical finite-size correction. For an elementary dyonic
giant magnon, we recover the correction given by arXiv:0903.3365. We then
extend this to calculate the next term in 1/h, giving a one-loop prediction.
Finally we also calculate F-terms for the various composite giant magnons, RP^3
and `big', again finding agreement to all orders.",1111.2839v3
2012-01-14,Effect of lattice geometry on magnon Hall effect in ferromagnetic insulators,"We have investigated the thermal Hall effect of magnons for various
ferromagnetic insulators. For pyrochlore ferromagnetic insulators
Lu$_2$V$_2$O$_7$, Ho$_2$V$_2$O$_7$, and In$_2$Mn$_2$O$_7$, finite thermal Hall
conductivities have been observed below the Curie temperature $T_C$ . From the
temperature and magnetic field dependences, it is concluded that magnons are
responsible for the thermal Hall effect. The Hall effect of magnons can be well
explained by the theory based on the Berry curvature in momentum space induced
by the Dzyaloshinskii-Moriya (DM) interaction. The analysis has been extended
to the transition metal (TM) oxides with perovskite structure. The thermal Hall
signal was absent or far smaller in La$_2$NiMnO$_6$ and YTiO$_3$, which have
the distorted perovskite structure with four TM ions in the unit cell. On the
other hand, a finite thermal Hall response is discernible below $T_C$ in
another ferromagentic perovskite oxide BiMnO$_3$, which shows orbital ordering
with a larger unit cell. The presence or absence of the thermal Hall effect in
insulating pyrochlore and perovskite systems reflect the geometric and
topological aspect of DM-induced magnon Hall effect.",1201.3002v2
2017-06-11,Dirac magnons in honeycomb ferromagnets,"The discovery of the Dirac electron dispersion in graphene led to the
question of the Dirac cone stability with respect to interactions. Coulomb
interactions between electrons were shown to induce a logarithmic
renormalization of the Dirac dispersion. With a rapid expansion of the list of
compounds and quasiparticle bands with linear band touching, the concept of
bosonic Dirac materials has emerged. We consider a specific case of
ferromagnets consisting of the Van der Waals-bonded stacks of honeycomb layers,
e.g chromium trihalides CrX3 (X = F, Cl, Br and I), that display two spin wave
modes with energy dispersion similar to that for the electrons in graphene. At
the single particle level, these materials resemble their fermionic
counterparts. However, how different particle statistics and interactions
affect the stability of Dirac cones has yet to be determined. To address the
role of interacting Dirac magnons, we expand the theory of ferromagnets beyond
the standard Dyson theory to a case of non-Bravais honeycomb layers. We
demonstrate that magnon-magnon interactions lead to a significant
momentum-dependent renormalization of the bare band structure in addition to
strongly momentum-dependent magnon lifetimes. We show that our theory
qualitatively accounts for hitherto unexplained anomalies in a nearly half
century old magnetic neutron scattering data for CrBr3. We also show that
honeycomb ferromagnets display dispersive surface and edge states, unlike their
electronic analogs.",1706.03384v1
2017-10-09,Laser-driven quantum magnonics and THz dynamics of the order parameter in antiferromagnets,"The impulsive generation of two-magnon modes in antiferromagnets by
femtosecond optical pulses, so-called femto-nanomagnons, leads to coherent
longitudinal oscillations of the antiferromagnetic order parameter that cannot
be described by a thermodynamic Landau-Lifshitz approach. We argue that this
dynamics is triggered as a result of a laser-induced modification of the
exchange interaction. In order to describe the oscillations we have formulated
a quantum mechanical description in terms of magnon pair operators and coherent
states. Such an approach allowed us to} derive an effective macroscopic
equation of motion for the temporal evolution of the antiferromagnetic order
parameter. An implication of the latter is that the photo-induced spin dynamics
represents a macroscopic entanglement of pairs of magnons with femtosecond
period and nanometer wavelength. By performing magneto-optical pump-probe
experiments with 10 femtosecond resolution in the cubic KNiF$_3$ and the
uniaxial K$_2$NiF$_4$ collinear Heisenberg antiferromagnets, we observed
coherent oscillations at the frequency of 22 THz and 16 THz, respectively. The
detected frequencies as a function of the temperature ideally fit the
two-magnon excitation up to the N\'eel point. The experimental signals are
described as dynamics of magnetic linear dichroism due to longitudinal
oscillations of the antiferromagnetic vector.",1710.03143v3
2017-12-12,Microwave to optical photon conversion by means of travelling-wave magnons in YIG films,"In this work we study theoretically the efficiency of a travelling magnon
based microwave to optical photon converter for applications in Quantum
Information (QI). The converter employs an epitaxially grown yttrium iron
garnet (YIG) film as the medium for propagation of travelling magnons (spin
waves). The conversion is achieved through coupling of magnons to guided
optical modes of the film. The total microwave to optical photon conversion
efficiency is found to be larger than in a similar process employing a YIG
sphere by at least 4 orders of magnitude. By creating an optical resonator of a
large length from the film (such that the traveling magnon decays before
forming a standing wave over the resonator length) one will be able to further
increase the efficiency by several orders of magnitude, potentially reaching a
value similar to achieved with opto-mechanical resonators. Also, as a spin-off
result, it is shown that isolation of more that 20 dB with direct insertion
losses about 5 dBm can be achieved with YIG film based microwave isolators for
applications in Quantum Information.
  An important advantage of the suggested concept of the QI devices based on
travelling spin waves is a perfectly planar geometry and a possibility of
implementing a the device as a hybrid opto-microwave chip.",1712.04304v2
2017-09-19,Criteria for accurate determination of the magnon relaxation length from the nonlocal spin Seebeck effect,"The nonlocal transport of thermally generated magnons not only unveils the
underlying mechanism of the spin Seebeck effect, but also allows for the
extraction of the magnon relaxation length ($\lambda_m$) in a magnetic
material, the average distance over which thermal magnons can propagate. In
this study, we experimentally explore in yttrium iron garnet (YIG)/platinum
systems much further ranges compared with previous investigations. We observe
that the nonlocal SSE signals at long distances ($d$) clearly deviate from a
typical exponential decay. Instead, they can be dominated by the nonlocal
generation of magnon accumulation as a result of the temperature gradient
present away from the heater, and decay geometrically as $1/d^2$. We emphasize
the importance of looking only into the exponential regime (i.e., the
intermediate distance regime) to extract $\lambda_m$. With this principle, we
study $\lambda_m$ as a function of temperature in two YIG films which are 2.7
and 50 $\mu$m in thickness, respectively. We find $\lambda_m$ to be around 15
$\mu$m at room temperature and it increases to 40 $\mu$m at $T=$ 3.5 K. Finite
element modeling results agree with experimental studies qualitatively, showing
also a geometrical decay beyond the exponential regime. Based on both
experimental and modeling results we put forward a general guideline for
extracting $\lambda_m$ from the nonlocal spin Seebeck effect.",1709.06321v1
2018-03-22,Field induced magnon excitation and in gap absorption of Kitaev candidate RuCl3,"We use time-domain terahertz spectroscopy to measure the low energy
conductivity and magnons in RuCl$_3$ under external magnetic field. At zero
field, an oscillation with a frequency of 0.62 THz is clearly observed in
time-domain spectrum below T$_N$, which is identified as a magnon excitation in
the magnetic order state. The magnon excitation is not affected by the external
magnetic field $\textbf{H}_{DC}$ when it is applied along the c-axis, but is
clearly suppressed when $\textbf{H}_{DC}$ is applied within ab plane. More
interestingly, when the magnetic component of THz wave $\textbf{h}(t)$ is
perpendicular to the applied in-plane magnetic field, we observe another
coherent oscillation at slightly higher energy scale at the field above 2 T,
which is eventually suppressed for $H_{DC}>$5 T. The measurement seems to
indicate that the in-plane magnetic field can lift the degeneracy of two
branches of low energy magnons at $\Gamma$ point. The low energy optical
conductivity calculated from the measured transmission spectrum is dominated by
a broad continuum contribution, which is not affected by changing either
temperature or external magnetic field. The continuum is likely to be related
to the fractional spin excitation due to dominated Kitaev interaction in the
material.",1803.08398v1
2019-01-02,Skyrmion Tubes as Magnonic Waveguides,"Various latest experiments have proven the theoretical prediction that domain
walls in planar magnetic structures can channel spin waves as outstanding
magnonic waveguides, establishing a superb platform for building magnonic
devices. Recently, three-dimensional nanomagnetism has been boosted up and
become a significant branch of magnetism, because three-dimensional magnetic
structures expose a lot of emerging physics hidden behind planar ones and will
inevitably provide broader room for device engineering. Skyrmions and
antiSkyrmions, as natural three-dimensional magnetic configurations, are not
considered yet in the context of spin-wave channeling and steering. Here, we
show that skyrmion tubes can act as nonplanar magnonic waveguides if excited
suitably. An isolated skyrmion tube in a magnetic nanoprism induces spatially
separate internal and edge channels of spin waves; the internal channel has a
narrower energy gap, compared to the edge channel, and accordingly can transmit
signals at lower frequencies. Additionally, we verify that those spin-wave
beams along magnetic nanoprism are restricted to the regions of potential
wells. Transmission of spin-wave signals in such waveguides results from the
coherent propagation of locally driven eigenmodes of skyrmions, i.e., the
breathing and rotational modes. Finally, we find that spin waves along the
internal channels are less susceptible to magnetic field than those along the
edge channels. Our work will open a new arena for spin-wave manipulation and
help bridge skyrmionics and magnonics.",1901.00253v1
2019-01-07,Abnormal anti-crossing effect in photon-magnon coupling,"We report the experimental demonstration of an abnormal, opposite
anti-crossing effect in a photon-magnon-coupled system that consists of an
Yttrium Iron Garnet film and an inverted pattern of split-ring resonator
structure (noted as ISRR) in a planar geometry. It is found that the normal
shape of anti-crossing dispersion typically observed in photon-magnon coupling
is changed to its opposite anti-crossing shape just by changing the
position/orientation of the ISRR's split gap with respect to the microstrip
line axis along which ac microwave currents are applied. Characteristic
features of the opposite anti-crossing dispersion and its linewidth evolution
are analyzed with the help of analytical derivations based on electromagnetic
interactions. The observed opposite anti-crossing dispersion is ascribed to the
compensation of both intrinsic damping and coupling-induced damping in the
magnon modes. This compensation is achievable by controlling the relative
strength and phase of oscillating magnetic fields generated from the ISRR's
split gap and the microstrip feeding line. The position/orientation of an
ISRR's split gap provides a robust means of controlling the dispersion shape of
anti-crossing and its damping in a photon-magnon coupling, thereby offering
more opportunity for advanced designs of microwave devices.",1901.01729v2
2019-01-10,Spin-wave Confinement and Coupling in Organic-Based Magnetic Nanostructures,"Vanadium tetracyanoethylene (V[TCNE]$_\text{x}$) is an organic-based
ferrimagnet that exhibits robust magnetic ordering (T$_\text{C}$ of over 600
K), high quality-factor (high-Q) microwave resonance (Q up to 3,500), and
compatibility with a wide variety of substrates and encapsulation technologies.
Here, we substantially expand the potential scope and impact of this emerging
material by demonstrating the ability to produce engineered nanostructures with
tailored magnetic anisotropy that serve as a platform for the exploration of
cavity magnonics, revealing strongly coupled quantum confined standing wave
modes that can be tuned into and out of resonance with an applied magnetic
field. Specifically, time-domain micromagnetic simulations of these
nanostructures faithfully reproduce the experimentally measured spectra,
including the quasi-uniform mode and higher-order spin-wave (magnon) modes.
Finally, when the two dominant magnon modes present in the spectra are brought
into resonance by varying the orientation of the in-plane magnetic field, we
observe anti-crossing behavior indicating strong coherent coupling between
these two magnon modes at room temperature. These results position
V[TCNE]$_\text{x}$ as a leading candidate for the development of coherent
magnonics, with potential applications ranging from microwave electronics to
quantum information.",1901.03286v2
2019-08-16,Antiferromagnetic cavity optomagnonics,"Currently, there is a growing interest in studying the coherent interaction
between magnetic systems and electromagnetic radiation in a cavity, prompted
partly by possible applications in hybrid quantum systems. We propose a
multimode cavity optomagnonic system based on antiferromagnetic insulators,
where optical photons couple coherently to the two homogeneous magnon modes of
the antiferromagnet. These have frequencies typically in the THz range, a
regime so far mostly unexplored in the realm of coherent interactions, and
which makes antiferromagnets attractive for quantum transduction from THz to
optical frequencies. We derive the theoretical model for the coupled system,
and show that it presents unique characteristics. In particular, if the
antiferromagnet presents hard-axis magnetic anisotropy, the optomagnonic
coupling can be tuned by a magnetic field applied along the easy axis. This
allows to bring a selected magnon mode into and out of a dark mode, providing
an alternative for a quantum memory protocol. The dynamical features of the
driven system present unusual behavior due to optically induced magnon-magnon
interactions, including regions of magnon heating for a red detuned driving
laser. The multimode character of the system is evident in a substructure of
the optomagnonically induced transparency window.",1908.06110v2
2019-11-18,Experimental observation of Josephson oscillations in a room-temperature Bose-Einstein magnon condensate,"The alternating current (ac) Josephson effect in a time-independent
spatially-inhomogeneous setting is manifested by the occurrence of Josephson
oscillations - periodic macroscopic phase-induced collective motions of the
quantum condensate. So far, this phenomenon was observed at cryogenic
temperatures in superconductors, in superfluid helium, and in Bose-Einstein
condensates (BECs) of trapped atoms. Here, we report on the discovery of the ac
Josephson effect in a magnon BEC carried by a room-temperature ferrimagnetic
film. The BEC is formed in a parametrically populated magnon gas in the spatial
vicinity of a magnetic trench created by a dc electric current. The appearance
of the Josephson effect is manifested by oscillations of the magnon BEC density
in the trench, caused by a coherent phase shift between this BEC and the BEC in
the nearby regions. Our findings advance the physics of room-temperature
macroscopic quantum phenomena and will allow for their application for data
processing in magnon spintronics devices.",1911.07802v3
2019-11-21,Modulation-induced long-range magnon bound states in one-dimensional optical lattices,"Ultracold two-level atoms in optical lattices offer an excellent experimental
platform to explore magnon excitations [Fukuhara et al 2013 Nat. Phys. 9, 235;
Fukuhara et al 2013 Nature 502, 76]. Here, we investigate how gradient magnetic
field and periodically modulated tunneling strength affect the two-magnon
excitations in these ultracold atomic systems. In the resonant condition where
the driving frequency matches and smooths the potential bias, the system gains
translational invariance in both space and time in the rotating frame, and thus
we can develop a Floquet-Bloch band theory for two magnons. We find a new kind
of bound states with relative distance no less than two sites, apart from the
conventional bound states with relative distance at one site, which indicates
the modulation-induced long-range interaction. We analytically derive an
effective Hamiltonian via the many-body perturbation theory for a deeper
understanding of such novel bound states and explore the interplay between
these two types of bound states. Moreover, we propose to probe
modulation-induced bound states via quantum walks. Our study not only provides
a scheme to form long-range magnon bound states, but also lays a cornerstone
for engineering exotic quantum states in multi-particle Floquet systems.",1911.09369v2
2019-12-11,Travelling photons mediated interactions between a magnon mode and a cavity photon mode,"We systematically study the indirect interaction between a magnon mode and a
cavity photon mode mediated by travelling photons of a waveguide. From a
general Hamiltonian, we derive the effective coupling strength between two
separated modes, and obtain the theoretical expression of system's
transmission. Accordingly, we design an experimental set-up consisting of a
shield cavity photon mode, microstrip line and a magnon system to test our
theoretical predictions. From measured transmission spectra, indirect
interaction, as well as mode hybridization, between two modes can be observed.
All experimental observations support our theoretical predictions. In this
work, we clarify the mechanism of travelling photon mediated interactions
between two separate modes. Even without spatial mode overlap, two separated
modes can still couple with each other through their correlated dissipations
into a mutual travelling photon bus. This conclusion may help us understand the
recently discovered dissipative coupling effect in cavity magnonics systems.
Additionally, the physics and technique developed in this work may benefit us
in designing new hybrid systems based on the waveguide magnonics.",1912.05478v1
2016-03-14,Topological honeycomb magnon Hall effect: A calculation of thermal Hall conductivity of magnetic spin excitations,"Quite recently, magnon Hall effect of spin excitations has been observed
experimentally on the kagome and pyrochlore lattices. Thermal Hall conductivity
$\kappa^{xy}$, changes sign as a function of magnetic field or temperature on
the kagome lattice, and $\kappa^{xy}$ changes sign upon reversing the sign of
the magnetic field on the pyrochlore lattice. Motivated by these recent
exciting experimental observations, we theoretically propose a simple
realization of magnon Hall effect in a two-band model on the honeycomb lattice.
The magnon Hall effect of spin excitations arises in the usual way via the
breaking of inversion symmetry of the lattice, however, by a
next-nearest-neighbour Dzyaloshinsky-Moriya (DM) interaction. We find that
$\kappa^{xy}$ has a fixed sign for all parameter regimes considered. These
results are in contrast to the Lieb, kagome and pyrochlore lattices. We further
show that the low-temperature dependence on the magnon Hall conductivity
follows a $T^{2}$ law, as opposed to the kagome and pyrochlore lattices. These
results suggest an experimental procedure to measure thermal Hall conductivity
within a class of 2D honeycomb quantum magnets and ultracold atoms trapped in
honeycomb optical lattice.",1603.04331v6
2016-03-21,Inelastic Magnon Scattering,"We study the worldsheet S-matrix of a string attached to a D-brane in
AdS$_5\times$S$^5$. The D-brane is either a giant graviton or a dual giant
graviton. In the gauge theory, the operators we consider belong to the
$su(2|3)$ sector of the theory. Magnon excitations of open strings can exhibit
both elastic (when magnons in the bulk of the string scatter) and inelastic
(when magnons at the endpoint of an open string participate) scattering. Both
of these $S$-matrices are determined (up to an overall phase) by the
$su(2|2)^2$ global symmetry of the theory. In this note we study the $S$-matrix
for inelastic scattering. We show that it exhibits poles corresponding to
boundstates of bulk and boundary magnons. A crossing equation is derived for
the overall phase. It reproduces the crossing equation for maximal giant
gravitons, in the appropriate limit. Finally, scattering in the $su(2)$ sector
is computed to two loops. This two loop result, which determines the overall
phase to two loops, will be useful when a unique solution to the crossing
equation is to be selected.",1603.06414v2
2016-10-27,Magnonic Analogs of Topological Dirac Semimetals,"In electronic topological Dirac semimetals the conduction and valence bands
touch at discrete points in the Brillouin zone and form Dirac cones. They are
robust against spin-orbit interaction (SOI) and protected by crystal
symmetries. They can be driven to different topological phases by breaking the
symmetries. In the low-temperature quantum magnetic systems the magnon
dispersions have similar band structures as the electron dispersions, but with
positive definite energies. In these magnetic systems SOI manifests in the form
of the Dzyaloshinskii-Moriya interaction (DMI). In this Communication, we
identify two types of {\it magnonic Dirac semimetals} in quasi-two-dimensional
quantum magnets. The first type is a consequence of topological phase
transition between trivial and topological magnon insulators and the second
type is intrinsic and protected by crystal symmetries. They are robust against
DMI and can be driven to a topological magnon phase by breaking the symmetries.
They can be manipulated by an external magnetic field and accessible by the
bulk sensitive inelastic neutron scattering experiments.",1610.08869v14
2017-07-08,Magnetic ground state and magnon-phonon interaction in multiferroic h-YMnO$_3$,"Inelastic neutron scattering has been used to study the magneto-elastic
excitations in the multiferroic manganite hexagonal YMnO$_3$. An avoided
crossing is found between magnon and phonon modes close to the Brillouin zone
boundary in the $(a,b)$-plane. Neutron polarization analysis reveals that this
mode has mixed magnon-phonon character. An external magnetic field along the
$c$-axis is observed to cause a linear field-induced splitting of one of the
spin wave branches. A theoretical description is performed, using a Heisenberg
model of localized spins, acoustic phonon modes and a magneto-elastic coupling
via the single-ion magnetostriction. The model quantitatively reproduces the
dispersion and intensities of all modes in the full Brillouin zone, describes
the observed magnon-phonon hybridized modes, and quantifies the magneto-elastic
coupling. The combined information, including the field-induced magnon
splitting, allows us to exclude several of the earlier proposed models and
point to the correct magnetic ground state symmetry, and provides an effective
dynamic model relevant for the multiferroic hexagonal manganites.",1707.02468v2
2020-06-20,Momentum-dependent magnon lifetime in the metallic non-collinear triangular antiferromagnet CrB2,"Non-collinear magnetic order arises for various reasons in several magnetic
systems and exhibits interesting spin dynamics. Despite its ubiquitous
presence, little is known of how magnons, otherwise stable quasiparticles,
decay in these systems, particularly in metallic magnets. Using inelastic
neutron scattering, we examine the magnetic excitation spectra in a metallic
non-collinear antiferromagnet CrB$_{2}$, in which Cr atoms form a triangular
lattice and display incommensurate magnetic order. Our data show intrinsic
magnon damping and continuum-like excitations that cannot be explained by
linear spin wave theory. The intrinsic magnon linewidth $\Gamma(q,E_{q})$ shows
very unusual momentum dependence, which our analysis shows to originate from
the combination of two-magnon decay and the Stoner continuum. By comparing the
theoretical predictions with the experiments, we identify where in the momentum
and energy space one of the two factors becomes more dominant. Our work
constitutes a rare comprehensive study of the spin dynamics in metallic
non-collinear antiferromagnets. It reveals, for the first time, definite
experimental evidence of the higher-order effects in metallic antiferromagnets.",2006.11530v2
2015-12-24,Ultra-High Cooperativity Interactions between Magnons and Resonant Photons in a YIG sphere,"Resonant photon modes of a 5mm diameter YIG sphere loaded in a cylindrical
cavity in the 10-30GHz frequency range are characterised as a function of
applied DC magnetic field at millikelvin temperatures. The photon modes are
confined mainly to the sphere, and exhibited large mode filling factors in
comparison to previous experiments, allowing ultrastrong coupling with the
magnon spin wave resonances. The largest observed coupling between photons and
magnons is $2g/2\pi=7.11$ GHz for a 15.5 GHz mode, corresponding to a
cooperativity of $C=1.51\pm0.47\times10^7$. Complex modifications beyond a
simple multi-oscillator model, of the photon mode frequencies were observed
between 0 and 0.1 Tesla. Between 0.4 to 1 Tesla, degenerate resonant photon
modes were observed to interact with magnon spin wave resonances with different
couplings strengths, indicating time reversal symmetry breaking due to the
gyrotropic permeability of YIG. Bare dielectric resonator mode frequencies were
determined by detuning magnon modes to significantly higher frequencies with
strong magnetic fields. By comparing measured mode frequencies at 7 Tesla with
Finite Element modelling, a bare dielectric permittivity of $15.96\pm0.02$ of
the YIG crystal has been determined at about $20$ mK.",1512.07773v5
2016-09-01,The role of the internal demagnetizing field in a surface-modulated magnonic crystal,"Magnonic crystals with locally alternating properties and specific
periodicities exhibit interesting effects, such as a multitude of different
spin-wave states and large band gaps. This work aims for demonstrating and
understanding the key role of local demagnetizing fields in such systems. To
achieve this, hybrid structures are investigated consisting of a continuous
thin film with a stripe modulation on top favorable due to the adjustability of
the magnonic effects with the modulation size. For a direct access to the spin
dynamics, a magnonic crystal was reconstructed from `bottom-up', i.e., the
structural shape as well as the internal field landscape of the structure were
experimentally obtained on the nanoscale using electron holography.
Subsequently, both properties were utilized to perform dynamic response
calculations. The simulations yield the frequency-field dependence as well as
the angular dependence of spin waves in a magnonic crystal and reveal the
governing role of the internal field landscape around the backward-volume
geometry. The complex angle-dependent spin-wave behavior is described for a 360
degree in-plane rotation of an external field by connecting the internal field
landscape with the individual spin-wave localization.",1609.00342v3
2016-09-12,Topological Magnon Insulator in NonCoplanar Kagomé Antiferromagnets: Supplemental Material of arXiv:1608.04561,"Topological magnon insulators in insulating Kagom\'e ferromagnets have been
extensively studied in a series of papers. It has been established that
Dzyaloshinskii-Moriya interaction (DMI) is the key ingredient to observe a
nontrivial topological magnon with edge modes. However, insulating
antiferromagnets on the Kagom\'e lattice are frustrated systems considered as a
playground for studying quantum spin liquid physics. In these systems the DMI
can induce a coplanar but noncollinear magnetic orders with a $\mathbf{q}=0$
propagating wavevector. We show that topological magnon bands are absent in
this coplanar spin texture in sharp contrast to collinear ferromagnets with
DMI. Hence, geometrically frustrated Kagom\'e antiferromagnets can be deemed
topologically trivial. The presence of an out-of-plane magnetic field in these
frustrated magnets induces noncoplanar spin textures exhibiting a nonzero spin
scalar chirality. We show that the field-induced spin chirality provides
topological magnon bands in Kagom\'e antiferromagnets without the need of DMI
and survives in the chiral spin liquid phase of frustrated magnets. Possible
experimentally material includes iron jarosite KFe$_3$(OH)$_{6}$(SO$_{4}$)$_2$.",1609.03563v5
2018-08-22,Long-distance supercurrent transport in a room-temperature Bose-Einstein magnon condensate,"The term supercurrent relates to a macroscopic dissipation-free collective
motion of a quantum condensate and is commonly associated with such famous
low-temperature phenomena as superconductivity and superfluidity. Another type
of motion of quantum condensates is second sound - a wave of the density of a
condensate. Recently, we reported on an enhanced decay of a parametrically
induced Bose-Einstein condensate (BEC) of magnons caused by a supercurrent
outflow of the BEC phase from the locally heated area of a room temperature
magnetic film. Here, we present the direct experimental observation of a
long-distance spin transport in such a system. The condensed magnons being
pushed out from the potential well within the heated area form a density wave,
which propagates through the BEC many hundreds of micrometers in the form of a
specific second sound pulse - Bogoliubov waves - and is reflected from the
sample edge. The discovery of the long distance supercurrent transport in the
magnon BEC further advances the frontier of the physics of quasiparticles and
allows for the application of related transport phenomena for low-loss data
transfer in perspective magnon spintronics devices.",1808.07407v1
2019-06-12,Frequency-Division Multiplexing in Magnonic Logic Networks Based on Caustic-Like Spin-Wave Beams,"Wave-based data processing by spin waves and their quanta, magnons, is a
promising technique to overcome the challenges which CMOS-based logic networks
are facing nowadays. The advantage of these quasi-particles lies in their
potential for the realization of energy efficient devices on the micro- to
nanometer scale due to their charge-less propagation in magnetic materials. In
this paper, the frequency dependence of the propagation direction of
caustic-like spin-wave beams in microstructured ferromagnets is studied by
micromagnetic simulations. Based on the observed alteration of the propagation
angle, an approach to spatially combine and separate spin-wave signals of
different frequencies is demonstrated. The presented magnetic structure
constitutes a prototype design of a passive circuit enabling frequency-division
multiplexing in magnonic logic networks. It is verified that spin-wave signals
of different frequencies can be transmitted through the device simultaneously
without any interaction or creation of spurious signals. Due to the wave-based
approach of computing in magnonic networks, the technique of frequency-division
multiplexing can be the basis for parallel data processing in single magnonic
devices, enabling the multiplication of the data throughput.",1906.04993v2
2019-07-05,Tunability of domain structure and magnonic spectra in antidot arrays of Heusler alloy,"Materials suitable for magnonic crystals demand low magnetic damping and long
spin wave (SW) propagation distance. In this context Co based Heusler compounds
are ideal candidates for magnonic based applications. In this work, antidot
arrays (with different shapes) of epitaxial
$\mathrm{Co}_2\mathrm{Fe}_{0.4}\mathrm{Mn}_{0.6}\mathrm{Si}$ (CFMS) Heusler
alloy thin films have been prepared using e-beam lithography and sputtering
technique. Magneto-optic Kerr effect and ferromagnetic resonance analysis have
confirmed the presence of dominant cubic and moderate uniaxial magnetic
anisotropies in the thin films. Domain imaging via x-ray photoemission electron
microscopy on the antidot arrays reveals chain like switching or correlated
bigger domains for different shape of the antidots. Time-resolved MOKE
microscopy has been performed to study the precessional dynamics and magnonic
modes of the antidots with different shapes. We show that the optically induced
spin-wave spectra in such antidot arrays can be tuned by changing the shape of
the holes. The variation in internal field profiles, pinning energy barrier,
and anisotropy modifies the spin-wave spectra dramatically within the antidot
arrays with different shapes. We further show that by combining the
magnetocrystalline anisotropy with the shape anisotropy, an extra degree of
freedom can be achieved to control the magnonic modes in such antidot lattices.",1907.02746v1
2019-10-19,Propagation of spin-waves packets in individual nano-sized yttrium iron garnet magnonic conduits,"Modern-days CMOS-based computation technology is reaching its fundamental
limitations. The emerging field of magnonics, which utilizes spin waves for
data transport and processing, proposes a promising path to overcome these
limitations. Different devices have been demonstrated recently on the macro-
and microscale, but the feasibility of the magnonics approach essentially
relies on the scalability of the structure feature size down to an extent of a
few 10 nm, which are typical sizes for the established CMOS technology. Here,
we present a study of propagating spin-wave packets in individual yttrium iron
garnet (YIG) conduits with lateral dimensions down to 50 nm. Space and time
resolved micro-focused Brillouin-Light-Scattering (BLS) spectroscopy is used to
characterize the YIG nanostructures and measure the spin-wave decay length and
group velocity directly. The revealed magnon transport at the scale comparable
to the scale of CMOS proves the general feasibility of a magnon-based data
processing.",1910.08801v2
2019-10-21,High-frequency magnon excitation due to femtosecond spin-transfer torques,"Femtosecond laser pulses can induce ultrafast demagnetization as well as
generate bursts of hot electron spin currents. In trilayer spin valves
consisting of two metallic ferromagnetic layers separated by a nonmagnetic one,
hot electron spin currents excited by an ultrashort laser pulse propagate from
the first ferromagnetic layer through the spacer reaching the second magnetic
layer. When the magnetizations of the two magnetic layers are noncollinear,
this spin current exerts a torque on magnetic moments in the second
ferromagnet. Since this torque is acting only within the sub-ps timescale, it
excites coherent high-frequency magnons as recently demonstrated in
experiments. Here, we calculate the temporal shape of the hot electron spin
currents using the superdiffusive transport model and simulate the response of
the magnetic system to the resulting ultrashort spin-transfer torque pulse by
means of atomistic spin-dynamics simulations. Our results confirm that the
acting spin-current pulse is short enough to excite magnons with frequencies
beyond 1 THz, a frequency range out of reach for current induced spin-transfer
torques. We demonstrate the formation of thickness dependent standing spin
waves during the first picoseconds after laser excitation. In addition, we vary
the penetration depth of the spin-transfer torque to reveal its influence on
the excited magnons. Our simulations clearly show a suppression effect of
magnons with short wavelengths already for penetration depths in the range of 1
nm confirming experimental findings reporting penetration depths below $2\,
{\rm nm}$.",1910.09412v1
2020-01-27,Resonant thermal energy transfer to magnons in a ferromagnetic nanolayer,"Energy harvesting is a modern concept which makes dissipated heat useful by
transferring thermal energy to other excitations. Most of the existing
principles for energy harvesting are realized in systems which are heated
continuously, for example generating DC voltage in thermoelectric devices. Here
we present the concept of high-frequency energy harvesting where the dissipated
heat in a sample excites resonant magnons in a 5-nm thick ferromagnetic metal
layer. The sample is excited by femtosecond laser pulses with a repetition rate
of 10 GHz which results in temperature modulation at the same frequency with
amplitude ~0.1 K. The alternating temperature excites magnons in the
ferromagnetic nanolayer which are detected by measuring the net magnetization
precession. When the magnon frequency is brought onto resonance with the
optical excitation, a 12-fold increase of the amplitude of precession indicates
efficient resonant heat transfer from the lattice to coherent magnons. The
demonstrated principle may be used for energy harvesting in various nanodevices
operating at GHz and sub-THz frequency ranges.",2001.09732v2
2020-04-22,Excitation of high-frequency magnon modes in magnetoelastic films by short strain pulses,"Development of energy efficient techniques for generation of spin waves
(magnons) is important for implementation of low-dissipation spin-wave-based
logic circuits and memory elements. A promising approach to achieve this goal
is based on the injection of short strain pulses into ferromagnetic films with
a strong magnetoelastic coupling between spins and strains. Here we report
micromagnetoelastic simulations of the magnetization and strain dynamics
excited in Fe$_{81}$Ga$_{19}$ films by picosecond and nanosecond acoustic
pulses created in a GaAs substrate by a transducer subjected to an optical or
electrical impulse. The simulations performed via the numerical solution of the
coupled Landau-Lifshitz-Gilbert and elastodynamic equations show that the
injected strain pulse induces an inhomogeneous magnetization precession in the
ferromagnetic film. The precession lasts up to 1 ns and can be treated as a
superposition of magnon modes having the form of standing spin waves. For
Fe$_{81}$Ga$_{19}$ films with nanoscale thickness, up to seven (six) distinct
modes have been revealed under free-surface (pinning) magnetic boundary
conditions. Remarkably, magnon modes with frequencies over 1 THz can be excited
by acoustic pulses with an appropriate shape and duration in the films
subjected to a moderate external magnetic field. This finding shows that short
strain pulses represent a promising tool for the generation of THz spin waves
necessary for the implementation of high-speed magnonic devices.",2004.10838v1
2020-04-30,Current-Controlled Nanomagnetic Writing for Reconfigurable Magnonic Crystals,"Strongly-interacting nanomagnetic arrays are crucial across an ever-growing
suite of technologies. Spanning neuromorphic computing, control over
superconducting vortices and reconfigurable magnonics, the utility and appeal
of these arrays lies in their vast range of distinct, stable magnetisation
states. Different states exhibit different functional behaviours, making
precise, reconfigurable state control an essential cornerstone of such systems.
However, few existing methodologies may reverse an arbitrary array element, and
even fewer may do so under electrical control, vital for device integration.
  We demonstrate selective, reconfigurable magnetic reversal of ferromagnetic
nanoislands via current-driven motion of a transverse domain wall in an
adjacent nanowire. The reversal technique operates under all-electrical control
with no reliance on external magnetic fields, rendering it highly suitable for
device integration across a host of magnonic, spintronic and neuromorphic logic
architectures. Here, the reversal technique is leveraged to realise two fully
solid-state reconfigurable magnonic crystals, offering magnonic gating,
filtering, transistor-like switching and peak-shifting without reliance on
global magnetic fields.",2004.14709v1
2020-05-23,Enhanced entanglement and asymmetric EPR steering between magnons,"The generation and manipulation of strong entanglement and
Einstein-Podolsky-Rosen (EPR) steering in macroscopic systems are outstanding
challenges in modern physics. Especially, the observation of asymmetric EPR
steering is important for both its fundamental role in interpreting the nature
of quantum mechanics and its application as resource for the tasks where the
levels of trust at different parties are highly asymmetric. Here, we study the
entanglement and EPR steering between two macroscopic magnons in a hybrid
ferrimagnet-light system. In the absence of light, the two types of magnons on
the two sublattices can be entangled, but no quantum steering occurs when they
are damped with the same rates. In the presence of the cavity field, the
entanglement can be significantly enhanced, and strong two-way asymmetric
quantum steering appears between two magnons with equal dispassion. This is
very different from the conventional protocols to produce asymmetric steering
by imposing additional unbalanced losses or noises on the two parties at the
cost of reducing steerability. The essential physics is well understood by the
unbalanced population of acoustic and optical magnons under the cooling effect
of cavity photons. Our finding may provide a novel platform to manipulate the
quantum steering and the detection of bi-party steering provides a knob to
probe the magnetic damping on each sublattice of a magnet.",2005.11471v1
2020-05-28,Spin-Pumping-Induced Non-Linear Electric Current on the Surface of a Ferromagnetic Topological Insulator,"We investigate the spin-pumping-induced electric current on the surface of a
three-dimensional topological insulator hybridized with a ferromagnet, namely,
ferromagnetic topological insulator. In order to do this, we establish the
microscopic formalism and construct the perturbation theory using a Keldysh
Green's function approach. We analyze how this electric current is generated by
an exchange interaction and an external ac magnetic field, which is the driving
force of ferromagnetic resonance as well as the spin pumping. The mechanism is
as follows. First, the ferromagnetic resonance is driven and a zero-momentum
magnon emerges. It is the fluctuation from the saturation magnetization
pointing parallel to the precession axis of the ferromagnetic resonance. After
then, the spin pumping is generated with the zero-momentum magnon being the
carrier of spin. The zero-momentum magnon and the topological insulator surface
state couples through the exchange interaction and the spin carried by the
magnon is transferred to it. Owing to the spin-momentum locking, the
transferred spin is converted into the momentum of topological insulator
surface state leading to the generation of electric current flowing
perpendicular to the precession axis of the ferromagnetic resonance. It is
quadratic in the amplitude of external ac magnetic field whereas it is linear
to the strength of the exchange interaction. The associated electric voltage is
described by the spectrum of zero-momentum magnon. The non-linearity of
spin-pumping-induced electric current in the ac magnetic field as well as the
linearity in the exchange-interaction strength reflects that the surface of
ferromagnetic topological insulator has a high-performing functionality of
generating the electric charge current by magnetic controlling.",2005.13850v1
2020-07-06,Stable Higgs mode in anisotropic quantum magnets,"Low-energy excitations associated with the amplitude fluctuation of an order
parameter in condensed matter systems can mimic the Higgs boson, an elementary
particle in the standard model, and are dubbed as Higgs modes. Identifying the
condensed-matter Higgs mode is challenging because it is known in many cases to
decay rapidly into other low-energy bosonic modes, which renders the Higgs mode
invisible. Therefore, it is desirable to find a way to stabilize the Higgs
mode, which can offer an insight into the stabilization mechanism of the Higgs
mode in condensed matter physics. In quantum magnets, magnetic order caused by
spontaneous symmetry breaking supports transverse (magnons) and longitudinal
(Higgs modes) fluctuations. When a continuous symmetry is broken, the Goldstone
magnon mode generally has a lower excitation energy than the Higgs mode,
causing a rapid decay of the latter. In this work, we show that a stable Higgs
mode exists in anisotropic quantum magnets near the quantum critical point
between the dimerized and magnetically ordered phases. We find that an easy
axis anisotropy increases the magnon gap such that the magnon mode is above the
Higgs mode near the quantum critical point, and the decay of the Higgs mode
into the magnon mode is forbidden kinematically. Our results suggest that the
anisotropic quantum magnets provide ideal platforms to explore the Higgs
physics in condensed matter systems.",2007.02498v1
2020-08-13,Magnetization Dynamics of Nanoscale Magnetic Materials: A Perspective,"Nanomagnets form the building blocks for a gamut of miniaturized
energy-efficient devices including data storage, memory, wave-based computing,
sensors and biomedical devices. They also offer a span of exotic phenomena and
stern challenges. The progress in the magnetization dynamics of single
nanomagnets and one- and two-dimensional arrays of nanostructures in the form
of dots, antidots, nanoparticles, binary and bicomponent structures and
patterned multilayers have been presented. Progress in unconventional and new
structures like artificial spin ice and three-dimensional nanomagnets and spin
textures like domain walls, vortex and skyrmions have been presented.
Furthermore, a huge variety of new topics in the magnetization dynamics of
magnetic nanostructures are rapidly emerging. An overview of the steadily
evolving topics like spatio-temporal imaging of fast dynamics of
nanostructures, dynamics of spin textures, artificial spin ice have been
discussed. In addition, dynamics of contemporary and newly transpired magnetic
architectures such as nanomagnet arrays with complex basis and symmetry,
magnonic quasicrystals, fractals, defect structures, novel three-dimensional
structures have been introduced. Effects of various spin-orbit coupling and
ensuing spin textures as well as quantum hybrid systems comprising of
magnon-photon, magnon-phonon and magnon-magnon coupling, antiferromagnetic
nanostructures are rapidly growing and are expected to dominate this research
field in the coming years. Finally, associated topics like nutation dynamics
and nanomagnet antenna are briefly discussed. Despite showing a great progress,
only a small fraction of nanomagnetism and its ancillary topics have been
explored so far and huge efforts are envisaged in this evergrowing research
area in the generations to come.",2008.05819v1
2020-11-17,Non-local magnon-based transport in yttrium iron garnet/platinum heterostructures at high temperatures,"The spin Hall effect in a heavy metal thin film allows to probe the magnetic
properties of an adjacent magnetic insulator via magnetotransport measurements.
Here, we investigate the magnetoresistive response of yttrium iron
garnet/platinum heterostructures from room temperature to beyond the Curie
temperature $T_\mathrm{C, YIG} \approx 560\,\mathrm{K}$ of the ferrimagnetic
insulator. We find that the amplitude of the (local) spin Hall
magnetoresistance decreases monotonically from $300\,\mathrm{K}$ towards
$T_\mathrm{C}$, mimicking the evolution of the saturation magnetization of
yttrium iron garnet. Interestingly, the spin Hall magnetoresistance vanishes
around $500\,\mathrm{K}$, well below $T_\mathrm{C}$, which we attribute to the
formation of a parasitic interface layer by interdiffusion. Around room
temperature the non-local magnon-mediated magnetoresistance exhibits a power
law scaling $T^{\alpha}$ with $\alpha = 3/2$, as already reported. The exponent
decreases gradually to $\alpha \sim 1/2$ at around $420\,\mathrm{K}$, before
the non-local magnetoresistance vanishes rapidly at a similar temperature as
the spin Hall magnetoresistance. We attribute the reduced $\alpha$ at high
temperatures to the increasing thermal magnon population which leads to
enhanced scattering of the non-equilibrium magnon population and a reduced
magnon diffusion length. Finally, we find a magnetic field independent offset
voltage in the non-local signal for $T > 470\,\mathrm{K}$ which we associate
with electronic leakage currents through the normally insulating yttrium iron
garnet film. Indeed, this non-local offset voltage is thermally activated with
an energy close to the band gap.",2011.08589v1
2021-06-15,Cavity magnon-polaritons in cuprate parent compounds,"Cavity control of quantum matter may offer new ways to study and manipulate
many-body systems. A particularly appealing idea is to use cavities to enhance
superconductivity, especially in unconventional or high-$T_c$ systems.
Motivated by this, we propose a scheme for coupling Terahertz resonators to the
antiferromagnetic fluctuations in a cuprate parent compound, which are believed
to provide the glue for Cooper pairs in the superconducting phase. First, we
derive the interaction between magnon excitations of the Ne\'el-order and polar
phonons associated with the planar oxygens. This mode also couples to the
cavity electric field, and in the presence of spin-orbit interactions mediates
a linear coupling between the cavity and magnons, forming hybridized
magnon-polaritons. This hybridization vanishes linearly with photon momentum,
implying the need for near-field optical methods, which we analyze within a
simple model. We then derive a higher-order coupling between the cavity and
magnons which is only present in bilayer systems, but does not rely on
spin-orbit coupling. This interaction is found to be large, but only couples to
the bimagnon operator. As a result we find a strong, but heavily damped,
bimagnon-cavity interaction which produces highly asymmetric cavity line-shapes
in the strong-coupling regime. To conclude, we outline several interesting
extensions of our theory, including applications to carrier-doped cuprates and
other strongly-correlated systems with Terahertz-scale magnetic excitations.",2106.07828v1
2021-06-17,Attraction from frustration in ladder systems,"We analyze the formation of multi-particle bound states in ladders with
frustrated kinetic energy in two component bosonic and two component fermionic
systems. We focus on the regime of light doping relative to insulating states
at half-filling, spin polarization close to 100 percent, and strong repulsive
interactions. A special feature of these systems is that the binding energy
scales with single particle tunneling $t$ rather than exchange interactions,
since effective attraction arises from alleviating kinetic frustration. For two
component Fermi systems on a zigzag ladder we find a bound state between a hole
and a flipped spin (magnon) with a binding energy that can be as large as
$0.6t$. We demonstrate that magnon-hole attraction leads to formation of
clusters comprised of several holes and magnons and expound on
antiferromagentic correlations for the transverse spin components inside the
clusters. We identify several many-body states that result from
self-organization of multi-particle bound states, including a Luttinger liquid
of hole-magnon pairs and a density wave state of two hole - three magnon
composites. We establish a symmetry between the spectra of Bose and Fermi
systems and use it to establish the existence of antibound states in two
component Bose mixtures with SU(2) symmetric repulsion on a zigzag ladder. We
also consider Bose and Fermi systems on a square ladder with flux and
demonstrate that both systems support bound states. We discuss experimental
signatures of multi-particle bound states in both equilibrium and dynamical
experiments. We point out intriguing connections between these systems and the
quark bag model in QCD.",2106.09600v1
2021-06-20,Parametric generation of spin waves in nano-scaled magnonic conduits,"The research feld of magnonics proposes a low-energy wave-logic computation
technology based on spin waves to complement the established CMOS technology
and provide a basis for emerging unconventional computation architectures.
However, magnetic damping is a limiting factor for all-magnonic logic circuits
and multi-device networks, ultimately rendering mechanisms to effciently
manipulate and amplify spin waves a necessity. In this regard, parallel pumping
is a versatile tool since it allows to selectively generate and amplify spin
waves. While extensively studied in microscopic systems, nano-scaled systems
are lacking investigation to assess the feasibility and potential future use of
parallel pumping in magnonics. Here, we investigate a longitudinally magnetized
100 nm-wide magnonic nano-conduit using space and time-resolved micro-focused
Brillouin-light-scattering spectroscopy. Employing parallel pumping to generate
spin waves, we observe that the non-resonant excitation of dipolar spin waves
is favored over the resonant excitation of short wavelength exchange spin
waves. In addition, we utilize this technique to access the effective spin-wave
relaxation time of an individual nano-conduit, observing a large relaxation
time up to (115.0 +- 7.6) ns. Despite the significant decrease of the pumping
effciency in the investigated nano-conduit, a reasonably small threshold is
found rendering parallel pumping feasible on the nano-scale.",2106.10727v2
2021-06-28,Exact solutions of few-magnon problems in the spin-$S$ periodic XXZ chain,"We solve few-magnon problems for a finite-size spin-$S$ periodic Heisenberg
XXZ chain with single-ion anisotropy through constructing sets of exact Bloch
states achieving block diagonalization of the system. Concretely, the
two-magnon (three-magnon) problem is converted to a single-particle one on a
one-dimensional (two-dimensional) effective lattice whose size depends linearly
(quadratically) on the total number of sites. For parameters lying within
certain ranges, various types of multimagnon bound states are manifested and
shown to correspond to edge states on the effective lattices. In the absence of
the single-ion anisotropy, we reveal the condition under which exact
zero-energy states emerge. As applications of the formalism, we calculate the
transverse dynamic structure factor for a higher-spin chain near saturation
magnetization and find signatures of the multimagnon bound states. We also
calculate the real-time three-magnon dynamics from certain localized states,
which are relevant to cold-atom quantum simulations, by simulating
single-particle quantum walks on the effective lattices. This provides a
physically transparent interpretation of the observed dynamics in terms of
propagation of bound state excitations. Our method can be directly applied to
more general spin or itinerant particle systems possessing translational
symmetry.",2106.14809v4
2021-06-30,Electron-magnon coupling and quasiparticle lifetimes on the surface of a topological insulator,"The fermionic self-energy on the surface of a topological insulator proximity
coupled to ferro- and antiferromagnetic insulators is studied. An enhanced
electron-magnon coupling is achieved by allowing the electrons on the surface
of the topological insulator to have a different exchange coupling to the two
sublattices of the antiferromagnet. Such a system is therefore seen as superior
to a ferromagnetic interface for the realization of magnon-mediated
superconductivity. The increased electron-magnon-coupling simultaneously
increases the self-energy effects. In this paper we show how the inverse
quasiparticle lifetime and energy renormalization on the surface of the
topological insulator can be kept low close to the Fermi level by using a
magnetic insulator with a sufficient easy-axis anisotropy. We find that the
antiferromagnetic case is most interesting from both a theoretical and an
experimental standpoint due to the increased electron-magnon coupling, combined
with a reduced need for easy-axis anisotropy compared to the ferromagnetic
case. We also consider a set of material and instrumental parameters where
these self-energies should be measurable in angle-resolved photoemission
spectroscopy experiments, paving the way for a measurement of the interfacial
exchange coupling strength.",2107.00028v2
2021-07-08,Topological magnon insulators in two-dimensional van der Waals ferromagnets CrSiTe$_3$ and CrGeTe$_3$: towards intrinsic gap-tunability,"The bosonic analogues of topological insulators have been proposed in
numerous theoretical works, but their experimental realization is still very
rare, especially for spin systems. Recently, two-dimensional (2D) honeycomb van
der Waals (vdW) ferromagnets have emerged as a new platform for topological
spin excitations. Here, via a comprehensive inelastic neutron scattering study
and theoretical analysis of the spin-wave excitations, we report the
realization of topological magnon insulators in CrXTe$_3$ (X=Si, Ge) compounds.
The nontrivial nature and intrinsic tunability of the gap opening at the magnon
band-crossing Dirac points are confirmed, while the emergence of the
corresponding in-gap topological edge states is demonstrated theoretically. The
realization of topological magnon insulators with intrinsic gap-tunability in
this class of remarkable 2D materials will undoubtedly lead to new and
fascinating technological applications in the domain of magnonics and
topological spintronics.",2107.03835v1
2022-01-27,Magnon-Polaron Driven Thermal Hall Effect in a Heisenberg-Kitaev Antiferromagnet,"The thermal Hall effect, defined as a heat current response transversal to an
applied temperature gradient, is a central experimental probe of exotic
electrically insulating phases of matter. A key question is how the interplay
between magnetic and structural degrees of freedom gives rise to a nonzero
thermal Hall conductivity (THC). Here, we present evidence for an intrinsic
thermal Hall effect in the Heisenberg-Kitaev antiferromagnet and spin-liquid
candidate Na$_2$Co$_2$TeO$_6$ brought about by the quantum-geometric Berry
curvature of so-called magnon polarons, resulting from magnon-phonon
hybridization. At low temperatures, our field- and temperature-dependent
measurements show a negative THC for magnetic fields below 10 T and a sign
change to positive THC above. Theoretically, the sign and the order of
magnitude of the THC cannot be solely explained with magnetic excitations. We
demonstrate that, by incorporating spin-lattice coupling into our theoretical
calculations, the Berry curvature of magnon polarons counteracts the purely
magnonic contribution, reverses the overall sign of the THC, and increases its
magnitude, which significantly improves agreement with experimental data. Our
work highlights the crucial role of spin-lattice coupling in the thermal Hall
effect.",2201.11396v2
2022-05-19,Resonant phonon-magnon interactions in free-standing metal-ferromagnet multilayer structures,"We analyze resonant magneto-elastic interactions between standing
perpendicular spin wave modes (exchange magnons) and longitudinal acoustic
phonon modes in free-standing hybrid metal-ferromagnet bilayer and trilayer
structures. Whereas the ferromagnetic layer acts as a magnetic cavity, all
metal layers control the frequencies and eigenmodes of acoustic vibrations. The
here proposed design allows for achieving and tuning the spectral and spatial
modes overlap between phonons and magnons that results in their strong resonant
interaction. Realistic simulations for gold-nickel multilayers show that
sweeping the external magnetic field should allow for observing resonantly
enhanced interactions between individual magnon and phonon modes in a broad
range of frequencies spanning from tens of GHz up to several hundreds of GHz,
which can be finely tuned through the multilayer design. Our results would
enable the systematic study and the deep understanding of resonantly enhanced
magneto-elastic coupling between individual phonon and magnon modes up to
frequencies of great contemporary fundamental and applied interest.",2205.09805v1
2022-10-05,Particle zoo in a doped spin chain: Correlated states of mesons and magnons,"It is a widely accepted view that the interplay of spin- and charge-degrees
of freedom in doped antiferromagnets (AFMs) gives rise to the rich physics of
high-temperature superconductors. Nevertheless, it remains unclear how
effective low-energy degrees of freedom and the corresponding field theories
emerge from microscopic models, including the $t-J$ and Hubbard Hamiltonians. A
promising view comprises that the charge carriers have a rich internal parton
structure on intermediate scales, but the interplay of the emergent partons
with collective magnon excitations of the surrounding AFM remains unexplored.
Here we study a doped one-dimensional spin chain in a staggered magnetic field
and demonstrate that it supports a zoo of various long-lived excitations. These
include magnons; mesonic pairs of spinons and chargons, along with their
ro-vibrational excitations; and tetra-parton bound states of mesons and
magnons. We identify these types of quasiparticles in various spectra using
DMRG simulations. Moreover, we introduce a strong-coupling theory describing
the polaronic dressing and molecular binding of mesons to collective magnon
excitations. The effective theory can be solved by standard tools developed for
polaronic problems, and can be extended to study similar physics in
two-dimensional doped AFMs in the future. Experimentally, the doped spin-chain
in a staggered field can be directly realized in quantum gas microscopes.",2210.02320v1
2022-11-12,Quantum control of a single magnon in a macroscopic spin system,"Non-classical quantum states are the pivotal features of a quantum system
that differs from its classical counterpart. However, the generation and
coherent control of quantum states in a macroscopic spin system remain an
outstanding challenge. Here we experimentally demonstrate the quantum control
of a single magnon in a macroscopic spin system (i.e., 1~mm-diameter
yttrium-iron-garnet sphere) coupled to a superconducting qubit via a microwave
cavity. By tuning the qubit frequency {\it in situ} via the Autler-Townes
effect, we manipulate this single magnon to generate its non-classical quantum
states, including the single-magnon state and the superposition state of a
single magnon and vacuum. Moreover, we confirm the deterministic generation of
these non-classical states by Wigner tomography. Our experiment offers the
first reported deterministic generation of the non-classical quantum states in
a macroscopic spin system and paves a way to explore its promising applications
in quantum engineering.",2211.06644v3
2022-12-22,Spin wave dispersion of ultra-low damping hematite ($α\text{-Fe}_2\text{O}_3$) at GHz frequencies,"Low magnetic damping and high group velocity of spin waves (SWs) or magnons
are two crucial parameters for functional magnonic devices. Magnonics research
on signal processing and wave-based computation at GHz frequencies focussed on
the artificial ferrimagnetic garnet Y$_3$Fe$_5$O$_{12}$ (YIG) so far. We report
on spin-wave spectroscopy studies performed on the natural mineral hematite
($\alpha\text{-Fe}_2\text{O}_3$) which is a canted antiferromagnet. By means of
broadband GHz spectroscopy and inelastic light scattering, we determine a
damping coefficient of $1.1\times10^{-5}$ and magnon group velocities of a few
10 km/s, respectively, at room temperature. Covering a large regime of wave
vectors up to $k\approx 24~{\rm rad}/\mu$m, we find the exchange stiffness
length to be relatively short and only about 1 \r{A}. In a small magnetic field
of 30 mT, the decay length of SWs is estimated to be 1.1 cm similar to the best
YIG. Still, inelastic light scattering provides surprisingly broad and partly
asymmetric resonance peaks. Their characteristic shape is induced by the large
group velocities, low damping and distribution of incident angles inside the
laser beam. Our results promote hematite as an alternative and sustainable
basis for magnonic devices with fast speeds and low losses based on a stable
natural mineral.",2212.11887v2
2023-01-25,Enhanced tripartite interactions in spin-magnon-mechanical hybrid systems,"Coherent tripartite interactions among degrees of freedom of completely
different nature are instrumental for quantum information and simulation
technologies, but they are generally difficult to realize and remain largely
unexplored. Here, we predict a tripartite coupling mechanism in a hybrid setup
comprising a single NV center and a micromagnet. We propose to realize direct
and strong tripartite interactions among single NV spins, magnons and phonons
via modulating the relative motion between the NV center and the micromagnet.
Specifically, by introducing a parametric drive (two-phonon drive) to modulate
the mechanical motion (such as the center-of-mass motion of a NV spin in
diamond trapped in an electrical trap or a levitated micromagnet in a magnetic
trap), we can obtain a tunable and strong spin-magnon-phonon coupling at the
single quantum level, with up to two orders of magnitude enhancement for the
tripartite coupling strength. This enables, for example, tripartite
entanglement among solid-state spins, magnons, and mechanical motions in
quantum spin-magnonics-mechanics with realistic experimental parameters. This
protocol can be readily implemented with the well-developed techniques in ion
traps or magnetic traps, and could pave the way for general applications in
quantum simulations and information processing based on directly and strongly
coupled tripartite systems.",2301.10424v1
2023-05-10,Giant spin Nernst effect in a two-dimensional antiferromagnet due to magnetoelastic coupling-induced gaps and interband transitions between magnon-like bands,"We analyze theoretically the origin of the spin Nernst and thermal Hall
effects in FePS3 as a realization of two-dimensional antiferromagnet (2D AFM).
We find that a strong magnetoelastic coupling, hybridizing magnetic excitation
(magnon) and elastic excitation (phonon), combined with
time-reversal-symmetry-breaking, results in a Berry curvature hotspots in the
region of anticrossing between the two distinct hybridized bands. Furthermore,
large spin Berry curvature emerges due to interband transitions between two
magnon-like bands, where a small energy gap is induced by magnetoelastic
coupling between such bands that are energetically distant from anticrossing of
hybridized bands. These nonzero Berry curvatures generate topological
transverse transport (i.e., the thermal Hall effect) of hybrid excitations,
dubbed magnon-polaron, as well as of spin (i.e., the spin Nernst effect)
carried by them, in response to applied longitudinal temperature gradient. We
investigate the dependence of the spin Nernst and thermal Hall conductivities
on the applied magnetic field and temperature, unveiling very large spin Nernst
conductivity even at zero magnetic field. Our results suggest FePS3 AFM, which
is already available in 2D form experimentally, as a promising platform to
explore the topological transport of the magnon-polaron quasiparticles at THz
frequencies.",2305.06020v2
2023-06-23,Phonon Frequency and its Modification by Magnon-Phonon Coupling from All-Temperature Theory of Magnon,"The all-temperature magnon (ATM) theory [J. Phys. Condens. Matter 21,
336003/1-14, 2009] has been used to analyze the temperature dependence of
magnetization as well as the internal energy components of a mono-domain
ferromagnetic solid. The critical exponents have been in better agreement with
experiment than their mean-field theory and critical phenomenon theory
counterparts, and unlike in the latter theories, vary from one ferromagnet to
another. Expressions have been derived for the thermally averaged spin-center
force constants and their break-up in terms of the base-line related (solid)
and exchange-cum-field mediated (magnetic) components. These components give
rise to expressions for phonon frequencies and their modifications by
magnon-phonon coupling. The derived expressions are suitable for a correct
quantum chemical evaluation of the involved properties. A detailed numerical
calculation using spin configurations at varying crystal geometries is hardly
possible even today and beyond the scope of the present work. The focus here is
on the correctness and explaining the trends of properties. It has been shown
that the frequency modification by magnon-phonon interaction can be negative
for certain phonon branches near the ferromagnetic transition temperature.
Also, the ratio of frequency modification and phonon frequency is approximately
proportional to the ratio of curvatures of the involved energy surfaces.",2306.13346v2
2023-07-12,Electrical detectability of magnon-mediated spin current shot noise,"A magnonic spin current crossing a ferromagnet-metal interface is accompanied
by spin current shot noise arising from the discrete quanta of spin carried by
magnons. In thin films, e.g., the spin of so-called squeezed magnons have been
shown to deviate from the common value $\hbar$, with corresponding changes in
the spin noise. In experiments, spin currents are typically converted to charge
currents via the inverse spin Hall effect. We here analyze the magnitude of the
spin current shot noise in the charge channel for a typical electrically
detected spin pumping experiment, and find that the voltage noise originating
from the spin current shot noise is much smaller than the inevitable
Johnson-Nyquist noise. Furthermore, we find that due to the local nature of the
spin-charge conversion, the ratio of spin current shot noise and
Johnson-Nyquist noise cannot be systematically enhanced by tuning the sample
geometry, in contrast to the linear increase in dc spin pumping voltage with
sample length. Instead, the ratio depends sensitively on material-specific
transport properties. Our analysis thus provides guidance for the experimental
detection of squeezed magnons through spin pumping shot noise.",2307.06103v2
2023-08-31,Magnon Orbital Angular Momentum of Ferromagnetic Honeycomb and Zig-Zag Lattices,"By expanding the gauge $\lambda_n(k)$ for magnon band $n$ in harmonics of
momentum ${\bf k} =(k,\phi )$, we demonstrate that the only observable
component of the magnon orbital angular momentum $O_n({\bf k})$ is its angular
average over all angles $\phi$, denoted by $F_n(k)$. For both the FM honeycomb
and zig-zag lattices, we show that $F_n(k)$ is nonzero in the presence of a
Dzyalloshinzkii-Moriya (DM) interaction. The FM zig-zag lattice model with
exchange interactions $0<J_1< J_2$ provides a new system where the effects of
orbital angular momentum are observable. For the zig-zag model with equal
exchange interactions $J_{1x}$ and $J_{1y}$ along the $x$ and $y$ axis, the
magnon bands are degenerate along the boundaries of the Brillouin zone with
$k_x-k_y =\pm \pi/a$ and the Chern numbers $C_n$ are not well defined. However,
a revised model with $J_{1y}\ne J_{1x}$ lifts those degeneracy and produces
well-defined Chern numbers of $C_n=\pm 1$ for the two magnon bands. When
$J_{1y}=J_{1x}$, the thermal conductivity $\kappa^{xy}(T)$ of the FM zig-zag
lattice is largest for $J_2/J_1>6$ but is still about four times smaller than
that of the FM honeycomb lattice at high temperatures. Due to the removal of
band degeneracies, $\kappa^{xy}(T)$ is slightly enhanced when $J_{1y}\ne
J_{1x}$.",2308.16832v2
2023-09-21,A Series Expansion Study for Large Negative Quantum Renormalization of Magnon Spectra in the $S=1/2$ Kagome-Lattice Heisenberg Antiferromagnet Cs$_{2}$Cu$_{3}$SnF$_{12}$,"The series expansion method is used to study magnon spectra of the kagome
system with nearest-neighbor exchange interaction $J$ and out-of-plane
Dzyaloshinskii-Moriya (DM) interaction $D^{\parallel}$, which is a minimal
model for Cs$_{2}$Cu$_{3}$SnF$_{12}$. Compared to the magnon spectra by the
linear spin wave (LSW) theory, we find that dispersions at high energy part
suffer downward deformation, which is similar to the triangle lattice case, in
addition to the reduction of the energy scale of about 40\% as pointed out in a
neutron-scattering study by Ono {\it et al.} Using a reliable estimation
$J=20.7$ meV in a previous study on the magnetic susceptibility of
Cs$_{2}$Cu$_{3}$SnF$_{12}$, we use $D^{\parallel}$ as the fitting parameter to
reproduce the experimental magnon spectra and obtain $D^{\parallel}=0.12J$. We
also report that a roton-like minimum occurs at the M point and a maximum at
points somewhat away from the $\Gamma$ point. Compression of the LSW magnon
band is commonly seen in the kagome and triangular-lattice systems, which may
be viewed as being pushed from above by the spinon continuum.",2309.11778v1
2023-10-24,Spin wave excitations in low dimensional systems with large magnetic anisotropy,"The low energy excitation spectrum of a two-dimensional ferromagnetic
material is dominated by single-magnon excitations that show a gapless
parabolic dispersion relation with the spin wave vector. This occurs as long as
magnetic anisotropy and anisotropic exchange are negligible compared to
isotropic exchange. However, to maintain magnetic order at finite temperatures,
it is necessary to have sizable anisotropy to open a gap in the spin wave
excitation spectrum. We consider four real two-dimensional systems for which
ferromagnetic order at finite temperature has been observed or predicted.
Density functional theory calculations of the total energy differences for
different spin configurations permit us to extract the relevant parameters and
connect them with a spin Hamiltonian. The corresponding values of the Curie
temperature are estimated using a simple model and found to be mostly
determined by the value of the isotropic exchange. The exchange and anisotropy
parameters are used in a toy model of finite-size periodic chains to study the
low-energy excitation spectrum, including single-magnon and two-magnon
excitations. At low energies we find that single-magnon excitations appear in
the spectrum together with two-magnon excitations. These excitations present a
gap that grows particularly for large values of the magnetic anisotropy or
anisotropic exchange, relative to the isotropic exchange.",2310.15942v1
2023-12-07,Magnon-Photon Coupling in an Opto-Electro-Magnonic Oscillator,"The opto-electronic oscillators (OEOs) hosting self-sustained oscillations by
a time delayed mechanism are of particular interest in long-haul signal
transmission and processing. On the other hand, owing to their unique
tunability and compatibility, magnons - as elementary excitations of spin waves
- are advantageous carriers for coherent signal transduction across different
platforms. In this work, we integrated an opto-electronic oscillator with a
magnonic oscillator consisting of a microwave waveguide and a yttrium iron
garnet sphere. We find that, in the presence of the magnetic sphere, the
oscillator power spectrum exhibits sidebands flanking the fundamental OEO
modes. The measured waveguide transmission reveals anti-crossing gaps, a
hallmark of the coupling between the opto-electronic oscillator modes and the
Walker modes of the sphere. Experimental results are well reproduced by a
coupled-mode theory that accounts for nonlinear magnetostrictive interactions
mediated by the magnetic sphere. Leveraging the advanced fiber-optic
technologies in opto-electronics, this work lays out a new, hybrid platform for
investigating long distance coupling and nonlinearity in coherent magnonic
phenomena.",2312.04676v2
2023-12-08,Engineering synthetic gauge fields through the coupling phases in cavity magnonics,"Cavity magnonics, which studies the interaction of light with magnetic
systems in a cavity, is a promising platform for quantum transducers, quantum
memories, and devices with non-reciprocal behaviour. At microwave frequencies,
the coupling between a cavity photon and a magnon, the quasi-particle of a spin
wave excitation, is a consequence of the Zeeman interaction between the
cavity's magnetic field and the magnet's macroscopic spin. For each
photon/magnon interaction, a coupling phase factor exists, but is often
neglected in simple systems. However, in ""loop-coupled"" systems, where there
are at least as many couplings as modes, the coupling phases become relevant
for the physics and lead to synthetic gauge fields. We present experimental
evidence of the existence of such coupling phases by considering two spheres
made of Yttrium-Iron-Garnet and two different re-entrant cavities. We predict
numerically the values of the coupling phases, and we find good agreement
between theory and the experimental data. Theses results show that in cavity
magnonics, one can engineer synthetic gauge fields, which can be useful for
cavity-mediated coupling, dark mode memories, and building nonreciprocal
devices.",2312.04915v3
2023-12-29,In-Plane Magnon Valve Effect in Magnetic Insulator/Heavy Metal/ Magnetic Insulator Device,"We propose an in-plane magnon valve (MV), a sandwich structure composed of
ferromagnetic insulator/heavy metal/ferromagnetic insulator (MI/HM/MI). When
the magnetizations of the two MI layers are parallel, the longitudinal
conductance in the HM layer is greater than that in the antiparallel state
according to the magnetic proximity effect, termed as the in-plane magnon valve
effect. We investigate the dependence of MV ratio (MVR), which is the relative
change in longitudinal conductance between the parallel and antiparallel MV
states, on the difference in electronic structure between magnetized and
non-magnetized metal atoms, revealing that MVR can reach 100%. Additionally,
the dependence of MVR on the thickness of metal layer is analyzed, revealing an
exponential decrease with increasing thickness. Then we investigate the
dependence of HM layer conductance on the relative angle between the
magnetizations of two MI layers, illustrating the potential of MV as a
magneto-sensitive magnonic sensor. We also investigate the effect of Joule
heating on the measurement signal based on the spin Seebeck effect. Two
designed configurations are proposed according to whether the electron current
is parallel or perpendicular to the magnetization of the MI layer. In the
parallel configuration, the transverse voltage differs between the parallel and
antiparallel MV states. While in the perpendicular configuration, the
longitudinal resistance differs. Quantitative numerical results indicate the
feasibility of detecting a voltage signal using the first configuration in
experiments. Our work contributes valuable insights for the design, development
and integration of magnon devices",2312.17413v1
2024-03-01,Microwave-to-optics conversion using magnetostatic modes and a tunable optical cavity,"Quantum computing, quantum communication and quantum networks rely on hybrid
quantum systems operating in different frequency ranges. For instance, the
superconducting qubits work in the gigahertz range, while the optical photons
used in communication are in the range of hundreds of terahertz. Due to the
large frequency mismatch, achieving the direct coupling and information
exchange between different information carriers is generally difficult.
Accordingly, a quantum interface is demanded, which serves as a bridge to
establish information linkage between different quantum systems operating at
distinct frequencies. Recently, the magnon mode in ferromagnetic spin systems
has received significant attention. While the inherent weak optomagnonic
coupling strength restricts the microwave-to-optical photon conversion
efficiency using magnons, the versatility of the magnon modes, together with
their readily achievable strong coupling with other quantum systems, endow them
with many distinct advantages. Here, we realize the magnon-based
microwave-light interface by adopting an optical cavity with adjustable free
spectrum range and different kinds of magnetostatic modes in two microwave
cavity configurations. By optimizing the parameters, an internal conversion
efficiency of $1.28 \times 10^{-7}$ is achieved. We analyze the impact of
various parameters on the microwave-to-optics conversion. The study provides
useful guidance and insights to further enhancing the microwave-to-optics
conversion efficiency using magnons.",2403.00345v2
2011-04-25,Magnetic excitations in L-edge resonant inelastic x-ray scattering from cuprate compounds,"We study the magnetic excitation spectra in L-edge resonant inelastic x-ray
scattering (RIXS) from undoped cuprates. We analyze the second-order dipole
allowed process that the strong perturbation works through the intermediate
state in which the spin degree of freedom is lost at the core-hole site. Within
the approximation neglecting the perturbation on the neighboring sites, we
derive the spin-flip final state in the scattering channel with changing the
polarization, which leads to the RIXS spectra expressed as the dynamical
structure factor of the transverse spin components. We assume a spherical form
of the spin-conserving final state in the channel without changing the
polarization, which leads to the RIXS spectra expressed as the 'exchange'-type
multi-spin correlation function. Evaluating numerically the transition
amplitudes to these final states on a finite-size cluster, we obtain a sizable
amount of the transition amplitude to the spin-conserving final state in
comparison with that to the spin-flip final state. We treat the itinerant
magnetic excitations in the final state by means of the 1/S-expansion method.
Evaluating the higher-order correction with 1/S, we find that the peak arising
from the one-magnon excitation is reduced with its weight, and the continuous
spectra arising from the three-magnon excitations come out. The interaction
between two magnons is treated by summing up the ladder diagrams. On the basis
of these results, we analyze the L_3-edge RIXS spectra in Sr_2CuO_2Cl_2 in
comparison with the experiment. It is shown that the three-magnon excitations
as well as the two-magnon excitations give rise to the intensity in the high
energy side of the one-magnon peak, making the spectral shape asymmetric with
wide width, in good agreement with the experiment.",1104.4683v2
2014-08-14,Quantum magnetism of spinor bosons in optical lattices with synthetic non-Abelian gauge fields,"We study quantum magnetism of interacting spinor bosons at integer fillings
hopping in a square lattice in the presence of non-Abelian gauge fields. In the
strong coupling limit, it leads to the Rotated ferromagnetic Heisenberg model
(RFHM) which is a new class of quantum spin model. We introduce Wilson loops to
characterize frustrations and gauge equivalent classes. For a special
equivalent class, we identify a new spin-orbital entangled commensurate ground
state. It supports not only commensurate magnons, but also a new gapped
elementary excitation: in-commensurate magnons with two gap minima continuously
tuned by the SOC strength. At low temperatures, these magnons lead to dramatic
effects in many physical quantities such as density of states, specific heat,
magnetization, uniform susceptibility, staggered susceptibility and various
spin correlation functions. The commensurate magnons lead to a pinned central
peak in the angle resolved light or atom Bragg spectroscopy. However, the
in-commensurate magnons split it into two located at their two gap minima. At
high temperatures, the transverse spin structure factors depend on the SOC
strength explicitly. The whole set of Wilson loops can be mapped out by
measuring the specific heat at the corresponding orders in the high temperature
expansion. We argue that one gauge may be realized in current experiments and
other gauges may also be realized in near future experiments. The results
achieved along the exact solvable line sets up the stage to investigate
dramatic effects when tuning away from it by various means. We sketch the
crucial roles to be played by these magnons at other equivalent classes, with
spin anisotropic interactions and in the presence of finite magnetic fields.
Various experimental detections of these new phenomena are discussed. Rotated
Anti-ferromagnetic Heisenberg model are also briefly mentioned.",1408.3399v3
2016-07-22,Magnon-induced long-range correlations and their neutron-scattering signature in quantum magnets,"We consider the coupling of the magnons in both quantum ferromagnets and
antiferromagnets to the longitudinal order-parameter fluctuations, and the
resulting nonanalytic behavior of the longitudinal susceptibility. In classical
magnets it is well known that long-range correlations induced by the magnons
lead to a singular wave-number dependence of the form $1/k^{4-d}$ in all
dimensions 2<d<4, for both ferromagnets and antiferromagnets. At zero
temperature we find a profound difference between the two cases. Consistent
with naive power counting, the longitudinal susceptibility in a quantum
antiferromagnet scales as $k^{d-3}$ for 1<d<3, whereas in a quantum ferromagnet
the analogous result, $k^{d-2}$, is absent due to a zero scaling function. This
absence of a nonanalyticity in the longitudinal susceptibility is due to the
lack of magnon number fluctuations in the ground state of a quantum
ferromagnet; correlation functions that are sensitive to other fluctuations do
exhibit the behavior predicted by simple power counting. Also of interest is
the dynamical behavior as expressed in the longitudinal part of the dynamical
structure factor, which is directly measurable via neutron scattering. For both
ferromagnets and antiferromagnets there is a logarithmic singularity at the
magnon frequency with a prefactor that vanishes as $T\to 0$. In the
antiferromagnetic case there also is a nonzero contribution at T=0 that is
missing for ferromagnets. Magnon damping due to quenched disorder restores the
expected scaling behavior of the longitudinal susceptibility in the
ferromagnetic case; it scales as $k^{d-2}$ if the order parameter is not
conserved, or as $k^d$ if it is. Detailed predictions are made for both two-
and three-dimensional systems at both T=0 and in the limit of low temperatures,
and the physics behind the various nonanalytic behaviors is discussed.",1607.06790v1
2017-06-17,Magnon-phonon coupling effects on the indirect K-edge resonant inelastic X-ray scattering spectrum of a 2D Heisenberg antiferromagnet,"We compute the effects of magnon-phonon coupling on the indirect K-edge
bimagnon resonant inelastic x-ray scattering (RIXS) intensity spectrum of a
square lattice Heisenberg antiferromagnet. We analyze the effects of competing
nearest and next--nearest magnetic and magnon-phonon coupling interaction in
the RIXS spectrum, for both the antiferromagnetic (AF) and the collinear
antiferromagnetic (CAF) phases of the model. Utilizing the Dyson-Maleev
representation of spin operators, the Bethe-Salpeter ladder approximation
scheme for the bimagnon interacting channel, and considering the lowest order
magnon-phonon-magnon scattering interaction we highlight distinct features in
the X-ray spectrum. Considering damping effects, arising due to the presence of
phonons, we find that in the AF phase the RIXS intensity spectrum attains a
maximum value primarily localized around the K $\left(\pm\frac{\pi}{2}, \pm
\frac{\pi}{2}\right)$ - point. For the CAF phase the intensity is broadly
distributed with a significant scattering intensity located around the Y
$\left(\pm\frac{\pi}{2}, 0\right)$ - point. Furthermore, in the CAF phase for
suitable anisotropy, nearest-, and next-nearest neighbor interaction parameters
the phonon effects can manifest itself as a distinct peak both below and above
the bimagnon peak. Such a feature is in contrast to the antiferromagnetic
spectrum where the effect due to the phonon peak was located consistently
beyond the bimagnon peak in the high energy end of the spectrum. Additionally,
in the CAF phase we find the RIXS bimagnon-phonon spectrum to be more sensitive
to anisotropy compared to its antiferromagnetic counterpart. We conclude that
the ultimate effect of magnon-phonon effects in the indirect K-edge RIXS
spectrum, in both the antiferromagnetic and the collinear antiferromagnetic
phase, is an observable effect.",1706.05478v2
2016-12-15,Magnons and Magnetodielectric Effects in CoCr$_2$O$_4$: Raman Scattering Studies,"Magnetoelectric materials have generated wide technological and scientific
interest because of the rich phenomena these materials exhibit, including the
coexistence of magnetic and ferroelectric orders, magnetodielectric behavior,
and exotic hybrid excitations such as electromagnons. The multiferroic spinel
material, CoCr$_2$O$_4$, is a particularly interesting example of a
multiferroic material, because evidence for magnetoelectric behavior in the
ferrimagnetic phase seems to conflict with traditional noncollinear-spin-driven
mechanisms for inducing a macroscopic polarization. This paper reports an
inelastic light scattering study of the magnon and phonon spectrum of
CoCr$_2$O$_4$ as simultaneous functions of temperature, pressure, and magnetic
field. Below the Curie temperature ($T_C \sim 94$ K) of CoCr$_2$O$_4$ we
observe a $\omega \sim 16 \,\text{cm}^{-1}$ $\boldsymbol q=0$ magnon having
T$_{1g}$-symmetry, which has the transformation properties of an axial vector.
The anomalously large Raman intensity of the T$_{1g}$-symmetry magnon is
characteristic of materials with a large magneto-optical response and likely
arises from large magnetic fluctuations that strongly modulate the dielectric
response in CoCr$_2$O$_4$. The Raman susceptibility of the T$_{1g}$-symmetry
magnon exhibits a strong magnetic-field dependence that is consistent with the
magnetodielectric response observed in CoCr$_2$O$_4$, suggesting that
magnetodielectric behavior in CoCr$_2$O$_4$ primarily arises from the
field-dependent suppression of magnetic fluctuations that are strongly coupled
to long-wavelength phonons. Increasing the magnetic anisotropy in CoCr$_2$O$_4$
with applied pressure decreases the magnetic field-dependence of the
T$_{1g}$-symmetry magnon Raman susceptibility in CoCr$_2$O$_4$, suggesting that
strain can be used to control the magnetodielectric response in CoCr$_2$O$_4$.",1612.05283v1
2020-06-28,Avoided quasiparticle decay and enhanced excitation continuum in the spin-1/2 near-Heisenberg triangular antiferromagnet Ba3CoSb2O9,"We explore the magnetic excitations of the spin-1/2 triangular
antiferromagnet Ba3CoSb2O9 in its 120 degree ordered phase using single-crystal
high-resolution inelastic neutron scattering. Sharp magnons with no decay are
observed throughout reciprocal space, with a strongly renormalized dispersion
and multiple soft modes compared to linear spin wave theory. We propose an
empirical parametrization that can quantitatively capture the complete
dispersions in the three-dimensional Brillouin zone and explicitly show that
the dispersion renormalizations have the direct consequence that one to two
magnon decays are avoided throughout reciprocal space, whereas such decays
would be allowed for the unrenormalized dispersions. At higher energies, we
observe a very strong continuum of excitations with highly-structured intensity
modulations extending up at least 4x the maximum one-magnon energy. The
one-magnon intensities decrease much faster upon increasing energy than
predicted by linear spin wave theory and the higher-energy continuum contains
much more intensity than can be accounted for by a two-magnon cross-section,
suggesting a significant transfer of spectral weight from the high-energy
magnons into the higher-energy continuum states. We attribute the strong
dispersion renormalizations and substantial transfer of spectral weight to
continuum states to the effect of quantum fluctuations and interactions beyond
the spin wave approximation, and make connections to theoretical approaches
that might capture such effects. Finally, through measurements in a strong
applied magnetic field, we find evidence for magnetic domains with opposite
senses for the spin rotation in the 120 degree ordered ground state, as
expected in the absence of Dzyaloshinskii-Moriya interactions, when the sense
of spin rotation is selected via spontaneous symmetry breaking.",2006.15554v1
2021-05-02,Steady state entanglement of distant nitrogen-vacancy centers in a coherent thermal magnon bath,"We investigate steady-state entanglement (SSE) between two nitrogen-vacancy
(NV) centers in distant nanodiamonds on an ultrathin Yttrium Iron Garnet (YIG)
strip. We determine the dephasing and dissipative interactions of the qubits
with the quanta of spin waves (magnon bath) in the YIG depending on the qubit
positions on the strip. We show that the magnon's dephasing effect can be
eliminated, and we can transform the bath into a multimode displaced thermal
state using external magnetic fields. Entanglement dynamics of the qubits in
such a displaced thermal bath have been analyzed by deriving and solving the
master equation. An additional electric field is considered to engineer the
magnon dispersion relation at the band edge to control the Markovian character
of the open system dynamics. We determine the optimum geometrical parameters of
the system of distant qubits and the YIG strip to get SSE. Furthermore,
parameter regimes for which the shared displaced magnon bath can sustain
significant SSE against the local dephasing and decoherence of NV centers to
their nuclear spin environments have been determined. Along with SSE, we
investigate the steady-state coherence (SSC) and explain the physical mechanism
of how delayed SSE appears following a rapid generation and sudden death of
entanglement using the interplay of decoherence-free subspace states, system
geometry, displacement of the thermal bath, and enhancement of the qubit
dissipation near the magnon band edge. A non-monotonic relation between bath
coherence and SSE is found, and critical coherence for maximum SSE is
determined. Our results illuminate the efficient use of system geometry, band
edge in bath spectrum, and reservoir coherence to engineer system-reservoir
interactions for robust SSE and SSC.",2105.00519v2
2022-12-10,Ring-Exchange Interaction Effects on Magnons in Dirac Magnet CoTiO$_3$,"In magnetically ordered materials with localized electrons, the fundamental
magnetic interactions are due to exchange of electrons [1-3]. Typically, only
the interaction between pairs of electrons' spins is considered to explain the
nature of the ground state and its excitations, whereas three-, four-, and
six-spin interactions are ignored. When these higher order processes occur in a
loop they are called cyclic or ring exchange. The ring-exchange interaction is
required to explain low temperature behavior in bulk and thin films of solid
$^3$He [4-8]. It also plays a crucial role in the quantum magnet La$_2$CuO$_4$
[9,10]. Here, we use a combination of time domain THz (TDTS) and magneto-Raman
spectroscopies to measure the low energy magnetic excitations in CoTiO$_3$, a
proposed Dirac topological magnon material [11,12] where the origin of the
energy gap in the magnon spectrum at the Brillouin zone center remains unclear.
We measured the magnetic field dependence of the energies of the two lowest
energy magnons and determine that the gap opens due to the ring-exchange
interaction between the six spins in a hexagon. This interaction also explains
the selection rules of the THz magnon absorption. Finally, we clarify that
topological surface magnons are not expected in CoTiO$_3$. Our study
demonstrates the power of combining TDTS and Raman spectroscopies with theory
to identify the microscopic origins of the magnetic excitations in quantum
magnets.",2212.05278v1
2023-02-22,Unveiling the impact of temperature on magnon diffuse scattering detection in the transmission electron microscope,"Magnon diffuse scattering (MDS) signals could be studied with high spatial
resolution in scanning transmission electron microscopy (STEM), thanks to
recent technological progress in electron energy loss spectroscopy. However,
detecting MDS signals in STEM is challenging due to their overlap with stronger
thermal diffuse scattering (TDS) signals. In bcc Fe at 300 K, MDS signals
greater than or comparable to TDS signals occur under the central Bragg disk,
into a currently inaccesible energy-loss region. Therefore, to detect MDS in
STEM, it is necessary to find conditions in which TDS and MDS signals can be
separated. Temperature may be a key factor due to the distinct thermal
signatures of magnon and phonon signals. In this work, we present a study on
the effects of temperature on MDS and TDS in bcc Fe -- considering a detector
outside the central Bragg disk and a fixed convergent electron probe -- using
the frozen phonon and frozen magnon multislice methods. Our study reveals that
neglecting the effects of atomic vibrations causes the MDS signal to grow
approximately linearly up to the Curie temperature of Fe, after which it
exhibits less variation. The MDS signal displays an alternating behavior due to
dynamical diffraction, instead of increasing monotonically as a function of
thickness. Including the effects of atomic vibrations through a complex atomic
electrostatic potential causes the linear growth of the MDS signal to change to
a non-linear behavior that exhibits a predominant peak for a sample of
thickness 16.072 nm at 1100 K. In contrast, the TDS signal grows more linearly
than the MDS signal but still exhibits appreciable dynamical diffraction
effects. An analysis of the signal-to-noise ratio (SNR) shows that the MDS
signal can be a statistically significant contribution to the total scattering
intensity under realizable measurement conditions and acquisition times.",2302.11462v2
2023-08-14,Investigation of Phonon Lifetimes and Magnon-Phonon Coupling in YIG/GGG Hybrid Magnonic Systems in the Diffraction Limited Regime,"Quantum memories facilitate the storage and retrieval of quantum information
for on-chip and long-distance quantum communications. Thus, they play a
critical role in quantum information processing and have diverse applications
ranging from aerospace to medical imaging fields. Bulk acoustic wave (BAW)
phonons are one of the most attractive candidates for quantum memories because
of their long lifetime and high operating frequency. In this work, we establish
a modeling approach that can be broadly used to design hybrid magnonic
high-overtone bulk acoustic wave resonator (HBAR) structures for high-density,
long-lasting quantum memories and efficient quantum transduction devices. We
illustrate the approach by investigating a hybrid magnonic system, where BAW
phonons are excited in a gadolinium iron garnet (GGG) thick film via coupling
with magnons in a patterned yttrium iron garnet (YIG) thin film. We present
theoretical and numerical analyses of the diffraction-limited BAW phonon
lifetimes, modeshapes, and their coupling strengths to magnons in planar and
confocal YIG/GGG HBAR structures. We utilize Fourier beam propagation and
Hankel transform eigenvalue problem methods and discuss the effectiveness of
the two methods to predict the HBAR phonons. We discuss strategies to improve
the phonon lifetimes, since increased lifetimes have direct implications on the
storage times of quantum states for quantum memory applications. We find that
ultra-high, diffraction-limited, cooperativities and phonon lifetimes on the
order of ~10^5 and ~10 milliseconds, respectively, could be achieved using a
CHBAR structure with 10mum lateral YIG dimension. Additionally, the confocal
HBAR structure will offer more than 100-fold improvement of integration
density. A high integration density of on-chip memory or transduction centers
is naturally desired for high-density memory or transduction devices.",2308.06896v2
2023-12-26,Observation of Magnon Damping Minimum Induced by Kondo Coupling in a van der Waals Ferromagnet Fe$_{3-x}$GeTe$_{2}$,"In heavy-fermion systems with $f$ electrons, there is an intricate interplay
between Kondo screening and magnetic correlations, which can give rise to
various exotic phases. Recently, similar interplay appears to also occur in
$d$-electron systems, but the underlying mechanism remains elusive. Here, using
inelastic neutron scattering, we investigate the temperature evolution of the
low-energy spin waves in a metallic van der Waals ferromagnet
Fe$_{3-x}$GeTe$_{2}$ (Curie temperature $T_{\rm C}\sim160$ K), where the
Kondo-lattice behavior emerges in the ferromagnetic phase below a
characteristic temperature $T^*\sim90$ K. We observe that the magnon damping
constant diverges at both low and high temperatures, exhibiting a minimum
coincidentally around $T^*$. Such an observation is analogous to the
resistivity minimum as due to the single-impurity Kondo effect. This unusual
behavior is described by a formula that combines logarithmic and power terms,
representing the dominant contributions from Kondo screening and thermal
fluctuations, respectively. Furthermore, we find that the magnon damping
increases with momentum below $T_{\rm C}$. These findings can be explained by
considering spin-flip electron-magnon scattering, which serves as a magnonic
analog of the Kondo-impurity scattering, and thus provides a measure of the
Kondo coupling through magnons. Our results provide critical insights into how
Kondo coupling manifests itself in a system with magnetic ordering and shed
light on the coexistence of and interplay between magnetic order and Kondo
effect in itinerant 3$d$-electron systems.",2312.15961v1
1993-11-09,S(k) for Haldane Gap Antiferromagnets: Large-scale Numerical Results vs. Field Theory and Experiment,"The structure function, S(k), for the s=1, Haldane gap antiferromagnetic
chain, is measured accurately using the recent density matrix renormalization
group method, with chain-length 100. Excellent agreement with the nonlinear
$\sigma$ model prediction is obtained, both at $k\approx \pi$ where a single
magnon process dominates and at $k\approx 0$ where a two magnon process
dominates. We repeat our calculation with crystal field anisotropy chosen to
model NENP, obtaining good agreement with both field theory predictions and
recent experiments. Correlation lengths, gaps and velocities are determined for
both polarizations.",9311021v2
1995-09-13,Spin-wave excitation spectra and spectral weights in square lattice antiferromagnets,"Using a recently developed method for calculating series expansions of the
excitation spectra of quantum lattice models, we obtain the spin-wave spectra
for square lattice, $S=1/2$ Heisenberg-Ising antiferromagnets. The calculated
spin-wave spectrum for the Heisenberg model is close to but noticeably
different from a uniformly renormalized classical (large-$S$) spectrum with the
renormalization for the spin-wave velocity of approximately $1.18$. The
relative weights of the single-magnon and multi-magnon contributions to neutron
scattering spectra are obtained for wavevectors throughout the Brillouin zone.",9509077v1
1995-10-12,"Spinons, Solitons and Magnons in One-dimensional Heisenberg-Ising Antiferromagnets","We calculate the excitation spectra for the one-$d$ Heisenberg-Ising
antiferromagnets by expansions around the Ising limit. For $S=1/2$, the
calculated expansion coefficients for the spinon-spectra agree term by term
with the solution of Johnson and McCoy. For $S=1$, the solitons become gapless
before the Heisenberg limit is reached, signalling a transition to the Haldane
phase. By applying a staggered field we calculate the one-magnon spectra for
the $S=1$ Heisenberg chain. For $S=3/2$ the quantum renormalization of the
spin-wave spectra is calculated to be approximately $1.16$.",9510064v1
1996-12-02,One-dimensional spin-liquid without magnon excitations,"It is shown that a sufficiently strong four-spin interaction in the spin-1/2
spin ladder can cause dimerization. Such interaction can be generated either by
phonons or (in the doped state) by the conventional Coulomb repulsion between
the holes. The dimerized phases are thermodynamically undistinguishable from
the Haldane phase, but have dramatically different correlation functions: the
dynamical magnetic susceptibility, instead of displaying a sharp single magnon
peak near $q = \pi$, shows only a two-particle threshold separated from the
ground state by a gap.",9612014v2
1997-04-10,Magnetic Excitations in the S=1/2 Alternating Chain Compound (VO)_2P_2O_7,"Magnetic excitations in an array of (VO)_2P_2O_7 single crystals have been
measured using inelastic neutron scattering. Until now, (VO)_2P_2O_7 has been
thought of as a two-leg antiferromagnetic Heisenberg spin ladder with chains
running in the a-direction. The present results show unequivocally that
(VO)_2P_2O_7 is best described as an alternating spin-chain directed along the
crystallographic b-direction. In addition to the expected magnon with magnetic
zone-center energy gap \Delta = 3.1 meV, a second excitation is observed at an
energy just below 2\Delta. The higher mode may be a triplet two-magnon bound
state.",9704092v1
1997-12-05,Spin flip scattering in magnetic junctions,"Processes which flip the spin of an electron tunneling in a junction made up
of magnetic electrodes are studied. It is found that: i) Magnetic impurities
give a contribution which increases the resistance and lowers the
magnetoresistance, which saturates at low temperatures. The conductance
increases at high fields. ii) Magnon assisted tunneling reduces the
magnetoresistance as $T^{3/2}$, and leads to a non ohmic contribution to the
resistance which goes as $V^{3/2}$, iii) Surface antiferromagnetic magnons,
which may appear if the interface has different magnetic properties from the
bulk, gives rise to $T^2$ and $V^2$ contributions to the magnetoresistance and
resistance, respectively, and, iv) Coulomb blockade effects may enhance the
magnetoresistance, when transport is dominated by cotunneling processes.",9712075v1
1999-01-21,Interplay between Mesoscopic and Microscopic Fluctuations in Ferromagnets,"A model of a ferromagnet is considered, in which there arise mesoscopic
fluctuations of paramagnetic phase. The presence of these fluctuations
diminishes the magnetization of the ferromagnet, softens the spin-wave
spectrum, increases the spin-wave attenuation, shortens the magnon free path,
lowers the critical point, and can change the order of phase transition. A
special attention is paid to the interplay between these mesoscopic
paramagnetic fluctuations and microscopic fluctuations due to magnons. One of
the main results of this interplay is an essential extension of the region of
parameters where the ferromagnet-paramagnet phase transition is of first order.",9901213v1
1999-01-30,The correlation length of the Heisenberg antiferromagnet with arbitrary spin S,"Experiments and numerical data on the correlation length for large S disagree
strongly with the theoretical prediction based on the effective field theory
prescription of the magnon physics. The reason is that for large S, at any
accessible correlation lengths, the cut-off effects from the non-magnon scales
become large and can not be treated by an effective field theory. We study
these effects in a spin-wave expansion. The corrected prediction connects the
renormalized classical and the classical scaling regions smoothly and comes
close to the data.",9901355v1
1999-08-09,Realization of Bose-Einstein condensation of dilute magnons in TlCuCl$_3$,"The recent observation [Oosawa et al. J. Phys. : Condens. Matter {\bf 11},
265 (1999)] of the field-induced N\'{e}el ordering in a spin-gap magnetic
compound TlCuCl$_3$ is interpreted as a Bose-Einstein Condensation of magnons.
A mean-field calculation based on this picture is shown to describe well the
temperature dependence of the magnetization. The present system opens a new
area for studying Bose-Einstein condensation of thermodynamically large number
of particles in a grand-canonical ensemble.",9908118v1
1999-10-07,Theory of anomalous magnon softening in ferromagnetic manganites,"In metallic manganites with low Curie temperatures, a peculiar softening of
the magnon spectrum close to the magnetic zone boundary has experimentally been
observed. Here we present a theory of the renormalization of the magnetic
excitation spectrum in colossal magnetoresistance compounds. The theory is
based on the modulation of magnetic exchange bonds by the orbital degree of
freedom of double-degenerate e_g electrons. The model considered is an
orbitally degenerate double-exchange system coupled to Jahn-Teller active
phonons which we treat in the limit of strong onsite repulsions. Charge and
coupled orbital-lattice fluctuations are identified as the main origin of the
unusual softening of the magnetic spectrum.",9910106v1
2000-05-24,Evolution of the Spin Gap Upon Doping a 2-Leg Ladder,"The evolution of the spin gap of a 2-leg ladder upon doping depends upon the
nature of the lowest triplet excitations in a ladder with two holes. Here we
study this evolution using various numerical techniques for a t-t'-J ladder as
the next-near-neighbor hopping t' is varied. We find that depending on the
value of t', the spin gap can evolve continuously or discontinuously and the
lowest triplet state can correspond to a magnon, a bound magnon-hole-pair, or
two separate quasi-particles. Previous experimental results on the
superconducting two-leg ladder Sr12Ca2Cu24O41 are discussed.",0005403v1
2000-09-04,Observation of anomalous single-magnon scattering in half-metallic ferromagnets by chemical pressure control,"Temperature variation of resistivity and specific heat have been measured for
prototypical half-metallic ferromagnets,
  R_0.6Sr_0.4MnO_3, with controlling the one-electron bandwidth W. We have
found variation of the temperature scalings in the resistivity from
  T^2 (R = La, and Nd) to T^3 (R = Sm), and have interpreted the $T^3-law in
terms of the anomalous single-magnon scattering (AMS) process in the
half-metallic system.",0009035v1
2000-09-04,Magnon scattering processes and low temperature resistivity in CMR manganites,"Low temperature resistivity of CMR manganites is investigated. At the ground
state, conduction electrons are perfectly spin polarized, which is called
half-metallic. From one-magnon scattering processes, it is discussed that the
resistivity of a half metal as a function of temperature scales as rho(T) -
rho(0) propto T^3. We take (Nd,Tb,Sr)MnO_3 as an example to compare theory and
experiments. The result is in a good agreement.",0009036v1
2000-10-30,Anisotropic charge transfer mechanism in La$_{2-x}$Sr$_x$CuO$_4$ and Bi$_2$Sr$_2$Ca$_{1-x}$Y$_x$Cu$_2$O$_{8+δ}$,"Raman spectra of La$_{2-x}$Sr$_x$CuO$_4$ are similar to those of
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ except for the split two-magnon peaks at the
stripe phase and the stronger step-like decrease in the low energy $B_{\rm 2g}$
spectra. The suppressed $B_{\rm 1g}$ spectral region below the two-magnon peak
decreases as the carrier density increases, whereas the energy of the step-like
suppression is little dependent on the carrier density. It suggests the
anisotropic charge transfer mechanism coupled with magnetic excitations along
the nearest neighbor Cu sites and the phononic excitations along the diagonal
direction.",0010472v1
2000-12-06,Magnons in the ferromagnetic Kondo-lattice model,"The magnetic properties of the ferromagnetic Kondo-lattice model (FKLM) are
investigated. Starting from an analysis of the magnon spectrum in the spin-wave
regime, we examine the ferromagnetic stability as a function of the occupation
of the conduction band $n$ and the strength $J$ of the coupling between the
localised moments and the conduction electrons. From the properties of the
spin-wave stiffness $D$ the ferromagnetic phase at zero temperature is derived.
Using an approximate formula the critical temperature $T_c$ is calculated as a
function of $J$ and $n$.",0012086v1
2000-12-19,Significance of the direct relaxation process in the low-energy spin dynamics of a one-dimensional ferrimagnet NiCu(C_7H_6N_2O_6)(H_2O)_3 2H_2O,"In response to recent nuclear-magnetic-resonance measurements on a
ferrimagnetic chain compound NiCu(C_7H_6N_2O_6)(H_2O)_3 2H_2O [Solid State
Commun. {\bf 113} (2000) 433], we calculate the nuclear spin-lattice relaxation
rate 1/T_1 in terms of a modified spin-wave theory. Emphasizing that the
dominant relaxation mechanism arises from the direct (single-magnon) process
rather than the Raman (two-magnon) one, we explain the observed temperature and
applied-field dependences of 1/T_1. Ferrimagnetic relaxation phenomena are
generally discussed and novel ferrimagnets with extremely slow dynamics are
predicted.",0012342v1
2002-01-04,Magnon modes for thin circular vortex state magnetic dot,"The magnetization in a magnetic microdot made from soft magnetic materials
can have a vortex-like ground state structure resulting from competition
between the exchange and dipolar interactions. Normal mode magnon frequencies
for such dots are calculated taking into account both exchange and
magnetostatic effects. The presence of a low-lying mode as well as doublet
structure with small splitting is demonstrated. Estimates of the mode
frequencies for permalloy dots are obtained, and the possibility of
experimental detection of such modes is discussed.",0201050v1
2002-09-17,Magnetic Order and Dynamics in an Orbitally Degenerate Ferromagnetic Insulator,"Neutron scattering was used to determine the spin structure and the magnon
spectrum of the Mott--Hubbard insulator YTiO$_3$. The magnetic structure is
complex, comprising substantial G-type and A-type antiferromagnetic components
in addition to the predominant ferromagnetic component. The magnon spectrum, on
the other hand, is gapless and nearly isotropic. We show that these findings
are inconsistent with the orbitally ordered states thus far proposed for
YTiO$_3$ and discuss general implications for a theoretical description of
exchange interactions in orbitally degenerate systems.",0209400v1
2002-11-22,Nonlinear microscopic relaxation of uniform magnetization precession,"Dynamic relaxation for nonlinear magnetization excitation is analyzed. For
direct processes, such as magnon-electron scattering and two-magnon scattering,
the relaxation rate is determined from the linear case simply by utilizing the
magnetization oscillation frequency for nonlinear excitation. For an indirect
process, such as slow-relaxing impurities, the analysis gives an additional
relaxation term proportional to the excitation level. In all cases the
effective magnetization damping is increased compared to
Landau-Lifshitz-Gilbert damping.",0211499v1
2003-01-28,Longitudinal magnon in the tetrahedral spin system Cu2Te2O5Br2 near quantum criticality,"We present a comprehensive study of the coupled tetrahedra-compound
Cu2Te2O5Br2 by theory and experiments in external magnetic fields. We report
the observation of a longitudinal magnon in Raman scattering in the ordered
state close to quantum criticality. We show that the excited
tetrahedral-singlet sets the energy scale for the magnetic ordering temperature
T_N. This energy is determined experimentally. The ordering temperature T_N has
an inverse-log dependence on the coupling parameters near quantum criticality.",0301546v1
2004-02-24,Dynamical model of the dielectric screening of conjugated polymers,"A dynamical model of the dielectric screening of conjugated polymers is
introduced and solved using the density matrix renormalization group method.
The model consists of a line of quantized dipoles interacting with a polymer
chain. The polymer is modelled by the Pariser-Parr-Pople (P-P-P) model. It is
found that: (1) Compared to isolated, unscreened single chains, the screened
1Bu- exciton binding energy is typically reduced by ca. 1 eV to just over 1 eV;
(2) Covalent (magnon and bi-magnon) states are very weakly screened compared to
ionic (exciton) states; (3) Screening of the 1Bu- exciton is closer to the
dispersion than solvation limit.",0402569v1
2004-05-26,Bose-Einstein condensation of magnons,"We use the Renormalization Group method to study the Bose-Einstein
condensation of the interacting dilute magnons which appears in three
dimensional spin systems in magnetic field. The obtained temperature dependence
of the critical field $H_c(T)-H_c(0) \sim T^{2}$ is different from the recent
self-consistent Hartree-Fock-Popov calculations (cond-mat/0405422) in which a
$T^{3/2}$ dependence was reported . The origin of this difference is discussed
in the framework of quantum critical phenomena.",0405619v1
2004-06-11,Bose-Einstein condensation of magnons in TlCuCl$_3$,"A quantitative study of the field-induced magnetic ordering in TlCuCl$_3$ in
terms of a Bose-Einstein condensation (BEC) of magnons is presented. It is
shown that the hitherto proposed simple BEC scenario is in quantitative and
qualitative disagreement with experiment. It is further shown that even very
small Dzyaloshinsky-Moriya interactions or a staggered $g$ tensor component of
a certain type can change the BEC picture qualitatively. Such terms lead to a
nonzero condensate density for all temperatures and a gapped quasiparticle
spectrum. Including this type of interaction allows us to obtain good agreement
with experimental data.",0406262v1
2004-12-08,Further Series Studies of the Spin-1/2 Heisenberg Antiferromagnet at T=0: Magnon Dispersion and Structure Factors,"We have extended our previous series studies of quantum antiferromagnets at
zero temperature by computing the one-magnon dispersion curves and various
structure factors for the linear chain, square and simple cubic lattices. Many
of these results are new; others are a substantial extension of previous work.
These results are directly comparable with neutron scattering experiments and
we make such comparisons where possible.",0412184v2
2005-03-17,Raman resonance in spin S two-leg ladder systems,"We argue that the Raman intensity in a spin S two-leg spin-ladder has a
pseudo-resonance peak, whose width is very small at large S. The
pseudo-resonance originates from the existence of a local minimum in the magnon
excitation spectrum, and is located slightly below twice the magnon energy at
the minimum. The physics behind the peak is similar to the excitonic scenario
for the neutron and Raman resonances in a d-wave superconductor.",0503453v1
2005-06-30,Magnon transport and spin current switching through quantum dots,"We study the nonequilibrium spin current through a quantum dot consisting of
two localized spin-1/2 coupled to two ferromagnetic insulators. The influence
of an intra-dot magnetic field and exchange coupling, different dot-reservoir
coupling configurations, and the influence of magnon chemical potential
differences vs. magnetic field gradients onto the spin current are examined. We
discuss various spin switching mechanisms and find that, in contrast to
electronic transport, the spin current is very sensitive to the specific
coupling configuration and band edges. In particular, we identify 1- and
2-magnon transport processes which can lead to resonances and antiresonances
for the spin current.",0506806v1
2005-10-21,Structural distortions of frustrated quantum spin lattices in high magnetic fields,"We study the stability of some strongly frustrated antiferromagnetic spin
lattices in high magnetic fields against lattice distortions. In particular, we
consider a spin-s anisotropic Heisenberg antiferromagnet on the
square-kagom\'{e} and kagom\'{e} lattices. The independent localized magnons
embedded in a ferromagnetic environment, which are the ground state at the
saturation field, imply lattice instabilities for appropriate lattice
distortions fitting to the structure of the localized magnons. We discuss in
detail the scenario of this spin-Peierls instability in high magnetic fields
which essentially depends on the values of the exchange interaction anisotropy
\Delta and spin s.",0510582v1
2005-11-11,Magnonics: Experiment to Prove the Concept,"An experimental scheme for studying spin wave propagation across thin film
samples is proposed. An experiment upon a periodically layered nanowire is
numerically simulated, while the sample might equally well be a continuous film
or an array of elements (e.g. nanowires) that either have uniform composition
or are periodically layered as in a magnonic crystal. The experiments could be
extended to study domain wall induced spin wave phase shifts, and used for
creation of the spin wave magnetic logic devices.",0511290v1
2006-01-18,Decay of quasiparticles in quantum spin liquids,"Magnetic excitations are studied in gapped quantum spin systems, for which
spontaneous two-magnon decays are allowed by symmetry. Interaction between one-
and two-particle states acquires nonanalytic frequency and momentum dependence
near the boundary of two-magnon continuum. This leads to a termination point of
single-particle branch for one-dimensional systems and to strong suppression of
quasiparticle weight in two dimensions. The momentum dependence of the decay
rate is calculated in arbitrary dimensions and effect of external magnetic
field is discussed.",0601405v1
2006-08-22,Clustering induced suppression of ferromagnetism in diluted magnets,"Ferromagnetism in diluted magnets in the compensated regime p << x is shown
to be suppressed by the formation of impurity spin clusters. The majority bulk
spin couplings are shown to be considerably weakened by the preferential
accumulation of holes in spin clusters, resulting in low-energy magnon
softening and enhanced low-temperature decay of magnetic order. A locally
self-consistent magnon renormalization analysis of spin dynamics shows that
although strong intra-cluster correlations tend to prolong global order, T_c is
still reduced compared to the ordered case.",0608474v2
2006-09-08,Peak resistance temperature and low temperature resistivity in thin film $La_{1-x} Ca_x Mn O_3$ mixtures for x <= 1/3,"The electrical resistivity of La$_{1-x}$Ca$_{x}$MnO$_{3}$ thin films grown on
(001) NdGaO$_{3}$ and (100) SrTiO$_{3}$ substrates by off-axis sputtering has
been studied as a function of the Calcium doping level. The samples have very
narrow rocking curves and excellent in plane registry with the substrate. A
strong correlation between the peak resistance temperature and the polaronic
hopping energy is seen which is not simply linear. The low temperature
resistivity is seen to be fit better by a model of single magnon scattering,
and a near linear correlation between the resistivity due to magnon scattering
and static impurities is observed.",0609206v1
2006-12-22,Magnon Dispersion and Anisotropies in SrCu$_2$(BO$_3$)$_2$,"We study the dispersion of the magnons (triplet states) in
SrCu$_2$(BO$_3$)$_2$ including all symmetry-allowed Dzyaloshinskii-Moriya
interactions. We can reduce the complexity of the general Hamiltonian to a new
simpler form by appropriate rotations of the spin operators. The resulting
Hamiltonian is studied by both perturbation theory and exact numerical
diagonalization on a 32-site cluster. We argue that the dispersion is dominated
by Dzyaloshinskii-Moriya interactions. We point out which combinations of these
anisotropies affect the dispersion to linear-order, and extract their
magnitudes.",0612598v2
2007-01-12,Momentum dependent light scattering in insulating cuprates,"We investigate the problem of inelastic x-ray scattering in the spin$-{1/2}$
Heisenberg model on the square lattice. We first derive a momentum dependent
scattering operator for the $A_{1g}$ and $B_{1g}$ polarization geometries. On
the basis of a spin-wave analysis, including magnon-magnon interactions and
exact-diagonalizations, we determine the qualitative shape of the spectra. We
argue that our results may be relevant to help interpret inelastic x-ray
scattering experiments in the antiferromagnetic phase of the cuprates.",0701289v1
2007-01-19,Excited states of quantum many-body interacting systems: A variational coupled-cluster description,"We extend recently proposed variational coupled-cluster method to describe
excitation states of quantum many-body interacting systems. We discuss, in
general terms, both quasiparticle excitations and quasiparticle-density-wave
excitations (collective modes). In application to quantum antiferromagnets, we
reproduce the well-known spin-wave excitations, i.e. quasiparticle magnons of
spin $\pm 1$. In addition, we obtain new, spin-zero magnon-density-wave
excitations which has been missing in Anserson's spin-wave theory. Implications
of these new collective modes are discussed.",0701482v1
1992-03-18,Quantum Galilei Group as Symmetry of Magnons,"Inhomogeneous quantum groups are shown to be an effective algebraic tool in
the study of integrable systems and to provide solutions equivalent to the
Bethe ansatz. The method is illustrated on the 1D Heisenberg ferromagnet whose
symmetry is shown to be the quantum Galilei group Gamma_q(1) here introduced.
Both the single magnon and the s=1/2 bound states of n-magnons are completely
described by the algebra.",9203048v1
2006-07-18,Magnon-Like Dispersion Relation from M-Theory,"We investigate classical rotating membranes in two different backgrounds.
First, we obtain membrane solution in $AdS_4\times S^7$ background, analogous
to the solution obtained by Hofman and Maldacena in the case of string theory.
We find a magnon type dispersion relation similar to that of Hofman and
Maldacena and to the one found by Dorey for the two spin case. In the appendix
of the paper, we consider membrane solutions in $AdS_7\times S^4$, which give
new relations between the conserved charges.",0607116v3
2006-09-29,A T-duality interpretation of the relationship between massive and massless magnonic TBA systems,"We propose an alternative understanding of the relationship between massive
and massless magnonic TBA systems, using the T-duality symmetries of the
Homogeneous sine-Gordon models. This is shown to be in agreement with a
previous treatment by Dorey, Dunning and Tateo, based on the properties of
Y-systems.",0609224v3
2006-10-27,The Asymptotic Spectrum of the N=4 Super Yang-Mills Spin Chain,"In this paper we discuss the asymptotic spectrum of the spin chain
description of planar N=4 SUSY Yang-Mills. The states appearing in the spectrum
belong to irreducible representations of the unbroken supersymmetry SU(2|2) x
SU(2|2) with non-trivial extra central extensions. The elementary magnon
corresponds to the bifundamental representation while boundstates of Q magnons
form a certain short representation of dimension 16Q^{2}. Generalising the
Beisert's analysis of the Q=1 case, we derive the exact dispersion relation for
these states by purely group theoretic means.",0610295v1
2006-12-30,Zero modes for the giant magnon,"We explicitly construct the eight fermion zero mode solutions for the
Hofman-Maldacena giant magnon. The solutions are naturally gauge fixed under
the \kappa-symmetry. Substituting the solutions back into the Lagrangian leads
to a simple expression that can be quantized directly. We also show how to
construct the SU(2|2)\times SU(2|2) superalgebra from these zero modes. For
completeness we also find the four bosonic zero mode solutions.",0701005v3
2004-06-02,Ultra-cold Neutron Production in Anti-ferromagnetic Oxygen Solid,"Spin waves, or magnons, in the anti-ferromagnetic $\alpha$ phase of solid
oxygen provide a novel mechanism for ultra-cold neutron (UCN) production.
Magnons dominate the energy exchange mechanisms for cold neutrons and UCN in
solid $\alpha$-oxygen, much in the same way as do phonons in solid deuterium
superthermal UCN sources. We present calculations of UCN production and
upscattering rates in S-O$_2$. The results indicate that S-O$_2$ is potentially
a much more efficient UCN source material than solid deuterium.",0406004v1
2000-04-11,Theory of acoustic analog of magneto-optic Kerr effect under magnon-phonon resonance,"Theory of the acoustic analog of the polar Kerr effect is developed for an
inclined incidence of the p-type wave on the interface between isotropic
non-magnetic medium and ferromagnetic cubic crystal. Magnetic field dependences
of ellipticity and rotation of the polarization plane of the reflected wave
under magnon-phonon resonance are analysed in a weak-coupling approximation for
interaction of magnetic and elastic subsystems.",0004017v1
2007-06-04,"Spiky Strings, Giant Magnons and beta-deformations","We study rigid string solutions rotating on the S^3 subspace of the
beta-deformed AdS_5xS^5 background found by Lunin and Maldacena. For particular
values of the parameters of the solutions we find the known giant magnon and
single spike strings. We present a single spike string solution on the deformed
S^3 and find how the deformation affects the dispersion relation. The possible
relation of this string solution to spin chains and the connection of the
solutions on the undeformed S^3 to the sine-Gordon model are briefly discussed.",0706.0442v2
2007-07-04,Quantum Scattering of Giant Magnons,"We perform a first-principles semi-classical computation of the one-loop
corrections to the dispersion relation and S-matrix of Giant Magnons in AdS_5 x
S^5 string theory. The results agree exactly with expectations based on the
strong coupling expansion of the exact Asymptotic Bethe Ansatz equations. In
particular we reproduce the Hernandez-Lopez term in the dressing phase.",0707.0668v2
2007-07-31,Giant magnon bound states from strongly coupled N=4 SYM,"We calculate in a very simple way the spectrum of giant magnon bound states
at strong coupling in N=4 SYM, by utilizing the description of the field theory
vacuum in terms of a condensate of eigenvalues of commuting matrices. We
further show that these calculations can be understood in terms of the central
charge extension that permits the calculation of BPS masses in the Coulomb
branch of N=4 SYM. This paper shows further evidence that the strong coupling
expansion of the maximally supersymmetric Yang-Mills theory in four dimensions
can be done systematically from first principles, without the assumption of
integrability.",0707.4669v1
2007-08-16,Wrapping interactions at strong coupling -- the giant magnon,"We derive generalized Luscher formulas for finite size corrections in a
theory with a general dispersion relation. For the AdS_5xS^5 superstring these
formulas encode leading wrapping interaction effects. We apply the generalized
mu-term formula to calculate finite size corrections to the dispersion relation
of the giant magnon at strong coupling. The result exactly agrees with the
classical string computation of Arutyunov, Frolov and Zamaklar. The agreement
involved a Borel resummation of all even loop-orders of the BES/BHL dressing
factor thus providing a strong consistency check for the choice of the dressing
factor.",0708.2208v2
2007-12-13,Bose-Einstein Condensation in Magnetic Insulators,"The elementary excitations in antiferromagnets are magnons, quasiparticles
with integer spin and Bose statistics. In an experiment their density is
controlled efficiently by an applied magnetic field and can be made finite to
cause the formation of a Bose-Einstein condensate (BEC). Studies of magnon
condensation in a growing number of magnetic materials provide a unique window
into an exciting world of quantum phase transitions (QPT) and exotic quantum
states.",0712.2250v1
2007-12-25,Singularities of the Magnon Boundstate S-Matrix,"We study the conjectured exact S-matrix for the scattering of BPS magnon
boundstates in the spin-chain description of planar N=4 SUSY Yang-Mills. The
conjectured S-matrix exhibits both simple and double poles at complex momenta.
Some of these poles lie parametrically close to the real axis in momentum space
on the branch where particle energies are positive. We show that all such poles
are precisely accounted for by physical processes involving one or more
on-shell intermediate particles belonging to the known BPS spectrum.",0712.4068v1
2008-01-08,Magnetic excitations in SrCu2O3: a Raman scattering study,"We investigated temperature dependent Raman spectra of the one-dimensional
spin-ladder compound SrCu2O3. At low temperatures a two-magnon peak is
identified at 3160+/-10 cm^(-1) and its temperature dependence analyzed in
terms of a thermal expansion model. We find that the two-magnon peak position
must include a cyclic ring exchange of J_cycl/J_perp=0.09-0.25 with a coupling
constant along the rungs of J_perp approx. 1215 cm^(-1) (1750 K) in order to be
consistent with other experiments and theoretical results.",0801.1303v1
2008-01-29,A new derivation of Luscher F-term and fluctuations around the giant magnon,"In this paper we give a new derivation of the generalized Luscher F-term
formula from a summation over quadratic fluctuations around a given soliton.
The result is very general providing that S-matrix is diagonal and is valid for
arbitrary dispersion relation. We then apply this formalism to compute the
leading finite size corrections to the giant magnon dispersion relation coming
from quantum fluctuations.",0801.4463v3
2008-05-15,Dynamical Casimir effect for magnons in a spinor Bose-Einstein condensate,"Magnon excitation in a spinor Bose-Einstein condensate by a driven magnetic
field is shown to have a close analogy with the dynamical Casimir effect. A
time-dependent external magnetic field amplifies quantum fluctuations in the
magnetic ground state of the condensate, leading to magnetization of the
system. The magnetization occurs in a direction perpendicular to the magnetic
field breaking the rotation symmetry. This phenomenon is numerically
demonstrated and the excited quantum field is shown to be squeezed.",0805.2210v1
2008-05-27,Scattering of backward spin waves in a one-dimensional magnonic crystal,"Scattering of backward volume magnetostatic spin waves from a one-dimensional
magnonic crystal, realized by a grating of shallow grooves etched into the
surface of an yttrium-iron garnet film, was experimentally studied. Rejection
frequency bands were clearly observed. The rejection efficiency and the
frequency width of the rejection bands increase with increasing groove depth. A
theoretical model based on the analogy of a spin-wave film-waveguide with a
microwave transmission line was used to interpret the obtained experimental
results.",0805.4142v1
2008-06-24,Magnon like solutions for strings in I-brane background,"We study the solutions for fundamental string rotating in a background
generated by a 1+1 dimensional intersection of two orthogonal stacks of
fivebranes in type IIB string theory. We show the existence of magnon like
solutions for the string moving simultaneously in the two spheres in this
background and find the relevant dispersion relation among the various
conserved charges.",0806.3879v2
2008-08-27,Heralded single-magnon quantum memory for photon polarization states,"We demonstrate a heralded quantum memory based on mapping of a photon
polarization state onto a single collective-spin excitation (magnon) shared
between two spatially overlapped atomic ensembles. The polarization fidelity is
measured by quantum state tomography to be above 90(2)% for any input
polarization, which far exceeds the classical limit of 2/3. The process also
constitutes a quantum non-destructive probe that detects and regenerates a
photon without touching its - potentially undetermined - polarization.",0808.3603v1
2008-10-03,Finite-Size Corrections of the $\mathbb{CP}^3$ Giant Magnons: the Lüscher terms,"We compute classical and first quantum finite-size corrections to the
recently found giant magnon solutions in two different subspaces of
$\mathbb{CP}^3$. We use the L\""{u}scher approach on the recently proposed exact
S-matrix for $\mathcal{N}=6$ superconformal Chern-Simons theory. We compare our
results with the string and algebraic curve computations and find agreement,
thus providing a non-trivial test for the new $AdS_4/CFT_3$ correspondence
within an integrability framework.",0810.0704v2
2008-10-13,Finite-size Effect of the Dyonic Giant Magnons in N=6 super Chern-Simons Theory,"We consider finite-size effects for the dyonic giant magnon of the type IIA
string theory on AdS_4 x CP^3 by applying Luscher mu-term formula which is
derived from a recently proposed S-matrix for the N=6 super Chern-Simons
theory. We compute explicitly the effect for the case of a symmetric
configuration where the two external bound states, each of A and B particles,
have the same momentum p and spin J_2. We compare this with the classical
string theory result which we computed by reducing it to the
Neumann-Rosochatius system. The two results match perfectly.",0810.2079v2
2008-10-20,Spiky Strings in AdS(4) X CP**3 with Neveu-Schwarz Flux,"We study general rotating string solution in the AdS(4) X CP**3 background
with a B_NS holonomy turned on over ${\bf CP}^1$ $\subset $ $ {\bf CP}^3$. We
find the giant magnon and single spike solutions for the string moving in this
background corresponding to open spin chain. We calculate the corresponding
dispersion relation among various conserved charges for both the cases. We
further study the finite size effect on both the giant magnon and single spike
solutions.",0810.3516v3
2008-11-03,Giant magnons and spiky strings on the conifold,"We find explicit solutions for giant magnons and spiky strings on the
squashed three dimensional sphere. For a special value of the squashing
parameter the solutions describe strings moving in a sector of the conifold,
while for another value of the squashing parameter we recover the known results
on the round three dimensional sphere. A new feature is that the energy and the
momenta enter in the dispersion relation of the conifold in a transcendental
way.",0811.0145v1
2008-12-15,Zero Modes for the Boundary Giant Magnons,"We study the fermion zero-mode dynamics for open strings ending on the giant
graviton branes. For the open string ending on the Z=0 brane, the quantization
of the fermion zero-modes of boundary giant magnons reproduces the 256 states
of the boundary degrees with the precise realization of the SU(2|2) X SU(2|2)
symmetry algebra. Also for the open string ending on the Y=0 brane, we
reproduce the unique vacuum state from the fermion zero-modes.",0812.2645v2
2009-02-19,Giant Magnons on CP^3 by Dressing Method,"We consider classical string spectrum of R x CP^3, and construct a family of
solutions with residual SU(2) symmetry by the dressing method on SU(4)/U(3)
sigma model. All of them obey the square-root type dispersion relation, as is
expected from the su(2|2) symmetry. A single dyonic giant magnon is not found
in this approach.",0902.3368v3
2009-05-08,Multipartite entangled magnon states as quantum communication channels,"We explicate conditions under which, the two magnon state becomes highly
entangled and is useful for several quantum communication protocols. This
state, which is experimentally realizable in quantum dots using Heisenberg
exchange interaction, is found to be suitable for carrying out deterministic
teleportation of an arbitrary two qubit composite system. Further, conditions
for which the channel capacity reaches ""Holevo bound"", allowing maximal amount
of classical information to be encoded, are derived. Later, an explicit
protocol for the splitting and sharing of a two qubit entangled state among two
parties, using this state as an entangled resource, is demonstrated.",0905.1233v2
2009-05-17,Physics of $π$-Meson Condensation and High Temperature Cuprate Superconductors,"The idea of condensation of the Goldstone $\pi$-meson field in nuclear matter
had been put forward a long time ago. However, it was established that the
normal nuclear density is too low, it is not sufficient to condensate
$\pi$-mesons. This is why the $\pi$-condensation has never been observed.
Recent experimental and theoretical studies of high temperature cuprate
superconductors have revealed condensation of Goldstone magnons, the effect
fully analogous to the $\pi$-condensation. The magnon condensation has been
observed. It is clear now that quantum fluctuations play a crucial role in the
condensation, in particular they drive a quantum phase transition that destroys
the condensate at some density of fermions.",0905.2717v1
2009-08-16,A note on strings in deformed AdS_4 x CP3: giant magnon and single spike solutions,"In this paper we study the solitonic string solutions of magnon and single
spike type in the beta-deformed AdS_4 x CP3 background. We find the dispersion
relations which are supposed to give the anomalous dimension of the gauge
theory operators.",0908.2246v3
2009-08-21,"Strings on the deformed T^{1,1}: giant magnon and single spike solutions","In this paper we find giant magnon and single spike string solutions in a
sector of the gamma-deformed conifold. We examine the dispersion relations and
find a behavior analogous to the undeformed case. The transcendental functional
relations between the conserved charges are shifted by certain gamma-dependent
term. The latter is proportional to the total momentum and thus qualitatively
different from known cases.",0908.3065v1
2009-10-28,Interacting spin waves in the ferromagnetic Kondo lattice model,"We present an new approach for the ferromagnetic, three-dimensional,
translational-symmetric Kondo lattice model which allows us to derive both
magnon energies and linewidths (lifetimes) and to study the properties of the
ferromagnetic phase at finite temperatures. Both ""anomalous softening"" and
""anomalous damping"" are obtained and discussed. Our method consists of mapping
the Kondo lattice model onto an effective Heisenberg model by means of the
""modified RKKY interaction"" and the ""interpolating self-energy approach"". The
Heisenberg model is approximatively solved by applying the Dyson-Maleev
transformation and using the ""spectral density approach"" with a broadened
magnon spectral density.",0910.5478v2
2009-11-23,Production of Ultra-Cold-Neutrons in Solid α-Oxygen,"Our recent neutron scattering measurements of phonons and magnons in solid
\alpha-oxygen have led us to a new understanding of the production mechanismen
of ultra-cold-neutrons (UCN) in this super-thermal converter. The UCN
production in solid \alpha-oxygen is dominated by the excitation of phonons.
The contribution of magnons to UCN production becomes only slightly important
above E >10 meV and at E >4 meV. Solid \alpha-oxygen is in comparison to solid
deuterium less effcient in the down-scattering of thermal or cold neutrons into
the UCN energy regime.",0911.4398v2
2010-03-17,Three-magnon bound states in exactly rung-dimerized spin ladders,"Three magnon bound states in all total spin sectors of general nonintegrable
exactly rung-dimerized spin ladder are obtained by Bethe Ansatz. Basing on this
result a dispersion law for $m$-magnon ($m>3$) bound states is conjectured. It
is suggested that (depending on correlations between coupling constants) a
behavior of the gas parameter (density of rung-triplets) on the boundary of the
rung-dimerized phase may either be smooth or have a leap.",1003.3472v2
2010-06-21,Magnonic spin-wave modes in CoFeB antidot lattices,"In this manuscript time-resolved magneto-optical Kerr effect experiments on
structured CoFeB films are presented. The geometries considered are two
dimensional square lattices of micrometer-sized antidots, fabricated by a
focused ion beam. The spin-wave spectra of these magnonic crystals show a novel
precessional mode, which can be related to a Bloch state at the zone boundary.
Additionally, another magnetic mode of different nature appears, whose
frequency displays no dependence on the externally applied magnetic field.
These findings are interpreted as delocalized and localized modes,
respectively.",1006.4038v1
2010-08-06,Ferromagnetic phases in spin-Fermion systems,"Spin-Fermion systems which obtain their magnetic properties from a system of
localized magnetic moments being coupled to conducting electrons are
considered. The dynamical degrees of freedom are spin-$s$ operators of
localized spins and spin-1/2 Fermi operators of itinerant electrons.
Renormalized spin-wave theory, which accounts for the magnon-magnon
interaction, and its extension are developed to describe the two ferrimagnetic
phases in the system: low temperature phase $0<T<T^{*}$, where all electrons
contribute the ordered ferromagnetic moment, and high temperature phase
$T^{*}<T<T_C$, where only localized spins form magnetic moment. The
magnetization as a function of temperature is calculated. The theoretical
predictions are utilize to interpret the experimentally measured
magnetization-temperature curves of $UGe_2$..",1008.1148v1
2010-10-12,"Phonon, Two-Magnon and Electronic Raman Scattering of Fe1+yTe1-xSex","We have measured Raman scattering spectra of single-crystalline FeTe0.6Se0.4
(T_c ~ 14.5 K) and its parent compound Fe1.074Te at various temperatures. In
the parent compound Fe1.074Te, A1g and B1g modes have been observed at 157.5
and 202.3 cm-1, respectively, at 5 K. These frequencies qualitatively agree
with the calculated results. Two-magnon excitation has been observed around
2300 cm-1 for both compounds. Temperature dependence between the electronic
Raman spectra below and above T_c has been observed and 2\Delta and
2\Delta/k_BT_C have been estimated as 5.0 meV and 4.0, respectively.",1010.2374v2
2010-10-26,Close to the Giant Magnons,"We consider the most general string configurations on the R_t x S^3 subspace
of AdS_5 xS^5, described by the Neumann-Rosochatius integrable system. Under
some restrictions on the parameters of the solution and in an appropriate
limit, they correspond to small deviation from the known finite-size giant
magnon solutions with one and two angular momenta. Analyzing the finite-size
effect on the dispersion relation, we find that the leading correction is
modified in a way similar to the gamma-deformed case R_t x S^3_gamma. The
subleading correction for a string with one angular momentum is also found. It
depends on the same parameter, which describes the generalization of the
leading correction.",1010.5465v1
2010-10-29,Magnetic and Electronic Raman Scattering at the Nodal Spin-Density-Wave Transition in BaFe2As2,"Two magnon excitations and the nodal spin density wave (SDW) gap were
observed in BaFe2As2 by Raman scattering. Below the SDW transition temperature
(TSDW) nodal SDW gap opens together with new excitations in reconstructed
electronic states. The two-magnon peak remains above TSDW and moreover the
energy increases a little. The change from the long-range ordered state to the
short-range correlated state is compared to the cuprate superconductors.",1010.6151v1
2010-12-15,Correlation functions of magnon and spike,"After considering a solitonic string moving in the Poincare $AdS$ and
two-dimensional sphere, we calculate the two-point correlation function of
magnon and spike in the semi-classical limit without any explicit solution. We
also calculate the three-point correlation functions between two heavy and one
light operators. We find that the coupling between two heavy and one light
operators in the string side is the exactly same as one obtained from the gauge
theory by using the RG analysis.",1012.3293v2
2011-01-14,Antiferromagnets at low Temperatures,"The low-temperature properties of the Heisenberg antiferromagnet in 2+1
space-time dimensions are analyzed within the framework of effective
Lagrangians. It is shown that the magnon-magnon interaction is very weak and
repulsive, manifesting itself through a term proportional to five powers of the
temperature in the pressure. The structure of the low-temperature series for
antiferromagnets in 2+1 dimensions is compared with the structure of the
analogous series for antiferromagnets in 3+1 dimensions. The model-independent
and systematic effective field theory approach clearly proves to be superior to
conventional condensed matter methods such as spin-wave theory.",1101.2919v1
2011-04-06,Three-point correlators for giant magnons,"Three-point correlation functions in the strong-coupling regime of the
AdS/CFT correspondence can be analyzed within a semiclassical approximation
when two of the vertex operators correspond to heavy string states having large
quantum numbers while the third vertex corresponds to a light state with fixed
charges. We consider the case where the heavy string states are chosen to be
giant magnon solitons with either a single or two different angular momenta,
for various different choices of light string states.",1104.1160v2
2011-04-11,Correlation function of dyonic strings,"We investigate the two- and three-point correlation functions of the dyonic
magnon and spike, which correspond to the solitonic string moving in the
Poincare AdS and three-dimensional sphere. We show that the coupling between
two dyonic magnons or spikes together with a marginal scalar operator in the
string theory is exactly the same as one obtained by the RG analysis in the
gauge theory.",1104.1896v1
2011-05-24,Long-lived magnons throughout the Brillouin zone of the strong-leg spin ladder (C$_7$H$_{10}$N)$_2$CuBr$_4$,"Inelastic neutron scattering is used to measure spin excitations in fully
deuterated single crystal samples of the strong-leg antiferromagnetic S=1/2
spin ladder compound (C$_7$H$_{10}$N)$_2$CuBr$_4$. Sharp resolution-limited
magnons are observed across the entire one-dimensional Brillouin zone. The
results validate the previously proposed {\it symmetric} spin ladder model and
provide a reliable estimate of the relevant exchange interactions.",1105.4832v1
2011-06-16,Thermomagnonic spin transfer and Peltier effects in insulating magnets,"We study the coupled magnon energy transport and collective magnetization
dynamics in ferromagnets with magnetic textures. By constructing a
phenomenological theory based on irreversible thermodynamics, we describe
motion of domain walls by thermal gradients and generation of heat flows by
magnetization dynamics. From microscopic description based on magnon kinetics,
we estimate the transport coefficients and analyze the feasibility of
energy-related applications in insulating ferromagnets, such as yttrium iron
garnet and europium oxide.",1106.3135v2
2011-06-28,Three-point Correlation Function of Giant Magnons in the Lunin-Maldacena background,"We compute semiclassical three-point correlation function, or structure
constant, of two finite-size (dyonic) giant magnon string states and a light
dilaton mode in the Lunin-Maldacena background, which is the \gamma-deformed,
or TsT-transformed AdS_5 x S_{\gamma}^5, dual to N = 1 super Yang-Mills theory.
We also prove that an important relation between the structure constant and the
conformal dimension, checked for the N = 4 super Yang-Mills case, still holds
for the \gamma-deformed string background.",1106.5656v1
2011-07-05,Storage-recovery phenomenon in magnonic crystal,"The phenomenon of wave trapping in an artificial crystal with limited number
of periods is demonstrated experimentally using spin waves in a magnonic
crystal. The information stored in the crystal is recovered afterwards by
parametric amplification of the trapped wave. The storage process is based on
the excitation of standing internal crystal modes and differs principally from
the well-known phenomenon of deceleration of light in photonic crystals.",1107.0984v1
2011-07-29,Temperature dependence of spin currents in one- and three-dimensional insulators,"The temperature dependence of spin currents in insulators at the finite
temperature near zero Kelvin is theoretically studied. The spin currents are
carried by Jordan-Wigner fermions and magnons in one- and three- dimensional
insulators. The quasiparticle description of one-dimensional spin systems is
valid only in the finite temperature near zero Kelvin. These spin currents are
generated by the external magnetic field gradient along the quantization axis
and also by the two-particle interaction gradient. In one-dimensional
insulators, quantum fluctuations are strong and the spin current carried by
Jordan-Wigner fermions shows the stronger dependence on temperatures than the
one by magnons.",1107.5867v1
2011-09-22,Intrinsic coupling of orbital excitations to spin fluctuations in Mott insulators,"We show how the general and basic asymmetry between two fundamental degrees
of freedom present in strongly correlated oxides, spin and orbital, has very
profound repercussions on the elementary spin and orbital excitations. Whereas
the magnons remain largely unaffected, orbitons become inherently coupled with
spin fluctuations in spin-orbital models with antiferromagnetic and
ferroorbital ordered ground states. The composite orbiton-magnon modes that
emerge fractionalize again in one dimension, giving rise to spin-orbital
separation in the peculiar regime where spinons are faster than orbitons.",1109.4745v1
2011-10-18,Life times and chirality of spin-waves in antiferromagnetic and ferromagnetic FeRh: time depedent density functional theory perspective,"The study of the spin excitations in antiferromagnetic (AFM) and
ferromagnetic (FM) phases of FeRh is reported. We demonstrate that although the
Fe atomic moments are well defined there is a number of important phenomena
absent in the Heisenberg description: Landau damping of spin waves, large Rh
moments induced by the AFM magnons, the formation of the optical magnons
terminated by Stoner excitations. We relate the properties of the spin-wave
damping to the features of the Stoner continuum and compare the chirality of
the spin excitations in AFM, FM and paramagnetic (PM) systems.",1110.3913v1
2012-05-31,Bose-Einstein Condensation of Magnons in NiCl2-4SC(NH2)2,"A Bose-Einstein condensation (BEC) has been observed in magnetic insulators
in the last decade. The bosons that condensed are magnons, associated with an
ordered magnetic phase induced by a magnetic field. We review the experiments
in the spin-gap compound NiCl2-4SC(NH2)2, in which the formation of BEC occurs
by applying a magnetic field at low temperatures. This is a contribution to the
celebration for the 50th anniversary of the Solid State and Low Temperature
Laboratory of the University of S\~ao Paulo, where this compound was first
magnetically characterized.",1206.0035v1
2012-11-22,Observation of Phonon-Assisted Magnon Absorption in Spin-Orbit Coupling Induced Mott Insulator Sr$_{2}$IrO$_{4}$,"The infrared transmission measurement was performed for a two-dimensional
spin-orbit coupling induced Mott insulator Sr$_{2}$IrO$_{4}$ with a
layered-perovskite structure. A phonon-assisted magnon mode was observed at
around 1870 cm$^{-1}$ in the absorption spectrum below 180 K. The
nearest-neighbor superexchange interaction $J$ was estimated to be 57 meV,
which is about a half of the $J$ value of a copper oxide La$_{2}$CuO$_{4}$. Our
results indicate the possibility that nontrivial properties such as
superconductivity are realized at rather high temperature in iridium oxides.",1211.5241v2
2013-01-24,Phonon-Magnon coupling in CoF$_2$ investigated by time-of-flight neutron spectroscopy,"We report the results of inelastic neutron scattering investigation on the
model antiferromagnet CoF$_2$ by time-of-flight neutron spectroscopy. We
measured the details of the scattering function $S(Q,\omega)$ as a function of
temperature with two different incident neutron wavelengths. The temperature
and Q dependence of the measured scattering function suggests the presence of
magnon-phonon coupling in almost all branches. The present results are in
agreement with the strong magnetoelastic effects observed previously.",1301.5788v1
2013-02-04,Gravity waves on the surface of topological superfluid 3He-B,"We have observed waves on the free surface of 3He-B sample at temperatures
below 0.2mK. The waves are excited by vibrations of the cryostat and detected
by coupling the surface to the Bose-Einstein condensate of magnon
quasiparticles in the superfluid. The two lowest gravity-wave modes in our
cylindrical container are identified. Damping of the waves increases with
temperature linearly with the density of thermal quasiparticles, as expected.
Additionally finite damping of the waves in the zero-temperature limit and
enhancement of magnetic relaxation of magnon condensates by the surface waves
are observed. We discuss whether the latter effects may be related to Majorana
fermions bound to the surface of the topological superfluid.",1302.0764v1
2013-12-24,S matrix from matrix product states,"We use the matrix product state formalism to construct stationary scattering
states of elementary excitations in generic one-dimensional quantum lattice
systems. Our method is applied to the spin-1 Heisenberg antiferromagnet, for
which we calculate the full magnon-magnon S matrix for arbitrary momenta and
spin, the two-particle contribution to the spectral function and the
magnetization curve. As our method provides an accurate microscopic
representation of the interaction between elementary excitations, we envisage
the description of low-energy dynamics of one-dimensional spin chains in terms
of these particlelike excitations.",1312.6793v2
2014-08-14,Design of a spin-wave majority gate employing mode selection,"The design of a microstructured, fully functional spin-wave majority gate is
presented and studied using micromagnetic simulations. This all-magnon logic
gate consists of three-input waveguides, a spin-wave combiner and an output
waveguide. In order to ensure the functionality of the device, the output
waveguide is designed to perform spin-wave mode selection. We demonstrate that
the gate evaluates the majority of the input signals coded into the spin-wave
phase. Moreover, the all-magnon data processing device is used to perform logic
AND-, OR-, NAND- and NOR- operations.",1408.3235v1
2014-09-24,Dissipationless Multiferroic Magnonics,"We propose that the magnetoelectric effect in multiferroic insulators with
coplanar antiferromagnetic spiral order, such as BiFeO$_{3}$, enables
electrically controlled dissipationless magnonics. Applying an oscillating
electric field in these materials with frequency as low as household frequency
can activate Goldstone modes that manifests fast planar rotations of spins,
whose motion is not obstructed by crystalline anisotropy. Combining with spin
ejection mechanisms, such a fast planar rotation can deliver electricity at
room temperature over a distance of the magnetic domain, which is free from the
energy loss due to Gilbert damping.",1409.6900v2
2014-11-10,Magnon spectrum in ferromagnets with a skyrmion,"The analysis of the spin wave excitations in two-dimensional isotropic
Heisenberg ferromagnet is performed with a single skyrmion in the ground state.
We employ the ideas of semiclassical quantization method, duly modified for the
use of the lattice model and Maleyev-Dyson boson representation of spin
operators. The resulting Schr\""odinger equation for magnons describes the
dispersion and wave functions of spin-wave excitations with strictly
non-negative spectrum. In contrast to usual ferromagnet, we demonstrate the
existence of three zero modes, corresponding to conformal symmetries
spontaneously violated by the skyrmion configuration.",1411.2452v2
2014-12-20,A HHL 3-point correlation function in the η-deformed AdS_5 x S^5,"We derive the 3-point correlation function between two giant magnons heavy
string states and the light dilaton operator with zero momentum in the
\eta-deformed AdS_5 x S^5 valid for any J_1 and \eta in the semiclassical
limit. We show that this result satisfies a consistency relation between the
3-point correlation function and the conformal dimension of the giant magnon.
We also provide a leading finite J_1 correction explicitly.",1412.6668v1
2015-01-09,Creation and Amplification of Electro-Magnon Solitons by Electric Field in Nanostructured Multiferroics,"We develop a theoretical description of electro-magnon solitons in a coupled
ferroelectric-ferromagnetic heterostructure. The solitons are considered in the
weakly nonlinear limit as a modulation of plane waves corresponding to two,
electric- and magnetic-like branches in the spectrum. Emphasis is put on
magnetic-like envelope solitons that can be created by an alternating electric
field. It is shown also that the magnetic pulses can be amplified by an
electric field with a frequency close to the band edge of the magnetic branch.",1501.02064v1
2015-01-30,The open Heisenberg chain under boundary fields: a magnonic logic gate,"We study the spin transport in the quantum Heisenberg spin chain subject to
boundary magnetic fields and driven out of equilibrium by Lindblad dissipators.
An exact solution is given in terms of matrix product states, which allows us
to calculate exactly the spin current for any chain size. It is found that the
system undergoes a discontinuous spin-valve-like quantum phase transition from
ballistic to sub-diffusive spin current, depending on the value of the boundary
fields. Thus, the chain behaves as an extremely sensitive magnonic logic gate
operating with the boundary fields as the base element.",1501.07732v2
2015-06-18,Dispersive read-out of ferromagnetic resonance for strongly coupled magnons and microwave photons,"We demonstrate the dispersive measurement of ferromagnetic resonance in a
yttrium iron garnet sphere embedded within a microwave cavity. The reduction in
the longitudinal magnetization at resonance is measured as a frequency shift in
the cavity mode coupled to the sphere. This measurement is a result of the
intrinsic non-linearity in magnetization dynamics, indicating a promising route
towards experiments in magnon cavity quantum electro-dynamics.",1506.05631v1
2015-06-30,Spin-wave localization in disordered magnets,"The effect of disorder on magnonic transport in low-dimensional magnetic
materials is studied in the framework of a classical spin model. Numerical
investigations give insight into scattering properties of the systems and show
the existence of Anderson localization in 1D and weak localization in 2D,
potentially affecting the functionality of magnonic devices.",1506.09045v2
2015-09-09,Spin wave vortex from the scattering on Bloch point solitons,"The interaction of a spin wave with a stationary Bloch point is studied. The
topological non-trivial structure of the Bloch point manifests in the
propagation of spin waves endowing them with a gauge potential that resembles
the one associated with the interaction of a magnetic monopole and an electron.
By pursuing this analogy, we are led to the conclusion that the scattering of
spin waves and Bloch points is accompanied by the creation of a magnon vortex.
Interference between such a vortex and a plane wave leads to dislocations in
the interference pattern that can be measurable by means of magnon holography.",1509.02891v1
2015-10-08,Nematic ordering in pyrochlore antiferromagnets: high-field phase of chromium spinel oxides,"Motivated by recent observation of a new high field phase near saturation in
chromium spinels $A$Cr$_2$O$_4$ ($A=$ Zn, Cd, Hg), we study the $S = 3/2$
pyrochlore Heisenberg antiferromagnet with biquadratic interactions. Magnon
instability analysis at the saturation field reveals that a very small
biquadratic interaction can induce magnon pairing in pyrochlore
antiferromagnets, which leads to the emergence of a ferro-quadrupolar phase, or
equivalently a spin nematic phase, below the saturation field. We present the
magnetic phase diagram in an applied field, studying both $S=3/2$ and $S=1$
spin systems. The relevance of our result to chromium spinels is discussed.",1510.02373v1
2016-04-25,Bias-free spin-wave phase shifter for magnonic logic,"A design of a magnonic phase shifter operating without an external bias
magnetic field is proposed. The phase shifter uses a localized collective spin
wave mode propagating along a domain wall ""waveguide"" in a dipolarly-coupled
magnetic dot array existing in a chessboard antiferromagnetic (CAFM) ground
state. It is demonstrated numerically that remagnetization of a single magnetic
dot adjacent to the domain wall waveguide introduces a controllable phase shift
in the propagating spin wave mode without significant change of the mode
amplitude. It is also demonstrated that a logic XOR gate can be realized in the
same system.",1604.07337v1
2016-07-16,"Comment on ""Supercurrent in a room temperature Bose-Einstein magnon condensate""","The comment explains that the preprint arXiv:1503.0042 has not presented
persuasive theoretical or experimental arguments of existence of spin
supercurrents in a magnon condensate prepared in a room temperature
yttrium-iron-garnet magnetic film because the authors did not check known
criteria for existence of spin supercurrents in magnetically ordered materials.
Also they did not compare their supercurrent interpretation with a competing
and more realistic scenario of transport by spin diffusion.",1607.04720v2
2018-04-09,Microwave non-reciprocity of magnon excitations in a non-centrosymmetric antiferromagnet Ba$_2$MnGe$_2$O$_7$,"We have investigated the microwave non-reciprocity for a non-centrosymmetric
antiferromagnet Ba$_2$MnGe$_2$O$_7$. The magnon modes expected by the
conventional spin wave theory for staggered antiferromagnets are certainly
observed. The magnitudes of exchange interaction and magnetic anisotropy are
obtained by the comparison with the theory. The microwave non-reciprocity is
identified for one of these mode. The relative magnitude of microwave
non-reciprocity can be explained with use of spin wave theory and Kubo formula.",1804.03250v1
2013-08-16,Influence of ferromagnetic ordering on Raman scattering in CoS$_2$,"The effects of phonon anharmonicity, phonon-magnon and electron-phonon
interactions on the temperature dependence of Raman optical phonon modes are
theoretically investigated. Besides of the Klemens result for the phonon width
due to anharmonicity, the corresponding lineshift is derived. We argue that the
phonon decay into two magnons has very low intensity in ferromagnets with low
Curie temperatures. Therefore, the electron interband transitions accompanied
with the ferromagnetic ordering are included in considerations to get a good
quantitative agreement with experiments.",1308.3571v1
2017-01-16,Hierarchy of the low-lying excitations for the $(2+1)$-dimensional $q=3$ Potts model in the ordered phase,"The $(2+1)$-dimensional $q=3$ Potts model was simulated with the exact
diagonalization method. In the ordered phase, the elementary excitations
(magnons) are attractive, forming a series of bound states in the low-energy
spectrum. We investigate the low-lying spectrum through a dynamical
susceptibility, which is readily tractable with the exact diagonalization
method via the continued-fraction expansion. As a result, we estimate the
series of (scaled) mass gaps, $m_{2,3,4}/m_1$ ($m_1$: single-magnon mass), in
proximity to the transition point.",1701.04232v1
2008-07-16,Spiky Strings on AdS(4) X CP**3,"We study a giant magnon and a spike solution for the string rotating on
AdS(4) X CP**3 geometry. We consider rigid rotating fundamental string in the
SU(2) X SU(2) sector inside the CP**3 and find out the general form of all the
conserved charges. We find out the dispersion relation corresponding to both
the known giant magnon and the new spike solutions. We further study the finite
size correction in both cases.",0807.2559v3
2008-07-17,"Giant magnons in AdS_4/CFT_3: dispersion, quantization and finite--size corrections","We study giant magnon solutions in AdS4 \times CP3. We compute quantum
corrections to their dispersion relation. We find out that the one--loop
correction vanishes in infinite volume. This implies that the interpolating
function h(\lambda) between strong and weak coupling regimes does not have a
constant term \lambda^0 at strong coupling. We also compute first nonvanishing
finite volume correction to the one--loop expression. When compared to the
L\""{u}sher formula, our results could provide a nontrivial check of the AdS4
\times CP3 S--matrix proposed recently in arXiv:0807.1924.",0807.2861v3
2008-07-21,Neumann-Rosochatius integrable system for strings on AdS_4 x CP^3,"We use the reduction of the string dynamics on AdS_4 x CP^3 to the
Neumann-Rosochatius integrable system. All constraints can be expressed simply
in terms of a few parameters. We analyze the giant magnon and single spike
solutions on R_t x CP^3 with two angular momenta in detail and find the
energy-charge relations. The finite-size effects of the giant magnon and single
spike solutions are analyzed.",0807.3134v2
2011-11-10,Magnonic band structure of a two-dimensional magnetic superlattice,"The frequencies and linewidths of spin waves in a two-dimensional periodic
superlattice of magnetic materials are found, using the Landau-Lifshitz-Gilbert
equations. The form of the exchange field from a surface-torque-free boundary
between magnetic materials is derived, and magnetic-material combinations are
identified which produce gaps in the magnonic spectrum across the entire
superlattice Brillouin zone for hexagonal and square-symmetry superlattices.",1111.2506v1
2012-01-12,Finite-size effects of beta-deformed AdS_5/CFT_4 at strong coupling,"We compute both classical and quantum finite-size corrections at leading
order in the strong coupling limit for the (dyonic) giant magnon in the
Lunin-Maldacena background. Based on the exact S-martix conjectured for the
deformed theory, we generalize the Luscher formula to include twisted boundary
conditions and show that the results match with those derived both by
finite-size classical solutions of the giant magnon and by algebraic curve
analysis.",1201.2635v1
2014-05-14,Generation of magnonic spin wave traps,"Spatially resolved measurements of the magnetization dynamics induced by an
intense laser pump-pulse reveal that the frequencies of resulting spin wave
modes depend strongly on the distance to the pump center. This can be
attributed to a laser generated temperature profile. On a CoFeB thin film
magnonic crystal, Damon-Eshbach modes are expected to propagate away from the
point of excitation. The experiments show that this propagation is frustrated
by the strong temperature gradient",1405.3470v1
2014-05-27,Strongly coupled magnons and cavity microwave photons,"We realize a cavity magnon-microwave photon system in which magnetic dipole
interaction mediates strong coupling between collective motion of large number
of spins in a ferrimagnet and the microwave field in a three-dimensional
cavity. By scaling down the cavity size and increasing number of spins, an
ultrastrong coupling regime is achieved with a cooperativity reaching 12600.
Interesting dynamic features including classical Rabi oscillation, magnetically
induced transparency, and Purcell effect are demonstrated in this highly
versatile platform, highlighting its great potential for coherent information
processing.",1405.7062v1
2014-06-10,Spontaneous chiral symmetry breaking on a fully polarized frustrated magnet at finite temperature,"Frustration can introduce more-than-two minima in a spin dispersion relation
even in a fully polarized magnet under high magnetic field. We generally
discuss, on the fully polarized phase, the possibility of the chiral symmetry
breaking where the number of magnons pumped by finite temperature deviates to
one side of minima. We study this phase by constructing the Ginzburg-Landau
energy which is controlled by the external magnetic field and interactions
between magnons near the dispersion minima. This chirality breaking phase
accompanies, not a magnetization perpendicular to the external field, but the
vector chirality $S_m \times S_n$. We also discuss the possibility of the
chirality breaking phase on LiCuVO4 slightly above the saturation field.",1406.2617v1
2017-10-24,Indirect coupling of magnons by cavity photons,"The interaction between two magnetic spheres in microwave cavities is studied
by Mie scattering theory beyond the magnetostatic and rotating wave
approximations. We demonstrate that two spatially separated dielectric and
magnetic spheres can be strongly coupled over a long distance by standing
cavity modes. The interactions splits acoustical (dark) and optical (bright)
modes in a way that can be mapped on a molecular orbital theory of the hydrogen
molecule. Breaking the symmetry by assigning different radii to the two spheres
introduces ""ionic"" character to the magnonic bonds. These results illustrate
the coherent and controlled energy exchange between objects in microwave
cavities.",1710.08907v1
2017-12-08,Giant magnons and spiky strings in the Schrodinger/dipole-deformed CFT correspondence,"We construct semi-classical string solutions of the Schr\""odinger $Sch_5
\times S^5$ spacetime, which is conjectured to be the gravity dual of a
non-local dipole-deformed CFT. They are the counterparts of the giant magnon
and spiky string solutions of the undeformed $AdS_5 \times S^5$ to which they
flow when the deformation parameter is turned off. They live in an $S^3$
subspace of the five-sphere along the directions of which the $B$-field has
non-zero components having also extent in the $Sch_5$ part of the metric.
Finally, we speculate on the form of the dual field theory operators.",1712.03091v2
2018-09-03,Loschmidt-amplitude wave function spectroscopy and the physics of dynamically driven phase transitions,"We introduce the Loschmidt amplitude as a powerful tool to perform
spectroscopy of generic many-body wave functions and use it to interrogate the
wave function obtained after ramping the transverse field quantum Ising model
through its quantum critical point. Previous results are confirmed and a more
complete understanding of the population of defects and of the effects of
magnon-magnon interaction or finite-size corrections is obtained. The influence
of quantum coherence is clarified.",1809.00733v2
2018-09-23,The antiferromagnetic phase of the Floquet-driven Hubbard model,"A saddle point plus fluctuations analysis of the periodically driven
half-filled two-dimensional Hubbard model is performed. For drive frequencies
below the equilibrium gap, we find discontinuous transitions to time-dependent
solutions. A highly excited, generically non-thermal distribution of magnons
occurs even for drive frequencies far above the gap. Above a critical drive
amplitude, the low-energy magnon distribution diverges as the frequency tends
to zero and antiferromagnetism is destroyed, revealing the generic importance
of collective mode excitations arising from a non-equilibrium drive.",1809.08607v2
2019-09-12,Quantum computation of magnon spectra,"We demonstrate quantum computation of two-point correlation functions for a
Heisenberg spin chain. Using the IBM Q 20 quantum machines, we find that for
two sites the correlation functions produce the exact results reliably. For
four sites, results from the IBM Q 20 Tokyo quantum computer are noisy due to
read out errors and decoherence. Nevertheless, the correlation functions retain
the correct spectral information. This is illustrated in the frequency domain
by accurately extracting the magnon energies from peaks in the spectral
function.",1909.05701v2
2017-05-03,Controlling Chiral Domain Walls in Antiferromagnets Using Spin-Wave Helicity,"In antiferromagnets, the Dzyaloshinskii-Moriya interaction lifts the
degeneracy of left- and right-circularly polarized spin waves. This
relativistic coupling increases the efficiency of spin-wave-induced domain wall
motion and leads to higher drift velocities. We show that in biaxial
antiferromagnets, the spin-wave helicity controls both the direction and
magnitude of the magnonic force on chiral domain walls. By contrast, in
uniaxial antiferromagnets, the magnonic force is propulsive with a helicity
dependent strength.",1705.01572v2
2017-05-11,Bethe Ansatz for two-magnon scattering states in 2D and 3D Heisenberg-Ising ferromagnets,"Various versions of the Bethe ansatz are suggested for evaluation of
scattering two-magnon states in 2D and 3D Heisenberg-Ising ferromagnets. It is
shown that for 2D square (3D qubic) finite-periodic or infinite lattices about
a half (3/4) of states have a correctly 2D- (3D-) generalized Bethe form. The
remaining scattering states are treated (on the infinite lattices only) within
the degenerative discrete-diffractive modification of the Bethe ansatz
previously suggested by the author.",1705.04117v2
2017-05-25,Prediction of ultra-narrow Higgs resonance in magnon Bose-condensates,"Higgs resonance modes in condensed matter systems are generally broad;
meaning large decay widths or short relaxation times. This common feature has
obscured and limited their observation to a select few systems. Contrary to
this, the present work predicts that Higgs resonances in magnetic field
induced, three-dimensional magnon Bose-condensates have vanishingly small decay
widths. Specifically for parameters relating to TlCuCl$_3$, we find an energy
($\Delta_H$) to width ($\Gamma_H$) ratio $\Delta_H/\Gamma_H\sim500$, making
this the narrowest predicted Higgs mode in a condensed matter system, some two
orders of magnitude `narrower' than the sharpest condensed matter Higgs
observed so far.",1705.09007v1
2019-01-02,Fermion zero modes for the mixed-flux AdS$_3$ giant magnon,"We explicitly construct the four and two fermion zero modes for the
mixed-flux generalization of the Hofman-Maldacena giant magnon on two of the
AdS$_3$ backgrounds with maximal amount of supersymmetry, AdS$_3 \times$S$^3
\times$T$^4$ and AdS$_3 \times$S$^3 \times$S$^3 \times$S$^1$. We also show how
to get the $\mathfrak{psu}(1|1)^4$ and $\mathfrak{su}(1|1)^2$ superalgebras
from the semiclassically quantized fermion zero modes.",1901.00530v2
2019-02-07,Intrinsic Spin Decay Length in Antiferromagnetic Insulator,"We report intrinsic spin decay length of an antiferromagnetic insulator. We
found that at an antiferromagnetic/ferromagnetic interface, a spin current
generated by spin pumping is strongly suppressed by two-magnon scattering. By
eliminating the two-magnon contribution, we discovered that the characteristic
length of spin decay in NiO changes by two-orders of magnitude through the
paramagnetic to antiferromagnetic transition. The spin decay length in the
antiferromagnetic state is longer than 100 nm, which is an order of magnitude
longer than previously believed. These results provide a crucial piece of
information for the fundamental understanding of the physics of spin transport.",1902.02462v1
2019-02-08,Magnetic excitations and transport properties in frustrated ferromagnetic chain,"To clarify the transport property mediated by bound magnons in a spin nematic
state, we investigate the spin Drude weight of a spin-1/2 $J_{1}$-$J_{2}$
Heisenberg chain with ferromagnetic $J_{1}$ and antiferromagnetic $J_{2}$ in a
magnetic field by numerical diagonalization. At zero magnetic field, numerical
results with up to 24 sites suggest that the spin Drude weight would be zero in
the thermodynamic limit, indicating diffusive spin transport. With increasing
the magnetic field, the spin Drude weight is enhanced at low temperatures,
indicating that bound magnons would contribute to the spin transport.",1902.02932v1
2019-03-28,Spin Seebeck and Spin Nernst Effects of Magnons in Noncollinear Antiferromagnetic Insulators,"Our joint theoretical and computer experimental study of heat-to-spin
conversion reveals that noncollinear antiferromagnetic insulators are promising
materials for generating magnon spin currents upon application of a temperature
gradient: they exhibit spin Seebeck and spin Nernst effects. Using Kubo theory
and spin dynamics simulations, we explicitly evaluate these effects in a single
kagome sheet of potassium iron jarosite, KFe$_3$(OH)$_6$(SO$_4$)$_2$, and
predict a spin Seebeck conversion factor of $0.2 \mu\mathrm{V}/\mathrm{K}$ at a
temperature of $20 \mathrm{K}$.",1903.11896v1
2019-04-03,Frustrated quantum Heisenberg double-tetrahedral and octahedral chains at high magnetic fields,"We consider the spin-1/2 antiferromagnetic Heisenberg model on two
one-dimensional frustrated lattices, double-tetrahedral chain and octahedral
chain, with almost dispersionless (flat) lowest magnon band in a strong
magnetic field. Using the localized magnons picture, we construct an effective
description of the one-dimensional chains with triangle and square traps within
the strong coupling approximation. We perform extensive exact diagonalization
and density matrix renormalization group calculations to check the validity of
the obtained effective Hamiltonians by comparison with the initial models with
special focus on the magnetization and specific heat at high magnetic fields.",1904.02240v2
2019-04-24,Layered optomagnonic structures: Time Floquet scattering-matrix approach,"A fully dynamic theoretical approach to layered optomagnonic structures,
based on a time Floquet scattering-matrix method, is developed. Its
applicability is demonstrated on a simple design of a dual photonic-magnonic
cavity, formed by sandwiching a magnetic garnet thin film between two
dielectric Bragg mirrors, subject to continuous excitation of a perpendicular
standing spin wave. Some remarkable phenomena, including nonlinear
photon-magnon interaction effects and enhanced inelastic light scattering in
the strong-coupling regime, fulfilling a triple-resonance condition, are
analyzed and the limitations of the quasistatic adiabatic approximation are
established.",1904.10714v1
2019-08-03,On the Regge limit of Fishnet correlators,"We study the Regge trajectories of the Mellin amplitudes of the $0-,1-$ and
$2-$ magnon correlators of the Fishnet theory. Since fishnet theory is both
integrable and conformal, the correlation functions are known exactly. We find
that while for $0$ and $1$ magnon correlators, the Regge poles can be exactly
determined as a function of coupling, $2$-magnon correlators can only be dealt
with perturbatively. We evaluate the resulting Mellin amplitudes at weak
coupling, while for strong coupling we do an order of magnitude calculation.",1908.01123v3
2019-08-28,Effect of inhomogeneous Dzyaloshinskii-Moriya interaction on antiferromagnetic spin-wave propagation,"We investigate the effect of inhomogeneous Dzyaloshinskii-Moriya interaction
(DMI) on antiferromagnetic spin-wave propagation theoretically and numerically.
We find that antiferromagnetic spin waves can be amplified at a boundary where
the DMI varies. The inhomogeneous DMI also provides a way to construct a
magnonic crystal with forbidden and allowed antiferromagnetic spin-wave bands
in terahertz frequency ranges. In contrast to ferromagnetic spin waves,
antiferromagnetic spin waves experience a polarization-dependent phase shift
when passing through the inhomogeneous DMI, offering a magnonic crystal that
also serves as a spin-wave polarizer.",1908.10650v1
2016-10-13,Electronic control of magnonic and spintronic devices,"Nanometric magnonic and spintronic devices need magnetic field control in
addition to conventional electronic control. In this work we review ways to
replace magnetic field control by an electronic one in order to circumvent
appearance of stray magnetic fields or the difficulty of creating large
magnetic fields over nanometric distances. Voltage control is compared to
current control and corresponding devices are compared from their energetic
efficiency point of view.",1610.04011v1
2017-07-03,Theory of magnon-mediated tunnel magneto-Seebeck effect,"The tunnel magneto-Seebeck effect is the dependence of the thermopower of
magnetic tunnel junctions on the magnetic configuration. It is conventionally
interpreted in terms of a thermoelectric generalization of the tunnel
magnetoresistance. Here, we investigate the heat-driven electron transport in
these junctions associated with electron-magnon scattering, using stochastic
Landau-Lifshitz phenomenology and quantum kinetic theory. Our findings
challenge the widely accepted single-electron picture of the tunneling
thermopower in magnetic junctions.",1707.00674v1
2020-06-03,Circulating cavity magnon polaritons,"We predict magnon polariton states circulating unidirectionally in a
microwave cavity when loaded by a number of magnets on special lines. Realistic
finite-element numerical simulations, including dielectric, time-dependent and
non-linear effects, confirm the validity of the approximations of a fully
analytical input-output model. We find that a phased antenna array can focus
all power into a coherent microwave beam with controlled direction and an
intensity that scales with the number of magnets.",2006.02203v1
2020-06-15,Open giant magnons suspended between dual giant gravitons in ${\cal N}=4$ SYM,"We study classical solutions to the Nambu-Goto string on $AdS_2 \times S^1$
and $AdS_3 \times S^1$ corresponding to strings stretched between wrapped
branes extending in $AdS$. The solutions are obtained by analytic continuation
of giant magnon solutions, cut at the position of the branes.
  These solutions carry one or two $SO(2,4)$ charges and a single $SO(6)$
charge. We compute their energies and show their relation to $\frac{1}{2}$-BPS
geometries. Their relevance to the $SL(2)$ sector of $\mathcal{N}=4$ SYM is
also discussed.",2006.08649v1
2021-02-12,Impossibility of Increasing N$\acute{\textrm{e}}$el Temperature in Zigzag Graphene Nanoribbon by Electric Field and Carrier Doping,"We investigated the dependence of N$\acute{\textrm{e}}$el temperature as a
critical temperature on the electric field and hole-electron doping in the
antiferromagnetically ordered zigzag graphene nanoribbon. The temperature was
calculated by averaging the magnon energy in the Brillouin zone within the
mean-field approximation. We employed the generalized Bloch theorem instead of
the supercell approach to reduce the computational cost significantly to obtain
the magnon spectrum. We showed that the N$\acute{\textrm{e}}$el temperature
reduces when increasing both the electric field and the hole-electron doping,
thus these treatments will never enhance the N$\acute{\textrm{e}}$el
temperature.",2102.06508v1
2015-12-14,Gauge and emergent electromagnetic fields for moving magnetic topological solitons,"We apply the general conception of non-Abelian gauge fields for description
of magnetic soliton excitations. We show that the component of the gauge field
along the soliton local magnetization (Abelian part of the gauge potential)
determines dynamics of spin fluctuations over the soliton background in a
ferromagnet. Assumption that the gauge field is a pure gauge allows calculating
the gauge field components and finding simple expressions for the emergent
electromagnet fields related to the soliton motion. The gauge field results in
soliton-magnon interaction leading to renormalization of the soliton and magnon
dynamics. The presented approach allows reaching more deep understanding of a
relationship of the field theory and condensed matter magnetism.",1512.04396v1
2016-09-07,Thermally induced magnon accumulation in two-sublattice magnets,"We present a temperature dependent study of the thermal excitation of
magnonic spin currents in two-sublattice magnetic materials. Using atomistic
spin model simulations, we study the local magnetization profiles in the
vicinity of a temperature step in antiferromagnets, as well as in ferrimagnets.
It is shown that the strength of the excitation of the spin currents in these
systems scales with the derivative of the magnetization with respect to the
temperature.",1609.02161v1
2016-09-15,Route toward high-speed nano-magnonics,"We study experimentally the possibility to utilize pulses of pure spin
current, produced via the nonlocal spin injection mechanism, to generate short
packets of spin waves propagating in nanoscale magnetic waveguides. The
spatially and time-resolved micro-focus Brillouin light scattering spectroscopy
measurements demonstrate that the excitation by spin current results in
extremely fast transient response, enabling efficient generation of short
spin-wave packets with duration down to a few nanoseconds. The proposed method
opens a route for the implementation of high-speed magnonic systems for
transmission and processing of information on the nanoscale.",1609.04526v1
2016-11-05,Magnonic analog of relativistic Zitterbewegung in an antiferromagnetic spin chain,"We theoretically investigate the spin wave (magnon) excitations in a
classical antiferromagnetic spin chain with easy-axis anisotropy. We obtain a
Dirac-like equation by linearizing the Landau- Lifshitz-Gilbert equation in
this antiferromagnetic system, in contrast to the ferromagnetic system in which
a Schr\""{o}dinger equation is derived. The Hamiltonian operator in the
Dirac-like equation is a pseudo-Hermitian. We compute and demonstrate the
relativistic Zitterbewegung (trembling motion) in the antiferromagnetic spin
chain by measuring the expectation values of the wave packet position.",1611.01512v2
2019-10-25,Voltage-Controlled Magnonic Spin Tunneling Junction,"We theoretically investigate the effective exchange interaction,
$J_\mathrm{eff}$, mediated by conductive electrons within a nonmagnetic metal
spacer, in the presence of a bias voltage, sandwiched by two ferromagnetic
insulators. On the basis of the tight-binding model, we show the voltage and
spacer thickness dependences of $J_\mathrm{eff}$, and its contorollability is
demonstrated. We also propose a new magnonic device with the functions of both
field effect transistor and non-volatile memory.",1910.11549v2
2020-02-07,Order Parameter and Magnetization of Antiferromagnets in Mutually Parallel Staggered and Magnetic Fields,"We explore the behavior of the order parameter and the magnetization of
antiferromagnetic solids subjected to mutually parallel staggered and magnetic
fields. The effective field theory analysis of the partition function is taken
up to the two-loop level, where the magnon-magnon interaction comes into play.
These interaction effects, however, are small. A phenomenon that comes rather
unexpectedly is that the finite-temperature magnetization increases with
temperature when the strengths of the staggered and magnetic field are held
constant.",2002.02958v1
2020-03-21,Low-temperature asymptotic of the transverse dynamical structure factor for a magnetically polarized XX chain,"Dyson equation for the real two-time commutator retarded one-magnon Green
function of the ferromagnetically polarized XX chain is suggested following the
Plakida-Tserkovnikov algorithm. Starting from this result a low-temperature
integral representation for the corresponding magnon self energy is obtained by
the truncated form factor expansion however without any resummations. Within
the suggested approach the low-temperature asymptotics of the transverse
dynamical structure factor may be readily studied. Some obtained line shapes
are presented.",2003.09743v3
2020-03-28,Magnon binding in BaCdVO(PO$_4$)$_2$,"The bond-nematic state has both long-range ordering of magnetic quadrupoles
and the high entanglement typical of spin liquids, and as such can be thought
of as a spin liquid crystal. One of the most promising materials in which to
find such a state is BaCdVO(PO$_4$)$_2$, in which a magnetically silent phase
has been found between a magnetically ordered low-field phase and the
high-field polarised phase. Here I study the magnetic Hamiltonian of
BaCdVO(PO$_4$)$_2$, and, for the Hamiltonian-parameters determined by fits to
inelastic neutron scattering experiments, show that magnons condense out of the
polarised state as bound pairs. This is direct theoretical support for the
existence of a bond-nematic state just below the saturation field.",2003.12747v1
2020-07-23,Magnetic characteristics of epitaxial NiO films studied by Raman spectroscopy,"Raman spectroscopy is utilized to study the magnetic characteristics of
heteroepitaxial NiO thin films grown by plasma-assisted molecular beam epitaxy
on MgO(100) substrates. For the determination of the N\'eel temperature, we
demonstrate a reliable approach by analyzing the temperature dependence of the
Raman peak originating from second-order scattering by magnons. The
antiferromagnetic coupling strength is found to be strongly influenced by the
growth conditions. The low-temperature magnon frequency and the N\'eel
temperature are demonstrated to depend on the biaxial lattice strain and the
degree of structural disorder which is dominated by point defects.",2007.11982v1
2020-10-04,Bethe ansatz on a quantum computer?,"We consider the feasibility of studying the anisotropic Heisenberg quantum
spin chain with the Variational Quantum Eigensolver (VQE) algorithm, by
treating Bethe states as variational states, and Bethe roots as variational
parameters. For short chains, we construct exact one-magnon trial states that
are functions of the variational parameter, and implement the VQE calculations
in Qiskit. However, exact multi-magnon trial states appear to be out out of
reach.",2010.01609v2
2020-10-16,Zero-magnon sound in quantum Heisenberg ferromagnets,"Using a functional renormalization group approach we show that at low
temperatures and in a certain range of magnetic fields the longitudinal dynamic
structure factor of quantum Heisenberg ferromagnets in dimensions $D\leq 2$
exhibits a well-defined quasi-particle peak with linear dispersion that we
identify as zero-magnon sound. In $D>2$ the larger phase space available for
the decay into transverse spin waves leads only to a broad hump centered at
zero frequency whose width scales linearly in momentum.",2010.08425v3
2021-04-05,Two-magnon Raman scattering in antiferromagnetic phases of frustrated spin models on the honeycomb lattice,"We calculate the two-magnon Raman scattering spectra in antiferromagnetic
phases of several frustrated spin models defined on the honeycomb lattice.
These include the N\'{e}el antiferromagnetic phase of a $J_1$-$J_2$-$J_3$ model
and the stripe phase of the Heisenberg-Kitaev model. We show that both the
magnetic frustration and the anisotropy of interactions may significantly
affect the Raman spectra. We further discuss the implications of our results to
the magnetic excitations of the iron-based compound BaFe$_2$Se$_2$O and show
how the magnetic interactions can be extracted from fit to the Raman spectrum.",2104.01903v1
2021-11-05,Multiple optical gaps and laser with magnonic pumping in 2D Ising superconductors,"Ising superconductivity has been recently discovered in 2D transition metal
dichalcogenides. We report that such superconductors have unusual optical
properties controlled by the in-plane Zeeman field. First, we find several
optical gaps visible as peaks of the conductivity and the Raman susceptibility.
Moreover, we find that the Ising spin splitting in the spectrum of Bogolubov
quasiparticles enables strong population inversion generated by the
time-dependent Zeeman field. Ultimately this leads to the possibility of the
superconducting laser with magnonic pumping which can be realized in the van
der Waals structures consisting of the Ising superconductor and the
ferromagnetic insulator layers.",2111.03623v1
2022-06-30,Single magnon excited states of a Heisenberg spin-chain using a quantum computer,"Excited states of spin-chains play an important role in condensed matter
physics. We present a method of calculating the single magnon excited states of
the Heisenberg spin-chain that can be efficiently implemented on a quantum
processor for small spin chains. Our method involves finding the stationary
points of the energy vs wavenumber curve. We implement our method for 4-site
and 8-site Heisenberg Hamiltonians using numerical techniques as well as using
an IBM quantum processor. Finally, we give an insight into the circuit
complexity and scaling of our proposed method.",2206.15300v1
2022-08-29,Experimental generation of circulating cavity magnon polaritons,"We experimentally realize circularly polarised unidirectional cavity magnon
polaritons in a torus-shaped microwave cavity loaded by a small magnetic
sphere. At special positions the clockwise and counterclockwise modes are
circularly polarized, such that only one of them couples to the magnet, which
breaks the mode degeneracy. We reveal the chiral nature of the spectral energy
and angular momentum flow by observing and modelling non-reciprocities of the
microwave scattering matrix.",2208.13408v3
2022-12-02,Interplay between spin wave and magnetic vortex,"In this paper, the interplay between spin wave and magnetic vortex is
studied. We find three types of magnon scatterings: skew scattering, symmetric
side deflection and back reflection, which associate with respectively magnetic
topology, energy density distribution and linear momentum transfer torque
within vortex. The vortex core exhibits two translational modes: the intrinsic
circular mode and a coercive elliptical mode, which can be excited based on
permanent and periodic magnon spin-transfer torque effects of spin wave.
Lastly, we propose a vortex-based spin wave valve in which via inhomogeneity
modulation we access arbitrary control of the phase shift.",2212.01172v1
2023-01-03,Amplitude representation of Landau-Lifshitz equation and its application to ferromagnetic films,"The purpose of this article is to develop a systematic approach to the
solution of the Landau-Lifshitz equation in terms of the magnon wave function
{\psi} (r) and apply it to physical phenomena in a thin ferromagnetic film.
This article has also a purpose to represent the modern state of art for the
properties of ferromagnetic films and the pumping-induced Bose-Einstein
condensation of magnons in them at room temperature. Thus, it can be considered
as a review on basic principles and the recent advances in the field.",2301.01391v1
2023-02-15,Coherent Microwave Emission of a Gain-Driven Polariton,"By developing a gain-embedded cavity magnonics platform, we create
gain-driven polariton (GDP) that is activated by an amplified electromagnetic
field. Distinct effects of gain-driven light-matter interaction, such as
polariton auto-oscillations, polariton phase singularity, self-selection of a
polariton bright mode, and gain-induced magnon-photon synchronization, are
theoretically studied and experimentally manifested. Utilizing the
gain-sustained photon coherence of the GDP, we demonstrate polariton-based
coherent microwave amplication (~ 40 dB) and achieve high-quality coherent
microwave emission (Q > 10^9).",2302.08904v1
2023-02-22,Magnons in the fan phase of anisotropic frustrated antiferromagnets,"Elementary excitations spectrum of the fan phase of anisotropic frustrated
antiferromagnets is discussed analytically. In the linear spin-wave
approximation, the spectrum is determined by the Hamiltonian, including normal,
anomalous, and umklapp terms. The latter mix states with momenta which differ
by two modulation vectors of the fan structure. This mixing leads to essential
rearrangement of the low-energy part of the spectrum, which is shown to consist
of a gapless phason branch with linear dispersion and a gapped ``optical''
branch, which corresponds to the fan structure amplitude oscillations. In the
high-energy part of the spectrum, the effect of the umklapps is negligible, and
the excitations are similar to the magnons of the fully polarized phase.",2302.11235v1
2023-05-22,Acousto-magnonic Hall effect in honeycomb antiferromagnets,"The recently discovered van der Waals antiferromagnets have suffered from the
lack of a comprehensive method to study their magnetic properties. Here, we
propose a dissipationless magnon spin Hall current driven by surface acoustic
waves as a novel probe for such antiferromagnets. Our results pave the way
towards mechanical detection and manipulation of the magnetic order in
two-dimensional antiferromagnets. Furthermore, they will overcome the
difficulties with weak magnetic responses inherent in the use of
antiferromagnets and hence provide a building block for future
antiferromagnetic spintronics.",2305.13375v1
2023-07-10,Surface magnon spectra of nodal loop semimetals,"In this paper we establish a connection between the bulk topological
structure and the magnetic properties of drumhead surface states of nodal loop
semimetals. We identify the magnetic characteristics of the surface states and
compute the system's magnon spectrum by treating electron-electron interactions
on a mean-field level. We draw attention to a subtle connection between a
Lifshitz-like transition of the surface states driven by mechanical distortions
and the magnetic characteristics of the system. Our findings may be
experimentally verified e.g. by spin polarized electron energy loss
spectroscopy of nodal semimetal surfaces.",2307.04620v2
2023-08-16,Two-dimensional altermagnets: Superconductivity in a minimal microscopic model,"We propose a minimal toy model for a two-dimensional altermagnet. The model
unravels altermagnetic properties at a microscopic level. We find spin-split
electron- and non-degenerate magnon bands with a $d$-wave symmetry. We use the
model to explore magnon-mediated superconductivity in altermagnets. The
dominant superconducting state is spin-polarized with a $p$-wave symmetry. The
state adopts its characteristics from the spin-split electron bands.
Furthermore, we find that the superconducting critical temperature of
altermagnets can be significantly enhanced by tuning the chemical potential.",2308.08606v2
2023-09-07,Electric Analog of Magnons in Order-Disorder Ferroelectrics,"We analyze the ``ferron"" excitations in order-disorder ferroelectrics by a
microscopic pseudo-spin model. We demonstrate that analogous to magnons, the
quanta of spin waves in magnetic materials, ferrons carry both static and
oscillating electric dipole moments, exhibit a Stark effect, and may be
parametrically excited by THz radiation. The anti-crossing gap of the
ferron-photon hybrid depends strongly on propagation direction and an applied
static electric field. We predict ferron diffusion lengths that can reach
centimeters, which implies efficient transport of electric polarization by
temperature gradients. These properties suggest that ferroelectric materials
may be useful for information technology beyond data storage applications.",2309.03691v1
2023-10-06,A Physical Theory of Two-stage Thermalization,"One indication of thermalization time is subsystem entanglement reaching
thermal values. Recent studies on local quantum circuits reveal two exponential
stages with decay rates $r_1$ and $r_2$ of the purity before and after
thermalization. We provide an entanglement membrane theory interpretation, with
$r_1$ corresponding to the domain wall free energy. Circuit geometry can lead
to $r_1 < r_2$, producing a ``phantom eigenvalue"". Competition between the
domain wall and magnon leads to $r_2 < r_1$ when the magnon prevails. However,
when the domain wall wins, this mechanism provides a practical approach for
measuring entanglement growth through local correlation functions.",2310.04491v1
2023-11-16,Unidirectional propagation of zero-momentum magnons,"We report on experimental observation of unidirectional propagation of
zero-momentum magnons in synthetic antiferromagnet consisting of strained
CoFeB/Ru/CoFeB trilayer. Inherent non-reciprocity of spin waves in synthetic
antiferromagnets with uniaxial anisotropy results in smooth and monotonous
dispersion relation around Gamma point, where the direction of the phase
velocity is reversed, while the group velocity direction is conserved. The
experimental observation of this phenomenon by intensity-, phase-, and
time-resolved Brillouin light scattering microscopy is corroborated by
analytical models and micromagnetic simulations.",2311.10044v1
2023-11-21,Magnonic spin current shot noise in an itinerant Fermi gas,"Spin transport phenomena at strongly-correlated interfaces play central roles
in fundamental physics as well as spintronic applications. To anatomize
spin-transport carriers, we propose the detection of the spin current noise in
interacting itinerant fermions. The Fano factor given by the ratio between the
spin current and its noise reflects elementary carriers of spin transport at
the interface of spin-polarized Fermi gases realized in ultracold atoms. The
change of the Fano factor microscopically evinces a crossover from the
quasiparticle transport to magnon transport in itinerant fermionic systems.",2311.12383v2
2023-12-05,Topological superconductivity in quantum wires mediated by helical magnons,"Triplet superconductivity arises in systems where the effective electronic
attraction favors Cooper pairs with a symmetric spin structure, as may occur in
non-collinear magnetic systems. Here we show that topological triplet
superconductivity can be realized in a one-dimensional quantum wire through the
interaction with magnetic fluctuations of a helical magnet. The magnon-mediated
attraction favors triplet superconductivity over a large sector of magnetic
parameters, and stabilizes a topological phase with a superconducting gap of
the order of 1 meV over an extended region of chemical potentials.",2312.02655v1
2024-01-28,Dynamical theory of angle-resolved electron energy loss and gain spectroscopies of phonons and magnons including multiple scattering effects,"We present a method for computing angle-resolved electron-energy-loss and
gain spectroscopies for phonon and magnon excitations in transmission electron
microscopy. Fractional scattering intensities are derived from the
temperature-dependent time auto-correlation of the electron beam wave function.
This method captures both single and multiple scattering processes, as well as
dynamical diffraction effects. Our method remains computationally efficient,
and it is easy to parallelize.",2401.15599v2
2024-01-29,Topological magnon-polarons in honeycomb antiferromagnets with spin-flop transition,"We theoretically investigate the thermal Hall transport of magnon-polarons in
a two-dimensional honeycomb antiferromagnetic insulator under the influence of
a perpendicular magnetic field, varying in strength. The application of a
perpendicular magnetic field induces a magnetic phase transition from the
collinear antiferromagnetic phase to the spin-flop phase, leading to a
significant alteration in Hall transport across the transition point. In this
paper, our focus is on the intrinsic contribution to thermal Hall transport
arising from the magnetoelastic interaction. To facilitate experimental
verification of our theoretical results, we present the dependence of thermal
Hall conductivity on magnetic field strength and temperature.",2401.15888v1
2024-02-16,Parametric instability in a magnomechanical system,"We study parametric instability in a magnomechanical system, specifically
examining magnon tunneling between moving ferromagnetic insulators. Our
analysis reveals that quantum fluctuations generate spin currents above a
critical velocity threshold, while no spin currents occur below this threshold
at low temperatures. The critical velocity depends on magnon stiffness and
Zeeman energy. Approaching the threshold, the spin current becomes divergent,
linked to the PT-symmetry-breaking transition. This enhanced behavior could
offer sensitive measurements and signal amplification in quantum technology.",2402.10796v1
2024-03-18,Current-induced magnon trapping in spin torque oscillation,"Spin torque nano-oscillators realized by magnetization dynamics trapped in a
current-induced potential are reported. We fabricated Ni$_{81}$Fe$_{19}$/Pt
nanostructures and measured current-induced microwave emission from the
structures. The result shows an increase in the magnitude and spectral
narrowing of the microwave emission. We demonstrate that the current-induced
magnetic field suppresses magnon radiation loss and significantly reduces the
linewidth and the threshold current required for the spin torque oscillation.",2403.11618v1
1997-12-15,Spin Tunneling in Conducting Oxides,"Direct tunneling in ferromagnetic junctions is compared with
impurity-assisted, surface state assisted, and inelastic contributions to a
tunneling magnetoresistance (TMR). Theoretically calculated direct tunneling in
iron group systems leads to about a 30% change in resistance, which is close to
experimentally observed values. It is shown that the larger observed values of
the TMR might be a result of tunneling involving surface polarized states. We
find that tunneling via resonant defect states in the barrier radically
decreases the TMR (down to 4% with Fe-based electrodes), and a resonant tunnel
diode structure would give a TMR of about 8%. With regards to inelastic
tunneling, magnons and phonons exhibit opposite effects: one-magnon emission
generally results in spin mixing and, consequently, reduces the TMR, whereas
phonons are shown to enhance the TMR. The inclusion of both magnons and phonons
reasonably explains an unusual bias dependence of the TMR.
  The model presented here is applied qualitatively to half-metallics with 100%
spin polarization, where one-magnon processes are suppressed and the change in
resistance in the absence of spin-mixing on impurities may be arbitrarily
large. Even in the case of imperfect magnetic configurations, the resistance
change can be a few 1000 percent. Examples of half-metallic systems are
CrO$_2$/TiO$_2$ and CrO$_2$/RuO$_2$, and an account of their peculiar band
structures is presented. The implications and relation of these systems to CMR
materials which are nearly half-metallic, are discussed.",9712170v2
1998-03-12,Static and dynamic structure factors in the Haldane phase of the bilinear-biquadratic spin-1,"The excitation spectra of the T=0 dynamic structure factors for the spin,
dimer, and trimer fluctuation operators as well as for the newly defined center
fluctuation operator in the one-dimensional S=1 Heisenberg model wi th
isotropic bilinear $(J\cos\theta)$ and biquadratic $(J\sin\theta)$ exchange are
investigated via the recursion method for systems with up to N=18 site s over
the predicted range, $-\pi/4<\theta\lesssim\pi/4$, of the topologically ordered
Haldane phase. The four static and dynamic structure factors probe t he
ordering tendencies in the various coupling regimes and the elementary and
composite excitations which dominate the T=0 dynamics. At $\theta =
\arctan{1/3}$ (VBS point), the dynamically relevant spectra in the invariant
subspaces with total spin $S_T = 0,1,2$ are dominated by a branch of magnon
states $(S_T = 1)$, by continua of two-magnon scattering states $(S_T =
0,1,2)$, and by discrete branches of two-magnon bound states with positive
interaction energy $(S_T = 0,2)$. The dimer and trimer spectra at $q=\pi$ ar e
found to consist of single modes with $N$-independent excitation energies
$\omega_\lambda^D/|e_0|=5$ and $\omega_\lambda^T/|e_0|=6$, where $e_0=E_0/N$ is
the ground-state energy per site. The basic structure of the dynamically
relevant excitation spectrum remains the same over a substantial parameter
range within the Haldane phase. At the transition to the dimerized phase
($\theta=-\pi/4$), the two-magnon excitations turn into two-spinon excitations.",9803147v1
2002-12-20,Magnon Exchange Mechanism of Ferromagnetic Superconductivity,"The magnon exchange mechanism of ferromagnetic superconductivity
(FM-superconductivity) was developed to explain in a natural way the fact that
the superconductivity in $UGe_2$, $ZrZn_2$ and $URhGe$ is confined to the
ferromagnetic phase.The order parameter is a spin anti-parallel component of a
spin-1 triplet with zero spin projection. The transverse spin fluctuations are
pair forming and the longitudinal ones are pair breaking. In the present paper,
a superconducting solution, based on the magnon exchange mechanism, is obtained
which closely matches the experiments with $ZrZn_2$ and $URhGe$. The onset of
superconductivity leads to the appearance of complicated Fermi surfaces in the
spin up and spin down momentum distribution functions. Each of them consist of
two pieces, but they are simple-connected and can be made very small by varying
the microscopic parameters. As a result, it is obtained that the specific heat
depends on the temperature linearly, at low temperature, and the coefficient
$\gamma=\frac {C}{T}$ is smaller in the superconducting phase than in the
ferromagnetic one. The absence of a quantum transition from ferromagnetism to
ferromagnetic superconductivity in a weak ferromagnets $ZrZn_2$ and $URhGe$ is
explained accounting for the contribution of magnon self-interaction to the
spin fluctuations' parameters. It is shown that in the presence of an external
magnetic field the system undergoes a first order quantum phase transition.",0212520v1
2003-11-13,Magnon squeezing in an antiferromagnet: reducing the spin noise below the standard quantum limit,"At absolute zero temperature, thermal noise vanishes when a physical system
is in its ground state, but quantum noise remains as a fundamental limit to the
accuracy of experimental measurements. Such a limitation, however, can be
mitigated by the formation of squeezed states. Quantum mechanically, a squeezed
state is a time-varying superposition of states for which the noise of a
particular observable is reduced below that of the ground state at certain
times. Quantum squeezing has been achieved for a variety of systems, including
the electromagnetic field, atomic vibrations in solids and molecules, and
atomic spins, but not so far for magnetic systems. Here we report on an
experimental demonstration of spin wave (i.e., magnon) squeezing. Our method
uses femtosecond optical pulses to generate correlations involving pairs of
magnons in an antiferromagnetic insulator, MnF2. These correlations lead to
quantum squeezing in which the fluctuations of the magnetization of a
crystallographic unit cell vary periodically in time and are reduced below that
of the ground state quantum noise. The mechanism responsible for this squeezing
is stimulated second order Raman scattering by magnon pairs. Such squeezed
states have important ramifications in the emerging fields of spintronics and
quantum computing involving magnetic spin states or the spin-orbit coupling
mechanism.",0311329v1
2004-02-19,Self-energy corrections in an antiferromagnet -- interplay of classical and quantum effects on quasiparticle dispersion,"Self-energy corrections due to fermion-magnon interaction are studied in the
antiferromagnetic state of the $t-t'-t''$ Hubbard model within the rainbow
(noncrossing) approximation in the full $U$ range from weak to strong coupling.
The role of classical (mean-field) features of fermion and magnon dispersion,
associated with finite $U,t',t''$, are examined on quantum corrections to
quasiparticle energy, weight, one-particle density of states etc. A finite-$U$
induced classical dispersion term, absent in the $t-J$ model, is found to play
an important role in suppressing the quasiparticle weight for states near ${\bf
k}=(0,0)$, as seen in cuprates. For intermediate $U$, the renormalized AF band
gap is found to be nearly half of the classical value, and the weak coupling
limit is quite non-trivial due to strongly suppressed magnon amplitude. For
finite $t'$, the renormalized AF band gap is shown to vanish at a critical
interaction strength $U_c$, yielding a spin fluctuation driven first-order AF
insulator - PM metal transition. Quasiparticle dispersion evaluated with the
same set of Hubbard model cuprate parameters, as obtained from a recent magnon
spectrum fit, provides excellent agreement with ARPES data for $\rm Sr_2 Cu O_2
Cl_2 $.",0402487v3
2006-02-14,Electron-phonon coupling in the self-consistent Born approximation of the t-J model,"We study an undoped t-J model with electron-phonon interaction using the
self-consistent Born approximation (SCBA). By neglecting vertex corrections,
the SCBA solves a boson-holon model, where a holon couples to phonons and
magnons. Comparison with exact diagonalization results for the t-J model
suggests that the SCBA describes the electron-phonon interaction fairly
accurately over a substantial range of J/t values. Exact diagonalization of the
boson-holon model shows that the deviations are mainly due to the neglect of
vertex corrections for small J/t and due to the replacement of the t-J model by
the boson-holon model for large J/t. For typical values of J/t, the
electron-phonon part Sigma_ep of the electron self-energy has comparable
contributions from the second order diagram in the electron-phonon interaction
and a phonon induced change of magnon diagrams. A very simple approximation to
Sigma_ep gives a rather accurate effective mass. Using this approximation, we
study the factors influencing the electron-phonon interaction. Typically, we
find that the magnons nominally have a stronger coupling to the holon than the
phonons. The phonons, nevertheless, drive the formation of small polarons
(self-localization) due to important differences between the character of the
phonon and magnon couplings.",0602327v1
2006-02-24,Magnetization dynamics in dysprosium orthoferrites via inverse Faraday effect,"The ultrafast non-thermal control of magnetization has recently become
feasible in canted antiferromagnets through photomagnetic instantaneous pulses
[A.V. Kimel {\it et al.}, Nature {\bf 435}, 655 (2005)]. In this experiment
circularly polarized femtosecond laser pulses set up a strong magnetic field
along the wave vector of the radiation through the inverse Faraday effect,
thereby exciting non-thermally the spin dynamics of dysprosium orthoferrites. A
theoretical study is performed by using a model for orthoferrites based on a
general form of free energy whose parameters are extracted from experimental
measurements. The magnetization dynamics is described by solving coupled
sublattice Landau-Lifshitz-Gilbert equations whose damping term is associated
with the scattering rate due to magnon-magnon interaction. Due to the inverse
Faraday effect and the non-thermal excitation, the effect of the laser is
simulated by magnetic field Gaussian pulses with temporal width of the order of
hundred femtoseconds. When the field is along the z-axis, a single resonance
mode of the magnetization is excited. The amplitude of the magnetization and
out-of-phase behavior of the oscillations for fields in z and -z directions are
in good agreement with the cited experiment. The analysis of the effect of the
temperature shows that magnon-magnon scattering mechanism affects the decay of
the oscillations on the picosecond scale. Finally, when the field pulse is
along the x-axis, another mode is excited, as observed in experiments. In this
case the comparison between theoretical and experimental results shows some
discrepancies whose origin is related to the role played by anisotropies in
orthoferrites.",0602593v1
2006-06-29,Two-Hole Bound States from a Systematic Low-Energy Effective Field Theory for Magnons and Holes in an Antiferromagnet,"Identifying the correct low-energy effective theory for magnons and holes in
an antiferromagnet has remained an open problem for a long time. In analogy to
the effective theory for pions and nucleons in QCD, based on a symmetry
analysis of Hubbard and t-J-type models, we construct a systematic low-energy
effective field theory for magnons and holes located inside pockets centered at
lattice momenta (\pm pi/2a,\pm pi/2a). The effective theory is based on a
nonlinear realization of the spontaneously broken spin symmetry and makes
model-independent universal predictions for the entire class of lightly doped
antiferromagnetic precursors of high-temperature superconductors. The
predictions of the effective theory are exact, order by order in a systematic
low-energy expansion. We derive the one-magnon exchange potentials between two
holes in an otherwise undoped system. Remarkably, in some cases the
corresponding two-hole Schr\""odinger equations can even be solved analytically.
The resulting bound states have d-wave characteristics. The ground state wave
function of two holes residing in different hole pockets has a d_{x^2-y^2}-like
symmetry, while for two holes in the same pocket the symmetry resembles d_{xy}.",0606766v1
2006-08-07,Magnon Bound-state Scattering in Gauge and String Theory,"It has been shown that, in the infinite length limit, the magnons of the
gauge theory spin chain can form bound states carrying one finite and one
strictly infinite R-charge. These bound states have been argued to be
associated to simple poles of the multi-particle scattering matrix and to world
sheet solitons carrying the same charges. Classically, they can be mapped to
the solitons of the complex sine-Gordon theory.
  Under relatively general assumptions we derive the condition that simple
poles of the two-particle scattering matrix correspond to physical bound states
and construct higher bound states ``one magnon at a time''. We construct the
scattering matrix of the bound states of the BDS and the AFS S-matrices. The
bound state S-matrix exhibits simple and double poles and thus its analytic
structure is much richer than that of the elementary magnon S-matrix. We also
discuss the bound states appearing in larger sectors and their S-matrices. The
large 't Hooft coupling limit of the scattering phase of the bound states in
the SU(2) sector is found to agree with the semiclassical scattering of world
sheet solitons. Intriguingly, the contribution of the dressing phase has an
independent world sheet interpretation as the soliton-antisoliton scattering
phase shift. The small momentum limit provides independent tests of these
identifications.",0608049v3
2009-05-10,Incoherent bound states in an infinite $XXZ$ chain at $Δ=-1/2$,"For an infinite $XXZ$ chain with $\Delta=-1/2$ we have obtained a family of
translationary invariant three-magnon states which do not satisfy the string
conjecture. All of them have the same energy.",0905.1494v1
2010-10-13,Electric-field control of spin waves at room temperature in multiferroic BiFeO3,"To face the challenges lying beyond current CMOS-based technology, new
paradigms for information processing are required. Magnonics proposes to use
spin waves to carry and process information, in analogy with photonics that
relies on light waves, with several advantageous features such as potential
operation in the THz range and excellent coupling to spintronics. Several
magnonic analog and digital logic devices have been proposed, and some
demonstrated. Just as for spintronics, a key issue for magnonics is the large
power required to control/write information (conventionally achieved through
magnetic fields applied by strip lines, or by spin transfer from large
spin-polarized currents). Here we show that in BiFeO3, a room-temperature
magnetoelectric material, the spin wave frequency (>600 GHz) can be tuned
electrically by over 30%, in a non-volatile way and with virtually no power
dissipation. Theoretical calculations indicate that this effect originates from
a linear magnetoelectric effect related to spin-orbit coupling induced by the
applied electric field. We argue that these properties make BiFeO3 a promising
medium for spin wave generation, conversion and control in future magnonics
architectures.",1010.2678v1
2011-01-03,The building blocks of magnonics,"Novel material properties can be realized by designing waves' dispersion
relations in artificial crystals. The crystal's structural length scales may
range from nano- (light) up to centimeters (sound waves). Because of their
emergent properties these materials are called metamaterials. Different to
photonics, where the dielectric constant dominantly determines the index of
refraction, in a ferromagnet the spin-wave index of refraction can be
dramatically changed already by the magnetization direction. This allows a
different flexibility in realizing dynamic wave guides or spin-wave switches.
The present review will give an introduction into the novel functionalities of
spin-wave devices, concepts for spin-wave based computing and magnonic
crystals. The parameters of the magnetic metamaterials are adjusted to the
spin-wave k-vector such that the magnonic band structure is designed. However,
already the elementary building block of an antidot lattice, the singular hole,
owns a strongly varying internal potential determined by its magnetic dipole
field and a localization of spin-wave modes. Photo-magnonics reveal a way to
investigate the control over the interplay between localization and
delocalization of the spin-wave modes using femtosecond lasers, which is a
major focus of this review. We will discuss the crucial parameters to realize
free Bloch states and how, by contrast, a controlled localization might allow
to gradually turn on and manipulate spin-wave interactions in spin-wave based
devices in the future.",1101.0479v2
2011-02-01,Yangian symmetry of boundary scattering in AdS/CFT and the explicit form of bound state reflection matrices,"The reflection matrices of multi magnon bound states are obtained explicitely
by exploiting the Yangian symmetry of boundary scattering on the Y=0 maximal
giant graviton brane.",1102.0122v1
2012-02-21,Quantum critical behavior in three-dimensional one-band Hubbard model at half filling,"One-band Hubbard model with hopping parameter $t$ and Coulomb repulsion $U$
is considered at half filling. By means of the Schwinger bosons and slave
Fermions representation of the electron operators and integrating out the
spin-singlet Fermi fields an effective Heisenberg model with antiferromagnetic
exchange constant is obtained for vectors which identifies the local
orientation of the spin of the itinerant electrons. The amplitude of the spin
vectors is an effective spin of the itinerant electrons accounting for the fact
that some sites, in the ground state, are doubly occupied or empty. Accounting
adequately for the magnon-magnon interaction the N\'{e}el temperature is
calculated. When the ratio $\frac tU$ is small enough ($\frac tU\leq 0.09$) the
effective model describes a system of localized electrons. Increasing the ratio
increases the density of doubly occupied states which in turn decreases the
effective spin and N\'{e}el temperature. The phase diagram in plane of
temperature $\frac {T_N}{U}$ and parameter $\frac tU$ is presented. The quantum
critical point ($T_N=0$) is reached at $\frac tU=0.9$. The magnons in the
paramagnetic phase are studied and the contribution of the magnons'
fluctuations to the heat capacity is calculated. At N\'{e}el temperature the
heat capacity has a peak which is suppressed when the system approaches quantum
critical point.",1202.4627v2
2013-07-25,Relaxation of Bose-Einstein Condensates of Magnons in Magneto-Textural Traps in Superfluid $^3$He-B,"In superfluid $^3$He-B externally pumped quantized spin-wave excitations or
magnons spontaneously form a Bose-Einstein condensate in a 3-dimensional trap
created with the order-parameter texture and a shallow minimum in the
polarizing field. The condensation is manifested by coherent precession of the
magnetization with a common frequency in a large volume. The trap shape is
controlled by the profile of the applied magnetic field and by the condensate
itself via the spin-orbit interaction. The trapping potential can be
experimentally determined with the spectroscopy of the magnon levels in the
trap. We have measured the decay of the ground state condensates after
switching off the pumping in the temperature range $(0.14\div
0.2)T_{\mathrm{c}}$. Two contributions to the relaxation are identified: (1)
spin-diffusion with the diffusion coefficient proportional to the density of
thermal quasiparticles and (2) the approximately temperature-independent
radiation damping caused by the losses in the NMR pick-up circuit. The measured
dependence of the relaxation on the shape of the trapping potential is in a
good agreement with our calculations based on the magnetic field profile and
the magnon-modified texture. Our values for the spin diffusion coefficient at
low temperatures agree with the theoretical prediction and earlier measurements
at temperatures above $0.5T_{\mathrm{c}}$.",1307.6782v2
2013-11-07,Giant magnon solution and dispersion relation in string theory in AdS_3 x S^3 x T^4 with mixed flux,"We address the question about the exact form of the dispersion relation for
light-cone string excitations in string theory in AdS3 x S3 x T4 with mixed R-R
and NS-NS 3-form fluxes. The analogy with string theory in AdS5 x S5 suggests
that in addition to the data provided by the perturbative near-BMN expansion
and the symmetry algebra considerations there is also another source of
information about the dispersion relation -- the semiclassical giant magnon
solution. In earlier work in arXiv:1303.1037 and arXiv:1304.4099 it was found
that the symmetry algebra constraints consistent with perturbative expansion do
not completely determine the form of the dispersion relation. The aim of the
present paper is to fix it by constructing a generalization of the known dyonic
giant magnon soliton on S3 to the presence of a non-zero NS-NS flux described
by a WZ term in the string action. We find that the angular momentum of this
soliton gets shifted by a term linear in world-sheet momentum. We also discuss
the symmetry algebra of the string light-cone S-matrix and show that the exact
dispersion relation, which should have the correct perturbative BMN and
semiclassical giant magnon limits, should also contain such a linear momentum
term. The simplicity of the resulting bound-state picture provides a strong
argument in favour of this dispersion relation.",1311.1794v2
2013-11-29,Magnon radiation by moving Abrikosov vortices in ferromagnetic superconductors and superconductor-ferromagnet multilayers,"In systems combining type-II superconductivity and magnetism the
non-stationary magnetic field of moving Abrikosov vortices may excite spin
waves, or magnons. This effect leads to the appearance of an additional damping
force acting on the vortices. By solving the London and Landau-Lifshitz-Gilbert
equations we calculate the magnetic moment induced force acting on vortices in
ferromagnetic superconductors and superconductor/ferromagnet superlattices. If
the vortices are driven by a dc force, magnon generation due to the Cherenkov
resonance starts as the vortex velocity exceeds some threshold value. For an
ideal vortex lattice this leads to an anisotropic contribution to the
resistivity and to the appearance of resonance peaks on the current voltage
characteristics. For a disordered vortex array the current will exhibit a
step-like increase at some critical voltage. If the vortices are driven by an
ac force with a frequency \omega, the interaction with magnetic moments will
lead to a frequency-dependent magnetic contribution \eta_M to the vortex
viscosity. If \omega is below the ferromagnetic resonance frequency \omega_F,
vortices acquire additional inertia. For \omega > \omega_F dissipation is
enhanced due to magnon generation. The viscosity \eta_M can be extracted from
the surface impedance of the ferromagnetic superconductor. Estimates of the
magnetic force acting on vortices for the U-based ferromagnetic superconductors
and cuprate/manganite superlattices are given.",1311.7620v1
2014-09-19,Study of strong photon-magnon coupling in a YIG-film split-ring resonant system,"By using the stripline Microwave Vector Network Analyzer Ferromagnetic
Resonance and Pulsed Inductive Microwave Magnetometry spectroscopy techniques,
we study a strong coupling regime of magnons to microwave photons in the planar
geometry of a lithographically formed split-ring resonator (SRR) loaded by a
single-crystal epitaxial yttrium-iron garnet (YIG) film. Strong anti-crossing
of the photon modes of SRR and of the magnon modes of the YIG film is observed
in the applied-magnetic-field resolved measurements. The coupling strength
extracted from the experimental data reaches 9 percent at 3 GHz.
  Theoretically, we propose an equivalent circuit model of an SRR loaded by a
magnetic film. This model follows from the results of our numerical simulations
of the microwave field structure of the SRR and of the magnetization dynamics
in the YIG film driven by the microwave currents in the SRR. The equivalent
circuit model is in good agreement with the experiment. It provides a simple
physical explanation of the process of mode anti-crossing.
  Our findings are important for future applications in microwave quantum
photonic devices as well as in magnetically tunable metamaterials exploiting
the strong coupling of magnons to microwave photons.",1409.5499v1
2014-11-14,Light Higgs channel of the resonant decay of magnon condensate in superfluid $^3$He-B,"In superfluids the order parameter, which describes spontaneous symmetry
breaking, is an analogue of the Higgs field in the Standard Model of particle
physics. Oscillations of the field amplitude are massive Higgs bosons, while
oscillations of the orientation are massless Nambu-Goldstone bosons. The
125~GeV Higgs boson, discovered at Large Hadron Collider, is light compared to
electroweak energy scale, which led to a suggestion of the ``little Higgs''
extension of the Standard Model, in which the light Higgs appears as a NG mode
acquiring mass due to violation of a hidden symmetry. Here we show that such
light Higgs exists in superfluid $^3$He-B, where one of three Nambu-Goldstone
spin-wave modes acquires small mass due to the spin-orbit interaction. Other
modes become optical and acoustic magnons. We observe parametric decay of
Bose-Einstein condensate of optical magnons to light Higgs modes and decay of
optical to acoustic magnons. Formation of a light Higgs from a Nambu-Goldstone
mode observed in $^3$He-B opens a possibility that such scenario can be
realized in other systems, where violation of some hidden symmetry is possible,
including the Standard Model.",1411.3983v4
2015-04-10,Long range pure magnon spin diffusion observed in a non-local spin-Seebeck geometry,"The spin diffusion length for thermally excited magnon spins is measured by
utilizing a non-local spin-Seebeck effect measurement. In a bulk single crystal
of yttrium iron garnet (YIG) a focused laser thermally excites magnon spins.
The spins diffuse laterally and are sampled using a Pt inverse spin Hall effect
detector. Thermal transport modeling and temperature dependent measurements
demonstrate the absence of spurious temperature gradients beneath the Pt
detector and confirm the non-local nature of the experimental geometry.
Remarkably, we find that thermally excited magnon spins in YIG travel over 120
$\mu$m at 23 K, indicating that they are robust against inelastic scattering.
The spin diffusion length is found to be at least 47 $\mu$m and as high as 73
$\mu$m at 23 K in YIG, while at room temperature it drops to less than 10
$\mu$m. Based on this long spin diffusion length, we envision the development
of thermally powered spintronic devices based on electrically insulating, but
spin conducting materials.",1504.02808v2
2015-06-19,Origin of the thickness-dependent low-temperature enhancement of spin Seebeck effect in YIG films,"The temperature dependent longitudinal spin Seebeck effect (SSE) in heavy
metal (HM)/ Y3Fe5O12 (YIG) bilayers is investigated as a function of different
magnetic field strength, different HM detection material, and YIG thickness
ranging from nm to mm. A large enhancement of the SSE signal is observed at low
temperatures leading to a peak of the signal amplitude. We demonstrate that
this enhancement shows a clear dependence on the film thickness, being more
pronounced for thicker films and vanishing for films thinner than 600 nm. The
peak temperature depends on the applied magnetic field strength as well as on
the detection material and interface, revealing a more complex behavior beyond
the currently discussed phonon-magnon coupling mechanism that considers only
bulk effects. While the thickness dependence and magnetic field dependence can
be well explained in the framework of the magnon-driven SSE by taking into
account the frequency dependent propagation length of thermally excited magnons
in the bulk material, the temperature dependence of the SSE is significantly
influenced by the interface coupling to an adjacent detection layer. This
indicates that previously neglected interface effects play a key role and that
the spin current traversing the interface and being detected in the HM depends
differently on the magnon frequency for different HMs.",1506.06037v1
2016-04-14,"Longitudinal magnon, inversion breaking and magnetic instabilities in the pseudo-Kagome francisites Cu3Bi(SeO3)2O2X with X=Br, Cl","We performed Raman studies and a dielectric characterization of the
pseudo-kagome Cu3Bi(SeO3)2O2X (X = Cl, Br). These compounds share competing
nearest-neighbour ferromagnetic exchange and frustrating next-nearest-neighbour
antiferromagnetic exchange as well as highly noncollinear magnetic ground
state. However, at low temperature they differ with respect to the existence of
inversion symmetry. For both compounds there exists a pronounced interplay of
polar phonon modes with quantum magnetic fluctuations. A novel Raman mode
appears for temperatures below the Neel temperature with a Fano lineshape and
an enormous intensity that exceeds most of the phonon lines. We discuss a
possible contribution of longitudinal magnons to this signal. In contrast, one
magnon scattering based on linear transvers magnons is excluded based on a
symmetry analysis of spin wave representations and Raman tensor calculations.
There exists evidence that in these pseudo-kagome compounds magnetic quantum
fluctuations carry an electric dipole moment. Our data as well as a comparison
with previous far-infrared spectra allow us to conclude that Cu3Bi(SeO3)2O2Cl
changes its symmetry most likely from Pmmn to P21mn with a second order
structural phase transition at T*=120 K and becomes multiferroic.
Cu3Bi(SeO3)2O2Br represents an interesting counter part as it does not show
this instability and stays inversion symmetric down to lowest temperatures,
investigated.",1604.04249v1
2017-03-02,Diversity of quantum ground states and quantum phase transitions of a spin-1/2 Heisenberg octahedral chain,"The spin-1/2 Heisenberg octahedral chain with regularly alternating monomeric
and square-plaquette sites is investigated using various analytical and
numerical methods: variational technique, localized-magnon approach, exact
diagonalization (ED) and density-matrix renormalization group (DMRG) method.
The model belongs to the class of flat-band systems and it has a rich
ground-state phase diagram including phases with spontaneously broken
translational symmetry. Moreover, it exhibits an anomalous low-temperature
thermodynamics close to continuous or discontinuous field-driven quantum phase
transitions between three quantum ferrimagnetic phases, tetramer-hexamer phase,
monomer-tetramer phase, localized-magnon phase and two different spin-liquid
phases. If the intra-plaquette coupling is at least twice as strong as the
monomer-plaquette coupling, the variational method furnishes a rigorous proof
of the monomer-tetramer ground state in a low-field region and the
localized-magnon approach provides an exact evidence of a single magnon trapped
at each square plaquette in a high-field region. In the rest of parameter space
we have numerically studied the ground-state phase diagram and magnetization
process using DMRG and ED methods. It is shown that the zero-temperature
magnetization curve may involve up to four intermediate plateaus at zero,
one-fifth, two-fifth and three-fifth of the saturation magnetization, while the
specific heat exhibits a striking low-temperature peak in a vicinity of
discontinuous quantum phase transitions.",1703.00715v2
2017-03-09,Magnon mode selective spin transport in compensated ferrimagnets,"We investigate the generation of magnonic thermal spin currents and their
mode selective spin transport across interfaces in insulating, compensated
ferrimagnet/normal metal bilayer systems. The spin Seebeck effect signal
exhibits a non-monotonic temperature dependence with two sign changes of the
detected voltage signals. Using different ferrimagnetic garnets, we demonstrate
the universality of the observed complex temperature dependence of the spin
Seebeck effect. To understand its origin, we systematically vary the interface
between the ferrimagnetic garnet and the metallic layer, and by using different
metal layers we establish that interface effects play a dominating role. They
do not only modify the magnitude of the spin Seebeck effect signal but in
particular also alter its temperature dependence. By varying the temperature,
we can select the dominating magnon mode and we analyze our results to reveal
the mode selective interface transmission probabilities for different magnon
modes and interfaces. The comparison of selected systems reveals
semi-quantitative details of the interfacial coupling depending on the
materials involved, supported by the obtained field dependence of the signal.",1703.03218v2
2017-08-07,Field dependence of non-reciprocal magnons in chiral MnSi,"Spin waves in chiral magnetic materials are strongly influenced by the
Dzyaloshinskii-Moriya interaction resulting in intriguing phenomena like
non-reciprocal magnon propagation and magnetochiral dichroism. Here, we study
the non-reciprocal magnon spectrum of the archetypical chiral magnet MnSi and
its evolution as a function of magnetic field covering the field-polarized and
conical helix phase. Using inelastic neutron scattering, the magnon energies
and their spectral weights are determined quantitatively after deconvolution
with the instrumental resolution. In the field-polarized phase the imaginary
part of the dynamical susceptibility $\chi''(\varepsilon, {\bf q})$ is shown to
be asymmetric with respect to wavevectors ${\bf q}$ longitudinal to the applied
magnetic field ${\bf H}$, which is a hallmark of chiral magnetism. In the
helimagnetic phase, $\chi''(\varepsilon, {\bf q})$ becomes increasingly
symmetric with decreasing ${\bf H}$ due to the formation of helimagnon bands
and the activation of additional spinflip and non-spinflip scattering channels.
The neutron spectra are in excellent quantitative agreement with the low-energy
theory of cubic chiral magnets with a single fitting parameter being the
damping rate of spin waves.",1708.02098v2
2017-08-09,Chiral liquid phase of simple quantum magnets,"We study a $T=0$ quantum phase transition between a quantum paramagnetic
state and a magnetically ordered state for a spin $S=1$ XXZ Heisenberg
antiferromagnet on a two-dimensional triangular lattice. The transition is
induced by an easy plane single-ion anisotropy $D$. At the mean-field level,
the system undergoes a direct transition at a critical $D = D_c$ between a
paramagnetic state at $D > D_c$ and an ordered state with broken U(1) symmetry
at $D < D_c$. We show that beyond mean field the phase diagram is very
different and includes an intermediate, partially ordered chiral liquid phase.
Specifically, we find that inside the paramagnetic phase the Ising ($J_z$)
component of the Heisenberg exchange binds magnons into a two-particle bound
state with zero total momentum and spin. This bound state condenses at $D >
D_c$, before single-particle excitations become unstable, and gives rise to a
chiral liquid phase, which spontaneously breaks spatial inversion symmetry, but
leaves the spin-rotational U(1) and time-reversal symmetries intact. This
chiral liquid phase is characterized by a finite vector chirality without long
range dipolar magnetic order. In our analytical treatment, the chiral phase
appears for arbitrarily small $J_z$ because the magnon-magnon attraction
becomes singular near the single-magnon condensation transition. This phase
exists in a finite range of $D$ and transforms into the magnetically ordered
state at some $D<D_c$. We corroborate our analytic treatment with numerical
density matrix renormalization group calculations.",1708.02980v2
2018-11-27,Magnon topology and thermal Hall effect in trimerized triangular lattice antiferromagnet,"The non-trivial magnon band topology and its consequent responses have been
extensively studied in two-dimensional magnetisms. However, the triangular
lattice antiferromagnet (TLAF), the best-known frustrated two-dimensional
magnet, has received less attention than the closely related Kagome system,
because of the spin-chirality cancellation in the umbrella ground state of the
undistorted TLAF. In this work, we study the band topology and the thermal Hall
effect (THE) of the TLAF with (anti-)trimerization distortion under the
external perpendicular magnetic field using the linearized spin wave theory. We
show that the spin-chirality cancellation is removed in such case, giving rise
to the non-trivial magnon band topology and the finite THE. Moreover, the
magnon bands exhibit band topology transitions tuned by the magnetic field. We
demonstrate that such transitions are accompanied by the logarithmic divergence
of the first derivative of the thermal Hall conductivity. Finally, we examine
the above consequences by calculating the THE in the hexagonal manganite
YMnO$_3$, well known to have anti-trimerization.",1811.11201v2
2018-05-15,Thermal Hall effect and topological edge states in a square lattice antiferromagnet,"We show that the two dimensional spatial inversion-symmetry (SIS) broken
square lattice antiferromagnet with easy-plane spin anisotropy exhibits a
thermal Hall effect and the edge modes characterized by the Z 2 topological
invariant. These topological properties require a nonzero Berry curvature, and
its origin is ascribed to the Dzyaloshinskii-Moriya (DM) interactions or the
noncoplanar magnetic ordering generating a U(1) gauge field that couples to the
kinetic motion of magnons. Although this picture is established in ferromagnets
on the kagome and pyrochlore lattices, it does not apply to our square lattice
model since such gauge field cancels out in an edge shared geometry. Instead,
our case has an analogy with the anomalous Hall effect of Rashba electronic
system where the spin orbit coupling generates an SU(2) gauge field. The two
species of magnons defined on antiferromagnetic sublattices can be regarded as
the pseudo-spin degrees of freedom of magnons. The DM interactions that emerge
due to the SIS breaking serve as a pseudo-spin orbit coupling of magnons and
generate a Berry curvature, when the direction of the magnetic moments is
properly controlled by the magnetic field. The thermal Hall conductivity of our
antiferromagnet shows rapid growth in temperature T beyond the power-law,
reflecting the almost gapless low energy branch of the antiferromagnet, which
distinctively differs from the T^7/2 -dependence in the pyrochlore
ferromagnets. The present system serves as a standard model for the
noncentrosymmetric crystals Ba2MnGe2O7 and Ba2CoGe2O7.",1805.05872v3
2011-11-13,Thermal spin pumping and magnon-phonon-mediated spin-Seebeck effect,"The spin-Seebeck effect (SSE) in ferromagnetic metals and insulators has been
investigated systematically by means of the inverse spin-Hall effect (ISHE) in
paramagnetic metals. The SSE generates a spin voltage as a result of a
temperature gradient in a ferromagnet, which injects a spin current into an
attached paramagnetic metal. In the paramagnet, this spin current is converted
into an electric field due to the ISHE, enabling the electric detection of the
SSE. The observation of the SSE is performed in longitudinal and transverse
configurations consisting of a ferromagnet/paramagnet hybrid structure, where
thermally generated spin currents flowing parallel and perpendicular to the
temperature gradient are detected, respectively. Our results explain the SSE in
terms of a two-step process: (1) the temperature gradient creates a
non-equilibrium state in the ferromagnet governed by both magnon and phonon
propagations and (2) the non-equilibrium between magnons in the ferromagnet and
electrons in the paramagnet at the contact interface leads to ""thermal spin
pumping"" and the ISHE signal. The non-equilibrium state of metallic magnets
(e.g. Ni81Fe19) under a temperature gradient is governed mainly by the phonons
in the sample and the substrate, while in insulating magnets (e.g. Y3Fe5O12)
both magnon and phonon propagations appear to be important. The phonon-mediated
non-equilibrium that drives the thermal spin pumping is confirmed also by
temperature-dependent measurements, giving rise to a giant enhancement of the
SSE signals at low temperatures.",1111.3036v2
2014-06-13,Low Energy Electrodynamics of Novel Spin Excitations in the Quantum Spin Ice Yb$_2$Ti$_2$O$_7$,"In condensed matter systems, the formation of long range order (LRO) with
broken symmetry is often accompanied by new types of excitations. However, in
many magnetic pyrochlore oxides, geometrical frustration suppresses
conventional LRO while at the same time non-trivial spin correlations are
observed. For such materials, a natural question to ask then is what is the
nature of the excitations in this highly correlated state without broken
symmetry? Frequently the application of a symmetry breaking field can stabilize
excitations whose properties still reflect certain aspects of the anomalous
state without long-range order. Here we report evidence of novel magnetic
excitations in the quantum spin ice material Yb$_2$Ti$_2$O$_7$, obtained from
time-domain terahertz spectroscopy (TDTS). At large fields, both magnon and
two-magnon-like excitations are observed in a $<$001$>$ directed magnetic field
illustrating the stabilization of a field induced LRO state. The unique ability
of TDTS to measure complex response functions allows a direct study of magnetic
responses in different polarization channels, revealing the existence of an
unusual left-hand polarized magnon. The g-factors of these excitations are
dramatically enhanced in the low-field limit, showing a cross-over of these
one- and two-magnon states into features consistent with quantum string-like
excitations proposed to exist in quantum spin ice in a small $<$001$>$ applied
field.",1406.3576v1
2018-09-07,Electron-magnon scattering in elementary ferromagnets from first principles: lifetime broadening and band anomalies,"We study the electron-magnon scattering in bulk Fe, Co, and Ni within the
framework of many-body perturbation theory implemented in the full-potential
linearized augmented-plane-wave method. To this end, a $\mathbf{k}$-dependent
self-energy ($GT$ self-energy) describing the scattering of electrons and
magnons is constructed from the solution of a Bethe-Salpeter equation for the
two-particle (electron-hole) Green function, in which single-particle Stoner
and collective spin-wave excitations (magnons) are treated on the same footing.
Partial self-consistency is achieved by the alignment of the chemical
potentials. The resulting renormalized electronic band structures exhibit
strong spin-dependent lifetime effects close to the Fermi energy, which are
strongest in Fe. The renormalization can give rise to a loss of quasiparticle
character close to the Fermi energy, which we attribute to electron scattering
with spatially extended spin waves. This scattering is also responsible for
dispersion anomalies in conduction bands of iron and for the formation of
satellite bands in nickel. Furthermore, we find a band anomaly at a binding
energy of 1.5~eV in iron, which results from a coupling of the quasihole with
single-particle excitations that form a peak in the Stoner continuum. This band
anomaly was recently observed in photoemission experiments. On the theory side,
we show that the contribution of the Goldstone mode to the $GT$ self-energy is
expected to (nearly) vanish in the long-wavelength limit. We also present an
in-depth discussion about the possible violation of causality when an
incomplete subset of self-energy diagrams is chosen.",1809.02395v2
2020-12-23,Hot electron relaxation in normal state of iron pnictides: memory function approach,"This study leads to the investigation of the non-equilibrium electron
relaxation in the normal state of iron pnictides. Here we consider the
relaxation of electrons due to their coupling with magnons and phonons in the
metallic state of iron pnictides using the memory function approach. In the
present model, electrons live at a higher temperature than that of the phonon
and magnon baths, mimicking a non-equilibrium steady state situation. Further
we analyze theoretically the generalized Drude scattering rate within the
framework of Two Temperature Model and study the full frequency and temperature
behavior for it. In zero frequency regime, the rate of electron-magnon
scattering and electron-phonon scattering shows a linear temperature dependence
at higher temperature values greater than Bloch-Gr\""{u}neisen temperature.
Whereas at lower temperature values, $T\ll\Theta_{BG}$, corresponding
scattering rates follow the temperature behavior as ($1/\tau_{e-p} \varpropto
T^3$) and ($1/\tau_{e-m} \varpropto T^{3/2}$), respectively. In the AC regime,
we compute that $1/\tau \propto \omega^2$ for $\omega\ll\omega_{BG}$ and for
the values greater than the Bloch-Gr\""{u}neisen frequency, it is
$\omega$-independent. Also, in lower frequency and zero temperature limit, we
have observed the different frequency scale of electron-magnon and
electron-phonon scattering i.e ($1/\tau \propto \omega^{3/2}$) and
($1/\tau\propto \omega^{3}$). These results can be viewed with the pump-probe
experimental setting for the normal state of iron pnictides.",2012.12650v1
2017-05-05,Neutron-scattering study of yttrium iron garnet,"The nuclear and magnetic structure and full magnon dispersions of yttrium
iron garnet Y$_3$Fe$_5$O$_{12}$ have been studied by neutron scattering. The
refined nuclear structure is distorted to a trigonal space group of $R\bar{3}$.
The highest-energy dispersion extends up to 86 meV. The observed dispersions
are reproduced by a simple model with three nearest-neighbor-exchange integrals
between 16$a$ (octahedral) and 24$d$ (tetrahedral) sites, $J_{aa}$, $J_{ad}$,
and $J_{dd}$, which are estimated to be 0.00$\pm$0.05, $-$2.90$\pm$0.07, and
$-$0.35$\pm$0.08 meV, respectively. The lowest-energy dispersion below 14 meV
exhibits a quadratic dispersion as expected from ferromagnetic magnons. The
imaginary part of $q$-integrated dynamical spin susceptibility $\chi$""($E$)
exhibits a square-root energy-dependence in the low energies. The magnon
density of state is estimated from the $\chi$""($E$) obtained on an absolute
scale. The value is consistent with a single polarization mode for the magnon
branch expected theoretically.",1705.02167v5
2017-05-12,Pseudo-Goldstone magnons in the frustrated S=3/2 Heisenberg helimagnet ZnCr2Se4 with a pyrochlore magnetic sublattice,"Low-energy spin excitations in any long-range ordered magnetic system in the
absence of magnetocrystalline anisotropy are gapless Goldstone modes emanating
from the ordering wave vectors. In helimagnets, these modes hybridize into the
so-called helimagnon excitations. Here we employ neutron spectroscopy supported
by theoretical calculations to investigate the magnetic excitation spectrum of
the isotropic Heisenberg helimagnet ZnCr2Se4 with a cubic spinel structure, in
which spin-3/2 magnetic Cr3+ ions are arranged in a geometrically frustrated
pyrochlore sublattice. Apart from the conventional Goldstone mode emanating
from the (0 0 q) ordering vector, low-energy magnetic excitations in the
single-domain proper-screw spiral phase show soft helimagnon modes with a small
energy gap of ~0.17 meV, emerging from two orthogonal wave vectors (q 0 0) and
(0 q 0) where no magnetic Bragg peaks are present. We term them
pseudo-Goldstone magnons, as they appear gapless within linear spin-wave theory
and only acquire a finite gap due to higher-order quantum-fluctuation
corrections. Our results are likely universal for a broad class of symmetric
helimagnets, opening up a new way of studying weak magnon-magnon interactions
with accessible spectroscopic methods.",1705.04642v3
2019-01-17,Observation of topological magnon insulator states in a superconducting circuit,"Searching topological states in artificial systems has recently become a
rapidly growing field of research. Meanwhile, significant experimental
progresses on observing topological phenomena have been made in superconducting
circuits. However, topological insulator states have not yet been reported in
this system. Here, for the first time, we experimentally realize a tunable
dimerized spin chain model and observe the topological magnon insulator states
in a superconducting qubit chain. Via parametric modulations of the qubit
frequencies, we show that the qubit chain can be flexibly tuned into
topologically trivial or nontrivial magnon insulator states. Based on
monitoring the quantum dynamics of a single-qubit excitation in the chain, we
not only measure the topological winding numbers, but also observe the
topological magnon edge and defect states. Our experiment exhibits the great
potential of tunable superconducting qubit chain as a versatile platform for
exploring non-interacting and interacting symmetry-protected topological
states.",1901.05683v2
2019-01-27,Observation of band narrowing and mode conversion in two-dimensional binary magnonic crystal,"We introduce a new type of binary magnonic crystal, where Ni$_{80}$Fe$_{20}$
nanodots of two different sizes are diagonally connected forming a unit and
those units are arranged in a square lattice. The magnetization dynamics of the
sample is measured by using time-resolved magneto-optical Kerr effect
microscope with varying magnitude and in-plane orientation ($\phi$) of the bias
magnetic field. Interestingly, at $\phi=0^{\circ}$, the spin-wave mode profiles
show frequency selective spatial localization of spin-wave power within the
array. With the variation of $\phi$ in the range $0^{\circ}<\phi\leq
45^{\circ}$, we observe band narrowing due to localized to extended spin-wave
mode conversion. Upon further increase of $\phi$, the spin-wave modes slowly
lose the extended nature and become fully localized again at 90$^{\circ}$. We
have extensively demonstrated the role of magnetostatic stray field
distribution on the rotational symmetries obtained for the spin-wave modes.
From micromagnetic simulations, we find that the dipole-exchange coupling
between the nano-dots leads to remarkable modifications of the spin-wave mode
profiles when compared with arrays of individual small and large dots.
Numerically, we also show that the physical connection between the nano-dots
provides more control points over the spin-wave propagation in the lattice at
different orientations of bias magnetic field. This new type of binary magnonic
crystal may find potential applications in magnonic devices such as spin-wave
waveguide, filter, coupler, and other on-chip microwave communication devices.",1901.09325v1
2019-02-11,Spin-wave phase inverter upon a single nanodefect,"Local modification of magnetic properties of nanoelements is a key to design
future-generation magnonic devices, in which information is carried and
processed via spin waves. One of the biggest challenges here is to fabricate
simple and miniature phase-controlling elements with broad tunability. Here, we
successfully realize such spin-wave phase shifter upon a single nanogroove
milled by focused ion beam in a Co-Fe microsized magnonic waveguide. By varying
the groove depth and the in-plane bias magnetic field we continuously tune the
spin-wave phase and experimentally evidence a complete phase inversion. The
microscopic mechanism of the phase shift is based on the combined action of the
nanogroove as a geometrical defect and the lower spin-wave group velocity in
the waveguide under the groove where the magnetization is reduced due to the
incorporation of Ga ions during the ion-beam milling. The proposed phase
shifter can easily be on-chip integrated with spin-wave logic gates and other
magnonic devices. Our findings are crucial for designing nano-magnonic circuits
and for the development of spin-wave nano-optics.",1902.03758v1
2019-03-04,Consideration of Thermal Hall Effect in Undoped Cuprates,"A recent observation of thermal Hall effect of magnetic origin in underdoped
cuprates calls for critical re-examination of low-energy magnetic dynamics in
undoped antiferromagnetic compound on square lattice, where traditional,
renormalized spin-wave theory was believed to work well. Using
Holstein-Primakoff boson formalism, we find that magnon-based theories can lead
to finite Berry curvature in the magnon band once the Dzyaloshinskii-Moriya
spin interaction is taken into account explicitly, but fail to produce non-zero
thermal Hall conductivity. Assuming accidental doping by impurities and magnon
scattering off of such impurity sites fails to predict skew scattering at the
level of Born approximation. Local formation of skyrmion defects is also found
incapable of generating magnon thermal Hall effect. Turning to spinon-based
scenario, we write down a simple model by adding spin-dependent diagonal
hopping to the well-known {\pi}-flux model of spinons. The resulting two-band
model has Chern number in the band structure, and generates thermal Hall
conductivity whose magnetic field and temperature dependences mimic closely the
observed thermal Hall signals. In disclaimer, there is no firm microscopic
basis of this model and we do not claim to have found an explanation of the
data, but given the unexpected nature of the experimental observation, it is
hoped this work could serve as a first step towards reaching some level of
understanding.",1903.01125v3
2019-03-31,Control of the Coupling Strength and the Linewidth of a Cavity-Magnon Polariton,"The full coherent control of hybridized systems such as strongly coupled
cavity photon-magnon states is a crucial step to enable future information
processing technologies. Thus, it is particularly interesting to engineer
deliberate control mechanisms such as the full control of the coupling strength
as a measure for coherent information exchange. In this work, we employ cavity
resonator spectroscopy to demonstrate the complete control of the coupling
strength of hybridized cavity photon-magnon states. For this, we use two
driving microwave inputs which can be tuned at will. Here, only the first input
couples directly to the cavity resonator photons, whilst the second tone
exclusively acts as a direct input for the magnons. For these inputs, both the
relative phase $\phi$ and amplitude $\delta_0$ can be independently controlled.
We demonstrate that for specific quadratures between both tones, we can
increase the coupling strength, close the anticrossing gap, and enter a regime
of level merging. At the transition, the total amplitude is enhanced by a
factor of 1000 and we observe an additional linewidth decrease of $13\%$ at
resonance due to level merging. Such control of the coupling, and hence
linewidth, open up an avenue to enable or suppress an exchange of information
and bridging the gap between quantum information and spintronics applications.",1904.00393v1
2019-04-25,Skyrmion solids in monolayer graphene,"Partially filled Landau levels host competing orders, with electron solids
prevailing close to integer fillings before giving way to fractional quantum
Hall liquids as the Landau level fills. Here, we report the observation of an
electron solid with noncolinear spin texture in monolayer graphene, consistent
with solidification of skyrmions---topological spin textures characterized by
quantized electrical charge. In electrical transport, electron solids reveal
themselves through a rapid metal-insulator transition in the bulk electrical
conductivity as the temperature is lowered, accompanied by the emergence of
strongly nonlinear dependence on the applied bias voltage. We probe the spin
texture of the solids using a modified Corbino geometry that allows
ferromagnetic magnons to be launched and detected. We find that magnon
transport is highly efficient when one Landau level is filled ($\nu=1$),
consistent with quantum Hall ferromagnetic spin polarization; however, even
minimal doping immediately quenches the the magnon signal while leaving the
vanishing low-temperature charge conductivity unchanged. Our results can be
understood by the formation of a solid of charged skyrmions near $\nu=1$, whose
noncolinear spin texture leads to rapid magnon decay. Data near fractional
fillings further shows evidence for several fractional skyrmion crystals,
suggesting that graphene hosts a vast and highly tunable landscape of coupled
spin and charge orders.",1904.11485v1
2019-04-29,Magnon-Mediated Indirect Exciton Condensation through Antiferromagnetic Insulators,"Electrons and holes residing on the opposing sides of an insulating barrier
and experiencing an attractive Coulomb interaction can spontaneously form a
coherent state known as an indirect exciton condensate. We study a trilayer
system where the barrier is an antiferromagnetic insulator. The electrons and
holes here additionally interact via interfacial coupling to the
antiferromagnetic magnons. We show that by employing magnetically uncompensated
interfaces, we can design the magnon-mediated interaction to be attractive or
repulsive by varying the thickness of the antiferromagnetic insulator by a
single atomic layer. We derive an analytical expression for the critical
temperature $T_c$ of the indirect exciton condensation. Within our model,
anisotropy is found to be crucial for achieving a finite $T_c$, which increases
with the strength of the exchange interaction in the antiferromagnetic bulk.
For realistic material parameters, we estimate $T_c$ to be around 7 K, the same
order of magnitude as the current experimentally achievable exciton
condensation where the attraction is solely due to the Coulomb interaction. The
magnon-mediated interaction is expected to cooperate with the Coulomb
interaction for condensation of indirect excitons, thereby providing a means to
significantly increase the exciton condensation temperature range.",1904.12699v3
2019-08-01,Quasi-Two-Dimensional Magnon Identification in Antiferromagnetic FePS3 via Magneto-Raman Spectroscopy,"Recently it was discovered that van der Waals-bonded magnetic materials
retain long range magnetic ordering down to a single layer, opening many
avenues in fundamental physics and potential applications of these fascinating
materials. One such material is FePS3, a large spin (S=2) Mott insulator where
the Fe atoms form a honeycomb lattice. In the bulk, FePS3 has been shown to be
a quasi-two-dimensional-Ising antiferromagnet, with additional features in the
Raman spectra emerging below the Neel temperature of approximately 120 K. Using
magneto-Raman spectroscopy as an optical probe of magnetic structure, we show
that one of these Raman-active modes in the magnetically ordered state is
actually a magnon with a frequency of of approximately 3.7 THz (122 cm-1).
Contrary to previous work, which interpreted this feature as a phonon, our
Raman data shows the expected frequency shifting and splitting of the magnon as
a function of temperature and magnetic field, respectively, where we determine
the g-factor to be approximately 2. In addition, the symmetry behavior of the
magnon is studied by polarization-dependent Raman spectroscopy and explained
using the magnetic point group of FePS3.",1908.00608v1
2019-11-29,Real space observation of magnon interaction with driven space-time crystals,"The concept of Space-Time Crystals (STC), i.e. translational symmetry
breaking in time and space, was recently proposed and experimentally
demonstrated for quantum systems. Here, we transfer this concept to magnons and
experimentally demonstrate a driven STC at room temperature. The STC is
realized by strong homogeneous micro-wave pumping of a micron-sized permalloy
(Py) stripe and is directly imaged by Scanning Transmission X-ray Microscopy
(STXM). For a fundamental understanding of the formation of the STC,
micromagnetic simulations are carefully adapted to model the experimental
findings. Beyond the mere generation of a STC, we observe the formation of a
magnonic band structure due to back folding of modes at the STC's Brillouin
zone boundaries. We show interactions of magnons with the STC that appear as
lattice scattering. This results in the generation of ultra short spin waves
down to 100 nm wavelength that cannot be described by classical dispersion
relations for linear spin wave excitations. We expect that room temperature
STCs will be a useful tool to investigate non-linear wave physics, as they can
be easily generated and manipulated to control their spatial and temporal band
structure.",1911.13192v1
2019-12-16,Dynamics of reconfigurable artificial spin ice: towards magnonic functional materials,"Over the past few years, the study of magnetization dynamics in artificial
spin ices has become a vibrant field of study. Artificial spin ices are
ensembles of geometrically arranged, interacting magnetic nanoislands, which
display frustration by design. These were initially created to mimic the
behavior in rare earth pyrochlore materials and to study emergent behavior and
frustration using two-dimensional magnetic measurement techniques. Recently, it
has become clear that it is possible to create artificial spin ices, which can
potentially be used as functional materials. In this Perspective, we review the
resonant behavior of spin ices (which is in the GHz frequency range), focusing
on their potential application as magnonic crystals. In magnonic crystals, spin
waves are functionalized for logic applications by means of band structure
engineering. While it has been established that artificial spin ices can
possess rich mode spectra, the applicability of spin ices to create magnonic
crystals hinges upon their reconfigurability. Consequently, we describe recent
work aiming to develop techniques and create geometries allowing full
reconfigurability of the spin ice magnetic state. We also discuss experimental,
theoretical, and numerical methods for determining the spectral response of
artificial spin ices, and give an outlook on new directions for reconfigurable
spin ices.",1912.07280v2
2016-03-18,Magnons in Sr$_2$CuO$_3$: possible evidence for Goldstone-Higgs interaction in a weakly ordered spin-1/2 chain antiferromagnet,"The Goldstone theorem mandates that a spontaneous symmetry breaking entails
the emergence of gap(mass)less excitations. In the case where a rotational
invariance of a system of spin magnetic moments is broken by an
antiferromagnetic order, these are well-known transverse spin waves. The
interaction of such Goldstone magnons with the Higgs amplitude mode of the
order parameter is usually discarded, even though glimpses of Higgs physics
have recently been reported in a quantum magnet, a topological insulator, and
ferroelectric and disordered superconductor systems. The Goldstone-Higgs
interactions could be expected to grow in importance near a quantum critical
point (QCP), where the symmetry-breaking order is weak, and its amplitude
fluctuations are significant. Here we report an electron spin resonance (ESR)
study of a nearly one-dimensional spin-1/2 chain system, Sr$_2$CuO$_3$, which
presents exactly such a case. The ESR spectra at $T > T_N$, in the disordered
Luttinger-spin-liquid phase with unconfined spinons reveal ideal
Heisenberg-chain behavior with only very small, field-independent linewidth,
$\sim 1/T$. In the ordered state, below $T_N$, we identify antiferromagnetic
resonance (AFMR) modes, which are well described by pseudo-Goldstone magnons in
the model of a collinear biaxial antiferromagnet with two gaps, $\Delta_1 =
23.0$ GHz and $\Delta_2 = 13.3$ GHz. Additionally, we observe a major resonant
response of special nature, which we attribute to magnon interaction with the
Higgs amplitude mode in a weakly ordered antiferromagnet. Its unusual field
dependence indicates the presence of a quantum phase transition at $\mu_0 H
\simeq 9.4$ T.",1603.05869v2
2016-12-06,Thermal conductivity of local moment models with strong spin-orbit coupling,"We study the magnetic and lattice contributions to the thermal conductivity
of electrically insulating strongly spin-orbit coupled magnetically ordered
phases on a two-dimensional honeycomb lattice using the Kitaev-Heisenberg
model. Depending on model parameters, such as the relative strength of the
spin-orbit induced anisotropic coupling, a number of magnetically ordered
phases are possible. In this work, we study two distinct regimes of thermal
transport depending on whether the characteristic energy of the phonons or the
magnons dominates, and focus on two different relaxation mechanisms, boundary
scattering and magnon-phonon scattering. For spatially anisotropic magnetic
phases, the thermal conductivity tensor can be highly anisotropic when the
magnetic energy scale dominates, since the magnetic degrees of freedom dominate
the thermal transport for temperatures well below the magnetic transition
temperature. In the opposite limit in which the phonon energy scale dominates,
the thermal conductivity will be nearly isotropic, reflecting the isotropic (at
low temperatures) phonon dispersion assumed for the honeycomb lattice. We
further discuss the extent to which thermal transport properties are influenced
by strong spin-orbit induced anisotropic coupling in the local moment regime of
insulating magnetic phases. The developed methodology can be applied to any 2D
magnon-phonon system, and more importantly to systems where an analytical
Bogoliubov transformation cannot be found and magnon bands are not necessarily
isotropic.",1612.01658v1
2018-10-10,Photo-induced Floquet Weyl magnons in noncollinear antiferromagnets,"We study periodically driven insulating noncollinear stacked kagome
antiferromagnets with a conventional symmetry-protected three-dimensional (3D)
in-plane $120^\circ$ spin structure, with either positive or negative vector
chirality. We show that the symmetry protection of the in-plane $120^\circ$
spin structure can be broken in the presence of an off-resonant circularly or
linearly polarized electric field propagating parallel to the in-plane
$120^\circ$ spin structure (say along the $x$ direction). Consequently,
topological Floquet Weyl magnon nodes with opposite chirality are photoinduced
along the $k_x$ momentum direction. They manifest as the monopoles of the
photoinduced Berry curvature. We also show that the system exhibits a
photoinduced magnon thermal Hall effect for circularly polarized electric
field. Furthermore, we show that the photoinduced chiral spin structure is a
canted 3D in-plane $120^\circ$ spin structure, which was recently observed in
the equilibrium noncollinear antiferromagnetic Weyl semimetals Mn$_3$Sn\slash
Ge. Our result not only paves the way towards the experimental realization of
Weyl magnons and photoinduced thermal Hall effects, but also provides a
powerful mechanism for manipulating the intrinsic properties of 3D topological
antiferromagnets.",1810.04675v5
2019-05-03,Optical excitation of magnons in an easy-plane antiferromagnet: Application to Sr$_2$IrO$_4$,"We study the interaction of a (classical) light field with the magnetic
degrees of freedom in the two-dimensional antiferromagnet Sr$_2$IrO$_4$. The
reduced space group symmetry of the crystal allows for several channels for
spin-operator bilinears to couple to the electric field. Integrating out
high-energy degrees of freedom in a Keldysh framework, we derive induced
effective fields which enter the equations of motion of the low-energy mode of
in-plane rotations which couple to the out-of-plane magnetization. Considering
a pump-probe protocol, these induced fields excite magnetization oscillations
which can subsequently probed, e.g. using Kerr rotation. We discuss how the
induced fields depend on polarization and frequency of the driving light, and
our study applies to both resonant and non-resonant regimes. Crucially, the
induced fields depend on the two-magnon density of states, thus allowing for
further insight into properties of the magnetic excitation spectrum.
Furthermore, these effects rely upon (weak) magnon-interactions, and so are
beyond a ""Floquet magnon"" description.",1905.01313v3
2019-05-27,Influence of nonmagnetic dielectric spacers on the spin wave response of one-dimensional planar magnonic crystals,"The one-dimensional planar magnonic crystals are usually fabricated as a
sequence of stripes intentionally or accidentally separated by non-magnetic
spacers. The influence of spacers on shaping the spin wave spectra is complex
and still not completely clarified. We performed the detailed numerical studies
of the one-dimensional single- and bi-component magnonic crystals comprised of
a periodic array of thin ferromagnetic stripes separated by non-magnetic
spacers. We showed that the dynamic dipolar interactions between the stripes
mediated by non-magnetic spacer, even ultra-narrow, significantly shift up the
frequency of the ferromagnetic resonance and simultaneously reduce the spin
wave group velocity, which is manifested by the flattening of the magnonic
band. We attributed these changes in the spectra to the modifications of
dipolar pinning and shape anisotropy both dependent on the width of the spacers
and the thickness of the stripes, as well as to the dynamical magnetic volume
charges formed due to inhomogeneous spin wave amplitude.",1905.11016v4
2019-07-31,Ferromagnetism and its stability from the one-magnon spectrum in twisted bilayer graphene,"We study ferromagnetism and its stability in twisted bilayer graphene. We
work with a Hubbard-like interaction that corresponds to the screened Coulomb
interaction in a well-defined limit where the Thomas-Fermi screening length
$l_\text{TF}$ is much larger than monolayer graphene's lattice spacing $l_g \ll
l_\text{TF}$ and much smaller than the Moir\'e super lattice's spacing $
l_\text{TF} \ll l_{\text{Moir\'e}}$. We show that in the perfectly flat band
""chiral"" limit and at filling fractions $\pm 3/4$, the saturated ferromagnetic
(spin and valley polarized) states are ideal ground states candidates in the
large band-gap limit. By assuming a large enough substrate (hBN) induced
sub-lattice potential, the same argument can be applied to filling fractions
$\pm 1/4$. We estimate the regime of stability of the ferromagnetic phase
around the chiral limit by studying the exactly calculated spectrum of
one-magnon excitations. The instability of the ferromagnetic state is signaled
by a negative magnon excitation energy. This approach allows us to deform the
results of the idealized chiral model (by increasing the bandwidth and/or
modified interactions) towards more realistic systems. Furthermore, we use the
low energy part of the exact one-magnon spectrum to calculate the
spin-stiffness of the Goldstone modes throughout the ferromagnetic phase. The
calculated value of spin-stiffness can determine the excitation energy of
charged skyrmions.",1907.13633v1
2020-01-15,Nonreciprocal surface acoustic wave propagation via magneto-rotation coupling,"One of the most fundamental forms of magnon-phonon interaction is an
intrinsic property of magnetic materials, the ""magnetoelastic coupling"". This
particular form of interaction has been the basis for describing magnetic
materials and their strain related applications, where strain induces changes
of internal magnetic fields. Different from the magnetoelastic coupling, more
than 40 years ago, it was proposed that surface acoustic waves may induce
surface magnons via rotational motion of the lattice in anisotropic magnets.
However, a signature of this magnon-phonon coupling mechanism, termed
magneto-rotation coupling, has been elusive. Here, we report the first
observation and theoretical framework of the magneto-rotation coupling in a
perpendicularly anisotropic ultra-thin film Ta/CoFeB(1.6 nm)/MgO, which
consequently induces nonreciprocal acoustic wave attenuation with a
unprecedented ratio up to 100$\%$ rectification at the theoretically predicted
optimized condition. Our work not only experimentally demonstrates a
fundamentally new path for investigating magnon-phonon coupling, but also
justify the feasibility of the magneto-rotation coupling based application.",2001.05135v3
2020-03-03,Van Hove singularity in the magnon spectrum of the antiferromagnetic quantum honeycomb lattice,"The magnetic excitation spectrum of the quantum magnet YbCl$_3$ is studied
with inelastic neutron scattering. The spectrum exhibits an unusually sharp
feature within a broad continuum, as well as conventional spin waves. By
including both transverse and longitudinal channels of the neutron response,
linear spin wave theory with a single Heisenberg interaction on the honeycomb
lattice reproduces all of the key features in the spectrum. In particular, the
broad continuum corresponds to a two-magnon contribution from the longitudinal
channel, while the sharp feature within this continuum is identified as a Van
Hove singularity in the joint density of states, which indicates the
two-dimensional nature of the two-magnon continuum. We term these singularities
magneto-caustic features in analogy with caustic features in ray optics where
focused envelopes of light are generated when light passes through or reflects
from curved or distorted surfaces. The experimental demonstration of a sharp
Van Hove singularity in a two-magnon continuum is important because analogous
features in potential two-spinon continua could distinguish quantum spin
liquids from merely disordered systems. These results establish YbCl$_3$ as a
nearly ideal two-dimensional honeycomb lattice material hosting strong quantum
effects in the unfrustrated limit.",2003.01754v1
2020-03-15,Magnetic-field-induced tunability of spin Hamiltonians: Resonances and Efimov states in Yb$_2$Ti$_2$O$_7$,"Universality is a powerful concept that arises from the divergence of a
characteristic length scale. For condensed matter systems, this length scale is
typically the correlation length, which diverges at critical points separating
two different phases. Few-particle systems exhibit a simpler form of
universality when the $s$-wave scattering length diverges. A prominent example
of universal phenomena is the emergence of an infinite tower of three-body
bound states obeying discrete scale invariance, known as the Efimov effect,
which has been subject to extensive research in chemical, atomic, nuclear and
particle physics. In principle, these universal phenomena can also emerge in
the excitation spectrum of condensed matter systems, such as quantum
magnets~[Y. Nishida, Y. Kato, and C. Batista, Nat. Phys. 9, 93 (2013)].
However, the limited tunability of the effective inter-particle interaction
relative to the kinetic energy has precluded so far their observation. Here we
demonstrate that a high degree of magnetic-field-induced tunability can also be
achieved in quantum magnets with strong spin-orbit coupling: a two-magnon
resonance condition can be achieved in Yb$_2$Ti$_2$O$_7$ with a field of $\sim$
13~T along the [110] direction, which leads to the formation of Efimov states
in the three-magnon spectrum of this material. Raman scattering experiments can
reveal the field-induced two-magnon resonance, as well as the Efimov
three-magnon bound states that emerge near the resonance condition.",2003.06738v2
2020-04-16,A Magnon Scattering Platform,"Scattering experiments have revolutionized our understanding of nature.
Examples include the discovery of the nucleus, crystallography, and the
discovery of the double helix structure of DNA. Scattering techniques differ by
the type of the particles used, the interaction these particles have with
target materials and the range of wavelengths used. Here, we demonstrate a new
2-dimensional table-top scattering platform for exploring magnetic properties
of materials on mesoscopic length scales. Long lived, coherent magnonic
excitations are generated in a thin film of YIG and scattered off a magnetic
target deposited on its surface. The scattered waves are then recorded using a
scanning NV center magnetometer that allows sub-wavelength imaging and
operation under conditions ranging from cryogenic to ambient environment. While
most scattering platforms measure only the intensity of the scattered waves,
our imaging method allows for spatial determination of both amplitude and phase
of the scattered waves thereby allowing for a systematic reconstruction of the
target scattering potential. Our experimental results are consistent with
theoretical predictions for such a geometry and reveal several unusual features
of the magnetic response of the target, including suppression near the target
edges and gradient in the direction perpendicular to the direction of surface
wave propagation. Our results establish magnon scattering experiments as a new
platform for studying correlated many-body systems.",2004.07763v1
2020-08-12,Strongly two-dimensional exchange interactions in the in-plane metallic antiferromagnet Fe$_2$As probed by inelastic neutron scattering,"To understand spin interactions in materials of the Cu$_2$Sb structure type,
inelastic neutron scattering of Fe$_2$As single crystals was examined at
different temperatures and incident neutron energies. The experimental phonon
spectra match well with the simulated phonon spectra obtained from density
functional theory (DFT) calculations. The measured magnon spectra were compared
to the simulated magnon spectra obtained via linear spin wave theory with the
exchange coupling constants calculated using the spin polarized, relativistic
Korringa-Kohn-Rostoker method in Zhang et al. (2013). The simulated magnon
spectra broadly agree with the experimental data although, the energy values
are underestimated along the $K$ direction. Exchange coupling constants between
Fe atoms were refined by fits to the experimental magnon spectra, revealing
stronger nearest neighbor Fe1-Fe1 exchange coupling than previously reported.
The strength of this exchange coupling is almost an order of magnitude higher
than other exchange interactions despite the three-dimensional nature of the
phonon interactions. The lack of scattering intensity at energies above 60 meV
makes unconstrained determination of the full set of exchange interactions
difficult, which may be a fundamental challenge in metallic antiferromagnets.",2008.05050v2
2020-08-21,Magnons Parametric Pumping in Bulk Acoustic Waves Resonator,"We report on the experimental observation of excitation and detection of
parametric spin waves and spin currents in the bulk acoustic wave resonator.
The hybrid resonator consists of ZnO piezoelectric film, yttrium iron garnet
(YIG) films on gallium gadolinium garnet substrate, and a heavy metal Pt layer.
Shear bulk acoustic waves are electrically excited in the ZnO layer due to
piezoeffect at the resonant frequencies of the resonator. The magnetoelastic
interaction in the YIG film emerges magnons (spin waves) excitation by acoustic
waves either on resonator's eigenfrequencies or the half-value frequencies at
supercritical power. We investigate acoustic pumping of magnons at the
half-value frequencies and acoustic spin pumping from parametric magnons, using
the inverse spin Hall effect in the Pt layer. The constant electric voltage in
the Pt layer, depending on the frequency, the magnetic field, and the pump
power, was systematically studied. We explain the low threshold obtained (~0.4
mW) by the high efficiency of electric power transmission into the acoustic
wave in the resonator.",2008.09520v1
2020-08-24,Ultrastrong Magnon-Magnon Coupling Dominated by Antiresonant Interactions,"Exotic quantum vacuum phenomena are predicted in cavity quantum
electrodynamics (QED) systems with ultrastrong light-matter interactions. Their
ground states are predicted to be vacuum squeezed states with suppressed
quantum fluctuations. The source of such phenomena are antiresonant terms in
the Hamiltonian, yet antiresonant interactions are typically negligible
compared to resonant interactions in light-matter systems. We report an unusual
coupled matter-matter system of magnons that can simulate a unique cavity QED
Hamiltonian with coupling strengths that are easily tunable into the
ultrastrong coupling regime and with dominant antiresonant terms. We found a
novel regime where vacuum Bloch-Siegert shifts, the hallmark of antiresonant
interactions, greatly exceed analogous frequency shifts from resonant
interactions. Further, we theoretically explored the system's ground state and
calculated up to 5.9 dB of quantum fluctuation suppression. These observations
demonstrate that magnonic systems provide an ideal platform for simulating
exotic quantum vacuum phenomena predicted in ultrastrongly coupled light-matter
systems.",2008.10721v2
2020-11-19,An insulating doped antiferromagnet with low magnetic symmetry as a room temperature spin conduit,"We report room temperature long-distance spin transport of magnons in
antiferromagnetic thin film hematite doped with Zn. The additional dopants
significantly alter the magnetic anisotropies, resulting in a complex
equilibrium spin structure that is capable of efficiently transporting spin
angular momentum at room temperature without the need for a well-defined, pure
easy-axis or easy-plane anisotropy. We find intrinsic magnon spin-diffusion
lengths of up to 1.5 {\mu}m, and magnetic domain governed decay lengths of 175
nm for the low frequency magnons, through electrical transport measurements
demonstrating that the introduction of non-magnetic dopants does not strongly
reduce the transport length scale showing that the magnetic damping of hematite
is not significantly increased. We observe a complex field dependence of the
non-local signal independent of the magnetic state visible in the local
magnetoresistance and direct magnetic imaging of the antiferromagnetic domain
structure. We explain our results in terms of a varying and
applied-field-dependent ellipticity of the magnon modes reaching the detector
electrode allowing us to tune the spin transport.",2011.09755v1
2021-01-29,Comparison of Spin-Wave Modes in Connected and Disconnected Artificial Spin Ice Nanostructures Using Brillouin Light Scattering Spectroscopy,"Artificial spin ice systems have seen burgeoning interest due to their
intriguing physics and potential applications in reprogrammable memory, logic
and magnonics. Integration of artificial spin ice with functional magnonics is
a relatively recent research direction, with a host of promising results. As
the field progresses, direct in-depth comparisons of distinct artificial spin
systems are crucial to advancing the field. While studies have investigated the
effects of different lattice geometries, little comparison exists between
systems comprising continuously connected nanostructures, where spin-waves
propagate via dipole-exchange interaction, and systems with nanobars
disconnected at vertices where spin-wave propagation occurs via stray
dipolar-field. Gaining understanding of how these very different coupling
methods affect both spin-wave dynamics and magnetic reversal is key for the
field to progress and provides crucial system-design information including for
future systems containing combinations of connected and disconnected elements.
Here, we study the magnonic response of two kagome spin ices via Brillouin
light scattering, a continuously connected system and a disconnected system
with vertex gaps. We observe distinct high-frequency dynamics and magnetization
reversal regimes between the systems, with key distinctions in spin-wave
localization and mode quantization, microstate-trajectory during reversal and
internal field-profiles. These observations are pertinent for the fundamental
understanding of artificial spin systems and broader design and engineering of
reconfigurable functional magnonic crystals.",2101.12619v2
2021-03-10,Eigenmodes of a disordered FeCo magnonic crystal at finite temperatures,"In this report we present a systematic study of the magnonic modes in the
disordered Fe$_{0.5}$Co$_{0.5}$ alloy based on the Heisenberg Hamiltonian using
two complementary approaches. In order to account for substitutional disorder,
on the one hand we directly average the transverse magnetic susceptibility in
real space over different disorder configurations and on the other hand we use
the coherent potential approximation (CPA). While the method of direct
averaging is numerically exact, it is computationally expensive and limited by
the maximal size of the supercell which can be simulated on a computer. On the
contrary the CPA does not suffer from this drawback and yields a cheap
numerical scheme. Therefore, we additionally compare the results of these two
approaches and show that the CPA gives very good results for most of the
magnetic properties, including the magnon energies and the spatial shape of the
eigenmodes. However, it turns out that while reproducing the general trend, the
CPA systematically underestimates the disorder induced damping of the magnons.
This provides evidence that the physics of impurity scattering in this system
is governed by non-local effects missing in the CPA. Finally, we study the real
space eigenmodes of the system, including their spatial shapes, and analyze
their temperature dependence within the random phase approximation.",2103.05967v2
2021-03-29,Electrically switchable entanglement channel in van der Waals magnets,"Two dimensional layered van der Waals (vdW) magnets have demonstrated their
potential to study both fundamental and applied physics due to their remarkable
electronic properties. However, the connection of vdW magnets to spintronics as
well as quantum information science is not clear. In particular, it remains
elusive whether there are novel magnetic phenomena only belonging to vdW
magnets, but absent in the widely studied crystalline magnets. Here we consider
the quantum correlations of magnons in a layered vdW magnet and identify an
entanglement channel of magnons across the magnetic layers, which can be
effectively tuned and even deterministically switched on and off by both
magnetic and electric means. This is a unique feature of vdW magnets in which
the underlying physics is well understood in terms of the competing roles of
exchange and anisotropy fields that contribute to the magnon excitation.
Furthermore, we show that such a tunable entanglement channel can mediate the
electrically controllable entanglement of two distant qubits, which also
provides a protocol to indirectly measure the entanglement of magnons. Our
findings provide a novel avenue to electrically manipulate the qubits and
further open up new opportunities to utilize vdW magnets for quantum
information science.",2103.15899v1
2021-06-29,Neutron Scattering Study on Yttrium Iron Garnet for Spintronics,"Spin current -- a flow of the spin degree of freedom in matter -- has vital
importance in spintronics. Propagation of the spin current ranges over a whole
momentum space; however, generated spin currents are mainly detected in the
long-wavelength limit. To facilitate practical uses of spintronics and
magnonics, microscopic understanding of the spin current is necessary. We here
address yttrium iron garnet, which is a well-employed ferrimagnet for
spintronics, and review {\it in re} the momentum- and energy-resolved
characteristics of its magnetism. Using {\it unpolarized} neutrons, we refined
its detailed crystal and magnetic structure, and examined magnetic excitations
through four decades (10~$\mu$eV-100~meV) using chopper spectrometers in
J-PARC, Japan. We also measured mode-resolved directions of the precessional
motion of the magnetic moment, i.e., magnon polarization, which carries the
spin current in insulators through {\it polarized} neutron scattering, using a
triple-axis spectrometer in ILL, France. The magnon polarization is a hitherto
untested fundamental property of magnets, affecting the thermodynamic
properties of the spin current. Our momentum- and energy-resolved experimental
findings provide an intuitive understanding of the spin current and demonstrate
the importance of neutron scattering techniques for spintronics and magnonics.",2106.15752v1
2021-06-30,Origin of Nonlinear Damping due to Mode Coupling in Auto-Oscillatory Modes Strongly Driven by Spin-Orbit Torque,"We investigate the physical origin of nonlinear damping due to mode coupling
between several auto-oscillatory modes driven by spin-orbit torque in
constricted Py/Pt heterostructures by examining the dependence of
auto-oscillation on temperature and applied field orientation. We observe a
transition in the nonlinear damping of the auto-oscillation modes extracted
from the total oscillation power as a function of drive current, which
coincides with the onset of power redistribution amongst several modes and the
crossover from linewidth narrowing to linewidth broadening in all individual
modes. This indicates the activation of another relaxation process by nonlinear
magnon-magnon scattering within the modes. We also find that both nonlinear
damping and threshold current in the mode-interaction damping regime at high
drive current after transition are temperature independent, suggesting that the
mode coupling occurs dominantly through a non-thermal magnon scattering process
via a dipole or exchange interaction rather than thermally excited
magnon-mediated scattering. This finding presents a promising pathway to
overcome the current limitations of efficiently controlling the interaction
between two highly nonlinear magnetic oscillators to prevent mode crosstalk or
inter-mode energy transfer and deepens understanding of complex nonlinear spin
dynamics in multimode spin wave systems.",2107.00150v2
2021-07-14,Frequency fluctuations of ferromagnetic resonances at milliKelvin temperatures,"Unwanted fluctuations over time, in short, noise, are detrimental to device
performance, especially for quantum coherent circuits. Recent efforts have
demonstrated routes to utilizing magnon systems for quantum technologies, which
are based on interfacing single magnons to superconducting qubits. However, the
coupling of several components often introduces additional noise to the system,
degrading its coherence. Researching the temporal behavior can help to identify
the underlying noise sources, which is a vital step in increasing coherence
times and the hybrid device performance. Yet, the frequency noise of the
ferromagnetic resonance (FMR) has so far been unexplored. Here, we investigate
such FMR frequency fluctuations of a YIG sphere down to mK-temperatures, and
find them independent of temperature and drive power. This suggests that the
measured frequency noise in YIG is dominated by so far undetermined noise
sources, which properties are not consistent with the conventional model of
two-level systems, despite their effect on the sample linewidth. Moreover, the
functional form of the FMR frequency noise power spectral density (PSD) cannot
be described by a simple power law. By employing time-series analysis, we find
a closed function for the PSD that fits our observations. Our results underline
the necessity of coherence improvements to magnon systems for useful
applications in quantum magnonics.",2107.06531v1
2021-08-04,Theory of quantum entanglement and the structure of two-mode squeezed antiferromagnetic magnon vacuum,"Recent investigations of the quantum properties of an antiferromagnet in the
spin wave approximation have identified the eigenstates as two-mode squeezed
sublattice states. The uniform squeezed vacuum and one-magnon states were shown
to display a massive sublattice entanglement. Here we expand this investigation
and study the squeezing properties of all sublattice Fock states throughout the
magnetic Brillouin zone. We derive the full statistics of the sublattice magnon
number with wave number $\vec k$ in the ground state and show that magnons are
created in pairs with opposite wavevectors, hence, resulting in entanglement of
both modes. To quantify the degree of entanglement we apply the
Duan-Giedke-Cirac-Zoller inequality and show that it can be violated for all
modes. The degree of entanglement decrease towards the corners of the Brillouin
zone. We relate the entanglement to measurable correlations of components of
the N\'eel and the magnetization vectors, thus, allowing to experimentally test
the quantum nature of the squeezed vacuum. The distinct $\vec k$-space
structure of the probabilites shows that the squeezed vacuum has a nonuniform
shape that is revealed through the $\vec k$-dependent correlators for the
magnetization and the N\'eel vectors.",2108.01942v3
2021-08-23,Dzyaloshinskii-Moriya anisotropy effect on field-induced magnon condensation in kagome antiferromagnet $α-Cu_3Mg(OH)_6Br_2$,"We performed a comprehensive electron spin resonance, magnetization and heat
capacity study on the field-induced magnetic phase transitions in the kagome
antiferromagnet $\alpha-Cu_3Mg(OH)_6Br_2$. With the successful preparation of
single crystals, we mapped out the magnetic phase diagrams under the $c$-axis
and $ab$-plane directional magnetic fields $B$. For $B\|c$, the
three-dimensional (3D) magnon Bose-Einstein condensation (BEC) is evidenced by
the power law scaling of the transition temperature, $T_c\propto
(B_c-B)^{2/3}$. For $B\|ab$, the transition from the canted antiferromagetic
(CAFM) state to the fully polarized (FP) state is a crossover rather than phase
transition, and the characteristic temperature has a significant deviation from
the 3D BEC scaling. The different behaviors of the field-induced magnetic
transitions for $B\|c$ and $B\|ab$ could result from the Dzyaloshinkii-Moriya
(DM) interaction with the DM vector along the $c$-axis, which preserves the
$c$-axis directional spin rotation symmetry and breaks the spin rotation
symmetry when $B\|ab$. The 3D magnon BEC scaling for $B\|c$ is immune to the
off-stoichiometric disorder in our sample
$\alpha-Cu_{3.26}Mg_{0.74}(OH)_6Br_2$. Our findings have the potential to shed
light on the investigations of the magnetic anisotropy and disorder effects on
the field-induced magnon BEC in the quantum antiferromagnet.",2108.10020v1
2021-10-29,4D visualization of the photoexcited coherent magnon by an X-ray free electron laser,"X-ray free electron lasers (XFEL) create femtosecond X-ray pulses with high
brightness and high longitudinal coherence allowing to extend X-ray
spectroscopy and scattering techniques into the ultrafast time-domain. These
X-rays are a powerful probe for studying coherent quasiparticle excitations in
condensed matter triggered by an impulsive optical laser pump. However, unlike
coherent phonons, other quasiparticles have been rarely observed due to small
signal changes and lack of standards for the identification. Here, we exploit
resonant magnetic X-ray diffraction using an XFEL to visualize a photoexcited
coherent magnon in space and time. Large intensity oscillations in
antiferromagnetic and ferromagnetic Bragg reflections from precessing moment
are observed in a multiferroic Y-type hexaferrite. The precession trajectory
reveals that a large, long-lived, photoinduced magnetic-field changes the net
magnetization substantially through the large-amplitude of the magnon. This
work demonstrates an efficient XFEL probe for the coherent magnon in the
spotlight for opto-spintronics application.",2110.15626v1
2022-01-26,Multi-band Bose-Einstein condensate at four-particle scattering resonance,"Superfluidity and superconductivity are macroscopic manifestations of quantum
mechanics, which have fascinated scientists since their discoveries roughly a
century ago. Ever since the initial theories of such quantum fluids were
formulated, there has been speculation as to the possibility of multi-component
quantum order. A particularly simple multi-component condensate is built from
particles occupying different quantum states, or bands, prior to condensation.
The particles in one or both bands may undergo condensation, as seen for
certain solids and anticipated for certain cold atom systems. For bulk solids,
the different bands always order simultaneously, with conventional pairing
characterized by complex order parameters describing the condensates in each
band. Another type of condensate, notably occurring at room temperature, has
been identified for magnons, the magnetic analogue of lattice vibrations,
injected by microwaves into yttrium iron garnet. Here we show that magnon
quantization for thin samples results in a new multi-band magnon condensate. We
establish a phase diagram, as a function of microwave drive power and frequency
relative to the magnon bands, revealing both single and multi-band
condensation. The most stable multi-band condensate is found in a narrow regime
favoured on account of a resonance in the scattering between two bands. Our
discovery introduces a flexible non-equilibrium platform operating at room
temperature for a well-characterised material, exploiting a Feshbach-like
resonance, for examining multi-band phenomena. It points to qualitatively new
ways to engineer and control condensates and superconducting states in
multiband systems and potential devices containing multiple interacting
condensates.",2201.11043v1
2022-03-24,Writable spin wave nanochannels in an artificial-spin-ice-mediated ferromagnetic thin film,"Magnonics, which employs spin-waves to transmit and process information, is a
promising venue for low-power data processing. One of the major challenges is
the local control of the spin-wave propagation path. Here, we introduce the
concept of writable magnonics by taking advantage of the highly flexible
reconfigurability and rewritability of artificial spin ice systems. Using
micromagnetic simulations, we show that globally switchable spin-wave
propagation and the locally writable spin-wave nanochannels can be realized in
a ferromagnetic thin film underlying an artificial pinwheel spin ice. The
rewritable magnonics enabled by reconfigurable spin wave nanochannels provides
a unique setting to design programmable magnonic circuits and logic devices for
ultra-low power applications.",2203.12812v2
2022-04-26,A puzzling insensitivity of magnon spin diffusion to the presence of 180$^\circ$ domain walls in a ferrimagnetic insulator,"We present room-temperature measurements of magnon spin diffusion in
epitaxial ferrimagnetic insulator MgAl$_{0.5}$Fe$_{1.5}$O$_{4}$ (MAFO) thin
films near zero applied magnetic field where the sample forms a multi-domain
state. Due to a weak uniaxial magnetic anisotropy, the domains are separated
primarily by 180$^\circ$ domain walls. We find, surprisingly, that the presence
of the domain walls has very little effect on the spin diffusion -- nonlocal
spin transport signals in the multi-domain state retain at least 95% of the
maximum signal strength measured for the spatially-uniform magnetic state, over
distances at least five times the typical domain size. This result is in
conflict with simple models of interactions between magnons and static domain
walls, which predict that the spin polarization carried by the magnons reverses
upon passage through a 180$^\circ$ domain wall.",2204.12377v1
2022-09-21,Dynamics of photo-induced ferromagnetism in oxides with orbital degeneracy,"By using intense coherent electromagnetic radiation, it may be possible to
manipulate the properties of quantum materials very quickly, or even induce new
and potentially useful phases that are absent in equilibrium. For instance,
ultrafast control of magnetic dynamics is crucial for a number of proposed
spintronic devices and can also shed light on the possible dynamics of
correlated phases out of equilibrium. Inspired by recent experiments on
spin-orbital ferromagnet YTiO$_3$ we consider the nonequilibrium dynamics of
Heisenberg ferromagnetic insulator with low-lying orbital excitations. We model
the dynamics of the magnon excitations in this system following an optical
pulse which resonantly excites infrared-active phonon modes. As the phonons
ring down they can dynamically couple the orbitals with the low-lying magnons,
leading to a dramatically modified effective bath for the magnons. We show this
transient coupling can lead to a dynamical acceleration of the magnetization
dynamics, which is otherwise bottlenecked by small anisotropy. Exploring the
parameter space more we find that the magnon dynamics can also even completely
reverse, leading to a negative relaxation rate when the pump is blue-detuned
with respect to the orbital bath resonance. We therefore show that by using
specially targeted optical pulses, one can exert a much greater degree of
control over the magnetization dynamics, allowing one to optically steer
magnetic order in this system. We conclude by discussing interesting parallels
between the magnetization dynamics we find here and recent experiments on
photo-induced superconductivity, where it is similarly observed that depending
on the initial pump frequency, an apparent metastable superconducting phase
emerges.",2209.10567v1
2022-11-21,Magnetic monopole induced polarons in atomic superlattices,"Magnetic monopoles have been realized as emergent quasiparticles in both
condensed matter and ultracold atomic platforms, with growing interests in the
coupling effects between the monopole and different magnetic quasiparticles. In
this work, interaction effects between monopoles and magnons are investigated
for an atomic pseudospin chain. We reveal that the monopole can excite a
virtual magnon cloud in the paramagnetic chain, thereby giving rise to a new
type of polaron, the monopole-cored polaron (McP). The McP is composed of the
monopole as the impurity core and the virtual magnon excitation as the dressing
cloud. The magnon dressing facilitates the Dirac string excitation and impacts
the monopole hopping. This induces an anti-trapping effect of the McP, which
refers to the fact that the dressing enhances the mobility of the McP, in
contrast to the self-trapping of the common polarons. Moreover, heterogeneous
bipolarons are shown to exist under the simultaneous doping of a north and a
south monopole. The heterogeneous bipolaron possesses an inner degree of
freedom composed of two identical impurities. Our investigation sheds light on
the understanding of how the coupling between the impurity core and the
dressing cloud can engineer the property of the polaron",2211.11481v1
2023-01-16,Antiferromagnetic magnon spintronic based on non-reciprocal and non-degenerated ultra-fast spin-waves in the canted antiferromagnet α-Fe2O3,"Spin-waves in antiferromagnets hold the prospects for the development of
faster, less power-hungry electronics, as well as promising physics based on
spin-superfluids and coherent magnon-condensates. For both these perspectives,
addressing electrically coherent antiferromagnetic spin-waves is of importance,
a prerequisite that has so far been elusive, because unlike
ferromagnets,antiferromagnets couple weakly to radiofrequency fields. Here, we
demonstrate the detection of ultra-fast non-reciprocal spin-waves in the
dipolar-exchange regime of a canted antiferromagnet using both inductive and
spintronic transducers. Using time-of-flight spin-wave spectroscopy on hematite
({\alpha}-Fe2O3), we find that the magnon wave packets can propagate as fast as
20 km/s for reciprocal bulk spin-wave modes and up to 6 km/s for surface-spin
waves propagating parallel to the antiferromagnetic Neel vector. We finally
achieve efficient electrical detection of non-reciprocal spin-wave transport
using non-local inverse spin-Hall effects. The electrical detection of coherent
non-reciprocal antiferromagnetic spin waves paves the way for the development
of antiferromagnetic and altermagnet-based magnonic devices.",2301.06329v2
2023-01-27,Magnomechanical backaction corrections due to coupling to higher order Walker modes and Kerr nonlinearities,"The radiation pressure-like coupling between magnons and phonons in magnets
can modify the phonon frequency (magnomechanical spring effect) and decay rate
(magnomechanical decay) via dynamical backaction. Such effects have been
recently observed by coupling the uniform magnon mode of a magnetic sphere (the
Kittel mode) to a microwave cavity. In particular, the ability to evade
backaction effects was demonstrated [C.A. Potts et al., arXiv:2211.13766
[quant-ph] (2022)], a requisite for applications such as magnomechanical based
thermometry. However, deviations were observed from the predicted
magnomechanical decay rate within the standard theoretical model. In this work,
we account for these deviations by considering corrections due to (i) magnetic
Kerr nonlinearities and (ii) the coupling of phonons to additional magnon
modes. Provided that such additional modes couple weakly to the driven cavity,
our model yields a correction proportional to the average Kittel magnon mode
occupation. We focus our results on magnetic spheres, where we show that the
magnetostatic Walker modes couple to the relevant mechanical modes as
efficiently as the Kittel mode. Our model yields excellent agreement with the
experimental data.",2301.11920v4
2023-04-17,Emergent SU(3) magnons and thermal Hall effect in the antiferromagnetic skyrmion lattice,"Complexity of quantum phases of matter is often understood by the underlying
gauge structures, as was recognized by the $\mathbb{Z}_2$ and U(1) gauge theory
description of spin liquid in frustrated magnets. Anomalous Hall effect of
conducting electrons can intrisically arise from U(1) gauges expressing the
spatial modulation of ferromagnetic moments or from SU(2) gauges representing
the spin-orbit coupling effect. Similarly, in insulating ferro and
antiferromagnets, the magnon excitations can contribute to anomalous transports
by feeling the U(1) and SU(2) gauges arising from the features of ordered
moments or interactions. In this work, we report the emergent higher rank SU(3)
gauge structure in the magnon transport based on the thermal conductivity
measurements of MnSc$_2$S$_4$ in an applied field up to 14\,T. The thermal Hall
coefficient takes a substantial value when the material enters a
three-sublattice antiferromagnetic skyrmion phase, which is confirmed by the
large-scale spin wave theory. The excited magnons are dressed with SU(3) gauge
field, which is a mixture of three species of U(1) gauge fields originating
from the slowly varying magnetic moments on these sublattices.",2304.08029v1
2023-04-19,Real-time observation of magnetization and magnon dynamics in a two-dimensional topological antiferromagnet MnBi2Te4,"Atomically thin van der Waals magnetic materials have not only provided a
fertile playground to explore basic physics in the two-dimensional (2D) limit
but also created vast opportunities for novel ultrafast functional devices.
Here we systematically investigate ultrafast magnetization dynamics and spin
wave dynamics in few-layer topological antiferromagnetic MnBi2Te4 crystals as a
function of layer number, temperature, and magnetic field. We find
laser-induced (de)magnetization processes can be used to accurately track the
distinct magnetic states in different magnetic field regimes, including showing
clear odd-even layer number effects. In addition, strongly field-dependent
antiferromagnetic magnon modes with tens of gigahertz frequencies are optically
generated and directly observed in the time domain. Remarkably, we find that
magnetization and magnon dynamics can be observed in not only the time-resolved
magneto-optical Kerr effect but also the time resolved reflectivity, indicating
strong correlation between the magnetic state and electronic structure. These
measurements present the first comprehensive overview of ultrafast spin
dynamics in this novel 2D antiferromagnet, paving the way for potential
applications in 2D antiferromagnetic spintronics and magnonics as well as
further studies of ultrafast control of both magnetization and topological
quantum states.",2304.09390v2
2023-05-16,Magnon Hall effect in antiferromagnetic lattices,"Topology applied to condensed matter is an important area of research and
technology, and topological magnetic excitations have recently become an active
field of study. This paper presents a general discussion of magnon Hall
transport in two-dimensional antiferromagnets. Although the Chern number is
zero for a collinear antiferromagnet, we offer a general discussion that can be
used in the more general case. First, we study the Union Jack lattice, where an
effective time-reversal symmetry is broken, making the system display the
magnon Hall effect. Then, we investigate the brick-wall lattice where such
symmetry is present. Consequently, we have a phenomenon similar to the quantum
spin Hall effect in electronic systems. Both lattices have not yet been studied
from the topological point of view. The coexistence of opposite spin
polarization in an antiferromagnet resembles the electron spin in various
transport phenomena. We study magnon transport in the lattices mentioned above
with Dzyaloshinskii-Moriya interaction and easy-axis single-ion anisotropy. We
calculate the Berry curvature from the eigenvalues of the Hamiltonian. From
that, we plot the spin Hall and thermal Hall conductivities, as well as the
spin Nernst coefficient, as functions of the temperature. In the Union Jack
lattice, we treat the effect of anharmonic interactions using a mean-field spin
wave theory where the Hamiltonian becomes implicitly temperature-dependent. We
determine self-consistently the renormalized dispersion and the staggered
magnetization as a function of temperature. Our calculations can be applied to
other antiferromagnetic lattices.",2305.09077v2
2023-05-16,Non-Hermitian Casimir Effect of Magnons,"There has been a growing interest in non-Hermitian quantum mechanics. The key
concepts of quantum mechanics are quantum fluctuations. Quantum fluctuations of
quantum fields confined in a finite-size system induce the zero-point energy
shift. This quantum phenomenon, the Casimir effect, is one of the most striking
phenomena of quantum mechanics in the sense that there are no classical analogs
and has been attracting much attention beyond the hierarchy of energy scales,
ranging from elementary particle physics to condensed matter physics, together
with photonics. However, the non-Hermitian extension of the Casimir effect and
the application to spintronics have not yet been investigated enough, although
exploring energy sources and developing energy-efficient nanodevices are its
central issues. Here we fill this gap. By developing a magnonic analog of the
Casimir effect into non-Hermitian systems, we show that this non-Hermitian
Casimir effect of magnons is enhanced as the Gilbert damping constant (i.e.,
the energy dissipation rate) increases. When the damping constant exceeds a
critical value, the non-Hermitian Casimir effect of magnons exhibits an
oscillating behavior, including a beating one, as a function of the film
thickness and is characterized by the exceptional point. Our result suggests
that energy dissipation serves as a key ingredient of Casimir engineering.",2305.09231v1
2023-06-03,Manipulating chiral-spin transport with ferroelectric polarization,"A collective excitation of the spin structure in a magnetic insulator can
transmit spin-angular momentum with negligible dissipation. This quantum of a
spin wave, introduced more than nine decades ago, has always been manipulated
through magnetic dipoles, (i.e., timereversal symmetry). Here, we report the
experimental observation of chiral-spin transport in multiferroic BiFeO3, where
the spin transport is controlled by reversing the ferroelectric polarization
(i.e., spatial inversion symmetry). The ferroelectrically controlled magnons
produce an unprecedented ratio of up to 18% rectification at room temperature.
The spin torque that the magnons in BiFeO3 carry can be used to efficiently
switch the magnetization of adja-cent magnets, with a spin-torque efficiency
being comparable to the spin Hall effect in heavy metals. Utilizing such a
controllable magnon generation and transmission in BiFeO3, an alloxide,
energy-scalable logic is demonstrated composed of spin-orbit injection,
detection, and magnetoelectric control. This observation opens a new chapter of
multiferroic magnons and paves an alternative pathway towards low-dissipation
nanoelectronics.",2306.02185v1
2023-11-06,Effects of dilution in a 2D topological magnon insulator,"We study the effect of diluting a two-dimensional ferromagnetic insulator
hosting a topological phase in the clean limit. By considering the
ferromagnetic Heisenberg model in the honeycomb lattice with second
nearest-neighbor Dzyanshikii-Moriya interaction, and working in the linear
spin-wave approximation, we establish the topological phase diagram as a
function of the fraction $p$ of diluted magnetic atoms. The topological phase
with Chern number $C=1$ is robust up to a moderate dilution $p_{1}^{*}$, while
above a higher dilution $p_{2}^{*}>p_{1}^{*}$ the system becomes trivial.
Interestingly, both $p_{1}^{*}$ and $p_{2}^{*}$ are below the classical
percolation threshold $p_{c}$ for the honeycomb lattice, which gives physical
significance to the obtained phases. In the topological phase for
$p<p_{1}^{*}$, the magnon spectrum is gapless but the states filling the
topological, clean-limit gap region are spatially localized. For energies above
and below the region of localized states, there are windows composed of
extended states. This is at odds with standard Chern insulators, where extended
states occur only at single energies. For dilutions $p_{1}^{*}<p<p_{2}^{*}$,
the two regions of extended states merge and a continuum of delocalized states
appears around the middle of the magnon spectrum. For this range of dilutions
the Chern number seems to be ill defined in the thermodynamic limit, and only
for $p>p_{2}^{*}$, when all states become localized, the system shows $C=0$ as
expected for a trivial phase. Replacing magnetic with non-magnetic atoms in a
systems hosting a magnon Chern insulator in the clean limit puts all the three
phases within experimental reach.",2311.03457v1
2023-12-26,All solution grown epitaxial magnonic crystal of thulium iron garnet thin film,"Magnonics has shown the immense potential of compatibility with CMOS devices
and the ability to be utilized in futuristic quantum computing. Therefore, the
magnonic crystals, both metallic and insulating, are under extensive
exploration. The presence of high spin-orbit interaction induced by the
presence of rare-earth elements in thulium iron garnet (TmIG) increases its
potential in magnonic applications. Previously, TmIG thin films were grown
using ultra-high vacuum-based techniques. Here, we present a cost-effective
solution-based approach that enables the excellent quality interface and
surface roughness of the epitaxial TmIG/GGG. The deposited TmIG (12.2 nm) thin
film's physical and spin dynamic properties are investigated in detail. The
confirmation of the epitaxy using X-ray diffraction in $\phi$-scan geometry
along with the X-ray reflectivity and atomic force for the thickness and
roughness analysis and topography, respectively. The epitaxial TmIG/GGG have
confirmed the perpendicular magnetic anisotropy utilizing the
polar-magneto-optic Kerr effect. Analyzing the ferromagnetic resonance study of
TmIG/GGG thin films provides the anisotropy constant K$_U$ = 20.6$\times$10$^3$
$\pm$ 0.2$\times$10$^3$ N/m$^2$ and the Gilbert damping parameter $\alpha$ =
0.0216 $\pm$ 0.0028. The experimental findings suggest that the
solution-processed TmIG/GGG thin films have the potential to be utilized in
device applications.",2312.15973v1
2024-01-03,Observation of the Magnonic Dicke Superradiant Phase Transition,"Two-level atoms coupled with single-mode cavity photons are predicted to
exhibit a quantum phase transition when the coupling strength exceeds a
critical value, entering a phase in which atomic polarization and photonic
field are finite even at zero temperature and without external driving.
However, this phenomenon, the superradiant phase transition (SRPT), is
forbidden by a no-go theorem due to the existence of the diamagnetic term in
the Hamiltonian. Here, we present spectroscopic evidence for a magnonic SRPT in
ErFeO$_3$, where the role of the photonic mode (two-level atoms) in the
photonic SRPT is played by an Fe$^{3+}$ magnon mode (Er$^{3+}$ spins). The
absence of the diamagnetic term in the Fe$^{3+}$-Er$^{3+}$ exchange coupling
ensures that the no-go theorem does not apply. Terahertz and gigahertz
magnetospectroscopy experiments revealed the signatures of the SRPT -- a kink
and a softening, respectively, of two spin-magnon hybridized modes at the
critical point.",2401.01873v1
2023-12-01,Large enhancement of spin-orbit torques under a MHz modulation due to phonon-magnon coupling,"The discovery of spin-orbit torques (SOTs) generated through the spin Hall or
Rashba effects provides an alternative write approach for magnetic
random-access memory (MRAM), igniting the development of spin-orbitronics in
recent years. Quantitative characterization of SOTs highly relies on the
SOT-driven ferromagnetic resonance (ST-FMR), where a modulated microwave
current is used to generate ac SOTs and the modulation-frequency is usually
less than 100 kHz (the limit of conventional lock-in amplifiers). Here we have
investigated the SOT of typical SOT material/ferromagnet bilayers in an
extended modulation-frequency range, up to MHz, by developing the ST-FMR
measurement. Remarkably, we found that the measured SOTs are enhanced about
three times in the MHz range, which cannot be explained according to present
SOT theory. We attribute the enhancement of SOT to additional magnon
excitations due to phonon-magnon coupling, which is also reflected in the
slight changes of resonant field and linewidth in the acquired ST-FMR spectra,
corresponding to the modifications of effective magnetization and damping
constant, respectively. Our results indicate that the write current of SOT-MRAM
may be reduced with the assistant of phonon-magnon coupling.",2401.02967v1
2024-01-10,Transverse and longitudinal magnons in strongly anisotropic antiferromagnet FePSe3,"FePSe3 is a collinear honeycomb antiferromagnet with an easy-axis anisotropy
and large spins S=2. It belongs to a family of magnetic van der Waals
materials, which recently attracted a considerable attention. In this work we
present an experimental magneto-optical study of the low-energy excitation
spectrum in FePSe3, together with its theoretical description. The observed
response contains several types of magnon excitations. Two of them are
conventional transverse magnons described by a classical theory of
antiferromagnetic resonance. Two other modes are identified as multimagnon
hexadecapole excitations with an anomalous g factor approximately equal to four
times the g factor of a single Fe^2+ ion. These quasiparticles correspond to
full reversals of iron spins that coherently propagate in the up-down
antiferromagnetic structure. They constitute a novel type of collective
excitations in anisotropic magnetic solids, called longitudinal magnons.
Comparison between theory and experiment allows us to estimate the microscopic
parameters of FePSe3 including exchange coupling constants and the single-ion
anisotropy.",2401.05557v2
2024-01-11,"Two-dimensional altermagnets from high throughput computational screening: symmetry requirements, chiral magnons and spin-orbit effects","We present a high throughput computational search for altermagnetism in
two-dimensional (2D) materials based on the Computational 2D Materials Database
(C2DB). We start by showing that the symmetry requirements for altermagnetism
in 2D are somewhat more strict compared to bulk materials and applying these
yields a total of 7 altermagnets in the C2DB. The collinear ground state in
these monolayers are verified by spin spiral calculations using the generalized
Bloch theorem. We focus on four $d$-wave altermagnetic materials belonging to
the $P2_1'/c'$ magnetic space group - RuF$_4$, VF$_4$, AgF$_2$ and OsF$_4$. The
first three of these are known experimentally as van der Waals bonded bulk
materials and are likely to be exfoliable from their bulk parent compounds. We
perform a detailed analysis of the electronic structure and non-relativistic
spin splitting in $k$-space exemplified by RuF$_4$. The magnon spectrum of
RuF$_4$ is calculated from the magnetic force theorem and it is shown that the
symmetries that enforce degenerate magnon bands in anti-ferromagnets are absent
in altermagnets and give rise to the obtained non-degenerate magnon spectrum.
We then include spin-orbit effects and show that these will dominate the
splitting ofmagnons in RuF$_4$. Finally, we provide an example of $i$-wave
altermagnetism in the 2H phase of FeBr$_3$.",2401.05992v1
2024-02-06,Magnon mediated spin pumping by coupled ferrimagnetic garnets heterostructure,"Spin pumping has significant implications for spintronics, providing a
mechanism to manipulate and transport spins for information processing.
Understanding and harnessing spin currents through spin pumping is critical for
the development of efficient spintronic devices. The use of a magnetic
insulator with low damping, enhances the signal-to-noise ratio in crucial
experiments such as spin-torque ferromagnetic resonance (FMR) and spin pumping.
A magnetic insulator coupled with a heavy metal or quantum material offers a
more straight forward model system, especially when investigating spin-charge
interconversion processes to greater accuracy. This simplicity arises from the
absence of unwanted effects caused by conduction electrons unlike in
ferromagnetic metals. Here, we investigate the spin pumping in coupled
ferrimagnetic (FiM) Y3Fe5O12 (YIG)/Tm3Fe5O12 (TmIG) bilayers combined with
heavy-metal (Pt) using the inverse spin Hall effect (ISHE). It is observed that
magnon transmission occurs at both of the FiMs FMR positions. The enhancement
of spin pumping voltage (Vsp) in the FiM garnet heterostructures is attributed
to the strong interfacial exchange coupling between FiMs. The modulation of Vsp
is achieved by tuning the bilayer structure. Further, the spin mixing
conductance for these coupled systems is found to be 10^18 m^-2. Our findings
describe a novel coupled FiM system for the investigation of magnon coupling
providing new prospects for magnonic devices.",2402.03734v1
2024-02-19,Designed spin-texture-lattice to control anisotropic magnon transport in antiferromagnets,"Spin waves in magnetic materials are promising information carriers for
future computing technologies due to their ultra-low energy dissipation and
long coherence length. Antiferromagnets are strong candidate materials due, in
part, to their stability to external fields and larger group velocities.
Multiferroic aniferromagnets, such as BiFeO$_3$ (BFO), have an additional
degree of freedom stemming from magnetoelectric coupling, allowing for control
of the magnetic structure, and thus spin waves, with electric field.
Unfortunately, spin-wave propagation in BFO is not well understood due to the
complexity of the magnetic structure. In this work, we explore long-range spin
transport within an epitaxially engineered, electrically tunable,
one-dimensional (1D) magnonic crystal. We discover a striking anisotropy in the
spin transport parallel and perpendicular to the 1D crystal axis. Multiscale
theory and simulation suggests that this preferential magnon conduction emerges
from a combination of a population imbalance in its dispersion, as well as
anisotropic structural scattering. This work provides a pathway to
electrically-reconfigurable magnonic crystals in antiferromagnets.",2402.12341v1
2024-04-05,Field-dependent Magnons in a Honeycomb Antiferromagnet CoTiO$_3$,"We report field-dependent high-resolution inelastic neutron scattering (INS)
measurements on the honeycomb lattice magnet, CoTiO$_3$, to study the evolution
of its magnon excitations across a spin reorientation transition driven by an
in-plane magnetic field. By carrying out elastic neutron scattering in a
magnetic field, we show that the sample transitions from a collinear
antiferromagnetic state with multiple magnetic domains at a low field to a
mono-domain state with a canted magnetic structure at a high field. Concurrent
with this transition, we observed significant changes in both the energy and
the width of the zone center magnon peak. The observed width change is argued
to be consistent with an unusual zero-field state with extended domain walls.
On the other hand, the magnon spectra near the $\mathbf{K}$ point of the
Brillouin zone boundary are found to be largely insensitive to the changes in
the ordered moment directions and the domain configuration. We argue that this
observation is difficult to explain within the framework of the bond-dependent
model proposed in a recent INS study [Elliot \textit{et\,al}, Nat. Commun.,
\textbf{12}, 3936 (2021)]. Our study therefore calls for alternative
explanations for the observed $\mathbf{K}$-point gap in CoTiO$_3$.",2404.04390v1
1995-01-16,Spin-Waves in the Mid-Infrared Spectrum of Antiferromagnetic YBa$_2$Cu$_3$O$_{6.0}$,"The mid-infrared spin-wave spectrum of antiferromagnetic
YBa$_2$Cu$_3$O$_{6.0}$\ was determined by infrared transmission and reflection
measurements ($\bbox{k} \!\! \parallel $c) at $T\!=\!10\!$~K.\@ Excitation of
single magnons of the optical branch was observed at
$E_{\text{op}}\!=\!178.0\!$~meV.\@ Two further peaks at $346\!$~meV
($\approx\!1.94\,E_{\text{op}}$) and $470\!$~meV
($\approx\!2.6\,E_{\text{op}}$) both belong to the two-magnon spectrum. Linear
spin wave theory is in good agreement with the measured two-magnon spectrum,
and allows to determine the exchange constant $J$ to be about $120\!$~meV,
whereas the intrabilayer coupling $J_{12}$ is approximately $0.55\,J$.",9501065v1
1995-08-23,Spin-ordering and magnon collective modes for two-dimensional electron lattices in strong magnetic fields,"We study the spin-ordering and the magnon collective modes of the
two-dimensional Wigner crystal state at strong magnetic fields. Our work is
based on the Hartree-Fock approximation for the ground state and the
time-dependent Hartree-Fock approximation for the collective modes. We find
that the ground state is ferromagnetic, i.e that all spins are aligned at T=0
even when the electronic g-factor is negligibly small. The magnon calculations
show that the spin-stiffness is much smaller in the crystal state than in fluid
states which occur at nearby Landau level filling factors.",9508100v1
1996-06-03,Magnetic excitation spectrum of dimerized antiferromagnetic chains,"Motivated by recent measurements on CuGeO$_3$ the spectrum of magnetic
excitations of an antiferromagnetic $S=\frac{1}{2}$ chain with alternating
coupling strength is investigated. Wave vector dependent magnons and a
continuum with square root behavior at the band edges are found. The spectral
density of the continua is calculated. Spin rotation symmetry fixes the gap of
the continuum to be twice the elementary magnon gap. This is in excellent
agreement with experimental results. In addition, the existence of bound states
of two magnons is predicted: below the continuum a singlet and a triplet, above
the continuum an ``anti-bound'' quintuplet. The results are based on field
theoretic arguments, RPA calculations, and consideration of the limit of strong
alternation.",9606001v1
1996-07-08,A Neutron Scattering Study of Magnetic Excitations in the Spin Ladder (VO)2P2O7,"In this letter we report results from inelastic neutron scattering
experiments on powder samples of vanadyl pyrophosphate, (VO)2P2O7. We see
evidence for three magnetic excitations, at 3.5 meV, 6.0 meV and 14 meV. The
intensity of the 3.5 meV mode is strong near Q=0.8 A-1, consistent with the
one-magnon gap mode reported previously and predicted by the spin-ladder model.
The magnetic scattering at 14 meV may be due to the top of the one-magnon band
or the two-magnon continuum predicted in the ladder model. The 6.0 meV mode
also peaks in intensity at Q=0.8 A-1. This mode has not been reported
previously and was not anticipated by existing theoretical treatments of the
spin ladder.",9607059v1
1998-01-21,A Study of the S=1/2 Alternating Chain using Multiprecision Methods,"In this paper we present results for the ground state and low-lying
excitations of the $S=1/2$ alternating Heisenberg antiferromagnetic chain. Our
more conventional techniques include perturbation theory about the dimer limit
and numerical diagonalization of systems of up to 28 spins. A novel application
of multiple precision numerical diagonalization allows us to determine
analytical perturbation series to high order; the results found using this
approach include ninth-order perturbation series for the ground state energy
and one magnon gap, which were previously known only to third order. We also
give the fifth-order dispersion relation and third-order exclusive neutron
scattering structure factor for one-magnon modes and numerical and analytical
binding energies of S=0 and S=1 two-magnon bound states.",9801224v1
1998-03-09,Disorder-enhanced delocalization and local-moment quenching in a disordered antiferromagnet,"The interplay of disorder and spin-fluctuation effects in a disordered
antiferromagnet is studied. In the weak-disorder regime (W \le U), while the
energy gap decreases rapidly with disorder, the sublattice magnetization,
including quantum corrections, is found to remain essentially unchanged in the
strong correlation limit. Magnon energies and Neel temperature are enhanced by
disorder in this limit. A single paradigm of disorder-enhanced delocalization
qualitatively accounts for all these weak disorder effects. Vertex corrections
and magnon damping, which appear only at order (W/U)^4, are also studied. With
increasing disorder a crossover is found at W \sim U, characterized by a rapid
decrease in sublattice magnetization due to quenching of local moments, and
formation of spin vacancies. The latter suggests a spin-dilution behavior,
which is indeed observed in softened magnon modes, lowering of Neel
temperature, and enhanced transverse spin fluctuations.",9803094v3
1998-05-10,Assisted Tunneling in Ferromagnetic Junctions and Half-Metallic Oxides,"Different mechanisms of spin-dependent tunneling are analyzed with respect to
their role in tunnel magnetoresistance (TMR). Microscopic calculation within a
realistic model shows that direct tunneling in iron group systems leads to
about a 30% change in resistance, which is close but lower than experimentally
observed values. The larger observed values of the tunnel magnetoresistance
(TMR) might be a result of tunneling involving surface polarized states. It is
found that tunneling via resonant defect states in the barrier radically
decreases the TMR by order of magnitude. One-magnon emission is shown to reduce
the TMR, whereas phonons increase the effect. The inclusion of both magnons and
phonons reasonably explains an unusual bias dependence of the TMR. The model
presented here is applied qualitatively to half-metallics with 100% spin
polarization, where one-magnon processes are suppressed and the change in
resistance in the absence of spin-mixing on impurities may be arbitrarily
large. Even in the case of imperfect magnetic configurations, the resistance
change can be a few 1000 percent. Examples of half-metallic systems are
CrO2/TiO2 and CrO2/RuO2.",9805114v1
1998-07-16,Low temperature resistivity in a nearly half-metallic ferromagnet,"We consider electron transport in a nearly half-metallic ferromagnet, in
which the minority spin electrons close to the band edge at the Fermi energy
are Anderson-localized due to disorder. For the case of spin-flip scattering of
the conduction electrons due to the absorption and emission of magnons, the
Boltzmann equation is exactly soluble to the linear order. From this solution
we calculate the temperature dependence of the resistivity due to single magnon
processes at sufficiently low temperature, namely $k_BT\ll D/L^2$, where $L$ is
the Anderson localization length and $D$ is the magnon stiffness. And depending
on the details of the minority spin density of states at the Fermi level, we
find a $T^{1.5}$ or $T^{2}$ scaling behavior for resistivity. Relevance to the
doped perovskite manganite systems is discussed.",9807244v1
1999-07-23,Unconventional one-magnon scattering resistivity in half metals,"Low-temperature resistivity of half-metals is investigated. To date it has
been discussed that the one-magnon scattering process in half-metals is
irrelevant for low-temperature resistivity, due to the fully spin-polarized
electronic structure at the ground state. If one takes into account the
non-rigid-band behavior of the minority band due to spin fluctuations at finite
temperatures, however, the unconventional one-magnon scattering process is
shown to be most relevant and gives T^3 dependence in resistivity. This
behavior may be used as a crucial test in the search for half-metallic
materials which are potentially important for applications. Comparison with
resistivity data of
  La_1-x Sr_x MnO_3 as candidates for half-metals shows good agreement.",9907363v2
1999-08-02,Magnon-Paramagnon Effective Theory of Itinerant Ferromagnets,"The present work is devoted to the derivation of an effective
magnon-paramagnon theory starting from a microscopic lattice model of
ferromagnetic metals. For some values of the microscopic parameters it
reproduces the Heisenberg theory of localized spins. For small magnetization
the effective model describes the physics of weak ferromagnets in accordance
with the experimental results. It is written in a way which keeps O(3) symmetry
manifest,and describes both the order and disordered phases of the system.
  Analytical expression for the Curie temperature,which takes the magnon
fluctuations into account exactly, is obtained. For weak ferromagnets $T_c$ is
well below the Stoner's critical temperature and the critical temperature
obtained within Moriya's theory.",9908024v3
2000-03-14,Two-magnon Raman scattering in insulating cuprates: Modifications of the effective Raman operator,"Calculations of Raman scattering intensities in spin 1/2 square-lattice
Heisenberg model, using the Fleury-Loudon-Elliott theory, have so far been
unable to describe the broad line shape and asymmetry of the two magnon peak
found experimentally in the cuprate materials. Even more notably, the
polarization selection rules are violated with respect to the
Fleury-Loudon-Elliott theory. There is comparable scattering in $B_{1g}$ and
$A_{1g}$ geometries, whereas the theory would predict scattering in only
$B_{1g}$ geometry. We review various suggestions for this discrepency and
suggest that at least part of the problem can be addressed by modifying the
effective Raman Hamiltonian, allowing for two-magnon states with arbitrary
total momentum. Such an approach based on the Sawatzsky-Lorenzana theory of
optical absorption assumes an important role of phonons as momentum sinks. It
leaves the low energy physics of the Heisenberg model unchanged but
substantially alters the Raman line-shape and selection rules, bringing the
results closer to experiments.",0003248v2
2001-01-11,Internal Modes and Magnon Scattering on Topological Solitons in 2d Easy-Axis Ferromagnets,"We study the magnon modes in the presence of a topological soliton in a 2d
Heisenberg easy-axis ferromagnet. The problem of magnon scattering on the
soliton with arbitrary relation between the soliton radius R and the ""magnetic
length"" Delta_0 is investigated for partial modes with different values of the
azimuthal quantum numbers m. Truly local modes are shown to be present for all
values of m, when the soliton radius is enough large. The eigenfrequencies of
such internal modes are calculated analytically on limiting case of a large
soliton radius and numerically for arbitrary soliton radius. It is demonstrated
that the model of an isotropic magnet, which admits an exact analytical
investigation, is not adequate even for the limit of small radius solitons,
R<<Delta_0: there exists a local mode with nonzero frequency. We use the data
about local modes to derive the effective equation of soliton motion; this
equation has the usual Newtonian form in contrast to the case of the easy-plane
ferromagnet. The effective mass of the soliton is found.",0101150v1
2001-01-31,Bulk and Edge excitations in a $ν=1$ quantum Hall ferromagnet,"In this article, we shall focus on the collective dynamics of the fermions in
a $\nu = 1$ quantum Hall droplet. Specifically, we propose to look at the
quantum Hall ferromagnet. In this system, the electron spins are ordered in the
ground state due to the exchange part of the Coulomb interaction and the Pauli
exclusion principle. The low energy excitations are ferromagnetic magnons. To
provide a means for describing these magnons, we shall discuss a method of
introducing collective coordinates in the Hilbert space of many-fermion
systems. These collective coordinates are bosonic in nature. They map a part of
the fermionic Hilbert space into a bosonic Hilbert space. Using this technique,
we shall interpret the magnons as bosonic collective ex citations in the
Hilbert space of the many-electron Hall system. By considering a Hall droplet
of finite extent, we shall also obtain the effective Lagrangian governing the
spin collective excitations at the edge of the sample.",0101475v1
2001-05-31,Bulk and edge excitations of a $ν=1$ Hall ferromagnet,"In this article, we shall focus on the collective dynamics of the fermions in
a $\nu = 1$ quantum Hall droplet. Specifically, we propose to look at the
quantum Hall ferromagnet. In this system, the electron spins are ordered in the
ground state due to the exchange part of the Coulomb interaction and the Pauli
exclusion principle. The low energy excitations are ferromagnetic magnons. In
order to obtain an effective Lagrangian for these magnons, we shall introduce
bosonic collective coordinates in the Hilbert space of many-fermion systems.
These collective coordinates describe a part of the fermionic Hilbert space.
Using this technique, we shall interpret the magnons as bosonic collective
excitations in the Hilbert space of the many-electron Hall system. Furthermore,
by considering a Hall droplet of finite extent, we shall also obtain the
effective Lagrangian governing the spin collective excitations at the edge of
the sample.",0105626v1
2001-08-29,Macroscopic magnetization jumps due to independent magnons in frustrated quantum spin lattices,"For a class of frustrated spin lattices including the kagome lattice we
construct exact eigenstates consisting of several independent, localized
one-magnon states and argue that they are ground states for high magnetic
fields. If the maximal number of local magnons scales with the number of spins
in the system, which is the case for the kagome lattice, the effect persists in
the thermodynamic limit and gives rise to a macroscopic jump in the
zero-temperature magnetization curve just below the saturation field. The
effect decreases with increasing spin quantum number and vanishes in the
classical limit. Thus it is a true macroscopic quantum effect.",0108498v2
2001-11-06,Direct Measurement of the Spin Hamiltonian and Observation of Condensation of Magnons in the 2D Frustrated Quantum Magnet Cs2CuCl4,"We propose a method for measuring spin Hamiltonians and apply it to the
spin-1/2 Heisenberg antiferromagnet Cs2CuCl4, which shows a 2D fractionalized
RVB state at low fields. By applying strong fields we fully align the spin
moment of Cs2CuCl4 transforming it into an effective ferromagnet. In this phase
the excitations are conventional magnons and their dispersion relation measured
using neutron scattering give the exchange couplings directly, which are found
to form an anisotropic triangular lattice with small Dzyaloshinskii-Moriya
terms. Using the field to control the excitations we observe Bose condensation
of magnons into an ordered ground state.",0111079v1
2002-06-27,Magnon-Mediated Superconductivity in Itinerant Ferromagnets,"The present paper discusses magnon-mediated superconductivity in
ferromagnetic metals. The mechanism explains in a natural way the fact that the
superconductivity in UGe_2, ZrZn_2 and URhGe is apparently confined to the
ferromagnetic phase.The order parameter is a spin anti-parallel component ofa
spin-1 triplet with zero spin projection. The transverse spinfluctuations are
pair forming and the longitudinal ones are pair breaking.The competition
between magnons and paramagnons explains the existence of two successive
quantum phase transitions in UGe_2, from ferromagnetism to ferromagnetic
superconductivity, and at higher pressure to paramagnetism. The maximum T_{SC}
results from the suppression of the paramagnon contribution. To form a Cooper
pair an electron transfers from one Fermi surface to the other. As a result,
the onset of superconductivity leads to the appearance of two Fermi surfaces in
each of the spin up and spin down momentum distribution functions. This fact
explains the linear temperaturedependence at low temperature of the specific
heat, and the experimental results for UGe_2.",0206534v2
2003-02-11,Magnon and phonon assisted tunneling in a high-magnetoresistance tunnel junction using Co$_{75}$Fe$_{25}$ ferromagnetic electrodes,"Magnetoelectric properties of the spin-valve-type tunnel junction of Ta(5\
nm)/Ni$_{79}$Fe$_{21}$ (25 nm)/Ir$_{22}$Mn$_{78}$ (10 nm)/Co$_{75}$Fe$_{25}$ (4
nm)/Al (0.8 nm)-oxide/Co$_{75}$Fe$_{25}$ (4 nm)/Ni$_{79}$Fe$_{21}$ (20 nm)/Ta
(5 nm) are investigated both experimentally and theoretically. It is shown that
both magnon and phonon excitations contribute to the tunneling process.
Moreover, we show that there are two branches of magnon with spin $S=1/2$ and
3/2 respectively. The theoretical results are in good agreement with the
experimental data.",0302210v2
2003-09-29,Magnons in CMR pyrochlore Tl2Mn2O7,"Well defined spin waves were observed when the spin dynamics of Tl2Mn2O7, the
first pyrochlore compound found to exhibit colossal magnetoresistance, was
measured [J.W.Lynn et al., Phys.Rev.Lett. 80,4582(1998)], in stark contrast
with the experimental results on the larger family of magnetoresistive
manganites with perovskite structure. In this work, we present our calculation
for the spin waves in Tl2Mn2O7, which we described using the microscopic
generic model proposed recently for this compound [C.I.Ventura and M.A.Gusmao,
Phys.Rev.B 65, 14422(2002)]. We have employed a canonical transformation to
determine perturbatively the effective spin-wave Hamiltonian, obtaining
therefrom the renormalization of the ferromagnetic spin waves related to the
localized Mn$^{4+}$ spins, due to their coupling with the conduction electrons
present. We have calculated the magnon dispersion relations along different
paths in the first Brillouin zone, comparing them with those which are obtained
for an ideal isotropic ferromagnet. This comparison evidences an agreement
between the ferromagnetic magnons obtained from the generic model and the bare
spin waves, such as had been found in neutron scattering experiments.",0309684v1
2003-12-02,Drastic enhancement of magnon thermal conductivity in the Bose-Einstein condensed state of TlCuCl$_3$,"We have measured the thermal conductivity of a TlCuCl$_3$ single crystal in
magnetic fields up to 14 T. It has been found that the temperature dependence
of the thermal conductivity exhibits a sharp peak at 4 K in zero field, which
is suppressed by the application of magnetic fields up to 7 T. The peak is
concluded to be attributable to the enhancement of the thermal conductivity due
to phonons because of the formation of a spin-gap state. In high magnetic
fields above 7 T, on the other hand, another sharp peak appears around 4 K and
this is enhanced with increasing magnetic field. This peak is regarded as being
attributable to the enhancement of the thermal conductivity due to magnons
and/or phonons because of the drastic extension of the mean free path of
magnons and/or phonons in the Bose-Einstein condensed state.",0312063v2
2004-01-09,Thermal Spin Valves,"The ability of an insulating solid to conduct heat is rarely effected by the
application of a magnetic field. We have found, however, that the low
temperature heat conduction of some solids increases by more than a factor of
two with the application of a modest magnetic field. The effect occurs in
low-dimensional magnetically ordered materials when a small gap, \delta, in the
acoustic magnon (spin wave) spectra is closed using a magnetic field H >
\delta/g\mu_B. Since all magnetically ordered materials must have a gap in the
magnon spectra for magnons with k = 0, this may be a very general effect. Extra
heat is carried through the solid only when the magentic field exceeds the
critical value (H > \delta/g\mu_B). At this critical field the tiny atomic
magnets in the solid abruptly change the direction they point which results in
more heat flowing through the material. The magnetic field thus acts as a heat
switch. We have observed this effect in three quite different magnetically
ordered materials: K_2V_3O_8, Nd_2CuO_4 and Pr_2CuO_4. Several possible
explanations for these effects will be discussed.",0401154v1
2004-09-08,Multi-Triplet Magnons in SrCu$_2$(BO$_3$)$_2$ Studied by Thermal Conductivity Measurements in Magnetic Fields,"We have measured the thermal conductivity parallel to the a-axis of the
Zn-free and 1% Zn-substituted SrCu$_{2-x}$Zn$_x$(BO$_3$)$_2$ in magnetic fields
up to 14 T, in order to examine the thermal conductivity due to the
multi-triplet magnons. It has been found that the thermal conductivity peak
observed in the spin gap state is suppressed by the substitution of Zn for Cu
in high magnetic fields above 6 T, while it is not changed in low magnetic
fields below 6 T. The results suggest that the thermal conductivity peak in the
spin-gap state of SrCu$_2$(BO$_3$)$_2$ is composed of not only thermal
conductivity due to phonons but also that due to the multi-triplet magnons in
high fields above 6 T.",0409178v2
2005-04-04,Theory of Spin-Wave Frequency Gaps in 3D Magnonic Crystals. Application to Manganites,"his study is an investigation of spin wave spectrum in macrostructures
composed of two ferromagnetic materials and showing a 3D periodicity: spherical
ferromagnetic grains disposed in the nodes of a 3D crystal lattice are embedded
in a matrix with different ferromagnetic properties. Frequency ranges forbidden
to spin wave propagation are found in the calculated magnonic spectra. Both the
position and the width of the gaps are found to depend on the magnetic
(exchange and magnetization) contrasts in the composite material, as well as on
its structural parameters (filling fraction, crystal lattice type). Having
applied our theory to interpretation of the existence of a spin wave gap in
doped manganites, recently revealed in neutron scattering experiments by S.
Hennion {\em et al.}, we obtained a good (though approximate) quantitative
agreement with the experimental results. A working hypothesis is proposed on
this basis, supposing that certain manganites can be treated as natural 3D
magnonic crystals.",0504073v1
2005-05-03,Bose-Einstein Condensation of Magnons in Cs2CuCl4,"We report on results of specific heat measurements on single crystals of the
frustrated quasi-2D spin-1/2 antiferromagnet Cs_2CuCl_4 (T_N=0.595 K) in
external magnetic fields B<12 T and for temperatures T>30 mK. Decreasing B from
high fields leads to the closure of the field-induced gap in the magnon
spectrum at a critical field B_c = 8.51 T and a magnetic phase transition is
clearly seen below B_c. In the vicinity to B_c, the phase transition boundary
is well described by the power-law T_c(B)\propto (B_c-B)^{1/\phi} with the
measured critical exponent \phi\simeq 1.5. These findings are interpreted as a
Bose-Einstein condensation of magnons.",0505058v1
2005-06-14,Systematic Low-Energy Effective Theory for Magnons and Charge Carriers in an Antiferromagnet,"By electron or hole doping quantum antiferromagnets may turn into
high-temperature superconductors. The low-energy dynamics of antiferromagnets
are governed by their Nambu-Goldstone bosons -- the magnons -- and are
described by an effective field theory analogous to chiral perturbation theory
for the pions in strong interaction physics. In analogy to baryon chiral
perturbation theory -- the effective theory for pions and nucleons -- we
construct a systematic low-energy effective theory for magnons and electrons or
holes in an antiferromagnet. The effective theory is universal and makes
model-independent predictions for the entire class of antiferromagnetic
cuprates. We present a detailed analysis of the symmetries of the Hubbard model
and discuss how these symmetries manifest themselves in the effective theory. A
complete set of linearly independent leading contributions to the effective
action is constructed. The coupling to external electromagnetic fields is also
investigated.",0506324v2
2005-06-20,Hole and electron dynamics in the triangular-lattice antiferromagnet -- interplay of frustration and spin fluctuations,"Single-particle dynamics in the 120$^{\circ}$ ordered antiferromagnetic state
of the triangular-lattice Hubbard model is studied using a physically
transparent fluctuation approach in terms of multiple magnon emission and
absorption processes within the noncrossing approximation. Hole and electron
spectral features are evaluated at intermediate $U$, and analyzed in terms of a
competition between the frustration-induced direct hopping and the virtual
hopping terms. Finite $U$-induced competing interactions and frustration
effects contributing through the magnon dispersion are also discussed. Finite
contribution to self-energy correction from long-wavelength (Goldstone) modes,
together with the high density of electron scattering states in the narrow,
sharp peak in the upper band, result in strong fermion-magnon scattering
leading to pronounced incoherent behaviour in the electron dynamics. The
fluctuation-induced first-order metal-insulator transition due to vanishing
band gap is also discussed.",0506488v2
2005-07-05,One-magnon Raman scattering in La(2)CuO(4): the origin of the field-induced mode,"We investigate the one-magnon Raman scattering in the layered
antiferromagnetic La(2)CuO(4) compound. We find that the Raman signal is
composed by two one-magnon peaks: one in the B1g channel, corresponding to the
Dzyaloshinskii-Moryia (DM) mode, and another in the B3g channel, corresponding
to the XY mode. Furthermore, we show that a peak corresponding to the XY mode
can be induced in the planar (RR) geometry when a magnetic field is applied
along the easy axis for the sublattice magnetization. The appearance of such
field-induced mode (FIM) signals the existence of a new magnetic state above
the Neel temperature T_N, where the direction of the weak-ferromagnetic moment
(WFM) lies within the CuO(2) planes.",0507103v2
2005-11-04,Magnetic excitations and phonons in the spin-chain compound NaCu2O2,"We report an inelastic light scattering study of single-crystalline
NaCu$_2$O$_2$, a spin-chain compound known to exhibit a phase with helical
magnetic order at low temperatures. Phonon excitations were studied as a
function of temperature and light polarization, and the phonon frequencies are
compared to the results of ab-initio lattice dynamical calculations, which are
also reported here. The good agreement between the observed and calculated
modes allows an assignment of the phonon eigenvectors. Two distinct high-energy
two-magnon features as well as a sharp low-energy one-magnon peak were also
observed. These features are discussed in terms of the magnon modes expected in
a helically ordered state. Their polarization dependence provides evidence of
substantial exchange interactions between two closely spaced spin chains within
a unit cell. At high temperatures, the spectral features attributable to
magnetic excitations are replaced by a broad, quasielastic mode due to
overdamped spin excitations.",0511116v1
2006-02-16,Finite Temperature Dynamics of the Spin 1/2 Bond Alternating Heisenberg Antiferromagnetic Chain,"We present results for the dynamic structure factor of the S=1/2 bond
alternating Heisenberg chain over a large range of frequencies and
temperatures. Data are obtained from a numerical evaluation of thermal averages
based on the calculation of all eigenvalues and eigenfunctions for chains of up
to 20 spins. Interpretation is guided by the exact temperature dependence in
the noninteracting dimer limit which remains qualitatively valid up to an
interdimer exchange $\lambda \approx 0.5$. The temperature induced central peak
around zero frequency is clearly identified and aspects of the crossover to
spin diffusion in its variation from low to high temperatures are discussed.
The one-magnon peak acquires an asymmetric shape with increasing temperature.
The two-magnon peak is dominated by the S=1 bound state which remains well
defined up to temperatures of the order of J. The variation with temperature
and wavevector of the integrated intensity for one and two magnon scattering
and of the central peak are discussed.",0602387v2
2006-07-19,Multiple Magnon Modes and Consequences for the Bose-Einstein Condensed Phase in BaCuSi2O6,"The compound BaCuSi2O6 is a quantum magnet with antiferromagnetic dimers of S
= 1/2 moments on a quasi-2D square lattice. We have investigated its spin
dynamics by inelastic neutron scattering experiments on single crystals with an
energy resolution considerably higher than in an earlier study. We observe
multiple magnon modes, indicating clearly the presence of magnetically
inequivalent dimer sites. This more complex spin Hamiltonian leads to a
distinct form of magnon Bose-Einstein condensate (BEC) phase with a spatially
modulated condensate amplitude.",0607465v2
2006-08-25,Excitations from a Bose-Einstein condensate of magnons in coupled spin ladders,"The weakly coupled quasi-one-dimensional spin ladder compound
(CH$_3$)$_2$CHNH$_3$CuCl$_3$ is studied by neutron scattering in magnetic
fields exceeding the critical field of Bose-Einstein condensation of magnons.
Commensurate long-range order and the associated Goldstone mode are detected
and found to be similar to those in a reference 3D quantum magnet. However, for
the upper two massive magnon branches the observed behavior is totally
different, culminating in a drastic collapse of excitation bandwidth beyond the
transition point.",0608566v3
2006-10-19,Bose-Einstein condensation of magnons in magnets with predominant ferromagnetic interaction,"We discuss Bose-Einstein condensation of magnons (BEC) in magnets with
predominant ferromagnetic (FM) interaction in magnetic field $H$ near
saturation ($H_c$). Because $H_c$ is independent of FM couplings, magnetic
materials of this type can have small $H_c$ that makes them promising
candidates for experimental investigation of BEC. Ferromagnets with easy-plane
anisotropy and antiferromagnets (AFs) containing weakly coupled FM planes or
chains are discussed in detail. We observe small effective interaction between
magnons near the QCP in such magnets, in contrast to AFs with strong AF
coupling previously discussed. In particular, this smallness allows us to find
crossovers in the critical temperature $T_c(H)\propto (H_c-H)^{1/\phi}$ from
$\phi=3/2$ to $\phi=1$ in quasi-1D magnets, and from $\phi=3/2$ to
$\phi\approx1$ ($T_c\ln T_c\propto H_c-H$) in quasi-2D ones.",0610536v4
2006-12-14,Systematic Low-Energy Effective Field Theory for Electron-Doped Antiferromagnets,"In contrast to hole-doped systems which have hole pockets centered at $(\pm
\frac{\pi}{2a},\pm \frac{\pi}{2a})$, in lightly electron-doped antiferromagnets
the charged quasiparticles reside in momentum space pockets centered at
$(\frac{\pi}{a},0)$ or $(0,\frac{\pi}{a})$. This has important consequences for
the corresponding low-energy effective field theory of magnons and electrons
which is constructed in this paper. In particular, in contrast to the
hole-doped case, the magnon-mediated forces between two electrons depend on the
total momentum $\vec P$ of the pair. For $\vec P = 0$ the one-magnon exchange
potential between two electrons at distance $r$ is proportional to $1/r^4$,
while in the hole case it has a $1/r^2$ dependence. The effective theory
predicts that spiral phases are absent in electron-doped antiferromagnets.",0612363v1
2006-12-31,Excitation spectra and ground state properties of the layered spin-1/2 frustrated antiferromagnets Cs_2CuCl_4 and Cs_2CuBr_4,"We use series expansion methods to study ground- and excited-state properties
in the helically ordered phase of spin-1/2 frustrated antiferromagnets on an
anisotropic triangular lattice. We calculate the ground state energy, ordering
wavevector, sublattice magnetization and one-magnon excitation spectrum for
parameters relevant to Cs_2CuCl_4 and Cs_2CuBr_4. Both materials are modeled in
terms of a Heisenberg model with spatially anisotropic exchange constants; for
Cs_2CuCl_4 we also take into account the additional Dzyaloshinskii-Moriya (DM)
interaction. We compare our results for Cs_2CuCl_4 with unpolarized neutron
scattering experiments and find good agreement. In particular, the large
quantum renormalizations of the one-magnon dispersion are well accounted for in
our analysis, and inclusion of the DM interaction brings the theoretical
predictions for the ordering wavevector and the magnon dispersion closer to the
experimental results.",0701014v1
2007-01-18,On the theory of Bose-Einstein condensation of quasiparticles: on the possibility of condensation of magnons at high temperatures,"Bose condensation of quasiparticles in physical systems of finite size iz
considered for the case of ferromagnetic thin films. It is shown that in
accordance with present-day experimental capabilities which permit one to
achieve densities of long-wave spin excitation of 1018-1019 cm-3, in such films
the formation of condensate of such quasiparticles begins at temperatures T~100
K (room one including). It is found that Bose condensation is accompanied by
scaling phenomenon according to which the main thermodynamic variable is not
the number N of particles but the ratio N/T. This indicates that condensation
of magnons can be observed at relatively low magnon densities. The roles played
by the shape of the spectrum and film thickness are analyzed.",0701437v1
2006-12-11,Connecting giant magnons to the pp-wave: An interpolating limit of $AdS_5 \times S^5$,"We consider a particular large-radius limit of the worldsheet $S$-matrix for
strings propagating on $AdS_5 \times S^5$. This limiting theory interpolates
smoothly between the so-called plane-wave and giant-magnon regimes of the
theory. The sigma model in this region simplifies; it stands as a toy model of
the full theory, and may be easier to solve directly. The $S$ matrix of the
limiting theory is non-trivial, and receives contributions to all orders in the
$\alpha'$ expansion. We analyze a guess for the full worldsheet $S$ matrix that
was formulated recently by Beisert, Hernandez and Lopez, and Beisert, Eden, and
Staudacher, and take the corresponding limit. After doing a Borel resummation
we find that the proposed $S$ matrix reproduces the expected results in the
giant-magnon region. In addition, we rely on general considerations to draw
some basic conclusions about the analytic structure of the $S$ matrix.",0612079v4
2007-02-06,Gauge invariant finite size spectrum of the giant magnon,"It is shown that the finite size corrections to the spectrum of the giant
magnon solution of classical string theory, computed using the uniform
light-cone gauge, are gauge invariant and have physical meaning. This is seen
in two ways: from a general argument where the single magnon is made gauge
invariant by putting it on an orbifold as a wrapped state obeying the level
matching condition as well as all other constraints, and by an explicit
calculation where it is shown that physical quantum numbers do not depend on
the uniform light-cone gauge parameter. The resulting finite size effects are
exponentially small in the $R$-charge and the exponent (but not the prefactor)
agrees with gauge theory computations using the integrable Hubbard model.",0702043v3
2007-04-17,q-Deformed Supersymmetry and Dynamic Magnon Representations,"It was recently noted that the dispersion relation for the magnons of planar
N=4 SYM can be identified with the Casimir of a certain deformation of the
Poincare algebra, in which the energy and momentum operators are supplemented
by a boost generator J. By considering the relationship between J and su(2|2) x
R^2, we derive a q-deformed super-Poincare symmetry algebra of the kinematics.
Using this, we show that the dynamic magnon representations may be obtained by
boosting from a fixed rest-frame representation. We comment on aspects of the
coalgebra structure and some implications for the question of boost-covariance
of the S-matrix.",0704.2069v2
2007-04-26,Colossal magnon-phonon coupling in multiferroic Eu$_{0.75}$Y$_{0.25}$MnO$_3$,"We report the spectra of magnetically induced electric dipole absorption in
Eu$_{0.75}$Y$_{0.25}$MnO$_3$ from temperature dependent far infrared
spectroscopy (10-250 cm$^{-1}$). These spectra, which occur only in the $e||a$
polarization, consist of two relatively narrow electromagnon features that
onset at $T_{FE}=30$ K and a broad absorption band that persists to
temperatures well above $T_N=47$ K. The observed excitations account for the
step up of the static dielectric constant in the ferroelectric phase. The
electromagnon at 80 cm$^{-1}$ is observed to be strongly coupled to the nearby
lowest optical phonon which transfers more than 1/2 of its spectral weight to
the magnon. We attribute the origin of the broad background absorption to the
two magnon emission decay process of the phonon.",0704.3632v4
2007-06-08,Optical coupling to spin waves in the cycloidal multiferroic BiFeO3,"The magnon and optical phonon spectrum of an incommensurate multiferroic such
as BiFeO3 is considered in the framework of a phenomenological Landau theory.
The resulting spin wave spectrum is quite distinct from commensurate substances
due to soft mode anisotropy and magnon zone folding. The former allows
electrical control of spin wave propagation via reorientation of the
spontaneous ferroelectric moment. The latter gives rise to multiple
magneto-dielectric resonances due to the coupling of optical phonons at zero
wavevector to magnons at integer multiples of the cycloid wavevector. These
results show that the optical response of a multiferroic reveals much more
about its magnetic excitations than previously anticipated on the basis of
simpler models.",0706.1260v3
2007-06-11,"Spiral phases and two-particle bound states from a systematic low-energy effective theory for magnons, electrons, and holes in an antiferromagnet","We have constructed a systematic low-energy effective theory for hole- and
electron-doped antiferromagnets, where holes reside in momentum space pockets
centered at $(\pm\frac{\pi}{2a},\pm\frac{\pi}{2a})$ and where electrons live in
pockets centered at $(\frac{\pi}{a},0)$ or $(0,\frac{\pi}{a})$. The effective
theory is used to investigate the magnon-mediated binding between two holes or
two electrons in an otherwise undoped system. We derive the one-magnon exchange
potential from the effective theory and then solve the corresponding
two-quasiparticle Schr\""odinger equation. As a result, we find bound state wave
functions that resemble $d_{x^2-y^2}$-like or $d_{xy}$-like symmetry. We also
study possible ground states of lightly doped antiferromagnets.",0706.1423v1
2007-11-21,The Ising model and planar N=4 Yang-Mills,"The scattering-matrix for planar Yang-Mills with N=4 supersymmetry relies on
the assumption that integrability holds to all orders in perturbation theory.
In this note we define a map from the spectral variables x^{\pm},
parameterizing the long-range magnon momenta, to couplings in a two-dimensional
Ising model. Under this map integrability of planar N=4 Yang-Mills becomes
equivalent to the Yang-Baxter equation for the two-dimensional Ising model, and
the long-range variables x^{\pm} translate into the entries of the Ising
transfer matrices. We explore the Ising correlation length which equals the
inverse magnon momentum in the small momentum limit. The critical regime is
thus reached for vanishing magnon momentum. We also discuss the meaning of the
Kramers-Wannier duality transformation on the gauge theory, together with that
of the Ising model critical points.",0711.3404v2
2008-01-06,Finite-Size Effects for Dyonic Giant Magnons,"We compute finite-size corrections to dyonic giant magnons in two ways. One
is by examining the asymptotic behavior of helical strings of hep-th/0609026 as
elliptic modulus k goes to unity, and the other is by applying the generalized
Luscher formula for mu-term of arXiv:0708.2208 to the situation in which
incoming particles are boundstates. By careful choice of poles in the
su(2|2)^2-invariant S-matrix, we find agreement of the two results, which makes
possible to predict the (leading) finite-size correction for dyonic giant
magnons to all orders in the 't Hooft coupling.",0801.0747v5
2008-01-24,Quantum Wrapped Giant Magnon,"Understanding the finite-size corrections to the fundamental excitations of a
theory is the first step towards completely solving for the spectrum in finite
volume. We compute the leading exponential correction to the quantum energy of
the fundamental excitation of the light-cone gauged string in AdS(5) x S(5),
which is the giant magnon solution. We present two independent ways to obtain
this correction: the first approach makes use of the algebraic curve
description of the giant magnon. The second relies on the purely
field-theoretical Luscher formulas, which depend on the world-sheet S-matrix.
We demonstrate the agreement to all orders in g/Delta of these approaches,
which in particular presents a further test of the S-matrix. We comment on
generalizations of this method of computation to other string configurations.",0801.3671v1
2008-03-24,Spin-Charge-Lattice Coupling through Resonant Multi-Magnon Excitations in Multiferroic BiFeO3,"Spin-charge-lattice coupling mediated by multi-magnon processes is
demonstrated in multiferroic BiFeO3. Experimental evidence of two and three
magnons excitations as well as multimagnon coupling at electronic energy scales
and high temperatures are reported. Temperature dependent Raman experiments
show up to five resonant enhancements of the 2-magnon excitation below the Neel
temperature. These are shown to be collective interactions between on-site Fe
d-d electronic resonance, phonons and multimagnons",0803.3473v2
2008-03-31,Vibrating giant spikes and the large-winding sector,"The single spike is a rigidly rotating classical string configuration closely
related to the giant magnon. We calculate bosonic and fermionic modes of this
solution, from which we see that it is not supersymmetric. It can be viewed as
an excitation above a hoop of string wound around the equator, in the same
sense that the magnon is an excitation above an orbiting point particle. We
find the operator which plays the role of the Hamiltonian for this sector,
which compared to the magnon's E-J has the angular momentum replaced by a
winding charge. The single spike solution is unstable, and we use the modes to
attempt a semi-classical computation of its lifetime.",0803.4222v2
2008-06-10,Ferrimagnetism of MnV_2O_4 spinel,"The spinel MnV_2O_4 is a two-sublattice ferrimagnet, with site A occupied by
the Mn^{2+} ion and site B by the V^{3+} ion. The magnon of the system, the
transversal fluctuation of the total magnetization, is a complicated mixture of
the sublattice A and B transversal magnetic fluctuations. As a result, the
magnons' fluctuations suppress in a different way the manganese and vanadium
magnetic orders and one obtains two phases. At low temperature (0,T^*) the
magnetic orders of the Mn and V ions contribute to the magnetization of the
system, while at the high temperature (T^*,T_N), the vanadium magnetic order is
suppressed by magnon fluctuations, and only the manganese ions have non-zero
spontaneous magnetization. A modified spin-wave theory is developed to describe
the two phases and to calculate the magnetization as a function of temperature.
The anomalous $M(T)$ curve reproduces the experimentally obtained ZFC
magnetization.",0806.1641v1
2008-06-27,Observation of Spin Fluctuations in a High-Tc Parent Compound Using Resonant Inelastic X-ray Scattering,"We report the first observation of soft-x-ray scattering from spin
fluctuations in a high-Tc parent compound. An antiferromagnetic charge transfer
insulator, CaCuO2, was irradiated by Cu M-edge soft x-rays. Ultra-high
resolution measurements of scattered intensity revealed magnon-magnon
excitations, due to spin exchange scattering, as low-energy loss features. The
process is analogous to optical Raman scattering. The spectra provide the first
measurement of the two-magnon excitation energy and the antiferromagnetic
exchange parameter in infinite-layer CaCuO2. The results reveal resonant
inelastic soft x-ray scattering as a novel probe of the spin dynamics in
cuprates.",0806.4432v1
2008-06-30,The SU(2) x SU(2) sector in the string dual of N=6 superconformal Chern-Simons theory,"We examine the string dual of the recently constructed $\mathcal{N}=6$
superconformal Chern-Simons theory of Aharony, Bergman, Jafferis and Maldacena
(ABJM theory). We focus in particular on the $SU(2)\times SU(2)$ sector. We
find a sigma-model limit in which the resulting sigma-model is two
Landau-Lifshitz models added together. We consider a Penrose limit for which we
can approach the $SU(2)\times SU(2)$ sector. Finally, we find a new Giant
Magnon solution in the $SU(2)\times SU(2)$ sector corresponding to one magnon
in each $SU(2)$. We put these results together to find the full magnon
dispersion relation and we compare this to recently found results for ABJM
theory at weak coupling.",0806.4959v4
2008-10-02,Doubly excited ferromagnetic spin-chain as a pair of coupled kicked rotors,"We show that the dynamics of a doubly-excited 1D Heisenberg ferromagnetic
chain, subject to short pulses from a parabolic magnetic field may be analyzed
as a pair of quantum kicked rotors. By focusing on the two-magnon dynamics in
the kicked XXZ model we investigate how the anisotropy parameter - which
controls the strength of the magnon-magnon interaction - changes the nature of
the coupling between the two ""image"" coupled Kicked Rotors. We investigate
quantum state transfer possibilities and show that one may control whether the
spin excitations are transmitted together, or separate from each other.",0810.0454v3
2008-11-04,Physical Origin and Generic Control of Magnonic Band Gaps of Dipole-Exchange Spin Waves in Width-Modulated-Nanostrip Waveguides,"We report, for the first time, on a novel planar structure of
magnonic-crystal waveguides, made of a single magnetic material, in which the
allowed and forbidden bands of propagating dipole-exchange spin-waves can be
manipulated by the periodic modulation of different widths in thin-film
nanostrips. The origin of the presence of several magnonic wide band gaps and
the crucial parameters for controlling those band gaps of the order of ~10 GHz
are found by micromagnetic numerical and analytical calculations. This work can
offer a route to the potential application to broad-band spin-wave filters in
the GHz frequency range.",0811.0411v3
2008-12-17,The antiferromagnetic spin-1/2 Heisenberg model on the square lattice in a magnetic field,"We study the field dependence of the antiferromagnetic spin-1/2 Heisenberg
model on the square lattice by means of exact diagonalizations. In a first
part, we calculate the spin-wave velocity, the spin-stiffness, and the magnetic
susceptibility and thus determine the microscopic parameters of the low-energy
long-wavelength description. In a second part, we present a comprehensive study
of dynamical spin correlation functions for magnetic fields ranging from zero
up to saturation. We find that at low fields, magnons are well defined in the
whole Brillouin zone, but the dispersion is substantially modified by quantum
fluctuations compared to the classical spectrum. At higher fields, decay
channels open and magnons become unstable with respect to multi-magnon
scattering. Our results directly apply to inelastic neutron scattering
experiments.",0812.3420v3
2008-12-19,Direct Observation of Magnon Fractionalization in the Quantum Spin Ladder,"We measure by inelastic neutron scattering the spin excitation spectra as a
function of applied magnetic field in the quantum spin-ladder material
(C5H12N)2CuBr4. Discrete magnon modes at low fields in the quantum disordered
phase and at high fields in the saturated phase contrast sharply with a spinon
continuum at intermediate fields characteristic of the Luttinger-liquid phase.
By tuning the magnetic field, we drive the fractionalization of magnons into
spinons and, in this deconfined regime, observe both commensurate and
incommensurate continua.",0812.3880v1
2009-02-13,"Magnons, their Solitonic Avatars and the Pohlmeyer Reduction","We study the solitons of the symmetric space sine-Gordon theories that arise
once the Pohlmeyer reduction has been imposed on a sigma model with the
symmetric space as target. Under this map the solitons arise as giant magnons
that are relevant to string theory in the context of the AdS/CFT
correspondence. In particular, we consider the cases S^n, CP^n and SU(n) in
some detail. We clarify the construction of the charges carried by the solitons
and also address the possible Lagrangian formulations of the symmetric space
sine-Gordon theories. We show that the dressing, or Backlund, transformation
naturally produces solitons directly in both the sigma model and the symmetric
space sine-Gordon equations without the need to explicitly map from one to the
other. In particular, we obtain a new magnon solution in CP^3. We show that the
dressing method does not produce the more general ""dyonic"" solutions which
involve non-trivial motion of the collective coordinates carried by the
solitons.",0902.2405v1
2009-04-02,Current-controlled dynamic magnonic crystal,"We demonstrate a current-controlled, dynamic magnonic crystal. It consists of
a ferrite film whose internal magnetic field exhibits a periodic, cosine-like
variation. The field modulation is created by a direct current flowing through
an array of parallel wires placed on top of a spin-wave waveguide. A single,
pronounced rejection band in the spin-wave transmission characteristics is
formed due to spin-wave scattering from the inhomogeneous magnetic field. With
increasing current the rejection band depth and its width increase strongly.
The magnonic crystal allows a fast control of its operational characteristics
via the applied direct current. Simulations confirm the experimental results.",0904.0332v1
2009-05-04,Magnon BEC and various phases of 3D quantum helimagnets under high magnetic field,"We study high-field phase diagram and low-energy excitations of
three-dimensional quantum helimagnets. Slightly below the saturation field, the
emergence of magnetic order may be viewed as Bose-Einstein condensation (BEC)
of magnons. The method of dilute Bose gas enables a quantitative analysis of
quantum effects in these helimagnets and thereby three phases are found: cone,
coplanar fan and a phase-separated one. As an application, we map out the phase
diagram of a 3D helimagnet which consists of frustrated J1-J2 chains as a
function of frustration and an interchain coupling. Moreover, we also calculate
the stability of the 2-magnon bound state to investigate the possibility of the
bound-magnon BEC.",0905.0249v2
2009-05-04,Direct current control of three magnon scattering processes in spin-valve nanocontacts,"We have investigated the generation of spin waves in the free layer of an
extended spin-valve structure with a nano-scaled point contact driven by both
microwave and direct electric current using Brillouin light scattering
microscopy. Simultaneously with the directly excited spin waves, strong
nonlinear effects are observed, namely the generation of eigenmodes with
integer multiple frequencies (2 \emph{f}, 3 \emph{f}, 4 \emph{f}) and modes
with non-integer factors (0.5 \emph{f}, 1.5 \emph{f}) with respect to the
excitation frequency \emph{f}. The origin of these nonlinear modes is traced
back to three magnon scattering processes. The direct current influence on the
generation of the fundamental mode at frequency \emph{f} can be related to the
spin-transfer torque, while the efficiency of three-magnon-scattering processes
is controlled by the Oersted field as an additional effect of the direct
current.",0905.0323v1
2009-10-28,Scattering of Giant Magnons in CP^3,"We study classical scattering phase of CP^2 dyonic giant magnons in R_t x
CP^3. We construct two-soliton solutions explicitly by the dressing method.
Using these solutions, we compute the classical time delays for the scattering
of giant magnons, and compare them to boundstate S-matrix elements derived from
the conjectured AdS_4/CFT_3 S-matrix by Ahn and Nepomechie in the strong
coupling limit. Our result is consistent with the conjectured S-matrix. The
dyonic solutions play an essential role in revealing the polarization
dependence of scattering phase.",0910.5315v5
2010-03-10,Dynamical properties of a three-dimensional diluted Heisenberg model,"We study the magnetic excitation spectrum in three-dimensional diluted
ferromagnetic nearest-neighbor systems down to the percolation threshold. The
disorder effects resulting from the dilution are handled accurately within
self-consistent local random phase approximation approach. The calculations are
performed using relatively large systems containing typically 20 000 localized
spins, a systematic average over many configurations of disorder is performed.
We analyze in details the change in the magnon spectrum and magnon density of
states as we increase the dilution. The zone of stability of the well-defined
magnon modes is shown to shrink drastically as we approach the percolation
threshold. We also calculate the spin stiffness which appears to vanish at the
percolation threshold exactly. A comparison with available data, based on a
different theoretical approach, is also provided. We hope that this study will
motivate new experimental studies based on inelastic neutron-scattering
measurements.",1003.2077v2
2010-04-12,Brillouin light scattering studies of planar metallic magnonic crystals,"The application of Brillouin light scattering to the study of the spin-wave
spectrum of one- and two-dimensional planar magnonic crystals consisting of
arrays of interacting stripes, dots and antidots is reviewed. It is shown that
the discrete set of allowed frequencies of an isolated nanoelement becomes a
finite-width frequency band for an array of identical interacting elements. It
is possible to tune the permitted and forbidden frequency bands, modifying the
geometrical or the material magnetic parameters, as well as the external
magnetic field. From a technological point of view, the accurate fabrication of
planar magnonic crystals and a proper understanding of their magnetic
excitation spectrum in the GHz range is oriented to the design of filters and
waveguides for microwave communication systems.",1004.1881v1
2010-06-03,Bose Glass-BEC Transition of Magnons in Tl$_{1-x}$K$_x$CuCl$_3$,"We report the magnetic-field induced Bose glass$-$BEC transition of magnons
in Tl$_{1-x}$K$_x$CuCl$_3$ and its critical behavior investigated through
specific heat and ESR measurements. The field dependence of the BEC transition
temperature $T_{\rm N}$ can be described by the power law $\left[H\,{-}\,H_{\rm
c}\right]\,{\propto}\,T_{\rm N}^{\phi}$ near the quantum critical point $H_{\rm
c}\,{\sim}\,3.5$ T. The critical exponent ${\phi}$ tends to reach a value
smaller than 1/2 with decreasing fitting window in contrast with
${\phi}\rightarrow 3/2$ for the standard BEC in pure system. At sufficiently
low temperatures, the ESR line shape for $H\,{\simeq}\,H_{\rm c}$ is
intermediate between Gaussian and Lorentzian. This indicates the localization
of magnons for $H\,{<}H_{\rm c}$ at T=0.",1006.0584v1
2010-08-05,Influence of spin waves on transport through a quantum-dot spin valve,"We study the influence of spin waves on transport through a single-level
quantum dot weakly coupled to ferromagnetic electrodes with noncollinear
magnetizations. Side peaks appear in the differential conductance due to
emission and absorption of spin waves. We, furthermore, investigate the
nonequilibrium magnon distributions generated in the source and drain lead. In
addition, we show how magnon-assisted tunneling can generate a fullly
spin-polarized current without an applied transport voltage. We discuss the
influence of spin waves on the current noise. Finally, we show how the magnonic
contributions to the exchange field can be detected in the finite-frequency
Fano factor.",1008.0948v2
2010-12-15,Theory of two magnon Raman scattering in antiferromagnetic iron pnictides,"Although the parent iron-based pnictides and chalcogenides are itinerant
antiferromagnets, the use of local moment picture to understand their magnetic
properties is still widespread. We study magnetic Raman scattering from a local
moment perspective for various quantum spin models proposed for this new class
of superconductors. These models vary greatly in the level of magnetic
frustration and show a vastly different two-magnon Raman response. Light
scattering by two-magnon excitations thus provides a robust and independent
measure of the underlying spin interactions. In accord with other recent
experiments, our results indicate that the amount of magnetic frustration in
these systems may be small.",1012.3254v2
2011-02-01,One-dimentional magnonic crystal as a medium with magnetically tunable disorder on a periodical lattice,"We show that periodic magnetic nanostructures (magnonic crystals) represent
an ideal system for studying excitations on disordered periodical lattices
because of the possibility of controlled variation of the degree of disorder by
varying the applied magnetic field. Ferromagnetic resonance (FMR) data
collected inside minor hysteresis loops for a periodic array of Permalloy
nanowires of alternating width and magnetic force microscopy images of the
array taken after running each of these loops were used to establish convincing
evidence that there is a strong correlation between the type of FMR response
and the degree of disorder of the magnetic ground state. We found two types of
dynamic responses: anti-ferromagnetic (AFM) and ferromagnetic (FM), which
represent collective spin wave modes or collective magnonic states. Depending
on the history of sample magnetization either AFM or FM state is either the
fundamental FMR mode or represents a state of a magnetic defect on the
artificial crystal. A fundamental state can be transformed into a defect one
and vice versa by controlled magnetization of the sample.",1102.0069v1
2011-04-07,Width of the longitudinal magnon in the vicinity of the O(3) quantum critical point,"We consider a three-dimensional quantum antiferromagnet in the vicinity of a
quantum critical point separating the magnetically ordered and the magnetically
disordered phases. A specific example is TlCuCl$_3$ where the quantum phase
transition can be driven by hydrostatic pressure and/or by external magnetic
field. As expected two transverse and one longitudinal magnetic excitation have
been observed in the pressure driven magnetically ordered phase. According to
the experimental data, the longitudinal magnon has a substantial width, which
has not been understood and has remained a puzzle. In the present work, we
explain the mechanism for the width, calculate the width and relate value of
the width with parameters of the Bose condensate of magnons observed in the
same compound. The method of an effective quantum field theory is employed in
the work.",1104.1245v1
2011-04-07,Consequences of current conservation in systems with partial magnetic order,"We discuss the consequences of spin current conservation in systems with
SU(2) spin symmetry that is spontaneously broken by partial magnetic order,
using a momentum-space approach. The long-distance interaction is mediated by
Goldstone magnons, whose interaction is expressed in terms of the electron
Green's functions. There is also a Higgs mode, whose excitation energy can be
calculated. The case of fast magnons obeying linear dispersion relation in
three spatial dimensions admits nonperturbative treatment using the Gribov
equation, and the solution exhibits singular behaviour which has an
interpretation as a tower of spin-1 electronic excitations. This occurs near
the Mott insulator state. The electrons are more free in the case of slow
magnons, where the perturbative corrections are less singular at the
thresholds. We then turn our attention to the problem of high-Tc
superconductivity, through the discussion of the stability of the
antiferromagnetic ground state in two spatial dimensions. We argue that this is
caused by an effective mixing of the Goldstone and Higgs modes, which in turn
is caused by an effective Goldstone-boson condensation. The instability of the
antiferromagnetic system is analyzed by studying the non-perturbative behaviour
of the Higgs boson self-energy using the Dyson-Schwinger equations.",1104.1247v1
2011-04-19,"Determination of Boundary Scattering, Intermagnon Scattering, and the Haldane Gap in Heisenberg Chains","Low-lying magnon dispersion in a S=1 Heisenberg antiferromagnetic (AF) chain
is analyzed using the non-Abelian DMRG method. The scattering length $a_{\rm
b}$ of the boundary coupling and the inter-magnon scattering length $a$ are
determined. The scattering length $a_{\rm b}$ is found to exhibit a
characteristic diverging behavior at the crossover point. In contrast, the
Haldane gap $\Delta$, the magnon velocity $v$, and $a$ remain constant at the
crossover. Our method allowed estimation of the gap of the S=2 AF chain to be
$\Delta = 0.0891623(9)$ using a chain length longer than the correlation length
$\xi$.",1104.3633v2
2011-05-05,Antiferromagnet with two coupled antiferromagnetic sublattices in a magnetic field,"We discuss the magnon spectrum of an antiferromagnet (AF) in a magnetic field
$h$ consisting of two interpenetrating AF sublattices coupled by the exchange
interaction at T=0. One-ion easy-plane anisotropy is also taken into account.
We calculate using the 1/S expansion the gap in the spectrum which is a
manifestation of the order-by-disorder effect in this system and the optical
magnon mode splitting. Both of these phenomena originate from the
inter-sublattice interaction. We calculate also the gap value at $h\approx h_c$
in the leading order of the small parameter $(h_c-h)/h_c$ using the magnon
Bose-Einstein condensation treatment, where $h_c$ is the saturation field. By
comparing results obtained within these two approaches we conclude that the 1/S
expansion gives a qualitatively correct result at $h\sim h_c$ even at large
one-ion anisotropy but it overestimates the gap value. The application is
discussed of these results to the actively studied AF of the considered type
$\rm NiCl_2$-$\rm 4SC(NH_2)_2$ (DTN).",1105.1044v1
2011-05-10,Spontaneous magnon decays in planar ferromagnet,"We predict that spin-waves in an easy-plane ferromagnet have a finite
lifetime at zero temperature due to spontaneous decays. In zero field the
damping is determined by three-magnon decay processes, whereas decays in the
two-particle channel dominate in a transverse magnetic field. Explicit
calculations of the magnon damping are performed in the framework of the
spin-wave theory for the $XXZ$ square-lattice ferromagnet with an anisotropy
parameter $\lambda<1$. In zero magnetic field the decays occur for
$\lambda^*<\lambda<1$ with $\lambda^*\approx 1/7$. We also discuss possibility
of experimental observation of the predicted effect in a number of
ferromagnetic insulators.",1105.1893v1
2011-05-24,Multi-Frequency Magnonic Logic Circuits for Parallel Data Processing,"We describe and analyze magnonic logic circuits enabling parallel data
processing on multiple frequencies. The circuits combine bi-stable (digital)
input/output elements and an analog core. The data transmission and processing
within the analog part is accomplished by the spin waves, where logic 0 and 1
are encoded into the phase of the propagating wave. The latter makes it
possible to utilize a number of bit carrying frequencies as independent
information channels. The operation of the magnonic logic circuits is
illustrated by numerical modeling. We also present the estimates on the
potential functional throughput enhancement and compare it with scaled CMOS.
The described multi-frequency approach offers a fundamental advantage over the
transistor-based circuitry and may provide an extra dimension for the Moor's
law continuation. The shortcoming and potentials issues are also discussed.",1105.4671v1
2011-09-07,Systematic Low-Energy Effective Field Theory for Magnons and Holes in an Antiferromagnet on the Honeycomb Lattice,"Based on a symmetry analysis of the microscopic Hubbard and t-J models, a
systematic low-energy effective field theory is constructed for hole-doped
antiferromagnets on the honeycomb lattice. In the antiferromagnetic phase,
doped holes are massive due to the spontaneous breakdown of the $SU(2)_s$
symmetry, just as nucleons in QCD pick up their mass from spontaneous chiral
symmetry breaking. In the broken phase the effective action contains a
single-derivative term, similar to the Shraiman-Siggia term in the square
lattice case. Interestingly, an accidental continuous spatial rotation symmetry
arises at leading order. As an application of the effective field theory we
consider one-magnon exchange between two holes and the formation of two-hole
bound states. As an unambiguous prediction of the effective theory, the wave
function for the ground state of two holes bound by magnon exchange exhibits
$f$-wave symmetry.",1109.1419v2
2011-10-04,Disorder instability of the magnon condensate in a frustrated spin ladder,"The effect of disorder is studied on the field-induced quantum phase
transition in the frustrated spin-ladder compound H8C4SO2Cu2(Cl[1-x]Brx)4 using
bulk magnetic and thermodynamic measurements. The parent material (x=0) is a
quantum spin liquid, which in applied fields is known to form a magnon
condensate with long-range helimagnetic order. We show that bond randomness
introduced by a chemical substitution on the non-magnetic halogene site
destroys this phase transition at very low concentrations, already for x=0.01.
The extreme fragility of the magnon condensate is attributed to random
frustration in the incommensurate state.",1110.0806v3
2011-12-21,Direct detection of magnon spin transport by the inverse spin Hall effect,"Conversion of traveling magnons into an electron carried spin current is
demonstrated in a time resolved experiment using a spatially separated
inductive spin-wave source and an inverse spin Hall effect (ISHE) detector. A
short spin-wave packet is excited in a yttrium-iron garnet (YIG) waveguide by a
microwave signal and is detected at a distance of 3 mm by an attached Pt layer
as a delayed ISHE voltage pulse. The delay in the detection appears due to the
finite spin-wave group velocity and proves the magnon spin transport. The
experiment suggests utilization of spin waves for the information transfer over
macroscopic distances in spintronic devices and circuits.",1112.4969v1
2012-03-07,Dynamical structure factor of quasi-2D antiferromagnet in high fields,"We study high-field magnon dynamics and examine the dynamical structure
factor in the quasi-2D tetragonal Heisenberg antiferromagnet with interlayer
coupling corresponding to realistic materials. Within spin-wave theory, we show
that a non-zero interlayer coupling mitigates singular corrections to the
excitation spectrum occurring in the high-field regime that would otherwise
require a self-consistent approach beyond the 1/S approximation. For the fields
between the threshold for decays and saturation field we observe widening of
the two-magnon sidebands with significant shifting of the spectral weight away
from the quasiparticle peak. We find spectrum broadening throughout large
regions of the Brillouin zone, dramatic redistributions of spectral weight to
the two-magnon continuum, two-peak structures and other features clearly unlike
conventional single-particle peaks.",1203.1621v2
2012-07-27,"Half-Metallic Ferromagnetism in the Heusler Compound Co$_2$FeSi revealed by Resistivity, Magnetoresistance, and Anomalous Hall Effect measurements","We present electrical transport data for single-crystalline Co$_2$FeSi which
provide clear-cut evidence that this Heusler compound is truly a half-metallic
ferromagnet, i.e. it possesses perfect spin-polarization. More specifically,
the temperature dependence of $\rho$ is governed by electron scattering off
magnons which are thermally excited over a sizeable gap $\Delta\approx 100 K$
($\sim 9 meV$) separating the electronic majority states at the Fermi level
from the unoccupied minority states. As a consequence, electron-magnon
scattering is only relevant at $T\gtrsim\Delta$ but freezes out at lower
temperatures, i.e., the spin-polarization of the electrons at the Fermi level
remains practically perfect for $T\lesssim\Delta$. The gapped magnon population
has a decisive influence on the magnetoresistance and the anomalous Hall effect
(AHE): i) The magnetoresistance changes its sign at $T\sim 100 K$, ii) the
anomalous Hall coefficient is strongly temperature dependent at $T\gtrsim 100
K$ and compatible with Berry phase related and/or side-jump electronic
deflection, whereas it is practically temperature-independent at lower
temperatures.",1207.6611v1
2012-08-20,Reversal and Termination of Current-Induced Domain Wall Motion via Magnonic Spin-Transfer Torque,"We investigate the domain wall dynamics of a ferromagnetic wire under the
combined influence of a spin-polarized current and magnonic spin-transfer
torque generated by an external field, taking also into account Rashba
spin-orbit coupling interactions. It is demonstrated that current-induced
motion of the domain wall may be completely reversed in an oscillatory fashion
by applying a magnonic spin-transfer torque as long as the spin-wave velocity
is sufficiently high. Moreover, we show that the motion of the domain wall may
be fully terminated by means of the generation of spin-waves, suggesting the
possibility to pin the domain-walls to predetermined locations. We also discuss
how strong spin-orbit interactions modify these results.",1208.4108v1
2012-08-27,Photo-magnonics,"In the framework of magnonics all-optical femtosecond laser experiments are
used to study spin waves and their relaxation paths. Magnonic crystal
structures based on antidots allow the control over the spin-wave modes. In
these two-dimensional magnetic metamaterials with periodicities in the
wave-length range of dipolar spin waves the spin-wave bands and dispersion are
modified. Hence, a specific selection of spin-wave modes excited by laser
pulses is possible. Different to photonics, the modes depend strongly on the
strength of the magneto-static potential at around each antidot site - the
dipolar field. While this may lead to a mode localization, also for filling
fractions around or below 10%, Bloch states are found in low damping
ferromagnetic metals. In this chapter, an overview of these mechanisms is given
and the connection to spin-wave band spectra calculated from an analytical
model is established. Namely, the plane-wave method yields flattened bands as
well as band gaps at the antidot lattice Brillouin zone boundary.",1208.5383v1
2012-08-29,Electron spin resonance shifts in S=1 antiferromagnetic chains,"We discuss electron spin resonance (ESR) shifts in spin-1 Heisenberg
antiferromagnetic chains with a weak single-ion anisotropy based on several
effective field theories, the O(3) nonlinear sigma model (NLSM) in the Haldane
phase, free fermion theories around the lower and the upper critical fields. In
the O(3) NLSM, the single-ion anisotropy corresponds to a composite operator
which creates two magnons at the same time and position. Therefore, even inside
a parameter range where free magnon approximation is valid, we have to take
interactions among magnons into account. Though the O(3) NLSM is only valid in
the Haldane phase, an appropriate translation of Faddeev-Zamolodchikov
operators of the O(3) NLSM to fermion operators enables one to treat ESR shifts
near the lower critical field in a similar manner to discussions in Haldane
phase. We present that our theory gives quantitative agreements with recent ESR
experimental results on an spin-1 chain compounds NDMAP.",1208.6017v2
2012-10-12,"Spin superfluidity, coherent spin precession, and magnon BEC","Spin superfluidity, coherent spin precession, and magnon BEC are intensively
investigated theoretically and experimentally nowadays. Meanwhile, clear
definition and differentiation between these related phenomena is needed. It is
argued that spin stiffness, which leads to existence of coherent spin
precession and dissipationless spin supercurrents, is a necessary but not
sufficient condition for spin superfluidity. The latter is defined as a
possibility of spin transport on macroscopical distances with sufficiently
large spin supercurrents. This possibility is realized at special topology of
the magnetic-order-parameter space, such as, e.g., that in easy-plane
antiferromagnets. It is argued that an arbitrarily chosen formal criterion for
the existence of magnon BEC has no connection with conditions for observation
of macroscopic dissipationless spin transport.",1210.3451v1
2012-11-26,Magnon Supersolid and Anomalous Hysteresis in Spin Dimers on a Triangular Lattice,"We study the magnetic phase diagram and hysteresis behavior of weakly coupled
spin dimers on a triangular lattice using the cluster mean-field method with
cluster-size scaling. We find that the magnetization curve has plateaus at 1/3
and 2/3 of the total magnetization, in which local singlet and triplet states
form a superlattice pattern. Moreover, if increasing (decreasing) the magnetic
field from the 1/3 (2/3) plateau, the Bose-Einstein condensation (BEC) of
triplons occurs on the superlattice background, leading to the transition into
magnon supersolid phase. We also find that the first-order transition between
these solid states and the standard magnon BEC state exhibits an anomalous
hysteresis upon cycling the magnetic field; the transition can occur only from
solid to BEC, and the system cannot return to the initial solid state in the
reverse process.",1211.5880v2
2012-12-07,Brillouin Light Scattering Spectra as local Temperature Sensors for Thermal Magnons and Acoustic Phonons,"We demonstrate the use of the micro-Brillouin light scattering (micro-BLS)
technique as a local temperature sensor for magnons in a Permalloy thin film
and phonons in the glass substrate. A systematic shift in the frequencies of
two thermally excited perpendicular standing spin wave modes as the film is
uniformly heated allows us to achieve a temperature resolution better than 2.5
K. We demonstrate that the micro-BLS spectra can be used to measure the local
temperatures of phonons and magnons across a thermal gradient. Such local
temperature sensors are useful for investigating spin caloritronic and thermal
transport phenomena in general.",1212.1520v2
2012-12-11,Raman scattering in a Heisenberg S=1/2 antiferromagnet on the anisotropic triangular lattice,"We investigate the two-magnon Raman scattering from an anisotropic S=1/2
triangular Heisenberg antiferromagnet Cs2CuCl4. We find that the Raman response
is very sensitive to magnon-magnon interactions and to scattering geometries, a
feature that is in remarkable contrast with the polarization-independent Raman
signal from the isotropic triangular Heisenberg antiferromagnet. Since a
spin-liquid ground state gives rise to a similar rotationally invariant Raman
response, our results on the polarization dependence of the scattering spectrum
suggest that Raman spectroscopy provides a useful probe, complementary to
neutron scattering, of the ground-state properties of Cs2CuCl4, particularly
whether the time-reversal symmetry is broken in the ground state.",1212.2286v2
2013-01-10,"Comment on ""Physical Origin and Generic Control of Magnonic Band Gaps of Dipole-Exchange Spin Waves in Width-Modulated Nanostrip Waveguides"" [K.-S. Lee, D.-S. Han, and S.-K. Kim, PRL 102, 127202 (2009), arXiv:0811.0411]","In Ref. [PRL 102, 127202 (2009)] Lee et al. reported the existence of large
magnonic bandgaps in one-dimensional width-modulated Permalloy nanostripe
waveguides based on OOMMF simulations. However, as the symmetry of the magnetic
field pulse they applied to excite the spin waves (SWs) was not general, the
entire set of SW branches with A symmetry was omitted from the magnonic band
structures (see below). This omission has unfortunately led to misleading
conclusions of, for instance, the number, width and position of the bandgaps.
We present here the full band structure based on three different theoretical
approaches that gave consistent predictions, thus corroborating the methods
employed, namely, a microscopic approach, OOMMF simulations, and a method based
on the linearized Landau-Lifshitz equation. Further, we provide a physical
interpretation using group theory.",1301.2036v1
2013-02-25,Varied Perturbation Theory for the Dispersion Dip in the Two-Dimensional Heisenberg Quantum Antiferromagnet,"We study the roton-like dip in the magnon dispersion at the boundary of the
Brillouin zone in the isotropic S=1/2 Heisenberg quantum antiferromagnet. This
high-energy feature is sometimes seen as indication of a fractionalization of
the magnons to spinons. In this article, we provide evidence that the
description of the dip in terms of magnons can be improved significantly by
applying more advanced evaluation schemes. In particular, we illustrate the
usefulness of the application of the principle of minimal sensitivity in varied
perturbation theory. Thereby, we provide an example for the application of this
approach to an extended condensed matter problem governed by correlations which
can trigger analogous investigations for many other systems.",1302.6201v2
2013-03-05,Dynamics and relaxation in spin nematics,"We study dynamics and relaxation of elementary excitations (magnons) in the
spin nematic (quadrupole ordered) phase of S=1 magnets. We develop a general
phenomenological theory of spin dynamics and relaxation for spin-1 systems.
Results of the phenomenological approach are compared to those obtained by
microscopic calculations for the specific S=1 model with isotropic bilinear and
biquadratic exchange interactions. This model exhibits a rich behavior
depending on the ratio of bilinear and biquadratic exchange constants,
including several points with an enhanced symmetry. It is shown that symmetry
plays an important role in relaxation. Particularly, at the SU(3) ferromagnetic
point the magnon damping $\Gamma$ depends on its wavevector k as $\Gamma\propto
k^{4}$, while a deviation from the high-symmetry point changes the behavior of
the leading term to $\Gamma\propto k^{2}$. We point out a similarity between
the behavior of magnon relaxation in spin nematics to that in an isotropic
ferromagnet.",1303.1194v2
2013-03-14,Spin-current order in anisotropic triangular antiferromagnets,"We analyze instabilities of the collinear up-up-down state of a
two-dimensional quantum spin-S spatially anisotropic triangular lattice
antiferromagnet in a magnetic field. We find, within large-S approximation,
that near the end point of the plateau, the collinear state becomes unstable
due to condensation of two-magnon bound pairs rather than single magnons. The
two-magnon instability leads to a novel 2D vector chiral phase with alternating
spin currents but no magnetic order in the direction transverse to the field.
This phase breaks a discrete Z_2 symmetry but preserves a continuous U(1) one
of rotations about the field axis. It possesses orbital antiferromagnetism and
displays a magnetoelectric effect.",1303.3519v2
2013-03-26,Double zigzag spin chain in strong magnetic field close to saturation,"We study the ground state phase diagram of a frustrated spin tube in a strong
external magnetic field. This model can be viewed as two coupled zigzag spin
chains, or as a two-leg spin ladder with frustrating next-nearest-neighbor
couplings along the legs, and its study is motivated by the physics of such
materials as Sulfolane-Cu_{2}Cl_{4} and BiCu_{2}PO_{6}. In magnetic fields
right below the saturation, the system can be effectively represented as a
dilute gas of two species of bosonic quasiparticles that correspond to magnons
with inequivalent incommensurate momenta at two degenerate minima of the magnon
dispersion. Using the method previously proposed and tested for frustrated spin
chains, we calculate effective interactions in this two-component Bose gas. On
this basis, we establish the phase diagram of nearly-saturated frustrated spin
tube, which is shown to include the two-component Luttinger liquid, two types
of vector chiral phases, and phases whose physics is determined by the presence
of bound magnons. We study the phase diagram of the model numerically by means
of the density matrix renormalization group technique, and find a good
agreement with our analytical predictions.",1303.6662v1
2013-04-08,Intrinsic and extrinsic mirror symmetry breaking in anti-dot spin-wave waveguides,"We theoretically study the spin-wave spectra in magnonic waveguides
periodically patterned with square anti-dots in nanoscale with pinned
magnetization at the edges. We show that the breaking of the mirror symmetry of
the waveguide by the structural changes can result in a magnonic band gap
closing. These intrinsic symmetry breaking can be compensated by properly
chosen asymmetric external bias magnetic field, i.e. in an extrinsic way. As a
result the magnonic gaps existing in the ideal symmetric structure can be
recovered. The model used for the explanation also suggests that this idea
could be generic both for exchange and dipolar interaction regimes of
spin-waves and also for other types of waves, e.g., electrons in the graphene
ribbons.",1304.1799v1
2013-05-22,Spin-wave nonreciprocity based on interband magnonic transitions,"We theoretically demonstrate linear spin-wave nonreciprocity in a Ni80Fe20
nanostripe waveguide, based on interband magnonic transitions induced by a
time-reversal and spatialinversion symmetry breaking magnetic field. An
analytical coupled-mode theory of spin waves, developed to describe the
transitions which are accompanied by simultaneous frequency and wavevector
shifts of the coupled spin waves, is well corroborated by numerical
simulations. Our findings could pave the way for the realization of spin-wave
isolation and the dynamic control of spin-wave propagation in nanoscale
magnonic integrated circuits via an applied magnetic field.",1305.5018v3
2013-07-18,Gutzwiller Approach for Elementary Excitations in $S=1$ Antiferromagnetic Chains,"In a previous paper [Phys. Rev. B 85,195144 (2012)], variational Monte Carlo
method (based on Gutzwiller projected states) was generalized to $S=1$ systems.
This method provided very good trial ground states for the gapped phases of
$S=1$ bilinear-biquadratic (BLBQ) Heisenberg chain. In the present paper, we
extend the approach to study the low-lying elementary excitations in $S=1$
chains. We calculate the one-magnon and two-magnon excitation spectra of the
BLBQ Heisenberg chain and the results agree very well with recent data in
literature. In our approach, the difference of the excitation spectrum between
the Haldane phase and the dimer phase (such as the even/odd size effect) can be
understood from their different topology of corresponding mean field theory. We
especially study the Takhtajan-Babujian critical point. Despite the fact that
the `elementary excitations' are spin-1 magnons which are different from the
spin-1/2 spinons in Bethe solution, we show that the excitation spectrum,
critical exponent ($\eta=0.74$) and central charge ($c=1.45$) calculated from
our theory agree well with Bethe ansatz solution and conformal field theory
predictions.",1307.4958v2
2013-11-13,Fokker-Planck approach to the theory of magnon-driven spin Seebeck effect,"Following the theoretical approach by Xiao et al [Phys. Rev. B 81, 214418
(2010)] to the spin Seebeck effect, we calculate the mean value of the total
spin current flowing through a normalmetal/ ferromagnet interface. The spin
current emitted from the ferromagnet to the normal metal is evaluated in the
framework of the Fokker-Planck approach for the stochastic
Landau-Lifshitz-Gilbert equation. We show that the total spin current depends
not only on the temperature difference between the electron and the magnon
baths, but also on the external magnetic field and magnetic anisotropy. Apart
from this, the spin current is shown to saturate with increasing magnon
temperature, and the saturation temperature increases with increasing magnetic
field and/or magnetic anisotropy.",1311.3117v1
2013-11-19,Observation of the spin Peltier effect,"We report the observation of the spin Peltier effect (SPE) in the
ferrimagnetic insulator Yttrium Iron Garnet (YIG), i.e. a heat current
generated by a spin current flowing through a Platinum (Pt)|YIG interface. The
effect can be explained by the spin torque that transforms the spin current in
the Pt into a magnon current in the YIG. Via magnon-phonon interactions the
magnetic fluctuations modulate the phonon temperature that is detected by a
thermopile close to the interface. By finite-element modelling we verify the
reciprocity between the spin Peltier and spin Seebeck effect. The observed
strong coupling between thermal magnons and phonons in YIG is attractive for
nanoscale cooling techniques.",1311.4772v1
2013-12-04,Transient Carrier Dynamics in a Mott Insulator with Antiferromagnetic Order,"We study transient dynamics of hole carriers injected at a certain time into
a Mott insulator with antiferromagnetic long range order. This is termed
``dynamical hole doping"" as contrast with chemical hole doping. Theoretical
framework for the transient carrier dynamics are presented based on the two
dimensional $t-J$ model. Time dependences of the optical conductivity spectra
as well as the one-particle excitation spectra are calculated based on the
Keldysh Green's function formalism at zero temperature combined with the
self-consistent Born approximation. At early stage after dynamical hole doping,
the Drude component appears, and then incoherent components originating from
hole-magnon scatterings start to grow. Fast oscillatory behavior due to
coherent magnon, and slow relaxation dynamics are confirmed in the spectra.
Time profiles are interpreted as that doped bare holes are dressed by magnon
clouds, and are relaxed into spin polaron quasi-particle states. Characteristic
relaxation times for Drude and incoherent peaks strongly depend on momentum of
a dynamically doped hole, and the exchange constant. Implications to the recent
pump-probe experiments are discussed.",1312.1077v1
2014-01-20,On the theory of inhomogeneous Bose-Einstein condensation of magnons in yttrium garnet,"The Bose-Einstein condensation (BEC) of magnons created by a strong pumping
in ferromagnetic thin films of yttrium iron garnet used as systems of finite
size is considered analytically. Such a peculiarity, typical for this magnetic
material, as the presence of a minimum in the spectrum of spin waves at a
finite value of the wave vector is taken into account. The definition of
hightemperature BEC is introduced and its characteristics are discussed. A role
of boundary conditions for spin variables is analyzed, and it is shown that in
the case of free spins on the boundary the magnon lattice can form in the
system. The factors responsible for its appearance are discussed.",1401.4854v1
2014-02-27,Magneto- to electro-active transmutation of spin waves in ErMnO3,"The low energy dynamical properties of the multiferroic hexagonal perovskite
ErMnO3 have been studied by inelastic neutron scattering as well as terahertz
and far infrared spectroscopies on synchrotron source. From these complementary
techniques, we have determined the magnon and crystal field spectra and
identified a zone center magnon only excitable by the electric field of an
electromagnetic wave. Using comparison with the isostructural YMnO3 compound
and crystal field calculations, we propose that this dynamical magnetoelectric
process is due to the hybridization of a magnon with an electro-active crystal
field transition.",1402.7018v1
2014-04-01,Raman study of magnetic excitations and magneto-elastic coupling in alpha-SrCr2O4,"Using Raman spectroscopy, we investigate the lattice phonons, magnetic
excitations, and magneto-elastic coupling in the distorted triangular-lattice
Heisenberg antiferromagnet alpha-SrCr2O4, which develops helical magnetic order
below 43 K. Temperature dependent phonon spectra are compared to predictions
from density functional theory calculations which allows us to assign the
observed modes and identify weak effects arising from coupled lattice and
magnetic degrees of freedom. Raman scattering associated with two-magnon
excitations is observed at 20 meV and 40 meV. These energies are in general
agreement with our ab-initio calculations of exchange interactions and earlier
theoretical predictions of the two-magnon Raman response of triangular-lattice
antiferromagnets. The temperature dependence of the two-magnon excitations
indicates that spin correlations persist well above the N\'eel temperature.",1404.0355v2
2014-09-09,Magnetization pumping and dynamics in a Dzyaloshinskii-Moriya magnet,"We formulate a phenomenological description of thin ferromagnetic layers with
inversion asymmetry where the single-domain magnetic dynamics experiences
magnon current-induced torques and leads to magnon-motive forces. We first
construct a phenomenological theory based on irreversible thermodynamics,
taking into account the symmetries of the system. Furthermore, we confirm that
these effects originate from Dzyaloshinskii-Moriya interactions from the
analysis based on the stochastic Landau-Lifshitz-Gilbert equation. Our
phenomenological results generalize to a general form of Dzyaloshinskii-Moriya
interactions and to other systems, such as pyrochlore crystals and chiral
magnets. Possible applications include spin current generation, magnetization
reversal and magnonic cooling.",1409.2846v3
2014-09-25,Tuning the Band Structures of a 1D Width-Modulated Magnonic Crystal by a Transverse Magnetic Field,"Theoretical studies, based on three independent techniques, of the band
structure of a one-dimensional width-modulated magnonic crystal under a
transverse magnetic field are reported. The band diagram is found to display
distinct behaviors when the transverse field is either larger or smaller than a
critical value. The widths and center positions of bandgaps exhibit unusual
non-monotonic and large field-tunability through tilting the direction of
magnetization. Some bandgaps can be dynamically switched on and off by simply
tuning the strength of such a static field. Finally, the impact of the lowered
symmetry of the magnetic ground state on the spin-wave excitation efficiency of
an oscillating magnetic field is discussed. Our finding reveals that the
magnetization direction plays an important role in tailoring magnonic band
structures and hence in the design of dynamic spin-wave switches.",1409.7183v1
2014-12-11,Magnon Hall effect and anisotropic thermal transport in NiFe and YIG ferromagnets,"The Righi-Leduc effect refers to the thermal analogue of the Hall effect, for
which the electric current is replaced by the heat current and the electric
field by the temperature gradient. In both cases, the magnetic field generates
a transverse force that deviates the carriers (electron, phonon, magnon) in the
direction perpendicular to the current. In a ferromagnet, the magnetization
plays the role of the magnetic field, and the corresponding effect is called
anomalous Hall effect. Furthermore, a second transverse contribution due to the
anisotropy, the planar Hall effect, is superimposed to the anomalous Hall
effect. We report experimental evidence of the thermal counterpart of the Hall
effects in ferromagnets, namely the magnon Hall effect (or equivalently the
anomalous Righi-Leduc effect) and the planar Righi-Leduc effect, measured on
ferromagnets that are either electrical conductor (NiFe) or insulator (YIG).
The study shows the universal character of these new thermokinetic effects,
related to the intrinsic chirality of the anisotropic ferromagnetic degrees of
freedom.",1412.3723v1
2015-01-06,Sensing magnetic nanoparticles using nano-confined ferromagnetic resonances in a magnonic crystal,"We demonstrate the use of the magnetic-field-dependence of highly spatially
confined, GHz-frequency ferromagnetic resonances in a ferromagnetic
nanostructure for the detection of adsorbed magnetic nanoparticles. This is
achieved in a large area magnonic crystal consisting of a thin ferromagnetic
film containing a periodic array of closely spaced, nano-scale anti-dots. Stray
fields from nanoparticles within the anti-dots modify resonant dynamic
magnetisation modes in the surrounding magnonic crystal, generating easily
measurable resonance peak shifts. The shifts are comparable to the resonance
linewidths for high anti-dot filling fractions with their signs and magnitudes
dependent upon the modes' localisations (in agreement with micromagnetic
simulation results). This is a highly encouraging result for the development of
frequency-based nanoparticle detectors for high speed nano-scale biosensing.",1501.01171v1
2015-02-06,Thermal Hall conductivity in the frustrated pyrochlore magnet Tb2Ti2O7,"In a ferromagnet, the spin excitations are the well-studied magnons. In
frustrated quantum magnets, long-range magnetic order fails to develop despite
a large exchange coupling between the spins. In contrast to the magnons in
conventional magnets, their spin excitations are poorly understood. Are they
itinerant or localized? Here we show that the thermal Hall conductivity
$\kappa_{xy}$ provides a powerful probe of spin excitations in the ""quantum
spin ice"" pyrochlore Tb$_2$Ti$_2$O$_7$. The thermal Hall response is large even
though the material is transparent. The Hall response arises from spin
excitations with specific characteristics that distinguish them from magnons.
At low temperature ($T<$ 1 K), the thermal conductivity imitates that of a
dirty metal. Using the Hall angle, we construct a phase diagram showing how the
excitations are suppressed by a magnetic field.",1502.02006v1
2015-03-05,Analytical soliton solution for the Landau-Lifshitz equation of one dimensional magnonic crystal,"Nonlinear localized magnetic excitations in one dimensional magnonic crystal
is investigated under periodic magntic field. The governing Landau-Lifshitz
equation is transformed into variable coefficient nonlinear Schrodinger
equation(VCNLS) using sterographic projection. The VCNLS equation is in general
nonintegrable, by using painleve analysis necessary conditions for the VCNLS
equation to pass Weiss-Tabor-Carnevale (WTC) Painleve test are obtained. A
sufficient integrability condition is obtained by further exploring a
transformation, which can map the VCNLS equation into the well-known standard
nonlinear Schrodinger equation. The transformation built a systematic
connection between the solution of the standard nonlinear Schrodinger equation
and VC-NLS equation. The results shows the excitation of magnetization in the
form of soliton has spatialperiod exists on the background of spin Bloch waves.
Such solution exisits only certain constrain conditions on the coefficient of
the VCNLS equation are satisfied. The analytical results suggest a way to
control the dynamics of magnetization in the form of solitons by an appropriate
spatial modulation of the nonlinearity coefficient in the governing VCNLS
equation which is determined by the ferromagnetic materials which forms the
magnonic crystal.",1503.01559v1
2015-03-30,Magnon contribution to the magnetoresistance of iron nanowires deposited using pulsed electrodeposition,"Iron nanowires with a square cross section are grown by pulsed
electrodeposition within a newly developed nanochannel template that allows for
easy characterization. Measurements of the magnetoresistance as a function of
magnetic field and temperature are performed within a large parameter window
allowing for the investigation of the magnonic contribution to the
magnetoresistance of electrodeposited iron nanowires. Values for the
temperature dependent magnon stiffness D(T) are extracted.",1503.08595v1
2015-06-03,In-plane angular dependence of the spin-wave nonreciprocity of an ultrathin film with Dzyaloshinskii-Moriya interaction,"The nonreciprocal propagation of spin waves in an ultrathin Pt/Co/Ni film has
been measured by Brillouin light scattering. The frequency nonreciprocity, due
to the interfacial Dzyaloshinskii-Moriya interaction (DMI), has a sinusoidal
dependence on the in-plane angle between the magnon wavevector and the applied
magnetic field. The results, which are in good agreement with analytical
predictions reported earlier, yield a value of the DMI constant which is the
same as that obtained previously from a study of the magnon dispersion
relations. We have demonstrated that our magnon-dynamics based method can
experimentally ascertain the DMI constant of multilayer thin films.",1506.01134v1
2015-09-14,Spectral characteristics of time resolved magnonic spin Seebeck effect,"Spin Seebeck effect (SSE) holds promise for new spintronic devices with
low-energy consumption. The underlying physics, essential for a further
progress, is yet to be fully clarified. This study of the time resolved
longitudinal SSE in the magnetic insulator yttrium iron garnet (YIG) concludes
that a substantial contribution to the spin current stems from small
wave-vector subthermal exchange magnons. Our finding is in line with the recent
experiment by S. R. Boona and J. P. Heremans, Phys. Rev. B 90, 064421 (2014).
Technically, the spin-current dynamics is treated based on the
Landau-Lifshitz-Gilbert (LLG) equation also including magnons back-action on
thermal bath, while the formation of the time dependent thermal gradient is
described self-consistently via the heat equation coupled to the magnetization
dynamics",1509.04018v1
2015-09-18,Theory and experiment on cavity magnon polariton in the 1D configuration,"We have theoretically and experimentally investigated the dispersion of the
cavity-magnon-polariton (CMP) in a 1D configuration, created by inserting a low
damping magnetic insulator into a high-quality 1D microwave cavity. By
simplifying the full-wave simulation based on the transfer matrix approach in
the long wavelength limit, an analytic approximation of the CMP dispersion has
been obtained. The resultant coupling strength of the CMP shows different
dependence on the sample thickness as well as the permittivity of the sample,
determined by the parity of the cavity modes. These scaling effects of the
cavity and material parameters are confirmed by experimental data. Our work
provide a detailed understanding of the 1D CMP, which could help to engineer
coupled magnon-photon system.",1509.05804v1
2015-10-07,Cavity optomagnonics with spin-orbit coupled photons,"We experimentally implement a system of cavity optomagnonics, where a sphere
of ferromagnetic material supports whispering gallery modes (WGMs) for photons
and the magnetostatic mode for magnons. We observe pronounced nonreciprocity
and asymmetry in the sideband signals generated by the magnon-induced Brillouin
scattering of light. The spin-orbit coupled nature of the WGM photons, their
geometric birefringence and the time-reversal symmetry breaking in the magnon
dynamics impose the angular-momentum selection rules in the scattering process
and account for the observed phenomena. The unique features of the system may
find interesting applications at the crossroad between quantum optics and
spintronics.",1510.01837v4
2015-10-13,Optomagnonic whispering gallery microresonators,"Magnons in ferrimagnetic insulators such as yttrium iron garnet (YIG) have
recently emerged as promising candidates for coherent information processing in
microwave circuits. Here we demonstrate optical whispering gallery modes of a
YIG sphere interrogated by a silicon nitride photonic waveguide, with quality
factors approaching $10^6$ in the telecom c-band after surface treatments.
Moreover, in contrast to conventional Faraday setup, this implementation allows
input photon polarized colinearly to the magnetization to be scattered to a
sideband mode of orthogonal polarization. This Brillouin scattering process is
enhanced through triply resonant magnon, pump and signal photon modes - all of
whispering gallery nature - within an ""optomagnonic cavity"". Our results show
the potential use of magnons for mediating microwave-to-optical carrier
conversion.",1510.03545v1
2015-11-12,Thermally-driven spin torques in layered magnetic insulators,"Thermally-driven spin-transfer torques have recently been reported in
electrically insulating ferromagnet$|$normal-metal heterostructures. In this
paper, we propose two physically distinct mechanisms for such torques. The
first is a local effect: out-of-equilibrium, thermally-activated magnons in the
ferromagnet, driven by a spin Seebeck effect, exert a torque on the
magnetization via magnon-magnon scattering with coherent dynamics. The second
is a nonlocal effect which requires an additional magnetic layer to provide the
symmetry breaking necessary to realize a thermal torque. The simplest structure
in which to induce a nonlocal thermal torque is a spin valve composed of two
insulating magnets separated by a normal metal spacer; there, a thermal flux
generates a pure spin current through the spin valve, which results in a torque
when the magnetizations of the layers are misaligned.",1511.04104v1
2016-01-04,Unidirectional terahertz light absorption in the pyroelectric ferrimagnet CaBaCo4O7,"Spin excitations were studied by absorption spectroscopy in CaBaCo4O7 which
is a type-I multiferroic compound with the largest magnetic-order induced
ferroelectric polarization ({\Delta}P=17mC/m2) reported, so far. We observed
two optical magnon branches: a solely electric dipole allowed one and a mixed
magnetoelectric resonance. The entangled magnetization and polarization
dynamics of the magnetoelectric resonance gives rise to unidirectional light
absorption, i.e. that magnon mode absorbs the electromagnetic radiation for one
propagation direction but not for the opposite direction. Our systematic study
of the magnetic field and temperature dependence of magnon modes provides
information about the energies and symmetries of spin excitations, which is
required to develop a microscopic spin model of CaBaCo4O7.",1601.00444v1
2016-01-21,Spin pumping in strongly coupled magnon-photon systems,"We experimentally investigate magnon-polaritons, arising in ferrimagnetic
resonance experiments in a microwave cavity with a tuneable quality factor. To
his end, we simultaneously measure the electrically detected spin pumping
signal and microwave reflection (the ferrimagnetic resonance signal) of a
yttrium iron garnet (YIG) / platinum (Pt) bilayer in the microwave cavity. The
coupling strength of the fundamental magnetic resonance mode and the cavity is
determined from the microwave reflection data. All features of the magnetic
resonance spectra predicted by first principle calculations and an input-output
formalism agree with our experimental observations. By changing the decay rate
of the cavity at constant magnon-photon coupling rate, we experimentally tune
in and out of the strong coupling regime and successfully model the
corresponding change of the spin pumping signal. Furthermore, we observe the
coupling and spin pumping of several spin wave modes and provide a quantitative
analysis of their coupling rates to the cavity.",1601.05681v1
2016-01-26,LLM Magnons,"We consider excitations of LLM geometries described by coloring the LLM plane
with concentric black rings. Certain closed string excitations are localized at
the edges of these rings. The string theory predictions for the energies of
magnon excitations of these strings depends on the radii of the edges of the
rings. In this article we construct the operators dual to these closed string
excitations and show how to reproduce the string theory predictions for magnon
energies by computing one loop anomalous dimensions. These operators are linear
combinations of restricted Schur polynomials. The distinction between what is
the background and what is the excitation is accomplished in the choice of the
subgroup and the representations used to construct the operator.",1601.06914v1
2016-01-27,Observation of magnon-mediated current drag in Pt/yttrium iron garnet/Pt(Ta) trilayers,"Pure spin current, a flow of spin angular momentum without flow of any
companying net charge, is generated in two common ways. One makes use of the
spin Hall effect in normal metals (NM) with strong spin-orbit coupling, such as
Pt or Ta. The other utilizes the collective motion of magnetic moments or spin
waves with the quasi-particle excitations called magnons. A popular material
for the latter is yttrium iron garnet, a magnetic insulator (MI). Here we
demonstrate in NM/MI/NM trilayers that these two types of spin currents are
interconvertible across the interfaces, predicated as the magnon-mediated
current drag phenomenon. The transmitted signal scales linearly with the
driving current without a threshold and follows the power-law with n ranging
from 1.5 to 2.5. Our results indicate that the NM/MI/NM trilayer structure can
serve as a scalable pure spin current valve device which is an essential
ingredient in spintronics.",1601.07429v1
2016-02-11,Probing ultrafast spin dynamics in the antiferromagnetic multiferroic HoMnO$_3$ through a magnon resonance,"We demonstrate a new approach for directly measuring the ultrafast energy
transfer between elec- trons and magnons, enabling us to track spin dynamics in
an antiferromagnet (AFM). In multiferroic HoMnO3, optical photoexcitation
creates hot electrons, after which changes in the spin order are probed with a
THz pulse tuned to a magnon resonance. This reveals a photoinduced
transparency, which builds up over several picoseconds as the spins heat up due
to energy transfer from hot elec- trons via phonons. This spin-lattice
thermalization time is ?10 times faster than that of typical ferromagnetic (FM)
manganites. We qualitatively explain the fundamental differences in
spin-lattice thermalization between FM and AFM systems and apply a Boltzmann
equation model for treating AFMs. Our work gives new insight into spin-lattice
thermalization in AFMs and demonstrates a new approach for directly monitoring
the ultrafast dynamics of spin order in these systems.",1602.03872v2
2016-04-25,Magnon based logic in a multi-terminal YIG/Pt nanostructure,"Boolean logic is the foundation of modern digital information processing.
Recently, there has been a growing interest in phenomena based on pure spin
currents, which allow to move from charge to spin based logic gates. We study a
proof-of-principle logic device based on the ferrimagnetic insulator Yttrium
Iron Garnet (YIG), with Pt strips acting as injectors and detectors for
nonequilibrium magnons. We experimentally observe incoherent superposition of
magnons generated by different injectors. This allows to implement a fully
functional majority gate, enabling multiple logic operations (AND and OR) in
one and the same device. Clocking frequencies of the order of several GHz and
straightforward down-scaling make our device promising for applications.",1604.07262v1
2016-06-06,Spin Nernst Effect of Magnons in Collinear Antiferromagnets,"In a collinear antiferromagnet with easy-axis anisotropy, symmetry guarantees
that the spin wave modes are doubly degenerate. The two modes carry opposite
spin angular momentum and exhibit opposite chirality. Using a honeycomb
antiferromagnet in the presence of the Dzyaloshinskii-Moriya interaction, we
show that a longitudinal temperature gradient can drive the two modes to
opposite transverse directions, realizing a spin Nernst effect of magnons with
vanishing thermal Hall current. We find that magnons around the {\Gamma}-point
and the K-point contribute oppositely to the transverse spin transport, and
their competition leads to a sign change of the spin Nernst coefficient at
finite temperature. Possible material candidates are discussed.",1606.01952v3
2016-06-09,"Competing effects at Pt/YIG interfaces: spin Hall magnetoresistance, magnon excitations and magnetic frustration","We study the spin Hall magnetoresistance (SMR) and the magnon spin transport
(MST) in Pt/Y3Fe5O12(YIG)-based devices with intentionally modified interfaces.
Our measurements show that the surface treatment of the YIG film results in a
slight enhancement of the spin-mixing conductance and an extraordinary increase
in the efficiency of the spin-to-magnon excitations at room temperature. The
surface of the YIG film develops a surface magnetic frustration at low
temperatures, causing a sign change of the SMR and a dramatic suppression of
the MST. Our results evidence that SMR and MST could be used to explore
magnetic properties of surfaces, including those with complex magnetic
textures, and stress the critical importance of the non-magnetic/ferromagnetic
interface properties in the performance of the resulting spintronic devices.",1606.02968v2
2016-07-08,Magnon Polarons in the Spin Seebeck Effect,"Sharp structures in magnetic field-dependent spin Seebeck effect (SSE)
voltages of Pt/Y$_{3}$Fe$_{5}$O$_{12}$ (YIG) at low temperatures are attributed
to the magnon-phonon interaction. Experimental results are well reproduced by a
Boltzmann theory that includes the magnetoelastic coupling (MEC). The SSE
anomalies coincide with magnetic fields tuned to the threshold of
magnon-polaron formation. The effect gives insight into the relative quality of
the lattice and magnetization dynamics.",1607.02312v2
2016-07-25,Damping of parametrically excited magnons in the presence of the longitudinal spin Seebeck effect,"The impact of the longitudinal spin Seebeck effect (LSSE) on the magnon
damping in magnetic-insulator/nonmagnetic-metal bilayers was recently discussed
in several reports. However, results of those experiments can be blurred by
multimode excitation within the measured linewidth. In order to avoid possible
intermodal interference, we investigated the damping of a single magnon group
in a platinum covered Yttrium Iron Garnet (YIG) film by measurement of the
threshold of its parametric excitation. Both dipolar and exchange spin-wave
branches were probed. It turned out that the LSSE-related modification of
spin-wave damping in a micrometer-thick YIG film is too weak to be observed in
the entire range of experimentally accessible wavevectors. At the same time,
the change in the mean temperature of the YIG layer, which can appear by
applying a temperature gradient, strongly modifies the damping value.",1607.07274v1
2016-07-27,Field-induced decays in $XXZ$ triangular-lattice antiferromagnets,"We investigate field-induced transformations in the dynamical response of the
$XXZ$ model on the triangular lattice that are associated with the anharmonic
magnon coupling and decay phenomena. A set of concrete theoretical predictions
is made for a close physical realization of the spin-1/2 $XXZ$ model,
Ba$_3$CoSb$_2$O$_9$. We demonstrate that dramatic modifications in magnon
spectrum must occur in low out-of-plane fields that are easily achievable for
this material. The hallmark of the effect is a coexistence of the clearly
distinct well-defined magnon excitations with significantly broadened ones in
different regions of the ${\bf k}-\omega$ space. The field-induced decays are
generic for this class of models and become more prominent at larger
anisotropies and in higher fields.",1607.08238v2
2017-02-13,Collective spin excitations of helices and magnetic skyrmions: review and perspectives of magnonics in non-centrosymmetric magnets,"Magnetic materials hosting correlated electrons play an important role for
information technology and signal processing. The currently used ferro-, ferri-
and antiferromagnetic materials provide microscopic moments (spins) that are
mainly collinear. Recently more complex spin structures such as spin helices
and cycloids have regained a lot of interest. The interest has been initiated
by the discovery of the skyrmion lattice phase in non-centrosymmetric helical
magnets. In this review we address how spin helices and skyrmion lattices
enrich the microwave characteristics of magnetic materials. When discussing
perspectives for microwave electronics and magnonics we focus particularly on
insulating materials as they avoid eddy current losses, offer low spin-wave
damping, and might allow for electric field control of collective spin
excitations. Thereby, they further fuel the vision of magnonics operated at low
energy consumption.",1702.03668v1
2017-02-22,Domain wall motion by localized temperature gradients,"Magnetic domain wall (DW) motion induced by a localized Gaussian temperature
profile is studied in a Permalloy nanostrip within the framework of the
stochastic Landau-Lifshitz-Bloch equation. The different contributions to
thermally induced DW motion, entropic torque and magnonic spin transfer torque,
are isolated and compared. The analysis of magnonic spin transfer torque
includes a description of thermally excited magnons in the sample. A third
driving force due to a thermally induced dipolar field is found and described.
Finally, thermally induced DW motion is studied under realistic conditions by
taking into account the edge roughness. The results give quantitative insights
into the different mechanisms responsible for domain wall motion in temperature
gradients and allow for comparison with experimental results.",1702.06725v1
2017-02-22,Is spin superfluidity possible in YIG films?,"Recently it was suggested that stationary spin supercurrents (spin
superfluidity) are possible in the magnon condensate observed in
yttrium-iron-garnet (YIG) magnetic films under strong external pumping. Here we
analyze this suggestion. From topology of the equilibrium order parameter in
YIG one must not expect energetic barriers making spin supercurrents
metastable. However some small barriers of dynamical origin are possible
nevertheless. The critical phase gradient (analog of the Landau critical
velocity in superfluids) is proportional to intensity of the coherent spin wave
(number of condensed magnons). The conclusion is that although spin
superfluidity in YIG films is possible in principle, the published claim of its
observation is not justified.
  The analysis revealed that the widely accepted spin-wave spectrum in YIG
films with magnetostatic and exchange interaction required revision. This led
to revision of non-linear corrections, which determine stability of the magnon
condensate with and without spin supercurrents.",1702.06994v2
2017-02-27,Breakdown of Magnons in a Strongly Spin-Orbital Coupled Magnet,"The description of quantized collective excitations stands as a landmark in
the quantum theory of condensed matter. A prominent example occurs in
conventional magnets, which support bosonic magnons - quantized harmonic
fluctuations of the ordered spins. In striking contrast is the recent discovery
that strongly spin-orbital coupled magnets, such as $\alpha$-RuCl$_3$, may
display a broad excitation continuum inconsistent with conventional magnons.
Due to incomplete knowledge of the underlying interactions unraveling the
nature of this continuum remains challenging. The most discussed explanation
refers to a coherent continuum of fractional excitations analogous to the
celebrated Kitaev spin liquid. Here we present a more general scenario. We
propose that the observed continuum represents incoherent excitations
originating from strong magnetic anharmoniticity that naturally occurs in such
materials. This scenario fully explains the observed inelastic magnetic
response of $\alpha$-RuCl$_3$ and reveals the presence of nontrivial
excitations in such materials extending well beyond the Kitaev state.",1702.08466v3
2017-03-09,Overcoming thermal noise in non-volatile spin wave logic,"Spin waves are propagating disturbances in magnetically ordered materials,
analogous to lattice waves in solid systems and are often described from a
quasiparticle point of view as magnons. The attractive advantages of
Joule-heat-free transmission of information, utilization of the phase of the
wave as an additional degree of freedom and lower footprint area compared to
conventional charge-based devices have made spin waves or magnon spintronics a
promising candidate for beyond-CMOS wave-based computation. However, any
practical realization of an all-magnon based computing system must undergo the
essential steps of a careful selection of materials and demonstrate robustness
with respect to thermal noise or variability. Here, we aim at identifying
suitable materials and theoretically demonstrate the possibility of achieving
error-free clocked non-volatile spin wave logic device, even in the presence of
thermal noise and clock jitter or clock skew.",1703.03460v2
2017-03-13,Influence of the Dzyaloshinskii-Moriya interaction on the FMR spectrum of magnonic crystals and confined structures,"We study the effect of surface-induced Dzyaloshinskii-Moriya interaction
(DMI) on the ferromagnetic resonance (FMR) spectrum of thickness-modulated
one-dimensional magnonic crystals and isolated stripes. The DMI is found to
substantially increases the intensity of absorption peaks and shifts the
frequencies of the laterally quantized modes. The role of the DMI is determined
by analyzing the amplitude and phase distributions of dynamic magnetic
excitations calculated with frequency- and time-domain calculation methods. We
propose experimentally realizable magnonic crystals and confined structures
with multiple FMR absorption peaks. The frequency or magnetic field separation
between FMR lines is exploited to propose a method for estimation of the DMI
strength.",1703.04463v1
2017-03-29,Magnon dispersion in Ca2RuO4: impact of spin-orbit coupling and oxygen moments,"The magnon dispersion of Ca$_2$RuO$_4$ has been studied by polarized and
unpolarized neutron scattering experiments on crystals containing 0, 1 and 10 %
of Ti. The entire dispersion of transverse magnons can be well described by a
conventional spin-wave model with interaction and anisotropy parameters that
agree with density functional theory calculations. Spin-orbit coupling strongly
influences the magnetic excitations, which is most visible in large energies of
the magnetic zone-center modes arising from magnetic anisotropy. We find
evidence for a low-lying additional mode that exhibits strongest scattering
intensity near the antiferromagnetic zone center. This extra signal can be
explained by a sizable magnetic moment of 0.11 Bohr magnetons on the apical
oxygens parallel to the Ru moment, which is found in the density functional
theory calculations. The energy and the signal strength of the additional
branch are well described by taking into account this oxygen moment with weak
ferromagnetic coupling between Ru and O moments.",1703.10017v1
2018-04-17,Reprogrammable magnonic band structure of layered Permalloy/Cu/Permalloy nanowires,"Reprogrammability of magnonic band structure in layered
Permalloy/Cu/Permalloy nanowires is demonstrated to depend on the relative
orientation of the two layers magnetization. By using Brillouin light
spectroscopy, we show that when the layers are aligned parallel two dispersive
modes, with positive and negative group velocity, are observed while when the
magnetic layers are aligned anti-parallel, only one dispersive mode, with
positive group velocity, is detected. Our findings are successfully compared
and interpreted in terms of a microscopic (Hamiltonian-based) method. An
explanation for the observed behavior can be attributed to mode-mixing (or
hybridization) effect when the two magnetic layers are aligned anti-parallel.
This work opens the path to magnetic field-controlled reconfigurable magnonic
crystals with multi-modal frequency transmission characteristics.",1804.06217v1
2018-11-08,Calculated magnetic exchange interactions in Dirac magnon material Cu3TeO6,"Recently topological aspects of magnon band structure have attracted much
interest, and especially, the Dirac magnons in Cu3TeO6 have been observed
experimentally. In this work, we calculate the magnetic exchange interactions
J's using the first-principles linear-response approach and find that these J's
are short-range and negligible for the Cu-Cu atomic pair apart by longer than 7
Angstrom. Moreover there are only 5 sizable magnetic exchange interactions, and
according to their signs and strengths, modest magnetic frustration is
expected. Based on the obtained magnetic exchange couplings, we successfully
reproduce the experimental spin-wave dispersions. The calculated neutron
scattering cross section also agrees very well with the experiments. We also
calculate Dzyaloshinskii-Moriya interactions (DMIs) and estimate the canting
angle (about 1.3{\deg}) of the magnetic non-collinearity based on the
competition between DMIs and J's, which is consistent with the experiment. The
small canting angle agrees with that the current experiments cannot distinguish
the DMI induced nodal line from a Dirac point in the spin-wave spectrum.
Finally we analytically prove that the ""sum rule"" conjectured in [Nat. Phys.
14, 1011 (2018)] holds but only up to the 11th nearest neighbour.",1811.03603v1
2018-11-14,Tunable space-time crystal in room-temperature magnetodielectrics,"We report the experimental realization of a space-time crystal with tunable
periodicity in time and space in the magnon Bose-Einstein Condensate (BEC),
formed in a room-temperature Yttrium Iron Garnet (YIG) film by radio-frequency
space-homogeneous magnetic field. The magnon BEC is prepared to have a well
defined frequency and non-zero wavevector. We demonstrate how the crystalline
""density"" as well as the time and space textures of the resulting crystal may
be tuned by varying the experimental parameters: external static magnetic
field, temperature, thickness of the YIG film and power of the radio-frequency
field. The proposed space-time crystals provide a new dimension for exploring
dynamical phases of matter and can serve as a model nonlinear Floquet system,
that brings in touch the rich fields of classical nonlinear waves, magnonics
and periodically driven systems.",1811.05801v1
2018-11-15,Criticality of the magnon-bound-state hierarchy for the quantum Ising chain with the long-range interactions,"The quantum Ising chain with the interaction decaying as a power law
$1/r^{1+\sigma}$ of the distance between spins $r$ was investigated
numerically. A particular attention was paid to the low-energy spectrum,
namely, the single-magnon and two-magnon-bound-state masses, $m_{1,2}$,
respectively, in the ordered phase. It is anticipated that for each $\sigma$,
the scaled bound-state mass $m_2/m_1$ should take a universal constant
(critical amplitude ratio) in the vicinity of the critical point. In this
paper, we calculated the amplitude ratio $m_2/m_1$ with the exact
diagonalization method, which yields the spectral information such as $m_{1,2}$
directly. As a result, we found that the scaled mass $m_2/m_1$ exhibits a
non-monotonic dependence on $\sigma$; that is, the bound state is stabilized by
an intermediate value of $\sigma$. Such a feature is accordant with a recent
observation based on the non-perturbative-renormalization-group method.",1811.06241v1
2018-11-18,Topological thermal Hall effect driven by fluctuation of spin chirality in frustrated antiferromagnets,"By revealing an underlying relation between the Dzyaloshinskii-Moriya
interaction (DMI) and the scalar spin chirality, we develop the theory of
magnon thermal Hall effects in antiferromagnetic systems. The dynamic
fluctuation of the scalar chirality is shown to directly respond to the
nontrivial topology of magnon bands. In materials such as the jarosites
compounds KFe$_3$(OH)$_6$(SO$_4$)$_2$ and veseignite BaCu$_3$V$_2$O$_8$(OH)$_2$
in the presence of in-plane DMI, the time-reversal symmetry can be broken by
the fluctuations of scalar chirality even in the case of coplanar
$\mathbf{q}=0$ magnetic configuration. The spin-wave Hamiltonian is influenced
by a fictitious magnetic flux determined by the in-plane DMI. Topological
magnon bands and corresponding nonzero Chern numbers are presented without the
need of a canted non-coplanar magnetic ordering. The canting angle dependence
of thermal Hall conductivity is discussed in detail as well. These results
offer a clear principle of chirality-driven topological effects in
antiferromagnetically coupled systems.",1811.07319v3
2010-05-04,S-matrix for magnons in the D1-D5 system,"We show that integrability and symmetries of the near horizon geometry of the
D1-D5 system determine the S-matrix for the scattering of magnons with
polarizations in AdS3 $\times$ S3 completely up to a phase. Using
semi-classical methods we evaluate the phase to the leading and to the one-loop
approximation in the strong coupling expansion. We then show that the phase
obeys the unitarity constraint implied by the crossing relations to the
one-loop order. We also verify that the dispersion relation obeyed by these
magnons is one-loop exact at strong coupling which is consistent with their BPS
nature.",1005.0501v4
2014-03-11,Overcoming damping in spin wave propagation: A continuous excitation approach to determine time-dependent dispersion diagrams in 2D magnonic crystals,"We propose an alternative micromagnetic approach to determine the spin wave
dispersion relations in magnonic structures. Characteristic of the method is
that a limited area of the system is continuously excited with a spatially
uniform oscillating field, tuned at a given frequency. After a transitory time,
the regime magnetization dynamics is collected and a spatial Fourier analysis
on it determines the frequency vs wave vector relation. Combining several
simulations in any predetermined range of frequencies, at any resolution, we
investigate the dispersion relations for different kinds of magnonic crystals:
a dot array, an antidot array, and a bicomponent film. Especially compared to
traditional pulse-excitation methods this technique has many advantages. First,
the excitation power is concentrated at a single frequency, allowing the
corresponding spin waves to propagate with very low attenuation, resulting in a
higher k-space resolution. Second, the model allows to include very large wave
vector components, necessary to describe the high-frequency response of
non-quantized spin waves in quasi-continuous systems. Finally, we address some
possible experimental opportunities with respect to excitation/detection
techniques over large distances and the observation of the odd/even symmetry of
spin waves using Brillouin light scattering.",1403.2549v1
2017-01-10,Temperature dependence of the non-local spin Seebeck effect in YIG/Pt nanostructures,"We study the transport of thermally excited non-equilibrium magnons through
the ferrimagnetic insulator YIG using two electrically isolated Pt strips as
injector and detector. The diffusing magnons induce a non- local inverse spin
Hall voltage in the detector corresponding to the so-called non-local spin
Seebeck effect (SSE). We measure the non-local SSE as a function of temperature
and strip separation. In experiments at room temperature we observe a sign
change of the non-local SSE voltage at a characteristic strip separation d0, in
agreement with previous investigations. At lower temperatures however, we find
a strong temperature dependence of d0. This suggests that both the angular
momentum transfer across the YIG/Pt interface as well as the transport
mechanism of the magnons in YIG as a function of temperature must be taken into
account to describe the non-local spin Seebeck effect.",1701.02635v1
2017-01-14,Paving Spin-Wave Fibers in Magnonic Nanocircuits Using Spin-Orbit Torque,"Recent studies have revealed that domain walls in magnetic nanostructures can
serve as compact, energy-efficient spin-wave waveguides for building magnonic
devices that are considered promising candidates for overcoming the challenges
and bottlenecks of today's CMOS technologies. However, imprinting long
strip-domain walls into magnetic nanowires remains a challenge, especially in
curved geometries. Here, through micromagnetic simulations, we present a method
for writing strip-domain walls into curved magnetic nanowires using spin-orbit
torque. We employ Y-shaped magnetic nanostructures as well as an S-shaped
magnetic nanowire to demonstrate the injection process. In addition, we verify
that the Y-shaped nanostructures that incorporate strip-domain walls can
function as superior spin-wave multiplexers, and that spin-wave propagation
along each conduit can be controllably manipulated. This spin-wave multiplexer
based on strip-domain walls is expected to become a key signal-processing
component in magnon spintronics.",1701.03886v1
2017-01-31,Tunable magnon-photon coupling in a compensating ferrimagnet - from weak to strong coupling,"We experimentally study the magnon-photon coupling in a system consitsing of
the compensating ferrimagnet gadolinium iron garnet (GdIG) and a
three-dimensional microwave cavity. The temperature is varied in order to tune
the GdIG magnetization and to observe the transition from the weak coupling
regime to the strong coupling regime. By measuring and modelling the complex
reflection parameter of the system the effective coupling rate g eff and the
magnetization M eff of the sample are extracted. Comparing g eff with the
magnon and the cavity decay rate we conclude that the strong coupling regime is
easily accessible using GdIG. We show that the effective coupling strength
follows the predicted square root dependence on the magnetization.",1701.08969v3
2018-05-04,Effective damping enhancement in noncollinear spin structures,"Damping mechanisms in magnetic systems determine the lifetime, diffusion and
transport properties of magnons, domain walls, magnetic vortices, and
skyrmions. Based on the phenomenological Landau-Lifshitz-Gilbert equation, here
the effective damping parameter in noncollinear magnetic systems is determined
describing the linewidth in resonance experiments or the decay parameter in
time-resolved measurements. It is shown how the effective damping can be
calculated from the elliptic polarization of magnons, arising due to the
noncollinear spin arrangement. It is concluded that the effective damping is
larger than the Gilbert damping, and it may significantly differ between
excitation modes. Numerical results for the effective damping are presented for
the localized magnons in isolated skyrmions, with parameters based on the
Pd/Fe/Ir(111) model-type system.",1805.01815v2
2018-05-21,Inverse Edelstein effect induced by magnon - phonon coupling,"We demonstrate a spin to charge current conversion via magnon-phonon coupling
and inverse Edelstein effect on the hybrid device Ni/Cu(Ag)/Bi$_{2}$O$_{3}$.
The generation of spin current ($J_{s}\approx 10^{8}A/m^{2}$) due to magnon -
phonon coupling reveals the viability of acoustic spin pumping as mechanism for
the development of spintronic devices. A full in-plane magnetic field angle
dependence of the power absorption and a combination of longitudinal and
transverse voltage detection reveals the symmetric and asymmetric components of
the inverse Edelstein effect voltage induced by Rayleigh type surface acoustic
waves. While the symmetric components are well studied, asymmetric components
are widely unexplored. We assign the asymmetric contributions to the
interference between longitudinal and shear waves and an anisotropic charge
distribution in our hybrid device.",1805.07900v1
2018-05-27,Dynamical structure factors in the nematic phase of frustrated ferromagnetic spin chains,"Frustrated spin systems can show phases with spontaneous breaking of
spin-rotational symmetry without the formation of local magnetic order. We
study the dynamic response of the spin-nematic phase of one-dimensional
spin-1/2 systems, characterized by slow large-distance decay of quadrupolar
correlations, by numerically computing one-spin and two-spin dynamical
structure factors at zero temperature using time-dependent density matrix
renormalization group methods. We interpret the results in terms of an
effective theory of gapped magnon excitations interacting with a
quasi-condensate of bound magnon pairs. This employs an extension of the
well-known Tomonaga-Luttinger liquid theory which includes the magnon states as
a mobile impurity. A good qualitative understanding of the characteristic
thresholds and their intensity in the structure factors is obtained this way.
Our results are useful in the interpretation of inelastic neutron scattering
and resonant inelastic x-ray scattering experiments.",1805.10612v2
2008-07-25,Frustration-induced quantum phase transitions in a quasi-one-dimensional ferrimagnet: Hard-core boson map and the Ton ks-Girardeau limit,"We provide evidence of a superfluid-insulator transition (SIT) of magnons in
a quasi-one-dimensional quantum ferrimagnet with {\it isotropic} competing
antiferromagnetic spin interactions. This SIT occurs between two distinct
ferrimagnetic phases due to the frustration-induced closing of the gap to a
magnon excitation. It thus causes a coherent superposition of singlet and
triplet states at lattice unit cells and a power-law decay on the staggered
spin correlation function along the transverse direction to the spontaneous
magnetization. A hard-core boson map suggests that asymptotically close to the
SIT the magnons attain the Tonks-Girardeau limit. The quantized nature of the
condensed singlets is observed before a first-order transition to a singlet
magnetic spiral phase accompanied by critical antiferromagnetic ordering. In
the limit of strong frustration, the system undergoes a decoupling transition
to an isolated gapped two-leg ladder and a critical single linear chain.",0807.4153v1
2011-11-10,Electronic Pumping of Quasiequilibrium Bose-Einstein Condensed Magnons,"We theoretically investigate spin transfer between a system of
quasiequilibrated Bose-Einstein condensed magnons in an insulator in direct
contact with a conductor. While charge transfer is prohibited across the
interface, spin transport arises from the exchange coupling between insulator
and conductor spins. In normal insulator phase, spin transport is governed
solely by the presence of thermal and spin-diffusive gradients; the presence of
Bose-Einstein condensation (BEC), meanwhile, gives rise to a
temperature-independent condensate spin current. Depending on the thermodynamic
bias of the system, spin may flow in either direction across the interface,
engendering the possibility of a dynamical phase transition of magnons. We
discuss experimental feasibility of observing a BEC steady state (fomented by a
spin Seebeck effect), which is contrasted to the more familiar spin-transfer
induced classical instabilities.",1111.2382v2
2013-09-09,Sub-microsecond fast temporal evolution of the spin Seebeck effect,"We present temporal evolution of the spin Seebeck effect in a YIG|Pt bilayer
system. Our findings reveal that this effect is a sub-microseconds fast
phenomenon governed by the temperature gradient and the thermal magnons
diffusion in the magnetic materials. A comparison of experimental results with
the thermal-driven magnon-diffusion model shows that the temporal behavior of
this effect depends on the time development of the temperature gradient in the
vicinity of the YIG|Pt interface. The effective thermal-magnon diffusion length
for YIG|Pt systems is estimated to be around 700nm.",1309.2164v1
2014-06-28,Magnetic excitation spectrum of LuFe2O4 measured with inelastic neutron scattering,"We report neutron inelastic scattering measurements and analysis of the
spectrum of magnons propagating within the Fe2O4 bilayers of LuFe2O4. The
observed spectrum is consistent with six magnetic modes and a single prominent
gap, which is compatible with a single bilayer magnetic unit cell containing
six spins. We model the magnon dispersion by linear spin-wave theory and find
very good agreement with the domain-averaged spectrum of a spin-charge bilayer
superstructure comprising one Fe3+ -rich monolayer and one Fe2+ -rich
monolayer. These findings indicate the existence of polar bilayers in LuFe2O4,
contrary to recent studies that advocate a charge-segregated non-polar bilayer
model. Weak scattering observed below the magnon gap suggests that a fraction
of the bilayers contain other combinations of charged monolayers not included
in the model. Refined values for the dominant exchange interactions are
reported.",1406.7418v1
2016-05-25,Chirality-induced magnon transport in AA-stacked bilayer honeycomb chiral magnets,"In this Letter, we study the magnetic transport in AA-stacked bilayer
honeycomb chiral magnets coupled either ferromagnetically or
antiferromagnetically. For both couplings, we observe chirality-induced gaps,
chiral protected edge states, magnon Hall and magnon spin Nernst effects of
magnetic spin excitations. For ferromagnetically coupled layers, thermal Hall
and spin Nernst conductivities do not change sign as function of magnetic field
or temperature similar to single-layer honeycomb ferromagnetic insulator. In
contrast, for antiferromagnetically coupled layers, we observe a sign change in
the thermal Hall and spin Nernst conductivities as the magnetic field is
reversed. We discuss possible experimental accessible honeycomb bilayer quantum
materials in which these effects can be observed.",1605.07971v5
2017-06-05,Terahertz-driven magnetism dynamics in the orthoferrite DyFeO3,"Terahertz driven magnetization dynamics are explored in the orthoferrite
DyFeO3. A high-field, single cycle THz pulse is used to excite magnon modes in
the crystal together with other resonances. Both quasi-ferromagnetic and
quasi-antiferromagnetic magnon modes are excited and appear in time-resolved
measurements of the Faraday rotation. Other modes are also observed in the
measurements of the time-resolved linear birefringence. Analysis of the
excitation process reveals that despite larger than expected electro-optical
susceptibility it is mainly the THz magnetic field that couples to the
quasi-ferromagnetic and quasi-antiferromagnetic magnon branches.",1706.01545v1
2017-06-06,Antiferromagnetic Domain Wall as Spin Wave Polarizer and Retarder,"As a collective quasiparticle excitation of the magnetic order in magnetic
materials, spin wave, or magnon when quantized, can propagate in both
conducting and insulating materials. Like the manipulation of its optical
counterpart, the ability to manipulate spin wave polarization is not only
important but also fundamental for magnonics. With only one type of magnetic
lattice, ferromagnets can only accommodate the right-handed circularly
polarized spin wave modes, which leaves no freedom for polarization
manipulation. In contrast, antiferromagnets, with two opposite magnetic
sublattices, have both left and right circular polarizations, and all linear
and elliptical polarizations. Here we demonstrate theoretically and confirm by
micromagnetic simulations that, in the presence of Dzyaloshinskii-Moriya
interaction, an antiferromagnetic domain wall acts naturally as a spin wave
polarizer or a spin wave retarder (waveplate). Our findings provide extremely
simple yet flexible routes toward magnonic information processing by harnessing
the polarization degree of freedom of spin wave.",1706.01617v1
2017-06-10,Mapping the magnonic landscape in patterned magnetic structures,"We report the development of a hybrid numerical / analytical model capable of
mapping the spatially-varying distributions of the local ferromagnetic
resonance (FMR) frequency and dynamic magnetic susceptibility in a wide class
of patterned and compositionally modulated magnetic structures. Starting from
the numerically simulated static micromagnetic state, the magnetization is
deliberately deflected orthogonally to its equilibrium orientation, and the
magnetic fields generated in response to this deflection are evaluated using
micromagnetic software. This allows us to calculate the elements of the
effective demagnetizing tensor, which are then used within a linear analytical
formalism to map the local FMR frequency and dynamic magnetic susceptibility.
To illustrate the typical results that one can obtain using this model, we
analyze three micromagnetic systems boasting non-uniformity in either one or
two dimensions, and successfully explain the spin-wave emission observed in
each case, demonstrating the ubiquitous nature of the Schl\""omann excitation
mechanism underpinning the observations. Finally, the developed model of local
FMR frequency could be used to explain how spin waves could be confined and
steered using magnetic non-uniformities of various origins, rendering it a
powerful tool for the mapping of the graded magnonic index in magnonics.",1706.03212v1
2017-06-19,Orbiton-magnon interplay in the spin-orbital polarons of KCuF3 and LaMnO3,"We present a quasi-analytical solution of a spin-orbital model of KCuF$_{3}$,
using the variational method for Green's functions. By analyzing the spectra
for different partial bosonic compositions as well as the full solution, we
show that hole propagation needs both orbiton and magnon excitations to
develop, but the orbitons dominate the picture. We further elucidate the role
of the different bosons by analyzing the self-energies for simplified models,
establishing that because of the nature of the spin-orbital ground state,
magnons alone do not produce a full quasiparticle band, in contrast to
orbitons. Finally, using the electron-hole transformation between the $e_g$
states of KCuF$_3$ and LaMnO$_3$ we suggest the qualitative scenario for
photoemission experiments in LaMnO$_3$.",1706.06071v1
2017-06-27,Evidence for the role of the magnon energy relaxation length in the Spin Seebeck Effect,"Temperature-dependent spin-Seebeck effect data on Pt|YIG
(Y$_3$Fe$_5$O$_{12}$)|GGG (Gd$_3$Ga$_5$O$_{12}$) are reported for YIG films of
various thicknesses. The effect is reported as a spin-Seebeck resistivity
(SSR), the inverse spin-Hall field divided by the heat flux, to circumvent
uncertainties about temperature gradients inside the films. The SSR is a
non-monotonic function of YIG thickness. A diffusive model for magnon transport
demonstrates how these data give evidence for the existence of two distinct
length scales in thermal spin transport, a spin diffusion length and a magnon
energy relaxation length.",1706.09021v2
2017-06-30,Spin-wave propagation in cubic anisotropic materials,"The information carrier of modern technologies is the electron charge whose
transport inevitably generates Joule heating. Spin-waves, the collective
precessional motion of electron spins, do not involve moving charges and thus
avoid Joule heating. In this respect, magnonic devices in which the information
is carried by spin-waves attract interest for low-power computing. However
implementation of magnonic devices for practical use suffers from low spin-wave
signal and on/off ratio. Here we demonstrate that cubic anisotropic materials
can enhance spin-wave signals by improving spin-wave amplitude as well as group
velocity and attenuation length. Furthermore, cubic anisotropic material shows
an enhanced on/off ratio through a laterally localized edge mode, which closely
mimics the gate-controlled conducting channel in traditional field-effect
transistors. These attractive features of cubic anisotropic materials will
invigorate magnonics research towards wave-based functional devices.",1706.10057v1
2017-10-06,Spectral Weight of Resonant Inelastic X-Ray Scattering in Doped Cuprates: Effect of Core-Hole Lifetime,"We examine the effect of core-hole lifetime on the spectral weight of
resonant inelastic x-ray scattering (RIXS) in hole-doped cuprates. We calculate
the spectral weight by using exact diagonalization technique for a 4x4 doped
Hubbard lattice and find that spin-flip channel detecting single-magnon
excitation is less sensitive to the core-hole lifetime while in non-spin-flip
channel the spectral weight is strongly dependent on the lifetime. In the
latter, charge and two-magnon excitations predominately contribute to RIXS for
short and long core-hole lifetimes, respectively. For a realistic value of the
core-hole lifetime in cuprates, both the charge and two-magnon excitations are
expected to contribute to non-spin-flip channel in RIXS when the
incident-photon energy is tuned to the main peak of x-ray absorption spectrum.",1710.02250v1
2017-10-12,Magnon spin Hall magnetoresistance of a gapped quantum paramagnet,"Motivated by recent experimental work, we consider spin transport between a
normal metal and a gapped quantum paramagnet. We model the latter as the
magnonic Mott-insulating phase of an easy-plane ferromagnetic insulator. We
evaluate the spin current mediated by the interface exchange coupling between
the ferromagnet and the adjacent normal metal. For the strongly interacting
magnons that we consider, this spin current gives rise to a spin Hall
magnetoresistance that strongly depends on the magnitude of the magnetic field,
rather than its direction. This Letter may motivate electrical detection of the
phases of quantum magnets and the incorporation of such materials into
spintronic devices.",1710.04431v2
2017-12-15,Inplane anisotropy of longitudinal thermal conductivities and weak localization of magnons in a disordered spiral magnet,"We demonstrate the inplane anisotropy of longitudinal thermal conductivities
and the weak localization of magnons in a disordered screw-type spiral magnet
on a square lattice. We consider a disordered spin system, described by a spin
Hamiltonian for the antiferromagnetic Heisenberg interaction and the
Dzyaloshinsky-Moriya interaction with the mean-field type potential of
impurities. We derive longitudinal thermal conductivities for the disordered
screw-type spiral magnet in the weak-localization regime by using the
linear-response theory with the linear-spin-wave approximation and performing
perturbation calculations. We show that the inplane longitudinal thermal
conductivities are anisotropic due to the Dzyaloshinsky-Moriya interaction.
This anisotropy may be useful for experimentally estimating the magnitude of a
ratio of the Dzyaloshinsky-Moriya interaction to the Heisenberg interaction. We
also show that the main correction term gives a logarithmic suppression with
the length scale due to the critical back scattering. This suggests that the
weak localization of magnons is ubiquitous for the disordered two-dimensional
magnets having global time-reversal symmetry. We finally discuss several
implications for further research.",1712.05575v2
2018-09-04,Separation of the two-magnon scattering contribution to damping for the determination of the spin mixing conductance,"We present angle dependent measurements of the damping properties of
epitaxial Fe layers with MgO, Al and Pt capping layers. Based on the
preferential distribution of lattice defects following the crystal symmetry, we
make use of a model of the defect density to separate the contribution of
two-magnon scattering to the damping from the isotropic contribution
originating in the spin pumping effect, the viscous Gilbert damping and the
magnetic proximity effect. The separation of the two-magnon contribution, which
depends strongly on the defect density, allows for the measurement of a value
of the effective spin mixing conductance which is closer to the value
exclusively due to spin pumping. The influence of the defect density for
bilayers systems due to the different capping layers and to the unavoidable
spread in defect density from sample to sample is thus removed. This shows the
potential of studying spin pumping phenomena in fully ordered systems in which
this separation is possible, contrary to polycrystalline or amorphous metallic
thin films.",1809.01042v1
2018-09-17,On the speed of domain walls in thin nanotubes: the transition from the linear to the magnonic regime,"Numerical simulations of domain wall propagation in thin nanotubes when an
external magnetic field is applied along the nanotube axis have shown an
unexpected behavior described as a transition from a linear to a magnonic
regime. As the applied magnetic field increases, the initial regime of linear
growth of the speed with the field is followed by a sudden change in slope
accompanied by the emission of spin waves. In this work an analytical formula
for the speed of the domain wall that explains this behavior is derived by
means of an asymptotic study of the Landau Lifshitz Gilbert equation for thin
nanotubes. We show that the dynamics can be reduced to a one dimensional
hyperbolic reaction diffusion equation, namely, the damped double Sine Gordon
equation, which shows the transition to the magnonic regime as the domain wall
speed approaches the speed of spin waves. This equation has been previously
found to describe domain wall propagation in weak ferromagnets with the
mobility proportional to the Dzyaloshinskii-Moriya interaction constant, for
Permalloy nanotubes the mobility is proportional to the nanotube radius.",1809.06278v3
2019-08-31,High-fidelity magnonic gates for surface spin waves,"We study the propagation of surface spin waves in two wave guides coupled
through the dipole-dipole interaction. Essential for the observations made here
is the magneto-electric coupling between the spin waves and the effective
ferroelectric polarization. This allows an external electric field to act on
spin waves and to modify the band gaps of magnonic excitations in individual
layers. By an on/off switching of the electric field and/or varying its
strength or direction with respect to the equilibrium magnetization, it is
possible to permit or ban the propagation of the spin waves in selected
waveguide. We propose experimentally feasible nanoscale device operating as a
high fidelity surface wave magnonic gate.",1909.00162v1
2019-09-01,Magnetic field-dependent low-energy magnon dynamics in $α$-RuCl3,"Revealing the spin excitations of complex quantum magnets is key to
developing a minimal model that explains the underlying magnetic correlations
in the ground state. We investigate the low-energy magnons in $\alpha$-RuCl$_3$
by combining time-domain terahertz spectroscopy under an external magnetic
field and model Hamiltonian calculations. We observe two absorption peaks
around 2.0 and 2.4 meV, which we attribute to zone-center spin waves. Using
linear spin-wave theory with only nearest-neighbor terms of the exchange
couplings, we calculate the antiferromagnetic resonance frequencies and reveal
their dependence on an external field applied parallel to the nearest-neighbor
Ru-Ru bonds. We find that the magnon behavior in an applied magnetic field can
be understood only by including an off-diagonal $\Gamma$ exchange term to the
minimal Heisenberg-Kitaev model. Such an anisotropic exchange interaction that
manifests itself as a result of strong spin-orbit coupling can naturally
account for the observed mixing of the modes at higher fields strengths.",1909.00462v1
2019-09-06,The interplay of large two-magnon ferromagnetic resonance linewidths and low Gilbert damping in Heusler thin films,"We report on broadband ferromagnetic resonance linewidth measurements
performed on epitaxial Heusler thin films. A large and anisotropic two-magnon
scattering linewidth broadening is observed for measurements with the
magnetization lying in the film plane, while linewidth measurements with the
magnetization saturated perpendicular to the sample plane reveal low Gilbert
damping constants of $(1.5\pm0.1)\times 10^{-3}$, $(1.8\pm0.2)\times 10^{-3}$,
and $<8\times 10^{-4}$ for Co$_2$MnSi/MgO, Co$_2$MnAl/MgO, and Co$_2$FeAl/MgO,
respectively. The in-plane measurements are fit to a model combining Gilbert
and two-magnon scattering contributions to the linewidth, revealing a
characteristic disorder lengthscale of 10-100 nm.",1909.02738v2
2019-09-12,Magnetostrictively induced stationary entanglement between two microwave fields,"We present a scheme to entangle two microwave fields by using the nonlinear
magnetostrictive interaction in a ferrimagnet. The magnetostrictive interaction
enables the coupling between a magnon mode (spin wave) and a mechanical mode in
the ferrimagnet, and the magnon mode simultaneously couples to two microwave
cavity fields via the magnetic dipole interaction. The magnon-phonon coupling
is enhanced by directly driving the ferrimagnet with a strong red-detuned
microwave field, and the driving photons are scattered onto two sidebands
induced by the mechanical motion. We show that two cavity fields can be
prepared in a stationary entangled state if they are respectively resonant with
two mechanical sidebands. The present scheme illustrates a new mechanism for
creating entangled states of optical fields, and enables potential applications
in quantum information science and quantum tasks that require entangled
microwave fields.",1909.05936v3
2019-09-17,SU(3) Topology of Magnon-Phonon Hybridization in 2D Antiferromagnets,"Magnon-phonon hybrid excitations are studied theoretically in a
two-dimensional antiferromagnet with an easy axis normal to the plane. We show
that two magnon bands and one phonon band are intertwined by the magnetoelastic
coupling through a nontrivial SU(3) topology, which can be intuitively
perceived by identifying a skyrmion structure in the momentum space. We develop
a continuum theory as the long-wavelength approximation to the tight-binding
model, showing our results are insensitive to lattice details and generally
applicable to two-dimensional antiferromagnets. The theoretical results can be
probed by measuring the thermal Hall conductance as a function of the
temperature and the magnetic field. We envision that the magnetoelastic
coupling in antiferromagnets can be a promising venue in search of various
topological excitations, which cannot be found in magnetic or elastic models
alone.",1909.08031v2
2020-12-21,Inverse Faraday Effect in an Optomagnonic Waveguide,"Single-mode high-index-contrast waveguides have been ubiquitously exploited
in optical, microwave, and phononic structures for achieving enhanced
wave-matter interactions. Although micro-scale optomechanical and
electro-optical devices have been widely studied, optomagnonic devices remain a
grand challenge at the microscale. Here, we introduce a planar optomagnonic
waveguide platform based on a ferrimagnetic insulator that simultaneously
supports single transverse mode of spin waves (magnons) and highly confined
optical modes. The co-localization of spin and light waves gives rise to
enhanced inverse Faraday effect, and as a result, magnons are excited by an
effective magnetic field generated by interacting optical photons. Moreover,
the strongly enhanced optomagnonic interaction allows us to observe such effect
using low-power (milliwatt level) light signals in the continuous-wave form, as
opposed to high-intensity (megawatt peak power) light pulses that are typically
required in magnetic bulk materials or thin films. The optically-driven magnons
are detected electrically with preserved phase coherence, showing the
feasibility for launching spin waves with low-power continuous optical fields.",2012.11119v2
2020-12-31,Soft magnon contributions to dielectric constant in spiral magnets with domain walls,"Competing magnetic exchange interactions often result in non-collinear
magnetic states, such as spin spirals, which break the inversion symmetry and
induce ferroelectric polarization. The resulting strong interactions between
magnetic and dielectric degrees of freedom lead to a technologically important
possibility to control magnetic order by electric fields and to electromagnons,
magnetic excitations that can be excited by an electric dipole of the
electromagnetic field. Here we study the effects of chiral domain walls on
magnetoelectric properties of spiral magnets. We use a quasi-1D model
Hamiltonian with competing Heisenberg exchange interactions, leading to a spin
spiral, and Dzyaloshinskii-Moriya interactions, that couple spins and electric
dipoles and mix magnon and phonon excitations. The results suggest that low
frequency dielectric anomalies in spiral magnets, such as TbMnO3 and MnWO4, may
originate from hybrid magnon - polar phonon excitations associated with domain
walls.",2012.15383v1
2012-05-03,Beliaev theory of spinor Bose-Einstein condensates,"By generalizing the Green's function approach proposed by Beliaev [1, 2], we
investigate the effect of quantum depletion on the energy spectra of elementary
excitations in an F = 1 spinor Bose-Einstein condensate, in particular, of 87Rb
atoms in an external magnetic field. We find that quantum depletion increases
the effective mass of magnons in the spin-wave excitations with quadratic
dispersion relations. The enhancement factor turns out to be the same for both
ferromagnetic and polar phases, and also independent of the magnitude of the
external magnetic field. The lifetime of these magnons in a 87Rb spinor BEC is
shown to be much longer than that of phonons. We propose an experimental setup
to measure the effective mass of these magnons in a spinor Bose gas by
exploiting the effect of a nonlinear dispersion relation on the spatial
expansion of a wave packet of transverse magnetization. This type of
measurement has practical applications, for example, in precision magnetometry.",1205.0657v1
2017-09-12,Green's function formalism for spin transport in metal-insulator-metal heterostructures,"We develop a Green's function formalism for spin transport through
heterostructures that contain metallic leads and insulating ferromagnets. While
this formalism in principle allows for the inclusion of various magnonic
interactions, we focus on Gilbert damping. As an application, we consider
ballistic spin transport by exchange magnons in a metal-insulator-metal
heterostructure with and without disorder. For the former case, we show that
the interplay between disorder and Gilbert damping leads to spin current
fluctuations. For the case without disorder, we obtain the dependence of the
transmitted spin current on the thickness of the ferromagnet. Moreover, we show
that the results of the Green's function formalism agree in the clean and
continuum limit with those obtained from the linearized stochastic
Landau-Lifshitz-Gilbert equation. The developed Green's function formalism is a
natural starting point for numerical studies of magnon transport in
heterostructures that contain normal metals and magnetic insulators.",1709.03775v1
2017-09-29,Thermodynamic properties of Ba$_2$CoSi$_2$O$_6$Cl$_2$ in strong magnetic field: Realization of flat-band physics in a highly frustrated quantum magnet,"The search for flat-band solid-state realizations is a crucial issue to
verify or to challenge theoretical predictions for quantum many-body flat-band
systems. For frustrated quantum magnets flat bands lead to various
unconventional properties related to the existence of localized many-magnon
states. The recently synthesized magnetic compound Ba$_2$CoSi$_2$O$_6$Cl$_2$
seems to be an almost perfect candidate to observe these features in
experiments. We develop a theory for Ba$_2$CoSi$_2$O$_6$Cl$_2$ by adapting the
localized-magnon concept to this compound. We first show that our theory
describes the known experimental facts and then we propose new experimental
studies to detect a field-driven phase transition related to a
Wigner-crystal-like ordering of localized magnons at low temperatures.",1709.10356v2
2018-03-06,Evolution of Magnetic Excitations Across the Metal-Insulator Transition in a Pyrochlore Iridate Eu$_{2}$Ir$_{2}$O$_{7}$,"We report Resonant Inelastic X-ray Scattering (RIXS) study of the magnetic
excitation spectrum in a highly insulating Eu$_{2}$Ir$_{2}$O$_{7}$ single
crystal that exhibits a metal-insulator transition at $T_{MI}$ = 111(7) K. A
propagating magnon mode with 20 meV bandwidth and 28 meV magnon gap is found in
the excitation spectrum at 7 K, which is expected in the all-in-all-out (AIAO)
magnetically ordered state. This magnetic excitation exhibits substantial
softening as temperature is raised towards $T_{MI}$, and turns into highly
damped excitation in the paramagnetic phase. Remarkably, the softening occurs
throughout the whole Brillouin zone including the zone boundary. This
observation is inconsistent with magnon renormalization expected in a local
moment system, and indicates that the strength of electron correlation in
Eu$_{2}$Ir$_{2}$O$_{7}$ is only moderate, so that electron itinerancy should be
taken into account in describing its magnetism.",1803.02248v1
2018-03-09,Spin transport across antiferromagnets induced by the spin Seebeck effect,"For prospective spintronics devices based on the propagation of pure spin
currents, antiferromagnets are an interesting class of materials that
potentially entail a number of advantages as compared to ferromagnets. Here, we
present a detailed theoretical study of magnonic spin current transport in
ferromagnetic-antiferromagnetic multilayers by using atomistic spin dynamics
simulations. The relevant length scales of magnonic spin transport in
antiferromagnets are determined. We demonstrate the transfer of angular
momentum from a ferromagnet into an antiferromagnet due to the excitation of
only one magnon branch in the antiferromagnet. As an experimental system, we
ascertain the transport across an antiferromagnet in
YIG$|$Ir$_{20}$Mn$_{80}|$Pt heterostructures. We determine the spin transport
signals for spin currents generated in the YIG by the spin Seebeck effect and
compare to measurements of the spin Hall magnetoresistance in the
heterostructure stack. By means of temperature-dependent and
thickness-dependent measurements, we deduce conclusions on the spin transport
mechanism across IrMn and furthermore correlate it to its
paramagnetic-antiferromagnetic phase transition.",1803.03416v1
2018-03-11,Skyrmions Driven by Intrinsic Magnons,"We study the dynamics of a skyrmion in a magnetic insulating nanowire in the
presence of time-dependent oscillating magnetic field gradients. These ac
fields act as a net driving force on the skyrmion via its own intrinsic
magnetic excitations. In a microscopic quantum field theory approach we include
the unavoidable coupling of the external field to the magnons, which gives rise
to time-dependent dissipation for the skyrmion. We demonstrate that the
magnetic ac field induces a super-Ohmic to Ohmic crossover behavior for the
skyrmion dissipation kernels with time-dependent Ohmic terms. The ac driving of
the magnon bath at resonance results in a unidirectional helical propagation of
the skyrmion in addition to the otherwise periodic bounded motion.",1803.04001v2
2018-03-13,Spin wave localization and guiding by magnon band structure engineering in yttrium iron garnet,"In spintronics the propagation of spin-wave excitations in magnetically
ordered materials can also be used to transport and process information. One of
the most popular materials in this regard is the ferrimagnetic insulator
yttrium-iron-garnet due its exceptionally small spin-wave damping parameter.
While the small relaxation rate allows for large propagation length of magnetic
excitations, it also leads to non-locality of the magnetic properties. By
imaging spin waves their band structure is mapped. In doing so wave vector
selection is shown to suppress dispersion effects to a large extent allowing
for local measurements of spin relaxation. Moreover we demonstrate even higher
control of magnon propagation by employing the wave vector selectivity near an
avoided crossing of different spin-wave modes where the group velocity
approaches zero. Here local engineering of the dispersion allows constructing
magnonic waveguides and at the same time reveals the local relaxation
properties.",1803.04943v3
2018-03-16,Two-dimensional Dirac nodal loop magnons in collinear antiferromagnets,"We study the nontrivial linear magnon band crossings in the collinear
antiferromagnets on the two-dimensional (2D) CaVO lattice, also realized in
some iron-based superconductors such as AFe$_{1.6+x}$Se$_2$ (A = K, Rb, Cs). It
is shown that the combination of space-inversion and time-reversal symmetry
($\mathcal{PT}$-symmetry) leads to doubly-degenerate eight magnon branches,
which cross each other linearly along a one-dimensional loop in the 2D
Brillouin zone. We show that the Dirac nodal loops (DNLs) are not present in
the collinear ferromagnet on this lattice. Thus, the current 2D
antiferromagnetic DNLs are symmetry-protected and they provide a novel platform
to search for their analogs in 2D electronic antiferromagnetic systems.",1803.06349v3
2018-03-28,Unidirectional Magnon-Driven Domain Wall Motion Due to the Interfacial Dzyaloshinskii-Moriya Interaction,"We demonstrate a unidirectional motion of a quasiparticle without an explicit
symmetry breaking along the space-time coordinate of the particle motion. This
counterintuitive behavior originates from a combined action of two intrinsic
asymmetries in the other two directions. We realize this idea with the
magnon-driven motion of a magnetic domain wall in thin films with interfacial
asymmetry. Contrary to previous studies, the domain wall moves along the same
direction regardless of the magnon-flow direction. Our general symmetry
analysis and numerical simulation reveal that the odd order contributions from
the interfacial asymmetry is unidirectional, which is dominant over
bidirectional contributions in the realistic regime. We develop a simple
analytic theory on the unidirectional motion, which provides an insightful
description of this counterintuitive phenomenon.",1803.10776v3
2018-06-06,Excitation of magnon spin photocurrents in antiferromagnetic insulators,"In the circular photogalvanic effect, circularly polarized light can produce
a direct electron photocurrent in metals and the direction of the current
depends on the polarization. We suggest that an analogous nonlinear effect
exists for antiferromagnetic insulators wherein the total spin of light and
spin waves is conserved. In consequence, a spin angular momentum is expected to
be transfered from photons to magnons so that a circularly polarized
electromagnetic field will generate a direct magnon spin current. The direction
of the current is determined by the helicity of the light. We show that this
resonant effect appears as a second order light-matter interaction. We find
also a geometric contribution to the spin photocurrent, which appears for
materials with complex lattice structures and Dzyaloshinskii-Moriya
interactions.",1806.02442v2
2018-12-04,Spin transport in a magnetic insulator with zero effective damping,"Applications based on spin currents strongly profit from the control and
reduction of their effective damping and their transport properties. We here
experimentally observe magnon mediated transport of spin (angular) momentum
through a 13.4 nm thin yttrium iron garnet film with full control of the
magnetic damping via spin-orbit torque. Above a critical spin-orbit torque, the
fully compensated damping manifests itself as an increase of magnon
conductivity by almost two orders of magnitude. We compare our results to
theoretical expectations based on recently predicted current induced magnon
condensates and discuss other possible origins of the observed critical
behaviour.",1812.01334v3
2018-12-12,Laser-Irradiated CrI$_3$: When Chiral Photons Meet Topological Magnons,"Insulating honeycomb ferromagnet CrI$_3$ has recently attracted considerable
attention due to its potential use for dissipationless spintronics
applications. Recently, topological spin excitations have been observed
experimentally in bulk CrI$_3$ by L. Chen, et al. [Phys. Rev. X ${\bf 8}$,
041028 (2018)] using inelastic neutron scattering. This suggest that bulk
CrI$_3$ has strong spin-orbit coupling and its spin Hamiltonian should include
a next-nearest neighbour Dzyaloshinskii-Moriya (DM) interaction. Inspired by
this experiment, we study non-equilibrium emergent photon-dressed topological
spin and thermal Hall transports in laser-irradiated CrI$_3$ with and without
the DM interaction. We show that the spin excitations can be manipulated into
different topological phases with different Chern numbers. Most importantly, we
show that the emergent photon-dressed spin and thermal Hall response can be
switched to different signs. Hence, the generated magnon spin photocurrents can
be manipulated by the laser field, which is of great interest in ultrafast spin
current generation and could pave the way for future applications of CrI$_3$ to
topological opto-spintronics and opto-magnonics.",1812.05101v2
2019-01-18,Magnonic Weyl states in Cu2OSeO3,"The multiferroic ferrimagnet Cu$_2$OSeO$_3$ with a chiral crystal structure
attracted a lot of recent attention due to the emergence of magnetic skyrmion
order in this material. Here, the topological properties of its magnon
excitations are systematically investigated by linear spin-wave theory and
inelastic neutron scattering. When considering Heisenberg exchange interactions
only, two degenerate Weyl magnon nodes with topological charges $\pm$2 are
observed at high-symmetry points. Each Weyl point splits into two as the
symmetry of the system is further reduced by including into consideration the
nearest-neighbor Dzyaloshinsky-Moriya interaction, crucial for obtaining an
accurate fit to the experimental spin-wave spectrum. The predicted topological
properties are verified by surface state and Chern number analysis.
Additionally, we predict that a measurable thermal Hall conductivity can be
associated with the emergence of the Weyl points, the position of which can be
tuned by changing the crystal symmetry of the material.",1901.06192v1
2019-01-22,Control of the magnon-photon level attraction in a planar cavity,"A resistive coupling circuit is used to model the recently discovered
dissipative coupling in a hybridized cavity photon-magnon system. With this
model as a basis we have designed a planar cavity in which a controllable
transition between level attraction and level repulsion can be achieved. This
behaviour can be quantitatively understood using an LCR circuit model with a
complex coupling strength. Our work therefore develops and verifies a circuit
method to model level repulsion and level attraction and confirms the
universality of dissipative coupling in the cavity photon-magnon system. The
realization of both coherent and dissipative couplings in a planar cavity may
provide new avenues for the design and adaptation of dissipatively coupled
systems for practical applications in information processing.",1901.07633v2
2019-01-26,Unconventional Thermal Magnon Hall Effect in a Ferromagnetic Topological Insulator,"We present theoretically the thermal Hall effect of magnons in a
ferromagnetic lattice with a Kekul\'e-O coupling (KOC) modulation and a
Dzyaloshinskii-Moriya interaction (DMI). Through a strain-based mechanism for
inducing the KOC modulation, we identify four topological phases in terms of
the KOC parameter and DMI strength. We calculate the thermal magnon Hall
conductivity ${\kappa^{xy}}$ at low temperature in each of these phases. We
predict an unconventional conductivity due to a non-zero Berry curvature
emerging from band proximity effects in the topologically trivial phase. We
find sign changes of ${\kappa^{xy}}$ as a function of the model parameters,
associated with the local Berry curvature and occupation probability of the
bulk bands. Throughout, ${\kappa^{xy}}$ can be easily tuned with external
parameters such as the magnetic field and temperature.",1901.09213v2
2019-01-29,The effects of three magnons interactions in the magnon-density waves of triangular spin lattices,"We investigate the magnon-density waves proposed as the longitudinal
excitations in triangular lattice antiferromagnets by including the cubic and
quartic corrections in the large-s expansion. The longitudinal excitation
spectra for the two-dimensional (2D) triangular antiferromagnetic model and
quasi-one dimensional (quasi-1D) antiferromagnetic materials have been obtained
for a general quantum spin number $s$. For the 2D triangular lattice model, we
find a significant reduction (about 40 %) in the energy spectra at the zone
boundaries due to both the cubic and quartic corrections. For the quasi-1D
antiferromagnets, since the cubic term comes from the very weak couplings on
the hexagonal planes, they make very little correction to the energy spectra,
whereas the major correction contribution comes from the quartic terms in the
couplings along the chains with the numerical values for the energy gaps in
good agreement with the experimental results as reported earlier (Ref.~41).",1901.11007v2
2019-01-30,Surface dynamics of rough magnetic films,"The chirality of Damon-Eshbach (DE) magnons affects the transport of energy
and angular momentum at the surface of magnetic films and spheres. We calculate
the surface-disorder-limited dephasing and transport lifetimes of surface modes
of sufficiently thick high-quality magnetic films such as yttrium iron garnet.
In spite of their chirality surface magnons are not protected, but interact
strongly with smooth surface roughness. Nevertheless, for long-range disorder
the transport is much less affected by the suppressed back scattering (vertex
correction). Moreover, in the presence of roughness ferromagnetic resonance
under a \textit{uniform} microwave field can gennerate a considerable amount of
surface magnons.",1901.11046v2
2019-02-25,Topological spin Hall effects and tunable skyrmion Hall effects in uniaxial antiferromagnetic insulators,"Recent advances in the physics of current-driven antiferromagnetic skyrmions
have observed the absence of a Magnus force. We outline the symmetry reasons
for this phenomenon, and show that this cancellation will fail in the case of
spin polarized currents. Pairing micromagnetic simulations with semiclassical
spin wave transport theory, we demonstrate that skyrmions produce a
spin-polarized transverse magnon current, and that spin-polarized magnon
currents can in turn produce transverse motion of antiferromagnetic skyrmions.
We examine qualitative differences in the frequency dependence of the skyrmion
Hall angle between ferromagnetic and antiferromagnetic cases, and close by
proposing a simple skyrmion-based magnonic device for demultiplexing of spin
channels.",1902.09382v2
2019-03-04,Enhancement of superconductivity mediated by antiferromagnetic squeezed magnons,"We investigate theoretically magnon-mediated superconductivity in a
heterostructure consisting of a normal metal and a two-sublattice
antiferromagnetic insulator. The attractive electron-electron pairing
interaction is caused by an interfacial exchange coupling with magnons residing
in the antiferromagnet, resulting in p-wave, spin-triplet superconductivity in
the normal metal. Our main finding is that one may significantly enhance the
superconducting critical temperature by coupling the normal metal to only one
of the two antiferromagnetic sublattices employing, for example, an
uncompensated interface. Employing realistic material parameters, the critical
temperature increases from vanishingly small values to values significantly
larger than 1 K as the interfacial coupling becomes strongly
sublattice-asymmetric. We provide a general physical picture of this
enhancement mechanism based on the notion of squeezed bosonic eigenmodes.",1903.01470v3
2019-03-05,Optimal mode matching in cavity optomagnonics,"Inelastic scattering of photons is a promising technique to manipulate
magnons but it suffers from weak intrinsic coupling. We theoretically discuss
an idea to increase optomagnonic coupling in optical whispering gallery mode
cavities, by generalizing previous analysis to include the exchange
interaction. We predict that the optomagnonic coupling constant to surface
magnons in yttrium iron garnet (YIG) spheres with radius $300\,\mathrm{\mu}$m
can be up to $40$ times larger than that to the macrospin Kittel mode. Whereas
this enhancement falls short of the requirements for magnon manipulation in
YIG, nanostructuring and/or materials with larger magneto-optical constants can
bridge this gap.",1903.01718v2
2019-03-05,Strong Coupling between Microwave Photons and Nanomagnet Magnons,"Coupled microwave photon-magnon hybrid systems offer promising applications
by harnessing various magnon physics. At present, in order to realize high
coupling strength between the two subsystems, bulky ferromagnets with large
spin numbers are utilized, which limits their potential applications for
scalable quantum information processing. In this paper, by enhancing single
spin coupling strength using lithographically defined superconducting
resonators, we report high cooperativities between a resonator mode and a
Kittel mode in nanometer thick Permalloy wires. The on-chip, lithographically
scalable, and superconducting quantum circuit compatible design provides a
direct route towards realizing hybrid quantum systems with nanomagnets, whose
coupling strength can be precisely engineered and dynamic properties can be
controlled by various mechanisms derived from spintronic studies.",1903.01887v3
2019-03-07,Interplay between intra- and inter-nanowires dynamic dipolar interactions in the spin wave band structure of Py/Cu/Py nanowires,"We have studied both experimentally and theoretically the reprogrammable spin
wave band structure in Permalloy(10nm)/Cu(5nm)/Permalloy(30nm) nanowire arrays
of width w=280 nm and inter-wire separation in the range from 80 to 280 nm. We
found that, depending on the inter-wire separation, the anti-parallel
configuration, where the magnetizations of the two Permalloy layers point in
opposite directions, is stabilized over specific magnetic field ranges thus
enabling us to directly compare the band structure with that of the parallel
alignment. We show that collective spin waves of the Bloch type propagate
through the arrays with different magnonic bandwidths as a consequence of the
interplay between the intra- and inter-nanowire dynamic dipolar interactions. A
detailed understanding, e.g. whether they have a stationary or propagating
character, is achieved by considering the phase relation (in-phase or
out-of-phase) between the dynamic magnetizations in the two ferromagnetic
layers and their average value. This work opens the path to magnetic
field-controlled reconfigurable layered magnonic crystals that can be used for
future nanoscale magnon spintronic devices.",1903.02935v1
2019-03-21,Room temperature and low-field resonant enhancement of spin Seebeck effect in partially compensated magnets,"Resonant enhancement of spin Seebeck effect (SSE) due to phonons was recently
discovered in Y3Fe5O12 (YIG). This effect is explained by hybridization between
the magnon and phonon dispersions. However, this effect was observed at low
temperatures and high magnetic fields, limiting the scope for applications.
Here we report observation of phonon-resonant enhancement of SSE at room
temperature and low magnetic field. We observed in Lu2BiFe4GaO12 and
enhancement 700 % greater than that in a YIG film and at very low magnetic
fields around 10-1 T, almost one order of magnitude lower than that of YIG. The
result can be explained by the change in the magnon dispersion induced by
magnetic compensation due to the presence of non-magnetic ion substitutions.
Our study provides a way to tune the magnon response in a crystal by chemical
doping with potential applications for spintronic devices.",1903.09007v1
2019-03-29,Quantum Simulation of the Fermion-Boson Composite Quasi-Particles with a Driven Qubit-Magnon Hybrid Quantum System,"We experimentally demonstrate strong coupling between the ferromagnetic
magnons in a small yttrium-iron-garnet (YIG) sphere and the drive-field-induced
dressed states of a superconducting qubit, which gives rise to the double
dressing of the superconducting qubit. The YIG sphere and the superconducting
qubit are embedded in a microwave cavity and the effective coupling between
them is mediated by the virtual cavity photons. The theoretical results fit the
experimental observations well in a wide region of the drive-field power
resonantly applied to the superconducting qubit and reveal that the driven
qubit-magnon hybrid quantum system can be harnessed to emulate a
particle-hole-symmetric pair coupled to a bosonic mode. This hybrid quantum
system offers a novel platform for quantum simulation of the composite
quasi-particles consisting of fermions and bosons.",1903.12498v1
2019-04-01,Inelastic spin-wave scattering by Bloch domain wall flexure oscillations,"The calculations of the inelastic spin wave scattering by flexure vibrations
of the Bloch domain wall (Winters magnons) in thin magnetic films are
presented. The approach is based on the interaction of the propagating spin
waves with the dynamical emergent electromagnetic field generated by the moving
inhomogeneous magnetization texture (domain wall). The probability of the spin
wave scattering for the Winters magnon emission and absorption processes
essentially rises with the spin wave scattering angle increase up to 900. The
angular dependence of the scattering probability is essentially stronger for
the magnon absorption processes that allow distinguishing these elementary
emission/absorption processes experimentally.",1904.00596v1
2019-04-12,Spin excitations of magnetoelectric LiNiPO$_4$ in multiple magnetic phases,"Spin excitations of magnetoelectric LiNiPO$_4$ are studied by infrared
absorption spectroscopy in the THz spectral range as a function of magnetic
field through various commensurate and incommensurate magnetically ordered
phases up to 33\,T. Six spin resonances and a strong two-magnon continuum are
observed in zero magnetic field. Our systematic polarization study reveals that
some of the excitations are usual magnetic-dipole active magnon modes, while
others are either electromagnons, electric-dipole active, or magnetoelectric,
both electric- and magnetic-dipole active spin excitations. Field-induced
shifts of the modes for all three orientations of the field along the
orthorhombic axes allow us to refine the values of the relevant exchange
couplings, single-ion anisotropies, and the Dzyaloshinskii-Moriya interaction
on the level of a four-sublattice mean-field spin model. This model also
reproduces the spectral shape of the two-magnon absorption continuum, found to
be electric-dipole active in the experiment.",1904.06106v1
2019-04-18,Magnonic Analogue Black/White Hole Horizon in Superfluid $^3$He-B: experiment,"We provide experimental details of the first experiment made in zero
temperature limit ($\sim$ 600\,$\mu$K) studying the magnonic black/white hole
horizon analogue using absolutely pure physical system based on the spin
superfluidity in superfluid $^3$He-B. We show that spin precession waves
propagating on the background of the spin super-currents in a channel between
two Bose-Einstein condensates of magnons in form of homogeneously precessing
domains mimic the properties of the black/white horizon. Once the white hole
horizon is formed and blocks the propagation of the spin-precession waves
between two domains, we observed an amplification effect, i.e. when the energy
of the spin precession waves reflected from the horizon is higher than the
energy of the excited spin precession waves before horizon was formed.
Moreover, the estimated temperature of the spontaneous Hawking radiation in
this model system is about four orders of magnitude lower than the system's
background temperature what makes it a promising tool to study the effect of
spontaneous Hawking radiation.",1904.09183v1
2019-08-09,Sustained coherent spin wave emission using frequency combs,"We demonstrate sustained coherent emission of spin waves in NiFe films using
rapid demagnetization from high repetition rate femtosecond laser pulse trains.
As the pulse separation is shorter than the magnon decay time, magnons having a
frequency equal to a multiple of the 1 GHz repetition-rate are coherently
amplified. Using scanning micro-Brillouin Light Scattering (BLS) we observe
this coherent amplification as strong peaks spaced 1 GHz apart. The BLS counts
vs. laser power exhibit a stronger than parabolic dependence consistent with
counts being proportional to the square of the magnetodynamic amplitude, and
the demagnetization pulse strength being described by a Bloch law. Spatial spin
wave mapping demonstrates how both localized and propagating spin waves can be
excited, and how the propagation direction can be directly controlled. Our
results demonstrate the versatility of BLS spectroscopy for rapid
demagnetization studies and enable a new platform for photo-magnonics where
sustained coherent spin waves can be utilized.",1908.03388v2
2019-08-13,Anti-chiral edge states in Heisenberg ferromagnet on a honeycomb lattice,"We demonstrate the emergence of anti-chiral edge states in a Heisenberg
ferromagnet with Dzyaloshinskii-Moriya interaction(DMI) on a honeycomb lattice
with in-equivalent sub-lattices. The DMI, which acts between atoms of the same
species, differs in magnitude for the two sub-lattices, resulting in a shifting
of the energy of the magnon bands in opposite directions at the two Dirac
points. The chiral symmetry is broken and for sufficiently strong asymmetry,
the band shifting leads to anti-chiral edge states (in addition to the normal
chiral edge states) in a rectangular strip where the magnon current propagates
in the same direction along the two edges. This is compensated by a
counter-propagating bulk current that is enabled by the broken chiral symmetry.
We analyze the resulting magnon current profile across the width of the system
in details and suggest realistic experimental probes to detect them. Finally,
we propose a material that can potentially exhibit such anti-chiral edge
states.",1908.04580v2
2019-08-23,Magnon Pairs and Spin-Nematic Correlation in the Spin-Seebeck Effect,"Investigating exotic magnetic materials with spintronic techniques is
effective at advancing magnetism as well as spintronics. In this work, we
report unusual field-induced suppression of the spin-Seebeck effect (SSE) in a
quasi one-dimensional frustrated spin-$\frac{1}{2}$ magnet LiCuVO$_4$, known to
exhibit spin-nematic correlation in a wide range of external magnetic field
$B$. The suppression takes place above $|B| > 2$ T in spite of the $B$-linear
isothermal magnetization curves in the same $B$ range. The result can be
attributed to the growth of the spin-nematic correlation while increasing $B$.
The correlation stabilizes magnon pairs carrying spin-2, thereby suppressing
the interfacial spin injection of SSE by preventing the spin-1 exchange between
single magnons and conduction electrons at the interface. This interpretation
is supported by integrating thermodynamic measurements and theoretical analysis
on the SSE.",1908.08701v1
2019-08-26,Disentangle magnon magnetoresistance from anisotropic and spin Hall magnetoresistance in NiFe/Pt bilayers,"We conducted a systematic angular dependence study of nonlinear
magnetoresistance in NiFe/Pt bilayers at variable temperature and field using
the Wheatstone bridge method. We successfully disentangled magnon
magnetoresistance from other types of magnetoresistances based on their
different temperature and field dependences. Both the spin Hall/anisotropic and
magnon magnetoresistances contain sine phi and sine 3 phi components with phi
the angle between current and magnetization, but they exhibit different field
and temperature dependence. The competition between different types of
magnetoresistances leads to a sign reversal of sine 3 phi component at a
specific magnetic field, which was not reported previously. The
phenomenological model developed is able to account for the experimental
results for both NiFe/Pt and NiFe/Ta samples with different layer thicknesses.
Our results demonstrate the importance of disentangling different types of
magnetoresistances when characterizing the charge-spin interconversion process
in magnetic heterostructures.",1908.09571v2
2019-08-29,Magnon crystals and magnetic phases in a Kagomé-stripe antiferromagnet,"In this work we analyze the magnetization properties of an antiferromagnetic
Kagom\'e stripe lattice, motivated by the recent synthesis of materials
exhibiting this structure. By employing a variety of techniques that include
numerical methods as Density Matrix Renormalization Group and Monte Carlo
simulations, as well as analytical techniques, as perturbative low energy
effective models and exact solutions, we characterize the magnetization process
and magnetic phase diagram of a Kagom\'e stripe lattice. The model captures a
variety of behaviors present in the two dimensional Kagom\'e lattice, which are
described here by analytical models and numerically corroborated. In addition
to the characterization of semiclassical intermediate plateaus, it is worth
noting the determination of an exact magnon crystal phase which breaks the
underlying symmetry of the lattice. This magnon crystal phase generalizes
previous findings and according to our knowledge is reported here for the first
time.",1908.11269v2
2019-11-22,Thermodynamic Bethe Ansatz for Fishnet CFT,"We present the TBA equations and the Y-system for the exact spectrum of
general multi-magnon local operators in the $D$-dimensional anisotropic version
of the bi-scalar fishnet CFT. The mixing matrix of such operators is given in
terms of fishnet planar graphs of multi-wheel and multi-spiral type. These
graphs probe the two main building blocks of the TBA approach that are the
magnon dispersion relation and the magnon scattering matrix and which we both
obtain by diagonalising suitable graph-building operators. We also obtain the
dual version of the TBA equations, which relates, in the continuum limit,
$D$-dimensional graphs to two dimensional sigma models in $AdS_{D+1}$. It
allows us to verify a general formula obtained by A.~Zamolodchikov for the
critical coupling.",1911.10213v1
2019-12-15,Magnon-photon strong coupling for tunable microwave circulators,"We present a generic theoretical framework to describe non-reciprocal
microwave circulation in a multimode cavity magnonic system and assess the
optimal performance of practical circulator devices. We show that high
isolation (> 56 dB), extremely low insertion loss (< 0.05 dB), and flexible
bandwidth control can be potentially realized in high-quality-factor
superconducting cavity based magnonic platforms. These circulation
characteristics are analyzed with materials of different spin densities. For
high-spin-density materials such as yttrium iron garnet, strong coupling
operation regime can be harnessed to obtain a broader circulation bandwidth. We
also provide practical design principles for a highly integratible
low-spin-density material (vanadium tetracyanoethylene) for narrow-band
circulator operation, which could benefit noise-sensitive quantum microwave
measurements. This theory can be extended to other coupled systems and provide
design guidelines for achieving tunable microwave non-reciprocity for both
classical and quantum applications.",1912.07128v2
2019-12-16,Magnon-mediated superconductivity on the surface of a topological insulator,"We study superconductivity on the surface of a topological insulator,
mediated by magnetic fluctuations in an adjacent ferromagnetic or
antiferromagnetic insulator. Superconductivity can arise from effective
interactions between helical fermions induced by interfacial fermion-magnon
interactions. For both ferromagnetic and antiferromagnetic insulators, these
fermion-fermion interactions have the correct structure to facilitate pairing
between particles located on the same side of the Fermi surface, also known as
Amperean pairing. In antiferromagnets, the strength of the induced interactions
can be enhanced by coupling the topological insulator asymmetrically to the two
sublattices of the antiferromagnet. This effect is further amplified by next
nearest neighbor frustration in the antiferromagnetic insulator. The
enhancement makes the induced interactions significantly stronger in the
antiferromagnetic case, as compared to the ferromagnetic case. These results
indicate that an uncompensated antiferromagnetic interface might be a better
candidate than a ferromagnetic interface for proximity-induced magnon-mediated
superconductivity on the surface of a topological insulator.",1912.07607v2
2019-12-17,Topological magnons in ferromagnetic Kitaev-Heisenberg model on CaVO lattice,"A number of topological phases are found to emerge in the ferromagnetic
Kitaev-Heisenberg model on CaVO lattice in the presence of
Dzyaloshinskii-Moriya interaction. Heisenberg and Kitaev terms have been
considered on nearest and next-nearest neighbor bonds in a variety of ways.
Both isotropic and anisotropic couplings are taken into account. Topological
phases are characterized by Chern numbers for the distinct magnon bands as well
as the number of modes for topologically protected gapless magnon edge states.
Band structure, dispersion relation along the high-symmetric points of first
Brillouin zone, density of states and thermal Hall conductance have been
evaluated for every phase. An extensive Phase diagram has been constructed.
Topological phase transition in the parameter space is also studied.",1912.07877v2
2019-12-18,Theory of Quantum Acoustomagnonics and Acoustomechanics with a Micromagnet,"Recently [1], we proposed a new way to engineer a flexible acoustomechanical
coupling between the center-of-mass motion of an isolated micromagnet and one
of its internal acoustic phonons by using a magnon as a passive mediator. In
our approach, the coupling is enabled by the strong magnetoelastic interaction
between magnons and acoustic phonons which originates from the small particle
size. Here, we substantially extend our previous work. First, we provide the
full theory of the quantum acoustomagnonic interaction in small micromagnets
and analytically calculate the magnon-phonon coupling rates. Second, we fully
derive the acoustomechanical Hamiltonian presented in Ref. [1]. Finally, we
extend our previous results for the fundamental acoustic mode to higher order
modes. Specifically, we show the cooling of the center-of-mass motion with a
range of internal acoustic modes. Additionally, we derive the power spectral
densities of the center-of-mass motion which allow to probe the same acoustic
modes.",1912.08745v2
2015-08-05,Thermal conductivity in large-$J$ two-dimensional antiferromagnets: Role of phonon scattering,"Motivated by the recent heat transport experiments in 2D antiferromagnets,
such as La$_2$CuO$_4$, where the exchange coupling $J$ is larger than the Debye
energy $\Theta_{\rm D}$, we discuss different types of relaxation processes for
magnon heat current with a particular focus on coupling to 3D phonons. We study
thermal conductivity by these in-plane magnetic excitations using two distinct
techniques, Boltzmann formalism within the relaxation-time approximation and
memory-function approach. Within these approaches, a close consideration is
given to the scattering of magnons by both acoustic and optical branches of
phonons. A remarkable accord between the two methods with regards to the
asymptotic behavior of the effective relaxation rates is demonstrated.
Additional scattering mechanisms, due to grain boundaries, impurities, and
finite correlation length in the paramagnetic phase, are discussed and included
in the calculations of the thermal conductivity $\kappa(T)$. Again, we
demonstrate a close similarity of the results from the two techniques of
calculating $\kappa(T)$. Our complementary approach strongly suggests that
scattering from optical or zone-boundary phonons is important for magnon heat
current relaxation in a high temperature window of $\Theta_D\lesssim T \ll J$.",1508.01221v1
2015-08-20,Superstrong Coupling of a Microwave Cavity to YIG Magnons,"Multiple-post reentrant 3D lumped cavity modes have been realized to design
the concept of discrete Whispering Gallery and Fabry-Perot-like Modes for
multimode microwave Quantum Electrodynamics experiments. Using a magnon
spin-wave resonance of a submillimeter-sized Yttrium-Iron-Garnet sphere at
milliKelvin temperatures and a four-post cavity, we demonstrate the
ultra-strong coupling regime between discrete Whispering Gallery Modes and a
magnon resonance with strength of 1.84 GHz. By increasing the number of posts
to eight and arranging them in a D$_4$ symmetry pattern, we expand the mode
structure to that of a discrete Fabry-Perot cavity and modify the Free Spectral
Range (FSR). We reach the superstrong coupling regime, where spin-photon
coupling strength is larger than FSR, with coupling strength in the 1.1 to 1.5
GHz range.",1508.04967v3
2016-03-31,Enhanced spin Seebeck effect signal due to spin-momentum locked topological surface states,"Spin-momentum locking in protected surface states enables efficient
electrical detection of magnon decay at a
magnetic-insulator/topological-insulator heterojunction. Here we demonstrate
this property using the spin Seebeck effect, i.e. measuring the transverse
thermoelectric response to a temperature gradient across a thin film of yttrium
iron garnet, an insulating ferrimagnet, and forming a heterojunction with
(BixSb1-x)2Te3, a topological insulator. The non-equilibrium magnon population
established at the interface can decay in part by interactions of magnons with
electrons near the Fermi energy of the topological insulator. When this decay
channel is made active by tuning (BixSb1-x)2Te3 to a bulk insulator, a large
electromotive force emerges in the direction perpendicular to the in-plane
magnetization of yttrium iron garnet. The enhanced, tunable spin Seebeck effect
which occurs when the Fermi level lies in the bulk gap offers unique advantages
over the usual spin Seebeck effect in metals and therefore opens up exciting
possibilities in spintronics.",1603.09427v1
2016-10-26,Time-resolved measurements of surface spin-wave pulses at millikelvin temperatures,"In this work, we experimentally investigate the propagation of pulsed
magnetostatic surface spin-wave (magnon) signals in an yttrium iron garnet
(YIG) waveguide at millikelvin temperatures. Our measurements are performed in
a dilution refrigerator at microwave frequencies. The excellent signal-to-noise
ratio afforded by the low-temperature environment allows the propagation of the
pulses to be observed in detail. The work gives insight both into
low-temperature magnon dynamics in YIG and the potential application of systems
of propagating magnons to solid-state quantum information processing.",1610.08402v2
2016-10-31,Strong coupling of magnons in a YIG sphere to photons in a planar superconducting resonator in the quantum limit,"We report measurements of a superconducting coplanar waveguide resonator
(CPWR) coupled to a sphere of yttrium-iron garnet. The non-uniform CPWR field
allows us to excite various magnon modes in the sphere. Mode frequencies and
relative coupling strengths are consistent with theory. Strong coupling is
observed to several modes even with, on average, less than one excitation
present in the CPWR. The time response to square pulses shows oscillations at
the mode splitting frequency. These results indicate the feasibility of
combining magnonic and planar superconducting quantum devices.",1610.09963v2
2016-12-13,Noninteger-spin magnonic excitations in untextured magnets,"Interactions are responsible for intriguing physics, e.g. emergence of exotic
ground states and excitations, in a wide range of systems. Here we
theoretically demonstrate that dipole-dipole interaction leads to bosonic
eigen-excitations with average spin ranging from zero to above $\hbar$ in
magnets with uniformly magnetized ground states. These exotic excitations can
be interpreted as quantum coherent conglomerates of spin $\hbar$ magnons, the
eigen-excitations when the dipolar interactions are disregarded. We further
find that the eigenmodes in an easy-axis antiferromagnet are spin-zero
quasiparticles instead of the widely believed spin $\pm \hbar$ magnons. The
latter re-emerge when the symmetry is broken by a sufficiently large applied
magnetic field. The average spin greater than $\hbar$ is accompanied by vacuum
fluctuations and may be considered to be a weak form of frustration.",1612.04163v2
2017-07-04,Effective Minkowski to Euclidean signature change of the magnon BEC pseudo-Goldstone mode in polar 3He,"We discuss the effective metric experienced by the Nambu-Goldstone mode
propagating in the broken symmetry spin-superfluid state of coherent precession
of magnetization. This collective mode represents the phonon in the RF driven
or pulsed out-of-equilibrium Bose-Einstein condensate (BEC) of optical magnons.
We derive the effective BEC free energy and consider the phonon spectrum when
the spin superfluid BEC is formed in the anisotropic polar phase of superfluid
3He, experimentally observed in uniaxial aerogel 3He-samples. The coherent
precession of magnetization experiences an instability at a critical value of
the tilting angle of external magnetic field with respect to the anisotropy
axis. From the action of quadratic deviations around equilibrium, this
instability is interpreted as a Minkowski-to-Euclidean signature change of the
effective phonon metric. We also note the similarity between the magnon BEC in
the unstable region and an effective vacuum scalar ""ghost"" condensate.",1707.00905v4
2017-11-08,Bose-Einstein condensation of magnons and spin superfluidity in the polar phase of $^3$He,"The polar phase of $^3$He, which is topological spin-triplet superfluid with
the Dirac nodal line in the spectrum of Bogolubov quasiparticles, has been
recently stabilized in a nanoconfined geometry. We pump magnetic excitations
(magnons) into the sample of polar phase and observe how they form a
Bose-Einstein condensate, revealed by coherent precession of the magnetization
of the sample. Spin superfluidity, which supports this coherence, is associated
with the spontaneous breaking of U(1) symmetry by the phase of precession. We
observe the corresponding Nambu-Goldstone boson and measure its mass emerging
when applied rf field violates the U(1) symmetry explicitly. We suggest that
the magnon BEC in the polar phase is a powerful probe for topological objects
such as vortices and solitons and topological nodes in the fermionic spectrum.",1711.02915v1
2018-10-02,Spin-dependent (inverse) spin Hall effect in Co$_{60}$Fe$_{20}$B$_{20}$,"In ferromagnetic metals, the interconversion of spin and charge currents via
the spin Hall effect and its inverse can depend on the angle between the
ferromagnets magnetization and the spin current polarization direction. Here,
such a spin-dependent (inverse) spin Hall effect is found in the ferromagnetic
alloy Co$_{60}$Fe$_{20}$B$_{20}$. In a nonlocal magnon transport experiment,
Co$_{60}$Fe$_{20}$B$_{20}$ is used to both excite and detect magnonic spin
currents flowing in the ferrimagnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$. We
find that the signal amplitude is significantly modulated by tuning the
direction of the Co$_{60}$Fe$_{20}$B$_{20}$ magnetization. We design a sample
structure that completely prevents direct magnonic coupling between the
ferromagnets. Thus, we can identify unambiguously an intrinsic electronic
origin of the observed effect.",1810.01227v3
2018-10-23,Magnonic Analogue of Black/White Hole Horizon in Superfluid $^3$He-B,"We report on theoretical model and experimental results of the experiment
made in a limit of absolute zero temperature ($\sim$ 600\,$\mu$K) studying the
spin wave analogue of black/white hole horizon using spin (magnonic)
superfluidity in superfluid $^3$He-B. As an experimental tool simulating the
properties of the black/white horizon we used the spin-precession waves
propagating on the background of the spin super-currents between two
Bose-Einstein condensates of magnons in form of homogeneously precessing
domains. We provide experimental evidence of the white hole formation for spin
precession waves in this system, together with observation of an amplification
effect. Moreover, the estimated temperature of the spontaneous Hawking
radiation in this system is about four orders of magnitude lower than the
system's background temperature what makes it a promising tool to study the
effect of spontaneous Hawking radiation.",1810.09890v2
2019-05-11,Spin Seebeck effect from antiferromagnetic magnons and critical spin fluctuations in epitaxial FeF2 films,"We report a longitudinal spin Seebeck effect (SSE) study in epitaxially grown
FeF2(110) antiferromagnetic (AFM) thin films with strong uniaxial anisotropy
over the temperature range of 3.8 - 250 K. Both the magnetic field- and
temperature-dependent SSE signals below the N\'eel temperature (TN=70 K) of the
FeF2 films are consistent with a theoretical model based on the excitations of
AFM magnons without any net induced static magnetic moment. In addition to the
characteristic low-temperature SSE peak associated with the AFM magnons, there
is another SSE peak at TN which extends well into the paramagnetic phase. All
the SSE data taken at different magnetic fields up to 12 T near and above the
critical point TN follow the critical scaling law very well with the critical
exponents for magnetic susceptibility of 3D Ising systems, which suggests that
the AFM spin correlation is responsible for the observed SSE near TN.",1905.04408v1
2019-05-16,Three-dimensional topological magnon systems,"We propose a class of models for a magnonic analog of topological insulators
in three dimensions. The models have pseudo-time-reversal symmetry which
ensures the existence of bosonic Kramers pairs. We define a set of
$\mathbb{Z}_2$ topological invariants that characterizes different topological
phases and determines the presence or absence of surface Dirac cones. This is
demonstrated by considering a bosonic counterpart of the Fu-Kane-Mele model on
a diamond lattice. The model is found to exhibit three distinct phases
analogous to strong topological, weak topological, and trivial insulator phases
of the original fermionic model. We also discuss a possible realization of the
thermal Hall effect of surface magnons in the presence of a magnetic field in
proximity to a normal ferromagnet.",1905.06748v3
2019-05-31,Detecting Light Dark Matter with Magnons,"Scattering of light dark matter with sub-eV energy deposition can be detected
with collective excitations in condensed matter systems. When dark matter has
spin-independent couplings to atoms or ions, it has been shown to efficiently
excite phonons. Here we show that, if dark matter couples to the electron spin,
magnon excitations in materials with magnetic dipole order offer a promising
detection path. We derive general formulae for single magnon excitation rates
from dark matter scattering, and demonstrate as a proof of principle the
projected reach of a yttrium iron garnet target for several dark matter models
with spin-dependent interactions. This highlights the complementarity of
various collective excitations in probing different dark matter interactions.",1905.13744v2
2020-06-12,Symmetry Approach to Chiral Optomagnonics in Antiferromagnetic Insulators,"We discuss several aspects of chiral optomagnonics in antiferromagnetic
insulators by considering common symmetries between the electromagnetic field
and spin excitations. This approach allows us to look at optical and magnetic
materials from similar perspectives, and discuss useful analogies between them.
We show that spin waves in collinear antiferromagnets and the electromagnetic
field in vacuum are both invariant under the same eight-dimensional algebra of
symmetry transformations. By such analogy, we can extend the concept of optical
chirality to antiferromagnetic insulators, and demonstrate that the spin-wave
dynamics in these materials in the presence of a spin current is similar to
that of the light inside chiral metamaterials. Photo-excitation of magnonic
spin currents is also discussed from the symmetry point of view. It is
demonstrated that a direct magnonic spin photocurrent can be exited by
circularly polarized light, which can be considered as a magnonic analogue of
the photogalvanic effect. We also note that the Zitterbewegung process should
appear and may play a role in photo-excitation processes.",2006.07399v1
2020-06-20,Stratonovich-Ito integration scheme in ultrafast spin caloritronics,"The magnonic spin Seebeck effect is a key element of spin caloritronic, a
field that exploits thermal effects for spintronic applications. Early studies
were focused on investigating the steady-state nonequilibrium magnonic spin
Seebeck current, and the underlying physics of the magnonic spin Seebeck effect
is now relatively well established. However, the initial steps of the formation
of the spin Seebeck current are in the scope of recent interest. To address
this dynamical aspect theoretically we propose here a new approach to the
time-resolved spin Seebeck effect. Our method exploits the supersymmetric
theory of stochastics and Ito - Stratonovich integration scheme. We found that
in the early step the spin Seebeck current has both nonzero transversal and
longitudinal components. As the magnetization dynamics approaches the
steady-state, the transversal components decay through dephasing over the
dipole-dipole reservoir. The time scale for this process is typically in the
sub-nanoseconds pointing thus to the potential of an ultrafast control of the
dynamical spin Seebeck during its buildup.",2006.11471v1
2020-06-25,Transition magnon modes in thin ferromagnetic nanogratings,"This work presents micromagnetic simulations in ferromagnetic nanogratings
for the full range of directions of an applied in-plane external magnetic
field. We focus on the modification of the magnon mode characteristics when the
magnetic field orientation is gradually changed between the classical
Damon-Eshbach (DE) and backward-volume (BV) geometries. We found that in a
specific range of field directions, the magnon mode parameters differ
significantly from the parameters in the classical cases, namely, the modes are
characterized by complex spatial distributions and have low group velocities.
The center of this range corresponds to the direction of the external magnetic
field, which gives the maximal nonuniform distribution of the static
magnetization in the nanogratings.",2006.14394v2
2020-09-09,Sub-pico-liter magneto-optical cavities,"Microwave-to-optical conversion via ferromagnetic magnons has so-far been
limited by the optical coupling rates achieved in mm-scale whispering gallery
mode resonators. Towards overcoming this limitation, we propose and demonstrate
an open magneto-optical cavity containing a thin-film of yttrium iron garnet
(YIG). We achieve a 0.1 pL (100 $\mu$m$^{3}$) optical mode volume, $\sim$50
times smaller than previous devices. From this, we estimate the magnon
single-photon coupling rate is $G\approx50$ Hz. This open cavity design offers
the prospect of wavelength scale mode volumes, small polarization splittings,
and good magneto-optical mode overlap. With achievable further improvements and
optimization, efficient microwave-optical conversion and magnon cooling devices
become a realistic possibility.",2009.04162v1
2020-09-09,Effect of dipolar interactions on cavity magnon-polaritons,"The strong photon-magnon coupling between an electromagnetic cavity and two
yttrium iron garnet (YIG) spheres has been investigated in the context of a
strong mutual dipolar interaction between the spheres. A decrease in the
coupling strength between the YIG spheres and the electromagnetic cavity is
observed, along with an increase of the total magnetic losses, as the distance
between the spheres is decreased. A model of inhomogeneous broadening of the
ferromagnetic resonance linewidth, partly mitigated by the dipolar narrowing
effect, reproduces the reduction in the coupling strength observed
experimentally. These findings have important implications for the
understanding of strongly coupled photon-magnon system involving densely packed
magnetic objects, such as ferromagnetic nanowires arrays, in which the total
coupling strength with an electromagnetic cavity might become limited due to
mutual dipolar interactions.",2009.04557v1
2020-09-10,Sensing chiral magnetic noise via quantum impurity relaxometry,"We present a theory for quantum impurity relaxometry of magnons in thin
films, exhibiting quantitative agreement with recent experiments without
needing arbitrary scale factors used in theoretical models thus far. Our theory
reveals that chiral coupling between prototypical spin>1/2 quantum impurities
and magnons plays a central role in determining impurity relaxation, which is
further corroborated by our experiments on nickel films interfaced with
nitrogen-vacancy centers. Along with advancing magnonics and understanding
decoherence in hybrid quantum platforms with magnets, the ability of a quantum
impurity spin to sense chiral magnetic noise presents an opportunity to probe
chiral phenomena in condensed matter.",2009.05060v1
2020-09-16,Dynamical mass generation for ferromagnetic skyrmions in two dimensions,"Magnetic skyrmions are topological magnetization textures that are
characterized by the homotopy group of two dimensional spheres. Despite years
of intensive research on skyrmions, the fundamental problem of the inertia of a
skyrmion in driven motion remains unresolved. By properly taking into account a
direct coupling between skyrmion motion and the correspondingly excited
magnons, we identify a dynamical mass for the skyrmion in motion. The direct
coupling between skyrmion motion and magnons can be employed to engineer
skyrmion dynamics with magnons through ingenious material and geometry design.",2009.07486v4
2020-09-17,Spin-wave focusing induced skyrmion generation,"We propose a new method to generate magnetic skyrmions through spin-wave
focusing in chiral ferromagnets.A lens is constructed to focus spin waves by a
curved interface between two ferromagnetic thin films with different
perpendicular magnetic anisotropies. Based on the principle of identical
magnonic path length, we derive the lens contour that can be either elliptical
or hyperbolical depending on the magnon refractive index. Micromagnetic
simulations are performed to verify the theoretical design. It is found that
under proper condition magnetic skyrmions emerge near the focus point of the
lens where the spin-wave intensity has been significantly enhanced. A close
investigation shows that a magnetic droplet first forms and then converts to
the skyrmion accompanying with a change of topological charge. Phase diagram
about the amplitude and duration-time of the exciting field for skyrmion
generation is obtained. Our findings would be helpful for designing novel
spintronic devices combining the advantages of skyrmionics and magnonics.",2009.08091v1
2021-02-09,Nonlinear Decay of Quantum Confined Magnons in Itinerant Ferromagnets,"Quantum confinement leads to the emergence of several magnon modes in
ultrathin layered magnetic structures. We probe the lifetime of these quantum
confined modes in a model system composed of three atomic layers of Co grown on
different surfaces. We demonstrate that the quantum confined magnons exhibit
nonlinear decay rates, which strongly depend on the mode number, in sharp
contrast to what is assumed in the classical dynamics. Combining the
experimental results with those of linear-response density functional
calculations we provide a quantitative explanation for this nonlinear damping
effect. The results provide new insights into the decay mechanism of spin
excitations in ultrathin films and multilayers and pave the way for tuning the
dynamical properties of such structures.",2102.04956v1
2021-02-11,Magnonics in collinear magnetic insulating systems,"In the last decades, collinear magnetic insulating systems have emerged as
promising energy-saving information carriers. Their elementary collective spin
excitations, i.e., magnons, can propagate for long distances bypassing the
Joule heating effects that arise from electron scattering in metal-based
devices. This tutorial article provides an introduction to theoretical and
experimental advances in the study of magnonics in collinear magnetic
insulating systems. We start by outlining the quantum theory of spin waves in
ferromagnetic and antiferromagnetic systems and we discuss their quantum
statistics. We review the phenomenology of spin and heat transport of the
coupled coherent and incoherent spin dynamics and the interplay between
magnetic excitations and lattice degrees of freedom. Finally, we introduce the
reader to the key ingredients of two experimental probes of magnetization
dynamics, spin transport and NV-center relaxometry setups, and discuss
experimental findings relevant to the outlined theory.",2102.08829v1
2021-02-24,Phase-controlled pathway interferences and switchable fast-slow light in a cavity-magnon polariton system,"We study the phase controlled transmission properties in a compound system
consisting of a 3D copper cavity and an yttrium iron garnet (YIG) sphere. By
tuning the relative phase of the magnon pumping and cavity probe tones,
constructive and destructive interferences occur periodically, which strongly
modify both the cavity field transmission spectra and the group delay of light.
Moreover, the tunable amplitude ratio between pump-probe tones allows us to
further improve the signal absorption or amplification, accompanied by either
significantly enhanced optical advance or delay. Both the phase and
amplitude-ratio can be used to realize in-situ tunable and switchable fast-slow
light. The tunable phase and amplitude-ratio lead to the zero reflection of the
transmitted light and an abrupt fast-slow light transition. Our results confirm
that direct magnon pumping through the coupling loops provides a versatile
route to achieve controllable signal transmission, storage, and communication,
which can be further expanded to the quantum regime, realizing coherent-state
processing or quantum-limited precise measurements.",2102.12181v1
2013-06-05,Dynamical structure factor of triangular-lattice antiferromagnet,"We elucidate the role of magnon interaction and spontaneous decays in the
spin dynamics of the triangular-lattice Heisenberg antiferromagnet by
calculating its dynamical structure factor within the spin-wave theory.
Explicit theoretical results for neutron-scattering intensity are provided for
spins S=1/2 and S=3/2. The dynamical structure factor exhibits unconventional
features such as quasiparticle peaks broadened by decays, non-Lorentzian
lineshapes, and significant spectral weight redistribution to the two-magnon
continuum. This rich excitation spectrum illustrates the complexity of the
triangular-lattice antiferromagnet and provides distinctive qualitative and
quantitative fingerprints for experimental observation of decay-induced magnon
dynamics.",1306.1231v3
2013-06-18,"Magnon, phonon and electron temperature profiles and the spin Seebeck effect in magnetic insulator/normal metal hybrid structures","We calculate the phonon, electron and magnon temperature profiles in yttrium
iron garnet/platinum bilayers by diffusive theory with appropriate boundary
conditions, in particular taking into account interfacial thermal resistances.
Our calculations show that in thin film hybrids, the interface magnetic heat
conductance qualitatively affects the magnon temperature. Based on published
material parameters we assess the degree of non-equilibrium at the yttrium iron
garnet/platinum interface. The magnitude of the spin Seebeck effect derived
from this approach compares well with experimental results for the longitudinal
spin Seebeck effect. Additionally we address the temperature profiles in the
transverse spin Seebeck effect.",1306.4292v2
2015-07-22,Spin-current emission governed by nonlinear spin dynamics,"Coupling between conduction electrons and localized magnetization is
responsible for a variety of phenomena in spintronic devices. This coupling
enables to generate spin currents from dynamical magnetization. Due to the
nonlinearity of magnetization dynamics, the spin-current emission through the
dynamical spin-exchange coupling offers a route for nonlinear generation of
spin currents. Here, we demonstrate spin-current emission governed by nonlinear
magnetization dynamics in a metal/magnetic insulator bilayer. The spin-current
emission from the magnetic insulator is probed by the inverse spin Hall effect,
which demonstrates nontrivial temperature and excitation power dependences of
the voltage generation. The experimental results reveal that nonlinear
magnetization dynamics and enhanced spin-current emission due to magnon
scatterings are triggered by decreasing temperature. This result illustrates
the crucial role of the nonlinear magnon interactions in the spin-current
emission driven by dynamical magnetization, or nonequilibrium magnons, from
magnetic insulators.",1507.06081v1
2015-07-30,Quasi-soliton scattering in quantum spin chains,"The quantum scattering of magnon bound states in the anisotropic Heisenberg
spin chain is shown to display features similar to the scattering of solitons
in classical exactly solvable models. Localized colliding Gaussian wave packets
of bound magnons are constructed from string solutions of the Bethe equations
and subsequently evolved in time, relying on an algebraic Bethe ansatz based
framework for the computation of local expectation values in real space-time.
The local magnetization profile shows the trajectories of colliding wave
packets of bound magnons, which obtain a spatial displacement upon scattering.
Analytic predictions on the displacements for various values of anisotropy and
string lengths are derived from scattering theory and Bethe ansatz phase
shifts, matching time evolution fits on the displacements. The time evolved
block decimation (TEBD) algorithm allows for the study of scattering
displacements from spin-block states, showing similar scattering displacement
features.",1507.08624v2
2015-12-16,Interacting Double Coset Magnons,"We consider the anomalous dimensions of restricted Schur polynomials
constructed using n~O(N) complex adjoint scalars Z and m complex adjoint
scalars Y. We fix m<<n so that our operators are almost half BPS. At leading
order in m/n this system corresponds to a dilute gas of m free magnons. Adding
the first correction of order m/n to the anomalous dimension, which arises at
two loops, we find non-zero magnon interactions. The form of this new operator
mixing is studied in detail for a system of two giant gravitons with four
strings attached.",1512.05019v1
2016-11-01,Topological invariant and cotranslational symmetry in strongly interacting multi-magnon systems,"It is still an outstanding challenge to characterize and understand the
topological features of strongly interacting states such as bound-states in
interacting quantum systems. Here, by introducing a cotranslational symmetry in
an interacting multi-particle quantum system, we systematically develop a
method to define a Chern invariant, which is a generalization of the well-known
Thouless-Kohmoto-Nightingale-den Nijs invariant, for identifying strongly
interacting topological states. As an example, we study the topological
multi-magnon states in a generalized Heisenberg XXZ model, which can be
realized by the currently available experiment techniques of cold atoms [Phys.
Rev. Lett. \textbf{111}, 185301 (2013); Phys. Rev. Lett. \textbf{111}, 185302
(2013)]. Through calculating the two-magnon excitation spectrum and the defined
Chern number, we explore the emergence of topological edge bound-states and
give their topological phase diagram. We also analytically derive an effective
single-particle Hofstadter superlattice model for a better understanding of the
topological bound-states. Our results not only provide a new approach to
defining a topological invariant for interacting multi-particle systems, but
also give insights into the characterization and understanding of strongly
interacting topological states.",1611.00205v2
2016-11-06,Terahertz-frequency magnon-phonon-polaritons in the strong coupling regime,"Strong coupling between light and matter occurs when the two interact
strongly enough to form new hybrid modes called polaritons. Here we report on
the strong coupling of both the electric and magnetic degrees of freedom to an
ultrafast terahertz (THz) frequency electromagnetic wave. In our system,
optical phonons in a slab of ferroelectric lithium niobate (LiNbO$_3$) are
strongly coupled to a THz electric field to form phonon-polaritons, which are
simultaneously strongly coupled to magnons in an adjacent slab of canted
antiferromagnetic erbium orthoferrite (ErFeO$_3$) via the THz magnetic field.
The strong coupling leads to the formation of new magnon-phonon-polariton
modes, which we experimentally observe in the wavevector-frequency dispersion
curve as an avoided crossing and in the time-domain as a normal-mode beating.
Our simple yet versatile on-chip waveguide platform provides a promising avenue
by which to explore both ultrafast THz spintronics applications and the quantum
nature of the interaction.",1611.01814v3
2016-11-22,Control and Local Measurement of the Spin Chemical Potential in a Magnetic Insulator,"The spin chemical potential characterizes the tendency of spins to diffuse.
Probing the spin chemical potential could provide insight into materials such
as magnetic insulators and spin liquids and aid optimization of spintronic
devices. Here, we introduce single-spin magnetometry as a generic platform for
non-perturbative, nanoscale characterization of spin chemical potentials. We
use this platform to investigate magnons in a magnetic insulator, surprisingly
finding that the magnon chemical potential can be efficiently controlled by
driving the system's ferromagnetic resonance. We introduce a symmetry-based
two-fluid theory describing the underlying magnon processes, realize the first
experimental determination of the local thermomagnonic torque, and illustrate
the detection sensitivity using electrically controlled spin injection. Our
results open the way for nanoscale control and imaging of spin transport in
mesoscopic spin systems.",1611.07408v1
2016-11-26,Specific Heat of Spin Excitations Measured by FerromagneticResonance,"Using ferromagnetic-resonance spectroscopy (FMR), we investigate the
anisotropic properties of epitaxial 3 nmPt/2 nmAg/10 nmFe/10 nmAg/GaAs(001)
films in fully saturated meta-stable states at temperatures ranging from 70 K
to 280 K. By comparison to spin-wave theory calculations, we identify the role
of thermal fluctuation of magnons in overcoming the energy barrier associated
with these meta-stable states. We show that the energy associated with the size
of the barrier that bounds the meta-stable regime is proportional to the heat
stored in the magnonic bath. Our findings offer the possibility to measure the
magnonic contribution to the heat capacity by FMR, independent of other
contributions at temperatures ranging from 0 K to ambient temperature and
above. The only requirement being that the selected sample exhibits magnetic
anisotropy, here, magnetocrystalline anisotropy.",1611.08713v2
2017-04-17,Raman scattering from Higgs mode oscillations in the two-dimensional antiferromagnet Ca$_2$RuO$_4$,"We present and analyze Raman spectra of the Mott insulator Ca$_2$RuO$_4$,
whose quasi-two-dimensional antiferromagnetic order has been described as a
condensate of low-lying spin-orbit excitons with angular momentum $J_{eff}=1$.
In the $A_g$ polarization geometry, the amplitude (Higgs) mode of the
spin-orbit condensate is directly probed in the scalar channel, thus avoiding
infrared-singular magnon contributions. In the $B_{1g}$ geometry, we observe a
single-magnon peak as well as two-magnon and two-Higgs excitations. Model
calculations using exact diagonalization quantitatively agree with the
observations. Together with recent neutron scattering data, our study provides
strong evidence for excitonic magnetism in Ca$_2$RuO$_4$ and points out new
perspectives for research on the Higgs mode in two dimensions.",1704.04991v2
2017-04-25,Probing length-scale separation of thermal and spin currents by nanostructuring YIG,"We have fabricated bulk nanostructured ferrimagnetic materials with different
grain sizes by sintering ball-milled Y3Fe5O12 (YIG) nanoparticles and measured
the grain-size dependence of the thermal conductivity and spin Seebeck
thermopower. The nanostructuring reduces both thermal conductivity and
thermopower, but the reduction of the latter was found to be considerably
stronger despite the moderate difference in magnetization, which suggests that
the length scales of transport of magnons and phonons contributing to the spin
Seebeck effect are significantly larger than that of phonons carrying thermal
current. This is consistent with the measurements of high-magnetic-field
response of the spin Seebeck thermopower and low-temperature thermal
conductivity, where the quenching of magnons seen in single-crystalline YIG was
not observed in nanostructured YIG due to scattering of long-range low
frequency magnons.",1704.07568v1
2018-01-30,Edge on-site potential effects in a honeycomb topological magnon insulator,"The difference between the edge on-site potential and the bulk values in a
magnonic topological honeycomb lattice leads to the formation of edge states in
a bearded boundary, and the same difference is found to be the responsible for
the absence of edge states in a zig-zag termination. In a finite lattice, the
intrinsic on-site interactions along the boundary sites generate an effective
defect and Tamm-like edge states appear for both zig-zag and bearded
terminations. If a non-trivial gap is induced, Tamm-like and topologically
protected edge states appear in the band structure. The effective defect can be
strengthened by an external on-site potential and the dispersion relation,
velocity and magnon-density of the edge states become tunable.",1801.09945v1
2018-08-02,Quantum kinetic theory of spin-transfer torque and magnon-assisted transport in nanostructures,"We theoretically investigate the role of spin fluctuations in charge
transport through a magnetic junction. Motivated by recent experiments that
measure a nonlinear dependence of the current on electrical bias, we develop a
systematic understanding of the interplay of charge and spin dynamics in
nanoscale magnetic junctions. Our model captures two distinct features arising
from these fluctuations: magnon-assisted transport and the effect of
spin-transfer torque on the magnetoconductance. The latter stems from magnetic
misalignment in the junction induced by spin-current fluctuations. As the
temperature is lowered, inelastic quantum scattering takes over thermal
fluctuations, exhibiting signatures that make it readily distinguishable from
magnon-assisted transport.",1808.00777v2
2018-08-19,Quantized excitation spectra by magnon confinement in quasi-one-dimensional S=1 spin systems,"We apply the infinite time-evolving-block-decimation algorithm to calculate
the dynamical spin-structure factors of the quasi-one-dimensional (q1D) S=1
antiferromagnetic spin system with the single-ion anisotropy and the bond
alternation. We find that excitation continuum originating from magnons is
quantized, when the staggered field induced by the weak inter-chain interaction
is taken into account. The excitation energies of the quantized excitation
spectra are well explained by negative zeros of the Airy functions, when the
easy-axis anisotropy is strong and the ground state is located deep in the
N\'eel phase. This quantization of the magnon continuum is a counterpart of the
spinon confinement, which has been recently discussed in q1D S=1/2
antiferromagnets. We further show that, when the staggered field exists, the
quantized excitation spectra appear the phase boundary between the Haldane
phase and the N\'eel phase of the phase diagram without the staggered field.
However, the quantized excitation spectra disappear in the singlet dimer phase.",1808.06270v2
2019-06-05,Phonon laser in a cavity magnomechanical system,"The phonon analog of an optical laser has become the focus of research. We
theoretically study phonon laser in a cavity magnomechanical system, which
consist of a microwave cavity, a small ferromagnetic sphere and an uniform
external bias magnetic field. This system can realize the phonon-magnon
coupling and the cavity photon-magnon coupling via magnetostrictive interaction
and magnetic dipole interaction respectively, the magnons are driven directly
by a strong microwave field simultaneously. Frist, the intensity of driving
magnetic field which can reach the threshold condition of phonon laser is
given. Then, we demonstrate that the phonon laser can be well controlled by an
adjustable external magnetic field without changing other parameters, which
provides an additional degree of freedom compared to the phonon laser in
optomechanical systems. Finally, with the experimentally feasible parameters,
threshold power in our scheme is close to the case of optomechanical systems.
Our results provide a theoretical basis for the realization of phonon lasers in
magnomechanical systems.",1906.01871v3
2019-06-18,Magnon-polaron excitations in the noncollinear antiferromagnet Mn$_3$Ge,"We present the detailed inelastic neutron scattering measurements of the
noncollinear antiferromagnet Mn$_3$Ge. Time-of-flight and triple-axis
spectroscopy experiments were conducted at the temperature of 6~K, well below
the high magnetic ordering temperature of 370~K. The magnetic excitations have
a 5-meV gap and display an anisotropic dispersive mode reaching $\simeq 90$~meV
at the boundaries of the magnetic Brillouin zone. The spectrum at the zone
center shows two additional excitations that demonstrate characteristics of
both magnons and phonons. The \textit{ab initio} lattice-dynamics calculations
show that these can be associated with the magnon-polaron modes resulting from
the hybridization of the spin fluctuations and the low-energy optical phonons.
The observed magnetoelastic coupling agrees with the previously found negative
thermal expansion in this compound and resembles the features reported in the
spectroscopic studies of other antiferromagnets with the similar noncollinear
spin structures.",1906.07640v1
2019-06-28,Ground-state cooling of an magnomechanical resonator induced by magnetic damping,"Quantum manipulation of mechanical resonators has been widely applied in
fundamental physics and quantum information processing. Among them, cooling the
mechanical system to its quantum ground state is regarded as a key step. In
this work, we propose a scheme which one can realize ground-state cooling of
resonator in a cavity magnomechanical system. The system consists of a
microwave cavity and a small ferromagnetic sphere, in which phonon-magnon
coupling and cavity photon-magnon coupling can be achieved via magnetostrictive
interaction and magnetic dipole interaction, respectively. After adiabatically
eliminating the cavity mode, an effective Hamiltonian which consists of magnon
and mechanical modes is obtained. Within experimentally feasible parameters, we
demonstrate that the ground-state cooling of the magnomechanical resonator can
be achieved by extra magnetic damping. Unlike optomechanical cooling,
magnomechanical interaction is utilized to realize the cooling of resonators.
We further illustrate the ground-state cooling can be effectively controlled by
the external magnetic field.",1906.12081v1
2019-07-01,Chemical potential of an antiferromagnetic magnon gas,"Understanding the statistics of quasi-particle excitations in magnetic
systems is essential for exploring new magnetic phases and collective quantum
phenomena. While the chemical potential of a ferromagnetic gas has been
extensively investigated both theoretically and experimentally, its
antiferromagnetic counterpart remains uncharted. Here, we derive the statistics
of a two-component U(1)-symmetric Bose gas and apply our results to an
axially-symmetric antiferromagnetic insulator. We find that the two magnon
eigenmodes of the system are described by an equal and opposite chemical
potential, in analogy with a particle-antiparticle pair. Furthermore, we derive
the thermomagnonic torques describing the interaction between the coherent and
incoherent antiferromagnetic spin dynamics. Our results show that the magnitude
and sign of the chemical potential can be tuned via an AC magnetic field
driving resonantly one of the magnon modes. Finally, we propose NV-center
relaxometry as a method to experimentally test our predictions.",1907.00931v1
2019-07-05,Theory for shift current of bosons: Photogalvanic spin current in ferrimagnetic and antiferromagnetic insulators,"We theoretically study the optical generation of dc spin current (i.e., a
spin-current solar cell) in ordered antiferromagnetic and ferrimagnetic
insulators, motivated by a recent study on the laser-driven spinon spin current
in noncentrosymmetric quantum spin chains [H. Ishizuka and M. Sato, Phys. Rev.
Lett. 122, 197702 (2019)]. Using a non-linear response theory for magnons, we
analyze the dc spin current generated by a linearly-polarized electromagnetic
wave (typically, terahertz or gigahertz waves). Considering noncentrosymmetric
two-sublattice magnets as an example, we find a finite dc spin current
conductivity at $T=0$, where no thermally-excited magnons exist; this is in
contrast to the case of the spinon spin current, in which the optical
transition of the Fermi degenerate spinons plays an essential role. We find
that the dc spin-current conductivity is insensitive to the Gilbert damping,
i.e., it may be viewed as a shift current carried by bosonic particles
(magnons). Our estimate shows that an electric-field intensity of
$E\sim10^4-10^6$ V/cm is sufficient for an observable spin current. Our theory
indicates that the linearly-polarized electromagnetic wave generally produces a
dc spin current in noncentrosymmetric magnetic insulators.",1907.02734v1
2019-07-14,Realization of Attractive Level Crossing via a Dissipative Mode,"The new field of spin cavitronics focuses on the interaction between the
magnon excitation of a magnetic element and the electromagnetic wave in a
microwave cavity. In strong interaction regime, such interaction usually gives
rise to the level anti-crossing for the magnonic and the electromagnetic mode.
Recently, the attractive level crossing has been observed, and is explained by
a non-Hermitian model Hamiltonian. However, the mechanism of the such
attractive coupling is still unclear. Here we reveal the secret by using a
simple model with two harmonic oscillators coupled to a third oscillator with
large dissipation. We further identify this dissipative third-party as the
invisible cavity mode with large leakage in the cavity-magnon experiments. This
understanding enables designing dissipative coupling in all sorts of coupled
systems.",1907.06222v2
2019-10-09,Magnetic field dependence of the nonlocal spin Seebeck effect in Pt/YIG/Pt systems at low temperatures,"We report the nonlocal spin Seebeck effect (nlSSE) in a lateral configuration
of Pt/Y$_3$Fe$_5$O$_{12}$(YIG)/Pt systems as a function of the magnetic field
$B$ (up to 10 T) at various temperatures $T$ (3 K < $T$ < 300 K). The nlSSE
voltage decreases with increasing $B$ in a linear regime with respect to the
input power (the applied charge-current squared $I^2$). The reduction of the
nlSSE becomes substantial when the Zeeman energy exceeds thermal energy at low
temperatures, which can be interpreted as freeze-out of magnons relevant for
the nlSSE. Furthermore, we found the non-linear power dependence of the nlSSE
with increasing $I$ at low temperatures ($T$ < 20 K), at which the $B$-induced
signal reduction becomes less visible. Our experimental results suggest that in
the non-linear regime high-energy magnons are over populated than those
expected from the thermal energy. We also estimate the magnon spin diffusion
length as functions of $B$ and $T$.",1910.04046v1
2019-10-17,Phonon Transport Controlled by Ferromagnetic Resonance,"The resonant coupling of phonons and magnons is important for the
interconversion of phononic and spin degrees of freedom. We studied the phonon
transmission in LiNbO3 manipulated by the dynamic magnetization in a Ni thin
film. It was observed that the phonons could be absorbed strongly through
resonant magnon-phonon coupling, which was realized by optimizing the
interfacial coupling between Ni and LiNbO3. The line shapes of phonon
transmission were further investigated considering the magnon-phonon
interconversion in the elastically driven ferromagnetic resonance process. The
results promote unique routes for phonon manipulation and detection in the
presence of magnetization dynamics.",1910.08148v1
2019-10-18,Magnetic field control of topological magnon-polaron bands in two-dimensional ferromagnets,"We theoretically study magnon-phonon hybrid excitations in a square lattice
ferromagnet subjected to a magnetic field by varying the field direction. The
bulk bands of hybrid excitations, which are referred to as magnon-polarons, are
investigated by considering all three phonon modes: vertical phonon, transverse
phonon, and longitudinal phonon. We show that the topological proprieties of
three hybridizations are different in terms of the Berry curvature and the
Chern numbers. We also find that the topological properties of the bands can be
controlled by changing the direction of the magnetic field, exhibiting one or
more topological phase transitions. The dependence of thermal Hall conductivity
as a function of magnetic field direction is proposed as an experiment probe of
our theoretical results.",1910.08603v1
2019-10-30,The magnon-mediated attraction between two holes doped in a CuO$_2$ layer,"Using a realistic multi-band model for two holes doped into a CuO$_2$ layer,
we devise a method to turn off the magnon-mediated interaction between the
holes. This allows us to verify that this interaction is attractive, and
therefore could indeed be (part of) the superconducting glue. We derive its
analytical expression and show that it consists of a novel kind of
pair-hopping+spin-exchange terms. Its coupling constant is fitted from the
ground state energy obtained with variational exact diagonalization, and it
faithfully reproduces the effect of the magnon-mediated attraction in the
entire Brillouin zone. For realistic parameter values, this effective
interaction is borderline strong enough to bind the holes into preformed pairs.",1910.14069v1
2020-01-02,Origin of Strong Two-Magnon Scattering in Heavy Metal/Ferromagnet/Oxide Heterostructures,"We experimentally investigate the origin of the two-magnon scattering (TMS)
in heavy-metal (HM)/ferromagnet (FM)/oxide heterostructures (FM = Co, Ni81Fe19,
or Fe60Co20B20) by varying the materials located above and below the FM layers.
We show that strong TMS in HM/FM/oxide systems arises primarily at the HM/FM
interface and increases with the strength of interfacial spin-orbit coupling
and magnetic roughness at this interface. TMS at the FM/oxide interface is
relatively weak, even in systems where spin-orbit coupling at this interface
generates strong interfacial magnetic anisotropy. We also suggest that the
spin-current-induced excitation of non-uniform short-wavelength magnon at the
HM/FM interface may function as a mechanism of spin memory loss for the
spin-orbit torque exerted on the uniform mode.",2001.00300v1
2020-01-29,Detecting Light Boson Dark Matter through Conversion into Magnon,"Light boson dark matter such as axion or hidden photon can be resonantly
converted into a magnon in a magnetic insulator under the magnetic field, which
can be detected experimentally. We provide a quantum mechanical formulation for
the magnon event rate and show that the result is consistent with that obtained
by a classical calculation. Besides, it is pointed out that the experimental
setup of the QUAX proposal for the axion detection also works as a detector of
hidden photon dark matter. It has good sensitivity in the mass range around 1
meV, which is beyond astrophysical constraints.",2001.10666v3
2020-03-10,Dynamics of a two-dimensional quantum spin-orbital liquid: spectroscopic signatures of fermionic magnons,"We provide an exact study of dynamical correlations for the quantum
spin-orbital liquid phases of an SU(2)-symmetric Kitaev honeycomb lattice
model. We show that the spin dynamics in this Kugel-Khomskii type model is
exactly the density-density correlation function of S=1 fermionic magnons,
which could be probed in resonant inelastic x-ray scattering experiments. We
predict the characteristic signatures of spin-orbital fractionalization in
inelastic scattering experiments and compare them to the ones of the
spin-anisotropic Kitaev honeycomb spin liquid. In particular, the resonant
inelastic x-ray scattering response shows a characteristic momentum dependence
directly related to the dispersion of fermionic excitations. The neutron
scattering cross section displays a mixed response of fermionic magnons as well
as spin-orbital excitations. The latter has a bandwidth of broad excitations
and a vison gap that is three times larger than that of the spin-1/2 Kitaev
model.",2003.04835v3
2020-03-25,Asymmetric scattering behaviors of spin wave dependent on magnetic vortex chirality,"In this letter, an asymmetric spin wave scattering behaviors caused by vortex
chirality are investigated in cross-shaped ferromagnetic system. In the system,
four scattering behaviors are found, 1) asymmetric skew scattering, depending
on the polarity of vortex core, 2) back scattering (reflection), depending on
the vortex core stiffness, 3) side deflection scattering, depending on
structural symmetry of the vortex circulation, and 4) geometrical scattering,
depending on waveguide structure. The first and second scattering behaviors are
attributed to nonlinear topological magnon spin Hall effect related to magnon
spin-transfer torque effect, which has value for magnonic exploration and
application.",2003.11193v2
2020-04-05,Magnon spectrum and electron spin resonance in antiferromagnet with large single-ion easy plane anisotropy,"Motivated by recent experiments on quantum magnet
NiCl$_{2}$-4SC(NH$_{2}$)$_{2}$ (DTN) and its Br-doped counterpart DTNX we
propose a theoretical description of optical magnon branch in the
antiferromagnet with large single-ion anisotropy in the magnetically ordered
phase. In the framework of the 1/S expansion we derive analytical expressions
for optical magnon with $\mathbf{k}=0$ energy magnetic field dependence
$\Delta(h)$. It is shown that in the linear spin wave approximation $\Delta(h)$
is monotonic without extrema whereas first order in 1/S corrections makes it
drastically different function with a minimum near the center of magnetically
ordered phase. The latter behaviour was observed in ESR experiments. Moreover,
we show that $\Delta(h)$ has nontrivial dependence on the system parameters. It
solves the discrepancy between inelastic neutron scattering data where the
growth of interaction constants in DTNX with small Br concentration was
observed and ESR experiments showing almost unchanged $\Delta(h)$ in comparison
with pure DTN.",2004.02170v2
2020-04-06,Resonant inelastic x-ray scattering study of vector chiral ordered kagome antiferromagnet,"We study the resonant inelastic x-ray scattering (RIXS) features of vector
chiral ordered kagome antiferromagnets. Utilizing a group theoretical formalism
that respects lattice site symmetry, we calculated the $L$ -edge magnon
contribution for the vesignieite compound BaCu$_{3}$V$_{2}$O$_{8}$(OH)$_{2}$.
We show that polarization dependence of the $L$ -edge RIXS spectrum can be used
to track magnon branches. We predict a non-zero $L$ -edge signal in the
non-cross $\pi-\pi$ polarization channel. At the $K$ -edge, we derived the
two-site effective RIXS and Raman scattering operator for two-magnon excitation
in vesignieite using the Shastry-Shraiman formalism. Our derivation considers
spin-orbit coupling effects in virtual hopping processes. We find vector chiral
correlation (four-spin) contribution that is proportional to the RIXS spectrum.
Our scattering operator formalism can be applied to a host of non-collinear
non-coplanar magnetic materials at both the $L$ and $K$ -edge. We demonstrate
that vector chiral correlations can be accessed by RIXS experiments.",2004.02871v2
2020-04-21,Optical observation of quasiperiodic Heisenberg antiferromagnets in two dimensions,"We calculate magnetic Raman spectra of Heisenberg antiferromagnets on the
two-dimensional Penrose lattice. We follow the Shastry-Shraiman formulation of
Raman scattering in a strongly correlated Hubbard system and obtain the second-
and fourth-order effective Raman operators. The second-order Raman intensity
comes from the E2 mode, and it is invariant under an arbitrary rotation of
polarization vectors. The fourth-order Raman intensities consist of A1 and A2,
as well as E2, modes and therefore yield strong polarization dependence. In
particular, the A2 mode intensity directly detects the dynamical spin-chirality
fluctuations. Employing linearly and circularly polarized lights, we can
separately extract every irreducible representation from the observations. We
further discuss effects of magnon-magnon interactions on the magnetic Raman
scattering. Our theory provides a reasonable explanation for the two-magnon
scattering process.",2004.09850v1
2020-05-04,Spin-orbit-torque magnonics,"The field of magnonics, which utilizes propagating spin waves for nano-scale
transmission and processing of information, has been significantly advanced by
the advent of the spin-orbit torque. The latter phenomenon can allow one to
overcome two main drawbacks of magnonic devices - low energy efficiency of
conversion of electrical signals into spin wave signals, and fast spatial decay
of spin waves in thin-film waveguiding structures. At first glance, the
excitation and amplification of spin waves by spin-orbit torques can seem to be
straightforward. Recent research indicates, however, that the lack of the
mode-selectivity in the interaction of spin currents with dynamic magnetic
modes and the onset of dynamic nonlinear phenomena represent significant
obstacles. Here, we discuss the possible route to overcoming these limitations,
based on the suppression of nonlinear spin-wave interactions in magnetic
systems with perpendicular magnetic anisotropy. We show that this approach
enables efficient excitation of coherent magnetization dynamics and propagating
spin waves in extended spatial regions, and is expected to enable practical
implementation of complete compensation of spin-wave propagation losses.",2005.01391v1
2020-05-08,Observation of terahertz magnon of Kaplan-Kittel exchange resonance in yttrium-iron garnet by Raman spectroscopy,"Backscattering Raman spectroscopic investigations were performed on an
yttrium-iron garnet single crystal using linearly and circularly polarized
light. A terahertz (THz) magnon of the Kaplan-Kittel (KK) exchange resonance
was discovered, which had been regarded as unobservable via optical methods.
The KK exchange resonance had a 7.8-THz frequency at 80 K, and the polarization
selection rule led to an antisymmetric Raman tensor of the $A_{2}$ mode.
Moreover, the assignment of all the Raman-active phonon modes, $3A_{1g}$,
$8E_{g}$, and $14T_{2g}$, was proposed. This study will stimulate further
investigation of the coupling of THz magnons and phonons and pave the way
toward THz optomagnonics.",2005.03852v2
2020-05-20,Einstein-de Haas Effect of Topological Magnons,"We predict the existence of Einstein-de Haas effect in topological magnon
insulators. Temperature variation of angular momentum in the topological state
shows a sign change behavior, akin to the low temperature thermal Hall
conductance response. This manifests itself as a macroscopic mechanical
rotation of the material hosting topological magnons. We show that an
experimentally observable Einstein-de Haas effect can be measured in the
square-octagon, the kagome, and the honeycomb lattices. Albeit, the effect is
the strongest in the square-octagon lattice. We treat both the low and the high
temperature phases using spin wave and Schwinger boson theory, respectively. We
propose an experimental set up to detect our theoretical predictions. We
suggest candidate square-octagon materials where our theory can be tested.",2005.10273v2
2020-05-22,Dipolar spin waves in uniaxial easy-axis antiferromagnets: A natural topological nodal-line semimetal,"The existence of the magnetostatic surface spin waves in ferromagnets, known
as Damon-Eshbach mode, was recently demonstrated to originate from the topology
of the dipole-dipole interaction. In this work, we study the topological
characteristics of magnons in easy-axis antiferromagnets with uniaxial
anisotropy. The dipolar spin waves are found to be, driven by the dipole-dipole
interaction, in a topological nodal-line semimetal phase, which hosts
Damon-Eshbach-type surface modes due to the bulk-edge correspondence. The long
wavelength character of dipolar spin waves makes our proposal valid for any
natural uniaxial easy-axis antiferromagnet, and thus enriches the candidates of
topological magnonic materials. In contrast to the nonreciprocal property in
ferromagnetic case, the surface modes with opposite momentum coexist at each
surface, but with different chiralities. Such a chirality-momentum or
spin-momentum locking, similar to that of electronic surface states in
topological insulators, offers the opportunity to design novel chirality-based
magnonic devices in antiferromagnets.",2005.10971v1
2020-07-06,Hippopede curves for modelling radial spin waves in an azimuthal graded magnonic landscape,"We propose a mathematical model for describing radially propagating spin
waves emitted from the core region in a magnetic patch with n vertices in a
magnetic vortex state. The azimuthal anisotropic propagation of surface spin
waves (SSW) into the domain, and confined spin waves (or Winter's Magnons, WM)
in domain walls increases the complexity of the magnonic landscape. In order to
understand the spin wave propagation in these systems, we first use an approach
based on geometrical curves called 'hippopedes', however it provides no insight
into the underlying physics. Analytical models rely on generalized expressions
from the dispersion relation of SSW with an arbitrary angle between
magnetization M and wavenumber k. The derived algebraic expression for the
azimuthal dispersion is found to be equivalent to that of the 'hippopede'
curves. The fitting curves from the model yield a spin wave wavelength for any
given azimuthal direction, number of patch vertices and excitation frequency,
showing a connection with fundamental physics of exchange dominated surface
spin waves. Analytical results show good agreement with micromagnetic
simulations and can be easily extrapolated to any n-corner patch geometry.",2007.02646v2
2020-07-23,Density Wave Mediated Dzyaloshinskii-Moriya Interactions,"We investigate the effect that density wave states have on the localized
spins of a square lattice. We find that topologically nontrivial density wave
states can induce stable Dzyaloshinskii-Moriya (DM) interactions among the
localized spins of the lattice in the presence of an external magnetic field,
and we study the resulting spin models for both antiferromagnetic and
ferromagnetic backgrounds. While the density wave state itself can contribute
to the the thermal Hall effect, as shown by Li & Lee (arXiv:1905.04248v3),
symmetry considerations preclude the resulting spin excitations from inducing a
further thermal Hall effect. We utilize a Holstein-Primakoff (HP) substitution
about the classical mean-field ground state to calculate the magnon dispersion
for LSCO and find that the density wave induces a weak $d_{x^2-y^2}$
anisotropy; upon calculating the non-Abelian Berry curvature for this magnon
branch we show explicitly that the magnon contribution to $\kappa_{xy}$ is
zero. Finally, we calculate corrections to the magnetic ground state energy,
spin canting angles, and the spin-wave dispersion due to the topological
density wave for ferromagnetic backgrounds. We find that terms linear in the HP
bosons can affect the critical behavior, a point previously overlooked in the
literature.",2007.11719v1
2020-07-26,Fractionalized Spin Excitations in the Edge Ferromagnetic State of Graphene: Signature of the Ferromagnetic Luttinger Liquid,"The elementary excitations from the conventional magnetic ordered states,
such as ferromagnets and antiferromagnets, are magnons. Here, we elaborate a
case where the well-defined magnons are absent completely and the spin
excitation spectra exhibit an entire continuum in the itinerant edge
ferromagnetic state of graphene arising from the flatband edge electronic
states. Based on the further studies of the entanglement entropy and
finite-size analysis, we show that the continuum other than the Stoner part
results from the spin-1/2 spinons deconfined from magnons. The spinon continuum
in a magnetically ordered state is ascribed to a ferromagnetic Luttinger liquid
in this edge ferromagnet. The investigation is carried out by using the
numerical exact diagonalization method with a projection of the interacting
Hamiltonian onto the flat band.",2007.13142v1
2020-07-30,Resonant subwavelength control of the phase of spin waves reflected from a ferromagnetic film edge,"Using frequency-domain finite element calculations cross-checked with
micromagnetic simulations, we demonstrate that the phase of spin waves
reflected from an interface between a permalloy film and a bilayer can be
controlled by changing dimensions of the bilayer. Treating the bilayer formed
by the permalloy film and a ferromagnetic stripe as a segment of a multi-mode
waveguide, we show that spin-wave Fabry-Perot resonances of one of its modes
are responsible for the high sensitivity of the phase of reflected waves to
stripe width and the stripe-film separation. Thus, the system is a unique
realization of a fully magnonic Gires-Tournois interferometer based on a
two-modes resonator, which can be treated as a magnonic counterpart of a
metasurface, since it enables manipulation of the phase of spin waves at
subwavelength distances. Knowledge gained from these calculations might be used
to design magnonic devices such as flat lenses or magnetic particle detectors.",2007.15226v5
2020-07-30,Coherent multi-mode conversion from microwave to optical wave via a magnon-cavity hybrid system,"Coherent conversion from microwave to optical wave opens new research avenues
towards long distant quantum network covering quantum communication, computing,
and sensing out of the laboratory. Especially multi-mode enabled system is
essential for practical applications. Here we experimentally demonstrate
coherent multi-mode conversion from the microwave to optical wave via
collective spin excitation in a single crystal yttrium iron garnet (YIG,
Y3Fe5O12) which is strongly coupled to a microwave cavity mode in a
three-dimensional rectangular cavity. Expanding collective spin excitation mode
of our magnon-cavity hybrid system from Kittel to multi magnetostatic modes, we
verify that the size of YIG sphere predominantly plays a crucial role for the
microwave-to-optical multi-mode conversion efficiency at resonant conditions.
We also find that the coupling strength between multi magnetostatic modes and a
cavity mode is manipulated by the position of a YIG inside the cavity. It is
expected to be valuable for designing a magnon hybrid system that can be used
for coherent conversion between microwave and optical photons.",2007.15299v1
2020-08-04,Bose-Einstein condensate of Dirac magnons: Pumping and collective modes,"We explore the formation and collective modes of Bose-Einstein condensate of
Dirac magnons (Dirac BEC). While we focus on two-dimensional Dirac magnons, an
employed approach is general and could be used to describe Bose-Einstein
condensates with linear quasiparticle spectrum in various systems. By using a
phenomenological multicomponent model of pumped boson population together with
bosons residing at Dirac nodes, the formation and time evolution of condensates
of Dirac bosons is investigated. The condensate coherence and its
multicomponent nature are manifested in the Rabi oscillations whose period is
determined by the gap in the spin-wave spectrum. A Dirac nature of the
condensates could be also probed by the spectrum of collective modes. It is
shown that the Haldane gap provides an efficient means to tune between the
gapped and gapless collective modes as well as controls their stability.",2008.01328v2
2020-08-05,Magnon-driven dynamics of a hybrid system excited with ultrafast optical pulses,"The potential of photon-magnon hybrid systems as building blocks for quantum
information science has been widely demonstrated, and it is still the focus of
much research. We leverage the strengths of this unique heterogeneous physical
system in the field of precision physics beyond the standard model, where the
sensitivity to the so-called ""invisibles"" is currently being boosted by quantum
technologies. Here, we demonstrate that quanta of spin waves, driven by
tiniest, effective magnetic field, can be detected in a large frequency band
using a hybrid system as transducer. This result can be applied to the search
of cosmological signals related, for example, to cold Dark Matter, which may
directly interact with magnons. Our model of the transducer is based on a
second-quantisation two-oscillators hybrid system, it matches the observations,
and can be easily extended to thoroughly describe future large-scale
ferromagnetic haloscopes.",2008.02317v1
2020-08-13,Concurrence of topological electrons and magnons in the Kagome ferromagnet CoCu3(OH)6Cl2,"Spin and charge are two interrelated properties of electrons. However, most
of previous works on topological matter study the electronic and magnonic
excitations separately. In this paper, by combining density functional theory
calculations with the Schwinger boson method, we determine the topological
electronic band structures and topological magnon spectrum simultaneously in
the ferromagnetic ground state of the narrow-band-gap CoCu3(OH)6Cl2, which is
an ABC stacking Kagome lattice material with fractional occupancy on Cu sites.
This material provides an ideal platform to study the interplay of different
types of topological excitations. Our work also proposes a useful method to
deal with correlated topological magnetic systems with narrow band gaps.",2008.05692v1
2020-08-21,Spin waves in skyrmionic structures with various topological charges,"Equilibrium properties and localised magnon excitations are investigated in
topologically distinct skyrmionic textures. The observed shape of the
structures and their orientation on the lattice is explained based on their
vorticities and the symmetry of the crystal. The transformation between
different textures and their annihilation as a function of magnetic field is
understood based on the energy differences between them. The angular momentum
spin-wave eigenmodes characteristic of cylindrically symmetric structures are
combined in the distorted spin configurations, leading to avoided crossings in
the magnon spectrum. The susceptibility of the skyrmionic textures to
homogeneous external fields is calculated, revealing that a high number of
modes become detectable due to the hybridization between the angular momentum
eigenmodes. These findings should contribute to the observation of spin waves
in distorted skyrmionic structures via experiments and numerical simulations,
widening the range of their possible applications in magnonic devices.",2008.09392v2
2020-08-21,Quantum sensing of a coherent single spin excitation in a nuclear ensemble,"The measurement of single quanta in a collection of coherently interacting
objects is transformative in the investigations of emergent quantum phenomena.
An isolated nuclear-spin ensemble is a remarkable platform owing to its
coherence, but detecting its single spin excitations has remained elusive.
Here, we use an electron spin qubit in a semiconductor quantum dot to sense a
single nuclear-spin excitation (a nuclear magnon) with 1.9-ppm precision via
the 200-kHz hyperfine shift on the 28-GHz qubit frequency. We demonstrate this
single-magnon precision across multiple modes identified by nuclear species and
polarity. Finally, we monitor the coherent dynamics of a nuclear magnon and the
emergence of quantum correlations competing against decoherence. A direct
extension of this work is to probe engineered quantum states of the ensemble
including long-lived memory states.",2008.09541v1
2020-08-25,Short-range Thermal Magnon Diffusion in Magnetic Garnet,"Using the spin Seebeck effect (SSE), we study the propagation distance of
thermal spin currents inside a magnetic insulator thin film in the short-range
regime. We disambiguate spin currents driven by temperature and chemical
potential gradients by comparing the SSE signal before and after adding a
thermalization capping layer on the same device. We report that the measured
spin decay behavior near the heat source is well accounted for by a diffusion
model where the magnon diffusion length is in submicron range, \textit{i.e.}
two orders of magnitude smaller than previous estimates inferred from the
long-range behavior. Our results highlight the caveat in applying a diffusive
theory to describe thermal magnon transport, where a single decay length may
not capture the behavior on all length scales.",2008.10942v1
2020-10-21,Determination of magnon mass of neodymium magnet by temperature dependence of spontaneous magnetization,"The effective mass of a magnon in a neodymium magnet is evaluated using Bloch
temperature dependence of spontaneous magnetization. We used the Hall effect
magnetometer and thermo-couple thermometer for measuring of the temperature of
the water in which the encapsulated commercial magnet is immersed. The
experiment can be repeated in every school and methodological remarks for
experimental data processing are given with great detail and attention. The
purpose of the paper is to illustrate the simple relation between the quantum
spectrum of magnons and the easily measured magnetic field of a permanent
magnet.",2010.11229v2
2020-11-09,Microwave Quantum Illumination via Cavity Magnonics,"Quantum illumination (QI) is a quantum sensing protocol mainly for target
detection which uses entangled signal-idler photon pairs to enhance the
detection efficiency of low-reflectivity objects immersed in thermal noisy
environments. Especially, due to the naturally occurring background radiation,
the photon emitted toward potential targets more appropriately lies in the
microwave region. Here, we propose a hybrid quantum source based on cavity
magnonics for microwave QI, where the medium that bridges the optical and the
microwave modes is magnon, the quanta of spin wave. Within experimentally
accessible parameters, significant microwave-optical quantum resources of
interest can be generated, leading to orders of magnitude lower detecting error
probability compared with the electro-optomechanical prototype quantum radar
and any classical microwave radar with equal transmitted energy.",2011.04301v2
2020-11-29,Cross-sublattice Spin Pumping and Magnon Level Attraction in van der Waals Antiferromagnets,"We theoretically study spin pumping from a layered van der Waals
antiferromagnet in its canted ground state into an adjacent normal metal. We
find that the resulting dc spin pumping current bears contributions along all
spin directions. Our analysis allows for detecting intra- and cross-sublattice
spin-mixing conductances via measuring the two in-plane spin current
components. We further show that sublattice symmetry-breaking Gilbert damping
can be realized via interface engineering and induces a dissipative coupling
between the optical and acoustic magnon modes. This realizes magnon level
attraction and exceptional points in the system. Furthermore, the dissipative
coupling and cross-sublattice spin pumping contrive to produce an
unconventional spin current in the out-of-plane direction. Our findings provide
a route to extract the spin mixing conductance matrix and uncovers the unique
opportunities, such as level attraction, offered by van der Waals
antiferromagnet-normal metal hybrids.",2011.14314v1
2021-01-12,Temperature dependence of the mean magnon collision time in a spin Seebeck device,"Based on the relaxation time approximation, the mean collision time for
magnon scattering $\tau_c(T)$ is computed from the experimental spin Seebeck
coefficient of a bulk YIG / Pt bilayer. The scattering results to be composed
by two processes: the low temperature one, with a $T^{-1/2}$ dependence, is
attributed to the scattering by defects and provides a mean free path around 10
$\mu$m; the high temperature one, depending on $T^{-4}$, is associated to the
scattering by other magnons. The results are employed to predict the thickness
dependence of the spin Seebeck coefficient for thin films.",2101.04405v1
2021-03-09,Spin-Space Groups and Magnon Band Topology,"Band topology is both constrained and enriched by the presence of symmetry.
The importance of anti-unitary symmetries such as time reversal was recognized
early on leading to the classification of topological band structures based on
the ten-fold way. Since then, lattice point group and non-symmorphic symmetries
have been seen to lead to a vast range of possible topologically nontrivial
band structures many of which are realized in materials. In this paper we show
that band topology is further enriched in many physically realizable instances
where magnetic and lattice degrees of freedom are wholly or partially
decoupled. The appropriate symmetry groups to describe general magnetic systems
are the spin-space groups. Here we describe cases where spin-space groups are
essential to understand the band topology in magnetic materials. We then focus
on magnon band topology where the theory of spin-space groups has its simplest
realization. We consider magnetic Hamiltonians with various types of coupling
including Heisenberg and Kitaev couplings revealing a hierarchy of enhanced
magnetic symmetry groups depending on the nature of the lattice and the
couplings. We describe, in detail, the associated representation theory and
compatibility relations thus characterizing symmetry-enforced constraints on
the magnon bands revealing a proliferation of nodal points, lines, planes and
volumes.",2103.05656v1
2021-03-11,Spin Nernst Effect of Antiferromagnetic Magnons in the Presence of Spin Diffusion,"Magnon spin Nernst effect was recently proposed as an intrinsic effect in
antiferromagnets, where spin diffusion and boundary spin transmission have been
ignored. However, diffusion processes are essential to convert a bulk spin
current into boundary spin accumulation, which determines the spin injection
rate into detectors through imperfect transmission. We formulate a diffusive
theory to describe the detection of magnon spin Nernst effect with boundary
conditions reflecting real device geometry. Thanks to the spin diffusion
effect, the output signals in both electronic and optical detection grow
rapidly with an increasing system size in the transverse dimension, which
eventually saturate. Counterintuitively, the measurable signals are even
functions of magnetic field, yielding optical detection more favorable than
electronic detection.",2103.06870v3
2021-03-18,Cherenkov radiation of spin waves by ultra-fast moving magnetic flux quanta,"Despite theoretical predictions for a Cherenkov-type radiation of spin waves
(magnons) by various propagating magnetic perturbations, fast-enough moving
magnetic field stimuli have not been available so far. Here, we experimentally
realize the Cherenkov radiation of spin waves in a Co-Fe magnonic conduit by
fast-moving (>1 km/s) magnetic flux quanta (Abrikosov vortices) in an adjacent
Nb-C superconducting strip. The radiation is evidenced by the microwave
detection of spin waves propagating a distance of 2 micrometers from the
superconductor and it is accompanied by a magnon Shapiro step in its
current-voltage curve. The spin-wave excitation is unidirectional and
monochromatic, with sub-40 nm wavelengths determined by the period of the
vortex lattice. The phase-locking of the vortex lattice with the excited spin
wave limits the vortex velocity and reduces the dissipation in the
superconductor.",2103.10156v2
2021-04-22,Dynamical Backaction Magnomechanics,"Dynamical backaction resulting from radiation pressure forces in
optomechanical systems has proven to be a versatile tool for manipulating
mechanical vibrations. Notably, dynamical backaction has resulted in the
cooling of a mechanical resonator to its ground-state, driving phonon lasing,
the generation of entangled states, and observation of the optical-spring
effect. In certain magnetic materials, mechanical vibrations can interact with
magnetic excitations (magnons) via the magnetostrictive interaction, resulting
in an analogous magnon-induced dynamical backaction. In this article, we
directly observe the impact of magnon-induced dynamical backaction on a
spherical magnetic sample's mechanical vibrations. Moreover, dynamical
backaction effects play a crucial role in many recent theoretical proposals;
thus, our work provides the foundation for future experimental work pursuing
many of these theoretical proposals.",2104.11218v2
2021-04-22,Theory for electrical detection of the magnon Hall effect induced by dipolar interactions,"We derive the anomalous Hall contributions arising from dipolar interactions
to diffusive spin transport in magnetic insulators. Magnons, the carriers of
angular momentum in these systems, are shown to have a non-zero Berry
curvature, resulting in a measurable Hall effect. For yttrium iron garnet (YIG)
thin films we calculate both the anomalous and magnon spin conductivities. We
show that for a magnetic field perpendicular to the film the anomalous Hall
conductivity is finite. This results in a non-zero Hall signal, which can be
measured experimentally using Permalloy strips arranged like a Hall bar on top
of the YIG thin film. We show that electrical detection and injection of spin
is possible, by solving the resulting diffusion-relaxation equation for a Hall
bar. We predict the experimentally measurable Hall coefficient for a range of
temperatures and magnetic field strengths. Most strikingly, we show that there
is a sign change of the Hall coefficient associated with increasing the
thickness of the film.",2104.11304v1
2021-04-27,Magnon modes as a joint effect of surface ferromagnetism and spin-orbite coupling in CoSi chiral topological semimetal,"CoSi single crystal is a known realization of a chiral topological semimetal
with simultaneously broken mirror and inversion symmetries. In addition to the
symmetry-induced spin-orbit coupling, surface ferromagnetism is known in
nominally diamagnetic CoSi structures, which appears due to the distorted bonds
and ordered vacancies near the surface. We experimentally investigate electron
transport through a thin CoSi flake at high current density. Surprisingly, we
demonstrate $dV/dI(I)$ curves which are qualitatively similar to ones for
ferromagnetic multilayers with characteristic $dV/dI$ magnon peaks and
unconventional magnetic field evolution of the peaks' positions. We understand
these observations as a result of current-induced spin polarization due to the
significant spin-orbit coupling in CoSi. Scattering of non-equilibrium
spin-polarized carriers within the surface ferromagnetic layer is responsible
for the precessing spin-wave excitations, so the observed magnon modes are the
joint effect of surface ferromagnetism and spin-orbit coupling in a CoSi chiral
topological semimetal. Thus, thin CoSi flakes behave as magnetic conductors
with broken inversion symmetry, which is important for different spintronic
phenomena.",2104.13252v1
2021-05-12,Magnon spin current induced by triplet Cooper pair supercurrents,"At the interface between a ferromagnetic insulator and a superconductor there
is a coupling between the spins of the two materials. We show that when a
supercurrent carried by triplet Cooper pairs flows through the superconductor,
the coupling induces a magnon spin current in the adjacent ferromagnetic
insulator. The effect is dominated by Cooper pairs polarized in the same
direction as the ferromagnetic insulator, so that charge and spin supercurrents
produce similar results. Our findings demonstrate a way of converting Cooper
pair supercurrents to magnon spin currents.",2105.05861v2
2021-05-16,Tunable anisotropic quantum Rabi model via a magnon--spin-qubit ensemble,"The ongoing rapid progress towards quantum technologies relies on new hybrid
platforms optimized for specific quantum computation and communication tasks,
and researchers are striving to achieve such platforms. We study theoretically
a spin qubit exchange-coupled to an anisotropic ferromagnet that hosts magnons
with a controllable degree of intrinsic squeezing. We find this system to
physically realize the quantum Rabi model from the isotropic to the
Jaynes-Cummings limit with coupling strengths that can reach the deep-strong
regime. We demonstrate that the composite nature of the squeezed magnon enables
concurrent excitation of three spin qubits coupled to the same magnet. Thus,
three-qubit Greenberger-Horne-Zeilinger and related states needed for
implementing Shor's quantum error-correction code can be robustly generated.
Our analysis highlights some unique advantages offered by this hybrid platform,
and we hope that it will motivate corresponding experimental efforts.",2105.07430v2
2021-05-24,Cavity magnomechanics with surface acoustic waves,"Magnons, namely spin waves, are collective spin excitations in ferromagnets,
and their control through coupling with other excitations is a key technology
for future hybrid spintronic devices. Although strong coupling has been
demonstrated with microwave photonic structures, an alternative approach
permitting high density integration and minimized electromagnetic crosstalk is
required. Here we report a planar cavity magnomechanical system, where the
cavity of surface acoustic waves enhances the spatial and spectral power
density to thus implement magnon-phonon coupling at room temperature.
Excitation of spin-wave resonance involves significant acoustic power
absorption, whereas the collective spin motion reversely exerts a back-action
force on the cavity dynamics. The cavity frequency and quality-factor are
significantly modified by the back-action effect, and the resultant
cooperativity exceeds unity, suggesting coherent interaction between magnons
and phonons. The demonstration of a chip-scale magnomechanical system paves the
way to the development of novel spin-acoustic technologies for classical and
quantum applications.",2105.11127v1
2021-05-28,Implementation and enhancement of nonreciprocal quantum synchronization with strong isolation in antiferromagnet-cavity systems,"Sensitive signal detection and processing in classical world, especially in
quantum regime, require nonreciprocal manipulation. In this paper we show how
to achieve nonreciprocal quantum synchronization for two magnon modes in a
two-sublattice antiferromagnet with strong isolation.The antiferromagnet is
trapped in a cavity with two posts so that the two magnon modes not only couple
to each other through a parametric-type interaction, but also interact with a
same cavity respectively in a beam splitter-type and parametric-type ways.
Under the condition of system's stability, we show that nonreciprocal quantum
synchronization between two magnon modes is mainly dependent on resonance
frequency of the cavity caused by direction of input currents. In
addition,quantum synchronization is enhanced by the increase of interaction
strength between two Bogoliubov modes and cavity mode. Moreover, numerical
simulation results with parameters commonly used in current experiments show
that the present scheme may be feasible.",2105.13525v1
2021-06-15,Two-dimensional spin models with macroscopic degeneracy,"We consider a class of anisotropic spin-$\frac{1}{2}$ models with competing
ferro- and antiferromagnetic interactions on two-dimensional Tasaki and kagome
lattices consisting of corner sharing triangles. For certain values of the
interactions the ground state is macroscopically degenerated in zero magnetic
field. In this case the ground state manifold consists of isolated magnons as
well as the bound magnon complexes. The ground state degeneracy is estimated
using a special form of exact wave function which admits arrow configuration
representation on two-dimensional lattice. The comparison of this estimate with
the result for some special exactly solved models shows that the used approach
determines the number of the ground states with exponential accuracy. It is
shown that the main contribution to the ground state degeneracy and the
residual entropy is given by the bound magnon complexes.",2106.07965v1
2021-06-19,Approaching to the deep-strong photon-to-magnon coupling,"In this work, the ultra-strong photon-to-magnon coupling is demonstrated for
on-chip multilayered superconductor/ferromagnet/insulator hybrid thin film
structures reaching the coupling strength above 6 GHz, the coupling ratio about
0.6, the single-spin coupling strength about 350 Hz, and cooperativity about
10^4. High characteristics of coupling are achieved owing to a radical
suppression of the photon phase velocity in electromagnetic resonator. With
achieved coupling the spectrum reveals inapplicability of the Dicke model, and
evidences contribution of the diamagnetic A^2 interaction term in the
Hamiltonian of the system, which satisfies the Thomas-Reiche-Kuhn sum rule. The
contribution of the A^2 term denotes validity of the Hopfield quantum model and
manifests observation of a different hybrid polariton quasi-particle, namely,
the plasmon-magnon polariton.",2106.10450v1
2021-07-07,Polarization dependent scattering in cavity optomagnonics,"The polarization dependence of magnon-photon scattering in an optical
microcavity is reported. Due to the short cavity length, the mode-matching
conditions found in previously explored, large path-length whispering gallery
resonators are absent. Nonetheless, for cross-polarized scattering a strong and
broadband suppression of one side-band is observed. This arises due to an
interference between the Faraday and second-order Cotton-Mouton effects. To
fully account for the suppression of the cross-polarized scattering, it is
necessary to consider the squeezing of magnon modes intrinsic to thin-film
geometry. A co-polarized scattering due to Cotton-Mouton effect is also
observed. In addition, the magnon modes involved are identified as
Damon-Eshbach surface modes, whose non-reciprocal propagation could be
exploited in devices applications. This work experimentally demonstrates the
important role of second order Cotton-Mouton effect for optomagnonic devices.",2107.03214v1
2021-07-12,Instabilities of heavy magnons in an anisotropic magnet,"The search for new elementary particles is one of the most basic pursuits in
physics, spanning from subatomic physics to quantum materials. Magnons are the
ubiquitous elementary quasiparticle to describe the excitations of
fully-ordered magnetic systems. But other possibilities exist, including
fractional and multipolar excitations. Here, we demonstrate that strong quantum
interactions exist between three flavors of elementary quasiparticles in the
uniaxial spin-one magnet FeI2. Using neutron scattering in an applied magnetic
field, we observe spontaneous decay between conventional and heavy magnons and
the recombination of these quasiparticles into a super-heavy bound-state. Akin
to other contemporary problems in quantum materials, the microscopic origin for
new physics in FeI2 is the quasi-flat nature of excitation bands and the
presence of Kitaev anisotropic magnetic exchange interactions.",2107.05694v3
2021-07-23,"Dirac magnons, nodal lines, and nodal plane in elemental gadolinium","We investigate the magnetic excitations of elemental gadolinium (Gd) using
inelastic neutron scattering, showing that Gd is a Dirac magnon material with
nodal lines at $K$ and nodal planes at half integer $\ell$. We find an
anisotropic intensity winding around the $K$-point Dirac magnon cone, which is
interpreted to indicate Berry phase physics. Using linear spin wave theory
calculations, we show the nodal lines have non-trivial Berry phases, and
topological surface modes. We also discuss the origin of the nodal plane in
terms of a screw-axis symmetry, and introduce a topological invariant
characterizing its presence and effect on the scattering intensity. Together,
these results indicate a highly nontrivial topology, which is generic to
hexagonal close packed ferromagnets. We discuss potential implications for
other such systems.",2107.11372v3
2021-08-07,Strong Momentum-Dependent Electron-Magnon Renormalization of a Surface Resonance on Iron,"The coupling of fermionic quasiparticles to magnons is essential for a wide
range of processes, from ultrafast magnetization dynamics in ferromagnets to
Cooper pairing in superconductors. Although magnon energies are generally much
larger than phonon energies, up to now their electronic band renormalization
effect in ferromagnetic metals suggests a significantly weaker quasiparticle
interaction. Here, using spin- and angle-resolved photoemission, we show an
extraordinarily strong renormalization leading to replica-band formation of an
iron surface resonance at ~200 meV. Its strong magnetic linear dichroism
unveils the magnetic nature and momentum dependence of the energy
renormalization. By determining the frequency- and momentum-dependent
self-energy due to generic electron-boson interaction to compute the resultant
electron spectral function, we show that the surface-state replica formation is
consistent with strong coupling to an optical spin wave in a Fe thin film.",2108.03421v1
2021-08-10,Fragmentation and emergent integrable transport in the weakly tilted Ising chain,"We investigate emergent quantum dynamics of the tilted Ising chain in the
regime of a weak transverse field. Within the leading order perturbation
theory, the Hilbert space is fragmented into exponentially many decoupled
sectors. We find that the sector made of isolated magnons is integrable with
dynamics being governed by a constrained version of the XXZ spin Hamiltonian.
As a consequence, when initiated in this sector, the Ising chain exhibits
ballistic transport on unexpectedly long times scales. We quantitatively
describe its rich phenomenology employing exact integrable techniques such as
Generalized Hydrodynamics. Finally, we initiate studies of
integrability-breaking magnon clusters whose leading-order transport is
activated by scattering with surrounding isolated magnons.",2108.04845v2
2021-08-12,Magnetic interactions and spin excitations in van der Waals ferromagnet VI$_3$,"Using a combination of density functional theory (DFT) and spin-wave theory
methods, we investigate the magnetic interactions and spin excitations in
semiconducting VI$_3$. Exchange parameters of monolayer, bilayer, and bulk
forms are evaluated by mapping the magnetic energies of various spin
configurations, calculated using DFT+$U$, onto the Heisenberg model. The
intralayer couplings remain largely unchanged in three forms of VI$_3$, while
the interlayer couplings show stronger dependence on the dimensionality of the
materials. We calculate the spin-wave spectra within a linear spin-wave theory
and discuss how various exchange parameters affect the magnon bands. The
magnon-magnon interaction is further incorporated, and the Curie temperature is
estimated using a self-consistently renormalized spin-wave theory. To
understand the roles of constituent atoms on magnetocrystalline anisotropy
energy (MAE), we resolve MAE into sublattices and find that a strong negative
V-I inter-sublattice contribution is responsible for the relatively small
easy-axis MAE in VI$_3$.",2108.05528v1
2021-08-19,Intrinsic topological magnons in arrays of magnetic dipoles,"We study a simple magnetic system composed of periodically modulated magnetic
dipoles with an easy axis. Upon adjusting the modulation amplitude alone,
chains and two-dimensional stacked chains exhibit a rich magnon spectrum where
frequency gaps and magnon speeds are easily manipulable. The blend of
anisotropy due to dipolar interactions between magnets and geometrical
modulation induces a magnetic phase with fractional Zak number in infinite
chains and end states in open one-dimensional systems. In two dimensions it
gives rise to topological modes at the edges of stripes. Tuning the amplitude
in two-dimensional lattices causes a band touching, which triggers the exchange
of the Chern numbers of the volume bands and switches the sign of the thermal
conductivity.",2108.08459v2
2021-09-01,Magnonic Proximity Effect in Insulating Ferro- and Antiferromagnetic Trilayers,"The design of spin-transport based devices such as magnon transistors or spin
valves will require multilayer systems composed of different magnetic materials
with different physical properties. Such layered structures can show various
interface effects, one class of which being proximity effects, where a certain
physical phenomenon that occurs in the one layers leaks into another one. In
this work a magnetic proximity effect is studied in trilayers of different
ferro- and antiferromagnetic materials within an atomistic spin model. We find
the magnetic order in the central layer - with lower critical temperature -
enhanced, even for the case of an antiferromagnet surrounded by ferromagnets.
We further characterize this proximity effect via the magnon spectra which are
specifically altered, especially for the case of the antiferromagnet in the
central layer.",2109.00286v1
2021-09-23,Quantum Interference between Photons and Single Quanta of Stored Atomic Coherence,"Essential for building quantum networks over remote independent nodes, the
indistinguishability of photons has been extensively studied by observing the
coincidence dip in the Hong-Ou-Mandel interferometer. However,
indistinguishability is not limited to the same type of bosons. For the first
time, we hereby observe quantum interference between flying photons and a
single quantum of stored atomic coherence (magnon) in an atom-light beam
splitter interface. We demonstrate that the Hermiticity of this interface
determines the type of quantum interference between photons and magnons.
Consequently, not only the bunching behavior that characterizes bosons is
observed, but counterintuitively, fermionlike antibunching as well. The hybrid
nature of the demonstrated magnon-photon quantum interface can be applied to
versatile quantum memory platforms, and can lead to fundamentally different
photon distributions from those occurring in boson sampling.",2109.11148v2
2021-09-27,Entanglement of magnon excitations in spin chains,"We calculate exactly the entanglement content of magnon excited states in the
integrable spin-1/2 XXX and XXZ chains in the scaling limit. In particular, we
show that as far as the number of excited magnons with respect to the size of
the system is small one can decompose the entanglement content, in the scaling
limit, to the sum of the entanglement of particular excited states of free
fermionic or bosonic theories. In addition we conjecture that the entanglement
content of the generic translational invariant free fermionic and bosonic
Hamiltonians can be also classified, in the scaling limit, with respect to the
entanglement content of the fermionic and bosonic chains with the number
operator as the Hamiltonian in certain circumstances. Our results effectively
classify the entanglement content of wide range of integrable spin chains in
the scaling limit.",2109.12826v2
2021-10-05,The impact of defects on excitations in two-dimensional bipartite uniaxial antiferromagnet insulators,"We address scatterings of spin waves off uncorrelated defects in
two-dimensional (2D) easy-axis antiferromagnet (AFM) insulators. Although an
onsite magnetic anisotropy leads gapped Goldstone modes, such that a long range
(AFM) order can be established in 2D, lattice imperfections tend to weaken, and
eventually destroy the magnetic ordering. Here, the impact of defects is
considered within two limits, single and multiple defects. Using Green's
function, we perform self consistent simulations to study magnon properties
such as density of states and lifetime of induced resonances. Our findings show
that repulsive defects decrease the magnon density of states while attractive
ones may enhance it. We provide a comprehensive analysis of how defects can
result in a reduction and even closing, the anisotropy induced gap, which
weakens the long range (AFM) order parameter in the 2D state. We conclude that
a small concentration of random defects can fill the gap of magnon spectrum.",2110.02045v3
2021-10-06,Magnon Interference Tunneling Spectroscopy as a Probe of 2D Magnetism,"Probing two-dimensional single-layer quantum magnets remains a significant
challenge. In this letter, we propose exploiting tunneling spectroscopy in the
presence of magnetic impurities to obtain information about the magnon
dispersion relations in analogy to quasiparticle interference in non-magnetic
materials. We show this technique can be used to establish the dispersion
relations even for frustrated magnets, where the presence of an impurity
generally leads to a nontrivial spin texture. Finally, we consider the problem
of establishing Chern magnon bands in 2D magnets showing how tuneable
impurities allow probing the nature of the surface states.",2110.02662v1
2021-10-13,Fractional spin excitations in the infinite-layer cuprate CaCuO$_2$,"We use resonant inelastic x-ray scattering (RIXS) to investigate the magnetic
dynamics of the infinite-layer cuprate CaCuO2. We find that close to the
(1/2,0) point the single magnon decays into a broad continuum of excitations
accounting for 80% of the total magnetic spectral weight. Polarization resolved
RIXS spectra reveal the overwhelming dominance of spin-flip ($\Delta S = 1$)
characterof this continuum with respect to the $\Delta S= 0$ multimagnon
contributions. Moreover, its incident energy dependence is identical to that of
the magnon, supporting a common physical origin. We propose that the continuum
originates from the decay of the magnon into spinon pairs, and we relate it to
the exceptionally high ring exchange $J_\mathrm{c} \sim J_1$ of CaCuO2. In the
infinite layer cuprates long-range and multi-site hopping integrals are pivotal
and amplify the 2D quantum magnetism effects in spite the 3D antiferromagnetic
N\'eel order.",2110.06666v1
2021-10-14,Influence of static correlation on the magnon dynamics of an itinerant ferromagnet with competing exchange interactions -- a first principles study of MnBi,"We present first principles calculations of the dynamic susceptibility in
strained and doped ferromagnetic MnBi using time-dependent density functional
theory. In spite of being a metal, MnBi exhibits signatures of strong
correlation and a proper description in the framework of density functional
theory requires Hubbard corrections to the Mn $d$-orbitals. To permit
calculations of the dynamic susceptibility with Hubbard corrections applied to
the ground state electronic structure, we use a consistent rescaling of the
exchange-correlation kernel maintaining the delicate balance between the magnon
dispersion and the Stoner continuum. We find excellent agreement with the
experimentally observed magnon dispersion for pristine MnBi and show that the
material undergoes a phase transition to helical order under application of
either doping or strain. The presented methodology paves the way for future
LR-TDDFT studies of magnetic phase transitions, also for the wide range of
materials with pronounced static correlation effects that are not accounted for
at the LDA level.",2110.07282v3
2021-10-21,Massless Dirac magnons in the two dimensional van der Waals honeycomb magnet CrCl3,"Two dimensional van der Waals ferromagnets with honeycomb structures are
expected to host the bosonic version of Dirac particles in their magnon
excitation spectra. Using inelastic neutron scattering, we study spin wave
excitations in polycrystalline CrCl$_3$, which exhibits ferromagnetic honeycomb
layers with antiferromagnetic stackings along the $c$-axis. For comparison,
polycrystal samples of CrI$_3$ with different grain sizes are also studied. We
find that the powder-averaged spin wave spectrum of CrCl$_3$ at $T$ = 2 K can
be adequately explained by the two dimensional spin Hamiltonian including
in-plane Heisenberg exchanges only. The observed excitation does not exhibit
noticeable broadening in energy, which is in remarkable contrast to the
substantial broadening observed in CrI$_3$. Based on these results, we conclude
that the ferromagnetic phase of CrCl$_3$ hosts massless Dirac magnons and is
thus not topological.",2110.10894v1
2021-11-04,Control of the phase of reflected spin-waves from magnonic Gires-Tournois interferometer of subwavelength width,"The phase is one of the fundamental properties of a wave that allows to
control interference effects and can be used to efficiently encode information.
We examine numerically a magnonic resonator of the Gires-Tournois
interferometer type, which enables the control of the phase of spin waves
reflected from the edges of the ferromagnetic film. The considered
interferometer consists of a Py thin film and a thin, narrow Py stripe placed
above its edge, both coupled magnetostatically. We show that the resonances and
the phase of the reflected spin waves are sensitive for a variation of the
geometrical parameters of this bi-layerd part of the system. The high
sensitivity to film, stripe, and non-magnetic spacer thicknesses, offers a
prospect for developing magnonic metasurfaces and sensors.",2111.02678v1
2021-11-14,Single-sideband microwave-to-optical conversion in high-Q ferrimagnetic microspheres,"Coherent conversion of microwave and optical photons can significantly expand
the ability to control the information processing and communication systems.
Here, we experimentally demonstrate the microwave-to-optical frequency
conversion in a magneto-optical whispering gallery mode microcavity. By
applying a magnetic field parallel to the microsphere equator, the intra-cavity
optical field will be modulated when the magnon is excited by the microwave
drive, leading to microwave-to-optical conversion via the magnetic Stokes and
anti-Stokes scattering processes. The observed single sideband conversion
phenomenon indicates a non-trivial optical photon-magnon interaction mechanism,
which is derived from the magnon induced both the frequency shift and modulated
coupling rate of optical modes. In addition, we demonstrate the single-sideband
frequency conversion with an ultrawide tuning range up to 2.5GHz, showing its
great potential in microwave-to-optical conversion.",2111.07325v1
2021-12-06,Unified formulation of interfacial magnonic pumping from non-collinear magnets,"We establish a general formulation for spin current pumped by magnons at the
interface between a normal metal and a magnetic insulator, valid for any
non-collinear magnetic configuration in the linear spin wave regime. This
current is generated by driving the system in a non-equilibrium state, covering
setups such as thermal spin injection (spin Seebeck effect) or spin voltage by
irradiation of the insulator (spin pumping). We illustrate the formula in the
special case of a honeycomb topological ferromagnet, for simplicity, and cover
both the spin Seebeck and spin-pumping setups. We show how the topological
parameters influence the spin current and propose a way to obtain a
contribution mainly from the topological edge magnons in the spin pumping case.",2112.02946v1
2021-12-23,Electric-field-induced parametric excitation of exchange magnons in a CoFeB/MgO junction,"Electric-field controlled magnetization dynamics is an important integrant in
low power spintronic devices. In this letter, we demonstrate electric-field
induced parametric excitation for CoFeB/MgO junctions by using interfacial
in-plane magnetic anisotropy. When the in-plane magnetic anisotropy and the
external magnetic field are parallel to each other, magnons are efficiently
excited by using electric-field induced parametric excitation. Its wavelength
and wavenumber are tuned by changing input power and frequency of the applied
voltage. A generalized phenomenological model is developed to explain the
underlying role of the electric-field torque. Electrical excitation with no
Joule heating offers a good opportunity for developing magnonic devices and
exploring various nonlinear dynamics in magnetic systems.",2112.12308v2
2022-01-12,The angular dependence of magnetization dynamics induced by a GHz range strain pulse,"The dynamics of magnetization is important in spintronics, where the coupling
between phonon and magnon attracts much attention. In this work, we study the
angular dependence of the coupling between longitudinal-wave phonon and magnon.
We investigated the magnetization dynamics using the time-resolved
magneto-optical Kerr effect, which allows measuring spin-wave resonances and
the magnetic echo signal. The frequency, mode number, and amplitude of the
spin-wave resonance change with the out-of-plane angle of the external magnetic
field. The amplitude of the magnetic echo signal caused by the strain pulse
also changes with the angle. We calculate these angular dependences based on
the Landau-Lifshitz-Gilbert equation and find that the angles of the external
field and magnetic moment are important factors for the phonon-magnon coupling
when phonon propagates in the thickness direction under the out-of-plane
magnetic field.",2201.04396v1
2022-01-29,Accelerated adiabatic passage in cavity magnomechanics,"Cavity magnomechanics provides a readily-controllable hybrid system, that
consisted of cavity mode, magnon mode, and phonon mode, for quantum state
manipulation. To implement a fast-and-robust state transfer between the hybrid
photon-magnon mode and the phonon mode, we propose two accelerated
adiabatic-passage protocols individually based on the counterdiabatic
Hamiltonian for transitionless quantum driving and the Levis-Riesenfeld
invariant for inverse engineering. Both the counterdiabatic Hamiltonian and the
Levis-Riesenfeld invariant generally apply to the continuous-variable systems
with arbitrary target states. It is interesting to find that our
counterdiabatic Hamiltonian can be constructed in terms of the creation and
annihilation operators rather than the system-eigenstates and their
time-derivatives. Our protocol can be optimized with respect to the stability
against the systematic errors of coupling strength and frequency detuning. It
contributes to a quantum memory for photonic and magnonic quantum information.
We also discuss the effects from dissipation and the counter-rotating
interactions.",2201.12536v1
2022-02-07,Spin-transfer-assisted parametric pumping of magnons in yttrium iron garnet,"The combination of parametric pumping and spin-transfer torque is a powerful
approach that enables high-level control over magnetic excitations in thin-film
ferromagnets. The excitation parameters, such as pumping power and external
field strength, affect the instabilities of individual magnon modes. We
theoretically explore how the simultaneous effects of parametric pumping and
spin transfer torque influence these magnetic instabilities in a thin-film
ferromagnet. Within the Landau-Lifshitz-Gilbert framework, we perform
micromagnetic simulations of magnon excitations in yttrium iron garnet by
pumping, spin transfer torque, and a combination of the two. We find that
consistent with experimental results, the magnitude and direction of the
spin-transfer torque tune the parametric instability thresholds.",2202.03247v2
2022-02-07,Magnetization dynamics affected by phonon pumping,"""Pumping"" of phonons by a dynamic magnetization promises to extend the range
and functionality of magnonic devices. We explore the impact of phonon pumping
on room-temperature ferromagnetic resonance (FMR) spectra of bilayers of thin
yttrium iron garnet (YIG) films on thick gadolinium gallium garnet substrates
over a wide frequency range. At low frequencies the Kittel mode hybridizes
coherently with standing ultrasound waves of a bulk acoustic resonator to form
magnon polarons that induce rapid oscillations of the magnetic susceptibility,
as reported before. At higher frequencies, the phonon resonances overlap,
merging into a conventional FMR line, but with an increased line width. The
frequency dependence of the increased line broadening follows the predictions
from phonon pumping theory in the thick substrate limit. In addition, we find
substantial magnon-phonon coupling of a perpendicular standing spin wave (PSSW)
mode. This evidences the importance of the mode overlap between the acoustic
and magnetic modes, and provides a route towards engineering the magnetoelastic
mode coupling.",2202.03331v1
2022-02-10,Amplifying spin waves along Néel domain wall by spin-orbit torque,"Traveling spin waves in magnonic waveguides undergo severe attenuation, which
tends to result in a finite propagation length of spin waves, even in magnetic
materials with the accessible lowest damping constant, heavily restricting the
development of magnonic devices. Compared with the spin waves in traditional
waveguides, propagating spin waves along strip domain wall are expected to
exhibit enhanced transmission. Here, we demonstrate, theoretically and through
micromagnetic simulations, that spin-orbit torque associated with a
ferromagnet/heavy metal bilayer can efficiently control the attenuation of spin
waves along a N\'eel-type strip domain wall, despite the complexity in the
ground-state magnetization configuration. The direction of the electric current
applied to the heavy-metal layer determines whether these spin waves are
amplified or further attenuated otherwise. Remarkably, our simulations reveal
that the effective current densities required to efficiently tune the decay of
such spin waves are just ~10^10 Am-2, roughly an order smaller than those
required in conventional spin waveguides. Our results will enrich the toolset
for magnonic technologies.",2202.05181v1
2022-02-23,Spin canting in nonlinear terahertz magnon dynamics revealed by magnetorefractive probing in orthoferrite,"We excite the spin precession in rare-earth orthoferrite YFeO3 by the
magnetic field of intense terahertz pulse and probe its dynamics by transient
absorption change in the near infrared. The observed waveforms contain
quasi-ferromagnetic-mode magnon oscillation and its second harmonics with a
comparably strong amplitude. The result can be explained by dielectric function
derived from magnetorefractive Hamiltonian. We reveal that the strong second
harmonic signal microscopically originates from novel dynamics of the
quasi-ferromagnetic mode magnon at nonlinear regime, wherein spin canting angle
periodically oscillates.",2202.11365v1
2022-02-24,Magnon corner states in twisted bilayer honeycomb magnets,"Search for higher-order topological insulators, characterized by
topologically protected gapless boundary states of codimension higher than one,
in bosonic systems has attracted growing interest. Here, we establish twisted
bilayer honeycomb magnets as a new platform for hosting second-order
topological magnon insulators (SOTMIs) without fine-tuning. We employ a simple,
minimal Heisenberg spin model to describe misaligned bilayer sheets of
honeycomb ferromagnetic magnets with a large commensurate twist angle. We found
that the higher-order topology in this bilayer system shows a significant
dependence on the interlayer exchange coupling. The SOTMI, featuring
topologically protected magnon corner states, appears for ferromagnetic
interlayer couplings, while the twisted bilayer exhibits a nodal phase in the
case of antiferromagnetic interlayer coupling.",2202.12151v2
2022-03-03,The fate of the spin polaron in the 1D antiferromagnets,"The stability of the spin polaron quasiparticle, well established in studies
of a single hole in the 2D antiferromagnets, is investigated in the 1D
antiferromagnets using a $t$-$J$ model. We perform an exact slave fermion
transformation to the holon-magnon basis, and diagonalize numerically the
resulting model in the presence of a single hole. We prove that the spin
polaron is stable for any strength of the magnon-magnon interaction except for
the unique value of the SU(2)-symmetric 1D $t$-$J$ model. Fine-tuning to this
unique value is extremely unlikely to occur in quasi-1D antiferromagnets,
therefore the spin polaron is the stable quasiparticle of real 1D materials.
Our results lead to a new interpretation of the ARPES spectra of quasi-1D
antiferromagnets in the spin polaron language.",2203.01846v4
2022-03-07,Switchable selective interactions in a Dicke Model with Driven Biased term,"In this work, we propose a method to investigate controllable qubit-resonator
interactions in a Dicke model with driven biased term. The nonlinearity of
spectrum, which can be induced by qubit-resonator interactions, plays an
important role in such controllable interactions. To gain insight into
mechanism of the nonlinearity, we perform a unitary transformation to the
Hamiltonian. The results show that the nonlinearity of the transformed
Hamiltonian depends on the qubit-resonator coupling strength. The general forms
of the effective Hamiltonians are discussed in detail based on the frequency
modulation approach. The dynamical evolution can be switched on and off by
adjusting the modulation parameters. By utilizing such controllable
interactions, we discuss the creation of Dicke states and arbitrary
superposition of Dicke states. We also consider the nonlinearity of energy
level for the limit of large qubit numbers. In the thermodynamics limit, the
kerr type nonlinearity is induced from ""magnon""-resonator coupling, and the
selective preparation of ""magnon"" Fock states can be studied under ""magnon""
scenario.",2203.03301v1
2022-03-08,Multi-Center Magnon Excitations Open the Entire Brillouin Zone to Terahertz Magnetometry of Quantum Magnets,"Due to the small photon momentum, optical spectroscopy commonly probes
magnetic excitations only at the center of the Brillouin zone; however, there
are ways to override this restriction. In the case of the distorted kagome
quantum magnet Y-kapellasite, Y$_3$Cu$_9$(OH)$_{19}$Cl$_8$, under scrutiny
here, the magnon density of states can be accessed over the entire Brillouin
zone through three-center magnon excitations. This mechanism is aided by the
three different magnetic sublattices and strong short-range correlations in the
distorted kagome lattice. The results of THz time-domain experiments agree
remarkably well with linear spin-wave theory. Relaxing the conventional
zone-center constraint of photons gives a new aspect to probe magnetism in
matter.",2203.04343v1
2022-03-30,Nonvolatile Electric Field Control of Thermal Magnons in the Absence of an Applied Magnetic Field,"Spin transport through magnetic insulators has been demonstrated in a variety
of materials and is an emerging pathway for next-generation spin-based
computing. To modulate spin transport in these systems, one typically applies a
sufficiently strong magnetic field to allow for deterministic control of
magnetic order. Here, we make use of the well-known multiferroic
magnetoelectric, BiFeO3, to demonstrate non-volatile, hysteretic,
electric-field control of thermally excited magnon current in the absence of an
applied magnetic field. These findings are an important step toward
magnon-based devices, where electric-field-only control is highly desirable.",2203.16519v2
2022-04-10,Unveiling a Pump-Induced Magnon Mode via its Strong Interaction with Walker Modes,"We observe a power-dependent anticrossing of Walker spin-wave modes under
microwave pumping when a ferrimagnet is placed in a microwave waveguide that
does not support any discrete photon mode. We interpret this unexpected
anticrossing as the generation of a pump-induced magnon mode that couples
strongly to the Walker modes of the ferrimagnet. This anticrossing inherits an
excellent tunability from the pump, which allows us to control the anticrossing
via the pump power, frequency, and waveform. Further, we realize a remarkable
functionality of this anticrossing, namely, a microwave frequency comb, in
terms of the nonlinear interaction that mixes the pump and probe frequencies.
Such a frequency comb originates from the magnetic dynamics and thereby does
not suffer from the charge noise. The unveiled hybrid magnonics driven away
from its equilibrium enriches the utilization of anticrossing for coherent
information processing.",2204.04590v2
2022-04-12,Loschmidt echo and Momentum Distribution in a Kitaev Spin Chain,"We investigate the Loschmidt echo in a one-dimensional spin chain having
Kitaev-type interaction in constant and kicked magnetic fields. The Loschmidt
echo for the initial states having different magnon excitations shows long-time
revivals for smaller chains and has short-time revival peaks for the longer
chains. The system near the critical point shows peculiarly long-time revival
peaks of the Loschmidt echo for relatively larger chains. The presence of a
magnon in the initial state affects the Loschmidt echo revival peaks. The
momentum distribution function exhibits maxima for a few momenta that are
associated with the momentum of the magnon excitation present in the initial
states. The probability maxima decay as O(1/N ) with the system size. For the
Hamiltonian with kicked magnetic fields, the Loschmidt echo depends on the kick
period. For a special kick period, the Loschmidt echo shows no evolution at all
irrespective of the system size.",2204.05564v1
2022-05-16,Twisting the near-field radiative heat switch in hyperbolic antiferromagnets,"We study the twisted control of the near-field radiative heat transfer
between two hyperbolic antiferromagnetic insulators under external magnetic
fields. We show that the near-field heat flux can be affected by both the twist
angle $\theta$ and the magnitude of the applied magnetic field with different
broken symmetries. Irrespective of twist angle, the external magnetic field
causes the radiative heat flux to change nonmonotonically, and the minimum heat
flux can be found with the magnetic fields of about 1.5 T. Such nonmonotonic
behavior is due to the fact that the magnetic field can radically change the
nature of the magnon polaritons with time reversal symmetry breaking. The field
not only affects the topological structure of surface magnon polaritons, but
also induces the volume magnon polaritons that progressively dominate the heat
transfer as the field increases. We further propose a twist-induced thermal
switch device with inversion symmetry breaking, which can severely regulate
radiative heat flux through different magnetic fields. Our findings account for
a characteristic modulation of radiative heat transfer with implications for
applications in dynamic thermal management.",2205.07648v1
2022-05-20,Tunable large spin Nernst effect in a two-dimensional magnetic bilayer,"We theoretically investigate topological spin transport of the
magnon-polarons in bilayer magnet with two-dimensional square lattices. Our
theory is motivated by recent reports on the van der Waals magnets which show
the reversible electrical switching of the interlayer magnetic order between
antiferromagnetic and ferromagnetic orders. The magnetoelastic interaction
opens band gaps and allows the interband transition between different
excitation states. In the layered antiferromagnet, due to the interband
transition between the magnon-polaron states, the spin Berry curvature which
allows the topological spin transport occurs even if the time-reversal symmetry
is preserved. We find that the spin Berry curvature in the layered
antiferromagnet is very large due to the small energy spacing between two
magnon-like states. As a result, the spin Nernst conductivity shows sudden
increase (or decrease) at the phase transition point between ferromagnetic and
antiferromagnetic phases. Our result suggest that the ubiquity of tunable
topological spin transports in two dimensional magnetic systems.",2205.09970v1
2022-05-25,Quantum topological phase transitions in skyrmion crystals,"Topological order is important in many aspects of condensed matter physics,
and has been extended to bosonic systems. In this Letter we report on the
nontrivial topology of the magnon bands in two distinct quantum skyrmion
crystals appearing in zero external magnetic field. This is revealed by nonzero
Chern numbers for some of the bands. As a bosonic analog of the quantum
anomalous Hall effect, we show that topological magnons can appear in skyrmion
crystals without explicitly breaking time-reversal symmetry with an external
magnetic field. By tuning the value of the easy-axis anisotropy at zero
temperature, we find eight quantum topological phase transitions signaled by
discontinuous jumps in certain Chern numbers. We connect these quantum
topological phase transitions to gaps closing and reopening between magnon
bands.",2205.12965v2
2022-06-02,Diagonal and off-diagonal thermal conduction with resonant phonon scattering in $Ni_3TeO_6$,"The coupling between phonon and magnon is ubiquitous in magnetic materials
and plays a crucial role in many aspects of magnetic properties, most notably
in spintronics. Yet, this academically and technologically interesting problem
still poses a severe challenge to a general understanding of the issue in
certain materials. We report that Ni3TeO6 exhibits clear evidence of
significant magnon-phonon coupling in both longitudinal thermal conductivity
($\kappa_{xx}$) and thermal Hall coefficient ($\kappa_{xy}$). The
Debye-Callaway model, a phenomenological description for phonon heat
conduction, can explain the measured magnetic field dependence of
$\kappa_{xx}(H)$: phonon scattering from spin fluctuation in the paramagnetic
phase and additional scattering due to magnon-phonon coupling in the collinear
antiferromagnetic phase. We further suggest that a similar approach could be
applied to understand the finite $\kappa_{xy}$ values in $Ni_3TeO_6$.",2206.00837v2
2022-06-05,Evidence of Magnon-Mediated Orbital Magnetism in a Quasi-2D Topological Magnon Insulator,"We explore spin dynamics in Cu(1,3-bdc), a quasi-2D topological magnon
insulator. The results show that the thermal evolution of Land\'e $g$-factor
($g$) is anisotropic: $g_\textrm{in-plane}$ reduces while
$g_\textrm{out-plane}$ increases with increasing temperature $T$. Moreover, the
anisotropy of the $g$-factor ($\Delta g$) and the anisotropy of saturation
magnetization ($\Delta M_\textrm{s}$) are correlated below 4 K, but they
diverge above 4 K. We show that the electronic orbital moment contributes to
the $g$ anisotropy at lower $T$, while the topological orbital moment induced
by thermally excited spin chirality dictates the $g$ anisotropy at higher $T$.
Our work suggests an interplay among topology, spin chirality, and orbital
magnetism in Cu(1,3-bdc).",2206.02191v1
2022-06-21,Dipolar spin-waves and tunable band gap at the Dirac points in the 2D magnet ErBr3,"Topological magnon insulators constitute a growing field of research for
their potential use as information carriers without heat dissipation. We report
an experimental and theoretical study of the magnetic ground-state and
excitations in the van der Waals two-dimensional honeycomb magnet ErBr3. We
show that the magnetic properties of this compound are entirely governed by the
dipolar interactions which generate a continuously degenerate non-collinear
ground-state on the honeycomb lattice with spins confined in the plane. We find
that the magnon dispersion exhibits Dirac-like cones when the magnetic moments
in the ground-state are related by time-reversal and inversion symmetries
associated with a Berry phase \pi as in single-layer graphene. A magnon band
gap opens when the dipoles are rotated away from this state, entailing a finite
Berry curvature in the vicinity of the K and K' Dirac points. Our results
illustrate that the spin-wave dispersion of dipoles on the honeycomb lattice
can be reversibly controlled from a magnetic phase with Dirac cones to a
topological antiferromagnetic insulator with non-trivial valley Chern number.",2206.10168v1
2022-06-26,Tripartite high-dimensional magnon-photon entanglement in PT -symmetry broken phases of a non-Hermitian hybrid system,"Hybrid systems that combine spin ensembles and superconducting circuits
provide a promising platform for implementing quantum information processing.
We propose a non-Hermitian magnoncircuit-QED hybrid model consisting of two
cavities and an yttrium iron garnet (YIG) sphere placed in one of the cavities.
Abundant exceptional points (EPs), parity-time (PT )-symmetry phases and PT
-symmetry broken phases are investigated in the parameter space. Tripartite
highdimensional entangled states can be generated steadily among modes of the
magnon and photons in PT -symmetry broken phases, corresponding to which the
stable quantum coherence exists. Results show that the tripartite
high-dimensional entangled state is robust against the dissipation of hybrid
system, independent of a certain initial state, and insensitive to the
fluctuation of magnonphoton coupling. Further, we propose to simulate the
hybrid model with an equivalent LCR circuit. This work may provide prospects
for realizing multipartite high-dimensional entangled states in the
magnon-circuit-QED hybrid system.",2206.12769v1
2022-06-29,Reservoir engineering strong quantum entanglement in cavity magnomechanical systems,"We construct a hybrid cavity magnomechanical system to transfer the bipartite
entanglements and achieve the strong microwave photon-phonon entanglement based
on the reservoir engineering approach. The magnon mode is coupled to the
microwave cavity mode via magnetic dipole interaction, and to the phonon mode
via magnetostrictive force (optomechanical-like). It is shown that the initial
magnon-phonon entanglement can be transferred to the photon-phonon subspace in
the case of these two interactions cooperating. In reservoir-engineering
parameter regime, the initial entanglement is directionally transferred to the
photon-phonon subsystem, so we obtain a strong bipartite entanglement in which
the magnon mode acts as the cold reservoir to effectively cooling the
Bogoliubov mode delocalized over the cavity and the mechanical deformation
mode. Moreover, as the dissipation ratio between the cold reservoir mode and
the target mode increases, we can achieve greater quantum entanglement and
better cooling effect. Our results indicate that the steady-state entanglement
is robust against temperature. The scheme may provides potential applications
for quantum information processing, and is expected to be extended to other
three-mode systems.",2206.14365v2
2022-07-21,Unidirectional Spin Hall Magnetoresistance in Antiferromagnetic Heterostructures,"Unidirectional spin Hall magnetoresistance (USMR) has been widely reported in
the heavy metal / ferromagnet (HM/FM) bilayer systems. We observe the USMR in
the Pt/{\alpha}-Fe2O3 bilayers where the {\alpha}-Fe2O3 is an antiferromagnetic
(AFM) insulator. Systematic field and temperature dependent measurements
confirm the magnonic origin of the USMR. The appearance of AFM-USMR is driven
by the imbalance of creation and annihilation of AFM magnons by spin orbit
torque due to thermal random field. However, unlike its ferromagnetic
counterpart, theoretical modeling reveals that the USMR in Pt/{\alpha}-Fe2O3 is
determined by the antiferromagtic magnon number, and with a non-monotonic field
dependence. Our findings extend the generality of the USMR which pave the ways
for the highly sensitive detection of AF spin state.",2207.10346v1
2022-08-05,Frequency-dependent Phonon-mediated Unidirectional Magnetoresistance in a Metal on an Insulator with Highly Nonequilibrium Magnons,"Heavy metal (HM)/magnet bilayers host many magnetoresistances (MR) and spin
caloritronic effects. Here we show that the spin Peltier effect and
electron-phonon scattering produce much larger unidirectional MR of an HM on a
magnetic insulator than existing theories that neglect the interplay between MR
and spin caloritronic effects. By accounting for local nonequilibrium in both
the magnon chemical potential and temperature, our analytical model attributes
the observed frequency dependence of the spin Peltier MR and the spin Seebeck
effect to the reduction of the thermal penetration depth, which approaches the
1 micron scale magnon spin diffusion length at high frequencies.",2208.03388v1
2022-09-13,Topological magnons in Kitaev magnets with finite Dzyaloshinskii-Moriya interaction at high field,"There have been intensive studies on Kitaev materials for the sake of the
realization of exotic states such as quantum spin liquid and topological
orders. In realistic materials, the Kitaev interaction may coexist with the
Dzyaloshinskii-Moriya interaction (DMI), and it is a challenge to distinguish
their magnitudes separately. Here, we study the topological magnon excitations
and related thermal Hall conductivity of kagome magnet exhibiting Heisenberg,
Kitaev and DM interactions exposed to a magnetic field. In a strong magnetic
field perpendicular to the plane of the lattice ([111] direction) that bring
the system into the fully polarized paramagnetic phase, we find that the magnon
bands carry nontrivial Chern numbers in the full region of the phase diagram.
Furthermore, there are phase transitions relate two topological phases with
opposite Chern numbers, which lead to the sign changes of the thermal Hall
conductivity. In the phase with negative thermal conductivity, the Kitaev
interaction is relatively large and the width of the phase increases with the
strength of DMI. Hence the study here will contribute to the understanding of
related compounds.",2209.05813v1
2022-09-19,Unidirectional orbital magnetoresistance in light metal/ferromagnet bilayers,"We report the observation of a unidirectional magnetoresistance (UMR) that
originates from the nonequilibrium orbital momentum induced by an electric
current in a naturally oxidized Cu/Co bilayer. The orbital-UMR scales with the
torque efficiency due to the orbital Rashba-Edelstein effect upon changing the
Co thickness and temperature, reflecting their common origin. We attribute the
UMR to orbital-dependent electron scattering and orbital-to-spin conversion in
the ferromagnetic layer. In contrast to the spin-current induced UMR, the
magnon contribution to the orbital-UMR is absent in thin Co layers, which we
ascribe to the lack of coupling between low energy magnons and orbital current.
The magnon contribution to the UMR emerges in Co layers thicker than about 5
nm, which is comparable to the orbital-to-spin conversion length. Our results
provide insight into orbital-to-spin momentum transfer processes relevant for
the optimization of spintronic devices based on light metals and orbital
transport.",2209.09355v1
2022-09-21,Tailoring spin waves in 2D transition metal phosphorus trichalcogenides via atomic-layer substitution,"The family of two-dimensional (2D) van der Waals transition metal phosphorus
trichalcogenides has received a renewed interest due to their intrinsic 2D
antiferromagnetism, which proves them as unprecedented and highly tunable
building blocks for spintronics and magnonics at the single-layer limit.
Herein, motivated by the exciting potential of atomic-substitution demonstrated
in Janus transition metal dichalcogenides, we investigate the crystal,
electronic and magnetic structure of selenized Janus monolayers based on
MnPS$_3$ and NiPS$_3$ from first-principles. In addition, we calculate the
magnon dispersion and perform real-time real-space atomistic dynamic
simulations to explore the propagation of spin waves in MnPS$_3$, NiPS$_3$,
MnPS$_{1.5}$Se$_{1.5}$ and NiPS$_{1.5}$Se$_{1.5}$. Our calculations predict a
drastic enhancement of magnetic anisotropy and the emergence of large
Dzyaloshinskii-Moriya interactions, which arises from the induced broken
inversion symmetry in the 2D Janus layers. These results pave the way to the
development of Janus 2D transition metal phosphorus trichalcogenides and
highlight their potential for magnonic applications.",2209.10356v1
2022-10-18,Simultaneous multitone microwave emission by DC-driven spintronic nano-element,"Current-induced self-sustained magnetization oscillations in spin-torque
nano-oscillators (STNOs) are promising candidates for ultra-agile microwave
sources or detectors. While usually STNOs behave as a monochrome source, we
report here clear bimodal simultaneous emission of incommensurate microwave
oscillations, where the two tones correspond to two parametrically coupled
eigenmodes with tunable splitting. The emission range is crucially sensitive to
the change in hybridization of the eigenmodes of free and fixed layers, for
instance, through a slight tilt of the applied magnetic field from the normal
of the nano-pillar. Our experimental findings are supported both analytically
and by micromagnetic simulations, which ascribe the process to four-magnon
scattering between a pair of radially symmetric magnon modes and a pair of
magnon modes with opposite azimuthal index. Our findings open up new
possibilities for cognitive telecommunications and neuromorphic systems that
use frequency multiplexing to improve communication performance.",2210.09752v1
2022-10-24,Optimal anti-ferromagnets for light dark matter detection,"We propose anti-ferromagnets as optimal targets to hunt for sub-MeV dark
matter with spin-dependent interactions. These materials allow for multi-magnon
emission even for very small momentum transfers, and are therefore sensitive to
dark matter particles as light as the keV. We use an effective theory to
compute the event rates in a simple way. Among the materials studied here, we
identify nickel oxide (a well-assessed anti-ferromagnet) as an ideal candidate
target. Indeed, the propagation speed of its gapless magnons is very close to
the typical dark matter velocity, allowing the absorption of all its kinetic
energy, even through the emission of just a single magnon.",2210.13516v2
2022-11-07,Dynamic self-organisation and pattern formation by magnon-polarons,"Nonlinear dynamics can give rise, via the processes of self-organisation and
pattern formation, to the spontaneous manifestation of order in open and
complex systems far from equilibrium. Self-organising systems, transforming the
inflow of energy into information, are ubiquitously found in current topical
areas of science ranging from brainwave entrainment and neuromorphic computing
to energy-efficient data storage technologies. In the latter, magnetic
materials play a pivotal role combining very fast switching with permanent
retention of information. However, it has been shown that, at very short time
scales, magnetisation dynamics become chaotic due to internal instabilities,
resulting in incoherent spin-wave excitations that ultimately destroy magnetic
ordering. Here, contrary to all expectations, we show that such chaos gives
rise to a periodic pattern of reversed magnetic domains, with a feature size
far smaller than the spatial extent of the excitation. We explain this pattern
as a result of phase-synchronisation of magnon-polaron waves, driven by strong
coupling of magnetic and elastic modes. Our results reveal not only the
peculiar formation and evolution of magnon-polarons at short time-scales, but
also present a novel mechanism of magnetization reversal driven by coherent
packets of short-wavelength quasiparticles.",2211.03723v1
2022-11-23,Anatomy of spin wave driven magnetic texture motion via magnonic torques,"The interplay between spin wave and magnetic texture represents the
information exchange between the fast and slow dynamical parts of magnetic
systems. Here we formulate a set of magnonic torques acting on background
magnetic texture, by extracting time-invariant information from the fast
precessing spin waves. Under the frame of magnonic torques, we use theoretical
formulations and micromagnetic simulations to investigate the spin wave driven
domain wall motion in two typical symmetry-breaking situations: the rotational
symmetry broken by the Dzyaloshinkii-Moriya interaction, and the translational
symmetry broken by magnetic damping. The torque-based microscopic analyses
provide compact yet quantitative tools to reinterpret the magnetic texture
dynamics induced by spin wave, beyond the conventional framework of global
momentum conservation.",2211.12958v1
2022-11-27,Entangled atomic ensemble and YIG sphere in coupled microwave cavities,"We present a scheme to generate distant bipartite and tripartite entanglement
between an atomic ensemble and a yttrium iron garnet (YIG) sphere in coupled
microwave cavities. The system we consider has five excitation modes namely
cavity-1 photons, atomic ensemble, cavity-2 photons, magnon and phonon modes in
YIG sphere. We show that significant bipartite entanglement exists between
indirectly coupled subsystems in the cavities, which is robust against
temperature. Moreover, we present suitable detuning parameters for a
significant tripartite entanglement of ensemble, magnon, and phonon modes. We
also demonstrate the existence of tripartite entanglement between magnon and
phonon modes of YIG sphere with indirectly coupled cavity photons. Further, we
show that cavity-cavity coupling strength affects both the degree and transfer
of quantum entanglement between various subsystems. It follows that an
appropriate cavity-cavity coupling optimizes the distant entanglement by
increasing the entanglement strength and critical temperature for its
existence.",2211.14914v1
2022-12-01,Nonreciprocal Phonon Propagation in a Metallic Chiral Magnet,"The phonon magnetochiral effect (MChE) is the nonreciprocal acoustic and
thermal transports of phonons caused by the simultaneous breaking of the mirror
and time-reversal symmetries. So far, the phonon MChE has been observed only in
a ferrimagnetic insulator Cu2OSeO3, where the nonreciprocal response disappears
above the Curie temperature of 58 K. Here, we study the nonreciprocal acoustic
properties of a room-temperature ferromagnet Co9Zn9Mn2 for unveiling the phonon
MChE close to the room temperature. Surprisingly, the nonreciprocity in this
metallic compound is enhanced at higher temperatures and observed up to 250 K.
This clear contrast between insulating Cu2OSeO3 and metallic Co9Zn9Mn2 suggests
that metallic magnets have a mechanism to enhance the nonreciprocity at higher
temperatures. From the ultrasound and microwave-spectroscopy experiments, we
conclude that the magnitude of the phonon MChE of Co9Zn9Mn2 mostly depends on
the magnon bandwidth, which increases at low temperatures and hinders the
magnon-phonon hybridization. Our results suggest that the phonon nonreciprocity
could be further enhanced by engineering the magnon band of materials.",2212.00225v2
2022-12-18,Micromagnetic study of spin transport in easy-plane antiferromagnetic insulators,"Magnon eigenmodes in easy-plane antiferromagnetic insulators are linearly
polarized and are not expected to carry any net spin angular momentum.
Motivated by recent nonlocal spin transport experiments in the easy-plane phase
of hematite, we perform a series of micromagnetic simulations in a nonlocal
geometry at finite temperatures. We show that by tuning an external magnetic
field, we can control the magnon eigenmodes and the polarization of the spin
transport signal in these systems. We argue that a coherent beating oscillation
between two orthogonal linearly polarized magnon eigenmodes is the mechanism
responsible for finite spin transport in easy-plane antiferromagnetic
insulators. The sign of the detected spin signal is also naturally explained by
the proposed coherent beating mechanism. Our finding opens a path for on-demand
control of the spin signal in a large class of easy-plane antiferromagnetic
insulators.",2212.09024v3
2022-12-22,Hybrid Magnonic-Oscillator System,"We propose a hybrid magnonic-oscillator system based on the combination of a
spin transfer auto-oscillator and a magnonic waveguide to open new perspectives
for spin-wave based circuits. The system is composed of a spin transfer
oscillator based on a vortex state which is dipolarly coupled to a nanoscale
spin-wave waveguide with longitudinal magnetization. In its auto-oscillating
regime, the oscillator emits coherent spin waves with tunable and controllable
frequencies, directions and amplitudes into the waveguide. We demonstrate the
principle of this method using micromagnetic simulations and show that
reconfiguration of the system is possible by changing the chirality and
polarity of the magnetic vortex. Spin waves are emitted into the waveguide with
high non-reciprocity and the preferred direction depends on the core polarity
of the vortex. In contrast, different vortex chiralities lead to different
amplitudes of the emitted waves. Our findings open up a novel way to design an
agile spintronic device for the coherent and tunable generation of propagating
spin waves.",2212.11532v1
2023-01-05,Ultrafast spontaneous spin switching in an antiferromagnet,"Owing to their high magnon frequencies, antiferromagnets are key materials
for future high-speed spintronics. Picosecond switching of antiferromagnetic
order has been viewed a milestone for decades and pursued only by using
ultrafast external perturbations. Here, we show that picosecond spin switching
occurs spontaneously due to thermal fluctuations in the antiferromagnetic
orthoferrite Sm0.7Er0.3FeO3. By analysing the correlation between the
pulse-to-pulse polarisation fluctuations of two femtosecond optical probes, we
extract the autocorrelation of incoherent magnon fluctuations. We observe a
strong enhancement of the magnon fluctuation amplitude and the coherence time
around the critical temperature of the spin reorientation transition. The
spectrum shows two distinct modes, one corresponding to the quasi-ferromagnetic
mode and another one which has not been previously reported in pump-probe
experiments. Comparison to a stochastic spin dynamics simulation reveals this
new mode as smoking gun of ultrafast spontaneous spin switching within the
double-well anisotropy potential.",2301.02006v1
2023-01-09,Long distance magnon transport in the van der Waals antiferromagnet CrPS$_4$,"We demonstrate the potential of van der Waals magnets for spintronic
applications by reporting long-distance magnon spin transport in the
electrically insulating antiferromagnet chromium thiophosphate (CrPS$_4$) with
perpendicular magnetic anisotropy. We inject and detect magnon spins
non-locally by Pt contacts and monitor the non-local resistance as a function
of an in-plane magnetic field up to 7 Tesla. We observe a non-local resistance
over distances up to at least a micron below the Neel temperature (T$_{\rm N}$
= 38 Kelvin) close to magnetic field strengths that saturate the sublattice
magnetizations.",2301.03268v1
2023-02-19,Laser Cooling of Nuclear Magnons,"The initialization of nuclear spin to its ground state is challenging due to
its small energy scale compared with thermal energy, even at cryogenic
temperature. In this Letter, we propose an opto-nuclear quadrupolar effect,
whereby two-color optical photons can efficiently interact with nuclear spins.
Leveraging such an optical interface, we demonstrate that nuclear magnons, the
collective excitations of nuclear spin ensemble, can be cooled down optically.
Under feasible experimental conditions, laser cooling can suppress the
population and entropy of nuclear magnons by more than two orders of magnitude,
which could facilitate the application of nuclear spins in quantum information
science.",2302.09615v2
2023-02-22,Advanced magnon-optic effects with spin-wave leaky modes,"We numerically demonstrate the excitation of leaky spin waves (SWs) guided
along a ferromagnetic stripe by an obliquely incident SW beam on the thin film
edge placed below the stripe. During propagation, leaky waves emit energy back
to the layer in the form of plane waves and several laterally shifted parallel
SW beams. This resonance excitation, combined with interference effects of the
reflected and re-emitted waves, results in the magnonic Woods anomaly and
significant increase of the Goos-Hanchen shift magnitude. Hence, we provide a
unique platform to control SW reflection and to transfer SWs from a 2D platform
into the 1D guiding mode that can be used to form a transdimensional magnonic
router.",2302.11507v3
2023-02-25,Magnonic Hall Effect and Magnonic Holography of Hopfions,"Hopfions are localized and topologically non-trivial magnetic configurations
that have received considerable attention in recent years. Through a
micromagnetic approach, we analyze the scattering of spin waves by magnetic
hopfions. We show that the spin waves experience an emergent electromagnetic
field related to the topological properties of the hopfion. We find that spin
waves propagating along the hopfion symmetry axis are deflected by the magnetic
texture, which acts as a convergent or divergent lens, depending on the spin
wave propagation direction. The effect differs for spin waves propagating along
the plane perpendicular to the symmetry axis. In the last case, they respond
with a skew scattering and a closely related Aharonov-Bohm effect. This allows
probing the existence of a magnetic hopfion by magnonic holography.",2302.13190v1
2023-03-08,Nonreciprocity in Cavity Magnonics at Milikelvin Temperature,"Incorporating cavity magnonics has opened up a new avenue in controlling
non-reciprocity. This work examines a yttrium iron garnet sphere coupled to a
planar microwave cavity at milli-Kelvin temperature. Non-reciprocal device
behavior results from the cooperation of coherent and dissipative coupling
between the Kittel mode and a microwave cavity mode. The device's
bi-directional transmission was measured and compared to the theory derived
previously in the room temperature experiment. Investigations are also
conducted into key performance metrics such as isolation, bandwidth, and
insertion loss. The findings point to the coexistence of coherent and
dissipative interactions at cryogenic conditions, and one can leverage their
cooperation to achieve directional isolation. This work foreshadows the
application of a cavity magnonic isolator for on-chip readout and signal
processing in superconducting circuitry.",2303.04358v1
2023-03-22,Phonon and magnon jets above the critical current in nanowires with planar domain walls,"We show through non-equilibrium non-adiabatic electron-spin-lattice
simulations that above a critical current in magnetic atomic wires with a
narrow domain wall (DW), a couple of atomic spaces in width, the electron flow
triggers violent stimulated emission of phonons and magnons with an almost
complete conversion of the incident electron momentum flux into a phonon and
magnon flux. Just below the critical levels of the current flow, the DW
achieves maximal velocity of about $3\times 10^{4}$ m/s, entering a strongly
non-adiabatic regime of DW propagation, followed by a breakdown at higher
biases. Above this threshold a further increase of the current with the applied
bias is impossible -- the electronic current suffers a heavy suppression and
the DW stops. This poses a fundamental limit to the current densities
attainable in atomic wires. At the same time it opens up an exciting way of
generating the alternative quasi-particle currents, described above, once the
requisite electronic-structure properties are met.",2303.12879v2
2023-03-23,Majorana-magnon interactions in topological Shiba chains,"A chain of magnetic impurities deposited on the surface of a superconductor
can form a topological Shiba band that supports Majorana zero modes and holds a
promise for topological quantum computing. Yet, most experiments scrutinizing
these zero modes rely on transport measurements, which only capture local
properties. Here we propose to leverage the intrinsic dynamics of the magnetic
impurities to access their non-local character. We use linear response theory
to determine the dynamics of the uniform magnonic mode in the presence of
external $ac$ magnetic fields and coupling the Shiba electrons. We demonstrate
that this mode, which spreads over the entire chain of atoms, becomes imprinted
with the parity of the ground state and, moreover, can discriminate between
Majorana and trivial zero modes located at the ends of the chain. Our approach
offers a non-invasive alternative to the scanning tunneling microscopy
techniques used to probe Majorana zero modes. Conversely, the magnons could
facilitate the manipulation of Majorana zero modes in topological Shiba chains.",2303.13513v2
2023-03-26,Compact localised states in magnonic Lieb lattices,"Lieb lattice is one of the simplest bipartite lattices where compact
localized states (CLS) are observed. This type of localisation is induced by
the peculiar topology of the unit cell, where the modes are localized only on
one sublattice due to the destructive interference of partial waves. The CLS
exist in the absence of defects and are associated with the flat bands in the
dispersion relation. The Lieb lattices were successfully implemented as optical
lattices or photonic crystals. This work demonstrates the possibility of
magnonic Lieb lattice realization where the flat bands and CLS can be observed
in the planar structure of sub-micron in-plane sizes. Using forward volume
configuration, we investigated numerically (using the finite element method)
the Ga-dopped YIG layer with cylindrical inclusions (without Ga content)
arranged in a Lieb lattice of the period 250 nm. We tailored the structure to
observe, for the few lowest magnonic bands, the oscillatory and evanescent spin
waves in inclusions and matrix, respectively. Such a design reproduces the Lieb
lattice of nodes (inclusions) coupled to each other by the matrix with the CLS
in flat bands.",2303.14843v1
2023-04-14,Gauge-Invariant Measure of the Magnon Orbital Angular Momentum,"Unlike the Berry phase, the orbital angular momentum (OAM) of magnons with
two-dimensional wavevector k in band n is not gauge invariant for arbitrary
phase lambda_n(k). However, by integrating the OAM over the orientation $\phi $
of wavevector k, we construct a gauge-invariant function F_n(k). Like F_n(k),
the average OAM for magnon band n in a circle of radius k is also gauge
invariant. We demonstrate these results for a ferromagnet on a honeycomb
lattice with Dzyalloshinskii-Moriya interactions between next-nearest neighbor
spins. With wavevectors k restricted to the first Brillouin zone, the angular
averaged OAM F_n(k) then has opposite signs for lower and upper bands n=1 and 2
for all k.",2304.07379v2
2023-04-19,Nonreciprocal ultrastrong magnon-photon coupling in the bandgap of photonic crystals,"We observe a nonreciprocal ultrastrong magnon-photon coupling in the bandgap
of photonic crystals by introducing a single crystal YIG cylinder into copper
photonic crystals cavity as a point defect. The coupling strength reaches up to
1.18 GHz, which constitutes about 10.9% of the photon energy compared to the
photon frequency around 10.8 GHz. It is fascinating that the coupling achieves
unidirectional signal transmission in the whole bandgap. This study
demonstrates the possibility of controlling nonreciprocal magnon-photon
coupling by manipulating the structure of photonic crystals, providing new
methods to investigate the influence of magnetic point defects on microwave
signal transmission.",2304.09627v1
2023-05-14,Quantum-Enhanced Metrology in Cavity Magnomechanics,"Magnons, as fundamental quasiparticles emerged in elementary spin
excitations, hold a big promise for innovating quantum technologies in
information coding and processing. Here we discover subtle roles of
entanglement in a metrological scheme based on an experimentally feasible
cavity magnomechanical system, where the magnons are responsible for sensing a
weak magnetic field whereas the cavity field carries out a precision
measurement of the weak field. By establishing exact relations between the
Fisher information and entanglement, we show that for the weak coupling case
the measurement precision can reach the Heisenberg limit, whereas quantum
criticality enables us to enhance measurement precision for the strong coupling
case. In particular, we also find that the entanglement between magnons and
photons is of crucial importance during the dynamical encoding process, but the
presence of such an entanglement in the measurement process dramatically
reduces the final measurement precision.",2305.08045v2
2023-05-29,Intrinsic nonlinear thermal Hall transport of magnons: A Quantum kinetic theory approach,"We present a systematic study of the nonlinear thermal Hall responses in
bosonic systems using the quantum kinetic theory framework. We demonstrate the
existence of an intrinsic nonlinear boson thermal current, arising from the
quantum metric which is a wavefunction dependent band geometric quantity. In
contrast to the nonlinear Drude and nonlinear anomalous Hall contributions, the
intrinsic nonlinear thermal conductivity is independent of the scattering
timescale. We demonstrate the dominance of this intrinsic thermal Hall response
in topological magnons in a two-dimensional ferromagnetic honeycomb lattice
without Dzyaloshinskii-Moriya interaction. Our findings highlight the
significance of band geometry induced nonlinear thermal transport and motivate
experimental probe of the intrinsic nonlinear thermal Hall response with
implications for quantum magnonics.",2305.18127v2
2023-05-29,Light-induced weak ferromagnetism through nonlinear magnonic rectification,"Rectification describes the generation of a quasistatic component from an
oscillating field, such as an electric polarization in optical rectification,
or a structural distortion in nonlinear phononic rectification. Here, we
present a third fundamental process for magnetization, in which spin precession
is rectified along the coordinates of a nonlinearly driven magnon mode in an
antiferromagnet. We demonstrate theoretically that a quasistatic magnetization
can be induced by transient spin canting in response to the coherent excitation
of a chiral phonon mode that produces an effective magnetic field for the
spins. This mechanism, which we call nonlinear magnonic rectification, is
generally applicable to magnetic systems that exhibit infrared-active chiral
phonon modes. Our results serve as an example of light-induced weak
ferromagnetism and open a promising avenue towards creating dynamical spin
configurations that are not accessible in equilibrium.",2305.18656v2
2023-06-13,Magnon gap excitations and spin-entangled optical transition in van der Waals antiferromagnet NiPS3,"Optical magneto-spectroscopy methods (Raman scattering, far-infrared
transmission, and photoluminescence) have been applied to investigate the
properties of the NiPS3 semiconducting antiferromagnet. The fundamental magnon
gap excitation in this van der Waals material has been found to be split into
two components, in support of the biaxial character of the NiPS3
antiferromagnet. Photoluminescence measurements in the near-infrared spectral
range show that the intriguing 1.475 eV-excitation unique to the NiPS3
antiferromagnetic phase splits upon the application of the in-plane magnetic
field. The observed splitting patterns are correlated with properties of magnon
excitations and reproduced with the simple model proposed. Possible routes
toward a firm identification of the spin-entangled 1.475 eV-optical excitation
in NiPS3, which can hardly be recognized as a coherent Zhang-Rice exciton, are
discussed.",2306.07660v1
2023-07-01,Edge and corner skin effects of chirally coupled magnons characterized by a topological winding tuple,"We investigate a long-ranged coupled and non-Hermitian two-dimensional array
of nanomagnets, fabricated on a thin magnetic substrate and subjected to an
in-plane magnetic field. We predict topology-driven edge and corner skin
effects of magnetic eigenmodes with the localization position at boundaries
precisely characterized by a topological winding tuple $({\cal W}_1,{\cal
W}_2)$. By varying the direction of the in-plane field, all magnon states pile
up either at different edges of the array with $({\cal W}_1=\pm 1,{\cal
W}_2=0)$ or $({\cal W}_1=0,{\cal W}_2=\pm 1)$, or at different corners
characterized by $({\cal W}_1=\pm 1,{\cal W}_2=\pm 1)$. Exploiting the
non-Hermitian topology is potentially helpful for designing useful magnonic
metasurface in the future.",2307.00353v2
2023-07-08,Magnon influence on the superconducting density of states in superconductor$-$ferromagnetic-insulator bilayers,"Superconductor$-$ferromagnetic-insulator heterostructures are paradigmatic
systems for studying the mutual influence of superconductivity and magnetism
via proximity effects. In particular, spin-split superconductivity is realized
in such structures. Recent experiments and theories demonstrate a rich variety
of transport phenomena occurring in devices based on such heterostructures that
suggest direct applications in thermoelectricity, low-dissipative spintronics,
radiation detection, and sensing. In this work we investigate the influence of
the electron-magnon interaction at the superconductor$-$ferromagnetic-insulator
interface on the spin-split superconductivity. It is predicted that due to the
magnon-mediated electron spin-flip processes the spin-split quasiparticle
branches are partially mixed and reconstructed, and the BCS-like spin-split
shape of the superconducting density of states, which is typical for
superconductors in the effective exchange field, is strongly modified. An
odd-frequency superconducting order parameter admixture to the leading singlet
order parameter is also found. These findings expand the physical picture of
spin-split superconductivity beyond the mean-field description of the
ferromagnet exchange field.",2307.03954v2
2023-07-14,Achieving unidirectional propagation of twisted magnons in a magnetic nanodisk array,"Twisted magnons (TMs) have great potential applications in communication and
computing owing to the orbital angular momentum (OAM) degree of freedom.
Realizing the unidirectional propagation of TMs is the key to design functional
magnonics devices. Here we theoretically study the propagation of TMs in
one-dimensional magnetic nanodisk arrays. By performing micromagnetic
simulations, we find that the one-dimensional nanodisk array exhibits a few
bands due to the collective excitations of TMs. A simple model by considering
the exchange interaction is proposed to explain the emerging multiband
structure and theoretical results agree well with micromagnetic simulations.
Interestingly, for a zigzag structure, the dispersion curves and propagation
images of TMs show obvious nonreciprocity for specific azimuthal quantum number
($l$), which originates from a geometric effect depending on the phase
difference of TMs and the relative angle between two adjacent nanodisks.
Utilizing this feature, one can conveniently realize the unidirectional
propagation of TMs with arbitrary nonzero $l$. Our work provides important
theoretical references for controlling the propagation of TMs.",2307.07347v1
2023-07-18,Distant entanglement via photon hopping in a coupled magnomechanical system,"We theoretically propose a scheme to generate distant bipartite entanglement
between various subsystems in coupled magnomechanical systems where both the
microwave cavities are coupled through single photon hopping parameter. Each
cavity also contains a magnon mode and phonon mode and this gives five
excitation modes in our model Hamiltonian which are cavity-1 photons, cavity-2
photons, magnon, and phonon modes in both YIG spheres. We found that
significant bipartite entanglement exists between indirectly coupled subsystems
in coupled microwave cavities for an appropriate set of parameters regime.
Moreover, we also obtain suitable cavity and magnon detuning parameters for a
significant distant bipartite entanglement in different bipartitions. In
addition, it can be seen that a single photon hopping parameter significantly
affects both the degree as well as the transfer of quantum entanglement between
various bipartitions. Hence, our present study related to coupled microwave
cavity magnomechanical configuration will open new perspectives in coherent
control of various quantum correlations including quantum state transfer among
macroscopic quantum systems",2307.09424v1
2023-08-16,Nontrivial Aharonov-Bohm effect and alternating dispersion of magnons in cone-state ferromagnetic rings,"Soft magnetic dots in the form of thin rings have unique topological
properties. They can be in a vortex state with no vortex core. Here, we study
the magnon modes of such systems both analytically and numerically. In an
external magnetic field, magnetic rings are characterized by easy-cone
magnetization and shows a giant splitting of doublets for modes with the
opposite value of the azimuthal mode quantum number. The effect of the
splitting can be refereed as a magnon analog of the topology-induced
Aharonov-Bohm effect. For this we develop an analytical theory to describe the
non-monotonic dependence of the mode frequencies on the azimuthal mode number,
influenced by the balance between the local exchange and non-local dipole
interactions.",2308.08486v3
2023-09-07,Gænice: a general model for magnon band structure of artificial spin ices,"Arrays of artificial spin ices exhibit reconfigurable ferromagnetic resonance
frequencies that can be leveraged and designed for potential
applications.However, analytical and numerical studies of the frequency
response of artificial spin ices have remained somewhat limited due to the need
of take into account nonlocal dipole fields in theoretical calculations or by
long computation times in micromagnetic simulations. Here, we introduce
Gaenice, a framework to compute magnon dispersion relations of arbitrary
artificial spin ice configurations. Gaenice makes use of a tight-binding
approach to compute the magnon bands. It also provides the user complete
control of the interaction terms included, e.g., external field, anisotropy,
exchange, and dipole, making it useful also to compute ferromagnetic resonances
for a variety of structures, such as multilayers and ensembles of weakly or
non-interacting nanoparticles. Because it relies on a semi-analytical model,
Gaenice is computationally inexpensive and efficient, making it an attractive
tool for the exploration of large parameter spaces.",2309.03826v2
2023-09-10,Cavity-Magnon-Polariton spectroscopy of strongly hybridized electro-nuclear spin excitations in LiHoF4,"We first present a formalism that incorporates the input-output formalism and
the linear response theory to employ cavity-magnon-polariton coupling as a
spectroscopic tool for investigating strongly hybridized electro-nuclear spin
excitations. A microscopic relation between the generalized susceptibility and
the scattering parameter |S11| in strongly hybridized cavity-magnon-polariton
systems has been derived without resorting to semi-classical approximations.
The formalism is then applied to both analyze and simulate a specific systems
comprising a model quantum Ising magnet (LiHoF4) and a high-finesse 3D
re-entrant cavity resonator. Quantitative information on the electro-nuclear
spin states in LiHoF4 is extracted, and the experimental observations across a
broad parameter range were numerically reproduced, including an external
magnetic field titraversing a quantum critical point. The method potentially
opens a new avenue not only for further studies on the quantum phase transition
in LiHoF4 but also for a wide range of complex magnetic systems.",2309.05051v1
2023-09-14,Phase control of magnons in the van der Waals antiferromagnet NiPS$_3$,"We demonstrate phase control of magnons in the van der Waals antiferromagnet
NiPS$_3$ using optical excitation by polarized light. The sign of the coherent
precession of spin amplitude changes upon (1) reversing the helicity of a
circularly polarized pump beam, or (2) rotating the polarization of a linearly
polarized pump by $\pi/2$. Because these two excitation pathways have
comparable generation efficiency, the phase of spin precession can be
continuously tuned from 0 to $2\pi$ by controlling the polarization state of
the pump pulse. The ability to excite magnons with a desired phase has
potential applications in the design of a spin-wave phased array and ultrafast
spin information processing.",2309.07451v1
2023-09-22,Strongly Coupled Spin Waves and Surface Acoustic Waves at Room Temperature,"Here, we report the observation of strong coupling between magnons and
surface acoustic wave (SAW) phonons in a thin CoFeB film constructed in an
on-chip SAW resonator by analyzing SAW phonon dispersion anticrossings. Our
device design provides the tunability of the film thickness with a fixed phonon
wavelength, which is a departure from the conventional approach in strong
magnon--phonon coupling research. We detect a monotonic increase in the
coupling strength by expanding the film thickness, which agrees with our
theoretical model. Our work offers a significant way to advance fundamental
research and the development of devices based on magnon--phonon hybrid
quasiparticles.",2309.12690v1
2023-12-05,Giant natural optical rotation from chiral electromagnons in a collinear antiferromagnet,"In NiTe$_3$O$_6$ with a chiral crystal structure, we report on a giant
natural optical rotation of the lowest-energy magnon. This polarization
rotation, as large as 140 deg/mm, corresponds to a path difference between
right and left circular polarizations that is comparable to the sample
thickness. Natural optical rotation, being a measure of structural chirality,
is highly unusual for long-wavelength magnons. The collinear antiferromagnetic
order of NiTe$_3$O$_6$ makes this giant effect even more peculiar: Chirality of
the crystal structure does not affect the magnetic ground state but is strongly
manifested in the lowest excited state. We show that the dynamic
magnetoelectric effect, turning this magnon to a magnetic- and electric-dipole
active hybrid mode, generates the giant natural optical rotation. In finite
magnetic fields, it also leads to a strong optical magnetochiral effect.",2312.02733v1
2023-12-06,Magnon interactions in the quantum paramagnetic phase of CoNb$_2$O$_6$,"In this work, we study effects of magnon interactions in the excitation
spectrum of CoNb$_2$O$_6$ in the quantum paramagnetic phase in transverse
field, where the $1/S$ spin-wave theory exhibits unphysical divergences at the
critical field. We propose a self-consistent Hartree-Fock approach that
eliminates such unphysical singularities while preserving the integrity of the
singular threshold phenomena of magnon decay and spectrum renormalization that
are present in both theory and experiment. With the microscopic parameters
adopted from previous studies, this method yields a close quantitative
agreement with the available experimental data for CoNb$_2$O$_6$ in the
relevant regime. Insights into the general structure of the spin-anisotropic
model of CoNb$_2$O$_6$ and related zigzag chain materials are also provided and
a discussion of the effects of additional longitudinal field on the spectrum is
given.",2312.03829v3
2023-12-08,Towards an experimental proof of the magnonic Aharonov$-$Casher effect,"Controlling the phase and amplitude of spin waves in magnetic insulators with
an electric field opens the way to fast logic circuits with ultra-low power
consumption. One way to achieve such control is to manipulate the magnetization
of the medium via magnetoelectric effects. In experiments with magnetostatic
spin waves in an yttrium iron garnet film, we have obtained the first evidence
of a theoretically predicted phenomenon: The change of the spin-wave phase due
to the magnonic Aharonov$-$Casher effect$-$the geometric accumulation of the
magnon phase as these quasiparticles propagate through an electric field
region.",2312.05113v1
2023-12-13,Defect-sensitive High-frequency Modes in a Three-Dimensional Artificial Magnetic Crystal,"Modern three-dimensional nanofabrication methods make it possible to generate
arbitrarily shaped nanomagnets, including periodic networks of interconnected
magnetic nanowires. Structurally similar to optical or acoustic metamaterials,
these arrays could represent magnetic variants of such artificial materials.
Using micromagnetic simulations, we investigate a three-dimensional array of
interconnected magnetic nanowires with intersection points corresponding to
atomic positions of a diamond lattice. The high-frequency excitation spectrum
of this artificial magnetic crystal (AMC) is governed by its microstructure
and, to a lesser extent, by the magnetic configuration. The magnetic system
displays characteristics of three-dimensional artificial spin ice. It can
contain Dirac-type magnetic defect structures, which modify the magnonic
spectrum of the AMC similarly as defect sites in a natural diamond crystal
influence optical absorption spectra. Our study opens new perspectives for
applying such materials in high-density magnonic devices and shows that AMCs
represent a promising category of magnonic materials with tunable properties.",2312.08415v1
2023-12-23,Pulse-driven depinning of magnetic gap modes in ferromagnetic films,"Manipulation of magnons in artificial magnonic crystals (MCs) leads to
fascinating nonlinear wave phenomena such as the generation of gap solitons,
which has been mostly limited to one-dimensional systems. Here, we propose a
model system for the magnetization in two-dimensional MCs subjected to a
periodic external magnetic field, describing the dynamics of magnetic gap
solitons (MGSs) formed by nonlinear self-trapping. We show the formation,
stability, and dynamics for various two-dimensional gap modes, including gap
solitons and vortical ones. Their existence regions depend on the anisotropic
axis orientation of the ferromagnetic film. The Bloch oscillation and depinning
propagation of MGSs under constant spin-current injections are discovered and
characterized. We design a scheme of pulse current injection to achieve
distortionless propagation of MGSs. These findings show that the 2D magnonic
crystals can be viewed as a building block for MGSs-based storage and
transmission, where the propagation and localization are variously controlled
and reconfigurable.",2312.15200v1
2024-01-15,Transient Magnetoelastic Coupling in CrSBr,"Recent research has revealed remarkable properties of the two-dimensional
(2D) van der Waals layered crystal CrSBr, which is both a semiconductor and an
A-type antiferromagnet. Here we show the role of strong magnetoelastic coupling
in the generation and propagation of coherent magnons in CrSBr. Time and
spatially resolved magneto-optical Kerr effect (tr-MOKE) microscopy reveals two
time-varying transient strain fields induced by out-of-plane transverse and
in-plane longitudinal lattice displacements. These transient strain fields
launch coherent wavepackets of magnons, optical and acoustic at 24.6 GHz and
33.4 GHz, respectively. These findings suggest mechanisms for controlling and
manipulating coherent magnons from distinct magnetoelastic couplings in this 2D
van der Waals magnetic semiconductor.",2401.07828v1
2024-01-19,Excess heat capacity in magnetically ordered Ce heavy fermion metals,"We study the magnetic heat capacity of a series of magnetically ordered
Ce-based heavy fermion materials, which show an anomalous $T^3$ heat capacity
in excess of the phonon contribution in many materials. For compounds for which
magnon models have been worked out, we show that the local-moment magnon heat
capacity derived from the measured magnon spectra underestimates the
experimental specific heat. The excess heat capacity reveals increasing density
of states with increasing energy, akin to a pseudogap. We show that this
anomalous temperature-dependent term is not associated with proximity to a
quantum critical point (QCP), but is strongly correlated with $T_N$, indicating
the anomalous excitations are governed by the magnetic exchange interaction.
This insight may hold key information for understanding magnetically ordered
heavy fermions.",2401.10718v1
2024-01-22,Generating magnon Bell states via parity measurement,"We propose a scheme to entangle two magnon modes based on parity measurement.
In particular, we consider a system that two yttrium-iron-garnet spheres are
coupled to a $V$-type superconducting qutrit through the indirect interactions
mediated by cavity modes. An effective parity-measurement operator that can
project the two macroscopic spin systems to the desired subspace emerges when
the ancillary qutrit is projected to the ground state. Consequently,
conventional and multi-excitation magnon Bell states can be generated from any
separable states with a nonvanishing population in the desired subspace. The
target state can be distilled with a near-to-unit fidelity only by several
rounds of measurements and can be stabilized in the presence of decoherence. In
addition, a single-shot version of our scheme is obtained by shaping the
detuning in the time domain. Our scheme that does not rely on any nonlinear
effect brings insight to the entangled-state generation in massive
ferrimagnetic materials via quantum measurement.",2401.11684v1
2024-01-23,Topological magnons in a non-coplanar magnetic order on the triangular lattice,"The bond-dependent Kitaev interaction $K$ is familiar in the effective spin
model of transition metal compounds with octahedral ligands. In this work, we
find a peculiar non-coplanar magnetic order can be formed with the help of $K$
and next nearest neighbor Heisenberg coupling $J_2$ on triangular lattice. It
can be seen as a miniature version of skyrmion lattice, since it has nine spins
and integer topological number in a magnetic unit cell. The magnon excitations
in such an order is studied by the linear spin-wave theory. Of note is that the
change in the relative size of $J_2$ and $K$ will produce topological magnon
phase transitions although the topological number remains unchanged. We also
calculated the experimentally observable thermal Hall conductivity, and found
that the signs of thermal Hall conductivity will change with topological phase
transitions or temperature changes in certain regions.",2401.12505v2
2024-02-26,Distinct Optical Excitation Mechanisms of a Coherent Magnon in a van der Waals Antiferromagnet,"The control of antiferromagnets with ultrashort optical pulses has emerged as
a prominent field of research. Tailored laser excitation can launch coherent
spin waves at terahertz frequencies, yet a comprehensive description of their
generation mechanisms is still lacking despite extensive efforts. Using
terahertz emission spectroscopy, we investigate the generation of a coherent
magnon mode in the van der Waals antiferromagnet NiPS$_3$ under a range of
photoexcitation conditions. By tuning the pump photon energy from transparency
to resonant with a $d$-$d$ transition, we reveal a striking change in the
coherent magnon's dependence on the pump polarization, indicating two distinct
excitation mechanisms. Our findings provide a strategy for the manipulation of
magnetic modes via photoexcitation around sub-gap electronic states.",2402.17041v2
2024-02-27,Semiclassical approach to spin dynamics of a ferromagnetic S=1 chain,"Motivated by recent experimental progress in the quasi-one-dimensional
quantum magnet NiNb$_2$O$_6$, we study the spin dynamics of an S=1
ferromagnetic Heisenberg chain with single-ion anisotropy by using a
semiclassical molecular dynamics approach. This system undergoes a quantum
phase transition from a ferromagnetic to a paramagnetic state under a
transverse magnetic field, and the magnetic responses reflecting this
transition is well described by our semiclassical method. We show that at
low-temperature the transverse component of the dynamical structure factor
depicts clearly the magnon dispersion, and the longitudinal component exhibits
two continua associated with single- and two-magnon excitations, respectively.
These spin excitation spectra show interesting temperature dependence as
effects of magnon interactions.Our findings shed light on experimental
detection of spin excitations in a large class of quasi-one-dimensional
magnets.",2402.17416v1
2024-03-20,Thermal Hall effect driven by phonon-magnon hybridization in a honeycomb antiferromagnet,"The underlying mechanism of the thermal Hall effect (THE) generated by
phonons in a variety of insulators is yet to be identified. Here, we report on
a sizeable thermal Hall conductivity in NiPS$_3$, a van de Waals stack of
honeycomb layers with a zigzag antiferromagnetic order below $T_N$ = 155 K. The
longitudinal ($\kappa_{aa}$) and the transverse ($\kappa_{ab}$) thermal
conductivities peak at the same temperature and the thermal Hall angle
($\kappa_{ab}/\kappa_{aa}/B$) respects a previously identified bound. The
amplitude of $\kappa_{ab}$ is extremely sensitive to the amplitude of
magnetization along the $b$-axis, in contrast to the phonon mean free path,
which is not at all. We show that the magnon and acoustic phonon bands cross
each other along the $b^\ast$ orientation in the momentum space. The
exponential temperature dependence of $\kappa_{ab}$ above its peak reveals an
energy scale on the order of magnitude of the gap expected to be opened by
magnon-phonon hybridization. This points to an intrinsic scenario for THE with
possible relevance to other magnetic insulators.",2403.13306v1
2024-03-25,Magnon-microwave backaction noise evasion in cavity magnomechanics,"In cavity magnomechanical systems, magnetic excitations couple simultaneously
with mechanical vibrations and microwaves, combining the tunability of the
magnetization, the long lifetimes of mechanical modes and the whole measurement
toolbox of microwave systems. Such hybrid systems have been proposed for
applications ranging from thermometry to entanglement generation. However,
backaction noise can hinder the measurement of the mechanical vibrations,
potentially rendering such applications infeasible. In this paper, we
investigate the noise introduced in a mechanical mode of a cavity
magnomechanical system in a one-tone drive scheme and propose a scheme for
realizing backaction evasion measurements of the mechanical vibrations. Our
proposal consists of driving the microwave cavity with two tones separated by
twice the phonon frequency and with amplitudes balanced to generate equal
numbers of coherent magnons. We demonstrate that different configurations of
such a scheme are possible and show that drives centered around the lower
frequency magnon-microwave polariton in a triple resonance scheme add the
minimum imprecision noise in the measurement, even though such configuration is
not the most robust to imperfections.",2403.17185v1
2024-03-31,Quantum Weak Force Sensing with Squeezed Magnomechanics,"Cavity magnomechanics, exhibiting remarkable experimental tunability, rich
magnonic nonlinearities, and compatibility with various quantum systems, has
witnessed considerable advances in recent years. However, the potential
benefits of using cavity magnomechanical (CMM) systems in further improving the
performance of quantum-enhanced sensing for weak forces remain largely
unexplored. Here we show that the performance of a quantum CMM sensor can be
significantly enhanced beyond the standard quantum limit (SQL), by squeezing
the magnons. We find that, for comparable parameters, two orders of enhancement
in force sensitivity can be achieved in comparison with the case without the
magnon squeezing. Moreover, we show optimal parameter regimes of homodyne angle
for minimizing added quantum noise. Our findings provide a promising approach
for highly tunable and compatible quantum force sensing using hybrid CMM
devices, with potential applications ranging from quantum precision
measurements to quantum information processing.",2404.00723v1
2024-04-03,Enhancement of Magnon Transport by Superconductor Meissner Screening,"Recent experiments observe the spin-wave-Meissner-current modes in
ferromagnetic insulator-superconductor heterostructures, in which the
coherently excited spin waves seemingly do not decay as usual beneath the
superconductor strip [Borst et al., Science 382, 430 (2023)]. We interpret this
phenomenon by demonstrating that the stray magnetic field emitted by the
magnetization dynamics is reflected, focused, and enhanced inside the
ferromagnet by the supercurrent induced in the superconductor, such that the
group velocity of spin waves is strongly enhanced. Analytical and numerical
calculations based on this model predict that the coherent transport of magnons
is enhanced by close to 500% for yttrium iron garnet capped by superconducting
NbN with a decay length exceeding millimeters. Our finding may augment the
performance of magnons in quantum information and quantum transport processing.",2404.02598v1
2024-04-05,Magnetoroton in a two-dimensional Bose-Bose mixture,"We extend our theory of slow magnons in a two-component Bose-Einstein
condensate to the case of two spatial dimensions (2D). We provide a detailed
discussion of polaronic corrections to the magnon branch of the elementary
excitation spectrum in a weakly- and strongly-interacting regimes. In a dilute
system, the latter may be achieved by adjusting inter-species attraction such
as to obtain a resonance in the $p$-wave scattering channel. Resonantly
enhanced $p$-wave attraction results in the self-localization of magnons and
formation of a magnetoroton. The nature of a ground state beyond the
magnetoroton instability remains to be explored. We suggest a dilute p-wave
crystal of alternating polarization as a possible candidate. In contrast to
three dimensions (3D), the required strength of the attractive potential in the
$s$-wave channel here corresponds to tight binding, and we suggest a potential
realization of our model with excitons in 2D semiconductors.",2404.04440v1
2024-04-09,Robustness of topological magnons in disordered arrays of skyrmions,"The effects of disorder on the robustness of topological magnon states of
two-dimensional ferromagnetic skyrmions is investigated. It is diagnosed by
evaluating a real space topological invariant, the bosonic Bott index (BI). The
disorder simultaneously breaks the axially symmetric shape and the crystalline
ordering of the skyrmions array. The corresponding magnonic fluctuations and
band spectrum are determined in terms of magnetic field and strength of
disorder. We observe the closing of the existing band gaps as the individual
skyrmions start to occupy random positions. The analysis reveals that
topological states (TSs) persist beyond the perturbative limit when skyrmions
reach the glassy phase. In addition, the localization of topologically
protected edge states is weakened by the disordered skyrmion structure with
increasing localization length. Our findings shed light on the physical
understanding of the coexistence of disordered magnetic textures and their
topological spin fluctuations.",2404.06541v1
2006-02-28,Field dependence of the magnetic spectrum in anisotropic and Dzyaloshinskii-Moriya antiferromagnets: II. Raman spectroscopy,"We compare the theoretical predictions of the previous article [L. Benfatto
and M. B. Silva Neto, cond-mat/0602419], with Raman spectroscopy experiments in
Sr(2)CuO(2)Cl(2) and untwinned La(2)CuO(4) single crystals. We construct the
magnetic point group for the magnetically ordered phase of the two compounds,
Sr(2)CuO(2)Cl(2) and La(2)CuO(4), and we classify all the Raman active
one-magnon excitations according to the irreducible co-representations for the
associated magnetic point group. We then measure the evolution of the
one-magnon Raman energies and intensities for low and moderate magnetic fields
along the three crystallographic directions. In the case of La(2)CuO(4), we
demonstrate that from the jump of the Dzyaloshinskii-Moriya gap at the critical
magnetic field H_c ~ 6.6 T for the weak-ferromagnetic transition one can
determine the value of the interlayer coupling J_\perp/J ~ 3.2 x 10^-5. We
furthermore determine the components of the anisotropic gyromagnetic tensor as
g_s^a=2.0, g_s^b=2.08, and the upper bound g_s^c=2.65. For the case of
Sr(2)CuO(2)Cl(2), we compare the Raman data obtained in an in-plane magnetic
field with previous magnon-gap measurements done by ESR. Using the very low
magnon gap estimated by ESR (~ 0.05 meV), the data for the one-magnon Raman
energies agree reasonably well with the theoretical predictions for the case of
a transverse field (only hardening of the gap).",0602664v2
2010-09-21,Twisting the Mirror TBA,"We study finite-size corrections to the magnon dispersion relation in three
models which differ from string theory on AdS5 x S5 in their boundary
conditions. Asymptotically, this is accomplished by twisting the transfer
matrix in a way which manifestly preserves integrability. In model I all
world-sheet fields are periodic, whereas model II represents a particular
orbifold of AdS5 x S5 and model III is a beta-deformed theory. For models I and
II we construct the one-particle TBA equations and use them to determine the
leading finite-size correction to the asymptotic Bethe equation. We also make
some interesting observations concerning the quantization conditions for the
momentum. For the same models we compute the leading and for model II the
next-to-leading order finite-size corrections to the asymptotic magnon
dispersion relation. Furthermore, we apply L\""uscher's formulae to compute the
leading finite-size corrections in beta-deformed theory. In addition to
reproducing known results, we provide new predictions for two-particle states
from the sl(2) sector, to be confronted with explicit field-theoretic
calculations in the dual gauge theory. Finally, we prove that the leading
finite-size correction to the energy of an sl(2) magnon in orbifold theory is
the same as the one for an su(2) magnon in beta-deformed theory for special
values of beta. We also speculate that for these values of beta our result for
the next-to-leading order correction in the orbifold model might coincide with
the corresponding correction to the energy of su(2) magnon in beta-deformed
theory.",1009.4118v3
2012-04-23,Spin nematic phase in (quasi-)one-dimensional frustrated magnet in strong magnetic field,"We discuss spin-1/2 one-dimensional (1D) and quasi-1D magnets with competing
ferromagnetic nearest-neighbor $J_1$ and antiferromagnetic
next-nearest-neighbor $J$ exchange interactions in strong magnetic field $H$.
It is well known that due to attraction between magnons quantum phase
transitions (QPTs) take place at $H=H_s$ from the fully polarized phase to
nematic ones if $J>|J_1|/4$. Such a transition at $J>0.368|J_1|$ is
characterized by softening of the two-magnon bound-state spectrum. Using a bond
operator formalism we propose a bosonic representation of the spin Hamiltonian
containing, aside from bosons describing one-magnon spin-1 excitations, a boson
describing spin-2 excitations which spectrum coincides at $H\ge H_s$ with the
two-magnon bound-state spectrum obtained before. The presence of the bosonic
mode in the theory that softens at $H=H_s$ makes substantially standard the QPT
consideration. In 1D case at $H<H_s$, we find an expression for the
magnetization which describes well existing numerical data. Expressions for
spin correlators are obtained which coincide with those derived before either
in the limiting case of $J\gg|J_1|$ or using a phenomenological theory. In
quasi-1D magnets, we find that the boundary in the $H$--$T$ plane between the
fully polarized and the nematic phases is given by $H_s(0)-H_s(T)\propto
T^{3/2}$. Simple expressions are obtained in the nematic phase for static spin
correlators, spectra of magnons and the soft mode, magnetization and the
nematic order parameter. All static two-spin correlation functions are short
ranged with the correlation length proportional to $1/\ln(1+|J_1|/J)$.
Dynamical spin susceptibilities are discussed and it is shown that the soft
mode can be observed experimentally in the longitudinal channel.",1204.4983v3
2019-03-12,Magnon interactions in the frustrated pyrochlore ferromagnet Yb$_2$Ti$_2$O$_7$,"The frustrated rare-earth pyrochlore Yb$_2$Ti$_2$O$_7$ is remarkable among
magnetic materials: despite a ferromagnetically ordered ground state it
exhibits a broad, nearly gapless, continuum of excitations. This broad
continuum connects smoothly to the sharp one-magnon excitations expected, and
indeed observed, at high magnetic fields, raising the question: how does this
picture of sharp magnons break down as the field is lowered? In this paper, we
consider the effects of magnon interactions in Yb$_2$Ti$_2$O$_7$, showing that
their inclusion greatly extends the reach of spin-wave theory. First, we show
that magnon interactions shift the phase boundary between the (splayed)
ferromagnet (SFM) and the antiferromagnetic $\Gamma_5$ phase so that
Yb$_2$Ti$_2$O$_7$ lies very close to it. Next, we show how the high-field limit
connects to lower fields; this includes corrections to the critical fields for
the $[111]$ and $[1\bar{1}0]$ directions, bringing them closer to the observed
experimental values, as well as accounting for the departures from linear spin
wave theory that appear in $[001]$ applied fields below 3 T [Thompson et al.,
Phys. Rev. Lett. 119, 057203 (2017)]. Turning to low-fields, though the extent
of the experimentally observed broadening is not quite reproduced, we find a
rough correspondence between non-linear spin-wave theory and inelastic neutron
scattering data on both a single-crystal sample, as well as on a powder sample
[Pe\c{c}anha-Antonio et al., Phys. Rev. B, 96, 214415 (2017)]. We conclude with
an outlook on implications for future experimental and theoretical work on
Yb$_2$Ti$_2$O$_7$ and related materials, highlighting the importance of
proximity to the splayed ferromagnet-$\Gamma_5$ phase boundary and its
potential role in intrinsic or extrinsic explanations of the low-field physics
of Yb$_2$Ti$_2$O$_7$.",1903.05098v1
2016-12-21,Bose-Einstein Condensation of Quasi-Particles by Rapid Cooling,"The fundamental phenomenon of Bose-Einstein Condensation (BEC) has been
observed in different systems of real and quasi-particles. The condensation of
real particles is achieved through a major reduction in temperature while for
quasi-particles a mechanism of external injection of bosons by irradiation is
required. Here, we present a novel and universal approach to enable BEC of
quasi-particles and to corroborate it experimentally by using magnons as the
Bose-particle model system. The critical point to this approach is the
introduction of a disequilibrium of magnons with the phonon bath. After heating
to an elevated temperature, a sudden decrease in the temperature of the
phonons, which is approximately instant on the time scales of the magnon
system, results in a large excess of incoherent magnons. The consequent
spectral redistribution of these magnons triggers the Bose-Einstein
condensation.",1612.07305v3
2017-11-02,Topological spin excitations observed in a three-dimensional antiferromagnet,"Band topology, or global wave-function structure that enforces novel
properties in the bulk and on the surface of crystalline materials, is
currently under intense investigations for both fundamental interest and its
technological promises. While band crossing of non-trivial topological nature
was first studied in three dimensions for electrons, the underlying physical
idea is not restricted to fermionic excitations. In fact, experiments have
confirmed the possibility to have topological band crossing of electromagnetic
waves in artificial structures. Fundamental bosonic excitations in real
crystals, however, have not been observed to exhibit the counterpart under
ambient pressure and magnetic field, where the difficulty is in part because
natural materials cannot be precisely engineered like artificial structures.
Here, we use inelastic neutron scattering to reveal the presence of topological
spin excitations (magnons) in a three-dimensional antiferromagnet, Cu3TeO6,
which features a unique lattice of magnetic spin-1/2 Cu2+ ions. Beyond previous
understanding, we find that the material's spin lattice possesses a variety of
exchange interactions, with the interaction between the ninth-nearest
neighbours being as strong as that between the nearest neighbours. Although
theoretical analysis indicates that the presence of topological magnon band
crossing is independent of model details, Cu3TeO6 turns out to be highly
favourable for the experimental observation, as its optical magnons are
spectrally sharp and intense due to the highly interconnected spin network and
the large magnetic cell. The observed magnon band crossing generally has the
form of a special type of Z2-topological nodal lines that are yet to be found
in fermion systems, rendering magnon systems a fertile ground for exploring
novel band topology.",1711.00632v1
2018-10-25,Inhomogeneous magnon scattering during ultrafast demagnetization,"Ni$_{0.8}$Fe$_{0.2}$ (Py) and Py alloyed with Cu exhibit intriguing ultrafast
demagnetization behavior, where the Ni magnetic moment shows a delayed response
relative to the Fe, an effect which is strongly enhanced by Cu alloying. We
have studied a broad range of Cu concentrations to elucidate the effects of Cu
alloying in Py. The orbital/spin magnetic moment ratios are largely unaffected
by Cu alloying, signifying that Cu-induced changes in the ultrafast
demagnetization are not related to spin-orbit interactions. We show that magnon
diffusion can explain the delayed Ni response, which we attribute to an
enhanced magnon generation rate in the Fe sublattice relative to the Ni
sublattice. Furthermore, Py exhibits prominent RKKY-like exchange interactions,
which are strongly enhanced between Fe atoms and diminished between Ni atoms by
Cu alloying. An increased Fe magnon scattering rate is expected to occur
concurrently with this increased Fe-Fe exchange interaction, supporting the
results obtained from the magnon diffusion model.",1810.10994v1
2021-02-01,Strong-Magnetic-Field Magnon Transport in Monolayer Graphene,"At high magnetic fields, monolayer graphene hosts competing phases
distinguished by their breaking of the approximate SU(4) isospin symmetry.
Recent experiments have observed an even denominator fractional quantum Hall
state thought to be associated with a transition in the underlying isospin
order from a spin-singlet charge density wave at low magnetic fields to an
antiferromagnet at high magnetic fields, implying that a similar transition
must occur at charge neutrality. However, this transition does not generate
contrast in typical electrical transport or thermodynamic measurements and no
direct evidence for it has been reported, despite theoretical interest arising
from its potentially unconventional nature. Here, we measure the transmission
of ferromagnetic magnons through the two dimensional bulk of clean monolayer
graphene. Using spin polarized fractional quantum Hall states as a benchmark,
we find that magnon transmission is controlled by the detailed properties of
the low-momentum spin waves in the intervening Hall fluid, which is highly
density dependent. Remarkably, as the system is driven into the
antiferromagnetic regime, robust magnon transmission is restored across a wide
range of filling factors consistent with Pauli blocking of fractional quantum
hall spin-wave excitations and their replacement by conventional ferromagnetic
magnons confined to the minority graphene sublattice. Finally, using devices in
which spin waves are launched directly into the insulating charge-neutral bulk,
we directly detect the hidden phase transition between bulk insulating charge
density wave and a canted antiferromagnetic phases at charge neutrality,
completing the experimental map of broken-symmetry phases in monolayer
graphene.",2102.01061v1
2018-01-04,Complex temperature dependence of coupling and dissipation of cavity-magnon polaritons from milliKelvin to room temperature,"Hybridized magnonic-photonic systems are key components for future
information processing technologies such as storage, manipulation or conversion
of data both in the classical (mostly at room temperature) and quantum
(cryogenic) regime. In this work, we investigate a YIG sphere coupled strongly
to a microwave cavity over the full temperature range from $290\,\mathrm{K}$
down to $30\,\mathrm{mK}$. The cavity-magnon polaritons are studied from the
classical to the quantum regime where the thermal energy is less than one
resonant microwave quanta, i.e. at temperatures below $1\,\mathrm{K}$. We
compare the temperature dependence of the coupling strength $g_{\rm{eff}}(T)$,
describing the strength of coherent energy exchange between spin ensemble and
cavity photon, to the temperature behavior of the saturation magnetization
evolution $M_{\rm{s}}(T)$ and find strong deviations at low temperatures. The
temperature dependence of magnonic disspation is governed at intermediate
temperatures by rare earth impurity scattering leading to a strong peak at
$40\,$K. The linewidth $\kappa_{\rm{m}}$ decreases to $1.2\,$MHz at $30\,$mK,
making this system suitable as a building block for quantum electrodynamics
experiments. We achieve an electromagnonic cooperativity in excess of $20$ over
the entire temperature range, with values beyond $100$ in the milliKelvin
regime as well as at room temperature. With our measurements, spectroscopy on
strongly coupled magnon-photon systems is demonstrated as versatile tool for
spin material studies over large temperature ranges. Key parameters are
provided in a single measurement, thus simplifying investigations
significantly.",1801.01439v1
2018-08-03,Decoupled spin dynamics in the rare-earth orthoferrite YbFeO$_3$: Evolution of magnetic excitations through the spin-reorientation transition,"In this paper we present a comprehensive study of magnetic dynamics in the
rare-earth orthoferrite YbFeO$_3$ at temperatures below and above the
spin-reorientation (SR) transition $T_{\mathrm{SR}}=7.6$ K, in magnetic fields
applied along the $a, b$ and $c$ axes. Using single-crystal inelastic neutron
scattering, we observed that the spectrum of magnetic excitations consists of
two collective modes well separated in energy: 3D gapped magnons with a
bandwidth of $\sim$60 meV, associated with the antiferromagnetically (AFM)
ordered Fe subsystem, and quasi-1D AFM fluctuations of $\sim$1 meV within the
Yb subsystem, with no hybridization of those modes. The spin dynamics of the Fe
subsystem changes very little through the SR transition and could be well
described in the frame of semiclassical linear spin-wave theory. On the other
hand, the rotation of the net moment of the Fe subsystem at $T_{\mathrm{SR}}$
drastically changes the excitation spectrum of the Yb subsystem, inducing the
transition between two regimes with magnon and spinon-like fluctuations. At $T
< T_{\mathrm{SR}}$, the Yb spin chains have a well defined field-induced
ferromagnetic (FM) ground state, and the spectrum consists of a sharp
single-magnon mode, a two-magnon bound state, and a two-magnon continuum,
whereas at $T > T_{\mathrm{SR}}$ only a gapped broad spinon-like continuum
dominates the spectrum. In this work we show that a weak quasi-1D coupling
within the Yb subsystem $J_\text{Yb-Yb}$, mainly neglected in previous studies,
creates unusual quantum spin dynamics on the low energy scales. The results of
our work may stimulate further experimental search for similar compounds with
several magnetic subsystems and energy scales, where low-energy fluctuations
and underlying physics could be ""hidden"" by a dominating interaction.",1808.01269v2
2020-03-28,Theory of the anomalous spin dynamics of spin-$\frac{1}{2}$ triangular lattice Heisenberg antiferromagnet and its application to Ba$_3$CoSb$_2$O$_9$,"Although it is well accepted that the spin-$\frac{1}{2}$ triangular lattice
Heisenberg antiferromagnet(TLHAF) has a long range ordered ground state, a
thorough understanding of its spin dynamics is still missing. While the linear
spin wave theory(LSWT) predicts three branches of magnon mode in the magnetic
Brillouin zone(MBZ), the 1/S expansion at the next order is found to break down
in a large portion of the MBZ centered around the M point, leaving the fate of
the magnon modes there undecided. Recent neutron scattering measurement on
Ba$_3$CoSb$_2$O$_9$, an ideal realization of the spin-$\frac{1}{2}$ TLHAF,
provides a surprising answer to this issue. Two, rather than three branches of
magnon mode are observed around the M point, whose dispersion are strongly
renormalized with respect to the LSWT prediction and exhibit pronounced
roton-like minimum at the M point. This is accompanied by a strong spin
fluctuation continuum at higher energy, inside which two strong and broad
spectral peaks of unknown origin are observed. In this work, we propose a
simple picture for these spectral anomalies by invoking the resonating valence
bond(RVB) physics in the description of the ground state of the system. We find
that the roton-like minimum in the magnon dispersion can be explained by the
coupling between the collective spin fluctuation and the continuum of Dirac
spinon excitation moving in a $\pi$-flux background. We also propose that the
two broad peaks in the continuum can be understood respectively as the Landau
damped third magnon mode and the Landau damped longitudinal mode. Such a
picture can be verified by studying the polarization character of the various
spectral features.",2003.12688v1
2020-08-06,Magnon versus electron mediated spin-transfer torque exerted by spin currents across antiferromagnetic insulator to switch magnetization of adjacent ferromagnetic metal,"The recent experiment [Y. Wang et al., Science 366, 1125 (2019)] on
magnon-mediated spin-transfer torque (MSTT) was interpreted in terms of a
picture where magnons are excited within an antiferromagnetic insulator (AFI),
by applying nonequilibrium electronic spin density at one of its surfaces, so
that their propagation across AFI deprived of conduction electrons eventually
leads to reversal of magnetization of a ferromagnetic metal (FM) attached to
the opposite surface of AFI. We employ a recently developed time-dependent
nonequilibrium Green functions combined with the Landau-Lifshitz-Gilbert
equation (TDNEGF+LLG) formalism to evolve conduction electrons
quantum-mechanically while they interact via self-consistent back-action with
localized magnetic moments described classically by atomistic spin dynamics
solving a system of LLG equations. Upon injection of square current pulse as
the initial condition, TDNEGF+LLG simulations of FM-polarizer/AFI/FM-analyzer
junctions show that reversal of localized magnetic moments within FM-analyzer
is less efficient, in the sense of requiring larger pulse height and its longer
duration, than conventional electron-mediated STT (ESTT) driving magnetization
switching in standard FM-polarizer/normal-metal/FM-analyzer spin valve. Since
both electronic, generated by spin pumping from AFI, and magnonic, generated by
direct transmission from AFI, spin currents are injected into the FM-analyzer,
its localized magnetic moments will experience combined MSTT and ESTT.
Nevertheless, by artificially turning off ESTT we demonstrate that MSTT plays a
dominant role whose understanding, therefore, paves the way for
all-magnon-driven magnetization switching devices with no electronic parts.",2008.02794v3
2021-07-19,Multimagnon dynamics and thermalization in the $S=1$ easy-axis ferromagnetic chain,"Quasiparticles are physically motivated mathematical constructs for
simplifying the seemingly complicated many-body description of solids. A
complete understanding of their dynamics and the nature of the effective
interactions between them provides rich information on real material properties
at the microscopic level. In this work, we explore the dynamics and
interactions of magnon quasiparticles in a ferromagnetic spin-1 Heisenberg
chain with easy-axis onsite anisotropy, a model relevant for the explanation of
recent terahertz optics experiments on NiNb$_2$O$_6$ [P. Chauhan et al., Phys.
Rev. Lett. 124, 037203 (2020)],and nonequilibrium dynamics in ultracold atomic
settings [W.C. Chung et al., Phys. Rev. Lett. 126, 163203 (2021)]. We build a
picture for the properties of clouds of a few magnons with the help of exact
diagonalization and density matrix renormalization group calculations supported
by physically motivated Jastrow wavefunctions. We show how the binding energy
of magnons effectively reduces with their number and explain how this energy
scale is of direct relevance for dynamical magnetic susceptibility
measurements. This understanding is used to make predictions for
ultracold-atomic platforms which are ideally suited to study the thermalization
of multimagnon states. We simulate the nonequilibrium dynamics of these chains
using the matrix product state based time-evolution block decimation algorithm
and explore the dependence of revivals and thermalization on magnon density and
easy-axis onsite anisotropy (which controls the strength of effective magnon
interactions). We observe behaviors akin to those reported for many-body
quantum scars which we explain with an analytic approximation that is accurate
in the limit of small anisotropy.",2107.09105v2
2022-02-04,Unusual dynamics of spin-1/2 antiferromagnets on the triangular lattice in magnetic field,"We theoretically discuss dynamical properties of spin-1/2 Heisenberg
antiferromagnet on the triangular lattice in magnetic field $\bf H$. We use the
recently proposed bond-operator theory which operates with quantum states of
the whole magnetic unit cell containing three spins. This technique describes
accurately short-range spin correlations and provides a quantitative
description of elementary excitations which appear in other approaches as bound
states of conventional low-energy quasiparticles (e.g., magnons). In
quantitative agreement with previous numerical and analytical findings, we
observe four phases with coplanar spin arrangements upon the field increasing:
the three-sublattice Y-phase, the collinear ""up-up-down"" (UUD) state, the
non-collinear V-phase, and the collinear fully polarized (FP) state. We
demonstrate that apart from magnons there are spin-0 elementary excitations in
the UUD state one of which is long lived and its spectrum lies below magnon
branches. This mode originates from a high-energy quasiparticle at $H=0$ and it
produces anomalies only in the longitudinal spin correlator. In the V-phase, we
obtain multiple short-wavelength spin excitations which have no counterparts in
the semiclassical spin-wave theory. We demonstrate a highly nontrivial field
evolution of quasiparticles spectra on the way from one collinear state (UUD)
to another one (FP) via the non-collinear V-phase (in which the longitudinal
and the transverse channels are mixed). In particular, some parts of the spin-0
branch in the UUD state become parts of the spin-1 (magnon) branch in the FP
phase whereas some parts of one magnon branch turn into parts of spin-2 branch.
Such evolution would be very difficult to find by any conventional analytical
approach. Our results are in good agreement with neutron experimental data
obtained recently in $\rm Ba_3CoSb_2O_9$, $\rm KYbSe_2$, and $\rm CsYbSe_2$.",2202.02120v3
2022-10-13,Single- and Multimagnon Dynamics in Antiferromagnetic $α$-Fe$_2$O$_3$ Thin Films,"Understanding the spin dynamics in antiferromagnetic (AFM) thin films is
fundamental for designing novel devices based on AFM magnon transport. Here, we
study the magnon dynamics in thin films of AFM $S=5/2$ $\alpha$-Fe$_2$O$_3$ by
combining resonant inelastic x-ray scattering, Anderson impurity model plus
dynamical mean-field theory, and Heisenberg spin model. Below 100 meV, we
observe the thickness-independent (down to 15 nm) acoustic single-magnon mode.
At higher energies (100-500 meV), an unexpected sequence of equally spaced,
optical modes is resolved and ascribed to $\Delta S_z = 1$, 2, 3, 4, and 5
magnetic excitations corresponding to multiple, noninteracting magnons. Our
study unveils the energy, character, and momentum-dependence of single and
multimagnons in $\alpha$-Fe$_2$O$_3$ thin films, with impact on AFM magnon
transport and its related phenomena. From a broader perspective, we generalize
the use of L-edge resonant inelastic x-ray scattering as a multispin-excitation
probe up to $\Delta S_z = 2S$. Our analysis identifies the spin-orbital mixing
in the valence shell as the key element for accessing excitations beyond
$\Delta S_z = 1$, and up to, e.g., $\Delta S_z = 5$. At the same time, we
elucidate the novel origin of the spin excitations beyond the $\Delta S_z = 2$,
emphasizing the key role played by the crystal lattice as a reservoir of
angular momentum that complements the quanta carried by the absorbed and
emitted photons.",2210.06911v4
2022-12-05,Phonon-mediated strong coupling between a three-dimensional topological insulator and a two-dimensional antiferromagnetic material,"Van der Waals antiferromagnetic and topological insulator materials provide
powerful platforms for modern optical, electronic, and spintronic devices
applications. The interaction between an antiferromagnet (AFM) and a
topological insulator (TI), if sufficiently strong, could offer emergent hybrid
material properties that enable new functionality exceeding what is possible in
any individual material constituent. In this work, we study strong coupling
between THz excitations in a three dimensional (3D) topological insulator and a
quasi-two dimensional (2D) antiferromagnetic material resulting in a new
hybridized mode, namely a surface Dirac plasmon-phonon-magnon polariton. We
find that the interaction between a surface Dirac plasmon polariton in the 3D
TI and a magnon polariton in the 2D AFM is mediated by the phonon coupling in
the 3D TI material. The coupling of phonons with an electromagnetic wave
propagating in the 3D TI enhances the permittivity of the TI thin film in a way
that results in a strong correlation between the dispersion of Dirac plasmon
polaritons on the surfaces of the TI with the thickness of the TI. As a result,
the dispersion of surface Dirac plasmon polaritons in the TI can be tuned
toward resonance with the magnon polariton in the AFM material by varying the
TI's thickness, thereby enhancing the strength of the coupling between the
excitations in the two materials. The strength of this coupling, which results
in the surface Dirac plasmon-phonon-magnon polariton, can be parameterized by
the amplitude of the avoided-crossing splitting between the two polariton
branches at the magnon resonance frequency...",2212.02656v1
2023-04-25,Riemann meets Goldstone: magnon scattering off quantum Hall skyrmion crystals probes interplay of symmetry breaking and topology,"We introduce a model to study magnon scattering in skyrmion crystals,
sandwiched between ferromagnets which act as the source of magnons. Skyrmions
are topological objects while skyrmion crystals break internal and
translational symmetries, thus our setup allows us to study the interplay of
topology and symmetry breaking. Starting from a basis of holomorphic theta
functions, we construct an analytical ansatz for such a junction with finite
spatially modulating topological charge density in the central region and
vanishing in the leads. We then construct a suitably defined energy functional
for the junction and derive the resulting equations of motion, which resemble a
Bogoliubov-de Gennes-like equation. Using analytical techniques, field theory,
heuristic models and microscopic recursive transfer-matrix numerics, we
calculate the spectra and magnon transmission properties of the skyrmion
crystal. We find that magnon transmission can be understood via a combination
of low-energy Goldstone modes and effective emergent Landau levels at higher
energies. The former manifests in discrete low-energy peaks in the transmission
spectrum which reflect the nature of the Goldstone modes arising from symmetry
breaking. The latter, which reflect the topology, lead to band-like
transmission features, from the structure of which further details of the
excitation spectrum of the skyrmion crystal can be inferred. Such
characteristic transmission features are absent in competing phases of the
quantum Hall phase diagram, and hence provide direct signatures of skyrmion
crystal phases and their spectra. Our results directly apply to quantum Hall
heterojunction experiments in monolayer graphene with the central region doped
slightly away from unit filling, a $\nu = 1:1 \pm \delta \nu: 1$ junction and
are also relevant to junctions formed by metallic magnets or in junctions with
artificial gauge fields.",2304.13049v1
2023-06-12,Field-induced bound-state condensation and spin-nematic phase in SrCu$_2$(BO$_3$)$_2$ revealed by neutron scattering up to 25.9 T,"Bose-Einstein condensation (BEC) underpins exotic forms of order ranging from
superconductivity to superfluid 4 He. In quantum magnetic materials, ordered
phases induced by an applied magnetic field can be described as the BEC of
magnon excitations. With sufficiently strong magnetic frustration, exemplified
by the system SrCu$_2$(BO$_3$)$_2$ , no clear magnon BEC is observed and the
complex spectrum of multi-magnon bound states may allow a different type of
condensation, but the high fields required to probe this physics have remained
a barrier to detailed investigation. Here we exploit the first purpose-built
high-field neutron scattering facility to measure the spin excitations of
SrCu$_2$(BO$_3$)$_2$ up to 25.9 T and use cylinder matrix-product-states (MPS)
calculations to reproduce the experimental spectra with high accuracy. Multiple
unconventional features point to a condensation of $S = 2$ bound states into a
spin-nematic phase, including the gradients of the one-magnon branches, the
presence of many novel composite two- and three-triplon excitations and the
persistence of a one-magnon spin gap. This gap reflects a direct analogy with
superconductivity, suggesting that the spin-nematic phase in
SrCu$_2$(BO$_3$)$_2$ is best understood as a condensate of bosonic Cooper
pairs. Our results underline the wealth of unconventional states yet to be
found in frustrated quantum magnetic materials under extreme conditions.",2306.07389v1
2023-12-01,Exploring kinetically induced bound states in triangular lattices with ultracold atoms: spectroscopic approach,"Quantum simulations with ultracold fermions in triangular optical lattices
have recently emerged as a new platform for studying magnetism in frustrated
systems. Experimental realizations of the Fermi Hubbard model revealed striking
contrast between magnetism in bipartite and triangular lattices. In bipartite
lattices magnetism peaks at half filling, and doped charge carriers tend to
suppress magnetic correlations. In triangular lattices for large $U/t$,
magnetism is enhanced by doping away from $n=1$ because kinetic energy of
dopants can be lowered through developing magnetic correlations. This
corresponds to formation of magnetic polarons, with hole and doublon doping
resulting in antiferro- and ferromagnetic polarons respectively. Snapshots
obtained with quantum gas microscopes revealed formation of magnetic polarons
around dopants at temperatures exceeding the superexchange energy scale. In
this work we discuss theoretically that additional insight into properties of
magnetic polarons can be achieved using spectroscopic experiments with
ultracold atoms. We consider starting from a spin polarized state with small
hole doping and applying a two-photon Raman photoexcitation, which transfers
atoms into a different spin state. We show that such magnon injection spectra
exhibit a separate peak corresponding to formation of a bound state between a
hole and a magnon. This polaron peak is separated from the simple magnon
spectrum by energy proportional to single particle tunneling and can be easily
resolved with currently available experimental techniques. For some momentum
transfer there is an additional peak corresponding to photoexciting a bound
state between two holes and a magnon. We point out that in two component Bose
mixtures in triangular lattices one can also create dynamical magnetic
polarons, with one hole and one magnon forming a repulsive bound state.",2312.00768v1
2024-02-19,Magnetic anisotropy and GGG substrate stray field in YIG films down to millikelvin temperatures,"Quantum magnonics investigates the quantum-mechanical properties of magnons
such as quantum coherence or entanglement for solid-state quantum information
technologies at the nanoscale. The most promising material for quantum
magnonics is the ferrimagnetic yttrium iron garnet (YIG), which hosts magnons
with the longest lifetimes. YIG films of the highest quality are grown on a
paramagnetic gadolinium gallium garnet (GGG) substrate. The literature has
reported that ferromagnetic resonance (FMR) frequencies of YIG/GGG decrease at
temperatures below 50 K despite the increase in YIG magnetization. We
investigated a 97 nm-thick YIG film grown on 500 $\mathrm{\mu}$m-thick GGG
substrate through a series of experiments conducted at temperatures as low as
30 mK, and using both analytical and numerical methods. Our findings suggest
that the primary factor contributing to the FMR frequency shift is the stray
magnetic field created by the partially magnetized GGG substrate. This stray
field is antiparallel to the applied external field and is highly
inhomogeneous, reaching up to 40 mT in the center of the sample. At
temperatures below 500 mK, the GGG field exhibits a saturation that cannot be
described by the standard Brillouin function for a paramagnet. Including the
calculated GGG field in the analysis of the FMR frequency versus temperature
dependence allowed the determination of the cubic and uniaxial anisotropies. We
find that the total anisotropy increases more than three times with the
decrease in temperature down to 2 K. Our findings enable accurate predictions
of the YIG/GGG magnetic systems behavior at low and ultra-low millikelvin
temperatures, crucial for developing quantum magnonic devices.",2402.12112v1
1997-09-21,The dispersion of a single hole in an antiferromagnet,"We revisit the problem of the dispersion of a single hole injected into a
quantum antiferromagnet. We applied a spin-density-wave formalism extended to a
large number of orbitals and obtained an integral equation for the full
quasiparticle Green's function in the self-consistent ""non-crossing"" Born
approximation. We found that for $t/J \gg 1$, the bare fermionic dispersion is
completely overshadowed by the self-energy corrections. We obtain a broad
incoherent continuum which extends over a frequency range of $\sim 6t$ and a
narrow region of width $O(JS)$ below the top of the valence band, where the
excitations are mostly coherent, though with a small quasiparticle residue $Z
\sim J/t$. Furthermore, we argue in this paper that two-magnon Raman scattering
as well as neutron scattering experiments strongly suggest that the zone
boundary magnons are not free particles since a substantial portion of their
spectral weight is transferred into an incoherent background. We modeled this
effect by introducing a cutoff $q_c$ in the integration over magnon momenta. We
found analytically that for small $q_c$, the strong coupling solution for the
Green's function is universal, and both effective masses are equal to
$(4JS)^{-1}$. We further computed the full fermionic dispersion for $J/t =0.4$
relevant for $Sr_2CuO_2Cl_2$, and $t^\prime=-0.4J$ and found not only that the
masses are both equal to $(2J)^{-1}$, but also that the energies at $(0,0)$ and
$(0,\pi)$ are equal, the energy at $(0,\pi/2)$ is about half of that at
$(0,0)$, and the bandwidth for the coherent excitations is around $3J$. All of
these results are in full agreement with the experimental data.",9709235v1
2000-03-06,Modeling of the Magnetic Susceptibilities of the Ambient- and High-Pressure Phases of (VO)_{2}P_{2}O_{7},"The magnetic susceptibilities chi versus temperature T of powders and single
crystals of the ambient-pressure (AP) and high-pressure (HP) phases of
(VO)_{2}P_{2}O_{7} are analyzed using an accurate theoretical prediction of
chi(T, J1, J2) for the spin-1/2 antiferromagnetic alternating-exchange (J1, J2)
Heisenberg chain. The results are consistent with recent models with two
distinct types of alternating-exchange chains in the AP phase and a single type
in the HP phase. The spin gap for each type of chain is derived from the
respective set of two fitted alternating exchange constants and the one-magnon
dispersion relation for each of the two types of chains in the AP phase is
predicted. The influences of interchain coupling on the derived intrachain
exchange constants, spin gaps, and dispersion relations are estimated using a
mean-field approximation for the interchain coupling. The accuracies of the
spin gaps obtained using fits to the low-T chi(T) data by theoretical low-T
approximations are determined. The results of these studies are compared with
previously reported estimates of the exchange couplings and spin gaps in the AP
and HP phases and with the magnon dispersion relations in the AP phase measured
previously using inelastic neutron scattering.",0003065v2
2002-09-14,"Magnon-polaron and Spin-polaron Signatures in the Specific Heat and Electrical Resistivity of $La_{0.6}Y_{0.1}Ca_{0.3}MnO_3$ in Zero Magnetic Field, and the Effect of $Mn-O-Mn$ Bond Environment","$La_{0.6}Y_{0.1}Ca_{0.3}MnO_{3}$, an $ABO_{3}$ perovskite manganite oxide,
exhibits a non trivial behavior in the vicinity of the sharp peak found in the
resistivity $\rho$ as a function of temperature $T$ in zero magnetic field. The
various features seen on $d\rho/dT$ are discussed in terms of competing phase
transitions. They are related to the $Mn-O-Mn$ bond environment depending on
the content of the $A$ crystallographic site. A Ginzburg-Landau type theory is
presented for incorporating concurrent phase transitions. The specific heat $C$
of such a compound is also examined from 50 till 200 K. A log-log analysis
indicates different regimes. In the low temperature conducting ferromagnetic
phase, a collective magnon signature ($C \simeq T^{3/2}$) is found as for what
are called magnon-polaron excitations. A $C \simeq T^{2/3}$ law is found at
high temperature and discussed in terms of the fractal dimension of the
conducting network of the weakly conducting (so-called insulating) phase and
Orbach estimate of the excitation spectral behaviors. The need of considering
both independent spin scattering and collective spin scattering is thus
emphasized. The report indicates a remarkable agreement for the Fisher-Langer
formula, i.e. $C$ $\sim$ $d\rho/dT$ at second order phase transitions. Within
the Attfield model, we find an inverse square root relationship between the
critical temperature(s) and the total local $Mn-O-Mn$ strain.",0209340v1
2003-03-28,Magnons in the half-doped manganites,"Recently, based on the refined crystal structure of Pr0.6Ca0.4MnO3 from
neutron diffraction, Daoud-Aladine et al.[PRL89,97205(2002)] have proposed a
new ground state structure for the half-doped manganites R0.5Ca0.5MnO3, where R
is a trivalent ion like Bi,La,Pr,Sm or Y. Their proposal describes the CE
magnetic structure attributed to these materials as an arrangement of dimers
along the ferromagnetic Mn zig-zag chains that form it. However, the dimers'
proposal is in conflict with the Goodenough-Kanamori-Anderson rules, which give
a coherent description of many transition metal insulating compounds and
predict the coexistence of Mn3+ and Mn4+ ions in equal parts in the half-doped
manganites. On the other hand, Rivadulla et al.[PRB 66, 174432 (2002)] have
studied several single crystal samples of half-doped manganites and propose a
phase diagram in terms of the tolerance factor which contains both types of
structures. In the present work we have calculated the magnon dispersion
relations for the CE magnetic structure, arising for each type of proposal: the
charge ordered and the dimer phases, respectively. We consider a
three-dimensional unit cell containing 16 spins, and compare the magnetic
excitations along different paths in the first Brillouin zone. We conclude that
measurement of the magnon dispersion relations should allow a clear distinction
between the two proposals, predicting qualitative differences arising along
specific directions of propagation in the first Brillouin zone.",0303613v1
2003-07-01,Observation of extended scattering continua characteristic of spin fractionalization in the 2D frustrated quantum magnet Cs2CuCl4 by neutron scattering,"The magnetic excitations of the quasi-2D spin-1/2 Heisenberg antiferromagnet
on an anisotropic triangular lattice Cs2CuCl4 are explored throughout the 2D
Brillouin zone using inelastic neutron scattering. In the spin liquid phase
above the transition to magnetic order extended excitation continua are
observed, characteristic of fractionalization of S=1 spin waves into pairs of
deconfined S=1/2 spinons and the hallmark of a resonating-valence-bond (RVB)
state. The weak inter-layer couplings stabilize incommensurate spiral order at
low temperatures and in this phase sharp magnons carrying a small part of the
total scattering weight are observed at low energies below the continuum lower
boundary. Linear spin-wave theory including one- and two-magnon processes can
describe the sharp magnon excitation, but not the dominant continuum
scattering, which instead is well described by a parameterized two-spinon
cross-section. Those results suggest a cross-over in the nature of excitations
from S=1 spin waves at low energies to S=1/2 spinons at medium to high
energies, which could be understood if Cs2CuCl4 was in the close proximity of a
transition between a fractional spin liquid and a magnetically-ordered phase.",0307025v1
2003-09-18,Field- and pressure-induced magnetic quantum phase transitions in TlCuCl_3,"Thallium copper chloride is a quantum spin liquid of S = 1/2 Cu^2+ dimers.
Interdimer superexchange interactions give a three-dimensional magnon
dispersion and a spin gap significantly smaller than the dimer coupling. This
gap is closed by an applied hydrostatic pressure of approximately 2kbar or by a
magnetic field of 5.6T, offering a unique opportunity to explore the both types
of quantum phase transition and their associated critical phenomena. We use a
bond-operator formulation to obtain a continuous description of all disordered
and ordered phases, and thus of the transitions separating these. Both
pressure- and field-induced transitions may be considered as the Bose-Einstein
condensation of triplet magnon excitations, and the respective phases of
staggered magnetic order as linear combinations of dimer singlet and triplet
modes. We focus on the evolution with applied pressure and field of the
magnetic excitations in each phase, and in particular on the gapless
(Goldstone) modes in the ordered regimes which correspond to phase fluctuations
of the ordered moment. The bond-operator description yields a good account of
the magnetization curves and of magnon dispersion relations observed by
inelastic neutron scattering under applied fields, and a variety of
experimental predictions for pressure-dependent measurements.",0309440v1
2005-03-22,Anisotropy effects on the magnetic excitations of a ferromagnetic monolayer below and above the Curie temperature,"The field-driven reorientation transition of an anisotropic ferromagnetic
monolayer is studied within the context of a finite-temperature Green's
function theory. The equilibrium state and the field dependence of the magnon
energy gap $E_0$ are calculated for static magnetic field $H$ applied in plane
along an easy or a hard axis. In the latter case, the in-plane reorientation of
the magnetization is shown to be continuous at T=0, in agreement with free spin
wave theory, and discontinuous at finite temperature $T>0$, in contrast with
the prediction of mean field theory. The discontinuity in the orientation angle
creates a jump in the magnon energy gap, and it is the reason why, for $T>0$,
the energy does not go to zero at the reorientation field. Above the Curie
temperature $T_C$, the magnon energy gap $E_0(H)$ vanishes for H=0 both in the
easy and in the hard case. As $H$ is increased, the gap is found to increase
almost linearly with $H$, but with different slopes depending on the field
orientation. In particular, the slope is smaller when $H$ is along the hard
axis. Such a magnetic anisotropy of the spin-wave energies is shown to persist
well above $T_C$ ($T \approx 1.2 T_C$).",0503538v4
2005-10-16,Magnetic vortex dynamics in a 2D easy plane ferromagnet,"In this thesis, we consider the dynamics of vortices in the easy plane
insulating ferromagnet in two dimensions. In addition to the quasiparticle
excitations, here spin waves or magnons, this magnetic system admits a family
of vortex solutions carrying two topological invariants, the winding number or
vorticity, and the polarization. A vortex is approximately described as a
particle moving about the system, endowed with an effective mass and acted upon
by a variety of forces. Classically, the vortex has an inter-vortex potential
energy giving a Coulomb-like force (attractive or repulsive depending on the
relative vortex vorticity), and a gyrotropic force, behaving as a self-induced
Lorentz force, whose direction depends on both topological indices. Expanding
semiclassically about a many-vortex solution, the vortices are quantized by
considering the scattered magnon states, giving a zero point energy correction
and a many-vortex mass tensor. The vortices cannot be described as independent
particles--that is, there are off-diagonal mass terms, such as 1/2 Mij vi vj,
that are non-negligible. This thesis examines the full vortex dynamics in
further detail by evaluating the Feynman-Vernon influence functional, which
describes the evolution of the vortex density matrix after the magnon modes
have been traced out. In addition to the set of forces already known, we find
new damping forces acting both longitudinally and transversely to the vortex
motion. The vortex motion within a collective cannot be entirely separated:
there are damping forces acting on one vortex due to the motion of another. The
effective damping forces have memory effects: they depend not only on the
current motion of the vortex collection but also on the motion history.",0510414v1
2006-06-10,Topological solitons in highly anisotropic two dimensional ferromagnets,"e study the solitons, stabilized by spin precession in a classical
two--dimensional lattice model of Heisenberg ferromagnets with non-small
easy--axis anisotropy. The properties of such solitons are treated both
analytically using the continuous model including higher then second powers of
magnetization gradients, and numerically for a discrete set of the spins on a
square lattice. The dependence of the soliton energy $E$ on the number of spin
deviations (bound magnons) $N$ is calculated. We have shown that the
topological solitons are stable if the number $N$ exceeds some critical value
$N_{\rm{cr}}$. For $N < N_{\rm{cr}}$ and the intermediate values of anisotropy
constant $K_{\mathrm{eff}} <0.35J$ ($J$ is an exchange constant), the soliton
properties are similar to those for continuous model; for example, soliton
energy is increasing and the precession frequency $ \omega (N)$ is decreasing
monotonously with $N$ growth. For high enough anisotropy $K_{\mathrm{eff}} >
0.6 J$ we found some fundamentally new soliton features absent for continuous
models incorporating even the higher powers of magnetization gradients. For
high anisotropy, the dependence of soliton energy E(N) on the number of bound
magnons become non-monotonic, with the minima at some ""magic"" numbers of bound
magnons. Soliton frequency $\omega (N)$ have quite irregular behavior with
step-like jumps and negative values of $\omega $ for some regions of $N$. Near
these regions, stable static soliton states, stabilized by the lattice effects,
exist.",0606263v1
2006-08-18,Effects of dilution and disorder on magnetism in diluted spin systems,"The influence of configurational disorder on the magnetic properties of
diluted Heisenberg spin systems is studied with regard to the ferromagnetic
stability of diluted magnetic semiconductors. The equation of motion of the
magnon Green's function is decoupled by Tyablikov approximation. With supercell
approach, the concentrations of magnetic ions are determined by the size of the
supercell in which there is only one magnetic ion per supercell in our method.
In order to distinguish the influence of dilution and disorder, there are two
kinds of supercells being used: the \textit{diluted and ordered} case and the
\textit{diluted and disordered} case. The configurational averaging of magnon
Green function due to disorder is treated in the augmented space formalism. The
random exchange integrals between two supercells are treated as a matrix. The
obtained magnon spectral densities are used to calculate the temperature
dependence of magnetization and Curie temperature. The results are shown as
following: (i) dilution leads to increasing the averaged distance of two
magnetic ions, further decreases the effective exchange integrals and is main
reason to reduce Curie temperature; (ii) spatial position disorder of magnetic
ions results in the dispersions of the exchange integrals between two
supercells and slightly changes ferromagnetic transition temperature; (iii) the
exponential damping of distance dependence obviously reduces Curie temperature
and should be set carefully in any phenomenological model.",0608418v2
2007-05-02,Magnetic Excitations in La2CuO4 probed by Indirect Resonant Inelastic X-ray Scattering,"Recent experiments on La$_2$CuO$_4$ suggest that indirect resonant inelastic
X-ray scattering (RIXS) might provide a probe for transversal spin dynamics. We
present in detail a systematic expansion of the relevant magnetic RIXS cross
section by using the ultrashort core-hole lifetime (UCL) approximation. We
compute the scattering intensity and its momentum dependence in leading order
of the UCL expansion. The scattering is due to two-magnon processes and is
calculated within a linear spin-wave expansion of the Heisenberg spin model for
this compound, including longer range and cyclic spin interactions. We observe
that the latter terms in the Hamiltonian enhance the first moment of the
spectrum if they strengthen the antiferromagnetic ordering. The theoretical
spectra agree very well with experimental data, including the observation that
scattering intensity vanishes for the transferred momenta ${\bf q} = (0,0)$ and
${\bf q} = (\pi,\pi)$. We show that at finite temperature there is an
additional single-magnon contribution to the scattering with a spectral weight
proportional to $T^3$. We also compute the leading corrections to the UCL
approximation and find them to be small, putting the UCL results on a solid
basis. All this univocally points to the conclusion that the observed low
temperature RIXS intensity in La$_2$CuO$_4$ is due to two-magnon scattering.",0705.0263v4
2008-01-17,Towards the theory of ferrimagnetism,"Two-sublattice ferrimagnet, with spin-$s_1$ operators $\bf{S_{1i}}$ at the
sublattice $A$ site and spin-$s_2$ operators $\bf{S_{2i}}$ at the sublattice
$B$ site, is considered. The magnon of the system, the transversal fluctuation
of the total magnetization, is a complicate mixture of the transversal
fluctuations of the sublattice $A$ and $B$ spins. As a result, the magnons'
fluctuations suppress in a different way the magnetic orders of the $A$ and $B$
sublattices and one obtains two phases. At low temperature $(0,T^*)$ the
magnetic orders of the $A$ and $B$ spins contribute to the magnetization of the
system, while at the high temperature $(T^*,T_N)$, the magnetic order of the
spins with a weaker intra-sublattice exchange is suppressed by magnon
fluctuations, and only the spins with stronger intra-sublattice exchange has
non-zero spontaneous magnetization. The $T^*$ transition is a transition
between two spin-ordered phases in contrast to the transition from spin-ordered
state to disordered state ($T_N$-transition). There is no additional symmetry
breaking, and the Goldstone boson has a ferromagnetic dispersion in both
phases. A modified spin-wave theory is developed to describe the two phases.
All known Neel's anomalous $M(T)$ curves are reproduced, in particular that
with ""compensation point"". The theoretical curves are compared with
experimental ones for sulpho-spinel $MnCr2S_{4-x}Se_{x}$ and rare earth iron
garnets.",0801.2651v1
2008-03-27,Aspects of Integrability in AdS/CFT Duality,"In this dissertation, we discuss how our understanding of the large-N
spectrum of AdS/CFT has been deepened by integrability-based approaches. We
begin with a comprehensive review of the integrability of the gauge theory
spin-chain and that of the string sigma model. In the light of the AdS/CFT
duality, they should be just two ways of describing the same underlying
integrability, and it is believed that the unified integrability can be
characterised by a set of Bethe ansatz equations which is valid for all values
of the 't Hooft coupling. By studying the asymptotic spectrum of the AdS/CFT in
the infinite spin/R-charge limit, we first identify the corresponding solitonic
counterparts in the context of the AdS/CFT, which are the so-called dyonic
giant magnons and the SYM magnon boundstates. Then we show that the S-matrix
computed directly from the string solitons scattering precisely reproduces the
prediction from the conjecture. We further perform an analyticity test by
studying the singularities of the conjectured magnon boundstate S-matrix and
checking the physicality conditions. These tests give strong positive supports
for the integrability of large-N AdS/CFT as well as the specific form of the
conjectured Bethe ansatz equations. Concerning the string theory integrability,
we also provide a detailed study of certain classical string solutions on AdS_5
x S^5. These are constructed in such a way they correspond to generic soliton
solutions of (Complex) sine/sinh-Gordon equations via the so-called Pohlmeyer
reduction procedure. Furthermore, we describe them in terms of
algebro-geometric data as finite-gap solutions, giving a complete map of the
elliptic string solutions.",0803.3999v2
2010-02-07,Magnetic nature of the 500 meV peak in $\rm La_{2-x}Sr_{x}CuO_4$ observed with resonant inelastic x-ray scattering at the Cu $K$-edge,"We present a comprehensive study of the temperature and doping dependence of
the 500 meV peak observed at ${\bf q}=(\pi,0)$ in resonant inelastic x-ray
scattering (RIXS) experiments on $\rm La_2CuO_4$. The intensity of this peak
persists above the N\'eel temperature (T$_{N}$=320 K), but decreases gradually
with increasing temperature, reaching zero at around T=500 K. The peak energy
decreases with temperature in close quantitative accord with the behavior of
the two-magnon $\rm B_{1g}$ Raman peak in $\rm La_2CuO_4$, and with suitable
rescaling, agrees with the Raman peak shifts in $\rm EuBa_2Cu_3O_6$ and $\rm
K_2NiF_4$. The overall dispersion of this excitation in the Brillouin zone is
found to be in agreement with theoretical calculations for a two-magnon
excitation. Upon doping, the peak intensity decreases analogous to the Raman
mode intensity and appears to track the doping dependence of the spin
correlation length. Taken together, these observations strongly suggest that
the 500 meV mode is magnetic in character and is likely a two-magnon
excitation.",1002.1440v1
2010-06-18,The Relativistic Avatars of Giant Magnons and their S-Matrix,"The motion of strings on symmetric space target spaces underlies the
integrability of the AdS/CFT correspondence. Although these theories, whose
excitations are giant magnons, are non-relativistic they are classically
equivalent, via the Polhmeyer reduction, to a relativistic integrable field
theory known as a symmetric space sine-Gordon theory. These theories can be
formulated as integrable deformations of gauged WZW models. In this work we
consider the class of symmetric spaces CP^{n+1} and solve the corresponding
generalized sine-Gordon theories at the quantum level by finding the exact
spectrum of topological solitons, or kinks, and their S-matrix. The latter
involves a trignometric solution of the Yang-Baxer equation which exhibits a
quantum group symmetry with a tower of states that is bounded, unlike for
magnons, as a result of the quantum group deformation parameter q being a root
of unity. We test the S-matrix by taking the semi-classical limit and comparing
with the time delays for the scattering of classical solitons. We argue that
the internal CP^{n-1} moduli space of collective coordinates of the solitons in
the classical theory can be interpreted as a q-deformed fuzzy space in the
quantum theory. We analyse the n=1 case separately and provide a further test
of the S-matrix conjecture in this case by calculating the central charge of
the UV CFT using the thermodynamic Bethe Ansatz.",1006.3667v2
2011-02-25,Elastic constants and ultrasonic attenuation in the cone state of the frustrated antiferromagnet Cs_2CuCl_4,"In an external magnetic field perpendicular to the plane of the layers, the
quasi two-dimensional frustrated antiferromagnet Cs_2CuCl_4 exhibits a
magnetically ordered ""cone state"" at low temperatures. In this state the
component of the magnetic moments in field direction is finite, while their
projection onto the plane of the layers forms a spiral. We present both
theoretical and experimental results for the magnetic field dependence of the
elastic constants and the ultrasonic attenuation rate in the cone state. Our
theoretical analysis is based on the usual spin-wave expansion around the
classical ground state of a Heisenberg model on an anisotropic triangular
lattice with Dzyaloshinskii-Moriya interactions. Magnon-phonon interactions are
modeled by expanding the exchange interactions up to second order in powers of
the phonon coordinates. As long as the external magnetic field is not too close
to the critical field where the cone state becomes unstable, we obtain
reasonable agreement between theory and experiment, suggesting that at least in
this regime magnons behave as well-defined quasiparticles. We also show that
the assumption of well-defined magnons implies that at long wavelengths the
ultrasonic attenuation rate in the cone state of Cs_2CuCl_4 is proportional to
the fourth power of the phonon momentum.",1102.5342v2
2011-07-04,A Relativistic Relative of the Magnon S-Matrix,"We construct a relativistic scattering theory based on a q deformation and
large string tension limit of the magnon S-matrix of the string world sheet
theory in AdS_5 x S^5. The S-matrix falls naturally into a previously studied
class associated to affine quantum groups, in this case for a twisted affine
loop superalgebra associated to an outer automorphism of sl(2|2). This infinite
algebra includes the celebrated triply extended psl(2|2) x R^3 algebra, but
only two of the centres, the lightcone components of the 2-momentum, are
non-vanishing. The algebra has the interpretation as an extended supersymmetry
algebra including a non-trivial R-symmetry. The representation theory of this
algebra has some complications in that tensor products are reducible but
indecomposable, however, we find that structure meshes perfectly with the
bootstrap, or fusion, equations of S-matrix theory. The bootstrap equations can
then be used inductively to generate the complete S-matrix. Unlike the magnon
theory, the relativistic theory only has a finite set of states and we find
that - at least when the deformation parameter q is a root of unity - the
spectrum matches precisely the soliton spectrum of the relativistic theory
underlying the Pohlmeyer reduction of the string world sheet theory known as
the semi-symmetric space sine-Gordon theory.",1107.0628v2
2011-08-05,Magnons and electromagnons in a spin-lattice-coupled frustrated magnet CuFeO2 as seen via inelastic neutron scattering,"We have investigated spin-wave excitations in a four-sublattice (4SL)
magnetic ground state of a frustrated magnet CuFeO2, in which `electromagnon'
(electric-field-active magnon) excitation has been discovered by recent
terahertz time-domain spectroscopy [Seki et al. Phys. Rev. Lett. 105 097207
(2010)]. In previous study, we have identified two spin-wave branches in the
4SL phase by means of inelastic neutron scattering measurements under applied
uniaxial pressure. [T. Nakajima et al. J. Phys. Soc. Jpn. 80 014714 (2011) ] In
the present study, we have performed high-energy-resolution inelastic neutron
scattering measurements in the 4SL phase, resolving fine structures of the
lower-energy spin-wave branch near the zone center. Taking account of the
spin-driven lattice distortions in the 4SL phase, we have developed a model
Hamiltonian to describe the spin-wave excitations. The determined Hamiltonian
parameters have successfully reproduced the spin-wave dispersion relations and
intensity maps obtained in the inelastic neutron scattering measurements. The
results of the spin-wave analysis have also revealed physical pictures of the
magnon and electromagnon modes in the 4SL phase, suggesting that collinear and
noncollinear characters of the two spin-wave modes are the keys to understand
the dynamical coupling between the spins and electric dipole moments in this
system.",1108.1297v1
2012-07-31,Spin excitations in a single La$_2$CuO$_4$ layer,"The dynamics of S=1/2 quantum spins on a 2D square lattice lie at the heart
of the mystery of the cuprates
\cite{Hayden2004,Vignolle2007,Li2010,LeTacon2011,Coldea2001,Headings2010,Braicovich2010}.
In bulk cuprates such as \LCO{}, the presence of a weak interlayer coupling
stabilizes 3D N\'{e}el order up to high temperatures. In a truly 2D system
however, thermal spin fluctuations melt long range order at any finite
temperature \cite{Mermin1966}. Further, quantum spin fluctuations transfer
magnetic spectral weight out of a well-defined magnon excitation into a
magnetic continuum, the nature of which remains controversial
\cite{Sandvik2001,Ho2001,Christensen2007,Headings2010}. Here, we measure the
spin response of \emph{isolated one-unit-cell thick layers} of \LCO{}. We show
that coherent magnons persist even in a single layer of \LCO{} despite the loss
of magnetic order, with no evidence for resonating valence bond (RVB)-like spin
correlations \cite{Anderson1987,Hsu1990,Christensen2007}. Thus these
excitations are well described by linear spin wave theory (LSWT). We also
observe a high-energy magnetic continuum in the isotropic magnetic response.
This high-energy continuum is not well described by 2 magnon LSWT, or indeed
any existing theories.",1208.0018v1
2013-10-01,Linear Spin Wave Analysis for General Magnetic Orders in the Kondo Lattice Model,"We extend the formulation of the spin wave theory for the Kondo lattice
model, which was mainly used for the ferromagnetic metallic state, to general
magnetic orders including complex noncollinear and noncoplanar orders. The 1/S
expansion is reformulated in the matrix form depending on the size of the
magnetic unit cell. The noncollinearity and noncoplanarity of the localized
moments are properly taken into account by the matrix elements of the
para-unitary matrix used in the diagonalization of the Bogoliubov-de Gennes
type Hamiltonian for magnons. We apply the formulation within the linear spin
wave approximation to a typical noncollinear case, the $120^{\circ}$ N{\'e}el
order on a triangular lattice at half filling. We calculate the magnon
excitation spectrum and the quantum correction to the magnitude of ordered
moments as functions of the strength of the Hund's-rule coupling, $J_{\rm
H}/t$. We find that the magnon excitation shows softening at $J_{\rm H}/t
\simeq 2.9$, which indicates that the $120^{\circ}$ order is destabilized for
smaller $J_{\rm H}/t$. On the other hand, we show that the $120^{\circ}$ order
is stable in the entire range of $J_{\rm H}/t \gtrsim 2.9$, and, in the limit
of $J_{\rm H}/t \to \infty$, the form of the spin wave spectrum approaches that
for the antiferromagnetic Heisenberg model, while the bandwidth is proportional
to $t^2/J_{\rm H}$. The reduction of the ordered moment is smaller than that
for the spin-only model, except in the vicinity of the softening.",1310.0176v1
2014-01-19,Topological excitations in a Kagomé magnet,"It is shown here that a Kagom\'e magnet, with Heisenberg and
Dzyaloshinskii-Moriya interactions causes non trivial topological and chiral
magnetic properties. Chirality---that is, left or right handedness---is a very
important concept in a broad range of scientific areas, and particularly, in
condensed matter physics. Inversion symmetry breaking relates chirality with
skyrmions, that are protected field configurations with particle-like and
topological properties. Here, the reported numerical simulations and
theoretical considerations reveal that the magnetic excitations of the Kagom\'e
magnet can both be of regular bulk magnon character, as well as, having a
non-trivial topological nature. We also find that under special circumstances,
skyrmions emerge as excitations, having stability even at room temperature.
Chiral magnonic edge states of a Kagom\'e magnet offer, in addition, a
promising way to create, control and manipulate skyrmions. This has potential
for applications in spintronics, magnonics and skyrmionics, i.e., for
information storage or as logic devices based on the transportation and control
of these particles. Collisions between these particle-like excitations are
found to be elastic in the skyrmion-skyrmion channel, albeit without
mass-conservation for an individual skyrmion. Skyrmion-antiskyrmion collisions
are found to be more complex, where annihilation and creation of these objects
have a distinct non-local nature.",1401.4757v2
2014-10-22,The connection between vortex-like topological excitations and conventional excitations in quantum ferromagnetic spin systems on two dimensional lattice and their stability,"We present a scheme for the construction of quantum states of vortex like
topological excitations corresponding to spin- 1/2 strongly XY anisotropic
nearest neighbor Heisenberg Ferromagnet on two dimensional lattice. The
procedure involving Pauli spin basis states is carried out corresponding to
both infinite dilute limit and finite density limit of vortex/anti-vortex. It
is found that the corresponding quantum mechanical states representing charge 1
quantum vortices/ anti-vortices can be expressed as linear combinations of
single magnon states, composite multi-magnon states and the ground state.
Detailed calculations show that these states are quantum mechanically stable
states of the Hamiltonian only when the system size exceeds certain threshold
value. Our analysis indicates that the interactions between different magnon
modes can very well generate these topological excitations. Possible
applications of our calculations to real magnetic systems are also discussed.
Magnetic measurements probing spin dynamics may be undertaken to verify the
existence of the threshold size for the stability of vortices.",1410.5921v3
2014-10-24,Macroscopic quantum superposition of spin ensembles with ultra-long coherence times via superradiant masing,"Macroscopic quantum phenomena such as lasers, Bose-Einstein condensates,
superfluids, and superconductors are of great importance in foundations and
applications of quantum mechanics. In particular, quantum superposition of a
large number of spins in solids is highly desirable for both quantum
information processing and ultrasensitive magnetometry. Spin ensembles in
solids, however, have rather short collective coherence time (typically less
than microseconds). Here we demonstrate that under realistic conditions it is
possible to maintain macroscopic quantum superposition of a large spin ensemble
(such as about ~10^{14} nitrogen-vacancy center electron spins in diamond) with
an extremely long coherence time ~10^8 sec under readily accessible conditions.
The scheme, following the mechanism of superradiant lasers, is based on
superradiant masing due to coherent coupling between collective spin
excitations (magnons) and microwave cavity photons. The coherence time of the
macroscopic quantum superposition is the sum of the magnon life time and the
cavity lifetime, further elongated by the total number of coherent magnons and
photons, which have macroscopic values when masing occurs. The macroscopic
quantum coherence of spin ensembles can be exploited for magnetometry with
sensitivity ~10 fT/(Hz)^{1/2}. The long-living collective states of spin
ensembles in solids will provide a new platform for studying macroscopic
quantum phenomena and are useful for quantum technologies.",1410.6565v1
2014-11-19,Magnons and Excitation Continuum in XXZ triangular antiferromagnetic model: Application to $Ba_3CoSb_2O_9$,"We investigate the excitation spectrum of the triangular-lattice
antiferromagnetic $XXZ$ model using series expansions and mean field Schwinger
bosons approaches. The single-magnon spectrum computed with series expansions
exhibits rotonic minima at the middle points of the edges of the Brillouin
zone, for all values of the anisotropy parameter in the range $0\leq
J^z/J\leq1$. Based on the good agreement with series expansions for the
single-magnon spectrum, we compute the full dynamical magnetic structure factor
within the mean field Schwinger boson approach to investigate the relevance of
the $XXZ$ model for the description of the unusual spectrum found recently in
$Ba_3CoSb_2O_9$. In particular, we obtain an extended continuum above the spin
wave excitations, which is further enhanced and brought closer to those
observed in $Ba_3CoSb_2O_9$ with the addition of a second neighbor exchange
interaction approximately 15% of the nearest-neighbor value. Our results
support the idea that excitation continuum with substantial spectral-weight are
generically present in two-dimensional frustrated spin systems and
fractionalization in terms of {\it bosonic} spinons presents an efficient way
to describe them.",1411.5298v2
2015-04-22,Thermally Driven Ratchet Motion of Skyrmion Microcrystal and Topological Magnon Hall Effect,"Spontaneously emergent chirality is an issue of fundamental importance across
the natural sciences. It has been argued that a unidirectional (chiral)
rotation of a mechanical ratchet is forbidden in thermal equilibrium, but
becomes possible in systems out of equilibrium. Here we report our finding that
a topologically nontrivial spin texture known as a skyrmion - a particle-like
object in which spins point in all directions to wrap a sphere - constitutes
such a ratchet. By means of Lorentz transmission electron microscopy we show
that micron-sized crystals of skyrmions in thin films of Cu2OSeO3 and MnSi
display a unidirectional rotation motion. Our numerical simulations based on a
stochastic Landau-Lifshitz-Gilbert equation suggest that this rotation is
driven solely by thermal fluctuations in the presence of a temperature
gradient, whereas in thermal equilibrium it is forbidden by the Bohr-van
Leeuwen theorem. We show that the rotational flow of magnons driven by the
effective magnetic field of skyrmions gives rise to the skyrmion rotation,
therefore suggesting that magnons can be used to control the motion of these
spin textures.",1504.05860v1
2015-09-01,Magnon-driven longitudinal spin Seebeck effect in F|N and N|F|N structures: role of asymmetric in-plane magnetic anisotropy,"The influence of an asymmetric in-plane magnetic anisotropy on the thermally
activated spin current is studied theoretically for two different systems; (i)
the system consisting of a ferromagnetic insulator in a direct contact with a
nonmagnetic metal, and the sandwich structure consisting of a ferromagnetic
insulating part sandwiched between two nonmagnetic metals. It is shown that
when the difference between the temperatures of the two nonmagnetic metals in a
structure is not large, the spin pumping currents from the magnetic part to the
nonmagnetic ones are equal in amplitude and have opposite directions, so only
the spin torque current contributes to the total spin current. The spin current
flows then from the nonmagnetic metal with the higher temperature to the
nonmagnetic metal having a lower temperature. Its amplitude varies linearly
with the difference in temperatures. In addition, we have found that if the
magnetic anisotropy is in the layer plane, then the spin current increases with
the magnon temperature, while in the case of an out-of-plane magnetic
anisotropy the spin current decreases when the magnon temperature enhances.
Enlarging the difference between the temperatures of the nonmagnetic metals,
the linear response becomes important, as confirmed by analytical expressions
inferred from the Fokker-Planck approach and by the results obtained upon a
full numerical integration of the stochastic Landau-Lifshitz-Gilbert equation.",1509.00376v1
2016-01-07,Hubbard model with Rashba or Dresselhaus spin-orbit coupling and Rotated Anti-ferromagnetic Heisenberg Model,"In this work, we investigate the possible dramatic effects of Rashba or
Dresselhaus spin-orbit coupling (SOC) on fermionic Hubbard model in a 2d square
lattice. In the strong coupling limit, it leads to the Rotated
Anti-ferromagnetic Heisenberg model which is a new class of quantum spin model.
For a special equivalent class, we identify a new spin-orbital entangled
commensurate ground ( Y-y ) state subject to strong quantum fluctuations at
$T=0$. We evaluate the quantum fluctuations by the spin wave expansion up to
order $ 1/S^2 $. In some SOC parameter regime, the Y-y state supports a massive
relativistic in-commensurate magnon ( C-IC ) with its two gap minima positions
continuously tuned by the SOC parameters. The C-IC magnons dominate all the low
temperature thermodynamic quantities and also lead to the separation of the
peak positions between the longitudinal and the transverse spin structure
factors. In the weak coupling limit, any weak repulsive interaction also leads
to a weak Y-y state. There is only a crossover from the weak to the strong
coupling. High temperature expansions of the specific heats in both weak and
strong coupling are presented. The dramatic roles to be played by these C-IC
magnons at generic SOC parameters or under various external probes are hinted.
Experimental applications to both layered noncentrosymmetric materials and cold
atom are discussed.",1601.01642v2
2017-03-14,Finite-Temperature Dynamics and Thermal Intraband Magnon Scattering in Haldane Spin-One Chains,"The antiferromagnetic spin-one chain is considerably one of the most
fundamental quantum many-body systems, with symmetry protected topological
order in the ground state. Here, we present results for its dynamical spin
structure factor at finite temperatures, based on a combination of exact
numerical diagonalization, matrix-product-state calculations and quantum Monte
Carlo simulations. Open finite chains exhibit a sub-gap band in the thermal
spectral functions, indicative of localized edge-states. Moreover, we observe
the thermal activation of a distinct low-energy continuum contribution to the
spin spectral function with an enhanced spectral weight at low momenta and its
upper threshold. This emerging thermal spectral feature of the Haldane spin-one
chain is shown to result from intra-band magnon scattering due to the thermal
population of the single-magnon branch, which features a large bandwidth-to-gap
ratio. These findings are discussed with respect to possible future studies on
spin-one chain compounds based on inelastic neutron scattering.",1703.04652v2
2018-02-05,Integrable Subsectors from Holography,"We consider operators in ${\cal N}=4$ super Yang-Mills theory dual to closed
string states propagating on a class of LLM geometries. The LLM geometries we
consider are specified by a boundary condition that is a set of black rings on
the LLM plane. When projected to the LLM plane, the closed strings are polygons
with all corners lying on the outer edge of a single ring. The large $N$ limit
of correlators of these operators receives contributions from non-planar
diagrams even for the leading large $N$ dynamics. Our interest in these
fluctuations is because a previous weak coupling analysis argues that the net
effect of summing the huge set of non-planar diagrams, is a simple rescaling of
the 't Hooft coupling. We carry out some nontrivial checks of this proposal.
Using the $su(2|2)^2$ symmetry we determine the two magnon $S$-matrix and
demonstrate that it agrees, up to two loops, with a weak coupling computation
performed in the CFT. We also compute the first finite size corrections to both
the magnon and the dyonic magnon by constructing solutions to the Nambu-Goto
action that carry finite angular momentum. These finite size computations
constitute a strong coupling confirmation of the proposal.",1802.01367v2
2018-02-09,Monocrystalline free standing 3D yttrium iron garnet magnon nano resonators,"Nano resonators in which mechanical vibrations and spin waves can be coupled
are an intriguing concept that can be used in quantum information processing to
transfer information between different states of excitation. Until now, the
fabrication of free standing magnetic nanostructures which host long lived spin
wave excitatons and may be suitable as mechanical resonators seemed elusive. We
demonstrate the fabrication of free standing monocrystalline yttrium iron
garnet (YIG) 3D nanoresonators with nearly ideal magnetic properties. The
freestanding 3D structures are obtained using a complex lithography process
including room temperature deposition and lift-off of amorphous YIG and
subsequent crystallization by annealing. The crystallization nucleates from the
substrate and propagates across the structure even around bends over distances
of several micrometers to form e.g. monocrystalline resonators as shown by
transmission electron microscopy. Spin wave excitations in individual
nanostructures are imaged by time resolved scanning Kerr microscopy. The narrow
linewidth of the magnetic excitations indicates a Gilbert damping constant of
only $\alpha = 2.6 \times 10^{-4}$ rivalling the best values obtained for
epitaxial YIG thin film material. The new fabrication process represents a leap
forward in magnonics and magnon mechanics as it provides 3D YIG structures of
unprecedented quality. At the same time it demonstrates a completely new route
towards the fabrication of free standing crystalline nano structures which may
be applicable also to other material systems.",1802.03176v2
2018-02-20,Magnon-photon coupling in a non-collinear magnetic insulator Cu$_2$OSeO$_3$,"Anticrossing behavior between magnons in a non-collinear chiral magnet
Cu$_2$OSeO$_3$ and a two-mode X-band microwave resonator was studied in the
temperature range 5-100K. In the field-induced ferrimagnetic phase, we observed
a strong coupling regime between magnons and two microwave cavity modes with a
cooperativity reaching 3600. In the conical phase, cavity modes are
dispersively coupled to a fundamental helimagnon mode, and we demonstrate that
the magnetic phase diagram of Cu$_2$OSeO$_3$ can be reconstructed from the
measurements of the cavity resonance frequency. In the helical phase, a
hybridized state of a higher-order helimagnon mode and a cavity mode - a
helimagnon polariton - was found. Our results reveal a new class of magnetic
systems where strong coupling of microwave photons to non-trivial spin textures
can be observed.",1802.07113v3
2018-02-26,Magnon-induced superconductivity in a topological insulator coupled to ferro- and antiferromagnetic insulators,"We study the effective interactions between Dirac fermions on the surface of
a three-dimensional topological insulator due to the proximity coupling to the
magnetic fluctuations in a ferromagnetic or antiferromagnetic insulator. Our
results show that the magnetic fluctuations can mediate attractive interactions
between Dirac fermions of both Amperean and BCS type. In the ferromagnetic
case, we find pairing between fermions with parallel momenta, so-called
Amperean pairing, whenever the effective Lagrangian for the magnetic
fluctuations does not contain a quadratic term. The pairing interaction also
increases with increasing Fermi momentum, and is in agreement with previous
studies in the limit of high chemical potential. If a quadratic term is
present, the pairing is instead of BCS type above a certain chemical potential.
In the antiferromagnetic case, BCS pairing occurs when the ferromagnetic
coupling between magnons on the same sublattice exceeds the antiferromagnetic
coupling between magnons on different sublattices. Outside this region in
parameter space, we again find that Amperean pairing is realized.",1802.09524v4
2018-11-23,Broadening Frequency Range of a Ferromagnetic Axion Haloscope with Strongly Coupled Cavity-Magnon Polaritons,"With the axion being a prime candidate for dark matter, there has been some
recent interest in direct detection through a so called `Ferromagnetic
haloscope.' Such devices exploit the coupling between axions and electrons in
the form of collective spin excitations of magnetic materials with the readout
through a microwave cavity. Here, we present a new, general, theoretical
treatment of such experiments in a Hamiltonian formulation for strongly coupled
magnons and photons, which hybridise as cavity-magnon polaritons. Such strongly
coupled systems have an extended measurable dispersive regime. Thus, we extend
the analysis and operation of such experiments into the dispersive regime,
which allows any ferromagnetic haloscope to achieve improved bandwidth with
respect to the axion mass parameter space. This experiment was implemented in a
cryogenic setup, and initial search results are presented setting laboratory
limits on the axion-electron coupling strength of $g_{aee}>3.7\times10^{-9}$ in
the range $33.79~\mu$eV$< m_a<33.94~\mu$eV with $95\%$ confidence. The
potential bandwidth of the Ferromagnetic haloscope was calculated to be in two
bands, the first of about $1$GHz around $8.24$GHz (or $4.1~\mu$eV mass range
around $34.1~\mu$eV) and the second of about $1.6$GHz around $10$GHz
($6.6~\mu$eV mass range around $41.4~\mu$eV). Frequency tuning may also be
easily achieved via an external magnetic field which changes the ferromagnetic
resonant frequency with respect to the cavity frequency. The requirements
necessary for future improvements to reach the DFSZ axion model band are
discussed in the paper.",1811.09348v3
2014-03-04,Spin excitations of half-doped bilayer manganites: intermediate phase,"The ground state of half-doped manganites involves intricate spin, charge and
orbital orderings, which are difficult to discern experimentally. In this work,
we resort to the theoretical analysis of the spin fluctuation spectrum of the
half-doped bilayer Pr(Ca$_{0.9}$Sr$_{0.1}$)$_2$ Mn$_2$O$_7$ in order to get an
insight of its electronic ground state. By means of the linear spin wave
approximation, we compute the magnon dispersion for a family of localized spin
models, which can describe several phases proposed for the ground state of
half-doped manganites, like the intermediate one proposed by Efremov et al.
[Nat.Mats. 3, 853, (2004)] along with its particular cases corresponding to
Goodenough's CE phase [Phys. Rev. 100, 564, (1955)] and Zener polaron or dimer
phases. We obtain an excellent agreement between theory and experiment when the
ground state is assumed to be a generalized Goodenough's CE phase, with a
Mn-charge disproportionation inside the experimentally expected range. As
essential ingredients for our improved fit of the upper and lower magnon
branches measured around the gap, we identified two next-nearest neighbour
exchange interactions between the planar Mn zig-zag chains, one for each type
of Mn ion present. In connection with this finding, we revisited the magnetic
excitations of the laminar related compounds, focusing on the upper magnon
branches. Here we prove that their measurement would provide the key to
identify unambiguously the ground state present in the layered half-doped
manganites.",1403.0971v2
2017-01-10,Motion of skyrmions in nanowires driven by magnonic momentum-transfer forces,"We study the motion of magnetic skyrmions in a nanowire induced by a
spin-wave current $J$ flowing out of a driving layer close to the edge of the
wire. By applying micromagnetic simulation and an analysis of the effective
Thiele equation, we find that the skyrmion trajectory is governed by an
interplay of both forces due to the magnon current and the wire boundary. The
skyrmion is attracted to the driving layer and is accelerated by the repulsive
force due to the wire boundary. We consider both cases of a driving
longitudinal and transverse to the nanowire, but a steady-state motion of the
skyrmion is only obtained for a transverse magnon current. For the latter case,
we find in the limit of low current densities $J$ the velocity-current relation
$v \sim J/\alpha$ where $v$ is the skyrmion velocity and $\alpha$ is the
Gilbert damping. For large $J$ in case of strong driving, the skyrmion is
pushed into the driving layer resulting in a drop of the skyrmion velocity and,
eventually, the destruction of the skyrmion.",1701.02430v2
2016-05-20,Two-dimensional terahertz magnetic resonance spectroscopy of collective spin waves,"Nonlinear manipulation of nuclear and electron spins is the basis for all
advanced methods in magnetic resonance including multidimensional nuclear
magnetic and electron spin resonance spectroscopies, magnetic resonance
imaging, and in recent years, quantum control over individual spins. The
methodology is facilitated by the ease with which the regime of strong coupling
can be reached between radiofrequency or microwave magnetic fields and nuclear
or electron spins respectively, typified by sequences of magnetic pulses that
control the magnetic moment directions. The capabilities meet a bottleneck,
however, for far-infrared magnetic resonances characteristic of correlated
electron materials, molecular magnets, and proteins that contain high-spin
transition metal ions. Here we report the development of two-dimensional
terahertz magnetic resonance spectroscopy and its use for direct observation of
the nonlinear responses of collective spin waves (magnons). The spectra show
magnon spin echoes and 2-quantum signals that reveal pairwise correlations
between magnons at the Brillouin zone center. They also show resonance-enhanced
second-harmonic and difference-frequency signals. Our methods are readily
generalizable to multidimensional magnetic resonance spectroscopy and nonlinear
coherent control of terahertz-frequency spin systems in molecular complexes,
biomolecules, and materials.",1605.06476v3
2016-05-29,Heisenberg Necklace Model in Magnetic Field,"Motivated by the experimental realizations of nearly one-dimensional spin-1/2
Heisenberg model found in chain cuprates SrCuO$_2$ and Sr$_2$CuO$_3$, which
remain in a quantum-critical Luttinger liquid state down to temperatures that
are much lower than in-chain exchange coupling, we consider the perturbation to
this state caused by interactions with nuclear spins on the same sites. We
study the low-energy sector of the Heisenberg Necklace model and estimate the
effect of the coupling between the nuclear and the electronic spins on the
overall spins dynamics and its dependence on the magnetic field. We find that
the Necklace model has a characteristic energy scale, $\Lambda \sim
J^{1/3}(\gamma I)^{2/3}$, at which the coupling between spins of the necklace
and the spins of the Heisenberg chain becomes strong. In the strong magnetic
field $\mu_B B > \Lambda$ the low energy spectrum is gapless, but two gapless
bosonic modes have different velocities whose ratio at strong fields approaches
a universal number, $\sqrt 2 +1$. In the case of Sr$_2$CuO$_3$ the energy scale
$\Lambda $ is sizable and comparable to the Neel ordering temperature induced
by the inter-chain coupling, and thus could noticeably modify the low
temperature magnon dynamics. We further find that the above energy scale is
insensitive to strong magnetic field, $\mu_B B \gg \Lambda \sim J^{1/3}(\gamma
I)^{2/3}$, and therefore the interaction with nuclear spins cannot lead to
unusually strong magnetic field dependence of the magnon spectrum observed by
ESR in Sr$_2$CuO$_3$, which has been attributed to the magnon interaction with
the Higgs mode.",1605.09014v2
2017-06-23,Ferroic collinear multilayer magnon spin valve,"Information transport and processing by pure magnonic spin currents in
insulators is a promising alternative to conventional charge-current driven
spintronic devices. The absence of Joule heating as well as the reduced spin
wave damping in insulating ferromagnets has been suggested to enable the
implementation of efficient logic devices. After the proof of concept for a
logic majority gate based on the superposition of spin waves has been
successfully demonstrated, further components are required to perform complex
logic operations. A key component is a switch that corresponds to a
conventional magnetoresistive spin valve. Here, we report on magnetization
orientation dependent spin signal detection in collinear magnetic multilayers
with spin transport by magnonic spin currents. We find in Y3Fe5O12|CoO|Co
tri-layers that the detected spin signal depends on the relative alignment of
Y3Fe5O12 and Co. This demonstrates a spin valve behavior with an effect
amplitude of 120% in our systems. We demonstrate the reliability of the effect
and investigate the origin by both temperature and power dependent
measurements, showing that spin rectification effects and a magnetic layer
alignment dependent spin transport effect result in the measured signal.",1706.07592v1
2017-11-30,Effective interactions in a quantum Bose-Bose mixture,"We generalize the Beliaev diagrammatic theory of an interacting spinless
Bose-Einstein condensate to the case of a binary mixture. We derive a set of
coupled Dyson equations and find analytically the Green's functions of the
system. The elementary excitation spectrum consists of two branches, one of
which takes the characteristic parabolic form in the limit of a
spin-independent interaction. We observe renormalization of the magnon mass and
the spin-wave velocity due to the Andreev-Bashkin entrainment effect. For a 3D
weakly-interacting gas the spectrum can be obtained by applying the Bogoliubov
transformation to the second-quantized Hamiltonian in which the microscopic
two-body potentials in each channel are replaced by the corresponding off-shell
scattering amplitudes. The superfluid drag density can be calculated by
considering a mixture of phonons and magnons interacting via the effective
potentials. We show that this problem is identical to the second-order
perturbative treatment of a Bose polaron. In 2D the drag contributes to the
magnon dispersion already in the first approximation. Our consideration
provides a basis for systematic study of emergent phases in quantum degenerate
Bose-Bose mixtures.",1712.00039v3
2019-09-11,Discovering momentum-dependent magnon spin texture in insulating antiferromagnets: Role of the Kitaev interaction,"We theoretically show that the Kitaev interaction generates a novel class of
spin texture in the excitation spectrum of the antiferromagnetic insulator
found in the Kitaev-Heisenberg-$\Gamma$ model. In conducting electronic
systems, there is a series of vortex type of spin texture along the Fermi
surface induced by Rashba and Dresselhaus spin-orbit coupling. Such spin
textures are rarely found in magnetic insulators, since there had been no
systematic ways to control the kinetics of its quasi-particle called magnon
using a magnetic field or spacially asymmetric exchange couplings. Here, we
propose a general framework to explore such spin textures in arbitrary
insulating antiferromagnets. We introduce an analytical method to transform any
complicated Hamiltonian to the simple representation based on pseudo-spin
degrees of freedom. The direction of the pseudo-spin on a Bloch sphere
describes the degree of contributions from the two magnetic sublattices to the
spin moment carried by the magnon. The momentum dependent fictitious ""Zeeman
field"" determines the direction of the pseudo-spin and thus becomes the control
parameter of the spin texture, which is explicitly described by the original
model parameters. The framework enabled us to clarify the uncovered aspect of
the Kitaev interaction, and further provides a tool to easily design or explore
materials with intriguing magnetic properties. Since these spin textures can be
a source of a pure spin current, the Kitaev materials $A_{2}$PrO$_{3}$ ($A$
=Li, Na) shall become a potential platform of power-saving spintronics devices.",1909.05174v2
2017-05-08,Experimental Verification of the Universality of Magnetic-Field-Induced Bose-Einstein Condensation of Magnons,"CsFeBr$_3$ is an $S\,{=}\,1$ hexagonal antiferromagnet that has a singlet
ground state owing to its large easy-plane single-ion anisotropy. The critical
behavior of the magnetic-field-induced phase transition for a magnetic field
parallel to the $c$ axis, which can be described by the Bose-Einstein
condensation (BEC) of magnons under the $U(1)$ symmetry, was investigated via
magnetization and specific heat measurements down to 0.1$\,$K. For the specific
heat measurement, we have developed a method of effectively suppressing the
torque acting on a sample with strong anisotropy that uses the spin dimer
compound Ba$_2$CoSi$_2$O$_6$Cl$_2$ with large and anisotropic Van Vleck
paramagnetism. The temperature dependence of the transition field $H_{\rm
c}(T)$ was found to follow the power-law $H_{\rm c}(T)\,{-}\,H_{\rm
c}\,{\propto}\,T^{\phi}$ with a critical exponent of
${\phi}\,{=}\,1.50\,{\pm}\,0.02$ and critical field of $H_{\rm c}\,{=}\,2.60$
T. This result verifies the universality of the three-dimensional BEC of
magnons described by ${\phi}_{\rm BEC}\,{=}\,3/2$.",1705.02739v2
2017-08-31,Antiferromagnetic magnons from fractionalized excitations,"We develop an approach to describe antiferromagnetic magnons on a bipartite
lattice supporting the N\'{e}el state using fractionalized degrees of freedom
typically inherent to quantum spin liquids. In particular we consider a
long-range magnetically ordered state of interacting two-dimensional quantum
spin$-1/2$ models using the Chern-Simons (CS) fermion representation of
interacting spins. The interaction leads to Cooper instability and pairing of
CS fermions, and to CS superconductivity which spontaneously violates the
continuous $\mathrm{U}(1)$ symmetry generating a linearly-dispersing gapless
Nambu-Goldstone mode due to phase fluctuations. We evaluate this mode and show
that it is in high-precision agreement with magnons of the corresponding
N\'{e}el antiferromagnet irrespective to the lattice symmetry. Using the
fermion formulation of a system with competing interactions, we show that the
frustration gives raise to nontrivial long-range four, six, and higher-leg
interaction vertices mediated by the CS gauge field, which are responsible for
restoring of the continuous symmetry at sufficiently strong frustration. We
identify these new interaction vertices and discuss their implications to
unconventional phase transitions. We also apply the proposed theory to a model
of anyons that can be tuned continuously from fermions to bosons.",1709.00060v2
2018-03-06,Magnon-assisted tunnelling in van der Waals heterostructures based on CrBr3,"The growing family of two-dimensional (2D) materials that are now available
can be used to assemble van der Waals heterostructures with a wide range of
properties. Of particular interest are tunnelling heterostructures, which have
been used to study the electronic states both in the tunnelling barrier and in
the emitter and collector contacts. Recently, 2D ferromagnets have been studied
theoretically and experimentally. Here we investigate electron tunnelling
through a thin (2-6 layers) ferromagnetic CrBr3 barrier. For devices with
non-magnetic barriers, conservation of momentum can be relaxed by
phonon-assisted tunnelling or by tunnelling through localised states. In the
case of our ferromagnetic barrier the dominant tunnelling mechanisms are the
emission of magnons at low temperatures or scattering of electrons on localised
magnetic excitations above the Curie temperature. Tunnelling with magnon
emission offers the possibility of injecting spin into the collector electrode.",1803.02120v2
2018-06-03,Field evolution of magnons in $α$-RuCl$_3$ by high-resolution polarized terahertz spectroscopy,"The Kitaev quantum spin liquid (KSL) is a theoretically predicted state of
matter whose fractionalized quasiparticles are distinct from bosonic magnons,
the fundamental excitation in ordered magnets. The layered honeycomb
antiferromagnet $\alpha$-RuCl$_3$ is a KSL candidate material, as it can be
driven to a magnetically disordered phase by application of an in-plane
magnetic field, with $H_c \sim 7$ T. Here we report a detailed characterization
of the magnetic excitation spectrum of this material by high-resolution
time-domain terahertz (THz) spectroscopy. We observe two sharp magnon
resonances whose frequencies and amplitudes exhibit a discontinuity as a
function of applied magnetic field, as well as two broader peaks at higher
energy. Below the N\'eel temperature, we find that linear spin wave theory can
account for all of these essential features of the spectra when a
$C_3$-breaking distortion of the honeycomb lattice and the presence of
structural domains are taken into account.",1806.00855v2
2019-01-22,High-harmonic generation in quantum spin systems,"We theoretically study the high-harmonic generation (HHG) in one-dimensional
spin systems. While in electronic systems the driving by AC electric fields
produces radiation from the dynamics of excited charges, we consider here the
situation where spin systems excited by a magnetic field pulse generate
radiation via a time-dependent magnetization. Specifically, we study the
magnetic dipole radiation in two types of ferromagnetic spin chain models, the
Ising model with static longitudinal field and the XXZ model, and reveal the
structure of the spin HHG and its relation to spin excitations. For weak laser
amplitude, a peak structure appears which can be explained by time-dependent
perturbation theory. With increasing amplitude, plateaus with well-defined
cutoff energies emerge. In the Ising model with longitudinal field, the
thresholds of the multiple plateaus in the radiation spectra can be explained
by the annihilation of multiple magnons. In the XXZ model, which retains the
$\mathbf{Z}_{2}$ symmetry, the laser magnetic field can induce a phase
transition of the ground state when it exceeds a critical value, which results
in a drastic change of the spin excitation character. As a consequence, the
first cutoff energy in the HHG spectrum changes from a single-magnon to a
two-magnon energy at this transition. Our results demonstrate the possibility
of generating high-harmonic radiation from magnetically ordered materials and
the usefulness of high-harmonic signals for extracting information on the spin
excitation spectrum.",1901.07588v2
2019-01-31,Triplet superconductivity in ferromagnets due to magnon exchange,"We consider the superconducting pairing induced by spin waves exchange in a
ferromagnet with both conduction and localized electrons, the latter being
described as spins. We use the microscopic Eliashberg theory to describe the
pairing of conducting electrons and the RPA approach to treat the localized
spins assuming an exchange coupling between the conducting electrons and spins.
In the framework of non relativistic Hamiltonian twe found that he spin wave
exchange results in equal spin electron pairing described by the two components
of the order parameter, $\Delta^{\uparrow}$ (both spins up) and
$\Delta^{\downarrow}$ (both spins down). Due to the conservation of total spin
projection on the axis of the spontaneous ferromagnetic moment, the spin wave
exchange at low temperatures includes an emission of magnons and an absorption
of thermal magnons by the conduction electrons. The absorption and emission
processes depend differently on the temperature, with the absorption being
progressively suppressed as the temperature drops. As a result, the
superconducting pairing exists only if the electron-spin wave exchange
parameter $g$ exceeds some critical value $g_c$. At $g>g_c$ pairing vanishes if
the temperature drops below the lowest point $T_{cl}$ or increases above the
upper critical point $T_{ch} \approx T_m$ (the Curie temperature) where the
spin waves cease to exist. This behavior inherent to the spin carrying glue is
in an obvious disagreement with the results of conventional BCS approach which
assumes that the effective electron-electron attraction is simply proportional
to the static magnetic susceptibility.",1901.11248v1
2019-02-28,Finite field regime for a quantum spin liquid in $α$-RuCl$_3$,"An external magnetic field can induce a transition in $\alpha$-RuCl$_3$ from
an ordered zigzag state to a disordered state that is possibly related to the
Kitaev quantum spin liquid. Here we present new field dependent inelastic
neutron scattering and magnetocaloric effect measurements implying the
existence of an additional transition out of the quantum spin liquid phase at
an upper field limit $B_u$. The neutron scattering shows three distinct regimes
of magnetic response. In the low field ordered state the response shows magnon
peaks; the intermediate field regime shows only continuum scattering, and above
$B_u$ the response shows sharp magnon peaks at the lower bound of a strong
continuum. Measurable dispersion of magnon modes along the $(0,0,L)$ direction
implies non-negligible inter-plane interactions. Combining the magnetocaloric
effect measurements with other data a $T-B$ phase diagram is constructed. The
results constrain the range where one might expect to observe quantum spin
liquid behavior in $\alpha$-RuCl$_3$.",1903.00056v3
2019-03-02,Excitation of unidirectional exchange spin waves by a nanoscale magnetic grating,"Magnon spintronics is a prosperous field that promises beyond-CMOS technology
based on elementary excitations of the magnetic order that act as information
carriers for future computational architectures. Unidirectional propagation of
spin waves is key to the realization of magnonic logic devices. However,
previous efforts to enhance the Damon-Eshbach-type nonreciprocity did not
realize (let alone control) purely unidirectional propagation. Here we
experimentally demonstrate excitations of unidirectional exchange spin waves by
a nanoscale magnetic grating consisting of Co nanowires fabricated on an
ultrathin yttrium iron garnet film. We explain and model the nearly perfect
unidirectional excitation by the chirality of the magneto-dipolar interactions
between the Kittel mode of the nanowires and the exchange spin waves of the
film. Reversal of the magnetic configurations of film and nanowire array from
parallel to antiparallel changes the direction of the excited spin waves. Our
results raise the prospect of a chiral magnonic logic without the need for
fragile surface states.",1903.00638v1
2019-03-15,The Krein-Schrödinger Formalism of Bosonic BdG and Certain Classical Systems and Their Topological Classification,"To understand recent works on classical and quantum spin equations and their
topological classification, we develop a unified mathematical framework for
bosonic BdG systems and associated classical wave equations; it applies not
just to equations that describe quantized spin excitations in magnonic crystals
but more broadly to other systems that are described by a BdG hamiltonian.
Because here the generator of dynamics, the analog of the hamiltonian, is
para-, aka Krein-, hermitian but not hermitian, the theory of Krein spaces
plays a crucial role. For systems which are thermodynamically stable, the
classical equations can be expressed as a Schr\""odinger equation with a
hermitian hamiltonian. We then proceed to apply the Cartan-Altland-Zirnbauer
classification scheme: to properly understand what topological class these
equations belong to, we need to conceptually distinguish between symmetries and
constraints. Complex conjugation enters as a particle-hole constraint (as
opposed to a symmetry), since classical waves are necessarily real-valued.
Because of this distinction only commuting symmetries enter in the topological
classification. Our arguments show that the equations for spin waves in
magnonic crystals are a system of class A, the same topological class as
quantum hamiltonians describing the Integer Quantum Hall Effect. Consequently,
the magnonic edge modes first predicted by Shindou et al. are indeed analogs of
the Quantum Hall Effect, and their net number is topologically protected.",1903.06354v1
2019-04-04,Resonant inelastic x-ray scattering study of bond order and spin excitations in nickelate thin-film structures,"We used high-resolution resonant inelastic x-ray scattering (RIXS) at the Ni
$L_3$ edge to simultaneously investigate high-energy interband transitions
characteristic of Ni-O bond ordering and low-energy collective excitations of
the Ni spins in the rare-earth nickelates $R$NiO$_3$ ($R$ = Nd, Pr, La) with
pseudocubic perovskite structure. With the support of calculations based on a
double-cluster model we quantify bond order (BO) amplitudes for different thin
films and heterostructures and discriminate short-range BO fluctuations from
long-range static order. Moreover we investigate magnetic order and exchange
interactions in spatially confined $R$NiO$_3$ slabs by probing dispersive
magnon excitations. While our study of superlattices (SLs) grown in the (001)
direction of the perovskite structure reveals a robust non-collinear spin
spiral magnetic order with dispersive magnon excitations that are essentially
unperturbed by BO modulations and spatial confinement, we find magnons with
flat dispersions and strongly reduced energies in SLs grown in the
$(111)_{\text{pc}}$ direction that exhibit collinear magnetic order. These
results give insight into the interplay of different collective ordering
phenomena in a prototypical 3$d$ transition metal oxide and establish RIXS as a
powerful tool to quantitatively study several order parameters and the
corresponding collective excitations within one experiment.",1904.02782v2
2019-04-19,Quantum Brownian Motion of a Magnetic Skyrmion,"Within a microscopic theory, we study the quantum Brownian motion of a
skyrmion in a magnetic insulator coupled to a bath of magnon-like quantum
excitations. The intrinsic skyrmion-bath coupling gives rise to damping terms
for the skyrmion center-of-mass, which remain finite down to zero temperature
due to the quantum nature of the magnon bath. We show that the quantum version
of the fluctuation-dissipation theorem acquires a non-trivial temperature
dependence. As a consequence, the skyrmion mean square displacement is finite
at zero temperature and has a fast thermal activation that scales quadratically
with temperature, contrary to the linear increase predicted by the classical
phenomenological theory. The effects of an external oscillating drive which
couples directly on the magnon bath are investigated. We generalize the
standard quantum theory of dissipation and we show explicitly that additional
time-dependent dissipation terms are generated by the external drive. From
these we emphasize a friction and a topological charge renormalization term,
which are absent in the static limit. The skyrmion response function inherits
the time periodicity of the driving field and it is thus enhanced and lowered
over a driving cycle. Finally, we provide a generalized version of the
nonequilibrium fluctuation-dissipation theorem valid for weakly driven baths.",1904.09215v1
2019-08-15,Steering between Level Repulsion and Attraction: Broad tunability of Two-Port Driven Cavity Magnon-Polaritons,"Cavity-magnon polaritons (CMPs) are the associated quasiparticles of the
hybridization between cavity photons and magnons in a magnetic sample placed in
a microwave resonator. In the strong coupling regime, where the macroscopic
coupling strength exceeds the individual dissipation, there is a coherent
exchange of information. This renders CMPs as promising candidates for future
applications such as in information processing. Recent advances on the study of
the CMP now allow not only for creation of CMPs on demand, but also for tuning
of the coupling strength - this can be thought of as enhancing or suppressing
of information exchange. Here, we go beyond standard single-port driven CMPs
and employ a two-port driven CMP. We control the coupling strength by the
relative phase $\phi$ and amplitude field ratio $\delta_0$ between both ports.
Specifically, we derive a new expression from Input-Output theory for the study
of the two-port driven CMP and discuss the implications on the coupling
strength. Furthermore, we examine intermediate cases where the relative phase
is tuned between its maximal and minimal value and, in particular, the high
$\delta_0$ regime, which has not been yet explored.",1908.05439v2
2019-12-18,Gravitational realization of magnons in a ferromagnetic spin chain,"A gravitational model of magnons in thermal equilibrium with a ferromagnetic
spin chain is developed in a phenomenological bottom-up approach. A large
Schwarzschild-AdS black hole background is used as the thermal reservoir and
the magnon dynamics is obtained by scalar fields and branes in the bulk. The
key feature of this model is that the coupling of the spin chain is related
with the radial position in which the brane is located. We further study a
ferromagnetic spin chain with a competing interaction and find that the
couplings are related by the difference of positions of the branes. We show how
to obtain the model from a weak limit of a dynamical gravitational system. This
allows us to embed the model into a holographic system. The couplings can be
related to entanglement entropy at finite temperature of the CFT since the
turning point of minimal surfaces coincides with the position of the branes.
The difference of entropy is used to define a notion of distance between the
chain couplings.",1912.08761v2
2019-12-20,Slow-wave based magnonic diode,"Spin waves, the collective excitations of the magnetic order parameter, and
magnons, the associated quasiparticles, are envisioned as possible data
carriers in future wave-based computing architectures. On the road towards
spin-wave computing, the development of a diode-like device capable of
transmitting spin waves in only one direction, thus allowing controlled signal
routing, is an essential step. Here, we report on the design and experimental
realization of a microstructured magnonic diode in a ferromagnetic bilayer
system. Effective unidirectional propagation of spin waves is achieved by
taking advantage of nonreciprocities produced by dynamic dipolar interactions
in transversally magnetized media, which lack symmetry about their horizontal
midplane. More specifically, dipolar-induced nonreciprocities are used to
engineer the spin-wave dispersion relation of the bilayer system so that the
group velocity is reduced to very low values for one direction of propagation,
and not for the other, thus producing unidirectional slow spin waves. Brillouin
light scattering and propagating spin-wave spectroscopy are used to demonstrate
the diode-like behavior of the device, the composition of which was previously
optimized through micromagnetic simulations. simulations.",1912.09735v1
2016-03-24,Persistent paramagnons deep in the metallic phase of Sr$_{2-x}$La$_x$IrO$_4$,"We have studied the magnetic excitations of electron-doped
Sr$_{2-x}$La$_x$IrO$_4$ ($0 \leq x \leq 0.10$) using resonant inelastic x-ray
scattering (RIXS) at the Ir L$_3$-edge. The long range magnetic order is
rapidly lost with increasing $x$, but two-dimensional short-range order (SRO)
and dispersive magnon excitations with nearly undiminished spectral weight
persist well into the metallic part of the phase diagram. The magnons in the
SRO phase are heavily damped and exhibit anisotropic softening. Their
dispersions are well described by a pseudospin-1/2 Heisenberg model with
exchange interactions whose spatial range increases with doping. We also find a
doping-independent high-energy magnetic continuum, which is not described by
this model. The spin-orbit excitons arising from the pseudospin-3/2 manifold of
the Ir ions broaden substantially in the SRO phase, but remain largely
separated from the low-energy magnons. Pseudospin-1/2 models are therefore a
good starting point for the theoretical description of the low-energy magnetic
dynamics of doped iridates.",1603.07547v2
2016-12-20,Magnons in a two dimensional transverse field XXZ model,"The XXZ model on a square lattice in the presence of a transverse magnetic
field is studied within the spin wave theory to investigate the resulting
canted antiferromagnet. The small and large field regimes are probed separately
both for easy-axis and easy-plane scenarios which reveal an unentangled
factorized ground state at an intermediate value of the field. Goldstone modes
are obtained for the field-free $XY$ antiferromagnet as well as for the
isotropic antiferromagnet with field up to its saturation value. Moreover, for
an easy-plane anisotropy, we find that there exists a non-zero field, where
magnon degeneracy appears as a result of restoration of an U(1) sublattice
symmetry and that, across that field, there occurs a magnon band crossing. For
completeness, we then obtain the system phase diagram for $S=1/2$ via large
scale quantum Monte Carlo simulations using the stochastic series expansion
technique. Our numerical method is based on a quantization of spin along the
direction of the applied magnetic field and does not suffer from a
sign-problem, unlike comparable algorithms based on a spin quantization along
the axis of anisotropy. With this formalism, we are also able to obtain powder
averages of the transverse and longitudinal magnetizations, which may be useful
for understanding experimental measurements on polycrystalline samples.",1612.06608v3
2017-11-20,Continuum of quantum fluctuations in a three-dimensional $S\!=\!1$ Heisenberg magnet,"Conventional crystalline magnets are characterized by symmetry breaking and
normal modes of excitation called magnons with quantized angular momentum
$\hbar$. Neutron scattering correspondingly features extra magnetic Bragg
diffraction at low temperatures and dispersive inelastic scattering associated
with single magnon creation and annihilation. Exceptions are anticipated in
so-called quantum spin liquids as exemplified by the one-dimensional spin-1/2
chain which has no magnetic order and where magnons accordingly fractionalize
into spinons with angular momentum $\hbar/2$. This is spectacularly revealed by
a continuum of inelastic neutron scattering associated with two-spinon
processes and the absence of magnetic Bragg diffraction. Here, we report
evidence for these same key features of a quantum spin liquid in the
three-dimensional Heisenberg antiferromagnet NaCaNi$_2$F$_7$. Through specific
heat and neutron scattering measurements, Monte Carlo simulations, and analytic
approximations to the equal time correlations, we show that NaCaNi$_2$F$_7$ is
an almost ideal realization of the spin-1 antiferromagnetic Heisenberg model on
a pyrochlore lattice with weak connectivity and frustrated interactions.
Magnetic Bragg diffraction is absent and 90\% of the spectral weight forms a
continuum of magnetic scattering not dissimilar to that of the spin-1/2 chain
but with low energy pinch points indicating NaCaNi$_2$F$_7$ is in a Coulomb
phase. The residual entropy and diffuse elastic scattering points to an exotic
state of matter driven by frustration, quantum fluctuations and weak exchange
disorder.",1711.07509v1
2020-09-14,Effective field theory of magnon: Dynamics in chiral magnets and Schwinger mechanism,"We develop the effective field theoretical descriptions of spin systems in
the presence of symmetry-breaking effects: the magnetic field, single-ion
anisotropy, and Dzyaloshinskii-Moriya interaction. Starting from the lattice
description of spin systems, we show that the symmetry-breaking terms
corresponding to the above effects can be incorporated into the effective field
theory as a combination of a background (or spurious) $\SO(3)$ gauge field and
a scalar field in the symmetric tensor representation, which are eventually
fixed at their physical values. We use the effective field theory to
investigate mode spectra of inhomogeneous ground states, with focusing on
one-dimensionally inhomogeneous states, such as helical and spiral states.
Although the helical and spiral ground states share a common feature of
supporting the gapless Nambu-Goldstone modes associated with the translational
symmetry breaking, they have qualitatively different dispersion relations:
isotropic in the helical phase while anisotropic in the spiral phase. We also
discuss the magnon production induced by an inhomogeneous magnetic field, and
find a formula akin to the Schwinger formula. Our formula for the magnon
production gives a finite rate for antiferromagnets, and a vanishing rate for
ferromagnets, whereas that for ferrimagnets interpolates between the two cases.",2009.06694v1
2020-09-16,Unconventional magnon excitation by off-resonant microwaves,"It is widely recognized that a physical system can only respond to a periodic
driving significantly when the driving frequency matches the normal mode
frequency of the system, which leads to resonance. Off-resonant phenomena are
rarely considered because of the difficulty to realize strong coupling between
physical systems and off-resonant waves. Here we examine the response of a
magnetic system to squeezed light and surprisingly find that the magnons are
maximally excited when the effective driving frequency is several orders of
magnitude larger than the resonant frequency. The generated magnons are
squeezed which brings the advantage of tunable squeezing through an external
magnetic field. Furthermore, we demonstrate that such off-resonant
quasi-particle excitation is universal in all the hybrid systems in which the
coherent and parametric interaction of bosons exists and that it is purely a
quantum effect, which is rooted in the quantum fluctuations of particles in the
squeezed vacuum. Our findings may provide an unconventional route to study
off-resonant phenomena and may further benefit the use of hybrid matter-light
systems in continuous variable quantum information.",2009.07948v2
2020-09-22,Magnon-mediated spin currents in Tm3Fe5O12/Pt with perpendicular magnetic anisotropy,"The control of pure spin currents carried by magnons in magnetic insulator
(MI) garnet films with a robust perpendicular magnetic anisotropy (PMA) is of
great interest to spintronic technology as they can be used to carry, transport
and process information. Garnet films with PMA present labyrinth domain
magnetic structures that enrich the magnetization dynamics, and could be
employed in more efficient wave-based logic and memory computing devices. In
MI/NM bilayers, where NM being a normal metal providing a strong spin-orbit
coupling, the PMA benefits the spin-orbit torque (SOT) driven magnetization's
switching by lowering the needed current and rendering the process faster,
crucial for developing magnetic random-access memories (SOT-MRAM). In this
work, we investigated the magnetic anisotropies in thulium iron garnet (TIG)
films with PMA via ferromagnetic resonance measurements, followed by the
excitation and detection of magnon-mediated pure spin currents in TIG/Pt driven
by microwaves and heat currents. TIG films presented a Gilbert damping constant
{\alpha}~0.01, with resonance fields above 3.5 kOe and half linewidths broader
than 60 Oe, at 300 K and 9.5 GHz. The spin-to-charge current conversion through
TIG/Pt was observed as a micro-voltage generated at the edges of the Pt film.
The obtained spin Seebeck coefficient was 0.54 {\mu}V/K, confirming also the
high interfacial spin transparency.",2009.10299v1
2021-02-13,Accumulation and control of spin waves in magnonic dielectric microresonators by a comb of ultrashort laser pulses,"Spin waves in magnetic microresonators are at the core of modern magnonics.
Here we demonstrate a new method of tunable excitation of different spin wave
modes in magnetic microdisks by using a train of laser pulses coming at a
repetition rate higher than the decay rate of spin precession. The microdisks
are etched in a transparent bismuth iron garnet film and the light pulses
influence the spins nonthermally through the inverse Faraday effect. The high
repetition rate of the laser stimulus of 10 GHz establishes an interplay
between the spin wave resonances in the frequency and momentum domains. As a
result, scanning of the focused laser spot near the disk boarder changes
interference pattern of the magnons and leads to a resonant dependence of the
spin wave amplitude on the external magnetic field. Apart from that, we
achieved a switching between volume and surface spin waves by a small variation
of the external magnetic field.",2102.07021v4
2021-02-16,Magnonic band structure in CoFeB/Ta/NiFe meander-shaped magnetic bilayers,"In this work, we investigate the spin-wave propagation in three-dimensional
nanoscale CoFeB/Ta/NiFe meander structures fabricated on a structured SiO2/Si
substrate. The magnonic band structure has been experimentally determined by
wavevector-resolved Brillouin light scattering (BLS) spectroscopy and a set of
stationary modes interposed by two dispersive modes of Bloch type have been
identified. The results could be understood by micromagnetic and finite element
simulations of the mode distributions in both real space and the frequency
domain. The dispersive modes periodically oscillate in frequency over the
Brillouin zones and correspond to modes, whose spatial distributions extend
over the entire sample and are either localized exclusively in the CoFeB layer
or the entire CoFeB/Ta/NiFe magnetic bilayer. Stationary modes are mainly
concentrated in the vertical segments of the CoFeB and NiFe layers and show
negligible amplitudes in the horizon-tal segments. The findings are compared
with those of single-layer CoFeB meander structures with the same geometry
parameters, which reveals the influence of the dipolar coupling between the two
ferromagnetic layers on the magnonic band structure.",2102.08223v2
2013-06-04,Determination of the origin of the spin Seebeck effect - bulk vs. interface effects,"The observation of the spin Seebeck effect in insulators has meant a
breakthrough for spin caloritronics due to the unique ability to generate pure
spin currents by thermal excitations in insulating systems without moving
charge carriers. Since the recent first observation, the underlying mechanism
and the origin of the observed signals have been discussed highly
controversially. Here we present a characteristic dependence of the
longitudinal spin Seebeck effect amplitude on the thickness of the insulating
ferromagnet (YIG). Our measurements show that the observed behavior cannot be
explained by any effects originating from the interface, such as magnetic
proximity effects in the spin detector (Pt). Comparison to theoretical
calculations of thermal magnonic spin currents yields qualitative agreement for
the thickness dependence resulting from the finite effective magnon propagation
length so that the origin of the effect can be traced to genuine bulk magnonic
spin currents ruling out parasitic interface effects.",1306.0784v1
2015-07-10,Theoretical Study of $L$-edge Resonant Inelastic X-ray Scattering in La$_2$CuO$_4$ on the Basis of Detailed Electronic Band Structure,"We study theoretically resonant inelastic x-ray scattering (RIXS) at the Cu
$L_3$-edge in a typical parent compound of high-$T_c$ cuprate superconductors
La$_2$CuO$_4$ on the basis of a detailed electronic band structure. We
construct a realistic and precise tight-binding model by employing the
maximally-localized Wannier functions derived from a first-principles
electronic structure calculation, and then take account of the Coulomb
repulsion between d electrons at each Cu site. The antiferromagnetic ground
state is described within the Hartree-Fock approximation, and take account of
electron correlations in the intermediate states of RIXS within the
random-phase approximation (RPA). Calculated RIXS spectra agree well with the
experimentally observed features including low-energy magnon excitation,
$d$-$d$ excitations, and charge-transfer excitations, over a wide
excitation-energy range. In particular, we stress the importance of photon
polarization dependence: the intensity of magnon excitation and the spectral
structure of $d$-$d$ excitations depend significantly not only on the
polarization direction of incident incoming photons but also that of outgoing
photons. It is demonstrated that the single-magnon excitation intensity is
maximized when the polarization directions of incoming and outgoing photons are
perpendicular to each other.",1507.02795v2
2016-09-07,Magnonic Interferometric Switch for Multi-Valued Logic Circuits,"We investigated a possible use of the magnonic interferometric switches in
multi-valued logic circuits. The switch is a three-terminal device consisting
of two spin channels where input, control, and output signals are spin waves.
Signal modulation is achieved via the interference between the source and gate
spin waves. We report experimental data on a micrometer scale prototype based
on Y3Fe2(FeO4)3 structure. The output characteristics are measured at different
angles of the bias magnetic field. The On/Off ratio of the prototype exceeds 36
dB at room temperature. Experimental data is complemented by the theoretical
analysis and the results of micro magnetic simulations showing spin wave
propagation in a micrometer size magnetic junction. We also present the results
of numerical modeling illustrating the operation of a nanometer-size switch
consisting of just 20 spins in the source-drain channel. The utilization of
spin wave interference as a switching mechanism makes it possible to build
nanometer-scale logic gates, and minimize energy per operation, which is
limited only by the noise margin. The utilization of phase in addition to
amplitude for information encoding offers an innovative route towards
multi-state logic circuits. We describe possible implementation of the
three-value logic circuits based on the magnonic interferometric switches. The
advantages and shortcomings inherent in interferometric switches are also
discussed.",1609.02217v1
2016-09-12,"Spontaneous decays of magneto-elastic excitations in noncollinear antiferromagnet (Y,Lu)MnO3","When magnons and phonons, the fundamental quasiparticles of the solid, are
coupled to one another, they form a new hybrid quasi-particle, leading to novel
phenomena and interesting applications. Despite its wide-ranging importance,
however, detailed experimental studies on the underlying Hamiltonian is rare
for actual materials. Moreover, the anharmonicity of such magnetoelastic
excitations remains largely unexplored although it is essential for a proper
understanding of their diverse thermodynamic behaviour as well as intrinsic
zero-temperature decay. Here we show that in noncollinear antiferromagnets, a
strong magnon-phonon coupling can significantly enhance the anharmonicity,
resulting in the creation of magnetoelastic excitations and their spontaneous
decay. By measuring the spin waves over the full Brillouin zone and carrying
out anharmonic spin wave calculations using a Hamiltonian with an explicit
magnon-phonon coupling, we have identified a hybrid magnetoelastic mode in
(Y,Lu)MnO3 and quantified its decay rate and the exchange-striction coupling
term required to produce it. Our work has wide implications for understanding
of the spin-phonon coupling and the resulting excitations of the broad classes
of noncollinear magnets.",1609.03262v1
2016-09-12,Topological Magnon Bands and Unconventional Superconductivity in Pyrochlore Iridate Thin Films,"We theoretically study the magnetic properties of pyrochlore iridate bilayer
and trilayer thin films grown along the $[111]$ direction using a strong
coupling approach. We find the ground state magnetic configurations on a mean
field level and carry out a spin-wave analysis about them. In the trilayer case
the ground state is found to be the all-in-all-out (AIAO) state, whereas the
bilayer has a deformed AIAO state. For all parameters of the spin-orbit coupled
Hamiltonian we study, the lowest magnon band in the trilayer case has a nonzero
Chern number. In the bilayer case we also find a parameter range with nonzero
Chern numbers. We calculate the magnon Hall response for both geometries,
finding a striking sign change as function of temperature. Using a slave-boson
mean-field theory we study the doping of the trilayer system and discover an
unconventional time-reversal symmetry broken $d+id$ superconducting state. Our
study complements prior work in the weak coupling limit and suggests that the
$[111]$ grown thin film pyrochlore iridates are a promising candidate for
topological properties and unconventional orders.",1609.03612v3
2018-08-08,Kink far below the Fermi level reveals new electron-magnon scattering channel in Fe,"Many properties of real materials can be modeled using ab initio methods
within a single-particle picture. However, for an accurate theoretical
treatment of excited states, it is necessary to describe electron-electron
correlations including interactions with bosons: phonons, plasmons, or magnons.
In this work, by comparing spin- and momentum-resolved photoemission
spectroscopy measurements to many-body calculations carried out with a newly
developed first-principles method, we show that a kink in the electronic band
dispersion of a ferromagnetic material can occur at much deeper binding
energies than expected (E_b=1.5 eV). We demonstrate that the observed spectral
signature reflects the formation of a many-body state that includes a photohole
bound to a coherent superposition of renormalized spin-flip excitations. The
existence of such a many-body state sheds new light on the physics of the
electron-magnon interaction which is essential in fields such as spintronics
and Fe-based superconductivity.",1808.02682v2
2018-08-20,Calculating critical temperatures for ferromagnetic order in two-dimensional materials,"Magnetic order in two-dimensional (2D) materials is intimately coupled to
magnetic anisotropy (MA) since the Mermin-Wagner theorem implies that
rotational symmetry cannot be spontaneously broken at finite temperatures in
2D. Large MA thus comprises a key ingredient in the search for magnetic 2D
materials that retains the magnetic order above room temperature. Magnetic
interactions are typically modeled in terms of Heisenberg models and the
temperature dependence on magnetic properties can be obtained with the Random
Phase Approximation (RPA), which treats magnon interactions at the mean-field
level. In the present work we show that large MA gives rise to strong
magnon-magnon interactions that leads to a drastic failure of the RPA. We then
demonstrate that classical Monte Carlo (MC) simulations correctly describe the
critical temperatures in the large MA limit and agree with RPA when the MA
becomes small. A fit of the MC results leads to a simple expression for the
critical temperatures as a function of MA and exchange coupling constants,
which significantly simplifies the theoretical search for new 2D magnetic
materials with high critical temperatures. The expression is tested on a
monolayer of CrI$_3$, which were recently observed to exhibit ferromagnetic
order below 45 K and we find excellent agreement with the experimental value.",1808.06400v3
2019-07-03,Dirac magnons in a honeycomb lattice quantum XY magnet CoTiO3,"The discovery of massless Dirac electrons in graphene and topological
Dirac-Weyl materials has prompted a broad search for bosonic analogues of such
Dirac particles. Recent experiments have found evidence for Dirac magnons above
an Ising-like ferromagnetic ground state in a two-dimensional (2D) kagome
lattice magnet and in the van der Waals layered honeycomb crystal CrI$_3$, and
in a 3D Heisenberg magnet Cu$_3$TeO$_6$. Here we report on our inelastic
neutron scattering investigation on large single crystals of a stacked
honeycomb lattice magnet CoTiO$_3$, which is part of a broad family of ilmenite
materials. The magnetically ordered ground state of CoTiO$_3$ features
ferromagnetic layers of Co$^{2+}$, stacked antiferromagnetically along the
$c$-axis. We discover that the magnon dispersion relation exhibits strong
easy-plane exchange anisotropy and hosts a clear gapless Dirac cone along the
edge of the 3D Brillouin zone. Our results establish CoTiO$_3$ as a model
pseudospin-$1/2$ material to study interacting Dirac bosons in a 3D quantum XY
magnet.",1907.02061v1
2019-10-08,Magnon Bose-Einstein Condensation and Superconductivity in a Frustrated Kondo Lattice,"Motivated by recent experiments on magnetically frustrated heavy fermion
metals, we theoretically study the phase diagram of the Kondo lattice model
with a nonmagnetic valence bond solid ground state on a ladder. A similar
physical setting may be naturally occurring in YbAl$_3$C$_3$, CeAgBi$_2$, and
TmB$_4$ compounds. In the insulating limit, the application of a magnetic field
drives a quantum phase transition to an easy-plane antiferromagnet, which is
described by a Bose-Einstein condensation of magnons. Using a combination of
field theoretical techniques and density matrix renormalization group
calculations we demonstrate that in one dimension this transition is stable in
the presence of a metallic Fermi sea and its universality class in the local
magnetic response is unaffected by the itinerant gapless fermions. Moreover, we
find that fluctuations about the valence bond solid ground state can mediate an
attractive interaction that drives unconventional superconducting correlations.
We discuss the extensions of our findings to higher dimensions and argue that,
depending on the filling of conduction electrons, the magnon Bose-Einstein
condensation transition can remain stable in a metal also in dimensions two and
three.",1910.03589v2
2020-01-31,Temperature dependence of spin pinning and spin-wave dispersion in nanoscopic ferromagnetic waveguides,"The field of magnonics attracts significant attention due to the possibility
of utilizing information coded into the spin-wave phase or amplitude to perform
computation operations on the nanoscale. Recently, spin waves were investigated
in Yttrium Iron Garnet (YIG) waveguides with widths ranging down to 50 nm and
aspect ratios thickness over width approaching unity. A critical width was
found, below which the exchange interaction suppresses the dipolar pinning
phenomenon and the system becomes unpinned. Here we continue these
investigations and analyse the pinning phenomenon and spin-wave dispersions as
a function of temperature, thickness and material of choice. Higher order
modes, the influence of a finite wavevector along the waveguide and the impact
of the pinning phenomenon on the spin-wave lifetime are discussed as well as
the influence of a trapezoidal cross section and edge roughness of the
waveguides. The presented results are of particular interest for potential
applications in magnonic devices and the incipient field of quantum magnonics
at cryogenic temperatures.",2002.00003v1
2020-02-05,Phonon angular momentum Hall effect,"Spin Hall effect is the transverse flow of the electron spin in conductors
under external electric field. Similarly, thermal gradient in magnetic
insulators can drive a transverse flow of the spin angular momentum of magnons,
which provides a thermal alternative for spin manipulation. Recently, the
phonon angular momentum (PAM), which is the angular momentum of atoms as a
result of their orbital motion around their equilibrium positions, has garnered
attention as a quantity analogous to the magnon spin. However, can we
manipulate PAM like magnon spin? Here, we show that temperature gradient
generally induces a transverse flow of PAM, which we term the phonon angular
momentum Hall effect (PAMHE). The PAMHE relies only on the presence of
transverse and longitudinal acoustic phonons, and it is therefore ubiquitous in
condensed matter systems. As a consequence of the PAMHE, PAM accumulates at the
edges of a crystal. When the atoms in the crystal carry nonzero Born effective
charge, the edge PAM induces edge magnetization, which may be observed through
optical measurement. We believe that PAMHE provides a new principle for the
manipulation of angular momenta in insulators and opens up an avenue for
developing functional materials based on phonon engineering.",2002.01659v2
2020-04-05,Spin wave based tunable switch between superconducting flux qubits,"Quantum computing hardware has received world-wide attention and made
considerable progress recently. YIG thin film have spin wave (magnon) modes
with low dissipation and reliable control for quantum information processing.
However, the coherent coupling between a quantum device and YIG thin film has
yet been demonstrated. Here, we propose a scheme to achieve strong coupling
between superconducting flux qubits and magnon modes in YIG thin film. Unlike
the direct $\sqrt{N}$ enhancement factor in coupling to the Kittel mode or
other spin ensembles, with N the total number of spins, an additional spatial
dependent phase factor needs to be considered when the qubits are magnetically
coupled with the magnon modes of finite wavelength. To avoid undesirable
cancelation of coupling caused by the symmetrical boundary condition, a CoFeB
thin layer is added to one side of the YIG thin film to break the symmetry. Our
numerical simulation demonstrates avoided crossing and coherent transfer of
quantum information between the flux qubits and the standing spin waves in YIG
thin films. We show that the YIG thin film can be used as a tunable switch
between two flux qubits, which have modified shape with small direct inductive
coupling between them. Our results manifest that it is possible to couple flux
qubits while suppressing undesirable cross-talk.",2004.02156v1
2020-05-20,Spin excitations in the frustrated triangular lattice antiferromagnet NaYbO$_2$,"Here we present a neutron scattering-based study of magnetic excitations and
magnetic order in NaYbO$_2$ under the application of an external magnetic
field. The crystal electric field-split $J = 7/2$ multiplet structure is
determined, revealing a mixed $|m_z>$ ground state doublet and is consistent
with a recent report Ding et al. [1]. Our measurements further suggest
signatures of exchange effects in the crystal field spectrum, manifested by a
small splitting in energy of the transition into the first excited doublet. The
field-dependence of the low-energy magnetic excitations across the transition
from the quantum disordered ground state into the fluctuation-driven ordered
regime is analyzed. Signs of a first-order phase transition into a noncollinear
ordered state are revealed at the upper-field phase boundary of the ordered
regime, and higher order magnon scattering, suggestive of strong magnon-magnon
interactions, is resolved within the previously reported $up-up-down$ phase.
Our results reveal a complex phase diagram of field-induced order and spin
excitations within NaYbO$_2$ and demonstrate the dominant role of quantum
fluctuations cross a broad range of fields within its interlayer frustrated
triangular lattice.",2005.10375v1
2020-05-29,Long-distance spin-transport across the Morin phase transition up to room temperature in ultra-low damping single crystals of the antiferromagnet α-Fe2O3,"Antiferromagnetic materials can host spin-waves with polarizations ranging
from circular to linear depending on their magnetic anisotropies. Until now,
only easy-axis anisotropy antiferromagnets with circularly polarized spin-waves
were reported to carry spin-information over long distances of micrometers. In
this article, we report long-distance spin-transport in the easy-plane canted
antiferromagnetic phase of hematite and at room temperature, where the linearly
polarized magnons are not intuitively expected to carry spin. We demonstrate
that the spin-transport signal decreases continuously through the easy-axis to
easy-plane Morin transition, and persists in the easy-plane phase through
current induced pairs of linearly polarized magnons with dephasing lengths in
the micrometer range. We explain the long transport distance as a result of the
low magnetic damping, which we measure to be below 0.0001 as in the best
ferromagnets. All of this together demonstrates that long-distance transport
can be achieved across a range of anisotropies and temperatures, up to room
temperature, highlighting the promising potential of this insulating
antiferromagnet for magnon-based devices.",2005.14414v5
2021-01-10,Optical magnons with dominant bond-directional exchange interactions in a honeycomb lattice iridate $α$-Li$_{2}$IrO$_{3}$,"We have used resonant inelastic x-ray scattering to reveal optical magnons in
a honeycomb lattice iridate $\alpha$-Li$_{2}$IrO$_{3}$. The spectrum in the
energy region 20-25 meV exhibits momentum dependence, of which energy is
highest at the location of the magnetic Bragg peak, ($\textit{h}, \textit{k}$)
= ($\pm$0.32, 0), and lowered toward (0, 0) and ($\pm$1, 0). We compare our
data with a linear spin-wave theory based on a generic nearest-neighbor spin
model. We find that a dominant bond-directional Kitaev interaction of order 20
meV is required to explain the energy scale observed in our study. The observed
excitations are understood as stemming from optical magnon modes whose
intensity is modulated by a structure factor, resulting in the apparent
momentum dependence. We also observed diffuse magnetic scattering arising from
the short-range magnetic correlation well above $\textit{T}_{N}$. In contrast
to Na$_{2}$IrO$_{3}$, this diffuse scattering lacks the $C_3$ rotational
symmetry of the honeycomb lattice, suggesting that the bond anisotropy is far
from negligible in $\alpha$-Li$_{2}$IrO$_{3}$.",2101.03471v1
2021-01-19,Reconfigurable magnonic mode-hybridisation and spectral control in a bicomponent artificial spin ice,"Strongly-interacting nanomagnetic arrays are finding increasing use as model
host systems for reconfigurable magnonics. The strong inter-element coupling
allows for stark spectral differences across a broad microstate space due to
shifts in the dipolar field landscape. While these systems have yielded
impressive initial results, developing rapid, scaleable means to access abroad
range of spectrally-distinct microstates is an open research problem.We present
a scheme whereby square artificial spin ice is modified by widening a
'staircase' subset of bars relative to the rest of the array, allowing
preparation of any ordered vertex state via simple global-field protocols.
Available microstates range from the system ground-state to high-energy
'monopole' states, with rich and distinct microstate-specific magnon spectra
observed. Microstate-dependent mode-hybridisation and anticrossings are
observed at both remanence and in-field with dynamic coupling strength tunable
via microstate-selection. Experimental coupling strengths are found up to g /
2$\pi$ = 0.15 GHz. Microstate control allows fine mode-frequency shifting, gap
creation and closing, and active mode number selection.",2101.07767v3
2021-01-23,Antiferromagnets with random vacancies and substitutional spins on the triangular lattice,"We discuss theoretically static and dynamical properties of $XY$ and
Heisenberg antiferromagnets on triangular lattice with random vacancies and
substitutional spins. It is shown that the distortion of $120^\circ$ magnetic
order produced by a single defect is described by electrostatic equations for a
field of an electrically neutral complex of six charges located around the
impurity. The first finite term in the multipole expansion of this field is the
octupole moment which decays as $1/r^3$ with the distance $r$. The linearity of
equations allows to describe analytically the distortion of the long-range
magnetic order at a small concentration $c$ of defects. We obtain analytically
renormalization of the elastic neutron scattering cross section and the magnon
spectrum $\epsilon_{\bf k}$ in the leading order in $c$. We find that the
scattering on impurities renormalizes weakly the bare spectrum $\epsilon_{\bf
k}\propto k$ at $k\gg\sqrt c$. However the renormalization is substantial of
the long-wavelength magnon spectrum at $k\ll\sqrt c$: $\epsilon_{\bf k}\propto
\sqrt{c /\ln(1/k)}$ at $k\to0$ and there is a parametrically large region in
which magnons with not too small momenta are overdamped and localized. This
strong modification of the long-wavelength spectrum leads to the stabilization
of the slightly distorted magnetic long-range order at $T<T_N\sim
S^2J/\ln(1/c)$ and to the considerable change in the density of states and in
the specific heat. The overdamped modes arise also in quasi-2D spin systems on
a stacked triangular lattice.",2101.09511v3
2021-02-28,Topological excitations in quasi two-dimensional quantum magnets with weak interlayer interactions,"The study of topological magnetic excitations has attracted widespread
attention in the past few years. In this thesis, I have studied some examples
of novel topological magnonic phases/phenomena in low-dimensional quantum
magnets. The first chapter motivates the research based on the research gap in
this field of study. The second chapter is written to make the thesis
self-sufficient and the concepts are explained through examples. In the second
chapter, the following formalisms and physical observables are described:
Holstein-Primakoff, bond operator, Schwinger boson, Bogoliubov-Valatin, Group
theory, Berry-phase, Berry-curvature, Chern number, thermal Hall conductance,
Nernst conductivity, dynamical spin structure factor, edge-current. The main
results of the thesis are shown in the third, fourth, and fifth chapters. In
the third chapter, I have shown that anti-chiral edge states (co-propagating
edge states) arise in the ferromagnetic Heisenberg model on the honeycomb
lattice with Dzyaloshinskii-Moriya (DM) interactions. My results suggest that
such anti-chiral edge states may be induced in certain realistic models of
quantum magnets. In the fourth chapter, I have found the emergence of many
magnon band-topological phases in the flux state of the Shastry-Sutherland
model. I have derived a simple analytical form of the temperature dependence of
derivative of thermal Hall conductivity near the band topological transition
point which I propose to be experimentally useful. In the fifth chapter, I have
investigated the emergence of Weyl triplons due to inter-layer DM-interaction
in a microscopic model of SrCu2(BO3)2, a widely studied frustrated quantum
magnet. I have shown that the thermal magnon Hall conductivity has a
quasi-linear dependence as a function of the magnetic field in a Weyl-triplon
region.",2103.00401v1
2021-03-15,Determining the range of magnetic interactions from the relations between magnon eigenvalues at high-symmetry k points,"Magnetic exchange interactions (MEIs) define networks of coupled magnetic
moments and lead to a surprisingly rich variety of their magnetic properties.
Typically MEIs can be estimated by fitting experimental results. But how many
MEIs need to be included in the fitting process for a material is not clear a
priori, which limits the quality of results obtained by these conventional
methods. In this paper, based on linear spin-wave theory but without performing
matrix diagonalization, we show that for a general quadratic spin Hamiltonian,
there is a simple relation between the Fourier transform of MEIs and the sum of
square of magnon energies (SSME). We further show that according to the
real-space distance range within which MEIs are considered relevant, one can
obtain the corresponding relationships between SSME in momentum space. We also
develop a theoretical tool for tabulating the rule about SSME. By directly
utilizing these characteristics and the experimental magnon energies at only a
few high-symmetry k points in the Brillouin zone, one can obtain strong
constraints about the range of exchange path beyond which MEIs can be safely
neglected. Our methodology is also general applicable for other Hamiltonian
with quadratic Fermi or Boson operators.",2103.08486v1
2021-04-14,Finite-element dynamic-matrix approach for spin-wave dispersions in magnonic waveguides with arbitrary cross section,"We present a numerical approach to efficiently calculate spin-wave
dispersions and spatial mode profiles in magnetic waveguides of arbitrarily
shaped cross section with any non-collinear equilibrium magnetization which is
translationally invariant along the waveguide. Our method is based on the
propagating-wave dynamic-matrix approach by Henry et al. and extends it to
arbitrary cross sections using a finite-element method. We solve the linearized
equation of motion of the magnetization only in a single waveguide cross
section which drastically reduces computational effort compared to common
three-dimensional micromagnetic simulations. In order to numerically obtain the
dipolar potential of individual spin-wave modes, we present a plane-wave
version of the hybrid finite-element/boundary-element method by Frekdin and
Koehler which, for the first time, we extend to a modified version of the
Poisson equation. Our method is applied to several important examples of
magnonic waveguides including systems with surface curvature, such as magnetic
nanotubes, where the curvature leads to an asymmetric spin-wave dispersion. In
all cases, the validity of our approach is confirmed by other methods. Our
method is of particular interest for the study of curvature-induced or
magnetochiral effects on spin-wave transport but also serves as an efficient
tool to investigate standard magnonic problems.",2104.06943v1
2021-04-19,Magnetic coupling in Y$_3$Fe$_5$O$_{12}$/Gd$_3$Fe$_5$O$_{12}$ heterostructures,"Ferrimagnetic Y$_3$Fe$_5$O$_{12}$ (YIG) is the prototypical material for
studying magnonic properties due to its exceptionally low damping. By
substituting the yttrium with other rare earth elements that have a net
magnetic moment, we can introduce an additional spin degree of freedom. Here,
we study the magnetic coupling in epitaxial
Y$_3$Fe$_5$O$_{12}$/Gd$_3$Fe$_5$O$_{12}$ (YIG/GIG) heterostructures grown by
pulsed laser deposition. From bulk sensitive magnetometry and surface sensitive
spin Seebeck effect (SSE) and spin Hall magnetoresistance (SMR) measurements,
we determine the alignment of the heterostructure magnetization through
temperature and external magnetic field. The ferromagnetic coupling between the
Fe sublattices of YIG and GIG dominates the overall behavior of the
heterostructures. Due to the temperature dependent gadolinium moment, a
magnetic compensation point of the total bilayer system can be identified. This
compensation point shifts to lower temperatures with increasing thickness of
YIG due the parallel alignment of the iron moments. We show that we can control
the magnetic properties of the heterostructures by tuning the thickness of the
individual layers, opening up a large playground for magnonic devices based on
coupled magnetic insulators. These devices could potentially control the magnon
transport analogously to electron transport in giant magnetoresistive devices.",2104.09592v1
2021-06-18,Inelastic spin-wave beam scattering by edge-localized spin waves in ferromagnetic thin film,"Spin waves are promising chargeless information carriers for the future,
energetically efficient beyond-CMOS systems. Among many advantages there are
the ease of achieving nonlinearity, the variety of possible interactions, and
excitation types. Although the rapidly developing magnonic research has already
yielded impressive realizations, multi-mode nonlinear effects, particularly
with the propagating waves and their nanoscale realizations, are still an open
research problem. We study theoretically the dynamic interactions of the spin
waves confined to the edge of a thin ferromagnetic film with the spin-wave beam
incident at this edge. We found the inelastically scattered spin-wave beams at
frequencies increased and decreased by the frequency of the edge spin-wave
relative to the specularly reflected beam. We observed a strong dependence of
the angular shift of the inelastic scattered spin-wave beam on the edge-mode
frequency, which allowed us to propose a magnonic demultiplexing of the signal
encoded in spin waves propagating along the edge. Since dynamic magnetostatic
interactions, which are ubiquitous in the spin-wave dynamics, are decisive in
this process, this indicates the possibility of implementing the presented
effects, also in other configurations and their use in magnonic systems.",2106.09954v3
2021-06-24,Magnetic excitations of the field induced states in BaCo2(AsO4)2 probed by time-domain terahertz spectroscopy,"Searching for Kitaev quantum spin liquid (QSL) is a fascinating and
challenging problem. Much effort has been devoted to honeycomb lattice
candidates with strong spin-orbit coupling in 5d-electron iridates and
4delectron RuCl3. Recently, theoretical studies suggested that the 3d7 Co-based
honeycomb materials with high spin state S=3/2 and effective orbital angular
momentum L=1 could also be promising candidates of Kitaev QSL. One of the
candidates, BaCo2(AsO4)2, was revisited recently. The long range magnetic order
in BaCo2(AsO4)2 can be suppressed by very weak in-plane magnetic field,
suggesting its proximity to Kitaev QSL. Here we perform time domain terahertz
spectroscopy measurement to study the magnetic excitations on BaCo2(AsO4)2. We
observe different magnon excitations upon increasing external magnetic field.
In particular, the system is easily driven to a field-polarized paramagnetic
phase, after the long range magnetic order is suppressed by a weak field Hc 2.
The spectra beyond Hc2 are dominated by single magnon and two magnon
excitations without showing signature of QSL. We discuss the similarity and
difference of the excitation spectra between BaCo2(AsO4)2 and the widely
studied Kitaev QSL candidate RuCl3.",2106.13100v1
2021-09-02,Magnon heat transport in a two-dimensional Mott insulator,"Whether or not anomalies in the thermal conductivity in insulating cuprates
can be attributed to antiferromagnetic order and magnons in a 2D Mott insulator
remains an intriguing open question. To shed light on this issue, we
investigate the thermal conductivity $\kappa$ and its relationship with the
specific heat $c_v$ in the half-filled 2D single-band Hubbard model, using the
numerically exact determinant quantum Monte Carlo algorithm and maximum entropy
analytic continuation. At low temperatures where the charge degrees of freedom
are gapped-out and $c_v$ exhibits a clear magnon peak, we observe that thermal
conductivity $\kappa$ also tends to form a peak at similar temperatures.
Reducing temperature further produces a sharp upturn in $\kappa$, associated
with an increasing mean-free path. We identify this as the high-temperature
side of the anomalous peak in insulating cuprates, where the mean-free path
eventually is cut-off by other scattering effects, including phonons, disorder,
and physical size. Different scattering effects in our model are identified and
analyzed in the thermal diffusivity.",2109.01119v2
2021-12-10,"Observation and control of collective spin-wave mode-hybridisation in chevron arrays and square, staircase and brickwork artificial spin ices","Dipolar magnon-magnon coupling has long been predicted in nano-patterned
artificial spin systems. However, observation of such phenomena and related
collective spin-wave signatures have until recently proved elusive or limited
to low-power edge-modes which are difficult to measure experimentally. Here we
describe the requisite conditions for dipolar mode-hybridisation, how it may be
controlled, why it was not observed earlier and how strong coupling may occur
between nanomagnet bulk-modes. We experimentally investigate four
nano-patterned artificial spin system geometries: `chevron' arrays, `square',
`staircase' and `brickwork' artificial spin ices. We observe significant
dynamic dipolar-coupling in all systems with relative coupling strengths and
avoided-crossing gaps supported by micromagnetic-simulation results. We
demonstrate reconfigurable mode-hybridisation regimes in each system via
microstate control, and in doing so elucidate the underlying dynamics governing
dynamic dipolar-coupling with implications across reconfigurable magnonics. We
demonstrate that confinement of the bulk-modes via edge effects play a critical
role in dipolar hybridised-modes, and treating nanoislands as a coherently
precessing macro-spins or standing spin-waves are insufficient to capture
experimentally-observed coupling phenomena. Finally, we present a
parameter-space search detailing how coupling strength may be tuned via
nanofabrication-dimensions and material properties.",2112.05354v1
2021-12-10,Impact of the interfacial Dzyaloshinskii-Moriya interaction on the band structure of one-dimensional artificial magnonic crystals: a micromagnetic study,"We present the results of a systematic micromagnetic study of the effect of
the Dzyaloshinskii-Moriya interaction (DMI) on the spin wave band structure of
two one-dimensional magnonic crystals (MCs), both with the same periodicity
p=300 nm, but different implementation of the DMI modulation. In the first
system the artificial periodicity was achieved by modulating the interfacial
DMI constant D, while in the second system also the sample morphology was
modulated. Due to the folding property of the band structure in the dispersion
relations of the magnonic crystals it is possible to extend the sensitivity of
Brillouin light scattering towards weak DMI strength (D in the range from 0 to
0.5 mJ/m2), by measuring the frequency splitting of folded modes in high-order
artificial Brillouin zones, since the splitting increases almost linearly with
the band index. For relatively large values of the DMI (D in the range from 1.0
to 2.0 mJ/m2) the spin waves dispersion relations present flat modes for
positive wavevectors, separated by forbidden frequency gaps whose amplitude
depend on the value of D. These frequency gaps are more pronounced for the
sample with morphology modulation. The non-reciprocal, localised, spatial
profiles of these modes in both MCs are discussed with reference to spin waves
in plain films and in isolated stripes of the same thickness.",2112.05360v1
2021-12-11,Anisotropic long-range spin transport in canted antiferromagnetic orthoferrite YFeO$_3$,"In antiferromagnets, the efficient propagation of spin-waves has until now
only been observed in the insulating antiferromagnet hematite, where circularly
(or a superposition of pairs of linearly) polarized spin-waves propagate over
long distances. Here, we report long-distance spin-transport in the
antiferromagnetic orthoferrite YFeO$_3$, where a different transport mechanism
is enabled by the combined presence of the Dzyaloshinskii-Moriya interaction
and externally applied fields. The magnon decay length is shown to exceed
hundreds of nano-meters, in line with resonance measurements that highlight the
low magnetic damping. We observe a strong anisotropy in the magnon decay
lengths that we can attribute to the role of the magnon group velocity in the
propagation of spin-waves in antiferromagnets. This unique mode of transport
identified in YFeO$_3$ opens up the possibility of a large and technologically
relevant class of materials, i.e., canted antiferromagnets, for long-distance
spin transport.",2112.05947v1
2021-12-15,Decoupling of static and dynamic criticality in a driven Mott insulator,"Dynamically driven interacting quantum many-body systems have the potential
to exhibit properties that defy the laws of equilibrium statistical mechanics.
A widely studied model is the impulsively driven antiferromagnetic Mott
insulator, which is predicted to realize exotic transient phenomena including
dynamical phase transitions into thermally forbidden states and highly
non-thermal magnon distributions. However such far-from-equilibrium regimes,
where conventional time-dependent Ginzburg-Landau descriptions fail, are
experimentally challenging to prepare and to probe especially in solid state
systems. Here we use a combination of time-resolved second harmonic optical
polarimetry and coherent magnon spectroscopy to interrogate $n$-type
photo-doping induced ultrafast magnetic order parameter dynamics in the Mott
insulator Sr$_2$IrO$_4$. We uncover an unusual far-from-equilibrium critical
regime in which the divergences of the magnetic correlation length and
relaxation time are decoupled. This violation of conventional thermal critical
behavior arises from the interplay of photo-doping and non-thermal magnon
population induced demagnetization effects. Our findings, embodied in a
non-equilibrium ""phase diagram"", provide a blueprint for engineering the
out-of-equilibrium properties of quantum matter, with potential applications to
terahertz spintronics technologies.",2112.08397v1
2021-12-28,"Phonon, Electron, and Magnon Excitations in Antiferromagnetic L1$_{0}$-type MnPt","Antiferromagnetic L1$_{0}$-type MnPt is a material with relatively simple
crystal and magnetic structure, recently attracting interest due to its high
N{\'{e}}el temperature and wide usage as a pinning layer in magnetic devices.
While it is experimentally well characterized, the theoretical understanding is
much less developed, in part due to the challenging accuracy requirements
dictated by the small underlying energy scales that govern magnetic ordering in
antiferromagnetic metals. In this work, we use density functional theory, the
Korringa-Kohn-Rostoker formalism, and a Heisenberg model to establish a
comprehensive theoretical description of antiferromagnetic L1$_{0}$-type MnPt,
along with accuracy limits, by thoroughly comparing to available literature
data. Our simulations show that the contribution of the magnetic dipole
interaction to the magnetocrystalline anisotropy energy of $K_{1}$=1.07$\times
10^{6}$\,J/m$^3$ is comparable in magnitude to the spin-orbit contribution.
Using our result for the magnetic susceptibility of $5.25\times10^{-4}$, a
lowest magnon frequency of about 2.02\,THz is predicted, confirming THz spin
dynamics in this material. From our data for electron, phonon, and magnon
dispersion we compute the individual contributions to the total heat capacity
and show that the dominant term at or above 2\,K arises from phonons. From the
Landau-Lifshitz-Gilbert equation, we compute a N\'{e}el temperature of
990--1070 K. Finally, we quantify the magnitude of the magneto-optical Kerr
effect generated by applying an external magnetic field. Our results provide
insight into the underlying physics, which is critical for a deep understanding
of fundamental limits of the time scale of spin dynamics, stability of the
magnetic ordering, and the possibility of magneto-optical detection of
collective spin motion.",2112.13954v1
2022-03-22,"Towards lattice-gas description of low-temperature properties above the Haldane and cluster-based Haldane ground states of a mixed spin-(1,1/2) Heisenberg octahedral chain","The rich ground-state phase diagram of the mixed spin-(1,1/2) Heisenberg
octahedral chain was previously elaborated from effective mixed-spin Heisenberg
chains, which were derived by employing a local conservation of a total spin on
square plaquettes of an octahedral chain. Here we present a comprehensive
analysis of the thermodynamic properties of this model. In the highly
frustrated parameter region the lowest-energy eigenstates of the mixed-spin
Heisenberg octahedral chain belong to flat bands, which allow a precise
description of low-temperature magnetic properties within the localized-magnon
approach exploiting a classical lattice-gas model of hard-core monomers. The
present article provides a more comprehensive version of the localized-magnon
approach, which extends the range of its validity down to a less frustrated
parameter region involving the Haldane and cluster-based Haldane ground states.
A comparison between results of the developed localized-magnon theory and
accurate numerical methods such as full exact diagonalization and
finite-temperature Lanczos technique convincingly evidence that the
low-temperature magnetic properties above the Haldane and the cluster-based
Haldane ground states can be extracted from a classical lattice-gas model of
hard-core monomers and dimers, which is additionally supplemented by a
hard-core particle spanned over the whole lattice representing the gapped
Haldane phase.",2203.11569v1
2022-04-04,Measuring a population of spin waves from the electrical noise of an inductively coupled antenna,"We study how a population of spin waves can be characterized from the
analysis of the electrical microwave noise delivered by an inductive antenna
placed in its vicinity. The measurements are conducted on a synthetic
antiferromagnetic thin stripe covered by a micron-sized antenna that feeds a
spectrum analyser after amplification. The antenna noise contains two
contributions. The population of incoherent spin waves generates a fluctuating
field that is sensed by the antenna: this is the ""magnon noise"". The antenna
noise also contains the contribution of the electronic fluctuations: the
Johnson-Nyquist noise. The latter depends on all impedances within the
measurement circuit, which includes the antenna self-inductance. As a result,
the electronic noise contains information about the magnetic susceptibility of
the stripe, though it does not inform on the absolute amplitude of the magnetic
fluctuations. For micrometer-sized systems at thermal equilibrium, the
electronic noise dominates and the pure magnon noise cannot be determined. If
in contrast the spinwave bath is not at thermal equilibrium with the
measurement circuit, and if the spin wave population can be changed then one
could measure a mode-resolved effective magnon temperature provided specific
precautions are implemented.",2204.01427v2
2022-05-24,Emission of coherent THz magnons in an antiferromagnetic insulator triggered by ultrafast spin-phonon interactions,"Antiferromagnetic materials have been proposed as new types of narrowband THz
spintronic devices owing to their ultrafast spin dynamics. Manipulating
coherently their spin dynamics, however, remains a key challenge that is
envisioned to be accomplished by spin-orbit torques or direct optical
excitations. Here, we demonstrate the combined generation of broadband THz
(incoherent) magnons and narrowband (coherent) magnons at 1 THz in low damping
thin films of NiO/Pt. We evidence, experimentally and through modelling, two
excitation processes of magnetization dynamics in NiO, an off-resonant
instantaneous optical spin torque and a strain-wave-induced THz torque induced
by ultrafast Pt excitation. Both phenomena lead to the emission of a THz signal
through the inverse spin Hall effect in the adjacent heavy metal layer. We
unravel the characteristic timescales of the two excitation processes found to
be < 50 fs and > 300 fs, respectively, and thus open new routes towards the
development of fast opto-spintronic devices based on antiferromagnetic
materials.",2205.11965v1
2022-07-08,Measuring the magnon-photon coupling in shaped ferromagnets: tuning of the resonance frequency,"Cavity photons and ferromagnetic spins excitations can exchange information
coherently in hybrid architectures, at speeds set by their mutual coupling
strength. Speed enhancement is usually achieved by optimizing the geometry of
the electromagnetic cavity. Here we show that the geometry of the ferromagnet
plays also an important role, by setting the fundamental frequency of the
magnonic resonator. Using focused ion beam patterning, we vary the aspect ratio
of different Permalloy samples reaching operation frequencies above 10 GHz
while working at low external magnetic fields. Additionally, we perform broad
band ferromagnetic resonance measurements and cavity experiments that
demonstrate that the magnon-photon coupling strength can be estimated using
either open transmission lines or resonant cavities, yielding very good
agreement. Finally, we describe a simple theoretical framework based on
electromagnetic and micromagnetic simulations that successfully accounts for
the experimental results. This approach can be used to design hybrid quantum
systems exploiting whatsoever magnetostatic mode excited in ferromagnets of
arbitrary size and shape and to tune their operation conditions.",2207.03829v3
2022-09-09,Thermal Hall conductivity with sign change in the Heisenberg-Kitaev kagome magnet,"The Heisenberg-Kitaev (HK) model on various lattices have attracted a lot of
attention because they may lead to exotic states such as quantum spin liquid
and topological orders. The rare-earth-based kagome lattice (KL) compounds
$\rm{Mg_2RE_3Sb_3O_{14}}$ $\rm{(RE=Gd, Er)}$ and $\rm{(RE=Nd)}$ have ${\bf
q=0}$, $120^\circ$ order and canted ferromagnetic (CFM) order, respectively.
Interestingly, the HK model on the KL has the same ground state long-range
orders. In the theoretical phase diagram, the CFM phase resides in a continuous
parameter region and there is no phase change across special parameter points,
such as the Kitaev ferromagnetic (KFM) point, the ferromagnetic (FM) point and
its dual FM point. However, a ground state property cannot distinguish a system
with or without topological nontrivial excitations and related phase
transitions. Here, we study the topological magnon excitations and related
thermal Hall conductivity in the HK model on the KL with CFM order. The CFM
phase can be divided into two regions related by the Klein duality, with the
self dual KFM point as their boundary. We find that the scalar spin chirality
which is intrinsic in the CFM order changes sign across the KFM point. This
leads to the opposite Chern numbers of corresponding magnon bands in the two
regions, and also the sign change of the magnon thermal Hall conductivity.",2209.04133v1
2022-09-12,Unidirectional propagation of spin waves excited by femtosecond laser pulses in a planar waveguide,"Low-energy magnonic logic circuits are an actively developing field of modern
magnetism. The potential benefits of magnonics for data processing are vitally
dependent on units based on non-reciprocal propagation of spin waves in analogy
to semiconductor diodes and transistors in electronics. In this article, we
suggest the approach to realize non-reciprocal propagation of spin waves in a
ferromagnetic metallic waveguide by exciting them with femtosecond laser pulse.
Using micromagnetic modeling, we show that the combination of an external
magnetic field and the position of the excitation laser spot across the
waveguide leads to unidirectional propagation of the excited spin-wave packet.
The results are crucial for the design of hybrid magnonic-photonic circuits in
future generations of data processing devices.",2209.05611v3
2022-11-21,Excitonic instability towards a Potts-nematic quantum paramagnet,"Magnetic frustration can lead to peculiar magnetic orderings that break a
discrete symmetry of the lattice in addition to the fundamental magnetic
symmetries (i.e., spin rotation invariance and time-reversal symmetry). In this
work, we focus on frustrated quantum magnets and study the nature of the
quantum phase transition between a paramagnet and a magnetically ordered state
with broken threefold ($Z_3$) crystal rotation symmetry. We predict the
transition to happen in two stages, giving rise to an intermediate nematic
phase in which rotation symmetry is broken but the system remains magnetically
disordered. The nematic transition is described by the three-state Potts model.
This prediction is based on an analysis of bound states formed from two-magnon
excitations in the paramagnet, which become gapless while single-magnon
excitations remain gapped. By considering three different lattice models, we
demonstrate a generic instability towards two-magnon bound state formation in
the Potts-nematic nematic channel. We present both numerical results and a
general analytical perturbative formula for the bound state binding energy
similar to BCS theory. We further discuss a number of different materials that
realize key features of the model considered, and thus provide promising venues
for possible experimental observation.",2211.11739v1
2023-01-07,High-efficiency entanglement of microwave fields in cavity opto-magnomechanical systems,"We demonstrate a scheme to realize high-efficiency entanglement of two
microwave fields in a dual opto-magnomechanical system. The magnon mode
simultaneously couples with the microwave cavity mode and phonon mode via
magnetic dipole interaction and magnetostrictive interaction, respectively.
Meanwhile, the phonon mode couples with the optical cavity mode via radiation
pressure. Each magnon mode and optical cavity mode adopts a strong red detuning
driving field to activate the beam splitter interaction. Therefore, the
entangled state generated by the injected two-mode squeezed light in optical
cavities can be eventually transferred into two microwave cavities. A
stationary entanglement E_{a_{1}a_{2}}=0.54 is obtained when the input two-mode
squeezed optical field has a squeezing parameter r=1. The entanglement
E_{a_{1}a_{2}} increases as the squeezing parameter r increases, and it shows
the flexible tunability of the system. Meanwhile, the entanglement survives up
to an environmental temperature about 385 mK, which shows high robustness of
the scheme. The proposed scheme provides a new mechanism to generate entangled
microwave fields via magnons, which enables the degree of the prepared
microwave entanglement to a more massive scale. Our result is useful for
applications which require high entanglement of microwave fields like quantum
radar, quantum navigation, quantum teleportation, quantum wireless fidelity
(Wi-Fi) network, etc.",2301.02808v2
2023-01-18,"Magnons, Phonons, and Thermal Hall Effect in Candidate Kitaev Magnet $α$-RuCl$_3$","We study the nature of the debated thermal Hall effect in the candidate
Kitaev material $\alpha$-RuCl$_3$. Without assuming the existence of a gapped
spin liquid, we show that a realistic minimal spin model in the canted zigzag
phase suffices, at the level of linear spin-wave theory, to qualitatively
explain the observed temperature and magnetic field dependence of the
non-quantized thermal Hall conductivity $\kappa_{xy}$, with its origin lying in
the Berry curvature of the magnon bands. The magnitude of the effect is however
too small compared to the measurement by Czajka et al. [Nat. Mater. 22, 36-41
(2023)], even after scanning a broad range of model parameters so as to
maximize $\kappa_{xy}/T$. Recent experiments suggest that phonons play an
important role, which we show couple to the spins, endowing phonons with
chirality. The resulting intrinsic contribution, from both magnons and phonons,
is however still insufficient to explain the observed magnitude of the Hall
signal. After careful analysis of the extrinsic phonon mechanisms, we use the
recent experimental data on thermal transport in $\alpha$-RuCl$_3$ by
Lefran\c{c}ois et al. [Phys. Rev. X 12, 021025 (2022)] to determine the
phenomenological ratio of the extrinsic and intrinsic contributions $\eta\equiv
\kappa_{xy}^{E}/\kappa_{xy}^{I}$. We find $\eta=1.2\pm 0.5$, which when
combined with our computed intrinsic value, explains quantitavely both the
magnitude and detailed temperature dependence of the experimental thermal Hall
effect in $\alpha$-RuCl$_3$.",2301.07401v2
2023-02-02,Ultrastrong Magnon-Photon Coupling Achieved by Magnetic Films in Contact with Superconducting Resonators,"Coherent coupling between spin wave excitations (magnons) and microwave
photons in a cavity may disclose new paths to unconventional phenomena as well
as for novel applications. Here, we present a systematic investigation on YIG
(Yttrium Iron Garnet) films on top of coplanar waveguide resonators made of
superconducting YBCO. We first show that spin wave excitations with frequency
higher than the Kittel mode can be excited by putting in direct contact a
5~$\mu$m thick YIG film with the YBCO coplanar resonator (cavity frequency
$\omega_c/2 \pi = 8.65$~GHz). With this configuration, we obtain very large
values of the collective coupling strength $\lambda/2 \pi \approx 2$~GHz and
cooperativity $C=5 \times 10^4$. Transmission spectra are analyzed by a
modified Hopfield model for which we provide an exact solution that allows us
to well reproduce spectra by introducing a limited number of free parameters.
It turns out that the coupling of the dominant magnon mode with photons exceeds
0.2 times the cavity frequency, thus demonstrating the achievement of the
ultrastrong coupling regime with this architecture. Our analysis also shows a
vanishing contribution of the diamagnetic term which is a peculiarity of pure
spin systems.",2302.00804v2
2023-04-25,Bose-Einstein condensations of magnons in quantum magnets with spin-orbit coupling in a Zeeman field,"We study the response of a quantum magnet with spin-orbit coupling (SOC) to a
Zeeman field by constructing effective actions and performing Renormalization
Group (RG) analysis. There are several novel classes of quantum phase
transitions at a low $ h_{c1} $ and an upper critical field $ h_{c2} $ driven
by magnon condensations at commensurate (C-) or in-commensurate (IC-) momenta $
0 < k_0 < \pi $. The intermediate IC- Skyrmion crystal (IC-SkX) phase is
controlled by a line of fixed points in the RG flows labeled by $ k_0 $. We
derive the relations between the quantum spin and the order parameters of the
effective actions which determine the spin-orbital structures of the IC-SkX
phase. We also analyze the operator contents near $ h_{c1} $ and $ h_{c2} $
which determine the exotic excitation spectra inside the IC-SkX. The intrinsic
differences between the magnon condensations at the C- and IC- momenta are
explored. The two critical fields $ h_{c1} < h_{c2} $ and the intermediate
IC-SkX phase could be a generic feature to any quantum magnets with SOC in a
Zeeman field. Experimental implications to some materials or cold atom systems
with SOC in a Zeeman field are presented.",2304.12612v1
2023-04-26,Bond-dependent anisotropy and magnon decay in cobalt-based Kitaev triangular antiferromagnet,"The Kitaev model, a honeycomb network of spins with bond-dependent
anisotropic interactions, is a rare example of having a quantum spin liquid
ground state. Although most Kitaev model candidate materials eventually order
magnetically due to inevitable non-Kitaev terms, their bond-dependent
anisotropy manifests in unusual spin dynamics. It has recently been suggested
that bond-dependent anisotropy can stabilise novel magnetic phases and exotic
spin dynamics on the geometrically frustrated triangular lattice. However, few
materials have been identified with simultaneous geometric frustration and
bond-dependent anisotropy. Here, we report a frustrated triangular lattice with
bond-dependent anisotropy in the cobalt-based triangular van der Waals
antiferromagnet CoI2. Its momentum and energy-resolved spin dynamics exhibit
substantial magnon breakdown and complex level repulsion, as measured by
inelastic neutron scattering. A thorough examination of excitations in both the
paramagnetic and magnetically ordered states reveals that the bond-dependent
anisotropy is the origin of the spiral order and the magnon breakdown found in
CoI2. Our result paves the way toward a new research direction for the Kitaev
model with geometrical frustration.",2304.13236v3
2023-04-29,Observation of linear magnetoelectric effect in a Dirac magnon antiferromagnet Cu$_3$TeO$_6$,"Cu$_3$TeO$_6$, a three-dimensional antiferromagnet forming a unique spin-web
lattice of spin-1/2 Cu2+ ions below the Neel temperature T$_N$ = 62 K, has
recently been found to exhibit topological Dirac or nodal magnon dispersion. In
this study, we report the discovery of the linear magnetoelectric (ME) effects
in Cu$_3$TeO$_6$ below TN. Our pyroelectric current measurements at a constant
magnetic field (H) reveal a linear increase of electric polarization (P) with H
for both P // H and P $\perp$ H configurations; a maximum P // [110] = 20
$\mu$C/m$^2$ is obtained at u0H // [1-10] = 14 T, corresponding to a linear ME
coefficient 1.8 ps/m. Magnetic point group analysis and Monte-Carlo simulations
confirm that finite linear ME coefficients are allowed in the off-diagonal and
diagonal ME tensor components, consistent with the magnetic point group of
$\bar{3}'$. As the parity-time symmetry can be broken in the presence of H or
electric field E in the linear ME materials, we envisage that Cu$_3$TeO$_6$
should exhibit a H- or E-induced transformation in the topological magnon
dispersion from a Dirac point/nodal line type into two Weyl point types.",2305.00215v1
2023-08-10,Ferromagnetic monolayer with interfacial Dzyaloshinskii-Moriya interaction: magnon spectrum and effect of quenched disorder,"We discuss theoretically a ferromagnetic monolayer with an interfacial
Dzyaloshinskii-Moriya interaction (iDMI) and a small axial anisotropy. It is
shown that the system has a long-period cycloid magnetic order slightly
distorted by the anisotropy whose modulation vector $\bf k$ can have several
orientations. We find that due to iDMI-induced umklapp terms in the
Hamiltonian, the spectrum of long-wavelength magnons is essentially
anisotropic: it is linear and quadratic for momenta directed along and
perpendicular to $\bf k$, respectively. Due to such a quasi-1D spectrum, the
temperature correction to the mean spin value has a power-law singularity
hampering the magnetic ordering at $T\ne0$. We demonstrate that the umklapps
lead to a peculiar band structure of the magnon spectrum similar to electronic
bands in solids. We discuss also the effect of vacancies and defect bonds on
system properties at $T=0$. Owing to the quasi-1D character of the spectrum,
the distortion of the cycloid structure by a single defect bond is described by
the field of 1D electric dipole. As a result, even infinitesimal concentrations
of defects $c$ destroy the long-range order and establish a short-range order
whose correlation length shows a power-law dependence on $1/c$. Our findings
should also be applicable to ultra-thin films with strong enough iDMI.",2308.05283v1
2023-08-24,Dynamics of quantum spin-nematics: Comparisons with canted antiferromagnets,"The identification of spin-nematic states is challenging due to the absence
of Bragg peaks. However, the study of dynamical physical quantities provides a
promising avenue for characterizing these states. In this study, we investigate
the dynamical properties of spin-nematic states in three-dimensional quantum
spin systems in a magnetic field, using a two-component boson theory that
incorporates magnons and bi-magnons. Our particular focus lies on the dynamical
spin structure factor at zero temperature and the nuclear magnetic resonance
(NMR) relaxation rate at finite temperatures. Our findings reveal that the
dynamical structure factor does not exhibit any diverging singularity across
momentum and frequency while providing valuable information about the form
factor of bi-magnon states and the underlying structure of spin-nematic order.
Furthermore, we find a temperature dependence in the NMR relaxation rate
proportional to $T^3$ at low temperatures, similar to canted antiferromagnets.
A clear distinction arises as there is no critical divergence of the NMR
relaxation rate at the spin-nematic transition temperature. Our theoretical
framework provides a comprehensive understanding of the excitation spectrum and
the dynamical properties of spin-nematic states, covering arbitrary spin values
$S$ and encompassing site and bond nematic orders. Additionally, we apply the
same methodology to analyze these dynamical quantities in a canted
antiferromagnetic state and compare the results with those in the spin-nematic
states.",2308.12569v2
2023-09-06,Universal spin wavepacket transport in van der Waals antiferromagnets,"Antiferromagnets (AFMs) are promising platforms for the transmission of
quantum information via magnons (the quanta of spin waves), offering advantages
over ferromagnets with regard to dissipation, speed of response, and immunity
to external fields. Recently, it was shown that in the insulating van der Waals
(vdW) semiconductor, CrSBr, strong spin-exciton coupling enables readout of
magnon density and propagation using photons of visible light. This exciting
observation came with a puzzle: photogenerated magnons were observed to
propagate 10$^3$ times faster than the velocity inferred from neutron
scattering, leading to a conjecture that spin wavepackets are carried along by
coupling to much faster elastic modes. Here we show, through a combination of
theory and experiment, that the propagation mechanism is, instead, coupling
within the magnetic degrees of freedom through long range dipole-dipole
coupling. This mechanism is an inevitable consequence of Maxwell's equations,
and as such, will dominate the propagation of spin at long wavelengths in the
entire class of vdW magnets currently under intense investigation. Moreover,
identifying the mechanism of spin propagation provides a set of optimization
rules, as well as caveats, that are essential for any future applications of
these promising systems.",2309.03278v1
2023-10-06,Antiferromagnetic magnonic charge current generation via ultrafast optical excitation,"N\'eel spin-orbit torque allows a charge current pulse to efficiently
manipulate the N\'eel vector in antiferromagnets, which offers a unique
opportunity for ultrahigh density information storage with high speed. However,
the reciprocal process of N\'eel spin-orbit torque, the generation of ultrafast
charge current in antiferromagnets has not been demonstrated. Here, we report
the experimental observation of charge current generation in antiferromagnetic
metallic Mn2Au thin films using ultrafast optical excitation. The ultrafast
laser pulse excites antiferromagnetic magnons, resulting in instantaneous
non-equilibrium spin polarization at the antiferromagnetic spin sublattices
with broken spatial symmetry. Then the charge current is generated directly via
spin-orbit fields at the two sublattices, which is termed as the reciprocal
phenomenon of N\'eel spin-orbit torque, and the associated THz emission can be
detected at room temperature. Besides the fundamental significance on the
Onsager reciprocity, the observed magnonic charge current generation in
antiferromagnet would advance the development of antiferromagnetic THz emitter.",2310.03987v1
2023-10-09,Mapping of Spin-Wave Transport in Thulium Iron Garnet Thin Films Using Diamond Quantum Microscopy,"Spin waves, collective dynamic magnetic excitations, offer crucial insights
into magnetic material properties. Rare-earth iron garnets offer an ideal
spin-wave (SW) platform with long propagation length, short wavelength,
gigahertz frequency, and applicability to magnon spintronic platforms. Of
particular interest, thulium iron garnet (TmIG) has attracted a huge interest
recently due to its successful growth down to a few nanometers, observed
topological Hall effect and spin orbit torque-induced switching effects.
However, there is no direct spatial measurement of its SW properties. This work
uses diamond nitrogen vacancy (NV) magnetometry in combination with SW
electrical transmission spectroscopy to study SW transport properties in TmIG
thin films. NV magnetometry allows probing spin waves at the sub-micrometer
scale, seen by the amplification of the local microwave magnetic field due to
the coupling of NV spin qubits with the stray magnetic field produced by the
microwave-excited spin waves. By monitoring the NV spin resonances, the SW
properties in TmIG thin films are measured as function of the applied magnetic
field, including their amplitude, decay length (~ 50 um), and wavelength (0.8 -
2 um). These results pave the way for studying spin qubit-magnon interactions
in rare-earth magnetic insulators, relevant to quantum magnonics applications.",2310.06188v4
2023-10-23,Microscopic theory of spin Seebeck effect in antiferromagnets,"We develop a microscopic theory for the spin Seebeck effect (SSE) in N\'eel
and canted phases of antiferromagnetic insulators. We calculate the DC spin
current tunneling from the antiferromagnet to an attached metal, incorporating
the spin-wave theory and the non-equilibrium Green's function approach. Our
result shows a sign change of the spin current at the spin-flop phase
transition between N\'eel and canted phases, which is in agreement with a
recent experiment for the SSE of $\rm Cr_2O_3$ in a semi-quantitative level.
The sign change can be interpreted from the argument based on the density of
states of up- and down-spin magnons, which is related to the polarized-neutron
scattering spectra. The theory also demonstrates that the spin current in the
N\'eel phase is governed by the magnon correlation, while that in the canted
phase consists of two parts: Contributions from not only the magnon dynamics
but also the static transverse magnetization. This result leads to a prediction
that at sufficiently low temperatures, the spin current non-monotonically
changes as a function of magnetic field in the canted phase. Towards a more
unified understanding of the SSE in antiferromagnets, we further discuss some
missing links of theories of SSE: Interface properties, effects of the
transverse spin moment in the canted phase, the spin-orbit coupling in the
metal, etc. Finally, we compare the SSE of antiferromagnets with those of
different magnetic phases such as ferromagnets, ferrimagnets, an
one-dimensional spin liquid, a spin-nematic liquid, and a spin-Peierls
(dimerized) phase.",2310.15292v2
2023-11-22,Quantum spin dynamics due to strong Kitaev interactions in the triangular-lattice antiferromagnet CsCeSe$_2$,"The extraordinary properties of the Kitaev model have motivated an intense
search for new physics in materials that combine geometrical and bond
frustration. In this work, we employ inelastic neutron scattering, spin wave
theory, and exact diagonalization to study the spin dynamics in the perfect
triangular-lattice antiferromagnet (TLAF) CsCeSe$_2$. This material orders into
a stripe phase, which is demonstrated to arise as a consequence of the
off-diagonal bond-dependent terms in the spin Hamiltonian. By studying the spin
dynamics at intermediate fields, we identify an interaction between the
single-magnon state and the two-magnon continuum that causes decay of coherent
magnon excitations, level repulsion, and transfer of spectral weight to the
continuum that are controlled by the strength of the magnetic field. Our
results provide a microscopic mechanism for the stabilization of the stripe
phase in TLAF and show how complex many-body physics can be present in the spin
dynamics in a magnet with strong Kitaev coupling even in an ordered ground
state.",2311.13146v2
2023-11-28,Quantum Fluctuations in a Weakly Correlated Mott Insulator,"Quantum fluctuations in low-dimensional systems and near quantum phase
transitions have significant influences on material properties. Yet, it is
difficult to experimentally gauge the strength and importance of quantum
fluctuations. Here we provide a resonant inelastic x-ray scattering study of
magnon excitations in Mott insulating cuprates. From the thin film of
SrCuO$_2$, single- and bi-magnon dispersions are derived. Using an effective
Heisenberg Hamiltonian generated from the Hubbard model, we show that the
single magnon dispersion is only described satisfactorily when including
significant renormalization stemming from quantum fluctuations. Comparative
results on La$_2$CuO$_4$ indicate that quantum fluctuations are much stronger
in SrCuO$_2$ suggesting closer proximity to a magnetic quantum critical point.
Monte Carlo calculations suggest an exotic incommensurate magnetic order as the
ground state that competes with the antiferromagnetic N\'eel order. Our results
indicate that SrCuO$_2$ -- due to strong quantum fluctuations -- is a unique
starting point for the exploration of novel magnetic ground states.",2311.16553v1
2023-11-29,Nonlinear erasing of propagating spin-wave pulses in thin-film Ga:YIG,"Nonlinear phenomena are key for magnon-based information processing, but the
nonlinear interaction between two spin-wave signals requires their
spatio-temporal overlap which can be challenging for directional processing
devices. Our study focuses on a gallium-substituted yttrium iron garnet film,
which exhibits an exchange-dominated dispersion relation and thus provides a
particularly broad range of group velocities compared to pure YIG. Using time-
and space-resolved Brillouin light scattering spectroscopy, we demonstrate the
excitation of time-separated spin-wave pulses at different frequencies from the
same source, where the delayed pulse catches up with the previously excited
pulse and outruns it due to its higher group velocity. By varying the
excitation power of the faster pulse, the outcome can be finely tuned from a
linear superposition to a nonlinear interaction of both pulses, resulting in a
full attenuation of the slower pulse. Therefore, our findings demonstrate the
all-magnonic erasing process of a propagating magnonic signal, which enables
the realization of complex temporal logic operations with potential
application, e.g., in inhibitory neuromorphic functionalities.",2311.17821v2
2024-02-24,Steerable current-driven emission of spin waves in magnetic vortex pairs,"The efficient excitation of spin waves is a key challenge in the realization
of magnonic devices. We demonstrate the current-driven generation of spin waves
in antiferromagnetically coupled magnetic vortices. We employ time-resolved
scanning transmission X-ray microscopy (TR-STXM) to directly image the emission
of spin waves upon the application of an alternating current flowing directly
through the magnetic stack. Micromagnetic simulations allow us to identify the
origin of the excitation to be the current-driven Oersted field, which in the
present system proves to be orders of magnitude more efficient than the
commonly used excitation via stripline antennas. Our numerical studies also
reveal that the spin-transfer torque can lead to the emission of spin waves as
well, yet only at much higher current amplitudes. By using magnetostrictive
materials, we futhermore demonstrate that the direction of the magnon
propagation can be steered by increasing the excitation amplitude, which
modifies the underlying magnetization profile through an additional anisotropy
in the magnetic layers. The demonstrated methods allow for the efficient and
tunable excitation of spin waves, marking a significant advance in the
generation and control of spin waves in magnonic devices.",2402.15831v1
2024-03-14,"Stoner ferromagnetism, correlated metal and thermoelectricity in partially flat-band materials","Recent discovery of correlated electronic phases in twisted heterostructures
raised a surge of interests in studying models and materials with flat bands
where the electronic excitations are nearly dispersionless in momentum space.
As such, the kinetic energy is quenched and the correlations are enhanced,
giving rise to a plethora of unusual magnetic, superconducting and transport
behaviors. Finding materials whose energy bands are completely flat is rather
challenging, yet those whose dispersion is flat only in a portion of the
momentum space might be more accessible in material search. In this work, we
propose a partially flat-band system on a square lattice. Using the Hubbard
model, it is demonstrated that the suppression of the electronic kinetic energy
in the flat portion of the band dispersion drives the system to Stoner
ferromagnetism even at very weak interactions, i.e., much smaller than the
bandwidth, with significantly enhanced Curie temperature. While the low-energy
magnon modes are well defined collective excitations, flat magnon bands can be
observed at high energies. We show that the strong interaction leads to
reduction of the flat portion of the magnon band. However, tuning the chemical
potential at a strong interaction regime may lead to spin density wave at
finite wave vectors. Then, focusing on the non-magnetic correlated phase and
using dynamical mean-field theory, we demonstrate the appearance of a flat-band
induced sharp peak in the density of states in addition to the
correlation-induced Mott bands. Furthermore, the large seebeck coefficient and
the figure of merit of the proposed partially flat-band model, compared to
symmetric regular band models, put them in the category of efficient
thermoelectric materials.",2403.09147v1
1993-06-16,Large-Scale Numerical Evidence for Bose Condensation in the S=1 Antiferromagnetic Chain in a Strong Field,"Using the recently proposed density matrix renormalization group technique we
show that the magnons in the S=1 antiferromagnetic Heisenberg chain effectively
behaves as bosons that condense at a critical field h_c.",9306036v1
1997-11-10,Numerical Calculations of the B1g Raman Spectrum of the Two-Dimensional Heisenberg Model,"The B1g Raman spectrum of the two-dimensional S=1/2 Heisenberg model is
discussed within Loudon-Fleury theory at both zero and finite temperature. The
exact T=0 spectrum for lattices with up to 6*6 sites is computed using Lanczos
exact diagonalization. A quantum Monte Carlo (QMC) method is used to calculate
the corresponding imaginary-time correlation function and its first two
derivatives for lattices with up to 16*16 spins. The imaginary-time data is
continued to real frequency using the maximum-entropy method, as well as a fit
based on spinwave theory. The numerical results are compared with spinwave
calculations for finite lattices. There is a surprisingly large change in the
exact spectrum going from 4*4 to 6*6 sites. In the former case there is a
single dominant two-magnon peak at frequency w/J appr. 3.0, whereas in the
latter case there are two approximately equal-sized peaks at w/J appr. 2.7 and
3.9. This is in good qualitative agreement with the spinwave calculations
including two-magnon processes on the same lattices. Both the Lanczos and the
QMC results indicate that the actual infinite-size two-magnon profile is
broader than the narrow peak obtained in spinwave theory, but the positions of
the maxima agree to within a few percent. The higher-order contributions
present in the numerical results are merged with the two-magnon profile and
extend up to frequencies w/J appr. 7. The first three frequency cumulants of
the spectrum are in excellent agreement with results previously obtained from a
series expansion around the Ising limit. Typical experimental B1g$ spectra for
La2CuO4 are only slightly broader than what we obtain here. The exchange
constant extracted from the peak position is J appr. 1400K, in good agreement
with values obtained from neutron scattering and NMR experiments.",9711083v1
1998-04-01,The two-leg t-J ladder: a spin liquid generated by Gutzwiller projection of magnetic bands,"The ground state of the two-leg Heisenberg ladder is identified as an RVB
type spin liquid, which is generated by Gutzwiller projection of tight-binding
bands with flux pi per plaquet. Explicit trial wave functions for the magnon
and hole excitations are formulated in terms of spinons and holons.",9804002v3
1999-03-09,Skyrmion dynamics in quantum Hall ferromagnets,"Exploring a classical solution of the non-linear sigma model for a quantum
Hall ferromagnet, a skyrmion-magnon effective hamiltonian is obtained via the
collective coordinates method. Using the Feynman-Vernon functional integral
formalism for this model we find the temperature dependent transport
coefficients which characterize a single skyrmion dynamics.",9903143v1
1999-08-26,Spin-1/2 periodic nonuniform XX chains and the spin-Peierls instability,"Using continued fractions we obtained the exact result for the density of
magnon states of the regularly alternating spin-1/2 XX chain with
Dzyaloshinskii-Moriya interaction. We examined the stability of the magnetic
chain with respect to the spin-Peierls dimerization.",9908385v1
2000-01-03,Spin-Peierls instability in the spin-1/2 transverse XX chain with Dzyaloshinskii-Moriya interaction,"We calculate exactly the density of magnon states of the regularly
alternating spin-1/2 XX chain with Dzyaloshinskii-Moriya interaction. The
obtained results permit us to examine the stability of the chain with respect
to spin-Peierls dimerization. We found that depending on the dependences of
Dzyaloshinskii-Moriya interaction on distortion amplitude it may act either in
favour of the dimerization or against the dimerization.",0001014v1
2002-03-13,Linear energy bounds for Heisenberg spin systems,"Recently obtained results on linear energy bounds are generalized to
arbitrary spin quantum numbers and coupling schemes. Thereby the class of
so-called independent magnon states, for which the relative ground-state
property can be rigorously established, is considerably enlarged. We still
require that the matrix of exchange parameters has constant row sums, but this
can be achieved by means of a suitable gauge and need not be considered as a
physical restriction.",0203270v1
2005-06-02,N-leg integer-spin ladders and tubes in commensurate external fields -Nonlinear sigma model approach-,"We investigate the low-energy properties of N-leg integer-spin ladders and
tubes with an antiferromagnetic (AF) intrachain coupling. In the odd-leg tubes,
the AF rung coupling causes the frustration. To treat all ladders and tubes
systematically, we apply S\'en\'echal's method [Phys. Rev. B 52, 15319 (1995)],
based on the nonlinear sigma model. This strategy is valid in the weak
interchain (rung) coupling regime. We show that all frustrated tubes possess
six-fold degenerate spin-1 magnon bands, as the lowest excitations, while other
ladders and tubes have a standard triply degenerate bands. We also consider
effects of four kinds of Zeeman terms: uniform, staggered only along the rung,
only along the chain, or both directions. The above prediction of the no-field
case implies that a sufficiently strong uniform field yields a two-component
Tomonaga-Luttinger liquid (TLL) due to the condensation of doubly degenerate
lowest magnons in frustrated tubes. In contrast, the field induces a standard
one-component TLL in all other systems. This is supported by symmetry and
bosonization arguments based on the Ginzburg-Landau theory. The bosonization
also suggests that the two-component TLL vanishes and a one-component TLL
appears, when the uniform field becomes larger for the second lowest magnon
bands to touch the zero-energy line. This transition could be observed as a
cusp singularity in the magnetization process. All the analyses for the systems
with a staggered Zeeman term suggest that the emergence of the doubly
degenerate transverse magnons and the single longitudinal one is universal for
the one-dimensional AF spin systems with weak staggered field.",0506049v3
2007-01-24,Critical properties and Bose Einstein Condensation in dimer spin systems,"We analyze the spin relaxation time $1/T_1$ for a system made of weakly
coupled one dimensional ladders.This system allows to probe the dimensional
crossover between a Luttinger liquid and a Bose-Einstein condensateof magnons.
We obtain the temperature dependence of $1/T_1$ in the various dimensional
regimes, and discuss the experimental consequences.",0701590v1
1999-03-02,Remarks on Goldstone bosons and hard thermal loops,"The hard thermal loop effective action for Goldstone bosons is deduced by
symmetry arguments from the corresponding result for gauge bosons. Pseudoscalar
mesons in Chromodynamics and magnons in an antiferromagnet are discussed as
special cases, including the hard thermal loop contribution to their
scattering.",9903232v1
2002-01-15,Magnon-mediated NMR quantum gates in a 1-D antiferromagnet,"We propose a method of controlling a quantum logic gate in a solid-state NMR
quantum computer. A switchable inter-qubit coupling can be generated by using
the longitudinal component of the Suhl-Nakamura interaction induced by a local
singlet-triplet excitation in a 1-D antiferromagnet with a spin gap.",0201060v1
2005-05-17,Qubit entanglement in multimagnon states,"The qubit entanglement induced by quasiparticle excitations in the Heisenberg
spin chain and its relationship to the Bethe Ansatz structure of the eigenmodes
is studied. A phenomenon called entanglement quenching, which suppresses
eigenstate entanglement, is described and shown to be mediated by Goldstone
magnons. Scattering states are characterized by short-range entanglement, and
never exhibit entanglement at the longest range. In contrast, bound states have
complex long-range entanglement structures.",0505124v1
2010-07-30,Energy relaxation in the Gross-Pitaevskii equation,"We introduce a dissipation term in the Gross-Pitaevskii equation that
describes the stimulated relaxation of condensed bosons due to scattering with
a different type of particles. This situation applies to Bose-Einstein
condensates of quasi-particles in the solid state, such as magnons and
excitons. Our model is compatible with the phenomenology of superfluidity:
supercurrents are stable up to a critical speed and decay when they are faster.",1007.5453v1
2010-08-02,Entanglement perturbation theory for the elementary excitation in one dimension,"The entanglement perturbation theory is developed to calculate the excitation
spectrum in one dimension. Applied to the spin-$\frac{1}{2}$ antiferromagnetic
Heisenberg model, it reproduces the des Cloiseaux-Pearson Bethe ansatz result.
As for spin-1, the spin-triplet magnon spectrum has been determined for the
first time for the entire Brillouin zone, including the Haldane gap at $k=\pi$.",1008.0346v1
2011-06-28,Classical strings in $AdS_4\times\mathbb{CP}^3$ with three angular momenta,"In this paper, rotating strings in three directions of $AdS_4 \times
\mathbb{CP}^3$ geometry are studied; its divergent energy limit, and conserved
charges are also determined. An interpretation of these configurations as
either giant magnons or spiky strings are discussed.",1106.5684v2
2011-10-17,Divergent energy strings in $AdS_5\times S^5$ with three angular momenta,"In this paper, novel solutions for strings with three angular momenta in
$AdS_5 \times S^5$ geometry are presented; the divergent energy limit and the
corresponding conserved charges, as well as dispersion relation are also
determined. Interpretations of these configurations as either a giant magnon
(GM) or a spiky string (SS) are discussed.",1110.3682v2
2013-04-02,Entanglement of one-magnon Schur-Weyl states,"We investigate the entanglement properties of symmetry states of the
Schur-Weyl duality. Our approach based on reduced two-qubit density matrices,
and concurrence as the measure of entanglement. We show that all kinds of
entangled graphs, which describe the entanglement structure in Schur-Weyl
states are completely coded in the corresponding Young tableau.",1304.0590v1
2013-10-27,Fundamental limitations of half-metallicicity in spintronic materials,"Zero-point spin fluctuations are shown to strongly influence the ground state
of ferromagnetic metals and to impose limitations for the fully spin polarized
state assumed in half-metallic ferromagnets, which may influence their
applications in spintronics. This phenomenon leads to the low-frequency Stoner
excitations and cause strong damping and softening of magnons in
magnetoresistive manganites observed experimentally.",1310.7174v1
2014-02-25,Transverse magneto-optical Kerr effect in subwavelength dielectric gratings,"We demonstrate theoretically a large transverse magneto-optical Kerr effect
(TMOKE) in subwavelength gratings consisting of alternating magneto-insulating
and nonmagnetic dielectric nanostripes. The reflectivity of the grating reaches
$96\%$ at the frequencies corresponding to the maximum of the TMOKE response.
The combination of a large TMOKE response and high reflectivity is important
for applications in $3$D imaging, magneto-optical data storage, and magnonics.",1402.6075v1
2015-04-24,Signatures of indirect K-edge resonant inelastic x-ray scattering on magnetic excitations in triangular lattice antiferromagnet,"We compute the K-edge indirect resonant inelastic x-ray scattering (RIXS)
spectrum of a triangular lattice antiferromagnet in its ordered coplanar 3-
sublattice 120 degree magnetic state. By considering the first order
self$-$energy corrections to the spin wave spectrum, magnon decay rate,
bimagnon interactions within the ladder approximation Bethe-Salpeter scheme,
and the effect of three-magnon contributions up to 1/S- order we find that the
RIXS spectra is non-trivially affected. For a purely isotropic triangular
lattice model, the peak splitting mechanism and the appearance of a multipeak
RIXS structure is primarily dictated by the damping of magnon modes. At a
scattering wavevector corresponding to the zone center \Gamma point and at the
roton point q=M, where the magnon decay rate is zero, a stable single peak
forms. At the $\Gamma$ point, the contribution is purely trimagnon at the 1/S
level and occurs approximately at the trimagnon energy of 6JS. The roton peak
occurs at a lower energy of 4JS. The K-edge single peak RIXS spectra at the
roton momentum can be utilized as an experimental signature to detect the
presence of roton excitations. A unique feature of the triangular lattice
K-edge RIXS spectra is the nonvanishing RIXS intensity at both the zone center
$\Gamma$ point and the antiferromagnetic wavevector K point. This result is in
sharp contrast to the vanishing K-edge RIXS intensity of the collinear magnetic
phases on the square lattice. We find that including XXZ anisotropy leads to
additional peak splitting, including at the roton scattering wavevector where
the single peak destabilizes towards a two-peak structure. The observed
splitting is consistent with our earlier theoretical prediction of the effects
of spatial anisotropy on the RIXS spectra of a frustrated quantum magnet [Luo,
Datta, and Yao, Phys. Rev. B 89, 165103 (2014)].",1504.06609v1
2012-01-25,Spin-Density-Wave Gap with Dirac Nodes and Two-Magnon Raman Scattering in BaFe2As2,"Raman selection rules for electronic and magnetic excitations in BaFe2As2
were theoretically investigated and applied them to the separate detection of
the nodal and anti-nodal gap excitations at the spin density wave (SDW)
transition and the separate detection of the nearest and the next nearest
neighbor exchange interaction energies. The SDW gap has Dirac nodes, because
many orbitals participate in the electronic states near the Fermi energy. Using
a two-orbital band model the electronic excitations near the Dirac node and the
anti-node are found to have different symmetries. Applying the symmetry
difference to Raman scattering the nodal and anti-nodal electronic excitations
are separately obtained. The low-energy spectra from the anti-nodal region have
critical fluctuation just above TSDW and change into the gap structure by the
first order transition at TSDW, while those from the nodal region gradually
change into the SDW state. The selection rule for two-magnon scattering from
the stripe spin structure was obtained. Applying it to the two-magnon Raman
spectra it is found that the magnetic exchange interaction energies are not
presented by the short-range superexchange model, but the second derivative of
the total energy of the stripe spin structure with respect to the moment
directions. The selection rule and the peak energy are expressed by the
two-magnon scattering process in an insulator, but the large spectral weight
above twice the maximum spin wave energy is difficult to explain by the decayed
spin wave. It may be explained by the electronic scattering of itinerant
carriers with the magnetic self-energy in the localized spin picture or the
particle-hole excitation model in the itinerant spin picture. The magnetic
scattering spectra are compared to the insulating and metallic cuprate
superconductors whose spins are believed to be localized.",1201.5207v1
2019-02-01,Quantum thermodynamics of complex ferrimagnets,"High-quality magnets such as yttrium iron garnet (YIG) are electrically
insulating and very complex. By implementing a quantum thermostat into
atomistic spin dynamics we compute YIG's key thermodynamic properties, viz. the
magnon power spectrum and specific heat, for a large temperature range. The
results differ (sometimes spectacularly) from simple models and classical
statistics, but agree with available experimental data.",1902.00449v1
2019-03-13,"Dynamical structure factor of the $J_1-J_2$ Heisenberg model on the triangular lattice: magnons, spinons, and gauge fields","Understanding the nature of the excitation spectrum in quantum spin liquids
is of fundamental importance, in particular for the experimental detection of
candidate materials. However, current theoretical and numerical techniques have
limited capabilities, especially in obtaining the dynamical structure factor,
which gives a crucial characterization of the ultimate nature of the quantum
state and may be directly assessed by inelastic neutron scattering. In this
work, we investigate the low-energy properties of the $S=1/2$ Heisenberg model
on the triangular lattice, including both nearest-neighbor $J_1$ and
next-nearest-neighbor $J_2$ super-exchanges, by a dynamical variational Monte
Carlo approach that allows accurate results on spin models. For $J_2=0$, our
calculations are compatible with the existence of a well-defined magnon in the
whole Brillouin zone, with gapless excitations at $K$ points (i.e., at the
corners of the Brillouin zone). The strong renormalization of the magnon branch
(also including roton-like minima around the $M$ points, i.e., midpoints of the
border zone) is described by our Gutzwiller-projected state, where Abrikosov
fermions are subject to a non-trivial magnetic $\pi$-flux threading half of the
triangular plaquettes. When increasing the frustrating ratio $J_2/J_1$, we
detect a progessive softening of the magnon branch at $M$, which eventually
becomes gapless within the spin-liquid phase. This feature is captured by the
band structure of the unprojected wave function (with $2$ Dirac points for each
spin component). In addition, we observe an intense signal at low energies
around the $K$ points, which cannot be understood within the unprojected
picture and emerges only when the Gutzwiller projection is considered,
suggesting the relevance of gauge fields for the low-energy physics of spin
liquids.",1903.05691v3
2016-10-20,Magnetic excitations in quasi-one dimensional helimagnets: Magnon decays and in uence of the inter-chain interactions,"We present a comprehensive study of the magnetic properties of the long-range
ordered quasi-one dimensional $J_{1}$-$J_{2}$ systems with a newly developed
torque equilibrium spin-wave expansion approach, which can describe the spin
Casimir and magnon decay effects in a unified framework. While the framework
does not lose the generality, our discussion will be restricted to two
representative types of inter-chain coupling systems: $J_{3}$- or
$J_{4}$-system respectively. In spite of the long-range spiral order, the
dynamical properties of these systems turn out to be highly nontrivial due to
the incommensurate noncollinear spin configuration and the strong quantum
fluctuation effects enhanced by the frustration and low-dimensionality. Both
the systems show prominent spin Casimir effects induced by the vacuum
fluctuation of the spin waves and related modification of the ordering vector,
Lifshitz point's position and sublattice magnetization. Significant and
spontaneous magnon decay effects are manifested in the quantum corrections to
the excitation spectrum. By adjusting the strength of magnetic anisotropy and
varying the approximation scheme, it is revealed that these striking distinct
features are quite robust and have deep connection with both the spin Casimir
and the magnon decay effects. Thus these special consequences of the
inter-chain coupling on the spin wave dynamics may be served as a set of probes
for different types of inter-chain couplings in experiments. At last, to guide
experimental measurements such as inelastic neutron scattering in realistic
materials and complement our theoretical framework, we develop the analytical
theory of the dynamical structure factor within the torque equilibrium
formulism and provide the explicit results of the quasi-one dimensional
$J_{1}$-$J_{2}$ systems.",1610.06365v2
2019-05-10,Experimental Implementations of Cavity-Magnon Systems: from Ultra Strong Coupling to Applications in Precision Measurement,"Several experimental implementations of cavity-magnon systems are presented.
First an Yttrium Iron Garnet (YIG) block is placed inside a re-entrant cavity
where the resulting hybrid mode is measured to be in the ultra strong coupling
regime. When fully hybridised the ratio between the coupling rate and uncoupled
mode frequencies is determined to be $g/\omega=0.46$. Next a thin YIG cylinder
is placed inside a loop gap cavity. The bright mode of this cavity couples to
the YIG sample and is similarly measured to be in the ultra strong coupling
regime with ratio of coupling rate to uncoupled mode frequencies as
$g/\omega=0.34$. A larger spin density medium such as lithium ferrite (LiFe) is
expected to improve couplings by a factor of 1.46 in both systems as coupling
strength is shown to be proportional to the square root of spin density and
magnetic moment. Such strongly coupled systems are potentially useful for
cavity QED, hybrid quantum systems and precision dark matter detection
experiments. The YIG disc in the loop gap cavity, is, in particular, shown to
be a strong candidate for dark matter detection. Finally, a LiFe sphere inside
a two post re-entrant cavity is considered. In past work it was shown that the
magnon mode in the sample has a turnover point in frequency. Additionally, it
was predicted that if the system was engineered such that it fully hybridised
at this turnover point the cavity-magnon polariton (CMP) transition frequency
would become insensitive to both first and second order magnetic bias field
fluctuations, a result useful for precision frequency applications. This work
implements such a system by engineering the cavity mode frequency to near this
turnover point, with suppression in sensitivity to second order bias magnetic
field fluctuations shown.",1905.04002v2
2021-01-29,Topological magnons for thermal Hall transport in frustrated magnets with bond-dependent interactions,"Thermal transport in topologically-ordered phases of matter provides valuable
insights as it can detect the charge-neutral quasiparticles that would not
directly couple to electromagnetic probes. An important example is edge heat
transport of Majorana fermions in a chiral spin liquid, which leads to a
half-quantized thermal Hall conductivity. This signature is precisely what has
recently been measured in $\alpha$-RuCl$_3$ under external magnetic fields. The
plateau-like behavior of the half-quantized thermal Hall conductivity as a
function of external magnetic field, and the peculiar sign change depending on
the magnetic field orientation, has been proposed as strong evidence for the
non-Abelian Kitaev spin liquid. Alternatively, for in-plane magnetic fields, it
was theoretically shown that such a sign structure can also arise from
topological magnons in the field-polarized state. In this work, we investigate
the full implications of topological magnons as heat carriers on thermal
transport measurements. We first prove analytically that for any commensurate
order with a finite magnetic unit cell, reversing the field direction leads to
a sign change in the magnon thermal Hall conductivity in two-dimensional
systems. We verify this proof numerically with nontrivial magnetic orders as
well as the field-polarized state in Kitaev magnets subjected to an in-plane
field. In the case of a tilted magnetic field, whereby there exist both finite
in-plane and out-of-plane field components, we find that the plateau-like
behavior of the thermal Hall conductivity and the sign change upon reversing
the in-plane component of the magnetic field arise in the partially-polarized
state, as long as the in-plane field contribution to the Zeeman energy is
significant. While these results are consistent with the experimental
observations, we comment on other aspects requiring investigation in future
studies.",2102.00014v2
2018-08-17,Collective excitations in spin-1/2 magnets through bond-operator formalism designed both for paramagnetic and ordered phases,"We present a bond-operator theory (BOT) suitable for description both
magnetically ordered phases and paramagnetic phases with singlet ground states
in spin-1/2 magnets. Proposed BOT provides a regular expansion of physical
quantities in powers of 1/n, where n is the maximum number of bosons which can
occupy a unit cell (physical results correspond to n=1). Two variants of BOT
are suggested: for two and for four spins in the unit cell (two-spin and
four-spin BOTs, respectively). We consider spin-1/2 Heisenberg antiferromagnet
(HAF) on simple square lattice bilayer by the two-spin BOT. Ground-state energy
E, staggered magnetization M, and quasiparticles spectra found within the first
order in 1/n are in good quantitative agreement with previous results both in
paramagnetic and in ordered phases not very close to the quantum critical point
between the phases. By doubling the unit cell in two directions, we discuss
spin-1/2 HAF on square lattice using the suggested four-spin BOT. Magnon
spectrum, E, and M found in the first order in 1/n are in good quantitative
agreement with previous numerical and experimental results. We observe a
special moderately damped spin-0 quasiparticle (""singlon"" for short) whose
energy is smaller than the energy of the Higgs mode in the most part of the
Brillouin zone. By considering HAF with Izing-type anisotropy, we find that
both Higgs and ""singlon"" modes stem from two-magnon bound states which merge
with two-magnon continuum not far from the isotropic limit. We demonstrate that
""singlons"" appear explicitly in ""scalar"" correlators one of which describes the
Raman intensity in $B_{1g}$ symmetry. The latter is expressed in the leading
order in 1/n via the ""singlon"" Green's function at zero momentum which shows an
asymmetric peak. The position of this peak coincides with the position of
""two-magnon"" peak observed experimentally in, e.g., layered cuprates.",1808.05841v2
2020-01-17,Thermal spin transport and spin in thermoelectrics,"This article reviews the principles that govern the combined transport of
spin, heat, and charge. The extensive thermodynamic quantity associated with
spin transport is the magnetization; its Onsager-conjugate force is in general
the derivative of the free energy with respect to the magnetization. Spins are
carried in one of two ways: (1) by spin-polarized free electrons in magnetic
metals and doped semiconductors, or (2) by spin waves (magnons) that reside on
localized electrons on unfilled d- or f-shells of transition metal or
rare-earth elements. The paper covers both cases in separate chapters. In both
cases, it is possible to define a spin chemical potential whose gradient is the
more practical conjugate force to spin transport. The paper further describes
the anomalous Hall, spin Hall, and inverse spin Hall effects in magnetic and
non-magnetic solids with strong spin-orbit coupling because these effects are
used to generate and measure spin fluxes. Spin transport across interfaces is
described next, and includes spin pumping and spin transfer torque. The final
chapter then puts all these concepts together to describe the spin-Seebeck,
spin-Peltier, and magnon-drag effects, which exist in ferromagnetic,
antiferromagnetic, and even paramagnetic solids. Magnon-drag, in particular, is
a high-temperature effect that boosts the thermopower of metals by an order of
magnitude and that of semiconductors by a factor of 2 or 3 above the electronic
diffusion thermopower. This is the only example where a spin-driven effect is
larger than a charge-driven effect. Magnon drag leads a simple binary
paramagnetic semiconductor, MnTe, to have zT > 1 without optimization. This
shows how adding spin as an additional design parameter in thermoelectrics
research is a new and promising approach toward the quest for high-zT
materials.",2001.06366v1
2020-04-12,Spin vortex lattice in the Landau vortex-free state of rotating superfluids,"We show that the Landau vortex-free state in rotating container may give rise
to the lattice of spin vortices. We consider this effect on example of spin
vortices in magnon Bose-Einstein condensate (the phase coherent spin
precession) in the B-phase of superfluid $^3$He, and on example of spin
vortices in the polar phase of $^3$He.",2004.05583v1
2020-07-15,"Relativistic exchange interactions in CrX$_3$ (X=Cl, Br, I) monolayers","It has been predicted theoretically and indirectly confirmed experimentally
that single-layer CrX$_3$ (X=Cl, Br, I) might be the prototypes of topological
magnetic insulators (TMI). In this work, by using first-principles calculations
combined with atomistic spin dynamics we provide a complete picture of the
magnetic interactions and magnetic excitations in CrX$_3$. The focus is here on
the two most important aspects for the actual realization of TMI, namely the
relativistic magnetic interactions and the finite-size (edge) effects. We
compute the full interaction tensor, which includes both Kitaev and
Dzyaloshinskii-Moriya terms, which are considered as the most likely mechanisms
for stabilizing topological magnons. First, we instigate the properties of bulk
CrI$_3$ and compare the simulated magnon spectrum with the experimental data
[Phys. Rev. X 8, 041028 (2018)]. Our results suggest that a large size of
topological gap, seen in experiment ($\approx$ 4 meV), can not be explained by
considering pair-wise spin interactions only. We identify several possible
reasons for this disagreement and suggest that a pronounced magneto-elastic
coupling should be expected in this class of materials. The magnetic
interactions in the monolayers of CrX$_3$ are also investigated. The strength
of the anisotropic interactions is shown to scale with the position of halide
atom in the Periodic Table, the heavier the element the larger is the
anisotropy. Comparing the magnons for the bulk and single-layer CrI$_3$, we
find that the size of the topological gap becomes smaller in the latter case.
Finally, we investigate finite-size effects in monolayers and demonstrate that
the anisotropic couplings between Cr atoms close to the edges are much stronger
than those in ideal periodic structure. This should have impact on the dynamics
of the magnon edge modes in this class of materials.",2007.07611v2
2021-05-10,Magnon-phonon interactions enhance the gap at the Dirac point in the spin-wave spectra of CrI$_3$ two-dimensional magnets,"Recent neutron-diffraction experiments in honeycomb CrI$_3$ quasi-2D
ferromagnets have evinced the existence of a gap at the Dirac point in their
spin-wave spectra. The existence of this gap has been attributed to strong
in-plane Dzyaloshinskii-Moriya or Kitaev (DM/K) interactions and suggested to
set the stage for topologically protected edge states to sustain
non-dissipative spin transport. We perform state-of-the-art simulations of the
spin-wave spectra in monolayer CrI$_3$, based on time-dependent
density-functional perturbation theory (TDDFpT) and fully accounting for
spin-orbit couplings (SOC) from which DM/K interactions ultimately stem. While
our results are in qualitative agreement with experiments, the computed TDDFpT
magnon gap at the Dirac point is found to be 0.47~meV, roughly 6 times smaller
than the most recent experimental estimates, so questioning that intralayer
anisotropies alone can explain the observed gap. Lattice-dynamical
calculations, performed within density-functional perturbation theory (DFpT),
indicate that a substantial degeneracy and a strong coupling between
vibrational and magnetic excitations exist in this system, providing a possible
additional gap-opening mechanism in the spin-wave spectra. In order to pursue
this path, we introduce an interacting magnon-phonon Hamiltonian featuring a
linear coupling between lattice and spin fluctuations, enabled by the magnetic
anisotropy induced by SOC. Upon determination of the relevant interaction
constants by DFpT and supercell calculations, this model allows us to propose
magnon-phonon interactions as an important microscopic mechanism responsible
for the enhancement of the gap in the range of $\approx 4$~meV around the Dirac
point of the CrI$_3$ monolayer.",2105.04531v4
2022-05-15,Surface plasmon-phonon-magnon polariton in a topological insulator-antiferromagnetic bilayer structure,"We present a robust technique for computationally studying surface polariton
modes in hybrid materials. We use a semi-classical model that allows us to
understand the physics behind the interactions between collective excitations
of the hybrid system and develop a scattering and transfer matrix method that
imposes the proper boundary conditions to solve Maxwell equations and derive a
general equation describing the surface polariton in a heterostructure
consisting of N constituent materials. We apply this method to a test structure
composed of a topological insulator (TI) and an antiferromagnetic material
(AFM) to study the resulting surface Dirac plasmon-phonon-magnon polariton
(DPPMP). We find that interactions between the excitations of the two
constituents result in the formation of hybridized modes and the emergence of
avoided-crossing points in the dispersion relations for the DPPMP. For the
specific case of a Bi2Se3 TI material, the polariton branch with low frequency
below 2 THz redshifts upon increasing the thickness of TI thin film, which
leads to an upper bound on the thickness of the TI layer that will allow an
observable signature of strong coupling and the emergence of hybridized states.
We also find that the strength of the coupling between the TI and the AFM,
which is parameterized by the amplitude of the avoided-crossing splitting
between the two polariton branches at the magnon resonance frequency, depends
on the magnitude of the magnetic dipole and the line width of the magnon in the
AFM material as well as on the Fermi energy of Dirac plasmon in the TI.
Finally, we predict that materials with extremely high quality, i.e. low
scattering loss rate, are essential to achieve an experimentally-observable
strong coupling between a TI and AFM.",2205.07367v1
2022-08-14,Spin-wave theory in a randomly disordered lattice: A Heisenberg ferromagnet,"Starting from the hamiltonian for the Heisenberg ferromagnet which comprise
randomly distributed nonmagnetic ions as impurities in a Bravais lattice, we
express the spin operators by means of the Dyson-Maleev transformation in terms
of the Bose operators of the second quantization. Then by using methods of
quantum statistical field theory, we derive the partition function and the free
energy for the system. We adopt the Matsubara thermal perturbation method to a
portion of the hamiltonian which describes the interaction between magnons and
the stationary field of nonmagnetic ions. Upon averaging over all possible
distributions of impurities, we express the free energy of the system as a
function of the mean impurity concentration. Subsequently, we set up the
double-time single particle Green function at temperature T in the momentum
space in terms of magnon operators and derive the equation of motion for the
Green function through the Heisenberg equation of motion and then solve the
resulting equation. From this, we calculate the self-energy and then the
spectral density for the system. We apply the formalism to the case of the
simple cubic lattice and compute the density of states, the spectral density
function and the lifetime of the magnons as a function of energy for several
values of the mean concentration of nonmagnetic ions in the lattice. We
calculate magnon energy spectrum as a function of average impurity
concentration fraction c, which shows that for low lying states, the excitation
energy increases continuously with c in the studied range 0.1 < c < 0.7. We use
the spectral density function to compute some thermal quantities. We have
obtained closed form expressions for the configurationally averaged physical
quantities of interest in a unified fashion as functions of c to any order of c
applicable below a critical percolation concentration.",2208.06910v1
2022-12-05,Spin excitation continuum to topological magnon crossover and thermal Hall conductivity in Kitaev magnets,"There has been great interest in identifying a Kitaev quantum spin liquid
state in frustrated magnets with bond-dependent interactions. In particular,
the experimental report of a half-quantized thermal Hall conductivity in
$\alpha$-RuCl$_3$ in the presence of a magnetic field has generated excitement
as it could be strong evidence for a field-induced chiral spin liquid. More
recent experiments, however, provide a conflicting interpretation advocating
for topological magnons in the field-polarized state as the origin of the
non-quantized thermal Hall conductivity observed in their experiments. An
inherent difficulty in distinguishing between the two scenarios is the phase
transition between a putative two-dimensional spin liquid and the
field-polarized state exists only at zero temperature, while the behaviour at
finite temperature is mostly crossover phenomena. In this work, we provide
insights into the finite temperature crossover behavior between the spin
excitation continuum in a quantum spin liquid and topological magnons in the
field-polarized state in three different theoretical models with large Kitaev
interactions. These models allow for a field-induced phase transition from a
spin liquid (or an intermediate field-induced spin liquid) to the
field-polarized state in the quantum model. We obtain the dynamical spin
structure factor as a function of magnetic field using molecular dynamics
simulations and compute thermal Hall conductivity in the field-polarized
regime. We demonstrate the gradual evolution of the dynamical spin structure
factor exhibiting crossover behaviour near magnetic fields where
zero-temperature phase transitions occur in the quantum model. We also examine
nonlinear effects on topological magnons and the validity of thermal Hall
conductivity computed using linear spin wave theory. We discuss the
implications of our results to existing and future experiments.",2212.02516v1
2023-01-09,Thermal Hall conductivity of $α$-RuCl$_3$,"Thermal Hall conductivity originating from topological magnons is observed in
the Kitaev candidate $\alpha$-RuCl$_3$ in broad intervals of temperature and
in-plane magnetic field, raising questions on the role of the Majorana mode in
heat conduction.",2301.03526v1
2023-04-24,Perspective on non-Hermitian physics in magnetic systems,"A perspective on non-Hermitian physics in magnetic systems is addressed in
this short article, including exceptional points, exceptional nodal phases, the
non-Hermitian SSH model, and the non-Hermitian skin effect.",2304.11948v2
2023-06-16,Bose-Einstein condensation of a two-magnon bound state in a spin-one triangular lattice,"Interactions of collective excitations often lead to rich emergent phenomena
in many-particle quantum systems. In ordered magnets, the elementary
excitations are spin waves (magnons), which obey Bose-Einstein statistics.
Similar to the Cooper pairs in superconductors, magnons can be paired into
bound states under attractive interactions. Even more interestingly, the Zeeman
coupling to a magnetic field acts as a chemical potential that can tune the
particle density through a quantum critical point (QCP), beyond which a
``hidden order'' is predicted to exist. However, experimental confirmation of
this QCP and the associated new state of matter remain elusive. Here we report
direct observation of the Bose-Einstein condensation (BEC) of the two-magnon
bound state in Na$_2$BaNi(PO$_4$)$_2$. Comprehensive thermodynamic measurements
confirmed the existence of a two-dimensional BEC-QCP at the saturation field.
Inelastic neutron scattering experiments were performed to accurately establish
the magnetic exchange model. An exact solution of the model found stable
2-magnon bound states that were further confirmed by an electron spin resonance
(ESR) experiment, demonstrating that the QCP is due to the pair condensation
and the phase below saturation field is the long-sought-after spin nematic (SN)
phase.",2306.09695v1
2024-02-22,Laser patterning of magnonic structure via local crystallization of Yittrium Iron Garnet,"The fabrication and integration of high-quality structures of Yttrium Iron
Garnet (YIG) is critical for magnonics.Films with excellent properties are
obtained only on single crystal Gadolinium Gallium Garnet (GGG) substrates
using high-temperature processes. The subsequent realization of magnonic
structures via lithography and etching is not straightforward as it requires a
tight control of the edge roughness, to avoid magnon scattering, and
planarization in case of multilayer devices. In this work we describe a
different approach based on local laser annealing of amorphous YIG films,
avoiding the need for subjecting the entire sample to high thermal budgets and
for physical etching. Starting from amorphous and paramagnetic YIG films grown
by pulsed laser deposition at room temperature on GGG, a 405 nm laser is used
for patterning arbitrary shaped ferrimagnetic structures by local
crystallization. In thick films (160 nm) the laser induced surface corrugation
prevents the propagation of spin-wave modes in patterned conduits. For thinner
films (80 nm) coherent propagation is observed in 1.2 micron wide conduits
displaying an attenuation length of 5 micron which is compatible with a damping
coefficient of about 5e-3. Possible routes to achieve damping coefficients
compatible with state-of-the art epitaxial YIG films are discussed.",2402.14444v1
1995-05-03,Magnetic Raman Scattering in Two-Dimensional Spin-1/2 Heisenberg Antiferromagnets: Spectral Shape Anomaly and Magnetostrictive Effects,"We calculate the Raman spectrum of the two-dimensional (2D) spin-1/2
Heisenberg antiferromagnet by exact diagonalization and quantum Monte Carlo
techniques on clusters of up to 144 sites and, on a 16-site cluster, by
considering the phonon-magnon interaction which leads to random fluctuations of
the exchange integral. Results are in good agreement with experiments on
various high-T_c precursors, such as La_2CuO_4 and YBa_2Cu_3O_{6.2}. In
particular, our calculations reproduce the broad lineshape of the two-magnon
peak, the asymmetry about its maximum, the existence of spectral weight at high
energies, and the observation of nominally forbidden A_{1g} scattering.",9505012v1
1995-10-26,Properties of lightly doped t-J two-leg ladders,"We have numerically investigated the doped t-J ladder using exact
diagonalization. We have studied both the limit of strong inter-chain coupling
and isotropic coupling. The ladder scales to the Luther-Emery liquid regime in
the strong inter-chain coupling limit. In this strong coupling limit there is a
simple picture of the excitation spectrum that can be continued to explain the
behavior at isotropic coupling. At J=0 we have indications of a ferromagnetic
ground state. At a large $J/t$ the ladder is phase separated into holes and a
Heisenberg ladder. At intermediate coupling the ground state shows hole pairing
with a modified d-wave symmetry. The excitation spectrum separates into a
limited number of quasiparticles which carry charge $+|e|$ and spin ${1\over
2}$ and a triplet magnon mode. At half-filling the former vanish but the latter
evolves continuously into the magnon band of the spin liquid. At low doping the
quasiparticles form a dilute Fermi gas with a strong attraction but
simultaneously the Fermi wave vector, as would be measured in photoemission, is
large. The dynamical structure factors are calculated and are found to be very
similar to calculations on 2D clusters.",9510150v1
1996-11-14,Heisenberg model with Dzyaloshinskii-Moriya interaction: A Schwinger boson study,"We present a Schwinger-boson approach to the Heisenberg model with
Dzyaloshinskii-Moriya interaction. We write the anisotropic interactions in
terms of Schwinger bosons keeping the correct symmetries present in the spin
representation, which allows us to perform a conserving mean-field
approximation. Unlike previous studies of this model by linear spin-wave
theory, our approach takes into account magnon-magnon interactions and includes
the effects of three-boson terms characteristic of noncollinear phases. The
results reproduce the linear spin-wave predictions in the semiclassical large-S
limit, and show a small renormalization in the strong quantum limit S=1/2. For
the sake of definiteness, we specialize the calculations for the pattern of
Moriya vectors corresponding to the orthorhombic phase in La_2CuO_4, and give a
fairly detailed account of the behavior of ground-state energy, anisotropy gap,
and net ferromagnetic moment. In the last part of this work we generalize our
approach to describe the geometry of the intermediate phase in
La_{2-x}Nd_xCuO_4, and discuss the effects of including nondegenerate 2p_z
oxygen orbitals in the calculations.",9611116v1
1997-08-11,Excitations and Possible Bound States in the S=1/2 Alternating Chain Compound (VO)2P2O7,"Magnetic excitations in an array of (VO)2P2O7 single crystals have been
measured using inelastic neutron scattering. Until now, (VO)2P2O7 has been
thought of as a two-leg antiferromagnetic Heisenberg spin ladder with chains
running in the a-direction. The present results show unequivocally that
(VO)2P2O7 is best described as an alternating spin-chain directed along the
crystallographic b-direction. In addition to the expected magnon with magnetic
zone-center energy gap DE = 3.1$ meV, a second excitation is observed at an
energy just below 2DE. The higher mode may be a triplet two-magnon bound state.
Numerical results in support of bound modes are presented.",9708078v1
1997-11-06,Observation of three-magnon light scattering in CuGeO_3,"Temperature and magnetic field dependent polarized Brillouin spectra of
CuGeO_3 are reported. In addition to a bound singlet state at 30 cm^{-1}, a new
feature has been observed at 18 cm^{-1}. This feature is interpreted in terms
of a novel three-magnon light scattering process between excited triplet
states.",9711047v1
1998-02-13,Spin and orbital excitations in magnetic insulators with Jahn-Teller ions,"The elementary excitations of a model Hamiltonian that captures the low
energy behavior of a simple two-fold degenerate Hubbard Hamiltonian with Hund's
rule coupling, is studied. The phase diagram in the mean-field limit and in a
two-site approach reveals a rich variety of phases where both the orbital and
the spin degrees of freedom are ordered. We show that besides usual spin waves
(magnons) there exist also orbital waves (orbitons) and, most interestingly, in
a completely ferromagnetically coupled system, a combined spin-orbital
excitation which can be visualized as a bound state of magnons and orbitons.
For a completely degenerate system the bound states are found to be the lowest
lying elementary excitations, both in one- and two-dimensions. Finally we
extend our treatment to almost-degenerate systems. This can serve as an example
that the elementary excitations in orbitally degenerate strongly correlated
electron systems in general carry both spin and orbital character.",9802146v1
1998-04-09,Charged Magnons in NaV2O5,"We investigated the temperature-dependent optical conductivity of NaV2O5 in
the energy range 4 meV-4 eV. The intensities and the polarization dependence of
the detected electronic excitations give a direct indication for a
broken-parity electronic ground-state and for a non-centrosymmetric crystal
structure of the system in the high-temperature phase. A direct two-magnon
optical absorption process, proposed in this Letter, is in quantitative
agreement with the optical data. By analyzing the optically allowed phonons at
various temperatures above and below the phase transition, we conclude that a
second-order change to a larger unit cell takes place below 34 K.",9804110v3
1999-04-06,Non-Fermi-liquid behavior in the Kondo lattices induced by peculiarities of magnetic ordering and spin dynamics,"A scaling consideration of the Kondo lattices is performed with account of
singularities in the spin excitation spectral function. It is shown that a
non-Fermi-liquid (NFL) behavior between two critical values of the bare $s-f$
coupling constant occurs naturally for complicated magnetic structures with
several magnon branches. This may explain the fact that a NFL behavior takes
place often in the heavy-fermion systems with peculiar spin dynamics. Another
kind of a NFL-like state (with different critical exponents) can occur for
simple antiferromagnets with account of magnon damping, and for paramagnets,
especially with two-dimensional character of spin fluctuations. The mechanisms
proposed lead to some predictions about behavior of specific heat, resistivity,
magnetic susceptibility, and anisotropy parameter, which can be verified
experimentally.",9904072v3
1999-08-13,Spin Wave Theory of Double Exchange Ferromagnets,"We construct the 1/S spin-wave expansion for double exchange ferromagnets at
T=0. It is assumed that the value of Hund's rule coupling, J_H, is sufficiently
large, resulting in a fully saturated, ferromagnetic half-metallic ground
state. We evaluate corrections to the magnon dispersion law, and we also find
that, in contrast to earlier statements in the literature, magnon-electron
scattering does give rise to spin wave damping. We analyse the momentum
dependence of these quantities and discuss the experimental implications for
colossal magnetoresistance compounds.",9908202v3
1999-09-06,Excitation Spectra of Structurally Dimerized and Spin-Peierls Chains in a Magnetic Field,"The dynamical spin structure factor and the Raman response are calculated for
structurally dimerized and spin-Peierls chains in a magnetic field, using exact
diagonalization techniques. In both cases there is a spin liquid phase composed
of interacting singlet dimers at small fields h < h_c1, an incommensurate
regime (h_c1 < h < h_c2) in which the modulation of the triplet excitation
spectra adapts to the applied field, and a fully spin polarized phase above an
upper critical field h_c2. For structurally dimerized chains, the spin gap
closes in the incommensurate phase, whereas spin-Peierls chains remain gapped.
In the spin liquid regimes, the dominant feature of the triplet spectra is a
one-magnon bound state, separated from a continuum of states at higher
energies. There are also indications of a singlet bound state above the
one-magnon triplet.",9909093v1
2000-04-25,Can a frustrated spin-cluster model describe the low-temperature physics of NaV_2O_5 ?,"Recent experimental evidence suggest the existence of three distinct
V-valence states (V^{+4}, V^{+4.5} and V^{+5}) in the low-temperature phase of
NaV_2O_5 in apparent discrepancy with the observed spin-gap. We investigate a
novel spin cluster model, consisting of weakly coupled, frustrated four-spin
clusters aligned along the crystallographic b-axis that was recently proposed
to reconcile these experimental observations. We have studied the phase diagram
and the magnon dispersion relation of this model using DMRG, exact
diagonalization and a novel cluster-operator theory. We find a spin-gap for all
parameter values and two distinct phases, a cluster phase and a Haldane phase.
We evaluate the size of the gap and the magnon dispersion and find no parameter
regime which would reproduce the experimental results. We conclude that this
model is inappropriate for the low-temperature regime of NaV_2O_5.",0004404v1
2000-04-27,Two-magnon Raman scattering in spin-ladder geometries and the ratio of rung and leg exchange constants,"We discuss ways in which the ratio of exchange constants along the rungs and
legs of a spin-ladder material influences the two-magnon Raman scattering
spectra and hence can be determined from it. We show that within the
Fleury-Loudon-Elliott approach, the Raman line-shape does not change with
polarization geometries. This lineshape is well known to be difficult to
calculate accurately from theory. However, the Raman scattering intensities do
vary with polarization geometries, which are easy to calculate. With some
assumptions about the Raman scattering Hamiltonian, the latter can be used to
estimate the ratio of exchange constants. We apply these results to Sugai's
recent measurements of Raman scattering from spin-ladder materials such as
La$_6$Ca$_8$Cu$_{24}$O$_{41}$ and Sr$_{14}$Cu$_{24}$O$_{41}$.",0004477v3
2001-01-19,Nature of Spin Excitations in Two-dimensional Mott Insulators: Undoped Cuprates and Other Materials,"We investigate the excitation spectrum of a two-dimensional resonating
valence bond (RVB) state. Treating the $pi$-flux phase with antiferromagnetic
correlations as a variational ground state, we recover the long wavelength
magnon as an ""RVB exciton"". However, we find that this excitation does NOT
exhaust the entire spectral weight and the high energy spectrum is dominated by
fermionic excitations. The latter can be observed directly by inelastic neutron
scattering and we predict their characteristic energy scales along different
high symmetry directions in the magnetic Brillouin zone. We also interpret
experimental results on two magnon Raman scattering and mid-infrared absorption
within this scenario.",0101298v2
2001-06-22,A Spin-Resolved Photoemission Study of Photohole Lifetimes in Ferromagnetic Gadolinium,"High resolution spin-resolved photoemission is used to probe the properties
of a Gd(0001) surface state. The state shows both a spin-mixing behavior
reflecting the exchange of magnons with the local moments and a reduction of
the exchange splitting with increasing temperature. The surface state
polarization at low T suggests that the surface layer has an enhanced Tc of
365K or greater. Measurements of the photoemission linewidths show that at low
temperatures, the lifetime of a majority spin photohole is predominantly
limited by electron-phonon scattering and that of a minority spin photohole by
electron-magnon scattering. Since similar behavior may be expected for bulk
states close to the Fermi level, the transport properties of this material will
also be determined by different decay mechanisms in the two channels.",0106453v1
2001-06-24,Spontaneous Magnetization of the O(3) Ferromagnet at Low Temperatures,"We investigate the low-temperature behavior of ferromagnets with a
spontaneously broken symmetry O(3) $\to$ O(2). The analysis is performed within
the perspective of nonrelativistic effective Lagrangians, where the dynamics of
the system is formulated in terms of Goldstone bosons. Unlike in a
Lorentz-invariant framework (chiral perturbation theory), where loop graphs are
suppressed by two powers of momentum, loops involving ferromagnetic spin waves
are suppressed by three momentum powers. The leading coefficients of the
low-temperature expansion for the partition function are calculated up to order
$p^{10}$. In agreement with Dyson's pioneering microscopic analysis of the
cubic ferromagnet, we find that, in the spontaneous magnetization, the
magnon-magnon interaction starts manifesting itself only at order $T^4$. The
striking difference with respect to the low-temperature properties of the O(3)
antiferromagnet is discussed from a unified point of view, relying on the
effective Lagrangian technique.",0106492v1
2001-08-06,Plaquette Boson-Fermion Model of Cuprates,"The strongly interacting Hubbard model on the square lattice is reduced to
the low energy Plaquette Boson Fermion Model (PBFM). The four bosons (an
antiferromagnon triplet and a d-wave hole pair), and the fermions are defined
by the lowest plaquette eigenstates. We apply the Contractor Renormalization
method of Morningstar and Weinstein to compute the boson effective
interactions. The range-3 truncation error is found to be very small, signaling
short hole-pair and magnon coherence lengths. The pair-hopping and magnon
interactions are comparable, which explains the rapid destruction of
antiferromagnetic order with emergence of superconductivity, and validates a
key assumption of the projected SO(5) theory. A vacuum crossing at larger
doping marks a transition into the overdoped regime. With hole fermions
occupying small Fermi pockets and Andreev coupled to hole pair bosons, the PBFM
yields several testable predictions for photoemmission, tunneling asymmetry and
entropy measurements.",0108087v2
2001-12-24,Stability of the doped antiferromagnetic state of the t-t'-Hubbard model,"The next-nearest-neighbour hopping term t' is shown to stabilize the AF state
of the doped Hubbard model with respect to transverse perturbations in the
order- parameter by strongly suppressing the intraband particle-hole processes.
For a fixed sign of t', this stabilization is found to be significantly
different for electron and hole doping, which qualitatively explains the
observed difference in the degree of robustness of the AF state in the
electron-doped (Nd_{2-x}Ce_{x}CuO_{4}) and hole-doped (La_{2-x}Sr_{x}CuO_{4})
cuprates. The t'-U phase diagram is obtained for both signs of the t' term,
showing the different regions of stability and instability of the doped
antiferromagnet. Doping is shown to suppress the t'-induced frustration due to
the competing interaction J'. A study of transverse spin fluctuations in the
metallic AF state reveals that the decay of magnons into particle-hole
excitations yields an interesting low-energy result \Gamma \sim \omega for
magnon damping.",0112438v1
2002-05-18,Resonant Two-Magnon Raman Scattering and Photoexcited States in Two-Dimensional Mott Insulators,"We investigate the resonant two-magnon Raman scattering in two-dimensional
(2D) Mott insulators by using a half-filled 2D Hubbard model in the strong
coupling limit. By performing numerical diagonalization calculations for small
clusters, we find that the Raman intensity is enhanced when the incoming photon
energy is not near the optical absorption edge but well above it, being
consistent with experimental data. The absence of resonance near the gap edge
is associated with the presence of background spins, while photoexcited states
for resonance are found to be characterized by the charge degree of freedom.
The resonance mechanism is different from those proposed previously.",0205388v2
2002-07-01,Spin fluctuations in a metallic antiferromagnet,"The magnon energy and amplitude renormalization due to intraband
particle-hole excitations are studied in a metallic antiferromagnet. The change
in sign of the intraband contribution with $\omega$ results in significant
differences between static and dynamical behaviours. For electron doping, while
the coherent magnon peak looses spectral weight and is shifted to higher energy
with doping, the low-energy incoherent part becomes increasingly prominent as
it narrows in width and shifts to lower energy. Implications for spin dynamics
in the electron-doped cuprate $\rm Nd_{2-x} Ce_x Cu O_4$ are discussed. Due to
an exact cancellation of two logarithmically divergent spin-fluctuation
processes, the AF order parameter is unaffected to leading order. For hole
doping, short wavelength transverse perturbations ($q > q^* \sim \sqrt{x}$) are
found to be stable, implying short-range AF order with a spin correlation
length $\xi/a \sim 1/\sqrt{x}$.",0207032v1
2002-11-25,Resonant two-magnon Raman scattering in two-dimensional and ladder-type Mott insulators,"We investigate the resonant two-magnon Raman scattering in the
two-dimensional (2D) and ladder-type Mott insulators by using a half-filled
Hubbard model in the strong coupling limit. By performing numerical
diagonalization calculations for small clusters, we find that the model can
reproduce the experimental features in the 2D that the Raman intensity is
enhanced when the incoming photon energy is not near the absorption edge but
well above it. In the ladder-type Mott insulators, the Raman intensity is found
to resonate with absorption spectrum in contrast to the 2D system. The
difference between 2D and the ladder systems is explained by taking into
account the fact that the ground state in 2D is a spin-ordered state while that
in ladder is a spin-gapped one.",0211552v1
2003-02-26,Field-induced structural evolution in the spin-Peierls compound CuGeO$_3$: high-field ESR study,"The dimerized-incommensurate phase transition in the spin-Peierls compound
CuGeO$_3$ is probed using multifrequency high-resolution electron spin
resonance (ESR) technique, in magnetic fields up to 17 T. A field-induced
development of the soliton-like incommensurate superstructure is clearly
indicated as a pronounced increase of the ESR linewidth $\Delta B$ (magnon
excitations), with a $\Delta B_{max}$ at $B_{c}\sim$ 13.8 T. The anomaly is
explained in terms of the magnon-soliton scattering, and suggests that the
soliton-like phase exists close to the boundary of the dimerized-incommensurate
phase transition. In addition, magnetic excitation spectra in 0.8% Si-doped
CuGeO$_3$ are studied. Suppression of the $\Delta B$ anomaly observed in the
doped samples suggests a collapse of the long-range-ordered soliton states upon
doping, that is consistent with high-field neutron scattering experiments.",0302542v2
2003-09-10,Spectrum and transition rates of the XX chain analyzed via Bethe ansatz,"As part of a study that investigates the dynamics of the s=1/2 XXZ model in
the planar regime |Delta|<1, we discuss the singular nature of the Bethe ansatz
equations for the case Delta=0 (XX model). We identify the general structure of
the Bethe ansatz solutions for the entire XX spectrum, which include states
with real and complex magnon momenta. We discuss the relation between the
spinon or magnon quasiparticles (Bethe ansatz) and the lattice fermions
(Jordan-Wigner representation). We present determinantal expressions for
transition rates of spin fluctuation operators between Bethe wave functions and
reduce them to product expressions. We apply the new formulas to two-spinon
transition rates for chains with up to N=4096 sites.",0309247v1
2003-12-09,Magnetic-field induced spin-Peierls instability in strongly frustrated quantum spin lattices,"For a class of frustrated antiferromagnetic spin lattices (in particular, the
square-kagom\'{e} and kagom\'{e} lattices) we discuss the impact of recently
discovered exact eigenstates on the stability of the lattice against
distortions. These eigenstates consist of independent localized magnons
embedded in a ferromagnetic environment and become ground states in high
magnetic fields. For appropriate lattice distortions fitting to the structure
of the localized magnons the lowering of magnetic energy can be calculated
exactly and is proportional to the displacement of atoms leading to a
spin-Peierls lattice instability. Since these localized states are present only
for high magnetic fields, this instability might be driven by magnetic field.
The hysteresis of the spin-Peierls transition is also discussed.",0312216v2
2004-02-26,Unifying Magnons and Triplons in Stripe-Ordered Cuprate Superconductors,"Based on a two-dimensional model of coupled two-leg spin ladders, we derive a
unified picture of recent neutron scattering data of stripe-ordered
La_(15/8)Ba_(1/8)CuO_4, namely of the low-energy magnons around the
superstructure satellites and of the triplon excitations at higher energies.
The resonance peak at the antiferromagnetic wave vector Q_AF in the
stripe-ordered phase corresponds to a saddle point in the dispersion of the
magnetic excitations. Quantitative agreement with the neutron data is obtained
for J= 130-160 meV and J_cyc/J = 0.2-0.25.",0402659v2
2004-03-12,Consequences of spin-orbit coupling for the Bose-Einstein condensation of magnons,"In the first part we discuss how the BEC picture for magnons is modified by
anisotropies induced by spin-orbit coupling. In particular we focus on the
effects of antisymmetric spin interactions and/or a staggered component of the
$g$ (gyromagnetic) tensor. Such terms lead to a gapped quasiparticle spectrum
and a nonzero condensate density for all temperatures so that no phase
transition occurs. We contrast this to the effect of crystal field anisotropies
which are also induced by spin-orbit coupling. In the second part we study the
field-induced magnetic ordering in TlCuCl$_3$ on a quantitative level. We show
that the usual BEC picture does not allow for a good description of the
experimental magnetisation data and argue that antisymmetric spin interactions
and/or a staggered $g$ tensor component are still crucial, although both are
expected to be tiny in this compound due to crystal symmetries. Including this
type of interaction we obtain excellent agreement with experimental data.",0403311v2
2004-05-18,Bose-Einstein Condensation of Magnons in TlCuCl3: Phase diagram and specific heat from a self-consistent Hartee-Fock calculation with a realistic dispersion relation,"We extend the self-consistent Hartree-Fock-Popov calculations by Nikuni et
al. [Phys. Rev. Lett. 84, 5868 (2000)] concerning the Bose-Eistein condensation
of magnons in TlCuCl3 to include a realistic dispersion of the excitations. The
result for the critical field as a function of temperature behaves as
Hc(T)-Hc(0) T^{3/2} below 2K but deviates from this simple power-law at higher
temperature and is in very good agreement with the experimental results. The
specific heat is computed as a function of temperature for different values of
the magnetic field. It shows a lambda-like shape at the transition and is in
good qualitative agreement with the results of Oosawa et al. [Phys. Rev. B 63,
134416 (2001)].",0405422v2
2004-06-21,Nonquasiparticle states in half-metallic ferromagnets,"Anomalous magnetic and electronic properties of the half-metallic
ferromagnets (HMF) have been discussed. The general conception of the HMF
electronic structure which take into account the most important correlation
effects from electron-magnon interactions, in particular, the spin-polaron
effects, is presented. Special attention is paid to the so called
non-quasiparticle (NQP) or incoherent states which are present in the gap near
the Fermi level and can give considerable contributions to thermodynamic and
transport properties. Prospects of experimental observation of the NQP states
in core-level spectroscopy is discussed. Special features of transport
properties of the HMF which are connected with the absence of one-magnon
spin-flip scattering processes are investigated. The temperature and magnetic
field dependences of resistivity in various regimes are calculated. It is shown
that the NQP states can give a dominate contribution to the temperature
dependence of the impurity-induced resistivity and in the tunnel junction
conductivity. First principle calculations of the NQP-states for the prototype
half-metallic material NiMnSb within the local-density approximation plus
dynamical mean field theory (LDA+DMFT) are presented.",0406487v1
2004-07-08,Study of Field-Induced Magnetic Order in Singlet-Ground-State Magnet CsFeCl$_3$,"The field-induced magnetic order in the singlet-ground-state system
CsFeCl$_3$ has been studied by measuring magnetization and neutron diffraction.
The field dependence of intensity for the neutron magnetic reflection has
clearly demonstrated that the field-induced ordered phase is described by the
order parameter $<S_x>$. A condensate growth of magnons is investigated through
the temperature dependence of $M_z$ and $M_{\perp}$, and this ordering is
discussed in the context of a magnon Bose-Einstein condensation. Development of
the coherent state and the static correlation length has been observed in the
incommensurate phase in the field region of $5 < H < 6$ T at 1.8 K. At $H >
H_{\rm c}$, a satellite peak was found in coexistence with the commensurate
peak at the phase boundary around 10 T, which indicates that the tilt of the
c-axis would be less than $\sim 0.5^{\circ}$ in the whole experiments.",0407196v2
2004-07-09,Magnetically Stabilized Nematic Order I: Three-Dimensional Bipartite Optical Lattices,"We study magnetically stabilized nematic order for spin-one bosons in optical
lattices. We show that the Zeeman field-driven quantum transitions between
non-nematic Mott states and quantum spin nematic states in the weak hopping
limit are in the universality class of the ferromagnetic XXZ (S=1/2) spin
model. We further discuss these transitions as condensation of interacting
magnons. The development of O(2) nematic order when external fields are applied
corresponds to condensation of magnons, which breaks a U(1) symmetry.
Microscopically, this results from a coherent superposition of two non-nematic
states at each individual site. Nematic order and spin wave excitations around
critical points are studied and critical behaviors are obtained in a dilute gas
approximation. We also find that spin singlet states are unstable with respect
to quadratic Zeeman effects and Ising nematic order appears in the presence of
any finite quadratic Zeeman coupling. All discussions are carried out for
states in three dimensional bipartite lattices.",0407259v1
2004-10-16,Low-lying magnon excitations in integer-spin ladders and tubes,"We consider low-energy excitation structures of N-leg integer-spin ladder and
tube systems with an antiferromagnetic (AF) intrachain coupling and a uniform
external field. The tube means the ladder with the periodic boundary condition
along the interchain (rung) direction. Odd-leg AF-rung tubes have the
frustration. In order to analyze all systems including frustrated tubes, we
apply a field-theoretical method based on the nonlinear sigma model. We mainly
focus on the systems without any external fields. In this case, it is shown
that the lowest bands of frustrated tubes always consist of six-fold degenerate
magnon excitations, while those of all other systems are triply degenerate.
This result implies that the ground states of frustrated tubes (all
non-frustrated systems) become a two (one)-component Tomonaga-Luttinger liquid,
when a sufficiently strong uniform field is applied.",0410419v3
2005-05-16,Spin Waves in a Periodically Layered Magnetic Nanowire,"We report a simple theoretical derivation of the spectrum and damping of spin
waves in a cylindrical periodically structured magnetic nanowire (cylindrical
magnonic crystal) in the ""effective medium"" approximation. The dependence of
the ""effective"" magnetic parameters upon the individual layer parameters is
shown to be different from the arithmetic average over the volume of the
superlattice. The formulae that are obtained can be applied firstly in the
description of spin wave dispersion in the first allowed band of the structure;
and secondly in the design of a magnonic crystal with band-gaps in an arbitrary
part of the spin wave spectrum.",0505382v1
2005-06-23,Magnon Heat Conductivity and Mean Free Paths in Two-Leg Spin Ladders: A Model-Independent Determination,"The magnon thermal conductivity $\kappa_{\mathrm{mag}}$ of the spin ladders
in $\rm Sr_{14}Cu_{24-x}Zn_xO_{41}$ has been investigated at low doping levels
$x=0$, 0.125, 0.25, 0.5 and 0.75. The Zn-impurities generate nonmagnetic
defects which define an upper limit for $l_{\mathrm{mag}}$ and therefore allow
a clear-cut relation between $l_{\mathrm{mag}}$ and $\kappa_{\mathrm{mag}}$ to
be established independently of any model. Over a large temperature range we
observe a progressive suppression of $\kappa_{\mathrm{mag}}$ with increasing
Zn-content and find in particular that with respect to pure $\rm
Sr_{14}Cu_{24}O_{41}$ $\kappa_{\mathrm{mag}}$ is strongly suppressed even in
the case of tiny impurity densities where $l_{\mathrm{mag}}\lesssim 374$~{\AA}.
This shows unambiguously that large $l_{\mathrm{mag}}\approx 3000$~{\AA} which
have been reported for $\rm Sr_{14}Cu_{24}O_{41}$ and $\rm
La_{5}Ca_9Cu_{24}O_{41}$ on basis of a kinetic model are in the correct order
of magnitude.",0506595v2
2005-07-22,Neutron Scattering Study of Quantum Phase Transitions in Integral Spin Chains,"Quite a few low-dimensional magnets are quantum-disordered ``spin liquids''
with a characteristic gap in the magnetic excitation spectrum. Among these are
antiferromagnetic chains of integer quantum spins. Their generic feature are
long-lived massive (gapped) excitations (magnons) that are subject to Zeeman
splitting in external magnetic fields. The gap in one of the magnon branches
decreases with field, driving a soft-mode quantum phase transition. The system
then enters a qualitatively new high-field phase. The actual properties at high
fields, particularly the spin dynamics, critically depend on the system under
consideration. Recent neutron scattering studies of organometallic polymer
crystals NDMAP (Haldane spin chains with anisotropy) and NTENP (dimerized S=1
chains) revealed rich and unique physics.",0507534v1
2005-10-06,New light on magnetic excitations: indirect resonant inelastic X-ray scattering on magnons,"Recent experiments show that indirect resonant inelastic X-ray scattering
(RIXS) is a new probe of spin dynamics. Here I derive the cross-section for
magnetic RIXS and determine the momentum dependent four-spin correlation
function that it measures. These results show that this technique offers
information on spin dynamics that is complementary to e.g. neutron scattering.
The RIXS spectrum of Heisenberg antiferromagnets is calculated. It turns out
that only scattering processes that involve at least two magnons are allowed.
Other selection rules imply that the scattering intensity vanishes for specific
transferred momenta ${\bf q}$, in particular for ${\bf q}=0$. The calculated
spectra agree very well with the experimental data.",0510140v3
2005-11-01,Possibility of a Two-Component Tomonaga-Luttinger Liquid in Frustrated Integer-Spin Tubes,"Uniform-field effects for frustrated odd-leg integer-spin tubes (cylinder
type spin systems) are investigated in the weak interchain-coupling regime. We
predict that, as the field exceeds the spin gap, a two-component
Tomonaga-Luttinger liquid (TLL) appears due to the condensation of the doubly
degenerate lowest magnons. Furthermore, it is argued that when the uniform
field is so strong that the second lowest magnons are also condensed, the
two-component TLL is destroyed and a new one-component TLL emerges. This
quantum phase transition may be detected as a magnetization cusp.",0511024v2
2005-11-16,Field-induced Bose-Einstein condensation of interacting dilute magnons in three-dimensional spin systems: A renormalization-group study,"We use the Renormalization Group method to study the magnetic field influence
on the Bose-Einstein condensation of interacting dilute magnons in three
dimensional spin systems. We first considered a model with SU(2) symmetry
(universality class $z=1$) and we obtain for the critical magnetic field a
power law dependence on the critical temperature, $[H_c(T)-H_c(0)]\sim T^2$. In
the case of U(1) symmetry (universality class $z=2$) the dependence is
different, and the magnetic critical field depends linearly on the critical
temperature, $[H_c(T)-H_c(0)]\sim T$. By considering a more relevant model,
which includes also the system's anisotropy, we obtain for the same symmetry
class a $T^{3/2}$ dependence of the magnetic critical field on the critical
temperature. We discuss these theoretical predictions of the renormalization
group in connection with experimental results reported in the literature.",0511388v1
2005-12-09,On the Bose-Einstein Condensation of Magnons in Cs2CuCl4,"In a recent paper \cite{Radu}, Radu \textit{et al.} report experimental
results they claim to support Bose-Einstein condensation (BEC) of magnons in
Cs$_2$CuCl$_4$. It is true that an experimentally measured critical power law
scaling exponent in agreement with the BEC universality class would support the
realization of a BEC in magnetic systems that order as a canted
antiferromagnet. It can be shown, however, that the claim of Radu {\it et al.}
is overstated in this instance, because their determination of the critical
exponent $\phi$ relies on a model-dependent theoretical approximation to the
critical field $H_{\textrm{c1}}$ for which the associated errors are neglected.
We show that when these errors are included, the uncertainty in the obtained
exponent is so large that the available experimental data cannot be used to
differentiate between contending universality classes.",0512193v1
2006-01-02,Three--body Correlation Effects on the Spin Dynamics of Double--Exchange Ferromagnets,"We present a variational calculation of the spin wave excitation spectrum of
double--exchange ferromagnets in different dimensions. Our theory recovers the
Random Phase approximation and 1/S expansion results as limiting cases and can
be used to study the intermediate exchange coupling and electron concentration
regime relevant to the manganites. In particular, we treat exactly the long
range three--body correlations between a Fermi sea electron--hole pair and a
magnon excitation and show that they strongly affect the spin dynamics in the
parameter range relevant to experiments in the manganites. The manifestations
of these correlations are many-fold. We demonstrate that they significantly
change the ferromagnetic phase boundary. In addition to a decrease in the
magnon stiffness, we obtain an instability of the ferromagnetic state against
spin wave excitations close to the Brillouin zone boundary.Within a range of
intermediate concentrations, we find a strong softening of the spin wave
dispersion as compared to the Heisenberg ferromagnet with the same stiffness,
which changes into hardening for other concentrations. We discuss the relevance
of these results to experiments in colossal magnetoresistance ferromagnets.",0601025v2
2006-04-02,Universal low-temperature behavior of frustrated quantum antiferromagnets in the vicinity of the saturation field,"We study the low-temperature thermodynamic properties of a number of
frustrated quantum antiferromagnets which support localized magnon states in
the vicinity of the saturation field. For this purpose we use 1) a mapping of
the low-energy degrees of freedom of spin systems onto the hard-core object
lattice gases and 2) an exact diagonalization of finite spin systems of up to
N=30 sites. The considered spin systems exhibit universal behavior which is
determined by a specific hard-core object lattice gas representing the
independent localized magnon states. We test the lattice gas description by
comparing its predictions with the numerical results for low-lying energy
states of finite spin systems. For all frustrated spin systems considered we
find a strong variation of the low-temperature specific heat passing the
saturation field and a maximum in the isothermal entropy at saturation field
resulting in an enhanced magnetocaloric effect.",0604023v1
2006-04-03,Canted Ferromagnetism in Double Exchange Model with on-site Coulomb Repulsion,"The double exchange model with on-site Coulomb repulsion is considered.
Schwinger-bosons representation of the localized spins is used and two
spin-singlet Fermion operators are introduced. In terms of the new Fermi fields
the on-site Hund's interaction is in a diagonal form and the true magnons of
the system are identified. The singlet fermions can be understood as electrons
dressed by a cloud of repeatedly emitted and reabsorbed magnons. Rewritten in
terms of Schwinger-bosons and spin-singlet fermions the theory is U(1) gauge
invariant. We show that spontaneous breakdown of the gauge symmetry leads to
\emph{\textbf{canted ferromagnetism with on-site spins of localized and
delocalized electrons misaligned}}. On-site canted phase emerges in double
exchange model when Coulomb repulsion is large enough. The quantum phase
transition between ferromagnetism and canted phase is studied varying the
Coulomb repulsion for different values of parameters in the theory such as
Hund's coupling and chemical potential.",0604047v1
2006-04-27,Raman scattering from phonons and magnons in RFe3)BO3)4,"Inelastic light scattering spectra of several members of the RFe3(BO3)4
family reveal a cascade of phase transitions as a function of temperature,
starting with a structural, weakly first order, phase transition followed by
two magnetic phase transitions. Those consist of the ordering of the Fe-spin
sublattice revealed by all the compound, and a subsequent spin-reorientational
transition for GdFe3(BO3)4. The Raman data evidence a strong coupling between
the lattice and magnetic degrees of freedom in these borates. The Fe-sublattice
ordering leads to a strong suppression of the low energy magnetic scattering,
and a multiple peaked two-magnon scattering continuum is observed. Evidence for
short-range correlations is found in the `paramagnetic' phase by the
observation of a broad magnetic continuum in the Raman data, which persists up
to surprisingly high temperatures.",0604615v1
2006-05-25,Magnon bands of N-leg integer-spin antiferromagnetic systems in the weak interchain-coupling regime,"Using the exact results of the O(3) nonlinear sigma model (NLSM) and a few
quantitative numerical data for integer-spin antiferromagnetic (AF) chains, we
systematically estimate all magnon excitation energies of N-leg integer-spin AF
ladders and tubes in the weak-interchain-coupling regime. Our method is based
on a first-order perturbation theory for the strength of the interchain
coupling. It can deal with any kind of interchain interactions, in principle.
We confirm that results of the perturbation theory are in good agreement with
those of a quantum Monte Carlo simulation and with our recent study based on a
saddle-point approximation of the NLSM [Phys. Rev. B 72, 104438 (2005)]. Our
theory further supports the existence of a Haldane (gapped) phase even in a
d-dimensional (d\geq 2) spatially anisotropic integer-spin AF model, if the
exchange coupling in one direction is sufficiently strong compared with those
in all the other directions. The strategy in this paper is applicable to other
N-leg systems consisting of gapped chains which low-energy physics is exactly
or quantitatively known.",0605635v2
2006-07-27,Magnon Bose condensation in symmetry breaking magnetic field,"Magnon Bose condensation (BC)in the symmetry breaking magnetic field is a
result of unusual form of the Zeeman energy, which has terms linear in the
spin-wave operators and terms mixing excitations differ in the Wave-vector of
the magnetic structure. The following examples are considered: simple
easy-plane tetragonal antiferromagnets (AF), frustrated AF family$R_2Cu O_4$
where $R=Pr,Nd$ etc. and cubic magnets with the Dzyaloshinskii-Moriya
interaction ($Mn Si$ etc.). In all cases the BC becomes important when the
magnetic field becomes comparable with the spin-wave gap. The theory is
illustrated by existing experimental results.",0607711v2
2006-08-01,Magnon Decay in Noncollinear Quantum Antiferromagnets,"Instability of the excitation spectrum of an ordered noncollinear Heisenberg
antiferromagnet (AF) with respect to spontaneous two-magnon decays is
investigated. We use a spin-1/2 AF on a triangular lattice as an example and
examine the characteristic long- and short wave-length features of its
zero-temperature spectrum within the 1/S-approximation. The kinematic
conditions are shown to be crucial for the existence of decays and for overall
properties of the spectrum. The XXZ and the J-J' generalizations of the model,
as well as the role of higher-order corrections are discussed.",0608035v4
2006-08-31,Momentum-dependent light scattering in a 2D Heisenberg antiferromagnet,"Motivated by the achievements of the $X$-ray scattering technique, we
analyzed the profile of the light scattering intensity $R(q, \omega)$ at a
finite $q$ in a 2D Heisenberg antiferromagnet. Previous Raman scattering
studies at $q=0$ identified the two-magnon peak in $B_{1g}$ scattering
geometry. We found that the $B_{1g}$ peak disperses downwards at a finite $q$,
and its intensity increases, reaching its maximum at $q= q_0 = (0, \pi)$ and
symmetry related points. In addition, the intensity in the $A_{1g}$ geometry
becomes non-zero at a finite $q$, and also displays a two-magnon peak which
gains strength and disperses to larger frequencies with increasing $q$, and
reaches its highest intensity at $q_0$. We found that the profile of $R(q_0,
\omega)$ is equivalent in $A_{1g}$ and $B_{1g}$ geometries.",0609002v1
2006-10-20,Probing Anomalous Longitudinal Fluctuations of the Interacting Bose Gas via Bose-Einstein Condensation of Magnons,"The emergence of a finite staggered magnetization in quantum Heisenberg
antiferromagnets subject to a uniform magnetic field can be viewed as
Bose-Einstein condensation of magnons. Using non-perturbative results for the
infrared behavior of the interacting Bose gas, we present exact results for the
staggered spin-spin correlation functions of quantum antiferromagnets in a
magnetic field at zero temperature. In particular, we show that in dimensions 1
< D <= 3 the longitudinal dynamic structure factor S_parallel(q,omega)
describing staggered spin fluctuations in the direction of the staggered
magnetization exhibits a critical continuum whose weight can be controlled
experimentally by varying the magnetic field.",0610575v3
2006-10-23,Magnetic excitations in multiferroic TbMnO3,"The magnetic excitations in multiferroic TbMnO3 have been studied by
inelastic neutron scattering in the spiral and sinusoidally ordered phases. At
the incommensurate magnetic zone center of the spiral phase, we find three
low-lying magnons whose character has been fully determined using
neutron-polarization analysis. The excitation at the lowest energy is the
sliding mode of the spiral, and two modes at 1.1 and 2.5meV correspond to
rotations of the spiral rotation plane. These latter modes are expected to
couple to the electric polarization. The 2.5meV-mode is in perfect agreement
with recent infra-red-spectroscopy data giving strong support to its
interpretation as an hybridized phonon-magnon excitation.",0610620v1
2006-12-12,Universal properties of highly frustrated quantum magnets in strong magnetic fields,"The purpose of the present paper is two-fold. On the one hand, we review some
recent studies on the low-temperature strong-field thermodynamic properties of
frustrated quantum spin antiferromagnets which admit the so-called
localized-magnon eigenstates. One the other hand, we provide some complementary
new results. We focus on the linear independence of the localized-magnon
states, the estimation of their degeneracy with the help of auxiliary classical
lattice-gas models and the analysis of the contribution of these states to
thermodynamics.",0612281v2
2007-01-09,Twenty years of magnon Bose condensation and spin current superfluidity in 3He-B,"20 years ago a new quantum state of matter was discovered and identified. The
observed dynamic quantum state of spin precession in superfluid 3He-B bears the
properties of spin current superfluidity, Bose condensation of spin waves --
magnons, off-diagonal long-range order and related phenomena of quantum
coherence",0701180v13
2007-01-30,Magnetic Excitations in the Spin-1 Anisotropic Heisenberg Antiferromagnetic Chain System NiCl$_2$-4SC(NH$_2$)$_2$,"NiCl$_2$-4SC(NH$_2$)$_2$ (DTN) is a quantum S=1 chain system with strong
easy-pane anisotropy and a new candidate for the Bose-Einstein condensation of
the spin degrees of freedom. ESR studies of magnetic excitations in DTN in
fields up to 25 T are presented. Based on analysis of the single-magnon
excitation mode in the high-field spin-polarized phase and previous
experimental results [Phys. Rev. Lett. 96, 077204 (2006)], a revised set of
spin-Hamiltonian parameters is obtained. Our results yield $D=8.9$ K, $J_c=2.2$
K, and $J_{a,b}=0.18$ K for the anisotropy, intrachain, and interchain exchange
interactions, respectively. These values are used to calculate the
antiferromagnetic phase boundary, magnetization and the frequency-field
dependence of two-magnon bound-state excitations predicted by theory and
observed in DTN for the first time. Excellent quantitative agreement with
experimental data is obtained.",0701750v1
2007-03-07,Magnon condensation into Q-ball in 3He-B,"The theoretical prediction of Q-balls in relativistic quantum fields is
realized here experimentally in superfluid 3He-B. The condensed-matter analogs
of relativistic Q-balls are responsible for an extremely long lived signal of
magnetic induction -- the so-called Persistent Signal -- observed in NMR at the
lowest temperatures. This Q-ball is another representative of a state with
phase coherent precession of nuclear spins in 3He-B, similar to the well known
Homogeneously Precessing Domain which we interpret as Bose condensation of spin
waves -- magnons. At large Q the effect of self-localization is observed. In
the language of relativistic quantum fields it is caused by interaction between
the charged and neutral fields, where the neutral field provides the potential
for the charged one. In the process of self-localization the charged field
modifies locally the neutral field so that the potential well is formed in
which the charge is condensed.",0703183v3
2006-04-13,Higher Loop Bethe Ansatz for Open Spin-Chains in AdS/CFT,"We propose a perturbative asymptotic Bethe ansatz (PABA) for open spin-chain
systems whose Hamiltonians are given by matrices of anomalous dimension for
composite operators, and apply it to two types of composite operators related
to two different brane configurations. One is an AdS_4 \times S^2-brane in the
bulk AdS_5 \times S^5 which gives rise to a defect conformal field theory
(dCFT) in the dual field theory, and the other is a giant graviton system with
an open string excitation. In both cases, excitations on open strings attaching
to D-branes (a D5-brane for the dCFT case, and a spherical D3-brane for the
giant graviton case) can be represented by magnon states in the spin-chains
with appropriate boundary conditions, in which informations of the D-branes are
encoded. We concentrate on single-magnon problems, and explain how to calculate
boundary S-matrices via the PABA technique. We also discuss the energy spectrum
in the BMN limit.",0604100v3
2006-12-27,Emergent Classical Strings from Matrix Model,"Generalizing the idea of hep-th/0509015 by Berenstein, Correa, and Vazquez,
we study many-magnon states in an SU(2) sector of a reduced matrix quantum
mechanics obtained from N=4 SU(N) super Yang-Mills on R x S^3. Generic Q-magnon
states are described as a chain of ``string-bits'' joining Q+1 eigenvalues of
background matrices which form a 1/2 BPS circular droplet in the large N limit.
We will concentrate on infinitely long states whose first and last eigenvalues
localize at the edge of the droplet. Each constituent string-bit has a complex
quasi-momentum in general, while the total quasi-momentum P of the state is
real. For given Q and P, the minimum energy of the chain of string-bits is
realized when the Q+1 eigenvalues are equally spaced on one and the same line
segment joining the two outmost eigenvalues localized on the edge with angular
difference P. Such configuration of bound string-bits precisely reproduces the
dispersion relation for dyonic giant magnons in classical string theory. We
also show the emergence of two-spin folded/circular strings in special infinite
spin limit as particular configurations of closed chains of string-bits.",0612269v1
2007-07-09,"Evidence for strong 5d electron correlations and electron-magnon coupling in a pyrochlore, Y2Ir2O7","We report the observation of an unusual behavior of highly extended 5d
electrons in Y2Ir2O7 belonging to pyrochlore family of great current interest
using high resolution photoemission spectroscopy. The experimental bulk spectra
reveal an intense lower Hubbard band in addition to weak intensities in the
vicinity of the Fermi level, e_F. This provides a direct evidence for strong
electron correlation among the 5d electrons, despite their highly extended
nature. The high resolution spectrum at room temperature exhibits a pseudogap
at e_F and |e - e_F|^2 dependence demonstrating the importance of electron
correlation in this system. Remarkably, in the magnetically ordered phase (T <
150 K), the spectral lineshape evolves to a |e - e_F|^1.5 dependence
emphasizing the dominant role of electron-magnon coupling.",0707.1179v1
2007-07-24,Strong orientational effect of stretched aerogel on the 3He order parameter,"Deformation of aerogel strongly modifies the orientation of the order
parameter of superfluid 3He confined in aerogel. We used a radial squeezing of
aerogel to keep the orbital angular momentum of the 3He Cooper pairs in the
plane perpendicular to the magnetic field. We did not find strong evidence for
a ""polar"" phase, with a nodal line along the equator of the Fermi surface,
predicted to occur at large radial squeezing. Instead we observed 3He-A with a
clear experimental evidence of the destruction of the long-range order by
random anisotropy -- the Larkin-Imry-Ma effect. In 3He-B we observed and
identified new modes of NMR, which are impossible to obtain in bulk 3He-B. One
of these modes is characterized by a repulsive interaction between magnons,
which is suitable for the magnon Bose-Einstein condensation (BEC).",0707.3544v3
2007-07-26,Curie temperature of the two band double exchange model for manganites,"We consider two-band double exchange model and calculate the critical
temperature in ferromagnetic regime (Curie temperature). The localized spins
are represented in terms of the Schwinger-bosons, and two spin-singlet Fermion
operators are introduced. In terms of the new Fermi fields the on-site Hund's
interactions are in a diagonal form and one accounts for them exactly.
Integrating out the spin-singlet fermions we derive an effective Heisenberg
type model for a vector which describes the local orientations of the total
magnetization. The transversal fluctuations of the vector are the true magnons
in the theory, which is a base for Curie temperature calculation. The critical
temperature is calculated employing the Schwinger-bosons mean-field theory.
While approximate, this technic of calculation captures the essentials of the
magnon fluctuations in the theory, and for 2D systems one obtains zero Curie
temperature, in accordance with Mermin-Wagner theorem.",0707.3891v1
2007-08-17,Magnetic Phase Diagram of Spin-1/2 Two-Leg Ladder with Four-Spin Ring Exchange,"We study the spin-1/2 two-leg Heisenberg ladder with four-spin ring exchanges
under a magnetic field. We introduce an exact duality transformation which is
an extension of the spin-chirality duality developed previously and yields a
new self-dual surface in the parameter space. We then determine the magnetic
phase diagram using the numerical approaches of the density-matrix
renormalization-group and exact diagonalization methods. We demonstrate the
appearance of a magnetization plateau and the Tomonaga-Luttinger liquid with
dominant vector-chirality quasi-long-range order for a wide parameter regime of
strong ring exchange. A ""nematic"" phase, in which magnons form bound pairs and
the magnon-pairing correlation functions dominate, is also identified.",0708.2338v1
2007-09-26,On the anatomy of multi-spin magnon and single spike string solutions,"We study rigid string solutions rotating in $AdS_5\times S^5$ background. For
particular values of the parameters of the solutions we find multispin
solutions corresponding to giant magnons and single spike strings. We present
an analysis of the dispersion relations in the case of three spin solutions
distributed only in $S^5$ and the case of one spin in $AdS_5$ and two spins in
$S^5$. The possible relation of these string solutions to gauge theory
operators and spin chains are briefly discussed.",0709.4231v3
2007-10-11,Scattering of single spikes,"We apply the dressing method to a string solution given by a static string
wrapped around the equator of a three-sphere and find that the result is the
single spike solution recently discussed in the literature. Further application
of the method allows the construction of solutions with multiple spikes. In
particular we construct the solution describing the scattering of two single
spikes and compute the scattering phase shift. As a function of the dressing
parameters, the result is exactly the same as the one for the giant magnon, up
to non-logarithmic terms. This suggests that the single spikes should be
described by an integrable spin chain closely related to the one associated to
the giant magnons. The field theory interpretation of such spin chain however
is still unclear.",0710.2300v2
2007-11-22,A note on the Near Flat Limit for strings in the Maldacena-Nunez background,"Recently Maldacena and Swanson suggested a new limit of string theory on the
$AdS_5\times S^5$ background, the so called near flat space limit. The
resulting reduced theory interpolates between the pp-wave limit and giant
magnon type string solutions. It was shown that the reduced model possess many
features of the original theory. On the other hand, theories with less
supersymmetry are of great importance for the string/gauge theory
correspondence. In this paper we study the near flat limit reduction of string
theory on the Maldacena-Nunez background, which is dual to $\N=1$ Yang-Mills
theory. The reduced model interpolates between the pp-wave limit and a certain
magnon type subsector of the theory. The similarity of the structures of the
reduced model obtained here and that by Maldacena and Swanson indicates the
possibility of existence of integrable subsectors of strings on the
Maldacena-Nunez background.",0711.3600v4
2008-01-11,Melting of magnetic correlations in charge-orbital ordered La(0.5)Sr(1.5)MnO(4) : competition of ferro and antiferromagnetic states,"The magnetic correlations in the charge- and orbital-ordered manganite
La(0.5)Sr(1.5)MnO(4) have been studied by elastic and inelastic neutron
scattering techniques. Out of the well-defined CE-type magnetic structure with
the corresponding magnons a competition between CE-type and ferromagnetic
fluctuations develops. Whereas ferromagnetic correlations are fully suppressed
by the static CE-type order at low temperature, elastic and inelastic CE-type
correlations disappear with the melting of the charge-orbital order at high
temperature. In its charge-orbital disordered phase, La(0.5)Sr(1.5)MnO(4)
exhibits a dispersion of ferromagnetic correlations which remarkably resembles
the magnon dispersion in ferromagnetically ordered metallic perovskite
manganites.",0801.1787v1
2008-04-10,Magneto-elastic interaction in cubic helimagnets with B20 structure,"The magneto-elastic interaction in cubic helimagnets with B20 symmetry is
considered. It is shown that this interaction is responsible for negative
contribution to the square of the spin-wave gap $\Delta$ which is alone has to
disrupt assumed helical structure. It is suggested that competition between
positive part of $\Delta^2_I$ which stems from magnon-magnon interaction and
its negative magneto-elastic part leads to the quantum phase transition
observed at high pressure in $Mn Si$ and $Fe Ge$. This transition has to occur
when $\Delta^2=0$. For $Mn Si$ from rough estimations at ambient pressure both
parts $\Delta_I$ and $|\Delta_{ME}|$ are comparable with the experimentally
observed gap. The magneto-elastic interaction is responsible also for $2\m k$
modulation of the lattice where $\m k$ is the helix wave-vector and
contribution to the magnetic anisotropy.
  Experimental observation by $x$-ray and neutron scattering the lattice
modulation allows determine the strength of anisotropic part of the
magneto-elastic interaction responsible for above phenomena and the lattice
helicity.",0804.1678v1
2008-04-11,"Two-magnon Raman scattering from the Cu3O4 layers in (Sr,Ba)2Cu3O4Cl2","(Sr$_{2}$,Ba$_{2}$)Cu$_{3}$O$_{4}$Cl$_{2}$ are antiferromagnetic insulators
which are akin to the parent compounds of the cuprate superconductors but with
two distinct magnetic ordering temperatures related to two magnetic Cu$_{I}$
and Cu$_{II}$ spin sublattices. Here we present a study of these materials by
means of Raman spectroscopy. Following the temperature and polarization
dependence of the data we readily identify two distinct features at around 3000
cm$^{-1}$ and 300 cm$^{-1}$ that are related to two-magnon scattering from the
two sublattices. The estimated spin-exchange coupling constants for the
Cu$_{I}$ and Cu$_{II}$ sublattices are found to be J$_{I}\sim$139-143(132-136)
meV and J$_{II}\sim$14(11) meV for Sr(Ba) compounds. Moreover, we observe modes
at around 480 and 445 cm$^{-1}$ for the Sr and Ba containing samples
respectively, that disappears at the ordering temperature of the Cu$_{II}$. We
argue that this modes may also be of magnetic origin and possibly related to
interband transitions between the Cu$_{I}$-Cu$_{II}$ sublattices.",0804.1859v2
2008-04-18,On the giant magnon and spike solutions for strings on AdS$_3\times$ S$^3$,"We study solutions for the rotating strings on the sphere with a background
NS-NS field and on the Anti-de-Sitter spacetime. We show the existence of
magnon and single spike solutions on R$\times$S$^2$ in the presence of constant
magnetic field as two limiting cases. We also study the solution for strings on
AdS$_3\times$ S$^3$ with Melvin deformation. The dispersion relations among
various conserved charges are shown to receive finite corrections due to the
deformation parameter. We further study the rotating string on AdS$_3 \times$
S$^3$ geometry with two conserved angular momenta on S$^3$ and one spin along
the AdS$_3$. We show that there exists two kinds of solutions: a circular
string solution and a helical string. We find out the dispersion relation among
various charges and give physical interpretation of these solutions.",0804.2923v3
2008-06-01,Anomalous frequency and intensity scaling of collective and local modes in a coupled spin tetrahedron system,"We report on the magnetic excitation spectrum of the coupled spin tetrahedral
system Cu$_{2}$Te$_{2}$O$_{5}$Cl$_{2}$ using Raman scattering on single
crystals. The transition to an ordered state at T$_{N}^{Cl}$=18.2 K evidenced
from thermodynamic data leads to the evolution of distinct low-energy magnetic
excitations superimposed by a broad maximum. These modes are ascribed to
magnons with different degree of localization and a two-magnon continuum. Two
of the modes develop a substantial energy shift with decreasing temperature
similar to the order parameter of other Neel ordered systems. The other two
modes show only a negligible temperature dependence and dissolve above the
ordering temperature in a continuum of excitations at finite energies. These
observations point to a delicate interplay of magnetic inter- and
intra-tetrahedra degrees of freedom and an importance of singlet fluctuations
in describing a spin dynamics.",0806.0147v1
2008-08-04,Reflecting magnons from D7 and D5 branes,"We obtain the reflection matrices for the scattering of elementary magnons
from certain open boundaries, corresponding to open strings ending on D7 and D5
branes in $AdS_5\times S^5$. In each case we consider two possible orientations
for the vacuum state. We show that symmetry arguments are sufficient to
determine the reflection matrices up to at most two unknown functions. The D7
reflection matrices obey the boundary Yang Baxter-Equation. This is automatic
for one vacuum orientation, and requires a natural choice of ratio between two
unknowns for the other. In contrast, the D5 reflection matrices do not obey the
boundary Yang Baxter-Equation. In both cases we show consistency with the
existent weak and strong coupling results.",0808.0452v3
2008-08-15,Collective excitations in two-dimensional antiferromagnet in strong magnetic field,"We discuss spin-$\frac12$ two-dimensional (2D) Heisenberg antiferromagnet
(AF) on a square lattice at T=0 in strong magnetic field H near its saturation
value $H_c$. A perturbation approach is proposed to obtain spectrum of magnons
with momenta not very close to AF vector in the leading order in small
parameter $(H_c-H)/H_c$. We find that magnons are well-defined quasi-particles
at $H>0.9H_c$ although the damping is quite large near the zone boundary. A
characteristic rotonlike local minimum in the spectrum is observed at ${\bf
k}=(\pi,0)$ accompanied by decrease of the damping near $(\pi,0)$. The
suggested approach can be used in discussion of short-wavelength excitations in
other 2D Bose gases of particles or quasi-particles.",0808.2127v3
2008-11-25,Electric-dipole active two-magnon excitation in {\textit{ab}} spiral spin phase of a ferroelectric magnet Gd$_{\textbf{0.7}}$Tb$_{\textbf{0.3}}$MnO$_{\textbf 3}$,"A broad continuum-like spin excitation (1--10 meV) with a peak structure
around 2.4 meV has been observed in the ferroelectric $ab$ spiral spin phase of
Gd$_{0.7}$Tb$_{0.3}$MnO$_3$ by using terahertz (THz) time-domain spectroscopy.
Based on a complete set of light-polarization measurements, we identify the
spin excitation active for the light $E$ vector only along the a-axis, which
grows in intensity with lowering temperature even from above the magnetic
ordering temperature but disappears upon the transition to the $A$-type
antiferromagnetic phase. Such an electric-dipole active spin excitation as
observed at THz frequencies can be ascribed to the two-magnon excitation in
terms of the unique polarization selection rule in a variety of the
magnetically ordered phases.",0811.4098v1
2009-01-18,Stable topological textures in a classical 2D Heisenberg model,"We show that stable localized topological soliton textures (skyrmions) with
$\pi_2$ topological charge $\nu \geq 1$ exist in a classical 2D Heisenberg
model of a ferromagnet with uniaxial anisotropy. For this model the soliton
exist only if the number of bound magnons exceeds some threshold value $N_{\rm
cr}$ depending on $\nu $ and the effective anisotropy constant $K_{\rm eff}$.
We define soliton phase diagram as the dependence of threshold energies and
bound magnons number on anisotropy constant. The phase boundary lines are
monotonous for both $\nu=1$ and $\nu >2$, while the solitons with $\nu=2$
reveal peculiar nonmonotonous behavior, determining the transition regime from
low to high topological charges. In particular, the soliton energy per
topological charge (topological energy density) achieves a minimum neither for
$\nu=1$ nor high charges, but rather for intermediate values $\nu=2$ or
$\nu=3$.",0901.2707v1
2009-01-22,Magnon-Mediated Pairing and Isotope Effect in Iron-based Superconductors,"Within a minimal model for the iron-based superconductors in which itinerant
electrons interact with a band of local moments, we derive a a general
conclusion for multi-band superconductivity. In a multi-band superconductor,
due to the Adler theorem, the inter-band scattering dominates the intra-band
scattering at the long wave length limit as long as both interactions are
induced by Goldstone boson (which is magnon in our case) and the transfered
momentum is nonzero. Such kind of interaction leads to a well-known
sigh-reversing superconductivity even if the inter-band and intra-band
interaction are repulsive. This effect can be modeled as arising from an
internal Josephson link between the Fermi surface sheets. Our model is also
consistent with the recently discovered coexistence of superconductivity and
magnetic order in iron-pnictides. Although the experimentally observed isotope
effect is large, $\alpha=0.4$, we show that it is consistent with a
non-phononic mechanism in which it is the isotope effects result in a change in
the lattice constant and as a consequence the zero-point motion of the Fe
atoms.",0901.3538v2
2009-01-28,A phason disordered two dimensional quantum antiferromagnet,"We examine a novel type of disorder in quantum antiferromagnets. Our model
consists of localized spins with antiferromagnetic exchanges on a bipartite
quasiperiodic structure, which is geometrically disordered in such a way that
no frustration is introduced. In the limit of zero disorder, the structure is
the perfect Penrose rhombus tiling. This tiling is progressively disordered by
augmenting the number of random ""phason flips"" or local tile-reshuffling
operations. The ground state remains N\'eel ordered, and we have studied its
properties as a function of increasing disorder using linear spin wave theory
and quantum Monte Carlo. We find that the ground state energy decreases,
indicating enhanced quantum fluctuations with increasing disorder. The magnon
spectrum is progressively smoothed, and the effective spin wave velocity of low
energy magnons increases with disorder. For large disorder, the ground state
energy as well as the average staggered magnetization tend towards limiting
values characteristic of this type of randomized tilings.",0901.4441v1
2009-02-08,Three-magnon problem for exactly rung-dimerized spin ladders: from general outlook to Bethe Ansatze,"Three-magnon problem for exactly rung-dimerized spin ladder is brought up
separately at all total spin sectors. At first a special duality transformation
of the $\rm Schr\ddot odinger$ equation is found within general outlook. Then
the problem is treated within Coordinate Bethe Ansatze. A straightforward
approach is developed to obtain pure scattering states. At values S=0 and S=3
of total spin the $\rm Schr\ddot odinger$ equation has the form inherent in the
$XXZ$ chain. For $S=1,2$ solvability holds only in five previously found {\it
completely integrable} cases. Nevertheless a partial S=1 Bethe solution always
exists even for general non integrable model. Pure scattering states for all
total spin sectors are presented explicitly.",0902.1335v1
2009-03-21,Scattering of surface and volume spin waves in a magnonic crystal,"The operational characteristics of a magnonic crystal, which was fabricated
as an array of shallow grooves etched on a surface of a magnetic film, were
compared for magnetostatic surface spin waves and backward volume magnetostatic
spin waves. In both cases the formation of rejection frequency bands was
studied as a function of the grooves depth. It has been found that the
rejection of the volume wave is considerably larger than of the surface one.
The influences of the nonreciprocity of the surface spin waves as well as of
the scattering of the lowest volume spin-wave mode into higher thickness volume
modes on the rejection efficiency are discussed.",0903.3686v1
2009-04-17,Control of bound-pair transport by periodic driving,"We investigate the effect of periodic driving by an external field on systems
with attractive pairing interactions. These include spin systems (like the
ferromagnetic XXZ model) as well as ultracold fermionic atoms described by the
attractive Hubbard model. We show that a well-known phenomenon seen in
periodically driven systems--the renormalization of the exchange coupling
strength--acts selectively on bound-pairs of spins/atoms, relative to
magnon/bare atom states. Thus one can control the direction and speed of
transport of bound-pair relative to magnon/unpaired atom states, and thus
coherently achieve spatial separation of these components. Applications to
recent experiments on transport with fermionic atoms in optical lattices which
consist of mixtures of bound-pairs and bare atoms are discussed.",0904.2614v6
2009-04-22,Theory of Thermal Hall Effect in Quantum Magnets,"We present a theory of the thermal Hall effect in insulating quantum magnets,
where the heat current is totally carried by charge-neutral objects such as
magnons and spinons. Two distinct types of thermal Hall responses are
identified. For ordered magnets, the intrinsic thermal Hall effect for magnons
arises when certain conditions are satisfied for the lattice geometry and the
underlying magnetic order. The other type is allowed in a spin liquid which is
a novel quantum state since there is no order even at zero temperature. For
this case, the deconfined spinons contribute to the thermal Hall response due
to Lorentz force. These results offer a clear experimental method to prove the
existence of the deconfined spinons via a thermal transport phenomenon.",0904.3427v3
2009-05-07,Towards the theory of ferrimagnetism II,"The present paper is a sequel to the paper by Karchev (2008
J.Phys.:Condens.Matter {\bf 20} 325219). A two-sublattice ferrimagnet, with
spin-$s_1$ operators $\bf{S_{1i}}$ at the sublattice $A$ site and spin-$s_2$
operators $\bf{S_{2i}}$ at the sublattice $B$ site, is considered. Renormalized
spin-wave theory, which accounts for the magnon-magnon interaction, and its
extension are developed to describe the two ferrimagnetic phases $(0,T^*)$ and
$(T^*,T_N)$ in the system, and to calculate the magnetization as a function of
temperature.
  The influence of the parameters in the theory on the characteristic
temperatures $T_N$ and $T^*$ is studied. It is shown that, increasing the
inter-sublattice exchange interaction, the ratio $T_N/T^*>1$ decreases
approaching one, and above some critical value of the exchange constant there
is only one phase $T_N = T^*$, and the magnetization-temperature curve has the
typical Curie-Weiss profile. When the intra-exchange constant of sublattice
with stronger intra-exchange interaction increases the $Ne\grave{e}l$
temperature increases while $T^*$ remains unchanged. Finally, when the magnetic
order of the sublattice with smaller magnetic order decreases, $T^*$ decreases.
The theoretical predictions are utilize to interpret the experimentally
measured magnetization-temperature curves.",0905.1036v1
2009-08-17,Magnon dispersion to four loops in the ABJM and ABJ models,"The ABJM model is a superconformal Chern-Simons theory with N=6 supersymmetry
which is believed to be integrable in the planar limit. However, there is a
coupling dependent function that appears in the magnon dispersion relation and
the asymptotic Bethe ansatz that is only known to leading order at strong and
weak coupling. We compute this function to four loops in perturbation theory by
an explicit Feynman diagram calculation for both the ABJM model and the ABJ
extension. We find that all coefficients have maximal transcendentality. We
then compute the four-loop wrapping correction for a scalar operator in the 20
of SU(4) and find that it agrees with a recent prediction from the ABJM
Y-system of Gromov, Kazakov and Vieira. We also propose a limit of the ABJ
model that might be perturbatively integrable at all loop orders but has a
short range Hamiltonian.",0908.2463v3
2009-09-19,Cooperative order and excitation spectra in the bicomponent spin networks,"A ferrimagnetic spin model composed of $S=1/2$ spin-dimers and $S=5/2$
spin-chains is studied by combining the bond-operator representation (for
$S=1/2$ spin-dimers) and Holstein-Primakoff transformation (for $S=5/2$ spins).
A finite interaction $J_{\rm DF}$ between the spin-dimer and the spin chain
makes the spin chains ordered antiferromagnetically and the spin dimers
polarized. The effective interaction between the spin chains, mediated by the
spin dimers, is calculated up to the third order. The staggered magnetization
in the spin dimer is shown proportional to $J_{\rm DF}$. It presents an
effective staggered field reacting on the spin chains. The degeneracy of the
triplons is lifted due to the chain magnetization and a mode with longitudinal
polarization is identified. Due to the triplon-magnon interaction, the
hybridized triplon-like excitations show different behaviors near the vanishing
$J_{\rm DF}$. On the other hand, the hybridized magnon-like excitations open a
gap $\Delta_A\sim J_{\rm DF}$. These results consist well with the experiments
on Cu$_{2}$Fe$_{2}$Ge$_{4}$O$_{13}$.",0909.3576v2
2009-11-10,Spin-wave propagation in a microstructured magnonic crystal,"Transmission of microwave spin waves through a microstructured magnonic
crystal in the form of a permalloy waveguide of a periodically varying width
was studied experimentally and theoretically. The spin wave characteristics
were measured by spatially-resolved Brillouin light scattering microscopy. A
rejection frequency band was clearly observed. The band gap frequency was
controlled by the applied magnetic field. The measured spin-wave intensity as a
function of frequency and propagation distance is in good agreement with a
model calculation.",0911.1920v1
2010-01-11,Magnon softening in a ferromagnetic monolayer: a first-principles spin dynamics study,"We study the Fe/W(110) monolayer system through a combination of first
principles calculations and atomistic spin dynamics simulations. We focus on
the dispersion of the spin waves parallel to the [001] direction. Our results
compare favorably with the experimental data of Prokop et al. [Phys. Rev. Lett.
102, 177206], and correctly capture a drastic softening of the magnon spectrum,
with respect to bulk bcc Fe. The suggested shortcoming of the itinerant
electron model, in particular that given by density functional theory, is
refuted. We also demonstrate that finite temperature effects are significant,
and that atomistic spin dynamics simulations represent a powerful tool with
which to include these.",1001.1669v2
2010-01-22,Theory of electromagnon in the multiferroic Mn perovskites: Vital role of higher harmonic components of the spiral spin order,"We study theoretically the electromagnon and its optical spectrum (OS) of the
terahertz-frequency regime in the magnetic-spiral-induced multiferroic phases
of the rare-earth (R) Mn perovskites, RMnO3, taking into account the elliptical
deformation or the higher harmonics of the spiral spin configuration, which has
been missed so far. A realistic spin Hamiltonian, which gives phase diagrams in
agreement with experiments, resolves a long standing puzzle, i.e., the
double-peak structure of the OS with a larger low-energy peak originating from
magnon modes hybridized with the zone-edge state. We also predict the magnon
branches associated with the electromagnon, which can be tested by
neutron-scattering experiment.",1001.3905v1
2010-02-12,Instability of Magnons in Two-dimensional Antiferromagnet at High Magnetic Fields,"Spin dynamics of the square lattice Heisenberg antiferromagnet, \BaMnGeO, is
studied by a combination of bulk measurements, neutron diffraction, and
inelastic neutron scattering techniques. Easy plane type antiferromagnetic
order is identified at $T \le 4.0$ K. The exchange interactions are estimated
as $J_1$ = 27.8(3)${\mu}$eV and $J_2$ = 1.0(1) ${\mu}$eV, and the saturation
field $H_{\rm C}$ is 9.75 T. Magnetic excitation measurements with high
experimental resolution setup by triple axis neutron spectrometer reveals the
instability of one magnon excitation in the field range of $0.7H_{\rm C}
\lesssim H \lesssim 0.85H_{\rm C}$.",1002.2470v1
2010-02-19,Electromagnons in the multiferroic state of perovskite manganites with symmetric-exchange striction,"We have investigated electrically-active magnetic excitations
(electromagnons) in perovskite manganites with the $E$-type (up-up-down-down)
spin structure by terahertz spectroscopy. Eu$_{1-x}$Y$_x$MnO$_3$ (0.1$\le
x\le$1) and Y$_{1-y}$Lu$_y$MnO$_3$ (0$\le y\le$1) without magnetic $f$-moments,
which host collinear sinusoidal, $A$-type, cycloidal, and $E$-type spin orders,
are used to examine the systematics of possible electromagnons. Three-peak
structures (23, 35, 45 cm$^{-1}$) of magnetic origin show up in the $E$-type
phase with little composition ($y$) dependence of frequencies, making a
contrast with the electromagnons observed in the cycloidal-spin ($x\le0.8$)
phases. One of these electromagnon is ascribed to the zone-edge magnon mode
based on the calculated magnon dispersions.",1002.3676v1
2010-06-27,Spin wave resonances in antiferromagnets,"Spin wave resonances with enormously large wave numbers corresponding to wave
vectors 10^5-10^6 cm^{-1} are observed in thin plates of FeBO3. The study of
spin wave resonances allows one to obtain information about the spin wave
spectrum. The temperature dependence of a non-uniform exchange constant is
determined for FeBO3. Considerable softening of the magnon spectrum resulting
from the interaction of magnons, is observed at temperatures above 1/3 of the
Neel temperature. The excitation level of spin wave resonances is found to
depend significantly on the inhomogeneous elastic distortions artificially
created in the sample. A theoretical model to describe the observed effects is
proposed.",1006.5192v1
2010-07-09,Quantum finite-size effects for dyonic magnons in the AdS_4 x CP^3,"We compute quantum corrections to finite-size effects for various dyonic
giant magnons in the AdS_4 x CP^3 in two different approaches. The off-shell
algebraic curve method is used to quantize the classical string configurations
in semi-classical way and to compute the corrections to the string energies.
These results are compared with the F-term L\""uscher formula based on the
S-matrix of the AdS_4 / CFT_3. The fact that the two results match exactly
provides another stringent test for the all-loop integrability conjecture and
the exact S-matrix based on it.",1007.1598v2
2010-07-19,Anomalously large damping of long-wavelength quasiparticles caused by long-range interaction,"We demonstrate that long-range interaction in a system can lead to a very
strong interaction between long-wavelength quasiparticles and make them heavily
damped. In particular, we discuss magnon spectrum using 1/S expansion in 3D
Heisenberg ferromagnet (FM) with arbitrary small dipolar forces at T<<T_C. We
obtain that a fraction of long-wavelength magnons with energies e_k<T has
anomalously large damping G_k (ratio G_k/e_k reaches 0.3 for certain k). This
effect is observed both in quantum and classical FMs. Remarkably, this result
contradicts expectation of the quasiparticle concept according which a weakly
excited state of a many-body system can be represented as a collection of
weakly interacting elementary excitations. Particular materials are pointed out
which are suitable for corresponding experiments.",1007.3081v2
2010-07-19,Bose-Einstein condensation at finite momentum and magnon condensation in thin film ferromagnets,"We use the Gross-Pitaevskii equation to determine the spatial structure of
the condensate density of interacting bosons whose energy dispersion epsilon_k
has two degenerate minima at finite wave-vectors q. We show that in general the
Fourier transform of the condensate density has finite amplitudes for all
integer multiples of q. If the interaction is such that many Fourier components
contribute, the Bose condensate is localized at the sites of a one-dimensional
lattice with spacing 2pi/q; in this case Bose-Einstein condensation resembles
the transition from a liquid to a crystalline solid. We use our results to
investigate the spatial structure of the Bose condensate formed by magnons in
thin films of ferromagnets with dipole-dipole interactions.",1007.3200v3
2010-08-09,Observation of the Magnon Hall Effect,"The Hall effect usually occurs when the Lorentz force acts on a charge
current in a conductor in the presence of perpendicular magnetic field. On the
other hand, neutral quasi-particles such as phonons and spins can carry heat
current and potentially show the Hall effect without resorting to the Lorentz
force. We report experimental evidence for the anomalous thermal Hall effect
caused by spin excitations (magnons) in an insulating ferromagnet with a
pyrochlore lattice structure. Our theoretical analysis indicates that the
propagation of the spin wave is influenced by the Dzyaloshinskii-Moriya
spin-orbit interaction, which plays the role of the vector potential as in the
intrinsic anomalous Hall effect in metallic ferromagnets.",1008.1564v1
2010-08-31,Magnetic excitations in the low-temperature ferroelectric phase of multiferroic YMn2O5 using inelastic neutron scattering,"We studied magnetic excitations in a low-temperature ferroelectric phase of
the multiferroic YMn2O5 using inelastic neutron scattering (INS). We identify
low-energy magnon modes and establish a correspondence between the magnon peaks
observed by INS and electromagnon peaks observed in optical absorption [1].
Furthermore, we explain the microscopic mechanism, which results in the
lowest-energy electromagnon peak, by comparing the inelastic neutron spectral
weight with the polarization in the commensurate ferroelectric phase.",1008.5354v2
2010-09-02,Theory of magnon-driven spin Seebeck effect,"The spin Seebeck effect is a spin-motive force generated by a temperature
gradient in a ferromagnet that can be detected via normal metal contacts
through the inverse spin Hall effect [K. Uchida {\it et al.}, Nature {\bf 455},
778-781 (2008)]. We explain this effect by spin pumping at the contact that is
proportional to the spin-mixing conductance of the interface, the inverse of a
temperature-dependent magnetic coherence volume, and the difference between the
magnon temperature in the ferromagnet and the electron temperature in the
normal metal [D. J. Sanders and D. Walton, Phys. Rev. B {\bf 15}, 1489 (1977)].",1009.0318v2
2010-10-27,Magnetic spin excitations in Mn doped GaAs : A model study,"We provide a quantitative theoretical model study of the dynamical magnetic
properties of optimally annealed Ga$_{1-x}$Mn$_x$As. This model has already
been shown to reproduce accurately the Curie temperatures for
Ga$_{1-x}$Mn$_x$As. Here we show that the calculated spin stiffness are in
excellent agreement with those which were obtained from ab-initio based
studies. In addition, an overall good agreement is also found with available
experimental data. We have also evaluated the magnon density of states and the
typical density of states from which the ""mobility edge"", separating the
extended from localized magnon states, was determined. The power of the model
lies in its ability to be generalized for a broad class of diluted magnetic
semiconductor materials, thus it bridges the gap between first principle
calculations and model based studies.",1010.5763v2
2010-11-30,Phonons and their Coupling to Magnons in n=2 Ruddlesden-Popper Compounds,"The number of absorption and Raman optical modes for each Wyckoff orbit in
the high-temperature tetragonal (I4/mmm) parent lattice of the
Ruddlesden-Popper compounds Ca$_3$X$_2$O$_7$, with X=Mn or X=Ti is given. We
analyze the effect of sequential perturbations which lower the symmetry to Cmcm
and Cmc2$_1$ and finally include magnetic ordering. We determine the power law
behavior (within mean field theory) for the cross section for photon absorption
and Raman scattering of modes which appear as the symmetry is successively
lowered. In the Cmc2$_1$ phase we give a symmetry analysis to discuss the
magnon-phonon coupling which in other systems gives rise to ""electromagnons.""
From our results we suggest several experiments to clarify the phase diagram
and other properties of these systems.",1011.6672v2
2010-12-21,Non-Volatile Magnonic Logic Circuits Engineering,"We propose a concept of magnetic logic circuits engineering, which takes an
advantage of magnetization as a computational state variable and exploits spin
waves for information transmission. The circuits consist of magneto-electric
cells connected via spin wave buses. We present the result of numerical
modeling showing the magneto-electric cell switching as a function of the
amplitude as well as the phase of the spin wave. The phase-dependent switching
makes it possible to engineer logic gates by exploiting spin wave buses as
passive logic elements providing a certain phase-shift to the propagating spin
waves. We present a library of logic gates consisting of magneto-electric cells
and spin wave buses providing 0 or p phase shifts. The utilization of phases in
addition to amplitudes is a powerful tool which let us construct logic circuits
with a fewer number of elements than required for CMOS technology. As an
example, we present the design of the magnonic Full Adder Circuit comprising
only 5 magneto-electric cells. The proposed concept may provide a route to more
functional wave-based logic circuitry with capabilities far beyond the limits
of the traditional transistor-based approach.",1012.4768v1
2011-02-14,Thermal conductivity of pure and Zn-doped LiCu_2O_2 single crystals,"We report a study of the low-temperature thermal conductivity (\kappa) of
pure and Zn-doped LiCu_2O_2 single crystals. The \kappa(T) of pure LiCu_2O_2
single crystal shows a double-peak behavior, with two peaks locating at 48 K
and 14 K, respectively. The different dependences of the peaks on the Zn
concentration indicate that the high-T peak is likely due to the phonon
transport while the low-T one is attributed to the magnon transport in the spin
spiral ordering state. In addition, the magnetic field can gradually suppress
the low-T peak but does not affect the high-T one; this further confirms that
the low-T peak is originated from the magnon heat transport.",1102.2788v1
2011-05-27,Resonances in a dilute gas of magnons and metamagnetism of isotropic frustrated ferromagnetic spin chains,"We show that spin-S chains with SU(2)-symmetric, ferromagnetic
nearest-neighbor and frustrating antiferromagnetic next-nearest-neighbor
exchange interactions exhibit metamagnetic behavior under the influence of an
external magnetic field for small S, in the form of a first-order transition to
the fully polarized state. The corresponding magnetization jump increases
gradually starting from an S-dependent critical value of exchange couplings and
takes a maximum in the vicinity of a ferromagnetic Lifshitz point. The
metamagnetism results from resonances in the dilute magnon gas caused by an
interplay between quantum fluctuations and frustration.",1105.5626v2
2011-06-22,Tunable Magnonic Frequency and Damping in [Co/Pd]8 Multilayers with Variable Co Layer Thickness,"We report the experimental observation of collective picosecond magnetization
dynamics in [Co/Pd]8 multilayers with perpendicular magnetic anisotropy. The
precession frequency shows large and systematic variation from about 5 GHz to
about 90 GHz with the decrease in the Co layer thickness from 1.0 nm to 0.22 nm
due to the linear increase in the perpendicular magnetic anisotropy. The
damping coefficient 'alpha' is found to be inversely proportional to the Co
layer thickness and a linear relation between the perpendicular magnetic
anisotropy and 'alpha' is established. We discuss the possible reasons behind
the enhanced damping as the d-d hybridization at the interface and spin
pumping. These observations are significant for the applications of these
materials in spintronics and magnonic crystals.",1106.4491v1
2011-07-13,More three-point correlators of giant magnons with finite size,"In the framework of the semiclassical approach, we compute the normalized
structure constants in three-point correlation functions, when two of the
vertex operators correspond to heavy string states, while the third vertex
corresponds to a light state. This is done for the case when the heavy string
states are finite-size giant magnons with one or two angular momenta, and for
two different choices of the light state, corresponding to dilaton operator and
primary scalar operator. The relevant operators in the dual gauge theory are
Tr(F_{\mu\nu}^2 Z^j+...) and Tr(Z^j). We first consider the case of AdS_5 x S^5
and N = 4 super Yang-Mills. Then we extend the obtained results to the
gamma-deformed AdS_5 x S^5_\gamma, dual to N = 1 super Yang-Mills theory,
arising as an exactly marginal deformation of N = 4 super Yang-Mills.",1107.2645v1
2011-08-16,Magnetic rogue wave in a perpendicular anisotropic ferromagnetic nanowire with spin-transfer torque,"We present the current controlled motion of dynamic soliton embedded in spin
wave background in ferromagnetic nanowire. With the stronger breather character
we get the novel magnetic rogue wave and clarify its formation mechanism. The
generation of magnetic rogue wave is mainly arose from the accumulation of
energy and magnons toward to its central part. We also observe that the
spin-polarized current can control the exchange rate of magnons between
envelope soliton and background, and the critical current condition is obtained
analytically. Even more interesting is that the spin-transfer torque plays the
completely opposite role for the cases of below and above the critical value.",1108.3252v2
2011-08-18,Three-point correlators: finite-size giant magnons and singlet scalar operators on higher string levels,"In the framework of the semiclassical approach, we compute the normalized
structure constants in three-point correlation functions, when two of the
vertex operators correspond to ""heavy"" string states, while the third vertex
corresponds to a ""light"" state. This is done for the case when the ""heavy""
string states are finite-size giant magnons, carrying one or two angular
momenta. The ""light"" states are taken to be singlet scalar operators on higher
string levels. We first consider the case of string theory on AdS_5xS^5 dual to
N = 4 super Yang-Mills. Then we extend the obtained results to the
gamma-deformed AdS_5x S^5, corresponding to N = 1 super Yang-Mills theory,
appearing as an exactly marginal deformation of N=4 super Yang-Mills.",1108.3812v1
2011-09-03,Low-temperature heat transport of the layered spin-dimer compound Ba_3Mn_2O_8,"We report the study on the low-temperature heat transport of Ba_3Mn_2O_8
single crystal, a layered spin-dimer compound exhibiting the
magnetic-field-induced magnetic order or the magnon Bose-Einstein condensation.
The thermal conductivities (\kappa) along both the ab plane and the c axis show
nearly isotropic dependence on magnetic field, that is, \kappa is strongly
suppressed with increasing field, particularly at the critical fields of
magnetic phase transitions. These results indicate that the magnetic
excitations play a role of scattering phonons and the scattering effect is
enhanced when the magnetic field closes the gap in the spin spectrum. In
addition, the magnons in the BEC state of this materials do not show notable
ability of carrying heat.",1109.0587v1
2011-09-12,Single-ion anisotropy in Haldane chains and form factor of the O(3) nonlinear sigma model,"We consider spin-1 Haldane chains with single-ion anisotropy, which exists in
known Haldane chain materials. We develop a perturbation theory in terms of
anisotropy, where magnon-magnon interaction is important even in the low
temperature limit. The exact two-particle form factor in the O(3) nonlinear
sigma model leads to quantitative predictions on several dynamical properties
including dynamical structure factor and electron spin resonance frequency
shift. These agree very well with numerical results, and with experimental data
on the Haldane chain material Ni(C$_5$H$_{14}$N$_2$)$_2$N$_3$(PF$_6$).",1109.2607v2
2011-10-04,Isospin Dynamics in Sr2IrO4 : Forging Links to Cuprate Superconductivity,"We used resonant inelastic x-ray scattering to reveal the nature of magnetic
interactions in Sr2IrO4, a 5d transition-metal oxide with a spin-orbit
entangled ground state and Jeff=1/2 magnetic momemts, referred to as
'isospins'. The magnon dispersion in Sr2IrO4 is well described by an
antiferromagnetic Heisenberg model with isospin one-half moments on a square
lattice, which renders the low-energy effective physics of Sr2IrO4 much akin to
that in superconducting cuprates. This is further supported by the observation
of exciton modes in Sr2IrO4 whose dispersion is strongly renormalized by
magnons, which can be understood by analogy to the hole propagation in the
background of antiferromagnetically ordered spins in the cuprates.",1110.0759v1
2011-12-15,"Selective spin wave excitation in ferromagnetic (Ga,Mn)As layers by picosecond strain pulses","We demonstrate selective excitation of a spin wave mode in a ferromagnetic
(Ga,Mn)As film by picosecond strain pulses. For a certain range of magnetic
fields applied in the layer plane only a single frequency is detected for the
magnetization precession. We explain this selectivity of spin mode excitation
by the necessity of spatial matching of magnon and phonon eigenfunctions, which
represents a selection rule analogous to momentum conservation for
magnon-phonon interaction in bulk ferromagnetic materials.",1112.3394v1
2012-06-07,Semiquantitative theory for high-field low-temperature properties of a distorted diamond spin chain,"We consider the antiferromagnetic Heisenberg model on a distorted diamond
chain and use the localized-magnon picture adapted to a distorted geometry to
discuss some of its high-field low-temperature properties. More specifically,
in our study we assume that the partition function for a slightly distorted
geometry has the same form as for ideal geometry, though with slightly
dispersive one-magnon energies. We also discuss the relevance of such a
description to azurite.",1206.1441v2
2012-08-25,Magnon-mediated interactions between fermions depend strongly on the lattice structure,"We propose two new methods to calculate exactly the spectrum of two
spin-${1\over 2}$ charge carriers moving in a ferromagnetic background, at zero
temperature. We find that if the spins are located on a different sublattice
than that on which the fermions move, magnon-mediated effective interactions
are very strong and can bind the fermions into low-energy bipolarons with
triplet character. This never happens in models where spins and charge carriers
share the same lattice, whether they are in the same band or in different
bands. This proves that effective one-lattice models do not describe correctly
the low-energy part of the two-carrier spectrum of a two-sublattice model, even
though they may describe the low-energy single-carrier spectrum appropriately.",1208.5144v1
2012-08-25,The role of the lattice structure in determining the magnon-mediated interactions between charge carriers doped into a magnetically ordered background,"We use two recently proposed methods to calculate exactly the spectrum of two
spin-${1\over 2}$ charge carriers moving in a ferromagnetic background, at zero
temperature, for three types of models. By comparing the low-energy states in
both the one-carrier and the two-carrier sectors, we analyze whether complex
models with multiple sublattices can be accurately described by simpler
Hamiltonians, such as one-band models. We find that while this is possible in
the one-particle sector, the magnon-mediated interactions which are key to
properly describe the two-carrier states of the complex model are not
reproduced by the simpler models. We argue that this is true not just for
ferromagnetic, but also for antiferromagnetic backgrounds. Our results question
the ability of simple one-band models to accurately describe the low-energy
physics of cuprate layers.",1208.5146v1
2012-11-23,Comment on Strings in AdS3 x S3 x S3 x S1 at One Loop,"This paper studies semiclassical strings in AdS3 x S3 x S3 x S1 using the
algebraic curve. Calculating one-loop corrections to the energy of the giant
magnon fixes the constant term c in the expansion of the coupling h(\lambda).
Comparing these to similar corrections for long spinning strings gives a
prediction for the one-loop term f_1 in the expansion of the cusp anomalous
dimension f(h), for all \alpha (where \alpha --> 1 is the AdS3 x S3 x T4
limit). For these semiclassical mode sums there is a similar choice of
regularisation prescriptions to that encountered in AdS4 x CP3. However at
\alpha \neq 1/2 they lead to different values of f_1 and are therefore not
related by a simple change of the coupling. The algebraic curve is also used to
calculate various finite-size corrections for giant magnons, which are
well-behaved as \alpha --> 1, and can be compared to the recently published
S-matrices.",1211.5587v3
2012-12-12,Highly Dispersive Scattering From Defects In Non-Collinear Magnets,"We demonstrate that point-like defects in non-collinear magnets give rise to
a highly dispersive structure in the magnon scattering, violating a standard
paradigm of its momentum independence. For a single impurity spin coupled to a
prototypical non-collinear antiferromagnet we find that the resolvent is
dominated by a distinct dispersive structure with its momentum-dependence set
by the magnon dispersion and shifted by the ordering vector. This feature is a
consequence of umklapp scattering off the impurity-induced spin texture, which
arises due to the non-collinear ground state of the host system. Detailed
results for the staggered and uniform magnetization of this texture as well as
the T-matrix from numerical linear spin-wave theory are presented.",1212.2972v1
2012-12-24,Excited states of the quasi-one-dimensional hexagonal quantum antiferromagnets,"We investigate the excited states of the quasi-one-dimensional quantum
antiferromagnets on hexagonal lattices, including the longitudinal modes based
on the magnon-density waves. A model Hamiltonian with a uniaxial single-ion
anisotropy is first studied by a spin-wave theory based on the one-boson
method; the ground state thus obtained is employed for the study of the
longitudinal modes. The full energy spectra of both the transverse modes (i.e.,
magnons) and the longitudinal modes are obtained as functions of the
nearest-neighbor coupling and the anisotropy constants. We have found two
longitudinal modes due to the non-collinear nature of the triangular
antiferromagnetic order, similar to that of the phenomenological field theory
approach by Affleck. The excitation energy gaps due to the anisotropy and the
energy gaps of the longitudinal modes without anisotropy are then investigated.
We then compare our results for the longitudinal energy gaps at the magnetic
wavevectors with the experimental results for several antiferromagnetic
compounds with both integer and non-integer spin quantum numbers, and we find
good agreement after the higher-order contributions are included in our
calculations.",1212.5924v3
2013-01-10,Magnetic structure and magnon dispersion in LaSrFeO$_4$,"We present elastic and inelastic neutron scattering data on LaSrFeO$_4$. We
confirm the known magnetic structure with the magnetic moments lying in the
tetragonal basal plane, but contrarily to previous reports our macroscopic and
neutron diffraction data do not reveal any additional magnetic phase transition
connected to a spin reorientation or to a redistribution of two irreducible
presentations. Our inelastic neutron scattering data reveals the magnon
dispersion along the main-symmetry directions [0 $\xi$ 0] and [$\xi$ -$\xi$ 0].
The dispersion can be explained within linear spin-wave theory yielding an
antiferromagnetic nearest-neighbour interaction parameter $J_{1}=7.4(1)$ meV
and a next-nearest neighbour interaction parameter $J_{2}=0.4(1)$ meV. The
dispersion is gapped with the out-of-plane anisotropy gap found at
$\Delta_{out}=5.26(2)$ meV, while evidence is present that the in-plane
anisotropy gap lies at lower energies, where it cannot be determined due to
limited instrument resolution.",1301.2106v1
2013-02-15,A novel magnetic phase prior to a saturation moment in frustrated spinel oxides in ultra-high magnetic fields,"We have investigated the magnetic-field induced phases of a typical
three-dimensional frustrated magnet, CdCr$_2$O$_4$, in magnetic fields of up to
120 T that is generated by the single-turn coil techniques. We focused on
magnetic phase transitions in proximity of a saturated magnetization moment. We
utilized both the electromagnetic induction method using magnetic pick-up coils
and magneto-optical spectroscopies of the $d$-$d$ transitions and the
exciton-magnon-phonon transitions to study the magnetic properties subjected to
ultra-high magnetic fields. Anomalies were observed in magneto-optical
absorption intensity as well as differential magnetization prior to a fully
polarized magnetic phase (a vacuum state in the magnon picture), revealing a
novel magnetic phase associated with changes in both crystal and magnetic
structures accompanied by the first order phase transition. Magnetic superfluid
state such as an umbrella-like magnetic structure or a spin nematic state, is
proposed as a candidate for the novel magnetic phase, which is found universal
in the series of chromium spinel oxides, $A$Cr$_2$O$_4$ ($A$ = Zn, Cd, Hg).",1302.3664v1
2013-03-07,Theory of atomistic simulation of spin-transfer torque in nanomagnets,"In spin-transfer torque (STT) for technological applications, the
miniaturization of the magnet may reach the stage of requiring a fully
quantum-mechanical treatment. We present an STT theory which uses the quantum
macrospin ground and excited (magnon) states of the nanomagnet. This allows for
energy and angular momentum exchanges between the current electron and the
nano-magnet. We develop a method of magnetization dynamics simulation which
captures the heating effect on the magnet by the spin-polarized current and the
temperature-dependence in STT. We also discuss the magnetostatics effect on
magnon scattering for ferromagnetic relaxation in a thin film. Our work
demonstrates a realistic step towards simulation of quantum spin-transfer
torque physics in nano-scale magnets.",1303.1775v1
2013-03-12,Worldsheet one-loop energy correction to IIA Giant Magnon,"We compute one-loop corrections to the energy of a IIA giant magnon solution
in the $AdS_4 \times CP^3$ background by using the standard quantum field
theory (QFT) techniques. The string action is expanded around the solution to
the quadratic order in the fluctuation fields. The resulting action has 2D
coordinate dependent-coefficients, a feature that complicates the analysis. The
solution contains a worldsheet velocity parameter $v$, and is expanded in terms
of the parameter. A perturbative analysis is carried out by treating the
$v$-dependent parts as vertices. The energy is computed by first putting the
system in a box of length $L$ and Fourier-transforming the fields into the
discrete momentum modes. We compare our result with the results obtained by the
algebraic curve method.",1303.2743v3
2013-03-18,Wave modes of collective vortex gyration in dipolar-coupled-dot-array magnonic crystals,"Lattice vibration modes are collective excitations in periodic arrays of
atoms or molecules. These modes determine novel transport properties in solid
crystals. Analogously, in periodical arrangements of magnetic vortex-state
disks, collective vortex motions have been predicted. Here, we experimentally
observe wave modes of collective vortex gyration in one-dimensional (1D) chains
of periodic disks using time-resolved scanning transmission x-ray microscopy.
The observed modes are interpreted based on micromagnetic simulation and
numerical calculation of coupled Thiele equations. Dispersion of the modes is
found to be strongly affected by both vortex polarization and chirality
ordering, as revealed by the explicit analytical form of 1D infinite chains. A
thorough understanding thereof is fundamental both for lattice vibrations and
vortex dynamics, which we demonstrate for 1D magnonic crystals. Such magnetic
disk arrays with vortex-state ordering, referred to as magnetic metastructure,
offer potential implementation into information processing devices.",1303.4170v1
2013-04-10,"Lee, Han, and Kim Reply","In Letter [1], we reported our finding on the physical origin of wide
band-gap opening in planar nanostrips, as derived by application of periodic
width modulations in the magnetic waveguides. K. Di et al. in their Comment[2],
however, argued that the band gap can be reduced remarkably by applying a
linear combination of symmetric and antisymmetric fields (see 'A+B' in the
inset of Fig. 1(a)). They also insisted that they could find a complete set of
magnonic bands based on all of the mode symmetries. However, their work does
not constitute grounds for judging our method ""wrong"" and our conclusions
""erroneous"". Moreover, the excitation field alluded to in the Comment is not
sufficiently general to obtain the complete set of magnonic band structures,
but rather remains specific. The grounds of these conclusions, drawn from our
further work, are the following.",1304.2859v1
2013-04-24,Observation of an inter-sublattice exchange magnon in CoCr$_2$O$_4$ and analysis of magnetic ordering,"We report on an investigation of optical properties of multiferroic
CoCr$_{2}$O$_{4}$ at terahertz frequencies in magnetic fields up to 30 T. Below
the ferrimagnetic transition (94 K), the terahertz response of
CoCr$_{2}$O$_{4}$ is dominated by a magnon mode, which shows a steep
magnetic-field dependence. We ascribe this mode to an exchange resonance
between two magnetic sublattices with different $g$-factors. In the framework
of a simple two-sublattice model (the sublattices are formed by Co$^{2+}$ and
Cr$^{3+}$ ions), we find the inter-sublattice coupling constant, $\lambda = -
(18 \pm 1)$ K, and trace the magnetization for each sublattice as a function of
field. We show that the Curie temperature of the Cr$^{3+}$ sublattice,
$\Theta_{2}$ = $(49 \pm 2)$ K, coincides with the temperature range, where
anomalies of the dielectric and magnetic properties of CoCr$_{2}$O$_{4}$ have
been reported in literature.",1304.6525v1
2013-04-26,Landau-Lifshitz theory of the longitudinal spin Seebeck effect,"Thermal-bias-induced spin angular momentum transfer between a paramagnetic
metal and ferromagnetic insulator is studied theoretically based on the
stochastic Landau-Lifshitz-Gilbert (LLG) phenomenology. Magnons in the
ferromagnet establish a nonequilibrium steady state by equilibrating with
phonons via bulk Gilbert damping and electrons in the paramagnet via spin
pumping, according to the fluctuation-dissipation theorem. Subthermal magnons
and the associated spin currents are treated classically, while the appropriate
quantum crossover is imposed on high-frequency magnetic fluctuations. We
identify several length scales in the ferromagnet, which govern qualitative
changes in the dependence of the thermally-induced spin current on the magnetic
film thickness.",1304.7295v2
2013-05-07,Micromagnetic modelling of anisotropic damping in ferromagnet,"We report a numerical implementation of the Landau-Lifshitz-Baryakhtar
theory, which dictates that the micromagnetic relaxation term obeys the
symmetry of the magnetic crystal, i. e. replacing the single intrinsic damping
constant with a tensor of corresponding symmetry. The effect of anisotropic
relaxation is studied in thin saturated ferromagnetic disk and ellipse with and
without uniaxial magneto-crystalline anisotropy. We investigate the angular
dependency of the linewidth of magnonic resonances with respect to the given
structure of the relaxation tensor. The simulations suggest that the anisotropy
of the magnonic linewidth is determined by only two factors: the projection of
the relaxation tensor onto the plane of precession and the ellipticity of the
later.",1305.1641v2
2013-05-20,The 'Higgs' amplitude mode in weak ferromagnetic metals,"Using Ferromagnetic Fermi liquid theory, Bedell and Blagoev derived the
collective low-energy excitations of a weak ferromagnet. They obtained the
well-known magnon (Nambu-Goldstone) mode and found a new gapped mode that was
never studied in weak ferromagnetic metals. In this article we have identified
this mode as the Higgs boson (amplitude mode) of a ferromagnetic metal. This is
identified as the Higgs since it can be show that it corresponds to a
fluctuation of the amplitude of the order parameter. We use this model to
describe the itinerant-electron ferromagnetic material MnSi. By fitting the
model with the existing experimental results, we calculate the dynamical
structure function and see well-defined peaks contributed from the magnon and
the Higgs. Our estimates of the relative intensity of the Higgs amplitude mode
suggest that it can be seen in neutron scattering experiments on MnSi.",1305.4674v2
2013-05-28,Magnonic band gaps in waveguides with a periodic variation of the saturation magnetization,"We present a micromagnetic analysis of spin-wave propagation in a magnonic
crystal realized as a permalloy spin-wave waveguide with a spatial periodical
variation of its saturation magnetization. Frequency band gaps were clearly
observed in the spin-wave transmission spectra and their origin is traced back
to an overlap of individual band gaps of the fundamental and the higher order
spin-wave width modes. The control of the depth, width and the position in
frequency and space of the rejection band gaps by the width areas with a
reduced magnetization and by the modulation level, are discussed in this study.",1305.6619v1
2013-07-14,Controlling frustrated liquids and solids with an applied field in a kagome Heisenberg antiferromagnet,"Quantum spin-1/2 kagome Heisenberg antiferromagnet is the representative
frustrated system possibly hosting a spin liquid. Clarifying the nature of this
elusive topological phase is a key challenge in condensed matter, however, even
identifying it still remains unsettled. Here, we apply a magnetic field and
discover a series of spin gapped phases appearing at five different fractions
of magnetization by means of grand canonical density matrix renormalization
group, an unbiased state-of-art numerical technique. The magnetic field dopes
magnons and first gives rise to a possible Z3 spin liquid plateau at
1/9-magnetization. Higher field induces a self-organized super-lattice-unit, a
six-membered ring of quantum spins, resembling an atomic orbital structure.
Putting magnons into this unit one by one yields three quantum solid plateaus.
We thus find that the magnetic field could control the transition between
various emergent phases by continuously releasing the frustration.",1307.3710v2
2013-10-24,Spin Excitations in Half-Doped Manganites,"We investigate magnetic excitations in half-doped colossal magnetoresistance
manganites. In particular, we focus on spin excitations in the CE phase
originally proposed by Goodenough (Phys. Rev. 100, 564 (1955)). Using a
localized spin model we calculated magnons for 3D-perovskite compounds such as
La$_{1-x}$M$_x$MnO$_3$, where M=Ca,Sr,Ba, and for their 2D-laminar
counterparts. We compared them with predictions for the spin excitations
corresponding to other phases proposed. For the laminar half-doped manganite
La$_{0.5}$Sr$_{1.5}$MnO$_4$, for which magnon measurements by inelastic neutron
scattering exist, as well as an estimation of the magnetic couplings, our
calculations agree well with the experimental data.",1310.6714v1
2013-11-12,Leggett's Modes in Magnetic Systems with Jahn-Teller distortion,"Leggett's mode is a collective excitation corresponding to the oscillation of
the relative phase of the order parameters in a two band superconductor, with
frequency proportional to interband coupling. We report on the existence of
modes, similar to Leggett's mode, in magnetic systems with Jahn-Teller
distortion. The minimal Kugel-Khomskii model, which describes simultaneously
both the spin and the orbital order, is studied. The dynamical degrees of
freedom are spin-$s$ operators of localized spins and pseudospin-$\tau$
operators, which respond to the orbital degeneracy and satisfy the similar
commutation relation with those of the spin operators. In the case of
""antiferro"" spin and pseudospin order the system possesses two
antiferromagnetic magnons with equal spin-wave velocities and two Leggett's
modes with equal gaps proportional to the square root of the spin-pseudospin
interaction constant. In the case of ""ferro"" spin and pseudospin order the
system possesses one ferromagnetic magnon and one Leggett's mode with gap
proportional to the spin-pseudospin interaction constant.",1311.2733v3
2013-11-12,On the Possibility of Reversible Magnonic Logic Gates,"We propose and develop a concept of magnonic logic gates enabling reversible
computing. The gates consist of passive elements: waveguides, cross-junctions
and phase shifters. Logical 0 and 1 are encoded in the relative phase of the
propagating spin wave packets (0 or {\pi}). The gates contain several possible
trajectories for each packet to propagate from the input to the output.
Re-direction of the spin wave packets among the possible trajectories is due to
the interference in the magnetic cross-junctions. Two wave packets coming to
the cross-junction in-phase propagate through the junction without reflection.
Two packets coming out-of-phase to the junction are completely reflected back.
The operation of the cross-junction is illustrated by numerical modeling. We
estimate the power dissipation in the proposed circuits and the feasibility of
cascading such magnetic devices in large circuits. The proposed gates may
potentially provide a route to magnetic reversible logic circuitry with power
dissipation less than kT per operation.",1311.2918v1
2013-12-04,The square-kagome quantum Heisenberg antiferromagnet at high magnetic fields: The localized-magnon paradigm and beyond,"We consider the spin-1/2 antiferromagnetic Heisenberg model on the
two-dimensional square-kagome lattice with almost dispersionless lowest magnon
band. For a general exchange coupling geometry we elaborate low-energy
effective Hamiltonians which emerge at high magnetic fields. The effective
model to describe the low-energy degrees of freedom of the initial frustrated
quantum spin model is the (unfrustrated) square-lattice spin-1/2 $XXZ$ model in
a $z$-aligned magnetic field. For the effective model we perform quantum Monte
Carlo simulations to discuss the low-temperature properties of the
square-kagome quantum Heisenberg antiferromagnet at high magnetic fields. We
pay special attention to a magnetic-field driven
Berezinskii-Kosterlitz-Thouless phase transition which occurs at low
temperatures.",1312.1111v1
2013-12-17,Real-Space Visualization of Remnant Mott Gap and Magnon Excitations,"We demonstrate the ability to visualize real-space dynamics of charge gap and
magnon excitations in the Mott phase of the single-band Hubbard model and the
remnants of these excitations with hole or electron doping. At short times, the
character of magnetic and charge excitations is maintained even for large
doping away from the Mott and antiferromagnetic phases. Doping influences both
the real-space patterns and long timescales of these excitations with a clear
carrier asymmetry attributable to particle-hole symmetry breaking in the
underlying model. Further, a rapidly-oscillating charge density wave-like
pattern weakens, but persists as a visible demonstration of a sub-leading
instability at half-filling which remains upon doping. The results offer an
approach to analyzing the behavior of systems where momentum-space is either
inaccessible or poorly defined.",1312.5312v2
2014-01-21,Magnonic Holographic Memory,"Holography is a technique based on the wave nature of light which allows us
to utilize wave interference between the object beam and the coherent
background. Holography is usually associated with images being made from light,
however, this is only a narrow field of holography. The first holograms were
designed for use with electron microscopes and it was not until a decade later
with the advent of the laser that optical holographic images were popularized.
Multiple other fields have had major contributions made by using wave
interference to produce holograms, including acoustic holograms used in seismic
applications, and microwave holography used in radar systems. Holography has
been also recognized as a future data storing technology with unprecedented
data storage capacity and ability to write and read a large number of data in a
highly parallel manner. Here, we present a magnonic holographic memory device
exploiting spin wave interference.",1401.5133v1
2014-02-13,High-Energy Continuum of Magnetic Excitations in the Two-Dimensional Quantum Antiferromagnet Sr2CuO2Cl2,"We have measured the magnetic excitation spectrum of the model square-lattice
spin-1/2 antiferromagnet $\rm Sr_2CuO_2Cl_2$ over a broad range of energy and
momentum using high-resolution inelastic neutron scattering (INS). The magnon
dispersion along the zone boundary was accurately measured to be a 43 meV
between (1/2,0) and (3/4,1/4) indicating the importance of coupling beyond
nearest neighbors in the spin Hamiltonian. We observe a strong momentum
dependent damping of the zone-boundary magnons at (1/2,0) revealing a high
energy continuum of magnetic excitations. A direct comparison between our INS
measurements and resonant inelastic X-ray scattering (RIXS) measurements shows
that the RIXS spectrum contains significant contributions from higher energy
excitations not previously considered. Our observations demonstrate that this
high-energy continuum of magnetic fluctuations is a ubiquitous feature of
insulating monolayer cuprates, apparent in both inelastic neutron and light
scattering measurements.",1402.3282v1
2014-02-25,Delta chain with ferromagnetic and antiferromagnetic interactions at the critical point,"We investigate the spin-1/2 Heisenberg model on the delta chain (sawtooth
chain) with ferromagnetic nearest-neighbor and antiferromagnetic next-neighbor
interactions. For a special ratio between these interactions there is a class
of exact ground states formed by localized magnons and the ground state is
macroscopically degenerate with a large residual entropy per spin
$s_0=\frac{1}{2}\ln 2$. An important feature of this model is a sharp decrease
of the gaps for excited states with an increase of the number of magnons. These
excitations give an essential contribution to the low-temperature
thermodynamics. The behavior of the considered model is compared with that of
the delta chain with both antiferromagnetic interactions.",1402.6087v1
2014-02-26,Improved Domain Wall Dynamics and Magnonic Torques using Topological Insulators,"We investigate the magnetization dynamics that arise when a thin-film
ferromagnet is deposited on a topological insulator (TI), focusing in
particular on domain-wall motion via current and the possibility of a spin-wave
torque acting on the magnetization. We show analytically that the coupling
between the magnetic domain wall and the TI removes the degeneracy of the wall
profile with respect to its chirality and topological charge. Moreover, we find
that the threshold for Walker breakdown of domain wall motion is substantially
increased and determined by the interaction with the TI, allowing for higher
attainable wall velocities than in the conventional case where the hard axis
anisotropy determines the Walker threshold. Finally, we show that the allowed
modes of spin-wave excitations and the ensuing magnetization dynamics in the
presence of a TI coupling enable a magnonic torque acting even on homogeneous
magnetization textures. Our results indicate that the TI-ferromagnet
interaction has a similar effect on the magnetization dynamics as an intrinsic
Dzyaloshinskii-Moriya interaction in ferromagnets.",1402.6701v2
2014-02-27,On the longitudinal spin current induced by a temperature gradient in a ferromagnetic insulator,"Based on the solution of the stochastic Landau-Lifshitz-Gilbert equation
discretized for a ferromagnetic chain subject to a uniform temperature
gradient, we present a detailed numerical study of the spin dynamics with a
focus particularly on finite-size effects. We calculate and analyze the net
longitudinal spin current for various temperature gradients, chain lengths, and
external static magnetic fields. In addition, we model an interface formed by a
nonuniformly magnetized finite-size ferromagnetic insulator and a normal metal
and inspect the effects of enhanced Gilbert damping on the formation of the
space-dependent spin current within the chain. A particular aim of this study
is the inspection of the spin Seebeck effect beyond the linear response regime.
We find that within our model the microscopic mechanism of the spin Seebeck
current is the magnon accumulation effect quantified in terms of the exchange
spin torque. According to our results, this effect drives the spin Seebeck
current even in the absence of a deviation between the magnon and phonon
temperature profiles. Our theoretical findings are in line with the recently
observed experimental results by M. Agrawal et al., Phys. Rev. Lett. 111,
107204 (2013).",1402.6899v1
2014-02-28,Lattice and spin excitations in multiferroic h-YMnO3,"We used Raman and terahertz spectroscopies to investigate lattice and
magnetic excitations and their cross-coupling in the hexagonal YMnO3
multiferroic. Two phonon modes are strongly affected by the magnetic order.
Magnon excitations have been identified thanks to comparison with neutron
measurements and spin wave calculations but no electromagnon has been observed.
In addition, we evidenced two additional Raman active peaks. We have compared
this observation with the anti-crossing between magnon and acoustic phonon
branches measured by neutron. These optical measurements underly the unusual
strong spin-phonon coupling.",1402.7239v1
2014-04-01,Semi-classical scalar products in the generalised SU(2) model,"In these notes we review the field-theoretical approach to the computation of
the scalar product of multi-magnon states in the Sutherland limit where the
magnon rapidities condense into one or several macroscopic arrays. We formulate
a systematic procedure for computing the 1/M expansion of the
on-shell/off-shell scalar product of M-magnon states in the generalised
integrable model with SU(2)-invariant rational R-matrix. The coefficients of
the expansion are obtained as multiple contour integrals in the rapidity plane.",1404.0235v3
2014-04-08,Resonant inelastic x-ray scattering on single-magnons at oxygen $K$-edges,"The recent discovery that resonant inelastic x-ray scattering can probe
single-magnon (SM) dispersions in transition metal (TM) oxides when the x-ray
energy is tuned to the TM $L$-edge, has put this technique on a par with
inelastic neutron scattering. It is generally presumed that selection rules
forbid SM-scattering at oxygen (O) $K$-edges. However, based on a symmetry
analysis and exact diagonalization study, we show that SM-scattering at O
K-edges becomes allowed when (i) spin-orbit coupling is present in the TM
d-shell and (ii) inversion symmetry at the O-site is broken. For cuprates the
resulting SM-amplitude is very weak but in iridates both prerequisites can
amply be fulfilled.",1404.2040v1
2014-07-25,Analysis of resonant inelastic x-ray scattering from Sr$_2$IrO$_4$ in an itinerant-electron approach,"We analyze the resonant x-ray scattering (RIXS) spectra from Sr$_2$IrO$_4$ in
an itinerant electron approach. Employing a multi-orbital tight-binding model
on the square lattice, we calculate the one-electron energy band within the
Hartree-Fock approximation, which leads to an antiferromagnetic ground state.
We then evaluate the two-particle Green's functions for the particle-hole pair
excitations within the random phase approximation, which are connected to the
RIXS spectra within the fast collision approximation. The calculated RIXS
spectra exhibit two-peak structure with slightly different energies in the
low-energy region, which are originated from the bound states in the
two-particle Green's function. They may be interpreted as the split modes of
magnon. We also obtain several $\delta$-function peaks, which arise from the
bound states around the bottom of energy continuum. They may be called as the
exciton modes. These characteristics are in qualitative agreement with the RIXS
experiment, demonstrating that the weak coupling theory could explain both the
magnon and the exciton peaks in the RIXS spectra on an equal footing.",1407.6766v1
2014-07-28,Magneto Seebeck effect in REFeAsO (RE=rare earth) compounds: probing the magnon drag scenario,"We investigate Seebeck effect in REFeAsO (RE=rare earth)compounds as a
function of temperature and magnetic field up to 30T. The Seebeck curves are
characterized by a broad negative bump around 50K, which is sample dependent
and strongly enhanced by the application of a magnetic field. A model for the
temperature and field dependence of the magnon drag contribution to the Seebeck
effect by antiferromagnetic (AFM) spin fluctuation is developed. It accounts
for the magnitude and scaling properties of such bump feature in our
experimental data. This analysis allows to extract precious information on the
coupling between electrons and AFM spin fluctuations in these parent compound
systems, with implications on the pairing mechanism of the related
superconducting compounds.",1407.7358v1
2014-08-13,Temperature-Dependent Interplay of Dzyaloshinskii-Moriya Interaction and Single-Ion Anisotropy in Multiferroic BiFeO$_3$,"Low-energy magnon excitations in multiferroic BiFeO$_3$ were measured in
detail as a function of temperature around several Brillouin zone centers by
inelastic neutron scattering experiments on single crystals. Unique features
around 1 meV are directly associated with the interplay of the
Dzyaloshinskii-Moriya interaction and a small single-ion anisotropy. The
temperature dependence of these and the exchange interactions were determined
by fitting the measured magnon dispersion with spin-wave calculations. The
spectra best fits an easy-axis type magnetic anisotropy and the deduced
exchange and anisotropy parameters enable us to determine the anharmonicity of
the magnetic cycloid. We then draw a direct connection between the changes in
the parameters of spin Hamiltonian with temperature and the physical properties
and structural deformations of BiFeO$_3$.",1408.2942v2
2014-10-01,Holes Localized on a Skyrmion in a Doped Antiferromagnet on the Honeycomb Lattice: Symmetry Analysis,"Using the low-energy effective field theory for hole-doped antiferromagnets
on the honeycomb lattice, we study the localization of holes on Skyrmions, as a
potential mechanism for the preformation of Cooper pairs. In contrast to the
square lattice case, for the standard radial profile of the Skyrmion on the
honeycomb lattice, only holes residing in one of the two hole pockets can get
localized. This differs qualitatively from hole pairs bound by magnon exchange,
which is most attractive between holes residing in different momentum space
pockets. On the honeycomb lattice, magnon exchange unambiguously leads to
$f$-wave pairing, which is also observed experimentally. Using the
collective-mode quantization of the Skyrmion, we determine the quantum numbers
of the localized hole pairs. Again, $f$-wave symmetry is possible, but other
competing pairing symmetries cannot be ruled out.",1410.0218v1
2014-10-06,Possible coupling between magnons and phonons in multiferroic CaMn7O12,"Spin and lattice dynamics of CaMn7O12 ceramics were investigated using
infrared, THz and inelastic neutron scattering (INS) spectroscopies in the
temperature range 2 to 590 K, and, at low temperatures, in applied magnetic
fields of up to 12 T. On cooling, we observed phonon splitting accompanying the
structural phase transition at Tc = 450K as well as the onset of the
incommensurately modulated structure at 250 K. In the two antiferromagnetic
phases below T_N1 = 90K and T_N2 = 48 K, several infrared-active excitations
emerge in the meV range; their frequencies correspond to the maxima in the
magnon density of states obtained by INS. At the magnetic phase transitions,
these modes display strong anomalies and for some of them, a transfer of
dielectric strength from the higher-frequency phonons is observed. We propose
that these modes are electromagnons. Remarkably, at least two of these modes
remain active also in the paramagnetic phase; for this reason, we call them
paraelectromagnons. In accordance with this observation, quasielastic neutron
scattering revealed short-range magnetic correlations persisting within
temperatures up to 500K above T_N1.",1410.1394v1
2014-10-30,Quantum versus classical polarons in a ferromagnetic CuO$_3$-like chain,"We present an exact solution for an itinerant hole added into the oxygen
orbitals of a CuO$_{3}$-like ferromagnetic chain. Using the Green's function
method, the quantum polarons obtained for the Heisenberg SU(2) interaction
between localized Cu spins are compared with the polarons in the Ising chain.
We find that magnons with large energy are favorable towards quasiparticle
existence, even in the case of relatively modest electron-magnon coupling. We
observe two quasiparticle states with dispersion $\sim 2t$ each, which emerge
from the incoherent continuum when the exchange coupling $J$ increases. Quantum
fluctuations in the spin system modify the incoherent part of the spectrum and
change the spectral function qualitatively, beyond the bands derived from the
perturbation theory.",1410.8481v1
2014-11-14,Modelling Dynamical Fluorescent Micro Thermal Imaging of the Heat Diffusion in the La5Ca9Cu24O41 Spin Ladder Compound,"The dynamical fluorescent microthermal imaging (FMI) experiment has been used
to investigate the phonon-magnon interaction in the 1D Heisenberg
antiferromagnet La5Ca9Cu24O41. This material shows highly anisotropic heat
conductivity due to the efficient magnetic heat transport along the spin
ladders in the compound. To extract information on the phonon-magnon
interaction we modelled the dynamic heat transport experiment using a two
temperature model approach, taking both the crystal as well as the PMMA/EuTTA
fluorescent heat imaging layer into account. The simulations are carried out by
the finite element method using COMSOL Multiphysics Heat Transfer Module. The
results of the numerical calculations are expected to be used for the data
analysis of the experimental studies.",1411.3872v2
2014-12-05,Magnetic order and electromagnon excitations in DyMnO3 studied by neutron scattering experiments,"Magnetic order and excitations in multiferroic DyMnO3 were studied by neutron
scattering experiments using a single crystal prepared with enriched 162Dy
isotope. The ordering of Mn moments exhibits pronounced hysteresis arising from
the interplay between Mn and Dy magnetism which possesses a strong impact on
the ferroelectric polarization. The magnon dispersion resembles that reported
for TbMnO3. We identify the excitations at the magnetic zone center and near
the zone boundary in the b direction, which can possess electromagnon
character. The lowest frequency of the zone-center magnons is in good agreement
with a signal in a recent optical measurement so that this mode can be
identified as the electromagnon coupled by the same Dzyaloshinski-Moriya
interaction as the static multiferroic phase.",1412.1938v1
2014-12-11,High-field magnetic resonance of spinons and magnons in a triangular lattice S=1/2 antiferromagnet Cs2CuCl4,"The electron spin resonance doublet indicating the width of the two spinon
continuum in a spin-1/2 triangular-lattice Heisenberg antiferromagnet Cs2CuCl4
was studied in high magnetic field. The doublet was found to collapse in a
magnetic field of a half of the saturation field. The collapse of the doublet
occurs via vanishing of the high frequency component in a qualitative agreement
with the theoretical prediction for the S=1/2 chain. The field of the collapse
is, however, much lower than expected for the S=1/2 chain. This is proposed to
be due to the destruction of frustration of interchain exchange bonds in a
magnetic field, which restores the 2D character of this spin system. In the
saturated phase the mode with the Larmor frequency and a much weaker mode
downshifted for 119~GHz are observed. The weak mode is of exchange origin, it
demonstrates a positive frequency shift at heating corresponding to the
repulsion of magnons in the saturated phase.",1412.3776v1
2015-01-23,Giant gap in the magnon excitations of the quasi-1D chain compound Sr3NiIrO6,"Inelastic neutron scattering on the spin-chain compound Sr$_3$NiIrO$_6$
reveals gapped quasi-1D magnetic excitations. The observed one-magnon band
between 29.5 and 39 meV consists of two dispersive modes. The spin wave
spectrum could be well fitted with an antiferromagnetic anisotropic exchange
model and single ion anisotropy on the Ni site. The extracted dominant
anisotropic antiferromagnetic intra-chain exchange interaction between Ir and
Ni ions are $J_z=19.5$ meV and $J_{xy}=12.1$ meV. These values justify previous
electronic structure calculations, showing the importance of Ir spin orbit
coupling on the electron correlations. The magnetic excitations survive up to
200 K well above the magnetic ordering temperature of $T_N \sim 70$ K, also
indicating a quasi-1D nature of the magnetic interactions in Sr$_3$NiIrO$_6$.
Our results not only support the idea of the existence of a new temperature
scale well above $T_N$, but also emphasize the need to consider new exchange
paths complicated by the SOC, resulting in additional characteristic
temperatures in such spin-chain systems.",1501.05735v1
2015-02-18,Where are the roots of the Bethe Ansatz equations?,"Changing the variables in the Bethe Ansatz Equations (BAE) for the XXZ
six-vertex model we had obtained a coupled system of polynomial equations. This
provided a direct link between the BAE deduced from the Algebraic Bethe Ansatz
(ABA) and the BAE arising from the Coordinate Bethe Ansatz (CBA). For two
magnon states this polynomial system could be decoupled and the solutions given
in terms of the roots of some self-inversive polynomials. From theorems
concerning the distribution of the roots of self-inversive polynomials we made
a thorough analysis of the two magnon states, which allowed us to find the
location and multiplicity of the Bethe roots in the complex plane, to discuss
the completeness and singularities of Bethe's equations, the ill-founded
string-hypothesis concerning the location of their roots, as well as to find an
interesting connection between the BAE with Salem's polynomials.",1502.05316v2
2015-04-02,Electron-Magnon Scattering in Magnetic Heterostructures Far Out-of-Equilibrium,"We present a theory of out-of-equilibrium ultrafast spin dynamics in magnetic
heterostructures based on the s-d model of ferromagnetism. Both in the bulk and
across interfaces, the exchange processes between the itinerant s and the
localized d electrons are described by kinetic rate equations for
electron-magnon spin-flop scattering. The principal channel for dissipation of
angular momentum is provided by spin relaxation of the itinerant electrons. Our
theory extends interfacial spin phenomena such as torques, pumping, and the
Peltier and Seebeck effects to address laser-induced rapid spin dynamics, in
which the effective electron temperature may approach or even exceed the Curie
temperature.",1504.00511v1
2015-04-02,Thermodynamic Magnon Recoil for Domain Wall Motion,"We predict a thermodynamic magnon recoil effect for domain wall motions in
the presence of temperature gradients. All current thermodynamic theories
assert that a magnetic domain wall must move toward the hotter side, based on
equilibrium thermodynamic arguments. Microscopic calculations on the other hand
show that a domain wall can move either along or against the direction of heat
currents, depending on how strong the heat currents are reflected by the domain
wall. We have resolved the inconsistency between these two approaches by
augmenting the theory in the presence of thermal gradients by incorporating in
the free energy of domain walls by a heat current term present in
nonequilibrium steady states. The condition to observe a domain wall
propagation toward the colder regime is derived analytically and can be tested
by future experiments.",1504.00651v2
2015-04-07,Thermal conductivity of IPA-CuCl_3: Evidences of ballistic magnon transport and limited applicability of the Bose-Einstein condensation model,"The heat transport of the spin-gapped material (CH_3)_2CHNH_3CuCl_3
(IPA-CuCl_3), a candidate quantum magnet with Bose-Einstein condensation (BEC),
is studied at ultra-low temperatures and in high magnetic fields. Due to the
presence of the spin gap, the zero-field thermal conductivity (\kappa) is
purely phononic and shows a ballistic behavior at T < 1 K. When the gap is
closed by magnetic field at H = H_{c1}, where a long-range antiferromanetic
(AF) order of Cu^{2+} moments is developed, the magnons contribute
significantly to heat transport and exhibit a ballistic T^3 behavior at T < 600
mK. In addition, the low-T \kappa(H) isotherms show sharp peaks at H_{c1},
which indicates a gap re-opening in the AF state (H > H_{c1}) and demonstrates
limited applicability of the BEC model to IPA-CuCl_3.",1504.01465v1
2015-05-22,Thermometry and cooling of a Bose-Einstein condensate to 0.02 times the critical temperature,"Ultracold gases promise access to many-body quantum phenomena at convenient
length and time scales. However, it is unclear whether the entropy of these
gases is low enough to realize many phenomena relevant to condensed matter
physics, such as quantum magnetism. Here we report reliable single-shot
temperature measurements of a degenerate $^{87}$Rb gas by imaging the momentum
distribution of thermalized magnons, which are spin excitations of the atomic
gas. We record average temperatures as low as
$0.022(1)_\text{stat}(2)_\text{sys}$ times the Bose-Einstein condensation
temperature, indicating an entropy per particle, $S/N\approx0.001\, k_B$ at
equilibrium, that is well below the critical entropy for antiferromagnetic
ordering of a Bose-Hubbard system. The magnons themselves can reduce the
temperature of the system by absorbing energy during thermalization and by
enhancing evaporative cooling, allowing low-entropy gases to be produced within
deep traps.",1505.06196v1
2015-06-02,Scattering particles in quantum spin chains,"A variational approach for constructing an effective particle description of
the low-energy physics of one-dimensional quantum spin chains is presented.
Based on the matrix product state formalism, we compute the one- and
two-particle excitations as eigenstates of the full microscopic Hamiltonian. We
interpret the excitations as particles on a strongly-correlated background with
non-trivial dispersion relations, spectral weights and two-particle S matrices.
Based on this information, we show how to describe a finite density of
excitations as an interacting gas of bosons, using its approximate
integrability at low densities. We apply our framework to the Heisenberg
antiferromagnetic ladder: we compute the elementary excitation spectrum and the
magnon-magnon S matrix, study the formation of bound states and determine both
static and dynamic properties of the magnetized ladder.",1506.01008v2
2015-06-03,Magnetization reversal using excitation of collective modes in nanodot matrices,"The large arrays of magnetic dots are the building blocks of magnonic
crystals and the emerging bit patterned media for future recording technology.
In order to fully utilize the functionalities of high density magnetic
nanodots, a method for the selective reversal of a single nanodot in a matrix
of dots is desired. We have proposed a method for magnetization reversal of a
single nanodot with microwave excitation in a matrix of magneto-statically
interacting dots. The method is based on the excitation of collective modes and
the spatial anomaly in the microwave power absorption. We perform numerical
simulations to demonstrate the possibility of switching a single dot from any
initial state of a 3 by 3 matrix of dots, and develop a theoretical model for
the phenomena. We discuss the applicability of the proposed method for
introducing defect modes in magnonic crystals as well as for future magnetic
recording.",1506.01359v1
2015-06-23,Chirality-dependent transmission of spin waves through domain walls,"Spin-wave technology (magnonics) has the potential to further reduce the size
and energy consumption of information processing devices. In the submicrometer
regime (exchange spin waves), topological defects such as domain walls may
constitute active elements to manipulate spin waves and perform logic
operations. We predict that spin waves that pass through a domain wall in an
ultrathin perpendicular-anisotropy film experience a phase shift that depends
on the orientation of the domain wall (chirality). The effect, which is absent
in bulk materials, originates from the interfacial Dzyaloshinskii-Moriya
interaction and can be interpreted as a geometric phase. We demonstrate
analytically and by means of micromagnetic simulations that the phase shift is
strong enough to switch between constructive and destructive interference. The
two chirality states of the domain wall may serve as a memory bit or spin-wave
switch in magnonic devices.",1506.07038v2
2015-09-01,Spin dynamics of antiferromagnetically coupled bilayers - the case of Cr_2TeO_6,"Understanding the dynamics of interacting quantum spins has been one of the
active areas of condensed matter physics research. Recently, extensive
inelastic neutron scattering measurements have been carried out in an
interesting class of systems, Cr_2(W, Te, Mo)O_6. These systems consist of
bilayers of Cr^{3+} spins (S=3/2) with strong antiferromagnetic inter-bilayer
coupling (J) and tuneable intra-bilayer coupling (j) from ferro (for W and Mo)
to antiferro (for Te). In the limit when J>|j|, the system reduces to weakly
interacting quantum spin-3/2 dimers. In this paper we discuss the
low-temperature magnetic properties of Cr_2TeO_6 systems where both intra-layer
and inter-layer exchange couplings are antiferromagnetic, i.e. J,j>0. Using
linear spin-wave theory we obtain the magnon dispersion, sublattice
magnetization, two-magnon density of states, and longitudinal spin-spin
correlation function.",1509.00101v3
2015-09-11,Two-magnon Raman scattering and pseudospin-lattice interactions in Sr2IrO4 and Sr3Ir2O7,"We have used Raman scattering to investigate the magnetic excitations and
lattice dynamics in the prototypical spin-orbit Mott insulators Sr2IrO4 and
Sr3Ir2O7. Both compounds exhibit pronounced two-magnon Raman scattering
features with different energies, lineshapes, and temperature dependencies,
which in part reflect the different influence of long-range frustrating
exchange interactions. Additionally, we find strong Fano asymmetries in the
lineshapes of low-energy phonon modes in both compounds, which disappear upon
cooling below the antiferromagnetic ordering temperatures. These unusual phonon
anomalies indicate that the spin-orbit coupling in Mott-insulating iridates is
not sufficiently strong to quench the orbital dynamics in the paramagnetic
state.",1509.03396v1
2015-09-14,Highly anisotropic magnon dispersion in Ca2RuO4: evidence for strong spin orbit coupling,"The magnon disp ersion in Ca2RuO4 has been determined by inelastic neutron
scattering on single crytals containing 1% of Ti. The dispersion is well
described by a conventional Heisenberg model suggesting a local moment model
with nearest neighbor interaction of J=8 meV. Nearest and next-nearest neighbor
interaction as well as inter-layer coupling parameters are required to properly
describe the entire dispersion. Spin-orbit coupling induces a very large
anisotropy gap in the magnetic excitations in apparent contrast with a simple
planar magnetic model. Orbital ordering breaking tetragonal symmetry, and
strong spin-orbit coupling can thus be identified as important factors in this
system.",1509.03999v2
2015-11-29,Ultrafast electron-phonon-magnon interactions at noble metal-ferromagnet interfaces,"Ultrafast optical excitation of gold-cobalt bilayers triggers the nontrivial
interplay between the electronic, acoustic, and magnetic degrees of freedom.
Laser-heated electrons generated at the gold-air interface diffuse through the
layer of gold and strongly overheat the lattice in cobalt resulting in the
emission of ultrashort acoustic pulses and generation of exchange-coupled
magnons. Time-resolved optical measurements allow for extracting the thermal
boundary (Kapitza) resistances at metal/metal interfaces and the hot electron
diffusion length in ferromagnetic materials. Both the experimental data and the
analytical treatment of the two-temperature model reveal the role of the
Kapitza resistance in transient lattice overheating.",1511.09060v3
2016-01-26,Edge instabilities and skyrmion creation in magnetic layers,"We study both analytically and numerically the edge of two-dimensional
ferromagnets with Dzyaloshinskii-Moriya (DM) interactions, considering both
chiral magnets and magnets with interface-induced DM interactions. We show that
in the field-polarized ferromagnetic phase magnon states exist which are bound
to the edge, and we calculate their spectra within a continuum field theory.
Upon lowering an external magnetic field, these bound magnons condense at a
finite momentum and the edge becomes locally unstable. Micromagnetic
simulations demonstrate that this edge instability triggers the creation of a
helical phase which penetrates the field-polarized state within the bulk. A
subsequent increase of the magnetic field allows to create skyrmions close to
the edge in a controlled manner.",1601.06922v2
2016-02-12,Spin dynamics in the high-field phases of volborthite,"We report single-crystal 51V NMR studies on volborthite Cu3V2O7(OH)2 2H2O,
which is regarded as a quasi-two-dimensional frustrated magnet with competing
ferromagnetic and antiferromagnetic interactions. In the 1/3 magnetization
plateau above 28 T, the nuclear spin-lattice relaxation rate 1/T1 indicates an
excitation gap with a large effective g factor in the range of 4.6-5.9,
pointing to magnon bound states. Below 26 T where the gap has closed, the NMR
spectra indicate small internal fields with a Gaussian-like distribution,
whereas 1/T1 shows a power-law-like temperature dependence in the paramagnetic
state, which resembles a slowing down of spin fluctuations associated with
magnetic order. We discuss the possibility of an exotic spin state caused by
the condensation of magnon bound states below the magnetization plateau.",1602.04028v2
2016-04-08,Spin-Superfluidity and Spin-Current Mediated Non-Local Transport,"Some strategies for reducing energy consumption in information processing
devices involve the use of spin rather than charge to carry information. This
idea is especially attractive when the spin current is a collective one carried
by the condensate of a magnetically ordered state, rather than a quasiparticle
current carried by electrons or magnons. In this Chapter we explain how
easy-plane magnets can be viewed as Bose-Einstein condensates of magnons,
defined in terms of quanta of the spin-component perpendicular to the easy
plane, and how they can carry dissipationless spin-currents that induce
non-local interactions between electrically isolated conducting channels. We
comment specifically on important differences between superconductivity in
normal/superconducting/normal circuits and spin-superfluidity in
normal/magnetic/normal circuits.",1604.02429v1
2016-04-22,Frustrated Ising chains on the triangular lattice in Sr$_3$NiIrO$_6$,"Inelastic neutron scattering study on the spin-chain compound Sr$_3$NiIrO$_6$
reveals gapped quasi-1D magnetic excitations. The observed one-magnon band
between 29.5 and 39 meV consists of magnon modes of the Ni$^{2+}$ ions. The
fitting of the spin wave spectrum reveals strongly coupled Ising-like chains
along the $c$-axis that are weakly coupled into a frustrated triangular lattice
in the $ab$-plane. The magnetic excitations survive up to 200 K well above the
magnetic ordering temperature of $T_N \sim 75$ K, also indicating a quasi-1D
nature of the magnetic interactions in Sr$_3$NiIrO$_6$. Our microscopic model
is in good agreement with ab initio electronic structure calculations and
explains the giant spin flip field observed in bulk magnetization measurements.",1604.06569v1
2016-04-28,"Magnetization Saturation Process in the Magnonic Anti-dot Structures Based on (Ga,Mn)As: A Magnetometric Study","Applicability of dilute magnetic semiconductors (DMS) in electronic devices
relies upon the understanding and control of their magnetic anisotropy. This
paper explores one of the ways in engineering magnetic anisotropy in epitaxial
layers of DMS by forming them into magnonic structures. For this purpose the
canonical ferromagnetic DMS, namely (Ga,Mn)As, is employed. The anti-dot
systems based on (Ga,Mn)As layers of various thicknesses are fabricated with
focused ion beam apparatus and studied by means of microscopy as well as
magnetometry. The magnetometric data - collected along two nonequivalent
in-plane crystallographic directions of (Ga,Mn)As: [110] and [1-10] - shows the
effect of structuring on high-field magnetization process, whereas no
significant change of the width of hysteresis loop in anti-dot samples is
observed.",1604.08607v1
2016-06-23,Spin-Hall nano-oscillator with oblique magnetization and Dzyaloshinskii-Moriya interaction as generator of skyrmions and nonreciprocal spin-waves,"Spin-Hall oscillators are promising sources of spin-wave signals for
magnonics applications, and can serve as building blocks for magnonic logic in
ultralow power computation devices. Here, we analytically and micromagnetically
study magnetization dynamics excited in a Spin-Hall oscillator with oblique
magnetization when the spin-Hall effect and interfacial Dzyaloshinskii-Moriya
interaction act simultaneously. Our key results are (i) excitation of
nonreciprocal spin-waves propagating perpendicularly to the in-plane projection
of the static magnetization, (ii) skyrmions generation by pure spin-current,
(iii) excitation of a new spin-wave mode with a spiral spatial profile
originating from a gyrotropic rotation of a dynamical skyrmion. These results
demonstrate that Spin-Hall oscillators can be used as generators of magnetic
skyrmions and different types of propagating spin-waves for magnetic data
storage and signal processing applications.",1606.07280v1
2016-07-08,Control of magnon-photon coupling strength in a planar resonator/YIG thin film configuration,"A systematic study of the coupling at room temperature between ferromagnetic
resonance (FMR) and a planar resonator is presented. The chosen magnetic
material is a ferrimagnetic insulator (Yttrium Iron Garnet: YIG) which is
positioned on top of a stop band (notch) filter based on a stub line
capacitively coupled to a 50 $\Omega$ microstrip line resonating at 4.731 GHz.
Control of the magnon-photon coupling strength is discussed in terms of the
microwave excitation configuration and the YIG thickness from 0.2 to 41 $\mu$m.
From the latter dependence, we extract a single spin-photon coupling of
g$_{0}$/2$\pi$=162$\pm$6 mHz and a maximum of an effective coupling of 290 MHz.",1607.02358v3
2017-02-16,"Magnetization dynamics in magnonic structures with different geometries: interfaces, notches and waveguides","The discovery of the ultrafast magnetization dynamics twenty years ago has
led to a broad variety of experimental techniques to explore phenomena in
magnetic materials with high temporal resolution. In the present article we
present a study dealing with broadband excitation of spin-wave packets at
different magnonic crystal continuous magnetic film interfaces. Similar to
protected conducting states on the surfaces of topological band insulators,
these interfaces exhibit surface spin-wave modes that propagate out of the
crystal into the continuous film. The propagation distance depends on the
direction of the applied magnetic field as well as the surface geometry of the
crystal.",1702.05041v1
2017-02-27,Dynamics of Bound Magnon Pairs in the Quasi-One-Dimensional Frustrated Magnet LiCuVO_4,"We report on the dynamics of the spin-1/2 quasi-one-dimensional frustrated
magnet LiCuVO$\mathrm{_4}$ measured by nuclear spin relaxation in high magnetic
fields 10--34 T, in which the ground state has spin-density-wave order. The
spin fluctuations in the paramagnetic phase exhibit striking anisotropy with
respect to the magnetic field. The transverse excitation spectrum probed by
$^{51}$V nuclei has an excitation gap, which increases with field. On the other
hand, the gapless longitudinal fluctuations sensed by $^7$Li nuclei grow with
lowering temperature, but tend to be suppressed with increasing field. Such
anisotropic spin dynamics and its field dependence agree with the theoretical
predictions and are ascribed to the formation of bound magnon pairs, a
remarkable consequence of the frustration between ferromagnetic nearest
neighbor and antiferromagnetic next-nearest-neighbor interactions.",1702.08573v2
2017-03-11,Confined states in photonic-magnonic crystals with complex unit cell,"We have investigated multifunctional periodic structures in which
electromagnetic waves and spin waves can be confined in the same areas. Such
simultaneous localization of both sorts of excitations can potentially enhance
the interaction between electromagnetic waves and spin waves. The system we
considered has a form of one dimensional photonic-magnonic crystal with two
types of magnetic layers (thicker and thinner ones) separated by sections of
the dielectric photonic crystals. We focused on the electromagnetic defect
modes localized in the magnetic layers (areas where spin waves can be exited)
and decaying in the sections of conventional (nonmagnetic) photonic crystals.
We showed how the change of relative thickness of two types of the magnetic
layers can influence on the spectrum of spin waves and electromagnetic defect
modes, both localized in magnetic parts of the system.",1703.04013v1
2017-08-19,Current induced multi-mode propagating spin waves in a spin transfer torque nano-contact with strong perpendicular magnetic anisotropy,"Current induced spin wave excitations in spin transfer torque nano-contacts
are known as a promising way to generate exchange-dominated spin waves at the
nano-scale. It has been shown that when these systems are magnetized in the
film plane, broken spatial symmetry of the field around the nano-contact
induced by the Oersted field opens the possibility for spin wave mode
co-existence including a non-linear self-localized spin-wave bullet and a
propagating mode. By means of micromagnetic simulations, here we show that in
systems with strong perpendicular magnetic anisotropy (PMA) in the free layer,
two propagating spin wave modes with different frequency and spatial
distribution can be excited simultaneously. Our results indicate that in-plane
magnetized spin transfer nano-contacts in PMA materials do not host a solitonic
self-localized spin-wave bullet, which is different from previous studies for
systems with in plane magnetic anisotropy. This feature renders them
interesting for nano-scale magnonic waveguides and crystals since magnon
transport can be configured by tuning the applied current.",1708.05886v1
2017-08-30,Propagation of self-localised Q-ball solitons in the $^3$He universe,"In relativistic quantum field theories, compact objects of interacting bosons
can become stable owing to conservation of an additive quantum number $Q$.
Discovering such $Q$-balls propagating in the Universe would confirm
supersymmetric extensions of the standard model and may shed light on the
mysteries of dark matter, but no unambiguous experimental evidence exists. We
report observation of a propagating long-lived $Q$-ball in superfluid $^3$He,
where the role of $Q$-ball is played by a Bose-Einstein condensate of magnon
quasiparticles. We achieve accurate representation of the $Q$-ball Hamiltonian
using the influence of the number of magnons, corresponding to the charge $Q$,
on the orbital structure of the superfluid $^3$He order parameter. This
realisation supports multiple coexisting $Q$-balls which in future allows
studies of $Q$-ball dynamics, interactions, and collisions.",1708.09224v1
2018-02-05,Micromagnetic view on ultrafast magnon generation by femtosecond spin current pulses,"In this Article we discuss a micromagnetic modelling approach to describe the
ultrafast spin-transfer torque excitation of coherent and incoherent magnons on
the nanoscale. Implementing the action of a femtosecond spin current pulse
entering an orthogonally magnetized thin ferromagnetic film, we reproduce
recent experimental results and reveal the factors responsible for the unequal
excitation efficiency of various spin waves. Our findings are in an excellent
agreement with the results of an analytical description of spin-wave excitation
based on classical kinetic equations. Furthermore, we suggest an experimental
design allowing for the excitation of laterally propagating spin waves beyond
the optical diffraction limit. Our findings demonstrate that the classical
micromagnetic picture retains its predictive and interpretative power on
femtosecond temporal and nanometer spatial scales.",1802.01305v1
2018-04-18,Spin dynamics from time-dependent density functional perturbation theory,"We present a new method to model spin-wave excitations in magnetic solids,
based on the Liouville-Lanczos approach to time-dependent density functional
perturbation theory. This method avoids computationally expensive sums over
empty states and naturally deals with the coupling between spin and charge
fluctuations, without ever explicitly computing charge-density
susceptibilities. Spin-wave excitations are obtained with one Lanczos chain per
magnon wave-number and polarization, avoiding the solution of the
linear-response problem for every individual value of frequency, as other
state-of-the-art approaches do. Our method is validated by computing magnon
dispersions in bulk Fe and Ni, resulting in agreement with previous theoretical
studies in both cases, and with experiment in the case of Fe. The disagreement
in the case of Ni is also comparable with that of previous computations.",1804.06573v2
2018-11-08,Quasistatic oscillations in subwavelength particles: Can one observe energy eigenstates?,"In increasing the capabilities of the optical and microwave techniques
further into the subwavelength regime, quasistatic resonant structures has
attracted considerable interest. Electromagnetic responses of electrostatic
(ES) plasmon resonances in optics and magnetostatic (MS) magnon resonances in
microwaves give rise to a strong enhancement of local fields near the surfaces
of subwavelength particles. In the near-field regions of subwavelength
particles one can only measure the electric or the magnetic field with
accuracy. Such uncertainty in definition of the electric or magnetic field
components raises the question of energy eigenstates of quasistatic
oscillations. The energy eigenstate problem can be properly formulated when
potential functions, used in the quasistatic-resonance problems, are introduced
as scalar wave functions. In this case, one should observe quasistatic-wave
retardation effects still staying in frames of the quasistatic description of
oscillations in a subwavelength particle. In this paper, we analyze the problem
of energy quantization of ES resonances in subwavelength optical metallic
structures with plasmon oscillations and MS resonances in subwavelength
microwave ferrite particles with magnon oscillations. We show that in a case of
MS-potential scalar wave function one can observe quasistatic retardation
effects and obtain a proper formulation of the energy eigenstate problem.",1811.03314v1
2018-11-24,Goos-Hänchen effect in light transmission through biperiodic photonic-magnonic crystals,"We present a theoretical investigation of the Goos-H\""anchen effect, i.e.,
the lateral shift of the light beam transmitted through one-dimensional
biperiodic multilayered photonic systems consisting of equidistantmagnetic
layers separated by finite size dielectric photonic crystals. We show that the
increase of the number of periods in the photonic-magnonic structure leads to
increase of the Goos-H\""anchen shift in the vicinity of the frequencies of
defect modes located inside the photonic band gaps. Presence of the linear
magnetoelectric coupling in the magnetic layers can result in a vanishing of
the positive maxima of the cross-polarized contribution to the Goos-H\""anchen
shift.",1811.09865v1
2018-11-24,Faraday effect in bi-periodic photonic-magnonic crystals,"We present a theoretical investigation of the polarization plane rotation at
light transmission - Faraday effect, through one-dimensional multilayered
magneto-photonic systems consisting of periodically distributed magnetic and
dielectric layers. We calculate Faraday rotation spectra of photonic-magnonic
crystals, where cell (or supercell) is composed of magnetic layer and
dielectric layer (or section of dielectric photonic crystal). We found that the
Faraday rotation of p-polarized incident light is increasing in the
transmission band with the number of magnetic supercells. The increase of
Faraday rotation is observed also in vicinity of the band-gap modes localized
in magnetic layers but the maximal polarization plane rotation angles are
reached at minimal transmittivity. We show that presence of linear
magneto-electric interaction in the magnetic layers leads to significant
increase of the Faraday rotation angles of s-polarized incident light in the
vicinity of the fine-structured modes inside the photonic-band-gap.",1811.09866v1
2010-05-10,Spectrum of short-wavelength magnons in two-dimensional quantum Heisenberg antiferromagnet on a square lattice: third order expansion in 1/S,"The spectrum of short-wavelength magnons in two-dimensional quantum
Heisenberg antiferromagnet on a square lattice is calculated in the third order
in $1/S$ expansion. It is shown that $1/S$ series for $S=1/2$ converges fast in
the whole Brillouin zone except for the neighborhood of the point ${\bf
k}=(\pi,0)$, at which absolute values of the third and the second order
$1/S$-corrections are approximately equal to each other. It is shown that the
third order corrections make deeper the roton-like local minimum at ${\bf
k}=(\pi,0)$ improving the agreement with the recent experiments and numerical
results in the neighborhood of this point. It is suggested that $1/S$ series
converges slowly near ${\bf k}=(\pi,0)$ also for $S=1$ although the spectrum
renormalization would be small in this case due to very small values of
high-order $1/S$ corrections.",1005.1568v1
2010-05-12,Ferromagnetism of $UGe_2$,"Magnetism of $UGe_2$ is due to the magnetic ordered moments of $5f$ uranium
electrons. The strong spin-orbit coupling splits them into two groups. The
magnetization is investigated in terms of two vector fields ${\bf M}_{1i}$ and
${\bf M}_{2i}$ which identify the local orientation of the magnetization of the
two groups of $f$ electrons. Renormalized spin-wave theory, which accounts for
the magnon-magnon interaction, and its extension are developed to describe two
ferromagnetic phases in the system: low temperature large moment phase
$0<T<T^{*}$ (FM2), where all $5f$ electrons contribute the ordered
ferromagnetic moment, and high temperature low-moment phase $T^{*}<T<T_C$
(FM1), where $f$ electrons are partially ordered. Both phases are strictly
ferromagnetic in accordance with experiment. The magnetization as a function of
temperature is calculated. The anomalous temperature dependence of the ordered
moment, known from the experiments with $UGe_2$, is very well reproduced
theoretically. Below $T_x$ ($T^*$ in the present paper) the ferromagnetic
moment increases in an anomalous way. The new understanding of the anomalous
$FM2\to FM1$ transition, as a result of the magnetic order of two well
separated groups of $f$ electrons, yields the key to an understanding of the
ferromagnetism and transport properties in these compounds.",1005.2032v1
2010-05-15,Coupling between magnon and ligand-field excitations in magnetoelectric Tb3Fe5O12 garnet,"The spectra of far-infrared transmission in Tb3Fe5O12 magnetoelectric single
crystals have been studied in the range between 15 and 100 cm-1, in magnetic
fields up to 10 T, and for temperatures between 5 and 150 K. We attribute some
of the observed infrared-active excitations to electric-dipole transitions
between ligand-field split states of Tb3+ ions. Anticrossing between the
magnetic exchange excitation and the ligand-field transition occurs at the
temperature between 60 and 80 K. The corresponding coupling energy for this
interaction is 6 cm-1. Temperature-induced softening of the hybrid IR
excitation correlates with the increase of the static dielectric constant. We
discuss the possibility for hybrid excitations of magnons and ligand-field
states and their possible connection to the magnetoelectric effect in
Tb3Fe5O12.",1005.2705v1
2013-08-06,Two magnon bound state causes ultrafast thermally induced magnetisation switching,"There has been much interest recently in the discovery of thermally induced
magnetisation switching, where a ferrimagnetic system can be switched
deterministically without and applied magnetic field. Experimental results
suggest that the reversal occurs due to intrinsic material properties, but so
far the microscopic mechanism responsible for reversal has not been identified.
Using computational and analytic methods we show that the switching is caused
by the excitation of two magnon bound states, the properties of which are
dependent on material factors. This discovery allows us to accurately predict
the switching behaviour and the identification of this mechanism will allow new
classes of materials to be identified or designed to use this switching in
memory devices in the THz regime.",1308.1314v1
2013-08-12,AC current generation in chiral magnetic insulators and skyrmion motion induced by the spin Seebeck effect,"We show that a temperature gradient induces an ac electric current in
multiferroic insulators when the sample is embedded in a circuit. We also show
that a thermal gradient can be used to move magnetic skyrmions in insulating
chiral magnets: the induced magnon flow from the hot to the cold region drives
the skyrmions in the opposite direction via a magnonic spin transfer torque.
Both results are combined to compute the effect of skyrmion motion on the ac
current generation and demonstrate that skyrmions in insulators are a promising
route for spin caloritronics applications.",1308.2634v3
2013-08-30,Spin-motive force and orbital-motive force: from magnon BEC to chiral Weyl superfluids,"Spin-motive geometric force acting on electrons in metallic ferromagnets is
extended to spin-motive force in magnon BEC, which is represented by
phase-coherent precession of magnetization, and to the orbital-motive force in
superfluid 3He-A. In 3He-A there are two contributions to the orbital-motive
force. One of them comes from the chiral nature of this liquid. Another one
originates from chiral Weyl fermions living in the vicinity of the
topologically protected Weyl points, and is related to the phenomenon of chiral
anomaly.",1308.6700v3
2014-03-23,Magnon dispersion and Single hole motion in 2D frustrated antiferromagnets with four-sublattice structures,"We study a two dimensional spin-$\frac{1}{2}$ $J_1-J_2$ antiferromagnet in a
square lattice using the linearized spin wave theory recognizing the
4-sublattice nature of the underlying magnetic lattice. Multiple magnon modes
with optical and acoustic branches about the stable Neel ordered and double
acoustic branches about the columnar reference states are obtained for small
and large values of $\lambda(=J_2/J_1)$ respectively. An additional uniaxial
anisotropy, for large $\lambda$, can lead to distinct spin gaps in such
systems, as also witnessed experimentally. The single hole spectral behavior in
a 2D $t-J_1-J_2$ model, for small frustration, is then calculated within the
non-crossing approximation. Our results match fairly well with exact
diagonalization results from a $4\times4$ cluster. Hole spectral features and
their evolution with $\lambda$ resulting in ""water-fall""-like smooth spectral
weight transfer are discussed. Hole energy bands are identified and the
corresponding energy-shift and reduction in width with spin-frustration are
indicated.",1403.5742v3
2017-01-05,Excitation and detection of short-waved spin waves in ultrathin Ta/CoFeB/MgO-layer system suitable for spin-orbit-torque magnonics,"We report on the excitation and detection of short-waved spin waves with wave
vectors up to about $40\,\mathrm{rad}\,\mu\mathrm{m}^{-1}$ in spin-wave
waveguides made from ultrathin, in-plane magnetized Co$_{8}$Fe$_{72}$B$_{20}$
(CoFeB). The CoFeB is incorporated in a layer stack of Ta/CoFeB/Mgo, a layer
system featuring large spin orbit torques and a large perpendicular magnetic
anisotropy constant. The short-waved spin waves are excited by nanometric
coplanar waveguides and are detected via spin rectification and microfocussed
Brillouin light scattering spectroscopy. We show that the large perpendicular
magnetic anisotropy benefits the spin-wave lifetime greatly, resulting in a
lifetime comparable to bulk systems without interfacial damping. The presented
results pave the way for the successful extension of magnonics to ultrathin
asymmetric layer stacks featuring large spin orbit torques.",1701.01399v1
2017-01-16,Four-Point Functions with a Twist,"We study the OPE of correlation functions of local operators in planar N=4
super Yang-Mills theory. The considered operators have an explicit spacetime
dependence that is defined by twisting the translation generators with certain
R-symmetry generators. We restrict to operators that carry a small number of
excitations above the twisted BMN vacuum. The OPE limit of the four-point
correlator is dominated by internal states with few magnons on top of the
vacuum. The twisting directly couples all spacetime dependence of the
correlator to these magnons. We analyze the OPE in detail, and single out the
extremal states that have to cancel all double-trace contributions.",1701.04424v3
2017-01-17,Mutually attracting spin waves in the square-lattice quantum antiferromagnet,"The Heisenberg model for S=1/2 describes the interacting spins of electrons
localized on lattice sites due to strong repulsion. It is the simplest
strong-coupling model in condensed matter physics with wide-spread
applications. Its relevance has been boosted further by the discovery of curate
high-temperature superconductors. In leading order, their undoped parent
compounds realize the Heisenberg model on square-lattices. Much is known about
the model, but mostly at small wave vectors, i.e., for long-range processes,
where the physics is governed by spin waves (magnons), the Goldstone bosons of
the long-range ordered antiferromagnetic phase. Much less, however, is known
for short-range processes, i.e., at large wave vectors. Yet these processes are
decisive for understanding high-temperature superconductivity. Recent reports
suggest that one has to resort to qualitatively different fractional
excitations, spinons. By contrast, we present a comprehensive picture in terms
of dressed magnons with strong mutual attraction on short length scales. The
resulting spectral signatures agree strikingly with experimental data",1701.04730v2
2017-01-27,Collective dynamics in atomistic models with coupled translational and spin degrees of freedom,"Using an atomistic model that simultaneously treats the dynamics of
translational and spin degrees of freedom, we perform combined molecular and
spin dynamics simulations to investigate the mutual influence of the phonons
and magnons on their respective frequency spectra and lifetimes in
ferromagnetic bcc iron. By calculating the Fourier transforms of the space- and
time-displaced correlation functions, the characteristic frequencies and the
linewidths of the vibrational and magnetic excitation modes were determined.
Comparison of the results with that of the stand-alone molecular dynamics and
spin dynamics simulations reveals that the dynamic interplay between the
phonons and magnons leads to a shift in the respective frequency spectra and a
decrease in the lifetimes. Moreover, in the presence of lattice vibrations,
additional longitudinal magnetic excitations were observed with the same
frequencies as the longitudinal phonons.",1701.07906v1
2017-01-31,Spin waves in planar quasicrystal of Penrose tiling,"We investigated two-dimensional magnonic structures which are the
counterparts of photonic quasicrystals forming Penrose tiling. We considered
the slab composed of Ni (or Py) disks embedded in Fe (or Co) matrix. The disks
were arranged in quasiperiodic Pernose-like structure. The infinite
quasicrystal was approximated by its rectangular section with periodic boundary
conditions applied. This approach allowed us to use the plane wave method to
find the frequency spectrum of eigenmodes for spin waves and their spatial
profiles. The calculated integrated density of states shows more distictive
magnonic gaps for the structure composed of materials of high magnetic contrast
(Ni and Fe) and relatively high filling fraction. This proves the impact of
quasiperiodic long-range order on the spectrum of spin waves. We also
investigated the localization of SW eingenmodes resulting from the
quasipeiodicity of the structure.",1701.09125v1
2018-05-01,Spontaneous Exact Spin-Wave Fractals in Magnonic Crystals,"Exact fractals of nonlinear waves that rely on strong dispersion and
nonlinearity and arise spontaneously out of magnetic media were observed for
the first time. The experiments make use of a microwave to excite a spin wave
in a quasi-one-dimensional magnonic crystal. When the power of the input
microwave (Pin) is low, the output signal has a power-frequency spectrum that
consists of a single peak. When Pin is increased to a certain level, the new
side modes are generated through modulational instability (MI), resulting in a
comb-like frequency spectrum. With a further increase in Pin, each peak in the
frequency comb can evolve into its own, finer comb through the MI. As Pin is
increased further, one can observe yet another set of finer frequency combs.
Such a frequency-domain fractal manifests itself as multiple layers of
amplitude modulation in the time-domain signal.",1805.00517v2
2018-05-03,Sculpting the Spin-Wave Response of Artificial Spin Ice via Microstate Selection,"The spin-wave dynamics of the ferromagnetic nanoarrays termed artificial spin
ice (ASI) are known to vary depending on their magnetic microstate. However,
little work has been done to characterise this relationship. Recent advances in
control over the magnetic configuration of ASI bring designs harnessing the
interplay between spin-wave eigenmodes and the microstate within reach,
offering diverse applications including reconfigurable magnonic crystals,
microwave filters and microstate read-out probes. These designs hinge on a
strong understanding of the underlying spin wave-microstate correspondence.
  Here, we analyse the effects of the magnetic microstate on spin-wave spectra
of honeycomb ASI systems via micromagnetic simulation. We find the spin-wave
spectrum to be highly-tunable via the microstate to an enhanced degree relative
to existing magnonic crystals, with mode shifting and (de)activation realised
by reversing individual nanoislands. Symmetries of ASI systems and the
chirality of magnetic defects are found to play important roles in determining
the high-frequency response.",1805.01397v3
2018-05-09,Programmable control of spin-wave transmission in a domain-wall spin valve,"Active manipulation of spin waves is essential for the development of
magnon-based technologies. Here, we demonstrate programmable spin-wave
filtering by resetting the spin structure of a pinned 90$^\circ$ N\'{e}el
domain wall in a continuous CoFeB film with abrupt rotations of uniaxial
magnetic anisotropy. Using phase-resolved micro-focused Brillouin light
scattering and micromagnetic simulations, we show that broad 90$^\circ$
head-to-head or tail-to-tail magnetic domain walls are transparent to spin
waves over a broad frequency range. In contrast, magnetic switching to a
90$^\circ$ head-to-tail configuration produces much narrower domain walls at
the same pinning locations. Spin waves are strongly reflected by a resonance
mode in these magnetic domain walls. Based on these results, we propose a
magnetic spin-wave valve with two parallel domain walls. Switching the
spin-wave valve from an open to a close state changes the transmission of spin
waves from nearly 100% to 0% at the resonance frequency. This active control
over spin-wave transport could be utilized in magnonic logic devices or
non-volatile memory elements.",1805.03470v1
2018-05-21,Nonreciprocal superposition state in antiferromagnetic optospintronics,"The absence of net magnetization, which forbids any stray magnetic fields, is
one of the greatest advantages of antiferromagnets in device applications. In
conventional antiferromagnets, however, spin current cannot be extracted
without the aid of a static magnetic field. Here, we develop a theory of
antiferromagnetic opto-spintronics to resolve this fundamental dilemma. By
coupling a linearly polarized photon and nonreciprocal magnon bands, we
construct a superposition state of left- and right-handed magnon states with
opposite group velocities. We numerically demonstrate that by using this
superposition state, an antiferromagnetic spin current can be efficiently
generated without a net magnetic field including net magnetization. We also
find that the breakdown of the superposition state induces the stripe
superfluid phase of a two-component Bose-Einstein condensate. Our results lay
the foundation for manipulating the superposition states of emergent particles
in devices.",1805.08226v3
2018-05-22,Non-reciprocal magnons in non-centrosymmetric MnSi,"Using two cold-neutron triple-axis spectrometers we have succeeded in fully
mapping out the field-dependent evolution of the non-reciprocal magnon
dispersion relations in all magnetic phases of MnSi. The non-reciprocal nature
of the dispersion manifests itself in a full asymmetry (non-reciprocity) of the
dynamical structure factor $S(q, E, \mu_0 H_{int})$ with respect to flipping
either the direction of the applied magnetic field $\mu_0 H_{int}$, the reduced
momentum transfer $q$, or the energy transfer $E$.",1805.08750v2
2018-05-23,Spectral signatures of fractionalization in the frustrated Heisenberg model on the square lattice,"We employ a variational Monte Carlo approach to efficiently obtain the
dynamical structure factor for the spin-1/2 $J_1-J_2$ Heisenberg model on the
square lattice. Upon increasing the frustrating ratio $J_2/J_1$, the ground
state undergoes a continuous transition from a N\'eel antiferromagnet to a
$\mathbb{Z}_{2}$ gapless spin liquid. We identify the characteristic spectral
features in both phases and highlight the existence of a broad continuum of
excitations in the proximity of the spin-liquid phase. The magnon branch, which
dominates the spectrum of the unfrustrated Heisenberg model, gradually loses
its spectral weight, thus releasing nearly-deconfined spinons, whose signatures
are visible even in the magnetically ordered state. Our results show how free
spinons emerge across a quantum critical point, providing evidence for the
fractionalization of magnons into deconfined spinons.",1805.09287v2
2009-12-10,Magnonic crystal based forced dominant wavenumber selection in a spin-wave active ring,"Spontaneous excitation of the dominant mode in a spin-wave active ring -- a
self-exciting positive-feedback system incorporating a spin-wave transmission
structure -- occurs at a certain threshold value of external gain. In general,
the wavenumber of the dominant mode is extremely sensitive to the properties
and environment of the spin-wave transmission medium, and is almost impossible
to predict. In this letter, we report on a backward volume magnetostatic
spin-wave active ring system incorporating a magnonic crystal. When mode
enhancement conditions -- readily predicted by a theoretical model -- are
satisfied, the ring geometry permits highly robust and consistent forced
dominant wavenumber selection.",0912.2029v3
2009-12-11,"Orbital order, stacking defects and spin-fluctuations in the $p$-electron molecular solid RbO$_2$","We examine magnon and orbiton behavior in localized O$_2$ anti-bonding
molecular $\pi^*$ orbitals using an effective Kugel-Khomskii Hamiltonian
derived from a two band Hubbard model with hopping parameters taken from {\em
ab initio} density functional calculations. The considerable difference between
intraband and interband hoppings leads to a strong coupling between the spin
wave dispersion and the orbital ground state, providing a straightforward way
of experimentally determining the orbital ground state from the measured magnon
dispersion. The near degeneracy of different orbital ordered states leads to
stacking defects which further modulate spin-fluctuation spectra. Proliferation
of orbital domains disrupts long-range magnetic order, thus causing a
significant reduction in the observed N\'eel temperature.",0912.2334v2
2011-11-14,Nematic phase and phase separation near saturation field in frustrated ferromagnets,"We study the effects of quantum fluctuations in magnetic properties of
quantum frustrated ferromagnets in a magnetic field. It is shown that a
non-classical phase or a phase separation appears due to quantum fluctuations
below the saturation field in a parameter range close to the classical
zero-field phase boundary between ferromagnetic and antiferromagnetic phases,
for the case that the classical antiferromagnetic state is not an eigenstate of
the quantum model. As an example to which this argument is applicable, we study
the S=1/2 J1-J2 Heisenberg model with ferromagnetic J1 (J1<0) on the bcc
lattice using a dilute Bose gas approach. For -1.50097 < J1/J2 < -1.389,
magnons form f-wave two-magnon bound states, leading to a spin nematic phase,
and for -1.389 < J1/J2 < -0.48 a canted coplanar antiferromagnetic phase
appears accompanied with a phase separation below the saturation field.",1111.3184v3
2011-11-14,Superatom Representation of High-TC Superconductivity Revisited,"A ""super-atom"" conceptual interface between chemistry and physics is proposed
in order to assist in the search for higher TC superconductors. High-TC
superconductivity HTSC is articulated as the entanglement of two disjoint
electronic manifolds in the vicinity of a common Fermi energy. The resulting
HTSC ground state couples near-degenerate protected local ""super-atom"" states
to virtual magnons in an antiferromagnetic AFM embedding. The composite Cooper
pairs emerge as the interaction particles for virtual magnons mediated
""self-coherent entanglement"" of super-atom states. A H\""uckel type resonating
valence bond RVB formalism is employed in order to illustrate the real-space
Cooper pairs as well as their delocalization and Bose Einstein condensation BEC
on a ring of super-atoms. The chemical potential \mu(BEC) for Cooper pairs
joining the condensate is formulated in terms of the super-exchange
interaction, and consequently the TC in terms of the Ne\'el temperature. A
rationale for the robustness of the HTSC ground state is proposed: achieving
local maximum ""electron correlation entropy"" at the expense of non-local phase
rigidity.",1111.3190v1
2011-11-21,Magnetic excitations in the spin-1 anisotropic antiferromagnet $NiCl_2-4SC(NH_2)_2$,"The spin-1 anisotropic antiferromagnet NiCl_2-4SC(NH2)_2 exhibits a
field-induced quantum phase transition that is formally analogous to
Bose-Einstein condensation. Here we present results of systematic high-field
electron spin resonance (ESR) experimental and theoretical studies of this
compound with a special emphasis on single-ion two-magnon bound states. In
order to clarify some remaining discrepancies between theory and experiment,
the frequency-field dependence of magnetic excitations in this material is
reanalyzed. In particular, a more comprehensive interpretation of the
experimental signature of single-ion two-magnon bound states is shown to be
fully consistent with theoretical results. We also clarify the structure of the
ESR spectrum in the so-called intermediate phase.",1111.5017v2
2012-04-05,The Magnon Spectrum in the Domain Ferromagnetic State of Antisite Disordered Double Perovskites,"In their ideal structure, double perovskites like Sr_2FeMoO_6 have
alternating Fe and Mo along each cubic axes, and a homogeneous ferromagnetic
metallic ground state. Imperfect annealing leads to the formation of structural
domains. The moments on mislocated Fe atoms that adjoin each other across the
domain boundary have an antiferromagnetic coupling between them. This leads to
a peculiar magnetic state, with ferromagnetic domains coupled
antiferromagnetically. At short distance the system exhibits ferromagnetic
correlation while at large lengthscales the net moment is strongly suppressed
due to inter-domain cancellation. We provide a detailed description of the spin
wave excitations of this complex magnetic state, obtained within a 1/S
expansion, for progressively higher degree of mislocation, i.e., antisite
disorder. At a given wavevector the magnons propagate at multiple energies,
related, crudely, to `domain confined' modes with which they have large
overlap. We provide a qualitative understanding of the trend observed with
growing antisite disorder, and contrast these results to the much broader
spectrum that one obtains for uncorrelated antisites.",1204.1194v1
2012-04-11,A short note on spin pumping theory with Landau-Lifshitz-Gilbert equation under quantum fluctuation; necessity for quantization of localized spin,"We would like to point out the blind spots of the approach combining the spin
pumping theory proposed by Tserkovnyak et al. with the Landau-Lifshitz-Gilbert
equation; this method has been widely used for interpreting vast experimental
results. The essence of the spin pumping effect is the quantum fluctuation.
Thus, localized spin degrees of freedom should be quantized, i.e. be treated as
magnons not as classical variables. Consequently, the precessing ferromagnet
can be regarded as a magnon battery. This point of view will be useful for
further progress of spintronics.",1204.2339v1
2013-08-30,Superconducting instability of a non-magnetic metallic band in an antiferromagnetic background,"It is shown that a non-magnetic metallic band in the presence of an
antiferromagnetic background coupled only by the exchange interaction develops
a superconducting instability similar to the one described by BCS theory plus
additional terms that strongly renormalize the spin excitation spectra. A
Bardeen-Pines-like hole-hole interaction Hamiltonian, mediated by magnetic
excitations, is deduced from a microscopic model of a fermion band and a spin
band that interact with each other only via the exchange interaction. The
theory shows the appearance of an attractive interaction when the Fermi
velocity in the non-magnetic band is larger than the magnon velocity in the
magnetic band. The electron-magnon scattering is suppressed by the appearance
of a spin gap simultaneously with the superconducting state. Although this
model may well describe a general class of materials to be discovered, the
possibility that this theory could describe superconducting cuprates is
discussed.",1309.0032v1
2013-09-12,Cluster Functional Renormalization Group,"Functional renormalization group (FRG) has become a diverse and powerful tool
to derive effective low-energy scattering vertices of interacting many-body
systems. Starting from a non-interacting expansion point of the action, the
flow of the RG parameter Lambda allows to trace the evolution of the effective
one-particle and two-particle vertices towards low energies by taking into
account the vertex corrections between all parquet channels in an unbiased
fashion. In this work, we generalize the expansion point at which the
diagrammatic resummation procedure is initiated from a free UV limit to a
cluster product state. We formulate a cluster FRG scheme where the
non-interacting building blocks (i.e., decoupled spin clusters) are treated
exactly, and the inter-cluster couplings are addressed via RG. As a benchmark
study, we apply our cluster FRG scheme to the spin-1/2 bilayer Heisenberg model
(BHM) on a square lattice where the neighboring sites in the two layers form
the individual 2-site clusters. Comparing with existing numerical evidence for
the BHM, we obtain reasonable findings for the spin susceptibility, magnon
dispersion, and magnon quasiparticle weight even in coupling regimes close to
antiferromagnetic order. The concept of cluster FRG promises applications to a
large class of interacting electron systems.",1309.3262v1
2013-09-26,Magnetic frustration in BaCuSi2O6 released,"Han Purple (BaCuSi2O6) is not only an ancient pigment, but also a valuable
model material for studying Bose-Einstein condensation (BEC) of magnons in high
magnetic fields. Using precise low-temperature structural data and extensive
density-functional calculations, we elucidate magnetic couplings in this
compound. The resulting magnetic model comprises two types of nonequivalent
spin dimers, in excellent agreement with the (63,65)Cu nuclear magnetic
resonance data. We further argue that leading interdimer couplings connect the
upper site of one dimer to the bottom site of the contiguous dimer, and not the
upper-to-upper and bottom-to-bottom sites, as assumed previously. This finding
is verified by inelastic neutron scattering data and implies the lack of
magnetic frustration in BaCuSi2O6, thus challenging existing theories of the
magnon BEC in this compound.",1309.6762v1
2014-05-02,Weyl fermions induced Magnon electrodynamics in Weyl semimetal,"Weyl fermions, which are fermions with definite chiralities, can give rise to
anomalous breaking of the symmetry of the physical system which they are a part
of. In their (3+1)-dimensional realizations in condensed matter systems, i.e.,
the so-called Weyl semimetals, this anomaly gives rise to topological
electromagnetic response of magnetic fluctuations, which takes the form of
non-local interaction between magnetic fluctuations and electromagnetic fields.
We study the physical consequences of this non-local interaction, including
electric field assisted magnetization dynamics, an extra gapless magnon
dispersion, and polariton behaviors that feature ""sibling"" bands in small
magnetic fields.",1405.0491v2
2014-05-06,Hybridizing ferromagnetic magnons and microwave photons in the quantum limit,"We demonstrate large normal-mode splitting between a magnetostatic mode (the
Kittel mode) in a ferromagnetic sphere of yttrium iron garnet and a microwave
cavity mode. Strong coupling is achieved in the quantum regime where the
average number of thermally or externally excited magnons and photons is less
than one. We also confirm that the coupling strength is proportional to the
square root of the number of spins. A nonmonotonic temperature dependence of
the Kittel-mode linewidth is observed below 1 K and is attributed to the
dissipation due to the coupling with a bath of two-level systems.",1405.1913v2
2014-05-20,Triplet excitations in graphene-based systems,"In this article we investigate the excitations in a single graphene layer and
in a single-walled carbon nanotube, i.e. the spectrum of magnetic excitations
is calculated. In the absence of interactions in these systems there is a
unique gap in the electron-hole continuum. We show that in the presence of
Coulomb correlations new states, magnons, appear in this forbidden region.
Coulomb interaction is examined in the context of Pariser-Parr-Pople (PPP)
model which takes into account long range nature of interaction. The energy of
new bound states depends on the strength of Coulomb forces. The calculations
are performed for arbitrary electron-hole ($e-h$) momentum $\textbf{q}$ what
allows to find the magnons dispersion law $\varepsilon(\textbf{q})$, effective
mass $m^*$ and velocity $v_{gr}$. Finally, we determine the critical values of
system parameters when this type of excitations can exist.",1405.5150v2
2014-06-08,Stability and magnetization curve of spin-nematic phase slightly below saturation field,"We discuss the magnetization process slightly below the saturation field in
frustrated magnets. A condensation of bound magnons on the spin-polarized state
induces either a spin nematic phase or a state with phase separation. The
(effective) interaction between the bound magnon pairs not only is crucial to
the stability of the nematic phase, but also determines the slope of the
magnetization curve near saturation. We generally derive the expression of this
interaction by using the perturbative scattering theory. By applying the method
to coupled zigzag chains LiCuVO4, we find the positive pair-pair interaction
implying the stability of the spin nematic phase. We also point out that the
magnetization curve of LiCuVO4 is almost vertical (i.e. very large dM/dH) near
the saturation exhibiting one-dimensional feature despite non-negligible
interchain couplings.",1406.1960v1
2014-06-14,Electric-field coupling to spin waves in a centrosymmetric ferrite,"We experimentally demonstrate that the spin-orbit interaction can be utilized
for direct electric-field tuning of the propagation of spin waves in a
single-crystal yttrium iron garnet magnonic waveguide. Magnetoelectric coupling
not due to the spin-orbit interaction, and hence an order of magnitude weaker,
leads to electric-field modification of the spin-wave velocity for waveguide
geometries where the spin-orbit interaction will not contribute. A theory of
the phase shift, validated by the experiment data, shows that, in the exchange
spin wave regime, this electric tuning can have high efficiency. Our findings
point to an important avenue for manipulating spin waves and developing
electrically tunable magnonic devices.",1406.3675v1
2014-06-19,Band structure of magnonic crystals with defects: Brillouin spectroscopy and micromagnetic simulations,"Using Brillouin spectroscopy, the first observation has been made of the band
structures of nanostructured defect magnonic crystals. The samples are
otherwise one-dimensional periodic arrays of equal-width Ni80Fe20 and cobalt
nanostripes, where the defects are stripes of a different width. A
dispersionless defect branch emerges within the bandgap with a frequency
tunable by varying the defect stripe width, while the other branches observed
are similar to those of a defect-free crystal. Micromagnetic and finite-element
simulations performed unveil additional tiny bandgaps and the
frequency-dependent localization of the defect mode in the vicinity of the
defects.",1406.4981v2
2014-06-23,Magnon-Driven Domain-Wall Motion with the Dzyaloshinskii-Moriya Interaction,"We study domain wall (DW) motion induced by spin waves (magnons) in the
presence of Dzyaloshinskii-Moriya interaction (DMI). The DMI exerts a torque on
the DW when spin waves pass through the DW, and this torque represents a linear
momentum exchange between the spin wave and the DW. Unlike angular momentum
exchange between the DW and spin waves, linear momentum exchange leads to a
rotation of the DW plane rather than a linear motion. In the presence of an
effective easy plane anisotropy, this DMI induced linear momentum transfer
mechanism is significantly more efficient than angular momentum transfer in
moving the DW.",1406.5997v2
2016-08-02,Magnetic Domain Wall Floating on a Spin Superfluid,"We theoretically investigate the transfer of angular momentum between a spin
superfluid and a domain wall in an exchange coupled easy-axis and easy-plane
magnetic insulator system. A domain wall in the easy-axis magnet absorbs spin
angular momentum via disrupting the flow of a superfluid spin current in the
easy-plane magnet. Focusing on an open geometry, where the spin current is
injected electrically via a nonequilibrium spin accumulation, we derive
analytical expressions for the resultant superfluid-mediated motion of the
domain wall. The analytical results are supported by micromagnetic simulations.
The proposed phenomenon extends the regime of magnon-driven domain-wall motion
to the case when the magnons are condensed and exhibit superfluidity.
Furthermore, by controlling the pinning of the domain wall, we propose a
realization of a reconfigurable spin transistor. The long-distance
dissipationless character of spin superfluids can thus be exploited for
manipulating soliton-based memory and logic devices.",1608.00683v1
2016-08-17,Quenched Magnon excitations by oxygen sublattice reconstruction in (SrCuO$_2$)$_n$/(SrTiO$_3$)$_2$ superlattices,"The recently discovered structural reconstruction in the cuprate superlattice
(SrCuO$_2$)$_n$/(SrTiO$_3$)$_2$ has been investigated across the critical value
of $n=5$ using resonant inelastic x-ray scattering (RIXS). We find that at the
critical value of $n$, the cuprate layer remains largely in the bulk-like
two-dimensional structure with a minority of Cu plaquettes being reconstructed.
The partial reconstruction leads to quenching of the magnons starting at the
$\Gamma$-point due to the minority plaquettes acting as scattering points.
Although comparable in relative abundance, the doped charge impurities in
electron-doped cuprate superconductors do not show this quenching of magnetic
excitations.",1608.05092v1
2017-06-07,Making the Dzyaloshinskii-Moriya interaction visible,"Brillouin light spectroscopy is a powerful and robust technique for measuring
the interfacial Dzyaloshinskii-Moriya interaction in thin films with broken
inversion symmetry. Here we show that the magnon visibility, i.e. the intensity
of the inelastically scattered light, strongly depends on the thickness of the
dielectric seed material - SiO$_2$. By using both, analytical thin-film optics
and numerical calculations, we reproduce the experimental data. We therefore
provide a guideline for the maximization of the signal by adapting the
substrate properties to the geometry of the measurement. Such a boost-up of the
signal eases the magnon visualization in ultrathin magnetic films, speeds-up
the measurement and increases the reliability of the data.",1706.02258v1
2017-06-08,Equations of Motion and Frequency Dependence of Magnon-Induced Domain Wall Motion,"Spin waves can induce domain wall motion in ferromagnets. We derive the
equations of motion for a transverse domain wall driven by spin waves. Our
calculations show that the magnonic spin-transfer torque does not cause
rotation-induced Walker breakdown. The amplitude of spin waves that are excited
by a localized microwave field depends on the spatial profile of the field and
the excitation frequency. By taking this frequency dependence into account, we
show that a simple one-dimensional model may reproduce much of the puzzling
frequency dependence observed in early numerical studies.",1706.02709v2
2017-06-27,Noise in tunneling spin current across coupled quantum spin chains,"We theoretically study the spin current and its dc noise generated between
two spin-1/2 spin chains weakly coupled at a single site in the presence of an
over-population of spin excitations and a temperature elevation in one
subsystem relative to the other, and compare the corresponding transport
quantities across two weakly coupled magnetic insulators hosting magnons. In
the spin chain scenario, we find that applying a temperature bias exclusively
leads to a vanishing spin current and a concomitant divergence in the spin Fano
factor, defined as the spin current noise-to-signal ratio. This divergence is
shown to have an exact analogy to the physics of electron scattering between
fractional quantum Hall edge states and not to arise in the magnon scenario. We
also reveal a suppression in the spin current noise that exclusively arises in
the spin chain scenario due to the fermion nature of the spin-1/2 operators. We
discuss how the spin Fano factor may be extracted experimentally via the
inverse spin Hall effect used extensively in spintronics.",1706.09030v2
2017-06-30,Raman scattering study of vibrational and magnetic excitations in Sr$_{2-x}$La$_x$IrO$_4$,"We have measured the doping and temperature dependence of lattice vibrations
and magnetic excitations in the prototypical doped spin-orbit Mott insulator
Sr$_{2-x}$La$_x$IrO$_4$ (x=0, 0.015, and 0.10). Our findings show that the
pseudospin-lattice coupling -- responsible for the renormalization of several
low energy phonon modes -- is preserved even when long-range magnetic order is
suppressed by doping. In our most highly doped sample, the single magnon
(Gamma-point) excitation disappears while the two-magnon mode softens and
becomes heavily damped. Doping induced electron-phonon coupling is also
observed in a higher energy phonon mode. We observe two different
electron-phonon interaction channels, which provide evidence of the coexistence
of fluctuating magnetic moments and mobile carriers in doped iridates.",1706.10103v2
2017-10-25,Co- and contra-directional vertical coupling between ferromagnetic layers with grating for short-wavelength spin wave generation,"The possibility to generate short spin waves is of great interest in the
field of magnonics nowadays. We present an effective and technically affordable
way of conversion of long spin waves, which may be generated by conventional
microwave antenna, to the short, sub-micrometer waves. It is achieved by
grating-assisted resonant dynamic dipolar interaction between two ferromagnetic
layers separated by some distance. We analyze criteria for the optimal
conversion giving a semi-analytical approach for the coupling coefficient. We
show by the numerical calculations the efficient energy transfer between layers
which may be either of co-directional or contra-directional type. Such a system
may operate either as a short spin wave generator or a frequency filter, moving
foreward possible application of magnonics.",1710.09138v1
2017-10-25,Collective spin waves in arrays of Permalloy nanowires with single-side periodically modulated width,"We have experimentally and numerically investigated the dispersion of
collective spin waves prop-agating through arrays of longitudinally magnetized
nanowires with periodically modulated width. Two nanowire arrays with
single-side modulation and different periodicity of modulation were studied and
compared to the nanowires with homogeneous width. The spin-wave dispersion,
meas-ured up to the third Brillouin zone of the reciprocal space, revealed the
presence of two dispersive modes for the width-modulated NWs, whose amplitude
of magnonic band depends on the modula-tion periodicity, and a set of
nondispersive modes at higher frequency. These findings are different from
those observed in homogeneous width NWs where only the lowest mode exhibits
sizeable dis-persion. The measured spin-wave dispersion has been satisfactorily
reproduced by means of dynam-ical matrix method. Results presented in this work
are important in view of the possible realization of frequency tunable magnonic
device.",1710.09163v1
2017-12-04,"Faster, farther, stronger: spin transfer torque driven high order propagating spin waves in nano-contact magnetic tunnel junctions","Short wave-length exchange-dominated propagating spin waves will enable
magnonic devices to operate at higher frequencies and higher data transmission
rates.1 While GMR based magnetic nano-contacts are highly efficient injectors
of propagating spin waves2,3, the generated wave lengths are 2.6 times the
nano-contact diameter4, and the electrical signal strength remains much too
weak for practical applications. Here we demonstrate nano-contact based spin
wave generation in magnetic tunnel junction stacks, and observe large discrete
frequency steps consistent with the hitherto ignored possibility of second and
third order propagating spin waves with wave lengths of 120 and 74 nm, i.e.
much smaller than the 150 nm nano-contact. These higher-order propagating spin
waves will not only enable magnonic devices to operate at much higher
frequencies, but also greatly increase their transmission rates and spin wave
propagating lengths, both proportional to the much higher group velocity.",1712.00954v2
2017-12-08,Magnon-induced superconductivity in field-cooled spin-1/2 antiferromagnets,"If, during the preparation, an external magnetic field is applied upon
cooling we say it has been field cooled. A novel mechanism for insulator-metal
transition and superconductivity in field-cooled spin-$1/2$ antiferromagnets on
bcc lattice is discussed. Applying a magnetic field along the sublattice B
magnetization, we change the magnetic and transport properties of the material.
There is a critical value $H_{cr1}$. When the magnetic field is below the
critical one $H<H_{cr1}$ the prepared material is a spin$-1/2$
antiferromagnetic insulator. When $H>H_{cr1}$ the sublattice A electrons are
delocalized and the material is metal. There is a second critical value
$H_{cr2}>H_{cr1}$. When $H=H_{cr2}$, it is shown that the Zeeman splitting of
the sublattice A electrons is zero and they do not contribute to the
magnetization of the system. At this quantum partial order point (QPOP) the
sublattice B transversal spin fluctuations (magnons) interact with sublattice A
electrons inducing spin anti-parallel \emph{p}-wave superconductivity which
coexists with magnetism. At zero temperature the magnetic moment of sublattice
B electrons is maximal. Below the N\'{e}el temperature $(T_N)$ the gap is
approximately constant with a small increase when the system approaches $T_N$.
It abruptly falls down to zero at temperatures above $T_N$.",1712.02983v1
2017-12-09,Magnonic waveguide based on exchange-spring magnetic structure,"We propose to use a soft/hard exchange-spring coupling bilayer magnetic
structure to introduce a narrow channel for spin-wave propagation. We show by
micromagnetic simulations that broad-band Damon-Eshbach geometry spin waves can
be strongly localized into the channel and propagate effectively with a proper
high group velocity. The beamwidth of the bound mode spin waves is almost
independent from the frequency and is smaller than 24nm. For a low-frequency
excitation, we further investigate the appearance of two other spin beams in
the lateral of the channel. In contrast to a domain wall, the channel formed by
exchange-spring coupling can be easier to realize in experimental scenarios and
holds stronger immunity to surroundings. This work is expected to open new
possibilities for energy-efficient spin-wave guiding as well as to help shape
the field of beam magnonics.",1712.03392v1
2017-12-13,Mobile bound states of Rydberg excitations in a lattice,"Spin lattice models play central role in the studies of quantum magnetism and
non-equilibrium dynamics of spin excitations -- magnons. We show that a spin
lattice with strong nearest-neighbor interactions and tunable long-range
hopping of excitations can be realized by a regular array of laser driven
atoms, with an excited Rydberg state representing the spin-up state and a
Rydberg-dressed ground state corresponding to the spin-down state. We find
exotic interaction-bound states of magnons that propagate in the lattice via
the combination of resonant two-site hopping and non-resonant second-order
hopping processes. Arrays of trapped Rydberg-dressed atoms can thus serve as a
flexible platform to simulate and study fundamental few-body dynamics in spin
lattices.",1712.04713v1
2017-12-19,Observation of time quasicrystal and its transition to superfluid time crystal,"We report experimental realization of a quantum time quasicrystal, and its
transformation to a quantum time crystal. We study Bose-Einstein condensation
of magnons, associated with coherent spin precession, created in a flexible
trap in superfluid $^3$He-B. Under a periodic drive with an oscillating
magnetic field, the coherent spin precession is stabilized at a frequency
smaller than that of the drive, demonstrating spontaneous breaking of discrete
time translation symmetry. The induced precession frequency is incommensurate
with the drive, and hence the obtained state is a time quasicrystal. When the
drive is turned off, the self-sustained coherent precession lives a
macroscopically-long time, now representing a time crystal with broken symmetry
with respect to continuous time translations. Additionally, the magnon
condensate manifests spin superfluidity, justifying calling the obtained state
a time supersolid or a time super-crystal.",1712.06877v5
2017-12-22,Nanomagnonic waveguides based on reconfigurable spin-textures for spin computing,"Magnonics is gaining momentum as an emerging technology for information
processing. The wave character and Joule heating-free propagation of spin-waves
hold promises for highly efficient analog computing platforms, based on
integrated magnonic circuits. Miniaturization is a key issue but, so far, only
few examples of manipulation of spin-waves in nanostructures have been
demonstrated, due to the difficulty of tailoring the nanoscopic magnetic
properties with conventional fabrication techniques. In this Letter, we
demonstrate an unprecedented degree of control in the manipulation of
spin-waves at the nanoscale by using patterned reconfigurable spin-textures. By
space and time-resolved scanning transmission X-ray microscopy imaging, we
provide direct evidence for the channeling and steering of propagating
spin-waves in arbitrarily shaped nanomagnonic waveguides based on patterned
domain walls, with no need for external magnetic fields or currents.
Furthermore, we demonstrate a prototypic nanomagnonic circuit based on two
converging waveguides, allowing for the tunable spatial superposition and
interaction of confined spin-waves modes.",1712.08293v1
2018-09-06,Statistical Thermodynamics of the Fröhlich-Bose-Einstein Condensation of Magnons out of Equilibrium,"A non-equilibrium statistical-thermodynamic approach to the study of a
Fr\""ohlich-Bose-Einstein condensation of magnons under radio-frequency
radiation pumping is presented. Such a system displays a complex behavior
consisting in steady-state conditions to the emergence of a synergetic
dissipative structure resembling the Bose-Einstein condensation of systems in
equilibium. A kind of ""two fluid model"" arises: the ""normal"" non-equilibrium
structure and Fr\""ohlich condensate or ""non-equilibrium"" one, which is shown to
be an attractor to the system. We analyze some aspects of the irreversible
thermodynamics of this dissipative complex system, namely, its informational
entropy, expressions for the fluctuations in non-equilibrium conditions, the
associated Maxwell relations and the formulation of a generalized
$\mathscr{H}$-theorem. We also study the informational entropy production of
the system, an order parameter is introduced and Glansdorff-Prigogine criteria
for evolution and (in)stability are verified.",1809.03335v1
2018-09-19,Photocontrol of magnetic structure in an itinerant magnet,"We study the photoinduced magnetic transition in an itinerant magnet
described by the double-exchange model, in which conduction electrons couple
with localized spins through the ferromagnetic (FM) Hund coupling. It is shown
that intense light applied to the FM ground state induces an antiferromagnetic
(AFM) order, in contrast to the AFM-to-FM transition due to the photocarrier
injection. In particular, we focus on the mechanism for instability of the FM
structure by the light irradiation. The magnon spectrum in the Floquet state is
formulated on the basis of the pertrubative expansion of the Floquet Green
function. The magnon dispersion shows softening at momentum $(\pi,\pi)$ in the
square lattice with increasing the light amplitude, implying photoinduced AFM
instability. This result is mainly attributed to a nonequilibrium electron
distribution, which promotes low-energy Stoner excitations. The transient
optical conductivity spectra characterized by interband excitations and Floquet
sidepeaks are available to identify the photoinduced AFM state.",1809.07132v1
2019-09-20,Effects of Dzyaloshinskii-Moriya interactions in volborthite: Magnetic orders and thermal Hall effect,"Volborthite offers an interesting example of a highly frustrated quantum
magnet in which ferromagnetic and antiferromagnetic interactions compete on
anisotropic kagome lattices. A recent density functional theory calculation has
provided a magnetic model based on coupled trimers, which is consistent with a
broad 1/3-magnetization plateau observed experimentally. Here we study the
effects of Dzyaloshinskii-Moriya (DM) interactions in volborthite. We derive an
effective model in which pseudospin-1/2 moments emerging on trimers form a
network of an anisotropic triangular lattice. Using the effective model, we
show that for a magnetic field perpendicular to the kagome layer, magnon
excitations from the 1/3-plateau feel a Berry curvature due to the DM
interactions, giving rise to a thermal Hall effect. Our magnon Bose gas theory
can explain qualitative features of the magnetization and the thermal Hall
conductivity measured experimentally. A further quantitative comparison with
experiment poses constraints on the coupling constants in the effective model,
promoting a quasi-one-dimensional picture. Based on this picture, we analyze
low-temperature magnetic phase diagrams using effective field theory, and point
out their crucial dependence on the field direction.",1909.09425v2
2019-09-20,Tuning entanglement by squeezing magnons in anisotropic magnets,"We theoretically study the entanglement between two arbitrary spins in a
magnetic material, where magnons naturally form a general squeezed coherent
state, in the presence of an applied magnetic field and axial anisotropies.
Employing concurrence as a measure of entanglement, we demonstrate that spins
are generally entangled in thermodynamic equilibrium, with the amount of
entanglement controlled by the external fields and anisotropies. As a result,
the magnetic medium can serve as a resource to store and process quantum
information. We, furthermore, show that the entanglement can jump
discontinuously when decreasing the applied magnetic field. This tunable
entanglement can be potentially used as an efficient switch in
quantum-information processing tasks.",1909.09653v2
2020-12-03,Total Moment Sum Rule for Magnets in the Vicinity of Quantum Critical Point,"It is known that the longitudinal and transverse excitation modes can exist
in the vicinity of a quantum critical point in the ordered phase of quantum
magnetic systems. The total moment sum rule for such systems is derived on the
basis of the extended spin-wave theory, where both longitudinal and transverse
magnetic excitations are taken into account. The sum rule is resolved into
elastic, one-magnon, and two-magnon components. The formulation is applicable
to spin systems with the longitudinal mode, such as $S=1$ systems with
single-ion anisotropy of easy-plane type and spin dimer systems. The result
helps us analyze and understand measured data of inelastic neutron scattering.",2012.01669v1
2000-06-20,Microscopic Theory of Dipole-Exchange Spin Waves in Ferromagnetic Films: Linear and Nonlinear Processes,"The linear and nonlinear processes in ferromagnetic films at low temperatures
T<< Tc are studied in a microscopic theory. Both the long-range magnetic
dipole-dipole and the Heisenberg exchange interactions to nearest and
next-nearest neighbors are included. The results obtained for the linearized
spin-wave spectrum are compared with previous macroscopic theories. For
ultrathin films (or for large wave vectors) the microscopic theory provides
important corrections. The nonlinear dynamics of the spin waves are studied
through a finite-temperature perturbation theory based on Feynman diagrams. We
obtain explicit results for the energy shift and damping (or reciprocal
lifetime) of the dipole-exchange spin waves due to all possible three-magnon
and four-magnon processes involving combinations of the surface and quantized
bulk spin waves at low temperatures. To investigate different dipole
interaction strengths (relative to the exchange) numerical results are
presented using parameters for Fe, EuO, and GdCl3.",0006308v1
2017-05-26,"Low-energy spin dynamics of orthoferrites AFeO$_3$ (A = Y, La, Bi)","YFeO$_3$ and LaFeO$_3$ are members of the rare-earth orthoferrites family
with \textit{Pbnm} space group. Using inelastic neutron scattering, the
low-energy spin excitations have been measured around magnetic Brillouin zone
center. Splitting of magnon branches and non-zero magnon gap is observed for
both compounds, which is similar to the behavior observed in multiferroic
BiFeO$_3$. Spin wave calculations which include both Dzyaloshinsky-Moriya
interactions and single-ion anisotropy in the spin-Hamiltonian comprehensively
accounts for all the experimentally observed features. Our results offer
insight into the unifying physics underlying the Fe$^{3+}$-based perovskites as
well as their distinguishing characteristics.",1705.09441v3
2017-09-19,Spin Seebeck effect in a polar antiferromagnet $α$-Cu$_{2}$V$_{2}$O$_{7}$,"We have studied the longitudinal spin Seebeck effect in a polar
antiferromagnet $\alpha$-Cu$_{2}$V$_{2}$O$_{7}$ in contact with a Pt film.
Below the antiferromagnetic transition temperature of
$\alpha$-Cu$_{2}$V$_{2}$O$_{7}$, spin Seebeck voltages whose magnetic field
dependence is similar to that reported in antiferromagnetic MnF$_{2}$$\mid$Pt
bilayers are observed. Though a small weak-ferromagnetic moment appears owing
to the Dzyaloshinskii-Moriya interaction in $\alpha$-Cu$_{2}$V$_{2}$O$_{7}$,
the magnetic field dependence of spin Seebeck voltages is found to be
irrelevant to the weak ferromagnetic moments. The dependences of the spin
Seebeck voltages on magnetic fields and temperature are analyzed by a magnon
spin current theory. The numerical calculation of spin Seebeck voltages using
magnetic parameters of $\alpha$-Cu$_{2}$V$_{2}$O$_{7}$ determined by previous
neutron scattering studies reveals that the magnetic-field and temperature
dependences of the spin Seebeck voltages for
$\alpha$-Cu$_{2}$V$_{2}$O$_{7}$$\mid$Pt are governed by the changes in magnon
lifetimes with magnetic fields and temperature.",1709.06330v1
2018-03-08,Quantum Interactions of Topological Solitons from Electrodynamics,"The Casimir energy for the classically stable configurations of the
topological solitons in 2D quantum antiferromagnets is studied by performing
the path-integral over quantum fluctuations. The magnon fluctuation around the
solitons saturating the Bogomol'nyi inequality may be viewed as a charged
scalar field coupled with an effective magnetic field induced by the solitons.
The magnon-soliton couping is closely related to the Pauli Hamiltonian, with
which the effective action is calculated by adapting the worldline formulation
of the derivative expansion for the 2+1d quantum electrodynamics in an external
field. The resulting framework is more flexible than the conventional
scattering analysis based on the Dashen-Hasslacher-Neveu formula. We obtain a
short-range attractive well and a universal long-range $1/r$-type repulsive
potential between two solitons.",1803.03181v5
2018-03-12,Probing ultrafast spin-relaxation and precession dynamics in a cuprate Mott insulator with 7-fs optical pulses,"A charge excitation in a two-dimensional Mott insulator is strongly coupled
with the surrounding spins, which is observed as magnetic-polaron formations of
doped carriers and a magnon sideband in the Mott-gap transition spectrum.
However, the dynamics related to the spin sector are difficult to measure.
Here, we show that pump-probe reflection spectroscopy with 7-fs laser pulses
can detect the optically induced spin dynamics in Nd$_2$CuO$_4$, a cuprate Mott
insulator. The bleaching signal at the Mott-gap transition is enhanced at
$\sim$18 fs, which corresponds to the spin-relaxation time in magnetic-polaron
formations and is characterized by the exchange interaction. More importantly,
ultrafast coherent oscillations appear in the time evolutions of the
reflectivity changes, and their frequencies (1400-2700 cm$^{-1}$) are equal to
the probe energy measured from the Mott-gap transition peak. These oscillations
originate from interferences between charge excitations with two magnons and
provide direct evidence for charge-spin coupling.",1803.04158v1
2018-03-20,Polarization-dependent resonant inelastic X-ray scattering study at the Cu L and O K-edge of YBa$_2$Cu$_3$O$_{7-x}$,"We present a study on the high-Tc superconductor (HTSC) YBa2Cu3O7-x (YBCO)
using polarization-dependent X-ray absorption and resonant inelastic X-ray
scattering. High-resolution measurements using synchrotron-radiation are
compared with calculations using a quasi-atomic multiplet approach performed at
the Cu 2p3/2-edge of YBCO. We use a multiplet approach within the single
impurity Anderson model to reproduce and understand the character of the
localized low-energy excitations in YBCO. We observe a distinct peak at about
0.5 eV in O K RIXS. This peak shows dependence on doping, incident energy, and
momentum transfer that suggests that it has a different origin than the
previously discussed cuprate bi-magnons. Therefore, we assign it to
multi-magnon excitations between the Zhang Rice bands and/or the Upper Hubbard
bands, respectively.",1803.07507v1
2018-03-26,Tunable Snell's law for spin waves in heterochiral magnetic films,"Thin ferromagnetic films with an interfacially induced DMI exhibit nontrivial
asymmetric dispersion relations that lead to unique and useful magnonic
properties. Here we derive an analytical expression for the magnon propagation
angle within the micromagnetic framework and show how the dispersion relation
can be approximated with a comprehensible geometrical interpretation in the
k-space of the propagation of spin waves. We further explore the refraction of
spin waves at DMI interfaces in heterochiral magnetic films, after deriving a
generalized Snell's law tunable by an in-plane magnetic field, that yields
analytical expressions for critical incident angles. The found asymmetric
Brewster angles at interfaces of regions with different DMI strengths,
adjustable by magnetic field, support the conclusion that heterochiral
ferromagnetic structures are an ideal platform for versatile spin-wave guides.",1803.09648v1
2018-03-29,Giant resonant nonlinear damping in nanoscale ferromagnets,"Magnetic damping is a key metric for emerging technologies based on magnetic
nanoparticles, such as spin torque memory and high-resolution biomagnetic
imaging. Despite its importance, understanding of magnetic dissipation in
nanoscale ferromagnets remains elusive, and the damping is often treated as a
phenomenological constant. Here we report the discovery of a giant
frequency-dependent nonlinear damping that strongly alters the response of a
nanoscale ferromagnet to spin torque and microwave magnetic field. This novel
damping mechanism originates from three-magnon scattering that is strongly
enhanced by geometric confinement of magnons in the nanomagnet. We show that
the giant nonlinear damping can invert the effect of spin torque on a
nanomagnet leading to a surprising current-induced enhancement of damping by an
antidamping torque. Our work advances understanding of magnetic dynamics in
nanoscale ferromagnets and spin torque devices.",1803.10925v1
2018-06-01,Optical determination of the exchange stiffness constant in an iron garnet,"Brillouin light scattering measurements were performed in the backscattering
geometry on a Bi-substituted rare earth iron garnet. We observed two different
peaks, one attributed to a surface spin wave in the dipole-exchange regime. The
other is referred to as a backscattering magnon mode, because the incident
light in this case is scattered backward by exchange-dominated spin wave inside
the material. We propose a method to estimate the exchange stiffness constant
from the frequency of the backscattering magnon mode. The obtained value is
comparable with the previously reported values for Y$ _3 $Fe$ _5 $O$ _{12} $.",1806.00228v2
2018-06-13,Origins of the unidirectional spin Hall magnetoresistance in metallic bilayers,"Recent studies evidence the emergence of asymmetric electron transport in
layered conductors owing to the interplay between electrical conductivity,
magnetization, and the spin Hall or Rashba- Edelstein effects. Here, we
investigate the unidirectional magnetoresistance (UMR) caused by the
current-induced spin accumulation in Co/Pt and CoCr/Pt bilayers. We identify
three competing mechanisms underpinning the resistance asymmetry, namely
interface and bulk spin-dependent electron scattering and electron-magnon
scattering. Our measurements provide a consistent description of the current,
magnetic field, and temperature dependence of the UMR and show that both
positive and negative UMR can be obtained by tuning the interface and bulk
spin-dependent scattering terms relative to the magnon population.",1806.05305v1
2018-06-23,Magnetic and magnetoresistive behavior of the ferromagnetic heavy fermion YbNi$_2$,"We present a study on the magnetic susceptibility $\chi(T)$ and electrical
resistance, as a function of temperature and magnetic field $R(T,H)$, of the
ferromagnetic heavy fermion YbNi$_2$. The X-ray diffraction analysis shows that
the synthesized polycrystalline samples crystallizes in the cubic Laves phase
structure C15, with a spatial group $Fd\overline{3}m$. The magnetic
measurements indicate a ferromagnetic behavior with transition temperature at 9
K. The electrical resistance is metallic-like at high temperatures and no
signature of Kondo effect was observed. In the ferromagnetic state, the
electrical resistance can be justified by electron-magnon scattering
considering the existence of an energy gap in the magnonic spectrum. The energy
gap was determined for various applied magnetic fields. Magnetoresistance as a
function of applied magnetic field, subtracted from the $R(T,H)$ curves at
several temperatures, is negative from 2 K until about 40 K for all applied
magnetic fields. The negative magnetoresistance originates from the suppression
of magnetic disorder by the magnetic field.",1806.08881v1
2018-06-27,Site-selective Probe of Magnetic Excitations in Rare-earth Nickelates using Resonant Inelastic X-ray Scattering,"We have used high-resolution resonant inelastic x-ray scattering (RIXS) to
study a thin film of NdNiO$_3$, a compound whose unusual spin- and bond-ordered
electronic ground state has been of long-standing interest. Below the magnetic
ordering temperature, we observe well-defined collective magnon excitations
along different high-symmetry directions in momentum space. The magnetic
spectra depend strongly on the incident photon energy, which we attribute to
RIXS coupling to different local electronic configurations of the expanded and
compressed NiO$_6$ octahedra in the bond-ordered state. Both the noncollinear
magnetic ground state and the observed site-dependent magnon excitations are
well described by a model that assumes strong competition between the
antiferromagnetic superexchange and ferromagnetic double-exchange interactions.
Our study provides direct insight into the magnetic dynamics and exchange
interactions of the rare-earth nickelates, and demonstrates that RIXS can serve
as a site-selective probe of magnetism in these and other materials.",1806.10499v1
2018-07-06,Quantum Dynamics of the Square-Lattice Heisenberg Model,"Despite nearly a century of study of the $S=1/2$ Heisenberg model on the
square lattice, there is still disagreement on the nature of its high-energy
excitations. By tuning toward the Heisenberg model from the exactly soluble
Ising limit, we find that the strongly attractive magnon interactions of the
latter naturally account for a number of spectral features of the Heisenberg
model. This claim is backed up both numerically and analytically. Using the
density matrix renormalization group method, we obtain the dynamical structure
factor for a cylindrical geometry, allowing us to continuously connect both
limits. Remarkably, a semi-quantitative description of certain observed
features arises already at the lowest non-trivial order in perturbation theory
around the Ising limit. Moreover, our analysis uncovers that high-energy
magnons are localized on a single sublattice, which is related to the
entanglement properties of the ground state.",1807.02399v2
2018-07-09,Spin wave dynamics in artificial anti spin-ice systems: experimental and theoretical investigations,"Reversed structures of artificial spin-ice systems, where elongated holes
with elliptical shape (antidots) are arranged into a square array with two
orthogonal sublattices, are referred to as anti-squared spin-ice. Using
Brillouin light scattering spectroscopy and plane wave method calculations, we
investigate the spin wave propagation perpendicular to the applied field
direction for two 20 nm thick Permalloy nanostructures which differ by the
presence of single and double elliptical antidots. For the spin waves
propagation along the principal antidot lattice axis, the spectrum consists of
flat bands separated by several frequency gaps which are the effect of spin
wave amplitude confinement in the regions between antidots. Contrarily, for
propagation direction at 45 degrees with respect to the antidot symmetry axis,
straight and narrow channels of propagation are formed, leading to broadening
of bands and closing of the magnonics gaps. Interestingly, in this case, extra
magnonic band gaps occur due to the additional periodicity along this
direction. The width and the position of these gaps depend on the presence of
single or double antidots. In this context, we discuss possibilities for the
tuning of spin wave spectra in anti-squared spin ice structures.",1807.03016v1
2018-07-10,Spin Wave Analysis of Low-Temperature Thermal Hall Effect in the Candidate Kitaev Spin Liquid $α$-RuCl${}_3$,"Proposed effective Hamiltonians from the literature for the material
$\alpha$-RuCl${}_3$ are used to compute the magnon thermal Hall conductivity,
$\kappa_{xy}$, using linear spin wave theory for the magnetically ordered
state. No model previously proposed that was tested explains published
experimental data. Models with Kitaev interaction $K>0$ are seen to predict
$\kappa_{xy}\gtrsim 0$, which is inconsistent with the data. Fluctuations
toward a Kitaev-type spin liquid would have the wrong sign to explain the data.
However, a slight variant of a previously proposed model predicts a large
$\kappa_{xy}$, demonstrating that the low-temperature thermal Hall effect could
be generated exclusively by the Berry curvature of the magnon bands. The
experimental data of $\kappa_{xy}$ can therefore serve as another method to
constrain a proposed effective Hamiltonian.",1807.03857v1
2018-07-11,Electron-phonon coupling in undoped cuprate $YBa_2Cu_3O_6$ estimated from Raman and optical conductivity spectra,"We study experimentally the Raman response of the undoped high-Tc parent
compound $YBa_2Cu_3O_6$, and give a unified theory of the two-magnon Raman peak
and optical conductivity based on the Hubbard-Holstein model with
electron-phonon coupling (EPC). The Hubbard model without EPC can qualitatively
account for the experimentally observed resonance of the Raman response, but
only the Hubbard-Holstein model (i) reproduces asymmetry of the Raman spectrum,
(ii) validates experimental visibility of the two-magnon peak, and (iii)
predicts the correct shape and energy of the lower edge of the charge transfer
gap in optical conductivity. Comparison of experiments with the theory gives
the EPC strength $\lambda$ = 0.6. This result convincingly indicates the vital
role of EPC in high-Tc cuprates providing a clue to the mechanism of high-Tc.",1807.04256v1
2018-07-17,Magnon-phonon conversion experiment and phonon spin,"Recent experiment demonstrates magnon to phonon conversion in a YIG film
under the application of a non-uniform magnetic field. Light scattered from
phonons is observed to change its polarization state interpreted by the authors
signifying phonon spin. In this note we argue that the experimental data merely
shows the exchange of angular momentum $\pm \hbar$ per photon. We suggest that
it has physical origin in the orbital angular momentum of phonons. The
distinction between spin and orbital parts of the total angular momentum, and
between phonons and photons with added emphasis on their polarizations is
explained. The main conclusion of the present note is that phonon spin
hypothesis is unphysical.",1807.08614v2
2018-07-25,Anharmonic magnon excitations in noncollinear and charge-ordered RbFe$^{2+}$Fe$^{3+}$F$_6$,"RbFe$^{2+}$Fe$^{3+}$F$_6$ is an example of a charge ordered antiferromagnet
where iron sites, with differing valences, are structurally separated into two
interpenetrating sublattices. The low temperature magnetically ordered
Fe$^{2+}$ ($S$=2) and Fe$^{3+}$ ($S$=5/2) moments form a noncollinear
orthogonal structure with the Fe$^{3+}$ site displaying a reduced static
ordered moment. Neutron spectroscopy on single crystals finds two distinct spin
wave branches with a dominant coupling along the Fe$^{3+}$ chain axis
($b$-axis). High resolution spectroscopic measurements find an intense energy
and momentum broadened magnetic band of scattering bracketing a momentum-energy
region where two magnon processes are kinematically allowed. These anharmonic
excitations are enhanced in this non collinear magnet owing to the orthogonal
spin arrangement.",1807.09605v1
2018-12-15,Novel magnetic field tuning of quantum spin excitations in a weakly coupled $S=$1/2 Heisenberg spin chain as seen from NMR,"We report our NMR study of the spin excitations in the quasi-one dimensional
(1D) $S=1/2$ quantum magnet CH$_3$NH$_3$Cu(HCOO)$_3$ lying in the 1D-3D
dimensional crossover regime with Dzyaloshinskii-Moriya(DM) interactions. Above
$T_N$, the spinon excitation is observed from the constant $1/T_1$ at low
temperatures contributed from the staggered spin susceptibility. At low
temperatures well below $T_N$, the $1/T_1$ begins to flatten out under weak
magnetic fields. With the increasing field intensity, $1/T_1$ tends to show a
power-law temperature dependence gradually, with the index increasing from zero
to $\sim3$, and finally $\sim5$, which are the respective typical
characteristic for the dominating two-magnon Raman process and three-magnon
scattering contributions to the nuclear relaxation in a conventional 3D magnet.
A possible physical mechanism for this novel magnetic field tuning of quantum
spin excitations related with the enhanced effective staggered field created by
the DM interactions under magnetic field is discussed.",1812.06261v2
2019-01-17,Magnonic band structure in a Co/Pd stripe domain system investigated by Brillouin light scattering and micromagnetic simulations,"By combining Brillouin Light Scattering and micromagnetic simulations we
studied the spin-wave dynamics of a Co/Pd thin film multilayer, features a
stripe domain structure at remanence. The periodic up and down domains are
separated by cork-screw type domain walls. The existence of these domains
causes a scattering of the otherwise bulk and surface spin-wave modes, which
form mode families, similar to a one dimensional magnonic crystal. The
dispersion relation and mode profiles of spin waves are measured for
transferred wave vector parallel and perpendicular to the domain axis.",1901.05985v1
2019-01-21,"Magnonics: Spin Waves Connecting Charges, Spins and Photons","Spin waves (SW) are the excitation of the spin system in a ferromagnetic
condensed matter body. They are collective excitations of the electron system
and, from a quasi-classical point of view, can be understood as a coherent
precession of the electrons' spins. Analogous to photons, they are also
referred to as magnons indicating their quasi-particle character. The
collective nature of SWs is established by the short-range exchange interaction
as well as the non-local magnetic dipolar interaction, resulting in coherence
of SWs from mesoscopic to even macroscopic length scales. As one consequence of
this collective interaction, SWs are ""charge current free"" and, therefore, less
subject to dissipation caused by scattering with impurities on the atomic
level. This is a clear advantage over diffusive transport in spintronics that
not only uses the charge of an electron but also its spin degree of freedom.
Any (spin) current naturally involves motion and, thus, scattering of electrons
leading to excessive heating as well as losses. This renders SWs a promising
alternative to electric (spin) currents for the transport of spin information -
one of the grand challenges of condensed matter physics.",1901.07021v1
2019-01-25,Focused ion beam modification of non-local magnon-based transport in yttrium iron garnet/platinum heterostructures,"We study the impact of Ga ion exposure on the local and non-local
magnetotransport response in heterostructures of the ferrimagnetic insulator
yttrium iron garnet and platinum. In particular, we cut the yttrium iron garnet
layer in between two electrically separated wires of platinum using a Ga ion
beam, and study the ensuing changes in the magnetoresistive response. We find
that the non-local magnetoresistance signal vanishes when the yttrium iron
garnet film between the Pt wires is fully cut, although the local spin Hall
magnetoresistance signal remains finite. This observation corroborates the
notion that pure spin currents carried by magnons are crucial for the non-local
magnetotransport effects observed in magnetic insulator/metal nanostructures.",1901.08894v1
2019-01-28,Flat-band spin dynamics and phonon anomalies of the saw-tooth spin-chain system Fe$_2$O(SeO$_3$)$_2$,"Fe$^{3+}$ $S = 5/2$ ions form saw-tooth like chains along the $a$ axis of the
oxo-selenite Fe$_2$O(SeO$_3$)$_2$ and an onset of long-range magnetic order is
observed for temperatures below $T_C = 105$ K. This order leads to distinct
fingerprints in phonon mode linewidths and energies as resolved by Raman
scattering. In addition, new excitations with small linewidths emerge below $T
= 150$ K, and are assigned to two-magnon scattering processes with the
participation of flat-band and high energy magnon branches. From this a set of
exchange coupling constants is estimated. The specific ratio of the saw-tooth
spine-spine and spine-vertex interactions may explain the instability of the
dimer quantum ground state against an incommensurate 3D magnetic order.",1901.09617v1
2019-01-30,Exceptional magnetic sensitivity of PT-symmetric cavity magnon polaritons,"Achieving magnetometers with ultrahigh sensitivity at room temperature is an
outstanding problem in physical sciences and engineering. Recently developed
non-Hermitian cavity spintronics offers new possibilities. In this work we
predict an exceptional magnetic sensitivity of cavity magnon polaritons with
the peculiar parity-time (PT) symmetry. Based on the input-output formalism, we
demonstrate a ""Z""-shape spectrum including two sideband modes and a dark-state
branch with an ultranarrow linewidth in the exact PT phase. The spectrum
evolves to a step function when the polariton touches the third-order
exceptional point, accompanied by an ultrahigh sensitivity with respect to the
detuning. The estimated magnetic sensitivity can approach 10^{-15}T Hz^{-1/2}
in the strong coupling region, which is two orders of magnitude higher than
that of the state-of-the-art magnetoelectric sensor. We derive the condition
for the noiseless sensing performance. Purcell-like effect is observed when the
PT symmetry is broken. A possible experimental scheme to realize our proposal
is also discussed.",1901.10685v2
2019-02-25,Driving magnetization dynamics in an on-demand magnonic crystal by magneto-elastic interaction,"Using spatial light interference of ultrafast laser pulses, we generate a
lateral modulation in the magnetization profile of an otherwise uniformly
magnetized film, whose magnetic excitation spectrum is monitored via the
coherent and resonant interaction with elastic waves. We find an unusual
dependence of the magnetoelastic coupling as the externally applied magnetic
field is angle- and field-tuned relative to the wave vector of the
magnetization modulation, which can be explained by the emergence of spatially
inhomogeneous spin-wave modes. In this regard, the spatial light interference
methodology can be seen as a user-configurable, temporally windowed, on-demand
magnonic crystal, potentially of arbitrary two-dimensional shape, which allows
control and selectivity of the spatial distribution of spin waves. Calculations
of spin waves using a variety of methods, demonstrated here using the
plane-wave method and micromagnetic simulation, can identify the spatial
distribution and associated energy scales of each excitation, which opens the
door to a number of excitation methodologies beyond our chosen elastic wave
excitation.",1902.09186v1
2019-02-26,A new class of nonreciprocal spin waves on the edges of 2D antiferromagnetic honeycomb nanoribbons,"Antiferromagnetic two-dimensional (2D) materials are currently under
intensive theoretical and experimental investigations in view of their
potential applications in antiferromagnet-based magnonic and spintronic
devices. Recent experimental studies revealed the importance of magnetic
anisotropy and Dzyaloshinskii-Moriya interactions (DMI) on the ordered ground
state and the magnetic excitations in these materials. Here we present a robust
classical field theory approach to analyze the effect of magnetic anisotropy
and Dzyaloshinskii-Moriya interactions (DMI) on the edge and bulk spin waves in
2D antiferromagnetic nanoribbons. We predict the existence of a new class of
nonreciprocal edge spin waves characterized by opposite polarizations in
opposite directions. These novel edge spin waves are induced by the DMI and are
fundamentally different from conventional nonreciprocal spin waves for which
the polarization is independent of the propagation direction. Aside this
breakthrough in the field of antiferromagnetic spin waves, the study further
analysis the effect of the edges structure on the magnetic excitations. In
particular, we show that anisotropic bearded edges nanoribbons act as magnetic
topological insulators with exceptionally interesting potentials for
applications in magnonics.",1902.09704v3
2019-03-02,Enhanced thermal Hall conductivity below 1 Kelvin in the pyrochlore magnet Yb$_2$Ti$_2$O$_7$,"In this letter, we report the gigantic thermal Hall effect $\kappa_{xy}$ in
the low-field correlated-paramagnetic state of the frustrated pyrochlore
Yb$_2$Ti$_2$O$_7$. We observed a record magnitude for the thermal Hall angle in
an insulator, $\left|\kappa_{xy}\right|/\kappa_{xx}\sim 2\,\%$. The signal
onsets at $T\sim 3\,$K and is severely weakened around the transition to canted
ferromagnetic order at $T_\text{CFM}=0.275\,$K. Besides the large
$\kappa_{xy}>0$ of the fluctuating regime, a sign change towards negative
$\kappa_{xy}$ occurs at the lowest temperatures and in moderate field, where
sharp magnon excitations appear in the inelastic neutron scattering spectra. We
analyze the magnon-Hall signal and its suppression with field
semi-quantitatively. A contribution of phonon skew scattering to $\kappa_{xy}$
is ruled out by a comparison of $\kappa_{xx}$ for Tb-, Yb-, and Y-based rare
earth pyrochlore titanates. These results represent the first report of
non-vanishing $\kappa_{xy}$ measured in a dilution refrigerator ($T <
0.29\,$K).",1903.00595v2
2019-03-03,Goos-Hanchen Shift of a Spin-Wave Beam at the Interface Between Two Ferromagnets,"Spin waves are promising information carriers which can be used in modern
magnonic devices, characterized by higher performance and lower energy
consumption than presently used electronic circuits. However, before practical
application of spin waves, the efficient control over spin wave amplitude and
phase needs to be developed. We analyze analytically reflection and refraction
of the spin waves at the interface between two ferromagnetic materials. In the
model we consider the system consisting of two semi-infinite ferromagnetic
media, separated by the ultra-narrow interface region with the magnetic
anisotropy. We have found the Goos-Hanchen shift for spin waves in transmission
and reflection, and performed detailed investigations of its dependence on the
anisotropy at the interface and materials surrounding the interface. We have
demonstrated possibility of obtaining Goos-Hanchen shift of several wavelengths
in reflection for realistic material parameters. That proves the possibility
for change of the spin waves phase in ferromagnetic materials at subwavelength
distances, which can be regarded as a metasurface for magnonics.",1903.00861v1
2019-03-20,Spin dynamics of antiferromagnetically coupled ferromagnetic bilayers -- the case of Cr$_2$WO$_6$ and Cr$_2$MoO$_6$,"Recent inelastic neutron diffraction measurements on Cr$_2$(Te, W, Mo)O$_6$
have revealed that these systems consist of bilayers of spin-3/2 Cr$^{3+}$ ions
with strong antiferromagnetic inter-bilayer coupling and tuneable intra-bilayer
coupling from ferro (for W and Mo) to antiferro (for Te). These measurements
have determined the ground state spin structure and the values of sublattice
magnetization, which shows significant reduction of sublattice magnetization
from the atomic spin value of 3.0$\mu_B$ for Cr$^{3+}$ atoms. In an earlier
paper we theoretically investigated the low temperature spin dynamics of
Cr$_2$TeO$_6$ bilayer system where both the intra and inter-bilayer couplings
are antiferromagnetic. In this paper we investigate Cr$_2$WO$_6$ and
Cr$_2$MoO$_6$ systems where intra-bilayer exchange couplings are ferromagnetic
but the inter-bilayer exchange couplings are antiferromagnetic. We obtain the
magnon dispersion, sublattice magnetization, two-magnon density of states,
longitudinal spin-spin correlation function, and its powder average and compare
the results for these systems with results for Cr$_2$TeO$_6$.",1903.08512v1
2019-03-28,Topological phases of higher Chern numbers in Kitaev-Heisenberg ferromagnet with further-neighbor interactions,"Emergence of multiple topological phases with a series of Chern numbers, $\pm
1$, $\mp 1$, $\pm 2$, $\mp 2$, $\pm 3$ and $\mp 4$, are observed in a
ferromagnetic Kitaev-Heisenberg-spin-anisotropic model on honeycomb lattice
with further neighbor interactions in the presence of an external magnetic
field. Magnon Chern insulating dispersions of this two-band model are studied
by using linear spin-wave theory formulated on the exact ferromagnetic ground
state. Magnon edge states are obtained for the respective topological phases
along with density of states. A topological phase diagram of this model is
presented. Behavior of thermal Hall conductivity for those phases is studied.
Sharp jumps of thermal hall conductivity is noted near the vicinity of phase
transition points. Topological phases of the Kitaev spin-liquid compounds,
$\alpha$-RuCl$_3$, X$_2$IrO$_3$, X = (Na, Li) and CrY$_3$, Y = (Cl, Br, I) are
characterized based on this theoretical findings.",1903.11882v2
2019-04-08,Quantum entanglement between two magnon modes via Kerr nonlinearity,"We propose a scheme to entangle two magnon modes via Kerr nonlinear effect
when driving the systems far-from-equilibrium. We consider two macroscopic
yttrium iron garnets (YIGs) interacting with a single-mode microcavity through
the magnetic dipole coupling. The Kittel mode describing the collective
excitations of large number of spins are excited through driving cavity with a
strong microwave field. We demonstrate how the Kerr nonlineraity creates the
entangled quantum states between the two macroscopic ferromagnetic samples,
when the microcavity is strongly driven by a blue-detuned microwave field. Such
quantum entanglement survives at the steady state. Our work offers new insights
and guidance to designate the experiments for observing the entanglement in
massive ferromagnetic materials. It can also find broad applications in
macroscopic quantum effects and magnetic spintronics.",1904.04167v1
2019-04-12,Bosonic Dirac Materials on a honeycomb antiferromagnetic Ising model,"Motivated by the recent proposal of Bosonic Dirac materials (BDM), we
revisited the Ising model on a honeycomb lattice in the presence of the
longitudinal and transverse fields. We apply linear spin-wave theory to obtain
the magnon dispersion and its degenerated points. These special degenerated
points emerge on the excitation spectrum as a function of the external fields
and can be identified as Bosonic Dirac Points (BDP). Since that, in the
vicinity of these points the Magnons becomes massless with a linear energy
spectrum as well as insensible in relation to weak impurity, exactly as it
occurs with a Fermionic Dirac point. We also have calculated the quantum and
thermal fluctuations over the ground state of the system using Effective Field
Theory. Our results point out that this simple model can host Bosonic Dirac
points and therefore is a suitable prototype to build a Bosonic Dirac material
only controlled by external field.",1904.06323v1
2019-04-16,Multiple magnon modes in the Co$_3$Sn$_2$S$_2$ Weyl semimetal candidate,"We experimentally investigate electron transport in kagome-lattice
ferromagnet Co$_3$Sn$_2$S$_2$, which is regarded as a time-reversal symmetry
broken Weyl semimetal candidate. We demonstrate $dV/dI(I)$ curves with
pronounced asymmetric $dV/dI$ spikes, similar to those attributed to
current-induced spin-wave excitations in ferromagnetic multilayers. In contrast
to multilayers, we observe several $dV/dI$ spikes' sequences at low,
$\approx$10$^4$ A/cm$^2$, current densities for a thick single-crystal
Co$_3$Sn$_2$S$_2$ flake in the regime of fully spin-polarized bulk. The spikes
at low current densities can be attributed to novel magnon branches in magnetic
Weyl semimetals, which are predicted due to the coupling between two magnetic
moments mediated by Weyl fermions. Presence of spin-transfer effects at low
current densities in Co$_3$Sn$_2$S$_2$ makes the material attractive for
applications in spintronics.",1904.07762v2
2019-04-18,Transient Response of the Cavity-Magnon-Polariton,"The Rabi oscillations of a coupled spin-photon system are experimentally
studied using time domain microwave transmission measurements. An external
magnetic bias field is applied to control the amplitude and frequency of Rabi
oscillations, which are a measure of the coherent spin-photon energy exchange
and coupling strength respectively. The amplitude and frequency control is
described in the time domain as the transient response of a two-level
spin-photon system. This approach reveals the link between steady state and
transient behaviour of the cavity-magnon-polariton, which is necessary to
interpret time domain cavity-spintronic experiments. This transient model also
allows us to understand and quantify the polariton mode composition, which can
be controlled by the bias field.",1904.08591v1
2019-08-07,Compressive-strain induced enhancement of exchange interactions and short-range magnetic order in Sr$_2$IrO$_4$ investigated by Raman spectroscopy,"We have carried out Raman spectroscopy experiments to investigate two-magnon
excitations in epitaxial thin films of the quasi-two-dimensional
antiferromagnetic Mott insulator Sr$_2$IrO$_4$ under in-plane misfit strain.
With in-plane biaxial compression, the energy of the two-magnon peak increases,
and the peak remains observable over a wider temperature range above the N\'eel
temperature, indicating strain-induced enhancement of the superexchange
interactions between $\it{J}_{eff}$ = 1/2 pseudospins. From density functional
theory calculations, we have found an increase of the nearest-neighbor hopping
parameter and exchange interaction with increasing biaxial compressive strain,
in agreement with the experimental observations. Our experimental and
theoretical results provide perspectives for systematic, theory-guided strain
control of the primary exchange interactions in 5$\it{d}$ transition metal
oxides.",1908.02716v1
2019-08-13,Magnetic structure and excitations of the topological semimetal YbMnBi$_2$,"We investigated the magnetic structure and dynamics of YbMnBi$_2$, with
elastic and inelastic neutron scattering, to shed light on the topological
nature of the charge carriers in the antiferromagnetic phase. We confirm C-type
antiferromagnetic ordering of the Mn spins below $T_{\rm N} = 290$ K, and
determine that the spins point along the $c$-axis to within about $3^\circ$.
The observed magnon spectrum can be described very well by the same effective
spin Hamiltonian as was used previously to model the magnon spectrum of
CaMnBi$_2$. Our results show conclusively that the creation of Weyl nodes in
YbMnBi$_2$ by the time-reversal-symmetry breaking mechanism can be excluded in
the bulk.",1908.04872v1
2019-08-15,Collective Excitations of Quantum Anomalous Hall Ferromagnets in Twisted Bilayer Graphene,"We present a microscopic theory for collective excitations of quantum
anomalous Hall ferromagnets (QAHF) in twisted bilayer graphene. We calculate
the spin magnon and valley magnon spectra by solving Bethe-Salpeter equations,
and verify the stability of QAHF. We extract the spin stiffness from the
gapless spin wave dispersion, and estimate the energy cost of a
skyrmion-antiskyrmion pair, which is found to be comparable in energy with the
Hartree-Fock gap. The valley wave mode is gapped, implying that the valley
polarized state is more favorable compared to the valley coherent state. Using
a nonlinear sigma model, we estimate the valley ordering temperature, which is
considerably reduced from the mean-field transition temperature due to thermal
excitations of valley waves.",1908.05417v2
2019-08-16,Scaling Analysis of Anomalous Hall Resistivity in the Co$_{2}$TiAl Heusler Alloy,"A comprehensive magnetotransport study including resistivity ($\rho_{xx}$) at
various fields, isothermal magnetoresistance and Hall resistivity ($\rho_{xy}$)
has been carried out at different temperatures on the Co$_{2}$TiAl Heusler
alloy. Co$_{2}$TiAl alloy shows a paramagnetic (PM) to ferromagnetic (FM)
transition below the curie temperature (T$_{C}$) $\sim$ 125 K. In the FM
region, resistivity and magnetoresistance reveals a spin flip electron-magnon
scattering and the Hall resistivity unveils the anomalous Hall resistivity
($\rho_{xy}^{AH}$). Scaling of anomalous Hall resistivity with resistivity
establishes the extrinsic scattering process responsible for the anomalous hall
resistivity; however Skew scattering is the dominant mechanism compared to the
side-jump contribution. A one to one correspondence between magnetoresistance
and side-jump contribution to anomalous Hall resistivity verifies the
electron-magnon scattering being the source of side-jump contribution to the
anomalous hall resistivity.",1908.05974v1
2019-08-24,Magnetic order and anisotropic interactions induced by mixing between the $J=1/2$ and $3/2$ sectors in spin-orbit coupled honeycomb-lattice compounds,"Novel magnetic ordering on the honeycomb lattice due to emergent weak
anisotropic interactions generated by the mixing between the $J=1/2$ sector and
the magnetically inactive 3/2 sector is investigated in a three-orbital
interacting electron model in the absence of Hund's coupling. Self-consistent
determination of magnetic order yields anisotropic N\'{e}el and zigzag orders
for different parameter regimes, highlighting the effect of the emergent
single-ion anisotropy. Study of magnon excitations shows extremely small magnon
energy scale compared to the hopping energy scale, and enhancement of
anisotropy effects for smaller spin-orbit coupling. These results account for
several features of the honeycomb lattice compounds such as $\rm Na_2 Ir O_3$
and $\rm Ru Cl_3$, where the leading order anisotropic interactions within the
magnetically active $J=1/2$ sector are completely quenched due to the
edge-sharing octahedra.",1908.09130v1
2019-11-01,Giant unidirectional magnetoresistance in topological insulator -- ferromagnetic semiconductor heterostructures,"The unidirectional magnetoresistance (UMR) is one of the most complex
spin-dependent transport phenomena in ferromagnet/non-magnet bilayers, which
involves spin injection and accumulation due to the spin Hall effect (SHE) or
Rashba-Edelstein effect (REE), spin-dependent scattering, and magnon scattering
at the interface or in the bulk of the ferromagnet. While UMR in metallic
bilayers has been studied extensively in very recent years, its magnitude is as
small as 10$^-$$^5$, which is too small for practical applications. Here, we
demonstrate a giant UMR effect in a heterostructure of BiSb topological
insulator -- GaMnAs ferromagnetic semiconductor. We obtained a large UMR ratio
of 1.1%, and found that this giant UMR is governed not by the giant
magnetoresistance (GMR)-like spin-dependent scattering, but by magnon
emission/absorption and strong spin-disorder scattering in the GaMnAs layer.
Our results provide new insight into the complex physics of UMR, as well as a
strategy for enhancing its magnitude for device applications.",1911.00247v1
2019-11-09,The magnonic superfluid droplet at room temperature,"We declare the observation of spin superfluid state in Yttrium Iron Garnet
(YIG) at room temperature. It is similar to a Homogeneous Precessing State
(HPD), observed earlier in antiferromagnetic superfluid $^3$He-B. The formation
of this state explains by the repulsive interaction between magnons, which is
required as a prior condition for the spin superfluidity. It establishes an
energy gap, which stabilizes the long range superfluid transport of
magnetization and determines the Ginzburg-Landau coherence length. This
discovery paves a way to many quantum applications of supermagnonics at room
temperature, such as magnetic Josephson effect, long distance spin transport,
Q-bit, quantum logics, magnetic sensors and others.",1911.03708v1
2019-11-30,Microwave dynamics of pure and doped anisotropic S=1 chain antiferromagnet NiCl2-4SC(NH2)2,"We studied electron spin resonance in a quantum magnet NiCl2-4SC(NH2)2,
demonstrating a field-induced quantum phase transition from a
quantum-disordered phase to an antiferromagnet. We observe two branches of the
antiferromagnetic resonance of the ordered phase, one of them has a gap and the
other is a Goldstone mode with zero frequency at a magnetic field along the
four-fold axis. This zero frequency mode acquires a gap at a small tilting of
the magnetic field with respect to this direction. The upper gap was found to
be reduced in the doped compound Ni(Cl(1-x)Br(x))2-4SC(NH2)2 with $x=0.21$.
This reduction is unexpected because of the previously reported rise of the
main exchange constant in a doped compound. Further, a nonresonant diamagnetic
susceptibility $\chi^{\prime}$ was found for the ordered phase in a wide
frequency range above the quasi-Goldstone mode. This dynamic diamagnetism is as
large as the dynamic susceptibility of the paramagnetic resonance. We speculate
that it originates from a two-magnon absorption band of low-frequency
dispersive magnon branch.",1912.00172v1
2019-12-27,Multi-particle finite-volume effects for hexagon tessellations,"Correlation functions of gauge-invariant composite operators in N=4 super
Yang-Mills theory can be computed by integrability using triangulations. The
elementary tile in this process is the hexagon, which should be glued by
appropriately inserting resolutions of the identity involving virtual
(""mirror"") magnons. We consider this problem for five-point functions of
protected operators. At one-loop in the 't Hooft coupling, it is necessary to
glue three adjacent tiles which involves two virtual magnons scattering among
each other. We show that the result can be simplified by using an adapted
mirror rotation and employing appropriate summation techniques. The
mirror-particle contributions then yield hyperlogarithms of weight two.
Finally, we use these results to investigate braiding prescriptions introduced
in earlier work on the problem.",1912.12231v1
2015-08-06,"Phenomenological description of the nonlocal magnetization relaxation in magnonics, spintronics, and domain-wall dynamics","A phenomenological equation called Landau-Lifshitz-Baryakhtar (LLBar)
equation, which could be viewed as the combination of Landau-Lifshitz (LL)
equation and an extra ""exchange damping"" term, was derived by Baryakhtar using
Onsager's relations. We interpret the origin of this ""exchange damping"" as
nonlocal damping by linking it to the spin current pumping. The LLBar equation
is investigated numerically and analytically for the spin wave decay and domain
wall motion. Our results show that the lifetime and propagation length of
short-wavelength magnons in the presence of nonlocal damping could be much
smaller than those given by LL equation. Furthermore, we find that both the
domain wall mobility and the Walker breakdown field are strongly influenced by
the nonlocal damping.",1508.01478v1
2016-03-15,A Realization of the Haldane-Kane-Mele Model in a System of Localized Spins,"We theoretically study a spin Hamiltonian for spin-orbit-coupled ferromagnets
on the honeycomb lattice. We find that the effective Hamiltonian for magnons, a
quanta of spin-wave excitations from ordered states, is equivalent to the
Haldane model for electrons in the honeycomb lattice, which indicates the
nontrivial topology of the magnon bands and the existence of the associated
edge state. We also take the Schwinger boson (or bosonic spinon)
representations of spins that are applicable both to ordered and disordered
phases. We show that the spinon bands within the mean-field treatment are
topologically nontrivial, which causes a spin Nernst effect, by adopting the
properties of the Kane-Mele model. A feasible experimental realization of the
spin Hamiltonian is proposed.",1603.04827v2
2016-03-17,Spin-Wave Fiber,"Spin waves are collective excitations propagating in the magnetic medium with
ordered magnetizations. Magnonics, utilizing the spin wave (magnon) as
information carrier, is a promising candidate for low-dissipation computation
and communication technologies. We discover that, due to the
Dzyaloshinskii-Moriya interaction, the scattering behavior of spin wave at a
magnetic domain wall follows a generalized Snell's law, where two magnetic
domains work as two different mediums. Similar to optical total reflection that
occurs at the water-air interfaces, spin waves may experience total reflection
at magnetic domain walls when their incident angle larger than a critical
value. We design a spin wave fiber using a magnetic domain structure with two
domain walls, and demonstrate that such a spin wave fiber can transmit spin
waves over long distance by total internal reflections, in analogy to an
optical fiber. Our design of spin wave fiber opens up new possibilities in pure
magnetic information processing.",1603.05325v1
2016-03-18,Entanglement Entropy of the N=4 SYM spin chain,"We present a detailed study of the Entanglement Entropy (EE) of excited
states in all closed rank one subsectors of N=4 SYM, namely SU(2), SU(1|1) and
SL(2). Exploiting the techniques of the Coordinate and the Algebraic Bethe
Ansatz we obtain the EE for spin chains with up to seven magnons, at leading
order in the coupling expansion but exact in the length of the spin chain and
of the part of it that we cut. Focusing on the superconformal primary operator
with two magnons in the BMN limit, we derive analytic and exact, in the
coupling $\lambda'$, expressions for the Renyi and the EE. The interpolating
functions for the Renyi and the EE monotonically increase as the coupling
increases from the weak coupling $\lambda' \rightarrow 0$ regime to the strong
coupling $\lambda' \rightarrow \infty$ regime. This results to a violation of a
certain bound for the EE that is present at weak coupling and confirms the
physical intuition that entanglement increases when the coupling increases.",1603.05929v2
2016-03-21,Topological hard-core bosons on the honeycomb lattice,"This paper presents a connection between the topological properties of
hardcore bosons and that of magnons in quantum spin magnets. We utilize the
Haldane-like hardcore bosons on the honeycomb lattice as an example. We show
that this system maps to a spin-$1/2$ quantum XY model with a
next-nearest-neighbour Dzyaloshinsky-Moriya interaction. We obtain the magnon
excitations of the quantum spin model and compute the edge states, Berry
curvature, thermal and spin Nernst conductivities. Due to the mapping from spin
variables to bosons, the hardcore bosons possess the same nontrivial
topological properties as those in quantum spin system. These results are
important in the study of magnetic excitations in quantum magnets and they are
also useful for understanding the control of ultracold bosonic quantum gases in
honeycomb optical lattices, which is experimentally accessible.",1603.06495v4
2016-03-25,Magnon edge states in hardcore-Bose-Hubbard model,"Quantum Monte Carlo (QMC) simulation has uncovered nonzero Berry curvature
and bosonic edge states in hardcore-Bose-Hubbard model on the gapped honeycomb
lattice. The competition between the chemical potential and staggered onsite
potential leads to an interesting quantum phase diagram comprising superfluid
phase, Mott insulator, and charge density wave insulator. In this paper, we
present a semiclassical perspective of this system by mapping to a spin-$1/2$
quantum XY model. We give an explicit analytical origin of the quantum phase
diagram, the Berry curvatures, and the edge states using semiclassical
approximations. We find very good agreements between the semiclassical analyses
and the QMC results. Our results show that the topological properties of
hardcore-Bose-Hubbard model are the same as those of magnon in the
corresponding quantum spin system. Our results are applicable to systems of
ultracold bosonic atoms trapped in honeycomb optical lattices.",1603.07989v4
2016-03-29,Current-induced asymmetric magnetoresistance due to energy transfer via quantum spin-flip process,"Current-induced magnetization excitation is a core phenomenon for
next-generation magnetic nanodevices, and has been attributed to the
spin-transfer torque (STT) that originates from the transfer of the spin
angular momentum between a conduction electron and a local magnetic moment
through the exchange coupling. However, the same coupling can transfer not only
spin but also energy, though the latter transfer mechanism has been largely
ignored. Here we report on experimental evidence concerning the energy transfer
in ferromagnet/heavy metal bilayers. The magnetoresistance (MR) is found to
depend significantly on the current direction down to low in-plane currents,
for which STT cannot play any significant role. Instead we find that the
observed MR is consistent with the energy transfer mechanism through the
quantum spin-flip process, which predicts short wavelength,
current-direction-dependent magnon excitations in the THz frequency range. Our
results unveil another aspect of current-induced magnetic excitation, and open
a channel for the dc-current-induced generation of THz magnons.",1603.08746v1
2016-10-07,Local thermomagnonic torques in two-fluid spin dynamics,"We develop a general phenomenology describing the interplay between coherent
and incoherent dynamics in ferromagnetic insulators. Using the Onsager
reciprocity and Neumann's principle, we derive expressions for the local
thermomagnonic torques exerted by thermal magnons on the order-parameter
dynamics and the reciprocal pumping processes, which are in close analogy to
the spin-transfer torque and spin pumping at metallic interfaces. Our formalism
is applicable to general long-wavelength dynamics and, although here we
explicitly focus on ferromagnetic insulators possessing U(1) symmetry, our
approach can be easily extended to other classes of magnetic materials. As an
illustrative example, we apply our theory to investigate a domain wall floating
over a spin superfluid, whose dynamics is triggered thermally at the system's
edge. Our results demonstrate that the local pumping of coherent spin dynamics
by a thermal magnon gas offers an alternative route - with no need for
conducting components and thus devoid of Ohmic losses - for the control and
manipulation of topological solitons.",1610.02224v1
2016-12-20,Spin liquid mediated RKKY interaction,"We propose an RKKY-type interaction that is mediated by a spin liquid. If a
spin liquid ground state exists such an interaction could leave a fingerprint
by ordering underlying localized moments such as nuclear spins. This
interaction has a unique phenomenology that is distinct from the RKKY
interaction found in fermionic systems; most notably the lack of a Fermi
surface and absence of the requirement for itinerant electrons, since most spin
liquids are insulators. As a working example we investigate the two-dimensional
spin-1/2 kagome antiferromagnet (KAFM), although the treatment remains general
and can be extended to other spin liquids and dimensions. We find that several
different nuclear spin orderings minimize the RKKY-type energy induced by the
KAFM but are unstable due to a zero-energy flat magnon band. Despite this we
show that a small magnetic field is able to gap out this magnon spectrum for
some of the orderings resulting in an intricate nuclear magnetism.",1612.06868v2
2017-11-01,Spin current noise of the spin Seebeck effect and spin pumping,"We theoretically investigate the fluctuation of a pure spin current induced
by the spin Seebeck effect and spin pumping in a normal metal (NM)/ferromagnet
(FM) bilayer system. Starting with a simple FI--NM interface model with both
spin-conserving and spin-non-conserving processes, we derive general
expressions of the spin current and the spin-current noise at the interface
within second-order perturbation of the FI--NM coupling strength, and estimate
them for an yttrium iron garnet (YIG) --platinum interface. We show that the
spin-current noise can be used to determine the effective spin carried by a
magnon, modified by the spin-non-conserving process at the interface. In
addition, we show that it provides information on the effective spin of a
magnon, heating at the interface under spin pumping, and spin Hall angle of the
NM.",1711.00237v2
2017-11-09,Pairing from strong repulsion in triangular lattice Hubbard model,"We propose a paring mechanism between holes in the dilute limit of doped
frustrated Mott insulators. Hole pairing arises from a hole-hole-magnon
three-body bound state. This pairing mechanism has its roots on single-hole
kinetic energy frustration, which favors antiferromagnetic (AFM) correlations
around the hole. We demonstrate that the AFM polaron (hole-magnon bound state)
produced by a single-hole propagating on a field-induced polarized background
is strong enough to bind a a second hole. The effective interaction between
these three-body bound states is repulsive, implying that this pairing
mechanism is relevant for superconductivity.",1711.03233v2
2017-11-27,Characterization of Spin-Transfer-Torque effect induced magnetization dynamics driven by short current pulses,"We present a time-resolved study of the magnetization dynamics in a
microstructured Cr$|$Heusler$|$Pt waveguide driven by the Spin-Hall-Effect and
the Spin-Transfer-Torque effect via short current pulses. In particular, we
focus on the determination of the threshold current at which the spin-wave
damping is compensated. We have developed a novel method based on the temporal
evolution of the magnon density at the beginning of an applied current pulse at
which the magnon density deviates from the thermal level. Since this method
does not depend on the signal-to-noise ratio, it allows for a robust and
reliable determination of the threshold current which is important for the
characterization of any future application based on the Spin-Transfer-Torque
effect.",1711.09647v1
2017-11-28,Spin eigen-excitations of the magnetic skyrmion and problem of the effective mass,"Properties of magnon modes localized on a ferromagnetic skyrmion are studied.
Three types of possible asymptotic behavior of the modes eigenfrequencies are
found for the case of large skyrmion radius $R_s$, namely $\omega_0\sim
R_s^{-2}$ for the breathing mode, $\omega_{-|\mu|}\sim R_s^{-1}$ and
$\omega_{|\mu|}\sim R_s^{-3}$ for modes with negative and positive azimuthal
quantum numbers, respectively. A number of properties of the magnon
eigenfunctions are determined. This enables us to demonstrate that the skyrmion
dynamics based on the traveling wave model is described by the massless Thiele
equation.",1711.10461v1
2018-10-23,Higher-order exceptional point in a cavity magnonics system,"We propose to realize the pseudo-Hermiticity in a cavity magnonics system
consisting of the Kittel modes in two small yttrium-iron-garnet spheres coupled
to a microwave cavity mode. The effective gain of the cavity can be achieved
using the coherent perfect absorption of the two input fields fed into the
cavity. With certain constraints of the parameters, the Hamiltonian of the
system has the pseudo-Hermiticity and its eigenvalues can be either all real or
one real and other two constituting a complex-conjugate pair. By varying the
coupling strengths between the two Kittel modes and the cavity mode, we find
the existence of the third-order exceptional point in the parameter space, in
addition to the usual second-order exceptional point existing in the system
with parity-time symmetry. Also, we show that these exceptional points can be
demonstrated by measuring the output spectrum of the cavity.",1810.09689v3
2019-05-04,Identification and time-resolved study of YIG spin wave modes in a MW cavity in strong coupling regime,"Recently, the hybridization of microwave-frequency cavity modes with
collective spin excitations attracted large interest for the implementation of
quantum computation protocols, which exploit the transfer of information among
these two physical systems. Here, we investigate the interaction among the
magnetization precession modes of a small YIG sphere and the MW electromagnetic
modes, resonating in a tridimensional aluminum cavity. In the strong coupling
regime, anti-crossing features were observed in correspondence of various
magnetostatic modes, which were excited in a magnetically saturated sample.
Time-resolved studies show evidence of Rabi oscillations, demonstrating
coherent exchange of energy among photons and magnons modes. To facilitate the
analysis of the standing spin-wave patterns, we propose here a new procedure,
based on the introduction of a novel functional variable. The resulting easier
identification of magnetostatic modes can be exploited to investigate, control
and compare many-levels hybrid systems in cavity- and opto-magnonics research.",1905.01522v2
2019-05-09,Bidirectional spin-wave-driven domain wall motion in antiferromagnetically coupled ferrimagnets,"We investigate ferrimagnetic domain wall dynamics induced by circularly
polarized spin waves theoretically and numerically. We find that the direction
of domain wall motion depends on both the circular polarization of spin waves
and the sign of net spin density of ferrimagnet. Below the angular momentum
compensation point, left- (right-) circularly polarized spin waves push a
domain wall towards (away from) the spin-wave source. Above the angular
momentum compensation point, on the other hand, the direction of domain wall
motion is reversed. This bidirectional motion originates from the fact that the
sign of spin-wave-induced magnonic torque depends on the circular polarization
and the subsequent response of the domain wall to the magnonic torque is
governed by the net spin density. Our finding provides a way to utilize a spin
wave as a versatile driving force for bidirectional domain wall motion.",1905.03521v1
2019-05-20,Quantum drives produce strong entanglement between YIG samples without using intrinsic nonlinearities,"We show how to generate an entangled pair of yttrium iron garnet (YIG)
samples in a cavity-magnon system without using any nonlinearities which are
typically very weak. This is against the conventional wisdom which necessarily
requires strong Kerr like nonlinearity. Our key idea, which leads to
entanglement, is to drive the cavity by a weak squeezed vacuum field generated
by a flux-driven Josephson parametric amplifier (JPA). The two YIG samples
interact via the cavity. For modest values of the squeezing of the pump, we
obtain significant entanglement. This is the principal feature of our scheme.
We discuss entanglement between macroscopic spheres using several different
quantitative criteria. We show the optimal parameter regimes for obtaining
entanglement which is robust against temperature. We also discuss squeezing of
the collective magnon variables.",1905.07884v2
2019-05-21,High-Field Expansion Approach to Kagome Antiferromagnets with Dzyaloshinskii-Moriya Interactions,"We apply linked cluster expansion techniques to study the polarized
high-field phase of a spin-half antiferromagnet on the Kagome lattice with
Heisenberg and Dzyaloshinskii-Moriya interactions (DMI). We find that the Dirac
points of the single-magnon spectrum without DMI are robust against arbitrary
DMI when the magnetic field lies in the Kagome plane. Unlike the typical case
where DMI gaps the spectrum, here we find that varying the DMI merely shifts
the location of the Dirac points. In contrast, a magnetic field with a
component out of the Kagome plane gaps the spectrum, leading to topological
magnon bands. We map out a topological phase diagram as the couplings are
varied by computing the band Chern numbers. A pair of phase transitions are
observed and we find an enhanced thermal Hall conductivity near the phase
boundary.",1905.08805v2
2020-06-01,Strong magnon-photon coupling within a tunable cryogenic microwave cavity,"The ability to achieve strong-coupling has made cavity-magnon systems an
exciting platform for the development of hybrid quantum systems and the
investigation of fundamental problems in physics. Unfortunately, current
experimental realizations are constrained to operate at a single frequency,
defined by the geometry of the microwave cavity. In this article we realize a
highly-tunable, cryogenic, microwave cavity strongly coupled to magnetic spins.
The cavity can be tuned in situ by up to 1.5 GHz, approximately 15% of its
original 10 GHz resonance frequency. Moreover, this system remains within the
strong-coupling regime at all frequencies with a cooperativity of approximately
800.",2006.01223v2
2020-06-15,Femtosecond laser pulse driven caustic spin wave beams,"Controlling the directionality of spin waves is a key ingredient in
wave-based computing methods such as magnonics. In this paper, we demonstrate
this particular aspect by using an all-optical point-like source of continuous
spin waves based on frequency comb rapid demagnetization. The emitted spin
waves contain a range of k-vectors and by detuning the applied magnetic field
slightly off the ferromagnetic resonance (FMR), we observe X-shaped caustic
spin-wave patterns at $70^{\circ}$ propagation angles as predicted by theory.
When the harmonic of the light source approaches theFMR, the caustic pattern
gives way to uniaxial spin-wave propagation perpendicular to the in-plane
component of the applied field. This field-controlled propagation pattern and
directionality of optically emitted short-wavelength spin waves provide
additional degrees of freedom when designing magnonic devices.",2006.08219v1
2020-06-27,A Comprehensible Review: Magnonic Magnetoelectric Coupling in Ferroelectric/ Ferromagnetic Composites,"Composite materials consisting of coupled magnetic and ferroelectric layers
hold the promise for new emergent properties such as controlling magnetism with
electric fields. Obviously, the interfacial coupling mechanism plays a crucial
role and its understanding is the key for exploiting this material class for
technological applications. This short review is focused on the magnonic-based
magnetoelectric coupling that forms at the interface of a metallic ferromagnet
with a ferroelectric insulator. After analyzing the physics behind this
coupling, the implication for the magnetic, transport, and optical properties
of these composite materials is discussed. Furthermore, examples for the
functionality of such interfaces are illustrated by the electric field
controlled transport through ferroelectric/ferromagnetic tunnel junctions, the
electrically and magnetically controlled optical properties, and the generation
of electromagnon solitons for the use as reliable information carriers.",2006.15290v2
2020-09-10,Current jumps in flat band ladders with Dzyaloshinskii-Moriya interactions,"Localized magnons states, due to flat bands in the spectrum, is an intensely
studied phenomenon and can be found in many frustrated magnets of different
spatial dimensionality. The presence of Dzyaloshinskii-Moriya (DM) interactions
may change radically the behavior in such systems. In this context, we study a
paradigmatic example of a one-dimensional frustrated antiferromagnet, the
sawtooth chain in the presence of DM interactions. Using both path integrals
methods and numerical Density Matrix Renormalization Group, we revisit the
physics of localized magnons and determine the consequences of the DM
interaction on the ground state. We have studied the spin current behavior,
finding three different regimes. First, a Luttinger-liquid regime where the
spin current shows a step behavior as a function of parameter $D$, at a low
magnetic field. Increasing the magnetic field, the system is in the Meissner
phase at the $m = 1/2$ plateau, where the spin current is proportional to the
DM parameter. Finally, further increasing the magnetic field and for finite $D$
there is a small stiffness regime where the spin current shows, at fixed
magnetization, a jump to large values at $D = 0$, a phenomenon also due to the
flat band.",2009.04970v2
2020-09-30,Electronic structure and finite temperature magnetism of yttrium iron garnet,"Yttrium iron garnet is a complex ferrimagnetic insulator with 20 magnon modes
which is used extensively in fundamental experimental studies of magnetisation
dynamics. As a transition metal oxide with moderate gap (2.8 eV), yttrium iron
garnet requires a careful treatment of electronic correlation. We have applied
quasiparticle self-consistent GW to provide a fully ab initio description of
the electronic structure and resulting magnetic properties, including the
parameterisation of a Heisenberg model for magnetic exchange interactions.
Subsequent spin dynamical modelling with quantum statistics extends our
description to the magnon spectrum and thermodynamic properties such as the
Curie temperature, finding favourable agreement with experimental measurements.
This work provides a snapshot of the state-of-the art in modelling of complex
magnetic insulators.",2009.14601v1
2021-02-01,Magnon-assisted dynamics of a hole doped in a cuprate superconductor,"We calculate the quasiparticle dispersion and spectral weight of the
quasiparticle that results when a hole is added to an antiferromagnetically
ordered CuO$_2$ plane of a cuprate superconductor. We also calculate the magnon
contribution to the quasiparticle spectral function. We start from a multiband
model for the cuprates considered previously [Nat. Phys. \textbf{10}, 951
(2014)]. We map this model and the operator for creation of an O hole to an
effective one-band generalized $t-J$ model, without free parameters. The
effective model is solved using the state of the art self-consistent Born
approximation. Our results reproduce all the main features of experiments. They
also reproduce qualitatively the dispersion of the multiband model, giving
better results for the intensity near wave vector $(\pi,\pi)$, in comparison
with the experiments. In contrast to what was claimed in [Nat. Phys.
\textbf{10}, 951 (2014)], we find that spin fluctuations play an essential role
in the dynamics of the quasiparticle, and hence in both its weight and
dispersion.",2102.01177v1
2021-02-02,"Coupled spin-orbital fluctuations in a three orbital model for $4d$ and $5d$ oxides with electron fillings $n=3,4,5$ -- Application to $\rm NaOsO_3$, $\rm Ca_2RuO_4$, and $\rm Sr_2IrO_4$","A unified approach is presented for investigating coupled spin-orbital
fluctuations within a realistic three-orbital model for strongly spin-orbit
coupled systems with electron fillings $n=3,4,5$ in the $t_{2g}$ sector of
$d_{yz},d_{xz},d_{xy}$ orbitals. A generalized fluctuation propagator is
constructed which is consistent with the generalized self-consistent
Hartree-Fock approximation where all Coulomb interaction contributions
involving orbital diagonal and off-diagonal spin and charge condensates are
included. Besides the low-energy magnon, intermediate-energy orbiton and
spin-orbiton, and high-energy spin-orbit exciton modes, the generalized
spectral function also shows other high-energy excitations such as the Hund's
coupling induced gapped magnon modes. We relate the characteristic features of
the coupled spin-orbital excitations to the complex magnetic behavior resulting
from the interplay between electronic bands, spin-orbit coupling, Coulomb
interactions, and structural distortion effects, as realized in the compounds
$\rm NaOsO_3$, $\rm Ca_2RuO_4$, and $\rm Sr_2IrO_4$.",2102.01400v2
2021-02-02,Topological Aspects of Antiferromagnets,"The long fascination antiferromagnetic materials have exerted on the
scientific community over about a century has been entirely renewed recently
with the discovery of several unexpected phenomena including various classes of
anomalous spin and charge Hall effects and unconventional magnonic transport,
but also homochiral magnetic entities such as skyrmions. With these
breakthroughs, antiferromagnets standout as a rich playground for the
investigation of novel topological behaviors, and as promising candidate
materials for disruptive low-power microelectronic applications. Remarkably,
the newly discovered phenomena are all related to the topology of the magnetic,
electronic or magnonic ground state of the antiferromagnets. This review
exposes how non-trivial topology emerges at different levels in
antiferromagnets and explores the novel mechanisms that have been discovered
recently. We also discuss how novel classes of quantum magnets could enrich the
currently expanding field of antiferromagnetic spintronics and how spin
transport can in turn favor a better understanding of exotic quantum
excitations.",2102.01632v3
2021-02-05,Symmetry-Driven Spin-Wave Gap Modulation in Nanolayered SrRuO3/SrTiO3 Heterostructures: Implications for Spintronic Applications,"A strong correlation between magnetic interaction and topological symmetries
leads to unconventional magneto-transport behavior. Weyl fermions induce
topologically protected spin-momentum locking, which is closely related to
spin-wave gap formation in magnetic crystals. Ferromagnetic SrRuO3, regarded as
a strong candidate for Weyl semimetal, inherently possesses a nonzero spin-wave
gap owing to its strong magnetic anisotropy. In this paper, we propose a method
to control the spin-wave dynamics by nanolayer designing of the SrRuO3/SrTiO3
superlattices. In particular, the six-unit-cell-thick SrRuO3 layers within the
superlattices undergo a phase transition in crystalline symmetry from
orthorhombic to tetragonal, as the thickness of the SrTiO3 layers is modulated
with atomic-scale precision. Consequently, the magnetic anisotropy, anomalous
Hall conductivity, and spin-wave gap could be systematically manipulated. Such
customization of magnetic anisotropy via nanoscale heterostructuring offers a
novel control knob to tailor the magnon excitation energy for future spintronic
applications, including magnon waveguides and filters. Our nanolayer approach
unveils the important correlation between the tunable lattice degrees of
freedom and spin dynamics in topologically non-trivial magnetic materials.",2102.03008v1
2021-02-08,Multiple-magnon excitations shape the spin spectrum of cuprate parent compounds,"Thanks to high resolution and polarization analysis, resonant inelastic x-ray
scattering (RIXS) magnetic spectra of La2CuO4, Sr2CuO2Cl2 and CaCuO2 reveal a
rich set of properties of the spin 1/2 antiferromagnetic square lattice of
cuprates. The leading single-magnon peak energy dispersion is in excellent
agreement with the corresponding inelastic neutron scattering measurements.
However, the RIXS data unveil an asymmetric lineshape possibly due to odd
higher order terms. Moreover, a sharp bimagnon feature emerges from the
continuum at (1/2,0), coincident in energy with the bimagnon peak detected in
optical spectroscopy. These findings show that the inherently complex spin
spectra of cuprates, an exquisite manifestation of quantum magnetism, can be
effectively explored by exploiting the richness of RIXS cross sections.",2102.04078v1
2021-02-23,Tunable Cooperativity in Coupled Spin--Cavity Systems,"We experimentally study the tunability of the cooperativity in coupled
spin--cavity systems by changing the magnetic state of the spin system via an
external control parameter. As model system, we use the skyrmion host material
Cu$_2$OSeO$_3$ coupled to a microwave cavity resonator. In the different
magnetic phases we measure a dispersive coupling between the resonator and the
magnon modes and model our results by using the input--output formalism. Our
results show a strong tunability of the normalized coupling rate by magnetic
field, allowing us to change the magnon--photon cooperativity from 1 to 60 at
the phase boundaries of the skyrmion lattice state.",2102.11713v1
2013-06-05,Dynamical mean-field theory for quantum spin systems: Test of solutions for magnetically ordered states,"A spin version of dynamical mean-field theory is extended for magnetically
ordered states in the Heisenberg model. The self-consistency equations are
solved with high numerical accuracy by means of the continuous-time quantum
Monte Carlo with bosonic baths coupled to the spin. The resultant solution is
critically tested by known physical properties. In contrast with the mean-field
theory, soft paramagnons appear near the transition temperature. Moreover, the
Nambu-Goldstone mode (magnon) in the ferromagnetic phase is reproduced
reasonably well. However, antiferromagnetic magnons have an energy gap in
contradiction to the Nambu-Goldstone theorem. The origin of this failure is
discussed in connection with an artificial first-order nature of the
transition.",1306.1186v2
2016-09-23,Electromagnon resonance in a collinear spin state of a polar antiferromagnet Fe2Mo3O8,"Magnetic excitations are investigated for a hexagonal polar magnet Fe2Mo3O8
by terahertz spectroscopy. We observed magnon modes including an electric-field
active magnon, electromagnon, in the collinear antiferromagnetic phase with
spins parallel to the c axis. We unravel the nature of these excitations by
investigating the correlation between the evolution of the mode profile and the
magnetic transition from antiferromagnetic to ferrimagnetic order induced by
magnetic field or Zn-doping. We propose that the observed electromagnon mode
involves the collective precession of the spins with oscillating in-plane
electric polarization through the mechanism of the linear magnetoelectric
effect.",1609.07230v2
2016-09-26,Observables in the Guarino-Jafferis-Varela/CS-SYM duality,"We study various semiclassical observables in the duality proposed by
Guarino, Jafferis and Varela, between a warped $AdS_4\times$ squashed $S^6$
gravitational solution and a 3 dimensional ${\cal N}=2$ SYM-CS$_k$ conformal
gauge theory, deformed from the maximal $SU(N)$ ${\cal N}=8$ SYM. Baryon
vertices corresponding to particle-like branes have unusual behaviour with $N$
and $k$ and present strong evidence for a certain level-rank duality. Wilson
loops and the anomalous dimensions of operators of high spin scale like
$(N/k)^{3/2}$. The entanglement entropy behaves like in a usual CFT. Giant
magnon operators obey the same law as in 4 dimensional ${\cal N}=4$ SYM, and
giant gravitons are also sub-determinant operators.",1609.08008v2
2016-09-24,Optical manipulation of a magnon-photon hybrid system,"We demonstrate an all-optical method for manipulating the magnetization in a
1-mm YIG (yttrium-iron-garnet) sphere placed in a $\sim0.17\,$T uniform
magnetic field. An harmonic of the frequency comb delivered by a multi-GHz
infrared laser source is tuned to the Larmor frequency of the YIG sphere to
drive magnetization oscillations, which in turn give rise to a radiation field
used to thoroughly investigate the phenomenon. The radiation damping issue that
occurs at high frequency and in the presence of highly magnetizated materials,
has been overcome by exploiting magnon-photon strong coupling regime in
microwave cavities. Our findings demonstrate an effective technique for
ultrafast control of the magnetization vector in optomagnetic materials via
polarization rotation and intensity modulation of an incident laser beam. We
eventually get a second-order susceptibility value of $\sim10^{-7}$ cm$^2$/MW
for single crystal YIG.",1609.08147v1
2017-04-07,Reconfigurable nano-scale spin-wave directional coupler,"A spin-wave (SW) directional coupler comprised of two laterally parallel
nano-scale dipolarly-coupled SW waveguides is proposed and studied using
micromagnetic simulations and analytical theory. The energy of a SW excited in
one of the waveguides in the course of propagation is periodically transferred
to the other waveguide and back, and the spatial half-period of this transfer
is defined as the coupling length. The coupling length is determined by the
dipolar coupling between the waveguides, and the fraction of the SW energy
transferred to the other waveguide at the device output can be varied with the
SW frequency, bias magnetic field, and relative orientation of the waveguide's
static magnetizations. The proposed design of a directional coupler can be used
in digital computing-oriented magnonics as a connector (multiplexer) of
magnonic conduits without a direct contact, or in the analog microwave signal
processing as a reconfigurable nano-scale power divider and/or frequency
separator.",1704.02255v1
2017-04-28,Thermal Control of the Magnon-Photon Coupling in a Notch Filter coupled to a Yttrium-Iron-Garnet/Platinum System,"We report thermal control of mode hybridization between the ferromagnetic
resonance (FMR) and a planar resonator (notch filter) working at 4.74 GHz. The
chosen magnetic material is a ferrimagnetic insulator (Yttrium Iron Garnet:
YIG) covered by 6 nm of platinum (Pt). A current induced heating method has
been used in order to enhance the temperature of the YIG/Pt system. The device
permits us to control the transmission spectra and the magnon-photon coupling
strength at room temperature. These experimental findings reveal potentially
applicable tunable microwave filtering function.",1704.08896v3
2018-01-22,Localization of magnon modes in a curved magnetic nanowire,"Spin waves in magnetic nanowires can be bound by a local bending of the wire.
The eigenfrequency of a truly local magnon mode is determined by the curvature:
a general analytical expression is established for any infinitesimally weak
localized curvature of the wire. The interaction of the local mode with spin
waves, propagating through the bend, results in scattering features, which is
well confirmed by spin-lattice simulations.",1801.06945v3
2018-01-31,Finite-temperature dynamic structure factor of the spin-1 XXZ chain with single-ion anisotropy,"Improving matrix-product state techniques based on the purification of the
density matrix, we are able to accurately calculate the finite-temperature
dynamic response of the infinite spin-1 XXZ chain with single-ion anisotropy in
the Haldane, large-$D$ and antiferromagnetic phases. Distinct thermally
activated scattering processes make a significant contribution to the spectral
weight in all cases. In the Haldane phase intraband magnon scattering is
prominent, and the onsite anisotropy causes the magnon to split into singlet
and doublet branches. In the large-$D$ phase response, the intraband signal is
separated from an exciton-antiexciton continuum. In the antiferromagnetic
phase, holons are the lowest-lying excitations, with a gap that closes at the
transition to the Haldane state. At finite temperatures, scattering between
domain-wall excitations becomes especially important and strongly enhances the
spectral weight for momentum transfer $\pi$.",1801.10572v2
2018-08-08,Exact Correlation Functions in Conformal Fishnet Theory,"We compute exactly various 4-point correlation functions of shortest scalar
operators in bi-scalar planar four-dimensional ""fishnet"" CFT. We apply the OPE
to extract from these functions the exact expressions for the scaling
dimensions and the structure constants of all exchanged operators with an
arbitrary Lorentz spin. In particular, we determine the conformal data of the
simplest unprotected two-magnon operator analogous to the Konishi operator, as
well as of the one-magnon operator. We show that at weak coupling 4-point
correlation functions can be systematically expanded in terms of harmonic
polylogarithm functions and verify our results by explicit calculation of
Feynman graphs at a few orders in the coupling. At strong coupling we obtain
that the correlation functions exhibit the scaling behaviour typical for
semiclassical description hinting at the existence of the holographic dual.",1808.02688v1
2018-08-20,Gilbert damping of [Co/Pd]n/Py multilayer thin films,"Understanding the Gilbert damping in exchange-coupled multilayer materials is
particularly important to develop future fast switching spintronics devices.
Here, we report an experimental investigation of temperature-dependent Gilbert
damping in [Co/Pd]n/Py multilayer films of varying the number of Co/Pd
repetitions by ferromagnetic resonance. The results demonstrate that three
independent contributions to the Gilbert damping are identified, namely the
intrinsic Gilbert damping, the inhomogeneous linewidth broadening and the
two-magnon scattering contribution. Of particular interest, the two-magnon
scattering intensity increases as the enlargement of number repetitions of
Co/Pd due to the larger pinning effect at the interface between Py and the
Co/Pd layers. The Gilbert damping increases monotonically as the temperature
decreases from 300K to 50K. Our findings open the door to comprehend the
physical origin of the Gilbert damping in ultrathin exchange-coupled multilayer
films.",1808.06515v2
2018-08-30,Coexisting spinons and magnons in frustrated zigzag spin-1/2 chain compound $β$-TeVO$_4$,"We investigate magnetic excitations in the frustrated zigzag spin-1/2 chain
compound $\beta$-TeVO$_4$ by inelastic neutron scattering. In the magnetically
ordered ground state, the excitation spectrum exhibits coexisting magnon
dispersion, characteristic of long-range magnetic order, and a spinon-like
continuum that prevails above 2 meV, indicating the dominance of intrachain
interactions. Combining linear-spin-wave-theory and pre-calculated
spinon-continuum results, we reproduce the experimental spectrum. Our analysis
offers a minimal exchange-network model which determines dominant intrachain
interactions, their anisotropies and weak interchain interactions. The obtained
parameters explain the magnetic ordering vector and spin excitations in the
magnetic ground state.",1808.10174v1
2019-06-01,Coherent ac spin current transmission across an antiferromagnetic CoO insulator,"The recent discovery of spin-current transmission through antiferromagnetic
(AFM) insulating materials opens up unprecedented opportunities for fundamental
physics and spintronics applications. The great mystery currently surrounding
this topic is: how could THz AFM magnons mediate a GHz spin current? This
mis-match of frequencies becomes particularly critical for the case of coherent
ac spin-current, raising the fundamental question of whether a GHz ac
spin-current can ever keep its coherence inside an AFM insulator and so drive
the spin precession of another FM layer coherently? Utilizing element- and
time-resolved x-ray pump-probe measurements on Py/Ag/CoO/Ag/Fe75Co25/MgO(001)
heterostructures, we demonstrate that a coherent GHz ac spin current pumped by
the permalloy (Py) ferromagnetic resonance (FMR) can transmit coherently across
an antiferromagnetic CoO insulating layer to drive a coherent spin precession
of the FM Fe75Co25 layer. Further measurement results favor thermal magnons
rather than evanescent spin waves as the mediator of the coherent ac spin
current in CoO.",1906.00155v1
2019-06-04,Thickness dependence of spin Peltier effect visualized by thermal imaging technique,"Magnon propagation length in a ferrimagnetic insulator yttrium iron garnet
(YIG) has been investigated by measuring and analyzing the YIG-thickness t_YIG
dependence of the spin Peltier effect (SPE) in a Pt/YIG junction system. By
means of the lock-in thermography technique, we measured the spatial
distribution of the SPE-induced temperature modulation in the Pt/YIG system
with the t_YIG gradation, allowing us to obtain the accurate t_YIG dependence
of SPE with high t_YIG resolution. Based on the t_YIG dependence of SPE, we
verified the applicability of several phenomenological models to estimate the
magnon diffusion length in YIG.",1906.01560v3
2019-06-13,Magnetoelasticity of $\mathrm{Co_{25}}\mathrm{Fe_{75}}$ thin films,"We investigate the magnetoelastic properties of
$\mathrm{Co_{25}}\mathrm{Fe_{75}}$ and $\mathrm{Co_{10}}\mathrm{Fe_{90}}$ thin
films by measuring the mechanical properties of a doubly clamped string
resonator covered with multi-layer stacks containing these films. For the
magnetostrictive constants we find
$\lambda_{\mathrm{Co_{25}}\mathrm{Fe_{75}}}=(-20.68\pm0.25)\times10^{-6}$ and
$\lambda_{\mathrm{Co_{10}}\mathrm{Fe_{90}}}=(-9.80\pm0.12)\times10^{-6}$ at
room temperature. In stark contrast to the positive magnetostriction previously
found in bulk CoFe crystals. $\mathrm{Co_{25}}\mathrm{Fe_{75}}$ thin films
unite low damping and sizable magnetostriction and are thus a prime candidate
for micromechanical magnonic applications, such as sensors and hybrid
phonon-magnon systems.",1906.05543v1
2019-06-17,Ferromagnetism and superconductivity in twisted double bilayer graphene,"We present a theory of competing ferromagnetic and superconducting orders in
twisted double bilayer graphene (TDBG). In our theory, ferromagnetism is
induced by Coulomb repulsion, while superconductivity with intervalley
equal-spin pairing can be mediated by electron-acoustic phonon interactions. We
calculate the transition temperatures for ferromagnetism and superconductivity
as a function of moir\'e band filling factor, and find that superconducting
domes can appear on both the electron and hole sides of the ferromagnetic
insulator at half filling. We show that the ferromagnetic insulating gap has a
dome shape dependence on the layer potential difference, which provides an
explanation to the experimental observation that the ferromagnetic insulator
only develops over a finite range of external displacement field. We also
verify the stability of the half-filled ferromagnetic insulator against two
types of collective excitations, i.e., spin magnons and valley magnons.",1906.07302v2
2019-06-19,String theory on the Schrodinger pp-wave background,"We study string theory on the pp-wave geometry obtained by taking the Penrose
limit around a certain null geodesic of the non-supersymmetric Schrodinger
background. We solve for the spectrum of bosonic excitations and find
compelling agreement with the dispersion relation of the giant magnons in the
Schrodinger background obtained previously in arXiv:1712.03091. Inspired by the
pp-wave spectrum we conjecture an exact in the t'Hooft coupling dispersion
relation for the magnons in the original Schrodinger background. We show that
the pp-wave background admits exactly 16 Killing spinors. We use the explicit
form of the latter in order to derive the supersymmetry algebra of the
background which explicitly depends on the deformation parameter. Its bosonic
subalgebra is of the Newton-Hooke type.",1906.08269v2
2019-07-09,Quantum Acoustomechanics with a Micromagnet,"We theoretically show how to strongly couple the center-of-mass motion of a
micromagnet in a harmonic potential to one of its acoustic phononic modes. The
coupling is induced by a combination of an oscillating magnetic field gradient
and a static homogeneous magnetic field. The former parametrically couples the
center-of-mass motion to a magnonic mode while the latter tunes the magnonic
mode in resonance with a given acoustic phononic mode. The magnetic fields can
be adjusted to either cool the center-of-mass motion to the ground state, or to
enter into the strong quantum coupling regime. The center-of-mass can thus be
used to probe and manipulate an acoustic mode, thereby opening new
possibilities for out-of-equilibrium quantum nanophysics. Our results hold for
experimentally feasible parameters and apply to levitated micromagnets as well
as micromagnets deposited on a clamped nanomechanical oscillator.",1907.04039v2
2019-07-17,Majorana-Magnon Crossover by a Magnetic Field in the Kitaev Model: Continuous-Time Quantum Monte Carlo Study,"Kitaev quantum spin liquids host Majorana fermions via the fractionalization
of spins. In a magnetic field, the Majorana fermions were predicted to comprise
a topological state, which has attracted great attention by the discovery of
the half-quantized thermal Hall conductivity. Nevertheless, a reliable theory
remains elusive for the field effect, especially at finite temperature. Here we
present unbiased large-scale numerical results for the Kitaev model in a wide
range of magnetic field and temperature. We find that the unconventional
paramagnetic region showing fractional spin dynamics extends at finite
temperature, far beyond the field range where the topological state is expected
at zero temperature. Our results show the confinement-deconfinement behavior
between the fractional Majorana excitations and the conventional magnons.",1907.07299v2
2019-09-30,Magnonic Analogue of Edelstein Effect in Antiferromagnetic Insulators,"We investigate the nonequilibrium spin polarization due to a temperature
gradient in antiferromagnetic insulators, which is the magnonic analogue of the
inverse spin-galvanic effect of electrons. We derive a linear response theory
of a temperature-gradient-induced spin polarization for collinear and
noncollinear antiferromagnets, which comprises both extrinsic and intrinsic
contributions. We apply our theory to several noncentrosymmetric
antiferromagnetic insulators, i.e., to a one-dimensional antiferromagnetic spin
chain, a single layer of kagome noncollinear antiferromagnet, e.g.,
$\text{KFe}_3(\text{OH})_6(\text{SO}_4)_2$, and a noncollinear breathing
pyrochlore antiferromagnet, e.g., LiGaCr$_4$O$_8$. The shapes of our
numerically evaluated response tensors agree with those implied by the magnetic
symmetry. Assuming a realistic temperature gradient of $10 \text{K}/\text{mm}$,
we find two-dimensional spin densities of up to $\sim 10^6\hbar/\text{cm}^2$
and three-dimensional bulk spin densities of up to $\sim
10^{14}\hbar/\text{cm}^3$, encouraging an experimental detection.",1910.00143v1
2019-10-16,Spin-Wave frequency division multiplexing in an yttrium iron garnet microstripe magnetized by inhomogeneous field,"Spin waves are promising candidates for information processing and
transmission in a broad frequency range. In the realization of magnonic
devices, the frequency depended division of the spin wave frequencies is a
critical function for parallel information processing. In this work, we
demonstrate a proof-of-concept spin-wave frequency division multiplexing method
by magnetizing a homogenous magnetic microstripe with an inhomogeneous field.
The symmetry breaking additional field is introduced by a permalloy stripe
simply placed in lateral proximity to the waveguide. Spin waves with different
frequencies can propagate independently, simultaneously and separately in space
along the shared waveguide. This work brings new potentials for parallel
information transmission and processing in magnonics.",1910.07136v1
2019-10-18,Universal Characterisation of Cavity--Magnon Polariton Coupling Strength Verified in Modifiable Microwave Cavity,"A comprehensive study of the frequency dependence of the photon-magnon
coupling for different magnetic samples is made possible with a tuneable
3D--printed re--entrant cavity. Strong coupling is achieved, with values
ranging between 20--140 MHz. The reworked theory, experimentally verified for
the first time here, enables coupling values to be calculated from simulations
alone, enabling future experiments with exotic cavity designs to be precisely
engineered, with no limitations on sample and cavity geometry. Finally, the
requirements of the deep strong coupling regime are shown to be achievable in
such experiments.",1910.08333v3
2019-10-31,Ultrafast magnetic dynamics in insulating YBa$_2$Cu$_3$O$_{6+x}$ revealed by time resolved two-magnon Raman Scattering,"Measurement and control of magnetic order and correlations in real time is a
rapidly developing scientific area relevant for magnetic memory and spintronics
[1,15]. In these experiments an ultra-short laser pulse (pump) is first
absorbed by excitations carrying electric dipole moment. These then give their
energy to the magnetic subsystem monitored by a time-resolved probe. A lot of
progress has been made in investigations of ferromagnets but antiferromagnets
are more challenging. Here we introduce time-resolved two-magnon Raman
scattering as a novel real time probe of magnetic correlations especially
well-suited for antiferromagnets. Its application to the antiferromagnetic
charge transfer insulator YBa$_2$Cu$_3$O$_{6+x}$ revealed rapid demagnetization
within 90fs of photoexcitation. The relaxation back to thermal equilibrium is
characterized by much slower timescales. We interpret these results in terms of
slow relaxation of the charge sector and rapid equilibration of the magnetic
sector to a prethermal state characterized by parameters that change slowly as
the charge sector relaxes.",1910.14585v3
2020-01-16,Evidence of standing spin-waves in a van der Waals magnetic material,"Spin-waves have been studied for data storage, communication and logic
circuits in the field of spintronics based on their potential to substitute
electrons. Recent discovery of magnetism in two-dimensional (2D) systems such
as monolayer CrI$_3$ and Cr$_2$Ge$_2$Te$_6$ has led to a renewed interest in
such applications of magnetism in the 2D limit. Here we present direct evidence
of standing spin-waves along with the uniform precessional resonance modes in
van der Waals magnetic material, CrCl$_3$. Our experiment is the first direct
observation of standing spin-wave modes, set up across a thickness of 20
$\mu$m, in a van der Waals material. We detect standing spin-waves in the
vicinity of both, optical and acoustic, branches of the antiferromagnetic
resonance. We also observe magnon-magnon coupling, softening of resonance modes
with temperature and extract the evolution of interlayer exchange field as a
function of temperature.",2001.05981v1
2020-01-24,Propagating magnetic droplet solitons as moveable nanoscale spin-wave sources with tunable direction of emission,"Magnetic droplets are strongly nonlinear and localized spin-wave solitons
that can be formed in current-driven nanocontacts. Here, we propose a simple
way to launch droplets in an inhomogeneous nanoscopic waveguide. We use the
drift motion of a droplet and show that in a system with broken translational
symmetry, the droplet acquires a linear momentum and propagates. We find that
the droplet velocity can be tuned via the strength of the break in symmetry and
the size of the nanocontact. In addition, we demonstrate that the launched
droplet can propagate up to several micrometers in a realistic system with
reasonable damping. Finally, we demonstrate how an annihilating droplet
delivers its momentum to a highly nonreciprocal spin-wave burst with a tunable
wave vector with nanometer wavelengths. Such a propagating droplet can be used
as a moveable spin-wave source in nanoscale magnonic networks. The presented
method enables full control of the spin-wave emission direction, which can
largely extend the freedom to design integrated magnonic circuits with a single
spin-wave source.",2001.08967v1
2020-01-29,Magnon-Phonon Quantum Correlation Thermometry,"A large fraction of quantum science and technology requires low-temperature
environments such as those afforded by dilution refrigerators. In these
cryogenic environments, accurate thermometry can be difficult to implement,
expensive, and often requires calibration to an external reference. Here, we
theoretically propose a primary thermometer based on measurement of a hybrid
system consisting of phonons coupled via a magnetostrictive interaction to
magnons. Thermometry is based on a cross-correlation measurement in which the
spectrum of back-action driven motion is used to scale the thermomechanical
motion, providing a direct measurement of the phonon temperature independent of
experimental parameters. Combined with a simple low-temperature compatible
microwave cavity read-out, this primary thermometer is expected to become a
popular thermometer for experiments below 1 K.",2001.11124v1
2020-01-30,An anomalous refraction of spin waves as a way to guide signals in curved magnonic multimode waveguides,"We present a method for efficient spin wave guiding within the magnonic
nanostructures. Our technique is based on the anomalous refraction in the
metamaterial flat slab. The gradual change of the material parameters
(saturation magnetization or magnetic anisotropy) across the slab allows
tilting the wavefronts of the transmitted spin waves and controlling the
refraction. Numerical studies of the spin wave refraction are preceded by the
analytical calculations of the phase shift acquired by the spin wave due to the
change of material parameters in a confined area. We demonstrate that our
findings can be used to guide the spin waves smoothly in curved waveguides,
even through sharp bends, without reflection and scattering between different
waveguide's modes, preserving the phase -- the quantity essential for wave
computing.",2001.11356v1
2020-02-03,Quantum walk versus classical wave: Distinguishing ground states of quantum magnets by spacetime dynamics,"We investigate the wavepacket spreading after a single spin flip in
prototypical two-dimensional ferromagnetic and antiferromagnetic quantum spin
systems. We find characteristic spatial magnon density profiles: While the
ferromagnet shows a square-shaped pattern reflecting the underlying lattice
structure, as exhibited by quantum walkers, the antiferromagnet shows a
circular-shaped pattern which hides the lattice structure and instead resembles
a classical wave pattern. We trace these fundamentally different behaviors back
to the distinctly different magnon energy-momentum dispersion relations and
also provide a real-space interpretation. Our findings point to new
opportunities for real-time, real-space imaging of quantum magnets both in
materials science and in quantum simulators.",2002.00812v3
2020-02-08,Higher-order exceptional points in all-magnetic structures,"Magnetometers with exceptional sensitivity are highly demanded in solving a
variety of physical and engineering problems, such as measuring Earth's weak
magnetic fields and prospecting mineral deposits and geological structures. It
has been shown that the non-Hermitian degeneracy at exceptional points (EPs)
can provide a new route for that purpose, because of the nonlinear response to
external perturbations. One recent work [H. Yang et al., Phys. Rev. Lett. 121,
197201 (2018)] has made the first step to realize the second-order magnonic EP
in ferromangetic bilayers respecting the parity-time symmetry. In this paper,
we generalize the idea to higher-order cases by considering ferromagnetic
trilayers consisting of a gain, a neutral, and a (balanced-)loss layer. We
observe both second- and third-order magnonic EPs by tuning the interlayer
coupling strength, the external magnetic field, and the gain-loss parameter. We
show that the magnetic sensitivity can be enhanced by $3$ orders of magnitude
comparing to the conventional magnetic tunneling junction based sensors. Our
results pave the way for studying high-order EPs in purely magnetic system and
for designing magnetic sensors with ultrahigh sensitivity.",2002.03085v1
2020-02-14,Spin-wave diode and circulator based on unidirectional coupling,"In magnonics, an emerging branch of wave physics characterized by low-energy
consumption, it is highly desirable to realize circuit elements within the
scope of spin-wave computing. Here, based on numerical simulations, we
demonstrate the functionality of the spin-wave diode and the circulator to
steer and manipulate spin waves over a wide range of frequency in the GHz
regime. They take advantage of the unidirectional coupling induced by the
interfacial Dzyaloshinskii-Moriya interaction to transfer the spin wave between
thin ferromagnetic layers in only one direction of propagation. Using the
multilayered structure consisting of Py and Co in direct contact with Pt, we
obtain sub-micrometer-size devices of high efficiency. In the diode, the power
loss ratio between forward and reverse direction reaches 22 dB, while in the
four-port circulator, the efficiency exceeds 13 dB. Thus, our work contributes
to the emerging branch of energy-efficient magnonic logic devices, where,
thanks to short wavelength of spin waves, it is possible to realize nanoscale
devices.",2002.06096v1
2020-02-26,Control of spin dynamics in artificial honeycomb spin-ice-based nanodisks,"We report the experimental and theoretical characterization of the
angular-dependent spin dynamics in arrays of ferromagnetic nanodisks arranged
on a honeycomb lattice. The magnetic field and microwave frequency dependence,
measured by broadband ferromagnetic resonance, reveal a rich spectrum of modes
that is strongly affected by the microstate of the network. Based on symmetry
arguments with respect to the external field, we show that certain parts of the
ferromagnetic network contribute to the detected signal. A comparison of the
experimental data with micromagnetic simulations reveals that different
subsections of the lattice predominantly contribute to the high-frequency
response of the array. This is confirmed by optical characterizations using
microfocused Brillouin light scattering. Furthermore, we find indications that
nucleation and annihilation of vortex-like magnetization configurations in the
low-field range affect the dynamics, which is different from clusters of
ferromagnetic nanoellipses. Our work opens up new perspectives for designing
magnonic devices that combine geometric frustration in gyrotropic vortex
crystals at low frequencies with magnonic crystals at high frequencies.",2002.11694v1
2020-03-06,Spin-accumulation induced magnetic texture in a metal-insulator bilayer,"We consider the influence of a spin accumulation in a normal metal on the
magnetic statics and dynamics in an adjacent magnetic insulator. In particular,
we focus on arbitary angles between the spin accumulation and the easy-axis of
the magnetic insulator. Based on Landau-Lifshitz-Gilbert phenomenology
supplemented with magnetoelectronic circuit theory, we find that the magnetic
texture twists into a stable configuration that turns out to be described by a
virtual, or image, domain wall configuration, i.e., a domain wall outside the
ferromagnet. We show that even when the spin accumulation is perpendicular to
the anisotropy axis, the magnetic texture develops a component parallel to the
spin accumulation for sufficiently large spin bias. The emergence of this
parallel component gives rise to threshold behavior in the spin Hall
magnetoresistance and nonlocal magnon transport. This threshold can be used to
design novel spintronic and magnonic devices that can be operated without
external magnetic fields.",2003.03226v1
2020-03-12,Double Yang-Baxter deformation of spinning strings,"We study the reduction of classical strings rotating in the deformed
three-sphere truncation of the double Yang-Baxter deformation of the
$\hbox{AdS}_3 \times \hbox{S}^3 \times \hbox{T}^4$ background to an integrable
mechanical model. The use of the generalized spinning-string ansatz leads to an
integrable deformation of the Neumann-Rosochatius system. Integrability of this
system follows from the fact that the usual constraints for the Uhlenbeck
constants apply to any deformation that respects the isometric coordinates of
the three-sphere. We construct solutions to the system in terms of the
underlying ellipsoidal coordinate. The solutions depend on the domain of the
deformation parameters and the reality conditions of the roots of a fourth
order polynomial. We obtain constant-radii, giant-magnon and trigonometric
solutions when the roots degenerate, and analyze the possible solutions in the
undeformed limit. In the case where the deformation parameters are purely
imaginary and the polynomial involves two complex-conjugated roots, we find a
new class of solutions. The new class is connected with twofold giant-magnon
solutions in the degenerate limit of infinite period.",2003.05724v2
2020-03-13,AC Josephson effect between two superfluid time crystals,"Quantum time crystals are systems characterised by spontaneously emerging
periodic order in the time domain. A range of such phases has been reported.
The concept has even been discussed in popular literature, and deservedly so:
while the first speculation on a phase of broken time translation symmetry did
not use the name ""time crystal"", it was later adopted from 1980's popular
culture. For the physics community, however, the ultimate qualification of a
new concept is its ability to provide predictions and insight. Confirming that
time crystals manifest the basic dynamics of quantum mechanics is a necessary
step in that direction. We study two adjacent quantum time crystals
experimentally. The time crystals, realised by two magnon condensates in
superfluid $^3$He-B, exchange magnons leading to opposite-phase oscillations in
their populations -- AC Josephson effect -- while the defining periodic motion
remains phase coherent throughout the experiment.",2003.06313v1
2020-03-25,Magnon Trap by Chiral Spin Pumping,"Chiral spin pumping is the generation of a unidirectional spin current in
half of ferromagnetic films or conductors by dynamic dipolar stray fields from
close-by nanomagnets. We formulate a general theory of long-range chiral
interactions between magnets mediated by unidirectional traveling waves, e.g.,
spin waves in a magnetic film or microwaves in a waveguide. The traveling waves
emitted by an excited magnet can be perfectly trapped by a second, initially
passive, magnet by a dynamical interference effect. When both magnets are
excited by a uniform microwave, the chiral interaction between them creates a
large imbalance in their magnon numbers.",2003.11581v2
2020-04-13,Impact of the surface phase transition on magnon and phonon excitations in BiFeO3 nanoparticles,"We have performed Raman scattering measurements on BiFeO3 nanoparticles and
studied both magnetic and lattice modes. We reveal strong anomalies between 140
K and 200 K in the frequency of magnon and E(LO1), E(TO1) and A1(LO1) phonon
modes. These anomalies are related to a surface expansion and are enhanced for
nanoparticle sizes approaching the spin cycloidal length. These observations
point out the strong interplay between the surface, the lattice, and the
magnetism for sizes of BiFeO3 nanoparticles close to the cycloid periodicity.",2004.06028v1
2020-04-20,Ultra Thin Films of Yttrium Iron Garnet with Very Low Damping: A Review,"Thin Yttrium Iron Garnet (YIG) is a promising material for integrated
magnonics. In order to introduce YIG into nanofabrication processes it is
necessary to fabricate very thin YIG films with a thickness well below 100 nm
while retaining the extraordinary magnetic properties of the material,
especially its long magnon lifetime and spin wave propagation length. Here, we
give a brief introduction into the topic and we review and discuss the various
results published over the last decade in this area. Especially for ultrathin
films it turns out that pulsed layer deposition and sputtering are the most
promising candidates. In addition, we discuss the application of room
temperature deposition and annealing for lift-off based nanopatterning and the
properties of nanostructures obtained by this method over the past years.",2004.09094v1
2020-05-13,Waveguide cavity optomagnonics for broadband multimode microwave-to-optics conversion,"Cavity optomagnonics has emerged as a promising platform for studying
coherent photon-spin interactions as well as tunable microwave-to-optical
conversion. However, current implementation of cavity optomagnonics in
ferrimagnetic crystals remains orders of magnitude larger in volume than
state-of-the-art cavity optomechanical devices, resulting in very limited
magneto-optical interaction strength. Here, we demonstrate a cavity
optomagnonic device based on integrated waveguides and its application for
microwave-to-optical conversion. By designing a ferrimagnetic rib waveguide to
support multiple magnon modes with maximal mode overlap to the optical field,
we realize a high magneto-optical cooperativity which is three orders of
magnitude higher compared to previous records obtained on polished YIG spheres.
Furthermore, we achieve tunable conversion of microwave photons at around 8.45
GHz to 1550 nm light with a broad conversion bandwidth as large as 16.1 MHz.
The unique features of the system point to novel applications at the crossroad
between quantum optics and magnonics.",2005.06429v1
2020-05-15,Multiple magnon modes in spin-1/2 Heisenberg antiferromagnet on simple square lattice in strong magnetic field,"We discuss spin-$\frac12$ Heisenberg antiferromagnet on simple square lattice
in magnetic field $H$ using recently proposed bond-operator technique. It is
well known that magnetically ordered phases of quantum magnets are well
described at least qualitatively by the conventional spin-wave theory that only
introduces quantum corrections into the classical solution of the problem. We
observe that quantum fluctuations change drastically dynamical properties of
the considered model at $H$ close to its saturation value: the dynamical
structure factor shows anomalies corresponding to Green's function poles which
have no counterparts in the spin-wave theory. That is, quantum fluctuations
produce multiple short-wavelength magnon modes not changing qualitatively the
long-wavelength spin dynamics. Our results are in agreement with previous
quantum Monte-Carlo simulations and exact diagonalization of finite clusters.",2005.07537v3
2020-05-18,From deconfined spinons to coherent magnons in an antiferromagnetic Heisenberg chain with long range interactions,"We study the spectrum and the nature of the excitations of an
antiferromagnetic (AFM) Heisenberg chain with staggered long range
interactions, both numerically using the time-dependent density matrix
renormalization group (tDMRG) method and by means of a multi-spinon
approximation that qualitatively explains its main features. The unfrustrated
long-range nature of the exchange effectively increases the dimensionality of
the system and the chain is able to undergo true symmetry breaking and develop
long range order, transitioning from a gapless spin liquid to a gapless ordered
AFM phase. We calculated the momentum resolved spin dynamical structure factor
and found that for weakly decaying interactions the emergence of N\'eel order
can be associated to the formation of bound states of spinons that become
coherent magnons. The quasiparticle band leaks out from the two-spinon
continuum that is pushed up to higher energies. Our physical picture is also
supported by an analysis of the behavior of the excitations in real-time.",2005.09086v2
2020-05-20,Quantum Geometry and Stability of Moiré Flatband Ferromagnetism,"Several moir\'e systems created by various twisted bilayers have manifested
magnetism under flatband conditions leading to enhanced interaction effects. We
theoretically study stability of moir\'e flatband ferromagnetism against
collective excitations, with a focus on the effects of Bloch band quantum
geometry. The spin magnon spectrum is calculated using different approaches,
including Bethe-Salpeter equation, single mode approximation, and an analytical
theory. One of our main results is an analytical expression for the spin
stiffness in terms of the Coulomb interaction potential, the Berry curvatures,
and the quantum metric tensor, where the last two quantities characterize the
quantum geometry of moir\'e bands. This analytical theory shows that Berry
curvatures play an important role in stiffening the spin magnons. Furthermore,
we construct an effective field theory for the magnetization fluctuations, and
show explicitly that skyrmion excitations bind an integer number of electrons
that is proportional to the Bloch band Chern number and the skyrmion winding
number.",2005.10254v2
2020-07-06,"Triplons, Magnons, and Spinons in a Single Quantum Spin System: SeCuO3","Quantum spin systems exhibit an enormous range of collective excitations, but
their spin waves, gapped triplons, fractional spinons, or yet other modes are
generally held to be mutually exclusive. Here we show by neutron spectroscopy
on SeCuO$_3$ that magnons, triplons, and spinons are present simultaneously. We
demonstrate that this is a consequence of a structure consisting of two coupled
subsystems and identify all the interactions of a minimal magnetic model. Our
results serve qualitatively to open the field of multi-excitation spin systems
and quantitatively to constrain the complete theoretical description of one
member of this class of materials.",2007.02972v2
2020-07-15,Local Probes for Charge-Neutral Edge States in Two-Dimensional Quantum Magnets,"The bulk-boundary correspondence is a defining feature of topological states
of matter. However, for quantum magnets such as spin liquids or topological
magnon insulators a direct observation of topological surface states has proven
challenging because of the charge-neutral character of the excitations. Here we
propose spin-polarized scanning tunneling microscopy as a spin-sensitive local
probe to provide direct information about charge neutral topological edge
states. We show how their signatures, imprinted in the local structure factor,
can be extracted by specifically employing the strengths of existing
technologies. As our main example, we determine the dynamical spin correlations
of the Kitaev honeycomb model with open boundaries. We show that by contrasting
conductance measurements of bulk and edge locations, one can extract direct
signatures of the existence of fractionalized excitations and non-trivial
topology. The broad applicability of this approach is corroborated by a second
example of a kagome topological magnon insulator.",2007.07912v1
2020-07-20,"Topology of many-body edge and extended quantum states in an open spin chain: 1/3--plateau, Kosterlitz-Thouless transition, and Luttinger liquid","Quantum many-body edge and extended magnon excitations from the 1/3 --
plateau of the anisotropic Heisenberg model on an open AB$_2$ chain in a
magnetic field $h$ are unveiled using the density matrix renormalization group
and exact diagonalization. By tuning both the anisotropy and $h$ in the rich
phase diagram, the edge states penetrate in the bulk, whose gap closes in a
symmetry-protected topological Kosterlitz-Thouless transition. Also, we witness
the squeezed chain effect, the breaking of the edge states degeneracy, and a
topological change of the excitations from gapped magnons with quadratic
long-wavelength dispersion to a linear spinon dispersion in the Luttinger
liquid gapless phase as the anisotropy $\lambda$ approaches the critical point
from the $\lambda>0$ side of the phase diagram.",2007.10414v1
2020-07-22,Nuclear Magnetic Relaxation Time near Compensation Temperature in Ferrimagnetic Insulator,"The nuclear magnetic relaxation time $T_1$ in ferrimagnetic insulators is
calculated by a Raman process of hyperfine interaction with a meanfield
approximation. It is found that the 1/$T_1$ on one site rapidly increases near
the compensation temperature $T_0$, whereas that on another site does not
increase up to Curie temperature $T_c$. This is due to that the band width of
soft magnon becomes comparable to $T_0$. The increasing behavior of 1/$T_1$
below $T_c$ is found also in another type ferrimagnet, which shows hump
structure in the temperature dependence of magnetization instead of
compensation. Also in this case, we find the rapid increase of 1/$T_1$ below
$T_c$, even though the magnetization does not show the compensation. Such a
coexistence of soft and hard magnons will lead to remarkable properties of
ferrimagnet.",2007.11214v3
2020-08-05,Torque equilibrium spin wave theory of Raman scattering in an anisotropic triangular lattice antiferromagnet with Dzyaloshinskii-Moriya interaction,"We apply torque equilibrium spin wave theory (TESWT) to investigate an
anisotropic XXZ antiferromagnetic model with Dzyaloshinskii-Moriya (DM)
interaction in a triangular lattice. Considering the quasiparticle vacuum as
our reference, we provide an accurate analysis of the non-collinear ground
state of a frustrated triangular lattice magnet using the TESWT formalism. We
elucidate the effects of quantum fluctuations on the ordering wave vector based
on model system parameters. We study the single magnon dispersion, the
two-magnon continuum using the spectral function, and the Raman spectrum of
bimagnon and trimagnon excitations. We present our results for the $HH, VV$,
and the $HV$ polarization Raman geometry dependence of the bimagnon and the
trimagnon excitation spectrum where $H (V)$ represents horizontal (vertical)
polarization. Our calculations show that both the $HH$ and the $HV$
polarization spectrum can be used to determine the degree of anisotropy of our
system. We calculate the Raman spectra of Ba$_3$CoSb$_2$O$_9$ and
Cs$_2$CuCl$_4$.",2008.02287v2
2020-08-10,Entanglement enhanced and one-way steering in PT -symmetric cavity magnomechanics,"We study creation of entanglement and quantum steering in a parity-time- (PT
-) symmetric cavity magnomechanical system. There is magnetic dipole
interaction between the cavity and photon-magnon, and there is also
magnetostrictive interaction which is induced by the phononmagnon coupling in
this system. By introducing blue-detuned driving microwave field to the system,
the bipartite entanglement of the system with PT -symmetry is significantly
enhanced versus the case in the conventional cavity magnomechanical systems
(loss-loss systems). Moreover, the one-way quantum steering between
magnon-phonon and photon-phonon modes can be obtained in the unbroken-PT
-symmetric regime. The boundary of stability is demonstrated and this show that
the steady-state solutions are more stable in the gain and loss systems. This
work opens up a route to explore the characteristics of quantum entanglement
and steering in magnomechanical systems, which might have potential
applications in quantum state engineering and quantum information.",2008.03870v1
2020-09-30,Phase-modulated cavity magnon polaritons as a precise magnetic field probe,"We describe and operate a novel spin-magnetometer based on the phase
modulation of cavity magnon polaritons. In this scheme a rf magnetic field is
detected through the sidebands it induces on a pump, and the experimental
configuration allows for a negligible pump noise and a high frequency readout.
The demonstrator setup, based on a copper cavity coupled to an yttrium iron
garnet sphere hybrid system, reached a sensitivity of
$2.0\,\mathrm{pT/\sqrt{Hz}}$, evading the pump noise and matching the
theoretical previsions. An optimized setup can attain a rf magnetic field
sensitivity of about $8\,\mathrm{fT/\sqrt{Hz}}$ at room temperature. An orders
of magnitude improvement is expected at lower temperatures, making this
instrument one of the few magnetometers accessing the sub-fT limit. Due to its
natural applications, miniaturization and multiplexing are eventually
discussed.",2010.00093v3
2020-10-13,Antiphase Oscillations in the Time-Resolved Spin Structure Factor of a Photoexcited Mott Insulator,"Motivated by the recent development of time-resolved resonant-inelastic x-ray
scattering (TRRIXS) in photoexcited antiferromagnetic Mott insulators, we
numerically investigate momentum-dependent transient spin dynamics in a
half-filled Hubbard model on a square lattice. After turning off a pumping
photon pulse, the intensity of a dynamical spin structure factor temporally
oscillates with frequencies determined by the energy of two magnons in the
antiferromagnetic Mott insulator. We find an antiphase behavior in the
oscillations between two orthogonal momentum directions, parallel and
perpendicular to the electric field of a pump pulse. The phase difference comes
from the $B_{1g}$ channel of the two-magnon excitation. Observing the antiphase
oscillations will be a big challenge for TRRIXS experiments when their time
resolution will be improved by more than an order of magnitude.",2010.06151v2
2020-10-19,Controllable optical response and tunable sensing based on self interference in waveguide QED systems,"We study the self interference effect of a resonator coupled with a bent
waveguide at two separated ports. Such interference effects are shown to be
similar for the cases of standing-wave and traveling-wave resonators, while in
the system of two separated resonators indirectly coupled via a waveguide, the
coupling forms and the related interference effects depend on which kind of
resonators is chosen. Due to the self interference, controllable optical
responses including tunable linewidth and frequency shift, and optical dark
state can be achieved. Moreover, we consider a self-interference photon-magnon
hybrid model and show phase-dependent Fano-like line shapes which have
potential applications in frequency sensing. The photon-magnon hybridization
can not only enhance the sensitivity and provide tunable working region, but
also enables optical readout of the magnetic field strength in turn. The
results in this paper provide a deeper insight into the self interference
effect and its potential applications.",2010.09319v2
2020-10-19,Theory of the low-temperature longitudinal spin Seebeck effect,"Using a simplified microscopic model of coupled spin and lattice excitations
in a ferromagnetic insulator we evaluate the magnetic-field dependence of the
spin Seebeck effect at low temperatures. The model includes Heisenberg exchange
coupling, a harmonic lattice potential, and a pseudo-dipolar exchange
interaction. Our approach goes beyond previous work [Phys. Rev. B 98, 134421
(2018)] in that it does not rely on the a priori assumption of a fast
equilibration of the magnon and phonon distributions. Our theory shows that
singular features in the magnetic-field dependence of the spin Seebeck effect
at low temperatures observed by Kikkawa et al. [Phys. Rev. Lett. 117, 207203
(2016)] are independent of the relative strength of magnon-impurity and
phonon-impurity scattering.",2010.09571v2
2020-10-28,Strong coupling of quantized spin waves in ferromagnetic bilayers,"We formulate a strong-coupling theory for perpendicular standing spin waves
(PSSWs) in ferromagnetic bilayers with the interlayer exchange coupling (IEC).
Employing the Hoffmann boundary condition and the energy-flow continuity across
the interface, we show that the PSSWs are still quantized but with non-integral
quantum numbers, in sharp contrast to that of a single layer. The magnon-magnon
coupling is characterized by the spectrum splitting which is linear with the
IEC in the weak-coupling region, but getting saturated in the strong-coupling
limit. Analytical predictions are verified by full micromagnetic simulations
with good agreement.",2010.14981v2
2020-11-09,The effect of confinement on thermally induced fluctuations in nanomagnets,"We study the magnetization dynamics in nanomagnets excited by stochastic
magnetic fields to mimic temperature in a micromagnetic framework. The effect
of confinement arising from the finite size of the structures is investigated,
and we visualise the spatial extension of the internal magnon modes.
Furthermore, we determine the temperature dependence of the magnon modes, and
focus specifically on the low frequency edge modes, which are found to display
fluctuations associated with switching between C- and S-states, thus posing an
energy barrier. We classify this fluctuating behaviour in three different
regimes, and calculate the associated energy barriers using the Arrhenius law.",2011.04494v3
2020-11-09,Spin-wave dynamics and symmetry breaking in an artificial spin ice,"Artificial spin ices are periodic arrangements of interacting nanomagnets
successfully used to investigate emergent phenomena in the presence of
geometric frustration. Recently, it has been shown that artificial spin ices
can be used as building blocks for creating functional materials, such as
magnonic crystals, and support a large number of programmable magnetic states.
We investigate the magnetization dynamics in a system exhibiting anisotropic
magnetostatic interactions owing to locally broken structural inversion
symmetry. We find a rich spin-wave spectrum and investigate its evolution in an
external magnetic field. We determine the evolution of individual modes, from
building blocks up to larger arrays, highlighting the role of symmetry breaking
in defining the mode profiles. Moreover, we demonstrate that the mode spectra
exhibit signatures of long-range interactions in the system. These results
contribute to the understanding of magnetization dynamics in spin ices beyond
the kagome and square ice geometries and are relevant for the realization of
reconfigurable magnonic crystals based on spin ices.",2011.04505v2
2020-11-12,"Edge states in quantum spin chains: the interplay among interaction, gradient magnetic field and Floquet engineering","We explore the edge defects induced by spin-spin interaction in a finite
paradigmatic Heisenberg spin chain. By introducing a gradient magnetic field
and a periodically-modulated spin-exchange strength, the resonance between
modulation frequency and magnetic field gradient can also induce asymmetric
defects at the edges, dubbed as Floquet-Wannier-Zeeman edge defects. The
interplay between these two types of edge defects allows us to manipulate the
magnon edge states from an isolated band into a continuum one. In the
high-frequency regime, we analytically derive effective models to interpret the
formation mechanisms for edge states by employing the multiscale perturbation
analysis. Our study offers new insights to understand and control the magnon
edge states governed by the interplay between edge defects induced by the
spin-spin interaction and the Floquet-Wannier-Zeeman manipulation.",2011.06164v4
2020-11-16,Magnonic Goos-Hanchen effect induced by one dimensional solitons,"The magnon spectral problem is solved in terms of the spectrum of a
diagonalizable operator for a generic class of magnetic states that includes
several types of domain walls and the chiral solitons of monoaxial helimagnets.
Focusing on the isolated solitons of monoaxial helimagnets, it is shown that
the spin waves scattered (reflected and transmitted) by the soliton suffer a
lateral displacement analogous to the Goos-Hanchen effect of optics. The
displacement is a fraction of the wavelength, but can be greatly enhanced by
using an array of well separated solitons. Contrarily to the Goos-Hanchen
effect recently studied in some magnetic systems, which takes place at
interfaces between different magnetic systems, the effect predicted here takes
place at the soliton position, what it is interesting from the point of view of
applications since solitons can be created at different places and moved across
the material. This kind of Goos-Hanchen effect is not particular of monoaxial
helimagnets, but it is generic of a class of magnetic states, including domain
walls in systems with interfacial Dzyaloshinskii-Moriya interaction.",2011.07886v1
2020-11-17,Brillouin-Mandelstam Light Scattering Spectroscopy: Applications in Phononics and Spintronics,"Recent years witnessed much broader use of Brillouin inelastic light
scattering spectroscopy for the investigation of phonons and magnons in novel
materials, nanostructures, and devices. Driven by developments in
instrumentation and the strong need for accurate knowledge of energies of
elemental excitations, the Brillouin - Mandelstam spectroscopy is rapidly
becoming an essential technique, complementary to the Raman inelastic light
scattering spectroscopy. We provide an overview of recent progress in the
Brillouin light scattering technique, focusing on the use of this photonic
method for the investigation of confined acoustic phonons, phononic
metamaterials, magnon propagation and scattering. The Review emphasizes
emerging applications of the Brillouin - Mandelstam spectroscopy for phonon
engineered structures and spintronic devices and concludes with a perspective
for future directions.",2011.08352v1
2021-01-13,Fast and robust magnon transport in a spin chain,"A protocol for fast and robust magnon transport in a one-dimensional spin
chain is devised. Employing an approximate mapping between the chain and a
single harmonically trapped particle, we exploit the known analytic control
protocols for the latter and adopt them to achieve fast, high-fidelity
transport in the chain. We compare the performance with finite time adiabatic
protocols, establishing that the designed scheme allows for significantly
faster and more stable transport. Furthermore, we show that a sharp transition
exists between regions in which the protocol is effective and when it breaks
down, giving rise to a heuristic speed limit for the process.",2101.04995v2
2021-01-25,Nonreciprocal Transmission and Entanglement in a cavity-magnomechanical system,"Quantum entanglement, a key element for quantum information is generated with
a cavity-magnomechanical system. It comprises of two microwave cavities, a
magnon mode and a vibrational mode, and the last two elements come from a YIG
sphere trapped in the second cavity. The two microwave cavities are connected
by a superconducting transmission line, resulting in a linear coupling between
them. The magnon mode is driven by a strong microwave field and coupled to
cavity photons via magnetic dipole interaction, and at the same time interacts
with phonons via magnetostrictive interaction. By breaking symmetry of the
configuration, we realize nonreciprocal photon transmission and one-way
bipartite quantum entanglement. By using current experimental parameters for
numerical simulation, it is hoped that our results may reveal a new strategy to
built quantum resources for the realization of noise-tolerant quantum
processors, chiral networks, and so on.",2101.09931v1
2021-03-23,Ultralow threshold bistability and generation of long-lived mode in a dissipatively coupled nonlinear system: application to magnonics,"The prospect of a system possessing two or more stable states for a given
excitation condition is of topical interest with applications in information
processing networks. In this work, we establish the remote transfer of
bistability from a nonlinear resource in a dissipatively coupled two-mode
system. As a clear advantage over coherently coupled settings, the dissipative
nature of interaction is found to support a lower pumping threshold for
bistable signals. For comparable parameters, the bistability threshold for
dissipatively coupled systems is lower by a factor of about five. The resulting
hysteresis can be studied spectroscopically by applying a probe field through
the waveguide and examining the polariton character of the transmitted field.
Our model is generic, apropos of an extensive set of quantum systems, and we
demonstrate our results in the context of magnonics where experimental interest
has flourished of late. As a consequence of dissipative coupling and the
nonlinearity, a long-lived mode emerges, which is responsible for heightened
transmission levels and pronounced sensitivity in signal propagation through
the fiber.",2103.12861v1
2021-04-11,Ultrafast Measurements of the Interfacial Spin Seebeck Effect in Au and Rare-Earth Iron Garnet Bilayers,"We investigate picosecond spin-currents across Au/iron-garnet interfaces in
response to ultrafast laser heating of the electrons in the Au film. In the
picoseconds after optical heating, interfacial spin currents occur due to an
interfacial temperature difference between electrons in the metal and magnons
in the insulator. We report measurements of this interfacial longitudinal spin
Seebeck effect between Au and rare-earth iron-garnet insulators, i.e. RE$_3$
Fe$_5$O$_{12}$, where RE is Y, Eu, Tb, Tm. We use time domain thermoreflectance
(TDTR) measurements to characterize the thermal response of the bilayer to
ultrafast optical heating. We use time-resolved magneto-optic Kerr effect
(TR-MOKE) measurements of the Au layer to measure the time-evolution of spin
accumulation in the Au film. We observe a spin Seebeck effect between Au/TmIG
that is three times larger than for an Au/YIG bilayer. The interfacial thermal
conductance between electrons in the Au and magnons in the TmIG layer is ~ 3
$\frac{MW}{m^2 K}$.",2104.05075v2
2021-04-13,Theory of spin-Hall magnetoresistance in the AC (terahertz) regime,"In bilayers consisting of a normal metal (N) with spin-orbit coupling and a
ferromagnet (F), the combination of the spin-Hall effect, the spin-transfer
torque, and the inverse spin-Hall effect gives a small correction to the
in-plane conductivity of N, which is referred to as spin-Hall magnetoresistance
(SMR). We here present a theory of the SMR and the associated off-diagonal
conductivity corrections for frequencies up to the terahertz regime. We show
that the SMR signal has pronounced singularities at the spin-wave frequencies
of F, which identifies it as a potential tool for all-electric spectroscopy of
magnon modes. A systematic change of the magnitude of the SMR at lower
frequencies is associated with the onset of a longitudinal magnonic
contribution to spin transport across the F-N interface.",2104.06518v2
2021-04-23,Electric polarization and nonlinear optical effects in noncentrosymmetric magnets,"We study electric polarization and nonlinear optical effects in spin systems
with broken inversion symmetry. We apply strong coupling expansion to the
underlying electronic Hamiltonians, and systematically derive expressions for
electric polarization in spin systems that are represented in terms of spin
operators. The magnon representation of the obtained electric polarization
operator allows us to compute linear and nonlinear optical responses by the
standard diagrammatic method. We apply our formalism to Heisenberg model with
alternating coupling constants and $J_1$-$J_2$ model with inversion symmetry
breaking. We demonstrate that these inversion broken spin systems support dc
current flow upon magnon excitations which arises from the shift current
mechanism.",2104.11418v2
2021-04-23,Spin-wave frequency combs,"We experimentally demonstrate the generation of spin-wave frequency combs
based on the nonlinear interaction of propagating spin waves in a
microstructured waveguide. By means of time and space-resolved Brillouin light
scattering spectroscopy, we show that the simultaneous excitation of spin waves
with different frequencies leads to a cascade of four-magnon scattering events
which ultimately results in well-defined frequency combs. Their spectral weight
can be tuned by the choice of amplitude and frequency of the input signals.
Furthermore, we introduce a model for stimulated four-magnon scattering which
describes the formation of spin-wave frequency combs in the frequency and time
domain.",2104.11491v2
2021-04-27,Exceptional field dependence of antiferromagnetic magnons in LiFePO$_4$,"Low-energy magnon excitations in magnetoelectric LiFePO$_4$ have been
investigated by high-frequency high-field electron spin resonance spectroscopy
in magnetic fields up to B = 58 T and frequencies up to f = 745 GHz. For
magnetic fields applied along the easy magnetic axis, the excitation gap
softens and vanishes at the spin-flop field of BSF = 32 T before hardening
again at higher fields. In addition, for B smaller than BSF we observe a
resonance mode assigned to excitations due to Dzyaloshinskii-Moriya
(DM)-interactions, thereby evidencing sizable DM interaction of approx 150
micro eV in LiFePO4. Both the magnetisation and the excitations up to high
magnetic fields are described in terms of a mean-field theory model which
extends recent zero field inelastic neutron scattering results. Our results
imply that magnetic interactions as well as magnetic anisotropy have a sizable
quadratic field dependence which we attribute to significant magnetostriction.",2104.13275v1
2021-05-06,Short spin waves detection by means of the magneto-optical intensity effect,"It is proposed a novel method of the spectrally selective detection of the
short spin waves (or magnons) by means of the transverse magneto-optical (MO)
intensity effect in transmission in the magnetoplasmonic nanostructure. The
method is based on the analysis of the MO effect spectrum versus the modulation
of the sample magnetization (i.e. spin wave) and related spatial symmetry
breaking in the magnetic layer. The spatial symmetry breaking leads to the
appearance of MO effect modulation at the normal incidence of light in the
spectral range of the optical states (SPP and waveguide modes) and the breaking
of the antisymmetry of the effect with respect to the sign of the incidence
angle of light. Besides it is revealed that the magnitude of the MO effect
varies with respect to the phase shift between the spin wave and the plasmonic
grating as a harmonic function with a period equal to the magnon wavelength.
All these facts allow to detect the spin waves of the certain wavelength
propagating in the nanostructure by measuring of the MO effect in the
nanostructure.",2105.02539v1
2021-05-07,Field-angle dependence of thermal Hall conductivity in magnetically ordered Kitaev-Heisenberg system,"We study magnetic excitations and thermal Hall effect on the
Kitaev-Heisenberg model under magnetic fields. By employing the spin-wave
theory for the magnetic orders realized in this model, we examine the
topological nature of the spin-wave dispersions and calculate the thermal Hall
conductivity. The comprehensive investigations on the field-angle dependence
clarify that the thermal Hall conductivity is sensitive to the spin ordered
pattern and excitation spectra of magnons; this quantity is enhanced by the
noncoplanar spin configurations and small magnon gap in the excitation
spectrum. On the other hand, we also find a common feature in the field-angle
dependence of the thermal Hall conductivity. It vanishes when the magnetic
field is on the planes spanned by the spin axes. We reveal that the behavior is
intrinsic to the Kitaev -Heisenberg model in an applied field and demonstrate
that the introduction of the off-diagonal spin interaction causes the
disappearance of the feature in the thermal Hall conductivity.",2105.03113v1
2021-05-26,Antiferromagnetic magnons and local anisotropy: dynamical mean-field study,"We present a dynamical mean-field study of antiferromagnetic magnons in one-,
two- and three-orbital Hubbard model of square and bcc cubic lattice at
intermediate coupling strength. Weinvestigate the effect of anisotropy
introduced by an external magnetic field or single-ion anisotropy.For the
latter we tune continuously between the easy-axis and easy-plane models. We
also analyzea model with spin-orbit coupling in cubic site-symmetry setting.
The ordered states as well as themagnetic excitations are sensitive to even a
small breaking ofSU(2)symmetry of the model andfollow the expectations of
spin-wave theory as well as general symmetry considerations.",2105.12468v2
2021-05-28,Anisotropic Magnon Spin Transport in Ultra-thin Spinel Ferrite Thin Films -- Evidence for Anisotropy in Exchange Stiffness,"We report measurements of magnon spin transport in a spinel ferrite,
magnesium aluminum ferrite $\mathrm{MgAl_{0.5}Fe_{1.5}O_4}$ (MAFO), which has a
substantial in-plane four-fold magnetic anisotropy. We observe spin diffusion
lengths $> 0.8$ $\mathrm{\mu m}$ at room temperature in 6 nm films, with spin
diffusion length 30% longer along the easy axes compared to the hard axes. The
sign of this difference is opposite to the effects just of anisotropy in the
magnetic energy for a uniform magnetic state. We suggest instead that
accounting for anisotropy in exchange stiffness is necessary to explain these
results.",2105.13943v1
2021-06-04,Neutron diffraction of field-induced magnon condensation in the spin-dimerized antiferromagnet Sr$_{3}$Cr$_{2}$O$_{8}$,"In this work, we investigate the evolution and settling of magnon
condensation in the spin-1/2 dimer system Sr$_{3}$Cr$_{2}$O$_{8}$ using a
combination of magnetostriction in pulsed fields and inelastic neutron
scattering in a continuous magnetic field. The magnetic structure in the
Bose-Einstein condensation (BEC) phase was probed by neutron diffraction in
pulsed magnetic fields up to 39~T. The magnetic structure in this phase was
confirmed to be an XY-antiferromagnetic structure validated by irreducible
representational analysis. The magnetic phase diagram as a function of an
applied magnetic field for this system is presented. Furthermore, zero-field
neutron diffraction results indicate that dimerization plays an important role
in stabilizing the low-temperature crystal structure.",2106.02360v1
2021-06-07,Voltage-control of damping constant in magnetic-insulator/topological-insulator bilayers,"The magnetic damping constant is a critical parameter for magnetization
dynamics and the efficiency of memory devices and magnon transport. Therefore,
its manipulation by electric fields is crucial in spintronics. Here, we
theoretically demonstrate the voltage-control of magnetic damping in ferro- and
ferrimagnetic-insulator (FI)/topological-insulator (TI) bilayers. Assuming a
capacitor-like setup, we formulate an effective dissipation torque induced by
spin-charge pumping at the FI/TI interface as a function of an applied voltage.
By using realistic material parameters, we find that the effective damping for
a FI with 10nm thickness can be tuned by one order of magnitude under the
voltage with 0.25V. Also, we provide perspectives on the voltage-induced
modulation of the magnon spin transport on proximity-coupled FIs.",2106.03332v1
2021-06-08,Spectral-fingerprinting: Microstate readout via remanence ferromagnetic resonance in artificial spin systems,"Artificial spin ices are magnetic metamaterials comprising
geometrically-tiled interacting nanomagnets. There is significant interest in
these systems for reconfigurable magnonics due to their vast microstate
landscape. Studies to-date have focused on the in-field GHz spin-wave response,
convoluting effects from applied field, nanofabrication-imperfections
('quenched disorder') and microstate-dependent dipolar field landscapes. Here,
we study artificial spin ices in pure and disordered microstates and evaluate
their zero-field spectra. Removing the applied field allows us to deconvolute
contributions to reversal dynamics and spin-wave spectra, directly measuring
the dipolar field landscape and quenched disorder. Mode-amplitude provides
population readout of nanomagnet magnetisation direction, and hence net
magnetisation as well as local vertex populations. We demonstrate
microstate-fingerprinting via distinct spectral-readout of three microstates
with identical (zero) magnetisation, supported by simulation. These results
establish remanence 'spectral-fingerprinting' as a rapid, scalable on-chip
readout of both magnetic state and nanoscale dipolar field texture, a critical
step in realising next-generation functional magnonic devices.",2106.04406v2
2021-06-24,Magnon Transport in the Presence of Antisymmetric Exchange in a Weak Antiferromagnet,"The Dzyaloshinskii-Moriya interaction (DMI) is at the heart of many modern
developments in the research field of spintronics. DMI is known to generate
noncollinear magnetic textures, and can take two forms in antiferromagnets:
homogeneous or inter-sublattice, leading to small, canted moments and
inhomogeneous or intra-sublattice, leading to formation of chiral structures.
In this work, we first determine the strength of the effective field created by
the DMI, using SQUID based magnetometry and transport measurements, in thin
films of the antiferromagnetic iron oxide hematite, $\alpha$-Fe$_2$O$_3$. We
demonstrate that DMI additionally introduces reconfigurability in the long
distance magnon transport in these films under different orientations of a
magnetic field. This arises as a hysteresis centred around the easy-axis
direction for an external field rotated in opposing directions whose width
decreases with increasing magnetic field as the Zeeman energy competes with the
effective field created by the DMI.",2106.12853v2
2021-06-24,Magnetic skyrmion generation by reflective spin-wave focusing,"We propose a method to generate magnetic skyrmions by focusing spin waves
totally reflected by a curved film edge. Based on the principle of identical
magnonic path length, we derive the edge contour that is parabolic and
frequency-independent. Micromagnetic simulations are performed to verify our
theoretical design. It is found that under proper conditions, magnetic droplet
first emerges near the focal point where the spin-wave intensity has been
significantly enhanced, and then converts to magnetic skyrmion accompanied by a
change of the topological charge. The phase diagram about the amplitude and
frequency of the driving field for skyrmion generation is obtained. Our finding
would be helpful for the designment of spintronic devices combing the advantage
of skyrmionics and magnonics.",2106.12876v1
2021-07-01,Novel elementary excitations in spin-1/2 antiferromagnets on the triangular lattice,"We discuss spin-$\frac12$ Heisenberg antiferromagnet on the triangular
lattice using the recently proposed bond-operator technique (BOT). We use the
variant of the BOT which takes into account all spin degrees of freedom in the
magnetic unit cell containing three spins. Apart from conventional magnons
known from the spin-wave theory (SWT), there are novel high-energy collective
excitations in the BOT which are built from high-energy excitations of the
magnetic unit cell. We obtain also another novel high-energy quasiparticle
which has no counterpart not only in the SWT but also in the harmonic
approximation of the BOT. All observed elementary excitations produce visible
anomalies in dynamical spin correlators. We show that quantum fluctuations
considerably change properties of conventional magnons predicted by the SWT.
The effect of a small easy-plane anisotropy is discussed. The anomalous spin
dynamics with multiple peaks in the dynamical structure factor is explained
that was observed recently experimentally in $\rm Ba_3CoSb_2O_9$ and which the
SWT could not describe even qualitatively.",2107.00256v5
2021-07-08,Nonequilibrium phase transition in a driven-dissipative quantum antiferromagnet,"A deeper theoretical understanding of driven-dissipative interacting systems
and their nonequilibrium phase transitions is essential both to advance our
fundamental physics understanding and to harness technological opportunities
arising from optically controlled quantum many-body states. This paper provides
a numerical study of dynamical phases and the transitions between them in the
nonequilibrium steady state of the prototypical two-dimensional Heisenberg
antiferromagnet with drive and dissipation. We demonstrate a nonthermal
transition that is characterized by a qualitative change in the magnon
distribution, from subthermal at low drive to a generalized Bose-Einstein form
including a nonvanishing condensate fraction at high drive. A finite-size
analysis reveals static and dynamical critical scaling at the transition, with
a discontinuous slope of the magnon number versus driving field strength and
critical slowing down at the transition point. Implications for experiments on
quantum materials and polariton condensates are discussed.",2107.03841v3
2021-07-10,The field-angle anisotropy of proximate Kitaev systems under an in-plane magnetic field,"We have investigated the field-angle behaviors of magnetic excitations under
an in-plane magnetic field for proximate Kitaev systems. By employing the exact
diagonalization method in conjunction with the linear spin wave theory, we have
demonstrated that the magnetic excitation gap in the polarized phase is
determined by the magnon excitation at $M$ points and has a strong anisotropy
with respect to the field direction in the vicinity of the critical field
limit. The specific heat from this magnon excitation bears qualitatively the
same anisotropic behaviors as expected one for the non-Abelian spin liquid
phase in the Kitaev model and experimentally observed one of the intermediate
phases in $\alpha$-RuCl$_3$.",2107.04785v2
2021-07-12,Nonlinear two-level dynamics of quantum time crystals,"A time crystal is a macroscopic quantum system in periodic motion in its
ground state, stable only if isolated from energy exchange with the
environment. For this reason, coupling separate time crystals is challenging,
and time crystals in a dynamic environment have yet not been studied. In our
experiments, two coupled time crystals made of spin-wave quasiparticles
(magnons) form a macroscopic two-level system. The two levels evolve in time as
determined intrinsically by a nonlinear feedback. Magnons move from the ground
level to the excited level driven by the Landau-Zener effect, combined with
Rabi population oscillations. We thus demonstrate how to arrange spontaneous
dynamics between interacting time crystals. Our experiments allow access to
every aspect and detail of the interaction in a single run of the experiment,
inviting technological exploitation-- potentially even at room temperature.",2107.05236v1
2021-07-19,Damped Dirac magnon in a metallic kagome antiferromagnet FeSn,"The kagome lattice is a fertile platform to explore topological excitations
with both Fermi-Dirac and Bose-Einstein statistics. While relativistic Dirac
Fermions and flat-bands have been discovered in the electronic structure of
kagome metals, the spin excitations have received less attention. Here we
report inelastic neutron scattering studies of the prototypical kagome magnetic
metal FeSn. The spectra display well-defined spin waves extending up to 120
meV. Above this energy, the spin waves become progressively broadened,
reflecting interactions with the Stoner continuum. Using linear spin wave
theory, we determine an effective spin Hamiltonian that reproduces the measured
dispersion. This analysis indicates that the Dirac magnon at the K-point
remarkably occurs on the brink of a region where well-defined spin waves become
unobservable. Our results emphasize the influential role of itinerant carriers
on the topological spin excitations of metallic kagome magnets.",2107.08915v1
2021-07-20,Quantitative Determination of the Confinement and Deconfinement of spinons in the anomalous spectra of Antiferromagnets via the Entanglement Entropy,"We introduce an entanglement entropy analysis to quantitatively identify the
confinement and deconfinement of the spinons in the spin excitations of quantum
magnets. Our proposal is implemented by the parton construction of a
honeycomb-lattice antiferromagnet exhibiting high-energy anomalous spectra. To
obtain the quasiparticles of spin excitations for entanglement entropy
calculations, we develop an effective Hamiltonian using the random phase
approximation. We elaborate quantitatively the deconfinement-to-confinement
transition of spinons in the anomalous spectra with the increase of the Hubbard
interaction, indicating the avoided fractionalization of magnons in the strong
interaction regime. Meanwhile, the Higgs mode at the {\Gamma}0 point is
fractionalized into four degenerate spinon pairs, although it appears as a
sharp well-defined peak in the spectra. Our work extends our understanding of
the deconfinement of the spinon and its coexistence with the magnon in quantum
magnets.",2107.09241v1
2021-07-29,Magnon band structure of skyrmion crystals and stereographic projection approach,"Using semiclassical method combined with stereographic projection approach,
we investigate the magnetic dynamics of the skyrmion crystal (SkX), formed in
planar ferromagnet with both Dzyaloshinskii-Moriya interaction and uniform
magnetic field. The topologically non-trivial ground state of SkX is described
in stereographic projection by the complex valued function with simple poles at
skyrmions' positions. We use the earlier proposed ansatz for this ground state
function in the form of the sum of individual skyrmions. The dynamics follows
from the second variation of the classical action. Numerical analysis yields
the magnon band structure of tight-binding form in accordance with previously
known results. There are two sets of bands, one set with a flat dispersion,
topologically trivial and rapidly evolving with magnetic field. Another set is
robust to magnetic field, characterized by pronounced dispersion and with the
Berry curvature which may be sign-reversal in the Brillouin zone. The developed
theory can be straightforwardly generalized for the analysis of magnetic
dynamics in topological spin structures of other types.",2107.14335v1
2021-08-03,Unusual spin pseudogap behavior in the spin web lattice Cu$_3$TeO$_6$ probed by $^{125}$Te nuclear magnetic resonance,"We present a $^{125}$Te nuclear magnetic resonance (NMR) study in the
three-dimensional spin web lattice Cu$_3$TeO$_6$, which harbors topological
magnons. The $^{125}$Te NMR spectra and the Knight shift $\mathcal{K}$ as a
function of temperature show a drastic change at $T_\text{S}\sim 40$ K much
lower than the N\'eel ordering temperature $T_\text{N}\sim 61$ K, providing
evidence for the first-order structural phase transition within the
magnetically ordered state. Most remarkably, the temperature dependence of the
spin-lattice relaxation rate $T_1^{-1}$ unravels spin-gap-like magnetic
excitations, which sharply sets in at $T^*\sim 75$ K, the temperature well
above $T_\text{N}$. The spin gap behavior may be understood by weakly
dispersive optical magnon branches of high-energy spin excitations originating
from the unique corner-sharing Cu hexagon spin-1/2 network with low
coordination number.",2108.01288v1
2021-08-05,Magnonic thermal transport using the quantum Boltzmann equation,"We present a formula for thermal transport in the bulk of Bose systems based
on the quantum Boltzmann equation (QBE). First, starting from the quantum
kinetic equation and using the Born approximation for impurity scattering, we
derive the QBE of Bose systems and provide a formula for thermal transport
subjected to a temperature gradient. Next, we apply the formula to magnons.
Assuming a relaxation time approximation and focusing on the linear response
regime, we show that the longitudinal thermal conductivity of the QBE exhibits
the different behavior from the conventional. The thermal conductivity of the
QBE reduces to the Drude-type in the limit of the quasiparticle approximation,
while not in the absence of the approximation. Finally, applying the
quasiparticle approximation to the QBE, we find that the correction to the
conventional Boltzmann equation is integrated as the self-energy into the
spectral function of the QBE, and this enhances the thermal conductivity. Thus
we shed light on the thermal transport property of the QBE beyond the
conventional.",2108.02875v1
2021-08-14,Laser-controlled real- and reciprocal-space topology in multiferroic insulators,"Magnetic materials in which it is possible to control the topology of their
magnetic order in real space or the topology of their magnetic excitations in
reciprocal space are highly sought-after as platforms for alternative data
storage and computing architectures. Here we show that multiferroic insulators,
owing to their magneto-electric coupling, offer a natural and advantageous way
to address these two different topologies using laser fields. We demonstrate
that via a delicate balance between the energy injection from a high-frequency
laser and dissipation, single skyrmions -- archetypical topological magnetic
textures -- can be set into motion with a velocity and propagation direction
that can be tuned by the laser field amplitude and polarization, respectively.
Moreover, we uncover an ultrafast Floquet magnonic topological phase transition
in a laser-driven skyrmion crystal and we propose a new diagnostic tool to
reveal it using the magnonic thermal Hall conductivity.",2108.06535v2
2021-08-16,Lattice-driven femtosecond magnon dynamics in $α$-MnTe,"The light-induced femtosecond dynamics of the sublattice magnetizations in
the antiferromagnetically ordered phase of the semiconductor $\alpha$-MnTe is
investigated theoretically as function of an external driving field. The
electromagnetic field is coupled to optical modes and the concomitant atomic
displacements modulate the Heisenberg exchange couplings. We derive the
equations of motion for the time-dependent sublattice magnetization in spin
wave theory and analyze the contributions from the driven magnon modes. The
antiferromagnetic order parameter exhibits coherent longitudinal oscillations
determined by the external driving frequency which decay due to dephasing.
Including a phenomenological dissipative term to mimic spin-lattice relaxation
processes leads to relaxation back to thermal equilibrium. We provide
approximate analytic solutions of the resulting differential equations which
allow us to understand the effect of the driving light pulse on the amplitude,
frequency, and lifetime of the coherent spin dynamics.",2108.07079v2
2021-09-13,Model of the low temperature magnetic phases of gadolinium gallium garnet,"The magnetic behaviour of gadolinium gallium garnet in an external magnetic
field at zero temperature is considered. For high fields a classical spin model
of the gadolinium ions predicts a spin configuration that is periodic at the
level of the smallest repeating unit cell. The quantum version of the model is
treated via a truncated Holstein-Primakoff transformation with axes defined by
the classical spin configuration, and the magnon excitation bands are
calculated. The model predicts a transition in the field range of $1.9$ --
$2.1\mathrm{T}$, sensitive to the direction of the applied field, which is
caused by one or more magnon modes becoming soft as the field is decreased. In
general the soft modes occur at incommensurate wavevectors and therefore break
the periodicity of the spin configuration below the transition. One exception
occurs when the field aligns with one of the principle crystal axes, in which
case periodicity of the spin configuration is found to be maintained on a
larger crystallographic cubic cell even below the transition. This simple case
is studied in more detail. Comparisons are drawn with existing experimental
data, and further experimental tests of the model are suggested.",2109.05827v1
2021-09-15,Type-II Weyl Excitation in Vortex Arrays,"Weyl-like magnon excitations in ordered magnets have attracted significant
recent attention. Despite of the tantalizing physics and application prospects,
the experimental observation of Weyl magnons is still challenging owing to
their extraordinarily high frequency that is not accessible to the microstrip
antenna technique. Here we predict gigahertz Weyl excitations in the collective
dynamics of dipolar-coupled magnetic vortices arranged in a three-dimensional
stacked honeycomb lattice. It is found that the inversion symmetry breaking
leads to the emergence of the type-II Weyl semimetal (WSM) state with tilted
dispersion. We derive the full phase diagram of the vortex arrays that support
WSMs with both single and double pairs of Weyl nodes, and the topological
insulator phase. We observe robust arc surface states in a dual-segment fashion
due to the tilted nature of type-II WSMs. Our findings uncover the
low-frequency WSM phase in magnetic texture based crystals that are
indispensable for future Weyltronic applications.",2109.07083v1
2021-09-23,Magnon Landau levels in the strained antiferromagnetic honeycomb nanoribbons,"The pseudo-magnetic field created by a non-uniform unaxial strain is
introduced into the antiferromagnetic honeycomb nanoribbons. The formation of
magnon pseudo-Landau levels, which appear from the upper end of the spectrum
and whose level spacings are proportional to the square root of the level
index, is revealed by the linear spin-wave theory. The antiferromagnetic order
is gradually weakened along the $y$-direction by the strain. At large enough
strength, the system is decoupled into isolated zigzag chains near the upper
boundary, and demonstrates one-dimensional magnetic property there. While the
quantum Monte Carlo simulations also predict such a transition, this exact
method gives a critical point deeper in the bulk. We also investigate the $XY$
antiferromagnetic honeycomb nanoribbons, and find similar pseudo-Landau levels
and antiferromagnetic evolution. Our results unveil the effect of a non-uniform
unaxial strain on the spin excitaions, and may be realized experimentally based
on two-dimensional quantum magnetic materials.",2109.11189v2
2021-09-28,"Spin wave propagation in uniform waveguide: effects, modulation and its application","With the advent of the post-Moore era, researches on beyond-Complementary
Metal Oxide Semiconductor (CMOS) approaches have been attracted more and more
attention. Magnonics, or spin wave is one of the most promising technology
beyond CMOS, which magnons-quanta for spin waves-process the information
analogous to electronic charges in electronics. Information transmission by
spin waves, which uses the frequency, amplitude and (or) phase to encode
information, has a great many of advantages such as extremely low energy loss
and wide-band frequency. Moreover, using the nonlinear characteristics of spin
waves for information transmission can increase the extra degree of freedom of
information. This review provides a tutorial overview over the effects of spin
wave propagation and recent research progress in uniform spin wave waveguide.
The propagation characteristics of spin waves in uniform waveguides and some
special propagation phenomena such as spin wave beam splitting and
self-focusing are described by combining experimental phenomena and theoretical
formulas. Furthermore, we summarize methods for modulating propagation of spin
wave in uniform waveguide, and comment on the advantages and limitations of
these methods. The review may promote the development of information
transmission technology based on spin waves.",2109.13464v1
2021-10-15,Frustrated magnetism of spin-1/2 Heisenberg diamond and octahedral chains as a statistical-mechanical monomer-dimer problem,"It is evidenced that effective lattice-gas models of hard-core monomers and
dimers afford a proper description of low-temperature features of spin-1/2
Heisenberg diamond and octahedral chains. Besides monomeric particles assigned
within the localized-magnon theory to bound one- and two-magnon eigenstates,
the effective monomer-dimer lattice-gas model additionally includes dimeric
particles assigned to a singlet-tetramer (singlet-hexamer) state as a
cornerstone of dimer-tetramer (tetramer-hexamer) ground state of a spin-1/2
Heisenberg diamond (octahedral) chain. A feasibility of the effective
description is confirmed through the exact diagonalization and
finite-temperature Lanczos methods. Both quantum spin chains display rich
ground-state phase diagrams including discontinuous as well as continuous
field-driven phase transitions, whereby the specific heat shows in vicinity of
the former phase transitions an extraordinary low-temperature peak coming from
a highly-degenerate manifold of low-lying excitations.",2110.07958v3
2021-11-18,Magnons and Spinons in $\mathrm{Ba}_2\mathrm{CoTeO}_6 $: A Composite System of Isolated Spin-$1/2$ Triangular Heisenberg-like and Frustrated Honeycomb Ising-like Antiferromagnets,"We report the neutron scattering results on magnetic orderings and
excitations in $\mathrm{Ba}_2\mathrm{CoTeO}_6$ composed of two almost isolated
subsystems A and B, which are described as an $S\,{=}\,1/2$ triangular
Heisenberg-like antiferromagnet and a frustrated honeycomb Ising-like
antiferromagnet, respectively. Stripy ordering of subsystem B was confirmed
below $T_{\rm N1}\,{=}\,12.0$ K, whereas sharp streaks were observed along
$(1/3, 1/3, L)$ and $(2/3, 2/3, L)$ at 0.3 K (${\ll}\,T_{\rm N2}\,{=}\,3.0$ K).
This indicates the two-dimensional nature of ordering in subsystem A. It was
found that the excitation spectra of both subsystems are well separated and
independent of each other. The excitation spectrum of subsystem A is composed
of two single-magnon branches with roton-like minima at the M point and a
clearly structured intense continuum, as similarly observed in
$\mathrm{Ba}_3\mathrm{CoSb}_2\mathrm{O}_9$, which is strongly indicative of
spinon excitations. Dispersion curves for subsystem B can be described by
linear spin wave theory within the third-neighbor exchange interaction.",2111.09558v1
2021-11-19,Terahertz-Light Driven Coupling of Antiferromagnetic Spins to Lattice,"Understanding spin-lattice coupling represents a key challenge in modern
condensed matter physics, with crucial importance and implications for
ultrafast and 2D-magnetism. The efficiency of angular momentum and energy
transfer between spins and the lattice imposes fundamental speed limits on the
ability to control spins in spintronics, magnonics and magnetic data storage.
We report on an efficient nonlinear mechanism of spin-lattice coupling driven
by THz light pulses. A nearly single-cycle THz pulse resonantly interacts with
a coherent magnonic state in the antiferromagnet CoF2 and excites the
Raman-active THz phonon. The results reveal the unique functionality of
antiferromagnets allowing ultrafast spin-lattice coupling using light.",2111.10186v3
2021-11-26,A modal approach to modelling spin wave scattering,"Efficient numerical methods are required for the design of optimised devices.
In magnonics, the primary computational tool is micromagnetic simulations,
which solve the Landau-Lifshitz equation discretised in time and space.
However, their computational cost is high, and the complexity of their output
hinders insight into the physics of the simulated system, especially in the
case of multimode propagating wave-based devices. We propose a finite-element
modal method allowing an efficient solution of the scattering problem for
dipole-exchange spin waves propagating perpendicularly to the magnetisation
direction. The method gives direct access to the scattering matrix of the whole
system and its components. We extend the formula for the power carried by a
magnetostatic mode in the Damon-Eshbach configuration to the case with
exchange, allowing the scattering coefficients to be normalised to represent
the fraction of the input power transferred to each output channel. We apply
the method to the analysis of spin-wave scattering on a basic functional block
of magnonic circuits, consisting of a resonator dynamically coupled to a thin
film. The results and the method are validated by comparison with micromagnetic
simulations.",2111.13448v2
2021-12-03,Quasiparticle metamorphosis in the random t-J model,"Motivated by the pseudogap-Fermi liquid transition in doped Mott insulators,
we examine the excitations of a $t$-$J$ model with random and all-to-all
hopping and exchange. The stability of quasiparticles such as spin-1/2
fermions, spin-1 magnons, and emergent Jordan-Wigner (JW) spinless fermions is
cast as a problem of localization in the many-body Hilbert space, which is
studied by the FEAST eigensolver algorithm. At low dopings, magnons and JW
fermions are better defined than spin-1/2 fermions, which are unstable. Upon
crossing a critical value of doping around $p_c$ = 1/3, their stabilities are
interchanged. Near the critical doping, these quasiparticles are all found to
be ill-defined. The critical point is thus associated with a localization
transition in the many-body Hilbert space",2112.01760v2
2021-12-08,Intertwining of lasing and superradiance under spintronic pumping,"We introduce a quantum optics platform featuring the minimal ingredients for
the description of a spintronically pumped magnon condensate, which we use to
promote driven-dissipative phase transitions in the context of spintronics. We
consider a Dicke model weakly coupled to an out-of-equilibrium bath with a
tunable spin accumulation. The latter is pumped incoherently in a fashion
reminiscent of experiments with magnet-metal heterostructures. The core of our
analysis is the emergence of a hybrid lasing-superradiant regime that does not
take place in an ordinary pumped Dicke spin ensemble, and which can be traced
back to the spintronics pumping scheme. We interpret the resultant
non-equilibrium phase diagram from both a quantum optics and a spintronics
standpoint, supplying a conceptual bridge between the two fields. The outreach
of our results concern dynamical control in magnon condensates and
frequency-dependent gain media in quantum optics.",2112.04509v2
2021-12-15,Spin Waves and Dirac Magnons in a Honeycomb Lattice Zig-zag Antiferromagnet BaNi2(AsO4)2,"The topological properties of massive and massless fermionic quasiparticles
have been intensively investigated over the past decade in topological
materials without magnetism. Recently, the bosonic analogs of such
quasiparticles arising from spin waves have been reported in the
two-dimensional (2D) honeycomb lattice ferromagnet/antiferromagnet and the 3D
antiferromagnet. Here we use time-of-flight inelastic neutron scattering to
study spin waves of the S = 1 honeycomb lattice antiferromagnet BaNi2(AsO4)2,
which has a zig-zag antiferromagnetic (AF) ground state identical to that of
the Kitaev quantum spin liquid candidate alpha-RuCl3. We determine the magnetic
exchange interactions in the zig-zag AF ordered phase, and show that spin waves
in BaNi2(AsO4)2 have symmetry-protected Dirac points inside the Brillouin zone
boundary. These results provide a microscopic understanding of the zig-zag AF
order and associated Dirac magnons in honeycomb lattice magnets, and are also
important for establishing the magnetic interactions in Kitaev quantum spin
liquid candidates.",2112.07915v1
2021-12-27,An Effective Field Theory of Magneto-Elasticity,"We utilize the coset construction to derive the effective field theory of
magnon-phonon interactions in (anti)-ferromagnetic and ferrimagnetic insulating
materials. The action is used to calculate the equations of motion which
generalize the Landau-Lifshitz and stress equations to allow for
magneto-acoustic couplings to all orders in the fields at lowest order in the
derivative expansion. We also include the symmetry breaking effects due to
Zeeman, and Dzyaloshinsky-Moriya interactions. This effective theory is a
toolbox for the study of magneto-elastic phenomena from first principles. As an
example we use this theory to calculate the leading order contribution to the
magnon decay width due to its the decay into phonons.",2112.13873v3
2022-01-17,Mode attraction in Floquet systems with memory: application to magnonics,"Level attraction is a type of mode hybridization in open systems where
instead of forming a hybridization gap, the energy spectrum of two modes
coalesce in a region bounded by exceptional points. We demonstrate that this
phenomenon can be realized in a Floquet system with memory, which appears in
describing linear excitations in a nonlinear driven system with a limit cycle.
Linear response of the system in this state is different from its response near
thermodynamic equilibrium. We develop a general formalism and provide an
example in the context of cavity magnonics, where we show that magnetic
excitations in systems driven far from the equilibrium may show level
attraction with cavity photons. Our approach works equally well for quantum and
semiclassical magnetic dynamics. The theory is formulated so that it can be
used in combination with micromagnetic simulations to explore a wide range of
experimentally interesting systems.",2201.06547v2
2022-01-20,Superfluid-Mott insulator quantum phase transition in a cavity optomagnonic system,"The emerging hybrid cavity optomagnonic system is a very promising quantum
information processing platform for its strong or ultrastrong photon-magnon
interaction on the scale of micrometers in the experiment. In this paper, the
superfluid-Mott insulator quantum phase transition in a two-dimensional cavity
optomagnonic array system has been studied based on this characteristic. The
analytical solution of the critical hopping rate is obtained by the mean field
approach, second perturbation theory and Landau second order phase transition
theory. The numerical results show that the increasing coupling strength and
the positive detunings of the photon and the magnon favor the coherence and
then the stable areas of Mott lobes are compressed correspondingly. Moreover,
the analytical results agree with the numerical ones when the total excitation
number is lower. Finally, an effective repulsive potential is constructed to
exhibit the corresponding mechanism. The results obtained here provide an
experimentally feasible scheme for characterizing the quantum phase transitions
in a cavity optomagnonic array system, which will offer valuable insight for
quantum simulations.",2201.08086v1
2022-01-22,Dissipative generation of significant amount of photon-phonon asymmetric steering in magnomechanical interfaces,"We propose an effective approach for generating significant amount of
entanglement and asymmetric steering between photon and phonon in a cavity
magnomechanical system which consists of a microwave cavity and a yttrium iron
garnet sphere. By driving the magnon mode of the yttrium iron garnet sphere
with blue-detuned microwave field, the magnon mode can be acted as an
engineered resevoir cools the Bogoliubov modes of microwave cavity mode and
mechanical mode via beam-splitter-like interaction. In this way, the microwave
cavity mode and mechanical mode are driven to two-mode squeezed states in the
stationary limit. In particular, strong two-way and one-way asymmetric quantum
steering between the photon and phonon modes can be obtained with even equal
dissipation. It is very different from the conventional proposal of asymmetric
quantum steering, where additional unbalanced losses or noises on the two
subsystems has been imposed. Our finding may be significant to expand our
understanding of the essential physics of asymmetric steering and extend the
potential application of the cavity spintronics to device-independent quantum
key distribution.",2201.08965v1
2022-01-27,Effect of vertex corrections on the enhancement of Gilbert damping in spin pumping into a two-dimensional electron gas,"We theoretically consider the effect of vertex correction on spin pumping
from a ferromagnetic insulator (FI) into a two-dimensional electron gas (2DEG)
in which the Rashba and Dresselhaus spin-orbit interactions coexist. The
Gilbert damping in the FI is enhanced by elastic spin-flipping or magnon
absorption. We show that the Gilbert damping due to elastic spin-flipping is
strongly enhanced by the vertex correction when the ratio of the two spin-orbit
interactions is near a special value at which the spin relaxation time diverges
while that due to magnon absorption shows only small modification. We also show
that the shift in the resonant frequency due to elastic spin-flipping is
strongly enhanced in a similar way as the Gilbert damping.",2201.11498v3
2022-01-30,Nonreciprocal Spin Waves Driven by Left-Hand Microwaves,"It is a conventional wisdom that a left-hand microwave cannot efficiently
excite the spin wave (SW) in ferromagnets, due to the constraint of angular
momentum conservation. In this work, we show that the left-hand microwave can
drive nonreciprocal SWs in the presence of a strong ellipticity-mismatch
between the microwave and precessing magnetization. A critical frequency is
predicted, at which the left-hand microwave cannot excite SWs. Away from it the
SW amplitude sensitively depends on the ellipticity of left-hand microwaves, in
sharp contrast to the case driven by right-hand ones. By tuning the microwave
frequency, we observe a switchable SW non-reciprocity in a ferromagnetic single
layer. A mode-dependent mutual demagnetizing factor is proposed to explain this
finding. Our work advances the understanding of the photon-magnon conversion,
and paves the way to designing diode-like functionalities in nano-scaled
magnonics.",2202.00780v3
2022-03-06,Elongated Skyrmion as Spin Torque Nano-Oscillator and Magnonic Waveguide,"Spin torque nano-oscillator has been extensively studied both theoretically
and experimentally in recent decades due to its potential applications in
future microwave communication technology and neuromorphic computing. In this
work, we present a skyrmion-based spin torque nano-oscillator driven by a
spatially uniform direct current, where the skyrmion is confined by two pinning
sites. Different from other skyrmion-based oscillators that arise from the
circular motion or the breathing mode of a skyrmion, the steady-state
oscillatory motions are produced by the periodic deformation of an elongated
skyrmion. Through micromagnetic simulations, we find that the oscillation
frequency depends on the driving current, the damping constant as well as the
characteristics of pinning sites. This nonlinear response to direct current
turns out to be universal and can also appear in the case of antiskyrmions,
skyrmioniums and domain walls. Furthermore, the elongated skyrmion possesses a
rectangle-like domain wall, which could also serve as a magnonic waveguide.
Utilizing the propagation of spin waves in this waveguide, we propose a device
design of logic gate and demonstrate its performance.",2203.02969v2
2022-03-15,Magnetic field-controlled lattice thermal conductivity in MnBi2Te4,"Phonon properties and the lattice thermal conductivity are not generally
understood to be sensitive to magnetic fields. Using an applied field to change
MnBi2Te4 between antiferromagnetic (AFM), canted (CAFM) and ferromagnetic (FM)
phases we discovered a new way to control the lattice thermal conductivity,
generating both a positive and a negative magnetic field dependence. We report
the field dependence of the thermal conductivity, k, in the in-plane direction
under an applied magnetic field along the cross-plane direction in MnBi2Te4
from 2K to 30K. k decreases with field in the AFM phase, saturates in the CAFM
phase, and increases with field in the FM phase. We explain this in terms of
the field-induced changes of the magnon gap which modifies which magnon-phonon
scattering processes are allowed by energy conservation. We also report
magneto-Seebeck coefficient, Nernst coefficient and thermal Hall data measured
in the same configuration. This finding may open a way to design magnetically
controlled heat switches.",2203.08032v2
2022-04-12,Stabilization and application of asymmetric Néel skyrmions in hybrid nanostructures,"Increasing amounts of information force the continuous improvement of
information storage and processing technologies, further device
miniaturization, and their efficiency increase. Magnetic skyrmions, topological
quasiparticles, and the smallest stable magnetic textures possess intriguing
properties and potential for data storage applications. Hybrid nanostructures
with elements of different magnetization orientations can offer additional
advantages for developing skyrmion-based spintronic and magnonic devices. We
show that an N\'eel-type skyrmion confined within a nanodot placed on top of a
ferromagnetic stripe produces a unique and compelling platform for exploring
mutual coupling between magnetization textures. The skyrmion induces an imprint
upon the stripe, which, in turn, asymmetrically squeezes the skyrmion in the
dot, increasing their size and the range of skyrmion stability for small values
of DMI, as well as introducing skyrmion bi-stability. At the end, we present a
proof-of-concept technique for unconstrained transport of a skyrmion along a
racetrack based on proposed hybrid systems. Our results demonstrate a hybrid
structure that is promising for applications in magnonics and spintronics.",2204.05620v1
2022-04-16,Accelerated Magnonic Motional Cooling with Deep Reinforcement Learning,"Achieving fast cooling of motional modes is a prerequisite for leveraging
such bosonic quanta for high-speed quantum information processing. In this
work, we address the aspect of reducing the time limit for cooling below that
constrained by the conventional sideband cooling techniques; and propose a
scheme to apply deep reinforcement learning (DRL) to achieve this. In
particular, we have shown how the scheme can be used effectively to accelerate
the dynamic motional cooling of a macroscopic magnonic sphere, and how it can
be uniformly extended for more complex systems, for example, a tripartite
opto-magno-mechanical system to obtain cooling of the motional mode below the
time bound of coherent cooling. While conventional sideband cooling methods do
not work beyond the well-known rotating wave approximation (RWA) regimes, our
proposed DRL scheme can be applied uniformly to regimes operating within and
beyond the RWA, and thus this offers a new and complete toolkit for rapid
control and generation of macroscopic quantum states for application in quantum
technologies.",2204.07710v1
2022-04-19,Role of shape anisotropy on thermal gradient-driven domain wall dynamics in magnetic nanowires,"We investigate the magnetic domain wall (DW) dynamics in uniaxial/biaxial
nanowires under a thermal gradient (TG). The findings reveal that the DW
propagates toward the hotter region in both nanowires. The main physics of such
observations is the magnonic angular momentum transfer to the DW. The hard
(shape) anisotropy exists in biaxial nanowire, which contributes an additional
torque, hence DW speed is larger than that in uniaxial nanowire. With lower
damping, the DW velocity is smaller and DW velocity increases with damping
which is opposite to usual expectation. To explain this, it is predicted that
there is a probability to form the standing spin-waves (which do not carry net
energy/momentum) together with travelling spin-waves if the propagation length
of thermally-generated spin-waves is larger than the nanowire length. For
larger-damping, DW decreases with damping since the magnon propagation length
decreases. Therefore, the above findings might be useful in realizing the
spintronic (racetrack memory) devices.",2204.09101v2
2022-05-02,Switching of the information backflow between a helical spin system and non-Markovian bath,"The dissipative dynamics of the spin chain coupled to the non-Markovian
magnonic reservoir was studied. The chirality of the chain is formed due to the
magnetoelectric coupling. We explored the sign of the trace distance derivative
and found the alternating positive/negative periods in system's time evolution.
The negative sign is associated with the flow of information from the system to
the bath and decrease in states distinguishability, while the positive sign is
related to the flow of the information in the opposite direction and increase
in distinguishability. We found the distinct effect of the applied electric and
magnetic fields. While the Dzyaloshinskii-Moriya interaction and external
electric field lead to reshuffling of the periods, the applied magnetic field
leads to the swift positive-negative transitions. Thus, in the helical quantum
rings coupled to the non-Markovian magnonic baths, it is possible to control
the directions of information flow through the external fields.",2205.00985v1
2022-05-17,"Spin Dynamics, Loop Formation and Cooperative Reversal in Artificial Quasicrystals with Tailored Exchange Coupling","Aperiodicity and un-conventional rotational symmetries allow quasicrystalline
structures to exhibit unprecedented physical and functional properties. In
magnetism, artificial ferromagnetic quasicrystals exhibited knee anomalies
suggesting reprogrammable magnetic properties via nonstochastic switching.
However, the decisive roles of short-range exchange and long-range dipolar
interactions have not yet been clarified for optimized reconfigurable
functionality. We report broadband spin-wave spectroscopy and X-ray
photoemission electron microscopy on different quasicrystal lattices consisting
of ferromagnetic Ni81Fe19 nanobars arranged on aperiodic Penrose and Ammann
tilings with different exchange and dipolar interactions. We imaged the
magnetic states of partially reversed quasicrystals and analyzed their
configurations in terms of the charge model, geometrical frustration and the
formation of flux-closure loops. Only the exchange-coupled lattices are found
to show aperiodicity-specific collective phenomena and non-stochastic
switching. Both, exchange and dipolarly coupled quasicrystals show magnonic
excitations with narrow linewidths in minor loop measurements. Thereby
reconfigurable functionalities in spintronics and magnonics become realistic.",2205.08224v1
2022-07-19,Discrete time crystal made of topological edge magnons,"We report the emergence of time-crystalline behavior in the \pi-Berry phase
protected edge states of a Heisenberg ferromagnet in the presence of an
external driving field. The magnon amplification due to the external field
spontaneously breaks the discrete time-translational symmetry, resulting in a
discrete time crystal with a period that is twice that of the applied EM field.
We discuss the nature and symmetry protection of the time crystalline edge
states and their stability against various perturbations that are expected in
real quantum magnets. We propose an experimental signature to unambiguously
detect the time crystalline behavior and identify two recently discovered
quasi-2D magnets as potential hosts. We present a first-of-its-kind realization
of time crystals at topological edge states, which can be generalized and
extrapolated to other bosonic quasi-particle systems that exhibit parametric
pumping and topological edge states.",2207.09077v2
2022-08-02,Finite-frequency spin conductance of a ferro-/ferrimagnetic-insulator|normal-metal interface,"The interface between a ferro-/ferrimagnetic insulator and a normal metal can
support spin currents polarized collinear with and perpendicular to the
magnetization direction. The flow of angular momentum perpendicular to the
magnetization direction (""transverse"" spin current) takes place via spin torque
and spin pumping. The flow of angular momentum collinear with the magnetization
(""longitudinal"" spin current) requires the excitation of magnons. In this
article we extend the existing theory of longitudinal spin transport [Bender
and Tserkovnyak, Phys. Rev. B 91, 140402(R) (2015)] in the zero-frequency
weak-coupling limit in two directions: We calculate the longitudinal spin
conductance non-perturbatively (but in the low-frequency limit) and at finite
frequency (but in the limit of low interface transparency). For the
paradigmatic spintronic material system YIG|Pt, we find that non-perturbative
effects lead to a longitudinal spin conductance that is ca. 40% smaller than
the perturbative limit, whereas finite-frequency corrections are relevant at
low temperatures < 100 K only, when only few magnon modes are thermally
occupied.",2208.01420v1
2022-08-15,Entanglement enhancement in cavity magnomechanics by an optical parametric amplifier,"We propose a method to enhance bipartite and tripartite entanglement in
cavity magnomechanics using an optical parametric amplifier (OPA). We analyze
this system and identified parametric regimes where different types of
entanglement are enhanced. We show that a proper choice of the phase of the
parametric amplifier leads to the enhancement of the bipartite entanglements.
Moreover, the tripartite entanglement is also significantly enhanced in the
presence of OPA. The OPA not only enhances the strength of entanglement but
also increases the domain of entanglement over a wider space of detunings as
compared to the system when no OPA is present. Similarly, the robustness of
entanglement against temperature is also enhanced. Another important
consequence of OPA is the fact that it relaxes the requirement of strong
magnon-phonon coupling to generate cavity-magnon entanglement which is
necessary for the case when OPA is not present. We believe that the presented
scheme is a step forward to realize robust quantum entanglement using current
technology.",2208.07207v1
2022-08-17,Competing Incommensurate Spin Fluctuations and Magnetic Excitations in Infinite-Layer Nickelate Superconductors,"The recently discovered infinite-layer nickelates show great promise in
helping to disentangle the various cooperative mechanisms responsible for
high-temperature superconductivity. However, lack of antiferromagnetic order in
the pristine nickelates presents a challenge for connecting the physics of the
cuprates and nickelates. Here, by using a quantum many-body Green's
function-based approach to treat the electronic and magnetic structures, we
unveil the presence of many two- and three-dimensional magnetic stripe
instabilities that are shown to persist across the phase diagram of LaNiO$_2$.
Our analysis indicates that the magnetic properties of the infinite-layer
nickelates are closer to those of the doped cuprates which host inhomogeneous
ground states rather than the undoped cuprates. The computed magnon spectrum in
LaNiO$_2$ is found to contain an admixture of contributions from localized and
itinerant carriers. The theoretically obtained magnon dispersion is in accord
with the results of the corresponding RIXS experiments. Our study gives insight
into the origin of inhomogeneity in the infinite-layer nickelates and their
relationship with the cuprates.",2208.08375v1
2022-08-23,Microwave-optics Entanglement via Cavity Optomagnomechanics,"Microwave-optics entanglement is a vital component for building hybrid
quantum networks. Here, a new mechanism for preparing stationary entanglement
between microwave and optical cavity fields in a cavity optomagnomechanical
system is proposed. It consists of a magnon mode in a ferrimagnetic crystal
that couples directly to a microwave cavity mode via the magnetic dipole
interaction, and indirectly to an optical cavity through the deformation
displacement of the crystal. The mechanical displacement is induced by the
magnetostrictive force and coupled to the optical cavity via radiation
pressure. Both the opto- and magnomechanical couplings are dispersive.
Magnon-phonon entanglement is created via magnomechanical parametric
down-conversion, which is further distributed to optical and microwave photons
via simultaneous optomechanical beamsplitter interaction and electromagnonic
state-swap interaction, yielding stationary microwave-optics entanglement. The
microwave-optics entanglement is robust against thermal noise, which will find
broad potential applications in quantum networks and quantum information
processing with hybrid quantum systems.",2208.10703v2
2022-09-05,Topological edge states in dipolar zig-zag stripes,"We study the magnon spectrum of stacked zig-zag chains of point magnetic
dipoles with an easy axis. The anisotropy due to the dipolar interactions and
the two-point basis of the zig-zag chain unit cell combine to give rise to
topologically non-trivial magnon bands in 2D zig-zag lattices. Adjusting the
distance between the two sublattice sites in the unit cell causes a band
touching, which triggers the exchange of the Chern numbers of volume bands
switching the sign of the thermal conductivity and the sense of motion of edges
modes in zig-zag stripes. We show that these topological features survive when
the range of the dipolar interactions is truncated up to the second nearest
neighbors.",2209.02019v1
2022-09-08,Terahertz Magnetic and Lattice Excitations in van der Waals Ferromagnet VI3,"We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT
calculations to shed light on the magnetic and lattice properties of VI3. The
structural transition at TS1 = 79 K is accompanied by a large splitting of
polar phonon modes. Below TS1, strong ferromagnetic fluctuations are observed.
The variations of phonon frequencies at 55 K induced by magnetoelastic coupling
enhanced by spin-orbit interaction indicate the proximity of long-range
ferromagnetic order. Below TC = 50 K, two Raman modes simultaneously appear and
show dramatic softening in the narrow interval around the temperature TS2 of
the second structural transition associated with the order-order magnetic phase
transition. Below TS2, a magnon in the THz range appears in Raman spectra. The
THz magnon observed in VI3 indicates the application potential of 2D van der
Waals ferromagnets in ultrafast THz spintronics, which has previously been
considered an exclusive domain of antiferromagnets.",2209.03725v2
2022-09-28,Spin triangle chain with ferromagnetic and antiferromagnetic interactions on the transition line,"We investigate spin-$\frac{1}{2}$ anisotropic model of a linear chain of
triangles with competing ferro- and antiferromagnetic interactions and
ferromagneic Heisenberg interactions between triangles. For a certain ratio
between interactions the one-magnon excitation band is dispersionless leading
to an existence of localized-magnon states which form macroscopically
degenerated ground states. The spectrum of excitations has a specific structure
depending on the value of the triangle-triangle interaction. Such specific
structure determines the low-temperature thermodynamics and, in particular, the
temperature dependence of a specific heat. In the limit of strong anisotropy of
interactions the spectrum has a multi-scale structure which consists of subsets
rank-ordered on small parameter. Each subset is responsible for the appearance
of the peak in the temperature dependence of the specific heat.",2209.14459v1
2022-10-03,Chirality-induced one-way quantum steering between two waveguide-mediated ferrimagnetic microspheres,"One-way quantum steering is of importance for quantum technologies, such as
secure quantum teleportation. In this paper, we study the generation of one-way
quantum steering between two distant yttrium iron garnet (YIG) microspheres in
chiral waveguide electromagonics. We consider that the magnon mode with the
Kerr nonlinearity in each YIG sphere is chirally coupled to left- and
right-propagating guided photons in the waveguide. We find that quantum
steering between the magnon modes is absent with non-chirality but is present
merely in the form of one way (i.e., one-way steering) when the chirality
occurs. The maximal achievable steering is obviously improved as the chirality
degree increases. We further find that when the waveguide's outputs are
subjected to continuous homodyne detection, the steering can be considerably
enhanced and asymmetric steering with strong entanglement can also be achieved
by tuning the chirality. Our study shows that chirality can be explored to
effectively realize one-way quantum steering. Compared to other studies on
achieving asymmetric steering via controlling intrinsic dissipation, e.g.
cavity loss rates, our scheme merely depends on the chirality enabled via
positioning the micromagnets in the waveguide and is continuously adjustable
and experimentally more feasible.",2210.00710v1
2022-10-12,Electrical Seebeck-Contrast Observation of Magnon Hall Effect in Topological Ferromagnet Lu$_2$V$_2$O$_7$/Heavy Metal Heterostructures,"The observation of the magnon Hall effect (MHE) has relied solely on the
challenging measurement of the thermal Hall conductivity. Here, we report a
highly sensitive electrical Seebeck-contrast method for the observation of MHE
in Lu$_2$V$_2$O$_7$/heavy metal heterostructures, that is highly desirable for
the exploration of new MHE materials and their applications. Using measuring
wires with very different Seebeck coefficients, we established a general method
that can separate contributions (e.g., MHE) that generates a lateral
temperature drop, from those [e.g., anomalous Nernst effect (ANE) and spin
Seebeck effect (SSE)] that generate a lateral electric field. We show that a
suitable heavy metal overlayer can eliminate the inherent ANE and SSE signals
from the semiconducting Lu$_2$V$_2$O$_7$. The MHE in Lu$_2$V$_2$O$_7$ is
quasi-isotropic among crystals with different orientations. In addition to the
previously reported transverse MHE under an in-plane temperature gradient, we
have uncovered longitudinal MHE under an out-of-plane temperature gradient.",2210.06606v2
2022-10-26,Nonreciprocal collective magnetostatic wave modes in geometrically asymmetric bilayer structure with nonmagnetic spacer,"Nonreciprocity, i.e. inequivalence in amplitudes and frequencies of spin
waves propagating in opposite directions, is a key property underlying
functionality in prospective magnonic devices. Here we demonstrate
experimentally and theoretically a simple approach to induce frequency
nonreciprocity in a magnetostatically coupled ferromagnetic bilayer structure
with a nonmagnetic spacer by its geometrical asymmetry. Using Brillouin light
scattering, we show the formation of two collective spin wave modes in
Fe$_{81}$Ga$_{19}$/Cu/Fe$_{81}$Ga$_{19}$ structure with different thicknesses
of ferromagnetic layers. Experimental reconstruction and theoretical modeling
of the dispersions of acoustic and optical collective spin wave modes reveal
that both possess nonreciprocity reaching several percent at the wavenumber of
$22~\cdot~10^4$ rad cm$^{-1}$. The analysis demonstrates that the shift of the
amplitudes of counter-propagating coupled modes towards either of the layers is
responsible for the nonreciprocity because of the pronounced dependence of spin
wave frequency on the layers thickness. The proposed approach enables the
design of multilayered ferromagnetic structures with a given spin wave
dispersion for magnonic logic gates.",2210.14882v2
2022-10-27,Spin-current driven Dzyaloshinskii-Moriya interaction in the multiferroic BiFeO3 from first-principles,"The electrical control of magnons opens up new ways to transport and process
information for logic devices. In magnetoelectrical multiferroics, the
Dzyaloshinskii-Moriya (DM) interaction directly allow for such a control and,
hence, is of major importance. We determine the origin and the strength of the
(converse) spin current DM interaction in the R3c bulk phase of the
multiferroic BiFeO3 based on density functional theory. Our data supports only
the existence of one DM interaction contribution originating from the spin
current model. By exploring then magnon dispersion in the full Brillouin Zone,
we show that the exchange is isotropic, but the DM interaction and anisotropy
prefer any propagation and any magnetization direction within the full (111)
plane. Our work emphasizes the significance of the asymmetric potential induced
by the spin current over the structural asymmetry induced by the anionic
octahedron in multiferroics such as BiFeO3.",2210.15591v1
2022-11-02,Disentangling Electron-Boson Interactions on the Surface of a Familiar Ferromagnet,"We report energy renormalizations from electron-phonon and electron-magnon
interactions in spin minority surface resonances on Ni(111). The different
interactions are identified, disentangled, and quantified from the
characteristic signatures they provide to the complex self-energy and the
largely different binding energies at which they occur. The observed
electron-magnon interactions exhibit a strong dependence on momentum and energy
band position in the bulk Brillouin zone. In contrast, electron-phonon
interactions from the same bands appear to be relatively momentum- and
symmetry-independent. Additionally, a moderately strong ($\lambda>0.5$)
electron-phonon interaction is distinguished from a near-parabolic spin
majority band not crossing the Fermi level.",2211.01431v2
2022-11-07,"Magnetic excitations, phase diagram and order-by-disorder in the extended triangular-lattice Hubbard model","The dynamical structure factor is an important observable of quantum magnets
but due to numerical and theoretical limitations, it remains a challenge to
make predictions for Hubbard-like models beyond one dimension. In this work, we
study the magnetic excitations of the triangular lattice Hubbard model
including next-nearest neighbor hopping. Starting from the 120$^{\circ}$ and
stripe magnetic orders we compute the magnon spectra within a self-consistent
random phase approximation. In the stripe phase, we generically find accidental
zero modes related to a classical degeneracy known from the corresponding
$J_1$-$J_2$ Heisenberg model. We extend the order-by-disorder mechanism to
Hubbard systems and show how quantum fluctuations stabilize the stripe order.
In addition, the frustration-induced condensation of magnon modes allows us to
map out the entire phase diagram which is in remarkable agreement with recent
numerical works. We discuss connections to experiments on triangular lattice
compounds and the relation of our results to the proposed chiral spin liquid
phase.",2211.03654v2
2022-11-17,Optically-mediated remote entanglement generation in magnon-cavity systems,"We study the remote entanglement generation between macroscopic microwave
magnon modes in a coupled cavity system. The cavities are connected via an
optical fiber, which necessitates the use of a frequency conversion inside the
cavity. The converter may be implemented via a rare-earth doped crystal acting
like an effective three-level system. The entanglement dynamics of the system
is analytically studied, and an active optimal control method is also proposed
where one may generate maximally entangled Bell states on demand with a given
evolution time. The system dynamics and its control have also been studied in a
generic non-Markovian open system framework, and the generated entanglement is
found to be robust against environmental noises.",2211.09315v1
2022-11-20,Plasmon-magnon interactions in two-dimensional honeycomb magnets,"Two-dimensional honeycomb ferromagnets offer the unprecedented opportunity to
study interactions between collective modes that in standard bulk ferromagnets
do not cross paths. Indeed, they harbor an optical spin-wave branch, i.e. a
spin wave which disperses weakly near the Brillouin zone center. When doped
with free carriers, they also host the typical gapless plasmonic mode of 2D
itinerant electron/hole systems. When the plasmon branch meets the optical
spin-wave branch, energy and momentum matching occurs, paving the way for
interactions between the charge and spin sector. In this Letter we present a
microscopic theory of such plasmon-magnon interactions, which is based on a
double random phase approximation. We comment on the possibility to unveil this
physics in recently isolated 2D honeycomb magnets such as ${\rm Cr}_2{\rm
Ge}_2{\rm Te}_6$.",2211.11098v1
2022-11-24,Dynamical Backaction Evading Magnomechanics,"The interaction between magnons and mechanical vibrations dynamically modify
the properties of the mechanical oscillator, such as its frequency and decay
rate. Known as dynamical backaction, this effect is the basis for many
theoretical protocols, such as entanglement generation or mechanical
ground-state cooling. However, dynamical backaction is also detrimental for
specific applications. Here, we demonstrate the implementation of a cavity
magnomechanical measurement that fully evades dynamical backaction effects.
Through careful engineering, the magnomechanical scattering rate into the
hybrid magnon-photon modes can be precisely matched, eliminating dynamical
backaction damping. Backaction evasion is confirmed via the measurement of a
drive-power-independent mechanical linewidth.",2211.13766v3
2022-11-30,Enhancement of magnon-photon-phonon entanglement in a cavity magnomechanics with coherent feedback loop,"We propose a scheme to improve magnon-photon-phonon entanglement in cavity
magnomechanics using coherent feedback loop. In addition, we prove that the
steady state and dynamical state of the system is a genuine tripartite
entanglement state. We use the logarithmic negativity as the witness of quantum
correlations to quantify the entanglement of all bipartite subsystem, and
genuine tripartite entanglement via the nonzero minimum residual contangle, in
steady and dynamical regime. We consider the feasible experiment parameters to
realize the tripartite entanglement. We show that the entanglement can be
significantly improved with coherent feedback using a suitable tuning of the
reflective parameter of the beam splitter. The entanglement is robust against
the thermal effects. Our proposal scheme to improve the entanglement can be of
interest to applications in quantum information.",2211.17052v1
2022-12-02,Surface Ferron Excitations in Ferroelectrics and Their Directional Routing,"The duality between the electric and magnetic dipoles inspires recent
comparisons between ferronics and magnonics. Here we predict surface
polarization waves or ``ferrons"" in ferroelectric insulators, taking the
long-range dipolar interaction into account. We predict properties that are
strikingly different from the magnetic counterpart, \textit{i.e.} the surface
``Damon-Eshbach"" magnons in ferromagnets. The dipolar interaction pushes the
ferron branch with locked circular polarization and momentum to the ionic
plasma frequency. The low-frequency modes are on the other hand in-plane
polarized normal to their wave vectors. The strong anisotropy of the lower
branch renders directional emissions of electric polarization and chiral near
fields when activated by a focused laser beam, allowing optical routing in
ferroelectric devices.",2212.01047v2
2022-12-08,Exploring High Frequency Gravitational Waves with Magnons,"Detecting gravitational waves with frequencies higher than 10 kHz requires
new strategies. In previous papers, we proposed magnon gravitational wave
detectors and gave the first limit on GHz gravitational waves by reinterpreting
the existing data from axion dark matter experiments. In this paper, we show
that the sensitivity can be improved by constructing the detector specific to
gravitational waves. In particular, we employ an infinite sum of terms in the
expansion of Fermi normal coordinates to probe gravitational waves with a
wavelength comparable to the detector size. As a consequence, we obtain
sensitivity of around $h_c \sim 10^{-20}$.",2212.04094v2
2022-12-14,High-angular momentum excitations in collinear antiferromagnet FePS$_3$,"We report on magneto-optical studies of the quasi-two-dimensional van der
Waals antiferromagnet FePS$_3$. Our measurements reveal an excitation that
closely resembles the antiferromagnetic resonance mode typical of easy-axis
antiferromagnets, nevertheless, it displays an unusual, four-times larger
Zeeman splitting in an applied magnetic field. We identify this excitation with
an $|S_z|=4$ multipolar magnon -- a single-ion 4-magnon bound state -- that
corresponds to a full reversal of a single magnetic moment of the Fe$^{2+}$
ion. We argue that condensation of multipolar magnons in large-spin materials
with a strong magnetic anisotropy can produce new exotic states.",2212.07221v1
2022-12-18,Ground-state cooling of a massive mechanical oscillator by feedback in cavity magnomechanics,"Cooling the motion of a massive mechanical oscillator into its quantum ground
state plays an essential role in observing macroscopic quantum effects in
mechanical systems. Here we propose a measurement-based feedback cooling
protocol in cavity magnomechanics that is able to cool the mechanical vibration
mode of a macroscopic ferromagnet into its ground state. The mechanical mode
couples to a magnon mode via a dispersive magnetostrictive interaction, and the
latter further couples to a microwave cavity mode via the magnetic-dipole
interaction. A feedback loop is introduced by measuring the amplitude of the
microwave cavity output field and applying a force onto the mechanical
oscillator that is proportional to the amplitude fluctuation of the output
field. We show that by properly designing the feedback gain, the mechanical
damping rate can be significantly enhanced while the mechanical frequency
remains unaffected. Consequently, the vibration mode can be cooled into its
quantum ground state in the unresolved-sideband regime at cryogenic
temperatures. The protocol is designed for cavity magnomechanical systems using
ferromagnetic materials which possess strong magnetostriction along with large
magnon dissipation.",2212.09002v1
2023-02-06,Thermal Hall effect in a van der Waals triangular magnet FeCl2,"Thermal transport is a pivotal probe for studying low-energy, charge-neutral
quasi-particles in insulating magnets. In this Letter, we report an observation
of large magneto-thermal conductivity and thermal Hall effect (THE) in a van
der Waals antiferromagnet FeCl2. The magneto-thermal conductivity reaches over
~700%, indicating strong magnon-phonon coupling. Furthermore, we find an
appreciable thermal Hall signal which changes sign concurrently with the
spin-flip transition from the antiferromagnetic state to the polarized
ferromagnetic state. Our theoretical calculations suggest that, in addition to
the Berry curvature induced at the anticrossing points of the hybridized magnon
and acoustic phonon modes of FeCl2, other mechanisms are needed to account for
the magnitude of the observed THE.",2302.02855v1
2023-02-15,Superradiant phase transition induced by the indirect Rabi interaction,"We theoretically study the superradiant phase transition (SPT) in an indirect
Rabi model, where both a two-level system and a single mode bosonic field
couple to an auxiliary bosonic field. We find that the indirect spin-field
coupling induced by the virtual excitation of the auxiliary field can allow the
occurrence of a SPT at a critical point, and the influence of the so-called
$A^2$ term in the normal Rabi model is naturally avoided. In the large detuning
regime, we present the analytical expression of quantum critical point in terms
of the original system parameters. The critical atom-field coupling strength is
tunable, which will loosen the conditions on realizing the SPT. Considering a
hybrid magnon-cavity-qubit system, we predict the squeezed cat state of magnon
generated with feasible experimental parameters, which has potential
applications in quantum metrology and quantum information processing.",2302.07506v1
2023-02-20,Optimizing the magnon-phonon cooperativity in planar geometries,"Optimizing the cooperativity between two distinct particles is an important
feature of quantum information processing. Of particular interest is the
coupling between spin and phonon, which allows for integrated long range
communication between gates operating at GHz frequency. Using local light
scattering, we show that, in magnetic planar geometries, this attribute can be
tuned by adjusting the orientation and strength of an external magnetic field.
The coupling strength is enhanced by about a factor of 2 for the out-of-plane
magnetized geometry where the Kittel mode is coupled to circularly polarized
phonons, compared to the in-plane one where it couples to linearly polarized
phonons. We also show that the overlap between magnon and phonon is maximized
by matching the Kittel frequency with an acoustic resonance that satisfies the
half-wave plate condition across the magnetic film thickness. Taking the
frequency dependence of the damping into account, a maximum cooperativity of
about 6 is reached in garnets for the normal configuration near 5.5 GHz.",2302.09936v2
2023-02-21,Aluminium substituted yttrium iron garnet thin films with reduced Curie temperature,"Magnetic garnets such as yttrium iron garnet (Y$_3$Fe$_5$O$_{12}$, YIG) are
widely used in spintronic and magnonic devices. Their magnetic and
magneto-optical properties can be modified over a wide range by tailoring their
chemical composition. Here, we report the successful growth of Al-substituted
yttrium iron garnet (YAlIG) thin films via radio frequency sputtering in
combination with an ex situ annealing step. Upon selecting appropriate process
parameters, we obtain highly crystalline YAlIG films with different Al$^{3+}$
substitution levels on both, single crystalline Y$_3$Al$_5$O$_{12}$ (YAG) and
Gd$_3$Ga$_5$O$_{12}$ (GGG) substrates. With increasing Al$^{3+}$ substitution
levels, we observe a reduction of the saturation magnetisation as well as a
systematic decrease of the magnetic ordering temperature to values well below
room temperature. YAlIG thin films thus provide an interesting material
platform for spintronic and magnonic experiments in different magnetic phases.",2302.10517v2
2023-03-15,Acoustically driven magnon-phonon coupling in a layered antiferromagnet,"Harnessing the causal relationships between mechanical and magnetic
properties of van der Waals materials presents a wealth of untapped opportunity
for scientific and technological advancement, from precision sensing to novel
memories. This can, however, only be exploited if the means exist to
efficiently interface with the magnetoelastic interaction. Here, we demonstrate
acoustically-driven spin-wave resonance in a crystalline antiferromagnet,
chromium trichloride, via surface acoustic wave irradiation. The resulting
magnon-phonon coupling is found to depend strongly on sample temperature and
external magnetic field orientation, and displays a high sensitivity to
extremely weak magnetic anisotropy fields in the few~mT range. Our work
demonstrates a natural pairing between power-efficient strain-wave technology
and the excellent mechanical properties of van der Waals materials,
representing a foothold towards widespread future adoption of dynamic
magneto-acoustics.",2303.08305v1
2023-03-27,Non-linear magnon transport in a bilayer van der Waals antiferromagnets,"In this paper, we study the Berry curvature induced linear and nonlinear
magnon transport in bilayer van der Waals antiferromagnets, where we deduce
forms for the spin and energy currents within the semiclassical Boltzmann
formalism under the relaxation time approximation. Even in the absence of the
Dzyaloshinskii-Moriya interaction, if we turn on the layer-dependent
electrostatic doping (ED) potential and anisotropy in the Heisenberg
interactions, the linear response remains zero, whereas, we obtain a nonzero
nonlinear thermal Hall response resulting from higher moments of the Berry
curvature. We show that, there is a sign reversal of nonlinear thermal Hall
conductivity with varying strength of ED potential, which can be potentially
useful in spin-based technologies. We also comment on the momentum and
temperature dependence of the relaxation time which can influence the transport
properties.",2303.15426v1
2023-04-05,Pinch points and half moons encode Berry curvature,"""Half moons"", distinctive crescent patterns in the dynamical structure
factor, have been identified in inelastic neutron scattering experiments for a
wide range of frustrated magnets. In an earlier paper [H. Yan et al., Phys.
Rev. B 98, 140402(R) (2018)] we have shown how these features are linked to the
local constraints realized in classical spin liquids. Here we explore their
implication for the topology of magnon bands. The presence of half moons
indicates a separation of magnetic degrees of freedom into irrotational and
incompressible components. Where bands satisfying these constraints meet, it is
at a singular point encoding Berry curvature of $\pm 2\pi$. Interactions which
mix the bands open a gap, resolving the singularity, and leading to bands with
finite Berry curvature, accompanied by characteristic changes to half--moon
motifs. These results imply that inelastic neutron scattering can, in some
cases, be used to make rigorous inference about the topological nature of
magnon bands.",2304.02203v2
2023-04-05,Control of 4-magnon-scattering in a magnonic waveguide by pure spin current,"We use a pure spin current originating from the spin Hall effect to generate
a spin-orbit torque (SOT) strongly reducing the effective damping in an
adjacent ferromagnet. Due to additional microwave excitation, large spin-wave
amplitudes are achieved exceeding the threshold for 4-magnon scattering, thus
resulting in additional spin-wave signals at discrete frequencies. Two or more
modes are generated below and above the directly pumped mode with equal
frequency spacing. It is shown how this nonlinear process can be controlled in
magnonic waveguides by the applied dc current and the microwave pumping power.
The sudden onset of the nonlinear effect after exceeding the thresholds can be
interpreted as spiking phenomenom which makes the effect potentially
interesting for neuromorphic computing applications. Moreover, we investigated
this effect under microwave frequency and external field variation. The
appearance of the additional modes was investigated in the time-domain
revealing a time delay between the directly excited and the simultaneously
generated nonlinear modes. Furthermore, spatially resolved measurements show
different spatial decay lengths of the directly pumped mode and nonlinear
modes.",2304.02708v1
2023-05-04,Integrating Magnons for Quantum Information,"Magnons, the quanta of collective spin excitations in magnetically ordered
materials, have distinct properties that make them uniquely appealing for
quantum information applications. They can have ultra-small wavelengths down to
the nanometer scale even at microwave frequencies. They can provide coupling to
a diverse set of other quantum excitations, and their inherently gyrotropic
dynamics forms the basis for pronounced non-reciprocities. In this article we
discuss what the current research challenges are for integrating magnetic
materials into quantum information systems and provide a perspective on how to
address them.",2305.03164v1
2023-05-05,Coexistence of Dirac Fermions and Magnons in a Layered Two-Dimensional Semiquinoid Metal-Organic Framework,"We predict the magnetic and electronic properties of a novel metal-organic
framework. By combining density functional theory and density matrix
renormalization group approaches, we find the diatomic Kagome crystal structure
of the metal-semiquinoid framework (H$_2$NMe$_2$)$_2$M$_2$(Cl$_2$dhbq)$_3$ (M =
Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) to host a rich variety of
antiferromagnetic (AFM) and ferromagnetic (FM) Dirac semimetallic,
spin-polarized Dirac fermions, and flat band magnetic insulators and metallic
phases. Concomitantly, the spin excitation spectrum of the various magnetic
systems display multiple Dirac-like and nodal-ring crossings. This suggests
that the metal-semiquinoid system is an ideal platform for examining the
intertwining of Dirac fermions and magnons.",2305.03867v2
2023-05-07,Lectures on spintronics and magnonincs,"In this series of lectures, we discuss the basic theoretical concepts of
magnonics and spintronics. We first briefly recall the relevant topics from
quantum mechanics, electrodynamics of continuous media, and basic theory of
magnetism. We then discuss the classical theory of magnetic dynamics:
ferromagnetic and antiferromagnetic resonance, dynamic susceptibilities, and
spin waves. We open the main discussion with phenomena of spin and exchange
spin currents, spin torques, the spin Hall effect, and the spin Hall and Hanle
magnetoresistance. Special emphasis is given to the effects of spin transfer
torque and spin pumping, where we follow the celebrated derivation utilizing
Landauer quantum multi-channel scattering matrix approach. Finally, we outline
the most important features distinguishing antiferromagnetic dynamics from
ferromagnetic one, which make antiferromagnets particularly promising material
candidates for spintronics and magnonics.",2305.04385v1
2023-05-10,Investigation of Spin-Wave Dynamics in Gyroid Nanostructures,"A new concept in magnonics studies the dynamics of spin waves (SWs) in
three-dimensional nanosystems. It is a natural evolution from conventionally
used planar systems to explore magnetization configurations and dynamics in 3D
nanostructures with lengths near intrinsic magnetic scales. In this work, we
perform broadband ferromagnetic resonance (BBFMR) measurements and
micromagnetic simulations of nanoscale magnetic gyroids - a periodic chiral
structure consisting entirely of chiral triple junctions. Our results show
unique properties of the network, such as the localization of the SW modes,
evoking their topological properties, and the substantial sensitivity to the
direction of the static magnetic field. The presented results open a wide range
of applications in the emerging field of 3D magnonic crystals and spintronics.",2305.06319v2
2023-05-31,Magnetization dynamics in a three-dimensional interconnected nanowire array,"Three-dimensional magnetic nanostructures have recently emerged as artificial
magnetic material types with unique properties bearing potential for
applications, including magnonic devices. Interconnected magnetic nanowires are
a sub-category within this class of materials that is attracting particular
interest. We investigate the high-frequency magnetization dynamics in a cubic
array of cylindrical magnetic nanowires through micromagnetic simulations based
on a frequency-domain formulation of the linearized Landau-Lifshitz-Gilbert
equation. The small-angle high-frequency magnetization dynamics excited by an
external oscillatory field displays clear resonances at distinct frequencies.
These resonances are identified as oscillations connected to specific geometric
features and micromagnetic configurations. The geometry- and
configuration-dependence of the nanowire array's absorption spectrum
demonstrates the potential of such magnetic systems for tuneable and
reprogrammable magnonic applications.",2306.00174v1
2023-06-15,Efficient Spin Seebeck and Spin Nernst Effects of Magnons in Altermagnets,"We report two non-degenerate magnon modes with opposite spins or chiralities
in collinearly antiferromagnetic insulators driven by symmetry-governed
anisotropic exchange couplings. The consequent giant spin splitting contributes
to spin Seebeck and spin Nernst effects generating longitudinal and transverse
spin currents when the temperature gradient applies along and away from the
main crystal axis, without requiring any external magnetic field and spin-orbit
coupling. Based on first-principle calculations, we predict feasible material
candidates holding robust altermagnetic spin configurations and
room-temperature structural stability to efficiently transport spin. The spin
Seebeck conductivity is comparable to the records of antiferromagnets that
require the magnetic field, and the spin Nernst conductivity is two orders in
magnitude larger than that in antiferromagnetic monolayers that need Berry
curvature.",2306.08976v2
2023-06-23,Spin transport between polarized Fermi gases near the ferromagnetic phase transition,"We theoretically study the spin current between two polarized Fermi gases
with repulsive interactions near the itinerant ferromagnetic phase transition.
We consider a two-terminal model where the left reservoir is fixed to be fully
polarized while the polarization of the right reservoir is tuned through a
fictitious magnetic field defined by the chemical-potential difference between
different atomic hyperfine states. We calculate the spectra of the spin-flip
susceptibility function, which displays a magnon dispersion emerging from the
Stoner continuum at low momentum in the ferromagnetic phase. Based on the
spin-flip susceptibility and using Keldysh Green's function formalism, we
investigate the spin current induced by quasiparticle and spin-flip tunneling
processes, respectively, and show their dependence on the polarization bias
between two reservoirs. The one-body (quasiparticle) tunneling demonstrates a
linear dependence with respect to the polarization bias. In contrast, the
spin-flip process manifests a predominantly cubic dependence on the bias. While
indicating an enhanced magnon tunneling in the strong-coupling regime, our
results also demonstrate a characteristic behavior around the critical
repulsive strength for ferromagnetic phase transition at low temperatures.",2306.13536v2
2023-07-13,Magnon-magnon coupling in synthetic ferrimagnets,"Magnetic multilayers with interlayer exchange coupling have been widely
studied for both static and dynamic regimes. Their dynamical responses depend
on the exchange coupling strength and magnetic properties of individual layers.
Magnetic resonance spectra in such systems are conveniently discussed in terms
of coupling of acoustic and optical modes. At a certain value of applied
magnetic field, the two modes come close to being degenerate and the spectral
gap indicates the strength of mode hybridisation. In this work, we
theoretically and experimentally study the mode hybridisation of
interlayer-exchange-coupled moments with dissimilar magnetisation and thickness
of two ferromagnetic layers. In agreement with symmetry analysis for
eigenmodes, our low-symmetry multilayers exhibit sizable spectral gaps for all
experimental conditions. The spectra agree well with the predictions from the
Landau-Lifshitz-Gilbert equation at the macrospin limit whose parameters are
independently fixed by static measurements.",2307.06888v2
2023-08-22,Observation of Current-induced Nonlinear Spin Polarization in Pt-Py Bilayers,"We experimentally observe nonlinear spin polarization in metallic bilayers of
platinum and permalloy by means of spin-torque ferromagnetic resonance (ST-FMR)
with the spin-Hall effects. The ST-FMR results under massive dc current
injection contain striking features, which are not caused by extrinsic Joule
heating, but by intrinsic nonlinear spin polarization. The emergence of
nonlinear spin polarization is consistent with observation of unidirectional
spin-Hall magnetoresistance due to magnon generation/annihilation. Moreover,
the magnon generation (annihilation) leads to effective magnetization shrinkage
(expansion) revealed by the ST-FMR measurements. The present study paves a way
to spin-Hall effect based nonlinear spintronic devices as well as
6th-generation mobile communication light sources.",2308.11156v2
2023-08-28,Spin-wave spectral analysis in crescent-shaped ferromagnetic nanorods,"The research on the properties of spin waves (SWs) in three-dimensional
nanosystems is an innovative idea in the field of magnonics. Mastering and
understanding the nature of magnetization dynamics and binding of SWs at
surfaces, edges, and in-volume parts of three-dimensional magnetic systems
enables the discovery of new phenomena and suggests new possibilities for their
use in magnonic and spintronic devices. In this work, we use numerical methods
to study the effect of geometry and external magnetic field manipulations on
the localization and dynamics of SWs in crescent-shaped (CS) waveguides. It is
shown that changing the magnetic field direction in these waveguides breaks the
symmetry and affects the localization of eigenmodes with respect to the static
demagnetizing field. This in turn has a direct effect on their frequency.
Furthermore, CS structures were found to be characterized by significant
saturation at certain field orientations, resulting in a cylindrical
magnetization distribution. Thus, we present chirality-based nonreciprocal
dispersion relations for high-frequency SWs, which can be controlled by the
field direction (shape symmetry) and its amplitude (saturation).",2308.14678v1
2023-09-13,Looking at the magnetic properties of half metal from the paramagnetic phase: DFT+DMFT study of exchange interactions in CrO$_2$,"We study magnetic properties of CrO$_2$ within the density functional theory
plus dynamical mean-field theory (DFT+DMFT) approach in the paramagnetic phase.
We consider the 3-orbital (per Cr site) model, containing only $t_{2g}$ states,
the $5$-orbital model, including all $d$-states, as well as the model including
also the oxygen $p$-states. Using the recently proposed approach of calculation
of exchange interactions in paramagnetic phase, we extract exchange interaction
parameters and magnon dispersions for these models. While the magnon dispersion
in the 3-orbital model possesses negative branches in accordance with previous
studies in ferromagnetic phase, this drawback is removed in the $5$-orbital
model. The model containing oxygen states overestimates the exchange
interactions and spin stiffness. Therefore, the $5$-orbital model appears as
most adequate for describing magnetic properties of CrO$_2$, in accordance with
the recent experimental data of fixed valence of chromium in this compound. The
possibility of describing magnetic properties of this material starting from
paramagnetic phase points to the correspondence of magnetic properties in this
phase and ferromagnetic phase and possible important contributions of RKKY
exchange interactions in paramagnetic phase of CrO$_2$.",2309.06892v1
2023-10-19,Micromagnetic Simulation of Three-dimensional Nanoarchitectures,"The thesis discusses micromagnetic simulation studies on high-frequency
magnetic dynamics in three-dimensional ferromagnetic nanoarchitectures made of
interconnected magnetic nanowire networks. Such artificial magnetic materials
with nanoscale features have recently emerged as a vivid topic of research, as
their geometry has a decisive impact on their magnetic properties. By studying
their static magnetization structure, we find that these systems display a
behavior analogous to that of 3D artificial spin ice lattices, with frustrated
interactions and the emergence of monopole-like defect structures at the wires'
intersection points. Our simulations reveal a high activity of these defect
sites in the magnonic high-frequency spectrum. We study various 3D
nanoarchitectures and show that their geometry and magnetization state results
in characteristic high-frequency signatures. Controlling these features could
open new pathways for magnonics research and reprogrammable magnetic
metamaterials.",2310.12839v1
2023-11-03,Cryogenic spin Peltier effect detected by a RuO$_2$-AlO$_x$ on-chip microthermometer,"We report electric detection of the spin Peltier effect (SPE) in a bilayer
consisting of a Pt film and a Y$_{3}$Fe$_5$O$_{12}$ (YIG) single crystal at the
cryogenic temperature $T$ as low as 2 K based on a RuO$_2$$-$AlO$_x$ on-chip
thermometer film. By means of a reactive co-sputtering technique, we
successfully fabricated RuO$_2$$-$AlO$_x$ films having a large temperature
coefficient of resistance (TCR) of $\sim 100\% ~\textrm{K}^{-1}$ at around
$2~\textrm{K}$. By using the RuO$_2$$-$AlO$_x$ film as an on-chip temperature
sensor for a Pt/YIG device, we observe a SPE-induced temperature change on the
order of sub-$\mu \textrm{K}$, the sign of which is reversed with respect to
the external magnetic field $B$ direction. We found that the SPE signal
gradually decreases and converges to zero by increasing $B$ up to
$10~\textrm{T}$. The result is attributed to the suppression of magnon
excitations due to the Zeeman-gap opening in the magnon dispersion of YIG,
whose energy much exceeds the thermal energy at 2 K.",2311.01711v1
2023-11-13,Microwave-to-Optical Quantum Transduction Utilizing the Topological Faraday Effect of Topological Insulator Heterostructures,"The quantum transduction between microwave and optical photons is essential
for realizing scalable quantum computers with superconducting qubits. Due to
the large frequency difference between microwave and optical ranges, the
transduction needs to be done via intermediate bosonic modes or nonlinear
processes. So far, the transduction efficiency $\eta$ via the magneto-optic
Faraday effect (i.e., the light-magnon interaction) in the ferromagnet YIG has
been demonstrated to be small as $\eta\sim 10^{-8} \mathrm{-} 10^{-15}$ due to
the sample size limitation inside the cavity. Here, we take advantage of the
fact that three-dimensional topological insulator thin films exhibit a
topological Faraday effect that is independent of the sample thickness. This
leads to a large Faraday rotation angle and therefore enhanced light-magnon
interaction in the thin film limit. We show theoretically that the transduction
efficiency can be greatly improved to $\eta\sim10^{-4}$ by utilizing the
heterostructures consisting of topological insulator thin films such as
Bi$_2$Se$_3$ and ferromagnetic insulator thin films such as YIG.",2311.07293v1
2023-12-13,Detection of Geometric Phases in Spin Waves using Nitrogen-Vacancy Centers,"Due to their robustness, the implementation of geometric phases provides a
reliable and controllable way to manipulate the phase of a spin wave, thereby
paving the way towards functional magnonics-based data processing devices.
Moreover, geometric phases in spin waves are interesting from a fundamental
perspective as they contain information about spin wave band structures and
play an important role in magnon Hall effects. In this paper we propose to
directly measure geometric phases in spin wave systems using the magnetic field
sensing capabilities of nitrogen-vacancy (NV) centers. We demonstrate the
general principles of this method on two systems in which spin waves acquire a
geometric phase, namely a wire with a magnetic domain wall and a system with
position-dependent anisotropy axes, and explicitly show how this phase can be
deduced from the NV center signal.",2312.08137v1
2023-12-18,Fractional Topological Charges in 2D Magnets,"Magnetic skyrmions and antiskyrmions are characterised by an integer
topological charge $\mathcal Q =\mp 1$, describing the winding of the magnetic
orientation. Half-integer winding numbers, $\mathcal Q=\pm \frac{1}{2}$, can be
obtained for magnetic vortices (merons). Here, we discuss the physics of
magnets with fractional topological charge which is neither integer nor
half-integer. We argue that in ferromagnetic films with cubic anisotropy,
textures with $\mathcal Q=\pm\frac{1}{6} $ or $\pm\frac{1}{8}$ arise naturally
when three or more magnetic domains meet. We also show that a single magnetic
skyrmion with $\mathcal Q =-1$ can explode into four fractional defects, each
carrying charge $\mathcal Q=-\frac{1}{4}$. Additionally, we investigate a point
defect with a non-quantised fractional charge ($\mathcal Q\neq \frac{n}{m},
n,m\in\mathbb{Z}$) which can move parallel to a magnetic domain wall. Only
defects with fractional charge lead to an Aharonov-Bohm effect for magnons. We
investigate the resulting forces on a fractional defect due to magnon currents.",2312.11435v1
2023-12-20,Beyond the Tensionless Limit: Integrability in the Symmetric Orbifold,"The symmetric orbifold of $\mathbb{T}^4$ is exactly dual to string theory on
$\mathrm{AdS}_3\times \mathrm{S}^3 \times \mathbb{T}^4$ with minimal ($k=1$)
NS-NS flux. In this paper we study the perturbation of the symmetric orbifold
that is dual to switching on R-R flux, and hence to deforming the theory away
from the tensionless point. More specifically, we determine systematically the
action of a centrally extended supersymmetry algebra on the CFT states, and
deduce from it the anomalous conformal dimensions. In the $w$-twisted sector
with large $w$ the structure is similar to what was found for $\mathcal{N}=4$
SYM: the basic excitations are multi-magnons whose individual dispersion
relation is fixed by symmetry, and the comparison with the BMN answer suggests
that the result is true to all orders in perturbation theory. Finally we show
that the multi-magnon states interact via an integrable $S$-matrix and possess
a natural family of bound states.",2312.13288v2
2023-12-25,Entangling two Dicke states in a periodic modulated quantum system,"We propose a theoretical approach for entangling two Dicke states in a
periodic modulated quantum system. By considering two qubit ensembles that are
nonuniformly coupled to a common resonator, we can derive an effective
Hamiltonian whose energy levels depend nonlinearly on the excitation number of
each qubit ensemble. More simplified effective Hamiltonian can be obtained by
selecting appropriate driving parameters and initial state. Based on the
dynamic evolution of the effective Hamiltonian, we can selectively achieve
Dicke state transitions and generate entangled Dicke states controllably. For a
special case, we can obtain ensemble-ensemble entangled states by performing a
projective even-odd cat measurement. By implementing Gaussian soft temporal
modulation, we can effectively suppress off-resonant contributions in the
interaction and enhance the fidelity of target states. Furthermore, by
utilizing the Holstein-Primakoff transformation, we study the
resonator-ensemble coupling system in the thermodynamic limit and investigate
the generation of entangled magnon states. Additionally, we propose a scheme of
creating magnon NOON states through frequency modulation and study the
influence of decoherence on the fidelity of target states.",2312.15568v1
2024-01-16,"Influence of temperature, doping, and amorphization on the electronic structure and magnetic damping of iron","Hybrid magnonic quantum systems have drawn increased attention in recent
years for coherent quantum information processing, but too large magnetic
damping is a persistent concern when metallic magnets are used. Their intrinsic
damping is largely determined by electron-magnon scattering induced by
spin-orbit interactions. In the low scattering limit, damping is dominated by
intra-band electronic transitions, which has been theoretically shown to be
proportional to the electronic density of states at the Fermi level. In this
work, we focus on body-centered-cubic iron as a paradigmatic ferromagnetic
material. We comprehensively study its electronic structure using
first-principles density functional theory simulations and account for finite
lattice temperature, boron (B) doping, and structure amorphization. Our results
indicate that temperature induced atomic disorder and amorphous atomic
geometries only have a minor influence. Instead, boron doping noticeably
decreases the density of states near the Fermi level with an optimal doping
level of 6.25%. In addition, we show that this reduction varies significantly
for different atomic geometries and report that the highest reduction
correlates with a large magnetization of the material. This may suggest
materials growth under external magnetic fields as a route to explore in
experiment.",2401.08076v1
2024-01-25,Enhancing Weak magnetic field sensing of cavity-magnon system with dual frequency modulation,"The crucial limitation of improving the sensitivity of the detection of weak
magnetic fields is the unavoidable measurement noise. In this paper, we propose
a scheme to achieve precise sensing robust against additional noise by
employing a dual-frequency bias field modulation within a cavity magnon system.
We find that the anti-rotating wave term can amplify the signal of the detected
magnetic field, but this amplification effect must coexist with the rotating
wave term. In particular, by the bias field modulation, we find the robustness
against cavity field thermal noise is substantially enhanced, quantum noise and
cavity field thermal noise is greatly reduced, and the external magnetic field
signal is amplified, thereby improving the weak magnetic field sensing system's
sensitivity. Compared with the previous scheme, our scheme requires neither an
ultra- or deep-strong coupling mechanism nor the suppression of the additional
noise by increasing the electromagnetic cooperativity. Our scheme could provide
a valuable candidate for weak magnetic field sensing.",2401.13879v1
2024-02-07,Broadband squeezed light field by magnetostriction in an opto-magnomechanical,"We present a novel mechanism for generating a wide bandwidth squeezed optical
output field in an opto-magnomechanical system. In this system, the magnon
(mechanical) mode in the yttrium-iron-garnet crystal is coupled to the
microwave field (optical field) through magnetic dipole (radiation pressure)
interaction. The magnetostrictive force induced by the yttrium-iron-garnet
crystal causes a mechanical displacement and creates a quadrature squeezed
magnon mode. Eventually, this quadrature squeezed mechanical mode is
transferred to the output optical field through state-swap interaction. Our
results demonstrate the optimal parameter range for obtaining a stable squeezed
optical output field with a wide bandwidth. Moreover, the squeezed light field
exhibits strong robustness to environmental temperature. The new scheme we
propose has potential applications in quantum precision measurements, quantum
wireless networks, quantum radar, etc.",2402.04983v1
2024-02-12,Continuum of spin excitations in an ordered magnet,"Continuum of spin excitations observed in inelastic neutron scattering
experiments are often considered as a strong evidence of quantum spin liquid
formation. When quantum spin liquid is indeed the ground state of a
disorder-free magnetic compound, the elementary excitation is no longer the
conventional spin waves (magnons). Instead, the magnons fractionalize into
spinons, leaving only a two-spinon continuum detectable in inelastic neutron
scattering experiments. For a clean ordered antiferromagnet, it was unclear if
we can observe a continuous spectrum similar to the ones in a quantum spin
liquid state. Here we show that the magnetically ordered state in
Na$_2$BaCo(PO$_4$)$_2$ is able to host a spin excitation continuum induced by
strong quantum fluctuations. Thus, a second thought is necessary when
concluding such continuum as signature of quantum spin liquid in new material
explorations.",2402.07730v1
2024-02-12,Photoinduced Topological Phase Transitions in a Kitaev kagome magnet,"Based on the magnetic Floquet-Bloch theory, we investigate topological
natures of Floquet magnons in a laser-irradiated kagome magnet exhibiting
Heisenberg, Kitaev and Dzyaloshinskii Moriya interactions in the presence of an
external magnetic field applied along directions of [111]. In a strong magnetic
field perpendicular to the lattice plane [111] direction, the system is in the
fully polarized paramagnetic phase and the magnon band carries an asymmetric
Chern number throughout the region of the phase diagram. Our results show that
periodically driven intrinsic topological magnetic materials can be manipulated
into different topological phases with different thermal conductivities and
Berry curvature signatures by adjusting the light intensity throughout the
phase diagram region.",2402.07731v1
2024-02-28,"SU(3) altermagnetism: Lattice models, chiral magnons, and flavor-split bands","Altermagnetism, a type of magnetic order that combines properties of
ferromagnets and antiferromagnets, has generated significant interest recently,
in particular due to its potential spintronics applications. Here, we show that
altermagnetic states, which typically involve collinear magnetic order of SU(2)
spins, can be generalized to higher SU($N$) symmetry groups. We construct a
two-dimensional Heisenberg model for a three-sublattice SU(3) altermagnet and
analyze its excitation spectrum via flavor-wave theory. By deriving a general
formula for the chirality of the excitations, we show that the SU(3)
altermagnet harbors chiral magnons that are split in energy. We also compute
the electronic band structure for a metallic system of the same symmetry and
map out the polarization of the resulting flavor-split bands. We conclude by
discussing further generalizations.",2402.18629v1
2024-03-09,Hybrid Spin-Orbit Exciton-Magnon Excitations in FePS$_3$,"FePS$_3$ is a layered van der Waals (vdW) Ising antiferromagnet that has
recently been studied in the context of true 2D magnetism, and emerged as an
ideal material platform for investigating strong spin-phonon coupling, and
non-linear magneto-optical phenomena. In this work, we demonstrate an important
unresolved role of spin-orbit coupling (SOC) in the ground state and
excitations of this compound. Combining first principles calculations with
Linear Flavor Wave Theory (LFWT), we find strong mixing and spectral overlap of
different spin-orbital single-ion states. As such, the low-lying excitations
form complex mixtures of local degrees of freedom most accurately viewed as
hybrid spin-orbit exciton-magnon modes. Complete parameterization of the
resulting low-energy model including all such degrees of freedom requires
nearly half a million coupling constants. Despite this complexity, we show that
such a model can be inexpensively derived using local many-body-based
approaches, which yield quantitative agreement with recent experiments. The
results highlight the importance of SOC even in first row transition metals,
and provide essential insight into the properties of 2D magnets with unquenched
orbital moments.",2403.05958v1
2024-03-19,Acoustoelectric non-local spin wave power detector for studying magnon-phonon coupling,"We have developed a simple detection scheme to study spin waves excited by
surface acoustic wave (SAW) in ferromagnetic thin films. Metallic antennas made
of Ta and a ferromagnetic element are placed along the SAW propagation path.
The SAW excites spin waves in the ferromagnetic element and induces
acoustoelectric current in the antennas, which are detected as a DC voltage.
The DC voltage takes an extremum at the spin wave resonance condition, which
demonstrates that the antenna acts as a non-local spin wave detector. The
antennas placed before and after the ferromagnetic element along the SAW
propagation path can probe spin wave excitation from reflected and transmitted
SAWs, respectively. Interestingly, we find characteristics of spin wave
excitations that are different for the reflected and transmitted SAWs: the
former excites spin waves with larger frequency with broader linewidth compared
to the latter. The results show that the antennas act as a non-local spin wave
power detector and can be used to map out the spin wave spectra in a unique
way, providing insights into the magnon-phonon coupling in magnetic
nanostructures fabricated on phononic SAW devices.",2403.12745v1
2024-03-20,Dipole-dipole-interaction-induced entanglement between two-dimensional ferromagnets,"We investigate the viability of dipole-dipole interaction as a means of
entangling two distant ferromagnets. To this end we make use of the Bogoliubov
transformation as a symplectic transformation. We show that the coupling of the
uniform magnon modes can be expressed using four squeezing parameters which we
interpret in terms of hybridization, one-mode and two-mode squeezing. We
utilize the expansion in terms of the squeezing parameters to obtain an
analytic formula for the entanglement in the magnon ground state using the
logarithmic negativity as entanglement measure. Our investigation predicts that
for infinitely large two-dimensional ferromagnets, the dipole-dipole
interaction does not lead to significant long-range entanglement. However, in
the case of finite ferromagnets, finite entanglement can be expected",2403.13354v1
2024-03-26,Magnonic inverse-design processor,"Artificial Intelligence (AI) technology has revolutionized our everyday lives
and research. The concept of inverse design, which involves defining a
functionality by a human and then using an algorithm to search for the device's
design, opened new perspectives for information processing. A specialized
AI-driven processor capable of solving an inverse problem in real-time offers a
compelling alternative to the time and energy-intensive CMOS computations.
Here, we report on a magnon-based processor that uses a complex reconfigurable
medium to process data in the gigahertz range, catering to the demands of 5G
and 6G telecommunication. Demonstrating its versatility, the processor solves
inverse problems using two algorithms to realize RF notch filters and
demultiplexers. The processor also exhibits potential for binary, reservoir,
and neuromorphic computing paradigms.",2403.17724v1
2024-03-27,Collective excitations in competing phases in two and three dimensions,"We investigate the superconducting (SC), charge-density wave (CDW), and
antiferromagnetic (AFM) phases in the extended Hubbard model at zero
temperature and half-filling. We employ the iterated equations of motion
approach to compute the two-particle Green's functions and their spectral
densities. This renders a comprehensive analysis of the behavior of collective
excitations possible as the model's parameters are tuned across phase
transitions. We identify the well-known amplitude (Higgs) and phase
(Anderson-Bogoliubov) modes within the superconducting phase and observe a
similar excitation (cooperon) in the CDW phase which shifts towards zero energy
as the system approaches the phase transition to the SC phase. In the CDW
phase, close to the phase transition to the AFM phase, we find a collective
mode, an exciton, that does not change significantly and another mode, a
longitudinal magnon, that emerges from the two-particle continuum as the system
approaches the phase transition to the AFM phase. It becomes identical with the
former at the transition. In the AFM phase, their roles are reversed.
Additionally, we find a transversal Goldstone magnon located at zero energy.",2403.18701v1
2024-03-28,Influence of disorder at Insulator-Metal interface on spin transport,"Motivated by experimental work showing enhancement of spin transport between
Yttrium Iron Garnet and Platinum by a thin antiferromagnetic insulator between
them, we consider spin transport through the interface of a non-magnetic metal
and compensated antiferromagnetically ordered insulator and focus on the
significance of the interface itself. The spin transport is carried by
spin-polarized electrons in the metal and by magnons in the insulator. We
compute the spin current in the presence of a spin accumulation in the metal,
cause by the spin Hall effect, and a thermal gradient using Fermi's Golden Rule
in the presence of interfacial disorder. For a perfectly clean interface, the
in-plane momentum is conserved by the electron-magnon scattering events that
govern the spin transport through the interface. We calculate how
disorder-induced broadening of scattering matrix elements with respect to the
in-plane momentum influences the spin current. As a general result, we observe
that for many experimental setups, specifically for high temperatures, one
should expect a rather small effect of interface disorder on the measured spin
current, while for small temperatures there is a significant reduction of a
spin current with increasing disorder.",2403.19277v1
2004-05-28,Macroscopic entanglement of many-magnon states,"We study macroscopic entanglement of various pure states of a one-dimensional
N-spin system with N>>1. Here, a quantum state is said to be macroscopically
entangled if it is a superposition of macroscopically distinct states. To judge
whether such superposition is hidden in a general state, we use an essentially
unique index p: A pure state is macroscopically entangled if p=2, whereas it
may be entangled but not macroscopically if p<2. This index is directly related
to the stability of the state. We calculate the index p for various states in
which magnons are excited with various densities and wavenumbers. We find
macroscopically entangled states (p=2) as well as states with p=1. The former
states are unstable in the sense that they are unstable against some local
measurements. On the other hand, the latter states are stable in the senses
that they are stable against local measurements and that their decoherence
rates never exceed O(N) in any weak classical noises. For comparison, we also
calculate the von Neumann entropy S(N) of a subsystem composed of N/2 spins as
a measure of bipartite entanglement. We find that S(N) of some states with p=1
is of the same order of magnitude as the maximum value N/2. On the other hand,
S(N) of the macroscopically entangled states with p=2 is as small as O(log N)<<
N/2. Therefore, larger S(N) does not mean more instability. We also point out
that these results are analogous to those for interacting many bosons.
Furthermore, the origin of the huge entanglement, as measured either by p or
S(N), is discussed to be due to the spatial propagation of magnons.",0405173v2
2010-09-30,A New Method of Calculating the Spin-Wave Velocity $c$ of Spin-1/2 Antiferromagnets With $O(N)$ Symmetry in a Monte Carlo Simulation,"Motivated by the so-called cubical regime in magnon chiral perturbation
theory, we propose a new method to calculate the low-energy constant, namely
the spin-wave velocity $c$ of spin-1/2 antiferromagnets with $O(N)$ symmetry in
a Monte Carlo simulation. Specifically we suggest that $c$ can be determined by
$c = L/\beta$ when the squares of the spatial and temporal winding numbers are
tuned to be the same in the Monte Carlo calculations. Here $\beta$ and $L$ are
the inverse temperature and the box size used in the simulations when this
condition is met. We verify the validity of this idea by simulating the quantum
spin-1/2 XY model. The $c$ obtained by using the squares of winding numbers is
given by $c = 1.1348(5)Ja$ which is consistent with the known values of $c$ in
the literature. Unlike other conventional approaches, our new idea provides a
direct method to measure $c$. Further, by simultaneously fitting our Monte
Carlo data of susceptibilities $\chi_{11}$ and spin susceptibilities $\chi$ to
their theoretical predictions from magnon chiral perturbation theory, we find
$c$ is given by $c = 1.1347(2)Ja$ which agrees with the one we obtain by the
new method of using the squares of winding numbers. The low-energy constants
magnetization density ${\cal M}$ and spin stiffenss $\rho$ of quantum spin-1/2
XY model are determined as well and are given by ${\cal M} = 0.43561(1)/a^2$
and $\rho = 0.26974(5)J$, respectively. Thanks to the prediction power of
magnon chiral perturbation theory which puts a very restricted constraint among
the low-energy constants for the model considered here, the accuracy of ${\cal
M}$ we present in this study is much precise than previous Monte Carlo result.",1009.6122v1
2014-11-11,Magnonic Charge Pumping via Spin-Orbit Coupling,"The interplay between spin, charge, and orbital degrees of freedom has led to
the development of spintronic devices like spin-torque oscillators, spin-logic
devices, and spin-transfer torque magnetic random-access memories. In this
development spin pumping, the process where pure spin-currents are generated
from magnetisation precession, has proved to be a powerful method for probing
spin physics and magnetisation dynamics. The effect originates from direct
conversion of low energy quantised spin-waves in the magnet, known as magnons,
into a flow of spins from the precessing magnet to adjacent normal metal leads.
The spin-pumping phenomenon represents a convenient way to electrically detect
magnetisation dynamics, however, precessing magnets have been limited so far to
pump pure spin currents, which require a secondary spin-charge conversion
element such as heavy metals with large spin Hall angle or multi-layer layouts
to be detectable. Here, we report the experimental observation of charge
pumping in which a precessing ferromagnet pumps a charge current, demonstrating
direct conversion of magnons into high-frequency currents via the relativistic
spin-orbit interaction. The generated electric current, differently from spin
currents generated by spin-pumping, can be directly detected without the need
of any additional spin to charge conversion mechanism and amplitude and phase
information about the relativistic current-driven magnetisation dynamics. The
charge-pumping phenomenon is generic and gives a deeper understanding of the
recently observed spin-orbit torques, of which it is the reciprocal effect and
which currently attract interest for their potential in manipulating magnetic
information. Furthermore, charge pumping provides a novel link between
magnetism and electricity and may find application in sourcing alternating
electric currents.",1411.2779v1
2016-04-27,Collective spin excitations in bi-component magnonic crystals consisting of bi-layer Permalloy/Fe nanowires,"In the developing field of magnonics,it is very important to achieve
tailoring of spin wave propagation by both a proper combination of materials
with different magnetic properties and their nanostructuring on the
submicrometric scale. We have exploited deep ultra-violet lithography, in
combination with tilted shadow deposition technique, to fabricate arrays of
closely spaced bi-layer nanowires (NWs), with separation d=100 nm and
periodicity a=440 nm, having bottom and top layers made of Permalloy and Iron,
respectively. The NWs have either rectangular cross section (bottom and upper
layers of equal width) or L-shaped cross section (upper layer of half width).
The frequency dispersion of collective spin wave excitations in the above
bi-layered NWs arrays has been measured by the Brillouin light scattering
technique while sweeping the wave vector perpendicularly to the wire length
over three Brillouin zones of the reciprocal space. For the rectangular NWs,
the lowest frequency fundamental mode, characterized by a quasi- uniform
profile of the amplitude of the dynamic magnetization across the NW width,
exhibits a sizeable and periodic frequency dispersion. A similar dispersive
mode is also present in L-shaped NWs, but the mode amplitude is concentrated in
the thin side of the NWs. The width and the center frequency of the magnonic
band associated with the above fundamental modes has been analyzed, showing
that both can be tuned by varying the external applied field. Moreover, for the
L-shaped NWs it is shown that there is also a second dispersive mode,at higher
frequency,characterized by an amplitude concentrated in the thick side of the
NW. These experimental results have been quantitatively reproduced by an
original numerical model that includes two-dimensional Green's function of the
dipole field of the dynamic magnetization and interlayer exchange coupling
between the layers.",1604.07950v1
2018-02-19,Dynamical structure factor of the triangular antiferromagnet: the Schwinger boson theory beyond the mean field approach,"We compute the zero temperature dynamical structure factor $S({\bf
q},\omega)$ of the triangular lattice Heisenberg model (TLHM) using a Schwinger
boson approach that includes the Gaussian fluctuations ($1/N$ corrections) of
the saddle point solution. While the ground state of this model exhibits a
well-known 120$^{\circ}$ magnetic ordering, experimental observations have
revealed a strong quantum character of the excitation spectrum. We conjecture
that this phenomenon arises from the proximity of the ground state of the TLHM
to the quantum melting point separating the magnetically ordered and spin
liquid states. Within this scenario, magnons are described as collective modes
(two spinon-bound states) of a spinon condensate (Higgs phase) that
spontaneously breaks the SU(2) symmetry of the TLHM. Crucial to our results is
the proper account of this spontaneous symmetry breaking. The main qualitative
difference relative to semi-classical treatments ($1/S$ expansion) is the
presence of a high-energy spinon continuum extending up to about three times
the single-magnon bandwidth. In addition, the magnitude of the ordered moment
($m=0.224$) agrees very well with numerical results and the low energy part of
the single-magnon dispersion is in very good agreement with series expansions.
Our results indicate that the Schwinger boson approach is an adequate starting
point for describing the excitation spectrum of some magnetically ordered
compounds that are near the quantum melting point separating this Higgs phase
from the {\it deconfined} spin liquid state.",1802.06878v4
2018-04-06,Magnon excitations and quantum critical behavior of the ferromagnet U$_4$Ru$_7$Ge$_6$,"We present an extensive study of the ferromagnetic heavy fermion compound
U$_4$Ru$_7$Ge$_6$. Measurements of electrical resistivity, specific heat and
magnetic properties show that U$_4$Ru$_7$Ge$_6$ orders ferromagnetically at
ambient pressure with a Curie temperature $T_{C} = 6.8 \pm 0.3$ K. The low
temperature magnetic behavior of this soft ferromagnet is dominated by the
excitation of gapless spin-wave modes. Our results on the transport properties
of U$_4$Ru$_7$Ge$_6$ under pressures up to $2.49$ GPa suggest that
U$_4$Ru$_7$Ge$_6$ has a putative ferromagnetic quantum critical point (QCP) at
$P_c \approx 1.7 \pm 0.02$ GPa. In the ordered phase, ferromagnetic magnons
scatter the conduction electrons and give rise to a well defined power law
temperature dependence in the resistivity. The coefficient of this term is
related to the spin-wave stiffness and measurements of the very low temperature
resistivity allow to accompany the behavior of this quantity as the the
ferromagnetic QCP is approached. We find that the spin-wave stiffness decreases
with increasing pressure implying that the transition to the non-magnetic Fermi
liquid state is driven by the softening of the magnons. The observed quantum
critical behavior of the magnetic stiffness is consistent with the influence of
disorder in our system. At quantum criticality ($P = P_c \approx 1.7 \pm 0.02$
GPa), the resistivity shows the behavior expected for an itinerant metallic
system near a ferromagnetic QCP.",1804.02420v1
2018-11-26,Cavity Spintronics: An Early Review of Recent Progress in the Study of Magnon-Photon Level Repulsion,"Light-matter interactions lie at the heart of condensed matter physics,
providing physical insight into material behaviour while enabling the design of
new devices. Perhaps this is most evident in the push to develop quantum
information and spintronic technologies. On the side of quantum information,
engineered light-matter interactions offer a powerful means to access and
control quantum states, while at the same time new insights into spin-photon
manipulation will benefit the development of spintronic technologies. In this
context the recent discovery of hybridization between ferromagnets and cavity
photons has ushered in a new era of light-matter exploration at the crossroads
of quantum information and spintronics. The key player in this rapidly
developing field of cavity spintronics is a quasiparticle, the
cavity-magnon-polariton. In this early review of recent work, the fundamental
behaviour of the cavity-magnon-polariton is summarized and related to the
development of new spintronic applications. In the last few years a
comprehensive theoretical framework of spin-photon hybridization has been
developed. On an intuitive level many features can be described by a model of
coupled oscillators, however the origin of hybridization is only revealed by
considering a comprehensive electrodynamic framework. Here both approaches are
summarized and a quantum description using the input-output formalism is
outlined. Based on this foundation, in depth experimental investigations of the
coupled spin-photon system have been performed. For example, the influence of
hybridization on spin current generation has been revealed and several in-situ
coupling control mechanisms have been developed. The many recent developments
within this field represent only the first steps in what appears to be a bright
future for cavity spintronics.",1811.10659v1
2018-09-11,Neutron Scattering Investigation of Rhenium Orbital Ordering in $3d-5d$ Double Perovskite Ca$_2$FeReO$_6$,"We have carried out inelastic neutron scattering experiments to study
magnetic excitations in ordered double perovskite Ca$_2$FeReO$_6$. We found a
well-defined magnon mode with a bandwidth of $\sim$50meV below the
ferri-magnetic ordering temperature ($T_c\sim$520K), similar to previously
studied Ba$_2$FeReO$_6$. The spin excitation is gapless for most temperatures
within the magnetically ordered phase. However, a spin gap of $\sim$10meV opens
up below $\sim$150K, which is well below the magnetic ordering temperature but
coincides with a previously reported metal-insulator transition and onset of
structural distortion. The observed temperature dependence of spin gap provides
strong evidence for ordering of Re orbitals at $\sim$150~K, in accordance with
earlier proposal put forward by Oikawa $\it{et.\,al}$ based on neutron
diffraction [J. Phys. Soc. Jpn., $\bf{72}$, 1411 (2003)] as well as recent
theoretical work by Lee and Marianetti [Phys. Rev. B, $\bf{97}$, 045102
(2018)]. The presence of separate orbital and magnetic ordering in
Ca$_2$FeReO$_6$ suggests weak coupling between spin and orbital degrees of
freedom and hints towards a sub-dominant role played by spin orbit coupling in
describing its magnetism. In addition, we observed only one well-defined magnon
band near magnetic zone boundary, which is incompatible with simple
ferrimagnetic spin waves arising from Fe and Re local moments, but suggests a
strong damping of Re magnon mode.",1809.03647v2
2019-08-07,Analysis of the linear relationship between asymmetry and magnetic moment at the M-edge of 3d transition metals,"The magneto-optical response of Fe and Ni during ultrafast demagnetization is
studied experimentally and theoretically. We have performed pump-probe
experiments in the transverse magneto-optical Kerr effect (T-MOKE) geometry
using photon energies that cover the M-absorption edges of Fe and Ni between 40
to 72 eV. The asymmetry was detected by measuring the reflection of light for
two different orientations of the sample magnetization. Density functional
theory (DFT) wasused to calculate the magneto-optical response of different
magnetic configurations, representing different types of excitations:
long-wavelength magnons, short wavelength magnons, and Stoner excitations. In
the case of Fe, we find that the calculated asymmetry is strongly dependent on
the specific type of magnetic excitation. Our modelling also reveals that
during remagnetization Fe is, to a reasonable approximation, described by
magnons, even though small non-linear contributions could indicate some degree
of Stoner excitations as well. In contrast, we find that the calculated
asymmetry in Ni is rather insensitive to the type of magnetic excitations.
However, there is a weak non-linearity in the relation between asymmetry and
the off-diagonal component of the dielectric tensor, which does not originate
from the modifications of the electronic structure. Our experimental and
theoretical results thus emphasize the need of considering a coupling between
asymmetry and magnetization that may be more complex that a simple linear
relationship. This insight is crucial for the microscopic interpretation of
ultrafast magnetization experiments.",1908.02872v1
2019-11-13,Unveiling domain wall dynamics of ferrimagnets in thermal magnon currents: competition of angular momentum transfer and entropic torque,"Control of magnetic domain wall motion holds promise for efficient
manipulation and transfer of magnetically stored information. Thermal magnon
currents, generated by temperature gradients, can be used to move magnetic
textures, from domain walls, to magnetic vortices and skyrmions. In the last
years, theoretical studies have centered in ferro- and antiferromagnetic spin
structures, where domain walls always move towards the hotter end of the
thermal gradient. Here we perform numerical studies using atomistic spin
dynamics simulations and complementary analytical calculations to derive an
equation of motion for the domain wall velocity. We demonstrate that in
ferrimagnets, domain wall motion under thermal magnon currents shows a much
richer dynamics. Below the Walker breakdown, we find that the temperature
gradient always pulls the domain wall towards the hot end by minimizating its
free energy, in agreement with the observations for ferro- and antiferromagnets
in the same regime. Above Walker breakdown, the ferrimagnetic domain wall can
show the opposite, counterintuitive behavior of moving towards the cold end. We
show that in this case, the motion to the hotter or the colder ends is driven
by angular momentum transfer and therefore strongly related to the angular
momentum compensation temperature, a unique property of ferrimagnets where the
intrinsic angular momentum of the ferrimagnet is zero while the sublattice
angular momentum remains finite. In particular, we find that below the
compensation temperature the wall moves towards the cold end, whereas above it,
towards the hot end. Moreover, we find that for ferrimagnets, there is a torque
compensation temperature at which the domain wall dynamics shows similar
characteristics to antiferromagnets, that is, quasi-inertia-free motion and the
absence of Walker breakdown.",1911.05393v1
2019-12-19,Magnon-driven chiral charge and spin pumping and electron-magnon scattering from time-dependent quantum transport combined with atomistic spin dynamics theory,"Using newly developed quantum-classical hybrid framework, we investigate
interaction between spin-polarized conduction electrons and a single spin wave
(SW) coherently excited within a metallic ferromagnetic nanowire. When the
nanowire hosting SW is attached to two normal metal (NM) leads, with no dc bias
voltage applied between them, the SW pumps chiral electronic charge and spin
currents into the leads---their direction is tied to the direction of SW
propagation and they scale linearly with the frequency of the precession. This
is in contrast to: standard pumping by the uniform precession mode with
identical spin currents flowing in both directions and no accompanying charge
current; or experimentally observed [C.~Ciccarelli et al., Nat. Nanotech. 10,
50 (2014); M.~Evelt et al., Phys. Rev. B 95, 024408 (2017)] magnonic charge
pumping which requires spin-orbit coupling spin-orbit coupling. The mechanism
behind our prediction is nonadiabaticity due to time-retardation
effects---motion of localized magnetic moment affects conduction electron spin
in a retarded way, so that it takes a finite time until the electron spin
reacts to the motion of the classical vector. Upon injecting dc spin-polarized
charge current from the left NM lead, electrons interact with SW where
outflowing charge and spin current into the right NM lead are changed due to
both scattering off time-dependent potential generated by the SW and
superposition with the currents pumped by the SW itself. Using Lorentzian
voltage pulse to excite leviton out of the Fermi sea, which carries one
electron charge with no accompanying electron-hole pairs and behaves as
soliton-like quasiparticle, we describe how a single electron interacts with a
single SW.",1912.09452v2
2017-11-30,Topological magnons in a kagome lattice spin system with $XXZ$ and Dzyaloshinskii-Moriya interactions,"We study the phases of a spin system on the Kagome lattice with
nearest-neighbor $XXZ$ interactions with anisotropy ratio $\Delta$ and
Dzyaloshinsky-Moriya interactions with strength $D$. In the classical limit
where the spin $S$ at each site is very large, we find a rich phase diagram of
the ground state as a function of $\Delta$ and $D$. There are five distinct
phases which correspond to different ground state spin configurations in the
classical limit. We use spin wave theory to find the bulk energy bands of the
magnons in some of these phases. We also study a strip of the system which has
infinite length and finite width; we find modes which are localized on one of
the edges of the strip with energies which lie in the gaps of the bulk modes.
In the ferromagnetic phase in which all the spins point along the $+ \hat z$ or
$- \hat z$ direction, the bulk bands are separated from each other by finite
energy gaps. This makes it possible to calculate the Berry curvature at all
momenta, and hence the Chern numbers for every band; the number of edge states
is related to the Chern numbers. Interestingly, we find that there are four
different regions in this phase where the Chern numbers are different. Hence
there are four distinct topological phases even though the ground state spin
configuration is identical in all these phases. We calculate the thermal Hall
conductivity of the magnons as a function of the temperature in the above
ferromagnetic phase; we find that this can distinguish between the various
topological phases. These results are valid for all values of $S$.In the other
phases, there are no gaps between the different bands; hence the edge states
are not topologically protected.",1711.11232v3
2020-09-28,"Magnetic-field dependence of low-energy magnons, anisotropic heat conduction, and spontaneous relaxation of magnetic domains in the cubic helimagnet ZnCr2Se4","Anisotropic low-temperature properties of the cubic spinel helimagnet
ZnCr2Se4 in the single-domain spin-spiral state are investigated by a
combination of neutron scattering, thermal conductivity, ultrasound velocity,
and dilatometry measurements. In an applied magnetic field, neutron
spectroscopy shows a complex and nonmonotonic evolution of the spin-wave
spectrum across the quantum-critical point that separates the spin-spiral phase
from the field-polarized ferromagnetic phase at high fields. A tiny spin gap of
the pseudo-Goldstone magnon mode, observed at wave vectors that are
structurally equivalent but orthogonal to the propagation vector of the spin
helix, vanishes at this quantum critical point, restoring the cubic symmetry in
the magnetic subsystem. The anisotropy imposed by the spin helix has only a
minor influence on the lattice structure and sound velocity but has a much
stronger effect on the heat conductivities measured parallel and perpendicular
to the magnetic propagation vector. The thermal transport is anisotropic at T <
2 K, highly sensitive to an external magnetic field, and likely results
directly from magnonic heat conduction. We also report long-time thermal
relaxation phenomena, revealed by capacitive dilatometry, which are due to
magnetic domain motion related to the destruction of the single-domain magnetic
state, initially stabilized in the sample by the application and removal of
magnetic field. Our results can be generalized to a broad class of helimagnetic
materials in which a discrete lattice symmetry is spontaneously broken by the
magnetic order.",2009.13424v1
2020-09-28,Effective Field Theory of Dark Matter Direct Detection With Collective Excitations,"We develop a framework for computing light dark matter direct detection rates
through single phonon and magnon excitations via general effective operators.
Our work generalizes previous calculations focused on spin-independent
interactions involving the total nucleon and electron numbers $N$ (the usual
route to excite phonons) and spin-dependent interactions involving the total
electron spin $S$ (the usual route to excite magnons), leading us to identify
new responses involving the orbital angular momenta $L$, as well as spin-orbit
couplings $L\otimes S$ in the target. All four types of responses can excite
phonons, while couplings to electron's $S$ and $L$ can also excite magnons. We
apply the effective field theory approach to a set of well-motivated
relativistic benchmark models, including (pseudo-)scalar mediated interactions,
and models where dark matter interacts via a multipole moment, such as a dark
electric dipole, magnetic dipole or anapole moment. We find that couplings to
point-like degrees of freedom $N$ and $S$ often dominate dark matter detection
rates, implying that exotic materials with orbital $L$ order or large
spin-orbit couplings $L\otimes S$ are not necessary to have strong reach to a
broad class of DM models. We highlight that phonon based crystal experiments in
active R&D (such as SPICE) will probe light dark matter models well beyond
those having a simple spin-independent interaction, including e.g. models with
dipole and anapole interactions. Lastly, we make publicly available a code,
PhonoDark, which computes single phonon production rates in a wide variety of
materials with the effective field theory framework.",2009.13534v2
2019-10-08,Highly dispersive magnons with spin-gap like features in the frustrated ferromagnetic S=1/2 chain compound Ca2Y2Cu5O10 detected by inelastic neutron scattering,"We report inelastic neutron scattering experiments in Ca2Y2Cu5O10 and map out
the full one magnon dispersion which extends up to a record value of 53 meV for
frustrated ferromagnetic (FM) edge-sharing CuO2 chain (FFESC) cuprates. A
homogeneous spin-1/2 chain model with a FM nearest-neighbor (NN), an
antiferromagnetic (AFM) next-nearest-neighbor (NNN) inchain, and two diagonal
AFM interchain couplings (ICs) analyzed within linear spin-wave theory (LSWT)
reproduces well the observed strong dispersion along the chains and a weak one
perpendicularly. The ratio R=|J_{a2}/J_{a1}| of the FM NN and the AFM NNN
couplings is found as ~0.23, close to the critical point Rc=1/4 which separates
ferromagnetically and antiferromagnetically correlated spiral magnetic ground
states in single chains, whereas Rc>0.25 for coupled chains is considerably
upshifted even for relatively weak IC. Although the measured dispersion can be
described by homogeneous LSWT, the scattering intensity appears to be
considerably reduced at ~11.5 and ~28 meV. The gap-like feature at 11.5 meV is
attributed to magnon-phonon coupling whereas based on DMRG simulations of the
dynamical structure factor the gap at 28 meV is considered to stem partly from
quantum effects due to the AFM IC. Another contribution is ascribed to the
intrinsic superstructure from the distorting incommensurate pattern of CaY
cationic chains adjacent to the CuO2 ones. It gives rise to non-equivalent CuO4
units and Cu-O-Cu bond angles Phi and a resulting distribution of all exchange
integrals. The J's fitted by homogeneous LSWT are regarded as average values.
The record value of the FM NN integral J1=24 meV among FFESC cuprates can be
explained by a non-universal Phi (not 90 deg.) and Cu-O bond length dependent
anisotropic mean direct FM Cu-O exchange K_{pd}~120 meV. Enhanced K_{pd} values
are also needed to compensate a significant AFM J_{dd} > ~6 meV.",1910.03209v1
2020-01-08,Dirac Hamiltonians for bosonic spectra,"Dirac materials are of great interest as condensed matter realizations of the
Dirac and Weyl equations. In particular, they serve as a starting point for the
study of topological phases. This physics has been extensively studied in
electronic systems such as graphene, Weyl- and Dirac semi-metals. In contrast,
recent studies have highlighted several examples of Dirac-like cones in
collective excitation spectra, viz. in phonon, magnon and triplon bands. These
cannot be directly related to the Dirac or Weyl equations as they are bosonic
in nature with pseudo-unitary band bases. In this article, we show that any
Dirac-like equation can be smoothly deformed into a form that is applicable to
bosonic bands. The resulting bosonic spectra bear a two-to-one relation to that
of the parent Dirac system. Their dispersions inherit several interesting
properties including conical band touching points and a gap-opening-role for
`mass' terms. The relationship also extends to the band eigenvectors with the
bosonic states carrying the same Berry connections as the parent fermionic
states. The bosonic bands thus inherit topological character as well. If the
parent fermionic system has non-trivial topology that leads to mid-gap surface
states, the bosonic analogue also hosts surface states that lie within the
corresponding band gap. The proposed bosonic Dirac structure appears in several
known models. In materials, it is realized in Ba$_2$CuSi$_2$O$_6$Cl$_2$ and
possibly in CoTiO$_3$ as well as in paramagnetic honeycomb ruthenates. Our
results allow for a rigorous understanding of Dirac phononic and magnonic
systems and enable concrete predictions, e.g., of surface states in magnonic
topological insulators and Weyl semi-metals.",2001.02694v2
2020-04-08,Spin dynamics of a magnetic Weyl semimetal Sr$_{1-x}$Mn$_{1-y}$Sb$_2$,"Dirac matters provide a platform for exploring the interplay of their
carriers with other quantum phenomena. Sr$_{1-x}$Mn$_{1-y}$Sb$_2$ has been
proposed to be a magnetic Weyl semimetal and provides an excellent platform to
study the coupling between Weyl fermions and magnons. Here, we report
comprehensive inelastic neutron scattering (INS) measurements on single
crystals of Sr$_{1-x}$Mn$_{1-y}$Sb$_2$, which have been well characterized by
magnetization and magnetotransport measurements, both of which demonstrate that
the material is a topologically nontrivial semimetal. The INS spectra clearly
show a spin gap of $\sim6$ meV. The dispersion in the magnetic Mn layer extends
up to about 76 meV, while that between the layers has a narrow band width of 6
meV. We find that the linear spin-wave theory using a Heisenberg spin
Hamiltonian can reproduce the experimental spectra with the following
parameters: a nearest-neighbor ($SJ_1\sim28.0$ meV) and next-nearest-neighbor
in-plane exchange interaction ($SJ_2\sim9.3$ meV) , interlayer exchange
coupling ($SJ_c\sim-0.1$ meV), and spin anisotropy constant ($SD\sim-0.07$
meV). Despite the coexistence of Weyl fermions and magnons, we find no clear
evidence that the magnetic dynamics are influenced by the Weyl fermions in
Sr$_{1-x}$Mn$_{1-y}$Sb$_2$, possibly because that the Weyl fermions and magnons
reside in the Sb and Mn layers separately, and the interlayer coupling is weak
due to the quasi-two-dimensional nature of the material, as also evident from
the small $SJ_c$ of -0.1 meV.",2004.03893v1
2021-01-25,Long-range spin-wave propagation in transversely magnetized nano-scaled conduits,"Magnonics attracts increasing attention in the view of novel low-energy
computation technologies based on spin waves. Recently, spin-wave propagation
in longitudinally magnetized nano-scaled spin-wave conduits was demonstrated,
proving the fundamental feasibility of magnonics at the sub-100 nm scale.
Transversely magnetized nano-conduits, which are of great interest in this
regard as they offer a large group velocity and a potentially chirality-based
protected transport of energy, have not yet been investigated due to their
complex internal magnetic field distribution. Here, we present a study of
propagating spin waves in a transversely magnetized nanoscopic yttrium iron
garnet conduit of 50 nm width. Space and time-resolved micro-focused
Brillouin-light-scattering spectroscopy is employed to measure the spin-wave
group velocity and decay length. A long-range spin-wave propagation is observed
with a decay length of up to (8.0+-1.5) {\mu}m and a large spin-wave lifetime
of up to (44.7+-9.1) ns. The results are supported with micromagnetic
simulations, revealing a single-mode dispersion relation in contrast to the
common formation of localized edge modes for microscopic systems. Furthermore,
a frequency non-reciprocity for counter-propagating spin waves is observed in
the simulations and the experiment, caused by the trapezoidal cross-section of
the structure. The revealed long-distance spin-wave propagation on the
nanoscale is particularly interesting for an application in spin-wave devices,
allowing for long-distance transport of information in magnonic circuits, as
well as novel low-energy device architectures.",2101.10192v1
2022-06-17,Quantum Dynamics of Magnetic Skyrmions: Consistent Path Integral Formulation,"We present a path integral formalism for the intrinsic quantum dynamics of
magnetic skyrmions coupled to a thermal background of magnetic fluctuations.
Upon promoting the skyrmion's collective coordinate $\boldsymbol{R}$ to a
dynamic variable and integrating out the magnonic heat bath, we derive the
generalized equation of motion for $\boldsymbol{R}$ with a non-local damping
term that describes a steady-state skyrmion dynamics at finite temperatures.
Being essentially temperature dependent, the intrinsic damping is shown to
originate from the coupling of thermally activated magnon modes to the
adiabatic potential driven by a rigid skyrmion motion, which can be regarded as
another manifestation of emergent electrodynamics inherent to topological
magnetic textures. We further argue that the diagonal components of the damping
term act as the source of dissipation and inertia, while its off-diagonal
components modify the gyrotropic motion of a magnetic skyrmion. By means of
numerical calculations for the lattice spin model of chiral ferromagnets, we
study the temperature behavior of the intrinsic damping as a function of
magnetic field in periodic and confined geometries. The intrinsic damping is
demonstrated to be highly non-local, revealing its quantum-mechanical nature,
that becomes more pronounced with increasing temperature. At high temperatures
when the magnon occupation factors are large, the intrinsic damping is shown to
yield a modified Thiele's equation with the additional non-local dissipative
and mass terms that exhibit an almost linear temperature behavior. Our results
provide a microscopic background for semiclassical magnetization dynamics and
establish a framework for understanding spin caloritronics effects in
topological magnetic textures.",2206.08532v2
2022-08-11,Universal behavior of magnon-mediated spin transport in disordered NM/NM/FI heterostructure,"We numerically investigate the magnon-mediated spin transport and scaling
behaviors of a nonmagnetic metal/nonmagnetic metal/ferromagnetic insulator
(NM/NM/FI) heterostructure in the presence of Anderson disorders. For the
two-dimensional (2D) NM/NM/FI system, an enhancement of spin conductance in the
weak disorder regime is found due to the increasing of the interfacial density
of states (DOS) at NM/FI interface. As a result, a new scaling regime is
uncovered in the metallic regime where the spin conductance fluctuation rms(G)
scales linearly with the average spin conductance <G>, independent of system
parameters such as Fermi energies and temperatures. The competition between the
disorder-enhanced interfacial DOS and disorder-suppressed spin transport
results in a non-monotonic dependence of average spin conductance on disorder
strength. In the localized regime, the variance of lnG for different system
parameters follows a universal function that depends linearly on the average of
logarithm of spin conductance <lnG>.The distribution of lnG in the localized
regime is non-Gaussian whose deviation from Gaussian can be characterized by
the third and fourth order cumulants of lnG, denoted as k3 and k4,
respectively. We find that k3~<-lnG>^3/2 and k4~<lnG>^2. This suggests that the
spin conductance mediated by magnon in NM/NM/FI hybrid systems belongs to a
different universality class. For fixed <lnG>, the lnG distribution in the
localized regime for different system parameters is found to collapse onto a
single curve, suggesting the distribution P(lnG;<lnG>) is a universal function
that depends only on <lnG>. Furthermore, spin thermopower S in the presence of
disorder is studied and the variance of lnS is found to scale linearly with
<lnS> for different system parameters in the localized regime. Spin conductance
and spin thermopower for 1D NM/NM/FI are also studied.",2208.05759v2
2023-02-01,Ramification of complex magnetism in Nd$_2$Ir$_2$O$_7$ observed by Raman scattering spectroscopy,"Using Raman scattering spectroscopy, we uncover a complex magnetic behavior
of Nd$_2$Ir$_2$O$_7$, which stands out among magnetic pyrochlores by the lowest
temperature of the all-in-all-out (AIAO) Ir moments ordering
($T^\mathrm{N}_{\small\rm{Ir}}=33$~K) and the highest temperature at which AIAO
order of rare-earth Nd ions is detected ($T^\mathrm{*}_{\small\rm{Nd}}$=15~K).
Our work suggests that in the temperature range between 15~K and 33~K, Nd
magnetic moments exhibit characteristic spin ice fluctuations, which indicate a
strong enhancement of magnetic interactions between Nd ions. This complex
behavior emerges from the interplay of strong spin-orbit coupling, electronic
correlations, and geometric frustration on two magnetic pyrochlore sublattices
of Nd and Ir ions. The ordering of Ir magnetic moments is accompanied by an
appearance of one-magnon Raman modes at 26.3 and 29.6 meV compatible with the
AIAO order and of a broad mode at 14 meV, characteristic of the spinon
continuum originated from Nd spin ice fluctuations. While two one-magnon
excitations show minimal temperature evolution with decreasing temperature, the
14 meV mode shifts to higher frequencies as the temperature approaches a
crossover to Nd AIAO order, broadens, and disappears below 15~K. An additional
two-magnon excitation of the AIAO Nd order at around 33 meV appears in the
spectra at low temperatures. These rather high energies of magnetic excitations
of Nd moments make Nd$_2$Ir$_2$O$_7$ a particularly attractive playground to
study the rare-earth magnetism on the pyrochlore lattice.",2302.00579v2
2023-06-28,Degenerate and non-degenerate parametric excitation in YIG nanostructures,"We study experimentally the processes of parametric excitation in microscopic
magnetically saturated disks of nanometer-thick Yttrium Iron Garnet. We show
that, depending on the relative orientation between the parametric pumping
field and the static magnetization, excitation of either degenerate or
non-degenerate magnon pairs is possible. In the latter case, which is
particularly important for applications associated with the realization of
computation in the reciprocal space, a single-frequency pumping can generate
pairs of magnons whose frequencies correspond to different eigenmodes of the
disk. We show that, depending on the size of the disk and the modes involved,
the frequency difference in a pair can vary in the range 0.1-0.8 GHz. We
demonstrate that in this system, one can easily realize a practically important
situation where several magnon pairs share the same mode. We also observe the
simultaneous generation of up to six different modes using a fixed-frequency
monochromatic pumping. Our experimental findings are supported by numerical
calculations that allow us to unambiguously identify the excited modes. Our
results open new possibilities for the implementation of reciprocal-space
computing making use of low damping magnetic insulators.",2306.16094v1
2024-03-05,Anomalous continuum scattering and higher-order van Hove singularity in the strongly anisotropic S = 1/2 triangular lattice antiferromagnet,"The S = 1/2 triangular lattice antiferromagnet (TLAF) stands out as a
paradigmatic example of frustrated quantum magnetism. An ongoing challenge
involves understanding the influence of exchange anisotropy on the collective
behavior within such systems. Using inelastic neutron scattering (INS) and
advanced calculation techniques, we have studied the low and high temperature
spin dynamics of Ba2La2CoTe2O12 (BLCTO): a Co2+-based Jeff = 1/2 TLAF that
exhibits 120 deg order below TN = 3.26 K. The spin Hamiltonian was determined
by fitting the energy-resolved paramagnetic excitations measured at T > TN,
revealing an exceptionally strong easy-plane XXZ exchange anisotropy. Below TN,
the excitation spectrum exhibits a high energy continuum having a larger
spectral weight than the single-magnon modes. Combined with advanced
theoretical calculations of magnetic excitations based on magnons and spinons,
this observation suggests a scenario characterized by a spinon confinement
length that markedly exceeds the lattice spacing. We conjecture that this
phenomenon arises due to the proximity to a quantum melting point, which
persists even in the presence of strong easy-plane XXZ anisotropy. Finally, we
highlight characteristic flat features in the excitation spectrum, which are
connected to higher-order van Hove singularities in the magnon dispersion and
are directly induced by easy-plane XXZ anisotropy. Our results provide a rare
experimental insight into the nature of highly anisotropic S = 1/2 TLAFs
between the Heisenberg and XY limits.",2403.03210v1
1993-11-09,Spin Excitations and Sum Rules in the Heisenberg Antiferromagnet,"Various bounds for the energy of collective excitations in the Heisenberg
antiferromagnet are presented and discussed using the formalism of sum rules.
We show that the Feynman approximation significantly overestimates (by about
30\% in the $S={1\over2}$ square lattice) the spin velocity due to the non
negligible contribution of multi magnons to the energy weighted sum rule. We
also discuss a different, Goldstone type bound depending explicitly on the
order parameter (staggered magnetization). This bound is shown to be
proportional to the dispersion of classical spin wave theory with a
q-independent normalization factor. Rigorous bounds for the excitation energies
in the anisotropic Heisenberg model are also presented.",9311020v1
1993-11-16,Quantum Phases of the Shraiman-Siggia Model,"We examine phases of the Shraiman-Siggia model of lightly-doped, square
lattice quantum antiferromagnets in a self-consistent, two-loop, interacting
magnon analysis. We find magnetically-ordered and quantum-disordered phases
both with and without incommensurate spin correlations. The quantum disordered
phases have a pseudo-gap in the spin excitation spectrum. The quantum
transition between the magnetically ordered and commensurate quantum-disordered
phases is argued to have the dynamic critical exponent $z=1$ and the same
leading critical behavior as the disordering transition in the pure $O(3)$
sigma model. The relationship to experiments on the doped cuprates is
discussed.",9311037v1
1994-08-02,Composite-Fermion Picture for the Spin-Wave Excitation in the fractional quantum Hall system,"Spin-wave excitation mode from the spin-polarized ground state in the
fractional quantum Hall liquid with odd fractions ($\nu=1/3,1/5$) numerically
obtained by the exact diagonalization of finite systems is shown to be
accurately described, for wavelengths exceeding the magnetic length, in terms
of the composite-fermion mean-field approximation for the spin-wave (magnon)
theory formulated in the spherical geometry. This indicates that the composite
picture extends to excited states, and also provides the spin stiffness in
terms of peculiar exchange interactions.",9408004v1
1995-08-21,Two--magnon scattering and the spin--phonon interaction beyond the adiabatic approximation,"We consider a model of Raman scattering for a two--dimensional $S=1/2$
Heisenberg Anti-Ferromagnet which includes a {\it dynamical} spin--phonon
interaction. We observe a broadening of the line shape due to increased
coupling with excited high--energy spin states. Our results are close to a
model of random static exchange interactions, first introduced in this context
by Haas {\it et al.} [J. Appl. Phys. {\bf 75}, 6340, (1994)], which, when
extended to large numbers of spins, explains experiments in the parent
insulating compounds of high-$T_c$ superconductors.",9508089v1
1995-10-18,Zero field muon spin lattice relaxation rate in a Heisenberg ferromagnet at low temperature,"We provide a theoretical framework to compute the zero field muon spin
relaxation rate of a Heisenberg ferromagnet at low temperature. We use the
linear spin wave approximation. The rate, which is a measure of the spin
lattice relaxation induced by the magnetic fluctuations along the easy axis,
allows one to estimate the magnon stiffness constant.",9510096v1
1995-11-23,Sharp Photoemission Spactra in the Quantum Antiferromagnet,"The low energy photoemission spectra in quantum antiferromagnets are studied
by using several approximation-free calculations and rigorous theorems. The
important and measurable property found is that the hole eigenstates with
momenta differing by the antiferromagnetic wavevector $Q$ are equivalent and
degenerate in energy. However the corresponding eigenstates differs by the
presence or the absence of a well defined quasiparticle corresponding to a
singular -zero energy- magnon, carrying spin one and momentum $Q$. This
difference between the two eigenstates affects dramatically the spectral weight
as a function of the scattered momentum, since a sharp effect at the surface of
the magnetic Brillouin zone is predicted, in apparent agreement with recent
experimental data.",9511114v1
1995-12-22,Theory of Nuclear Magnetic Relaxation in Haldane Gap Materials: An Illustration of the Use of (1+1)-Dimensional Field Theory Techniques,"A comprehensive theory of nuclear magnetic relaxation is developed for
Haldane Gap materials using nonlinear sigma, boson and fermion models. We
include a thorough discussion of of the effects of interchain couplings,
nearest neighbour hyperfine couplings and crystal structure. We also introduce
a new theory of impurities corresponding to broken chain ends weakly coupled to
bulk magnons. The work is compared to recent experiments on NENP.",9512161v2
1996-01-29,Raman Response in Doped Antiferromagnets,"The resonant part of the $B_{1g}$ electronic Raman scattering response is
calculated within the $t-J$ model on a planar lattice as a function of
temperature and hole doping, using a finite-temperature diagonalization method
for small systems. Results, directly applicable to experiments on cuprates,
reveal on doping a very pronounced increase of the width of the two-magnon
Raman peak, accompanied by a decrease of the total intensity. At the same time
the peak position does not shift substantially in the underdoped regime.",9601136v2
1996-07-30,An extension of the characteristic angle method to the easy-plane spin-3/2 ferromagnet,"The Characteristic Angle (CA) method [Lei Zhou and Ruibao Tao, J. Phys. A,
{\bf 27} 5599] developed previously for the easy-plane spin-1 magnetic systems
has been successfully extended to the spin-3/2 case. A compact form of the CA
spin-3/2 operator transformation is given, then the ground state energy, the
magnon dispersion relation and the spontaneous magnetization are discussed for
an easy-plane spin-3/2 ferromagnet by using the CA method. Comparisons with the
old theoretical methods are made in the end.",9607218v1
1997-03-04,Magnetic and thermodynamic properties of Sr_{2}LaFe_{3}O_{9},"Using a Dirac-Heisenberg Hamiltonian with biquadratic exchange interactions,
we study the effect of iron disproportionation on the magnetic ordering, and
describe the first-order magnetic transition occurring in the perovskite
Sr_{2}LaFe_{3}O_{9}. Upon fitting the experimental data, we give an estimate of
the exchange integrals for the antiferromagntic and ferromagnetic interactions,
in agreement with previous works on kindered compounds. Spin-wave theory yields
a magnon spectrum with a gapless antiferromagnetic mode together with two
gapped ferromagnetic ones.",9703044v1
1997-05-26,Symmetry and dimension of the magnon dispersion of inorganic spin-Peierls systems,"The data on the dispersion of the magnetic excitations of CuGeO_3 in the
spin-Peierls dimerized phase are analyzed. On the basis of the lattice
structure it is shown that even along the chains the $d=2$ character cannot be
neglected. The symmetry of the dispersion differs from the one assumed so far.
The magnetic resonance data is reinterpreted. The possibility of interchain
rather than intrachain frustration is discussed.",9705258v1
1997-11-13,An SO(5) Symmetric Ladder,"We construct an SO(5) symmetric electron model on a two-chain ladder with
purely local interactions on a rung. The ground state phase diagram of this
model is determined in the strong-coupling limit. The relationship between the
spin-gap magnon mode of the spin-gap insulator and the $\pi$ resonance mode of
the d-wave pairing phase is discussed. We also present the exact ground state
for an SO(5) superspin model.",9711117v1
1997-12-08,Non-Haldane Spin Liquid Models with Exact Ground States,"We present a family of spin ladder models which admit exact solution for the
ground state and exhibit non-Haldane spin liquid properties as predicted
recently by Nersesyan and Tsvelik [Phys. Rev. Lett. v.78, 3939 (1997)], and
study their excitation spectrum using a simple variational ansatz. The
elementary excitation is neither a magnon nor a spinon, but a pair of
propagating triplet or singlet solitons connecting two spontaneously dimerized
ground states. Second-order phase transitions separate this phase from the
Haldane phase and the rung-dimer phase.",9712087v3
1998-02-04,Dynamical effects of phonons on soliton binding in spin-Peierls systems,"The role of dynamical magneto-elastic coupling in spin-Peierls chains is
investigated by numerical and analytical techniques. We show that a Heisenberg
spin chain coupled to dynamical optical phonons exhibits a transition towards a
spontaneously dimerized state in a wide range of parameter space. The low
energy excitations are characterized as solitons. No binding between solitons
occurs in the isolated spin-phonon chain and the dynamical spin structure
factor shows a broad magnon dispersion. However, elastic interchain coupling
can lead to the formation of bound states.",9802053v2
1998-03-14,Bound states of magnons in the S=1/2 quantum spin ladder,"We study the excitation spectrum of the two-leg antiferromagnetic S=1/2
Heisenberg ladder. Our approach is based on the description of the excitations
as triplets above a strong-coupling singlet ground state. The quasiparticle
spectrum is calculated by treating the excitations as a dilute Bose gas with
infinite on-site repulsion. We find singlet (S=0) and triplet (S=1)
two-particle bound states of the elementary triplets. We argue that bound
states generally exist in any dimerized quantum spin model.",9803180v1
1998-06-18,The symmetry problem in NaV2O5,"We discuss the symmetry of NaV2O5 in the high temperature phase on the basis
of optical conductivity data. Conclusive information cannot be obtained by
studying the optically allowed lattice vibrations. However, intensity and
polarization of the electronic excitations give a direct indication for a
broken-parity electronic ground-state. This is responsible for the detection of
charged bi-magnons in the optical spectrum.",9806222v1
1998-07-09,Magnitoelastic interaction and long-range magnetic ordering in two-dimesional ferromagnetics,"The influence of magnitoelastic (ME) interaction on the stabilization of
long-range magnetic order (LMO) in the two-dimensional easy-plane ferromagnetic
is investigated in this work. The account of ME exchange results in the root
dispersion law of magnons and appearance of ME gap in the spectra of elementary
excitations. Such a behavior of the spectra testifies to the stabilization of
LMO and finite Curie's temperature.",9807129v1
1998-08-24,Theory of magnetic excitations in the Kondo lattice,"We present a theory for the spin excitations of the Kondo lattice. We derive
an effective Hamiltonian, which describes Fermionic spin 1/2 charge
fluctuations interacting with Bosonic triplet spin fluctuations. We show that
already the simplest approximation possible, i.e. replacing the magnon
self-energy by the free polarization bubble for the charge excitation Greens
function, gives a quantitative description of the `spin gap' and `charge gap'
as obtained in numerical calculations for the Kondo insulator.",9808261v1
1998-12-09,Thermodynamics of the t-J Ladder: A Stable Finite Temperature Density Matrix Renormalization Group Calculation,"Accurate numerical simulations of a doped t-J model on a two-leg ladder are
presented for the particle number, chemical potential, magnetic susceptibility
and entropy in the limit of large exchange coupling on the rung using a finite
temperature density matrix renormalization group (TDMRG) method. This required
an improved algorithm to achieve numerical stability down to low temperatures.
The thermal dissociation of hole pairs and of the rung singlets are separately
observed and the evolution of the hole pair binding energy and magnon spin gap
with hole doping is determined.",9812144v1
1999-05-24,Temperature behavior of the magnon modes of the square lattice antiferromagnet,"A spin-wave theory of short-range order in the square lattice Heisenberg
antiferromagnet is formulated. With growing temperature from T=0 a gapless mode
is shown to arise simultaneously with opening a gap in the conventional
spin-wave mode. The spectral intensity is redistributed from the latter mode to
the former. For low temperatures the theory reproduces results of the modified
spin-wave theory by M.Takahashi, J.E.Hirsch et al. and without fitting
parameters gives values of observables in good agreement with Monte Carlo
results in the temperature range 0 <= T < 0.8J where J is the exchange
constant.",9905340v1
1999-06-16,Perturbation Theory by Flow Equations: Dimerized and Frustrated S=1/2 Chain,"The flow equation method (Wegner 1994) is used as continuous unitary
transformation to construct perturbatively effective Hamiltonians. The method
is illustrated in detail for dimerized and frustrated antiferromagnetic S=1/2
chains. The effective Hamiltonians conserve the number of elementary
excitations which are S=1 magnons for the dimerized chains. The sectors of
different number of excitations are clearly separated. Easy-to-use results for
the gap, the dispersion and the ground state energies of the chains are
provided.",9906243v1
1999-06-25,Structure and relevant dimension of the Heisenberg model and applications to spin rings,"For the diagonalization of the Hamilton matrix in the Heisenberg model
relevant dimensions are determined depending on the applicable symmetries.
Results are presented, both, by general formulae in closed form and by the
respective numbers for a variety of special systems. In the case of cyclic
symmetry, diagonalizations for Heisenberg spin rings are performed with the use
of so-called magnon states. Analytically solvable cases of small spin rings are
singled out and evaluated.",9906382v1
1999-08-26,Magnons in real materials from density-functional theory,"We present an implementation of the adiabatic spin-wave dynamics of Niu and
Kleinman. This technique allows to decouple the spin and charge excitations of
a many-electron system using a generalization of the adiabatic approximation.
The only input for the spin-wave equations of motion are the energies and Berry
curvatures of many-electron states describing frozen spin spirals. The latter
are computed using a newly developed technique based on constrained
density-functional theory, within the local spin density approximation and the
pseudo-potential plane-wave method. Calculations for iron show an excellent
agreement with experiments.",9908386v1
2000-02-04,Robust half-metallicity and metamagnetism in Fe_{x}Co_{1-x}S_{2},"The Fe_{x}Co_{1-x}S_{2} system is predicted, on the basis of density
functional calculations, to be a half metal for a large range of
concentrations. Unlike most known half metals, the half metallicity in this
system should be very stable with respect to crystallographic disorder and
other types of defects. The endmember of the series, CoS_{2}, is not a half
metal, but exhibits interesting and unusual magnetic properties which can,
however, be reasonably well understood within the density functional theory,
particularly with the help of the extended Stoner model. Calculations suggest
strong electron-phonon and electron-magnon coupling in the system, and probably
a bad metal behavior at high temperatures.",0002069v1
2000-03-21,Quantum renormalization of high energy excitations in the 2D Heisenberg antiferromagnet,"We find using Monte Carlo simulations of the spin-1/2 2D square lattice
nearest neighbour quantum Heisenberg antiferromagnet that the high energy peak
locations at (pi,0) and (pi/2,pi/2) differ by about 6%, (pi/2,pi/2) being the
highest. This is a deviation from linear spin wave theory which predicts equal
magnon energies at these points.",0003350v2
2000-05-09,Spin-Waves in itinerant ferromagnets,"We introduce a novel approach for the investigation of spin-wave excitations
in itinerant ferromagnets. Our theory is based on a variational treatment of
general multi-band Hubbard models which describe elements and compounds of
transition metals. The magnon dispersion is derived approximately as the energy
of a variational spin-wave state in the limit of large spatial dimensions. A
numerical evaluation of our results is feasible for general multi-band models.
As a first application we consider a model with two degenerate orbitals per
lattice site. From our results we can conclude that spin-wave excitations in
strong itinerant ferromagnets are very similar to those in ferromagnetic spin
systems.",0005154v1
2000-05-24,Andreev Peaks and Massive Magnons in Cuprate SNS junctions,"The projected SO(5) theory (pSO(5)) is used to resolve the puzzle of two
distinct energy gaps in high T$_c$ Superconductor-Normal-Superconductor
junctions. Counter to conventional theory of multiple Andreev reflections
(MAR), the differential resistance peaks are associated with the
antiferromagnetic resonance observed in neutron scattering, and not with Cooper
pair breaking. The pSO(5) and MAR theories differ by the expected tunneling
charges at the peaks. We propose that shot noise experiments could discriminate
against the conventional interpretation.",0005420v2
2000-08-01,Introduction to the Bethe Ansatz III,"Having introduced the magnon in part I and the spinon in part II as the
relevant quasi-particles for the interpretation of the spectrum of low-lying
excitations in the one-dimensional (1D) s=1/2 Heisenberg ferromagnet and
antiferromagnet, respectively, we now study the low-lying excitations of the
Heisenberg antiferromagnet in a magnetic field and interpret these collective
states as composites of quasi-particles from a different species. We employ the
Bethe ansatz to calculate matrix elements and show how the results of such a
calculation can be used to predict lineshapes for neutron scattering
experiments on quasi-1D antiferromagnetic compounds. The paper is designed as a
tutorial for beginning graduate students. It includes 11 problems for further
study.",0008018v1
2000-08-09,Bulk experimental evidence of half-metallic ferromagnetism in doped manganites,"We report precise measurements and quantitative data analysis on the
low-temperature resistivity of several ferromagnetic manganite films. We
clearly show that there exists a T^{4.5} term in low-temperature resistivity,
and that this term is in quantitative agreement with the quantum theory of
two-magnon scattering for half metallic ferromagnets. Our present results
provide the first bulk experimental evidence of half-metallic ferromagnetism in
doped manganites.",0008151v1
2000-08-10,Discussion of a spin-cluster model for the low temperature phase of NaV_2O_5,"We discuss magnetic excitations of a spin-cluster model which has been
suggested to describe the low temperature phase of alpha'-NaV_2O_5. This model
fulfills all symmetry criteria proposed by recent x-ray investigations. We find
that this model is not able to describe the occurence of two well separated
magnon lines perpendicular to the ladder direction as observed in INS
experiments. We suggest further experimental analysis to generally distinguish
between models with double reflection or inversion symmetry.",0008172v2
2000-10-19,Magnetic Collective Mode Dispersion in High Temperature Superconductors,"Recent neutron scattering experiments in the superconducting state of YBCO
have been interpreted in terms of a magnetic collective mode whose dispersion
relative to the commensurate wavevector has a curvature opposite in sign to a
conventional magnon dispersion. The purpose of this article is to demonstrate
that simple linear response calculations are in support of a collective mode
interpretation, and to explain why the dispersion has the curvature it does.",0010298v1
2000-10-20,Dynamical stripe structure in La$_{2-x}$Sr$_x$CuO$_4$ observed by Raman scattering,"The dynamical stripe structure relating to the ""1/8 problem"" was investigated
in La$_{2-x}$Sr$_x$CuO$_4$ utilizing the high frequency response of Raman
scattering. The split of the two-magnon peak due to the formation of the stripe
structure was observed at whole Sr concentration region from $x=0.035$ to 0.25
at low temperatures. Especially clear split was observed at low carrier
concentration region $x=0.035 - 0.06$ and at $x \sim 1/8$. The onset
temperatures of these stripe structures are as high as 300-350 K, which are
much higher than the temperatures measured by slow response probes.",0010305v1
2000-11-07,Electron Spin Flip Relaxation by One Magnon Processes: Application to the Gadolinium surface band,"The ""s-f model"", also known as the ferromagnetic Kondo lattice, contains a
description of band electrons coupled to localized spins which is an
appropriate description of the magnetic part of the low-energy physics of Gd
metal. Here the model is used to estimate the lifetime broadening of the
minority spin component of the surface electron band in ferromagnetic
gadolinium metal at temperatures below the Curie temperature. The low
temperature result 0.10 eV agrees nicely with a measurement by Fedorov et al.",0011121v2
2000-12-06,Dispersion of the neutron resonance in cuprate superconductors,"We argue that recently measured downward dispersion of the neutron resonance
peak in cuprate superconductors is naturally explained if the resonance is
viewed as a spin-1 collective mode in a d-wave superconductor. The reduction of
the resonant frequency away from the antiferromagnetic wave vector is a direct
consequence of the momentum dependence of the d-wave superconducting gap. When
the magnetic correlation length becomes large, the dispersion should become
magnon-like, i.e., curve upwards from (pi,pi).",0012065v2
2001-04-05,Nonlinear dynamics of the classical isotropic Heisenberg antiferromagnetic chain: the sigma model sector and the kink sector,"We identify two distinct low-energy sectors in the classical isotropic
antiferromagnetic Heisenberg spin-S chain. In the continuum limit, we show that
two types of rotation generators arise for the field in each sector. Using
these, the Lagrangian for sector I is shown to be that of the nonlinear sigma
model. Sector II has a null Lagrangian; Its Hamiltonian density is just the
Pontryagin term. Exact solutions are found in the form of magnons and
precessing pulses in I and moving kinks in II. The kink has `spin' S. Sector I
has a higher minimum energy than II.",0104093v1
2001-05-23,Field-Induced Magnetic Order in Quantum Spin Liquids,"We study magnetic field-induced three-dimensional ordering transitions in
low-dimensional quantum spin liquids, such as weakly coupled, antiferromagnetic
spin-1/2 Heisenberg dimers and ladders. Using stochastic series expansion
quantum Monte Carlo simulations, thermodynamic response functions are obtained
down to ultra-low temperatures. We extract the critical scaling exponents which
dictate the power-law dependence of the transition temperature on the applied
magnetic field. These are compared with recent experiments on candidate
materials and with predictions for the Bose-Einstein condensation of magnons
obtained in mean-field theory.",0105464v1
2001-05-31,Strong Enhancement of Superconducting T_c in Ferromagnetic Phases,"It is shown that the critical temperature for spin-triplet, p-wave
superconductivity mediated by spin fluctuations is generically much higher in a
Heisenberg ferromagnetic phase than in a paramagnetic one, due to the coupling
of the magnons to the longitudinal magnetic susceptibility. Together with the
tendency of the low-temperature ferromagnetic transition in very clean
Heisenberg magnets to be of first order, this qualitatively explains the phase
diagram recently observed in UGe_2.",0105627v2
2001-06-04,An analytical strong coupling approach in dynamical mean-field theory,"In the limit of infinite spatial dimensions a thermodynamically consistent
theory of the strongly correlated electron systems, which is valid for
arbitrary value of the Coulombic interaction ($U<\infty$), is built. For the
Hubbard model the total auxiliary single-site problem exactly splits into four
subspaces, which describe Fermi and non-Fermi liquid components. Such
analytical approach allows to construct different thermodynamically consistent
approximations: alloy-analogy approximation, Hartree-Fock approximation and
further, that describes the self-consistent renormalization of the bosonic
excitations (magnons and doublons).",0106044v2
2001-06-06,Low Voltage I-V Characteristics in Magnetic Tunnel Junctions,"We show that elastic currents that take into account variations of the tunnel
transmitivity with voltage and a large ratio of majority to minority spin
densities of states of the $s$ band, can account for the low voltage current
anomalies observed in magnet-oxide-magnet junctions. The anomalies can be
positive, negative or have a mixed form, depending of the position of the Fermi
level in the $s$ band, in agreement with observations. Magnon contribution is
negligible small to account for the sharp drop of the magnetoresistance with
the voltage bias.",0106123v1
2001-07-16,Continuous canonical transformation for the double exchange model,"The method of continuous canonical transformation is applied to the double
exchange model with a purpose to eliminate the interaction term responsible for
non conservation of magnon number. Set of differential equations for the
effective Hamiltonian parameters is derived. Within the lowest order
(approximate) solution we reproduce results of the standard (single step)
canonical transformation. Results of the selfconsistent numerical treatment are
compared with the other known studies for this model.",0107331v1
2001-08-06,Li Induced Spin and Charge Excitations in a Spin Ladder,"A lithium dopant in a cuprate spin ladder acts as a vacant (non-magnetic)
site accompanied by an extra hole bound by a Coulomb potential. We find that,
although the undoped ladder spin gap is not essentially altered by Li doping, a
dopant-magnon bound state appears within the gap. Contrary to the case of
Zn-doped ladders, we predict that, in the Li-doped ladder, the spin liquid
character is very robust against antiferromagnetism. We also predict the
spatial dependence of the density of states in the vicinity of the dopant which
could be measured by local spectroscopic probes.",0108099v1
2001-08-27,Independent magnon states on magnetic polytopes,"For many spin systems with constant isotropic antiferromagnetic
next-neighbour Heisenberg coupling the minimal energies E_{min}(S) form a
rotational band, i.e. depend approximately quadratically on the total spin
quantum number S, a property which is also known as Lande interval rule.
However, we find that for certain coupling topologies, including recently
synthesised icosidodecahedral structures this rule is violated for high total
spins. Instead the minimal energies are a linear function of total spin. This
anomaly results in a corresponding jump of the magnetisation curve which
otherwise would be a regular staircase.",0108432v1
2001-08-29,Order by distortion and string modes in pyrochlore antiferromagnet,"We study the effects of magnetoelastic couplings on pyrochlore
antiferromagnets. We employ Landau theory, extending an investigation begun by
Yamashita and Ueda for the case of S=1, and semiclassical analyses to argue
that such couplings generate bond order via a spin--Peierls transition. This is
followed by, or concurrent with, a transition into one of several possible
low-temperature Neel phases, with most simply collinear, but also coplanar or
mixed spin patterns. In a collinear Neel phase, a dispersionless string-like
magnon mode dominates the resulting excitation spectrum, providing a
distinctive signature of the parent geometrically frustrated state. We comment
on the experimental situation.",0108505v1
2001-10-16,"Spin Waves, Phase Separation, and Interphase Boundaries in Double Exchange Magnets","We study a classical double exchange magnet with direct antiferromagnetic
superexchange coupling, J, between the localized spins. It is shown that the
de-stabilization of the ferromagnetic ground state with increasing J leads to
phase separation; the latter always preempts the spin-wave instability
(softening of the magnon spectrum). It is also found that the boundaries
separating the ferromagnetic and antiferromagnetic areas of the sample tend to
be abrupt.",0110322v2
2001-12-24,Magnetic phase diagram of the half-filled t-t'-Hubbard model -- finite-U effects on competing interactions and frustration,"The magnon propagator is evaluated in the AF (\pi,\pi,\pi) and the F-AF
(0,\pi,\pi) states at the RPA level, and the spin-fluctuation corrections are
compared. Transverse spin fluctuations are sharply enhanced by the frustration-
inducing NNN hopping t', reducing the zero-temperature AF order in two
dimensions, and the Neel temperature in three dimensions. The phase boundary
between the insulating AF and F-AF states is obtained in the full range of
interaction strength, indicating that the AF state is interestingly stabilized
with decreasing U.",0112442v1
2002-04-04,"Boundaries, Cusps and Caustics in the Multimagnon Continua of 1D Quantum Spin Systems","The multimagnon continua of 1D quantum spin systems possess several
interesting singular features that may soon be accessible experimentally
through inelastic neutron scattering. These include cusps and composition
discontinuities in the boundary envelopes of two-magnon continuum states and
discontinuities in the density of states, ""caustics"", on and within the
continuum, which will appear as discontinuities in scattering intensity. In
this note we discuss the general origins of these continuum features, and
illustrate our results using the alternating Heisenberg antiferromagnetic chain
and two-leg ladder as examples.",0204115v1
2002-04-14,Raman scattering through surfaces having biaxial symmetry,"Magnetic Raman scattering in two-leg spin ladder materials and the
relationship between the anisotropic exchange integrals are analyzed by P. J.
Freitas and R. R. P. Singh in Phys. Rev. B, {\bf 62}, 14113 (2000). The angular
dependence of the two-magnon scattering is shown to provide information for the
magnetic anisotropy in the Sr_14Cu_24O_41 and La_6Ca_8Cu_24O_41 compounds. We
point out that the experimental results of polarized Raman measurements at
arbitrary angles with respect to the crystal axes have to be corrected for the
light ellipticity induced inside the optically anisotropic crystals. We refer
quantitatively to the case of Sr_14Cu_24O_41 and discuss potential implications
for spectroscopic studies in other materials with strong anisotropy.",0204312v1
2002-06-15,Effect of a Staggered Magnetic Field on the S=1 Haldane Chain with Single-ion Anisotropy,"We analyze the gaps in the excitation spectrum of a Haldane chain with
single-ion anisotropy in a staggered field. We show that the gap along the
direction of the field increases at a faster rate than the others, while its
spectral weight decreases, being transferred to a two-magnon continuum.",0206288v1
2002-07-04,Large N behavior of string solutions in the Heisenberg model,"We investigate the large $N$ behavior of the complex solutions for the two
magnon system in the S=1/2 Heisenberg XXZ model. The Bethe ansatz equations are
numerically solved for the string solutions with new iteration method. A clear
evidence of the violation of the string configurations is found at
$N=22,62,121,200,299,417$, but the broken states are still Bethe states. The
number of the Bethe states is consistent with the exact diagonalization, except
one singular state.",0207117v1
2002-07-04,Quantum spin liquid in antiferromagnetic chain S=1/2 with Acoustic Phonons,"A spin and phonon excitations spectrum are studied using quantum Monte Carlo
method in antiferromagnetic chain with spins $S=1/2$ coupled nonadiabaticity
with acoustic phonons . It is found the critical coupling exists to open gap in
the triplet excitation spectrum for any phonon velocity. The phase boundaries
of delocalized phonons and propagated the bound states of magnon and a phonon
are calculated. It is shown that the spherical symmetry of the spin-spin
correlation functions is broken . The magnetic and optical properties $CuGeO_3$
are explained without using spin-Peierls transition.",0207118v1
2002-10-15,Spin-wave Hamiltonian in double-exchange systems,"A simple derivation of the effective spin-wave Hamiltonian for a
double-exchange system with infinitely large Hund's-rule coupling is
demonstrated. The formalism can be applied to models with arbitrary range of
hopping as well as those with randomness. The result shows that, within the
leading order of the 1/S expansion, one magnon excitation spectrum can be
described by the Heisenberg model.",0210333v1
2003-01-23,Excitations of anisotropic spin-1 chains with matrix product ground state,"We investigate a large class of antiferromagnetic spin-1 chains with nearest
neighbour interaction and exactly known matrix product ground state. The
spectrum of low-lying excitations is calculated numerically by DMRG and exact
diagonalisation. Spin liquid behaviour with an excitation gap is observed
meeting the Haldane scenario. Further, we compare properties of the anisotropic
model with the well-known isotropic AKLT model and use the analytical single
mode approximation to obtain a quantitative understanding of the excitation
gap. The low-lying excited states can be interpreted in terms of a magnon-like
elementary excitation.",0301435v1
2003-03-20,About possible Phonon to Magnon alignment in 2 dimensions and theory of superconductivity in Copper-Oxide planes,"We suggest that the phonon dispersion in cuprates becomes strongly
anisotropic due to interaction with spin waves; moreover the phonon dispersion
becomes singular along $|k_x|=|k_y|$ directions. This would allow more
electrons to form Cooper pairs and increase temperature of the superconducting
transition. The interaction of phonons with spin waves is more important than
the interaction of phonons with free electrons, because spin waves do not have
the Fermi surface constrain.",0303402v5
2003-04-25,Peculiar roles of spins in the thermal conductivity of pure and doped La_{2}CuO_{4}: Comparison with CuGeO_{3},"In-plane and out-of-plane thermal conductivities (\kappa_{ab} and \kappa_{c})
are measured on single crystals of pure, 1%-hole-doped, and 1%-Zn-doped
La_{2}CuO_{4}. The roles of magnons and the spin stripes in the heat transport
in these samples are discussed. Comparison with the heat transport in
CuGeO_{3}, which shows similar \kappa(T) behavior as that of La_{2}CuO_{4},
gives us a lesson of how the heat transport can probe the difference in the
spin ground state.",0304569v1
2003-05-19,"Raman scattering studies of spin, charge, and lattice dynamics in Ca_{2-x}Sr_{x}RuO_{4} (0 =< x < 0.2)","We use Raman scattering to study spin, charge, and lattice dynamics in
various phases of Ca_{2-x}Sr_{x}RuO_{4}. With increasing substitution of Ca by
Sr in the range 0 =< x < 0.2, we observe (1) evidence for an increase of the
electron-phonon interaction strength, (2) an increased temperature-dependence
of the two-magnon energy and linewidth in the antiferromagnetic insulating
phase, and (3) evidence for charge gap development, and hysteresis associated
with the structural phase change, both of which are indicative of a first-order
metal-insulator transition (T_{MI}) and a coexistence of metallic and
insulating components for T < T_{MI}.",0305427v1
2003-06-25,Dimerization versus Orbital Moment Ordering in the Mott insulator YVO$_3$,"We use exact diagonalization combined with mean-field theory to investigate
the phase diagram of the spin-orbital model for cubic vanadates. The spin-orbit
coupling competes with Hund's exchange and triggers a novel phase, with the
ordering of $t_{2g}$ orbital magnetic moments stabilized by the tilting of
VO$_6$ octahedra. It explains qualitatively spin canting and reduction of
magnetization observed in YVO$_3$. At finite temperature an orbital Peierls
instability in the $C$-type antiferromagnetic phase induces modulation of
magnetic exchange constants even in absence of lattice distortions. The
calculated spin structure factor shows a magnon splitting due to the orbital
Peierls dimerization.",0306637v1
2003-07-07,Universal scaling at field-induced magnetic phase transitions,"We study field-induced magnetic order in cubic lattices of dimers with
antiferromagnetic Heisenberg interactions. The thermal critical exponents at
the quantum phase transition from a spin liquid to a magnetically ordered phase
are determined from Stochastic Series Expansion Quantum Monte Carlo
simulations. These exponents are independent of the interdimer coupling ratios,
and converge to the value obtained by considering the transition as a
Bose-Einstein condensation of magnons, alpha_(BEC) = 1.5. The scaling results
are of direct relevance to the spin-dimer systems TlCuCl_3 and KCuCl_3, and
explain the broad range of exponents reported for field-induced ordering
transitions.",0307126v1
2003-07-15,Signatures of pairing mechanisms and order parameters in ferromagnetic superconductors,"Two predictions are made for properties of the ferromagnetic superconductors
discovered recently. The first one is that spin-triplet, p-wave pairing in such
materials will give the magnons a mass inversely proportional to the square of
the magnetization. The second one is based on a specific mechanism for p-wave
pairing, and predicts that the observed broad anomaly in the specific heat of
URhGe will be resolved into a split transition with increasing sample quality.
These predictions will help discriminate between different possible mechanisms
for ferromagnetic superconductivity.",0307348v1
2003-11-03,Current-driven domain wall motion in thin ferromagnetic wires,"The coupling between a current and a Bloch wall is examined in the half-metal
limit of the double exchange model. The conduction electrons transfer angular
momentum to the Bloch wall with 100% efficiency in the absence of pinning. The
wall is displaced without distortion with velocity proportional to the current.
In the presence of a pinning potential either the angular momentum is destroyed
by the perpendicular component of the anisotropy field or is converted to
coherent magnons. A moving wall has its velocity reduced by pinning. The
expression for the velocity agrees surprisingly well with experiment.",0311039v2
2004-01-20,The spin excitation spectrum in striped bilayer compounds,"The spin dynamics of bilayer cuprate compounds are studied in a basic model.
The magnetic spectral properties are calculated in linear spin-wave theory for
several stripe configurations which differ by the relative location of the
stripes in the layers. We focus on the bilayer splitting of the magnon bands
near the incommensurate low energy peaks as well as near the $\pi$ resonance,
distinguishing between the odd and even channel. We find that a x-shaped
dispersion near the $\pi$ resonance is generic for stripes. By comparison of
our results to neutron scattering data for $\mathrm{YBa_2Cu_3O_{6+x}}$ we
conclude that the stripe model is consistent with characteristic features of
bilayer high-$T_c$ compounds.",0401354v1
2004-01-24,Normal metal to ferromagnetic superconductor tunneling,"We study the point-contact tunneling between normal metal and ferromagnetic
superconductor. In the case of magnon-induced pairing the tunneling conductance
is continuous and smooth function of the applied voltage. For small values of
the applied voltage the Ohm law holds. We show that one can obtain the
magnetization and the superconducting order parameter from the tunneling
conduc- tance. In the case of paramagnon-induced superconductivity the
tunneling does not depend on the magnetization. We argue that tunneling
experiment can unambiguously determine the correct pairing mechanism in the
ferromagnetic superconductors.",0401480v2
2004-01-31,Resonant magnetic mode in superconducting 2-leg ladders,"The spin dynamics of a doped 2-leg spin ladder is investigated by numerical
techniques. We show that a hole pair-magnon boundstate evolves at finite hole
doping into a sharp magnetic excitation below the two-particle continuum. This
is supported by a field theory argument based on a SO(6)-symmetric ladder.
Similarities and differences with the resonant mode of the high-T$_c$ cuprates
are discussed.",0402011v1
2004-02-20,Low-frequency incommensurate magnetic response in strongly correlated systems,"It is shown that in the t-J model of Cu-O planes at low frequencies the
dynamic spin structure factor is peaked at incommensurate wave vectors
(1/2+-delta,1/2)$, (1/2,1/2+-delta). The incommensurability is connected with
the momentum dependencies of the magnon frequency and damping near the
antiferromagnetic wave vector. The behavior of the incommensurate peaks is
similar to that observed in La_{2-x}(Ba,Sr)_xCuO_{4+y} and YBa_2Cu_3O_{7-y}:
for hole concentrations 0.02<x<=0.12 we find that delta is nearly proportional
to x, while for x>0.12 it tends to saturation. The incommensurability
disappears with increasing temperature. Generally the incommensurate magnetic
response is not accompanied by an inhomogeneity of the carrier density.",0402512v1
2004-02-25,Spin-polaron model: transport properties of EuB$_6$,"To understand anomalous transport properties of EuB$_6$, we have studied the
spin-polaron Hamiltonian incorporating the electron-phonon interaction.
Assuming a strong exchange interaction between the carriers and the localized
spins, the electrical conductivity is calculated. The temperature and magnetic
field dependence of the resistivity of EuB$_6$ are well explained. At low
temperature, magnons dominate the conduction process, whereas the lattice
contribution becomes significant at very high temperature due to the scattering
with the phonons. Large negative magnetoresistance near the ferromagnetic
transition is also reproduced as observed in EuB$_6$.",0402608v1
2004-04-15,Magnetic groundstate and Fermi surface of bcc Eu,"Using spin-spiral technique within the full potential linearized
augmented-plane-waves (LAPW) electronic structure method we investigate the
magnon spectrum and N\'eel temperature of bcc Eu. Ground state corresponding to
an incommensurate spin-spiral is obtained in agreement with experiment and
previous calculations. We demonstrate that the magnetic coupling is primarily
through the intra-atomic $f-s$ and $f-d$ exchange and
Ruderman-Kittel-Kasuya-Yosida mechanism. We show that the existence of this
spin-spiral is closely connected to a nesting feature of the Fermi surface
which was not noticed before.",0404369v1
2004-05-25,Exact low-temperature behavior of kagome antiferromagnet at high fields,"Low-energy degrees of freedom of a spin-1/2 kagome antiferromagnet in the
vicinity of the saturation field are mapped to a hard-hexagon model on a
triangular lattice. The latter model is exactly solvable. The presented mapping
allows to obtain quantitative description of the magnetothermodynamics of a
quantum kagome antiferromagnet up to exponentially small corrections as well as
predict the critical behavior for the transition into a magnon crystal state.
Analogous mapping is presented for the sawtooth chain, which is mapped onto a
model of classical hard dimers on a chain.",0405578v2
2004-06-21,"Magnetic susceptibility, exchange interactions and spin-wave spectra in the local spin density approximation","Starting from exact expression for the dynamical spin susceptibility in the
time-dependent density functional theory a controversial issue about exchange
interaction parameters and spin-wave excitation spectra of itinerant electron
ferromagnets is reconsidered. It is shown that the original expressions for
exchange integrals based on the magnetic force theorem (J. Phys. F14 L125
(1984)) are optimal for the calculations of the magnon spectrum whereas static
response function is better described by the ``renormalized'' magnetic force
theorem by P. Bruno (Phys. Rev. Lett. 90, 087205 (2003)). This conclusion is
confirmed by the {\it ab initio} calculations for Fe and Ni.",0406488v1
2004-07-09,Stability of the spiral phase in the 2D extended t-J model,"We analyze the t-t'-t''-J model at low doping by chiral perturbation theory
and show that the (1,0) spiral state is stabilized by the presence of t',t''
above critical values around 0.2J, assuming t/J=3.1. We find that the (magnon
mediated) hole-hole interactions have an important effect on the region of
charge stability in the space of parameters t',t'', generally increasing
stability, while the stability in the magnetic sector is guaranteed by the
presence of spin quantum fluctuations (order from disorder effect). These
conclusions are based on perturbative analysis performed up to two loops, with
very good convergence.",0407242v1
2004-08-18,Hole dynamics in spin and orbital ordered vanadium perovskites,"Hole dynamics in spin and orbital ordered vanadates with perovskite structure
is investigated. A mobile hole coupled to the spin excitation (magnon) in the
spin G-type and orbital C-type (SG/OC) ordered phase, and that to the orbital
excitation (orbiton) in the spin C-type and orbital G-type (SC/OG) one are
formulated on an equal footing. The observed fragile character of the (SG/OC)
order is attributed to the orbiton softening caused by a reduction of the
taggered magnetic order parameter. It is proposed that the qualitatively
different hole dynamics in the two spin-orbital ordered phases in vanadates can
be probed by the optical spectra.",0408395v1
2004-10-19,Haldane-gap chains in a magnetic field,"We consider quasi one dimensional spin-1 Heisenberg chains with crystal field
anisotropy in a uniform magnetic field. We determine the dynamical structure
factor in various limits and obtain a fairly complete qualitative picture of
how it changes with the applied field. In particular, we discuss how the width
of the higher energy single magnon modes depends on the field. We consider the
effects of a weak interchain coupling. We discuss the relevance of our results
for recent neutron scattering experiments on the quasi-1D Haldane-gap compound
NDMAP.",0410487v1
2004-10-20,Spin-Transfer Torque and Electron-Magnon Scattering,"According to the spin-torque model, current-driven magnetic dynamics in
ferromagnetic multilayers is determined by the transfer of electron spin
perpendicular to the layers' magnetizations. By separating the largest
contributions to the magnetic dynamics, we demonstrate that the dominant effect
of spin-torque is rather due to the electron spin parallel to the field. We
show that this effect can be equivalently described as stimulated
current-driven excitation of spin-waves, and discuss four specifically
quantum-mechanical aspects of spin-transfer, not described by the spin-torque.",0410520v1
2004-11-13,Existence of the Spin-Wave Gap in a Deformed Flat-Band Hubbard Model,"We consider a deformed flat-band Hubbard model under a periodic boundary
condition in arbitrary dimensions. We show that the ground state is only
all-spin-up or -down state. We obtain upper and lower bounds of the one-magnon
spin-wave energy with an arbitrary momentum. This dispersion relation is the
same as that in the XXZ model in the certain parameter region. Therefore the
spin wave has a finite energy gap. These results suggests the our model and the
XXZ model.",0411362v1
2004-11-28,Two-magnon Raman scattering in spin-ladders with exact singlet-rung ground state,"We suggest a family of spin-ladder models with exact singlet-rung vacuum. In
addition to the terms describing interactions along legs and rungs the
Hamiltonian has frustration and cyclic terms. Using coordinate Bethe ansatze
all one- and two-dimensional states are constructed. The explicit formula for
zero-temperature Raman scattering cross section is derived. The corresponding
line-shapes are strongly asymmetric and have no more than one van-Hove
singularity. The results are in good agreement with experimental data.",0411700v2
2004-12-08,"The Gap Function Phi(k,w) for a Two-leg t-J Ladder and the Pairing Interaction","The gap function phi(k,omega), determined from a Lanczos calculation for a
doped 2-leg t-J ladder, is used to provide insight into the spatial and
temporal structure of the pairing interaction. It implies that this interaction
is a local near-neighbor coupling which is retarded. The onset frequency of the
interaction is set by the energy of an S=1 magnon-hole-pair and it is spread
out over a frequency region of order the bandwith.",0412186v1
2004-12-09,Modified Spin-Wave Theory for Nanomagnets : Application to the Keplerate Molecule Mo(72)Fe(30),"We adapt Takahashi's modified spin-wave theory to the context of
nano-magnets, and apply it to the molecular compound based on the giant
magnetic molecule Mo(72)Fe(30). This involves solving numerically the
mean-field equations and then forcing the sublattice magnetizations to zero by
means of local chemical potentials for the magnons. We have thus constructed a
quantum state with no local magnetization at all temperatures, appropriate to a
finite-size system, but with strong correlations. We compare theoretical
results to specific heat and ESR measurements.",0412244v1
2005-01-20,Quantum Heisenberg antiferromagnets: a survey of the activity in Firenze,"Over the years the research group in Firenze has produced a number of
theoretical results concerning the statistical mechanics of quantum
antiferromagnetic models, which range from the theory of two-magnon Raman
scattering to the characterization of the phase transitions in quantum
low-dimensional antiferromagnetic models. Our research activity was steadily
aimed to the understanding of experimental observations.",0501496v1
2005-02-22,Large magnetic field-induced spectral weight enhancement of high-energy spin excitations in $La_{1.88}Sr_{0.12}CuO_{4}$,"We report electronic Raman scattering experiments on a superconducting ${\rm
La_{1.88}Sr_{0.12}CuO_{4}}$ single crystal in a magnetic field. At low
temperatures, the spectral weight of the high-energy two-magnon peak increases
linearly with field and is amplified by a factor of more than two at 14 T. The
effect disappears at elevated temperatures and is not present in undoped ${\rm
La_{2}CuO_{4}}$. This observation is discussed in terms of an electronically
inhomogeneous state in which the field enhances the volume fraction of a phase
with local antiferromagnetic order at the expense of the superconducting phase.",0502524v1
2005-02-24,Lifetime Widths of Surface States on Magnetic Lanthanide Metals,"Low-temperature scanning tunneling spectroscopy is used to study electronic
structure and dynamics of d-like surface states of trivalent lanthanide metals
from La to Lu. The magnetic exchange splitting of these states is found to
scale with the 4f spin multiplied by an effective exchange-coupling constant
that increases with 4f occupancy in an approximately linear way. The dynamics
of the surfaces states, as revealed by the lifetime width, is dominated by
electron-phonon scattering in the occupied region and by electron-magnon
scattering in the unoccupied region, respectively.",0502581v1
2005-04-01,Doped two-leg ladder with ring exchange,"The effect of a ring exchange on doped two-leg ladders is investigated
combining exact diagonalization (ED) and density matrix renormalization group
(DMRG) computations. We focus on the nature and weights of the low energy
magnetic excitations and on superconducting pairing. The stability with respect
to this cyclic term of a remarkable resonant mode originating from a hole
pair-magnon bound state is examined. We also find that, near the zero-doping
critical point separating rung-singlet and dimerized phases, doping reopens a
spin gap.",0504027v2
2005-04-26,"Magnetic exchange coupling and Curie temperature of Ni(1+x)MnSb (x=0, 0.25, 0.5, 0.75, 1) from first principles","We study the dependence of magnetic interactions and Curie temperature in
Ni(1+x)MnSb system on the Ni concentration within the framework of the
density-functional theory. The calculation of the exchange parameters is based
on the super-cell and frozen-magnon approaches. The Curie temperatures, Tc, are
calculated within the random-phase approximation. In agreement with experiment
we obtain decrease of the Curie temperature with increasing Ni content.",0504685v1
2005-05-31,Dynamical properties of S=1 bond-alternating Heisenberg chains at finite temperatures,"Dynamical structure factors of the S=1 bond-alternating spin chains in the
dimer phase are calculated at finite temperature, using the pair dynamical
correlated-effective-field approximation. At T=0, the delta-function-type peak
of the one-magnon mode appears. When temperature is increased, such a sharp
peak is broadened and the additional weak peak caused by the excitation from
the triplet state to the quintet state emerges in the higher energy region. The
results are discussed in comparison with those obtained by the exact
diagonalization method.",0505735v3
2005-06-29,Nonanalytic Corrections to Fermi-Liquid Behavior in Helimagnets,"The Goldstone mode in the ordered phase of itinerant helimagnets, such as
MnSi or FeGe, is determined and shown to have a strongly anisotropic dispersion
relation. The softness of this mode is, in a well-defined sense, in between
that of ferromagnetic and antiferromagnetic magnons, respectively. It is shown
that this soft mode leads to nonanalytic corrections to Fermi-liquid behavior,
with a T-contribution to the specific heat coefficient, and a
T^{5/2}-contribution to the resistivity. The quasi-particle inelastic lifetime
shows anisotropic behavior in momentum space.",0506770v1
2005-07-20,Persistent spin current in mesoscopic ferrimagnetic spin ring,"Using a semiclassical approach, we study the persistent magnetization current
of a mesoscopic ferrimagnetic ring in a nonuniform magnetic field. At zero
temperature, there exists persistent spin current because of the quantum
fluctuation of magnons, similar to the case of an antiferromagnetic spin ring.
At low temperature, the current shows activation behavior because of the
field-induced gap. At higher temperature, the magnitude of the spin current is
proportional to temperature T, similar to the reported result of a
ferromagnetic spin ring.",0507467v2
2005-11-06,Dynamics of composite Haldane spin chains in IPA-CuCl3,"Magnetic excitations in the quasi-one-dimensional antiferromagnet IPA-CuCl3
are studied by cold neutron inelastic scattering. Strongly dispersive gap
excitations are observed. Contrary to previously proposed models, the system is
best described as an asymmetric quantum spin ladder. The observed spectrum is
interpreted in terms of ``composite'' Haldane spin chains. The key difference
from actual S=1 chains is a sharp cutoff of the single-magnon spectrum at a
certain critical wave vector.",0511143v1
2006-03-05,Dynamical vertex approximation - a step beyond dynamical mean field theory,"We develop a diagrammatic approach with local and nonlocal self-energy
diagrams, constructed from the local irreducible vertex. This approach includes
the local correlations of dynamical mean field theory and long-range
correlations beyond. It allows for example to describe (para-)magnons and weak
localization effects in strongly correlated systems. As a first application, we
study the interplay between nonlocal antiferromagnetic correlations and the
strong local correlations emerging in the vicinity of a Mott-Hubbard
transition.",0603100v2
2006-03-14,Fingerprints of the Magnetic Polaron in Nonequilibrium Electron Transport through a Quantum Wire Coupled to a Ferromagnetic Spin Chain,"We study nonequilibrium quantum transport through a mesoscopic wire coupled
via local exchange to a ferromagnetic spin chain. Using the Keldysh formalism
in the self-consistent Born approximation, we identify fingerprints of the
magnetic polaron state formed by hybridization of electronic and magnon states.
Because of its low decoherence rate, we find coherent transport signals. Both
elastic and inelastic peaks of the differential conductance are discussed as a
function of external magnetic fields, the polarization of the leads and the
electronic level spacing of the wire.",0603374v2
2006-05-06,Evidence for multimagnon-mediated nuclear spin relaxation in the intertwining double-cain ferrimagnet $Ca_3 Cu_3 (PO_4)_4$,"The nuclear spin-lattice relaxation time $T_1$ of $^{31}P$ nuclei in the
title compound is measured for the first time and interpreted in terms of a
modified spin-wave theory. We establish a novel scenario for one-dimensional
ferrimagnetic spin dynamics -- it is three-magnon processes enhanced by
exchange scattering, rather than Raman processes, that make the major
contribution to $1/T_1$.",0605157v2
2006-05-22,Coupling of phonons and electromagnons in GdMnO_3,"The infrared and Terahertz properties of GdMnO_3 have been investigated as
function of temperature and magnetic field, with special emphasis on the phase
boundary between the incommensurate and the canted antiferromagnetic
structures. The heterogeneous incommensurate phase reveals strong
magnetodielectric effects, characterized by significant magnetoelectric
contributions to the static dielectric permittivity and by the existence of
electrically excited magnons (electromagnons). In the commensurate canted
antiferromagnetic phase the magnetoelectric contributions to the dielectric
constant and electromagnons are suppressed. The corresponding spectral weight
is transferred to the lowest lattice vibration demonstrating the strong
coupling of phonons with electromagnons.",0605523v1
2006-07-07,Broken axisymmetry phase of a spin-1 ferromagnetic Bose-Einstein condensate,"A spin-1 ferromagnetic Bose-Einstein condensate subject to a certain magnetic
field exhibits a broken-axisymmetry phase in which the magnetization tilts
against the applied magnetic field due to the competition between
ferromagnetism and linear and quadratic Zeeman effects. The Bogoliubov analysis
shows that in this phase two Goldstone modes associated with U(1) and SO(2)
symmetry breakings exist, in which phonons and magnons are coupled to restore
the two broken symmetries.",0607184v1
2006-09-17,Universal conductance fluctuations in epitaxial GaMnAs ferromagnets: structural and spin disorder,"Mesoscopic transport measurements reveal a large effective phase coherence
length in epitaxial GaMnAs ferromagnets, contrary to usual 3d-metal
ferromagnets. Universal conductance fluctuations of single nanowires are
compared for epilayers with a tailored anisotropy. At large magnetic fields,
quantum interferences are due to structural disorder only, and an unusual
behavior related to hole-induced ferromagnetism is evidenced, for both quantum
interferences and decoherence. At small fields, phase coherence is shown to
persist down to zero field, even in presence of magnons, and an additional spin
disorder contribution to quantum interferences is observed under domain walls
nucleation.",0609410v1
2006-09-25,"Light-Induced Magnetic Precession in (Ga,Mn)As Slabs: Hybrid Standing-Wave Damon-Eshbach Modes","Coherent oscillations associated with spin precessions were observed in
ultrafast optical experiments on ferromagnetic (Ga,Mn)As films. Using a
complete theoretical description of the processes by which light couples to the
magnetization, values for the exchange and anisotropy constants were obtained
from the field-dependence of the magnon frequencies and the
oscillation-amplitude ratios. Results reveal a relatively large negative
contribution to the energy due to surface anisotropy leading to excitations
that are a mixture of bulk waves and surface modes.",0609646v1
2006-10-23,Mechanism of temperature dependence of the magnetic anisotropy energy in ultrathin Cobalt and Nickel films,"Temperature dependent FMR-measurements of Ni and Co films are analysed using
a microscopic theory for ultrathin metallic systems. The mechanism governing
the temperature dependence of the magnetic anisotropy energy is identified and
discussed. It is reduced with increasing temperature. This behavior is found to
be solely caused by magnon excitations.",0610649v1
2006-10-27,Local moment fluctuations in an optimally-doped high Tc superconductor,"We present results of neutron scattering experiments on YBa2Cu3O6.95
(Tc=93K). Our results indicate that magnetic collective modes due to correlated
local moments are present both above and below Tc in optimally doped YBCO. The
magnon-like modes are robust and not overdamped by itinerant particle-hole
excitations, which may point at a substantial static or slowly fluctuating
charge inhomogeneity. We compare the experimental results to predictions of the
Fermi liquid (FL) theory in the Random Phase Approximation (RPA).",0610755v2
2006-12-12,Lattice gas description of pyrochlore and checkerboard antiferromagnets in a strong magnetic field,"Quantum Heisenberg antiferromagnets on pyrochlore and checkerboard lattices
in a strong external magnetic field are mapped onto hard-core lattice gases
with an extended exclusion region. The effective models are studied by the
exchange Monte Carlo simulations and by the transfer matrix method. The
transition point and the critical exponents are obtained numerically for a
square-lattice gas of particles with the second-neighbor exclusion, which
describes a checkerboard antiferromagnet. The exact structure of the magnon
crystal state is determined for a pyrochlore antiferromagnet.",0612283v2
2006-12-21,Spin wave excitations: The main source of the temperature dependence of Interlayer exchange coupling in nanostructures,"Quantum mechanical calculations based on an extended Heisenberg model are
compared with ferromagnetic resonance (FMR) experiments on prototype trilayer
systems Ni_7/Cu_n/Co_2/Cu(001) in order to determine and separate for the first
time quantitatively the sources of the temperature dependence of interlayer
exchange coupling. Magnon excitations are responsible for about 75% of the
reduction of the coupling strength from zero to room temperature. The remaining
25% are due to temperature effects in the effective quantum well and the
spacer/magnet interfaces.",0612568v1
2007-03-05,Non--Heisenberg Spin Dynamics of Double-Exchange Ferromagnets with Coulomb Repulsion,"With a variational three--body calculation we study the role of the interplay
between the onsite Coulomb, Hund's rule, and superexchange interactions on the
spinwave excitation spectrum of itinerant ferromagnets. We show that
correlations between a Fermi sea electron--hole pair and a magnon result in a
very pronounced zone boundary softening and strong deviations from the
Heisenberg spinwave dispersion. We show that this spin dynamics depends
sensitively on the Coulomb and exchange interactions and discuss its possible
relevance to experiments in the manganites.",0703099v2
2007-03-12,Effect of Polarized Current on the Magnetic State of Antiferromagnet,"We provide evidence for the effects of spin polarized current on a
nanofabricated antiferromagnet incorporated into a spin-valve structure.
Signatures of current-induced effects include bipolar steps in differential
resistance, current-induced changes of exchange bias correlated with these
steps, and deviations from the statistics expected for thermally activated
switching of spin valves. We explain our observations by a combination of spin
torque exerted on the interfacial antiferromagnetic moments, and
electron-magnon scattering in antiferromagnet.",0703281v2
2007-03-30,Field-induced paramagnons at the metamagnetic transition in Ca1.8Sr0.2RuO4,"The magnetic excitations in Ca1.8Sr0.2RuO4 were studied across the
metamagnetic transition and as a function of temperature using inelastic
neutron scattering. At low temperature and low magnetic field the magnetic
response is dominated by a complex superposition of incommensurate
antiferromagnetic fluctuations. Upon increasing the magnetic field across the
metamagnetic ransition, paramagnon and finally well-defined magnon scattering
is induced, partially suppressing the incommensurate signals. The high-field
phase in Ca1.8Sr0.2RuO4 has, therefore, to be considered as an intrinsically
ferromagnetic state stabilized by the magnetic field.",0703806v1
1992-12-18,Finite size and temperature effects in the AF Heisenberg model,"The low temperature and large volume effects in the d=2+1 antiferromagnetic
quantum Heisenberg model are dominated by magnon excitations. The leading and
next-to-leading corrections are fully controlled by three physical constants,
the spin stiffness, the spin wave velocity and the staggered magnetization.
Among others, the free energy, the ground state energy, the low lying
excitations, staggered magnetization, staggered and uniform susceptibilities
are studied here. The special limits of very low temperature and infinite
volume are considered also.",9212022v1
1993-11-10,Nonrelativistic effective Lagrangians,"Chiral perturbation theory is extended to nonrelativistic systems with
spontaneously broken symmetry. In the effective Lagrangian, order parameters
associated with the generators of the group manifest themselves as effective
coupling constants of a topological term, which is gauge invariant only up to a
total derivative. In the case of the ferromagnet, a term connected with the
Brouwer degree dominates the derivative expansion. The general analysis
includes antiferromagnetic magnons and phonons, while the effective field
theory of fluids or gases is beyond the scope of the method.",9311264v1
1999-07-26,The Casimir energy of skyrmions in the 2+1-dimensional O(3)-model,"One-loop quantum corrections to the classical vortices in 2+1 dimensional
O(3)-models are evaluated. Skyrme and Zeeman potential terms are used to
stabilize the size of topological solitons. Contributions from zero modes,
bound-states and scattering phase-shifts are calculated for vortices with
winding index n=1 and n=2. For both cases the S-matrix shows a pronounced
series of resonances for magnon-vortex scattering in analogy to the
well-established baryon resonances in hadron physics, while vortices with n>2
are already classically unstable against decay. The quantum corrections
destabilize the classically bound n=2 configuration. Approximate independence
of the results with respect to changes in the renormalization scale is
demonstrated.",9907492v1
1992-04-17,Heisenberg XXZ Model and Quantum Galilei Group,"The 1D Heisenberg spin chain with anisotropy of the XXZ type is analyzed in
terms of the symmetry given by the quantum Galilei group Gamma_q(1). We show
that the magnon excitations and the s=1/2, n-magnon bound states are determined
by the algebra. Thus the Gamma_q(1) symmetry provides a description that
naturally induces the Bethe Ansatz. The recurrence relations determined by
Gamma_q(1) permit to express the energy of the n-magnon bound states in a
closed form in terms of Tchebischeff polynomials.",9204054v1
1998-08-28,"Goldstone and Pseudo-Goldstone Bosons in Nuclear, Particle and Condensed-Matter Physics","These notes review the effective lagrangian treatment of Goldstone and
pseudo-Goldstone bosons, taking examples from high-energy/nuclear and
condensed-matter physics. The contents are:
  1. Goldstone Bosons
  2. Pions: A Relativistic Application
  3. Magnons: Nonrelativistic Applications
  4. SO(5) Invariance and Superconductors
  5. Bibliography",9808176v3
2005-04-19,"Notes on non-extremal, charged, rotating black holes in minimal D=5 gauged supergravity","We consider the non-extremal, charged, rotating black hole solution of five
dimensional minimal gauged supergravity of Cvetic, Lu and Pope [Phys.Lett. B
598 (2004) 273]. We compute the Ashtekar-Magnon-Das mass and show it agrees
with the thermodynamic mass. We find a reducible Killing tensor and integrate
the geodesic equation explicitly. We also compute the Euclidean action of the
black hole and show it satisfies the quantum statistical relation. Further we
present a Smarr relation. We end with a discussion of applications to string
theory.",0504158v1
2005-11-28,Controlling quantum state transfer in spin chain with the confined field,"As a demonstration of the spectrum-parity matching condition (SPMC) for
quantum state transfer, we investigate the propagation of single-magnon state
in the Heisenberg chain in the confined external tangent magnetic field
analytically and numerically. It shows that the initial Gaussian wave packet
can be retrieved at the counterpart location near-perfectly over a longer
distance if the dispersion relation of the system meets the SPMC approximately.",0511246v1
2006-03-14,Levinson theorem for Aharonov-Bohm scattering in two dimensions,"We apply the recently generalized Levinson theorem for potentials with
inverse square singularities [Sheka et al, Phys.Rev.A, v.68, 012707 (2003)] to
Aharonov-Bohm systems in two-dimensions. By this theorem, the number of bound
states in a given m-th partial wave is related to the phase shift and the
magnetic flux. The results are applied to 2D soliton-magnon scattering.",0603123v3
2007-04-26,Coupling of phonons and spin waves in triangular antiferromagnet,"We investigate the influence of the spin-phonon coupling in the triangular
antiferromagnet where the coupling is of the exchange-striction type. The
magnon dispersion is shown to be modified significantly at wave vector (2pi,0)
and its symmetry-related points, exhibiting a roton-like minimum and an
eventual instability in the dispersion. Various correlation functions such as
equal-time phonon correlation, spin-spin correlation, and local magnetization
are calculated in the presence of the coupling.",0704.3510v2
2007-05-22,Two interacting GL-equations in High-T$_c$ superconductivity and quantum chromodynamics,"The possible connection between High-T$_c$ superconductivity and quantum
chromodynamics is considered that is based on two interacting Ginzburg-Landau
equations. For High-T$_c$ superconductivity these two equations describe Cooper
electrons interacting with different kind of quasi particles (phonons, magnons,
excitons and so on). The interaction term describes a possible interaction
between different kind of quasi particles. For quantum chromodynamics the
equations describe two kinds of gauge condensates. The condensates describe a
gauge potential from a subalgebra of the SU(3) gauge group and the
corresponding coset. Regular solutions are found which describe the situation
where one field, $\psi_1$, is pushed out by another, $\psi_2$.",0705.3170v1
2007-06-14,Spin-waves in a complex magnetic system: nonextensive approach,"In this paper we analyze the spin-wave excitations (magnons) of an
inhomogeneous spin system within the Boltzmann-Gibbs framework and then connect
the results with the nonextensive approach (in the sense of Tsallis
statistics). Considering an equivalence between those two frameworks, we could
connect the entropic parameter q with moments of the distribution of exchange
integrals of the inhomogenous system. It ratifies the idea that the entropic
parameter is connected to the microscopic properties of the system.",0706.2201v2
2007-11-15,The QCD spin chain S matrix,"Beisert et al. have identified an integrable SU(2,2) quantum spin chain which
gives the one-loop anomalous dimensions of certain operators in large N QCD. We
derive a set of nonlinear integral equations (NLIEs) for this model, and
compute the scattering matrix of the various (in particular, magnon)
excitations.",0711.2415v3
2007-12-15,Dynamic and Static Excitations of a Classical Discrete Anisotropic Heisenberg Ferromagnetic Spin Chain,"Using Jacobi elliptic function addition formulas and summation identities we
obtain several static and moving periodic soliton solutions of a classical
anisotropic, discrete Heisenberg spin chain with and without an external
magnetic field. We predict the dispersion relations of these nonlinear
excitations and contrast them with that of magnons and relate these findings to
the materials realized by a discrete spin chain. As limiting cases, we discuss
different forms of domain wall structures and their properties.",0712.2503v1
2008-03-20,Interplay between magnetic and lattice excitations in BiFeO$_3$,"We have performed Raman scattering investigations on the high energy magnetic
excitations in a BiFeO$_3$ single crystal as a function of both temperature and
laser excitation energy. A strong feature observed at 1250 cm$^{-1}$ in the
Raman spectra has been previously assigned to two phonon overtone. We show here
that its unusual frequency shift with the excitation energy and its asymmetric
temperature dependent Fano lineshape reveal a strong coupling to magnetic
excitations. In the same energy range, we have also identified the two-magnon
excitation with a temperature dependence very similar to $\alpha$-Fe$_2$O$_3$
hematite.",0803.3015v1
2008-04-14,Dissipation characteristics of quantized spin waves in nano-scaled magnetic ring structures,"The spatial profiles and the dissipation characteristics of spin-wave
quasi-eigenmodes are investigated in small magnetic Ni$_{81}$Fe$_{19}$ ring
structures using Brillouin light scattering microscopy. It is found, that the
decay constant of a mode decreases with increasing mode frequency. Indications
for a contribution of three-magnon processes to the dissipation of higher-order
spin-wave quasi-eigenmodes are found.",0804.2200v1
2008-04-14,On the observation of the spin resonance in superconducting CeCoIn_5,"Recent observation of a resonance spin excitation at (1/2,1/2,1/2) in the
superconducting state of CeCoIn_5 [C. Stock et al., Phys. Rev. Lett. {\bf 100}
087001 (2008)] was interpreted as an evidence for d_{x^2-y^2} gap symmetry, by
analogy with the cuprates. This is true if the resonance is a spin exciton. We
argue that such description is undermined by the three-dimensionality of
CeCoIn_5. We show that in 3D systems the excitonic resonance only emerges at
strong coupling, and is weak. We argue in favor of the alternative, magnon
scenario, which does not require a d_{x^2-y^2} gap.",0804.2217v1
2008-05-11,Evolution of Spin-Orbital-Lattice Coupling in the $R$VO$_3$ Perovskites,"We introduce a microscopic model which unravels the physical mechanisms
responsible for the observed phase diagram of the $R$VO$_3$ perovskites. It
reveals a nontrivial interplay between superexchange, the orbital-lattice
coupling due to the GdFeO$_3$-like rotations of the VO$_6$ octahedra, and
orthorhombic lattice distortions. We find that the lattice strain affects the
onset of the magnetic and orbital order by partial suppression of orbital
fluctuations. The present approach provides also a natural explanation of the
observed reduction of magnon energies from LaVO$_3$ to YVO$_3$.",0805.1545v1
2008-05-20,The Exchange Parameters in Lanthanum Manganites,"The spin-wave spectrum in the metallic ferromagnetic lantha-num cubic
manganites was investigated. The interactions with four coordination spheres
(32 neighbors) were taken into ac-count. The Heisenberg superexchange between
all manganese ions as well as double exchange between the manganese ions with
different valence were under consideration. The obtained magnon spectrum
permits to investigate dispersion relations along an any crystallographic
directions. This possibility was used for the numerical evaluation of the
exchange parameters in the compounds La0.75Ca0.25MnO3 and La0.70Ca0.30MnO3 from
the inelastic neutron scattering data.",0805.3006v1
2008-06-12,Finite-size effects in the superconformal beta-deformed N=4 SYM,"We study finite size effects for composite operators in the SU(2) sector of
the superconformal beta-deformed N=4 SYM theory. In particular we concentrate
on the spectrum of one single magnon. Since in this theory one-impurity states
are non BPS we compute their anomalous dimensions including wrapping
contributions up to four loops and discuss higher order effects.",0806.2103v2
2008-06-13,Negative differential conductivity in far-from-equilibrium quantum spin chains,"We show that, when a finite anisotropic Heisenberg spin-1/2 chain in the
gapped regime is driven far from equilibrium, oppositely polarized
ferromagnetic domains build up at the edges of the chain, thus suppressing
quantum spin transport. As a consequence, a negative differential conductivity
regime arises, where increasing the driving decreases the current. The above
results are explained in terms of magnon localization and are shown to be
structurally stable against breaking of integrability.",0806.2236v2
2008-06-24,Spin dynamics in point contacts to single ferromagnetic films,"Excitation of magnons or spin-waves driven by nominally unpolarized transport
currents in point contacts of normal and ferromagnetic metals is probed by
irradiating the contacts with microwaves. Two characteristic dynamic effects
are observed: a rectification of off-resonance microwave current by spin-wave
nonlinearities in the point contact conductance, and a resonant stimulation of
spin-wave modes in the nano-contact core by the microwave field. These
observations provide a direct evidence that the magnetoconductance effects
observed are due to GHz spin dynamics at the ferromagnetic interface driven by
the spin transfer torque effect of the transport current.",0806.3805v1
2008-09-03,Detection of coherent magnons via ultrafast pump-probe reflectance spectroscopy in multiferroic Ba0.6Sr1.4Zn2Fe12O22,"We report the detection of a magnetic resonance mode in multiferroic
Ba0.6Sr1.4Zn2Fe12O22 using time domain pump-probe reflectance spectroscopy.
Magnetic sublattice precession is coherently excited via picosecond thermal
modification of the exchange energy. Importantly, this precession is recorded
as a change in reflectance caused by the dynamic magnetoelectric effect. Thus,
transient reflectance provides a sensitive probe of magnetization dynamics in
materials with strong magnetoelectric coupling, such as multiferroics,
revealing new possibilities for application in spintronics and ultrafast
manipulation of magnetic moments.",0809.0672v1
2008-09-28,Coherent terahertz needle radiation emission mechanism from nanostructured ferromagnetic metal films excited by femtosecond laser pulses,"A coherent terahertz needle radiation emission mechanism is proposed. This
mechanism can be realized in the experiments on femtosecond laser pulses
interacting with surface of nanostructured ferromagnetic metal film. The
proposed mechanism is based on exciting of coupled coherent phonon-magnon modes
on a nanostructured metal surface by laser femtosecond pulse. The value of the
Poynting vector for the terahertz radiation is estimated.",0809.4868v1
2008-11-27,Single impurity operators at critical wrapping order in the beta-deformed N=4 SYM,"We study the spectrum of one single magnon in the superconformal
beta-deformed N=4 SYM theory in the planar limit. We compute the anomalous
dimensions of one-impurity operators O_{1,L}= tr(phi Z^{L-1}), including
wrapping contributions at their critical order L.",0811.4594v2
2008-12-30,BPS ansatzes as electric form-factors,"We argue that BPS ansatzes, entering manifestly vacuum BPS monopole solutions
to equations of motion in the (Minkowskian) non-Abelian Higgs model play the
role of some electric form-factors and that this implies (soft) violating the
CP-invariance of the mentioned model, similar to taking place in the Euclidian
Yang-Mills (YM) theory with instantons, generating the $\theta$-term in the
appropriate effective Hamiltonian.",0812.5080v5
2009-01-11,Magnetic soliton transport over topological spin texture in chiral helimagnet with strong easy-plane anisotropy,"We show the existence of an isolated soliton excitation over the topological
ground state configuration in chiral helimagnet with the Dzyaloshinskii-Moryia
exchange and the strong easy-plane anisotropy. The magnetic field perpendicular
to the helical axis stabilizes the kink crystal state which plays a role of
""topological protectorate"" for the traveling soliton with a definite
handedness. To find new soliton solution, we use the B\""{a}cklund
transformation technique. It is pointed out that the traveling soliton carries
the magnon density and a magnetic solition transport may be realized.",0901.1423v1
2009-01-25,Sketching the Theory of Copper Oxide High Temperature Superconductivity,"The role of the anti-bonding state in the electron correlations in Copper
Oxide HTSC is analyzed. Then the t-J Hamiltonian is used to establish the
formation of the charge stripes in underdoped oxides. It is proposed that these
stripes make up the boundaries between the two degenerate anti-ferromagnetic
(AFM) states, and that they are a key factor in switching between these states.
We also provide a theoretical expression for the charge driven AFM magnons that
have been observed by Neutron scattering experiments. Finally, the double
correlation theory is applied to the stripe phase of holes to result in the
superconductive gap and in the ""pseudogap"".",0901.3896v2
2009-02-10,Low-temperature thermodynamics of one class of flat-band models,"We consider the antiferromagnetic Heisenberg model and the repulsive Hubbard
model for a class of frustrated lattices with a completely dispersionless
(flat) lowest one-particle (either one-magnon or one-electron) band. We
construct exact many-particle ground states for a wide range of particle
densities, calculate their degeneracy, and, as a result, obtain closed-form
expressions for the low-temperature thermodynamic quantities around a
particular value of the magnetic field $h_{\rm{sat}}$ or the chemical potential
$\mu_0$. We confirm our analytic findings by numerical data for finite
lattices.",0902.1627v1
2009-02-10,The Bethe Ansatz Equations for Reflecting Magnons,"We derive the Bethe Ansatz Equations on the half line for particles
interacting through factorized $S$-matrices invariant relative to the centrally
extended $su(2|2)$ Lie superalgebra and $su(1|2)$ open boundaries. These
equations may be of relevance for the study of the spectrum of open strings on
$AdS_{5} \times S^5$ background attached to Y=0 giant graviton branes. An
one-dimensional spin chain hamiltonian associated to this system is also
derived.",0902.1681v2
2009-02-25,Hydrodynamic relation in 2D Heisenberg antiferromagnet in a field,"The spin-stiffness \rho_s of a 2D Heisenberg antiferromagnet depends
non-analytically on external magnetic field. We demonstrate that the
hydrodynamic relation between \rho_s, the uniform susceptibility \chi, and the
spin-wave velocity $c$ is not violated by such a behavior because similar
non-analytic terms from all three quantities mutually cancel out. In this work,
explicit expressions for the field-dependent spin stiffness and for the magnon
velocity of the 2D square lattice antiferromagnet are obtained by direct
calculation to order 1/S and in the whole range of magnetic fields.",0902.4455v1
2009-03-04,On the wrapping correction to single magnon energy in twisted N=4 SYM,"We apply Zeilberger summation to derive a closed formula for the wrapping
correction to one-impurity states in the su(2) sector of the beta-deformed N=4
SYM theory at beta=1/2. As an application depending heavily on the result, we
compute the large volume expansion of the wrapping correction.",0903.0778v2
2009-04-02,Emergent geometry in N=6 Chern-Simons-matter theory,"We investigate a strong coupling expansion of N=6 superconformal Chern-Simons
theory obtained from the semiclassical analysis of low energy, effective
degrees of freedom given by the eigenvalues of a certain matrix model. We show
how the orbifolded sphere S^7/Z_k of the dual geometry emerges dynamically from
the distribution of the eigenvalues. As a test of this approach we compute the
energy of off-diagonal excitations, finding perfect agreement with the
dispersion relation of giant magnons.",0904.0449v2
2009-05-05,Edge exponent in the dynamic spin structure factor of the Yang-Gaudin model,"The dynamic spin structure factor $\mathcal{S}(k,\omega)$ of a system of
spin-1/2 bosons is investigated at arbitrary strength of interparticle
repulsion. As a function of $\omega$ it is shown to exhibit a power-law
singularity at the threshold frequency defined by the energy of a magnon at
given $k.$ The power-law exponent is found exactly using a combination of the
Bethe Ansatz solution and an effective field theory approach.",0905.0598v1
2009-05-12,Polar phonons and spin excitations coupling in multiferroic BiFeO3 crystals,"Raman scattering measurements on BiFeO3 single crystals show an important
coupling between the magnetic order and lattice vibrations. The temperature
evolution of phonons shows that the lowest energy E and A1 phonon modes are
coupled to the spin order up to the Neel temperature. Furthermore, low
temperature anomalies associated with the spin re-orientation are observed
simultaneously in both the E phonon and the magnon. These results suggest that
magnetostriction plays an important role in BiFeO3.",0905.1891v1
2009-08-18,Spectral collapse in ensembles of meta-molecules,"We report on a new collective phenomenon in metamaterials: spectral line
collapse with increasing number of the unit cell resonators (meta-molecules).
Resembling the behaviour of exotic states of matter, such as Bose-Einstein
condensates of excitons and magnons, this new effect is linked to the
suppression of radiation losses in periodic arrays. We demonstrate
experimentally spectral line collapse at microwave, terahertz and optical
frequencies. It emerges as a universal and truly scalable effect underpinned by
classical electromagnetic interactions between the excited meta-molecules.",0908.2533v1
2009-10-10,Spin Order and Excitations in Triangular Antiferromagnet La2Ca2MnO7,"We report a spin S = 3/2 triangular antiferromagnet with nearest-neighbor
coupling J = 0.29 meV in La2Ca2MnO7. A genuinely two-dimensional,
three-sublattice order develops below 2.80 K << the Weiss constant (25 K). The
spin excitations deviate substantially from linear spin-wave theory, suggesting
that magnon breakdown occurs in the material. Such a breakdown has been
anticipated in recent theoretical studies, although the excitation spectrum
remains to be accounted for.",0910.1904v1
2009-11-26,Bose-Einstein condensation of semi-hard bosons in S=1 dimerized organic compound F2PNNNO,"An analysis of the energy spectrum and the magnetization curve of
two-dimensional organic antiferromagnet F2PNNNO with a spin-one dimerized
structure shows that a behavior of the compound in an external magnetic field
can be explained within a lattice boson model with an extended Pauli's
exclusion principle, i.e. no more than two bosons per a dimer. The unusual
magnetization curve observed experimentally in the compound reflects a sequence
of phase transitions intrinsic for a lattice boson system with strong on-site
and inter-site repulsions due to a tuning of magnon density by the applied
magnetic field.",0911.5039v1
2010-01-25,Evidence for effective thermal boundary resistance from magnon/phonon disequilibrium,"We use the time-resolved magneto-optical Kerr effect (TRMOKE) to measure the
local temperature and heat flow dynamics in ferromagnetic SrRuO3 thin films.
After heating by a pump pulse, the film temperature decays exponentially,
indicating that the heat flow out of the film is limited by the film/substrate
interface. We show that this behavior is consistent with an effective boundary
resistance resulting from disequilibrium between the spin and phonon
temperatures in the film.",1001.4557v1
2010-03-04,Quantum critical surface of the zigzag spin chain under magnetic field: Application to superconducting quantum dots,"We analyze the exact ground state of XXZ zigzag spin chain with applied
magnetic field and find the quantum critical surface. Using the theorem of
positive semi-definite matrix, we can prove that the ground states for a
specific region, are fully polarized state and one magnon states. With Bethe
ansatz, we argue that this is the quantum critical surface in all cases. A
first in the literature, we derive the analytical expression of quantum
critical surface for superconducting quantum dots array in the presence of gate
voltage.",1003.0954v1
2010-04-01,Phenomenological Model of Longitudinal Spin Fluctuations in Itinerant Antiferromagnets,"We present the phenomenological analysis of the spectrum of longitudinal spin
fluctuations in isotropic itinerant electron antiferromagnets with account of
spin anharmonicity giving rise to coupling of transverse and longitudinal
normal modes. The spectrum consists of a quasielastic part forming a central
peak or a dip, depending on temperature and the Landau relaxation rate. Effects
of spin fluctuation coupling also give rise to an inelastic part of the
spectrum which has a form of resonances or antiresonances near the magnon
frequencies related to non-propagating longitudinal excitations.",1004.0136v1
2010-06-07,The t-J model on a semi-infinite lattice,"The hole spectral function of the t-J model on a two-dimensional
semi-infinite lattice is calculated using the spin-wave and noncrossing
approximations. In the case of small hole concentration and strong
correlations, $t\gg J$, several near-boundary site rows appear to be depleted
of holes. The reason for this depletion is a deformation of the magnon cloud,
which surrounds the hole, near the boundary. The hole depletion in the boundary
region leads to a more complicated spectral function in the boundary row in
comparison with its bulk shape.",1006.1215v1
2010-06-21,On the reflection of magnon bound states,"We investigate the reflection of two-particle bound states of a free open
string in the light-cone AdS_5 x S^5 string sigma model, for large angular
momentum J=J_56 and ending on a D7 brane which wraps the entire AdS_5 and a
maximal S^3 of S^5. We use the superspace formalism to analyse fundamental and
two-particle bound states in the cases of supersymmetry-preserving and
broken-supersymmetry boundaries. We find the boundary S-matrices corresponding
to bound states both in the bulk and on the boundary.",1006.4102v2
2010-06-22,Complete bond-operator theory of the two-chain spin ladder,"The discovery of the almost ideal, two-chain spin-ladder material
(C_5H_12N)_2CuBr_4 has once again focused attention on this most fundamental
problem in low-dimensional quantum magnetism. Within the bond-operator
framework, three qualitative advances are introduced which extend the theory to
all finite temperatures and magnetic fields in the gapped regime. This
systematic description permits quantitative and parameter-free experimental
comparisons, which are presented for the specific heat, and predictions for
thermal renormalization of the triplet magnon excitations.",1006.4196v1
2010-06-24,Field-induced decay dynamics in square-lattice antiferromagnet,"Dynamical properties of the square-lattice Heisenberg antiferromagnet in
applied magnetic field are studied for arbitrary value S of the spin. Above the
threshold field for two-particle decays, the standard spin-wave theory yields
singular corrections to the excitation spectrum with logarithmic divergences
for certain momenta. We develop a self-consistent approximation applicable for
S >= 1, which avoids such singularities and provides regularized magnon decay
rates. Results for the dynamical structure factor obtained in this approach are
presented for S = 1 and S = 5/2.",1006.4838v2
2010-07-23,Super-Atom Representation of High-TC Superconductivity,"A resonating valence bond RVB approach is taken to demonstrate formation of
real-space Cooper pairs and High-TC superconductivity HTS. Non-adiabatic
coupling between holes aggregates (super-atoms) and undoped anti-ferromagnet
cause virtual excitations in either system due to inter-system coupling. HTS is
said to reflect cooperative co-existence of two Bose-Einstein condensates in
terms of one real-space Cooper pair condensate, and a second magnon condensate,
which form at the same critical temperature. TC is formulated in terms of the
super-exchange interaction. Connection is made to an equivalent real-space BCS
formulation of HTS. Novel perspectives on the HTS in the electron-doped
Sr1-xLaxCuO2 and Nd2-xCexCuO4 emerge.",1007.4115v1
2010-08-02,Tunneling spectroscopy probing magnetic and nonmagnetic electrodes in tunnel junctions,"Tunneling spectroscopy is applied to tunnel junctions with only one or no
ferromagnetic electrode to study the excitation of quasi particles in magnetic
tunnel junctions. The bias dependence is investigated with high accuracy by
inelastic electron tunneling spectroscopy. Both types of junctions show a zero
bias anomaly that is different in size and sign compared to magnetic tunnel
junctions, i.e. junctions with two ferromagnetic electrodes. A pronounced
difference is also found depending on the material that is probed by the
tunneling electrons, which might be attributed to the excitation of magnons.",1008.0326v1
2010-08-27,Electromagnons in the spin collinear state of a triangular lattice antiferromagnet,"Terahertz time-domain spectroscopy was performed to directly probe the
low-energy (1-5 meV) electrodynamics of triangular lattice antiferromagnets
CuFe1-xGaxO2 (x = 0.00, 0.01, and 0.035). We discovered an electromagnon
(electric-field-active magnon) excitation at 2.3 meV in the paraelectric
up-up-down-down collinear magnetic phase, while this electromagnon vanishes in
the ferroelectric helimagnetic phase. Anti-correlation with noncollinear
magnetism excludes the exchange-striction mechanism as the origin of dynamical
magnetoelectric coupling, and hence evidences the observation of spin-orbit
coupling mediated electromagnon in the present compound.",1008.4709v1
2010-09-15,Anomalous High-Energy Spin Excitations in La2CuO4,"Inelastic neutron scattering is used to investigate the collective magnetic
excitations of the high-temperature superconductor parent antiferromagnet
La2CuO4. We find that while the lower energy excitations are well described by
spin-wave theory, including one- and two-magnon scattering processes, the
high-energy spin waves are strongly damped near the (1/2,0) position in
reciprocal space and merge into a momentum dependent continuum. This anomalous
damping indicates the decay of spin waves into other excitations, possibly
unbound spinon pairs.",1009.2915v1
2010-10-12,Linear-response theory of spin Seebeck effect in ferromagnetic insulators,"We formulate a linear response theory of the spin Seebeck effect, i.e., a
spin voltage generation from heat current flowing in a ferromagnet. Our
approach focuses on the collective magnetic excitation of spins, i.e., magnons.
We show that the linear-response formulation provides us with a qualitative as
well as quantitative understanding of the spin Seebeck effect observed in a
prototypical magnet, yttrium iron garnet.",1010.2325v2
2010-10-12,Coherent spin-current oscillations in transverse magnetic fields,"We address the coherence of the dynamics of spin-currents with components
transverse to an external magnetic field for the spin-1/2 Heisenberg chain. We
study current autocorrelations at finite temperatures and the real-time
dynamics of currents at zero temperature. Besides a coherent Larmor
oscillation, we find an additional collective oscillation at higher
frequencies, emerging as a coherent many-magnon effect at low temperatures.
Using numerical and analytical methods, we analyze the oscillation frequency
and decay time of this coherent current-mode versus temperature and magnetic
field.",1010.2351v2
2010-10-19,Yangian symmetry of the Y=0 maximal giant graviton,"We study the remnants of Yangian symmetry of AdS/CFT magnons reflecting from
boundaries with no degrees of freedom. We present the generalized twisted
boundary Yangian of open strings ending on boundaries which preserve only a
subalgebra h of the bulk algebra g, where (g,h) is a symmetric pair. This is
realized by open strings ending on the D3 brane known as the Y=0 maximal giant
graviton in AdS_5 x S^5. We also consider the Yangian symmetry of the boundary
which preserves an su(1|2) subalgebra only.",1010.3761v2
2011-01-22,Brief Note on AMD Conserved Quantities in Quadratic Curvature Theories,"Motivated by the recent work on critical gravity theories in dimensions D>3,
we reexamine the results in [arXiv:hep-th/0501044], where the conformal mass
definition of Ashtekar, Magnon and Das (AMD) for asymptotically AdS space-times
was generalized to incorporate curvature squared terms. The results in
[arXiv:hep-th/0501044] appear to contradict the findings in critical gravity,
where, using the methods of Deser and Tekin, black holes were shown to have
zero mass. We show that after correcting an error in [arXiv:hep-th/0501044],
the AMD approach actually produces results in complete agreement with those
obtained by using the methods of Deser and Tekin.",1101.4267v2
2011-02-04,Phonon driven spin distribution due to the spin-Seebeck effect,"Here we report on measurements of the spin-Seebeck effect of GaMnAs over an
extended temperature range alongside the thermal conductivity, specific heat,
magnetization, and thermoelectric power. The amplitude of the spin-Seebeck
effect in GaMnAs scales with the thermal conductivity of the GaAs substrate and
the phonon-drag contribution to the thermoelectric power of the GaMnAs,
demonstrating that phonons drive the spin redistribution. A phenomenological
model involving phonon-magnon drag explains the spatial and temperature
dependence of the measured spin distribution.",1102.1024v1
2011-03-31,Spiky Strings on NS5-branes,"We study rigidly rotating strings in the near horizon geometry of a stack of
Neveu-Schwarz (NS) 5-branes. We solve the Nambu-Goto action of the fundamental
string in the presence of a NS-NS two form $(B_{\mu\nu})$ and find out limiting
cases corresponding to magnon and spike like solutions.",1103.6153v3
2011-04-28,Spin and Orbital Characters of Excitations in Iron Arsenide Superconductors Revealed by Simulated Fe L-Edge RIXS,"We theoretically examine the orbital excitations coupled to the spin degree
of freedom in the parent state of the iron-arsenide superconductor, based on
the calculation in a five-band itinerant model. The calculated Fe $L_3$-edge
resonant inelastic x-ray scattering (RIXS) spectra disclose the presence of
spin-flip excitations involving several specific orbitals. Magnon excitations
predominantly composed of a single orbital component can be seen in
experiments, although its spectral weight is smaller than spin-flipped
interorbital high-energy excitations. The detailed polarization and momentum
dependence is also discussed with predictions for the experiments.",1104.5424v2
2011-05-30,Dynamical spin-flip susceptibility for a strongly interacting ultracold Fermi gas,"The Stoner model predicts that a two-component Fermi gas at increasing
repulsive interactions undergoes a ferromagnetic transition. Using the
random-phase approximation we study the dynamical properties of the interacting
Fermi gas. For an atomic Fermi gas under harmonic confinement we show that the
transverse (spin-flip) dynamical susceptibility displays a clear signature of
the ferromagnetic phase in a magnon peak emerging from the Stoner particle-hole
continuum. The dynamical spin susceptibilities could be experimentally explored
via spin-dependent Bragg spectroscopy.",1105.5997v1
2011-08-09,"Magnetic phases of a highly frustrated magnet, ZnCr2O4, up to an ultra-high magnetic field of 600 T","The Faraday rotation and magneto-optical absorption spectral measurements
were conducted to reveal the full-magnetization process and map out a magnetic
phase diagram of a typical geometrical frustrated magnet, ZnCr2O4, by using the
electromagnetic flux compression method in ultra-magnetic fields up to 600 T. A
fully polarized ferromagnetic phase is observed in which the absorption spectra
associated with an exciton-magnon-phonon transition disappears. Furthermore,
prior to the fully polarized ferromagnetic phase above 410 T, we found a novel
magnetic phase above 350 T followed by a canted 3:1 phase.",1108.1932v1
2011-09-12,Rotating Strings in AdS(4) X CP(3) with B-NS holonomy,"We study solutions for rigidly rotating strings on AdS$_4 \times$ CP$^3$ in
the presence of $B_{\rm NS}$ holonomy turned on over $CP^1 \subset CP^3$. We
construct general solutions for rotating strings with two and three angular
momenta in $CP^3$ and discuss various limits corresponding to giant magnon and
spike like solutions.",1109.2458v2
2011-12-14,Dynamics of Thermal Effects in the Spin-Wave Theory of Quantum Antiferromagnets,"We derive a master equation that allows us to study non-equilibrium dynamics
of a quantum antiferromagnet. By resorting to spin-wave theory, we obtain a
closed analytic form for the magnon decay rates. These turn out to be closely
related to form factors, which are experimentally accessible by means of
neutron and Raman scattering. Furthermore, we compute the time evolution of the
staggered magnetization showing that, for moderate temperatures, the magnetic
order is not spoiled even if the coupling is fully isotropic.",1112.3158v3
2012-06-06,Magnon-driven quantum-dot heat engine,"We investigate a heat- to charge-current converter consisting of a
single-level quantum dot coupled to two ferromagnetic metals and one
ferromagnetic insulator held at different temperatures. We demonstrate that
this nano engine can act as an optimal heat to spin-polarized charge current
converter in an antiparallel geometry, while it acts as a heat to pure spin
current converter in the parallel case. We discuss the maximal output power of
the device and its efficiency.",1206.1259v2
2012-06-12,Spiky Strings on I-brane,"We study rigidly rotating strings in the near horizon geometry of the 1+1
dimensional intersection of two orthogonal stacks of NS5-branes, the so called
I-brane background. We solve the equations of motion of the fundamental string
action in the presence of two form NS-NS fluxes that the I-brane background
supports and write down general form of conserved quantities. We further find
out two limiting cases corresponding to giant magnon and single spike like
strings in various parameter space of solutions.",1206.2539v2
2012-06-21,On Rotating and Oscillating Four-Spin Strings in AdS5 X S5,"We study rigidly rotating strings in AdS5 X S5 background with one spin along
AdS5 and three angular momenta along S5. We find dispersion relations among
various charges and interpret them as giant magnon and spiky string solutions
in various limits. Further we present an example of oscillating string which
oscillates in the radial direction of the AdS5 and at the same time rotates in
S5.",1206.4920v2
2012-07-13,Magnetic relaxation in bilayers of yttrium iron garnet/platinum due to the dynamic coupling at the interface,"We show that in ferromagnetic (FM)/normal metal (NM) bilayers the dynamic
coupling at the interface transfers an additional magnetic relaxation from the
heavily damped motion of the conduction electron spins in the NM layer to the
FM spins. While the FM relaxation rates due to two-magnon scattering and spin
pumping decrease rapidly with increasing FM film thickness, the damping due to
the dynamic coupling does not depend on the FM film thickness. The proposed
mechanism explains the very large broadening of ferromagnetic resonance lines
in thick films of yttrium iron garnet after deposition of a Pt layer.",1207.3330v1
2012-07-28,Excitations and spin correlations near the interface of two three-dimensional Heisenberg antiferromagnets,"Magnetic excitations and spin correlations near the interface of two
spin-$\frac12$ Heisenberg antiferromagnets are considered using the spin-wave
approximation. When the interaction between boundary spins differs essentially
from exchange constants inside the antiferromagnets, quasi-two-dimensional spin
waves appear in the near-boundary region. They eject bulk magnons from this
region, thereby dividing the antiferromagnets into areas with different
magnetic excitations. The decreased dimensionality of the near-boundary modes
leads to amplified nearest-neighbor spin correlations in the interface area.",1207.6688v1
2012-08-05,Quantum bath refrigeration towards absolute zero: unattainability principle challenged,"A minimal model of a quantum refrigerator (QR), i.e. a periodically
phase-flipped two-level system permanently coupled to a finite-capacity bath
(cold bath) and an infinite heat dump (hot bath), is introduced and used to
investigate the cooling of the cold bath towards the absolute zero (T=0).
Remarkably, the temperature scaling of the cold-bath cooling rate reveals that
it does not vanish as T->0 for certain realistic quantized baths, e.g. phonons
in strongly disordered media (fractons) or quantized spin-waves in ferromagnets
(magnons). This result challenges Nernst's third-law formulation known as the
unattainability principle.",1208.1015v1
2012-09-03,Longitudinal magnetic excitation in KCuCl3 studied by Raman scattering under hydrostatic pressures,"We measure Raman scattering in an interacting spin-dimer system KCuCl3 under
hydrostatic pressures up to 5 GPa mediated by He gas. In the pressure-induced
quantum phase, we observe a one-magnon Raman peak, which originates from the
longitudinal magnetic excitationand is observable through the second-order
exchange interaction Raman process. We report the pressure dependence of the
frequency, halfwidth and Raman intensity of this mode.",1209.0235v1
2012-09-25,Hydrodynamic Description of Spin-1 Bose-Einstein Condensates,"We establish a complete set of hydrodynamic equations for a spin-1
Bose-Einstein condensate (BEC), which are equivalent to the multi-component
Gross-Pitaevskii equations and expressed in terms of only observable physical
quantities: the spin density and the nematic (or quadrupolar) tensor in
addition to the density and the mass current that appear in the hydrodynamic
description of a scalar BEC. The obtained hydrodynamic equations involve a
generalized Mermin-Ho relation that is valid regardless of the spatiotemporal
dependence of the spin polarization. Low-lying collec- tive modes for phonons
and magnons are reproduced by linearizing the hydrodynamic equations. We also
apply the single-mode approximation to the hydrodynamic equations and find a
complete set of analytic solutions.",1209.5509v1
2012-11-01,Spin Seebeck effect in antiferromagnets and compensated ferrimagnets,"We theoretically investigate the spin Seebeck effect (SSE) in
antiferromagnets and ferrimagnets, and show that the SSE vanishes in
antiferromagnets but survives in ferrimagnets even at the magnetization
compensation point despite the absence of its saturation magnetization. The
non-vanishing SSE in ferrimagnets stems from two non-degenerate magnons. We
demonstrate that the magnitude of the SSE in ferrimagnets is unchanged across
the magnetization compensation point.",1211.0123v1
2012-12-14,Leading finite-size effects on some three-point correlators in AdS_5 x S^5,"In the framework of the semiclassical approach, we find the leading
finite-size effects on the normalized structure constants in some three-point
correlation functions in AdS_5 x S^5, expressed in terms of the conserved
string angular momenta J_1, J_2, and the worldsheet momentum p_w, identified
with the momentum p of the magnon excitations in the dual spin-chain arising in
N=4 SYM in four dimensions.",1212.3485v4
2013-02-06,p-wave superconductivity near a transverse saturation field,"We investigate reentrant superconductivity in an Ising ferromagnetic
superconductor URhGe under a transverse magnetic field h_x. The superconducting
transition temperature for p-wave order parameters T_{sc} is calculated and
shows two domes as a function of h_x. We find strong enhancement of T_{sc} in
the high-field dome near a saturation field h_s where the spins align in the
transverse direction. Soft magnons generate strong attractive interactions
there. Spin components of the pairing show a significant change between h_x<h_s
and h_x>h_s. We also discuss the appearance of superconductivity with zero-spin
pair due to cancellation between external and exchange fields.",1302.1364v1
2013-03-07,Thickness dependence of the degree of spin polarization of the electrical current in permalloy thin films,"Spin-polarized electrical transport is investigated in $
Al_{2}O_{3}/Ni_{80}Fe_{20}/Al_{2}O_{3}$ thin films for permalloy thickness
between 6 and 20nm. The degree of spin-polarization of the current flowing in
the plane of the film is measured through the current induced spin wave Doppler
shift. We find that it decreases as the film thickness decreases, from 0.72 at
20nm to 0.46 at 6nm. This decrease is attributed to a spin depolarization
induced by the film surfaces. A model is proposed which takes into account the
contributions of the different sources of electron scattering (alloy disorder,
phonons, thermal magnons, grain boundaries, film surfaces) to the measured
spin-dependent resistivities.",1303.1692v1
2013-04-08,Leading finite-size effects on some three-point correlators in TsT-deformed AdS_5 x S^5,"We compute the leading finite-size effects on the normalized structure
constants in semiclassical three-point correlation functions of two finite-size
giant magnon string states and three different types of ""light"" states -
primary scalar operators, dilaton operator with nonzero momentum and singlet
scalar operators on higher string levels. This is done for the case of
TsT-transformed, or gamma-deformed, AdS_5 x S^5 string theory background, dual
to N = 1 super Yang-Mills theory in four dimensions, arising as an exactly
marginal deformation of N = 4 super Yang-Mills.",1304.2139v2
2013-05-09,Spin Conductivity in Two-Dimensional Non-Collinear Antiferromagnets,"We propose a method to derive the spin current operator for non-collinear
Heisenberg antiferromagnets. We show that the spin conductivity calculated by
the spectral representation with the spin current satisfies the f-sum rule. We
also study the spin conductivity at T=0 within spin wave theory. We show how
the spin conductivity depends on the external magnetic field with changing
magnon spectrum. We also find that the spin Drude weight vanishes for any
external magnetic field at T=0.",1305.2072v3
2013-07-03,Numerical study of magnetization plateaux in the spin-1/2 kagome Heisenberg antiferromagnet,"We clarify the existence of several magnetization plateaux for the kagome
$S=1/2$ antiferromagnetic Heisenberg model in a magnetic field. Using
approximate or exact localized magnon eigenstates, we are able to describe in a
similar manner the plateau states that occur for magnetization per site
$m=1/3$, $5/9$, and $7/9$ of the saturation value. These results are confirmed
using large-scale Exact Diagonalization on lattices up to 63 sites.",1307.0975v2
2013-07-18,Semiclassical Strings on Curved Branes,"We study semiclassical strings in the near horizon geometry of certain curved
branes. We investigate the rigidly rotating strings in the near horizon
geometry of NS5-branes wrapped on AdS3 x S3 and in the presence of background
NS-NS flux. We study several string solutions corresponding to giant magnon,
single spike and more general folded strings for the fundamental string in this
background. We comment that in the S-dual background the situation changes
drastically.",1307.4926v1
2013-10-25,Goldstone-mode Instability leading to Fragmentation in a Spinor Bose-Einstein Condensate,"We apply the number-conserving Bogoliubov theory to spinor Bose-Einstein
condensates and show that the Goldstone magnon leads instability leading to
fragmentation. Unlike the dynamical instability, where modes with complex
eigenfrequencies grow exponentially, here the zero-energy mode exhibits
algebraic growth. We also point out that a small fraction of thermally excited
atoms enhances the fragmentation dynamics.",1310.6989v3
2013-11-01,Thermo-magnonic diode: rectification of energy and magnetization currents,"We investigate the dynamics of two coupled macrospins connected to thermal
baths at different temperatures. The system behaves like a diode which allows
the propagation of energy and mag- netization currents in one direction only.
This effect is described by a simple model of two coupled nonlinear oscillators
interacting with two independent reservoirs. It is shown that the rectification
phenomenon can be interpreted as a a stochastic phase synchronization of the
two spin-oscillators. A brief comparison with realistic micromagnetic
simulations is presented. This new effect yields promising opportunities in
spin-caloritronics devices.",1311.0123v2
2014-01-27,Spin chains and classical strings in rotating Rindler-AdS space,"In this paper, we study the spin chain and string excitation in the rotating
Rindler-$AdS_3$ proposed in [12]. We obtain a one-parameter deformed $SL(2)$
spin chain at the fast spin limit. Two-spin GKP-like solutions are studied at
short and long string limits. General ansatz for the giant magnons and the
spiky strings are analyzed in detail for various $\beta$. At last, we explore
its counterpart in analytic continuation and pp-wave limit.",1401.6915v1
2014-02-20,Transverse ultrasonic anomaly in La1/3Sr2/3MnO3,"The charge ordering (CO) transition in polycrystalline La1/3Sr2/3MnO3 has
been studied by measuring the resistivity, magnetization and transverse
ultrasonic velocity. At about 235K, a conspicuous increase in resistivity was
observed, while the magnetization shows a cusp structure, corresponding to an
antiferromagnetic charge ordering transition. Around this transition
temperature, dramatic anomaly in transverse sound velocity was observed. The
simultaneous occurrence of electron, magnon and phonon anomalous features
implies strong spin-phonon coupling and electron-phonon in La1/3Sr2/3MnO3. The
analysis suggests that the spin-phonon interaction is due to single-ion
magnetostriction, and electron-phonon coupling originates from the Jahn-Teller
effect of Mn3+.",1402.4857v2
2014-04-04,Conformal Mass in AdS gravity,"We show that the Ashtekar-Magnon-Das (AMD) mass and other conserved
quantities are equivalent to the Kounterterm charges in the asymptotically AdS
spacetimes that satisfy the Einstein equations, if we assume the same
asymptotic fall-off behavior of the Weyl tensor as considered by AMD. This
therefore implies that, in all dimensions, the conformal mass can be directly
derived from the bulk action and the boundary terms, which are written in terms
of the extrinsic curvature.",1404.1411v2
2014-04-18,Validity of spin wave theory for the quantum Heisenberg model,"Spin wave theory is a key ingredient in our comprehension of quantum spin
systems, and is used successfully for understanding a wide range of magnetic
phenomena, including magnon condensation and stability of patterns in dipolar
systems. Nevertheless, several decades of research failed to establish the
validity of spin wave theory rigorously, even for the simplest models of
quantum spins. A rigorous justification of the method for the three-dimensional
quantum Heisenberg ferromagnet at low temperatures is presented here. We derive
sharp bounds on its free energy by combining a bosonic formulation of the model
introduced by Holstein and Primakoff with probabilistic estimates and operator
inequalities.",1404.4717v1
2014-04-30,String solutions in AdS_3 x S^3 x T^4 with NS-NS B-field,"We develop an approach for solving the string equations of motion and
Virasoro constraints in any background which has some (unfixed) number of
commuting Killing vector fields. It is based on a specific ansatz for the
string embedding. We apply the above mentioned approach for strings moving
inAdS_3 x S^3 x T^4 with 2-form NS-NS B-field. We succeeded to find solutions
for a large class of string configurations on this background. In particular,
we derive dyonic giant magnon solutions in the R_t x S^3 subspace, and obtain
the leading finite-size correction to the dispersion relation.",1404.7644v1
2014-07-18,Microwave-induced spin currents in ferromagnetic-insulator|normal-metal bilayer system,"A microwave technique is employed to simultaneously examine the spin pumping
and the spin Seebeck effect processes in a YIG|Pt bilayer system. The
experimental results show that for these two processes, the spin current flows
in opposite directions. The temporal dynamics of the longitudinal spin Seebeck
effect exhibits that the effect depends on the diffusion of bulk
thermal-magnons in the thermal gradient in the
ferromagnetic-insulator|normal-metal system.",1407.4957v2
2014-07-29,Spin-Hall Nano-oscillator: a micromagnetic study,"This letter studies the dynamical behavior of spin-Hall nanoscillators from a
micromagnetic point of view. The model parameters have been identified by
reproducing recent experimental data quantitatively. Our results indicate that
a strongly localized mode is observed for in-plane bias fields such as in the
experiments, while predict the excitation of an asymmetric propagating mode for
large enough out-of plane bias field similarly to what observed in spin-torque
nanocontact oscillators. Our findings show that spin-Hall nanoscillators can
find application as spin-wave emitters for magnonic applications where spin
waves are used for transmission and processing information on nanoscale.",1407.7655v1
2014-08-20,Second cluster integral from the spectrum of an infinite XXZ spin chain,"First and second terms of the low-temperature cluster expansion for the free
energy density of a magnetically polarized $XXZ$ spin chain is obtained within
the propagator approach suggested by E. W. Montroll and J. C. Ward. All the
calculations employ only one- and two-magnon infinite-chain spectrums. In the
$XXX$-point the result reproduces the well known S. Katsura's formula obtained
50 years ago by finite-chain calculations.",1408.4586v2
2014-11-17,Curved magnonic waveguides based on domain walls,"The channeling of spin waves with domain walls in ultrathin ferromagnetic
films is demonstrated theoretically and through micromagnetics simulations. It
is shown that propagating excitations localized to the wall, which appear in
the frequency gap of bulk spin wave modes, can be guided effectively in curved
geometries and can propagate in close proximity to other channels with no
perceptible scattering or loss in coherence. For N\'eel-type walls arising from
an interfacial Dzyaloshinskii-Moriya interaction, the channeling is strongly
nonreciprocal and group velocities can exceed 1 km/s in the long wavelength
limit for certain propagation directions.",1411.4525v1
2014-11-28,Guided magnon transport in spin chains: transport speed and correcting for disorder,"High fidelity quantum information transport is necessary for most practical
models of quantum computation. By analogy with optical wave guides, a
spatio-temporally varying magnetic potential on a one dimensional spin chain
can achieve high fidelity transport of spin excitations. By comparing different
potential shapes, we establish the effects of potential shape on the fidelity
and transport speed. We incorporate disorder into our model and show methods to
minimise its effect on transport. Finally, we discuss implementations of our
scheme in several accessible systems based on hydrogenic approximations.",1411.7749v1
2014-12-22,Rotating and Orbiting Strings in Dp-brane background,"We probe the open fundamental strings in Dp-brane (p=1, 3, 5) backgrounds and
find new classes of rotating and orbiting string solutions. We show that for
various worldsheet embedding ansatz we get solutions of the string equations of
motion that correspond to the well known giant magnon and single spikes, in
addition to few new solutions corresponding to the orbiting strings. We make a
systematic study of both rigidly rotating and orbiting strings in D1, D3 and
D5-brane backgrounds.",1412.6941v1
2015-03-27,Giant dielectric nonlinearities at a magnetic Bose-Einstein condensation,"We experimentally investigate the dielectric response of the low-dimensional
gapped quantum magnet Cu$_2$Cl$_{4}\cdot$H$_8$C$_4$SO$_2$ near a magnetic
field-induced quantum critical point, which separates the quantum-disordered
and helimagnetic ground states. The observed magnetocapacitive effect
originates from an improper ferroelectric nature of the transition, which
itself is perhaps one of the best known realizations of Bose--Einstein
condensation of magnons. Despite that, we find that the magnetocapacitive
effect associated with the transition exhibits huge and very unusual
anharmonicities.",1503.08173v2
2015-05-13,Optical detection of magnetic orders in HgCr$_2$O$_4$ frustrated spin magnet under pulsed high magnetic fields,"A magneto-optical survey was conducted for HgCr$_2$O$_4$ powder samples under
pulsed high magnetic fields of up to 55 T. Intensity changes in magnetic fields
observed for the exciton-magnon-phonon optical transition spectra coincide well
with those of magnetization, lattice distortion from X-ray diffraction, and
electron-magnetic resonances. The last-ordered phase was detected prior to the
fully polarized magnetic phase, similarly to the other chromium spinel oxide,
ZnCr$_2$O$_4$ and CdCr$_2$O$_4$.",1505.03216v1
2015-06-23,Effects of magnetic anisotropy on spin dynamics of ferromagnetic frustrated chain,"By exploiting density-matrix renormalization group techniques, we investigate
the spin dynamics of a spin-1/2 one-dimensional J1-J2 XXZ model with competing
ferromagnetic J1 and antiferromagnetic J2 exchange couplings under applied
magnetic fields. Numerical results of spin excitation spectra show that in the
field-induced spin quadrupole regime, the longitudinal component has a gapless
mode and the transverse component has a gapped mode irrespective of the
exchange anisotropy. The excitation gap of the transverse spin excitation
increases as the exchange anisotropy increases over the XY-like and Ising-like
regions, demonstrating that two-magnon bound states are stabilized due to the
easy-axis anisotropy.",1506.06883v1
2015-09-08,Anomalous Hall effect driven by dipolar spin waves in uniform ferromagnets,"A new type of anomalous Hall effect is shown to arise from the interaction of
conduction electrons with dipolar spin waves in ferromagnets. This effect
exists even in homogeneous ferromagnets without relativistic spin-orbit
coupling. The leading contribution to the Hall conductivity is proportional to
the chiral spin correlation of dynamical spin textures and is physically
understood in terms of the skew scattering by dipolar magnons.",1509.02284v1
2015-10-18,Two-Fluid Theory for Spin Superfluidity in Magnetic Insulators,"We investigate coupled spin and heat transport in easy-plane magnetic
insulators. These materials display a continuous phase transition between
normal and condensate states that is controlled by an external magnetic field.
Using hydrodynamic equations supplemented by Gross-Pitaevski phenomenology and
magnetoelectric circuit theory, we derive a two-fluid model to describe the
dynamics of thermal and condensed magnons, and the appropriate boundary
conditions in a hybrid normal-metal|magnetic-insulator|normal-metal
heterostructure. We discuss how the emergent spin superfluidity can be
experimentally probed via a spin Seebeck effect measurement.",1510.05316v2
2015-11-25,Topological spin waves in the atomic-scale magnetic skyrmion crystal,"We study the spin waves of the triangular skyrmion crystal that emerges in a
two dimensional spin lattice model as a result of the competition between
Heisenberg exchange, Dzyalonshinkii-Moriya interactions, Zeeman coupling and
uniaxial anisotropy. The calculated spin wave bands have a finite Berry
curvature that, in some cases, leads to non-zero Chern numbers, making this
system topologically distinct from conventional magnonic systems. We compute
the edge spin-waves, expected from the bulk-boundary correspondence principle,
and show that they are chiral, which makes them immune to elastic
backscattering. Our results illustrate how topological phases can occur in
self-generated emergent superlattices at the mesoscale.",1511.08244v1
2016-01-02,Semiclassical structure constants in the eta-deformed AdS_5 x S^5: Leading finite-size corrections,"We consider the leading finite-size effects on some structure constants for
the $\eta$ - deformed $AdS_5\times S^5$ background in the framework of the
semiclassical approach. The leading finite-size corrections are derived for the
cases when we have two heavy string states represented by giant magnons and for
two different choices of the light states corresponding to dilaton operator
with nonzero momentum and primary scalar operators. Since the dual field theory
is still unknown, the results obtained here must be considered as conjectures
or as predictions from the string theory side.",1601.00127v2
2016-02-02,Magnetization Dynamics of Topological Defects and the Spin Solid in Kagome Artificial Spin Ice,"We report broadband spin-wave spectroscopy on kagome artificial spin ice
(ASI) made of large arrays of interconnected Ni$_{80}$Fe$_{20}$ nanobars.
Spectra taken in saturated and disordered states exhibit a series of resonances
with characteristic magnetic field dependencies. Making use of micromagnetic
simulations, we identify resonances that reflect the spin-solid-state and
monopole-antimonopole pairs on Dirac strings. The latter resonances allow for
the generation of highly-charged vertices in ASIs via microwave assisted
switching. Our findings open further perspectives for fundamental studies on
ASIs and their usage in reprogrammable magnonics.",1602.00918v1
2016-04-22,Swift thermal steering of domain walls in ferromagnetic MnBi stripes,"We predict a fast domain wall (DW) motion induced by a thermal gradient
across a nanoscopic ferromagnetic stripe of MnBi. The driving mechanism is an
exchange torque fueled by magnon accumulation at the DWs. Depending on the
thickness of the sample, both hot-to-cold and cold-to-hot DW motion directions
are possible. The finding unveils an energy efficient way to manipulate DWs as
an essential element in magnetic information processing such as racetrack
memory.",1604.06575v1
2016-04-28,Energy repartition in the nonequilibrium steady state,"The concept of temperature in nonequilibrium thermodynamics is an outstanding
theoretical issue. We propose an energy repartition principle that leads to a
spectral (mode-dependent) temperature in steady-state nonequilibrium systems.
The general concepts are illustrated by analytic solutions of the classical
Heisenberg spin chain connected to Langevin heat reservoirs with arbitrary
temperature profiles. Gradients of external magnetic fields are shown to
localize spin waves in a Wannier-Zeemann fashion, while magnon interactions
renormalize the spectral temperature. Our generic results are applicable to
other thermodynamic systems such as Newtonian liquids, elastic solids, and
Josephson junctions.",1604.08369v2
2016-07-21,Spin wave and spin flip in hexagonal LuMnO3 single crystal,"Manipulate and control of spin wave and spin flip are crucial for future
developments of magnonic and spintronic devices. We present that the spin wave
in hexagonal LuMnO3 single crystal can be selectively excited with laser
polarization perpendicular to the c-axis of hexagonal LuMnO3 and photon energy
~ 1.8 eV. The selective excitation of spin wave also suggests that the spin
flip can be selectively controlled in hexagonal manganites. In addition, the
physical origin of spin wave correlated with spin flip in hexagonal manganites
is discussed.",1607.06212v1
2018-04-27,Angular Dependent Magnetization Dynamics with Mirror-symmetric Excitations in Artificial Quasicrystalline Nanomagnet Lattices,"We report angle-dependent spin-wave spectroscopy on aperiodic
quasicrystalline magnetic lattices, i.e., Ammann, Penrose P2 and P3 lattices
made of large arrays of interconnected Ni$_{80}$Fe$_{20}$ nanobars. Spin-wave
spectra obtained in the nearly saturated state contain distinct sets of
resonances with characteristic angular dependencies for applied in-plane
magnetic fields. Micromagnetic simulations allow us to attribute detected
resonances to mode profiles with specific mirror symmetries. Spectra in the
reversal regime show systematic emergence and disappearance of spin wave modes
indicating reprogrammable magnonic characteristics.",1804.10630v1
2018-11-02,New magnetic states in nanorings created by anisotropy gradients,"Magnetic nanorings have been widely studied due to their potential
applications in spintronic and magnonic devices. In this work, by means of
analytical calculations and micromagnetic simulations we have analyzed the
magnetic energy of nanorings with variable anisotropy along their radius. Four
magnetic states, including two new magnetic configurations, here called meron
and knot-like states, are considered, looking to the relative lower energy
states as a function of anisotropy. Phase diagrams with this states are
presented.",1811.01069v1
2018-11-06,Parametrically squeezed states of microwave magnons in yttrium iron garnet films,"We demonstrate theoretically, and confirm experimentally, that nonlinear spin
waves excited in thin yttrium iron garnet films are good candidates for
squeezing vacuum quantum noise. The experimental demonstration is in the form
of a measurement of spin-wave induced modulation instability (IMI) conducted in
the classical regime. The experiment evidences strong phase locking of an idler
wave parametrically generated in the film with a deterministic small-signal
wave launched into the film from an external source. The theory predicts that
the same behaviour will be observed for vacuum quantum noise, resulting in
squeezing of the noise.",1811.02104v1
2010-05-24,Application of entanglement conditions to spin systems,"There have been numerous studies of entanglement in spin systems. These have
usually focussed on examining the entanglement between individual spins or
determining whether the state of the system is completely separable. Here we
present conditions that allow us to determine whether blocks of spins are
entangled. We show that sometimes these conditions can detect entanglement
better than conditions involving individual spins. We apply these conditions to
study entanglement in spin wave states, both when there are only a few magnons
present and also at finite temperature.",1005.4448v1
2014-03-24,Skyrmionic spin Seebeck effect via dissipative thermomagnonic torques,"We derive thermomagnonic torque and its ""$\beta$-type"" dissipative correction
from the stochastic Landau-Lifshitz-Gilbert equation. The ""$\beta$-type""
dissipative correction describes viscous coupling between magnetic dynamics and
magnonic current and it stems from spin mistracking of the magnetic order. We
show that thermomagnonic torque is important for describing temperature
gradient induced motion of skyrmions in helical magnets while dissipative
correction plays an essential role in generating transverse Magnus force. We
propose to detect such skyrmionic motion by employing the transverse spin
Seebeck effect geometry.",1403.6160v2
2017-01-13,Magnetic ground states for bent nanotubes,"Magnetic nanotubes have been widely studied because they are promising
candidates to be part in devices based on spintronic and magnonic technologies.
However, the experimental techniques used to prepare these elements could not
guarantee to have perfect nanostructures. Therefore, some geometric
imperfections can appear. In this direction, the bent of a nanotube could play
an essential role in the magnetic properties of a device. In this work, we
analyze the influence of curvature on the magnetic properties of a bent
nanotube, a topic scarcely studied, and that can have a strong impact on
applications.",1701.03656v2
2017-01-13,Delocalized charge carriers in strongly disordered t-J model,"We study the influence of the electron-magnon interaction on the particle
transport in strongly disordered systems. The analysis is based on results
obtained for a single hole in the one-dimensional t-J model. Unless there
exists a mechanism that localizes spin excitations, the charge carrier remains
delocalized even for a very strong disorder and shows subdiffusive motion up to
the longest accessible times. However, upon inspection of the propagation times
between neighboring sites as well as a careful finite-size scaling we
conjecture that the anomalous subdiffusive transport may be transient and
should eventually evolve into a normal diffusive motion.",1701.03659v1
2017-01-20,Probing of the interfacial Heisenberg and Dzyaloshinskii--Moriya exchange interaction by magnon spectroscopy,"This Topical Review presents an overview of the recent experimental results
on the quantitative determination of the magnetic exchange parameters in
ultrathin magnetic films and multilayers, grown on different substrates. The
experimental approaches to probe both the symmetric Heisenberg as well as the
antisymmetric Dzyaloshinskii--Moriya exchange interaction in ultrathin magnetic
films and at interfaces are discussed in detail. It is explained how the
experimental spectrum of magnetic excitations can be used to quantify the
strength of these interactions.",1701.05850v1
2011-11-23,Dispersion of orbital excitations in 2D quantum antiferromagnets,"We map the problem of the orbital excitation (orbiton) in a 2D
antiferromagnetic and ferroorbital ground state onto a problem of a hole in 2D
antiferromagnet. The orbiton turns out to be coupled to magnons and can only be
mobile on a strongly renormalized scale by dressing with magnetic excitations.
We show that this leads to a dispersion relation reflecting the two-site unit
cell of the antiferromagnetic background, in contrast to the predictions based
on a mean-field approximation and linear orbital-wave theory.",1111.5522v1
2011-11-28,Quasi-one-dimensional magnons in an intermetallic marcasite,"We present inelastic neutron scattering measurements and first principles
calculations examining the intermetallic marcasite CrSb2. The observed spin
wave dispersion implies that the magnetic interactions are strongly
one-dimensional with antiferromagnetic chains parallel to the crystalline
c-axis. Such low-dimensional excitations are unexpected in a semiconducting
intermetallic system. Moreover this material may be further interesting in that
the magnetic anisotropy may enhance thermoelectric properties along particular
crystallographic directions.",1111.6499v1
2012-01-11,Local magnetic anisotropy in BaFe$_2$As$_2$: a polarized inelastic neutron scattering study,"The anisotropy of the magnetic excitations in BaFe$_2$As$_2$ was studied by
polarized inelastic neutron scattering which allows one to separate the
components of the magnetic response. Despite the in-plane orientation of the
static ordered moment we find the in-plane polarized magnons to exhibit a
larger gap than the out-of-plane polarized ones indicating very strong
single-ion anisotropy within the layers. It costs more energy to rotate a spin
within the orthorhombic {\it a-b} plane than rotating it perpendicular to the
FeAs layers.",1201.2332v1
2012-04-09,Ising model: secondary phase transition,"Lttice-spin phonons are considered, which make the heat capacity at the
critical temperature satisfy experimental observations better. There is a BEC
phase transition in an Ising model attributable to the lattice-spin phonons. We
proved that the spin-wave theory only is available after BEC transition, and
the magnons have the same characteristics as the lattice-spin phonons',
resulting from quantum effect. Energy-level overlap effect at ultralow
temperature is found. A prediction of BEC phase transition in a crystal is put
forward as our theory generalization.",1204.1807v1
2014-05-21,Spiky strings on AdS3 x S3 with NS-NS flux,"We study rigidly rotating strings in the background of AdS3 x S3 with
Neveu-Schwarz (NS) fluxes. We find two interesting limiting cases corresponding
to the known giant magnon and the new single spike solution of strings in the
above background and write down the dispersion relations among various
conserved charges. We use proper regularization to find the correct relations
among them. We further study the circular and infinite spiky strings on AdS and
study their properties.",1405.5497v3
2014-06-13,On the Rotating and Oscillating strings in $(AdS_3\times S^3)_{\varkappa}$,"We study rigidly rotating strings in the $\varkappa$-deformed $AdS_3 \times
S^3$ background. We find out two classes of solutions corresponding to the
giant magnon and single spike solutions of the string rotating in two
$S^2_{\varkappa}$ subspace of rotations reduced along two different isometries.
We verify that the dispersion relations reduce to the well known relation in
the $\varkappa\rightarrow 0$ limit. We further study some oscillating string
solutions in the $S^3_{\varkappa}$ subspace.",1406.3642v4
2014-06-20,Thermoelectric energy harvesting with quantum dots,"We review recent theoretical work on thermoelectric energy harvesting in
multi-terminal quantum-dot setups. We first discuss several examples of
nanoscale heat engines based on Coulomb-coupled conductors. In particular, we
focus on quantum dots in the Coulomb-blockade regime, chaotic cavities and
resonant tunneling through quantum dots and wells. We then turn towards
quantum-dot heat engines that are driven by bosonic degrees of freedom such as
phonons, magnons and microwave photons. These systems provide interesting
connections to spin caloritronics and circuit quantum electrodynamics.",1406.5329v2
2016-05-12,Transmission of spin waves in ordered FeRh epitaxial thin films,"We report on B2-ordering dependence of magnetostatic surface spin waves in
ferromagnetic FeRh at room temperature. Spin waves transmit over a distance
longer than 21 {\mu}m in highly ordered FeRh alloys even with relatively large
spin-orbit interaction. The long-range transmission likely arises from the
induced Rh moments of the ordered FeRh due to ferromagnetic exchange
interaction between Fe and Rh. The results indicate a potential of using FeRh
in spintronic and magnonic applications by integrating with other fascinating
magnetic characteristics of FeRh such as electric field induced magnetic phase
transition.",1605.03798v1
2016-08-22,Spin transport in half-metallic ferromagnets,"We theoretically investigate spin transport in half-metallic ferromagnets at
finite temperatures. The side-jump and skew-scattering contributions to spin
Hall conductivity are derived using the Kubo formula. The electron-magnon
interaction causes a finite density of states in the energy gap of the
minority-spin band and induces spin Hall conductivity. We show that spin Hall
conductivity is proportional to $T^{3/2}$, with $T$ being temperature and is
sensitive to $T$. We propose that spin Hall conductivity may be a tool to study
the minority-spin state.",1608.06333v2
2017-06-01,Cavity mediated manipulation of distant spin currents using cavity-magnon-polariton,"Using electrical detection of a strongly coupled spin-photon system comprised
of a microwave cavity mode and two magnetic samples, we demonstrate the long
distance manipulation of spin currents. This distant control is not limited by
the spin diffusion length, instead depending on the interplay between the local
and global properties of the coupled system, enabling systematic spin current
control over large distance scales (several centimeters in this work). This
flexibility opens the door to improved spin current generation and manipulation
for cavity spintronic devices.",1706.00347v1
2017-12-13,Some semiclassical structure constants for AdS_4 x CP^3,"We compute structure constants in three-point functions of three string
states in AdS_4 x CP^3 in the framework of the semiclassical approach. We
consider HHL correlation functions where two of the states are ""heavy"" string
states of finite-size giant magnons carrying one or two angular momenta and the
other one corresponds to such ""light"" states as dilaton operators with non-zero
momentum, primary scalar operators, and singlet scalar operators with higher
string levels.",1712.04816v2
2018-09-24,Phonon Magnetochiral Effect,"Magnetochiral effect (MChE) of phonons, a nonreciprocal acoustic property
arising due to the symmetry principles, is demonstrated in a chiral-lattice
ferrimagnet Cu$_2$OSeO$_3$. Our high-resolution ultrasound experiments reveal
that the sound velocity differs for parallel and antiparallel propagation with
respect to the external magnetic field.The sign of the nonreciprocity depends
on the chirality of the crystal in accordance with the selection rule of the
MChE. The nonreciprocity is enhanced below the magnetic ordering temperature
and at higher ultrasound frequencies, which is quantitatively explained by a
proposed magnon-phonon hybridization mechanism.",1809.08775v1
2018-09-26,Resonant magnetic induction tomography of a magnetized sphere,"We demonstrate the structural imaging of magnetostatic spin-wave modes hosted
in a millimeter-sized ferromagnetic sphere. Unlike for low-dimensional magnetic
materials, there is no prior technique to image these modes in bulk magnetized
solid of revolution. Based on resonant magnetic induction tomography in the
microwave range, our approach ensures the robust identification of these
non-trivial spin-wave modes by establishing their azimuthal and polar
dependences, starting point of magnonic fundamental studies and hybrid systems
with complex spin textures well beyond the uniform precession mode.",1809.09785v2
2019-09-01,Electric-field control of collective spin excitations in Neel-type skyrmions,"We demonstrate that an electric field could activate the three low-energy
eigenmodes of the Neel-type skyrmion lattice via the electrically induced
Dzyaloshinskii-Moriya interaction. In particular, we predict that the relative
intensity of the clockwise rotation mode against the counter-clockwise rotation
mode is significantly enhanced for the electrical activation in comparison with
the magnetic activation. We also discover that the electrically and
magnetically active modes obey unique selection rules. These findings promise a
fresh pathway towards energy-efficient electrical manipulation of skyrmion
excitations for future skyrmion-based magnonics.",1909.00348v1
2019-09-26,Magnetic Doublon Bound States in the Kondo Lattice Model,"We present a novel pairing mechanism for electrons, mediated by magnons.
These paired bound states are termed ""magnetic doublons"". Applying numerically
exact techniques (full diagonalization and the density-matrix renormalization
group, DMRG) to the Kondo lattice model at strong exchange coupling $J$ for
different fillings and magnetic configurations, we demonstrate that magnetic
doublon excitations exist as composite objects with very weak dispersion. They
are highly stable, support a novel ""inverse"" colossal magnetoresistance and
potentially other effects.",1909.11896v2
2020-12-23,Analytic description of spin waves in dipolar/octupolar pyrochlore magnets,"We derive analytic forms for spin waves in pyrochlore magnets with
dipolar-octupolar interactions, such as ${\rm Nd}_2{\rm Zr}_2{\rm O}_7$. We
obtain full knowledge of the diagonalized magnonic Hamiltonian within the
linear spin wave approximation. We also consider the effect of a ""breathing
mode"" as a perturbation of this system. The breathing mode lifts the degeneracy
of the upper band of the spin wave dispersion along the direction $X\to W$ in
$k$-space.",2012.12500v1
2000-06-02,Dzyaloshinskii-Moriya interaction in NaV$_2$O$_5$: a microscopic study,"We present a unified account of magnetic exchange and Raman scattering in the
quasi-one-dimensional transition-metal oxide NaV$_2$O$_5$. Based on a
cluster-model approach explicit expressions for the exchange integral and the
Raman-operator are given. It is demonstrated that a combination of the
electronic-structure and the Dzyaloshinskii-Moriya interaction, allowed by
symmetry in this material, are responsible for the finite Raman cross-section
giving rise to both, one- and two-magnon scattering amplitudes.",0006036v1
2000-06-13,Coherent dipolar correlations in the ground-state of Kagome frustrated antiferromagnets,"We propose a new model for the nature of the low temperature phase of a
geometrically frustrated antiferromgnet (AFM) with a Kagome lattice,
SrCr$_{8-x}$Ga$_{4+x}$O$_{19}$. We propose that the long-range dipolar
interaction between the magnetic Cr$^{3+}$ ions introduces correlations in
their dynamics. The dipolar ground-state has the spins performing correlated
zero-point oscillations in a coherent state with a well defined global phase
and a complex order-parameter (i.e. Off-Diagonal Long Range Order). We
calculate the magnon excitations of such a dipolar array and we find good
agreement with the spin-wave velocities infered from measurements of the
specific-heat. Various experimental properties of these materials are naturally
explained by such a model.",0006215v1
2012-05-30,Exceptional Operators in N=4 super Yang-Mills,"We consider one particularly interesting class of composite gauge-invariant
operators in N=4 super Yang-Mills theory. An exceptional feature of these
operators is that in the Thermodynamic Bethe Ansatz approach the one-loop
rapidities of the constituent magnons are shown to be exact in the 't Hooft
coupling constant. This is used to propose the mirror TBA description for these
operators. The proposal is shown to pass several non-trivial checks.",1205.6660v1
2017-05-29,Rectifying full-counting statistics in a spin Seebeck engine,"We investigate the rectification and negative differential effects of
full-counting statistics of conjugate thermal spin transport in a spin Seebeck
engine. The engine is made by an electron/magnon interface diode driven by a
temperature bias, of which the scaled cumulant generating function is
formulated in the framework of nonequilibrium Green's function. The strongly
fluctuating interfacial electron density of states induced by quantum dot is
responsible for these intriguing properties. This work is relevant for
designing efficient spin caloritronic engine and can broaden our view in
nonequilibrium thermodynamics and the nonlinear phenomenon in quantum transport
system.",1705.10025v2
2017-09-05,Gate-controlled magnonic-assisted switching of magnetization in ferroelectric/ferromagnetic junctions,"Interfacing a ferromagnet with a polarized ferroelectric gate generates a
non-uniform, interfacial spin density coupled to the ferroelectric polarization
allowing so for an electric field control of effective transversal field to
magnetization. Here we study the dynamic magnetization switching behavior of
such a multilayer system based on the Landau-Lifshitz-Baryakhtar equation,
demonstrating that interfacial magnetoelectric coupling is utilizable as a
highly localized and efficient tool for manipulating magnetism.",1709.01286v1
2017-09-06,Edge states in a ferromagnetic honeycomb lattice with armchair boundaries,"We investigate the properties of magnon edge states in a ferromagnetic
honeycomb lattice with armchair boundaries. In contrast with fermionic
graphene, we find novel edge states due to the missing bonds along the boundary
sites. After introducing an external on-site potential at the outermost sites
we find that the energy spectra of the edge states are tunable. Additionally,
when a non-trivial gap is induced, we find that some of the edge states are
topologically protected and also tunable. Our results may explain the origin of
the novel edge states recently observed in photonic lattices. We also discuss
the behavior of these edge states for further experimental confirmations",1709.01855v1
2018-07-12,Finite Size Effect from Classical Strings in deformed AdS$_3\times$ S$^3$,"We study the finite size effect of rigidly rotating and spinning folded
strings in $(AdS_3\times S^3)_{\varkappa}$ background. We calculate the leading
order exponential corrections to the infinite size dispersion relation of the
giant magnon, and single spike solutions. For the spinning folded strings we
write the finite size effect in terms of the known Lambert $W$-function.",1807.04601v3
2019-01-10,Finite size effect from classical strings in $AdS_3 \times S^3$ with NS-NS flux,"We study the finite size effect of rigidly rotating strings and closed folded
strings in $AdS_3\times S^3$ geometry with NS-NS B-field. We calculate the
classical exponential corrections to the dispersion relation of infinite size
giant magnon and single spike in terms of Lambert $\mathbf{W}-$function. We
also write the analytic expression for the dispersion relation of finite size
Gubser-Klebanov-Polyakov (GKP) string in the form of Lambert
$\mathbf{W}-$function.",1901.03026v2
2019-01-14,Physics Beyond Colliders: QCD Working Group Report,"This report summarises the main findings of the QCD Working Group in the CERN
Physics Beyond Colliders Study.",1901.04482v1
2019-01-15,On soliton solutions of the time-discrete generalized lattice Heisenberg magnet model,"Generalized lattice Heisenberg magnet model is an integrable model exhibiting
soliton solutions. The model is physically important for describing the magnon
bound states (or soliton excitations) with arbitrary spin, in magnetic
materials. In this paper, a time-discrete generalized lattice Heisenberg magnet
(GLHM) model is investigated. By writing down the Lax pair representation of
the time-discrete GLHM model, we present explicitly the underlying integrable
structure like, the Darboux transformation and soliton solutions.",1901.04671v1
2019-03-22,Spin-wave coupling to electromagnetic cavity fields in dysposium ferrite,"Coupling of spin-waves with electromagnetic cavity field is demonstrated in
an antiferromagnet, dysprosium ferrite (DyFeO3). By measuring transmission at
0.2-0.35 THz and sweeping sample temperature, magnon-photon coupling signatures
were found at crossings of spin-wave resonances with Fabry-Perot cavity modes
formed in samples. The obtained spectra are explained in terms of classical
electrodynamics and a microscopic model.",1903.09590v2
2019-08-24,Chiral Pumping of Spin Waves,"We report a theory for the coherent and incoherent chiral pumping of spin
waves into thin magnetic films through the dipolar coupling with a local
magnetic transducer, such as a nanowire. The ferromagnetic resonance of the
nanowire is broadened by the injection of unidirectional spin waves that
generate a non-equilibrium magnetization in only half of the film. A
temperature gradient between the local magnet and film leads to a
unidirectional flow of incoherent magnons, i.e., a chiral spin Seebeck effect.",1908.09141v2
2019-11-05,Spin chains and classical strings in two parameters $q$-deformed AdS$_3\times$S$^3$,"In this paper, we study the spin chain and string excitation in the
two-parameters $q$-deformed AdS$_3\times$S$^3$ proposed by Hoare. We obtain the
deformed spin chain model at the fast spin limit for choices of deformed
parameters. General ansatz for giant magnons are studied in great detail and
complicated dispersion relation is treated perturbatively. We also study
several types of hanging string solutions and their charges and spins are
analyzed numerically. At last, we explore its pp-wave limit and find its
solution only depends on the difference of deformed parameters.",1911.01567v1
2019-11-07,Magnetic monopoles and toroidal moments in LuFeO$_3$ and related compounds,"Magnetic monopoles and toroidal order are compelling features that have long
been theorized but remain elusive in real materials. Multiferroic hexagonal
ferrites are an interesting realization of frustrated triangular lattice, where
magnetic order is coupled to ferroelectricity and trimerization. Here we
propose a mechanism, through which magnetic monopolar and toroidal orders
emerge from the combination of 120$^\circ$ antiferromagnetism and
trimerization, present in hexagonal manganites and ferrites. The experimentally
observable signatures of magnetic monopolar and toroidal orders are identified
in the inelastic neutron scattering cross section, simulated from a microscopic
model of LuFeO3. The non-reciprocal magnon propagation is demonstrated.",1911.02704v1
2019-11-25,Cavity Optomagnonics,"In the recent years a series of experimental and theoretical efforts have
centered around a new topic: the coherent, cavity-enhanced interaction between
optical photons and solid state magnons. The resulting emerging field of Cavity
Optomagnonics is of interest both at a fundamental level, providing a new
platform to study light-matter interaction in confined structures, as well as
for its possible relevance for hybrid quantum technologies. In this chapter I
introduce the basic concepts of Cavity Optomagnonics and review some
theoretical developments.",1911.11104v1
2019-12-04,Free energy asymptotics of the quantum Heisenberg spin chain,"We consider the ferromagnetic quantum Heisenberg model in one dimension, for
any spin $S\geq 1/2$. We give upper and lower bounds on the free energy,
proving that at low temperature it is asymptotically equal to the one of an
ideal Bose gas of magnons, as predicted by the spin-wave approximation. The
trial state used in the upper bound yields an analogous estimate also in the
case of two spatial dimensions, which is believed to be sharp at low
temperature.",1912.01967v2
2019-12-09,On the singularities of the R-R AdS_3 x S^3 x T^4 S matrix,"We investigate the analytic properties of the exact magnon S matrix of string
theory on AdS_3 x S^3 x T^4 with R-R flux. We show that the previously proposed
dressing factors have the exact double-pole/zero structure expected from Landau
box diagrams. This constitutes a strong consistency check of our dressing
factors, much as the Dorey-Hofman-Maldacena poles do for the all-loop dressing
factor in AdS_5 x S^5.",1912.04320v2
2019-12-13,Angular Momentum Conservation and Phonon Spin in Magnetic Insulators,"We develop a microscopic theory of spin-lattice interactions in magnetic
insulators, separating rigid-body rotations and the internal angular momentum,
or spin, of the phonons, while conserving the total angular momentum. In the
low-energy limit, the microscopic couplings are mapped onto experimentally
accessible magnetoelastic constants. We show that the transient phonon spin
contribution of the excited system can dominate over the magnon spin, leading
to nontrivial Einstein-de Haas physics.",1912.06616v2
2016-03-10,Resonant excitation of the spin-wave current in hybrid nanostructures,"Using the non-equilibrium statistical operator method (NSO), we have
investigated the spin transport through the interface in a
semiconductor/ferromagnetic insulator hybrid structure. We have analyzed the
approximation of effective parameters, when each of the considered subsystems
(conduction electrons, magnons, and phonons) is characterized by its effective
temperature. We have constructed the macroscopic equations, describing the
spin-wave current caused by both resonantly excited spin system of conduction
electrons and by an inhomogeneous thermal field in the ferromagnetic insulator.",1603.03207v1
2016-12-13,Theory of magnons in spin systems with Dzyaloshinskii-Moriya interaction,"We study in this paper magnetic properties of a system of quantum Heisenberg
spins interacting with each other via a ferromagnetic exchange interaction J
and an in-plane Dzyaloshinskii-Moriya interaction D. The non-collinear ground
state due to the competition between J and D is determined. We employ a
self-consistent Green'function theory to calculate the spin-wave spectrum and
the layer magnetizations at finite T in two and three dimensions as well as in
a thin film with surface effects. Analytical details and the validity of the
method are shown and discussed.",1612.04147v1
2017-07-27,Ultrafast nanoscale magnetic switching via intense picosecond electron bunches,"The magnetic field associated with a picosecond intense electron pulse is
shown to switch locally the magnetization of extended films and nanostructures
and to ignite locally spin waves excitations. Also, topologically protected
magnetic textures such as skyrmions can be imprinted swiftly in a sample with a
residual Dzyaloshinskii-Moriya spin-orbital coupling. Characteristics of the
created excitations such as the topological charge or the width of the magnon
spectrum can be steered via the duration and the strength of the electron
pulses. The study points to a possible way for a spatiotemporally controlled
generation of magnetic and skyrmionic excitations.",1707.08838v1
2017-11-07,Dzyaloshinskii-Moriya interaction induced extrinsic linewidth broadening of ferromagnetic resonance,"For a thin ferromagnetic film with the Dzyaloshinskii-Moriya interaction
(DMI), we derive an expression of the extrinsic ferromagnetic resonance (FMR)
linewidth in a quantum mechanical way, taking into account scatterings from
structural inhomogeneity. In the presence of the DMI, the magnon dispersion
exhibits rich resonant states, especially in small external magnetic fields and
strong DMI strength. It is found that the FMR linewidth shows several
characteristic features such as a finite linewidth at zero frequency and peaks
in the low frequency range.",1711.02263v1
2019-05-21,Collective spin dynamics of Z2 vortex crystals in triangular Kitaev-Heisenberg antiferromagnets,"We show that the mesoscopic incommensurate $\mathbb{Z}_2$ vortex crystals
proposed for layered triangular anisotropic magnets can be most saliently
identified by two distinctive signatures in dynamical spin response
experiments: The presence of pseudo-Goldstone `phonon' modes at low frequencies
$\omega$, associated with the collective vibrations of the vortex cores, and a
characteristic multi-scattered intensity profile at higher $\omega$, arising
from a large number of Bragg reflections and magnon bandgaps. These are direct
fingerprints of the large vortex sizes and magnetic unit cells and the
solitonic spin profile around the vortex cores.",1905.08694v2
2020-06-03,Finite size effects in classical string solutions of the Schrodinger geometry,"We study finite size corrections to the semiclassical string solutions of the
Schrodinger spacetime. We compute the leading order exponential corrections to
the infinite size dispersion relation of the single spin giant magnon and of
the single spin single spike solutions. The solutions live in a $S^3$ subspace
of the five-sphere and extent in the Schrodinger part of the metric. In the
limit of zero deformation the finite size dispersion relations flow to the
undeformed $AdS_5 \times S^5$ counterparts and in the infinite size limit the
correction term vanishes and the known infinite size dispersion relations are
obtained.",2006.02285v1
2020-06-22,Magnetic excitations of Sr$_3$Ir$_2$O$_7$ observed by inelastic neutron scattering measurement,"In this short note, we report on the first inelastic neutron scattering (INS)
study on high-energy magnetic excitations in Sr$_3$Ir$_2$O$_7$. We observed
excitations between $\sim$80 meV and $\sim$180 meV. The peak position of the
excitations is consistent with the dispersion of a single magnon determined by
resonant inelastic X-ray scattering (RIXS) measurement. Thus, our results
demonstrate that INS and RIXS observe identical excitations in iridate oxides,
as in La$_2$CuO$_4$.",2006.12074v2
2020-06-22,Macroscale non-local transfer of superconducting signatures to a ferromagnet in a cavity,"Cavity spintronics recently heralded non-local magnonic signal transfer
between magnetic samples. Here we show that by including superconductors in the
cavity, we can make use of these principles to bring composite
superconductor--ferromagnet systems to the macroscale. We analyze how a
superconductor's a.c. conductivity influences the spin dynamics of a spatially
separated magnet, and we discuss the potential impact on spintronic
applications.",2006.12516v2
2020-09-04,Detection of Ferromagnetic Resonance from 1 nm-thick Co,"To explore the further possibilities of nanometer-thick ferromagnetic films
(ultrathin ferromagnetic films), we investigated the ferromagnetic resonance
(FMR) of 1 nm-thick Co film. Whilst an FMR signal was not observed for the Co
film grown on a SiO2 substrate, the insertion of a 3 nm-thick amorphous Ta
buffer layer beneath the Co enabled the detection of a salient FMR signal,
which was attributed to the smooth surface of the amorphous Ta. This result
implies the excitation of FMR in an ultrathin ferromagnetic film, which can
pave the way to controlling magnons in ultrathin ferromagnetic films.",2009.01977v1
2021-02-23,Fast acquisition of spin-wave dispersion by compressed sensing,"For the realization of magnonic devices, spin-wave dispersions need to be
identified. Recently, the time-resolved pump-probe imaging method combined with
the Fourier transform was demonstrated for obtaining the dispersions in the
lower-wavenumber regime. However, the measurement takes a long time when the
sampling rate is sufficiently high. Here, we demonstrated the fast acquisition
of spin-wave dispersions by using the compressed sensing technique. Further, we
quantitatively evaluated the consistency of the results. Our results can be
applied to other various pump-probe measurements, such as observations based on
the electro-optical effects.",2102.11626v1
2013-06-07,Symmetric and asymmetric excitations of a strong-leg quantum spin ladder,"The zero-field excitation spectrum of the strong-leg spin ladder
(C$_7$H$_10$N)$_2$CuBr$_4$ (DIMPY) is studied with a neutron time-of-flight
technique. The spectrum is decomposed into its symmetric and asymmetric parts
with respect to the rung momentum and compared with theoretical results
obtained by the density matrix renormalization group method. Additionally, the
calculated dynamical correlations are shown for a wide range of rung and leg
coupling ratios in order to point out the evolution of arising excitations, as
e.g. of the two-magnon bound state from the strong to the weak coupling limit.",1306.1671v1
2016-09-15,Spin Transport through the metallic antiferromagnet FeMn,"We investigate spin transport through metallic antiferromagnets using
measurements based on spin pumping combined with inverse spin Hall effects in
Ni80Fe20/FeMn/W trilayers. The relatively large magnitude and opposite sign of
spin Hall effects in W compared to FeMn enable an unambiguous detection of spin
currents transmitted through the entire FeMn layer thickness. Using this
approach we can detect two distinctively different spin transport regimes,
which we associate with electronic and magnonic spin currents respectively. The
latter can extend to relatively large distances (up to 9 nm) and is enhanced
when the antiferromagnetic ordering temperature is close to the measurement
temperature.",1609.04834v1
2016-09-19,Large Unidirectional Magnetoresistance in a Magnetic Topological Insulator,"We report current-direction dependent or unidirectional magnetoresistance
(UMR) in magnetic/nonmagnetic topological insulator (TI) heterostructures,
Cr$_x$(Bi$_{1-y}$Sb$_y$)$_{2-x}$Te$_3$/(Bi$_{1-y}$Sb$_y$)$_2$Te$_3$, that is
several orders of magnitude larger than in other reported systems. From the
magnetic field and temperature dependence, the UMR is identified to originate
from the asymmetric scattering of electrons by magnons. In particular, the
large magnitude of UMR is an outcome of spin-momentum locking and a small Fermi
wavenumber at the surface of TI. In fact, the UMR is maximized around the Dirac
point with the minimal Fermi wavenumber.",1609.05906v1
2016-11-01,Spin-Mechanical Inertia in Antiferromagnet,"Angular momentum conservation has served as a guiding principle in the
interplay between spin dynamics and mechanical rotations. However, in an
antiferromagnet with vanishing magnetization, new fundamental rules are
required to properly describe spin-mechanical phenomena. Here we show that the
N\'eel order dynamics affects the mechanical motion of a rigid body by
modifying its inertia tensor in the presence of strong magnetocrystalline
anisotropy. This effect depends on temperature when magnon excitations are
considered. Such a spin-mechanical inertia can produce measurable consequences
at small scales.",1611.00100v3
2016-11-14,Low-temperature evolution of the spectral weight of a spin-up carrier moving in a ferromagnetic background,"We derive the lowest-temperature correction to the self-energy of a spin-up
particle injected in a ferromagnetic background. The background is modeled with
both Heisenberg and Ising Hamiltonians so that differences due to gapless vs.
gapped magnons can be understood. Beside the expected thermal broadening of the
quasiparticle peak as it becomes a resonance inside a continuum, we also find
that spectral weight is transferred to regions lying outside this continuum. We
explain the origin of this spectral weight transfer and its low-temperature
evolution.",1611.04575v1
2016-11-25,Electrical Detection of Spin Backflow from an Antiferromagnetic Insulator/Y3Fe5O12 Interface,"Spin Hall magnetoresistance (SMR) has been observed in Pt/NiO/Y3Fe5O12 (YIG)
heterostructures with characteristics very different from those in Pt/YIG. We
show that the SMR in Pt/NiO/YIG strongly correlates with spin conductance, both
sharing very strong temperature dependence due to antiferromagnetic magnons and
spin fluctuation. This phenomenon indicates that spin current generated by spin
Hall effect in the Pt transmits through the insulating NiO and is reflected
from the NiO/YIG interface. Inverted SMR has been observed below a temperature
which increases with the NiO thickness, suggesting spin-flip reflection from
the antiferromagnetic NiO exchange coupled with the YIG.",1611.08593v1
2016-11-30,Finite-size effect of η-deformed AdS_5 x S^5 at strong coupling,"We compute Luscher corrections for a giant magnon in the \eta-deformed
(AdS_5\times S^5)_{\eta} using the su(2|2)_q-invariant S-matrix at strong
coupling and compare with the finite-size effect of the corresponding string
state, derived previously. We find that these two results match and confirm
that the su(2|2)_q-invariant S-matrix is describing world-sheet excitations of
the \eta-deformed background.",1611.09992v2
2017-04-19,"Crystal field excitations and magnons: their roles in oxyselenides Pr2O2M2OSe2 (M = Mn, Fe)","We present the results of neutron scattering experiments to study the crystal
and magnetic structures of the Mott-insulating transition metal oxyselenides
Pr2O2M2OSe2 (M = Mn, Fe). The structural role of the non-Kramers Pr3+ ion is
investigated and analysis of Pr3+ crystal field excitations performed.
Long-range order of Pr3+ moments in Pr2O2Fe2OSe2 can be induced by an applied
magnetic field.",1704.05750v1
2018-01-14,Polar phase of superfluid $^3$He: Dirac lines in the parameter and momentum spaces,"The time reversal symmetric polar phase of the spin-triplet superfluid $^3$He
has two types of Dirac nodal lines. In addition to the Dirac loop in the
spectrum of the fermionic Bogoliubov quasiparticles in the momentum space
$(p_x,p_y,p_z)$, the spectrum of bosons (magnons) has Dirac loop in the 3D
space of parameters -- the components of magnetic field $(H_x,H_y,H_z)$. The
bosonic Dirac system lives on the border between the type-I and type-II.",1801.04576v2
2019-07-26,Thermodynamics of Antiferromagnetic Solids in Magnetic Fields,"We analyze the thermodynamic properties of antiferromagnetic solids subjected
to a combination of mutually orthogonal uniform magnetic and staggered fields.
Low-temperature series for the pressure, order parameter and magnetization up
to two-loop order in the effective expansion are established. We evaluate the
self-energy and the dispersion relation of the dressed magnons in order to
discuss the impact of spin-wave interactions on thermodynamic observables.",1907.11480v1
2019-10-02,Continuous Aharonov--Bohm effect,"The Aharonov--Bohm effect in a model system described by the generalized
Schr\""{o}dinger equation is considered. The scattering cross section is
calculated in the standard formulation: an electron beam impinges on a long
impenetrable solenoid encompassing a magnetic field. It is shown that the
incident wave is scattered regardless of whether the magnetic flux through the
solenoid is an integer of flux quanta whereby the scattering cross section
becomes a continuously nonzero function of the total magnetic flux. The problem
may find its application in investigations of the soliton-magnon interaction in
the low-dimensional magnetism.",1910.00890v1
2019-10-09,Scattering of exchange spin waves from regions of modulated magnetization,"We investigate the reflection coefficient of spin waves propagating in an
ultra-thin ferromagnetic film with regions where saturation magnetization is
modulated. We find analytically and using micromagnetic simulations that there
are transmission resonances that depend on the width of the regions and on the
energy of excitation. Our results resemble the quantum mechanical
Ramsauer-Townsend effect in which an electron with certain energies can
propagate above a potential field without scattering. Our findings are useful
for reconfigurable magnonic devices where the saturation magnetization can be
dynamically controlled via a thermal landscape.",1910.04303v1
2019-10-09,Thermally controlled confinement of spin wave field in a magnonic YIG waveguide,"Methods for detecting spin waves rely on electrodynamical coupling between
the spin wave dipolar field and an inductive probe. While this coupling is
usually treated as constant, in this work, we experimentally and theoretically
show that it is indeed temperature dependent. By measuring the spin wave
magnetic field as a function of temperature of, and distance to the sample, we
demonstrate that there is both a longitudinal and transversal confinement of
the field near the YIG-Air interface. Our results are relevant for spin wave
detection, in particular in the field of spin wave caloritronics",1910.04304v1
2020-01-10,Long-Range Phonon Spin Transport in Ferromagnet-Nonmagnetic Insulator Heterostructures,"We investigate phonon spin transport in an insulating
ferromagnet-nonmagnet-ferromagnet heterostructure. We show that the
magnetoelastic interaction between the spins and the phonons leads to nonlocal
spin transfer between the magnets. This transfer is mediated by a local phonon
spin current and accompanied by a phonon spin accumulation. The spin
conductance depends nontrivially on the system size, and decays over millimeter
length scales for realistic material parameters, far exceeding the decay
lengths of magnonic spin currents.",2001.03462v2
2020-01-19,Chiral Coupling to Magnetodipolar Radiation,"We review and extend the theory of chiral pumping of spin waves by
magnetodipolar stray fields that generate unidirectional spin currents and
asymmetric magnon densities. We illustrate the physical principles by two kinds
of chiral excitations of magnetic films, i.e., by the evanescent Oersted field
of a narrow metallic stripline with an AC current bias and a magnetic nanowire
under ferromagnetic resonance.",2001.06821v2
2020-02-28,Quantum Damping of Skyrmion Crystal Eigenmodes due to Spontaneous Quasiparticle Decay,"The elementary excitations of skyrmion crystals experience both emergent
magnetic fields and anharmonic interactions brought about by the topologically
nontrivial noncollinear texture. The resulting flat bands cause strong
spontaneous quasiparticle decay, dressing the eigenmodes of skyrmion crystals
with a finite zero-temperature quantum lifetime. Sweeping the flat bands
through the spectrum by changing the magnetic field leads to an externally
controllable energy-selective magnon breakdown. In particular, we uncover that
the three fundamental modes, i.e., the anticlockwise, breathing, and clockwise
mode, exhibit distinct decay behavior, with the clockwise (anticlockwise) mode
being the least (most) stable mode out of the three.",2002.12676v1
2020-04-28,More on Schrödinger holography,"We find explicit solutions for giant magnons and spiky strings living on the
Schr\""odinger $Schr_5 \times T^{1,1}$ and compute dispersion relations. The
holographic dual field theory is conjectured to be a non-local dipole-deformed
CFT at strong coupling. We find that the dependence between conserved charges
in the dispersion relations is transcendental, which is quite different from
the most symmetric case of spherical internal space. Keeping the squashing
parameter $b$ general allows us to take some limits and to compare our results
to known cases.",2004.13802v1
2020-10-29,Difference between angular momentum and pseudoangular momentum,"In condensed matter systems it is necessary to distinguish between the
momentum of the constituents of the system and the pseudomomentum of
quasiparticles. The same distinction is also valid for angular momentum and
pseudoangular momentum. Based on Noether's theorem, we demonstrate that the
recently discussed orbital angular momenta of phonons and magnons are
pseudoangular momenta. This conceptual difference is important for a proper
understanding of the transfer of angular momentum in condensed matter systems,
especially in spintronics applications.",2010.15616v2
2021-03-03,The studies on phonons and magnons in (Ni80Fe20/Au/Co/Au) multilayers of different number of repetitions,"In Ni80Fe20/Au/Co/Au deposited on silicon substrate the interaction between
spin waves and surface acoustic waves is observed by Brillion light scattering
spectroscopy. We show that by changing the number of receptions in the
multilayer we can tune the dispersion of surface acoustic waves (SAWs) not
affecting significantly fundamentals spin wave (SW) mode. As a result, we can
control (activate or deactivate) the magnetoelastic interaction between
fundamental SW mode and SAWs by the thickness of the magnetostrictive
multilayer.",2103.02686v2
2021-03-15,Magnetically switchable spin wave retarder with $90^\circ$ antiferromagnetic domain wall,"Polarization, denoting the precession direction with respect to the
background magnetization, is an intrinsic degree of freedom of spin wave. Using
magnetic textures to control the spin wave polarization is fundamental and
indispensable toward reprogrammable polarization-based magnonics. Here, we show
that due to the intrinsic cubic anisotropy, a $90^\circ$ antiferromagnetic
domain wall naturally acts as a spin wave retarder (wave-plate). Moreover, for
a $90^\circ$ domain wall pair developed by introducing a second domain in a
homogenous antiferromagnetic wire, the sign of retarding effect can be flipped
by simply switching the direction of the intermediate domain.",2103.08342v2
2021-03-15,Ferromagnetic Resonance in Permalloy Metasurfaces,"Permalloy films with one-dimensional (1D) profile modulation of submicron
periodicity are fabricated based on commercially available DVD-R discs and
studied using ferromagnetic resonance (FMR) method and micromagnetic numerical
simulations. The main resonance position shows in-plane angular dependence
which is strongly reminiscent of that in ferromagnetic films with uniaxial
magnetic anisotropy. The main signal and additional low field lines are
attributed to multiple standing spin wave resonances defined by the grating
period. The results may present interest in magnetic metamaterials and
magnonics applications.",2103.08704v1
2021-04-05,Thermal Hall Effect of Chiral Spin Fluctuations,"Using a two-dimensional square lattice Heisenberg model with a Rashba-type
Dzyaloshinskii-Moriya interaction, we demonstrate that chiral spin fluctuations
can give rise to a thermal Hall effect in the absence of any static spin
texture or momentum space topology. It is shown by means of Monte Carlo and
stochastic spin dynamics simulations that the thermal Hall response is finite
at elevated temperature outside of the linear spin wave regime and consistent
with the presence of thermal fluctuation-induced nontrivial topology. Our
result suggests that the high-fluctuation phases outside of the conventional
regime of magnonics may yet be a promising area of exploration for spin-based
electronics.",2104.02168v1
2021-04-06,Quantum States of a Skyrmion in a 2D Antiferromagnet,"Quantum states of a skyrmion in a 2D antiferromagnetic lattice are obtained
by quantizing the scaling parameter of Belavin-Polyakov model. Skyrmion
classical collapse due to violation of the translational invariance of the
continuous spin-field model by the lattice is replaced in quantum mechanics by
transitions between discrete energy levels of the skyrmion. Rates of
transitions due to the emission of magnons are computed. Ways of detecting
quantization of skyrmion states are discussed.",2104.02212v1
2021-08-13,Coupling the Higgs mode and ferromagnetic resonance in spin-split superconductors with Rashba spin-orbit coupling,"We consider the Higgs mode at nonzero momentum in superconductors and
demonstrate that in the presence of Rashba spin-orbit coupling, it couples
linearly with an external exchange field. The Higgs-spin coupling dramatically
modifies the spin susceptibility near the superconducting critical temperature
and consequently enhances the spin pumping effect in a ferromagnetic
insulator/superconductor bilayer system. We show that this effect can be
detected by measuring the magnon-induced voltage generated by the inverse spin
Hall effect.",2108.06202v2
2021-10-23,Optical interface for a hybrid magnon-photon resonator,"We study optical detection of magnetic resonance of a ferrimagnetic sphere
resonator, which is strongly coupled to a microwave loop gap resonator. Optical
fibers are employed for coupling the sphere resonator with light in the telecom
band. We find that magnetic resonance can be optically detected in the region
of anti-crossing between the loop gap and the ferrimagnetic resonances. By
measuring the response time of the optical detection we rule out the
possibility that microwave induced heating is responsible for the optical
detectability.",2110.13644v1
2021-11-04,Sequence of superconducting states in field cooled $FeCr_2S_4$,"In the present article we discuss theoretically the emergence of
superconductivity in field cooled $FeCr_2S_4$. The chromium electrons form a
triplet $t_{2g}$ states and due to antiferromagnetic exchange with the iron
spins have Zeeman splitting. Applied, during preparation, magnetic field along
the moment of iron ions, successively compensates the Zeeman splittings. The
chromium electrons with zero Zeeman energy form Cooper pairs induced by iron
magnons. In that way, we predict theoretically the existence of sequence of
superconducting states in field cooled $FeCr_2S_4$. Actually there are three
different superconductors prepared applying, during preparation, different
magnetic fields. In these compounds superconductivity coexist with the
saturated magnetism of iron ions.",2111.02765v1
2021-11-18,Antiferromagnetic Resonance Revisited: Dissipative Coupling without Dissipation,"The antiferromagnet is a closed Hermitian system, we find that its
excitations, even in the absence of dissipation, can be viewed as a
non-Hermitian system with dissipative coupling. Consequently, the
antiferromagnetic resonance spectrum does not show the typical level repulsion,
but shows the level attraction -- a characteristic behavior often observed in
non-Hermitian systems. Such behavior is because the antiferromagnetic ground
state is $\mathcal{PT}$-symmetric. This new understanding on antiferromagnetic
resonance also explains the mysterious enhancement of antiferromagnetic damping
rate. Being effectively non-Hermitian, antiferromagnetic magnons can be used
for quantum entanglement generation without introducing a third party like
external pumping.",2111.09682v1
2021-12-07,Curvilinear manipulation of polarized spin wave,"Polarization, the precession direction with respect to the background
magnetization, is an intrinsic degree of freedom of spin wave. Introducing
symmetry breaking mechanisms lies in the heart of lifting the degeneracy
between polarized spin wave modes, and is essential in constructing
polarization-based magnonic devices. Here we show that polarized spin waves can
be naturally harnessed in a curved antiferromagnetic wire via tuning its
curvature and torsion. Specifically, we investigate evolution of polarized spin
wave in a spin wave rotator and a spin wave interferometer based on magnetic
circular helices, and correlate these curvilinear effects to the Berry phase
accumulated along wires.",2112.03748v1
2022-03-12,Excitations of the ferroelectric order,"We identify the bosonic excitations in ferroelectrics that carry electric
dipoles from the phenomenological Landau-Ginzburg-Devonshire theory. The
""ferron"" quasi-particles emerge from the concerted action of anharmonicity and
broken inversion symmetry. In contrast to magnons, the transverse excitations
of the magnetic order, the ferrons in displacive ferroelectrics are
longitudinal with respect to the ferroelectric order. Based on the ferron
spectrum, we predict temperature dependent pyroelectric and electrocaloric
properties, electric-field-tunable heat and polarization transport, and
ferron-photon hybridization.",2203.06367v1
2022-06-01,Dynamic Fingerprints of Synthetic Antiferromagnet Nanostructures with Interfacial Dzyaloshinskii-Moriya Interaction,"Synthetic antiferromagnet (SAF) nanostructures with interfacial
Dzyaloshinskii-Moriya interaction can host topologically distinct spin textures
such as skyrmions and thus are regarded as promising candidates for both
spintronics and magnonics applications. Here, we present comprehensive
micromagnetic simulations of such material systems and discuss the rich phase
diagrams that contain various types of magnetic configurations. Aside from the
static properties, we further discuss the resonant excitations of the
calculated magnetic states which include individual skyrmions and skyrmioniums.
Finally, the internal modes of SAF skyrmion clusters are studied and discussed
in the context of magnetic sensing applications based on the dynamic
fingerprint in broadband ferromagnetic resonance measurements.",2206.00554v2
2023-02-25,A Perspective on Ferrons,"The duality between electric and magnetic dipoles in electromagnetism only
partly applies to condensed matter. In particular, the elementary excitations
of the magnetic and ferroelectric orders, namely magnons and ferrons,
respectively, have received asymmetric attention from the condensed matter
community in the past. In this perspective, we introduce and summarize the
current state of the budding field of ""ferronics"" and speculate about its
potential applications in thermal, information, and communication technology.",2302.12985v1
2023-03-22,Even-odd effect for spin current through thin antiferromagnetic insulator,"Magnon spin transport in a metal-antiferromagnetic insulator-ferromagnetic
insulator heterostructure is considered. The spin current is generated via the
spin Seebeck effect and in the limit of clean sample where the effects of
interface imperfections and lattice defects are excluded. For NiO as an
antiferromagnetic insulator we have a magnetic order of antiferromagnetically
combined planes which are internally in ferromagnetic order. We find that the
sign of the spin current depends on the magnetization direction of the plane
next to the metal resulting in an even-odd effect for the spin current.
Moreover, as long as damping is excluded, this even-odd effect is the only
remaining dependence on the NiO thickness for high temperatures.",2303.12282v1
2023-04-19,Thermoelectric effect of skyrmion crystal confined in a magnetic disk,"We theoretically propose that an electric voltage can be generated by thermal
gradient with a rotating skyrmion crystal confined in a magnetic disk. We find
that the rotation of skyrmion crystal induced by diffusive thermal magnon
currents in the presence of temperature gradient gives rise to spinmotive
forces in the radial direction through coupling to conduction-electron spins.
The amplitude of generated spinmotive force is larger for a larger temperature
gradient at a lower temperature. The proposed phenomenon can be exploited as
spintronics-based thermoelectric devices to realize the conversion of heat to
electricity.",2304.09503v1
2023-05-22,Stoner ferromagnetism in a momentum-confined interacting 2D electron gas,"In this work we investigate the ground state of a momentum-confined
interacting 2D electron gas, a momentum-space analog of an infinite quantum
well. The study is performed by combining analytical results with a numerical
exact diagonalization procedure. We find a ferromagnetic ground state near a
particular electron density and for a range of effective electron (or hole)
masses. We argue that this observation may be relevant to the generalized
Stoner ferromagnetism recently observed in multilayer graphene systems. The
collective magnon excitations exhibit a linear dispersion, which originates
from a diverging spin stiffness.",2305.13373v1
2023-05-30,Quantum spin fluctuations and the stability of atomically-sized Bloch points,"We reveal the role of the spin variables' zero-point fluctuations (ZPFs) on
the stability of Bloch point (BP) singularities. As topological solitons, BPs
are important in topological transitions in nanomagnets. BPs present a
singularity at their core, where the long-length-scale approximation fails. We
found that ZPFs bloom nearby this core, reducing the effective magnetic moment
and increasing the BP's stability. As suggested by classical models, the
magnonic eigenmodes found by our methods fit with the bound state of an
electron surrounding a dyon, with a magnetic and an electric charge.",2305.18704v1
2023-08-01,Overview of superconductivity in field-cooled magnetic materials,"Considerable experimental skills have been accumulated in the preparation of
field-cooled (FC) magnetic materials. This stimulates the search for FC
magnetic materials that are superconductors.
  The article overviews the recent proposed mechanism of superconductivity in
field-cooled magnetic materials.
  It is based on previously published results for magnon-induced
superconductivity in field-cooled spin-1/2 antiferromagnets $[PRB96,214409]$
(arXiv:1712.02983), Sequence of superconducting states in field cooled
$FeCr_2S_4$ $[JPCM33,495604]$
  (arXiv:2111.02765) and Partial order induced superconductivity in $Fe^{2+}$
iron. $[EPL126,47001]$ (arXiv:1902.02290).",2308.00470v1
2023-11-11,Emergence and enhancement of feedback control induced quantum entanglement,"We present a scheme for controlling quantum correlations by applying feedback
to the cavity mode that exits a cavity while interacting with a mechanical
oscillator and magnons. In a hybrid cavity magnomechanical system with a
movable mirror, the proposed coherent feedback scheme allows for the
enhancement of both bipartite and tripartite quantum correlations. Moreover, we
demonstrate that the resulting entanglement remains robust with respect to
ambient temperatures in the presence of coherent feedback control.",2311.06578v3
2023-12-15,Nonlinear enhancement of coherent magnetization dynamics,"Magnets are interesting materials for classical and quantum information
technologies. However, the short decoherence and dephasing times that determine
the scale and speed of information networks, severely limit the appeal of
employing the ferromagnetic resonance. Here we show that the lifetime and
coherence of the uniform Kittel mode can be enhanced by 3-magnon
interaction-induced mixing with the long-lived magnons at the minima of the
dispersion relation. Analytical and numerical calculations based on this model
explain recent experimental results and predict experimental signatures of
quantum coherence.",2312.09856v1
2024-01-30,Coherent and dissipative coupling in a magneto-mechanical system,"Hybrid elastic and spin waves hold promises for energy-efficient and
versatile generation and detection of magnetic signals, with potentially long
coherence times. Here we report on the combined elastic and magnetic dynamics
in a one-dimensional magneto-mechanical crystal composed of an array of
magnetic nanowires. Phononic and magnonic modes are impulsively excited by an
optical ultrafast trigger and their decay is monitored by time resolved Magneto
Optical Kerr Effect, with complementary Brillouin Light Scattering measurements
and micromagnetic simulations. The strength and degree of mixing of coherent
and dissipative coupling of the quasi-particles is determined quantitatively.",2401.16994v1
2024-02-05,Modelling nanomagnet vertex dynamics through Coulomb charges,"We investigate the magnetization dynamics in nanomagnet vertices often found
in artificial spin ices. Our analysis involves creating a simplified model that
depicts edge magnetization using magnetic charges. We utilize the model to
explore the energy landscape, its associated curvatures, and the fundamental
modes. Our study uncovers specific magnonic regimes and transitions between
magnetization states, marked by zero-modes, which can be understood within the
framework of Landau theory. To verify our model, we compare it with
micromagnetic simulations, demonstrating a noteworthy agreement.",2402.03036v1
1999-09-23,The Pseudogap in La(2-x)Sr(x)CuO(4): A Raman Viewpoint,"We report the results of Raman scattering experiments on single crystals of
La(2-x)Sr(x)CuO(4) [La214] as a function of temperature and doping. In
underdoped compounds low-energy B1g spectral weight is depleted in association
with the opening of a pseudogap on regions of the Fermi surface located near
(pi, 0) and (0, pi). The magnitude of the depletion increases with decreasing
doping, and in the most underdoped samples, with decreasing temperature. The
spectral weight that is lost at low-energies (omega < 800 cm-1) is transferred
to the higher energy region normally occupied by multi-magnon scattering. From
the normal state B2g spectra we have determined the scattering rate
Gamma(omega, T) of qausiparticles located near the diagonal directions in
k-space, (pi/2, pi/2) regions. In underdoped compounds, Gamma(omega, T) is
suppressed at low temperatures for energies less than Eg(x) ~ 800 cm-1. The
observed doping dependence of the two-magnon scattering and the scattering rate
suppression thus suggest that the pseudogap is characterized by an energy scale
Eg ~ J, where J is the antiferromagnetic super-exchange energy. Comparison with
the results from other techniques provides a consistent picture of the
pseudogap in La214.",9909342v3
2002-03-08,Low-temperature electrical transport in bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$,"The temperature $T$ and magnetic field $H$ dependence of anisotropic in-plane
$\rho_{ab}$ and out-of-plane $\rho_{c}$ resistivities have been investigated in
single crystals of the bilayer manganite La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$.
Below the Curie transition temperature $T_c=$ 125 K, $\rho_{ab}$ and $\rho_{c}$
display almost the same temperature dependence with an up-turn around 50 K. In
the metallic regime (50 K $\leq T \leq$ 110 K), both $\rho_{ab}(T)$ and
$\rho_{c}(T)$ follow a $T^{9/2}$ dependence, consistent with the two-magnon
scattering. We found that the value of the proportionality coefficient
$B_{ab}^{fit}$ and the ratio of the exchange interaction $J_{ab}/J_c$ obtained
by fitting the data are in excellent agreement with the calculated $B_{ab}$
based on the two-magnon model and $J_{ab}/J_c$ deduced from neutron scattering,
respectively. This provides further support for this scattering mechanism. At
even lower $T$, in the non-metallic regime ($T<$ 50 K), {\it both} the in-plane
$\sigma_{ab}$ and out-of-plane $\sigma_{c}$ conductivities obey a $T^{1/2}$
dependence, consistent with weak localization effects. Hence, this demonstrates
the three-dimensional metallic nature of the bilayer manganite
La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ at $T<T_c$.",0203197v1
2002-05-08,Quasiparticles governing the zero-temperature dynamics of the 1D spin-1/2 Heisenberg antiferromagnet in a magnetic field,"The T=0 dynamical properties of the one-dimensional (1D) $s=1/2$
  Heisenberg antiferromagnet in a uniform magnetic field are studied via Bethe
ansatz for cyclic chains of $N$ sites. The ground state at magnetization
$0<M_z<N/2$, which can be interpreted as a state with $2M_z$ spinons or as a
state of $M_z$ magnons, is reconfigured here as the vacuum for a different
species of quasiparticles, the {\em psinons} and {\em antipsinons}. We
investigate three kinds of quantum fluctuations, namely the spin fluctuations
parallel and perpendicular to the direction of the applied magnetic field and
the dimer fluctuations. The dynamically dominant excitation spectra are found
to be sets of collective excitations composed of two quasiparticles excited
from the psinon vacuum in different configurations. The Bethe ansatz provides a
framework for (i) the characterization of the new quasiparticles in relation to
the more familiar spinons and magnons, (ii) the calculation of spectral
boundaries and densities of states for each continuum, (iii) the calculation of
transition rates between the ground state and the dynamically dominant
collective excitations, (iv) the prediction of lineshapes for dynamic structure
factors relevant for experiments performed on a variety of quasi-1D
antiferromagnetic compounds, including KCuF$_3$,
  Cu(C$_4$H$_4$N$_2)(NO_3)_2$, and CuGeO$_3$.",0205142v1
2002-11-28,Low energy spin wave excitations in bilayered magnetic manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ ($0.30\leq{x}\leq{0.50}$),"We have studied the low-temperature behavior of the magnetization and the
specific heat of the bilayered perovskite system $%
La_{2-2x}Sr_{1+2x}Mn_{2}O_{7}$, for $0.30\leq x\leq 0.50$. Our analysis reveals
that below 30 K the temperature dependence of the magnetization of the
ferromagnetic samples, $x=0.30$, 0.32, and 0.36, in a field of 1T can be
interpreted in terms of the thermal excitations of a two-dimensional gas of
ferromagnetic magnons. The specific heat in zero field for these samples as for
the $x=0.50$ antiferromagnetic one, is linear with temperature between the
range of 1.8K $\leq $ T $\leq $ 10K. This behavior can be also explained by the
magnon gas model. By comparing specific heat measurements in zero field with
those taken in a field of 9T we are able to extract the lattice and electronic
contributions and determine the in-plane exchange interactions. That are found
to be in reasonable agreement with the values inferred from the analysis of the
magnetization data and also with the values reported by inelastic neutron
scattering studies. In addition, we found that the electronic density of states
obtained for the $x=0.50$ sample is in agreement with previous band structure
calculations.",0211677v3
2004-07-30,Central peak in the pseudogap of high T_c superconductors,"We study the effect of antiferromagnetic (AF) correlations in the three-band
Emery model, with respect to the experimental situation in weakly underdoped
and optimally doped BSCCO. In the vicinity of the vH singularity of the
conduction band there appears a central peak in the middle of a pseudogap,
which is in an antiadiabatic regime, insensitive to the time scale of the
mechanism responsible for the pseudogap. We find a quantum low-temperature
regime corresponding to experiment, in which the pseudogap is created by
zero-point motion of the magnons, as opposed to the usual semiclassical
derivation, where it is due to a divergence of the magnon occupation number.
Detailed analysis of the spectral functions along the (pi,0)-(pi,pi) line show
significant agreement with experiment, both qualitative and, in the principal
scales, quantitative. The observed slight approaching-then-receding of both the
wide and narrow peaks with respect to the Fermi energy is also reproduced. We
conclude that optimally doped BSCCO has a well-developed pseudogap of the order
of 1000 K. This is only masked by the narrow antiadiabatic peak, which provides
a small energy scale, unrelated to the AF scale, and primarily controlled by
the position of the chemical potential.",0407800v3
2004-10-28,Transport properties in manganite thin films,"The resistivity of thin $La_{0.7}A_{0.3}MnO_{3}$ films ($A=Ca, Sr$) is
investigated in a wide temperature range. The comparison of the resistivities
is made among films grown by different techniques and on several substrates
allowing to analyze samples with different amounts of disorder. In the
low-temperature nearly half-metallic ferromagnetic state the prominent
contribution to the resistivity scales as $T^{\alpha}$ with $\alpha \simeq 2.5$
for intermediate strengths of disorder supporting the theoretical proposal of
single magnon scattering in presence of minority spin states localized by the
disorder. For large values of disorder the low-temperature behavior of the
resistivity is well described by the law $T^{3}$ characteristic of anomalous
single magnon scattering processes, while in the regime of low disorder the
$\alpha$ exponent tends to a value near 2. In the high temperature insulating
paramagnetic phase the resistivity shows the activated behavior characteristic
of polaronic carriers. Finally in the whole range of temperatures the
experimental data are found to be consistent with a phase separation scenario
also in films doped with strontium ($A=Sr$).",0410743v1
2005-06-14,High field properties of geometrically frustrated magnets,"Above the saturation field, geometrically frustrated quantum antiferromagnets
have dispersionless low-energy branches of excitations corresponding to
localized spin-flip modes. Transition into a partially magnetized state occurs
via condensation of an infinite number of degrees of freedom. The ground state
below the phase transition is a magnon crystal, which breaks only translational
symmetry and preserves spin-rotations about the field direction. We give a
detailed review of recent works on physics of such phase transitions and
present further theoretical developments. Specifically, the low-energy degrees
of freedom of a spin-1/2 kagom\'e antiferromagnet are mapped to a hard hexagon
gas on a triangular lattice. Such a mapping allows to obtain a quantitative
description of the magnetothermodynamics of a quantum kagom\'e antiferromagnet
from the exact solution for a hard hexagon gas. In particular, we find the
exact critical behavior at the transition into a magnon crystal state, the
universal value of the entropy at the saturation field, and the position of
peaks in temperature- and field-dependence of the specific heat. Analogous
mapping is presented for the sawtooth chain, which is mapped onto a model of
classical hard dimers on a chain. The finite macroscopic entropies of
geometrically frustrated magnets at the saturation field lead to a large
magnetocaloric effect.",0506327v2
2006-02-17,Field dependence of the magnetic spectrum in anisotropic and Dzyaloshinskii-Moriya antiferromagnets: I. Theory,"We consider theoretically the effects of an applied uniform magnetic field on
the magnetic spectrum of anisotropic two-dimensional and Dzyaloshinskii-Moriya
layered quantum Heisenberg antiferromagnets. The first case is relevant for
systems such as the two-dimensional square lattice antiferromagnet
Sr(2)CuO(2)Cl(2), while the later is known to be relevant to the physics of the
layered orthorhombic antiferromagnet La(2)CuO(4). We first establish the
correspondence betwenn the low-energy spectrum obtained within the anisotropic
non-linear sigma model and by means of the spin-wave approximation for a
standard easy-axis antiferromagent. Then, we focus on the field-theory approach
to calculate the magnetic field dependence of the magnon gaps and spectral
intensities for magnetic fields applied along the three possible
crystallographic directions. We discuss the various possible ground states and
their evolution with temperature for the different field orientations, and the
occurrence of spin-flop transitions for fields perpendicular to the layers
(transverse fields) as well as for fields along the easy axis (longitudinal
fields). Measurements of the one-magnon Raman spectrum in Sr(2)CuO(2)Cl(2) and
La(2)CuO(4) and a comparison between the experimental results and the
predictions of the present theory will be reported in part II of this research
work [L. Benfatto et al., cond-mat/0602664].",0602419v3
2006-06-15,Exact eigenstates of highly frustrated spin lattices probed in high fields,"Strongly frustrated antiferromagnets such as the magnetic molecule
{Mo72Fe30}, the kagome, or the pyrochlore lattice exhibit a variety of
fascinating properties like low-lying singlets, magnetization plateaus as well
as magnetization jumps. During recent years exact many-body eigenstates could
be constructed for several of these spin systems. These states become ground
states in high magnetic fields, and they also lead to exotic behavior. A key
concept to an understanding of these properties is provided by independent
localized magnons. The energy eigenvalue of these n-magnon states scales
linearly with the number n of independent magnons and thus with the total
magnetic quantum number M=Ns-n. In an applied field this results in a giant
magnetization jump which constitutes a new macroscopic quantum effect. It will
be demonstrated that this behavior is accompanied by a massive degeneracy, an
extensive (T=0)-entropy, and thus a large magnetocaloric effect at the
saturation field. The connection to flat band ferromagnetism will be outlined.",0606401v2
2006-07-25,Spin dynamics in the diluted ferromagnetic Kondo lattice model,"The interplay of disorder and competing interactions is investigated in the
carrier-induced ferromagnetic state of the Kondo lattice model within a
numerical finite-size study in which disorder is treated exactly. Competition
between impurity spin couplings, stability of the ferromagnetic state, and
magnetic transition temperature are quantitatively investigated in terms of
magnon properties for different models including dilution, disorder, and
weakly-coupled spins. A strong optimization is obtained for T_c at hole doping
p << x, highlighting the importance of compensation in diluted magnetic
semiconductors. The estimated T_c is in good agreement with experimental
results for Ga_{1-x}Mn_x As for corresponding impurity concentration, hole
bandwidth, and compensation. Finite-temperature spin dynamics is quantitatively
studied within a locally self-consistent magnon renormalization scheme, which
yields a substantial enhancement in T_c due to spin clustering, and highlights
the nearly-paramagnetic spin dynamics of weakly-coupled spins. The large
enhancement in density of low-energy magnetic excitations due to disorder and
competing interactions results in a strong thermal decay of magnetization,
which fits well with the Bloch form M_0(1-BT^{3/2}) at low temperature, with B
of same order of magnitude as obtained in recent squid magnetization
measurements on Ga_{1-x}Mn_x As samples.",0607633v1
2006-08-29,Collective modes for an array of magnetic dots in the vortex state,"The dispersion relations for collective magnon modes for square-planar arrays
of vortex-state magnetic dots, having closure magnetic flux are calculated. The
array dots have no direct contact between each other, and the sole source of
their interaction is the magnetic dipolar interaction. The magnon formalism
using Bose operators along with translational symmetry of the lattice, with the
knowledge of mode structure for the isolated dot, allows the diagonalization of
the system Hamiltonian giving the dispersion relation. Arrays of vortex-state
dots show a large variety of collective mode properties, such as positive or
negative dispersion for different modes. For their description, not only
dipolar interaction of effective magnetic dipoles, but non-dipolar terms common
to higher multipole interaction in classical electrodynamics can be important.
The dispersion relation is shown to be non-analytic as the value of the
wavevector approaches zero for all dipolar active modes of the single dot. For
vortex-state dots the interdot interaction is not weak, because, the dynamical
part (in contrast to the static magnetization of the vortex state) dot does not
contain the small parameter, the ratio of vortex core size to the dot radius.
This interaction can lead to qualitative effects like the formation of modes of
angular standing waves instead of modes with definite azimuthal number known
for the insolated vortex state dot.",0608638v1
2006-11-09,"Ferromagnetism in the t-t' Hubbard model: interplay of lattice, band dispersion, and interaction effects studied within a Goldstone-mode preserving scheme","Ferromagnetism in the Hubbard model is investigated on sc, bcc, and fcc
lattices using a systematic inverse-degeneracy ($1/{\cal N}$) expansion which
incorporates self-energy and vertex corrections such that spin-rotation
symmetry and the Goldstone mode are explicitly preserved. First-order quantum
corrections to magnon energies are evaluated for several cases, providing a
comprehensive picture of the interplay of lattice, band dispersion, and
interaction effects on the stability of the ferromagnetic state with respect to
both long- and short-wavelength fluctuations. Our results support the belief
that ferromagnetism is a generic feature of the Hubbard model at intermediate
and strong coupling provided the DOS is sufficiently asymmetric and strongly
peaked near band edge, as for fcc lattice with finite $t'$. For
short-wavelength modes, behavior of a characteristic energy scale $\omega^*
\sim T_c$ (magnon-DOS-peak energy) is in excellent agreement with the $T_c$ vs.
$n$ behavior within DMFT, both with respect to the stable range of densities
($0.20 < n < 0.85$) as well as the optimal density $n=0.65$. However, our
finding of vanishing spin stiffness near optimal density highlights the role of
long-wavelength fluctuations in further reducing the stable range of densities.",0611238v1
2006-11-17,Field-driven phase transitions in a quasi-two-dimensional quantum antiferromagnet,"We report magnetic susceptibility, specific heat, and neutron scattering
measurements as a function of applied magnetic field and temperature to
characterize the $S=1/2$ quasi-two-dimensional frustrated magnet piperazinium
hexachlorodicuprate (PHCC). The experiments reveal four distinct phases. At low
temperatures and fields the material forms a quantum paramagnet with a 1 meV
singlet triplet gap and a magnon bandwidth of 1.7 meV. The singlet state
involves multiple spin pairs some of which have negative ground state bond
energies. Increasing the field at low temperatures induces three dimensional
long range antiferromagnetic order at 7.5 Tesla through a continuous phase
transition that can be described as magnon Bose-Einstein condensation. The
phase transition to a fully polarized ferromagnetic state occurs at 37 Tesla.
The ordered antiferromagnetic phase is surrounded by a renormalized classical
regime. The crossover to this phase from the quantum paramagnet is marked by a
distinct anomaly in the magnetic susceptibility which coincides with closure of
the finite temperature singlet-triplet pseudo gap. The phase boundary between
the quantum paramagnet and the Bose-Einstein condensate features a finite
temperature minimum at $T=0.2$ K, which may be associated with coupling to
nuclear spin or lattice degrees of freedom close to quantum criticality.",0611463v1
2007-02-27,Low temperature specific heat and possible gap to magnetic excitations in the Heisenberg pyrochlore antiferromagnet Gd2Sn207,"The Gd2Sn2O7 pyrochlore Heisenberg antiferromagnet displays a phase
transition to a four sublattice Neel ordered state at a temperature near 1 K.
Despite the seemingly conventional nature of the ordered state, the specific
heat has been found to be described in the temperature range 350-800 mK by an
anomalous T-squared power law. A similar temperature dependence has also been
reported for Gd2Ti2O7, another pyrochlore Heisenberg material. Such anomalous
T-squared behavior in Cv has been argued to be correlated to an unusual
energy-dependence of the density of states which also seemingly manifests
itself in low-temperature spin fluctuations found in muon spin relaxation
experiments. In this paper, we report calculations of Cv that consider spin
wave like excitations out of the Neel order observed in Gd2Sn2O7 and argue that
the parametric T-squared behavior does not reflect the true low-energy
excitations of Gd2Sn2O7. Rather, we find that the low-energy excitations of
this material are antiferromagnetic magnons gapped by single-ion and dipolar
anisotropy effects, and that the lowest temperature of 350 mK considered in
previous specific heat measurements accidentally happens to coincide with a
crossover temperature below which magnons become thermally activated and Cv
takes an exponential form. We argue that further specific heat measurements
that extend down to at least 100 mK are required in order to ascribe an
unconventional description of magnetic excitations out of the ground state of
Gd2Sn2O7 or to invalidate the standard picture of gapped excitations proposed
herein.",0702661v2
2004-02-19,Static black holes with a negative cosmological constant: Deformed horizon and anti-de Sitter boundaries,"Using perturbative techniques, we investigate the existence and properties of
a new static solution for the Einstein equation with a negative cosmological
constant, which we call the deformed black hole. We derive a solution for a
static and axisymmetric perturbation of the Schwarzschild-anti-de Sitter black
hole that is regular in the range from the horizon to spacelike infinity. The
key result is that this perturbation simultaneously deforms the two boundary
surfaces--i.e., both the horizon and spacelike two-surface at infinity. Then we
discuss the Abbott-Deser mass and the Ashtekar-Magnon one for the deformed
black hole, and according to the Ashtekar-Magnon definition, we construct the
thermodynamic first law of the deformed black hole. The first law has a
correction term which can be interpreted as the work term that is necessary for
the deformation of the boundary surfaces. Because the work term is negative,
the horizon area of the deformed black hole becomes larger than that of the
Schwarzschild-anti-de Sitter black hole, if compared under the same mass,
indicating that the quasistatic deformation of the Schwarzschild-anti-de Sitter
black hole may be compatible with the thermodynamic second law (i.e., the area
theorem).",0402082v2
2007-07-31,Renormalized spin wave excitations in the antiferromagnetic Heisenberg-Kondo model for heavy fermions,"Recent inelastic neutron scattering experiments in CeIn3 and CePd2Si2 single
crystals, measured spin wave excitations at low temperatures. These two heavy
fermion compounds exhibit antiferromagnetic long-range order, but a strong
competition between the Ruderman-Kittel-Kasuya-Yosida(RKKY) interaction and
Kondo effect is evidenced by their nearly equal Neel and Kondo temperatures.
Our aim is to show how magnons such as measured in the antiferromagnetic phase
of these Ce compounds, can be described with a microscopic Heisenberg-Kondo
model as introduced by J.R.Iglesias, C.Lacroix and B.Coqblin, used before for
studies of the non-magnetic phase. The model includes the correlated Ce-4f
electrons hybridized with the conduction band, and we consider competing RKKY
(Heisenberg-like J_{H}) and Kondo (J_{K}) antiferromagnetic couplings. Carrying
on a series of unitary transformations, we perturbatively derive a second-order
effective Hamiltonian which, projected onto the antiferromagnetic electron
ground state, describes the spin wave excitations, renormalized by their
interaction with correlated itinerant electrons. We numerically study how the
different parameters of the model influence the renormalization of the magnons,
yielding useful information for the analysis of inelastic neutron scattering
experiments in antiferromagnetic heavy fermion compounds. We also compare our
results with available experimental data, finding good agreement with the spin
wave measurements in cubic CeIn3.",0708.0023v1
2008-08-19,Three-dimensional N=6 SCFT's and their membrane dynamics,"We analyze several aspects of the recent construction of three-dimensional
conformal gauge theories by Aharony et al. in various regimes. We pay special
attention to understanding how the M-theory geometry and interpretation can be
extracted from the analysis of the field theory. We revisit the calculations of
the moduli space of vacua and the complete characterization of chiral ring
operators by analyzing the field theory compactified on a 2-sphere. We show
that many of the states dual to these operators can be interpreted as D-brane
states in the weak coupling limit. Also, various features of the dual AdS
geometry can be obtained by performing a strong coupling expansion around
moduli space configurations, even though one is not taking the planar
expansion. In particular, we show that at strong coupling the corresponding
weak coupling D-brane states of the chiral ring localize on particular
submanifolds of the dual geometry that match the M-theory interpretation. We
also study the massive spectrum of fields in the moduli space. We use this to
investigate the dispersion relation of giant magnons from the field theory
point of view. Our analysis predicts the exact functional form of the
dispersion relation as a function of the world-sheet momentum, independently of
integrability assumptions, but not the exact form with respect to the 't Hooft
coupling. We also get the dispersion relation of bound states of giant magnons
from first principles, providing evidence for the full integrability of the
corresponding spin chain model at strong 't Hooft coupling.",0808.2503v2
2009-09-28,Thermodynamics of O(N) Antiferromagnets in 2+1 Dimensions,"Within the framework of effective Lagrangians we calculate the free energy
density for an O($N$) antiferromagnet in 2+1 dimensions up to three-loop order
in the perturbative expansion and derive the low-temperature series for various
thermodynamic quantities. In particular, we show that the magnon-magnon
interaction in the O(3) antiferromagnet in $d$=2+1 -- the O(3)-invariant
quantum Heisenberg antiferromagnet on a square or a honeycomb lattice -- is
very weak and repulsive and manifests itself through a term proportional to
five powers of the temperature in the free energy density. Remarkably, the
corresponding coefficient is fully determined by the leading-order effective
Lagrangian ${\cal L}^2_{eff}$ and does not involve any higher order effective
constants from ${\cal L}^4_{eff}$ related to the anisotropies of the lattice --
the symmetries are thus very restrictive in $d$=2+1. We also compare our
results that apply to O($N$) antiferromagnets in 2+1 dimensions with the those
for O($N$) antiferromagnets in 3+1 dimensions. The present work demonstrates
the efficiency of the fully systematic effective Lagrangian method in the
condensed matter domain, which clearly proves to be superior to spin-wave
theory. We would like to emphasize that the structure of the low-temperature
series derived in the present work is model-independent and universal as it
only relies on symmetry considerations.",0909.5239v1
2010-05-31,Spin Chains in N=2 Superconformal Theories: from the Z_2 Quiver to Superconformal QCD,"In this paper we find preliminary evidence that N=2 superconformal QCD, the
SU(N_c) SYM theory with N_f= 2 N_c fundamental hypermultiplets, might be
integrable in the large N Veneziano limit. We evaluate the one-loop dilation
operator in the scalar sector of the N=2 superconformal quiver with SU(N_c) X
SU(N_{\check c}) gauge group, for N_c = N_{\check c}. Both gauge couplings g
and \check g are exactly marginal. This theory interpolates between the Z_2
orbifold of N=4 SYM, which corresponds to \check g=g, and N=2 superconformal
QCD, which is obtained for \check g -> 0. The planar one-loop dilation operator
takes the form of a nearest-neighbor spin-chain Hamiltonian. For superconformal
QCD the spin chain is of novel form: besides the color-adjoint fields
\phi^a_{b}, which occupy individual sites of the chain, there are ""dimers""
Q^a_{i} \bar Q^i_{b} of flavor-contracted fundamental fields, which occupy two
neighboring sites. We solve the two-body scattering problem of magnon
excitations and study the spectrum of bound states, for general \check g/g. The
dimeric excitations of superconformal QCD are seen to arise smoothly for \check
g -> 0 as the limit of bound wavefunctions of the interpolating theory. Finally
we check the Yang-Baxter equation for the two-magnon S-matrix. It holds as
expected at the orbifold point \check g = g. While violated for general \check
g \neq g, it holds again in the limit \check g -> 0, hinting at one-loop
integrability of planar N=2 superconformal QCD.",1006.0015v2
2010-08-10,Numerical study of the spin-1/2 Heisenberg antiferromagnet on a 48-site triangular lattice,"We numerically study the magnetization and the dispersion relation of a
frustrated quantum spin system. Our method, which is named the stochastic state
selection method, is a kind of Monte Carlo method to give eigenstates of the
system through statistical averaging processes.
  Using the stochastic state selection method with some constraints, we make a
successful study of the spin-1/2 Heisenberg antiferromagnet on a 48-site
triangular lattice. We calculate the sublattice magnetization and the static
structure function in the ground state. Our result on the sublattice
magnetization is consistent with the value given by the linear spin wave
theory. This adds an evidence for the analysis based on the spontaneous
symmetry breaking of the semi-classical Neel order in the ground state.
  We also evaluate the low-lying one magnon spectra of the model with all wave
vectors available on a 48-site triangular lattice. We find that at the ordering
wave vector there is a Goldstone mode, which is in good agreement with the
result from the spin wave analysis. The magnon spectra with other wave vectors,
however, are quite different from results obtained by the linear spin wave
theory. We observe a flat dispersion relation with a strong downward
renormalization. Our results are compatible with those recently reported in the
series expansion study and in the order 1/S calculation of the spin wave
analysis.",1008.1612v1
2010-11-25,Calculation of the Expected Zero Field Muon Relaxation Rate in the Geometrically Frustrated Rare Earth Pyrochlore Gd2Sn2O7 Antiferromagnet,"The magnetic insulator Gd2Sn2O7 is one of many geometrically frustrated
magnetic materials known to exhibit a nonzero muon spin polarization relaxation
rate, $\lambda(T)$, down to the lowest temperature (T) studied. Such behaviour
is typically interpreted as a significant level of persisting spin dynamics
(PSD) of the host material. In the case of Gd2Sn2O7, such PSD comes as a
surprise since magnetic specific heat measurements suggest conventional gapped
magnons, which would naively lead to an exponentially vanishing $\lambda(T)$ as
$T \rightarrow 0$. In contrast to most materials that display PSD, the ordered
phase of Gd2Sn2O7 is well characterized and both the nature and the magnitude
of the interactions have been inferred from the magnetic structure and the
temperature dependence of the magnetic specific heat. Based on this
understanding, the temperature dependence of the muon spin polarization
relaxation through the scattering of spin waves (magnons) is calculated. The
result explicitly shows that, despite the unusual extensive number of weakly
dispersive (gapped)excitations characterizing Gd2Sn2O7, a remnant of the zero
modes of the parent frustrated pyrochlore Heisenberg antiferromagnet, the
temperature dependence of the calculated $\lambda(T)$ differs dramatically from
the experimental one. Indeed, the calculation conforms to the naive expectation
of an exponential collapse of $\lambda(T)$ at temperatures below ~ 0.7K. This
result, for the first time, illustrates crisply and quantitatively the paradox
that presents itself with the pervasive occurrence of PSD in highly frustrated
magnetic systems as evinced by muon spin relaxation measurements.",1011.5689v1
2010-12-29,Finite-size corrections for quantum strings on AdS_4 x CP^3,"We revisit the calculation of curvature corrections to the pp-wave energy of
type IIA string states on AdS_4 x CP^3 initiated in arXiv:0807.1527. Using the
near pp-wave Hamiltonian found in arXiv:0912.2257, we compute the first
non-vanishing correction to the energy of a set of bosonic string states at
order 1/R^2, where R is the curvature radius of the background. The leading
curvature corrections give rise to cubic, order 1/R, and quartic, order 1/R^2,
terms in the Hamiltonian, for which we implement the appropriate normal
ordering prescription. Including the contributions from all possible fermionic
and bosonic string states, we find that there exist logarithmic divergences in
the sums over mode numbers which cancel between the cubic and quartic
Hamiltonian. We show that the cubic Hamiltonian naturally requires that the
cutoff for summing over heavy modes must be twice the one for light modes. With
this prescription the strong-weak coupling interpolating function h(\lambda),
entering the magnon dispersion relation, does not receive a one-loop
correction, in agreement with the algebraic curve spectrum. However, the single
magnon dispersion relation exhibits finite-size exponential corrections.",1101.0004v2
2011-03-01,Inelastic Neutron and X-ray Scattering from Incommensurate Magnetic Systems,"Neutrons and X-rays are powerful probes for studying magnetic and lattice
excitations in strongly correlated materials over very wide ranges of momentum
and energy transfers. In the focus of the present work are the incommensurate
magnetic systems MnSi and Cr. Under application of a magnetic field, helically
ordered MnSi transforms into a weak itinerant ferromagnet. Using polarized
neutrons we demonstrate that the Stoner excitations are spin flip excitations.
The amplitude (longitudinal) fluctuations associated with the magnon modes are
already strong far away from T_C. Interestingly, even the non spin flip
excitations associated with the Stoner modes are observable. In Cr, we have
observed Kohn anomalies in the phonon spectrum at those incommensurate
positions in reciprocal space, where the spin density wave is observed. The
corresponding phonon and magnon modes are not coupled. In addition, an
anomalous softening of a transverse phonon branch along the N-H zone boundary
line is observed that is caused by strong electron phonon coupling. High
resolution neutron scattering indicate that the low energy Fincher-Burke
excitations may rather correspond to localized modes in momentum and energy and
not to propagating collective modes. Finally, we demonstrate that in the near
future it may become feasible to investigate excitations in very small samples
thus allowing to measure the dynamics of strongly correlated materials under
extreme conditions and in the vicinity of quantum phase transitions.",1103.0161v1
2011-08-04,Luescher formula for GKP string,"We investigate finite-size corrections to anomalous dimensions of large-spin
twist-two operators in the planar maximally supersymmetric Yang-Mills theory.
We develop a framework for analysis of these corrections, that is complementary
to the conventional spin-chain approach, by making use of the hole rather than
the magnon picture. From the dual string theory perspective where the
large-spin operator is identified with the Gubser-Klebanov-Polyakov (GKP)
string, our approach is equivalent to constructing the first Luescher
correction to the energy of the GKP string by incorporating the contribution of
virtual excitations propagating on it. It allows us to propose a formula that
controls a particular class of large-spin corrections to the twist-two
anomalous dimension and holds at any value of the coupling constant. Compared
to wrapping corrections computed with magnons propagating on the spin chain,
the finite-size corrections that are encoded in our formalism start at a lower
loop level. Our formalism thus calls for modification of the asymptotic
contributions which are conventionally incorporated within the Asymptotic Bethe
Ansatz. An educated guess allows us to remedy this pitfall and successfully
confront our predictions with known results up to five loop accuracy at weak
coupling. Finally, our formula sheds light on the weak-to-strong coupling
transition for the subleading large-spin corrections under study and confirms
stringy expectations at strong coupling where they are found to be identical to
the first Luescher correction to the vacuum energy of the O(6) sigma model.",1108.0999v1
2011-09-15,Uncovering selective excitations using the resonant profile of indirect inelastic x-ray scattering in correlated materials: Observing two-magnon scattering and relation to the dynamical structure factor,"Resonant inelastic x-ray scattering (RIXS) is a spectroscopic technique which
has been widely used to study various elementary excitations in correlated and
other condensed matter systems. For strongly correlated materials, besides
boosting the overall signal the dependence of the resonant profile on incident
photon energy is still not fully understood. Previous endeavors in connecting
indirect RIXS, such as Cu K-edge for example where scattering takes place only
via the core-hole created as an intermediate state, with the charge dynamical
structure factor S(q,\omega) neglected complicated dependence on the
intermediate state configuration. To resolve this issue, we performed an exact
diagonalization study of the RIXS cross-section using the single-band Hubbard
model by fully addressing the intermediate state contribution. Our results are
relevant to indirect RIXS in correlated materials, such as high Tc cuprates. We
demonstrate that RIXS spectra can be reduced to S(q,\omega) when there is no
screening channel for the core-hole potential in the intermediate state. We
also show that two-magnon excitations are highlighted at the resonant photon
energy when the core-hole potential in the corresponding intermediate state is
poorly screened. Our results demonstrate that different elementary excitations
can be emphasized at different intermediate states, such that selecting the
exact incident energy is critical when trying to capture a particular
elementary excitation.",1109.3446v2
2011-09-28,"Different dimensionality trends in the Landau damping of magnons in iron, cobalt and nickel: time dependent density functional study","We study the Landau damping of ferromagnetic magnons in Fe, Co, and Ni as the
dimensionality of the system is reduced from three to two. We resort to the
\textit{ab initio} linear response time dependent density functional theory in
the adiabatic local spin density approximation. The numerical scheme is based
on the Korringa-Kohn-Rostoker Green's function method. The key points of the
theoretical approach and the implementation are discussed. We investigate the
transition metals in three different forms: bulk phases, free-standing thin
films and thin films supported on a nonmagnetic substrate. We demonstrate that
the dimensionality trends in Fe and Ni are opposite: in Fe the transition from
bulk bcc crystal to Fe/Cu(100) film reduces the damping whereas in Ni/Cu(100)
film the attenuation increases compared to bulk fcc Ni. In Co, the strength of
the damping depends relatively weakly on the sample dimensionality. We explain
the difference in the trends on the basis of the underlying electronic
structure. The influence of the substrate on the spin-wave damping is analyzed
by employing Landau maps representing wave-vector resolved spectral density of
the Stoner excitations.",1109.6217v2
2012-03-27,High frequency permeability of magnonic metamaterials with magnetic inclusions of complex shape,"We present a method of calculation of the effective magnetic permeability of
magnonic metamaterials containing periodically arranged magnetic inclusions of
arbitrary shapes. The spectrum of spin wave modes confined in the inclusions is
fully taken into account. Within the scope of the proposed method, we compare
two approaches. The first approach is based on a simple semi-analytical theory
that uses the numerically calculated susceptibility tensor of an isolated
inclusion as input data. Within the second approach, micromagnetic packages
with periodic boundary conditions (PBC) are used to calculate the
susceptibility of a single 2D periodic array of such inclusions, with the whole
3D metamaterial consisting of a stack of such arrays. To calculate the
susceptibility tensor of an isolated inclusion, we have implemented and
compared two different methods: (a) a micromagnetic method, in which we have
employed three different micromagnetic packages: the finite element package
NMAG and the two finite differences packages OOMMF and MicroMagus; and (b) the
modified dynamical matrix method. To illustrate the methodology, we have
calculated the effective permeability of a metamaterial consisting of a stack
of hexagonal arrays of magnetic nanodisks in a non-magnetic matrix. The range
of geometrical parameters for which such a metamaterial is characterized by the
negative permeability has been identified. The critical comparison of the
different micromagnetic packages and the dynamical matrix method (based on the
calculation of the susceptibility tensor of an isolated inclusion) has
demonstrated that their results agree to within 3 %.",1203.6116v1
2012-07-19,Difference in Bose-Einstein condensation of conserved and unconserved particles,"The peculiarities in the Bose-Einstein condensation of particles and
quasiparticles are discussed. The difference between the condensation of
conserved and unconserved particles is analyzed. A classification of
quasiparticles is given. The emphasis is made on the ability of particles and
quasiparticles to condense. Illustrations include: general Bose-condensed
atomic systems, such as ensembles of trapped atoms, Bose gases with conserved
and unconserved number of atoms, vibrating atoms in double-well lattices,
Holstein-Primakoff magnons, Schwinger bosons, slave bosons, and the
condensation of singletons and triplons. The basic difference is that the
system of particles, whose total number is conserved, can form equilibrium as
well as nonequilibrium condensates, while unconserved particles can condense
only in a nonequilibrium system subject to external pumping supporting the
density of these particles sufficient for their condensation. The examples of
such a nonequilibrium condensation of unconserved particles are the
Bose-Einstein condensation of excitons, polaritons, and photons. Elementary
collective excitations, such as bogolons and phonons, being self-consistently
defined, do not condense. Magnons cannot condense in equilibrium systems.
Controversies, existing in literature with regard to the Bose-Einstein
condensation of some quasiparticles, are explained. Pushing a system out of
equilibrium may favor the condensation of unconserved quasiparticles, but
suppresses the condensate fraction of conserved particles.",1207.4621v1
2013-01-07,Persistent high-energy spin excitations in iron pnictide superconductors,"Motivated by the premise that superconductivity in iron-based superconductors
is unconventional and mediated by spin fluctuations, an intense research effort
has been focused on characterizing the spin excitation spectrum in the
magnetically ordered parent phases of the Fe-pnictides2,3 and - chalcogenides4.
For these undoped materials it is well-established that the spin excitation
spectrum consists of a sharp, highly dispersive magnon spanning an energy range
of up to 200 meV (ref. 3). The fate of these high-energy magnetic modes upon
sizable doping is hitherto unresolved. Using resonant inelastic x-ray
scattering we show that optimally doped superconducting Ba0.6K0.4Fe2As2 retains
well defined, dispersive high-energy modes of magnetic origin. These paramagnon
modes are softer than, though as intense as, the magnon of undoped
antiferromagnetic BaFe2As2. The persistence of spin excitations well into the
superconducting phase suggests that, if spin fluctuations are responsible for
superconducting pairing, they originate from a distinctly correlated
spin-state. This connects Fe-pnictide superconductors to the high-Tc cuprates,
for which in spite of fundamental differences in the electronic structure,
similar paramagnon modes are present5.",1301.1289v1
2013-01-17,Spin-wave excitations in presence of nanoclusters of magnetic impurities,"Nanoscale inhomogeneities and impurity clustering are often found to
drastically affect the magnetic and transport properties in disordered/diluted
systems, giving rise to rich and complex phenomena. However, the physics of
these systems still remains to be explored in more details as can be seen from
the scarce literature available. We present a detailed theoretical analysis of
the effects of nanoscale inhomogeneities on the spin excitation spectrum in
diluted magnetic systems. The calculations are performed on relatively large
systems (up to $N$=$66^3$). It is found that even low concentrations of
inhomogeneities have drastic effects on both the magnon density of states and
magnon excitations. These effects become even more pronounced in the case of
short ranged magnetic interactions between the impurities. In contrast to the
increase of critical temperatures $T_C$, reported in previous studies, the
spin-stiffness $D$ is systematically suppressed in the presence of nanoscale
inhomogeneities. Moreover $D$ is found to strongly depend on the
inhomogeneities' concentration, the cluster size, as well as the range of the
magnetic interactions. The findings are discussed in the prospect of potential
spintronics applications. We believe that this detailed numerical work could
initiate future experimental studies to probe this rich physics with the most
appropriate tool, Inelastic Neutron Scattering (INS).",1301.4111v2
2013-03-21,Persistence of magnetic excitations in La2-xSrxCuO4 from the undoped insulator to the heavily overdoped non-superconducting metal,"One of the most intensely studied scenarios of high-temperature
superconductivity (HTS) postulates pairing by exchange of magnetic excitations
[1]. Indeed, such excitations have been observed up to around optimal doping in
the cuprates [2-7]. In the heavily overdoped regime neutron scattering
measurements indicate that magnetic excitations have effectively disappeared
[8-10], and this was argued to cause the demise of HTS with overdoping [1, 8,
10]. Here we use resonant inelastic x-ray scattering (RIXS), which is sensitive
to complementary parts of reciprocal space, to measure the evolution of the
magnetic excitations in La2-xSrxCuO4 across the entire phase diagram, from a
strongly correlated insulator (x = 0) to a non-superconducting metal (x =
0.40). For x = 0, well-defined magnon excitations are observed [11]. These
magnons broaden with doping, but they persist with a similar dispersion and
comparable intensity all the way to the non-superconducting, heavily overdoped
metallic phase. The destruction of HTS with overdoping is therefore caused
neither by the general disappearance nor by the overall softening of magnetic
excitations; other factor(s), such as the redistribution of spectral weight,
must be considered.",1303.5359v3
2013-04-15,Frustrated quantum Heisenberg antiferromagnets at high magnetic fields: Beyond the flat-band scenario,"We consider the spin-1/2 antiferromagnetic Heisenberg model on three
frustrated lattices (the diamond chain, the dimer-plaquette chain and the
two-dimensional square-kagome lattice) with almost dispersionless lowest magnon
band. Eliminating high-energy degrees of freedom at high magnetic fields, we
construct low-energy effective Hamiltonians which are much simpler than the
initial ones. These effective Hamiltonians allow a more extended analytical and
numerical analysis. In addition to the standard strong-coupling perturbation
theory we also use a localized-magnon based approach leading to a substantial
improvement of the strong-coupling approximation. We perform extensive exact
diagonalization calculations to check the quality of different effective
Hamiltonians by comparison with the initial models. Based on the
effective-model description we examine the low-temperature properties of the
considered frustrated quantum Heisenberg antiferromagnets in the high-field
regime. We also apply our approach to explore thermodynamic properties for a
generalized diamond spin chain model suitable to describe azurite at high
magnetic fields. Interesting features of these highly frustrated spin models
consist in a steep increase of the entropy at very small temperatures and a
characteristic extra low-temperature peak in the specific heat. The most
prominent effect is the existence of a magnetic-field driven
Berezinskii-Kosterlitz-Thouless phase transition occurring in the
two-dimensional model.",1304.4136v1
2013-12-17,Longitudinal Excitations in Bipartite and Hexagonal Antiferromagnetic Spin Lattices,"Based on our recently proposed magnon-density-waves using the microscopic
many-body approach, we investigate the longitudinal excitations in quantum
antiferromagnets by including the second order corrections in the large-$s$
expansion. The longitudinal excitation spectra for a general spin quantum
number using the antiferromagnetic Heisenberg Hamiltonian are obtained for
various spin lattice models. For bipartite lattice models, we find that the
numerical results for the energy gaps for the longitudinal modes at $q\to0$ and
the magnetic ordering wavevector $\bf Q$ are reduced by about $40-50\%$ after
including the second order corrections. Thus, our estimate of the energy gaps
for the quasi-one-dimensional (quasi-1D) antiferromagnetic compound KCuF$_3$ is
in better agreement with the experimental result. For the quasi-1D
antiferromagnets on hexagonal lattices, the full excitation spectra of both the
transverse modes (i.e., magnons) and the longitudinal modes are obtained as
functions of the nearest-neighbor coupling and the anisotropy constants. We
find two longitudinal modes due to the non-collinear nature of the triangular
antiferromagnetic order, similar to that of the phenomenological field theory
approach by Affleck. We compare our results for the longitudinal energy gaps at
the magnetic wavevectors with the experimental results for several
antiferromagnetic compounds with both integer and non-integer spin quantum
numbers, and also find good agreement after the higher-order contributions are
included in our calculations.",1312.4929v1
2013-12-24,Phonon and magnetic dimer excitations in Fe-based S=2 spin ladder compound BaFe$_2$Se$_2$O,"Raman scattering spectra of new Fe-based S=2 spin ladder compound
BaFe$_2$Se$_2$O are measured in a temperature range between 15 K and 623 K. All
six A$_{1g}$ and two B$_{1g}$ Raman active modes of BaFe$_2$Se$_2$O, predicted
by the factor-group analysis, have been experimentally observed at energies
that are in a rather good agreement with the lattice dynamics calculation. The
antiferromagnetic long-range spin ordering in BaFe$_2$Se$_2$O below $T_N$=240 K
leaves a fingerprint both in the A$_{1g}$ and B$_{1g}$ phonon mode linewidth
and energy. In the energy range between 400 and 650 cm$^{-1}$ we have observed
magnetic excitation related structure in the form of magnon continuum, with the
peaks corresponding to the singularities in the one dimensional density of
magnon states. The onset value of magnetic continuum (2$\Delta_{S}$) is found
at about 437 cm$^{-1}$ at 15 K. The magnetic continuum disappears at about 623
K, which lead us to conclude that the short-range magnetic ordering in
BaFe$_2$Se$_2$O exists apparently up to 2.6$T_N$.",1312.6844v1
2014-12-01,Coexistence of superconductivity and magnetism in spin-fermion model of ferrimagnetic spinel in an external magnetic field,"A two-sublattice spin-fermion model of ferrimagnetic spinel, with spin-$1/2$
itinerant electrons at the sublattice $A$ site and spin-$s$ localized electrons
at the sublattice $B$ site is considered. The exchange between itinerant and
localized electrons is antiferromanetic. As a result the external magnetic
field, applied along the magnetization of the localized electrons, compensates
the Zeeman splitting due to the spin-fermion exchange and magnon-fermion
interaction induces spin anti-parallel p-wave superconductivity which coexists
with magnetism. We have obtained five characteristic values of the applied
field (in units of energy) $H_{cr1}<H_3<H_0<H_4<H_{cr2}$. At $H_0$ the external
magnetic field compensates the Zeeman splitting. When $H_{cr1}<H<H_{cr2}$ the
spin antiparallel p-wave superconductivity with $T_{1u}$ configuration coexists
with magnetism. The superconductor to normal magnet transition at finite
temperature is second order when $H$ runs the interval $(H_3,H_4)$. It is an
abrupt transition when $H_{cr1}<H<H_3$ or $H_4<H<H_{cr2}$. This is proved
calculating the temperature dependence of the gap for three different values of
the external magnetic field $H_{cr1}<H<H_3$, $H_4<H<H_{cr2}$ and $H=H_0$. In
the first two cases the abrupt fall to zero of the gap at superconducting
critical temperature shows that the superconductor to normal magnet transition
is first order. The Hubbard term (Coulomb repulsion), in a weak coupling
regime, does not affect significantly the magnon induced superconductivity.
Relying on the above results one can formulate a recipe for preparing a
superconductor from ferrimagnetic spinel: i) hydrostatic pressure above the
critical value of insulator-metal transition. ii) external magnetic field along
the sublattice magnetization with higher amplitude.",1412.0450v1
2015-05-13,Interpolating relativistic and non-relativistic Nambu-Goldstone and Higgs modes,"When a continuous symmetry is spontaneously broken in non-relativistic
theories, there appear Nambu-Goldstone (NG) modes, whose dispersion relations
are either linear (type-I) or quadratic (type-II). We give a general framework
to interpolate between relativistic and non-relativistic NG modes, revealing a
nature of type-I and II NG modes in non-relativistic theories. The
interpolating Lagrangians have the nonlinear Lorentz invariance which reduces
to the Galilei or Schrodinger invariance in the non-relativistic limit. We find
that type-I and type-II NG modes in the interpolating region are accompanied
with a Higgs mode and a chiral NG partner, respectively, both of which are
gapful. In the ultra-relativistic limit, a set of a type-I NG mode and its
Higgs partner remains, while a set of type-II NG mode and gapful NG partner
turns to a set of two type-I NG modes. In the non-relativistic limit, the both
types of accompanied gapful modes become infinitely massive, disappearing from
the spectrum. The examples contain a phonon in Bose-Einstein condensates, a
magnon in ferromagnets, and a Kelvon and dilaton-magnon localized around a
skyrmion line in ferromagnets.",1505.03299v3
2015-09-08,Magnon spectrum of the helimagnetic insulator Cu2OSeO3,"Complex low-temperature ordered states in chiral magnets are typically
governed by a competition between multiple magnetic interactions. The
chiral-lattice multiferroic Cu2OSeO3 became the first insulating helimagnetic
material in which a long-range order of topologically stable spin vortices
known as skyrmions was established. Here we employ state-of-the-art inelastic
neutron scattering (INS) to comprehend the full three-dimensional spin
excitation spectrum of Cu2OSeO3 over a broad range of energies. Distinct types
of high- and low-energy dispersive magnon modes separated by an extensive
energy gap are observed in excellent agreement with the previously suggested
microscopic theory based on a model of entangled Cu4 tetrahedra. The comparison
of our INS data with model spin-dynamical calculations based on these
theoretical proposals enables an accurate quantitative verification of the
fundamental magnetic interactions in Cu2OSeO3 that are essential for
understanding its abundant low-temperature magnetically ordered phases.",1509.02432v2
2016-01-15,Bidirectional conversion between microwave and light via ferromagnetic magnons,"Coherent conversion of microwave and optical photons in the single-quantum
level can significantly expand our ability to process signals in various
fields. Efficient up-conversion of a feeble signal in the microwave domain to
the optical domain will lead to quantum-noise-limited microwave amplifiers.
Coherent exchange between optical photons and microwave photons will also be a
stepping stone to realize long-distance quantum communication. Here we
demonstrate bidirectional and coherent conversion between microwave and light
using collective spin excitations in a ferromagnet. The converter consists of
two harmonic oscillator modes, a microwave cavity mode and a magnetostatic mode
called Kittel mode, where microwave photons and magnons in the respective modes
are strongly coupled and hybridized. An itinerant microwave field and a
travelling optical field can be coupled through the hybrid system, where the
microwave field is coupled to the hybrid system through the cavity mode, while
the optical field addresses the hybrid system through the Kittel mode via
Faraday and inverse Faraday effects. The conversion efficiency is theoretically
analyzed and experimentally evaluated. The possible schemes for improving the
efficiency are also discussed.",1601.03908v3
2016-01-22,Study of the cavity-magnon-polariton transmission line shape,"We experimentally and theoretically investigate the microwave transmission
line shape of the cavity-magnon-polariton (CMP) created by inserting a low
damping magnetic insulator into a high quality 3D microwave cavity. While fixed
field measurements are found to have the expected Lorentzian characteristic, at
fixed frequencies the field swept line shape is in general asymmetric. Such
fixed frequency measurements demonstrate that microwave transmission can be
used to access magnetic characteristics of the CMP, such as the field line
width $\Delta H$. By developing an effective oscillator model of the microwave
transmission we show that these line shape features are general characteristics
of harmonic coupling. At the same time, at the classical level the underlying
physical mechanism of the CMP is electrodynamic phase correlation and a second
model based on this principle also accurately reproduces the experimental line
shape features. In order to understand the microscopic origin of the effective
coupled oscillator model and to allow for future studies of CMP phenomena to
extend into the quantum regime, we develop a third, microscopic description,
based on a Green's function formalism. Using this method we calculate the
transmission spectra and find good agreement with the experimental results.",1601.06049v2
2016-06-14,Solitary magnons in the $S={5\over2}$ antiferromagnet CaFe$_{2}$O$_{4}$,"CaFe$_{2}$O$_{4}$ is a $S={5\over 2}$ anisotropic antiferromagnet based upon
zig-zag chains having two competing magnetic structures, denoted as the A
($\uparrow \uparrow \downarrow \downarrow$) and B ($\uparrow \downarrow
\uparrow \downarrow$) phases, which differ by the $c$-axis stacking of
ferromagnetic stripes. We apply neutron scattering to demonstrate that the
competing A and B phase order parameters results in magnetic antiphase
boundaries along $c$ which freeze on the timescale of $\sim$ 1 ns at the onset
of magnetic order at 200 K. Using high resolution neutron spectroscopy, we find
quantized spin wave levels and measure 9 such excitations localized in regions
$\sim$ 1-2 $c$-axis lattice constants in size. We discuss these in the context
of solitary magnons predicted to exist in anisotropic systems. The magnetic
anisotropy affords both competing A+B orders as well as localization of spin
excitations in a classical magnet.",1606.04243v1
2016-07-19,Kitaev materials beyond iridates: order by quantum disorder and Weyl magnons in rare-earth double perovskites,"Motivated by the experiments on the rare-earth double perovskites, we propose
a generalized Kitaev-Heisenberg model to describe the generic interaction
between the spin-orbit-entangled Kramers' doublets of the rare-earth moments.
We carry out a systematic analysis of the mean-field phase diagram of this new
model. In the phase diagram, there exist large regions with a continuous U(1)
or O(3) degeneracy. Since no symmetry of the model protects such a continuous
degeneracy, we predict that the quantum fluctuation lifts the continuous
degeneracy and favors various magnetic orders in the phase diagram. From this
order by quantum disorder mechanism, we further predict that the magnetic
excitations of the resulting ordered phases are characterized by nearly gapless
pseudo-Goldstone modes. We find that there exist Weyl magnon excitations for
certain magnetic orders. We expect our prediction to inspire further study of
Kitaev physics, the order by quantum disorder phenomenon and topological spin
wave modes in the rare-earth magnets and the systems alike.",1607.05618v3
2017-02-15,Spin waves in periodic antidot waveguide of complex base,"We consider the planar magnonic waveguide with a periodic sequence of
antidots forming zig-zag pattern, where two neighboring antidots are shifted
towards the opposite edges of the waveguide. This system has a complex base
with two antidots in one unit cell. The Brillouin zone is here two-times
narrower than the Brillouin zone for the waveguide without displacement of
antidots. We have shown that for dispersion relation folded into narrower
Brillouin zone, new frequency gap can be opened and their width can be
controlled by the shift of the antidots. We found that, the different strength
of spin wave pinning at the edges of the periodic waveguide (and their
antidots)determines the dependence of the width of gap on the shift of
antidots. For the systems with completely free or ideally pinned magnetization,
these dependencies are qualitatively different. We have found an optimum shift
of antidot for maximzing the width of the gap for the system with pinned
magnetization. More interestingly, we notice that for this kind of geometry of
the structure, majority of the modes are doubly degenerate at the edge of
Brillouin zone and have a finite group velocity at the very close vicinity of
the edge of Brillouin zone, for larger values of antidot shift. This empowers
us to design magnonic waveguide to steer the spin waves.",1702.04667v2
2017-02-16,Mobile Spin Impurity in an Optical Lattice,"We investigate the Fermi polaron problem in a spin-1/2 Fermi gas in an
optical lattice for the limit of both strong repulsive contact interactions and
one dimension. In this limit, a polaronic-like behaviour is not expected, and
the physics is that of a magnon or impurity. While the charge degrees of
freedom of the system are frozen, the resulting tight-binding Hamiltonian for
the impurity's spin exhibits an intriguing structure that strongly depends on
the filling factor of the lattice potential. This filling dependency also
transfers to the nature of the interactions for the case of two magnons and the
important spin balanced case. At low filling, and up until near unit filling,
the single impurity Hamiltonian faithfully reproduces a single-band,
quasi-homogeneous tight-binding problem. As the filling is increased and the
second band of the single particle spectrum of the periodic potential is
progressively filled, the impurity Hamiltonian, at low energies, describes a
single particle trapped in a multi-well potential. Interestingly, once the
first two bands are fully filled, the impurity Hamiltonian is a near-perfect
realisation of the Su-Schrieffer-Heeger model. Our studies, which go well
beyond the single-band approximation, that is, the Hubbard model, pave the way
for the realisation of interacting one-dimensional models of condensed matter
physics.",1702.05097v2
2018-01-23,Super Scatterers Based on Artificial Localized Magnon Resonances,"The interaction between electromagnetic waves and objects is strongly
affected by the shape and material composition of the latter. Artificially
created materials, formed by a subwavelength structuring of their unit cells,
namely metamaterials, can exhibit peculiar responses to electromagnetic
radiation and provide additional powerful degrees of freedom to the scatterer
design. In particular, negative material susceptibilities give rise to strong
resonant interactions with deeply subwavelength particles. While the negative
electrical permittivity of natural noble metals manifests itself in localized
plasmon resonant oscillations, negative magnetic permeability is virtually
non-existent in nature. Here the concept of artificial magnon resonance in
subwavelength objects with effective negative permeability, designed based on
the metamaterial approach, is demonstrated. Strong localized oscillations of
the magnetic fields within an array of split ring resonators, forming a sphere,
hybridize in a collective mode of the structure. As a result, extremely high
scattering cross section, exceeding that of a steel sphere with the same radius
by four orders of magnitude, was demonstrated. Furthermore, the scattering
cross section of subwavelength metamaterial-based sphere was shown to be
comparable to the low frequency (MHz) radar signature of a big military
aircraft. Super scatterers, based on tunable resonances within artificially
created materials, can find use in a broad range of electromagnetic
applications, including wireless communications, radars, RFID, internet of
things hardware and many others.",1802.01663v1
2018-02-06,Spin transport in multilayer systems with fully epitaxial NiO thin films,"We report on the generation and transport of thermal spin currents in fully
epitaxial {\gamma}-Fe$_2$O$_3$/NiO(001)/Pt and Fe$_3$O$_4$/NiO(001)/Pt
trilayers. A thermal gradient, perpendicular to the plane of the sample,
generates a magnonic spin current in the ferrimagnetic maghemite
({\gamma}-Fe$_2$O$_3$) and magnetite (Fe$_3$O$_4$) thin films by means of the
spin Seebeck effect. The spin current propagates across the epitaxial,
antiferromagnetic insulating NiO layer, before being detected in the Pt layer
by the inverse spin Hall effect. The transport of the spin signal is studied as
a function of the NiO thickness, temperature and ferrimagnetic material where
the spin current is generated. In epitaxial NiO grown on maghemite, the spin
Seebeck signal decays exponentially as a function of the NiO thickness, with a
spin-diffusion length for thermally-generated magnons of {\lambda}$_{MSDL}$ =
$1.6 \pm 0.2$ nm, largely independent on temperature. We see no enhancement of
the spin current signal as previously reported for certain temperatures and
thicknesses of the NiO. In epitaxial NiO grown on magnetite, the
temperature-averaged spin diffusion length is {\lambda}$_{MSDL}$ = $3.8 \pm
0.3$ nm, and we observe an enhancement of the spin signal when the NiO
thickness is 0.8 nm, demonstrating that the growth conditions dramatically
affect the spin transport properties of the NiO even for full epitaxial growth.
In contrast to theoretical predictions for coherent spin transport, we do not
see vastly different spin diffusion lengths between epitaxial and
polycrystalline NiO layers.",1802.01844v1
2018-04-14,Interplay of Wave Localization and Turbulence in Spin Seebeck Effect,"One of the most important discoveries in spintronics is the spin Seebeck
effect (SSE) recently observed in both insulating and (semi-)conducting
magnets. However, the very existence of the effect in transverse configuration
is still a subject of current debates, due to conflicting results reported in
different experiments. Present understanding of the SSE is mainly based on a
particle-like picture with the local equilibrium approximation (LEA), i.e.,
spatially resolved temperature-field assumed to describe the system. In this
work, we abandon the LEA to some extent and develop a wave theory to explain
the SSE, by highlighting the interplay between wave localization and
turbulence. We show that the emerging SSE with a sign change in the
high/low-temperature regions is closely related to the extendedness of the spin
wave that senses an average temperature of the system. On the one hand,
ubiquitous disorders (or magnetic field gradients) can strongly suppress the
transverse spin Seebeck effect (TSSE) due to Anderson (or Wannier-Zeeman)
spin-wave localization. On the other hand, the competing wave turbulence of
interacting magnons tends to delocalize the wave, and thus remarkably revives
the TSSE before the magnon self-trapping. Our theory provides a promising route
to resolve the heated debate on TSSE with a clear experiment scheme to test it
in future spin caloritronic devices.",1804.05157v1
2014-03-03,Semi-classical analysis of the inner product of Bethe states,"We study the inner product of two Bethe states, one of which is taken
on-shell, in an inhomogeneous XXX chain in the Sutherland limit, where the
number of magnons is comparable with the length L of the chain and the magnon
rapidities arrange in a small number of macroscopically large Bethe strings.
The leading order in the large L limit is known to be expressed through a
contour integral of a dilogarithm. Here we derive the subleading term. Our
analysis is based on a new contour-integral representation of the inner product
in terms of a Fredholm determinant. We give two derivations of the sub-leading
term. Besides a direct derivation by solving a Riemann-Hilbert problem, we give
a less rigorous, but more intuitive derivation by field-theoretical methods.
For that we represent the Fredholm determinant as an expectation value in a
Fock space of chiral fermions and then bosonize. We construct a collective
field for the bosonized theory, the short wave-length part of which may be
evaluated exactly, while the long wave-length part is amenable to a $1/L$
expansion. Our treatment thus results in a systematic 1/L expansion of
structure factors within the Sutherland limit.",1403.0358v2
2016-12-30,Magnetization jump in one dimensional $J-Q_{2}$ model with anisotropic exchange,"We investigate the adiabatic magnetization process of the one-dimensional
$J-Q_{2}$ model with XXZ anisotropy $g$ in an external magnetic field $h$ by
using density matrix renormalization group (DMRG) method. According to the
characteristic of the magnetization curves, we draw a magnetization phase
diagram consisting of four phases. For a fixed nonzero pair coupling $Q$, i)
when $g<-1$, the ground state is always ferromagnetic in spite of $h$; ii) when
$g>-1$ but still small, the whole magnetization curve is continuous and smooth;
iii) if further increasing $g$, there is a macroscopic magnetization jump from
partially- to fully-polarized state; iv) for a sufficiently large $g$, the
magnetization jump is from non- to fully-polarized state. By examining the
energy per magnon and the correlation function, we find that the origin of the
magnetization jump is the condensation of magnons and the formation of magnetic
domains. We also demonstrate that while the experienced states are
Heisenberg-like without long-range order, all the \textit{jumped-over} states
have antiferromagnetic or N\'eel long-range orders, or their mixing.",1701.00013v3
2008-07-05,Vector chiral and multipolar orders in the spin-1/2 frustrated ferromagnetic chain in magnetic field,"We study the one-dimensional spin-1/2 Heisenberg chain with competing
ferromagnetic nearest-neighbor J_1 and antiferromagnetic next-nearest-neighbor
J_2 exchange couplings in the presence of magnetic field. We use both numerical
approaches (the density matrix renormalization group method and exact
diagonalization) and effective field-theory approach, and obtain the
ground-state phase diagram for wide parameter range of the coupling ratio
J_1/J_2. The phase diagram is rich and has a variety of phases, including the
vector chiral phase, the nematic phase, and other multipolar phases. In the
vector chiral phase, which appears in relatively weak magnetic field, the
ground state exhibits long-range order (LRO) of vector chirality which
spontaneously breaks a parity symmetry. The nematic phase shows a quasi-LRO of
antiferro-nematic spin correlation, and arises as a result of formation of
two-magnon bound states in high magnetic fields. Similarly, the higher
multipolar phases, such as triatic (p=3) and quartic (p=4) phases, are formed
through binding of p magnons near the saturation fields, showing quasi-LRO of
antiferro-multipolar spin correlations. The multipolar phases cross over to
spin density wave phases as the magnetic field is decreased, before
encountering a phase transition to the vector chiral phase at a lower field.
The implications of our results to quasi-one-dimensional frustrated magnets
(e.g., LiCuVO_4) are discussed.",0807.0858v2
2008-07-03,Spin wave excitations in the antiferromagnetic Heisenberg-Kondo model for heavy fermions,"Recent inelastic neutron scattering experiments in CeIn$_{3}$ and
CePd$_{2}$Si$_{2}$ single crystals measured spin wave excitations at low
temperatures. These two heavy fermion compounds exhibit antiferromagnetic
long-range order, but a strong competition between the
Ruderman-Kittel-Kasuya-Yosida(RKKY) interaction and Kondo effect is evidenced
by their nearly equal N\'eel and Kondo temperatures. Our aim is to show how
magnons such as measured in the antiferromagnetic phase of these Ce compounds,
can be described with a microscopic Heisenberg-Kondo model introduced by
J.R.Iglesias, C.Lacroix and B.Coqblin, used before for studies of the
non-magnetic phase. The model includes the correlated Ce-$4 f$ electrons
hybridized with the conduction band, where we also allow for correlations, and
we consider competing
  RKKY (Heisenberg-like $J_{H} $) and Kondo ($J_{K}$) antiferromagnetic
couplings. Carrying on a series of unitary transformations, we perturbatively
derive a second-order effective Hamiltonian which, projected onto the
antiferromagnetic electron ground state, describes the spin wave excitations,
renormalized by their interaction with correlated itinerant electrons. We
numerically study how the different parameters of the model influence the
renormalization of the magnons, yielding useful information for the analysis of
inelastic neutron scattering experiments in antiferromagnetic heavy fermion
compounds. We also compare our results with the available experimental data,
finding good agreement with the spin wave measurements in cubic CeIn$_3$.",0807.1246v1
2011-11-01,Phase diagrams of $\rm La_{1-x}Ca_xMnO_3$ in Double Exchange Model with added antiferromagnetic and Jahn-Teller interaction,"The phase diagram of the multivalent manganites $\rm La_{1-x}Ca_xMnO_3$, in
space of temperature and doping $x$, is a challenge for the theoretical
physics. It is an important test for the model used to study these compounds
and the method of calculation. To obtain theoretically this diagram for
$x<0.5$, we consider the two-band Double Exchange Model for manganites with
added Jahn-Teller coupling and antiferromagnetic Heisenberg term. In order to
calculate Curie and N\'{e}el temperatures we derive an effective Heisenberg
model for a vector which describes the local orientation of the total
magnetization of the system. The exchange constants of this model are different
for different space directions and depend on the density of $e_g$ electrons,
antiferromagnetic constants and the Jahn-Teller energy. To reproduce the well
known phase transitions from A-type antiferromagnetism to ferromagnetism at low
$x$ and C-type antiferromagnetism to G-type antiferromagnetism at large $x$, we
argue that the antiferromagnetic exchange constants should depend on the
lattice direction. We show that ferromagnetic to A-type antiferromagnetic
transition results from the Jahn-Teller distortion. Accounting adequately for
the magnon-magnon interaction, Curie and N\'{e}el temperatures are calculated.
The results are in very good agreement with the experiment and provide values
for the model parameters, which best describe the behavior of the critical
temperature for $x<0.5$.",1111.0267v1
2011-11-09,Terahertz and infrared spectroscopic evidence of phonon-paramagnon coupling in hexagonal piezomagnetic YMnO3,"Terahertz and far-infrared electric and magnetic responses of hexagonal
piezomagnetic YMnO3 single crystals are investigated. Antiferromagnetic
resonance is observed in the spectra of magnetic permeability mu_a [H(omega)
oriented within the hexagonal plane] below the Neel temperature T_N. This
excitation softens from 41 to 32 cm-1 on heating and finally disappears above
T_N. An additional weak and heavily-damped excitation is seen in the spectra of
complex dielectric permittivity epsilon_c within the same frequency range. This
excitation contributes to the dielectric spectra in both antiferromagnetic and
paramagnetic phases. Its oscillator strength significantly increases on heating
towards room temperature thus providing evidence of piezomagnetic or
higher-order couplings to polar phonons. Other heavily-damped dielectric
excitations are detected near 100 cm-1 in the paramagnetic phase in both
epsilon_c and epsilon_a spectra and they exhibit similar temperature behavior.
These excitations appearing in the frequency range of magnon branches well
below polar phonons could remind electromagnons; however, their temperature
dependence is quite different. We have used density functional theory for
calculating phonon dispersion branches in the whole Brillouin zone. A detailed
analysis of these results and of previously published magnon dispersion
branches brought us to the conclusion that the observed absorption bands stem
from phonon-phonon and phonon- paramagnon differential absorption processes.
The latter is enabled by a strong short-range in-plane spin correlations in the
paramagnetic phase.",1111.2161v1
2011-11-28,Strings in AdS_4 x CP^3: finite size spectrum vs. Bethe Ansatz,"We compute the first curvature corrections to the spectrum of light-cone
gauge type IIA string theory that arise in the expansion of $AdS_4\times
\mathbb{CP}^3$ about a plane-wave limit. The resulting spectrum is shown to
match precisely, both in magnitude and degeneration that of the corresponding
solutions of the all-loop Gromov--Vieira Bethe Ansatz. The one-loop dispersion
relation correction is calculated for all the single oscillator states of the
theory, with the level matching condition lifted. It is shown to have all
logarithmic divergences cancelled and to leave only a finite exponentially
suppressed contribution, as shown earlier for light bosons. We argue that there
is no ambiguity in the choice of the regularization for the self-energy sum,
since the regularization applied is the only one preserving unitarity.
Interaction matrices in the full degenerate two-oscillator sector are
calculated and the spectrum of all two light magnon oscillators is completely
determined. The same finite-size corrections, at the order 1/J, where $J$ is
the length of the chain, in the two-magnon sector are calculated from the all
loop Bethe Ansatz. The corrections obtained by the two completely different
methods coincide up to the fourth order in $\lambda' =\lambda/J^2$. We
conjecture that the equivalence extends to all orders in $\lambda$ and to
higher orders in 1/J.",1111.6628v3
2012-01-05,Longitudinal and transverse static spin fluctuations in layered ferro- and antiferromagnets,"We analyse the momentum dependence of static non-uniform susceptibilities of
layered local-moment systems below Curie (Neel) temperature within the
1/S-expansion, the renormalization-group approach, and first order of
1/N-expansion. We argue that the previously known results of the spin-wave
theory and renormalization-group approach for the transverse spin
susceptibility acquire strong corrections already at sufficiently low
temperatures, which appear due to the interaction of the incomping magnon
having momentum q with magnons with momenta k<q. Such corrections can not be
treated in the standard renormalization-group approach, but can be described by
both, 1/S and 1/N expansions. The results of these expansions can be
successfully extrapolated to the magnetic transition temperature T=T_M yielding
the correct weight of static spin fluctuations, determined by the O(3)
symmetry. For the longitudinal susceptibility, the summation of leading terms
of 1/S expansion within the parquet approach allows to fulfill the sumrule for
the weights of transverse and longitudinal fluctuations in a broad temperature
region below T_M outside the critical regime. We also discuss the effect of
longitudinal spin fluctuations on the (sublattice) magnetization of layered
systems.",1201.1148v4
2016-05-07,Prime Factorization Using Magnonic Holographic Device,"Determining the prime factors of a given number N is a problem, which
requires super-polynomial time for conventional digital computers. A
polynomial-time algorithm was invented by P. Shor for quantum computers.
However, the realization of quantum computers is associated with significant
technological challenges in achieving and preserving quantum entanglement.
Prime factorization can be also solved by using classical wave interference
without quantum entanglement. In this work, we present experimental data
showing prime factorization by utilizing spin wave interference. The prime
factorization includes three major steps. First, general type computer
calculates the sequence of numbers mkmod(N), where N is the number to be
factorized, m is a randomly chosen positive integer, and k=0,1,2,3,4,5,6 .. .
Next, the period of calculated sequence r is determined by exploiting spin wave
interference. Finally, the general type computer finds the primes based on the
obtained r. The experiment on the period finding was accomplished on the
6-terminal Y3Fe2(FeO4)3 device. We chose number 15 for a test and found its
primes in a sequence of measurements. The obtained data demonstrate an example
of solving a prime factorization problem using classical wave interference. We
discuss the physical and technological limitations of this approach, which
define the maximum size of N and the computational speed. Though this classical
approach cannot compete with the quantum algorithm in efficiency, magnonic
holographic devices can be potentially utilized as complementary logic units
aimed to speed up prime factorization with classical computers.",1605.02127v1
2017-06-16,Long-distance spin transport in a disordered magnetic insulator,"Spin transport through magnetic insulators via magnons has recently been
explored for a growing variety of magnetic systems with long-range order and
well-understood spin excitation spectra. Here we show dramatic effects of spin
transport through an amorphous magnetic insulator, which is both magnetically
and structurally disordered. We generate and detect spin flow though amorphous
yttrium-iron-garnet ($a$-YIG) thin films in a non-local geometry by use of the
spin Hall and inverse spin Hall effects in platinum strips separated by 10 or
more microns. By comparing non-local spin transport in $a$-YIG on suspended
micromachined thermal isolation platforms to the same experiment performed on a
bulk substrate, we show strong effects of in-plane thermal gradients on spin
transport in the disordered magnetic insulator. The resulting non-local voltage
signals are orders of magnitude larger than those seen in crystalline magnetic
insulators, with easily measurable spin signals seen even at distances in
excess of $100$ microns. In analogy to heat transport, where disordered
materials support a range of vibrational excitations that can allow large
thermal conductivities, we suggest that efficient spin transport in disordered
magnetic systems can occur via a similar spectrum of excitations that relies on
strong local exchange interactions and does not require long-range order. This
work not only opens a new area for fundamental experimental and theoretical
studies of spin transport, but also sets a new direction in materials science
for magnonic and spintronic devices.",1706.05196v1
2017-12-25,Effects of nuclear spins on the transport properties of the edge of two-dimensional topological insulators,"The electrons in the edge channels of two-dimensional topological insulators
can be described as a helical Tomonaga-Luttinger liquid. They couple to nuclear
spins embedded in the host materials through the hyperfine interaction, and are
therefore subject to elastic spin-flip backscattering on the nuclear spins. We
investigate the nuclear-spin-induced edge resistance due to such backscattering
by performing a renormalization-group analysis. Remarkably, the effect of this
backscattering mechanism is stronger in a helical edge than in nonhelical
channels, which are believed to be present in the trivial regime of InAs/GaSb
quantum wells. In a system with sufficiently long edges, the disordered nuclear
spins lead to an edge resistance which grows exponentially upon lowering the
temperature. On the other hand, electrons from the edge states mediate an
anisotropic Ruderman-Kittel-Kasuya-Yosida nuclear spin-spin interaction, which
induces a spiral nuclear spin order below the transition temperature. We
discuss the features of the spiral order, as well as its experimental
signatures. In the ordered phase, we identify two backscattering mechanisms,
due to charge impurities and magnons. The backscattering on charge impurities
is allowed by the internally generated magnetic field, and leads to an
Anderson-type localization of the edge states. The magnon-mediated
backscattering results in a power-law resistance, which is suppressed at zero
temperature. Overall, we find that in a sufficiently long edge the nuclear
spins, whether ordered or not, suppress the edge conductance to zero as the
temperature approaches zero.",1712.09040v2
2018-09-27,Low-temperature Atomistic Spin Relaxation and Non-equilibrium Intensive Properties Using Steepest-Entropy-Ascent Quantum-Inspired Thermodynamics Modeling,"The magnetization of body-centered cubic iron at low temperatures is
calculated with the steepest-entropy-ascent quantum thermodynamics (SEAQT)
framework. This framework assumes that a thermodynamic property in an isolated
system traces the path through state space with the greatest entropy
production. Magnetization is calculated from the expected value of a
thermodynamic ensemble of quantized spin waves based on the Heisenberg spin
model applied to an ensemble of coupled harmonic oscillators. A realistic
energy landscape is obtained from a magnon dispersion relation calculated using
spin-density-functional-theory. The equilibrium magnetization as well as the
evolution of magnetization from a non-equilibrium state to equilibrium are
calculated from the path of steepest entropy ascent determined from the SEAQT
equation of motion in state space. The framework makes it possible to model the
temperature- and time-dependence of magnetization without a detailed
description of magnetic damping. The approach is also used to define intensive
properties (temperature and magnetic field strength) that are fundamentally,
i.e., canonically or grand canonically, valid for any non-equilibrium state.
Given the assumed magnon dispersion relation, the SEAQT framework is used to
calculate the equilibrium magnetization at different temperatures and external
magnetic fields and the results are shown to closely agree with experiment for
temperatures less than 500 K. The time-dependent evolution of magnetization
from different initial states and interactions with a reservoir is also
predicted.",1809.10619v2
2020-12-02,"Synthesis, structure and magnetic properties of honeycomb-layered Li3Co2SbO6 with new data on its sodium precursor, Na3Co2SbO6","Li3Co2SbO6 is prepared by molten salt ion exchange and its structure refined
by the Rietveld method confirming the honeycomb-type Co/Sb ordering of its Na
precursor. Monoclinic rather than trigonal symmetry of Na3Co2SbO6 is directly
demonstrated for the first time by peak splitting in the high-resolution
synchrotron XRD pattern. The long-range antiferromagnetic order is established
at TN about 6.7 K and 9.9 K in Na3Co2SbO6 and Li3Co2SbO6, respectively,
confirmed by both the magnetic susceptibility and specific heat. Spin-wave
analysis of specific heat data indicates the presence of 3D AFM magnons in
Na3Co2SbO6 and 2D AFM magnons in Li3Co2SbO6. The field dependence of the
magnetization almost reaches saturation in moderate magnetic fields up to 9 T
and demonstrates characteristic features of magnetic field induced
spin-reorientation transitions for both A3Co2SbO6 (A = Na, Li). Overall
thermodynamic studies show that the magnetic properties of both compounds are
very sensitive to an external magnetic field, thus predicting a non-trivial
ground state with a rich magnetic phase diagram. The ground state spin
configuration of Li3Co2SbO6 has been determined by low-temperature neutron
powder diffraction. It represents a ferromagnetic arrangement of moments in the
honeycomb layers with antiferromagnetic coupling between adjacent layers.",2012.01093v1
2020-12-02,Engineered magnetization and exchange stiffness in direct-write Co-Fe nanoelements,"Media with engineered magnetization are essential building blocks in
superconductivity, magnetism and magnon spintronics. However, the established
thin-film and lithographic techniques insufficiently suit the realization of
planar components with on-demand-tailored magnetization in the lateral
dimension. Here, we demonstrate the engineering of the magnetic properties of
CoFe-based nanodisks fabricated by the mask-less technique of focused electron
beam induced deposition (FEBID). The material composition in the nanodisks is
tuned \emph{in-situ} via the e-beam waiting time in the FEBID process and their
post-growth irradiation with Ga ions. The magnetization $M_s$ and exchange
stiffness $A$ of the disks are deduced from perpendicular ferromagnetic
resonance measurements. The achieved $M_s$ variation in the broad range from
$720$ emu/cm$^3$ to $1430$ emu/cm$^3$ continuously bridges the gap between the
$M_s$ values of such widely used magnonic materials as permalloy and CoFeB. The
presented approach paves a way towards nanoscale 2D and 3D systems with
controllable and space-varied magnetic properties.",2012.01481v1
2018-03-07,"Voltage driven, local, and efficient excitation of nitrogen-vacancy centers in diamond","Magnetic sensing technology has found widespread application in industries as
diverse as transportation, medicine, and resource exploration. Such use cases
often require highly sensitive instruments to measure the extremely small
magnetic fields involved, relying on difficult to integrate Superconducting
Quantum Interference Device (SQUID) and Spin-Exchange Relaxation Free (SERF)
magnetometers. A potential alternative, nitrogen vacancy (NV) centers in
diamond, has shown great potential as a high sensitivity and high resolution
magnetic sensor capable of operating in an unshielded, room-temperature
environment. Transitioning NV center based sensors into practical devices,
however, is impeded by the need for high power RF excitation to manipulate
them. Here we report an advance that combines two different physical phenomena
to enable a highly efficient excitation of the NV centers: magnetoelastic drive
of ferromagnetic resonance (FMR) and NV-magnon coupling. Our work demonstrates
a new pathway to combine acoustics and magnonics that enables highly energy
efficient and local excitation of NV centers without the need for any external
RF excitation, and thus could lead to completely integrated, on-chip, atomic
sensors.",1803.02863v2
2018-03-27,Generation and annihilation time of magnetic droplet solitons,"Magnetic droplet solitons were first predicted to occur in materials with
uniaxial magnetic anisotropy due to a long-range attractive interaction between
elementary magnetic excitations, magnons. A non-equilibrium magnon population
provided by a spin-polarized current in nanocontacts enables their creation and
there is now clear experimental evidence for their formation, including direct
images obtained with scanning x-ray transmission microscopy. Interest in
magnetic droplets is associated with their unique magnetic dynamics that can
lead to new types of high frequency nanometer scale oscillators of interest for
information processing, including in neuromorphic computing. However, there are
no direct measurements of the time required to nucleate droplet solitons or
their lifetime---experiments to date only probe their steady-state
characteristics, their response to dc spin currents. Here we determine the
timescales for droplet annihilation and generation using current pulses.
Annihilation occurs in a few nanoseconds while generation can take several
nanoseconds to a microsecond depending on the pulse amplitude. Micromagnetic
simulations show that there is an incubation time for droplet generation that
depends sensitively on the initial magnetic state of the nanocontact. An
understanding of these processes is essential to utilizing the unique
characteristics of magnetic droplet solitons oscillators, including their high
frequency, tunable and hysteretic response.",1803.10334v1
2018-06-04,Magnetic Order with Fractionalized Excitations: Applications to $\mathrm{Yb}_2 \mathrm{Ti}_2 \mathrm{O}_7$,"A recent inelastic neutron scattering experiment on $\mathrm{Yb}_2
\mathrm{Ti}_2 \mathrm{O}_7$ uncovers an unusual scattering continuum in the
spin excitation spectrum despite the splayed ferromagnetic order in the ground
state. While there exist well defined spin wave excitations at high magnetic
fields, the one magnon modes and the two magnon continuum start to strongly
overlap upon decreasing the field, and eventually they become the scattering
continuum at zero field. Motivated by these observations, we investigate the
possible emergence of a magnetically ordered ground state with fractionalized
excitations in the spin model with the exchange parameters determined from two
previous experiments. Using the fermionic parton mean field theory, we show
that the magnetically ordered state with fractionalized excitations can arise
as a stable mean field ground state in the presence of sufficiently strong
quantum fluctuations. The spin excitation spectrum in such a ground state is
computed and shown to have the scattering continuum. Upon increasing the
magnetic field, the fractionalized magnetically ordered state is suppressed,
and is eventually replaced by the conventional magnetically ordered phase at
high fields, which is consistent with the experimental data. We discuss further
implications of these results to the experiments and possible improvements on
the theoretical analysis.",1806.01276v1
2018-06-06,Photo-induced Entanglement in a Magnonic Floquet Topological Insulator,"When irradiated via high frequency circularly polarized light, the
stroboscopic dynamics in a Heisenberg spin system on a honeycomb lattice
develops a next nearest neighbor (NNN) Dzyaloshinskii-Moriya (DM) type
term\cite{owerre}, making it a magnonic Floquet topological insulator. We
investigate the entanglement generation and its evolution on such systems -
particularly an irradiated ferromagnetic XXZ spin-$\frac{1}{2}$ model in a
honeycomb lattice as the system parameters are optically tuned. In the high
frequency limit, we compute the lowest quasi-energy state entanglement in terms
of the concurrence between nearest neighbor (NN) and NNN pair of spins and
witness the entanglement transitions occurring there. For the easy axis
scenario, the unirradiated system forms a product state but entanglement grows
between the NNN spin pairs beyond some cut-off DM strength. Contrarily in easy
planar case, NN and NNN spins remain already entangled in the unirradiated
limit. It then goes through an entanglement transition which causes decrease
(increase) of the NN (NNN) concurrences down to zero (up to some higher value)
at some critical finite DM interaction strength. For a high frequency of
irradiation and a suitably chosen anisotropy parameter, we can vary the field
strength to witness sudden death and revival of entanglement in the Floquet
system. Both exact diagonalization and modified Lanczos techniques are used to
obtain the results upto 24 site lattice. We also calculate the thermal
entanglement and obtain estimates for the threshold temperatures below which
non-zero concurrence can be expected in the system.",1806.02125v3
2018-06-21,"Flat Bands, Indirect Gaps, and Unconventional Spin-Wave Behavior Induced by a Periodic Dzyaloshinskii-Moriya Interaction","Periodically patterned metamaterials are known for exhibiting wave properties
similar to the ones observed in electronic band structures in crystal lattices.
In particular, periodic ferromagnetic materials are characterized by the
presence of bands and bandgaps in their spin-wave spectrum at tunable GHz
frequencies. Recently, the fabrication of magnets hosting Dzyaloshinskii-Moriya
interactions has been pursued with high interest since properties such as the
stabilization of chiral spin textures and nonreciprocal spin-wave propagation
emerge from this antisymmetric exchange coupling. In this context, to further
engineer the magnon band structure, we propose the implementation of magnonic
crystals with periodic Dzyaloshinskii-Moriya interactions, which can be
obtained, for instance, via patterning of periodic arrays of heavy-metals wires
on top of an ultrathin magnetic film. We demonstrate through theoretical
calculations and micromagnetic simulations that such systems show an unusual
evolution of the standing spin waves around the gaps in areas of the film that
are in contact with the heavy-metal wires. We also predict the emergence of
indirect gaps and flat bands and, effects that depend on the strength of the
Dzyaloshinskii-Moriya interaction. This study opens new routes towards
engineered metamaterials for spin-wave-based devices.",1806.08333v2
2018-07-02,Emission and Propagation of Multi-Dimensional Spin Waves with nanoscale wavelengths in Anisotropic Spin Textures,"Spin waves offer intriguing novel perspectives for computing and signal
processing, since their damping can be lower than the Ohmic losses in
conventional CMOS circuits. For controlling the spatial extent and propagation
of spin waves on the actual chip, magnetic domain walls show considerable
potential as magnonic waveguides. However, low-loss guidance of spin waves with
nanoscale wavelengths, in particular around angled tracks, remains to be shown.
Here we experimentally demonstrate that such advanced control of propagating
spin waves can be obtained using natural features of magnetic order in an
interlayer exchange-coupled, anisotropic ferromagnetic bilayer. Using Scanning
Transmission X-Ray Microscopy, we image generation of spin waves and their
propagation across distances exceeding multiple times the wavelength, in
extended planar geometries as well as along one-dimensional domain walls, which
can be straight and curved. The observed range of wavelengths is between 1
{\mu}m and 150 nm, at corresponding excitation frequencies from 250 MHz to 3
GHz. Our results show routes towards practical implementation of magnonic
waveguides employing domain walls in future spin wave logic and computational
circuits.",1807.00897v2
2018-07-11,Strong coupling of a single photon to a magnetic vortex,"Strong light-matter coupling is a necessary condition for exchanging
information in quantum information protocols. It is used to couple different
qubits (matter) via a quantum bus (photons) or to communicate different type of
excitations, e.g. transducing between light and phonons or magnons. An
unexplored, so far, interface is the coupling between light and topologically
protected particle like excitations as magnetic domain walls, skyrmions or
vortices. Here, we show theoretically that a single photon living in a
superconducting cavity can be coupled strongly to the gyrotropic mode of a
magnetic vortex in a nanodisc. We combine numerical and analytical calculations
for a superconducting coplanar waveguide resonator and different realizations
of the nanodisc (materials and sizes). We show that, for enhancing the
coupling, constrictions fabricated in the resonator are beneficial, allowing to
reach the strong coupling in CoFe discs of radius $200-400$ nm having resonance
frequencies of few GHz. The strong coupling regime permits to exchange
coherently a single photon and quanta of vortex excitations. Thus, our
calculations show that the device proposed here serves as a transducer between
photons and gyrating vortices, opening the way to complement superconducting
qubits with topologically protected spin-excitations like vortices or
skyrmions. We finish by discussing potential applications in quantum data
processing based on the exploitation of the vortex as a short-wavelength magnon
emitter.",1807.04075v2
2019-03-21,Non-reciprocity of spin waves in magnetic nanotubes with helical equilibrium magnetization,"Spin waves (SWs) in magnetic nanotubes have shown interesting nonreciprocal
properties in their dispersion relation, group velocity, frequency linewidth
and attenuation lengths. The reported chiral effects are similar to those
induced by the Dzyaloshinskii-Moriya interaction, but originating from the
dipole-dipole interaction. Here we show, that the isotropic-exchange
interaction can also induce chiral effects in the SW transport; the so-called
Berry phase of SWs. We demonstrate that with the application of magnetic
fields, the nonreciprocity of the different SW modes can be tuned between the
fully dipolar governed and the fully exchange governed cases, as they are
directly related to the underlaying equilibrium state. In the helical state,
due to the combined action of the two effects every single sign combination of
the azimuthal and axial wave vectors leads to different dispersion, allowing
for a very sophisticated tuning of the SW transport. A disentanglement of the
dipole-dipole and exchange contributions so far was not reported for the SW
transport in nanotubes. Furthermore, we propose a device based on coplanar
waveguides that would allow to selectively measure the exchange or dipole
induced SW nonreciprocities. In the context of magnonic applications, our
results might encourage further developments in the emerging field of 3D
magnonic devices using curved magnetic membranes.",1903.09062v2
2019-03-25,Quantum Many-Body Scars and Space-Time Crystalline Order from Magnon Condensation,"We study the eigenstate properties of a nonintegrable spin chain that was
recently realized experimentally in a Rydberg-atom quantum simulator. In the
experiment, long-lived coherent many-body oscillations were observed only when
the system was initialized in a particular product state. This pronounced
coherence has been attributed to the presence of special ""scarred"" eigenstates
with nearly equally-spaced energies and putative nonergodic properties despite
their finite energy density. In this paper we uncover a surprising connection
between these scarred eigenstates and low-lying quasiparticle excitations of
the spin chain. In particular, we show that these eigenstates can be accurately
captured by a set of variational states containing a macroscopic number of
magnons with momentum $\pi$. This leads to an interpretation of the scarred
eigenstates as finite-energy-density condensates of weakly interacting
$\pi$-magnons. One natural consequence of this interpretation is that the
scarred eigenstates possess long-range order in both space and time, providing
a rare example of the spontaneous breaking of continuous time-translation
symmetry. We verify numerically the presence of this space-time crystalline
order and explain how it is consistent with established no-go theorems
precluding its existence in ground states and at thermal equilibrium.",1903.10517v2
2019-03-31,Magnetic skyrmion spectrum under voltage excitation and its linear modulation,"Magnetic skyrmions are topological quasiparticles with great potential for
applications in future information storage and processing devices because of
their nanoscale size, high stability, and large velocity. Recently, the
high-frequency properties of skyrmions have been explored for magnon
nanodevices. Here we systematically study the dynamics of an isolated skyrmion
under voltage excitation through the voltage-controlled magnetic anisotropy
effect in a circular thin film. A theoretical model considering the
demagnetization energy, which has often been neglected or treated superficially
in previous skyrmion research but is demonstrated to have importance in
determining the skyrmion dynamic state, is developed. With our model, the
periodic oscillation of the skyrmion radius can be solved numerically with
similar precision compared to micromgnetic simulations, and the characteristic
frequency of the skyrmion breathing can be determined analytically with greater
precision than previous studies. Furthermore, we find that the breathing
skyrmion can be analogized as a modulator by investigating its linear
modulation functionality under sinusoidal-form voltage excitation. Different
from the conventional modulation system with complex CMOS circuits, this
skyrmion modulator device integrates with both the modulation and carrier wave
generation functionality, thus showing greater convenience and efficiency in
applications. Our findings can provide useful guidance for both theoretical and
experimental skyrmions research as well as the development of skyrmion-based
magnonic devices with significant potential applicability in future
communication system.",1904.00404v2
2019-04-21,Brillouin-Mandelstam Spectroscopy of Stress-Modulated Spatially Confined Spin Waves in Ni Thin Films on Piezoelectric Heterostructures,"We report results of micro-Brillouin-Mandelstam light scattering spectroscopy
of thermal magnons in the two-phase synthetic multiferroic structure consisting
of a piezoelectric (PMN-PT) substrate and a Ni thin film with the thickness of
64 nm. The experimental data reveal the first two modes of the perpendicular
standing spin waves (PSSW) spatially confined across the Ni thin film. A
theoretical analysis of the frequency dependence of the PSSW peaks on the
external magnetic field reveals the asymmetric boundary condition, i.e.
pinning, for variable magnetization at different surfaces of the Ni thin film.
The strain field induced by applying DC voltage to PMN-PT substrate leads to a
down shift of PSSW mode frequency owing to the magneto-elastic effect in Ni,
and corresponding changes in the spin wave resonance conditions. The observed
non-monotonic dependence of the PSSW frequency on DC voltage is related to an
abrupt change of the pinning parameter at certain values of the voltage. The
obtained results are important for understanding the thermal magnon spectrum in
ferromagnetic films and development of the low-power spin-wave devices.",1904.09553v1
2019-08-29,High-temperature spin dynamics in the Heisenberg chain: Magnon propagation and emerging KPZ-scaling in the zero-magnetization limit,"The large-scale dynamics of quantum integrable systems is often dominated by
ballistic modes due to the existence of stable quasi-particles. We here
consider as an archetypical example for such a system the spin-$\frac{1}{2}$
XXX Heisenberg chain that features magnons and their bound states. An
interesting question, which we here investigate numerically, arises with
respect to the fate of ballistic modes at finite temperatures in the limit of
zero magnetization $m{=}0$. At a finite magnetization density $m$, the spin
autocorrelation function $\Pi(x,t)$ (at high temperatures) typically exhibits a
trimodal behavior with left- and right-moving quasi-particle modes and a broad
center peak with slower dynamics. The broadening of the fastest propagating
modes exhibits a sub-diffusive $t^{1/3}$ scaling at large magnetization
densities, $m {\rightarrow} \frac{1}{2}$, familiar from non-interacting models;
it crosses over into a diffusive scaling $t^{1/2}$ upon decreasing the
magnetization to smaller values. The behavior of the center peak appears to
exhibit a crossover from transient super-diffusion to ballistic relaxation at
long times. In the limit $m{\to}0$, the weight carried by the propagating peaks
tends to zero; the residual dynamics is carried only by the central peak; it is
sub-ballistic and characterized by a dynamical exponent $z$ close to the value
$\frac{3}{2}$ familiar from Kardar-Parisi-Zhang (KPZ) scaling. We confirm,
employing elaborate finite-time extrapolations, that the spatial scaling of the
correlator $\Pi$ is in excellent agreement with KPZ-type behavior and analyze
the corresponding corrections.",1908.11432v2
2019-11-01,Role of spin mixing conductance in determining thermal spin pumping near the ferromagnetic phase transition in EuO_{1-x} and La2NiMnO6,"We present a comprehensive study of the temperature (T) dependence of the
longitudinal spin Seebeck effect (LSSE) in Pt/EuO_{1-x} and Pt/La2NiMnO6 (LNMO)
hybrid structures across their Curie temperatures (Tc). Both systems host
ferromagnetic interaction below Tc, hence present optimal conditions for
testing magnon spin current based theories against ferrimagnetic YIG. Notably,
we observe an anomalous Nernst effect (ANE) generated voltage in bare
EuO_{1-x}, however, we find LSSE predominates the thermal signals in the
bilayers with Pt. The T-dependence of the LSSE in small T-range near Tc could
be fitted to a power law of the form (Tc-T)^P. The derived critical exponent,
P, was verified for different methods of LSSE representation and sample
crystallinity. The results are explained based on the magnon-driven thermal
spin pumping mechanism that relate the T-dependence of LSSE to the spin mixing
conductance (Gmix) at the heavy metal/ferromagnet (HM/FM) interface, which in
turn is known to vary in accordance with the square of the spontaneous
magnetization (Ms). Additionally, the T-dependence of the real part of Gmix
derived from spin Hall magnetoresistance measurements at different temperatures
for the Pt/LNMO structure, further establish the interdependence.",1911.00211v1
2019-11-21,Low damping and microstructural perfection of sub-40nm-thin yttrium iron garnet films grown by liquid phase epitaxy,"The field of magnon spintronics is experiencing an increasing interest in the
development of solutions for spin-wave-based data transport and processing
technologies that are complementary or alternative to modern CMOS
architectures. Nanometer-thin yttrium iron garnet (YIG) films have been the
gold standard for insulator-based spintronics to date, but a potential process
technology that can deliver perfect, homogeneous large-diameter films is still
lacking. We report that liquid phase epitaxy (LPE) enables the deposition of
nanometer-thin YIG films with low ferromagnetic resonance losses and
consistently high magnetic quality down to a thickness of 20 nm. The obtained
epitaxial films are characterized by an ideal stoichiometry and perfect film
lattices, which show neither significant compositional strain nor geometric
mosaicity, but sharp interfaces. Their magneto-static and dynamic behavior is
similar to that of single crystalline bulk YIG. We found, that the Gilbert
damping coefficient alpha is independent of the film thickness and close to 1 x
10-4, and that together with an inhomogeneous peak-to-peak linewidth broadening
of delta H0|| = 0.4 G, these values are among the lowest ever reported for YIG
films with a thickness smaller than 40 nm. These results suggest, that
nanometer-thin LPE films can be used to fabricate nano- and micro-scaled
circuits with the required quality for magnonic devices. The LPE technique is
easily scalable to YIG sample diameters of several inches.",1911.09400v1
2019-12-16,Spin-current manipulation of photoinduced magnetization dynamics in heavy metal / ferromagnet double layer based nanostructures,"Spin currents offer a way to control static and dynamic magnetic properties,
and therefore they are crucial for next-generation MRAM devices or spin-torque
oscillators. Manipulating the dynamics is especially interesting within the
context of photo-magnonics. In typical $3d$ transition metal ferromagnets like
CoFeB, the lifetime of light-induced magnetization dynamics is restricted to
about 1 ns, which e.g. strongly limits the opportunities to exploit the wave
nature in a magnonic crystal filtering device. Here, we investigate the
potential of spin-currents to increase the spin wave lifetime in a functional
bilayer system, consisting of a heavy metal (8 nm of $\beta$-Tantalum
(Platinum)) and 5 nm CoFeB. Due to the spin Hall effect, the heavy metal layer
generates a transverse spin current when a lateral charge current passes
through the strip. Using time-resolved all-optical pump-probe spectroscopy, we
investigate how this spin current affects the magnetization dynamics in the
adjacent CoFeB layer. We observed a linear spin current manipulation of the
effective Gilbert damping parameter for the Kittel mode from which we were able
to determine the system's spin Hall angles. Furthermore, we measured a strong
influence of the spin current on a high-frequency mode. We interpret this mode
an an exchange dominated higher order spin-wave resonance. Thus we infer a
strong dependence of the exchange constant on the spin current.",1912.07728v1
2019-12-17,Antiferromagnetic CuMnAs: Ab initio description of finite temperature magnetism and resistivity,"Noncollinear magnetic moments in antiferromagnets (AFM) lead to a complex
behavior of electrical transport, even to a decreasing resistivity due to an
increasing temperature. Proper treatment of such phenomena is required for
understanding AFM systems at finite temperatures; however first-principles
description of these effects is complicated. With ab initio techniques, we
investigate three models of spin fluctuations (magnons) influencing the
transport in AFM CuMnAs; the models are numerically feasible and easily
implementable to other studies. We numerically justified a fully relativistic
collinear disordered local moment approach and we present its uncompensated
generalization. A saturation or a decrease of resistivity caused by magnons,
phonons, and their combination (above approx. 400 K) was observed and explained
by changes in electronic structure. Within the coherent potential
approximation, our finite-temperature approaches may be applied also to systems
with impurities, which are found to have a large impact not only on residual
resistivity, but also on canting of magnetic moments from the AFM to the
ferromagnetic (FM) state.",1912.08025v4
2016-03-07,Superfluidity and spin superfluidity in spinor Bose gases,"We show that spinor Bose gases subject to a quadratic Zeeman effect exhibit
coexisting superfluidity and spin superfluidity, and study the interplay
between these two distinct types of superfluidity. To illustrate that the basic
principles governing these two types of superfluidity are the same, we describe
the magnetization and particle-density dynamics in a single hydrodynamic
framework. In this description spin and mass supercurrents are driven by their
respective chemical potential gradients. As an application, we propose an
experimentally accessible stationary state, where the two types of
supercurrents counterflow and cancel each other, thus resulting in no mass
transport. Furthermore, we propose a straightforward setup to probe spin
superfluidity by measuring the in-plane magnetization angle of the whole cloud
of atoms. We verify the robustness of these findings by evaluating the
four-magnon collision time, and find that the time scale for coherent
(superfluid) dynamics is separated from that of the slower incoherent dynamics
by one order of magnitude. Comparing the atom and magnon kinetics reveals that
while the former can be hydrodynamic, the latter is typically collisionless
under most experimental conditions. This implies that, while our
zero-temperature hydrodynamic equations are a valid description of spin
transport in Bose gases, a hydrodynamic description that treats both mass and
spin transport at finite temperatures may not be readily feasible.",1603.01996v2
2016-12-20,Current-nonlinear Hall effect and spin-orbit torque magnetization switching in a magnetic topological insulator,"Precise estimation of spin Hall angle as well as successful maximization of
spin-orbit torque (SOT) form a basis of electronic control of magnetic
properties with spintronic functionality. Until now, current-nonlinear Hall
effect, or second harmonic Hall voltage has been utilized as one of the methods
for estimating spin Hall angle, which is attributed to the magnetization
oscillation by SOT. Here, we argue the second harmonic Hall voltage in
magnetic/nonmagnetic topological insulator (TI) heterostructures,
Cr$_x$(Bi$_{1-y}$Sb$_y$)$_{2-x}$Te$_3$/(Bi$_{1-y}$Sb$_y$)$_2$Te$_3$. From the
angular, temperature and magnetic field dependence, it is unambiguously shown
that the large second harmonic Hall voltage in TI heterostructures is governed
not by SOT but mainly by asymmetric magnon scattering mechanism without
magnetization oscillation. Thus, this method does not allow an accurate
estimation of spin Hall angle when magnons largely contribute to electron
scattering. Instead, the SOT contribution in a TI heterostructure is
exemplified by current pulse induced non-volatile magnetization switching,
which is realized with a current density of $\sim 2.5 \times 10^{10}
\mathrm{A/m}^2$, showing its potential as spintronic materials.",1612.06862v2
2017-07-12,X-Ray Microscopy of Spin Wave Focusing using a Fresnel Zone Plate,"Magnonics, i.e. the artificial manipulation of spin waves, is a flourishing
field of research with many potential uses in data processing within reach.
Apart from the technological applications the possibility to directly influence
and observe these types of waves is of great interest for fundamental research.
Guidance and steering of spin waves has been previously shown and lateral spin
wave confinement has been achieved. However, true spin wave focusing with both
lateral confinement and increase in amplitude has not been shown before. Here,
we show for the first time spin wave focusing by realizing a Fresnel zone plate
type lens. Using x-ray microscopy we are able to directly image the propagation
of spin waves into the nanometer sized focal spot. Furthermore, we observe that
the focal spot can be freely moved in a large area by small variations of the
bias field. Thus, this type of lens provides a steerable intense nanometer
sized spin wave source. Potentially, this could be used to selectively
illuminate magnonic devices like nano oscillators with a steerable spin wave
beam.",1707.03664v1
2017-11-14,Magnon Splitting Induced by Charge Transfer in the Three-Orbital Hubbard Model,"Understanding spin excitations and their connection to unconventional
superconductivity have remained a central issue since the discovery of the
cuprates. Direct measurement of the dynamical spin structure factor in the
parent compounds can provide key information on important interactions relevant
in the doped regime, and variations in the magnon dispersion have been linked
closely to differences in crystal structure between families of cuprate
compounds. Here, we elucidate the relationship between spin excitations and
various controlling factors thought to be significant in high-$T_c$ materials
by systematically evaluating the dynamical spin structure factor for the
three-orbital Hubbard model, revealing differences in the spin dispersion along
the Brillouin zone axis and the diagonal. Generally, we find that the absolute
energy scale and momentum dependence of the excitations primarily are sensitive
to the effective charge-transfer energy, while changes in the on-site Coulomb
interactions have little effect on the details of the dispersion. In
particular, our result highlights the splitting between spin excitations along
the axial and diagonal directions in the Brillouin zone. This splitting
decreases with increasing charge-transfer energy and correlates with changes in
the apical oxygen position, and general structural variations, for different
cuprate families.",1711.04931v2
2018-10-02,Strong quantum interactions prevent quasiparticle decay,"Quantum states of matter---such as solids, magnets and topological
phases---typically exhibit collective excitations---phonons, magnons, anyons.
These involve the motion of many particles in the system, yet, remarkably, act
like a single emergent entity---a quasiparticle. Known to be long-lived at the
lowest energies, common wisdom says that quasiparticles become unstable when
they encounter the inevitable continuum of many-particle excited states at high
energies. Whilst correct for weak interactions, we show that this is far from
the whole story: strong interactions generically stabilise quasiparticles by
pushing them out of the continuum. This general mechanism is straightforwardly
illustrated in an exactly solvable model. Using state-of-the-art numerics, we
find it at work also in the spin-$\frac{1}{2}$ triangular lattice Heisenberg
antiferromagnet (TLHAF) near the isotropic point---this is surprising given the
common expectation of magnon decay in this paradigmatic frustrated magnet.
Turning to existing experimental data, we identify the detailed phenomenology
of avoided decay in the TLHAF material Ba$_3$CoSb$_2$O$_9$, and even in liquid
helium---one of the earliest instances of quasiparticle decay. Our work unifies
various phenomena above the universal low-energy regime in a comprehensive
description. This broadens our window of understanding of many-body
excitations, and provides a new perspective for controlling and stabilising
quantum matter in the strongly-interacting regime.",1810.01422v1
2018-10-16,Extreme enhancement of spin relaxation mediated by surface magnon polaritons,"Polaritons in metals, semimetals, semiconductors, and polar insulators, with
their extreme confinement of electromagnetic energy, provide many promising
opportunities for enhancing typically weak light-matter interactions such as
multipolar radiation, multiphoton spontaneous emission, Raman scattering, and
material nonlinearities. These highly confined polaritons are
quasi-electrostatic in nature, with most of their energy residing in the
electric field. As a result, these ""electric"" polaritons are far from optimized
for enhancing emission of a magnetic nature, such as spin relaxation, which is
typically many orders of magnitude slower than corresponding electric decays.
Here, we propose using surface magnon polaritons in negative magnetic
permeability materials such as MnF$_2$ and FeF$_2$ to strongly enhance
spin-relaxation in nearby emitters in the THz spectral range. We find that
these magnetic polaritons in 100 nm thin-films can be confined to lengths over
10,000 times smaller than the wavelength of a photon at the same frequency,
allowing for a surprising twelve orders of magnitude enhancement in magnetic
dipole transitions. This takes THz spin-flip transitions, which normally occur
at timescales on the order of a year, and forces them to occur at sub-ms
timescales. Our results suggest an interesting platform for polaritonics at THz
frequencies, and more broadly, a new way to use polaritons to control
light-matter interactions.",1810.06761v1
2018-10-16,Superfluid spin transport in ferro- and antiferromagnets,"This paper focuses on spin superfluid transport, observation of which was
recently reported in antiferromagnet Cr$_2$O$_3$ [Yuan et al., Sci. Adv. 4,
eaat1098 (2018)]. This paper analyzes the role of dissipation in transformation
of spin current injected with incoherent magnons to a superfluid spin current
near the interface where spin is injected. The Gilbert damping parameter in the
Landau-Lifshitz-Gilbert theory does not describe dissipation properly, and the
dissipation parameters are calculated from the Boltzmann equation for magnons
scattered by defects. The two-fluid theory is developed similar to the
two-fluid theory for superfluids. This theory shows that the influence of
temperature variation in bulk on the superfluid spin transport (bulk Seebeck
effect) is weak at low temperatures. The scenario that the results of Yuan et
al. are connected with the Seebeck effect at the interface between the spin
detector and the sample is also discussed.
  The Landau criterion for an antiferromagnet put in a magnetic field is
derived from the spectrum of collective spin modes. The Landau instability
starts in the gapped mode earlier than in the Goldstone gapless mode, in
contrast to easy-plane ferromagnets where the Goldstone mode becomes unstable.
The structure of the magnetic vortex in the geometry of the experiment is
determined. The vortex core has the skyrmion structure with finite
magnetization component normal to the magnetic field. This magnetization
creates stray magnetic fields around the exit point of the vortex line from the
sample, which can be used for experimental detection of vortices.",1810.07020v4
2018-10-25,Exchange dependent ultrafast magnetization dynamics in Fe$_{1-x}$Ni$_{x}$ alloys,"Element specific ultrafast demagnetization was studied in Fe$_{1-x}$Ni$_{x}$
alloys, covering the concentration range between $0.1<x<0.9$. For all
compositions, we observe a delay in the onset of Ni demagnetization relative to
the Fe demagnetization. We find that the delay is correlated to the Curie
temperature and hence also the exchange interaction. The temporal evolution of
demagnetization is fitted to a magnon diffusion model based on the
presupposition of enhanced ultrafast magnon generation in the Fe sublattice.
The spin wave stiffness extracted from this model correspond well to known
experimental values.",1810.11001v1
2019-05-25,Large-$S$ limit of the large-$N$ theory for the triangular antiferromagnet,"Large-$S$ and large-$N$ theories (spin value $S$ and spinor component number
$N$) are complementary, and sometimes conflicting, approaches to quantum
magnetism. While large-$S$ spin-wave theory captures the correct semiclassical
behavior, large-$N$ theories, on the other hand, emphasize the quantumness of
spin fluctuations. In order to evaluate the possibility of the non-trivial
recovery of the semiclassical magnetic excitations within a large-$N$ approach,
we compute the large-$S$ limit of the dynamic spin structure of the triangular
lattice Heisenberg antiferromagnet within a Schwinger boson spin
representation. We demonstrate that, only after the incorporation of Gaussian
($1/N$) corrections to the saddle-point ($N=\infty$) approximation, we are able
to exactly reproduce the linear spin wave theory results in the large-$S$
limit. The key observation is that the effect of $1/N$ corrections is to cancel
out exactly the main contribution of the saddle-point solution; while the
collective modes (magnons) consist of two spinon bound states arising from the
poles of the RPA propagator. This result implies that it is essential to
consider the interaction of the spinons with the emergent gauge fields and that
the magnon dispersion relation should not be identified with that of the
saddle-point spinons.",1905.10689v3
2020-06-23,Imaging non-collinear antiferromagnetic textures via single spin relaxometry,"Antiferromagnetic materials are promising platforms for next-generation
spintronics owing to their fast dynamics and high robustness against parasitic
magnetic fields. However, nanoscale imaging of the magnetic order in such
materials with zero net magnetization remains a major experimental challenge.
Here we show that non-collinear antiferromagnetic spin textures can be imaged
by probing the magnetic noise they locally produce via thermal populations of
magnons. To this end, we perform nanoscale, all-optical relaxometry with a
scanning quantum sensor based on a single nitrogen-vacancy (NV) defect in
diamond. Magnetic noise is detected through an increase of the spin relaxation
rate of the NV defect, which results in an overall reduction of its
photoluminescence signal under continuous laser illumination. As a
proof-of-concept, the efficiency of the method is demonstrated by imaging
various spin textures in synthetic antiferromagnets, including domain walls,
spin spirals and antiferromagnetic skyrmions. This imaging procedure could be
extended to a large class of intrinsic antiferromagnets and opens up new
opportunities for studying the physics of localized spin wave modes for
magnonics.",2006.13130v1
2020-09-08,Chiral phase transition and thermal Hall effect in an anisotropic spin model on the kagome lattice,"We present a study of the thermal Hall effect in the extended Heisenberg
model with $XXZ$ anisotropy in the kagome lattice. This model has the
particularity that, in the classical case, and for a broad region in parameter
space, an external magnetic field induces a chiral symmetry breaking: the
ground state is a doubly degenerate $q=0$ order with either positive or
negative net chirality. Here, we focus on the effect of this chiral phase
transition in the thermal Hall conductivity using Linear-Spin-Waves theory. We
explore the topology and calculate the Chern numbers of the magnonic bands,
obtaining a variety of topological phase transitions. We also compute the
magnonic effect to the critical temperature associated with the chiral phase
transition ($T_c^{SW}$). Our main result is that, the thermal Hall
conductivity, which is null for $T>T_c^{SW}$, becomes non-zero as a consequence
of the spontaneous chiral symmetry breaking at low temperatures. Therefore, we
present a simple model where it is possible to ""switch"" on/off the thermal
transport properties introducing a magnetic field and heating or cooling the
system.",2009.03941v2
2021-02-17,Axion search with quantum nondemolition detection of magnons,"The axion provides a solution for the strong CP problem and is one of the
leading candidates for dark matter. This paper proposes an axion detection
scheme based on quantum nondemolition detection of magnon, i.e., quanta of
collective spin excitations in solid, which is expected to be excited by the
axion-electron interaction predicted by the Dine-Fischer-Srednicki-Zhitnitsky
(DFSZ) model. The prototype detector is composed of a ferrimagnetic sphere as
an electronic spin target and a superconducting qubit. Both of these are
embedded inside a microwave cavity, which leads to a coherent effective
interaction between the uniform magnetostatic mode in the ferrimagnetic crystal
and the qubit. An upper limit for the coupling constant between an axion and an
electron is obtained as $g_{aee}<2.6\times10^{-6}$ at the 95% confidence level
for the axion mass of $33.117$$\mu$eV $<m_{a}<33.130$$\mu$eV.",2102.08764v4
2021-02-26,Beyond magnons in Nd2ScNbO7: An Ising pyrochlore antiferromagnet with all in all out order and random fields,"We report the low temperature magnetic properties of Nd$^{3+}$ pyrochlore
$\rm Nd_2ScNbO_7$. Susceptibility and magnetization show an easy-axis moment,
and heat capacity reveals a phase transition to long range order at
$T_N=371(2)$ mK with a fully recovered $\Delta S = R \ln(2)$, 53\% of it
recovered for $T>T_N$. Elastic neutron scattering shows a long-range
all-in-all-out magnetic order with low-$Q$ diffuse elastic scattering.
Inelastic neutron scattering shows a low-energy flat-band, indicating a
magnetic Hamiltonian similar to $\rm Nd_2Zr_2O_7$. Nuclear hyperfine
excitations measured by ultra-high-resolution neutron backscattering indicates
a distribution of static electronic moments below $T_N$, which may be due to
B-site disorder influencing Nd crystal electric fields. Analysis of heat
capacity data shows an unexpected $T$-linear or $T^{3/2}$ term which is
inconsistent with conventional magnon quasiparticles, but is consistent with
fractionalized spinons or gapless local spin excitations. We use legacy data to
show similar behavior in $\rm Nd_2Zr_2O_7$. Comparing local static moments also
reveals a suppression of the nuclear Schottky anomaly in temperature,
evidencing a fraction of Nd sites with nearly zero static moment, consistent
with exchange-disorder-induced random singlet formation. Taken together, these
measurements suggest an unusual fluctuating magnetic ground state which mimics
a spin-liquid -- but may not actually be one.",2102.13656v1
2015-12-03,Cavity quantum electrodynamics with ferromagnetic magnons in a small yttrium-iron-garnet sphere,"Hybridizing collective spin excitations and a cavity with high cooperativity
provides a new research subject in the field of cavity quantum electrodynamics
and can also have potential applications to quantum information. Here we report
an experimental study of cavity quantum electrodynamics with ferromagnetic
magnons in a small yttrium-iron-garnet (YIG) sphere at both cryogenic and room
temperatures. We observe for the first time a strong coupling of the same
cavity mode to both a ferromagnetic-resonance (FMR) mode and a magnetostatic
(MS) mode near FMR in the quantum limit. This is achieved at a temperature ~ 22
mK, where the average microwave photon number in the cavity is less than one.
At room temperature, we also observe strong coupling of the cavity mode to the
FMR mode in the same YIG sphere and find a slight increase of the damping rate
of the FMR mode. These observations reveal the extraordinary robustness of the
FMR mode against temperature. However, the MS mode becomes unobservable at room
temperature in the measured transmission spectrum of the microwave cavity
containing the YIG sphere. Our numerical simulations show that this is due to a
drastic increase of the damping rate of the MS mode.",1512.00983v1
2016-09-19,Geometrical complexity of the antidots unit cell effect on the spin wave excitations spectra,"We have investigated theoretically (with micromagnetic simulations and plane
wave method) and experimentally (with ferromagnetic resonance and Brillouin
light scattering) three types of antidot lattices (ADLs) based on permalloy
thin films with increased complexity of the unit cell: simple square,
bi-component square and wave-like ADL. We have found that placing a small
additional antidot in the center of the unit cell of the square ADL modify
significantly the spin wave spectrum and its dependence on the orientation of
the in-plane magnetic field. We also check the further changes of spin wave
spectrum resulting from the introduction of air-gaps connecting small and large
antidots. In particular, the presence of small antidots change the dependence
of the frequency of the fundamental mode on the angle of the in-plane applied
magnetic field. The air-gaps strongly discriminates the propagation of spin
waves in two principal direction of ADL lattice, orthogonal to each other. In
spite of these spectral changes, the spatial distribution of the spin wave
amplitude generally preserves some similarities for all three structures. We
also highlighted out the role of defects in the ADL in the observed spectra.
The obtained results can be interesting for the magnonics applications of the
magnonic crystals.",1609.05663v1
2016-09-27,Identical Spin Rotation Effect and Electron Spin Waves in Quantum Gas of Atomic Hydrogen,"We present an experimental study of electron spin waves in atomic hydrogen
gas compressed to high densities of $\sim 5 \times 10^{18}$ cm$^{-3}$ at
temperatures ranging from 0.26 to 0.6 K in strong magnetic field of 4.6 T.
Hydrogen gas is in a quantum regime when the thermal de Broglie wavelength is
much larger than the s-wave scattering length. In this regime the identical
particle effects play major role in atomic collisions and lead to the Identical
Spin Rotation effect (ISR). We observed a variety of spin wave modes caused by
this effect with strong dependence on the magnetic potential caused by
variations of the polarizing magnetic field. We demonstrate confinement of the
ISR modes in the magnetic potential and manipulate their properties by changing
the spatial profile of magnetic field. We have found that at a high enough
density of H gas the magnons accumulate in their ground state in the magnetic
trap and exhibit long coherence, which has a profound effect on the electron
spin resonance spectra. Such macroscopic accumulation of the ground state
occurs at a certain critical density of hydrogen gas, where the chemical
potential of the magnons becomes equal to the energy of their ground state in
the trapping potential.",1609.08432v2
2016-11-17,Low-temperature heat transport of spin-gapped quantum magnets,"This article reviews low-temperature heat transport studies of spin-gapped
quantum magnets in the last few decades. Quantum magnets with small spins and
low dimensionality exhibit a variety of novel phenomena. Among them, some
systems are char-acteristic of having quantum-mechanism spin gap in their
magnetic excitation spectra, including spin-Peierls systems, S = 1 Haldane
chains, S = 1/2 spin ladders, and spin dimmers. In some particular spin-gapped
systems, the XY-type antiferromagnetic state induced by magnetic field that
closes the spin gap can be described as a magnon Bose-Einstein condensation
(BEC). Heat transport is effective in probing the magnetic excitations and
magnetic phase transitions, and has been extensively studied for the
spin-gapped systems. A large and ballistic spin thermal conductivity was
observed in the two-leg Heisenberg S = 1/2 ladder compounds. The characteristic
of magnetic thermal transport of the Haldane chain systems is quite
controversial on both the theoretical and experimental results. For the
spin-Peierls system, the spin excitations can also act as heat carriers. In
spin-dimer compounds, the magnetic excitations mainly play a role of scattering
phonons. The magnetic excitations in the magnon BEC systems displayed dual
roles, carrying heat or scattering phonons, in different materials.",1611.05578v1
2019-07-02,Three-point functions at strong coupling in the BMN limit,"We consider structure constants of single-trace operators at strong coupling
in planar $\mathcal{N}=4$ SYM theory using the hexagon formalism. We
concentrate on heavy-heavy-light correlators where the heavy operators are BMN
operators, with large R-charges and finite anomalous dimensions, and the light
one is a finite-charge chiral primary operator. They describe the couplings
between two highly boosted strings and a supergravity mode in the bulk dual. In
the hexagon framework, two sums over virtual magnons are needed to bind the
hexagons together around the light operator. We evaluate these sums explicitly
at strong coupling, for a certain choice of BMN operators, and show that they
factorise into a ratio of Gamma functions and a simple stringy prefactor. The
former originates from giant mirror magnons scanning the AdS geometry while the
latter stems from small fluctuations around the BMN vacuum. The resulting
structure constants have poles at positions where an enhanced mixing with
double-trace operators is expected and zeros whenever the process is forbidden
by supersymmetry. We also discuss the transition to the classical regime, when
the length of the light operator scales like the string tension, where we
observe similitudes with the Neumann coefficients of the pp-wave String Field
Theory vertex.",1907.01534v1
2019-10-02,Distinct magneto-Raman signatures of spin-flip phase transitions in CrI3,"The discovery of 2-dimensional (2D) materials, such as CrI3, that retain
magnetic ordering at monolayer thickness has resulted in a surge of research in
2D magnetism from both pure and applied perspectives. Here, we report a
magneto-Raman spectroscopy study on multilayered CrI3, focusing on two new
features in the spectra which appear at temperatures below the magnetic
ordering temperature and were previously assigned to high frequency magnons. We
observe a striking evolution of the Raman spectra with increasing magnetic
field in which clear, sudden changes in intensities of the modes are attributed
to the interlayer ordering changing from antiferromagnetic to ferromagnetic at
a critical magnetic field. Our work highlights the sensitivity of the Raman
modes to weak interlayer spin ordering in CrI3. In addition, we theoretically
examine potential origins for the new modes, which we deduce are unlikely
single magnons.",1910.01237v3
2019-10-08,Field evolution of low-energy excitations in the hyperhoneycomb magnet $β$-Li$_2$IrO$_3$,"$^7$Li nuclear magnetic resonance (NMR) and terahertz (THz) spectroscopies
are used to probe magnetic excitations and their field dependence in the
hyperhoneycomb Kitaev magnet $\beta$-Li$_2$IrO$_3$. Spin-lattice relaxation
rate ($1/T_1$) measured down to 100\,mK indicates gapless nature of the
excitations at low fields (below $H_c\simeq 2.8$\,T), in contrast to the gapped
magnon excitations found in the honeycomb Kitaev magnet $\alpha$-RuCl$_3$ at
zero applied magnetic field. At higher temperatures in $\beta$-Li$_2$IrO$_3$,
$1/T_1$ passes through a broad maximum without any clear anomaly at the N\'eel
temperature $T_N\simeq 38$\,K, suggesting the abundance of low-energy
excitations that are indeed observed as two peaks in the THz spectra, both
correspond to zone-center magnon excitations. At higher fields (above $H_c$),
an excitation gap opens, and a re-distribution of the THz spectral weight is
observed without any indication of an excitation continuum, in contrast to
$\alpha$-RuCl$_3$ where an excitation continuum was reported.",1910.03251v2
2019-10-30,Magnon spectrum of the Weyl semimetal half-Heusler compound GdPtBi,"The compound GdPtBi is known as a material where the non-trivial topology of
electronic bands interplays with an antiferromagnetic order, which leads to the
emergence of many interesting magnetotransport phenomena. Although the magnetic
structure of the compound has previously been reliably determined, the magnetic
interactions responsible for this type of order remained controversial. In the
present study, we employed time-of-flight inelastic neutron scattering to map
out the low-temperature spectrum of spin excitations in single-crystalline
GdPtBi. The observed spectra reveal two spectrally sharp dispersive spin-wave
modes, which reflects the multi-domain state of the $\mathbf{k} =
(\frac{1}{2}\frac{1}{2}\frac{1}{2})$ face-centred cubic antiferromagnet in the
absence of a symmetry-breaking magnetic field. The magnon dispersion reaches an
energy of $\sim 1.1$~meV and features a gap of $\sim 0.15$~meV. Using linear
spin-wave theory, we determined the main magnetic microscopic parameters of the
compound that provide good agreement between the simulated spectra and the
experimental data. We show that GdPtBi is well within the
($\frac{1}{2}\frac{1}{2}\frac{1}{2}$) phase and is dominated by second-neighbor
interactions, thus featuring low frustration.",1910.13845v2
2020-02-17,Laser-Induced Transient Magnons in Sr3Ir2O7 Throughout the Brillouin Zone,"Although ultrafast manipulation of magnetism holds great promise for new
physical phenomena and applications, targeting specific states is held back by
our limited understanding of how magnetic correlations evolve on ultrafast
timescales. Using ultrafast resonant inelastic x-ray scattering we demonstrate
that femtosecond laser pulses can excite transient magnons at large wavevectors
in gapped antiferromagnets, and that they persist for several picoseconds which
is opposite to what is observed in nearly gapless magnets. Our work suggests
that materials with isotropic magnetic interactions are preferred to achieve
rapid manipulation of magnetism.",2002.07301v3
2020-03-02,Non-Hermitian topology of one-dimensional spin-torque oscillator arrays,"Magnetic systems have been extensively studied both from a fundamental
physics perspective and as building blocks for a variety of applications. Their
topological properties, in particular those of excitations, remain relatively
unexplored due to their inherently dissipative nature. The recent introduction
of non-Hermitian topological classifications opens up new opportunities for
engineering topological phases in dissipative systems. Here, we propose a
magnonic realization of a non-Hermitian topological system. A crucial
ingredient of our proposal is the injection of spin current into the magnetic
system, which alters and can even change the sign of terms describing
dissipation. We show that the magnetic dynamics of an array of spin-torque
oscillators can be mapped onto a non-Hermitian Su-Schrieffer-Heeger model
exhibiting topologically protected edge states. Using exact diagonalization of
the linearized dynamics and numerical solutions of the non-linear equations of
motion, we find that a topological magnonic phase can be accessed by tuning the
spin current injected into the array. In the topologically nontrivial regime, a
single spin-torque oscillator on the edge of the array is driven into
auto-oscillation and emits a microwave signal, while the bulk oscillators
remain inactive. Our findings have practical utility for memory devices and
spintronics neural networks relying on spin-torque oscillators as constituent
units.",2003.01152v1
2020-05-20,Controlling the propagation of dipole-exchange spin waves using local inhomogeneity of the anisotropy,"Spin waves are promising candidates to carry, transport, and process
information. Controlling the propagation characteristics of spin waves in
magnetic materials is an essential ingredient for designing spin-wave based
computing architectures. Here, we study the influence of surface
inhomogeneities on the spin-wave signals transmitted through thin films. We use
micromagnetic simulations to study the spin-wave dynamics in an in-plane
magnetized yttrium iron garnet thin film with a thickness in the nanometre
range in the presence of surface defects in the form of locally introduced
uniaxial anisotropies. These defects are used to demonstrate that the Backward
Volume Magnetostatic Spin Waves (BVMSW) are more responsive to backscattering
in comparison to Magnetostatic Surface Spin Waves (MSSWs). For this particular
defect type, the reason for this behavior can be quantitatively related to the
difference in the magnon band structures for the two types of spin waves. To
demonstrate this, we develop a quasi-analytical theory for the scattering
process. It shows an excellent agreement with the micromagnetic simulations,
sheds light on the backscattering processes and provides a new way to analyze
the spin-wave transmission rates in the presence of surface inhomogeneities in
sufficiently thin films, for which the role of exchange energy in the spin-wave
dynamics is significant. Our study paves the way to designing magnonic logic
devices for data processing which rely on a designed control of the spin-wave
transmission.",2005.09965v2
2020-07-17,Coherent coupling between multiple ferrimagnetic spheres and a microwave cavity in the quantum-limit,"The spin resonance of electrons can be coupled to a microwave cavity mode to
obtain a photon-magnon hybrid system. These quantum systems are widely studied
for both fundamental physics and technological quantum applications. In this
article, the behavior of a large number of ferrimagnetic spheres coupled to a
single cavity is put under test. We use second-quantization modeling of
harmonic oscillators to theoretically describe our experimental setup and
understand the influence of several parameters. The magnon-polariton dispersion
relation is used to characterize the system, with a particular focus on the
vacuum Rabi mode splitting due to multiple spheres. We combine the results
obtained with simple hybrid systems to analyze the behavior of a more complex
one, and show that it can be devised in such a way to minimize the degrees of
freedom needed to completely describe it. By studying single-sphere coupling
two possible size-effects related to the sample diameter have been identified,
while multiple-spheres configurations reveal how to upscale the system. This
characterization is useful for the implementation of an
axion-to-electromagnetic field transducer in a ferromagnetic haloscope for dark
matter searches. Our dedicated setup, consisting in ten 2 mm-diameter YIG
spheres coupled to a copper microwave cavity, is used for this aim and studied
at mK temperatures. Moreover, we show that novel applications of
optimally-controlled hybrid systems can be foreseen for setups embedding a
large number of samples.",2007.08908v3
2020-07-27,Magnonic band structure in vertical meander-shaped CoFeB thin films,"The dispersion of spin waves in vertical meander-shaped CoFeB thin films
consisting of segments located at 90{\deg} angles with respect to each other is
investigated by Brillouin light scattering spectroscopy. We reveal the periodic
character of several dispersive branches as well as alternating frequency
ranges where spin waves are allowed or forbidden to propagate. Noteworthy is
the presence of the frequency band gaps between each couple of successive modes
only for wave numbers k=n$\pi$/a, where n is an even integer number and a is
the size of the meander unit cell, whereas the spectra show propagating modes
in the orthogonal film segments for the other wavenumbers. The micromagnetic
simulations and analytical calculations allow us to understand and explain the
results in terms of the mode spatial localization and symmetry. The obtained
results demonstrate the wave propagation in three dimensions opening the path
for multi-level magnonic architectures for signal processing.",2007.13707v2
2020-08-24,Regge amplitudes in Generalized Fishnet and Chiral Fishnet Theories,"We extend the analysis of \cite{Chowdhury:2019hns} to study the Regge
trajectories of the Mellin amplitudes of the $0-$ and $1-$ magnon correlators
of the generalized Fishnet theory in $d$ dimensions and one type of correlators
of chiral fishnet theory in $4$ dimensions. We develop a systematic procedure
to perturbatively study the Regge trajectories and subsequently perform the
spectral integral. Our perturbative method is very generic and in principle can
be applied to correlators whose perturbative Regge trajectories obey some
structural conditions which we list down. Our $d$ dimensional results reduce to
previously known results in $d=4$ for 0-magnon and 1- magnon. As a non trivial
check, we show that the results for 1-magnon correlator in $d=8$, when
evaluated using the exact techniques in \cite{Chowdhury:2019hns,
Korchemsky:2018hnb} are in perfect agreement with our $d$ dimensional
perturbative results. We also perturbatively compute the Regge trajectories and
Regge-Mellin amplitudes of the chiral fishnet correlator $\langle{\rm
Tr}[\phi_1(x_1)\phi_1(x_2)]{\rm
Tr}[\phi_1^\dagger(x_3)\phi_1^\dagger(x_4)]\rangle$ using the techniques
developed in this paper. Since this correlator has two couplings $\kappa$ and
$\omega$, we have obtained closed-form results in the limit $\kappa \to 0,
\omega \to 0$ with $\kappa/\omega$ held constant. We verify this computation
with an independent method of computing the same and obtain perfect agreement.",2008.10201v3
2020-08-26,Theory of three-magnon interaction in a vortex-state magnetic nanodot,"We use vector Hamiltonian formalism (VHF) to study theoretically three-magnon
parametric interaction (or three-wave splitting) in a magnetic disk existing in
a magnetic vortex ground state. The three-wave splitting in a disk is found to
obey two selection rules: (i) conservation of the total azimuthal number of the
resultant spin-wave modes, and (ii) inequality for the radial numbers of
interacting modes, if the mode directly excited by the driving field is
radially symmetric (i.e. if the azimuthal number of the directly excited mode
is $m = 0$). The selection rule (ii), however, is relaxed in the ""small""
magnetic disks, due to the influence of the vortex core. We also found, that
the efficiency of the three-wave interaction of the directly excited mode
strongly depends on the azimuthal and radial mode numbers of the resultant
modes, that becomes determinative in the case when several splitting channels
(several pairs of resultant modes) simultaneously approximately satisfy the
resonance condition for the splitting. The good agreement of the VHF analytic
calculations with the experiment and micromagnetic simulations proves the
capability of the VHF formalism to predict the actual splitting channels and
the magnitudes of the driving field thresholds for the three-wave splitting.",2008.11812v2
2020-08-28,Sign structure of thermal Hall conductivity for in-plane-field polarized Kitaev magnets,"The appearance of half-quantized thermal Hall conductivity in
$\alpha$-RuCl$_3$ in the presence of in-plane magnetic fields has been taken as
a strong evidence for Kitaev spin liquid. Apart from the quantization, the
observed sign structure of the thermal Hall conductivity is also consistent
with predictions from the exact solution of the Kitaev model. Namely, the
thermal Hall conductivity changes sign when the field direction is reversed
with respect to the heat current, which is perpendicular to one of the three
nearest neighbor bonds on the honeycomb lattice. On the other hand, it is
almost zero when the field is applied along the bond direction. Here, we show
that such a peculiar sign structure of the thermal Hall conductivity is a
generic property of the polarized state in the presence of in-plane
magnetic-fields. In this case, thermal Hall effect arises from topological
magnons with finite Chern numbers and the sign structure follows from the
symmetries of the momentum space Berry curvature. Using a realistic spin model
with bond-dependent interactions, we show that the thermal Hall conductivity
can have a magnitude comparable to that observed in the experiments. Hence the
sign structure alone cannot make a strong case for Kitaev spin liquid. The
quantization at very low temperatures, however, will be a decisive test as the
magnon contribution vanishes in the zero temperature limit.",2008.12788v1
2020-10-10,Controlling the anisotropy of a van der Waals antiferromagnet with light,"Magnetic van der Waals materials provide an ideal playground for exploring
the fundamentals of low-dimensional magnetism and open new opportunities for
ultrathin spin processing devices. The Mermin-Wagner theorem dictates that as
in reduced dimensions isotropic spin interactions cannot retain long-range
correlations; the order is stabilized by magnetic anisotropy. Here, using
ultrashort pulses of light, we demonstrate all-optical control of magnetic
anisotropy in the two-dimensional van der Waals antiferromagnet NiPS$_3$.
Tuning the photon energy in resonance with an orbital transition between
crystal-field split levels of the nickel ions, we demonstrate the selective
activation of a sub-THz two-dimensional magnon mode. The pump polarization
control of the magnon amplitude confirms that the activation is governed by the
instantaneous magnetic anisotropy axis emergent in response to photoexcitation
of orbital states with a lowered symmetry. Our results establish pumping of
orbital resonances as a universal route for manipulating magnetic order in
low-dimensional (anti)ferromagnets.",2010.05062v1
2020-11-11,Colossal band renormalization and stoner ferromagnetism induced by electron-antiferromagnetic-magnon coupling,"The interactions between electrons and antiferromagnetic magnons (AFMMs) are
important for a large class of correlated materials. For example, they are the
most plausible pairing glues in high-temperature superconductors, such as
cuprates and iron pnictides. However, unlike electron-phonon interactions
(EPIs), clear-cut observations regarding how electron-AFMM interactions (EAIs)
affect the band structure are still lacking. Consequently, critical information
on the EAIs, such as its strength and doping dependence, remains elusive. Here
we directly observe that EAIs induces a kink structure in the band dispersion
in Ba$_{1-x}$K$_x$Mn$_2$As$_2$, and subsequently unveil several key
characteristics of EAIs. We found that the coupling constant of EAIs can be as
large as 6, and it shows huge doping dependence and temperature dependence, all
in stark contrast to the behaviors of EPI and beyond our current understanding
of EAIs. Such a colossal renormalization of electronic bands by EAIs drives the
system to the Stoner criteria, giving the intriguing ferromagnetic state in
Ba$_{1-x}$K$_x$Mn$_2$As$_2$. Our results expand the current knowledge of EAIs,
which may facilitate the further understanding of many correlated materials
where EAIs play a critical role, such as high-temperature superconductors.",2011.05683v1
2020-11-11,Single-quadrature quantum magnetometry in cavity electromagnonics,"A scheme of an ultra-sensitive magnetometer in the cavity quantum
electromagnonics where the intracavity microwave mode coupled to a magnonic
mode via magnetic dipole interaction is proposed. It is shown that by driving
both magnonic and microwave modes with external classical fields and
controlling the system parameters, one can reduce the added noise of magnetic
field measurement below the standard quantum limit (SQL). Surprisingly, we show
that beyond the rotating wave approximation (RWA), not only the added noise can
be suppressed, but also the output cavity response to the input signal can be
substantially amplified in order to achieve a precise magnetic-field
measurement. The estimated theoretical sensitivity of the proposed magnetic
amplifier-sensor is approximately in the order of $10^{-18}T/\sqrt{Hz}$ which
is competitive compared to the current state-of-the-art magnetometers like
superconducting quantum interference devices (SQUIDs) and atomic magnetometers.
The advantage of the proposed sensor in comparison with the other magnetometers
is its high sensitivity at room temperature and sensing in a wide range of
frequency up to MHz as well as its capability to signal-response amplification.",2011.06081v5
2020-11-27,Perfect Transfer of enhanced entanglement and asymmetric steering in a cavity magnomechanical system,"We propose a hybrid cavity magnomechanical system to realize and transfer the
bipartite entanglements and Einstein-Podolsky-Rosen (EPR) steerings between
magnons, photons, and phonons in the regime of stability of the system. As a
parity-time-symmetric-like structure exhibiting the natural magnetostrictive
magnon-phonon interaction, our passive-active cavity system can be explored to
enhance the robust distant quantum entanglement and generate the relatively
obvious asymmetric (even directional) EPR steering that is useful for the task
with the highly asymmetric trusts of the bidirectional local measurements
between two entangled states. It is of great interest that, based on such a
tunable magnomechanical system, the perfect transfer between near and distant
entanglements and steerings of different mode pairs is realized by adjusting
the coupling parameters; in particular, we propose a perfect transfer scheme of
steerings. These transferring processes suggest indeed an alternative method
for quantum information storage and manipulation. In addition, the
entanglements and steerings can also be exchanged between different mode pairs
by adjusting the detunings between different modes. This work may provide a
potential platform for distant and asymmetric quantum modulation.",2011.13515v2
2021-01-14,Laser-induced THz magnetism of antiferromagnetic CoF$_2$,"Excitation, detection and control of coherent THz magnetic excitation in
antiferromagnets are challenging problems that can be addressed using ever
shorter laser pulses. We study experimentally excitation of magnetic dynamics
at THz frequencies in an antiferromagnetic insulator CoF$_2$ by sub-10 fs laser
pulses. Time-resolved pump-probe polarimetric measurements at different
temperatures and probe polarizations reveal laser-induced transient circular
birefringence oscillating at the frequency of 7.45 THz and present below the
N\'eel temperature. The THz oscillations of circular birefringence are ascribed
to oscillations of the magnetic moments of Co$^{2+}$ ions induced by the
laser-driven coherent E$_g$ phonon mode via the THz analogue of the transverse
piezomagnetic effect. It is also shown that the same pulse launches coherent
oscillations of the magnetic linear birefringence at the frequency of 3.4 THz
corresponding to the two-magnon mode. Analysis of the probe polarization
dependence of the transient magnetic linear birefringence at the frequency of
the two-magnon mode enables identifying its symmetry.",2101.05622v5
2021-01-28,Universal field dependence of magnetic resonance near zero frequency,"Magnetic resonance is a widely-established phenomenon that probes magnetic
properties such as magnetic damping and anisotropy. Even though the typical
resonance frequency of a magnet ranges from gigahertz to terahertz, experiments
also report the resonance near zero frequency in a large class of magnets. Here
we revisit this phenomenon by analyzing the symmetry of the system and find
that the resonance frequency ($\omega$) follows a universal power law $\omega
\varpropto |H-H_c|^p$, where $H_c$ is the critical field at which the resonance
frequency is zero. When the magnet preserves the rotational symmetry around the
external field ($H$), $p = 1$. Otherwise, $p=1/2$. The magnon excitations are
gapped above $H_c$, gapless at $H_c$ and gapped again below $H_c$. The zero
frequency is often accompanied by a reorientation transition in the
magnetization. For the case that $p=1/2$, this transition is described by a
Landau theory for second-order phase transitions. We further show that the spin
current driven by thermal gradient and spin-orbit effects can be significantly
enhanced when the resonance frequency is close to zero, which can be measured
electrically by converting the spin current into electric signals. This may
provide an experimentally accessible way to characterize the critical field.
Our findings provide a unified understanding of the magnetization dynamics near
the critical field, and may, furthermore, inspire the study of magnon transport
near magnetic transitions.",2101.12292v1
2021-02-27,Curvature-driven homogeneous Dzyaloshinskii-Moriya interaction and emergent weak ferromagnetism in anisotropic antiferromagnetic spin chains,"Chiral antiferromagnets are currently considered for broad range of
applications in spintronics, spin-orbitronics and magnonics. In contrast to the
established approach relying on materials screening, the anisotropic and chiral
responses of low-dimensional antifferromagnets can be tailored relying on the
geometrical curvature. Here, we consider an achiral, anisotropic
antiferromagnetic spin chain and demonstrate that these systems possess
geometry-driven effects stemming not only from the exchange interaction but
also from the anisotropy. Peculiarly, the anisotropy-driven effects are
complementary to the curvature effects stemming from the exchange interaction
and rather strong as they are linear in curvature. These effects are
responsible for the tilt of the equilibrium direction of vector order
parameters and the appearance of the homogeneous Dzyaloshinskii-Moriya
interaction. The latter is a source of the geometry-driven weak ferromagnetism
emerging in curvilinear antiferromagnetic spin chains. Our findings provide a
deeper fundamental insight into the physics of curvilinear antiferromagnets
beyond the $\sigma$-model and offer an additional degree of freedom in the
design of spintronic and magnonic devices.",2103.00169v1
2021-03-08,Quantifying the charge carrier interaction in metallic twisted graphene superlattices,"The mechanism of charge carrier interaction in twisted bilayer graphene (TBG)
remains an unresolved problem, where some researchers proposed the dominance of
the electron-phonon interaction, while the others showed evidence for
electron-electron or electron-magnon interactions. Here we propose to resolve
this problem by generalizing the Bloch-Gr\""uneisen equation and using it for
the analysis of the temperature dependent resistivity in TBG. It is a
well-established theoretical result that the Bloch-Gr\""uneisen equation
power-law exponent, n, exhibits exact integer values for certain mechanisms.
For instance, n=5 implies the electron-phonon interaction, n=3 is associated
with electron-magnon interaction and n=2 applies to the electron-electron
interaction. Here we interpret the linear temperature-dependent resistance,
widely observed in TBG, as n$\rightarrow$1, which implies the quasielastic
charge interaction with acoustic phonons. Thus, we fitted TBG resistance curves
to the Bloch-Gr\""uneisen equation, where we propose that n is a free-fitting
parameter. We found that TBGs have a smoothly varied n-value (ranging from 1.4
to 4.4) depending on the Moir\'e superlattice constant, $\lambda$, or the
charge carrier concentration. This implies that different mechanisms smoothly
transition from one to another. The proposed generalized Bloch-Gr\""uneisen
equation is applicable to a wide range of problems, including the Earth
geology.",2103.04810v1
2021-03-17,Emergence of a noncollinear magnetic state in twisted bilayer CrI3,"The emergence of two-dimensional (2D) magnetic crystals and moir\'e
engineering has opened the door for devising new magnetic ground states via
competing interactions in moir\'e superlattices. Although a suite of
interesting phenomena, including multi-flavor magnetic states, noncollinear
magnetic states, moir\'e magnon bands and magnon networks, has been predicted,
nontrivial magnetic ground states in twisted bilayer magnetic crystals have yet
to be realized. Here, by utilizing the stacking-dependent interlayer exchange
interactions in CrI3, we demonstrate in small-twist-angle bilayer CrI3 a
noncollinear magnetic ground state. It consists of both antiferromagnetic (AF)
and ferromagnetic (FM) domains and is a result of the competing interlayer AF
coupling in the monoclinic stacking regions of the moir\'e superlattice and the
energy cost for forming AF-FM domain walls. Above the critical twist angle of ~
3{\deg}, the noncollinear state transitions abruptly to a collinear FM ground
state. We further show that the noncollinear magnetic state can be controlled
by gating through the doping-dependent interlayer AF interaction. Our results
demonstrate the possibility of engineering moir\'e magnetism in twisted bilayer
magnetic crystals, as well as gate-voltage-controllable high-density magnetic
memory storage.",2103.09850v1
2021-04-16,Strong magnon-photon coupling with chip-integrated YIG in the zero-temperature limit,"The cross-integration of spin-wave and superconducting technologies is a
promising method for creating novel hybrid devices for future information
processing technologies to store, manipulate, or convert data in both classical
and quantum regimes. Hybrid magnon-polariton systems have been widely studied
using bulk Yttrium Iron Garnet (Y$_{3}$Fe$_{5}$O$_{12}$, YIG) and
three-dimensional microwave photon cavities. However, limitations in YIG growth
have thus far prevented its incorporation into CMOS compatible technology such
as high quality factor superconducting quantum technology. To overcome this
impediment, we have used Plasma Focused Ion Beam (PFIB) technology -- taking
advantage of precision placement down to the micron-scale -- to integrate YIG
with superconducting microwave devices. Ferromagnetic resonance has been
measured at millikelvin temperatures on PFIB-processed YIG samples using planar
microwave circuits. Furthermore, we demonstrate strong coupling between
superconducting resonator and YIG ferromagnetic resonance modes by maintaining
reasonably low loss while reducing the system down to the micron scale. This
achievement of strong coupling on-chip is a crucial step toward fabrication of
functional hybrid quantum devices that advantage from spin-wave and
superconducting components.",2104.08068v3
2021-04-29,The dominance of non-electron-phonon charge carrier interaction in highly-compressed superhydrides,"The primary mechanism governing the emergence of near-room-temperature
superconductivity in superhydrides is widely accepted to be the electron-phonon
interaction. If so, the temperature dependent resistance, R(T), in these
materials should obey the Bloch-Gr\""uneisen equation, where the power-law
exponent, p, should be equal to the exact integer value of p=5. On the other
hand, there is a well-established theoretical result that pure electron-magnon
interaction should be manifested by p=3, and p=2 is the value for pure
electron-electron interaction. Here we aimed to reveal the type of charge
carrier interaction in the layered transition metal dichalcogenides PdTe2,
high-entropy alloy (ScZrNb)0.65[RhPd]0.35, and highly-compressed elemental
boron and superhydrides H3S, LaHx, PrH9 and BaH12 by fitting the temperature
dependent resistance of these materials to the Bloch-Gr\""uneisen equation where
the power-law exponent, p, is a free-fitting parameter. In the result, we
showed that the high-entropy alloy (ScZrNb)0.65[RhPd]0.35 exhibited pure
electron-phonon mediated superconductivity with p = 4.9. Unexpectedly we
revealed that all studied superhydrides exhibit 1.8 < p < 3.2. This implies
that it is unlikely that the electron-phonon interaction is the primary
mechanism for the Cooper pairs formation in highly-compressed superhydrides and
alternative pairing mechanisms, for instance, the electron-magnon, the
electron-polaron, the electron-electron or other, should be considered as the
origin for the emergence of near-room-temperature superconductivity in these
compounds.",2104.14145v3
2021-05-05,Ni$_{80}$Fe$_{20}$ Nanotubes with Optimized Spintronic Functionalities Prepared by Atomic Layer Deposition,"Permalloy Ni$_{80}$Fe$_{20}$ is one of the key magnetic materials in the
field of magnonics. Its potential would be further unveiled if it could be
deposited in three dimensional (3D) architectures of sizes down to the
nanometer. Atomic Layer Deposition, ALD, is the technique of choice for
covering arbitrary shapes with homogeneous thin films. Early successes with
ferromagnetic materials include nickel and cobalt. Still, challenges in
depositing ferromagnetic alloys reside in the synthesis via decomposing the
consituent elements at the same temperature and homogeneously. We report
plasma-enhanced ALD to prepare permalloy Ni$_{80}$Fe$_{20}$ thin films and
nanotubes using nickelocene and iron(III) tert-butoxide as metal precursors,
water as the oxidant agent and an in-cycle plasma enhanced reduction step with
hydrogen. We have optimized the ALD cycle in terms of Ni:Fe atomic ratio and
functional properties. We obtained a Gilbert damping of 0.013, a resistivity of
28 $\mu\Omega$cm and an anisotropic magnetoresistance effect of 5.6 $\%$ in the
planar thin film geometry. We demonstrate that the process also works for
covering GaAs nanowires, resulting in permalloy nanotubes with high aspect
ratios and diameters of about 150 nm. Individual nanotubes were investigated in
terms of crystal phase, composition and spin-dynamic response by microfocused
Brillouin Light Scattering. Our results enable NiFe-based 3D spintronics and
magnonic devices in curved and complex topology operated in the GHz frequency
regime.",2105.01969v1
2021-05-11,On the connection between magnetic interactions and the spin-wave gap of the insulating phase of NaOsO$_{3}$,"The scenario of a metal-insulator transition driven by the onset of
antiferromagnetic order in NaOsO$_3$ calls for a trustworthy derivation of the
underlying effective spin Hamiltonian. To determine the latter we rely on {\it
ab initio} electronic-structure calculations, linear spin-wave theory, and
comparison to experimental data of the corresponding magnon spectrum. We arrive
this way to Heisenberg couplings that are $\lesssim$45\% to$\lesssim$63\%
smaller than values presently proposed in the literature and
Dzyaloshinskii-Moriya interactions in the region of 15\% of the Heisenberg
exchange $J$. These couplings together with the symmetric anisotropic exchange
interaction and single-ion magnetocrystalline anisotropy successfully reproduce
the magnon dispersion obtained by resonant inelastic X-ray scattering
measurements. In particular, the spin-wave gap fully agrees with the measured
one. We find that the spin-wave gap is defined from a subtle interplay between
the single-ion anisotropy, the Dzyaloshinskii-Moriya exchange and the symmetric
anisotropic exchange interactions. The results reported here underpin the
local-moment description of NaOsO$_3$, when it comes to analyzing the magnetic
excitation spectra. Interestingly, this comes about from a microscopic theory
that describes the electron system as Bloch states, adjusted to a mean-field
solution to Hubbard-like interactions.",2105.04893v2
2021-05-24,Role of two-dimensional Ising superconductivity in the non-equilibrium quasiparticle spin-to-charge conversion efficiency,"Non-equilibrium studies of two-dimensional (2D) superconductors (SCs) with
Ising spin-orbit coupling are prerequisite for their successful application to
equilibrium spin-triplet Cooper pairs and, potentially, Majorana fermions. By
taking advantage of the recent discoveries of 2D SCs and their compatibility
with any other materials, we fabricate here non-local magnon devices to examine
how such 2D Ising superconductivity affects the conversion efficiency of magnon
spin to quasiparticle charge in superconducting flakes of 2H-NbSe2 transferred
onto ferrimagnetic insulating Y3Fe5O12. Comparison with a reference device
based on a conventionally paired superconductor shows that the Y3Fe5O12-induced
in-plane (IP) exchange spin-splitting in the NbSe2 flake is hindered by its
inherent out-of-plane (OOP) spin-orbit-field, which, in turn, limits the
transition-state enhancement of the spin-to-charge conversion efficiency. Our
out-of-equilibrium study highlights the significance of symmetry matching
between underlying Cooper pairs and exchange-induced spin-splitting for the
giant transition-state spin-to-charge conversion and may have implications
towards proximity-engineered spin-polarized triplet pairing via tuning the
relative strength of IP exchange and OOP spin-orbit fields in ferromagnetic
insulator/2D Ising SC bilayers.",2105.11159v4
2021-06-04,Non-Loudon-Fleury Raman scattering in spin-orbit coupled Mott insulators,"We revisit the theory of magnetic Raman scattering in Mott insulators with
strong spin-orbit coupling, with a major focus on Kitaev materials. We show
that Kitaev materials with bond-anisotropic interactions are generally expected
to show both one- and two-magnon responses. It is further shown that, in order
to obtain the correct leading contributions to the Raman vertex operator
$\mc{R}$, one must take into account the precise, photon-assisted microscopic
hopping processes of the electrons and that, in systems with multiple hopping
paths, $\mc{R}$ contains terms beyond those appearing in the traditional
Loudon-Fleury theory. Most saliently, a numerical implementation of the revised
formalism to the case of the three-dimensional hyperhoneycomb Kitaev material
$\beta$-Li$_2$IrO$_3$ reveals that the non-Loudon-Fleury scattering terms
actually dominate the Raman intensity. In addition, they induce a qualitative
modification of the polarization dependence, including, e.g., the emergence of
a sharp one-magnon peak at low energies which is not expected in the
traditional Loudon-Fleury theory. This peak is shown to arise from microscopic
photon-assisted tunneling processes that are of similar type with the ones
leading to the symmetric off-diagonal interaction $\Gamma$ (known to be present
in many Kitaev materials), but take the form of a bond-directional magnetic
dipole term in the Raman vertex. These results are expected to apply across all
Kitaev materials and mark a drastic change of paradigm for the understanding of
Raman scattering in materials with strong spin-orbit coupling and multiple
exchange paths.",2106.02645v2
2021-06-08,Quantum Monte Carlo study of honeycomb antiferromagnets under a triaxial strain,"The honeycomb antiferromagnet under a triaxial strain is studied using the
quantum Monte Carlo simulation. The strain dimerizes the exchange couplings
near the corners, thus destructs the antiferromagnetic order therein. The
antiferromagnetic region is continuously reduced by the strain. For the same
strain strength, the exact numerical results give a much smaller
antiferromagnetic region than the linear spin-wave theory. We then study the
strained $XY$ antiferromagnet, where the magnon pseudo-magnetic field behaves
quite differently. The $0$th Landau level appears in the middle of the
spectrum, and the quantized energies above (below) it are proportional to
$n^{\frac{1}{3}} (n^{\frac{2}{3}})$, which is in great contrast to the
equally-spaced ones in the Heisenberg case. Besides, we find the
antiferromagnetic order of the $XY$ model is much more robust to the
dimerization than the Heisenberg one. The local susceptibility of the
Heisenberg case is extracted by the numerical analytical continuation, and no
sign of the pseudo-Landau levels is resolved. It is still not sure whether the
result is due to the intrinsic problem of the numerical analytical
continuation. Thus the existence of the magnon pseudo-Landau levels in the
spin-$\frac{1}{2}$ strained Heisenberg Hamiltonian remains an open question.
Our results are closely related to the two-dimensional van der Waals quantum
antiferromagnets and may be realized experimentally.",2106.04358v3
2021-06-15,Dynamical Spin Chains in 4D $\mathcal{N}=2$ SCFTs,"This is the first in a series of papers devoted to the study of spin chains
capturing the spectral problem of 4d $\mathcal{N}=2$ SCFTs in the planar limit.
At one loop and in the quantum plane limit, we discover a quasi-Hopf symmetry
algebra, defined by the $R$-matrix read off from the superpotential. This
implies that when orbifolding the $\mathcal{N}=4$ symmetry algebra down to the
$\mathcal{N}=2$ one and then marginaly deforming, the broken generators are not
lost, but get upgraded to quantum generators. Importantly, we demonstrate that
these chains are dynamical, in the sense that their Hamiltonian depends on a
parameter which is dynamically determined along the chain. At one loop we map
the holomorphic SU(3) scalar sector to a dynamical 15-vertex model, which
corresponds to an RSOS model, whose adjacency graph can be read off from the
gauge theory quiver/brane tiling. One scalar SU(2) sub-sector is described by
an alternating nearest-neighbour Hamiltonian, while another choice of SU(2)
sub-sector leads to a dynamical dilute Temperley-Lieb model. These sectors have
a common vacuum state, around which the magnon dispersion relations are
naturally uniformised by elliptic functions. Concretely, for the $\mathbb{Z}_2$
quiver theory we study these dynamical chains by solving the one- and
two-magnon problems with the coordinate Bethe ansatz approach. We confirm our
analytic results by numerical comparison with the explicit diagonalisation of
the Hamiltonian for short closed chains.",2106.08449v1
2021-06-23,Field-Induced Spin Excitations in the Spin-1/2 Triangular-Lattice Antiferromagnet CsYbSe$_2$,"A layered triangular lattice with spin-1/2 ions is an ideal platform to
explore highly entangled exotic states like quantum spin liquid (QSL). Here, we
report a systematic in-field neutron scattering study on a perfect
two-dimensional triangular-lattice antiferromagnet, CsYbSe$_2$, a member of the
large QSL candidate family rare-earth chalcogenides. The elastic neutron
scattering measured down to 70 mK shows that there is a short-range
120$^{\circ}$ magnetic order at zero field. In the field-induced ordered
states, the spin-spin correlation lengths along the $c$ axis are relatively
short, although the heat capacity results indicate long-range magnetic orders
at 3 T $-$ 5 T. The inelastic neutron scattering spectra evolve from highly
damped continuum-like excitations at zero field to relatively sharp spin wave
modes at the plateau phase. Our extensive large-cluster density-matrix
renormalization group calculations with a Heisenberg triangular-lattice
nearest-neighbor antiferromagnetic model reproduce the essential features of
the experimental spectra, including continuum-like excitations at zero field,
series of sharp magnons at the plateau phase as well as two-magnon excitations
at high energy. This work presents comprehensive experimental and theoretical
overview of the unconventional field-induced spin dynamics in
triangular-lattice Heisenberg antiferromagnet and thus provides valuable
insight into quantum many-body phenomena.",2106.12451v2
2021-07-09,Excitations and ergodicity of critical quantum spin chains from non-equilibrium classical dynamics,"We study a quantum spin-1/2 chain that is dual to the canonical problem of
non-equilibrium Kawasaki dynamics of a classical Ising chain coupled to a
thermal bath. The Hamiltonian is obtained for the general disordered case with
non-uniform Ising couplings. The quantum spin chain (dubbed Ising-Kawasaki) is
stoquastic, and depends on the Ising couplings normalized by the bath's
temperature. We give its exact ground states. Proceeding with uniform
couplings, we study the one- and two-magnon excitations. Solutions for the
latter are derived via a Bethe Ansatz scheme. In the antiferromagnetic regime,
the two-magnon branch states show intricate behavior, especially regarding
their hybridization with the continuum. We find that that the gapless chain
hosts multiple dynamics at low energy as seen through the presence of multiple
dynamical critical exponents. Finally, we analyze the full energy level spacing
distribution as a function of the Ising coupling. We conclude that the system
is non-integrable for generic parameters, or equivalently, that the
corresponding non-equilibrium classical dynamics are ergodic.",2107.04615v3
2021-07-18,Straintronics: Manipulating the Magnetization of Magnetostrictive Nanomagnets with Strain for Energy-Efficient Applications,"The desire to perform information processing, computation, communication,
signal generation and related tasks, while dissipating as little energy as
possible, has inspired many ideas and paradigms. One of the most powerful among
them is the notion of using magnetostrictive nanomagnets as the primitive units
of the hardware platforms and manipulating their magnetizations with
electrically generated static or time varying mechanical strain to elicit
myriad functionalities. This approach has two advantages. First, information
can be retained in the devices after powering off since the nanomagnets are
non-volatile unlike charge-based devices such as transistors. Second, the
energy expended to perform a given task is exceptionally low since it takes
very little energy to alter magnetization states with strain. This field is now
known as ""straintronics"", in analogy with electronics, spintronics,
valleytronics, etc. We review the recent advances and trends in straintronics,
including digital information processing (logic), information storage (memory),
domain wall devices operated with strain, control of skyrmions with strain,
non-Boolean computing and machine learning with straintronics, signal
generation (microwave sources) and communication (ultra-miniaturized acoustic
and electromagnetic antennas) implemented with strained nanomagnets, hybrid
straintronics-magnonics, and interaction between phonons and magnons in
straintronic systems. We identify key challenges and opportunities, and lay out
pathways to advance this field to the point where it might become a mainstream
technology for energy-efficient systems.",2107.08497v1
2021-07-20,Spin-wave dispersion measurement by variable-gap propagating spin-wave spectroscopy,"Magnonics is seen nowadays as a candidate technology for energy-efficient
data processing in classical and quantum systems. Pronounced nonlinearity,
anisotropy of dispersion relations and phase degree of freedom of spin waves
require advanced methodology for probing spin waves at room as well as at mK
temperatures. Yet, the use of the established optical techniques like Brillouin
light scattering (BLS) or magneto optical Kerr effect (MOKE) at ultra-low
temperatures is forbiddingly complicated. By contrast, microwave spectroscopy
can be used at all temperatures but is usually lacking spatial and wavenumber
resolution. Here, we develop a variable-gap propagating spin-wave spectroscopy
(VG-PSWS) method for the deduction of the dispersion relation of spin waves in
wide frequency and wavenumber range. The method is based on the phase-resolved
analysis of the spin-wave transmission between two antennas with variable
spacing, in conjunction with theoretical data treatment. We validate the method
for the in-plane magnetized CoFeB and YIG thin films in $k\perp B$ and
$k\parallel B$ geometries by deducing the full set of material and spin-wave
parameters, including spin-wave dispersion, hybridization of the fundamental
mode with the higher-order perpendicular standing spin-wave modes and surface
spin pinning. The compatibility of microwaves with low temperatures makes this
approach attractive for cryogenic magnonics at the nanoscale.",2107.09363v1
2021-08-30,Bright and dark states of two distant macrospins strongly coupled by phonons,"We study the collective dynamics of two distant magnets coherently coupled by
acoustic phonons that are transmitted through an intercalated crystal. By
tuning the ferromagnetic resonances of the two magnets to an acoustic resonance
of the crystal, we control a coherent three levels system. We show that the
parity of the phonon mode governs the nature of the indirect coupling between
the magnets: the resonances with odd / even phonon modes correspond to
out-of-phase / in-phase lattice displacements at the magnets, leading to bright
/ dark states in response to uniform microwave magnetic fields, respectively.
The sample is a tri-layer garnet consisting of two thin magnetic layers
epitaxially grown on both sides of a half-millimeter thick non-magnetic single
crystal. In spite of the relatively weak magneto-elastic interaction, the long
lifetimes of the magnon and phonon modes in the sample are the key to unveil
this long range strong coupling. This demonstrates that garnets are a great
platform to study multi-partite hybridization process between magnon and
phonons at microwave frequencies.",2108.13272v2
2021-09-23,Dynamics in an exact solvable quantum magnet: benchmark for quantum computer,"Quantum magnets are never short of novel and fascinating dynamics, yet its
simulation by classical computers requires exponentially-scaled computation
resources, which renders the research on large-scale many-body dynamics
fiendishly difficult. In this letter, we explore the dynamic behavior of 2D
large-scale ferromagnetic J1-J2 Heisenberg model both theoretically and
experimentally. First, the analytical solution of magnon dynamics is obtained
to show an obvious ballistic propagation of magnon, which is typical for
quantum walk. Then, we verify the dynamic behavior of the system through
numerical approach of exact diagonalization and tensor network method. We also
calculate out-of-time ordered correlators and butterfly velocities among
different lattice points, finding that they can well depict the competition
between different couplings. Finally, a quantum walk experiment is designed and
conducted on the basis of IBM programmable quantum processors, and the
experimental results are in consistence with our theoretical predictions. Since
the analytical results can be used, in principle, to predict the behavior of
large-scale quantum many-body systems and even those infinitely large, this
work will help facilitate further research on quantum walk and quantum
many-body dynamics in large-scale lattice systems, guide future design of
quantum computers, as well as popularize quantum computers until they are known
and available to every household in the world.",2109.11371v5
2021-10-05,Thermal transport of the frustrated spin-chain mineral linarite: Magnetic heat transport and strong spin-phonon scattering,"The mineral linarite (PbCuSO$_4$(OH)$_2$) forms a monoclinic structure where
a sequence of Cu(OH)$_2$ units forms a spin-$\frac{1}{2}$ chain. Competing
ferromagnetic nearest-neighbor ($J_1$) and antiferromagnetic
next-nearest-neighbor interactions ($J_2$) in this quasi-one-dimensional spin
structure imply magnetic frustration and lead to magnetic ordering below $T_N
=$2.8 K in a mutliferroic elliptical spin-spiral ground state. Upon the
application of a magnetic field along the spin-chain direction, distinct
magnetically ordered phases can be induced. We studied the thermal conductivity
$\kappa$ in this material across the magnetic phase diagram as well as in the
paramagnetic regime in the temperature ranges 0.07-1 K and 9-300 K. We found
that in linarite the heat is carried mainly by phonons but shows a peculiar
non-monotonic behavior in field. In particular, $\kappa$ is highly suppressed
at the magnetic phase boundaries, indicative of strong scattering of the
phonons off critical magnetic fluctuations. Even at temperatures far above the
magnetically ordered phases, the phononic thermal conductivity is reduced due
to scattering off magnetic fluctuations. The mean free path due to spin-phonon
scattering ($l_{\text{spin-phonon}}$) was determined as function of
temperature. A power law behavior was observed mainly above 0.5 K indicating
the thermal activation of spin fluctuations. In the critical regime close to
the saturation field, $l_{\text{spin-phonon}}$ shows a $1/T$ dependence.
Furthermore, a magnon thermal transport channel was verified in the helical
magnetic phase. We estimate a magnon mean free path which corresponds to about
1000 lattice spacings.",2110.01865v1
2021-12-10,Low-energy excitations and magnetic anisotropy of the layered van der Waals antiferromagnet Ni$_2$P$_2$S$_6$,"The quasi-two-dimensional antiferromagnet Ni$_2$P$_2$S$_6$ belongs to the
family of magnetic van der Waals compounds that provides a rich material base
for the realization of fundamental models of quantum magnetism in low
dimensions. Here, we report high-frequency/high-magnetic field electron spin
resonance measurements on single crystals of Ni$_2$P$_2$S$_6$. The results
enable one to reliably determine the positive, ""easy""-plane type of the single
ion anisotropy of the Ni$^{2+}$ ions with the upper limit of its magnitude
$\lesssim 0.7$ meV. The resonance response reveals strongly anisotropic spin
fluctuations setting in shortly above the N\'eel temperature $T_{\rm N} = 158$
K and extending in the antiferromagnetically ordered state down to low
temperatures. There, a low-energy magnon excitation gapped from the ground
state by only $\sim 1$ meV was found. This magnon mode may explain unusual low
temperature relaxation processes observed in a latest nuclear magnetic
resonance experiments and together with the estimate of the single ion
anisotropy they should enable setting up a realistic spin model for an accurate
description of magnetic properties of Ni$_2$P$_2$S$_6$.",2112.05699v3
2021-12-11,Optical excitation of electromagnons in hexaferrite,"Understanding ultrafast magnetization dynamics on the microscopic level is of
strong current interest due to the potential for applications in information
storage. In recent years, the spin-lattice coupling has been recognized to be
essential for ultrafast magnetization dynamics. Magnetoelectric multiferroics
of type II possess intrinsic correlations among magnetic sublattices and
electric polarization (P) through spin-lattice coupling, enabling fundamentally
coupled dynamics between spins and lattice. Here we report on ultrafast
magnetization dynamics in a room-temperature multiferroic hexaferrite
possessing ferrimagnetic and antiferromagnetic sublattices, revealed by
time-resolved resonant x-ray diffraction. A femtosecond above-bandgap
excitation triggers a coherent magnon in which the two magnetic sublattices
entangle and give rise to a transient modulation of P. A novel microscopic
mechanism for triggering the coherent magnon in this ferrimagnetic insulator
based on the spin-lattice coupling is proposed. Our finding opens up a novel
but general pathway for ultrafast control of magnetism.",2112.05961v1
2022-01-25,Frustration model and spin excitations in the helimagnet FeP,"The metallic compound FeP belongs to the class of materials that feature a
complex noncollinear spin order driven by magnetic frustration. While its
double-helix magnetic structure with a period $\lambda_{\text{s}} \approx 5c$,
where $c$ is the lattice constant, was previously well determined, the relevant
spin-spin interactions that lead to that ground state remain unknown. By
performing extensive inelastic neutron scattering measurements, we obtained the
spin-excitation spectra in a large part of the momentum-energy space. The
spectra show that the magnons are gapped with a gap energy of $\sim$5 meV.
Despite the 3D crystal structure, the magnon modes display strongly anisotropic
dispersions, revealing a quasi-one-dimensional character of the magnetic
interactions in FeP. The physics of the material, however, is not determined by
the dominating exchange, which is ferromagnetic. Instead, the weaker
two-dimensional antiferromagnetic interactions between the rigid ferromagnetic
spin chains drive the magnetic frustration. Using linear spin-wave theory, we
were able to construct an effective Heisenberg Hamiltonian with an anisotropy
term capable of reproducing the observed spectra. This enabled us to quantify
the exchange interactions in FeP and determine the mechanism of its magnetic
frustration.",2201.10358v2
2022-04-07,Spin dynamics and exchange interaction in orthoferrite TbFeO$_3$ with non-Kramers rare-earth ion,"The low-temperature spin dynamics of the orthorhombic TbFeO$_3$ perovskite
has been studied. It has been found that the inelastic neutron scattering (INS)
spectrum contains two modes corresponding to different sublattices in the
compound. The iron subsystem orders antiferromagnetically at T$_N$ = 632 K and
exhibits the high-energy magnon dispersion. Magnetic dynamics of this subsystem
has been described using the linear spin wave theory and our solution yields
sizable anisotropy between in-plane and out-of-plane exchange interactions.
This approach was previously used to describe the magnon dispersion in the
TmFeO$_3$ compound. Three non-dispersive crystal electric field levels
corresponding to Tb$^{3+}$ ions have been established in the region below 40
meV at about 17, 26, and 35 meV. Study of diffuse scattering at different
temperatures has elucidated the behavior of the magnetic correlation length.
The behavior of the Tb$^{3+}$ ion subsystem has been numerically described in
the framework of the point charge model. The numerical data agree
satisfactorily with the experiment and with the general concept of the
single-ion approximation applied to the rare-earth subsystem of orthorhombic
perovskites.",2204.03239v1
2022-04-19,Unravelling the Nature of Spin Excitations Disentangled from Charge Contributions in a Doped Cuprate Superconductor,"The nature of the spin excitations in superconducting cuprates is a key
question toward a unified understanding of the cuprate physics from long-range
antiferromagnetism to superconductivity. The intense spin excitations up to the
over-doped regime revealed by resonant inelastic X-ray scattering bring new
insights as well as questions like how to understand their persistence or their
relation to the collective excitations in ordered magnets (magnons). Here, we
study the evolution of the spin excitations upon hole-doping the
superconducting cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ by disentangling the
spin from the charge excitations in the experimental cross section. We compare
our experimental results against density matrix renormalization group
calculations for a $t$-$J$-like model on a square lattice. Our results
unambiguously confirm the persistence of the spin excitations, which are
closely connected to the persistence of short-range magnetic correlations up to
high doping. This suggests that the spin excitations in hole-doped cuprates are
related to magnons -- albeit short-ranged.",2204.08749v2
2022-04-22,Controlling magnetism with light in a zero orbital angular momentum antiferromagnet,"Antiferromagnetic materials feature intrinsic ultrafast spin dynamics, making
them ideal candidates for future magnonic devices operating at THz frequencies.
A major focus of current research is the investigation of optical methods for
the efficient generation of coherent magnons in antiferromagnetic insulators.
In magnetic lattices endowed with orbital angular momentum, spin-orbit coupling
enables spin dynamics through the resonant excitation of low-energy electric
dipoles such as phonons and orbital resonances which interact with spins.
However, in magnetic systems with zero orbital angular momentum, microscopic
pathways for the resonant and low-energy optical excitation of coherent spin
dynamics are lacking. Here, we consider experimentally the relative merits of
electronic and vibrational excitations for the optical control of zero orbital
angular momentum magnets, focusing on a limit case: the antiferromagnet
manganese thiophoshate (MnPS3), constituted by orbital singlet Mn2+ ions. We
study the correlation of spins with two types of excitations within its band
gap: a bound electron orbital excitation from the singlet orbital ground state
of Mn2+ into an orbital triplet state, which causes coherent spin precession,
and a vibrational excitation of the crystal field that causes thermal spin
disorder. Our findings cast orbital transitions as key targets for magnetic
control in insulators constituted by magnetic centers of zero orbital angular
momentum.",2204.10574v1
2022-06-06,Probing spin dynamics of ultra-thin van der Waals magnets via photon-magnon coupling,"Layered van der Waals (vdW) magnets can maintain a magnetic order even down
to the single-layer regime and hold promise for integrated spintronic devices.
While the magnetic ground state of vdW magnets was extensively studied, key
parameters of spin dynamics, like the Gilbert damping, crucial for designing
ultra-fast spintronic devices, remains largely unexplored. Despite recent
studies by optical excitation and detection, achieving spin wave control with
microwaves is highly desirable, as modern integrated information technologies
predominantly are operated with these. The intrinsically small numbers of
spins, however, poses a major challenge to this.
  Here, we present a hybrid approach to detect spin dynamics mediated by
photon-magnon coupling between high-Q superconducting resonators and ultra-thin
flakes of Cr$_2$Ge$_2$Te$_6$ (CGT) as thin as 11\,nm. We test and benchmark our
technique with 23 individual CGT flakes and extract an upper limit for the
Gilbert damping parameter. These results are crucial in designing on-chip
integrated circuits using vdW magnets and offer prospects for probing spin
dynamics of monolayer vdW magnets.",2206.02460v2
2022-06-11,Chirality as Generalized Spin-Orbit Interaction in Spintronics,"This review focuses on the chirality observed in the excited states of the
magnetic order, dielectrics, and conductors that hold transverse spins when
they are evanescent. Even without any relativistic effect, the transverse spin
of the evanescent waves are locked to the momentum and the surface normal of
their propagation plane. This chirality thereby acts as a generalized
spin-orbit interaction, which leads to the discovery of various chiral
interactions between magnetic, phononic, electronic, photonic, and plasmonic
excitations in spintronics that mediate the excitation of quasiparticles into a
single direction, leading to phenomena such as chiral spin and phonon pumping,
chiral spin Seebeck, spin skin, magnonic trap, magnon Doppler, and spin diode
effects. Intriguing analogies with electric counterparts in the nano-optics and
plasmonics exist. After a brief review of the concepts of chirality that
characterize the ground state chiral magnetic textures and chirally coupled
magnets in spintronics, we turn to the chiral phenomena of excited states. We
present a unified electrodynamic picture for dynamical chirality in spintronics
in terms of generalized spin-orbit interaction and compare it with that in
nano-optics and plasmonics. Based on the general theory, we subsequently review
the theoretical progress and experimental evidence of chiral interaction, as
well as the near-field transfer of the transverse spins, between various
excitations in magnetic, photonic, electronic and phononic nanostructures at
GHz time scales. We provide a perspective for future research before concluding
this article.",2206.05535v2
2022-07-30,Excitation Spectra of one-dimensional spin-1/2 Fermi gas with an attraction,"Using exact Bethe ansatz solution, we rigorously study excitation spectra of
the spin-1/2 Fermi gas (called Yang-Gaudin model) with an attractive
interaction. Elementary excitations of this model involve particle-hole
excitations, hole excitations and adding particles in the Fermi seas of pairs
and unpaired fermions. The gapped magnon excitations in spin sector show a
ferromagnetic coupling to the Fermi sea of the single fermions. By numerically
and analytically solving the Bethe ansatz equations and the thermodynamic Bethe
ansatz equations of this model, we obtain excitation energies for various
polarizations in the phase of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like
state. For a small momentum (long-wavelength limit) and in the strong
interaction regime, we analytically obtained their linear dispersions with
curvature corrections, effective masses as well as velocities in particle-hole
excitations of pairs and unpaired fermions. Such a type of particle-hole
excitations display a novel separation of collective motions of bosonic modes
within paired and unpaired fermions. Finally, we also discuss magnon
excitations in spin sector and the application of the Bragg spectroscopy for
testing such separated charge excitation modes of pairs and single fermions.",2208.00212v2
2022-08-17,The importance of the interface for picosecond spin pumping in antiferromagnet-heavy metal heterostructures,"Interfaces between heavy metals (HMs) and antiferromagnetic insulators (AFIs)
have recently become highly investigated and debated systems in the effort to
create spintronic devices able to function at terahertz frequencies. Such
heterostructures have great technological potential because AFIs can generate
sub-picosecond spin currents which the HMs can convert into charge signals. In
this work we demonstrate an optically induced picosecond spin transfer at the
interface between AFIs and Pt using time-resolved THz emission spectroscopy. We
select two antiferromagnets in the same family of fluoride cubic perovskites,
KCoF3 and KNiF3, whose magnon frequencies at the centre of the Brillouin zone
differ by an order of magnitude. By studying their behaviour with temperature
we correlate changes in the spin transfer efficiency across the interface to
the opening of a gap in the magnon density of states below the N\'eel
temperature. Our observations are reproduced in a model based on the spin
exchange between the localized electrons in the antiferromagnet and the free
electrons in Pt. These results constitute an important step in the rigorous
investigation and understanding of the physics of AFIs/HMs interfaces on the
ultrafast timescale.",2208.08332v1
2022-09-20,Terahertz spin dynamics in rare-earth orthoferrites,"Recent interest in developing fast spintronic devices and laser-controllable
magnetic solids has sparked tremendous experimental and theoretical efforts to
understand and manipulate ultrafast dynamics in materials. Studies of spin
dynamics in the terahertz (THz) frequency range are particularly important for
elucidating microscopic pathways toward novel device functionalities. Here, we
review THz phenomena related to spin dynamics in rare-earth orthoferrites, a
class of materials promising for antiferromagnetic spintronics. We expand this
topic into a description of four key elements. (1) We start by describing THz
spectroscopy of spin excitations for probing magnetic phase transitions in
thermal equilibrium. While acoustic magnons are useful indicators of spin
reorientation transitions, electromagnons that arise from dynamic
magnetoelectric couplings serve as a signature of inversion-symmetry-breaking
phases at low temperatures. (2) We then review the strong laser driving
scenario, where the system is excited far from equilibrium and thereby subject
to modifications to the free energy landscape. Microscopic pathways for
ultrafast laser manipulation of magnetic order are discussed. (3) Furthermore,
we review a variety of protocols to manipulate coherent THz magnons in time and
space, which are useful capabilities for antiferromagnetic spintronic
applications. (4) Finally, new insights on the connection between dynamic
magnetic coupling in condensed matter and the Dicke superradiant phase
transition in quantum optics are provided. By presenting a review on an array
of THz spin phenomena occurring in a single class of materials, we hope to
trigger interdisciplinary efforts that actively seek connections between
subfields of spintronics, which will facilitate the invention of new protocols
of active spin control and quantum phase engineering.",2209.09468v1
2022-09-27,Tailoring crosstalk between localized 1D spin-wave nanochannels using focused ion beams,"1D spin-wave conduits are envisioned as nanoscale components of
magnonics-based logic and computing schemes for future generation electronics.
`A-la-carte methods of versatile control of the local magnetization dynamics in
such nanochannels are highly desired for efficient steering of the spin waves
in magnonic devices. Here, we present a study of localized dynamical modes in
1-$\mu$m-wide Permalloy conduits probed by microresonator ferromagnetic
resonance technique. We clearly observe the lowest-energy edge mode in the
microstrip after its edges were finely trimmed by means of focused Ne$^+$ ion
irradiation. Furthermore, after milling the microstrip along its long axis by
focused ion beams, creating consecutively $\sim$50 and $\sim$100 nm gaps,
additional resonances emerge and are attributed to modes localized at the inner
edges of the separated strips. To visualize the mode distribution, spatially
resolved Brillouin light scattering microscopy was used showing an excellent
agreement with the ferromagnetic resonance data and confirming the mode
localization at the outer/inner edges of the strips depending on the magnitude
of the applied magnetic field. Micromagnetic simulations confirm that the
lowest-energy modes are localized within $\sim$15-nm-wide regions at the edges
of the strips and their frequencies can be tuned in a wide range (up to 5 GHz)
by changing the magnetostatic coupling (i.e. spatial separation) between the
microstrips.",2209.13180v2
2022-10-11,"Specifics of the Elemental Excitations in ""True One-Dimensional"" MoI$_3$ van der Waals Nanowires","We report on the temperature evolution of the polarization-dependent Raman
spectrum of exfoliated MoI$_3$, a van der Waals material with a ""true
one-dimensional"" crystal structure that can be exfoliated to individual atomic
chains. The temperature evolution of several Raman features reveals anomalous
behavior suggesting a phase transition of a magnetic origin. Theoretical
considerations indicate that MoI$_3$ is an easy-plane antiferromagnet with
alternating spins along the dimerized chains and with inter-chain helical spin
ordering. The calculated frequencies of the phonons and magnons are consistent
with the interpretation of the experimental Raman data. The obtained results
shed light on the specifics of the phononic and magnonic states in MoI$_3$ and
provide a strong motivation for future study of this unique material with
potential for spintronic device applications.",2210.05081v1
2022-10-25,Planar Thermal Hall Effects in Kitaev Spin Liquid Candidate Na2Co2TeO6,"We investigate both the longitudinal thermal conductivity ($\kappa_{xx}$) and
the planar thermal Hall conductivity ($\kappa_{xy}$) in the Kitaev spin liquid
candidate of Co-based honeycomb antiferromagnet Na$_2$Co$_2$TeO$_6$ in a
magnetic field ($B$) applied along the $a$ and $a^*$ axes. A finite
$\kappa_{xy}$ is resolved for both field directions in the antiferromagnetic
(AFM) phase below the N\'eel temperature of 27 K. The temperature dependence of
$\kappa_{xy}/T$ shows the emergence of topological bosonic excitations. In
addition, the field dependence of $\kappa_{xy}$ shows sign reversals at the
critical fields in the AFM phase, suggesting the changes in the Chern number
distribution of the topological magnons. Remarkably, a finite $\kappa_{xy}$ is
observed in $B \parallel a^*$ between the first-order transition field in the
AFM phase and the saturation field, which is prohibited in a disordered state
by the two-fold rotation symmetry around the $a^*$ axis of the honeycomb
lattice, showing the presence of a magnetically ordered state that breaks the
two-fold rotation symmetry. Our results demonstrate the presence of topological
magnons in this compound in the whole field range below the saturation field.",2210.13798v1
2022-10-29,Micromagnetic frequency-domain simulation methods for magnonic systems,"We present efficient numerical methods for the simulation of small
magnetization oscillations in three-dimensional micromagnetic systems.
Magnetization dynamics is described by the Landau-Lifshitz-Gilbert (LLG)
equation, linearized in the frequency domain around a generic equilibrium
configuration, and formulated in a special operator form that allows leveraging
large-scale techniques commonly used to evaluate the effective field in
time-domain micromagnetic simulations. By using this formulation, we derive
numerical algorithms to compute the free magnetization oscillations (i.e., spin
wave eigenmodes) as well as magnetization oscillations driven by ac
radio-frequency fields for arbitrarily shaped nanomagnets. Moreover,
semi-analytical perturbation techniques based on the computation of a reduced
set of eigenmodes are provided for fast evaluation of magnetization frequency
response and absorption spectra as a function of damping and ac field. We
present both finite difference and finite element implementations and
demonstrate their effectiveness on a test case. These techniques open the
possibility to study generic magnonic systems discretized with several hundred
thousand (or even millions) of computational cells in a reasonably short time.",2210.16564v3
2022-11-17,Observation of an unexpected negative magnetoresistance in magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$,"Time-reversal symmetry breaking allows for a rich set of magneto-transport
properties related to electronic topology. Focusing on the magnetic Weyl
semimetal Co$_3$Sn$_2$S$_2$, we prepared micro-ribbons and investigated their
transverse and longitudinal transport properties from 100 K to 180 K in
magnetic fields $\mu_0 H$ up to 2T. We establish the presence of a
magnetoresistance (MR) up to 1 % with a strong anisotropy depending the
projection of $H$ on the easy-axis magnetization, which exceeds all other
magnetoresistive effects. Based on detailed phenomenological modeling, we
attribute the observed results with unexpected form of anisotropy to magnon MR
resulting from magnon-electron coupling. Moreover, a similar angular dependence
is also found in the transverse resistivity which we show to originate from the
combination of ordinary Hall and anomalous Hall effects. Thus the interplay of
magnetic and topological properties governs the magnetotransport features of
this magnetic Weyl system.",2211.09798v1
2022-11-30,A proposal for leaky integrate-and-fire neurons by domain walls in antiferromagnetic insulators,"Brain-inspired neuromorphic computing is a promising path towards next
generation analogue computers that are fundamentally different compared to the
conventional von Neumann architecture. One model for neuromorphic computing
that can mimic the human brain behavior are spiking neural networks (SNNs), of
which one of the most successful is the leaky integrate-and-fire (LIF) model.
Since conventional complementary metal-oxide-semiconductor (CMOS) devices are
not meant for modelling neural networks and are energy inefficient in network
applications, recently the focus shifted towards spintronic-based neural
networks. In this work, using the advantage of antiferromagnetic insulators, we
propose a non-volatile magnonic neuron that could be the building block of a
LIF spiking neuronal network. In our proposal, an antiferromagnetic domain wall
in the presence of a magnetic anisotropy gradient mimics a biological neuron
with leaky, integrating, and firing properties. This single neuron is
controlled by polarized antiferromagnetic magnons, activated by either a
magnetic field pulse or a spin transfer torque mechanism, and has properties
similar to biological neurons, namely latency, refraction, bursting and
inhibition. We argue that this proposed single neuron, based on
antiferromagnetic domain walls, is faster and has more functionalities compared
to previously proposed neurons based on ferromagnetic systems.",2211.16845v2
2022-12-05,Propagating spin-wave spectroscopy in nanometer-thick YIG films at millikelvin temperatures,"Performing propagating spin-wave spectroscopy of thin films at millikelvin
temperatures is the next step towards the realisation of large-scale integrated
magnonic circuits for quantum applications. Here we demonstrate spin-wave
propagation in a $100\,\mathrm{nm}$-thick yttrium-iron-garnet film at the
temperatures down to $45 \,\mathrm{mK}$, using stripline nanoantennas deposited
on YIG surface for the electrical excitation and detection. The clear
transmission characteristics over the distance of $10\,\mu \mathrm{m}$ are
measured and the subtracted spin-wave group velocity and the YIG saturation
magnetisation agree well with the theoretical values. We show that the
gadolinium-gallium-garnet substrate influences the spin-wave propagation
characteristics only for the applied magnetic fields beyond $75\,\mathrm{mT}$,
originating from a GGG magnetisation up to $47 \,\mathrm{kA/m}$ at $45
\,\mathrm{mK}$. Our results show that the developed fabrication and measurement
methodologies enable the realisation of integrated magnonic quantum
nanotechnologies at millikelvin temperatures.",2212.02257v3
2022-12-24,Anatomy of ultrafast quantitative magneto-acoustics in freestanding nickel thin films,"We revisit the quantitative analysis of the ultrafast magneto-acoustic
experiment in a freestanding nickel thin film by Kim and Bigot [1] by applying
our recently proposed approach of magnetic and acoustic eigenmodes
decomposition by Vernik et al. [2]. We show that the application of our
modeling to the analysis of time-resolved reflectivity measurements allows for
the determination of amplitudes and lifetimes of standing perpendicular
acoustic phonon resonances with unprecedented accuracy. The acoustic damping is
found to scale as $\propto\omega^2$ for frequencies up to 80~GHz and the peak
amplitudes reach $10^{-3}$. The experimentally measured magnetization dynamics
for different orientations of an external magnetic field agrees well with
numerical solutions of magneto-elastically driven magnon harmonic oscillators.
Symmetry-based selection rules for magnon-phonon interactions predicted by our
modeling approach allow for the unambiguous discrimination between spatially
uniform and non-uniform modes, as confirmed by comparing the resonantly
enhanced magneto-elastic dynamics simultaneously measured on opposite sides of
the film. Moreover, the separation of time scales for (early) rising and (late)
decreasing precession amplitudes provide access to magnetic (Gilbert) and
acoustic damping parameters in a single measurement.",2212.12673v1
2023-01-10,Magnetic anisotropy and low-energy spin dynamics in the van der Waals compounds Mn$_{2}$P$_{2}$S$_{6}$ and MnNiP$_{2}$S$_{6}$,"We report the detailed high-field and high-frequency electron spin resonance
(HF-ESR) spectroscopic study of the single-crystalline van der Waals compounds
Mn$_{2}$P$_{2}$S$_{6}$ and MnNiP$_{2}$S$_{6}$. Analysis of magnetic excitations
shows that in comparison to Mn$_{2}$P$_{2}$S$_{6}$ increasing the Ni content
yields a larger magnon gap in the ordered state and a larger g-factor value and
its anisotropy in the paramagnetic state. The studied compounds are found to be
strongly anisotropic having each the unique ground state and type of magnetic
order. Stronger deviation of the g-factor from the free electron value in the
samples containing Ni suggests that the anisotropy of the exchange is an
important contributor to the stabilization of a certain type of magnetic order
with particular anisotropy. At temperatures above the magnetic order, we have
analyzed the spin-spin correlations resulting in a development of slowly
fluctuating short-range order. They are much stronger pronounced in
MnNiP$_{2}$S$_{6}$ compared to Mn$_{2}$P$_{2}$S$_{6}$. The enhanced spin
fluctuations in MnNiP$_{2}$S$_{6}$ are attributed to the competition of
different types of magnetic order. Finally, the analysis of the temperature
dependent critical behavior of the magnon gaps below the ordering temperature
in Mn$_{2}$P$_{2}$S$_{6}$ suggests that the character of the spin wave
excitations in this compound undergoes a field induced crossover from a 3D-like
towards 2D XY regime.",2301.04239v2
2023-02-10,Traveling Salesman Problem solution using Magnonic Combinatorial Device,"Traveling Salesman Problem (TSP) is a decision-making problem that is
essential for a number of practical applications. Today, this problem is solved
on digital computers exploiting Boolean-type architecture by checking one by
one a number of possible routes. In this work, we describe a special type of
hardware for the TSP solution. It is a magnonic combinatorial device comprising
magnetic and electric parts connected in the active ring circuit. There is a
number of possible propagation routes in the magnetic mesh made of phase
shifters, frequency filters, and attenuators. The phase shifters mimic cities
in TSP while the distance between the cities is encoded in the signal
attenuation. The set of frequency filters makes the waves on different
frequencies propagate through the different routes. The principle of operation
is based on the classical wave superposition. There is a number of waves coming
in all possible routes in parallel accumulating different phase shifts and
amplitude damping. However, only the wave(s) that accumulates the certain phase
shift will be amplified by the electric part. The amplification comes first to
the waves that possess the minimum propagation losses. It makes this type of
device suitable for TSP solution, where waves are similar to the salesmen
traveling in all possible routes at a time. We present the results of numerical
modeling illustrating the TSP solutions for four and six cities. Also, we
present experimental data for the TSP solution with four cities.",2302.04996v1
2023-03-29,Stability of superconducting gap symmetries arising from antiferromagnetic magnons,"We consider a planar heterostructure consisting of a normal metal in
proximity to an antiferromagnetic insulator, with an interlayer exchange
coupling between the metal and the insulator. The coupling to the two
sublattices of the antiferromagnetic insulator is allowed to be asymmetric. An
effective electron-electron interaction in the normal metal, mediated by
antiferromagnetic magnons in the insulator, is derived to second order in the
interlayer exchange coupling. Particular emphasis is placed on including
analytically derived expressions for the effective interactions including
Umklapp processes in the solutions to the superconducting gap equation. The gap
equation is first solved at the critical temperature as an eigenvalue problem
by linearizing the gap equation. The eigenvectors yield information on the
symmetry of the superconducting gap at the onset of superconductivity, and we
derive a phase diagram for the order parameter in this case. In the various
regimes of the phase diagram, we find $p$-wave, $f$-wave, and $d$-wave
superconductivity, with $p$-wave superconductivity in the dominant part of the
phase diagram. Umklapp processes, that come into play with increasing size of
the Fermi surface, yield $f$- and $d$-wave symmetries as the preferred
symmetries when band filling approaches half filling. To investigate the
stability of this order parameter symmetry as the temperature is lowered, we
also consider the nonlinear gap equation at zero temperature. We conclude that
the phase diagram and the symmetries of the superconducting order parameter
essentially are left intact as the temperature is lowered to zero temperature.",2303.16916v2
2023-04-19,Dipole superfluid hydrodynamics,"We construct a theory of hydrodynamic transport for systems with conserved
dipole moment, U(1) charge, energy, and momentum. These models have been
considered in the context of fractons, since their elementary and isolated
charges are immobile by symmetry, and have two known translation-invariant
gapless phases: a ""p-wave dipole superfluid"" phase where the dipole symmetry is
spontaneously broken and a ""s-wave dipole superfluid"" phase where both the U(1)
and dipole symmetries are spontaneously broken. We argue on grounds of symmetry
and thermodynamics that there is no transitionally-invariant gapless fluid with
unbroken dipole symmetry. In this work, we primarily focus on the hydrodynamic
description of p-wave dipole superfluids, including leading dissipative
corrections. That theory has, in a sense, a dynamical scaling exponent $z=2$,
and its spectrum of fluctuations includes novel subdiffusive modes $\omega \sim
-i k^4$ in the shear sector and magnon-like sound mode $\omega\sim \pm k^2 -i
k^2$. By coupling the fluid to background fields, we find response functions of
the various symmetry currents. We also present a preliminary generalization of
our work to s-wave dipole superfluids, which resemble $z=1$ fluids and feature
sound waves and diffusive shear modes, as in an ordinary fluid. However, the
spectrum also contains a magnon-like second-sound mode $\omega\sim \pm k^2 \pm
k^4 -i k^4$ with subdiffusive attenuation.",2304.09852v2
2023-05-15,Scattering of magnetostatic surface modes of ferromagnetic films by geometric defects,"Magnonics, an emerging field of Magnetism, studies spin waves (SWs) in
nano-structures, with an aim towards possible applications. As information may
be eventually transmitted with efficiency stored in the phase and amplitude of
spin waves, a topic of interest within Magnonics is the propagation of SW
modes. Thus, understanding mechanisms that may influence SW propagation is of
interest. Here the effect of localized surface geometric defects on
magnetostatic surface modes propagation is studied in ferromagnetic films and
semi-infinite media. Theoretical results are developed that allow to calculate
the scattering of these surface or Damon-Eshbach (DE) modes. A Green-Extinction
theorem is used to determine the scattering of incident surface modes, through
the determination of phase shifts of associated modes that are symmetric and
anti-symmetric under inversion in the same geometry with geometric defects.
Choosing localized symmetric depressions as geometric defects, scattering
transmission coefficients are determined that show perfect transmission at
specific frequencies or wave-lengths, that we associate with resonances in the
system. Interestingly the system shows the appearance of localized modes in the
depression regions, with associated discrete frequencies immersed in the
continuum spectrum of these surface DE modes. These localized modes have a
short wave-length content, and appear similarly in semi-infinite surfaces with
depressions. The latter indicates that these types of scattering effects should
appear in all surfaces with roughness or more pronounced geometric defects.",2305.08749v1
2023-06-27,"Three-dimensional spin-wave dynamics, localization and interference in a synthetic antiferromagnet","Spin waves are collective perturbations in the orientation of the magnetic
moments in magnetically ordered materials. Their rich phenomenology is
intrinsically three dimensional, from the trajectory of the spin precession
during their propagation, to the profiles of the spin-wave mode throughout the
volume of the magnetic system. This gives rise to novel complex phenomena with
high potential for applications in the field of magnonics. However, the
three-dimensional imaging of spin waves, key to understanding and harnessing
these phenomena, has so far not been possible. Here, we image the
three-dimensional dynamics of spin waves excited in a synthetic
antiferromagnet, with nanoscale spatial resolution and sub-ns temporal
resolution, using time-resolved magnetic laminography. In this way, we map the
distribution of the spin-wave modes throughout the volume of the structure,
revealing unexpected depth-dependent profiles originating from the interlayer
dipolar interaction. We experimentally demonstrate the existence of complex
three-dimensional interference patterns, and analyze them via micromagnetic
modelling. We find that these patterns are generated by the superposition of
spin waves with non-uniform amplitude profiles, and that their features can be
controlled by tuning the composition and structure of the magnetic system. Our
results open unforeseen possibilities for the study of complex spin-wave modes
and their interaction within nanostructures, and for the generation and
manipulation of three-dimensional spin-wave landscapes for the design of novel
functions in magnonic devices.",2306.15404v1
2023-07-06,Spin transport from order to disorder,"Schwinger boson mean-field theory (SBMFT) is a non-perturbative approach
which treats ordered and disordered phases of magnetic systems on equal
footing. We leverage its versatility to evaluate the spin correlators which
determine thermally-induced spin transport (the spin Seebeck effect) in
Heisenberg ferromagnets (FMs) and antiferromagnets (AFs), at arbitrary
temperatures. In SBMFT, the spin current, $J_s$, is made up of
particle-hole-like excitations which carry integral spin angular momentum. Well
below the ordering temperature, $J_s$ is dominated by a magnonic contribution,
reproducing the behavior of a dilute-magnon gas. Near the transition
temperature, an additional, paramagnetic-like contribution becomes significant.
In the AF, the two contributions come with opposite signs, resulting in a
signature, rapid inversion of the spin Seebeck coefficient as a function of
temperature. Ultimately, at high temperatures, the low-field behavior of the
paramagnetic SSE reduces to Curie-Weiss physics. Analysis based on our theory
confirms that in recent experiments on gadolinium gallium garnet, the low-field
spin Seebeck coefficient $\mathcal{S}(T) \propto \chi(T)$, the spin
susceptibility, down to the Curie-Weiss temperature. At lower temperatures in
the disordered phase, our theory shows a deviation of $\mathcal{S}(T)$ relative
to $\chi(T)$ in both FMs and AFs, which increases with decreasing temperature
and arises due to a paramagnetic liquid phase in our theory. These results
demonstrate that the SSE can be a probe of the short-ranged magnetic
correlations in disordered correlated spin systems and spin liquids.",2307.02734v1
2023-07-12,Anisotropic in-plane heat transport of Kitaev magnet Na$_2$Co$_2$TeO$_6$,"We report a study on low-temperature heat transport of Kitaev magnet
Na$_2$Co$_2$TeO$_6$, with the heat current and magnetic fields along the
honeycomb spin layer (the $ab$ plane). The zero-field thermal conductivity of
$\kappa^a_{xx}$ and $\kappa^{a*}_{xx}$ display similar temperature dependence
and small difference in their magnitudes; whereas, their magnetic field
(parallel to the heat current) dependence are quite different and are related
to the field-induced magnetic transitions. The $\kappa^a_{xx}(B)$ data for $B
\parallel a$ at very low temperatures have an anomaly at 10.25--10.5 T, which
reveals an unexplored magnetic transition. The planar thermal Hall conductivity
$\kappa^a_{xy}$ and $\kappa^{a*}_{xy}$ show very weak signals at low fields and
rather large values with sign change at high fields. This may point to a
possible magnetic structure transition or the change of the magnon band
topology that induces a radical change of magnon Berry curvature distribution
before entering the spin polarized state. These results put clear constraints
on the high-field phase and the theoretical models for Na$_2$Co$_2$TeO$_6$.",2307.06316v1
2023-07-14,Magnonic Combinatorial Memory,"In this work, we consider a type of magnetic memory where information is
encoded into the mutual arrangements of magnets. The device is an active ring
circuit comprising magnetic and electronic parts connected in series. The
electric part includes a broad-band amplifier, phase shifters, and attenuators.
The magnetic part is a mesh of magnonic waveguides with magnets placed on the
waveguide junctions. There are amplitude and phase conditions for
auto-oscillations to occur in the active ring circuit. The frequency(s) of the
auto-oscillation and spin wave propagation route(s) in the magnetic part
depends on the mutual arrangement of magnets in the mesh. The propagation route
is detected with a set of power sensors. The correlation between circuit
parameters and spin wave route is the base of memory operation. The combination
of input/output switches connecting electric and magnetic parts, and electric
phase shifters constitute the memory address. The output of power sensors is
the memory state. We present experimental data on the proof-of-the-concept
experiments on the prototype with just three magnets placed on top of a
single-crystal yttrium iron garnet Y3Fe2(FeO4)3 (YIG) film. The results
demonstrate a robust operation with On/Off ratio for route detection exceeding
35 dB at room temperature. The number of propagation routes scales factorial
with the size of the magnetic part. Coding information in propagation routes
makes it possible to drastically increase the data storage density compared to
conventional memory devices. MCM with just 25 magnets can store as much as 25!
(10 Yotta) bits. Physical limits and constraints are also discussed.",2307.07464v1
2023-07-30,Spin dynamics in ordered phases of anisotropic triangular-lattice antiferromagnet Cs2CoBr4,"We study spin dynamics of ordered phases of Cs2CoBr4 in a magnetic field
using electron spin resonance (ESR) technique and theoretical analysis. This
material hosts weakly interacting distorted-triangular-lattice planes of
spin-3/2 Co(2+) ions which can be viewed as spin chains coupled by frustrating
interactions. Strong single-ion anisotropy allows to describe the low-energy
spin dynamics of this system by an effective strongly anisotropic
pseudospin-1/2 model. Our ESR data show up to seven branches of magnetic
resonance in four magnetic phases arising due to subtle interplay of
frustration, low dimensionality and strong anisotropy. In particular, in the
low-field collinear stripe phase, the field evolution of modes lying below 200
GHz is described reasonably good by spectra of spin-1 and spin-0 quasiparticles
which we obtain using the bond-operator technique. These well-defined
excitations can be treated as conventional magnons and bound states of two
magnons, respectively. In contrast, numerous excitations lying above 200 GHz
are not captured by our theory due to pronounced one-dimensional correlations
inside spin chains which govern the spin dynamics at high enough energies. As
it was shown before, these modes can be most naturally interpreted as bound
states of domain walls in individual chains and their sequence resembles the
so-called ""Zeeman ladder"" in anisotropic Ising-like spin chains. Thus, Cs2CoBr4
is a system showing spin-dynamics in ordered state characteristic of both
two-dimensional and one-dimensional magnets.",2307.16251v1
2023-08-15,Tuning the confinement potential between spinons in the Ising chain CoNb2O6 using longitudinal fields and quantitative determination of the microscopic Hamiltonian,"The Ising chain realizes the fundamental paradigm of spin fractionalization,
where locally flipping a spin creates two domain walls (spinons) that can
separate apart at no energy cost. In a quasi-one-dimensional system, the
mean-field effects of the weak three-dimensional couplings confine the spinons
into a Zeeman ladder of two-spinon bound states. Here, we experimentally tune
the confinement potential between spinons in the quasi-one-dimensional Ising
ferromagnet CoNb2O6 by means of an applied magnetic field with a large
component along the Ising direction. Using high-resolution single crystal
inelastic neutron scattering, we directly observe how the spectrum evolves from
the limit of very weak confinement at low field (with many closely-spaced bound
states with energies scaling as the field strength to the power 2/3) to very
strong confinement at high field (where it consists of a magnon and a
dispersive two-magnon bound state, with a linear field dependence). At
intermediate fields, we explore how the higher-order bound states disappear
from the spectrum as they move to higher energies and overlap with the
two-particle continuum. By performing a global fit to the observed spectrum in
zero field and high field applied along two orthogonal directions, combined
with a quantitative parameterization of the interchain couplings, we propose a
refined single chain and interchain Hamiltonian that quantitatively reproduces
all observed dispersions and their field dependence.",2308.07699v2
2023-10-17,Low-energy electronic interactions in ferrimagnetic Sr2CrReO6 thin films,"We reveal in this study the fundamental low-energy landscape in the
ferrimagnetic Sr2CrReO6 double perovskite and describe the underlying
mechanisms responsible for the three low-energy excitations below 1.4 eV. Based
on resonant inelastic x-ray scattering and magnetic dynamics calculations, and
experiments collected from both Sr2CrReO6 powders and epitaxially strained thin
films, we reveal a strong competition between spin-orbit coupling, Hund's
coupling, and the strain-induced tetragonal crystal field. We also demonstrate
that a spin-flip process is at the origin of the lowest excitation at 200 meV,
and we bring insights into the predicted presence of orbital ordering in this
material. We study the nature of the magnons through a combination of ab initio
and spin-wave theory calculations, and show that two nondegenerate magnon bands
exist and are dominated either by rhenium or chromium spins. The rhenium band
is found to be flat at about 200 meV ($\pm$25 meV) through X-L-W-U
high-symmetry points and is dispersive toward $\Gamma$",2310.11585v1
2023-11-06,Exploiting polarization dependence in two dimensional coherent spectroscopy: examples of Ce$_2$Zr$_2$O$_7$ and Nd$_2$Zr$_2$O$_7$,"Two dimensional coherent spectroscopy (2DCS) probes the nonlinear optical
response of correlated systems. An interesting application is the study of
fractionalized excitations, which are challenging to distinguish unambiguously
in linear response. Here we demonstrate how the sensitivity of optical matrix
elements to variations in the photon polarization allows one to probe different
aspects of low lying excitations in models of the candidate fractionalized
materials Ce$_2$Zr$_2$O$_7$ and Nd$_2$Zr$_2$O$_7$, which host effective
one-dimensional spin chains when subjected to a [110] magnetic field. We show
how both fractionalized spinon excitations or conventional magnons can be
picked out in the 2DCS response, and how the response from polarized spin
chains can be used to probe the dipolar-octupolar mixing angle $\theta$ through
the relative intensity of one- and two-magnon signals. Further, we find that a
$[001]$ polarization of the probe field is particularly sensitive to lower band
edge of the spinon continuum and can be used as a measure of the proximity of a
quantum critical point in Ce$_2$Zr$_2$O$_7$. 2DCS can thus be employed to
provide invaluable and detailed information both on the constituent degrees of
freedom of a quantum material and on their collective behaviour.",2311.03200v1
2023-11-23,Two-dimensional coherent spectrum of high-spin models via a quantum computing approach,"We present and benchmark a quantum computing approach to calculate the
two-dimensional coherent spectrum (2DCS) of high-spin models. Our approach is
based on simulating their real-time dynamics in the presence of several
magnetic field pulses, which are spaced in time. We utilize the adaptive
variational quantum dynamics simulation (AVQDS) algorithm for the study due to
its compact circuits, which enables simulations over sufficiently long times to
achieve the required resolution in frequency space. Specifically, we consider
an antiferromagnetic quantum spin model that incorporates Dzyaloshinskii-Moriya
interactions and single-ion anisotropy. The obtained 2DCS spectra exhibit
distinct peaks at multiples of the magnon frequency, arising from transitions
between different eigenstates of the unperturbed Hamiltonian. By comparing the
one-dimensional coherent spectrum with 2DCS, we demonstrate that 2DCS provides
a higher resolution of the energy spectrum. We further investigate how the
quantum resources scale with the magnitude of the spin using two different
binary encodings of the high-spin operators: the standard binary encoding and
the Gray code. At low magnetic fields both encodings require comparable quantum
resources, but at larger field strengths the Gray code is advantageous. Lastly,
we compare the numerical 2DCS with experimental results on a rare-earth
orthoferrite system. The observed strength of the magnonic high-harmonic
generation signals in the 2DCS of the quantum high-spin model aligns well with
the experimental data, showing significant improvement over the corresponding
mean-field results.",2311.14035v2
2023-12-14,"Nonlocal damping of spin waves in a magnetic insulator induced by normal, heavy, or altermagnetic metallic overlayer: a Schwinger-Keldysh field theory approach","Understanding spin wave (SW) damping, and how to control it to the point of
being able to amplify SW-mediated signals, is one of the key requirements to
bring the envisaged magnonic technologies to fruition. Even widely used
magnetic insulators with low magnetization damping in their bulk, such as
yttrium iron garnet, exhibit 100-fold increase in SW damping due to inevitable
contact with metallic layers in magnonic circuits, as observed in very recent
experiments [I. Bertelli et al., Adv. Quantum Technol. 4, 2100094 (2021)]
mapping SW damping in spatially-resolved fashion. Here, we provide microscopic
and rigorous understanding of wavevector-dependent SW damping using extended
Landau-Lifshitz-Gilbert equation with nonlocal damping tensor, instead of
conventional local scalar Gilbert damping, as derived from Schwinger-Keldysh
nonequilibrium quantum field theory. In this picture, the origin of nonlocal
magnetization damping and thereby induced wavevector-dependent SW damping is
interaction of localized magnetic moments of magnetic insulator with conduction
electrons from the examined three different types of metallic overlayers --
normal, heavy, and altermagnetic. Due to spin-split energy-momentum dispersion
of conduction electrons in the latter two cases, the nonlocal damping is
anisotropic in spin and space, and it can be dramatically reduced by changing
the relative orientation of the two layers when compared to the usage of normal
metal overlayer.",2312.09140v1
2023-12-16,Topological phase transitions and thermal Hall effect in a noncollinear spin texture,"The noncollinear spin textures provide promising avenues to stabilize exotic
magnetic phases and excitations. They have attracted vast attention owning to
their nontrivial band topology in the past decades. Distinct from the
conventional route of involving the Dzyaloshinskii-Moriya interaction in a
honeycomb magnet, the interplay of bond-dependent Kitaev and $\Gamma$
interactions, originating from the spin-orbit coupling and octahedra crystal
field in real materials, has demonstrated to be another source to generate
noncollinear spin textures with multiple spins in a magnetic unit cell.
Notably, earlier works have revealed a triple-meron crystal (TmX) consisting of
eighteen spins in the frustrated Kitaev-$\Gamma$ model. Aligning with previous
efforts, here we attempt to identity that the TmX hosts several peculiar
features with the help of the linear spin-wave theory. To begin with, the
symmetric anisotropic exchanges are beneficial for the existence of
nonreciprocal magnons, which are stabilized by an external magnetic field.
Further, within the regime of TmX, successive topological phase transitions
occur, accompanied by the changes of Chern number in value and thermal Hall
conductivity in sign. In addition, topological nature of magnons is also
verified by the onset of chiral edge modes in a nanoribbon geometry. Our
findings pave the way to study topological phenomena of noncollinear spin
textures in potential Kitaev materials.",2312.10473v1
2024-01-12,Scanning spin probe based on magnonic vortex quantum cavities,"Performing nanoscale scanning electron paramagnetic resonance (EPR) requires
three essential ingredients. First, a static magnetic field together to field
gradients to Zeeman split the electronic energy levels with spatial resolution.
Second, a radiofrequency (rf) magnetic field capable of inducing spin
transitions. Finally, a sensitive detection method to quantify the energy
absorbed by spins. This is usually achieved by combining externally applied
magnetic fields with inductive coils or cavities, fluorescent defects or
scanning probes. Here, we theoretically propose the realization of a EPR
scanning sensor merging all three characteristics into a single device: the
vortex core stabilized in ferromagnetic thin-film discs. On one hand, the
vortex ground state generates a significant static magnetic field and field
gradients. On the other hand, the precessional motion of the vortex core around
its equilibrium position produces a circularly polarized oscillating magnetic
field, which is enough to produce spin transitions. Finally, the spin-magnon
coupling broadens the vortex gyrotropic frequency, suggesting a direct measure
of the presence of unpaired electrons. Moreover, the vortex core can be
displaced by simply using external magnetic fields of a few mT, enabling EPR
scanning microscopy with large spatial resolution. Our numerical simulations
show that, by using low damping magnets, it is theoretically possible to detect
single spins located on the disc's surface. Vortex nanocavities could also
attain strong coupling to individual spin molecular qubits, with potential
applications to mediate qubit-qubit interactions or to implement qubit readout
protocols.",2401.06549v1
2024-01-25,Thermal and Magnetoelastic Properties of the van der Waals Ferromagnet Fe$_{3-δ}$GeTe$_2$: Anisotropic Spontaneous Magnetostriction and Ferromagnetic Magnon Excitations,"By determining the lattice parameters as a function of temperature of the
hexagonal van der Waals ferromagnet Fe$_{2.92(1)}$Ge$_{1.02(3)}$Te$_2$ we
obtain the temperature dependence of the spontaneous in-plane magnetostriction
in the ferromagnetic and the linear thermal expansion coefficients in the
paramagnetic state. The spontaneous magnetostriction is clearly seen in the
temperature dependence of the in-plane lattice parameter $a(T)$, but less well
pronounced perpendicular to the planes along $c$. Below $T_{\rm C}$ the
spontaneous magnetostriction follows the square of the magnetization and leads
to an expansion of the hexagonal layers. Extrapolating to $T\rightarrow$ 0~K we
obtain a spontaneous in-plane saturation magnetostriction of $\lambda_{{\rm
sp},a}(T \rightarrow 0) \approx-220 ~\times~10^{-6}$. In the paramagnetic state
the linear thermal expansion coefficients amount to
13.9(1)$\times$10$^{-6}$~K$^{-1}$ and to 23.2(2)$\times$10$^{-6}$~K$^{-1}$ for
the in-plane and out-of-plane direction, respectively, indicating a linear
volume thermal expansion coefficient of 50.8(4)$\times$10$^{-6}$K$^{-1}$ which
we use to estimate the volume thermal expansion contribution to the heat
capacity determined at constant pressure. A Sommerfeld-type linear term in the
low-temperature heat capacities can be quantitatively ascribed to 2dim
ferromagnetic magnon excitations.",2401.14050v1
2024-01-31,Magnetic polarons beyond linear spin-wave theory: Mesons dressed by magnons,"When a mobile hole is doped into an antiferromagnet, its movement will
distort the surrounding magnetic order and yield a magnetic polaron. The
resulting complex interplay of spin and charge degrees of freedom gives rise to
very rich physics and is widely believed to be at the heart of high-temperature
superconductivity in cuprates. In this paper, we develop a quantitative
theoretical formalism, based on the phenomenological parton description, to
describe magnetic polarons in the strong coupling regime. We construct an
effective Hamiltonian with weak coupling to the spin-wave excitations in the
background, making the use of standard polaronic methods possible. Our starting
point is a single hole doped into an AFM described by a 'geometric string'
capturing the strongly correlated hopping processes of charge and spin degrees
of freedom, beyond linear spin-wave approximation. Subsequently, we introduce
magnon excitations through a generalized 1/S expansion and derive an effective
coupling of these spin-waves to the hole plus the string (the meson) to arrive
at an effective polaron Hamiltonian with density-density type interactions.
After making a Born-Oppenheimer-type approximation, this system is solved using
the self-consistent Born approximation to extract the renormalized polaron
properties. We apply our formalism (i) to calculate beyond linear spin-wave
ARPES spectra, (ii) to reveal the interplay of ro-vibrational meson
excitations, and (ii) to analyze the pseudogap expected at low doping.
Moreover, our work paves the way for exploring magnetic polarons out-of
equilibrium or in frustrated systems, where weak-coupling approaches are
desirable and going beyond linear spin-wave theory becomes necessary.",2402.00130v1
2024-02-02,Structural and optical characterization of NiO polycrystalline thin films fabricated by spray-pyrolysis,"Nickel (II) oxide, NiO, a wide band gap Mott insulator characterized by
strong Coulomb repulsion between d-electrons and displaying antiferromagnetic
order at room temperature, has gained attention in recent years as a very
promising candidate for applications in a broad set of areas, including
chemistry and metallurgy to spintronics and energy harvesting. Here, we report
on the synthesis of polycrystalline NiO fabricated using spray-pyrolysis
technique, which is a deposition technique able to produce quite uniform films
of pure and crystalline materials without the need of high vacuum or inert
atmospheres. We then characterized the composition and structure of our NiO
thin films using X-ray diffraction, and atomic force and scanning electron
microscopies, respectively. We completed our study by looking at the phononic
and magnonic properties of our NiO thin films via Raman spectroscopy, and at
the ultrafast electron dynamics by using optical pump probe spectroscopy. We
found that our NiO samples display the same phononic and magnonic dispersion
expected for single crystal NiO at room temperature, and that electron dynamics
in our system is similar to those of previously reported NiO mono- and
poli-crystalline systems synthesized with different techniques. These results
prove that spray-pyrolysis can be used as affordable and large-scale
fabrication technique to synthetize strongly correlated materials for a large
set of applications.",2402.01437v1
2024-02-29,Resonant generation of propagating second-harmonic spin waves in nano-waveguides,"Generation of second-harmonic waves is one of the universal nonlinear
phenomena that have found numerous technical applications in many modern
technologies, in particular, in photonics. This phenomenon also has great
potential in the field of magnonics, which considers the use of spin waves in
magnetic nanostructures to implement wave-based signal processing and
computing. However, due to the strong frequency dependence of the phase
velocity of spin waves, resonant phase-matched generation of second-harmonic
spin waves has not yet been achieved in practice. Here, we show experimentally
that such a process can be realized using a combination of different modes of
nano-sized spin-wave waveguides based on low-damping magnetic insulators. We
demonstrate that our approach enables efficient spatially-extended energy
transfer between interacting waves, which can be controlled by the intensity of
the initial wave and the static magnetic field. The demonstrated approach can
be used for the generation of short-wavelength spin waves that are difficult to
excite directly, as well as for the implementation of novel devices for
magnonic logic and unconventional computing.",2402.18964v1
2024-03-15,"Microscopic Modeling, Multi-magnon condensation and Strain effect in chain compound NaVOPO$_4$","Employing first-principles modelling and many-body methods, the magnetic
properties of spin-1/2 chain compound NaVOPO$_4$ are explored. The extensive
first-principles calculations establish an intricate three-dimensionally
coupled model that consists of weakly alternating $J$-$J^{'}$ antiferromagnetic
chains running along cris-cross directions between two consecutive $ab$ planes,
connected via two subleading couplings, a ferromagnetic exchange along the $c$
direction ($J_c$) and a weaker antiferromagnetic exchange ($J_a$) along the
body diagonal direction. The exact diagonalization and density matrix
renormalized group study have been carried out on a two-dimensional spin model
with $J$-$J^{'}$-$J_c$ and effective $J_d$ couplings, constructed based on the
full model, for numerical ease. The $J_c$-$J_d$ phase diagram is found to host
a {\it disorder} phase with finite spin gap for comparable values of $J_c$ and
$J_d$, arising out of the competing nature of these two interactions, other
than two ordered phases. The predominant manifestation of $J_c$ and $J_d$ in
the disorder phase happens in stabilisation of a multi-magnon condensed phase,
upon gap closing by application of an external magnetic field. We further
explore the effect of tensile uniaxial strain, which is found to drive the
system from gapful to gapless ground state.",2403.10025v1
2024-04-03,Discovery of universal phonon thermal Hall effect in crystals,"Thermal Hall effect (THE) in insulator is a remarkable phenomenon that arises
from the motion of chargeless quasi-particles under a magnetic field. While
magnons or exotic spin excitations were considered as the origin of THE in some
magnetic materials, there are more and more evidences suggest that phonons play
a significant role. However, the mechanism behind phonon THE is still unknown.
Here we report the observation of THE, including planar THE, in a broad range
of non-magnetic insulators and semiconductor: SrTiO$_3$, SiO$_2$ (quartz), MgO
and Si. While the presence of antiferrodistortive domains in SrTiO$_3$ and
chiral phonons in SiO$_2$ may complicate the interpretation of THE, the
striking observations of THE in the trivial insulator MgO and high-purity
intrinsic semiconductor Si demonstrate that phonon THE is a universal property
of crystals. Without other effects on phonons such as from magnons, this
universal phonon THE is characterized by a scaling law of |$\kappa_{xy}$|
$\sim$ $\kappa_{xx}^2$. Our results experimentally discover a fundamental
physics of phonons in magnetic field, which must come from the direct coupling
between atom vibrations and the field. Starting from this universal phonon THE
in crystals, all previous interpretations of THE in magnetic or non-magnetic
materials need to be reconsidered.",2404.02863v1
2024-04-06,Non-volatile spin transport in a single domain multiferroic,"Antiferromagnets have attracted significant attention in the field of
magnonics, as promising candidates for ultralow-energy carriers for information
transfer for future computing. The role of crystalline orientation distribution
on magnon transport has received very little attention. In multiferroics such
as BiFeO$_3$ the coupling between antiferromagnetic and polar order imposes yet
another boundary condition on spin transport. Thus, understanding the
fundamentals of spin transport in such systems requires a single domain, a
single crystal. We show that through Lanthanum(La) substitution, a single
ferroelectric domain can be engineered with a stable, single-variant spin
cycloid, controllable by an electric field. The spin transport in such a single
domain displays a strong anisotropy, arising from the underlying spin cycloid
lattice. Our work shows a pathway to understand the fundamental origins of spin
transport in such a single domain multiferroic.",2404.04746v1
1997-05-06,Resonant Raman Scattering in Antiferromagnets,"Two-magnon Raman scattering provides important information about electronic
correlations in the insulating parent compounds of high-$T_c$ materials. Recent
experiments have shown a strong dependence of the Raman signal in $B_{1g}$
geometry on the frequency of the incoming photon. We present an analytical and
numerical study of the Raman intensity in the resonant regime. It has been
previously argued by one of us (A.Ch) and D. Frenkel that the most relevant
contribution to the Raman vertex at resonance is given by the triple resonance
diagram. We derive an expression for the Raman intensity in which we
simultaneously include the enhancement due to the triple resonance and a final
state interaction. We compute the two-magnon peak height (TMPH) as a function
of incident frequency and find two maxima at $\omega^{(1)}_{res} \approx
2\Delta + 3J$ and $\omega^{(2)}_{res} \approx 2\Delta + 8J$. We argue that the
high-frequency maximum is cut only by a quasiparticle damping, while the
low-frequency maximum has a finite amplitude even in the absence of damping. We
also obtain an evolution of the Raman profile from an asymmetric form around
$\omega^{(1)}_{res}$ to a symmetric form around $\omega^{(2)}_{res}$. We
further show that the TMPH depends on the fermionic quasiparticle damping, the
next-nearest neighbor hopping term $t^{\prime}$ and the corrections to the
interaction vertex between light and the fermionic current. We discuss our
results in the context of recent experiments by Blumberg et al. on
$Sr_2CuO_2Cl_2$ and $YBa_2Cu_3O_{6.1}$ and R\""{u}bhausen et al. on
$PrBa_2Cu_3O_7$ and show that the triple resonance theory yields a qualitative
and to some extent also quantitative understanding of the experimental data.",9705051v1
1999-06-03,Optical spectroscopic study of the interplay of spin and charge in NaV2O5,"We investigate the temperature dependent optical properties of NaV2O5, in the
energy range 4meV-4eV. The symmetry of the system is discussed on the basis of
infrared phonon spectra. By analyzing the optically allowed phonons at
temperatures below and above the phase transition, we conclude that a
second-order change to a larger unit cell takes place below 34 K, with a
fluctuation regime extending over a broad temperature range. In the high
temperature undistorted phase, we find good agreement with the recently
proposed centrosymmetric space group Pmmn. On the other hand, the detailed
analysis of the electronic excitations detected in the optical conductivity,
provides direct evidence for a charge disproportionated electronic
ground-state, at least on a locale scale: A consistent interpretation of both
structural and optical conductivity data requires an asymmetrical charge
distribution on each rung, without any long range order. We show that, because
of the locally broken symmetry, spin-flip excitations carry a finite electric
dipole moment, which is responsible for the detection of direct two-magnon
optical absorption processes for E parallel to the a axis. The charged-magnon
model, developed to interpret the optical conductivity of NaV2O5, is described
in detail, and its relevance to other strongly correlated electron systems,
where the interplay of spin and charge plays a crucial role in determining the
low energy electrodynamics, is discussed.",9906042v2
2005-10-07,Magnetic and Charge Correlations in La{2-x-y}Nd_ySr_xCuO_4: Raman Scattering Study,"Two aspects in connection with the magnetic properties of
La_{2-x-y}Nd_ySr_xCuO_4 single crystals are discussed. The first is related to
long wavelength magnetic excitations in x = 0, 0.01, and 0.03 La_{2-x}Sr_xCuO_4
detwinned crystals as a function of doping, temperature and magnetic field. Two
magnetic modes were observed within the AF region of the phase diagram. The one
at lower energies was identified with the spin-wave gap induced by the
antisymmetric DM interaction and its anisotropic properties in magnetic field
could be well explained using a canonical form of the spin Hamiltonian. A new
finding was a magnetic field induced mode whose dynamics allowed us to discover
a spin ordered state outside the AF order which was shown to persist in a 9 T
field as high as 100 K above the N\'eel temperature T_N for x = 0.01. For these
single magnon excitations we map out the Raman selection rules in magnetic
fields and demonstrate that their temperature dependent spectral weight is
peaked at the N\'eel temperature. The second aspect is related to phononic and
magnetic Raman scattering in La_{2-x-y}Nd_ySr_xCuO_4 with three doping
concentrations: x = 1/8, y = 0; x = 1/8, y = 0.4; and x = 0.01, y = 0. We
observed that around 1/8 Sr doping and independent of Nd concentration there
exists substantial disorder in the tilt pattern of the CuO_6 octahedra in both
the orthorhombic and tetragonal phases which persist down to 10 K and are
coupled to bond disorder in the cation layers. The weak magnitude of existing
charge/spin modulations in the Nd doped structure did not allow us to detect
specific Raman signatures on lattice dynamics or two-magnon scattering around
2200 cm-1.",0510191v1
2005-12-08,High-field Magnetic Torque Measurements in (CH$_3$)$_2$CHNH$_3$CuCl$_3$,"High-field magnetic torque measurements were carried out in the spin gap
system (CH$_3$)$_2$CHNH$_3$CuCl$_3$. It was observed that the magnitude of the
magnetic torque $\tau$ is almost zero until the critical field and then
increases rapidly, which indicates the existence of the magnetic quantum phase
transition from the spin-gap phase to the field-induced magnetic ordered phase.
  From the temperature dependence of the magnetic torque $\tau$, the cusp-like
minimum indicative of the Bose-Einstein condensation (BEC) of magnons was
observed for $H \perp$ C-plane, while the kink-anomaly was observed for $H
\perp$ A-plane at the field-induced transition temperature. It was found that
this different behavior between $H \perp$ A-plane and $H \perp$ C-plane can be
interpreted as the breaking of the magnon BEC picture due to the rotational
symmetry breaking. The additional anomaly was also observed in the
field-induced magnetic ordered phase at the intermediate angle of the applied
magnetic field in the A-plane and discussed in terms of the spin-flop
transition, the spin-reorientation transition and the spin-lattice
correlations. In order to investigate the spin-lattice correlations in
(CH$_3$)$_2$CHNH$_3$CuCl$_3$, we performed the magnetostriction measurements
and observed that the magnetostriction appears by vanishing of the spin gap.",0512164v2
2008-08-25,Low energy theory of the t-t'-t''-U Hubbard Model at half-filling: interaction strengths in cuprate superconductors and an effective spin-only description of La_2CuO_4,"Spin-only descriptions of the half-filled one-band Hubbard model are relevant
for a wide range of Mott insulators. In addition to the usual Heisenberg
exchange, many new types of interactions, including ring exchange, appear in
the effective Hamiltonian in the intermediate coupling regime. In order to
improve on the quantitative description of magnetic excitations in the
insulating antiferromagnetic phase of copper-oxide (cuprate) materials, and to
be consistent with band structure calculations and photoemission experiments on
these systems, we include second and third neighbor hopping parameters, t' and
t'', into the Hubbard Hamiltonian. A unitary transformation method is used to
find systematically the effective Hamiltonian and any operator in the spin-only
representation. The results include all closed, four hop electronic pathways in
the canonical transformation. The method generates many ring exchange terms
that play an important role in the comparison with experiments on La2CuO4.
Performing a spin wave analysis, we calculate the magnon dispersion as a
function of U,t,t' and t''. The four parameters are estimated by fitting the
magnon dispersion to the experimental results of Coldea et al. [Phys. Rev.
Lett. 86, 5377, {2001}] for La2CuO4. The ring exchange terms are found
essential, in particular to determine the relative sign of t' and t'', with the
values found in good agreement with independent theoretical and experimental
estimates for other members of the cuprate family. The zero temperature
sublattice magnetization is calculated using these parameters and also found to
be in good agreement with the experimental value estimated by Lee et al. [Phys.
Rev. B 60, 3643 (1999)]. We find a value of the interaction strength U~8t
consistent with Mott insulating behavior.",0808.3167v1
2009-10-02,"On the room temperature multiferroic BiFeO3: Magnetic, dielectric and thermal properties","Magnetic dc susceptibility between 1.5 and 800 K, ac susceptibility and
magnetization, thermodynamic properties, temperature dependence of radio and
audio-wave dielectric constants and conductivity, contact-free dielectric
constants at mm-wavelengths, as well as ferroelectric polarization are reported
for single crystalline BiFeO3. A well developed anomaly in the magnetic
susceptibility signals the onset of antiferromagnetic order close to 635 K.
Beside this anomaly no further indications of phase or glass transitions are
indicated in the magnetic dc and ac susceptibilities down to the lowest
temperatures. The heat capacity has been measured from 2 K up to room
temperature and significant contributions from magnon excitations have been
detected. From the low-temperature heat capacity an anisotropy gap of the
magnon modes of the order of 6 meV has been determined. The dielectric
constants measured in standard two-point configuration are dominated by
Maxwell-Wagner like effects for temperatures T > 300 K and frequencies below 1
MHz. At lower temperatures the temperature dependence of the dielectric
constant and loss reveals no anomalies outside the experimental errors,
indicating neither phase transitions nor strong spin phonon coupling. The
temperature dependence of the dielectric constant was measured contact free at
microwave frequencies. At room temperature the dielectric constant has an
intrinsic value of 53. The loss is substantial and strongly frequency dependent
indicating the predominance of hopping conductivity. Finally, in small thin
samples we were able to measure the ferroelectric polarization between 10 and
200 K. The saturation polarization is of the order of 40 microC/cm^2,
comparable to reports in literature.",0910.0385v2
2013-10-25,Quantum magnons of the intermediate phase of half-doped manganite oxides,"At half doping, the ground state of three-dimensional manganite perovskite
oxides like R$_{1-x}$Ca$_x$MnO$_3$, where R is a trivalent ion such as La, Pr,
etc, is still unclear. Many experimental findings agree better with the
combined magnetic, charge, and orbital order characteristic of the
""intermediate phase"", introduced by Efremov et al. in 2004 [Nature Mats. 3,
853]. This phase consists of spin dimers (thus incorporating aspects of the
Zener polaron phase (ZP) proposed in 2002 by Daoud-Aladine et al. [Phys. Rev.
Lett. 89, 097205]), though formed by a pair of parallel Mn spins of different
magnitude, in principle (thereby allowing for a degree of Mn charge
disproportionation: not necessarily as large as that of Mn$^{3+}$-Mn$^{4+}$ in
Goodenough's original CE phase [Phys. Rev. 100, 564 (1955)]). In the
intermediate phase, consecutive spin dimers localed along the planar zig-zag
chains are oriented at a constant relative angle $\Phi$ between them. Varying
Mn-charge disproportionation and $\Phi$, the intermediate phase should allow to
continuously interpolate between the two limiting cases of the CE phase and the
dimer phase denoted as ""orthogonal intermediate $\pi/2-$phase"". It is not easy
to find a microscopic model able to describe the phenomenological intermediate
phase adequately for the spin, charge, and orbital degrees of freedom
simultaneously. Here, we study the quantum spin excitations of a planar model
of interacting localized spins, which we found can stabilize the intermediate
phase classically. We compare the quantum magnons of the intermediate phase
with those of the CE and orthogonal $\pi/2$ phases, in the context of recent
experimental results.",1310.6954v1
2015-11-30,Generalized isotropic Lipkin-Meshkov-Glick models: ground state entanglement and quantum entropies,"We introduce a new class of generalized isotropic Lipkin-Meshkov-Glick models
with su$(m+1)$ spin and long-range non-constant interactions, whose
non-degenerate ground state is a Dicke state of su$(m+1)$ type. We evaluate in
closed form the reduced density matrix of a block of $L$ spins when the whole
system is in its ground state, and study the corresponding von Neumann and
R\'enyi entanglement entropies in the thermodynamic limit. We show that both of
these entropies scale as $a\log L$ when $L$ tends to infinity, where the
coefficient $a$ is equal to $(m-k)/2$ in the ground state phase with $k$
vanishing su$(m+1)$ magnon densities. In particular, our results show that none
of these generalized Lipkin-Meshkov-Glick models are critical, since when
$L\to\infty$ their R\'enyi entropy $R_q$ becomes independent of the parameter
$q$. We have also computed the Tsallis entanglement entropy of the ground state
of these generalized su$(m+1)$ Lipkin-Meshkov-Glick models, finding that it can
be made extensive by an appropriate choice of its parameter only when
$m-k\ge3$. Finally, in the su$(3)$ case we construct in detail the phase
diagram of the ground state in parameter space, showing that it is determined
in a simple way by the weights of the fundamental representation of su$(3)$.
This is also true in the su$(m+1)$ case; for instance, we prove that the region
for which all the magnon densities are non-vanishing is an $(m+1)$-simplex in
$\mathbf R^m$ whose vertices are the weights of the fundamental representation
of su$(m+1)$.",1511.09346v2
2016-04-28,"Highly-anisotropic exchange interactions of $j_{\rm eff}=1/2$ iridium moments on the fcc lattice in La$_2B$IrO$_6$ ($B$ $=$ Mg, Zn)","We have performed inelastic neutron scattering (INS) experiments to
investigate the magnetic excitations in the weakly distorted
face-centered-cubic (fcc) iridate double perovskites La$_2$ZnIrO$_6$ and
La$_2$MgIrO$_6$, which are characterized by A-type antiferromagnetic ground
states. The powder inelastic neutron scattering data on these geometrically
frustrated $j_{\rm eff}=1/2$ Mott insulators provide clear evidence for gapped
spin wave excitations with very weak dispersion. The INS results and
thermodynamic data on these materials can be reproduced by conventional
Heisenberg-Ising models with significant uniaxial Ising anisotropy and sizeable
second-neighbor ferromagnetic interactions. Such a uniaxial Ising exchange
interaction is symmetry-forbidden on the ideal fcc lattice, so that it can only
arise from the weak crystal distortions away from the ideal fcc limit. This may
suggest that even weak distortions in $j_{\rm eff}=1/2$ Mott insulators might
lead to strong exchange anisotropies. More tantalizingly, however, we find an
alternative viable explanation of the INS results in terms of spin models with
a dominant Kitaev interaction. In contrast to the uniaxial Ising exchange, the
highly-directional Kitaev interaction is a type of exchange anisotropy which is
symmetry-allowed even on the ideal fcc lattice. The Kitaev model has a magnon
gap induced by quantum order-by-disorder, while weak anisotropies of the Kitaev
couplings generated by the symmetry-lowering due to lattice distortions, can
pin the order and enhance the magnon gap. Our findings highlight how even
conventional magnetic orders in heavy transition metal oxides may be driven by
highly-directional exchange interactions rooted in strong spin-orbit coupling.",1604.08431v1
2018-02-08,Numerical Algorithm for Exact Finite Temperature Spectra and Its Application to Frustrated Quantum Spin Systems,"A numerical algorithm to calculate exact finite-temperature spectra of
many-body lattice Hamiltonians is formulated by combining the typicality
approach and the shifted Krylov subspace method. The combined algorithm, which
we name finite-temperature shifted Krylov subspace method for simulating
spectra (FTK$\omega$), efficiently reproduces the canonical-ensemble
probability distribution at finite temperatures with the computational cost
proportional to the Fock space dimension. The present FTK$\omega$ enables us to
exactly calculate finite-temperature spectra of many-body systems whose system
sizes are twice larger than those handled by the canonical ensemble average and
allows us to access the frequency domain without sequential real-time evolution
often used in previous studies. By employing the reweighting method with the
present algorithm, we obtain significant reduction of the numerical costs for
temperature sweeps. Application to the Kiteav-Heisenberg model (KHM) on a
honeycomb lattice demonstrates the capability of the FTK$\omega$. The KHM shows
quantum phase transitions from the quantum spin liquid (QSL) phase to
magnetically ordered phases when the finite Heisenberg exchange coupling is
introduced. We examine temperature dependence of dynamical spin structure
factors of the KHM in proximity to the QSL. It is clarified that the crossover
from a spin-excitation continuum, which is a characteristic of the QSL, to a
damped high-energy magnon mode occurs at temperatures higher than the energy
scale of the Heisenberg couplings or the spin gap that is a signature of the
QSL at zero temperature. The crossover and the closeness to the Kitaev's QSL
are quantitatively measured by the width of the excitation continuum or the
magnon spectrum. The present results shed new light on analysis of neutron
scattering and other spectroscopy measurements on QSL candidates.",1802.02854v1
2018-11-05,Reply to Comment on Optical detection of transverse spin-Seebeck effect in permalloy film using Sagnac interferometer microscopy (arXiv:1810.13220),"In their Comment, Kimling and Kuschel (hereafter the commenters) challenge
our original interpretation of the magneto-optical Kerr effect (MOKE)
measurements using ultrasensitive Sagnac interferometer, claiming that the
transverse spin-Seebeck effect (TSSE) is not the only contribution to the
measured change in the Kerr rotation angle from a Ni80Fe20 (NiFe) ferromagnetic
(FM) slab subjected to a lateral temperature gradient in the presence of a
magnetic field. The authors assert that: (1) the TSSE, in general, has not been
completely proven so far, and that the existing theories, in particular the
phonon magnon drag model that we have used in our publication, in fact cannot
explain the original work of Uchida et al. (2) The commenters based their
critique on an estimate of the magnitude of the measured effect, arguing that
to observe the TSSE that we originally claimed, the temperature gradient in our
measurements should have been much larger than the value we measured. In this
Reply, firstly, we summarize previous literature reports on the TSSE response
in FM metallic systems. Secondly, we dispute the estimate of the necessary
large temperature gradient made by the commenters. Importantly, the Kerr effect
sensitivity to spin accumulation (via the TSSE response) has not been
recognized by the commenters, in fact it was mistakenly assumed that the Kerr
effect sensitivity to the spin accumulation by the TSSE is the same as the Kerr
sensitivity to the bulk magnetization change due to the temperature change. In
conclusion, we show that the TSSE is the only viable interpretation of our
original measurements, and that the phonon-magnon drag model is indeed capable
of explaining our results.",1811.01877v1
2014-03-02,Magnons and a two-component spin gap in FeV2O4,"The spinel vanadates have become a model family for exploring orbital order
on the frustrated pyrochlore lattice, and recent debate has focused on the
symmetry of local crystal fields at the cation sites. Here, we present neutron
scattering measurements of the magnetic excitation spectrum in
$\mathrm{FeV_2O_4}$, a recent example of a ferrimagnetic spinel vanadate which
is available in single crystal form. We report the existence of two emergent
magnon modes at low temperatures, which draw strong parallels with the closely
related material, $\mathrm{MnV_2O_4}$. We were able to reproduce the essential
elements of both the magnetic ordering pattern and the dispersion of the
inelastic modes with semi- classical spin wave calculations, using a minimal
model that implies a sizeable single-ion anisotropy on the vanadium sublattice.
Taking into account the direction of ordered spins, we associate this
anisotropy with the large trigonal distortion of $\mathrm{VO_6}$ octahedra,
previously observed via neutron powder diffraction measurements. We further
report on the spin gap, which is an order-of-magnitude larger than that
observed in $\mathrm{MnV_2O_4}$. By looking at the overall temperature
dependence, we were able to show that the gap magnitude is largely associated
with the ferro-orbital order known to exist on the iron sublattice, but the
contribution to the gap from the vanadium sublattice is in fact comparable to
what is reported in the Mn compound. This reinforces the conclusion that the
spin canting transition is associated with the ordering of vanadium orbitals in
this system, and closer analysis indicates closely related physics underlying
orbital transitions in $\mathrm{FeV_2O_4}$ and $\mathrm{MnV_2O_4}$.",1403.0269v2
2018-05-22,Frequency linewidth and decay length of spin waves in curved magnetic membranes,"The curvature of a magnetic membrane was presented as a means of inducing
nonreciprocities in the spin-wave (SW) dispersion relation (see [Ot\'alora et
al. Phys. Rev. Lett., 2016 117, 227203] and [Ot\'alora et al. Phys. Rev. B.,
2017 95, 184415]), thereby expanding the toolbox for controlling SWs. In this
paper, we further complement this toolbox by analytically showing that the
membrane curvature is also manifested in the absorption of SWs, leading to a
difference in the frequency linewidth (or lifetime) of counterpropagating
magnons. Herein, we studied the nanotubular case, predicting changes of
approximately greater than 10% and up to 20% in the frequency linewidth of
counterpropagating SWs for a wide range of nanotube radii ranging from 30 nm to
260 nm and with a thickness of 10 nm. These percentages are comparable to those
that can be extracted from experiments on heavy metal/magnetic metal
sandwiches, wherein linewidth asymmetry results from an interfacial
Dzyaloshinskii-Moriya interaction (DMI). We also show that the interplay
between the frequency linewidth and group velocity leads to asymmetries in the
SW decay length, presenting changes between 10\% and 22\% for
counterpropagating SWs in the frequency range of 2 - 10 GHz. For the case of
the SW dispersion relation, the predicted effects are identified as the
classical dipole-dipole interaction, and the analytical expression of the
frequency linewidth has the same mathematical form as in thin films with the
DMI. Furthermore, we present limiting cases of a tubular geometry with
negligible curvature such that our analytical model converges to the case of a
planar thin film known from the literature. Our findings represent a step
forward toward the realization of three-dimensional curvilinear magnonic
devices.",1805.08827v1
2017-06-30,Comprehensive study of the dynamics of a classical Kitaev spin liquid,"We study the spin-$S$ Kitaev model in the classical ($S \to \infty$) limit
using Monte Carlo simulations combined with semi-classical spin dynamics. We
discuss differences and similarities in the dynamical structure factors of the
spin-$1/2$ and the classical Kitaev liquids. Interestingly, the low-temperature
and low-energy spectrum of the classical model exhibits a finite energy peak,
which is the precursor of the one produced by the Majorana modes of the $S=1/2$
model. The classical peak is spectrally narrowed compared to the quantum result
and can be explained by magnon excitations within fluctuating one-dimensional
manifolds (loops). Hence the difference from the classical limit to the quantum
limit can be understood by the fractionalization of magnons propagating in
one-dimensional manifolds. Moreover, we show that the momentum space
distribution of the low-energy spectral weight of the $S=1/2$ model follows the
momentum space distribution of zero modes of the classical model.",1706.10242v2
2017-10-17,Spin - Phonon Coupling in Nickel Oxide Determined from Ultraviolet Raman Spectroscopy,"Nickel oxide (NiO) has been studied extensively for various applications
ranging from electrochemistry to solar cells [1,2]. In recent years, NiO
attracted much attention as an antiferromagnetic (AF) insulator material for
spintronic devices [3-10]. Understanding the spin - phonon coupling in NiO is a
key to its functionalization, and enabling AF spintronics' promise of
ultra-high-speed and low-power dissipation [11,12]. However, despite its status
as an exemplary AF insulator and a benchmark material for the study of
correlated electron systems, little is known about the spin - phonon
interaction, and the associated energy dissipation channel, in NiO. In
addition, there is a long-standing controversy over the large discrepancies
between the experimental and theoretical values for the electron, phonon, and
magnon energies in NiO [13-23]. This gap in knowledge is explained by NiO
optical selection rules, high Neel temperature and dominance of the magnon band
in the visible Raman spectrum, which precludes a conventional approach for
investigating such interaction. Here we show that by using ultraviolet (UV)
Raman spectroscopy one can extract the spin - phonon coupling coefficients in
NiO. We established that unlike in other materials, the spins of Ni atoms
interact more strongly with the longitudinal optical (LO) phonons than with the
transverse optical (TO) phonons, and produce opposite effects on the phonon
energies. The peculiarities of the spin - phonon coupling are consistent with
the trends given by density functional theory calculations. The obtained
results shed light on the nature of the spin - phonon coupling in AF insulators
and may help in developing innovative spintronic devices.",1710.06386v1
2018-09-06,Spin-current-mediated rapid magnon localisation and coalescence after ultrafast optical pumping of ferrimagnetic alloys,"Sub-picosecond magnetisation manipulation via femtosecond optical pumping has
attracted wide attention ever since its original discovery in 1996. However,
the spatial evolution of the magnetisation is not yet well understood, in part
due to the difficulty in experimentally probing such rapid dynamics. Here, we
find evidence of rapid magnetic order recovery in materials with perpendicular
magnetic anisotropy via nonlinear magnon processes. We identify both
localisation and coalescence regimes, whereby localised magnetic textures
nucleate and subsequently evolve in accordance with a power law formalism.
Coalescence is observed for optical excitations both above and below the
switching threshold. Simulations indicate that the ultrafast generation of
noncollinear magnetisation via optical pumping establishes exchange-mediated
spin currents with an equivalent 100% spin polarised charge current density of
$10^8$ A/cm$^2$. Such large spin currents precipitate rapid recovery of
magnetic order after optical pumping. These processes suggest an ultrafast
optical route for the stabilization of desired meta-stable states, e.g.,
isolated skyrmions.",1809.02076v2
2015-08-06,Physical properties and electronic structure of Sr$_2$Cr$_3$As$_2$O$_2$ containing CrO$_2$ and Cr$_2$As$_2$ square-planar lattices,"We report the physical properties and electronic structure calculations of a
layered chromium oxypnictide, Sr$_2$Cr$_3$As$_2$O$_2$, which crystallizes in a
Sr$_2$Mn$_3$As$_2$O$_2$-type structure containing both CrO$_2$ planes and
Cr$_2$As$_2$ layers. The newly synthesized material exhibits a metallic
conduction with a dominant electron-magnon scattering. Magnetic and
specific-heat measurements indicate at least two intrinsic magnetic transitions
below room temperature. One is an antiferromagnetic transition at 291 K,
probably associated with a spin ordering in the Cr$_2$As$_2$ layers. Another
transition is broad, occurring at around 38 K, and possibly due to a
short-range spin order in the CrO$_2$ planes. Our first-principles calculations
indicate predominant two-dimensional antiferromagnetic exchange couplings, and
suggest a KG-type (i.e. K$_2$NiF$_4$ type for CrO$_2$ planes and G type for
Cr$_2$As$_2$ layers) magnetic structure, with reduced moments for both Cr
sublattices. The corresponding electronic states near the Fermi energy are
mostly contributed from Cr-3$d$ orbitals which weakly (modestly) hybridize with
the O-2$p$ (As-4$p$) orbitals in the CrO$_2$ (Cr$_2$As$_2$) layers. The bare
bandstructure density of states at the Fermi level is only $\sim$1/4 of the
experimental value derived from the low-temperature specific-heat data,
consistent with the remarkable electron-magnon coupling. The title compound is
argued to be a possible candidate to host superconductivity.",1508.01371v1
2016-03-14,Field-driven quantum phase transitions in $S=1/2$ spin chains,"We study the magnetization process of a 1D extended Heisenberg model, the
$J$-$Q$ model, as a function of an external magnetic field. In this model, $J$
represents the traditional antiferromagnetic Heisenberg exchange and $Q$ is the
strength of a competing four-spin interaction. Without external field, this
system hosts a twofold-degenerate dimerized (valence-bond solid) state above a
critical value $q_c\approx 0.85$ where $q\equiv Q/J$. The dimer order is
destroyed and replaced by a partially polarized translationally invariant state
at a critical field value. We find magnetization jumps (metamagnetism) between
the partially polarized and fully polarized state for $q>q_{\rm min}$, where we
have calculated $q_{\rm min}=2/9$ exactly. For $q>q_{\rm min}$ two magnons
(flipped spins on a fully polarized background) attract and form a bound state.
Quantum Monte Carlo studies confirm that the bound state corresponds to the
first step of an instability leading to a finite magnetization jump for
$q>q_{\rm min}$. Our results show that neither geometric frustration nor
spin-anisotropy are necessary conditions for metamagnetism. Working in the
two-magnon subspace, we also find evidence pointing to the existence of
metamagnetism in the unfrustrated $J_1$-$J_2$ chain ($J_1>0$, $J_2<0$), but
only if $J_2$ is spin-anisotropic. We also investigate quantum-critical scaling
near the transition into the fully polarized state for $q\le q_{\rm min}$ at
$T>0$. While the expected `zero-scale-factor' universality is clearly seen for
$q=0$ and $q\ll q_{\rm min}$; closer to $q_{\rm min}$ we find that extremely
low temperatures are required to observe the asymptotic behavior, due to the
influence of the tricritical point at $q_{\rm min}$, which leads to a
cross-over at a temperature $T^*(q)$ between logarithmic tricritical scaling
and zero-scale-factor universality, with $T^*(q)\to 0$ when $q\to q_{\rm min}$.",1603.04359v3
2020-06-17,Recent Progress on Electrical Excitation and Manipulation of Spin-Waves in Spin Hall Nano-Oscillators,"Spin waves (SWs), the collective precessional motion of spins in a magnetic
system, have been proposed as a promising alternative system with low-power
consumption for encoding information. Spin Hall nano-oscillator (SHNO), a
new-type spintronic nano-device, can electrically excite and control spin waves
in both nanoscale magnetic metals and insulators with low damping by the spin
current due to spin Hall effect. Here, we will review recent progress about
spin-wave excitation and experimental parameters dependent spectrum in SHNOs.
The nanogap SHNOs based on in-plane magnetization Py/Pt exhibits a nonlinear
self-localized bullet soliton localized at the center of the gap between the
electrodes and a secondary high-frequency mode which coexists with the primary
bullet mode at higher currents. While in the nanogap SHNOs with strong
perpendicular magnetic anisotropy (PMA), besides both nonlinear bullet soliton
and propagating spin-wave mode are achieved and controlled by varying the
external magnetic field and current, the magnetic bubble skyrmion mode also can
be excited at a low in-plane magnetic field. These SW modes show
thermal-induced mode hopping behavior at high temperature due to the coupling
between modes mediated by thermal-magnon-mediated scattering. Moreover, thanks
to PMA-induced effective field, a single coherent mode also can be achieved
without applying an external magnetic field. The strong nonlinear effect of
spin-waves makes SHNOs easy to achieve synchronization with external microwave
signals or mutual synchronization between multiple oscillators with improving
the coherence and power of oscillation modes significantly. Spin-waves in SHNOs
with an external free magnetic layer have a wide range of applications from as
a nanoscale signal source of low-power consumption magnonic devices to
spin-based neuromorphic computing systems in the field of artificial
intelligence.",2006.09737v2
2021-02-04,Magnon-spinon dichotomy in the Kitaev hyperhoneycomb $β$-Li$_2$IrO$_3$,"The family of edge-sharing tri-coordinated iridates and ruthenates has
emerged in recent years as a major platform for Kitaev spin liquid physics,
where spins fractionalize into emergent magnetic fluxes and Majorana fermions
with Dirac-like dispersions. While such exotic states are usually pre-empted by
long-range magnetic order at low temperatures, signatures of Majorana fermions
with long coherent times have been predicted to manifest at intermediate and
higher energy scales, similar to the observation of spinons in quasi-1D spin
chains. Here we present a Resonant Inelastic X-ray Scattering study of the
magnetic excitations of the hyperhoneycomb iridate $\beta$-Li$_2$IrO$_3$ under
a magnetic field with a record-high-resolution spectrometer. At
low-temperatures, dispersing spin waves can be resolved around the predicted
intertwined incommensurate spiral and field-induced zigzag orders, whose
excitation energy reaches a maximum of 16meV. A 2T magnetic field softens the
dispersion around ${\bf Q}=0$. The behavior of the spin waves under magnetic
field is consistent with our semiclassical calculations for the ground state
and the dynamical spin structure factor, which further predicts that the ensued
intertwined uniform states remain robust up to very high fields (100 T). Most
saliently, the low-energy magnon-like mode is superimposed by a broad continuum
of excitations, centered around 35meV and extending up to 100meV. This
high-energy continuum survives up to at least 300K -- well above the ordering
temperature of 38K -- and gives evidence for pairs of long-lived Majorana
fermions of the proximate Kitaev spin liquid.",2102.02714v2
2017-04-13,Exact diagonalization and cluster mean-field study of triangular-lattice XXZ antiferromagnets near saturation,"Quantum magnetic phases near the magnetic saturation of triangular-lattice
antiferromagnets with XXZ anisotropy have been attracting renewed interest
since it has been suggested that a nontrivial coplanar phase, called the
$\pi$-coplanar or $\Psi$ phase, could be stabilized by quantum effects in a
certain range of anisotropy parameter $J/J_z$ besides the well-known 0-coplanar
(known also as $V$) and umbrella phases. Recently, Sellmann $et$ $al$. [Phys.
Rev. B {\bf 91}, 081104(R) (2015)] claimed that the $\pi$-coplanar phase is
absent for $S=1/2$ from an exact-diagonalization analysis in the sector of the
Hilbert space with only three down-spins (three magnons). We first reconsider
and improve this analysis by taking into account several low-lying eigenvalues
and the associated eigenstates as a function of $J/J_z$ and by sensibly
increasing the system sizes (up to 1296 spins). A careful identification
analysis shows that the lowest eigenstate is a chirally antisymmetric
combination of finite-size umbrella states for $J/J_z\gtrsim 2.218$ while it
corresponds to a coplanar phase for $J/J_z\lesssim 2.218$. However, we
demonstrate that the distinction between 0-coplanar and $\pi$-coplanar phases
in the latter region is fundamentally impossible from the symmetry-preserving
finite-size calculations with fixed magnon number.} Therefore, we also perform
a cluster mean-field plus scaling analysis for small spins $S\leq 3/2$. The
obtained results, together with the previous large-$S$ analysis, indicate that
the $\pi$-coplanar phase exists for any $S$ except for the classical limit
($S\rightarrow \infty$) and the existence range in $J/J_z$ is largest in the
most quantum case of $S=1/2$.",1704.04024v2
2017-04-13,Low energy magnon dynamics and magneto-optics of the skyrmionic Mott insulator Cu$_2$OSeO$_3$,"In this work, we present a comprehensive study of the low energy optical
magnetic response of the skyrmionic Mott insulator Cu$_2$OSeO$_3$ via high
resolution time-domain THz spectroscopy. In zero field, a new magnetic
excitation not predicted by spin-wave theory with frequency $f$ = 2.03 THz is
observed and shown, with accompanying time-of-flight neutron scattering
experiments, to be a zone folded magnon from the $\mathrm{R}$ to
$\mathrm{\Gamma}$ points of the Brillouin zone. Highly sensitive polarimetry
experiments performed in weak magnetic fields, $\mu_0$H $<$ 200 mT, observe
Faraday and Kerr rotations which are proportional to the sample magnetization,
allowing for optical detection of the skyrmion phase and construction of a
magnetic phase diagram. From these measurements, we extract a critical exponent
of $\beta$ = 0.35 $\pm$ 0.04, in good agreement with the expected value for the
3D Heisenberg universality class of $\beta$ = 0.367. In large magnetic fields,
$\mu_0$H $>$ 5 T, we observe the magnetically active uniform mode of the
ferrimagnetic field polarized phase whose dynamics as a function of field and
temperature are studied. In addition to extracting a $g_\text{eff}$ = 2.08
$\pm$ 0.03, we observe the uniform mode to decay through a non-Gilbert damping
mechanism and to possesses a finite spontaneous decay rate, $\Gamma_0$
$\approx$ 25 GHz, in the zero temperature limit. Our observations are
attributed to Dzyaloshinkii-Moriya interactions, which have been proposed to be
exceptionally strong in Cu$_2$OSeO$_3$ and are expected to impact the low
energy magnetic response of such chiral magnets.",1704.04228v1
2020-01-26,Magnetic field-induced quantum phase transitions in a van der Waals magnet,"Exploring new parameter regimes to realize and control novel phases of matter
has been a main theme in modern condensed matter physics research. The recent
discovery of 2D magnetism in nearly freestanding monolayer atomic crystals has
already led to observations of a number of novel magnetic phenomena absent in
bulk counterparts. Such intricate interplays between magnetism and crystalline
structures provide ample opportunities for exploring quantum phase transitions
in this new 2D parameter regime. Here, using magnetic field and temperature
dependent circularly polarized Raman spectroscopy of phonons and magnons, we
map out the phase diagram of CrI3 that has been known to be a layered AFM in
its 2D films and a FM in its 3D bulk. We, however, reveal a novel mixed state
of layered AFM and FM in 3D CrI3 bulk crystals where the layered AFM survives
in the surface layers and the FM appears in deeper bulk layers. We then show
that the surface layered AFM transits into the FM at a critical magnetic field
of 2 T, similar to what was found in the few layer case. Interestingly,
concurrent with this magnetic phase transition, we discover a first-order
structural phase transition that alters the crystallographic point group from
C3i to C2h and thus, from a symmetry perspective, this monoclinic structural
phase belongs to the 3D nematic order universality class. Our result not only
unveils the complex single magnon behavior in 3D CrI3, but also settles down
the puzzle of how CrI3 transits from a bulk FM to a thin layered AFM
semiconductor, despite recent efforts in understanding the origin of layered
AFM in CrI3 thin layer, and reveals the intimate relationship between the
layered AFM-to-FM and the crystalline rhombohedral-to-monoclinic phase
transitions. These findings further open up opportunities for future 2D
magnet-based magneto-mechanical devices.",2001.09536v1
2020-05-15,"Thermal generation, manipulation and detection of skyrmions","Recent years have witnessed significant progresses in realizing skyrmions in
chiral magnets1-4 and asymmetric magnetic multilayers5-13, as well as their
electrical manipulation2,7,8,10. Equally important, thermal generation,
manipulation and detection of skyrmions can be exploited for prototypical new
architecture with integrated computation14 and energy harvesting15. It has yet
to verify if skyrmions can be purely generated by heating16,17, and if their
resultant direction of motion driven by temperature gradients follows the
diffusion or, oppositely, the magnonic spin torque17-21. Here, we address these
important issues in microstructured devices made of multilayers:
(Ta_CoFeB_MgO)15, (Pt_CoFeB_MgO_Ta)15 and (Pt_Co_Ta)15 integrated with on-chip
heaters, by using a full-field soft X-ray microscopy. The thermal generation of
densely packed skyrmions is attributed to the low energy barrier at the device
edge, together with the thermally induced morphological transition from stripe
domains to skyrmions. The unidirectional diffusion of skyrmions from the hot
region towards the cold region is experimentally observed. It can be
theoretically explained by the combined contribution from repulsive forces
between skyrmions, and thermal spin-orbit torques in competing with magnonic
spin torques17,18,20,21 and entropic forces22. These thermally generated
skyrmions can be further electrically detected by measuring the accompanied
anomalous Nernst voltages23. The on-chip thermoelectric generation,
manipulation and detection of skyrmions could open another exciting avenue for
enabling skyrmionics, and promote interdisciplinary studies among spin
caloritronics15, magnonics24 and skyrmionics3,4,12.",2005.07441v1
2020-08-29,Exploring a quantum-information-relevant magnonic material: Ultralow damping at low temperature in the organic ferrimagnet V[TCNE]x,"Quantum information science and engineering requires novel low-loss magnetic
materials for magnon-based quantum-coherent operations. The search for low-loss
magnetic materials, traditionally driven by applications in microwave
electronics near room-temperature, has gained additional constraints from the
need to operate at cryogenic temperatures for many applications in quantum
information science and technology. Whereas yttrium iron garnet (YIG) has been
the material of choice for decades, the emergence of molecule-based materials
with robust magnetism and ultra-low damping has opened new avenues for
exploration. Specifically, thin-films of vanadium tetracyanoethylene (V[TCNE]x)
can be patterned into the multiple, connected structures needed for hybrid
quantum elements and have shown room-temperature Gilbert damping ({\alpha} = 4
\times 10^-5) that rivals the intrinsic (bulk) damping otherwise seen only in
highly-polished YIG spheres (far more challenging to integrate into arrays).
Here, we present a comprehensive and systematic study of the low-temperature
magnetization dynamics for V[TCNE]x thin films, with implications for their
application in quantum systems. These studies reveal a temperature-driven,
strain-dependent magnetic anisotropy that compensates the thin-film shape
anisotropy, and the recovery of a magnetic resonance linewidth at 5 K that is
comparable to room-temperature values (roughly 2 G at 9.4 GHz). We can account
for these variations of the V[TCNE]x linewidth within the context of scattering
from very dilute paramagnetic impurities, and anticipate additional linewidth
narrowing as the temperature is further reduced.",2008.13061v3
2021-04-15,Quantum many-body states and Green functions of nonequilibrium electron-magnon systems: Localized spin operators vs. their mapping to Holstein-Primakoff bosons,"The operators of localized spins within a magnetic material commute at
different sites of its lattice and anticommute on the same site, so they are
neither fermionic nor bosonic operators. Thus, to construct diagrammatic
many-body perturbation theory, the spin operators are usually mapped to the
bosonic ones with Holstein-Primakoff (HP) transformation being the most widely
used in magnonics and spintronics literature. However, to make calculations
tractable, the square root of operators in the HP transformation is expanded
into a Taylor series truncated to some low order. This poses a question on the
range of validity of truncated HP transformation when describing nonequilibrium
dynamics of localized spins interacting with each other or with conduction
electron spins. Here we apply exact diagonalization techniques to Hamiltonian
of fermions (i.e., electrons) interacting with HP bosons vs. Hamiltonian of
fermions interacting with the original localized spin operators in order to
compare their many-body states and one-particle equilibrium or nonequilibrium
Green functions. The Hamiltonian of fermions interacting with HP bosons gives
incorrect ground state and electronic spectral function, unless large number of
terms are retained in truncated HP transformation. Furthermore, tracking
nonequilibrium dynamics of localized spins over longer time intervals requires
progressively larger number of terms in truncated HP transformation. Finally,
we show that recently proposed [M. Vogl et al., Phys. Rev. Research 2, 043243
(2020); J. K\""{o}nig et al., SciPost Phys. 10, 007 (2021)] resummed HP
transformation resolves the trouble with truncated HP transformation, while
allowing us to derive an exact (manifestly Hermitian) Hamiltonian consisting of
finite and fixed number of boson-boson and electron-boson interacting terms.",2104.07657v2
2021-04-23,Roadmap of spin-orbit torques,"Spin-orbit torque (SOT) is an emerging technology that enables the efficient
manipulation of spintronic devices. The initial processes of interest in SOTs
involved electric fields, spin-orbit coupling, conduction electron spins and
magnetization. More recently interest has grown to include a variety of other
processes that include phonons, magnons, or heat. Over the past decade, many
materials have been explored to achieve a larger SOT efficiency. Recently,
holistic design to maximize the performance of SOT devices has extended
material research from a nonmagnetic layer to a magnetic layer. The rapid
development of SOT has spurred a variety of SOT-based applications. In this
Roadmap paper, we first review the theories of SOTs by introducing the various
mechanisms thought to generate or control SOTs, such as the spin Hall effect,
the Rashba-Edelstein effect, the orbital Hall effect, thermal gradients,
magnons, and strain effects. Then, we discuss the materials that enable these
effects, including metals, metallic alloys, topological insulators,
two-dimensional materials, and complex oxides. We also discuss the important
roles in SOT devices of different types of magnetic layers. Afterward, we
discuss device applications utilizing SOTs. We discuss and compare
three-terminal and two-terminal SOT-magnetoresistive random-access memories
(MRAMs); we mention various schemes to eliminate the need for an external
field. We provide technological application considerations for SOT-MRAM and
give perspectives on SOT-based neuromorphic devices and circuits. In addition
to SOT-MRAM, we present SOT-based spintronic terahertz generators,
nano-oscillators, and domain wall and skyrmion racetrack memories. This paper
aims to achieve a comprehensive review of SOT theory, materials, and
applications, guiding future SOT development in both the academic and
industrial sectors.",2104.11459v2
2021-05-18,Computational Micromagnetics based on Normal Modes: bridging the gap between macrospin and full spatial discretization,"The Landau-Lifshitz equation governing magnetization dynamics is written in
terms of the amplitudes of normal modes associated with the micromagnetic
system's appropriate ground state. This results in a system of nonlinear
ordinary differential equations (ODEs), the right-hand side of which can be
expressed as the sum of a linear term and nonlinear terms with increasing order
of nonlinearity (quadratic, cubic, etc.). The application of the method to
nanostructured magnetic systems demonstrates that the accurate description of
magnetization dynamics requires a limited number of normal modes, which results
in a considerable improvement in computational speed. The proposed method can
be used to obtain a reduced-order dynamical description of magnetic
nanostructures which allows to adjust the accuracy between low-dimensional
models, such as macrospin, and micromagnetic models with full spatial
discretization. This new paradigm for micromagnetic simulations is tested for
three problems relevant to the areas of spintronics and magnonics: directional
spin-wave coupling in magnonic waveguides, high power ferromagnetic resonance
in a magnetic nanodot, and injection-locking in spin-torque nano-oscillators.
The case studies considered demonstrate the validity of the proposed approach
to systematically obtain an intermediate order dynamical model based on normal
modes for the analysis of magnetic nanosystems. The time-consuming calculation
of the normal modes has to be done only one time for the system. These modes
can be used to optimize and predict the system response for all possible
time-varying external excitations (magnetic fields, spin currents). This is of
utmost importance for applications where fast and accurate system simulations
are required, such as in electronic circuits including magnetic devices.",2105.08829v2
2021-06-15,Low-energy spin dynamics in rare-earth perovskite oxides,"We review recent studies of spin dynamics in rare-earth orthorhombic
perovskite oxides of the type $RM$O$_3$, where $R$ is a rare-earth ion and $M$
is a transition-metal ion, using single-crystal inelastic neutron scattering
(INS). After a short introduction to the magnetic INS technique in general, the
results of INS experiments on both transition-metal and rare-earth subsystems
for four selected compounds (YbFeO$_3$, TmFeO$_3$, YFeO$_3$, YbAlO$_3$) are
presented. We show that the spectrum of magnetic excitations consists of two
types of collective modes that are well separated in energy: gapped magnons
with a typical bandwidth of $<$70 meV, associated with the
antiferromagnetically (AFM) ordered transition-metal subsystem, and AFM
fluctuations of $<$5 meV within the rare-earth subsystem, with no hybridization
of those modes. We discuss the high-energy conventional magnon excitations of
the 3$d$ subsystem only briefly, and focus in more detail on the spectacular
dynamics of the rare-earth sublattice in these materials. We observe that the
nature of the ground state and the low-energy excitation strongly depends on
the identity of the rare-earth ion. In the case of non-Kramers ions, the
low-symmetry crystal field completely eliminates the degeneracy of the
multiplet state, creating a rich magnetic field-temperature phase diagram. In
the case of Kramers ions, the resulting ground state is at least a doublet,
which can be viewed as an effective quantum spin-1/2. Equally important is the
fact that in Yb-based materials the nearest-neighbor exchange interaction
dominates in one direction, despite the three-dimensional nature of the
orthoperovskite crystal structure. The observation of a fractional spinon
continuum and quantum criticality in YbAlO$_3$ demonstrates that Kramers
rare-earth based magnets can provide realizations of various aspects of quantum
low-dimensional physics.",2106.08378v1
2021-10-09,Imaging the ultrafast coherent control of a skyrmion crystal,"Exotic magnetic textures emerging from the subtle interplay between
thermodynamic and topological fluctuation have attracted intense interest due
to their potential applications in spintronic devices. Recent advances in
electron microscopy have enabled the imaging of random photo-generated
individual skyrmions. However, their deterministic and dynamical manipulation
is hampered by the chaotic nature of such fluctuations and the intrinsically
irreversible switching between different minima in the magnetic energy
landscape. Here, we demonstrate a method to coherently control the rotation of
a skyrmion crystal by discrete amounts at speeds which are much faster than
previously observed. By employing circularly polarized femtosecond laser pulses
with an energy below the bandgap of the Mott insulator Cu2OSeO3, we excite a
collective magnon mode via the inverse Faraday effect. This triggers coherent
magnetic oscillations that directly control the rotation of a skyrmion crystal
imaged by cryo-Lorentz Transmission Electron Microscopy. The manipulation of
topological order via ultrafast laser pulses shown here can be used to engineer
fast spin-based logical devices.",2110.04548v3
2021-11-26,Enhanced superconductivity and various edge modes in modulated $t$-$J$ chains,"We numerically investigate the ground state of the extended $t$-$J$
Hamiltonian with periodic local modulations in one dimension by using the
density-matrix renormalization group method. Examining charge and spin
excitation gaps, as well as the pair binding energy, with extrapolated results
to the thermodynamic limit, we obtain a rich ground-state phase diagram
consisting of the metallic state, the superconducting state, the phase
separation, and insulating states at commensurate fillings. Compared to the
homogeneous 1D $t$-$J$ model, the superconductivity is greatly enhanced and
stabilized by the flat-band structure. This superconducting state in
quasi-periodic chains shares similar properties with ladder systems:
significant negative pair binding energy occurs, and the singlet pairing
correlation function dominates with the algebraic decay while the
single-particle Green's function and spin correlation function decay
exponentially. On the other hand, quasi-periodicity leads to nontrivial
topological nature in insulating states, characterized by different integer
Chern numbers at different fillings. Due to the interplay among the topology,
the interaction, and the 1D confinement, gapless edge modes show strong
spin-charge separation and in different regions can relate to different
collective modes, which are the charge of a single fermion, the magnon, and the
singlet-pair. We also find two interaction driven topological transitions: i)
at particle filling $\rho=1/2$, the low-energy edge excitations change from the
magnon to singlet-pair, accompanied with pair formation in bulk; and ii) at
$\rho=3/4$, while the gapless edge mode remains the charge of a single fermion,
there is a gap-closing point and a $\pi$-phase shift in the quasi-particle
spectrum.",2111.13718v1
2022-01-10,Ultrafast manipulation of the NiO antiferromagnetic order via sub-gap optical excitation,"Wide-band-gap insulators such as NiO offer the exciting prospect of
coherently manipulating electronic correlations with strong optical fields.
Contrary to metals where rapid dephasing of optical excitation via electronic
processes occurs, the sub-gap excitation in charge-transfer insulators has been
shown to couple to low-energy bosonic excitations. However, it is currently
unknown if the bosonic dressing field is composed of phonons or magnons. Here
we use the prototypical charge-transfer insulator NiO to demonstrate that 1.5
eV sub-gap optical excitation leads to a renormalised NiO band-gap in
combination with a significant reduction of the antiferromagnetic order. We
employ element-specific X-ray reflectivity at the FLASH free-electron laser to
demonstrate the reduction of the upper band-edge at the O 1s-2p core-valence
resonance (K-edge) whereas the antiferromagnetic order is probed via X-ray
magnetic linear dichroism (XMLD) at the Ni 2p-3d resonance (L2-edge). Comparing
the transient XMLD spectral line shape to ground-state measurements allows us
to extract a spin temperature rise of 65 +/- 5 K for time delays longer than
400 fs while at earlier times a non-equilibrium spin state is formed. We
identify transient mid-gap states being formed during the first 200 fs
accompanied by a band-gap reduction lasting at least up to the maximum measured
time delay of 2.4 ps. Electronic structure calculations indicate that magnon
excitations significantly contribute to the reduction of the NiO band gap.",2201.03280v1
2022-04-24,Ground-state phase diagram and thermodynamics of coupled trimer chains,"The density matrix renormalization group and quantum Monte Carlo method are
used to describe coupled trimer chains in a magnetic field h. The Hamiltonian
contains exchange terms involving the intra-trimer coupling J1 (taken as the
unit of energy) and the inter-trimer coupling J2, plus the Zeeman interaction
for a magnetic field h along the z direction. Results for the magnetization per
trimer m are calculated in regimes of positive and negative values of the ratio
J = J2 / J1, from which the rich field-induced ground state phase diagram h vs.
J is derived, with the presence of Luttinger liquid, the 1/3-plateau (m = 1/2),
and the one of fully polarized magnetization (m = 3/2). Also, the zero-field
Lanczos calculation of spin-wave dispersion from the 1/3-plateau for $S^z = 1$
is shown at the previous regimes of J values. In addition, we also report on
the decay of correlation functions of trimers along open chains, as well as the
average two-magnon distribution. The ground state is ferrimagnetic for $0<J\leq
1$, and is a singlet for $-1\leq J<0$. In the singlet phase, the spin
correlation functions along the legs present an antiferromagnetic power-law
decay, similar to the spin-1/2 linear chain, thus suggesting that the ground
state is made of three coupled antiferromagnetically oriented chains. In the
singlet phase, the dimensionless thermal magnetic susceptibility per site
normalized by 1/|J| gets closer to $1/\pi^2$ as the temperature $T\rightarrow
0$. For the ferrimagnetic phase, we fit the susceptibility to the experimental
data for the compound Pb$_3$Cu$_3$(PO$_4$)$_4$ and estimate the model exchange
couplings: J1=74.8 K and J=0.4. These values imply a range of energies for the
magnon excitations that are in accord with the data from neutron scattering
experiments on Pb$_3$Cu$_3$(PO$_4$)$_4$ for two excitation modes. The
1/3-plateau closes only at 1/|J|=0 with J<0.",2204.11186v1
2022-07-29,Non-linear magnons and exchange Hamiltonians of delafossite proximate quantum spin liquids,"Quantum spin liquids (QSL) are theoretical states of matter with long-range
entanglement and exotic quasiparticles. However, they generally elude
quantitative theory, rendering their underlying phases mysterious and hampering
efforts to identify experimental QSL states. Here we study triangular lattice
resonating valence bond QSL candidate materials KYbSe$_2$ and NaYbSe$_2$. We
measure the magnon modes in their 1/3 plateau phase, where quantitative theory
is tractable, using inelastic neutron scattering and fit them using nonlinear
spin wave theory. We also fit the KYbSe$_2$ heat capacity using high
temperature series expansion. Both KYbSe$_2$ fits yield the same magnetic
Hamiltonian to within uncertainty, confirming previous estimates and showing
the Heisenberg $J_2/J_1$ to be an accurate model for these materials. Most
importantly, comparing KYbSe$_2$ and NaYbSe$_2$ shows that smaller $A$-site
Na$^+$ ion has a larger $J_2/J_1$ ratio. However, hydrostatic pressure applied
to KYbSe$_2$ increases the ordering temperature (a result consistent with
density functional theory calculations), indicating that pressure decreases
$J_2/J_1$. These results show how periodic table and hydrostatic pressure can
tune the $A$YbSe$_2$ materials in a controlled way.",2207.14785v3
2023-03-30,Fate of entanglement in magnetism under Lindbladian or non-Markovian dynamics and conditions for their transition to Landau-Lifshitz-Gilbert classical dynamics,"It is commonly assumed in spintronics and magnonics that localized spins
within antiferromagnets are in the N\'{e}el ground state (GS), as well as that
such state evolves, when pushed out of equilibrium by current or external
fields, according to the Landau-Lifshitz-Gilbert (LLG) equation viewing
localized spins as classical vectors of fixed length. On the other hand, the
true GS of antiferromagnets is highly entangled, as confirmed by very recent
neutron scattering experiments witnessing their entanglement. Although GS of
ferromagnets is always unentangled, their magnonic low-energy excitation are
superpositions of many-body spin states and, therefore, entangled. In this
study, we initialize quantum Heisenberg ferro- or antiferromagnetic chains
hosing localized spins $S=1/2$, $S=1$ or $S=5/2$ into unentangled pure state
and then evolve them by quantum master equations (QMEs) of Lindblad or
non-Markovian type, derived by coupling localized spins to a bosonic bath (such
as due to phonons) or by using additional ``reaction coordinate'' in the latter
case. The time evolution is initiated by applying an external magnetic field,
and entanglement of time-evolving {\em mixed} quantum states is monitored by
computing its logarithmic negativity. We find that non-Markovian dynamics
maintains some degree of entanglement, which shrinks the length of the vector
of spin expectation values, thereby making the LLG equation inapplicable.
Conversely, Lindbladian (i.e., Markovian) dynamics ensures that entanglement
goes to zero, thereby enabling quantum-to-classical (i.e., to LLG) transition
in all cases -- $S=1/2$, $S=1$ and $S=5/2$ ferromagnet or $S=5/2$
antiferromagnet -- {\em except} for $S=1/2$ and $S=1$ antiferromagnet. We also
investigate the stability of entangled antiferromagnetic GS upon suddenly
coupling it to the bosonic bath.",2303.17596v3
2023-08-14,Temperature Evolution of Magnon Propagation Length in Tm$_3$Fe$_5$O$_{12}$ Thin Films: Roles of Magnetic Anisotropy and Gilbert Damping,"The magnon propagation length ($\langle\xi\rangle$) of a ferro/ferrimagnet
(FM) is one of the key factors that controls the generation and propagation of
thermally-driven spin current in FM/heavy metal (HM) bilayer based
spincaloritronic devices. Theory predicts that for the FM layer,
$\langle\xi\rangle$ is inversely proportional to the Gilbert damping ($\alpha$)
and the square root of the effective magnetic anisotropy constant ($K_{\rm
eff}$). However, direct experimental evidence of this relationship is lacking.
To experimentally confirm this prediction, we employ a combination of
longitudinal spin Seebeck effect (LSSE), transverse susceptibility, and
ferromagnetic resonance experiments to investigate the temperature evolution of
$\langle\xi\rangle$ and establish its correlation with the effective magnetic
anisotropy field, $H_K^{\rm eff}$ ($\propto K_{\rm eff}$) and $\alpha$ in
Tm$_3$Fe$_5$O$_{12}$ (TmIG)/Pt bilayers. We observe concurrent drops in the
LSSE voltage and $\langle\xi\rangle$ below 200$^\circ$K in TmIG/Pt bilayers
regardless of TmIG film thickness and substrate choice and attribute it to the
noticeable increases in $H_K^{\rm eff}$ and $\alpha$ that occur within the same
temperature range. From the TmIG thickness dependence of the LSSE voltage, we
determined the temperature dependence of $\langle\xi\rangle$ and highlighted
its correlation with the temperature-dependent $H_K^{\rm eff}$ and $\alpha$ in
TmIG/Pt bilayers, which will be beneficial for the development of rare-earth
iron garnet-based efficient spincaloritronic nanodevices.",2308.07236v3
2023-08-18,Large thermo-spin effects in Heusler alloy based spin-gapless semiconductor thin films,"Recently, Heusler alloys-based spin gapless semiconductors (SGSs) with high
Curie temperature (TC) and sizeable spin polarization have emerged as potential
candidates for tunable spintronic applications. We report comprehensive
investigation of the temperature dependent ANE and intrinsic longitudinal spin
Seebeck effect (LSSE) in CoFeCrGa thin films grown on MgO substrates. Our
findings show the anomalous Nernst coefficient for the MgO/CoFeCrGa (95 nm)
film is $\cong 1.86$ micro V/K at room temperature which is nearly two orders
of magnitude higher than that of the bulk polycrystalline sample of CoFeCrGa (=
0.018 micro V/K) but comparable to that of the magnetic Weyl semimetal Co2MnGa
thin film (2-3 micro V/K). Furthermore, the LSSE coefficient for our
MgO/CoFeCrGa(95nm)/Pt(5nm) heterostructure is $\cong 20.5$ $\mu$V/K/$\Omega$ at
room temperature which is twice larger than that of the half-metallic
ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_3$ thin films ($\cong$ 20.5
$\mu$V/K/$\Omega$). We show that both ANE and LSSE coefficients follow
identical temperature dependences and exhibit a maximum at $\cong$ 225 K which
is understood as the combined effects of inelastic magnon scatterings and
reduced magnon population at low temperatures. Our analyses not only indicate
that the extrinsic skew scattering is the dominating mechanism for ANE in these
films but also provide critical insights into the functional form of the
observed temperature dependent LSSE at low temperatures. Furthermore, by
employing radio frequency transverse susceptibility and broadband ferromagnetic
resonance in combination with the LSSE measurements, we establish a correlation
among the observed LSSE signal, magnetic anisotropy and Gilbert damping of the
CoFeCrGa thin films, which will be beneficial for fabricating tunable and
highly efficient Heusler alloys based spincaloritronic nanodevices.",2308.09843v1
2023-12-02,Interplay of topology and antiferromagnetic order in two-dimensional van der Waals crystals of (NixFe1-x)2P2S6,"Mermin-Wagner theorem forbid spontaneous symmetry breaking of spins in
one/two-dimensional systems at finite temperature and rules out the
stabilization of this ordered state. However, it does not apply to all types of
phase transitions in low dimensions such as topologically ordered phase
rigorously shown by Berezinskii-Kosterlitz-Thouless (BKT) and experimentally
realized in very limited systems such as superfluids, superconducting thin
films. Quasi 2D van der Waals magnets provide an ideal platform to investigate
the fundamentals of low-dimensional magnetism. We explored the 2D honeycomb
antiferromagnetic single crystals of (NixFe1-x)2P2S6 with varying spins ( )
using in depth temperature dependent Raman measurements supported by
first-principles calculations of the phonon frequencies. As a function of
doping, a tunable transition from paramagnetic to antiferromagnetic ordering is
shown via phonons reflected in the strong renormalization of the self-energy
parameters of the Raman active phonon modes. An anomalously broad magnetic
continuum attributed to two-magnon excitations is observed and its coupling
with the phonons is revealed in the observation a Fano line asymmetry.
Interestingly, the two-magnon continuum is observed only for the finite doping
understood invoking underlying nature of insulator these materials belongs to,
i.e. exchange interaction between transition metals via surrounding ligands
(sulphur) and the resonance involving phonon modes associated with the (P2S6)
cage. Quite surprisingly, we also observed renormalization of the phonon modes
much below the long-range magnetic ordered temperature attributed to the
topological ordered state, namely the BKT phase, which is also found to change
as a function of doping. The extracted critical exponent of the order-parameter
evince the signature of topologically active state driven by vortex-antivortex
excitations.",2312.01098v1
1993-08-23,The Quartet State of the Two-Dimensional Heisenberg Model with Spin 1/2 on a Square Lattice,"The low-energy properties of the two-dimensional Heisenberg model with
spin-$\frac{1}{2}$ on a square lattice are investigated on the basis of the
local dimer order. The lattice is divided into square blocks consisting of the
quartet of spins. The spin variables and the Heisenberg Hamiltonian are
expressed in terms of the low-energy quartet variables. On the basis of the
Dyson-Maleev representation the spin-wave theory of the quartet state is
developed. The spectrum of the lower magnon excitations consists of three
degenerate modes with the energy gap $\Delta =0.17J$. The ground state energy
per spin $E/N =-0.6J$. This preprint repeats in the main the previous one but
it contains calculations of the basic corrections and therefore has complete
character.",9308030v1
1994-07-06,"Exact spectra, spin susceptibilities and order parameter of the quantum Heisenberg antiferromagnet on the triangular lattice","Exact spectra of periodic samples are computed up to $ N=36 $.
  Evidence of an extensive set of low lying levels, lower than the softest
magnons, is exhibited.
  These low lying quantum states are degenerated in the thermodynamic limit;
their symmetries and dynamics as well as their finite-size scaling are strong
arguments in favor of N\'eel order.
  It is shown that the N\'eel order parameter agrees with first-order spin-wave
calculations. A simple explanation of the low energy dynamics is given as well
as the numerical determinations of the energies, order parameter and spin
susceptibilities of the studied samples. It is shown how suitable boundary
conditions, which do not frustrate N\'eel order, allow the study of samples
with $ N=3p+1 $ spins.
  A thorough study of these situations is done in parallel with the more
conventional case $ N=3p $.",9407028v1
1994-11-14,$XXZ$ model as effective Hamiltonian for generalized Hubbard models with broken $η$-symmetry,"We consider the limit of strong Coulomb attraction for generalized Hubbard
models with $\eta$-symmetry. In this limit these models are equivalent to the
ferromagnetic spin-1/2 Heisenberg quantum spin chain. In order to study the
behaviour of the superconducting phase in the electronic model under
perturbations which break the $\eta$-symmetry we investigate the ground state
of the ferromagnetic non-critical $XXZ$-chain in the sector with fixed
magnetization. It turns out to be a large bound state of $N$ magnons. We find
that the perturbations considered here lead to the disappearance of the
off-diagonal longe-range order.",9411051v2
1995-08-25,Inverse photoemission in strongly correlated electron systems,"Based on exact results for small clusters of 2D t-J model we demonstrate the
existence of several distinct `channels' in its inverse photoemission (IPES)
spectrum. Hole-like quasiparticles can either be annihilated completely, or
leave behind a variable number of spin excitations, which formed the `dressing
cloud' of the annihilated hole. In the physical parameter regime the latter
processes carry the bulk of IPES weight and although the Fermi surface takes
the form of hole pockets, the distribution of spectal weight including these
`magnon-bands' in the IPES spectrum is reminiscent of free electrons. The
emerging scenario for Fermiology and spectral weight distribution is shown to
be consistent with photoemission, inverse photemission and de Haas--van Alphen
experiments on cuprate superconductors.",9508114v1
1995-12-15,The spectral gap of the ferromagnetic XXZ chain,"We prove that the spectral gap of the spin-1/2 ferromagnetic XXZ chain with
Hamiltonian $H=-\sum_x S^{(1)}_xS^{(1)}_{x+1}+S^{(2)}_xS^{(2)}_{x+1} +\Delta
S^{(3)}_xS^{(3)}_{x+1}$, is given by $\Delta-1$ for all $\Delta\geq 1$. This is
the gap in the spectrum of the infinite chain in any of its ground states, the
translation invariant ones as well as the kink ground states, which contain an
interface between an {\it up} and a {\it down} region. In particular, this
shows that the lowest magnon energy is not affected by the presence of a domain
wall. This surprising fact is a consequence of the $SU_q(2)$ quantum group
symmetry of the model.",9512120v1
1996-04-30,Investigation of the Spin-Peierls transition in CuGeO_3 by Raman scattering,"Raman experiments on the spin-Peierls compound CuGeO$_3$ and the substituted
(Cu$_{1- x}$,Zn$_x$)GeO$_3$ and Cu(Ge$_{1-x}$,Ga$_x$)O$_3$ compounds were
performed in order to investigate the response of specific magnetic excitations
of the one-dimensional spin-1/2 chain to spin anisotropies and
substitution-induced disorder. In pure CuGeO$_3$, in addition to normal phonon
scattering which is not affected at all by the spin-Peierls transition, four
types of magnetic scattering features were observed. Below T$_{SP}$=14 K a
singlet-triplet excitation at 30 cm$^{-1}$, two-magnon scattering from 30 to
227 cm$^{-1}$ and folded phonon modes at 369 and 819 cm$^{-1}$ were identified.
They were assigned by their temperature dependence and lineshape. For
temperatures between the spin-Peierls transition T$_{SP}$ and approximately 100
K a broad intensity maximum centered at 300 cm$^{-1}$ is observed.",9604181v1
1996-06-04,On the Bilayer Coupling in the Yttrium-Barium Family of High Temperature Superconductors,"We derive the expressions needed to interpret experiments relating to
interplane magnetic coupling in YBa$_{2}$Cu$_{3}$O$_{6+x}$ and related
materials, and use the results to interpret measurements of the optical magnon
energy in YBa_{2}Cu_{3}O_{6.2} and of the NMR ``cross-relaxation"" rate in
Y_2Ba_{4}Cu_{7}O_{15}. We estimate $J_{\perp}\sim 14$ meV in both materials,
and $\chi_{max}/\mu^2_{B} \sim 100$ {states}/{eV}-Cu in Y_2Ba_4Cu_7O_{15} at
T=100K. We show that there is at present no obvious contradiction between the
results of a widely-used analysis of NMR experiments and the results of neutron
scattering experiments in the Y-Ba system. We argue that the 41 meV excitation
observed in superconducting YBa_{2}Cu_{3}O_{7} is a collective mode pulled down
below the superconducting gap by interactions, and that the observed
antisymmetry under interchange of planes follows from the non-negligible value
of $J_{\perp}$.",9606008v2
1996-11-04,The J_1-J_2 model revisited : Phenomenology of CuGeO_3,"We present a mean field solution of the antiferromagnetic Heisenberg chain
with nearest (J_1) and next to nearest neighbor (J_2) interactions. This
solution provides a way to estimate the effects of frustration. We calculate
the temperature-dependent spin-wave velocity, v_s(T) and discuss the
possibility to determine the magnitude of frustration J_2/J_1 present in quasi
1D compounds from measurements of v_s(T). We compute the thermodynamic
susceptibility at finite temperatures and compare it with the observed
susceptibility of the spin-Peierls compound CuGeO_3. We also use the method to
study the two-magnon Raman continuum observed in CuGeO_3 above the spin-Peierls
transition.",9611018v1
1996-11-07,Substitution effects on spin fluctuations in the spin-Peierls compound CuGeO_3,"Using Raman scattering we studied the effect of substitutions on 1D spin
fluctuations in CuGeO_3 observed as a spinon continuum in frustration induced
exchange scattering. For temperatures below the spin-Peierls transition
(T_{SP}=14K) the intensity of this continuum at 120-500 cm^{-1} is
exponentially suppressed and transferred into a 3D two-magnon density of
states. Besides a spin-Peierls gap-induced mode at 30 cm^{-1} and additional
modes at 105 and 370 cm^{-1} are observed. Substitution of Zn on the Cu-site
and Si on the Ge-site of CuGeO_3 quenches easily the spin-Peierls state.
Consequently a suppression of the spin-Peierls gap observable below T_{SP}=14K
as well as a change of the temperature dependence of the spinon continuum are
observed. These effects are discussed in the context of a dimensional crossover
of this compound below T_{SP} and strong spin-lattice interaction.",9611053v1
1996-12-24,Spin polarons in the t-J model in an unconstrained representation,"The report discusses the slave-fermion representations of the t-J model and
describes another representation, in which fermions and bosons are completely
commuting and in which the properties of fermions are directly related to the
properties of physical holes. For a study of the system in the new
representation at half-filling, interaction of fermions with two magnons is
treated in mean-field theory. The obtained effective model, in comparison to
that of the usual slave-fermion representation, has an additional bare hole
dispersion due to the hole moving by using quantum spin fluctuations present in
the undoped antiferromagnetic ground state. The single-hole Green's function at
half-filling is then found numerically using the self-consistent Born
approximation. For all studied quantities good or excellent agreement with
numerical data is observed in the entire parameter range, noticeably better
than in the studies with the slave-fermion representation. Using the same
effective model, the two-hole problem is also studied by solving numerically
the Bethe-Salpeter equation with noncrossing diagrams.",9612215v2
1997-02-19,Electronic structure and magnetism in Ru based perovskites,"The magnetic properties of (Ca,Sr)RuO_3 and Sr_2YRuO_6 are studied within the
context of band structure based Stoner theory. LSDA calculations without
recourse to strong correlation effects yield the correct magnetic behavior and
order in all cases. Insulating character of Sr_2YRuO_6 is reproduced. The
different magnetic states of SrRuO_3 and CaRuO_3 are shown to be due to the
different structural distortions. CaRuO_3 is found to be on the verge of a
ferromagnetic instability. O p states hybridize strongly with Ru d states in
all three compounds, and O, through this hybridization plays an unusually large
role in the magnetic properties. Transport properties of CaRuO_3 and SrRuO_3
are analyzed using the calculated Fermiology. Unusually large magnon and
paramagnon couplings are found, which are consistent with reported measurements
of the low temperature specific heat and the resistivity coefficient. We also
demonstrate how the calculated band structures can be understood in terms of a
nearest neighbor tight binding model.",9702168v2
1997-04-02,Dynamical properties of the spin-Peierls compound α'--NaV2O5,"Dynamical properties of the novel inorganic spin-Peierls compound
\alpha'--NaV2O5 are investigated using a one-dimensional dimerized Heisenberg
model. By exact diagonalizations of chains with up to 28 sites, supplemented by
a finite-size scaling analysis, the dimerization parameter \delta is determined
by requiring that the model reproduces the experimentally observed spin gap
\Delta. The dynamical and static spin structure factors are calculated. As for
CuGeO3, the existence of a low energy magnon branch separated from the
continuum is predicted. The present calculations also suggest that a large
magnetic Raman scattering intensity should appear above an energy threshold of
1.9 \Delta. The predicted photoemission spectrum is qualitatively similar to
results for an undimerized chain due to the presence of sizable short-range
antiferromagnetic correlations.",9704015v3
1997-05-05,Dynamical properties of low dimensional CuGeO3 and NaV2O5 systems,"Properties of low-dimensional spin-Peierls systems are described by using a
one dimensional S=1/2 antiferromagnetic Heisenberg chain linearly coupled to a
single phonon mode of wave vector pi (whose contribution is expected to be
dominant). By exact diagonalizations of small rings with up to 24 sites
supplemented by a finite size scaling analysis, static and dynamical properties
are investigated. Numerical evidences are given for a spontaneous discrete
symmetry breaking towards a spin gapped phase with a frozen lattice
dimerization. Special emphasis is put on the comparative study of the two
inorganic spin-Peierls compounds CuGeO3 and NaV2O5 and the model parameters are
determined from a fit of the experimental spin gaps. We predict that the
spin-phonon coupling is 2 or 3 times larger in NaV2O5 than in CuGeO3. Inelastic
neutron scattering spectra are calculated and similar results are found in the
single phonon mode approximation and in the model including a static
dimerization. In particular, the magnon S=1 branch is clearly separated from
the continuum of triplet excitations by a finite gap.",9705032v2
1997-09-27,"Low-temperature electrical transport and double exchange in La(Pb,Ca)MnO","The resistivity in the ferromagnetic state of flux-grown
La_{2/3}(Pb,Ca)_{1/3}MnO_3 single crystals, measured in magnetic fields up to 7
T, reveals a strong quadratic temperature dependence at and above 50 K. At
lower temperatures, this contribution drops precipitously leaving the
resistivity essentially temperature independent below 20 K. The Seebeck
coefficient also reflects a change of regime at the same temperature. We
attribute this behavior to a cut-off of single magnon scattering processes at
long wavelengths due to the polarized bands of a double-exchange ferromagnet.",9709305v3
1997-12-19,Domain excitations in spin-Peierls systems,"We study a model of a Spin-Peierls material consisting of a set of
antiferromagnetic Heisenberg chains coupled with phonons and interacting among
them via an inter-chain elastic coupling. The excitation spectrum is analyzed
by bosonization techniques and the self-harmonic approximation. The elementary
excitation is the creation of a localized domain structure where the dimerized
order is the opposite to the one of the surroundings. It is a triplet
excitation whose formation energy is smaller than the magnon gap. Magnetic
internal excitations of the domain are possible and give the further
excitations of the system. We discuss these results in the context of recent
experimental measurements on the inorganic Spin-Peierls compound CuGeO$_3$",9712239v2
1998-01-28,Exact SO(8) Symmetry in the Weakly-Interacting Two-Leg Ladder,"A perturbative renormalization group analysis of interacting electrons on a
two-leg ladder reveals that at half-filling any weakly repulsive system scales
onto an exactly soluble Gross-Neveu model with a hidden SO(8) symmetry. The
half-filled ground state is a Mott insulator with short-range d-wave pair
correlations. We extract the exact energies, degeneracies, and quantum numbers
of *all* the low energy excited multiplets. One energy (mass) m octets contains
Cooper pair, magnon, and density-wave excitations, two more octets contain
single-particle excitations, and a mass \sqrt{3}m antisymmetric tensor contains
28 ""bound states"". Exact single-particle and spin gaps are found for the
lightly-doped (d-wave paired one-dimension Bose fluid) system. We also
determine the four other robust phases occuring at half-filling for partially
attractive interactions. All 5 phases have distinct SO(8) symmetries, but share
S.C. Zhang's SO(5) as a common subgroup.",9801285v1
1998-02-27,Quantum spin chains in a magnetic field,"We demonstrate that the ``worm'' algorithm allows very effective and precise
quantum Monte Carlo (QMC) simulations of spin systems in a magnetic field, and
its auto-correlation time is rather insensitive to the value of H at low
temperature. Magnetization curves for the $s=1/2$ and $s=1$ chains are
presented and compared with existing Bethe ansatz and exact diagonalization
results. From the Green function analysis we deduce the magnon spectra in the
s=1 system, and directly establish the ""relativistic"" form E(p)=(\Delta ^2 +v^2
p^2)^{1/2} of the dispersion law.",9802294v2
1998-04-19,Impurities and Inelastic Processes in Magnetic Tunnel Junctions,"We have studied tunnel magnetoresistance (TMR) in junctions with 3d
ferromagnetic electrodes. Previously we predicted that defects in the barrier
would result in reduced effective polarization P of the impurity assisted
current. This is confirmed experimentally in the present work: introductions of
defects into the barrier drastically decreases the TMR. The degradation of
magnetoresistance with bias has also been studied and shows universal features,
attributed to effects of tunneling assisted by magnons and phonons, whose
different role is described. Details of the bias dependence of the TMR depend
on preparation procedures and well described by the model which includes
assisted tunneling. Non-linear features, seen at low biases, are related to
excitation of bulk modes by tunneling electrons. The analysis of factors
resulting in fall-off of the TMR with bias is presented.",9804208v1
1998-04-24,Perturbation Theory for Spin Ladders Using Angular-Momentum Coupled Bases,"We compute bulk properties of Heisenberg spin-1/2 ladders using
Rayleigh-Schr\""odinger perturbation theory in the rung and plaquette bases. We
formulate a method to extract high-order perturbative coefficients in the bulk
limit from solutions for relatively small finite clusters. For example, a
perturbative calculation for an isotropic $2\times 12$ ladder yields an
eleventh-order estimate of the ground-state energy per site that is within
0.02% of the density-matrix-renormalization-group (DMRG) value. Moreover, the
method also enables a reliable estimate of the radius of convergence of the
perturbative expansion. We find that for the rung basis the radius of
convergence is $\lambda_c\simeq 0.8$, with $\lambda$ defining the ratio between
the coupling along the chain relative to the coupling across the chain. In
contrast, for the plaquette basis we estimate a radius of convergence of
$\lambda_c\simeq 1.25$. Thus, we conclude that the plaquette basis offers the
only currently available perturbative approach which can provide a reliable
treatment of the physically interesting case of isotropic $(\lambda=1)$ spin
ladders. We illustrate our methods by computing perturbative coefficients for
the ground-state energy per site, the gap, and the one-magnon dispersion
relation.",9804261v1
1998-05-08,Physical properties of the double exchange and its relevance to the specific heat of double perovskite materials,"In response to the experimental discovery of double perovskite Ru- base
material, The magnetic and electronic properties of the double exchange
mechanism are studied theoretically. The electron band structure and the spin
wave spectrum response at zero temperature with the canting background are
computed from the first principle Hamiltonian. The long range Coulomb
interaction between charge carriers is found to be crucial in stabilizing the
canting order. A Monte Carlo simulation is performed on de-Gennes's effective
magnetic Hamiltonian with classical spin approximation. The peaks of the
specific heat as a function of temperature, which correspond to FM(AF) and
canting transitions, and its response to the external magnetic field are
consistent with the observation. For certain region of parameters, a more
complex canting order than layer type is found at sufficiently low temperature.
The effective interaction between carriers via exchanging a magnon wave is
found to be rather similar to that via exchanging an acoustic phonon. The pros
and cons to a genuine superconducting order in this system is analyzed.",9805094v1
1998-05-29,Scaling theory of magnetic ordering in the Kondo lattices with anisotropic exchange interactions,"The lowest-order scaling consideration of the magnetic state formation in the
Kondo lattices is performed within the $s-f$ model with inclusion of anisotropy
for both the $f-f$ coupling and $s-f$ exchange interaction. The Kondo
renormalizations of the effective transverse and longitudinal $s-f$ coupling
parameters, spin-wave frequency, gap in the magnon spectrum and ordered moment
are calculated in the case of both ferro- and antiferromagnets. The
anisotropy-driven change of the scaling behavior (e.g., critical value of $g$
for entering the strong-coupling region and the corresponding critical
exponents) is investigated numerically for N=2 and analytically in the large-N
limit. The dependence of the effective Kondo temperature on the bare $s-f$
coupling parameter $g$ weakens in the presence of anisotropy. The relative
anisotropy parameters for both the $s-f$ and $f-f$ coupling are demonstrated to
decrease during the renormalization process. The role of next-nearest exchange
interactions for this effect in the antiferromagnet is discussed.",9805390v2
1998-06-13,Nonanalytic Magnetization Dependence of the Magnon Effective Mass in Itinerant Quantum Ferromagnets,"The spin wave dispersion relation in both clean and disordered itinerant
quantum ferromagnets is calculated. It is found that effects akin to
weak-localization physics cause the frequency of the spin-waves to be a
nonanalytic function of the magnetization m. For low frequencies \Omega, small
wavevectors k, and small m, the dispersion relation is found to be of the form
\Omega ~ m^{1-\alpha} k^2, with \alpha = (4-d)/2 (2<d<4) for disordered
systems, and \alpha = (3-d) (1<d<3) for clean ones. In d=4 (disordered) and d=3
(clean), \Omega ~ m ln(1/m) k^2. Experiments to test these predictions are
proposed.",9806168v3
1998-06-22,Magnetic excitation spectra of $NaV_2O_5$,"We present low temperature Raman spectra of $NaV_2O_5$ single crystal. The
assignation of the observed modes is done on the basis of Monte Carlo
calculation of excited states of Heisenberg dimerized Hamiltonian, without
frustration. We found a good agreement between measured and calculated magnetic
excitations for $\delta=0.048$ and $J=452 K$. The singlet-triplet,
singlet-singlet excitations as well as two-magnon mode are observed to be at
66, 127 and 132 cm^{-1}.",9806258v2
1998-08-07,Electromagnetic Interactions in the Quantum Hall Ferromagnet,"The $\nu =1$ quantum Hall ground state in samples like GaAs is well known to
be ferromagnetic. The global SU(2) spin symmetry of the microscopic action is
broken down to a U(1) symmetry by the ground state. The Goldstone bosons
corresponding to this spontaneous breaking of symmetry are the ferromagnetic
magnons which are neutral spin waves. In addition, there are topologically
nontrivial, electrically charged spin excitations known as spin skyrmions,
which in these samples are the favoured charge carriers. In this letter, we
look into the electromagnetic coupling of these spin excitations. The discrete
symmetries P and T are also broken by the ground state and we find that to the
leading order, the electromagnetic interaction of the spin waves occurs through
a nonminimal coupling given by a Chern-Simons like term containing both the
electromagnetic potentials and the two Euler angles that specify the coset
SU(2)/U(1) and thereby, the Goldstone bosons.",9808081v1
1998-10-30,Phase diagram and magnons in quasi-one-dimensional dipolar antiferromagnets,"We investigate antiferromagnetic spin chains, which are coupled by a weak
antiferromagnetic exchange interaction on a hexagonal lattice. We particulary
study the role of the dipole-dipole interaction within the framework of a
Heisenberg model with nearest-neighbor exchange and additional dipolar
interaction. We find several commensurate and incommensurate phases depending
on the ratio of dipolar energy to interchain-exchange energy due to their
competing qualtity. The ground-state analysis is supplemented by a stability
analysis by means of a linear spin-wave theory. In comparison with experiments
(CsMnBr_3, RbMnBr_3) we obtain good agreement for the energy gaps. From this we
conclude, that the dipolar interaction is the most important source of
anisotropy in these Mn-compounds.",9810402v1
1998-12-17,Incommensurability in the magnetic excitations of the bilinear-biquadratic spin-1 chain,"We study the magnetic excitation spectrum of the S=1 quantum Heisenberg spin
chain with Hamiltonian : H = sum_i cos(theta) S_i S_i+1 + sin(theta) (S_i
S_i+1)^2. We focus on the range -pi/4 < theta < +pi/4 where the spin chain is
in the gapped Haldane phase. The excitation spectrum and static structure
factor is studied using direct Lanczos diagonalization of small systems and
density-matrix renormalization group techniques combined with the single-mode
approximation. The magnon dispersion has a minimum at q=pi until a critical
value theta_c = 0.38 is reached at which the curvature (velocity) vanishes.
Beyond this point, which is distinct from the VBS point and the Lifshitz point,
the minimum lies at an incommensurate value that goes smoothly to 2pi/3 when
theta approaches pi/4, the Lai-Sutherland point. The mode remains isolated from
the other states: there is no evidence of spinon deconfinement before the point
theta =+pi/4. These findings explain recent observation of the magnetization
curve M approx (H -H_c)^1/4 for theta =theta_c.",9812296v1
1998-12-18,"Spinon signatures in the critical phase of the (1,1/2) ferrimagnet in a magnetic field","We propose an effective theory for the critical phase of a quantum
ferrimagnetic chain with alternating spins 1 and 1/2 in an external magnetic
field. With the help of the matrix product variational approach, the system is
mapped to a spin-1/2 XXZ chain in an (effective) magnetic field; as a
byproduct, we obtain an excellent description of the optical magnon branch in
the gapped phase. Recent finite-temperature DMRG results for the
low-temperature part of the specific heat are well described by the present
approach, and the ``pop-up'' peaks, developing near the critical field values
and in the middle of the critical phase, are identified with the contributions
from two different spinon bands of the effective spin-1/2 chain. The effect
should be as well observable in other spin-gap systems in an external field,
particularly in spin ladders.",9812326v1
1999-01-22,The Effective Theory of Hole Doped Spin-1 Chain,"An effective theory for the hole doped spin-1 antiferromagnetic chain is
proposed in this paper. The two branches of low energy quasipaticle excitation
is obtained by the diagrammic technique. In the large t limit(in which t is the
hole hopping term), the lower band is essentially the bound state of one hole
and one magnon and the other band is the sigle hole state. We find a critical
value of t, $t_c=0.21\Delta_H$(in which $\Delta_H$ is the Haldane gap).For
$t>t_c$, with the decrement of t, the mixing of these two bands become stronger
and stronger, and at the same time the effective band mass becomes larger and
larger. When $t<t_c$ the minimum of the lower band moves away from zero the
another point between zero and $\pi/2$. The spin strcture factor is also
calculated in this paper, and we find that for large t limit the main
contribution is from the inter-band transition which induce a resonant peak in
the Haldane gap. While for small t limit the main contribution is from the
intra-band transition which only cause a diffusion like broad bump in the
Haldan gap.",9901234v1
1999-01-28,From Magnons to the Resonance Peak: Spin Dynamics in High-T_C Superconducting Cuprates by Inelastic Neutron Scattering,"The spin dynamics of high temperature superconductors measured by inelastic
neutron scattering is reviewed. The spin susceptibility evolves a lot with
increasing doping from the undoped insulating state to the overdoped metallic
state. In the superconducting state, a strong magnetic resonance peak occurs in
the spin excitation spectrum whose energy is proportional to the
superconducting temperature. In underdoped and optimally doped regimes, normal
state antiferromagnetic fluctuations are observed, proving the existence of
strong electronic correlations in cuprates. They are characterized by a spin
pseudo-gap. In contrast, these dynamical spin correlations vanish in overdoped
samples.",9901333v2
1999-02-19,Ground State and Excitations of Spin Chain with Orbital Degeneracy,"The one dimensional Heisenberg model in the presence of orbital degeneracy is
studied at the SU(4) symmetric viewpoint by means of Bethe ansatz. Following
Sutherland's previous work on an equivalent model, we discuss the ground state
and the low-lying excitations more extensively in connection to the spin
systems with orbital degeneracy. We show explicitly that the ground state is a
SU(4) singlet. We study the degeneracies of the elementary excitations and the
spectra of the generalized magnons consisting of these excitations. We also
discuss the complex 2-strings in the context of the Bethe ansatz solutions.",9902269v3
1999-02-26,The Effect of Ni and Zn Doping in Bi2212 from Tunneling Measurements. The MCS model of the High-Tc Superconductivity in hole-doped cuprates,"We use Ni and Zn impurities doped into Bi2212 to test the MS (Magnetic
polaron - Spinon) model proposed earlier. We present electron-tunneling
spectroscopy of pure, Ni- and Zn-doped Bi2212 single crystals below Tc using a
break-junction technique. We show that the magnetic (Ni) and nonmagnetic (Zn)
impurities doped into CuO2 planes affect both Tc and the density of states in
Bi2212 but do it differently. In order to explain the data, we had to modify
the MS model. Thus, we present a Magnetic Coupling of Stripes (MCS) model. In
the MCS model, there is only one superconductivity - a spinon superconductivity
along charged stripes. The coherent state of the spinon superconductivity is
established by magnons which are excited by motions of charged stripes. So, in
the MCS model, the superconductivity has two mechanisms: along charged stripes
and perpendicular to stripes.",9902355v1
1999-03-10,Functional Schrödinger Representations of Holstein-Primakoff Boson and Slave-Boson Theories for Heisenberg Antiferromagnets,"We present functional Schr\""odinger representations of Holstein-Primakoff
boson and slave-boson theories for the Heisenberg Hamiltonian and the $t-J$
Hamiltonian respectively. Based on these representations we obtain the
dispersion relations of magnons for two dimensional antiferromagnets. By
applying the functional Schr\""odinger representation of the Holstein-Primakoff
boson theory to the Heisenberg Hamiltonian, the exchange energy is correctly
predicted and the self-energy of quasi-hole is obtained. From the use of the
functional Schr\""odinger representation of the $t-J$ Hamiltonian it is shown
that at half-filling the dispersion relation obtained from the slave-boson
theory leads to that obtained from the Holstein-Primakoff boson approach.",9903159v1
1999-04-06,On the estimate of the spin-gap in quasi-1D Heisenberg antiferromagnets from nuclear spin-lattice relaxation,"We present a careful analysis of the temperature dependence of the nuclear
spin-lattice relaxation rate $1/T_1$ in gapped quasi-1D Heisenberg
antiferromagnets. It is found that in order to estimate the value of the gap
correctly from $1/T_1$ the peculiar features of the dispersion curve for the
triplet excitations must be taken into account. The temperature dependence of
$1/T_1$ due to two-magnon processes, is reported for different values of the
ratio $r=J_{\perp}/J_{\parallel}$ between the superexchange constants in a
2-leg-ladder. As an illustrative example we compare our results to the
experimental findings for $^{63}Cu 1/T_1$ in the dimerized chains and
2-leg-ladders contained in Sr$_{14}Cu_{24}O_{41}$",9904074v1
1999-06-23,The underscreened Kondo effect in ladder systems,"We introduce a new Kondo ladder model by coupling the well-known
Takhtajan-Babujan S=1 chain to a half-filled one-dimensional electron gas, and
we solve it using quantum field theory techniques. Through the so-called
underscreened Kondo effect, this gives novel insights into low-dimensional spin
liquid systems. For instance, the optical conductivity reveals only a pseudogap
at $k=\pi$ despite of the insulating state, a coherent magnon peak in the
dynamical spin susceptibility coexists with an incoherent spinon background,
the magnetization yields a fractional-like quantization. More generally, this
problem belongs to the class of Luttinger liquids in active environments, a
topic of importance not only to the Kondo physics, but also to striped phases
or carbon nanotubes. Experimental consequences on heavy-fermion systems are
also discussed.",9906345v1
1999-07-13,On the nature of the transition from the spontaneously dimerized to the Neel phase in the two-dimensional J1-J2 model,"We analyze the spectrum of the 2D S=1/2 frustrated Heisenberg model near the
transition from the spontaneously dimerized spin-liquid phase into the Neel
ordered phase. Two excitation branches: the triplet magnon, and the collective
singlet mode, both become gapless at the transition point. However we find that
the length scales associated with these modes are well separated at the quantum
transition. While in the quantum disordered phase the singlet excitation has
finite spectral weight and reflects the existence of spontaneous dimer order,
near the transition point the size of the singlet bound state grows
exponentially with the correlation length, and hence the quasiparticle residue
is exponentially small. Therefore the critical dynamics remains in the O(3)
universality class in spite of the four gapless modes.",9907178v1
1999-07-22,Charge carrier density collapse in La_0.67Ca_0.33MnO_3 and La_0.67Sr_0.33MnO_3 epitaxial thin films,"We measured the temperature dependence of the linear high field Hall
resistivity of La_0.67Ca_0.33MnO_3 (T_C=232K) and La_0.67Sr_0.33MnO_3
(T_C=345K) thin films in the temperature range from 4K up to 360K in magnetic
fields up to 20T. At low temperatures we find a charge carrier density of 1.3
and 1.4 holes per unit cell for the Ca- and Sr-doped compound, respectively. In
this temperature range electron-magnon scattering contributes to the
longitudinal resistivity. At the ferromagnetic transition temperature T_C a
dramatic drop in the number of current carriers $n$ down to 0.6 holes per unit
cell, accompanied by an increase in unit cell volume, is observed. Corrections
of the Hall data due to a non saturated magnetic state will lead a more
pronounced charge carrier density collapse.",9907346v1
1999-10-18,Three-Dimensional Ordering in Weakly Coupled Antiferromagnetic Ladders and Chains,"A theoretical description is presented for low-temperature magnetic-field
induced three-dimensional (3D) ordering transitions in strongly anisotropic
quantum antiferromagnets, consisting of weakly coupled antiferromagnetic
spin-1/2 chains and ladders. First, effective continuum field theories are
derived for the one-dimensional subsystems. Then the Luttinger parameters,
which determine the low-temperature susceptibilities of the chains and ladders,
are calculated from the Bethe ansatz solution for these effective models. The
3D ordering transition line is obtained using a random phase approximation for
the weak inter-chain (inter-ladder) coupling. Finally, considering a Ginzburg
criterion, the fluctuation corrections to this approach are shown to be small.
The nature of the 3D ordered phase resembles a Bose condensate of integer-spin
magnons. It is proposed that for systems with higher spin degrees of freedom,
e.g. N-leg spin-1/2 ladders, multi-component condensates can occur at high
magnetic fields.",9910259v1
1999-11-03,Does the Heisenberg model describe the multimagnon spin dynamics in antiferromagnetic CuO layers ?,"We compute the absorption spectrum for multimagnon excitations assisted by
phonons in insulating layered cuprates using exact diagonalization in clusters
of up to 32 sites. The resulting line shape is very sensitive to the underlying
magnetic Hamiltonian describing the spin dynamics. For the usual Heisenberg
description of undoped Cu-O planes we find, in accordance with experiment, a
two-magnon peak followed by high energy side bands. However the relative weight
of the side bands is too small to reproduce the experiment. An extended
Heisenberg model including a sizable four-site cyclic exchange term is shown to
be consistent with the experimental data.",9911037v1
1999-11-09,Nanoconstriction Microscopy of the Giant Magnetoresistance in Cobalt/Copper Spin Valves,"We use nanometer-sized point contacts to a Co/Cu spin valve to study the
giant magnetoresistance (GMR) of only a few Co domains. The measured data show
strong device-to-device differences of the GMR curve, which we attribute to the
absence of averaging over many domains. The GMR ratio decreases with increasing
bias current. For one particular device, this is accompanied by the development
of two distinct GMR plateaus, the plateau level depending on bias polarity and
sweep direction of the magnetic field. We attribute the observed behavior to
current-induced changes of the magnetization, involving spin transfer due to
incoherent emission of magnons and self-field effects.",9911133v1
1999-11-18,Time-resolved optical observation of spin-wave dynamics,"We have created a nonequilibrium population of antiferromagnetic spin-waves
in Cr2O3, and characterized its dynamics, using frequency- and time-resolved
nonlinear optical spectroscopy of the exciton-magnon transition. We observe a
time-dependent pump-probe line shape, which results from excitation induced
renormalization of the spin-wave band structure. We present a model that
reproduces the basic characteristics of the data, in which we postulate the
optical nonlinearity to be dominated by interactions with long-wavelength
spin-waves, and the dynamics to be due to spin-wave thermalization.",9911294v1
1999-11-30,Two-dimensional frustrated Heisenberg model: Variational study,"The stability of the ferromagnetic phase of the 2D quantum spin-1/2 model
with nearest-neighbor ferro- and next-nearest neighbor antiferromagnetic
interactions is studied. It turns out that values of exchange integrals at
which the ferromagnetic state becomes unstable with respect to a creation of
one and two magnon are different. This difference shows that the classical
approximation is inapplicable to the study of the transition from the
ferromagnetic to the singlet state in contrast with 1D case. This problem is
investigated using a variational function of new type. It is based on the boson
representation of spin operators which is different from the Holstein-Primakoff
approximation. This allows us to obtain the accurate estimate of the transition
point and to study the character of the phase transition.",9911491v1
2000-01-21,Electron momentum distribution of a single mobile hole in the t-J model,"We investigate the electron momentum distribution function (EMDF) for the
two-dimensional t-J model. The results are based on the self-consistent Born
approximation (SCBA) for the self-energy and the wave function. In the Ising
limit of the model we give the results in a closed form, in the Heisenberg
limit the results are obtained numerically. An anomalous momentum dependence of
EMDF is found and the anomaly is in the lowest order in number of magnons
expressed analitycally. We interpret the anomaly as a fingerprint of an
emerging large Fermi surface coexisting with hole pockets.",0001306v1
2000-02-28,Spin stripe dimerization in the t-J model,"In the present work we demonstrate that the spin columnar dimerized phase
(conducting spin stripe liquid) is stable at doping $x_{c1} < x < x_{c2}$. At
$x < x_{c1}$ the system undergoes phase transition to the Neel state, and at $x
> x_{c2}$ it is Normal Fermi liquid. At t/J=3 the critical concentrations are
$x_{c1}\approx 0.09$ and $x_{c2}\sim 0.36$. To prove stability of the stripe
phase and to calculate critical concentrations we employ two approaches. The
first one is more technically involved, and it gives more accurate value of the
critical concentration. The approach is based on the calculation of the magnon
Green's function. Imaginary poles in the Green's function indicate transition
to the Neel state. The second approach consists in direct comparison of ground
state energies of the Neel state and the columnar dimerized state. Both
approaches demonstrate stability of the spin stripe phase and give close values
of the critical concentrations.",0002421v1
2000-03-02,On the dynamics of coupled S=1/2 antiferromagnetic zig-zag chains,"We investigate the elementary excitations of quasi one-dimensional S=1/2
systems built up from zig-zag chains with general isotropic exchange constants,
using exact (Lanczos) diagonalization for 24 spins and series expansions
starting from the decoupled dimer limit. For the ideal one-dimensional zig-zag
chain we discuss the systematic variation of the basic (magnon) triplet
excitation with general exchange parameters and in particular the presence of
practically flat dispersions in certain regions of phase space. We extend the
dimer expansion in order to include the effects of 3D interactions on the
spectra of weakly interacting zig-zag chains. In an application to KCuCl_3 we
show that this approach allows to determine the exchange interactions between
individual pairs of spins from the spectra as determined in recent neutron
scattering experiments.",0003028v2
2000-03-07,Diagonalization of a bosonic quadratic form using CCM: Application on a system with two interpenetrating square lattice antiferromagnets,"While the diagonalization of a quadratic bosonic form can always be done
using a Bogoliubov transformation, the practical implementation for systems
with a large number of different bosons is a tedious analytical task. Here we
use the coupled cluster method (CCM) to exactly diagonalise such complicated
quadratic forms. This yields to a straightforward algorithm which can easily be
implemented using computer algebra even for a large number of different bosons.
We apply this method on a Heisenberg system with two interpenetrating square
lattice antiferromagnets, which is a model for the quasi 2D antiferromagnet
Ba_2Cu_3O_4Cl_2. Using a four-magnon spin wave approximation we get a
complicated Hamiltonian with four different bosons, which is treated with CCM.
Results are presented for magnetic ground state correlations.",0003105v1
2000-04-10,Quantum Impurity in a Nearly Critical Two Dimensional Antiferromagnet,"We describe the spin dynamics of an arbitrary localized impurity in an
insulating two dimensional antiferromagnet, across the host transition from a
paramagnet with a spin gap to a Neel state. The impurity spin susceptibility
has a Curie-like divergence at the quantum-critical coupling, but with a
universal, effective spin which is neither an integer nor a half-odd-integer.
In the Neel state, the transverse impurity susceptibility is a universal number
divided by the host spin stiffness (which determines the energy cost to slow
twists in the orientation of the Neel order). These, and numerous other results
for the thermodynamics, Knight shift, and magnon damping have significant
applications to experiments on layered transition metal oxides.",0004156v1
2000-05-23,Critical properties of projected SO(5) models at finite temperatures,"We consider the projected SO(5) bosonic model introduced in order to connect
the SO(5) theory of high-T$_c$ superconductivity with the physics of the
Mott-insulating gap, and derive the corresponding effective functional
describing low-energy degrees of freedom. At the
antiferromagnetic-superconducting transition, SO(5) symmetry-breaking effects
due to the gap are purely quantum mechanical and become irrelevant in the
neighborhood of a possible finite-temperature multicritical point separating
the normal from the antiferromagnetic and the superconducting phases. A
difference in the magnon and hole-pair mobility always takes the system away
from the SO(5)-symmetric fixed point towards a region of instability, and the
phase transition between the normal and the two ordered phases becomes first
order before merging into the antiferromagnetic-superconducting line. Quantum
fluctuations at intermediate temperatures, while introducing symmetry-breaking
terms in the case of equal mobilities, tend to cancel the symmetry-breaking
effects in the case of different mobilities.",0005385v1
2000-07-27,Oscillatory Curie temperature of two-dimensional ferromagnets,"The effective exchange interactions of magnetic overlayers Fe/Cu(001) and
Co/Cu(001) covered by a Cu-cap layer of varying thickness were calculated in
real space from first principles. The effective two-dimensional Heisenberg
Hamiltonian was constructed and used to estimate magnon dispersion laws,
spin-wave stiffness constants, and overlayer Curie temperatures within the
mean-field and random-phase approximations. Overlayer Curie temperature
oscillates as a function of the cap-layer thickness in a qualitative agreement
with a recent experiment.",0007446v3
2000-08-22,Elementary excitations in one-dimensional spin-orbital models: neutral and charged solitons and their bound states,"We study, both numerically and variationally, the interplay between different
types of elementary excitations in the model of a spin chain with anisotropic
spin-orbit coupling, in the vicinity of the ""dimer line"" with an exactly known
dimerized ground state. Our variational treatment is found to be in a
qualitative agreement with the exact diagonalization results. Soliton pairs are
shown to be the lowest excitations only in a very narrow region of the phase
diagram near the dimer line, and the phase transitions are always governed by
magnon-type excitations which can be viewed as soliton-antisoliton bound
states. It is shown that when the anisotropy exceeds certain critical value, a
new phase boundary appears. In the doped model on the dimer line, the exact
elementary charge excitation is shown to be a hole bound to a soliton. Bound
states of those ""charged solitons"" are studied; exact solutions for N-hole
bound states are presented.",0008321v1
2000-09-12,Magnetization jump in the XXZ chain with next-nearest-neighbor exchange,"We study the dependence of the magnetization M with magnetic field B at zero
temperature in the spin-1/2 XXZ chain with nearest-neighbor (NN) J1 and next-NN
J2 exchange interactions, with anisotropies Delta1 and Delta2 respectively. The
region of parameters for which a jump in M(B) exists is studied using numerical
diagonalization, and analytical results for two magnons on a ferromagnetic
background in the thermodynamic limit. We find a line in the parameter space
(J2/J1, Delta1/J1, Delta2/J2) (determined by two simple equations) at which the
ground state is highly degenerate. M(B) has a jump near this line, and at or
near the isotropic case with ferromagnetic J1 and antiferromagnetic J2, with
|J2/J1| near 1/4. These results are relevant for some systems containing CuO
chains with edge-sharing CuO4 units.",0009175v1
2000-09-27,Projected SO(5) Hamiltonian for Cuprates and Its Applications,"The projected SO(5) (pSO(5)) Hamiltonian incorporates the quantum spin and
superconducting fluctuations of underdoped cuprates in terms of four bosons
moving on a coarse grained lattice. A simple mean field approximation can
explain some key feautures of the experimental phase diagram: (i) The Mott
transition between antiferromagnet and superconductor, (ii) The increase of T_c
and superfluid stiffness with hole concentration x and (iii) The increase of
antiferromagnetic resonance energy as sqrt{x-x_c} in the superconducting phase.
We apply this theory to explain the ``two gaps'' problem found in underdoped
cuprate Superconductor-Normal- Superconductor junctions. In particular we
explain the sharp subgap Andreev peaks of the differential resistance, as
signatures of the antiferromagnetic resonance (the magnon mass gap). A critical
test of this theory is proposed. The tunneling charge, as measured by shot
noise, should change by increments of Delta Q= 2e at the Andreev peaks, rather
than by Delta Q=e as in conventional superconductors.",0009421v1
2000-10-11,Magnetic properties of (VO)_2P_2O_7: two-plane structure and spin-phonon interactions,"Detailed experiments on single-crystal (VO)_2P_2O_7 continue to reveal new
and unexpected features. We show that a model composed of two, independent
planes of spin chains with frustrated magnetic coupling is consistent with
nuclear magnetic resonance and inelastic neutron scattering measurements. The
pivotal role of PO_4 groups in mediating intrachain exchange interactions
explains both the presence of two chain types and their extreme sensitivity to
certain lattice vibrations, which results in the strong magnetoelastic coupling
observed by light scattering. We compute the respective modifications of the
spin and phonon dynamics due to this coupling, and illustrate their observable
consequences on the phonon frequencies, magnon dispersions, static
susceptibility and specific heat.",0010168v1
2000-10-25,Specific Heat Study of the Field-Induced Magnetic Ordering in the Spin Gap System TlCuCl$_3$,"Specific heat measurements have been performed in the coupled dimer system
TlCuCl$_3$, which has a singlet ground state with the excitation gap $\Delta
\simeq 7.5$K. The cusplike anomaly indicative of the 3D magnetic ordering was
clearly observed in magnetic fields higher than the critical field $H_c$
corresponding to the gap $\Delta$. The phase boundary determined by the present
specific heat measurements coincides with that determined by previous
magnetization measurements. The phase boundary can be described by the power
law $[ H_{\rm c}(T)-H_{\rm g} ] \propto T^{\phi}$ with $\phi=2.1(1)$. This
result supports the magnon Bose condensation picture for the field-induced
magnetic ordering in TlCuCl$_3$ [Nikuni {\it et al}., Phys. Rev. Lett. {\bf 84}
(2000) 5868].",0010383v2
2000-10-26,Fingerprints of spin-fermion pairing in cuprates,"We demonstrate that the feedback effect from bosonic excitations on fermions,
which in the past allowed one to verify the phononic mechanism of a
conventional, $s-$wave superconductivity, may also allow one to experimentally
detect the ``fingerprints'' of the pairing mechanism in cuprates. We argue that
for spin-mediated $d-$wave superconductivity, the fermionic spectral function,
the density of states, the tunneling conductance through an insulating
junction, and the optical conductivity are affected by the interaction with
collective spin excitations, which below $T_c$ are propagating, magnon-like
quasiparticles with gap $\Delta_s$.
  We show that the interaction with a propagating spin excitation gives rise to
singularities at frequencies $\Delta + \Delta_s$ for the spectral function and
the density of states, and at
  $2\Delta + \Delta_s$ for tunneling and optical conductivities, where $\Delta$
is the maximum value of the $d-$wave gap. We further argue that recent optical
measurements also allow one to detect subleading singularities at $4\Delta$ and
$2\Delta + 2\Delta_s$. We consider the experimental detection of these
singularities as a strong evidence in favor of the magnetic scenario for
superconductivity in cuprates.",0010403v1
2001-01-10,Soliton dynamics in 3D ferromagnets,"We study the dynamics of solitons in a Landau-Lifshitz equation describing
the magnetization of a three-dimensional ferromagnet with an easy axis
anisotropy. We numerically compute the energy dispersion relation and the
structure of moving solitons, using a constrained minimization algorithm. We
compare the results with those obtained using an approximate form for the
moving soliton, valid in the small momentum limit. We also study the
interaction and scattering of two solitons, through a numerical simulation of
the (3+1)-dimensional equations of motion. We find that the force between two
solitons can be either attractive or repulsive depending on their relative
internal phase and that in a collision two solitons can form an unstable
oscillating loop of magnons.",0101129v1
2001-01-18,Observation of Field-Induced Transverse Néel Ordering in the Spin Gap System TlCuCl$_3$,"Neutron elastic scattering experiments have been performed on the spin gap
system TlCuCl$_3$ in magnetic fields parallel to the $b$-axis. The magnetic
Bragg peaks which indicate the field-induced N\'{e}el ordering were observed
for magnetic field higher than the gap field $H_{\rm g}\approx 5.5$ T at $Q=(h,
0, l)$ with odd $l$ in the $a^*-c^*$ plane. The spin structure in the ordered
phase was determined. The temperature and field dependence of the Bragg peak
intensities and the phase boundary obtained were discussed in connection with a
recent theory which describes the field-induced N\'{e}el ordering as a
Bose-Einstein condensation of magnons.",0101276v2
2001-02-12,Quantum effects in the quasiparticle structure of the ferromagnetic Kondo lattice model,"A new ``Dynamical Mean-field theory'' based approach for the Kondo lattice
model with quantum spins is introduced. The inspection of exactly solvable
limiting cases and several known approximation methods, namely the second-order
perturbation theory, the self-consistent CPA and finally a moment-conserving
decoupling of the equations of motion help in evaluating the new approach. This
comprehensive investigation gives some certainty to our results: Whereas our
method is somewhat limited in the investigation of the J<0-model, the results
for J>0 reveal important aspects of the physics of the model: The energetically
lowest states are not completely spin-polarized.A band splitting, which occurs
already for relatively low interaction strengths, can be related to distinct
elementary excitations, namely magnon emission (absorption) and the formation
of magnetic polarons. We demonstrate the properties of the ferromagnetic Kondo
lattice model in terms of spectral densities and quasiparticle densities of
states.",0102210v1
2001-03-09,Intermediate phase in the spiral antiferromagnet Ba_2CuGe_2O_7,"The magnetic compound Ba_2CuGe_2O_7 has recently been shown to be an
essentially two-dimensional spiral antiferromagnet that exhibits an
incommensurate-to-commensurate phase transition when a magnetic field applied
along the c-axis exceeds a certain critical value H_c. The T=0 dynamics is
described here in terms of a continuum field theory in the form of a nonlinear
sigma model. We are thus in a position to carry out a complete calculation of
the low-energy magnon spectrum for any strength of the applied field throughout
the phase transition. In particular, our spin-wave analysis reveals
field-induced instabilities at two distinct critical fields H_1 and H_2 such
that H_1 < H_c < H_2. Hence we predict the existence of an intermediate phase
whose detailed nature is also studied to some extent in the present paper.",0103217v2
2001-03-21,"Observation of two-magnon bound states in the two-leg ladders of (Ca,La)14Cu24O41","Phonon-assisted 2-magnon absorption is studied at T=4 K in the spin-1/2
two-leg ladders of Ca_14-x La_x Cu_24 O_41 (x=5 and 4) for polarization of the
electrical field parallel to the legs and the rungs, respectively. Two peaks at
about 2140 and 2800 1/cm reflect van-Hove singularities in the density of
states of the strongly dispersing 2-magnon singlet bound state, and a broad
peak at about 4000 1/cm is identified with the 2-magnon continuum. Two
different theoretical approaches (Jordan-Wigner fermions and perturbation
theory) describe the data very well for J_parallel = 1050 - 1100 1/cm and
J_parallel / J_perp = 1 - 1.1. A striking similarity of the high-energy
continuum absorption of the ladders and of the undoped high T_c cuprates is
observed.",0103438v1
2001-03-24,Interaction-round-a-face density-matrix renormalization-group method,"A brief review of the interaction-round-a-face (IRF) density-matrix
renormalization-group (DMRG) method. We have demonstrated the numerical
superiority of IRF-DMRG method applied to SU(2) invariant quantum spin chains
over the conventional DMRG. The ground state energy densities and the gap
energies of both $S = 1$ and $S = 2$ spin chains can be calculated using the
IRF-DMRG without extensive computations. We have also studied the effect of
tuning boundary interaction $J_{\rm end}^{~}$ at both ends of the chain from
the IRF view point. It is clearly observed that the magnon distribution is
uniform when the best $J_{\rm end}^{~}$ is chosen.",0103508v1
2001-04-27,Raman scattering study of phonon and spin modes in (TMTSF)2PF6,"We have studied polarized Raman spectra of (TMTSF)2PF6 single crystals in the
wide spectral and temperature range using different laser energies. We observed
and assigned 23 Raman active modes. The carbon C=C in-phase and out-of-phase
stretching modes at 1464 cm-1 and 1602 cm-1 show strong
electron-molecular-vibration coupling. The mode at about 1565 cm-1 is
temperature independent below 50 K due to the methyl group motion freezing. For
(TMTSF)2PF6 in the spin density wave phase for a temperature induced
dimensionality 1D (quarter filled dimerized linear chain with correlated
electrons) the spin gapless magnon spectra have been calculated. No evidence of
spin-lattice coupling or spin-related modes is found in the Raman spectra in
the spin density wave phase.",0104528v1
2001-05-03,Zero-field Time Correlation Functions of Four Classical Heisenberg Spins on a Ring,"A model relevant for the study of certain molecular magnets is the ring of
N=4 classical spins with equal near-neighbor isotropic Heisenberg exchange
interactions. Assuming classical Heisenberg spin dynamics, we solve explicitly
for the time evolution of each of the spins. Exact triple integral
representations are derived for the auto, near-neighbor, and
next-nearest-neighbor time correlation functions for any temperature. At
infinite temperature, the correlation functions are reduced to quadrature. We
then evaluate the Fourier transforms of these functions in closed form, which
are double integrals. At low temperatures, the Fourier transform functions
explicitly demonstrate the presence of magnons. Our exact results for the
infinite temperature correlation functions in the long-time asymptotic limit
differ qualitatively from those obtained assuming diffusive spin dynamics.
Whether such explicitly non-hydrodynamic behavior would be maintained for
large-N rings is discussed.",0105050v1
2001-08-01,A microscopic model for the structural transition and spin gap formation in alpha'-NaV2O5,"We present a microscopic model for alpha'-NaV2O5. Using an extended Hubbard
model for the vanadium layers we derive an effective low-energy model
consisting of pseudospin Ising chains and Heisenberg chains coupled to each
other. We find a ``spin-Peierls-Ising'' phase transition which causes charge
ordering on every second ladder and superexchange alternation on the other
ladders. This transition can be identified with the first transition of the two
closeby transitions observed in experiment. Due to charge ordering the
effective coupling between the lattice and the superexchange is enhanced. This
is demonstrated within a Slater-Koster approximation. It leads to a second
instability with superexchange alternation on the charge-ordered ladders due to
an alternating shift of the O sites on the rungs of that ladder. We can explain
within our model the observed spin gap, the anomalous BCS ratio, and the
anomalous shift of the critical temperature of the first transition in a
magnetic field. To test the calculated superstructure we determine the
low-energy magnon dispersion and find agreement with experiment.",0108008v1
2001-08-27,Superconductivity and Quantum Phase Transitions in Weak Itinerant Ferromagnets,"It is argued that the phase transition in low-T_c clean itinerant
ferromagnets is generically of first order, due to correlation effects that
lead to a nonanalytic term in the free energy. A tricritical point separates
the line of first order transitions from Heisenberg critical behavior at higher
temperatures. Sufficiently strong quenched disorder suppresses the first order
transition via the appearance of a critical endpoint. A semi-quantitative
discussion is given in terms of recent experiments on MnSi and UGe_2. It is
then shown that the critical temperature for spin-triplet, p-wave
superconductivity mediated by spin fluctuations is generically much higher in a
Heisenberg ferromagnetic phase than in a paramagnetic one, due to the coupling
of magnons to the longitudinal magnetic susceptibility. This qualitatively
explains the phase diagram recently observed in UGe_2 and ZrZn_2.",0108443v1
2001-08-30,Spin dynamics of Sr_14Cu_24O_41 two-leg ladder studied by Raman spectroscopy,"The two-magnon (2M) excitation at 3000 cm-1 in Sr_14Cu_24O_41 two-leg ladder
is studied by Raman scattering. A slight anisotropy of the superexchange
coupling J_{\perp}/J_{||} \approx 0.8 with J_{||} = 110 \pm 20 meV is proposed
from the analysis of the magnetic scattering. The resonant coupling across the
charge transfer gap increases the 2M intensity by orders of magnitude. The
anisotropy of Raman scattering is dependent upon the excitation energy. The 2M
relaxation is found to be correlated with the temperature dependent electronic
Raman continuum at low frequencies.",0108507v2
2002-02-07,Carrier induced ferromagnetism in diluted magnetic semi-conductors,"We present a theory for carrier induced ferromagnetism in diluted magnetic
semi-conductor (DMS). Our approach treats on equal footing quantum fluctuations
within the RPA approximation and disorder within CPA. This method allows for
the calculation of $T_c$, magnetization and magnon spectrum as a function of
hole, impurity concentration and temperature. It is shown that, sufficiently
close to $T_c$, and within our decoupling scheme (Tyablicov type) the CPA for
the itinerant electron gas reduces to the Virtual Crystal Approximation. This
allows, in the low impurity concentration and low density of carriers to
provide analytical expression for $T_c$. For illustration, we consider the case
of $Ga_{1-c}Mn_{c}As$ and compare our results with available experimental data.",0202111v1
2002-02-07,Resistivity extrema in double exchange ferromagnetic nondegenerate semiconductors,"A version of the magnetoimpurity theory of the colossal magnetoresistance
materials suitable for the double exchange ferromagnetic nondegenerate
semiconductors is presented. It provides an explanation of the nonmonotonic
temperature dependence for the charge carrier density in them when it displays
first a maximum and then a minimum, on increase in temperature. Respectively,
the resistivity displays first a minimum and then a maximum. The theory is
based on the relation between the charge carrier activation energy and the
change in the magnon free energy caused by the ionization of an impurity. This
is tantamount to the relation between the charge carrier density and the so
called giant red shift of the optical absorption edge.",0202126v1
2002-02-09,Ferromagnets and antiferromagnets in the effective Lagrangian perspective,"Nonrelativistic systems exhibiting collective magnetic behavior are analyzed
within the framework of effective Lagrangians. The method, which formulates the
dynamics of the system in terms of Goldstone bosons, allows to investigate the
consequences of spontaneous symmetry breaking from a unified point of view.
Analogies and differences with respect to the Lorentz-invariant situation
(chiral perturbation theory) are pointed out. We then consider the
low-temperature expansion of the partition function both for ferro- and
antiferromagnets, where the spin waves or magnons represent the Goldstone
bosons of the spontaneously broken symmetry $O(3) \to O(2)$. In particular, the
low-temperature series of the staggered magnetization for antiferromagnets and
the spontaneous magnetization for ferromagnets are compared with the condensed
matter literature.",0202153v1
2002-03-14,Random Bond Effect in the Quantum Spin System (Tl$_{1-x}$K$_{x}$)CuCl$_3$,"The effect of exchange bond randomness on the ground state and the
field-induced magnetic ordering was investigated through magnetization
measurements in the spin-1/2 mixed quantum spin system
(Tl$_{1-x}$K$_{x}$)CuCl$_3$ for $x<0.36$. Both parent compounds TlCuCl$_3$ and
KCuCl$_3$ are coupled spin dimer systems, which have the singlet ground state
with excitation gaps ${\Delta}/k_{\rm B}=7.7$ K and 31 K, respectively. Due to
bond randomness, the singlet ground state turns into the magnetic state with
finite susceptibility, nevertheless, the excitation gap remains. Field-induced
magnetic ordering, which can be described by the Bose condensation of excited
triplets, magnons, was observed as in the parent systems. The phase transition
temperature is suppressed by the bond randomness. This behavior may be
attributed to the localization effect.",0203293v2
2002-05-17,Thermodynamic Properties of Weakly Anisotropic Disordered Magnetic Chains in the Large-S Limit,"we investigate the thermodynamic properties of weakly anisotropic disordered
magnetic chains where the nearest-neighbor Heisenberg exchange coupling is the
dominant spin-spin interaction. In addition to the exchange interaction, there
is single-ion anisotropy with the direction of the easy axis being the same for
all spins. The analysis is carried out in the limit S >> 1, where S denotes the
ionic spin, and in the temperature range T << JaveS, where Jave is the average
magnitude of the exchange interaction. It is assumed that the independent boson
picture is appropriate so that the thermodynamic properties of the chain are
those of a gas of weakly interacting magnons whose frequencies are obtained
from linearized equations of motion for the spins. The =+/-J model is studied
in detail with both random and non-random anisotropy. Particular emphasis is
placed on the existence",0205372v1
2002-05-21,Field-Induced Magnetic Ordering in the Quantum Spin System KCuCl$_3$,"KCuCl$_3$ is a three-dimensional coupled spin-dimer system and has a singlet
ground state with an excitation gap ${\Delta}/k_{\rm B}=31$ K. High-field
magnetization measurements for KCuCl$_3$ have been performed in static magnetic
fields of up to 30 T and in pulsed magnetic fields of up to 60 T. The entire
magnetization curve including the saturation region was obtained at $T=1.3$ K.
  From the analysis of the magnetization curve, it was found that the exchange
parameters determined from the dispersion relations of the magnetic excitations
should be reduced, which suggests the importance of the renormalization effect
in the magnetic excitations. The field-induced magnetic ordering accompanied by
the cusplike minimum of the magnetization was observed as in the isomorphous
compound TlCuCl$_3$. The phase boundary was almost independent of the field
direction, and is represented by the power law. These results are consistent
with the magnon Bose-Einstein condensation picture for field-induced magnetic
ordering.",0205429v1
2002-05-27,Resistivity and thermoelectric power measurements on CeFe$_2$ and its pseudobinaries,"Resistivity and thermoelectric power (TEP) measurements on CeFe$_2$ and two
of its pseudo-binaries Ce(Fe, 5% Ir)$_2$ and Ce(Fe, 7% Ru)$_2$ between 78K and
275K are reported. The resistivity data are analysed in terms of contributions
from scattering due to phonon, magnon, spin fluctuation and lattice defects,
and also from interband scattering. Attempts are made to analyze the TEP data
in terms of these resistivity components. Thermal hysteresis is observed in the
temperature dependence of TEP in the Ir and Ru doped CeFe$_2$ samples around
the ferromagnetic to antiferromagnetic transition, indicating the first order
nature of this transition.",0205539v1
2002-06-24,"Pressure-Tuned Collapse of the Mott-Like State in Ca_{n+1}Ru_nO_{3n+1} (n=1,2): Raman Spectroscopic Studies","We report a Raman scattering study of the pressure-induced collapse of the
Mott-like phases of Ca_3Ru_2O_7 (T_N=56 K) and Ca_2RuO_4 (T_N=110 K). The
pressure-dependence of the phonon and two-magnon excitations in these materials
indicate: (i) a pressure-induced collapse of the antiferromagnetic (AF)
insulating phase above P* ~ 55 kbar in Ca_3Ru_2O_7 and P* ~ 5-10 kbar in
Ca_2RuO_4, reflecting the importance of Ru-O octahedral distortions in
stabilizing the AF insulating phase; and (ii) evidence for persistent AF
correlations above the critical pressure of Ca_2RuO_4, suggestive of phase
separation involving AF insulator and ferromagnetic metal phases.",0206455v2
2002-07-02,Exchange interactions and Curie temperature in (GaMn)As,"We use supercell and frozen-magnon approaches to study the dependence of the
magnetic interactions in (Ga,Mn)As on the Mn concentration. We report the
parameters of the exchange interaction between Mn spins and the estimates of
the Curie temperature within the mean-field and random-phase approximations. In
agreement with experiment we obtain a nonmonotonous dependence of the Curie
temperature on the Mn concentration. We estimate the dependence of the Curie
temperature on the concentration of the carries in the system and show that the
decrease of the number of holes in the valence band leads to fast decrease of
the Curie temperature. We show that the hole states of the valence band are
more efficient in mediating the exchange interaction between Mn spins than the
electron states of the conduction band.",0207050v1
2002-07-30,Microscopic relaxation mechanisms and linear magnetization dynamics,"Linear magnetization dynamics in the presense of a thermal bath is analyzed
for two general classes of microscopic damping mechanisms. The resulting
stochastic differential equations are always in the form of a damped harmonic
oscillator driven by a thermal field. The damping term contains both the
interaction mechanisms and the symmetry of the magnetic system. Back
transformation from the oscillator coordinates to the magnetization variables
results in a macroscopic tensor form of damping that reflects the system
anisotropy. Scalar Landau-Lifshitz-Gilbert damping term is valid only for
systems with axial symmetry. Analysis of FMR linewith measurements versus
frequency, temperature, and film thickness in NiFe films shows good agreement
with a combination of slow-relaxing impurity and magnon-electron confluence
processes.",0207721v1
2002-09-23,Magnetization transport and quantized spin conductance,"We analyze transport of magnetization in insulating systems described by a
spin Hamiltonian. The magnetization current through a quasi one-dimensional
magnetic wire of finite length suspended between two bulk magnets is determined
by the spin conductance which remains finite in the ballistic limit due to
contact resistance. For ferromagnetic systems, magnetization transport can be
viewed as transmission of magnons and the spin conductance depends on the
temperature T. For antiferromagnetic isotropic spin-1/2 chains, the spin
conductance is quantized in units of order $(g \mu_B)^2/h$ at T=0.
Magnetization currents produce an electric field and hence can be measured
directly. For magnetization transport in electric fields phenomena analogous to
the Hall effect emerge.",0209521v2
2002-11-26,Spin dynamics of stripes,"The spin dynamics of stripes in high-temperature superconductors and related
compounds is studied in the framework of a spin-wave theory for a simple
spin-only model. The magnon dispersion relation and the magnetic structure
factor are calculated for diagonal and vertical stripes. Acoustical as well as
optical bands are included in the analysis. The incommensurability and the
$\pi$ resonance appear as complementary features of the band structure at
different energy scales. The dependence of spin-wave velocities and resonance
frequencies on the stripe spacing and coupling is calculated. At low doping,
the resonance frequency is found to scale roughly inversely proportional to the
stripe spacing. The favorable comparison of the results with experimental data
suggests that the spin-only model provides a suitable and simple basis for
calculating and understanding the spin dynamics of stripes.",0211593v1
2003-01-10,Quantum Spin Chains with Nonlocally-Correlated Random Exchange Coupling and Random-Mass Dirac Fermions,"S=1/2 quantum spin chains and ladders with random exchange coupling are
studied by using an effective low-energy field theory and transfer matrix
methods. Effects of the nonlocal correlations of exchange couplings are
investigated numerically. In particular we calculate localization length of
magnons, density of states, correlation functions and multifractal exponents as
a function of the correlation length of the exchange couplings. As the
correlation length increases, there occurs a ""phase transition"" and the above
quantities exhibit different behaviors in two phases. This suggests that the
strong-randomness fixed point of the random spin chains and random-singlet
state get unstable by the long-range correlations of the random exchange
couplings.",0301139v2
2003-04-15,Ferromagnetic Kondo-Lattice Model,"We present a many-body approach to the electronic and magnetic properties of
the (multiband) Kondo-lattice model with ferromagnetic interband exchange. The
coupling between itinerant conduction electrons and localized magnetic moments
leads, on the one hand, to a distinct temperature-dependence of the electronic
quasiparticle spectrum and, on the other hand, to magnetic properties, as
e.~g.the Curie temperature T_C or the magnon dispersion, which are strongly
influenced by the band electron selfenergy and therewith in particular by the
carrier density. We present results for the single-band Kondo-lattice model in
terms of quasiparticle densities of states and quasiparticle band structures
and demonstrate the density-dependence of the self-consistently derived Curie
temperature. The transition from weak-coupling (RKKY) to strong-coupling
(double exchange) behaviour is worked out.
  The multiband model is combined with a tight-binding-LMTO bandstructure
calculation to describe real magnetic materials. As an example we present
results for the archetypal ferromagnetic local-moment systems EuO and EuS. The
proposed method avoids the double counting of relevant interactions and takes
into account the correct symmetry of atomic orbitals.",0304338v1
2003-04-24,Ferromagnetism in a dilute magnetic semiconductor -- Generalized RKKY interaction and spin-wave excitations,"Carrier-mediated ferromagnetism in a dilute magnetic semiconductor has been
studied using i) a single-impurity based generalized RKKY approach which goes
beyond linear response theory, and ii) a mean-field-plus-spin-fluctuation
(MF+SF) approach within a (purely fermionic) Hubbard-model representation of
the magnetic impurities, which incorporates dynamical effects associated with
finite frequency spin correlations in the ordered state. Due to a competition
between the magnitude of the carrier spin polarization and its oscillation
length scale, the ferromagnetic spin coupling is found to be optimized with
respect to both hole doping concentration and impurity-carrier spin coupling
energy $J$ (or equivalently $U$). The ferromagnetic transition temperature
$T_c$, deteremined within the spin-fluctuation theory, corresponds closely with
the observed $T_c$ values. Positional disorder of magnetic impurities causes
significant stiffening of the high-energy magnon modes. We also explicitly
study the stability/instability of the mean-field ferromagnetic state, which
highlights the role of competing AF interactions causing spin twisting and
noncollinear ferromagnetic ordering.",0304557v2
2003-05-21,Disorder Effect on Spin Excitation in Double Exchange Systems,"Spin excitation spectrum is studied in the double exchange model in the
presence of disorder. Spin wave approximation is applied in the lowest order of
1/S expansion. The disorder causes anomalies in the spin excitation spectrum
such as broadening, branching, anticrossing with gap opening. The origin of the
anomalies is the Friedel oscillation, in which the perfectly polarized
electrons form the charge density wave to screen the disorder effect. Near the
zone center $q = 0$, the linewidth has a $q$ linear component while the
excitation energy scales to $q^2$, which indicates that the magnon excitation
is incoherent. As $q$ increases, there appears a crossover from this incoherent
behavior to the marginally coherent one in which both the linewidth and the
excitation energy are proportional to $q^2$. The results are compared with
experimental results in colossal magnetoresistance manganese oxides.
Quantitative comparison of the linewidth suggests that spatially-correlated or
mesoscopic-scale disorder is more relevant in real compounds than local or
atomic-scale disorder. Comparison with other theoretical proposals is also
discussed. Experimental tests are proposed for the relevance of disorder.",0305488v2
2003-06-05,Quasiparticle vanishing driven by geometrical frustration,"We investigate the single hole dynamics in the triangular t-J model. We study
the structure of the hole spectral function, assuming the existence of a 120
magnetic Neel order. Within the self-consistent Born approximation (SCBA) there
is a strong momentum and t sign dependence of the spectra, related to the
underlying magnetic structure and the particle-hole asymmetry of the model. For
positive t, and in the strong coupling regime, we find that the low energy
quasiparticle excitations vanish outside the neighbourhood of the magnetic
Goldstone modes; while for negative t the quasiparticle excitations are always
well defined. In the latter, we also find resonances of magnetic origin whose
energies scale as (J/t)^2/3 and can be identified with string excitations. We
argue that this complex structure of the spectra is due to the subtle interplay
between magnon-assisted and free hopping mechanisms. Our predictions are
supported by an excellent agreement between the SCBA and the exact results on
finite size clusters. We conclude that the conventional quasiparticle picture
can be broken by the effect of geometrical magnetic frustration.",0306147v3
2003-07-10,Non-dissipative thermal transport in the massive regimes of the XXZ chain,"We present exact results on the thermal conductivity of the one-dimensional
spin-1/2 XXZ model in the massive antiferromagnetic and ferromagnetic regimes.
The thermal Drude weight is calculated by a lattice path integral formulation.
Numerical results for wide ranges of temperature and anisotropy as well as
analytical results in the low and high temperature limits are presented. At
finite temperature, the thermal Drude weight is finite and hence there is
non-dissipative thermal transport even in the massive regime. At low
temperature, the thermal Drude weight behaves as $D(T)\sim
\exp(-\delta/T)/\sqrt{T}$ where $\delta$ is the one-spinon (respectively
one-magnon) excitation energy for the antiferromagnetic (respectively
ferromagnetic) regime.",0307227v1
2003-07-10,Vortex behavior near a spin vacancy in 2D XY-magnets,"The dynamical behavior of anisotropic two dimensional Heisenberg models is
still a matter of controversy. The existence of a central peak at all
temperatures and a rich structure of magnon peaks are not yet understood. It
seems that the central peaks are related, in some way, to structures like
vortices. In order to contribute to the discussion of the dynamical behavior of
the model we use Monte Carlo and spin dynamics simulations as well analytical
calculations to study the behavior of vortices in the presence of nonmagnetic
impurities. Our simulations show that vortices are attracted and trapped by the
impurities. Using this result we show that if we suppose that vortices are not
very much disturbed by the presence of the impurities, then they work as an
attractive potential to the vortices explaining the observed behavior in our
simulations.",0307245v1
2003-07-16,Diffusive energy transport in the S=1 Haldane chain compound AgVP2S6,"We present the results of measurements of the thermal conductivity $\kappa$
of the spin S=1 chain compound AgVP_2S_6 in the temperature range between 2 and
300 K and with the heat flow directed either along or perpendicular to the
chain direction. The analysis of the anisotropy of the heat transport allowed
for the identification of a small but non-negligible magnon contribution
$\kappa_m$ along the chains, superimposed on the dominant phonon contribution
$\kappa_ph$. At temperatures above about 100 K the energy diffusion constant
D_E(T), calculated from the $\kappa_m(T)$ data, exhibits similar features as
the spin diffusion constant D_S(T), previously measured by NMR. In this regime,
the behaviour of both transport parameters is consistent with a diffusion
process that is caused by interactions inherent to one-dimensional S=1 spin
systems.",0307383v1
2003-08-14,Inhomogeneous CuO_{6} Tilt Distribution and Charge/Spin Correlations in La_{2-x-y}Nd_{y}Sr_{x}CuO$_{4} around commensurate hole concentration,"Phononic and magnetic Raman scattering are studied in
La$_{2-x-y}$Nd$_{y}$Sr$_{x}$CuO$_{4}$ with three doping concentrations: x ~
1/8, y = 0; x ~ 1/8, y = 0.4; and x = 0.01, y = 0. We observe strong disorder
in the tilt pattern of the CuO_{6} octahedra in both the orthorhombic and
tetragonal phases which persist down to 10 K and are coupled to bond disorder
in the cation layers around 1/8 doping independent of Nd concentration. The
weak magnitude of existing charge/spin modulations in the Nd doped structure
does not allow us to detect the specific Raman signatures on lattice dynamics
or two-magnon scattering around 2200 cm-1.",0308295v1
2003-08-26,Dynamics of an anisotropic Haldane antiferromagnet in strong magnetic field,"We report the results of elastic and inelastic neutron scattering experiments
on the Haldane gap quantum antiferromagnet Ni(C5D14N2)2N3(PF6) performed at mK
temperatures in a wide range of magnetic field applied parallel to the S = 1
spin chains. Even though this geometry is closest to an ideal axially symmetric
configuration, the Haldane gap closes at the critical field Hc~4T, but reopens
again at higher fields. The field dependence of the two lowest magnon modes is
experimentally studied and the results are compared with the predictions of
several theoretical models. We conclude that of several existing theories, only
the recently proposed model [Zheludev et al., cond-mat/0301424 ] is able to
reproduce all the features observed experimentally for different field
orientations.",0308545v1
2003-09-18,Magnetization plateaus in frustrated antiferromagnetic quantum spin models,"Plateaus can be observed in the zero-temperature magnetization curve of
quantum spin systems at rational values of the magnetization. In one dimension,
the appearance of a plateau is controlled by a quantization condition for the
magnetization which involves the length of the local spin and the volume of a
translational unit cell of the ground state. We discuss examples of
geometrically frustrated quantum spin systems with large (in general unbounded)
periodicities of spontaneous breaking of translational symmetry in the ground
state.
  In two dimensions, we discuss the square, triangular and Kagome lattices
using exact diagonalization (ED) for up to N=40 sites. For the spin-1/2 XXZ
model on the triangular lattice we study the nature and stability region of a
plateau at one third of the saturation magnetization. The Kagome lattice gives
rise to particularly rich behaviour with several plateaus in the magnetization
curve and a jump due to local magnon excitations just below saturation.",0309425v2
2003-10-09,Exchange interactions in (ZnMn)Se,"One of the remarkable properties of the II-VI diluted magnetic semiconductor
(ZnMn)Se is the giant spin splitting of the valence band states under
application of the magnetic field (giant Zeeman splitting). This splitting
reveals strong exchange interaction between Mn moments and semiconductor
states. On the other hand, no magnetic phase transition has been observed for
systems with small Mn content up to very low temperatures. The latter property
shows weakness of the exchange interaction between Mn moments. In this paper,
the local density approximation (LDA) and the LDA+U techniques are employed to
study exchange interactions in (ZnMn)Se. Supercell and frozen-magnon approaches
applied earlier to III-V diluted magnetic semiconductors are used. It is found
that both LDA and LDA+U describe successfully the combination of the strong
Zeeman splitting and weak interatomic exchange. However, the physical pictures
provided by two techniques differ strongly. A detailed analysis shows that the
LDA+U method provides the description of the system which is much closer to the
experimental data.",0310206v1
2003-10-23,On implementation of ferrite magnetostatic/magnetoelectric particles for quantum computation,"We consider an implementation of quantum gates for quantum computation using
magnetostatic/magnetoelectric (MS/ME) macroscopically quantized states in small
ferrite disks. Confinement phenomena for MS oscillations in a normally
magnetized ferrite disk show typical atomic properties like discrete energy
levels. Because of discrete energy eigenstates of MS oscillations, the
oscillating system is described as a collective motion of quasi-particles - the
light magnons. A macroscopic quantum analysis of MS oscillations underlines the
physics of quantized ME oscillating spectrums in ferrite disks with surface
electrodes. We discuss possible technologies for physical realization of new
logic gates based on MS/ME-particle qubits.",0310558v1
2003-11-22,Influence of a Uniform Current on Collective Magnetization Dynamics in a Ferromagnetic Metal,"We discuss the influence of a uniform current, $\vec{j} $, on the
magnetization dynamics of a ferromagnetic metal. We find that the magnon energy
$\epsilon(\vec{q})$ has a current-induced contribution proportional to
$\vec{q}\cdot \vec{\cal J}$, where $\vec{\cal J}$ is the spin-current, and
predict that collective dynamics will be more strongly damped at finite ${\vec
j}$. We obtain similar results for models with and without local moment
participation in the magnetic order. For transition metal ferromagnets, we
estimate that the uniform magnetic state will be destabilized for $j \gtrsim
10^{9} {\rm A} {\rm cm}^{-2}$. We discuss the relationship of this effect to
the spin-torque effects that alter magnetization dynamics in inhomogeneous
magnetic systems.",0311522v1
2003-11-24,Charge-Density-Wave Formation in the Doped Two-Leg Extended Hubbard Ladder,"We investigate electronic properties of the doped two-leg Hubbard ladder with
both the onsite and the nearest-neighbor Coulomb repulsions, by using the the
weak-coupling renormalization-group method. It is shown that, for strong
nearest-neighbor repulsions, the charge-density-wave state coexisting with the
p-density-wave state becomes dominant fluctuation where spins form intrachain
singlets. By increasing doping rate, we have also shown that the effects of the
nearest-neighbor repulsions are reduced and the system exhibits a quantum phase
transition into the d-wave-like (or rung-singlet) superconducting state. We
derive the effective fermion theory which describes the critical properties of
the transition point with the gapless excitation of magnon. The phase diagram
of the two-leg ladder compound, Sr_{14-x}Ca_xCu_{24}O_{41}, is discussed.",0311534v2
2003-12-02,Frustrated antiferromagnets at high fields: the Bose-Einstein condensation in degenerate spectra,"Quantum phase transition at the saturation field is studied for a class of
frustrated quantum antiferromagnets. The considered models include (i) the
$J_1$-$J_2$ frustrated square-lattice antiferromagnet with $J_2={1/2}J_1$ and
(ii) the nearest-neighbor Heisenberg antiferromagnet on a face centered cubic
lattice. In the fully saturated phase the magnon spectra for the two models
have lines of degenerate minima. Transition into partially magnetized state is
treated via a mapping to a dilute gas of hard core bosons and by complementary
spin-wave calculations. Momentum dependence of the exact four-point boson
vertex removes the degeneracy of the single-particle excitation spectra and
selects the ordering wave-vectors at $(\pi,\pi)$ and $(\pi,0,0)$ for the two
models. The asymptotic behavior of the magnetization curve differs
significantly from that of conventional antiferromagnet in $d$-spatial
dimensions. We predict a unique form for the magnetization curve $\Delta
M=S-M\simeq \mu^{(d-1)/2}(\log\mu)^{(d-1)}$, where $\mu$ is a distance from the
quantum critical point.",0312076v2
2003-12-17,Goldstone-Mode Relaxation in a Quantized Hall Ferromagnet in the Presence of Smooth Random Potential,"We discuss the spin relaxation of a strongly correlated two-dimensional (2D)
electron gas (2DEG) in the quantized Hall regime when the filling factor is
close to an odd-integer. As the initial state we consider a coherent deviation
of the spin system from the ${\bf B}$ direction and investigate a break-down of
this Goldstone-mode state due to the spin-orbit (SO) coupling and smooth
disorder. The spin relaxation (SR) process is considered in terms of
annihilation transitions in the system of spin excitons (magnons).",0312427v5
2003-12-23,Ferromagnetic tunneling junctions at low voltages: elastic versus inelastic scattering at $T=0 K$,"In this paper we analyze different contributions to the magnetoresistance of
magnetic tunneling junctions at low voltages. A substantial fraction of the
resistance drop with voltage can be ascribed to variations of the density of
states and the barrier transmission with the bias. However, we found that the
anomaly observed at zero bias and the magnetoresistance behavior at very small
voltages, point to the contribution of inelastic magnon-assisted tunneling. The
latter is described by a transfer parameter $T^{J}$, which is one or two orders
of magnitude smaller than $T^{d}$, the direct transmission for elastic
currents. Our theory is in excellent agreement with experimental data, yielding
estimated values of $T^{J}$ which are of the order of $T^{d}$ / $T^{J}$ ~ 40.",0312608v1
2004-04-07,"First-principles calculation of the intersublattice exchange interactions and Curie temperatures of full Heusler alloys Ni2MnX (X=Ga, In, Sn, Sb)","The interatomic exchange interactions and Curie temperatures in Ni-based full
Heusler alloys Ni2MnX with X=Ga, In, Sn and Sb are studied within the framework
of the density-functional theory. The calculation of the exchange parameters is
based on the frozen-magnon approach. Despite closeness of the experimental
Curie temperatures for all four systems their magnetism appeared to differ
strongly. This difference involves both the Mn-Mn and Mn-Ni exchange
interactions. The Curie temperatures, Tc, are calculated within the mean-field
approximation by solving a matrix equation for a multi-sublattice system. Good
agreement with experiment for all four systems is obtained. The role of
different exchange interactions in the formation of Tc of the systems is
discussed.",0404162v1
2004-04-08,Finite low-temperature entropy of some strongly frustrated quantum spin lattices in the vicinity of the saturation field,"For a class of highly frustrated antiferromagnetic quantum spin lattices the
ground state exhibits a huge degeneracy in high magnetic fields due to the
existence of localized magnon states. For some of these spin lattices (in
particular, the 1D dimer-plaquette, sawtooth and kagom\'{e}-like chains as well
as the 2D kagom\'{e} lattice) we calculate rigorously the ground-state entropy
at the saturation field. We find that the ground-state entropy per site remains
finite at saturation. This residual ground-state entropy produces a maximum in
the field dependence of the isothermal entropy at low temperatures. By
numerical calculation of the field dependence of the low-temperature entropy
for the sawtooth chain we find that the enhancement of isothermal entropy is
robust against small deviations in exchange constants. Moreover, the effect is
most pronounced in the extreme quantum case of spin 1/2.",0404204v1
2004-05-17,Itinerant Ferromagnetism and Superconductivity,"Superconductivity has again become a challenge following the discovery of
unconventional superconductivity. Resistance-free currents have been observed
in heavy-fermion materials, organic conductors and copper oxides. The discovery
of superconductivity in a single crystal of $UGe_2$, $ZrZn_2$ and $URhGe$
revived the interest in the coexistence of superconductivity and
ferromagnetism. The experiments indicate that: i)The superconductivity is
confined to the ferromagnetic phase. ii)The ferromagnetic order is stable
within the superconducting phase (neutron scattering experiments). iii) The
specific heat anomaly associated with the superconductivity in these materials
appears to be absent. The specific heat depends on the temperature linearly at
low temperature.
  I present a review of the recent experimental results and the basic
theoretical ideas concerning ferromagnetic superconductivity
(FM-superconductivity) induced by the ferromagnetic spin fluctuations. A
particular attention is paid to the magnon exchange mechanism of
FM-superconductivity.",0405371v2
2004-06-04,Absence of magnetic order for the spin-half Heisenberg antiferromagnet on the star lattice,"We study the ground-state properties of the spin-half Heisenberg
antiferromagnet on the two-dimensional star lattice by spin-wave theory, exact
diagonalization and a variational mean-field approach. We find evidence that
the star lattice is (besides the \kagome lattice) a second candidate among the
11 uniform Archimedean lattices where quantum fluctuations in combination with
frustration lead to a quantum paramagnetic ground state. Although the classical
ground state of the Heisenberg antiferromagnet on the star exhibits a huge
non-trivial degeneracy like on the \kagome lattice, its quantum ground state is
most likely dimerized with a gap to all excitations. Finally, we find several
candidates for plateaux in the magnetization curve as well as a macroscopic
magnetization jump to saturation due to independent localized magnon states.",0406103v2
2004-08-02,Disorder in Diluted Spin Systems,"The influence of substitutional disorder on the magnetic properties of
diluted Heisenberg spin systems is studied with regard to the magnetic
stability of ferromagnetic diluted semiconductors (DMS). The equation of motion
for the magnon Green's function within Tyablikov approximation is solved
numerically for finite systems. The resulting spectral density is then used to
estimate the magnetization and Curie temperature of an infinite system. This
method is suitable for any form of a ferromagnetic exchange interaction.
Besides different lattices and spin magnitude $S$, exchange interactions of
different range are examined. The results show that, for short-range
interaction, no magnetic order exists below the critical percolation
concentration, whereas a linear dependence of the Curie temperature on the
concentration of spins is found for ferromagnetic long-range interaction.",0408039v1
2004-08-02,Topological Theory of Electron-Phonon Interactions in High Temperature Superconductors,"There are large isotope effects in the phonon kinks observed in photoemission
spectra (ARPES) of optimally doped cuprate high temperature superconductors
(HTSC), but they are quite different (Gweon et al. 2004) from those expected
for a nearly free electron metal with strong electron-phonon interactions (Tang
et al. 2003). These differences, together with many other anomalies in infrared
spectra, seem to suggest that other particles (such as magnons) must be
contributing to HTSC. Here we use topological (non-Hamiltonian) methods to
discuss the data, emphasizing nanoscale phase separation and the importance of
a narrow band of quantum percolative states near the Fermi energy that is
spatially pinned to a self-organized filamentary dopant array. Topological
discrete, noncontinuum, nonperturbative methods have previously explained the
form of HTSC phase diagrams without involving detailed microscopic assumptions,
and they are especially useful in the presence of strong nanoscale disorder.
These methods also explain the ``miracle'' of an ideal nearly free electron
phonon kink in sharply defined nodal quasiparticle states in LSCO at the
metal-insulator transition. Finally the universality of the kink energy and
Fermi velocity in different cuprates is the result of the marginally elastic
nature of these materials, and specifically the isostatic character of the CuO2
planes.",0408040v1
2004-09-04,Stochastic dynamic simulations of fast remagnetization processes: recent advances and applications,"Numerical simulations of fast remagnetization processes using the stochastic
dynamics are widely used to study various magnetic systems. In this paper we
first address several crucial methodological problems of such simulations: (i)
the influence of the finite-element discretization on the simulated dynamics,
(ii) choice between Ito and Stratonovich stochastic calculi by the solution of
micromagnetic stochastic equations of motion and (iii) non-trivial correlation
properties of the random (thermal) field. Next we discuss several examples to
demonstrate the great potential of the Langevin dynamics for studying fast
remagnetization processes in technically relevant applications: we present
numerical analysis of equilibrium magnon spectra in patterned structures, study
thermal noise effects on the magnetization dynamics of nanoelements in pulsed
fields and show some results for a remagnetization dynamics induced by a
spin-polarized current.",0409098v1
2004-09-21,The field-induced magnetic ordering transition in TlCuCl$_3$,"In the first part we investigate in detail if the Bose-Einstein condensation
scenario for magnons can quantitatively explain the observed field-induced
magnetic ordering in TlCuCl$_3$. We use a bond-operator approach to map the
spin system onto hard-core bosons and exactly account for the hard-core
constraint in the dilute limit. We solve the hard-core model within the
Hartree-Fock-Popov approximation and discuss its validity and the consequences
of this approximation for the critical properties. In the second part the
effects of spin-phonon and spin-orbit coupling are discussed within this
framework. We show that the experimental magnetization and specific heat data
are well described if a certain type of anisotropy is included. We also present
predictions for the quasiparticle gap which might be tested in the future.",0409530v1
2004-10-01,Magnetic Properties of Cuprate Perovskites,"The magnetic susceptibility of underdoped yttrium and lanthanum cuprates is
interpreted based on the self-consistent solution of the t-J model of a Cu-O
plane. The calculations reproduce correctly the frequency dependencies of the
susceptibility in YBa2Cu3O_{7-y} and La_{2-x}Sr_xCuO4 attributing their
dissimilarity to the difference in the damping of spin excitations. In
YBa2Cu3O_{7-y} these excitations are well defined at the antiferromagnetic wave
vector Q=(\pi,\pi) even in the normal state which manifests itself in a
pronounced maximum -- the resonance peak -- in the susceptibility. In
La_{2-x}Sr_xCuO4 the spin excitations are overdamped which leads to a broad
low-frequency feature in the susceptibility. The low-frequency
incommensurability in the magnetic response is attributed to a dip in the
magnon damping at Q. The calculated concentration and temperature dependencies
of the incommensurability parameter conform with experimental observations.
Generally the incommensurate magnetic response is not accompanied with an
inhomogeneity of the carrier density.",0410009v1
2004-10-07,Quantum Fluctuations and Excitations in Antiferromagnetic Quasicrystals,"We study the effects of quantum fluctuations and the excitation spectrum for
the antiferromagnetic Heisenberg model on a two-dimensional quasicrystal, by
numerically solving linear spin-wave theory on finite approximants of the
octagonal tiling. Previous quantum Monte Carlo results for the distribution of
local staggered magnetic moments and the static spin structure factor are
reproduced well within this approximate scheme. Furthermore, the magnetic
excitation spectrum consists of magnon-like low-energy modes, as well as
dispersionless high-energy states of multifractal nature. The dynamical spin
structure factor, accessible to inelastic neutron scattering, exhibits
linear-soft modes at low energies, self-similar structures with bifurcations
emerging at intermediate energies, and flat bands in high-energy regions. We
find that the distribution of local staggered moments stemming from the
inhomogeneity of the quasiperiodic structure leads to a characteristic energy
spread in the local dynamical spin susceptibility, implying distinct nuclear
magnetic resonance spectra, specific for different local environments.",0410180v1
2004-11-26,The spin-half Heisenberg antiferromagnet on the square-kagomé lattice: Ground state and low-lying excitations,"We discuss the ground state and the low-lying excitations of the spin-half
Heisenberg antiferromagnet on the two-dimensional square-kagome lattice. This
magnetic system belongs to the class of highly frustrated spin systems with an
infinite non-trivial degeneracy of the classical ground state as it is known
also for the Heisenberg antiferromagnet on the kagome and on the star lattice.
The quantum ground state of the spin-half system is a quantum paramagnet likely
with a finite spin gap and with a large number of non-magnetic excitations
within this gap. The magnetization versus field curve shows plateaux as well as
a macroscopic magnetization jump to saturation due to independent localized
magnon states.",0411673v1
2004-12-02,Interplay between SDW and induced local moments in URu2Si2,"Theoretical model for magnetic ordering in the heavy-fermion metal URu2Si2 is
suggested. The 17.5K transition in this material is ascribed to formation of a
spin-density wave, which develops due to a partial nesting between electron and
hole parts of the Fermi surface and has a negligibly small form-factor.
Staggered field in the SDW state induces tiny antiferromagnetic order in the
subsystem of localized singlet-singlet levels. Unlike the other models our
scenario is based on coexistence of two orderings with the same
antiferromagnetic dipole symmetry.The topology of the pressure phase diagram
for such a two order parameter model is studied in the framework of the Landau
theory. The field dependences of the staggered magnetization and the magnon gap
are derived from the microscopic theory and found to be in good quantitative
agreement with experiment.",0412055v2
2004-12-10,Evidence for very strong electron-phonon coupling in YBa_{2}Cu_{3}O_{6},"From the observed oxygen-isotope shift of the mid-infrared two-magnon
absorption peak of YBa$_{2}$Cu$_{3}$O$_{6}$, we evaluate the oxygen-isotope
effect on the in-plane antiferromagnetic exchange energy $J$. The exchange
energy $J$ in YBa$_{2}$Cu$_{3}$O$_{6}$ is found to decrease by about 0.9% upon
replacing $^{16}$O by $^{18}$O, which is slightly larger than that (0.6%) in
La$_{2}$CuO$_{4}$. From the oxygen-isotope effects, we determine the lower
limit of the polaron binding energy, which is about 1.7 eV for
YBa$_{2}$Cu$_{3}$O$_{6}$ and 1.5 eV for La$_{2}$CuO$_{4}$, in quantitative
agreement with angle-resolved photoemission data, optical conductivity data,
and the parameter-free theoretical estimate. The large polaron binding energies
in the insulating parent compounds suggest that electron-phonon coupling should
also be strong in doped superconducting cuprates and may play an essential role
in high-temperature superconductivity.",0412262v3
2005-01-03,Magnon and Hole Excitations in the Two-Dimensional Half-filled Hubbard Model,"Spin and hole excitation spectra and spectral weights are calculated for the
half-filled Hubbard model, as a function of $t/U$. We find that the high energy
spin spectra are sensitive to charge fluctuations. The energy difference
$\Delta(\pi,0)- \Delta(\pi/2,\pi/2)$, which is negative for the Heisenberg
model, changes sign at a fairly small $t/U\approx 0.053(5)$. The hole bandwidth
is proportional to $J$, and considerably larger than in the t-J models. It has
a minimum at ($\pi/2,\pi/2$) and a very weak dispersion along the
antiferromagnetic zone boundary. A good fit to the measured spin spectra in
La$_2$CuO$_4$ at $T=10K$ is obtained with the parameter values $U=3.1{\rm eV}$,
$t=0.35{\rm eV}$.",0501029v1
2005-01-10,Spin excitations under fields in an anisotropic bond-alternating quantum S=1 chain: contrast with Haldane spin chains,"Inelastic neutron scattering experiments on the S=1 quasi-one-dimensional
bond-alternating antiferromagnet Ni(C9D24N4)(NO2)ClO4 have been performed under
magnetic fields below and above a critical field Hc at which the energy gap
closes. Normal field dependece of Zeeman splitting of the excited triplet modes
below Hc has been observed, but the highest mode is unusually small and smears
out with increasing field. This can be explained by an interaction with a
low-lying two magnon continuum at q=pi that is present in dimerized chains but
absent in uniform ones. Above Hc, we find only one excited mode, in stark
contrast with three massive excitations previously observed in the structurally
similar Haldane-gap material NDMAP [A. Zheludev et al., Phys. Rev. B 68, 134438
(2003)].",0501207v1
2005-01-20,Magnetically Stabilized Order. II: Critical states and algebraically ordered nematic spin liquids in one-dimensional optical lattices,"We investigate the Zeeman-field-driven quantum phase transitions between
singlet spin liquids and algebraically ordered O(2) nematic spin liquids of
spin-one bosons in one-dimensional optical lattices. We find that the critical
behavior is characterized by condensation of hardcore bosons instead of ideal
magnons in high dimensional lattices. Critical exponents are strongly
renormalized by hardcore interactions and critical states are equivalent to the
free Fermion model up to the Friedal oscillations. We also find that the
algebraically ordered nematic spin liquids close to critical points are fully
characterized by the Luttinger liquid dynamics with Luttinger liquid parameters
magnetically tunable. The Bethe Ansatz solution has been applied to determine
the critical magnetization and nematic correlations.",0501490v2
2005-02-01,Can Frustration Preserve a Quasi-Two-Dimensional Spin Fluid?,"Using spin-wave theory, we show that geometric frustration fails to preserve
a two-dimensional spin fluid. Even though frustration can remove the interlayer
coupling in the ground-state of a classical anti-ferromagnet, spin layers
innevitably develop a quantum-mechanical coupling via the mechanism of ``order
from disorder''. We show how the order from disorder coupling mechanism can be
viewed as a result of magnon pair tunneling, a process closely analogous to
pair tunneling in the Josephson effect. In the spin system, the Josephson
coupling manifests itself as a a biquadratic spin coupling between layers, and
for quantum spins, these coupling terms are as large as the inplane coupling.
An alternative mechanism for decoupling spin layers occurs in classical XY
models in which decoupled ""sliding phases"" of spin fluid can form in certain
finely tuned conditions. Unfortunately, these finely tuned situations appear
equally susceptible to the strong-coupling effects of quantum tunneling,
forcing us to conclude that in general, geometric frustration cannot preserve a
two-dimensional spin fluid.",0502003v2
2005-02-11,An ab initio effective Hamiltonian for magnetism including longitudinal spin fluctuations,"We discuss the use of the magnetic force theorem (MFT) using different
reference states upon which the perturbative approach is based. Using a
disordered local moment (DLM) state one finds goo d Curie (or Ne\'el)
temperatures, and good energetics for planar spin spirals in the 3d magnets Fe,
Co (fcc), Mn, Cr. On the other hand the ferromagnetic reference state provides
excellent energetics for small $\theta$ spin spirals in Fe, Co and Ni, and by
extension magnon energies under the assumption of adiabacity. However, planar
spin spiral energetics and transition temperatures for Ni, Fe, Mn, and Cr show
worse agreement. The reasons for this, and for the case of fcc Co whe re both
approaches work very well are discussed. We further provide an extension of the
mapping of the quantum problem to include longitudinal fluctuations, and
discuss the role they will play in magnetic phase transitions. This constru
ction is tested using planar spin spirals where ${\bf q}$ is fixed but the
moment is allowed to rel ax. It is demonstrated that results from the magnetic
force theorem approach and directly calculated {\it ab initio} values agree
very well.",0502291v1
2005-02-18,Frustrated Quantum Magnets,"A description of different phases of two dimensional magnetic insulators is
given. The first chapters are devoted to the understanding of the symmetry
breaking mechanism in the semi-classical Neel phases. Order by disorder
selection is illustrated. All these phases break SU(2) symmetry and are gapless
phases with magnon excitations. Different gapful quantum phases exist in two
dimensions: the Valence Bond Crystal phases (VBC) which have long range order
in local S=0 objects (either dimers in the usual Valence Bond acception or
quadrumers..), but also Resonating Valence Bond Spin Liquids (RVBSL), which
have no long range order in any local order parameter and an absence of
susceptibility to any local probe. VBC have gapful S = 0, or 1 excitations,
RVBSL on the contrary have deconfined spin-1/2 excitations. Examples of these
two kinds of quantum phases are given in chapters 4 and 5. A special class of
magnets (on the kagome or pyrochlore lattices) has an infinite local degeneracy
in the classical limit: they give birth in the quantum limit to different
behaviors which are illustrated and questionned in the last lecture.",0502464v1
2005-02-18,High-energy Cu spin excitations in PrBa2Cu3O6+x,"This paper describes high-energy neutron inelastic scattering measurements of
propagating magnetic excitations in PrBa2Cu3O6+x (x approx 0.2 and 0.93). The
measurements probe the acoustic and optic modes of the
antiferromagnetically-ordered copper--oxygen bilayers in the energy range
50-150 meV. The observed magnon dispersion can be described satisfactorily in
this energy range by a spin wave model including intra- and inter-layer
nearest-neighbour exchange constants J_parallel and J_perp. We find J_parallel
= 127 +/- 10 meV and J_perp = 5.5 +/- 0.9 meV. The value of J_parallel is
virtually the same as that found in YBa2Cu3O6.2, but J_perp is a factor of two
smaller. To within experimental error the values of J_parallel and J_perp for
PrBa2Cu3O6+x do not vary with oxygen doping.",0502467v1
2005-02-21,On the finite temperature Drude weight of the anisotropic Heisenberg chain,"We present a study of the Drude weight $D(T)$ of the spin-1/2 $XXZ$ chain in
the gapless regime. The thermodynamic Bethe ansatz (TBA) is applied in two
different ways. In the first application we employ the particle basis of
magnons and their bound states. In this case we rederive and considerably
extend earlier work in the literature. However, in the course of our
investigation we find arguments that cast doubt on the applicability of the TBA
in this case. In a second application by use of the spinon and anti-spinon
particle basis we obtain completely different results. Only for anisotropy
parameter $\Delta$ close to 0 we find that $D(T)$ is a monotonously decaying
function of temperature. For $\Delta$ close to 1 the behaviour is entirely
different showing a finite temperature maximum. Also for the isotropic
antiferromagnetic chain ($\Delta=1$) the results for $D(T)$ are finite for T=0
as well as for $T>0$ with an infinite positive slope at T=0.",0502516v1
2005-03-03,Quantum Phase Transitions of Frustrated Heisenberg Antiferromagnets: a Comprehensive Renormalization Approach,"We develop a novel renormalization approach for frustrated quantum
antiferromagnets. It is designed to consistently treat spin-wave interactions
all over the magnetic Brillouin zone, including high-energy modes in outer
regions as well as low energy modes in the center. Focusing on the paradigmatic
$J_1$-$J_2$ model, we find a unifying description of the second-order
transition between the N\'eel phase and the paramagnetic phase and the
first-order transition between the N\'eel phase and the columnar phase. Our
approach provides explicit results for the renormalized values of the spin
stiffness and spin-wave velocity characterizing the low-energy magnons in the
N\'eel phase.",0503076v2
2005-03-17,Quantum Criticality in an Organic Magnet,"Exchange interactions between $S=\frac{1}{2}$ sites in piperazinium
hexachlorodicuprate produce a frustrated bilayer magnet with a singlet ground
state. We have determined the field-temperature phase diagram by high field
magnetization and neutron scattering experiments. There are two quantum
critical points: $H_{c1}=7.5$ T separates a quantum paramagnet phase from a
three dimensional, antiferromagnetically-ordered state while $H_{c2}=37$ T
marks the onset of a fully polarized state. The ordered phase, which we
describe as a magnon Bose-Einstein condensate (BEC), is embedded in a quantum
critical regime with short range correlations. A low temperature anomaly in the
BEC phase boundary indicates that additional low energy features of the
material become important near $H_{c1}$.",0503450v2
2005-03-30,Stability of ferromagnetism in the half-metallic pnictides and similar compounds: A first-principles study,"Based on first-principles electron structure calculations and employing the
frozen-magnon approximation we study the exchange interactions in a series of
transition-metal binary alloys crystallizing in the zinc-blende structure and
calculate the Curie temperature within both the mean-field approximation (MFA)
and random-phase approximation (RPA). We study two Cr compounds, CrAs and CrSe,
and four Mn compounds: MnSi, MnGe, MnAs and MnC. MnC, MnSi and MnGe are
isovalent to CrAs and MnAs is isoelectronic with CrSe. Ferromagnetism is
particular stable for CrAs, MnSi and MnGe: All three compounds show Curie
temperatures around 1000 K. On the other hand, CrSe and MnAs show a tendency to
antiferromagnetism when compressing the lattice. In MnC the half-metallic gap
is located in the majority-spin channel contrary to the other five compounds.
The large half-metallic gaps, very high Curie temperatures, the stability of
the ferromagnetism with respect to the variation of the lattice parameter and a
coherent growth on semiconductors make MnSi and CrAs most promising candidates
for the use in spintronics devises.",0503713v1
2005-03-30,Spintronic transport and Kondo effect in quantum dots,"We investigate the spin-dependent transport properties of quantum-dot based
structures where Kondo correlations dominate the electronic dynamics. The
coupling to ferromagnetic leads with parallel magnetizations is known to give
rise to nontrivial effects in the local density of states of a single quantum
dot. We show that this influence strongly depends on whether charge
fluctuations are present or absent in the dot. This result is confirmed with
numerical renormalization group calculations and perturbation theory in the
on-site interaction. In the Fermi-liquid fixed point, we determine the
correlations of the electric current at zero temperature (shot noise) and
demonstrate that the Fano factor is suppressed below the Poissonian limit for
the symmetric point of the Anderson Hamiltonian even for nonzero lead
magnetizations. We discuss possible avenues of future research in this field:
coupling to the low energy excitations of the ferromagnets (magnons), extension
to double quantum dot systems with interdot antiferromagnetic interaction and
effect of spin-polarized currents on higher symmetry Kondo states such as
SU(4).",0503716v1
2005-04-26,Pressure dependence of the Curie temperature in Ni2MnSn Heusler alloy: A first-principles study,"The pressure dependence of electronic structure, exchange interactions and
Curie temperature in ferromagnetic Heusler alloy Ni2MnSn has been studied
theoretically within the framework of the density-functional theory. The
calculation of the exchange parameters is based on the frozen--magnon approach.
The Curie temperature, Tc, is calculated within the mean-field approximation by
solving the matrix equation for a multi-sublattice system. In agrement with
experiment the Curie temperature increased from 362K at ambient pressure to 396
at 12 GPa. Extending the variation of the lattice parameter beyond the range
studied experimentally we obtained non-monotonous pressure dependence of the
Curie temperature and metamagnetic transition. We relate the theoretical
dependence of Tc on the lattice constant to the corresponding dependence
predicted by the empirical interaction curve. The Mn-Ni atomic interchange
observed experimentally is simulated to study its influence on the Curie
temperature.",0504644v1
2005-04-26,"First-principles study of exchange interactions and Curie temperatures of half-metallic ferrimagnetic full Heusler alloys Mn2VZ (Z=Al, Ge)","We report the parameter-free, density functional theory calculations of
interatomic exchange interactions and Curie temperatures of half-metallic
ferrimagnetic full Heusler alloys Mn2VZ (Z=Al, Ge). To calculate the
interatomic exchange interactions we employ the frozen-magnon approach. The
Curie temperatures are calculated within the mean-field approximation to the
classical Heisenberg Hamiltonian by solving a matrix equation for a
multi-sublattice system. Our calculations show that, although a large magnetic
moment is carried by Mn atoms, competing ferromagnetic (inter sublattice) and
antiferromagnetic (intra sublattice) Mn-Mn interactions in Mn2VAl almost cancel
each other in the mean-field experienced by the Mn atoms. In Mn2VGe the leading
Mn-Mn exchange interaction is antiferromagnetic. In both compounds the
ferromagnetism of the Mn subsystem is favored by strong antiferromagnetic Mn-V
interactions. The obtained value of the Curie temperature of Mn2VAl is in good
agrement with experiment. For Mn2VGe there is no experimental information
available and our calculation is a prediction.",0504679v1
2005-04-27,A generating functional approach to the Hubbard model,"The method of generating functional is generalized to the case of strongly
correlated systems, and applied to the Hubbard model. For the electronic
Green's function constructed for Hubbard operators, an equation using
variational derivatives with respect to the fluctuating fields has been derived
and its multiplicative form has been determined. Corrections for the electronic
self-energy are calculated up to the second order with respect to the parameter
W/U (W width of the band), and a mean field type approximation was formulated,
including both charge and spin static fluctuations. The equations for the
Bose-like Green's functions have been derived, describing the collective modes:
the magnons and doublons. The properties of the poles of the doublon Green's
functions depend on electronic filling. The investigation of the special case
n=1 demonstrates that the doublon Green's function has a soft mode at the wave
vector Q=(pi,pi,...), indicating possible instability of the uniform
paramagnetic phase relatively to the two sublattices charge ordering. However
this instability should compete with an instability to antiferromagnetic
ordering.",0504723v1
2005-05-11,"Above-room-temperature ferromagnetism in half-metallic Heusler compounds NiCrP, NiCrSe, NiCrTe and NiVAs: A first-principles study","We study the interatomic exchange interactions and Curie temperatures in
half-metallic semi Heusler compounds NiCrZ (Z=P, Se, Te) and NiVAs. The study
is performed within the framework of density functional theory. The calculation
of exchange parameters is based on the frozen-magnon approach. It is shown that
the exchange interactions in NiCrZ vary strongly depending on the Z
constituent. The Curie temperature, Tc, is calculated within the mean field and
random phase approximations. The difference between two estimations is related
to the properties of the exchange interactions. The predicted Curie
temperatures of all four systems are considerably higher than room temperature.
The relation between the half-metallicity and the value of the Curie
temperature is discussed. The combination of a high spin-polarization of charge
carriers and a high Curie temperature makes these Heusler alloys interesting
candidates for spintronics applications.",0505299v1
2005-06-04,Exact soliton solution of Spin Chain with a external magnetic field in linear wave background,"Employing a simple, straightforward Darboux transformation we construct exact
N-soliton solution for anisotropic spin chain driven by a external magnetic
field in linear wave background. As a special case the explicit one- and
two-soliton solution dressed by the linear wave corresponding to magnon in
quantum theory is obtained analytically and its property is discussed in
detail. The dispersion law, effective soliton mass, and the energy of each
soliton are investigated as well. Our result show that the stability criterion
of soliton is related with anisotropic parameter and the amplitude of the
linear wave.",0506104v1
2005-06-24,Spin dynamics of the quasi two dimensional spin-1/2 quantum magnet Cs_2CuCl_4,"We study dynamical properties of the anisotropic triangular quantum
antiferromagnet Cs_2CuCl_4. Inelastic neutron scattering measurements have
established that the dynamical spin correlations cannot be understood within a
linear spin wave analysis. We go beyond linear spin wave theory by taking
interactions between magnons into account in a 1/S expansion. We determine the
dynamical structure factor and carry out extensive comparisons with
experimental data. We find that compared to linear spin wave theory a
significant fraction of the scattering intensity is shifted to higher energies
and strong scattering continua are present. However, the 1/S expansion fails to
account for the experimentally observed large quantum renormalization of the
exchange energies.",0506667v1
2005-07-18,Coupled ferro-antiferromagnetic Heisenberg bilayers investigated by many-body Green's function theory,"A theory of coupled ferro- and antiferromagnetic Heisenberg layers is
developed within the framework of many-body Green's function theory (GFT) that
allows non-collinear magnetic arrangements by introducing sublattice
structures. As an example, the coupled ferro- antiferromagnetic (FM-AFM)
bilayer is investigated. We compare the results with those of bilayers with
purely ferromagnetic or antiferromagnetic couplings. In each case we also show
the corresponding results of mean field theory (MFT), in which magnon
excitations are completely neglected. There are significant differences between
GFT and MFT. A remarkable finding is that for the coupled FM-AFM bilayer the
critical temperature decreases with increasing interlayer coupling strength for
a simple cubic lattice, whereas the opposite is true for an fcc lattice as well
as for MFT for both lattice types.",0507408v1
2005-07-22,Structural instability of two- and three-dimensional pyrochlore spin lattices in high magnetic fields,"We consider the quantum spin-$s$ $XXZ$ Heisenberg antiferromagnet on the two-
and three-dimensional pyrochlore lattices and examine a spin-Peierls mechanism
of lowering the total energy by a lattice distortion in a high magnetic field.
For the exact eigenstates consisting of several independent localized magnons
in a ferromagnetic environment we show the existence of a spin-Peierls
instability by rigorous analytical calculations. In addition we report on exact
diagonalization data for finite two-dimensional pyrochlore lattices up to N=64
sites. We discuss a scenario of the field-tuned changes in the ground-state
properties of the frustrated spin lattices due to the coupling between spin and
lattice degrees of freedom.",0507538v1
2005-07-28,Exchange interactions and temperature dependence of the magnetization in half--metallic Heusler alloys,"We study the exchange interactions in half-metallic Heusler alloys using
first-principles calculations in conjunction with the frozen-magnon
approximation. The Curie temperature is estimated within both mean-field (MF)
and random-phase-approximation (RPA) approaches. For the half-Heusler alloys
NiMnSb and CoMnSb the dominant interaction is between the nearest Mn atoms. In
this case the MF and RPA estimations differ strongly. The RPA approach provides
better agreement with experiment. The exchange interactions are more complex in
the case of full-Heusler alloys Co$_2$MnSi and Co$_2$CrAl where the dominant
effects are the inter-sublattice interactions between the Mn(Cr) and Co atoms
and between Co atoms at different sublattices. For these compounds we find that
both MF and RPA give very close values of the Curie temperature slightly
underestimating experimental quantities. We study the influence of the lattice
compression on the magnetic properties. The temperature dependence of the
magnetization is calculated using the RPA method within both quantum mechanical
and classical approaches.",0507697v2
2005-09-08,Frustration effects in magnetic molecules,"Besides being a fascinating class of new materials, magnetic molecules
provide the opportunity to study concepts of condensed matter physics in zero
dimensions. This contribution will exemplify the impact of molecular magnetism
on concepts of frustrated spin systems. We will discuss spin rings and the
unexpected rules that govern their low-energy behavior. Rotational bands, which
are experimentally observed in various molecular magnets, provide a useful,
simplified framework for characterizing the energy spectrum, but there are also
deviations thereof with far-reaching consequences. It will be shown that
localized independent magnons on certain frustrated spin systems lead to giant
magnetization jumps, a new macroscopic quantum effect. In addition a
frustration-induced metamagnetic phase transitions will be discussed, which
demonstrates that hysteresis can exist without anisotropy. Finally, it is
demonstrated that frustrated magnetic molecules could give rise to an enhanced
magnetocaloric effect.",0509204v1
2005-10-07,Quantum effects in a half-polarized pyrochlore antiferromagnet,"We study quantum effects in a spin-3/2 antiferromagnet on the pyrochlore
lattice in an external magnetic field, focusing on the vicinity of a plateau in
the magnetization at half the saturation value, observed in CdCr$_2$O$_4$, and
HgCr$_2$O$_4$. Our theory, based on quantum fluctuations, predicts the
existence of a symmetry-broken state on the plateau, even with only
nearest-neighbor microscopic exchange. This symmetry broken state consists of a
particular arrangement of spins polarized parallel and antiparallel to the
field in a 3:1 ratio on each tetrahedron. It quadruples the lattice unit cell,
and reduces the space group from $Fd\bar{3}m$ to $P4_332$. We also predict that
for fields just above the plateau, the low temperature phase has transverse
spin order, describable as a Bose-Einstein condensate of magnons. Other
comparisons to and suggestions for experiments are discussed.",0510202v1
2005-10-12,Magnetic susceptibility of the body-centred orthorhombic La$_2$CuO$_4$ system,"A model Hamiltonian representing the Cu spins in La$_2$CuO$_4$ in its
low-temperature body-centred orthorhombic phase, that includes both spin-orbit
generated Dzyaloshinskii-Moriya interactions and interplanar exchange, is
examined within the RPA utilizing a Tyablikov decoupling of various high-order
Green's functions. The magnetic susceptibility is evaluated as a function of
temperature and the parameters quantifying these interactions, and compared to
recently obtained experimental data of Lavrov, Ando and collaborators. An
effective Hamiltonian corresponding to a simple tetragonal structure is shown
to reproduce both the magnon spectra and the susceptibility of the more
complicated body-centred orthorhombic model.",0510328v1
2005-11-08,Spin-wave interactions in quantum antiferromagnets,"We study spin-wave interactions in quantum antiferromagnets by expressing the
usual magnon annihilation and creation operators in terms of Hermitian field
operators representing transverse staggered and ferromagnetic spin
fluctuations. In this parameterization, which was anticipated by Anderson in
1952, the two-body interaction vertex between staggered spin fluctuations
vanishes at long wavelengths. We derive a new effective action for the
staggered fluctuations only by tracing out the ferromagnetic fluctuations. To
one loop order, the renormalization group flow agrees with the
nonlinear-$\sigma$-model approach.",0511199v3
2005-12-12,Spin-polarized scanning tunneling microscopy of half-metallic ferromagnets: Non-quasiparticle contributions,"The role of the many-body (spin-polaronic) effects in the scanning tunneling
spectroscopy of half-metallic ferromagnets (HMF) is considered. It is shown
that the non-quasiparticle (NQP) states exist in the majority or minority spin
gap in the presence of arbitrary external potential and, in particular, at the
surfaces and interfaces. Energy dependence of the NQP density of states is
obtained in various models of HMF, an important role of the hybridization
nature of the energy gap being demonstrated. The corresponding temperature
dependence of spin polarization is calculated. It is shown that the NQP states
result in a sharp bias dependence of the tunneling conductance near zero bias.
Asymmetry of the NQP states with respect to the Fermi energy provides an
opportunity to separate phonon and magnon peaks in the inelastic spectroscopy
by STM.",0512245v1
2005-12-14,Spin-wave dispersion in orbitally ordered La(0.5)Sr(1.5)MnO(4),"The magnon dispersion in the charge, orbital and spin ordered phase in
La(0.5)Sr(1.5)MnO(4) has been studied by means of inelastic neutron scattering.
We find an excellent agreement with a magnetic interaction model basing on the
CE-type superstructure. The magnetic excitations are dominated by ferromagnetic
exchange parameters revealing a nearly-one dimensional character at high
energies. The nearest neighbor ferromagnetic interaction in
La(0.5)Sr(1.5)MnO(4) is significantly larger than the one in the metallic
ferromagnetically ordered manganites. The large ferromagnetic interaction in
the charge/orbital ordered phase appears to be essential for the capability of
manganites to switch between metallic and insulating phases.",0512305v1
2005-12-15,Nematic order in square lattice frustrated ferromagnets,"We present a new scenario for the breakdown of ferromagnetic order in a
two-dimensional quantum magnet with competing ferromagnetic and
antiferromagnetic interactions. In this, dynamical effects lead to the
formation of two-magnon bound states, which undergo Bose-Einstein condensation,
giving rise to bond-centered nematic order. This scenario is explored in some
detail for an extended Heisenberg model on a square lattice. In particular, we
present numerical evidence confirming the existence of a state with d-wave
nematic correlations but no long range magnetic order, lying between the
saturated ferromagnetic and collinear antiferromagnetic phases of the
ferromagnetic J1-J2 model. We argue by continuity of spectra that this phase is
also present in a model with 4-spin cyclic exchange.",0512349v1
2005-12-16,Evolution of spin-wave excitations in ferromagnetic metallic manganites,"Neutron scattering results are presented for spin-wave excitations of three
ferromagnetic metallic $A_{1-x}A^{\prime}_{x}$MnO$_3$ manganites (where $A$ and
$A^\prime$ are rare- and alkaline-earth ions), which when combined with
previous work elucidate systematics of the interactions as a function of
carrier concentration $x$, on-site disorder, and strength of the lattice
distortion. The long wavelength spin dynamics show only a very weak dependence
across the series. The ratio of fourth to first neighbor exchange ($J_4/J_1$)
that controls the zone boundary magnon softening changes systematically with
$x$, but does not depend on the other parameters. None of the prevailing models
can account for these behaviors.",0512368v1
2005-12-22,Magnetic properties of a disordered Heisenberg binary spin system with long-range exchange,"The influence of substitutional disorder on the magnetic properties of
disordered Heisenberg binary spin systems with long-range exchange integrals is
studied. The equation of motion for the magnon Green's function which is
decoupled by the Tyablikov approximation is solved in the
Blackman-Esterling-Berk(BEB) coherent potential approximation(CPA) framework,
where the environmental disorder term is treated by virtual crystal
approximation. The long-range exchange integrals include a power-law decaying
and an oscillating Ruderman-Kittel-Kasuya-Yosida(RKKY) exchange interaction.
The resulting spectral density, which is calculated by CPA self-consistent
equation, is then used to estimate the magnetization and Curie temperature. The
results show, in the case of the three-dimensional simple cubic systems, a
strong influence of ferromagnetic long-range exchange integrals on the
magnetization and Curie temperature of the systems, which is obviously
different from the calculation of short-range interaction.",0512585v1
2005-12-29,Electron-magnon coupling and nonlinear tunneling transport in magnetic nanoparticles,"We present a theory of single-electron tunneling transport through a
ferromagnetic nanoparticle in which particle-hole excitations are coupled to
spin collective modes. The model employed to describe the interaction between
quasiparticles and collective excitations captures the salient features of a
recent microscopic study. Our analysis of nonlinear quantum transport in the
regime of weak coupling to the external electrodes is based on a rate-equation
formalism for the nonequilibrium occupation probability of the nanoparticle
many-body states. For strong electron-boson coupling, we find that the
tunneling conductance as a function of bias voltage is characterized by a large
and dense set of resonances. Their magnetic field dependence in the large-field
regime is linear, with slopes of the same sign. Both features are in agreement
with recent tunneling experiments.",0512697v2
2006-01-31,Spin-lattice interaction in the quasi one-dimensional helimagnet LiCu_2O_2,"The field dependence of the electron spin resonance in a helimagnet,
LiCu_2O_2, was investigated for the first time. In the paramagnetic state a
broad resonance line was observed corresponding to a g-factor of 2.3. In the
critical regime around the paramagnetic to helimagnetic phase transition the
resonance broadens and shifts to higher frequencies. A narrow signal is
recovered at low temperature, corresponding to a spin gap of 1.5 meV in zero
field. A comprehensive model of the magnons is presented, using exchange
parameters from neutron scattering (T. Masuda et al,
  Phys. Rev. B 72, 014405), and the spin anisotropy determined here. The role
of the quantum fluctuations is discussed.",0601701v1
2006-02-01,The metal-insulator transition of the ferromagnetic La7/8Sr1/8MnO3 probed by spin waves: a 2D stripe superstructure,"We report a study of spin waves in hole-doped ferromagnetic La7/8Sr1/8MnO3,
in the metallic state (165K) below TC (181K) and through the puzzling
metal-insulator transition which occurs at TO'O''=159K. They reveal very
unusual excitations. Propagating spin waves are observed in the small q-range
up to q=0.25 (lambda=4a), and, beyond, four dispersionless levels. Both types
of excitations have a quasi two-dimensional (2D) character. The transition is
revealed by a folding of the dispersed magnon branch at q=1/8. In the metallic
state, the dispersionless levels reveal ferromagnetic domains with 4 lattice
spacings for their size along a and b. They lead to a picture of charge
segregation with hole-poor domains surrounded with hole-rich paths. Within this
description, the transition appears as the ordering of domains, which can be
interpreted in terms of a 2D superstructure of orthogonal stripes.",0602026v1
2006-02-07,Possible evidence for electromagnons in multiferroic manganites,"Magnetodielectric materials are characterized by a strong coupling of
magnetic and dielectric properties and in rare cases simultaneously exhibit
both, magnetic and polar order. Among other multiferroics, TbMnO3 and GdMnO3
reveal a strong magneto-dielectric (ME) coupling and as a consequence
fundamentally new spin excitations exist: Electro-active magnons, or
electromagnons, i. e. spin waves which can be excited by ac electric fields.
Here we show that these excitations appear in the phase with an incommensurate
(IC) magnetic structure of the manganese spins. In external magnetic fields
this IC structure can be suppressed and the electromagnons are wiped out,
thereby inducing considerable changes in the index of refraction from dc up to
THz frequencies. Hence, besides adding a new creature to the zoo of fundamental
excitations, the refraction index can be tuned by moderate magnetic fields,
which allows the design of a new generation of optical switches and
optoelectronic devices.",0602173v1
2006-02-15,Low-energy excitations in the magnetized state of the bond-alternating quantum S=1 chain system NTENP,"High intensity inelastic neutron scattering experiments on the S=1
quasi-one-dimensional bond-alternating antiferromagnet Ni(C9D24N4)(NO2)ClO4
(NTENP) are performed in magnetic fields of up to 14.8~T. Excitation in the
high field magnetized quantum spin solid (ordered) phase are investigated. In
addition to the previously observed coherent long-lived gap excitation [M.
Hagiwara et al., Phys. Rev. Lett 94, 177202 (2005)], a broad continuum is
detected at lower energies. This observation is consistent with recent
numerical studies, and helps explain the suppression of the lowest-energy gap
mode in the magnetized state of NTENP. Yet another new feature of the
excitation spectrum is found at slightly higher energies, and appears to be
some kind of multi-magnon state.",0602357v1
2006-03-22,Electronic structure and exchange constants in magnetic semiconductors digital alloys: chemical and band-gap effects,"First-principles simulations have been performed for [001]-ordered Mn/Ge and
Mn/GaAs ""digital alloys"", focusing on the effects of i) a larger band-gap and
ii) a different semiconducting host on the electronic structure of the magnetic
semiconductors of interest. Our results for the exchange constants in Mn/Ge,
evaluated using a frozen-magnon scheme, show that a larger band-gap tends to
give a stronger nearest-neighbor ferromagnetic coupling and an overall enhanced
in-plane ferromagnetic coupling even for longer-ranged coupling constants. As
for the chemical effects on the exchange constants, we show that Mn/GaAs shows
a smaller nearest-neighbor ferromagnetic coupling than Mn/Ge, but exchange
constants for higher Mn-Mn distance show an overall increased ferromagnetic
behavior in Mn/GaAs. As a result, from the magnetic-coupling point of view, the
two systems behave on average rather similarly.",0603589v1
2006-03-23,Condensation of magnons and spinons in a frustrated ladder,"Motivated by the ever-increasing experimental effort devoted to the
properties of frustrated quantum magnets in a magnetic field, we present a
careful and detailed theoretical analysis of a one-dimensional version of this
problem, a frustrated ladder with a magnetization plateau at m=1/2. We show
that even for purely isotropic Heisenberg interactions, the magnetization curve
exhibits a rather complex behavior that can be fully accounted for in terms of
simple elementary excitations. The introduction of anisotropic interactions
(e.g., Dzyaloshinskii-Moriya interactions) modifies significantly the picture
and reveals an essential difference between integer and fractional plateaux. In
particular, anisotropic interactions generically open a gap in the region
between the plateaux, but we show that this gap closes upon entering fractional
plateaux. All of these conclusions, based on analytical arguments, are
supported by extensive Density Matrix Renormalization Group calculations.",0603609v2
2006-05-05,Bose-Einstein condensation and entanglement in magnetic systems,"We present a study of magnetic field induced quantum phase transitions in
insulating systems. A generalized scaling theory is used to obtain the
temperature dependence of several physical quantities along the quantum
critical trajectory ($H=H_{C}$, $T\to0$) where $H$ is a longitudinal external
magnetic field and $H_{C}$ the critical value at which the transition occurs.
We consider transitions from a spin liquid at a critical field $H_{C1}$ and
from a fully polarized paramagnet, at $H_{C2}$, into phases with long range
order in the transverse components. The transitions at $H_{C1}$ and $H_{C2}$
can be viewed as Bose-Einstein condensations of magnons which however belong to
different universality classes since they have different values of the dynamic
critical exponent $z$. Finally, we use that the magnetic susceptibility is an
entanglement witness to discuss how this type of correlation sets in as the
system approaches the quantum critical point along the critical trajectory,
$H=H_{C2}$, $T\to0$.",0605148v2
2006-05-08,Frustrated ferromagnetic spin-1/2 chain in a magnetic field: The phase diagram and thermodynamic properties,"The frustrated ferromagnetic spin-1/2 Heisenberg chain is studied by means of
a low-energy field theory as well as the density-matrix renormalization group
and exact diagonalization methods. Firstly, we study the ground-state phase
diagram in a magnetic field and find an `even-odd' (EO) phase characterized by
bound pairs of magnons in the region of two weakly coupled antiferromagnetic
chains. A jump in the magnetization curves signals a first-order transition at
the boundary of the EO phase, but otherwise the curves are smooth. Secondly, we
discuss thermodynamic properties at zero field, where we confirm a double-peak
structure in the specific heat for moderate frustrating next-nearest neighbor
interactions.",0605204v2
2006-05-10,Spin-flip processes and ultrafast magnetization dynamics in Co - unifying the microscopic and macroscopic view of femtosecond magnetism,"The femtosecond magnetization dynamics of a thin cobalt film excited with
ultrashort laser pulses has been studied using two complementary pump-probe
techniques, namely spin-, energy- and time-resolved photoemission and
time-resolved magneto-optical Kerr effect. Combining the two methods it is
possible to identify the microscopic electron spin-flip mechanisms responsible
for the ultrafast macroscopic magnetization dynamics of the cobalt film. In
particular, we show that electron-magnon excitation does not affect the overall
magnetization even though it is an efficient spin-flip channel on the sub-200
fs timescale. Instead we find experimental evidence for the relevance of
Elliott-Yafet type spin-flip processes for the ultrafast demagnetization taking
place on a time scale of 300 fs.",0605272v2
2006-05-25,Bosonic representation of spin operators in the field-induced critical phase of spin-1 Haldane chains,"A bosonized-field-theory representation of spin operators in the
uniform-field-induced Tomonaga-Luttinger-liquid (TLL) phase in spin-1 Haldane
chains is formulated by means of the non-Abelian (Tsvelik's Majorana fermion
theory) and the Abelian bosonizations and Furusaki-Zhang technique [Phys. Rev.
B {\bf 60}, 1175 (1999)]. It contains massive magnon fields as well as massless
boson fields. From it, asymptotic forms of spin correlation functions in the
TLL phase are completely determined. Applying the formula, we further discuss
effects of perturbations (bond alternation, single-ion anisotropy terms,
staggered fields, etc) for the TLL state, and the string order parameter.
Throughout the paper, we often consider in some detail how the symmetry
operations of the Haldane chains are translated in the low-energy effective
theory.",0605636v3
2006-06-06,Hole dynamics in canted antiferromagnets,"We have analyzed the dynamics of a single hole doped in a canted
antiferromagnet using the t-J model. Within the self consistent Born
approximation we have found that the hole propagates at two different energy
scales along the antiferromagnetic and the ferromagnetic components of the
canted order, respectively. While the many body quasiparticle excitation has
its origin in the coherent coupling of the hole with the magnon excitations of
the antiferromagnetic component, the ferromagnetic component gives rise to a
free like hole motion at higher energies. We have found a non-trivial behavior
of the hole spectral function with the canting angle. In particular, in the
strong coupling regime, the quasiparticle weight strongly depends on the
momenta, vanishing inside the magnetic Brillouin zone for a canting angle
greater than 60 degree.",0606157v1
2006-06-14,Dynamic spin-response function of the high-temperature Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ superconductor form angle resolved photoemission spectra,"We introduce a formalism for calculating dynamic response functions using
experimental single particle Green's functions derived from angle resolved
photoemission spectroscopy (ARPES). As an illustration of this procedure we
estimate the dynamic spin response of the cuprate superconductor
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. We find good agreement with superconducting
state neutron data, in particular the $(\pi,\pi)$ resonance with its unusual
`reversed magnon' dispersion. We anticipate our formalism will also be of
useful in interpreting results from other spectroscopies, such as optical and
Raman responses.",0606346v2
2006-07-18,Magentic-Field Induced Quantum Phase Transition and Critical Behavior in a Gapped Spin System TlCuCl$_3$,"Magnetization measurements were performed on TlCuCl$_3$ with gapped ground
state. The critical density and the magnetic phase diagram were obtained. The
interacting constant was obtained as $U/k_{\rm B} = 313$ K. The experimental
phase boundary for $T < 5$ K agrees perfectly with the magnon BEC theory based
on the Hartree-Fock approximation with realistic dispersion relations and
$U/k_{\rm B} = 320 $ K. The exponent $\phi$ obtained with all the data points
for $T < 5$ K is $\phi = 1.99$, which is somewhat larger than theoretical
exponent $\phi_{\rm BEC} =3/2$. However, it was found that the exponent
converges at $\phi_{\rm BEC} =3/2$ with decreasing fitting window.",0607439v1
2006-07-18,Exact spin dynamics of the 1/r^2 supersymmetric t-J model in a magnetic field,"The dynamical spin structure factor S^{zz}(Q,omega) in the small momentum
region is derived analytically for the one-dimensional supersymmetric t-J model
with 1/r^2 interaction. Strong spin-charge separation is found in the spin
dynamics. The structure factor S^{zz}(Q,omega) with a given spin polarization
does not depend on the electron density in the small momentum region. In the
thermodynamic limit, only two spinons and one antispinon (magnon) contribute to
S^{zz}(Q,omega). These results are derived via solution of the SU(2,1)
Sutherland model in the strong coupling limit.",0607444v1
2006-07-30,"The Resolution of the Diamond Problem After 200 Years: Phonon, Roton and Magnon Induced Spin-Orbital Dynamics, Subshell Rehybridization and Shell Rotation for the Little Effect","The problem of the physicochemical synthesis of diamond spans more than 200
years, involving many giants of science. Many technologies have been
discovered, realized and used to resolve this diamond problem. Here the origin,
definition and cause of the diamond problem are presented. The Resolution of
the diamond problem is then discussed on the basis of the Little Effect,
involving novel roton-phonon driven (antisymmetrical) multi-spin induced
orbital orientation, subshell rehybridization and valence shell rotation of
radical complexes in quantum fluids under magnetization across thermal,
pressure, compositional, and spinor gradients in both space and time. Some
experimental evidence of this magnetic quantum Resolution is briefly reviewed
and integrated with this recent fruitful discovery. Furthermore, the
implications of the Little Effect in comparison to the Woodward-Hoffman Rule
are considered. The distinction of the Little Effect from the prior radical
pair effect is clarified. The better compatibility of radicals, dangling bonds
and magnetism with the diamond lattice relative to the graphitic lattice is
discussed. Finally, these novel physicochemical phenomena for the Little Effect
are compared with the natural diamond genesis.",0607793v1
2006-08-01,Flat spin wave dispersion in a triangular antiferromagnet,"The excitation spectrum of a S=1/2 2D triangular quantum antiferromagnet is
studied using 1/S expansion. Due to the non-collinearity of the classical
ground state significant and non-trivial corrections to the spin wave spectrum
appear already in the first order in 1/S in contrast to the square lattice
antiferromagnet. The resulting magnon dispersion is almost flat in a
substantial portion of the Brillouin zone. Our results are in quantitative
agreement with recent series expansion studies by Zheng, Fjaerestad, Singh,
McKenzie, and Coldea [PRL 96, 057201 (2006) and cond-mat/0608008].",0608002v2
2006-08-01,Excitation spectra of the spin-1/2 triangular-lattice Heisenberg antiferromagnet,"We use series expansion methods to calculate the dispersion relation of the
one-magnon excitations for the spin-1/2 triangular-lattice nearest-neighbor
Heisenberg antiferromagnet above a three-sublattice ordered ground state.
Several striking features are observed compared to the classical (large-S)
spin-wave spectra. Whereas at low energies the dispersion is only weakly
renormalized by quantum fluctuations, significant anomalies are observed at
high energies. In particular, we find roton-like minima at special wave-vectors
and strong downward renormalization in large parts of the Brillouin zone,
leading to very flat or dispersionless modes. We present detailed comparison of
our calculated excitation energies in the Brillouin zone with the spin-wave
dispersion to order 1/S calculated recently by Starykh, Chubukov, and Abanov
[cond-mat/0608002]. We find many common features but also some quantitative and
qualitative differences. We show that at temperatures as low as 0.1J the
thermally excited rotons make a significant contribution to the entropy.
Consequently, unlike for the square lattice model, a non-linear sigma model
description of the finite-temperature properties is only applicable at
extremely low temperatures.",0608008v1
2006-08-15,Excitations with fractional spin less than 1/2 in frustrated magnetoelastic chains,"We study the magnetic excitations on top of the plateaux states recently
discovered in spin-Peierls systems in a magnetic field. We show by means of
extensive density matrix renormalization group (DMRG) computations and an
analytic approach that one single spin-flip on top of $M=1-\frac2N$
($N=3,4,...$) plateau decays into $N$ elementary excitations each carrying a
fraction $\frac1N$ of the spin. This fractionalization goes beyond the
well-known decay of one magnon into two spinons taking place on top of the M=0
plateau. Concentrating on the $\frac13$ plateau (N=3) we unravel the
microscopic structure of the domain walls which carry fractional
spin-$\frac13$, both from theory and numerics. These excitations are shown to
be noninteracting and should be observable in x-ray and nuclear magnetic
resonance experiments.",0608326v2
2006-08-31,Magnetic phase diagram of the semi-Heusler alloys from first-principles,"The magnetic phase diagram of the Mn-based semi-Heusler alloys is determined
at T=0 using first-principles calculations in conjunction with the
frozen-magnon approximation. We show that the magnetism in these systems
strongly depends on the number of conduction electrons, their spin polarization
and the position of the unoccupied Mn 3d states with respect to Fermi energy.
Various magnetic phases are obtained depending on these characteristics. The
conditions leading to diverse magnetic behavior are identified. We find that in
the case of a large conduction electron spin polarization and the unoccupied Mn
3d states lying far above the Fermi level, an RKKY-like ferromagnetic
interaction is dominating. On the other hand, the antiferromagnetic
superexchange becomes important in the presence of large peaks of the
unoccupied Mn 3d states lying close to the Fermi energy. The overall magnetic
behavior depends on the competition of these two exchange mechanisms. The
obtained results are in very good agreement with the available experimental
data.",0609001v1
2006-09-28,Homogeneous versus Spiral Phases of Hole-doped Antiferromagnets: A Systematic Effective Field Theory Investigation,"Using the low-energy effective field theory for magnons and holes -- the
condensed matter analog of baryon chiral perturbation theory for pions and
nucleons in QCD -- we study different phases of doped antiferromagnets. We
systematically investigate configurations of the staggered magnetization that
provide a constant background field for doped holes. The most general
configuration of this type is either constant itself or it represents a spiral
in the staggered magnetization. Depending on the values of the low-energy
parameters, a homogeneous phase, a spiral phase, or an inhomogeneous phase is
energetically favored. The reduction of the staggered magnetization upon doping
is also investigated.",0609731v1
2006-09-29,Magnetic Excitation Spectrum of the Square Lattice S=1/2 Heisenberg Antiferromagnet K2V3O8,"We have explored the magnetic excitation spectrum of the S=1/2 square lattice
Heisenberg antiferromagnet, K2V3O8 using both triple-axis and time-of-flight
inelastic neutron scattering. The long-wavelength spin waves are consistent
with the previously determined Hamiltonian for this material. A small energy
gap of 72+/-9 micro-eV is observed at the antiferromagnetic zone center and the
near-neighbor exchange constant is determined to be 1.08+/-0.03 meV. A finite
ferromagnetic interplanar coupling is observed along the crystallographic
c-axis with a magnitude of Jc=-0.0036+/-0.006 meV. However, upon approaching
the zone boundary, the observed excitation spectrum deviates significantly from
the expectation of linear spin wave theory resulting in split modes at the
(pi/2,pi/2) zone boundary point. The effects of magnon-phonon interaction,
orbital degrees of freedom, multimagnon scattering, and dilution/site
randomness are considered in the context of the mode splitting. Unfortunately,
no fully satisfactory explanation of this phenomenon is found and further
theoretical and experimental work is needed.",0609775v2
2006-10-17,Magnetic spin excitations in diluted ferromagnetic systems: the case of $Ga_{1-x}Mn_{x}As$,"We propose a theory which allow to calculate the magnetic excitation spectrum
in diluted ferromagnetic systems. The approach is rather general and based on
the Self-Consistent local Random Phase Approximation in which disorder
(dilution) and thermal fluctuations are properly treated. To illustrate its
reliability and accuracy we calculate the magnetic excitation in the diluted
III-V magnetic semiconductor $Ga_{1-x}Mn_{x}As$. It is shown that dilution has
a drastic effect on the excitation spectrum, indeed well defined magnon
excitations exist only in a small region of the Brillouin zone centered around
the $\Gamma$ point. We also calculate the spin stiffness in optimally annealed
sample as a function of $Mn^{2+}$ concentration. A comparison to available
measurements is done. We find a very good agreement for both the Curie
temperature and the spin stiffness measured in well annealed samples and
provide a plausible explanation for the very small values measured in as grown
samples.",0610465v2
2006-11-07,Impurity susceptibility and the fate of spin-flop transitions in lightly-doped La(2)CuO(4),"We investigate the occurrence of a two-step spin-flop transition and spin
reorientation when a longitudinal magnetic field is applied to lightly
hole-doped La(2)CuO(4). We find that for large and strongly frustrating
impurities, such as Sr in La(2-x)Sr(x)CuO(4), the huge enhancement of the
longitudinal susceptibility suppresses the intermediate flop and the
reorientation of spins is smooth and continuous. Contrary, for small and weakly
frustrating impurities, such as O in La(2)CuO(4+y), a discontinuous spin
reorientation (two-step spin-flop transition) takes place. Furthermore, we show
that for La(2-x)Sr(x)CuO(4) the field dependence of the magnon gaps differs
qualitatively from the La(2)CuO(4) case, a prediction to be verified with Raman
spectroscopy or neutron scattering.",0611183v2
2007-01-11,Transfer-matrix renormalization group study of the spin ladders with cyclic four-spin interactions,"The temperature dependence of the specific heat and spin susceptibility of
the spin ladders with cyclic four-spin interactions in the rung-singlet phase
is explored by making use of the transfer-matrix renormalization group method.
The values of spin gap are extracted from the specific heat and susceptibility,
respectively. It is found that for different relative strength between
interchain and intrachain interactions, the spin gap is approximately linear
with the cyclic four-spin interaction in the region far away from the critical
point. Furthermore, we show that the dispersion for the one-triplet magnon
branch can be obtained by numerically fitting on the partition function.",0701243v1
2007-02-13,Centipede ladder at quarter filling,"We study the ground state and excitation spectrum of a quasi one-dimensional
nanostructure consisting of a pole and rungs oriented in the opposite
directions (""centipede ladder"", CL) at quarter filling. The spin and charge
excitation spectra are found in the limits of small and large longitudinal
hopping $t_\|$ compared to the on-rung hopping rate $t_\perp$ and exchange
coupling $I_\perp$. At small $t_\|$ the system with ferromagnetic on-rung
exchange demonstrates instability against dimerization. Coherent propagation of
charge transfer excitons is possible in this limit. At large $t_\|$ CL behaves
like two-orbital Hubbard chain, but the gap opens in the charge excitation
spectrum thus reducing the symmetry from SU(4) to SU(2). The spin excitations
are always gapless and their dispersion changes from quadratic magnon-like for
ferromagnetic on-rung exchange to linear spinon-like for antiferromagnetic
on-rung exchange in weak longitudinal hopping limit.",0702307v1
2007-02-15,Large Magnetoresistance in Co/Ni/Co Ferromagnetic Single Electron Transistors,"We report on magnetotransport investigations of nano-scaled ferromagnetic
Co/Ni/Co single electron transistors. As a result of reduced size, the devices
exhibit single electron transistor characteristics at 4.2K. Magnetotransport
measurements carried out at 1.8K reveal tunneling magnetoresistance (TMR)
traces with negative coercive fields, which we interpret in terms of a
switching mechanism driven by the shape anisotropy of the central wire-like Ni
island. A large TMR of about 18% is observed within a finite source-drain bias
regime. The TMR decreases rapidly with increasing bias, which we tentatively
attribute to excitation of magnons in the central island.",0702370v1
2007-03-09,Infrared-active excitations related to Ho3+ ligand-field splitting at the commensurate-incommensurate magnetic phase transition in HoMn2O5,"Linearly polarized spectra of far-infrared (IR) transmission in HoMn2O5
multiferroic single crystals have been studied in the frequency range between
8.5 and 105 cm-1 and for temperatures between 5 K and 300 K. Polarization of
IR-active excitations depends on the crystallographic directions in HoMn2O5 and
is sensitive to the magnetic phase transitions. We attribute some of the
infrared-active excitations to electric-dipole transitions between ligand-field
split states of Ho3+ ions. For light polarization along crystalline b-axis, the
oscillator strength of electric dipoles at low frequencies (10.5, 13, and 18
cm-1) changes significantly at the commensurate-incommensurate
antiferromagnetic phase transition at T3 = 19 K. This effect shows a strong
correlation with the pronounced steps of the b-directional static dielectric
function. We propose that the ligand field (LF) on Ho3+ connects the magnetism
and dielectric properties of this compound through coupling with the Mn spin
structure. We comment on the possibility for composite excitations of magnons
and excited LF states.",0703255v2
2007-03-16,Distribution of local Lyapunov exponents in spin-glass dynamics,"We investigate the statistical properties of local Lyapunov exponents which
characterize magnon localization in the $d=1$ Heisenberg-Mattis spin glass
(HMSG) at zero temperature, by means of a connection to a suitable version of
the Fokker-Planck (F-P) equation. We consider the local Lyapunov exponents
(LLE), in particular the case of {\em instantaneous} LLE. We establish a
connection between the transfer-matrix recursion relation for the problem, and
an F-P equation governing the evolution of the probability distribution of the
instantaneous LLE. The closed-form (stationary) solutions to the F-P equation
are in excellent accord with numerical simulations, for both the unmagnetized
and magnetized versions of the HMSG. Scaling properties for non-stationary
conditions are derived from the F-P equation in a special limit (in which
diffusive effects tend to vanish), and also shown to provide a close
description to the corresponding numerical-simulation data.",0703444v2
1994-04-11,The Positivity of Energy for Asymptotically Anti-de Sitter Spacetimes,"We use the formulation of asymptotically anti-de Sitter boundary conditions
given by Ashtekar and Magnon to obtain a coordinate expression for the general
asymptotically AdeS metric in a neighbourhood of infinity. From this, we are
able to compute the time delay of null curves propagating near infinity. If the
gravitational mass is negative, so will be the time delay (relative to null
geodesics at infinity) for certain null geodesics in the spacetime. Following
closely an argument given by Penrose, Sorkin, and Woolgar, who treated the
asymptotically flat case, we are then able to argue that a negative time delay
is inconsistent with non-negative matter-energies in spacetimes having good
causal properties. We thereby obtain a new positive mass theorem for these
spacetimes. The theorem may be applied even when the matter flux near the
boundary-at-infinity falls off so slowly that the mass changes, provided the
theorem is applied in a time-averaged sense. The theorem also applies in
certain spacetimes having local matter-energy that is sometimes negative, as
can be the case in semi-classical gravity.",9404019v1
1995-07-11,Particle Interpretations and Green Functions for a Free Scalar Field,"The formalism of Ashtekar and Magnon \cite{AshtekarMagnon:1975} for the
definition of particles in quantum field theory in curved spacetime is further
developed. The relation between basic objects of this formalism (e.g., the
complex structure) and different Green functions is found. It allows one to
derive composition laws for Green functions. The relation of two definitions of
particles is reformulated in the formalism and the base-independent Bogoljubov
transformation is expressed using quantities which are derivable directly from
the ``in-out'' Green function.",9507023v2
1995-10-01,Large enhancement of deuteron polarization with frequency modulated microwaves,"We report a large enhancement of 1.7 in deuteron polarization up to values of
0.6 due to frequency modulation of the polarizing microwaves in a two liters
polarized target using the method of dynamic nuclear polarization. This target
was used during a deep inelastic polarized muon-deuteron scattering experiment
at CERN. Measurements of the electron paramagnetic resonance absorption spectra
show that frequency modulation gives rise to additional microwave absorption in
the spectral wings. Although these results are not understood theoretically,
they may provide a useful testing ground for the deeper understanding of
dynamic nuclear polarization.",9509014v1
2003-08-18,Order from disorder in lattice QCD at high density,"We investigate the properties of the ground state of strong coupling lattice
QCD at finite density. Our starting point is the effective Hamiltonian for
color singlet objects, which looks at lowest order as an antiferromagnet, and
describes meson physics with a fixed baryon number background. We concentrate
on uniform baryon number backgrounds (with the same baryon number on all
sites), for which the ground state was extracted in an earlier work, and
calculate the dispersion relations of the excitations. Two types of Goldstone
boson emerge. The first, antiferromagnetic spin waves, obey a linear dispersion
relation. The others, ferromagnetic magnons, have energies that are quadratic
in their momentum. These energies emerge only when fluctuations around the
large-N_c ground state are taken into account, along the lines of ``order from
disorder'' in frustrated magnetic systems. Unlike other spectrum calculations
in order from disorder, we employ the Euclidean path integral. For comparison,
we also present a Hamiltonian calculation using a generalized
Holstein-Primakoff transformation. The latter can only be constructed for a
subset of the cases we consider.",0308018v4
1992-07-21,A New Family of Diagonal Ade-Related Scattering Theories,"We propose the factorizable S-matrices of the massive excitations of the
non-unitary minimal model $M_{2,11}$ perturbed by the operator $\Phi_{1,4}$.
The massive excitations and the whole set of two particle S-matrices of the
theory is simply related to the $E_8$ unitary minimal scattering theory. The
counting argument and the Thermodynamic Bethe Ansatz (TBA) are applied to this
scattering theory in order to support this interpretation. Generalizing this
result, we describe a new family of NON UNITARY and DIAGONAL $ADE$-related
scattering theories. A further generalization suggests the magnonic TBA for a
large class of non-unitary $\G\otimes\G/\G$ coset models
($\G=A_{odd},D_n,E_{6,7,8}$) perturbed by $\Phi_{id,id,adj}$, described by
non-diagonal S-matrices.",9207069v1
1996-05-07,Ground state and low excitations of an integrable chain with alternating spins,"An anisotropic integrable spin chain, consisting of spins $s=1$ and
$s=\frac{1}{2}$, is investigated \cite{devega}. It is characterized by two real
parameters $\bar{c}$ and $\tilde{c}$, the coupling constants of the spin
interactions. For the case $\bar{c}<0$ and $\tilde{c}<0$ the ground state
configuration is obtained by means of thermodynamic Bethe ansatz. Furthermore
the low excitations are calculated. It turns out, that apart from free magnon
states being the holes in the ground state rapidity distribution, there exist
bound states given by special string solutions of Bethe ansatz equations (BAE)
in analogy to \cite{babelon}. The dispersion law of these excitations is
calculated numerically.",9605040v1
1997-07-10,Excited states in the twisted XXZ spin chain,"We compute the finite size spectrum for the spin 1/2 XXZ chain with twisted
boundary conditions, for anisotropy in the regime $0< \gamma <\pi/2$, and
arbitrary twist $\theta$. The string hypothesis is employed for treating
complex excitations. The Bethe Ansatz equtions are solved within a coupled
non-linear integral equation approach, with one equation for each type of
string. The root-of-unity quantum group invariant periodic chain reduces to the
XXZ_1/2 chain with a set of twist boundary conditions ($\pi/\gamma\in Z$,
$\theta$ an integer multiple of $\gamma$). For this model, the restricted
Hilbert space corresponds to an unitary conformal field theory, and we recover
all primary states in the Kac table in terms of states with specific twist and
strings.",9707098v2
2002-04-24,Positive Mass from Holographic Causality,"For n+1 dimensional asymptotically AdS spacetimes which have holographic
duals on their n dimensional conformal boundaries, we show that the imposition
of causality on the boundary theory is sufficient to prove positivity of mass
for the spacetime when n > 2, without the assumption of any local energy
condition. We make crucial use of a generalization of the time-delay formula
calculated in gr-qc/9404019, which relates the time delay of a bulk null curve
with respect to a boundary null geodesic to the Ashtekar-Magnon mass of the
spacetime. We also discuss holographic causality for the negative mass AdS
soliton and its implications for the positive energy conjecture of Horowitz and
Myers.",0204198v2
2004-08-30,The First Law of Thermodynamics for Kerr-Anti-de Sitter Black Holes,"We obtain expressions for the mass and angular momenta of rotating black
holes in anti-de Sitter backgrounds in four, five and higher dimensions. We
verify explicitly that our expressions satisfy the first law of thermodynamics,
thus allowing an unambiguous identification of the entropy of these black holes
with $\ft14$ of the area. We find that the associated thermodynamic potential
equals the background-subtracted Euclidean action multiplied by the
temperature. Our expressions differ from many given in the literature. We find
that in more than four dimensions, only our expressions satisfy the first law
of thermodynamics. Moreover, in all dimensions we show that our expression for
the mass coincides with that given by the conformal conserved charge introduced
by Ashtekar, Magnon and Das. We indicate the relevance of these results to the
AdS/CFT correspondence.",0408217v3
2006-06-15,Infinite spin limit of semiclassical string states,"Motivated by recent works of Hofman and Maldacena and Dorey we consider a
special infinite spin limit of semiclassical spinning string states in AdS5 x
S5. We discuss examples of known folded and circular 2-spin string solutions
and demonstrate explicitly that the 1-loop superstring correction to the
classical expression for the energy vanishes in the limit when one of the spins
is much larger that the other. We also give a general discussion of this limit
at the level of integral equations describing finite gap solutions of the
string sigma model and argue that the corresponding asymptotic form of the
string and gauge Bethe equations is the same.",0606145v2
2006-07-25,"BPS Condensates, Matrix Models and Emergent String Theory","A prescription is given for computing anomalous dimensions of single trace
operators in SYM at strong coupling and large $N$ using a reduced model of
matrix quantum mechanics. The method involves treating some parts of the
operators as ""BPS condensates"" which, in certain limit, have a dual description
as null geodesics on the $S^5$. In the gauge theory, the condensate is similar
to a representative of the chiral ring and it is described by a background of
commuting matrices. Excitations around these condensates correspond to
excitations around this background and take the form of ""string bits"" which are
dual to the ""giant magnons"" of Hofman and Maldacena. In fact, the matrix model
approach gives a {\it quantum} description of these string configurations and
explains why the infinite momentum limit suppresses the quantum effects. This
method allows, not only to derive part of the classical sigma model Hamiltonian
of the dual string (in the infinite momentum limit), but also its quantum
canonical structure. Therefore, it provides an alternative method of testing
the AdS/CFT correspondence without the need of integrability.",0607204v2
2006-09-04,A Perspective on Classical Strings from Complex Sine-Gordon Solitons,"We study a family of classical string solutions with large spins on R x S^3
subspace of AdS_5 x S^5 background, which are related to Complex sine-Gordon
solitons via Pohlmeyer's reduction. The equations of motion for the classical
strings are cast into Lame equations and Complex sine-Gordon equations. We
solve them under periodic boundary conditions, and obtain analytic profiles for
the closed strings. They interpolate two kinds of known rigid configurations
with two spins: on one hand, they reduce to folded or circular
spinning/rotating strings in the limit where a soliton velocity goes to zero,
while on the other hand, the dyonic giant magnons are reproduced in the limit
where the period of a kink-array goes to infinity.",0609026v4
2006-09-06,A Crossing-Symmetric Phase for AdS_5 x S^5 Strings,"We propose an all-order perturbative expression for the dressing phase of the
AdS_5 x S^5 string S-matrix at strong coupling. Moreover, we are able to sum up
large parts of this expression. This allows us to start the investigation of
the analytic structure of the phase at finite coupling revealing a few
surprising features. The phase obeys all known constraints including the
crossing relation and it matches with the known physical data at strong
coupling. In particular, we recover the bound states of giant magnons recently
found by Hofman and Maldacena as poles of the scattering matrix. At weak
coupling our proposal seems to differ with gauge theory. A possible solution to
this disagreement is the inclusion of additional pieces in the phase not
contributing to crossing, which we also study.",0609044v2
2007-01-22,Quantum deformed magnon kinematics,"The dispersion relation for planar N=4 supersymmetric Yang-Mills is
identified with the Casimir of a quantum deformed two-dimensional kinematical
symmetry, E_q(1,1). The quantum deformed symmetry algebra is generated by the
momentum, energy and boost, with deformation parameter q=e^{2\pi i/\lambda}.
Representing the boost as the infinitesimal generator for translations on the
rapidity space leads to an elliptic uniformization with crossing
transformations implemented through translations by the elliptic half-periods.
This quantum deformed algebra can be interpreted as the kinematical symmetry of
a discrete integrable model with lattice spacing given by the BMN length
a=2\pi/\sqrt{\lambda}. The interpretation of the boost generator as the corner
transfer matrix is briefly discussed.",0701200v2
2007-03-05,Strong coupling limit of Bethe Ansatz equations,"We develop a method to analyze the strong coupling limit of the Bethe ansatz
equations supposed to give the spectrum of anomalous dimensions of the planar
${\cal N}=4$ gauge theory. This method is particularly adapted for the three
rank-one sectors, $su(2)$, $su(1|1)$ and $sl(2)$. We use the elliptic
parametrization of the Bethe ansatz variables, which degenerates to a
hyperbolic one in the strong coupling limit. We analyze the equations for the
highest excited states in the su(2) and $su(1|1)$ sectors and for the state
corresponding to the twist-two operator in the $sl(2)$ sector, both without and
with the dressing kernel. In some cases we were able to give analytic
expressions for the leading order magnon densities. Our method reproduces all
existing analytical and numerical results for these states at the leading
order.",0703031v2
2006-06-27,Periodic structure of spin-transfer current in ferromagnetic multilayers,"We show that the drift-diffusion mechanism in a normal-metal layer in
combination with the resonance electron-magnon interactions at
ferromagnet-normal interface of F-N-F heterostucture creates spatial
instability modes and, out of these modes, a quasi-stable periodic structure of
spin-transfer d.c. current can arise with certain channel step and
step-to-radius ratio. The ferromagnetic resonance conditions determine
spin-transfer current density. Independent nano-oscillators creating
intersecting arrays of channels can phase-lock on sub-micrometer distance,
which depends on multilayer geometry and applied fields. By decreasing the
layer thickness, the number of channels affected by each independent
d.c.-current source and their radius may be diminished. Phase-locking of
multiple independent nano-oscillators can be used for enhancement of output
power.",0606064v1
2003-10-21,Phonons and Magnetic Excitations in Mott-Insulator LaTiO$_3$,"The polarized Raman spectra of stoichiometric LaTiO$_3$ (T$_N = 150$ K) were
measured between 6 and 300 K. In contrast to earlier report on half-metallic
LaTiO$_{3.02}$, neither strong background scattering, nor Fano shape of the
Raman lines was observed. The high frequency phonon line at 655 cm$^{-1}$
exhibits anomalous softening below T$_N$: a signature for structural
rearrangement. The assignment of the Raman lines was done by comparison to the
calculations of lattice dynamics and the nature of structural changes upon
magnetic ordering are discussed. The broad Raman band, which appears in the
antiferromagnetic phase, is assigned to two-magnon scattering. The estimated
superexchange constant $J = 15.4\pm0.5$ meV is in excellent agreement with the
result of neutron scattering studies.",0310097v1
2002-05-22,A switchable controlled-NOT gate in a spin-chain NMR quantum computer,"A method of switching a controlled-NOT gate in a solid-stae NMR quantum
computer is presented. Qubits of I=1/2 nuclear spins are placed periodically
along a quantum spin chain (1-D antiferromagnet) having a singlet ground state
with a finite spin gap to the lowest excited state caused by some quantum
effect. Irradiation of a microwave tuned to the spin gap energy excites a
packet of triplet magnons at a specific part of the chain where control and
target qubits are involved. The packet switches on the Suhl-Nakamura
interaction between the qubits, which serves as a controlled NOT gate. The
qubit initialization is achieved by a qubit initializer consisting of
semiconducting sheets attached to the spin chain, where spin polarizations
created by the optical pumping method in the semiconductors are transferred to
the spin chain. The scheme allows us to separate the initialization process
from the computation, so that one can optimize the computation part without
being restricted by the initialization scheme, which provides us with a wide
selection of materials for a quantum computer.",0205135v1
2002-12-09,Entanglement sharing in one-particle states,"Entanglement sharing among sites of one-particle states is considered using
the measure of concurrence. These are the simplest in an hierarchy of
number-specific states of many qubits and corresponds to ``one-magnon'' states
of spins. We study the effects of onsite potentials that are both integrable
and nonintegrable. In the integrable case we point to a metal-insulator
transition that reflects on the way entanglement is shared. In the
nonintegrable case the average entanglement content increases and saturates
along with a transition to classical chaos. Such quantum chaotic states are
shown to have universal concurrence distributions that are modified Bessel
functions derivable within random matrix theory. Time-reversal breaking and
time evolving states are shown to possess significantly higher entanglement
sharing capacity that eigenstates of time-reversal symmetric systems. We use
the ordinary Harper and kicked Harper Hamiltonians as model systems.",0212049v3
2003-07-18,Entanglement dynamics and quantum state transport in spin chains,"We study the dynamics of a Heisenberg-XY spin chain with an unknown state
coded into one qubit or a pair of entangled qubits, with the rest of the spins
being in a polarized state. The time evolution involves magnon excitations, and
through them the entanglement is transported across the channel. For a large
number of qubits, explicit formulae for the concurrences, measures for
two-qubit entanglements, and the fidelity for recovering the state some
distance away are calculated as functions of time. Initial states with an
entangled pair of qubits show better fidelity, which takes its first maximum
value at earlier times, compared to initial states with no entangled pair. In
particular initial states with a pair of qubits in an unknown state (alpha
up-up + beta down-down) are best suited for quantum state transport.",0307135v2
2004-09-18,Spin wave based quantum storage of electronic spin with a ring array of nuclear spins,"We propose a solid state based protocol to implement the universal quantum
storage for electronic spin qubit. The quantum memory in this scheme is the
spin wave excitation in the ring array of nuclei in a quantum dot. We show that
the quantum information carried by an arbitrary state of the electronic spin
can be coherently mapped onto the spin wave excitations or the magnon states.
With an appropriate external control, the stored quantum state in quantum
memory can be read out reversibly. We also explore in detail the quantum
decoherence mechanism due to the inhomogeneous couplings between the electronic
spin and the nuclear spins.",0409120v3
2006-06-04,Dynamic generation of entangling wave packets in XY spin system with decaying long range couplings,"We study the dynamic generation of spin entanglement between two distant
sites in an XY model with $1/r^{2}$-decay long range couplings. Due to the
linear dispersion relation $\epsilon (k)\sim |k|$ of magnons in such model, we
show that a well-located spin state can be dynamically split into two moving
entangled local wave packets without changing their shapes. Interestingly, when
such two wave packets meet at the diametrically opposite site after the fast
period $\tau =\pi $, the initial well-located state can be recurrent
completely. Numerical calculation is performed to confirm the analytical result
even the ring system of sizes $N$ up to several thousands are considered. The
truncation approximation for the coupling strengths is also studied. Numerical
simulation shows that the above conclusions still hold even the range of the
coupling strength is truncated at a relative shorter scale comparing to the
size of the spin system.",0606029v1
2006-12-11,Quantum chaos with spin-chains in pulsed magnetic fields,"Recently it was found that the dynamics in a Heisenberg spin-chain subjected
to a sequence of periodic pulses from an external, parabolic, magnetic field
can have a close correspondence with the quantum kicked rotor (QKR). The QKR is
a key paradigm of quantum chaos; it has as its classical limit the well-known
Standard Map. It was found that a single spin excitation could be converted
into a pair of non-dispersive, counter-propagating spin coherent states
equivalent to the accelerator modes of the Standard Map. Here we consider how
other types of quantum chaotic systems such as a double-kicked quantum rotor or
a quantum rotor with a double-well potential might be realized with spin
chains; we discuss the possibilities regarding manipulation of the one-magnon
spin waves.",0612074v1
2007-04-23,Neumann and Neumann-Rosochatius integrable systems from membranes on AdS_4xS^7,"It is known that large class of classical string solutions in the type IIB
AdS_5xS^5 background is related to the Neumann and Neumann-Rosochatius
integrable systems, including spiky strings and giant magnons. It is also
interesting if these integrable systems can be associated with some membrane
configurations in M-theory. We show here that this is indeed the case by
presenting explicitly several types of membrane embedding in AdS_4xS^7 with the
searched properties.",0704.3082v4
2007-05-14,Polaronic features in the optical properties of the Holstein-t-J model,"We derive the exact solution for the optical conductivity $\sigma(\omega)$ of
one hole in the Holstein-t-J model in the framework of dynamical mean-field
theory (DMFT). We investigate the magnetic and phonon features associated with
polaron formation as a function of the exchange coupling $J$, of the
electron-phonon interaction $\lambda$ and of the temperature. Our solution
directly relates the features of the optical conductivity to the excitations in
the single-particle spectral function, revealing two distinct mechanisms of
closing and filling of the optical pseudogap that take place upon varying the
microscopic parameters. We show that the optical absorption at the polaron
crossover is characterized by a coexistence of a magnon peak at low frequency
and a broad polaronic band at higher frequency. An analytical expression for
$\sigma(\omega)$ valid in the polaronic regime is presented.",0705.1944v2
2007-05-17,Single spike solutions for strings on S2 and S3,"We study solutions for rigidly rotating strings on a two sphere. Among them
we find two limiting cases that have a particular interest, one is the already
known giant magnon and the other we call the single spike solution. The
limiting behavior of this last solution is a string infinitely wrapped around
the equator. It differs from that solution by the existence of a single spike
of height theta that points toward the north pole.
  We study its properties and compute its energy E and angular momentum J as a
function of theta. We further generalize the solution by adding one angular
momentum to obtain a solution on S3. We find a spin chain interpretations of
these results in terms of free fermions and the Hubbard model but the exact
relation with the same models derived from the field theory is not clear.",0705.2429v2
2007-05-17,Thermodynamics of the Up-Up-Down Phase of the S = 1/2 Triangular-Lattice Antiferromagnet Cs$_2$CuBr$_4$,"Specific heat and the magnetocaloric effect are used to probe the
field-induced up-up-down phase of Cs2CuBr4, a quasi-two-dimensional spin-1/2
triangular antiferromagnet with near-maximal frustration. The shape of the
magnetic phase diagram shows that the phase is stabilized by quantum
fluctuations, not by thermal fluctuations as in the corresponding phase of
classical spins. The magnon gaps determined from the specific heat are
considerably larger than those expected for a Heisenberg antiferromagnet,
probably due to the presence of a small Dzyaloshinskii-Moriya interaction.",0705.2473v1
2007-06-21,Quantum and thermal fluctuations in a two-dimensional correlated band ferromagnet -- Goldstone-mode preserving investigation with self-energy and vertex corrections,"Ferromagnetism in the t-t' Hubbard model is investigated on a square lattice.
Correlation effects in the form of self-energy and vertex corrections are
systematically incorporated within a spin-rotationally-symmetric scheme which
explicitly preserves the Goldstone mode and is therefore in accord with the
Mermin-Wagner theorem. Interplay of band dispersion and correlation effects on
ferromagnetic-state stability are highlighted with respect to both long- and
short-wavelength fluctuations, which are shown to have substantially different
behaviour. Our approach provides a novel understanding of the enhancement of
ferromagnetism near van Hove filling for t'~0.5 in terms of strongly suppressed
saddle-point contribution to the destabilizing exchange part of spin stiffness.
Finite-temperature electron spin dynamics is investigated directly in terms of
spectral-weight transfer across the Fermi energy due to electron-magnon
coupling. Relevant in the context of recent magnetization measurements on
ultrathin films, the role of strong thermal spin fluctuations in low dimensions
is highlighted, in the anisotropy-stabilized ordered state, by determining the
thermal decay of magnetization and T_c within a renormalized spin-fluctuation
theory.",0706.3088v1
2007-08-03,Strong spin-orbit induced Gilbert damping and g-shift in iron-platinum nanoparticles,"The shape of ferromagnetic resonance spectra of highly dispersed, chemically
disordered Fe_{0.2}Pt_{0.8} nanospheres is perfectly described by the solution
of the Landau-Lifshitz-Gilbert (LLG) equation excluding effects by crystalline
anisotropy and superparamagnetic fluctuations. Upon decreasing temperature, the
LLG damping $\alpha(T)$ and a negative g-shift, g(T)-g_0, increase proportional
to the particle magnetic moments determined from the Langevin analysis of the
magnetization isotherms. These novel features are explained by the scattering
of the $q \to 0$ magnon from an electron-hole (e/h) pair mediated by the
spin-orbit coupling, while the sd-exchange can be ruled out. The large
saturation values, $\alpha(0)=0.76$ and $g(0)/g_0-1=-0.37$, indicate the
dominance of an overdamped 1 meV e/h-pair which seems to originate from the
discrete levels of the itinerant electrons in the d_p=3 nm nanoparticles.",0708.0463v1
2007-08-08,The anomalous Hall Effect and magnetoresistance in the layered ferromagnet Fe_{1/4}TaS_2: the inelastic regime,"The large magnetic anisotropy in the layered ferromagnet Fe_{1/4}TaS_2 leads
to very sharp reversals of the magnetization $\bf M$ at the coercive field. We
have exploited this feature to measure the anomalous Hall effect (AHE),
focussing on the AHE conductivity $\sigma^A_{xy}$ in the inelastic regime. At
low temperature T (5-50 K), $\sigma^A_{xy}$ is T-independent, consistent with
the Berry-phase/Karplus-Luttinger theory. Above 50 K, we extract an inelastic
AHE conductivity $\sigma^{in}_{xy}$ that scales as the square of $\Delta\rho$
(the T dependent part of the resistivity $\rho$). The term $\sigma^{in}_{xy}$
clarifies the T dependence and sign-reversal of the AHE coefficient R_s(T). We
discuss the possible ubiquity of $\sigma^{in}_{xy}$ in ferromagnets, and ideas
for interpreting its scaling with $(\Delta\rho)^2$. Measurements of the
magnetoresistance (MR) reveal a rich pattern of behavior vs. T and field
tilt-angle. We show that the 2 mechanisms, the anisotropic MR effect and
field-suppression of magnons, account for the intricate MR behavior, including
the bow-tie features caused by the sharp reversals in $\bf M$.",0708.1143v1
2007-08-16,Reflecting magnons,"We study the worldsheet reflection matrix of a string attached to a D-brane
in $AdS_5 \times S^5$. The D-brane corresponds to a maximal giant graviton and
it wraps an $S^3$ inside $S^5$. In the gauge theory, the open string is
described by a spin chain with boundaries. We study an open string with a large
SO(6) charge, which allows us to focus on one boundary at a time and to define
an asymptotic boundary reflection matrix. We consider two cases corresponding
to two possible relative orientations for the charges of the giant graviton and
the open string. Using the symmetries of the problem we compute the boundary
reflection matrix up to a phase. These matrices obey the boundary Yang Baxter
equation. A crossing equation is derived for the overall phase. We perform weak
coupling computations up to two loops and obtain results that are consistent
with integrability. Finally, we determine the phase factor at strong coupling
using classical solutions.",0708.2272v2
2007-10-01,Strongly coupled large N spectrum of two matrices coupled via a Yang-Mills interaction,"We consider the large N spectrum of the quantum mechanical hamiltonian of two
hermitean matrices coupled via a Yang-Mills interaction. In a framework where
one of the matrices is treated exactly and the other is treated as a creation
operator impurity, the difference equation associated with the Yang-Mills
interaction is derived and solved exactly for two impurities. In this case, the
full string tension corrected spectrum depends on two momenta. For a specific
value of one of these momenta, the spectrum has the same structure as that of
giant magnon bound states. States with general number of impurities are also
discussed.",0710.0073v2
2007-09-30,D1-brane in beta-Deformed Background,"We study various configurations of rotating and wound D1-brane in AdS_5\times
S^5 background and in its beta deformed version. We find giant magnon and spike
solutions on the world-volume of D1-brane in AdS_5\times S^5 background. We
also analyse the equations of motion of D1-brane in beta-deformed background.
We show that in the limit of large electric flux on world-volume of D1-brane
they reduce to the equations that describe collection of large number of
fundamental strings. We also construct rotating and wound D1-brane solution
that has two equal spins on S^5_\gamma.",0710.0148v3
2007-10-02,Ground-state phase diagram and magnetic properties of a tetramerized spin-1/2 J_1-J_2 model: BEC of bound magnons and absence of the transverse magnetization,"We study the ground state and the magnetization process of a spin-1/2
$J_1$-$J_2$ model with a plaquette structure by using various methods. For
small inter-plaquette interaction, this model is expected to have a spin-gap
and we computed the first- and the second excitation energies. If the gap of
the lowest excitation closes, the corresponding particle condenses to form
magnetic orders. By analyzing the quintet gap and magnetic interactions among
the quintet excitations, we find a spin-nematic phase around $J_1/J_2\sim -2$
due to the strong frustration and the quantum effect. When high magnetic moment
is applied, not the spin-1 excitations but the spin-2 ones soften and dictate
the magnetization process. We apply a mean-field approximation to the effective
Hamiltonian to find three different types of phases (a conventional BEC phase,
``striped'' supersolid phases and a 1/2-plateau). Unlike the BEC in spin-dimer
systems, this BEC phase is not accompanied by transverse magnetization.
Possible connection to the recently discovered spin-gap compound (CuCl)LaNb2O7
is discussed.",0710.0463v2
2007-10-21,From an Antiferromagnet to a Valence Bond Solid: Evidence for a First Order Phase Transition,"Using a loop-cluster algorithm we investigate the spin 1/2 Heisenberg
antiferromagnet on a square lattice with exchange coupling $J$ and an
additional four-spin interaction of strength $Q$. We confirm the existence of a
phase transition separating antiferromagnetism at $J/Q > J_c/Q$ from a valence
bond solid (VBS) state at $J/Q < J_c/Q$. Although our Monte Carlo data are
consistent with those of previous studies, we do not confirm the existence of a
deconfined quantum critical point. Instead, using a flowgram method on lattices
as large as $80^2$, we find evidence for a weak first order phase transition.
We also present a detailed study of the antiferromagnetic phase. For $J/Q >
J_c/Q$ the staggered magnetization, the spin stiffness, and the spinwave
velocity of the antiferromagnet are determined by fitting Monte Carlo data to
analytic results from the systematic low-energy effective field theory for
magnons. Finally, we also investigate the physics of the VBS state at $J/Q <
J_c/Q$, and we show that long but finite antiferromagnetic correlations are
still present.",0710.3926v1
2007-11-06,Half-metallic ferromagnets: From band structure to many-body effects,"A review of new developments in theoretical and experimental electronic
structure investigations of half-metallic ferromagnets (HMF) is presented.
Being semiconductors for one spin projection and metals for another ones, these
substances are promising magnetic materials for applications in spintronics
(i.e., spin-dependent electronics). Classification of HMF by the peculiarities
of their electronic structure and chemical bonding is discussed. Effects of
electron-magnon interaction in HMF and their manifestations in magnetic,
spectral, thermodynamic, and transport properties are considered. Especial
attention is paid to appearance of non-quasiparticle states in the energy gap,
which provide an instructive example of essentially many-body features in the
electronic structure. State-of-art electronic calculations for correlated
$d$-systems is discussed, and results for specific HMF (Heusler alloys,
zinc-blende structure compounds, CrO$_{2},$ Fe$_{3}$O$_{4}$) are reviewed.",0711.0872v1
2007-11-09,Chirality and $Z_2$ vortices in an Heisenberg spin model on the kagomé lattice,"The phase diagram of the classical \jj model on the \kag lattice is
investigated using extensive \mc simulations. In a realistic range of
parameters, this model has a low-temperature chiral-ordered phase without
long-range spin order. We show that the critical transition marking the
destruction of chiral order is preempted by the first order proliferation of
\Zdeux point defects. The core energy of these vortices appears to vanish when
approaching the T=0 phase boundary, where both \Zdeux defects and gapless
magnons contribute to disordering the system at very low temperature. This
situation might be typical of a large class of frustrated magnets. Possible
relevance for real materials is also discussed.",0711.1497v3
2007-11-26,Quasi-universal finite-$T$ scaling in gapped one-dimensional quantum magnets,"Temperature dependencies of gap energies and magnon lifetimes are measured in
the quasi-1-dimensional S=1/2 gapped quantum magnets IPA-CuCl3 and Sul-Cu2Cl4
using inelastic neutron scattering. The results are compared to those found in
literature for S=1 Haldane spin chain materials and to theoretical calculations
for the O(3)- and O(N)- quantum non-linear sigma-models. It is found that when
the T=0 energy gap Delta is used as the temperature scale, all experimental and
theoretical curves are identical to within system-dependent but
temperature-independent scaling factors of the order of unity. This
quasi-universality extends over a surprising broad T range, at least up to
kappa T ~ 1.5 Delta.",0711.4094v2
2007-12-13,Electromagnon excitations in modulated multiferroics,"The phenomenological theory of ferroelectricity in spiral magnets presented
in [M. Mostovoy, Phys. Rev. Lett. 96, 067601 (2006)] is generalized to describe
consistently states with both uniform and modulated-in-space ferroelectric
polarizations. A key point in this description is the symmetric part of the
magnetoelectric coupling since, although being irrelevant for the uniform
component, it plays an essential role for the non-uniform part of the
polarization. We illustrate this importance in generic examples of modulated
magnetic systems: longitudinal and transverse spin-density wave states and
planar cycloidal phase. We show that even in the cases with no uniform
ferroelectricity induced, polarization correlation functions follow to the soft
magnetic behavior of the system due to the magnetoelectric effect. Our results
can be easily generalized for more complicated types of magnetic ordering, and
the applications may concern various natural and artificial systems in
condensed matter physics (e.g., magnon properties could be extracted from
dynamic dielectric response measurements).",0712.2138v2
2007-12-18,Observation of electromagnon excitations in the BiFe0$_3$ spiral magnet,"Recently, oxide multiferroics have attracted much attention due to their
large magnetoelectric effect which allows the tuning of magnetic properties
with electric field and vice versa and open new venues for future spintronic
applications such as multiple-state memory devices with dual magnetic and
electric control. BiFeO$_3$ (BFO) belongs to this new class of materials and
shows both ferroelectric and antiferromagnetic orders at room temperature with
a large electric polarizationassociated with a cycloidal spiral magnetic
ordering. The incommensurate magnetic order induces magnon zone folding and
allows investigations by optical probes of unusual spin waves which couples to
optical phonons, the so called ""`electromagnons""'. Here, we unravel for the
first time the electromagnon spectra of BFO by means low energy inelastic light
scattering technique. We show the existence of two species of electromagnons
corresponding to spin wave excitations in and out of the cycloidal plane. The
present observations present an unique opportunity to study the interplay
between ferroelectric and magnetic orders.",0712.3044v1
2008-01-17,Uncovering a pressure-tuned electronic transition in BiSrYCu2O8 using Raman scattering and x-ray diffraction,"We report pressure tuned Raman and x-ray diffraction data of
Bi1.98Sr2.06Y0.68Cu2O8 revealing a critical pressure at 21 GPa with anomalies
in six physical quantities: electronic Raman background, electron-phonon
coupling, spectral weight transfer from high to low frequency, density
dependent behaviour of phonon and magnon frequencies, and a compressibility
change in the c-axis. For the first time in a cuprate, mobile charge carriers,
lattice, and magnetism all show anomalies at a distinct critical pressure in
the same experimental setting. Furthermore, the Raman spectral changes are
similar to that seen traversing the superconducting dome with doping,
suggesting that the critical pressure at 21 GPa is related to the much
discussed critical point at optimal doping.",0801.2772v1
2008-01-17,Bose-Einstein condensation in antiferromagnets close to the saturation field,"At zero temperature and strong applied magnetic fields the ground sate of an
anisotropic antiferromagnet is a saturated paramagnet with fully aligned spins.
We study the quantum phase transition as the field is reduced below an upper
critical $H_{c2}$ and the system enters a XY-antiferromagnetic phase. Using a
bond operator representation we consider a model spin-1 Heisenberg
antiferromagnetic with single-ion anisotropy in hyper-cubic lattices under
strong magnetic fields. We show that the transition at $H_{c2}$ can be
interpreted as a Bose-Einstein condensation (BEC) of magnons. The theoretical
results are used to analyze our magnetization versus field data in the organic
compound $NiCl_2$-$4SC(NH_2)_2$ (DTN) at very low temperatures. This is the
ideal BEC system to study this transition since $H_{c2}$ is sufficiently low to
be reached with static magnetic fields (as opposed to pulsed fields). The
scaling of the magnetization as a function of field and temperature close to
$H_{c2}$ shows excellent agreement with the theoretical predictions. It allows
to obtain the quantum critical exponents and confirm the BEC nature of the
transition at $H_{c2}$.",0801.2795v2
2008-01-22,Quantum Oscillations of Tunnel Magnetoresistance Induced by Spin-Wave Excitations in Ferromagnet-Ferromagnet-Ferromagnet Double Barrier Tunnel Junctions,"The possibility of quantum oscillations of the tunnel conductance and
magnetoresistance induced by spin-wave excitations in a
ferromagnet-ferromagnet-ferromagnet double barrier tunnel junction, when the
magnetizations of the two side ferromagnets are aligned antiparallel to that of
the middle ferromagnet, is investigated in a self-consistent manner by means of
Keldysh nonequilibrium Green function method. It has been found that owing to
the s-d exchange interactions between conduction electrons and the spin density
induced by spin accumulation in the middle ferromagnet, the differential
conductance and the TMR indeed oscillate with the increase of bias voltage,
being consistent with the phenomenon that is observed recently in experiments.
The effects of magnon modes, the energy levels of electrons as well as the
molecular field in the central ferromagnet on the oscillatory transport
property of the system are also discussed.",0801.3350v1
2008-03-12,Electron transport and thermoelectric properties of layered perovskite LaBaCo2O5.5,"We have investigated the systematic transport properties of the layered
112-type cobaltite LaBaCo2O5.5 by means of electrical resistivity,
magnetoresistance, electroresistance and thermoelectric measurements in various
conditions. In order to understand the complex conduction mechanism of
LaBaCo2O5.5, the transport data have been analyzed using different theoretical
models. The system shows semiconductor-semiconductor like transition (TSC)
around 326K, corresponding to ferromagnetic transition and in the low
temperature region resistivity data follows the Motts variable range hopping
model. Interestingly, near and below the room temperature this compound depicts
significant change in electro- and magnetoresistance behavior, the latter one
is noteworthy near the magnetic phase boundary. The temperature dependence of
thermopower, S(T), exhibits p-type polaronic conductivity in the temperature
range of 60-320K and reaches a maximum value of 303 uV/K (at 120K). In the low
temperature AFM region, the unusual S(T) behavior, generally observed for the
cobaltite series LnBaCo2O5.5 (Ln = Rare Earth), is explained by the electron
magnon scattering mechanism as previously described for perovskite manganites.",0803.1806v1
2008-04-07,"Energy equilibriation processes of electrons, magnons and phonons on the femtosecond timescale","By means of time-resolved Kerr spectroscopy experiments we relate the energy
dissipation processes on the femtosecond (electron-spin relaxation time
$\tau_{el-sp}$) and nanosecond timescale (Gilbert relaxation $\tau_{\alpha}$)
and compare the results to the first microscopic model, which was proposed by
Koopmans. For both energy dissipation processes, Elliot-Yafet scattering is
proposed as the dominant contributor. We controllably manipulate the energy
dissipation processes by transition metal doping (Pd) and rare earth doping
(Dy) of a Permalloy film and find that while a change of $\tau_{\alpha}$ of
more than a factor two is observed, \tau_{el-sp}$ remains constant, contrary to
the predictions of the model. We explain the discrepancies by relaxation
channels not considered in the original microscopic model and identify thereby
the applicability of the model and possible necessary extensions to the model.",0804.0985v1
2008-06-19,Magnetoelastic-magnetoelectric phase transitions in multiferroic BiFeO3,"Our measured dielectric constant and mechanical response of multiferroic
BiFeO3 indicate four phase transitions below room temperature. Features
correlate with those reported at 50K (from a peak in the zero-field-cooled
magnetic susceptibility) and 230K (from splitting between field-cooled and
zero-field-cooled magnetic data[1], and 200K (from magnon light scattering
cross sections[2]). The primary order parameter is not the polarization in any
of the low-T transitions. Instead, the transition near 230 K shows strong
elastic coupling, while that at 50K is fundamentally magnetic, but
magnetostrictively coupled to the the lattice. The low-T phase transitions
display glassy behaviour. A further anomaly at 140K interpreted as spin
reorientation[2,3] shows only weakly in dielectric and mechanical studies,
indicating that it is predominantly magnetic with little coupling to any of the
other order parameters.",0806.3241v2
2008-08-21,High energy spin excitations in BaFe2As2,"We report neutron scattering measurements of cooperative spin excitations in
antiferromagnetically ordered BaFe2As2, the parent phase of an iron pnictide
superconductor. The data extend up to ~100meV and show that the spin excitation
spectrum is sharp and highly dispersive. By fitting the spectrum to a linear
spin-wave model we estimate the magnon bandwidth to be in the region of 0.17eV.
The large characteristic spin fluctuation energy suggests that magnetism could
play a role in the formation of the superconducting state.",0808.2836v2
2008-09-02,Parametrically-stimulated recovery of a microwave signal using standing spin-wave modes of a magnetic film,"The phenomenon of storage and parametrically-stimulated recovery of a
microwave signal in a ferrite film has been studied both experimentally and
theoretically. The microwave signal is stored in the form of standing spin-wave
modes existing in the film due to its finite thickness. Signal recovery is
performed by means of frequency-selective amplification of one of these
standing modes by double- requency parametric pumping process. The time of
recovery, as well as the duration and magnitude of the recovered signal, depend
on the timing and amplitudes of both the input and pumping pulses. A mean-field
theory of the recovery process based on the competitive interaction of the
signal-induced standing spin-wave mode and thermal magnons with the parametric
pumping field is developed and compared to the experimental data.",0809.0395v1
2008-09-29,Systematic Effective Field Theory Investigation of Spiral Phases in Hole-Doped Antiferromagnets on the Honeycomb Lattice,"Motivated by possible applications to the antiferromagnetic precursor of the
high-temperature superconductor Na$_x$CoO$_2\cdot$yH$_2$O, we use a systematic
low-energy effective field theory for magnons and holes to study different
phases of doped antiferromagnets on the honeycomb lattice. The effective action
contains a leading single-derivative term, similar to the Shraiman-Siggia term
in the square lattice case, which gives rise to spirals in the staggered
magnetization. Depending on the values of the low-energy parameters, either a
homogeneous phase with four or a spiral phase with two filled hole pockets is
energetically favored. Unlike in the square lattice case, at leading order the
effective action has an accidental continuous spatial rotation symmetry.
Consequently, the spiral may point in any direction and is not necessarily
aligned with a lattice direction.",0809.4998v1
2008-10-05,Brillouin light scattering study of Co$_{2}$Cr$_{0.6}$Fe$_{0.4}$Al and Co$_{2}$FeAl Heusler compounds,"The thermal magnonic spectra of Co$_{2}$Cr$_{0.6}$Fe$_{0.4}$Al (CCFA) and
Co$_2$FeAl were investigated using Brillouin light scattering spectroscopy
(BLS). For CCFA, the exchange constant A (exchange stiffness D) is found to be
0.48 $\mu$erg/cm (203 meV A$^2$), while for Co$_2$FeAl the corresponding values
of 1.55 $\mu$erg/cm (370 meV A$^2$) were found. The observed asymmetry in the
BLS spectra between the Stokes and anti-Stokes frequencies was assigned to an
interplay between the asymmetrical profiles of hybridized Damon-Esbach and
perpendicular standing spin-wave modes, combined with the optical sensitivity
of the BLS signal to the upper side of the CCFA or Co$_2$FeAl film.",0810.0838v2
2008-10-11,Near Flat Space limit of strings on AdS_4 x CP^3,"The non-linear nature of string theory on non-trivial backgrounds related to
the AdS/CFT correspondence suggests to look for simplifications. Two such
simplifications proved to be useful in studying string theory. These are the
pp-wave limit which describes point-like strings and the so called ""near flat
space"" limit which connects two different sectors of string theory -- pp-waves
and ""giant magnons"". Recently another example of AdS/CFT duality emerged -
$AdS_4/CFT_3$, which suggests duality between $\mathcal N=6$ CS theory and
superstring theory on $AdS_4\times \cp$. In this paper we study the ""near flat
space"" limit of strings on the $AdS_4\times \cp$ background and discuss
possible applications of the reduced theory.",0810.2008v2
2008-10-13,M2-brane Perspective on N=6 Super Chern-Simons Theory at Level k,"Recently, O. Aharony, O. Bergman, D. L. Jafferis and J. Maldacena (ABJM)
proposed three-dimensional super Chern-Simons-matter theory, which at level k
is supposed to describe the low energy limit of N M2-branes. For large N and k,
but fixed 't Hooft coupling \lambda=N/k, it is dual to type IIA string theory
on AdS_4 x CP^3. For large N but finite k, it is dual to M theory on AdS_4 x
S^7/Z_k. In this paper, relying on the second duality, we find exact giant
magnon and single spike solutions of membrane configurations on AdS_4 x S^7/Z_k
by reducing the system to the Neumann-Rosochatius integrable model. We derive
the dispersion relations and their finite-size corrections with explicit
dependence on the level k.",0810.2171v2
2008-10-13,Antiferromagnetism in two-dimensional quasicrystals,"Quasiperiodic structures possess long range positional order, but are freed
of constraints imposed by translational invariance. For spins interacting via
Heisenberg couplings, one may expect therefore to find novel magnetic
configurations in such structures. We have studied magnetic properties for
simple two dimensional models, as a first step towards understanding
experimentally studied magnetic quasicrystals such as the Zn-Mg-R(rare earth)
compounds. We analyse properties of the antiferromagnetic ground state and
magnon excitation modes for bipartite tilings such as the octagonal and Penrose
tilings. In the absence of frustration, one has an inhomogeneous Neel-ordered
ground state, with local quantum fluctuations of the local staggered magnetic
order parameter. We study the spin wave spectrum and wavefunctions of such
antiferromagnets within the linear spin wave approximation. Some results for
spin-spin correlations in these systems are discussed.",0810.2312v1
2008-11-17,Dynamical properties of the one-dimensional spin-1/2 Bose-Hubbard model near Mott-insulator to ferromagnetic liquid transition,"We investigate the dynamics of the one-dimensional strongly repulsive
spin-1/2 Bose-Hubbard model for filling $\nu\le1.$ While at $\nu=1$ the system
is a Hubbard-Mott insulator exhibiting dynamical properties of the Heisenberg
ferromagnet, at $\nu<1$ it is a ferromagnetic liquid with complex spin
dynamics. We find that close to the insulator-liquid transition the system
admits for a complete separation of spin and density degrees of freedom valid
at {\it all} energy and momentum scales within the $t-J$ approximation. This
allows us to derive the propagator of transverse spin waves and the shape of
the magnon peak in the dynamic spin structure factor.",0811.2676v2
2009-01-05,Constraint Effective Potential of the Staggered Magnetization in an Antiferromagnet,"We employ an improved estimator to calculate the constraint effective
potential of the staggered magnetization in the spin $\tfrac{1}{2}$ quantum
Heisenberg model using a loop-cluster algorithm. The first and second moment of
the probability distribution of the staggered magnetization are in excellent
agreement with the predictions of the systematic low-energy magnon effective
field theory. We also compare the Monte Carlo data with the universal shape of
the constraint effective potential of the staggered magnetization and study its
approach to the convex effective potential in the infinite volume limit. In
this way the higher-order low-energy parameter $k_0$ is determined from a fit
to the numerical data.",0901.0445v1
2009-01-14,Charge and spin transport in strongly correlated one-dimensional quantum systems driven far from equilibrium,"We study the charge conductivity in one-dimensional prototype models of
interacting particles, such as the Hubbard and the t-V spinless fermion model,
when coupled to some external baths injecting and extracting particles at the
boundaries. We show that, if these systems are driven far from equilibrium, a
negative differential conductivity regime can arise. The above electronic
models can be mapped into Heisenberg-like spin ladders coupled to two magnetic
baths, so that charge transport mechanisms are explained in terms of quantum
spin transport. The negative differential conductivity is due to oppositely
polarized ferromagnetic domains which arise at the edges of the chain, and
therefore inhibit spin transport: we propose a qualitative understanding of the
phenomenon by analyzing the localization of one-magnon excitations created at
the borders of a ferromagnetic region. We also show that negative differential
conductivity is stable against breaking of integrability. Numerical simulations
of non-equilibrium time evolution have been performed by employing a
Monte-Carlo wave function approach and a matrix product operator formalism.",0901.2032v3
2009-01-15,Wave function for odd frequency superconductors,"We revisit the question of nature of odd-frequency superconductors, first
proposed by Berezinskii in 1974. \cite{berezinskii1974} We start with the
notion that order parameter of odd-frequency superconductors can be thought of
as a time derivative of the odd-time pairing operator. It leads to the notion
of the composite boson condensate.\cite{abrahams1995} To elucidate the nature
of broken symmetry state in odd-frequency superconductors, we consider a wave
function that properly captures the coherent condensate of composite charge
$2e$ bosons in an odd-frequency superconductor. We consider the Hamiltonian
which describes the equal-time composite boson condensation as proposed earlier
in Phys. Rev. B $\textbf{52}$, 1271 (1995). We propose a BCS-like wave function
that describes a composite condensate comprised of a spin-0 Cooper pair and a
spin-1 magnon excitation. We derive the quasiparticle dispersion, the
self-consistent equation for the order parameter and the density of states. We
show that the coherent wave function approach recovers all the known
proposerties of odd-frequency superconductors: the quasi-particle excitations
are gapless and the superconducting transition requires a critical coupling.",0901.2323v1
2009-01-16,Evidence of spin-density-wave order in RFeAsO from measurements of thermoelectric power,"Data on the magneto-thermopower and specific heat of three compounds
belonging to '1111' oxypnictides family are reported. One specimen
(SmAsFeO0.8F0.2) is a superconductor with Tc = 53 K, while two others (SmAsFeO
and NdAsFeO) are nonsuperconducting parent compounds. Our results confirm that
spin density wave (SDW) order is present in SmAsFeO and NdAsFeO. In these two
samples a strict connection between the thermoelectric power and electronic
specific heat is found in the vicinity of SDW transition, what indicates that
the chemical potential of charge carriers strongly depends on temperature in
this region. Low temperature data suggest presence of significant contribution
magnon-drag to the thermoelectric power.",0901.2472v2
2009-01-19,Chiral asymmetry of the spin-wave spectra in ultrathin magnetic films,"We raise the possibility that the chiral degeneracy of the magnons in
ultrathin films can be lifted due to the presence of Dzyaloshinskii-Moriya
interactions. By using simple symmetry arguments, we discuss under which
conditions such a chiral asymmetry occurs. We then perform relativistic first
principles calculations for an Fe monolayer on W(110) and explicitly reveal the
asymmetry of the spin-wave spectrum in case of wave-vectors parallel to the
(001) direction. Furthermore, we quantitatively interpret our results in terms
of a simplified spin-model by using calculated Dzyaloshinskii-Moriya vectors.
Our theoretical prediction should inspire experiments to explore the asymmetry
of spin-waves, with a particular emphasis on the possibility to measure the
Dzyaloshinskii-Moriya interactions in ultrathin films.",0901.2671v1
2009-01-30,"Twist-three at five loops, Bethe Ansatz and wrapping","We present a formula for the five-loop anomalous dimension of N=4 SYM
twist-three operators in the sl(2) sector. We obtain its asymptotic part from
the Bethe Ansatz and finite volume corrections from the generalized Luescher
formalism, considering scattering processes of spin chain magnons with virtual
particles that travel along the cylinder. The complete result respects the
expected large spin scaling properties and passes non-trivial tests including
reciprocity constraints. We analyze the pole structure and find agreement with
a conjectured resummation formula. In analogy with the twist-two anomalous
dimension at four-loops, wrapping effects are of order log^2 M/M^2 for large
values of the spin.",0901.4864v2
2009-02-02,Fermionic spin excitations in two and three-dimensional antiferromagnets,"Spin excitations in an ordered Heisenberg magnet are magnons--bosons with
spin 1. That may change when frustration and quantum fluctuations suppress the
order and restore the spin-rotation symmetry. We show that spin excitations in
the $S=1/2$ Heisenberg antiferromagnet on kagome are spinons--fermions with
spin 1/2. In the ground state the system can be described as a collection of
small, heavy pairs of spinons with spin 0. A magnetic excitation of lowest
energy amounts to breaking up a pair into two spinons at a cost of $0.06 J$.",0902.0378v4
2009-02-24,Can immobile magnetic vortex nucleate a vortex-antivortex pair?,"It is shown that under the action of rotating magnetic field an immobile
vortex, contrary to a general belief, can nucleate a vortex-antivortex pair and
switch its polarity. Two different kinds of OOMMF micromagnetic modeling are
used: (i) the original vortex is pinned by the highly--anisotropic easy-axes
impurity at the disk center, (ii) the vortex is pinned by artificially fixing
the magnetization inside the vortex core in the planar vortex distribution. In
both types of simulations a dip creation with a consequent vortex-antivortex
pair nucleation is observed. Polarity switching occurs in the former case only.
Our analytical approach is based on the transformation to the rotating frame of
reference, both in the real space and in the magnetization space, and on the
observation that the physical reason for the dip creation is softening of the
dipole magnon mode due to magnetic field rotation.",0902.4114v1
2009-02-27,Monte Carlo Determination of the Low-Energy Constants of a Spin 1/2 Heisenberg Model with Spatial Anisotropy,"Motivated by the possible mechanism for the pinning of the electronic liquid
crystal direction in YBCO as proposed in \cite{Pardini08}, we use the first
principles Monte Carlo method to study the spin 1/2 Heisenberg model with
antiferromagnetic couplings $J_{1}$ and $J_{2}$ on the square lattice. The
corresponding low-energy constants, namely the spin stiffness $\rho_s$, the
staggered magnetization density ${\cal M}_s$, the spin wave velocity $c$, as
well as the ground state energy density $e_0$ are determined by fitting the
Monte Carlo data to the predictions of magnon chiral perturbation theory. In
particular, the spin stiffnesses $\rho_{s1}$ and $\rho_{s2}$ are investigated
as a function of the ratio $J_{2}/J_{1}$ of the couplings. Although we find a
good agreement between our results with those obtained by the series expansion
method in the weakly anisotropic regime, for strong anisotropy we observe
discrepancies.",0902.4861v3
2009-03-13,"Spin-wave velocities, density of magnetic excitations, and NMR relaxation in ferro-pnictides","We perform an analysis of the experimentally known temperature dependence of
the staggered magnetization in the antiferromagnetic phase. This analysis
allows us to put an upper limit on the unknown value of the spin wave velocity
along the stripes of equal spin direction (spin stripes). The velocity is about
ten times smaller than the velocity perpendicular to the spin stripes. The
strongly anisotropic spin-wave dispersion implies a high density of low energy
magnetic excitations. We demonstrate that this high density strongly enhances
the $^{75}$As NMR spin-lattice relaxation via the Raman scattering of magnons.
We derive the polarization dependence of this relaxation channel and find very
good agreement with experimental data. The high density of low energy magnetic
excitations deduced from our phenomenological analysis supports the scenario
that ferro-pnictides are close to a quantum phase transition.",0903.2313v2
2009-03-17,Microscopic spin-wave theory for yttrium-iron garnet films,"Motivated by recent experiments on thin films of the ferromagnetic insulator
yttrium-iron garnet (YIG), we have developed an efficient microscopic approach
to calculate the spin-wave spectra of these systems. We model the
experimentally relevant magnon band of YIG using an effective quantum
Heisenberg model on a cubic lattice with ferromagnetic nearest neighbor
exchange and long-range dipole-dipole interactions. After a bosonization of the
spin degrees of freedom via a Holstein-Primakoff transformation and a
truncation at quadratic order in the bosons, we obtain the spin-wave spectra
for experimentally relevant parameters without further approximation by
numerical diagonalization, using efficient Ewald summation techniques to carry
out the dipolar sums. We compare our numerical results with two different
analytic approximations and with predictions based on the phenomenological
Landau-Lifshitz equation.",0903.2847v2
2009-03-24,Nematicity as a route to a magnetic field-induced spin density wave order; application to the high temperature cuprates,"The electronic nematic order characterized by broken rotational symmetry has
been suggested to play an important role in the phase diagram of the high
temperature cuprates. We study the interplay between the electronic nematic
order and a spin density wave order in the presence of a magnetic field. We
show that a cooperation of the nematicity and the magnetic field induces a
finite coupling between the spin density wave and spin-triplet staggered flux
orders. As a consequence of such a coupling, the magnon gap decreases as the
magnetic field increases, and it eventually condenses beyond a critical
magnetic field leading to a field-induced spin density wave order. Both
commensurate and incommensurate orders are studied, and the experimental
implications of our findings are discussed.",0903.3993v2
2009-03-30,Universal Holographic Chiral Dynamics in an External Magnetic Field,"In this work we further extend the investigation of holographic gauge
theories in external magnetic fields, continuing earlier work. We study the
phenomenon of magnetic catalysis of mass generation in 1+3 and 1+2 dimensions,
using D3/D7- and D3/D5-brane systems, respectively. We obtain the low energy
effective actions of the corresponding pseudo Goldstone bosons and study their
dispersion relations. The D3/D7 system exhibits the usual
Gell-Mann--Oakes--Renner (GMOR) relation and a relativistic dispersion
relation, while the D3/D5 system exhibits a quadratic non-relativistic
dispersion relation and a modified linear GMOR relation. The low energy
effective action of the D3/D5 system is related to that describing magnon
excitations in a ferromagnet. We also study properties of general Dp/Dq systems
in an external magnetic field and verify the universality of the magnetic
catalysis of dynamical symmetry breaking.",0903.5345v2
2009-04-14,Electron-lattice interaction and its impact on high Tc superconductivity,"In this Colloquium, the main features of the electron-lattice interaction are
discussed and high values of the critical temperature up to room temperature
could be provided. While the issue of the mechanism of superconductivity in the
high Tc cuprates continues to be controversial, one can state that there have
been many experimental results demonstrating that the lattice makes a strong
impact on the pairing of electrons. The polaronic nature of the carriers is
also a manifestation of strong electron-lattice interaction. One can propose an
experiment that allows an unambiguous determination of the intermediate boson
(phonon, magnon, exciton, etc.) which provides the pairing. The
electron-lattice interaction increases for nanosystems, and this is due to an
effective increase in the density of states.",0904.2038v1
2009-04-28,Low-temperature thermal conductivity of BaFe2As2: Parent compound of iron-arsenide superconductors,"We report low-temperature thermal conductivity down to 40 mK of the
antiferromagnet BaFe$_2$As$_{2}$, which is the parent compound of recently
discovered iron-based superconductors. In the investigated temperature range
below 4 K, the thermal conductivity $\kappa$ is well described by the
expression $\kappa$ = $aT$ + $bT^{2.22}$. We attribute the ``$aT$''-term to an
electronic contribution which is found to satisfy the Wiedemann-Franz law in
the $T$ $\to$ 0 K limit, and the remaining thermal conductivity, $\sim$
$T^{2.22}$, is attributed to phonon conductivity. A small influence on thermal
conductivity by magnetic fields up to 8 T is well accounted by the observed
magnetoresistance. The result is consistent with a fully gapped magnon
spectrum, inferred previously from inelastic neutron scattering measurements.",0904.4470v3
2009-05-18,Ferro-Orbital Order and Strong Magnetic Anisotropy in the Parent Compounds of Iron-Pnictide Superconductors,"The puzzling nature of magnetic and lattice phase transitions of iron
pnictides is investigated via a first-principles Wannier function analysis of
representative parent compound LaOFeAs. A rare ferro-orbital ordering is found
to give rise to the recently observed highly anisotropic magnetic coupling, and
drive the phase transitions--without resorting to widely employed frustration
or nesting picture. The revealed necessity of the additional orbital physics
leads to a correlated electronic structure fundamentally distinct from that of
the cuprates. In particular, the strong coupling to the magnons advocates
active roles of light orbitons in spin dynamics and electron pairing in iron
pnictides.",0905.2957v2
2009-05-26,Scattering Theory of Charge-Current Induced Magnetization Dynamics,"In ferromagnets, charge currents can excite magnons via the spin-orbit
coupling. We develop a novel and general scattering theory of charge current
induced macrospin magnetization torques in normal metal$|$ferromagnet$|$normal
metal layers. We apply the formalism to a dirty GaAs$|$(Ga,Mn)As$|$GaAs system.
By computing the charge current induced magnetization torques and solving the
Landau-Lifshitz-Gilbert equation, we find magnetization switching for current
densities as low as $ 5\times 10^{6}$~A/cm$^2$. Our results are in agreement
with a recent experimental observation of charge-current induced magnetization
switching in (Ga,Mn)As.",0905.4170v2
2009-05-28,Resonant Nonlinear Damping of Quantized Spin Waves in Ferromagnetic Nanowires,"We use spin torque ferromagnetic resonance to measure the spectral properties
of dipole-exchange spin waves in permalloy nanowires. Our measurements reveal
that geometric confinement has a profound effect on the damping of spin waves
in the nanowire geometry. The damping parameter of the lowest-energy quantized
spin wave mode depends on applied magnetic field in a resonant way and exhibits
a maximum at a field that increases with decreasing nanowire width. This
enhancement of damping originates from a nonlinear resonant three-magnon
confluence process allowed at a particular bias field value determined by
quantization of the spin wave spectrum in the nanowire geometry.",0905.4699v2
2009-05-29,Ferromagnetic resonance linewidth in ultrathin films with perpendicular magnetic anisotropy,"Transition metal ferromagnetic films with perpendicular magnetic anisotropy
(PMA) have ferromagnetic resonance (FMR) linewidths that are one order of
magnitude larger than soft magnetic materials, such as pure iron (Fe) and
permalloy (NiFe) thin films. A broadband FMR setup has been used to investigate
the origin of the enhanced linewidth in Ni$|$Co multilayer films with PMA. The
FMR linewidth depends linearly on frequency for perpendicular applied fields
and increases significantly when the magnetization is rotated into the film
plane. Irradiation of the film with Helium ions decreases the PMA and the
distribution of PMA parameters. This leads to a great reduction of the FMR
linewidth for in-plane magnetization. These results suggest that fluctuations
in PMA lead to a large two magnon scattering contribution to the linewidth for
in-plane magnetization and establish that the Gilbert damping is enhanced in
such materials ($\alpha \approx 0.04$, compared to $\alpha \approx 0.002$ for
pure Fe).",0905.4779v2
2009-06-17,Model for twin electromagnons and magnetically induced oscillatory polarization in multiferroic RMnO$_3$,"We propose a model for the pair of electromagnon excitations observed in the
class of multiferroic materials {\it R}MnO$_3$ ({\it R} is a rare-earth ion).
The model is based on a harmonic cycloid ground state interacting with a
zone-edge magnon and its twin excitation separated in momentum space by two
times the cycloid wave vector. The pair of electromagnons is activated by cross
coupling between magnetostriction and spin-orbit interactions. Remarkably, the
spectral weight of the twin electromagnon is directly related to the presence
of a magnetically induced oscillatory polarization in the ground state. This
leads to the surprising prediction that TbMnO$_3$ has an oscillatory
polarization with amplitude 50 times larger than its uniform polarization.",0906.3059v2
2009-08-01,Magnetic solitons in frustrated ferromagnetic spin chain,"We study the classical anisotropic ferromagnetic spin chain with frustration.
The behavior of soliton and kink solutions in the vicinity of the ground state
phase transition from the ferromagnetic to the spiral phase is studied. The
dependence of the soliton energy on small anisotropy parameter is established
using scaling estimates and numerical minimization of the energy functional.
Conditions of the existence of the solitons are determined. It is shown that
solitons survive in the spiral phase though with some restrictions on their
size. A comparison of the energies of the classical solitons and the bound
magnon complexes in the quantum model shows the functional similarity between
them. The influence of the finite-size effects on the soliton states is studied
and it is shown that the localized solitons originate from the uniform state
when the system size exceeds some critical value depending on the anisotropy.",0908.0068v1
2009-08-09,Microscopic Model versus Systematic Low-Energy Effective Field Theory for a Doped Quantum Ferromagnet,"We consider a microscopic model for a doped quantum ferromagnet as a test
case for the systematic low-energy effective field theory for magnons and
holes, which is constructed in complete analogy to the case of quantum
antiferromagnets. In contrast to antiferromagnets, for which the effective
field theory approach can be tested only numerically, in the ferromagnetic case
both the microscopic and the effective theory can be solved analytically. In
this way the low-energy parameters of the effective theory are determined
exactly by matching to the underlying microscopic model. The low-energy
behavior at half-filling as well as in the single- and two-hole sectors is
described exactly by the systematic low-energy effective field theory. In
particular, for weakly bound two-hole states the effective field theory even
works beyond perturbation theory. This lends strong support to the quantitative
success of the systematic low-energy effective field theory method not only in
the ferromagnetic but also in the physically most interesting antiferromagnetic
case.",0908.1225v1
2009-08-30,Magnetic-dipolar-mode vortices and microwave subwavelength metamaterials,"There has been a surge of interest in the subwavelength confinement of the
electromagnetic fields. It is well known that in optics the subwavelength
confinement can be obtained due to surface plasmon (quasielectrostatic)
oscillations. In this paper we propose to realize the subwavelength confinement
in microwaves due to dipolar-mode (quasimagnetostatic) magnon oscillations in
ferrite particles. Our studies of interactions between microwave
electromagnetic fields and small ferrite particles with magnetic-dipolar-mode
(MDM) oscillations show strong localization of electromagnetic energy. MDM
oscillations in a ferrite disk are origins of topological singularities
resulting in Poynting-vector vortices and symmetry breakings of the microwave
near fields. We show that new subwavelength microwave metamaterials can be
realized based on a system of interacting MDM ferrite disks. The volume- and
surface-wave propagation of electromagnetic signals in the proposed dense
metamaterials will be characterized by topological phase variations. The
MDM-particle-metamaterial concept opens a significantly new area of research.
In particular, there is a perspective for creation of engineered
electromagnetic fields with unique symmetry properties.",0908.4383v1
2009-09-18,Excitations from a chiral magnetized state of a frustrated quantum spin liquid,"We study excitations in weakly interacting pairs of quantum spin ladders
coupled through geometrically frustrated bonds. The ground state is a
disordered spin liquid, that at high fields is replaced by an ordered chiral
helimagnetic phase. The spectra observed by high-field inelastic neutron
scattering experiments on the prototype compound Sul-Cu2Cl4 are qualitatively
different from those in the previously studied frustration-free spin liquids.
Beyond the critical field Hc=3.7 T, the soft mode that drives the quantum phase
transition spawns two separate excitations: a gapless Goldstone mode and a
massive magnon. Additional massive quasiparticles are clearly visible below Hc,
but are destroyed in the ordered phase. In their place one observes a sharply
bound excitation continuum.",0909.3355v1
2009-11-03,Evidence for Dynamic Phase Separation and High Energy Spin Dynamics in underdoped La$_{2-x}$Sr$_x$CuO$_4$ from Resonant Inelastic X-Ray Scattering,"We probe the collective magnetic modes of La$_2$CuO$_4$ (LCO) and underdoped
La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) by momentum resolved Resonant Inelastic X-ray
Scattering (RIXS) at the Cu $L_3$ edge. In LCO the single magnon dispersion
measured by RIXS coincides with the one determined by inelastic neutron
scattering. For LSCO the spin dynamics shows a branch dispersing up to
$\sim$400 meV coexisting with a branch at lower energy. Only the latter has
been observed with neutrons so far and is considered a key signature of doped
cuprates. The presence of the high-energy branch indicates that LSCO is in a
dynamic inhomogeneous spin state.",0911.0621v1
2009-11-11,Dilute-Bose-Gas Approach to ground state phases of 3D quantum helimagnets under high magnetic field,"We study high-field phase diagram and low-energy excitations of
three-dimensional quantum helimagnets. Slightly below the saturation field, the
emergence of magnetic order may be mathematically viewed as Bose-Einstein
condensation (BEC) of magnons. The method of dilute Bose gas enables an
unbiased quantitative analysis of quantum effects in three-dimensional
helimagnets and thereby three phases are found: cone, coplanar fan and an
attraction-dominant one. To investigate the last phase, we extend the usual BEC
approach so that we can handle 2-magnon bound states. In the case of 2-magnon
BEC, the transverse magnetization vanishes and long-range order occurs in the
quadrupolar channel (spin-nematic phase). As an application, we map out the
phase diagram of a 3D helimagnet which consists of frustrated J1-J2 chains
coupled by an interchain interaction J3.",0911.2186v2
2009-11-13,Infrared optical absorption spectra of CuO single crystals: Fermion-spinon band and dimensional crossover of the antiferromagnetic order,"We have obtained mid-infrared optical absorption spectra of the S=1/2 quasi
one-dimensional CuO using polarized transmission measurement and interpreted
the spectra in terms of phonon assisted magnetic excitations. When the electric
field is parallel to the main antiferromagnetic direction a Delta shaped peak
is observed with the maximum at 0.23eV which is attributed to spinons along
Cu-O chains. At low temperatures in the antiferromagnetic phase another peak
appears at 0.16eV which is attributed to two-magnon absorption but the spinon
peak remains. This behavior is interpreted as due to a dimensional crossover
where the low temperature three-dimensional magnetic phase keeps short range
characteristics of a one-dimensional magnet.",0911.2573v1
2009-11-27,Multiple vortex-antivortex pair generation in magnetic nanodots,"The interaction of a magnetic vortex with a rotating magnetic field causes
the nucleation of a vortex--antivortex pair leading to a vortex polarity
switching. The key point of this process is the creation of a dip, which can be
interpreted as a nonlinear resonance in the system of certain magnon modes with
nonlinear coupling. The usually observed single-dip structure is a particular
case of a multidip structure. The dynamics of the structure with $n$ dips is
described as the dynamics of nonlinearly coupled modes with azimuthal numbers
$m=0,\pm n,\pm 2n$. The multidip structure with arbitrary number of vortex
antivortex pairs can be obtained in vortex-state nanodisk using a space- and
time-varying magnetic field. A scheme of a possible experimental setup for
multidip structure generation is proposed.",0911.5353v2
2010-01-03,Spin flexoelectricity in multiferroics,"Various phenomena related to inhomogeneous magnetoelectric interaction are
considered. The interrelation between spatial modulation of order parameter and
electric polarization, known as flexoelectric effect in liquid crystals, in the
case of magnetic media appears in a form of electric polarization induced by
spin modulation and vice versa. This flexomagnetoelectric interaction is also
related to the influence of ferroelectric domain structure on antiferromagnetic
vector distribution, and to the magnetoelectric properties of micromagnetic
structures. The influence of inhomogeneous magnetoelectric interaction on
dynamic properties of multiferroics, particularly magnon spectra is also
considered.",1001.0254v2
2010-01-19,Surface and bulk polaritons in a linear magnetoelectric multiferroic with canted spins: Transverse Electric polarisation,"Some magnetoelectric multiferroics have a canted spin structure that can be
described by a Dzyaloshinkii-Moriya coupling. We calculate properties and
features expected for surface and bulk magnon polaritons in such media with a
linear magnetoelectric interaction for the case of transverse electric
polarisation. The dielectric polarisation and magnetisation of weak
ferromagnetism are constrained to lie in the plane parallel to the surface. We
examine a geometry with the polarisation oriented in the film plane and present
numerical results for the transverse electric polarisation. Particular
attention is given to non-reciprocal surface modes, which exist in frequency
between two bulk bands, and show how these modes can be modified by external
magnetic field. Results for attenuated total reflection are presented, and
discussed in relation to nonreciprocity. Example results are calculated for the
canted antiferromagnet BaMnF4.",1001.3197v1
2010-02-08,A Triplet Resonance in Superconducting FeSe0.4Te0.6,"From heavy fermions to cuprates and iron pnictides, a spin resonance is a
staple of nearly magnetic superconductors. Possible explanations include a
two-particle bound state or loss of magnon damping in the superconductor. While
both scenarios suggest a central role for magnetic fluctuations, distinguishing
them is important to identify the right theoretical framework and to guide the
search for higher temperature superconductors. Using inelastic neutron
scattering technique, we show that the spin resonance in optimally doped
Fe(Se,Te) splits into three peaks in a high magnetic field, a signature of a
two-particle triplet bound state",1002.1617v2
2010-02-26,Correlation Effects in the Stochastic Landau-Lifshitz-Gilbert Equation,"We analyze the Landau-Lifshitz-Gilbert equation when the precession motion of
the magnetic moments is additionally subjected to an uniaxial anisotropy and is
driven by a multiplicative coupled stochastic field with a finite correlation
time $\tau$. The mean value for the spin wave components offers that the
spin-wave dispersion relation and its damping is strongly influenced by the
deterministic Gilbert damping parameter $\alpha$, the strength of the
stochastic forces $D$ and its temporal range $\tau$. The spin-spin-correlation
function can be calculated in the low correlation time limit by deriving an
evolution equation for the joint probability function. The stability analysis
enables us to find the phase diagram within the $\alpha-D$ plane for different
values of $\tau$ where damped spin wave solutions are stable. Even for zero
deterministic Gilbert damping the magnons offer a finite lifetime. We detect a
parameter range where the deterministic and the stochastic damping mechanism
are able to compensate each other leading to undamped spin-waves. The onset is
characterized by a critical value of the correlation time. An enhancement of
$\tau$ leads to an increase of the oscillations of the correlation function.",1002.4958v1
2010-03-03,Spin Oscillations in Antiferromagnetic NiO Triggered by Circularly Polarized Light,"Coherent spin oscillations were non-thermally induced by circularly polarized
pulses in fully compensated antiferromagnetic NiO. This effect is attributed to
an entirely new mechanism of the action, on the spins, of the effective
magnetic field generated by an inverse Faraday effect. The novelty of this
mechanism is that spin oscillations are driven by the time derivative of the
effective magnetic field acting even on ""pure"" antiferromagnets with zero net
magnetic moment in the ground state. The measured frequencies (1.07 THz and 140
GHz) of the spin oscillations correspond to the out-of-plane and in-plane modes
of antiferromagnetic magnons.",1003.0820v1
2010-03-13,"Magnetic, thermal and transport properties of Cd doped CeIn$_3$","We have investigated the effect of Cd substitution on the archetypal heavy
fermion antiferromagnet CeIn$_3$ via magnetic susceptibility, specific heat and
resistivity measurements. The suppression of the Neel temperature, T$_{N}$,
with Cd doping is more pronounced than with Sn. Nevertheless, a doping induced
quantum critical point does not appear to be achievable in this system. The
magnetic entropy at $T_N$ and the temperature of the maximum in resistivity are
also systematically suppressed with Cd, while the effective moment and the
Curie-Weiss temperature in the paramagnetic state are not affected. These
results suggest that Cd locally disrupts the AFM order on its neighboring Ce
moments, without affecting the valence of Ce. Moreover, the temperature
dependence of the specific heat below $T_N$ is not consistent with 3D magnons
in pure as well as in Cd-doped CeIn$_3$, a point that has been missed in
previous investigations of CeIn$_3$ and that has bearing on the type of quantum
criticality in this system.",1003.2723v1
2010-03-31,Magnonic Crystal with Two-Dimensional Periodicity as a Waveguide for Spin Waves,"We describe a simple method of including dissipation in the spin wave band
structure of a periodic ferromagnetic composite, by solving the Landau-Lifshitz
equation for the magnetization with the Gilbert damping term. We use this
approach to calculate the band structure of square and triangular arrays of Ni
nanocylinders embedded in an Fe host. The results show that there are certain
bands and special directions in the Brillouin zone where the spin wave lifetime
is increased by more than an order of magnitude above its average value. Thus,
it may be possible to generate spin waves in such composites decay especially
slowly, and propagate especially large distances, for certain frequencies and
directions in ${\bf k}$-space.",1003.6092v1
2010-03-31,Probing Non-Toric Geometry with Rotating Membranes,"Recently Martelli and Sparks presented the first non-toric AdS_4/CFT_3
duality relation between M-theory on AdS_4 x V_{5,2}/Z_k and a class of
three-dimensional N=2 quiver Chern-Simons-matter theories. V_{5,2} is a
seven-dimensional homogeneneous Sasaki-Einstein manifold with isometry group
SO(5)xU(1)_R, which is in general broken to SU(2)xU(1)xU(1)_R by the orbifold
projection if k>1. The dual field theory is described by the A_1 quiver, U(N)_k
x U(N)_{-k} gauge group, four bifundamentals, two adjoint chiral multiplets
interacting via a cubic superpotential. We explore this proposal by studying
various classical membrane solutions moving in V_{5,2}. Rotating membrane
solutions of folded, wrapped, spike, and giant magnon types are presented with
their dispersion relations. We also discuss their dual operators in the
Chern-Simons-matter theory.",1003.6111v1
2010-04-14,Heat capacity uncovers physics of a frustrated spin tube,"We report on refined experimental results concerning the low-temperature
specific heat of the frustrated spin tube material [(CuCl2tachH)3Cl]Cl2. This
substance turns out to be an unusually perfect spin tube system which allows to
study the physics of quasi-one dimensional antiferromagnetic structures in
rather general terms. An analysis of the specific heat data demonstrates that
at low enough temperatures the system exhibits a Tomonaga-Luttinger liquid
behavior corresponding to an effective spin-3/2 antiferromagnetic Heisenberg
chain with short-range exchange interactions. On the other hand, at somewhat
elevated temperatures the composite spin structure of the chain is revealed
through a Schottky-type peak in the specific heat located around 2 K. We argue
that the dominating contribution to the peak originates from gapped magnon-type
excitations related to the internal degrees of freedom of the rung spins.",1004.2373v1
2010-04-21,Thermal Transport Properties of Bechgaard Salts (TMTSF)2PF6 and (TMTSF)2ClO4: Implication of Spin-Charge Separation,"We report thermal transport measurements performed on the
quasi-one-dimensional Bechgaard salts (TMTSF)2ClO4 and (TMTSF)2PF6 along the
a-direction. For both salts, magnon-drag effects are found to contribute
considerably to thermopower above 80 K. These results imply spin-charge
separation in the metallic state for both salts. Moreover, a linear
temperature-dependent thermal conductivity is found to be unaffected by anion
disorder below an anion ordering transition temperature TAO~25 K in
(TMTSF)2ClO4.",1004.3614v1
2010-06-11,Quantum Strings and the AdS4/CFT3 Interpolating Function,"The existence of a nontrivial interpolating function h(\lambda) is one of the
novel features of the new AdS4/CFT3 correspondence involving ABJM theory. At
strong coupling, most of the investigation of semiclassical effects so far has
been for strings in the AdS4 sector. Several cutoff prescriptions have been
proposed, leading to different predictions for the constant term in the
expansion h(\lambda)=\sqrt{\lambda/2} + c + ... . We calculate quantum
corrections for giant magnons, using the algebraic curve, and show by comparing
to the dispersion relation that the same prescriptions lead to the same values
of c in this CP3 sector. We then turn to finite-J effects, where a comparison
with the Luescher F-term correction shows a mismatch for one of the three sum
prescriptions. We also compute some dyonic and higher F-terms for future
comparisons.",1006.2174v3
2010-06-21,Vortex core contribution to textural energy in 3He-B below 0.4Tc,"Vortex lines affect the spatial order-parameter distribution in superfluid
3He-B owing to superflow circulating around vortex cores and due to the
interaction of the order parameter in the core and in the bulk as a result of
superfluid coherence over the whole volume. The step-like change of the latter
contribution at 0.6Tc (at a pressure of 29bar) signifies the transition from
axisymmetric cores at higher temperatures to broken-symmetry cores at lower
temperatures. We extended earlier measurements of the core contribution to
temperatures below 0.2Tc, in particular searching for a possible new core
transition to lower symmetries. As a measuring tool we track the energy levels
of magnon condensate states in a trap formed by the order-parameter texture.",1006.4108v1
2010-06-26,Terahertz Hall Measurements On Optimally Doped Single Crystal Bi-2212,"The infrared Hall angle in optimally doped single crystal $\rm Bi_2 Sr_2 Ca
Cu_2 O_{8+x}$ was measured from 3.05 to 21.75 meV as a continuous function of
temperature from 25 to 300\,K. In the normal state, the temperature dependence
of the real part of the cotangent of the infrared Hall angle obeys the same
power law as dc measurements. The measured Hall frequency $\rm \omega_H$ is
significantly larger than the expected value based upon ARPES data analyzed in
terms of the relaxation time approximation. This discrepancy as well as the
temperature dependence of $\rm Re(\cot{\theta_H})$ and $\omega_H$ is well
described by a Fermi liquid theory in which current vertex corrections produced
by electron-magnon scattering are included.",1006.5106v1
2010-07-12,Ferromagnetic Excitations in La$_{0.82}$Sr$_{0.18}$CoO$_{3}$ Observed Using Neutron Inelastic Scattering,"Polarized neutron inelastic scattering has been used to measure spin
excitations in ferromagnetic La$_{0.82}$Sr$_{0.18}$CoO$_{3}$. The magnon
spectrum of these spin excitations is well defined at low energies but becomes
heavily damped at higher energies, and can be modeled using a quadratic
dispersion. We determined a spin wave stiffness constant of $D=94\pm
3$\,meV\,\AA$^{2}$. Assuming a nearest-neighbor Heisenberg model we find
reasonable agreement between the exchange determined from D and the bulk Curie
temperature. Several possible mechanisms to account for the observed spin-wave
damping are discussed.",1007.1919v1
2010-07-30,Magnetic excitations in multiferroic LuMnO3 studied by inelastic neutron scattering,"We present data on the magnetic and magneto-elastic coupling in the hexagonal
multiferroic manganite LuMnO3 from inelastic neutron scattering, magnetization
and thermal expansion measurements. We measured the magnon dispersion along the
main symmetry directions and used this data to determine the principal exchange
parameters from a spin-wave model. An analysis of the magnetic anisotropy in
terms of the crystal field acting on the Mn is presented. We compare the
results for LuMnO3 with data on other hexagonal RMnO3 compounds.",1007.5490v2
2010-09-17,Dynamics of coupled vortices in a pair of ferromagnetic disks,"We here experimentally demonstrate that coupled gyration modes can be
resonantly excited primarily by the ac current in a pair of ferromagnetic disks
with varied separating distance. The sole gyrotropic mode clearly splits into a
higher and a lower frequency modes for different configurations of polarities
via dipolar interaction. These experimental results indicate that the
magnetostatically coupled pair of vortices behaves similar to a diatomic
molecule with bonding and anti-bonding states. These findings lead to the
possible extension of designing the magnonic band structure in a chain or an
array of vortices.",1009.3363v3
2010-09-28,The magnonic limit of domain wall propagation in ferromagnetic nanotubes,"We report a study on the field-driven propagation of vortex-like domain walls
in ferromagnetic nanotubes. This particular geometry gives rise to a special
feature of the static wall configuration, which significantly influences its
dynamics. Unlike domain walls in flat strips, the left-right symmetry of domain
wall propagation is broken. Furthermore, the domain wall velocity is not
limited by the Walker breakdown. Under sufficiently large magnetic fields, the
domain wall velocity reaches the velocity of spin waves (about 1000 m/s) and is
thereafter connected with a direct emission of spin waves. The moving domain
wall maintains its main structure but has characteristic spin-wave tails
attached. The spatial profile of this topological soliton is determined by the
spin-wave dispersion.",1009.5552v1
2010-09-30,Magnetic Phase Diagram of Alternating Chain Compound Pb2V3O9,"Pb2V3O9 was found to have antiferromagnetic alternating chains of S=1/2 and
show the antiferromagnetic long range ordered (AFLRO) state under the magnetic
field between 4 and 38 T. We succeeded in growing single crystals of Pb2V3O9 by
floating zone method and determined the asymmetric H-T phase diagram of Pb2V3O9
from measurements of the magnetization and the heat capacity. Previous reports
focused on the AFLRO state in the low magnetic field region and that in high
magnetic field region was not discussed so much. In this paper, we examined the
AFLRO state in terms of the mean field theory and the picture of Bose-Einstein
condensation of magnons to discuss the whole phase diagram. Consequently, the
asymmetry of the phase boundary in Pb2V3O9 is explained by the mixing
contribution of excited states and by interchain interactions.",1009.6026v1
2010-10-08,Superspace calculation of the four-loop spectrum in N=6 supersymmetric Chern-Simons theories,"Using N=2 superspace techniques we compute the four-loop spectrum of single
trace operators in the SU(2) x SU(2) sector of ABJM and ABJ supersymmetric
Chern-Simons theories. Our computation yields a four-loop contribution to the
function h^2(\lambda) (and its ABJ generalization) in the magnon dispersion
relation which has fixed maximum transcendentality and coincides with the
findings in components given in the revised versions of arXiv:0908.2463 and
arXiv:0912.3460. We also discuss possible scenarios for an all-loop function
h^2(\lambda) that interpolates between weak and strong couplings.",1010.1756v2
2010-10-12,Revealing the degree of magnetic frustration by non-magnetic impurities,"Imaging the magnetic fields around a non-magnetic impurity can provide a
clear benchmark for quantifying the degree of magnetic frustration. Focusing on
the strongly frustrated $J_1$-$J_2$ model and the spatially anisotropic
$J_{1a}$-$J_{1b}$-$J_2$ model, very distinct low energy behaviors reflect
different levels of magnetic frustration. In the $J_1$-$J_2$ model, bound
magnons appear trapped near the impurity in the ground state and strongly
reduce the ordered moments for sites proximal to the impurity. In contrast,
local moments in the $J_{1a}$-$J_{1b}$-$J_2$ model are enhanced on the impurity
neighboring sites. These theoretical predictions can be probed by experiments
such as nuclear magnetic resonance and scanning tunneling microscopy, and the
results can elucidate the role of frustration in antiferromagnets and help
narrow the possible models to understand magnetism in the iron pnictdies.",1010.2917v2
2010-10-18,"Fermionic magnons, non-Abelian spinons, and spin quantum Hall effect from an exactly solvable Kitaev Hamiltonian with SU(2) symmetry","We introduce an exactly solvable SU(2) symmetric Kitaev model with exotic
spin excitations. With time reversal symmetry (TRS), the ground state is a spin
liquid with gapless or gapped spin-1 but fermionic excitations. When TRS is
broken, the resulting spin liquid exhibits deconfined vortex excitations which
carry spin-1/2 quantum number and obey non-Abelian statistics. We show that
this SU(2) invariant non-Abelian spin liquid exhibits spin quantum Hall effect
with quantized spin Hall conductivity \sigma^s_{xy}=\hbar/2\pi, and that the
spin response is effectively described the SO(3) level-1 Chern-Simons theory at
low energy. We further propose that a SU(2) level-2 Chern-Simons theory is the
effective field theory describing the topological structure of the non-Abelian
SU(2) invariant spin liquid.",1010.3724v2
2010-10-22,"Single-ion anisotropy, Dzyaloshinskii-Moriya interaction and negative magnetoresistance of the spin-1/2 pyrochlores R2V2O7","The electronic and magnetic properties of spin-1/2 pyrochlores R2V2O7 were
investigated on the basis of density-functional calculations. Contrary to the
common belief, the spin-1/2 V4+ ions are found to have a substantial easy-axis
single-ion anisotropy. The |D/J| ratio deduced from the magnon quantum Hall
effect of Lu2V2O7, where J is the nearest-neighbor spin exchange and D is the
Dzyaloshinskii-Moriya parameter, is much greater than the value estimated from
our calculations (i.e., 0.32 vs. 0.05). We show that this discrepancy is due to
the neglect of the single-ion anisotropy of the V4+ ions, and the negative
magnetoresistance observed for R2V2O7 arises from a new mechanism.",1010.4606v1
2010-10-29,Field-Induced Gap in a Quantum Spin-1/2 Chain in a Strong Magnetic Field,"Magnetic excitations in copper pyrimidine dinitrate, a spin-1/2
antiferromagnetic chain with alternating $g$-tensor and Dzyaloshinskii-Moriya
interactions that exhibits a field-induced spin gap, are probed by means of
pulsed-field electron spin resonance spectroscopy. In particular, we report on
a minimum of the gap in the vicinity of the saturation field $H_{sat}=48.5$ T
associated with a transition from the sine-Gordon region (with soliton-breather
elementary excitations) to a spin-polarized state (with magnon excitations).
This interpretation is fully confirmed by the quantitative agreement over the
entire field range of the experimental data with the DMRG investigation of the
spin-1/2 Heisenberg chain with a staggered transverse field.",1010.6141v1
2010-11-06,Effect of anisotropy on the field induced quantum critical properties of the three dimensional s=1/2 Heisenberg model,"The field induced quantum critical properties of the three dimensional
spin-1/2 anisotropic antiferromagnetic Heisenberg model has been studied. We
have investigated the quantum phase transition between the spiral order and
field induced ferromagnetic order by means of Bose-Einstein condensation of
magnons in terms of a bosonic representation. The effect of in-plane anisotropy
on the critical properties has been studied via the bosonic model by Green's
function approach. We have found an analytic expression for the gap exponent in
addition to numerical results for the critical magnetic field in terms of
anisotropy parameter. The in-plane anisotropy breaks the U(1) symmetry
explicitly which changes the universal behavior by a drastic change on the gap
exponent. Moreover, the critical magnetic field depends strongly on the
in-plane anisotropies. The divergence of the transverse structure factor at the
antiferromagnetic wave vector confirms the onset of the magnetic order which
scales with the negative value of gap exponent as the magnetic field approaches
the critical one. The transverse staggered magnetization as an order parameter
vanishes with exponent $\beta=0.5$ when the magnetic field reaches its critical
value in low field region.",1011.1574v1
2010-11-14,Scattering universality classes of side jump in anomalous Hall effect,"The anomalous Hall conductivity has an important extrinsic contribution known
as side jump contribution, which is independent of both scattering strength and
disorder density. Nevertheless, we discover that side jump has strong
dependence on the spin structure of the scattering potential. We propose three
universality classes of scattering for the side jump contribution, having the
characters of being spin-independent, spin-conserving and spin-flip
respectively. For each individual class, the side jump contribution takes a
different unique value. When two or more classes of scattering are present, the
value of side jump is no longer fixed but varies as a function of their
relative disorder strength. As system control parameter such as temperature
changes, due to the competition between different classes of disorder
scattering, the side jump Hall conductivity could flow from one class dominated
limit to another class dominated limit. Our result indicates that magnon
scattering plays a role distinct from normal impurity scattering and phonon
scattering in the anomalous Hall effect because they belong to different
scattering classes.",1011.3239v1
2010-11-26,Modes of magnetic resonance of S=1 dimer chain compound NTENP,"The spin dynamics of a quasi one dimensional $S=1$ bond alternating spin-gap
antiferromagnet Ni(C$_9$H$_{24}$N$_4$)NO$_2$(ClO$_4$) (abbreviated as NTENP) is
studied by means of electron spin resonance (ESR) technique. Five modes of ESR
transitions are observed and identified: transitions between singlet ground
state and excited triplet states, three modes of transitions between spin
sublevels of collective triplet states and antiferromagnetic resonance
absorption in the field-induced antiferromagnetically ordered phase.
Singlet-triplet and intra-triplet modes demonstrate a doublet structure which
is due to two maxima in the density of magnon states in the low-frequency
range. A joint analysis of the observed spectra and other experimental results
allows to test the applicability of the fermionic and bosonic models. We
conclude that the fermionic approach is more appropriate for the particular
case of NTENP.",1011.5742v1
2010-12-01,Persistence of magnons in a site-diluted dimerized frustrated antiferromagnet,"We present inelastic neutron scattering and thermodynamic measurements
characterizing the magnetic excitations in a disordered non-magnetic
substituted spin-liquid antiferromagnet. The parent compound Ba3Mn2O8 is a
dimerized, quasi-two-dimensional geometrically frustrated quantum disordered
antiferromagnet. We substitute this compound with non-magnetic vanadium for the
S = 1 manganese atoms, Ba3(Mn1-xVx)2O8, and find that the singlet-triplet
excitations which dominate the spectrum of the parent compound persist for the
full range of substitution examined, x = 0.02 to 0.3. We also observe
additional low-energy magnetic fluctuations which are enhanced at the greatest
substitution values. These excitations may be a precursor to a low-temperature
random singlet phase which may exist in Ba3(Mn1-xVx)2O8",1012.0099v1
2010-12-17,"Review of AdS/CFT Integrability, Chapter IV.3: N=6 Chern-Simons and Strings on AdS4xCP3","We review the duality and integrability of N=6 superconformal Chern-Simons
theory in three dimensions and IIA superstring theory on the background
AdS4xCP3. We introduce both of these models and describe how their degrees of
freedom are mapped to excitations of a long-range integrable spin-chain.
Finally, we discuss the properties of the Bethe equations, the S-matrix and the
algebraic curve that are special to this correspondence and differ from the
case of N=4 SYM theory and strings on AdS5xS5.",1012.3999v5
2010-12-21,Phase diagram and continuous pair-unbinding transition of the bilinear-biquadratic S=1 Heisenberg chain in a magnetic field,"We investigate the properties of the Heisenberg S=1 chain with bilinear and
biquadratic interactions in a magnetic field using the Density Matrix
Renormalization Group, Bethe ansatz and field theoretical considerations. In a
large region of the parameter space, we identify a magnetized ferroquadrupolar
Luttinger liquid consisting of a quasi-condensate of bound magnon pairs. This
liquid undergoes a continuous pair unbinding transition to a more conventional
Luttinger liquid region obtained by polarizing the system above the Haldane gap
region. This pair unbinding transition is shown to be in the Ising universality
class on top of a Luttinger liquid, leading to an effective central charge 3/2.
We also revisit the nature of the partially polarized Luttinger liquid around
and above the Uimin-Lai-Sutherland point. Our results confirm that this is a
two-component liquid and rule out the formation of a single-component vector
chiral phase.",1012.4518v2
2011-01-06,Quantum spin ladders of non-Abelian anyons,"Quantum ladder models, consisting of coupled chains, form intriguing systems
bridging one and two dimensions and have been well studied in the context of
quantum magnets and fermionic systems. Here we consider ladder systems made of
more exotic quantum mechanical degrees of freedom, so-called non-Abelian
anyons, which can be thought of as certain quantum deformations of ordinary
SU(2) spins. Such non-Abelian anyons occur as quasiparticle excitations in
topological quantum fluids, including p_x + i p_y superconductors, certain
fractional quantum Hall states, and rotating Bose-Einstein condensates. Here we
use a combination of exact diagonalization and conformal field theory to
determine the phase diagrams of ladders with up to four chains. We discuss how
phenomena familiar from ordinary SU(2) spin ladders are generalized in their
anyonic counterparts, such as gapless and gapped phases, odd/even effects with
the ladder width, and elementary `magnon' excitations. Other features are
entirely due to the topological nature of the anyonic degrees of freedom.",1101.1186v2
2011-05-03,Classical integrability in the BTZ black hole,"Using the fact the BTZ black hole is a quotient of AdS_3 we show that
classical string propagation in the BTZ background is integrable. We construct
the flat connection and its monodromy matrix which generates the non-local
charges. From examining the general behaviour of the eigen values of the
monodromy matrix we determine the set of integral equations which constrain
them. These equations imply that each classical solution is characterized by a
density function in the complex plane. For classical solutions which correspond
to geodesics and winding strings we solve for the eigen values of the monodromy
matrix explicitly and show that geodesics correspond to zero density in the
complex plane. We solve the integral equations for BMN and magnon like
solutions and obtain their dispersion relation. Finally we show that the set of
integral equations which constrain the eigen values of the monodromy matrix can
be identified with the continuum limit of the Bethe equations of a twisted
SL(2, R) spin chain at one loop.",1105.0480v2
2011-05-10,Longitudinal excitations in quantum antiferromagnets,"By extending our recently proposed magnon-density-waves to low dimensions, we
investigate, using a microscopic many-body approach, the longitudinal
excitations of the quasi-one-dimensional (quasi-1d) and quasi-2d Heisenberg
antiferromagnetic systems on a bipartite lattice with a general spin quantum
number. We obtain the full energy spectrum of the longitudinal mode as a
function of the coupling constants in the original lattice Hamiltonian and find
that it always has a non-zero energy gap if the ground state has a long-range
order and becomes gapless for the pure isotropic 1d model. The numerical value
of the minimum gap in our approximation agrees with that of a longitudinal mode
observed in the quasi-1d antiferromagnetic compound KCuF${}_3$ at low
temperature. It will be interesting to compare values of the energy spectrum at
other momenta if their experimental results are available.",1105.1959v3
2011-05-11,Theory of Raman Scattering in One-Dimensional Quantum Spin-1/2 Antiferromagnets,"We study theoretically the Raman scattering spectra in the one-dimensional
(1D) quantum spin-1/2 antiferromagnets. The analysis reveals that their
low-energy dynamics is exquisitely sensitive to various perturbations to the
Heisenberg chain with nearest-neighbor exchange interactions, such as magnetic
anisotropy, longer-range exchange interactions, and bond dimerization. These
weak interactions are mainly responsible for the Raman scattering and give rise
to different types of spectra as functions of frequency, temperature, and
external field. In contrast to the Raman spectra in higher dimensions in which
the two-magnon process is dominant, those in 1D antiferromagnets provide much
richer information on these perturbations.",1105.2259v2
2011-05-12,Coupled spin-lattice fluctuations in a compound with orbital degrees of freedom: the Cr based dimer system Sr3Cr2O8,"We report on an extended fluctuation regime in the spin dimer system Sr3Cr2O8
based on anomalies in Raman active phonons and magnetic scattering. The
compound has two characteristic temperatures, TS = 275 K, related to a
Jahn-Teller transition with structural distortions and orbital ordering and a
second, T*= 150 K, which is due to further changes in the orbital sector. Below
TS quasielastic scattering marks strong fluctuations and in addition phonon
anomalies are observed. For temperatures below T* we observe an exponential
decrease of one phonon linewidth and determine a gap of the orbital
excitations. At low temperatures the observation of two- and three-magnon
scattering allows the determination of the spin excitation gap.",1105.2369v1
2011-05-17,From N=4 gauge theory to N=2 conformal QCD: three-loop mixing of scalar composite operators,"We derive the planar dilatation operator in the closed subsector of scalar
composite operators of an N=2 superconformal quiver gauge theory to three
loops. By tuning the ratio of its two gauge couplings we interpolate between a
Z_2 orbifold of N=4 SYM theory and N=2 superconformal QCD. We find zeta(3)
contributions at three loops that disappear when the theory is at the orbifold
point. They are responsible for imaginary contributions to the dispersion
relation of a single scalar excitation in the spin-chain picture. This points
towards an interpretation of the individual scalar excitations as effective
rather than as elementary magnons. We argue that the elementary excitations
should be associated with certain fermions and covariant derivatives, and that
integrability in the respective subsectors should persist at least to two
loops.",1105.3487v1
2011-05-18,Integrable achiral D5-brane reflections and asymptotic Bethe equations,"We study the reflection of magnons from a D5-brane in the framework of the
AdS/CFT correspondence. We consider two possible orientations of the D5-brane
with respect to the reference vacuum state, namely vacuum states aligned along
""vertical"" and ""horizontal"" directions. We show that the reflections are of the
achiral type. We also show that the reflection matrices satisfy the boundary
Yang-Baxter equations for both orientations. In the horizontal case the
reflection matrix can be interpreted in terms of a bulk S-matrix, S(p, -p), and
factorizability of boundary scattering therefore follows from that of bulk
scattering. Finally, we solve the nested coordinate Bethe ansatz for the system
in the vertical case to find the Bethe equations. In the horizontal case, the
Bethe equations are of the same form as those for the closed string.",1105.3707v2
2011-05-23,Scattering of Giant Holes,"We study scalar excitations of high spin operators in N=4 super Yang-Mills
theory, which are dual to solitons propagating on a long folded string in AdS_3
x S^1. In the spin chain description of the gauge theory, these are associated
to holes in the magnon distribution in the sl(2,R) sector. We compute the
all-loop hole S-matrix from the asymptotic Bethe ansatz, and expand in leading
orders at weak and strong coupling. The worldsheet S-matrix of solitonic
excitations on the GKP string is calculated using semiclassical quantization.
We find an exact agreement between the gauge theory and string theory results.",1105.4596v2
2011-06-19,Oscillatory Energy Exchange Between Waves Coupled by a Dynamic Artificial Crystal,"We describe a general mechanism of controllable energy exchange between waves
propagating in a dynamic artificial crystal. We show that if a spatial
periodicity is temporarily imposed on the transmission properties of a
wave-carrying medium whilst a wave is inside, this wave is coupled to a
secondary counter-propagating wave and energy oscillates between the two. The
oscillation frequency is determined by the width of the spectral band gap
created by the periodicity and the frequency difference between the coupled
waves. The effect is demonstrated with spin waves in a dynamic magnonic
crystal.",1106.3722v1
2011-07-04,Magnetic field induced dehybridization of the electromagnons in multiferroic TbMnO3,"We have studied the impact of the magnetic field on the electromagnon
excitations in TbMnO3 crystal. Applying magnetic field along the c axis, we
show that the electromagnons transform into pure antiferromagnetic modes,
losing their polar character. Entering in the paraelectric phase, we are able
to track the spectral weight transfer from the electromagnons to the magnon
excitations and we discuss the magnetic excitations underlying the
electromagnons. We also point out the phonons involved in the phase transition
process. This reveals that the Mn-O distance plays a key role in understanding
the ferroelectricity and the polar character of the electromagnons. Magnetic
field measurements along the b axis allow us to detect a new electromagnon
resonance in agreement with a Heisenberg model.",1107.0593v1
2011-07-29,"Spin-wave excitations in the ferromagnetic-metallic and in the charge, orbital and spin ordered states in Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ with x$\approx$0.5","Inelastic neutron scattering experiments have been performed on single
crystals of Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ with x$\approx$0.5. Colossal
magnetoresistance (CMR) in the manganites arises from the interplay between a
ferromagnetic metallic and antiferromagnetic charge and orbital ordered
insulating state. Therefore, it appears important to compare these phases
concerning their underlying magnetic interaction parameters. Our investigations
of the spin-wave disperion in the AFM ordered state of
Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ exhibits a strongly anisotropic stiffness. The
sign of the anisotropy is characteristic for the site-centered model for charge
and orbital ordering in half-doped manganites. Within this model, linear
spin-wave theory yields a perfect description of the experimental dispersion.
Furthermore, magnetic excitations in the ferromagnetic metallic state of
Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ with x=0.49 and x=0.50 exhibit nearly the same
magnon dispersion which can be described with a Heisenberg model including
nearest-neighbor interactions.",1107.5882v1
2011-08-07,Supergraphs and the cubic Leigh-Strassler model,"We discuss supergraphs and their relation to ""chiral functions"" in N=4 Super
Yang-Mills. Based on the magnon dispersion relation and an explicit three-loop
result of Sieg's we make an all loop conjecture for the rational contributions
of certain classes of supergraphs. We then apply superspace techniques to the
""cubic"" branch of Leigh-Strassler N=1 superconformal theories. We show that
there are order 2^L/L single trace operators of length L which have zero
anomalous dimensions to all loop order in the planar limit. We then compute the
anomalous dimensions for another class of single trace operators we call
one-pair states. Using the conjecture we can find a simple expression for the
rational part of the anomalous dimension which we argue is valid at least up to
and including five-loop order. Based on an explicit computation we can compute
the anomalous dimension for these operators to four loops.",1108.1583v2
2011-08-22,Acoustic spin pumping: Direct generation of spin currents from sound waves in Pt/Y3Fe5O12 hybrid structures,"Using a Pt/Y3Fe5O12 (YIG) hybrid structure attached to a piezoelectric
actuator, we demonstrate the generation of spin currents from sound waves. This
""acoustic spin pumping"" (ASP) is caused by the sound wave generated by the
piezoelectric actuator, which then modulates the distribution function of
magnons in the YIG layer and results in a pure-spin-current injection into the
Pt layer across the Pt/YIG interface. In the Pt layer, this injected spin
current is converted into an electric voltage due to the inverse spin-Hall
effect (ISHE). The ISHE voltage induced by the ASP is detected by measuring
voltage in the Pt layer at the piezoelectric resonance frequency of the
actuator coupled with the Pt/YIG system. The frequency-dependent measurements
enable us to separate the ASP-induced signals from extrinsic heating effects.
Our model calculation based on the linear response theory provides us with a
qualitative and quantitative understanding of the ASP in the Pt/YIG system.",1108.4238v1
2011-08-27,Localized states on triangular traps and low-temperature properties of the antiferromagnetic Heisenberg and repulsive Hubbard models,"We consider the antiferromagnetic Heisenberg and the repulsive Hubbard model
on two $N$-site one-dimensional lattices, which support dispersionless
one-particle states corresponding to localized states on triangular trapping
cells. We calculate the degeneracy of the ground states in the subspaces with
$n\le n_{\max}$, $n_{\max}\propto N$ magnons or electrons as well as the
contribution of these states (independent localized states) to thermodynamic
quantities. Moreover, we discuss another class of low-lying eigenstates
(so-called interacting localized states) and calculate their contribution to
the partition function. We also discuss the effect of extra interactions, which
lift the degeneracy present due to the chirality of the localized states on
triangles. The localized states set an extra low-energy scale in the system and
lead to a nonzero residual ground-state entropy and to one (or more) additional
low-temperature peak(s) in the specific heat. Low-energy degrees of freedom in
the presence of perturbations removing degeneracy owing to the chirality can be
described in terms of an effective (pseudo)spin-1/2 transverse $XX$ chain.",1108.5453v1
2011-08-30,Quantum criticality of dipolar spin chains,"We show that a chain of Heisenberg spins interacting with long-range dipolar
forces in a magnetic field h perpendicular to the chain exhibits a quantum
critical point belonging to the two-dimensional Ising universality class.
Within linear spin-wave theory the magnon dispersion for small momenta k is
[Delta^2 + v_k^2 k^2]^{1/2}, where Delta^2 \propto |h - h_c| and v_k^2 \propto
|ln k|. For fields close to h_c linear spin-wave theory breaks down and we
investigate the system using density-matrix and functional renormalization
group methods. The Ginzburg regime where non-Gaussian fluctuations are
important is found to be rather narrow on the ordered side of the transition,
and very broad on the disordered side.",1108.6064v3
2011-09-13,Effects of semiclassical spiral fluctuations on hole dynamics,"We investigate the dynamics of a single hole coupled to the spiral
fluctuations related to the magnetic ground states of the antiferromagnetic
J_1-J_2-J_3 Heisenberg model on a square lattice. Using exact diagonalization
on finite size clusters and the self consistent Born approximation in the
thermodynamic limit we find, as a general feature, a strong reduction of the
quasiparticle weight along the spiral phases of the magnetic phase diagram. For
an important region of the Brillouin Zone the hole spectral functions are
completely incoherent, whereas at low energies the spectral weight is
redistributed on several irregular peaks. We find a characteristic value of the
spiral pitch, Q=(0.7,0.7)\pi, for which the available phase space for hole
scattering is maximum. We argue that this behavior is due to the non trivial
interference of the magnon assisted and the free hopping mechanism for hole
motion, characteristic of a hole coupled to semiclassical spiral fluctuations.",1109.2786v1
2011-10-03,Weyl problem and Casimir effects in spherical shell geometry,"We compute the generic mode sum that quantifies the effect on the spectrum of
a harmonic field when a spherical shell is inserted into vacuum. This
encompasses a variety of problems including the Weyl spectral problem and the
Casimir effect of quantum electrodynamics. This allows us to resolve several
long-standing controversies regarding the question of universality of the
Casimir self-energy; the resolution comes naturally through the connection to
the Weyl problem. Specifically we demonstrate that in the case of a scalar
field obeying Dirichlet or Neumann boundary conditions on the shell surface the
Casimir self-energy is cutoff-dependent while in the case of the
electromagnetic field perturbed by a conductive shell the Casimir self-energy
is universal. We additionally show that an analog non-relativistic Casimir
effect due to zero-point magnons takes place when a non-magnetic spherical
shell is inserted inside a bulk ferromagnet.",1110.0421v2
2011-10-06,Spin pumping with coherent elastic waves,"We show that the resonant coupling of phonons and magnons can be exploited to
generate spin currents at room temperature. Surface acoustic wave (SAW) pulses
with a frequency of 1.55 GHz and duration of 300 ns provide coherent elastic
waves in a ferromagnetic thin film/normal metal (Co/Pt) bilayer. We use the
inverse spin Hall voltage in the Pt as a measure for the spin current and
record its evolution as a function of time and external magnetic field
magnitude and orientation. Our experiments show that a spin current is
generated in the exclusive presence of a resonant elastic excitation. This
establishes acoustic spin pumping as a resonant analogue to the spin Seebeck
effect.",1110.1187v2
2011-10-16,Signatures of asymmetric and inelastic tunneling on the spin torque bias dependence,"The influence of structural asymmetries (barrier height and exchange
splitting), as well as inelastic scattering (magnons and phonons) on the bias
dependence of the spin transfer torque in a magnetic tunnel junction is studied
theoretically using the free electron model. We show that they modify the
""conventional"" bias dependence of the spin transfer torque, together with the
bias dependence of the conductance. In particular, both structural asymmetries
and bulk (inelastic) scattering add {\em antisymmetric} terms to the
perpendicular torque ($\propto V$ and $\propto j_e|V|$), while the interfacial
inelastic scattering conserves the junction symmetry and only produces {\em
symmetric} terms ($\propto |V|^n$, $n\in\mathbb{N}$). The analysis of spin
torque and conductance measurements displays a signature revealing the origin
(asymmetry or inelastic scattering) of the discrepancy.",1110.3492v1
2011-10-18,Anharmonic Phonons and Magnons in BiFeO3,"The phonon density of states (DOS) and magnetic excitation spectrum of
polycrystalline BiFeO$_3$ were measured for temperatures $200 \leq T \leq
750\,$K, using inelastic neutron scattering (INS). Our results indicate that
the magnetic spectrum of BiFeO$_3$ closely resembles that of similar Fe
perovskites, such as LaFeO$_3$, despite the cycloid modulation in BiFeO$_3$. We
do not find any evidence for a spin gap. A strong $T$-dependence of the phonon
DOS was found, with a marked broadening of the whole spectrum, providing
evidence of strong anharmonicity. This anharmonicity is corroborated by
large-amplitude motions of Bi and O ions observed with neutron diffraction.
These results highlight the importance of spin-phonon coupling in this
material.",1110.3873v3
2011-10-24,"Synthesis, Characterization and Investigation of Electrical Transport in Metal Nanowires and Nanotubes","This thesis is dedicated to the synthesis, characterization and the study of
electrical transport through metal nanowires and nanotubes. The metal
nanowires(Ni, Cu) and nanotubes(Cu) are synthesised by electrochemical
deposition in nanoporous templates. In the synthesis front, electrochemical
deposition schemes were developed to achieve single crystallinity and synthesis
of tubular nanostructures. The nanostructures are characterized using
structural characterization techniques such as X-ray diffraction, Scanning
electron microscopy and Transmission electron microscopy. The magnetic
nanowires are also characterized using VSM and SQUID magnetometers. The
electrical transport measurements performed in a wide range of temperature
(3K-700K) to understand the interactions of electrons with phonon, magnon and
surface when the electron mean free path is limited by the dimension of these
nanostructures. The measurements were performed in the wide range of
temperature to encompass and study the ferromagnetic to paramagnetic phase
transition in case of magnetic nanowires. We observed systematic scaling of
these interactions and the characteristic temperatures such as the Debye
temperature(\theta_R) and Curie temperature(T_C) as a function of diameter. We
also observed how these interactions change as we move from nanowires to
nanotubes of metals.",1110.5260v1
2011-10-25,Wilson ratio of a Tomonaga-Luttinger liquid in a spin-1/2 Heisenberg ladder,"Using micromechanical force magnetometry, we have measured the magnetization
of the strong-leg spin-1/2 ladder compound (C$_7$H$_{10}$N)$_2$CuBr$_2$ at
temperatures down to 45 mK. Low-temperature magnetic susceptibility as a
function of field exhibits a maximum near the critical field H_c at which the
magnon gap vanishes, as expected for a gapped one-dimensional antiferromagnet.
Above H_c a clear minimum appears in the magnetization as a function of
temperature as predicted by theory. In this field region, the susceptibility in
conjunction with our specific heat data yields the Wilson ratio R_W. The result
supports the relation R_W=4K, where K is the Tomonaga-Luttinger-liquid
parameter.",1110.5653v2
2011-10-26,SpinWaves in Ferromagnetic Dots 2D Honeycomb Lattice Stripes,"In this work, the spin wave calculations were carried out using the
Heisenberg Hamiltonian to study the allowed spin waves of zigzag and armchair
edged stripes for ferromagnetic nanodots arrayed in a 2D honeycomb lattice
\cite{Selim2011}. The Hamiltonian is used to construct the $\mathbf{E}$ matrix
which encodes the exchange flow of magnons in the stripes. It is found that the
allowed spin wave modes are the eigenvalues of the $\mathbf{E}$ matrix and
therefore it is used to study the effects of the stripe width, edge exchange,
the edge uniaxial anisotropy, and impurities on the allowed spin waves of
stripes. The obtained results almost coincide with the results of graphene
nanoribbons described by tight binding Hamiltonian for electronic excitations.
Therefore, we suggest the fabrication of the magnetic counterpart to graphene
as a new technology in the field of spintronic devices and magnetic
applications.",1110.5716v1
2011-12-12,Theory of laser-induced demagnetization at high temperatures,"Laser-induced demagnetization is theoretically studied by explicitly taking
into account interactions among electrons, spins and lattice. Assuming that the
demagnetization processes take place during the thermalization of the
sub-systems, the temperature dynamics is given by the energy transfer between
the thermalized interacting baths. These energy transfers are accounted for
explicitly through electron-magnons and electron-phonons interaction, which
govern the demagnetization time scale. By properly treating the spin system in
a self-consistent random phase approximation, we derive magnetization dynamic
equations for a broad range of temperature. The dependence of demagnetization
on the temperature and pumping laser intensity is calculated in detail. In
particular, we show several salient features for understanding magnetization
dynamics near the Curie temperature. While the critical slowdown in dynamics
occurs, we find that an external magnetic field can restore the fast dynamics.
We discuss the implication of the fast dynamics in the application of heat
assisted magnetic recording.",1112.2428v1
2011-12-12,"Relationship among superconductivity, pseudogap, and high-energy magnetic fluctuations in a model high-Tc superconductor from electronic Raman scattering","We use electronic Raman scattering to study the model single-layer cuprate
superconductor HgBa2CuO4+d. In an overdoped sample, we observe a pronounced
amplitude enhancement of a high-energy peak related to two-magnon excitations
in insulating cuprates upon cooling below the critical temperature Tc. This
effect is accompanied by the appearance of the superconducting gap and a
pairing peak above the gap in the Raman spectrum, and it can be understood as a
consequence of feedback of the Cooper pairing interaction on the high-energy
magnetic fluctuations. All of these effects occur already above Tc in two
underdoped samples, demonstrating a related feedback mechanism associated with
the pseudogap.",1112.2725v1
2011-12-20,Four-loop anomalous dimensions in Leigh-Strassler deformations,"We determine the scalar part of the four-loop chiral dilatation operator for
Leigh-Strassler deformations of N=4 super Yang-Mills. This is sufficient to
find the four-loop anomalous dimensions for operators in closed scalar
subsectors. This includes the SU(2) subsector of the (complex)
beta-deformation, where we explicitly compute the anomalous dimension for
operators with a single impurity. It also includes the ""3-string null""
operators of the cubic Leigh-Strassler deformation. Our four-loop results show
that the rational part of the anomalous dimension is consistent with a
conjecture made in arXiv:1108.1583 based on the three-loop result of
arXiv:1008.3351 and the N=4 magnon dispersion relation. Here we find additional
zeta(3) terms.",1112.4787v1
2012-02-06,High-temperature spin-wave propagation in BiFeO3: relation to the Polomska transition,"In bismuth ferrite thin films the cycloidal spiral spin structure is
suppressed, and as a result the spin-wave magnon branches of long wavelength
are reduced from a dozen to one, at \omega = 19.2 cm-1 (T=4K). This spin wave
has not been measured previously above room temperature, but in the present
work we show via Raman spectroscopy that it is an underdamped propagating wave
until 455 K. This has important room-temperature device implications. The data
show that \omega(T) follows an S=5/2 Brillouin function and hence its Fe+3 ions
are in the high-spin 5/2 state and not the low-spin S=1/2 state. The spin wave
cannot be measured as a propagating wave above 455 K. We also suggest that
since this temperature is coincident with the mysterious ""Polomska transition""
(M. Polomska et al., Phys. Stat. Sol. A 23, 567, (1974)) at 458+/-5 K, that
this may be due to overdamping.",1202.1040v1
2012-02-17,Spin-stretching modes in anisotropic magnets: spin-wave excitations in the multiferroic Ba2CoGe2O7,"We studied spin excitations of the multiferroic Ba2CoGe2O7 in high magnetic
fields up to 33 T. In the electron spin resonance and far infrared absorption
spectra we found several spin excitations beyond the two conventional magnon
modes expected for such a two-sublattice antiferromagnet. We show that a
multi-boson spin-wave theory can capture these unconventional modes, that
include spin-stretching modes associated with an oscillating magnetic dipole
(or only quadrupole) moment. The lack of the inversion symmetry allows these
modes to become electric dipole active. We expect that the spin-stretching
modes can be generally observed in inelastic neutron scattering and light
absorption experiments in a broad class of ordered S > 1/2 spin systems with
strong single-ion anisotropy and/or non-centrosymmetric lattice structure.",1202.3996v2
2012-02-23,Tc is insensitive to magnetic interactions in high-Tc superconductors,"A quarter of a century after their discovery the mechanism that pairs
carriers in the cuprate high-Tc superconductors (HTS) still remains uncertain.
Despite this the general consensus is that it is probably magnetic in origin
[1] so that the energy scale for the pairing boson is governed by J, the
antiferromagnetic exchange interaction. Recent studies using resonant inelastic
X-ray scattering strongly support these ideas [2]. Here as a further test we
vary J (as measured by two-magnon Raman scattering) by more than 60% by
changing ion sizes in the model HTS system LnA2Cu3O7-{\delta} where A=(Ba,Sr)
and Ln=(La, Nd, Sm, Eu, Gd, Dy, Yb, Lu). Such changes are often referred to as
""internal"" pressure. Surprisingly, we find Tcmax anticorrelates with J where
internal pressure is the implicit variable. This is the opposite to the effect
of external pressure and suggests that J is not the dominant energy scale
governing Tcmax.",1202.5078v1
2012-03-08,First principles calculation of the phonons modes in the hexagonal $\rm YMnO_3$ ferroelectric and paraelectric phases,"The lattice dynamics of the $\rm YMnO_3$ magneto-electric compound has been
investigated using density functional calculations, both in the ferroelectric
and the paraelectric phases. The coherence between the computed and
experimental data is very good in the low temperature phase. Using group
theory, modes continuity and our calculations we were able to show that the
phonons modes observed by Raman scattering at 1200K are only compatible with
the ferroelectric $P6_{3} cm$ space group, thus supporting the idea of a
ferroelectric to paraelectric phase transition at higher temperature. Finally
we proposed a candidate for the phonon part of the observed electro-magnon.
This mode, inactive both in Raman scattering and in Infra-Red, was shown to
strongly couple to the Mn-Mn magnetic interactions.",1203.1752v1
2012-05-31,Bound States of the q-Deformed AdS5 x S5 Superstring S-matrix,"The investigation of the q deformation of the S-matrix for excitations on the
string world sheet in AdS5 x S5 is continued. We argue that due to the lack of
Lorentz invariance the situation is more subtle than in a relativistic theory
in that the nature of bound states depends on their momentum. At low enough
momentum |p|<E the bound states transform in the anti-symmetric representation
of the super-algebra symmetry and become the solitons of the Pohlmeyer reduced
theory in the relativistic limit. At a critical momentum |p|=E they become
marginally unstable, and at higher momenta the stable bound states are in the
symmetric representation and become the familiar magnons in the string limit as
q->1. This subtlety fixes a problem involving the consistency of crossing
symmetry with the relativistic limit found in earlier work. With mirror
kinematics, obtained after a double Wick rotation, the bound state structure is
simpler and there are no marginally unstable bound states.",1206.0010v1
2012-05-31,Two-particle response in Cluster Dynamical Mean-Field Theory: Formalism and application to the Raman Response of High-temperature Superconductors,"A method is presented for the unbiased numerical computation of two-particle
response functions of correlated electron materials via a solution of the
dynamical mean-field equations in the presence of a perturbing field. The power
of the method is demonstrated via a computation of the Raman $B_{1g}$ and
$B_{2g}$ scattering intensities of the two dimensional Hubbard model, in
parameter regimes believed to be relevant to high-temperature
superconductivity. The theory reproduces the `two-magnon' peak characteristic
of the Raman intensity of the insulating parent compounds of high-$T_c$ copper
oxide superconductors and shows how it evolves to a quasiparticle response as
carriers are added. The method can be applied in any situation where a solution
of the equilibrium dynamical mean-field equations is feasible.",1206.0023v1
2012-07-06,Ferromagnetic Quantum critical behavior in three-dimensional Hubbard model with transverse anisotropy,"One-band Hubbard model with transverse anisotropy is considered at density of
electrons $n=0.4$. It is shown that when the anisotropy is appropriately
chosen, the ground state is ferromagnetic with magnetic order perpendicular to
the anisotropy. The increasing of the ratio $\frac tU$, where $t$ is the
hopping parameter and $U$ is the Coulomb repulsion, decreases the Curie
temperature, and the system arrives at the quantum critical point $(T_C=0)$.
The result is obtained introducing Schwinger bosons and slave Fermions
representation of the electron operators. Integrating out the spin-singlet
Fermi fields an effective Heisenberg model with ferromagnetic exchange constant
is obtained for vectors which identifies the local orientation of the spin of
the itinerant electrons. The amplitude of the spin vectors is an effective spin
of the itinerant electrons accounting for the fact that some sites, in the
ground state, are doubly occupied or empty. Owing to the anisotropy, the magnon
fluctuations drive the system to quantum criticality and when the effective
spin is critically small these fluctuations suppress the magnetic order.",1207.1674v1
2012-07-11,Inner products of Bethe states as partial domain wall partition functions,"We study the inner product of Bethe states in the inhomogeneous periodic XXX
spin-1/2 chain of length L, which is given by the Slavnov determinant formula.
We show that the inner product of an on-shell M-magnon state with a generic
M-magnon state is given by the same expression as the inner product of a
2M-magnon state with a vacuum descendent. The second inner product is
proportional to the partition function of the six-vertex model on a rectangular
Lx2M grid, with partial domain-wall boundary conditions.",1207.2562v3
2012-07-17,The Free Energy of the Quantum Heisenberg Ferromagnet at Large Spin,"We consider the spin-S ferromagnetic Heisenberg model in three dimensions, in
the absence of an external field. Spin wave theory suggests that in a suitable
temperature regime the system behaves effectively as a system of
non-interacting bosons (magnons). We prove this fact at the level of the
specific free energy: if $ S \to \infty $ and the inverse temperature $ \beta
\to 0 $ in such a way that $ \beta S $ stays constant, we rigorously show that
the free energy per unit volume converges to the one suggested by spin wave
theory. The proof is based on the localization of the system in small boxes and
on upper and lower bounds on the local free energy, and it also provides
explicit error bounds on the remainder.",1207.4050v1
2012-07-30,Variations on Slavnov's scalar product,"We consider the rational six-vertex model on an L-by-L lattice with domain
wall boundary conditions and restrict N parallel-line rapidities, N < L/2, to
satisfy length-L XXX spin-1/2 chain Bethe equations. We show that the partition
function is an (L-2N)-parameter extension of Slavnov's scalar product of a
Bethe eigenstate and a generic state, with N magnons each, on a length-L XXX
spin-1/2 chain.
  Decoupling the extra parameters, we obtain a third determinant expression for
the scalar product, where the first is due to Slavnov [1], and the second is
due to Kostov and Matsuo [2]. We show that the new determinant is a discrete KP
tau-function in the inhomogeneities, and consequently that tree-level N = 4 SYM
structure constants that are known to be determinants, remain determinants at
1-loop level.",1207.6871v1
2012-08-06,Spin-state Crossover Model for the Magnetism of Iron Pnictides,"We propose a minimal model describing magnetic behavior of Fe-based
superconductors. The key ingredient of the model is a dynamical mixing of
quasi-degenerate spin states of Fe2+ ion by intersite electron hoppings,
resulting in an effective local spin Seff. The moments Seff tend to form
singlet pairs, and may condense into a spin nematic phase due to the emergent
biquadratic exchange couplings. The long-range ordered part m of Seff varies
widely, 0<=m<=Seff, but magnon spectra are universal and scale with Seff,
resolving the puzzle of large but fluctuating Fe-moments. Unusual temperature
dependences of a local moment and spin susceptibility are also explained.",1208.1197v3
2012-08-19,Anderson localization of spinons in a spin-1/2 antiferromagnetic Heisenberg chain,"Anderson localization is a general phenomenon of wave physics, which stems
from the interference between multiple scattering paths1,2. It was originally
proposed for electrons in a crystal, but later was also observed for light3-5,
microwaves6, ultrasound7,8, and ultracold atoms9-12. Actually, in a crystal,
besides electrons there may exist other quasiparticles such as magnons and
spinons. However the search for Anderson localization of these magnetic
excitations is rare so far. Here we report the first observation of spinon
localization in copper benzoate, an ideal compound of spin-1/2
antiferromagnetic Heisenberg chain, by ultra-low-temperature specific heat and
thermal conductivity measurements. We find that while the spinon specific heat
Cs displays linear temperature dependence down to 50 mK, the spinons thermal
conductivity ks only manifests the linear temperature dependence down to 300
mK. Below 300 mK, ks/T decreases rapidly and vanishes at about 100 mK, which is
a clear evidence for Anderson localization. Our finding opens a new window for
studying such a fundamental phenomenon in condensed matter physics.",1208.3803v1
2012-08-30,Quantum-critical spin dynamics in quasi-one-dimensional antiferromagnets,"By means of nuclear spin-lattice relaxation rate 1/T1, we follow the spin
dynamics as a function of the applied magnetic field in two gapped
one-dimensional quantum antiferromagnets: the anisotropic spin-chain system
NiCl2-4SC(NH2)2 and the spin-ladder system (C5H12N)2CuBr4. In both systems,
spin excitations are confirmed to evolve from magnons in the gapped state to
spinons in the gapples Tomonaga-Luttinger-liquid state. In between, 1/T1
exhibits a pronounced, continuous variation, which is shown to scale in
accordance with quantum criticality. We extract the critical exponent for 1/T1,
compare it to the theory, and show that this behavior is identical in both
studied systems, thus demonstrating the universality of quantum critical
behavior.",1208.6117v1
2012-09-17,Nonlinear emission of spin-wave caustics from an edge mode of a micro-structured Co2Mn0.6Fe0.4Si waveguide,"Magnetic Heusler materials with very low Gilbert damping are expected to show
novel magnonic transport phenomena. We report nonlinear generation of higher
harmonics leading to the emission of caustic spin-wave beams in a low-damping,
micro-structured Co2Mn0.6Fe0.4Si Heusler waveguide. The source for the higher
harmonic generation is a localized edge mode formed by the strongly
inhomogeneous field distribution at the edges of the spin-wave waveguide. The
radiation characteristics of the propagating caustic waves observed at twice
and three times the excitation frequency are described by an analytical
calculation based on the anisotropic dispersion of spin waves in a magnetic
thin film.",1209.3669v2
2012-09-20,Anisotropic spin-dynamics in the Kondo semiconductor CeRu2Al10,"Spin dynamics in the new Kondo insulator compound CeRu2Al10 has been studied
using unpolarized and polarized neutron scattering on single crystals. In the
unconventional ordered phase forming below T0 = 27.3 K, two excitation branches
are observed with significant intensities, the lower one of which has a gap of
4.8 +/- 0.3 meV and a pronounced dispersion up to about 8.5 meV. Comparison
with RPA magnon calculations assuming crystal-field and anisotropic exchange
couplings captures major aspects of the data, but leaves unexplained
discrepancies, pointing to a key role of direction-specific hybridization
between 4f and conduction band states in this compound.",1209.4613v1
2012-09-28,Theory of the Spin Seebeck Effect,"The spin Seebeck effect refers to the generation of a spin voltage caused by
a temperature gradient in a ferromagnet, which enables the thermal injection of
spin currents from the ferromagnet into an attached nonmagnetic metal over a
macroscopic scale of several millimeters. The inverse spin Hall effect converts
the injected spin current into a transverse charge voltage, thereby producing
electromotive force as in the conventional charge Seebeck device. Recent
theoretical and experimental efforts have shown that the magnon and phonon
degrees of freedom play crucial roles in the spin Seebeck effect. In this
article, we present the theoretical basis for understanding the spin Seebeck
effect and briefly discuss other thermal spin effects.",1209.6407v2
2012-10-03,Easy-axis ferromagnetic chain on a metallic surface,"The phases and excitation spectrum of an easy-axis ferromagnetic chain of
S=1/2 magnetic impurities built on the top of a clean metallic surface are
studied. As a function of the (Kondo) coupling to the metallic surface and at
low temperatures, the spin chain exhibits a quantum phase transition from an
Ising ferromagnetic phase with long-range order to a paramagnetic phase where
quantum fluctuations destroy the magnetic order. In the paramagnetic phase, the
system consists of a chain of Kondo-singlets where the impurities are
completely screened by the metallic host. In the ferromagnetic phase, the
excitations above the Ising gap are damped magnons, with a finite lifetime
arising due to the coupling to the substrate. We discuss the experimental
consequences of our results to spin-polarized electron energy loss spectroscopy
(SPEELS), and we finally analyze possible extensions to spin chains with S>1/2.",1210.1195v1
2012-10-22,"Thickness-dependent structural, magnetic and transport properties of epitaxial Co2FeAl Heusler alloy thin films","We report on a systematic study of the structural, magnetic properties and
the anomalous Hall effect, in the Heusler alloy Co2FeAl (CFA) epitaxial films
on MgO(001), as a function of film thickness. It was found that the epitaxial
CFA films show a highly ordered B2 structure with an in-plane uniaxial magnetic
anisotropy. An analysis of the electrical transport properties reveals that the
lattice and magnon scattering contributions to the longitudinal resistivity.
Independent on the thickness of films, the anomalous Hall resistivity of CFA
films is found to be dominated by skew scattering only. Moreover, the anomalous
Hall resistivity shows weakly temperature dependent behavior, and its absolute
value increases as the thickness decreases. We attribute this temperature
insensitivity in the anomalous Hall resistivity to the weak temperature
dependent of tunneling spin-polarization in the CFA films, while the thickness
dependence behavior is likely due to the increasing significance of interface
or free surface electronic states.",1210.5807v1
2012-10-26,Ground state in a half-doped manganite distinguished by neutron spectroscopy,"We have measured the spin-wave spectrum of the half-doped bilayer manganite
Pr(Ca,Sr)2Mn2O7 in its spin, charge, and orbital ordered phase. The
measurements, which extend throughout the Brillouin zone and cover the entire
one-magnon spectrum, are compared critically with spin-wave calculations for
different models of the electronic ground state. The data are described very
well by the Goodenough model, which has weakly interacting ferromagnetic
zig-zag chains in the CE-type arrangement. A model that allows ferromagnetic
dimers to form within the zigzags is inconsistent with the data. The analysis
conclusively rules out the strongly bound dimer (Zener polaron) model.",1210.7108v2
2012-10-30,Quantum phases of a frustrated four-leg spin tube,"We study the ground state phase diagram of a frustrated spin-1/2 four-leg
tube. Using a variety of complementary techniques, namely density matrix
renormalization group, exact diagonalization, Schwinger boson mean field
theory, quantum Monte-Carlo and series expansion, we explore the parameter
space of this model in the regime of all-antiferromagnetic exchange. In
contrast to unfrustrated four-leg tubes we uncover a rich phase diagram. Apart
from the Luttinger liquid fixed point in the limit of decoupled legs, this
comprises several gapped ground states, namely a plaquette, an incommensurate,
and an antiferromagnetic quasi spin-2 chain phase. The transitions between
these phases are analyzed in terms of total energy and static structure factor
calculations and are found to be of (weak) first order. Despite the absence of
long range order in the quantum case, remarkable similarities to the classical
phase diagram are uncovered, with the exception of the icommensurate regime,
which is strongly renormalized by quantum fluctuations. In the limit of large
leg exchange the tube exhibits a deconfinement cross-over from gapped magnon
like excitations to spinons.",1210.8199v1
2012-11-01,On the Integrability of Planar N=2 Superconformal Gauge Theories,"We study the integrability properties of planar N=2 superconformal field
theories in four dimensions. We show that the spin chain associated to the
planar dilation operator of N=2 superconformal QCD fails to be integrable at
two loops. In our analysis we focus on a closed SU(2|1) sector, whose two-loop
spin chain we fix by symmetry arguments (up to a few undetermined
coefficients). It turns out that the Yang-Baxter equation for magnon scattering
is not satisfied in this sector. On the other hand, we suggest that the closed
SU(2,1|2) sector, which exists in any N=2 superconformal gauge theory, may be
integrable to all loops.",1211.0271v2
2012-11-07,Lattice and spin excitations in multiferroic h-YbMnO3,"Lattice and spin excitations have been studied by Raman scattering in
hexagonal YbMnO3 single crystals. The temperature dependences of the phonon
modes show that the E2 mode at 256 cm-1 related to the displacement of Mn and O
ions in a-b plane is coupled to the spin order. The A1 phonon mode at 678 cm-1
presents a soft mode behavior at the Neel temperature. Connected to the motion
of the apical oxygen ions along the c direction, this mode controls directly
the Mn-Mn interactions between adjacent Mn planes and the superexchange path.
Crystal field and magnon mode excitations have been identified. The temperature
investigation of the spin excitations shows that the spin structure is strongly
influence by the Yb-Mn interaction. Under a magnetic field along the c axis, we
have investigated the magnetic reordering and its impact on the spin
excitations.",1211.1540v1
2012-11-11,Electric polarization of magnetic textures: new horizons of micromagnetism,"A common scenario of magnetoelectric coupling in multiferroics is the
electric polarization induced by spatially modulated spin structures. It is
shown in this paper that the same mechanism works in magnetic dielectrics with
inhomogeneous magnetization distribution: the domain walls and magnetic
vortexes can be the sources of electric polarization. The electric field driven
magnetic domain wall motion is observed in iron garnet films. The electric
field induced nucleation of vortex state of magnetic nanodots is theoretically
predicted and numerically simulated. From the practical point of view the
electric field control of micromagnetic structures is promising for
applications in low-power-consumption spintronic and magnonic devices.",1211.2403v1
2012-11-13,Atomistic spin dynamics of low-dimensional magnets,"We investigate the magnetic properties of a range of low-dimensional
ferromagnets using a combination of first-principles calculations and atomistic
spin dynamics simulations. This approach allows us to evaluate the ground state
and finite temperature properties of experimentally well characterized systems
such as Co/Cu(111), Co/Cu(001), Fe/Cu(001) and Fe/W(110), for different
thicknesses of the magnetic layer. We compare our calculated spin wave spectra
with experimental data available in the literature, and find a good
quantitative agreement. We also predict magnon spectra for systems for which no
experimental data exist at the moment, and estimate the role of temperature
effects.",1211.2964v2
2012-11-20,Microscopic magnetic structuring of a spin-wave waveguide by ion implantation in a Ni(81)Fe(19) layer,"We investigate the spin-wave excitation in microscopic waveguides fabricated
by localized Cr+ ion implantation in a ferromagnetic Ni(81)Fe(19) film. We
demonstrate that spin-wave waveguides can be conveniently made by this
technique. The magnetic patterning technique yields an increased damping and a
reduction in saturation magnetization in the implanted regions that can be
extracted from Brillouin light scattering measurements of the spin-wave
excitation spectra. Furthermore, the waveguide performance as well as the
internal field of the waveguide depend on the doping fluence. The results prove
that localized ion implantation is a powerful tool for the patterning of magnon
spintronic devices.",1211.4786v1
2012-11-21,A dynamic su(1|1)^2 S-matrix for AdS3/CFT2,"We derive the S-matrix for the d(2,1;alpha)^2 symmetric spin-chain of
AdS3/CFT2 by considering the centrally extended su(1|1)^2 algebra acting on the
spin-chain excitations. The S-matrix is determined uniquely up to four scalar
factors, which are further constrained by a set of crossing relations. The
resulting scattering includes non-trivial processes between magnons of
different masses that were previously overlooked.",1211.5119v3
2012-11-22,Generating string solutions in BTZ,"Integrability of classical strings in the BTZ black hole enables the
construction and study of classical string propagation in this background. We
first apply the dressing method to obtain classical string solutions in the BTZ
black hole. We dress time like geodesics in the BTZ black hole and obtain open
string solutions which are pinned on the boundary at a single point and whose
end points move on time like geodesics. These strings upon regularising their
charge and spins have a dispersion relation similar to that of giant magnons.
We then dress space like geodesics which start and end on the boundary of the
BTZ black hole and obtain minimal surfaces which can penetrate the horizon of
the black hole while being pinned at the boundary. Finally we embed the giant
gluon solutions in the BTZ background in two different ways. They can be
embedded as a spiral which contracts and expands touching the horizon or a
spike which originates from the boundary and touches the horizon.",1211.5382v1
2012-11-26,Quantum corrections to spinning superstrings in AdS_3 x S^3 x M^4: determining the dressing phase,"We study the leading quantum string correction to the dressing phase in the
asymptotic Bethe Ansatz system for superstring in AdS_3 x S^3 x T^4 supported
by RR flux. We find that the phase should be different from the BES phase
appearing in the AdS_5 x S^5 case. We use the simplest example of a rigid
circular string with two equal spins in S^3 and also consider the general
approach based on the algebraic curve description. We also discuss the case of
the AdS_3 x S^3 x S^3 x S^1 theory and find the dependence of the 1-loop
correction to the effective string tension function h(\lambda) (expected to
enter the magnon dispersion relation) on the parameters alpha related to the
ratio of the two 3-sphere radii. This correction vanishes in the AdS_3 x S^3 x
T^4 case.",1211.6090v3
2012-12-17,The first-principles study of thermodynamical properties of random magnetic overlayers on fcc-Cu(001) substrate,"We present the theoretical study of thermodynamical properties of fcc-Cu(001)
substrate covered by iron-cobalt monolayer as well as by incomplete iron layer.
The effective two-dimensional Heisenberg Hamiltonian is constructed from first
principles and properties of exchange interactions are investigated. The Curie
temperatures are estimated using the Monte-Carlo (MC) simulations and compared
with a simplified approach using the random-phase approximation (RPA) in
connection with the virtual-crystal approach (VCA) to treat randomness in
exchange integrals. Calculations indicate a weak maximum of the Curie
temperature as a function of composition of the iron-cobalt overlayer. While a
good quantitative agreement between RPA-VCA and MC was found for iron-cobalt
monolayer, the RPA-VCA approach fails quantitatively for low coverage due to
the magnetic percolation effect. We also present the study of the effect of
alloy disorder on the shape of magnon spectra of random overlayers.",1212.4057v1
2013-01-06,Discrete time quantum walk with nitrogen-vacancy centers in diamond coupled to a superconducting flux qubit,"We propose a quantum-electrodynamics scheme for implementing the
discrete-time, coined quantum walk with the walker corresponding to the phase
degree of freedom for a quasi-magnon field realized in an ensemble of
nitrogen-vacancy centres in diamond. The coin is realized as a superconducting
flux qubit. Our scheme improves on an existing proposal for implementing
quantum walks in cavity quantum electrodynamics by removing the cumbersome
requirement of varying drive-pulse durations according to mean quasiparticle
number. Our improvement is relevant to all indirect-coin-flip cavity
quantum-electrodynamics realizations of quantum walks. Our numerical analysis
shows that this scheme can realize a discrete quantum walk under realistic
conditions.",1301.1050v2
2013-01-15,Giant gravitons: a collective coordinate approach,"In this paper I describe a collective coordinate approach to the study of
giant graviton states and their excitations in various field theories. The
method simplifies considerably the understanding of emergent gauge symmetry of
these configurations, as well as the calculation of the spectrum of strings
stretched between the giant gravitons. There is a limit where these results
reproduce the one loop dispersion relation for giant magnons. I also show that
this method gives rise to a simple geometric interpretation of a Higgs
mechanism for the emergent gauge symmetry which parallels the holographic dual
realization of these sates: the effective Higgs condensate is the geometric
separation of D-branes in the collective coordinate geometry.",1301.3519v3
2013-01-17,Excitation spectra of disordered dimer magnets near quantum criticality,"For coupled-dimer magnets with quenched disorder, we introduce a
generalization of the bond-operator method, appropriate to describe both
singlet and magnetically ordered phases. This allows for a numerical
calculation of the magnetic excitations at all energies across the phase
diagram, including the strongly inhomogeneous Griffiths regime near quantum
criticality. We apply the method to the bilayer Heisenberg model with bond
randomness and characterize both the broadening of excitations and the transfer
of spectral weight induced by disorder. Inside the antiferromagnetic phase this
model features the remarkable combination of sharp magnetic Bragg peaks and
broad magnons, the latter arising from the tendency to localization of
low-energy excitations.",1301.4223v2
2013-02-03,"Spectral non-uniform temperature, non-local heat transfer, and the spin Seebeck effect","We present a theory of the spin Seebeck effect driven by subthermal non-local
phonon heat transfer and spectral non-uniform temperature distribution. The
theory explains the non-local behavior of the effect arising from the fact that
phonons that store the energy (thermal) and the phonons that transfer it
(subthermal) are located in different parts of the spectrum and have very
different kinetics. This gives rise to a spectral phonon distribution function
that deviates from local equilibrium along the substrate and is sensitive to
boundary conditions. The theory also predicts a non-magnon origin of the effect
in ferromagnetic metals in agreement with observations in recent experiments.
Equilibration of the heat flow out of the substrate to the Pt probe and
backwards leads to a measurable vertical spin-current produced by the spin
polarized electrons dragged by the local thermal phonons. We predict specific
sample length limits and other dependencies that can be probed experimentally,
and obtain the correct magnitude of the effect.",1302.0499v1
2013-03-07,THz Magneto-electric atomic rotations in the chiral compound Ba$_3$NbFe$_3$Si$_2$O$_{14}$,"We have determined the terahertz spectrum of the chiral langasite
Ba$_3$NbFe$_3$Si$_2$O$_{14}$ by means of synchrotron-radiation measurements.
Two excitations are revealed that are shown to have a different nature. The
first one, purely magnetic, is observed at low temperature in the magnetically
ordered phase and is assigned to a magnon. The second one persits far into the
paramagnetic phase and exhibits both an electric and a magnetic activity at
slightly different energies. This magnetoelectric excitation is interpreted in
terms of atomic rotations and requires a helical electric polarization.",1303.1706v2
2013-03-11,Local quantum criticality out of equilibrium - effective temperatures and scaling in the steady state regime,"We study the out of equilibrium steady state properties of the
Bose-Fermi-Kondo model, describing a local magnetic moment coupled to two
ferromagnetic leads that support bosonic (magnons) and fermionic (Stoner
continuum electrons) low energy excitations. This model describes the
destruction of the Kondo effect as the coupling to the bosons is increased. Its
phase diagram comprises three non-trivial fixed points. Using a dynamical
large-$N$approach on the Keldysh contour, we study two different
non-equilibrium setups: (a) a finite bias voltage and (b) a finite temperature
gradient, imposed across the leads. The scaling behavior of the charge and
energy currents is identified and characterized for all fixed points. We report
the existence of a fixed-point-dependent effective temperature, defined though
the fluctuation dissipation relations of the local spin-susceptibility in the
scaling regime, which permits to recover the equilibrium behavior of both
dynamical and static spin susceptibilities.",1303.2518v1
2013-03-11,Measurement of Two Low-Temperature Energy Gaps in the Electronic Structure of Antiferromagnetic USb2 Using Ultrafast Optical Spectroscopy,"Ultrafast optical spectroscopy is used to study the antiferromagnetic
f-electron system USb2. We observe the opening of two charge gaps at low
temperatures (<45 K), arising from renormalization of the electronic structure.
Analysis of our data indicates that one gap is due to hybridization between
localized f-electron and conduction electron bands, while band renormalization
involving magnons leads to the emergence of the second gap. These experiments
thus enable us to shed light on the complex electronic structure emerging at
the Fermi surface in f-electron systems.",1303.2694v2
2013-03-16,Dispersive spin excitations in highly overdoped cuprates revealed by resonant inelastic x-ray scattering,"Using resonant inelastic x-ray scattering (RIXS) at the Cu $L$-absorption
edge, we have observed intense, dispersive spin excitations in highly overdoped
Tl$_2$Ba$_2$CuO$_{6+\delta}$ (superconducting $T_c =6$ K), a compound whose
normal-state charge transport and thermodynamic properties have been studied
extensively and shown to exhibit canonical Fermi-liquid behavior. Complementary
RIXS experiments on slightly overdoped Tl$_2$Ba$_2$CuO$_{6+\delta}$ ($T_c =89$
K) and on Y$_{1-x}$Ca$_{x}$Ba$_2$Cu$_3$O$_{6+\delta}$ compounds spanning a wide
range of doping levels indicate that these excitations exhibit energies and
energy-integrated spectral weights closely similar to those of
antiferromagnetic magnons in undoped cuprates. The surprising coexistence of
Fermi-liquid-like charge excitations and high-energy spin excitations
reminiscent of antiferromagnetic insulators in highly overdoped compounds poses
a challenge to current theoretical models of the cuprates.",1303.3947v1
2013-03-22,Bose-Einstein Condensation of Photons in a Microscopic Optical Resonator: Towards Photonic Lattices and Coupled Cavities,"Bose-Einstein condensation has in the last two decades been observed in cold
atomic gases and in solid-state physics quasiparticles, exciton-polaritons and
magnons, respectively. The perhaps most widely known example of a bosonic gas,
photons in blackbody radiation, however exhibits no Bose-Einstein condensation,
because the particle number is not conserved and at low temperatures the
photons disappear in the system's walls instead of massively occupying the
cavity ground mode. This is not the case in a small optical cavity, with a
low-frequency cutoff imprinting a spectrum of photon energies restricted to
values well above the thermal energy. The here reported experiments are based
on a microscopic optical cavity filled with dye solution at room temperature.
Recent experiments of our group observing Bose-Einstein condensation of photons
in such a setup are described. Moreover, we discuss some possible applications
of photon condensates to realize quantum manybody states in periodic photonic
lattices and photonic Josephson devices.",1303.5772v1
2013-04-08,YIG thickness and frequency dependence of the spin-charge current conversion in YIG/Pt systems,"We report the frequency dependence of the spin current emission in a hybrid
ferrimagnetic insulator/normal metal system as function of the insulating layer
thickness. The system is based on a yttrium iron garnet (YIG) film [0.2, 1, and
3 \mu m] grown by liquid-phase-epitaxy coupled with a spin current detector of
platinum [6 nm]. A strong YIG thickness dependence of the efficiency of the
spin pumping has been observed. The highest conversion factor \Delta V/P_{abs}
has been demonstrated for the thinner YIG (1.79 and 0.55 mV/mW^{-1} at 2.5 and
10 GHz, respectively) which presents an interest for the realisation of
YIG-based devices. A strong YIG thickness dependence of the efficiency of the
spin pumping has been also observed and we demonstrate the threshold frequency
dependence of the three-magnon splitting process.",1304.2190v1
2013-04-15,Neutron Scattering and Its Application to Strongly Correlated Systems,"Neutron scattering is a powerful probe of strongly correlated systems. It can
directly detect common phenomena such as magnetic order, and can be used to
determine the coupling between magnetic moments through measurements of the
spin-wave dispersions. In the absence of magnetic order, one can detect diffuse
scattering and dynamic correlations. Neutrons are also sensitive to the
arrangement of atoms in a solid (crystal structure) and lattice dynamics
(phonons). In this chapter, we provide an introduction to neutrons and neutron
sources. The neutron scattering cross section is described and formulas are
given for nuclear diffraction, phonon scattering, magnetic diffraction, and
magnon scattering. As an experimental example, we describe measurements of
antiferromagnetic order, spin dynamics, and their evolution in the
La(2-x)Ba(x)CuO(4) family of high-temperature superconductors.",1304.4214v1
2013-04-24,Magnonlike dispersion of spin resonance in Ni-doped BaFe$_2$As$_2$,"Inelastic neutron scattering measurements on
Ba(Fe$_{0.963}$Ni$_{0.037}$)$_2$As$_2$ manifest a neutron spin resonance in the
superconducting state with anisotropic dispersion within the Fe layer. Whereas
the resonance is sharply peaked at Q$_{AFM}$ along the orthorhombic a axis, the
resonance disperses upwards away from Q$_{AFM}$ along the b axis. In contrast
to the downward dispersing resonance and hour-glass shape of the spin
excitations in superconducting cuprates, the resonance in electron-doped
BaFe$_2$As$_2$ compounds possesses a magnon-like upwards dispersion.",1304.6657v1
2013-05-01,Predicted Rectification and Negative Differential Spin Seebeck Effect at Magnetic Interfaces,"We study the nonequilibrium Seebeck spin transport across metal-magnetic
insulator interfaces. The conjugate-converted thermal-spin transport is
assisted by the exchange interaction at the interface, between conduction
electrons in the metal lead and localized spins in the insulating magnet lead.
We predict the rectification and negative differential spin Seebeck effect and
resolve their microscopic mechanism, as a consequence of the
strongly-fluctuated electronic density of states in the metal lead. The
rectification of spin Peltier effect is also discussed. The phenomena predicted
here are relevant for designing efficient spin/magnon diode and transistor,
which could play crucial roles in controlling energy and information in
functional devices.",1305.0301v4
2013-05-07,Interatomic exchange interactions in non-collinear magnets,"We derive ab inito exchange parameters for general non-collinear magnetic
configurations, in terms of a multiple scattering formalism. We show that the
general exchange formula has an anisotropic-like term even in the absence of
spin-orbit coupling, and that this term is large, for instance for collinear
configuration in bcc Fe, whereas for fcc Ni it is quite small. We demonstrate
that keeping this term leads to that one should consider a biquadratic
effective spin Hamiltonian even in case of collinear arrangement. In
non-collinear systems this term results in new tensor elements, that are
important for exchange interactions at finite temperatures but they have less
importance at low temperature. To illustrate our results in practice, we
calculate for bcc Fe magnon spectra obtained from configuration dependent
exchange parameters, where the configurations are determined by finite
temperature effects. Our theory results in the same quantitative results as the
finite temperature neutron scattering experiments.",1305.1447v1
2013-05-14,Electromagnon in ferrimagnetic eps-Fe2O3 nanograin ceramics,"Electromagnons are known from multiferroics as spin waves excited by the
electric component of electromagnetic radiation. We report the discovery of an
excitation in the far-infrared spectra of eps-Fe2O3 which we attribute to an
electromagnon appearing below 110 K, where the ferrimagnetic structure becomes
incommensurately modulated. Inelastic neutron scattering shows that the
electromagnon energy corresponds to that of a magnon from the Brillouin zone
boundary. Dielectric measurements did not reveal any sign of ferroelectricity
in eps-Fe2O3 down to 10 K, despite its acentric crystal structure. This shows
that the activation of an electromagnon requires, in addition to the polar
ferrimagnetic structure, a modulation of the magnetic structure. We demonstrate
that a combination of inelastic neutron scattering with infrared and / or
terahertz spectroscopies allows detecting electromagnons in ceramics, where no
crystal-orientation analysis of THz and infrared spectra is possible.",1305.3064v3
2013-07-01,Elementary excitations probed by L-edge resonant inelastic x-ray scattering in systems with weak and intermediate electron correlations,"We develop a formalism to calculate the L-edge resonant x-ray scattering
(RIXS) spectra from transition-metal compounds. Using a multi-orbital
tight-binding model, we derive useful formulas to calculate the spectra by
collecting up the ladder diagrams on the basis of the Keldysh scheme, without
relying on the fast collision approximation. They are feasible in the weak and
intermediate coupling regimes of itinerant electron systems, where the charge
and the magnetic excitations are mixed together. We examine the difference
between the present formulas and those of the fast collision approximation.
Finally, to demonstrate how our theory works, we employ the formulas to study
the RIXS spectra on a simple model, the single-orbital Hubbard model on the
square lattice at half-filling. The intensities originated from the magnon and
from the continuous states are obtained on the equal footing in the
antiferromagnetic ground state.",1307.0255v1
2013-07-01,Doping-Dependent Raman Resonance in the Model High-Temperature Superconductor HgBa2CuO4+d,"We study the model high-temperature superconductor HgBa2CuO4+d with
electronic Raman scattering and optical ellipsometry over a wide doping range.
The resonant Raman condition which enhances the scattering cross section of
""two-magnon"" excitations is found to change strongly with doping, and it
corresponds to a rearrangement of inter-band optical transitions in the 1-3 eV
range seen by ellipsometry. This unexpected change of the resonance condition
allows us to reconcile the apparent discrepancy between Raman and x-ray
detection of magnetic fluctuations in superconducting cuprates. Intriguingly,
the strongest variation occurs across the doping level where the antinodal
superconducting gap reaches its maximum.",1307.0325v1
2013-07-09,Anisotropic spin fluctuations in the quasi one-dimensional frustrated magnet LiCuVO_4,"We report results of NMR experiments on a single crystal of the quasi
one-dimensional frustrated magnet LiCuVO_4. The NMR spectra of ^7Li and ^{51}V
nuclei indicate a helical spin order in a magnetic field of 4 T with the
helical spin plane perpendicular to the field and a spin-density-wave (SDW)
order at 10 T with modulation in the magnitude of the moments aligned along the
field, in agreement with earlier reports. The nuclear spin-lattice relaxation
rate 1/T_1 at ^{51}V nuclei, which is selectively coupled to the transverse
spin fluctuations perpendicular to the field, shows a pronounced peak near the
helical ordering temperature in the field of 4 T applied along the a-axis. In
the field of 10 T, however, such a peak is absent. Instead 1/T_1 at ^7Li nuclei
probing longitudinal spin fluctuations shows divergent behavior towards the SDW
ordering temperature. These results are qualitatively consistent with the
theoretical description that the SDW correlation is due to bound magnon pairs,
which produce an energy gap in the transverse spin excitation spectrum.",1307.2346v1
2013-10-09,Phase Transitions Driven by QuasiParticle Interactions,"Quasiparticles and collective effects may have seemed exotic when first
proposed in the 1930s, but their status has blossomed with their confirmation
by todays sophisticated experiment techniques. Evidence has accumulated about
the interactions of, say, magnons and rotons and with each other and also other
quasiparticles. We briefly review the conjectures of their existence necessary
to provide quantitative agreement with experiment which in the early period was
their only reason for being. Phase transitions in the Anderson model, the Kondo
effect, roton roton interactions, and highly correlated systems such as
helium4, the Quantum Hall Effect, and BEC condensates are discussed. Some
insulator and superconductor theories seem to suggest collective interactions
of several quasiparticles may be necessary to explain the behavior. We conclude
with brief discussions of the possibility of using the parameter to detect
quantum critical points and some background on bound states emerging from the
continuum. Finally, we present a summary and conclusions and also discuss
possible future directions.",1310.2522v1
2013-11-05,Quench Dynamics of the Anisotropic Heisenberg Model,"We develop an analytic approach for the study of the quench dynamics of the
anisotropic Heisenberg model (XXZ model) on the infinite line. We present the
exact time-dependent wavefunctions after a quench in an integral form for any
initial state and for any anisotropy $\Delta$ by means of a generalized Yudson
contour representation. We calculate the evolution of several observables from
two particular initial states: starting with a local N\`eel state we calculate
the time evolution of the antiferromagnetic order parameter--staggered
magnetization; starting with a state with consecutive flipped spins we
calculate the propagation of magnons and bound state excitations, and the
induced spin currents. We also show how the ""string"" solution of Bethe Ansatz
equations emerge naturally from the contour approach. We confront our results
with experiments and numerical methods where possible.",1311.1118v1
2013-11-06,Magnetization dynamics of cobalt grown on graphene,"Ferromagnetic resonance (FMR) spin pumping is a rapidly growing field which
has demonstrated promising results in a variety of material systems. This
technique utilizes the resonant precession of magnetization in a ferromagnet to
inject spin into an adjacent non-magnetic material. Spin pumping into graphene
is attractive on account of its exceptional spin transport properties. This
article reports on FMR characterization of cobalt grown on CVD graphene and
examines the validity of linewidth broadening as an indicator of spin pumping.
In comparison to cobalt samples without graphene, direct contact
cobalt-on-graphene exhibits increased FMR linewidth--an often used signature of
spin pumping. Similar results are obtained in Co/MgO/graphene structures, where
a 1nm MgO layer acts as a tunnel barrier. However, SQUID, MFM, and Kerr
microscopy measurements demonstrate increased magnetic disorder in cobalt grown
on graphene, perhaps due to changes in the growth process and an increase in
defects. This magnetic disorder may account for the observed linewidth
enhancement due to effects such as two-magnon scattering or mosaicity. As such,
it is not possible to conclude successful spin injection into graphene from FMR
linewidth measurements alone.",1311.1441v1
2013-11-11,Color Magnetism in Non-Abelian Vortex Matter,"We propose color magnetism as a generalization of the ordinary Heisenberg
(anti-)ferro magnets on a triangular lattice. Vortex matter consisting of an
Abrikosov lattice of non-Abelian vortices with color magnetic fluxes shows a
color ferro or anti-ferro magnetism, depending on the interaction among the
vortex sites. A prime example is a non-Abelian vortex lattice in rotating dense
quark matter, showing a color ferromagnetism. We show that the low-energy
effective theory for the vortex lattice system in the color ferromagnetic phase
is described by a 3+1 dimensional $CP^{N-1}$ nonlinear sigma model with
spatially anisotropic couplings. We identify gapless excitations independent
from Tkachenko modes as color magnons, that is, Nambu-Goldstone modes
propagating in the vortex lattice with an anisotropic linear dispersion
relation $\omega_p^2 = c_{xy}^2(p_x^2+p_y^2) + c_z^2 p_z^2$. We calculate the
transition temperature between the ordered and disordered phases, and apply it
to dense quark matter. We also identify the order parameter spaces for color
anti-ferromagnets.",1311.2399v2
2013-11-21,Mechanism of Ultrafast Relaxation of a Photo-Carrier in Antiferromagnetic Spin Background,"We study the relaxation mechanism of a highly excited carrier propagating in
the antiferromagnetic background modeled by the $t$-$J$ Hamiltonian on a square
lattice. We show that the relaxation consists of two distinct stages. The
initial ultrafast stage with the relaxation time $\tau\sim
(\hbar/t_0)(J/t_0)^{-2/3}$ (where $t_0$ is the hopping integral and $J$ is the
exchange interaction) is based on generation of string states in the close
proximity of the carrier. This unusual scaling of $\tau$ is obtained by means
of comparison of numerical results with a simplified $t$-$J_z$ model on a Bethe
lattice. In the subsequent (much slower) stage local spin excitations are
carried away by magnons. The relaxation time on the two-leg ladder system is an
order of magnitude longer due to the lack of string excitations. This further
reinforces the importance of string excitations for the ultrafast relaxation in
the two-dimensional system.",1311.5574v2
2013-11-22,Large-Spin Expansions of GKP Strings,"We demonstrate that the large-spin expansion of the energy of
Gubser-Klebanov-Polyakov (GKP) strings that rotate in RxS2 and AdS3 can be
expressed in terms of Lambert's W-function. We compute the leading, subleading
and next-to-subleading series of exponential corrections to the infinite-volume
dispersion relation of GKP strings that rotate in RxS2. These strings are dual
to certain long operators of N=4 SYM theory and provide their scaling
dimensions at strong coupling. We also show that the strings obey a short-long
(strings) duality. For the folded GKP strings that spin inside AdS3 and are
dual to twist-2 operators, we confirm the known formulas for the leading and
next-to-leading coefficients of their anomalous dimensions and derive the
corresponding expressions for the next-to-next-to-leading coefficients.",1311.5800v3
2013-11-25,Spin-wave excitation and propagation in microstructured waveguides of yttrium iron garnet (YIG)/Pt bilayers,"We present an experimental study of spin-wave excitation and propagation in
microstructured waveguides patterned from a 100 nm thick yttrium iron garnet
(YIG)/platinum (Pt) bilayer. The life time of the spin waves is found to be
more than an order of magnitude higher than in comparably sized metallic
structures despite the fact that the Pt capping enhances the Gilbert damping.
Utilizing microfocus Brillouin light scattering spectroscopy, we reveal the
spin-wave mode structure for different excitation frequencies. An exponential
spin-wave amplitude decay length of 31 {\mu}m is observed which is a
significant step towards low damping, insulator based micro-magnonics.",1311.6305v1
2013-12-03,Stoner instability revisited: Emergence of local quantum criticality?,"We revisit Stoner instability, an old problem but in a modern point of view.
An idea is to extract out dynamics of directional fluctuations of spins
explicitly, resorting to the CP$^{1}$ representation and integrating over their
amplitude fluctuations. As a result, we derive an effective field theory for
ferromagnetic quantum phase transitions in terms of bosonic spinons and
fermionic holons. We show that this effective field theory reproduces
overdamped spin dynamics in a paramagnetic Fermi liquid and magnon spectrum in
a ferromagnetic Fermi liquid. An interesting observation is that the velocity
of spinons becomes zero, approaching the ferromagnetic quantum critical point,
which implies emergence of local quantum criticality. Based on this scenario,
we predict the $\omega/T$ scaling behavior near ferromagnetic quantum
criticality beyond the conventional scenario of the weak-coupling approach.",1312.0807v2
2013-12-14,Tuning Magnetic Coupling in Sr$_2$IrO$_4$ Thin Films with Epitaxial Strain,"We report x-ray resonant magnetic scattering (XRMS) and resonant inelastic
x-ray scattering (RIXS) studies of epitaxially-strained $\mathrm{Sr_2IrO_4}$
thin films. The films were grown on $\mathrm{SrTiO_3}$ and
$\mathrm{(LaAlO_3)_{0.3}(Sr_2AlTaO_6)_{0.7}}$ substrates, under slight tensile
and compressive strains, respectively. Although the films develop a magnetic
structure reminiscent of bulk $\mathrm{Sr_2IrO_4}$, the magnetic correlations
are extremely anisotropic, with in-plane correlation lengths significantly
longer than the out-of-plane correlation lengths. In addition, the compressive
(tensile) strain serves to suppress (enhance) the magnetic ordering temperature
$\mathrm{T_N}$, while raising (lowering) the energy of the zone boundary
magnon. Quantum chemical calculations show that the tuning of magnetic energy
scales can be understood in terms of strain-induced change in bond lengths.",1312.4005v2
2013-12-17,Superconducting pairing in the spin-density-wave phase of iron pnictides,"Some of the iron pnictides show coexisting superconductivity and
spin-density-wave order. We study the superconducting pairing instability in
the spin-density-wave phase. Assuming that the pairing interaction is due to
spin fluctuations, we calculate the effective pairing interactions in the
singlet and triplet channels by summing the bubble and ladder diagrams taking
the reconstructed band structure into account. The leading pairing
instabilities and the corresponding superconducting gap structures are then
obtained from the superconducting gap equation. We illustrate this approach for
a minimal two-band model of the pnictides. Analytical and numerical results
show that the existence of propagating magnons in the spin-density-wave phase
strongly enhances the pairing in both the singlet and the spin s_z=0 triplet
channel. Over a limited parameter range, a spin s_z=0 triplet p_x-wave state is
the dominant instability. It competes with various singlet states, which have
mostly s^\pm-type structures. We analyze the effect of various symmetry-allowed
interactions on the pairing in some detail.",1312.4720v2
2013-12-18,"Multipolar, magnetic and vibrational lattice dynamics in the low temperature phase of uranium dioxide","We report the results of inelastic neutron scattering experiments performed
with triple-axis spectrometers to investigate the low-temperature collective
dynamics in the ordered phase of uranium dioxide. The results are in excellent
agreement with the predictions of mean-field RPA calculations emphasizing the
importance of multipolar superexchange interactions. By comparing neutron
scattering intensities in different polarization channels and at equivalent
points in different Brillouin zones, we show the mixed
magneto-vibrational-quadrupolar character of the observed excitations. The high
energy resolution afforded by the cold triple-axis spectrometer allowed us to
study in detail the magnon-phonon interaction giving rise to avoided crossings
along the $[00\xi]$ reciprocal space direction.",1312.5113v1
2014-01-05,Spin torque building blocks,"The discovery of the spin torque effect has made magnetic nanodevices
realistic candidates for active elements of memory devices and applications.
Magnetoresistive effects allow the read-out of increasingly small magnetic
bits, and the spin torque provides an efficient tool to manipulate - precisely,
rapidly and at low energy cost - the magnetic state, which is in turn the
central information medium of spintronic devices. By keeping the same magnetic
stack, but by tuning a device's shape and bias conditions, the spin torque can
be engineered to build a variety of advanced magnetic nanodevices. Here we show
that by assembling these nanodevices as building blocks with different
functionalities, novel types of computing architectures can be envisisaged. We
focus in particular on recent concepts such as magnonics and spintronic neural
networks.",1401.0874v1
2014-02-14,Spin liquid phase in a spatially anisotropic frustrated antiferromagnet,"We explore the effect of the third nearest-neighbors on the magnetic
properties of the Heisenberg model on an anisotropic triangular lattice. We
obtain the phase diagram of the model using Schwinger-boson mean-field theory.
Competition between N\'eel, spiral and collinear magnetically ordered phases is
found as we vary the on the ratios of the nearest, J1, next-nearest, J2, and
third-nearest, J_3, neighbor exchange couplings. A spin liquid phase is
stabilized between the spiral and collinear ordered states when J2/J1 < 1.8 for
rather small J3/J1 < 0.1. The lowest energy two-spinon dispersions relevant to
neutron scattering experiments are analyzed and compared to semiclassical
magnon dispersions finding significant differences in the spiral and collinear
phases between the two approaches. The results are discussed in the context of
the anisotropic triangular materials: Cs2CuCl4 and Cs2CuBr4 and layered organic
materials, kappa-(BEDT-TTF)2X and Y[Pd(dmit)2]2.",1402.3463v1
2014-02-24,Spectrally-resolved femtosecond reflectivity relaxation dynamics in undoped SDW 122-structure iron based pnictides,"We systematically investigate temperature- and spectrally-dependent optical
reflectivity dynamics in AAs$_{2}$Fe$_{2}$, (A=Ba, Sr and Eu), iron-based
superconductors parent spin-density-wave (SDW) compounds. Two different
relaxation processes are identified. The behavior of the slower process, which
is strongly sensitive to the magneto-structural transition, is analyzed in the
framework of the relaxation-bottleneck model involving magnons. The results are
compared to recent time resolved angular photoemission results (TR-ARPES) and
possible alternative assignment of the slower relaxation to the
magneto-structural order parameter relaxation is discussed.",1402.5811v1
2014-02-25,Unusual Two-stage Dynamics of the Spin-Lattice Polaron Formation,"We follow the formation of a spin-lattice polaron after a quantum quench that
simulates absorption of the pump{pulse in the time-resolved experiments. We
discover a two-stage relaxation where spin and lattice degrees of freedom
represent an integral part of the relaxation mechanism. In the first stage the
kinetic energy of the spin-lattice polaron relaxes towards its ground state
value while relaxation processes via spin and phonon degrees of freedom remain
roughly independent. In the second, typically much longer stage, a subsequent
energy transfer between lattice and spin degrees of freedom via the charge
carrier emerges. The excess local spin energy radiates away via magnon
excitations.",1402.6104v1
2014-02-27,Relation between cuprate superconductivity and magnetism: A Raman study of (CaLa)(BaLa)$_{2}$Cu$_{3}$O$_{y}$,"We present an investigation of charge-compensated antiferromagnetic
(Ca$_{x}$La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x}$)Cu$_{3}$O$_{y}$ single crystals
using Raman scattering as well as muon spin rotation. In this system the
parameter $x$ controls the Cu-O-Cu superexchange interaction via bond distances
and buckling angles. The oxygen content $y$ controls the charge doping. In the
absence of doping the two-magnon peak position is directly proportional to the
superexchange strength $J$. We find that both $x$ and $y$ affect the peak
position considerably. The N\'{e}el temperature determined from muon spin
rotation on the same samples independently confirms the strong dependence of
the magnetic interaction on $x$ and $y$. We find a considerable increase in the
maximum superconducting transition temperature $T_{c}^{max}$ with $J$. This is
strong evidence of the importance of orbital overlap to superconductivity in
this family of cuprates.",1402.7053v2
2014-07-10,Nonsinglet pentagons and NMHV amplitudes,"Scattering amplitudes in maximally supersymmetric gauge theory receive a dual
description in terms of the expectation value of the super Wilson loop
stretched on a null polygonal contour. This makes the analysis amenable to
nonperturbative techniques. Presently, we elaborate on a refined form of the
operator product expansion in terms of pentagon transitions to compute
twist-two contributions to NMHV amplitudes. To start with, we provide a novel
derivation of scattering matrices starting from Baxter equations for flux-tube
excitations propagating on magnon background. We propose bootstrap equations
obeyed by pentagon form factors with nonsinglet quantum numbers with respected
to the R-symmetry group and provide solutions to them to all orders in 't Hooft
coupling. These are then successfully confronted against available perturbative
calculations for NMHV amplitudes to three-loop order.",1407.2853v3
2014-07-14,Electronic excitations and structure of Li2IrO3 thin films grown on ZrO$_{2}$:Y (001) substrates,"Thin films are a prerequisite for application of the emergent exotic ground
states in iridates that result from the interplay of strong spin-orbit coupling
and electronic correlations. We report on pulsed laser deposition of
Li$_{2}$IrO$_{3}$ films on ZrO$_{2}$:Y(001) single crystalline substrates.
X-ray diffraction confirms preferential (001) and (10-1) out-of-plane
crystalline orientations with well defined in-plane orientation. Resistivity
between 35 and 300 K is dominated by a three-dimensional variable range hopping
mechanism. The dielectric function is determined by means of spectroscopic
ellipsometry and, complemented by Fourier transform infrared transmission
spectroscopy, reveals a small optical gap of $\approx$ 300 meV, a splitting of
the $5d$-$t_{2g}$ manifold, and several in-gap excitations attributed to
phonons and possibly magnons.",1407.3596v4
2014-08-04,Excitonic quasiparticles in a spin-orbit Mott insulator,"In condensed matter systems, out of a large number of interacting degrees of
freedom emerge weakly coupled particles, in terms of which most physical
properties are described. For example, Landau quasiparticles (QP) determine all
electronic properties of a normal metal. The lack of identification of such QPs
is major barrier for understanding myriad exotic properties of correlated
electrons, such as unconventional superconductivity and non-Fermi liquid
behaviours. Here, we report the observation of a composite particle in a Mott
insulator Sr2IrO4---and exciton dressed with magnons---that propagates with the
canonical characteristics of a QP: a finite QP residue and a lifetime longer
than the hopping time scale. The dynamics of this charge-neutral bosonic
excitation mirrors the fundamental process of the analogous one-hole
propagation in the background of ordered spins, for which a well-defined QP has
never been observed. The much narrower linewidth of the exciton reveals the
same intrinsic dynamics that is obscured for the hole and is intimately related
to the mechanism of high temperature superconductivity.",1408.0804v1
2014-08-21,Anyonic Liquids in Nearly Saturated Spin Chains,"Most Heisenberg-like spin chains flow to a universal free-fermion fixed point
near the magnetic-field induced saturation point. Here we show that an exotic
fixed point, characterized by two species of low-energy excitations with mutual
anyonic statistics, may also emerge in such spin chains if the dispersion
relation has two minima. By using bosonization, two-magnon exact calculations,
and numerical density-matrix-renormalization-group, we demonstrate the
existence of this anyonic-liquid fixed point in an XXZ spin chain with up to
second neighbor interactions. We also identify a range of microscopic
parameters, which support this phase.",1408.5151v2
2014-09-05,Electric field-induced Skyrmion distortion and giant lattice rotation in the magnetoelectric insulator Cu2OSeO3,"Uniquely in Cu2OSeO3, the Skyrmions, which are topologically protected
magnetic spin vortex-like objects, display a magnetoelectric coupling and can
be manipulated by externally applied electric (E) fields. Here, we explore the
E-field coupling to the magnetoelectric Skyrmion lattice phase, and study the
response using neutron scattering. Giant E-field induced rotations of the
Skyrmion lattice are achieved that span a range of $\sim$25$^{\circ}$.
Supporting calculations show that an E-field-induced Skyrmion distortion lies
behind the lattice rotation. Overall, we present a new approach to Skyrmion
control that makes no use of spin-transfer torques due to currents of either
electrons or magnons.",1409.1756v1
2014-09-08,Dynamic control of spin wave spectra using spin-polarized currents,"We describe a method of controlling the spin wave spectra dynamically in a
uniform nanostripe waveguide through spin-polarized currents. A stable periodic
magnetization structure is observed when the current flows vertically through
the center of nanostripe waveguide. After being excited, the spin wave is
transmitted at the sides of the waveguide. Numerical simulations of spin-wave
transmission and dispersion curves reveal a single, pronounced band gap.
Moreover, the periodic magnetization structure can be turned on and off by the
spin-polarized current. The switching process from full rejection to full
transmission takes place within less than 3ns. Thus, this type magnonic
waveguide can be utilized for low-dissipation spin wave based filters.",1409.2421v2
2014-09-15,Frustration and Dzyaloshinsky-Moriya anisotropy in the kagome francisites Cu$_3$Bi(SeO$_3)_2$O$_2$X,"We investigate the antiferromagnetic canting instability of the spin-1/2
kagome ferromagnet, as realized in the layered cuprates
Cu$_3$Bi(SeO$_3)_2$O$_2$X (X=Br, Cl, and I). While the local canting can be
explained in terms of competing exchange interactions, the direction of the
ferrimagnetic order parameter fluctuates strongly even at short distances on
account of frustration which gives rise to an infinite ground state degeneracy
at the classical level. In analogy with the kagome antiferromagnet, the
accidental degeneracy is fully lifted only by non-linear 1/S corrections,
rendering the selected uniform canted phase very fragile even for spins-1/2, as
shown explicitly by coupled-cluster calculations. To account for the observed
ordering, we show that the minimal description of these systems must include
the microscopic Dzyaloshinsky-Moriya interactions, which we obtain from
density-functional band-structure calculations. The model explains all
qualitative properties of the kagome francisites, including the detailed nature
of the ground state and the anisotropic response under a magnetic field. The
predicted magnon excitation spectrum and quantitative features of the
magnetization process call for further experimental investigations of these
compounds.",1409.4335v1
2014-10-03,Neutron Scattering Studies of the Ferroelectric Distortion and Spin Dynamics in the Type-1 Multiferroic Perovskite Sr0.56Ba0.44MnO3,"The magnetic order, spin dynamics, and crystal structure of the multiferroic
Sr0.56Ba0.44MnO3 have been investigated using neutron and x-ray scattering.
Ferroelectricity develops at TC=305 K with a polarization of 4.2 microC/cm2
associated with the displacements of the Mn ions, while the Mn4+ spins order
below TN = 200 K into a simple G-type commensurate magnetic structure. Below TN
the ferroelectric order decreases dramatically demonstrating that the two order
parameters are strongly coupled. The ground state spin dynamics are
characterized by a spin gap of 4.6(5) meV and the magnon density of states
peaking at 43 meV. Detailed spin wave simulations with a gap and isotropic
exchange of J=4.8(2) meV describe the excitation spectrum well. Above TN strong
spin correlations coexist with robust ferroelectric order.",1410.0922v1
2014-10-03,On a family of symmetric rational functions,"This paper is about a family of symmetric rational functions that form a
one-parameter generalization of the classical Hall-Littlewood polynomials. We
introduce two sets of (skew and non-skew) functions that are akin to P and Q
Hall-Littlewood polynomials. We establish (a) a combinatorial formula that
represents our functions as partition functions for certain path ensembles in
the square grid; (b) symmetrization formulas for non-skew functions; (c)
identities of Cauchy and Pieri type; (d) explicit formulas for principal
specializations; (e) two types of orthogonality relations for non-skew
functions.
  Our construction is closely related to the half-infinite volume, finite
magnon sector limit of the higher spin six-vertex (or XXZ) model, with both
sets of functions representing higher spin six-vertex partition functions
and/or transfer-matrices for certain domains.",1410.0976v1
2014-10-24,Non-linear spin-wave excitation at low bias fields,"Non-linear magnetization dynamics is essential for the operation of many
spintronics devices. For microwave assisted switching of magnetic elements the
low field regime is of particular interest. In addition a large number of
experiments uses high amplitude FMR in order to generate d.c. currents via spin
pumping mechanism. Here we use time resolved X-ray magnetic circular dichroism
experiments to determine the number density of excited magnons in magnetically
soft Ni_80Fe_20 thin films at small bias fields and large rf-excitation
amplitudes. Our data shows that the common model of non-linear ferromagnetic
resonance is not suitable to describe the low bias field limit. Here we derive
a new model of parametric spin-wave excitation which correctly predicts
threshold amplitudes and decay rates also at low bias fields. In fact a new
series of critical modes with amplitude phase oscillations is found,
generalizing the theory of parametric spin-wave excitation.",1410.6693v1
2014-11-04,A micro-sized parametric spin-wave amplifier,"We present the experimental observation of the localized amplification of
externally excited spin waves in a transversely in-plane magnetized
Ni$_{81}$Fe$_{19}$ magnonic waveguide by means of parallel pumping. By
employing microfocussed Brillouin light scattering spectroscopy, we analyze the
dependency of the amplification on the applied pumping power and on the delay
between the input spin-wave packet and the pumping pulse. We show that there
are two different operation regimes: At large pumping powers, the spin-wave
packet needs to enter the amplifier before the pumping is switched on in order
to be amplified while at low powers the spin-wave packet can arrive at any time
during the pumping pulse.",1411.0830v1
2014-11-12,Electric-Field Control over Spin-Wave and Current Induced Domain Wall Motion and Magnonic Torques in Multiferroics,"We discover that the way spin-waves exert magnetic torques in multiferroic
materials can cause not only domain wall motion, but also magnetization
dynamics for homogeneous magnetization textures. Interestingly, the domain wall
motion can be controlled via purely electrical means with the spin-waves being
generated by an ac electric field $E$ while the direction of the wall motion
also is sensitive to an applied dc $E$ field. Moreover, we determine the
interaction between spin-transfer torque from an electric current and a
magnetic domain wall in multiferroics and show that the Walker breakdown
threshold scales with the magnitude of a perpendicular electric field, offering
a way to control the properties of domain wall propagation via electric gating.",1411.3327v2
2014-12-11,Non-collinear Magnetic Configurations at Finite Temperature in Thin Films,"The finite-temperature magnetism of a monolayer on a bcc (110) surface was
examined using a model Hamiltonian containing ferromagnetic or
antiferromagnetic exchange interactions, Dzyaloshinsky-Moriya interactions and
easy-axis on-site anisotropy. We examined the competition between the collinear
ground state parallel to the easy axis and the spin spiral state in the plane
perpendicular to this axis preferred by the Dzyaloshinsky-Moriya interaction.
Using approximative methods to calculate the magnon spectrum at finite
temperatures, it was found that even if the ground state is collinear,
increasing the Dzyaloshinsky-Moriya interaction strongly decreases the critical
temperature where this collinear order disappears. Using atomistic spin
dynamics simulations it was found that at this critical temperature the system
transforms into the non-collinear state. Including external magnetic field
helps stabilising the ferromagnetic state. An effect due to the finite size of
the magnetic monolayer was included in the model by considering a different
value for the anisotropy at the edges of the monolayer. This effect was shown
to stabilize the spin spiral state by fixing the phase at the ends of the
stripe.",1412.3591v1
2014-12-12,Asymmetric spin-wave dispersion due to Dzyaloshinskii-Moriya interaction in an ultrathin Pt/CoFeB film,"Employing Brillouin spectroscopy, strong interfacial Dzyaloshinskii-Moriya
interactions have been observed in an ultrathin Pt/CoFeB film. Our
micromagnetic simulations show that spin-wave nonreciprocity due to asymmetric
surface pinning is insignificant for the 0.8nmthick CoFeB film studied. The
observed high asymmetry of the monotonic spin wave dispersion relation is thus
ascribed to strong Dzyaloshinskii-Moriya interactions present at the Pt/CoFeB
interface. Our findings should further enhance the significance of CoFeB as an
important material for magnonic, spintronic and skyrmionic applications.",1412.3907v2
2014-12-18,Exchange Magnon-Polaritons in Microwave Cavities,"We formulate a scattering theory to study magnetic films in microwave
cavities beyond the independent-spin and rotating wave approximations of the
Tavis-Cummings model. We demonstrate that strong coupling can be realized not
only for the ferromagnetic resonance (FMR) mode, but also for spin wave
resonances (SWRs); the coupling strengths are mode dependent and decrease with
increasing mode index. The strong coupling regime can be also accessed
electrically by spin pumping into a metal contact.",1412.5809v3
2015-01-08,Fractional excitations in the square-lattice quantum antiferromagnet,"The square-lattice quantum Heisenberg antiferromagnet displays a pronounced
anomaly of unknown origin in its magnetic excitation spectrum. The anomaly
manifests itself only for short wavelength excitations propagating along the
direction connecting nearest neighbors. Using polarized neutron spectroscopy,
we have fully characterized the magnetic fluctuations in the model
metal-organic compound CFTD, revealing an isotropic continuum at the anomaly
indicative of fractional excitations. A theoretical framework based on the
Gutzwiller projection method is developed to explain the origin of the
continuum at the anomaly. This indicates that the anomaly arises from
deconfined fractional spin-1/2 quasiparticle pairs, the 2D analog of 1D
spinons. Away from the anomaly the conventional spin-wave spectrum is recovered
as pairs of fractional quasiparticles bind to form spin-1 magnons. Our results
therefore establish the existence of fractional quasiparticles in the simplest
model two dimensional antiferromagnet even in the absence of frustration.",1501.01767v1
2015-01-16,"Spectroscopic Studies of Iron-based Superconductors, Multi-ferroic Oxides and Double-perovskite: Phonons, Electronic and Spin Excitations","Raman spectroscopy is a very powerful probe to study the nature of
quasi-particle excitations in condensed matter physics. The work presented in
this thesis is focused on two different families of novel materials, namely the
iron-based superconductors (FeBS), multiferroic oxides and double perovskite.
Although the properties of these two systems are quite different, some
comparison can still be drawn between them. For instance, in both of these
systems magnetism plays a crucial role and intricate coupling between phononic,
magnetic and orbital degrees of freedom is crucial to understand their
underlying physics responsible for their various exotic physical properties.
Understanding the microscopic origin of quasi-particle excitations, such as
phonons, magnons, orbitons, plasmons etc., and coupling between them in these
complex materials, has been an intense field of research because it is believed
that these excitations hold the key for explaining their rich physics. The
systems studied in the thesis include (A) FeBS - (i) FeSe0.82 (ii)
Ce1-zYzFeAsO1-xFx (z = 0, 0.4; x = 0.1, 0.2) (iii) Ca4Al2O5.7Fe2As2 (iv)
Ca(Fe1-xCox)2As2 (x = 0.03, 0.05). (B) Multiferroic oxides - (i) AlFeO3 (ii)
TbMnO3 and double perovskite (iii) La2NiMnO6.",1501.04055v1
2015-01-27,Conformal mass in Einstein-Gauss-Bonnet AdS gravity,"In this paper, we show that the physical information given by conserved
charges for asymptotically AdS spacetimes in Einstein-Gauss-Bonnet AdS gravity
is encoded in the electric part of the Weyl tensor. This result generalizes the
conformal mass definition by Ashtekar-Magnon-Das (AMD) to a gravity theory with
a Gauss-Bonnet term. This proof makes use of the Noether charges obtained from
an action renormalized by the addition of counterterms which depend on the
extrinsic curvature (Kounterterms). If the asymptotic fall-off behaviour of the
Weyl tensor is same as the one considered in the AMD method, then the
Kounterterm charges and the AMD charges agree in any dimension.",1501.06861v2
2015-02-02,Some three-point correlation functions in the eta-deformed AdS_5 x S^5,"We compute some normalized structure constants in the $\eta$-deformed
$AdS_5\times S^5$ in the framework of the semiclassical approach. This is done
for the cases when the ""heavy"" string states are finite-size giant magnons
carrying one angular momentum and for three different choices of the ""light""
state: primary scalar operators, dilaton operator with nonzero momentum,
singlet scalar operators on higher string levels. Since the dual field theory
is still unknown, the results obtained here must be considered as conjectures
or as predictions from the string theory side.
  Keywords: Gauge/string duality, Correlation functions
  PACS:11.25.-w, 11.25.Tq",1502.00610v4
2015-02-19,The thermal Hall effect of spin excitations in a Kagome magnet,"At low temperatures, the thermal conductivity of spin excitations in a
magnetic insulator can exceed that of phonons. However, because they are charge
neutral, the spin waves are not expected to display a thermal Hall effect in a
magnetic field. Recently, this semiclassical notion has been upended in quantum
magnets in which the spin texture has a finite chirality. In the Kagome
lattice, the chiral term generates a Berry curvature. This results in a thermal
Hall conductivity $\kappa_{xy}$ that is topological in origin. Here we report
observation of a large $\kappa_{xy}$ in the Kagome magnet Cu(1-3, bdc) which
orders magnetically at 1.8 K. The observed $\kappa_{xy}$ undergoes a remarkable
sign-reversal with changes in temperature or magnetic field, associated with
sign alternation of the Chern flux between magnon bands. We show that thermal
Hall experiments probe incisively the effect of Berry curvature on heat
transport.",1502.05688v1
2015-02-20,Magneto-Plasmonics and Resonant Interaction of Light with Dynamic Magnetisation in Metallic and All-Dielectric Nanostructures (Review),"A significant interest in combining plasmonics and magnetism at the nanoscale
gains momentum in both photonics and magnetism sectors that are concerned with
the resonant enhancement of light-magnetic-matter interaction in
nanostructures. These efforts result in a considerable amount of literature,
which is difficult to collect and digest in limited time. Furthermore, there is
insufficient exchange of results between the two research sectors.
Consequently, the goal of this review paper is to bridge this gap by presenting
an overview of recent progress in the field of magneto-plasmonics from two
different points of view: magneto-plasmonics, and magnonics and magnetisation
dynamics. It is expected that this presentation style will make this review
paper of particular interest to both general physical audience and specialists
conducting research on photonics, plasmonics, Brillouin light scattering
spectroscopy of magnetic nanostructures and magneto-optical Kerr effect
magnetometry, as well as ultrafast all-optical and THz-wave excitation of spin
waves. Moreover, readers interested in a new, rapidly emerging field of
all-dielectric nanophotonics will find a section about all-magneto-dielectric
nanostructures.",1502.05783v1
2015-03-05,Casimir effect mechanism of pairing between fermions in the vicinity of a magnetic quantum critical point,"We consider two immobile spin $1/2$ fermions in a two-dimensional magnetic
system that is close to the $ O(3)$ magnetic quantum critical point (QCP) which
separates magnetically ordered and disordered phases. Focusing on the
disordered phase in the vicinity of the QCP, we demonstrate that the
criticality results in a strong long range attraction between the fermions,
with potential $V(r) \propto -1/r^{\nu}$, $\nu \approx 0.75$, where $r$ is
separation between the fermions. The mechanism of the enhanced attraction is
similar to Casimir effect and corresponds to multi-magnon exchange processes
between the fermions. While we consider a model system, the problem is
originally motivated by recent establishment of magnetic QCP in hole doped
cuprates under the superconducting dome at doping of about 10\%. We suggest the
mechanism of magnetic critical enhancement of pairing in cuprates.",1503.01554v1
2015-03-06,Thermal conductivity through the quantum critical point in YbRh2Si2 at very low temperature,"The thermal conductivity of YbRh2Si2 has been measured down to very low
temperatures under field in the basal plane. An additional channel for heat
transport appears below 30 mK, both in the antiferromagnetic and paramagnetic
states, respectively below and above the critical field suppressing the
magnetic order. This excludes antiferromagnetic magnons as the origin of this
additional contribution to thermal conductivity. Moreover, this low temperature
contribution prevails a definite conclusion on the validity or violation of the
Wiedemann-Franz law at the field-induced quantum critical point. At high
temperature in the paramagnetic state, the thermal conductivity is sensitive to
ferromagnetic fluctuations, previously observed by NMR or neutron scattering
and required for the occurrence of the sharp electronic spin resonance
fracture.",1503.01991v1
2015-03-18,Observation of huge thermal spin currents in magnetic multilayers,"Thermal spin pumping constitutes a novel mechanism for generation of spin
currents; however their weak intensity constitutes a major roadblock for its
usefulness. We report a phenomenon that produces a huge spin current in the
central region of a multilayer system, resulting in a giant spin Seebeck effect
in a structure formed by repetition of ferromagnet/metal bilayers. The result
is a consequence of the interconversion of magnon and electron spin currents at
the multiple interfaces. This work opens the possibility to design thin film
heterostructures that may boost the application of thermal spin currents in
spintronics.",1503.05594v1
2015-03-19,Half-Quantum Vortices in an Antiferromagnetic Spinor Bose-Einstein Condensate,"We report on the observation of half-quantum vortices (HQVs) in the
easy-plane polar phase of an antiferromagnetic spinor Bose-Einstein condensate.
Using in situ magnetization-sensitive imaging, we observe that pairs of HQVs
with opposite core magnetization are generated when singly charged quantum
vortices are injected into the condensate. The dynamics of HQV pair formation
is characterized by measuring the temporal evolutions of the pair separation
distance and the core magnetization, which reveals the short-range nature of
the repulsive interactions between the HQVs. We find that spin fluctuations
arising from thermal population of axial magnon excitations do not
significantly affect the HQV pair formation dynamics. Our results demonstrate
the instability of a singly charged vortex in the antiferromagnetic spinor
condensate.",1503.05648v3
2015-04-23,Three-Dimensional Crystallization of Vortex Strings in Frustrated Quantum Magnets,"We demonstrate that frustrated exchange interactions can produce exotic 3D
crystals of vortex strings near the saturation field (H=Hsat) of body- and
face-centered cubic Mott insulators. The combination of cubic symmetry and
frustration leads to a magnon spectrum of the fully polarized spin state
(H>Hsat) with degenerate minima at multiple noncoplanar Q vectors. This
spectrum becomes gapless at the quantum critical point H=Hsat and the magnetic
ordering below Hsat can be formally described as a condensate of a dilute gas
of bosons. By expanding in the lattice gas parameter, we find that different
vortex crystals span sizable regions of the phase diagrams for isotropic
exchange and are further stabilized by symmetric exchange anisotropy.",1504.06270v2
2015-05-04,Landau-Lifshitz theory of the thermomagnonic torque,"We derive the thermomagnonic torque associated with smooth magnetic textures
subjected to a temperature gradient, in the framework of the stochastic
Landau-Lifshitz-Gilbert equation. Our approach captures on equal footing two
distinct contributions: (1) A local entropic torque that is caused by a
temperature dependence of the effective exchange field, the existence of which
had been previously suggested based on numerics and (2) the well-known
spin-transfer torque induced by thermally-induced magnon flow. The dissipative
components of two torques have the same structure, following a common
phenomenology, but opposite signs, with the twice larger entropic torque
leading to a domain-wall motion toward the hotter region. We compare the
efficiency of the torque-driven domain-wall motion with the recently proposed
Brownian thermophoresis.",1505.00818v1
2015-05-25,Integrable open spin-chains in AdS3/CFT2 correspondences,"We study integrable open boundary conditions for d(2,1;\alpha)^2 and
psu(1,1|2)^2 spin-chains. Magnon excitations of these open spin-chains are
mapped to massive excitations of type IIB open superstrings ending on D-branes
in the AdS_3 x S^3 x S^3 x S^1 and AdS_3 x S^3 x T^4 supergravity geometries
with pure R-R flux. We derive reflection matrix solutions of the boundary
Yang-Baxter equation which intertwine representations of a variety of boundary
coideal subalgebras of the bulk Hopf superalgebra. Many of these integrable
boundaries are matched to D1- and D5-brane maximal giant gravitons.",1505.06767v3
2015-06-07,Experimental observation of the interaction of propagating spin waves with Néel domain walls in a Landau domain structure,"The interaction of propagating dipolar spin waves with magnetic domain walls
is investigated in square-shaped microstructures patterned from the Heusler
compound Co$_2$Mn$_{0.6}$Fe$_{0.4}$Si. Using magnetic force microscopy, the
reversible preparation of a Landau state with four magnetic domains separated
by N\'eel domain walls is confirmed. A local spin-wave excitation using a
microstructured antenna is realized in one of the domains. It is shown by
Brillouin light scattering microscopy (BLS) that the domain structure in the
remanence state has a strong influence on the spin-wave excitation and
propagation. The domain walls strongly reflect the spin waves and can be used
as spin-wave reflectors. A comparison with micromagnetic simulations shows that
the strong reflection is due to the long-range dipolar interaction which has
important implications for the use of these spin waves for exerting an
all-magnonic spin-transfer torque.",1506.02303v1
2015-06-18,Anomalous polarization conversion in arrays of ultrathin ferromagnetic nanowires,"We study optical properties of arrays of ultrathin nanowires by means of the
Brillouin scattering of light on magnons. We employ the Stokes/anti-Stokes
scattering asymmetry to probe the circular polarization of a local electric
field induced inside nanowires by linearly polarized light waves. We observe
the anomalous polarization conversion of the opposite sign than that in a bulk
medium or thick nanowires with a great enhancement of the degree of circular
polarization attributed to an unconventional refraction in the nanowire medium.",1506.05594v1
2015-06-19,Spin superfluidity and long-range transport in thin-film ferromagnets,"In ferromagnets, magnons may condense into a single quantum state. Analogous
to superconductors, this quantum state may support transport without
dissipation. Recent works suggest that longitudinal spin transport through a
thin-film ferromagnet is an example of spin superfluidity. Although intriguing,
this tantalizing picture ignores long-range dipole interactions; we demonstrate
that such interactions dramatically affect spin transport. In single-film
ferromagnets, ""spin superfluidity"" only exists at length scales (a few hundred
nanometers in yttrium iron garnet) somewhat larger than the exchange length.
Over longer distances, dipolar interactions destroy spin superfluidity.
Nevertheless, we predict re-emergence of spin superfluidity in tri-layer
ferromagnet--normal metal--ferromagnet films of $\sim 1\, \mu$m in size. Such
systems also exhibit other types of long-range spin transport in samples
several micrometers in size.",1506.06029v1
2015-06-22,Heat transport between antiferromagnetic insulators and normal metals,"Antiferromagnetic insulators can become active spintronics components by
controlling and detecting their dynamics via spin currents in adjacent metals.
This cross-talk occurs via spin-transfer and spin-pumping, phenomena that have
been predicted to be as strong in antiferromagnets as in ferromagnets. Here, we
demonstrate that a temperature gradient drives a significant heat flow from
magnons in antiferromagnetic insulators to electrons in adjacent normal metals.
The same coefficients as in the spin-transfer and spin-pumping processes also
determine the thermal conductance. However, in contrast to ferromagnets, the
heat is not transferred via a spin Seebeck effect which is absent in
antiferromagnetic insulator-normal metal systems. Instead, the heat is
transferred via a large staggered spin Seebeck effect.",1506.06705v1
2015-09-02,All-phononic Digital Transistor on the Basis of Gap-Soliton Dynamics in Anharmonic Oscillator Ladder,"A conceptual mechanism of amplification of phonons by phonons on the basis of
nonlinear band-gap transmission (supratransmission) phenomenon is presented. As
an example a system of weakly coupled chains of anharmonic oscillators is
considered. One (source) chain is driven harmonically by boundary with a
frequency located in the upper band close to the band edge of the ladder
system. Amplification happens when a second (gate) chain is driven by a small
signal in the counter phase and with the same frequency as first chain. If the
total driving of both chains overcomes the band-gap transmission threshold the
large amplitude band-gap soliton emerges and amplification scenario is
realized. The mechanism is interpreted as nonlinear superposition of evanescent
and propagating nonlinear modes manifesting in a single or double soliton
generation working in band-gap or band-pass regimes, respectively. The results
could be straightforwardly generalized for all-optical or all-magnonic contexts
and has all the promises for logic gate operations.",1509.00679v1
2015-09-11,Thermal generation of spin current in epitaxial CoFe2O4 thin films,"The longitudinal spin Seebeck effect (LSSE) has been investigated in
high-quality epitaxial CoFe2O4 (CFO) thin films. The thermally excited spin
currents in the CFO films are electrically detected in adjacent Pt layers due
to the inverse spin Hall effect (ISHE). The LSSE signal exhibits a linear
increase with increasing temperature gradient, yielding a LSSE coefficient of
~100 nV/K at room temperature. The temperature dependence of the LSSE is
investigated from room temperature down to 30 K, showing a significant
reduction at low temperatures, revealing that the total amount of thermally
generated magnons decreases. Furthermore, we demonstrate that the spin Seebeck
effect is an effective tool to study the magnetic anisotropy induced by
epitaxial strain, especially in ultrathin films with low magnetic moments.",1509.03601v1
2015-09-16,Microwave excitation of spin wave beams in thin ferromagnetic films,"We present an approach enabling generation of narrow spin wave beams in thin
homogeneous ferromagnetic films. The main idea is to match the wave vector of
the spin wave with that corresponding to the spectral maximum of the exciting
microwave magnetic field only locally, in the region of space from which the
beam should be launched. We show that this can be achieved with the aid of a
properly designed coplanar waveguide transducer generating a nonuniform
microwave magnetic field. The resulting two-dimensional spin wave beams
obtained in micromagnetic simulations propagate over distances of several
micrometers. The proposed approach requires neither inhomogeneity of the
ferromagnetic film nor nonuniformity of the biasing magnetic field, and it can
be generalized to yield multiple spin wave beams of different width at the same
frequency. Other possible excitation scenarios and applications of spin wave
beam magnonics are also discussed.",1509.05061v2
2015-09-17,A spin wave diode,"A diode, a device allowing unidirectional signal transmission, is a
fundamental element of logic structures and lies in the heart of modern
information systems. Spin wave or magnon, representing a collective
quasi-particle excitation of the magnetic order in magnetic materials, is a
promising candidate of information carrier for the next generation
energy-saving technologies. Here we propose a scalable and reprogrammable pure
spin wave logic hardware architecture using domain walls and surface anisotropy
stripes as waveguides on a single magnetic wafer. We demonstrate theoretically
the design principle of the simplest logic component, a spin wave diode,
utilizing the chiral bound states in a magnetic domain wall with
Dzyaloshiskii-Moriya interaction, and confirm its performance through
micromagnetic simulations. Our findings open a new vista for realizing
different types of pure spin wave logic components and finally achieving an
energy-efficient and hardware-reprogrammable spin wave computer.",1509.05095v1
2015-09-17,Antiferromagnetic Spin Wave Field-Effect Transistor,"In a collinear antiferromagnet with easy-axis anisotropy, symmetry dictates
that the spin wave modes must be doubly degenerate. Theses two modes,
distinguished by their opposite polarization and available only in
antiferromagnets, give rise to a novel degree of freedom to encode and process
information. We show that the spin wave polarization can be manipulated by an
electric field induced Dzyaloshinskii-Moriya interaction and magnetic
anisotropy. We propose a prototype spin wave field-effect transistor which
realizes a gate-tunable magnonic analog of the Faraday effect, and demonstrate
its application in THz signal modulation. Our findings open up the exciting
possibility of digital data processing utilizing antiferromagnetic spin waves
and enable the direct projection of optical computing concepts onto the
mesoscopic scale.",1509.05295v3
2015-09-21,Crystal growth and intrinsic magnetic behavior of Sr$_2$IrO$_4$,"We report on the growth of stoichiometric Sr$_2$IrO$_4$ single crystals,
which allow us to unveil their intrinsic magnetic properties. The effect of
different growth conditions has been investigated for crystals grown by the
flux method. We find that the magnetic response depends very sensitively on the
details of the growth conditions. We assess the defect concentration based on
magnetization, X-ray diffraction, Raman scattering, and optical conductivity
measurements. We find that samples with a low concentration of electronically
active defects show much reduced in-gap spectral weight in the optical
conductivity and a pronounced two-magnon peak in the Raman scattering spectrum.
A prolonged exposure at high temperature during the growth leads to higher
defect concentration likely due to creation of oxygen vacancies. We further
demonstrate a systematic intergrowth of Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$
phases by varying the growth temperature. Our results thus emphasize that
revealing the intrinsic magnetic properties of Sr$_2$IrO$_4$ and related
materials requires a scrupulous control of the crystal growth process.",1509.06342v2
2015-09-24,Magnetic behavior of volborthite Cu3V2O7(OH)2(H2O)2 determined by coupled trimers rather than frustrated chains,"Motivated by recent experiments on volborthite single crystals showing a wide
1/3-magnetization plateau, we perform microscopic modeling by means of density
functional theory (DFT) with the single-crystal structural data as a starting
point. Using DFT+U, we find four leading magnetic exchanges: antiferromagnetic
J and J2, as well as ferromagnetic J' and J1. Simulations of the derived spin
Hamiltonian show good agreement with the experimental low-field magnetic
susceptibility and high-field magnetization data. The 1/3-plateau phase
pertains to polarized magnetic trimers formed by strong J bonds. An effective
J$\rightarrow\infty$ model shows a tendency towards condensation of magnon
bound states preceding the plateau phase.",1509.07333v2
2015-10-07,"Tunable damping, saturation magnetization, and exchange stiffness of half-Heusler NiMnSb thin films","The half-metallic half-Heusler alloy NiMnSb is a promising candidate for
applications in spintronic devices due to its low magnetic damping and its rich
anisotropies. Here we use ferromagnetic resonance (FMR) measurements and
calculations from first principles to investigate how the composition of the
epitaxially grown NiMnSb influences the magnetodynamic properties of saturation
magnetization $M_S$, Gilbert damping $\alpha$, and exchange stiffness $A$.
$M_S$ and $A$ are shown to have a maximum for stoichiometric composition, while
the Gilbert damping is minimum. We find excellent quantitative agreement
between theory and experiment for $M_S$ and $\alpha$. The calculated $A$ shows
the same trend as the experimental data, but has a larger magnitude.
Additionally to the unique in-plane anisotropy of the material, these
tunabilities of the magnetodynamic properties can be taken advantage of when
employing NiMnSb films in magnonic devices.",1510.01894v1
2015-10-21,Raman study of spin excitations in the tunable quantum spin ladder Cu(Qnx)(Cl$_{1-x}$Br$_x$)$_2$,"Raman spectroscopy is used to study magnetic excitations in the quasi one
dimensional $S=1/2$ quantum spin systems Cu(Qnx)(Cl$_{1-x}$Br$_x$)$_2$. The low
energy spectrum is found to be dominated by a two-magnon continuum as expected
from the numerical calculations for the Heisenberg spin ladder model. The
continuum shifts to higher energies as more Br is introduced. The cutoff of the
scattering increases faster than the onset indicating that the increase of
exchange constant along the leg is the main effect on the magnetic properties.
The upper and lower continuum thresholds are measured as a function of Br
content across the entire range and compared to estimates based on previous
bulk studies. We observe small systematic deviations that are discussed.",1510.06360v2
2015-10-21,Reconfigurable spin wave band structure of artificial square spin ice,"Artificial square spin ices are structures composed of magnetic elements
arranged on a geometrically frustrated lattice and located on the sites of a
two-dimensional square lattice, such that there are four interacting magnetic
elements at each vertex. Using a semi-analytical approach, we show that square
spin ices exhibit a rich spin wave band structure that is tunable both by
external magnetic fields and the configuration of individual elements. Internal
degrees of freedom can give rise to equilibrium states with bent magnetization
at the edges leading to characteristic excitations; in the presence of
magnetostatic interactions these form separate bands analogous to impurity
bands in semiconductors. Full-scale micromagnetic simulations corroborate our
semi-analytical approach. Our results show that artificial square spin ices can
be viewed as reconfigurable and tunable magnonic crystals that can be used as
metamaterials for spin-wave-based applications at the nanoscale.",1510.06385v2
2016-01-06,All-electrical detection of spin dynamics in magnetic antidot lattices by the inverse spin Hall effect,"The understanding of spin dynamics in laterally confined structures on
sub-micron length scales has become a significant aspect of the development of
novel magnetic storage technologies. Numerous ferromagnetic resonance
measurements, optical characterization by Kerr microscopy and Brillouin light
scattering spectroscopy and x-ray studies were carried out to detect the
dynamics in patterned magnetic antidot lattices. Here, we investigate
Oersted-field driven spin dynamics in rectangular Ni80Fe20/Pt antidot lattices
with different lattice parameters by electrical means and compare them to
micromagnetic simulations. When the system is driven to resonance, a dc voltage
across the length of the sample is detected that changes its sign upon field
reversal, which is in agreement with a rectification mechanism based on the
inverse spin Hall effect. Furthermore, we show that the voltage output scales
linearly with the applied microwave drive in the investigated range of powers.
Our findings have direct implications on the development of engineered
magnonics applications and devices.",1601.01221v1
2016-01-12,Spin-dependent Bragg spectroscopy of a spinor Bose gas,"We develop a general theory of spin-dependent Bragg spectroscopy for spinor
Bose-Einstein condensates. This spectroscopy involves using a density and
spin-coupled optical probe to excite the system. We show that within the linear
response regime the momentum or energy transferred by the probe is determined
by a set of density and spin-density dynamic structure factors. We derive a set
of $f$-sum rules that provide rigorous constraints for the first energy moments
of these structure factors. As an application we compute the dynamic structure
factors for cases within all four distinct phases of a spin-1 condensate using
Bogoliubov theory. Our results demonstrate that spin-dependent Bragg
spectroscopy can be used to selectively investigate the various phonon and
magnon excitation branches and will be a useful tool for advancing our
understanding of spinor condensates.",1601.02716v2
2016-01-28,Spin-Cherenkov effect in a magnetic nanostrip with interfacial Dzyaloshinskii-Moriya interaction,"Spin-Cherenkov effect enables strong excitations of spin waves (SWs) with
nonlinear wave dispersions. The Dzyaloshinskii-Moriya interaction (DMI) results
in anisotropy and nonreciprocity of SWs propagation. In this work, we study the
effect of the interfacial DMI on SW Cherenkov excitations in permalloy
thin-film strips within the framework of micromagnetism. By performing
micromagnetic simulations, it is shown that coherent SWs are excited when the
velocity of a moving magnetic source exceeds the propagation velocity of the
SWs. Moreover, the threshold velocity of the moving magnetic source with finite
DMI can be reduced compared to the case of zero DMI. It thereby provides a
promising route towards efficient SW generation and propagation, with potential
applications in spintronic and magnonic devices.",1601.07755v2
2016-02-04,Investigation of the unidirectional spin heat conveyer effect in a 200nm thin Yttrium Iron Garnet film,"We have investigated the unidirectional spin wave heat conveyer effect in
sub-micron thick yttrium iron garnet (YIG) films using lock-in thermography
(LIT). Although the effect is small in thin layers this technique allows us to
observe asymmetric heat transport by magnons which leads to asymmetric
temperature profiles differing by several mK on both sides of the exciting
antenna, respectively. Comparison of Damon-Eshbach and backward volume modes
shows that the unidirectional heat flow is indeed due to non-reciprocal
spin-waves. Because of the finite linewidth, small asymmetries can still be
observed when only the uniform mode of ferromagnetic resonance is excited. The
latter is of extreme importance for example when measuring the inverse
spin-Hall effect because the temperature differences can result in
thermovoltages at the contacts. Because of the non-reciprocity these
thermovoltages reverse their sign with a reversal of the magnetic field which
is typically deemed the signature of the inverse spin-Hall voltage.",1602.01662v1
2016-02-12,Internal dipolar field and soft magnons in periodic nanocomposite magnets,"We study spin wave excitations in a three-dimensional nanocomposite magnet of
exchange coupled hard (SmCo$_5$) and soft (FeCo) phases. The dipolar
interaction splits the spin wave energies into the upper and lower branches of
the spin wave manifold. When the amount of the soft phase is increased the
energy of low-lying spin excitations is considerably softened due to two
reasons: (i) the low- lying mode locked into the soft phase region with a spin
wave gap at ${\bf k}= 0$ which scales approximately proportional to the
anisotropy constant of the soft phase and (ii) the internal dipolar field which
comes from magnetic charges forming at hard-soft boundaries with normals
parallel to the magnetization displaces the spin wave manifold toward the lower
energies. With adding more soft phase the spin wave gap closes and the system
moves to another ground state characterized by the magnetization mismatch
between spins of the hard and soft phases.",1602.04111v1
2016-02-12,Non-equilibrium itinerant-electron magnetism: a time-dependent mean-field theory,"We study the dynamical magnetic susceptibility of a strongly correlated
electronic system in the presence of a time-dependent hopping field, deriving a
generalized Bethe-Salpeter equation which is valid also out of equilibrium.
Focusing on the single-orbital Hubbard model within the time-dependent
Hartree-Fock approximation, we solve the equation in the non-equilibrium
adiabatic regime, obtaining a closed expression for the transverse magnetic
susceptibility. From this, we provide a rigorous definition of non-equilibrium
(time-dependent) magnon frequencies and exchange parameters, expressed in terms
of non-equilibrium single-electron Green functions and self-energies. In the
particular case of equilibrium, we recover previously known results.",1602.04150v3
2016-02-17,Fermionic response from fractionalization in an insulating two-dimensional magnet,"Conventionally ordered magnets possess bosonic elementary excitations, called
magnons. By contrast, no magnetic insulators in more than one dimension are
known whose excitations are not bosons but fermions. Theoretically, some
quantum spin liquids (QSLs) -- new topological phases which can occur when
quantum fluctuations preclude an ordered state -- are known to exhibit Majorana
fermions as quasiparticles arising from fractionalization of spins. Alas,
despite much searching, their experimental observation remains elusive. Here,
we show that fermionic excitations are remarkably directly evident in
experimental Raman scattering data across a broad energy and temperature range
in the two-dimensional material $\alpha$-RuCl$_3$. This shows the importance of
magnetic materials as hosts of Majorana fermions. In turn, this first
systematic evaluation of the dynamics of a QSL at finite temperature emphasizes
the role of excited states for detecting such exotic properties associated with
otherwise hard-to-identify topological QSLs.",1602.05277v1
2016-02-17,Quantum Rotors on the AB$_2$ Chain with Competing Interactions,"We present the ground state phase diagram of $q = 1/2$ quantum-rotor chains
with competing interactions (frustration) calculated through cluster
variational mean field approaches. We consider two interaction patterns, named
F$_1$ and F$_2$ models, between the quantum-rotor momentum and position
operators, which follow exchange patterns of known one-dimensional spin-1/2
systems with a ferrimagnetic state in their phase diagrams. The spin-1/2 F$_1$
model is known as the diamond chain and is related to the azurite compound,
while the spin-1/2 F$_2$ model was recently shown to present a
frustration-induced condensation of magnons. We provide a detailed comparison
between the quantum-rotor phase diagrams, in single- and multi-site mean-field
approaches, and known results for the spin-1/2 models, including exact
diagonalization and density matrix renormalization group data for these
systems, as well as phase diagrams of the associated classical models.",1602.05383v1
2016-02-24,Spin Seebeck effect at microwave frequencies,"We experimentally study the transient voltage response of yttrium iron
garnet/platinum bilayer samples subject to periodic heating up to gigahertz
frequencies. We observe an intrinsic cutoff frequency of the induced
thermopower voltage, which characteristically depends on the thickness of the
yttrium iron garnet film. The cutoff frequency reaches values of up to 350 MHz
in a 50 nm thick yttrium iron garnet film, but drops to below 1 MHz for
several-micrometer-thick films. These data corroborate the notion that the
magnon spectrum responsible for the spin current emission in the spin Seebeck
effect can be shaped by tuning the thickness of the ferromagnetic layer.",1602.07442v2
2016-02-24,Super-Poissonian shot noise of squeezed-magnon mediated spin transport,"The magnetization of a ferromagnet (F) driven out of equilibrium injects pure
spin current into an adjacent conductor (N). Such F$|$N bilayers have become
basic building blocks in a wide variety of spin based devices. We evaluate the
shot noise of the spin current traversing the F$|$N interface when F is
subjected to a coherent microwave drive. We find that the noise spectrum is
frequency independent up to the drive frequency, and increases linearly with
frequency thereafter. The low frequency noise indicates super-Poissonian spin
transfer, which results from quasi-particles with effective spin $\hbar^* =
\hbar (1 + \delta)$. For typical ferromagnetic thin films, $\delta \sim 1$ is
related to the dipolar interaction-mediated squeezing of F eigenmodes.",1602.07513v2
2016-02-25,Magnetic Mass in 4D AdS Gravity,"We provide a fully-covariant expression for the diffeomorphic charge in 4D
anti-de Sitter gravity, when the Gauss-Bonnet and Pontryagin terms are added to
the action. The couplings of these topological invariants are such that the
Weyl tensor and its dual appear in the on-shell variation of the action, and
such that the action is stationary for asymptotic (anti) self-dual solutions in
the Weyl tensor. In analogy with Euclidean electromagnetism, whenever the
self-duality condition is global, both the action and the total charge are
identically vanishing. Therefore, for such configurations the magnetic mass
equals the Ashtekhar-Magnon-Das definition.",1602.07975v2
2016-04-06,Temperature Dependence of Angular Momentum Transport Across Interfaces,"Angular momentum transport in magnetic multilayered structures plays a
central role in spintronic physics and devices. The angular momentum currents
or spin currents are carried by either quasi-particles such as electrons and
magnons, or by macroscopic order parameters such as local magnetization of
ferromagnets. Based on the generic interface exchange interaction, we develop a
microscopic theory that describes interfacial spin conductance for various
interfaces among non-magnetic metals, ferromagnetic and antiferromagnetic
insulators. Spin conductance and its temperature dependence are obtained for
different spin batteries including spin pumping, temperature gradient and spin
Hall effect. As an application of our theory, we calculate the spin current in
a trilayer made of a ferromagnetic insulator, an antiferromagnetic insulator
and a non-magnetic heavy metal. The calculated results on the temperature
dependence of spin conductance quantitatively agree with the existing
experiments.",1604.01714v2
2016-04-07,Signatures of strong correlation effects in RIXS on Cuprates,"Recently, spin excitations in doped cuprates are measured using the resonant
inelastic X-ray scattering (RIXS). The paramagnon dispersions show the large
hardening effect in the electron-doped systems and seemingly
doping-independence in the hole-doped systems, with the energy scales
comparable to that of the antiferromagnetic magnons. This anomalous hardening
effect was partially explained by using the strong coupling t-J model but with
a three-site term(Nature communications 5, 3314 (2014)). However we show that
hardening effect is a signature of strong coupling physics even without
including this extra term. By considering the t-t'-t""-J model and using the
Slave-Boson (SB) mean field theory, we obtain, via the spin-spin
susceptibility, the spin excitations in qualitative agreement with the
experiments. These anomalies is mainly due to the doping-dependent bandwidth.
We further discuss the interplay between particle-hole-like and paramagnon-like
excitations in the RIXS measurements.",1604.01858v3
2016-04-09,Interaction Corrections to Spin-Wave Theory in the Large-S Limit of the Quantum Heisenberg Ferromagnet,"The Quantum Heisenberg Ferromagnet can be naturally reformulated in terms of
interacting bosons (called spin waves or magnons) as an expansion in the
inverse spin size. We calculate the first order interaction correction to the
free energy, as an upper bound in the limit where the spin size $S \to \infty$
and $\beta S$ is fixed ($\beta$ being the inverse temperature). Our result is
valid in two and three spatial dimensions. We extrapolate our result to compare
with Dyson's low-temperature expansion. While our first-order correction has
the expected temperature dependance, in higher orders of the perturbation
theory cancellations are necessary.",1604.02548v2
2016-04-12,Ab initio determination of spin Hamiltonians with anisotropic exchange interactions: the case of the pyrochlore ferromagnet Lu$_2$V$_2$O$_7$,"We present a general framework for deriving effective spin Hamiltonians of
correlated magnetic systems based on a combination of relativistic ab initio
density functional theory calculations (DFT), exact diagonalization of a
generalized Hubbard Hamiltonian on finite clusters and spin projections onto
the low-energy subspace. A key motivation is to determine anisotropic bilinear
exchange couplings in materials of interest. As an example, we apply this
method to the pyrochlore Lu$_2$V$_2$O$_7$ where the vanadium ions form a
lattice of corner-sharing spin-1/2 tetrahedra. In this compound, anisotropic
Dzyaloshinskii-Moriya interactions (DMI) play an essential role in inducing a
magnon Hall effect. We obtain quantitative estimates of the nearest-neighbor
Heisenberg exchange, the DMI, and the symmetric part of the anisotropic
exchange tensor. Finally, we compare our results with experimental ones on the
Lu$_2$V$_2$O$_7$ compound.",1604.03472v1
2016-04-21,A novel method for the injection and manipulation of magnetic charge states in nanostructures,"Realising the promise of next-generation magnetic nanotechnologies is
contingent on the development of novel methods for controlling magnetic states
at the nanoscale. There is currently demand for simple and flexible techniques
to access exotic magnetisation states without convoluted fabrication and
application processes. 360 degree domain walls (metastable twists in
magnetisation separating two domains with parallel magnetisation) are one such
state, which is currently of great interest in data storage and magnonics.
Here, we demonstrate a straightforward and powerful process whereby a moving
magnetic charge, provided experimentally by a magnetic force microscope tip,
can write and manipulate magnetic charge states in ferromagnetic nanowires. The
method is applicable to a wide range of nanowire architectures with
considerable benefits over existing techniques. We confirm the method's
efficacy via the injection and spatial manipulation of 360 degree domain walls
in Py and Co nanowires. Experimental results are supported by micromagnetic
simulations of the tip-nanowire interaction.",1604.06374v3
2016-04-21,All-in all-out magnetic order and propagating spin-waves in Sm2Ir2O7,"Using resonant magnetic x-ray scattering we address the unresolved nature of
the magnetic groundstate and the low-energy effective Hamiltonian of
Sm$_2$Ir$_2$O$_7$, a prototypical pyrochlore iridate with a finite temperature
metal-insulator transition. Through a combination of elastic and inelastic
measurements, we show that the magnetic ground state is an all-in all-out
(AIAO) antiferromagnet. The magnon dispersion indicates significant electronic
correlations and can be well-described by a minimal Hamiltonian that includes
Heisenberg exchange ($J=27.3(6)$ meV) and Dzyaloshinskii-Moriya interaction
($D=4.9(3)$ meV), which provides a consistent description of the magnetic order
and excitations. In establishing that Sm$_2$Ir$_2$O$_7$ has the requisite
inversion symmetry preserving AIAO magnetic groundstate, our results support
the notion that pyrochlore iridates may host correlated Weyl semimetals.",1604.06401v1
2016-04-22,Nanoscale control of heat and spin conductance in artificial spin chains,"We describe a mechanism to control energy and magnetisation currents in an
artificial spin-chain, consisting of an array of Permalloy nano-disks coupled
through the magneto-dipolar interaction. The chain is kept out of equilibrium
by two thermal baths with different temperatures connected to its ends, which
control the current propagation. Transport is enhanced by applying a uniform
radio frequency pump field resonating with some of the spin-wave modes of the
chain. Moreover, the two currents can be controlled independently by tuning the
static field applied on the chain. Thus we describe two effective means for the
independent control of coupled currents and the enhancement of thermal and
spin-wave conductivity in a realistic magnonics device, suggesting that similar
effects could be observed in a large class of nonlinear oscillating systems.",1604.06572v3
2016-04-24,Coupled Spin-Light dynamics in Cavity Optomagnonics,"Experiments during the past two years have shown strong resonant
photon-magnon coupling in microwave cavities, while coupling in the optical
regime was demonstrated very recently for the first time. Unlike with
microwaves, the coupling in optical cavities is parametric, akin to
optomechanical systems. This line of research promises to evolve into a new
field of optomagnonics, aimed at the coherent manipulation of elementary
magnetic excitations by optical means. In this work we derive the microscopic
optomagnonic Hamiltonian. In the linear regime the system reduces to the
well-known optomechanical case, with remarkably large coupling. Going beyond
that, we study the optically induced nonlinear classical dynamics of a
macrospin. In the fast cavity regime we obtain an effective equation of motion
for the spin and show that the light field induces a dissipative term
reminiscent of Gilbert damping. The induced dissipation coefficient however can
change sign on the Bloch sphere, giving rise to self-sustained oscillations.
When the full dynamics of the system is considered, the system can enter a
chaotic regime by successive period doubling of the oscillations.",1604.07053v3
2016-06-09,Snell's Law for Spin Waves,"We report the experimental observation of Snell's law for magneto-static spin
waves in thin ferromagnetic Permalloy films by imaging incident, refracted and
reflected waves. We use a thickness step as the interface between two media
with different dispersion relation. Since the dispersion relation for
magneto-static waves in thin ferromagnetic films is anisotropic, deviations
from the isotropic Snell's law known in optics are observed for incidence
angles larger than 25\textdegree{} with respect to the interface normal between
the two magnetic media. Furthermore, we can show that the thickness step
modifies the wavelength and the amplitude of the incident waves. Our findings
open up a new way of spin wave steering for magnonic applications.",1606.02895v1
2016-06-16,Spin dynamics of counterrotating Kitaev spirals via duality,"Incommensurate spiral order is a common occurrence in frustrated magnetic
insulators. Typically, all magnetic moments rotate uniformly, through the same
wavevector. However the honeycomb iridates family Li2IrO3 shows an
incommensurate order where spirals on neighboring sublattices are
counter-rotating, giving each moment a different local environment.
Theoretically describing its spin dynamics has remained a challenge: the Kitaev
interactions proposed to stabilize this state, which arise from strong
spin-orbit effects, induce magnon umklapp scattering processes in spin-wave
theory. Here we propose an approach via a (Klein) duality transformation into a
conventional spiral of a frustrated Heisenberg model, allowing a direct
derivation of the dynamical structure factor. We analyze both Kitaev and
Dzyaloshinskii-Moriya based models, both of which can stabilize counterrotating
spirals, but with different spin dynamics, and we propose experimental tests to
identify the origin of counterrotation.",1606.05356v2
2016-06-28,Nature of Bosonic Excitations revealed by high-energy charge carriers,"We address a long standing problem concerning the origin of bosonic
excitations that strongly interact with charge carriers. We show that the
time-resolved pump-probe experiments are capable to distinguish between regular
bosonic degrees of freedom, e.g. phonons, and the hard-core bosons, e.g.,
magnons. The ability of phonon degrees of freedom to absorb essentially
unlimited amount of energy renders relaxation dynamics nearly independent on
the absorbed energy or the fluence. In contrast, the hard core effects pose
limits on the density of energy stored in the bosonic subsystems resulting in a
substantial dependence of the relaxation time on the fluence and/or excitation
energy. Very similar effects can be observed also in a different setup when the
system is driven by multiple pulses of equal energy.",1606.08669v1
2016-07-11,Triple-resonant Brillouin light scattering in magneto-optical cavities,"An enhancement in Brillouin light scattering of optical photons with magnons
is demonstrated in magneto-optical whispering gallery mode resonators tuned to
a triple resonance point. This occurs when both the input and output optical
modes are resonant with those of the whispering gallery resonator, with a
separation given by the ferromagnetic resonance (FMR) frequency. The
identification and excitation of specific optical modes allows us to gain a
clear understanding of the mode-matching conditions. A selection rule due to
wavevector matching leads to an intrinsic single-sideband excitation. Strong
suppression of one sideband is essential for one-to-one frequency mapping in
coherent optical-to-microwave conversion.",1607.02985v2
2016-07-11,Low loss spin wave resonances in organic-based ferrimagnet vanadium tetracyanoethylene thin films,"We experimentally demonstrate high quality factor spin wave resonances in an
encapsulated thin film of the organic-based ferrimagnet vanadium
tetracyanoethylene (V[TCNE]$_\textit{x~2}$) coated on an a-plane sapphire
substrate by low temperature chemical vapor deposition. The thickness standing
wave modes are observed in a broad frequency range (1 GHz ~ 5 GHz) with high
quality factor exceeding 3,200 in ambient air at room temperature, rivaling
those of inorganic magnetic materials. The exchange constant of
V[TCNE]$_\textit{x~2}$, a crucial material parameter for future study and
device design of the V[TCNE]$_\textit{x~2}$, is extracted from the measurement
with a value of $(4.61\pm 0.35)\times 10^{-16} \mathrm{\: m^{2}}$. Our result
establishes the feasibility of using organic-based materials for building
hybrid magnonic devices and circuits.",1607.03041v2
2016-07-19,Electrical detection of spiral spin structures in Pt|Cu_2OSeO_3 heterostructures,"The interaction between the itinerant spins in metals and localized spins in
magnetic insulators thus far has only been explored in collinear spin systems,
such as garnets. Here, we report the spin-Hall magnetoresistance (SMR)
sensitive to the surface magnetization of the spin-spiral material, Cu_2OSeO_3.
We experimentally demonstrate that the angular dependence of the SMR changes
drastically at the transition between the helical spiral and the conical spiral
phases. Furthermore, the sign and magnitude of the SMR in the conical spiral
state are controlled by the cone angle. We show that this complex behaviour can
be qualitatively explained within the SMR theory initially developed for
collinear magnets. In addition, we studied the spin Seebeck effect (SSE), which
is sensitive to the bulk magnetization. It originates from the conversion of
thermally excited low-energy spin waves in the magnet, known as magnons, into
the spin current in the adjacent metal contact (Pt). The SSE displays
unconventional behavior where not only the magnitude but also the phase of the
SSE vary with the applied magnetic field.",1607.05630v1
2017-02-02,Parameterisation of non-collinear energy landscapes in itinerant magnets,"The magnetic force theorem provides convenient ways to study exchange
interactions in magnetic systems. However, it is well known that short range
interactions in itinerant magnetic systems are poorly described with the
conventional use of the theorem and numerous strategies have been developed
over the years to overcome this deficiency. In this study, we discuss this
issue in the context of the frozen magnon method and find that a
self-consistent approach is in general preferable. Moreover, an extended
Heisenberg model is suggested in order to better describe finite deviations
from the magnetic ground state and is shown through cross-validation to give a
superior description of the interactions in non-collinear magnetic
configurations compared to the regular Heisenberg model. The present study thus
supplies a fully self-consistent method for systematic investigations of
exchange interactions beyond the standard Heisenberg model. This may prove
relevant to high-throughput computational materials science, e.g., in
developing high moment materials for the magnetic storage industry.",1702.00599v3
2017-02-08,Monte Carlo determination of the low-energy constants for a two-dimensional spin-1 Heisenberg model with spatial anisotropy,"The low-energy constants, namely the spin stiffness $\rho_s$, the staggered
magnetization density ${\cal M}_s$ per area, and the spinwave velocity $c$ of
the two-dimensional (2D) spin-1 Heisenberg model on the square and rectangular
lattices are determined using the first principles Monte Carlo method. In
particular, the studied models have antiferromagnetic couplings $J_1$ and $J_2$
in the spatial 1- and 2-directions, respectively. For each considered
$J_2/J_1$, the aspect ratio of the corresponding linear box sizes $L_2/L_1$
used in the simulations is adjusted so that the squares of the two spatial
winding numbers take the same values. In addition, the relevant finite-volume
and -temperature predictions from magnon chiral perturbation theory are
employed in extracting the numerical values of these low-energy constants. Our
results of $\rho_{s1}$ are in quantitative agreement with those obtained by the
series expansion method over a broad range of $J_2/J_1$. This in turn provides
convincing numerical evidence for the quantitative correctness of our approach.
The ${\cal M}_s$ and $c$ presented here for the spatially anisotropic models
are new and can be used as benchmarks for future related studies.",1702.02436v1
2017-02-13,Kagome-like chains with anisotropic ferromagnetic and antiferromagnetic interactions,"We consider a spin-$\frac{1}{2}$ kagome-like chain with competing ferro- and
antiferromagnetic anisotropic exchange interactions. The ground state phase
diagram of this model consists of the ferromagnetic and ferrimagnetic phases.
We study the ground state and the low-temperature properties on the phase
boundary between these phases. The ground state on this phase boundary is
macroscopically degenerate and consists of localized magnon states. We
calculate the ground state degeneracy and corresponding residual entropy. The
spontaneous magnetization has a jump on the phase boundary confirming the
first-order type of the phase transition. In the limit of a strong anisotropy
the spectrum of the low-energy excitations has multi-scale structure governing
the peculiar features of the specific heat behavior.",1702.03915v1
2017-02-20,Parametric pumping of spin waves by acoustic waves,"The linear and nonlinear interactions between spin waves (magnons) and
acoustic waves (phonons) in magnetostrictive materials provide an exciting
opportunity for realizing novel microwave signal processing devices and
spintronic circuits. Here we demonstrate the parametric pumping of spin waves
by acoustic waves, the possibility of which has long been theoretically
anticipated but never experimentally realized. Spin waves propagating in a thin
film of yttrium iron garnet (YIG), a magnetostrictive ferrimagnet with low spin
and acoustic wave damping, are pumped using an acoustic resonator driven at
frequencies near twice the spin wave frequency. The observation of a
counter-propagating idler wave and a distinct pump threshold that increases
quadratically with frequency non-degeneracy are evidence of a nonlinear
parametric pumping process consistent with classical theory. This demonstration
of acoustic parametric pumping lays the groundwork for developing new
spintronic and microwave signal processing devices based on amplification and
manipulation of spin waves by efficient, spatially localized acoustic
transducers.",1702.06038v1
2017-03-02,On the nonlinear NMR and magnon BEC in antiferromagnetic materials with coupled electron-nuclear spin precession,"We present a new study of nonlinear NMR and Bose-Einstein Condensation (BEC)
of nuclear spin waves in antiferromagnetic MnCO3 with coupled electron and
nuclear spins. In particular, we show that the observed behaviour of NMR
signals strongly contradicts the conventional description of paramagnetic
ensembles of noninteracting spins based on the phenomenological Bloch
equations. We present a new theoretical description of the coupled
electron-nuclear spin precession, which takes into account an indirect
relaxation of nuclear spins via the electron subsystem. We show that the
magnitude of the nuclear magnetization is conserved for arbitrary large
excitation powers, which is drastically different from the conventional heating
scenario derived from the Bloch equations. This provides strong evidence that
the coherent precession of macroscopic nuclear magnetization observed
experimentally can be identified with BEC of nuclear spin waves with k=0.",1703.00614v2
2017-03-06,"Spindynamics in the antiferromagnetic phases of the Dirac metals $A$MnBi$_2$ ($A=$ Sr, Ca)","The square Bi layers in $A$MnBi$_2$ ($A =$ Sr, Ca) host Dirac fermions which
coexist with antiferromagnetic order on the Mn sublattice below $T_\mathrm{N} =
290\,$K (Sr) and $270\,$K (Ca). We have measured the spin-wave dispersion in
these materials by triple-axis neutron spectroscopy. The spectra show
pronounced spin gaps of 10.2(2)$\,$meV (Sr) and 8.3(8)$\,$meV (Ca) and extend
to a maximum energy transfer of 61 - 63$\,$meV. The observed spectra can be
accurately reproduced by linear spin-wave theory from an Heisenberg effective
spin Hamiltonian. Detailed global fits of the full magnon dispersion are used
to determine the in-plane and inter-layer exchange parameters as well as on the
magnetocrystalline anisotropy constant. To within experimental error we find no
evidence that the magnetic dynamics are influenced by the Dirac fermions.",1703.01849v1
2017-03-08,"Interplay of Dirac electrons and magnetism in AMnBi2 (A=Ca, Sr)","Dirac materials exhibit intriguing low-energy carrier dynamics that offer a
fertile ground for novel physics discovery. Of particular interest is the
interplay of Dirac carriers with other quantum phenomena, such as magnetism.
Here we report on a two-magnon Raman scattering study of AMnBi2 (A=Ca, Sr), a
prototypical magnetic Dirac system comprising alternating Dirac-carrier and
magnetic layers. We present the first accurate determination of the exchange
energies in these compounds and, by comparison to the reference compound
BaMn2Bi2, we show that the Dirac-carrier layers in AMnBi2 significantly enhance
the exchange coupling between the magnetic layers, which in turn drives a
charge-gap opening along the Dirac locus. Our findings break new grounds in
unveiling the fundamental physics of magnetic Dirac materials, which offer a
novel platform for probing a distinct type of spin-Fermion interaction. The
outstanding properties of these materials allow a delicate manipulation of the
interaction between the Dirac carriers and magnetic moments, thus holding great
promise for applications in magnetic Dirac devices.",1703.02712v1
2017-03-08,Terahertz sum-frequency excitation of a Raman-active phonon,"In stimulated Raman scattering, two incident optical waves induce a force
oscillating at the difference of the two light frequencies. This process has
enabled important applications such as the excitation and coherent control of
phonons and magnons by femtosecond laser pulses. Here, we experimentally and
theoretically demonstrate the so far neglected up-conversion counterpart of
this process: THz sum-frequency excitation of a Raman-active phonon mode, which
is tantamount to two-photon absorption by an optical transition between two
adjacent vibrational levels. Coherent control of an optical lattice vibration
of diamond is achieved by an intense terahertz pulse whose spectrum is centered
at half the phonon frequency of 40 THz. Remarkably, the carrier-envelope phase
of the driving pulse is directly imprinted on the lattice vibration. New
prospects in infrared spectroscopy, light storage schemes and lattice
trajectory control in the electronic ground state emerge.",1703.02869v1
2017-03-27,Anisotropic exchange and spin-wave damping in pure and electron-doped Sr$_2$IrO$_4$,"The collective magnetic excitations in the spin-orbit Mott insulator
(Sr$_{1-x}$La$_x$)$_2$IrO$_4$ ($x=0,\,0.01,\,0.04,\, 0.1$) were investigated by
means of resonant inelastic x-ray scattering. We report significant magnon
energy gaps at both the crystallographic and antiferromagnetic zone centers at
all doping levels, along with a remarkably pronounced momentum-dependent
lifetime broadening. The spin-wave gap is accounted for by a significant
anisotropy in the interactions between $J_\text{eff}=1/2$ isospins, thus
marking the departure of Sr$_2$IrO$_4$ from the essentially isotropic
Heisenberg model appropriate for the superconducting cuprates.",1703.09051v4
2017-03-28,Temperature dependent magnetic damping of yttrium iron garnet spheres,"We investigate the temperature dependent microwave absorption spectrum of an
yttrium iron garnet sphere as a function of temperature (5 K to 300 K) and
frequency (3 GHz to 43.5 GHz). At temperatures above 100 K, the magnetic
resonance linewidth increases linearly with temperature and shows a
Gilbert-like linear frequency dependence. At lower temperatures, the
temperature dependence of the resonance linewidth at constant external magnetic
fields exhibits a characteristic peak which coincides with a non-Gilbert-like
frequency dependence. The complete temperature and frequency evolution of the
linewidth can be modeled by the phenomenology of slowly relaxing rare-earth
impurities and either the Kasuya-LeCraw mechanism or the scattering with
optical magnons. Furthermore, we extract the temperature dependence of the
saturation magnetization, the magnetic anisotropy and the g-factor.",1703.09444v2
2017-03-30,Thermal Transport in the Kitaev Model,"In conventional insulating magnets, heat is carried by magnons and phonons.
In contrast, when the magnets harbor a quantum spin liquid state, emergent
quasiparticles from the fractionalization of quantum spins can carry heat.
Here, we investigate unconventional thermal transport yielded by such exotic
carriers, in both longitudinal and transverse components, for the Kitaev model,
whose ground state is exactly shown to be a quantum spin liquid with fractional
excitations described as itinerant Majorana fermions and localized $Z_2$
fluxes. We find that the longitudinal thermal conductivity exhibits a broad
peak at very different temperatures between the zero and nonzero frequency
components, reflecting the spin fractionalization. On the other hand, the
transverse thermal conductivity induced by the magnetic field shows
nonmonotonic temperature dependence, due to thermal excitations of the
localized $Z_2$ fluxes. In the low-temperature limit, the temperature-linear
coefficient rapidly approaches a quantized value, as expected from the
topologically nontrivial nature of itinerant Majorana fermions. The
characteristic behaviors provide experimentally-accessible evidences of
fractional excitations in the proximity to Kitaev quantum spin liquid.",1703.10395v1
2017-08-02,Realistic finite temperature simulations of magnetic systems using quantum statistics,"We have performed realistic atomistic simulations at finite temperatures
using Monte Carlo and atomistic spin dynamics simulations incorporating quantum
(Bose-Einstein) statistics. The description is much improved at low
temperatures compared to classical (Boltzmann) statistics normally used in
these kind of simulations, while at higher temperatures the classical
statistics are recovered. This corrected low-temperature description is
reflected in both magnetization and the magnetic specific heat, the latter
allowing for improved modeling of the magnetic contribution to free energies. A
central property in the method is the magnon density of states at finite
temperatures and we have compared several different implementations for
obtaining it. The method has no restrictions regarding chemical and magnetic
order of the considered materials. This is demonstrated by applying the method
to elemental ferromagnetic systems, including Fe and Ni, as well as Fe-Co
random alloys and the ferrimagnetic system GdFe$_3$ .",1708.00709v1
2017-08-05,Crystal growth with oxygen partial pressure of the $BaCuSi_2O_6$ and $Ba_{1-x}Sr_xCuSi_2O_6$ spin dimer compounds,"$BaCuSi_2O_6$ is a quasi-two dimensional spin dimer system and a model
material for studying Bose-Einstein condensation (BEC) of magnons in high
magnetic fields. The new $Ba_{1-x}Sr_xCuSi_2O_6$ mixed system, which can be
grown with x < 0.3, and $BaCuSi_2O_6$, both grown by using a crystal growth
method with enhanced oxygen partial pressure, have the same tetragonal
structure ($I4_1/acd$) at room temperature. The mixed system shows no
structural phase transition, so that the tetragonal structure is stable down to
low temperatures. The oxygen partial pressure acts as control parameter for the
growth process. A detailed understanding of the crystal structure depending on
the oxygen content will enable the study of the spin dynamics of field-induced
order states in this model magnetic compound of high current interest with only
one type of dimer layers, which shows the same distance between the Cu atoms,
in the structure.",1708.01739v1
2017-08-05,Spin Injection and Detection via the Anomalous Spin Hall Effect in a Ferromagnetic Metal,"We report a novel spin injection and detection mechanism via the anomalous
Hall effect in a ferromagnetic metal. The anomalous spin Hall effect (ASHE)
refers to the transverse spin current generated within the ferromagnet. We
utilize the ASHE and its reciprocal effect to electrically inject and detect
magnons in a magnetic insulator in a non-local geometry. Our experiments reveal
that permalloy can have a higher spin injection and detection efficiency to
that of platinum, owing to the ASHE. We also demonstrate the tunability of the
ASHE via the orientation of the permalloy magnetization, thus creating new
possibilities for spintronic applications.",1708.01752v1
2017-08-09,Weak localization of magnons in chiral magnets,"We report on the impact of the Dzyaloshinskii-Moriya interaction on the
coherent backscattering of spin waves in a disordered magnetic material. This
interaction breaks the inversion symmetry of the spin-wave dispersion relation,
such that $\omega_\mathbf{k} = \omega_{2\mathbf{K}^\mathrm{I}-\mathbf{k}} \neq
\omega_{-\mathbf{k}}$, where $\mathbf{K}^\mathrm{I}$ is related to the
Dzyaloshinskii-Moriya vectors. As a result of numerical investigations we find
that the backscattering peak of a wave packet with initial wave vector
$\mathbf{k}_0$ shifts from $-\mathbf{k}_0$ to
$2\mathbf{K}^\mathrm{I}-\mathbf{k}_0$, such that the backscattering wave vector
and the initial wave vector are in general no longer antiparallel. The shifted
coherence condition is explained by a diagrammatic approach and opens up an
avenue to measure sign and magnitude of the Dzyaloshinskii-Moriya interaction
in weakly disordered chiral magnets.",1708.02807v1
2017-08-11,Goldstone-like phonon modes in a (111)-strained perovskite,"Goldstone modes are massless particles resulting from spontaneous symmetry
breaking. Although such modes are found in elementary particle physics as well
as in condensed matter systems like superfluid helium, superconductors and
magnons - structural Goldstone modes are rare. Epitaxial strain in thin films
can induce structures and properties not accessible in bulk and has been
intensively studied for (001)-oriented perovskite oxides. Here we predict
Goldstone-like phonon modes in (111)-strained SrMnO3 by first-principles
calculations. Under compressive strain the coupling between two in-plane
rotational instabilities give rise to a Mexican hat shaped energy surface
characteristic of a Goldstone mode. Conversely, large tensile strain induces
in-plane polar instabilities with no directional preference, giving rise to a
continuous polar ground state. Such phonon modes with U(1) symmetry could
emulate structural condensed matter Higgs modes. The mass of this Higgs boson,
given by the shape of the Mexican hat energy surface, can be tuned by strain
through proper choice of substrate.",1708.03490v1
2017-08-30,Strain and Magnetic Field Induced Spin-Structure Transitions in Multiferroic BiFeO3,"The magnetic-field-dependent spin ordering of strained BiFeO3 films is
determined using nuclear resonant scattering and Raman spectroscopy. The
critical field required to destroy the cycloidal modulation of the Fe spins is
found to be significantly lower than in the bulk, with appealing implications
for field-controlled spintronic and magnonic devices.",1708.09183v1
2018-02-01,Theory of spin and lattice wave dynamics excited by focused laser pulses,"We develop a theory of the spin wave dynamics excited by ultrafast focused
laser pulses in a magnetic film. We take into account both volume and surface
spin wave modes in the presence of applied, dipolar and magnetic anisotropy
fields and include the dependence on laser spot exposure size and magnetic
damping. We show that the sound waves generated by local heating by an
ultrafast focused laser pulse can excite a wide spectrum of spin waves (on top
of a dominant magnon-phonon contribution). Good agreement with recent
experiments supports the validity of the model.",1802.00178v2
2018-02-05,Cooper-Pair Spin Current in a Strontium Ruthenate Heterostructure,"It has been recognized that the condensation of spin-triplet Cooper pairs
requires not only the broken gauge symmetry but also the spin ordering as well.
One consequence of this is the possibility of the Cooper-pair spin current
analogous to the magnon spin current in magnetic insulators, the analogy also
extending to the existence of the Gilbert damping of the collective
spin-triplet dynamics. The recently fabricated heterostructure of the thin film
of the itinerant ferromagnet SrRuO3 on the bulk Sr2RuO4, the best-known
candidate material for the spin-triplet superconductor, offers a promising
platform for generating such spin current. We will show how such
heterostructure allows us to not only realize the long-range spin valve but
also electrically drive the collective spin mode of the spin-triplet order
parameter. Our proposal represents both a new realization of the spin
superfluidity and a transport signature of the spin-triplet superconductivity.",1802.01599v1
2018-02-22,Dirac cones and mass terms in bosonic spectra,"The notion of Dirac cones, wherein two or more bands become degenerate at a
certain momentum, is the starting point for the study of topological phases.
Dirac cones have been thoroughly explored in fermionic systems such as
graphene, Weyl semimetals, etc. The underlying mathematical structure in these
systems is a Clifford algebra -- a rule for identifying sets of matrices that
span the Hamiltonian. This structure allows for the identification of suitable
`mass' terms to open band gaps. In this article, we extend these ideas to
bosonic systems. Due to the pseudo-orthogonal nature of eigenvectors, the
algebra of matrices takes a very different form. Taking the honeycomb XY
ferromagnet as a prototype, we show that a Dirac cone emerges in the magnon
spectrum. A gap can be opened by a suitable mass term involving next-nearest
neighbour interactions. We next construct a one-dimensional ladder model with
triplon excitations. Using the new Clifford algebra, we define winding number
as a topological invariant. In analogy with the Su-Schrieffer-Heeger model,
topological transitions occur when the band gap closes, leading to the
appearance (or disappearance) of protected edge states. Our results suggest a
new route to studying band touching and band topology in bosonic systems.",1802.08269v1
2018-04-01,Dynamics of a single hole in the Heisenberg-Kitaev model: a self-consistent Born approximation study,"The magnetic properties of 4d and 5d transition-metal insulating compounds
with the honeycomb structure are believed to be described by the
Heisenberg-Kitaev model, which contains both the isotropic Heisenberg
interaction J and anisotropic Kitaev interaction K. In this paper, to
investigate the charge dynamics in these materials, we study the single-hole
propagation of the t-J-K model in various magnetically ordered phases by the
self-consistent Born approximation. We find that there are low-energy coherent
quasiparticle (QP) excitations in all of these phases which appear firstly
around the K point in the Brillouin zone (BZ), but the band-widths of these QPs
are very small due to the hole-magnon coupling. Interestingly, in the zigzag
phase relevant to recent experiments, though the QP weights are largely
suppressed in the physical spectra in the first BZ, we find that they recover
in the extended BZs. Moreover, our results reveal that the low-energy QP
spectra are reduced with the increase of K.",1804.00265v1
2018-04-01,The silver route to cuprate analogs,"The parent compound of high-Tc superconducting cuprates is a unique Mott
state consisting of layers of spin-1/2 ions arranged on a square lattice and
with a record high antiferromagnetic coupling within the layers. Compounds with
similar characteristics have long been searched for. Nickelates and iridates
had been proposed as cuprate analogs but so far have not reached a satisfactory
similarity. Here we use a combination of experimental and theoretical tools to
show that the commercial compound AgF2 is an excellent cuprate analog with
remarkably similar electronic parameters to La2CuO4 but larger buckling of
planes. Two-magnon Raman scattering reveals a superexchange constant which
reaches 70% of that of a typical cuprate. We argue that structures that reduce
or eliminate the buckling of the AgF2 planes could have an antiferromagnetic
coupling that matches or surpasses the cuprates.",1804.00329v1
2018-04-03,Selection rules for cavity-enhanced Brillouin light scattering from magnetostatic modes,"We experimentally identify the magnetostatic modes active for Brillouin light
scattering in the optical whispering gallery modes of a yttrium iron garnet
sphere. Each mode is identified by magnetic field dispersion of
ferromagnetic-resonance spectroscopy and coupling strength to the known field
distribution of the microwave drive antenna. Our optical measurements confirm
recent predictions that higher-order magnetostatic modes can also generate
optical scattering, according to the selection rules derived from the axial
symmetry. From this we summarize the selection rules for Brillouin light
scattering. We give experimental evidence that the optomagnonic coupling to
non-uniform magnons can be higher than that of the uniform Kittel mode.",1804.00965v2
2018-04-06,Quantum-impurity relaxometry of magnetization dynamics,"Prototypes of quantum impurities (QI), such as NV and SiV centers in diamond,
have been recently growing in popularity due to their minimally invasive and
high-resolution magnetic field sensing. Here, we focus on quantum-impurity
relaxometry as a method to probe collective excitations in magnetic insulators.
We develop a general framework that relates the experimentally-measurable
quantum-impurity relaxation rates to the properties of a magnetic system via
the noise emitted by the latter. We suggest that, when the quantum-impurity
frequency lies within the spin-wave gap, quantum-impurity relaxometry can be
effectively deployed to detect signatures of the coherent spin dynamics, such
as magnon condensation, both in ferromagnetic and antiferromagnetic systems, as
well as open prospects to nonintrusively probe spin-wave transport regimes in
magnetic insulators.",1804.02417v1
2018-04-09,General method for atomistic spin-lattice dynamics with first principles accuracy,"We present a computationally efficient general first-principles based method
for spin-lattice simulations for solids. Our method is based on a combination
of atomistic spin dynamics and molecular dynamics, expressed through a
spin-lattice Hamiltonian where the bilinear magnetic term is expanded to second
order in displacement, and all parameters are computed using density functional
theory. The effect of first-order spin-lattice coupling on the magnon and
phonon dispersion in bcc Fe is reported as an example, and is seen to be in
good agreement with previous simulations performed with an empirical potential
approach. In addition, we also illustrate the abilities of our method on a more
conceptual level, by exploring dissipation-free spin and lattice motion in
small magnetic clusters (a dimer, trimer and quadmer). Our method opens the
door for quantitative description and understanding of the microscopic origin
of many fundamental phenomena of contemporary interest, such as ultrafast
demagnetization, magnetocalorics, and spincaloritronics.",1804.03119v2
2018-11-05,Quantum spin liquid at finite temperature: proximate dynamics and persistent typicality,"Quantum spin liquids are long-range entangled states of matter with emergent
gauge fields and fractionalized excitations. While candidate materials, such as
the Kitaev honeycomb ruthenate $\alpha$-RuCl$_3$, show magnetic order at low
temperatures $T$, here we demonstrate numerically a dynamical crossover from
magnon-like behavior at low $T$ and frequencies $\omega$ to long-lived
fractionalized fermionic quasiparticles at higher $T$ and $\omega$. This
crossover is akin to the presence of spinon continua in quasi-1D spin chains.
It is further shown to go hand in hand with persistent typicality down to very
low $T$. This aspect, which has also been observed in the spin-1/2 kagome
Heisenberg antiferromagnet, is a signature of proximate spin liquidity and
emergent gauge degrees of freedom more generally, and can be the basis for the
numerical study of many finite-$T$ properties of putative spin liquids.",1811.01671v2
2010-05-06,A contractor-renormalization study of Hubbard plaquette clusters,"We implement the contractor-renormalization method to study the checkerboard
Hubbard model on various finite-size clusters as function of the
inter-plaquette hopping t' and the on-site repulsion U at low hole doping. We
find that the pair-binding energy and the spin gap exhibit a pronounced maximum
at intermediate values of t' and U, thus indicating that moderate inhomogeneity
of the type considered here substantially enhances the formation of hole pairs.
The rise of the pair-binding energy for t'<t'_max is kinetic-energy driven and
reflects the strong resonating valence bond correlations in the ground state
that facilitate the motion of bound pairs as compared to single holes.
Conversely, as t' is increased beyond t'_max antiferromagnetic magnons
proliferate and reduce the potential energy of unpaired holes and with it the
pairing strength. For the periodic clusters that we study the estimated phase
ordering temperature at t'=t'_max is a factor of 2-7 smaller than the pairing
temperature.",1005.0978v2
2010-04-20,Wide-range wavevector selectivity of magnon gases in Brillouin light scattering spectroscopy,"Brillouin light scattering spectroscopy is a powerful technique for the study
of fast magnetization dynamics with both frequency- and wavevector resolution.
Here, we report on a distinct improvement of this spectroscopic technique
towards two-dimensional wide-range wavevector selectivity in a backward
scattering geometry. Spin-wave wavevectors oriented perpendicular to the bias
magnetic field are investigated by tilting the sample within the magnet gap.
Wavevectors which are oriented parallel to the applied magnetic field are
analyzed by turning the entire setup, including the magnet system. The setup
features a wide selectivity of wavevectors up to 2.04\cdot 10E5 rad/cm for both
orientations, and allows selecting and measuring wavevectors of dipole- and
exchange-dominated spin waves of any orientation to the magnetization
simultaneously.",1005.5084v1
2010-05-28,The Kondo-lattice state and non-Fermi-liquid behavior in the presence of Van Hove singularities,"A scaling consideration of the Kondo lattices is performed with account of
logarithmic Van Hove singularities (VHS) in the electron density of states. The
scaling trajectories are presented for different magnetic phases. It is
demonstrated that VHS lead to a considerable increase of the non-Fermi-liquid
behavior region owing to softening of magnon branches during the
renormalization process. Although the effective coupling constant remains
moderate, renormalized magnetic moment and spin-fluctuation frequency can
decrease by several orders of magnitude. A possible application to f-systems
and weak itinerant magnets is discussed.",1005.5248v3
2010-05-30,Quasi-1d quantum helimagnets: The fate of multipolar phases,"Coupled frustrated spin-1/2 chains in high magnetic fields described within
the ferro- antiferromagnetic J1-J2 Heisenberg model are studied by the DMRG,
the hard core boson, and the spin wave theory approaches. Multipolar phases
related to magnon bound states are destroyed (supported) by weak
antiferromagnetic (ferromagnetic) interchain couplings Jic. We show that
quantum spin nematics might be found for LiVCuO4 whereas for Li(Na)Cu2O2 it is
prevented by a sizeable antiferromagnetic Jic. Also for Li2ZrCuO4 with a small
antiferromagnetic Jic expected triatic or quartic phases are unlikely, too. The
saturation field is found to be strongly affected even by a relatively small
Jic.",1005.5500v2
2013-08-01,Resonant X-ray scattering and the $j_{\mathrm{eff}}=1/2$ electronic ground state in iridate perovskites,"The resonant X-ray scattering (magnetic elastic, RXMS, and inelastic, RIXS)
of Ir$^{4+}$ at the L$_{2,3}$ edges relevant to spin-orbit Mott insulators
A$_{n+1}$Ir$_{n}$O$_{3n+1}$ (A=Sr, Ba, etc.) are calculated using a single-ion
model which treats the spin-orbit and tetragonal crystal-field terms on an
equal footing. Both RXMS and RIXS in the spin-flip channel are found to display
a non-trivial dependence on the direction of the magnetic moment,
$\boldsymbol\mu$. Crucially, we show that for $\boldsymbol\mu$ in the
\emph{ab}-plane, RXMS at the L$_2$ edge is zero \emph{irrespective} of the
tetragonal crystal-field; spin-flip RIXS, relevant to measurements of magnons,
behaves reciprocally being zero at L$_2$ when $\boldsymbol\mu$ is perpendicular
to the \emph{ab}-plane. Our results provide important insights into the
interpretation of X-ray data from the iridates, including that a
$j_{\mathrm{eff}}=1/2$ ground state cannot be assigned on the basis of
L$_2$/L$_3$ intensity ratio alone.",1308.0128v1
2013-08-10,Dynamics of an Insulating Skyrmion under a Temperature Gradient,"We study the Skyrmion dynamics in thin films under a temperature gradient.
Our numerical simulations show that both single and multiple Skyrmions in a
crystal move towards the high temperature region, which is contrary to particle
diffusion. Noticing a similar effect in the domain wall motion, we employ a
theory based on magnon dynamics to explain this counterintuitive phenomenon.
Unlike the temperature driven domain wall motion, the Skyrmion's topological
charge plays an important role, and a transverse Skyrmion motion is observed.
Our theory turns out to be in agreement with numerical simulations, both
qualitatively and quantitatively. Our calculation indicated that a very
promising Skyrmion dynamic phenomenon can be observed in experiments.",1308.2343v1
2013-08-23,Impact of eddy currents on the dispersion relation of surface spin waves in thin conducting magnetic films,"We propose a rigorous solution to a long-standing problem of the impact of
eddy currents on the dispersion relation of surface spin waves propagating in
thin conducting magnetic films. Our results confirm the prediction of the
Almeida-Mill's exchange-free theory that the inclusion of the eddy current
contribution results in a deviation of the dispersion curve for the fundamental
mode from the Damon-Eshbach law and a substantial linewidth broadening in a
large wave vector range. We show that the decrease in the spin wave frequency
is due to an increase in the in-plane component of the dynamic magnetic field
within the conducting film. The decrease in the frequency is accompanied by a
drastic change in the asymmetry of the modal profiles for the waves. This
effect is not observable in magneto-insulating films and therefore it is
unambiguously attributed to eddy currents that appear in conducting films only.
We also show that the wave vector range in which eddy currents affect the
dispersion curve is strongly correlated with the value of the film
conductivity. This result holds for conducting films with the thickness 10-100
nm, which are considered promising for future magnonic and spintronic
applications.",1308.5040v1
2013-08-27,Soft confinement in a 3-d spin system,"We consider a 1+3 dimensional spin system. The spin-wave (magnon) field is
described by the O(3) non-linear sigma model with a symmetry-breaking
potential. This interacts with a slow spin SU(2) doublet Schrodinger fermion.
The interaction is described by a generalized nonperturbative Yukawa coupling,
and the self-consistency condition is solved with the aid of a non-relativistic
Gribov equation. When the Yukawa coupling is sufficiently strong, the solution
exhibits supercriticality and soft confinement, in a way that is quite
analogous to Gribov's light-quark confinement theory.
  The solution corresponds to a new type of spin polaron, whose condensation
may lead to exotic superconductivity.",1308.5731v1
2014-03-04,One dimensional zone center phonons in $O_{h}$ space group,"Motivated by experiments, we performed a systematic study on the one
dimensional zone center phonons in $O_{h}$ space groups. All the one
dimensional phonon modes for different Wyckoff positions are tabulated. We show
that at least four (inequivalent) atoms (in one set of Wyckoff positions) are
needed to carry a single one dimensional phonon. A general restriction rule on
the number of atoms and the number of one dimensional phonons is obtained. The
same restriction applies to phonons of all cubic crystal systems ($T$, $T_{h}$,
$T_{d}$, $O$, $O_{h}$) and to magnons for crystals whose unitary symmetry
elements are in ($T$, $T_{h}$, $T_{d}$, $O$, $O_{h}$). Crystals with $A15$
structure are found to satisfy experimental requirements (to have a
$\Gamma_{2}^{+}$ phonon) while most crystals of $O_{h}$ space groups do not.
$A15$ is also used to demonstrate the rules we found regarding the phonon
structure in $O_{h}$ space groups.",1403.0672v1
2014-03-07,Symplectic Deformations of Integrable Field Theories and AdS/CFT,"Relativistic integrable field theories like the sine-Gordon equation have an
infinite set of conserved charges. In a light-front formalism these conserved
charges are closely related to the integrable modified KdV hierarchy at the
classical level. The latter hierarchy admits a family of symplectic structures
which we argue can be viewed as deformations of the relativistic sine-Gordon
symplectic structure. These deformed theories are integrable but no longer
relativistic and the basic excitations of the theory, the solitons, have an
interesting non-relativistic dispersion relation that in a certain limit
becomes the dispersion relation of dyonic giant magnons of string theory in the
AdS/CFT correspondence. We argue that the deformed classical theories can be
lifted to quantum theories when the sine-Gordon theory is embedded in a larger
theory that describes the string world-sheet sigma model in AdS(5)xS(5).",1403.1899v1
2014-03-17,Entangled tetrahedron ground state and excitations of the magneto-electric skyrmion material Cu$_2$OSeO$_3$,"The strongly correlated cuprate Cu$_2$OSeO$_3$ has recently been identified
as the first insulating system exhibiting a skyrmion lattice phase. Using a
microscopic multi-boson theory for its magnetic ground state and excitations,
we establish the presence of two distinct types of modes: a low energy manifold
that includes a gapless Goldstone mode and a set of weakly dispersive
high-energy magnons. These spectral features are the most direct signatures of
the fact that the essential magnetic building blocks of Cu$_2$OSeO$_3$ are not
individual Cu spins, but rather weakly-coupled Cu$_4$ tetrahedra. Several of
the calculated excitation energies are in nearly perfect agreement with
reported Raman and far-infrared absorption data, while the magneto-electric
effect determined within the present quantum-mechanical framework is also fully
consistent with experiments, giving strong evidence in the entangled Cu$_4$
tetrahedra picture of Cu$_2$OSeO$_3$. The predicted dispersions along with the
dynamical dipole and quadrupole spin structure factors call for further
experimental tests of this picture.",1403.4081v2
2014-03-18,All-loop worldsheet S matrix for AdS_3 x S^3 x T^4,"We obtain the all-loop worldsheet S matrix for fundamental excitations on
AdS_3 x S^3 x T^4 by studying the off-shell symmetry algebra of the superspace
action in lightcone gauge. The massless modes, unaccounted for in earlier
works, are automatically included in our treatment. Their exact dispersion
relation is found to be non-relativistic, of giant-magnon form and their
scattering is naturally well-defined. This opens the way to a complete
investigation of AdS_3/CFT_2 integrability.",1403.4543v2
2014-03-24,On the exact spectrum and mirror duality of the (AdS_5 x S^5)_eta superstring,"We discuss the spectrum of a string propagating on eta-deformed AdS_5 x S^5
by treating its world-sheet theory as an integrable quantum field theory. The
exact S-matrix of this field theory is given by a q-deformation of the AdS_5 x
S^5 world-sheet S-matrix with real deformation parameter. By considering mirror
(double Wick-rotated) versions of these world-sheet theories we give the
Thermodynamic Bethe Ansatz description of their exact finite size spectra.
Interestingly, this class of models maps onto itself under the mirror
transformation. At the level of the string this appears to say that the
light-cone worldsheet theories of strings on particular pairs of backgrounds
are related by a double Wick rotation, a feature we call 'mirror duality'. We
provide a partial check of these statements at the level of the sigma model by
considering reduced actions and their corresponding (mirror) giant magnon
solutions.",1403.6104v2
2016-12-31,First-Principles Study of Exchange Interactions of Yttrium Iron Garnet,"Yttrium Iron Garnet is the ubiquitous magnetic insulator used for studying
pure spin currents. The exchange constants reported in the literature vary
considerably between different experiments and fitting procedures. Here we
calculate them from first-principles. The local Coulomb correction (U - J) of
density functional theory is chosen such that the parameterized spin model
reproduces the experimental Curie temperature and a large electronic band gap,
ensuring an insulating phase. The magnon spectrum calculated with our
parameters agrees reasonably well with that measured by neutron scattering. A
residual disagreement about the frequencies of optical modes indicates the
limits of the present methodology.",1701.00110v1
2017-01-19,Terahertz Dielectric Analysis and Spin-Phonon Coupling in Multiferroic GeV$_4$S$_8$,"We present an investigation of the multiferroic lacunar spinel compound
GeV$_4$S$_8$ using time-domain terahertz spectroscopy. We find three
absorptions which either appear or shift at the antiferromagnetic transition
temperature, T$_N = 17$ K, as S=1 magnetic moments develop on vanadium
tetrahedra. Two of these absorptions are coupled to the magnetic state and one
only appears below the N\'{e}el temperature, and is interpreted as a magnon. We
also observe isosbestic points in the dielectric constant in both the
temperature and frequency domains. Further, we perform analysis on the
isosbestic features to reveal an interesting collapse into a single curve as a
function of both frequency and temperature, behavior which exists throughout
the phase transitions. This analysis suggests the importance of spectral
changes in the terahertz range which are linear in frequency and temperature.",1701.05539v2
2017-01-25,Stable high-temperature paramagnons in a three-dimensional antiferromagnet near quantum criticality: Application to TlCuCl$_3$,"The complete set of hallmarks of the three-dimensional antiferromagnet near
the quantum critical point has been recently observed in the spin dimer
compound TlCuCl$_3$. Nonetheless, the mechanism, responsible for several
distinct features of the experimental data, has remained a puzzle, namely: (i)
the paramagnons exhibit remarkable robustness to thermal damping and are stable
up to high temperatures, where $k_B T$ is comparable with the excitation
energy, (ii) the width to mass ratios of the high-temperature paramagnons are,
within the error bars, equal to that of the low-temperature amplitude (or
Higgs) mode. We propose such a mechanism and identify two principal factors,
contributing to the scaling between width to mass ratios of the paramagnon and
the amplitude mode: (i) the emergence of the thermal mass scale reorganizing
the paramagnon decay processes, and (ii) substantial renormalization of the
multi-magnon interactions by thermal fluctuations. The study is carried out for
the general case of a $D= 3 + 1$ quantum antiferromagnet within the framework
of the $\varphi^4$ model using the hybrid Callan-Symanzik + Wilson thermal
renormalization group method. Our approach is tested by demonstrating a good
quantitative agreement with available experimental data across the phase
diagram of TlCuCl$_3$.",1701.07406v1
2017-01-30,Quantum transition between magnetically ordered and Mott glass phases,"We discuss a quantum transition from a superfluid to a Mott glass phases in
disordered Bose-systems by the example of an isotropic spin-$\frac12$
antiferromagnet with spatial dimension $d\ge2$ and with disorder in tunable
exchange couplings. Our analytical consideration is based on general properties
of a system in critical regime, on the assumption that the magnetically order
part of the system shows fractal properties near the transition, and on a
hydrodynamic description of long-wavelength magnons in the magnetically ordered
(""superfluide"") phase. Our results are fully consistent with a scaling theory
based on an ansatz for the free energy proposed by M.P. Fisher et al. (Phys.
Rev. B 40, 546 (1989)). We obtain $z=d-\beta/\nu$ for the dynamical critical
exponent and $\phi = z\nu$, where $\phi$, $\beta$, and $\nu$ are critical
exponents of the critical temperature, the order parameter, and the correlation
length, respectively. The density of states of localized excitations (fractons)
is found to show a superuniversal (i.e., independent of $d$) behavior.",1701.08624v2
2017-01-27,A unified description of collective magnetic excitations,"In this work, we define a set of analytic tools to describe the dynamic
response of the magnetization to small perturbations, which can be used on its
own or in combination with micromagnetic simulations and does not require
saturation. We present a general analytic description of the ferromagnetic high
frequency susceptibility tensor to describe angular as well as frequency
dependent ferromagnetic resonance spectra and account for asymmetries in the
line shape. Furthermore, we expand this model to reciprocal space and show how
it describes the magnon dispersion. Finally we suggest a trajectory dependent
solving tool to describe the equilibrium states of the magnetization.",1701.09078v2
2018-05-04,Spin wave effects in transport between a ferromagnet and a Weyl semimetal surface,"We experimentally investigate spin-polarized transport between a
ferromagnetic Ni electrode and a surface of Weyl semimetal, realized in a thick
WTe$_2$ single crystal. For highly-transparent Ni-WTe$_2$ planar junctions, we
observe non-Ohmic $dV/dI(I)$ behavior with an overall increase of differential
resistance $dV/dI$ with current bias, which is accomplished by current-induced
switchings. This behavior is inconsistent with trivial interface scattering,
but it is well known for spin-polarized transport with magnon emission. Thus,
we interpret the experimental results in terms of spin wave excitation in spin
textures in the WTe$_2$ topological surface states, which is supported by the
obtained magnetic field and temperature $dV/dI(I)$ dependencies.",1805.01838v2
2018-04-25,Evolution of the magnetic excitations in NaOsO$_3$ through its metal-insulator transition,"The temperature dependence of the excitation spectrum in NaOsO$_{\text{3}}$
through its metal-to-insulator transition (MIT) at 410 K has been investigated
using resonant inelastic X-ray scattering (RIXS) at the Os L$_{\text{3}}$ edge.
High resolution ($\Delta E \sim$ 56 meV) measurements show that the
well-defined, low energy magnons in the insulating state weaken and dampen upon
approaching the metallic state. Concomitantly, a broad continuum of excitations
develops which is well described by the magnetic fluctuations of a nearly
antiferromagnetic Fermi liquid. By revealing the continuous evolution of the
magnetic quasiparticle spectrum as it changes its character from itinerant to
localized, our results provide unprecedented insight into the nature of the MIT
in NaOsO$_{\text{3}}$.",1805.03176v1
2018-05-09,Nature of the Magnetic Interactions in Sr$_3$NiIrO$_6$,"Iridates abound with interesting magnetic behaviours because of their strong
spin-orbit coupling. Sr$_3$NiIrO$_6$ brings together the spin-orbital
entanglement of the Ir$^{4+}$ ion with a 3$d$ Ni cation and a one-dimensional
crystal structure. It has a ferrimagnetic ground state with a 55 T coercive
field. We perform a theoretical study of the magnetic interactions in this
compound, and elucidate the role of anisotropic symmetric exchange as the
source of its strong magnetic anisotropy. Our first-principles calculations
reproduce the magnon spectra of this compound and predict a signature in the
cross sections that can differentiate the anisotropic exchange from single-ion
anisotopy.",1805.03733v2
2018-05-10,Anomalous spin Hall magnetoresistance in Pt/Co bilayers,"We have studied the spin Hall magnetoresistance (SMR), the magnetoresistance
within the plane transverse to the current flow, of Pt/Co bilayers. We find
that the SMR increases with increasing Co thickness: the effective spin Hall
angle for bilayers with thick Co exceeds the reported values of Pt when a
conventional drift-diffusion model is used. An extended model including spin
transport within the Co layer cannot account for the large SMR. To identify its
origin, contributions from other sources are studied. For most bilayers, the
SMR increases with decreasing temperature and increasing magnetic field,
indicating that magnon-related effects in the Co layer play little role.
Without the Pt layer, we do not observe the large SMR found for the Pt/Co
bilayers with thick Co. Implementing the effect of the so-called interface
magnetoresistance and the textured induced anisotropic scattering cannot
account for the Co thickness dependent SMR. Since the large SMR is present for
W/Co but its magnitude reduces in W/CoFeB, we infer its origin is associated
with a particular property of Co.",1805.03843v1
2018-05-10,New Compact Construction of Eigenstates for Supersymmetric Spin Chains,"The problem of separation of variables (SoV) in supersymmetric spin chains is
closely related to the calculation of correlation functions in N=4 SYM theory
which is integrable in the planar limit. To address this question we find a
compact formula for the spin chain eigenstates, which does not have any sums
over auxiliary roots one usually gets in the widely adopted nested Bethe
ansatz. Our construction only involves one application of a simple Bg(u_k)
operator to the reference state for each of the magnons, in complete analogy
with the su(2) algebraic Bethe ansatz. This generalizes our SoV based
construction for su(n) to the supersymmetric su(1|2) case.",1805.03927v2
2018-05-21,Spin nematics next to spin singlets,"We provide a route to generate nematic order in a spin-1/2 system. Unlike the
well-known magnon-binding mechanism, our spin nematics requires neigher the
frustration effect nor a spin polarization in a high field or in the vicinity
of a ferromagnet, but instead appears next to the spin singlet phase. We start
from a state consisting of a quantum spin-1/2 singlet dimer placed on each site
of a triangular lattice, and show that inter-dimer ring exchange interactions
efficiently dope the SU(2) triplets that itinerate and interact, easily driving
a stable singlet state to either Bose Einstein condensates or a triplet
crystal, some hosting a spin nematic order. A variety of roles the ring
exchange serves include the generation of a bilinear-biquadratic interaction
between nearby triplets, which is responsible for the emergent nematic order
separated from the singlet phase by a first order transition.",1805.07910v1
2018-05-21,Spectral properties of the square-lattice antiferromagnetic J1-J2 Heisenberg model: confinement and deconfinement of spinons,"Based on the mapping between $s=1/2$ spin operators and hard-core bosons, we
extend the cluster perturbation theory to spin systems and study the whole
excitation spectrum of the antiferromagnetic $J_{1}$-$J_{2}$ Heisenberg model
on the square lattice. In the N\'eel phase for $J_{2}\lesssim0.4J_{1}$, in
addition to the dominant magnon excitation, there is an obvious continuum close
to $(\pi,0)$ in the Brillouin zone indicating the deconfined spin-1/2 spinon
excitations. In the stripe phase for $J_{2}\gtrsim0.6J_{1}$, we find similar
high-energy two-spinon continuums at $(\pi/2,\pi/2)$ and $(\pi/2,\pi)$,
respectively. The intermediate phase is characterized by a spectrum with
completely deconfined broad continuum, which is attributed to a $Z_{2}$ quantum
spin liquid with the aid of a variational-Monte-Carlo analysis.",1805.07915v1
2018-05-29,Efficient injection and detection of out-of-plane spins via the anomalous spin Hall effect in permalloy nanowires,"We report a novel mechanism for the electrical injection and detection of
out-of-plane spin accumulation via the anomalous spin Hall effect (ASHE), where
the direction of the spin accumulation can be controlled by manipulating the
magnetization of the ferromagnet. This mechanism is distinct from the spin Hall
effect (SHE), where the spin accumulation is created along a fixed direction
parallel to an interface. We demonstrate this unique property of the ASHE in
nanowires made of permalloy (Py), to inject and detect out-of-plane spin
accumulation in a magnetic insulator, yttrium iron garnet (YIG). We show that
the efficiency for the injection/detection of out-of-plane spins can be up to
50% of that of in-plane spins. We further report the possibility to detect spin
currents parallel to the Py/YIG interface for spins fully oriented in the
out-of-plane direction, resulting in a sign reversal of the non-local magnon
spin signal. The new mechanisms that we have demonstrated are highly relevant
for spin torque devices and applications.",1805.11575v1
2008-07-03,Finite-size effects of Membranes on AdS_4 x S_7,"We consider semi-classical solution of membranes on the AdS_4 x S^7. This is
supposed to be dual to the N=6 super Chern-Simons theory with k=1 in a planar
limit recently proposed by Aharony, Bergmann, Jafferis, and Maldacena (ABJM).
We have identified giant magnon and single spike states on the membrane by
reducing them to the Neumamm - Rosochatius integrable system. We also connect
these to the complex sine-Gordon integrable model. Based on this approach, we
find finite-size membrane solutions and obtain their images in the complex
sine-Gordon system along with the leading finite-size corrections to the
energy-charge relations.",0807.0566v2
2008-07-11,Dynamical vertex approximation -- an introduction,"We give an elementary introduction to a recent diagrammatic extension of
dynamical mean field theory (DMFT) coined dynamical vertex approximation
(D$\Gamma$A). This approach contains the important local correlations of DMFT,
giving, among others, rise to quasiparticle renormalizations, Mott-Hubbard
transitions and magnetism, but also non-local correlations beyond. The latter
are at the very essence of many physical phenomena in strongly correlated
elecectron systems. As correlations are treated equally on all length scales,
D$\Gamma$A allows us to describe physical phenomena such as magnons, quantum
criticality, and the interplay between antiferromagnetism and
superconductivity. We review results hitherto obtained for the Hubbard model in
dimensions d=3, 2, and 1.",0807.1860v1
2008-07-16,Spin currents and spin superfluidity,"The present review analyzes and compares various types of dissipationless
spin transport: (1) Superfluid transport, when the spin-current state is a
metastable state (a local but not the absolute minimum in the parameter space).
(2) Ballistic spin transport, when spin is transported without losses simply
because sources of dissipation are very weak. (3) Equilibrium spin currents,
i.e., genuine persistent currents. (4) Spin currents in the spin Hall effect.
Since superfluidity is frequently connected with Bose condensation, recent
debates about magnon Bose condensation are also reviewed. For any type of spin
currents simplest models were chosen for discussion in order to concentrate on
concepts rather than details of numerous models. The various hurdles on the way
of using the concept of spin current (absence of the spin-conservation law,
ambiguity of spin current definition, etc.) were analyzed. The final conclusion
is that the spin-current concept can be developed in a fully consistent manner,
and is a useful language for description of various phenomena in spin dynamics.",0807.2524v3
2008-07-29,Effect of magnesium doping on the orbital and magnetic order in LiNiO2,"In LiNiO2, the Ni3+ ions, with S=1/2 and twofold orbital degeneracy, are
arranged on a trian- gular lattice. Using muon spin relaxation (MuSR) and
electron spin resonance (ESR), we show that magnesium doping does not stabilize
any magnetic or orbital order, despite the absence of interplane Ni2+. A
disordered, slowly fluctuating state develops below 12 K. In addition, we find
that magnons are excited on the time scale of the ESR experiment. At the same
time, a g factor anisotropy is observed, in agreement with $| 3z^{2}-r^{2}>$
orbital occupancy.",0807.4651v1
2008-07-30,Efficient precision quantization in AdS/CFT,"Understanding finite-size effects is one of the key open questions in solving
planar AdS/CFT. In this paper we discuss these effects in the AdS_5xS^5 string
theory at one-loop in the world-sheet coupling. First we provide a very
general, efficient way to compute the fluctuation frequencies, which allows to
determine the energy shift for very general multi-cut solutions. Then we apply
this to two-cut solutions, in particular the giant magnon and determine the
finite-size corrections at subleading order. The latter are then compared to
the finite-size corrections from Luscher-Klassen-Melzer formulas and found to
be in perfect agreement.",0807.4752v2
2009-12-08,Bound states in two-dimensional spin systems near the Ising limit: A quantum finite-lattice study,"We analyze the properties of low-energy bound states in the transverse-field
Ising model and in the XXZ model on the square lattice. To this end, we develop
an optimized implementation of perturbative continuous unitary transformations.
The Ising model is studied in the small-field limit which is found to be a
special case of the toric code model in a magnetic field. To analyze the XXZ
model, we perform a perturbative expansion about the Ising limit in order to
discuss the fate of the elementary magnon excitations when approaching the
Heisenberg point.",0912.1463v2
2009-12-17,Anomalous dimensions at four loops in N=6 superconformal Chern-Simons theories,"In arXiv:0908.2463 we computed the four-loop correction to a function
depending on the 't Hooft coupling(s) that appears in the magnon dispersion
relation of the spin chains derived from single trace operators in N=6
superconformal Chern-Simons theories. In this paper we give detailed
descriptions of this calculation and the computation of the four-loop wrapping
corrections for a length four operator in the 20 of SU(4), the R-symmetry group
for these theories. Here, we give all relevant Feynman diagrams and loop
integrals explicitly, and also demonstrate the cancellation of double poles in
the logarithm of the renormalization constant.",0912.3460v2
2011-11-08,Analysis of the Collective Behavior of a 10 by 10 Array of Fe3O4 Dots in a Large Micromagnetic Simulation,"We report a full (3D) micromagnetic simulation of a set of 100 ferrite
(Fe$_3$O$_4$) cylindrical dots, arranged in a 10 by 10 square (planar) array of
side 3.27 $\mu$m, excited by an external in-plane magnetic field. The resulting
power spectrum of magnetic excitations and the dynamical magnetization field at
the resulting resonance modes were investigated. The absorption spectrum
deviates considerably from that of a single particle reference simulation,
presenting a mode-shifting and splitting effect. We found an inversion symmetry
through the center of the array, in the sense that each particle and its
inversion counterpart share approximately the same magnetization mode behavior.
Magnonic designs aiming at synchronous or coherent tunings of spin-wave
excitations at given spatially separated points within a regular square array
may benefit from the new effects here described.",1111.1866v1
2011-11-08,Time-dependent spin-wave theory,"We generalize the spin-wave expansion in powers of the inverse spin to
time-dependent quantum spin models describing rotating magnets or magnets in
time-dependent external fields. We show that in these cases, the spin operators
should be projected onto properly defined rotating reference frames before the
spin components are bosonized using the Holstein-Primakoff transformation. As a
first application of our approach, we calculate the reorganization of the
magnetic state due to Bose-Einstein condensation of magnons in the magnetic
insulator yttrium-iron garnet; we predict a characteristic dip in the
magnetization which should be measurable in experiments.",1111.2052v2
2011-11-09,Propagating two-dimensional magnetic droplets,"Propagating, solitary magnetic wave solutions of the Landau-Lifshitz equation
with uniaxial, easy-axis anisotropy in thin (two-dimensional) magnetic films
are investigated. These localized, nontopological wave structures, parametrized
by their precessional frequency and propagation speed, extend the stationary,
coherently precessing ""magnon droplet"" to the moving frame, a non-trivial
generalization due to the lack of Galilean invariance. Propagating droplets
move on a spin wave background with a nonlinear droplet dispersion relation
that yields a limited range of allowable droplet speeds and frequencies. An
iterative numerical technique is used to compute the propagating droplet's
structure and properties. The results agree with previous asymptotic
calculations in the weakly nonlinear regime. Furthermore, an analytical
criterion for the droplet's orbital stability is confirmed. Time-dependent
numerical simulations further verify the propagating droplet's robustness to
perturbation when its frequency and speed lie within the allowable range.",1111.2310v2
2011-11-14,Nambu-Goldstone dynamics and generalized coherent-state functional integrals,"The present paper gives a new method of attack on the Nambu-Goldstone
dynamics in spontaneously broken theories. Since the target space of the
Nambu-Goldstone fields is a group coset space, their effective quantum dynamics
can be naturally phrased in terms of generalized coherent-state functional
integrals. As an explicit example of this line of reasoning we construct a
low-energy effective Lagrangian for the Heisenberg ferromagnet in broken phase.
The leading field configuration in the WKB approximation leads to the
Landau-Lifshitz equation for quantum ferromagnet. The corresponding linearized
equations allow to identify the Nambu-Goldstone boson with ferromagnetic
magnon.",1111.3228v2
2011-12-30,Large magnetic circular dichroism in resonant inelastic x-ray scattering at the Mn L-edge of Mn-Zn ferrite,"We report resonant inelastic x-ray scattering (RIXS) excited by circularly
polarized x-rays on Mn-Zn ferrite at the Mn L2,3-resonances. We demonstrate
that crystal field excitations, as expected for localized systems, dominate the
RIXS spectra and thus their dichroic asymmetry cannot be interpreted in terms
of spin-resolved partial density of states, which has been the standard
approach for RIXS dichroism. We observe large dichroic RIXS at the L2-resonance
which we attribute to the absence of metallic core hole screening in the
insulating Mn-ferrite. On the other hand, reduced L3-RIXS dichroism is
interpreted as an effect of longer scattering time that enables spin-lattice
core hole relaxation via magnons and phonons occurring on a femtosecond time
scale.",1201.0091v1
2012-01-02,Localized domain-wall excitations in patterned magnetic dots probed by broadband ferromagnetic resonance,"We investigate the magnetization dynamics in circular Permalloy dots with
spatially separated magnetic vortices interconnected by domain walls (double
vortex state). We identify a novel type of quasi one-dimensional (1D) localised
spin wave modes confined along domain walls, connecting each of two vortex
cores with two edge half-antivortices. Variation of the mode eigenfrequencies
with the dot size is in quantitative agreement with the developed model, which
considers a dipolar origin of the localized 1D spin waves or so-called
Winter\'s magnons [J.M. Winter, Phys.Rev. 124, 452 (1961)]. These spin waves
are analogous to the displacement waves of strings, and could be excited in a
wide class of patterned magnetic nanostructures possessing domain walls, namely
in triangular, square, circular or elliptic magnetic dots.",1201.0482v1
2012-01-20,"Vortex Dynamics in Ferromagnetic Superconductors: Vortex Clusters, Domain Walls and Enhanced Viscosity","We demonstrate that there is a long-range vortex-vortex attraction in
ferromagnetic superconductors due to polarization of the magnetic moments.
Vortex clusters are then stabilized in the ground state for low vortex
densities. The motion of vortex clusters driven by the Lorentz force excites
magnons. This regime becomes unstable at a threshold velocity above which
domain walls are generated for slow relaxation of the magnetic moments and the
vortex configuration becomes modulated. This dynamics of vortices and magnetic
moments can be probed by transport measurements.",1201.4195v3
2012-01-31,Nonreciprocal Directional Dichroism and Toroidalmagnons in Helical Magnets,"We investigate a dynamical magnetoelectric effect due to a magnetic resonance
in helical spin structures through the coupling between magnetization and
electric polarization via a spin current mechanism. We show that the magnon has
both the dynamical magnetic moment $\Delta M^\omega$ and the electric moment
$\Delta P^\omega$ ($\perp \Delta M^\omega$), i.e., a dynamical toroidal moment,
under external magnetic fields, and thus it is named the {\em toroidalmagnon}.
The toroidalmagnon exists in most conical spin structures owing to the
generality of the spin current mechanism. In the absorption of electromagnetic
waves, the toroidalmagnon excitation process generally induces a nonreciprocal
directional dichroism as a consequence of an interference of the magnetic and
electric responses.",1201.6458v1
2012-04-18,On Form Factors in nested Bethe Ansatz systems,"We investigate form factors of local operators in the multi-component Quantum
Non-linear Schr\""odinger model, a prototype theory solvable by the so-called
nested Bethe Ansatz. We determine the analytic properties of the infinite
volume form factors using the coordinate Bethe Ansatz solution and we establish
a connection with the finite volume matrix elements. In the two-component
models we derive a set of recursion relations for the ""magnonic form factors"",
which are the matrix elements on the nested Bethe Ansatz states. In certain
simple cases (involving states with only one spin-impurity) we obtain explicit
solutions for the recursion relations.",1204.4037v2
2012-04-20,Size-dependent electronic-transport mechanism and sign reversal of magnetoresistance in Nd0.5Sr0.5CoO3,"A detailed investigation of electronic-transport properties of Nd0.5Sr0.5CoO3
has been carried out as a function of grain size ranging from micrometer order
down to an average size of 28 nm. Interestingly, we observe a size induced
metal-insulator transition in the lowest grain size sample while the bulk-like
sample is metallic in the whole measured temperature regime. An analysis of the
temperature dependent resistivity in the metallic regime reveals that the
electron-electron interaction is the dominating mechanism while other processes
like electron-magnon and electron-phonon scatterings are also likely to be
present. The fascinating observation of enhanced low temperature upturn and
minimum in resistivity on reduction of grain size is found due to
electron-electron interaction (quantum interference effect). This effect is
attributed to enhanced disorder on reduction of grain size. Interestingly, we
observed a cross over from positive to negative magnetoresistance in the low
temperature regime as the grain size is reduced. This observed sign reversal is
attributed to enhanced phase separation on decreasing the grain size of the
cobaltite.",1204.4572v1
2012-04-24,Nonlocal feedback in ferromagnetic resonance,"Ferromagnetic resonance in thin films is analyzed under the influence of
spatiotemporal feedback effects. The equation of motion for the magnetization
dynamics is nonlocal in both space and time and includes isotropic, anisotropic
and dipolar energy contributions as well as the conserved Gilbert- and the
non-conserved Bloch-damping. We derive an analytical expression for the
peak-to-peak linewidth. It consists of four separate parts originated by
Gilbert damping, Bloch-damping, a mixed Gilbert-Bloch component and a
contribution arising from retardation. In an intermediate frequency regime the
results are comparable with the commonly used Landau-Lifshitz-Gilbert theory
combined with two-magnon processes. Retardation effects together with Gilbert
damping lead to a linewidth the frequency dependence of which becomes strongly
nonlinear. The relevance and the applicability of our approach to ferromagnetic
resonance experiments is discussed.",1204.5342v1
2012-04-27,Coupling of spin and lattice modes in the S=1/2 two-dimensional antiferromagnet K$_{2}$V$_{3}$O$_{8}$ with magneto-dielectric couplings,"Lattice dynamics and magnetic excitations are investigated to elucidate the
origin of magneto-dielectric effects in the S=1/2 two-dimensional quantum spin
compound K$_2$V$_3$O$_8$. We find evidence for lattice instabilities at 110 K
and 60 K as optical phonon anomalies and a soft mode at 26 cm$^{-1}$ in A$_1$
symmetry. Two-magnon excitations in B$_1$ symmetry show an unconventional
double-peak structure and temperature dependence. This suggests the existence
of a split mode near the zone boundary caused by a mixing of spin and lattice
modes.",1204.6210v1
2012-04-28,"Coupled Phonons, Magnetic Excitations and Ferroelectricity in AlFeO3: Raman and First-principles Studies","We determine the nature of coupled phonons and magnetic excitations in AlFeO3
using inelastic light scattering from 5 K to 315 K covering a spectral range
from 100-2200 cm-1 and complementary first-principles density functional
theory-based calculations. A strong spin-phonon coupling and magnetic ordering
induced phonon renormalization are evident in (a) anomalous temperature
dependence of many modes with frequencies below 850 cm-1, particularly near the
magnetic transition temperature Tc ~ 250 K, (b) distinct changes in band
positions of high frequency Raman bands between 1100-1800 cm-1, in particular a
broad mode near 1250 cm-1 appears only below Tc attributed to the two-magnon
Raman scattering. We also observe weak anomalies in the mode frequencies at ~
100 K, due to a magnetically driven ferroelectric phase transition.
Understanding of these experimental observations has been possible on the basis
of first-principles calculations of phonons spectrum and their coupling with
spins.",1204.6387v1
2012-04-30,Chiral gauge field and axial anomaly in a Weyl semi-metal,"Weyl fermions are two-component chiral fermions in (3+1)-dimensions. When
coupled to a gauge field, the Weyl fermion is known to have an axial anomaly,
which means the current conservation of the left-handed and right-handed Weyl
fermions cannot be preserved separately. Recently, Weyl fermions have been
proposed in condensed matter systems named as ""Weyl semi-metals"". In this paper
we propose a Weyl semi-metal phase in magnetically doped topological
insulators, and study the axial anomaly in this system. We propose that the
magnetic fluctuation in this system plays the role of a ""chiral gauge field""
which minimally couples to the Weyl fermions with opposite charges for two
chiralities. We study the anomaly equation of this sytem and discuss its
physical consequences, including one-dimensional chiral modes in a
ferromagnetic vortex line, and a novel plasmon-magnon coupling.",1204.6551v1
2013-09-16,Time resolved spin Seebeck effect experiments,"In this Letter, we present the results of transient thermopower experiments,
performed at room temperature on yttrium iron garnet/platinum bilayers. Upon
application of a time-varying thermal gradient, we observe a characteristic
low-pass frequency response of the ensuing thermopower voltage with cutoff
frequencies of up to 37 MHz. We interpret our results in terms of the spin
Seebeck effect, and argue that small wavevector magnons are of minor importance
for the spin Seebeck effect in our thin film hybrid structures.",1309.3986v2
2014-05-06,Multifrequency transverse Faraday effect in single magneto-dielectric microspheres,"We propose using a single magneto-dielectric microsphere as a device for
enhancing the transverse Faraday effect at multiple wavelengths at the same
time. Although the diameter of the sphere can be $<1$ $\mu$m, the numerically
predicted strength of its magneto-optical (MO) response can be an order of
magnitude stronger than in MO devices based on thick magnetic plates. The MO
response of a microsphere is also comparable with that of subwavelength
magneto-dielectric gratings which, however, operate at a single wavelength and
occupy a large area. In contrast to gratings and thick plates, the compact size
of the microsphere and its capability to support spin-wave excitations make it
suitable for applications in nanophotonics, imaging systems, and magnonics.",1405.1149v1
2014-05-09,Magnetic excitations and anomalous spin wave broadening in multiferroic FeV2O4,"We report on the different roles of two orbital-active Fe$^{2+}$ at the A
site and V$^{3+}$ at the B site in the magnetic excitations and on the
anomalous spin wave broadening in FeV$_{2}$O$_{4}$. FeV$_{2}$O$_{4}$ exhibits
three structural transitions and successive paramagnetic (PM)-collinear
ferrimagnetic (CFI)-noncollinear ferrimagnetic (NCFI) transitions. The
high-temperature tetragonal/PM -orthorhombic/CFI transition is accompanied by
the appearance of an energy gap with a high magnitude in the magnetic
excitations due to strong spin-orbit coupling induced anisotropy at the
Fe$^{2+}$ site. While there is no measurable increase in the energy gap from
the orbital ordering of V$^{3+}$ at the orthorhombic/CFI-tetragonal/NCFI
transition, anomalous spin wave broadening is observed in the orthorhombic/CFI
state due to V$^{3+}$ spin fluctuations at the B site. The spin wave broadening
is also observed at the zone boundary without softening, which is discussed in
terms of magnon-phonon coupling.",1405.2272v2
2014-05-09,Spin wave spectrum of the quantum ferromagnet on the pyrochlore lattice Lu2V2O7,"Neutron inelastic scattering has been used to probe the spin dynamics of the
quantum (S=1/2) ferromagnet on the pyrochlore lattice Lu2V2O7. Well-defined
spin waves are observed at all energies and wavevectors, allowing us to
determine the parameters of the Hamiltonian of the system. The data are found
to be in excellent overall agreement with a minimal model that includes a
nearest- neighbour Heisenberg exchange J = 8:22(2) meV and a
Dzyaloshinskii-Moriya interaction (DMI) D =1:5(1) meV. The large DMI term
revealed by our study is broadly consistent with the model developed by Onose
et al. to explain the magnon Hall effect they observed in Lu2V2O7 [1], although
our ratio of D=J = 0:18(1) is roughly half of their value and three times
larger than calculated by ab initio methods [2].",1405.2284v1
2014-05-23,Magnetic Excitations in Spin-Orbit Coupled $d^4$ Mott Insulator on Square Lattice,"We study the magnetic order and excitations in strong spin-orbit coupled, Van
Vleck-type, $d^4$ Mott insulators on a square lattice. Extending the previous
work, we include the tetragonal crystal field splitting and explore its effects
on magnetic phase diagram and magnon spectra. Two different ordered phases,
with in-plane and out-of-plane orientation of the staggered moments, are found
for the higher and lower values of the crystal field splitting, respectively.
The magnetic excitation spectra for paramagnetic and magnetically ordered
phases are calculated and discussed in the context of a candidate spin-orbit
$d^4$ Mott insulator Ca$_2$RuO$_4$.",1405.6062v2
2014-05-23,Finite-gap equations for strings on AdS_3 x S^3 x T^4 with mixed 3-form flux,"We study superstrings on AdS_3 x S^3 x T^4 supported by a combination of
Ramond-Ramond and Neveu-Schwarz-Neveu-Schwarz three form fluxes, and construct
a set of finite-gap equations that describe the classical string spectrum.
Using the recently proposed all-loop S-matrix we write down the all-loop Bethe
ansatz equations for the massive sector. In the thermodynamic limit the Bethe
ansatz reproduces the finite-gap equations. As part of this derivation we
propose expressions for the leading order dressing phases. These phases differ
from the well-known Arutyunov-Frolov-Staudacher phase that appears in the pure
Ramond-Ramond case. We also consider the one-loop quantization of the algebraic
curve and determine the one-loop corrections to the dressing phases. Finally we
consider some classical string solutions including finite size giant magnons
and circular strings.",1405.6087v2
2014-06-23,"Phonon Anomalies, Orbital-Ordering and Electronic Raman Scattering in iron-pnictide Ca(Fe0.97Co0.03)2As2: Temperature-dependent Raman Study","We report inelastic light scattering studies on Ca(Fe0.97Co0.03)2As2 in a
wide spectral range of 120-5200 cm-1 from 5K to 300K, covering the tetragonal
to orthorhombic structural transition as well as magnetic transition at Tsm ~
160K. The mode frequencies of two first-order Raman modes B1g and Eg, both
involving displacement of Fe atoms, show sharp increase below Tsm.
Concomitantly, the linewidths of all the first-order Raman modes show anomalous
broadening below Tsm, attributed to strong spin-phonon coupling. The high
frequency modes observed between 400-1200 cm-1 are attributed to the electronic
Raman scattering involving the crystal field levels of d-orbitals of Fe2+. The
splitting between xz and yz d-orbital levels is shown to be ~ 25 meV which
increases as temperature decreases below Tsm. A broad Raman band observed at ~
3200 cm-1 is assigned to two-magnon excitation of the itinerant Fe 3d
antiferromagnet.",1406.5844v1
2014-06-27,Observation of propagating edge spin waves modes,"Broadband magnetization response of equilateral triangular 1000 nm Permalloy
dots has been studied under an in-plane magnetic field, applied parallel
(buckle state) and perpendicular (Y state) to the triangles base. Micromagnetic
simulations identify edge spin waves (E-SWs) in the buckle state as SWs
propagating along the two adjacent edges. These quasi one-dimensional spin
waves emitted by the vertex magnetic charges gradually transform from
propagating to standing due to interference and are weakly affected by dipolar
interdot interaction and variation of the aspect ratio. Spin waves in the Y
state have a two dimensional character. These findings open perspectives for
implementation of the E-SWs in magnonic crystals and thin films.",1406.7186v1
2016-05-01,Magnetoelectric fields for microwave chirality discrimination in enantiomeric liquids,"Chirality discrimination is of a fundamental interest in biology, chemistry,
and metamaterial studies. In optics, near-field plasmon-resonance spectroscopy
with superchiral probing fields is effectively applicable for analyses of large
biomolecules with chiral properties. We show possibility for microwave
near-field chirality discrimination analysis based on magnon-resonance
spectroscopy. Newly developed capabilities in microwave sensing using
magnetoelectric (ME) probing fields originated from multiresonance
magnetic-dipolar-mode (MDM) oscillations in quasi-2D yttrium-iron-garnet (YIG)
disks, provide a potential for unprecedented measurements of chemical and
biological objects. We report on microwave near-field chirality discrimination
for aqueous D- and L-glucose solutions. The shown ME-field sensing is addressed
to microwave biomedical diagnostics and pathogen detection and to deepening our
understanding of microwave-biosystem interactions. It can be also important for
an analysis and design of microwave chiral metamaterials.",1605.00212v1
2016-05-07,Order-by-disorder effects in antiferromagnets on face-centered cubic lattice,"We discuss the role of quantum fluctuations in Heisenberg antiferromagnets on
face-centered cubic lattice with small dipolar interaction in which the
next-nearest-neighbor exchange coupling dominates over the nearest-neighbor
one. It is well known that a collinear magnetic structure which contains (111)
ferromagnetic planes arranged antiferromagnetically along one of the space
diagonals of the cube is stabilized in this model via order-by-disorder
mechanism. On the mean-field level, the dipolar interaction forces spins to lie
within (111) planes. By considering 1/S - corrections to the ground state
energy, we demonstrate that quantum fluctuations lead to an anisotropy within
(111) planes favoring three equivalent directions for the staggered
magnetization (e.g., $[11\overline{2}]$, $[1\overline{2}1]$, and
$[\overline{2}11]$ directions for (111) plane). Such in-plane anisotropy was
obtained experimentally in related materials MnO, $\alpha$-MnS, $\alpha$-MnSe,
EuTe, and EuSe. We find that the order-by-disorder mechanism can contribute
significantly to the value of the in-plane anisotropy in EuTe. Magnon spectrum
is also derived in the first order in 1/S.",1605.02208v2
2016-05-10,Topological spin-transfer drag driven by skyrmion diffusion,"We study the spin-transfer drag mediated by the Brownian motion of skyrmions.
The essential idea is illustrated in a two-terminal geometry, in which a thin
film of a magnetic insulator is placed in between two metallic reservoirs. An
electric current in one of the terminals pumps topological charge into the
magnet via a spin-transfer torque. The charge diffuses over the bulk of the
system as stable skyrmion textures. By Onsager's reciprocity, the topological
charge leaving the magnet produces an electromotive force in the second
terminal. The voltage signal decays algebraically with the separation between
contacts, in contrast to the exponential suppression of the spin drag driven by
non-protected excitations like magnons. We show how this topological effect can
be used as a tool to characterize the phase diagram of chiral magnets and thin
films with interfacial Dzyaloshinskii-Moriya interactions.",1605.02857v2
2016-05-17,Direct observation of dynamic modes excited in a magnetic insulator by pure spin current,"Excitation of magnetization dynamics by pure spin currents has been recently
recognized as an enabling mechanism for spintronics and magnonics, which allows
implementation of spin-torque devices based on low-damping insulating magnetic
materials. Here we report the first spatially-resolved study of the dynamic
modes excited by pure spin current in nanometer-thick microscopic insulating
Yttrium Iron Garnet disks. We show that these modes exhibit nonlinear
self-broadening preventing the formation of the self-localized magnetic bullet,
which plays a crucial role in the stabilization of the single-mode
magnetization oscillations in all-metallic systems. This peculiarity associated
with the efficient nonlinear mode coupling in low-damping materials can be
among the main factors governing the interaction of pure spin currents with the
dynamic magnetization in high-quality magnetic insulators.",1605.05211v1
2016-05-20,Rewritable Artificial Magnetic Charge Ice,"Artificial ices enable the study of geometrical frustration by design and
through direct observation. However, it has proven difficult to achieve
tailored long-range ordering of their diverse configurations, limiting both
fundamental and applied research directions. We designed an artificial spin
structure that produces a magnetic charge ice with tunable long-range ordering
of eight different configurations. We also developed a technique to precisely
manipulate the local magnetic charge states and demonstrate write-read-erase
multifunctionality at room temperature. This globally reconfigurable and
locally writable magnetic charge ice could provide a setting for designing
magnetic monopole defects, tailoring magnonics, and controlling the properties
of other two-dimensional materials.",1605.06209v1
2016-08-02,Picosecond spin Seebeck effect,"We report time-resolved magneto-optic Kerr effect measurements of the
longitudinal spin Seebeck effect driven by an interfacial temperature
difference between itinerant electrons and magnons. The measured time-evolution
of spin accumulation induced by laser-excitation indicates transfer of angular
momentum across Au/Y$_3$Fe$_5$O$_{12}$ and Cu/Y$_3$Fe$_5$O$_{12}$ interfaces on
a picosecond time-scale. The product of spin-mixing conductance and interfacial
spin Seebeck coefficient determined is of the order of $10^8$ A m$^{-2}$
K$^{-1}$.",1608.00702v1
2016-08-11,Pumping approximately integrable systems,"Weak perturbations can drive an interacting many-particle system far from its
initial equilibrium state if one is able to pump into degrees of freedom
approximately protected by conservation laws. This concept has for example been
used to realize Bose-Einstein condensates of photons, magnons, and excitons.
Integrable quantum systems, like the one-dimensional Heisenberg model, are
characterized by an infinite set of conservation laws. Here we develop a theory
of weakly driven integrable systems and show that pumping can induce large spin
or heat currents even in the presence of integrability breaking perturbations,
since it activates local and quasi-local approximate conserved quantities. The
resulting steady state is qualitatively captured by a truncated generalized
Gibbs ensemble with Lagrange parameters that depend on the structure but not on
the overall amplitude of perturbations nor the initial state. We suggest to use
spin-chain materials driven by terahertz radiation to realize
integrability-based spin and heat pumps.",1608.03563v2
2016-08-16,Giant Spin Gap and Magnon Localization in the Disordered Heisenberg Antiferromagnet Sr2Ir1-xRuxO4,"We study the evolution of magnetic excitations in the disordered
two-dimensional antiferromagnet Sr2Ir1-xRuxO4. A gigantic magnetic gap greater
than 40 meV opens at x = 0.27 and increases with Ru concentration, rendering
the dispersive magnetic excitations in Sr2IrO4 almost momentum independent. Up
to a Ru concentration of x = 0.77, both experiments and first-principles
calculations show the Ir Jeff = 1/2 state remains intact. The magnetic gap
arises from the local interaction anisotropy in the proximity of the Ru
disorder. Under the coherent potential approximation, we reproduce the
experimental magnetic excitations using the disordered Heisenberg
antiferromagnetic model with suppressed next-nearest neighbor ferromagnetic
coupling.",1608.04640v2
2016-08-23,Ultrasonic elastic responses in monopole lattice,"The latest experimental advances have extended the scenario of coupling
mechanical degrees of freedom in chiral magnets (MnSi/MnGe) to the
topologically nontrivial skyrmion crystal and even monopole lattices. Equipped
with a spin-wave theory highlighting the topological features, we devise an
interacting model for acoustic phonons and magnons to explain the experimental
findings in a monopole lattice with a topological phase transition, i.e.,
annihilation of monopole-antimonopole pairs. We reproduce the anisotropic
magnetoelastic modulations of elastic moduli: drastic ultrasonic softening
around the phase transition and a multi-peak-and-trench fine structure for
sound waves parallel and orthogonal to the magnetic field, respectively.
Comparison with experiments indicates that the magnetoelastic coupling induced
by Dzyaloshinskii-Moriya interaction is comparable to that induced by the
exchange interaction. Other possibilities such as elastic hardening are also
predicted. The study implies that the monopole defects and their motion in MnGe
play a crucial role.",1608.06362v2
2016-08-25,Temperature-field phase diagram of geometrically frustrated CePdAl,"From the electrical resistivity and ac susceptibility measurements down to
$T$ = 40 mK, a rich temperature-field phase diagram has been constructed for
the geometrically frustrated heavy-fermion compound CePdAl. In the limit of
zero temperature, the field-induced suppression of the antiferromagnetism in
this compound results in a sequence of phase transitions/corssovers, rather
than a single critical point, in a critical region of field 3$-$5 T. In the
lowest temperature region, a power-law description of the temperature-dependent
resistivity yields $A$$\cdot$$T^n$ with $n$$>$2 for all applied fields.
However, a quadratic-in-temperature resistivity term can be extracted when
assuming an additional electron scattering channel from magnon-like
excitations, with $A$ diverging upon approaching the critical regime. Our
observations suggest CePdAl to be a new playground for exotic physics arising
from the competition between Kondo screening and geometrical frustration.",1608.07123v1
2017-06-06,Observation of spin superfluidity: YIG magnetic films and beyond,"From topology of the order parameter of the magnon condensate observed in
yttrium-iron-garnet (YIG) magnetic films one must not expect energetic barriers
making spin supercurrents metastable. But we show that some barriers of
dynamical origin are possible nevertheless until the gradient of the phase
(angle of spin precession) does not exceed the critical value (analog of the
Landau critical velocity in superfluids). On the other hand, recently published
claims of experimental detection of spin superfluidity in YIG films and
antiferromagnets are not justified, and spin superfluidity in magnetically
ordered solids has not yet been experimentally confirmed.",1706.01932v2
2017-06-08,Penrose limits and spin chains in the GJV/CS-SYM duality,"We examine Penrose limits of the duality proposed by Guarino, Jafferis and
Varela between a type IIA massive background of the type of a warped, squashed
$AdS_4\times S^6$, and a 2+1 dimensional IR fixed point of ${\cal N}=8$ super
Yang-Mills deformed by Chern-Simons terms to ${\cal N}=2$ supersymmetry. One
type of Penrose limit for closed strings corresponds to a large charge closed
spin chain, and another, for open strings on giant graviton D-branes,
corresponds to an open spin chain on sub-determinant operators. For the first
limit, we find that like in the ABJM case, there are functions $f_a(\lambda)$
that interpolate between the perturbative and nonperturbative (string) regions
for the magnon energy. For the second, we are unable to match the gravity
result with the expected field theory result, making this model more
interesting than ones with more supersymmetry.",1706.02711v2
2017-06-09,Structure constants of operators on the Wilson loop from integrability,"We study structure constants of local operators inserted on the Wilson loop
in ${\cal N}=4$ super Yang-Mills theory. We compute the structure constants in
the SU(2) sector at tree level using the correspondence between operators on
the Wilson loop and the open spin chain. The results are interpreted as the
summation over all possible ways of changing the signs of magnon momenta in the
hexagon form factors. This is consistent with a holographic description of the
correlator as the cubic open string vertex, which consists of one hexagonal
patch and three boundaries. We then conjecture that a similar expression should
hold also at finite coupling.",1706.02989v3
2017-06-11,Magnetization spin dynamics in a (LuBi)3Fe5O12 (BLIG) epitaxial film,"Bismuth substituted lutetium iron garnet (BLIG) films exhibit larger Faraday
rotation, and have a higher Curie temperature than yttrium iron garnet. We have
observed magnetic stripe domains and measured domain widths of 1.4 {\mu}{\mu}m
using Fourier domain polarization microscopy, Faraday rotation experiments
yield a coercive field of 5 Oe. These characterizations form the basis of
micromagnetic simulations that allow us to estimate and compare spin wave
excitations in BLIG films. We observed that these films support thermal magnons
with a precessional frequency of 7 GHz with a line width of 400 MHz. Further,
we studied the dependence of precessional frequency on the externally applied
magnetic field. Brillouin light scattering experiments and precession
frequencies predicted by simulations show similar trend with increasing field.",1706.03340v1
2017-06-14,Broadband conversion of microwaves into propagating spin waves in patterned magnetic structures,"We have used time-resolved scanning Kerr microscopy (TRSKM) and micromagnetic
simulations to demonstrate that, when driven by spatially uniform microwave
field, the edges of patterned magnetic samples represent both efficient and
highly tunable sources of propagating spin waves. The excitation is due to the
local enhancement of the resonance frequency induced by the non-uniform dynamic
demagnetizing field generated by precessing magnetization aligned with the
edges. Our findings represent a crucial step forward in the design of nanoscale
spin-wave sources for magnonic architectures, and are also highly relevant to
the understanding and interpretation of magnetization dynamics driven by
spatially-uniform magnetic fields in patterned magnetic samples.",1706.04409v2
2017-06-20,Magnetization process and low-temperature thermodynamics of a spin-1/2 Heisenberg octahedral chain,"Low-temperature magnetization curves and thermodynamics of a spin-1/2
Heisenberg octahedral chain with the intraplaquette and monomer-plaquette
interactions are examined within a two-component lattice-gas model of hard-core
monomers, which takes into account all low-lying energy modes in a highly
frustrated parameter space involving the monomer-tetramer, localized
many-magnon and fully polarized ground states. It is shown that the developed
lattice-gas model satisfactorily describes all pronounced features of the
low-temperature magnetization process and the magneto-thermodynamics such as
abrupt changes of the isothermal magnetization curves, a double-peak structure
of the specific heat or a giant magnetocaloric effect.",1706.06370v1
2017-06-21,Phase Diagram and Soliton Picture of an Ideal Spin-Peierls Compound D-F$_{5}$PNN,"We investigate magnetic properties of an $S$=1/2 quasi-one dimensional
organic antiferromagnet, D-F$_{5}$PNN using magnetization measurements taken at
temperatures as low as 0.5 K. Three distinct phases were observed consisting of
uniform, dimerized (D), and incommensurate (I) spin structures in the magnetic
field versus temperature plane, where a significant hysteresis appears between
D-I transitions in the field scan measurements. A combination of magnon ($S$=1)
and soliton ($S$=1/2) excitations have successfully reproduced the observed
magnetic susceptibility. In addition, such excitations provide a reasonable
interpretation of the temperature dependent electron spin resonance (ESR)
spectra. By comparison with the theoretical study, we conclude that
D-F$_{5}$PNN is an ideal compound for investigating the spin-Peierls
transition.",1706.06797v1
2017-06-27,Generalized Lipkin-Meshkov-Glick models of Haldane-Shastry type,"We introduce a class of generalized Lipkin-Meshkov-Glick (gLMG) models with
su$(m)$ interactions of Haldane-Shastry type. We have computed the partition
function of these models in closed form by exactly evaluating the partition
function of the restriction of a spin chain Hamiltonian of Haldane-Shastry type
to subspaces with well-defined magnon numbers. As a byproduct of our analysis,
we have obtained strong numerical evidence of the Gaussian character of the
level density of the latter restricted Hamiltonians, and studied the
distribution of the spacings of consecutive unfolded levels. We have also
discussed the thermodynamic behavior of a large family of su(2) and su(3) gLMG
models, showing that it is qualitatively similar to that of a two-level system.",1706.08751v2
2017-10-10,Lectures on Three-point Functions in N=4 Supersymmetric Yang-Mills Theory,"This is a pedagogical review on the integrability-based approach to the
three-point function in N=4 supersymmetric Yang-Mills theory. We first discuss
the computation of the structure constant at weak coupling and show that the
result can be recast as a sum over partitions of the rapidities of the magnons.
We then introduce a non-perturbative framework, called the ""hexagon approach"",
and explain how one can use the symmetries (i.e. superconformal and gauge
symmetries) and integrability to determine the structure constants. This
article is based on the lectures given in Les Houches Summer School
""Integrability: From statistical systems to gauge theory"" in June 2016.",1710.03853v2
2017-10-15,Itinerant ferromagnetism of the Pd-terminated polar surface of PdCoO$_2$,"We study the electronic structure of the Pd-terminated surface of the
non-magnetic delafossite oxide metal PdCoO$_2$. Combining angle-resolved
photoemission spectroscopy and density-functional theory, we show how an
electronic reconstruction driven by surface polarity mediates a Stoner-like
magnetic instability towards itinerant surface ferromagnetism. Our results
reveal how this leads to a rich multi-band surface electronic structure, and
provide spectroscopic evidence for an intriguing sample-dependent coupling of
the surface electrons to a bosonic mode which we attribute to electron-magnon
interactions. Moreover, we find similar surface state dispersions in PdCrO$_2$,
suggesting surface ferromagnetism persists in this sister compound despite its
bulk antiferromagnetic order.",1710.05392v1
2017-10-27,Colour-dressed hexagon tessellations for correlation functions and non-planar corrections,"We continue the study of four-point correlation functions by the hexagon
tessellation approach initiated in 1611.05436 and 1611.05577. We consider
planar tree-level correlation functions in $\mathcal{N} = 4$ supersymmetric
Yang-Mills theory involving two non-protected operators. We find that, in order
to reproduce the field theory result, it is necessary to include $SU(N)$ colour
factors in the hexagon formalism; moreover, we find that the hexagon approach
as it stands is naturally tailored to the single-trace part of correlation
functions, and does not account for multi-trace admixtures. We discuss how to
compute correlators involving double-trace operators, as well as more general
$1/N$ effects; in particular we compute the whole next-to-leading order in the
large-$N$ expansion of tree-level BMN two-point functions by tessellating a
torus with punctures. Finally, we turn to the issue of ""wrapping"",
L\""uscher-like corrections. We show that $SU(N)$ colour-dressing reproduces an
earlier empirical rule for incorporating single-magnon wrapping, and we provide
a direct interpretation of such wrapping processes in terms of $\mathcal{N}=2$
supersymmetric Feynman diagrams.",1710.10212v2
2017-12-10,Switching of magnons by electric and magnetic fields in multiferroic borates,"Electric manipulation of magnetic properties is a key problem of materials
research. To fulfil the requirements of modern electronics, these processes
must be shifted to high frequencies. In multiferroic materials this may be
achieved by electric and magnetic control of their fundamental excitations.
Here we identify magnetic vibrations in multiferroic iron-borates which are
simultaneously sensitive to external electric and magnetic fields. Nearly 100%
modulation of the terahertz radiation in an external field is demonstrated for
SmFe3(BO3)4. High sensitivity can be explained by a modification of the spin
orientation which controls the excitation conditions in multiferroic borates.
These experiments demonstrate the possibility to alter terahertz magnetic
properties of materials independently by external electric and magnetic fields.",1712.03501v1
2017-12-11,Topological Mott Insulator with Bosonic Edge Modes in 1D Fermionic Superlattices,"We investigate topological phase transitions driven by interaction and
identify a novel topological Mott insulator state in one-dimensional fermionic
optical superlattices through numerical density matrix renormalization group
(DMRG) method. Remarkably, the low-energy edge excitations change from spin-1/2
fermionic single-particle modes to spin-1 bosonic collective modes across the
phase transition. Due to spin-charge separation, the low-energy theory is
governed by an effective spin superexchange model, whereas the charge degree of
freedom is fully gapped out. Such topological Mott state can be characterized
by a spin Chern number and gapless magnon modes protected by a finite spin gap.
The proposed experimental setup is simple and may pave the way for the
experimental observation of exotic topological Mott states.",1712.03671v1
2017-12-22,Magnetically Ordered Insulators for Advanced Spintronics,"Magnetically ordered, electrically insulating materials pave the way towards
novel spintronic devices. In these materials the flow of magnetic excitations
such as magnons results in pure spin currents. These spin currents can be
driven by gradients of the spin chemical potential and/or temperature such that
they can play the same role in novel spintronic devices as charge currents in
traditional electronic circuits. Connecting spin current based and charge
current based devices requires spin to charge interconversion. This has been
achieved by the spin Hall effect with an efficiency of several 10%. The recent
progress in materials development and understanding of pure spin current
physics opens up a plethora of novel device concepts and opportunities for
fundamental studies.",1712.08517v1
2018-09-25,PT-Symmetric magnetic Chaos in cavity magnomechanics,"Here, we research a novel cavity magnomechanical system, where magnon driven
by a microwave field couples with a phonon mode with a nonlinear
magneostrictive interaction (radiation pressure-like). Based on this
interaction, we numerically demonstrate PT-symmetric chaos in this system. With
only one monochrome driving, the chaotic threshold is lowered to a rather low
levels, this is due to the dynamical enhancement of nonlinearity in the
PT-symmetry broken phase. Moreover, by simply manipulating the phase transition
between PT-symmetry phase and PT-symmetry broken phase, we can switch the
system between into and out of chaotic regimes. Our work may broaden the cavity
magnomechanics and provide a promising application for magnetic chaos-related
security communications.",1809.09309v1
2019-09-05,The critical nature of the Ni spin state in doped NdNiO$_2$,"Superconductivity with $T_c \approx 15K$ was recently found in doped
NdNiO$_2$. The Ni$^{1+}$O$_2$ layers are expected to be Mott insulators so hole
doping should produce Ni$^{2+}$ with $S=1$, incompatible with robust
superconductivity. We show that the NiO$_2$ layers fall inside a ``critical''
region where the large $pd$ hybridization favors a singlet $^1\!A_1$ hole-doped
state like in CuO$_2$. However, we find that the superexchange is about one
order smaller than in cuprates, thus a magnon ``glue'' is very unlikely and
another mechanism needs to be found.",1909.02557v3
2019-09-10,Anisotropic magnetic excitations of a frustrated bilinear-biquadratic spin model -- Implications for spin waves of detwinned iron pnictides,"Elucidating the nature of spin excitations is important to understanding the
mechanism of superconductivity in the iron pnictides. Motivated by recent
inelastic neutron scattering measurements in the nearly 100% detwinned
BaFe$_{2}$As$_{2}$, we study the spin dynamics of an $S=1$ frustrated
bilinear-biquadratic Heisenberg model in the antiferromagnetic phase with
wavevector $(\pi,0)$. The biquadratic interactions are treated in a dynamical
way using a flavor-wave theory in an $SU(3)$ representation. Besides the magnon
(dipolar) excitations, the biquadratic interactions give rise to quadrupolar
excitations at high energies. We find that the quadrupolar wave significantly
influences, in an energy dependent way, the anisotropy between the spin
excitation spectra along the $(\pi,0)$ and $(0,\pi)$ directions in the wave
vector space. Our theoretical results capture the essential behavior of the
spin dynamics measured in the antiferromagnetic phase of the detwinned
BaFe$_2$As$_2$. More generally, our results underscore the importance of
electron correlation effects for the microscopic physics of the iron pnictides.",1909.04555v2
2019-09-11,Spin dynamics of the planar kagome lattice ferromagnet with four-site ring exchange processes,"By means of quantum Monte Carlo simulations, combined with a stochastic
analytic continuation, we examine the spin dynamics of the spin-1/2 planar (XY)
ferromagnet on the kagome lattice with additional four-site ring exchange
terms. Such exchange processes were previously considered to lead into an
extended $Z_2$ quantum spin liquid phase beyond a quantum critical point from
the XY-ferromagnet. We examine the dynamical spin structure factor in the
non-magnetic regime and probe for signatures of spin fractionalization.
Furthermore, we contrast our findings and the corresponding energy scales of
the excitation gaps in the ring exchange model to those emerging in a related
Balents-Fisher-Girvin model with a $Z_2$ quantum spin liquid phase, and monitor
the softening of the magnon mode upon approaching the quantum critical point
from the XY-ferromagnetic regime.",1909.04877v1
2019-09-27,Thermally driven two-magnet nano-oscillator with large spin-charge conversion,"Next-generation spintronic applications require material properties that can
be hardly met by one material candidate. Here we demonstrate that by combining
insulating and metallic magnets, enhanced spin-charge conversion and
energy-efficient thermal spin currents can be realized. We develop a nanowire
device consisting of an yttrium iron garnet and permalloy bi-layer. An
interfacial temperature gradient drives the nanowire magnetization into
auto-oscillations at gigahertz frequencies. Interfacial spin coupling and
magnetoresistance of the permalloy layer translate spin dynamics into sizable
microwave signals. The results show prospect for energy-efficient spintronic
devices and present an experimental realization of magnon condensation in a
heterogeneous magnetic system.",1909.12445v1
2019-09-27,Neumann-Rosochatius system for strings in ABJ Model,"Neumann-Rosochatius system is a well known one dimensional integrable system.
We study the rotating and pulsating string in $AdS_4 \times \mathbb{CP}^3$ with
a $B_{\rm{NS}}$ holonomy turned on over $\mathbb{CP}^1 \subset \mathbb{CP}^3$,
or the so called Aharony-Bergman-Jafferis (ABJ) background. We observe that the
string equations of motion in both cases are integrable and the Lagrangians
reduce to a form similar to that of deformed Neuman-Rosochatius system. We find
out the scaling relations among various conserved charges and comment on the
finite size effect for the dyonic giant magnons on $R_{t}\times
\mathbb{CP}^{3}$ with two angular momenta. For the pulsating string we derive
the energy as function of oscillation number and angular momenta along
$\mathbb{CP}^{3}$.",1909.12632v3
2020-12-01,Hole in the 2D Ising Antiferromagnet: Origin of the Incoherent Spectrum,"We develop a ""self-avoiding walks"" approximation and use it to calculate the
spectral function of a single hole introduced into the 2D square lattice Ising
antiferromagnet. The obtained local spectral function qualitatively agrees with
the exact diagonalisation result and is largely incoherent. Such a result stays
in contrast with the spectrum obtained on a Bethe lattice, which consists of
the well-separated quasiparticle-like peaks and stems from the motion of a hole
in an effective linear potential. We determine that this onset of the
incoherent spectrum on a square lattice (i) is not triggered by the so-called
Trugman loops but (ii) originates in the warping of the linear potential by the
interactions between magnons created along the tangential paths of the moving
hole.",2012.00395v2
2020-12-05,Theory of magnetism in the van der Waals magnet CrI3,"We study the microscopical origin of anisotropic ferromagnetism in the van
der Waals magnet CrI3. We conclude that the nearest neighbors exchange is well
described by the Heisenberg-Kitaev-Gamma (HKGamma) model, and we also found a
nonzero Dzyaloshinskii-Moriya interaction (DMI) on next nearest neighbors. Both
Kitaev and DMI are known to generate a non-trivial topology of the magnons in
the honeycomb lattice and have been used separately to describe the low energy
regime of this material. We discuss that including one or the other leads to
different signs of the Chern number. Furthermore, the topological gap at
K-point seems to be mainly produced by DMI, despite it is one order of
magnitude smaller than Kitaev. Finally, we show that by applying an external
electric field perpendicular to the crystal plane, it is possible to induce DMI
on nearest neighbors, and this could have consequences in non-collinear spin
textures, such as domain walls and skyrmions.",2012.03099v2
2020-12-11,Compositional effect on auto-oscillation behavior of Ni100-xFex/Pt spin Hall nano-oscillators,"We demonstrate the compositional effect on the magnetodynamic and
auto-oscillations properties of Ni100-xFex/Pt (x= 10 to 40) nanoconstriction
based spin Hall nano-oscillators. Using spin-torque ferromagnetic resonance
(ST-FMR) performed on microstrips, we measure a significant reduction in both
damping and spin Hall efficiency with increasing Fe content, which lowers the
spin pumping contribution. The strong compositional effect on spin Hall
efficiency is primarily attributed to the increased saturation magnetization in
Fe-rich devices. As a direct consequence, higher current densities are required
to drive spin-wave auto-oscillations at higher microwave frequencies in Fe-rich
nano-constriction devices. Our results establish the critical role of the
compositional effect in engineering the magnetodynamic and auto-oscillation
properties of spin Hall devices for microwav eand magnonic applications.",2012.06121v2
2020-12-11,From one- to two-magnon excitations in the S=3/2 magnet $β$-CaCr$_2$O$_4$,"We apply neutron spectroscopy to measure the magnetic dynamics in the S=3/2
magnet $\beta$-CaCr$_2$O$_4$ (T$_N$=21 K). The low-energy fluctuations, in the
ordered state, resemble large-S linear spin-waves from the incommensurate
ground state. However, at higher energy transfers, these semi-classical and
harmonic dynamics are replaced by an energy and momentum broadened continuum of
excitations. Applying kinematic constraints required for energy and momentum
conservation, sum rules of neutron scattering, and comparison against exact
diagonalization calculations, we show that the dynamics at high-energy
transfers resemble low-S one-dimensional quantum fluctuations.
$\beta$-CaCr$_2$O$_4$ represents an example of a magnet at the border between
classical N\'eel and quantum phases, displaying dual characteristics.",2012.06314v2
2020-12-12,Magnetization Dynamics of Fibonacci-Distorted Kagome Artificial Spin Ice,"We present results of ferromagnetic resonance (FMR) experiments and
micromagnetic simulations for a distorted, 2D Kagome artificial spin ice. The
distorted structure is created by continuously modulating the 2D primitive
lattice translation vectors of a periodic honeycomb lattice, according to an
aperiodic Fibonacci sequence used to generate 1D quasicrystals. Experimental
data and micromagnetic simulations show the Fibonacci distortion causes
broadening and splitting of FMR modes into multiple branches, which accompany
the increasing number of segment lengths and orientations that develop with
increasing distortion. When the applied field is increased in the opposite
direction to the net magnetization of a segment, spin wave modes appear,
disappear or suddenly shift, to signal segment magnetization reversal events.
These results show the complex behavior of reversal events, as well as
well-defined frequencies and frequency-field slopes of FMR modes, can be
precisely tuned by varying the severity of the aperiodic lattice distortion.
This type of distorted structure could therefore provide a new tool for the
design of complicated magnonic systems.",2012.06837v1
2020-12-14,Non-collinear ground state from a four-spin chiral exchange in a tetrahedral magnet,"We propose a quartic chiral term $m_x m_y m_z \nabla \cdot \mathbf{m}$ for
the energy density of a cubic ferromagnet with broken parity symmetry (point
group $T_d$). We demonstrate that this interaction causes a phase transition
from a collinear ferromagnetic state to a non-collinear magnetic cone ground
state provided its strength exceeds the geometric mean of magnetic exchange and
cubic anisotropy. The corresponding non-collinear ground state may also be
additionally stabilized by an external magnetic field pointing along certain
crystallographic directions. The four-spin chiral exchange does also manifest
itself in peculiar magnon spectra and favors spin waves with the wave vector
that is perpendicular to the average magnetization direction.",2012.07666v1
2020-12-17,Geometric magnonics with chiral magnetic domain walls,"Spin wave, the collective excitation of magnetic order, is one of the
fundamental angular momentum carriers in magnetic systems. Understanding the
spin wave propagation in magnetic textures lies in the heart of developing pure
magnetic information processing schemes. Here we show that the spin wave
propagation across a chiral domain wall follows simple geometric trajectories,
similar to the geometric optics. And the geometric behaviors are qualitatively
different in normally magnetized film and tangentially magnetized film. We
identify the lateral shift, refraction, and total reflection of spin wave
across a ferromagnetic domain wall. Moreover, these geometric scattering
phenomena become polarization-dependent in antiferromagnets, indicating the
emergence of spin wave birefringence inside antiferromagnetic domain wall.",2012.09420v1
2020-12-18,Breaking Space Inversion-Symmetry to Obtain Asymmetric Spin-Wave Excitation in Systems with Nonuniform Magnetic Exchange,"We report on the consequences of non-uniform exchange in magnetic systems.
The quantum mechanical exchange interaction between spins is responsible for
the phenomenon of magnetic order, and is generally considered to be uniform
across bulk magnetic systems. Partly inspired by the Dzyaloshinskii-Moriya
interaction--also known as antisymmetric exchange--we use a linearly varying
exchange interaction along a magnetic strip as a route to spatial inversion
symmetry-breaking. We find that, in addition to asymmetric modes and
localization, spatially-varying exchange can be used to design nonreciprocal
magnetic signal excitation at frequencies that are tunable. Moreover, our work
predicts nonreciprocity to occur across a vast range of frequencies up to
hundreds of GHz. Such spin wave engineering is a key area of ongoing research
in the fields of magnonics and spintronics, which are expected to enable the
next generation of communication technology. Analogous nonreciprocity is
expected to occur in other wave systems with gradient properties.",2012.10381v2
2020-12-21,Effective quantum dynamics induced by a driven two-level-system bath,"We derive a Born-Markov master equation describing the dissipation induced by
a bath of lossy but coherently driven two-level systems (TLS) coupled to a
bosonic system via Jaynes-Cummings interaction. We analytically derive all the
master equation rates. We characterize these rates for the particular case of a
single-mode system coupled to identical TLS. We study the steady state of the
system and its exotic properties stemming from the non-thermal stationary state
of the driven TLS bath. These properties include dissipative amplification,
bath-induced linear instability, and both coherent and dissipative squeezing.
The master equation is valid for arbitrarily strong TLS driving, and it can be
generalized to include multi-level systems or other system-bath interaction
terms, among others. Our work provides a tool to study and characterize
TLS-induced decoherence, a key limiting factor in quantum technological devices
based on, for instance, superconducting circuits, magnonic systems, or quantum
acoustics.",2012.11235v2
2020-12-29,Topological to magnetically ordered quantum phase transition in antiferromagnetic spin ladders with long-range interactions,"We study a generalized quantum spin ladder with staggered long range
interactions that decay as a power-law with exponent $\alpha$. Using large
scale quantum Monte Carlo (QMC) and density matrix renormalization group (DMRG)
simulations, we show that this model undergoes a transition from a rung-dimer
phase characterized by a non-local string order parameter, to a symmetry broken
N\'eel phase. We find evidence that the transition is second order. In the
magnetically ordered phase, the spectrum exhibits gapless modes, while
excitations in the gapped phase are well described in terms of triplons --
bound states of spinons across the legs. We obtain the momentum resolved spin
dynamic structure factor numerically and find a well defined triplon band that
evolves into a gapless magnon dispersion across the transition. We further
discuss the possibility of deconfined criticality in this model.",2012.14908v5
2012-05-04,Longitudinal excitations in triangular lattice antiferromagnets,"We study the longitudinal excitations of quantum antiferromagnets on a
triangular lattice by a recently proposed microscopic many-body approach based
on magnon-density waves. We calculate the full longitudinal excitation spectra
of the antiferromagnetic Heisenberg model for a general spin quantum number in
the isotropic limit. Similar to the square lattice model, we find that, at the
center of the first hexagonal Brillouin zone $\Gamma(\mathbf q=0)$ and at the
magnetic ordering wavevectors $\pm[\mathbf Q= (4\pi/3,0)]$, the excitation
spectra become gapless in the thermodynamic limit, due to the slow, logarithmic
divergence of the structure factor. However, these longitudinal modes on
two-dimensional models may be considered as quasi-gapped, as any finite-size
effect or small anisotropy will induce a large energy gap, when compared with
the counterpart of the transverse spin-wave excitations. We also discuss a
possible second longitudinal mode in the triangular lattice model due to the
noncollinear nature of its magnetic order.",1205.0977v2
2012-05-16,Softened magnetic excitations in the s = 3/2 distorted triangular antiferromagnet alpha-CaCr2O4,"The spin dynamics and magnetic excitations of the slightly distorted
triangular s = 3/2 system alpha-CaCr2O4 are investigated by means of Raman
spectroscopy and electron spin resonance (ESR) to elucidate its peculiar
magnetic properties. Two-magnon excitations in circular RL symmetry show a
multi-maximum structure with a dominant spectral weight at low energies. The
temperature dependence of the ESR linewidth is described by a critical
broadening DeltaHpp(T) ~ (T - T_N)^{-p} with the exponent p = 0.30(3) - 0.38(5)
for temperatures above T_N = 42.6 K. The exponent is much smaller than that of
other s = 3/2 triangular lattices. This is ascribed to soft roton-like modes,
indicative of the instability of a helical 120{\deg} phase. As an origin we
discuss a complex spin topology formed by four inequivalent nearest neighbor
and sizable next-nearest neighbor interactions.",1205.3640v2
2012-05-16,Symmetry Analysis of Holes Localized on a Skyrmion in a Doped Antiferromagnet,"We use the low-energy effective field theory for holes coupled to the
staggered magnetization in order to investigate the localization of holes on a
Skyrmion in a square lattice antiferromagnet. When two holes get localized on
the same Skyrmion, they form a bound state. The quantum numbers of the bound
state are determined by the quantization of the collective modes of the
Skyrmion. Remarkably, for p-wave states the quantum numbers are the same as
those of a hole-pair bound by one-magnon exchange. Two holes localized on a
Skyrmion with winding number $n = 1$ or 2 may have s- or d-wave symmetry as
well. Possible relations with preformed Cooper pairs of high-temperature
superconductors are discussed.",1205.3677v1
2012-05-29,Hybrid paramagnon phonon modes at elevated temperatures in EuTiO3,"EuTiO3 (ETO) has recently experienced an enormous revival of interest because
of its possible multiferroic properties which are currently in the focus of
research. Unfortunately ETO is an unlikely candidate for enlarged
multifunctionality since the mode softening - typical for ferroelectrics -
remains incomplete, and the antiferromagnetic properties appear at 5.5K only.
However, a strong coupling between lattice and Eu spins exists and leads to the
appearance of a magnon-phonon-hybrid mode at elevated temperatures as evidenced
by electron paramagnetic resonance (EPR), muon spin rotation ({\mu}SR)
experiments and model predictions based on a coupled spin-polarizability
Hamiltonian. This novel finding supports the notion of strong
magneto-dielectric (MD) effects being realized in ETO and opens new strategies
in material design and technological applications.",1205.6287v1
2017-05-10,Hierarchy of correlations for the Ising model in the Majorana representation,"We study the quantum Ising model in D dimensions with the equation of motion
technique, in combination with the Majorana representation for spins. The
decoupling scheme used for the Green's functions is based on the hierarchy of
correlations in position space. To lowest order this method reproduces the well
known mean field phase diagram and critical exponents. When correlations
between spins are included, we show how the appearance of thermal fluctuations
and magnons strongly affect the physical properties. We demonstrate that in 1D
and for B=0, thermal fluctuations completely destroy the ordered phase, and
that near the quantum critical point, the quantum model displays different
behavior than its classical counterpart. We discuss the connection with the
Dyson's equation formalism and the explicit form of the self-energies.",1705.03961v2
2017-05-17,Quantum criticality in the coupled two-leg spin ladder Ba2CuTeO6,"We report on zero-field muon spin rotation, electron spin resonance and
polarized Raman scattering measurements of the coupled quantum spin ladder
Ba2CuTeO6. Zero-field muon spin rotation and electron spin resonance probes
disclose a successive crossover from a paramagnetic through a spin-liquid-like
into a magnetically ordered state with decreasing temperature. More
significantly, the two-magnon Raman response obeys a T-linear scaling relation
in its peak energy, linewidth and intensity. This critical scaling behavior
presents an experimental signature of proximity to a quantum critical point
from an ordered side in Ba2CuTeO6.",1705.06035v1
2017-05-18,Topological quantum paramagnet in a quantum spin ladder,"It has recently been found that bosonic excitations of ordered media, such as
phonons or spinons, can exhibit topologically nontrivial band structures. Of
particular interest are magnon and triplon excitations in quantum magnets, as
they can easily be manipulated by an applied field. Here we study triplon
excitations in an S=1/2 quantum spin ladder and show that they exhibit
nontrivial topology, even in the quantum-disordered paramagnetic phase. Our
analysis reveals that the paramagnetic phase actually consists of two separate
regions with topologically distinct triplon excitations. We demonstrate that
the topological transition between these two regions can be tuned by an
external magnetic field. The winding number that characterizes the topology of
the triplons is derived and evaluated. By the bulk-boundary correspondence, we
find that the non-zero winding number implies the presence of localized triplon
end states. Experimental signatures and possible physical realizations of the
topological paramagnetic phase are discussed.",1705.06764v3
2017-05-29,High pressure Raman study of the quantum magnet (C$_4$H$_{12}$N$_2$)Cu$_2$Cl$_6$,"Magnetic and lattice excitations in the quantum antiferromagnet
(C$_4$H$_{12}$N$_2$)Cu$_2$Cl$_6$ (PHCC) are studied across two pressure-induced
phase transition at $P_c=4.3~\mathrm{kbar}$ and $P_1=13.4~\mathrm{kbar}$ using
Raman spectroscopy. It is confirmed that neither transition is a result of a
structural transformation. Magnetic scattering is detected. It shows a
pronounced pressure dependence and undergoes substantial changes at both
transitions. The results are in clear contradiction with previous neutron
studies, which detected only minor changes of the magnon spectrum at $P_1$. A
number of phonons show anomalous frequency shifts at low temperatures. This
effect is pressure dependent and for two of the observed phonons dramatically
reverses sign at around $P_1$. The anomalous behavior is attributed to strong
magnetoelastic coupling in PHCC.",1705.10235v3
2017-05-30,Communication and information processing in magnetic nanostructures with edge spin waves,"Low dissipation data processing with spins is one of the promising directions
for future information and communication technologies. Despite a signifcant
progress, the available magnonic devices are not broadband yet and have
restricted capabilities to redirect spin waves. Here we propose a breakthrough
approach to the spin wave manipulation in patterned magnetic nanostructures
with unmatched characteristics, which exploits spin waves analogous to edge
waves propagating along a water-wall boundary. Using theory, micromagnetic
simulations and experiment we investigate spin waves propagating along the
edges in magnetic structures, under an in-plane DC magnetic field inclined with
respect to the edge. The proposed edge spin waves overcome important challenges
faced by previous technologies such as the manipulation of the spin wave
propagation direction, and they substantially improve the capability of
transmitting information at frequencies exceeding 10 GHz. The concept of the
edge spin waves allows to design broad range of logic devices such as
splitters, interferometers, or edge spin wave transistors with unprecedented
characteristics and potentially strong impact on information technologies.",1705.10536v2
2017-09-26,Magnetic-field-induced suppression of spin Peltier effect in Pt/${\rm Y_{3}Fe_{5}O_{12}}$ system at room temperature,"We report the observation of magnetic-field-induced suppression of the spin
Peltier effect (SPE) in a junction of a paramagnetic metal Pt and a
ferrimagnetic insulator ${\rm Y_{3}Fe_{5}O_{12}}$ (YIG) at room temperature.
For driving the SPE, spin currents are generated via the spin Hall effect from
applied charge currents in the Pt layer, and injected into the adjacent thick
YIG film. The resultant temperature modulation is detected by a commonly-used
thermocouple attached to the Pt/YIG junction. The output of the thermocouple
shows sign reversal when the magnetization is reversed and linearly increases
with the applied current, demonstrating the detection of the SPE signal. We
found that the SPE signal decreases with the magnetic field. The observed
suppression rate was found to be comparable to that of the spin Seebeck effect
(SSE), suggesting the dominant and similar contribution of the low-energy
magnons in the SPE as in the SSE.",1709.08997v1
2017-09-29,Non-local Gilbert damping tensor within the torque-torque correlation model,"An essential property of magnetic devices is the relaxation rate in magnetic
switching which depends strongly on the damping in the magnetisation dynamics.
It was recently measured that damping depends on the magnetic texture and,
consequently, is a non-local quantity. The damping enters the
Landau-Lifshitz-Gilbert equation as the phenomenological Gilbert damping
parameter $\alpha$, that does not, in a straight forward formulation, account
for non-locality. Efforts were spent recently to obtain Gilbert damping from
first principles for magnons of wave vector $\mathbf{q}$. However, to the best
of our knowledge, there is no report about real space non-local Gilbert damping
$\alpha_{ij}$. Here, a torque-torque correlation model based on a tight binding
approach is applied to the bulk elemental itinerant magnets and it predicts
significant off-site Gilbert damping contributions, that could be also
negative. Supported by atomistic magnetisation dynamics simulations we reveal
the importance of the non-local Gilbert damping in atomistic magnetisation
dynamics. This study gives a deeper understanding of the dynamics of the
magnetic moments and dissipation processes in real magnetic materials. Ways of
manipulating non-local damping are explored, either by temperature, material's
doping or strain.",1709.10365v1
2018-03-05,Low-Energy Excitations in Quantum Spin-Liquids Identified by Optical Spectroscopy,"The electrodynamic response of organic spin liquids with highly-frustrated
triangular lattices has been measured in a wide energy range. While the overall
optical spectra of these Mott insulators are governed by transitions between
the Hubbard bands, distinct in-gap excitations can be identified at low
temperatures and frequencies which we attribute to the quantum spin liquid
state. For the strongly correlated
$\beta^{\prime}$-EtMe$_3$\-Sb\-[Pd(dmit)$_2$]$_2$, we discover enhanced
conductivity below $175~{\rm cm}^{-1}$, comparable to the energy of the
magnetic coupling $J\approx 250$ K. For $\omega\rightarrow 0$ these
low-frequency excitations vanish faster than the charge-carrier response
subject to Mott-Hubbard correlations, resulting in a dome-shape band peaked at
100~\cm. Possible relations to spinons, magnons and disorder are discussed.",1803.01553v2
2018-03-12,Focused ion beam direct writing of magnetic patterns with controlled structural and magnetic properties,"Focused ion beam irradiation of metastable Fe$_{78}$Ni$_{22}$ thin films
grown on Cu(100) substrates is used to create ferromagnetic,
body-centered-cubic patterns embedded into paramagnetic, face-centered-cubic
surrounding. The structural and magnetic phase transformation can be controlled
by varying parameters of the transforming gallium ion beam. The focused ion
beam parameters as ion dose, number of scans, and scanning direction can be
used not only to control a degree of transformation, but also to change the
otherwise four-fold in-plane magnetic anisotropy into the uniaxial anisotropy
along specific crystallographic direction. This change is associated with a
preferred growth of specific crystallographic domains. The possibility to
create magnetic patterns with continuous magnetization transitions and at the
same time to create patterns with periodical changes in magnetic anisotropy
makes this system an ideal candidate for rapid prototyping of a large variety
of nanostructured samples. Namely spin-wave waveguides and magnonic crystals
can be easily combined into complex devices in a single fabrication step.",1803.04348v1
2018-03-18,Enhancement of photon blockade effect via quantum interference,"We study the photon blockade effect in a coupled cavity system, which is
formed by a linear cavity coupled to a Kerr-type nonlinear cavity via a
photon-hopping interaction. We explain the physical phenomenon from the
viewpoint of the conventional and unconventional photon blockade effects. The
corresponding physical mechanisms of the two kinds of photon blockade effects
are based on the anharmonicity in the eigenenergy spectrum and the destructive
quantum interference between two different transition paths, respectively. In
particular, we find that the photon blockade via destructive quantum
interference also exists in the conventional photon blockade regime, and that
the unconventional photon blockade occurs in both the weak- and strong-Kerr
nonlinearity cases. The photon blockade effect can be observed by calculating
the second-order correlation function of the cavity field. This model is
general and hence it can be implemented in various experimental setups such as
coupled optical-cavity systems, coupled photon-magnon systems, and coupled
superconducting-resonator systems. We present some discussions on the
experimental feasibility.",1803.06642v2
2018-06-02,Ultra-low damping insulating magnetic thin films get perpendicular,"A magnetic material combining low losses and large Perpendicular Magnetic
Anisotropy (PMA) is still a missing brick in the magnonic and spintronic
fields. We report here on the growth of ultrathin Bismuth doped
Y$_{3}$Fe$_{5}$O$_{12}$ (BiYIG) films on Gd$_{3}$Ga$_{5}$O$_{12}$ (GGG) and
substituted GGG (sGGG) (111) oriented substrates. A fine tuning of the PMA is
obtained using both epitaxial strain and growth induced anisotropies. Both
spontaneously in-plane and out-of-plane magnetized thin films can be
elaborated. Ferromagnetic Resonance (FMR) measurements demonstrate the high
dynamic quality of these BiYIG ultrathin films, PMA films with Gilbert damping
values as low as 3 10$^{-4}$ and FMR linewidth of 0.3 mT at 8 GHz are achieved
even for films that do not exceed 30 nm in thickness. Moreover, we measure
Inverse Spin Hall Effect (ISHE) on Pt/BiYIG stacks showing that the magnetic
insulator$'$s surface is transparent to spin current making it appealing for
spintronic applications.",1806.00658v1
2018-06-05,Backscattering immunity of dipole-exchange magnetostatic surface spin waves,"The existence of backscattering-immune spin-wave modes is demonstrated in
magnetic thin films of nano-scale thickness. Our results reveal that chiral
Magneto Static Surface Waves (cMSSWs), which propagate perpendicular to the
magnetization direction in an in-plane magnetized thin film, are robust against
backscattering from surface defects. cMSSWs are protected against various types
of surface inhomogeneities and defects as long as their frequency lies inside
the gap of the volume modes. Our explanation is independent of the topology of
the modes and predicts that this robustness is a consequence of symmetry
breaking of the dynamic magnetic fields of cMSSWs due to the off-diagonal part
of the dipolar interaction tensor, which is present both for long- (dipole
dominated) and short-wavelength (exchange dominated) spin waves. Micromagnetic
simulations confirm the robust character of the cMSSWs. Our results open a new
direction in designing highly efficient magnonic logic elements and devices
employing cMSSWs in nano-scale thin films.",1806.01554v3
2018-06-06,Spin liquid fingerprints in the thermal transport of a Kitaev-Heisenberg ladder,"We identify fingerprints of a proximate quantum spin-liquid (QSL), observable
by finite-temperature dynamical thermal transport within a minimal version of
the idealized Kitaev model on a two-leg ladder, if subjected to inevitably
present Heisenberg couplings. Using exact diagonalization and quantum
typicality, we uncover (i) an insulator-conductor crossover induced by fracton
recombination at infinitesimal Heisenberg coupling, (ii) low- and high-energy
signatures of fractons, which survive far off the pure QSL point, and (iii) a
non-monotonous current life-time versus Heisenberg couplings. Guided by
perturbation theory, we find (iv) a Kitaev-exchange induced `one-magnon'
contribution to the dynamical heat transport in the strong Heisenberg rung
limit.",1806.02344v1
2018-06-07,On the magnetic and electronic properties of NpPdSn,"We have studied NpPdSn by means of the heat capacity, electrical resistivity,
Seebeck and Hall effect, $^{237}$Np M\""{o}ssbauer spectroscopy, and neutron
diffraction measurements in the temperature range 2-300 K and under magnetic
fields up to 14 T. NpPdSn orders antiferromagnetically below the N\'eel
temperature $T_N$ = 19 K and shows localized magnetism of Np$^{3+}$ ion with a
a doubly degenerate ground state. In the magnetic state the electrical
resistivity and heat capacity are characterized by electron-magnon scattering
with spin-waves spectrum typical of anisotropic antiferromagnets. An enhanced
Sommerfeld coefficient and typical behavior of magnetorestistivity, Seebeck and
Hall coefficients are all characteristic of systems with strong electronic
correlations. The low temperature antiferromagnetic state of NpPdSn is verified
by neutron diffraction and $^{237}$Np M\""{o}ssbauer spectroscopy and possible
magnetic structures are discussed.",1806.02879v1
2018-06-13,Nature of the spin liquid in underdoped cuprate superconductors,"In the present work we address a long standing problem of the magnetic ground
state and magnetic excitations in underdoped cuprates. Modelling cuprates by
the extended $t-J$ model we show that there is a hidden dimensionless parameter
$\lambda$ which drives magnetic criticality at low doping $x$. Hence we derive
the zero temperature $\lambda-x$ phase diagram of the model. It is argued that
all underdoped cuprates are close to the quantum tricritical point $x=0$,
$\lambda=1$. The three phases ""meet"" at the tricritical point: (i) N\'eel
antiferromagnet, (ii) spin spiral with antinodal direction of the spiral wave
vector, (iii) algebraic spin liquid. We argue that underdoped cuprates belong
either to the spin liquid phase or they are on the borderline between the spin
liquid and the spin spiral. We calculate the energy position $E_{cross}$ of the
inelastic neutron scattering response maximum at ${\bm q}=(\pi,\pi)$ and
compare our results with experiments. We also explain softening of magnons in
the intermediate regime observed in inelastic neutron scattering.",1806.04855v1
2018-06-13,Effective Ising model for correlated systems with charge ordering,"Collective electronic fluctuations in correlated materials give rise to
various important phenomena, such as existence of the charge ordering,
superconductivity, Mott insulating and magnetic phases, plasmon and magnon
modes, and other interesting features of such systems. Unfortunately,
description of these correlation effects requires significant efforts, since
they almost entirely rely on strong local and nonlocal electron-electron
interactions. Some collective phenomena, such as magnetism, can be sufficiently
described by a simple Heisenberg-like models that are formulated in terms of
bosonic variables. This fact suggests that other many-body excitations can also
be described by simple bosonic models in spirit of the Heisenberg theory. Here
we derive an effective bosonic action for charge degrees of freedom for the
extended Hubbard model and define a physical regime where the obtained action
reduces to a classical Hamiltonian of an effective Ising model.",1806.04927v3
2018-06-14,Magnetism of the N=42 kagome lattice antiferromagnet,"For the paradigmatic frustrated spin-half Heisenberg antiferromagnet on the
kagome lattice we performed large-scale numerical investigation of
thermodynamic functions by means of the finite-temperature Lanczos method for
system sizes of up to N=42. We present the dependence of magnetization as well
as specific heat on temperature and external field and show in particular that
a finite-size scaling of specific heat supports the appearance of a
low-temperature shoulder below the major maximum. This seems to be the result
of a counterintuitive motion of the density of singlet states towards higher
energies. Other interesting features that we discuss are the asymmetric melting
of the 1/3 magnetization plateau as well the field dependence of the specific
heat that exhibits characteristic features caused by the existence of a flat
one-magnon band. By comparison with the unfrustrated square-lattice
antiferromagnet the tremendous role of frustration in a wide temperature range
is illustrated.",1806.05541v1
2018-06-15,Positivity of hexagon perturbation theory,"The hexagon-form-factor program was proposed as a way to compute three- and
higher-point correlation functions in $\mathcal{N}=4$ super-symmetric
Yang-Mills theory and in the dual AdS$_5\times$S$^5$ superstring theory, by
exploiting the integrability of the theory in the 't Hooft limit. This approach
is reminiscent of the asymptotic Bethe ansatz in that it applies to a
large-volume expansion. Finite-volume corrections can be incorporated through
L\""uscher-like formulae, though the systematics of this expansion is largely
unexplored so far. Strikingly, finite-volume corrections may feature negative
powers of the 't Hooft coupling $g$ in the small-$g$ expansion, potentially
leading to a breakdown of the formalism. In this work we show that the
finite-volume perturbation theory for the hexagon is positive and thereby
compatible with the weak-coupling expansion for arbitrary $n$-point functions.",1806.06051v2
2018-06-18,Cavity optomagnonics with magnetic textures: coupling a magnetic vortex to light,"Optomagnonic systems, where light couples coherently to collective
excitations in magnetically ordered solids, are currently of high interest due
to their potential for quantum information processing platforms at the
nanoscale. Efforts so far, both at the experimental and theoretical level, have
focused on systems with a homogeneous magnetic background. A unique feature in
optomagnonics is however the possibility of coupling light to spin excitations
on top of magnetic textures. We propose a cavity-optomagnonic system with a non
homogeneous magnetic ground state, namely a vortex in a magnetic microdisk. In
particular we study the coupling between optical whispering gallery modes to
magnon modes localized at the vortex. We show that the optomagnonic coupling
has a rich spatial structure and that it can be tuned by an externally applied
magnetic field. Our results predict cooperativities at maximum photon density
of the order of $\mathcal{C}\approx10^{-2}$ by proper engineering of these
structures.",1806.06727v2
2018-06-21,Holographic three-point correlators in the Schrodinger/dipole CFT correspondence,"We calculate, for the first time, three-point correlation functions involving
""heavy"" operators in the Schrodinger/null-dipole CFT correspondence at strong
coupling. In particular, we focus on the three-point functions of the dilaton
modes and two ""heavy"" operators. The heavy states are dual to the single spin
and dyonic magnon, the single spin and dyonic spike solutions or to two novel
string solutions which do not have an undeformed counterpart. Our results
provide the leading term of the correlators in the large $\lambda$ expansion
and are in perfect agreement with the form of the correlator dictated by
non-relativistic conformal invariance. We also specify the scaling function
which can not be fixed by using conformal invariance.",1806.08181v1
2018-07-03,Antiferromagnetism emerging in a ferromagnet with gain,"We present a theoretical mapping to show that a ferromagnet with gain (loss)
is equivalent to an antiferromagnet with an equal amount of loss (gain). Our
finding indicates a novel first-order ferromagnet-antiferromagnet phase
transition by tuning the gain-loss parameter. As an appealing application, we
demonstrate the realization as well as the manipulation of the
antiferromagnetic skyrmion, a stable topological quasiparticle not yet observed
experimentally, in a chiral ferromagnetic thin film with gain. We also consider
ferromagnetic bilayers with balanced gain and loss, and show that the
antiferromagnetic skyrmion can be found only in the cases with broken
parity-time symmetry phase. Our results pave a way for investigating the
emerging antiferromagnetic spintronics and parity-time symmetric magnonics in
ferromagnets.",1807.01048v2
2018-07-08,Hybrid quantum system with nitrogen-vacancy centers in diamond coupled to surface phonon polaritons in piezomagnetic superlattices,"We investigate a hybrid quantum system where an ensemble of nitrogen-vacancy
(NV) centers in diamond is interfaced with a piezomagnetic superlattice that
supports surface phonon polaritons (SPhPs). We show that the strong magnetic
coupling between the collective spin waves in the NV spin ensemble and the
quantized SPhPs can be realized, thanks to the subwavelength nature of the
SPhPs and relatively long spin coherence times. The magnon-polariton coupling
allows different modes of the SPhPs to be mapped and orthogonally stored in
different spatial modes of excitation in the solid medium. Because of its easy
implementation and high tunability, the proposed hybrid structure with NV spins
and piezoactive superlattices could be used for quantum memory and quantum
computation.",1807.02750v1
2018-07-16,Magnetic freezing transition in a CoO/Permalloy bilayer revealed by transverse ac susceptibility,"We utilize variable-temperature, variable-frequency magneto-optical
transverse magnetic susceptibility technique to study the static and dynamical
magnetic properties of a thin-film CoO/Permalloy bilayer. Our measurements
demonstrate that in the studied system, the directional asymmetry of the
hysteresis loop is associated mainly with the difference in the reversal
mechanisms between the two reversed states of magnetization stabilized by the
exchange-induced uniaxial anisotropy. The latter is found to be much larger
than the exchange-induced unidirectional anisotropy of the ferromagnet. We also
observe an abrupt variation of the frequency-dependent imaginary part of ac
susceptibility near the exchange bias blocking temperature, consistent with the
magnetic freezing transition inferred from the previous time-domain studies of
magnetic aging in similar systems. The developed measurement approach enables
precise characterization of the dynamical and static characteristics of
thin-film magnetic heterostructures that can find applications in
reconfigurable magnonic and neuromorphic circuits.",1807.05990v2
2018-07-19,Spin-wave amplification and lasing driven by inhomogeneous spin transfer torques,"We show that an inhomogeneity in the spin-transfer torques in a metallic
ferromagnet under suitable conditions strongly amplifies incoming spin waves.
Moreover, at nonzero temperatures the incoming thermally occupied spin waves
will be amplified such that the region with inhomogeneous spin transfer torques
emits spin waves spontaneously, thus constituting a spin-wave laser. We
determine the spin-wave scattering amplitudes for a simplified model and
set-up, and show under which conditions the amplification and lasing occurs.
Our results are interpreted in terms of a so-called black-hole laser, and could
facilitate the field of magnonics, that aims to utilize spin waves in logic and
data-processing devices.",1807.07312v1
2018-07-25,Left-handed polarized spin waves in ferromagnets induced by spin-transfer torque,"Polarization is a fundamental property of waves that refers to the
orientation of the oscillations. It has been widely used to encode information
in photonics and phononics. However, the polarization of spin waves is rarely
used yet in magnonics. The reason for this is that only the right-handed
polarized spin waves can be accommodated in ferromagnets. Here, we report that
stable left-handed polarized spin waves can be introduced into ferromagnets if
a spin-polarized electrical current is presented. The right-handed and
left-handed polarized spin waves coexist when the current density is larger
than a critical value while the system keeps stable. The results are confirmed
by micromagnetic simulations. This work provides new playgrounds to study spin
waves and points to new findings for future experimental studies.",1807.09456v2
2018-07-27,Optimization of Multi-Frequency Magnonic Waveguides with Enhanced Group Velocities by Exchange Coupled Ferrimagnet/Ferromagnet Bilayers,"We report broadband spectroscopy and numerical analysis by which we explore
propagating spin waves in a magnetic bilayer consisting of a 23 nm thick
permalloy film deposited on 130 nm thick $Y_{3}Fe_{5}O_{12}$. In the bilayer,
we observe a characteristic mode that exhibits a considerably larger group
velocity at small in-plane magnetic field than both the magnetostatic and
perpendicular standing spin waves. Using the finite element method, we confirm
the observations by simulating the mode profiles and dispersion relations. They
illustrate the hybridization of spin wave modes due to exchange coupling at the
interface. The high-speed propagating mode found in the bilayer can be utilized
to configure multi-frequency spin wave channels enhancing the performance of
spin wave based logic devices.",1807.10508v2
2018-07-30,Topological spin excitations in honeycomb ferromagnet CrI$_3$,"In two dimensional honeycomb ferromagnets, bosonic magnon quasiparticles
(spin waves) may either behave as massless Dirac fermions or form topologically
protected edge states. The key ingredient defining their nature is the
next-nearest neighbor Dzyaloshinskii-Moriya (DM) interaction that breaks the
inversion symmetry of the lattice and discriminates chirality of the associated
spin-wave excitations. Using inelastic neutron scattering, we find that spin
waves of the insulating honeycomb ferromagnet CrI$_3$ ($T_C=61$ K) have two
distinctive bands of ferromagnetic excitations separated by a $\sim$4 meV gap
at the Dirac points. These results can only be understood by considering a
Heisenberg Hamiltonian with DM interaction, thus providing experimental
evidence that spin waves in CrI$_3$ can have robust topological properties
useful for dissipationless spintronic applications.",1807.11452v3
2018-12-04,Optical excitation of single- and multi-mode magnetization precession in Galfenol nanolayers,"We demonstrate a variety of precessional responses of the magnetization to
ultrafast optical excitation in nanolayers of Galfenol (Fe,Ga), which is a
ferromagnetic material with large saturation magnetization and enhanced
magnetostriction. The particular properties of Galfenol, including cubic
magnetic anisotropy and weak damping, allow us to detect up to 6 magnon modes
in a 120-nm layer, and a single mode with effective damping ${\alpha}_{eff}$ =
0.005 and frequency up to 100 GHz in a 4-nm layer. This is the highest
frequency observed to date in time-resolved experiments with metallic
ferromagnets. We predict that detection of magnetization precession approaching
THz frequencies should be possible with Galfenol nanolayers.",1812.01237v1
2018-12-08,Microwave to optical photon conversion via fully concentrated rare-earth ion crystals,"Most investigations of rare earth ions in solids for quantum information have
used rare earth ion doped crystals. Here we analyse the conversion of quantum
information from microwave photons to optical frequencies using crystals where
the rare earth ions, rather than being dopants, are part of the host crystal.
The potential of large ion densities and small linewidths makes such systems
very attractive in this application. We show that, as well as high efficiency,
large bandwidth conversion is possible. In fact, the collective coupling
between the rare earth ions and the optical and microwave cavities is large
enough that the limitation on the bandwidth of the devices will instead be the
spacing between magnon mode modes in the crystal.",1812.03246v1
2018-12-14,The Higgs Mode of Planar Coupled Spin-Ladders and its Observation in C$_9$H$_{18}$N$_2$CuBr$_4$,"Polarized inelastic neutron scattering experiments recently identified the
amplitude (Higgs) mode in C$_9$H$_{18}$N$_2$CuBr$_4$, a two-dimensional
near-quantum-critical spin-1/2 two-leg ladder compound, which exhibits a weak
easy-axis exchange anisotropy. Here, we theoretically examine the dynamic spin
structure factor of such planar coupled spin-ladder systems using large-scale
quantum Monte Carlo simulations. This allows us to provide a quantitative
account of the experimental neutron scattering data within a consistent quantum
spin model. Moreover, we trance the details of the continuous evolution of the
amplitude mode from a two-particle bound state of coupled ladders in the
classical Ising limit all the way to the quantum spin-1/2 Heisenberg limit with
fully restored SU(2) symmetry, where it gets overdamped by the two-magnon
continuum in neutron scattering.",1812.05853v1
2018-12-18,Quantum Interface of an Electron and a Nuclear Ensemble,"Coherent excitation of an ensemble of quantum objects underpins quantum
many-body phenomena, and offers the opportunity to realize a quantum memory to
store information from a qubit. Thus far, a deterministic and coherent
interface between a single quantum system, e.g. a qubit, and such an ensemble
has remained elusive. We first use an electron to cool the mesoscopic
nuclear-spin ensemble of a semiconductor quantum dot to the nuclear
sideband-resolved regime. We then implement an all-optical approach to access
these individual quantized electronic-nuclear spin transitions. Finally, we
perform coherent optical rotations of a single collective nuclear spin
excitation corresponding to a spin wave called a nuclear magnon. These results
constitute the building blocks of a dedicated local memory per quantum-dot spin
qubit and promise a solid-state platform for quantum-state engineering of
isolated many-body systems.",1812.07540v1
2018-12-22,Atomic antiferromagnetic domain wall propagation beyond the relativistic limit,"We theoretically investigate the dynamics of atomic domain walls (DWs) in
antiferromagnets driven by a spin-orbit field. For a DW with the width of a few
lattice constants, we identify a Peierls-like pinning effect, with the
depinning field exponentially decaying with the DW width, so that a spin-orbit
field moderately larger than the threshold can drive the propagation of an
atomic DW in a step-wise manner. For a broad DW, the Peierls pinning is
negligibly small. However, the external spin-orbit field can induce a fast DW
propagation, accompanied by a significant shrinking of its width down to atomic
scales. Before stepping into the pinning region, noticeable spin waves are
emitted at the tail of the DW. The spin-wave emission event not only broadens
the effective width of the DW, but also pushes the DW velocity over the
magnonic barrier, which is generally believed to be the relativistic limit of
the DW speed. While the existing dynamic theory based on the continuum
approximation fails in the atomic scale, we develop an energy conversion theory
to interpret the DW dynamics beyond the relativistic limit.",1812.09540v1
2019-01-07,Simultaneous Optical and Electrical Spin-Torque Magnetometry with Stroboscopic Detection of Spin-Precession Phase,"Spin-based coherent information processing and encoding utilize the
precession phase of spins in magnetic materials. However, the detection and
manipulation of spin precession phases remain a major challenge for advanced
spintronic functionalities. By using simultaneous electrical and optical
detection, we demonstrate the direct measurement of the precession phase of
Permalloy ferromagnetic resonance driven by the spin-orbit torques from
adjacent heavy metals. The spin Hall angle of the heavy metals can be
independently determined from concurrent electrical and optical signals. The
stroboscopic optical detection also allows spatially measuring local
spin-torque parameters and the induced ferromagnetic resonance with
comprehensive amplitude and phase information. Our study offers a route towards
future advanced characterizations of spin-torque oscillators, magnonic
circuits, and tunnelling junctions, where measuring the current-induced spin
dynamics of individual nanomagnets are required.",1901.01923v1
2019-01-10,Quantum skyrmions in frustrated ferromagnets,"We develop a quantum theory of magnetic skyrmions and antiskyrmions in a
spin-1/2 Heisenberg magnet with frustrating next-nearest neighbor interactions.
Using exact diagonalization we show numerically that a quantum skyrmion exists
as a stable many-magnon bound state and investigate its quantum numbers. We
then derive a phenomenological Schr\""odinger equation for the quantum skyrmion
and its internal degrees of freedom. We find that quantum skyrmions have highly
unusual properties. Their bandwidth is exponentially small and arises from
tunneling processes between skyrmion and antiskyrmion. The bandstructure
changes both qualitatively and quantitatively when a single spin is added or
removed from the quantum skyrmion, reflecting a locking of angular momentum and
spin quantum numbers characteristic for skyrmions. Additionally, while for weak
forces the quantum skyrmion is accelerated parallel to the force, it moves in a
perpendicular direction for stronger fields.",1901.03343v3
2019-01-11,"Spin pumping as a generic probe for linear spin fluctuations: demonstration with ferromagnetic and antiferromagnetic orders, metallic and insulating electrical states","We investigated spin injection by spin pumping from a spin-injector(NiFe)
into a spin-sink to detect spin fluctuations in the spin-sink. By scanning the
ordering-temperature of several magnetic transitions, we found that enhanced
spin pumping due to spin fluctuations applies with several ordering states:
ferromagnetic(Tb) and antiferromagnetic(NiO, NiFeOx, BiFeO3, exchange-biased
and unbiased IrMn). Results also represent systematic experimental
investigation supporting that the effect is independent of the metallic and
insulating nature of the spin-sink, and is observed whether the spin current
probe involves electronic or magnonic transport, facilitating advances in
material characterization and engineering for spintronic applications.",1901.03539v1
2019-01-14,Controllable excitation of quasi-linear and bullet modes in a spin-Hall nano-oscillator,"We experimentally demonstrate that both quasi-linear and nonlinear
self-localized bullet modes of magnetization auto-oscillation can be excited by
dc current in the nano-gap spin Hall nano-oscillator, by utilizing the geometry
with an extended gap. The quasi-linear mode is stable at low driving currents,
while the bullet mode is additionally excited at larger currents, and becomes
increasingly dominant with increasing current. Time-resolved measurements show
that the formation of the bullet mode is delayed relative to the quasi-linear
mode by up to 100 nanoseconds, demonstrating that the mechanisms of the
formation of these modes are fundamentally different. We discuss the
relationship between the observed behaviors and the formation of an unstable
nonlinear magnon condensate.",1901.04197v1
2019-01-17,"Influences of interfacial oxidization on surface magnetic energy, magnetic damping and spin-orbit-torques in Pt / ferromagnet / capping structures","We investigate the effect of capping layer (CAP) on the interfacial magnetic
anisotropy energy density (K_S), magnetic damping ({\alpha}), and spin-orbit
torques (SOTs) in heavy-metal (Pt) / ferromagnet (Co or Py) / CAP (MgO/Ta,
HfOx, or TaN). At room temperature (RT) the CAP materials influence the
effective magnitude of K_S, which is associated with a formation of interfacial
magnetic oxides. The dynamical dissipation parameters of Co are considerably
influenced by the CAP (especially MgO) while those of Py are not. This is
possibly due to an extra magnetic damping via spin-pumping process across the
Co/CoO interface and incoherent magnon generation (spin fluctuation) in the
interfacial CoO. It is also observed that both anti-damping and field-like SOT
efficiencies vary marginally with the CAP in the thickness ranges we examined.
Our results reveal the crucial role of interfacial oxides on the perpendicular
magnetic anisotropy, magnetic damping, and SOTs.",1901.05777v1
2019-02-08,Field-tuned spin excitation spectrum of $kπ$-skyrmion,"We study spin-wave excitation modes of $k\pi$ skyrmion in a magnetic nanodot
under an external magnetic field along $z$-direction using micromagnetic
simulations based on Landau-Lifshitz-Gilbert equation. We find that a
transition of $k\pi$ skyrmion to other skyrmion-like structures appears under
some critical external field, the corresponding spin-wave spectra are simulated
for each state in the process of applying magnetic field. For skyrmion, the
frequencies of excitation modes increases and then decreases with the low
frequency modes splitting at a critical magnetic field. In addition to the well
known two in-plane rotational modes and a out-of-plane breathing mode of
skyrmion, a higher number of excitation modes are found with increasing $k$
($k=2, 3$). The excitation modes vary as a function of magnetic field, and the
excitation frequencies for different modes exhibit a rapid or slight change
depending on the field induced change of magnetization profile. Our study
indicates the rich spin-wave excitations for $k\pi$ skyrmion and opens a
possibility in theoretical or experimental investigation of magnonics
application.",1902.02901v2
2019-02-12,Thermally assisted Skyrmion drag in a nonuniform electric field,"Magnetic skyrmions are topologically protected excitations of the
magnetization vector field with promising applications in spintronics and
spin-caloritronics, particularly due to their high mobility. Skyrmions can be
steered by a spin-polarized charge current or by exposure to a magnonic spin
current. Here, we propose a further method for driving skyrmions by applying an
inhomogeneous electric field and a homogeneous thermal bias. We show that the
inhomogeneous electric torque leads to an efficient skyrmionic drag which can
be thermally assisted as to enhance the skyrmion velocity. The calculations and
analysis are limited to insulating samples; for conducting materials the
influence of the inhomogeneous electric field on the charge carriers need to be
taken also into account.",1902.04432v1
2019-02-11,Onset of spin wave time-domain fractals in a dynamic artificial crystal,"We report on the first observation of an exact fractal pattern in the
time-domain arising spontaneously from a dynamic artificial crystal (DAC). The
all-magnon process occurs in a passive, unlithographed magnetic waveguide. The
DAC was created by a standing spin wave of frequency $f_{\mathrm{DAC}}$ in a
region of nonlinear waveguide, resulting in a spatio-temporally periodic
potential. The interaction of travelling spin waves of frequency $f'$ with the
DAC resulted in a series of new modes appearing in a comb with intervals
$\Delta f^{(0)} = \mid f_{\mathrm{DAC}}-f'\mid$. As $H$ was increased a 1st
pre-fractal pattern was observed with frequency interval $\Delta f^{(1)}
=\Delta f^{(0)}/2$. Finally, the onset of a 2nd pre-fractal was observed with
frequency interval $\Delta f^{(2)} =\Delta f^{(1)}/2 = \Delta f^{(0)}/4$. The
magnetic field dependence of the nonlinear signals effect matched that of the
DAC. The fractal-like behavior was demonstrated for different values of $f'$.",1902.04445v2
2019-02-12,Ultra-low damping in lift-off structured yttrium iron garnet thin films,"We show that using maskless photolithography and the lift-off technique,
patterned yttrium iron garnet thin films possessing ultra-low Gilbert damping
can be accomplished. The films of 70 nm thickness were grown on (001)-oriented
gadolinium gallium garnet by means of pulsed laser deposition, and they exhibit
high crystalline quality, low surface roughness, and the effective
magnetization of 127 emu/cm3. The Gilbert damping parameter is as low as
5x10-4. The obtained structures have well-defined sharp edges which along with
good structural and magnetic film properties pave a path in the fabrication of
high-quality magnonic circuits and oxide-based spintronic devices.",1902.04605v1
2019-02-14,Quantized ΔS=2 Excitation Spectra by Confinement in an S=1 Spin Chain,"We calculate the dynamical spin-structure factor of the $S=1$ Ising spin
chain with negative single-ion anisotropy in magnetic fields using the infinite
time-evolving-block-decimation algorithm. We show that when a transverse
magnetic field is applied, both the $\Delta S=2$ excitation continuum and
one-magnon mode appear in the low-lying excitation. When a longitudinal
magnetic field is further applied, the excitation continuum changes into
quantized excitation spectra. The quantized $\Delta S=2$ excitation spectra
originate from the confinement of two domain walls, each of which carries
$\Delta S=1$. The quantized excitation energies are explained by the negative
zeros in the Airy function.",1902.05169v3
2019-02-20,Direct photoluminescence probing of ferromagnetism in monolayer two-dimensional CrBr3,"Atomically thin magnets are the key element to build up spintronics based on
two-dimensional materials. The surface nature of two-dimensional ferromagnet
opens up opportunities to improve the device performance efficiently. Here, we
report the intrinsic ferromagnetism in atomically thin monolayer CrBr3,
directly probed by polarization resolved magneto-photoluminescence. The
spontaneous magnetization persists in monolayer CrBr3 with a Curie temperature
of 34 K. The development of magnons by the thermal excitation is in line with
the spin-wave theory. We attribute the layer-number dependent hysteresis loops
in thick layers to the magnetic domain structures. As a stable monolayer
material in air, CrBr3 provides a convenient platform for fundamental physics
and pushes the potential applications of the two-dimensional ferromagnetism.",1902.07446v1
2019-02-25,Nonreciprocal nano-optics with spin-waves in synthetic antiferromagnets,"Integrated optically-inspired wave-based processing is envisioned to
outperform digital architectures in specific tasks, such as image processing
and speech recognition. In this view, spin-waves represent a promising route
due to their nanoscale wavelength in the GHz frequency range and rich
phenomenology. Here, we realize a versatile optically-inspired platform using
spin-waves, demonstrating the wavefront engineering, focusing, and robust
interference of spin-waves with nanoscale wavelength. In particular, we use
magnonic nanoantennas based on tailored spin-textures for launching spatially
shaped coherent wavefronts, diffraction-limited spin-wave beams, and generating
robust multi-beam interference patterns, which spatially extend for several
times the spin-wave wavelength. Furthermore, we show that intriguing features,
such as resilience to back-reflection, naturally arise from the spin-wave
nonreciprocity in synthetic antiferromagnets, preserving the high quality of
the interference patterns from spurious counterpropagating modes. This work
represents a fundamental step towards the realization of nanoscale
optically-inspired devices based on spin-waves.",1902.09420v2
2019-02-28,Building Magnetic Hysteresis in Holography,"We study the spontaneous magnetization and the magnetic hysteresis using the
gauge/gravity duality. We first propose a novel and general formula to compute
the magnetization in a large class of holographic models. By using this
formula, we compute the spontaneous magnetization in a model like a holographic
superconductor. Furthermore, we turn on the external magnetic field and build
the hysteresis curve of magnetization and charge density. To our knowledge,
this is the first holographic model realizing the hysteresis accompanied with
spontaneous symmetry breaking. By considering the Landau-Ginzburg type
effective potential in the symmetry broken phase, we obtain the mass of the
magnon from the bulk geometry data.",1902.10929v2
2019-03-03,Dynamical mode coupling and coherence in spin Hall nano-oscillator with perpendicular magnetic anisotropy,"We experimentally study the dynamical modes excited by spin current in Spin
Hall nano-oscillators based on the Pt/[Co/Ni] multilayers with perpendicular
magnetic anisotropy. Both propagating spin wave and localized solitonic modes
of the oscillation are achieved and controlled by varying the applied magnetic
field and current. At room temperature, the generation linewidth broadening
associated with mode hopping was observed at currents close to the transition
between different modes and in the mode coexistence regimes. The mode hopping
was suppressed at cryogenic temperatures, confirming that the coupling between
modes is mediated by thermal magnons. We also demonstrate that coherent
single-mode oscillations with linewidth of 5 MHz can be achieved without
applying external magnetic field. Our results provide insight into the
mechanisms controlling the dynamical coherence in nanomagnetic oscillators, and
guidance for optimizing their applications in spin wave-based electronics.",1903.00822v1
2019-03-03,Spin wave damping in periodic and quasiperiodic magnonic structures,"We investigated the lifetime of spin wave eigenmodes in periodic and
quasiperiodic sequences of Py and Co wires. Those materials differ
significantly in damping coefficients, therefore, the spatial distribution of
the mode amplitude within the structure is important for the lifetime of
collective spin wave excitations. Modes of the lower frequencies prefer to
concentrate in Py wires, because of the lower FMR frequency for this material.
This inhomogeneous distribution of amplitude of modes (with lower amplitude in
material of higher damping and with higher amplitude in material of lower
damping) is preferable for extending the lifetime of the collective excitations
beyond the volume average of lifetimes for solid materials. We established the
relation between the profile of the mode and its lifetime for periodic and
quasiperiodic structures. We performed also the comparative studies in order to
find the differences resulting from complexity of the structure and enhancement
of localization in quasiperiodic system on the lifetime of spin waves.",1903.00856v1
2019-03-06,Honeycomb rare-earth magnets with anisotropic exchange interactions,"We study the rare-earth magnets on a honeycomb lattice, and are particularly
interested in the experimental consequences of the highly anisotropic spin
interaction due to the spin-orbit entanglement. We perform a high-temperature
series expansion using a generic nearest-neighbor Hamiltonian with anisotropic
interactions, and obtain the heat capacity, the parallel and perpendicular spin
susceptibilities, and the magnetic torque coefficients. We further examine the
electron spin resonance linewidth as an important signature of the anisotropic
spin interactions. Due to the small interaction energy scale of the rare-earth
moments, it is experimentally feasible to realize the strong-field regime.
Therefore, we perform the spin-wave analysis and study the possibility of
topological magnons when a strong field is applied to the system. The
application and relevance to the rare-earth Kitaev materials are discussed.",1903.02530v3
2019-03-07,Interplay between spin dynamics and crystal field in multiferroic compound HoMnO$_3$,"In the multiferroic hexagonal manganite HoMnO3, inelastic neutron scattering
and synchrotron based THz spectroscopy have been used to investigate the spin
waves associated to the Mn order together with Ho crystal field excitations.
While the Mn order sets in first below 80 K, a spin reorientation occurs below
37 K, a rare feature in the rare earth manganites. We show that severalHo
crystal field excitations are present in the same energy range as the magnons,
and that they are all affected by the spin reorientation. Moreover, several
anomalous features are observed in the excitations at low temperature. Our
analysis and calculations for the Mn spin waves and Ho crystal field
excitations support Mn-Ho coupling mechanisms as well as coupling to the
lattice affecting the dynamics.",1903.03009v1
2019-03-19,Giant localised spin-Peltier effect due to ultrafast domainwalls motion in antiferromagnetic metals,"Spin thermo-electric phenomena have attracted wide attention recently, e.g.
the Spin Peltier effect (SPE) heat generation by magnonic spin currents. Here
we find that the Spin Peltier ef-fect also manifests as a heat wave
accompanying fast moving magnetic textures. High speed andextreme magnetic
excitation localisation are paramount for efficient transfer of energy from
thespin-degrees of freedom to electrons and lattice. While satisfying both
conditions is subject to se-vere restrictions in ferromagnets, we find that
domain walls in antiferomagnets can overcome theselimitations due to their
potential ultrahigh mobility and ultra-small widths originating from the
rel-ativistic contraction. To illustrate our findings, we show that electric
current driven domain wallmotion in the antiferromagnetic metal Mn2Au can carry
a localised heat wave with the maximumamplitude up to 1 K . Since domain walls
are well localised nanoscale magnetic objects, this effecthas the potential for
nanoscale heating sensing and functionalities.",1903.08034v1
2019-03-19,Composition dependence of magnetoresistance in Fe$_{1-x}$Ni$_{x}$ alloys,"Resistance of Fe$_{1-x}$Ni$_x$(x=0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.9)
has been measured using four probe method from 5K to 300K with and without a
longitudinal magnetic field of 8T. The zero field resistivity of x=0.1 and 0.9
alloys, predominant contribution to resistivity above near room temperature is
due to electron-phonon scattering, whereas for x=05 and 0.7 alloys
electron-magnon scattering is dominant. Alloys with x=0.1 and 0.9 exhibit
positive magnetoresistance(MR) from 5K to 300K. For x=0.5 and 0.7 alloys,
magnetoresistance changes sign from positive to negative with increase in
temperature. The temperature at which sign changes increase with Ni
concentration in the alloy. The field dependent magnetoresistance is positive
for x=0.1, 0.7 and 0.9 alloys whereas it is negative for x=0.5 alloy. MR
follows linear behaviour with field for x=0.1 alloy. MR of all other alloys
follow a second order polynomial in field.",1903.08230v3
2019-03-20,Tracking the ultrafast motion of an antiferromagnetic order parameter,"The unique functionalities of antiferromagnets offer promising routes to
advance information technology. Their compensated magnetic order leads to spin
resonances in the THz-regime, which suggest the possibility to coherently
control antiferromagnetic (AFM) devices orders of magnitude faster than
traditional electronics. However, the required time resolution, complex
sublattice interations and the relative inaccessibility of the AFM order
parameter pose serious challenges to studying AFM spin dynamics. Here, we
reveal the temporal evolution of an AFM order parameter directly in the time
domain. We modulate the AFM order in hexagonal YMnO$_\mathrm{3}$ by coherent
magnon excitation and track the ensuing motion of the AFM order parameter using
time-resolved optical second-harmonic generation (SHG). The dynamic symmetry
reduction by the moving order parameter allows us to separate electron dynamics
from spin dynamics. As transient symmetry reductions are common to coherent
excitations, we have a general tool for tracking the ultrafast motion of an AFM
order parameter.",1903.08604v1
2019-03-22,Exceptional points in classical spin dynamics,"Non-conservative physical systems admit a special kind of spectral
degeneracy, known as exceptional point (EP), at which eigenvalues and
eigenvectors of the corresponding non-Hermitian Hamiltonian coalesce. Dynamical
parametric encircling of the EP can lead to non-adiabatic evolution associated
with a state flip, a sharp transition between the resonant modes. Physical
consequences of the dynamical encircling of EPs in open dissipative systems
have been explored in optics and photonics. Building on the recent progress in
understanding the parity-time (PT)-symmetric dynamics in spin systems, we use
topological properties of EPs to implement chiral non-reciprocal transmission
of a spin through the material with non-uniform magnetization, like helical
magnet. We consider an exemplary system, spin-torque-driven single spin
described by the time-dependent non-Hermitian Hamiltonian. We show that
encircling individual EPs in parameter space results in non-reciprocal spin
dynamics and find the range of optimal protocol parameters for high-efficiency
asymmetric spin filter based on this effect. Our findings offer a platform for
non-reciprocal spin devices for spintronics and magnonics.",1903.09729v2
2019-03-29,Putative spin-nematic phase in BaCdVO(PO$_{4}$)$_{2}$,"We report neutron scattering and AC magnetic susceptibility measurements of
the 2D spin-1/2 frustrated magnet BaCdVO(PO$_{4}$)$_{2}$. At temperatures well
below $T_{\sf N}\approx 1K$, we show that only 34 % of the spin moment orders
in an up-up-down-down strip structure. Dominant magnetic diffuse scattering and
comparison to published $\mu$sr measurements indicates that the remaining 66 %
is fluctuating. This demonstrates the presence of strong frustration,
associated with competing ferromagnetic and antiferromagnetic interactions, and
points to a subtle ordering mechanism driven by magnon interactions. On
applying magnetic field, we find that at $T=0.1$ K the magnetic order vanishes
at 3.8 T, whereas magnetic saturation is reached only above 4.5 T. We argue
that the putative high-field phase is a realization of the long-sought
bond-spin-nematic state.",1903.12462v1
2019-04-01,Anomalous relaxation and the high-temperature structure factor of XXZ spin chains,"We compute the spin structure factor of XXZ spin chains in the Heisenberg and
gapped (Ising) regimes in the high-temperature limit for nonzero magnetization,
within the framework of generalized hydrodynamics including diffusive
corrections. The structure factor shows a hierarchy of timescales in the gapped
phase, owing to $s$-spin magnon bound states (`strings') of various sizes.
Although short strings move ballistically, long strings move primarily
diffusively as a result of their collisions with short strings. The interplay
between these effects gives rise to anomalous power-law decay of the spin
structure factor, with continuously varying exponents, at any fixed separation
in the late-time limit. We elucidate the crossover to diffusion (in the gapped
phase) and to superdiffusion (at the isotropic point) in the half-filling
limit. We verify our results via extensive matrix product operator
calculations.",1904.01039v1
2019-04-09,Ferromagnetic Resonance Studies of Strain tuned Bi:YIG Films,"Bismuth-doped Yttrium iron garnet (Bi:YIG) thin films known for large
Magneto-optical activity with low losses still needs to get probed for its
magnetization dynamics. We demonstrate a controlled tuning of
magnetocrystalline anisotropy in Bi-doped Y_3 Fe_5 O_12 (Bi:YIG) films of high
crystalline quality using growth induced epitaxial strain on [111]-oriented
Gd_3 Ga_5 O_12 (GGG) substrate. We optimize a growth protocol to get thick
highly-strained epitaxial films showing large magneto-crystalline anisotropy,
compare to thin films prepared using a different protocol. Ferromagnetic
resonance measurements establish a linear dependence of the out-of-plane
uniaxial anisotropy on the strain induced rhombohedral distortion of Bi:YIG
lattice. Interestingly, the enhancement in the magnetoelastic constant due to
an optimum substitution of Bi^(3+) ions with strong spin orbit coupling does
not strongly affect the precessional damping (~2x10^(-3) ). Large
magneto-optical activity, reasonably low damping, large magnetocrystalline
anisotropy and large magnetoelastic coupling in BiYIG are the properties that
may help BiYIG emerge as a possible material for photo-magnonics and other
spintronics applications.",1904.04800v2
2019-04-10,Dynamics of Phase Separated States in the Double Exchange Model,"We present extensive large-scale dynamical simulations of phase-separated
states in the double exchange model. These inhomogeneous electronic states that
play a crucial role in the colossal magnetoresistance phenomenon are composed
of ferromagnetic metallic clusters embedded in an antiferromagnetic insulating
matrix. We compute the dynamical structure factor of these nanoscale textures
using an efficient real-space formulation of coupled spin and electron
dynamics. Dynamical signatures of the various underlying magnetic structures
are identified. At small hole doping, the structure factor exhibits a
dominating signal of magnons from the background N\'eel order and localized
modes from magnetic polarons. A low-energy continuum due to large-size
ferromagnetic clusters emerges at higher doping levels. Implications for
experiments on magnetoresistive manganites are discussed.",1904.05252v2
2019-04-16,Plasmon Soft Mode in an Organic-Inorganic Hybrid Perovskite,"We report inelastic light scattering from underdamped plasmons in azetidinium
lead bromide (AzPbBr3). The plasmons are very strongly temperature dependent
and serve as a soft mode for the semiconductor-insulator phase transition near
TC {\guillemotright} 150 K, demonstrating a continuous decrease in hole
concentration np(T) by at least a factor of four and implying a nearly
tricritical transition. The plasmon frequency and linewidth agree with
independent measurements, and the impedance analysis reveals a frequency
dependence (modelled by a constant phase element, CPE) that can be identified
as due to electron-phonon coupling. The dependence of plasmon frequency upon
(TC-T) is analogous to that for magnons in magnetic insulators or soft
transverse optical phonons in ferroelectrics and ferroelastics, or for phasons
in incommensurately modulated insulators.",1904.07573v1
2019-04-24,Magnetic Susceptibility and Simulated Neutron Signal in the 2D Hubbard Model,"We compute dynamic spin susceptibilities in the two-dimensional Hubbard model
using the method of Dual Fermions and provide comparison to lattice Monte Carlo
and cluster dynamical mean field theory. We examine the energy dispersion
identified by peaks in ${\rm Im}\chi(\omega,q)$ which define spin modes and
compare the exchange scale and magnon dispersion to neutron experiments on the
parent La$_2$CuO$_4$ cuprate. We present the evolution of the spin excitations
as a function of Hubbard interaction strengths and doping and explore the
particle-hole asymmetry of the spin excitations. We also study the correlation
lengths and the spin excitation dispersion peak structure and find a `Y'-shaped
dispersion similar to neutron results on doped HgBa$_2$CuO$_{4+\delta}$.",1904.10782v1
2019-04-25,Controlled interconversion of quantized spin wave modes via local magnetic fields,"In the emerging field of magnonics, spin waves are considered for information
processing and transmission at high frequencies. Towards this end, the
manipulation of propagating spin waves in nanostructured waveguides for novel
functionality has recently been attracting increasing focus of research.
Excitations with uniform magnetic fields in such waveguides favors symmetric
spin wave modes with odd quantization numbers. Interference between multiple
odd spin wave modes leads to a periodic self-focusing effect of the propagating
spin waves. Here we demonstrate, how antisymmetric spin wave modes with even
quantization numbers can be induced by local magnetic fields in a
well-controlled fashion. The resulting interference patterns are discussed
within an analytical model and experimentally demonstrated using microfocused
Brillouin light scattering ({\mu}-BLS).",1904.11613v1
2019-04-29,Unconventional spin currents in magnetic films,"A spin current - a flow of spin angular momentum - can be carried either by
spin polarised free electrons or by magnons, the quanta of a moving collective
oscillation of localised electron spins - a spin wave. Traditionally, it was
assumed, that a spin wave in a magnetic film with spin-sink-free surfaces can
transfer energy and angular momentum only along its propagation direction. In
this work, using Brillouin light scattering spectroscopy in combination with a
theory of dipole-exchange spin-wave spectra, we show that in obliquely
magnetized free magnetic films the in-plane propagation of spin waves is
accompanied by a transverse spin current along the film normal without any
corresponding transverse transport of energy.",1904.12610v1
2019-08-06,Spectral properties of spin-orbital polarons as a fingerprint of orbital order,"Transition metal oxides are a rich group of materials with very interesting
physical properties that arise from the interplay of the charge, spin, orbital,
and lattice degrees of freedom. One interesting consequence of this,
encountered in systems with orbital degeneracy, is the coexistence of long
range magnetic and orbital order, and the coupling between them. In this paper
we develop and study an effective spin-orbital superexchange model for $e_g^3$
systems and use it to investigate the spectral properties of a charge (hole)
injected into the system, which is relevant for photoemission spectroscopy.
Using an accurate, semi-analytical, magnon expansion method, we gain insight
into various physical aspects of these systems and demonstrate a number of
subtle effects, such as orbital to magnetic polaron crossover, the coupling
between orbital and magnetic order, as well as the orbital order driving the
system towards one-dimensional quantum spin liquid behavior. Our calculations
also suggest a potentially simple experimental verification of the character of
the orbital order in the system, something that is not easily accessible
through most experimental techniques.",1908.02232v1
2019-08-06,Goldstone modes in the emergent gauge fields of a frustrated magnet,"We consider magnon excitations in the spin-glass phase of geometrically
frustrated antiferromagnets with weak exchange disorder, focussing on the
nearest-neighbour pyrochlore-lattice Heisenberg model at large spin. The
low-energy degrees of freedom in this system are represented by three copies of
a U(1) emergent gauge field, related by global spin-rotation symmetry. We show
that the Goldstone modes associated with spin-glass order are excitations of
these gauge fields, and that the standard theory of Goldstone modes in
Heisenberg spin glasses (due to Halperin and Saslow) must be modified in this
setting.",1908.02271v2
2019-08-27,Inelastic light scattering in the spin cluster Mott insulator Cu$_{2}$OSeO$_{3}$,"Clusters of single spins form the relevant spin entities in the formation of
long-range magnetic order in spin cluster Mott insulators. Such type of spin
order bears resemblance to molecular crystals, and we therefore may expect a
prototypical spin wave spectrum which can be divided into low-energy external
and high-energy internal cluster spin wave modes. Here, we study high-energy
spin cluster excitations in the spin cluster Mott insulator Cu$_{2}$OSeO$_{3}$
by means of spontaneous Raman scattering. Multiple high-energy optical magnon
modes are observed, of which the Raman-activity is shown to originate in the
Elliot-Loudon scattering mechanism. Upon crossing the long-range ordering
transition temperature the magnetic modes significantly broaden, corresponding
to scattering from localized spin excitations within the spin clusters.
Different optical phonon modes show a strong temperature dependence, evidencing
a strong magnetoelectric coupling between optical phonons and the high-energy
spin cluster excitations. Our results support the picture that
Cu$_{2}$OSeO$_{3}$ can be regarded as a solid-state molecular crystal of spin
nature.",1908.10279v1
2019-08-28,Raman scattering study of lattice and magnetic excitations in CrAs,"We report on the lattice and spin dynamics in the monopnictide CrAs using
polarized Raman scattering. This system exhibits a first-order
magneto-structural phase transition at T_N~ 265 K, below which the magnetic
moments of Cr form an incommensurate double helical magnetic structure while
the unit cell volume abruptly expands. Across the transition, the frequencies
of the fully symmetric Ag phonons strongly renormalize, which along with the
results of first-principle calculations suggest the presence of a sizeable
coupling of the phonons to the magnetic degrees of freedom in this compound. In
addition, we observe two broad modes around 350 and 1700 cm-1 in the magnetic
phase, which we associate with two-magnon Raman scattering. This work provides
one of the few examples of magnetic Raman scattering from a non-collinear
magnet.",1908.10908v2
2019-11-02,Tuning Non-Gilbert-type damping in FeGa films on MgO(001) via oblique deposition,"The ability to tailor the damping factor is essential for spintronic and
spin-torque applications. Here, we report an approach to manipulate the damping
factor of FeGa/MgO(001) films by oblique deposition. Owing to the defects at
the surface or interface in thin films, two-magnon scattering (TMS) acts as a
non-Gilbert damping mechanism in magnetization relaxation. In this work, the
contribution of TMS was characterized by in-plane angular dependent
ferromagnetic resonance (FMR). It is demonstrated that the intrinsic Gilbert
damping is isotropic and invariant, while the extrinsic mechanism related to
TMS is anisotropic and can be tuned by oblique deposition. Furthermore, the two
and fourfold TMS related to the uniaxial magnetic anisotropy (UMA) and
magnetocrystalline anisotropy were discussed. Our results open an avenue to
manipulate magnetization relaxation in spintronic devices.",1911.00728v1
2019-11-08,Core-level x-ray photoemission and Raman spectroscopy studies on electronic structures in Mott-Hubbard type nickelate oxide NdNiO$_2$,"We perform core-level X-ray photoemission spectroscopy (XPS) and electronic
Raman scattering studies of electronic structures and spin fluctuations in the
bulk samples of the nickelate oxide NdNiO$_2$. According to Nd $3d$ and O $1s$
XPS spectra, we conclude that NdNiO$_2$ has a large transfer energy. From the
analysis of the main line of the Ni $2p_{3/2}$ XPS, we confirm the NiO$_2$
planes in NdNiO$_2$ are of Mott-Hubbard type in the Zaanen-Sawatzky-Allen
scheme. The two-magnon peak in the Raman scattering provides direct evidence
for the strong spin-fluctuation in NdNiO$_2$. The peak position determines the
antiferromagnetic exchange $J=25$~meV. Our experimental results agree well with
our previous theoretical results.",1911.03177v2
2019-11-12,Parametric generation of propagating spin-waves in ultra thin yttrium iron garnet waveguides,"We present the experimental demonstration of the parallel parametric
generation of spin-waves in a microscaled yttrium iron garnet waveguide with
nanoscale thickness. Using Brillouin light scattering microscopy, we observe
the excitation of the first and second waveguide modes generated by a stripline
microwave pumping source. Micromagnetic simulations reveal the wave vector of
the parametrically generated spin-waves. Based on analytical calculations,
which are in excellent agreement with our experiments and simulations, we prove
that the spin-wave radiation losses are the determinative term of the
parametric instability threshold in this miniaturized system. The used method
enables the direct excitation and amplification of nanometer spin-waves
dominated by exchange interactions. Our results pave the way for integrated
magnonics based on insulating nano-magnets.",1911.04926v2
2019-11-19,Unidirectional Seebeck effect in magnetic topological insulators,"We theoretically investigate the temperature gradient-dependent or
unidirectional Seebeck effect (USE) in a magnetic/ nonmagnetic topological
insulator (TI) heterostructure with in-plane magnetization in terms of the
semiclassical electron dynamics and Fermi golden rule. The USE has a quantum
origin arising from the magnon asymmetric scattering of surface Dirac electrons
on TI. We discuss the USE in the heterostructures,
Cr$_{x}$(Bi$_{1-y}$Sb$_y$)$_{2-x}$Te$_{3}$/ (Bi$_{1-y}$Sb$_{y})_2$Te$_{3}$. The
USE exhibits $\cos\phi$-dependence (measured from $y$-direction) on the
orientation of magnetization. It's found that the sign of USE stays unchanged
when the system is transferred from $p$-doping to $n$-doping. The USE shows on
inverse-linearly temperature dependent at high temperature.",1911.08122v1
2019-11-28,First and Second Order Topological Phases on Ferromagnetic Breathing Kagome Lattice,"In this work, topological properties of a ferromagnetic Heisenberg model on a
breathing kagome lattice are investigated extensively in the presence of
  Dzyaloshinskii-Moriya interaction. While the kagome ferromagnet hosts only a
single first order topological phase, the breathing kagome system exhibits
multiple first and second order topological phases along with their
coexistence. Magnon dispersion relation is obtained by using linear spin wave
theory. Flat band and Dirac cones are obtained in the absence of
  Dzyaloshinskii-Moriya interaction. A topological phase diagram is presented
where several first and second order phases as well as their overlap are
identified. Values of thermal Hall conductivity for all the first order phases
are obtained. Distinct first order phases are characterized by different sets
of Chern numbers in association with the necessary chiral edge states in
accordance to the first order bulk-boundary-correspondence rule. Second order
phase is characterized by polarization along with the emergence of corner
states. Violation of the second order bulk-corner-correspondence rule has been
noted in some regions.",1911.12620v2
2019-11-29,Scaling theory of magnetism in frustrated Kondo lattices,"A scaling theory of the Kondo lattices with frustrated exchange interactions
is developed, criterium of antiferromagnetic ordering and quantum-disordered
state being investigated. The calculations taking into account magnon and
incoherent spin dynamics are performed. Depending on the bare model parameters,
one or two quantum phase transitions into non-magnetic spin-liquid and Kondo
Fermi-liquid ground states can occur with increasing the bare coupling
constant. Whereas the renormalization of the magnetic moment in the ordered
phase can reach orders of magnitude, spin fluctuation frequency and coupling
constant are moderately renormalized in the spin-liquid phase. This justifies
application of the scaling approach. Possibility of a non-Fermi-liquid behavior
is treated.",1911.13083v1
2019-12-28,A journey into the tuneable antiferromagnetic spin textures of BiFeO3,"Antiferromagnetic thin films are currently generating considerable excitement
for low dissipation magnonics and spintronics. However, while tuneable
antiferromagnetic textures form the backbone of functional devices, they are
virtually unknown at the submicron scale. Here we image a wide variety of
antiferromagnetic spin textures in multiferroic BiFeO3 thin films that can be
tuned by strain and manipulated by electric fields through room temperature
magnetoelectric coupling. Using piezoresponse force microscopy and scanning NV
magnetometry in self-organized ferroelectric patterns of BiFeO3, we reveal how
strain stabilizes different types of non-collinear antiferromagnetic states
(bulk-like and exotic spin cycloids) as well as collinear antiferromagnetic
textures. Beyond these local-scale observations, resonant elastic X-ray
scattering confirms the existence of both types of spin cycloids. Finally, we
show that electric-field control of the ferroelectric landscape induces
transitions either between collinear and non-collinear states or between
different cycloids, offering perspectives for the design of reconfigurable
antiferromagnetic spin textures on demand.",1912.12470v1
2019-12-31,Onsager's Scars in Disordered Spin Chains,"We propose a class of non-integrable quantum spin chain models that exhibit
quantum many-body scars even in the presence of disorder. With the use of the
so-called Onsager symmetry, we construct such scarred models for arbitrary spin
quantum number $ S $. There are two types of scar states, namely, coherent
states associated to an Onsager-algebra element and one-magnon scar states.
While both of them are highly-excited states, they have area-law entanglement
and can be written as a matrix product state. Therefore, they explicitly
violate the eigenstate thermalization hypothesis. We also investigate the
dynamics of the fidelity and entanglement entropy for several initial states.
The results clearly show that the scar states are trapped in a perfectly
periodic orbit in the Hilbert subspace and never thermalize, whereas other
generic states do rapidly. To our knowledge, our model is the first explicit
example of disordered quantum many-body scarred model.",1912.13399v3
2015-08-24,Origin of robust interaction of spin waves with a single skyrmion in perpendicularly magnetized nanostripes,"We studied interactions between propagating spin waves (SWs) and a single
skyrmion in a perpendicularly magnetized CoFeB nanostripe where the magnetic
layer is interfaced with W and MgO. Micromagnetic numerical calculations
revealed that robust interactions between the incident SWs and the skyrmion
give rise to considerable forward skyrmion motions for specific SW frequencies
(e.g., here: fsw = 12 - 19 GHz). Additionally, it was found that there exists a
sufficiently low threshold field amplitude, e.g., 0.1 kOe for the fsw = 15 GHz
SWs. This frequency-dependent interaction originated from the robust coupling
of the SWs with the internal modes of the skyrmion, through the SWs' linear
momentum transfer torque acting on the skyrmion. This work provides for
all-magnetic control of skyrmion motions without electronic currents, and
facilitates further understanding of the interactions between magnons and
topological solitons in constricted geometries.",1508.05682v1
2015-08-25,Non-local magnetoresistance in YIG/Pt nanostructures,"We study the local and non-local magnetoresistance of thin Pt strips
deposited onto yttrium iron garnet. The local magnetoresistive response,
inferred from the voltage drop measured along one given Pt strip upon
current-biasing it, shows the characteristic magnetization orientation
dependence of the spin Hall magnetoresistance. We simultaneously also record
the non-local voltage appearing along a second, electrically isolated, Pt
strip, separated from the current carrying one by a gap of a few 100 nm. The
corresponding non-local magnetoresistance exhibits the symmetry expected for a
magnon spin accumulation-driven process, confirming the results recently put
forward by Cornelissen et al. [1]. Our magnetotransport data, taken at a series
of different temperatures as a function of magnetic field orientation, rotating
the externally applied field in three mutually orthogonal planes, show that the
mechanisms behind the spin Hall and the non-local magnetoresistance are
qualitatively different. In particular, the non-local magnetoresistance
vanishes at liquid Helium temperatures, while the spin Hall magnetoresistance
prevails.",1508.06130v1
2016-03-01,Ferromagnetic resonance and magnetic damping in C-doped Mn5Ge3,"Ferromagnetic resonance (FMR) was used to investigate the static and dynamic
magnetic properties of carbon-doped Mn5Ge3 (C$_{0.1}$ and C$_{0.2}$) thin films
grown on Ge(111). The temperature dependence of magnetic anisotropy shows an
increased perpendicular magneto-crystalline contribution at 80K with an
in-plane easy axis due to the large shape contribution. We find that our
samples show a small FMR linewidth (corresponding to an intrinsic magnetic
damping parameter $\alpha$=0.005), which is a measure of the spin relaxation
and directly related with the magnetic and structural quality of the material.
In the perpendicular-to-plane geometry, the FMR linewidth shows a minimum at
around 200K for all the samples, which seems to be not correlated to the
C-doping. The magnetic relaxation parameters have been determined and indicate
the two-magnon scattering as the main extrinsic contribution. We observe a
change in the main contribution from scattering centres in Mn5Ge3C0.2 at low
temperatures, which could be related to the minimum in linewidth.",1603.00190v1
2016-03-14,Spin Hall Effect Induced Spin Transfer Through an Insulator,"When charge current passes through a normal metal that exhibits spin Hall
effect, spin accumulates at the edge of the sample in the transverse direction.
We predict that this spin accumulation, or spin voltage, enables quantum
tunneling of spin through an insulator or vacuum to reach a ferromagnet without
transferring charge. In a normal metal/insulator/ferromagnetic insulator
trilayer (such as Pt/oxide/YIG), the quantum tunneling explains the
spin-transfer torque and spin pumping that exponentially decay with the
thickness of the insulator. In a normal metal/insulator/ferromagnetic metal
trilayer (such as Pt/oxide/Co), the spin transfer in general does not decay
monotonically with the thickness of the insulator. Combining with the spin Hall
magnetoresistance, this tunneling mechanism points to the possibility of a new
type of tunneling spectroscopy that can probe the magnon density of states of a
ferromagnetic insulator in an all-electrical and noninvasive manner.",1603.04240v2
2016-10-11,Semiclassical analysis of a magnetization plateau in a 2D frustrated ferrimagnet,"We use a semiclassical large-$S$ expansion to study a plateau at $1/3$
saturation in the magnetization curve of a frustrated ferrimagnet on a
spatially anisotropic kagom\'{e} lattice. The spins have both ferromagnetic and
antiferromagnetic nearest-neighbor Heisenberg couplings, and a frustrating
next-nearest-neighbor coupling in one lattice direction. The magnetization
plateau appears at the classical level for a certain range of couplings, and
quantum fluctuations significantly broaden it at both ends. Near the region of
the phase diagram where the classical plateau destabilizes, we find an exotic
""chiral liquid"" phase that preserves translational and $U(1)$ spin symmetry, in
which bound pairs of magnons with opposite spins are condensed. We show how
this state is obtained naturally from a relativistic field theory formulation.
We comment on the relevance of the model to the material
$\text{Cu}_3\text{V}_2\text{O}_7\text{(OH)}_2 \cdot 2\text{H}_2\text{O}$
(volborthite).",1610.03135v3
2016-10-11,"$SL(2, Z)$ invariant rotating $(m,n)$ strings in $AdS_3\times S^3$ with mixed flux","We study rigidly rotating and pulsating $(m,n)$ strings in $AdS_3 \times S^3$
with mixed three form flux. The $AdS_3 \times S^3$ background with mixed three
form flux is obtained in the near horizon limit of $SL(2,Z)$-transformed
solution, corresponding to the bound state of NS5-branes and fundamental
strings. We study the probe $(m,n)-$string in this background by solving the
manifest $SL(2,Z)-$covariant form of the action. We find out the dyonic giant
magnon and single spike solutions corresponding to the equations of motion of a
probe string in this background and find out various relationships among the
conserved charges. We further study a class of pulsating $(m,n)$ string in
$AdS_3$ with mixed three form flux.",1610.03402v2
2016-10-13,Spin-wave spectra in periodically surface-modulated ferromagnetic thin films,"This article presents theoretical results for the dynamic response of
periodically surface-modulated ferromagnetic thin films. For such system, the
role of the periodic dipolar field induced by the modulation is addressed by
using the plane-wave method. By controlling the geometry of the modulated
volumes within the film, the frequency modes and spatial profiles of spin waves
can be manipulated. The angular dependence of the frequency band-gaps unveils
the influence of both dynamic and static magnetic charges, which reside in the
edges of the etching periodic zones, and it is stablished that band-gap widths
created by static magnetic charges are broader than the one created by dynamic
magnetic charges. To corroborate the validity of the model, the theoretical
results are compared with ferromagnetic resonance simulations, where a very
good agreement is achieved between both methods. The theoretical model allows
for a detailed understanding of the physics underlying these kind of systems,
thereby providing an outlook to potential applications associated with magnonic
crystals-based devices.",1610.04176v1
2016-10-17,Disorder-induced Revival of the Bose-Einstein Condensation in Ni(Cl$_{1-x}$Br$_x$)$_2$-4SC(NH$_2$)$_2$ at High Magnetic Fields,"Building on recent NMR experiments [Phys. Rev. Lett. 118, 067203], we
theoretically investigate the high magnetic field regime of the disordered
quasi-one-dimensional $S=1$ antiferromagnetic material
Ni(Cl$_{1-x}$Br$_x$)$_2$-4SC(NH$_2$)$_2$. The interplay between disorder,
chemically controlled by Br-doping, interactions, and the external magnetic
field, leads to a very rich phase diagram. Beyond the well-known
antiferromagnetically ordered regime, analog of a Bose condensate of magnons,
which disappears when $H\ge 12.3$ T, we unveil a resurgence of phase coherence
at higher field $H\sim 13.6$ T, induced by the doping. Interchain couplings
stabilize finite temperature long-range order whose extension in the field -
temperature space is governed by the concentration of impurities $x$. Such a
""mini-condensation"" contrasts with previously reported Bose-glass physics in
the same regime, and should be accessible to experiments.",1610.05136v2
2016-10-21,Spin transport and dynamics in all-oxide perovskite La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrRuO$_3$ bilayers probed by ferromagnetic resonance,"Thin films of perovskite oxides offer the possibility of combining emerging
concepts of strongly correlated electron phenomena and spin current in magnetic
devices. However, spin transport and magnetization dynamics in these complex
oxide materials are not well understood. Here, we experimentally quantify spin
transport parameters and magnetization damping in epitaxial perovskite
ferromagnet/paramagnet bilayers of La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrRuO$_3$
(LSMO/SRO) by broadband ferromagnetic resonance spectroscopy. From the SRO
thickness dependence of Gilbert damping, we estimate a short spin diffusion
length of $\lesssim$1 nm in SRO and an interfacial spin-mixing conductance
comparable to other ferromagnet/paramagnetic-metal bilayers. Moreover, we find
that anisotropic non-Gilbert damping due to two-magnon scattering also
increases with the addition of SRO. Our results demonstrate LSMO/SRO as a
spin-source/spin-sink system that may be a foundation for examining
spin-current transport in various perovskite heterostructures.",1610.06661v1
2016-10-22,Giant nonreciprocal emission of spin waves in Ta/Py bilayers,"Spin waves are propagating disturbances in the magnetization of magnetic
materials. One of their interesting properties is the nonreciprocity,
exhibiting that their amplitude depends on the magnetization direction.
Nonreciprocity in spin waves is of great interest in both fundamental science
and applications, as it offers an extra knob to control the flow of waves for
the technological fields of logics and switch applications. We show a high
nonreciprocity in spin waves from Ta/Py bilayer systems with out-of-plane
magnetic fields. The nonreciprocity depends on the thickness of Ta underlayer
which is found to induce an interfacial anisotropy. The origin of observed high
nonreciprocity is twofold; different polarities of the in-plane magnetization
due to different angles of canted out-of-plane anisotropy and the spin pumping
effect at the Ta/Py interface. Our findings provide an opportunity to engineer
highly efficient nonreciprocal spin wave based applications such as
nonreciprocal microwave devices, magnonic logic gates, and information
transports.",1610.07023v1
2016-10-27,On the Semi-Classical Limit of Scalar Products of the XXZ Spin Chain,"We study the scalar products between Bethe states in the XXZ spin chain with
anisotropy $|\Delta|>1$ in the semi-classical limit where the length of the
spin chain and the number of magnons tend to infinity with their ratio kept
finite and fixed. Our method is a natural yet non-trivial generalization of
similar methods developed for the XXX spin chain. The final result can be
written in a compact form as a contour integral in terms of Faddeev's quantum
dilogarithm function, which in the isotropic limit reduces to the classical
dilogarithm function.",1610.09006v2
2016-12-06,Increased low-temperature damping in yttrium iron garnet thin films,"We report measurements of the frequency and temperature dependence of
ferromagnetic resonance (FMR) for a 15-nm-thick yttrium iron garnet (YIG) film
grown by off-axis sputtering. Although the FMR linewidth is narrow at room
temperature (corresponding to a damping coefficient $\alpha$ = (9.0 $\pm$ 0.2)
$\times 10^{-4}$), comparable to previous results for high-quality YIG films of
similar thickness, the linewidth increases strongly at low temperatures, by a
factor of almost 30. This increase cannot be explained as due to two-magnon
scattering from defects at the sample interfaces. We argue that the increased
low-temperature linewidth is due to impurity relaxation mechanisms that have
been investigated previously in bulk YIG samples. We suggest that the
low-temperature linewidth is a useful figure of merit to guide the optimization
of thin-film growth protocols because it is a particularly sensitive indicator
of impurities.",1612.01954v1
2016-12-07,Gilbert damping of magnetostatic modes in a yttrium iron garnet sphere,"The magnetostatic mode (MSM) spectrum of a 300$\mu$m diameter single
crystalline sphere of yttrium iron garnet is investigated using broadband
ferromagnetic resonance (FMR). The individual MSMs are identified via their
characteristic dispersion relations and the corresponding mode number tuples
$(nmr)$ are assigned. Taking FMR data over a broad frequency and magnetic field
range allows to analyze both the Gilbert damping parameter~$\alpha$ and the
inhomogeneous line broadening contribution to the total linewidth of the MSMs
separately. The linewidth analysis shows that all MSMs share the same Gilbert
damping parameter $\alpha=2.7(5) \times 10^{-5}$ irrespective of their mode
index. In contrast, the inhomogeneous line broadening shows a pronounced mode
dependence. This observation is modeled in terms of two-magnon scattering
processes of the MSMs into the spin-wave manifold, mediated by surface and
volume defects.",1612.02360v1
2016-12-12,Anomalous thermal conductivity and magnetic torque response in the honeycomb magnet $α$-RuCl$_3$,"We report on the unusual behavior of the in-plane thermal conductivity
($\kappa$) and torque ($\tau$) response in the Kitaev-Heisenberg material
$\alpha$-RuCl$_3$. $\kappa$ shows a striking enhancement with linear growth
beyond H = 7 T, where magnetic order disappears, while $\tau$ for both of the
in-plane symmetry directions shows an anomaly at the same field. The
temperature- and field-dependence of $\kappa$ are far more complex than
conventional phonon and magnon contributions, and require us to invoke the
presence of unconventional spin excitations whose properties are characteristic
of a field-induced spin-liquid phase related to the enigmatic physics of the
Kitaev model in an applied magnetic field",1612.03881v3
2016-12-15,Theory of the Spin Seebeck Effect at a Topological-Insulator/Ferromagnetic-Insulator Interface,"The spin-Seebeck effect refers to voltage signals induced in metals by
thermally driven spin currents in adjacent magnetic systems. We present a
theory of the spin-Seebeck signal in the case where the conductor that supports
the voltage signal is the topologically protected two-dimensional surface-state
system at the interface between a ferromagnetic insulator (FI) and a
topological insulator (TI). Our theory uses a Dirac model for the TI
surface-states and assumes Heisenberg exchange coupling between the TI
quasiparticles and the FI magnetization. The spin-Seebeck voltage is induced by
the TI surface states scattering off the nonequilibrium magnon population at
the surface of the semi-infinite thermally driven FI. Our theory is readily
generalized to spin-Seebeck voltages in any two-dimensional conductor that is
exchange-coupled to the surface of a FI. Surface-state
carrier-density-dependent signal strengths calculated using Bi$_2$Te$_3$ and
yttrium iron garnet material parameters are consistent with recent experiments.",1612.04916v3
2016-12-19,Multispin magnons on deformed $ AdS_{3}\times S^{3} $,"In this paper, using the holographic prescription, we study multispin bound
states and their dispersion relations over the $ \kappa $-deformed $
AdS_{3}\times S^{3} $ background. In the first part of our analysis,
considering the conformal gauge conditions (associated with the Polyakov
action) we explore the dispersion relation associated with spin two bound
states at strong coupling. We solve corresponding world-sheet fluctuations and
compute all the conserved quantities associated with the stringy dynamics over
the deformed background. In the second part of our analysis, we perform similar
analysis for spin three configurations. In both cases, we observe the emergence
of non trivial background deformation that vanishes in the limit, $ \kappa
\rightarrow 0$.",1612.06217v3
2016-12-19,Dynamical Multiferroicity,"An appealing mechanism for inducing multiferroicity in materials is the
generation of electric polarization by a spatially varying magnetization that
is coupled to the lattice through the spin-orbit interaction. Here we describe
the reciprocal effect, in which a time-dependent electric polarization induces
magnetization even in materials with no existing spin structure. We develop a
formalism for this dynamical multiferroic effect in the case for which the
polarization derives from optical phonons, and compute the strength of the
phonon Zeeman effect, which is the solid-state equivalent of the
well-established vibrational Zeeman effect in molecules, using density
functional theory. We further show that a recently observed behavior -- the
resonant excitation of a magnon by optically driven phonons -- is described by
the formalism. Finally, we discuss examples of scenarios that are not driven by
lattice dynamics and interpret the excitation of Dzyaloshinskii-Moriya-type
electromagnons and the inverse Faraday effect from the viewpoint of dynamical
multiferroicity.",1612.06331v2
2016-12-24,Concomitant enhancement of longitudinal spin Seebeck effect with thermal conductivity,"We report a simultaneous measurement of a longitudinal spin Seebeck effect
(LSSE) and thermal conductivity in a Pt/${\rm Y_{3}Fe_{5}O_{12}}$ (YIG)/Pt
system in a temperature range from 10 to 300 K. By directly monitoring the
temperature difference in the system, we excluded thermal artifacts in the LSSE
measurements. It is found that both the LSSE signal and the thermal
conductivity of YIG exhibit sharp peaks at the same temperature, differently
from previous reports. The maximum LSSE coefficient is found to be
$S{\rm_{LSSE}}>10\ \mu{\rm V/K}$, one-order-of magnitude greater than the
previously reported values. The concomitant enhancement of the LSSE and thermal
conductivity of YIG suggests the strong correlation between magnon and phonon
transport in the LSSE.",1612.08142v1
2017-07-03,Heavy-mass magnetic modes in pyrochlore iridates due to dominant Dzyaloshinskii-Moriya interaction,"Materials with strong spin-orbit interactions are presently a main target in
the search for systems with novel magnetic properties. Magnetic anisotropies
can be very large in such compounds, ranging from strongly frustrated Kitaev
exchange in honeycomb iridates and the associated spin-liquid states to robust
antisymmetric couplings in square-lattice Sr_2IrO_4 . Here we predict from ab
initio quantum chemistry calculations that another highly unusual regime is
realized in pyrochlore iridium oxides: the nearest-neighbor Heisenberg
interaction can vanish whilst the antisymmetric Dzyaloshinskii- Moriya exchange
reaches values as large as 5 meV, a result which challenges common notions and
existing phenomenological models of magnetic superexchange. The resulting spin
excitation spectra reveal a very flat magnon dispersion in the Nd- and Tb-based
pyrochlore iridates, suggesting the possibility of using these modes to store
magnetic information. Indeed the magnetization dynamics indicates that these
modes are unable to propagate out of the excitation region.",1707.00500v1
2017-07-05,Magnetic Order in Laser-Irradiated Kagome Antiferromagnets,"Dispersionless ""zero energy mode'' is one of the hallmarks of frustrated
kagome antiferromagnets (KAFMs). It points to extensive classically degenerate
ground-states. The ""zero energy mode'' can be observed experimentally when
lifted to a flat mode at finite energy by a strong intrinsic magnetic
anisotropy. In this letter, we study the effects of irradiation of laser light
on the KAFMs. We adopt the magnon picture without loss of generality. It is
shown that circularly or linearly polarized light lifts the ""zero energy
mode'', stabilizes magnetic order, and induces energy gaps in the KAFMs. We
find that the circularly polarized light-induced anisotropies have similar
features as the intrinsic in-plane and out-of-plane Dzyaloshinskii-Moriya
interaction in KAFMs. The former stabilizes long-range magnetic order and the
latter induces spin canting out-of-plane with nonzero scalar spin chirality.
The Floquet thermal Hall effect shows that the synthetic magnetic excitation
modes in the case of circularly polarized light are topological, whereas those
of linearly polarized light are not.",1707.01502v3
2017-07-10,Critical spin superflow in a spinor Bose-Einstein condensate,"We investigate the critical dynamics of spin superflow in an easy-plane
antiferromagnetic spinor Bose-Einstein condensate. Spin-dipole oscillations are
induced in a trapped condensate by applying a linear magnetic field gradient
and we observe that the damping rate increases rapidly as the field gradient
increases above a certain critical value. The onset of dissipation is found to
be associated with the generation of dark-bright solitons due to the modulation
instability of the counterflow of two spin components. Spin turbulence emerges
as the solitons decay because of their snake instability. We identify another
critical point for spin superflow, in which transverse magnon excitations are
dynamically generated via spin-exchanging collisions, which leads to the
transient formation of axial polar spin domains.",1707.03054v1
2017-07-17,Indirect K-edge bimagnon resonant inelastic X-ray scattering spectrum of $α$-FeTe,"We calculate the K-edge indirect bimagnon resonant inelastic X-ray scattering
(RIXS) intensity spectra of the bicollinear antiferromagnetic order known to
occur in the $\alpha$-FeTe chalcogenide system. Utilizing linear spin wave
theory for this large-S spin system we find that the bimagnon spectrum contains
four scattering channels (two intraband and two interband). We find from our
calculations that for suitable energy-momentum combination the RIXS spectra can
exhibit a one-, two- or three- peak structure. The number of peaks provides a
clue on the various bimagnon excitation processes that can be supported both in
and within the acoustic and optical magnon branches of the bicollinear
antiferromagnet. Unlike the RIXS response of the antiferromagnetic or the
collinear antiferromagnetic spin ordering, the RIXS intensity spectrum of the
bicollinear antiferromagnet does not vanish at the magnetic ordering wave
vector $(\pi/2,-\pi/2)$. It is also sensitive to next-next nearest neighbor and
biquadratic coupling interactions. Our predicted RIXS spectrum can be utilized
to understand the role of multi-channel bimagnon spin excitations present in
the $\alpha$-FeTe chalcogenide.",1707.05057v1
2017-07-17,"Ab initio calculation of spin fluctuation spectra using time dependent density functional perturbation theory, planewaves, and pseudopotentials","We present an implementation of time-dependent density functional
perturbation theory for spin fluctuations, based on planewaves and
pseudopotentials. We compute the dynamic spin susceptibility self-consistently
by solving the time-dependent Sternheimer equation, within the adiabatic local
density approximation to the exchange and correlation kernel. We demonstrate
our implementation by calculating the spin susceptibility of representative
elemental transition metals, namely bcc Fe, fcc Ni and bcc Cr. The calculated
magnon dispersion relations of Fe and Ni are in agreement with previous work.
The calculated spin susceptibility of Cr exhibits a soft-paramagnon
instability, indicating the tendency of the Cr spins to condense in a
incommensurate spin density wave phase, in agreement with experiment.",1707.05219v1
2017-07-27,Perfect diode in quantum spin chains,"We study the rectification of spin current in $XXZ$ chains segmented in two
parts, each with a different anisotropy parameter. Using exact diagonalization
and a matrix product states algorithm we find that a large rectification (of
the order of $10^4$) is attainable even using a short chain of $N=8$ spins,
when one half of the chain is gapless while the other has large enough
anisotropy. We present evidence of diffusive transport when the current is
driven in one direction and of a transition to an insulating behavior of the
system when driven in the opposite direction, leading to a perfect diode in the
thermodynamic limit. The above results are explained in terms of matching of
spectrum of magnon excitations between the two halves of the chain.",1707.08823v3
2017-07-31,Spin wave beam propagation in ferromagnetic thin film with graded refractive index: mirage effect and prospective applications,"Using analysis of iso-frequency contours of the spin-wave dispersion
relation, supported by micromagnetic simulations, we study the propagation of
spin-wave (SW) beams in thin ferromagnetic films through the areas of the
inhomogeneous refractive index. We compare the transmission and reflection of
SWs in areas with gradual and step variation of the SW refractive index. In
particular, we show the mirage effect for SWs with narrowing SW beam width, and
an application of the gradual modulation of the SWs refractive index as a
diverging lens. Furthermore, we study the propagation of SWs in ferromagnetic
stripe with modulated refractive index. We demonstrate that the system can be
considered as the graded-index waveguide, which preserves the width of the SW
beam for a long distance-the property essential for prospective applications of
magnonics.",1707.09768v4
2017-11-06,Observation of Intrinsic Half-metallic Behavior of CrO$_2$ (100) Epitaxial Films by Bulk-sensitive Spin-resolved PES,"We have investigated the electronic states and spin polarization of
half-metallic ferromagnet CrO$_2$ (100) epitaxial films by bulk-sensitive
spin-resolved photoemission spectroscopy with a focus on non-quasiparticle
(NQP) states derived from electron-magnon interactions. We found that the
averaged values of the spin polarization are approximately 100% and 40% at 40 K
and 300 K, respectively. This is consistent with the previously reported result
[H. Fujiwara et al., Appl. Phys. Lett. 106, 202404 (2015).]. At 100 K, peculiar
spin depolarization was observed at the Fermi level ($E_{F}$), which is
supported by theoretical calculations predicting NQP states. This suggests the
possible appearance of NQP states in CrO$_2$. We also compare the temperature
dependence of our spin polarizations with that of the magnetization.",1711.01781v1
2017-11-17,Ginzburg-Landau-type theory of non-polarized spin superconductivity,"Since the concept of spin superconductor was proposed, all the related
studies concentrate on spin-polarized case. Here, we generalize the study to
spin-non-polarized case. The free energy of non-polarized spin superconductor
is obtained, and the Ginzburg-Landau-type equations are derived by using the
variational method. These Ginzburg-Landau-type equations can be reduced to the
spin-polarized case when the spin direction is fixed. Moreover, the expressions
of super linear and angular spin currents inside the superconductor are
derived. We demonstrate that the electric field induced by super spin current
is equal to the one induced by equivalent charge obtained from the second
Ginzburg-Landau-type equation, which shows self-consistency of our theory. By
applying these Ginzburg-Landau-type equations, the effect of electric field on
the superconductor is also studied. These results will help us get a better
understanding of the spin superconductor and the related topics such as
Bose-Einstein condensate of magnons and spin superfluidity.",1711.06416v1
2017-11-26,Brillouin light scattering by magnetic quasi-vortices in cavity optomagnonics,"A ferromagnetic sphere can support \textit{optical vortices} in forms of
whispering gallery modes and \textit{magnetic quasi-vortices} in forms of
magnetostatic modes with non-trivial spin textures. These vortices can be
characterized by their orbital angular momenta. We experimentally investigate
Brillouin scattering of photons in the whispering gallery modes by magnons in
the magnetostatic modes, zeroing in on the exchange of the orbital angular
momenta between the optical vortices and the magnetic quasi-vortices. We find
that the conservation of the orbital angular momentum results in different
nonreciprocal behaviors in the Brillouin light scattering. New avenues for
chiral optics and opto-spintronics can be opened up by taking the orbital
angular momenta as a new degree of freedom for cavity optomagnonics.",1711.09319v1
2018-10-01,Hybridization and Decay of Magnetic Excitations in two-dimensional Triangular Lattice Antiferromagnets,"Elementary quasiparticles in solids such as phonons and magnons occasionally
have nontrivial interactions between them, as well as among themselves. As a
result, their energy eigenvalues are renormalized, the quasiparticles
spontaneously decay into a multi-particle continuum state, or they are
hybridized with each other when their energies are close. As discussed in this
review, such anomalous features can appear dominantly in quantum magnets but
are not, a priori, negligible for magnetic systems with larger spin values and
noncollinear magnetic structures. We review the unconventional magnetic
excitations in two-dimensional triangular lattice antiferromagnets and discuss
their implications on related issues.",1810.00498v2
2018-10-04,Cryogenic Microwave Filter Cavity with a Tunability Greater than 5 GHz,"A wide variety of applications of microwave cavities, such as measurement and
control of superconducting qubits, magnonic resonators, and phase noise
filters, would be well served by having a highly tunable microwave resonance.
Often this tunability is desired in situ at low temperatures, where one can
take advantage of superconducting cavities. To date, such cryogenic tuning
while maintaining a high quality factor has been limited to $\sim500$ MHz. Here
we demonstrate a three-dimensional superconducting microwave cavity that shares
one wall with a pressurized volume of helium. Upon pressurization of the helium
chamber the microwave cavity is deformed, which results in in situ tuning of
its resonant frequency by more than 5 GHz, greater than 60% of the original 8
GHz resonant frequency. The quality factor of the cavity remains approximately
constant at $\approx7\times 10^{3}$ over the entire range of tuning. As a
demonstration of its usefulness, we implement a tunable cryogenic phase noise
filter, which reduces the phase noise of our source by approximately 10 dB
above 400 kHz.",1810.02404v1
2018-10-05,Remagnetization in the array of ferromagnetic nanowires with periodic and quasiperiodic order,"We investigate experimentally and theoretically the magnetization reversal
process in one-dimensional magnonic structures composed of permalloy nanowires
of the two different widths and finite length arranged in a periodic and
quasiperiodic order. The main features of the hysteresis loop are determined by
different shape anisotropies of the component elements and the dipolar
interactions between them. We showed, that the dipolar interactions between
nanowires forming a ribbon can be controlled by change a distance between the
neighboring ribbons. The quasiperiodic order can influence the hysteresis loop
by introduction additional tiny switching steps when the dipolar interactions
are sufficiently strong.",1810.02796v1
2018-10-08,The role of the rare earth in the lattice and magnetic coupling in multiferroic h-HoMnO3,"We used Raman scattering to study the lattice and magnetic excitations in the
hexagonal HoMnO3 single crystals. The E2 phonon mode at 237 cm-1 is affected by
the magnetic order. This mode is related to the displacement of Mn and O ions
in a-b plane and modulates the Mn-O-Mn bond angles in a-b plane and the
in-plane Mn-Mn superexchange interaction. The mode at 269 cm-1 associated to
the displacement of the apical Ho3+ ions along the c direction presents an
abrupt change of slope at TN showing that the role of the rare earth ions can
not be neglected in the magnetic transition. We have identified magnon and
crystal field excitations. The temperature dependence of the magnetic
excitations has been compared to the Mn and Ho moment and indicates that the
exchange interaction pattern between Mn and Ho atoms drives the uniaxial
anisotropy gap above the Mn-spin-rotation transition.",1810.03387v1
2018-10-15,Localized spin waves in isolated $kπ$ skyrmions,"The localized magnon modes of isolated $k\pi$ skyrmions on a field-polarized
background are analyzed based on the Landau-Lifshitz-Gilbert equation within
the terms of an atomistic classical spin model, with system parameters based on
the Pd/Fe biatomic layer on Ir(111). For increasing skyrmion order $k$ a higher
number of excitation modes are found, including modes with nodes in the radial
eigenfunctions. It is shown that at low fields $2\pi$ and $3\pi$ skyrmions are
destroyed via a burst instability connected to a breathing mode, while $1\pi$
skyrmions undergo an elliptic instability. At high fields all $k\pi$ skyrmions
collapse due to the instability of a breathing mode. The effective damping
parameters of the spin waves are calculated in the low Gilbert damping limit,
and they are found to diverge in the case of the lowest-lying modes at the
burst and collapse instabilities, but not at the elliptic instability. It is
shown that the breathing modes of $k\pi$ skyrmions may become overdamped at
higher Gilbert damping values.",1810.06471v1
2018-10-21,Triplon band splitting and topologically protected edge states in the dimerized antiferromagnet,"The search for topological insulators has been actively promoted in the field
of condensed matter physics for further development in energy-efficient
information transmission and processing. In this context, recent studies have
revealed that not only electrons but also bosonic particles such as magnons can
construct edge states carrying nontrivial topological invariants. Here we
demonstrate topological triplon bands in the spin-1/2 two-dimensional dimerized
quantum antiferromagnet Ba$_2$CuSi$_2$O$_6$Cl$_2$, which is closely related to
a pseudo-one-dimensional variant of the Su-Schrieffer-Heeger (SSH) model,
through inelastic neutron scattering experiments. The excitation spectrum
exhibits two triplon bands and a clear band gap between them due to a small
alternation in interdimer exchange interactions along the $a$-direction, which
is consistent with the crystal structure. The presence of topologically
protected edge states is indicated by a bipartite nature of the lattice.",1810.08931v1
2018-10-24,Strong Coupling of Magnons to Microwave Photons in Three-Dimensional Printed Resonators,"We report on ferromagnetic resonant mode hybridization in re-entrant cavities
made with a commercial three-dimensional (3D) printer, followed by conventional
3D metalization with copper and tin. The cavity volume was only 7% that of a
standard cavity resonating at the same frequency, while maintaining a high
quality factor. Simulations were in very good agreement. We obtained an
effective coupling of about 40 MHz in two cavities at room temperature. These
experimental results demonstrate the utility of tunable filters based on
complex 3D printed cavities.",1810.10305v3
2018-10-25,Spin waves near the edge of halogen substitution induced magnetic order in Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$,"We report an inelastic neutron scattering study of magnetic excitations in a
quantum paramagnet driven into a magnetically ordered state by chemical
substitution, namely Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$ with
$x=0.21(2)$. The measured spectrum is well accounted for by the generalized
spin wave theory (GSWT) approach [M. Matsumoto and M. Koga, [J. Phys. Soc. Jap.
76, 073709 (2007)]. This analysis allows us to determine the effective
Hamiltonian parameters for a direct comparison with those in the previously
studied parent compound and `underdoped' system. The issue of magnon lifetimes
due to structural disorder is also addressed.",1810.10912v2
2018-10-28,Magnetic excitations of the classical spin liquid MgCr2O4,"We report a comprehensive inelastic neutron-scattering study of the
frustrated pyrochlore antiferromagnet MgCr2O4 in its cooperative paramagnetic
regime. Theoretical modeling yields a microscopic Heisenberg model with
exchange interactions up to third-nearest neighbors, which quantitatively
explains all the details of the dynamic magnetic response. Our work
demonstrates that the magnetic excitations in paramagnetic MgCr2O4 are
faithfully represented in the entire Brillouin zone by a theory of magnons
propagating in a highly-correlated paramagnetic background. Our results also
suggest that MgCr2O4 is proximate to a spiral spin-liquid phase distinct from
the Coulomb phase, which has implications for the magneto-structural phase
transition in MgCr2O4 .",1810.11869v1
2019-05-02,Anomalous spin Hall angle of a metallic ferromagnet determined by a multiterminal spin injection/detection device,"We report on the determination of the anomalous spin Hall angle in the
ferromagnetic metal alloy cobalt-iron (Co$_{25}$Fe$_{75}$, CoFe). This is
accomplished by measuring the spin injection/detection efficiency in a
multiterminal device with nanowires of platinum (Pt) and CoFe deposited onto
the magnetic insulator yttrium iron garnet (Y$_3$Fe$_5$O$_{12}$, YIG). Applying
a spin-resistor model to our multiterminal spin transport data, we determine
the magnon conductivity in YIG, the spin conductance at the YIG/CoFe interface
and finally the anomalous spin Hall angle of CoFe as a function of its spin
diffusion length in a single device. Our experiments clearly reveal a negative
anomalous spin Hall angle of the ferromagnetic metal CoFe, but a vanishing
ordinary spin Hall angle. This is in contrast to the results reported for the
ferromagnetic metals Co and permalloy.",1905.00663v1
2019-05-05,Effective spin-mixing conductance of heavy-metal-ferromagnet interfaces,"The effective spin-mixing conductance (G_eff) of a heavy metal/ferromagnet
(HM/FM) interface characterizes the efficiency of the interfacial spin
transport.Accurately determining G_eff is critical to the quantitative
understanding of measurements of direct and inverse spin Hall effects. G_eff is
typically ascertained from the inverse dependence of magnetic damping on the FM
thickness under the assumption that spin pumping is the dominant mechanism
affecting this dependence.Here we report that, this assumption fails badly in
many in-plane magnetized prototypical HM/FM systems in the nm-scale thickness
regime. Instead, the majority of the damping is from two-magnon scattering at
the FM interface, while spin-memory-loss scattering at the interface can also
be significant.If these two effects are neglected, the results will be an
unphysical ""giant"" apparent G_eff and hence considerable underestimation of
both the spin Hall ratio and the spin Hall conductivity in inverse/direct spin
Hall experiments.",1905.01577v2
2019-05-06,Elementary excitation in the spin-stripe phase in quantum chains,"Elementary excitations in condensed matter capture the complex many-body
dynamics of interacting basic entities in a simple quasiparticle picture. In
magnetic systems the most established quasiparticles are magnons, collective
excitations that reside in ordered spin structures, and spinons, their
fractional counterparts that emerge in disordered, yet correlated spin states.
Here we report on the discovery of elementary excitation inherent to
spin-stripe order that represents a bound state of two phason quasiparticles,
resulting in a wiggling-like motion of the magnetic moments. We observe these
excitations, which we dub ""wigglons"", in the frustrated zigzag spin-1/2 chain
compound $\beta$-TeVO$_4$, where they give rise to unusual low-frequency spin
dynamics in the spin-stripe phase. This result provides insights into the
stripe physics of strongly-correlated electron systems.",1905.02079v1
2019-05-07,Temperature dependence of the Spin Seebeck effect in a mixed valent manganite,"We report on temperature dependent measurements of the Longitudinal Spin
Seebeck Effect (LSSE) in the mixed valent manganite
La$_{0.7}$Ca$_{0.3}$MnO$_3$. By disentangling the contribution arising due to
the Anisotropic Nernst effect, we observe that these two thermally driven
phenomena vary disparately with temperature. In a narrow low temperature
regime, the LSSE exhibits a $T^{0.55}$ dependence, which matches well with that
predicted by the magnon-driven spin current model. Across the double exchange
driven paramagnetic-ferromagnetic transition, the LSSE exponent is
significantly higher than the magnetization one. These observations highlights
the importance of individually ascertaining the temperature evolution of
different mechanisms which contribute to the measured spin Seebeck signal.",1905.02527v1
2019-05-08,Nanoscale spin-wave wake-up receiver,"We present the concept of a passive spin-wave device which is able to
distinguish different radio-frequency pulse trains and validate its
functionality using micromagnetic simulations. The information is coded in the
phase of the individual pulses which are transformed into spin-wave packets.
The device splits every incoming packet into two arms, one of which is coupled
to a magnonic ring which introduces a well-defined time delay and phase shift.
Since the time delay is matched to the pulse repetition rate, adjacent packets
interfere in a combiner which makes it possible to distinguish simple pulse
train patterns by the read-out of the time-integrated spin-wave intensity in
the output. Due to its passive construction, this device may serve as an
energy-efficient wake-up receiver used to activate the main receiver circuit in
power critical IoT applications.",1905.03006v1
2019-05-10,Long-wavelength emergent phonons in skyrmion crystals distorted by exchange anisotropy and tilted magnetic fields,"Skyrmion crystals (SkX) are periodic alignment of magnetic skyrmions, i.e., a
type of topologically protected spin textures. Compared with ordinary crystals,
they can be drastically deformed under anisotropic effects because they are
composed of field patterns whose deformation does not cause any bond-breaking.
This exotic ductility of SkX bring about great tunability of its collective
excitations called emergent phonons, which are vital for magnonics application.
The question is how to quantitatively determine the emergent phonons of
distorted SkX. Here we systematically study the long wavelength emergent
phonons of SkX distorted by (a) a negative exchange anisotropy, and (b) a
tilted magnetic field. In both cases, deformation and structural transitions of
SkX thoroughly influence the frequency, anisotropy of vibrational pattern and
dispersion relation, and coupling between lattice vibration and in-lattice
vibration for all modes. Tilted magnetic fields are very effective in tuning
the emergent phonons, such that all modes except the Goldstone mode can be
excited by AC magnetic fields when a tilted bias field is presented.",1905.03987v1
2019-05-26,Mass in Lovelock Unique Vacuum gravity theories,"We derive an expression for conserved charges in Lovelock AdS gravity for
solutions having $k$-fold degenerate vacua, making manifest a link between the
degeneracy of a given vacuum and the nonlinearity of the energy formula. We
show for a black hole solution to the field equations on a branch of
multiplicity $k$ that its mass comes from an expression that contains the
product of $k$ Weyl tensors. We prove that all divergent contributions of the
type (Weyl)$^q$, with $1\le q<k$, are suppressed. Our conserved charge
definition is a natural generalization of the Conformal Mass by Ashtekar,
Magnon and Das to the cases when $k>1$. Our results provide insight on the
holographic properties of degenerate Lovelock theories.",1905.10840v2
2019-05-27,Spin relaxation in Cs$_2$CuCl$_{4-x}$Br$_x$,"The quantum-spin S = 1=2 chain system Cs$_2$CuCl$_4$ is of high interest due
to competing anti-ferromagnetic intra-chain J and inter-chain exchange J'
interactions and represents a paramount example for Bose-Einstein condensation
of magnons [R. Coldea et al., Phys. Rev. Lett. 88, 137202 (2002)]. Substitution
of chlorine by bromine allows tuning the competing exchange interactions and
corresponding magnetic frustration. Here we report on electron spin resonance
(ESR) in single crystals of Cs$_2$CuCl$_{4-x}$Br$_x$ with the aim to analyze
the evolution of anisotropic exchange contributions. The main source of the ESR
linewidth is attributed to the uniform Dzyaloshinskii-Moriya interaction. The
vector components of the Dzyaloshinskii-Moriya interaction are determined from
the angular dependence of the ESR spectra using a high-temperature
approximation. The obtained results support the site selectivity of the Br
substitution suggested from the evolution of lattice parameters and magnetic
susceptibility dependent on the Br concentration.",1905.11186v1
2019-05-27,Magnetic Field Induced Competing Phases in Spin-Orbital Entangled Kitaev Magnets,"There has been a great interest in magnetic field induced quantum spin
liquids in Kitaev magnets after the discovery of neutron scattering continuum
and half quantized thermal Hall conductivity in the material $\alpha$-RuCl$_3$.
In this work, we provide a semiclassical analysis of the relevant theoretical
models on large system sizes, and compare the results to previous studies on
quantum models with small system sizes. We find a series of competing magnetic
orders with fairly large unit cells at intermediate magnetic fields, which are
most likely missed by previous approaches. We show that quantum fluctuations
are typically strong in these large unit cell orders, while their magnetic
excitations may resemble a scattering continuum and give rise to a large
thermal Hall conductivity. Our work provides an important basis for a thorough
investigation of emergent spin liquids and competing phases in Kitaev magnets.",1905.11408v2
2019-05-30,Predicting New Iron Garnet Thin Films with Perpendicular Magnetic Anisotropy,"Perpendicular magnetic anisotropy (PMA) is a necessary condition for many
spintronic applications like spin-orbit torques switching, logic and memory
devices. An important class of magnetic insulators with low Gilbert damping at
room temperature are iron garnets, which only have a few PMA types such as
terbium and samarium iron garnet. More and stable PMA garnet options are
necessary for researchers to be able to investigate new spintronic phenomena.
In this study, we predict 20 new substrate/magnetic iron garnet film pairs with
stable PMA at room temperature. The effective anisotropy energies of 10
different garnet films that are lattice-matched to 5 different commercially
available garnet substrates have been calculated using shape, magnetoelastic
and magnetocrystalline anisotropy terms. Strain type, tensile or compressive
depending on substrate choice, as well as the sign and the magnitude of the
magnetostriction constants of garnets determine if a garnet film may possess
PMA. We show the conditions in which Samarium, Gadolinium, Terbium, Holmium,
Dysprosium and Thulium garnets may possess PMA on the investigated garnet
substrate types. Guidelines for obtaining garnet films with low damping are
presented. New PMA garnet films with tunable saturation moment and field may
improve spin-orbit torque memory and compensated magnonic thin film devices.",1905.13042v1
2020-06-01,Spin-wave spectroscopy of individual ferromagnetic nanodisks,"The increasing demand for ultrahigh data storage densities requires
development of 3D magnetic nanostructures. In this regard, focused electron
beam induced deposition (FEBID) is a technique of choice for direct-writing of
various complex nano-architectures. However, intrinsic properties of
nanomagnets are often poorly known and can hardly be assessed by local optical
probe techniques. Here, we demonstrate spatially resolved spin-wave
spectroscopy of individual circular magnetic elements with radii down to 100
nm. The key component of the setup is a microwave antenna whose microsized
central part is placed over a movable substrate with well-separated CoFe-FEBID
nanodisks. The circular symmetry of the disks gives rise to standing spin-wave
resonances and allows for the deduction of the saturation magnetization and the
exchange stiffness of the material using an analytical theory. The presented
approach is especially valuable for the characterization of direct-write
elements opening new horizons for 3D nanomagnetism and magnonics.",2006.00763v1
2020-06-24,Nature of Magnetic Excitations in the High-Field Phase of $α$-RuCl$_3$,"We present comprehensive electron spin resonance (ESR) studies of in-plane
oriented single crystals of $\alpha$-RuCl$_3$, a quasi-two-dimensional material
with honeycomb structure, focusing on its high-field spin dynamics. The
measurements were performed in magnetic fields up to 16 T, applied along the
[110] and [100] directions. Several ESR modes were detected. Combining our
findings with recent inelastic neutron- and Raman-scattering data, we identify
most of the observed excitations. Most importantly, we show that the
low-temperature ESR response beyond the boundary of the magnetically ordered
region is dominated by single- and two-particle processes with magnons as
elementary excitations. The peculiarities of the excitation spectrum in the
vicinity of the critical field are discussed.",2006.13538v2
2020-06-30,Negative Gilbert damping in cavity optomagnonics,"Exceptional point (EP) associated with the parity-time (PT) symmetry breaking
is receiving considerable recent attention by the broad physics community. By
introducing balanced gain and loss, it has been realized in photonic, acoustic,
and electronic structures. However, the observation of magnonic EP remains
elusive. The major challenge is to experimentally generate the negative Gilbert
damping, which was thought to be highly unlikely but is demanded by the PT
symmetry. In this work, we study the magneto-optical interaction of
circularly-polarized lasers with a submicron magnet placed in an optical
cavity. We show that the off-resonant coupling between the driving laser and
cavity photon in the far-blue detuning can induce the magnetic gain (or
negative damping) exactly of the Gilbert type. A hyperbolic-tangent function
ansatz is found to well describe the time-resolved spin switching as the
intrinsic magnetization dissipation is overcome. When the optically pumped
magnet interacts with a purely lossy one, we observe a phase transition from
the imbalanced to passive PT symmetries by varying the detuning coeffcient. Our
findings provide a feasible way to manipulate the sign of the magnetic damping
parameter and to realize the EP in cavity optomagnonics.",2006.16510v1
2020-09-01,Mapping the stray fields of a nanomagnet using spin qubits in SiC,"We report the use of optically addressable spin qubits in SiC to probe the
magnetic stray fields generated by a ferromagnetic microstructure
lithographically patterned on the surface of a SiC crystal. The stray fields
cause shifts in the resonance frequency of the spin centers. The spin resonance
is driven by a micrometer-sized microwave antenna patterned adjacent to the
magnetic element. The patterning of the antenna is done to ensure that the
driving microwave fields are delivered locally and more efficiently compared to
conventional, millimeter-sized circuits. A clear difference in the resonance
frequency of the spin centers in SiC is observed at various distances to the
magnetic element, for two different magnetic states. Our results offer a
wafer-scale platform to develop hybrid magnon-quantum applications by deploying
local microwave fields and the stray field landscape at the micrometer
lengthscale.",2009.00347v1
2020-09-06,Exact solution of the Boltzmann equation for low-temperature transport coefficients in metals II: Scattering by ferromagnons,"In a previous paper (Paper I) we developed a technique for exactly solving
the linearized Boltzmann equation for the electrical and thermal transport
coefficients in metals in the low-temperature limit. Here we adapt this
technique to determine the magnon contribution to the electrical and thermal
conductivities, and to the thermopower, in metallic ferromagnets. For the
electrical resistivity $\rho$ at asymptotically low temperatures we find $\rho
\propto \exp{(-T_{\rm min}/T)}$, with $T_{\rm min}$ an energy scale that
results from the exchange gap and a temperature independent prefactor of the
exponential. The corresponding result for the heat conductivity is $\sigma_h
\propto T^3\,\exp{(T_{\rm min}/T)}$, and thermopower is $S \propto T$. All of
these results are exact, including the prefactors.",2009.02787v1
2020-09-07,Quantum Sensing of Spin Fluctuations of Magnetic Insulator Films with Perpendicular Anisotropy,"Nitrogen vacancy (NV) centers, optically active atomic defects in diamond,
have been widely applied to emerging quantum sensing, imaging, and network
efforts, showing unprecedented field sensitivity and nanoscale spatial
resolution. Many of these advantages derive from their excellent
quantum-coherence, controllable entanglement, and high fidelity of operations,
enabling opportunities to outperform the classical counterpart. Exploiting this
cutting-edge quantum metrology, we report noninvasive measurement of intrinsic
spin fluctuations of magnetic insulator thin films with a spontaneous
out-of-plane magnetization. The measured field dependence of NV relaxation
rates is well correlated to the variation of magnon density and band structure
of the magnetic samples, which are challenging to access by the conventional
magnetometry methods. Our results highlight the significant opportunities
offered by NV centers in diagnosing the noise environment of functional
magnetic elements, providing valuable information to design next-generation,
high-density, and scalable spintronic devices.",2009.02883v1
2020-09-07,"Exact solution of the Boltzmann equation for low-temperature transport coefficients in metals I: Scattering by phonons, antiferromagnons, and helimagnons","We present a technique for an exact solution of the linearized Boltzmann
equation for the electrical and thermal transport coefficients in metals in the
low-temperature limit. This renders unnecessary an uncontrolled approximation
that has been used in all previous solutions of the integral equations for the
transport coefficients. Applications include electron-phonon scattering in
nonmagnetic metals, as well as the magnon contribution to the electrical and
thermal conductivities, and to the thermopower, in metallic ferromagnets,
antiferromagnets, and helimagnets. In this paper, the first of a pair, we set
up the technique and apply it to the scattering of electrons by phonons,
antiferromagnons, and helimagnons. We show that the Bloch $T^5$ law for the
electrical resistivity, the $T^2$ law for the thermal resistivity, and the $T$
law for the thermopower due to phonon and antiferromagnon scattering are exact,
and determine the prefactors exactly. The corresponding exact results for
helimagnons are $T^{5/2}$, $T^{1/2}$, and $T$, respectively. In a second paper
we will consider the scattering by ferromagnons.",2009.03234v1
2020-09-12,Magnetic ground state and electron doping tuning of Curie temperature in Fe$_3$GeTe$_2$: first-principles studies,"Intrinsic magnetic van der Waals (vdW) materials have attracted much
attention, especially Fe$_{3}$GeTe$_{2}$ (FGT), which exhibits highly tunable
properties. However, despite vast efforts, there are still several challenging
issues to be resolved. Here using a first-principles linear-response approach,
we carry out comprehensive investigation of both bulk and monolayer FGT. We
find that although the magnetic exchange interactions in FGT are frustrated,
our Monte Carlo simulations agree with the total energy calculations,
confirming that the ground state of bulk FGT is indeed ferromagnetic (FM). A
tiny electron doping reduces the magnetic frustration, resulting in significant
increasing of the Curie temperature. We also calculate the spin-wave
dispersion, and find a small spin gap as well as a nearly flat band in the
magnon spectra. These features can be used to compare with the future neutron
scattering measurement and finally clarify the microscopic magnetic mechanism
in this two-dimensional family materials.",2009.05701v1
2020-09-27,Dynamical spin susceptibility in La2CuO4 studied by resonant inelastic x-ray scattering,"Resonant inelastic X-ray scattering (RIXS) is a powerful probe of elementary
excitations in solids. It is now widely applied to study magnetic excitations.
However, its complex cross-section means that RIXS has been more difficult to
interpret than inelastic neutron scattering (INS). Here we report
high-resolution RIXS measurements of magnetic excitations of La2CuO4, the
antiferromagnetic parent of one system of high-temperature superconductors. At
high energies (~2 eV), the RIXS spectra show angular-dependent dd orbital
excitations which are found to be in good agreement with single-site multiplet
calculations. At lower energies (<0.3 eV), we show that the
wavevector-dependent RIXS intensities are proportional to the product of the
single-ion spin-flip cross section and the dynamical susceptibility of the
spin-wave excitations. When the spin-flip crosssection is dividing out, the
RIXS magnon intensities show a remarkable resemblance to INS data. Our results
show that RIXS is a quantitative probe the dynamical spin susceptibility in
cuprate and therefore should be used for quantitative investigation of other
correlated electron materials.",2009.12925v1
2021-02-15,"Angular-Dependent Dynamic Response and Magnetization Reversal in Fibonacci-Distorted, Kagome Artificial Spin Ice","We have measured the angular dependence of ferromagnetic resonance (FMR)
spectra for Fibonacci-distorted, Kagome artificial spin ice (ASI). The number
of strong modes in the FMR spectra depend on the orientation of the applied DC
magnetic field. In addition, discontinuities observed in the FMR
field-frequency dispersion curves also depend on DC field orientation, and
signal a multi-step DC magnetization reversal, which is caused by the reduced
energy degeneracy of Fibonacci-distorted vertices. The results suggest the
orientation of applied magnetic field and severity of Fibonacci distortion
constitute control variables for FMR modes and multi-step reversal in future
magnonic devices and magnetic switching systems.",2102.07532v1
2021-02-16,Fully Spin-transparent magnetic interfaces enabled by insertion of a paramagnetic NiO layer,"Spin backflow and spin-memory loss have been well established to considerably
lower the interfacial spin transmissivity of metallic magnetic interfaces and
thus the energy efficiency of spin-orbit torque technologies. Here we report
that spin backflow and spin-memory loss at Pt-based heavy metal/ferromagnet
interfaces can be effectively eliminated by inserting an insulating
paramagnetic NiO layer of optimum thickness. The latter enables the thermal
magnon-mediated essentially unity spin-current transmission at room temperature
due to considerably enhanced effective spin-mixing conductance of the
interface. As a result, we obtain dampinglike spin-orbit torque efficiency per
unit current density of up to 0.8 as detected by the standard technology
ferromagnet FeCoB and others, which reaches the expected upper-limit spin Hall
ratio of Pt. We establish that Pt/NiO and Pt-Hf/NiO are two energy-efficient,
integration-friendly, and high-endurance spin-current generators that provide
>100 times greater energy efficiency than sputter-deposited topological
insulators BiSb and BiSe. Our finding will benefit spin-orbitronic research and
advance spin-torque technologies.",2102.08487v1
2021-02-18,Quantum control of bosonic modes with superconducting circuits,"Bosonic modes have wide applications in various quantum technologies, such as
optical photons for quantum communication, magnons in spin ensembles for
quantum information storage and mechanical modes for reversible
microwave-to-optical quantum transduction. There is emerging interest in
utilizing bosonic modes for quantum information processing, with circuit
quantum electrodynamics (circuit QED) as one of the leading architectures.
Quantum information can be encoded into subspaces of a bosonic superconducting
cavity mode with long coherence time. However, standard Gaussian operations
(e.g., beam splitting and two-mode squeezing) are insufficient for universal
quantum computing. The major challenge is to introduce additional nonlinear
control beyond Gaussian operations without adding significant bosonic loss or
decoherence. Here we review recent advances in universal control of a single
bosonic code with superconducting circuits, including unitary control, quantum
feedback control, driven-dissipative control and holonomic dissipative control.
Various approaches to entangling different bosonic modes are also discussed.",2102.09668v2
2021-02-20,Observation of Coherent Spin Waves in a Three-Dimensional Artificial Spin Ice Structure,"Harnessing high-frequency spin dynamics in three-dimensional (3D)
nanostructures may lead to paradigm-shifting, next generation devices including
high density spintronics and neuromorphic systems. Despite remarkable progress
in fabrication, the measurement and interpretation of spin dynamics in complex
3D structures remain exceptionally challenging. Here we take a first step and
measure coherent spin waves within a 3D artificial spin ice (ASI) structure
using Brillouin light scattering. The 3D-ASI was fabricated by using a
combination of two-photon lithography and thermal evaporation. Two spin-wave
modes were observed in the experiment whose frequencies showed a monotonic
variation with the applied field strength. Numerical simulations qualitatively
reproduced the observed modes. The simulated mode profiles revealed the
collective nature of the modes extending throughout the complex network of
nanowires while showing spatial quantization with varying mode quantization
numbers. The study shows a well-defined means to explore high-frequency spin
dynamics in complex 3D spintronic and magnonic structures.",2102.10270v1
2013-06-08,Systematic Effective Field Theory Analysis of the D=2+1 Quantum XY Model at Low Temperatures,"The low-temperature properties of the (2+1)-dimensional quantum XY model are
studied within the framework of effective Lagrangians up to three-loop order.
At zero temperature, the system is characterized by a spontaneously broken spin
rotation symmetry, O(2) $\to$ 1, where the corresponding Goldstone bosons are
the spin waves or magnons. Even though there is no spontaneously broken order
at finite $T$, the low-temperature behavior of the system is still governed by
the spin waves. The partition function is evaluated and various thermodynamic
quantities, including the order parameter, are derived in the presence of a
weak external field. In particular, we show that the spin-wave interaction is
repulsive at low temperatures, its magnitude depending on the strength of the
external field. We compare our results with those for the (2+1)-dimensional
antiferromagnetic Heisenberg model which, at T=0, exhibits the spontaneously
broken spin rotation symmetry O(3) $\to$ O(2).",1306.1944v1
2013-06-24,Spinon confinement: dynamics of weakly coupled Hubbard chains,"Using large-scale determinant quantum Monte Carlo simulations in combination
with the stochastic analytical continuation, we study two-particle dynamical
correlation functions in the anisotropic square lattice of weakly coupled
one-dimensional (1D) Hubbard chains at half-filling and in the presence of weak
frustration. The evolution of the static spin structure factor upon increasing
the interchain coupling is suggestive of the transition from the power-law
decay of spin-spin correlations in the 1D limit to long-range antiferromagnetic
order in the quasi-1D regime and at $T=0$. In the numerically accessible regime
of interchain couplings, the charge sector remains gapped. The low-energy
momentum dependence of the spin excitations is well described by the linear
spin-wave theory with the largest intensity located around the
antiferromagnetic wave vector. This magnon mode corresponds to a bound state of
two spinons. At higher energies the spinons deconfine and we observe signatures
of the two-spinon continuum which progressively fade away as a function of
interchain hopping.",1306.5775v2
2015-07-06,Delta chain with anisotropic ferromagnetic and antiferromagnetic interactions,"We consider analytically and numerically an anisotropic spin-$\frac{1}{2}$
delta-chain (sawtooth chain) with nearest-neighbor ferromagnetic and
next-nearest-neighbor antiferromagnetic interactions. For certain values of the
interactions a lowest one-particle band becomes flat and there is a class of
localized-magnon eigenstates which form a ground state with a macroscopic
degeneracy. In this case the model depends on a single parameter which can be
chosen as the anisotropy of the exchange interactions. When this parameter
changes from zero to infinity the model interpolates between the
one-dimensional isotropic ferromagnet and the frustrated Ising model on the
delta-chain. It is shown that the low-temperature thermodynamic properties in
these limiting cases are governed by the specific structure of the excitation
spectrum. In particular, the specific heat has one or infinite number of
low-temperature maxima for the small or the large anisotropy parameter,
correspondingly. Numerical calculations of finite chains demonstrate that this
behavior is generic for definite values of the anisotropy parameter.",1507.01389v1
2015-07-08,Excitations and the tangent space of projected entangled-pair states,"We develop tangent space methods for projected entangled-pair states (PEPS)
that provide direct access to the low-energy sector of strongly-correlated
two-dimensional quantum systems. More specifically, we construct a variational
ansatz for elementary excitations on top of PEPS ground states that allows for
computing gaps, dispersion relations, and spectral weights directly in the
thermodynamic limit. Solving the corresponding variational problem requires the
evaluation of momentum transformed two-point and three-point correlation
functions on a PEPS background, which we can compute efficiently by using a
contraction scheme. As an application we study the spectral properties of the
magnons of the Affleck-Kennedy-Lieb-Tasaki model on the square lattice and the
anyonic excitations in a perturbed version of Kitaev's toric code.",1507.02151v2
2015-07-21,Static and Dynamical Properties of the Spin-1/2 Equilateral Triangular-Lattice Antiferromagnet Ba$_3$CoSb$_2$O$_9$,"We present single-crystal neutron scattering measurements of the spin-1/2
equilateral triangular lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$. Besides
confirming that the Co$^{2+}$ magnetic moments lie in the ab plane for zero
magnetic field, we determine all the exchange parameters of the minimal
quasi-2D spin Hamiltonian, which confirms that Ba$_3$CoSb$_2$O$_9$ is an almost
perfect realization of the paradigmatic model of frustrated quantum magnetism.
A comparison with linear and nonlinear spin-wave theory reveals that quantum
fluctuations induce a strong downward renormalization of the magnon dispersion.",1507.05702v2
2015-07-26,Antiferromagnetic fluctuations in the quasi-two-dimensional organic superconductor detected by Raman spectroscopy,"Using Raman scattering, the quasi-two dimensional organic superconductor
k-(BEDT-TTF)2Cu[N(CN)2]Br (T_c=11.8 K) and the related antiferromagnet
k-(BEDT-TTF)2Cu[N(CN)2]Cl are studied. Raman scattering provides unique
spectroscopic information about magnetic degrees of freedom that has been
otherwise unavailable on such organic conductors. Below T=200 K a broad band at
about 500 cm-1 develops in both compounds. We identify this band with
two-magnon excitation. The position and the temperature dependence of the
spectral weight are similar in the antiferromagnet and in the metallic
Fermi-liquid. We conclude that antiferromagnetic correlations are similarly
present in the magnetic insulator and the Fermi-liquid state of the
superconductor.",1507.07178v2
2015-07-31,Topological spin transport by Brownian diffusion of domain walls,"We propose thermally-populated domain walls (DWs) in an easy-plane
ferromagnetic insulator as robust spin carriers between two metals. The
chirality of a DW, which serves as a topological charge, couples to the metal
spin accumulation via spin-transfer torque and results in the
chirality-dependent thermal nucleation rates of DWs at the interface. After
overpopulated DWs of a particular (net) chirality diffuse and leave the
ferromagnet at the other interface, they reemit the spin current by spin
pumping. The conservation of the topological charge supports an algebraic decay
of spin transport as the length of the ferromagnet increases; this is analogous
to the decaying behavior of superfluid spin transport but contrasts with the
exponential decay of magnon spin transport. We envision that similar spin
transport with algebraic decay may be implemented in materials with exotic spin
phases by exploiting topological characteristics and the associated conserved
quantities of their excitations.",1507.08767v2
2015-12-03,Large-$s$ expansions for the low-energy parameters of the honeycomb-lattice Heisenberg antiferromagnet with spin quantum number $s$,"The coupled cluster method (CCM) is employed to very high orders of
approximation to study the ground-state (GS) properties of the spin-$s$
Heisenberg antiferromagnet (with isotropic interactions, all of equal strength,
between nearest-neighbour pairs only) on the honeycomb lattice. We calculate
with high accuracy the complete set of GS parameters that fully describes the
low-energy behaviour of the system, in terms of an effective magnon field
theory, viz., the energy per spin, the magnetic order parameter (i.e., the
sublattie magnetization), the spin stiffness and the zero-field (uniform,
transverse) magnetic susceptibility, for all values of the spin quantum number
$s$ in the range $\frac{1}{2} \leq s \leq \frac{9}{2}$. The CCM data points are
used to calculate the leading quantum corrections to the classical ($s
\rightarrow \infty$) values of these low-energy parameters, considered as
large-$s$ asymptotic expansions.",1512.01063v2
2015-12-10,Magnetic ordering and non-Fermi-liquid behavior in the multichannel Kondo-lattice model,"Scaling equations for the Kondo lattice in the paramagnetic and magnetically
ordered phases are derived to two-loop order with account of spin dynamics. The
results are applied to describe various mechanisms of the non-Fermi-liquid
(NFL) behavior in the multichannel Kondo-lattice model where a fixed point
occurs in the weak-coupling region. The corresponding temperature dependences
of electronic and magnetic properties are discussed. The model describes
naturally formation of a magnetic state with soft boson mode and small moment
value. An important role of Van Hove singularities in the magnon spectral
function is demonstrated. The results are rather sensitive to the type of
magnetic ordering and space dimensionality, the conditions for NFL behavior
being more favorable in the antiferromagnetic and 2D cases.",1512.03161v3
2015-12-24,Collisional Dynamics of Half-Quantum Vortices in a Spinor Bose-Einstein Condensate,"We present an experimental study on the interaction and dynamics of
half-quantum vortices (HQVs) in an antiferromagnetic spinor Bose-Einstein
condensate. By exploiting the orbit motion of a vortex dipole in a trapped
condensate, we perform a collision experiment of two HQV pairs, and observe
that the scattering motions of the HQVs is consistent with the short-range
vortex interaction that arises from nonsingular magnetized vortex cores. We
also investigate the relaxation dynamics of turbulent condensates containing
many HQVs, and demonstrate that spin wave excitations are generated by the
collisional motions of the HQVs. The short-range vortex interaction and the
HQV-magnon coupling represent two characteristics of the HQV dynamics in the
spinor superfluid.",1512.07696v2
2016-09-01,Bound states and field-polarized Haldane modes in a quantum spin ladder,"The challenge of one-dimensional systems is to understand their physics
beyond the level of known elementary excitations. By high-resolution neutron
spectroscopy in a quantum spin ladder material, we probe the leading
multiparticle excitation by characterizing the two-magnon bound state at zero
field. By applying high magnetic fields, we create and select the singlet
(longitudinal) and triplet (transverse) excitations of the fully spin-polarized
ladder, which have not been observed previously and are close analogs of the
modes anticipated in a polarized Haldane chain. Theoretical modelling of the
dynamical response demonstrates our complete quantitative understanding of
these states.",1609.00198v1
2016-09-03,All-phononic Amplification in Coupled Cantilever Arrays based on Gap Soliton Dynamics,"We present a mechanism of amplification of phonons by phonons on the basis of
nonlinear band-gap transmission phenomenon. As a concept the idea may be
applied to the various number of systems, however we introduce the specific
idea of creating amplification scenario in the chain of coupled cantilever
arrays. One chain is driven at the constant frequency located in the upper band
of the ladder system, thus no wave enters the system. However the frequency is
specifically chosen to be very close to the maximum value of frequency
corresponding to dispersion relation of the system. Amplification scenario
happens when a counter phase pulse of same frequency with a small amplitude is
introduced to the second chain. If both signals exceed a threshold amplitude
for the band-gap transmission a large amplitude soliton enters the system -
therefore we have an amplifier. Although the concept may be applied in a
variety of contexts - all optical or all-magnonic systems, we choose the system
of coupled cantilever arrays and represent a clear example of the application
of presented conceptual idea. Logical operations is the other probable field,
where such mechanism could be used, which might significantly broaden the
horizon of considered applications of band-gap soliton dynamics.",1609.00855v1
2016-09-14,Post-quench dynamics and suppression of thermalization in an open half-filled Hubbard layer,"We study the time evolution of a half-filled Hubbard layer coupled to a
magnon bath after a quench of the Hubbard interaction. Qualitatively different
regimes, regarding the asymptotic long-time dynamics, are identified and
characterized within the mean-field approximation. In the absence of the bath,
the dynamics of the closed system is similar to that of a quenched BCS
condensate. Though the presence of the bath introduces an additional relaxation
mechanism, our numerical results and analytical arguments show that
equilibration with the bath is not necessarily attained within the
approximations used. Instead, nonequilibrium states, similar to the ones
observed in the closed system, can emerge at long times as a consequence of the
competition between the intrinsic relaxation mechanism (Landau damping, for
example), and the bath-induced dissipation.",1609.04151v3
2016-09-15,Low-damping sub-10-nm thin films of lutetium iron garnet grown by molecular-beam epitaxy,"We analyze the structural and magnetic characteristics of (111)-oriented
lutetium iron garnet (Lu$_3$Fe$_5$O$_{12}$) films grown by molecular-beam
epitaxy, for films as thin as 2.8 nm. Thickness-dependent measurements of the
in- and out-of-plane ferromagnetic resonance allow us to quantify the effects
of two-magnon scattering, along with the surface anisotropy and the saturation
magnetization. We achieve effective damping coefficients of $11.1(9) \times
10^{-4}$ for 5.3 nm films and $32(3) \times 10^{-4}$ for 2.8 nm films, among
the lowest values reported to date for any insulating ferrimagnetic sample of
comparable thickness.",1609.04753v1
2016-11-03,Spin caloritronic nano-oscillator,"Energy loss due to ohmic heating is a major bottleneck limiting down-scaling
and speed of nano-electronic devices, and harvesting ohmic heat for signal
processing is a major challenge in modern electronics. Here we demonstrate that
thermal gradients arising from ohmic heating can be utilized for excitation of
coherent auto-oscillations of magnetization and for generation of tunable
microwave signals. The heat-driven dynamics is observed in
$\mathrm{Y_{3}Fe_{5}O_{12}/Pt}$ bilayer nanowires where ohmic heating of the Pt
layer results in injection of pure spin current into the
$\mathrm{Y_{3}Fe_{5}O_{12}}$ layer. This leads to excitation of
auto-oscillations of the $\mathrm{Y_{3}Fe_{5}O_{12}}$ magnetization and
generation of coherent microwave radiation. Our work paves the way towards spin
caloritronic devices for microwave and magnonic applications.",1611.00887v2
2016-11-03,Scalable synchronization of spin-Hall oscillators in out-of-plane field,"A strategy for a scalable synchronization of an array of spin-Hall
oscillators (SHOs) is illustrated. In detail, we present micromagnetic
simulations of two and five SHOs realized by means of couples of triangular
golden contacts on the top of a Pt/CoFeB/Ta trilayer. Results highlight that
the synchronization occurs for the whole current region that gives rise to the
excitation of self-oscillations. This is linked to the role of the
magnetodipolar coupling, which is the phenomenon driving the synchronization
when the distance between oscillators is not too large. Synchronization turns
out to be also robust against geometrical differences of the contacts,
simulated by considering variable distances between the tips ranging from 100nm
to 200nm. Besides, it entails an enlargement of the radiation pattern that can
be useful for the generation of spin-waves in magnonics applications.
Simulations performed to study the effect of the interfacial
Dzyaloshinskii-Moriya interaction show nonreciprocity in spatial propagation of
the synchronized spin-wave. The simplicity of the geometry and the robustness
of the achieved synchronization make this design of array of SHOs scalable for
a larger number of synchronized oscillators.",1611.01227v1
2016-11-21,Coherence and stiffness of spin waves in diluted ferromagnets,"We present results of a numerical analysis of magnon spectra in supercells
simulating two-dimensional and bulk random diluted ferromagnets with
long-ranged pair exchange interactions. We show that low-energy spectral
regions for these strongly disordered systems contain a coherent component
leading to interference phenomena manifested by a pronounced sensitivity of the
lowest excitation energies to the adopted boundary conditions. The dependence
of configuration averages of these excitation energies on the supercell size
can be used for an efficient determination of the spin-wave stiffness D. The
developed formalism is applied to the ferromagnetic Mn-doped GaAs semiconductor
with optional incorporation of phosphorus; the obtained concentration trends of
D are found in reasonable agreement with recent experiments. Moreover, a
relation of the spin stiffness to the Curie temperature Tc has been studied for
Mn-doped GaAs and GaN semiconductors. It is found that the ratio Tc/D exhibits
qualitatively the same dependence on Mn concentration in both systems.",1611.06691v1
2016-11-23,Absence of magnetic thermal conductivity in the quantum spin liquid candidate YbMgGaO4,"We present the ultra-low-temperature specific heat and thermal conductivity
measurements on the single crystals of YbMgGaO$_4$, which was recently argued
to be a promising candidate for quantum spin liquid (QSL). In the zero magnetic
field, a large magnetic contribution of specific heat is observed, and exhibits
a power-law temperature dependence ($C_m \sim T^{0.74}$). On the contrary, we
do not observed any significant contribution of thermal conductivity from
magnetic excitations. In magnetic fields $H \ge$ 6 T, the exponential
$T$-dependence of $C_m$ and the enhanced thermal conductivity indicate a magnon
gap of the fully-polarized state. The absence of magnetic thermal conductivity
at the zero field in this QSL candidate puts a strong constraint on the
theories of its ground state.",1611.07683v1
2017-04-03,Effects of spatially engineered Dzyaloshinskii-Moriya interaction in ferromagnetic films,"The Dzyaloshinskii-Moriya interaction (DMI) is a chiral interaction that
favors formation of domain walls. Recent experiments and ab initio calculations
show that there are multiple ways to modify the strength of the interfacially
induced DMI in thin ferromagnetic films with perpendicular magnetic anisotropy.
In this paper we reveal theoretically the effects of spatially varied DMI on
the magnetic state in thin films. In such heterochiral 2D structures we report
several emergent phenomena, ranging from the equilibrium spin canting at the
interface between regions with different DMI, over particularly strong
confinement of domain walls and skyrmions within high-DMI tracks, to advanced
applications such as domain tailoring nearly at will, design of magnonic
waveguides, and much improved skyrmion racetrack memory.",1704.00770v1
2017-04-12,On the origin of magnetic anisotropy in two dimensional CrI$_3$,"The observation of ferromagnetic order in a monolayer of CrI$_3$ has been
recently reported, with a Curie temperature of 45 Kelvin and off-plane easy
axis. Here we study the origin of magnetic anisotropy, a necessary ingredient
to have magnetic order in two dimensions, combining two levels of modeling,
density functional calculations and spin model Hamiltonians. We find two
different contributions to the magnetic anisotropy of the material, both
favoring off-plane magnetization and contributing to open a gap in the spin
wave spectrum. First, ferromagnetic super-exchange across the $\simeq $ 90
degree Cr-I-Cr bonds, are anisotropic, due to the spin orbit interaction of the
ligand I atoms. Second, a much smaller contribution that comes from the single
ion anisotropy of the $S=3/2$ Cr atom. Our results permit to establish the XXZ
Hamiltonian, with a very small single ion anisotropy, as the adequate spin
model for this system. Using spin wave theory we estimate the Curie temperature
and we highlight the essential role played by the gap that magnetic anisotropy
induces on the magnon spectrum.",1704.03849v4
2017-04-13,Switching of magnetic ground states across the UIr1-xRhxGe alloy system,"We investigated the evolution of magnetism in the UIr1-xRhxGe system by the
systematic study of high-quality single crystals. Lattice parameters of both
parent compounds are very similar resulting in almost identical nearest
interatomic uranium distance close to the Hill limit. We established the x-T
phase diagram of the UIr1-xRhxGe system and found a discontinuous
antiferromagnetic/ferromagnetic boundary at xcrit = 0.56 where a local minimum
in ordering temperature and maximum of the Sommerfeld coefficient 175 mJ/mol K2
occurs in the UCoGe-URhGe-UIrGe system, signaling an increase in magnetic
fluctuations. However, a quantum critical point is not realized because of the
finite ordering temperature at xcrit. A magnon gap on the antiferromagnetic
side abruptly suppresses magnetic fluctuations. We find a field-induced first
order transition in the vicinity of the critical magnetic field along the b
axis in the entire UIr1-xRhxGe system including the ferromagnetic region
UCo0.6Rh0.4Ge - URhGe.",1704.04013v1
2017-04-24,Magnetic topological lithography: Gateway to the artificial spin ice manifold,"Nanomagnetic arrays are widespread in data storage and processing. As current
technologies approach fundamental limits on size and thermal stability,
extracting additional functionality from arrays is crucial to advancing
technological progress. One design exploiting the enhanced magnetic
interactions in dense arrays is the geometrically-frustrated metamaterial
'artificial spin ice' (ASI). Frustrated systems offer vast untapped potential
arising from their unique microstate landscapes, presenting intriguing
opportunities from reconfigurable logic to magnonic devices or hardware neural
networks. However, progress in such systems is impeded by the inability to
access more than a fraction of the total microstate space. Here, we present a
powerful surface-probe lithography technique, magnetic topological lithography,
providing access to all possible microstates in ASI and related nanomagnetic
arrays. We demonstrate the creation of two previously elusive states; the
spin-crystal ground state of dipolar kagome ASI and high-energy, low-entropy
'monopole-chain' states exhibiting negative effective temperatures.",1704.07439v1
2017-04-25,Analytical study of the edge states in the bosonic Haldane model,"We investigate the properties of magnon edge states in a ferromagnetic
honeycomb spin lattice with a Dzyaloshinskii-Moriya interaction (DMI). We
derive analytical expressions for the energy spectra and wavefunctions of the
edge states localized on the boundaries. By introducing an external on-site
potential at the outermost sites, we show that the bosonic band structure is
similar to that of the fermionic graphene. We investigate the region in the
momentum space where the bosonic edge states are well defined and we analyze
the width of the edge state and their dependence with DMI strength. Our
findings extend the predictions using topological arguments and they allow
size-dependent confirmation from possible experiments",1704.07746v3
2017-12-31,Proper dissipative torques in antiferromagnetic dynamics,"There is little doubt that the magnetization dynamics of ferromagnetic
systems is governed by the Landau-Lifshitz-Gilbert equation or its
generalization with various spin torques. In contrast, there are several sets
of dynamic equations for two-sublattice antiferromagnets (AFMs) in literature
that have different forms of dissipative torques and no proper dynamic
equations for multi-sublattice AFMs and ferrimagnets in general. Here we
introduce the general Rayleigh dissipation functional into the Lagrange
equation and derive the proper form of the dissipative torques in the
phenomenological equations for the AFMs with multiple sublattices. A new type
of dissipative torque arising from inter-sublattice drag effect is discovered
that has important influences on magnon lifetime and domain wall motion. In
particular, our theory unifies different dynamic equations of AFMs in
literature.",1801.00217v1
2018-01-02,Phonon driven Floquet matter,"A resonantly excited coherent phonon leads to a periodic oscillation of the
atomic lattice in a crystal structure bringing the material into a
non-equilibrium electronic configuration. Periodically oscillating quantum
systems can be understood in terms of Floquet theory and we show these concepts
can be applied to coherent lattice vibrations reflecting the underlying
coupling mechanism between electrons and bosonic modes. This coupling leads to
dressed quasi-particles imprinting specific signatures in the spectrum of the
electronic structure. Taking graphene as a paradigmatic material we show how
the phonon-dressed states display an intricate sideband structure revealing
electron-phonon coupling and topological ordering. This work establishes that
the recently demonstrated concept of light-induced non-equilibrium Floquet
phases can also be applied when using coherent phonon modes for the dynamical
control of material properties. The present results are generic for bosonic
time-dependent perturbations and similar phenomena can be observed for plasmon,
magnon or exciton driven materials.",1801.00599v1
2018-01-08,Low frequency spin dynamics in XY quantum spin ice Yb$_2$Pt$_2$O$_7$,"The XY pyrochlore Yb$_2$Ti$_2$O$_7$, with pseudo spin 1/2 at the Yb$^{3+}$
site, has been celebrated as potential host for the quantum spin ice state. The
substitution of non-magnetic Ti with Pt gives Yb$_2$Pt$_2$O$_7$, a system with
remarkably similar magnetic properties. The large nuclear gyromagnetic ratio
($\gamma_{n}/2 \pi = 9.15$~MHz/T) of $^{195}$Pt makes Yb$_2$Pt$_2$O$_7$ an
ideal material for NMR investigation of its unconventional magnetic properties.
Based on the $^{195}$Pt nuclear spin-lattice relaxation rate $1/T_1$ and the
magnetic specific heat $C_{p}$ measured in a broad range of magnetic field
$B_{ext}$, we demonstrate that the field-induced magnon gap linearly decreases
with $B_{ext}$ but additional low energy mode of spin excitations emerge below
$\sim 0.5$~T.",1801.02724v2
2018-01-29,Current-induced spin-orbit torques in ferromagnetic and antiferromagnetic systems,"Spin-orbit coupling in inversion-asymmetric magnetic crystals and structures
has emerged as a powerful tool to generate complex magnetic textures,
interconvert charge and spin under applied current, and control magnetization
dynamics. Current-induced spin-orbit torques mediate the transfer of angular
momentum from the lattice to the spin system, leading to sustained magnetic
oscillations or switching of ferromagnetic as well as antiferromagnetic
structures. The manipulation of magnetic order, domain walls and skyrmions by
spin-orbit torques provides evidence of the microscopic interactions between
charge and spin in a variety of materials and opens novel strategies to design
spintronic devices with potentially high impact in data storage, nonvolatile
logic, and magnonic applications. This paper reviews recent progress in the
field of spin-orbitronics, focusing on theoretical models, material properties,
and experimental results obtained on bulk noncentrosymmetric conductors and
multilayer heterostructures, including metals, semiconductors, and topological
insulator systems. Relevant aspects for improving the understanding and
optimizing the efficiency of nonequilibrium spin-orbit phenomena in future
nanoscale devices are also discussed.",1801.09636v2
2018-01-26,Probing magnetism in 2D van der Waals crystalline insulators via electron tunneling,"Magnetic insulators are a key resource for next-generation spintronic and
topological devices. The family of layered metal halides promises ultrathin
insulating multiferroics, spin liquids, and ferromagnets, but new
characterization methods are required to unlock their potential. Here, we
report tunneling through the layered magnetic insulator CrI3 as a function of
temperature and applied magnetic field. We electrically detect the magnetic
ground state and inter-layer coupling and observe a field-induced metamagnetic
transition. The metamagnetic transition results in magnetoresistances of 95%,
300%, and 550% for bilayer, trilayer, and tetralayer CrI3 barriers,
respectively. We further measure inelastic tunneling spectra for our junctions,
unveiling a rich spectrum of collective magnetic excitations (magnons) in CrI3.
Our results establish vertical tunneling as a versatile probe of magnetism in
atomically thin insulators.",1801.10075v1
2018-08-01,The generalized gradient approximation kernel in time-dependent density functional theory,"A complete understanding of a material requires both knowledge of the excited
states as well as of the ground state. In particular, the low energy
excitations are of utmost importance while studying the electronic, magnetic,
dynamical, and thermodynamical properties of the material. Time-Dependent
Density Functional Theory (TDDFT), within the linear regime, is a successful
\textit{ab-initio} method to access the electronic charge and spin excitations.
However, it requires an approximation to the exchange-correlation (XC) kernel
which encapsulates the effect of electron-electron interactions in the
many-body system. In this work we derive and implement the spin-polarized XC
kernel for semi-local approximations such as the adiabatic Generalized Gradient
Approximation (AGGA). This kernel has a quadratic dependence on the wavevector,
{\bf q}, of the perturbation, however the impact of this on the electron energy
loss spectra (EELS) is small. Although the GGA functional is good in predicting
structural properties, it generality overestimates the exchange spin-splitting.
This leads to higher magnon energies, as compared to both ALDA and experiment.
In addition, interaction with the Stoner spin-flip continuum is enhanced by
AGGA, which strongly suppresses the intensity of spin-waves.",1808.00215v1
2018-08-02,Impact of the regularization parameter in the Mean Free Path reconstruction method: Nanoscale heat transport and beyond,"The understanding of the mean free path (MFP) distribution of the energy
carriers in materials (e.g. electrons, phonons, magnons, etc.) is a key
physical insight into their transport properties. In this context, MFP
spectroscopy has become an important tool to describe the contribution of
carriers with different MFP to the total transport phenomenon. In this work, we
revise the MFP reconstruction technique and present a study on the impact of
the regularization parameter on the MFP distribution of the energy carriers. By
using the L-curve criterion, we calculate the optimal mathematical value of the
regularization parameter. The effect of the change from the optimal value in
the MFP distribution is analyzed in three case studies of heat transport by
phonons. These results demonstrate that the choice of the regularization
parameter has a large impact on the physical information obtained from the
reconstructed accumulation function, and thus cannot be chosen arbitrarily. The
approach can be applied to various transport phenomena at the nanoscale
involving carriers of different physical nature and behavior.",1808.00708v3
2018-08-06,Spin wave dispersion and intensity correlation in width-modulated nanowire arrays: A Brillouin light scattering study,"Using Brillouin light scattering spectroscopy and dynamical matrix method
calculations, we study collective spin waves in dense arrays of periodically
double-side width-modulated Permalloy nanowires. Width modulation is achieved
by creating a sequence of triangular notches on the two parallel nanowire
sides, with a periodicity of p=1000 nm, and tunable relative displacement (D)
of the notches sequence on the two lateral sides. Both symmetric (D=0) and
asymmetric (D= 250 and 500 nm) width-modulated nanowires were investigated. We
have found that the detected modes have Bloch-type character and belong to a
doublet deriving from the splitting of the modes characteristics of the
nanowire with homogeneous width. Interestingly, the amplitude of the magnonic
band, the frequency difference of the doublet, as well as their relative
scattering intensity, can be efficiently controlled by increasing D rather than
having single- or symmetric (D=0) double-side width-modulation.",1808.01732v1
2018-08-18,An overview of the NIRSPEC upgrade for the Keck II telescope,"NIRSPEC is a 1-5 micron echelle spectrograph in use on the Keck II Telescope
since 1999. The spectrograph is capable of both moderate (R~2,000) and high
(R~25,000) resolution observations and has been a workhorse instrument across
many astronomical fields, from planetary science to extragalactic observations.
In the latter half of 2018, we will upgrade NIRSPEC to improve the sensitivity
and stability of the instrument and increase its lifetime. The major components
of the upgrade include replacing the spectrometer and slit-viewing camera
detectors with Teledyne H2RG arrays and replacing all transputer-based
electronics. We present detailed design, testing, and analysis of the upgraded
instrument, including the finalized optomechanical design of the new 1-5 micron
slit-viewing camera, final alignment and assembly of the science array,
electronics systems, and updated software design.",1808.06024v1
2018-08-21,Pseudo Jahn-Teller Effect and Magnetoelastic Coupling in Spin-Orbit Mott Insulators,"The consequences of the Jahn-Teller (JT) orbital-lattice coupling for
magnetism of pseudospin J_{eff}=1/2 and J_{eff}=0 compounds are addressed. In
the former case, represented by Sr_2IrO_4, this coupling generates, through the
so-called pseudo-JT effect, orthorhombic deformations of a crystal concomitant
with magnetic ordering. The orthorhombicity axis is tied to the magnetization
and rotates with it under magnetic field. The theory resolves a number of
puzzles in Sr_2IrO_4 such as the origin of in-plane magnetic anisotropy and
magnon gaps, metamagnetic transition, etc. In J_{eff}=0 systems, the pseudo-JT
effect leads to spin-nematic transition well above magnetic ordering, which may
explain the origin of `orbital order' in Ca_2RuO_4",1808.06919v2
2018-08-23,The infrared imaging spectrograph (IRIS) for TMT: electronics-cable architecture,"The InfraRed Imaging Spectrograph (IRIS) is a first-light instrument for the
Thirty Meter Telescope (TMT). It combines a diffraction limited imager and an
integral field spectrograph. This paper focuses on the electrical system of
IRIS. With an instrument of the size and complexity of IRIS we face several
electrical challenges. Many of the major controllers must be located directly
on the cryostat to reduce cable lengths, and others require multiple bulkheads
and must pass through a large cable wrap. Cooling and vibration due to the
rotation of the instrument are also major challenges. We will present our
selection of cables and connectors for both room temperature and cryogenic
environments, packaging in the various cabinets and enclosures, and techniques
for complex bulkheads including for large detectors at the cryostat wall.",1808.07853v1
2018-08-27,Magnetostriction in Antiferromagnets,"We consider magnetostriction in magnetic materials. Starting from a
microscopic lattice model of the magneto-elastic coupling, we derive the
continuum magnetostriction Hamiltonian in cubic ferromagnets and
antiferromagnets. In ferromagnets, our results agree with established results.
In antiferromagnets, we find a new central contribution which reflects the spin
ordering. The additional term is essential in certain antiferromagnets where
magnetic moments order in antiparallel ferromagnetic planes characterized by a
normal axis. In such antiferromagnets, we predict that the magnetostrictive
strain depends on the direction of the normal axis. Our findings are relevant
for materials such as NiO and CoO, where there is a new spin-structure
dependent strain, but not in RbMnF$_3$. We propose an experimental procedure to
verify the importance of our findings in NiO that can be readily generalized to
other materials. We also generalize the theory to include long-wavelength
magnetoelastic coupling. We expect that the spin-structure dependent term
critically affects the magnon-phonon coupling.",1808.09013v2
2018-08-28,An Intrinsic Spin Orbit Torque Nano-Oscillator,"Spin torque and spin Hall effect nanooscillators generate high intensity spin
wave auto oscillations on the nanoscale enabling novel microwave applications
in spintronics, magnonics, and neuromorphic computing. For their operation,
these devices require externally generated spin currents either from an
additional ferromagnetic layer or a material with a high spin Hall angle. Here
we demonstrate highly coherent field and current tunable microwave signals from
nanoconstrictions in single 15 and 20 nm thick permalloy layers. Using a
combination of spin torque ferromagnetic resonance measurements, scanning
microBrillouin light scattering microscopy, and micromagnetic simulations, we
identify the autooscillations as emanating from a localized edge mode of the
nanoconstriction driven by spin orbit torques. Our results pave the way for
greatly simplified designs of auto oscillating nanomagnetic systems only
requiring a single ferromagnetic layer.",1808.09330v1
2019-06-01,Polarized inelastic neutron scattering of non-reciprocal spin waves in MnSi,"We report spin-polarized inelastic neutron scattering of the dynamical
structure factor of the conical magnetic helix in the cubic chiral magnet MnSi.
We find that the spectral weight of spin-flip scattering processes is
concentrated on single branches for wavevector transfer parallel to the helix
axis as inferred from well-defined peaks in the neutron spectra. In contrast,
for wavevector transfers perpendicular to the helix the spectral weight is
distributed among different branches of the magnon band structure as reflected
in broader features of the spectra. Taking into account the effects of
instrumental resolution, our experimental results are in excellent quantitative
agreement with parameter-free theoretical predictions. Whereas the dispersion
of the spin waves in MnSi appears to be approximately reciprocal at low
energies and small applied fields, the associated spin-resolved spectral weight
displays a pronounced non-reciprocity that implies a distinct non-reciprocal
response in the limit of vanishing uniform magnetization at zero magnetic
field.",1906.00221v3
2019-06-12,Unfolding the complexity of quasi-particle physics in disordered materials,"The concept of quasi-particles forms the theoretical basis of our microscopic
understanding of emergent phenomena associated with quantum mechanical
many-body interactions. However, quasi-particle theory in disordered materials
has proven difficult, resulting in the predominance of mean-field solutions.
Here we report first-principles phonon calculations and inelastic x-ray and
neutron scattering measurements on equiatomic alloys (NiCo, NiFe, AgPd, and
NiFeCo) with force constant dominant disorder - confronting a key 50-year-old
assumption in the Hamiltonian of all mean-field quasi-particle solutions for
off-diagonal disorder. Our results have revealed the presence of a large, and
heretofore unrecognized, impact of local chemical environments on the
distribution of the species-pair-resolved force constant disorder that can
dominate phonon scattering. This discovery not only identifies a critical
analysis issue that has broad implications for other elementary excitations
such as magnons and skyrmions in magnetic alloys, but also provides an
important tool for the design of materials with ultra-low thermal conductivity.",1906.04945v1
2019-06-14,Mechanisms of FMR line broadening in CoFeB-LiNbO$_3$ granular films in the vicinity of metal-insulator transition,"Metal-insulator (CoFeB)$_x$(LiNbO$_3$)$_{100-x}$ nanocomposite films with
different content of the ferromagnetic (FM) phase $x$ are investigated by
ferromagnetic resonance (FMR) technique. A strong change of the FMR line shape
is observed in the vicinity of metal-insulator transition (MIT) of the film,
where the hopping-type conductivity $\sigma$ modifies to the regime of a strong
intergranular tunnelling, characterized by a logarithmic dependence $\sigma(T)$
at high temperatures. It is shown that below MIT, the FMR linewidth is mainly
determined by the inhomogeneous distribution of the local anisotropy axes in
the film plane. Above MIT, the contribution of this inhomogeneity to the line
broadening decreases. At the same time, two-magnon magnetic relaxation
processes begin to play a significant role in the formation of the linewidth.
The observed behaviour indicates the critical role of interparticle exchange in
the tunnelling regime above MIT of the nanocomposite.",1906.06320v2
2019-06-17,Local Photothermal Control of Phase Transitions for On-demand Room-temperature Rewritable Magnetic Patterning,"The ability to make controlled patterns of magnetic structures within a
nonmagnetic background is essential for several types of existing and proposed
technologies. Such patterns provide the foundation of magnetic memory and logic
devices, allow the creation of artificial spin-ice lattices and enable the
study of magnon propagation. Here, we report a novel approach for magnetic
patterning that allows repeated creation and erasure of arbitrary shapes of
thin-film ferromagnetic structures. This strategy is enabled by epitaxial
Fe$_{0.52}$Rh$_{0.48}$ thin films designed so that both ferromagnetic and
antiferromagnetic phases are bistable at room temperature. Starting with the
film in a uniform antiferromagnetic state, we demonstrate the ability to write
arbitrary patterns of the ferromagnetic phase by local heating with a focused
laser. If desired, the results can then be erased by cooling with a
thermoelectric cooler and the material repeatedly re-patterned.",1906.07239v1
2019-06-19,Edge localization of spin waves in antidot multilayers with perpendicular magnetic anisotropy,"We study the spin-wave dynamics in nanoscale antidot lattices based on Co/Pd
multilayers with perpendicular magnetic anisotropy. Using time-resolved
magneto-optical Kerr effect measurements we demonstrate that the variation of
the antidot shape introduces significant change in the spin-wave spectra,
especially in the lower frequency range. By employing micromagnetic simulations
we show that additional peaks observed in the measured spectra are related to
narrow shell regions around the antidots, where the magnetic anisotropy is
reduced due to the Ga+ ion irradiation during the focused ion beam milling
process of the antidot fabrication. The results point at new possibilities for
exploitation of localized spin waves in out-of-plane magnetized thin films,
which are easily tunable and suitable for magnonics applications.",1906.08109v2
2019-06-24,Octahedral tilting induced isospin reorientation transition in iridate heterostructures,"Iridate heterostructures are gaining interest as their magnetic properties
are much more sensitive to structural distortion compared to pure spin systems
due to spin-orbital entanglement induced by strong spin-orbit coupling. While
bulk monolayer and bilayer iridates show $ab$-plane canted and $c$-axis
antiferromagnetic (AFM) order, recent experiments on layered iridate
superlattices (SL) have revealed striking properties, especially in the bilayer
SL. A spin model is presented including the tilting induced Kitaev type
interactions, which illustrates the proclivity towards $ab$-plane canted AFM
order. A realistic Hubbard model including spin-dependent hopping terms arising
from octahedral rotation and tilting is constructed for the bilayer SL in
isospin space, and magnetic excitations are investigated in the
self-consistently determined magnetic state. The Hubbard model analysis
confirms the spin model results and shows strongly reduced magnon energy gap
and an isospin reorientation transition from $c$-axis to $ab$-plane canted AFM
order with increasing tilting.",1906.09879v2
2019-06-28,Zero-field propagation of spin waves in waveguides prepared by focused ion beam direct writing,"Metastable face-centered-cubic Fe78Ni22 thin films grown on Cu(001)
substrates are excellent candidates for focused ion beam direct writing of
magnonic structures due to their favorable magnetic properties after
ion-beam-induced transformation. The focused ion beam transforms the originally
nonmagnetic fcc phase into the ferromagnetic bcc phase with additional control
over the direction of uniaxial magnetic in-plane anisotropy. The
magnetocrystalline anisotropy in transformed areas is strong enough to
stabilize the magnetization in transverse direction to the long axis of narrow
waveguides. Therefore, it is possible to propagate spin waves in these
waveguides without the presence of an external magnetic field in the favorable
Demon-Eshbach geometry. Phase-resolved micro-focused Brillouin light scattering
yields the dispersion relation of these waveguides in zero as well as in
nonzero external magnetic fields.",1906.12254v2
2019-07-12,Spin Seebeck effect and phonon energy transfer in heterostructures containing layers of a normal metal and a ferromagnetic insulator,"In the framework of the kinetic approach based on the Boltzmann equation for
the phonon distribution function, we analyze phonon heat transfer in a
heterostructure containing a layer of a normal metal ($ N $) and a layer of a
ferromagnetic insulator ($ F $). Two realistic methods for creating a
temperature gradient in such a heterostructure are considered: by heating of
the $N$-layer by an electric current and by placing the $N/F$-bilayer between
massive dielectrics with different temperatures. The electron temperature $ T_e
$ in the $ N $-layer and the magnon temperature $ T_m $ in the $ F $-layer are
calculated. The difference in these temperatures determines the voltage $
V_{ISHE} $ on the $ N $-layer in the Seebeck spin effect regime. The dependence
of $ V_{ISHE} $ on the bath temperature and on the thickness of the $ N $ and $
F $ layers is compared with the available experimental data.",1907.05739v1
2019-07-16,Polylogarithms from the bound state S-matrix,"Higher-point functions of gauge invariant composite operators in N=4 super
Yang-Mills theory can be computed via triangulation. The elementary tile in
this process is the hexagon introduced for the evaluation of structure
constants. A glueing procedure welding the tiles back together is needed to
return to the original object.
  In this note we present work in progress on n-point functions of BPS
operators. In this case, quantum corrections are entirely carried by the
glueing procedure. The lowest non-elementary process is the glueing of three
adjacent tiles by the exchange of two single magnons.
  This problem has been analysed before. With a view to resolving some
conceptional questions and to generalising to higher processes we are trying to
develop an algorithmic approach using the representation of hypergeometric sums
as integrals over Euler kernels.",1907.07014v1
2019-07-25,Order out of a Coulomb phase and Higgs transtion: frustrated transverse interactions of Nd2Zr2O7,"The pyrochlore material Nd$_2$Zr$_2$O$_7$ with an ""all-in-all-out"" (AIAO)
magnetic order shows novel quantum moment fragmentation with gapped flat
dynamical spin ice modes. The parameterized spin Hamiltonian with a dominant
frustrated ferromagnetic transverse term reveals a proximity to a U(1) spin
liquid. Here we study magnetic excitations of Nd$_2$Zr$_2$O$_7$ above the
ordering temperature ($T_\text{N}$) using high-energy-resolution inelastic
neutron scattering. We find strong spin ice correlations at zero energy with
the disappearance of gapped magnon excitations of the AIAO order. It seems that
the gap to the dynamical spin ice closes above $T_\text{N}$ and the system
enters a quantum spin ice state competing with and suppressing the AIAO order.
Classical Monte Carlo, molecular dynamics and quantum boson calculations
support the existence of a Coulombic phase above $T_\text{N}$. Our findings
relate the magnetic ordering of Nd$_2$Zr$_2$O$_7$ with the Higgs mechanism and
provide explanations for several previously reported experimental features.",1907.11056v1
2019-07-29,A Synthetic Skyrmion Platform with Robust Tunability,"Magnetic skyrmions are topologically non-trivial spin structure, and their
existence in ferromagnetically coupled multilayers has been reported with
disordered arrangement. In these multilayers, the heavy metal spacing layers
provide an interfacial Dzyaloshinskii-Moriya interaction (DMI) for stabilizing
skyrmions at the expense of interlayer exchanging coupling (IEC). To meet the
functional requirement of ordered/designable arrangement, in this work, we
proposed and experimentally demonstrated a scenario of skyrmion nucleation
using nanostructured synthetic antiferromagnetic (SAF) multilayers. Instead of
relying on DMI, the antiferromagnetic IEC in the SAF multilayers fulfills the
role of nucleation and stabilization of skyrmions. The IEC induced skyrmions
were identified directly imaged with MFM and confirmed by magnetometry and
magnetoresistance measurements as well as micromagnetic simulation.
Furthermore, the robustness of the proposed skyrmion nucleation scenario was
examined against temperature (from 4.5 to 300 K), device size (from 400 to 1200
nm), and different lattice designs. Hence, our results provide a synthetic
skyrmion platform meeting the functional needs in magnonic and spintronic
applications.",1907.12516v1
2019-10-02,Magnon bound states vs. anyonic Majorana excitations in the Kitaev honeycomb magnet $α$-RuCl$_3$,"The pure Kitaev honeycomb model harbors a quantum spin liquid in zero
magnetic fields, while applying finite magnetic fields induces a topological
spin liquid with non-Abelian anyonic excitations. This latter phase has been
much sought after in Kitaev candidate materials, such as $\alpha$-RuCl$_3$.
Currently, two competing scenarios exist for the intermediate field phase of
this compound ($B=7-10$ T), based on experimental as well as theoretical
results: (i) conventional multiparticle magnetic excitations of integer quantum
number vs. (ii) Majorana fermionic excitations of possibly non-Abelian nature
with a fractional quantum number. To discriminate between these scenarios a
detailed investigation of excitations over a wide field-temperature phase
diagram is essential. Here we present Raman spectroscopic data revealing
low-energy quasiparticles emerging out of a continuum of fractionalized
excitations at intermediate fields, which are contrasted by conventional
spin-wave excitations. The temperature evolution of these quasiparticles
suggests the formation of bound states out of fractionalized excitations.",1910.00800v1
2019-10-07,Spinon confinement and deconfinement in spin-1 chains,"Motivated by the rich phase diagrams recently unveiled in frustrated spin-1
chains with competing next-nearest neighbour and four-spin interactions, we
investigate the nature of the elementary excitations of spin-1 chains in the
vicinity of phase transitions using the matrix-product state (MPS)
representation of elementary excitations. First, we show that both spinons and
magnons naturally arise in SU(2) invariant spin chains when describing ground
states and elementary excitations using MPS. Then we investigate the nature of
the elementary excitations across the first-oder transition between the Haldane
phase and the topologically trivial next-nearest neighbor Haldane phase. We
show explicitly that spinons deconfine at the transition, and we calculate the
dispersion of these deconfined spinons with MPS. We also show that, immediately
away from the transition line, spinons confine, forming dispersive
spinon/anti-spinon bound states on both sides of the transition. Finally, we
show that deconfined spinons also appear at the transition between the Haldane
phase and the spontaneously dimerized phase when this transition is first
order.",1910.03064v2
2019-10-22,Controlled nonlinear magnetic damping in spin-Hall nano-devices,"Large-amplitude magnetization dynamics is substantially more complex compared
to the low-amplitude linear regime, due to the inevitable emergence of
nonlinearities. One of the fundamental nonlinear phenomena is the nonlinear
damping enhancement, which imposes strict limitations on the operation and
efficiency of magnetic nanodevices. In particular, nonlinear damping prevents
excitation of coherent magnetization auto-oscillations driven by the injection
of spin current into spatially extended magnetic regions. Here, we propose and
experimentally demonstrate that nonlinear damping can be controlled by the
ellipticity of magnetization precession. By balancing different contributions
to anisotropy, we minimize the ellipticity and achieve coherent magnetization
oscillations driven by spatially extended spin current injection into a
microscopic magnetic disk. Our results provide a novel route for the
implementation of efficient active spintronic and magnonic devices driven by
spin current.",1910.09801v1
2019-10-26,High-degeneracy points protected by site-permutation symmetries,"Space group symmetries dictate the energy degeneracy of quasiparticles (e.g.,
electronic, photonic) in crystalline structures. For spinless systems, there
can only be double or triple degeneracies protected by these symmetries, while
other degeneracies are usually taken as \textit{accidental}. In this Letter we
show that it is possible to design higher degeneracies exploring site
permutation symmetries. These design principles are shown to be satisfied in
previously studied lattices, and new structures are proposed with three, four
and five degeneracy points for spinless systems. The results are general and
apply to different quasiparticle models. Here, we focus on a tight-binding
approach for the electronic case as a proof of principle. The resulting
high-degeneracy points are protected by the site-permutation symmetries,
yielding pseudospin-1 and -2 Dirac fermions. The strategy proposed here can be
used to design lattices with high-degeneracy points in electronic (e.g.
metal-organic frameworks), photonic, phononic, magnonic and cold-atom systems.",1910.12144v2
2019-10-30,Strong coupling-enabled broadband non-reciprocity,"Non-reciprocity of signal transmission enhances capacity of communication
channels and protects transmission quality against possible signal
instabilities, thus becoming an important component ensuring coherent
information processing. However, non-reciprocal transmission requires breaking
time-reversal symmetry (TRS) which poses challenges of both practical and
fundamental character hindering the progress. Here we report a new scheme for
achieving broadband non-reciprocity using a specially engineered hybrid
microwave cavity. The TRS breaking is realized via strong coherent coupling
between a selected chiral mode in the microwave cavity and a single collective
spin excitation (magnon) in a ferromagnetic yttrium iron garnet (YIG) sphere.
The non-reciprocity in transmission is observed spanning nearly a 0.5 GHz
frequency band, which outperforms by two orders of magnitude the previously
achieved bandwidths. Our findings open new directions for robust coherent
information processing in a broad range of systems in both classical and
quantum regimes.",1910.14117v2
2020-01-01,Pseudo-spin rotation symmetry breaking by Coulomb interaction terms in spin-orbit coupled systems,"By transforming from the pure-spin-orbital ($t_{\rm 2g}$) basis to the
spin-orbital entangled pseudo-spin-orbital basis, the pseudo-spin rotation
symmetry of the different Coulomb interaction terms is investigated under SU(2)
transformation in pseudo-spin space. While the Hubbard and density interaction
terms are invariant, the Hund's coupling and pair-hopping interaction terms
explicitly break pseudo-spin rotation symmetry systematically. The form of the
symmetry-breaking terms obtained from the transformation of the Coulomb
interaction terms accounts for the easy $x$-$y$ plane anisotropy and magnon gap
for the out-of-plane mode, highlighting the importance of mixing with the
nominally non-magnetic $J$=3/2 sector, and providing a physically transparent
approach for investigating magnetic ordering and anisotropy effects in
perovskite ($\rm Sr_2 Ir O_4$) and other $d^5$ pseudo-spin compounds.",2001.00190v2
2020-01-15,Yttrium Iron Garnet Thickness Influence on the Spin Pumping in the Bulk Acoustic Wave Resonator,"The features of phonon-magnon interconversion in acoustic resonator determine
the efficiency of spin pumping from YIG into Pt that may be detected
electrically through the inverse spin Hall effect (ISHE). Based on the methods
developed in previous works for calculating resonator structures with a
piezoelectric (ZnO) and a magnetoelastic layer in contact with the heavy metal
(YIG/Pt), we present the results of numerical calculations of YIG film
thickness influence on acoustically driven spin waves. We obtain some YIG film
thickness regions with various behavior of dc ISHE voltage $U_{ISHE}$. At
micron and submicron thicknesses, the higher spin wave resonance (SWR) modes
(both even and odd) can be generated with efficiency comparable and even
exceeding that of the main mode. The absolute maximum of $U_{ISHE}$ is achieved
at the thickness about $s_1 \approx 208$ nm under the excitation of the first
SWR.",2001.05385v1
2020-01-15,Phonon magnetochiral effect of band-geometric origin in Weyl semimetals,"The phonon magnetochiral effect consists of a nonreciprocity in the velocity
or attenuation of acoustic waves when they propagate parallel and antiparallel
to an external magnetic field. The first experimental observation of this
effect has been reported recently in a chiral magnet and ascribed to the
hybridization between acoustic phonons and chiral magnons. Here, we predict a
potentially measurable phonon magnetochiral effect of electronic origin in
chiral Weyl semimetals. Caused by the Berry curvature and the orbital magnetic
moment, this effect is enhanced for longitudinal phonons by the chiral anomaly.",2001.05483v3
2020-01-18,Off-axial focusing of spin-wave lens in the presence of Dzyaloshinskii-Moriya interaction,"We theoretically study the effect of Dzyaloshinskii-Moriya interaction (DMI)
on the focusing of a spin-wave lens that is constructed by a circular interface
between two magnetic films. We analytically derive the generalized Snell's law
in the curved geometry and the position of the focal point which exhibits a
peculiar off-axial focusing behavior. We uncover a strong dependence of the
focal point on both the material parameters and the frequency of incident spin
waves. Full micromagnetic simulations compare well with theoretical
predictions. Our findings would be helpful to manipulate spin waves in chiral
magnets and to design functional magnonic devices.",2001.06604v1
2020-01-21,Nonresonant amplification of coherent spin waves through voltage-induced interface magnetoelectric effect and spin-transfer torque,"We present new mechanism for manipulation of the spin-wave amplitude through
the use of the dynamic charge-mediated magnetoelectric effect in ultrathin
multilayers composed of dielectric thin-film capacitors separated by a
ferromagnetic bilayer. Propagating spin waves can be amplified and attenuated
with rising and decreasing slopes of the oscillating voltage, respectively,
locally applied to the sample. The way the spin accumulation is generated makes
the interaction of the spin-transfer torque with the magnetization dynamics
mode-selective and restricted to some range of spin-wave frequencies, which is
in contrary to known types of the spin-transfer torque effects. The interfacial
nature of spin-dependent screening allows to reduce the thickness of the fixed
magnetization layer to a few nanometers, thus the proposed effect significantly
contributes toward realization of the magnonic devices and also miniaturization
of the spintronic devices.",2001.07474v1
2020-01-22,On two phases inside the Bose condensation dome of Yb$_2$Si$_2$O$_7$,"Recent experimental data on Bose-Einstein Condensation (BEC) of magnons in
the spin-gap compound Yb$_2$Si$_2$O$_7$ revealed an asymmetric BEC dome
arXiv:1810.13096v2. We examine modifications to the Heisenberg model on a
breathing honeycomb lattice, showing that this physics can be explained by
competing forms of weak anisotropy. We employ a gamut of analytical and
numerical techniques to show that the anisotropy yields a field driven phase
transition from a state with broken Ising symmetry to a phase which breaks no
symmetries and crosses over to the polarized limit.",2001.08219v2
2020-01-23,Dephasing-assisted macrospin transport,"Transport phenomena are ubiquitous in physics, and it is generally understood
that the environmental disorder and noise deteriorates the transfer of
excitations. There are however cases in which transport can be enhanced by
fluctuations. In the present work we show, by means of micromagnetics
simulations, that transport efficiency in a chain of classical macrospins can
be greatly increased by an optimal level of dephasing noise. We demonstrate
also the same effect in a simplified model, the dissipative Discrete Nonlinear
Schr\""odinger equation subject to phase noise. Our results point towards the
realisation of a large class of magnonics and spintronics devices, where
disorder and noise can be used to enhance spin-dependent transport efficiency.",2001.08624v2
2020-01-27,Acoustic ferromagnetic resonance and spin pumping induced by surface acoustic waves,"Voltage induced magnetization dynamics of magnetic thin films is a valuable
tool to study anisotropic fields, exchange couplings, magnetization damping and
spin pumping mechanism. A particularly well established technique is the
ferromagnetic resonance (FMR) generated by the coupling of microwave photons
and magnetization eigenmodes in the GHz range. Here we review the basic
concepts of the so-called acoustic ferromagnetic resonance technique (a-FMR)
induced by the coupling of surface acoustic waves (SAW) and magnetization of
thin films. Interestingly, additional to the benefits of the microwave excited
FMR technique, the coupling between SAW and magnetization also offers fertile
ground to study magnon-phonon and spin rotation couplings. We describe the
in-plane magnetic field angle dependence of the a-FMR by measuring the
absorption / transmission of SAW and the attenuation of SAW in the presence of
rotational motion of the lattice, and show the consequent generation of spin
current by acoustic spin pumping.",2001.09581v1
2020-02-09,Walker-like Domain Wall breakdown in layered Antiferromagnets driven by staggered spin-orbit fields,"Within linear continuum theory, no magnetic texture can propagate faster than
the maximum group velocity of its spin waves. Here we report a transient regime
due to the appearance of additional antiferromagnetic textures that breaks the
Lorentz translational invariance of the magnetic system by atomistic spin
dynamics simulations. This dynamical regime is akin to domain wall
Walker-breakdown in ferromagnets and involves the nucleation of an
antiferromagnetic domain wall pair. Subsequently, one of the nucleated
180$^{\circ}$ domain wall creates with the original domain wall a 360$^{\circ}$
spin-rotation which remains static even under the action of the spin-orbit
field. The other 180$^{\circ}$ domain wall becomes accelerated to
super-magnonic speeds. Under large spin-orbit fields, multiple domain wall
generation and recombination is obtained which may explain the recently
experimentally observed current pulse induce shattering of large domain
structures into small fragmented domains and the subsequent slow recreation of
large-scale domain formation prior current pulse.",2002.03332v1
2020-02-11,Sub-micrometer near-field focusing of spin waves in ultrathin YIG films,"We experimentally demonstrate tight focusing of a spin wave beam excited in
extended nanometer-thick films of Yttrium Iron Garnet by a simple microscopic
antenna functioning as a single-slit near-field lens. We show that the focal
distance and the minimum transverse width of the focal spot can be controlled
in a broad range by varying the frequency/wavelength of spin waves and the
antenna geometry. The experimental data are in good agreement with the results
of numerical simulations. Our findings provide a simple solution for
implementation of magnonic nano-devices requiring local concentration of the
spin-wave energy.",2002.04284v1
2020-02-12,Toward room-temperature nanoscale skyrmions in ultrathin films,"Breaking the dilemma between small size and room temperature stability is a
necessary prerequisite for skyrmion-based information technology. Here we
demonstrate by means of rate theory and an atomistic spin Hamiltonian that the
stability of isolated skyrmions in ultrathin ferromagnetic films can be
enhanced by the concerted variation of magnetic interactions while keeping the
skyrmion size unchanged. We predict film systems where the lifetime of sub-10
nm skyrmions can reach years at ambient conditions. The long lifetime of such
small skyrmions is due to exceptionally large Arrhenius pre-exponential and the
stabilizing effect of the energy barrier is insignificant at room temperature.
A dramatic increase in the pre-exponential is achieved thanks to softening of
magnon modes of the skyrmion, thereby increasing the entropy of the skyrmion
with respect to the transition state for collapse. Increasing the number of
skyrmion deformation modes should be a guiding principle for the realization of
nanoscale, room-temperature stable skyrmions.",2002.05285v2
2020-02-14,Low energy magnons in the chiral ferrimagnet $\text{Cu}_2\text{OSeO}_3$: a coarse-grained approach,"We report a comprehensive neutron scattering study of low energy magnetic
excitations in the breathing pyrochlore helimagnetic
$\text{Cu}_2\text{OSeO}_3$. Fully documenting the four lowest energy magnetic
modes that leave the ferrimagnetic configuration of the ""strong tetrahedra""
intact ($|\hbar\omega|<13$ meV), we find gapless quadratic dispersion at the
$\Gamma$ point for energies above 0.2 meV, two doublets separated by 1.6(2) meV
at the $R$ point, and a bounded continuum at the $X$ point. Our constrained
rigid spin cluster model relates these features to Dzyaloshinskii-Moriya (DM)
interactions and the incommensurate helical ground state. Combining
conventional spin wave theory with a spin cluster form-factor accurately
reproduces the measured equal time structure factor through multiple Brillouin
zones. An effective spin Hamiltonian describing the complex anisotropic
inter-cluster interactions is obtained.",2002.06283v2
2020-02-17,Enhancement of magnetization plateaus in low dimensional spin systems,"We study the low-energy properties and, in particular, the magnetization
process of a spin-1/2 Heisenberg $J_1-J_2$ sawtooth and frustrated chain (also
known as zig-zag ladder) with a spatially anisotropic $g$-factor. We treat the
problem both analytically and numerically while keeping the $J_2/J_1$ ratio
generic. Numerically, we use complete and Lanczos diagonalization as well as
the infinite time-evolving block decimation (iTEBD) method. Analytically we
employ (non-)Abelian bosonization. Additionally for the sawtooth chain, we
provide an analytical description in terms of flat bands and localized magnons.
By considering a specific pattern for the $g$-factor anisotropy for both
models, we show that a small anisotropy significantly enhances a magnetization
plateau at half saturation. For the magnetization of the frustrated chain, we
show the destruction of the $1/3$ of the full saturation plateau in favor of
the creation of a plateau at half-saturation. For large anisotropies, the
existence of an additional plateau at zero magnetization is possible. Here and
at higher magnetic fields, the system is locked in the half-saturation plateau,
never reaching full saturation.",2002.07190v1
2020-02-26,Magnon dispersion and dynamic spin response in three-dimensional spin models for $α$-RuCl$_3$,"In the search for experimental realizations of bond-anisotropic Kitaev
interactions and resulting spin-liquid phases, the layered magnet
$\alpha$-RuCl$_3$ is a prime candidate. Its modelling typically involves
Heisenberg, Kitaev, and symmetric off-diagonal $\Gamma$ interactions on the
two-dimensional honeycomb lattice. However, recent neutron-scattering
experiments point towards a sizeable magnetic interlayer coupling. Here we
study three-dimensional exchange models for $\alpha$-RuCl$_3$, for both
possible $R\bar{3}$ and $C2/m$ crystal structures. We discuss the symmetry
constraints on the interlayer couplings, construct minimal models, and use them
to compute the magnetic mode dispersion and the dynamical spin structure
factor, in both the zero-field zigzag phase and the paramagnetic high-field
phase. Our predictions for the interlayer mode dispersion shall guide future
experiments; they also call for a reevaluation of the quantitative model
parameters relevant for $\alpha$-RuCl$_3$.",2002.11727v2
2020-02-27,Ultrafast magnetization dynamics in half-metallic Co$_2$FeAl Heusler alloy,"We report on optically induced, ultrafast magnetization dynamics in the
Heusler alloy $\mathrm{Co_{2}FeAl}$, probed by time-resolved magneto-optical
Kerr effect. Experimental results are compared to results from electronic
structure theory and atomistic spin-dynamics simulations. Experimentally, we
find that the demagnetization time ($\tau_{M}$) in films of
$\mathrm{Co_{2}FeAl}$ is almost independent of varying structural order, and
that it is similar to that in elemental 3d ferromagnets. In contrast, the
slower process of magnetization recovery, specified by $\tau_{R}$, is found to
occur on picosecond time scales, and is demonstrated to correlate strongly with
the Gilbert damping parameter ($\alpha$). Our results show that
$\mathrm{Co_{2}FeAl}$ is unique, in that it is the first material that clearly
demonstrates the importance of the damping parameter in the remagnetization
process. Based on these results we argue that for $\mathrm{Co_{2}FeAl}$ the
remagnetization process is dominated by magnon dynamics, something which might
have general applicability.",2002.12255v1
2020-02-27,Spin Wave Radiation by a Topological Charge Dipole,"The use of spin waves (SWs) as data carriers in spintronic and magnonic logic
devices offers operation at low power consumption, free of Joule heating.
Nevertheless, the controlled emission and propagation of SWs in magnetic
materials remains a significant challenge. Here, we propose that
skyrmion-antiskyrmion bilayers form topological charge dipoles and act as
efficient sub-100 nm SW emitters when excited by in-plane ac magnetic fields.
The propagating SWs have a preferred radiation direction, with clear dipole
signatures in their radiation pattern, suggesting that the bilayer forms a SW
antenna. Bilayers with the same topological charge radiate SWs with spiral and
antispiral spatial profiles, enlarging the class of SW patterns. We demonstrate
that the characteristics of the emitted SWs are linked to the topology of the
source, allowing for full control of the SW features, including their
amplitude, preferred direction of propagation, and wavelength.",2002.12282v1
2020-03-04,Spin-wave Talbot effect in thin ferromagnetic film,"The Talbot effect has been known in optics since XIX century and found
various technological applications. In this paper, we demonstrate with the help
of micromagnetic simulations this self-imaging phenomenon for spin waves
propagating in a thin ferromagnetic film magnetized out-of-plane. We show that
the main features of the obtained Talbot carpets for spin waves can be
described, to a large extent, by the approximate analytical formulas yielded by
the general analysis of the wave phenomena. Our results indicate a route to a
feasible experimental realisation of the Talbot effect at low and high
frequencies and offer interesting effects and possible applications in
magnonics.",2003.02138v2
2020-03-09,Colossal quasiparticle radiation in the Lifshitz spin liquid phase of a two-dimensional quantum antiferromagnet,"Strong quantum fluctuations in magnetic systems can create disordered quantum
spin liquid phases of matter which are not predicted by classical physics. The
complexity of the exotic phenomena on display in spin liquids has led to a
great deal of theoretical and experimental interest. However, understanding the
fundamental nature of the excitations in these systems remains challenging. In
this work, we consider the Lifshitz quantum critical point in a two-dimensional
frustrated $XY$ antiferromagnet. At this point, quantum fluctuations destroy
long range order, leading to the formation of an algebraic Lifshitz spin
liquid. We demonstrate that the bosonic magnon excitations are long-lived and
well-defined in the Lifshitz spin liquid phase, though paradoxically, the
dynamic structure factor has a broad non-Lorentzian frequency distribution with
no single-particle weight. We resolve this apparent contradiction by showing
that the Lifshitz spin liquid suffers from an infrared catastrophe: An external
physical probe always excites an infinite number of arbitrarily low energy
quasiparticles, which leads to significant radiative broadening of the
spectrum.",2003.03936v1
2020-03-11,Spin Seebeck Effect near the Antiferromagnetic Spin-Flop Transition,"We develop a low-temperature, long-wavelength theory for the interfacial spin
Seebeck effect (SSE) in easy-axis antiferromagnets. The field-induced spin-flop
(SF) transition of N\'eel order is associated with a qualitative change in SSE
behavior: Below SF, there are two spin carriers with opposite magnetic moments,
with the carriers polarized along the field forming a majority magnon band.
Above SF, the low-energy, ferromagnetic-like mode has magnetic moment opposite
the field. This results in a sign change of the SSE across SF, which agrees
with recent measurements on Cr$_2$O$_3$/Pt and Cr$_2$O$_3$/Ta devices [Li
$\textit{et al.,}$ $\textit{Nature}$ $\textbf{578,}$ 70 (2020)]. In our theory,
SSE is due to a N\'eel spin current below SF and a magnetic spin current above
SF. Using the ratio of the associated N\'eel to magnetic spin-mixing
conductances as a single constant fitting parameter, we reproduce the field
dependence of the experimental data and partially the temperature dependence of
the relative SSE jump across SF.",2003.05232v1
2020-03-17,Odd-parity spin-loop-current order mediated by transverse spin fluctuations in cuprates and related electron systems,"Unconventional symmetry-breaking phenomena due to nontrivial order parameters
attract increasing attention in strongly correlated electron systems. Here, we
predict theoretically the occurrence of nanoscale spontaneous spin-current,
called the spin loop-current (sLC) order, as a promising origin of the
pseudogap and electronic nematicity in cuprates. We reveal that the sLC is
driven by the odd-parity electron-hole condensation that are mediated by
transverse spin fluctuations around the pseudogap temperature $T^*$. At the
same temperature, odd-parity magnon pair condensation occurs. The sLC order is
""hidden"" in that neither internal magnetic field nor charge density modulation
is induced, whereas the predicted sLC with finite wavenumber naturally gives
the Fermi arc structure. In addition, the fluctuations of sLC order work as
attractive pairing interaction between adjacent hot spots, which enlarges the
d-wave superconducting transition temperature $T_c$. The sLC state will be a
key ingredient in understanding the pseudogap, electronic nematicity as well as
superconductivity in cuprates and other strongly correlated metals.",2003.07556v3
2020-03-27,Electrical detection of unconventional transverse spin-currents in obliquely magnetized thin films,"In a typical experiment in magnonics, thin films are magnetized in-plane and
spin waves only carry angular momentum along their spatial propagation
direction. Motivated by the experiments of Bozhko et al. [Phys. Rev. Research
2, 023324 (2020)], we show theoretically that for obliquely magnetized thin
films, exchange-dipolar spin waves are accompanied by a transverse
spin-current. We propose an experiment to electrically detect this transverse
spin-current with Pt strips on top of a YIG film, by comparing the induced
spin-current for spin waves with opposite momenta. We predict the relative
difference to be of the order $10^{-4}$, for magnetic fields tilted at least
$30^{\circ}$ out of plane. This transverse spin-current is the result of the
long range dipole-dipole interaction and the inversion symmetry breaking of the
interface.",2003.12520v4
2020-04-03,Plasma-enhanced atomic layer deposition of nickel nanotubes with low resistivity and coherent magnetization dynamics for 3D spintronics,"We report plasma-enhanced atomic layer deposition (ALD) to prepare conformal
nickel thin films and nanotubes by using nickelocene as a precursor, water as
the oxidant agent and an in-cycle plasma enhanced reduction step with hydrogen.
The optimized ALD pulse sequence, combined with a post-processing annealing
treatment, allowed us to prepare 30 nm thick metallic Ni layers with a
resistivity of 8 $\mu\Omega$cm at room temperature and good conformality both
on the planar substrates and nanotemplates. Thereby we fabricated several
micrometer-long nickel nanotubes with diameters ranging from 120 to 330 nm. We
report on the correlation between ALD growth and functional properties of
individual Ni nanotubes characterized in terms of magneto-transport and the
confinement of spin wave modes. The findings offer novel perspectives for
Ni-based spintronics and magnonic devices operated in the GHz frequency regime
with a 3D device architecture.",2004.01592v2
2020-04-03,Spin-Phonon Coupling and Thermodynamic Behaviour in YCrO3 and LaCrO3: Inelastic Neutron Scattering and Lattice Dynamics,"We report detailed temperature-dependent inelastic neutron scattering and
ab-initio lattice dynamics investigation of magnetic perovskites YCrO3 and
LaCrO3. The magnetic neutron scattering from the Cr ions exhibits significant
changes with temperature and dominates at low momentum transfer regime.
Ab-inito calculations performed including magnetic interactions show that the
effect of magnetic interaction is very signicant on the low- as well as
high-energy phonon modes. We have also shown that the inelastic neutron
spectrum of YCrO3 mimics the magnon spectrum from a G-type antiferromagnetic
system, which is consistent with previously reported magnetic structure in the
compound. The ab-initio lattice dynamics calculations in both the compounds
exhibit anisotropic thermal expansion behaviour in the orthorhombic structure
and predict negative thermal expansion along the crystallographic a-axis at low
temperatures. We identify the anharmonic phonon modes responsible for this
anamolous behaviour in LaCrO3 involving low-energy La vibrations and
distortions of the CrO6 octahedra.",2004.01731v1
2020-04-11,Quasi-2D magnetic correlations in Ni$_2$P$_2$S$_6$ probed by ${}^{31}$P NMR,"Detailed ${}^{31}$P nuclear magnetic resonance (NMR) measurements are
presented on well-characterized single crystals of antiferromagnetic van der
Waals Ni$_2$P$_2$S$_6$. An anomalous breakdown is observed in the
proportionality of the NMR shift $K$ with the bulk susceptibility $\chi$. This
so-called $K$$-$$\chi$ anomaly occurs in close proximity to the broad peak in
$\chi(T)$, thereby implying a connection to quasi-2D magnetic correlations
known to be responsible for this maximum. Quantum chemistry calculations show
that crystal field energy level depopulation effects cannot be responsible for
the $K$$-$$\chi$ anomaly. Appreciable in-plane transferred hyperfine coupling
is observed, which is consistent with the proposed Ni$-$S$-$Ni super- and
Ni$-$S$-$S$-$Ni super-super-exchange coupling mechanisms. Magnetization and
spin$-$lattice relaxation rate ($T_1^{-1}$) measurements indicate little to no
magnetic field dependence of the N\'eel temperature. Finally, $T_1^{-1}(T)$
evidences relaxation driven by three-magnon scattering in the antiferromagnetic
state.",2004.05403v1
2020-04-21,Interlayer Coupling Induced Quasiparticles,"We present an exact treatment of layered many-body electronic systems in the
presence of interlayer coupling within the Schwinger functional derivative
approach on the Keldysh contour. Our transparent approach allows us to clarify
the definition of interlayer coupling by showing the independent roles
hybridization and interactions play in generating new electronic and magnetic
excitations. We find interlayer coupling to induce a variety of plasmons,
magnons, and excitons, residing within the a layer, traversing between layers,
or propagating along the interface. Moreover, we predict interfacial
excitations, including an electron-hole pairing pathway, facilitated by
previously ignored layer nonconserving interactions. Finally, we briefly
explore the consequence of interlayer coupling on a bilayer square lattice
system.",2004.10311v2
2020-04-22,Magnetic phase diagram of the linear quantum ferro-antiferromagnet Cs$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$,"A single-crystal sample of the frustrated quasi one-dimensional quantum
magnet Cs$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$ is investigated by magnetic and
thermodynamic measurements.A combination of specific heat and magnetic torque
measurements maps out the entire $H$-$T$ phase diagram for three
orientations.Remarkably, a new phase emerges below the saturation field,
irrespective of the crystal orientation. It is suggested that the presaturation
phase represents spin-nematic order or other multi-magnon condensate. The phase
diagrams within the long-range ordered dome are qualitatively different for
each geometry. In particular, multiple transitions are identified in the field
along the chain direction.",2004.10636v3
2020-04-27,Tunneling spectroscopy as a probe of fractionalization in 2D magnetic heterostructures,"In this paper we develop the theory for 2D-to-2D tunneling spectroscopy aided
by magnetic or quantum-order excitations, and apply it to the description of
van-der-Waals heterostructures of graphene/ultrathin $\alpha-{\rm RuCl}_3$. We
study the behavior of both the differential conductance and the inelastic
electron tunneling spectrum (IETS) of these heterostructures. The IETS in
particular exhibits features, such as the gap of continuum spinon excitations
and Majorana bound states, whose energies scale {\it cubicly} with the applied
magnetic field. Such scaling, which exists for a relatively wide range of
fields, is at odds with the linear one exhibited by conventional magnons and
can be used to prove the existence of Kitaev quantum spin liquids.",2004.13036v1
2020-04-28,Enhancement of acoustic spin pumping by acoustic distributed Bragg reflector cavity,"Surface acoustic waves (SAWs) in the GHz frequency range can inject spin
currents dynamically into adjacent nonmagnetic layers via spin pumping effect
associated with ferromagnetic resonance. Here, we demonstrate an enhancement of
acoustic ferromagnetic resonance and spin current generation by a pair of SAW
reflector gratings, which form an acoustic analogue of the distributed Bragg
reflector cavity. In the experiment, we confirmed 2.04 $\pm$ 0.02 times larger
SAW power absorption in a device with cavity than in case of no acoustic
cavity. We confirmed up to 2.96 $\pm$ 0.02 times larger spin current generation
by measuring electric voltages generated by the inverse Edelstein effect (IEE)
at the interface between Cu and Bi$_2$O$_3$. The results suggest that acoustic
cavities would be useful to enhance the conversion efficiency in SAW driven
coupled magnon-phonon dynamics.",2004.13885v3
2020-04-30,Flat-band physics in the spin-1/2 sawtooth chain,"We consider the strongly anisotropic spin-1/2 $XXZ$ model on the
sawtooth-chain lattice with ferromagnetic longitudinal interaction
$J^{zz}=\Delta J$ and aniferromagnetic transversal interaction
$J^{xx}=J^{yy}=J>0$. At $\Delta=-1/2$ the lowest one-magnon excitation band is
dispersionless (flat) leading to a massively degenerate set of ground states.
Interestingly, this model admits a three-coloring representation of the
ground-state manifold [H.~J.~Changlani et at., Phys. Rev. Lett. {\bf 120},
117202 (2018)]. We characterize this ground-state manifold and elaborate the
low-temperature thermodynamics of the system. We illustrate the manifestation
of the flat-band physics of the anisotropic model by comparison with two
isotropic flat-band Heisenberg sawtooth chains. Our analytical consideration is
complemented by exact diagonalization and finite-temperature Lanczos method
calculations.",2004.14730v1
2020-05-01,A Narrowband Spintronic Terahertz Emitter based on Magnetoelastic Heterostructures,"Narrowband terahertz (THz) radiation is crucial for high-resolution spectral
identification, but a narrowband THz source driven by femtosecond (fs) laser
has remained scarce. Here, it is computationally predicted that a
metal/dielectric/magnetoelastic heterostructure enables converting a fs laser
pulse into a multi-cycle THz pulse with a narrow linewidth down to ~1.5 GHz,
which is in contrast with the single-cycle, broadband THz pulse from the
existing fs-laser-excited emitters. It is shown that such narrowband THz pulse
originates from the excitation and long-distance transport of THz spin waves in
the magnetoelastic film, which can be enabled by a short strain pulse obtained
from fs laser irradiation of the metal film when the thicknesses of the metal
and magnetoelastic films both fall into a specific range. These results
therefore reveal an approach to achieving optical generation of narrowband THz
pulse based on heterostructure design, which also has implications in the
design of THz magnonic devices.",2005.00629v3
2020-05-06,Observation of evanescent spin waves in the magnetic dipole regime,"We observed spin-wave transmission through an air gap that works as a
prohibited region. The spin waves were excited by circularly polarized pump
pulses via the inverse Faraday effect, and their spatial propagation was
detected through the Faraday effect of probe pulses using a pump-probe imaging
technique. The experimentally observed spin-wave transmission was reproduced
using numerical calculations with a Green's function method and micromagnetic
simulation. We found that the amplitude of the spin waves decays exponentially
in the air gap, which indicates the existence of evanescent spin waves in the
magnetic dipole regime. This finding will pave the way for controllable
amplitudes and phases of spin waves propagating through an artificial magnonic
crystal.",2005.02559v1
2020-05-07,Low-energy physics of isotropic spin-1 chains in the critical and Haldane phases,"Using a matrix product state algorithm with infinite boundary conditions, we
compute high-resolution dynamic spin and quadrupolar structure factors in the
thermodynamic limit to explore the low-energy excitations of isotropic
bilinear-biquadratic spin-1 chains. Haldane mapped the spin-1 Heisenberg
antiferromagnet to a continuum field theory, the non-linear sigma model
(NL$\sigma$M). We find that the NL$\sigma$M fails to capture the influence of
the biquadratic term and provides only an unsatisfactory description of the
Haldane phase physics. But several features in the Haldane phase can be
explained by non-interacting multi-magnon states. The physics at the
Uimin-Lai-Sutherland point is characterized by multi-soliton continua. Moving
into the extended critical phase, we find that these excitation continua
contract, which we explain using a field-theoretic description. New excitations
emerge at higher energies and, in the vicinity of the purely biquadratic point,
they show simple cosine dispersions. Using block fidelities, we identify them
as elementary one-particle excitations and relate them to the integrable
Temperley-Lieb chain.",2005.03643v2
2020-05-10,Anisotropic Magnetocaloric Properties of The Ludwigite Single Crystal Cu2MnBO5,"We present the results of a thorough study of the specific heat and
magnetocaloric properties of a ludwigite crystal Cu2MnBO5 over a temperature
range of 60 - 350 K and in magnetic fields up to 18 kOe. It is found that at
temperatures below the Curie temperature (92 K), capacity possesses a linear
temperature-dependent behavior, which is associated with the predominance of
two-dimensional antiferromagnetic interactions of magnons. The temperature
independence of capacity is observed in the temperature range of 95 - 160 K,
which can be attributed to the excitation of the Wigner glass phase. The
magnetocaloric effect (i.e. the adiabatic temperature change) was assessed
through a direct measurement or an indirect method using the capacity data.
Owing to its strong magnetocrystalline anisotropy, an anisotropic MCE or the
rotating MCE is observed in Cu2MnBO5. A deep minimum in the rotating MCE near
the TC is observed and may be associated with the anisotropy of the
paramagnetic susceptibility.",2005.04572v1
2020-05-12,Curvilinear one-dimensional antiferromagnets,"Antiferromagnets host exotic quasiparticles, support high frequency
excitations and are key enablers of the prospective spintronic and
spin-orbitronic technologies. Here, we propose a concept of a curvilinear
antiferromagnetism where material responses can be tailored by a geometrical
curvature without the need to adjust material parameters. We show that an
intrinsically achiral one-dimensional (1D) curvilinear antiferromagnet behaves
as a chiral helimagnet with geometrically tunable Dzyaloshinskii--Moriya
interaction (DMI) and orientation of the N\'{e}el vector. The curvature-induced
DMI results in the hybridization of spin wave modes and enables a
geometrically-driven local minimum of the low frequency branch. This positions
curvilinear 1D antiferromagnets as a novel platform for the realization of
geometrically tunable chiral antiferromagnets for antiferromagnetic
spin-orbitronics and fundamental discoveries in the formation of coherent
magnon condensates in the momentum space.",2005.05835v2
2020-05-14,"Modelling spin waves in noncollinear antiferromagnets: spin-flop states, spin spirals, skyrmions and antiskyrmions","Spin waves in antiferromagnetic materials have great potential for
next-generation magnonic technologies. However, their properties and their
dependence on the type of ground-state antiferromagnetic structure are still
open questions. Here, we investigate theoretically spin waves in one- and
two-dimensional model systems with a focus on noncollinear antiferromagnetic
textures such as spin spirals and skyrmions of opposite topological charges. We
address in particular the nonreciprocal spin excitations recently measured in
bulk antiferromagnet $\alpha$--$\text{Cu}_2\text{V}_2\text{O}_7$ utilizing
inelastic neutron scattering experiments [Phys.\ Rev.\ Lett.\ \textbf{119},
047201 (2017)], where we help to characterize the nature of the detected
spin-wave modes. Furthermore, we discuss how the Dzyaloshinskii-Moriya
interaction can lift the degeneracy of the spin-wave modes in antiferromagnets,
resembling the electronic Rashba splitting. We consider the spin-wave
excitations in antiferromagnetic spin-spiral and skyrmion systems and discuss
the features of their inelastic scattering spectra. We demonstrate that
antiskyrmions can be obtained with an isotropic Dzyaloshinskii-Moriya
interaction in certain antiferromagnets.",2005.07250v1
2020-05-18,Local spin Seebeck imaging with scanning thermal probe,"In this work we present the results of an experiment to locally resolve the
spin Seebeck effect in a high-quality Pt/YIG sample. We achieve this by
employing a locally heated scanning thermal probe to generate a highly local
non-equilibrium spin current. To support our experimental results, we also
present a model based on the non-equilibrium thermodynamic approach which is in
a good agreement with experimental findings. To further corroborate our
results, we index the locally resolved spin Seebeck effect with that of the
local magnetisation texture by MFM and correlate corresponding regions. We
hypothesise that this technique allows imaging of magnetisation textures within
the magnon diffusion length and hence characterisation of spin caloritronic
materials at the nanoscale.",2005.08539v1
2020-05-21,Effect of Dzyaloshinskii-Moriya interaction on magnetic vortex switching driven by radial spin waves,"We theoretically investigate the radial-spin-wave induced magnetic vortex
switching in the presence of Dzyaloshinskii-Moriya interaction (DMI). From
micromagnetic simulations, we observe a circular-to-radial vortex phase
transition by increasing the DMI strength. The radial spin-wave excitation
spectrum for each magnetization configuration is analyzed, showing that the
frequency of spin-wave mode with a given radial node number monotonically
increases (decreases) with the DMI parameter of the radial (circular) vortex.
Interestingly, we find that the DMI can significantly facilitate the polarity
switching of the circular vortex driven by radial spin waves. Our work provides
a new insight into the DMI effect on the vortex dynamics and is helpful for
designing fast all-magnonic memory devices.",2005.10440v1
2020-05-21,Antiferromagnet-semiconductor van der Waals heterostructures: interlayer interplay of exciton with magnetic ordering,"Van der Waals (vdW) heterostructures have attracted great interest because of
their rich material combinations.The discovery of two-dimensional magnets has
provided a new platform for magnetic vdW heterointerfaces; however, research on
magnetic vdW heterointerfaces has been limited to those with ferromagnetic
surfaces. Here we report a magnetic vdW heterointerface using layered
intralayer-antiferromagnetic MPSe3 (M=Mn, Fe) and monolayer transition metal
dichalcogenides (TMDs). We found an anomalous upshift of the excitonic peak in
monolayer TMDs below the antiferromagnetic transition temperature in the MPSe3,
capturing a signature of the interlayer exciton-magnon coupling. This is a
concept extended from single materials to heterointerfaces. Moreover, this
coupling strongly depends on the in-plane magnetic structure and stacking
direction, showing its sensitivity to their magnetic interfaces. Our finding
offers an opportunity to investigate interactions between elementary
excitations in different materials across interfaces and to search for new
functions of magnetic vdW heterointerfaces.",2005.10727v1
2020-05-25,Evidence for biquadratic exchange in the quasi-two-dimensional antiferromagnet FePS$_3$,"FePS$_3$ is a van der Waals compound with a honeycomb lattice that is a good
example of a two-dimensional antiferromagnet with Ising-like anisotropy.
Neutron spectroscopy data from FePS3 were previously analysed using a
straight-forward Heisenberg Hamiltonian with a single-ion anisotropy. The
analysis captured most of the elements of the data, however some significant
discrepancies remained. The discrepancies were most obvious at the Brillouin
zone boundaries. The data are subsequently reanalysed allowing for unequal
exchange between nominally equivalent nearest-neighbours, which resolves the
discrepancies. The source of the unequal exchange is attributed to a
biquadratic exchange term in the Hamiltonian which most probably arises from a
strong magnetolattice coupling. The new parameters show that there are features
consistent with Dirac magnon nodal lines along certain Brillouin zone
boundaries.",2005.12031v1
2020-05-25,Dynamical signatures of quasiparticle interactions in quantum spin chains,"We study the transverse dynamical susceptibility of an antiferromagnetic
spin$-1/2$ chain in presence of a longitudinal Zeeman field. In the low
magnetization regime in the gapless phase, we show that the marginally
irrelevant backscattering interaction between the spinons creates a non-zero
gap between two branches of excitations at small momentum. We further
demonstrate how this gap varies upon introducing a second neighbor
antiferromagnetic interaction, vanishing in the limit of a non-interacting
""spinon gas"". In the high magnetization regime, as the Zeeman field approaches
the saturation value, we uncover the appearance of two-magnon bound states in
the transverse susceptibility. This bound state feature generalizes the one
arising from string states in the Bethe ansatz solution of the integrable case.
Our results are based on numerically accurate, unbiased matrix-product-state
techniques as well as analytic approximations.",2005.12399v1
2020-05-29,Microscopic Theory of the Spin Hall Magnetoresistance,"We consider a microscopic theory for the spin Hall magnetoresistance (SMR).
We generally formulate a spin conductance at an interface between a normal
metal and a magnetic insulator in terms of spin susceptibilities. We reveal
that SMR is composed of static and dynamic parts. The static part, which is
almost independent of the temperature, originates from spin flip caused by an
interfacial exchange coupling. However, the dynamic part, which is induced by
the creation or annihilation of magnons, has an opposite sign from the static
part. By the spin-wave approximation, we predict that the latter results in a
nontrivial sign change of the SMR signal at a finite temperature. In addition,
we derive the Onsager relation between spin conductance and thermal
spin-current noise.",2005.14494v3
2020-07-09,Fe3Se4: A Possible Ferrimagnetic Half-Metal?,"Half-metallic ferromagnets show 100% spin-polarization at the Fermi level and
are ideal candidates for spintronic applications. Despite the extensive
research in the field, very few materials have been discovered so far. Here we
present results of electronic band structure calculations based on density
functional theory and extensive physical-property measurements for Fe3Se4
revealing signatures of half-metallicity. The spin-polarized electronic band
structure calculations predict half-metallic ferrimagnetism for Fe3Se4. The
electrical resistivity follows exponentially suppressed electron-magnon
scattering mechanism in the low-temperature regime and show a magnetoresistance
effect that changes the sign from negative to positive with decreasing
temperature around 100 K. Other intriguing observations include the anomalous
behavior of Hall resistance below 100 K and an anomalous Hall coefficient that
roughly follows the \r{ho}2 behavior.",2007.04736v1
2020-07-10,Fermi Surface Geometry,"Motivated by the famous and pioneering mathematical works by Perelman,
Hamilton, and Thurston, we introduce the concept of using modern geometrical
mathematical classifications of multi-dimensional manifolds to characterize
electronic structures and predict non-trivial electron transport phenomena.
Here we develop the Fermi Surface Geometry Effect (FSGE), using the concepts of
tangent bundles and Gaussian curvature as an invariant. We develop an index,
$\mathbb{H}_F$, for describing the the ""hyperbolicity"" of the Fermi Surface
(FS) and show a universal correlation (R$^2$ = 0.97) with the experimentally
measured intrinsic anomalous Hall effect of 16 different compounds spanning a
wide variety of crystal, chemical, and electronic structure families, including
where current methods have struggled. This work lays the foundation for
developing a complete theory of geometrical understanding of electronic (and by
extension magnonic and phononic) structure manifolds, beginning with Fermi
surfaces. In analogy to the broad impact of topological physics, the concepts
begun here will have far reaching consequences and lead to a paradigm shift in
the understanding of electron transport, moving it to include geometrical
properties of the E vs k manifold as well as topological properties.",2007.05525v2
2020-07-23,Nonreciprocal surface magnetoelastic dynamics,"Motivated by recent experiments, we investigate the nonreciprocal
magnetoelastic interaction between the surface acoustic phonons of dielectric
non-magnetic substrates and magnons of proximity nanomagnets. The magnetization
dynamics exerts rotating forces at the edges of the nanomagnet that causes the
nonreciprocal interaction with surface phonons due to its rotation-momentum
locking. This coupling induces the nonreciprocity of the surface phonon
transmission and a nearly complete phonon diode effect by several (tens of)
magnetic nanowires of high (ordinary) magnetic quality. Phase-sensitive
microwave transmission is also nonreciprocal that can pick up clear signals of
the coherent phonons excited by magnetization dynamics. Nonreciprocal pumping
of phonons by precessing magnetization is predicted using Landauer-B\""uttiker
formalism.",2007.12240v3
2020-07-24,Voltage control of magnon spin currents in antiferromagnetic Cr2O3,"Voltage-controlled spintronic devices utilizing the spin degree of freedom
are desirable for future applications, and may allow energy-efficient
information processing. Pure spin current can be created by thermal excitations
in magnetic systems via the spin Seebeck effect (SSE). However, controlling
such spin currents, only by electrical means, has been a fundamental challenge.
Here, we investigate voltage control of the SSE in the antiferromagnetic
insulator Cr2O3. We demonstrate that the SSE response generated in this
material can be effectively controlled by applying a bias voltage, owing to the
sensitivity of the SSE to the orientation of the magnetic sublattices as well
as the existence of magnetoelectric couplings in Cr2O3. Our experimental
results are explained using a model based on the magnetoelectric effect in
Cr2O3.",2007.12774v1
2020-07-28,Electron correlation effects on exchange interactions and spin excitations in 2D van der Waals materials,"Despite serious effort, the nature of the magnetic interactions and the role
of electron-correlation effects in magnetic two-dimensional (2D) van der Waals
materials remain elusive. Using CrI$_3$ as a model system, we show that the
calculated electronic structure including nonlocal electron correlations yields
spin excitations consistent with inelastic neutron scattering measurements.
Remarkably, this approach identifies an unreported correlation-enhanced
interlayer super-superexchange, which rotates the magnon Dirac lines off, and
introduces a gap along, the high-symmetry $\Gamma$-$K$-$M$ path. This discovery
provides a different perspective on the gap opening mechanism observed in
CrI$_3$, which was previously associated with spin-orbit coupling through the
Dzyaloshinskii-Moriya interaction or Kitaev interaction. Our observation
elucidates the critical role of electron correlations on the spin ordering and
spin dynamics in magnetic van der Waals materials and demonstrates the
necessity of explicit treatment of electron correlations in the broad family of
2D magnetic materials.",2007.14518v2
2020-08-02,Magnetic field control of antiferromagnetic domain walls in a thermal gradient,"An antiferromagnetic domain wall in a thermal gradient is found to experience
a force towards colder regions upon the application of a uniform magnetic field
along the easy axis. This force increases with the strength of the applied
field and, for sufficiently high values, it overcomes the entropic force the
that pushes wall towards the hotter regions. The force is proportional to the
thermal gradient and it shows a linear dependence with the net magnetic moment
of the domain wall induced by the field. The origin of this force lies on the
increase of the domain wall reflectivity due the field-induced sizable break of
antiferromagnetic order inside it, which turns it into an efficient barrier for
magnons, which transfer linear momentum to the domain wall when they are
reflected on it",2008.00533v1
2020-08-05,Tunneling spectroscopy of quantum spin liquids,"We examine the spectroscopic signatures of tunneling through a Kitaev quantum
spin liquid (QSL) barrier in a number of experimentally relevant geometries. We
combine contributions from elastic and inelastic tunneling processes and find
that spin-flip scattering at the itinerant spinon modes gives rise to a gaped
contribution to the tunneling conductance spectrum. We address the spectral
modifications that arise in a magnetic field necessary to drive the candidate
material $\alpha$-RuCl$_3$ into a QSL phase, and we propose a lateral 1D tunnel
junction as a viable setup in this regime. The characteristic spin gap is an
unambiguous signature of the fractionalized QSL excitations, distinguishing it
from magnons or phonons. The results of our analysis are generically applicable
to a wide variety of topological QSL systems.",2008.02278v1
2020-08-10,Finite-temperature dynamics and the role of disorder in nearly critical Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$,"Inelastic neutron scattering is used to investigate the temperature
dependence of spin correlations in the 3-dimensional XY antiferromagnet
Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$, $ x = 0.14(1)$, tuned close to
the chemical-composition-induced soft-mode transition. The local dynamic
structure factor shows $\hbar\omega/T$ scaling behavior characteristic of a
quantum critical point. The deviation of the measured critical exponent from
spin wave theoretical expectations are attributed to disorder. Another effect
of disorder is local excitations above the magnon band. Their energy, structure
factor and temperature dependence are well explained by simple strong-bond
dimers associated with Br-impurity sites.",2008.04037v2
2020-08-12,Determination of Niobium Cavity Magnetic Field Screening via a Dispersively Hybridized Magnonic Sensor,"A method for determining the internal DC magnetic field inside a
superconducting cavity is presented. The method relies on the relationship
between magnetic field and frequency of the Kittel mode of a ferrimagnetic
sphere, hybridised in the dispersive regime of the superconducting cavity.
Results were used to experimentally determine the level of screening a
superconducting Nb cavity provides as it changes from perfect diamagnetism to
no screening. Two cavity geometries were tested, a cylinder and single post
re-entrant cavity. Both demonstrated a consistent value of field that enters
the cavity, expected to be the superheating critical field. Hysteresis in the
screened field during ramp up and ramp down of the external magnetic field due
to trapped vortices was also observed. Some abnormal behaviour was observed in
the cylindrical cavity in the form of plateaus in the internal field above the
first critical field, and we discuss the potential origin of this behaviour.
The measurement approach would be a useful diagnostic for axion dark matter
searches, which plan on using superconducting materials but need to know
precisely the internal magnetic field.",2008.05073v2
2020-08-21,Integration and characterization of micron-sized YIG structures with very low Gilbert damping on arbitrary substrates,"We present a novel process that allows the transfer of monocrystalline
yttrium-iron-garnet microstructures onto virtually any kind of substrate. The
process is based on a recently developed method that allows the fabrication of
freestanding monocrystalline YIG bridges on gadolinium-gallium-garnet. Here the
bridges' spans are detached from the substrate by a dry etching process and
immersed in a watery solution. Using drop casting the immersed YIG platelets
can be transferred onto the substrate of choice, where the structures finally
can be reattached and thus be integrated into complex devices or experimental
geometries. Using time resolved scanning Kerr microscopy and inductively
measured ferromagnetic resonance we can demonstrate that the structures retain
their excellent magnetic quality. At room temperature we find a ferromagnetic
resonance linewidth of $\mu_0\Delta H_{HWHM}\approx 195\,\mu T$ and we were
even able to inductively measure magnon spectra on a single micron-sized
yttrium-iron-garnet platelet at a temperature of 5 K. The process is flexible
in terms of substrate material and shape of the structure. In the future this
approach will allow for new types of spin dynamics experiments up to now
unthinkable.",2008.09390v1
2020-10-01,Modeling coupled spin and lattice dynamics,"A unified model of molecular and atomistic spin dynamics is presented
enabling simulations both in microcanonical and canonical ensembles without the
necessity of additional phenomenological spin damping. Transfer of energy and
angular momentum between the lattice and the spin systems is achieved by a
coupling term based upon the spin-orbit interaction. The characteristic spectra
of the spin and phonon systems are analyzed for different coupling strength and
temperatures. The spin spectral density shows magnon modes together with the
uncorrelated noise induced by the coupling to the lattice. The effective
damping parameter is investigated showing an increase with both coupling
strength and temperature. The model paves the way to understanding magnetic
relaxation processes beyond the phenomenological approach of the Gilbert
damping and the dynamics of the energy transfer between lattice and spins.",2010.00642v1
2020-10-05,Open giant magnons on LLM geometries,"We compute sigma model solutions for rigidly rotating open strings suspended
between giant gravitons in general LLM geometries. These solutions are confined
to the LLM plane. These all have a dispersion relation for $\Delta-J$ that is
consistent with saturation of a BPS bound of the centrally extended spin chain.
For the special case of circularly symmetric LLM geometries, we can further
evaluate the amount of angular momentum $J$ carried by these strings. This
quantity diverges for string configurations that try to move between different
""coloring regions"" in the LLM plane. All of these quantities have a
perturbative expansion in the t'Hooft coupling. For the strings suspended
between AdS giants, we can compute in field theory the leading result of $J$
carried by the string via an analytic continuation of the $SU(2)$ result, with
the help of the Bethe Ansatz for the $SL(2)$ sector. We thus provide additional
information on how the radial direction of $AdS$ arises from (open) spin chain
calculations.",2010.02236v1
2020-10-14,Semi-classical quantisation of magnetic solitons in the anisotropic Heisenberg quantum chain,"Using the algebro-geometric approach, we study the structure of
semi-classical eigenstates in a weakly-anisotropic quantum Heisenberg spin
chain. We outline how classical nonlinear spin waves governed by the
anisotropic Landau-Lifshitz equation arise as coherent macroscopic low-energy
fluctuations of the ferromagnetic ground state. Special emphasis is devoted to
the simplest types of solutions, describing precessional motion and elliptic
magnetisation waves. The internal magnon structure of classical spin waves is
resolved by performing the semi-classical quantisation using the
Riemann-Hilbert problem approach. We present an expression for the overlap of
two semi-classical eigenstates and discuss how correlation functions at the
semi-classical level arise from classical phase-space averaging.",2010.07232v6
2020-10-15,Topological Invariant for Bosonic Bogoliubov-de Gennes Systems with Disorder,"Using the method of noncommutative geometry, we define a topological
invariant in disordered bosonic Bogoliubov-de Gennes systems, which possess a
unique mathematical property---non-Hermiticity. To demonstrate the validity of
the definition, we investigate a disordered artificial spin ice model in two
dimensions numerically. In the clean limit, we clarify that the topological
index perfectly coincides with the Chern number. We also show that the
topological index is robust against disorder. The formula provides the
topological index $n_{\rm Ch}=1$ in the magnon Hall regime and $n_{\rm Ch}=0$
in a trivial localized one. We also show by example that our method can be
extended to other symmetry classes. Our results pave the way for further
studies on topological bosonic systems with disorder.",2010.07762v1
2020-10-19,On Choosing a Physically Meaningful Topological Classification for Non-Hermitian Systems and the Issue of Diagonalizability,"The topological classification of hermitian operators is solely determined by
the presence or absence of certain discrete symmetries. For non-hermitian
operators we in addition need to specify the type of spectral gap. They come in
the flavor of a point gap or a line gap. Since the presence of a line gap
implies the existence of a point gap, there is usually more than one
mathematical classification applicable to a physical system. That raises the
question: which of these gap-type classifications is physically meaningful?
  To decide this question, I propose a simple criterion, namely the choice of
physically relevant states. This generalizes the notion of Fermi projection
that plays a crucial role in the topological classification of fermionic
condensed matter systems, and enters as an auxiliary quantity in the bulk
classification of photonic and magnonic crystals. After that the classification
is entirely algorithmic, the system's topology is encoded in (pairs of)
projections with symmetries and constraints. A crucial point in my
investigation is the relevance of diagonalizability. Even for existing
topological classifications of non-hermitian systems diagonalizability needs to
be assumed to ensure that continuous deformations of the hamiltonian lead to
continuous deformations of the spectra, projections and unitaries.",2010.09261v1
2020-10-22,Spin-current version of solar cells in non-centrosymmetric magnetic insulators,"Photovoltaic effect, e.g., solar cells, converts light into DC electric
current. This phenomenon takes place in various setups such as in
noncentrosymmetric crystals and semiconductor pn junctions. Recently, we
proposed a theory for producing DC spin current in magnets using
electromagnetic waves, i.e., the spin-current counterpart of the solar cells.
Our calculation shows that the nonlinear conductivity for the spin current is
nonzero in a variety of noncentrosymmetric magnets, implying that the
phenomenon is ubiquitous in inversion-asymmetric materials with magnetic
excitations. Intuitively, this phenomenon is a bulk photovoltaic effect of
magnetic excitations, where electrons and holes, visible light, and
inversion-asymmetric semiconductors are replaced with magnons or spinons, THz
or GHz waves, and asymmetric magnetic insulators, respectively. We also show
that the photon-driven spin current is shift current type, and as a result, the
current is stable against impurity scattering. This bulk photovoltaic spin
current is in sharp contrast to that of well-known spin pumping that takes
place at the interface between a magnet and a metal.",2010.11795v1
2020-10-27,Vertex Dependent Dynamic Response of a Connected Kagome Artificial Spin Ice,"We present the dynamic response of a connected Kagome artificial spin ice
with emphasis on the effect of the vertex magnetization configuration on the
mode characteristics. We use broadband ferromagnetic resonance (FMR)
spectroscopy and micromagnetic simulations to identify and characterize
resonant modes. We find the mode frequencies of elongated, single-domain film
segments not only depend on the orientation of their easy-axis with respect to
the applied magnetic field, but also depend on the vertex magnetization
configuration, which suggests control over the FMR mode can be accomplished by
altering the vertex magnetization. Moreover, we study differences between the
vertex center mode (VCM) and the localized domain wall (LDW) mode. We show that
the LDW mode acts as a signature of the domain wall (DW) nucleation process and
the DW dynamics active during segment reversal events. The results show the VCM
and LDW modes can be controlled using a field protocol, which has important
implications for applications in magnonic and spintronic devices.",2010.13965v1
2020-10-28,Recent progress in antiferromagnetic dynamics,"Spintronics, since its inception, has mainly focused on ferromagnetic
materials for manipulating the spin degree of freedom in addition to the charge
degree of freedom, whereas much less attention has been paid to
antiferromagnetic materials. Thanks to the advances of micro-nano-fabrication
techniques and the electrical control of the N\'eel order parameter,
antiferromagnetic spintronics is booming as a result of abundant room
temperature materials, robustness against external fields and dipolar coupling,
and rapid dynamics in the terahertz regime. For the purpose of applications of
antiferromagnets, it is essential to have a comprehensive understanding of the
antiferromagnetic dynamics at the microscopic level. Here, we first review the
general form of equations that govern both antiferromagnetic and ferrimagnetic
dynamics. This general form unifies the previous theories in the literature. We
also provide a survey for the recent progress related to antiferromagnetic
dynamics, including the motion of antiferromagnetic domain walls and skyrmions,
the spin pumping and quantum antiferromagnetic spintronics. In particular, open
problems in several topics are outlined. Furthermore, we discuss the
development of antiferromagnetic quantum magnonics and its potential
integration with modern information science and technology.",2010.14940v1
2020-11-02,Spin wave propagation in corrugated waveguides,"Curvature-induced effects allow us to tailor the static and dynamic response
of a magnetic system with a high degree of freedom. We study corrugated
magnonic waveguides deposited on a sinusoidally modulated substrate prepared by
focused electron beam deposition. The surface curvature in films with
thicknesses comparable to the amplitude of modulation locally modifies the
contributions of dipolar and exchange energies and results in an effective
anisotropy term which can be tuned on-demand based on the exact geometry. We
show, by Brillouin light scattering microscopy, that without the presence of an
external magnetic field, spin waves propagate over a distance 10$\times$larger
in the corrugated waveguide than in the planar waveguide. Further, we analyze
the influence of the modulation amplitude on spin-wave propagation and conclude
that for moderate modulation amplitudes, the spin-wave decay length is not
affected. For larger amplitudes, the decay length decreases linearly with
increasing modulation.",2011.01274v1
2020-11-04,Level attraction and exceptional points in a resonant spin-orbit torque system,"Level attraction can appear in driven systems where instead of repulsion two
modes coalesce in a region separated by two exceptional points. This behavior
was proposed for optomechanical and optomagnonic systems, and recently observed
for dissipative cavity magnon-polaritons. We demonstrate that such a regime
exists in a spin-orbit torque system where a magnetic oscillator is resonantly
coupled to an electron reservoir. An instability mechanism necessary for mode
attraction can be provided by applying an electric field. The field excites
interband transitions between spin-orbit split bands leading to an instability
of the magnetic oscillator. Two exceptional points then appear in the
oscillator energy spectrum and the region of instability. We discuss conditions
under which this can occur and estimate the electric field strength necessary
for reaching the attraction region for a spin-orbit torque oscillator with
Rashba coupling. A proposal for experimental detection is made using magnetic
susceptibility measurements.",2011.02520v1
2020-11-10,Spin-waves in the collinear antiferromagnetic phase of Mn$_\bf{5}$Si$_\bf{3}$,"By combining two independent approaches, inelastic neutron scattering
measurements and density functional theory calculations, we study the
spin-waves in the high-temperature collinear antiferromagnetic phase (AFM2) of
Mn$_5$Si$_3$. We obtain its magnetic ground-state properties and electronic
structure. This study allowed us to determine the dominant magnetic exchange
interactions and magnetocrystalline anisotropy in the AFM2 phase of
Mn$_5$Si$_3$. Moreover, the evolution of the spin excitation spectrum is
investigated under the influence of an external magnetic field perpendicular to
the anisotropy easy-axis. The low energy magnon modes show a different magnetic
field dependence which is a direct consequence of their different precessional
nature. Finally, possible effects related to the Dzyaloshinskii-Moriya
interaction are also considered.",2011.05455v3
2020-11-11,Non-equilibrium heating path for utrafast laser-induced nucleation of skyrmion lattices,"We explore the helicity-independent light-induced nucleation of skyrmion
lattices in ferromagnetic cobalt-based trilayers with perpendicular magnetic
anisotropy. Using Atomistic Spin Dynamics simulations, we show that a high
temperature excitation followed by magnon drops and their non-equilibrium
relaxation, accessed by an ultrafast laser excitation with specific duration
and intensity, can lead to the generation of a skyrmion lattice stable at room
temperature. The nucleation window, the topological density and the skyrmion
polarity can be additionally manipulated by external magnetic fields. Our
results provide insight into the non-equilibrium nature of skyrmionic
excitations at non-zero temperatures and pave additional routes for their use
in information technologies.",2011.06093v2
2020-11-18,Field-Tuned Quantum Effects in a Triangular-Lattice Ising Magnet,"We report thermodynamic and neutron scattering measurements of the
triangular-lattice quantum Ising magnet TmMgGaO 4 in longitudinal magnetic
fields. Our experiments reveal a quasi-plateau state induced by quantum
fluctuations. This state exhibits an unconventional non-monotonic field and
temperature dependence of the magnetic order and excitation gap. In the high
field regime where the quantum fluctuations are largely suppressed, we observed
a disordered state with coherent magnon-like excitations despite the
suppression of the spin excitation intensity. Through detailed semi-classical
calculations, we are able to understand these behaviors quantitatively from the
subtle competition between quantum fluctuations and frustrated Ising
interactions.",2011.09376v3
2020-11-23,Asymmetric spin wave dispersion due to a saturation magnetization gradient,"We demonstrate using micromagnetic simulations and a theoretical model that a
gradient in the saturation magnetization ($M_s$) of a perpendicularly
magnetized ferromagnetic film induces a non-reciprocal spin wave propagation
and, consequently an asymmetric dispersion relation. The $M_s$ gradient adds a
linear potential to the spin wave equation of motion consistent with the
presence of a force. We consider a transformation from an inertial reference
frame in which the $M_s$ is constant to an accelerated reference frame where
the resulting inertial force corresponds to the force from the $M_s$ gradient.
As in the Doppler effect, the frequency shift leads to an asymmetric dispersion
relation. Additionally, we show that under certain circumstances,
unidirectional propagation of spin waves can be achieved which is essential for
the design of magnonic circuits. Our results become more relevant in light of
recent experimental works in which a suitable thermal landscape is used to
dynamically modulate the saturation magnetization.",2011.11148v2
2021-01-12,Double Nuclear Spin Relaxation in Hybrid Quantum Hall Systems,"Recent advances in quantum engineering have given us the ability to design
hybrid systems with novel properties normally not present in the regime they
operate in. The coupling of spin ensembles and magnons to microwave resonators
has for instance lead to a much richer understanding of collective effects in
these systems and their potential quantum applications. We can also hybridize
electron and nuclear spin ensembles together in the solid-state regime to
investigate collective effects normally only observed in the atomic, molecular
and optical world. Here we explore in the solid state regime the dynamics of a
double domain nuclear spin ensemble coupled to the Nambu-Goldstone boson in
GaAs semiconductors and show it exhibits both collective and individual
relaxation (thermalization) on very different time scales. Further the
collective relaxation of the nuclear spin ensemble is what one would expect
from superradiant decay. This opens up the possibility for the exploration of
novel collective behaviour in solid state systems where the natural energies
associated with those spins are much less than the thermal energy.",2101.04433v1
2021-01-21,Effects of the dynamical magnetization state on spin transfer,"We utilize simulations of electron scattering by a chain of dynamical quantum
spins, to analyze the interplay between the spin transfer effect and the
magnetization dynamics. We show that the complex interactions between the
spin-polarized electrons and the dynamical states of the local spins can be
decomposed into separate processes involving electron reflection and
transmission, as well as absorption and emission of magnons - the quanta of
magnetization dynamics. Analysis shows that these processes are substantially
constrained by the energy and momentum conversation laws, resulting in a
significant dependence of spin transfer on the electron's energy and the
dynamical state of the local spins. Our results suggest that exquisite control
of spin transfer efficiency and of the resulting dynamical magnetization states
may be achievable by tailoring the spectral characteristics of the conduction
electrons and of the magnetic systems.",2101.08868v1
2021-01-26,Enhancing the spin-photon coupling with a micromagnet,"Hybrid quantum systems involving solid-state spins and superconducting
microwave cavities play a crucial role in quantum science and technology, but
improving the spin-photon coupling at the single quantum level remains
challenging in such systems. Here, we propose a simple technique to strongly
couple a single solid-state spin to the microwave photons in a superconducting
coplanar waveguide (CPW) cavity via a magnetic microsphere. We show that,
strong coupling at the single spin level can be realized by virtual magnonic
excitations of a nearby micromagnet. The spin-photon coupling strength can be
enhanced up to typically four orders of magnitude larger than that without the
use of the micromagnet. This work can find applications in quantum information
processing with strongly coupled solid-state spin-photonic systems.",2101.10600v1
2021-01-29,Hierarchical excitations from correlated spin tetrahedra on the breathing pyrochlore lattice,"The hierarchy of the coupling strengths in a physical system often engenders
an effective model at low energies where the decoupled high-energy modes are
integrated out. Here, using neutron scattering, we show that the spin
excitations in the breathing pyrochlore lattice compound CuInCr$_4$S$_8$ are
hierarchical and can be approximated by an effective model of correlated
tetrahedra at low energies. At higher energies, intra-tetrahedron excitations
together with strong magnon-phonon couplings are observed, which suggests the
possible role of the lattice degree of freedom in stabilizing the spin
tetrahedra. Our work illustrates the spin dynamics in CuInCr$_4$S$_8$ and
demonstrates a general effective-cluster approach to understand the dynamics on
the breathing-type lattices.",2101.12363v1
2021-01-29,Unified First-Principles Study of the Anomalous Hall Effect Based on Exact Muffin-Tin Orbitals,"Based on the exact muffin-tin orbitals (EMTOs), we developed a
first-principles method to calculate the current operators and investigated the
anomalous Hall effect in bcc Fe as an example, with which we successfully
separated the skew scattering contribution from the side jump and intrinsic
contributions by fitting the scaling law with the introduction of sparse
impurities. By investigating the temperature dependence of the anomalous Hall
effect in bulk Fe, we predicted a fluctuated anomalous Hall angle as a function
of temperature when considering only phonons, which, in the future, can be
measured in experiments by suppressing magnon excitation, e.g., by applying a
high external magnetic field.",2101.12474v1
2021-03-04,X-Ray Scattering from Light-Driven Spin Fluctuations in a Doped Mott Insulator,"Manipulating spin fluctuations with ultrafast laser pulses is a promising
route to dynamically control collective phenomena in strongly correlated
materials. However, understanding how photoexcited spin degrees of freedom
evolve at a microscopic level requires a momentum- and energy-resolved
characterization of their nonequilibrium dynamics. Here, we study the
photoinduced dynamics of finite-momentum spin excitations in two-dimensional
Mott insulators on a square lattice. By calculating the time-resolved resonant
inelastic x-ray scattering cross-section, we show that an ultrafast pump above
the Mott gap induces a prompt softening of the spin excitation energy,
compatible with a transient renormalization of the exchange interaction. While
spin fluctuations in a hole-doped system (paramagnons) are well described by
Floquet theory, magnons at half filling are found to deviate from this picture.
Furthermore, we show that the paramagnon softening is accompanied by an
ultrafast suppression of $d$-wave pairing correlations, indicating a link
between the transient spin excitation dynamics and superconducting pairing far
from equilibrium.",2103.03369v2
2021-03-07,Phonon Hall Viscosity in Magnetic Insulators,"The Phonon Hall Viscosity is the leading term evincing time-reversal symmetry
breaking in the low energy description of lattice phonons. It may generate
phonon Berry curvature, and can be observed experimentally through the acoustic
Faraday effect and thermal Hall transport. We present a systematic procedure to
obtain the phonon Hall viscosity induced by phonon-magnon interactions in
magnetic insulators under an external magnetic field. We obtain a general
symmetry criterion that leads to non-zero Faraday rotation and Hall
conductivity, and clarify the interplay between lattice symmetry,
spin-orbit-coupling, external magnetic field and magnetic ordering. The
symmetry analysis is verified through a microscopic calculation. By
constructing the general symmetry-allowed effective action that describes the
spin dynamics and spin-lattice coupling, and then integrating out the spin
fluctuations, the leading order time-reversal breaking term in the phonon
effective action, i.e. the phonon Hall viscosity, can be obtained. The analysis
of the square lattice antiferromagnet for a cuprate Mott insulator,
Sr$_2$CuO$_2$Cl$_2$, is presented explicitly, and the procedure described here
can be readily generalized to other magnetic insulators.",2103.04223v1
2021-03-12,Temperature dependence of the interface spin Seebeck effect,"We performed temperature-dependent optical pump - THz emission measurements
in Y3Fe5O12 (YIG)|Pt from 5 K to room temperature in the presence of an
externally applied magnetic field. We study the temperature dependence of the
spin Seebeck effect and observe a continuous increase as temperature is
decreased, opposite to what is observed in electrical measurements where the
spin Seebeck effect is suppressed as 0K is approached. By quantitatively
analysing the different contributions we isolate the temperature dependence of
the spin-mixing conductance and observe features that are correlated to the
bands of magnon spectrum in YIG.",2103.07307v1
2021-03-14,Transverse spin dynamics in the anisotropic Heisenberg model realized with ultracold atoms,"In Heisenberg models with exchange anisotropy, transverse spin components are
not conserved and can decay not only by transport, but also by dephasing. Here
we utilize ultracold atoms to simulate the dynamics of 1D Heisenberg spin
chains, and observe fast, local spin decay controlled by the anisotropy.
Additionally, we directly observe an effective magnetic field created by
superexchange which causes an inhomogeneous decay mechanism due to variations
of lattice depth between chains, as well as dephasing within each chain due to
the twofold reduction of the effective magnetic field at the edges of the
chains and due to fluctuations of the effective magnetic field in the presence
of mobile holes. The latter is a new coupling mechanism between holes and
magnons. All these dephasing mechanisms, corroborated by extensive numerical
simulations, have not been observed before with ultracold atoms and illustrate
basic properties of the underlying Hubbard model.",2103.07866v1
2021-03-23,Low Temperature Suppression of the Spin Nernst Angle in Pt,"We demonstrate the low temperature suppression of the platinum (Pt) spin
Nernst angle in bilayers consisting of the antiferromagnetic insulator hematite
($\alpha$-Fe$_2$O$_3$) and Pt upon measuring the transverse spin Nernst
magnetothermopower (TSNM). We show that the observed signal stems from the
interplay between the interfacial spin accumulation in Pt originating from the
spin Nernst effect and the orientation of the N\'eel vector of
$\alpha$-Fe$_2$O$_3$, rather than its net magnetization. Since the latter is
negligible in an antiferromagnet, our device is superior to ferromagnetic
structures, allowing to unambiguously distinguish the TSNM from thermally
excited magnon transport (TMT), which usually dominates in ferri/ferromagnets
due to their non-zero magnetization. Evaluating the temperature dependence of
the effect, we observe a vanishing TSNM below ~100 K. We compare these results
with theoretical calculations of the temperature dependent spin Nernst
conductivity and find excellent agreement. This provides evidence for a
vanishing spin Nernst angle of Pt at low temperatures and the dominance of
extrinsic contributions to the spin Nernst effect.",2103.12697v1
2021-03-30,Non-planar universal anomalous dimension of twist-two operators with general Lorentz spin at four loops in N=4 SYM theory,"We compute the non-planar contribution to the universal anomalous dimension
of twist-two operators in N=4 supersymmetric Yang-Mills theory at four loops
through Lorentz spin eighteen. Exploiting the results of this and our previous
calculations along with recent analytic results for the cusp anomalous
dimension and some expected analytic properties, we reconstruct a general
expression valid for arbitrary Lorentz spin. We study various properties of
this general result, such as its large-spin limit, its small-x limit, and
others. In particular, we present a prediction for the non-planar contribution
to the anomalous dimension of the single-magnon operator in the beta-deformed
version of the theory.",2103.16420v1
2021-04-16,Electronic and magnetic properties of VOCl/FeOCl antiferromagnetic heterobilayers,"We study the electronic properties of the heterobilayer of vanadium and iron
oxychlorides, VOCl and FeOCl, two layered air stable van der Waals insulating
oxides with different types of antiferromagnetic order in bulk: VOCl monolayers
are ferromagnetic (FM) whereas the FeOCl monolayers are antiferromagnetic (AF).
We use density functional theory (DFT) calculations, with Hubbard correction
that is found to be needed to describe correctly the insulating nature of these
compounds. We compute the magnetic anisotropy and propose a spin model
Hamiltonian. Our calculations show that interlayer coupling in weak and
ferromagnetic so that magnetic order of the monolayers is preserved in the
heterobilayers providing thereby a van der Waals heterostructure that combines
two monolayers with different magnetic order. Interlayer exchange should lead
both to exchange bias and to the emergence of hybrid collective modes that that
combine FM and AF magnons. The energy band of the heterobilayer show a type II
band alignment, and feature spin-splitting of the states of the AF layer due to
the breaking of the inversion symmetry.",2104.08214v1
2021-04-22,Spin orbit torque nano-oscillators by dipole field-localized spin wave modes,"We demonstrate a high-quality spin orbit torque nano-oscillator comprised of
spin wave modes confined by the magnetic field by the strongly inhomogeneous
dipole field of a nearby micromagnet. This approach enables variable spatial
confinement and systematic tuning of magnon spectrum and spectral separations
for studying the impact of multi-mode interactions on auto-oscillations. We
find these dipole field-localized spin wave modes exhibit good characteristic
properties as auto-oscillators--narrow linewidth and large amplitude--while
persisting up to room temperature. We find that the linewidth of the
lowest-lying localized mode is approximately proportional to temperature in
good agreement with theoretical analysis of the impact of thermal fluctuations.
This demonstration of a clean oscillator with tunable properties provides a
powerful tool for understanding the fundamental limitations and linewidth
contributions to improve future spin-Hall oscillators.",2104.10838v3
2021-04-22,Bistable nanomagnet as programmable phase inverter for spin waves,"To realize spin wave logic gates programmable phase inverters are essential.
We image with phase-resolved Brillouin light scattering microscopy propagating
spin waves in a one-dimensional magnonic crystal consisting of dipolarly
coupled magnetic nanostripes. We demonstrate phase shifts upon a single
nanostripe of opposed magnetization. Using micromagnetic simulations we model
our experimental finding in a wide parameter space of bias fields and wave
vectors. We find that low-loss phase inversion is achieved, when the internal
field of the oppositely magnetized nanostripe is tuned such that the latter
supports a resonant standing spin wave mode with odd quantization number at the
given frequency. Our results are key for the realization of phase inverters
with optimized signal transmission.",2104.11015v1
2021-04-22,Regular and in-plane skyrmions and antiskyrmions from boundary instabilities,"We formulate a theory of skyrmion and antiskyrmion generation using magnetic
field and charge current pulses. We show that the topological defect can be
created at an edge of a system with Dzyaloshinskii-Moriya interaction (DMI) as
well as at a boundary between regions with different DMI. We consider both
perpendicular and in-plane (also known as magnetic bimerons) versions of
skyrmions and antiskyrmions. We show that the magnetization twist in the
vicinity of an edge or a boundary is described by a kink solution, the presence
of which can instigate the generation of topological defects. We study the
collective excitations of magnetization analytically and numerically, and
demonstrate that under application of magnetic field and charge current pulses
the magnon modes localized near boundaries can develop instabilities leading to
the formation of skyrmions or antiskyrmions. Due to the skyrmion and
antiskyrmion Hall effects, a properly chosen current direction can push the
topological defects away from the boundary, thus facilitating their generation.",2104.11194v2
2021-04-23,Collective Excitations in 2D Materials,"Research on 2D materials has been one of the fastest-growing fields in
condensed matter physics and materials science in the past 10 years. The low
dimensionality and strong correlations of 2D systems give rise to electronic
and structural properties, in the form of collective excitations, that do not
have counterparts in ordinary 3D materials used in modern technology. These 2D
materials present extraordinary opportunities for new technologies, such as in
flexible electronics. In this Review, we focus on plasmons, excitons, phonons
and magnons in 2D materials. We discuss the theoretical formalism of these
collective excitations and elucidate how they differ from their 3D
counterparts.",2104.11441v1
2021-04-30,Control of stripe domain wall magnetization in perpendicular anisotropy multilayers,"We report on the controlled switching of domain wall (DW) magnetization in
aligned stripe domain structures, stabilized in [Co (0.44 nm)/Pt (0.7 nm)]$_X$
($X = 48$, 100, 150) multilayers with perpendicular magnetic anisotropy. The
switching process, induced by an external magnetic field, is monitored by
measuring the evolution of the in-plane magnetization. % We show that the
remanent in-plane magnetization originates from the polarization of the
Bloch-type DWs. With micromagnetic simulations, we reveal that the reversal of
the DW polarization is the result of the emergence and collapse of horizontal
Bloch lines within the DWs at particular strengths of the external magnetic
field, applied opposite to the DW polarization. Our findings are relevant for
DW-based magnonics and bubble skyrmion applications in magnetic multilayers.",2104.14934v1
2021-05-18,Spin Spiral and Topological Hall Effect in a Metamagnet Fe$_{3}$Ga$_{4}$,"A new mechanism for Topological Hall Effect (THE) was recently proposed for
the spiral magnet YMn$_{6}$Sn$_{6},$ which requires transverse conical spiral
(TCS) magnetism, induced by external magnetic field, combined with thermally
excite helical spiral magnons. In principle, this mechanism should be
applicable to other itinerant spiral magnets as well. In this paper, we show
that another metamagnetic compound, Fe$_{3}$Ga$_{4}$, where THE was observed
experimentally before, in one of its phases satisfies this condition, and the
proposed theory of thermal-fluctuation driven THE is quantitatively consistent
with the experiment. This finding suggests that this mechanism is indeed rather
universal, and the effect may have been observed in other compounds before, but
overlooked.",2105.08773v1
2021-05-23,Multifunctional operation of the double-layer ferromagnetic structure coupled by a rectangular nanoresonator,"The use of spin waves as a signal carrier requires developing the functional
elements allowing for multiplexing and demultiplexing information coded at
different wavelengths. For this purpose, we propose a system of thin
ferromagnetic layers dynamically coupled by a rectangular ferromagnetic
resonator. We show that a single and double, clockwise and counter-clockwise,
circulating modes of the resonator offer a wide possibility of control of
propagating waves. Particularly, at frequency related to the double-clockwise
circulating spin-wave mode of the resonator, the spin wave excited in one layer
is transferred to the second one where it propagates in the backward direction.
Interestingly, the wave excited in the second layer propagates in the forward
direction only in that layer. This demonstrates add-drop filtering, as well as
circulator functionality. Thus, the proposed system can become an important
part of future magnonic technology for signal routing.",2105.10875v1
2021-06-10,4D Optically Reconfigurable Volumetric Metamaterials,"Metamaterials are artificially created media, which allow introducing
additional degrees of freedom into electromagnetic design by controlling
constitutive material parameters. Reconfigurable time-dependent metamaterials
can further enlarge those capabilities by introducing a temporal variable as an
additional controllable parameter. Here we demonstrate a first-of-its-kind
reconfigurable volumetric metamaterial-based scatterer, wherein the
electromagnetic properties are controlled dynamically with light. In
particular, hybridized resonances in arrays of split ring resonators give rise
to a collective mode that presents properties of artificial high-frequency
magnetism for centimeter waves. Resonant behavior of each individual ring is
controlled with a photocurrent, which allows the fast tuning of macroscopic
effective permeability. Thus, the artificial gigahertz magnon resonant
excitation within a subwavelength spherical scatterer is governed by light
intensity. Four-dimensional control over electromagnetic scattering in both
space and time opens new venues for modern applications, including wireless
communications and automotive radars to name just a few.",2106.05773v1
2021-06-10,Topological energy release from collision of relativistic antiferromagnetic solitons,"Magnetic solitons offer functionalities as information carriers in multiple
spintronic and magnonic applications. However, their potential for nanoscale
energy transport has not been revealed. Here we demonstrate that
antiferromagnetic solitons, e.g. domain walls, can uptake, transport and
release energy. The key for this functionality resides in their relativistic
kinematics; their self-energy increases with velocity due to Lorentz
contraction of the soliton and their dynamics can be accelerated up to the
effective speed of light of the magnetic medium. Furthermore, their
classification in robust topological classes allows to selectively release this
energy back into the medium by colliding solitons with opposite topology. Our
work uncovers important energy-related aspects of the physics of
antiferromagnetic solitons and opens up the attractive possibility for
spin-based nanoscale and ultra-fast energy transport devices.",2106.05804v1
2021-06-17,Magnetization process and ordering of the $S=1/2$ pyrochlore magnet in a field,"We study the $S=1/2$ pyrochlore Heisenberg antiferromagnet in a magnetic
field. Using large scale density-matrix renormalization group (DMRG)
calculations for clusters up to $128$ spins, we find indications for a finite
triplet gap, causing a threshold field to nonzero magnetization in the
magnetization curve. We obtain a robust saturation field consistent with a
magnon crystal, although the corresponding $5/6$ magnetization plateau is very
slim and possibly unstable. Most remarkably, there is a pronounced and
apparently robust $\frac 1 2$ magnetization plateau where the groundstate
breaks (real-space) rotational symmetry, exhibiting oppositely polarised spins
on alternating kagome and triangular planes. Reminiscent of the kagome ice
plateau of the pyrochlore Ising magnet known as spin ice, it arises via a much
more subtle `quantum order by disorder' mechanism.",2106.09722v2
2021-06-21,Negative correlation between the linear and the nonlinear conductance in magnetic tunnel junctions,"The current-voltage ($IV$) characteristics beyond the linear response regime
of magnetic tunnel junction (MTJ) is systematically investigated. We find a
clear negative correlation between the two coefficients to characterize the
linear ($I$$\propto$$V$) and the lowest-order nonlinear ($I$$\propto$$V^3$)
currents, which holds regardless of the temperature and the thickness of the
tunnel barrier. This observation cannot simply be explained by the standard
tunneling model such as the Brinkman model, suggesting a mechanism intrinsic to
MTJ. We propose a phenomenological model based on the Julli\'ere model that
attributes the observed negative correlation to the spin-flip tunneling
assisted by a magnon. These results suggest a fundamental law between the
linear and the nonlinear response of MTJ.",2106.11274v1
2021-06-21,A low-loss ferrite circulator as a tunable chiral quantum system,"Ferrite microwave circulators allow one to control the directional flow of
microwave signals and noise, and thus play a crucial role in present-day
superconducting quantum technology. They are typically viewed as a black-box,
and their internal structure is not specified, let alone used as a resource. In
this work, we demonstrate a low-loss waveguide circulator constructed with
single-crystalline yttrium iron garnet (YIG) in a 3D cavity, and analyze it as
a multi-mode hybrid quantum system with coupled photonic and magnonic
excitations. We show the coherent coupling of its chiral internal modes with
integrated superconducting niobium cavities, and how this enables tunable
non-reciprocal interactions between the intra-cavity photons. We also probe
experimentally the effective non-Hermitian dynamics of this system and its
effective non-reciprocal eigenmodes. The device platform provides a test bed
for implementing non-reciprocal interactions in open-system circuit QED.",2106.11283v2
2021-07-05,Identifying intrinsic and extrinsic mechanisms of anisotropic magnetoresistance with terahertz probes,"Identifying the intrinsic and extrinsic origins of magneto-transport in
spin-orbit coupled systems has long been a central theme in condensed matter
physics. However, it has been elusive owing to the lack of an appropriate
experimental tool. In this work, using terahertz time-domain spectroscopy, we
unambiguously disentangle the intrinsic and extrinsic contributions to the
anisotropic magnetoresistance (AMR) of a permalloy film. We find that the
scattering-independent intrinsic contribution to AMR is sizable and is as large
as the scattering-dependent extrinsic contribution to AMR. Moreover, the
portion of intrinsic contribution to total AMR increases with increasing
temperature due to the reduction of extrinsic contribution. Further
investigation reveals that the reduction of extrinsic contribution is caused by
the phonon/magnon-induced negative AMR. Our result will stimulate further
researches on other spin-orbit-interaction-induced phenomena for which
identifying the intrinsic and extrinsic contributions is important.",2107.01770v1
2021-07-23,Spin exchange Hamiltonian and topological degeneracies in elemental gadolinium,"We present a comprehensive study of the magnetic exchange Hamiltonian of
elemental Gadolinium. We use neutron scattering to measure the magnon spectrum
over the entire Brillouin zone, and fit the excitations to a spin wave model to
extract the first 26 nearest neighbor magnetic exchange interactions with
rigorously defined uncertainty. We find these exchange interactions to follow
RKKY behavior, oscillating from ferromagnetic to antiferromagnetic as a
function of distance. Finally, we discuss the topological features and
degeneracies in Gd, and HCP ferromagnets in general. We show theoretically how,
with asymmetric exchange, topological properties could be tuned with a magnetic
field.",2107.11370v3
2021-07-24,Electron-Phonon Scattering governs both Ultrafast and Precessional Magnetization Dynamics in Co-Fe Alloys,"Recent investigations have advanced the understanding of how
structure-property relationships in ferromagnetic metal alloys affect the
magnetization dynamics on nanosecond time-scales. A similar understanding for
magnetization dynamics on femto- to pico-second time-scales does not yet exist.
To address this, we perform time-resolved magneto optic Kerr effect (TRMOKE)
measurements of magnetization dynamics in Co-Fe alloys on femto- to nano-second
regimes. We show that Co-Fe compositions that exhibit low Gilbert damping
parameters also feature prolonged ultrafast demagnetization upon
photoexcitation. We analyze our experimental TR-MOKE data with the
three-temperature-model (3TM) and the Landau-Lifshitz-Gilbert equation. These
analyses reveal a strong compositional dependence of the dynamics across all
time-scales on the strength of electron-phonon interactions. Our findings are
beneficial to the spintronics and magnonics community, and will aid in the
quest for energy-efficient magnetic storage applications.",2107.11699v1
2021-08-05,Will Skyrmions Move to the Hot or Cold Region under Temperature Gradients?,"We obtain a dynamic equation at finite temperatures in the thermal bath of
magnons, electrons, and phonons. We further consider effects of the angular
momentum, momentum, magneto-crystalline anisotropy gradient, entropy equivalent
field, dipole field, and diffusion to extend this equation to thermal
gradients. Our findings are summarized as follows: skyrmions can move towards
the hot or cold region; the tangential velocity of skyrmions can be reversed;
two velocity components of skyrmions can have the same or different null
points; the velocity in the tangential direction can be greater than the heat
flow direction; the skyrmion Hall angle can be positive and negative; forces
acting on skyrmions produced by equivalent magnetic fields will decrease with
temperatures under uniform temperatures. Our results are consistent with
experiments and beneficial for spintronics.",2108.02621v1
2021-08-23,Reconfigurable 3D magnonic crystal: tunable and localized spin-wave excitations in CoFeB meander-shaped film,"In this work, we study experimentally by broadband ferromagnetic resonance
measurements, the dependence of the spin-wave excitation spectra on the
magnetic applied field in CoFeB meander-shaped films. Two different
orientations of the external magnetic field were explored, namely parallel or
perpendicular to the lattice cores. The interpretation of the field dependence
of the frequency and spatial profiles of major spin-wave modes were obtained by
micromagnetic simulations. We show that the vertical segments lead to the
easy-axis type of magnetic anisotropy and support the in-phase and out-of-phase
spin-wave precession amplitude in the vertical segments. The latter could
potentially be used for the design of tunable metasurfaces or in magnetic
memories based on meandering 3D magnetic films.",2108.10232v1
2021-08-24,Higher-order ferromagnetic resonances in periodic arrays of synthetic-antiferromagnet nanodiscs,"We investigate spin dynamics in nanodisc arrays of synthetic-antiferromagnets
(SAF) made of Py/NiCu/Py trilayers, where the NiCu spacer undergoes a Curie
transition at about 200 K. The observed ferromagnetic resonance spectra have
three distinct resonance modes at room temperature, which are fully recreated
in our micromagnetic simulations showing also how the intra-SAF asymmetry can
be used to create and control the higher-order resonances in the structure.
Below the Curie temperature of the spacer, the system effectively transitions
into a single-layer nanodisc array with only two resonance modes. Our results
show how multi-layering of nano-arrays can add tunable GHz functionality
relevant for such rapidly developing fields as magnetic meta-materials,
magnonic crystals, arrays of spin-torque oscillators and neuromorphic
junctions.",2108.10581v1
2021-08-24,Out-of-plane magnetic anisotropy in bulk ilmenite $\text{CoTiO}_3$,"Structural, electronic and magnetic properties of bulk ilmenite CoTiO$_3$ are
analyzed in the framework of density functional theory (DFT), using the
generalized gradient approximation (GGA) and Hubbard-corrected approaches. We
find that the G-type antiferromagnetic (G-AFM) structure, which consists of
antiferromagnetically coupled ferromagnetic $ab$ planes, is the ground-state of
the system, in agreement with experiments. Furthermore, cobalt titanates
present two critical temperatures related to the breaking of the inter- and
intra-layer magnetic ordering. This would result in the individual planes
remaining ferromagnetic even at temperatures above the N\'{e}el temperature.
When spin-orbit coupling is included in our calculations, we find an
out-of-plane magnetic anisotropy, which can be converted to an in-plane
anisotropy with a small doping of electrons corresponding to about 2.5% Ti
substitution for Co, consistent with experimental expectations. We thus present
a disorder-dependent study of the magnetic anisotropy in bulk $\text{CoTiO}_3$,
which will determine its magnon properties, including topological aspects.",2108.10847v2
2021-09-01,Dynamical properties of the Haldane chain with bond disorder,"By using Lanczos exact diagonalization and quantum Monte Carlo combined with
stochastic analytic continuation, we study the dynamical properties of the
$S=1$ antiferromagnetic Heisenberg chain with different strengths of bond
disorder. In the weak disorder region, we find weakly coupled bonds which can
induce additional low-energy excitation below the one-magnon mode. As the
disorder increases, the average Haldane gap closes at $\delta_{\Delta}\sim 0.5$
with more and more low-energy excitations coming out. After the critical
disorder strength $\delta_c\sim 1$, the system reaches a random-singlet phase
with prominent sharp peak at $\omega=0$ and broad continuum at $\omega>0$ of
the dynamic spin structure factor. In addition, we analyze the distribution of
random spin domains and numerically find three kinds of domains hosting
effective spin-1/2 quanta or spin-1 sites in between. These ""spins"" can form
the weakly coupled long-range singlets due to quantum fluctuation which
contribute to the sharp peak at $\omega=0$.",2109.00552v1
2021-09-03,Non-Hermitian skin effect in magnetic systems,"Far from being limited to a trivial generalization of their Hermitian
counterparts, non-Hermitian topological phases have gained widespread interest
due to their unique properties. One of the most striking non-Hermitian
phenomena is the skin effect, i.e., the localization of a macroscopic fraction
of bulk eigenstates at a boundary, which underlies the breakdown of the
bulk-edge correspondence. Here we investigate the emergence of the skin effect
in magnetic insulating systems by developing a phenomenological approach to
describing magnetic dissipation within a lattice model. Focusing on a
spin-orbit-coupled van der Waals (vdW) ferromagnet with spin-nonconserving
magnon-phonon interactions, we find that the magnetic skin effect emerges in an
appropriate temperature regime. Our results suggest that the interference
between Dzyaloshinskii-Moriya interaction (DMI) and nonlocal magnetic
dissipation plays a key role in the accumulation of bulk states at the
boundaries.",2109.01711v4
2021-09-30,Observation of the orbital Hall effect in a light metal Ti,"The orbital angular momentum is a core ingredient of orbital magnetism, spin
Hall effect, giant Rashba spin splitting, orbital Edelstein effect, and
spin-orbit torque. However, its experimental detection is tricky. In
particular, direct detection of the orbital Hall effect remains elusive despite
its importance for electrical control of magnetic nanodevices. Here we report
the direct observation of the orbital Hall effect in a light metal Ti. The Kerr
rotation by the accumulated orbital magnetic moment is measured at Ti surfaces,
whose result agrees with theoretical calculations semiquantitatively and is
supported by the orbital torque measurement in Ti-based magnetic
heterostructures. The results confirm the electron orbital angular momentum as
an essential dynamic degree of freedom, which may provide a novel mechanism for
the electric control of magnetism. The results may also deepen the
understanding of spin, valley, phonon, and magnon dynamics coupled with orbital
dynamics.",2109.14847v1
2021-09-30,Signatures of quartic asymmetric exchange in a class of two dimensional magnets,"Indirect quartic interaction of spins is suggested to play an important role
in two dimensional magnets with trigonal prismatic symmetry such as
Fe$_3$GeTe$_2$ monolayer. The proposed interaction is described by terms in
micromagnetic energy that are linear in magnetization gradients. Such terms
enable the stability of non-collinear magnetic textures. We investigate
signatures of the quartic interaction in magnon spectra in the presence of
anisotropy and external magnetic field. We also show how magnetic spirals,
which are induced by the proposed interaction, can be manipulated by external
field. Our analysis is based on symmetry considerations and can be used to
quantify the quartic interaction strength in experiments with magnetic
monolayers.",2109.15297v1
2021-10-04,Lumped circuit model for inductive antenna spin-wave transducers,"We derive a lumped circuit model for inductive antenna spin-wave transducers
in the vicinity of a ferromagnetic medium. The model considers the antenna's
Ohmic resistance, its inductance, as well as the additional inductance due to
the excitation of ferromagnetic resonance or spin waves in the ferromagnetic
medium. As an example, the additional inductance is discussed for a wire
antenna on top of a ferromagnetic waveguide, a structure that is characteristic
for many magnonic devices and experiments. The model is used to assess the
scaling properties and the energy efficiency of inductive antennas. Issues
related to scaling antenna transducers to the nanoscale and possible solutions
are also addressed.",2110.01318v3
2021-10-13,Dimensional reduction in quantum spin-1/2 system on a 1/7-depleted triangular lattice,"We study the magnetism of a quantum spin-1/2 antiferromagnet on a maple-leaf
lattice which is obtained by regularly depleting 1/7 of the sites of a
triangular lattice. Although the interactions are set to be spatially uniform,
the ground state shows a stripe Neel order and the temperature dependence of
magnetic susceptibility follows that of the one-dimensional XXZ model with a
finite spin gap. We examine the nature of frustration by mapping the low energy
degenerate manifold of states to the fully packed loop-string model on a dual
cluster-depleted honeycomb lattice, finding that the order-by-disorder due to
quantum fluctuation characteristic of highly frustrated magnets is responsible
for the emergent stripes. The excited magnons split into two spinons and
propagate in the one-dimensional direction along the stripe which is
reminiscent of the XXZ or Ising model in one dimension. Unlike most of the
previously studied dimensional reduction effects, our case is purely
spontaneous as the interactions of the Hamiltonian retains a spatially
isotropic two-dimensional structure.",2110.06491v2
2021-10-17,Understanding and Designing the Spin-Driven Thermoelectrics,"While the thermoelectric materials progress based on the engineering of
electronic and phononic characteristics is reaching a plateau, adding the spin
degree of freedom has the potential to open a new landscape for alternative
thermoelectric materials. Here we present the concepts, current understanding,
and guidelines for designing spin-driven thermoelectrics. We show that the
interplay between the spin and heat currents in entropy transport via charge
carriers can offer a strategic path to enhance the electronic thermopower. The
classical antiferromagnetic semiconductor manganese telluride (MnTe) is chosen
as the case study due to its significant spin-mediated thermoelectric
properties. We show that although the spin-disorder scattering reduces the
carrier mobility in magnetic materials, spin entropy, magnon, and paramagnon
carrier drags can dominate over and significantly enhance the thermoelectric
power factor and hence zT. Finally, several guidelines are drawn based on the
current understandings for designing high-performance spin-driven
thermoelectric materials.",2110.08897v1
2021-10-22,"Magnetic order, disorder, and excitations under pressure in the Mott insulator Sr$_2$IrO$_4$","Protected by the interplay of on-site Coulomb interactions and spin-orbit
coupling, Sr$_2$IrO$_4$ at high pressure is a rare example of a Mott insulator
with a paramagnetic ground state. Here, using optical Raman scattering, we
measure both the phonon and magnon evolution in Sr$_2$IrO$_4$ under pressure,
and identify three different magnetically-ordered phases, culminating in a
spin-disordered state beyond 18 GPa. A strong first-order structural phase
transition drives the magnetic evolution at $\sim$10 GPa with reduced
structural anisotropy in the IrO$_6$ cages, leading to increasingly isotropic
exchange interactions between the Heisenberg spins and a spin-flip transition
to $c$-axis-aligned antiferromagnetic order. In the disordered phase of
Heisenberg $J_\mathrm{eff}=1/2$ pseudospins, the spin excitations are
quasi-elastic and continuous to 10 meV, potentially hosting a gapless quantum
spin liquid in Sr$_2$IrO$_4$.",2110.11588v1
2021-10-22,Magnetization dynamics fingerprints of an excitonic condensate $t_{2g}^{4}$ magnet,"The competition between spin-orbit coupling $\lambda$ and electron-electron
interaction $U$ leads to a plethora of novel states of matter, extensively
studied in the context of $t_{2g}^4$ and $t_{2g}^5$ materials, such as
ruthenates and iridates. Excitonic magnets -- the antiferromagnetic state of
bounded electron-hole pairs -- is a prominent example of phenomena driven by
those competing energy scales. Interestingly, recent theoretical studies
predicted that excitonic magnets can be found in the ground-state of
spin-orbit-coupled $t_{2g}^4$ Hubbard models. Here, we present a detailed
computational study of the magnetic excitations in that excitonic magnet,
employing one-dimensional chains (via density matrix renormalization group) and
small two-dimensional clusters (via Lanczos). Specifically, first we show that
the low-energy spectrum is dominated by a dispersive (acoustic) magnonic mode,
with extra features arising from the $\lambda=0$ state in the phase diagram.
Second, and more importantly, we found a novel magnetic excitation forming a
high-energy optical mode with the highest intensity at wavevector $q\to 0$. In
the excitonic condensation regime at large $U$, we also have found a novel
high-energy $\pi$-mode composed solely of orbital excitations. These unique
fingerprints of the $t_{2g}^4$ excitonic magnet are important in the analysis
of neutron and RIXS experiments.",2110.11828v1
2021-10-27,Low temperature competing magnetic energy scales in the topological ferrimagnet TbMn6Sn6,"TbMn6Sn6 is a metallic ferrimagnet displaying signatures of both topological
electrons and topological magnons arising from ferromagnetism and spin-orbit
coupling within its Mn kagome layers. Inelastic neutron scattering measurements
find strong ferromagnetic (FM) interactions within the Mn kagome layer and
reveal a magnetic bandwidth of ~230 meV. The low-energy magnetic excitations
are characterized by strong FM Mn-Mn and antiferromagnetic (AFM) Mn-Tb
interlayer magnetic couplings. We observe weaker, competing long-range FM and
AFM Mn-Mn interlayer interactions similar to those driving helical magnetism in
the YMn6Sn6 system. Combined with density-functional theory calculations, we
find that competing Mn-Mn interlayer magnetic interactions occur in all RMn6Sn6
compounds with R= Y, Gd-Lu, resulting in magnetic instabilities and tunability
when Mn-R interactions are weak. In the case of TbMn6Sn6, strong AFM Mn-Tb
coupling ensures a highly stable three-dimensional ferrimagnetic network.",2110.14713v2
2021-10-30,Roadmap on Spin-Wave Computing,"Magnonics is a field of science that addresses the physical properties of
spin waves and utilizes them for data processing. Scalability down to atomic
dimensions, operations in the GHz-to-THz frequency range, utilization of
nonlinear and nonreciprocal phenomena, and compatibility with CMOS are just a
few of many advantages offered by magnons. Although magnonics is still
primarily positioned in the academic domain, the scientific and technological
challenges of the field are being extensively investigated, and many
proof-of-concept prototypes have already been realized in laboratories. This
roadmap is a product of the collective work of many authors that covers
versatile spin-wave computing approaches, conceptual building blocks, and
underlying physical phenomena. In particular, the roadmap discusses the
computation operations with Boolean digital data, unconventional approaches
like neuromorphic computing, and the progress towards magnon-based quantum
computing. The article is organized as a collection of sub-sections grouped
into seven large thematic sections. Each sub-section is prepared by one or a
group of authors and concludes with a brief description of the current
challenges and the outlook of the further development of the research
directions.",2111.00365v1
2021-10-31,Thermally induced all-optical ferromagnetic resonance in thin YIG films,"All-optical ferromagnetic resonance (AO-FMR) is a powerful tool for local
detection of micromagnetic parameters, such as magnetic anisotropy, Gilbert
damping or spin stiffness. In this work we demonstrate that the AO-FMR method
can be used in thin films of Yttrium Iron Garnet (YIG) if a metallic capping
layer (Au, Pt) is deposited on top of the film. Magnetization precession is
triggered by heating of the metallic layer with femtosecond laser pulses. The
heating modifies the magneto-crystalline anisotropy of the YIG film and shifts
the quasi-equilibrium orientation of magnetization, which results in
precessional magnetization dynamics. The laser-induced magnetization precession
corresponds to a uniform (Kittel) magnon mode, with the precession frequency
determined by the magnetic anisotropy of the material as well as the external
magnetic field, and the damping time set by a Gilbert damping parameter. The
AO-FMR method thus enables measuring local magnetic properties, with spatial
resolution given only by the laser spot size.",2111.00586v1
2021-11-02,Anti-PT-symmetry-enhanced interconversion between microwave and optical fields,"The intrinsic dissipation of systems into a shared reservoir introduces
coherence between two systems, enabling anti-Parity-Time (anti-PT) symmetry. In
this paper, we propose an anti-PT symmetric converter, consisting of a
microwave cavity coupled dissipatively to a ferromagnetic sphere, which
supports significant improvements in the conversion efficiency when compared to
coherently coupled setups. In particular, when only the ferrite sample is
driven, the strong coherence induced by the vacuum of the mediating channel
leads to much stronger enhancements in the intended conversion. The enhancement
is an inalienable artifact of the emergence of a long-lived, dark mode
associated with a quasi-real singularity of the hybrid system. In addition, we
observe considerable asymmetry in the efficiencies of microwave-to-optical and
optical-to-microwave conversions, in spite of the symmetrical structure of the
trilinear optomagnonic coupling stimulating both the transduction phenomena.
The nonreciprocity stems from the intrinsic asymmetry in the couplings of the
microwave and optical fields to the cavity-magnon network as well as the phase
coupling entailed by the spatial separation.",2111.01335v1
2021-11-03,Efficient geometrical control of spin waves in microscopic YIG waveguides,"We study experimentally and by micromagnetic simulations the propagation of
spin waves in 100-nm thick YIG waveguides, where the width linearly decreases
from 2 to 0.5 micrometers over a transition region with varying length between
2.5 and 10 micrometers. We show that this geometry results in a down-conversion
of the wavelength, enabling efficient generation of waves with wavelengths down
to 350 nm. We also find that this geometry leads to a modification of the group
velocity, allowing for almost-dispersionless propagation of spin-wave pulses.
Moreover, we demonstrate that the influence of energy concentration outweighs
that of damping in these YIG waveguides, resulting in an overall increase of
the spin-wave intensity during propagation in the transition region. These
findings can be utilized to improve the efficiency and functionality of
magnonic devices which use spin waves as an information carrier.",2111.02236v1
2021-11-09,Magnetotransport in ferromagnetic Fe$_2$Ge semimetallic thin films,"Thin films of the ferromagnet Fe$_2$Ge were grown via molecular beam epitaxy,
and their electrical and magneto-transport properties measured for the first
time. X-ray diffraction and vibrating sample magnetometry measurements
confirmed the crystalline ferromagnetic Fe$_2$Ge phase. The observed high
temperature maximum in the longitudinal resistivity, as well as the observed
suppression of electron-magnon scattering at low temperatures, point to the
presence of strong spin polarization in this material. Measurements of the Hall
resistivity, $\rho_{xy}$, show contributions from both the ordinary Hall effect
and anomalous Hall effect, $\rho_{xy}^{AH}$, from which we determined the
charge carrier concentration and mobility. Measurements also show a small
negative magnetoresistance in both the longitudinal and transverse geometries.
Fe$_2$Ge holds promise as a useful spintronic material, especially for its
semiconductor compatibility.",2111.05417v1
2021-11-17,Large photogalvanic spin current by magnetic resonance in bilayer Cr trihalides,"Magnetic materials show rich optical responses related to the magnetic order.
These phenomena reflect the nature of their excitations, providing a powerful
probe for the magnetic states and a way to control them. In recent years, such
studies were extended to the optical control of spin current using nonlinear
optical response similar to the photogalvanic effect. However, neither a
candidate material nor a general formula for calculating the photogalvanic spin
current is known so far. In this work, we develop a general theory for the
photogalvanic spin current through a magnetic resonance process. Using the
nonlinear response formalism, we find the nonlinear conductivity consists of
two contributions that involve one and two magnon bands; the latter is a
contribution unknown to date. We argue that the two-band process produces a
large photogalvanic spin current in the antiferromagnetic phase of bilayer
CrI$_3$ and CrBr$_3$, whose resonance frequency can be tuned between GHz-THz
range by an external magnetic field. Our findings open a route to the studies
on the photogalvanic effect of spin angular momentum in realistic setups.",2111.09007v1
2021-11-18,Topological to magnetically ordered quantum phase transition in antiferromagnetic spin ladders with long-range interactions,"We study a generalized quantum spin ladder with staggered long range
interactions that decay as a power-law with exponent $\alpha$. Using large
scale quantum Monte Carlo (QMC) and the density matrix renormalization group
(DMRG) simulations, we show that this model undergoes a transition from a
rung-dimer phase characterized by a non-local string order parameter, to a
symmetry broken N\'eel phase. We find evidence that the transition is second
order.In the magnetically ordered phase, the spectrum exhibits gapless modes,
while excitations in the gapped phase are well described in terms of triplons
-- bound states of spinons across the legs. We obtain the momentum resolved
spin dynamic structure factor numerically and find a well defined triplon band
evolves into a gapless magnon dispersion through the transition. We further
discuss the possibility of deconfined criticality in this model.",2111.09798v2
2021-11-21,Antiferromagnetic resonance in the cubic iridium hexahalides (NH$_4$)$_2$IrCl$_6$ and K$_2$IrCl$_6$,"We report on high-field electron spin resonance studies of two iridium
hexahalide compounds, (NH$_4$)$_2$IrCl$_6$ and K$_2$IrCl$_6$. In the
paramagnetic state, our measurements reveal isotropic $g$-factors $g = 1.79$(1)
for the Ir$^{4+}$ ions, in agreement with their cubic symmetries. Most
importantly, in the magnetically ordered state, we observe two magnon modes
with zero-field gaps of 11.3 and 14.2 K for (NH$_4$)$_2$IrCl$_6$ and
K$_2$IrCl$_6$, respectively. Based on that and using linear spin-wave theory,
we estimate the nearest-neighbor exchange couplings and anisotropic Kitaev
interactions, $J_1/k_B=10.3$ K, $K/k_B=0.7$ K for (NH$_4$)$_2$IrCl$_6$, and
$J_1/k_B=13.8$ K, $K/k_B=0.9$ K for K$_2$IrCl$_6$, revealing the
nearest-neighbor Heisenberg coupling as the leading interaction term, with only
a weak Kitaev anisotropy.",2111.10884v1
2021-11-28,Parametrically driven THz magnon-pairs: predictions towards ultimately fast and minimally dissipative switching,"Findings ways to achieve switching between magnetic states at the fastest
possible time scale that simultaneously dissipates the least amount of energy
is one of the main challenges in magnetism. Antiferromagnets exhibit intrinsic
dynamics in the THz regime, the highest among all magnets and are therefore
ideal candidates to address this energy-time dilemma. Here we study
theoretically THz-driven parametric excitation of antiferromagnetic
magnon-pairs at the edge of the Brillouin zone and explore the potential for
switching between two stable oscillation states. Using a semi-classical theory,
we predict that switching can occur at the femtosecond time scale with an
energy dissipation down to a few zepto Joule. This result touches the
thermodynamical bound of the Landauer principle, and approaches the quantum
speed limit up to 5 orders of magnitude closer than demonstrated with magnetic
systems so far.",2111.14202v1
2021-11-30,Nonreciprocity of spin waves in a double layer ferromagnet with Dzyaloshinskii-Moriya interactions,"In this paper, boundary conditions for Landau-Lifshitz equations at the
interface between two ferromagnets with different Dzyaloshinskii-Moriya
interactions are derived. We calculated and verified the analytical expression
for the energy flux density continuity for the spin-wave propagation through
the interface between two ferromagnets with/without Dzyaloshinskii-Moriya
interactions considering the boundary conditions mentioned. Analytical
expressions for reflection, transmission coefficients and the nonreciprocity
factor are calculated in the case of spin-wave propagation through a double
layer ferromagnet with/without Dzyaloshinskii-Moriya interactions in the
first/second layer. Two principally different types of nonreciprocity effects
for spin waves are revealed in such a double layer system. The material
parameters of a double layer ferromagnet with/without Dzyaloshinskii-Moriya
interactions in the first/second layer are found for which the extremely high
nonreciprocity factor (>10) is expected according to the results of
calculations. The results of the paper deepen the knowledge about the spin-wave
propagation control in magnonic devices.",2111.15244v1
2021-11-28,Tunable cryogenic THz cavity for strong light-matter coupling in complex materials,"We report here the realization and commissioning of an experiment dedicated
to the study of the optical properties of light matter hybrids constituted of
crystalline samples embedded in an optical cavity. The experimental assembly
developed offers the unique opportunity to study the weak and strong coupling
regime between a tunable optical cavity in cryogenic environment and low energy
degrees of freedom such as phonons, magnons or charge fluctuations. We describe
here the setup developed which allows the positioning of crystalline samples in
an optical cavity of different quality factor, the tuning of the cavity length
at cryogenic temperatures and its optical characterization with a broadband
time domain THz spectrometer (0.2-6 THz). We demonstrate the versatility of the
setup by studying the vibrational strong coupling in CuGeO3 single crystal at
cryogenic temperatures.",2112.01560v1
2021-12-03,Microwave Amplification in a PT -symmetric-like Cavity Magnomechanical System,"We propose a scheme that can generate tunable magnomechanically induced
amplification in a double-cavity parity-time-(PT -) symmetric-like
magnomechanical system under a strong control and weak probe field. The system
consists of a ferromagnetic-material yttrium iron garnet (YIG) sphere placed in
a passive microwave cavity which is connected with another active cavity. We
reveal that ideally induced amplification of the microwave probe signal may
reach the maximum value 1000000 when cavity-cavity, cavity-magnon and
magnomechanical coupling strengths are nonzero simultaneously. The phenomenon
might have potential applications in the field of quantum information
processing and quantum optical devices. Besides, we also find the phenomena of
slow-light propagation. In this case, group speed delay of the light can
achieve 0.000035s, which can enhance some nonlinear effect. Moreover, due to
the relatively flat dispersion curve, the proposal may be applied to sensitive
optical switches, which plays an important role in storing photons and quantum
optical chips.",2112.01727v1
2021-12-06,Charge and spin order dichotomy in NdNiO$_2$ driven by SrTiO$_3$ capping layer,"Superconductivity in infinite-layer nickelates holds exciting analogies with
that of cuprates, with similar structures and $3d$-electron count. Using
resonant inelastic x-ray scattering (RIXS) we studied electronic and magnetic
excitations and charge density correlations in Nd$_{1-x}$Sr$_{x}$NiO$_2$ thin
films with and without an SrTiO$_3$ capping layer. We observe dispersing
magnons only in the capped samples, progressively dampened at higher doping. In
addition, we detect an elastic resonant scattering peak in the uncapped $x=0$
compound at wave vector (1/3,0), remindful of the charge order signal in hole
doped cuprates. The peak weakens at $x=0.05$ and disappears in the
superconducting $x=0.20$ film. The uncapped samples also present a higher
degree of Ni$3d$-Nd$5d$ hybridization and a smaller anisotropy of the Ni$3d$
occupation with respect to the capped samples. The role of the capping on the
possible hydrogen incorporation or on other mechanisms responsible for the
electronic reconstruction far from the interface remains to be understood.",2112.03341v1
2021-12-21,Nonreciprocal spin-wave dynamics in Pt/Co/W/Co/Pt multilayers,"We present a detailed study of the spin-wave dynamics in single Pt/Co/W and
double Pt/Co/W/Co/Pt ferromagnetic layer systems. The dispersion of spin waves
was measured by wavevector-resolved Brillouin light scattering spectroscopy
while the in-plane and out-of-plane magnetization curves were measured by
alternating gradient field magnetometry. The interfacial Dzyaloshinskii-Moriya
interaction induced nonreciprocal dispersion relation was demonstrated for both
single and double ferromagnetic layers and explicated by numerical simulations
and theoretical formulas. The results indicate the crucial role of the order of
layers deposition on the magnetic parameters. A significant difference between
the perpendicular magnetic anisotropy constant in double ferromagnetic layer
systems conduces to the decline of the interlayer interactions and different
dispersion relations for the spin-wave modes. Our study provides a significant
contribution to the realization of the multifunctional nonreciprocal magnonic
devices based on ultrathin ferromagnetic/heavy-metal layer systems.",2112.11206v1
2021-12-23,NMR evidence against a spin-nematic nature of the presaturation phase in the frustrated magnet SrZnVO(PO4)2,"Using $^{31}$P nuclear magnetic resonance (NMR) we investigate the recently
discovered presaturation phase in the highly frustrated two-dimensional spin
system SrZnVO(PO$_4$)$_2$ [F. Landolt et al., Phys. Rev. B 104, 224435 (2021)].
Our data provide two pieces of evidence against the presumed spin-nematic
character of this phase: i) NMR spectra reveal that it hosts a dipolar spin
order and ii) the 1/$T_1$ relaxation rate data recorded above the saturation
field can be fitted by the sum of a single-magnon term, exponential in the gap,
and a critical second-order term, exponential in the triple gap, leaving no
space for a nematic spin dynamics, characterized by a double-gap exponential.
We explain the unexpectedly broad validity of the simple fit and the related
critical spin dynamics.",2112.12603v2
2021-12-24,Skyrmion nucleation on the surface of a topological insulator,"Skyrmion nucleation induced by spin-transfer torques at an interface of a
topological insulator and a ferromagnetic insulator is investigated. Due to
strong spin-orbit coupling on a surface of topological insulators, which
enhances the effect of spin torques, efficient manipulation of skyrmions is
expected, and therefore, topological insulators could provide the ideal
platform to achieve high-performance skyrmionic devices. Using micromagnetic
simulations and energetics, we evaluate properties of the skyrmion nucleation
on a surface of topological insulators, such as nucleation time, critical
electric field, and skyrmion numbers. We show that the nucleation time is
inversely proportional to the applied electric field. We also identify the
Gilbert damping and temperature dependencies of the critical field.
Furthermore, we analytically evaluate the effect of the Dzyaloshinskii-Moriya
interaction and demonstrate that the temperature dependence can be explained by
the reduction of a magnon excitation gap due to the self-energy corrections.",2112.12967v2
2021-12-19,Engineering spin wave spectra in thick Ni$_{80}$Fe$_{20}$ rings by using competition between exchange and dipolar fields,"Control of the spin wave dynamics in nanomagnetic elements is very important
for the realization of a broad range of novel magnonic devices. Here we study
experimentally the spin wave resonance in thick ferromagnetic rings (100 nm)
using perpendicular ferromagnetic resonance spectroscopy. Different from what
was observed for the continuous film of the same thickness, or from ring with
similar lateral dimensions but with lower thicknesses, the spectra of thick
patterned rings show a non-monotonic dependence of the mode intensity on the
resonance field for a fixed frequency. To explain this effect, the theoretical
approach by considering the dependence of the mode profiles on both the radial
and axial coordinates was developed. It was demonstrated that such unusual
behavior is a result of the competition between exchange and dipolar fields
acting at the spin excitations in the structure under study. The calculations
are in a good agreement with the experimental results.",2112.13657v1
2021-12-30,Cavity Quantum Materials,"The emergent field of cavity quantum materials bridges collective many-body
phenomena in solid-state platforms with strong light-matter coupling in cavity
quantum electrodynamics (cavity QED). This brief review provides an overview of
the state of the art of cavity platforms and highlights recent theoretical
proposals and first experimental demonstrations of cavity control of collective
phenomena in quantum materials. This encompasses light-matter coupling between
electrons and cavity modes, cavity superconductivity, cavity phononics and
ferroelectricity, correlated systems in a cavity, light-magnon coupling, cavity
topology and the quantum Hall effect, as well as superradiance. An outlook of
potential future developments is given.",2112.15018v2
2022-01-07,Control of spin waves by spatially modulated strain,"We suggest using spatially modulated strain for control of a spin wave
propagating inside a bulk magnet. The modulation with the wave vector $q=2k$,
by virtue of magnetoelasticity, mixes spin waves with wave vectors near $k$ and
$-k$. This leads to lifting the degeneracy of the symmetric and antisymmetric
eigenstate combinations of these waves. The resulting picture reminds one of a
tunneling particle in a symmetric double-well potential. Here, a moving spin
wave being subjected to the $2k$-lattice modulation after some time alters its
propagation direction to the opposite one, and so on. The effect can be
utilized for the control of the spin-wave propagation that can be useful for
spintronic and magnonic applications. The control may include a delay line
element, filtering, and waveguide of the spin waves.",2201.02691v1
2022-01-19,The planar thermal Hall conductivity in the Kitaev magnet α-RuCl3,"We report detailed measurements of the Onsager-like planar thermal Hall
conductivity $\kappa_{xy}$ in $\alpha$-RuCl$_3$, a spin-liquid candidate of
topical interest. With the thermal current ${\bf J}_{\rm Q}$ and magnetic field
$\bf B\parallel a$ (zigzag axis), the observed $\kappa_{xy}/T$ varies strongly
with temperature $T$ (1-10 K). The results are well-described by bosonic edge
excitations which evolve to topological magnons at large $B$. Fits to
$\kappa_{xy}/T$ yield a Chern number $\sim 1$ and a band energy $\omega_1\sim$1
meV, in agreement with sharp modes seen in electron spin-resonance experiments.
The bosonic character is incompatible with half-quantization of
$\kappa_{xy}/T$.",2201.07873v1
2022-01-25,Complex vortex-antivortex dynamics in the magnetic superconductor EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$,"We report on the investigation of the magnetic superconductor
EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$ based on muon-spin spectroscopy and ac
magnetic susceptibility ($\chi$) measurements. The dependence of the internal
field at the muon site on temperature is indicative of a ferromagnetic ordering
of Eu$^{2+}$ magnetic moments and only the conventional magnon scattering
governs the longitudinal relaxation rate at low temperatures. At the same time,
we observe a rich phenomenology for the imaginary component of the
susceptibility $\chi^{\prime\prime}$ by means of both standard ac
susceptibility and a novel technique based on a microwave coplanar waveguide
resonator. In particular, we detect activated trends for several features in
$\chi^{\prime\prime}$ over frequencies spanning ten orders of magnitude. We
interpret our results in terms of the complex dynamics of vortices and
antivortices influenced by the underlying structure of magnetic domains.",2201.10216v1
2022-01-25,Krein-unitary Schrieffer-Wolff transformation and band touchings in bosonic Bogoliubov-de Gennes and other Krein-Hermitian Hamiltonians,"Krein-Hermitian Hamiltonians, i.e., Hamiltonians Hermitian with respect to an
indefinite inner product, have emerged as an important class of non-Hermitian
Hamiltonians in physics, encompassing both single-particle bosonic
Bogoliubov-de Gennes (BdG) Hamiltonians and so-called ""$PT$-symmetric""
non-Hermitian Hamiltonians. In particular, they have attracted considerable
scrutiny owing to the recent surge in interest for boson topology. Motivated by
these developments, we formulate a perturbative Krein-unitary Schrieffer-Wolff
transformation for finite-size dynamically stable Krein-Hermitian Hamiltonians,
yielding an effective Hamiltonian for a subspace of interest. The effective
Hamiltonian is Krein Hermitian and, for sufficiently small perturbations, also
dynamically stable. As an application, we use this transformation to justify
codimension-based analyses of band touchings in bosonic BdG Hamiltonians, which
complement topological characterization. We use this simple approach based on
symmetry and codimension to revisit known topological magnon band touchings in
several materials of recent interest.",2201.10580v2
2022-01-31,Evidence of Two-Spinon Bound States in the Magnetic Spectrum of Ba$_3$CoSb$_2$O$_9$,"Recent inelastic neutron scattering (INS) experiments of the triangular
antiferromagnet Ba$_3$CoSb$_2$O$_9$ revealed strong deviations from
semiclassical theories. We demonstrate that key features of the INS data are
well reproduced by a parton Schwinger boson theory beyond the saddle-point
approximation. The measured magnon dispersion is well reproduced by the
dispersion of two-spinon bound states (poles of the emergent gauge fields
propagator), while the low-energy continuum scattering is reproduced by a
quasifree two-spinon continuum, suggesting that a free-spinon gas is a good
initial framework to study magnetically ordered states near a quantum melting
point.",2201.13369v4
2022-02-01,Signatures of non-Loudon-Fleury Raman scattering in the Kitaev magnet $β$-Li$_2$IrO$_3$,"We investigate the magnetic excitations of the hyperhoneycomb Kitaev magnet
$\beta$-$\text{Li}_2\text{IrO}_3$ by means of inelastic Raman scattering. The
spectra exhibits a coexistence of a broad scattering continuum and two sharp
low-energy peaks at 2.5 meV and 3 meV, with a distinctive polarization
dependence. While the continuum is suggestive of fractional quasi-particles
emerging from a proximate quantum spin liquid phase, the sharp peaks provide
the first experimental signature of the `non-Loudon-Fleury' one-magnon
scattering processes proposed recently [Phys. Rev. B 104, 144412 (2021)]. The
corresponding microscopic mechanism is similar to the one leading to the
symmetric off-diagonal exchange interaction $\Gamma$ (as it involves a
combination of both direct and ligand-mediated exchange paths), but is
otherwise completely unexpected within the traditional Loudon-Fleury theory of
Raman scattering. The present experimental verification therefore calls for a
drastic reevaluation of Raman scattering in similar systems with strong spin
orbit coupling and multiple exchange paths.",2202.00581v1
2022-02-08,Breakdown of Linear Spin-wave Theory and Existence of Spinon Bound States in the Frustrated Kagome Lattice Antiferromagnet,"Spin dynamics of the spin-1/2 kagome lattice antiferromagnet
Cs$_2$Cu$_3$SnF$_{12}$ was studied using high-resolution, time-of-flight
inelastic neutron scattering. The flat mode, a characteristic of the frustrated
kagome antiferromagnet, and the low-energy dispersive mode, which is dominated
by magnons, can be well described by the linear spin-wave theory. However, the
theory fails to describe three weakly dispersive modes between 9 and 14 meV.
These modes could be attributed to two-spinon bound states, which decay into
free spinons away from the zone center and at a high temperature, giving rise
to continuum scattering.",2202.03681v1
2022-02-10,Composite Spin Approach to the Blockade Effect in Rydberg Atom Arrays,"The Rydberg blockade induces strongly correlated many-body effects in Rydberg
atom arrays, including rich ground-state phases and many-body scar states in
the excitation spectrum. In this letter, we propose a composite spin
representation that can provide a unified description for major features in
this system. The composite spin combines Rydberg excitation and the auxiliary
fermions, which are introduced to implement the Rydberg blockade constraint
automatically. First, we focus on the PXP model describing one-dimensional
arrays with the Rydberg blocking radius being a lattice spacing. Using
composite spins, the ground state is simply a ferromagnetic product state of
composite spins, and the magnon excitations of these composite spins can
accurately describe the many-body scar states and signal the quantum phase
transition driven by detuning. Then, we show that Rydberg atom arrays with
different blocking radii can share a universal description of the composite
spin representation, and the difference between different blockade radii can be
absorbed in the formation of composite spins.",2202.04902v1
2022-02-20,Magneto-optical Spectroscopy with RAMBO: A Table-Top 30 T Magnet,"Optically probing materials in high magnetic fields can provide enlightening
insight into field-modified electronic states and phases, while optically
driving materials in high magnetic fields can induce novel nonequilibrium
many-body dynamics of spin and charge carriers. While there are high-field
magnets compatible with standard optical spectroscopy methods, they are
generally bulky and have limited optical access, which prohibit performing
state-of-the-art ultrafast and/or nonlinear optical experiments. The Rice
Advanced Magnet with Broadband Optics (RAMBO), a unique 30-T pulsed mini-coil
magnet system with direct optical access, has enabled previously challenging
experiments using femtosecond optical pulses, including time-domain terahertz
spectroscopy, in cutting-edge materials placed in strong magnetic fields. Here,
we review recent experimental advances made possible by the first-generation
RAMBO setup. After summarizing technological aspects of combining optical
spectroscopic techniques with the mini-coil magnet, we describe results of
magneto-optical studies of a wide variety of materials, providing new insight
into the states and dynamics of four types of quasiparticles in solids -
excitons, plasmons, magnons, and phonons - in high magnetic fields.",2202.09732v1
2022-03-01,turboMagnon -- A code for the simulation of spin-wave spectra using the Liouville-Lanczos approach to time-dependent density-functional perturbation theory,"We introduce turboMagnon, an implementation of the Liouville-Lanczos approach
to linearized time-dependent density-functional theory, designed to simulate
spin-wave spectra in solid-state materials. The code is based on the
noncollinear spin-polarized framework and the self-consistent inclusion of
spin-orbit coupling that allow to model complex magnetic excitations. The spin
susceptibility matrix is computed using the Lanczos recursion algorithm that is
implemented in two flavors - the non-Hermitian and the pseudo-Hermitian one.
turboMagnon is open-source software distributed under the terms of the GPL as a
component of Quantum ESPRESSO. As with other components, turboMagnon is
optimized to run on massively parallel architectures using native mathematical
libraries (LAPACK and FFTW) and a hierarchy of custom parallelization layers
built on top of MPI. The effectiveness of the code is showcased by computing
magnon dispersions for the CrI$_3$ monolayer, and the importance of the
spin-orbit coupling is discussed.",2203.01120v2
2022-03-09,Exchange spin waves in thin films with gradient composition,"We report investigation of ferromagnetic resonance phenomenon in
ferromagnetic thin films with essentially non-uniform composition. Epitaxial
Pd-Fe thin film with linear distribution of Fe content across the thickness is
used as the model material. Anomalous perpendicular standing spin waves are
observed and quantified using the collective dynamic equation. Numerical
analysis yields the exchange stiffness constant for diluted Pd-Fe alloy
$D=2A/\mu_0M_s=15$~T$\cdot$nm$^2$ and the ratio of the effective magnetization
to the saturation magnetization $M_{eff}/M_s=1.16$. It is demonstrated that,
overall, engineering of thin films with non-uniform composition across the
thickness can be used for high-frequency or low-field magnonic operations using
exchange spin waves.",2203.05014v1
2022-03-10,Dynamics of the collapse of a ferromagnetic skyrmion in a centrosymmetric lattice,"Time dependence of the size and chirality of a ferromagnetic skyrmion in a
Heisenberg model with the magnetic field on a square lattice has been studied
analytically and numerically. The lattice and the magnetic field generate
strong time dependence of the skyrmion chirality. Due to nonlinearity, the
lattice alone also generates strong intrinsic damping that leads to the
skyrmion collapse via the emission of spin waves. In the absence of the
magnetic field the collapse is slow for a large skyrmion but it becomes
exponentially fast in the presence of the Landau-Lifshitz damping when the
field is turned on. Magnons emitted by a collapsing skyrmion must have a
discrete spectrum due to the quantization of the skyrmion magnetic moment.",2203.05342v1
2022-03-19,Spin wave excitation and directional propagation in presence of magnetic charges in square artificial spin ice,"Artificial spin ice is a special class of engineered lattice of highly shape
anisotropic single domain magnetic nanostructures which is used as one of the
model systems to study the spin ice behavior observed in pyrochlore oxides. The
nanomagnets interact via dipolar interaction which results in correlated
magnetization dynamics exhibiting macroscopic spin configuration states. Here,
we exploit the interplay of underlying magnetic state and external bias field
orientation to study controlled spin wave propagation in square Artificial Spin
Ice (sASI) by performing detailed micromagnetic simulations. We report that
careful selection of vertices with local magnetic charges can effectively
direct the anisotropic spin wave in presence of an external field. Further, we
explore the influence of local charges due to the excited state in
even-coordinated vertices as well as uncompensated charges due to
odd-coordinated vertices on spin wave behavior. Our studies suggest that there
is no perceptible difference on spin wave dynamical behavior due to the origin
of local magnetic charge in sASI. Our results of controlled and directional
spin wave propagation in sASI system may be useful for low-power consumption
based all magnonic on-chip devices.",2203.10345v1
2022-03-24,Magnetic Excitations in Square Lattice Iridates: Contrast between Ba$_2$IrO$_4$ and Sr$_2$IrO$_4$,"We report a resonant inelastic x-ray scattering (RIXS) investigation of
ultra-thin epitaxial films of Ba$_2$IrO$_4$, and compare their low energy
magnetic and spin-orbit excitations to those of their sister compound
Sr$_2$IrO$_4$. Due to the 180$^\circ$ Ir-O-Ir bond, the bandwidth of the magnon
and spin-orbiton is significantly larger in Ba$_2$IrO$_4$, making it difficult
to describe these two types of excitations as separate well-defined
quasiparticles. Both types of excitations are found to be quite sensitive to
the effect of epitaxial strain. In addition, we find that the d-level inversion
observed in Sr$_2$IrO$_4$ is absent in Ba$_2$IrO$_4$, as predicted in recent
theoretical studies. Our results illustrate that the magnetic properties of
Ba$_2$IrO$_4$ are substantially different from those of Sr$_2$IrO$_4$,
suggesting that these materials need to be examined more carefully with
electron itinerancy taken into account.",2203.13102v1
2022-04-07,Domain wall formation and magnon localization in twisted chromium trihalides,"The rise of twistronics has revolutionized the field of condensed matter
physics, and more specifically the future applications of two-dimensional
materials. At small twist angles, the microscopic world becomes strongly
correlated, and unexpected physical phenomena such as superconductivity emerge.
For magnetic layers, stacking plays a crucial role in the magnetic exchange
coupling between the layers leading to non-trivial spin configurations and flat
spin-wave dispersion when twisted. In this work, we give a short overview of
the most recent theoretical and experimental works reporting the effect of
twist angles on two-dimensional magnets. Besides, we discuss the effect of the
twist angle and the local antiferromagnetic interlayer exchange coupling on the
formation of antiferromagnetic domains in chromium trihalides. Finally, we show
some preliminary results on the effect of the stacking and the twist angle on
the spin-wave dispersion of bilayer CrI$_3$",2204.03244v1
2022-04-13,Ground States and Dynamical Properties of $S>1/2$ Quantum Heisenberg Model on the 1/5-Depleted Square Lattice,"We study the $S>1/2$ antiferromagnetic Heisenberg model on the 1/5-depleted
square lattice as a function of the ratio of the intra-plaquette coupling to
the inter-plaquette coupling. Using stochastic series expansion quantum Monte
Carlo simulations, we numerically identify three quantum phases, including the
dimer phase, N\'eel phase and plaquette valence bond solid phase. We also
obtain the accurate quantum critical points that belong to the O(3)
universality class using the large-scale finite-size scaling. Most importantly,
we study the dynamic spin structure factors of different phases, which can be
measured by inelastic neutron scattering experiments. The low-energy
excitations can be explained as triplons in the dimer phase and plaquette
valence bond solid phase. While in the N\'eel phase, the more prominent magnon
mode can be found as the spin magnitude increases. Furthermore, we find a
broader continuum at smaller $S$, which may be the dynamical signature of
nearly deconfined spinon excitations.",2204.06573v1
2022-04-26,"Nonreciprocal electrical transport in multiferroic semiconductor (Ge,Mn)Te","We have investigated the nonreciprocal electrical transport, that is a
nonlinear resistance effect depending on the current direction, in multiferroic
Rashba semiconductor (Ge,Mn)Te. Due to coexistence of ferromagnetic and
ferroelectric orders, (Ge,Mn)Te provides a unique platform for exploring the
nonreciprocal electrical transport in a bulk form. (Ge,Mn)Te thin films shows a
large nonreciprocal resistance compared to GeTe, the nonmagnetic counterpart
with the same crystal structure. The magnetic-field-angle dependence of the
nonreciprocal resistance is maximized when magnetic field is orthogonal to both
current and electric polarization, in accord with the symmetry argument. From
the analysis of temperature and magnetic field dependence, we deduce that
inelastic scatterings of electrons mediated by magnons dominantly contribute to
the observed nonreciprocal response. Furthermore, the nonreciprocal resistance
is significantly enhanced by lowering hole density. The Fermi level dependence
is attributed to the deformation of the Rashba band in which the spin-momentum
locked single Fermi surface appears by exchange field from the in-plane
magnetization. The present study provides a key insight to the mechanisms of
novel transport phenomena caused by the interplay of ferroelectric and
ferromagnetic orders in a semiconductor.",2204.12427v1
2022-04-27,Refining magnetic interactions from the magnetic field dependence of spin-wave excitations in magnetoelectric LiFePO$_4$,"We investigated the spin excitations of magnetoelectric $\text{LiFePO}_4$ by
THz absorption spectroscopy in magnetic fields up to 33$\,$T. By studying their
selection rules, we found not only magnetic-dipole, but also electric-dipole
active (electromagnons) and magnetoelectric resonances. The magnetic field
dependence of four strong low-energy modes is reproduced well by our
four-sublattice spin model for fields applied along the three orthorhombic
axes. From the fit, we refined the exchange couplings, single-ion anisotropies,
and the Dzyaloshinskii-Moriya interaction parameters. Additional spin
excitations not described by the mean-field model are observed at higher
frequencies. Some of them shows a strong shift with magnetic field, up to
4$\,\text{cm}^{-1}/\text{T}$, when the field is applied along the easy axis.
Based on this field dependence, we attribute these high frequency resonances to
excitation of higher spin multipoles and of two magnons, which become
THz-active due to the low symmetry of the magnetically ordered state.",2204.12967v1
2022-05-24,Steady entangled-state generation via cross-Kerr effect in a ferrimagnetic crystal,"For solid-state spin systems, the collective spin motion in a single crystal
embodies multiple magnetostatic modes. Recently, it was found that the
cross-Kerr interaction between the higher-order magnetostatic mode and the
Kittel mode introduces a new operable degree of freedom. In this work, we
propose a scheme to entangle two magnon modes via the cross-Kerr nonlinearity
when the bias field is inhomogeneous and the system is driven. Quantum
entanglement persists at the steady state, as demonstrated by numerical results
using experimentally feasible parameters. Furthermore, we also demonstrate that
entangled states can survive better in the system where self-Kerr and
cross-Kerr nonlinearities coexist. Our work provides insights and guidance for
designing experiments to observe entanglement between different degrees of
freedom within a single ferrimagnetic crystal. Additionally, it may stimulate
potential applications in quantum information processing using spintronic
devices.",2205.11865v1
2022-05-27,Generation of long-lived $W$ states via reservoir engineering in dissipatively coupled systems,"Very recently, dissipative coupling was discovered, which develops and
broadens methods for controlling and utilizing light-matter interactions. Here,
we propose a scheme to generate the tripartite $W$ state in a dissipatively
coupled system, where one qubit and two resonators simultaneously interact with
a common reservoir. With appropriate parameters, we find the $W$ state is a
dark state of the system. By driving the qubit, the dissipatively coupled
system will evolve from the ground state to the tripartite $W$ state. Because
the initial state is the ground state of the system and no measurement is
required, our scheme is easy to implement in experiments. Moreover, the $W$
state decouples from the common reservoir and thus has a very long lifetime.
This scheme is applicable to a wide class of dissipatively coupled systems, and
we specifically illustrate how to prepare the $W$ state in a hybrid
qubit-photon-magnon system by using this scheme.",2205.13920v2
2022-06-01,Exceptional spin wave dynamics in an antiferromagnetic honeycomb lattice,"We theoretically investigate possible effects of electric current on the spin
wave dynamics for the N\'{e}el-type antiferromagnetic order in a honeycomb
lattice. Based on a general vector decomposition of the spin polarization of
conduction electrons, we find that there can exist reciprocal and nonreciprocal
terms in the current-induced torque acting on the local spins in the system.
Furthermore, we show that the reciprocal terms will cause the spin wave Doppler
effect, while the nonreciprocal terms can induce rich non-Hermitian topological
phenomena in the spin wave dynamics, including exceptional points, bulk Fermi
arc, non-Hermitian skin effect, etc. Our results indicate the capability to
manipulate non-Hermitian magnons in magnetic materials by electric current,
which could be important for both fundamental physics and technology
applications.",2206.00723v1
2022-06-03,Quasi-2D anomalous Hall Mott insulator of topologically engineered Jeff =1/2 electrons,"We investigate an experimental toy-model system of a pseudospin-half
square-lattice Hubbard Hamiltonian in [(SrIrO3)1/(CaTiO3)1] to include both
nontrivial complex hopping and moderate electronic correlation. While the
former induces electronic Berry phases as anticipated from the weak-coupling
limit, the later stabilizes an antiferromagnetic (AFM) Mott insulator ground
state in analogous to the strong-coupling limit. Their combined results in the
real system are found to be an anomalous Hall effect with a non-monotonic
temperature dependence due to the self-competition of the electron-hole pairing
in the Mott state, and an exceptionally large Ising anisotropy that is captured
as a giant magnon gap beyond the superexchange approach. The unusual phenomena
highlight the rich interplay of electronic topology and electronic correlation
in the intermediate-coupling regime that is largely unexplored and challenging
in theoretical modelling.",2206.01392v1
2022-06-05,Strain tuning the magnetic and transport properties of Mn$_3$Ge,"The kagome lattice antiferromagnet Mn$_3$Ge has a local hexagonal symmetry
with a 120$^\circ$ ordered ground state. This non-colinear ground state
engenders a strong anomalous Hall response. The main goal of this work is to
understand the effect of strain on this response. We derive an effective model
for the Hall vector which functions as our order parameter. Using this model we
show how both intra and inter planar strains can be used to switch the sign of
the Hall response at a constant magnetic field. Further, we also investigate
the effect of this strain on the spin wave band gaps and show that strain can
be used to effectively manipulate them, which can be used to tune the magnon
responses in this system.",2206.02219v2
2022-06-17,Melting of magnetization plateaus for kagome and square-kagome lattice antiferromagnets,"Unconventional features of the magnetization curve at zero temperature such
as plateaus or jumps are a hallmark of frustrated spin systems. Very little is
known about their behavior at non-zero temperatures. Here we investigate the
temperature dependence of the magnetization curve of the kagome lattice
antiferromagnet in particular at 1/3 of the saturation magnetization for large
lattice sizes of up to N=48 spins. We discuss the phenomenon of asymmetric
melting and trace it back to a combined effect of unbalanced magnetization
steps on either side of the investigated plateau as well as on the behavior of
the density of states across the plateau. We compare our findings to the
square-kagome lattice that behaves similarly at low temperatures at zero field,
but as we will demonstrate differently at 1/3 of the saturation magnetization.
Both systems possess a flat one-magnon band and therefore share with the class
of flat-band systems the general property that the plateau that precedes the
jump to saturation melts asymmetrically but now with a minimal susceptibility
that bends towards lower fields with increasing temperature.",2206.08710v3
2022-07-01,Texture fluctuations and emergent dynamics in coupled nanomagnets,"We analyse the thermal fluctuations of magnetization textures in two stray
field coupled elements, forming mesospins. To this end, the energy landscape
associated with the thermal dynamics of the textures is mapped out and
asymmetric energy barriers are identified. These barriers are modified by
changing the gap that separates the mesospins. Moreover, the coupling between
the edges leads to an anisotropy in the curvature of the energy surface at the
metastable minima. This yields a dynamic mode splitting of the edge modes and
affects the attempt switching frequencies. Thus, we elucidate the mechanism
with which the magnons in the thermal bath generate the stochastic fluctuations
of the magnetization at the edges.",2207.00395v3
2022-07-03,Deeply nonlinear excitation of self-normalised exchange spin waves,"Spin waves are ideal candidates for wave-based computing, but the
construction of magnetic circuits is blocked by a lack of an efficient
mechanism to excite long-running exchange spin waves with normalised
amplitudes. Here, we solve the challenge by exploiting the deeply nonlinear
phenomena of forward-volume spin waves in 200 nm wide nanoscale waveguides and
validate our concept with microfocused Brillouin light scattering spectroscopy.
An unprecedented nonlinear frequency shift of >2 GHz is achieved, corresponding
to a magnetisation precession angle of 55{\deg} and enabling the excitation of
exchange spin waves with a wavelength of down to ten nanometres with an
efficiency of >80%. The amplitude of the excited spin waves is constant and
independent of the input microwave power due to the self-locking nonlinear
shift, enabling robust adjustment of the spin wave amplitudes in future on-chip
magnonic integrated circuits.",2207.01121v1
2022-07-06,Filtering and imaging of frequency-degenerate spin waves using nanopositioning of a single-spin sensor,"Nitrogen-vacancy (NV) magnetometry is a new technique for imaging spin waves
in magnetic materials. It detects spin waves by their microwave magnetic stray
fields, which decay evanescently on the scale of the spin-wavelength. Here, we
use nanoscale control of a single-NV sensor as a wavelength filter to
characterize frequency-degenerate spin waves excited by a microstrip in a
thin-film magnetic insulator. With the NV-probe in contact with the magnet, we
observe an incoherent mixture of thermal and microwave-driven spin waves. By
retracting the tip, we progressively suppress the small-wavelength modes until
a single coherent mode emerges from the mixture. In-contact scans at low drive
power surprisingly show occupation of the entire iso-frequency contour of the
two-dimensional spin-wave dispersion despite our one-dimensional microstrip
geometry. Our distance-tunable filter sheds light on the spin-wave band
occupation under microwave excitation and opens opportunities for imaging
magnon condensates and other coherent spin-wave modes.",2207.02798v1
2022-07-12,Multitude of exceptional points in van der Waals magnets,"Several works have recently addressed the emergence of exceptional points
(EPs), i.e., spectral singularities of non-Hermitian Hamiltonians, in the
long-wavelength dynamics of coupled magnetic systems. Here, by focusing on the
driven magnetization dynamics of a van der Waals ferromagnetic bilayer, we show
that exceptional points can appear over extended portions of the first
Brillouin zone as well. Furthermore, we demonstrate that the effective
non-Hermitian magnon Hamiltonian, whose eigenvalues are purely real or come in
complex-conjugate pairs, respects an unusual wavevector-dependent
pseudo-Hermiticity. Finally, for both armchair and zigzag nanoribbon
geometries, we discuss both the complex and purely real spectra of the
topological edge states and their experimental implications.",2207.05326v3
2022-08-03,Ultrafast Demagnetization Dynamics Due to Electron-Electron Scattering and Its Relation to Momentum Relaxation in Ferromagnets,"We analyze theoretically the demagnetization dynamics in a ferromagnetic
model system due to the interplay of spin-orbit coupling and electron-electron
Coulomb scattering. We compute the $k$-resolved electronic reduced spin-density
matrix including precessional dynamics around internal spin-orbit and exchange
fields as well as the electron-electron Coulomb scattering for densities and
spin coherences. Based on a comparison with numerical solutions of the full
Boltzmann scattering integrals, we establish that the $k$-resolved reduced
spin-density matrix dynamics are well described using a simpler generalized
relaxation-time ansatz for the reduced spin-density matrix. This ansatz allows
one to relate the complicated scattering dynamics underlying the
demagnetization dynamics to a physically meaningful momentum relaxation time
$\tau$. Our approach reproduces the behaviors of the demagnetization time
$\tau_{m} \propto 1/\tau$ and $\tau_{m} \propto \tau$ for the limits of short
and long $\tau$, respectively, and is also valid for the intermediate regime.
The ansatz thus provides a tool to include the correct demagnetization behavior
in approaches that treat other contributions to the magnetization dynamics such
as transport or magnon/phonon dynamics.",2208.02356v1
2022-07-04,"Magnetic levitation within a microwave cavity: characterization, challenges, and possibilities","The low energy losses in the superconducting magnetic levitation make it
attractive for exciting applications in physics. Recently, superconducting
magnetic levitation has been realized as novel mechanical transduction for the
individual spin qubit in the nitrogen-vacancy center [1]. Furthermore, the
Meissner has been proposed for the study of modified gravitational wave
detection [2]. Meissner levitation within the microwave cavity could open
avenues for the novel cavity optomechanical system, readout for quantum object
such as the transmon, and magnon, gravitational wave detection, and
magnetomechanics [3]. This work characterized magnetic levitation within a
microwave. It also discusses possibilities, challenges, and room temperature
and cryogenic experiments of the cavity-magnet system.",2208.03220v1
2022-08-11,Magnetic Excitations in Strained Infinite-layer Nickelate PrNiO2,"Strongly correlated materials often respond sensitively to the external
perturbations. In the recently discovered superconducting infinite-layer
nickelates, the superconducting transition temperature can be dramatically
enhanced via only ~1% compressive strain-tuning enabled by substrate design.
However, the root of such enhancement remains elusive. While the
superconducting pairing mechanism is still not settled, magnetic Cooper pairing
- similar to the cuprates has been proposed. Using resonant inelastic x-ray
scattering, we investigate the magnetic excitations in infinite-layer PrNiO2
thin films for different strain conditions. The magnon bandwidth of PrNiO2
shows only marginal response to strain-tuning, in sharp contrast to the
striking enhancement of the superconducting transition temperature Tc in the
doped superconducting samples. These results suggest the enhancement of Tc is
not mediated by spin excitations and thus provide important empirics for the
understanding of superconductivity in infinite-layer nickelates.",2208.05614v1
2022-08-15,Tunable Strong Magnetic Anisotropy in Two-Dimensional van der Waals Antiferromagnets,"We show that anisotropic energy of a 2D antiferromagnet is greatly enhanced
via stacking on a magnetic substrate layer, arising from the
sublattice-dependent interlayer magnetic interaction that defines an effective
anisotropic energy. Interestingly, this effective energy couples strongly with
the interlayer stacking order and the magnetic order of the substrate layer,
providing unique mechanical and magnetic means to control the antiferromagnetic
order. These two types of control methods affect distinctly the sublattice
magnetization dynamics, with a change of the ratio of sublattice precession
amplitudes in the former and its chirality in the later. In moir\'{e}
superlattices formed by a relative twist or strain between the layers, the
coupling with stacking order introduces a landscape of effective anisotropic
energy across the moir\'{e}, which can be utilized to create nonuniform
antiferromagnetic textures featuring periodically localized low-energy magnons.",2208.06958v1
2022-08-17,A Green's function method for the two-dimensional frustrated spin-1/2 Heisenberg magnetic lattice,"The magnon Hedin's equations are derived via the Schwinger functional
derivative technique, and the resulting self-consistent Green's function method
is used to calculate ground state spin patterns and magnetic structure factors
for 2-dimensional magnetic systems with frustrated spin-1/2 Heisenberg exchange
coupling. Compared to random-phase approximation treatments, the inclusion of a
self-energy correction improves the accuracy in the case of scalar product
interactions, as shown by comparisons between our method and exact benchmarks
in homogeneous and inhomogeneous finite systems. We also find that for
cross-product interactions (e.g. antisymmetric exchange), the method does not
perform equally well, and an inclusion of higher corrections is in order. Aside
from indications for future work, our results clearly indicate that the Green's
function method in the form proposed here already shows potential advantages in
the description of systems with a large number of atoms as well as long-range
interactions.",2208.08359v1
2022-08-19,The $S=1$ dimer system K$_2$Ni(MoO$_4$)$_2$: a candidate for magnon Bose-Einstein condensation,"Dimerized quantum magnets provide a unique possibility to investigate
Bose-Einstein condensation of magnetic excitations in crystalline systems at
low temperature. Here, we model the low-temperature magnetic properties of the
recently synthesized spin $S=1$ dimer system K${}_2$Ni(MoO${}_4$)$_2$ and
propose it as a new candidate material for triplon and quintuplon condensation.
Based on a first principles analysis of its electronic structure, we derive an
effective spin-dimer model that we first solve within a mean-field
approximation to refine its parameters in comparison to experiment. Finally,
the model is solved by employing a numerically exact quantum Monte Carlo
technique which leads to magnetic properties in good agreement with
experimental magnetization and thermodynamic results. We discuss the emergent
spin model of K${}_2$Ni(MoO${}_4$)$_2$ in view of condensation of magnetic
excitations in a broad parameter regime. Finally, we comment on a geometrical
peculiarity of the proposed model and discuss how it could host a supersolid
phase upon structural distortions.",2208.09246v1
2022-08-29,Determining Kitaev interaction in spin-$S$ honeycomb Mott insulators,"The Kitaev interaction in a honeycomb lattice with higher-spin $S$ has been
one of the central attractions, as it may offer quantum spin liquids. A
microscopic theory showed that when the Hund's coupling at the transition metal
generates $S> \frac{1}{2}$, the spin-orbit coupling at the heavy ligands
provides a route to the Kitaev interaction. However, there have been debates
over its strength compared to other symmetry-allowed interactions.
Investigating the symmetry of the Hamiltonian for general $S$, we show the
magnon energies at two momentum points related by a broken mirror symmetry
reflect the Kitaev interaction when a magnetic field is in the mirror plane.
Applying the symmetry analysis to CrI$_3$ with $S=\frac{3}{2}$ together with
the available angle-dependent ferromagnetic resonance data, we estimate the
Kitaev interaction out of the full Hamiltonian and find that it is
sub-dominant. Our theory can be tested by inelastic neutron scattering on
candidate materials under the proposed magnetic field direction, which will
advance the search for general $S$ Kitaev materials.",2208.13807v2
2022-08-30,Spin-polarized hot electron transport versus spin pumping mediated by local heating,"A `toy model' - aimed at capturing the essential physics - is presented that
jointly describes spin-polarized hot electron transport and spin pumping driven
by local heating. These two processes both contribute to spin-current
generation in laser-excited magnetic heterostructures. The model is used to
compare the two contributions directly. The spin-polarized hot electron current
is modeled as one generation of hot electrons with a spin-dependent excitation
and relaxation scheme. Upon decay, the excess energy of the hot electrons is
transferred to a thermalized electron bath. The elevated electron temperature
leads to an increased rate of electron-magnon scattering processes and yields a
local accumulation of spin. This process is dubbed as spin pumping by local
heating. The built-up spin accumulation is effectively driven out of the
ferromagnetic system by (interfacial) electron transport. Within our model, the
injected spin current is dominated by the contribution resulting from spin
pumping, while the hot electron spin current remains relatively small. We
derive that this observation is related to the ratio between the Fermi
temperature and Curie temperature, and we show what other fundamental
parameters play a role.",2208.14342v1
2022-08-31,Frequency splitting of chiral phonons from broken time reversal symmetry in CrI$_3$,"Conventional approaches for lattice dynamics based on static interatomic
forces do not fully account for the effects of time-reversal-symmetry breaking
in magnetic systems. Recent approaches to rectify this involve incorporating
the first-order change in forces with atomic velocities under the assumption of
adiabatic separation of electronic and nuclear degrees of freedom. In this
work, we develop a first-principles method to calculate this velocity-force
coupling in extended solids, and show via the example of ferromagnetic CrI$_3$
that, due to the slow dynamics of the spins in the system, the assumption of
adiabatic separation can result in large errors for splittings of zone-center
chiral modes. We demonstrate that an accurate description of the lattice
dynamics requires treating magnons and phonons on the same footing.",2208.14593v1
2022-08-31,Small-angle neutron scattering in fully polarized phase of non-collinear magnets with interfacial-like DMI,"Spin waves in non-collinear magnets with $C_{nv}$ symmetry are discussed in
the context of inelastic neutron scattering in small-angle scattering geometry.
In the framework of a minimal model including exchange coupling,
Dzayloshinskii-Moriya interaction (DMI), and single-ion anisotropy, we consider
the system properties at moderate external magnetic fields and in the
high-field fully polarized phase. In the latter case, the magnon spectrum is
gapped and non-reciprocal due to DMI with the minimum perpendicular to the
field direction for the in-plane field and is ferromagnetic-like for the field
along the high-symmetry axis. Inelastic small-angle neutron scattering in the
fully polarized phase (SWSANS) is considered in four different geometries. It
is shown that analysis of the field dependence of the SWSANS cutoff feature in
these geometries allows determining all important parameters of the model. The
possibility of the proposed method utilization in thin films with interfacial
DMI is also discussed.",2208.14782v2
2022-09-07,Dynamical Signatures of Symmetry Broken and Liquid Phases in an $S=1/2$ Heisenberg Antiferromagnet on the Triangular Lattice,"We present the dynamical spin structure factor of the antiferromagnetic
spin-$\frac{1}{2}$ $J_1-J_2$ Heisenberg model on a triangular lattice obtained
from large-scale matrix-product state simulations. The high frustration due to
the combination of antiferromagnetic nearest and next-to-nearest neighbour
interactions yields a rich phase diagram. We resolve the low-energy excitations
both in the $120^{\circ}$-ordered phase and in the putative spin liquid phase
at $J_2/J_1 = 0.125$. In the ordered phase, we observe an avoided decay of the
lowest magnon-branch, demonstrating the robustness of this phenomenon in the
presence of gapless excitations. Our findings in the spin-liquid phase chime
with the field-theoretical predictions for a gapless Dirac spin liquid, in
particular the picture of low-lying monopole excitations at the corners of the
Brillouin zone. We comment on possible practical difficulties of distinguishing
proximate liquid and solid phases based on the dynamical structure factor.",2209.03344v2
2022-09-09,Control of magnetic states and spin interactions in bilayer CrCl$_{3}$ with strain and electric fields: an ab initio study,"Using ab initio density functional theory (DFT), we demonstrated the
possibility of controlling the magnetic ground-state properties of bilayer
CrCl$_{3}$ by means of mechanical strains and electric fields. In principle, we
investigated the influence of these two fields on parameters describing the
spin Hamiltonian of the system. The obtained results show that biaxial strains
change the magnetic ground state between ferromagnetic (FM) and
antiferromagnetic (AFM) phases. The mechanical strain also affects the
direction and amplitude of the uniaxial magnetic anisotropy (MAE). Importantly,
the direction and amplitude of the Dzyaloshinskii-Moriya vectors are also
highly tunable under external strain and electric fields. The competition
between nearest neighbor interaction, MAE, and Dzyaloshinskii-Moriya
interactions can lead to the stabilization of various exotic spin textures and
novel magnetic excitations. The high tunability of magnetic properties by
external fields makes bilayer CrCl$_{3}$ a promising candidate for application
in the emerging field of two-dimensional quantum spintronics and magnonics.",2209.04527v2
2022-09-12,State Space Geometry of the Spin-1 Antiferromagnetic Heisenberg Chain,"We study the phase diagram of the spin-1 antiferromagnetic Heisenberg chain
with uniaxial anisotropy and applied magnetic field in terms of the genuine
multipartite entanglement as witnessed by the mean quantum Fisher information
density. By generalizing the manifold studied in [1, 2] to the many body case
for spin 1, we connect the state space curvature in the vicinity of the ground
state of the Heisenberg chain to the genuine multipartite entanglement. Our
analysis demonstrates that the quantum critical points and symmetry protected
topological (SPT) phase exhibit large state space curvature, while the
separable phases are completely flat, offering insight into the physical
interpretation of state space curvature. We further show that the entanglement
in the SPT phase is enhanced by the presence of uniaxial anisotropy, and
undiminished in the presence of uniform magnetic fields. The magnon condensate
phase induced by large fields is shown to emanate from the Gaussian critical
point, and exhibits massive multipartite entanglement over a robust region of
the parameter space.",2209.05005v4
2022-09-21,Interactive Entanglement in Hybrid Opto-magno-mechanics System,"We present a novel cavity opto-magno-mechanical hybrid system to generate
entanglements among multiple quantum carriers, such as magnons, mechanical
resonators, and cavity photons in both the optical and microwave domains. Two
Yttrium iron garnet (YIG) spheres are embedded in two separate microwave
cavities which are joined by a communal mechanical resonator. Because the
microwave cavities are separate, the ferromagnetic resonate frequencies of two
YIG spheres can be tuned independently, as well as the cavity frequencies. We
show that entanglement can be achieved with experimentally reachable
parameters. The entanglement is robust against environmental thermal noise,
owing to the mechanical cooling process achieved by the optical cavity. The
maximum entanglement among different carriers is achieved by optimizing the
parameters of the system. The individual tunability of the separated cavities
allows us to independently control the entanglement properties of different
subsystems and establish quantum channels with different entanglement
properties in one system. This work could provide promising applications in
quantum metrology and quantum information tasks.",2209.10120v2
2022-09-26,Scanning tunneling spectroscopy of Majorana zero modes in a Kitaev spin liquid,"We describe scanning tunneling spectroscopic signatures of Majorana zero
modes (MZMs) in Kitaev spin liquids. The tunnel conductance is determined by
the dynamical spin correlations of the spin liquid, which we compute exactly,
and by spin-anisotropic cotunneling form factors. Near a $\mathbb{Z}_2$ vortex,
the tunnel conductance has a staircase voltage dependence, where conductance
steps arise from MZMs and (at higher voltages) from additional vortex
configurations. By scanning the probe tip position, one can detect the vortex
locations. Our analysis suggests that topological magnon bound states near
defects or magnetic impurities generate spectroscopic signatures that are
qualitatively different from those of MZMs.",2209.12930v2
2022-09-30,Perspectives on Antiferromagnetic Spintronics,"Although the development of spintronic devices has advanced significantly
over the past decade with the use of ferromagnetic materials, the extensive
implementation of such devices has been limited by the notable drawbacks of
these materials. Antiferromagnets claim to resolve many of these shortcomings
leading to faster, smaller, more energy-efficient, and more robust electronics.
Antiferromagnets exhibit many desirable properties including zero net
magnetization, imperviousness to external magnetic fields, intrinsic
high-frequency dynamics with a characteristic precession frequency on the order
of terahertz (THz), and the ability to serve as passive exchange-bias materials
in multiple magnetoresistance (MR)- based devices. In this Perspective article,
we will discuss the fundamental physics of magnetic structures in
antiferromagnets and their interactions with external stimuli such as spin
current, voltage, and magnons. A discussion on the challenges lying ahead is
also provided along with an outlook of future research directions of these
systems.",2209.15187v2
2022-10-03,Dynamics of mixed quantum-classical spin systems,"Mixed quantum-classical spin systems have been proposed in spin chain theory
and, more recently, in magnon spintronics. However, current models of
quantum-classical dynamics beyond mean-field approximations typically suffer
from long-standing consistency issues, and, in some cases, invalidate
Heisenberg's uncertainty principle. Here, we present a fully Hamiltonian theory
of quantum-classical spin dynamics that appears to be the first to ensure an
entire series of consistency properties, including positivity of both the
classical and the quantum density, so that Heisenberg's principle is satisfied
at all times. We show how this theory may connect to recent energy-balance
considerations in measurement theory and we present its Poisson bracket
structure explicitly. After focusing on the simpler case of a classical Bloch
vector interacting with a quantum spin observable, we illustrate the extension
of the model to systems with several spins, and restore the presence of orbital
degrees of freedom.",2210.00988v2
2022-10-12,"Spin Diffusion in Spin Glasses Require Two Magnetic Variables, $\vec{M}$ and $\vec{m}$","Experiment has established that spin-glasses can support a steady-state spin
current $\vec{j}_{i}$. However, the accepted theory of spin glass dynamics
permits oscillations but no steady-state spin current. Onsager's irreversible
thermodynamics implies that the spin current is proportional to the gradient of
a magnetization. We argue, however, that the magnon distribution function
associated with the local equilibrium magnetization $\vec{M}$ cannot diffuse
because it represents $10^{23}$ variables. We therefore invoke the
non-equilibrium magnetization $\vec{m}$, which in spintronics is called the
{\it spin accumulation}. Applying the theory of irreversible thermodynamics we
indeed find that it predicts spin diffusion, and we consider other experimental
consequences of the theory, including a wavelength-dependent coupling between
the reactive and the diffusive degrees of freedom.",2210.05865v2
2022-10-12,Observation of Vector Spin Seebeck Effect in a Noncollinear Antiferromagnet,"Spintronic phenomena to date have been established in magnets with collinear
moments, where the spin injection through the spin Seebeck effect (SSE) is
always along the out-of-plane direction. Here, we report the observation of a
vector SSE in a noncollinear antiferromagnet (AF) LuFeO$_3$, where temperature
gradient along the out-of-plane and also the in-plane directions can both
inject a pure spin current and generate a voltage in the heavy metal via the
inverse spin Hall effect (ISHE). We show that the thermovoltages are due to the
magnetization from canted spins in LuFeO$_3$. Furthermore, in contrast to the
challenges of generating, manipulating and detecting spin current in collinear
AFs, the vector SSE in LuFeO$_3$ is readily viable in zero magnetic field and
can be controlled by a small magnetic field of about 150 Oe at room
temperature. The noncollinear AFs expand new realms for exploring spin
phenomena and provide a new route to low-field antiferromagnetic spin
caloritronics and magnonics.",2210.06602v1
2022-10-14,Theory of edge states in graphene-like systems,"Systems that can be described with the same mathematical models that account
for the properties of electrons in graphene are known as graphene-like systems.
These include magnons, photons, polaritons, acoustic waves, and electrons in
honeycomb lattices, either natural or artificial. All of them feature an
outstanding property, the existence of states localized at the edges. We
distinguish two classes of edge states depending on whether or not a
topological band gap is present in the two-dimensional energy bands. We
introduce the theory of edge states in terms of tight-binding models and
discuss their properties, with an emphasis on the interplay between their
one-way character and the topological nature of the bulk phase.",2210.07568v1
2022-10-21,Optical spin-wave detection beyond the diffraction limit,"Spin waves are proposed as information carriers for next-generation computing
devices because of their low power consumption. Moreover, their wave-like
nature allows for novel computing paradigms. Conventional methods to detect
spin waves are based either on electrical induction, limiting the downscaling
and efficiency complicating eventual implementation, or on light scattering,
where the minimum detectable spin-wave wavelength is set by the wavelength of
the laser. In this Article we demonstrate magneto-optical detection of spin
waves beyond the diffraction limit using a metallic grating that selectively
absorbs laser light. Specifically, we demonstrate the detection of propagating
spin waves with a wavelength of 700 nm using a diffraction-limited laser spot
with a size of 10 $\mu$m in 20 nm thick Py strips. Additionally, we show that
this grating is selective to the wavelength of the spin wave, providing
wavevector-selective spin-wave detection. This should open up new avenues
towards the integration of the burgeoning fields of photonics and magnonics,
and aid in the optical detection of spin waves in the short-wavelength exchange
regime for fundamental research.",2210.12016v1
2022-11-02,Spin wave excitations in a nanowire spin-torque oscillator with perpendicular magnetic anisotropy,"Spin torque oscillators (STOs) are emerging microwave devices that can
potentially be used in spin-logic devices and the next-generation high-speed
computing architecture. Thanks to their non-linear nature, STOs are easily
tunable by the magnetic field and the dc current. Spin Hall nano-oscillators
(SHNOs) are promising types of STOs and most of the current studies focus on
localized modes that can be easily excited. Here, we study using micromagnetic
simulations, the nature of the spin-torque-induced excitations in nanowire
devices made of perpendicular magnetic anisotropy (PMA) material. Our results
showed that upon including PMA the excitation of localized and propagating spin
wave modes is feasible. We study the nature of the mode excitations as a
function of the PMA strength (\text{K}$_u$), and the current. Indeed, we
estimate a critical value of \text{K}$_u$ to allow for the excitation of the
propagating spin wave. We attribute this mode selectivity between localized and
propagating modes to a switch in the sign of the nonlinearity of the system
from negative to positive at a non-zero \text{K}$_u$ which is supported by
analytical calculations. Our results provide deep insight into engineering
reconfigurable microwave devices for future magnonic and computational
applications.",2211.01078v1
2022-11-09,Topological Superconductivity Mediated by Skyrmionic Magnons,"Topological superconductors are associated with the appearance of Majorana
bound states, with promising applications in topologically protected quantum
computing. In this Letter, we study a system where a skyrmion crystal is
interfaced with a normal metal. Through interfacial exchange coupling, spin
fluctuations in the skyrmion crystal mediate an effective electron-electron
interaction in the normal metal. We study superconductivity within a
weak-coupling approach and solve gap equations both close to the critical
temperature and at zero temperature. Special features in the effective
electron-electron interaction due to the noncolinearity of the magnetic ground
state yield topological superconductivity at the interface.",2211.05129v2
2022-11-11,Chiral Phonon Mediated High-Temperature Superconductivity,"Breaking down the traditional perception on phonons which are achiral, the
recent discovery of chiral phonon carrying angular momentum provides possible
ways to couple electron, photon, spin, magnon and exciton, etc. We
theoretically proposed an electron-chiral phonon interaction with two-phonon
process, in contrast to conventional electron-phonon interaction, and a kind of
effective Hubbard U through exchanging two chiral phonons is proposed. Taking
two-dimensional diatomic honeycomb lattice as an example, we found this
repulsive Hubbard U mediated by chiral phonons induces unconventional and
high-temperature superconductivity. Moreover, the numerical calculations show
an inverse isotope effect which is consistent with experimental observations in
high-$T_c$ superconductors. Our finding on electron-chiral phonon and the
associated Cooper pair provides a path to understand the high-$T_c$
superconductivity.",2211.06185v1
2022-11-14,Spin wave diffraction model for perpendicularly magnetized films,"We present a near-field diffraction model for spin waves in perpendicularly
magnetized films applicable in any geometries of excitation fields. This model
relies on Kalinikos-Slavin formalism to express the dynamic susceptibility
tensor in k-space, and calculate the diffraction patterns via inverse
2D-Fourier transform of the response functions. We show an excellent
quantitative agreement between our model and MuMax3 micro-magnetic simulations
on two different geometries of antennas. Our method benchmarks spin wave
diffraction in perpendicularly magnetized films, and is readily applicable for
future designs of magnon beamforming and interferometric devices.",2211.07231v2
2022-11-15,Self-selecting vapor growth of transition metal halide single crystals,"Transition metal halides can host a large variety of novel phenomena, such as
magnetism in the monolayer limit, quantum spin liquid and spiral spin liquid
states, and topological magnons and phonons. Sizeable high quality single
crystals are necessary for investigations of magnetic and lattice excitations
by, for example, inelastic neutron scattering. In this paper, we review a less
well-known vapor transport technique, self-selecting vapor growth, and report
our growths of transition metal halides using this technique. We report the
growth and characterizations of sizable single crystals of RuCl$_3$, CrCl$_3$,
Ru$_{1-x}$Cr$_x$Cl$_3$, and CrBr$_3$. In order to expedite the conversion of
starting powder to single crystals, we modified the technique by cooling the
growth ampoule through an appropriate temperature range. Our work shows that
the self-selecting vapor transport technique can provide large single crystals
of transition metal halides, demonstrating its potential for providing high
quality single crystals of quantum materials.",2211.07806v1
2022-11-20,Long-distance propagation of high-velocity antiferromagnetic spin waves,"We report on coherent propagation of antiferromagnetic (AFM) spin waves over
a long distance ($\sim$10 $\mu$m) at room temperature in a canted AFM
$\alpha$-Fe$_2$O$_3$ with the Dzyaloshinskii-Moriya interaction (DMI).
Unprecedented high group velocities (up to 22.5 km/s) are characterized by
microwave transmission using all-electrical spin wave spectroscopy. We derive
analytically AFM spin-wave dispersion in the presence of the DMI which accounts
for our experimental results. The AFM spin waves excited by nanometric coplanar
waveguides with large wavevectors enter the exchange regime and follow a
quasi-linear dispersion relation. Fitting of experimental data with our
theoretical model yields an AFM exchange stiffness length of 1.7 angstrom. Our
results provide key insights on AFM spin dynamics and demonstrate high-speed
functionality for AFM magnonics.",2211.10989v1
2022-11-20,Spin-wave spectra in antidot lattice with inhomogeneous perpendicular magnetocrystalline anisotropy,"Magnonic crystals are structures with periodically varied magnetic properties
that are used to control collective spin-wave excitations. With micromagnetic
simulations, we study spin-wave spectra in a 2D antidot lattice based on a
multilayered thin film with perpendicular magnetic anisotropy (PMA). We show
that the modification of the PMA near the antidot edges introduces interesting
modifications to the spin-wave spectra, even in a fully saturated state. In
particular, the spectra split in two types of excitations, bulk modes with
amplitude concentrated in a homogeneous part of antidot lattice, and edge modes
with an amplitude localized in the rims of reduced PMA at the antidot edges.
Their dependence on the geometrical or material parameters is distinct but at
resonance conditions fulfilled, we found strong hybridization between bulk and
radial edge modes. Interestingly, the hybridization between the fundamental
modes in bulk and rim is of magnetostatic origin but the exchange interactions
determine the coupling between higher-order radial rim modes and the
fundamental bulk mode of the antidot lattice.",2211.11002v1
2022-11-25,Microscopic parameters of the van der Waals CrSBr antiferromagnet from microwave absorption experiments,"Microwave absorption experiments employing a phase-sensitive external
resistive detection are performed for a topical van der Waals antiferromagnet
CrSBr. The field dependence of two resonance modes is measured in an applied
field parallel to the three principal crystallographic directions, revealing
anisotropies and magnetic transitions in this material. To account for the
observed results, we formulate a microscopic spin model with a bi-axial
single-ion anisotropy and inter-plane exchange. Theoretical calculations give
an excellent description of full magnon spectra enabling us to precisely
determine microscopic interaction parameters for CrSBr.",2211.14117v1
2022-11-25,Asymmetric Faraday Effect in a Magnetophotonic Crystal,"It is widely known that the magneto-optical Faraday effect is linear in
magnetization and therefore the Faraday angles for the states with opposite
magnetizations are of opposite sign but equal in modulus. Here we
experimentally study propagation of light through a one-dimensional all-garnet
magnetophotonic crystal to demonstrate an asymmetric Faraday effect (AFE) for
which Faraday angles for opposite magnetic states differ not only in sign but
in the absolute value as well. AFE appears in the vicinity of the cavity
resonance for an oblique incidence of light which plane of polarization is
inclined to the incidence plane. Under proper incidence and polarization angles
the magnitude of AFE could be very large reaching 30% of the absolute value of
the Faraday effect. The effect originates from the difference in Q-factors for
p- and s- polarized cavity modes that breaks the symmetry between the two
opposite directions of polarization rotation. The discovered AFE is of prime
importance for nanoscale magnonics and optomagnetism.",2211.14355v2
2022-12-02,Cavity-mediated coupling of antiferromagnetic spin waves,"Coupling of space-separated oscillators is interesting for quantum and
communication technologies. In this work, it is shown that two
antiferromagnetic oscillators placed inside an electromagnetic cavity couple
cooperatively to its terahertz modes and, in effect, hybridized
magnon-polariton modes are formed. This is supported by a systematic study of
reflection spectra from two parallel-plane slabs of hematite
($\alpha$-Fe$_2$O$_3$), measured as a function of their temperatures and
separation distance, and modeled theoretically. The mediating cavity was formed
by the crystals themselves and the experiment was performed in a practical
distance range of a few millimetres and above room temperature. Cavity-mediated
coupling allows for engineering of complex resonators controlled by their
geometry and by sharing properties of their components.",2212.01129v1
2022-12-06,Spin wavepacket propagation in quasi-2D antiferromagnets,"Antiferromagnets are attractive platforms for the propagation of information
via spin waves, offering advantages over ferromagnets in speed of response and
immunity to external fields. A recent study of the quasi-2D antiferromagnet
CrSBr reported that spin wavepackets propagate with group velocities that are
orders of magnitude higher than expected from the magnon dispersion obtained by
inelastic neutron scattering [1,2]. Here we show that the anomalous magnitude
and anisotropy of the group velocity, v_g, are naturally explained by
considering the long-range magnetic dipole-dipole interaction. We also
demonstrate that v_g can be tuned over orders of magnitude by applying an
external magnetic field or varying the sample thickness. Beyond spin wavepacket
propagation, the dipolar interaction creates the possibility of non-equilibrium
Bose-Einstein condensation in antiferromagnets, previously thought to be a
property unique to ferromagnets.",2212.03261v1
2022-12-13,Non-local correlation effects due to virtual spin-flip processes in itinerant electron ferromagnets,"We present an ab initio method for eletcronic structure calculations, which
accounts for the interaction of electrons and magnons in ferromagnets. While it
is based on a many body perturbation theory we approximate numerically complex
quantities with quantities from time dependent density functional theory. This
results in a simple and affordable algorithm which allows us to consider more
complex materials than those usually studied in this context ($3d$
ferromagnets) while still being able to account for the non-locality of the
self energy. Furthermore, our approach allows for a relatively simple way to
incorporate self-consistency. Our results are in a good agreement with
experimental and theoretical findings for iron and nickel. Especially the
experimental exchange splitting of nickel is predicted accurately within our
theory. Additionally, we study the halfmetallic ferromagnet NiMnSb concerning
its non-qusiparticle states appearing in the bandgap due to spin-flip
excitations.",2212.06420v3
2022-12-15,Giant spin ensembles in waveguide magnonics,"The dipole approximation is usually employed to describe light-matter
interactions under ordinary conditions. With the development of artificial
atomic systems, `giant atom' physics is possible, where the scale of atoms is
comparable to or even greater than the wavelength of the light they interact
with, and the dipole approximation is no longer valid. It reveals interesting
physics impossible in small atoms and may offer useful applications. Here, we
experimentally demonstrate the giant spin ensemble (GSE), where a ferromagnetic
spin ensemble interacts twice with the meandering waveguide, and the coupling
strength between them can be continuously tuned from finite (coupled) to zero
(decoupled) by varying the frequency. In the nested configuration, we
investigate the collective behavior of two GSEs and find extraordinary
phenomena that cannot be observed in conventional systems. Our experiment
offers a new platform for `giant atom' physics.",2212.07605v1
2022-12-24,Directionality between driven-dissipative resonators,"The notion of nonreciprocity, in essence when going forwards is different
from going backwards, emerges in all branches of physics from cosmology to
electromagnetism. Intriguingly, the breakdown of reciprocity is typically
associated with extraordinary phenomena, which may be readily capitalized on in
the design of (for example) nontrivial electromagnetic devices when Lorentz
reciprocity is broken. However, in order to enable the exploitation of
nonreciprocal-like effects in the next generation of quantum technologies,
basic quantum optical theories are required. Here we present a versatile model
describing a pair of driven-dissipative quantum resonators, where the relative
phase difference between the coherent and incoherent couplings induces an
asymmetry. The interplay between the diverse dissipative landscape - which
encompasses both intrinsic losses and dissipative couplings - and the coherent
interactions leads to some remarkable consequences including highly directional
(or even one-way) energy transport. Our work proffers the tantalizing prospect
of observing dissipation-induced quantum directionality in areas like photonics
or cavity magnonics (spin waves), which may aid the design of unconventional
nanoscopic devices.",2212.12777v1
2022-12-31,An implementation of the density functional perturbation theory in the PAW framework,"Quantifying materials' dynamical responses to external electromagnetic fields
is central to understanding their physical properties. Here we present an
implementation of the density functional perturbation theory for the
computation of linear susceptibilities using the projector augmented-wave
method. The Sternheimer equations are solved self-consistently through a nested
iterative procedure to compute the first-order wavefunctions, from which the
linear susceptibilities are obtained. As a demonstration, we compute the spin
wave spectral functions of two magnetic metals. The computed magnon spectra for
half-metallic CrO$_2$ and a Heusler intermetallic Cu$_2$MnAl show gapless
Goldstone modes when spin rotation symmetry is preserved and display reasonable
agreement with available experimental data. The Landau damping is computed to
be small in CrO$_2$, but significant in Cu$_2$MnAl producing an asymmetric
Lorentzian spectral lineshape. The access to linear susceptibilities as well as
first-order wavefunctions offers a range of novel possibilities in quantitative
understanding of materials' electronic properties from \textit{ab initio}
methods.",2301.00143v1
2023-01-03,"Uncovering the role of the light-induced reduction of the inter-atomic exchange in the ultrafast demagnetization of Fe, Co, and Ni","Time resolved measurements of the linear magneto-optical Kerr rotation reveal
that, in the {\it 3d} ferromagnets Fe, Co and Ni, the amplitude of the
demagnetization increases linearly with the fluence of the light. We
rationalize this phenomenon as being linearly driven by the increase in
temperature, the electron-phonon and electron-magnon scattering, and a
reduction of the inter-atomic exchange. The amplitude of the latter phenomenon,
which until the present study has been widely overlooked, is obtained through
\textit{ab initio} density functional theory calculations, and is argued to be
the principal source of demagnetization in Ni and Co, while still contributing
largely in Fe.",2301.01055v3
2023-01-05,Control of magnetic response in curved stripes by tailoring cross-section,"Curved magnetic architectures are key enablers of the prospective magnetic
devices with respect to size, functionality and speed. By exploring
geometry-governed magnetic interactions, curvilinear magnetism offers a number
of intriguing effects in curved magnetic wires and curved magnetic films. The
applicability of the current micromagnetic theory requires that the sample has
constant width and thickness, which does not correspond in many cases to
specificity of experimental sample preparation. Here, we put forth a
self-consistent micromagnetic framework of curvilinear magnetism of nanowires
and narrow stripes with spatially inhomogeneous cross-section. The influence of
the varying cross-section is exploited and illustrated by an example of the
simplest topological texture, which is a transversal head-to-head
(tail-to-tail) domain wall. The cross-section gradient becomes a source of
domain wall pinning which competes the curvature gradient. Eigenfrequencies of
the domain wall free oscillations at the pinning potential are determined by
both curvature and cross-section gradients. Prospects for curvilinear magnonics
and spintronics are discussed.",2301.02096v2
2023-01-06,Goldstone bosons and fluctuating hydrodynamics with dipole and momentum conservation,"We develop a Schwinger-Keldysh effective field theory describing the
hydrodynamics of a fluid with conserved charge and dipole moments, together
with conserved momentum. The resulting hydrodynamic modes are highly unusual,
including sound waves with quadratic (magnon-like) dispersion relation and
subdiffusive decay rate. Hydrodynamics itself is unstable below four spatial
dimensions. We show that the momentum density is, at leading order, the
Goldstone boson for a dipole symmetry which appears spontaneously broken at
finite charge density. Unlike an ordinary fluid, the presence or absence of
energy conservation qualitatively changes the decay rates of the hydrodynamic
modes. This effective field theory naturally couples to curved spacetime and
background gauge fields; in the flat spacetime limit, we reproduce the ""mixed
rank tensor fields"" previously coupled to fracton matter.",2301.02680v4
2023-01-16,Role of disorder and strong 5$d$ electron correlation in the electronic structure of Sr2TiIrO6,"Transport and magnetic properties along with high resolution valence band
photoemission study of disordered double perovskite Sr$_{2}$TiIrO$_{6}$ has
been investigated. Insulator to insulator transition along with a magnetic
transition concurrently occurs at 240 K. Comparison of valence band
photoemission with band structure calculations suggests that the spin orbit
coupling as well as electron correlation are necessary to capture the line
shape and width of the Ir 5$d$ band. Room temperature valence band
photoemission spectra show negligibly small intensity at Fermi energy, $E_{F}$.
Fermi cut-off is observed at low temperatures employing high resolution. The
spectral density of states at room temperature exhibits $|E-E_{F}|^{2}$ energy
dependence signifying the role of electron-electron interaction. This energy
dependence changes to $|E-E_{F}|^{3/2}$ below the magnetic transition
evidencing the role of electron-magnon coupling in magnetically ordered state.
The evolution of pseudogap ($\pm$12 meV) explains the sudden increase in
resistivity ($\rho$) below 50 K in this disordered system. The temperature
dependent spectral density of states at $E_{F}$ exhibiting $T^{1/2}$ behaviour
verifies Altshuler-Aronov theory for correlated disordered systems.",2301.06275v1
2023-01-18,"Perspectives on high-frequency nanomechanics, nanoacoustics, and nanophononics","Nanomechanics, nanoacoustics, and nanophononics refer to the engineering of
acoustic phonons and elastic waves at the nanoscale and their interactions with
other excitations such as magnons, electrons, and photons. This engineering
enables the manipulation and control of solid-state properties that depend on
the relative positions of atoms in a lattice. The access to advanced
nanofabrication and novel characterization techniques enabled a fast
development of the fields over the last decade. The applications of
nanophononics include thermal management, ultrafast data processing,
simulation, sensing, and the development of quantum technologies. In this
review, we cover some of the milestones and breakthroughs, and identify
promising pathways of these emerging fields.",2301.07451v1
2023-01-18,Cavity-controlled magneto-optical properties of a strongly coupled van der Waals magnet,"Controlling the properties of quantum materials with light is of fundamental
and technological importance. While high-power lasers may achieve this goal,
more practical strategies aim to exploit the strong coupling of light and
matter in optical cavities, which has recently been shown to affect elemental
physical phenomena, like superconductivity, phase transitions, and topological
protection. Here we report the capacity of strong light-matter coupling to
modify and control the magneto-optical properties of magnets. Tuning the
hybridization of magnetic excitons and cavity photons allows us to realize
distinct optical signatures of external magnetic fields and magnons in the
archetypal van der Waals magnetic semiconductor CrSBr. These results highlight
novel directions for cavity-controlled magneto-optics and the manipulation of
quantum material properties by strong light-matter coupling.",2301.07593v1
2023-02-02,Designing layered 2D skyrmion lattices in moiré magnetic hetero-structures,"Skyrmions are promising for the next generation of spintronic and magnonic
devices, but their zero-field stability and controlled nucleation through
chiral interactions remain challenging. In this theoretical study, we explore
the potential of moir\'e magnetic heterostructures to generate ordered skyrmion
lattices from the stacking-dependent magnetism in 2D magnets. We consider
heterostructures formed by twisting ultrathin CrBr_3 films on top of CrI_3
substrates, assuming a moderate interfacial Dzyaloshinskii-Moriya interaction.
At large moir\'e periodicity and appropriate substrate thickness, a moir\'e
skyrmion lattice emerges in the interfacial CrBr_3 layer due to the weaker
exchange interactions in CrBr_3 compared to CrI_3. This lattice is then
projected to the remaining layers of the CrBr_3 film via emergent chiral
interlayer fields. By varying the pristine stacking configurations within the
ultrathin CrBr_3 film, we realize layered ferromagnetic and antiferromagnetic
skyrmion lattices without the need for a permanent magnetic field. Our findings
suggest the possibility of creating colorful skyrmion lattices in moir\'e
magnetic heterostructures, enabling further exploration of their fundamental
properties and technological relevance.",2302.01074v2
2023-02-04,"Calculating spin-lattice interactions in ferro- and antiferromagnets: the role of symmetry, dimension and frustration","Recently, the interplay between spin and lattice degrees of freedom has
gained a lot of attention due to its importance for various fundamental
phenomena as well as for spintronic and magnonic applications. Examples are
ultrafast angular momentum transfer between the spin and lattice subsystems
during ultrafast demagnetization, frustration driven by structural distortions
in transition metal oxides, or in acoustically driven spin-wave resonances. In
this work, we provide a systematic analysis of spin-lattice interactions for
ferro- and antiferromagnetic materials and focus on the role of lattice
symmetries and dimensions, magnetic order, and the relevance of spin-lattice
interactions for angular momentum transfer as well as magnetic frustration. For
this purpose, we use a recently developed scheme which allows an efficient
calculation of spin-lattice interaction tensors from first principles. In
addition to that, we provide a more accurate and self consistent scheme to
calculate ab initio spin lattice interactions by using embedded clusters which
allows to benchmark the performance of the scheme introduced previously.",2302.02066v2
2023-02-06,Global solutions of the Landau--Lifshitz--Baryakhtar equation,"The Landau--Lifshitz--Baryakhtar (LLBar) equation is a generalisation of the
Landau--Lifshitz--Gilbert and the Landau--Lifshitz--Bloch equations which takes
into account contributions from nonlocal damping and is valid at moderate
temperature below the Curie temperature. Therefore, it is used to explain some
discrepancies between the experimental observations and the known theories in
various problems on magnonics and magnetic domain-wall dynamics. In this paper,
the existence and uniqueness of global weak, strong, and regular solutions to
LLBar equation are proven. H\""older continuity of the solution is also
discussed.",2302.02556v3
2023-02-07,Control of vortex chirality in a symmetric ferromagnetic ring using ferromagnetic nanoelement,"Controlling the vortex chirality in ferromagnetic nanodots and nanorings has
been a topic of investigation for the last few years. Many control methods have
been proposed and it has been found that the control is related to the breaking
of the circular symmetry. In this paper, we present a theoretical study
demonstrating the control of chirality in ferromagnetic nanoring without
directly breaking its symmetry, but instead by placing elongated ferromagnetic
nanoelement inside the ring, Here, the stray magnetostatic field exerted by the
asymmetrically placed nanoelement determines the movement of the domain walls
upon remagnetization of the nanoring and the resulting chirality in the
remanence. This approach allows the chirality of the vortex state to be
controlled and also promises its control in a dense array of nanorings, thus
suitable for spintronic and magnonic applications.",2302.03626v1
2023-02-08,Anisotropic skyrmion mass induced by surrounding conduction electrons: A Schwinger-Keldysh field theory approach,"The current-driven motion of magnetic skyrmions, as topologically protected
winding vector fields of local magnetization, has attracted considerable
attention due to both fundamental interest in the dynamics of topological
solitons, and potential spintronic applications. While widely-used Thiele
equation with zero skyrmion mass captures its rigid motion under the assumption
of preserved skyrmion shape, experiments on realistic skyrmions find deviation
due to inertial mass with numerous attempts to account for it by invoking
internal skyrmion dynamics, geometry of the nanowire, and interaction with
magnons or phonons. However, experimentally observed skyrmion mass is often
much larger than predicted by these theories. In this Letter, we employ
Schwinger-Keldysh field-theoretical approach to study conduction electrons
interacting with localized magnetic moments comprising a skyrmion to obtain a
stochastic differential equation of its motion. By analyzing its time-retarded
damping kernel, we extract an anisotropic mass governed by conduction electrons
and demonstrate its substantial effect on the current-driven skyrmion motion,
even in the absence of any skyrmion deformation.",2302.04220v1
2023-02-06,Chiral and flat-band magnetic cluster excitations in a ferromagnetic kagome metal,"TbMn6Sn6 is a metallic ferrimagnet that displays signatures of band topology
arising from a combination of uniaxial ferromagnetism and spin-orbit coupling
within its Mn kagome layers. Whereas the low energy magnetic excitations can be
described as collective spin waves using a local moment Heisenberg model,
sharply defined optical and flat-band collective magnon modes are not observed.
In their place, we find overdamped chiral and flat-band spin correlations that
are localized to hexagonal plaquettes within the kagome layer.",2302.04846v1
2023-02-18,Magnetoresistance signature of two-dimensional electronic states in Co$_3$Sn$_2$S$_2$,"Two-dimensional (2D) Dirac bands and flat bands are characteristics of a
kagome lattice. However, experimental studies on their electrical transport are
few, because three-dimensional (3D) bulk bands of kagome materials, consisting
of stacked 2D kagome layers, dominate the transport. We report a
magnetoresistance (MR) study of a kagome material, Co$_3$Sn$_2$S$_2$. Based on
analysis of the temperature, magnetic field, and field angle dependence of the
resistivity, we obtain a complete anatomy of MR. Besides a magnon MR, a
chirality-dependent MR, and a chiral-anomaly-induced MR, the most intriguing
feature is an orbital MR that scales only with the out-of-plane field, which
strongly indicates its 2D nature. We attribute it to the Dirac band of the
kagome lattice.",2302.09229v1
2023-02-21,Amplification and frequency conversion of spin waves using acoustic waves,"We numerically study the acoustic parametric amplification of spin waves
using surface acoustic waves (SAW) in a magnetic thin film. First, we
illustrate how the process of parametric spin-wave generation using short-waved
SAWs with a fixed frequency allows to tune frequencies of the generated spin
waves by the applied magnetic field. We further present the amplification of
microwave driven spin waves using this method. The decay length and the
amplitude of the driven spin waves can be amplified up to approximately 2.5 and
10 times compared to the reference signal, respectively. More importantly, the
proposed design can be used as a frequency converter, in which a low (high)
frequency spin-wave mode stimulates the excitation of a high (low) frequency
mode. Our results pave the way in designing highly flexible and efficient
hybrid magnonic device architectures for microwave data transport and
processing.",2302.10614v1
2023-02-21,Electron transport and scattering mechanisms in ferromagnetic monolayer Fe$_3$GeTe$_2$,"We study intrinsic charge-carrier scattering mechanisms and determine their
contribution to the transport properties of the two-dimensional ferromagnet
Fe$_3$GeTe$_2$. We use state-of-the-art first-principles calculations combined
with the model approaches to elucidate the role of the electron-phonon and
electron-magnon interactions in the electronic transport. Our findings show
that the charge carrier scattering in Fe$_3$GeTe$_2$ is dominated by the
electron-phonon interaction, while the role of magnetic excitations is
marginal. At the same time, the magnetic ordering is shown to effect
essentially on the electron-phonon coupling and its temperature dependence.
This leads to a sublinear temperature dependence of the electrical resistivity
near the Curie temperature, which is in line with experimental observations.
The room temperature resistivity is estimated to be $\sim$35 $\mu \Omega
\cdot$cm which may be considered as an intrinsic limit for monolayer
Fe$_3$GeTe$_2$.",2302.10974v1
2023-02-22,A spin-energy operator inequality for Heisenberg-coupled qubits,"We slightly strengthen an operator inequality identified by Correggi et al.
that lower bounds the energy of a Heisenberg-coupled graph of $s=1/2$ spins
using the total spin. In particular, $\Delta H \ge C \Delta\vec{S}^2$ for a
graph-dependent constant $C$, where $\Delta H$ is the energy above the ground
state and $\Delta\vec{S}^2$ is the amount by which the square of the total spin
$\vec{S} = \sum_i \vec{\sigma}_i/2$ falls below its maximum possible value. We
obtain explicit constants in the special case of a cubic lattice. We briefly
discuss the interpretation of this bound in terms of low-energy, approximately
non-interacting magnons in spin wave theory and contrast it with another
inequality found by B\""arwinkel et al.",2302.11267v1
2023-03-09,Antiferromagnetic resonances in superconductor-ferromagnet multilayers,"In this work, we study magnetization dynamics in superconductor-ferromagnet
(S-F) thin-film multilayer. Theoretical considerations supported by the
broad-band ferromagnetic resonance spectroscopy reveal development of acoustic
and optic resonance modes in S-F multilayers at significantly higher
frequencies in comparison to the Kittel mode of individual F-layers. These
modes are formed due to antiferromagnetic-like interaction between F-layers via
shared circulating superconducting currents in S-layers. The gap between
resonance modes is determined by the thickness and superconducting penetration
depth in S-layers. Overall, rich spectrum of S-F multilayers and its tunability
opens wide prospects for application of these multialyers in magnonics as well
as in various superconducting hybrid systems.",2303.05175v2
2023-03-09,Spin-order-dependent magneto-elastic coupling in two dimensional antiferromagnetic MnPSe$_3$ observed through Raman spectroscopy,"Layered antiferromagnetic materials have emerged as a novel subset of the
two-dimensional family providing a highly accessible regime with prospects for
layer-number-dependent magnetism. Furthermore, transition metal phosphorous
trichalcogenides, MPX3 (M = transition metal; X = chalcogen) provide a platform
for investigating fundamental interactions between magnetic and lattice degrees
of freedom providing new insights for developing fields of spintronics and
magnonics. Here, we use a combination of temperature dependent Raman
spectroscopy and density functional theory to explore
magnetic-ordering-dependent interactions between the manganese spin degree of
freedom and lattice vibrations of the non-magnetic sub-lattice via a
Kramers-Anderson super-exchange pathway in both bulk, and few-layer, manganese
phosphorous triselenide (MnPSe$_3$). We observe a nonlinear temperature
dependent shift of phonon modes predominantly associated with the non-magnetic
sub-lattice, revealing their non-trivial spin-phonon coupling below the
N{\'e}el temperature at 74 K, allowing us to extract mode-specific spin-phonon
coupling constants.",2303.05554v2
2023-03-10,Dynamic Exchange Coupling between Magnets Mediated by Attenuating Elastic Waves,"Coupling between spatially separated magnets can be mediated by excitations
such as photons and phonons, which can be characterized as coherent coupling
and dissipative coupling with real and imaginary coupling rate. We
theoretically predict the existence of dynamic exchange coupling in a closed
magneto-elastic system mediated by attenuating elastic waves and whose coupling
rate is complex in general, leading to alternating repulsive or attractive
spectrum depending on thickness of the elastic media. The presence of dynamic
exchange coupling and its competition with coherent coupling are numerically
verified according to the generalized Hooke's law in magneto-elastic systems.
The predicted mechanism provides a new strategy to synchronize precessing
magnets as well as other excitations over long distance and pave the way for
non-Hermitian engineering of collective modes in hybrid magnonics, phononics
and photonics.",2303.05778v1
2023-03-10,Ultrafast magnetization reversal in ferromagnetic spin-valves: an s-d model perspective,"We present an extension to simple s-d models, aiming at simulating ultrafast
magnetization dynamics and spin transport in metallic heterostructures. In
particular, we consider an alternative spin dissipation channel due to a finite
exchange splitting of the s band. From this theory, we show three different
mechanisms governing the dynamics of spin accumulation. On top of the already
widely discussed ""-dM/dt"" electron-magnon mechanism, we study the role of a
dynamic change of exchange splitting (of conduction electrons) as well as the
rotation of spins reflected at an interface with a ferromagnet. Finally, we use
the presented theory to explain the recent observation of subpicosecond
reversal of a ferromagnet in rare-earth free spin-valves. Our conclusion agrees
with the one of reference [1] favoring magnetization reversal due to the
rotation of the spin polarization of a reflected spin current.",2303.05907v1
2023-03-13,Coherent THz Spin Dynamics in Antiferromagnets Beyond the Approximation of the Néel vector,"Controlled generation of coherent spin waves with highest possible
frequencies and the shortest possible wavelengths is a cornerstone of
spintronics and magnonics. Here, using the Heisenberg antiferromagnet RbMF$_3$,
we demonstrate that laser-induced THz spin dynamics corresponding to pairs of
mutually coherent counter propagating spin waves with the wavevectors up to the
edge of the Brillouin zone cannot be understood in terms of magnetization and
antiferromagnetic (N\'eel) vectors, conventionally used to describe spin waves.
Instead, we propose to model such spin dynamics using the spin correlation
function. We derive a quantum-mechanical equation of motion for the latter and
emphasize that, unlike the magnetization and antiferromagnetic vectors the spin
correlations in antiferromagnets do not exhibit inertia.",2303.06996v4
2023-03-24,Shannon entropy in quasiparticle states of quantum chains,"In this paper, we investigate the Shannon entropy of the total system and its
subsystems, as well as the subsystem Shannon mutual information, in
quasiparticle excited states of free bosonic and fermionic chains and the
ferromagnetic phase of the spin-1/2 XXX chain. Our focus is on single-particle
and double-particle states, and we derive various analytical formulas for free
bosonic and fermionic chains in the scaling limit. These formulas are also
applicable to magnon excited states in the XXX chain under certain conditions.
We discover that, unlike entanglement entropy, Shannon entropy does not
separate when two quasiparticles have a large momentum difference. Moreover, in
the large momentum difference limit, we obtain universal results for quantum
spin chains that cannot be explained by a semiclassical picture of
quasiparticles.",2303.14132v1
2023-04-03,Spin waves in layered antiferromagnets with honeycomb structure,"We develop a description of spin waves in a $3D$ quantum $XY$ antiferromagnet
(AFM) in terms of macroscopic variables, magnetization and N\'eel vector
densities. We consider a layered AFM with spins located on the honeycomb
lattice. In the discussed system, the spectrum of spin waves consists of four
modes, all well captured by our macroscopic description. The gapless mode of
the spin waves, i.e., magnons, is described by a system of equations, which has
a structure general for the Goldstone mode in AFMs. We demonstrate that the
parameters in the spin Hamiltonian can be evaluated by fitting the experimental
data with the results obtained for the four modes using the macroscopic
variable approach. The description of AFM in terms of macroscopic variables can
be easily extended to the case when the lattice of the magnetic substance is
deformed by an external strain or acoustic wave.",2304.00700v1
2023-04-04,Unidirectional Seebeck effect from Rashba spin-orbit coupling on the subsurface of Ge(111),"A new nonlinear magnetothermal effect, namely unidirectional Seebeck
effect(USE), has been recently reported in magnetic/nonmagnetic topological
insulator (TI) heterostructure and is ascribed to the asymmetry magnon
scattering. Here, we show a new mechanism to generate USE in the Rashba two
dimensional electron gas (2DEG), in which the magnetism or magnetic order is
completely absent. It's found that the USE has a quantum origin from the
spin-momentum locking generated from Rashba spin-orbit coupling. We find that
the USE exhibits a sine dependence on the orientation of the magnetic field
with respect to the direction of the temperature gradient and is dominant from
the states above the Lifshitz point. The USE in the semiconductor Ge(111)
subsurface states has been theoretically and systematically investigated.",2304.01683v1
2023-04-04,Geometrical torque on magnetic moments coupled to a correlated antiferromagnet,"The geometrical spin torque mediates an indirect interaction of magnetic
moments, which are weakly exchange coupled to a system of itinerant electrons.
It originates from a finite spin-Berry curvature and leads to a non-Hamiltonian
magnetic-moment dynamics. We demonstrate that there is an unprecedentedly
strong geometrical spin torque in case of an electron system, where
correlations cause antiferromagnetic long-range order. The key observation is
that the anomalous torque is strongly boosted by low-energy magnon modes
emerging in the two-electron spin-excitation spectrum due to spontaneous
breaking of SU(2) spin-rotation symmetry. As long as single-electron
excitations are gapped out, the effect is largely universal, i.e., essentially
independent of the details of the electronic structure, but decisively
dependent on the lattice dimension and spatial and spin anisotropies. Analogous
to the reasoning that leads to the Mermin-Wagner theorem, there is a lower
critical dimension at and below which the spin-Berry curvature diverges.",2304.02071v1
2023-04-05,High-energy spin waves in the spin-1 square-lattice antiferromagnet La$_2$NiO$_4$,"Inelastic neutron scattering is used to study the magnetic excitations of the
$S=1$ square-lattice antiferromagnet La$_2$NiO$_4$. We find that the spin waves
cannot be described by a simple classical (harmonic) Heisenberg model with only
nearest-neighbor interactions. The spin-wave dispersion measured along the
antiferromagnetic Brillouin-zone boundary shows a minimum energy at the
$(1/2,0)$ position as is observed in some $S=1/2$ square-lattice
antiferromagnets. Thus, our results suggest that the quantum dispersion
renormalization effects or longer-range exchange interactions observed in
cuprates and other $S=1/2$ square-lattice antiferromagnets are also present in
La$_2$NiO$_4$. We also find that the overall intensity of the spin-wave
excitations is suppressed relative to linear spin-wave theory indicating that
covalency is important. Two-magnon scattering is also observed.",2304.02546v2
2023-04-06,Embedding Integrable Superspin Chain in String Theory,"Using results on topological line defects of 4D Chern-Simons theory and the
algebra/cycle homology correspondence in complex surfaces $\mathcal{S}$ with
ADE singularities, we study the graded properties of the $sl(m|n)$ chain and
its embedding in string theory. Because of the $\mathbb{Z}_{2}$-grading of $
sl(m|n)$, we show that the $\left( m+n\right) !/m!n!$ varieties of superspin
chains with underlying super geometries have different cycle homologies. We
investigate the algebraic and homological features of these integrable quantum
chains and give a link between graded 2-cycles and genus-g Rieman surfaces
$\Sigma _{g}$. Moreover, using homology language, we yield the brane
realisation of the $sl(m|n)$ chain in type IIA string and its uplift to
M-theory. Other \textrm{aspects} like graded complex surfaces with $sl(m|n)$
singularity as well as super magnons are also described.",2304.03152v1
2023-04-12,Machine-Learning Recognition of Dzyaloshinskii-Moriya Interaction from Magnetometry,"The Dzyaloshinskii-Moriya interaction (DMI), which is the antisymmetric part
of the exchange interaction between neighboring local spins, winds the spin
manifold and can stabilize non-trivial topological spin textures. Since
topology is a robust information carrier, characterization techniques that can
extract the DMI magnitude are important for the discovery and optimization of
spintronic materials. Existing experimental techniques for quantitative
determination of DMI, such as high-resolution magnetic imaging of spin textures
and measurement of magnon or transport properties, are time consuming and
require specialized instrumentation. Here we show that a convolutional neural
network can extract the DMI magnitude from minor hysteresis loops, or magnetic
""fingerprints"" of a material. These hysteresis loops are readily available by
conventional magnetometry measurements. This provides a convenient tool to
investigate topological spin textures for next-generation information
processing.",2304.05905v2
2023-04-13,Finite Temperature Dynamics of Spin Solitons with Applications in Thermocouples and Refrigerators,"The exploitation of spin Berry phases to generate emergent fields for
producing miniaturized and high-quality inductors has enjoyed considerable
popularity among proponents of quantum technologies [Nature 586, 202 (2020)}].
Inspired by this breakthrough, we extend its mechanism to spin thermoelectrics
by probing responses of ferrimagnetic domain walls (DWs) to thermal gradients.
Similarly, voltages here stem from DW-spin collective motion, in contrast to
normal electron transport phenomena. We further develop finite-temperature
dynamics to investigate thermoelectric figures of merit and attribute
corresponding quantum superiority to ultrafast spin evolution of ferrimagnetism
with tunable non-Abelian phases. We propose a more likely cause of DW motion
towards hot or cold regions (contrary to conclusions of previous reports) and
verify existence of efficient magnon-momentum transfers. These findings deepen
our understanding of heat-driven DW kinetics and suggest profitable new
directions in an emerging realm of spincaloritronics.",2304.06608v1
2023-04-26,Rapid-prototyping of microscopic thermal landscapes in Brillouin light scattering spectroscopy,"Since temperature and its spatial and temporal variations affect a wide range
of physical properties of material systems, they can be used to create
reconfigurable spatial structures of various types in physical and biological
objects. This paper presents an experimental optical setup for creating tunable
two-dimensional temperature patterns on a micrometer scale. As an example of
its practical application, we have produced temperature-induced magnetization
landscapes in ferrimagnetic yttrium iron garnet films and investigated them
using micro-focused Brillouin light scattering spectroscopy. It is shown that,
due to the temperature dependence of the magnon spectrum, temperature changes
can be visualized even for microscale thermal patterns.",2304.13658v2
2023-05-09,On spinning strings in I-brane background,"We revisit semiclassical strings, in particular we focus on rigidly rotating
strings, in the near horizon geometry of two orthogonal stacks of NS5-branes
(I-branes) using the string sigma model. We determine the conserved charges for
the probe string moving in the resulting $\mathbb{R}_t \times
S^3_{\theta_1}\times S^3_{\theta_2}$ background supported by NS-NS two-forms
and find a regularized dispersion relation for different values of integration
constants. Using configurations that move simultaneously on both spheres, we
obtain the giant magnon like dispersion relation for one particular set of
parameters, while other consistent set of values gives us a dispersion relation
reminiscent of the single spike.",2305.05164v3
2023-05-10,Symmetry and nonlinearity of spin wave resonance excited by focused surface acoustic waves,"The use of a complex ferromagnetic system to manipulate GHz surface acoustic
waves is a rich current topic under investigation, but the high-power nonlinear
regime is under-explored. We introduce focused surface acoustic waves, which
provide a way to access this regime with modest equipment. Symmetry of the
magneto-acoustic interaction can be tuned by interdigitated transducer design
which can introduce additional strain components. Here, we compare the impact
of focused acoustic waves versus standard unidirectional acoustic waves in
significantly enhancing the magnon-phonon coupling behavior. Analytical
simulation results based on modified Landau-Lifshitz-Gilbert theory show good
agreement with experimental findings. We also report nonlinear input power
dependence of the transmission through the device. This experimental
observation is supported by the micromagnetic simulation using mumax3 to model
the nonlinear dependence. These results pave the way for extending the
understanding and design of acoustic wave devices for exploration of
acoustically driven spin wave resonance physics.",2305.06259v1
2023-05-13,Suhl Instabilities in Nanoscopic Spheroids,"We simulate the magnetization dynamics of a permalloy spheroid of nanoscopic
size in zero external field, such that both dipolar and exchange interactions
are important. Low excitation power is used to obtain the frequencies and mode
patterns of many normal modes. At higher power, non-linear three and four mode
couplings between magnons carrying orbital angular momentum are observed to
give rise to Suhl instabilities. Suhl's analysis of the selection rules
governing the allowed processes is extended to initial states other than
uniform precession. These rules are studied and confirmed by the simulations.
Both down- and up-conversion are seen as well as three and four-mode processes.
General trends are inferred for preferred instabilities among those that are
allowed, although the thresholds for some instabilities appear to be very high.",2305.07986v1
2023-05-17,Quantum disordered ground state in the spin-orbit coupled Jeff = 1/2 distorted honeycomb magnet BiYbGeO5,"We delineate quantum magnetism in the strongly spin-orbit coupled, distorted
honeycomb-lattice antiferromagnet BiYbGeO$_{5}$. Our magnetization and heat
capacity measurements reveal that its low-temperature behavior is well
described by an effective $J_{\rm eff}=1/2$ Kramers doublet of Yb$^{3+}$. The
ground state is nonmagnetic with a tiny spin gap. Temperature-dependent
magnetic susceptibility, magnetization isotherm, and heat capacity could be
modeled well assuming isolated spin dimers with anisotropic exchange
interactions $J_{\rm Z} \simeq 2.6$~K and $J_{\rm XY} \simeq 1.3$~K. Heat
capacity measurements backed by muon spin relaxation suggest the absence of
magnetic long-range order down to at least 80\,mK both in zero field and in
applied fields. This sets BiYbGeO$_5$ apart from Yb$_2$Si$_2$O$_7$ with its
unusual regime of magnon Bose-Einstein condensation and suggests negligible
interdimer couplings, despite only a weak structural deformation of the
honeycomb lattice.",2305.10221v1
2023-05-18,Majorana-fermion origin of the planar thermal Hall effect in the Kitaev magnet $α$-RuCl$_3$,"The field-induced quantum disordered state of layered honeycomb magnet
$\alpha$-RuCl$_3$ is a prime candidate for Kitaev spin liquids hosting Majorana
fermions and non-Abelian anyons. Recent observations of anomalous planar
thermal Hall effect demonstrate a topological edge mode, but whether it
originates from Majorana fermions or bosonic magnons remains controversial.
Here we distinguish these origins from low-temperature measurements of
high-resolution specific heat and thermal Hall conductivity with rotating
in-plane fields. In the honeycomb bond direction, a distinct closure of the
low-energy bulk gap is observed concomitantly with the sign reversal of the
Hall effect. General discussions of topological bands show that this is the
hallmark of an angle-rotation-induced topological transition of fermions,
providing conclusive evidence for the Majorana-fermion origin of the thermal
Hall effect in $\alpha$-RuCl$_3$.",2305.10619v1
2023-05-21,Quantum estimation of tripartite coupling in Spin-Magnon-Mechanical Hybrid Systems,"Tripartite interactions play a fundamental role in the quantum information
processing and quantum technology. However, it is generally difficult to
realize strong tripartite coupling. We investigate the estimation of a
tripartite coupling strength in a hybrid setup composed of a single
nitrogen-vacancy (NV) center and a micromagnet. A time-independent parametric
drive can be utilized to increase the estimation precision of the tripartite
coupling strength. By calculating the quantum Fisher information (QFI), we can
obtain the optimal estimation precision by measuring the eigenstate of the
tripartite system. At the critical position, the QFI is divergent due to that
the preparation time of the eigenstate is divergent. When the system is
subjected to a dissipation, the QFI near the critical point of the
driven-dissipation phase transition is analytically obtained. The direct
intensity measurement is the optimal measurement near the dissipation phase
transition point. In addition, we quantify the robustness of an imperfect
measurement operator by the measurement noise susceptibility based on the error
propagation formula. We find that the direct intensity measurement is enough
robust against small measurement disturbance from a coherent drive. But it can
be disturbed by the nonlinear anti-harmonic measurement noise, especially near
the critical point.",2305.12435v1
2023-05-25,Niobium Quantum Interference Microwave Circuits with Monolithic Three-Dimensional (3D) Nanobridge Junctions,"Nonlinear microwave circuits are key elements for many groundbreaking
research directions and technologies, such as quantum computation and quantum
sensing. The majority of microwave circuits with Josephson nonlinearities to
date is based on aluminum thin films, and therefore they are severely
restricted in their operation range regarding temperatures and external
magnetic fields. Here, we present the realization of superconducting niobium
microwave resonators with integrated, three-dimensional (3D) nanobridge-based
superconducting quantum interference devices. The 3D nanobridges (constriction
weak links) are monolithically patterned into pre-fabricated microwave LC
circuits using neon ion beam milling, and the resulting quantum interference
circuits show frequency tunabilities, flux responsivities and Kerr
nonlinearities on par with comparable aluminum nanobridge devices, but with the
perspective of a much larger operation parameter regime. Our results reveal
great potential for application of these circuits in hybrid systems with e.g.
magnons and spin ensembles or in flux-mediated optomechanics.",2305.16276v1
2023-06-07,Gain assisted controllable fast light generation in cavity magnomechanics,"We study the controllable output field generation from a cavity
magnomechanical resonator system that consists of two coupled microwave
resonators. The first cavity interacts with a ferromagnetic yttrium iron garnet
(YIG) sphere providing the magnon-photon coupling. Under passive cavities
configuration, the system displays high absorption, prohibiting output
transmission even though the dispersive response is anamolous. We replace the
second passive cavity with an active one to overcome high absorption, producing
an effective gain in the system. We show that the deformation of the YIG sphere
retains the anomalous dispersion. Further, tuning the exchange interaction
strength between the two resonators leads to the system's effective gain and
dispersive response. As a result, the advancement associated with the
amplification of the probe pulse can be controlled in the close vicinity of the
magnomechanical resonance. Furthermore, we find the existence of an upper bound
for the intensity amplification and the advancement of the probe pulse that
comes from the stability condition. These findings may find potential
applications for controlling light propagation in cavity magnomechanics.",2306.04390v1
2023-06-24,Synchronization by Magnetostriction,"We show how to utilize magnetostriction to synchronize two mechanical
vibration modes in a cavity magnomechanical system. The dispersive
magnetostrictive interaction provides necessary nonlinearity required for
achieving synchronization. Strong phase correlation between two mechanical
oscillators can be established, leading to the synchronization robust against
thermal noise. We develop a theoretical framework to analyze the
synchronization by solving the constraint conditions of steady-state limit
cycles. We determine that the strong cavity-magnon linear coupling can enhance
and regulate the synchronization, which offers a new path to modulate
synchronization. The work reveals a new mechanism for achieving and modulating
synchronization and indicates that cavity magnomechanical systems can be an
ideal platform to explore rich synchronization phenomena.",2306.13942v2
2023-06-26,Quantum circuits to measure scalar spin chirality,"The scalar spin chirality is a three-body physical observable that plays an
outstanding role both in classical magnetism, characterizing non-coplanar spin
textures, and in quantum magnetism, as an order parameter for chiral spin
liquids. In quantum information, the scalar spin chirality is a witness of
genuine tripartite entanglement. Here we propose an indirect measurement
scheme, based on the Hadamard test, to estimate the scalar spin chirality for
general quantum states. We apply our method to study chirality in two types of
quantum states: generic one-magnon states of a ferromagnet, and the ground
state of a model with competing symmetric and antisymmetric exchange. We show a
single-shot determination of the scalar chirality is possible for chirality
eigenstates, via quantum phase estimation with a single auxiliary qutrit. Our
approach provides a unified theory of chirality in classical and quantum
magnetism.",2306.14804v1
2023-06-28,Dissipative Spin-wave Diode and Nonreciprocal Magnonic Amplifier,"We propose an experimentally feasible dissipative spin-wave diode comprising
two magnetic layers coupled via a non-magnetic spacer. We theoretically
demonstrate that the spacer mediates not only coherent interactions but also
dissipative coupling. Interestingly, an appropriately engineered dissipation
engenders a nonreciprocal device response, facilitating the realization of a
spin-wave diode. This diode permits wave propagation in one direction alone,
given that the coherent Dzyaloshinskii- Moriya (DM) interaction is balanced
with the dissipative coupling. The polarity of the diode is determined by the
sign of the DM interaction. Furthermore, we show that when the magnetic layers
undergo incoherent pumping, the device operates as a unidirectional spin-wave
amplifier. The amplifier gain is augmented by cascading multiple magnetic
bilayers. By extending our model to a one-dimensional ring structure, we
establish a connection between the physics of spin-wave amplification and
non-Hermitian topology. Our proposal opens up a new avenue for harnessing
inherent dissipation in spintronic applications.",2306.15916v1
2023-07-08,Enhanced Strong Coupling between Spin Ensemble and non-Hermitian Topological Edge States,"Light-matter interaction is crucial to both understanding fundamental
phenomena and developing versatile applications. Strong coupling, robustness,
and controllability are the three most important aspects in realizing
light-matter interactions. Topological and non-Hermitian photonics, have
provided frameworks for robustness and extensive control freedom, respectively.
How to engineer the properties of the edge state such as photonic density of
state, scattering parameters by using non-Hermitian engineering while ensuring
topological protection has not been fully studied. Here we construct a
parity-time-symmetric dimerized photonic lattice and generate complex-valued
edge states via spontaneous PT-symmetry breaking. The enhanced strong coupling
between the topological photonic edge mode and magnon mode in a ferromagnetic
spin ensemble is demonstrated. Our research reveals the subtle non-Hermitian
topological edge states and provides strategies for realizing and engineering
topological light-matter interactions.",2307.03944v1
2023-07-12,Microscopic origin of quantum supersonic phenomenon in one dimension,"Using the Bethe ansatz (BA) solution, we rigorously determine non-equilibrium
dynamics of quantum flutter and revival of an injected impurity with a large
initial momentum $Q$ into the one-dimensional (1D) interacting bosonic medium.
We show that two types of BA excited eigenstates drastically dominate the
oscillation nature of the quantum flutter with a periodicity which is simply
given by the charge and spin dressed energies $\varepsilon_{\rm c,s}(0)$ at
zero quasi-momentum $\tau_{\rm QF} = 2\pi/(|\varepsilon_{\rm c}(0)|-
|\varepsilon_{\rm s}(0)|)$. While we also determine quantum revival dynamics
with a larger periodicity $\tau_{L} = L/\left(v_{\rm c}(Q-k^*)-v_{\rm
s}(k^*)\right)$ than $\tau_{\rm QF}$, revealing the quantum reflection of
excitations induced by the periodic boundary conditions of a finite length $L$.
Here $v_{\rm c,s}$ are the sound velocities of charge and spin excitations,
respectively, and $k^*$ is determined by the rapidity of the impurity. Our
results reveals a microscopic origin of quantum supersonic phenomenon and shed
light on quantum magnon metrology for a measure of the gravitational force.",2307.05955v1
2023-07-18,Ultrafast spin-to-charge conversions of antiferromagnetic (111)-oriented $\mathrm{L1_2}$-$\mathrm{Mn_3Ir}$,"Antiferromagnetic $\mathrm{L1_2}$-$\mathrm{Mn_3Ir}$ combines outstanding
spin-transport properties with magnons in the terahertz (THz) frequency range.
However, the THz radiation emitted by ultrafast spin-to-charge conversion via
the inverse spin Hall effect remains unexplored. In this study, we measured the
THz emission and transmission of a permalloy/(111)-oriented
$\mathrm{L1_2}$-$\mathrm{Mn_3Ir}$ multilayer by THz time-domain spectroscopy.
The spin Hall angle was determined to be approximately constant at 0.035 within
a frequency range of 0.3-2.2 THz, in comparison with the THz spectroscopy of a
permalloy/Pt multilayer. Our results not only demonstrate the potential of
$\mathrm{L1_2}$-$\mathrm{Mn_3Ir}$ as a spintronic THz emitter but also provide
insights into the THz spin transport properties of
$\mathrm{L1_2}$-$\mathrm{Mn_3Ir}$.",2307.08928v3
2023-07-20,Robustness and eventual slow decay of bound states of interacting microwave photons in the Google Quantum AI experiment,"Integrable models are characterized by the existence of stable excitations
that can propagate indefinitely without decaying. This includes multi-magnon
bound states in the celebrated XXZ spin chain model and its integrable Floquet
counterpart. A recent Google Quantum AI experiment [A. Morvan et al., Nature
612, 240 (2022)] realizing the Floquet model demonstrated the persistence of
such collective excitations even when the integrability is broken: this
observation is at odds with the expectation of ergodic dynamics in generic
non-integrable systems. We here study the spectrum of the model realized in the
experiment using exact diagonalization and physical arguments. We find that
isolated bands corresponding to the descendants of the exact bound states of
the integrable model are clearly observable in the spectrum for a large range
of system sizes. However, our numerical analysis of the localization properties
of the eigenstates suggests that the bound states become unstable in the
thermodynamic limit. A perturbative estimate of the decay rate agrees with the
prediction of an eventual instability for large system sizes.",2307.11164v1
2023-07-24,Unidirectional spin wave emission by travelling pair of magnetic field profiles,"We demonstrate that the spin wave Cherenkov effect can be used to design the
unidirectional spin wave emitter with tunable frequency and switchable
direction of emission. In our numerical studies, we propose to use a pair of
traveling profiles of the magnetic field which generate the spin waves, for
sufficiently large velocity of their motion. In the considered system, the spin
waves of shorter (longer) wavelengths are induced at the front (back) of the
moving profiles and interfere constructively or destructively, depending on the
velocity of the profiles. Moreover, we showed that the spin waves can be
confined between the pair of traveling profiles of the magnetic field. This
work opens the perspectives for the experimental studies in hybrid
magnonic-superconducting systems where the magnetic vortices in a
superconductor can be used as moving sources of the magnetic field driving the
spin waves in the ferromagnetic subsystem.",2307.12653v2
2023-07-31,Gating ferromagnetic resonance of magnetic insulators by superconductors via modulating electric-field radiation,"We predict that ferromagnetic resonance in insulating magnetic film with
inplane magnetization radiates electric fields polarized along the
magnetization with opposite amplitudes at two sides of the magnetic insulator,
which can be modulated strongly by adjacent superconductors. With a single
superconductor adjacent to the magnetic insulator this radiated electric field
is totally reflected with a $\pi$-phase shift, which thereby vanishes at the
superconductor side and causes no influence on the ferromagnetic resonance.
When the magnetic insulator is sandwiched by two superconductors, this
reflection becomes back and forth, so the electric field exists at both
superconductors that drives the Meissner supercurrent, which in turn shifts
efficiently the ferromagnetic resonance. We predict an ultrastrong coupling
between magnons in the yttrium iron garnet and Cooper pairs in NbN with the
frequency shift achieving tens of percent of the bare ferromagnetic resonance.",2307.16472v2
2023-08-04,Experimental analysis of variability in WS$_2$-based devices for hardware security,"This work investigates the variability of tungsten disulfide (WS$_2$)-based
devices by experimental characterization in view of possible application in the
field of hardware security. To this aim, a preliminary analysis was performed
by measurements across voltages and temperatures on a set of seven
Si/SiO$_2$/WS$_2$ back-gated devices, also considering the effect of different
stabilization conditions on their conductivity. Obtained results show
appreciable variability in the conductivity, while also revealing similar
dependence on bias and temperature across tested devices. Overall, our analysis
demonstrates that WS$_2$-based devices can be potentially exploited to ensure
adequate randomness and robustness against environmental variations and then
used as building blocks for hardware security primitives.",2308.02265v1
2023-08-21,Photoinduced topological phase transitions in Kitaev-Heisenberg honeycomb ferromagnets with the Dzyaloshinskii-Moriya interaction,"We theoretically study topological properties of Floquet magnon in a
laser-irradiated Kitaev-Heisenberg honeycomb ferromagnet with the
Dzyaloshinskii-Moriya interaction by means of the Floquet-Bloch theory. It is
found that the Kitaev-Heisenberg ferromagnet can reveal two topological phases
with different Chern numbers when it is irradiated by a circular-polarized
light laser. Our results show that the topological phase of the system can be
switched from one topological phase to another one via varying the light
intensity. The intrinsic DMI plays a crucial role in the occurrence of
photoinduced topological phase transition. It is shown that the sign reversal
of the thermal hall conductivity is an important indicator on photoinduced
topological phase transitions in the Kitaev-Heisenberg honeycomb ferromagnet.",2308.11077v1
2023-08-24,Quantum interference between non-identical single particles,"Quantum interference between identical single particles reveals the intrinsic
quantum statistic nature of particles, which could not be interpreted through
classical physics. Here, we demonstrate quantum interference between
non-identical bosons using a generalized beam splitter based on a quantum
memory. The Hong-Ou-Mandel type interference between single photons and single
magnons with high visibility is demonstrated, and the crossover from the
bosonic to fermionic quantum statistics is observed by tuning the beam splitter
to be non-Hermitian. Moreover, multi-particle interference that simulates the
behavior of three fermions by three input photons is realized. Our work extends
the understanding of the quantum interference effects and demonstrates a
versatile experimental platform for studying and engineering quantum statistics
of particles.",2308.12677v1
2023-09-05,Signatures and characterization of dominating Kerr nonlinearity between two driven systems with application to a suspended magnetic beam,"We consider a model of two harmonically driven damped harmonic oscillators
that are coupled linearly and with a cross-Kerr coupling. We show how to
distinguish this combination of coupling types from the case where a coupling
of optomechanical type is present. This can be useful for the characterization
of various nonlinear systems, such as mechanical oscillators, qubits, and
hybrid systems. We then consider a hybrid system with linear and cross-Kerr
interactions and a relatively high damping in one of the modes. We derive a
quantum Hamiltonian of a doubly clamped magnetic beam, showing that the
cross-Kerr coupling is prominent there. We discuss, in the classical limit,
measurements of its linear response as well as the specific higher-harmonic
responses. These frequency-domain measurements can allow estimating the
magnitude of the cross-Kerr coupling or its magnon population.",2309.02204v2
2023-09-10,Resonant excitation of vortex gyrotropic mode via surface acoustic waves,"Finding new energy-efficient methods for exciting magnetization dynamics is
one of the key challenges in magnonics. In this work, we present an approach to
excite the gyrotropic dynamics of magnetic vortices through the phenomenon of
inverse magnetostriction, also known as the Villari effect. We develop an
analytical model based on the Thiele formalism that describes the gyrotropic
motion of the vortex core including the energy contributions due to inverse
magnetostriction. Based on this model, we predict excitations of the vortex
core resonances by surface acoustic waves whose frequency is resonant with the
frequency of the vortex core. We verify the model's prediction using
micromagnetic simulations, and show the dependence of the vortex core's
oscillation radius on the surface acoustic wave amplitude and the static bias
field. Our study contributes to the advancement of energy-efficient magnetic
excitations by relying on voltage-induced driven dynamics, which is an
alternative to conventional current-induced excitations.",2309.04987v1
2023-09-11,Probing Spin Wave Diffraction Patterns of Curved Antennas,"We report on the dependence of curvilinear shaped coplanar waveguides on the
near-field diffraction patterns of spin waves propagating in perpendicularly
magnetized thin films. Implementing the propagating spin waves spectroscopy
techniques on either concentrically or eccentrically shaped antennas, we show
how the link budget is directly affected by the spin wave interference, in good
agreement with near-field diffraction simulations. This work demonstrates the
feasibility to inductively probe a magnon interference pattern with a
resolution down to 1$\mu$m$^2$, and provides a methodology for shaping spin
wave beams from an antenna design. This methodology is successfully implemented
in the case study of a spin wave Young's interference experiment.",2309.05532v1
2023-09-12,Probing spatial variation of magnetic order in strained SrMnO$_3$ thin films using Spin Hall Magnetoresistance,"SrMnO$_{3}$ (SMO) is a magnetic insulator and predicted to exhibit a
multiferroic phase upon straining. Strained films of SMO display a wide range
of magnetic orders, ranging from G-type to C-and A-type, indicative of
competing magnetic interactions. The potential of spin Hall magnetoresistance
(SMR) is exploited as an electrical probe for detecting surface magnetic order,
to read surface magnetic moments in SMO and its spatial variation, by designing
and positioning electrodes of different sizes on the film. The findings
demonstrate antiferromagnetic domains with different magnetocrystalline
anisotropies along with a ferromagnetic order, where the magnetization arises
from double exchange mediated ferromagnetic order and canted antiferromagnetic
moments. Further, from a complete analysis of the SMR, a predominance of
antiferromagnetic domain sizes of 3.5 $\mu$m$^2$ is extracted. This work
enhances the applicability of SMR in unraveling the richness of correlation
effects in complex oxides, as manifested by the detection of coexisting and
competing ground states and lays the foundation for the study of magnon
transport for different magnetoelectric based computing applications.",2309.06279v1
2023-09-14,Exponentially Enhanced non-Hermitian Cooling,"Certain non-Hermitian systems exhibit the skin effect, whereby the
wavefunctions become exponentially localized at one edge of the system. Such
exponential amplification of wavefunction has received significant attention
due to its potential applications in e.g., classical and quantum sensing.
However, the opposite edge of the system, featured by the exponentially
suppressed wavefunctions, remains largely unexplored. Leveraging this
phenomenon, we introduce a non-Hermitian cooling mechanism, which is
fundamentally distinct from traditional refrigeration or laser cooling
techniques. Notably, non-Hermiticity will not amplify thermal excitations, but
rather redistribute them. Hence, thermal excitations can be cooled down at one
edge of the system, and the cooling effect can be exponentially enhanced by the
number of auxiliary modes, albeit with a lower bound that depends on the
dissipative interaction with the environment. Non-Hermitian cooling does not
rely on intricate properties such as exceptional points or non-trivial
topology, and it can apply to a wide range of Bosonic modes such as photons,
phonons, magnons, etc.",2309.07731v2
2023-09-18,Large Nonreciprocity of Shear-Horizontal Surface Acoustic Waves induced by Magnetoelastic Bilayers,"We report large nonreciprocity in the transmission of shear-horizontal
surface acoustic waves (SAWs) on LiTaO3 substrate coated with a FeCoSiB/NiFeCu
magnetoelastic bilayer. The large difference in saturation magnetization of the
two layers not only brings nonreciprocal spin waves (SWs), but also ensures the
phonon-magnon (SAWs-SWs) coupling at relatively low wavenumbers. It is found
that the angle between the magnetization and the wavevector play important
roles in determining the strength of magnetoelastic coupling and
nonreciprocity, simultaneously. A large nonreciprocal transmission of SAWs
about 30 dB (i.e. 60 dB/mm) is demonstrated at 2.33 GHz. In addition, the
dispersion relation between coupled SH-SAWs and nonreciprocal SWs is developed,
which provide a good insight into the observed phenomena. Our results offer a
convenient approach to implement nonreciprocal SAW isolators or circulators.",2309.09521v1
2023-09-21,Single-crystalline YIG nanoflakes with uniaxial in-plane anisotropy and diverse crystallographic orientations,"We study Y3Fe5O12 (YIG) nanoflakes that we produce via mechanical cleaving
and exfoliation of YIG single crystals. By characterizing their structural and
magnetic properties, we find that these YIG nanoflakes have surfaces oriented
along unusual crystallographic axes and uniaxial in-plane magnetic anisotropy
due to their shape, both of which are not commonly available in YIG thin films.
These physical properties, combined with the possibility of picking up the YIG
nanoflakes and stacking them onto nanoflakes of other van der Waals materials
or pre-patterned electrodes or waveguides, open unexplored possibilities for
magnonics and for the realization of novel YIG-based heterostructures and
devices.",2309.12477v1
2023-09-22,Electric control of optically-induced magnetization dynamics in a van der Waals ferromagnetic semiconductor,"Electric control of magnetization dynamics in two-dimensional (2D) magnetic
materials is an essential step for the development of novel spintronic
nanodevices. Electrostatic gating has been shown to greatly affect the static
magnetic properties of some van der Waals magnets, but the control over their
magnetization dynamics is still largely unexplored. Here we show that the
optically-induced magnetization dynamics in the van der Waals ferromagnet
Cr$_2$Ge$_2$Te$_6$ can be effectively controlled by electrostatic gates, with a
one order of magnitude change in the precession amplitude and over 10% change
in the internal effective field. In contrast to the purely thermally-induced
mechanisms previously reported for 2D magnets, we find that coherent
opto-magnetic phenomena play a major role in the excitation of magnetization
dynamics in Cr$_2$Ge$_2$Te$_6$. Our work sets the first steps towards electric
control over the magnetization dynamics in 2D ferromagnetic semiconductors,
demonstrating their potential for applications in ultrafast opto-magnonic
devices.",2309.12776v1
2023-09-26,Stable Bosonic Topological Edge Modes in the Presence of Many-Body Interactions,"Many magnetic materials are predicted to exhibit bosonic topological edge
modes in their excitation spectra, because of the nontrivial topology of their
magnon, triplon or other quasi-particle band structures. However, there is a
discrepancy between theory prediction and experimental observation, which
suggests some underlying mechanism that intrinsically suppresses the expected
experimental signatures, like the thermal Hall current. Many-body interactions
that are not accounted for in the non-interacting quasi-particle picture are
most often identified as the reason for the absence of the topological edge
modes. Here we report stable bosonic edge modes at the boundaries of a ladder
quantum paramagnet with gapped triplon excitations in the presence of the full
many-body interaction. For the first time, we use tensor network methods to
resolve topological edge modes in the time-dependent spin-spin correlations and
the dynamical structure factor, which is directly accessible experimentally. We
further show that these edge modes have anomalously long time coherence,
discuss the topological phase diagram of the model, demonstrate the
fractionalization of its low-lying excitations, and propose potential material
candidates.",2309.15113v1
2023-09-26,Predictive complexity of quantum subsystems,"We define predictive states and predictive complexity for quantum systems
composed of distinct subsystems. This complexity is a generalization of
entanglement entropy. It is inspired by the statistical or forecasting
complexity of predictive state analysis of stochastic and complex systems
theory, but is intrinsically quantum. Predictive states of a subsystem are
formed by equivalence classes of state vectors in the exterior Hilbert space
that effectively predict the same future behavior of that subsystem for some
time. As an illustrative example, we present calculations in the dynamics of an
isotropic Heisenberg model spin chain and show that, in comparison to the
entanglement entropy, the predictive complexity better signifies dynamically
important events, such as magnon collisions. We discuss how this quantity may
usefully characterize a variety of symmetries in quantum systems in an
information-theoretic way, and comment on possible applications and extensions.",2309.15200v3
2023-09-27,Goodenough-Kanamori-Anderson high-temperature ferromagnetism in tetragonal transition-metal xenes,"Seminal Goodenough-Kanamori-Anderson (GKA) rules provide the inceptive
understanding of the superexchange interaction of two magnetic metal ions
bridged with an anion, and suggest fostered ferromagnetic interaction for
orthogonal bridging bonds. However, there are no examples of two-dimensional
(2D) materials with structure that optimizes the GKA arguments towards enhanced
ferromagnetism and its critical temperature. Here we reveal that an ideally
planar GKA ferromagnetism is indeed stable in selected tetragonal
transition-metal xenes (tTMXs), with Curie temperature above 300~K found in CrC
and MnC. We provide the general orbitally-resolved analysis of magnetic
interactions that supports the claims and sheds light at the mechanisms
dominating the magnetic exchange process in these structures. With recent
advent of epitaxially-grown tetragonal 2D materials, our findings earmark tTMXs
for facilitated spintronic and magnonic applications, or as a desirable
magnetic constituent of functional 2D heterostructures.",2309.15445v1
2023-10-03,Optical excitation of multiple standing spin modes in 3D optomagnonic nanocavities,"We report the first experimental observation of multiple standing spin modes
in 3D optomagnonic nanocavity formed by nanometer-sized iron-garnet
nanocylinder. We show that launching of standing spin modes is achieved due to
a high confinement of the optically generated effective magnetic field caused
by the localized optical resonance. Quantization and spin-wave mode
inhomogeneity is achieved in each of the three spatial dimensions. The
presented approach opens new horizons of 3D optomagnonics by combining
nanophotonic and magnonic functionalities within a single nanocavity.",2310.01974v1
2023-10-04,Metadynamics calculations of the effect of thermal spin fluctuations on skyrmion stability,"The stability of magnetic skyrmions has been investigated in the past, but
mostly in the absence of thermal fluctuations. However, thermal spin
fluctuations modify the magnetic properties (exchange stiffness,
Dzyaloshinskii-Moriya interaction (DMI) and anisotropy) that define skyrmion
stability. Thermal magnons also excite internal skrymion dynamics, deforming
the skyrmion shape. Entropy has also been shown to modify skyrmion lifetimes in
experiments, but is absent or approximated in previous studies. Here we use
metadynamics to calculate the free energy surface of a magnetic thin film in
terms of the topological charge and magnetization. We identify the free energy
minima corresponding to different spin textures and the lowest energy paths
between the ferromagnetic and single skyrmion states. We show that at low
temperatures the lowest free energy barrier is a skyrmion collapse process.
However, this energy barrier increases with temperature. An alternative path,
where a singularity forms on the skrymion edge, has a larger free energy
barrier at low temperatures but decreases with increasing temperature and
eventually becomes the lowest energy barrier.",2310.03169v1
2023-10-20,Multiferroic kinks and spin-flop transition in Ni$_{2}$InSbO$_6$ from first principles,"Magnetoelectric multiferroics are key materials for next-generation devices
due to their entangled magnetic and ferroelectric properties. Spiral
multiferroics induce ferroelectric polarization and are particularly promising
for electric control of magnetism and magnetic control of ferroelectricity. In
this work, we uncover long-period incommensurate states characterized by unique
multiferroic kinks in corundum nickelate Ni$_{2}$InSbO$_6$. Utilizing a
2-orbital $S=1$ model, we derive formulas for Heisenberg and anisotropic
magnetic exchanges and magnetically-induced polarization, enabling their
calculations from first principles. Our theory reproduces experimentally
observed magnetic structures and polarization dependence on the magnetic field
and predicts a three-step magnetic phase transition (flat spiral $\rightarrow$
conical spiral $\rightarrow$ spin-flop $\rightarrow$ ferromagnet) under
external magnetic fields. Kinks in the spiral phases repel each other through a
Yukawa-like potential arising from exchange of massive magnons. Furthermore,
our continuum theory predicts that an electric field directed one way
stabilizes the ferromagnetic state, while the opposite direction stabilizes the
spiral layer state. Our findings open a new pathway to predictive
first-principles modelling of multiferroics and may lead to technological
applications based on multiferroic kinks.",2310.13301v1
2023-10-20,Surface-symmetry-driven Dzyaloshinskii--Moriya interaction and canted ferrimagnetism in collinear magnetoelectric antiferromagnet Cr$_2$O$_3$,"Antiferromagnets are normally thought of as materials with compensated
magnetic sublattices. This adds to their technological advantages but
complicates readout of the antiferromagnetic state. We demonstrate
theoretically the existence of a Dzyaloshinskii-Moriya interaction (DMI) which
is determined by the magnetic symmetry classes of Cr$_2$O$_3$ surfaces with an
in-plane magnetic easy axis. The DMI explains a previously predicted
out-of-plane magnetization at the nominally compensated surfaces of chromia,
leading to a surface-localized canted ferrimagnetism. This is in agreement with
magnetotransport measurements and with density functional theory predictions
which further allow us to quantify the strength of DMI. The temperature
dependence of the transversal resistance for these planes shows distinct
behavior in comparison with that of the Cr$_2$O$_3$ $c$ plane, which we
attribute to the influence of DMI. Our work provides a framework to analyze
surface-driven phenomena in antiferromagnets, and motivates the use of
nominally compensated chromia surfaces for antiferomagnetic spintronics and
magnonics.",2310.13438v1
2023-11-08,Quadrupolar Phases and Plateau States In Skewed Ladders,"Exotic quantum phases in low dimensional frustrated quantum magnets emerge
due to strong quantum fluctuations and competing spin exchanges. Two legged
skewed spin ladders with periodically missing rung bonds and periodically
placed slanted rung bonds are particularly interesting from this stand point.
Here, we study ground state (gs) properties of a spin- 1/2 Heisenberg model on
3/4, 3/5 and 5/5 skewed ladders in the presence of an axial magnetic field, B,
using exact diagonalization and the density matrix renormalization group
method. We note the existence of plateaus at m = 1/3 and 2/3 for 3/4 skewed
ladder, at m = 1/4, 1/2, and 3/4 for 3/5 skewed ladder, and at m = 0, 1/3, and
2/3 for 5/5 skewed ladder. The plateau state is always a gapped state and it
depends on the gap in the system. Surprisingly, the 3/4 and 5/5 skewed ladders
show interesting quadrupolar or n-type spin nematic phases below the 1/3rd
plateau, i.e, at very low magnetic fields. These two systems are unique as they
host both a plateau and a quadrupolar phase at low magnetic fields. The linear
variation of pitch angle of the spin with magnetization and behavior of binding
energy of magnon pairs as function of magnetic field are also studied in both
the systems.",2311.05031v1
2023-11-10,Relaxation to persistent currents in a Hubbard trimer coupled to fermionic baths,"We consider a ring of fermionic quantum sites, modeled by the Fermi-Hubbard
Hamiltonian, in which electrons can move and interact strongly via the Coulomb
repulsion. The system is coupled to fermionic cold baths which by the exchange
of particles and energy induce relaxation in the system. We eliminate the baths
and describe the system effectively by the Lindblad master equations in various
versions valid for different coupling parameter regimes. The early relaxation
phase proceeds in a universal way, irrespective of the relative couplings and
approximations. The system settles down to its low-energy sector and is
consecutively well approximated by the Heisenberg model. In the late
relaxation, different Lindblad approaches push the system towards different
final states with opposite, extreme spin orders, from ferromagenetic to
antiferromagnetic. Due to spin frustration in the trimer (a three site ring),
degenerate ground states are formed by spin waves (magnons). The system
described by the global coherent version of the Lindblad operators relaxes
towards the final states carrying directed persistent spin currents. We
numerically confirm these predictions.",2311.06331v1
2023-11-25,Spin pumping in YIG-Pt structures: the role of the van Hove singularities,"Spin pumping by surface and backward volume magnetostatic waves in YIG/Pt
structures is experimentally studied and analyzed. It is shown that at
frequencies corresponding to van Hove singularities in the density of states of
the spin wave spectrum, an increase in the efficiency of electron-magnon
scattering and spin current generation takes place. The obtained results are
important for spin wave-based spintronic devices development.",2311.15096v1
2023-11-27,Gilbert damping in two-dimensional metallic anti-ferromagnets,"A finite spin life-time of conduction electrons may dominate Gilbert damping
of two-dimensional metallic anti-ferromagnets or anti-ferromagnet/metal
heterostructures. We investigate the Gilbert damping tensor for a typical
low-energy model of a metallic anti-ferromagnet system with honeycomb magnetic
lattice and Rashba spin-orbit coupling for conduction electrons. We distinguish
three regimes of spin relaxation: exchange-dominated relaxation for weak
spin-orbit coupling strength, Elliot-Yafet relaxation for moderate spin-orbit
coupling, and Dyakonov-Perel relaxation for strong spin-orbit coupling. We
show, however, that the latter regime takes place only for the in-plane Gilbert
damping component. We also show that anisotropy of Gilbert damping persists for
any finite spin-orbit interaction strength provided we consider no spatial
variation of the N\'eel vector. Isotropic Gilbert damping is restored only if
the electron spin-orbit length is larger than the magnon wavelength. Our theory
applies to MnPS3 monolayer on Pt or to similar systems.",2311.16268v2
2023-12-06,Axion and Dark Fermion Electromagnetic Form Factors in Superfluid He-4,"Condensed matter materials have shown great potential in searching for light
dark matter (DM) via detecting the phonon or magnon signals induced by the
scattering of DMs off the materials. In this paper, we study the possibility of
detecting electromagnetic form factors of fermionic DM and axion-like particles
(ALPs) using superfluid Helium-4. The phonon induced by a sub-GeV fermionic DM
scattering off the superfluid can be described using the effective field theory
with the interaction between DM and the bulk ${}^4$He. Signals arising from the
electromagnetic form factors of light DM in the presence of an external
electric field are calculated. Projected constraints on the charge radius, the
anapole moment, and the magnetic moment of the DM are derived with 1
kg$\cdot$year exposure. The phonon signal induced by the scattering of ALPs off
the superfluid is also calculated, which can put competitive and the first
direct detection bounds on ALP-photon-dark photon couplings in the projected
experiments.",2312.03530v1
2023-12-12,Bulk Photovoltaic Effects in Helimagnets,"The bulk photovoltaic (BPV) effect that converts light into electric current
is highly sensitive to the system symmetry and its electronic Bloch
wavefunction. To create a sizable net electric current, it is necessary to
break the centrosymmetry P in its host material. Here, we investigate how
helical spin polarization breaks P that is otherwise kept in its crystalline
geometry. The magnetic helix widely exists in multiferroics and magnon
excitation in collinear magnets. We apply symmetry analysis and a tight-binding
model to show how the symmetry is reduced in helimagnetic platforms under
various degrees of freedom, namely, different types of spin spiral
configurations, spin winding angles, magnetic propagation vectors, and spin
chirality. They offer effective and versatile manipulation of BPV generation in
helimagnets. Our concept is further illustrated in the monolayer NiI2, a
typical helimagnetic material. It could host observable direction-dependent BPV
current, which serves as an opto-electric probe to track the subtle magnetic
configurations and potential ferroic nature. Our work reveals the fundamental
mechanism of the multiferroic order and the nonlinear optics process.",2312.06977v1
2023-12-12,Current-controlled chirality dynamics in a mesoscopic magnetic domain wall,"Chirality as internal degree of freedom of a mesoscopic domain wall inside a
quasi-one-dimensional fixture can be controlled by spin-polarized current for
ferro- as well as anti-ferro-magnetic domain walls. We show that the current
density required for the chirality manipulation can be significantly reduced in
the low-temperature regime where the chirality dynamics exhibits quantum
effects. In this quantum regime, weak currents can excite Bloch oscillations of
the domain wall angular rotation velocity, with the oscillation frequency
proportional to the current, modulated by a much higher magnon-range frequency.
In addition to that, the Wannier-Stark localization effects enable controlled
switching between different chiral states, suppressing inertial effects
characteristic for the classical regime. We also show that for recently
discovered novel class of magnetic materials -- altermagnets -- chirality
switching can be driven by the usual charge current (not spin-polarized).",2312.07487v2
2023-12-14,Exploration of magnetoelastic deformations in spin-chain compound CuBr$_2$,"We investigate a spin-$\frac{1}{2}$ antiferromagnet, CuBr$_2$, which has
quasi-one-dimensional structural motifs. The system has previously been
observed to exhibit unusual Raman modes possibly due to a locally deformed
crystal structure driven by the low-dimensional magnetism. Using hard X-ray
scattering and neutron total scattering, here we aim to verify a specific form
of tetramerized lattice deformation proposed in the previous study. Apart from
diffuse scattering signals which we can reproduce by performing a thorough
modeling of the lattice's thermal vibrations, we do not observe evidence for a
tetramerized lattice structure within our detection sensitivity. As a result,
it is more likely that the unusual Raman modes in CuBr$_2$ arise from classical
magnon-phonon hybridization, rather than from quantum spin-singlet-driven
lattice deformation.",2312.09055v1
2023-12-18,Doping-control of excitons and magnetism in few-layer CrSBr,"Magnetism in two-dimensional materials reveals phenomena distinct from bulk
magnetic crystals, with sensitivity to charge doping and electric fields in
monolayer and bilayer van der Waals magnet CrI3. Within the class of layered
magnets, semiconducting CrSBr stands out by featuring stability under ambient
conditions, correlating excitons with magnetic order and thus providing strong
magnon-exciton coupling, and exhibiting peculiar magneto-optics of
exciton-polaritons. Here, we demonstrate that both exciton and magnetic
transitions in bilayer and trilayer CrSBr are sensitive to voltage-controlled
field-effect charging, exhibiting bound exciton-charge complexes and
doping-induced metamagnetic transitions. Moreover, we demonstrate how these
unique properties enable optical probes of local magnetic order, visualizing
magnetic domains of competing phases across metamagnetic transitions induced by
magnetic field or electrostatic doping. Our work identifies few-layer CrSBr as
a rich platform for exploring collaborative effects of charge, optical
excitations, and magnetism.",2312.11041v1
2023-12-20,Non-equilibrium criticality at the onset of time-crystalline order in $O(N)$ models,"We explore the phase transitions at the onset of time-crystalline order in
$O(N)$ models driven out-of-equilibrium. The spontaneous breaking of time
translation symmetry and its Goldstone mode are captured by an effective
description with $O(N)\times SO(2)$ symmetry. Using the renormalization group
and the $\epsilon=4-d$ expansion in a leading two-loop analysis, we identify a
new non-equilibrium universality class. Strikingly, it controls the
long-distance physics no matter how small the microscopic breaking of
equilibrium conditions is. The $O(N=2)\times SO(2)$ symmetry group is realized
for magnon condensation in pumped yttirum iron garnet (YIG) films and in
exciton-polariton systems with a polarization degree of freedom.",2312.13372v1
2023-12-21,Direct evidence from high-field magnetotransport for a dramatic change of quasiparticle character in van der Waals ferromagnet Fe$_{3-x}$GeTe$_2$,"Magnetometry and magnetoresistance (MR) data taken on the van der Waals
ferromagnet Fe$_{3-x}$GeTe$_2$ (FGT) reveal three distinct contributions to the
MR: a linear negative component, a contribution from closed Fermi-surface
orbits, and a $H^2$ enhancement linked to a non-coplanar spin arrangement.
Contrary to earlier studies on FGT, by accounting for the field dependence of
the anomalous Hall effect, we find that the ordinary Hall coefficient decreases
markedly below 80 K, indicating a significant change in character of the
electrons and holes on the Fermi surface at this temperature. The resulting
altered ground state eventually causes the Hall coefficient to reverse sign at
35 K. Our Hall data support the proposal that Kondo-lattice behavior develops
in this $d$-electron material below 80 K. Additional evidence comes from the
negative linear component of the MR, which arises from electron-magnon
scattering with an atypical temperature dependence attributable to the onset of
Kondo screening.",2312.13894v1
2023-12-21,Pseudo-spectral Landau-Lifshitz description of magnetization dynamics,"Magnetic materials host a wealth of nonlinear dynamics, textures, and
topological defects. This is possible due to the competition between strong
nonlinearity and dispersion that act at the atomic scale as well as long-range
interactions. However, these features are difficult to analytically and
numerically study because of the vastly different temporal and spatial scales
involved. Here, we present a pseudo-spectral approach for the Landau-Lifshitz
equation that invokes energy and momentum conservation embodied in the magnon
dispersion relation to accurately describe both atomic and continuum limits.
Furthermore, this approach enables analytical study at every scale. We show the
applicability of this model in both the continuum and atomic limit by
investigating modulational instability and ultrafast evolution of magnetization
due to transient grating, respectively, in a 1D ferromagnetic chain with
perpendicular magnetic anisotropy. This model provides the possibility of
grid-independent multiscale numerical approaches that will enable the
description of singularities within a single framework.",2312.14068v1
2023-12-29,Understanding the magnetic interactions of the zig-zag honeycomb lattice: Application to $α$-RuCl$_3$,"This investigation covers the effects of variable exchange interactions on
the spin dynamics of the zig-zag honeycomb lattice. Using a Holstein-Primakoff
expansion of the Heisenberg Hamiltonian with easy-axis anisotropy, we
characterize the effects of multiple nearest-neighbor and next-nearest-neighbor
interactions with asymmetry within the context of a frustrated and
non-frustrated zig-zag magnetic configuration. Furthermore, we compare to the
known inelastic neutron scattering data for the proximate quantum spin liquid
$\alpha$-RuCl$_3$, and we provide insight into the evolution of the spin
dynamics, showing that the Heisenberg interaction dominates the majority of the
spin excitation behavior. By analyzing the frustrated system with multiple
interactions, direction-dependent Dirac nodes present themselves, and we can
demonstrate that a standard Heisenberg model can accurately describe the
observed magnon spectra.",2312.17433v1
2024-01-03,Synthetically enhanced sensitivity using higher-order exceptional point and coherent perfect absorption,"Sensors play a crucial role in advanced apparatuses and it is persistently
pursued to improve their sensitivities. Recently, the singularity of a
non-Hermitian system, known as the exceptional point (EP), has drawn much
attention for this goal. Response of the eigenfrequency shift to a perturbation
$\epsilon$ follows the $\epsilon^{1/n}$-dependence at an $n$th-order EP,
leading to significantly enhanced sensitivity via a high-order EP. However, due
to the requirement of increasingly complicated systems, great difficulties will
occur along the path of increasing the EP order to enhance the sensitivity.
Here we report that by utilizing the spectral anomaly of the coherent perfect
absorption (CPA), the sensitivity at a third-order EP can be further enhanced
owing to the cooperative effects of both CPA and EP. We realize this
synthetically enhanced sensor using a pseudo-Hermitian cavity magnonic system
composed of two yttrium iron garnet spheres and a microwave cavity. The
detectable minimum change of the magnetic field reaches $4.2\times10^{-21}$T.
It opens a new avenue to design novel sensors using hybrid non-Hermitian
quantum systems.",2401.01613v1
2024-01-10,Narrowly avoided spin-nematic phase in BaCdVO(PO$_4$)$_2$: NMR evidence,"We present a $^{31}$P nuclear magnetic resonance (NMR) investigation of
BaCdVO(PO$_4$)$_2$ focusing on the nearly saturated regime between
$\mu_0H_{c1}$ = 4.05 T and $\mu_0H_{c2}$ = 6.5 T, used to be considered as a
promising candidate for a spin-nematic phase. NMR spectra establish the absence
of any dipolar order there, whereas the weak field dependence of the
magnetization above $H_{c1}$ is accounted for by Dzyaloshinskii-Moriya
interaction terms. The low-energy spin dynamics (fluctuations), measured by
nuclear spin-lattice relaxation rate ($T_1^{-1}$), confirms the continuity of
this phase and the absence of any low-temperature phase transition.
Unexpectedly, the spin dynamics above $H_{c1}$ is largely dominated by
two-magnon processes, which is expected above the saturation field of a
spin-nematic phase, but not inside. This shows that BaCdVO(PO$_4$)$_2$ is
indeed close to a spin-nematic instability, however, this phase is not
stabilized. We thus confirm recent theoretical predictions that the
spin-nematic phase can be stabilized, at most, in an extremely narrow field
range close to saturation or is rather narrowly avoided [Jiang et al., Phys.
Rev. Lett. 130, 116701 (2023)].",2401.05269v1
2024-01-19,Non-Fermi-Liquid/Marginal-Fermi-Liquid Signatures Induced by Van Hove Singularity,"We theoretically study the two-dimensional metal that is coupled to critical
magnons and features van Hove singularities on the Fermi surface. When there is
only translationally invariant SYK-liked Yukawa interaction, van Hove points
suppress the contribution from the part of the Fermi surface away from them,
dominating and exhibiting non-Fermi-liquid behavior. When introducing
disordered Yukawa coupling, it leads to a crossover from non-Fermi-liquid to
marginal-Fermi-liquid, and the marginal-Fermi-liquid region exhibits the $T\ln
(1/T)$ specific heat and temperature-linear resistivity of strange metal. By
solving the gap equation, we provide the critical temperature for
superconductor induced by van Hove singularities and point out the possible
emergence of pair-density-wave superconductor. Our theory may become a new
mechanism for understanding non-Fermi-liquid or marginal-Fermi-liquid
phenomenons.",2401.10707v1
2024-01-22,Theory of momentum-resolved magnon electron energy loss spectra: The case of Yttrium Iron Garnet,"We explore the inelastic spectra of electrons impinging in a magnetic system.
The methodology here presented is intended to highlight the charge-dependent
interaction of the electron beam in a STEM-EELS experiment, and the local
vector potential generated by the magnetic lattice. This interaction shows an
intensity $10^{-2}$ smaller than the purely spin interaction, which is taken to
be functionally the same as in the inelastic neutron experiment. On the other
hand, it shows a strong scattering vector dependence ($\kappa^{-4}$) and a
dependence with the relative orientation between the probe wavevector and the
local magnetic moments of the solid. We present YIG as a case study due to its
high interest by the community.",2401.12302v2
2024-01-24,Progress and Prospects in Two-Dimensional Magnetism of van der Waals Materials,"Two-dimensional (2D) magnetism in van der Waals (vdW) atomic crystals and
moir\'e superlattices has emerged as a topic of tremendous interest in the
fields of condensed matter physics and materials science within the past
half-decade since its first experimental discovery in 2016 - 2017. It has not
only served as a powerful platform for investigating phase transitions in the
2D limit and exploring new phases of matter, but also provided new
opportunities for applications in microelectronics, spintronics, magnonics,
optomagnetics, and so on. Despite the flourishing developments in 2D magnetism
over this short period of time, further efforts are welcome in multiple
forefronts of 2D magnetism research for achieving the ultimate goal of
routinely implementing 2D magnets as quantum electronic components. In this
review article, we will start with basic concepts and properties of 2D
magnetism, followed by a brief overview of historical efforts in 2D magnetism
research and then a comprehensive review of vdW material-based 2D magnetism. We
will conclude with discussions on potential future research directions for this
growing field of 2D vdW magnetism.",2401.13781v1
2024-01-28,Magnetic interactions and excitations in SrMnSb$_2$,"The magnetic interactions in the antiferromagnetic (AFM) Dirac semimetal
candidate SrMnSb$_2$ are investigated using \textit{ab initio} linear response
theory and inelastic neutron scattering (INS). Our calculations reveal that the
first two nearest in-plane couplings ($J_1$ and $J_2$) are both AFM in nature,
indicating a significant degree of spin frustration, which aligns with
experimental observations. The orbital resolution of exchange interactions
shows that $J_1$ and $J_2$ are dominated by direct and superexchange,
respectively. In a broader context, a rigid-band model suggests that electron
doping fills the minority spin channel and results in a decrease in the AFM
coupling strength for both $J_1$ and $J_2$. To better compare with INS
experiments, we calculate the spin wave spectra within a linear spin wave
theory framework, utilizing the computed exchange parameters. The calculated
spin wave spectra exhibit overall good agreement with measurements from INS
experiments, although with a larger magnon bandwidth. Introducing additional
electron correlation within the Mn-$3d$ orbitals can promote electron
localization and reduce the magnetic coupling, further improving the agreement
with experiments.",2401.15572v1
2024-01-29,Dipole superfluid hydrodynamics II,"We present a dissipative hydrodynamic theory of ""s-wave dipole superfluids""
that arise in phases of translation-invariant and dipole-symmetric models in
which the U(1) symmetry is spontaneously broken. The hydrodynamic description
is subtle on account of an analogue of dangerously irrelevant operators, which
requires us to formalize an entirely new derivative counting scheme suitable
for these fluids. We use our hydrodynamic model to investigate the linearized
response of such a fluid, characterized by sound modes $\omega \sim \pm k -
ik^2$, shear modes $\omega\sim-ik^2$, and magnon-like propagating modes $\omega
\sim \pm k^2 - ik^4$ that are the dipole-invariant version of superfluid
""second sound"" modes. We find that these fluids can also admit equilibrium
states with ""dipole superflow"" that resemble a polarized medium. Finally, we
couple our theory to slowly varying background fields, which allows us to
compute response functions of hydrodynamic operators and Kubo formulas for
hydrodynamic transport coefficients.",2401.16385v1
2024-01-30,"Magneto-synthesis effect on magnetic order, phonons, and magnons in single-crystal Sr$_2$IrO$_4$","It was shown earlier that applying a magnetic field during the growth of
Sr$_2$IrO$_4$, also known as ""field-alteration"", induces significant changes to
its structural, magnetic, and transport properties. However, the microscopic
nature of these changes is enigmatic. In this study, we employed resonant
elastic and inelastic x-ray scattering, as well as Raman scattering, to
investigate samples from two batches of Sr$_2$IrO$_4$ grown in magnetic fields
of different strengths. Our findings reveal that samples grown in a weaker
magnetic field have similar magnetic order to non-altered samples, whereas
those grown in a stronger field show a different stacking of weak in-plane
ferromagnetic moments. Additionally, we observed significant softening and
broadening of select Raman-active phonons in the field altered samples, with a
stronger effect in the samples grown in the stronger field. We discuss insights
that our results provide into the microscopic nature of field-alteration in
Sr$_2$IrO$_4$.",2401.17407v1
2024-02-01,Spin wave-driven variable-phase mutual synchronization in spin Hall nano-oscillators,"Spin-orbit torque can drive auto-oscillations of propagating spin wave (PSW)
modes in nano-constriction spin Hall nano-oscillators (SHNOs). These modes
allow both long-range coupling and the potential of controlling its phase --
critical aspect for nano-magnonics, spin wave logic, and Ising machines. Here,
we demonstrate PSW-driven variable-phase coupling between two nano-constriction
SHNOs and study how their separation and the PSW wave vector impact their
mutual synchronization. In addition to ordinary in-phase mutual
synchronization, we observe, using both electrical measurements and
phase-resolved $\mu-$Brillouin Light Scattering microscopy, mutual
synchronization with a phase that can be tuned from 0 to $\pi$ using the drive
current or the applied field. Micromagnetic simulations corroborate the
experiments and visualize how the PSW patterns in the bridge connecting the two
nano-constrictions govern the coupling. These results advance the capabilities
of mutually synchronized SHNOs and open up new possibilities for applications
in spin wave logic, unconventional computing, and Ising Machines.",2402.00586v1
2024-02-09,Resonances involving integer magnons and spin-1/2 excitations in a magnetism modulated two-dimensional electron gas,"We conduct an experimental study of high-mobility two-dimensional electron
gas (2DEG) in GaAs/AlGaAs quantum wells modulated by strong magnetism at an
in-plane magnetic field ($B$). The modulated $B$-fields are performed via the
single stripe and gratings which are made of the heavy rare earth metal Terbium
(Tb) thin films on the sample surface. The robust ferromagnetic resonances
(FMRs) persist to the temperature of 50-70 K in both the stripe and grating
samples, for the ferromagnetism (FM) phase of Tb exists at above 100 K. The
high-order (with integer numbers of $j \equiv \hbar\omega/g\mu_{B}B = 1,
2,$...) magnetic resonances can also be observed in the stripe structure via
the microwave (MW) photovoltaic detection and magnetoresistance under microwave
irradiation. In addition, the resonance features around $j = 1/2$ are robust in
the single-stripe modulated sample, which suggests the spinons with spin-1/2
collective excitations in a 1D Heisenberg model.",2402.06792v1
2024-03-09,Superconducting Cavity-Based Sensing of Band Gaps in 2D Materials,"The superconducting coplanar waveguide (SCPW) cavity plays an essential role
in various areas like superconducting qubits, parametric amplifiers, radiation
detectors, and studying magnon-photon and photon-phonon coupling. Despite its
wide-ranging applications, the use of SCPW cavities to study various van der
Waals 2D materials is relatively unexplored. The resonant modes of the SCPW
cavity exquisitely sense the dielectric environment. In this work, we measure
the charge compressibility of bilayer graphene coupled to a half-wavelength
SCPW cavity. Our approach provides a means to detect subtle changes in the
capacitance of the bilayer graphene heterostructure, which depends on the
compressibility of bilayer graphene, manifesting as shifts in the resonant
frequency of the cavity. This method holds promise for exploring a wide class
of van der Waals 2D materials, including transition metal dichalcogenides
(TMDs) and their moir\'e where DC transport measurement is challenging.",2403.05867v1
2024-03-10,Spin Waves and Spin Currents in Magnon-Phonon Composite Resonator Induced by Acoustic Waves of Various Polarizations,"In this work, we present the results of a systematic experimental study of
linear and parametric spin wave resonant excitation accompanied by spin
currents (spin pumping) in a multifrequency composite bulk acoustic wave
resonator with a ZnO-YIG-GGG-YIG/Pt structure. The features of magnetic
dynamics excitation in YIG films due to magnetoelastic coupling with acoustic
thickness modes of various polarizations are studied. Acoustic spin waves and
spin pumping are detected by simultaneous frequency-field mapping of the
inverse spin Hall effect voltage and the resonant frequencies of thickness
extensional modes. In the parametric range of frequencies and fields, acoustic
spin pumping induced by both shear and longitudinal polarization modes was
observed. Linear acoustic spin waves are excited only by shear thickness
extensional modes because longitudinal acoustic waves do not couple with the
magnetic subsystem in linear regime.",2403.06274v1
2024-03-14,Topological responses from gapped Weyl points in 2D altermagnets,"Altermagnetism combines aspects of both ferromagnetism and
antiferromagnetism, breaking spin degeneracy while possessing no net
magnetization. In this work, we study the symmetry requirements for
topologically protected Weyl points in 2D altermagnets, involving bands with
the same spin quantum number. We classify all spin-wallpaper groups whose
symmetries protect 2D Weyl points and show that their nontrivial topology is
charactrized by a quantized $\pi$-Berry phase. Representative electronic
tight-binding and magnonic linear spin-wave models are constructed to
investigate the unusual transport characteristics of these 2D Weyl points.
Different mass terms, induced for example through strain or via coupling to
light or a substrate, gap out the Weyl points leading to emerging gapped
topological phases. Depending on the mass terms, these phases carry finite
Chern and/or spin Chern numbers and exhibit protected edge states as well as
anomalous electronic and thermal Hall responses. We calculate these Hall
currents for the different topological phases.",2403.09520v1
2024-03-14,Subharmonic lock-in detection and its optimisation for femtosecond noise correlation spectroscopy,"Although often viewed as detrimental, fluctuations carry valuable information
about the physical system from which they emerge. Femtosecond noise correlation
spectroscopy (FemNoC) has recently been established to probe the ultrafast
fluctuation dynamics of thermally populated magnons by measurement of their
amplitude autocorrelation. Subharmonic lock-in detection is the key technique
in this method, allowing to extract the pulse-to-pulse polarisation
fluctuations of two femtosecond optical pulse trains transmitted through a
magnetic sample. Here, we present a thorough technical description of the
subharmonic demodulation technique and of the FemNoC measurement system. We
mathematically model the data acquisition process and identify the essential
parameters which critically influence the signal-to-noise ratio of the signals.
Comparing the model calculations to real datasets allows validating the
predicted parameter dependences and provides a means to optimise FemNoC
experiments.",2403.09829v1
2024-03-15,"Structure, control, and dynamics of altermagnetic textures","We present a phenomenological theory of altermagnets, that captures their
unique magnetization dynamics and allows modelling magnetic textures in this
new magnetic phase. Focusing on the prototypical d-wave altermagnets, e.g.
RuO$_2$, we can explain intuitively the characteristic lifted degeneracy of
their magnon spectra, by the emergence of an effective sublattice-dependent
anisotropic spin stiffness arising naturally from the phenomenological theory.
We show that as a consequence the altermagnetic domain walls, in contrast to
antiferromagnets, have a finite gradient of the magnetization, with its
strength and gradient direction connected to the altermagnetic anisotropy, even
for 180$^\circ$ domain walls. This gradient generates a ponderomotive force in
the domain wall in the presence of a strongly inhomogeneous external magnetic
field, which may be achieved through magnetic force microscopy techniques. The
motion of these altermagentic domain walls is also characterized by an
anisotropic Walker breakdown, with much higher speed limits of propagation than
ferromagnets but lower than antiferromagnets.",2403.10218v2
2024-03-18,Uncovering the lowest thickness limit for room-temperature ferromagnetism of Cr$_{1.6}$Te$_{2}$,"Half-metallic ferromagnetic transition metal dichalcogenides have emerged as
important building blocks for scalable magnonics and memory applications.
Downscaling such systems to the ultra-thin limit is critical to integrate them
into technology. Here, we achieved layer-by-layer control over the transition
metal dichalcogenide Cr$_{1.6}$Te$_{2}$ by using pulsed laser deposition, and
we uncovered the minimum critical thickness above which room temperature
magnetic order is maintained. The electronic and magnetic structure is explored
experimentally and theoretically and it is shown that the films exhibit strong
in-plane magnetic anisotropy as a consequence of large spin-orbit effects. Our
study establishes Cr$_{1.6}$Te$_{2}$ as a platform material, viable for
ferromagnetic nanoscale devices and magnetic-memory architectures.",2403.11977v1
2024-03-20,Probing Hilbert Space Fragmentation with Strongly Interacting Rydberg Atoms,"Hilbert space fragmentation provides a mechanism to break ergodicity in
closed many-body systems. Here, we propose a realistic scheme to
comprehensively explore this exotic paradigm on a Rydberg quantum simulator. We
show that the Rydberg Ising model in the large detuning regime can be mapped to
a generalized folded XXZ model featuring a strongly fragmented Hilbert space.
The emergent Hamiltonian, however, displays distinct time scales for the
transport of a magnon and a hole excitation. This interesting property
facilitates a continuous tuning of the Krylov-subspace ergodicity, from the
integrable regime, to the Krylov-restricted thermal phase, and eventually to
the statistical bubble localization region. By further introducing nonlocal
interactions, we find that both the fragmentation behavior and the ergodicity
of the Krylov subspace can be significantly enriched. We also examine the role
of atomic position disorders and identify a symmetry-selective many-body
localization transition. We demonstrate that these phenomena manifest
themselves in quench dynamics, which can be readily probed in state-of-the-art
Rydberg array setups.",2403.13790v1
2024-03-25,Detection of spin pumping free of rectification and thermal artefacts in molecular-based ferromagnetic insulator V[TCNE]x~2,"The molecular-based ferrimagnetic insulator V(TCNE)x has gained recent
interest for efficient spin-wave excitation due to its low Gilbert damping
ratio a=4E-5, and narrow ferromagnetic resonance linewidth f=1Oe. Here we
report a clean spin pumping signal detected on V(TCNE)x/metal bilayer
structures, free from spin rectification or thermal artifacts. On-chip coupling
of microwave power is achieved via a coplanar waveguide to measure the in-plane
angle-dependence of the inverse spin-Hall effect under ferromagnetic resonance
conditions with respect to a constant external magnetic field. A signature of
pure spin current from V(TCNE)x is observed in both platinum and permalloy
metal layers, demonstrating the utility of V(TCNE)x for magnon spintronics
studies in molecule/solid-state heterostructures.",2403.16429v2
2024-03-27,Strong-coupling approach to temperature dependence of competing orders of superconductivity: Possible time-reversal symmetry breaking and nontrivial topology,"We use strong-coupling Eliashberg theory to study the competition of separate
superconducting orders at low temperatures. Specifically, we study
magnon-mediated superconductivity in a trilayer heterostructure with a thin
normal metal between two antiferromagnetic insulators. Spin-triplet $p$-wave,
spin-triplet $f$-wave, and spin-singlet $d$-wave superconducting gaps have been
predicted to occur close to the critical temperature for the superconducting
instability. The gap symmetry with the largest critical temperature depends on
parameters in the model. We confirm that the same gap symmetries appear at any
temperature below the critical temperature. Furthermore, we show that the
temperature can affect the competition between the different superconducting
orders. In addition, we consider time-reversal-symmetry-breaking, complex
linear combinations of candidate pairings, such as chiral $p$-, $f$-, and
$d$-wave gaps, as well as $p_x+if_y$-wave gaps. We find indications that some
of these time-reversal-symmetry-breaking, nodeless gaps offer a greater
condensation energy than the time-reversal symmetric gaps. This indicates that
superconducting states with spontaneously broken time-reversal symmetry and
nontrivial topology may be preferred in this system.",2403.18897v1
2024-04-02,Probing the band splitting near the Γ point in the van der Waals magnetic semiconductor CrSBr,"This study investigates the electronic band structure of Chromium Sulfur
Bromide (CrSBr) through comprehensive photoluminescence (PL) characterization.
We clearly identify low-temperature optical transitions between two closely
adjacent conduction-band states and two different valence-band states. The
analysis of the PL data robustly unveils energy splittings, bandgaps and
excitonic transitions across different thicknesses of CrSBr, ranging from
monolayer to bulk. Temperature-dependent PL measurements elucidate the
stability of the band splitting below the N\'eel temperature, suggesting that
magnons coupled with excitons are responsible for the symmetry breaking and
brightening of the transitions from the secondary conduction band minimum
(CBM2) to the global valence band maximum (VBM1). Collectively, these results
not only reveal band splitting in both the conduction and valence bands, but
also point to an intricate interplay between the optical, electronic and
magnetic properties of antiferromagnetic two-dimensional van der Waals
crystals.",2404.02927v1
2024-04-04,Temperature-dependent optical and magneto-optical spectra of ferromagnetic BCC Fe,"Optical and magneto-optical properties of magnetic materials have been widely
exploited to characterize magnetic structures and phenomena, however, their
temperature dependence is not well understood. This study implements the
supercell approach with thermal lattice and magnetic disorders to obtain
optical and magneto-optical spectra at finite temperatures based on
Williams-Lax theory. Our results show that large optical spectrum signals are
generated at photon energies below 1 eV, originating from the phonon- and
magnon-assisted intraband transitions as lattice and magnetic temperatures
increase. In addition, the prominent peak near 2.7 eV is redshifted
proportionally to magnetic temperature but depends much less on lattice
temperature. By analyzing unfolded bands, we show that the reduction of
exchange splitting due to the thermal demagnetization causes this redshift. Our
unfolded electronic band structure with magnetic disorder shows band kinks,
which are characteristic evidence of the coupling between electrons and
magnetic excitations. First-order magneto-optical spectra at finite temperature
are also predicted, but due to their small magnitude suffer more from sampling
errors. We discuss the effect of zero-point vibrations and the connection of
these simulations to the Drude model for intraband transitions.",2404.03545v1
2024-04-07,Time-resolved magneto-optical Kerr effect in the altermagnet candidate MnTe,"{\alpha}-MnTe is an antiferromagnetic semiconductor with above room
temperature TN = 310 K, which is promising for spintronic applications.
Recently, it was predicted to be an altermagnet, containing bands with
momentum-dependent spin splitting; time-resolved experimental probes of
magnetism in MnTe are therefore important both for understanding the magnetic
structure and potential device applications. We investigate ultrafast spin
dynamics in epitaxial MnTe(001)/InP(111) thin films using the time-resolved
magneto-optical Kerr effect. At room temperature, we observe an oscillation
mode at 55 GHz that does not appear at zero magnetic field. Combining field,
polarization, and temperature dependence, we identify this mode as an acoustic
phonon-coupled magnon, likely originating from inverse stimulated Raman
scattering. Additionally, we observe two optical phonons at 3.6 THz and 4.2
THz, which broaden and redshift with increasing temperature.",2404.05020v1
2024-04-11,Mode-resolved micromagnetics study of parametric spin wave excitation in thin-film disks,"We present a computational study of the parametric excitation of spin waves
in thin film disks with a mode-resolved approach. The method involves
projecting out the time-dependent magnetization, computed using micromagnetics
simulations, onto the spatial profile of the eigenmodes that are obtained from
the linearization of the equations of motion. Unlike spectral analysis in the
frequency domain, the projection allows for the analysis of transient mode
dynamics under parametric excitation. We apply this method to parallel pumping
of quantized spin wave modes in in-plane magnetized thin-film disks, where
phenomena like frequency pulling, mutual phase locking, and higher-order magnon
scattering processes are identified.",2404.07561v1
1993-07-19,Evolution of Magnetic Properties of Lightly Doped Copper Oxides,"We study how doping destroys the AF order in the layered cuprates within the
framework of the charge--transfer insulator concept. We use the criterion of
stability of the AF background to show that the stability problem is one of the
main issues in any correspondence between results for the $t-J$ model and, say,
the three--band model for the lightly--doped layered oxides. Provided a
phenomenological conduction band is chosen to satisfy the criterion of
stability, a detailed picture of how dopants influence the spin wave spectrum
at $T=0$ is presented. The critical concentration $x_c$ for the destruction of
the AF long range order is due to the Cherenkov effect when the Fermi velocity
first exceeds the spin wave velocity. We then discuss the overall spectrum of
spin excitations and find that the spin wave attenuation for $x < x_c$, $T=0$
due to Landau damping appears in the range of magnon momenta $k(x) = 2 m^* s
\pm \alpha \sqrt{x}$. We also argue that in the presence of superconductivity,
the Cherenkov effect is eliminated due to the gap in the spectrum. This may
restore the role of the AF fluctuations as the main source of dissipation at
the lowest temperatures.",9307040v1
1995-09-30,Integrability and Applications of the Exactly-Solvable Haldane-Shastry One-Dimensional Quantum Spin Chain,"Recently, the one dimensional model of $N$ spins with $S=\frac{1}{2}$ on a
circle, interacting with an exchange that falls off with the inverse square of
the separation: $H_{\rm ISE} =\sum_{i\neq j} \frac{1}{[\frac{N}{\pi}
\sin(\frac{i-j}{N}\pi)]^2}(\svec_i\cdot \svec_j-\frac{1}{4})$, or ISE-model,
has received ample attention. Its special features include: relatively simple
eigenfunctions, non-interacting elementary excitations that obey semionic
statistics (spinons), and a large ``quantum group'' symmetry algebra called the
Yangian. This model is fully integrable, albeit in a slightly different sense
than the more traditional nearest neighbor exchange (NNE) Heisenberg chain.
This thesis comes in 4 chapters. Chapter 1 introduces the model and presents
the construction of a subset of the eigenfunctions. The other eigenfunctions
are shown to be generated by the action of the Yangian symmetry algebra of
$H_{\rm ISE}$. Chapter 2 presents a method to construct the set of constants of
the motion of the ISE-model. The ISE-model is tractable enough to obtain its
zero-magnetic-field dynamical structure factors. Chapter 3 attempts to extend
this to a non-zero magnetic field, where, due to the presence of spinons in the
groundstate, more complicated excitations contribute: small numbers of magnons
and spinons. discuss the relation to the more complicated NNE-model. Finally
chapter 4 illustrates how a recently conjectured new form of Off Diagonal Long
Range Order in antiferromagnetic spin chains can be reinterpreted as a spinon
propagator in the ISE-model, and verified numerically. We briefly comment on
its relevance to stabilizing superconductivity in the layered cuprates.",9509178v1
1995-12-18,Raman scattering in a two-layer antiferromagnet,"Two--magnon Raman scattering is a useful tool to verify recent suggestions
concerning the value of the interplanar exchange constant in antiferromagnetic
two--layer systems, such as $YBa_2Cu_3O_{6+x}$. We present a theory for Raman
scattering in a two--layer antiferromagnet. We study the spectra for the
electronic and magnetic excitations across the charge transfer gap within the
one--band Hubbard model and derive the matrix elements for the Raman scattering
cross section in a diagrammatic formalism. We analyze the effect of the
interlayer exchange coupling $J_2$ for the Raman spectra in $A_{1g}$ and
$B_{1g}$ scattering geometries both in the non--resonant regime (when the
Loudon--Fleury model is valid), and at resonance. We show that within the
Loudon--Fleury approximation, a nonzero $J_2$ gives rise to a finite signal in
$A_{1g}$ scattering geometry. Both, in this approximation and at resonance, the
intensity in the $A_{1g}$ channel has a peak at {\it small} transferred
frequency equal to twice the gap in the spin--wave spectrum. We compare our
results with experiments in $YBa_2Cu_3O_{6.1}$ and $Sr_2CuO_2Cl_2$ compounds
and argue that the large value of $J_2$ suggested in a number of recent studies
is incompatible with Raman experiments in $A_{1g}$ geometry.",9512131v1
1996-09-19,Spectral properties of the t-J model in the presence of hole-phonon interaction,"We examine the effects of electron-phonon interaction on the dynamics of the
charge carriers doped in two-dimensional (2D) Heisenberg antiferromagnet. The
$t$-$J$ model Hamiltonian with a Fr\""ohlich term which couples the holes to a
dispersionless (optical) phonon mode is considered for low doping
concentration. The evolution of the spectral density function, the density of
states, and the momentum distribution function of the holes with an increase of
the hole-phonon coupling constant $g$ is studied numerically. As the coupling
to a phonon mode increases the quasiparticle spectral weight decreases and a
``phonon satellite'' feature close to the quasi-particle peak becomes more
pronounced. Furthermore, strong electron-phonon coupling smears the
multi-magnon resonances (``string states'') in the incoherent part of the
spectral function. The jump in the momentum distribution function at the Fermi
surface is reduced without changing the hole pocket volume, thereby providing a
numerical verification of Luttinger theorem for this strongly interacting
system. The vertex corrections due to electron- phonon interaction are
negligible in spite of the fact that the ratio of the phonon frequency to the
effective bandwidth is not small.",9609186v1
1996-10-01,Itinerant Antiferromagnetism in FeGe_2,"FeGe_2, and lightly doped compounds based on it, have a Fermi surface driven
instability which drive them into an incommensurate spin density wave state.
Studies of the temperature and magnetic field dependence of the resistivity
have been used to determine the magnetic phase diagram of the pure material
which displays an incommensurate phase at high temperatures and a commensurate
structure below 263 K in zero field. Application of a magnetic field in the
tetragonal basal plane decreases the range of temperatures over which the
incommensurate phase is stable. We have used inelastic neutron scattering to
measure the spin dynamics of FeGe_2. Despite the relatively isotropic transport
the magnetic dynamics is quasi-one dimensional in nature. Measurements carried
out on HET at ISIS have been used to map out the spin wave dispersion along the
c-axis up the 400 meV, more than an order of magnitude higher than the zone
boundary magnon for wavevectors in the basal plane.",9610009v1
1997-03-06,Effects of in-chain and off-chain substitutions on spin fluctuations in the spin-Peierls compound CuGeO_3,"The effect of in-chain and off-chain substitutions on 1D spin fluctuations in
the spin-Peierls compound CuGeO_3 has been studied using Raman scattering in
order to understand the interplay between defect induced states, enhanced
spin-spin correlations and the ground state of low dimensional systems.
In-chain and off-chain substitutions quench the spin-Peierls state and induce
3D antiferromagnetic order at T\leq 5 K. Consequently a suppression of a 1D
gap-induced mode as well as a constant intensity of a spinon continuum are
observed at low temperatures. A 3D two-magnon density of states now gradually
extends to higher temperatures T\leq 60K compared with pure CuGeO_3. This
effect is more pronounced in the case of off-chain substitutions (Si) for which
a N\'eel state occurs over a larger substitution range, starting at very low
concentrations. Besides, additional low energy excitations are induced. These
effects, i.e. the shift of a dimensional crossover to higher temperatures are
due to an enhancement of the spin-spin correlations induced by a small amount
of substitutions. The results are compared with recent Monte Carlo studies on
substituted spin ladders, pointing to a similar instability of coupled,
dimerized spin chains and spin ladders upon substitution.",9703060v1
1997-03-12,Scaling picture of magnetism formation in the anomalous f-systems: interplay of the Kondo effect and spin dynamics,"Formation of magnetically ordered state in the Kondo lattices is treated
within the degenerate $s-f$ exchange and Coqblin-Schrieffer models. The Kondo
renormalizations of the effective coupling parameter, magnetic moment and spin
excitation frequencies are calculated within perturbation theory. The results
of one-loop scaling consideration of the magnetic state in Kondo lattices are
analyzed. The dependence of the critical values of the bare model parameters on
the type of the magnetic phase and space dimensionality is investigated.
Renormalization of the effective Kondo temperature by the interatomic exchange
interactions is calculated. An important role of the character of spin dynamics
(existence of well-defined magnon excitations, presence of magnetic anisotropy
etc.) is demonstrated. The regime of strongly suppressed magnetic moments,
which corresponds to magnetic heavy-fermion system, may occur in a rather
narrow parameter region only. At the same time, in the magnetically ordered
phases the renormalized Kondo temperature depends weakly on the bare coupling
parameter in some interval. The critical behavior, corresponding to the
magnetic transition with changing the bare $s-f$ coupling parameter, is
investigated. In the vicinity of the strong coupling regime, the spectrum of
the Bose excitations becomes softened. Thus on the borderline of magnetic
instability the Fermi-liquid picture is violated in some temerature interval
due to scattering of electrons by these bosons. This may explain the fact that
a non-Fermi-liquid behavior often takes place in the heavy-fermion systems near
the onset of magnetic ordering.",9703125v1
1997-08-25,Effects of Phonons and Nuclear Spins on the Tunneling of a Domain Wall,"We consider the quantum dynamics of a magnetic domain wall at low
temperatures, where dissipative couplings to magnons and electrons are very
small. The wall motion is then determined by its coupling to phonons and
nuclear spins, and any pinning potentials. In the absence of nuclear spins
there is a dominant superOhmic 1-phonon coupling to the wall velocity, plus a
strongly T-dependent Ohmic coupling to pairs of phonons. There is also a
T-independent Ohmic coupling between single phonons and the wall chirality,
which suppresses ``chirality tunneling''. We calculate the effect of these
couplings on the T-dependent tunneling rate of a wall out of a pinning
potential. Nuclear spins have a very strong and hitherto unsuspected influence
on domain wall dynamics, coming from a hyperfine-mediated coupling to the
domain wall position. For $k_B T \gg \omega_{0}$ this coupling yields a
spatially random potential, fluctuating at a rate governed by the nuclear
$T_2$. When $k_B T \ll \omega_{0}$, the hyperfine potential fluctuates around a
linear binding potential. The wall dynamics is influenced by the fluctuations
of this potential, ie., by the nuclear spin dynamics. Wall tunneling can occur
when fluctuations open an occasional ``tunneling window''. This changes the
crossover to tunneling and also causes a slow ""wandering"", in time, of the
energy levels associated with domain wall motion inside the pinning potential.
This effect is fairly weak in $Ni$- and $Fe$- based magnets, and we give an
approximate treatment of its effect on the tunneling dynamics, as well as a
discussion of the relationship to recent domain wall tunneling experiments.",9708191v1
1998-06-30,"Transport, optical and electronic properties of the half metal CrO2","The electronic structure of CrO_2 is critically discussed in terms of the
relation of existing experimental data and well converged LSDA and GGA
calculations of the electronic structure and transport properties of this half
metal magnet, with a particular emphasis on optical properties. We find only
moderate manifestations of many body effects. Renormalization of the density of
states is not large and is in the typical for transition metals range. We find
substantial deviations from Drude behavior in the far-infrared optical
conductivity. These appear because of the unusually low energy of interband
optical transitions. The calculated mass renormalization is found to be rather
sensitive to the exchange-correlation functional used and varies from 10%
(LSDA) to 90% (GGA), using the latest specific heat data. We also find that
dressing of the electrons by spin fluctuations, because of their high energy,
renormalizes the interband optical transition at as high as 4 eV by about 20%.
Although we find no clear indications of strong correlations of the Hubbard
type, strong electron-magnon scattering related to the half metallic band
structure is present and this leads to a nontrivial temperature dependence of
the resistivity and some renormalization of the electron spectra.",9806378v1
1998-08-08,Pseudogaps and magnetic properties of the two-dimensional t-J model,"We apply the modified spin-wave theory with the constraint of zero staggered
magnetization to investigate normal-state spectral and magnetic properties of
the 2D t-J model in the paramagnetic state. A set of self-energy equations for
hole and magnon Green's functions is solved numerically in the self-consistent
Born approximation. The constraint can be fulfilled in the ranges of hole
concentrations 0.02 < x < 0.17 and temperatures T < 150 K. In this region the
hole spectrum differs from a conventional metallic spectrum which is manifested
in the variation with x of the quasiparticle weights of states and in the
violation of Luttinger's theorem. With decreasing x from x = 0.17 hidden parts
appear in the hole Fermi surface which can be interpreted as the opening of a
pseudogap near (pi,0). Obtained size, symmetry and concentration dependence of
the pseudogap are in agreement with photoemission data in Bi2212. Calculated
temperature dependencies of the spin correlation length, spin-lattice
relaxation times at the Cu and O sites, and static susceptibility are typical
for the quantum disordered regime with a pseudogap in the spectrum of magnetic
excitations. These quantities are in qualitative and in some cases in
quantitative agreement with experiment in underdoped YBa2Cu3O(6+y). At x > 0.12
the considered phase borders the phase of conventional metal.",9808087v1
1998-12-31,The Heisenberg antiferromagnet on an anisotropic triangular lattice: linear spin-wave theory,"We consider the effect of quantum spin fluctuations on the ground state
properties of the Heisenberg antiferromagnet on an anisotropic triangular
lattice using linear spin-wave theory. This model should describe the magnetic
properties of the insulating phase of the kappa-(BEDT-TTF)_2 X family of
superconducting molecular crystals. The ground state energy, the staggered
magnetization, magnon excitation spectra and spin-wave velocities are computed
as a function of the ratio between the second and first neighbours, J2/J1. We
find that near J2/J1 = 0.5, i.e., in the region where the classical spin
configuration changes from a Neel ordered phase to a spiral phase, the
staggered magnetization vanishes, suggesting the possibility of a quantum
disordered state. In this region, the quantum correction to the magnetization
is large but finite. This is in contrast to the frustrated Heisenberg model on
a square lattice, for which the quantum correction diverges logarithmically at
the transition from the Neel to the collinear phase. For large J2/J1, the model
becomes a set of chains with frustrated interchain coupling. For J2 > 4 J1, the
quantum correction to the magnetization, within LSW, becomes comparable to the
classical magnetization, suggesting the possibility of a quantum disordered
state. We show that, in this regime, quantum fluctuations are much larger than
for a set of weakly coupled chains with non-frustated interchain coupling.",9812429v2
2000-01-07,Theory of the spin bath,"The quantum dynamics of mesoscopic or macroscopic systems is always
complicated by their coupling to many ""environmental"" modes.At low T these
environmental effects are dominated by localised modes, such as nuclear and
paramagnetic spins, and defects (which also dominate the entropy and specific
heat). This environment, at low energies, maps onto a ""spin bath"" model. This
contrasts with ""oscillator bath"" models (originated by Feynman and Vernon)
which describe {\it delocalised} environmental modes such as electrons,
phonons, photons, magnons, etc. One cannot in general map a spin bath to an
oscillator bath (or vice-versa); they constitute distinct ""universality
classes"" of quantum environment. We show how the mapping to spin bath models is
made, and then discuss several examples in detail, including moving particles,
magnetic solitons, nanomagnets, and SQUIDs, coupled to nuclear and paramagnetic
spin environments. We show how to average over spin bath modes, using an
operator instanton technique, to find the system dynamics, and give analytic
results for the correlation functions, under various conditions. We then
describe the application of this theory to magnetic and superconducting
systems.Particular attention is given to recent work on tunneling magnetic
macromolecules, where the role of the nuclear spin bath in controlling the
tunneling is very clear; we also discuss other magnetic systems in the quantum
regime, and the influence of nuclear and paramagnetic spins on flux dynamics in
SQUIDs.",0001080v2
2000-05-04,Optical Properties of the Spin-Ladder Compound Sr_{14}Cu_{24}O_{41},"We report the measurements of the pseudodielectric function, far-infrared
reflectivity and Raman scattering spectra in Sr_{14}Cu_{24}O_{41} single
crystal. We study the lattice and the spin dynamics of the Cu$_2$O$_3$ spin
ladders and CuO$_2$ chains of this compound. The ellipsometric and the optical
reflectivity measurements yield the gap values of 1.4 eV, 1.86 eV, 2.34 eV (2.5
eV) for the ladders (chains) along the {\bf c}-axis and 2.4 eV along the {\bf
a}-axis. The electronic structure of the Cu$_2$O$_3$ ladders is analyzed using
tight-binding approach for the correlated electron systems. The correlation gap
value of 1.4 eV is calculated with the transfer energy (hopping) parameters
$t=t_{0}$=0.26 eV, along and perpendicular to legs, $t_{xy}$=0.026 eV
(interladder hopping) and U=2.1 eV, as a Coulomb repulsion. The optical
parameters of the infrared active phonons and plasmons are obtained by
oscillator fitting procedure of the reflectivity spectra. Raman scattering
spectra are measured at different temperatures using different laser line
energies. The two-magnon peak is observed at about 2880 cm$^{-1}$. At
temperatures below 150 K the new infrared and Raman modes appear due to the
charge ordering.",0005096v1
2001-04-24,Field-induced magnetic transitions in the quasi-two-dimensional heavy-fermion antiferromagnets Ce_{n}RhIn_{3n+2} (n=1 or 2),"We have measured the field-dependent heat capacity in the tetragonal
antiferromagnets CeRhIn$_{5}$ and Ce$_{2}$RhIn$_{8}$, both of which have an
enhanced value of the electronic specific heat coefficient $\gamma \sim 400$
mJ/mol-Ce K$^{2}$ above $T_{N}$. For $T<T_{N},$ the specific heat data at zero
applied magnetic field are consistent with the existence of an anisotropic
spin-density wave opening a gap in the Fermi surface for CeRhIn$_{5},$ while
Ce$_{2}$RhIn$_{8}$ shows behavior consistent with a simple antiferromagnetic
magnon. From these results, the magnetic structure, in a manner similar to the
crystal structure, appears more two-dimensional in CeRhIn$_{5}$ than in
Ce$_{2}$RhIn$_{8}$ where only about 12% of the Fermi surface remains ungapped
relative to 92% for Ce$_{2}$RhIn$_{8}$. When $B||c,$ both compounds behave in a
manner expected for heavy fermion systems as both $T_{N}$ and the electronic
heat capacity decrease as field is applied. When the field is applied in the
tetragonal basal plane ($B||a$), CeRhIn$_{5}$ and Ce$_{2}$RhIn$_{8}$ have very
similar phase diagrams which contain both first- and second-order field-induced
magnetic transitions .",0104446v1
2002-07-04,"Dynamics, Selection Rules and Dzyaloshinsky-Moriya interactions in Strongly Frustrated Magnets","Anisotropic spin-spin interactions of the symmetry described by Dzyaloshinsky
and Moriya are generally considered weak, as they depend on the spin-orbit
couplings. In frustrated spin systems with singlet ground states they can,
however, have rather strong effects. We discuss recent results related to two
gapped spin systems: $\rm CuGeO_3$ and $\rm SrCu_2(BO_3)_2$ in particular. In
the first compound the Dzyaloshinsky-Moriya interactions effectively lower the
symmetry of the magnetic unit cell and this leads to doubling of the low
frequency mode. In the second case, the Dzyaloshinsky-Moriya interactions also
split the lowest magnon mode linearly in the spin-orbit coupling. In addition,
the relatively weak Dzyaloshinsky-Moriya interactions can dominate the
dispersion.
  Consideration of the selection rules for optical transitions show that while
the Dzyaloshinsky-Moriya interactions can explain much of the dynamics, they do
not explain the observed transition amplitudes. This leads to a review of
recent calculations of anisotropic spin-phonon couplings. We discuss how this
leads to a novel mechanism to explain the ESR intensities in the spin gap
systems discussed. Selection rules for this novel mechanism involving coupling
to the electric field of the resonant probe are discussed and relation to
polarised neutron experiments briefly mentioned.",0207116v1
2003-01-17,Thermal conductivity of lightly Sr- and Zn-doped La$_2$CuO$_4$ single crystals,"Both ab-plane and c-axis thermal conductivities ($\kappa_{ab}$ and
$\kappa_c$) of lightly doped La$_{2-x}$Sr$_x$CuO$_4$ and
La$_2$Cu$_{1-y}$Zn$_y$O$_4$ single crystals ($x$ or $y$ = 0 -- 0.04) are
measured from 2 to 300 K. It is found that the low-temperature phonon peak (at
20 -- 25 K) is significantly suppressed upon Sr or Zn doping even at very low
doping, though its precise doping dependences show interesting differences
between the Sr and Zn dopants, or between the $ab$ plane and the c axis. Most
notably, the phonon peak in $\kappa_c$ decreases much more quickly with Sr
doping than with Zn doping, while the phonon-peak suppression in $\kappa_{ab}$
shows an opposite trend. It is discussed that the scattering of phonons by
stripes is playing an important role in the damping of the phonon heat
transport in lightly doped LSCO, in which static spin stripes has been observed
by neutron scattering. We also show $\kappa_{ab}$ and $\kappa_c$ data of
La$_{1.28}$Nd$_{0.6}$Sr$_{0.12}$CuO$_4$ and
La$_{1.68}$Eu$_{0.2}$Sr$_{0.12}$CuO$_4$ single crystals to compare with the
data of the lightly doped crystals for the discussion of the role of stripes.
At high temperature, the magnon peak (i.e., the peak caused by the spin heat
transport near the N\'{e}el temperature) in $\kappa_{ab}(T)$ is found to be
rather robust against Zn doping, while it completely disappears with only 1% of
Sr doping.",0301315v1
2003-02-25,"Magnon Specific Heat of Single Crystal Borocarbides RNi2B2C (R=Tm, Er, Ho, Dy, Tb, Gd)","Zero-field specific heat of the single crystals RNi2B2C (R= Er, Ho, Dy, Tb,
Gd) was measured within the temperature range 0.1 K<T<25 K. Linearized spin
wave analysis was successfully applied to account for and to rationalize the
thermal evolution of the low--temperature magnetic specific heats of all the
studied compounds (as well as the one reported for TmNi2B2C) in terms of only
two parameters, namely an energy gap, delta, and a characteristic temperature,
theta. The evolution of the gap and theta across the studied compounds
correlates very well with the known magnetic properties. Theta, as a measure of
the effective RKKY exchange couplings, scales reasonably well with the de
Gennes factor. The gap, on the other hand, reflects predominately the
anisotropic properties: ~2 K for GdNi2B2C, ~6 K for ErNi2B2C, ~7 K for
TbNi2B2C, and ~8 K for each of HoNi2B2C and DyNi2B2C. The equality in the gap
of HoNi2B2C and DyNi2B2C, coupled with the similarity in their magnetic
configurations, indicates that a variation of x in the solid solution
HoxDy1-xNi2B2C (x<0.8 and Tc<Tn) would not lead to any softening of the gap.
This supports the hypothesis of Cho et al (PRL 77,163(1996)) concerning the
influence of the collective magnetic excitations on the superconducting state.
This work underlines the importance of spin-wave excitations for a valid
description of low-temperature thermodynamics of borocarbides.",0302526v1
2003-03-18,On the significance of quantum effects and interactions for the apparent universality of Bloch laws for M_s(T),"The apparent universality of Bloch's T^{3/2}-law for the temperature
dependence of the spontaneous magnetization, and of generalizations thereof, is
considered. It is argued that in the derivation one should not only consider
the exchange interaction between the spins, but also the other interactions
between them, leading to elliptical spin precession and deviations from the
parabolic dispersion of magnons. Also interaction effects are important to
explain the apparent universality of generalized Bloch law exponents e_B,
defined by M_s(T)= M_s(0)-const. x T^{e_B}, valid in a wide temperature range
T_1 < T < T_2, and for dimensionalities d = 1, 2, and 3. The above-mentioned
temperature range, the 'Bloch range', lies above the quantum range, where
magnetic long-range order (e.g. in d=2 dimensions) is nontrivially enforced by
the additional interactions, but below the thermal critical region, where
universal 'anomalous scaling dimensions' apply. In contrast, for the Bloch
temperature region, the universality is only apparent, i.e. a
crossover-phenomenon, and simple scaling considerations with 'normal
dimensions' apply.
  However, due to interactions, the Bloch exponent e_B depends not only on the
dimensionality d of the system, but also on the spin quantum number s (mod
(1/2)) of the system, i.e. for given d the Bloch exponent e_B is different for
half-integer s and for integer s.",0303357v3
2003-06-24,First-order transition in the itinerant ferromagnet CoS$_{1.9}$Se$_{0.1}$,"Undoped CoS$_2$ is an isotropic itinerant ferromagnet with a continuous or
nearly continuous phase transition at $T_{\rm C} = 122$ K. In the doped
CoS$_{1.9}$Se$_{0.1}$ system, the Curie temperature is lowered to $T_{\rm C} =
90$ K, and the transition becomes clearly first order in nature. In particular
we find a discontinuous evolution of the spin dynamics as well as strong time
relaxation in the ferromagnetic Bragg intensity and small angle neutron
scattering in vicinity of the ferromagnetic transition. In the ordered state
the long-wavelength spin excitations were found to be conventional
ferromagnetic spin-waves with negligible spin-wave gap ($ < 0.04$ meV),
indicating that this system is also an excellent isotropic (soft) ferromagnet.
In a wide temperature range up to $0.9T_{\rm C}$, the spin-wave stiffness
$D(T)$ follows the prediction of the two-magnon interaction theory, $D(T) =
D(0)(1 - AT^{5/2})$, with $D(0) = 131.7 \pm 2.8$ meV-\AA$^{2}$. The stiffness,
however, does not collapse as $T \to T_{\rm C}$ from below. Instead a
quasielastic central peak abruptly develops in the excitation spectrum, quite
similar to results found in the colossal magnetoresistance oxides such as
(La-Ca)MnO$_3$.",0306618v1
2004-03-13,Interplay of structural and electronic phase separation in single crystalline La(2)CuO(4.05) studied by neutron and Raman scattering,"We report a neutron and Raman scattering study of a single-crystal of
La(2)CuO(4.05) prepared by high temperature electrochemical oxidation. Elastic
neutron scattering measurements show the presence of two phases, corresponding
to the two edges of the first miscibility gap, all the way up to 300 K. An
additional oxygen redistribution, driven by electronic energies, is identified
at 250 K in Raman scattering (RS) experiments by the simultaneous onset of
two-phonon and two-magnon scattering, which are fingerprints of the insulating
phase. Elastic neutron scattering measurements show directly an
antiferromagnetic ordering below a N\'eel temperature of T_N =210K. The opening
of the superconducting gap manifests itself as a redistribution of electronic
Raman scattering below the superconducting transition temperature, T_c = 24K. A
pronounced temperature-dependent suppression of the intensity of the (100)
magnetic Bragg peak has been detected below T_c. We ascribe this phenomenon to
a change of relative volume fraction of superconducting and antiferromagnetic
phases with decreasing temperature caused by a form of a superconducting
proximity effect.",0403340v1
2004-03-28,Fully polarized states and decoherence,"The aim of this review is to show how ``ferromagnetic'' states, that is,
states having a fully polarization, can produce intrinsic decoherence by
unitary evolution. This effect can give an understanding of recent experiments
on mesoscopic devices as quantum point contacts showing the 0.7 conductance
anomaly and the wide number of data about saturation of dephasing time observed
at very low temperatures, as a fully polarized two dimensional electron gas.
But similar effects can be seen in different area of physics as for example the
Dicke model describing the interaction of two-level systems with a radiation
mode. In this case one can show that decoherence is intrinsic and remove a
Schr\""odinger cat state leaving a single coherent state, collapsing the wave
function in the thermodynamic limit. So, saturation of dephasing time at low
temperatures in mesoscopic devices can be understood by a fully polarized two
dimensional electron gas that, by an exchange model, can be reduced to a
generalized form of the Dicke Hamiltonian and where the quasiparticles are spin
excitations interacting with magnons. In this way, one can see that several
experiments on nanowires and quantum dots can be satisfactorily explained. The
existence of intrinsic decoherence in the thermodynamic limit could have deep
implications in fundamental problems like quantum measurement and
irreversibility. Recent experiments with cavities with a large number of
photons and with nuclear magnetic resonance in organic molecular crystals give
a first strong support to this view.",0403678v1
2004-04-19,Néel-Dimer Transition in Antiferromagnetic Heisenberg Model and Deconfinement of Spinons at the Critical Point,"Quantum phase transition from the N\'eel to the dimer states in an
antiferromagnetic(AF) Heisenberg model on square lattice is studied. We
introduce a control parameter $\alpha$ for the exchange coupling which connects
the N\'eel ($\alpha=0$) and the dimer ($\alpha=1$) states. We employ the $CP^1$
(the Schwinger boson) representation of the $s={1\over 2}$ spin operator and
integrate out the half of the $CP^1$ variables at odd sites and we obtain a
$CP^1$ nonlinear $\sigma$ model. The effective coupling constant is a function
of $\alpha$ and at $\alpha=0$ the $CP^1$ model is in the ordered phase which
corresponds to the N\'eel state of the AF Heisenberg model. A phase transition
to the dimer state occurs at a certain critical value of $\alpha_C$ as $\alpha$
increases. In the N\'eel state, the dynamical composite U(1) gauge field in the
$CP^1$ model is in a Higgs phase and low-energy excitations are gapless spin
wave. In the dimer phase, a confinement phase of the gauge theory is realized
and low-energy excitations are $s=1$ magnons. For the critical point, we argue
that a deconfinement phase, which is similar to the Coulomb phase in 3 spatial
dimensions, is realized and $s={1\over 2}$ spinons appear as low-energy
excitations.",0404427v4
2004-05-19,Auxiliary-Fermion Approach to Critical Fluctuations in the 2D Quantum AF Heisenberg Model,"The nearest-neighbor quantum-antiferromagnetic (AF) Heisenberg model for spin
1/2 on a two-dimensional square lattice is studied in the auxiliary-fermion
representation. Expressing spin operators by canonical fermionic particles
requires a constraint on the fermion charge Q=1 on each lattice site, which is
imposed approximately through the thermal average. The resulting interacting
fermion system is first treated in mean-field theory (MFT), which yields an AF
ordered ground state and spin waves in quantitative agreement with conventional
spin-wave theory. At finite temperature a self-consistent approximation beyond
mean field is required in order to fulfill the Mermin-Wagner theorem. We first
discuss a fully self-consistent approximation, where fermions are renormalized
due to fluctuations of their spin density, in close analogy to FLEX. While
static properties like the correlation length come out correctly, the dynamical
response lacks the magnon-like peaks which would reflect the appearance of
short-range order at low T. This drawback, which is caused by overdamping, is
overcome in a `minimal self-consistent approximation' (MSCA), which we derive
from the equations of motion. The MSCA features dynamical scaling at small
energy and temperature and is qualitatively correct both in the regime of
order-parameter relaxation at long wavelengths and in the short-range-order
regime. We also discuss the impact of vertex corrections and the problem of
pseudo-gap formation in the single-particle density of states due to long-range
fluctuations. Finally we show that the (short-range) magnetic order in MFT and
MSCA helps to fulfill the constraint on the local fermion occupancy.",0405438v2
2004-06-02,Asymmetry of the electronic states in hole- and electron-doped cuprates: Exact diagonalization study of the t-t'-t''-J model,"We systematically examine the asymmetry of the electronic states in the hole-
and electron-doped cuprates by using the t-t'-t''-J model. Numerically exact
diagonalization method is employed for a 20-site square lattice. We impose
twisted boundary conditions (BC) instead of standard periodic BC. For static
and dynamical correlation functions, averaging procedure over the twisted BC is
used to reduce the finite-size effect. We find that antiferromagnetic spin
correlation remains strong in electron doping in contrast to the case of hole
doping, being similar to the case of the periodic BC. This leads to a
remarkable electron-hole asymmetry in the dynamical spin structure factor and
two-magnon Raman scattering. By changing the twist, the single-particle
spectral function is obtained for all momenta in the Brillouin zone. Examining
the spectral function in detail, we find a gap opening at around the k=(pi,0)
region for 10% doping of holes (the carrier concentration x=0.1), leading to a
Fermi arc that is consistent with experiments. In electron doping, however, a
gap opens at around k=(pi/2,pi/2) and persists up to x=0.2, being correlated
with the strength of the antiferromagnetic correlation. We find that the
magnitude of the gaps is sensitive to t' and t''. A pseudogap is also seen in
the optical conductivity for electron doping, and its magnitude is found to be
the same as that in the spectral function. We compare calculated quantities
with corresponding experimental data, and discuss similarities and differences
between them as well as their implications.",0406041v2
2005-02-08,Generalized kinetic and evolution equations in the approach of the nonequilibrium statistical operator,"The method of the nonequilibrium statistical operator developed by D. N.
Zubarev is employed to analyse and derive generalized transport and kinetic
equations. The degrees of freedom in solids can often be represented as a few
interacting subsystems (electrons, spins, phonons, nuclear spins, etc.).
Perturbation of one subsystem may produce a nonequilibrium state which is then
relaxed to an equilibrium state due to the interaction between particles or
with a thermal bath. The generalized kinetic equations were derived for a
system weakly coupled to a thermal bath to elucidate the nature of transport
and relaxation processes. It was shown that the ""collision term"" had the same
functional form as for the generalized kinetic equations for the system with
small interactions among particles. The applicability of the general formalism
to physically relevant situations is investigated. It is shown that some known
generalized kinetic equations (e.g. kinetic equation for magnons, Peierls
equation for phonons) naturally emerges within the NSO formalism. The
relaxation of a small dynamic subsystem in contact with a thermal bath is
considered on the basis of the derived equations. The Schrodinger-type equation
for the average amplitude describing the energy shift and damping of a particle
in a thermal bath and the coupled kinetic equation describing the dynamic and
statistical aspects of the motion are derived and analysed. The equations
derived can help in the understanding of the origin of irreversible behavior in
quantum phenomena.",0502194v1
2005-06-07,Effect of local charge fluctuations on spin physics in the Neel state of La$_2$CuO$_4$,"We explore the effect of local charge fluctuations on the spin response of a
Mott insulator by deriving an effective spin model, and studying it using
Schwinger boson mean field theory. Applying this to La$_2$CuO$_4$, we show that
an accurate fit to the magnon dispersion relation, measured by Coldea {\em et
al.} [Phys. Rev. Lett. {\bf 86}, 5377 (2001)] is obtained with Hubbard model
parameters $U \approx 2.34 eV$, and $t \approx 360 meV$. These parameters lead
to estimates of the staggered magnetization ($m_s \approx 0.25$), spin wave
velocity ($c\approx 800 meV$-\AA), and spin stiffness ($\rho_s \approx 24
meV$). In particular the staggered moment as well as the effective local moment
are renormalized to smaller values compared to the Heisenberg model due to
local charge fluctuations in the Hubbard model. The dynamical structure factor
shows considerable weight in the continuum along the zone boundary as well as
secondary peaks that may be observed in high resolution neutron scattering
experiments.",0506188v2
2005-10-07,"Collective Spin and Charge Excitations in (Sr,La)_{14-x}Ca_xCu_{24}O_{41} Quantum Spin Ladders","We study magnetic and electronic properties of two-leg ladder materials. We
observed a two-magnon (2M) resonance which we analyze in terms of symmetry,
relaxation and resonance properties. Our findings were contrasted to 2M Raman
measurements in other magnetic crystals. This comparison made us suggest that
the spin-spin correlations in a self-doped two leg ladder may have a modulated
component besides the exponential decay characteristic of a spin liquid ground
state. We found that the 2M intensity resonates at the Mott gap energy.
Interplane Sr substitution for Ca introduces strong disorder leading to
inhomogeneous broadening of the 2M resonance. The doped holes in the spin
liquid ground state further dilute the magnetic correlations, suppressing the
spectral weight of this excitation. At high Ca concentrations are
superconducting under pressure and hole pairing was proposed to be a robust
feature of doped ladders. The measured dielectric response in the microwave
region, the low energy Raman data, the non-linear transport properties along
with soft x-ray scattering allowed us to conclude that the ground state for a
wide range of Ca concentrations (x < 12) is characterized by charge density
wave correlations. This state seems to be driven not by phonons but by Coulomb
forces and many-body effects. We highlighted the similarity in the finite
frequency Raman response as opposed to the very different behavior of the DC
resistivity between undoped and doped ladders. We found that at high Ca
concentrations the carrier relaxation is characterized by the same large
activation energy (~2000 K) as in the self-doped compound. This observation
prompted us to suggest an unconventional metallic transport driven by
collective electronic response.",0510193v1
2006-01-23,Is room-temperature superconductivity with phonons possible?,"By recognizing the vital importance of two-hole Cooper pairs (CPs) in
addition to the usual two-electron ones in a strongly-interacting many-electron
system, the concept of CPs was re-examined with striking conclusions. Based on
this, Bose-Einstein condensation (BEC) theory has been generalized to include
not boson-boson interactions (also neglected in BCS theory) but rather
boson-fermion (BF)interaction vertices reminiscent of the Frohlich
electron-phonon interaction in metals. Unlike BCS theory, the GBEC model is not
a mean-field theory restricted to weak-coupling as it can be diagonalized
exactly. In weak coupling it reproduces the BCS condensation energy. Each kind
of CP is responsible for only half the condensation energy. The GBEC theory
reduces to all the old known statistical theories as special cases including
the so-called ""BCS-Bose crossover"" picture which in turn generalizes BCS theory
by not assuming that the electron chemical potential equals the Fermi energy.
Indeed, a BCS condensate is precisely the weak-coupling limit of a GBE
condensate with equal numbers of both types of CPs. With feasible Cooper/BCS
model interelectonic interaction parameter values, and even without BF
interactions, the GBEC theory yields transition temperatures [including
room-temperature superconductivity (RTSC)] substantially higher than the BCS
ceiling of around 45K, without relying on non-phonon dynamics involving
excitons, plasmons, magnons or otherwise purely-electronic mechanisms.",0601526v5
2006-09-26,Competing impurities and reentrant magnetism in La(2-x)Sr(x)Cu(1-z)Zn(z)O(4) revisited. The role of the Dzyaloshinskii-Moriya and XY anisotropies,"We study the order-from-disorder transition and reentrant magnetism in
La(2-x)Sr(x)Cu(1-z)Zn(z)O(4) within the framework of a long-wavelength
nonlinear sigma model that properly incorporates the Dzyaloshinskii-Moriya and
XY anisotropies. Doping with nonmagnetic impurities, such as Zn, is considered
according to classical percolation theory, whereas the effect of Sr, which
introduces charge carriers into the CuO(2) planes, is described as a dipolar
frustration of the antiferromagnetic order. We calculate several magnetic,
thermodynamic, and spectral properties of the system, such as the
antiferromagnetic order parameter, the Neel temperature, the spin-stiffness,
and the anisotropy gaps, as well as their evolution with both Zn and Sr doping.
We explain the nonmonotonic and reentrant behavior experimentally observed for
T_N by Hucker et al. in Phys. Rev. B 59, R725 (1999), as resulting from the
reduction, due to the nonmagnetic impurities, of the dipolar frustration
induced by the charge carriers (order-from-disorder). Furthermore, we find a
similar nonmonotonic and reentrant behavior for all the other observables
studied. Most remarkably, our results show that while for x=2% and z=0 the
Dzyaloshinskii-Moriya gap \Delta_{DM}=0, for z=15% it is approximately
\Delta_{DM} = 7.5 cm^(-1). The later is larger than the lowest low-frequency
cutoff for Raman spectroscopy (~ 5 cm^(-1)), and could thus be observed in
one-magnon Raman scattering.",0609653v1
2005-10-11,Mass of Rotating Black Holes in Gauged Supergravities,"The masses of several recently-constructed rotating black holes in gauged
supergravities, including the general such solution in minimal gauged
supergravity in five dimensions, have until now been calculated only by
integrating the first law of thermodynamics. In some respects it is more
satisfactory to have a calculation of the mass that is based directly upon the
integration of a conserved quantity derived from a symmetry principal. In this
paper, we evaluate the masses for the newly-discovered rotating black holes
using the conformal definition of Ashtekar, Magnon and Das (AMD), and show that
the results agree with the earlier thermodynamic calculations. We also consider
the Abbott-Deser (AD) approach, and show that this yields an identical answer
for the mass of the general rotating black hole in five-dimensional minimal
gauged supergravity. In other cases we encounter discrepancies when applying
the AD procedure. We attribute these to ambiguities or pathologies of the
chosen decomposition into background AdS metric plus deviations when scalar
fields are present. The AMD approach, involving no decomposition into
background plus deviation, is not subject to such complications. Finally, we
also calculate the Euclidean action for the five-dimensional solution in
minimal gauged supergravity, showing that it is consistent with the quantum
statistical relation.",0510081v3
2007-08-24,Disordered Correlated Kondo-lattice model,"We propose a self-consistent approximate solution of the disordered
Kondo-lattice model (KLM) to get the interconnected electronic and magnetic
properties of 'local-moment' systems like diluted ferromagnetic semiconductors.
Aiming at $(A_{1-x}M_x)$ compounds, where magnetic (M) and non-magnetic (A)
atoms distributed randomly over a crystal lattice, we present a theory which
treats the subsystems of itinerant charge carriers and localized magnetic
moments in a homologous manner. The coupling between the localized moments due
to the itinerant electrons (holes) is treated by a modified RKKY-theory which
maps the KLM onto an effective Heisenberg model. The exchange integrals turn
out to be functionals of the electronic selfenergy guaranteeing selfconsistency
of our theory. The disordered electronic and magnetic moment systems are both
treated by CPA-type methods.
  We discuss in detail the dependencies of the key-terms such as the long range
and oscillating effectice exchange integrals, 'the local-moment' magnetization,
the electron spin polarization, the Curie temperature as well as the electronic
and magnonic quasiparticle densities of states on the concentration $x$ of
magnetic ions, the carrier concentration $n$, the exchange coupling $J$, and
the temperature. The shape and the effective range of the exchange integrals
turn out to be strongly $x$-dependent. The disorder causes anomalies in the
spin spectrum especially in the low-dilution regime, which are not observed in
the mean field approximation.",0708.3361v1
2007-12-02,Role of the conduction electrons in mediating exchange interactions in Heusler alloys,"Because of large spatial separation of the Mn atoms in Heusler alloys the Mn
3d states belonging to different atoms do not overlap considerably. Therefore
an indirect exchange interaction between Mn atoms should play a crucial role in
the ferromagnetism of the systems. To study the nature of the ferromagnetism of
various Mn-based semi- and full-Heusler alloys we perform a systematic
first-principles calculation of the exchange interactions in these materials.
The calculation of the exchange parameters is based on the frozen-magnon
approach. The calculations show that the magnetism of the Mn-based Heusler
alloys depends strongly on the number of conduction electrons, their spin
polarization and the position of the unoccupied Mn 3d states with respect to
the Fermi level. Various magnetic phases are obtained depending on the
combination of these characteristics. The Anderson's s-d model is used to
perform a qualitative analysis of the obtained results. The conditions leading
to diverse magnetic behavior are identified. If the spin polarization of the
conduction electrons at the Fermi energy is large and the unoccupied Mn 3d
states lie well above the Fermi level, an RKKY-type ferromagnetic interaction
is dominating. On the other hand, the contribution of the antiferromagnetic
superexchange becomes important if unoccupied Mn 3d states lie close to the
Fermi energy. The resulting magnetic behavior depends on the competition of
these two exchange mechanisms. The calculational results are in good
correlation with the conclusions made on the basis of the Anderson s-d model
which provides useful framework for the analysis of the results of
first-principles calculations and helps to formulate the conditions for high
Curie temperature.",0712.0158v1
2007-12-25,Hierarchies of Geometric Entanglement,"We introduce a class of generalized geometric measures of entanglement. For
pure quantum states of $N$ elementary subsystems, they are defined as the
distances from the sets of $K$-separable states ($K=2,...,N$). The entire set
of generalized geometric measures provides a quantification and hierarchical
ordering of the different bipartite and multipartite components of the global
geometric entanglement, and allows to discriminate among the different
contributions. The extended measures are applied to the study of entanglement
in different classes of $N$-qubit pure states. These classes include $W$ and
$GHZ$ states, and their symmetric superpositions; symmetric multi-magnon
states; cluster states; and, finally, asymmetric generalized $W$-like
superposition states. We discuss in detail a general method for the explicit
evaluation of the multipartite components of geometric entanglement, and we
show that the entire set of geometric measures establishes an ordering among
the different types of bipartite and multipartite entanglement. In particular,
it determines a consistent hierarchy between $GHZ$ and $W$ states, clarifying
the original result of Wei and Goldbart that $W$ states possess a larger global
entanglement than $GHZ$ states. Furthermore, we show that all multipartite
components of geometric entanglement in symmetric states obey a property of
self-similarity and scale invariance with the total number of qubits and the
number of qubits per party.",0712.4085v3
2008-02-12,Doping dependence of phonon and quasiparticle heat transport of pure and Dy-doped Bi_2Sr_2CaCu_2O_{8+δ} single crystals,"The temperature and magnetic-field (H) dependences of thermal conductivity
(\kappa) of Bi_2Sr_2CaCu_2O_{8+\delta} (Bi2212) are systematically measured for
a broad doping range by using both pure Bi2212 single crystals with tuned
oxygen contents and Bi_2Sr_2Ca_{1-x}Dy_xCu_2O_{8+\delta} (Dy-Bi2212) single
crystals with different Dy contents x. In the underdoped samples, the
quasiparticle (QP) peak below T_c is strongly suppressed, indicating strong QP
scattering by impurities or oxygen defects, whereas the phonon conductivity is
enhanced in moderately Dy-doped samples and a phonon peak at 10 K is observed
for the first time in Bi2212 system, which means Dy^{3+} ions not only
introduce the impurities or point defects but also stabilize the crystal
lattice. The subkelvin data show that the QP heat conductivity gradually
decreases upon lowering the hole doping level. The magnetic-field dependence of
\kappa at temperature above 5 K is mainly due to the QP scattering off
vortices. While the underdoped pure Bi2212 show very weak field dependence of
\kappa, the Dy-doped samples present an additional ""dip""-like term of \kappa(H)
at low field, which is discussed to be related to the phonon scattering by free
spins of Dy^{3+} ions. For non-superconducting Dy-Bi2212 samples with x \simeq
0.50, an interesting ""plateau"" feature shows up in the low-T \kappa(H)
isotherms with characteristic field at 1 -- 2 T, for which we discuss the
possible revlevance of magnon excitations.",0802.1559v1
2008-04-21,Finite-size corrections to the rotating string and the winding state,"We compute higher order finite size corrections to the energies of the
circular rotating string on AdS_5 x S^5, of its orbifolded generalization on
AdS_5 x S^5/Z_M and of the winding state which is obtained as the limit of the
orbifolded circular string solution when J -> infinity and J/M^2 is kept fixed.
We solve, at the first order in lambda'=lambda/J^2, where lambda is the 't
Hooft coupling, the Bethe equations that describe the anomalous dimensions of
the corresponding gauge dual operators in an expansion in m/K, where m is the
winding number and K is the ""magnon number"", and to all orders in the angular
momentum J. The solution for the circular rotating string and for the winding
state can be matched to the energy computed from an effective quantum
Landau-Lifshitz model beyond the first order correction in 1/J. For the leading
1/J corrections to the circular rotating string in m^2 and m^4 and for the
subleading 1/J^2 corrections to the m^2 term, we find agreement. For the
winding state we match the energy completely up to, and including, the order
1/J^2 finite-size corrections. The solution of the Bethe equations
corresponding to the spinning closed string is also provided in an expansion in
m/K and to all orders in J.",0804.3301v2
2008-09-24,Quantum spinning strings in AdS_4 x CP^3: testing the Bethe Ansatz proposal,"Recently, an asymptotic Bethe Ansatz that is claimed to describe anomalous
dimensions of ""long"" operators in the planar N=6 supersymmetric
three-dimensional Chern-Simons-matter theory dual to quantum superstrings in
AdS_4 x CP^3 was proposed. It initially passed a few consistency checks but
subsequent direct comparison to one-loop string-theory computations created
some controversy. Here we suggest a resolution by pointing out that, contrary
to the initial assumption based on the algebraic curve considerations, the
central interpolating function h(\lambda) entering the BMN or magnon dispersion
relation receives a non-zero one-loop correction in the natural string-theory
computational scheme. We consider a basic example which has already played a
key role in the AdS_5 x S^5 case: a rigid circular string stretched in both
AdS_4 and along an S^1 of CP^3 and carrying two spins. Computing the leading
one-loop quantum correction to its energy allows us to fix the constant
one-loop term in h(\lambda) and also to suggest how one may establish a
correspondence with the Bethe Ansatz proposal, including the non-trivial
one-loop phase factor. We discuss some problems which remain in trying to match
a part of world-sheet contributions (sensitive to compactness of the string
direction) and their Bethe Ansatz counterparts.",0809.4038v2
2008-10-15,A Large Iron Isotope Effect in SmFeAsO1-xFx and Ba1-xKxFe2As2,"The recent discovery of superconductivity in oxypnictides with the critical
temperature (TC) higher than McMillan limit of 39 K (the theoretical maximum
predicted by Bardeen-Cooper-Schrieffer (BCS) theory) has generated great
excitement. Theoretical calculations indicate that the electron-phonon
interaction is not strong enough to give rise to such high transition
temperatures, while strong ferromagnetic/antiferromagnetic fluctuations have
been proposed to be responsible. However, superconductivity and magnetism in
pnictide superconductors show a strong sensitivity to the lattice, suggesting a
possibility of unconventional electron-phonon coupling. Here we report the
effect of oxygen and iron isotopic mass on Tc and the spin-density wave (SDW)
transition temperature (TSDW) in SmFeAsO1-xFx and Ba1-xKxFe2As2 systems. The
results show that oxygen isotope effect on TC and TSDW is very little, while
the iron isotope exponent alpha=-dlnTc/dlnM is about 0.35, being comparable to
0.5 for the full isotope effect. Surprisingly, the iron isotope exchange shows
the same effect on TSDW as TCc These results indicate that electron-phonon
interaction plays some role in the superconducting mechanism, but simple
electron-phonon coupling mechanism seems to be rather unlikely because a strong
magnon-phonon coupling is included. Sorting out the interplay between the
lattice and magnetic degrees of freedom is a key challenge for understanding
the mechanism of high-TC superconductivity.",0810.2694v2
2009-01-21,Instability of the collinear phase in two-dimensional ferromagnet in strong in-plane magnetic field,"It is well-known that in thin ferromagnetic film with a net magnetization
perpendicular to the film the collinear arrangement of spins is unstable in an
in-plane field $H$ smaller than its saturation value $H_c$. Existence of a
stripe phase was proposed with elongated domains of alternating direction of
magnetization component perpendicular to the film. We consider in the present
paper the strong-field regime $H<H_c$ and discuss the minimal microscopic model
describing this phenomenon, two-dimensional Heisenberg ferromagnet with strong
easy-axis anisotropy and dipolar forces. The noncollinear (stripe) phase is
discussed using the technique of the Bose-Einstein condensation of magnons.
Some previously unknown results are observed concerning the stripe phase.
Evolution is established of the spin arrangement in the noncollinear phase upon
the field rotation within the plane. We find a rapid decreasing of the period
of the stripe structure as the field decreases. We demonstrate that spins
components perpendicular to the film form a sinusoid in the noncollinear phase
at $H\approx H_c$ that transforms to a step-like profile upon the field
decreasing so that the domain wall density decreases from unity to a value much
smaller than unity. The spin-wave spectrum in the noncollinear phase is
discussed.",0901.3259v2
2009-05-14,Orbitally induced string formation in the spin-orbital polarons,"We study the spectral function of a single hole doped into the (a,b) plane of
the Mott insulator LaVO$_3$, with antiferromagnetic (AF) spin order of S=1
spins accompanied by alternating orbital (AO) order of active
$\{d_{yz},d_{zx}\}$ orbitals. Starting from the respective t-J model, with
spin-orbital superexchange and effective three-site hopping terms, we derive
the polaron Hamiltonian and show that a hole couples simultaneously to the
collective excitations of the AF/AO phase, magnons and orbitons. Next, we solve
this polaron problem using the self-consistent Born approximation and find a
stable quasiparticle solution -- a spin-orbital polaron. We show that the
spin-orbital polaron resembles the orbital polaron found in $e_g$ systems, as
e.g. in K$_2$CuF$_4$ or (to some extent) in LaMnO$_3$, and that the hole may be
seen as confined in a string-like potential. However, the spins also play a
crucial role in the formation of this polaron -- we explain how the orbital
degrees of freedom: (i) confine the spin dynamics acting on the hole as the
classical Ising spins, and (ii) generate the string potential which is of the
joint spin-orbital character. Finally, we discuss the impact of the results
presented here on the understanding of the phase diagrams of the lightly doped
cubic vanadates.",0905.2380v1
2009-07-31,Effects of Disorder and Interactions in the Quantum Hall Ferromagnet,"This work treats the effects of disorder and interactions in a quantum Hall
ferromagnet, which is realized in a two-dimensional electron gas (2DEG) in a
perpendicular magnetic field at Landau level filling factor equal one. We study
the problem by projecting the original fermionic Hamiltonian into magnon
states, which behave as bosons in the vicinity of the ferromagnetic ground
state. The approach permits the reformulation of a strongly interacting model
into a non-interacting one. The latter is a non-perturbative scheme that
consists in treating the two-particle neutral excitations of the electron
system as a bosonic single-particle. Indeed, the employment of bosonization
facilitates the inclusion of disorder in the study of the system. It has been
shown previously that disorder may drive a quantum phase transition in the Hall
ferromagnet. However, such studies have been either carried out in the
framework of nonlinear sigma model, as an effective low-energy theory, or
included the long-range Coulomb interaction in a quantum description only up to
the Hartree-Fock level. Here, we establish the occurrence of a disorder-driven
quantum phase transition from a ferromagnetic 2DEG to a spin glass phase by
taking into account interactions between electrons up to the random phase
approximation level in a fully quantum description.",0907.5513v1
2009-09-29,New pixelized Micromegas detector for the COMPASS experiment,"New Micromegas (Micro-mesh gaseous detectors) are being developed in view of
the future physics projects planned by the COMPASS collaboration at CERN.
Several major upgrades compared to present detectors are being studied:
detectors standing five times higher luminosity with hadron beams, detection of
beam particles (flux up to a few hundred of kHz/mm^2, 10 times larger than for
the present detectors) with pixelized read-out in the central part, light and
integrated electronics, and improved robustness. Studies were done with the
present detectors moved in the beam, and two first pixelized prototypes are
being tested with muon and hadron beams in real conditions at COMPASS. We
present here this new project and report on two series of tests, with old
detectors moved into the beam and with pixelized prototypes operated in real
data taking condition with both muon and hadron beams.",0909.5402v2
2009-10-24,Determining the Anisotropic Exchange Coupling of CrO_2 via First-Principles Density Functional Theory Calculations,"We report a study of the anisotropic exchange interactions in bulk CrO_2
calculated from first principles within density functional theory. We determine
the exchange coupling energies, using both the experimental lattice parameters
and those obtained within DFT, within a modified Heisenberg model Hamiltonian
in two ways. We employ a supercell method in which certain spins within a cell
are rotated and the energy dependence is calculated and a spin-spiral method
that modifies the periodic boundary conditions of the problem to allow for an
overall rotation of the spins between unit cells. Using the results from each
of these methods, we calculate the spin-wave stiffness constant D from the
exchange energies using the magnon dispersion relation. We employ a Monte Carlo
method to determine the DFT-predicted Curie temperature from these calculated
energies and compare with accepted values. Finally, we offer an evaluation of
the accuracy of the DFT-based methods and suggest implications of the competing
ferro- and antiferromagnetic interactions.",0910.4671v2
2010-01-12,Phonon Spectroscopy Near Phase Transition Temperatures in Multferroic BiFeO3 Epitaxial Thin Films,"We report a Raman scattering investigation of multiferroic bismuth ferrite
BiFeO3 epitaxial (c-axis oriented) thin films from -192 to 1000C. Phonon
anomalies have been observed in three temperature regions: in the gamma-phase
from 930C to 950C; at ~370C, Neel temperature (TN), and at ~123C, due to a
phase transition of unknown type (magnetic or structural). An attempt has been
made to understand the origin of the weak phonon-magnon coupling and the
dynamics of the phase sequence. The disappearance of several Raman modes at
~820C (Tc) is compatible with the known structural phase transition and the
Pbnm orthoferrite space group assigned by Arnold {\it et al.} \cite{arnold:09}.
The spectra also revealed a {\it non-cubic} $\beta$-phase from 820-930\dc and
the same {\it non-cubic} phase extends through the $\gamma$-phase between
930-950\dc, in agreement with Arnold {\it et al.} \cite{arnold2:09}, and an
evidence of a cubic $\delta$-phase around 1000\dc in thin films that is not
stable in powder and bulk. Such a cubic phase has been theoretically predicted
in \cite{vasquez:prb09}. Micro-Raman scattering and X-ray diffraction showed no
structural decomposition in thin films during the thermal cycling from
22-1000\dc.",1001.1997v1
2010-02-17,Measurement of Gilbert damping parameters in nanoscale CPP-GMR spin-valves,"In-situ, device level measurement of thermal mag-noise spectral linewidths in
60nm diameter CPP-GMR spin-valve stacks of IrMn/ref/Cu/free, with reference and
free layer of similar CoFe/CoFeGe alloy, are used to simultaneously determine
the intrinsic Gilbert damping for both magnetic layers. It is shown that
careful alignment at a ""magic-angle"" between free and reference layer static
equilibrium magnetization can allow direct measurement of the broadband
intrinsic thermal spectra in the virtual absence of spin-torque effects which
otherwise grossly distort the spectral line shapes and require linewidth
extrapolations to zero current (which are nonetheless also shown to agree well
with the direct method). The experimental magic-angle spectra are shown to be
in good qualitative and quantitative agreement with both macrospin calculations
and micromagnetic eigenmode analysis. Despite similar composition and
thickness, it is repeatedly found that the IrMn exchange pinned reference layer
has ten times larger intrinsic Gilbert damping (alpha ~ 0.1) than that of the
free-layer (alpha ~ 0.01). It is argued that the large reference layer damping
results from strong, off -resonant coupling to to lossy modes of an IrMn/ref
couple, rather than commonly invoked two-magnon processes.",1002.3295v1
2010-04-14,Measurement of magnetic excitations in the two-dimensional antiferromagnetic Sr2CuO2Cl2 insulator using resonant x-ray scattering:Evidence for extended interactions,"Using high-resolution resonant inelastic x-ray scattering (RIXS), we
performed a momentum-resolved study of magnetic excitations in the model
spin-1/2 2D antiferromagnetic insulator Sr_2CuCl_2O_2. We identify both a
single-spin-wave feature and a multi-magnon continuum, and show that the X-ray
polarization can be used to distinguish these two contributions in the
cross-section. The spin-waves display a large (70 meV) dispersion between the
zone-boundary points ($\pi$,0) and ($\pi$/2,$\pi$/2). Employing an extended
$t$-$t'$-$t""$-$U$ one-band Hubbard model, we find significant electronic
hopping beyond nearest-neighbor Cu ions. We conclude that sizeable extended
magnetic interactions are present in \scoc{} and probably important in all
undoped cuprates.",1004.2441v3
2010-04-22,Magnetic excitations in the metallic single-layer Ruthenates Ca(2-x)Sr(x)RuO(4) studied by inelastic neutron scattering,"By inelastic neutron scattering, we have analyzed the magnetic correlations
in the paramagnetic metallic region of the series Ca(2-x)Sr(x)RuO(4),
0.2<=x<=0.62. We find different contributions that correspond to 2D
ferromagnetic fluctuations and to fluctuations at incommensurate wave vectors
(0.11,0,0), (0.26,0,0) and (0.3,0.3,0). These components constitute the
measured response as function of the Sr-concentration x, of the magnetic field
and of the temperature. A generic model is applicable to metallic
Ca(2-x)Sr(x)RuO(4) close to the Mott transition, in spite of their strongly
varying physical properties. The amplitude, characteristic energy and width of
the incommensurate components vary only little as function of x, but the
ferromagnetic component depends sensitively on concentration, temperature and
magnetic field. While ferromagnetic fluctuations are very strong in
Ca1.38Sr0.62RuO4 with a low characteristic energy of 0.2 meV at T=1.5 K, they
are strongly suppressed in Ca1.8Sr0.2RuO4, but reappear upon the application of
a magnetic field and form a magnon mode above the metamagnetic transition. The
inelastic neutron scattering results document how the competition between
ferromagnetic and incommensurate antiferromagnetic instabilities governs the
physics of this system.",1004.3957v1
2010-06-04,"Magnetism and thermodynamics of spin-(1/2,1) decorated Heisenberg chain with spin-1 pendants","The magnetic and thermodynamic properties of a new ferrimagnetic decorated
spin-(1/2,1) Heisenberg chain with spin-1 pendant spins are investigated for
three cases: (A) J1,J2>0; (B) J1>0, J2<0; and (C) J1<0, J2>0, where J1 and J2
are the exchange couplings between spins in the chain and along the rung,
respectively. The low-lying and magnetic properties are explored jointly by the
real-space renormalization group, spin wave, and density-matrix renormalization
group methods, while the transfer-matrix renormalization group method is
invoked to study the thermodynamics. It is found that the magnon spectra
consist of a gapless and two gapped branches. Two branches in case (C) have
intersections. The coupling dependence of low-energy gaps are analyzed. In a
magnetic field, an m=3/2 (m is the magnetization per unit cell) plateau is
observed for case (A), while two plateaux at m=1/2 and 3/2 are observed for
cases (B) and (C). Between the two plateaux in cases (B) and (C), the
sublattice magnetizations for the spins coupled by ferromagnetic interactions
have novel decreasing regions with increasing the magnetic field. At finite
temperature, the zero-field susceptibility temperature product chi*T and
specific heat exhibit distinct exotic features with varying the couplings and
temperature for different cases. chi*T is found to converge as T approaches
zero, which is different from the divergent behavior in the spin-(1/2,1)
mixed-spin chain without pendants. The observed thermodynamic behaviors are
also discussed with the help of their low-lying excitations.",1006.0794v1
2010-06-25,"Comment on ""Two-spinon and four-spinon continuum in a frustrated ferromagnetic spin-1/2 chain"" by M. Enderle et al","We show that LiVCuO4 should be described by strongly ferromagnetically
coupled Heisenberg antiferromagnetic chains (HAC) in sharp contrast with the
effective exchange integrals Ji given in Enderle et al., Phys. Rev. Lett. vol.
104, 237207 (2010), and the main issues of that work, namely, (i) LiVCuO4 is
well described by two weakly ferromagnetically coupled interpenetrating
Heisenberg antiferromagnetic spin-1/2 chains, (ii) the extracted exchange
integrals J1, J2 agree with a previous spin-wave description (Enderle et al.,
Euphys. Lett. vol. 70, 237 (2005)), (iii) the spectral density of inelastic
neutron scattering (INS) above 10 meV is ascribed to a 4-spinon continuum.
Applying exact diagonalization and DMRG methods to fit their INS and
magnetization M(H) data, supported by two independent microscopic methods
(5-band Hubbard model and LSDA+U calculations), we demonstrate that LiCuVO4
exhibits strong inchain frustration with alpha =-J2/J1 < 1, i.e. strong
coupling of the HAC at odds with (i). An alternative phenomenological set in
accord with various experimental results is proposed. In view of the recent
possible discovery of quantum-spin nematics and Bose condensation of two-magnon
bound states (M. Zhitomirsky et al. arXiv:1003.4096v2 (2010), L. Svistov et al.
ibid. 1005.5668v2 (2010)) in LiCuVO4 precise knowledge of the main J-values is
of key importance.",1006.5070v2
2010-08-05,On three-point correlation functions in the gauge/gravity duality,"We study the effect of marginal and irrelevant deformations on the
renormalization of operators near a CFT fixed point. New divergences in a given
operator are determined by its OPE with the operator D that generates the
deformation. This provides a scheme to compute the couplings a_DAB between the
operator D and two arbitrary operators O_A and O_B. We exemplify for the case
of N=4 SYM, considering the simplest case of the exact Lagrangian deformation.
In this case the deformed anomalous dimension matrix is determined by the
derivative of the anomalous dimension matrix with respect to the coupling. We
use integrability techniques to compute the one-loop couplings a_LAB between
the Lagrangian and two distinct large operators built with Magnons, in the
SU(2) sector of the theory. Then we consider a_DAA at strong coupling, and show
how to compute it using the gauge/gravity duality, when D is a chiral operator
dual to any supergravity field and O_A is dual to a heavy string state. We
exemplify for the Lagrangian and operators O_A dual to heavy string states,
showing agreement with the prediction derived from the renormalization group
arguments.",1008.1070v2
2010-12-16,Coupled states of electromagnetic fields with magnetic-dipolar-mode vortices: MDM-vortex polaritons,"Under the influence of the material environment, electromagnetic fields in
the near-field regime exhibit quite different nature from those in the
far-field free space. A coupled state of an electromagnetic field with an
electric or magnetic dipole-carrying excitation is well known as a polariton.
Such a state is the result of the mixing of a photon with an excitation of a
material. The most discussed types of polaritons are phonon-polaritons,
exciton-polaritons, and surface plasmon-polaritons. Recently, it was shown that
in microwaves strong magnon-photon coupling can be achieved due to
magnetic-dipolar-mode (MDM) vortices in small thin-film ferrite disks. These
coupled states can be specified as MDM-vortex polaritons. In this paper we
study properties of MDM-vortex polaritons. We show that MDM-vortex polaritons
are characterized by helicity behaviors. For the observed frequency splits of
MDM resonances there are different-type helicities. In the split-resonance
states one has or localization, or cloaking of electromagnetic fields. We
analyze numerically a variety of the field topological structures of MDM-vortex
polaritons and give theoretical insights into the possible origin of such
topologically distinctive states. The shown properties of MDM-vortex polaritons
can be useful for realization of novel microwave metamaterial structures and
near-field sensing applications.",1012.3621v1
2010-12-29,Criticisms on and Comparison of Experimental Channel Backscattering Extraction Methods,"In this paper we critically review and compare experimental methods, based on
the Lundstrom model, to extract the channel backscattering ratio in nano
MOSFETs. Basically two experimental methods are currently used, the most common
of them is based on the measurement of the saturation drain current at
different temperatures. We show that this method is affected by very poor
assumptions and that the extracted backscattering ratio strongly underestimates
its actual value posing particular attention to the backscattering actually
extracted in nano devices. The second method is based on the direct measurement
of the inversion charge by CV characteristics and gets closer to the physics of
the backscattering model. We show, through measurements in high mobility
p-germanium devices, how the temperature based method gives the same result of
the CV based method once that its approximations are removed. Moreover we show
that the CV based method uses a number of approximations which are partially
inconsistent with the model. In particular we show, with the aid of 2D quantum
corrected device simulations, that the value of the barrier lowering obtained
through the CV based method is totally inconsistent with the barrier lowering
used to correct the inversion charge and that the extracted saturation
inversion charge is underestimated.",1012.5981v1
2011-03-21,Spontaneous Magnetization of an Ideal Ferromagnet: Beyond Dyson's Analysis,"Using the low-energy effective field theory for magnons, we systematically
evaluate the partition function of the O(3) ferromagnet up to three loops.
Dyson, in his pioneering microscopic analysis of the Heisenberg model, showed
that the spin-wave interaction starts manifesting itself in the low-temperature
expansion of the spontaneous magnetization of an ideal ferromagnet only at
order $T^4$. Although several authors tried to go beyond Dyson's result, to the
best of our knowledge, a fully systematic and rigorous investigation of higher
order terms induced by the spin-wave interaction, has never been achieved. As
we demonstrate in the present paper, it is straightforward to evaluate the
partition function of an ideal ferromagnet beyond Dyson's analysis, using
effective Lagrangian techniques. In particular, we show that the
next-to-leading contribution to the spontaneous magnetization resulting from
the spin-wave interaction already sets in at order $T^{9/2}$ -- in contrast to
all claims that have appeared before in the literature. Remarkably, the
corresponding coefficient is completely determined by the leading-order
effective Lagrangian and is thus independent of the anisotropies of the cubic
lattice. We also consider even higher-order corrections and thereby solve --
once and for all -- the question of how the spin-wave interaction in an ideal
ferromagnet manifests itself in the spontaneous magnetization beyond the Dyson
term.",1103.4110v1
2011-04-13,The AdS(5)xS(5) Semi-Symmetric Space Sine-Gordon Theory,"The generalized symmetric space sine-Gordon theories are a series of
1+1-integrable field theories that are classically equivalent to superstrings
on symmetric space spacetimes F/G. They are formulated in terms of a
semi-symmetric space as a gauged WZW model with fermions and a potential term
to deform it away from the conformal fixed point. We consider in particular the
case of PSU(2,2|4)/Sp(2,2)xSp(4) which corresponds to AdS(5)xS(5). We argue
that the infinite tower of conserved charges of these theories includes an
exotic N=(8,8) supersymmetry that is realized in a mildy non-local way at the
Lagrangian level. The supersymmetry is associated to a double central extension
of the superalgebra psu(2|2)+psu(2|2) and includes a non-trivial R symmetry
algebra corresponding to global gauge transformations, as well as 2-dimensional
spacetime translations. We then explicitly construct soliton solutions and show
that they carry an internal moduli superspace CP(2|1)xCP(2|1) with both bosonic
and Grassmann collective coordinates. We show how to semi-classical quantize
the solitons by writing an effective quantum mechanical system on the moduli
space which takes the form of a co-adjoint orbit of SU(2|2)xSU(2|2). The
spectrum consists of a tower of massive states in the short, or atypical,
symmetric representations, just as the giant magnon states of the string world
sheet theory, although here the tower is truncated.",1104.2429v2
2011-06-30,Hole spectral functions in lightly doped quantum antiferromagnets,"We study the hole and magnon spectral functions as a function of hole doping
in the two-dimensional (2D) t-J and t-t'-t""-J models working within the limits
of the spin-wave theory, by linearizing the hole-spin-deviation interaction and
by adapting the non-crossing approximation (NCA). We find that the staggered
magnetization decreases rather rapidly with doping and it goes to zero at a few
percent of hole concentration in both t-J and the t-t'-t""-J model. We find that
with doping, the residue of the quasiparticle peak at G=(pi/2,pi/2) decreases
rapidly with doping and the spectral function is in agreement with high
resolution angle-resolved photo-emission spectroscopy (ARPES) studies of the
copper-oxide superconductors. The observed large shift of the chemical
potential inside the Mott gap is found to be a result of broadening of the
quasiparticle peak. We find pockets centered at G, similar to those observed by
quantum oscillation measurements, (i) with an elliptical shape with large
eccentricity along the anti-nodal direction in the case of the t-J model, and
(ii) with an almost circular shape in the case of the t-t'-t""-J model. We also
find that the spectral intensity distribution in the doped antiferromagnet has
a waterfall-like patten along the nodal direction of the Brillouin zone, a
feature that is also seen in ARPES measurements.",1106.6332v1
2011-07-05,Deformed triangular lattice antiferromagnets in a magnetic field: role of spatial anisotropy and Dzyaloshinskii-Moriya interactions,"Recent experiments on the anisotropic spin-1/2 triangular antiferromagnet
Cs_2CuBr_4 have revealed a remarkably rich phase diagram in applied magnetic
fields, consisting of an unexpectedly large number of ordered phases. Motivated
by this finding, we study the role of spatial anisotropy, Dzyaloshinskii-Moriya
interactions, and quantum fluctuations on the magnetization process of a
triangular antiferromagnet, coming from the semiclassical limit. The richness
of the problem stems from two key facts: 1) the classical isotropic model
exhibits a large accidental ground state degeneracy, and 2) these three
ingredients compete with one another and split this degeneracy in opposing
ways. Using a variety of complementary approaches, including extensive Monte
Carlo numerics, spin-wave theory, and an analysis of Bose-Einstein condensation
of magnons at high fields, we find that their interplay gives rise to a complex
phase diagram consisting of numerous incommensurate and commensurate phases.
Our results shed light on the observed phase diagram for Cs_2CuBr_4 and suggest
a number of future theoretical and experimental directions that will be useful
for obtaining a complete understanding of this material's interesting
phenomenology.",1107.0772v4
2011-07-27,Melting of a frustration-induced dimer crystal and incommensurability in the J_1-J_2 two-leg ladder,"The phase diagram of an antiferromagnetic ladder with frustrating
next-nearest neighbor couplings along the legs is determined using numerical
methods (exact diagonalization and density-matrix renormalization group)
supplemented by strong-coupling and mean-field analysis. Interestingly, this
model displays remarkable features, bridging the physics of the J_1-J_2 chain
and of the unfrustated ladder. The phase diagram as a function of the
transverse coupling J_{\perp} and the frustration J_2 exhibits an Ising
transition between a columnar phase of dimers and the usual rung-singlet phase
of two-leg ladders. The transition is driven by resonating valence bond
fluctuations in the singlet sector while the triplet spin gap remains finite
across the transition. In addition, frustration brings incommensurability in
the real-space spin correlation functions, the onset of which evolves smoothly
from the J_1-J_2 chain value to zero in the large-J_{\perp} limit. The onset of
incommensurability in the spin structure-factor and in the dispersion relation
is also analyzed. The physics of the frustrated rung-singlet phase is well
understood using perturbative expansions and mean-field theories in the
large-J_{\perp} limit. Lastly, we discuss the effect of the non-trivial magnon
dispersion relation on the thermodynamical properties of the system. The
relation of this model and its physics to experimental observations on
compounds which are currently investigated, such as BiCu_2PO_6, is eventually
addressed.",1107.5498v2
2011-08-09,Thermal Equilibration and Thermally-Induced Spin Currents in a Thin-Film Ferromagnet on a Substrate,"Recent spin-Seebeck experiments on thin ferromagnetic films apply a
temperature difference $\Delta T_{x}$ along the length $x$ and measure a
(transverse) voltage difference $\Delta V_{y}$ along the width $y$. The
connection between these effects is complex, involving: (1) thermal
equilibration between sample and substrate; (2) spin currents along the height
(or thickness) $z$; and (3) the measured voltage difference. The present work
studies in detail the first of these steps, and outlines the other two steps.
Thermal equilibration processes between the magnons and phonons in the sample,
as well as between the sample and the substrate leads to two surface modes,
with surface lengths $\lambda$, to provide for thermal equilibration.
Increasing the coupling between the two modes increases the longer mode length
and decreases the shorter mode length. The applied thermal gradient along $x$
leads to a thermal gradient along $z$ that varies as $\sinh{(x/\lambda)}$,
which can in turn produce fluxes of the carriers of up- and down- spins along
$z$, and gradients of their associated \textit{magnetoelectrochemical
potentials} $\bar{\mu}_{\uparrow,\downarrow}$, which vary as
$\sinh{(x/\lambda)}$. By the inverse spin Hall effect, this spin current along
$z$ can produce a transverse (along $y$) voltage difference $\Delta V_y$, which
also varies as $\sinh{(x/\lambda)}$.",1108.1872v1
2011-09-27,Dispersion and damping of zone-boundary magnons in the noncentrosymmetric superconductor CePt3Si,"Inelastic neutron scattering (INS) is employed to study damped spin-wave
excitations in the noncentrosymmetric heavy-fermion superconductor CePt3Si
along the antiferromagnetic Brillouin-zone boundary in the low-temperature
magnetically ordered state. Measurements along the (1/2 1/2 L) and (H H 1/2-H)
reciprocal-space directions reveal deviations in the spin-wave dispersion from
the previously reported model. Broad asymmetric shape of the peaks in energy
signifies strong spin-wave damping by interactions with the particle-hole
continuum. Their energy width exhibits no evident anomalies as a function of
momentum along the (1/2 1/2 L) direction, which could be attributed to
Fermi-surface nesting effects, implying the absence of pronounced commensurate
nesting vectors at the magnetic zone boundary. In agreement with a previous
study, we find no signatures of the superconducting transition in the magnetic
excitation spectrum, such as a magnetic resonant mode or a superconducting spin
gap, either at the magnetic ordering wavevector (0 0 1/2) or at the zone
boundary. However, the low superconducting transition temperature in this
material still leaves the possibility of such features being weak and therefore
hidden below the incoherent background at energies ~0.1 meV, precluding their
detection by INS.",1109.5784v1
2011-10-03,Raman scattering study of delafossite magnetoelectric multiferroic compounds: CuFeO2 and CuCrO2,"Ultrasonic velocity measurements on the magnetoelectric multiferroic compound
CuFeO2 reveal that the antiferromagnetic transition observed at TN1 = 14 K
might be induced by an R-3m -> C2/m pseudoproper ferroelastic transition (G.
Quirion, M. J. Tagore, M. L. Plumer, O. A. Petrenko, Phys. Rev. B 77, 094111
(2008)). In that case, the group theory states that the order parameter
associated with the structural transition must belong to a two dimensional
irreducible representation Eg (x^2 - y^2, xy). Since this type of transition
can be driven by a Raman Eg mode, we performed Raman scattering measurements on
CuFeO2 between 5 K and 290 K. Considering that the isostructural multiferroic
compound CuCrO2 might show similar structural deformations at the
antiferromagnetic transition TN1 = 24.3 K, Raman measurements have also been
performed for comparison. At ambient temperature, the Raman modes in CuFeO2 are
observed at omega_Eg = 352 cm^-1 and omega_Ag = 692 cm^-1, while these modes
are detected at omega_E_g = 457 cm^-1 and omega_Ag = 709 cm^-1 in CuCrO2. The
analysis of the temperature dependence of modes shows that the frequency of all
modes increases down to 5 K. This typical behavior can be attributed to
anharmonic phonon-phonon interactions. These results clearly indicate that none
of the Raman active modes observed in CuFeO2 and CuCrO2 drive the pseudoproper
ferroelastic transition observed at the N\'eel temperature TN1. Finally, a
broad band at about 550 cm^-1 observed in the magnetoelectric phase of CuCrO2
below TN2 could be attributed to a magnon mode.",1110.0473v1
2011-10-10,"Ab-initio calculation of effective exchange interactions, spin waves, and Curie temperature in L2_1- and L1_2-type local moment ferromagnets","Employing first-principles electronic structure calculations in conjunction
with the frozen-magnon method we study the effective exchange interactions and
spin waves in local moment ferromagnets. As prototypes we have chosen three
L2$_1$-type full Heusler alloys Cu$_2$MnAl, Ni$_2$MnSn and Pd$_2$MnSn, and the
L1$_2$-type XPt$_3$ compounds with X= V, Cr and Mn. We have also included
CoPt$_3$ which is a usual ferromagnet. In all compounds due to the large
spatial separation ($\sim 4$ \AA) of the magnetic transition metal atoms, the
3\textit{d} states belonging to different atoms overlap weakly and as a
consequence the exchange coupling is indirect, mediated by the \textit{sp}
electrons. Calculated effective exchange parameters are long range and show
RKKY-type oscillations. The spin-wave dispersion curves are in reasonable
agreement with available experimental data. Using the calculated exchange
parameters we have estimated the Curie temperatures within both the mean-field
and the random-phase approximations. In local moment ferromagents deviations of
the estimated Curie temperature with respect to the available experimental data
occur when the ground-state electronic structure calculations overestimate the
values of the spin magnetic moments as in VPt$_3$.",1110.2156v1
2011-12-19,q-Deformation of the AdS5 x S5 Superstring S-matrix and its Relativistic Limit,"A set of four factorizable non-relativistic S-matrices for a multiplet of
fundamental particles are defined based on the R-matrix of the quantum group
deformation of the centrally extended superalgebra su(2|2). The S-matrices are
a function of two independent couplings g and q=exp(i\pi/k). The main result is
to find the scalar factor, or dressing phase, which ensures that the unitarity
and crossing equations are satisfied. For generic (g,k), the S-matrices are
branched functions on a product of rapidity tori. In the limit k->infinity, one
of them is identified with the S-matrix describing the magnon excitations on
the string world sheet in AdS5 x S5, while another is the mirror S-matrix that
is needed for the TBA. In the g->infinity limit, the rapidity torus
degenerates, the branch points disappear and the S-matrices become meromorphic
functions, as required by relativistic S-matrix theory. However, it is only the
mirror S-matrix which satisfies the correct relativistic crossing equation. The
mirror S-matrix in the relativistic limit is then closely related to that of
the semi-symmetric space sine-Gordon theory obtained from the string theory by
the Pohlmeyer reduction, but has anti-symmetric rather than symmetric bound
states. The interpolating S-matrix realizes at the quantum level the fact that
at the classical level the two theories correspond to different limits of a
one-parameter family of symplectic structures of the same integrable system.",1112.4485v1
2012-02-07,Effect of internal chemical pressure on transport properties and TCR for sensors applications,"In this work, the structural and transport properties of
(Nd0.7-xLax)Sr0.3MnO3 manganites with x = 0, 0.1 and 0.2 prepared by solid
state reaction route are studied. These compounds are found to be crystallized
in orthorhombic structural form. The influence of La substitution in place of
Nd at A-site shifts the metal to semiconductor/insulator transition temperature
(TMI) peak towards room temperature with x = 0, 0.1 and 0.2. A composition
prepared with the value of x = 0.2 in (Nd0.7-xLax)0.7Sr0.3MnO3 manganites (i.e.
(Nd0.5La0.2)0.7Sr0.3MnO3), TMI was observed at 289 K which is close to room
temperature. The maximum percentage of TCR values of compounds are increasing
with average radius <r_A> but %TCR are slightly equal in x = 0.1 and 0.2 as
compared to the parent compound. The maximum %TCR value is almost independent
with A-site average radius <r_A> in x = 0.1 and 0.2. The electrical resistivity
data are explored by different theoretical models and it has been concluded
that at low temperature (ferromagnetic metallic region) conduction mechanism
presumably due to the combined effect of electron-electron, electron-phonon and
electron-magnon scattering, while in paramagnetic semiconducting regime, the
variation of resistivity with temperature are explained by (1) Mott variable
range hopping mechanism, (2) Adiabatic small polaron hopping and (3) Thermally
activated hopping. The polaron hopping and thermal activation energies are
decreasing with increase of an average A-site ionic radius (<rA>). An
appropriate enlightenment for the observed behavior is discussed in detail.",1202.1355v1
2012-07-12,Fingerprints of spin-orbital entanglement in transition metal oxides,"The concept of spin-orbital entanglement on superexchange bonds in transition
metal oxides is introduced and explained on several examples. It is shown that
spin-orbital entanglement in superexchange models destabilizes the long-range
(spin and orbital) order and may lead either to a disordered spin-liquid state
or to novel phases at low temperature which arise from strongly frustrated
interactions. Such novel ground states cannot be described within the
conventionally used mean field theory which separates spin and orbital degrees
of freedom. Even in cases where the ground states are disentangled,
spin-orbital entanglement occurs in excited states and may become crucial for a
correct description of physical properties at finite temperature. As an
important example of this behaviour we present spin-orbital entanglement in the
$R$VO$_3$ perovskites, with $R$=La,Pr,...,Yb,Lu, where such finite temperature
properties of these compounds can be understood only using entangled states:
($i$) thermal evolution of the optical spectral weights, ($ii$) the dependence
of transition temperatures for the onset of orbital and magnetic order on the
ionic radius in the phase diagram of the $R$VO$_3$ perovskites, and ($iii$)
dimerization observed in the magnon spectra for the $C$-type antiferromagnetic
phase of YVO$_3$. Finally, it is shown that joint spin-orbital excitations in
an ordered phase with coexisting antiferromagnetic and alternating orbital
order introduces topological constraints for the hole propagation and will thus
radically modify transport properties in doped Mott insulators where hole
motion implies simultaneous spin and orbital excitations.",1207.3010v1
2012-07-20,Thermal evolution of the full three-dimensional magnetic excitations in the multiferroic BiFeO3,"The idea of embedding and transmitting information within the fluctuations of
the magnetic moments of spins (spin waves) has been recently proposed and
experimentally tested. The coherence of spin waves, which describes how well
defined these excitations are, is of course vital to this process, and themost
significant factor that affects the spin-wave coherence is temperature. Here we
present neutron inelastic scattering measurements of the full threedimensional
spin-wave dispersion in BiFeO3, which is one of the most promising functional
multiferroic material, for temperatures from 5K to 700K. Despite the presence
of strong electromagnetic coupling, the magnetic excitations behave like
conventional magnons over all parts of the Brillouin zone. At low temperature
the spin-waves are well-defined coherent modes, described by a classical model
for a G-type antiferromagnet. A spin-wave velocity softening is already present
at room temperature, and more pronounced damping occurs as the magnetic
ordering temperature TN \sim 640K is approached. In addition, a strong
hybridization of the Fe 3d and O 2p states is found to modify the distribution
of the spin-wave spectral weight significantly, which implies that the spins
are not restricted to the Fe atomic sites as previously believed.",1207.5078v1
2012-09-24,Effective quantum memory Hamiltonian from local two-body interactions,"In [Phys. Rev. A 88, 062313 (2013)] we proposed and studied a model for a
self-correcting quantum memory in which the energetic cost for introducing a
defect in the memory grows without bounds as a function of system size. This
positive behavior is due to attractive long-range interactions mediated by a
bosonic field to which the memory is coupled. The crucial ingredients for the
implementation of such a memory are the physical realization of the bosonic
field as well as local five-body interactions between the stabilizer operators
of the memory and the bosonic field. Here, we show that both of these
ingredients appear in a low-energy effective theory of a Hamiltonian that
involves only two-body interactions between neighboring spins. In particular,
we consider the low-energy, long-wavelength excitations of an ordered
Heisenberg ferromagnet (magnons) as a realization of the bosonic field.
Furthermore, we present perturbative gadgets for generating the required
five-spin operators. Our Hamiltonian involving only local two-body interactions
is thus expected to exhibit self-correcting properties as long as the noise
affecting it is in the regime where the effective low-energy description
remains valid.",1209.5289v3
2012-09-26,Classical and quantum integrability in AdS(2)/CFT(1),"We investigate the type IIA string on AdS(2)xS(2)xT(6) supported by RR-flux
which describes the gravitational side of the AdS(2)/CFT(1) correspondence.
While the four-dimensional part AdS(2)xS(2) can be realized as a supercoset,
the full superstring has both coset and non-coset excitations, the latter
giving rise to massless worldsheet modes, a somewhat novel feature in AdS/CFT.
The string is nevertheless known to be integrable at the classical level. In
this paper we perform several computations checking aspects of both classical
and quantum string integrability. At the classical level we compute energies
for the near BMN string and successfully match these against Bethe ansatz
predictions. Furthermore, integrability dictates a magnon dispersion relation
which we compare with the poles of loop corrected propagators, at both the one
and two-loop level. At one loop, where only tadpole diagrams contribute, we
find that the bosonic and fermionic contributions sum up to zero. Under the
assumption of worldsheet supersymmetry, we then compute the two-loop sunset
diagram in the near flat space limit. As in AdS(5)xS(5) we find that the result
fits nicely into the sine-square structure of the dispersion relation.",1209.6062v2
2012-10-08,Magnetic anisotropy in strained manganite films and bicrystal junctions,"Transport and magnetic properties of LSMO manganite thin films and bicrystal
junctions were investigated. Manganite films were epitaxially grown on STO,
LAO, NGO and LSAT substrates and their magnetic anisotropy were determined by
two techniques of magnetic resonance spectroscopy. Compare with cubic
substrates a small (about 0.3 persentage), the anisotropy of the orthorhombic
NGO substrate leads to a uniaxial anisotropy of the magnetic properties of the
films in the plane of the substrate. Samples with different tilt of
crystallographic basal planes of manganite as well as bicrystal junctions with
rotation of the crystallographic axes (RB - junction) and with tilting of basal
planes (TB - junction) were investigated. It was found that on vicinal NGO
substrates the value of magnetic anisotropy could be varied by changing the
substrate inclination angle from 0 to 25 degrees. Measurement of magnetic
anisotropy of manganite bicrystal junction demonstrated the presence of two
ferromagnetically ordered spin subsystems for both types of bicrystal
boundaries RB and TB. The magnitude of the magnetoresistance for TB - junctions
increased with decreasing temperature and with the misorientation angle even
misorientation of easy axes in the parts of junction does not change. Analysis
of the voltage dependencies of bicrystal junction conductivity show that the
low value of the magnetoresistance for the LSMO bicrystal junctions can be
caused by two scattering mechanisms with the spin- flip of spin - polarized
carriers due to the strong electron - electron interactions in a disordered
layer at the bicrystal boundary at low temperatures and the spin-flip by anti
ferromagnetic magnons at high temperatures.",1210.2261v1
2013-02-17,Raman Studies of Anisotropic Magnetic Excitations in Fluctuating Nematic Striped La2-xSrxCuO4 and the Comparison to Uniform Nd2-xCexCuO4,"The mechanism of the high temperature hole-doped superconductivity was
investigated by Raman scattering. The Raman selection rule is unique, so that
anisotropic magnetic excitations in a fluctuating spin-charge stripe can be
detected as if it is static. We use different Raman selection rules for two
kinds of magnetic Raman scattering processes, two-magnon scattering and
high-energy electronic scattering. In order to confirm the difference, the
Raman spectra of striped La2-xSrxCuO4 (LSCO) and non-striped Nd2-xCexCuO4
(NCCO) were compared. The main results in LSCO are (1) magnetic excitations are
presented by individual energy dispersions for the k\parallel stripe and the
k\perp stripe, (2) the charge transfer is allowed only in the direction
perpendicular to the stripe. The direction is the same as the Burgers vector of
an edge dislocation. Hence we assume that a charge moves together with the edge
dislocation of the charge stripe. The superconducting coherence length is close
to the inter-charge stripe distance at x < 0.2. Therefore we propose a model
that superconducting pairs are formed in the edge dislocations. The binding
energy is related to the stripe formation energy.",1302.4060v1
2013-05-03,"Co2 FeAl thin films grown on MgO substrates: Correlation between static, dynamic and structural properties","Co2FeAl (CFA) thin films with thickness varying from 10 nm to 115 nm have
been deposited on MgO(001) substrates by magnetron sputtering and then capped
by Ta or Cr layer. X-rays diffraction (XRD) revealed that the cubic $[001]$ CFA
axis is normal to the substrate and that all the CFA films exhibit full
epitaxial growth. The chemical order varies from the $B2$ phase to the $A2$
phase when decreasing the thickness. Magneto-optical Kerr effect (MOKE) and
vibrating sample magnetometer measurements show that, depending on the field
orientation, one or two-step switchings occur. Moreover, the films present a
quadratic MOKE signal increasing with the CFA thickness, due to the increasing
chemical order. Ferromagnetic resonance, MOKE transverse bias initial inverse
susceptibility and torque (TBIIST) measurements reveal that the in-plane
anisotropy results from the superposition of a uniaxial and of a fourfold
symmetry term. The fourfold anisotropy is in accord with the crystal structure
of the samples and is correlated to the biaxial strain and to the chemical
order present in the films. In addition, a large negative perpendicular
uniaxial anisotropy is observed. Frequency and angular dependences of the FMR
linewidth show two magnon scattering and mosaicity contributions, which depend
on the CFA thickness. A Gilbert damping coefficient as low as 0.0011 is found.",1305.0714v1
2013-05-10,Open spin chains for giant gravitons and relativity,"We study open spin chains for strings stretched between giant graviton states
in the N=4 SYM field theory in the collective coordinate approach. We study the
boundary conditions and the effective Hamiltonian of the corresponding spin
chain to two loop order.
  The ground states of the spin chain have energies that match the relativistic
dispersion relation characteristic of massive W boson particles on the
worldvolume of the giant graviton configurations, up to second order in the
limit where the momentum is much larger than the mass. We find evidence for a
non-renormalization theorem for the ground state wave function of this spin
chain system. We also conjecture a generalization of this result to all loop
orders which makes it compatible with a fully relativistic dispersion relation.
We show that the conjecture follows if one assumes that the spin chain admits a
central charge extension that is sourced by the giant gravitons, generalizing
the giant magnon dispersion relation for closed string excitations. This
provides evidence for ten dimensional local physics mixing AdS directions and
the five-sphere emerging from an N=4 SYM computation in the presence of a
non-trivial background (made of D-branes) that break the conformal field theory
of the system.",1305.2394v2
2013-05-21,Binding carriers to a non-magnetic impurity in a two-dimensional square Ising antiferromagnet,"A hole in a two-dimensional Ising antiferromagnet was believed to be
infinitely heavy due to the string of wrongly oriented spins it creates as it
moves, which should trap it near its original location. Trugman showed that, in
fact, the hole acquires a finite effective mass due to contributions from
so-called {\it Trugman loop} processes, where the hole goes nearly twice around
closed loops, first creating and then removing wrongly-oriented spins, and
ending up at a different lattice site. This generates effective second- and
third-nearest-neighbor hopping terms which keep the quasiparticle on the
sublattice it was created on. Here, we investigate the trapping of the
quasiparticle near a single attractive non-magnetic impurity placed at one
lattice site. We consider the two cases with the quasiparticle and impurity
being on the same versus on different sublattices. The main result is that even
though the quasiparticle can not see the bare disorder in the latter case, the
coupling to magnons generates an effective renormalized disorder on its own
sublattice which is strong enough to lead to bound states, which however have a
very different spectrum than when the quasiparticle and impurity are on the
same sublattice.",1305.4691v2
2013-05-29,Thermodynamic Properties of the Anisotropic Frustrated Spin-chain Compound Linarite PbCuSO$_4$(OH)$_2$,"We present a comprehensive macroscopic thermodynamic study of the
quasi-one-dimensional (1D) $s = \tfrac{1}{2}$ frustrated spin-chain system
linarite. Susceptibility, magnetization, specific heat, magnetocaloric effect,
magnetostriction, and thermal-expansion measurements were performed to
characterize the magnetic phase diagram. In particular, for magnetic fields
along the b axis five different magnetic regions have been detected, some of
them exhibiting short-range-order effects. The experimental magnetic entropy
and magnetization are compared to a theoretical modelling of these quantities
using DMRG and TMRG approaches. Within the framework of a purely 1D isotropic
model Hamiltonian, only a qualitative agreement between theory and the
experimental data can be achieved. Instead, it is demonstrated that a
significant symmetric anisotropic exchange of about 10% is necessary to account
for the basic experimental observations, including the 3D saturation field, and
which in turn might stabilize a triatic (three-magnon) multipolar phase.",1305.6731v3
2013-07-02,Electronic excitations in the edge-shared relativistic Mott insulator: Na$_2$IrO$_3$,"We have investigated the excitation spectra of $j_{eff}$=$\frac{1}{2}$ Mott
insulator Na$_2$IrO$_3$. Taking into account a relativistic multiplet structure
of Ir ions, we have calculated the optical conductivity $\sigma(\omega)$ and
resonant inelastic x-ray scattering (RIXS) spectra, which manifest different
features from those of a canonical $j_{eff}$=$\frac{1}{2}$ system
Sr$_2$IrO$_4$.Distinctly from the two-peak structure in Sr$_2$IrO$_4$,
$\sigma(\omega)$ in Na$_2$IrO$_3$ has a broad single peak dominated by
interband transitions from $j_{eff}$=$\frac{3}{2}$ to $\frac{1}{2}$. RIXS
spectra exhibit the spin-orbit (SO) exciton that has a two-peak structure
arising from the crystal-field effect, and the magnon peak at energies much
lower than in Sr$_2$IrO$_4$. In addition, a small peak near the optical
absorption edge is found in RIXS spectra, originating from the coupling between
the electron-hole ($e$-$h$) excitation and the SO exciton. Our findings
corroborate the validity of the relativistic electronic structure and
importance of both itinerant and local features in Na$_2$IrO$_3$.",1307.0614v3
2013-11-29,Controlling magnetic order and quantum disorder in one- and zero-dimensional molecule-based magnets,"We investigate the structural and magnetic properties of two molecule-based
magnets synthesized from the same starting components. Their different
structural motifs promote contrasting exchange pathways and consequently lead
to markedly different magnetic ground states. Through examination of their
structural and magnetic properties we show that
[Cu(pyz)(H$_{2}$O)(gly)$_{2}$](ClO$_{4}$)$_{2}$ may be considered a
quasi-one-dimensional quantum Heisenberg antiferromagnet while the related
compound [Cu(pyz)(gly)](ClO$_{4}$), which is formed from dimers of
antiferromagnetically interacting Cu$^{2+}$ spins, remains disordered down to
at least 0.03 K in zero field, but shows a field-temperature phase diagram
reminiscent of that seen in materials showing a Bose-Einstein condensation of
magnons.",1311.7614v1
2014-01-21,Observation of entanglement propagation in a quantum many-body system,"The key to explaining a wide range of quantum phenomena is understanding how
entanglement propagates around many-body systems. Furthermore, the controlled
distribution of entanglement is of fundamental importance for quantum
communication and computation. In many situations, quasiparticles are the
carriers of information around a quantum system and are expected to distribute
entanglement in a fashion determined by the system interactions. Here we report
on the observation of magnon quasiparticle dynamics in a one-dimensional
many-body quantum system of trapped ions representing an Ising spin model.
Using the ability to tune the effective interaction range, and to prepare and
measure the quantum state at the individual particle level, we observe new
quasiparticle phenomena. For the first time, we reveal the entanglement
distributed by quasiparticles around a many-body system. Second, for long-range
interactions we observe the divergence of quasiparticle velocity and breakdown
of the light-cone picture that is valid for short-range interactions. Our
results will allow experimental studies of a wide range of phenomena, such as
quantum transport, thermalisation, localisation and entanglement growth, and
represent a first step towards a new quantum-optical regime with on-demand
quasiparticles with tunable non-linear interactions.",1401.5387v3
2014-01-27,Direct determination of exchange parameters in Cs2CuBr4 and Cs2CuCl4: high-field ESR studies,"Spin-1/2 Heisenberg antiferromagnets Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$ with
distorted triangular-lattice structures are studied by means of electron spin
resonance spectroscopy in magnetic fields up to the saturation field and above.
In the magnetically saturated phase, quantum fluctuations are fully suppressed,
and the spin dynamics is defined by ordinary magnons. This allows us to
accurately describe the magnetic excitation spectra in both materials and,
using the harmonic spin-wave theory, to determine their exchange parameters.
The viability of the proposed method was proven by applying it to
Cs$_2$CuCl$_4$, yielding $J/k_B=4.7(2)$ K, $J'/k_B=1.42(7)$ K [$J'/J\simeq
0.30$] and revealing good agreement with inelastic neutron-scattering results.
For the isostructural Cs$_2$CuBr$_4$, we obtain $J/k_B=14.9(7)$ K,
$J'/k_B=6.1(3)$ K, [$J'/J\simeq 0.41$], providing exact and conclusive
information on the exchange couplings in this frustrated spin system.",1401.6793v2
2014-02-13,"Fractionalization, entanglement, and separation: understanding the collective excitations in a spin-orbital chain","Using a combined analytical and numerical approach, we study the collective
spin and orbital excitations in a spin-orbital chain under a crystal field.
Irrespective of the crystal field strength, these excitations can be
universally described by fractionalized fermions. The fractionalization
phenomenon persists and contrasts strikingly with the case of a spin chain,
where fractionalized spinons cannot be individually observed but confined to
form magnons in a strong magnetic field. In the spin-orbital chain, each of the
fractional quasiparticles carries both spin and orbital quantum numbers, and
the two variables are always entangled in the collective excitations. Our
result further shows that the recently reported separation phenomenon occurs
when crystal fields fully polarize the orbital degrees of freedom. In this
case, however, the spinon and orbiton dynamics are decoupled solely because of
a redefinition of the spin and orbital quantum numbers.",1402.3255v4
2014-06-30,Bound states and entanglement in the excited states of quantum spin chains,"We investigate entanglement properties of the excited states of the spin-1/2
Heisenberg (XXX) chain with isotropic antiferromagnetic interactions, by
exploiting the Bethe ansatz solution of the model. We consider eigenstates
obtained from both real and complex solutions (""strings"") of the Bethe
equations. Physically, the former are states of interacting magnons, whereas
the latter contain bound states of groups of particles. We first focus on the
situation with few particles in the chain. Using exact results and
semiclassical arguments, we derive an upper bound S_MAX for the entanglement
entropy. This exhibits an intermediate behavior between logarithmic and
extensive, and it is saturated for highly-entangled states. As a function of
the eigenstate energy, the entanglement entropy is organized in bands. Their
number depends on the number of blocks of contiguous Bethe-Takahashi quantum
numbers. In presence of bound states a significant reduction in the
entanglement entropy occurs, reflecting that a group of bound particles behaves
effectively as a single particle. Interestingly, the associated entanglement
spectrum shows edge-related levels. At finite particle density, the
semiclassical bound S_MAX becomes inaccurate. For highly-entangled states
S_A\propto L_c, with L_c the chord length, signaling the crossover to extensive
entanglement. Finally, we consider eigenstates containing a single pair of
bound particles. No significant entanglement reduction occurs, in contrast with
the few-particle case.",1407.0066v2
2014-08-07,Thermodynamics of AdS Black Holes in Einstein-Scalar Gravity,"We study the thermodynamics of $n$-dimensional static asymptotically AdS
black holes in Einstein gravity coupled to a scalar field with a potential
admitting a stationary point with an AdS vacuum. Such black holes with
non-trivial scalar hair can exist provided that the mass-squared of the scalar
field is negative, and above the Breitenlohner-Freedman bound. We use the Wald
procedure to derive the first law of thermodynamics for these black holes,
showing how the scalar hair (or ""charge"") contributes non-trivially in the
expression. We show in general that a black hole mass can be deduced by
isolating an integrable contribution to the (non-integrable) variation of the
Hamiltonian arising in the Wald construction, and that this is consistent with
the mass calculated using the renormalised holographic stress tensor and also,
in those cases where it is defined, with the mass calculated using the
conformal method of Ashtekar, Magnon and Das. Similar arguments can also be
given for the smooth solitonic solutions in these theories. Neither the black
hole nor the soliton solutions can be constructed explicitly, and we carry out
a numerical analysis to demonstrate their existence and to provide approximate
checks on some of our thermodynamic results.",1408.1514v2
2014-08-08,Stable Magnetic Droplet Solitons in Spin Transfer Nanocontacts,"Magnetic thin films with perpendicular magnetic anisotropy (PMA) have
localized excitations that correspond to reversed dynamically precessing
magnetic moments, known as magnetic droplet solitons. Fundamentally, these
excitations are associated with an attractive interaction between elementary
spin-excitations (i.e., magnons) and were predicted to occur in PMA materials
in the absence of damping [1,2]. While damping, present in all magnetic
materials, suppresses these excitations, it is now possible to compensate
damping by spin transfer torques through electrical current flow in nanometer
scale contacts to ferromagnetic thin films [3,4]. A theory predicts the
appearance of magnetic droplet solitons at a threshold current in nanocontacts
[5] and, recently, experimental signatures of droplet nucleation have been
reported [6]. However, thus far, they have been observed to be nearly
reversible excitations, with only partially reversed magnetization and to be
subject to instabilities that cause them to drift away from the nanocontacts
(i.e., drift instabilities) [6]. Here we show that magnetic droplet solitons
can be stabilized in a spin transfer nanocontact. Further, they exhibit a
strong hysteretic response to fields and currents and a nearly fully reversed
magnetization in the contact. These observations, in addition to their
fundamental interest, open up new applications for magnetic droplet solitons as
multi-state high frequency current and field tunable oscillators.",1408.1902v1
2014-08-19,Exploring quantum phase transition in Pd_{1-x}Ni_x nanoalloys,"Pd$_{1-x}$Ni$_x$ alloy system is an established ideal transition metal system
possessing a composition induced paramagnetic to ferromagnetic quantum phase
transition (QPT) at the critical concentration $x_c \sim$ 0.026 in bulk. A
low-temperature non-Fermi liquid (NFL) behaviour around $x_c$ usually indicates
the presence of quantum criticality (QC) in this system. In this work, we
explore the existence of such a QPT in nanoparticles of this alloy system. We
synthesized single-phase, polydispersed and 40-50 nm mean diameter crystalline
nanoparticles of Pd$_{1-x}$Ni$_x$ alloys, with $x$ near $x_c$ and beyond, by a
chemical reflux method. In addition to the determination of the size,
composition, phase and crystallinity of the alloys by microscopic and
spectroscopic techniques, the existence of a possible QPT was explored by
resistivity and DC magnetization measurements. A dip in the value of the
exponent $n$ near $x_c$, and a concomitant peak in the constant $A$, of the
$AT^n$ dependence of the low temperature ($T$) resistivity indicate the
presence of a quantum-like phase transition in the system. The minimum value of
$n$, however, remains within the Fermi liquid regime ($n >$ 2). The DC
magnetization results suggest an anticipatory presence of a superparamagnetic
to ferromagnetic QPT in the mean-sized nanoparticles. The observation of a
possible quantum critical NFL behaviour ($n <$ 2) through resistivity is argued
to be inhibited by the electron-magnon scatterings present in the smaller
nanoparticles.",1408.4316v1
2014-09-01,Statistics-dependent quantum co-walking of two particles in one-dimensional lattices with nearest-neighbor interactions,"We investigate continuous-time quantum walks of two indistinguishable
particles [bosons, fermions or hard-core bosons (HCBs)] in one-dimensional
lattices with nearest-neighbor interactions. The results for two HCBs are well
consistent with the recent experimental observation of two-magnon dynamics
[Nature 502, 76 (2013)]. The two interacting particles can undergo independent-
and/or co-walking depending on both quantum statistics and interaction
strength. Two strongly interacting particles may form a bound state and then
co-walk like a single composite particle with statistics-dependent walk speed.
Analytical solutions for the scattering and bound states, which appear in the
two-particle quantum walks, are obtained by solving the eigenvalue problem in
the two-particle Hilbert space. In the context of degenerate perturbation
theory, an effective single-particle model for the quantum co-walking is
analytically derived and the walk seep of bosons is found to be exactly three
times of the ones of fermions/HCBs. Our result paves the way for experimentally
exploring quantum statistics via two-particle quantum walks.",1409.0306v3
2014-09-30,Enhanced gyration-signal propagation speed in one-dimensional vortex-antivortex lattices and its control by perpendicular bias field,"We report on a micromagnetic simulation study of coupled core gyrations in
one-dimensional (1D) alternating vortex-antivortex (V-AV) lattices formed in
connected soft-magnetic-disk arrays (round-shaped modulated nanostrips). In the
V-AV lattices, we found very characteristic standing-wave modes of the coupled
gyrations as well as efficiently ultrafast gyration-signal propagation between
vortices through the neighboring antivortices, as originating from their
combined strong exchange and dipole interactions. Collective core oscillations
in the V-AV networks are characterized as unique two-branch magnonic bands that
are affected by the polarization ordering between the neighboring vortex and
antivortex and controllable by externally applied perpendicular fields each of
different field strength and direction. The gyration-signal propagation speed
is much faster than that for 1D disk arrays composed only of vortex states, and
the propagation speed for the parallel polarization ordering is increased,
remarkably, to more than 1 km/sec by application of perpendicular static
fields. This work provides a fundamental understanding of the coupled dynamics
of topological solitons as well as an additional mechanism for ultrafast
gyration-signal propagation; moreover, it offers an efficient means of
significant propagation-speed enhancement that is suitable for information
carrier applications in continuous nanostrips.",1409.8357v1
2014-10-14,Symmetry-dependent electron-electron interaction in coherent tunnel junctions resolved by zero bias anomaly measurements,"We provide conclusive experimental evidence that zero bias anomaly in the
differential resistance of magnetic tunnel junctions (MTJs) is due to
electron-electron interaction (EEI), clarifying a long standing issue. Magnon
effect that caused confusion is now excluded by measuring at low temperatures
down to 0.2 K and with reduced AC measurement voltages down to 0.06 mV. The
normalized change of conductance is proportional to $\ln{(eV/k_{B}T)}$,
consistent with the Altshuler-Aronov theory of tunneling that describes the
reduction of density of states due to EEI, but inconsistent with magnetic
impurity scattering. The slope of the $\ln{(eV/k_{B}T)}$ dependence is symmetry
dependent: the slopes for P and AP states are different for coherent tunnel
junctions with symmetry filtering, while nearly the same for those without
symmetry filtering (amorphous barriers). This observation may be helpful for
verifying symmetry preserved filtering in search of new coherent tunneling
junctions, and for probing and separating electron Bloch states of different
symmetries in other correlated systems.",1410.3636v2
2015-02-10,Magnetic-field and doping dependence of low-energy spin fluctuations in the antiferroquadrupolar compound Ce(1-x)La(x)B(6),"CeB(6) is a model compound exhibiting antiferroquadrupolar (AFQ) order, its
magnetic properties being typically interpreted within localized models. More
recently, the observation of strong and sharp magnetic exciton modes forming in
its antiferromagnetic (AFM) state at both ferromagnetic and AFQ wave vectors
suggested a significant contribution of itinerant electrons to the spin
dynamics. Here we investigate the evolution of the AFQ excitation upon the
application of an external magnetic field and the substitution of Ce with
non-magnetic La, both parameters known to suppress the AFM phase. We find that
the exciton energy decreases proportionally to T_N upon doping. In field, its
intensity is suppressed, while its energy remains constant. Its disappearance
above the critical field of the AFM phase is preceded by the formation of two
modes, whose energies grow linearly with magnetic field upon entering the AFQ
phase. These findings suggest a crossover from itinerant to localized spin
dynamics between the two phases, the coupling to heavy-fermion quasiparticles
being crucial for a comprehensive description of the magnon spectrum.",1502.03139v2
2015-03-04,Thermodynamics of quantum systems under dynamical control,"In this review the debated rapport between thermodynamics and quantum
mechanics is addressed in the framework of the theory of
periodically-driven/controlled quantum-thermodynamic machines. The basic model
studied here is that of a two-level system (TLS), whose energy is periodically
modulated while the system is coupled to thermal baths. When the modulation
interval is short compared to the bath memory time, the system-bath
correlations are affected, thereby causing cooling or heating of the TLS,
depending on the interval. In steady state, a periodically-modulated TLS
coupled to two distinct baths constitutes the simplest quantum heat machine
(QHM) that may operate as either an engine or a refrigerator, depending on the
modulation rate. We find their efficiency and power-output bounds and the
conditions for attaining these bounds. An extension of this model to multilevel
systems shows that the QHM power output can be boosted by the multilevel
degeneracy.
  These results are used to scrutinize basic thermodynamic principles: (i)
Externally-driven/modulated QHMs may attain the Carnot efficiency bound, but
when the driving is done by a quantum device (""piston""), the efficiency
strongly depends on its initial quantum state. Such dependence has been unknown
thus far. (ii) The refrigeration rate effected by QHMs does not vanish as the
temperature approaches absolute zero for certain quantized baths, e.g.,
magnons, thous challenging Nernst's unattainability principle. (iii)
System-bath correlations allow more work extraction under periodic control than
that expected from the Szilard-Landauer principle, provided the period is in
the non-Markovian domain. Thus, dynamically-controlled QHMs may benefit from
hitherto unexploited thermodynamic resources.",1503.01195v1
2015-03-20,Magnetic excitations of perovskite rare-earth nickelates: RNiO$_3$,"The perovskite nickelates RNiO$_3$ (R: rare-earth) have been studied as
potential multiferroic compounds. A certain degree of charge disproportionation
in the Ni ions has been confirmed by high resolution synchrotron power
diffraction: instead of the nominal Ni$^{3+}$ valence, they can have the
mixed-valence state Ni$^{(3-\delta)+}$ and Ni$^{(3+\delta)+}$, though agreement
has not been reached on the precise value of $\delta$ (e.g. for NdNiO$_3$,
$\delta=0.0$ and $\delta=0.29$ were reported). Also, the magnetic ground state
is not yet clear: collinear and non-collinear Ni-O magnetic structures have
been proposed to explain neutron diffraction and soft X-ray resonant
sccattering results in these compounds, and more recently a canted
antiferromagnetic spin arrangement was proposed on the basis of magnetic
susceptibility measurements. This scenario is reminiscent of the situation in
the half-doped manganites.
  In order to gain insight into the ground state of these compounds, we studied
the magnetic excitations of some of the different phases proposed, using a
localized spin model for a simplified spin chain which could describe these
compounds. We first analize the stability of the collinear, orthogonal, and
intermediate phases in the classical case. We then explore the quantum ground
state indirectly, calculating the spin excitations obtained for each phase,
using the Holstein-Primakoff transformation and the linear spin-wave
approximation. For the collinear and orthogonal ($\theta=\pi/2$) phases, we
predict differences in the magnon spectrum which would allow to distinguish
between them in future inelastic neutron scattering experiments.",1503.06102v1
2015-03-20,Control of spin current by a magnetic YIG substrate in NiFe/Al nonlocal spin valves,"We study the effect of a magnetic insulator (Yttrium Iron Garnet - YIG)
substrate on the spin transport properties of Ni$_{80}$Fe$_{20}$/Al nonlocal
spin valve (NLSV) devices. The NLSV signal on the YIG substrate is about 2 to 3
times lower than that on a non magnetic SiO$_2$ substrate, indicating that a
significant fraction of the spin-current is absorbed at the Al/YIG interface.
By measuring the NLSV signal for varying injector-to-detector distance and
using a three dimensional spin-transport model that takes spin current
absorption at the Al/YIG interface into account we obtain an effective
spin-mixing conductance $G_{\uparrow\downarrow}\simeq 5 - 8\times
10^{13}~\Omega^{-1}$m$^{-2}$. We also observe a small but clear modulation of
the NLSV signal when rotating the YIG magnetization direction with respect to
the fixed spin polarization of the spin accumulation in the Al. Spin relaxation
due to thermal magnons or roughness of the YIG surface may be responsible for
the observed small modulation of the NLSV signal.",1503.06108v1
2015-04-10,MCViNE -- An object oriented Monte Carlo neutron ray tracing simulation package,"MCViNE (Monte-Carlo VIrtual Neutron Experiment) is a versatile Monte Carlo
(MC) neutron ray-tracing program that provides researchers with tools for
performing computer modeling and simulations that mirror real neutron
scattering experiments. By adopting modern software engineering practices such
as using composite and visitor design patterns for representing and accessing
neutron scatterers, and using recursive algorithms for multiple scattering,
MCViNE is flexible enough to handle sophisticated neutron scattering problems
including, for example, neutron detection by complex detector systems, and
single and multiple scattering events in a variety of samples and sample
environments. In addition, MCViNE can take advantage of simulation components
in linear-chain-based MC ray tracing packages widely used in instrument design
and optimization, as well as NumPy-based components that make prototypes useful
and easy to develop. These developments have enabled us to carry out detailed
simulations of neutron scattering experiments with non-trivial samples in
time-of-flight inelastic instruments at the Spallation Neutron Source. Examples
of such simulations for powder and single-crystal samples with various
scattering kernels, including kernels for phonon and magnon scattering, are
presented. With simulations that closely reproduce experimental results,
scattering mechanisms can be turned on and off to determine how they contribute
to the measured scattering intensities, improving our understanding of the
underlying physics.",1504.02776v2
2015-05-14,"First-principles calculations of exchange interactions, spin waves, and temperature dependence of magnetization in inverse-Heusler-based spin gapless semiconductors","Employing first principles electronic structure calculations in conjunction
with the frozen-magnon method we calculate exchange interactions, spin-wave
dispersion, and spin-wave stiffness constants in inverse-Heusler-based spin
gapless semiconductor (SGS) compounds Mn$_2$CoAl, Ti$_2$MnAl, Cr$_2$ZnSi,
Ti$_2$CoSi and Ti$_2$VAs. We find that their magnetic behavior is similar to
the half-metallic ferromagnetic full-Heusler alloys, i.e., the intersublattice
exchange interactions play an essential role in the formation of the magnetic
ground state and in determining the Curie temperature, $T_\mathrm{c}$. All
compounds, except Ti$_2$CoSi possess a ferrimagnetic ground state. Due to the
finite energy gap in one spin channel, the exchange interactions decay sharply
with the distance, and hence magnetism of these SGSs can be described
considering only nearest and next-nearest neighbor exchange interactions. The
calculated spin-wave dispersion curves are typical for ferrimagnets and
ferromagnets. The spin-wave stiffness constants turn out to be larger than
those of the elementary 3$d$-ferromagnets. Calculated exchange parameters are
used as input to determine the temperature dependence of the magnetization and
$T_\mathrm{c}$ of the SGSs. We find that the $T_\mathrm{c}$ of all compounds is
much above the room temperature. The calculated magnetization curve for
Mn$_2$CoAl as well as the Curie temperature are in very good agreement with
available experimental data. The present study is expected to pave the way for
a deeper understanding of the magnetic properties of the inverse-Heusler-based
SGSs and enhance the interest in these materials for application in spintronic
and magnetoelectronic devices.",1505.03632v1
2015-10-14,Stability of skyrmion lattices and symmetries of quasi-two-dimensional chiral magnets,"Recently, there has been substantial interest in realizations of skyrmions,
in particular in 2D systems due to increased stability resulting from reduced
dimensionality. A stable skyrmion, representing the smallest realizable
magnetic texture, could be an ideal element for ultra-dense magnetic memories.
Here, we use the most general form of the 2D free energy with
Dzyaloshinskii-Moriya interactions constructed from general symmetry
considerations reflecting the underlying system. We predict that skyrmion phase
is robust and it is present even when the system lacks the in-plane rotational
symmetry. In fact, the lowered symmetry leads to increased stability of
vortex-antivortex lattices with four-fold symmetry and in-plane spirals, in
some instances even in the absence of an external magnetic field. Our results
relate different hexagonal and square cell phases to the symmetries of
materials used for realizations of skyrmions. This will give clear directions
for experimental realizations of hexagonal and square cell phases, and will
allow engineering of skyrmions with unusual properties. We also predict
striking differences in gyro-dynamics induced by spin currents for isolated
skyrmions and for crystals where spin currents can be induced by charge
carriers or by thermal magnons. We find that under certain conditions, isolated
skyrmions can move along the current without a side motion which can have
implications for realizations of magnetic memories.",1510.04262v3
2015-10-28,Raman and fluorescence characteristics of resonant inelastic X-ray scattering from doped superconducting cuprates,"Measurements of spin excitations are essential for an understanding of
spin-mediated pairing for superconductivity; and resonant inelastic X-ray
scattering (RIXS) provides a considerable opportunity to probe high-energy spin
excitations. However, whether RIXS correctly measures the collective spin
excitations of doped superconducting cuprates remains under debate. Here we
demonstrate distinct Raman- and fluorescence-like RIXS excitations of
Bi$_{1.5}$Pb$_{0.6}$Sr$_{1.54}$CaCu$_{2}$O$_{8+{\delta}}$ in the mid-infrared
energy region. Combining photon-energy and momentum dependent RIXS measurements
with theoretical calculations using exact diagonalization provides conclusive
evidence that the Raman-like RIXS excitations correspond to collective spin
excitations, which are magnons in the undoped Mott insulators and evolve into
paramagnons in doped superconducting compounds. In contrast, the
fluorescence-like shifts are due primarily to the continuum of particle-hole
excitations in the charge channel. Our results show that under the proper
experimental conditions RIXS indeed can be used to probe paramagnons in doped
high-$T_c$ cuprate superconductors.",1510.08403v1
2015-10-29,Thermal Conductivity in the Triangular-Lattice Antiferromagnet Ba3CoSb2O9,"We have measured the thermal conductivity in the ab-plane, $\kappa$ab, and
along the c-axis, $\kappa$c, of single crystals of the S = 1/2
triangular-lattice antiferromagnet Ba3CoSb2O9 in zero field and magnetic
fields. In zero field, it has been found that both $\kappa$ab and $\kappa$c
show a similar broad peak around 40 K, suggesting that the thermal conductivity
due to phonons is dominant in this compound and that the mean free path of
phonons is suppressed so much by strong magnetic-fluctuations due to the
magnetic frustration. It has also been found that both $\kappa$ab and $\kappa$c
show a kink at the antiferromagnetic transition temperature TN. In magnetic
fields parallel to the ab-plane up to 14 T, magnetic-field dependences of both
$\kappa$ab and $\kappa$c at temperatures below and above TN have been found to
be understood taking into account the phonon-magnon scattering in the so-called
120${\deg}$ structure phase and the up-up-down (UUD) phase, while the minima of
$\kappa$ab and $\kappa$c observed in the middle of the UUD phase have not been
understood. This suggests the occurrence of some change of the magnetic state
in the middle of the UUD phase.",1510.08635v1
2016-01-18,"Electromagnons, magnons, and phonons in Eu$_{1-x}$Ho$_x$MnO$_3$","Here we present a detailed study of the THz and FIR response of the mixed
perovskite manganite system Eu$_{1-x}$Ho$_x$MnO$_3$ for holmium concentrations
x = 0.1 and 0.3. We compare the magnetic excitations of the four different
magnetically ordered phases (A-type antiferromagnetic, sinusoidally modulated
collinear, helical phases with spin planes perpendicular to the
crystallographic a and c axes). The transition between the two latter phases
goes hand in hand with a switching of the ferroelectric polarization from P||a
to P||c. Special emphasis is paid to the temperature dependence of the
excitations at this transition. We find a significant change of intensity
indicating that the exchange striction mechanism cannot be the only mechanism
to induce dipolar weight to spin-wave excitations. We also focus on excitations
within the incommensurate collinear antiferromagnetic phase and find a so far
unobserved excitation close to 40 cm$^{-1}$. A detailed analysis of optical
weight gives a further unexpected result: In the multiferroic phase with P||c
all the spectral weight of the electromagnons comes from the lowest phonon
mode. However, for the phase with the polarization P||a additional spectral
weight must be transferred from higher frequencies.",1601.04486v1
2016-02-02,Non-Markovian Dynamics in Chiral Quantum Networks with Spins and Photons,"We study the dynamics of chiral quantum networks consisting of nodes coupled
by unidirectional or asymmetric bidirectional quantum channels. In contrast to
familiar photonic networks where driven two-level atoms exchange photons via 1D
photonic nanostructures, we propose and study a setup where interactions
between the atoms are mediated by spin excitations (magnons) in 1D $XX$ spin
chains representing spin waveguides. While Markovian quantum network theory
eliminates quantum channels as structureless reservoirs in a Born-Markov
approximation to obtain a master equation for the nodes, we are interested in
non-Markovian dynamics. This arises from the nonlinear character of the
dispersion with band-edge effects, and from finite spin propagation velocities
leading to time delays in interactions. To account for the non-Markovian
dynamics we treat the quantum degrees of freedom of the nodes and connecting
channels as a composite spin system with the surrounding of the quantum network
as a Markovian bath, allowing for an efficient solution with time-dependent
density matrix renormalization group techniques. We illustrate our approach
showing non-Markovian effects in the driven-dissipative formation of quantum
dimers, and we present examples for quantum information protocols involving
quantum state transfer with engineered elements as basic building blocks of
quantum spintronic circuits.",1602.00926v2
2016-02-03,Charge dynamics of the antiferromagnetically ordered Mott insulator,"We study the charge dynamics of the half-filled Hubbard model on the square
lattice at zero temperature. We employ a slave-fermion formulation in which the
charge degrees of freedom are represented by fermionic holons and doublons and
the bosonic spin degrees of freedom are assumed to be described by the
antiferromagnetic Heisenberg model. The ground state has long-ranged magnetic
(N\'eel) order and its Mott-insulating characteristics are the consequence of
holon-doublon bound-state formation. Within this framework, we calculate the
average double occupancy, the electron density of states, and the spectral
function in the self-consistent Born approximation. The lower and upper Hubbard
bands are reproduced, there is spectral-weight transfer into a coherent
quasiparticle band at their lower edges, and the Mott gap, associated with
holon-doublon binding, is renormalized due to the interactions of the holons
and doublons with the magnons. We compute the zeros of the Green function at
the chemical potential to discuss the Luttinger volume and show that the poles
of the self-energy reflect the underlying quasiparticle dispersion with a
spin-renormalized hopping parameter. Optical conductivity results relate the
optical gap to the Mott gap of the insulating system. We conclude that a
self-consistent treatment of the spin fluctuation effects on the charge degrees
of freedom captures all the essential physics of the antiferromagnetic
Mott-Hubbard insulator.",1602.01276v2
2016-04-20,Magnon Inflation: Slow Roll with Steep Potentials,"We find multi-scalar effective field theories (EFTs) that can achieve a slow
inflationary roll despite having a scalar potential that does not satisfy the
usual slow-roll condition (d V)^2 << V^2/Mp^2. They evade the usual slow-roll
conditions on $V$ because their kinetic energies are dominated by
single-derivative terms rather than the usual two-derivative terms. Single
derivatives dominate during slow roll and so do not require a breakdown of the
usual derivative expansion that underpins calculational control in much of
cosmology. The presence of such terms requires some sort of UV Lorentz-symmetry
breaking during inflation (besides the usual cosmological breaking).
Chromo-natural inflation provides an example of a UV theory that can generate
the multi-field single-derivative terms we consider, and we argue that the EFT
we find indeed captures the slow-roll conditions for the background evolution
for Chromo-natural inflation. We also show that our EFT can be understood as a
multi-field generalization of the single-field Cuscuton models. The multi-field
case introduces a new feature, however: the scalar kinetic terms define a
target-space 2-form, F_{ab}, whose antisymmetry gives new ways for slow roll to
be achieved.",1604.06048v2
2016-04-26,First principles studies of the Gilbert damping and exchange interactions for half-metallic Heuslers alloys,"Heusler alloys have been intensively studied due to the wide variety of
properties that they exhibit. One of these properties is of particular interest
for technological applications, i.e. the fact that some Heusler alloys are
half-metallic. In the following, a systematic study of the magnetic properties
of three different Heusler families $\textrm{Co}_2\textrm{Mn}\textrm{Z}$,
$\text{Co}_2\text{Fe}\text{Z}$ and $\textrm{Mn}_2\textrm{V}\textrm{Z}$ with
$\text{Z}=\left(\text{Al, Si, Ga, Ge}\right)$ is performed. A key aspect is the
determination of the Gilbert damping from first principles calculations, with
special focus on the role played by different approximations, the effect that
substitutional disorder and temperature effects. Heisenberg exchange
interactions and critical temperature for the alloys are also calculated as
well as magnon dispersion relations for representative systems, the
ferromagnetic $\textrm{Co}_2\textrm{Fe}\textrm{Si}$ and the ferrimagnetic
$\textrm{Mn}_2\textrm{V}\textrm{Al}$. Correlations effects beyond standard
density-functional theory are treated using both the local spin density
approximation including the Hubbard $U$ and the local spin density
approximation plus dynamical mean field theory approximation, which allows to
determine if dynamical self-energy corrections can remedy some of the
inconsistencies which were previously reported for these alloys.",1604.07552v1
2016-04-29,Spin Seebeck effect through antiferromagnetic NiO,"We report temperature-dependent spin-Seebeck measurements on Pt/YIG bilayers
and Pt/NiO/YIG trilayers, where YIG (Yttrium iron garnet, Y$_3$Fe$_5$O$_{12}$)
is an insulating ferrimagnet and NiO is an antiferromagnet at low temperatures.
The thickness of the NiO layer is varied from 0 to 10 nm. In the Pt/YIG
bilayers, the temperature gradient applied to the YIG stimulates dynamic spin
injection into the Pt, which generates an inverse spin Hall voltage in the Pt.
The presence of a NiO layer dampens the spin injection exponentially with a
decay length of $2 \pm 0.6$ nm at 180 K. The decay length increases with
temperature and shows a maximum of $5.5 \pm 0.8$ nm at 360 K. The temperature
dependence of the amplitude of the spin-Seebeck signal without NiO shows a
broad maximum of $6.5 \pm 0.5$ $\mu$V/K at 20 K. In the presence of NiO, the
maximum shifts sharply to higher temperatures, likely correlated to the
increase in decay length. This implies that NiO is most transparent to magnon
propagation near the paramagnet-antiferromagnet transition. We do not see the
enhancement in spin current driven into Pt reported in other papers when 1-2 nm
NiO layers are sandwiched between Pt and YIG.",1604.08659v2
2016-06-30,Correlations of zero-entropy critical states in the XXZ model: integrability and Luttinger theory far from the ground state,"Pumping a finite energy density into a quantum system typically leads to
`melted' states characterized by exponentially-decaying correlations, as is the
case for finite-temperature equilibrium situations. An important exception to
this rule are states which, while being at high energy, maintain a low entropy.
Such states can interestingly still display features of quantum criticality,
especially in one dimension. Here, we consider high-energy states in
anisotropic Heisenberg quantum spin chains obtained by splitting the ground
state's magnon Fermi sea into separate pieces. Using methods based on
integrability, we provide a detailed study of static and dynamical spin-spin
correlations. These carry distinctive signatures of the Fermi sea splittings,
which would be observable in eventual experimental realizations. Going further,
we employ a multi-component Tomonaga-Luttinger model in order to predict the
asymptotics of static correlations. For this effective field theory, we fix all
universal exponents from energetics, and all non-universal correlation
prefactors using finite-size scaling of matrix elements. The correlations
obtained directly from integrability and those emerging from the Luttinger
field theory description are shown to be in extremely good correspondence, as
expected, for the large distance asymptotics, but surprisingly also for the
short distance behavior. Finally, we discuss the description of dynamical
correlations from a mobile impurity model, and clarify the relation of the
effective field theory parameters to the Bethe Ansatz solution.",1606.09516v2
2016-07-04,Hamiltonian truncation approach to quenches in the Ising field theory,"In contrast to lattice systems where powerful numerical techniques such as
matrix product state based methods are available to study the non-equilibrium
dynamics, the non-equilibrium behaviour of continuum systems is much harder to
simulate. We demonstrate here that Hamiltonian truncation methods can be
efficiently applied to this problem, by studying the quantum quench dynamics of
the 1+1 dimensional Ising field theory using a truncated free fermionic space
approach. After benchmarking the method with integrable quenches corresponding
to changing the mass in a free Majorana fermion field theory, we study the
effect of an integrability breaking perturbation by the longitudinal magnetic
field. In both the ferromagnetic and paramagnetic phases of the model we find
persistent oscillations with frequencies set by the low-lying particle
excitations not only for small, but even for moderate size quenches. In the
ferromagnetic phase these particles are the various non-perturbative confined
bound states of the domain wall excitations, while in the paramagnetic phase
the single magnon excitation governs the dynamics, allowing us to capture the
time evolution of the magnetisation using a combination of known results from
perturbation theory and form factor based methods. We point out that the
dominance of low lying excitations allows for the numerical or experimental
determination of the mass spectra through the study of the quench dynamics.",1607.01068v3
2016-07-12,Thermodynamic transport theory of spin waves in ferromagnetic insulators,"We use the Boltzmann transport theory in the relaxation time approximation to
describe the thermal transport of spin waves in a ferromagnet. By treating spin
waves as magnon excitations we are able to compute analytically and numerically
the coefficients of the constitutive thermo-magnetic transport equations. As a
main result, we find that the absolute thermo-magnetic power coefficient
$\epsilon_M$, relating the gradient of the potential of the magnetization
current and the gradient of the temperature, in the limit of low temperature
and low field, is a constant $\epsilon_M = -0.6419 \, k_B/\mu_B$. The theory
correctly describes the low-temperature and magnetic-field dependencies of spin
Seebeck experiments. Furthermore, the theory predicts that in the limit of very
low temperatures the spin Peltier coefficient $\Pi_M$, relating the heat and
the magnetization currents, tends to a finite value which depends on the
amplitude of the magnetic field. This indicates the possibility to exploit the
spin Peltier effect as an efficient cooling mechanism in cryogenics.",1607.03301v3
2016-07-18,Robust Upward Dispersion of the Neutron Spin Resonance in the Heavy Fermion Superconductor Ce$_{1-x}$Yb$_{x}$CoIn$_5$,"The neutron spin resonance is a collective magnetic excitation that appears
in copper oxide, iron pnictide, and heavy fermion unconventional
superconductors. Although the resonance is commonly associated with a
spin-exciton due to the $d$($s^{\pm}$)-wave symmetry of the superconducting
order parameter, it has also been proposed to be a magnon-like excitation
appearing in the superconducting state. Here we use inelastic neutron
scattering to demonstrate that the resonance in the heavy fermion
superconductor Ce$_{1-x}$Yb$_{x}$CoIn$_5$ with $x=0,0.05,0.3$ has a ring-like
upward dispersion that is robust against Yb-doping. By comparing our
experimental data with random phase approximation calculation using the
electronic structure and the momentum dependence of the $d_{x^2-y^2}$-wave
superconducting gap determined from scanning tunneling microscopy for
CeCoIn$_5$, we conclude the robust upward dispersing resonance mode in
Ce$_{1-x}$Yb$_{x}$CoIn$_5$ is inconsistent with the downward dispersion
predicted within the spin-exciton scenario.",1607.05326v1
2016-07-20,Electromagnon in the Z-type hexaferrite $({\rm Ba}_{x}{\rm Sr}_{1-x})_3\rm Co_2Fe_{24}O_{41}$,"We studied experimentally the high-temperature magnetoelectric $({\rm
Ba}_{x}{\rm Sr}_{1-x})_3\rm Co_2Fe_{24}O_{41}$ prepared as ceramics (x = 0,
0.2) and a single crystal (x = 0.5) using inelastic neutron scattering, THz
time-domain, Raman and far-infrared spectroscopies. The spectra, measured with
varying temperature and magnetic field, reveal rich information about the
collective spin and lattice excitations. In the ceramics, we observed an
infrared-active magnon which is absent in $E^{\omega}\perp z$ polarized THz
spectra of the crystal, and we assume that it is an electromagnon active in
$E^{\omega} \| z$ polarized spectra. On heating from 7 to 250 K, the frequency
of this electromagnon drops from 36 to 25 cm$^{-1}$ and its damping gradually
increases, so it becomes overdamped at room temperature. Applying external
magnetic field has a similar effect on the damping and frequency of the
electromagnon, and the mode is no more observable in the THz spectra above 2 T,
as the transverse-conical magnetic structure transforms into a collinear one.
Raman spectra reveal another spin excitation with a slightly different
frequency and much higher damping. Upon applying magnetic field higher than 3
T, in the low-frequency part of the THz spectra, a narrow excitation appears
whose frequency linearly increases with magnetic field. We interpret this
feature as the ferromagnetic resonance.",1607.05878v1
2016-07-25,Goos-Hanchen shift of a spin-wave beam transmitted through anisotropic interface between two ferromagnets,"The main object of investigation in magnonics, spin waves (SWs) are promising
information carriers. Presently the most commonly studied are plane wave-like
SWs and SWs propagating in confined structures, such as waveguides. Here we
consider a Gaussian SW beam obliquely incident on an ultra-narrow interface
between two identical ferromagnetic materials. We use an analytical model and
micromagnetic simulations for an in-depth analysis of the influence of the
interface properties, in particular the magnetic anisotropy, on the
transmission of the SW beam. We derive analytical formulas for the reflectance,
transmittance, phase shift and Goos-Hanchen (GH) shift for beams reflected and
refracted by an interface between two semi-infinite ferromagnetic media; the
results for the refracted beam are the first to be reported to date. The GH
shifts in SW beam reflection and transmission are confirmed by micromagnetic
simulations in the thin-film geometry. We demonstrate the dependence of the
characteristic properties on the magnetic anisotropy at the interface, the
angle of incidence and the frequency of the SWs. We also propose a method for
the excitation of high-quality SW beams in micromagnetic simulations.",1607.07424v4
2017-02-16,Scalable Nernst Thermoelectric Power using a Coiled Galfenol Wire,"The Nernst thermopower usually is considered far too weak in most metals for
waste heat recovery. However, its transverse orientation gives it an advantage
over the Seebeck effect on non-flat surfaces. Here, we experimentally
demonstrate the scalable generation of a Nernst voltage in an air-cooled metal
wire coiled around a hot cylinder. In this geometry, a radial temperature
gradient generates an azimuthal electric field in the coil. A Galfenol
(Fe$_{0.85}$Ga$_{0.15}$) wire is wrapped around a cartridge heater, and the
voltage drop across the wire is measured as a function of axial magnetic field.
As expected, the Nernst voltage scales linearly with the length of the wire.
Based on heat conduction and fluid dynamic equations, finite-element method is
used to calculate the temperature gradient across the Galfenol wire and
determine the Nernst coefficient. A giant Nernst coefficient of -2.6
${\mu}$V/KT at room temperature is estimated, in agreement with measurements on
bulk Galfenol. We expect that the giant Nernst effect in Galfenol arises from
its magnetostriction, presumably through enhanced magnon-phonon coupling. Our
results demonstrate the feasibility of a transverse thermoelectric generator
capable of scalable output power from non-flat heat sources.",1702.04847v3
2017-02-20,Quantum criticality of spinons,"The free fermion nature of interacting spins in one dimensional (1D) spin
chains still lacks a rigorous study. In this letter we show that the length-$1$
spin strings significantly dominate critical properties of spinons, magnons and
free fermions in the 1D antiferromagnetic spin-1/2 chain. Using the Bethe
ansatz solution we analytically calculate exact scaling functions of thermal
and magnetic properties of the model, providing a rigorous understanding of the
quantum criticality of spinons. It turns out that the double peaks in specific
heat elegantly mark two crossover temperatures fanning out from the critical
point, indicating three quantum phases: the Tomonaga-Luttinger liquid (TLL),
quantum critical and fully polarized ferromagnetic phases. For the TLL phase,
the Wilson ratio $R_W=4K_s$ remains almost temperature-independent, here $K_s$
is the Luttinger parameter. Furthermore, applying our results we precisely
determine the quantum scalings and critical exponents of all magnetic
properties in the ideal 1D spin-1/2 antiferromagnet
Cu(C${}_4$H${}_4$N${}_2$)(NO${}_3$)${}_2$ recently studied in Phys. Rev. Lett.
{\bf 114}, 037202 (2015)]. We further find that the magnetization peak used in
experiments is not a good quantity to map out the finite temperature TLL phase
boundary.",1702.05903v2
2017-03-30,Anisotropy governed competition of magnetic phases in the honeycomb quantum magnet Na$_3$Ni$_2$SbO$_6$ studied by dilatometry and high-frequency ESR,"Thermodynamic properties as well as low-energy magnon excitations of $S=1$
honeycomb-layered Na$_3$Ni$_2$SbO$_6$ have been investigated by high-resolution
dilatometry, static magnetisation, and high-frequency electron spin resonance
studies in magnetic fields up to 16 T. At $T_{\rm N}$ = 16.5 K, there is a
tricritical point separating two distinct antiferromagnetic phases AF1 and AF2
from the paramagnetic regime. In addition, our data imply short-range
antiferromagnetic correlations at least up to $\sim 5\cdot T_{\rm N}$. Well
below $T_{\rm N}$, the magnetic field $B_{\rm C1}\approx$ 9.5 T is needed to
stabilize AF2 against AF1. The thermal expansion and magnetostriction anomalies
at $T_{\rm N}$ and $B_{\rm C1}$ imply significant magnetoelastic coupling, both
of which associated with a sign change of $\partial L/\partial B$. The
transition at $B_{\rm C1}$ is associated with softening of the
antiferromagnetic resonance modes observed in the electron spin resonance
spectra. The anisotropy gap $\Delta = 360$ GHz implies considerable uniaxial
anisotropy. We conclude the crucial role of axial anisotropy favoring the AF1
spin structure over the AF2 one. While the magnetostriction data disprove a
simple spin-flop scenario at $B_{\rm C1}$, the nature of a second transition at
$B_{\rm C2}\approx$ 13 T remains unclear. Both the sign of the magnetostriction
and Gr\""uneisen analysis suggest the short-range correlations at high
temperatures to be of AF2-type.",1703.10493v1
2017-08-23,Spin-memory loss due to spin-orbit coupling at ferromagnet/heavy-metal interfaces: Ab initio spin-density matrix approach,"Spin-memory loss (SML) of electrons traversing
ferromagnetic-metal/heavy-metal (FM/HM), FM/normal-metal (FM/NM) and HM/NM
interfaces is a fundamental phenomenon that must be invoked to explain
consistently large number of spintronic experiments. However, its strength
extracted by fitting experimental data to phenomenological semiclassical
theory, which replaces each interface by a fictitious bulk diffusive layer, is
poorly understood from a microscopic quantum framework and/or materials
properties. Here we describe an ensemble of flowing spin quantum states using
spin-density matrix, so that SML is measured like any decoherence process by
the decay of its off-diagonal elements or, equivalently, by the reduction of
the magnitude of polarization vector. By combining this framework with density
functional theory (DFT), we examine how all three components of the
polarization vector change at Co/Ta, Co/Pt, Co/Cu, Pt/Cu and Pt/Au interfaces
embedded within Cu/FM/HM/Cu vertical heterostructures. In addition, we use ab
initio Green's functions to compute spectral functions and spin textures over
FM, HM and NM monolayers around these interfaces which quantify interfacial
spin-orbit coupling and explain the microscopic origin of SML in long-standing
puzzles, such as why it is nonzero at Co/Cu interface; why it is very large at
Pt/Cu interface; and why it occurs even in the absence of disorder, intermixing
and magnons at the interface.",1708.07105v3
2018-02-23,Pure spin currents in magnetically ordered insulator/normal metal heterostructures,"Pure spin currents, i.e. the transport of angular momentum without an
accompanying charge current, represent a new, promising avenue in modern
spintronics from both a fundamental and an application point of view. Such pure
spin currents can not only flow in electrical conductors via mobile charge
carriers, but also in magnetically ordered electrical insulators as a flow of
spin excitation quanta. Over the course of the last years remarkable results
have been obtained in heterostructures consisting of magnetically ordered
insulators interfaced with a normal metal, where a pure spin current flows
across the interface. This topical review article deals with the fundamental
principles, experimental findings and recent developments in the field of pure
spin currents in magnetically ordered insulators. We here put our focus onto
four different manifestations of pure spin currents in such heterostructures:
The spin pumping effect, the longitudinal spin Seebeck effect, the spin Hall
magnetoresistance and the all-electrical detection of magnon transport in
non-local device concepts. In this article, we utilize a common theoretical
framework to explain all four effects and explain important material systems
(especially rare-earth iron garnets) used in the experiments. For each effect
we introduce basic measurement techniques and detection schemes and discuss
their application in the experiment. We account for the remarkable progress
achieved in each field by reporting the recent progress in each field and by
discussing research highlights obtained in our group. Finally, we conclude the
review article with an outlook on future challenges and obstacles in the field
of pure spin currents in magnetically ordered insulator / normal metal
heterostructures.",1802.08479v1
2018-04-03,Kinetic frustration induced supersolid in the $S=1/2$ kagome lattice antiferromagnet in a magnetic field,"We examine instabilities of the plateau phases in the spin-1/2 kagome-lattice
antiferromagnet in an applied field by means of degenerate perturbation theory,
and find some emergent supersolid phases below the $m=5/9$ plateau. The wave
functions of the plateau phases in a magnetic field have the particular
construction based on the building blocks of resonating hexagons and their
surrounding sites. Magnon excitations on each of these blocks suffer from a
kinetic frustration effect, namely, they cannot hop easily to the others since
the hopping amplitudes through the two paths destructively cancel out with each
other. The itineracy is thus weakened, and the system is driven toward the
strong coupling regime, which together with the selected paths allowed in real
space bears a supersolid phase. This mechanism is contrary to that proposed in
lattice-Bose gases, where the strong competing interactions suppress with each
other, allowing a small kinetic energy scale to attain the itinerancy.
Eventually, we find a supersolid state in which the pattern of resonating
hexagons are preserved from the plateau crystal state and only one-third of the
originally polarized spins outside the hexagons dominantly join the superfluid
component, or equivalently, participate in the magnetization process.",1804.00789v3
2018-04-11,Thermographic measurements of spin-current-induced temperature modulation in metallic bilayers,"Spin-to-heat current conversion effects have been investigated in bilayer
films consisting of a paramagnetic metal (PM; Pt, W, or Ta) and a ferromagnetic
metal (FM; CoFeB or permalloy). When a charge current is applied to the PM/FM
bilayer film, a spin current is generated across the PM/FM interface owing to
the spin Hall effect in PM. The spin current was found to exhibit cooling and
heating features depending on the sign of the spin Hall angle of PM, where the
spin-current-induced contribution is estimated by subtracting the contribution
of the anomalous Ettingshausen effect in FM monolayer films. We also found that
the magnitude of the spin-current-induced temperature modulation in the
Pt/CoFeB film is greater than but comparable to that in the Pt/permalloy film,
although the spin dependence of the Peltier coefficient for CoFeB is expected
to be greater than that for permalloy. We discuss the origin of the observed
behaviors with the aid of model calculations; the signals in the PM/FM films
may contain the contributions not only from the electron-driven spin-dependent
Peltier effect but also from the magnon-driven spin Peltier effect.",1804.04068v1
2018-04-13,Ni$_2$Mo$_3$O$_8$: zig-zag antiferromagnetic order an integer spin non-centrosymmetric honeycomb lattice,"Theoretical studies have predicted the existence of topological magnons in
honeycomb compounds with zig-zag antiferromagnetic (AFM) order. Here we report
the discovery of zig-zag AFM order in the layered and non-centrosymmetric
honeycomb nickelate Ni$_2$Mo$_3$O$_8$ through a combination of magnetization,
specific heat, x-ray and neutron diffraction and electron paramagnetic
resonance measurements. It is the first example of such order in an
integer-spin non-centrosymmetric structure ($P$$_6$3$mc$). Further, each of the
two distinct sites of the bipartite honeycomb lattice has a unique crystal
field environment, octahedral and tetrahedral Ni$^{2+}$ respectively, enabling
independent substitution on each sublattice. Replacement of Ni by Mg on the
octahedral site suppresses the long range magnetic order and results in a
weakly ferromagnetic state. Conversely, substitution of Fe for Ni enhances the
AFM ordering temperature. Thus Ni$_2$Mo$_3$O$_8$ provides a platform on which
to explore the rich physics of $S = 1$ on the honeycomb in the presence of
competing magnetic interactions with a non-centrosymmetric, formally
piezeo-polar, crystal structure.",1804.04776v1
2018-04-16,Magnetic interactions in PdCrO$_2$ and their effects on its magnetic structure,"We report a neutron scattering study of the metallic triangular lattice
antiferromagnet PdCrO$_2$. Powder neutron diffraction measurements confirm that
the crystalline space group symmetry remains $R\bar{3}m$ below $T_N$. This
implies that magnetic interactions consistent with the crystal symmetry do not
stabilise the non-coplanar magnetic structure which was one of two structures
previously proposed on the basis of single crystal neutron diffraction
measurements. Inelastic neutron scattering measurements find two gaps at low
energies which can be explained as arising from a dipolar-type exchange
interaction. This symmetric anisotropic interaction also stabilises a magnetic
structure very similar to the coplanar magnetic structure which was also
suggested by the single crystal diffraction study. The higher energy magnon
dispersion can be modelled by linear spin wave theory with exchange
interactions up to sixth nearest-neighbors, but discrepancies remain which hint
at additional effects unexplained by the linear theory.",1804.05573v1
2018-11-27,Hybrid magneto-dynamical modes in a single magnetostrictive nanomagnet on a piezoelectric substrate arising from magneto-elastic modulation of precessional dynamics,"Magneto-elastic (or ""straintronic"") switching has emerged as an extremely
energy-efficient mechanism for switching the magnetization of magnetostrictive
nanomagnets in magnetic memory, logic and non-Boolean circuits. Here, we
investigate the ultrafast magneto-dynamics associated with straintronic
switching in a single quasi-elliptical magnetostrictive Co nanomagnet deposited
on a piezoelectric PMN-PT substrate using time-resolved magneto-optical Kerr
effect (TR-MOKE) measurements. The pulsed laser pump beam in the TR-MOKE plays
a dual role: it causes precession of the nanomagnet's magnetization about an
applied bias magnetic field and it also generates surface acoustic waves (SAWs)
in the piezoelectric substrate that produce periodic strains in the
magnetostrictive nanomagnet and modulate the precessional dynamics. This
modulation gives rise to intriguing hybrid magneto-dynamical modes in the
nanomagnet, with rich spin wave texture. The characteristic frequencies of
these modes are 5-15 GHz, indicating that strain can affect magnetization in a
magnetostrictive nanomagnet in time scales much smaller than 1 ns (~100 ps).
This can enable ~10 GHz-range magneto-elastic nano-oscillators that are
actuated by strain instead of a spin-polarized current, as well as ultrafast
magneto-electric generation of spin waves for magnonic logic circuits,
holograms, etc.",1811.11297v1
2018-11-29,"High Saturation Magnetization, Low Coercivity and Fine YIG Nanoparticles Prepared by Modifying Co-Precipitation Method","Nanoparticles with their specific properties newly have drawn a great deal of
attention of researchers [1-3]Yttrium iron Garnet magnetic nanoparticles
(YIG-NPs) are promising materials with novel applications in microwave,
spintronics, magnonics, and magneto-optical devices. However, achieving stable
and remarkable magnetic YIG-NPs has been remaining as a great challenge. In
this paper, synthesized YIG-NPs by modifying co-precipitation (MCP) method is
reported. Structural and magnetic properties of final products are compared to
those of the materials prepared by citrate-nitrate (CN) method. Smaller
crystals and particle size have been found by MCP method comparing to that of
synthesized by CN method. Using a relatively low annealing temperatures for
both sets of samples (~700 {\deg}C), the final YIG samples prepared by MCP
method show more structural purity than those made by CN method. Higher
saturation magnetization (Ms) and lower coercivity (Hc) are observed in MCP YIG
sample (23.23 emu/g 36 and 30.1 Oe) than the CN prepared YIG sample (16.43
emu/g and 44.95 Oe). The Curie temperature is measured to be 569 {\deg}C for
the MCP YIG sample determined from set of Ms measurement at different
temperatures ranging from 80-600 K. These findings lead to significant
improvement in quality of synthesized (synthetic methods) of YIG-NPs.",1811.12511v1
2013-08-01,Magnetostatic wave analog of integer quantum Hall state in patterned magnetic films,"A magnetostatic spin wave analog of integer quantum Hall (IQH) state is
proposed in realistic patterned ferromagnetic thin films. Due to magnetic shape
anisotropy, magnetic moments in a thin film lie within the plane, while all
spin-wave excitations are fully gapped. Under an out-of-plane magnetic field,
the film acquires a finite magnetization, where some of the gapped magnons
become significantly softened near a saturation field. It is shown that, owing
to a spin-orbit locking nature of the magnetic dipolar interaction, these soft
spin-wave volume-mode bands become chiral volume-mode bands with finite
topological Chern integers. A bulk-edge correspondence in IQH physics suggests
that such volume-mode bands are accompanied by a chiral magnetostatic spin-wave
edge mode. The existence of the edge mode is justified both by micromagnetic
simulations and by band calculations based on a linearized Landau-Lifshitz
equation. Employing intuitive physical arguments, we introduce proper
tight-binding models for these soft volume-mode bands. Based on the
tight-binding models, we further discuss possible applications to other systems
such as magnetic ultrathin films with perpendicular magnetic anisotropy (PMA).",1308.0199v3
2013-08-17,Thickness and power dependence of the spin-pumping effect in Y3Fe5O12/Pt heterostructures measured by the inverse spin Hall effect,"The dependence of the spin-pumping effect on the yttrium iron garnet
(Y3Fe5O12, YIG) thickness detected by the inverse spin Hall effect (ISHE) has
been investigated quantitatively. Due to the spin-pumping effect driven by the
magnetization precession in the ferrimagnetic insulator YIG film a
spin-polarized electron current is injected into the Pt layer. This spin
current is transformed into electrical charge current by means of the ISHE. An
increase of the ISHE-voltage with increasing film thickness is observed and
compared to the theoretically expected behavior. The effective damping
parameter of the YIG/Pt samples is found to be enhanced with decreasing YIG
film thickness. The investigated samples exhibit a spin mixing conductance of
g=(7.43 \pm 0.36) \times 10^{18} m^{-2} and a spin Hall angle of theta_{ISHE} =
0.009 \pm 0.0008. Furthermore, the influence of nonlinear effects on the
generated voltage and on the Gilbert damping parameter at high excitation
powers are revealed. It is shown that for small YIG film thicknesses a
broadening of the linewidth due to nonlinear effects at high excitation powers
is suppressed because of a lack of nonlinear multi-magnon scattering channels.
We have found that the variation of the spin-pumping efficiency for thick YIG
samples exhibiting pronounced nonlinear effects is much smaller than the
nonlinear enhancement of the damping.",1308.3787v1
2013-08-21,Intense low-energy ferromagnetic fluctuations in the antiferromagnetic heavy-fermion metal CeB6,"Heavy-fermion metals exhibit a plethora of low-temperature ordering
phenomena, among them the so-called hidden-order phases that in contrast to
conventional magnetic order are invisible to standard neutron diffraction. One
of the oldest and structurally simplest hidden-order compounds, CeB6, became
famous for an elusive phase that was attributed to the antiferroquadrupolar
ordering of cerium-4f moments. In its ground state, CeB6 also develops a more
usual antiferromagnetic (AFM) order. Hence, its essential low-temperature
physics was always considered to be solely governed by AFM interactions between
the dipolar and multipolar Ce moments. Here we overturn this established
perspective by uncovering an intense ferromagnetic (FM) low-energy collective
mode that dominates the magnetic excitation spectrum of CeB6. Our inelastic
neutron-scattering data reveal that the intensity of this FM excitation by far
exceeds that of conventional spin-wave magnons emanating from the AFM wave
vectors, thus placing CeB6 much closer to a FM instability than could be
anticipated. This propensity of CeB6 to ferromagnetism may account for much of
its unexplained behavior, such as the existence of a pronounced electron spin
resonance, and should lead to a substantial revision of existing theories that
have so far largely neglected the role of FM interactions.",1308.4491v1
2014-03-17,Non-relativistic Nambu-Goldstone modes propagating along a skyrmion line,"We study Nambu-Goldstone (NG) modes or gapless modes propagating along a
skyrmion (lump) line in a relativistic and non-relativistic O(3) sigma model,
the latter of which describes isotropic Heisenberg ferromagnets. We show for
the non-relativistic case that there appear two coupled gapless modes with a
quadratic dispersion. In addition to the well-known Kelvin mode consisting of
two translational zero modes transverse to the skyrmion line, we show that the
other consists of a magnon and dilaton, that is, a NG mode for a spontaneously
broken internal U(1) symmetry and a quasi-NG mode for a spontaneously broken
scale symmetry of the equation of motion. We find that the commutation
relations of Noether charges admit a central extension between the dilatation
and phase rotation, in addition to the one between two translations found
recently. The counting rule is consistent with the Nielsen-Chadha and
Watanabe-Brauner relations only when we take into account quasi-NG modes.",1403.4031v4
2017-01-09,Spin-Wave versus Joule Heating in Spin-Hall-Effect/Spin-Transfer-Torque Driven Cr/Heusler/Pt Waveguides,"We present a time-resolved study of the DC-current driven magnetization
dynamics in a microstructured Cr/Heusler/Pt waveguide by means of Brillouin
light scattering. A reduction of the effective spin-wave damping via the
spin-transfer-torque effect leads to a strong increase in the magnon density.
This is accompanied by a decrease of the spin-wave frequencies. By evaluating
the time scales of these effects, the origin of this frequency shift can be
identified. However, recently, we found that the experimental setup partially
influences the decay of the spin-wave intensity after the current pulse is
switched off. Thus, further investigations on the presented effect are needed
to allow for a more detailed analysis. For this reason, we need to withdraw the
manuscript at this point and might publish an updated version later.",1701.02094v2
2017-01-27,The nature of spin excitations in the one-third magnetization plateau phase of Ba$_3$CoSb$_2$O$_9$,"Magnetization plateaus in quantum magnets---where bosonic quasiparticles
crystallize into emergent spin superlattices---are spectacular yet simple
examples of collective quantum phenomena escaping classical description. While
magnetization plateaus have been observed in a number of spin-1/2
antiferromagnets, the description of their magnetic excitations remains an open
theoretical and experimental challenge. Here, we investigate the dynamical
properties of the triangular-lattice spin-1/2 antiferromagnet
Ba$_3$CoSb$_2$O$_9$ in its one-third magnetization plateau phase using a
combination of nonlinear spin-wave theory and neutron scattering measurements.
The agreement between our theoretical treatment and the experimental data
demonstrates that magnons behave semiclassically in the plateau in spite of the
purely quantum origin of the underlying magnetic structure. This allows for a
quantitative determination of Ba$_3$CoSb$_2$O$_9$ exchange parameters. We
discuss the implication of our results to the deviations from semiclassical
behavior observed in zero-field spin dynamics of the same material and conclude
they must have an intrinsic origin.",1701.07971v4
2018-05-09,Dynamic spin-lattice coupling and nematic fluctuations in NaFeAs,"We use inelastic neutron scattering to study acoustic phonons and spin
excitations in single crystals of NaFeAs, a parent compound of iron pnictide
superconductors. NaFeAs exhibits a tetragonal-to-orthorhombic structural
transition at $T_s\approx 58$ K and a collinear antiferromagnetic (AF) order at
$T_N\approx 45$ K. While longitudinal and out-of-plane transverse acoustic
phonons behave as expected, the in-plane transverse acoustic phonons reveal
considerable softening on cooling to $T_s$, and then harden on approaching
$T_N$ before saturating below $T_N$. In addition, we find that spin-spin
correlation lengths of low-energy magnetic excitations within the FeAs layer
and along the $c$-axis increase dramatically below $T_s$, and show weak anomaly
across $T_N$. These results suggest that the electronic nematic phase present
in the paramagnetic tetragonal phase is closely associated with dynamic
spin-lattice coupling, possibly arising from the one-phonon-two-magnon
mechanism.",1805.03493v1
2018-05-10,"Magnetic excitations in bulk multiferroic two-dimensional triangular lattice antiferromagnet (Lu,Sc)FeO$_3$","Non-collinear two-dimensional triangular lattice antiferromagnets (2D TLAF)
are currently an area of very active research due to their unique magnetic
properties, which lead to non-trivial quantum effects that experimentally
manifest themselves in the spin excitation spectra. Recent examples of such
insulating 2D TLAF include (Y,Lu)MnO$_3$, LiCrO$_2$, and CuCrO$_2$. Hexagonal
LuFeO3 is a recently synthesized 2D TLAF which exhibits properties of an ideal
multiferroic material, partially because of the high spin ($S=5/2$) and strong
magnetic super-exchange interactions. We report the full range of spin dynamics
in a bulk single crystal of (Lu$_{0.6}$Sc$_{0.4}$)FeO$_3$ (Sc doping to
stabilize the hexagonal structure) measured via time-of-flight inelastic
neutron scattering. Modeling with linear spin wave theory yields a nearest
neighbor exchange coupling of $J$ = 4.0(2) meV (DFT calculations for
$h$-LuFeO$_3$ predicted a value of 6.31 meV) and anisotropy values of $K_D$ =
0.17(1) meV (easy plane) and $K_A$~=~-0.05(1)~meV (local easy axis). It is
observed that the magnon bandwidth of the spin wave spectra is twice as large
for $h$-(Lu,Sc)FeO$_3$ as it is for $h$-LuMnO$_3$.",1805.03912v2
2018-05-13,Giant peak of the Inverse Faraday effect in the band gap of magnetophotonic crystal,"Optical impact on the spin system in a magnetically ordered medium provides a
unique possibility for local manipulation of magnetization at subpicosecond
time scales that is very promising for magnetic data processing and other
magnonics applications. One of the mechanisms of the optical manipulation is
related to the inverse Faraday effect (IFE). Usually the IFE is observed in
crystals and magnetic films on a substrate. Here we demonstrate the IFE induced
by fs-laser pulses in the magnetic film inside the magnetophotonic crystal.
Spectral dependence of the IFE on the laser pulse wavelength in the band gap of
the magnetophotonic crystal has a sharp peak leading to a significant
enhancement of the IFE. This phenomenon is explained by strong confinement of
the electromagnetic energy and angular momentum within the magnetic film.
Calculated near field distribution of the IFE effective magnetic field
indicates its subwavelength localization within 30 nm along the film thickness.
These excited volumes can be shifted along the sample depth via e.g. changing
frequency of the laser pulses. The obtained results open a way for the new
applications in the areas of ultrafast spintronics and quantum information
processing.",1805.04862v1
2018-05-18,Quantum theory of spin waves for Helical ground states in Hollandite lattice,"We perform spin wave analysis of classical ground states of a model
Hamiltonian proposed earlier(Phys. Rev. B {\bf 90}, 104420(2014)) for
$\alpha-MnO_2$ compounds. It is known that the phase diagram of the Hollandite
lattice (lattice of $\alpha-MnO_2$ compounds) consists of four different
Helical phases(FH, A2H, C2H, CH phase) in the space of model parameters
$J_1,~J_2,~J_3$. The spin wave dispersion shows presence of gapless mode which
interpolates between quadratic to linear depending on phases and values of
$J_i$'s. In most cases, the 2nd lowest mode shows the existence of a roton
minima mainly from $X$ to $M$ and $M$ to $Z$ path. Few higher modes also show
roton minima. Each helical phase has its characteristic traits which can be
used to determine the phases itself. The analytical expressions of eigenmodes
at high symmetry points are obtained which can be utilized to extract the
values of $J_i$'s. Density of states, specific heat and susceptibilities at low
temperature has been studied within spin wave approximation. The specific heat
shows departure from $T^{1.5(3)}$ dependence found in three dimensional
unfrustrated ferromagnetic(anti-ferromagnetic) system which seems to be the
signature of incommensurate helical phase. The parallel susceptibility is
maximum for FH phase and minimum for CH phase at low temperature. The
perpendicular susceptibility is found to be independent of temperature at very
low temperature. Our study can be used to compare experimental results on
magnon spectrum, elastic neutron scattering, and finite temperature properties
mentioned above for clean $\alpha-MnO_2$ system",1805.07156v1
2018-05-23,Elliptic String Solutions on RxS^2 and Their Pohlmeyer Reduction,"We study classical string solutions on RxS^2 that correspond to elliptic
solutions of the sine-Gordon equation. In this work, these solutions are
systematically derived inverting Pohlmeyer reduction and classified with
respect to their Pohlmeyer counterparts. These solutions include the spiky
strings and other well-known solutions, such as the BMN particle, the GKP
string or the giant magnons, which arise as special limits, and reveal many
interesting features of the AdS/CFT correspondence. A mapping of the physical
properties of the string solutions to those of their Pohlmeyer counterparts is
established. An interesting element of this mapping is the correspondence of
the number of spikes of the string to the topological charge in the sine-Gordon
theory. In the context of the sine-Gordon/Thirring duality, the latter is
mapped to the Thirring model fermion number, leading to a natural
classification of the solutions to fermionic objects and bosonic condensates.
Finally, the convenient parametrization of the solutions, enforced by the
inversion of the Pohlmeyer reduction, facilitates the study of the string
dispersion relation. This leads to the identification of an infinite set of
trajectories in the moduli space of solutions, where the dispersion relation
can be expressed in a closed form by means of some algebraic operations,
arbitrarily far from the infinite size limit.",1805.09301v1
2011-11-17,Resonant magnetic exciton mode in the heavy-fermion antiferromagnet CeB6,"Resonant magnetic excitations are widely recognized as hallmarks of
unconventional superconductivity in copper oxides, iron pnictides, and
heavy-fermion compounds. Numerous model calculations have related these modes
to the microscopic properties of the pair wave function, but the mechanisms
underlying their formation are still debated. Here we report the discovery of a
similar resonant mode in the non-superconducting, antiferromagnetically ordered
heavy-fermion metal CeB6. Unlike conventional magnons, the mode is
non-dispersive, and its intensity is sharply concentrated around a wave vector
separate from those characterizing the antiferromagnetic order. The magnetic
intensity distribution rather suggests that the mode is associated with a
coexisting order parameter of the unusual antiferro-quadrupolar phase of CeB6,
which has long remained ""hidden"" to the neutron-scattering probes. The mode
energy increases continuously below the onset temperature for
antiferromagnetism, in parallel to the opening of a nearly isotropic spin gap
throughout the Brillouin zone. These attributes bear strong similarity to those
of the resonant modes observed in unconventional superconductors below their
critical temperatures. This unexpected commonality between the two disparate
ground states indicates the dominance of itinerant spin dynamics in the ordered
low-temperature phases of CeB6 and throws new light on the interplay between
antiferromagnetism, superconductivity, and ""hidden"" order parameters in
correlated-electron materials.",1111.4151v1
2012-04-13,Low temperature thermodynamic properties near the field-induced quantum critical point in DTN,"We present a comprehensive experimental and theoretical investigation of the
thermodynamic properties: specific heat, magnetization and thermal expansion in
the vicinity of the field-induced quantum critical point (QCP) around the lower
critical field $H_{c1} \approx 2$\,T in DTN . A $T^{3/2}$ behavior in the
specific heat and magnetization is observed at very low temperatures at
$H=H_{c1}$ that is consistent with the universality class of Bose-Einstein
condensation of magnons. The temperature dependence of the thermal expansion
coefficient at $H_{c1}$ shows minor deviations from the expected $T^{1/2}$
behavior. Our experimental study is complemented by analytical calculations and
Quantum Monte Carlo simulations, which reproduce nicely the measured
quantities. We analyze the thermal and the magnetic Gr\""{u}neisen parameters
that are ideal quantities to identify QCPs. Both parameters diverge at $H_{c1}$
with the expected $T^{-1}$ power law. By using the Ehrenfest relations at the
second order phase transition, we are able to estimate the pressure
dependencies of the characteristic temperature and field scales.",1204.3064v1
2013-09-07,On the broken time translation symmetry in macroscopic systems: precessing states and off-diagonal long-range order,"The broken symmetry state with off-diagonal long-range order (ODLRO), which
is characterized by the vacuum expectation value of the operator of creation of
the conserved quantum number Q, has the time-dependent order parameter.
However, the breaking of the time reversal symmetry is observable only if the
charge Q is not strictly conserved and may decay. This dihotomy is resolved in
systems with quasi-ODLRO. These systems have two well separated relaxation
times: the relaxation time \tau_Q of the charge Q and the energy relaxation
time \tau_E. If \tau_Q >> \tau_E, the perturbed system relaxes first to the
state with the ODLRO, which persists for a long time \tau_Q and finally relaxes
to the full equilibrium static state. In the limit \tau_Q -> \infty, but not in
the strict limit case when the charge Q is conserved, the intermediate ODLRO
state can be considered as the ground state of the system at fixed Q with the
observable spontaneously broken time reversal symmetry. Examples of systems
with quasi-ODLRO are provided by superfluid phase of liquid 4He, Bose-Einstein
condensation of magnons (phase coherent spin precession) and precessing
vortices.",1309.1845v2
2013-09-20,Internal modes of a skyrmion in the ferromagnetic background in chiral magnets,"A spin texture called skyrmion has been recently observed in certain chiral
magnets without inversion symmetry. The observed skyrmions are extended objects
with typical linear sizes of 10 nm to 100 nm that contain $10^3$ to $10^5$
spins and can be deformed in response to external perturbations. Weak
deformations are characterized by the internal modes which are localized around
the skyrmion center. Knowledge of internal modes is crucial to assess the
stability and rigidity of these topological textures. Here we compute the
internal modes of a skyrmion in the ferromagnetic background state by numerical
diagonalization of the dynamical matrix. We find several internal modes below
the magnon continuum, such as the mode corresponding to the translational
motion, and different kinds of breathing modes. The number of internal modes is
larger for lower magnetic fields. Indeed, several modes become gapless in the
low field region indicating that the single skyrmion solution becomes unstable,
although a skyrmion lattice is thermodynamically stable. On the other hand, at
high fields only three internal modes exist and the skyrmion is stable even at
higher fields when the ferromagnetic state is thermodynamically stable. We also
show that the presence of out-of-plane easy-axis anisotropy stabilizes the
single skyrmion solution. Finally, we discuss the effects of damping and
possible experimental observations of these internal modes.",1309.5168v2
2014-05-20,Origin of the spin Seebeck effect probed by temperature dependent measurements in Gd$_{3}$Fe$_{5}$O$_{12}$,"We probe the spin Seebeck effect in Gd$_{3}$Fe$_{5}$O$_{12}$/Pt hybrid
structures as a function of temperature and observe two sign changes of the
spin Seebeck signal with decreasing temperature. A first sign change occurs at
a temperature close to the Gd$_{3}$Fe$_{5}$O$_{12}$ magnetic compensation point
at around 280 K. There the spin Seebeck signal changes sign abruptly with
unaltered amplitude, indicating that the spin current is mainly caused by the
magnetic Fe sub-lattices, which reorient their directions at this temperature.
A second, more gradual sign change takes place around the ordering temperature
of the Gd sub-lattice in the range of 65-85 K, showing that the Gd magnetic
sub-lattice dominates the thermally driven spin current at lower temperatures.
These sign changes together with the non-monotonous dependence of the spin
Seebeck signal on the temperature demonstrate that the magnonic spin current is
not simply replicating the effective magnetization of Gd$_{3}$Fe$_{5}$O$_{12}$.
Rather, the thermally generated net spin current results from a complex
interplay of the three magnetic sub-lattices involved.",1405.4971v1
2016-05-22,Low Gilbert damping in Co2FeSi and Fe2CoSi films,"Thin highly textured Fe$_{\mathrm{1+x}}$Co$_{\mathrm{2-x}}$Si ($0 \leq$ x
$\leq 1$) films were prepared on MgO (001) substrates by magnetron
co-sputtering. The magneto-optic Kerr effect (MOKE) and ferromagnetic resonance
(FMR) measurements were used to investigate the composition dependence of the
magnetization, the magnetic anisotropy, the gyromagnetic ratio and the
relaxation of the films. The effective magnetization for the thin
Fe$_{\mathrm{1+x}}$Co$_{\mathrm{2-x}}$Si films, determined by FMR measurements,
are consistent with the Slater Pauling prediction. Both MOKE and FMR
measurements reveal a pronounced fourfold anisotropy distribution for all
films. In addition we found a strong influence of the stoichiometry on the
anisotropy as the cubic anisotropy strongly increases with increasing Fe
concentration. The gyromagnetic ratio is only weakly dependent on the
composition. We find low Gilbert damping parameters for all films with values
down to $0.0012\pm0.00012$ for Fe$_{1.75}$Co$_{1.25}$Si. The effective damping
parameter for Co$_2$FeSi is found to be $0.0018\pm 0.0004$. We also find a
pronounced anisotropic relaxation, which indicates significant contributions of
two-magnon scattering processes that is strongest along the easy axes of the
films. This makes thin Fe$_{\mathrm{1+x}}$Co$_{\mathrm{2-x}}$Si films ideal
materials for the application in STT-MRAM devices.",1605.06797v1
2016-05-31,The mechanical back-action of a spin-wave resonance in a magnetoelastic thin film on a surface acoustic wave,"Surface acoustic waves (SAWs) traveling on the surface of a piezoelectric
crystal can, through the magnetoelastic interaction, excite traveling spin-wave
resonance in a magnetic film deposited on the substrate. This spin-wave
resonance in the magnetic film creates a time dynamic surface stress of
magnetoelastic origin that acts back on the surface of the piezoelectric and
modifies the SAW propagation. Unlike previous analyses that treat the
excitation as a magnon-phonon polariton, here the magnetoelastic film is
treated as a perturbation modifying boundary conditions on the SAW. We use
acoustical perturbation theory to find closed form expressions for the
back-action surface stress and strain fields and the resultant SAW velocity
shifts and attenuation. We demonstrate that the shear stress fields associated
with this spin-wave back-action also generate effective surface currents on the
piezoelectric both in-phase and out-of-phase with the driving SAW potential.
Characterization of these surface currents and their applications in
determination of the magnetoelastic coupling are discussed. The perturbative
calculation is carried out explicitly to first order (a regime corresponding to
many experimental situations of current interest) and we provide a sketch of
the implications of the theory at higher order.",1605.09475v2
2016-08-22,Doping Evolution of Magnetic Order and Magnetic Excitations in (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$,"We use resonant elastic and inelastic X-ray scattering at the Ir-$L_3$ edge
to study the doping-dependent magnetic order, magnetic excitations and
spin-orbit excitons in the electron-doped bilayer iridate
(Sr$_{1-x}$La$_{x}$)$_3$Ir$_2$O$_7$ ($0 \leq x \leq 0.065$). With increasing
doping $x$, the three-dimensional long range antiferromagnetic order is
gradually suppressed and evolves into a three-dimensional short range order
from $x = 0$ to $0.05$, followed by a transition to two-dimensional short range
order between $x = 0.05$ and $0.065$. Following the evolution of the
antiferromagnetic order, the magnetic excitations undergo damping, anisotropic
softening and gap collapse, accompanied by weakly doping-dependent spin-orbit
excitons. Therefore, we conclude that electron doping suppresses the magnetic
anisotropy and interlayer couplings and drives
(Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$ into a correlated metallic state hosting
two-dimensional short range antiferromagnetic order and strong
antiferromagnetic fluctuations of $J_{\text{eff}} = \frac{1}{2}$ moments, with
the magnon gap strongly suppressed.",1608.06268v2
2017-06-05,Bose-Einstein Condensation in a Plasmonic Lattice,"Bose-Einstein condensation is a remarkable manifestation of quantum
statistics and macroscopic quantum coherence. Superconductivity and
superfluidity have their origin in Bose-Einstein condensation. Ultracold
quantum gases have provided condensates close to the original ideas of Bose and
Einstein, while condensation of polaritons and magnons have introduced novel
concepts of non-equilibrium condensation. Here, we demonstrate a Bose-Einstein
condensate (BEC) of surface plasmon polaritons in lattice modes of a metal
nanoparticle array. Interaction of the nanoscale-confined surface plasmons with
a room-temperature bath of dye molecules enables thermalization and
condensation in picoseconds. The ultrafast thermalization and condensation
dynamics are revealed by an experiment that exploits thermalization under
propagation and the open cavity character of the system. A crossover from BEC
to usual lasing is realized by tailoring the band structure. This new
condensate of surface plasmon lattice excitations has promise for future
technologies due to its ultrafast, room-temperature and on-chip nature.",1706.01528v3
2017-06-07,Evaluation of the operatorial Q-system for non-compact super spin chains,"We present an approach to evaluate the full operatorial Q-system of all
$\mathfrak{u}(p,q|r+s)$-invariant spin chains with representations of
Jordan-Schwinger type. In particular, this includes the super spin chain of
planar $\mathcal{N}=4$ super Yang-Mills theory at one loop in the presence of a
diagonal twist. Our method is based on the oscillator construction of
Q-operators. The Q-operators are built as traces over Lax operators which are
degenerate solutions of the Yang-Baxter equation. For non-compact
representations these Lax operators may contain multiple infinite sums that
conceal the form of the resulting functions. We determine these infinite sums
and calculate the matrix elements of the lowest level Q-operators. Transforming
the Lax operators corresponding to the Q-operators into a representation
involving only finite sums allows us to take the supertrace and to obtain the
explicit form of the Q-operators in terms of finite matrices for a given magnon
sector. Imposing the functional relations, we then bootstrap the other
Q-operators from those of the lowest level. We exemplify this approach for
non-compact spin $-s$ spin chains and apply it to $\mathcal{N}=4$ SYM at the
one-loop level using the BMN vacuum as an example.",1706.02320v3
2017-06-29,Reconfigurable topological spin wave beamsplitters and interferometers,"Conventional magnonic devices use three classes of magnetostatic waves that
require detailed manipulation of magnetization structure that makes the design
and the device/circuitry scalability difficult tasks. Here, we demonstrate that
devices based on topological exchange spin waves do not suffer from the problem
with additional nice features of nano-scale wavelength and high frequency. Two
results are reported. 1) A perpendicular ferromagnet on a honeycomb lattice is
generically a topological magnetic material in the sense that topologically
protected chiral edge spin waves exist in the band gap as long as spin-orbit
induced nearest-neighbor pseudodipolar interaction (and/or next-nearest
neighbor Dzyaloshinskii-Moriya interaction) is present. 2) As a proof of
concept, spin wave beamsplitters and spin wave interferometers are designed by
using domain walls to manipulate the propagation of topologically protected
chiral spin waves. Since magnetic domain walls can be controlled by magnetic
fields or electric current/fields, one can essentially draw, erase and redraw
different spin wave devices and circuitry on the same magnetic plate so that
the proposed devices are reconfigurable and tunable. Devices made from magnetic
topological materials are robust against both internal and external
perturbations such as the spin wave frequency variation and device geometry as
well as defects.",1706.09548v2
2017-10-02,Signatures of low-energy fractionalized excitations in $α$-RuCl$_3$ from field-dependent microwave absorption,"Topologically ordered states of matter are generically characterized by
excitations with quantum number fractionalization. A prime example is the spin
liquid realized in Kitaev's honeycomb-lattice compass model where spin-flip
excitations fractionalize into Majorana fermions and Ising gauge fluxes. While
numerous compounds have been proposed to be proximate to such a spin-liquid
phase, clear-cut evidence for fractionalized excitations is lacking. Here we
employ microwave absorption measurements to study the low-energy excitations in
$\alpha$-RuCl$_3$ over a wide range of frequencies, magnetic fields, and
temperatures, covering in particular the vicinity of the field-driven quantum
phase transition where long-range magnetic order disappears. In addition to
conventional gapped magnon modes we find a highly unusual broad continuum
characteristic of fractionalization which -- most remarkably -- extends to
energies below the lowest sharp mode and to temperatures significantly higher
than the ordering temperature, and develops a gap of a nontrivial origin in
strong magnetic fields. Our results unravel the signatures of fractionalized
excitations in $\alpha$-RuCl$_3$ and pave the way to a more complete
understanding of the Kitaev spin liquid and its instabilities.",1710.00670v3
2017-12-02,Coherent excitation of heterosymmetric spin waves with ultrashort wavelengths,"In the emerging field of magnonics, spin waves are foreseen as signal
carriers for future spintronic information processing and communication
devices, owing to both the very low power losses and a high device
miniaturisation potential predicted for short-wavelength spin waves. Yet, the
efficient excitation and controlled propagation of nanoscale spin waves remains
a severe challenge. Here, we report the observation of high-amplitude,
ultrashort dipole-exchange spin waves (down to 80 nm wavelength at 10 GHz
frequency) in a ferromagnetic single layer system, coherently excited by the
driven dynamics of a spin vortex core. We used time-resolved x-ray microscopy
to directly image such propagating spin waves and their excitation over a wide
range of frequencies. By further analysis, we found that these waves exhibit a
heterosymmetric mode profile, involving regions with anti-Larmor precession
sense and purely linear magnetic oscillation. In particular, this mode profile
consists of dynamic vortices with laterally alternating helicity, leading to a
partial magnetic flux closure over the film thickness, which is explained by a
strong and unexpected mode hybridisation. This spin-wave phenomenon observed is
a general effect inherent to the dynamics of sufficiently thick ferromagnetic
single layer films, independent of the specific excitation method employed.",1712.00681v3
2017-12-05,Peculiarities of the electronic transport in half-metallic Co-based Heusler alloys,"Electrical, magnetic and galvanomagnetic properties of half-metallic Heusler
alloys of Co$_2$YZ (Y = Ti, V, Cr, Mn, Fe, Ni, and Z = Al, Si, Ga, Ge, In, Sn,
Sb) were studied in the temperature range 4.2--900 K and in magnetic fields of
up to 100 kOe. It was found that varying Y in affects strongly the electric
resistivity and its temperature dependence $\rho(T)$, while this effect is not
observed upon changing Z. When Y is varied, extrema (maximum or minimum) are
observed in $\rho(T)$ near the Curie temperature $T_C$. At $T < T_C$, the
$\rho(T)$ behavior can be ascribed to a change in electronic energy spectrum
near the Fermi level. The coefficients of the normal and anomalous Hall effect
were determined. It was shown that the latter coefficient, $R_S$, is related to
the residual resistivity $\rho_0$ by a power law $R_S \sim \rho_0^k/M_S$ with
$M_S$ the spontaneous magnetization. The exponent $k$ was found to be 1.8 for
Co$_2$FeZ alloys, which is typical for asymmetric scattering mechanisms, and
2.9 for Co$_2$YAl alloys, which indicates an additional contribution to the
anomalous Hall effect. The temperature dependence of resistivity at low
temperatures is analyzed and discussed in the framework of the two-magnon
scattering theory.",1712.01584v1
2017-12-19,Dispersive Magnetic and Electronic Excitations in Iridate Perovskites Probed with Oxygen $K$-Edge Resonant Inelastic X-ray Scattering,"Resonant inelastic X-ray scattering (RIXS) experiments performed at the
oxygen-$K$ edge on the iridate perovskites {\SIOS} and {\SION} reveal a
sequence of well-defined dispersive modes over the energy range up to $\sim
0.8$ eV. The momentum dependence of these modes and their variation with the
experimental geometry allows us to assign each of them to specific collective
magnetic and/or electronic excitation processes, including single and
bi-magnons, and spin-orbit and electron-hole excitons. We thus demonstrated
that dispersive magnetic and electronic excitations are observable at the O-$K$
edge in the presence of the strong spin-orbit coupling in the $5d$ shell of
iridium and strong hybridization between Ir $5d$ and O $2p$ orbitals, which
confirm and expand theoretical expectations. More generally, our results
establish the utility of O-$K$ edge RIXS for studying the collective
excitations in a range of $5d$ materials that are attracting increasing
attention due to their novel magnetic and electronic properties. Especially,
the strong RIXS response at O-$K$ edge opens up the opportunity for
investigating collective excitations in thin films and heterostructures
fabricated from these materials.",1712.06895v1
2017-12-21,Exchange-torque-induced excitation of perpendicular standing spin waves in nanometer-thick YIG films,"Spin waves in ferrimagnetic yttrium iron garnet (YIG) films with ultralow
magnetic damping are relevant for magnon-based spintronics and low-power
wave-like computing. The excitation frequency of spin waves in YIG is rather
low in weak external magnetic fields because of its small saturation
magnetization, which limits the potential of YIG films for high-frequency
applications. Here, we demonstrate how exchange-coupling to a CoFeB film
enables efficient excitation of high-frequency perpendicular standing spin
waves (PSSWs) in nanometer-thick (80 nm and 295 nm) YIG films using uniform
microwave magnetic fields. In the 295-nm-thick YIG film, we measure intense
PSSW modes up to 10th order. Strong hybridization between the PSSW modes and
the ferromagnetic resonance mode of CoFeB leads to characteristic anti-crossing
behavior in broadband spin-wave spectra. A dynamic exchange torque at the
YIG/CoFeB interface explains the excitation of PSSWs. The localized torque
originates from exchange coupling between two dissimilar magnetization
precessions in the YIG and CoFeB layers. As a consequence, spin waves are
emitted from the YIG/CoFeB interface and PSSWs form when their wave vector
matches the perpendicular confinement condition. PSSWs are not excited when the
exchange coupling between YIG and CoFeB is suppressed by a Ta spacer layer.
Micromagnetic simulations confirm the exchange-torque mechanism.",1712.08204v1
2018-09-11,Detecting Crystallographic Lattice Chirality using Resonant Inelastic X-ray Scattering,"The control and detection of crystallographic chirality is an important and
challenging scientific problem. Chirality has wide ranging implications from
medical physics to cosmology including an intimate but subtle connection in
magnetic systems, for example Mn$_{1-x}$Fe$_{x}$Si. X-ray diffraction
techniques with resonant or polarized variations of the experimental setup are
currently utilized to characterize lattice chirality. We demonstrate using
theoretical calculations the feasibility of indirect $K$ -edge bimagnon
resonant inelastic X-ray scattering (RIXS) spectrum as a viable experimental
technique to distinguish crystallographic handedness. We apply spin wave theory
to the recently discovered $\sqrt {5}\times\sqrt {5}$ vacancy ordered
chalcogenide Rb$_{0.89}$Fe$_{1.58}$Se$_{2}$ for realistic X-ray experimental
set up parameters (incoming energy, polarization, and Bragg angle) to show that
the computed RIXS spectrum is sensitive to the underlying handedness (right or
left) of the lattice. A Flack parameter definition that incorporates the right-
and left- chiral lattice RIXS response is introduced. It is shown that the RIXS
response of the multiband magnon system RbFeSe arises both from inter- and
intra- band scattering processes. The extinction or survival of these RIXS
peaks are sensitive to the underlying chiral lattice orientation. This in turn
allows for the identification of the two chiral lattice orientations.",1809.04148v2
2018-09-13,Single-layer antiferromagnetic semiconductor CoS2 with the pentagonal structure,"Structure-property relationships have always been guiding principles in
discovering new materials. Here we explore the relationships to discover novel
two-dimensional (2D) materials with the goal of identifying 2D magnetic
semiconductors for spintronics applications. In particular, we report a density
functional theory + $U$ study of single-layer antiferromagnetic (AFM)
semiconductor CoS$_2$ with the pentagonal structure forming the so-called Cairo
Tessellation. We find that this single-layer magnet exhibits an indirect
bandgap of 1.06 eV with light electron and hole effective masses of 0.03 and
0.10 $m_0$, respectively, which may lead to high carrier mobilities. The hybrid
density functional theory calculations correct the bandgap to 2.24 eV. We also
compute the magnetocrystalline anisotropy energy (MAE), showing that the easy
axis of the AFM ordering is out of plane with a sizable MAE of 153 $\mu$eV per
Co ion. We further calculate the magnon frequencies at different spin-spiral
vectors, based on which we estimate the N$\acute{e}$el temperatures to be 20.4
and 13.3 K using the mean field and random phase approximations, respectively.
We then apply biaxial strains to tune the bandgap of single-layer pentagonal
CoS$_2$. We find that the energy difference between the ferromagnetic and AFM
structures strongly depends on the biaxial strain, but the ground state remains
the AFM ordering. Although the low critical temperature prohibits the magnetic
applications of single-layer pentagonal CoS$_2$ at room temperature, the
excellent electrical properties may find this novel single-layer semiconductor
applications in optoelectronic nanodevices.",1809.05055v1
2018-09-28,Isotropic non-local Gilbert damping driven by spin currents in epitaxial Pd/Fe/MgO(001) films,"Although both theoretical predications and experimental observations
demonstrated that the damping factor is anisotropic at
ferromagnet/semiconductor interface with robust interfacial spin-orbit
coupling, it is not well understood whether non-local Gilbert damping driven by
spin currents in heavy metal/ferromagnetic metal (HM/FM) bilayers is
anisotropic or not. Here, we investigated the in-plane angular- and frequency-
dependence of magnetic relaxation of epitaxial Fe/MgO(001) films with different
capping layers of Pd and Cu. After disentangling the parasitic contributions,
such as two-magnon scattering (TMS), mosaicity, and field-dragging effect, we
unambiguously observed that both local and non-local Gilbert damping are
isotropic in Fe(001) plane, suggesting that the pure spin currents absorption
is independent of Fe magnetization orientation in the epitaxial Pd/Fe
heterostructure. First principles calculation reveals that the effective spin
mixing conductance of Pd/Fe interface is nearly invariant for different
magnetization directions in good agreement with the experimental observations.
These results offer a valuable insight into the transmission and absorption of
pure spin currents, and facilitate us to utilize next-generation spintronic
devices.",1809.11020v1
2019-09-05,Extraction of Dzyaloshinksii-Moriya interaction from propagating spin waves validated,"The interfacial Dzyaloshinksii-Moriya interaction (iDMI) is of great interest
in thin-film magnetism because of its ability to stabilize chiral spin
textures. It can be quantified by investigating the frequency non-reciprocity
of oppositely propagating spin waves. However, as the iDMI is an interface
interaction the relative effect reduces when the films become thicker making
quantification more difficult. Here, we utilize all-electrical Propagating Spin
Wave Spectroscopy (PSWS) to disentangle multiple contributions to spin wave
frequency non-reciprocity to determine the iDMI. This is done by investigating
non-reciprocities across a wide range of magnetic layer thicknesses (from 4 to
26 nm) in Pt/Co/Ir, Pt/Co/Pt, and Ir/Co/Pt stacks. We find the expected sign
change in the iDMI when inverting the stack order, and a negligible iDMI for
the symmetric Pt/Co/Pt. We additionally extract a difference in surface
anisotropies and find a large contribution due to the formation of different
crystalline phases of the Co, which is corroborated using nuclear magnetic
resonance and high-resolution transmission-electron-microscopy measurements.
These insights will open up new avenues to investigate, quantify and
disentangle the fundamental mechanisms governing the iDMI, and pave a way
towards engineered large spin-wave non-reciprocities for magnonic applications.",1909.02467v1
2019-09-09,Intrinsic quantum Ising model on a triangular lattice magnet TmMgGaO$_{4}$ and beyond,"The rare-earth magnet TmMgGaO$_{4}$ is proposed to be an intrinsic quantum
Ising magnet described by the antiferromagnetic transverse field Ising model
(TFIM) on a triangular lattice, where the relevant degrees of freedom are the
non-degenerate dipole-multipole doublets of the Tm$^{3+}$ ions and the
transverse field has an intrinsic origin from the weak splitting of the
doublet. We compare this special doublet of Tm$^{3+}$ with the dipole-octupole
Kramers doublet. We study the proposed effective model for the Tm-based
triangular lattice and consider the effects of external magnetic fields and
finite temperatures. From the ""orthogonal operator approach"", we show that the
TFIM with the three-sublattice intertwined ordered state agrees with the
experiments and further clarify the discrepancy in the nubmers of the magnetic
sublattices and the measured magnon branches. We make specific predictions for
the evolution of the magnetic properties with the external magnetic field.
Furthermore, we demonstrate that an emergent U(1) symmetry emerges in thermal
melting of the underlying orders and at the criticality, and summarize the
previously known signatures related to the finite-temperature
Berezinskii-Kosterlitz-Thouless (BKT) physics. We discuss the broad relevance
of intrinsic quantum Ising magnets to many other systems, especially the
Tm-based materials.",1909.03608v3
2019-09-10,"Twisting and tweezing the spin wave: on vortices, skyrmions, helical waves, and the magnonic spiral phase plate","Spin waves are the low-energy excitations of magnetically ordered materials.
They are key elements in the stability analysis of the ordered phase and have a
wealth of technological applications. Recently, we showed that spin waves of a
magnetic nanowire may carry a definite amount of orbital angular momentum
components along the propagation direction. This helical, in addition to the
chiral, character of the spin waves is related to the spatial modulations of
the spin wave phase across the wire. It, however, remains a challenge to
generate and control such modes with conventional magnetic fields. Here, we
make the first proposal for a \textit{magnetic} spiral phase plate by
appropriately synthesizing two magnetic materials that have different speeds of
spin waves. It is demonstrated with full-numerical micromagnetic simulations
that despite the complicated structure of demagnetization fields, a homogeneous
spin wave passing through the spiral phase plate attains the required twist and
propagates further with the desired orbital angular momentum. While excitations
from the ordered phase may have a twist, the magnetization itself can be
twisted due to internal fields and forms what is known as a magnetic vortex. We
point out the differences between both types of magnetic phenomena and discuss
their possible interaction.",1909.04457v2
2019-09-10,Real-space imaging of confined magnetic skyrmion tubes,"Magnetic skyrmions are topologically nontrivial particles with a potential
application as information elements in future spintronic device architectures.
While they are commonly portrayed as two dimensional objects, in reality
magnetic skyrmions are thought to exist as elongated, tube-like objects
extending through the thickness of the sample. Study of this skyrmion tube
(SkT) state is highly relevant for investigating skyrmion metastability and for
implementation in recently proposed magnonic computing. However, direct
experimental imaging of skyrmion tubes has yet to be reported. Here, we
demonstrate the first real-space observation of skyrmion tubes in a lamella of
FeGe using resonant magnetic x-ray imaging and comparative micromagnetic
simulations, confirming their extended structure. The formation of these
structures at the edge of the sample highlights the importance of confinement
and edge effects in the stabilisation of the SkT state, opening the door to
further investigations into this unexplored dimension of the skyrmion spin
texture.",1909.04528v2
2019-09-11,Giant pressure-enhancement of multiferroicity in CuBr2,"Type-II multiferroic materials, in which ferroelectric polarization is
induced by inversion non-symmetric magnetic order, promise new and highly
efficient multifunctional applications based on the mutual control of magnetic
and electric properties. Although this phenomenon has to date been limited to
low temperatures, here we report a giant pressure-dependence of the
multiferroic critical temperature in CuBr$_2$. At 4.5 GPa, $T_\mathrm{C}$ is
enhanced from 73.5 to 162 K, to our knowledge the highest value yet reported
for a non-oxide type-II multiferroic. This growth shows no sign of saturating
and the dielectric loss remains small under these high pressures. We establish
the structure under pressure and demonstrate a 60\% increase in the two-magnon
Raman energy scale up to 3.6 GPa. First-principles structural and magnetic
energy calculations provide a quantitative explanation in terms of dramatically
pressure-enhanced interactions between CuBr$_2$ chains. These large,
pressure-tuned magnetic interactions motivate structural control in cuprous
halides as a route to applied high-temperature multiferroicity.",1909.04989v2
2019-09-24,Unconventional Hund Metal in a Weak Itinerant Ferromagnet,"The physics of weak itinerant ferromagnets is challenging due to their small
magnetic moments and the ambiguous role of local interactions governing their
electronic properties, many of which violate Fermi liquid theory. While
magnetic fluctuations play an important role in the materials' unusual
electronic states, the nature of these fluctuations and the paradigms through
which they arise remain debated. Here we use inelastic neutron scattering to
study magnetic fluctuations in the canonical weak itinerant ferromagnet MnSi.
Data reveal that short-wavelength magnons continue to propagate until a mode
crossing predicted for strongly interacting quasiparticles is reached, and the
local susceptibility peaks at a coherence energy predicted for a correlated
Hund metal by first-principles many-body theory. Scattering between electrons
and orbital and spin fluctuations in MnSi can be understood at the local level
to generate non-Fermi liquid character. These results provide crucial insight
into the role of interorbital Hund's exchange within the broader class of
enigmatic multiband itinerant, weak ferromagnets.",1909.11195v2
2020-12-07,Double-Exchange Enhanced Magnetic Blue-Shift of Mott Gaps,"A substantial energy gap of charge excitations induced by strong correlations
is the characteristic feature of Mott insulators. We study how the Mott gap is
affected by long-range antiferromagnetic order. Our key finding is that the
Mott gap is increased by the magnetic ordering: a magnetic blue-shift (MBS)
occurs. Thus, the effect is proportional to the exchange coupling in the
leading order in the Hubbard model. In systems with additional localized spins
the double-exchange mechanism induces an additional contribution to the MBS
which is proportional to the hopping in the leading order. The coupling between
spin and charge degrees of freedom bears the potential to enable spin-to-charge
conversion in Mott systems on extreme time scales determined by hopping and
exchange only, since a spin-orbit mediated transfer of angular momentum is not
involved in the process. In view of spintronic and magnonic applications, it is
highly promising to observe that several entire classes of compounds show
exchange and double-exchange effects. Exemplarily, we show that the magnetic
contribution to the band-gap blue-shift observed in the optical conductivity of
$\alpha$-MnTe is correctly interpreted as the MBS of a Mott gap.",2012.04018v2
2020-12-21,Development of short and long-range magnetic order in the double perovskite based frustrated triangular lattice antiferromagnet Ba2MnTeO6,"Oxide double perovskites wherein octahedra formed by both 3d elements and
sp-based heavy elements give rise to unconventional magnetic ordering and
correlated quantum phenomena crucial for futuristic applications. Here, by
carrying out experimental and first principles investigations, we present the
electronic structure and magnetic phases of Ba2MnTeO6, where Mn^2+ ions with S
= 5/2 spins constitute a perfect triangular lattice. The magnetic
susceptibility reveals a large Curie- Weiss temperature -152 K suggesting the
presence of strong antiferromagnetic interactions between Mn^2+ moments in the
spin lattice. A phase transition at 20 K is revealed by magnetic susceptibility
and specific heat which is attributed to the presence of a sizeable inter-plane
interactions. Below the transition temperature, the specific heat data show
antiferromagnetic magnon excitations with a gap of 1.4 K. Furthermore, muon
spin-relaxation reveals the presence of static internal fields in the ordered
state and provides strong evidence of short-range spin correlations for T > TN.
The DFT+U calculations and spin-dimer analysis infer that Heisenberg
interactions govern the inter and intra-layer spin-frustrations in this
perovskite. The inter and intra-layer exchange interactions are of comparable
strengths (J1 = 4.6 K, J2 = 0.92 J1). However, a weak third nearest-neighbor
ferromagnetic inter-layer interaction exists (J3=-0.04 J1) due to
double-exchange interaction via the linear path Mn-O-Te-O-Mn. The combined
effect of J2 and J3 interactions stabilizes a three dimensional long-range
magnetic ordering in this frustrated magnet.",2012.11292v1
2020-12-23,Monolayer CrCl$_3$ as an ideal Test Bed for the Universality Classes of 2D Magnetism,"The monolayer halides CrX$_3$ (X=Cl, Br, I) attract significant attention for
realizing 2D magnets with genuine long-range order (LRO), challenging the
Mermin-Wagner theorem. Here, we show that monolayer CrCl$_3$ has the unique
benefit of exhibiting tunable magnetic anisotropy upon applying a compressive
strain. This opens the possibility to use CrCl$_3$ for producing and studying
both ferromagnetic and antiferromagnetic 2D Ising-type LRO as well as the
Berezinskii-Kosterlitz-Thouless (BKT) regime of 2D magnetism with quasi-LRO.
Using state-of-the-art density functional theory, we explain how realistic
compressive strain could be used to tune the monolayer's magnetic properties so
that it could exhibit any of these phases. Building on large-scale quantum
Monte Carlo simulations, we compute the phase diagram of strained CrCl$_3$, as
well as the magnon spectrum with spin-wave theory. Our results highlight the
eminent suitability of monolayer CrCl$_3$ to achieve very high BKT transition
temperatures, around 50 K, due to their singular dependence on the weak
easy-plane anisotropy of the material.",2012.12801v2
2000-06-22,"Magnetic ordering and fluctuation in kagome lattice antiferromagnets, Fe and Cr jarosites","Jarosite family compounds, KFe_3(OH)_6(SO_4)_2, (abbreviate Fe jarosite), and
KCr_3(OH)_6(SO_4)_2, (Cr jarosite), are typical examples of the Heisenberg
antiferromagnet on the kagome lattice and have been investigated by means of
magnetization and NMR experiments. The susceptibility of Cr jarosite deviates
from Curie-Weiss law due to the short-range spin correlation below about 150 K
and shows the magnetic transition at 4.2 K, while Fe jarosite has the
transition at 65 K. The susceptibility data fit well with the calculated one on
the high temperature expansion for the Heisenberg antiferromagnet on the kagome
lattice. The values of exchange interaction of Cr jarosite and Fe jarosite are
derived to be J_Cr = 4.9 K and J_Fe = 23 K, respectively. The 1H-NMR spectra of
Fe jarosite suggest that the ordered spin structure is the q = 0 type with
positive chirality of the 120 degrees configuration. The transition is caused
by a weak single-ion type anisotropy. The spin-lattice relaxation rate, 1/T_1,
of Fe jarosite in the ordered phase decreases sharply with lowering the
temperature and can be well explained by the two-magnon process of spin wave
with the anisotropy.",0006343v1
2017-05-03,Linearly polarized GHz magnetization dynamics of spin helix modes in the ferrimagnetic insulator Cu$_{2}$OSeO$_{3}$,"Linear dichroism -- the polarization dependent absorption of electromagnetic
waves -- is routinely exploited in applications as diverse as structure
determination of DNA or polarization filters in optical technologies. Here
filamentary absorbers with a large length-to-width ratio are a prerequisite.
For magnetization dynamics in the few GHz frequency regime strictly linear
dichroism was not observed for more than eight decades. Here, we show that the
bulk chiral magnet Cu$_{2}$OSeO$_{3}$ exhibits linearly polarized magnetization
dynamics at an unexpectedly small frequency of about 2 GHz. Unlike optical
filters that are assembled from filamentary absorbers, the magnet provides
linear polarization as a bulk material for an extremely wide range of
length-to-width ratios. In addition, the polarization plane of a given mode can
be switched by 90$^\circ$ via a tiny variation in width. Our findings shed a
new light on magnetization dynamics in that ferrimagnetic ordering combined
with anisotropic exchange interaction offers strictly linear polarization and
cross-polarized modes for a broad spectrum of sample shapes. The discovery
allows for novel design rules and optimization of microwave-to-magnon
transduction in emerging microwave technologies.",1705.01582v1
2017-05-12,Switching dynamics of the spin density wave in superconducting CeCoIn5,"The ordering wave vector $\mathbf{Q}$ of a spin density wave (SDW),
stabilized within the superconducting state of $\mathrm{CeCoIn_5}$ in a high
magnetic field, has been shown to be hypersensitive to the direction of the
field. $\mathbf{Q}$ can be switched from a nodal direction of the $d$-wave
superconducting order parameter to a perpendicular node by rotating the
in-plane magnetic field through the antinodal direction within a fraction of a
degree. Here, we address the dynamics of the switching of $\mathbf{Q}$. We use
a free energy functional based on the magnetization density, which describes
the condensation of magnetic fluctuations of nodal quasiparticles, and show
that the switching process includes closing of the SDW gap at one $\mathbf{Q}$
and then reopening the SDW gap at another $\mathbf{Q}$ perpendicular to the
first one. The magnetic field couples to $\mathbf{Q}$ through the spin-orbit
interaction. Our calculations show that the width of the hysteretic region of
switching depends linearly on the deviation of magnetic field from the critical
field associated with the SDW transition, consistent with our thermal
conductivity measurements. The agreement between theory and experiment supports
our scenario of the hypersensitivity of the $Q$ phase on the direction of
magnetic field, as well as the magnon condensation as the origin of the SDW
phase in $\mathrm{CeCoIn_5}$.",1705.04655v1
2017-05-19,Impurity-induced magnetic droplet in unconventional superconductors near magnetic instability: Application to Nd doped $\mathbf{CeCoIn_5}$,"Beside the spin density wave (SDW) order inside the superconducting phase in
$\mathrm{CeCoIn_5}$ at high magnetic fields, recent neutron scattering
measurements have found Bragg peaks in $5\%$ Nd doped $\mathrm{CeCoIn_5}$ at
low fields. The intensity of Bragg peaks in low fields is suppressed by
increasing field. Based on the phenomenological and microscopic modeling, we
show that for the Pauli limited $d$-wave superconductors in the vicinity of SDW
instability relevant for $\mathrm{CeCoIn_5}$, magnetic impurities locally
induce droplets of SDW order. Because of the strong anisotropy in the momentum
space in the spin fluctuations guaranteed by the $d$-wave pairing symmetry,
sharp peaks in spin structure factor at $\mathbf{Q}$s are produced by the
impurities, even when the droplets of SDW do not order. At zero field, the Nd
impurity spins are along the $c$ axis due to the coupling to the conduction
electrons with an easy $c$ axis, besides their own crystal field effect. The
in-plane magnetic field cants the impurity moments toward the $ab$ plane, which
suppress the droplets of SDW order. At high fields, the long-range SDW inside
the superconducting phase is stabilized as a consequence of magnon
condensation. Our results are consistent with the recent neutron scattering and
thermal conductivity measurements.",1705.07174v1
2017-09-03,Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy,"Understanding the transfer of spin angular momentum is essential in modern
magnetism research. A model case is the generation of magnons in magnetic
insulators by heating an adjacent metal film. Here, we reveal the initial steps
of this spin Seebeck effect with <27fs time resolution using terahertz
spectroscopy on bilayers of ferrimagnetic yttrium-iron garnet and platinum.
Upon exciting the metal with an infrared laser pulse, a spin Seebeck current
$j_\textrm{s}$ arises on the same ~100fs time scale on which the metal
electrons thermalize. This observation highlights that efficient spin transfer
critically relies on carrier multiplication and is driven by conduction
electrons scattering off the metal-insulator interface. Analytical modeling
shows that the electrons' dynamics are almost instantaneously imprinted onto
$j_\textrm{s}$ because their spins have a correlation time of only ~4fs and
deflect the ferrimagnetic moments without inertia. Applications in material
characterization, interface probing, spin-noise spectroscopy and terahertz spin
pumping emerge.",1709.00768v5
2017-08-29,Novel magnetoelectric effects via penta-linear interactions,"Magnetoelectric multiferroic materials, particularly with the perovskite
structure, are receiving a lot of attention because of their inherent coupling
between electrical polarization and magnetic ordering. However, very few types
of direct coupling between polarization and magnetization are known, and it is
unclear whether they can be useful to the design of novel spintronic devices
exploiting the control of magnetization by electric fields. For instance, the
typical bi-quadratic coupling only allows to change the magnitude of the
magnetization by an electric field, but it does not permit an
electric-field-induced switching of the magnetization. Similarly, the so-called
Lifshitz invariants allow an electric-field control of complicated magnetic
orderings, but not of the magnetization. Here, we report the discovery of novel
direct couplings between polarization and magnetization in epitaxial perovskite
films, via the use of first-principles methods and the development of an
original Landau-type phenomenological theory. Our results feature penta-linear
interactions involving the ferromagnetic and anti-ferromagnetic vectors as well
as the polar distortions and oxygen octahedral tilting, and permit a number of
striking effects. Examples include a continuous electric-field control of the
magnetization magnitude and sign, and the discrete switching of the
magnetization magnitude. Thus, the high-order, penta-linear couplings
demonstrated in this work may open new paths towards novel magneto-electric
effects, as well as, spintronic and magnonic devices.",1709.01789v1
2017-09-08,Quartz-based flat-crystal resonant inelastic x-ray scattering spectrometer with sub-10 meV energy resolution,"Continued improvement of the energy resolution of resonant inelastic x-ray
scattering (RIXS) spectrometers is crucial for fulfilling the potential of this
technique in the study of electron dynamics in materials of fundamental and
technological importance. In particular, RIXS is the only alternative tool to
inelastic neutron scattering capable of providing fully momentum resolved
information on dynamic spin structures of magnetic materials, but is limited to
systems whose magnetic excitation energy scales are comparable to the energy
resolution. The state-of-the-art spherical diced crystal analyzer optics
provides energy resolution as good as 25 meV but has already reached its
theoretical limit. Here, we demonstrate a novel sub-10meV RIXS spectrometer
based on flat-crystal optics at the Ir-L$_3$ absorption edge (11.215$\sim$ keV)
that achieves an analyzer energy resolution of 3.9$\sim$meV, very close to the
theoretical value of 3.7$\sim$meV. In addition, the new spectrometer allows
efficient polarization analysis without loss of energy resolution. The
performance of the instrument is demonstrated using longitudinal acoustical and
optical phonons in diamond, and magnon in Sr$_3$Ir$_2$O$_7$. The novel
sub-10$\sim$meV RIXS spectrometer thus provides a window into magnetic
materials with small energy scales.",1709.02777v1
2018-03-15,Diffuson contribution to anomalous Hall effect in disordered Co2FeSi thin films,"A wide variation in the disorder strength, as inferred from an order of
magnitude variation in the longitudinal resistivity of Co2FeSi (CFS) Huesler
alloy thin films of fixed (50 nm) thickness, has been achieved by growing these
films on Si(111) substrates at substrate temperatures ranging from room
temperature (RT) to 600 C. An in-depth study of the influence of disorder on
anomalous Hall resistivity,longitudinal resistivity(LR) and magnetoresistance,
enabled by this approach, reveals the following. The side-jump mechanism gives
a dominant contribution to anomalous Hall resistivity (AHR) in the CFS thin
films, regardless of the degree of disorder present. A new and novel
contribution to both LR and AHR characterized by the logarithmic temperature
dependence at temperatures below the minimum, exclusive to the amorphous CFS
films, originates from the scattering of conduction electrons from the
diffusive hydrodynamic modes associated with the longitudinal component of
magnetization, called diffusons. In these amorphous CFS films, the
electron-diffuson, e d, scattering and weak localization (WL) mechanisms
compete with that arising from the inelastic electron magnon, e m, scattering
to produce the minimum in longitudinal resistivity, whereas the minimum in AHR
is caused by the competing contributions from the e d and e m scattering, as WL
does not make any contribution to AHR. In sharp contrast, in crystalline films,
enhanced electron electron Coulomb interaction (EEI), which is basically
responsible for the resistivity minimum, makes no contribution to AHR with the
result that AHR does not exhibit a minimum.",1803.05590v1
2018-06-25,"Boundary twists, instabilities, and creation of skyrmions and antiskyrmions","We formulate and study the general boundary conditions dictating the
magnetization profile in the vicinity of an interface between magnets with
dissimilar properties. Boundary twists in the vicinity of an edge due to
Dzyaloshinskii-Moriya interactions have been first discussed in [Wilson et al.,
Phys. Rev. B 88, 214420 (2013)] and in [Rohart and Thiaville, Phys. Rev. B 88,
184422 (2013)]. We show that in general case the boundary conditions lead to
the magnetization profile corresponding to the N\'eel, Bloch, or intermediate
twist. We explore how such twists can be utilized for creation of skyrmions and
antiskyrmions, e.g., in a view of magnetic memory applications. To this end, we
study various scenarios how skyrmions and antiskyrmions can be created from
interface magnetization twists due to local instabilities. We also show that a
judicious choice of Dzyaloshinskii-Moriya tensor (hence a carefully designed
material) can lead to local instabilities generating certain types of skyrmions
or antiskyrmions. The local instabilities are shown to appear in solutions of
the Bogoliubov-de-Gennes equations describing ellipticity of magnon modes bound
to interfaces. In one considered scenario, a skyrmion-antiskyrmion pair can be
created due to instabilities at an interface between materials with properly
engineered Dzyaloshinskii-Moriya interactions. We use micromagnetics
simulations to confirm our analytical predictions.",1806.09546v3
2018-07-02,A tunable time-resolved spontaneous Raman spectroscopy setup for probing ultrafast collective excitation and quasiparticle dynamics in quantum materials,"We present a flexible and efficient ultrafast time-resolved spontaneous Raman
spectroscopy setup to study collective excitation and quasi-particle dynamics
in quantum matter. The setup has a broad energy tuning range extending from the
visible to near infrared spectral regions for both the pump excitation and
Raman probe pulses. Additionally, the balance between energy and
time-resolution can be controlled. A high light collecting efficiency is
realized by high numerical aperture collection optics and a high-throughput
flexible spectrometer. We demonstrate the functionality of the setup with a
study of the zone-center longitudinal optical phonon and hole continuum
dynamics in silicon, and discuss the role of the Raman tensor in time-resolved
Raman scattering. In addition, we show evidence for unequal phonon softening
rates at different high symmetry points in the Brillouin zone of silicon by
means of detecting pump-induced changes in the two-phonon overtone spectrum.
Demagnetization dynamics in the helimagnet Cu2OSeO3 is studied by observing
softening and broadening of a magnon after photo-excitation, underlining the
unique power of measuring transient dynamics in the frequency domain, and the
feasibility to study phase transitions in quantum matter.",1807.00525v1
2018-07-20,Ising versus Potts criticality in a low-temperature magnetothermodynamics of a frustrated spin-1/2 Heisenberg triangular bilayer,"Low-temperature magnetization curves and thermodynamics of a frustrated
spin-1/2 Heisenberg triangular bilayer with the antiferromagnetic intradimer
interaction and either ferromagnetic or antiferromagnetic interdimer
interaction are investigated in a highly frustrated parameter region, where
localized many-magnon eigenstates provide the most dominant contribution to
magnetothermodynamics. Low-energy states of the highly frustrated spin-1/2
Heisenberg triangular bilayer can be accordingly found from a mapping
correspondence with an effective triangular-lattice spin-1/2 Ising model in a
field. A description based on the effective Ising model implies that the
frustrated Heisenberg triangular bilayer with the ferromagnetic interdimer
coupling displays in a zero-temperature magnetization curve discontinuous
magnetization jump, which is reduced upon increasing of temperature until a
continuous field-driven phase transition from the Ising universality class is
reached at a certain critical temperature. The frustrated Heisenberg triangular
bilayer with the antiferromagnetic interdimer coupling contrarily exhibits
multistep magnetization curve with intermediate plateaus at one-third and
two-thirds of the saturation magnetization, whereas discontinuous magnetization
jumps observable at zero temperature change to continuous field-driven phase
transitions from the universality class of three-state Potts model at
sufficiently low temperatures. Exact results and Monte Carlo simulations of the
effective Ising model are confronted with full exact diagonalization data for
the Heisenberg triangular bilayer in order to corroborate these findings.",1807.08042v2
2018-07-26,"Quantum Magnetism, Spin Waves, and Light","Both magnetic materials and light have always played a predominant role in
information technologies, and continue to do so as we move into the realm of
quantum technologies. In this course we review the basics of magnetism and
quantum mechanics, before going into more advanced subjects. Magnetism is
intrinsically quantum mechanical in nature, and magnetic ordering can only be
explained by use of quantum theory. We will go over the interactions and the
resulting Hamiltonian that governs magnetic phenomena, and discuss its
elementary excitations, denominated magnons. After that we will study
magneto-optical effects and derive the classical Faraday effect. We will then
move on to the quantization of the electric field and the basics of optical
cavities. This will allow us to understand a topic of current research
denominated Cavity Optomagnonics. These notes were written as the accompanying
material to the course I taught in the Summer Semester 2018 at the
Friedrich-Alexander University in Erlangen. The course is intended for Master
or advanced Bachelor students. Basic knowledge of quantum mechanics,
electromagnetism, and solid state at the Bachelor level is assumed. Each
section is followed by a couple of simple exercises which should serve as to
""fill in the blanks"" of what has been derived, plus specific references to
bibliography, and a couple of check-points for the main concepts developed. The
figures are pictures of the blackboard taken during the lecture.",1807.10626v2
2018-12-06,Interplay between non-Hermiticity and non-Abelian gauge potential in topological photonics,"Topological phases in spinless non-Hermitian models have been widely studied
both theoretically and experimentally in some artificial materials using
photonics, phononics and magnon. In this work, we investigate the interplay
between non-Hermitian loss and gain and non-Abelian gauge potential realized in
a two-component superconducting circuit. In our model, the non-Hermiticity
along only gives rise to trivial gain and loss to the states; while the
non-Abelian gauge along gives rise to flying butterfly spectra and associated
edge modes, which in photonics can be directly measured by the intensity of
photons at the boundaries. These two terms do not commute, and their interplay
can give rise to several intriguing non-Hermitian phases, including the fully
gapped quantum spin Hall (QSH) phase, gapless QSH phase, trivial gapped phase
and gapless metallic phase. The bulk-edge correspondence is absent and we find
that during the closing of energy gap in the gapped QSH phase, the system
enters the gapless QSH phase regime which still supports two
counter-propagating edge modes. We have also unveiled the intriguing role of
non-Hermiticity on the chiral symmetry and time-reversal symmetry of the
Hermitian models, which can be applied to other physical models.",1812.02610v2
2018-12-13,EDRIXS: An open source toolkit for simulating spectra of resonant inelastic x-ray scattering,"Resonant inelastic x-ray scattering (RIXS) has become a very powerful
experimental technique to probe a broad range of intrinsic elementary
excitations, for example, from low energy phonons and (bi-)magnons to high
energy $d$-$d$, charge-transfer and plasmons excitations in strongly correlated
electronic systems. Due to the complexity of the RIXS cross-section and strong
core-hole effects, theoretical simulation of the experimental RIXS spectra is
still a difficult task which hampers the understanding of RIXS spectra and the
development of the RIXS technique. In this paper, we present an open source
toolkit (dubbed EDRIXS) to facilitate the simulations of RIXS spectra of
strongly correlated materials based on exact diagonalization (ED) of certain
model Hamiltonians. The model Hamiltonian can be from a single atom, small
cluster or Anderson impurity model, with model parameters from density
functional theory plus Wannier90 or dynamical mean-field theory calculations.
The spectra of x-ray absorption spectroscopy (XAS) and RIXS are then calculated
using Krylov subspace techniques. This toolkit contains highly efficient ED,
XAS and RIXS solvers written in modern Fortran 90 language and a convenient
Python library used to prepare inputs and set up calculations. We first give a
short introduction to RIXS spectroscopy, and then we discuss the implementation
details of this toolkit. Finally, we show three examples to demonstrate its
usage.",1812.05735v2
2018-12-18,Thermal gradient driven domain wall dynamics,"The issue of whether a thermal gradient acts like a magnetic field or an
electric current in the domain wall (DW) dynamics is investigated. Broadly
speaking, magnetization control knobs can be classified as energy-driving or
angular-momentum driving forces. DW propagation driven by a static magnetic
field is the best-known example of the former in which the DW speed is
proportional to the energy dissipation rate, and the current-driven DW motion
is an example of the latter. Here we show that DW propagation speed driven by a
thermal gradient can be fully explained as the angular momentum transfer
between thermally generated spin current and DW. We found DW-plane rotation
speed increases as DW width decreases. Both DW propagation speed along the wire
and DW-plane rotation speed around the wire decrease with the Gilbert damping.
These facts are consistent with the angular momentum transfer mechanism, but
are distinct from the energy dissipation mechanism. We further show that
magnonic spin-transfer torque (STT) generated by a thermal gradient has both
damping-like and field-like components. By analyzing DW propagation speed and
DW-plane rotation speed, the coefficient ( \b{eta}) of the field-like STT
arising from the non-adiabatic process, is obtained. It is found that \b{eta}
does not depend on the thermal gradient; increases with uniaxial anisotropy
K_(||) (thinner DW); and decreases with the damping, in agreement with the
physical picture that a larger damping or a thicker DW leads to a better
alignment between the spin-current polarization and the local magnetization, or
a better adiabaticity.",1812.07244v2
2019-01-03,Colossal electromagnon excitation in the non-cycloidal phase of TbMnO3 under pressure,"The magnetoelectric coupling, i.e., cross-correlation between electric and
magnetic orders, is a very desirable property to combine functionalities of
materials for next-generation switchable devices. Multiferroics with
spin-driven ferroelectricity presents such a mutual interaction concomitant
with magneto- and electro-active excitations called electromagnons. TbMnO3 is a
paradigmatic material in which two electromagnons have been observed in the
cycloidal magnetic phase. However, their observation in TbMnO3 is restricted to
the cycloidal spin phase and magnetic ground states that can support the
electromagnon excitation are still under debate. Here, we show by performing
Raman spectroscopy measurements under pressure that the lower-energy
electromagnon (4 meV) disappears when the ground state enters from a cycloidal
phase to an antiferromagnetic phase (E-type). On the contrary, the
magnetoelectric activity of the higher-energy electromagnon (8 meV) increases
in intensity by one order of magnitude. Using microscopic model calculations,
we demonstrate that the lowerenergy electromagnon, observed in the cycloidal
phase, originates from a higher harmonic of the magnetic cycloid, and we
determine that the symmetric exchange-striction mechanism is at the origin of
the higher-energy electromagnon which survives even in the E-type phase. The
colossal enhancement of the electromagnon activity in TbMnO3 paves the way to
use multiferroics more efficiently for generation, conversion and control of
spin waves in magnonic devices.",1901.00919v1
2019-01-09,"The magnetic phase diagram of the frustrated spin chain compound linarite, PbCuSO$_4$(OH)$_2$, as seen by neutron diffraction and $^1$H-NMR","We report on a detailed neutron diffraction and $^1$H-NMR study on the
frustrated spin-1/2 chain material linarite, PbCuSO$_4$(OH)$_2$, where
competing ferromagnetic nearest neighbor and antiferromagnetic next-nearest
neighbor interactions lead to frustration. From the magnetic Bragg peak
intensity studied down to 60 mK, the magnetic moment per Cu atom is obtained
within the whole magnetic phase diagram for $H \parallel b$ axis. Further, we
establish the detailed configurations of the shift of the SDW propagation
vector in phase V with field and temperature. Finally, combining our neutron
diffraction results with those from a low-temperature/high-field NMR study we
find an even more complex phase diagram close to the quasi-saturation field
suggesting that bound two-magnon excitations are the lowest energy excitations
close to and in the quasi-saturation regime. Qualitatively and
semi-quantitatively, we relate such behavior to $XYZ$ exchange anisotropy and
contributions from the Dzyaloshinsky-Moriya interaction to affect the magnetic
properties of linarite.",1901.02842v1
2019-01-10,Giant thermal Hall conductivity from neutral excitations in the pseudogap phase of cuprates,"The nature of the pseudogap phase of cuprates remains a major puzzle.
Although there are indications that this phase breaks various symmetries, there
is no consensus on its fundamental nature. Although Fermi-surface, transport
and thermodynamic signatures of the pseudogap phase are reminiscent of a
transition into a phase with antiferromagnetic order, there is no evidence for
an associated long-range magnetic order. Here we report measurements of the
thermal Hall conductivity $\kappa_{\rm xy}$ in the normal state of four
different cuprates (Nd-LSCO, Eu-LSCO, LSCO, and Bi2201) and show that a large
negative $\kappa_{\rm xy}$ signal is a property of the pseudogap phase,
appearing with the onset of that phase at the critical doping $p^*$. Since it
is not due to charge carriers -- as it persists when the material becomes an
insulator, at low doping -- or magnons -- as it exists in the absence of
magnetic order -- or phonons -- since skew scattering is very weak, we
attribute this $\kappa_{\rm xy}$ signal to exotic neutral excitations,
presumably with spin chirality. The thermal Hall conductivity in the pseudogap
phase of cuprates is reminiscent of that found in insulators with spin-liquid
states. In the Mott insulator LCO, it attains the highest known magnitude of
any insulator.",1901.03104v1
2019-01-19,Resolving the nature of electronic excitations in resonant inelastic x-ray scattering,"The study of elementary bosonic excitations is essential toward a complete
description of quantum electronic solids. In this context, resonant inelastic
X-ray scattering (RIXS) has recently risen to becoming a versatile probe of
electronic excitations in strongly correlated electron systems. The nature of
the radiation-matter interaction endows RIXS with the ability to resolve the
charge, spin and orbital nature of individual excitations. However, this
capability has been only marginally explored to date. Here, we demonstrate a
systematic method for the extraction of the character of excitations as
imprinted in the azimuthal dependence of the RIXS signal. Using this novel
approach, we resolve the charge, spin, and orbital nature of elastic
scattering, (para-)magnon/bimagnon modes, and higher energy dd excitations in
magnetically-ordered and superconducting copper-oxide perovskites (Nd2CuO4 and
YBa2Cu3O6.75). Our method derives from a direct application of scattering
theory, enabling us to deconstruct the complex scattering tensor as a function
of energy loss. In particular, we use the characteristic tensorial nature of
each excitation to precisely and reliably disentangle the charge and spin
contributions to the low energy RIXS spectrum. This procedure enables to
separately track the evolution of spin and charge spectral distributions in
cuprates with doping. Our results demonstrate a new capability that can be
integrated into the RIXS toolset, and that promises to be widely applicable to
materials with intertwined spin, orbital, and charge excitations.",1901.06583v1
2019-01-20,Terahertz Strong-Field Physics without a Strong External Terahertz Field,"Traditionally, strong-field physics explores phenomena in matter (atoms,
molecules, and solids) driven by an extremely strong laser field
nonperturbatively. However, even in the complete absence of an external
electromagnetic field, strong-field phenomena can arise when matter strongly
couples with the zero-point field of the quantum vacuum state, i.e.,
fluctuating electromagnetic waves whose expectation value is zero. Some of the
most striking examples of this occur in a cavity setting, in which an ensemble
of two-level atoms resonantly interacts with a single photonic mode of vacuum
fields, producing vacuum Rabi splitting. In particular, the nature of the
matter-vacuum-field coupled system fundamentally changes when the coupling rate
(equal to one half of the vacuum Rabi splitting) becomes comparable to, or
larger than, the resonance frequency. In this so-called ultrastrong coupling
regime, a non-negligible number of photons exist in the ground state of the
coupled system. Furthermore, the coupling rate can be cooperatively enhanced
(via so-called Dicke cooperativity) when the matter is comprised of a large
number of identical two-level particles, and a quantum phase transition is
predicted to occur as the coupling rate reaches a critical value. Low-energy
electronic or magnetic transitions in many-body condensed matter systems with
large dipole moments are ideally suited for searching for these predicted
phenomena. Here, we discuss two condensed matter systems that have shown
cooperative ultrastrong interactions in the terahertz frequency range: a
Landau-quantized two-dimensional electron gas interacting with
high-quality-factor cavity photons, and an Er$^{3+}$ spin ensemble interacting
with Fe$^{3+}$ magnons in ErFeO$_3$.",1901.06749v1
2019-01-26,All optical detection of picosecond spin-wave dynamics in two-dimensional annular antidot lattice,"Novel magnetic structures with precisely controlled dimensions and shapes at
the nanoscale have potential applications in spin logic, spintronics and other
spin-based communication devices. We report the fabrication of two-dimensional
bi-structure magnonic crystal in the form of embedded nanodots in a periodic
Ni80Fe20 antidot lattice structure (annular antidot) by focused ion-beam
lithography. The spin-wave spectra of the annular antidot sample,studied for
the first time by a time-resolved magneto-optic Kerr effect microscopy show a
remarkable variation with bias field, which is important for the above device
applications. The optically induced spin-wave spectra show multiple modes in
the frequency range 14.7 GHz to 3.5 GHz due to collective interactions between
the dots and antidots as well as the annular elements within the whole array.
Numerical simulations qualitatively reproduce the experimental results, and
simulated mode profiles reveal the spatial distribution of the spin wave modes
and internal magnetic fields responsible for these observations. It is observed
that the internal field strength increases by about 200 Oe inside each dot
embedded within the hole of annular antidot lattice as compared to pure antidot
lattice and pure dot lattice. The stray field for the annular antidot lattice
is found to be significant (0.8kOe) as opposed to the negligible values of the
same for the pure dot lattice and pure antidot lattice. Our findings open up
new possibilities for development of novel artificial crystals.",1901.09152v1
2019-01-31,Fundamental Spin Interactions Underlying the Magnetic Anisotropy in the Kitaev Ferromagnet CrI$_3$,"We lay the foundation for determining the microscopic spin interactions in
two-dimensional (2D) ferromagnets by combining angle-dependent ferromagnetic
resonance (FMR) experiments on high quality CrI$_3$ single crystals with
theoretical modeling based on symmetries. We discover that the Kitaev
interaction is the strongest in this material with $K \sim -5.2$ meV, 25 times
larger than the Heisenberg exchange $J \sim -0.2$ meV, and responsible for
opening the $\sim$5 meV gap at the Dirac points in the spin-wave dispersion.
Furthermore, we find that the symmetric off-diagonal anisotropy $\Gamma \sim
-67.5$ $\mu$eV, though small, is crucial for opening a $\sim$0.3 meV gap in the
magnon spectrum at the zone center and stabilizing ferromagnetism in the 2D
limit. The high resolution of the FMR data further reveals a $\mu$eV-scale
quadrupolar contribution to the $S=3/2$ magnetism. Our identification of the
underlying exchange anisotropies opens paths toward 2D ferromagnets with higher
$T_\text{C}$ as well as magnetically frustrated quantum spin liquids based on
Kitaev physics.",1902.00077v2
2019-02-14,Polarization resolved Cu $L_3$-edge resonant inelastic x-ray scattering of orbital and spin excitations in NdBa$_{2}$Cu$_{3}$O$_{7-δ}$,"High resolution resonant inelastic x-ray scattering (RIXS) has proven
particularly effective in the determination of crystal field and spin
excitations in cuprates. Its strength lies in the large Cu $L_{3}$ resonance
and in the fact that the scattering cross section follows quite closely the
single-ion model predictions, both in the insulating parent compounds and in
the superconducting doped materials. However, the spectra become increasingly
broader with (hole) doping, hence resolving and assigning spectral features has
proven challenging even with the highest energy resolution experimentally
achievable. Here we have overcome this limitation by measuring the complete
polarization dependence of the RIXS spectra as function of momentum transfer
and doping in thin films of NdBa$_{2}$Cu$_{3}$O$_{7-\delta}$. Besides
confirming the previous assignment of $dd$ and spin excitations (magnon,
bimagnon) in the antiferromagnetic insulating parent compound, we unequivocally
single out the actual spin-flip contribution at all dopings. We also
demonstrate that the softening of $dd$ excitations is mainly attributed to the
shift of the $xy$ peak to lower energy loss. These results provide a definitive
assessment of the RIXS spectra of cuprates and demonstrate that RIXS
measurements with full polarization control are practically feasible and highly
informative.",1902.05471v1
2019-02-20,Spin-charge conversion in NiMnSb Heusler alloy films,"Half-metallic Heusler alloys are attracting considerable attention because of
their unique half-metallic band structures which exhibit high spin polarization
and yield huge magnetoresistance ratios. Besides serving as ferromagnetic
electrodes, Heusler alloys also have the potential to host spin-charge
conversion which has been recently demonstrated in other ferromagnetic metals.
Here, we report on the spin-charge conversion effect in the prototypical
Heusler alloy NiMnSb. Spin currents were injected from Y3Fe5O12 into NiMnSb
films by spin pumping, and then the spin currents were converted to charge
currents via spin-orbit interactions. Interestingly, an unusual charge signal
was observed with a sign change at low temperature, which can be manipulated by
film thickness and ordering structure. It is found that the spin-charge
conversion has two contributions. First, the interfacial contribution causes a
negative voltage signal, which is almost constant versus temperature. The
second contribution is temperature dependent because it is dominated by
minority states due to thermally excited magnons in the bulk part of the film.
This work provides a pathway for the manipulation of spin-charge conversion in
ferromagnetic metals by interface-bulk engineering for spintronic devices.",1902.07406v1
2019-02-21,Asymmetric Ferromagnetic Criticality in Pyrochlore Ferromagnet Lu$_2$V$_2$O$_7$,"Critical phenomenon at the phase transition reveals the universal and
long-distance properties of the criticality. We study the ferromagnetic
criticality of the pyrochlore magnet Lu$_2$V$_2$O$_7$ at the ferromagnetic
transition ${T_\text{c}\approx 70\, \text{K}}$ from the isotherms of
magnetization $M(H)$ via an iteration process and the Kouvel-Fisher method. The
critical exponents associated with the transition are determined as ${\beta =
0.32(1)}$, ${\gamma = 1.41(1)}$, and ${\delta = 5.38}$. The validity of these
critical exponents is further verified by scaling all the $M(H)$ data in the
vicinity of $T_\text{c}$ onto two universal curves in the plot of
$M/|\varepsilon|^\beta$ versus $H/|\varepsilon|^{\beta+\gamma}$, where
${\varepsilon = T/T_\text{c} -1}$. The obtained $\beta$ and $\gamma$ values
show asymmetric behaviors on the ${T < T_\text{c}}$ and the ${T > T_\text{c}}$
sides, and are consistent with the predicted values of 3D Ising and cubic
universality classes, respectively. This makes Lu$_2$V$_2$O$_7$ a rare example
in which the critical behaviors associated with a ferromagnetic transition
belong to different universality classes. We describe the observed criticality
from the Ginzburg-Landau theory with the quartic cubic anisotropy that
microscopically originates from the anti-symmetric Dzyaloshinskii-Moriya
interaction as revealed by recent magnon thermal Hall effect and theoretical
investigations.",1902.07924v1
2019-03-04,Evolution of interlayer and intralayer magnetism in three atomically thin chromium trihalides,"We conduct a comprehensive study of three different magnetic semiconductors,
CrI$_3$, CrBr$_3$, and CrCl$_3$, by incorporating both few- and bi-layer
samples in van der Waals tunnel junctions. We find that the interlayer magnetic
ordering, exchange gap, magnetic anisotropy, as well as magnon excitations
evolve systematically with changing halogen atom. By fitting to a spin wave
theory that accounts for nearest neighbor exchange interactions, we are able to
further determine a simple spin Hamiltonian describing all three systems. These
results extend the 2D magnetism platform to Ising, Heisenberg, and XY spin
classes in a single material family. Using magneto-optical measurements, we
additionally demonstrate that ferromagnetism can be stabilized down to
monolayer in more isotropic CrBr$_3$, with transition temperature still close
to that of the bulk.",1903.01409v2
2019-03-28,High spin-polarization in the low Curie temperature complex itinerant ferromagnet EuTi$_{1-x}$Nb$_x$O$_3$,"The physical systems with ferromagnetism and ""bad"" metallicity hosting
unusual transport properties are playgrounds of novel quantum phenomena.
Recently EuTi$_{1-x}$Nb$_x$O$_3$ emerged as a ferromagnetic system where
non-trivial temperature dependent transport properties are observed due to
coexistence and competition of various magnetic and non-magnetic scattering
processes. In the ferromagnetic state, the resistivity shows a $T^2$
temperature dependence possibly due to electron-magnon scattering and above the
Curie temperature $T_c$, the dependence changes to $T^{3/2}$ behaviour
indicating a correlation between transport and magnetic properties. In this
paper, we show that the transport spin-polarization in EuTi$_{1-x}$Nb$_x$O$_3$,
a low Curie temperature ferromagnet, is as high ($\sim 40\%$) as that in some
of the metallic ferromagnets with high Curie temperatures. In addition, owing
to the low Curie temperature of EuTi$_{1-x}$Nb$_x$O$_3$, the temperature ($T$)
dependence of $P_t$ could be measured systematically up to $T_c$ which revealed
a proportionate relationship with magnetization $M_s$ vs. $T$. This indicates
that such proportionality is far more universally valid than the ferromagnets
with ideal parabolic bands. Furthermore, our band structure calculations not
only helped understand the origin of such high spin polarization in
EuTi$_{1-x}$Nb$_x$O$_3$ but also provided a route to estimate the Hubbard $U$
parameter in complex metallic ferromagnets in general using experimental
inputs.",1903.12022v1
2019-04-10,Optical excitation of propagating magnetostatic waves in an epitaxial Galfenol film by an ultrafast magnetic anisotropy change,"Using a time-resolved optically-pumped scanning optical microscopy technique
we demonstrate the laser-driven excitation and propagation of spin waves in a
20-nm film of a ferromagnetic metallic alloy Galfenol epitaxially grown on a
GaAs substrate. In contrast to previous all-optical studies of spin waves we
employ laser-induced thermal changes of magnetocrystalline anisotropy as an
excitation mechanism. A tightly focused 70-fs laser pulse excites packets of
magnetostatic surface waves with a $e^{-1}$ propagation length of 3.4 $\mu$m,
which is comparable with that of permalloy. As a result, laser-driven
magnetostatic spin waves are clearly detectable at distances up to 10 $\mu$m,
which promotes epitaxial Galfenol films to the limited family of materials
suitable for magnonic devices. A pronounced in-plane magnetocrystalline
anisotropy of the Galfenol film offers an additional degree of freedom for
manipulating the spin waves' parameters. Reorientation of an in-plane external
magnetic field relative to the crystallographic axes of the sample tunes the
frequency, amplitude and propagation length of the excited waves.",1904.05171v2
2019-04-11,Spin dynamics of the antiferromagnetic Heisenberg model on a kagome bilayer,"We study the spin dynamics of classical Heisenberg antiferromagnet with
nearest neighbor interactions on a quasi-two-dimensional kagome bilayer. This
geometrically frustrated lattice consists of two kagome layers connected by a
triangular-lattice linking layer. By combining Monte Carlo with precessional
spin dynamics simulations, we compute the dynamical structure factor of the
classical spin liquid in kagome bilayer and investigate the thermal and
dilution effects. While the low frequency and long wavelength dynamics of the
cooperative paramagnetic phase is dominated by spin diffusion, weak magnon
excitations persist at higher energies, giving rise the half moon pattern in
the dynamical structure factor. In the presence of spin vacancies, the
dynamical properties of the diluted system can be understood within the two
population picture. The spin diffusion of the ""correlated"" spin clusters is
mainly driven by the zero-energy weather-van modes, giving rise to an
autocorrelation function that decays exponentially with time. On the other
hand, the diffusive dynamics of the quasi-free ""orphan"" spins leads to a
distinctive longer time power-law tail in the autocorrelation function. We
discuss the implications of our work for the glassy behaviors observed in the
archetypal frustrated magnet SrCr$_{9p}$Ga$_{12-9p}$O$_{19}$ (SCGO).",1904.05863v3
2019-04-15,Topological Hall effect at above room temperature in heterostructures composed of a magnetic insulator and a heavy metal,"Non-volatile memory and computing technology rely on efficient read and write
of ultra-tiny information carriers that do not wear out. Magnetic skyrmions are
emerging as a potential carrier since they are topologically robust nanoscale
spin textures that can be manipulated with ultralow current density. To date,
most of skyrmions are reported in metallic films, which suffer from additional
Ohmic loss and thus high energy dissipation. Therefore, skyrmions in magnetic
insulators are of technological importance for low-power information processing
applications due to their low damping and the absence of Ohmic loss. Moreover,
they attract fundamental interest in studying various magnon-skyrmion
interactions11. Skyrmions have been observed in one insulating material
Cu2OSeO3 at cryogenic temperatures, where they are stabilized by bulk
Dzyaloshinskii-Moriya interaction. Here, we report the observation of magnetic
skyrmions that survive above room temperature in magnetic insulator/heavy metal
heterostructures, i.e., thulium iron garnet/platinum. The presence of these
skyrmions results from the Dzyaloshinskii-Moriya interaction at the interface
and is identified by the emergent topological Hall effect. Through tuning the
magnetic anisotropy via varying temperature, we observe skyrmions in a large
window of external magnetic field and enhanced stability of skyrmions in the
easy-plane anisotropy regime. Our results will help create a new platform for
insulating skyrmion-based room temperature low dissipation spintronic
applications.",1904.07107v1
2019-04-15,Signatures of a Deconfined Phase Transition on the Shastry-Sutherland Lattice: Applications to Quantum Critical SrCu$_2$(BO$_3$)$_2$,"We study a possible deconfined quantum phase transition in a realistic model
of a two-dimensional Shastry-Sutherland quantum magnet, using both numerical
and field theoretic techniques. Using the infinite density matrix
renormalization group (iDMRG) method, we verify the existence of an
intermediate plaquette valence bond solid (pVBS) order, with two fold
degeneracy, between the dimer and N\'eel ordered phases. We argue that the
quantum phase transition between the N\'eel and pVBS orders may be described by
a deconfined quantum critical point (DQCP) with an emergent O(4) symmetry. By
analyzing the correlation length spectrum obtained from iDMRG, we provide
evidence for the DQCP and emergent O(4) symmetry in the lattice model. Such a
phase transition has been reported in the recent pressure tuned experiments in
the Shastry-Sutherland lattice material $\mathrm{SrCu_2 (BO_3)_2}$. The
non-symmorphic lattice structure of the Shastry-Sutherland compound leads to
extinction points in the scattering, where we predict sharp signatures of a
DQCP in both the phonon and magnon spectra associated with the spinon
continuum. The effect of weak interlayer couplings present in the three
dimensional material is also discussed. Our results should help guide the
experimental study of DQCP in quantum magnets.",1904.07266v3
2019-08-21,Half-metallic ferromagnetism and Ru-induced localization in quaternary Heusler alloy CoRuMnSi,"We report a combined theoretical and experimental investigation of
half-metallic ferromagnetism in equiatomic quaternary Heusler alloy CoRuMnSi.
Room temperature XRD analysis reveals that the alloy crystallizes in L21
disorder instead of pristine Y-type structure due to 50% swap disorder between
the tetrahedral sites, i.e., Co and Ru atoms. Magnetization measurements reveal
a net magnetization of 4 $\mu_B$ with Curie temperature of ~780 K. Resistivity
measurement reveals the presence of localization effect below 35 K while above
100 K, a linear dependence is observed. Resistivity behavior indicates the
absence of single magnon scattering, which indirectly supports the
half-metallic nature. The majority spin band near the Fermi level clearly
indicates the overlap of flat eg bands with sharply varying conduction bands
that are responsible for the localization. In-depth analysis of the projected
atomic d-orbital character of band structure reveals unusual bonding, giving
rise to the flat eg bands purely arising out of Ru ions. Co-Ru swap disorder
calculations indicate the robustness of half-metallic nature, even when the
structure changes from Y-type to L21-type, with no major change in the net
magnetization of the system. Thus, robust half-metallic nature, stable
structure, and high Curie temperature make this alloy quite a promising
candidate to be used as a source of highly spin-polarized currents in
spintronic applications.",1908.07804v1
2019-08-26,Amplitude and frequency sensing of microwave fields with a superconducting transmon qudit,"Experiments with superconducting circuits require careful calibration of the
applied pulses and fields over a large frequency range. This remains an ongoing
challenge as commercial semiconductor electronics are not able to probe signals
arriving at the chip due to its cryogenic environment. Here, we demonstrate how
the on-chip amplitude and frequency of a microwave signal can be inferred from
the ac Stark shifts of higher transmon levels. In our time-resolved
measurements we employ Ramsey fringes, allowing us to detect the amplitude of
the systems transfer function over a range of several hundreds of MHz with an
energy sensitivity on the order of $10^{-4}$. Combined with similar
measurements for the phase of the transfer function, our sensing method can
facilitate pulse correction for high fidelity quantum gates in superconducting
circuits. Additionally, the potential to characterize arbitrary microwave
fields promotes applications in related areas of research, such as quantum
optics or hybrid microwave systems including photonic, mechanical or magnonic
subsystems.",1908.09556v3
2019-08-30,"High-Field Quantum Disordered State in $α$-RuCl3: Spin Flips, Bound States, and a Multi-Particle Continuum","Layered $\alpha$-RuCl3 has been discussed as a proximate Kitaev spin liquid
compound. Raman and THz spectroscopy of magnetic excitations confirm that the
low-temperature antiferromagnetic ordered phase features a broad Raman
continuum, together with two magnon-like excitations at 2.7 and 3.6 meV,
respectively. The continuum strength is maximized as long-range order is
suppressed by an external magnetic field. The state above the field-induced
quantum phase transition around 7.5 T is characterized by a gapped
multi-particle continuum out of which a two-particle bound state emerges,
together with a well-defined single-particle excitation at lower energy. Exact
diagonalization calculations demonstrate that Kitaev and off-diagonal exchange
terms in the Fleury-Loudon operator are crucial for the occurrence of these
features in the Raman spectra. Our study firmly establishes the
partially-polarized quantum disordered character of the high-field phase.",1908.11617v1
2019-12-19,Dynamical properties of Néel and valence-bond phases in the $J_1-J_2$ model on the honeycomb lattice,"By using a variational Monte Carlo technique based upon Gutzwiller-projected
fermionic states, we investigate the dynamical structure factor of the
antiferromagnetic $S=1/2$ Heisenberg model on the honeycomb lattice, in
presence of first-neighbor ($J_1$) and second-neighbor ($J_2$) couplings, for
${J_2 < 0.5 J_1}$. The ground state of the system shows long-range
antiferromagnetic order for ${J_2/J_1 \lesssim 0.23}$, plaquette valence-bond
order for ${0.23 \lesssim J_2/J_1 \lesssim 0.36}$, and columnar dimer order for
${J_2/J_1 \gtrsim 0.36}$. Within the antiferromagnetic state, a well-defined
magnon mode is observed, whose dispersion is in relatively good agreement with
linear spin-wave approximation for $J_2=0$. When a nonzero second-neighbor
super-exchange is included, a roton-like mode develops around the $K$ point
(i.e., the corner of the Brillouin zone). This mode softens when $J_2/J_1$ is
increased and becomes gapless at the transition point, $J_2/J_1 \approx 0.23$.
Here, a broad continuum of states is clearly visible in the dynamical spectrum,
suggesting that nearly-deconfined spinon excitations could exist, at least at
relatively high energies. For larger values of $J_2/J_1$, valence-bond order is
detected and the spectrum of the system becomes clearly gapped, with a triplon
mode at low energies. This is particularly evident for the spectrum of the
dimer valence-bond phase, in which the triplon mode is rather well separated
from the continuum of excitations that appears at higher energies.",1912.09310v1
2015-08-12,Localization of Fe d-states in Ni-Fe-Cu alloys and implications for ultrafast demagnetization,"Ni$_{80}$Fe$_{20}$ (Py) and Py-Cu exhibit intriguing ultrafast
demagnetization behavior, where the Ni magnetic moment shows a delayed response
relative to the Fe [S. Mathias et al., PNAS {\bf 109}, 4792 (2012)]. To unravel
the mechanism responsible for this behavior, we have studied Py-Cu alloys for a
wide range of Cu concentrations using X-ray magnetic circular dichroism (XMCD).
The magnetic moments of Fe and Ni are found to respond very differently to Cu
alloying: Fe becomes a strong ferromagnet in Py, with the magnetic moment
largely unaffected by Cu alloying. In contrast, the Ni magnetic moment
decreases continuously with increasing Cu concentration. Our results are
corroborated by ab-initio calculations of the electronic structure, which we
discuss in the framework of virtual bound states (VBSs). For high Cu
concentrations, Ni exhibits VBSs below the Fermi level, which are likely
responsible for an increased orbital/spin magnetic ratio at high Cu
concentrations. Fe exhibits VBSs in the minority band, approximately 1 eV above
the Fermi level in pure Py, that move closer to the Fermi level upon Cu
alloying. A strong interaction between the VBSs and excited electrons above the
Fermi level enhances the formation of localized magnons at Fe sites, which
explains the different behavior between Fe and Ni during ultrafast
demagnetization.",1508.03015v1
2016-03-01,Magnetic field dependence of the neutron spin resonance in CeB6,"In zero magnetic field, the famous neutron spin resonance in the f-electron
superconductor CeCoIn5 is similar to the recently discovered exciton peak in
the non-superconducting CeB6. Magnetic field splits the resonance in CeCoIn5
into two components, indicating that it is a doublet. Here we employ inelastic
neutron scattering (INS) to scrutinize the field dependence of spin
fluctuations in CeB6. The exciton shows a markedly different behavior without
any field splitting. Instead, we observe a second field-induced magnon whose
energy increases with field. At the ferromagnetic zone center, however, we find
only a single mode with a non-monotonic field dependence. At low fields, it is
initially suppressed to zero together with the antiferromagnetic order
parameter, but then reappears at higher fields inside the hidden-order phase,
following the energy of an electron spin resonance (ESR). This is a unique
example of a ferromagnetic resonance in a heavy-fermion metal seen by both ESR
and INS consistently over a broad range of magnetic fields.",1603.00383v2
2016-10-17,"Structural, magnetic, electric, dielectric, and thermodynamic properties of multiferroic GeV4S8","The lacunar spinel GeV4S8 undergoes orbital and ferroelectric ordering at the
Jahn-Teller transition around 30 K and exhibits antiferromagnetic order below
about 14 K. In addition to this orbitally driven ferroelectricity, lacunar
spinels are an interesting material class, as the vanadium ions form V4
clusters representing stable molecular entities with a common electron
distribution and a well-defined level scheme of molecular states resulting in a
unique spin state per V4 molecule. Here we report detailed x-ray, magnetic
susceptibility, electrical resistivity, heat capacity, thermal expansion, and
dielectric results to characterize the structural, electric, dielectric,
magnetic, and thermodynamic properties of this interesting material, which also
exhibits strong electronic correlations. From the magnetic susceptibility, we
determine a negative Curie-Weiss temperature, indicative for antiferromagnetic
exchange and a paramagnetic moment close to a spin S = 1 of the V4 molecular
clusters. The low-temperature heat capacity provides experimental evidence for
gapped magnon excitations. From the entropy release, we conclude about strong
correlations between magnetic order and lattice distortions. In addition, the
observed anomalies at the phase transitions also indicate strong coupling
between structural and electronic degrees of freedom. Utilizing dielectric
spectroscopy, we find the onset of significant dispersion effects at the polar
Jahn-Teller transition. The dispersion becomes fully suppressed again with the
onset of spin order. In addition, the temperature dependencies of dielectric
constant and specific heat possibly indicate a sequential appearance of orbital
and polar order.",1610.05244v1
2016-10-28,Insulating nanomagnets driven by spin torque,"Magnetic insulators, such as yttrium iron garnet (Y$_3$Fe$_5$O$_{12}$), are
ideal materials for ultra-low power spintronics applications due to their low
energy dissipation and efficient spin current generation and transmission.
Recently, it has been realized that spin dynamics can be driven very
effectively in micrometer-sized Y$_3$Fe$_5$O$_{12}$/Pt heterostructures by
spin-Hall effects. We demonstrate here the excitation and detection of spin
dynamics in Y$_3$Fe$_5$O$_{12}$/Pt nanowires by spin-torque ferromagnetic
resonance. The nanowires defined via electron-beam lithography are fabricated
by conventional room temperature sputtering deposition on Gd$_3$Ga$_5$O$_{12 }$
substrates and lift-off. We observe field-like and anti-damping-like torques
acting on the magnetization precession, which are due to simultaneous
excitation by Oersted fields and spin-Hall torques. The Y$_3$Fe$_5$O$_{12}$/Pt
nanowires are thoroughly examined over a wide frequency and power range. We
observe a large change in the resonance field at high microwave powers, which
is attributed to a decreasing effective magnetization due to microwave
absorption. These heating effects are much more pronounced in the investigated
nanostructures than in comparable micron-sized samples. By comparing different
nanowire widths, the importance of geometrical confinements for magnetization
dynamics becomes evident: quantized spin-wave modes across the width of the
wires are observed in the spectra. Our results are the first stepping stones
toward the realization of integrated magnonic logic devices based on
insulators, where nanomagnets play an essential role.",1610.09360v1
2016-12-11,Phase Structure of XX0 Spin Chain and Nonintersecting Brownian Motion,"We study finite size and temperature XX0 Heisenberg spin chain in weak and
strong coupling regimes. By using an elegant connection of the model to
integrable combinatorics and probability, we explore and interpret a possible
phase structure of the model in asymptotic limit: the limit of large inverse
temperature and size. First, partition function and free energy of the model
are derived by using techniques and results from random matrix models and
nonintersecting Brownian motion. We show that, in the asymptotic limit,
partition function of the model, written in terms of matrix integral, is
governed by the Tracy-Widom distribution. Second, the exact analytic results
for the free energy, which is obtained by the asymptotic analysis of the
Tracy-Widom distribution, indicate a completely new and sophisticated phase
structure of the model. This phase structure consists of second- and
third-order phase transitions. Finally, to shed light on our new results, we
provide a possible interpretation of the phase structure in terms of dynamical
behavior of magnons in the spin chain. We demonstrate distinct features of the
phases with schematic spin configurations which have definite features in each
region of the phase diagram.",1612.03463v2
2016-12-18,Chiral limit of N = 4 SYM and ABJM and integrable Feynman graphs,"We consider a special double scaling limit, recently introduced by two of the
authors, combining weak coupling and large imaginary twist, for the
$\gamma$-twisted $\mathcal{N}=4$ SYM theory. We also establish the analogous
limit for ABJM theory. The resulting non-gauge chiral 4D and 3D theories of
interacting scalars and fermions are integrable in the planar limit. In spite
of the breakdown of conformality by double-trace interactions, most of the
correlators for local operators of these theories are conformal, with
non-trivial anomalous dimensions defined by specific, integrable Feynman
diagrams. We discuss the details of this diagrammatics. We construct the
doubly-scaled asymptotic Bethe ansatz (ABA) equations for multi-magnon states
in these theories. Each entry of the mixing matrix of local conformal operators
in the simplest of these theories - the bi-scalar model in 4D and tri-scalar
model in 3D - is given by a single Feynman diagram at any given loop order. The
related diagrams are in principle computable, up to a few scheme dependent
constants, by integrability methods (quantum spectral curve or ABA). These
constants should be fixed from direct computations of a few simplest graphs.
This integrability-based method is advocated to be able to provide information
about some high loop order graphs which are hardly computable by other known
methods. We exemplify our approach with specific five-loop graphs.",1612.05895v3
2016-12-20,Doping Dependence of Collective Spin and Orbital Excitations in Spin 1 Quantum Antiferromagnet La$_{2-x}$Sr$_x$NiO$_4$ Observed by X-rays,"We report the first empirical demonstration that resonant inelastic x-ray
scattering (RIXS) is sensitive to \emph{collective} magnetic excitations in
$S=1$ systems by probing the Ni $L_3$-edge of La$_{2-x}$Sr$_x$NiO$_4$ ($x = 0,
0.33, 0.45$). The magnetic excitation peak is asymmetric, indicating the
presence of single and multi spin-flip excitations. As the hole doping level is
increased, the zone boundary magnon energy is suppressed at a much larger rate
than that in hole doped cuprates. Based on the analysis of the orbital and
charge excitations observed by RIXS, we argue that this difference is related
to the orbital character of the doped holes in these two families. This work
establishes RIXS as a probe of fundamental magnetic interactions in nickelates
opening the way towards studies of heterostructures and ultra-fast pump-probe
experiments.",1612.06903v2
2016-12-22,Bismuth iron garnet Bi3Fe5O12: a room temperature magnetoelectric material,"The possibility to control the magnetic properties of a material with an
electric field at room temperature is highly desirable for modern applications.
Moreover, a coupling between magnetic and electric orders within a single
material presenting a wide range of exceptional physical properties, such as
bismuth iron garnet (BIG), may lead to great advances in the field of
spintronic applications. In particular, the combination of the magnetoelectric
(ME) coupling with the low damping of spin waves in BIG can allow the control
and manipulation of spin waves by an electric field in magnonic devices. Here
we report the unambiguous observation of linear magnetoelectric coupling above
300 K in BIG using ferromagnetic resonance technique with electric field
modulation. The measured coupling value is comparable with that observed for
prototypal magnetoelectric Cr2O3. On the basis of our experimental results, the
strength of this linear ME coupling is directly linked to the presence of
bismuth ions inducing strong spin orbit coupling and to the appearance of local
magnetic inhomogeneities related to the magnetic domain structure. The
unprecedented combination of magnetic, optical and magnetoelectrical properties
in BIG is expected to trigger significant interest for technological
applications as well as for theoretical studies.",1612.07531v3
2017-07-18,Crossover from itinerant to localized magnetic excitations through the metal-insulator transition in NaOsO$_{\text{3}}$,"NaOsO$_{\text{3}}$ undergoes a metal-insulator transition (MIT) at 410 K,
concomitant with the onset of antiferromagnetic order. The excitation spectra
have been investigated through the MIT by resonant inelastic x-ray scattering
(RIXS) at the Os L$_{\text{3}}$ edge. Low resolution ($\Delta E \sim$ 300 meV)
measurements over a wide range of energies reveal that local electronic
excitations do not change appreciably through the MIT. This is consistent with
a picture in which structural distortions do not drive the MIT. In contrast,
high resolution ($\Delta E \sim $ 56 meV) measurements show that the
well-defined, low energy magnons in the insulating state weaken and dampen upon
approaching the metallic state. Concomitantly, a broad continuum of excitations
develops which is well described by the magnetic fluctuations of a nearly
antiferromagnetic Fermi liquid. By revealing the continuous evolution of the
magnetic quasiparticle spectrum as it changes its character from itinerant to
localized, our results provide unprecedented insight into the nature of the MIT
in \naoso. In particular, the presence of weak correlations in the paramagnetic
phase implies a degree of departure from the ideal Slater limit.",1707.05551v2
2017-07-27,Magnetism in artificial Ruddlesden-Popper iridates leveraged by structural distortions,"We report on the tuning of magnetic interactions in superlattices composed of
single and bilayer SrIrO$_3$ inter-spaced with SrTiO$_3$. Magnetic scattering
shows predominately $c$-axis antiferromagnetic orientation of the magnetic
moments for the bilayer justifying these systems as viable artificial analogues
of the bulk Ruddlesden-Popper series iridates. Magnon gaps are observed in both
superlattices, with the magnitude of the gap in the bilayer being reduced to
nearly half that in its bulk structural analogue, Sr$_3$Ir$_2$O$_7$. We assign
this to modifications in the anisotropic exchange driven by bending of the
$c$-axis Ir-O-Ir bond and subsequent local environment changes, as detected by
x-ray diffraction and modeled using spin wave theory. These findings explain
how even subtle structural modulations driven by heterostructuring in iridates
are leveraged by spin orbit coupling to drive large changes in the magnetic
interactions.",1707.08910v1
2017-11-19,SU($N$) spin-wave theory: Application to spin-orbital Mott insulators,"We present the application of the SU($N$) ($N>2$) spin-wave theory to
spin-orbital Mott insulators whose ground states exhibit magnetic orders. When
taking both the spin and orbital degrees of freedom into account rather than
projecting onto the Kramers doublet, the lowest spin-orbital locking energy
levels, due to the inevitable spin-orbital multipole exchange interactions, the
SU($N$) spin-wave theory should take the place of the SU($2$) one. To implement
the application, we introduce an efficient general local mean field approach
which involves all the local fluctuations into the SU($N$) linear spin-wave
theory. Our approach is tested firstly by calculating the multipolar spin-wave
spectra of the SU($4$) antiferromagnetic model. Then we apply it to
spin-orbital Mott insulators. It is revealed that the Hund's coupling would
influence the effectiveness of the isospin-$1/2$ representation when the spin
orbital coupling is not large enough. Besides, we also calculate the spin-wave
spectra based on the first principle calculations for two concrete materials,
$\alpha$-RuCl$_3$ and Sr$_2$IrO$_4$. The SU($N$) spin-wave theory appropriately
depicts the low-energy magnons and the spin-orbital excitations qualitatively.",1711.07041v1
2017-11-28,Unconventional transport properties of an itinerant ferromagnet: EuTi$_{1-x}$Nb$_{x}$O$_3$ ($x$=0.10$-$0.20),"We report the temperature and magnetic field dependence of resistivity
($\rho$) for single-crystalline EuTi$_{1-x}$Nb$_{x}$O$_3$ ($x$=0.10$-$0.20), an
itinerant ferromagnetic system with very low Curie temperature ($T_C$). The
detailed analysis reveals that the charge conduction in
EuTi$_{1-x}$Nb$_{x}$O$_3$ is extremely sensitive to Nb concentration and
dominated by several scattering mechanisms. Well below the $T_C$, where the
spontaneous magnetization follows the Bloch's $T^{3/2}$ law, $\rho$ exhibits
$T^2$ dependence with a large coefficient $\sim$10$^{-8}$ $\Omega$ cm K$^{-2}$
due to the electron-magnon scattering. Remarkably, all the studied samples
exhibit a unique resistivity minimum at $T$$=$$T_{\rm min}$ below which $\rho$
shows logarithmic increment with $T$ (for $T_{\rm C}$$<$$T$$<$$T_{\rm min}$)
due to the Kondo scattering of Nb 4$d^1$ itinerant electrons by the localized
4$f$ moments of Eu$^{2+}$ ions which suppresses strongly with applied magnetic
field. In the paramagnetic state, $T^{2}$ and $T^{3/2}$ dependence of the
resistivity have been observed, suggesting an unusual crossover from a
Fermi-liquid to a non-Fermi-liquid behavior with increasing $T$. The observed
temperature and magnetic field dependence of resistivity has been analysed
using different theoretical models.",1711.10140v1
2019-05-10,Magnetic tunnel junctions with a B2-ordered CoFeCrAl equiatomic Heusler alloy,"The equiatomic quaternary Heusler alloy CoFeCrAl is a candidate material for
spin-gapless semiconductors (SGSs). However, to date, there have been no
experimental attempts at fabricating a junction device. This paper reports a
fully epitaxial (001)-oriented MgO barrier magnetic tunnel junction (MTJ) with
CoFeCrAl electrodes grown on a Cr buffer. X-ray and electron diffraction
measurements show that the (001) CoFeCrAl electrode films with atomically flat
surfaces have a $B2$-ordered phase. The saturation magnetization is 380
emu/cm$^3$, almost the same as the value given by the Slater--Pauling--like
rule, and the maximum tunnel magnetoresistance ratios at 300 K and 10 K are 87%
and 165%, respectively. Cross-sectional electron diffraction analysis shows
that the MTJs have MgO interfaces with fewer dislocations. The temperature- and
bias-voltage-dependence of the transport measurements indicates magnon-induced
inelastic electron tunneling overlapping with the coherent electron tunneling.
X-ray magnetic circular dichroism (XMCD) measurements show a ferromagnetic
arrangement of the Co and Fe magnetic moments of $B2$-ordered CoFeCrAl, in
contrast to the ferrimagnetic arrangement predicted for the $Y$-ordered state
possessing SGS characteristics. Ab-initio calculations taking account of the
Cr-Fe swap disorder qualitatively explain the XMCD results. Finally, the effect
of the Cr-Fe swap disorder on the ability for electronic states to allow
coherent electron tunneling is discussed.",1905.04070v1
2019-05-17,High-efficiency triple-resonant inelastic light scattering in planar optomagnonic cavities,"Optomagnonic cavities have recently been emerging as promising candidates for
implementing coherent microwave-to-optical conversion, quantum memories and
devices, and next generation quantum networks. A key challenge in the design of
such cavities is the attainment of high efficiencies, which could, e.g., be
exploited for efficient optical interfacing of superconducting qubits, as well
as the practicality of the final designs, which ideally should be planar and
amenable to on-chip integration. Here, on the basis of a novel time Floquet
scattering-matrix approach, we report on the design and optimization of a
planar, multilayer optomagnonic cavity, incorporating a Ce:YIG thin film,
magnetized in-plane, operating in the triple-resonant inelastic light
scattering regime. This architecture allows for conversion efficiencies of
about 5%, under realistic conditions, which is orders of magnitude higher than
alternative designs. Our results suggest a viable way forward for realizing
practical information inter-conversion between microwave photons and optical
photons, mediated by magnons, with efficiencies intrinsically greater than
those achieved in optomechanics and alternative related technologies, as well
as a platform for fundamental studies of classical and quantum dynamics in
magnetic solids, and implementation of futuristic quantum devices.",1905.07278v1
2019-05-23,Lattice frustration in spin-orbit Mott insulator Sr3Ir2O7 at high pressure,"The intertwined charge, spin, orbital, and lattice degrees of freedom could
endow 5d compounds with exotic properties. Current interest is focused on
electromagnetic interactions in these materials, whereas the important role of
lattice geometry remains to be fully recognized. For this sake, we investigate
pressure-induced phase transitions in the spin-orbit Mott insulator Sr3Ir2O7
with Raman, electrical resistance, and x-ray diffraction measurements. We
reveal an interesting magnetic transition coinciding with a structural
transition at 14.4 GPa, but without a concurrent insulator-metal transition.
The conventional correlation between magnetic and Mott insulating states is
thereby absent. The observed softening of the one-magnon mode can be explained
by a reduced tetragonal distortion, while the actual magnetic transition is
associated with tilting of the IrO6 octahedra. This work highlights the
critical role of lattice frustration in determining the high-pressure phases of
Sr3Ir2O7. The ability to control electromagnetic properties via manipulating
the crystal structure with pressure promises a new way to explore new quantum
states in spin-orbit Mott insulators.",1905.09637v1
2019-05-29,Thermal Tensor Network Simulations of the Heisenberg Model on the Bethe Lattice,"We have extended the canonical tree tensor network (TTN) method, which was
initially introduced to simulate the zero-temperature properties of quantum
lattice models on the Bethe lattice, to finite temperature simulations. By
representing the thermal density matrix with a canonicalized tree tensor
product operator, we optimize the TTN and accurately evaluate the thermodynamic
quantities, including the internal energy, specific heat, and the spontaneous
magnetization, etc, at various temperatures. By varying the anisotropic
coupling constant $\Delta$, we obtain the phase diagram of the spin-1/2
Heisenberg XXZ model on the Bethe lattice, where three kinds of magnetic
ordered phases, namely the ferromagnetic, XY and antiferromagnetic ordered
phases, are found in low temperatures and separated from the high-$T$
paramagnetic phase by a continuous thermal phase transition at $T_c$. The XY
phase is separated from the other two phases by two first-order phase
transition lines at the symmetric coupling points $ \Delta=\pm 1$. We have also
carried out a linear spin wave calculation on the Bethe lattice, showing that
the low-energy magnetic excitations are always gapped, and find the obtained
magnon gaps in very good agreement with those estimated from the TTN
simulations. Despite the gapped excitation spectrum, Goldstone-like transverse
fluctuation modes, as a manifestation of spontaneous continuous symmetry
breaking, are observed in the ordered magnetic phases with $|\Delta|\le 1$. One
remarkable feature there is that the prominent transverse correlation length
reaches $\xi_c=1/\ln{(z-1)}$ for $T\leq T_c$, the maximal value allowed on a
$z$-coordinated Bethe lattice.",1905.12478v1
2019-05-29,Coherent long-range transfer of angular momentum between magnon Kittel modes by phonons,"We report ferromagnetic resonance in the normal configuration of an
electrically insulating magnetic bilayer consisting of two yttrium iron garnet
(YIG) films epitaxially grown on both sides of a 0.5-mm-thick nonmagnetic
gadolinium gallium garnet (GGG) slab. An interference pattern is observed and
it is explained as the strong coupling of the magnetization dynamics of the two
YIG layers either in phase or out of phase by the standing transverse sound
waves, which are excited through a magnetoelastic interaction. This coherent
mediation of angular momentum by circularly polarized phonons through a
nonmagnetic material over macroscopic distances can be useful for future
information technologies.",1905.12523v3
2019-05-31,Fingerprints of Kitaev physics in the magnetic excitations of honeycomb iridates,"In the quest for realizations of quantum spin liquids, the exploration of
Kitaev materials - spin-orbit entangled Mott insulators with strong
bond-directional exchanges - has taken center stage. However, in these
materials the local spin-orbital j=1/2 moments typically show long-range
magnetic order at low temperature, thus defying the formation of a spin-liquid
ground state. Using resonant inelastic x-ray scattering (RIXS), we here report
on a proximate spin liquid regime with clear fingerprints of Kitaev physics in
the magnetic excitations of the honeycomb iridates alpha-Li2IrO3 and Na2IrO3.
We observe a broad continuum of magnetic excitations that persists up to at
least 300K, more than an order of magnitude larger than the magnetic ordering
temperatures. We prove the magnetic character of this continuum by an analysis
of the resonance behavior. RIXS measurements of the dynamical structure factor
for energies within the continuum show that dynamical spin-spin correlations
are restricted to nearest neighbors. Notably, these spectroscopic observations
are also present in the magnetically ordered state for excitation energies
above the conventional magnon excitations. Phenomenologically, our data agree
with inelastic neutron scattering results on the related honeycomb compound
RuCl3, establishing a common ground for a proximate Kitaev spin-liquid regime
in these materials.",1905.13590v1
2020-05-30,Magnetization dynamics in proximity-coupled superconductor/ferromagnet/superconductor multilayers,"In this work, magnetization dynamics is studied in
superconductor/ferromagnet/superconductor three-layered films in a wide
frequency, field, and temperature ranges using the broad-band ferromagnetic
resonance measurement technique. It is shown that in presence of both
superconducting layers and of superconducting proximity at both
superconductor/ferromagnet interfaces a massive shift of the ferromagnetic
resonance to higher frequencies emerges. The phenomenon is robust and
essentially long-range: it has been observed for a set of samples with the
thickness of ferromagnetic layer in the range from tens up to hundreds of
nanometers. The resonance frequency shift is characterized by proximity-induced
magnetic anisotropies: by the positive in-plane uniaxial anisotropy and by the
drop of magnetization. The shift and the corresponding uniaxial anisotropy grow
with the thickness of the ferromagnetic layer. For instance, the anisotropy
reaches 0.27~T in experiment for a sample with 350~nm thick ferromagnetic
layer, and about 0.4~T in predictions, which makes it a ferromagnetic film
structure with the highest anisotropy and the highest natural resonance
frequency ever reported. Various scenarios for the superconductivity-induced
magnetic anisotropy are discussed. As a result, the origin of the phenomenon
remains unclear. Application of the proximity-induced anisotropies in
superconducting magnonics is proposed as a way for manipulations with a
spin-wave spectrum.",2006.00348v1
2020-06-05,2D ferromagnetism at finite temperatures under quantum scrutiny,"Recent years have seen a tremendous rise of two-dimensional (2D) magnetic
materials, several of which verified experimentally. However, most of the
theoretical predictions to date rely on ab-initio methods, at zero temperature
and fluctuations-free, while one certainly expects detrimental quantum
fluctuations at finite temperatures. Here we present the solution of the
quantum Heisenberg model for honeycomb/hexagonal lattices with anisotropic
exchange interaction up to third nearest neighbors and in an applied field in
arbitrary direction, that answers the question whether long-range magnetization
can indeed survive in the ultrathin limit of materials, up to which
temperature, and what the characteristic excitation (magnon) frequencies are,
all essential to envisaged applications of magnetic 2D materials. We find that
long-range magnetic order persists at finite temperature for materials with
overall easy-axis anisotropy. We validate the calculations on the examples of
monolayer CrI3, CrBr3 and MnSe2. Moreover, we provide an easy-to-use tool to
calculate Curie temperatures of new 2D computational materials.",2006.03287v2
2020-06-08,Higher-order exchange interactions in two-dimensional magnets,"Magnetism in recently discovered van der Waals materials has opened new
avenues in the study of fundamental spin interactions in truly two-dimensions.
A paramount question is what effect higher-order interactions beyond bilinear
Heisenberg exchange have on the magnetic properties of few-atom thick
compounds. Here we demonstrate that biquadratic exchange interactions, which is
the simplest and most natural form of non-Heisenberg coupling, assume a key
role in the magnetic properties of layered magnets. Using a combination of
nonperturbative analytical techniques, non-collinear first-principles methods
and classical Monte Carlo calculations that incorporate higher-order exchange,
we show that several quantities including magnetic anisotropies, spin-wave gaps
and topological spin-excitations are intrinsically renormalized leading to
further thermal stability of the layers. We develop a spin Hamiltonian that
also contains antisymmetric exchanges (e.g. Dzyaloshinskii-Moriya interactions)
to successfully rationalize numerous observations currently under debate, such
as the non-Ising character of several compounds despite a strong magnetic
anisotropy, peculiarities of the magnon spectrum of 2D magnets, and the
discrepancy between measured and calculated Curie temperatures. Our results lay
the foundation of a universal higher-order exchange theory for novel 2D
magnetic design strategies.",2006.04891v1
2020-08-31,Electronic properties in itinerant ferromagnet SrRu$_{1-x}$Ti$_x$O$_3$,"Here, we study the electrical transport and specific heat in 4$d$ based
ferromagnetic material SrRuO$_3$ and its Ti substituted SrRu$_{1-x}$Ti$_x$O$_3$
series ($x$ $\le$ 0.7). The SrRuO$_3$ is a metal and shows itinerant
ferromagnetism with transition temperature $T_c$ $\sim$ 160 K. The nonmagnetic
Ti$^{4+}$ (3$d^0$) substitution would not only weaken the active Ru-O-Ru
channel but is also expected to tune the electronic density and electron
correlation effect. A metal to insulator transition has been observed around
$x$ $\sim$ 0.4. The nature of charge transport in paramagnetic-metallic state
($x$ $\leq$ 0.4) and in insulating state ($x$ $>$ 0.4) follows modified Mott's
variable range hopping model. In ferromagnetic-metallic state, resistivity
shows a $T^2$ dependence below $T_c$ which though modifies to $T^{3/2}$
dependence at low temperature. In Ti substituted samples, temperature range for
$T^{3/2}$ dependence extends to higher temperature. Interestingly, this
$T^{3/2}$ dependence dominates in whole ferromagnetic regime in presence of
magnetic field. This evolution of electronic transport behavior can be
explained within the framework of Fermi liquid theory and electron-magnon
scattering mechanism. The negative magnetoresistance exhibits a hysteresis and
a crossover between negative and positive value with magnetic field which is
connected with magnetic behavior in series. The decreasing electronic
coefficient of specific heat with $x$ supports the increasing insulating
behavior in present series. We calculate a high Kadowaki-Woods ratio ($x$
$\leq$ 0.3) for SrRuO$_3$ which increases with substitution concentration. This
signifies an increasing electronic correlation effect with substitution
concentration.",2009.00076v1
2020-09-01,Spin coherence on the ferromagnetic spherical surface,"Spintronics on flat surfaces has been studied over the years, and the
scenario is relatively well-known; however, there is a lack of information when
we consider non-flat surfaces. In this paper, we are concerned about the spin
dynamics of the ferromagnetic model on the spherical surface. We use the
Schwinger bosonic formalism for describing the thermodynamics of spin operators
in terms of spinon operators. Opposite to the flat two-dimensional model, which
is disordered at finite temperature, the curvature of the spherical surface
provides non-zero critical temperature for Schwinger boson condensation, which
characterizes order at finite temperature even in the absence of external
magnetic fields. The thermodynamics is then analyzed in the low-temperature
regime. In addition, we consider the presence of both static and oscillating
magnetic fields, the necessary condition for inducing the ferromagnetic
resonance, and we show systematically that the studied model is well-described
by $SU(2)$ coherent states, which provides the correct dynamics of the
magnetization. The archived results can be applied for describing a diversity
of experiments such as spin superfluidity, angular momentum injection by spin
pumping and spin-transfer torque in non-conventional junctions, magnon
dissipation, and magnetoelectronics on the spherical surface.",2009.00636v2
2020-09-04,Experimental observation of the curvature-induced asymmetric spin-wave dispersion in hexagonal nanotubes,"Theoretical and numerical studies on curved magnetic nano-objects predict
numerous exciting effects that can be referred to as magneto-chiral effects,
which do not originate from the intrinsic Dzyaloshinskii-Moriya interaction or
surface-induced anisotropies. The origin of these chiral effects is the
isotropic exchange or the dipole-dipole interaction present in all magnetic
materials but renormalized by the curvature. Here, we demonstrate
experimentally that curvature induced effects originating from the
dipole-dipole interaction are directly observable by measuring spin-wave
propagation in magnetic nanotubes with hexagonal cross section using time
resolved scanning transmission X-ray microscopy. We show that the dispersion
relation is asymmetric upon reversal of the wave vector when the propagation
direction is perpendicular to the static magnetization. Therefore
counter-propagating spin waves of the same frequency exhibit different
wavelenghts. Hexagonal nanotubes have a complex dispersion, resulting from
spin-wave modes localised to the flat facets or to the extremely curved regions
between the facets. The dispersion relations obtained experimentally and from
micromagnetic simulations are in good agreement. %The asymmetric spin-wave
transport is present for all modes, promoting hexagonal nanotubes for magnonic
applications. These results show that spin-wave transport is possible in 3D,
and that the dipole-dipole induced magneto-chiral effects are significant.",2009.02238v1
2020-09-29,Structural Phase Dependent Giant Interfacial Spin Transparency in W/CoFeB Thin Film Heterostructure,"Pure spin current has transfigured the energy-efficient spintronic devices
and it has the salient characteristic of transport of the spin angular
momentum. Spin pumping is a potent method to generate pure spin current and for
its increased efficiency high effective spin-mixing conductance (Geff) and
interfacial spin transparency (T) are essential. Here, a giant T is reported in
Sub/W(t)/Co20Fe60B20(d)/SiO2(2 nm) heterostructures in \beta-tungsten (\beta-W)
phase by employing all-optical time-resolved magneto-optical Kerr effect
technique. From the variation of Gilbert damping with W and CoFeB thicknesses,
the spin diffusion length of W and spin-mixing conductances are extracted.
Subsequently, T is derived as 0.81 \pm 0.03 for the \beta-W/CoFeB interface. A
sharp variation of Geff and T with W thickness is observed in consonance with
the thickness-dependent structural phase transition and resistivity of W. The
spin memory loss and two-magnon scattering effects are found to have negligible
contributions to damping modulation as opposed to spin pumping effect which is
reconfirmed from the invariance of damping with Cu spacer layer thickness
inserted between W and CoFeB. The observation of giant interfacial spin
transparency and its strong dependence on crystal structures of W will be
important for pure spin current based spin-orbitronic devices.",2009.14143v1
2021-02-14,Unusual exchange couplings and intermediate temperature Weyl state in Co3Sn2S2,"Understanding the magnetism and its possible correlations to the topological
properties has emerged as a forefront and difficult topic in studying magnetic
Weyl semimetals. Co$_{3}$Sn$_{2}$S$_{2}$ is a newly discovered magnetic Weyl
semimetal with a kagome lattice of cobalt ions and has triggered intense
interest for rich fantastic phenomena. Here, we report the magnetic exchange
couplings of Co$_{3}$Sn$_{2}$S$_{2}$ using inelastic neutron scattering and two
density functional theory (DFT) based methods: constrained magnetism and
multiple-scattering Green's function methods. Co$_{3}$Sn$_{2}$S$_{2}$ exhibits
highly anisotropic magnon dispersions and linewidths below $T_{C}$, and
paramagnetic excitations above $T_{C}$. The spin-wave spectra in the
ferromagnetic ground state is well described by the dominant third-neighbor
""across-hexagon"" $J_{d}$ model. Our density functional theory calculations
reveal that both the symmetry-allowed 120$^\circ$ antiferromagnetic orders
support Weyl points in the intermediate temperature region, with distinct
numbers and the locations of Weyl points. Our study highlights the important
role Co$_{3}$Sn$_{2}$S$_{2}$ can play in advancing our understanding of kagome
physics and exploring the interplay between magnetism and band topology.",2102.07050v2
2013-06-11,Bethe ansatz description of edge-localization in the open-boundary XXZ spin chain,"At large values of the anisotropy \Delta, the open-boundary Heisenberg
spin-1/2 chain has eigenstates displaying localization at the edges. We present
a Bethe ansatz description of this `edge-locking' phenomenon in the entire
\Delta>1 region. We focus on the simplest spin sectors, namely the highly
polarized sectors with only one or two overturned spins, i.e., one-particle and
two-particle sectors. Edge-locking is associated with pure imaginary solutions
of the Bethe equations, which are not commonly encountered in periodic chains.
In the one-particle case, at large anisotropies there are two eigenstates with
imaginary Bethe momenta, related to localization at the two edges. For any
finite chain size, one of the two solutions become real as the anisotropy is
lowered below a certain value. For two particles, a richer scenario is
observed, with eigenstates having the possibility of both particles locked on
the same or different edge, one locked and the other free, and both free either
as single magnons or as bound composites corresponding to `string' solutions.
For finite chains, some of the edge-locked spins get delocalized at certain
values of the anisotropy (`exceptional points'), corresponding to imaginary
solutions becoming real. We characterize these phenomena thoroughly by
providing analytic expansions of the Bethe momenta for large chains, large
anisotropy, and near the exceptional points. In the large-chain limit all the
exceptional points coalesce at the isotropic point (\Delta=1) and edge-locking
becomes stable in the whole \Delta>1 region.",1306.2666v3
2015-07-04,Spiral magnets with Dzyaloshinskii-Moriya interaction containing defect bonds,"We present a theory describing spiral magnets with Dzyaloshinskii-Moriya
interaction (DMI) subject to bond disorder at small concentration $c$ of
defects. It is assumed that both DMI and exchange coupling are changed on
imperfect bonds. Qualitatively the same physical picture is obtained in two
models which are considered in detail: B20 cubic helimagnets and layered
magnets in which DMI leads to a long-period spiral ordering perpendicular to
layers. We find that the distortion of the spiral magnetic ordering around a
single imperfect bond is long-range: values of additional turns of spins decay
with the distance $r$ to the defect as $1/r^2$ being governed by the Poisson's
equation for electric dipole. At finite concentration of randomly distributed
imperfect bonds, we calculate correction to the spiral vector. We show that
this correction can change the sign of spin chirality even at $c\ll1$ if
defects are strong enough. It is demonstrated that impurities lead to a diffuse
elastic neutron scattering which has power-law singularities at magnetic Bragg
peaks positions. Then, each Bragg peak acquires power-law decaying tails.
Corrections are calculated to the magnon energy and to its damping caused by
scattering on impurities.",1507.01139v2
2015-07-27,Physical and mathematical justification of the numerical Brillouin zone integration of the Boltzmann rate equation by Gaussian smearing,"Scatterings of electrons at quasiparticles or photons are very important for
many topics in solid state physics, e.g., spintronics, magnonics or photonics,
and therefore a correct numerical treatment of these scatterings is very
important. For a quantum-mechanical description of these scatterings Fermi's
golden rule is used in order to calculate the transition rate from an initial
state to a final state in a first-order time-dependent perturbation theory. One
can calculate the total transition rate from all initial states to all final
states with Boltzmann rate equations involving Brillouin zone integrations. The
numerical treatment of these integrations on a finite grid is often done via a
replacement of the Dirac delta distribution by a Gaussian. The Dirac delta
distribution appears in Fermi's golden rule where it describes the energy
conservation among the interacting particles. Since the Dirac delta
distribution is a not a function it is not clear from a mathematical point of
view that this procedure is justified. We show with physical and mathematical
arguments that this numerical procedure is in general correct, and we comment
on critical points.",1507.07420v1
2015-12-11,Ultra-low magnetic damping of a metallic ferromagnet,"The phenomenology of magnetic damping is of critical importance for devices
that seek to exploit the electronic spin degree of freedom since damping
strongly affects the energy required and speed at which a device can operate.
However, theory has struggled to quantitatively predict the damping, even in
common ferromagnetic materials. This presents a challenge for a broad range of
applications in spintronics and spin-orbitronics that depend on materials and
structures with ultra-low damping. Such systems enable many experimental
investigations that further our theoretical understanding of numerous magnetic
phenomena such as damping and spin-transport mediated by chirality and the
Rashba effect. Despite this requirement, it is believed that achieving
ultra-low damping in metallic ferromagnets is limited due to the scattering of
magnons by the conduction electrons. However, we report on a binary alloy of Co
and Fe that overcomes this obstacle and exhibits a damping parameter
approaching 0.0001, which is comparable to values reported only for
ferrimagnetic insulators. We explain this phenomenon by a unique feature of the
bandstructure in this system: The density of states exhibits a sharp minimum at
the Fermi level at the same alloy concentration at which the minimum in the
magnetic damping is found. This discovery provides both a significant
fundamental understanding of damping mechanisms as well as a test of
theoretical predictions.",1512.03610v1
2016-09-17,Influence of apical oxygen on the extent of in-plane exchange interaction in cuprate superconductors,"In high Tc superconductors the magnetic and electronic properties are
determined by the probability that valence electrons virtually jump from site
to site in the CuO2 planes, a mechanism opposed by on-site Coulomb repulsion
and favored by hopping integrals. The spatial extent of the latter is related
to transport properties, including superconductivity, and to the dispersion
relation of spin excitations (magnons). Here, for three antiferromagnetic
parent compounds (single-layer Bi2Sr0.99La1.1CuO6+delta, double-layer
Nd1.2Ba1.8Cu3O6 and infinite-layer CaCuO2) differing by the number of apical
atoms, we compare the magnetic spectra measured by resonant inelastic x-ray
scattering over a significant portion of the reciprocal space and with
unprecedented accuracy. We observe that the absence of apical oxygens increases
the in-plane hopping range and, in CaCuO2, it leads to a genuine 3D
exchange-bond network. These results establish a corresponding relation between
the exchange interactions and the crystal structure, and provide fresh insight
into the materials dependence of the superconducting transition temperature.",1609.05405v1
2016-09-20,Magnetic anisotropy and the phase diagram of chiral MnSb2O6,"The magnetic phase diagram and low-energy magnon excitations of structurally
and magnetically chiral MnSb2O6 are reported. The specific heat and the static
magnetization are investigated in magnetic fields up to 9 and 30 T,
respectively, while the dynamicmagnetic properties are probed by X-band as well
as tunable high-frequency electron spin-resonance spectroscopy. Below TN = 11.5
K, we observe antiferromagnetic resonance modes which imply small but finite
planar anisotropy showing up in a zero-field splitting of 20 GHz. The data are
well described by means of an easy-plane two-sublattice model with the
anisotropy field BA = 0.02 T. The exchange field BE = 13 T is obtained from the
saturation field derived from the pulsed-field magnetization. A crucial role of
the small anisotropy for the spin structure is reflected by competing
antiferromagnetic phases appearing, at T = 2 K, in small magnetic fields at BC1
$\approx$ 0.5 T and BC2 = 0.9 T. We discuss the results in terms of spin
reorientation and of small magnetic fields favoring helical spin structure over
the cycloidal ground state which, at B = 0, is stabilized by the planar
anisotropy. Above TN, short-range magnetic correlations up to >=60 K and
magnetic entropy changes well above TN reflect the frustrated triangular
arrangement of Mn2+ ions in MnSb2O6.",1609.06105v1
2016-09-22,Magnetization oscillations and waves driven by pure spin currents,"Recent advances in the studies of pure spin currents - flows of angular
momentum (spin) not accompanied by the electric currents - have opened new
horizons for the emerging technologies based on the electron's spin degree of
freedom, such as spintronics and magnonics. The main advantage of pure spin
current, as compared to the spin-polarized electric current, is the possibility
to exert spin transfer torque on the magnetization in thin magnetic films
without electrical current flow through the material. In addition to minimizing
Joule heating and electromigration effects, this characteristic enables the
implementation of spin torque devices based on the low-loss insulating magnetic
materials, and offers an unprecedented geometric flexibility. Here we review
the recent experimental achievements in investigations of magnetization
oscillations excited by pure spin currents in different magnetic nanosystems
based on metallic and insulating magnetic materials. We discuss the spectral
properties of spin-current nano-oscillators, and relate them to the spatial
characteristics of the excited dynamic magnetic modes determined by the
spatially-resolved measurements. We also show that these systems support
locking of the oscillations to external microwave signals, as well as their
mutual synchronization, and can be used as efficient nanoscale sources of
propagating spin waves.",1609.06899v1
2016-11-10,Magnetism and ultra-fast magnetization dynamics of Co and CoMn alloys at finite temperature,"Temperature-dependent magnetic experiments like pump-probe measurements
generated by a pulsed laser have become a crucial technique for switching the
magnetization in the picosecond time scale. Apart from having practical
implications on the magnetic storage technology, the research field of
ultrafast magnetization poses also fundamental physical questions. To correctly
describe the time evolution of the atomic magnetic moments under the influence
of a temperature-dependent laser pulse, it remains crucial to know if the
magnetic material under investigation has magnetic excitation spectrum that is
more or less dependent on the magnetic configuration, e.g. as reflected by the
temperature dependence of the exchange interactions. In this article, we
demonstrate from first-principles theory that the magnetic excitation spectra
in Co with fcc, bcc and hcp structures are nearly identical in a wide range of
non-collinear magnetic configurations. This is a curious result of a balance
between the size of the magnetic moments and the strength of the Heisenberg
exchange interactions, that in themselves vary with configuration, but put
together in an effective spin Hamiltonian results in a configuration
independent effective model. We have used such a Hamiltonian, together with
ab-initio calculated damping parameters, to investigate the magnon dispersion
relationship as well as the ultrafast magnetisation dynamics of Co and Co-rich
CoMn alloys.",1611.03541v2
2017-04-10,Tunable mode coupling in nano-contact spin torque oscillators,"Recent experiments on spin torque oscillators have revealed interactions
between multiple magnetodynamic modes, including mode-coexistence,
mode-hopping, and temperature-driven cross-over between modes. Initial
multimode theory has indicated that a linear coupling between several dominant
modes, arising from the interaction of the subdynamic system with a magnon
bath, plays an essential role in the generation of various multimode behaviors,
such as mode hopping and mode coexistence. In this work, we derive a set of
rate equations to describe the dynamics of coupled magnetodynamic modes in a
nano-contact spin torque oscillator. Expressions for both linear and nonlinear
coupling terms are obtained, which allow us to analyze the dependence of the
coupled dynamic behaviors of modes on external experimental conditions as well
as intrinsic magnetic properties. For a minimal two-mode system, we further map
the energy and phase difference of the two modes onto a two-dimensional phase
space, and demonstrate in the phase portraits, how the manifolds of periodic
orbits and fixed points vary with external magnetic field as well as with
temperature.",1704.03076v2
2017-04-20,Coupled mode theory for the acoustic wave and spin wave interaction in the magphonic crystals: Propagating magnetoelastic waves,"We have investigated co-directional and contra-directional couplings between
spin wave and acoustic wave in one-dimensional periodic structure (magphonic
crystal). The system consists of two ferromagnetic layers alternating in space.
We have taken into consideration materials commonly used in magnonics: yttrium
iron garnet, CoFeB, permalloy, and cobalt. The coupled mode theory (CMT)
formalism have been successfully implemented to describe magnetoelastic
interaction as a periodic perturbation in the magphonic crystal. The results of
CMT calculations have been verified by more rigorous simulations by
frequency-domain plane wave method and time-domain finite element method. The
presented resonant coupling in the magphonic crystal is an active in-space
mechanism which spatially transfers energy between propagating spin and
acoustic modes, thus creating propagating magnetoelastic wave. We have shown,
that CMT analysis of the magnetoelastic coupling is an useful tool to optimize
and design a spin wave - acoustic wave transducer based on a magphonic
crystals. The effect of spin wave damping has been included to the model to
discuss the efficiency of such a device. Our model shows that it is possible to
obtain forward conversion of the acoustic wave to the spin wave in case of
co-directional coupling and backward conversion in case of contra-directional
coupling.",1704.06118v1
2017-04-24,Antiferromagnetic resonance and terahertz continuum in $α-$RuCl$_3$,"We report measurements of optical absorption in the zig-zag antiferromagnet
$\alpha$-RuCl$_3$ as a function of temperature, $T$, magnetic field, $B$, and
photon energy, $\hbar\omega$ in the range $\sim$ 0.3 to 8.3 meV, using
time-domain terahertz spectroscopy. Polarized measurements show that 3-fold
rotational symmetry is broken in the honeycomb plane from 2 K to 300 K. We find
a sharp absorption peak at 2.56 meV upon cooling below the N\'eel temperature
of 7 K at $B=0$ that we identify as magnetic-dipole excitation of a
zero-wavevector magnon, or antiferromagnetic resonance (AFMR). With application
of $B$, the AFMR broadens and shifts to lower frequency as long-range magnetic
order is lost in a manner consistent with transitioning to a spin-disordered
phase. From direct, internally calibrated measurement of the AFMR spectral
weight, we place an upper bound on the contribution to the $dc$ susceptibility
from a magnetic excitation continuum.",1704.07357v3
2018-01-03,Magnetic structure and excitation spectrum of the hyperhoneycomb Kitaev magnet $β$-Li$_2$IrO$_3$,"We present a theoretical study of the static and dynamical properties of the
Kitaev magnet $\beta$-Li$_2$IrO$_3$ based on the minimal $J$-$K$-$\Gamma$
model. We argue that the incommensurate magnetic order of $\beta$-Li$_2$IrO$_3$
can be qualitatively understood in terms of a long-wavelength twisting of a
nearby commensurate, period-3 state, that shares the same symmetry, propagation
vector, counter-rotation of the two spin sublattices, and non-coplanarity. This
period-3 state shows very different structure when either the Kitaev
interaction $K$ or the off-diagonal exchange anisotropy $\Gamma$ is dominant,
featuring in particular ferromagnetic or antiferromagnetic spin dimers,
respectively. We compute the magnon excitation spectra and the dynamical spin
structure factor for both structures. The results for the momentum and energy
dependence of the neutron-scattering intensity provide distinctive qualitative
and quantitative fingerprints of the $K$-dominant or $\Gamma$-dominant states
and thus gives additional information on the relative strength of these
interactions in $\beta$-Li$_2$IrO$_3$.",1801.00874v3
2018-01-13,Lattice-Mediated Magnetic Order Melting in TbMnO3,"Recent ultrafast magnetic-sensitive measurements [Phys. Rev. B 92, 184429
(2015) and Phys. Rev. B 96, 184414 (2017)] have revealed a delayed melting of
the long-range cycloid spin-order in TbMnO$_3$ following photoexcitation across
the fundamental Mott-Hubbard gap. The microscopic mechanism behind this slow
transfer of energy from the photoexcited carriers to the spin degrees of
freedom is still elusive and not understood. Here, we address this problem by
combining spectroscopic ellipsometry, ultrafast broadband optical spectroscopy
and ab initio calculations. Upon photoexcitation, we observe the emergence of a
complex collective response, which is due to high-energy coherent optical
phonons coupled to the out-of-equilibrium charge density. This response
precedes the magnetic order melting and is interpreted as the fingerprint of
the formation of anti-Jahn Teller polarons. We propose that the charge
localization in a long-lived self-trapped state hinders the emission of magnons
and other spin-flip mechanisms, causing the energy transfer from the charge to
the spin system to be mediated by the reorganization of the lattice.
Furthermore, we provide evidence for the coherent excitation of a phonon mode
associated with the ferroelectric phase transition.",1801.04368v1
2018-01-16,Spin-orbit-torque and magnetic damping in tailored ferromagnetic bilayers,"We study spin-orbit-torque-driven ferromagnetic resonance (FMR) in
ferromagnetic (FM) bilayers, consisting of Co and permalloy (Py), sandwiched
between Pt and MgO layers. We find that the FM layer in contact with the Pt
layers dominantly determines that spin Hall angle, which is consistent with the
spin-transparency model. By contrast, the FMR linewidths are considerably
influenced not only by the spin-pumping effect across the Pt|FM in terface but
also by the spin relaxation such as two-magnon scattering at the FMMgO
interface.The CoMgO interface leads to notably increased FMR linewidths, while
the Py|MgO interface has less effect. This different contribution of each
interface to the spin Hall angel and dissipation parameter suggests that the
stack configuration of Pt|Co|Py|MgO requires less writing energy than
Pt|Py|Co|MgO in spin-orbit-torque-driven magnetic switching. Our approach
offers a promising method to optimize material parameters by engineering either
interfaces in contact with the heavy-metal or the oxide layer.",1801.05131v2
2018-08-04,The magnetic exchange parameters and anisotropy of the quasi-two dimensional antiferromagnet NiPS$_3$,"Neutron inelastic scattering has been used to measure the magnetic
excitations in powdered NiPS$_3$, a quasi-two dimensional antiferromagnet with
spin $S = 1$ on a honeycomb lattice. The spectra show clear, dispersive magnons
with a $\sim 7$ meV gap at the Brillouin zone center. The data were fitted
using a Heisenberg Hamiltonian with a single-ion anisotropy assuming no
magnetic exchange between the honeycomb planes. Magnetic exchange interactions
up to the third intraplanar nearest-neighbour were required. The fits show
robustly that NiPS$_3$ has an easy axis anisotropy with $\Delta = 0.3$ meV and
that the third nearest-neighbour has a strong antiferromagnetic exchange of
$J_3 = -6.90$ meV. The data can be fitted reasonably well with either $J_1 < 0$
or $J_1 > 0$, however the best quantitative agreement with high-resolution data
indicate that the nearest-neighbour interaction is ferromagnetic with $J_1 =
1.9$ meV and that the second nearest-neighbour exchange is small and
antiferromagnetic with $J_2 = -0.1$ meV. The dispersion has a minimum in the
Brillouin zone corner that is slightly larger than that at the Brillouin zone
center, indicating that the magnetic structure of NiPS$_3$ is close to being
unstable.",1808.01480v1
2018-08-05,"Dispersion, damping, and intensity of spin excitations in the single-layer (Bi,Pb)$_{2}$(Sr,La)$_{2}$CuO$_{6+δ}$ cuprate superconductor family","Using Cu-$L_3$ edge resonant inelastic x-ray scattering (RIXS) we measured
the dispersion and damping of spin excitations (magnons and paramagnons) in the
high-$T_\mathrm{c}$ superconductor (Bi,Pb)$_{2}$(Sr,La)$_{2}$CuO$_{6+\delta}$
(Bi2201), for a large doping range across the phase diagram ($0.03\lesssim
p\lesssim0.21$). Selected measurements with full polarization analysis
unambiguously demonstrate the spin-flip character of these excitations, even in
the overdoped sample. We find that the undamped frequencies increase slightly
with doping for all accessible momenta, while the damping grows rapidly, faster
in the (0,0)$\rightarrow$(0.5,0.5) nodal direction than in the
(0,0)$\rightarrow$(0.5,0) antinodal direction. We compare the experimental
results to numerically exact determinant quantum Monte Carlo (DQMC)
calculations that provide the spin dynamical structure factor
$S(\textbf{Q},\omega)$ of the three-band Hubbard model. The theory reproduces
well the momentum and doping dependence of the dispersions and spectral weights
of magnetic excitations. These results provide compelling evidence that
paramagnons, although increasingly damped, persist across the superconducting
dome of the cuprate phase diagram; this implies that long range
antiferromagnetic correlations are quickly washed away, while short range
magnetic interactions are little affected by doping.",1808.01682v1
2018-08-13,Single crystal investigation of proposed type-II Weyl semimetal CeAlGe,"We present details of materials synthesis, crystal structure, and anisotropic
magnetic properties of single crystals of CeAlGe, a proposed type-II Weyl
semimetal. Single-crystal x-ray diffraction confirms that CeAlGe forms in
noncentrosymmetric I4$_1$md space group, in line with predictions of
non-trivial topology. Magnetization, specific heat and electrical transport
measurements were used to confirm antiferromagnetic order below 5 K, with an
estimated magnon excitation gap of $\Delta$ = 9.11 K from heat capacity and
hole-like carrier density of 1.44 $\times$ 10$^{20}$ cm$^{-3}$ from Hall effect
measurements. The easy magnetic axis is along the [100] crystallographic
direction, indicating that the moment lies in the tetragonal $\it{ab}$-plane
below 7 K. A spin-flop transition to less than 1 $\mu_B$/Ce is observed to
occur below 30 kOe at 1.8 K in the $M(H)$ ($\bf{H}\|\bf{a}$) data. Small
magnetic fields of 3 kOe and 30 kOe are sufficient to suppress magnetic order
when applied along the $\it{a}$- and $\it{c}$-axes, respectively, resulting in
a complex $\it{T-H}$ phase diagram for $\bf{H}\|\bf{a}$ and a simpler one for
$\bf{H}\|\bf{c}$.",1808.04434v1
2019-06-14,Tunneling Magnetoresistance and Spin-Dependent Diode Performance in Fully Epitaxial Magnetic Tunnel Junctions with Rock-salt Type ZnO/MgO,"We fabricate fully epitaxial Fe/ZnO/MgO/Fe magnetic tunnel junctions (MTJs)
with a bilayer tunnel barrier, in which ZnO has a metastable rock-salt crystal
structure. We observe a high magnetoresistance ratio up to 96% at room
temperature (RT) and find that these MTJs have asymmetric current-voltage
characteristics, and their rectifying performances are largely dependent on the
magnetization alignments of the Fe electrodes. Diode responsibilities at a
zero-bias voltage ($\beta_{0}$), which is an important performance index for
harvesting applications, are observed up to 1.3 A/W at RT in the antiparallel
alignment of the magnetizations while maintaining rather low resistance-area
(RA) products (a few tens of k${\Omega\mu}$m$^2$). Even with the same top and
bottom electrodes (Fe), the obtained $\beta_{0}$ values are comparable to those
of reported high-performance tunnel diodes consisting of amorphous bilayer
tunnel barriers with polycrystalline dissimilar electrodes. This strongly
suggests that the epitaxial ZnO/MgO bilayer tunnel barrier is effective for
enhancing the $\beta_{0}$ without significant increase in the RA. In addition,
we demonstrate that a zero-bias anomaly in thetunnel conductance, which
originates from the magnon excitations at the Fe/barrier interfaces, plays a
crucial role in observed spin-dependent diode performance. The results indicate
that a fully epitaxial MTJ with a bilayer tunnel barrier is a promising
candidate to establish a high-performance high-frequency rectifying system.",1906.05981v1
2019-06-18,Dynamical and thermal magnetic properties of the Kitaev spin liquid candidate $α$-RuCl$_3$,"What is the correct low-energy spin Hamiltonian description of
$\alpha$-RuCl$_3$? The material is a promising Kitaev spin liquid candidate,
but is also known to order magnetically, the description of which necessitates
additional interaction terms. The nature of these interactions, their
magnitudes and even signs, remain an open question. In this work we
systematically investigate dynamical and thermodynamic magnetic properties of
proposed effective Hamiltonians. We calculate zero-temperature inelastic
neutron scattering (INS) intensities using exact diagonalization, and magnetic
specific heat using a thermal pure quantum states method. We find that no
single current model satisfactorily explains all observed phenomena of
$\alpha$-RuCl$_3$. In particular, we find that Hamiltonians derived from first
principles can capture the experimentally observed high-temperature peak in the
magnetic specific heat, while overestimating the magnon energy at the zone
center. In contrast, other models reproduce important features of the INS data,
but do not adequately describe the magnetic specific heat. To address this
discrepancy we propose a modified ab initio model that is consistent with both
magnetic specific heat and low-energy features of INS data.",1906.07579v3
2019-06-28,Lattice and magnetic dynamics in polar chiral incommensurate antiferromagnet Ni$_2$InSbO$_6$,"Complex systems with coexisting polarity, chirality and incommensurate
magnetism are of great interest because they open new degrees of freedom in
interaction between different subsystems and therefore they host a plethora of
intriguing physical properties. Here we report on optical properties and
lattice and spin dynamics of Ni$_2$InSbO$_6$ single crystals studied with the
use of polarized optical microscopy and micro-Raman spectroscopy in the
temperature range 10-300 K. Ni$_2$InSbO$_6$ crystallizes in a polar structure
described by the noncentrosymmetric space group R3 and two types of structural
domains were visualized due to natural optical activity of opposite chirality.
Raman tensor elements of most A and E phonons along with their symmetry were
determined. The manifestation of LO-TO splitting was observed for the A modes.
By tracking the temperature dependencies of phonon frequencies the well
pronounced spin-phonon interaction was observed for several modes below and
above the N\'eel transition temperature TN = 76 K. In antiferromagnetic phase a
wide excitation centred at 247 cm-1 was detected and assigned to the two-magnon
mode and this value was used for estimating exchange parameters through linear
spin-wave theory calculations.",1906.12327v1
2019-07-05,Propagation length of antiferromagnetic magnons governed by domain configurations,"The compensated magnetic order and characteristic, terahertz frequencies of
antiferromagnetic materials makes them promising candidates to develop a new
class of robust, ultra-fast spintronic devices. The manipulation of
antiferromagnetic spin-waves in thin films is anticipated to lead to new exotic
phenomena such as spin-superfluidity, requiring an efficient propagation of
spin-waves in thin films. However, the reported decay length in thin films has
so far been limited to a few nanometers. In this work, we achieve efficient
spin-wave propagation, over micrometer distances, in thin films of the
insulating antiferromagnet hematite with large magnetic domains whilst
evidencing much shorter attenuation lengths in multidomain thin films. Through
transport and magnetic imaging, we conclude on the role of the magnetic domain
structure and spin-wave scattering at domain walls to govern the transport. We
manipulate the spin transport by tailoring the domain configuration through
field cycle training. For the appropriate crystalline orientation, zero-field
spin-transport is achieved across micrometers, as required for device
integration.",1907.02751v2
2019-10-07,Chiral spin-wave velocities induced by all-garnet interfacial Dzyaloshinskii-Moriya interaction in ultrathin yttrium iron garnet films,"Spin waves can probe the Dzyaloshinskii-Moriya interaction (DMI) which gives
rise to topological spin textures, such as skyrmions. However, the DMI has not
yet been reported in yttrium iron garnet (YIG) with arguably the lowest damping
for spin waves. In this work, we experimentally evidence the interfacial DMI in
a 7~nm-thick YIG film by measuring the nonreciprocal spin wave propagation in
terms of frequency, amplitude and most importantly group velocities using all
electrical spin-wave spectroscopy. The velocities of propagating spin waves
show chirality among three vectors, i.e. the film normal direction, applied
field and spin-wave wavevector. By measuring the asymmetric group velocities,
we extract a DMI constant of 16~$\mu$J/m$^{2}$ which we independently confirm
by Brillouin light scattering. Thickness-dependent measurements reveal that the
DMI originates from the oxide interface between the YIG and garnet substrate.
The interfacial DMI discovered in the ultrathin YIG films is of key importance
for functional chiral magnonics as ultra-low spin-wave damping can be achieved.",1910.02599v2
2019-10-29,Origin of the Magnetic Spin Hall Effect: Spin Current Vorticity in the Fermi Sea,"The interplay of spin-orbit coupling (SOC) and magnetism gives rise to a
plethora of charge-to-spin conversion phenomena that harbor great potential for
spintronics applications. In addition to the spin Hall effect, magnets may
exhibit a magnetic spin Hall effect (MSHE), as was recently discovered [Kimata
\textit{et al.}, Nature \textbf{565}, 627-630 (2019)]. To date, the MSHE is
still awaiting its intuitive explanation. Here we relate the MSHE to the
vorticity of spin currents in the Fermi sea, which explains pictorially the
origin of the MSHE. For all magnetic Laue groups that allow for nonzero spin
current vorticities the related tensor elements of the MSH conductivity are
given. Minimal requirements for the occurrence of a MSHE are compatibility with
either a magnetization or a magnetic toroidal quadrupole. This finding implies
in particular that the MSHE is expected in all ferromagnets with sufficiently
large SOC. To substantiate our symmetry analysis, we present various models, in
particular a two-dimensional magnetized Rashba electron gas, that corroborate
an interpretation by means of spin current vortices. Considering thermally
induced spin transport and the magnetic spin Nernst effect in magnetic
insulators, which are brought about by magnons, our findings for electron
transport can be carried over to the realm of spincaloritronics, heat-to-spin
conversion, and energy harvesting.",1910.13375v1
2020-01-15,Analog spacetimes from nonrelativistic Goldstone modes in spinor condensates,"It is well established that linear dispersive modes in a flowing quantum
fluid behave as though they are coupled to an Einstein-Hilbert metric and
exhibit a host of phenomena coming from quantum field theory in curved space,
including Hawking radiation. We extend this analogy to any nonrelativistic
Goldstone mode in a flowing spinor Bose-Einstein condensate. In addition to
showing the linear dispersive result for all such modes, we show that the
quadratically dispersive modes couple to a special nonrelativistic spacetime
called a Newton-Cartan geometry. The kind of spacetime (Einstein-Hilbert or
Newton-Cartan) is intimately linked to the mean-field phase of the condensate.
To illustrate the general result, we further provide the specific theory in the
context of a pseudo-spin-1/2 condensate where we can tune between relativistic
and nonrelativistic geometries. We uncover the fate of Hawking radiation upon
such a transition: it vanishes and remains absent in the Newton-Cartan geometry
despite the fact that any fluid flow creates a horizon for certain wave
numbers. Finally, we use the coupling to different spacetimes to compute and
relate various energy and momentum currents in these analog systems. While this
result is general, present day experiments can realize these different
spacetimes including the magnon modes for spin-1 condensates such as $^{87}$Rb,
$^{7}$Li, $^{41}$K (Newton-Cartan), and $^{23}$Na (Einstein-Hilbert).",2001.05496v2
2020-01-24,Axion search with a quantum-limited ferromagnetic haloscope,"A ferromagnetic axion haloscope searches for Dark Matter in the form of
axions by exploiting their interaction with electronic spins. It is composed of
an axion-to-electromagnetic field transducer coupled to a sensitive rf
detector. The former is a photon-magnon hybrid system, and the latter is based
on a quantum-limited Josephson parametric amplifier. The hybrid system consists
of ten 2.1 mm diameter YIG spheres coupled to a single microwave cavity mode by
means of a static magnetic field. Our setup is the most sensitive rf
spin-magnetometer ever realized. The minimum detectable field is
$5.5\times10^{-19}\,$T with 9 h integration time, corresponding to a limit on
the axion-electron coupling constant $g_{aee}\le1.7\times10^{-11}$ at 95% CL.
The scientific run of our haloscope resulted in the best limit on DM-axions to
electron coupling constant in a frequency span of about 120 MHz, corresponding
to the axion mass range $42.4$-$43.1\,\mu$eV. This is also the first apparatus
to perform an axion mass scanning by changing the static magnetic field.",2001.08940v1
2020-02-25,Strain-Tuned Magnetic Anisotropy in Sputtered Thulium Iron Garnet Ultrathin Films and TIG/Au/TIG Valve Structures,"Defining the magnetic anisotropy for in-plane or out-of-plane easy axis in
ferrimagnetic insulators films by controlling the strain, while maintaining
high-quality surfaces, is desirable for spintronic and magnonic applications.
We investigate ways to tune the anisotropy of amorphous sputtered ultrathin
thulium iron garnet (TIG) films, and thus tailor their magnetic properties by
the thickness (7.5 to 60 nm), substrate choice (GGG and SGGG), and
crystallization process. We correlate morphological and structural properties
with the magnetic anisotropy of post-growth annealed films. 30 nm thick films
annealed at 600 {\deg}C show compressive strain favoring an in-plane magnetic
anisotropy (IPMA), whereas films annealed above 800 {\deg}C are under a tensile
strain leading to a perpendicular magnetic anisotropy (PMA). Air-annealed films
present a high degree of crystallinity and magnetization saturation close to
the bulk value. These results lead to successful fabrication of trilayers
TIG/Au/TIG, with coupling between the TIG layers depending on Au thickness.
These results will facilitate the use of TIG to create various in situ clean
hybrid structures for fundamental interface exchange studies, and towards the
development of complex devices. Moreover, the sputtering technique is
advantageous as it can be easily scaled up for industrial applications.",2002.10608v2
2020-02-26,Exceeding the Landau superflow speed limit with topological Bogoliubov Fermi surfaces,"A common property of topological systems is the appearance of topologically
protected zero-energy excitations. In a superconductor or superfluid such
states set the critical velocity of dissipationless flow $v_{\mathrm{cL}}$,
proposed by Landau, to zero. We check experimentally whether stable superflow
is nevertheless possible in the polar phase of p-wave superfluid $^3$He, which
features a Dirac node line in the energy spectrum of Bogoliubov quasiparticles.
The fluid is driven by rotation of the whole cryostat, and superflow breakdown
is seen as the appearance of single- or half-quantum vortices. Vortices are
detected using the relaxation rate of a Bose-Einstein condensate of magnons,
created within the fluid. The superflow in the polar phase is found to be
stable up to a finite critical velocity $v_{\rm c}\approx 0.2\,$cm/s, despite
the zero value of the Landau critical velocity. We suggest that the stability
of the superflow above $v_{\mathrm{cL}}$ but below $v_{\rm c}$ is provided by
the accumulation of the flow-induced quasiparticles into pockets in the
momentum space, bounded by Bogoliubov Fermi surfaces. In the polar phase this
surface has non-trivial topology which includes two pseudo-Weyl points.
Vortices forming above the critical velocity are strongly pinned in the
confining matrix, used to stabilize the polar phase, and hence stable
macroscopic superflow can be maintained even when the external drive is brought
to zero.",2002.11492v2
2020-03-17,Electrical generation and detection of terahertz signal based on spin-wave emission from ferrimagnets,"Terahertz (THz) signals, mainly generated by photonic or electronic
approaches, are being sought for various applications, whereas the development
of magnetic source might be a necessary step to harness the magnetic nature of
electromagnetic radiation. We show that the relativistic effect on the
current-driven domain-wall motion induces THz spin-wave emission in
ferrimagnets. The required current density increases dramatically in materials
with strong exchange interaction and rapidly exceeds 1012 A m-2, leading to the
device breakdown and thus the lack of experimental evidence. By translating the
collective magnetization oscillations into voltage signals, we propose a
three-terminal device for the electrical detection of THz spin wave. Through
material engineering, wide frequency range from 264 GHz to 1.1 THz and uniform
continuous signals with improved output power can be obtained. As a reverse
effect, the spin wave generated in this system is able to move ferrimagnetic
domain wall. Our work provides guidelines for the experimental verification of
THz spin wave, and could stimulate the design of THz spintronic oscillators for
wideband applications as well as the all-magnon spintronic devices.",2003.07750v1
2020-04-12,Hybridized quadrupolar excitations in the spin-anisotropic frustrated magnet FeI2,"Magnetic order is usually associated with well-defined magnon excitations.
Exotic magnetic fluctuations with fractional, topological or multipolar
character, have been proposed for radically different forms of magnetic matter
such as spin-liquids. As a result, considerable efforts have searched for, and
uncovered, low-spin materials with suppressed dipolar order at low
temperatures. Here, we report neutron-scattering experiments and quantitative
theoretical modeling of an exceptional spin-1 system -- the uniaxial triangular
magnet FeI2 -- where a bright and dispersive band of mixed dipolar-quadrupolar
fluctuations emerges just above a dipolar ordered ground-state. This excitation
arises from anisotropic exchange interactions that hybridize overlapping modes
carrying fundamentally different quantum numbers. Remarkably, a generalization
of spin-wave theory to local SU(3) degrees of freedom accounts for all details
of the low-energy dynamical response of FeI2 without going beyond quadratic
order. Our work highlights that quantum excitations without classical
counterparts can be realized even in presence of fully developed magnetic
order.",2004.05623v1
2020-04-22,Rethinking $α$-RuCl$_3$,"We argue that several empirical constraints strongly restrict parameters of
the effective microscopic spin model describing $\alpha$-RuCl$_3$. In
particular, such constraints dictate a substantial \emph{positive} off-diagonal
anisotropic coupling, $\Gamma^\prime\!>\!0$, not anticipated previously. The
renormalization by quantum fluctuations allows to reconcile larger values of
the advocated bare parameters with their earlier assessments and provides a
consistent description of the field evolution of spin excitations in the
paramagnetic phase. We assert that large anisotropic terms inevitably result in
strong anharmonic coupling of magnons, necessarily leading to broad features in
their spectra due to decays, in accord with the observations in
$\alpha$-RuCl$_3$. Using duality transformations, we explain the origin of the
pseudo-Goldstone mode that is ubiquitous to the studied parameter space and is
present in $\alpha$-RuCl$_3$. Our analysis offers a description of
$\alpha$-RuCl$_3$ as an easy-plane ferromagnet with antiferromagnetic
further-neighbor and strong off-diagonal couplings, which is in a fluctuating
zigzag ground state proximate to an incommensurate phase that is continuously
connected to a ferromagnetic one.",2004.10753v4
2020-05-05,Dimensional crossover in spin Hall oscillators,"Auto-oscillations of magnetization driven by direct spin current have been
previously observed in multiple quasi-zero-dimensional (0D) ferromagnetic
systems such as nanomagnets and nanocontacts. Recently, it was shown that pure
spin Hall current can excite coherent auto-oscillatory dynamics in
quasi-one-dimensional (1D) ferromagnetic nanowires but not in
quasi-two-dimensional (2D) ferromagnetic films. Here we study the 1D to 2D
dimensional crossover of current-driven magnetization dynamics in wire-based
Pt/$\mathrm{Ni}_{80}\mathrm{Fe}_{20}$ bilayer spin Hall oscillators via varying
the wire width. We find that increasing the wire width results in an increase
of the number of excited auto-oscillatory modes accompanied by a decrease of
the amplitude and coherence of each mode. We also observe a crossover from a
hard to a soft onset of the auto-oscillations with increasing the wire width.
The amplitude of auto-oscillations rapidly decreases with increasing
temperature suggesting that interactions of the phase-coherent auto-oscillatory
modes with incoherent thermal magnons plays an important role in suppression of
the auto-oscillatory dynamics. Our measurements set the upper limit on the
dimensions of an individual spin Hall oscillator and elucidate the mechanisms
leading to suppression of coherent auto-oscillations with increasing oscillator
size.",2005.01925v2
2020-05-07,Spin caloritronics in a CrBr$_3$-based magnetic van der Waals heterostructure,"The recently reported magnetic ordering in insulating two-dimensional (2D)
materials, such as chromium triiodide (CrI$_3$) and chromium tribromide
(CrBr$_3$), opens new possibilities for the fabrication of magneto-electronic
devices based on 2D systems. Inevitably, the magnetization and spin dynamics in
2D magnets are strongly linked to Joule heating. Therefore, understanding the
coupling between spin, charge and heat, i.e. spin caloritronic effects, is
crucial. However, spin caloritronics in 2D ferromagnets remains mostly
unexplored, due to their instability in air. Here we develop a fabrication
method that integrates spin-active contacts with 2D magnets through hBN
encapsulation, allowing us to explore the spin caloritronic effects in these
materials. The angular dependence of the thermal spin signal of the CrBr$_3$/Pt
system is studied, for different conditions of magnetic field and heating
current. We highlight the presence of a significant magnetic proximity effect
from CrBr$_3$ on Pt revealed by an anomalous Nernst effect in Pt, and suggest
the contribution of the spin Seebeck effect from CrBr$_3$. These results pave
the way for future magnonic devices using air-sensitive 2D magnetic insulators.",2005.03561v1
2020-05-08,Sublattice magnetizations of ultrathin ferrimagnetic lamellar nanostructures between cobalt leads,"In this work we model the salient magnetic properties of the alloy lamellar
ferrimagnetic nanostructures
$[Co_{1-c}Gd_c]_{\ell^{\prime}}[Co]_\ell[Co_{1-c}Gd_c]_{\ell^{\prime}}$ between
$Co$ semi-infinite leads. We have employed the Ising spin effective field
theory (EFT) to compute the reliable magnetic exchange constants for the pure
cobalt $J_{Co-Co}$ and gadolinium $J_{Gd-Gd}$ materials, in complete agreement
with their experimental data. The sublattice magnetizations of the $Co$ and
$Gd$ sites on the individual hcp atomic (0001) planes of the $Co-Gd$ layered
nanostructures are computed for each plane and corresponding sites, by using
the combined EFT and mean field theory (MFT) spin methods. The sublattice
magnetizations, effective site magnetic moments, and ferrimagnetic compensation
characteristics for the individual hcp atomic planes of the embedded
nanostructures, are computed as a function of temperature, and for various
stable eutectic concentrations in the range $c\leq$ 0.5. The theoretical
results for the sublattice magnetizations and the local magnetic variables of
these ultrathin ferrimagnetic lamellar nanostructured systems, between cobalt
leads, are necessary for the study of their magnonic transport properties, and
eventually their spintronic dynamic computations. The method developed in this
work is general and can be applied to comparable magnetic systems
nanostructured with other materials.",2005.03965v3
2020-05-18,Nature of the spin resonance mode in CeCoIn$_5$,"Spin-fluctuation-mediated unconventional superconductivity can emerge at the
border of magnetism, featuring a superconducting order parameter that changes
sign in momentum space. Detection of such a sign-change is experimentally
challenging, since most probes are not phase-sensitive. The observation of a
spin resonance mode (SRM) from inelastic neutron scattering is often seen as
strong phase-sensitive evidence for a sign-changing superconducting order
parameter, by assuming the SRM is a spin-excitonic bound state. Here, we show
that for the heavy fermion superconductor CeCoIn$_5$, its SRM defies
expectations for a spin-excitonic bound state, and is not a manifestation of
sign-changing superconductivity. Instead, the SRM in CeCoIn$_5$ likely arises
from a reduction of damping to a magnon-like mode in the superconducting state,
due to its proximity to magnetic quantum criticality. Our findings emphasize
the need for more stringent tests of whether SRMs are spin-excitonic, when
using their presence to evidence sign-changing superconductivity.",2005.08441v2
2020-05-28,Anomalous planar Hall effect in a kagome ferromagnet,"The macroscopic signature for Weyl nodes so far has been the negative
longitudinal magnetoresistance arising from the chiral anomaly. However,
negative longitudinal magnetoresistance is not unique to chiral anomaly and can
arise due to completely different mechanisms such as current jetting or in
ferromagnetic systems due to the suppression of scattering with magnons.
Therefore, a macroscopic effect that can be uniquely attributed to the presence
of Weyl nodes is desirable. Here we show that the planar Hall effect could be a
hallmark for Weyl nodes. We investigated the anisotropic magnetoresistance and
planar Hall effect in Fe$_3$Sn$_2$, which has a kagome lattice and has been
predicted to be a type II Weyl metal. We discover that the planar Hall effect
contains a field antisymmetric contribution in addition to the ordinary field
symmetric contribution. The field antisymmetric planar Hall effect has a 3-fold
rotational symmetry, distinctively different from the symmetric planar Hall
effect, but consistent with the 3-fold rotational degeneracy of the
magnetization in Fe$_3$Sn$_2$. The temperature and field dependence of the
antisymmetric planar Hall effect rules out an interpretation based on
contribution from the anomalous Hall effect and is different from the symmetric
planar Hall effect, pointing to a different origin. We attribute the
antisymmetric planar Hall effect to the topological nature of Fe$_3$Sn$_2$ due
to the presence of Weyl II nodes. Our finding offers a promising route for
macroscopically probing Weyl systems.",2005.14237v1
2020-07-02,Disorder-Free Localization and Many-Body Quantum Scars from Magnetic Frustration,"The concept of geometrical frustration has led to rich insights into
condensed matter physics, especially as a mechansim to produce exotic low
energy states of matter. Here we show that frustration provides a natural
vehicle to generate models exhibiting anomalous thermalization of various types
within high energy states. We consider three classes of non-integrable
frustrated spin models: (I) systems with local conserved quantities where the
number of symmetry sectors grows exponentially with the system size but more
slowly than the Hilbert space dimension, (II) systems with exact eigenstates
that are singlet coverings, and (III) flat band systems hosting magnon
crystals. We argue that several 1D and 2D models from class (I) exhibit
disorder-free localization in high energy states so that information
propagation is dynamically inhibited on length scales greater than a few
lattice spacings. We further show that models of class (II) and (III) exhibit
quantum many-body scars -- eigenstates of non-integrable Hamiltonians with
finite energy density and anomalously low entanglement entropy. Our results
demonstrate that magnetic frustration supplies a means to systematically
construct classes of non-integrable models exhibiting anomalous thermalization
in mid-spectrum states.",2007.01311v2
2020-07-08,Order-by-Disorder from Bond-Dependent Exchange and Intensity Signature of Nodal Quasiparticles in a Honeycomb Cobaltate,"Recent theoretical proposals have argued that cobaltates with edge-sharing
octahedral coordination can have significant bond-dependent exchange couplings
thus offering a platform in 3$d$ ions for such physics beyond the much-explored
realizations in 4$d$ and 5$d$ materials. Here we present high-resolution
inelastic neutron scattering data within the magnetically ordered phase of the
stacked honeycomb magnet CoTiO$_3$ revealing the presence of a finite energy
gap and demonstrate that this implies the presence of bond-dependent
anisotropic couplings. We also show through an extensive theoretical analysis
that the gap further implies the existence of a quantum order-by-disorder
mechanism that, in this material, crucially involves virtual crystal field
fluctuations. Our data also provide an experimental observation of a universal
winding of the scattering intensity in angular scans around linear
band-touching points for both magnons and dispersive spin-orbit excitons, which
is directly related to the non-trivial topology of the quasiparticle
wavefunction in momentum space near nodal points.",2007.04199v2
2020-07-23,Spin-waves in 2D honeycomb lattice $XXZ$-type van der Waals antiferromagnet CoPS$_3$,"The magnetic excitations in CoPS$_3$, a two-dimensional van der Waals (vdW)
antiferromagnet with spin $S=3/2$ on a honeycomb lattice, has been measured
using powder inelastic neutron scattering. Clear dispersive spin waves are
observed with a large spin gap of ~13 meV. The magnon spectra were fitted using
an $XXZ$-type $J_1-J_2-J_3$ Heisenberg Hamiltonian with a single-ion anisotropy
assuming no magnetic exchange between the honeycomb layers. The best-fit
parameters show ferromagnetic exchange $J_1=-2.08$ meV and $J_2=-0.26$ meV for
the nearest and second-nearest neighbors and a sizeable antiferromagnetic
exchange $J_3=4.21$ meV for the third-nearest neighbor with the strong
easy-axis anisotropy $K=-2.06$ meV. The suitable fitting could only be achieved
by the anisotropic $XXZ$-type Hamiltonian, in which the exchange interaction
for the out-of-plane component is smaller than that for the in-plane one by a
ratio $\alpha=J_z/J_x=0.6$. Moreover, the absence of spin-orbit exciton around
30 meV indicates that Co$^{2+}$ ions in CoPS$_3$ have a $S=3/2$ state rather
than a spin-orbital entangled $J_\rm{eff}=1/2$ ground state. Our result
directly shows that CoPS$_3$ is an experimental realization of the $XXZ$ model
with a honeycomb lattice in 2D vdW magnets.",2007.11791v1
2020-08-11,Enhanced spin correlations in the Bose-Einstein condensate compound Sr3Cr2O8,"Combined experimental and modeling studies of the magnetocaloric effect,
ultrasound, and magnetostriction were performed on single-crystal samples of
the spin-dimer system Sr$_3$Cr$_2$O$_8$ in large magnetic fields, to probe the
spin-correlated regime in the proximity of the field-induced XY-type
antiferromagnetic order also referred to as a Bose-Einstein condensate of
magnons. The magnetocaloric effect, measured under adiabatic conditions,
reveals details of the field-temperature ($H,T$) phase diagram, a dome
characterized by critical magnetic fields $H_{c1}$ = 30.4 T, $H_{c2}$ = 62 T,
and a single maximum ordering temperature $T_{{\rm max}}(45~$T$)\simeq$8 K. The
sample temperature was observed to drop significantly as the magnetic field is
increased, even for initial temperatures above $T_{{\rm max}}$, indicating a
significant magnetic entropy associated to the field-induced closure of the
spin gap. The ultrasound and magnetostriction experiments probe the coupling
between the lattice degrees of freedom and the magnetism in Sr$_3$Cr$_2$O$_8$.
Our experimental results are qualitatively reproduced by a minimalistic
phenomenological model of the exchange-striction by which sound waves
renormalize the effective exchange couplings.",2008.04547v1
2020-08-19,Strong Superexchange in a $d^{9-δ}$ Nickelate Revealed by Resonant Inelastic X-Ray Scattering,"The discovery of superconductivity in a $d^{9-{\delta}}$ nickelate has
inspired disparate theoretical perspectives regarding the essential physics of
this class of materials. A key issue is the magnitude of the magnetic
superexchange, which relates to whether cuprate-like high-temperature nickelate
superconductivity could be realized. We address this question using Ni L-edge
and O K-edge spectroscopy of the reduced trilayer nickelate $d^{9-1/3}$
La4Ni3O8 and associated theoretical modeling. A magnon energy scale of ~80 meV
resulting from a nearest-neighbor magnetic exchange of $J = 69(4)4$ meV is
observed, proving that $d^{9-{\delta}}$ nickelates can host a large
superexchange. This value, along with that of the Ni-O hybridization estimated
from our O K-edge data, implies that trilayer nickelates represent an
intermediate case between the infinite-layer nickelates and the cuprates, and
suggests that they represent a promising route towards higher-temperature
nickelate superconductivity.",2008.08209v2
2020-08-20,Magnetic Two-Dimensional Chromium Trihalides: A Theoretical Perspective,"The discovery of ferromagnetic order in monolayer 2D crystals has opened a
new venue in the field of two dimensional (2D) materials. 2D magnets are not
only interesting on their own, but their integration in van der Waals
heterostructures allows for the observation of new and exotic effects in the
ultrathin limit. The family of Chromium trihalides, CrI$_3$, CrBr$_3$ and
CrCl$_3$, is, so far, the most studied among magnetic 2D crystals. In this
mini-review, we provide a perspective of the state of the art of the
theoretical understanding of magnetic 2D trihalides, most of which will also be
relevant for other 2D magnets, such as vanadium trihalides. We discuss both the
well-established facts, such as the origin of the magnetic moment and magnetic
anisotropy and address as well open issues such as the nature of the
anisotropic spin couplings and the magnitude of the magnon gap. Recent
theoretical predictions on Moir\' e magnets and magnetic skyrmions are also
discussed. Finally, we give some prospects about the future interest of these
materials and possible device applications.",2008.08855v2
2020-08-23,Multispinon excitations in the spin S=1/2 antiferromagnetic Heisenberg model,"With the commutation relations of the spin operators, we first write out the
equations of motion of the spin susceptibility and related correlation
functions that have a hierarchical structure, then under the ""soft cut-off""
approximation, we give a set of equations of motion of spin susceptibilities
for a spin S=1/2 antiferromagnetic Heisenberg model, that is independent of
whether or not the system has a long range order in the low energy/temperature
limit. Applying for a chain, a square lattice and a honeycomb lattice,
respectively, we obtain the upper and the lowest boundaries of the low-lying
excitations by solving this set of equations. For a chain, the upper and the
lowest boundaries of the low-lying excitations are the same as that of the
exact ones obtained by the Bethe ansatz, where the elementary excitations are
the spinon pairs. For a square lattice, the spin wave excitation (magnons)
resides in the region close to the lowest boundary of the low-lying
excitations, and the multispinon excitations take place in the high energy
region close to the upper boundary of the low-lying excitations. For a
honeycomb lattice, we have one kind of ""mode"" of the low-lying excitation. The
present results obey the Lieb-Schultz-Mattis theorem, and they are also
consistent with recent neutron scattering observations and numerical
simulations for a square lattice.",2008.09974v1
2020-10-15,Dynamic fingerprint of fractionalized excitations in single-crystalline Cu$_3$Zn(OH)$_6$FBr,"Quantum spin liquid (QSL) represents a new class of condensed matter states
characterized by the long-range many-body entanglement of topological orders.
The most prominent feature of the elusive QSL state is the existence of
fractionalized spin excitations. Subject to the strong quantum fluctuations,
the spin-1/2 antiferromagnetic system on a kagome lattice is the promising
candidate for hosting a QSL ground state, but the structurally ideal
realization is rare. Here, we report Raman scattering on the single crystalline
Cu$_3$Zn(OH)$_6$FBr, and confirm that the ideal kagome structure remains down
to low temperatures without any lattice distortion by the angle-resolved
polarized Raman responses and second-harmonic-generation measurements.
Furthermore, at low temperatures the Raman scattering reveals a continuum of
the spin excitations in Cu$_3$Zn(OH)$_6$FBr, in contrast to the sharp magnon
peak in the ordered kagome antiferromagnet EuCu$_3$(OH)$_6$Cl$_3$. Such
magnetic Raman continuum, in particular, the substantial low-energy one-pair
spinon excitation serves as strong evidence for fractionalized spin excitations
in Cu$_3$Zn(OH)$_6$FBr.",2010.07900v1
2020-10-17,Tuning spin excitations in magnetic films by confinement,"Spin excitations of magnetic thin films are the founding element for novel
transport concepts in spintronics, magnonics, and magnetic devices in general.
While spin dynamics have been extensively studied in bulk materials, their
behaviour in mesoscopic films is less known due to experimental limitations.
Here, we employ Resonant Inelastic X-Ray Scattering to investigate the spin
excitation spectrum in mesoscopic Fe films, from bulk-like down to 3 unit cells
thick. In bulk-like samples, we find isotropic, dispersive ferromagnons
consistent with the dispersion observed by neutron scattering in bulk single
crystals. As the thickness is reduced, these ferromagnons survive and evolve
anisotropically: renormalising to lower energies along the out-of-plane
direction while retaining their dispersion in the in-plane direction. This
thickness dependence is captured by simple Heisenberg model calculations
accounting for the confinement in the out-of-plane direction through the loss
of Fe bonds. Our findings highlight the effects of mesoscopic scaling on spin
dynamics and identify thickness as a knob for fine-tuning and controlling
magnetic properties in films.",2010.08745v1
2020-10-21,Chiral switching and dynamic barrier reductions in artificial square ice,"Collective dynamics in lithographically-defined artificial spin ices offer
profound insights into emergent correlations and phase transitions of
geometrically-frustrated Ising spin systems. Their temporal and spatial
evolution are often simulated using kinetic Monte Carlo simulations, which rely
on the precise knowledge of the switching barriers to obtain predictive results
in agreement with experimental observations. In many cases, however, the
barriers are derived from simplified assumptions only, and do not take into
account the full physical picture of nanomagnetic switching. Here we describe
how the immediate magnetic environment of a nanomagnet reversing via
quasi-coherent rotation can induce clockwise and counter-clockwise switching
channels with different barrier energies. This barrier splitting for chiral
reversal channels can be sizeable and, as string-method micromagnetic
simulations show, is relevant for artificial spin ice systems made of both
exchange -- as well as magnetostatically --dominated units. Due to the barrier
splitting (and further reductions due to non-uniform reversal) transition rates
can be exponentially enhanced by several orders of magnitude compared to
mean-field predictions, especially in the limit of rare switching events where
thermal excitation is less likely. This leads to significantly faster
relaxation time scales and modified spatial correlations. Our findings are thus
of integral importance to achieve realistic kinetic Monte Carlo simulations of
emergent correlations in artificial spin systems, magnonic crystals, or the
evolution of nanomagnetic logic circuits.",2010.11291v1
2020-10-26,Spin Insulatronics,"Spin insulatronics covers efforts to generate, detect, control, and utilize
high-fidelity pure spin currents and excitations inside magnetic insulators.
Ultimately, the new findings may open doors for pure spin-based information and
communication technologies. The aim is to replace moving charges with dynamical
entities that utilize low-dissipation coherent and incoherent spin excitations
in antiferromagnetic and ferromagnetic insulators. The ambition is that the new
pure spin-based system will suffer reduced energy losses and operate at high
frequencies. In magnetic insulators, there are no mobile charge carriers that
can dissipate energy. Integration with conventional electronics is possible via
interface exchange interactions and spin-orbit couplings. In this way, the free
electrons in the metals couple to the localized spins in the magnetic
insulators. In turn, these links facilitate spin-transfer torques and
spin-orbit torques across metal-insulator interfaces and the associated
phenomena of spin-pumping and charge-pumping. The interface couplings also
connect the electron motion inside the metals with the spin fluctuations inside
the magnetic insulators. These features imply that the system can enable
unprecedented control of correlations resulting from the electron-magnon
interactions. We review recent developments to realize electric and thermal
generation, manipulation, detection, and control of pure spin information in
insulators.",2010.13512v1
2020-10-30,High-field thermal transport properties of the Kitaev quantum magnet alpha-RuCl3: evidence for low-energy excitations beyond the critical field,"We investigate the phononic in-plane longitudinal low-temperature thermal
conductivity kappa_ab of the Kitaev quantum magnet alpha-RuCl3 for large
in-plane magnetic fields up to 33 T. Our data reveal for fields larger than the
critical field Bc ~ 8 T, at which the magnetic order is suppressed, a dramatic
increase of kappa_ab at all temperatures investigated. The analysis of our data
shows that the phonons are not only strongly scattered by a magnetic mode at
relatively large energy which scales roughly linearly with the magnetic field,
but also by a small-energy mode which emerges near Bc with a square-root-like
field dependence. While the former is in striking agreement with recent spin
wave theory (SWT) results of the magnetic excitation spectrum at the Gamma
point, the energy of the latter is too small to be compatible with the
SWT-expected magnon gap at the M point, despite the matching field dependence.
Therefore, an alternative scenario based on phonon scattering off the thermal
excitation of random-singlet states is proposed.",2010.16282v1
2020-11-04,Spin Matrix Theory String Backgrounds and Penrose Limits of AdS/CFT,"Spin Matrix theory (SMT) limits provide a way to capture the dynamics of the
AdS/CFT correspondence near BPS bounds. On the string theory side, these limits
result in non-relativistic sigma models that can be interpreted as novel
non-relativistic strings. This SMT string theory couples to non-relativistic
$U(1)$-Galilean background geometries. In this paper, we explore the relation
between pp-wave backgrounds obtained from Penrose limits of AdS${}_5 \times
S^5$, and a new type of $U(1)$-Galilean backgrounds that we call flat-fluxed
(FF) backgrounds. These FF backgrounds are the simplest possible SMT string
backgrounds and correspond to free magnons from the spin chain perspective. We
provide a catalogue of the $U(1)$-Galilean backgrounds one obtains from SMT
limits of string theory on AdS${}_5 \times S^5$ and subsequently study large
charge limits of these geometries from which the FF backgrounds emerge. We show
that these limits are analogous to Penrose limits of AdS${}_5 \times S^5$ and
demonstrate that the large charge/Penrose limits commute with the SMT limits.
Finally, we point out that $U(1)$-Galilean backgrounds prescribe a symplectic
manifold for the transverse SMT string embedding fields. This is illustrated
with a Hamiltonian derivation for the SMT limit of a particle.",2011.02539v1
2020-11-05,Tailoring Dzyaloshinskii-Moriya interaction in a transition metal dichalcogenide by dual-intercalation,"Dzyaloshinskii-Moriya interaction (DMI) is vital to form various chiral spin
textures, novel behaviors of magnons and permits their potential applications
in energy-efficient spintronic devices. Here, we realize a sizable bulk DMI in
a transition metal dichalcogenide (TMD) 2H-TaS2 by intercalating Fe atoms,
which form the chiral supercells with broken spatial inversion symmetry and
also act as the source of magnetic orderings. Using a newly developed protonic
gate technology, gate-controlled protons intercalation could further change the
carrier density and intensely tune DMI via the Ruderman-Kittel-Kasuya-Yosida
mechanism. The resultant giant topological Hall resistivity of 1.4 uohm.cm at
-5.2V (about 460% of the zero-bias value) is larger than most of the known
magnetic materials. Theoretical analysis indicates that such a large
topological Hall effect originates from the two-dimensional Bloch-type chiral
spin textures stabilized by DMI, while the large anomalous Hall effect comes
from the gapped Dirac nodal lines by spin-orbit interaction. Dual-intercalation
in 2HTaS2 provides a model system to reveal the nature of DMI in the large
family of TMDs and a promising way of gate tuning of DMI, which further enables
an electrical control of the chiral spin textures and related electromagnetic
phenomena.",2011.02642v2
2020-11-09,Classical many-body chaos with and without quasiparticles,"We study correlations, transport and chaos in a Heisenberg magnet as a
classical model many-body system. By varying temperature and dimensionality, we
can tune between settings with and without symmetry breaking and accompanying
collective modes or quasiparticles. We analyse both conventional and
out-of-time-ordered spin correlators (`decorrelators') to track the spreading
of a spatiotemporally localised perturbation -- the wingbeat of the butterfly
-- as well as transport coefficients and Lyapunov exponents. We identify a
number of qualitatively different regimes. Trivially, at $T=0$, there is no
dynamics at all. In the limit of low temperature, $T=0^+$, integrability
emerges, with infinitely long-lived magnons; here the wavepacket created by the
perturbation propagates ballistically, yielding a lightcone at the spin wave
velocity which thus subsumes the butterfly velocity; inside the lightcone, a
pattern characteristic of the free spin wave spectrum is visible at short
times. On top of this, residual interactionslead to spin wave lifetimes which,
while divergent in this limit, remain finite at any nonzero $T$. At the longest
times, this leads to a `standard' chaotic regime; for this regime, we show that
the Lyapunov exponent is simply proportional to the inverse spin-wave lifetime.
Visibly strikingly, between this and the `short-time' integrable regimes, a
scarred regime emerges: here, the decorrelator is spatiotemporally highly
non-uniform, being dominated by rare and random scattering events seeding
secondary lightcones. As the spin correlation length decreases with increasing
$T$, the distinction between these regimes disappears and at high temperature
the previously studied chaotic paramagnetic regime emerges. For this, we
elucidate how, somewhat counterintuitively, the ballistic butterfly velocity
arises from a diffusive spin dynamics.",2011.04700v1
2020-11-10,Nonlinear level attraction of cavity axion polariton in antiferromagnetic topological insulator,"Strong coupling between cavity photons and various excitations in condensed
matters boosts the field of light-matter interaction and generates several
exciting sub-fields, such as cavity optomechanics and cavity magnon polariton.
Axion quasiparticles, emerging in topological insulators, were predicted to
strongly couple with the light and generate the so-called axion polariton.
Here, we demonstrate that there arises a gapless level attraction in cavity
axion polariton of antiferromagnetic topological insulators, which originates
from a nonlinear interaction between axion and the odd-order resonance of
cavity. Such a novel level attraction is essentially different from
conventional level attractions with the mechanism of either a linear coupling
or a dissipation-mediated interaction, and also different from the level
repulsion induced by the strong coupling in common polaritons. Our results
reveal a new mechanism of level attractions, and open up new roads for
exploring the axion polariton with cavity technologies. They have potential
applications for quantum information and dark matter research.",2011.04916v3
2020-11-18,Non-planar geometrical effects on the magnetoelectrical signal in a three-dimensional nanomagnetic circuit,"Expanding nanomagnetism and spintronics into three dimensions (3D) offers
great opportunities for both fundamental and technological studies. However,
probing the influence of complex 3D geometries on magnetoelectrical phenomena
poses important experimental and theoretical challenges. In this work, we
investigate the magnetoelectrical signals of a ferromagnetic 3D nanodevice
integrated into a microelectronic circuit using direct-write nanofabrication.
Due to the 3D vectorial nature of both electrical current and magnetisation, a
complex superposition of several magnetoelectrical effects takes place. By
performing electrical measurements under the application of 3D magnetic fields,
in combination with macrospin simulations and finite element modelling, we
disentangle the superimposed effects, finding how a 3D geometry leads to
unusual angular dependences of well-known magnetotransport effects such as the
anomalous Hall effect. Crucially, our analysis also reveals a strong role of
the noncollinear demagnetising fields intrinsic to 3D nanostructures, which
results in an angular dependent magnon magnetoresistance contributing strongly
to the total magnetoelectrical signal. These findings are key to the
understanding of 3D spintronic systems and underpin further fundamental and
device-based studies.",2011.09199v1
2021-01-20,Protected Long-Distance Guiding of Hypersound Underneath a Nano-Corrugated Surface,"Within a new paradigm for communications on the nanoscale, high-frequency
surface acoustic waves are becoming effective data carrier and encoder. On-chip
communications require acoustic wave propagation along nano-corrugated surfaces
which strongly scatter traditional Rayleigh waves. Here we propose the delivery
of information using subsurface acoustic waves with hypersound frequencies ~20
GHz, which is a nanoscale analogue of subsurface sound waves in the ocean. A
bunch of subsurface hypersound modes is generated by pulsed optical excitation
in a multilayer semiconductor structure with a metallic nanograting on top. The
guided hypersound modes propagate coherently beneath the nanograting, retaining
the surface imprinted information, on a distance of more than 50 {\mu}m which
essentially exceeds the propagation length of Rayleigh waves. The concept is
suitable for interfacing single photon emitters, such as buried quantum dots,
carrying coherent spin excitations in magnonic devices, and encoding the
signals for optical communications at the nanoscale.",2101.08189v1
2021-01-29,Dynamically stable negative-energy states induced by spin-transfer torques,"We investigate instabilities of the magnetic ground state in ferromagnetic
metals that are induced by uniform electrical currents, and, in particular, go
beyond previous analyses by including dipolar interactions. These instabilities
arise from spin-transfer torques that lead to Doppler shifted spin waves. For
sufficiently large electrical currents, spin-wave excitations have negative
energy with respect to the uniform magnetic ground state, while remaining
dynamically stable due to dissipative spin-transfer torques. Hence, the uniform
magnetic ground state is energetically unstable, but is not able to dynamically
reach the new ground state. We estimate this to happen for current densities $
j\gtrsim (1-D/D_c)10^{13} \mathrm{A/m^2} $ in typical thin film experiments,
with $ D $ the Dzyaloshinskii-Moriya interaction constant, and $ D_c $ the
Dzyaloshinskii-Moriya interaction that is required for spontaneous formation of
spirals or skyrmions. These current densities can be made arbitrarily small for
ultrathin film thicknesses at the order of nanometers, due to surface- and
interlayer effects. From an analogue gravity perspective, the stable negative
energy states are an essential ingredient to implement event horizons for
magnons -- the quanta of spin waves -- giving rise to e.g. Hawking radiation
and can be used to significantly amplify spin waves in a so-called black-hole
laser.",2101.12544v1
2021-02-26,Gapless Spin Wave Transport through a Quantum Canted-Antiferromagnet,"In the Landau levels of a two-dimensional electron system or when flat bands
are present, e.g. in twisted van der Waals bilayers, strong electron-electron
interaction gives rise to quantum Hall ferromagnetism with spontaneously broken
symmetries in the spin and isospin sectors. Quantum Hall ferromagnets support a
rich variety of low-energy collective excitations that are instrumental to
understand the nature of the magnetic ground states and are also potentially
useful as carriers of quantum information. Probing such collective excitations,
especially their dispersion {\omega}(k), has been experimentally challenging
due to small sample size and measurement constraints. In this work, we
demonstrate an all-electrical approach that integrates a Fabry-P\'erot cavity
with non-equilibrium transport to achieve the excitation, wave vector selection
and detection of spin waves in graphene heterostructures. Our experiments
reveal gapless, linearly dispersed spin wave excitations in the E = 0 Landau
level of bilayer graphene, thus providing direct experimental evidence for a
predicted canted antiferromagnetic order. We show that the gapless spin wave
mode propagates with a high group velocity of several tens of km/s and
maintains phase coherence over a distance of many micrometers. Its dependence
on the magnetic field and temperature agree well with the hydrodynamic theory
of spin waves. These results lay the foundation for the quest of spin
superfluidity in this high-quality material. The resonant cavity technique we
developed offers a powerful and timely method to explore the collective
excitation of many spin and isospin-ordered many-body ground states in van der
Waals heterostructures and open the possibility of engineering magnonic
devices.",2103.00018v1
2021-03-05,Why the Schwinger Boson mean field theory fails to describe the spin dynamics of the triangular lattice antiferromagnetic Heisenberg model?,"We find that the Schwinger Boson mean field theory(SBMFT) supplemented with
Gutzwiller projection provides an exceedingly accurate description for the
ground state of the spin-$\frac{1}{2}$ triangular lattice antiferromagnetic
Heisenberg model(spin-$\frac{1}{2}$ TLHAF). However, we find the SBMFT fails
even qualitatively in the description of the dynamical behavior of the system.
In particular, the SBMFT fails to predict the Goldstone mode in the magnetic
ordered phase. We show that the coherent peak in the two-spinon continuum in
the presence of spinon condensate should not be interpreted as a magnon mode.
The SBMFT also predicts incorrectly a gapless longitudinal spin fluctuation
mode in the magnetic ordered phase. We show that these failures are related to
the following facts: (1)Spinon condensation fails to provide a consistent
description of the order parameter manifold of the 120 degree ordered phase.
(2)There lacks in the SBMFT the coupling between the uncondensed spinon and the
spinon condensate, which breaks both the spin rotational and the translational
symmetry. (3)There lacks in the SBMFT the rigidity that is related to the no
double occupancy constraint on the spinon system. We show that such failures of
the SBMFT is neither restricted to the spin-$\frac{1}{2}$ TLHAF nor to the
magnetic ordered phase. We proposed a generalized SBMFT to resolve the first
two issues and a new formalism to address the third issue.",2103.03484v2
2021-03-08,Environmental Screening and Ligand-Field Effects to Magnetism in CrI$_3$ Monolayer,"We present a detailed study on the microscopic origin of magnetism in
suspended and dielectrically embedded CrI$_3$ monolayer. To this end, we
down-fold two distinct minimal generalized Hubbard models with different
orbital basis sets from \emph{ab initio} calculations using the constrained
random phase approximation. Within mean-field approximation, we show that these
models are capable of describing the formation of localized magnetic moments in
CrI$_3$ and of reproducing electronic properties of full \emph{ab initio}
calculations. We utilize the magnetic force theorem to study microscopic
magnetic exchange channels between the different orbital manifolds. We find a
multi-orbital super-exchange mechanism as the origin of magnetism in CrI$_3$
resulting from a detailed interplay between effective ferro- and
anti-ferromagnetic Cr-Cr $d$ coupling channels, which is decisively affected by
the ligand (I) $p$ orbitals. We show how environmental screening such as
resulting from encapsulation with hexagonal boron nitride (hBN) of the CrI$_3$
monolayer affects the Coulomb interaction in the film and how this successively
controls its magnetic properties. Driven by a non-monotonic interplay between
nearest and next-nearest neighbour exchange interactions we find the magnon
dispersion and the Curie temperature to be non-trivially affected by the
environmental dielectric screening.",2103.04686v1
2021-03-10,"Spin texture induced by non-magnetic doping and spin dynamics in 2D triangular lattice antiferromagnet h-Y(Mn,Al)O3","Novel effects induced by nonmagnetic impurities in frustrated magnets and
quantum spin liquid represent a highly nontrivial and interesting problem. A
theoretical proposal of extended modulated spin structures induced by doping of
such magnets, distinct from the well-known skyrmions has attracted significant
interest. Here, we demonstrate that nonmagnetic impurities can produce such
extended spin structures in h-YMnO3, a triangular antiferromagnet with
noncollinear magnetic order. Using inelastic neutron scattering (INS), we
measured the full dynamical structure factor in Al-doped h-YMnO3 and confirmed
the presence of magnon damping with a clear momentum dependence. Our
theoretical calculations can reproduce the key features of the INS data,
supporting the formation of the proposed spin textures. As such, our study
provides the first experimental confirmation of the impurity-induced spin
textures. It offers new insights and understanding of the impurity effects in a
broad class of noncollinear magnetic systems.",2103.05936v2
2021-03-15,Electron-plasmon interaction induced plasmonic-polaron band replication in epitaxial perovskite SrIrO$_3$ films,"Electron-boson interaction is fundamental to a thorough understanding of
various exotic properties emerging in many-body physics. In photoemission
spectroscopy, photoelectron emission due to photon absorption would trigger
diverse collective excitations in solids, including the emergence of phonons,
magnons, electron-hole pairs, and plasmons, which naturally provides a reliable
pathway to study electron-boson couplings. While fingerprints of
electron-phonon/-magnon interactions in this state-of-the-art technique have
been well investigated, much less is known about electron-plasmon coupling, and
direct observation of the band renormalization solely due to electron-plasmon
interactions is extremely challenging. Here by utilizing integrated oxide
molecular-beam epitaxy and angle-resolved photoemission spectroscopy, we
discover the long sought-after pure electron-plasmon coupling-induced low-lying
plasmonic-polaron replica bands in epitaxial semimetallic SrIrO$_3$ films, in
which the characteristic low carrier concentration and narrow bandwidth combine
to provide a unique platform where the electron-plasmon interaction can be
investigated kinematically in photoemission spectroscopy. This finding enriches
the forms of electron band normalization on collective modes in solids and
demonstrates that, to obtain a complete understanding of the quasiparticle
dynamics in 5d electron systems, the electron-plasmon interaction should be
considered on equal footing with the acknowledged electron-electron interaction
and spin-orbit coupling.",2103.08192v1
2021-03-26,Microwave resonances of magnetic skyrmions in thin film multilayers,"Non-collinear magnets exhibit a rich array of dynamic properties at microwave
frequencies. They can host nanometre-scale topological textures known as
skyrmions, whose spin resonances are expected to be highly sensitive to their
local magnetic environment. Here, we report a magnetic resonance study of an
[Ir/Fe/Co/Pt] multilayer hosting N\'eel skyrmions at room temperature.
Experiments reveal two distinct resonances of the skyrmion phase during
in-plane ac excitation, with frequencies between 6-12 GHz. Complementary
micromagnetic simulations indicate that the net magnetic dipole moment rotates
counterclockwise (CCW) during both resonances. The magnon probability
distribution for the lower-frequency resonance is localised within isolated
skyrmions, unlike the higher-frequency mode which principally originates from
areas between skyrmions. However, the properties of both modes depend
sensitively on the out-of-plane dipolar coupling, which is controlled via the
ferromagnetic layer spacing in our heterostructures. The gyrations of stable
isolated skyrmions reported in this room temperature study encourage the
development of new material platforms and applications based on skyrmion
resonances. Moreover, our material architecture enables the resonance spectra
to be tuned, thus extending the functionality of such applications over a
broadband frequency range.",2103.14524v1
2021-04-16,Implementing a magnonic time-delay reservoir computer model,"Recently we demonstrated experimentally that microwave oscillators based on
the time delay feedback provided by traveling spin waves could operate as
reservoir computers. In the present paper, we extend this concept by adding the
feature of time multiplexing made available by the large propagation
times/distances of traveling spin waves. The system utilizes the nonlinear
behavior of propagating magnetostatic surface spin waves in a yttrium-iron
garnet thin film and the time delay inherent in the active ring configuration
to process time dependent data streams. Higher reservoir dimensionality is
obtained through the time-multiplexing method, emulating ""virtual"" neurons as
temporally separated spin-wave pulses circulating in the active ring below the
auto-oscillation threshold. To demonstrate the efficacy of the concept, the
active ring reservoir computer is evaluated on the short-term memory and parity
check benchmark tasks, and the physical system parameters are tuned to optimize
performance. By incorporating a reference line to mix the input signal directly
onto the active ring output, both the amplitude and phase nonlinearity of the
spin waves can be exploited, resulting in significant improvement on the
nonlinear parity check task. We also find that the fading memory capacity of
the system can be easily tuned by controlling the active ring gain. Finally, we
show that the addition of a second spin-wave delay line configured to transmit
backward volume spin waves can partly compensate dispersive pulse broadening
and enhance the fading memory capacity of the active ring.",2104.07879v1
2021-04-20,Exceptional Point and Cross-Relaxation Effect in a Hybrid Quantum System,"Exceptional points (EPs) are exotic degeneracies of non-Hermitian systems,
where the eigenvalues and the corresponding eigenvectors simultaneously
coalesce in parameter space, and these degeneracies are sensitive to tiny
perturbations on the system. Here we report an experimental observation of the
EP in a hybrid quantum system consisting of dense nitrogen (P1) centers in
diamond coupled to a coplanar-waveguide resonator. These P1 centers can be
divided into three subensembles of spins, and cross relaxation occurs among
them. As a new method to demonstrate this EP, we pump a given spin subensemble
with a drive field to tune the magnon-photon coupling in a wide range. We
observe the EP in the middle spin subensemble coupled to the resonator mode,
irrespective of which spin subensemble is actually driven. This robustness of
the EP against pumping reveals the key role of the cross relaxation in P1
centers. It offers a novel way to convincingly prove the existence of the
cross-relaxation effect via the EP.",2104.09811v1
2021-05-03,Relaxation of a single defect spin by the low-frequency gyrotropic mode of a magnetic vortex,"We excite the gyrotropic mode of a magnetic vortex and observe the resulting
effect on the spin state of a nearby nitrogen-vacancy (NV) defect in diamond.
Thin permalloy disks fabricated on a diamond sample are magnetized in a vortex
state in which the magnetization curls around a central core. The magnetization
dynamics of this configuration are described by a discrete spectrum of confined
magnon modes, as well as a low-frequency gyrotropic mode in which the vortex
core precesses about its equilibrium position. Despite the spin transition
frequencies being far-detuned from the modes of the ferromagnet, we observe
enhanced relaxation of the NV spin when driving the gyrotropic mode. Moreover,
we map the spatial dependence of the interaction between the vortex and the
spin by translating the vortex core within the disk with an applied magnetic
field, resulting in steplike motion as the vortex is pinned and de-pinned. The
strong spin relaxation is observed when the vortex core is within approximately
250 nm of the NV center defect. We attribute this effect to the higher
frequencies in the spectrum of the magnetic fringe field arising from the
soliton-like nature of the gyrotropic mode when driven with sufficiently large
amplitude.",2105.00927v1
2021-05-10,Parameterization of magnetic vector potentials and fields for efficient multislice calculations of elastic electron scattering,"The multislice method, which simulates the propagation of the incident
electron wavefunction through a crystal, is a well-established method for
analyzing the multiple scattering effects that an electron beam may undergo.
The inclusion of magnetic effects into this method proves crucial towards
simulating magnetic differential phase contrast images at atomic resolution,
enhanced magnetic interaction of vortex beams with magnetic materials,
calculating magnetic Bragg spots, or searching for magnon signatures, to name a
few examples. Inclusion of magnetism poses novel challenges to the efficiency
of the multislice method for larger systems, especially regarding the
consistent computation of magnetic vector potentials and magnetic fields over
large supercells. We present in this work a tabulation of parameterized
magnetic values for the first three rows of transition metal elements computed
from atomic density functional theory calculations, allowing for the efficient
computation of approximate magnetic vector fields across large crystals using
only structural and magnetic moment size and direction information.
Ferromagnetic bcc Fe and tetragonal FePt are chosen as examples in this work to
showcase the performance of the parameterization versus directly obtaining
magnetic vector fields from the unit cell spin density by density functional
theory calculations, both for the quantities themselves and the resulting
magnetic signal from their respective use in multislice calculations.",2105.04195v1
2021-05-10,Switchable Magnetic Frustration in Buckyball Nanoarchitectures,"Recent progress in nanofabrication has led to the emergence of
three-dimensional magnetic nanostructures as a vibrant field of research. This
includes the study of three-dimensional arrays of interconnected magnetic
nanowires with tunable artificial spin-ice properties. Prominent examples of
such structures are magnetic buckyball nanoarchitectures, which consist of
ferromagnetic nanowires connected at vertex positions corresponding to those of
a C60 molecule. These structures can be regarded as prototypes for the study of
the transition from two- to three-dimensional spin-ice lattices. In spite of
their significance for three-dimensional nanomagnetism, little is known about
the micromagnetic properties of buckyball nanostructures. By means of
finite-element micromagnetic simulations, we investigate the magnetization
structures and the hysteretic properties of several sub-micron-sized magnetic
buckyballs. Similar to ordinary artificial spin ice lattices, the array can be
magnetized in a variety of zero-field states with vertices exhibiting different
degrees of magnetic frustration. Remarkably, and unlike planar geometries,
magnetically frustrated states can be reversibly created and dissolved by
applying an external magnetic field. This easiness to insert and remove
defect-like magnetic charges, made possible by the angle-selectivity of the
field-induced switching of individual nanowires, demonstrates a potentially
significant advantage of three-dimensional nanomagnetism compared to planar
geometries. The control provided by the ability to switch between ice-rule
obeying and magnetically frustrated structures could be an important feature of
future applications, including magnonic devices exploiting differences in the
fundamental frequencies of these configurations.",2105.04509v1
2021-05-17,Fluctuation-induced ferrimagnetism in sublattice-imbalanced antiferromagnets with application to SrCu$_2$(BO$_3$)$_2$ under pressure,"We show that a collinear Heisenberg antiferromagnet, whose sublattice
symmetry is broken at the Hamiltonian level, becomes a fluctuation-induced
ferrimagnet at any finite temperature $T$ below the N\'eel temperature $T_{\rm
N}$. We demonstrate this using a layered variant of a square-lattice
$J_1$-$J_2$ model. Linear spin-wave theory is used to determine the
low-temperature behavior of the uniform magnetization, and non-linear
corrections are argued to yield a temperature-induced qualitative change of the
magnon spectrum. We then consider a layered Shastry-Sutherland model,
describing a frustrated arrangement of orthogonal dimers. This model displays
an antiferromagnetic phase for large intra-dimer couplings. A lattice
distortion which breaks the glide symmetry between the two types of dimers
corresponds to broken sublattice symmetry and hence gives rise to
ferrimagnetism. Given indications that such a distortion is present in the
material SrCu$_2$(BO$_3$)$_2$ under hydrostatic pressure, we suggest the
existence of a fluctuation-induced ferrimagnetic phase in pressurized
SrCu$_2$(BO$_3$)$_2$. We predict a non-monotonic behavior of the uniform
magnetization as function of temperature.",2105.08088v2
2021-06-15,Exciton-driven antiferromagnetic metal in a correlated van der Waals insulator,"Collective excitations of bound electron-hole pairs -- known as excitons --
are ubiquitous in condensed matter, emerging in systems as diverse as band
semiconductors, molecular crystals, and proteins. Recently, their existence in
strongly correlated electron materials has attracted increasing interest due to
the excitons' unique coupling to spin and orbital degrees of freedom. The
non-equilibrium driving of such dressed quasiparticles offers a promising
platform for realizing unconventional many-body phenomena and phases beyond
thermodynamic equilibrium. Here, we achieve this in the van der Waals
correlated insulator NiPS$_3$ by photoexciting its newly discovered
spin-orbit-entangled excitons that arise from Zhang-Rice states. By monitoring
the time evolution of the terahertz conductivity, we observe the coexistence of
itinerant carriers produced by exciton dissociation and the long-wavelength
antiferromagnetic magnon that coherently precesses in time. These results
demonstrate the emergence of a transient metallic state that preserves
long-range antiferromagnetism, a phase that cannot be reached by simply tuning
the temperature. More broadly, our findings open an avenue toward the
exciton-mediated optical manipulation of magnetism.",2106.08355v1
2021-06-30,Classifying charge carrier interaction in highly-compressed elements and silane,"Since pivotal experimental discovery of the near-room-temperature
superconductivity (NRTS) in highly-compressed sulphur hydride by Drozdov et al
(2015 Nature 525 73-76), more than a dozen of binary and of ternary
hydrogen-rich phases exhibited superconducting transition above 100 K have been
discovered to date. There is a widely accepted theoretical point of view that
primary mechanism governing the emergence of superconductivity in hydrogen-rich
phases is the electron-phonon pairing. However, our recent analysis of
experimental temperature dependent resistance in $H_{3}S$, $LaH_{x}$, $PrH_{9}$
and $BaH_{12}$ (arXiv: 2104.14145) showed that these compounds exhibit the
dominance of non-electron-phonon charge carrier interaction and, thus, it is
unlikely that the electron-phonon pairing is the primary mechanism for the
emergence of superconductivity in these materials. Here we use the same
approach to reveal charge carrier interaction in highly-compressed lithium,
black phosphorous, sulfur, and silane. We found that all these superconductors
exhibit the dominance of non-electron-phonon charge carrier interaction. This
explains the failure of high-Tc values predicted for these materials by the
first-principles calculations which utilized the electron-phonon pairing as the
mechanism for the emergence of superconductivity in these materials. Our result
implies that alternative pairing mechanisms (i.e., electron-magnon,
electron-polaron, electron-electron, etc.) should be tested within
first-principles calculations approach as possible mechanisms for the emergence
of superconductivity in highly-compressed superconductors.",2106.15873v1
2021-07-09,Electric field driven flat bands: Enhanced magnetoelectric and electrocaloric effects in frustrated quantum magnets,"The $J_1$-$J_2$ quantum spin sawtooth chain is a paradigmatic one-dimensional
frustrated quantum spin system exhibiting unconventional ground-state and
finite-temperature properties. In particular, it exhibits a flat energy band of
one-magnon excitations accompanied by an enhanced magnetocaloric effect for two
singular ratios of the basal interactions $J_1$ and the zigzag interactions
$J_2$. In our paper, we demonstrate that one can drive the spin system into a
flat-band scenario by applying an appropriate electric field, thus overcoming
the restriction of fine-tuned exchange couplings $J_1$ and $J_2$ and allowing
one to tune more materials towards flat-band physics, that is to show a
macroscopic magnetization jump when crossing the magnetic saturation field, a
residual entropy at zero temperature as well as an enhanced magnetocaloric
effect. While the magnetic field acts on the spin system via the ordinary
Zeeman term, the coupling of an applied electric field with the spins is given
by the sophisticated Katsura-Nagaosa-Balatsky (KNB) mechanism, where the
electric field effectively acts as a Dzyaloshinskii-Moriya spin-spin
interaction. The resulting novel features are corresponding reciprocal effects:
We find a magnetization jump driven by the electric field as well as a jump of
the electric polarization driven by the magnetic field, i.e.\ the system
exhibits an extraordinarily strong magnetoelectric effect. Moreover, in analogy
to the enhanced magnetocaloric effect the system shows an enhanced
electrocaloric effect.",2107.04371v2
2021-07-16,Large Unidirectional Magnetoresistance in Metallic Heterostructures in the Spin Transfer Torque Regime,"A large unidirectional magnetoresistance (UMR) ratio of UMR/$R_{xx}\sim$
$0.36\%$ is found in W/CoFeB metallic bilayer heterostructures at room
temperature. Three different regimes in terms of the current dependence of UMR
ratio are identified: A spin-dependent-scattering mechanism regime at small
current densities $J \sim$ $10$$^{9}$A/m$^{2}$ (UMR ratio $\propto$ $J$), a
spin-magnon-interaction mechanism regime at intermediate $J \sim$
$10$$^{10}$A/m$^{2}$ (UMR ratio $\propto$ $J$$^{3}$), and a spin-transfer
torque (STT) regime at $J \sim$ $10$$^{11}$A/m$^{2}$ (UMR ratio independent of
$J$). We verify the direct correlation between this large UMR and the transfer
of spin angular momentum from the W layer to the CoFeB layer by both
field-dependent and current-dependent UMR characterizations. Numerical
simulations further confirm that the large STT-UMR stems from the tilting of
the magnetization affected by the spin Hall effect-induced spin-transfer
torques. An alternative approach to estimate damping-like spin-torque
efficiencies from magnetic heterostructures is also proposed.",2107.07780v1
2021-07-19,MoP$_3$SiO$_{11}$: a $4d^3$ honeycomb antiferromagnet with disconnected octahedra,"We report the crystal structure and magnetic behavior of the $4d^3$
spin-$\frac32$ silicophosphate MoP$_3$SiO$_{11}$ studied by high-resolution
synchrotron x-ray diffraction, neutron diffraction, thermodynamic measurements,
and ab initio band-structure calculations. Our data revise the crystallographic
symmetry of this compound and establish its rhombohedral space group ($R\bar
3c$) along with the geometrically perfect honeycomb lattice of the Mo$^{3+}$
ions residing in disconnected MoO$_6$ octahedra. Long-range antiferromagnetic
order with the propagation vector $\mathbf k=0$ observed below $T_N=6.8$ K is a
combined effect of the nearest-neighbor in-plane exchange coupling $J\simeq
2.6$ K, easy-plane single-ion anisotropy $D\simeq 2.2 $ K, and a weak
interlayer coupling $J_c\simeq 0.8$ K. The 12% reduction in the ordered
magnetic moment of the Mo$^{3+}$ ions and the magnon gap of $\Delta\simeq 7$ K
induced by the single-ion anisotropy further illustrate the impact of
spin-orbit coupling on the magnetism. Our analysis puts forward single-ion
anisotropy as an important ingredient of $4d^3$ honeycomb antiferromagnets
despite their nominally quenched orbital moment.",2107.08618v2
2021-07-19,Reconfigurable Training and Reservoir Computing in an Artificial Spin-Vortex Ice via Spin-Wave Fingerprinting,"Strongly-interacting artificial spin systems are moving beyond mimicking
naturally-occurring materials to emerge as versatile functional platforms, from
reconfigurable magnonics to neuromorphic computing. Typically artificial spin
systems comprise nanomagnets with a single magnetisation texture: collinear
macrospins or chiral vortices. By tuning nanoarray dimensions we achieve
macrospin/vortex bistability and demonstrate a four-state metamaterial
spin-system 'Artificial Spin-Vortex Ice' (ASVI). ASVI can host Ising-like
macrospins with strong ice-like vertex interactions, and weakly-coupled
vortices with low stray dipolar-field. Vortices and macrospins exhibit
starkly-differing spin-wave spectra with analogue-style mode-amplitude control
and mode-frequency shifts of df = 3.8 GHz.
  The enhanced bi-textural microstate space gives rise to emergent physical
memory phenomena, with ratchet-like vortex training and history-dependent
nonlinear fading memory when driven through global field cycles. We employ
spin-wave microstate fingerprinting for rapid, scaleable readout of vortex and
macrospin populations and leverage this for spin-wave reservoir computation.
ASVI performs linear and non-linear mapping transformations of diverse input
signals as well as chaotic time-series forecasting. Energy costs of machine
learning are spiralling unsustainably, developing low-energy neuromorphic
computation hardware such as ASVI is crucial to achieving a zero-carbon
computational future.",2107.08941v3
2021-08-03,Possibility to detect the bound state of the Heisenberg ferromagnetic chain at intermediate temperature,"Motivated by the lack of direct evidence with inelastic neutron scattering of
the well documented bound state of Heisenberg ferromagnets, we use the
time-dependent Thermal Density Matrix Renormalization Group algorithm to study
the temperature dependence of the dynamical spin structure factor of Heisenberg
ferromagnetic spin chains. For spin-1/2, we show that the bound state appears
as a well defined excitation with significant spectral weight in the
temperature range $J/12 \lesssim T \lesssim J/3$, pointing to the possibility
of detecting it with inelastic neutron scattering near $k=\pi$ provided the
temperature is neither too low nor too high - at low temperature, the spectral
weight only grows as $T^{3/2}$, and at high temperature the bound state peak
merges with the two-magnon continuum. For spin-1, the situation is more subtle
because the bound state with two neighboring spin flips competes with an
anti-bound state with two spin-flips on the same site. As a consequence, the
relative spectral weight of the bound state is smaller than for spin-1/2, and a
weak resonance due to the anti-bound state appears in the continuum. A clearer
signature of the bound state (resp. anti-bound state) can be obtained if a
negative (resp. positive) biquadratic interaction is present.",2108.01401v3
2021-08-11,Instabilities of spin-1 Kitaev spin liquid phase in presence of single-ion anisotropies,"We study the spin-one Kitaev model on the honeycomb lattice in the presence
of single-ion anisotropies. We consider two types of single ion anisotropies: A
$D_{111}$ anisotropy which preserves the symmetry between $X$, $Y$, and $Z$
bonds but violates flux conservation and a $D_{100}$ anisotropy that breaks the
symmetry between $X$, $Y$, and $Z$ bonds but preserves flux conservation. We
use series expansion methods, degenerate perturbation theory, and exact
diagonalization to study these systems. Large positive $D_{111}$ anisotropy
leads to a simple product ground state with conventional magnon-like
excitations, while large negative $D_{111}$ leads to a broken symmetry and
degenerate ground states. For both signs there is a phase transition at a small
$|D_{111}| \approx 0.12$ separating the more conventional phases from the
Kitaev spin liquid phase. With large $D_{100}$ anisotropy, the ground state is
a simple product state, but the model lacks conventional dispersive excitations
due to the large number of conservation laws. Large negative $D_{100}$ leads to
decoupled one-dimensional systems and many degenerate ground states. No
evidence of a phase transition is seen in our numerical studies at any finite
$D_{100}$. Convergence of the series expansion extrapolations all the way to
$D_{100}=0$ suggests that the nontrivial Kitaev spin liquid is a singular limit
of this type of single-ion anisotropy going to zero, which also restores
symmetry between the $X$, $Y$, and $Z$ bonds.",2108.05040v2
2021-08-18,Dispersionless propagation of ultra-short spin-wave pulses in ultrathin yttrium iron garnet waveguides,"We study experimentally the propagation of nanosecond spin-wave pulses in
microscopic waveguides made of nanometer-thick yttrium iron garnet films. For
these studies, we use micro-focus Brillouin light scattering spectroscopy,
which provides the possibility to observe propagation of the pulses with high
spatial and temporal resolution. We show that, for most spin-wave frequencies,
dispersion leads to broadening of the pulse by several times at propagation
distances of 10 micrometers. However, for certain frequency interval, the
dispersion broadening is suppressed almost completely resulting in a
dispersionless pulse propagation. We show that the formation of the
dispersion-free region is caused by the competing effects of the dipolar and
the exchange interaction, which can be controlled by the variation of the
waveguide geometry. These conclusions are supported by micromagnetic
simulations and analytical calculations. Our findings provide a simple solution
for the implementation of high-speed magnonic systems that require undisturbed
propagation of short information-carrying spin-wave pulses.",2108.08054v1
2021-09-08,Room-Temperature Intrinsic and Extrinsic Damping in Polycrystalline Fe Thin Films,"We examine room-temperature magnetic relaxation in polycrystalline Fe films.
Out-of-plane ferromagnetic resonance (FMR) measurements reveal Gilbert damping
parameters of $\approx$ 0.0024 for Fe films with thicknesses of 4-25 nm,
regardless of their microstructural properties. The remarkable invariance with
film microstructure strongly suggests that intrinsic Gilbert damping in
polycrystalline metals at room temperature is a local property of nanoscale
crystal grains, with limited impact from grain boundaries and film roughness.
By contrast, the in-plane FMR linewidths of the Fe films exhibit distinct
nonlinear frequency dependences, indicating the presence of strong extrinsic
damping. To fit our in-plane FMR data, we have used a grain-to-grain two-magnon
scattering model with two types of correlation functions aimed at describing
the spatial distribution of inhomogeneities in the film. However, neither of
the two correlation functions is able to reproduce the experimental data
quantitatively with physically reasonable parameters. Our findings advance the
fundamental understanding of intrinsic Gilbert damping in structurally
disordered films, while demonstrating the need for a deeper examination of how
microstructural disorder governs extrinsic damping.",2109.03684v2
2021-09-08,Schwinger boson theory of ordered magnets,"The Schwinger boson theory provides a natural path for treating quantum spin
systems with large quantum fluctuations. In contrast to semi-classical
treatments, this theory allows us to describe a continuous transition between
magnetically ordered and spin liquid states, as well as the continuous
evolution of the corresponding excitation spectrum. The square lattice
Heisenberg antiferromagnet is one of the first models that was approached with
the Schwinger boson theory. Here we revisit this problem to reveal several
subtle points that were omitted in previous treatments and that are crucial to
further develop this formalism. These points include the freedom for the choice
of the saddle point (Hubbard-Stratonovich decoupling and choice of the
condensate) and the $1/N$ expansion in the presence of a condensate. A key
observation is that the spinon condensate leads to Feynman diagrams that
include contributions of different order in $1/N$, which must be accounted to
get a qualitatively correct excitation spectrum. We demonstrate that a proper
treatment of these contributions leads to an exact cancellation of the
single-spinon poles of the dynamical spin structure factor, as expected for a
magnetically ordered state. The only surviving poles are the ones arising from
the magnons (two-spinon bound states), which are the true collective modes of
an ordered magnet.",2109.03964v2
2021-09-15,Spin-Current Instability at a Magnetic Domain Wall in a Ferromagnetic Superfluid: a Generation Mechanism of Eccentric Fractional Skyrmions,"Spinful superfluids of ultracold atoms are ideal for investigating the
intrinsic properties of spin current and texture because they are realized in
an isolated, nondissipative system free from impurities, dislocations, and
thermal fluctuations. This study theoretically reveals the impact of spin
current on a magnetic domain wall in spinful superfluids. An exact wall
solution is obtained in the ferromagnetic phase of a spin-1 Bose--Einstein
condensate with easy-axis anisotropy at zero temperature. The
bosonic-quasiparticle mechanics analytically show that the spin current along
the wall becomes unstable if the velocity exceeds the critical spin-current
velocities, leading to complicated situations because of the competition
between transverse magnons and ripplons. Our direct numerical simulation
reveals that this system has a mechanism to generate an eccentric fractional
skyrmion, which has a fractional topological charge, but its texture is not
similar to that of a meron. This mechanism is in contrast to the generation of
conventional skyrmions in easy-axis magnets. The theoretical findings can be
examined in the same situation as in a recent experiment on ultracold atoms. In
terms of the universality of spontaneous symmetry breaking, unexplored similar
phenomena are expected in different physical systems with the same broken
symmetry.",2109.07144v2
2021-09-16,Thermoelectric films and periodic structures and spin Seebeck effect systems: Facets of performance optimization,"The growing market for sensors, internet of things, and wearable devices is
fueling the development of low-cost energy-harvesting materials and systems.
Film based thermoelectric (TE) devices offer the ability to address the energy
requirements by using ubiquitously available waste-heat. This review narrates
recent advancements in fabricating high-performance TE films and superlattice
structures, from the aspects of microstructure control, doping, defects,
composition, surface roughness, substrate effect, interface control,
nanocompositing, and crystal preferred orientation realized by regulating
various deposition parameters and subsequent heat treatment. The review begins
with a brief account of heat conduction mechanism, quantum confinement effect
in periodic layers, film deposition processes, thin film configurations and
design consideration for TE in-plane devices, and characterization techniques.
It then proceeds to alayzing the latest findingd on the TE properties of
Bi2(Te,Se)3 and (Bi,Sb)2Te3, PbTe, GeTe, SnSe, SnTe, Cu2-xSe, and skutterudite
films, including superlattices and the performance of TE generators, sensors,
and cooling devices. Thickness dependent microstructure evolution and TE
characteristics of films in relation to temperature are also analyzed. In the
context of spin Seebeck effect (SSE) based systems, SSE mechanism analysis,
developments in enhancing the spin Seebeck signal since its first observation,
and recent developments are covered from the facets of new system design,
signal collection, magnetic manipulation, interface conditions,
thickness-dependent longitudinal spin Seebeck signal, and length scale of
phonon and magnon transport in longitudinal SSE (LSSE) in different bi-layer
systems. At the end, possible strategies for further enhancing zT of TE films
and spin Seebeck signals of many systems are addressed.",2109.08179v1
2021-09-17,Towards Exotic Layered Materials: 2D Cuprous Iodide,"Heterostructures composed of two-dimensional (2D) materials are already
opening many new possibilities in such fields of technology as electronics and
magnonics, but far more could be achieved if the number and diversity of 2D
materials is increased. So far, only a few dozen 2D crystals have been
extracted from materials that exhibit a layered phase in ambient conditions,
omitting entirely the large number of layered materials that may exist in other
temperatures and pressures. Here, we demonstrate how these structures can be
stabilized in 2D van der Waals stacks under room temperature via growing them
directly in graphene encapsulation by using graphene oxide as the template
material. Specifically, we produce an ambient stable 2D structure of copper and
iodine, a material that normally only occurs in layered form at elevated
temperatures between 645 and 675 K. Our results establish a route to the
production of more exotic phases of materials that would otherwise be difficult
or impossible to stabilize for experiments in ambient.",2109.08456v2
2021-09-17,Visualizing band selective enhancement of quasiparticle lifetime in a metallic ferromagnet,"Electrons navigate more easily in a background of ordered magnetic moments
than around randomly oriented ones. This fundamental quantum mechanical
principle is due to their Bloch wave nature and also underlies ballistic
electronic motion in a perfect crystal. As a result, a paramagnetic metal that
develops ferromagnetic order often experiences a sharp drop in the resistivity.
Despite the universality of this phenomenon, a direct observation of the impact
of ferromagnetic order on the electronic quasiparticles in a magnetic metal is
still lacking. Here we demonstrate that quasiparticles experience a significant
enhancement of their lifetime in the ferromagnetic state of the low-density
magnetic semimetal EuCd2As2, but this occurs only in selected bands and
specific energy ranges. This is a direct consequence of the magnetically
induced band splitting and the multi-orbital nature of the material. Our
detailed study allows to disentangle different electronic scattering mechanisms
due to non-magnetic disorder and magnon exchange. Such high momentum and energy
dependence quasiparticle lifetime enhancement can lead to spin selective
transport and potential spintronic applications.",2109.08538v1
2021-09-22,Bulk and edge dynamics of a 2D Affleck-Kennedy-Lieb-Tasaki model,"We study the dynamical properties of both bulk and edge spins of a
two-dimensional Affleck-Kennedy-Lieb-Tasaki (AKLT) model mainly by using the
stochastic series expansion quantum Monte Carlo method with stochastic analytic
continuation. In the deep AKLT phase, we obtain a spin spectrum with flat band,
which is a strong evidence for a localized state. Through the spectrum
analysis, we see a clear continuous phase transition from the AKLT phase to the
N\'eel phase in the model, and the energy gap becomes closed at the
corresponding momentum point. In comparison with linear spin-wave theory, the
differences show that there are strong interactions among magnons at high
energies. With open boundary condition, the gap of edge spins in the AKLT phase
closes at both the $\Gamma$ point and the $\pi$ point interestingly to emerge
into a flat-band-like Luttinger liquid phase, which can be explained by
symmetry and perturbation approximation. This paper helps us to better
understand the completely different dynamical behaviors of bulk and edge spins
in the symmetry protected topological phase.",2109.10901v2
2021-09-22,Optically- and thermally-driven huge lattice orbital and spin angular momenta from spinning fullerenes,"Lattice vibration in solids may carry angular momentum. But unlike the
intrinsic spin of electrons, the lattice vibration is rarely rotational. To
induce angular momentum, one needs to find a material that can accommodate a
twisted normal mode, two orthogonal modes or excitation of magnons. If
excitation is too strong, one may exceed the Lindemann limit, so the material
melts. Therefore these methods are not ideal. Here, we theoretically propose a
new route to phonon angular momentum in a molecular crystal $\rm C_{60}$. We
find that a single laser pulse is able to inject a significant amount of
angular momentum to $\rm C_{60}$, and the momentum transfer is
helicity-dependent. Changing from right-circularly polarized light to
left-circularly polarized light switches the direction of phonon angular
momentum. On the ultrafast time scale, the orbital angular momentum change
closely resembles the displacive excitation of coherent phonons, with a
cosine-function dependence on time, different from the spin counterpart. Atomic
displacements, even under strong laser excitation, remain far below the
Lindemann criterion. Under thermal excitation, spinning $\rm C_{60}$ even at
room temperature generates a huge angular momentum close to several hundred
$\hbar$. Our finding opens the door to a large group of fullerenes, from $\rm
C_{60}$, C$_{70}$ to their endohedral derivatives, where angular momentum can
be generated through light or temperature. This paves the way to the phononic
control electronic spin and harvesting thermal energy through phonon angular
momentum.",2109.11055v1
2021-10-01,Spin liquid in twisted homobilayers group-VI gichalcogenides,"Twisted transition metal dichalcogenide (TMD) homobilayers have recently
emerged as a powerful platform for studying correlated insulating states. In
the strongly correlated limit, we construct an effective spin Hamiltonian on a
honeycomb lattice that includes the Heisenberg interaction and nonsymmetric
interactions such as a Dzyaloshinskii-Moriya interaction and a Kane-Mele
coupling for the Mott-insulating phase at half-filling. For the twisted TMD
homobilayers, the spin-orbit coupling in the Hubbard model, which is expected
to induce the antisymmetric exchange couplings in the effective spin
Hamiltonian, is a highly tunable and experimentally accessible quantity that
can be tuned by an applied electric field. In this study, we investigate
classical and quantum phase diagrams of the effective spin Hamiltonian using
analytical and numerical methods. We show that the model exhibits a rich
classical phase diagram including an antiferromagnetic (AFM) phase, a planar
spiral ordered phase with high classical degeneracy, a $z$-AFM phase, a
noncoplanar phase, a noncollinear phase, and a 120$^{\circ}$-AFM phase. In the
quantum treatment, we calculate low-energy magnon excitation spectrum, ground
state energy, and static spin structure factor using linear spin-wave theory
and density matrix renormalization group methods to compose the quantum phase
diagram of the effective spin Hamiltonian. Beyond the Heisenberg interaction,
we find that the existence of these antisymmetric couplings is responsible for
the quantum spin liquid, $z$-AFM, noncoplanar, and 120$^{\circ}$ phases.
Twisted TMD homobilayers, therefore, offer rich platforms for realizing rich
phases of matter such as quantum spin liquid, noncoplanar, and 120$^{\circ}$,
resulting from the spin-orbit coupling.",2110.00536v1
2021-10-19,"Duality,Hidden Symmetry and Dynamic Isomerism in 2D Hinge Structures","Recently, a new type of duality was reported in some deformable mechanical
networks which exhibit Kramers-like degeneracy in phononic spectrum at the
self-dual point. In this work, we clarify the origin of this duality and
propose a design principle of 2D self-dual structures with arbitrary
complexity. We find that this duality originates from the (PCI) symmetry of the
hinge, which belongs to a more general end-fixed scaling transformation. This
symmetry gives the structure an extra degree of freedom without modifying its
dynamics. This results in , i.e., dissimilar 2D mechanical structures, either
periodic or aperiodic, having identical dynamic modes, based on which we
demonstrate a new type of wave-guide without reflection or loss. Moreover, the
PCI symmetry allows us to design various 2D periodic isostatic networks with
hinge duality. At last, by further studying a 2D non-mechanical magnonic
system, we show that the duality and the associated hidden symmetry should
exist in a broad range of Hamiltonian systems.",2110.09999v3
2021-11-01,Spectrum evolution and chirping of laser-induced spin wave packets in thin iron films,"We present the experimental study of ultrafast optical excitation of
magnetostatic surface spin wave (MSSW) packets and their spectral properties in
thin films of pure iron. As the packets leave the excitation area and propagate
in space, their spectra evolve non-trivially. Particularly, low or high
frequency components are suppressed at the border of the excitation area
depending on the orientation of the external magnetic field with respect to the
magnetocrystolline anisotropy axes of the film. The effect is ascribed to the
ultrafast local heating of the film. Further, the time resolution of the
implemented all-optical technique allows us to extract the chirp of the MSSW
packet in the time domain via wavelet analysis. The chirp is a result of the
group velocity dispersion of the MSSW and, thus, is controlled by the film
magnetic parameters, magnetization and anisotropy, and external field
orientation. The demonstrated tunable modulation of MSSW wave packets with
femtosecond laser pulses may find application in future magnonic-photonic
hybrid devices for wave-based data processing.",2111.00983v2
2021-11-02,Ultrafast coherent THz lattice dynamics coupled to spins in a van der Waals antiferromagnetic flake,"A coherent THz optical lattice mode is triggered by femtosecond laser pulses
in the antiferromagnetic van der Waals semiconductor FePS$_3$. The 380 nm thick
exfoliated flake was placed on a substrate and laser-driven lattice and spin
dynamics were investigated as a function of the excitation photon energy and
sample temperature. The pump-probe spectroscopic measurements reveal that the
photo-induced phonon is generated by a displacive mechanism. The amplitude of
the phononic signal decreases as the sample is heated up to the N\'eel
temperature and vanishes as the phase transition to the paramagnetic phase
occurs. This evidence confirms that the excited lattice mode is intimately
connected to the long-range magnetic order. Therefore our work discloses a
pathway towards a femtosecond coherent manipulation of the magneto-crystalline
anisotropy in a van der Waals antiferromagnet. In fact, it is reported that by
applying a magnetic field the induced phonon mode hybridizes via the
Kittel-mechanism with zone-centre magnons.",2111.01464v1
2021-11-05,Quadrupolar magnetic excitations in an isotropic spin-1 antiferromagnet,"The microscopic origins of emergent behaviours in condensed matter systems
are encoded in their excitations. In ordinary magnetic materials, single
spin-flips give rise to collective dipolar magnetic excitations called magnons.
Likewise, multiple spin-flips can give rise to multipolar magnetic excitations
in magnetic materials with spin $\mathbf{S} \boldsymbol{\ge} \mathbf{1}$.
Unfortunately, since most experimental probes are governed by dipolar selection
rules, collective multipolar excitations have generally remained elusive. For
instance, only dipolar magnetic excitations have been observed in isotropic
$\mathbf{S}\boldsymbol{=}\mathbf{1}$ Haldane spin systems. Here, we unveil a
hidden quadrupolar constituent of the spin dynamics in antiferromagnetic
$\mathbf{S}\boldsymbol{=}\mathbf{1}$ Haldane chain material
Y$_\mathbf{2}$BaNiO$_\mathbf{5}$ using Ni $\mathbf{L_3}$-edge resonant
inelastic x-ray scattering. Our results demonstrate that pure quadrupolar
magnetic excitations can be probed without direct interactions with dipolar
excitations or anisotropic perturbations. Originating from on-site double
spin-flip processes, the quadrupolar magnetic excitations in
Y$_\mathbf{2}$BaNiO$_\mathbf{5}$ show a remarkable dual nature of collective
dispersion. While one component propagates as non-interacting entities, the
other behaves as a bound quadrupolar magnetic wave. This result highlights the
rich and largely unexplored physics of higher-order magnetic excitations.",2111.03625v1
2021-11-05,Ultrafast optical excitation of magnetic dynamics in van der Waals magnets: Coherent magnons and BKT dynamics in NiPS$_3$,"Optical pump-probe experiments carried out in the time domain reveal both the
intrinsic low energy dynamics and its connections to higher energy excitations
in correlated electron systems. In this work, we propose two microscopic
mechanisms for the optical generation of coherent magnetic modes in van der
Waals magnets, and derive the corresponding effective light-spin interactions:
either through pumping atomic orbital excitations resonantly or via a
light-induced Floquet spin Hamiltonian, the ground state of the system is
driven out of equilibrium. The subsequent long-time relaxational dynamics can
then be probed using, e.g. the magneto-optical Kerr effect or transient grating
spectroscopy. As an example, we apply our framework to NiPS$_3$, which is
magnetically ordered in the bulk, and is conjectured to realize the XY model in
the monolayer limit. Our theory makes explicit how the material's low-energy
response depends sensitively on the microscopic details of the light-spin
coupling as well as pump fluence, frequency and polarization. For the case of
bulk NiPS$_3$, we find quantitative agreement with recent experiments by
Afanasiev et al. [Science Advances 7, abf3096 (2021)]. We further propose
pump-probe experiments for monolayer NiPS$_3$ and detail how anomalous
relaxational behaviour may reveal excitations of a (proximate) BKT phase in a
proposed effective XY model.",2111.03674v2
2021-11-08,Magnetic properties of a capped kagome molecule with 60 quantum spins,"We compute ground-state properties of the isotropic, antiferromagnetic
Heisenberg model on the sodalite cage geometry. This is a 60-spin spherical
molecule with 24 vertex-sharing tetrahedra which can be regarded as a molecular
analogue of a capped kagome lattice and which has been synthesized with
high-spin rare-earth atoms. Here, we focus on the $S=1/2$ case where quantum
effects are strongest. We employ the SU(2)-symmetric density-matrix
renormalization group (DMRG).
  We find a threefold degenerate ground state that breaks the spatial symmetry
and that splits up the molecule into three large parts which are almost
decoupled from each other. This stands in sharp contrast to the behaviour of
most known spherical molecules. On a methodological level, the disconnection
leads to ""glassy dynamics"" within the DMRG that cannot be targeted via standard
techniques.
  In the presence of finite magnetic fields, we find broad magnetization
plateaus at 4/5, 3/5, and 1/5 of the saturation, which one can understand in
terms of localized magnons, singlets, and doublets which are again nearly
decoupled from each other. At the saturation field, the zero-point entropy is
$S=\ln(182)\approx 5.2$ in units of the Boltzmann constant.",2111.04549v4
2021-11-09,Spin-lattice couplings in two-dimensional CrI$_3$ from first-principles study,"Since thermal fluctuations become more important as dimensions shrink, it is
expected that low-dimensional magnets are more sensitive to lattice distortions
and phonons than bulk systems are. Here we present a fully relativistic
first-principles study on the spin-lattice coupling, i.e. how the magnetic
interactions depend on local lattice distortions, of the prototypical
two-dimensional ferromagnet CrI$_3$. We extract an effective measure of the
spin-lattice coupling in CrI$_3$ which is up to ten times larger than what is
found for bcc Fe. The magnetic exchange interactions, including Heisenberg and
relativistic Dzyaloshinskii-Moriya interactions, are sensitive both to the
in-plane motion of Cr atoms and out-of-plane motion of ligand atoms. We find
that significant magnetic pair interactions change sign from ferromagnetic (FM)
to anti-ferromagnetic (AFM) for atomic displacements larger than 0.16 {\AA}. We
explain the observed strong spin-lattice coupling by analyzing the orbital
decomposition of isotropic exchange interactions, involving different
crystal-field-split Cr$-3d$ orbitals. The competition between the AFM t$_{2g}$
- t$_{2g}$ and FM t$_{2g}$ - e$_{g}$ contributions depends on the bond angle
formed by Cr and I atoms as well as Cr-Cr distance. In particular, if a Cr atom
is displaced, the FM-AFM sign change when the I-Cr-I bond angle approaches
90$^\circ$. The obtained spin-lattice coupling constants, along with the
microscopic orbital analysis can act as a guiding principle for further studies
of the thermodynamic properties and combined magnon-phonon excitations in
two-dimensional magnets.",2111.05382v1
2021-11-15,Relating spin-polarized STM imaging and inelastic neutron scattering in the van-der-Waals ferromagnet Fe3GeTe2,"Van-der-Waals (vdW) ferromagnets have enabled the development of
heterostructures assembled from exfoliated monolayers with spintronics
functionalities, making it important to understand and ultimately tune their
magnetic properties at the microscopic level. Information about the magnetic
properties of these systems comes so far largely from macroscopic techniques,
with little being known about the microscopic magnetic properties. Here, we
combine spin-polarized scanning tunneling microscopy and quasi-particle
interference imaging with neutron scattering to establish the magnetic and
electronic properties of the metallic vdW ferromagnet Fe3GeTe2. By imaging
domain walls at the atomic scale, we can relate the domain wall width to the
exchange interaction and magnetic anisotropy extracted from the magnon
dispersion as measured in inelastic neutron scattering, with excellent
agreement between the two techniques. From comparison with Density Functional
Theory calculations we can assign the quasi-particle interference to be
dominated by spin-majority bands. We find a dimensional dichotomy of the bands
at the Fermi energy: bands of minority character are predominantly
two-dimensional in character, whereas the bands of majority character are
three-dimensional. We expect that this will enable new design principles for
spintronics devices.",2111.08013v1
2021-11-29,Magnetic anisotropy and magnetic ordering of transition-metal phosphorus trisulfides,"Here, a magnetic model with an unprecedentedly large number of parameters was
determined from first-principles calculations for transition-metal phosphorus
trisulfides (TMPS$_3$'s), which reproduced the measured magnetic ground states
of bulk TMPS$_3$'s. Our Monte Carlo simulations for the critical temperature,
magnetic susceptibility, and specific heat of bulk and few-layer TMPS$_3$'s
agree well with available experimental data and show that the antiferromagnetic
order of FePS$_3$ and NiPS$_3$ persists down to monolayers. Remarkably, the
orbital polarization, which was neglected in recent first-principles studies,
dramatically enhances the magnetic anisotropy of FePS$_3$ by almost two orders
of magnitude. A recent Raman study [K. Kim et al., Nat. Commun. 10, 345 (2019)]
claimed that magnetic ordering is absent in monolayer NiPS$_3$ but
simultaneously reported a strong two-magnon continuum; we show that the
criterion used to judge magnetic ordering there is invalid in monolayer
NiPS$_3$, thus providing an understanding of the two seemingly contradictory
experimental results. The rich predictions on the magnetic susceptibility and
specific heat of few-layer FePS$_3$ and NiPS$_3$ await immediate experimental
verifications.",2111.14869v1
2021-11-30,Comparison of spin-wave transmission in parallel and antiparallel magnetic configurations,"Parallel (P) and antiparallel (AP) configurations are widely applied in
magnetic heterostructures and have significant impacts on the spin-wave
transmission in magnonic devices. In the present study, a theoretical
investigation was conducted into the transmission of exchange-dominated spin
waves with nanoscale wavelengths in a type of heterostructure including two
magnetic media, of which the magnetization state can be set to the P (AP)
configuration by ferromagnetic (antiferromagnetic) interfacial exchange
coupling (IEC). The boundary conditions in P and AP cases were derived, by
which the transmission and reflection coefficients of spin waves were
analytically given and numerically calculated. In the P configuration, a
critical angle $\theta_{\textrm{c}}$ always exists and has a significant
influence on the transmission. Spin waves are refracted and reflected when the
incident angle $\theta_{\textrm{i}}$ is smaller than the critical angle
($\theta_{\textrm{i}} < \theta_{\textrm{c}}$), while total reflection occurs as
$\theta_{\textrm{i}} \geq \theta_{\textrm{c}}$. In the AP configuration, the
spin-wave polarizations of medium 1 and 2 are inverse, that is, right-handed
(RH) and left-handed (LH), leading to the total reflection being independent of
$\theta_{\textrm{i}}$. As demonstrated by the difference in spin-wave
transmission properties between the P ($\theta_{\textrm{i}} <
\theta_{\textrm{c}}$) and AP cases, there is a polarization-dependent
scattering. However, as $\theta_{\textrm{i}}$ exceeds $\theta_{\textrm{c}}$,
the P ($\theta_{\textrm{i}} > \theta_{\textrm{c}}$) case exhibits similarities
with the AP case, where the transmitted waves are found to be evanescent in
medium 2 and their decay lengths are investigated.",2111.15091v2
2021-12-01,Low power continuous-wave all-optical magnetic switching in ferromagnetic nanoarrays,"All-optical magnetic switching promises ultrafast, high-resolution
magnetisation control with the technological attraction of requiring no
magnetic field. Existing all-optical switching schemes are driven by ultrafast
transient effects, typically requiring power-hungry femtosecond-pulsed lasers
and complex magnetic materials. Here, we demonstrate deterministic, all-optical
magnetic switching in simple ferromagnetic nanomagnets (Ni$_{81}$Fe$_{19}$,
Ni$_{50}$Fe$_{50}$) with sub-diffraction limit dimensions using a focused
low-power, linearly-polarised continuous-wave laser. Isolated nanomagnets are
switched across a range of dimensions, laser wavelengths and powers. All
square-geometry artificial spin ice vertex configurations are written,
including ground-state and energetically-unfavourable `monopole-like' states at
powers as low as 2.74 mW. Usually, magnetic switching with linearly polarised
light is symmetry-forbidden; however, here the laser spot has a similar size to
the nanomagnets, producing an absorption distribution dependent on the relative
nanoisland-spot displacement. We attribute the observed deterministic switching
to the transient dynamics of this asymmetric absorption. No switching is
observed in Co samples, suggesting the multi-species nature of NiFe alloys
plays a role in reversal. The results presented here usher in cheap, low-power
optically-controlled devices with impact across data storage, neuromorphic
computation and reconfigurable magnonics.",2112.00697v2
2021-12-06,Complex magnetic structure and spin waves of the noncollinear antiferromagnet Mn5Si3,"The investigations of the interconnection between micro- and macroscopic
properties of materials hosting noncollinear antiferromagnetic ground states
are challenging. These forefront studies are crucial for unraveling the
underlying mechanisms at play, which may prove beneficial in designing cutting
edge multifunctional materials for future applications. In this context, Mn5Si3
has regained scientific interest since it displays an unusual and complex
ground state, which is considered to be the origin of the anomalous transport
and thermodynamic properties that it exhibits. Here, we report the magnetic
exchange couplings of the noncollinear antiferromagnetic phase of Mn5Si3 using
inelastic neutron scattering measurements and density functional theory
calculations. We determine the ground-state spin configuration and compute its
magnon dispersion relations which are in good agreement with the ones obtained
experimentally. Furthermore, we investigate the evolution of the spin texture
under the application of an external magnetic field to demonstrate
theoretically the multiple field-induced phase transitions that Mn5Si3
undergoes. Finally, we model the stability of some of the material's magnetic
moments under a magnetic field and we find that very susceptible magnetic
moments in a frustrated arrangement can be tuned by the field.",2112.03368v1
2021-12-14,Enhanced valley polarization in WS$_2$/LaMnO$_3$ heterostructure,"Monolayer transition metal dichalcogenides have attracted great attentions
for potential applications in valleytronics. However, the valley polarization
degree is usually not high because of the intervalley scattering. Here, we
demonstrate a largely enhanced valley polarization up to 80\% in monolayer
WS$_2$ under non-resonant excitation at 4.2 K using WS$_2$/LaMnO$_3$ thin film
heterostructure, which is much higher than that for monolayer WS$_2$ on
SiO$_2$/Si substrate with a valley polarization of 15\%. Furthermore, the
greatly enhanced valley polarization can be maintained to a high temperature of
about 160 K with a valley polarization of 53\%. The temperature dependence of
valley polarization is strongly correlated with the thermomagnetic curve of
LaMnO$_3$, indicating an exciton-magnon coupling between WS$_2$ and LaMnO$_3$.
A simple model is introduced to illustrate the underlying mechanisms. The
coupling of WS$_2$ and LaMnO$_3$ is further confirmed with an observation of
two interlayer excitons with opposite valley polarizations in the
heterostructure, resulting from the spin-orbit coupling induced splitting of
the conduction bands in monolayer transition metal dichalcogenides. Our results
provide a pathway to control the valleytronic properties of transition metal
dichalcogenides by means of ferromagnetic van der Waals engineering, paving a
way to practical valleytronic applications.",2112.07321v1
2021-12-20,On the nature of valence charge and spin excitations via multi-orbital Hubbard models for infinite-layer nickelates,"Building upon the recent progress on the intriguing underlying physics for
the newly discovered infinite-layer nickelates, in this article we review an
examination of valence charge and spin excitations via multi-orbital Hubbard
models as way to determine the fundamental building blocks for Hamiltonians
that can describe the low energy properties of infinite-layer nickelates. We
summarize key results from density-functional approaches, and apply them to the
study of x-ray absorption to determine the valence ground states of
infinite-layer nickelates in their parent form, and show that a fundamental
$d^9$ configuration as in the cuprates is incompatible with a self-doped ground
state having holes in both $d_{x^2-y^2}$ and a rare-earth-derived axial
orbital. When doped, we determine that the rare-earth-derived orbitals empty
and additional holes form low spin $(S=0)$ $d^8$ Ni states, which can be
well-described as a doped single-band Hubbard model. Using exact
diagonalization for a 2-orbital model involving Ni and rare earth orbitals, we
find clear magnons at 1/2 filling that persist when doped, albeit with larger
damping, and with a dependence on the precise orbital energy separation between
the Ni- and rare-earth-derived orbitals. Taken together, a full two-band model
for infinite-layer nickelates can well describe the valence charge and spin
excitations observed experimentally.",2112.10758v1
2021-12-21,Fast long-wavelength exchange spin waves in partially-compensated Ga:YIG,"Spin waves in yttrium iron garnet (YIG) nano-structures attract increasing
attention from the perspective of novel magnon-based data processing
applications. For short wavelengths needed in small-scale devices, the group
velocity is directly proportional to the spin-wave exchange stiffness constant
$\lambda_\mathrm{ex}$. Using wave vector resolved Brillouin Light Scattering
(BLS) spectroscopy, we directly measure $\lambda_\mathrm{ex}$ in Ga-substituted
YIG thin films and show that it is about three times larger than for pure YIG.
Consequently, the spin-wave group velocity overcomes the one in pure YIG for
wavenumbers $k > 4$ rad/$\mu$m, and the ratio between the velocities reaches a
constant value of around 3.4 for all $k > 20$ rad/$\mu$m. As revealed by
vibrating-sample magnetometry (VSM) and ferromagnetic resonance (FMR)
spectroscopy, Ga:YIG films with thicknesses down to 59 nm have a low Gilbert
damping ($\alpha < 10^{-3}$), a decreased saturation magnetization $\mu_0
M_\mathrm{S}~\approx~20~$mT and a pronounced out-of-plane uniaxial anisotropy
of about $\mu_0 H_{\textrm{u1}} \approx 95 $ mT which leads to an out-of-plane
easy axis. Thus, Ga:YIG opens access to fast and isotropic spin-wave transport
for all wavelengths in nano-scale systems independently of dipolar effects.",2112.11348v1
2021-12-25,Lattice and magnetic dynamics in YVO$_{3}$ Mott insulator studied by neutron scattering and first-principles calculations,"The Mott insulator YVO$_{3}$ with $T_{N}$ = 118 K is revisited to explore the
role of spin, lattice and orbital correlations across the multiple structural
and magnetic transitions observed as a function of temperature. Upon cooling,
the crystal structure changes from orthorhombic to monoclinic at 200 K, and
back to orthorhombic at 77 K, followed by magnetic transitions. From the
paramagnetic high temperature phase, C-type ordering is first observed at 118
K, followed by a G-type spin re-orientation transition at 77 K. The dynamics of
the transitions were investigated via inelastic neutron scattering and first
principles calculations. An overall good agreement between the neutron data and
calculated spectra was observed. From the magnon density of states, the
magnetic exchange constants were deduced to be $J_{ab}$ = $J_{c}$ = -5.8 meV in
the G-type spin phase, and $J_{ab}$ = -3.8 meV, $J_{c}$ = 7.6 meV at 80 K and
$J_{ab}$ = -3.0 meV, $J_{c}$ = 6.0 meV at 100 K in the C-type spin phase.
Paramagnetic scattering was observed in the spin ordered phases, well below the
C-type transition temperature, that continuously increased above the
transition. Fluctuations in the temperature dependence of the phonon density of
states were observed between 50 and 80 K as well, coinciding with the G-type to
C-type transition. These fluctuations are attributed to optical oxygen modes
above 40 meV, from first principles calculations. In contrast, little change in
the phonon spectra is observed across $T_{N}$.",2112.13172v1
2022-01-12,Asymmetric modulation of Majorana excitation spectra and nonreciprocal thermal transport in the Kitaev spin liquid under a staggered magnetic field,"We present the theoretical results for the Majorana band structure and
thermal transport properties in the Kitaev honeycomb model under a staggered
magnetic field in addition to a uniform one. The Majorana band structure is
asymmetrically modulated in momentum space, and the valley degree of freedom is
activated and controlled by the magnitudes and directions of the uniform and
staggered magnetic fields; as a result, the Majorana excitation is either
gapped or gapless, the latter of which in general has Majorana Fermi surfaces,
in contrast to the point nodes in the absence of the fields. We show that the
asymmetric deformation of the Majorana band induces a nonlinear (nonreciprocal)
thermal transport, in addition to the linear one. The field dependence of the
linear thermal current is correlated with the magnitude of the Majorana gap,
while that of the nonlinear thermal current is associated with the asymmetry of
the Majorana excitation spectra. We show the estimates of the thermal currents
and discuss that the linear response is experimentally measurable, whereas
future improvement is required for the observation of the nonlinear one. We
also discuss how the thermal currents are modulated by other additional effects
of the magnetic field and contributions from conventional magnons, latter of
which are expected in heterostructures to realize an internal staggered
magnetic field.",2201.04274v2
2022-01-28,Spin Excitation Spectra of Anisotropic Spin-$1/2$ Triangular Lattice Heisenberg Antiferromagnets,"Investigation of dynamical excitations is difficult but crucial to the
understanding of many exotic quantum phenomena discovered in quantum materials.
This is particularly true for highly frustrated quantum antiferromagnets whose
dynamical properties deviate strongly from theoretical predictions made based
on the spin-wave or other approximations. Here we present a large-scale
numerical calculation on the dynamical correlation functions of spin-$1/2$
triangular Heisenberg model using a state-of-the-art tensor network
renormalization group method. The calculated results allow us to gain for the
first time a comprehensive picture on the nature of spin excitation spectra in
this highly frustrated quantum system. It provides a quantitative account for
all the key features of the dynamical spectra disclosed by inelastic neutron
scattering measurements for $\rm Ba_3CoSb_2O_9$, revealing the importance of
the interplay between low- and high-energy excitations and its renormalization
effect to the low-energy magnon bands and high-energy continuums. We identify
the longitudinal Higgs modes in the intermediate-energy scale and predict the
energy and momentum dependence of spectral functions along the three principal
axes that can be verified by polarized neutron scattering experiments.
Furthermore, we find that the spin excitation spectra weakly depend on the
anisotropic ratio of the antiferromagnetic interaction.",2201.12121v2
2022-02-09,Nonreciprocal transport in a bilayer of MnBi2Te4 and Pt,"MnBi2Te4 (MBT) is the first intrinsic magnetic topological insulator with the
interaction of spin-momentum locked surface electrons and intrinsic magnetism,
and it exhibits novel magnetic and topological phenomena. Recent studies
suggested that the interaction of electrons and magnetism can be affected by
the Mn-doped Bi2Te3 phase at the surface due to inevitable structural defects.
Here we report an observation of nonreciprocal transport, i.e.
current-direction-dependent resistance, in a bilayer composed of
antiferromagnetic MBT and nonmagnetic Pt. The emergence of the nonreciprocal
response below the N\'eel temperature confirms a correlation between
nonreciprocity and intrinsic magnetism in the surface state of MBT. The angular
dependence of the nonreciprocal transport indicates that nonreciprocal response
originates from the asymmetry scattering of electrons at the surface of MBT
mediated by magnon. Our work provides an insight into nonreciprocity arising
from the correlation between magnetism and Dirac surface electrons in intrinsic
magnetic topological insulators.",2202.05398v1
2022-02-21,Thermal Conductivity and Theory of Inelastic Scattering of Phonons by Collective Fluctuations,"We study the intrinsic scattering of phonons by a general quantum degree of
freedom, i.e. a fluctuating ""field"" $Q$, which may have completely general
correlations, restricted only by unitarity and translational invariance. From
the induced scattering rates, we obtain the consequences on the thermal
conductivity tensor of the phonons. We find that the lowest-order diagonal
scattering rate, which determines the longitudinal conductivity, is controlled
by two-point correlation functions of the $Q$ field, while the off-diagonal
scattering rates involve a minimum of three to four point correlation
functions. We obtain general and explicit forms for these correlations which
isolate the contributions to the Hall conductivity, and provide a general
discussion of the implications of symmetry and equilibrium. We evaluate these
two- and four-point correlation functions and hence the thermal transport for
the illustrative example of an ordered two dimensional antiferromagnet. In this
case the $Q$ field is a composite of magnon operators arising from spin-lattice
coupling. A numerical evaluation of the required integrals demonstrates that
the results satisfy all the necessary symmetry restrictions but otherwise lead
to non-vanishing scattering and Hall effects, and in particular that this
mechanism leads to comparable thermal Hall conductivity for thermal currents
within and normal to the plane of the antiferromagnetism.",2202.10366v3
2022-03-04,Exchange interactions and spin dynamics in the layered honeycomb ferromagnet CrI$_3$,"We derive the microscopic spin Hamiltonian for rhombohedral CrI$_3$ using
extensive first-principles density functional theory (DFT) calculations which
incorporate spin-orbit coupling and Hubbard U. Our calculations indicate a
dominant nearest-neighbor ferromagnetic Heisenberg exchange with weaker
further-neighbor Heisenberg terms. In addition, we find a Dzyaloshinskii-Moriya
interaction which primarily drives a topological gap in the spin-wave spectrum
at the Dirac point, and uncover a non-negligible antiferromagnetic Kitaev
coupling between the S=3/2 Cr moments. The out-of-plane magnetic moment is
stabilized by weak symmetric bond-dependent terms and a local single-ion
anisotropy. Using linear spin wave theory, we find that our exchange parameters
are in reasonably good agreement with inelastic neutron scattering (INS)
experiments. Employing classical Monte Carlo simulations, we study the magnetic
phase transition temperature $T_c$ and its evolution with an applied in-plane
magnetic field. We further demonstrate how future high-resolution INS
experiments on the magnon dispersion of single crystals in an in-plane magnetic
field may be used to quantitatively extract the strength of the
antiferromagnetic Kitaev exchange coupling.",2203.02567v1
2022-03-17,Tilings and Twist at 1/N^4,"We re-consider operator mixing in the so-called $SU(2)$ sector of ${\cal N}
\, = \, 4$ super Yang-Mills theory with gauge group $SU(N)$. Where possible,
single-trace operators of moderate length are completed by higher-trace
admixtures so as to yield large $N$ tree level eigenstates.
  We are particularly interested in parity pairs with three excitations. Since
parity is respected in the mixing, the odd single-trace operators at low length
cannot receive too many admixtures. We reproduce the tree-level norms of a set
of large $N$ eigenstates up to order $1/N^4$ by integrability methods. This
involves evaluating two-point functions on the sphere, the torus, and the
double-torus. A perfect match is found as long as descendents are absent from
the mixing.
  Using twist to make the descendents appear in the integrability picture
immediately leads to the question how to modify the entangled states occurring
in the hexagon tessellations. We take a closer look at the double-trace
admixtures to the parity even three-excitation operator at length seven, which
are both products of a primary state and a descendent. Their two-point
functions are sensitive to the twist introduced into the Bethe equations. For
transverse scalar excitations we succeed in recovering the corresponding field
theory results. For longitudinal magnons our methods fail, pointing at a
potential weakness of the formalism.",2203.09399v1
2022-03-21,Scattering of spin waves by a Bloch domain wall: effect of the dipolar interaction,"It is known that a Bloch domain wall in an anisotropic ferromagnet is
transparent to spin waves. This result is derived by approximating the dipolar
interaction between magnetic moments by an effective anisotropy interaction. In
this paper we study the the scattering of spin waves by a domain wall taking
into account the full complexity of the dipolar interaction, treating it
perturbatively in the distorted wave Born approximation. Due to the
peculiarities of the dipolar interaction, the implementation of this
approximation is not straightforward. The difficulties are circumvented here by
realizing that the contribution of the dipolar interaction to the spin wave
operator can be split into two terms: i) an operator that commutes with the
spin wave operator in absence of dipolar interaction, and ii) a local operator
suitable to be treated as a perturbation in the distorted wave Born
approximaton. We analyze the scattering parameters obtained within this
approach. It turns out that the reflection coefficient does not vanish in
general, and that the transmitted waves suffer a lateral shift even at normal
incidence. This lateral shift can be greatlty enhanced by making the spin wave
go through an array of well separated domain walls. The outgoing spin wave will
no be attenuated by the scattering at the domain walls since the reflection
coefficient vanishes at normal incidence. This effect may be very useful to
control the spin waves in magnonic devices.",2203.11140v1
2022-03-29,"Coupled frustrated ferromagnetic and antiferromagnetic quantum spin chains in the quasi-one-dimensional mineral antlerite, Cu$_3$SO$_4$(OH)$_4$","Magnetic frustration, the competition among exchange interactions, often
leads to novel magnetic ground states with unique physical properties which can
hinge on details of interactions that are otherwise difficult to observe. Such
states are particularly interesting when it is possible to tune the balance
among the interactions to access multiple types of magnetic order. We present
antlerite, Cu$_3$SO$_4$(OH)$_4$, as a potential platform for tuning
frustration. Contrary to previous reports, the low-temperature magnetic state
of its three-leg zigzag ladders is a quasi-one-dimensional analog of the
magnetic state recently proposed to exhibit spinon-magnon mixing in
botallackite. Density functional theory calculations indicate that antlerite's
magnetic ground state is exquisitely sensitive to fine details of the atomic
positions, with each chain independently on the cusp of a phase transition,
indicating an excellent potential for tunability.",2203.15343v2
2022-04-08,Probing the spin dimensionality in single-layer CrSBr van der Waals heterostructures by magneto-transport measurements,"Two-dimensional (2D) magnetic materials offer unprecedented opportunities for
fundamental physics and applied research in spintronics and magnonics. Beyond
the pioneering studies on 2D CrI3 and Cr2Ge2Te6, this emerging field has
expanded to 2D antiferromagnets exhibiting different spin anisotropies and
textures. Of particular interest is the layered metamagnet CrSBr, a relatively
air-stable semiconductor formed by antiferromagnetically-coupled ferromagnetic
layers (Tc~150 K) that can be exfoliated down to the single-layer. It presents
a complex magnetic behavior with a dynamic magnetic crossover leading to a
low-temperature hidden order below T*~40 K. Here, we inspect the
magneto-transport properties of CrSBr vertical heterostructures in the 2D
limit. Our results demonstrate the marked low-dimensional character of the
ferromagnetic monolayer, with short-range correlations above Tc and an
Ising-type in-plane anisotropy, being the spins spontaneously aligned along the
easy-axis b below Tc. By applying moderate magnetic fields along a and c axes,
a spin reorientation occurs, leading to a magnetoresistance enhancement below
T*. In multilayers, a spin-valve behavior is observed, with negative
magnetoresistance strongly enhanced along the three directions below T*. These
results show that CrSBr monolayer/bilayer provides an ideal platform for
studying and controlling field-induced phenomena in two-dimensions, offering
new insights regarding 2D magnets and their integration into vertical
spintronic devices.",2204.04095v2
2022-05-08,Spin interaction and magnetism in cobaltate Kitaev candidate materials: an $ab$ $initio$ and model Hamiltonian approach,"In the quest for materials hosting Kitaev spin liquids, much of the efforts
have been focused on the fourth- and fifth-row transition metal compounds,
which are spin-orbit coupling assisted Mott insulators. Here, we study the
structural and magnetic properties of 3$d$ transition metal oxides,
Na$_2$Co$_2$TeO$_6$ and Na$_3$Co$_2$SbO$_6$. The partial occupancy of sodium in
former compound is addressed using a cluster expansion, and a honeycomb lattice
of sodiums is found to be energetically favored. Starting from the \textit{ab
initio} band structures, a many-body second order perturbation theory leads to
a pseudospin-$\frac{1}{2}$ Hamiltonian with estimated magnetic interactions. We
show that the experimentally observed zigzag magnetic state is stabilized only
when the first neighbor Kitaev coupling dominates over the Heisenberg term,
both of which are highly suppressed due to presence of $e_g$ orbitals. A third
neighbor Heisenberg interaction is found dominant in both these compounds. We
also present a phase diagram for Na$_2$Co$_2$TeO$_6$ by varying the
electron-electron and spin-orbit interactions. The computed spin excitation
spectra are found to capture essential features of recent experimental magnon
spectrum, lending support to our results.",2205.03836v2
2022-05-16,Phase diagram and magnetic excitations of $J_1$-$J_3$ Heisenberg model on the square lattice,"We study the phase diagram and the dynamical spin structure factor of the
spin-1/2 J1-J3 Heisenberg model on the square lattice using density matrix
renormalization group, exact diagonalization (ED), and cluster perturbation
theory (CPT). By extrapolating the order parameters and studying the level
crossings of the low-lying energy and entanglement spectra, we obtain the phase
diagram of this model and identify a narrow region of quantum spin liquid (QSL)
phase followed by a plaquette valence-bond solid (PVBS) state in the
intermediate region, whose nature has been controversial for many years. More
importantly, we use CPT and ED to study the dynamical spin structure factor in
the QSL and the PVBS phase. In the QSL phase, the high-energy magnon mode
completely turns into some dispersive weak excitations around the X and M
points. For the PVBS phase, the low-energy spectrum is characterized by a
gapped triplet excitation, and at the high energy, we find another branch of
dispersive excitation with broad continua, which is unlike the plaquette phase
in the 2x2 checkerboard model. In the latter case, the second branch of
excitation is nearly flat due to the weak effective interactions between the
local excitations of the plaquettes. And in the J1-J3 Heisenberg model, the
uniform interactions and the spontaneously translational symmetry breaking of
the PVBS phase make the difference in the excitation spectra.",2205.07910v2
2022-05-25,Spin dynamics and continuum spectra of the honeycomb J1-J2 antiferromagnetic Heisenberg model,"We employ the spin cluster perturbation theory to investigate the dynamical
properties of the antiferromagnetic $J_{1}$-$J_{2}$ Heisenberg model on the
honeycomb lattice. We obtain the excitation spectra for all possible phases in
the phase diagram, including the N\'{e}el phase, plaquette valence-bond-solid
phase, dimer valence-bond-solid phase and stripe antiferromagnetic phase. In
the N\'{e}el phase, besides the obvious renormalization of the magnon
dispersion, we find that the spectrum exhibits a dome-shaped broad continuum
around the second Brillouin zone (BZ) and the additional strong continuum close
to the corner of the BZ. In the valence-bond-solid phases, the spectra are
dominated by a strong broad continuum all the way down to below $J_1$
coexisting with the lowest-energy triplon modes characterizing the plaquette
and dimer phases. We ascribe this strong broad continuum and the additional
continuum close to the BZ corner in the N\'{e}el phase to the contributions of
fractionalized spinon excitations. In the stripe phase, a clear difference from
the linear spin wave approximation is that the spectrum is gapped at the $M$
point while that obtained by the latter is gapless due to the strong quantum
fluctuations. We point out that the features observed in the N\'{e}el phase are
consistent with the recent neutron scattering experiments on YbCl$_{3}$ and
YbBr$_{3}$.",2205.12822v1
2022-05-26,Enhanced collisionless laser absorption in strongly magnetized plasmas,"Strongly magnetizing a plasma adds a range of waves that do not exist in
unmagnetized plasmas and enlarges the laser-plasma interaction (LPI) landscape.
In this paper, we use particle-in-cell (PIC) simulations to investigate
strongly magnetized LPI in one dimension under conditions relevant for
magneto-inertial fusion experiments, focusing on a regime where the
electron-cyclotron frequency is greater than the plasma frequency and the
magnetic field is at an oblique angle with respect to the wave vectors. We show
that when electron-cyclotron-like hybrid wave frequency is about half the laser
frequency, the laser light resonantly decays to magnetized plasma waves via
primary and secondary instabilities with large growth rates. These distinct
magnetic-field-controlled instabilities, which we collectively call two-magnon
decays, are analogous to two-plasmon decays in unmagnetized plasmas. Since
additional phase mixing mechanisms are introduced by the oblique magnetic
field, collisionless damping of large-amplitude magnetized waves substantially
broadens the electron distribution function, especially along the direction of
the magnetic field. During this process, energy is transferred efficiently from
the laser to plasma waves and then to electrons, leading to a large overall
absorptivity when strong resonances are present. The enhanced laser energy
absorption may explain hotter-than-expected temperatures observed in magnetized
laser implosion experiments and may also be exploited to develop more efficient
laser-driven x-ray sources.",2205.13478v2
2022-05-30,Geometrical frustration versus Kitaev interactions in BaCo$_2$(AsO$_4$)$_2$,"Recently, Co-based honeycomb magnets have been proposed as promising
candidate materials to host the Kitaev spin liquid state. One of the
front-runners is BaCo$_2$(AsO$_4$)$_2$ (BCAO), where it was suggested that the
exchange processes between Co$^{2+}$ ions via the surrounding edge-sharing
oxygen octahedra could give rise to bond-dependent Kitaev interactions. In this
work, we present and analyze comprehensive inelastic neutron scattering studies
of BCAO with fields in the honeycomb plane. Combining the constraints from the
magnon excitations in the high-field polarized state and the inelastic spin
structure factor measured in zero magnetic field, we examine two leading
theoretical models: the Kitaev-type \JKG model and the \XXZ model. We show that
the existing experimental data can be consistently accounted for by the \XXZ
model but not by the \JKG model, and we discuss the implications of these
results for the realization of a spin liquid phase in BCAO and more generally
for the realization of the Kitaev model in cobaltates.",2205.15262v1
2022-06-06,Quantitative theory of magnetic interactions in solids,"In this report we review the method of explicit calculations of interatomic
exchange interactions of magnetic materials. This involves exchange mechanisms
normally referred to as Heisenberg exchange, Dzyaloshinskii-Moriya interaction
and anisotropic symmetric exchange. The connection between microscopic theories
of the electronic structure, such as density functional theory or dynamical
mean field theory, and interatomic exchange, is given in detail. The different
aspects of extracting information for an effective spin Hamiltonian that
involves thousands of atoms, from electronic structure calculations considering
significantly fewer atoms (1-50) is highlighted. Examples of exchange
interactions of a large group of materials is presented, which involves heavy
elements of the 3d period, alloys between transition metals, Heusler compounds,
multilayer systems as well as overlayers and adatoms on a substrate, transition
metal oxides, 4f elements, magnetic materials in two dimensions and molecular
magnets. Where possible, a comparison to experimental data is made, that
naturally becomes focused on the magnon dispersion. The influence of relativity
is reviewed for a few cases, as is the importance of dynamical correlations.
Development to theories that handle out of equilibrium conditions is also
described here. The review ends with a short description of extensions of the
theories behind explicit calculations of interatomic exchange, to non-magnetic
situations, e.g. that describe chemical (charge) order and superconductivity.",2206.02415v2
2022-06-07,Non-volatile electric control of spin-orbit torques in an oxide two-dimensional electron gas,"Spin-orbit torques (SOTs) have opened a novel way to manipulate the
magnetization using in-plane current, with a great potential for the
development of fast and low power information technologies. It has been
recently shown that two-dimensional electron gases (2DEGs) appearing at oxide
interfaces provide a highly efficient spin-to-charge current interconversion.
The ability to manipulate 2DEGs using gate voltages could offer a degree of
freedom lacking in the classical ferromagnetic/spin Hall effect bilayers for
spin-orbitronics, in which the sign and amplitude of SOTs at a given current
are fixed by the stack structure. Here, we report the non-volatile
electric-field control of SOTs in an oxide-based Rashba-Edelstein 2DEG. We
demonstrate that the 2DEG is controlled using a back-gate electric-field,
providing two remanent and switchable states, with a large resistance contrast
of 1064%. The SOTs can then be controlled electrically in a non-volatile way,
both in amplitude and in sign. This achievement in a 2DEG-CoFeB/MgO
heterostructures with large perpendicular magnetization further validates the
compatibility of oxide 2DEGs for magnetic tunnel junction integration, paving
the way to the advent of electrically reconfigurable SOT MRAMS circuits, SOT
oscillators, skyrmion and domain-wall-based devices, and magnonic circuits.",2206.03068v1
2022-06-24,"Symmetry Resolved Entanglement of Excited States in Quantum Field Theory II: Numerics, Interacting Theories and Higher Dimensions","In a recent paper we studied the entanglement content of zero-density excited
states in complex free quantum field theories, focusing on the symmetry
resolved entanglement entropy (SREE). By zero-density states we mean states
consisting of a fixed, finite number of excitations above the ground state in
an infinite-volume system. The SREE is defined for theories that possess an
internal symmetry and provides a measure of the contribution to the total
entanglement of each symmetry sector. In our work, we showed that the ratio of
Fourier-transforms of the SREEs (i.e. the ratio of charged moments) takes a
very simple and universal form for these states, which depends only on the
number, statistics and symmetry charge of the excitations as well as the
relative size of the entanglement region with respect to the whole system's
size. In this paper we provide numerical evidence for our formulae by computing
functions of the charged moments in two free lattice theories: a 1D Fermi gas
and a complex harmonic chain. We also extend our results in two directions: by
showing that they apply also to excited states of interacting theories (i.e.
magnon states) and by developing a higher dimensional generalisation of the
branch point twist field picture, leading to results in (interacting)
higher-dimensional models.",2206.12223v2
2022-07-13,Marginally deformed Schrödinger/dipole CFT correspondence,"We construct and thoroughly study a new integrable example of the AdS/CFT
correspondence with Schr\""{o}dinger symmetry. On the gravity side, the
supergravity solution depends on two parameters and is obtained by marginally
deforming the internal space of the Schr\""{o}dinger background through a series
of TsT transformations. On the field theory side, we identify the dual field
theory which also depends on two parameters. We find a point-like string
solution and derive its dispersion relation. A non-trivial test of the
correspondence is provided by using the Landau-Lifshitz coherent state approach
to reproduce the leading, in the deformation parameters, terms of that
relation. Then, we calculate the Wilson loop, describing the quark/anti-quark
potential at strong coupling. It exhibits confining behaviour when the
separation length is much less than the Schr\""{o}dinger parameter. When the
separation length is much greater than the Schr\""{o}dinger parameter the
behaviour is that of a conformal theory. Subsequently, we take the Penrose
limit along a certain null geodesic of the constructed background and calculate
the bosonic spectrum. Based on that spectrum, we make an educated guess for the
exact, in the 't Hooft coupling, dispersion relation of the magnon excitations
in the original doubly deformed background. This provides us with an exact
prediction for the dimensions of the dual field theory operators.",2207.06092v1
2022-07-19,Static and dynamical signatures of Dzyaloshinskii-Moriya interactions in the Heisenberg model on the kagome lattice,"Motivated by recent experiments on Cs$_2$Cu$_3$SnF$_{12}$ and
YCu$_{3}$(OH)$_{6}$Cl$_{3}$, we consider the ${S=1/2}$ Heisenberg model on the
kagome lattice with nearest-neighbor super-exchange $J$ and (out-of-plane)
Dzyaloshinskii-Moriya interaction $J_D$, which favors (in-plane) ${\bf
Q}=(0,0)$ magnetic order. By using both variational Monte Carlo (based upon
Gutzwiller-projected fermionic wave functions) and tensor-network approaches
(built from infinite projected-entangled pair/simplex states), we show that the
ground state develops a finite magnetization for $J_D/J \gtrsim 0.03 - 0.04$,
while the gapless spin liquid remains stable for smaller values of the
Dzyaloshinskii-Moriya interaction. The relatively small value of $J_D/J$ for
which magnetic order sets in is particularly relevant for the interpretation of
low-temperature behaviors of kagome antiferromagnets, including
ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$. In addition, we assess the spin dynamical
structure factors and the corresponding low-energy spectrum, by using the
variational Monte Carlo technique. The existence of a continuum of excitations
above the magnon modes is reported within the magnetically ordered phase,
similarly to what has been detected by inelastic neutron scattering on
Cs$_{2}$Cu$_{3}$SnF$_{12}$.",2207.09274v3
2022-07-19,"Supersymmetry on the lattice: Geometry, Topology, and Spin Liquids","In quantum mechanics, supersymmetry (SUSY) posits an equivalence between two
elementary degrees of freedom, bosons and fermions. Here we show how this
fundamental concept can be applied to connect bosonic and fermionic lattice
models in the realm of condensed matter physics, e.g., to identify a variety of
(bosonic) phonon and magnon lattice models which admit topologically nontrivial
free fermion models as superpartners. At the single-particle level, the bosonic
and the fermionic models that are generated by the SUSY are isospectral except
for zero modes, such as flat bands, whose existence is undergirded by the
Witten index of the SUSY theory. We develop a unifying framework to formulate
these SUSY connections in terms of general lattice graph correspondences and
discuss further ramifications such as the definition of supersymmetric
topological invariants for generic bosonic systems. Notably, a Hermitian form
of the supercharge operator, the generator of the SUSY, can itself be
interpreted as a hopping Hamiltonian on a bipartite lattice. This allows us to
identify a wide class of interconnected lattices whose tight-binding
Hamiltonians are superpartners of one another or can be derived via squaring or
square-rooting their energy spectra all the while preserving band topology
features. We introduce a five-fold way symmetry classification scheme of these
SUSY lattice correspondences, including cases with a non-zero Witten index,
based on a topological classification of the underlying Hermitian supercharge
operator. These concepts are illustrated for various explicit examples
including frustrated magnets, Kitaev spin liquids, and topological
superconductors.",2207.09475v1
2022-07-26,Dynamics of spin-1/2 $J_1$-$J_2$ model on the triangular lattice,"We discuss spin-$\frac12$ $J_1$--$J_2$ model on the triangular lattice using
recently proposed bond-operator theory (BOT). In agreement with previous
discussions of this system, we obtain four phases upon $J_2$ increasing: the
phase with $120^\circ$ ordering of three sublattices, the spin-liquid phase,
the state with the collinear stripe order, and the spiral phase. The
$120^\circ$ and the stripe phases are discussed in detail. All calculated
static characteristics of the model are in good agreement with previous
numerical findings. In the $120^\circ$ phase, we observe the evolution of
quasiparticles spectra and dynamical structure factors (DSFs) upon approaching
the spin-liquid phase. Some of the considered elementary excitations were
introduced first in our recent study of this system at $J_2=0$ using the BOT.
In the stripe phase, we observe that the doubly degenerate magnon spectrum
known from the spin-wave theory (SWT) is split by quantum fluctuations which
are taken into account more accurately in the BOT. As compared with other known
findings of the SWT in the stripe state, we observe additional spin-1 and
spin-0 quasiparticles which give visible anomalies in the transverse and
longitudinal DSFs. We obtain also a special spin-0 quasiparticle named singlon
who produces a peak only in four-spin correlator and who is invisible in the
longitudinal DSF. We show that the singlon spectrum lies below energies of all
spin-0 and spin-1 excitations in some parts of the Brillouin zone. Singlon
spectrum at zero momentum can be probed by the Raman scattering.",2207.12708v3
2022-08-09,Hybrid spin Hall nano-oscillators based on ferromagnetic metal/ferrimagnetic insulator heterostructures,"Spin-Hall nano-oscillators (SHNOs) are promising spintronic devices to
realize current controlled GHz frequency signals in nanoscale devices for
neuromorphic computing and creating Ising systems. However, traditional SHNOs
-- devices based on transition metals -- have high auto-oscillation threshold
currents as well as low quality factors and output powers. Here we demonstrate
a new type of hybrid SHNO based on a permalloy (Py) ferromagnetic-metal
nanowire and low-damping ferrimagnetic insulator, in the form of epitaxial
lithium aluminum ferrite (LAFO) thin films. The superior characteristics of
such SHNOs are associated with the excitation of larger spin-precession angles
and volumes. We further find that the presence of the ferrimagnetic insulator
enhances the auto-oscillation amplitude of spin-wave edge modes, consistent
with our micromagnetic modeling. This hybrid SHNO expands spintronic
applications, including providing new means of coupling multiple SHNOs for
neuromorphic computing and advancing magnonics.",2208.04539v2
2022-08-10,Forward volume magnetoacoustic spin wave excitation with micron-scale spatial resolution,"The interaction between surface acoustic waves (SAWs) and spin waves (SWs) in
a piezoelectric-magnetic thin film heterostructure yields potential for the
realization of novel microwave devices and applications in magnonics. In the
present work, we characterize magnetoacoustic waves in three adjacent magnetic
micro-stripes made from CoFe+Ga, CoFe, and CoFe+Pt with a single pair of
tapered interdigital transducers (TIDTs). The magnetic micro-stripes were
deposited by focused electron beam-induced deposition (FEBID) and focused ion
beam-induced deposition (FIBID) direct-writing techniques. The transmission
characteristics of the TIDTs are leveraged to selectively address the
individual micro-stripes. Here, the external magnetic field is continuously
rotated out of the plane of the magnetic thin film and the forward volume SW
geometry is probed with the external magnetic field along the film normal. Our
experimental findings are well explained by an extended phenomenological model
based on a modified Landau-Lifshitz-Gilbert approach that considers SWs with
nonzero wave vectors. Magnetoelastic excitation of forward volume SWs is
possible because of the vertical shear strain $\varepsilon_{xz}$ of the
Rayleigh-type SAW.",2208.05205v1
2022-08-16,Easy-plane anisotropic-exchange magnets on a honeycomb lattice: quantum effects and dealing with them,"We provide analytical and numerical insights into the phase diagram and other
properties of the extended Kitaev-Heisenberg model on the honeycomb lattice in
the {\it easy-plane} limit, in which interactions are only between spin
components that belong to the plane of magnetic ions. This parameter subspace
allows for a much-needed systematic {\it quantitative} investigation of spin
excitations in the ordered phases and of their generic features. Specifically,
we demonstrate that in this limit one can consistently take into account magnon
interactions in both zero-field zigzag and field-polarized phases. For the
nominally polarized phase, we propose a regularization of the unphysical
divergences that occur at the critical field and are plaguing the
$1/S$-approximation in this class of models. For the explored parameter
subspace, all symmetry-allowed terms of the standard parametrization of the
extended Kitaev-Heisenberg model, such as $K$, $J$, and $\Gamma$, are
significant, making the offered consideration relevant to a much wider
parameter space. The dynamical structure factor near paramagnetic critical
point illustrates this relevance by showing features that are reminiscent of
the ones observed in $\alpha$-RuCl$_3$, underscoring that they are not unique
and should be common to a wide range of parameters of the model and, by an
extension, to other materials.",2208.07617v3
2022-08-16,Liger at Keck Observatory: Design of Imager Optical Assembly and Spectrograph Re-Imaging Optics,"Liger is an adaptive optics (AO) fed imager and integral field spectrograph
(IFS) designed to take advantage of the Keck All-sky Precision Adaptive-optics
(KAPA) upgrade for the W.M. Keck Observatory. We present the design and
analysis of the imager optical assembly including the spectrograph Re-Imaging
Optics (RIO) which transfers the beam path from the imager focal plane to the
IFS slicer module and lenslet array. Each imager component and the first two
RIO mechanisms are assembled and individually aligned on the same optical
plate. Baffling suppresses background radiation and scattered light, and a
pupil viewing camera allows the imager detector to focus on an image of the
telescope pupil. The optical plate mounts on an adapter frame for alignment of
the overall system. The imager and RIO will be characterized in a cryogenic
test chamber before installation in the final science cryostat.",2208.07936v1
2022-09-06,Dynamic coupling and spin-wave dispersions in a magnetic hybrid system made of an artificial spin-ice structure and an extended NiFe underlayer,"We present a combined experimental and numerical study of the spin-wave
dispersion in NiFe artificial spin-ice (ASI) system consisting of an array of
stadium-shaped nanoislands deposited on the top of a continuous NiFe film with
nonmagnetic spacer layers of varying thickness. The spin-wave dispersion,
measured by wavevector resolved Brillouin light scattering spectroscopy in the
Damon-Eshbach configuration, consists of a rich number of modes, with either
stationary or propagating character. We find that the lowest frequency mode
displays a bandwidth of approximately 0.5 GHz, which is independent of the
presence of the film underneath. On the contrary, the BLS intensity of some of
the detected modes strongly depends on the presence of the extended thin-film
underlayer. Micromagnetic simulations unveil the details of the dynamic
coupling between ASI lattice and film underlayer. Interestingly, the ASI
lattice facilitates dynamics of the film either specific wavelengths or
intensity modulation peculiar to the modes of the ASI elements imprinted in the
film. Our results demonstrate that propagating spin waves can be modulated at
the nanometer length scale by harnessing the dynamic mode coupling in the
vertical, i.e., the out-of-plane direction of suitably designed magnonic
structures.",2209.02783v1
2022-09-16,Unveiling new quantum phases in the Shastry-Sutherland compound SrCu2(BO3)2up to the saturation magnetic field,"Under magnetic fields, quantum magnets often undergo exotic phase transitions
with various kinds of order. The discovery of a sequence of fractional
magnetization plateaus in the Shastry-Sutherland compound SrCu2(BO3)2 has
played a central role in the high-field research on quantum materials, but so
far this system could only be probed up to half the saturation value of the
magnetization. Here, we report the first experimental and theoretical
investigation of this compound up to the saturation magnetic field of 140 T and
beyond. Using ultrasound and magnetostriction techniques combined with
extensive tensor-network calculations (iPEPS), several spin-supersolid phases
are revealed between the 1/2 plateau and saturation (1/1 plateau). Quite
remarkably, the sound velocity of the 1/2 plateau exhibits a drastic decrease
of -50%, related to the tetragonal-to-orthorhombic instability of the
checkerboard-type magnon crystal. The unveiled nature of this paradigmatic
quantum system is a new milestone for exploring exotic quantum states of matter
emerging in extreme conditions.",2209.07652v2
2022-09-21,Spin current driven by ultrafast magnetization of FeRh,"Laser-induced ultrafast demagnetization is an important phenomenon that
probes arguably ultimate limits of the angular momentum dynamics in solid.
Unfortunately, many aspects of the dynamics remain unclear except that the
demagnetization transfers the angular momentum eventually to the lattice. In
particular, roles of electron-carried spin current are debated. Here we
experimentally probe the spin current in the opposite phenomenon, i.e.,
laser-induced ultrafast magnetization of FeRh, where the laser pump pulse
initiates the angular momentum build-up rather than its dissipation. Using the
time-resolved magneto-optical Kerr effect, we directly measure the
ultrafast-magnetization-driven spin current in a FeRh/Cu heterostructure.
Strong correlation between the spin current and the net magnetization change
rate of FeRh is found even though the spin filter effect is negligible in this
opposite process. This result implies that the angular momentum build-up is
achieved by an angular momentum transfer from the electron bath (supplier) to
the magnon bath (receiver) and followed by the spatial transport of angular
momentum (spin current) and dissipation of angular momentum to the phonon bath
(spin relaxation).",2209.10290v1
2022-10-06,All hourglass bosonic excitations in the 1651 magnetic space groups and 528 magnetic layer groups,"The band connectivity as imposed by the compatibility relations between the
irreducible representations of little groups can give rise to the exotic
hourglass-like shape composed of four branches of bands and five band crossings
(BCs). Such an hourglass band connectivity could enforce the emergence of
nontrivial excitations like Weyl fermion, Dirac fermion or even beyond them. On
the other hand, the bosons, like phonons, magnons, and photons, were also shown
to possess nontrivial topology and a comprehensive symmetry classification of
the hourglass bosonic excitations would be of great significance to both
materials design and device applications. Here we firstly list all concrete
positions and representations of little groups in the Brillouin zone (BZ)
related with the hourglass bosonic excitations in all the 1651 magnetic space
groups and 528 magnetic layer groups, applicable to three dimensional (3D) and
two dimensional (2D) systems, respectively. 255 (42) MSGs (MLGs) are found to
essentially host such hourglass BCs: Here ``essentially'' means that the
bosonic hourglass BC exists definitely as long as the studied system is
crystallized in the corresponding MSG/MLG. We also perform first-principles
calculations on hundreds of 3D nonmagnetic materials essentially hosting
hourglass phonons and propose that the 2D material AlI can host hourglass
phonons. We choose AuX (X=Br and I) as illustrative examples to demonstrate
that two essential hourglass band structures can coexist in the phonon spectra
for both materials while for AuBr, an accidental band crossing sticking two
hourglasses is found interestingly. Our results of symmetry conditions for
hourglass bosonic excitations can provide a useful guide of designing
artificial structures with hourglass bosonic excitations.",2210.02954v1
2022-10-10,Complete field-induced spectral response of the spin-1/2 triangular-lattice antiferromagnet CsYbSe$_2$,"Fifty years after Anderson's resonating valence-bond proposal, the spin-1/2
triangular-lattice Heisenberg antiferromagnet (TLHAF) remains the ultimate
platform to explore highly entangled quantum spin states in proximity to
magnetic order. Yb-based delafossites are ideal candidate TLHAF materials,
which allow experimental access to the full range of applied in-plane magnetic
fields. We perform a systematic neutron scattering study of CsYbSe$_2$, first
proving the Heisenberg character of the interactions and quantifying the
second-neighbour coupling. We then measure the complex evolution of the
excitation spectrum, finding extensive continuum features near the
120$^{\circ}$-ordered state, throughout the 1/3-magnetization plateau and
beyond this up to saturation. We perform cylinder matrix-product-state (MPS)
calculations to obtain an unbiased numerical benchmark for the TLHAF and
spectacular agreement with the experimental spectra. The measured and
calculated longitudinal spectral functions reflect the role of multi-magnon
bound and scattering states. These results provide valuable insight into
unconventional field-induced spin excitations in frustrated quantum materials.",2210.04928v3
2022-10-11,Magnetic anisotropy reversal driven by structural symmetry-breaking in monolayer α-RuCl3,"Layered {\alpha}-RuCl3 is a promising material to potentially realize the
long-sought Kitaev quantum spin liquid with fractionalized excitations. While
evidence of this exotic state has been reported under a modest in-plane
magnetic field, such behavior is largely inconsistent with theoretical
expectations of Kitaev phases emerging only in out-of-plane fields. These
predicted field-induced states have been mostly out of reach due to the strong
easy-plane anisotropy of bulk crystals, however. We use a combination of
tunneling spectroscopy, magnetotransport, electron diffraction, and ab initio
calculations to study the layer-dependent magnons, anisotropy, structure, and
exchange coupling in atomically thin samples. Due to structural distortions,
the sign of the average off-diagonal exchange changes in monolayer
{\alpha}-RuCl3, leading to a reversal of magnetic anisotropy to easy-axis. Our
work provides a new avenue to tune the magnetic interactions in {\alpha}-RuCl3
and allows theoretically predicted quantum spin liquid phases for out-of-plane
fields to be more experimentally accessible.",2210.05733v1
2022-11-09,Magnetization reversal through an antiferromagnetic state,"Magnetization reversal in ferro- and ferrimagnets is a well-known archetype
of non-equilibrium processes, where the volume fractions of the oppositely
magnetized domains vary and perfectly compensate each other at the coercive
magnetic field. Here, we report on a fundamentally new pathway for
magnetization reversal that is mediated by an antiferromagnetic state.
Consequently, an atomic-scale compensation of the magnetization is realized at
the coercive field, instead of the mesoscopic or macroscopic domain
cancellation in canonical reversal processes. We demonstrate this unusual
magnetization reversal on the Zn-doped polar magnet Fe$_2$Mo$_3$O$_8$. Hidden
behind the conventional ferrimagnetic hysteresis loop, the surprising emergence
of the antiferromagnetic phase at the coercive fields is disclosed by a sharp
peak in the field-dependence of the electric polarization. In addition, at the
magnetization reversal our THz spectroscopy studies reveal the reappearance of
the magnon mode that is only present in the pristine antiferromagnetic state.
According to our microscopic calculations, this unusual process is governed by
the dominant intralayer coupling, strong easy-axis anisotropy and spin
fluctuations, which result in a complex interplay between the ferrimagnetic and
antiferromagnetic phases. Such antiferro-state-mediated reversal processes
offer novel concepts for magnetization control, and may also emerge for other
ferroic orders.",2211.05028v1
2022-11-10,Static and dynamic magnetic properties of the spin-5/2 triangle lattice antiferromagnet Na3Fe(PO4)2 studied by 31P NMR,"$^{31}$P nuclear magnetic resonance (NMR) measurements have been carried out
to investigate the magnetic properties and spin dynamics of Fe$^{3+}$ ($S$ =
5/2) spins in the two-dimensional triangular lattice (TL) compound
Na$_3$Fe(PO$_4$)$_2$. The temperature ($T$) dependence of nuclear spin-lattice
relaxation rates ($1/T_1$) shows a clear peak around N\'eel temperature,
$T_{\rm N} = 10.9$~K, corresponding to an antiferromagnetic (AFM) transition.
From the temparature dependence of NMR shift ($K$) above $T_{\rm N}$, an
exchange coupling between Fe$^{3+}$ spins was estimated to be $J/k_{\rm
B}\simeq 1.9$~K using the spin-5/2 Heisenberg isotropic-TL model. The
temperature dependence of $1/T_1T$ divided by the magnetic susceptibility
($\chi$), $1/T_1T\chi$, above $T_{\rm N}$ proves the AFM nature of spin
fluctuations below $\sim$ 50 K in the paramagnetic state. In the magnetically
ordered state below $T_{\rm N}$, the characteristic rectangular shape of the
NMR spectra is observed, indicative of a commensurate AFM state in its ground
state. The strong temperature dependence of 1/$T_1$ in the AFM state is well
explained by the two-magnon (Raman) process of the spin waves in a 3D
antiferromagnet with a spin-anisotropy energy gap of 5.7 K. The temperature
dependence of sublattice magnetization is also well reproduced by the spin
waves. Those results indicate that the magnetically ordered state of
Na$_3$Fe(PO$_4$)$_2$ is a conventional 3D AFM state, and no obvious spin
frustration effects were detected in its ground state.",2211.05921v1
2022-11-23,Raman scattering signatures of the strong spin-phonon coupling in the bulk magnetic van der Waals material CrSBr,"Magnetic excitations in layered magnetic materials that can be thinned down
the two-dimensional (2D) monolayer limit are of high interest from a
fundamental point of view and for applications perspectives. Raman scattering
has played a crucial role in exploring the properties of magnetic layered
materials and, even-though it is essentially a probe of lattice vibrations, it
can reflect magnetic ordering in solids through the spin-phonon interaction or
through the observation of magnon excitations. In bulk CrSBr, a layered A type
antiferromagnet (AF), we show that the magnetic ordering can be directly
observed in the temperature dependence of the Raman scattering response i)
through the variations of the scattered intensities, ii) through the activation
of new phonon lines reflecting the change of symmetry with the appearance of
the additional magnetic periodicity, and iii) through the observation, below
the Neel temperature (TN) of second order Raman scattering processes. We
additionally show that the three different magnetic phases encountered in
CrSBr, including the recently identified low temperature phase, have a
particular Raman scattering signature. This work demonstrates that magnetic
ordering can be observed directly in the Raman scattering response of bulk
CrSBr with in-plane magnetization, and that it can provide a unique insight
into the magnetic phases encountered in magnetic layered materials.",2211.12939v1
2022-12-06,Giant gate-tunable renormalization of spin-correlated flat-band states and bandgap in a 2D magnetic insulator,"Emergent quantum phenomena in two-dimensional van der Waal (vdW) magnets are
largely governed by the interplay between the exchange and Coulomb
interactions. The ability to tune the Coulomb interaction in such strongly
correlated materials enables the precise control of spin-correlated flat-band
states, bandgap (Eg) and unconventional magnetism, all of which are crucial for
next-generation spintronics and magnonics applications. Here, we demonstrate a
giant gate-tunable renormalization of spin-correlated flat-band states and
bandgap in magnetic chromium tribromide (CrBr3) monolayers grown on graphene.
Our gate-dependent scanning tunneling spectroscopy (STS) studies reveal that
the inter-flat-band spacing and bandgap of CrBr3 can be continuously tuned by
120 meV and 240 meV respectively via electrostatic injection of carriers into
the hybrid CrBr3/graphene system, equivalent to the modulation of the Cr
on-site Coulomb repulsion energy by 500 meV. This can be attributed to the
self-screening of CrBr3 arising from the gate-induced carriers injected into
CrBr3, which dominates over the opposite trend from the remote screening of the
graphene substrate. Precise tuning of the spin-correlated flat-band states and
bandgap in 2D magnets via electrostatic modulation of Coulomb interactions not
only provides new strategies for optimizing the spin transport channels but
also may exert a crucial influence on the exchange energy and spin-wave gap,
which could raise the critical temperature for magnetic order.",2212.02772v1
2022-12-16,Spin excitations in the kagome-lattice metallic antiferromagnet Fe$_{0.89}$Co$_{0.11}$Sn,"Kagome-lattice materials have attracted tremendous interest due to the broad
prospect for seeking superconductivity, quantum spin liquid states, and
topological electronic structures. Among them, the transition-metal kagome
lattices are high-profile objects for the combination of topological
properties, rich magnetism, and multiple-orbital physics. Here we report an
inelastic neutron scattering study on the spin dynamics of a kagome-lattice
antiferromagnetic metal Fe$_{0.89}$Co$_{0.11}$Sn. Although the magnetic
excitations can be observed up to $\sim$250 meV, well-defined spin waves are
only identified below $\sim$90 meV and can be modeled using Heisenberg exchange
with ferromagnetic in-plane nearest-neighbor coupling $J_1$, in-plane
next-nearest-neighbor coupling $J_2$, and antiferromagnetic (AFM) interlayer
coupling $J_c$ under linear spin-wave theory. Above $\sim$90 meV, the spin
waves enter the itinerant Stoner continuum and become highly damped
particle-hole excitations. At the K point of the Brillouin zone, we reveal a
possible band crossing of the spin wave, which indicates a potential Dirac
magnon. Our results uncover the evolution of the spin excitations from the
planar AFM state to the axial AFM state in Fe$_{0.89}$Co$_{0.11}$Sn, solve the
magnetic Hamiltonian for both states, and confirm the significant influence of
the itinerant magnetism on the spin excitations.",2212.08590v2
2022-12-19,Machine learning assisted derivation of effective low energy models for metallic magnets,"We consider the problem of extracting an effective low-energy spin model from
a Kondo Lattice Model (KLM) with classical localized moments. The non-analytic
dependence of the effective spin-spin interactions on the Kondo exchange $J$
excludes the possibility of using perturbation theory beyond the second order
Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction at zero temperature. Here we
introduce a Machine Learning (ML) assisted protocol to extract effective two-
and four-spin interactions by integrating out the conduction electrons of the
original KLM. The resulting effective spin model reproduces the phase diagram
obtained with the original KLM as a function of magnetic field and easy-axis
anisotropy and reveals the effective four-spin interactions that are
responsible for the field induced skyrmion crystal phase. Moreover, this
minimal spin model enables an efficient computation of static and dynamical
properties with a much lower numerical cost relative to the original KLM. A
comparison of the dynamical spin structure factor in the fully polarized phase
computed with the effective model and the original KLM reveals a good agreement
for the magnon dispersion despite the fact that this information was not
included in the training data set.",2212.09796v2
2022-12-22,Nonreciprocal magnetoacoustic waves in synthetic antiferromagnets with Dzyaloshinskii-Moriya interaction,"The interaction between surface acoustic waves (SAWs) and spin waves (SWs) in
a piezoelectric/magnetic thin film heterostructure yields potential for the
realization of novel microwave devices and applications in magnonics. In the
present work, we investigate the SAW-SW interaction in a Pt/Co(2 nm)/Ru(0.85
nm)/Co(4 nm)/Pt synthetic antiferromagnet (SAF) composed of two ferromagnetic
layers with different thicknesses separated by a thin nonmagnetic Ru spacer
layer. Because of the combined presence of interfacial Dzyaloshinskii--Moriya
interaction (iDMI) and interlayer dipolar coupling fields, the optical SW mode
shows a large nondegenerate dispersion relation for oppositely propagating SWs.
Due to SAW-SW interaction, we observe nonreciprocal SAW transmission in the
piezoelectric/SAF hybrid device. The equilibrium magnetization directions of
both Co layers are manipulated by an external magnetic field to set a
ferromagnetic, canted, or antiferromagnetic configuration. This has a strong
impact on the SW dispersion, its nonreciprocity, and SAW-SW interaction. The
experimental results are in agreement with a phenomenological SAW-SW
interaction model, which considers the interlayer exchange coupling, iDMI, and
interlayer dipolar coupling fields of the SWs.",2212.11838v1
2022-12-30,Selection rules for ultrafast laser excitation and detection of spin correlations dynamics in a cubic antiferromagnet,"Exchange interactions determine the correlations between microscopic spins in
magnetic materials. Probing the dynamics of these spin correlations on
ultrashort length and time scales is, however rather challenging, since it
requires simultaneously high spatial and high temporal resolution. Recent
experimental demonstrations of laser-driven two-magnon modes - zone-edge
excitations in antiferromagnets governed by exchange coupling - posed questions
about the microscopic nature of the observed spin dynamics, the mechanism
underlying its excitation, and their macroscopic manifestation enabling
detection. Here, on the basis of a simple microscopic model, we derive the
selection rules for cubic systems that describe the polarization of pump and
probe pulses required to excite and detect dynamics of nearest-neighbor spin
correlations, and can be employed to isolate such dynamics from other magnetic
excitations and magneto-optical effects. We show that laser-driven spin
correlations contribute to optical anisotropy of the antiferromagnet even in
the absence of spin-orbit coupling. In addition, we highlight the role of
subleading anisotropy in the spin system and demonstrate that the dynamics of
the antiferromagnetic order parameter occurs only in next-to-leading order,
determined by the smallness of the magnetic anisotropy as compared to the
isotropic exchange interactions in the system. We expect that our results will
stimulate and support further studies of magnetic correlations on the shortest
length and time scale.",2212.14698v2
2023-01-02,Dynamical properties of quantum many-body systems with long range interactions,"Employing large-scale quantum Monte Carlo simulations, we systematically
compute the energy spectra of the 2D spin-1/2 Heisenberg model with long-range
interactions. With the $1/r^{\alpha}$ ferromagnetic and staggered
antiferromagnetic interactions, we find the explicit range in $\alpha$ for
{\color{black} the short-range Goldstone-type (gapless), anomalous
Goldstone-type (gapless) and Higgs-type (gapped) spectra. Accompanied by the
spin wave analysis, our numerical results vividly reveal how the long-range
interactions alter the usual linear and quadratic magnon dispersions in 2D
quantum magnets and give rise to anomalous dynamical exponents. Moreover, we
find explicit case where the gapped excitation exists even when the Hamiltonian
is extensive. This work provides the first set of unbiased dynamical data} of
long-range quantum many-body systems and suggests that many universally
accepted low-energy customs for short-range systems need to be substantially
modified for long-range ones which are of immediate relevance to the ongoing
experimental efforts from quantum simulators to 2D quantum moir\'e materials.",2301.00829v3
2023-01-15,Exact solution of a non-Hermitian $\mathscr{PT}$-symmetric Heisenberg spin chain,"We construct the exact solution of a non-Hermitian $\mathscr{PT}$-symmetric
isotropic Heisenberg spin chain with integrable boundary fields. We find that
the system exhibits two types of phases we refer to as $A$ and $B$ phases. In
the $B$ type phase, the $\mathscr{PT}$- symmetry remains unbroken and it
consists of eigenstates with only real energies, whereas the $A$ type phase
contains a $\mathscr{PT}$-symmetry broken sector comprised of eigenstates with
only complex energies and a sector of unbroken $\mathscr{PT}$-symmetry with
eigenstates of real energies. The $\mathscr{PT}$-symmetry broken sector
consists of pairs of eigenstates whose energies are complex conjugates of each
other. The existence of two sectors in the $A$ type phase is associated with
the exponentially localized bound states at the edges with complex energies
which are described by boundary strings. We find that both $A$ and $B$ type
phases can be further divided into sub-phases which exhibit different ground
states. We also compute the bound state wavefunction in one magnon sector and
find that as the imaginary value of the boundary parameter is increased, the
exponentially localized wavefunction broadens thereby protruding more into the
bulk, which indicates that exponentially localized bound states may not be
stabilized for large imaginary values of the boundary parameter.",2301.06004v1
2023-02-01,A Brillouin light scattering study of the spin-wave magnetic field dependence in a magnetic hybrid system made of an artificial spin-ice structure and a film underlayer,"We present a combined Brillouin light scattering and micromagnetic simulation
investigation of the magnetic-field dependent spin-wave spectra in a hybrid
structure made of permalloy (NiFe) artificial spin-ice (ASI) systems, composed
of stadium-shaped nanoislands, deposited on the top of an unpatterned permalloy
film with a nonmagnetic spacer layer. The thermal spin-wave spectra were
recorded by Brillouin light scattering (BLS) as a function of the magnetic
field applied along the symmetry direction of the ASI sample. Magneto-optic
Kerr effect magnetometry was used to measure the hysteresis loops in the same
orientation as the BLS measurements. The frequency and intensity of several
spin-wave modes detected by BLS were measured as a function of the applied
magnetic field. Micromagnetic simulations enabled us to identify the modes in
terms of their frequency and spatial symmetry and to extract information about
the existence and strength of the dynamic coupling, relevant only to a few
modes of a given hybrid system. Using this approach, we suggest a way to
understand if dynamic coupling between ASI and film modes is present or not,
with interesting implications for the development of future three-dimensional
magnonic applications and devices.",2302.00483v1
2023-02-01,Temperature-independent ferromagnetic resonance shift in Bi-doped YIG garnets through magnetic anisotropy tuning,"Thin garnet films are becoming central for magnon-spintronics and
spin-orbitronics devices as they show versatile magnetic properties together
with low magnetic losses. These fields would benefit from materials in which
heat does not affect the magnetization dynamics, an effect known as the
non-linear thermal frequency shift. In this study, low damping Bi substituted
Iron garnet (Bi:YIG) ultra-thin films have been grown using Pulsed Laser
Deposition. Through a fine tuning of the growth parameters, the precise control
of the perpendicular magnetic anisotropy allows to achieve a full compensation
of the dipolar magnetic anisotropy. Strikingly, once the growth conditions are
optimized, varying the growth temperature from 405 {\deg}C to 475 {\deg}C as
the only tuning parameter induces the easy-axis to go from out-of-plane to
in-plane. For films that are close to the dipolar compensation, Ferromagnetic
Resonance measurements yield an effective magnetization $\mu _{0}M_{eff} (T)$
that has almost no temperature dependence over a large temperature range (260 K
to 400 K) resulting in an anisotropy temperature exponent of 2. These findings
put Bi:YIG system among the very few materials in which the temperature
dependence of the magnetic anisotropy varies at the same rate than the
saturation magnetization. This interesting behavior is ascribed
phenomenologically to the sizable orbital moment of $Bi^{3+}$.",2302.00585v1
2023-02-09,Structural changes induced by electric currents in a single crystal of Pr$_2$CuO$_4$,"We demonstrate a novel approach to the structural and electronic property
modification of perovskites, focusing on Pr$_2$CuO$_4$, an undoped parent
compound of a class of electron-doped copper-oxide superconductors. Currents
were passed parallel or perpendicular to the copper-oxygen layers with the
voltage ramped up until a rapid drop in the resistivity was achieved, a process
referred to as ""flash"". The current was then further increased tenfold in
current-control mode. This state was quenched by immersion into liquid
nitrogen. Flash can drive many compounds into different atomic structures with
new properties, whereas the quench freezes them into a long-lived state.
Single-crystal neutron diffraction of as-grown and modified Pr$_2$CuO$_4$
revealed a $\sqrt{10}$x$\sqrt{10}$ superlattice due to oxygen-vacancy order.
The diffraction peak intensities of the superlattice of the modified sample
were significantly enhanced relative to the pristine sample. Raman-active
phonons in the modified sample were considerably sharper. Measurements of
electrical resistivity, magnetization and two-magnon Raman scattering indicate
that the modification affected only the Pr-O layers, but not the Cu-O planes.
These results point to enhanced oxygen-vacancy order in the modified samples
well beyond what can be achieved without passing electrical current. Our work
opens a new avenue toward electric field/quench control of structure and
properties of layered perovskite oxides.",2302.04385v2
2023-02-12,Quantum Simulation of an Extended Dicke Model with a Magnetic Solid,"The Dicke model describes the cooperative interaction of an ensemble of
two-level atoms with a single-mode photonic field and exhibits a quantum phase
transition as a function of light--matter coupling strength. Extending this
model by incorporating short-range atom--atom interactions makes the problem
intractable but is expected to produce new phases. Here, we simulate such an
extended Dicke model using a crystal of ErFeO$_3$, where the role of atoms
(photons) is played by Er$^{3+}$ spins (Fe$^{3+}$ magnons). Through
magnetocaloric effect and terahertz magnetospectroscopy measurements, we
demonstrated the existence of a novel atomically ordered phase in addition to
the superradiant and normal phases that are expected from the standard Dicke
model. Further, we elucidated the nature of the phase boundaries in the
temperature--magnetic-field phase diagram, identifying both first-order and
second-order phase transitions. These results lay the foundation for studying
multiatomic quantum optics models using well-characterized many-body condensed
matter systems.",2302.06028v2
2023-02-14,Magnetic excitations in the topological semimetal $\mathrm{YbMnSb}_2$,"We report neutron scattering measurements on YbMnSb$_2$ which shed new light
on the nature of the magnetic moments and their interaction with Dirac
fermions. Using half-polarized neutron diffraction we measured the
field-induced magnetization distribution in the paramagnetic phase and found
that the magnetic moments are well localised on the Mn atoms. Using triple-axis
neutron scattering we measured the magnon spectrum throughout the Brillouin
zone in the antiferromagnetically ordered phase, and we determined the dominant
exchange interactions from linear spin-wave theory. The analysis shows that the
interlayer exchange is five times larger than in several related compounds
containing Bi instead of Sb. We argue that the coupling between the Mn local
magnetic moments and the topological band states is more important in
YbMnSb$_2$ than in the Bi compounds.",2302.07007v1
2023-02-15,Magnetically-dressed CrSBr exciton-polaritons in ultrastrong coupling regime,"The strong coupling between photons and matter excitations such as excitons,
phonons, and magnons is of central importance in the study of light-matter
interactions. Bridging the flying and stationary quantum states, the strong
light-matter coupling enables the coherent transmission, storage, and
processing of quantum information, which is essential for building photonic
quantum networks. Over the past few decades, exciton-polaritons have attracted
substantial research interest due to their half-light-half-matter bosonic
nature. Coupling exciton-polaritons with magnetic orders grants access to rich
many-body phenomena, but has been limited by the availability of material
systems that exhibit simultaneous exciton resonances and magnetic ordering.
Here we report magnetically-dressed microcavity exciton-polaritons in the van
der Waals antiferromagnetic (AFM) semiconductor CrSBr coupled to a Tamm plasmon
microcavity. Angle-resolved spectroscopy reveals an exceptionally high
exciton-polariton coupling strength attaining 169 meV, demonstrating
ultrastrong coupling that persists up to room temperature.
Temperature-dependent exciton-polariton spectroscopy senses the magnetic order
change from AFM to paramagnetism in CrSBr, confirming its magnetic nature. By
applying an out-of-plane magnetic field, an effective tuning of the polariton
energy is further achieved while maintaining the ultrastrong exciton-photon
coupling strength, which is attributed to the spin canting process that
modulates the interlayer exciton interaction. Our work proposes a hybrid
quantum platform enabled by robust opto-electronic-magnetic coupling, promising
for quantum interconnects and transducers.",2302.07616v1
2023-03-06,Phononic-magnetic dichotomy of the thermal Hall effect in the Kitaev-Heisenberg candidate material Na$_2$Co$_2$TeO$_6$,"Majorana fermions as emergent excitations of the Kitaev quantum spin liquid
ground state constitute a promising concept in fault tolerant quantum
computation. Experimentally, the recently reported topological half-quantized
thermal Hall effect in the Kitaev material $\alpha$-RuCl$_3$ seems to confirm
the Majorana nature of the material's magnetic excitations. It has been argued,
however, that the thermal Hall signal in $\alpha$-RuCl$_3$ rather stems from
phonons or topological magnons than from Majorana fermions. Here we investigate
the thermal Hall effect of the closely related Kitaev quantum material
Na$_2$Co$_2$TeO$_6$, and we show that the thermal Hall signal emerges from at
least two components, phonons and magnetic excitations. This dichotomy results
from our discovery that the transversal heat conductivity $\kappa_{xy}$ carries
clear signatures of the phononic $\kappa_{xx}$, but changes sign upon entering
the low-temperature, magnetically ordered phase. We systematically resolve the
two components by considering the detailed temperature and field dependence of
both $\kappa_{xy}$ and $\kappa_{xx}$. Our results demonstrate that uncovering a
genuinely quantized magnetic thermal Hall effect in a Kitaev topological
quantum spin liquid requires to disentangle phonon vs. magnetic contributions
where the latter include potentially fractionalized excitations such as the
expected Majorana fermions.",2303.03067v1
2023-03-15,Magnetization Dynamics and Peierls Instability in Topological Josephson Structures,"We study a long topological Josephson junction with a ferromagnetic strip
between two superconductors. The low-energy theory exhibits a non-local in time
and space interaction between chiral Majorana fermions, mediated by the
magnonic excitations in the ferromagnet. While short ranged interactions turn
out to be irrelevant by power counting, we show that sufficiently strong and
long-ranged interactions may induce a $\mathbb{Z}_2$-symmetry breaking. This
spontaneous breaking leads to a tilting of the magnetization perpendicular to
the Majorana propagation direction and the opening of a fermionic gap (Majorana
mass). It is analogous to the Peierls instability in the commensurate
Fr\""ohlich model and reflects the nontrivial interplay between Majorana modes
and magnetization dynamics. Within a Gaussian fluctuation analysis, we estimate
critical values for the temporal and spatial non-locality of the interaction,
beyond which the symmetry breaking is stable at zero temperature -- despite the
effective one-dimensionality of the model. We conclude that non-locality, i.e.,
the stiffness of the magnetization in space and time, stabilizes the symmetry
breaking. In the stabilized regime, we expect the current-phase relation to
exhibit an experimentally accessible discontinuous jump. At nonzero
temperatures, as usual in the 1D Ising model, the long-range order is destroyed
by solitonic excitations, which in our case carry each a Majorana zero mode. In
order to estimate the correlation length, we investigate the solitons within a
self-consistent mean-field approach.",2303.08321v1
2023-03-16,Antiferromagnet-mediated interlayer exchange: hybridization versus proximity effect,"We investigate the interlayer coupling between two thin ferromagnetic (F)
films mediated by an antiferromagnetic (AF) spacer in F*/AF/F trilayers and
show how it transitions between different regimes on changing the AF thickness.
Employing layer-selective Kerr magnetometry and ferromagnetic-resonance
techniques in a complementary manner enables us to distinguish between three
functionally distinct regimes of such ferromagnetic interlayer coupling. The F
layers are found to be individually and independently exchange-biased for thick
FeMn spacers - the first regime of no interlayer F-F* coupling. F-F* coupling
appears on decreasing the FeMn thickness below 9 nm. In this second regime
found in structures with 6.0-9.0 nm thick FeMn spacers, the interlayer coupling
exists only in a finite temperature interval just below the effective N\'eel
temperature of the spacer, which is due to magnon-mediated exchange through the
thermally softened antiferromagnetic spacer, vanishing at lower temperatures.
The third regime, with FeMn thinner than 4 nm, is characterized by a much
stronger interlayer coupling in the entire temperature interval, which is
attributed to a magnetic-proximity induced ferromagnetic exchange. These
experimental results, spanning the key geometrical parameters and thermal
regimes of the F*/AF/F nanostructure, complemented by a comprehensive
theoretical analysis, should broaden the understanding of the interlayer
exchange in magnetic multilayers and potentially be useful for applications in
spin-thermionics.",2303.09301v2
2023-04-04,Collective nature of orbital excitations in layered cuprates in the absence of apical oxygens,"We have investigated the 3d orbital excitations in CaCuO2 (CCO), Nd2CuO4
(NCO), and La2CuO4 (LCO) using high-resolution resonant inelastic x-ray
scattering. In LCO they behave as well-localized excitations, similarly to
several other cuprates. On the contrary, in CCO and NCO the dxy orbital clearly
disperse, pointing to a collective character of this excitation (orbiton) in
compounds without apical oxygen. We ascribe the origin of the dispersion as
stemming from a substantial next-nearest-neighbor (NNN) orbital superexchange.
Such an exchange leads to the liberation of orbiton from its coupling to
magnons, which is associated with the orbiton hopping between nearest neighbor
copper sites. We show that the exceptionally large NNN orbital superexchange
can be traced back to the absence of apical oxygens suppressing the charge
transfer energy.",2304.02115v3
2023-04-22,Lattice effect on the superexchange interaction in antiferromagnetic Bi$_2$Sr$_2$CaCu$_2$O$_8$,"The in-plane superexchange interaction $J$ of cuprate superconductors has
long been suggested to be an important parameter for exploring their
high-temperature superconductivity. The bilayer
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ is the most studied system with high-quality
single crystals in the wide doping range with the the same structure and phase.
So far, the lattice parameter dependence of $J$ in its antiferromagnetic parent
compound Bi$_2$Sr$_2$CaCu$_2$O$_{8}$ has not been established. By combining
Raman scattering and x-ray diffraction techniques on the same sample in the
same pressure environment, we obtain the evolution of both the two-magnon
spectrum and the structural parameters with pressure up to nearly 30 GPa, The
relationship between pressure or the in-plane lattice parameter and $J$ is thus
established for Bi$_2$Sr$_2$CaCu$_2$O$_{8}$. Over the studied pressure range,
superconductivity does not appear in this parent compound based on a sensitive
magnetic measurement technique. The effects of pressure and chemical doping on
the superexchange interaction and structure and their implications for
superconductivity are discussed from the comparison of the obtained
experimental data with the existing experiments. The results and findings
provide valuable information for the understanding of superconductivity and the
future theory developments for superconductivity in cuprates.",2304.11311v1
2023-04-28,"Noncollinear magnetic order, in-plane anisotropy, and magnetoelectric coupling in a pyroelectric honeycomb antiferromagnet Ni$_{2}$Mo$_{3}$O$_{8}$","Ni$_{2}$Mo$_{3}$O$_{8}$ is a pyroelectric honeycomb antiferromagnet
exhibiting peculiar changes of its electric polarization at magnetic
transitions. Ni$_{2}$Mo$_{3}$O$_{8}$ stands out from the isostructural magnetic
compounds, showing an anomalously low magnetic transition temperature and
unique magnetic anisotropy. We determine the magnetic structure of
Ni$_{2}$Mo$_{3}$O$_{8}$ utilizing high-resolution powder and single-crystal
neutron diffraction. A noncollinear stripy antiferromagnetic order is found in
the honeycomb planes. The magnetic space group is \textit{P$_C$na}2$_1$. The
in-plane magnetic connection is of the stripy type both for the $ab$-plane and
the $c$-axis spin components. This is a simpler connection than the one
proposed previously. The ferromagnetic interlayer order of the $c$-axis spin
components in our model is also distinct. The magnetic anisotropy of
Ni$_{2}$Mo$_{3}$O$_{8}$ is characterized by orientation-dependent magnetic
susceptibility measurements on a single crystal, consistent with neutron
diffraction analysis. The local magnetoelectric tensor analysis using our
magnetic models provides new insights into its magnetoelectric coupling and
polarization. Thus, our results deliver essential information for understanding
both the unusual magnetoelectric properties of Ni$_{2}$Mo$_{3}$O$_{8}$ and the
prospects for observation of exotic nonreciprocal, Hall, and magnonic effects
characteristic to this compound family.",2304.14641v2
2023-04-28,Persistent dynamic magnetic state in artificial honeycomb spin ice,"Topological magnetic charges, arising due to the non-vanishing magnetic flux
on spin ice vertices, serve as the origin of magnetic monopoles that traverse
the underlying lattice effortlessly. Unlike spin ice materials of atomic
origin, the dynamic state in artificial honeycomb spin ice is conventionally
described in terms of finite size domain wall kinetics that require magnetic
field or current application. Contrary to this common understanding, here we
show that thermally tunable artificial permalloy honeycomb lattice manifests a
perpetual dynamic state due to self-propelled magnetic charge defect relaxation
in the absence of any external tuning agent. Quantitative investigation of
magnetic charge defect dynamics using neutron spin echo spectroscopy reveals
sub-ns relaxation times that are comparable to monopole's relaxation in bulk
spin ices. Most importantly, the kinetic process remains unabated at low
temperature where thermal fluctuation is negligible. This suggests that dynamic
phenomena in honeycomb spin ice are mediated by quasi-particle type entities,
also confirmed by quantum Monte-Carlo simulations that replicate the kinetic
behavior. Our research unveils a new `macroscopic' magnetic particle that
shares many known traits of quantum particles, namely magnetic monopole and
magnon.",2305.00093v1
2023-05-05,Nonlinear terahertz Néel spin-orbit torques in antiferromagnetic Mn$_2$Au,"Antiferromagnets have large potential for ultrafast coherent switching of
magnetic order with minimum heat dissipation. In novel materials such as
Mn$_2$Au and CuMnAs, electric rather than magnetic fields may control
antiferromagnetic order by N\'eel spin-orbit torques (NSOTs), which have,
however, not been observed on ultrafast time scales yet. Here, we excite
Mn$_2$Au thin films with phase-locked single-cycle terahertz electromagnetic
pulses and monitor the spin response with femtosecond magneto-optic probes. We
observe signals whose symmetry, dynamics, terahertz-field scaling and
dependence on sample structure are fully consistent with a uniform in-plane
antiferromagnetic magnon driven by field-like terahertz NSOTs with a torkance
of (150$\pm$50) cm$^2$/A s. At incident terahertz electric fields above 500
kV/cm, we find pronounced nonlinear dynamics with massive N\'eel-vector
deflections by as much as 30{\deg}. Our data are in excellent agreement with a
micromagnetic model which indicates that fully coherent N\'eel-vector switching
by 90{\deg} within 1 ps is within close reach.",2305.03368v1
2023-05-08,Thermal Hall effects in quantum magnets,"In the recent years, the thermal Hall transport has risen as an important
diagnosis of the physical properties of the elementary excitations in various
quantum materials, especially among the Mott insulating systems where the
electronic transports are often featureless. Here we review the recent
development of thermal Hall effects in quantum magnets where all the relevant
excitations are charge-neutral. In addition to summarizing the existing
experiments, we pay a special attention to the underlying mechanisms of the
thermal Hall effects in various magnetic systems, and clarify the connection
between the microscopic physical variables and the emergent degrees of freedom
in different quantum phases. The external magnetic field is shown to modify the
intrinsic Berry curvature properties of various emergent and/or exotic
quasiparticle excitations in distinct fashions for different quantum systems
and quantum phases, contributing to the thermal Hall transports. These include,
for example, the conventional ones like the magnons in ordered magnets, the
triplons in dimerized magnets, the exotic and fractionalized quasparticles such
as the spinons and the magnetic monopoles in quantum spin liquids. We review
their contribution and discuss their presence in the thermal Hall conductivity
in different physical contexts. We expect this review to provide a useful
guidance for the physical mechanism of the thermal Hall transports in quantum
magnets.",2305.04830v1
2023-06-09,Strain-Tuned Magnetic Frustration in a Square Lattice $J_1$-$J_2$ Material,"Magnetic frustration is a route that can lead to the emergence of novel
ground states, including spin liquids and spin ices. Such frustration can be
introduced through either the geometry of lattice structures or by incompatible
exchange interactions. Identifying suitable strategies to control the degree of
magnetic frustration in real systems is an active field of research. In this
study, we devise a design principle for the tuning of frustrated magnetism on
the square lattice through the manipulation of nearest (NN) and next-nearest
neighbor (NNN) antiferromagnetic (AF) exchange interactions. By studying the
magnon excitations in epitaxially-strained La$_2$NiO$_4$ films using resonant
inelastic x-ray scattering (RIXS) we show that, in contrast to the cuprates,
the dispersion peaks at the AF zone boundary. This indicates the presence of an
AF-NNN spin interaction. Using first principles simulations and an effective
spin-model, we demonstrate the AF-NNN coupling to be a consequence of the
two-orbital nature of La$_2$NiO$_4$. Our results demonstrate that compressive
strain can enhance this coupling, providing a design principle for the
tunability of frustrated magnetism on a square lattice.",2306.05828v1
2023-06-29,Phonon thermal transport shaped by strong spin-phonon scattering in a Kitaev material Na$_2$Co$_2$TeO$_6$,"The recent report of a half-quantized thermal Hall effect in the Kitaev
material $\alpha$-RuCl$_3$ has sparked a strong debate on whether it is
generated by Majorana fermion edge currents or whether other more conventional
mechanisms involving magnons or phonons are at its origin. A more direct
evidence for Majorana fermions which could be expected to arise from a
contribution to the longitudinal heat conductivity $\kappa_{xx}$ at
$T\rightarrow0$ is elusive due to a very complex magnetic field dependence of
$\kappa_{xx}$. Here, we report very low temperature (below 1~K) thermal
conductivity ($\kappa$) of another candidate Kitaev material,
Na$_2$Co$_2$TeO$_6$. The application of a magnetic field along different
principal axes of the crystal reveals a strong directional-dependent
magnetic-field ($\bf B$) impact on $\kappa$. We show that no evidence for
mobile quasiparticles except phonons can be concluded at any field from 0~T to
the field polarized state. In particular, severely scattered phonon transport
is observed across the $B-T$ phase diagram, which is attributed to prominent
magnetic fluctuations. Cascades of phase transitions are uncovered for all $\bf
B$ directions by probing the strength of magnetic fluctuations via a precise
record of $\kappa$($B$). Our results thus rule out recent proposals for
itinerant magnetic excitations in Na$_2$Co$_2$TeO$_6$, and emphasise the
importance of discriminating true spin liquid transport properties from
scattered phonons in candidate materials.",2306.16963v1
2023-07-04,Dynamic paramagnon-polarons in altermagnets,"The combined rotational and time-reversal symmetry breakings that define an
altermagnet lead to an unusual d-wave (or g-wave) magnetization order
parameter, which in turn can be modeled in terms of multipolar magnetic
moments. Here, we show that such an altermagnetic order parameter couples to
the dynamics of the lattice even in the absence of an external magnetic field.
This coupling is analogous to the non-dissipative Hall viscosity and describes
the stress generated by a time-varying strain under broken time-reversal
symmetry. We demonstrate that this effect generates a hybridized
paramagnon-polaron mode, which allows one to assess altermagnetic excitations
directly from the phonon spectrum. Using a scaling analysis, we also
demonstrate that the dynamic strain coupling strongly affects the altermagnetic
phase boundary, but in different ways in the thermal and quantum regimes. In
the ground state for both 2D and 3D systems, we find that a hardening of the
altermagnon mode leads to an extended altermagnetic ordered regime, whereas for
non-zero temperatures in 2D, the softening of the phonon modes leads to
increased fluctuations that lower the altermagnetic transition temperature. In
3D even at finite temperatures the dominant effect is the suppression of
quantum fluctuations. We also discuss the application of these results to
standard ferromagnetic systems.",2307.01855v3
2023-07-11,Adiabatic dynamics of coupled spins and phonons in magnetic insulators,"In conventional \textit{ab initio} methodologies, phonons are calculated by
solving equations of motion involving static interatomic force constants and
atomic masses. The Born-Oppenheimer approximation, where all electronic degrees
of freedom are assumed to adiabatically follow the nuclear dynamics, is also
adopted. This approach does not fully account for the effects of broken
time-reversal symmetry in systems with magnetic order. Recent attempts to
rectify this involve the inclusion of the velocity dependence of the
interatomic forces in the equations of motion, which accounts for time-reversal
symmetry breaking, and can result in chiral phonon modes with non-zero angular
momentum even at the zone center. However, since the energy ranges of phonons
and magnons typically overlap, the spins cannot be treated as adiabatically
following the lattice degrees of freedom. Instead, phonon and spins must be
treated on a similar footing. Focusing on zone-center modes, we propose a
method involving Hessian matrices and Berry curvature tensors in terms of both
phonon and spin degrees of freedom, and describe a first-principles methodology
for calculating these. We then solve Lagrange's equations of motion to
determine the energies and characters of the mixed excitations, allowing us to
quantify, for example, the energy splittings between chiral pairs of phonons in
some cases, and the degree of magnetically induced mixing between infrared and
Raman modes in others. The approach is general, and can be applied to determine
the adiabatic dynamics of any mixed set of slow variables.",2307.05668v2
2023-07-14,Dark matter detection using nuclear magnetization in magnet with hyperfine interaction,"We consider the possibility to detect cosmic light dark matter (DM), i.e.,
axions and dark photons, of mass $\sim 10^{-6}$ eV and $\sim 10^{-4}$ eV, by
magnetic excitation in a magnet with strong hyperfine interaction. In
particular, we consider a canted anti-ferromagnet, MnCO$_3$, as a concrete
candidate material. With spin transfer between nuclear and electron spins
allowed by the hyperfine interaction, nuclear spins become naturally highly
polarized due to an effective (electron-spin-induced) magnetic field, and have
long-range interactions with each other. The collective precession of nuclear
spins, i.e., a nuclear magnon, can be generated by the DM field through the
nucleon-DM interaction, while they are also sensitive to the electron-DM
interaction through the electron-nuclear spin mixing. Compared with
conventional nuclear-spin precession experiments, this system as a DM sensor is
sensitive to higher frequency needing only a small static magnetic field
applied. The system also has collective precession of electron spins, mixed
with nuclear spins, as the additional channels that can be used for DM probes.
We estimate the sensitivity under appropriate readout setups such as an
inductive pick-up loop associated with an LC resonant circuit, or a photon
cavity with a photon counting device. We show that this method covers an
unexplored parameter region of light bosonic DM.",2307.08577v2
2023-07-24,Evidence of Charge-Phonon coupling in Van der Waals materials Ni1-xZnxPS3,"NiPS3 is a Van der Waals antiferromagnet that has been found to display
spin-charge and spin-phonon coupling in its antiferromagnetically ordered state
below TN = 155 K. Here, we study high-quality crystals of site-diluted
Ni1-xZnxPS3 (0 < x < 0.2) using temperature-dependent specific heat and Raman
spectroscopy probes. The site dilution suppresses the antiferromagnetic
ordering in accordance with the mean-field prediction. In NiPS3, we show that
the phonon mode P2 (176 cm-1) associated with Ni vibrations show a distinct
asymmetry due to the Fano resonance, which persists only in the paramagnetic
phase, disappearing below T_N = 155 K. This was further supported by
temperature-dependent Raman data on an 8% Zn-doped crystal (T_N = 135 K) where
Fano resonance similarly van in the magnetically ordered phase. This is
contrary to the behaviour of the Raman mode P9 (570 cm-1), which shows a Fano
resonance at low temperatures below T_N due to its coupling with the two-magnon
continuum. We show that the Fano resonance of P2 arises from its coupling with
an electronic continuum that weakens considerably upon cooling to low
temperatures. In the doped crystals, the Fano coupling is found to enhance with
Zn-doping. These observations suggest the presence of strong electron-phonon
coupling in the paramagnetic phase of NiPS3 due to charge density fluctuations
associated with the negative charge transfer state of Ni.",2307.12733v2
2023-07-24,The Spin Point Groups and their Representations,"The spin point groups are finite groups whose elements act on both real space
and spin space. Among these groups are the magnetic point groups in the case
where the real and spin space operations are locked to one another. The
magnetic point groups are central to magnetic crystallography for strong
spin-orbit coupled systems and the spin point groups generalize these to the
intermediate and weak spin-orbit coupled cases. The spin point groups were
introduced in the 1960's and enumerated shortly thereafter. In this paper, we
complete the theory of spin point groups by presenting an account of these
groups and their representation theory. Our main findings are that the
so-called nontrivial spin point groups (numbering 598 groups) have co-irreps
corresponding exactly to the (co-)irreps of regular or black and white groups
and we tabulate this correspondence for each nontrivial group. However a total
spin group, comprising the product of a nontrivial group and a spin-only group,
has new co-irreps in cases where there is continuous rotational freedom. We
provide explicit co-irrep tables for all these instances. We also discuss new
forms of spin-only group extending the Litvin-Opechowski classes. To exhibit
the usefulness of these groups to physically relevant problems we discuss a
number of examples from electronic band structures of altermagnets to magnons.",2307.12784v2
2023-06-25,Antiferromagnetic $α$-MnTe: Molten-Salt-Assisted Chemical Vapor Deposition Growth and Magneto-Transport Properties,"Antiferromagnetic (AF) materials are attracting increasing interest of
research in magnetic physics and spintronics. Here, we report controllable
synthesis of room-temperature AF $\alpha$-MnTe nanocrystals (N\'eel temperature
~ 307 K) via molten-salt-assisted chemical vapor deposition method. The growth
kinetics are investigated regarding the dependence of flake dimension and
macroscopic shape on growth time and temperature. The high crystalline quality
and atomic structure are confirmed by various crystallographic characterization
means. Cryogenic magneto-transport measurements reveal anisotropic
magnetoresistance (MR) response and a complicated dependence of MR on
temperature, owing to the subtle competition among multiple scattering
mechanisms of thermally excited magnetic disorders (magnon drag), magnetic
transition and thermally populated lattice phonons. Overall positive MR
behavior with twice transitions in magnitude is observed when out-of-plane
external magnetic field ($B$) is applied, while a transition from negative to
positive MR response is recorded when in-plane $B$ is applied. The rich
magnetic transport properties render $\alpha$-MnTe a promising material for
exploiting functional components in magnetic devices.",2307.13615v1
2023-08-03,Flavor-wave theory with quasiparticle damping at finite temperatures: Application to chiral edge modes in the Kitaev model,"We propose a theoretical framework to investigate elementary excitations at
finite temperatures within a localized electron model that describes the
interactions between multiple degrees of freedom, such as quantum spin models
and Kugel-Khomskii models. Thus far, their excitation structures have been
mainly examined using the linear flavor-wave theory, an SU($N$) generalization
of the linear spin-wave theory. These techniques introduce noninteracting
bosonic quasiparticles as elementary excitations from the ground state, thereby
elucidating numerous physical phenomena, including excitation spectra and
transport properties characterized by topologically nontrivial band structures.
Nevertheless, the interactions between quasiparticles cannot be ignored in
systems exemplified by $S=1/2$ quantum spin models, where strong quantum
fluctuations are present. Recent studies have investigated the effects of
quasiparticle damping at zero temperature in such models. In our study,
extending this approach to the flavor-wave theory for general localized
electron models, we construct a comprehensive method to calculate excitation
spectra with the quasiparticle damping at finite temperatures. We apply our
method to the Kitaev model under magnetic fields, a typical example of models
with topologically nontrivial magnon bands. Our calculations reveal that chiral
edge modes undergo significant damping in weak magnetic fields, amplifying the
damping rate by the temperature increase. This effect is caused by collisions
with thermally excited quasiparticles. Since our approach starts from a general
Hamiltonian, it will be widely applicable to other localized systems, such as
spin-orbital coupled systems derived from multi-orbital Hubbard models in the
strong correlation limit.",2308.01711v1
2023-08-21,Rotating Curved Spacetime Signatures from a Giant Quantum Vortex,"Gravity simulators are laboratory systems where small excitations like sound
or surface waves behave as fields propagating on a curved spacetime geometry.
The analogy between gravity and fluids requires vanishing viscosity, a feature
naturally realised in superfluids like liquid helium or cold atomic clouds.
Such systems have been successful in verifying key predictions of quantum field
theory in curved spacetime. In particular, quantum simulations of rotating
curved spacetimes indicative of astrophysical black holes require the
realisation of an extensive vortex flow in superfluid systems. Here we
demonstrate that despite the inherent instability of multiply quantised
vortices, a stationary giant quantum vortex can be stabilised in superfluid
$^4$He. Its compact core carries thousands of circulation quanta, prevailing
over current limitations in other physical systems such as magnons, atomic
clouds and polaritons. We introduce a minimally invasive way to characterise
the vortex flow by exploiting the interaction of micrometre-scale waves on the
superfluid interface with the background velocity field. Intricate wave-vortex
interactions, including the detection of bound states and distinctive analogue
black hole ringdown signatures, have been observed. These results open new
avenues to explore quantum-to-classical vortex transitions and utilise
superfluid helium as a finite temperature quantum field theory simulator for
rotating curved spacetimes.",2308.10773v4
2023-09-07,Coherent spin dynamics between electron and nucleus within a single atom,"The nuclear spin, being much more isolated from the environment than its
electronic counterpart, enables quantum experiments with prolonged coherence
times and presents a gateway towards uncovering the intricate dynamics within
an atom. These qualities have been demonstrated in a variety of nuclear spin
qubit architectures, albeit with limited control over the direct environment of
the nuclei. As a contrasting approach, the combination of electron spin
resonance (ESR) and scanning tunnelling microscopy (STM) provides a bottom-up
platform to study the fundamental properties of nuclear spins of single atoms
on a surface. However, access to the time evolution of these nuclear spins, as
was recently demonstrated for electron spins, remained a challenge. Here, we
present an experiment resolving the nanosecond coherent dynamics of a
hyperfine-driven flip-flop interaction between the spin of an individual
nucleus and that of an orbiting electron. We use the unique local
controllability of the magnetic field emanating from the STM probe tip to bring
the electron and nuclear spins in tune, as evidenced by a set of avoided level
crossings in ESR-STM. Subsequently, we polarize both spins through scattering
of tunnelling electrons and measure the resulting free evolution of the coupled
spin system using a DC pump-probe scheme. The latter reveals a complex pattern
of multiple interfering coherent oscillations, providing unique insight into
the atom's hyperfine physics. The ability to trace the coherent hyperfine
dynamics with atomic-scale structural control adds a new dimension to the study
of on-surface spins, offering a pathway towards dynamic quantum simulation of
low-dimensional magnonics.",2309.03749v1
2023-09-11,Singularity theory of Weyl-point creation and annihilation,"Weyl points (WP) are robust spectral degeneracies, which can not be split by
small perturbations, as they are protected by their non-zero topological
charge. For larger perturbations, WPs can disappear via pairwise annihilation,
where two oppositely charged WPs merge, and the resulting neutral degeneracy
disappears. The neutral degeneracy is unstable, meaning that it requires the
fine-tuning of the perturbation. Fine-tuning of more than one parameter can
lead to more exotic WP mergers. In this work, we reveal and analyze a
fundamental connection of the WP mergers and singularity theory: phase boundary
points of Weyl phase diagrams, i.e., control parameter values where Weyl point
mergers happen, can be classified according to singularity classes of maps
between manifolds of equal dimension. We demonstrate this connection on a
Weyl--Josephson circuit where the merger of 4 WPs draw a swallowtail
singularity, and in a random BdG Hamiltonian which reveal a rich pattern of
fold lines and cusp points. Our results predict universal geometrical features
of Weyl phase diagrams, and generalize naturally to creation and annihilation
of Weyl points in electronic (phononic, magnonic, photonic, etc) band-structure
models, where Weyl phase transitions can be triggered by control parameters
such as mechanical strain.",2309.05506v1
2023-09-20,Magnetic imaging and domain nucleation in CrSBr down to the 2D limit,"Recent advancements in 2D materials have revealed the potential of van der
Waals magnets, and specifically of their magnetic anisotropy that allows
applications down to the 2D limit. Among these materials, CrSBr has emerged as
a promising candidate, because its intriguing magnetic and electronic
properties have appeal for both fundamental and applied research in spintronics
or magnonics. Here, nano SQUID-on-tip (SOT) microscopy is used to obtain direct
magnetic imaging of CrSBr flakes with thicknesses ranging from monolayer (N=1)
to few-layer (N=5). The ferromagnetic order is preserved down to the monolayer,
while the antiferromagnetic coupling of the layers starts from the bilayer
case. For odd layers, at zero applied magnetic field, the stray field resulting
from the uncompensated layer is directly imaged. The progressive spin
reorientation along the out-of-plane direction (hard axis) is also measured
with a finite applied magnetic field, allowing to evaluate the anisotropy
constant, which remains stable down to the monolayer and is close to the bulk
value. Finally, by selecting the applied magnetic field protocol, the formation
of N\'eel magnetic domain walls is observed down to the single layer limit.",2309.11550v1
2023-09-25,Domain wall dynamics driven by a transient laser-induced magnetisation,"One of the fundamental effects of the laser-matter interaction is the
appearance of an induced transient magnetisation. While the underlying
phenomena differ in their microscopic origin and cover a diverse array of
materials, here we address a fundamental question about the possibility to
drive domain-wall dynamics on the femtosecond timescale of the exchange
interactions solely by longitudinal changes of the magnetic moments. We verify
the viability of this hypothesis in the case of a generic ferromagnetic system
described in the framework of the high-temperature micromagnetic model based on
the Landau-Lifshitz-Bloch equation. The effect is investigated in a 1D model at
constant temperature as well as in a full micromagnetic framework considering
realistic laser-induced heating. Our results demonstrate that domain-wall
deformation in a femtosecond timeframe leads to the displacement of the wall on
a larger timescale up to nanoseconds accompanied by a release of excess energy
in the form of spin waves. The domain wall deformation leads to the appearance
of a magnetisation gradient across the wall which promotes the motion towards
the region consisting of spins with decreased magnetisation length. The total
displacement is enhanced at larger temperatures and smaller damping due to an
increase of the longitudinal relaxation time which ensures the longer presence
of the induced magnetisation gradient. We also demonstrate an enhanced domain
wall motion in the presence of the Dzyaloshinskii-Moriya interaction attributed
to augmented magnonic torques. Our results are important towards the
understanding of ultrafast magnetism phenomena on the sub-picosecond timescale.",2309.14287v1
2023-09-29,Planar thermal Hall effect from phonons in a Kitaev candidate material,"Kitaev materials are a promising platform for the realization of quantum spin
liquid states. The thermal Hall effect has emerged as a potential probe of
exotic excitations within such states. In the Kitaev candidate material
$\alpha$-RuCl$_{3}$, however, the thermal Hall conductivity $\kappa_{xy}$ has
been attributed not only to exotic Majorana fermions or chiral magnons, but
also to phonons. It has been shown theoretically that the former two types of
heat carriers can generate a ""planar"" thermal Hall effect, whereby the magnetic
field is parallel to the heat current, as observed experimentally, but it is
unknown whether phonons also could. Here we show that a planar thermal Hall
effect is present in another Kitaev candidate material,
Na$_{2}$Co$_{2}$TeO$_{6}$. On the basis of a striking similarity between the
temperature and field dependence of $\kappa_{xy}$ and that of the
phonon-dominated thermal conductivity $\kappa_{xx}$, we argue that the planar
thermal Hall effect in Na$_{2}$Co$_{2}$TeO$_{6}$ is generated by phonons. The
phonon contributed planar $\kappa_{xy}$ also shows a strong sample dependence,
which indicates an extrinsic origin of the mechanism. By conducting a complete
study with different in-plane configurations of heat current $J$ and magnetic
field $H$, $i.e.$ $H$ $\|$ $J$ and $H$ $\bot$ $J$, we observe a large
difference in $\kappa_{xy}$ between these two configurations, which reveals
that the direction of the heat current $J$ may play an important role in
determining the planar thermal Hall effect. Our observation calls for a
re-evaluation of the planar thermal Hall effect observed in
$\alpha$-RuCl$_{3}$.",2309.17231v2
2023-10-01,Effect of Spin Fluctuations on Magnetoresistance and Anomalous Hall Effect in the Chiral Magnet Co8Zn8Mn4,"The beta Mn type Co-Zn-Mn alloys have seized significant attention due to
their ability to host skyrmions at room temperature. Here we analyse the
unconventional magneto-transport properties of Co8Zn8Mn4 single crystals with a
Curie temperature of 275 K. A negative magnetoresistance is obtained over a
wide temperature range of 50K to 300K. The deviation of the isothermal
magnetoresistance (MR) curves from linearity to non-linearity as one approaches
higher temperatures points towards the transition from the dominance of magnons
to spin fluctuations. In the paramagnetic phase, the change in the shape of the
MR curve has been explained using the Khosla and Fischer model. The
relationship between the anomalous Hall effect (AHE) and longitudinal
resistivity reveals the dominance of the skew-scattering mechanism, which is
inexplicable based on the theories of semi-classical magneto-transport. We
experimentally determine that the spin fluctuation is the source of the
skew-scattering mechanism in Co8Zn8Mn4. In general skew-scattering mechanisms
predominate in compounds with high conductivity, but our findings demonstrate
that this is not always the case and that other aspects also require equal
consideration. Our work throws new light on the predominant scattering
mechanism in chiral magnets with skyrmionics phase at low conductivity.",2310.00739v1
2023-10-11,Planar thermal Hall effect from phonons in cuprates,"A surprising ""planar"" thermal Hall effect, whereby the field is parallel to
the current, has recently been observed in a few magnetic insulators, and this
has been attributed to exotic excitations such as Majorana fermions or chiral
magnons. Here we investigate the possibility of a planar thermal Hall effect in
three different cuprate materials, in which the conventional thermal Hall
conductivity $\kappa_{\rm {xy}}$ (with an out-of-plane field perpendicular to
the current) is dominated by either electrons or phonons. Our measurements show
that the planar $\kappa_{\rm {xy}}$ from electrons in cuprates is zero, as
expected from the absence of a Lorentz force in the planar configuration. By
contrast, we observe a sizable planar $\kappa_{\rm {xy}}$ in those samples
where the thermal Hall response is due to phonons, even though it should in
principle be forbidden by the high crystal symmetry. Our findings call for a
careful re-examination of the mechanisms responsible for the phonon thermal
Hall effect in insulators.",2310.07696v2
2023-10-16,Implementation of a laser-neutron pump-probe capability at HYSPEC,"Exciting new fundamental scientific questions are currently being raised
regarding nonequilibrium dynamics in spin systems, as this directly relates to
low power and low loss energy transport for spintronics. Inelastic neutron
scattering (INS) is an indispensable tool to study spin excitations in complex
magnetic materials. However, conventional INS spectrometers currently only
perform steady-state measurements and probe averaged properties over many
collision events between spin excitations in thermodynamic equilibrium, while
the exact picture of re-equilibration of these excitations remains unknown. In
this work, we designed and implemented a time-resolved laser-neutron pump-probe
capability at HYSPEC (Hybrid Spectrometer, beamline 14-B) at the Spallation
Neutron Source (SNS) at Oak Ridge National Laboratory. This capability allows
us to excite out-of-equilibrium magnons with a nanosecond pulsed laser source
and probe the resulting dynamics using INS. Here, we discussed technical
aspects to implement such a capability in a neutron beamline, including choices
of suitable neutron instrumentation and material systems, laser excitation
scheme, experimental configurations, and relevant firmware and software
development to allow for time-synchronized pump-probe measurements. We
demonstrated that the laser-induced nonequilibrium structural factor is able to
be resolved by INS in a quantum magnet. The method developed in this work will
provide SNS with advanced capabilities for performing out-of-equilibrium
measurements, opening up an entirely new research direction to study
out-of-equilibrium phenomena using neutrons.",2310.10540v1
2023-10-27,Effect of interfacial Dzyaloshinskii-Moriya interaction in spin dynamics of an Antiferromagnet coupled Ferromagnetic double-barrier Magnetic Tunnel Junction,"In this work, we have studied the spin dynamics of a synthethic
Antiferromagnet (SAFM)$|$Heavy Metal (HM)$|$Ferromagnet (FM) double barrier
magnetic tunnel junction (MTJ) in presence of Ruderman-Kittel-Kasuya-Yoside
interaction (RKKYI), interfacial Dzyaloshinskii-Moriya interaction (iDMI),
N\'eel field and Spin-Orbit Coupling (SOC) with different Spin Transfer Torque
(STT). We employ Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation to
investigate the AFM dynamics of the proposed system. We found that the system
exhibits a transition from regular to damped oscillations with the increase in
strength of STT for systems with weaker iDMI than RKKYI while display sustained
oscillatons for system having same order of iDMI and RKKYI. On the other hand
the iDMI dominating system exhibits self-similar but aperiodic patterns in
absence of N\'eel field. In the presence of N\'eel field, the RKKYI dominating
systems exhibit chaotic oscillations for low STT but display sustained
oscillation under moderate STT. Our results suggest that the decay time of
oscillations can be controlled via SOC. The system can works as an oscillator
for low SOC but display nonlinear characteristics with the rise in SOC for
systems having weaker iDMI than RKKYI while an opposite characteristic are
noticed for iDMI dominating systems. We found periodic oscillations under low
external magnetic field in RKKYI dominating systems while moderate field are
necessary for sustained oscillation in iDMI dominating systems. Moreover, the
system exhibits saddle-node bifurcation and chaos under moderate N\'eel field
and SOC with suitable iDMI and RKKYI. In addition, our results indicate that
the magnon lifetime can be enhanced by increasing the strength of iDMI for both
optical and acoustic modes.",2310.18175v1
2023-11-09,Bose-Einstein condensation in systems with flux equilibrium,"We consider flux equilibrium in dissipative nonlinear wave systems subject to
external energy pumping. In such systems, the elementary excitations, or
quasiparticles, can create a Bose-Einstein condensate. We develop a theory on
the Bose-Einstein condensation of quasiparticles for various regimes of
external excitation, ranging from weak and stationary to ultra-strong pumping,
enabling us to determine the number of quasiparticles near the bottom of the
energy spectrum and their distribution along wave vectors. We identify physical
phenomena leading to condensation in each of the regimes. For weak stationary
pumping, where the distribution of quasiparticles deviates only slightly from
thermodynamic equilibrium, we define a range of pumping parameters where the
condensation occurs and estimate the density of the condensate and the fraction
of the condensed quasiparticles. As the pumping amplitude increases, a powerful
influx of injected quasiparticles is created by the Kolmogorov-Zakharov
scattering cascade, leading to their Bose-Einstein condensation. With even
stronger pumping, kinetic instability may occur, resulting in a direct transfer
of injected quasiparticles to the bottom of the spectrum. For the case of
ultra-strong parametric pumping, we have developed a stationary nonlinear
theory of kinetic instability. The theory agrees qualitatively with
experimental data obtained using Brillouin light scattering spectroscopy during
parametric pumping of magnons in room-temperature films of yttrium-iron garnet.",2311.05315v1
2023-11-13,Multiple topological phase transitions in magnetic systems: role of anisotropic Dzyaloshinskii-Moriya interaction,"In this work we study the topological properties of magnons in both
ferromagnetically and antiferromagnetically ordered systems on a
two-dimensional square lattice. While topological features emerge owing to the
violation of time reversal symmetry induced by the Dzyaloshinskii-Moriya
interaction (DMI), here we consider an anisotropic DMI and ascertain the role
of the anisotropy parameter on the topological phase transitions. In
particular, our phase plot for the ferromagnetic case demonstrates multiple
topological phase transitions, that is, from a topological phase to another
topological phase, and as well, from a topological phase to a trivial phase. In
the former, the topological invariant, namely the Chern number, discontinuously
changes from one finite value to another ($-1 \rightarrow +1$), while it
vanishes ($1 \rightarrow 0$) in the latter. Further, an extended trivial region
is observed where the DMI is non-zero, which is surprising since DMI is the
origin of the topological gap. The nature of the phases and the phase
transitions, therein are characterized by their band structure, presence (or
absence) of the chiral edge modes, computation of the topological Hall effect,
etc. In contrast, the antiferromagnetic case presents a more predictable
scenario, in the sense, it behaves as a trivial insulator in the absence of
DMI, although demonstrate a topological to trivial phase transition.",2311.07201v1
2023-11-17,Calibration Unit Design for High-Resolution Infrared Spectrograph for Exoplanet Characterization (HISPEC),"The latest generation of high-resolution spectrograph instruments on
10m-class telescopes continue to pursue challenging science cases.
Consequently, ever more precise calibration methods are necessary to enable
trail-blazing science methodology. We present the High-resolution Infrared
SPectrograph for Exoplanet Characterization (HISPEC) Calibration Unit (CAL),
designed to facilitate challenging science cases such as Doppler imaging of
exoplanet atmospheres, precision radial velocity, and high-contrast
high-resolution spectroscopy of nearby exoplanets. CAL builds upon the heritage
from the pathfinder instrument Keck Planet Imager and Characterizer (KPIC) and
utilizes four near-infrared (NIR) light sources encoded with wavelength
information that are coupled into single-mode fibers. They can be used
synchronously during science observations or asynchronously during daytime
calibrations. A hollow cathode lamp (HCL) and a series of gas absorption cells
provide absolute calibration from 0.98 {\mu}m to 2.5 {\mu}m. A laser frequency
comb (LFC) provides stable, time-independent wavelength information during
observation and CAL implements a lower finesse astro-etalon as a backup for the
LFC. Design lessons from instrumentation like HISPEC will serve to inform the
requirements for similar instruments for the ELTs in the future.",2311.10239v1
2023-11-20,Thermal Hall effects due to topological spin fluctuations in YMnO$_3$,"The thermal Hall effect in magnetic insulators has been considered a powerful
method for examining the topological nature of charge-neutral quasiparticles
such as magnons. Yet, unlike the kagome system, the triangular lattice has
received less attention for studying the thermal Hall effect because the scalar
spin chirality cancels out between adjacent triangles. However, such
cancellation cannot be perfect if the triangular lattice is distorted, which
could open the possibility of a non-zero thermal Hall effect. Here, we report
that the trimerized triangular lattice of multiferroic hexagonal manganite
YMnO$_3$ produces a highly unusual thermal Hall effect due to topological spin
fluctuations with the additional intricacy of a Dzyaloshinskii-Moriya
interaction under an applied magnetic field. We conclude the thermal Hall
conductivity arises from the system's topological nature of spin fluctuations.
Our theoretical calculations demonstrate that the thermal Hall conductivity is
also related in this material to the splitting of the otherwise degenerate two
chiralities, left and right, of its 120$^{\circ}$ magnetic structure. Our
result is one of the most unusual cases of topological physics due to this
broken $Z_2$ symmetry of the chirality in the supposedly paramagnetic state of
YMnO$_3$, with strong topological spin fluctuations. These new mechanisms in
this important class of materials are crucial in exploring new thermal Hall
physics and exotic excitations.",2311.11527v1
2023-12-20,An effective field theory of damped ferromagnetic systems,"Using the in-in formalism, we generalize the recently constructed
magnetoelastic EFT arXiv:2112.13873 [hep-th] to describe the damping dynamics
of ferromagnetic systems at long wavelengths. We find that the standard Gilbert
damping term naturally arises as the simplest leading-order symmetry-consistent
non-conservative contribution within the in-in framework. The EFT is easily
generalized to scenarios with anisotropy and inhomogeneity. In particular, we
find the classic Landau-Lifshitz damping term emerges when isotropy is broken
by a constant external background field. This provides a first principle
explanation for distinguishing the two types of damping dynamics that were
originally constructed phenomenologically. Furthermore, the EFT framework could
also incorporate intrinsic anisotropy of the material in a straightforward way
using the spurion method. For systems with inhomogeneity such as nontrivial
spin textures, we find that the leading order derivative correction yields the
generalized Gilbert damping equations that were found in condensed matter
literature. This shows that the EFT approach enables us to derive the form of
higher-derivative-order corrections in a systematic way. Lastly, using the
phonon-magnon coupling deduced in the magnetoelastic EFT, we are able to make a
prediction for the generic form of the phononic contribution to the damping
equation.",2312.13093v1
2024-01-05,Solutions to the Landau-Lifshitz-Gilbert equation in the frequency space: Discretization schemes for the dynamic-matrix approach,"The dynamic matrix method addresses the Landau-Lifshitz-Gilbert (LLG)
equation in the frequency domain by transforming it into an eigenproblem.
Subsequent numerical solutions are derived from the eigenvalues and
eigenvectors of the dynamic matrix. In this work we explore discretization
methods needed to obtain a matrix representation of the dynamic operator, a
fundamental counterpart of the dynamic matrix. Our approach opens a new set of
linear algebra tools for the dynamic matrix method and expose the
approximations and limitations intrinsic to it. Moreover, our discretization
algorithms can be applied to various discretization schemes, extending beyond
micromagnetism problems. We present some application examples, including a
technique to obtain the dynamic matrix directly from the magnetic free energy
function of an ensemble of macrospins, and an algorithmic method to calculate
numerical micromagnetic kernels, including plane wave kernels. We also show how
to exploit symmetries and reduce the numerical size of micromagnetic
dynamic-matrix problems by a change of basis. This procedure significantly
reduces the size of the dynamic matrix by several orders of magnitude while
maintaining high numerical precision. Additionally, we calculate analytical
approximations for the dispersion relations in magnonic crystals. This work
contributes to the understanding of the current magnetization dynamics methods,
and could help the development and formulations of novel analytical and
numerical methods for solving the LLG equation within the frequency domain.",2401.02933v2
2024-01-05,Dark Matter Candidates of a Very Low Mass,"We review dark matter (DM) candidates of a very low mass, appearing in the
window below the traditional weakly-interacting massive particle $m_\chi
\lesssim 10$ GeV and extending down to $m_\chi \gtrsim 1$ meV, somewhat below
the mass limit where DM becomes wavelike. Such candidates are motivated by
hidden sectors such as Hidden Valleys, which feature hidden forces and rich
dynamics, but have evaded traditional collider searches looking for New Physics
because of their relatively weak coupling to the Standard Model. Such sectors
can still be detected through dedicated low-energy colliders which, through
their intense beams, can have sensitivity to smaller couplings, or through
astrophysical observations of the evolution of DM halos and stellar structures
which, through the Universe's epochs, can be sensitive to small DM
interactions. We also consider mechanisms where the DM abundance is fixed
through the interaction with the SM, which directly motivates the search for
light DM in terrestrial experiments. The bulk of this review is dedicated to
the new ideas that have been proposed for directly detecting such DM candidates
of a low mass, through nuclear recoils, electronic excitations, or collective
modes such as phonons and magnons. The rich tapestry of materials and modes in
the Condensed Matter landscape is reviewed, along with specific prospects for
detection.",2401.03025v1
2024-01-09,Microwave magnetic excitations in U-type hexaferrite Sr$_4$CoZnFe$_{36}$O$_{60}$ ceramics,"Microwave (MW) spectra of ferrimagnetic U-hexaferrite
Sr$_4$CoZnFe$_{36}$O$_{60}$ ceramics were studied using several experimental
techniques from 100 MHz to 50 GHz at temperatures between 10 and 390 K. They
revealed 9 excitations, which exhibit remarkable temperature dependences near
the magnetic phase transitions at 145 K and 305 K. Some of them also change
under the application of a weak bias magnetic field at room temperature. Three
lowest-frequency modes seen between 200 MHz and 3 GHz were assigned to the
dynamics of the magnetic domains. The mode attributed to the natural
ferromagnetic resonance exhibits a dramatic critical slowing-down from 25 GHz
at 390 K to 5 GHz near the 305 K phase transition, and again a hardening to ~11
GHz on further cooling. The higher-frequency excitations are most likely spin
wave (magnon) modes arising from the complex ferrimagnetic structure of
Sr$_4$CoZnFe$_{36}$O$_{60}$. The high sensitivity of the MW spectra to the weak
magnetic bias field $H$ < 700 Oe at room temperature is shown to be caused by
the transformation of the polydomain magnetic structure in randomly oriented
ceramic grains to a monodomain one. The MW response measured above 2 GHz using
coplanar and microstrip lines with different electromagnetic field distribution
and sample coupling revealed the same excitations with similar temperature and
bias field dependences. It confirms the reliability of the results and proves
the effectiveness of the used MW techniques.",2401.04678v1
2024-01-12,Superorders and acoustic modes folding in BiFeO$_3$/LaFeO$_3$ superlattices,"Superlattices are materials created by the alternating growth of two
chemically different materials. The direct consequence of creating a
superlattice is the folding of the Brillouin zone which gives rise to
additional electronic bands and phonon modes. This has been successfully
exploited to achieve new transport and optical properties in semiconductor
superlattices, for example. Here, we show that multiferroic BiFeO$_3$/LaFeO$_3$
superlattices are more than just periodic chemical stacking. Using transmission
electron microscopy, X-ray diffraction and first-principles calculations, we
demonstrate the existence of a new order of FeO$_6$ octahedra, with a period
along the growth direction about twice that of the chemical supercell, i.e. a
superorder. The effect of this new structural order on the lattice dynamics is
studied with ultrafast optical pump-probe experiments. While a mode at 1.2 THz
is attributed solely to the chemical modulation of the superlattice, the
existence of another 0.7 THz mode seems to be explained only by a double
Brillouin zone folding in agreement with the structural description. Our work
shows that multiferroic BiFeO$_3$/LaFeO$_3$ superlattices can be used to tune
the spectrum of coherent THz phonons, and potentially that of magnons or
electromagnons.",2401.06647v1
2024-01-21,Exploring Intrinsic Magnetic Topological Insulators: The Case of EuIn$_2$As$_2$,"In this study, ultrafast optical spectroscopy was employed to elucidate the
intricate topological features of EuIn$_2$As$_2$, a promising candidate for a
magnetic topological-crystalline axion insulator. Our investigation, focusing
on the real-time evolution of topological states, unveiled a narrow surface
magnetic gap (2$\Delta_0$ $\simeq$ 8.2 meV)) emerging at the antiferromagnetic
transition temperature ($T_N$ $\approx$ 16 K). Below $T_N$, two extremely
low-energy collective modes, $\omega_1$ and $\omega_2$, with frequencies of
$\sim$9.9 and 21.6 GHz at $T$ = 4 K, respectively, were observed, exhibiting
strong temperature dependence. $\omega_1$ correlates with an acoustic phonon,
while $\omega_2$ is associated with a magnon. The results suggest that
EuIn$_2$As$_2$ has the potential to manifest a magnetic topological-crystalline
axion insulator, presenting a small magnetic energy gap on the (001) surface.
The findings further our understanding of the interplay between magnetism and
topology in this material, showcasing its potential for applications in quantum
information processing and spintronics.",2401.11386v1
2024-02-02,Skyrmions: A review on materials perspective for future electronic devices,"Recent years have witnessed an enormous rise in research interest in magnetic
skyrmions owing to their capability to improve over contemporary spintronic
devices. An overview of the various magnetic interactions responsible for the
formation of skyrmion together with distinct noncentrosymmetric and
centrosymmetric skyrmion candidates is given in this review article. The
magnetic interactions known as Dzyaloshinskii-Moriya interactions (DMI) have
been extensively studied over the years to better understand the mechanism of
skyrmions in chiral magnets that have larger skyrmion sizes. Because of their
low skyrmion size, the centrosymmetric frustrated magnets are dwelling to
skyrmions controlled by long-range interactions such as the
Ruderman-Kittel-Kasuya-Yosida interaction (RKKY), which may be useful in the
development of high-density memory devices. To lay a solid foundation for the
magnetic interactions involved in skyrmion formations and many other special
physical properties, more research in the field of centrosymmetric skyrmions is
required. Apart from studying candidates with low skyrmion sizes, one of the
main goals for the future is to better understand the dynamics of skyrmion
using polarized magnons, which has the potential to be extremely beneficial for
spintronic applications.",2402.01503v3
2024-02-05,Tracing d-d transitions in FePS$_{3}$ on ultrafast time scales,"Excitations between localized 3d states of transition metal ions within
crystalline solids, commonly known as d-d transitions, play a pivotal role in
diverse phenomena across solid state physics, materials science, and chemistry.
These transitions contribute to the coloration in transition metal oxides,
catalytic processes on oxide surfaces, and high-temperature superconductivity.
They also couple optical excitation to quantized collective phenomena such as
phonons and magnons in magnetic systems. Until now, an experimental method to
unravel the complex quasiparticle dynamics associated with d-d transitions has
remained elusive. We bridge this gap by demonstrating that d-d transitions can
be distinctly traced in momentum space and time using time- and angle-resolved
photoelectron spectroscopy (trARPES). Through this approach, we can assign
specific momentum-dependent characteristics and elucidate the decay mechanisms
of specific d-d transitions in FePS$_{3}$, a two-dimensional van der Waals
antiferromagnet with a rich array of quantum phenomena stemming from d-d
transitions. This study pioneers the use of ARPES in probing the dynamics of
d-d transitions across a wide spectrum of solid-state systems.",2402.03018v1
2024-03-06,"Crystal, ferromagnetism, and magnetoresistance with sign reversal in a EuAgP semiconductor","We synthesized the ferromagnetic EuAgP semiconductor and conducted a
comprehensive study of its crystalline, magnetic, heat capacity, band gap, and
magnetoresistance properties. Our investigation utilized a combination of X-ray
diffraction, optical, and PPMS DynaCool measurements. EuAgP adopts a hexagonal
structure with the $P6_3/mmc$ space group. As the temperature decreases, it
undergoes a magnetic phase transition from high-temperature paramagnetism to
low-temperature ferromagnetism. We determined the ferromagnetic transition
temperature to be $T_{\textrm{C}} =$ 16.45(1) K by fitting the measured
magnetic susceptibility using a Curie-Weiss law. Heat capacity analysis of
EuAgP considered contributions from electrons, phonons, and magnons, revealing
$\eta$ = 0.03 J/mol/$\textrm{K}^\textrm{2}$, indicative of semiconducting
behavior. Additionally, we calculated a band gap of $\sim$ 1.324(4) eV based on
absorption spectrum measurements. The resistivity versus temperature of EuAgP
measured in the absence of an applied magnetic field shows a pronounced peak
around $T_{\textrm{C}}$, which diminishes rapidly with increasing applied
magnetic fields, ranging from 1 to 14 T. An intriguing phenomenon emerges in
the form of a distinct magnetoresistance transition, shifting from positive
(e.g., 1.95\% at 300 K and 14 T) to negative (e.g., -30.73\% at 14.25 K and 14
T) as the temperature decreases. This behavior could be attributed to
spin-disordered scattering.",2403.03650v1
2024-03-12,Crystal design of altermagnetism,"Symmetry plays a fundamental role in condensed matter. The unique
entanglement between magnetic sublattices and alternating crystal environment
in altermagnets provides a unique opportunity for designing magnetic space
symmetry. There have been extensive experimental efforts concentrated on tuning
the Neel vector to reconstruct altermagnetic symmetry. However, it remains
challenging to modulate the altermagnetic symmetry through the crystal aspect.
Here, the crystal design of altermagnetism is successfully realized, by
breaking glide mirrors and magnetic mirrors of the (0001) crystallographic
plane in CrSb films via crystal distortion. We establish a locking relationship
between altermagnetic symmetry and the emergent Dzyaloshinskii-Moriya (DM)
vectors in different CrSb films, realizing unprecedentedly room-temperature
spontaneous anomalous Hall effect in an altermagnetic metal. The concept of
exchange-coupling torques is broadened to include both antiferromagnetic
exchange-coupling torque and DM torque. Their relationship is designable,
determining electrical manipulation modes, e.g., field-assisted switching for
CrSb(1-100)/Pt and field-free switching for W/CrSb(11-20). Particularly, the
unprecedentedly field-free 100-percent switching of Neel vectors is realized by
making these two torques parallel or antiparallel, dependent on Neel vector
orientation. Besides unravelling the rich mechanisms for electrical
manipulation of altermagnetism rooted in broadened concept of exchange-coupling
torques, we list other material candidates and propose that crystal design of
altermagnetism would bring rich designability to magnonics, topology, etc.",2403.07396v1
2024-03-12,A compact approach to higher-resolution resonant inelastic X-ray scattering detection using photoelectrons,"The detection of inelastically scattered soft X-rays with high energy
resolution usually requires large grating spectrometers. Recently,
photoelectron spectrometry for analysis of X-rays (PAX) has been rediscovered
for modern spectroscopy experiments at synchrotron light sources. By converting
scattered photons to electrons and using an electron energy analyser, the
energy resolution for resonant inelastic X-ray scattering (RIXS) becomes
decoupled from the X-ray spot size and instrument length. In this work, we
develop PAX towards high energy resolution using a modern photoemission
spectroscopy setup studying Ba2Cu3O4Cl2 at the Cu L3-edge. We measure a
momentum transfer range of 24% of the first Brillouin zone simultaneously. Our
results hint at the observation of a magnon excitation below 100 meV energy
transfer and show intensity variations related to the dispersion of
dd-excitations. With dedicated setups, PAX can become an alternative to the
best and largest RIXS instruments, while at the same time opening new
opportunities to acquire RIXS at a range of momentum transfers simultaneously
and combine it with angle-resolved photoemission spectroscopy in a single
instrument.",2403.07452v1
2024-03-17,Merons and magnetoelectric switching in centrosymmetric spiral magnets,"Spiral multiferroics exhibit a strong coupling between magnetic and
ferroelectric orders, allowing cross-control. Since the seminal work of Kimura
et al. in 2003, these materials have attracted great interest galvanized by the
prospect of new high-efficiency memory devices, where magnetic (electric) bits
are switched via an external electric (magnetic) field. Nevertheless, the
mechanism underlying such a switching process - the electric field-driven
dynamics of domain walls (DWs) - is still poorly understood. We address this
problem for meron DWs, which represent one of the main DW types in spiral
multiferroics and consist of an array of meron (half-skyrmion) strings. Minimum
energy walls feature merons with alternating topological charges and move as
relativistic massive particles, with the limiting velocity set by magnon speed.
Low-energy defects in this alternating charge sequence, which appear during
domain nucleation, can lead to DWs with net topological charge. This induces a
peculiar non-local dynamics where all the spins in the system rotate, merons
translate within the wall, and the DW mobility is suppressed. The topological
charge of the wall and the meron helicity can be easily modified via an
external magnetic and electric field, respectively, offering fine control over
DW dynamics. Defects within the meron strings, analogous to Bloch points, have
hedgehog-like spin texture and are strongly pinned to the lattice. The
fascinating interplay between domain wall motion, translation of merons within
the wall, and precession of spins in the entire domains opens a new playground
for the electric manipulation of topological spin textures.",2403.11195v1
2024-03-25,Giant tunability of magnetoelasticity in Fe$_4$N system: Platform for unveiling correlation between magnetostriction and magnetic damping,"Flexible spintronics has opened new avenue to promising devices and
applications in the field of wearable electronics. Particularly, miniaturized
strain sensors exploiting the spintronic function have attracted considerable
attention, in which the magnetoelasticity linking magnetism and lattice
distortion is a vital property for high-sensitive detection of strain. This
paper reports the demonstration that the magnetoelastic properties of Fe$_4$N
can be significantly varied by partially replacing Fe with Co or Mn. The high
quality Fe$_4$N film exhibits large negative magnetostriction along the [100]
direction ($\lambda_{100}$) of -121 ppm while Fe$_{3.2}$Co$_{0.8}$N shows
$\lambda_{100}$ of +46 ppm. This wide-range tunability of $\lambda_{100}$ from
-121 to +46 across 0 allows us to thoroughly examine the correlation between
the magnetoelasticity and other magnetic properties. The strong correlation
between $\lambda_{100}$ and magnetic damping ($\alpha$) is found. The enhanced
extrinsic term of $\alpha$ is attributable to the large two magnon scattering
coming from the large magnetostriction. In addition, the density of states at
the Fermi level plays a primal role to determine both $\lambda_{100}$ and the
intrinsic term of $\alpha$. Thanks to the giant tunability and the bipolarity
of magnetoelasticity, magnetic nitrides are candidate materials for
high-sensitive spintronic strain sensors.",2403.16679v1
2024-04-04,Waveguide QED at the onset of spin-spin correlations,"We experimentally explore the competition between light-mediated and direct
matter-matter interactions in waveguide quantum electrodynamics. For this, we
couple a superconducting line to a model magnetic material, made of organic
free radical DPPH molecules with a spin $S=1/2$ and a $g_{S}$ factor very close
to that of a free electron. The microwave transmission has been measured in a
wide range of temperatures ($0.013$ K $\leq T \leq 2$ K), magnetic fields
($0\leq B \leq 0.5$ T) and frequencies ($0 \leq \omega/2 \pi \leq 14$ GHz). We
find that molecules belonging to the crystal sublattice B form one-dimensional
spin chains. Temperature then controls intrinsic spin correlations along the
chain in a continuous and monotonic way. In the paramagnetic region ($T > 0.7$
K), the microwave transmission shows evidences for the collective coupling of
quasi-identical spins to the propagating photons, with coupling strengths that
reach values close to the dissipation rates. As $T$ decreases, the growth of
intrinsic spin correlations, combined with the anisotropy in the spin-spin
exchange constants, break down the collective spin-photon coupling. In this
regime, the temperature dependence of the spin resonance visibility reflects
the change in the nature of the dominant spin excitations, from single spin
flips to bosonic magnons, which is brought about by the magnetic correlation
growth.",2404.03727v1
2000-01-11,Magnetic Susceptibilities of Spin-1/2 Antiferromagnetic Heisenberg Ladders and Applications to Ladder Oxide Compounds,"A comprehensive theoretical and experimental study is presented of the
magnetic susceptibility versus temperature \chi(T) of spin S = 1/2 two- and
three-leg Heisenberg ladders and ladder oxide compounds. Extensive quantum
Monte Carlo simulations of \chi(T) were carried out for both isolated and
coupled two-leg ladders with spatially anisotropic intraladder exchange.
Accurate fits to these and related literature QMC data were obtained. We have
also calculated the one- and two-magnon dispersion relations and the dynamical
spin structure factor for anisotropic isolated 2 x 12 ladders. The exchange
constants in the two-leg ladder compound SrCu2O3 are estimated from LDA+U
calculations. We report the detailed crystal structure of SrCu2O3 and of the
three-leg ladder compound Sr2Cu3O5. New experimental \chi(T) data are reported
for the two-leg ladder cuprates SrCu2O3 and LaCuO_{2.5}, and for the
(nominally) two-leg ladder vanadates CaV2O5 and MgV2O5. The new and literature
\chi(T) data for these compounds and for Sr2Cu3O5 are modeled using our QMC
\chi(T) simulation fits, and the exchange coupling constants between the
spins-1/2 are thereby estimated for each material. The surpisingly strong
spatial anisotropy of the bilinear intraladder exchange constants in the
cuprate compounds is discussed together with the results of other experiments
sensitive to this anisotropy. Recent theoretical predictions are discussed
including those which indicate that a four-spin cyclic exchange interaction
within a Cu4 plaquette is important to determining the magnetic properties and
which can significantly influence the exchange interactions estimated from
\chi(T) data assuming the presence of only bilinear exchange.",0001147v1
2000-01-13,Extensive infrared spectroscopic study of CuO: signatures of strong spin-phonon interaction and structural distortion,"Optical properties of single-crystal monoclinic CuO in the range 70 - 6000
\cm were studied at temperatures from 7 to 300 K. Normal reflection spectra
were obtained from the (001) and (010) crystal faces thus giving for the first
time separate data for the $A_{u}$ and $B_{u}$ phonon modes excited in the
purely transverse way (TO modes). Mode parameters, including polarizations of
the $B_{u}$ modes not determined by the crystal symmetry, were extracted by the
dispersion analysis of reflectivity curves as a function of temperature.
Spectra of all the components of the optical conductivity tensor were obtained
using the Kramers-Kronig method recently extended to the case of the
low-symmetry crystals. The number of strong phonon modes is in agreement with
the factor-group analysis for the crystal structure, currently accepted for the
CuO. However, several ""extra"" modes of minor intensity are detected. Comparison
of frequencies of ""extra"" modes with the available phonon dispersion curves
points to possible ""diagonal"" doubling of the unit cell \{{\bf a}, {\bf b},
{\bf c}\} $\to$ \{{\bf a}+{\bf c}, {\bf b}, {\bf a}-{\bf c}\} and formation of
the superlattice. The previously reported softening of the $A^{3}_{u}$ mode
($\sim$ 400 \cm) with cooling at $T_{N}$ is found to be $\sim$ 10 % for the TO
mode. The mode is very broad at high temperatures and strongly narrows in the
AFM phase. We attribute this effect to strong resonance coupling of this mode
to optical or acoustic bi-magnons and reconstruction of the magnetic
excitations spectrum at the N\'eel point. A significant anisotropy of
$\epsilon^{\infty}$ is observed: it was found to be 5.9 along the {\bf b}-axis,
6.2 along the {[}101{]} chains and 7.8 the {[}10$\bar{1}${]} chains. The
""transverse"" effective charge is value is about 2 electrons.",0001176v2
2001-01-08,Influence of s-d interfacial scattering on the magnetoresistance of magnetic tunnel junctions,"We propose the two-band s-d model to describe theoretically a diffuse regime
of the spin-dependent electron transport in magnetic tunnel junctions (MTJ's)
of the form F/O/F where F's are 3d transition metal ferromagnetic layers and O
is the insulating spacer. We aim to explain the strong interface sensitivity of
the tunneling properties of MTJ's and investigate the influence of electron
scattering at the nonideal interfaces on the degradation of the TMR magnitude.
The generalized Kubo formalism and the Green's functions method were used to
calculate the conductance of the system. The vertex corrections to the
conductivity were found with the use of ""ladder"" approximation combined with
the coherent-potential approximation (CPA) that allowed to consider the case of
strong electron scattering. It is shown that the Ward identity is satisfied in
the framework of this approximation that provides the necessary condition for a
conservation of a tunneling current. Based on the known results of ab-initio
calculations of the TMR for ballistic junctions, we assume that exchange split
quasi-free s-like electrons with the density of states being greater for the
majority spin sub-band give the main contribution to the TMR effect. We show
that, due to interfacial inter-band scattering, the TMR can be substantially
reduced even down to zero value. This is related to the fact that delocalized
quasi-free electrons can scatter into the strongly localized d sub-band with
the density of states at the Fermi energy being larger for minority spins
compared to majority spins. It is also shown that spin-flip electron scattering
on the surface magnons within the interface leads to a further decrease of the
TMR at finite temperature.",0101090v2
2001-10-26,(Sr/Ca)_{14}Cu_{24}O_{41} spin ladders studied by NMR under pressure,"(63)Cu-NMR measurements have been performed on two-leg hole-doped spin
ladders Sr_{14-x}Ca_{x}Cu_{24}O_{41} single crystals (0-x-12) at several
pressures up to the pressure domain where the stabilization of a
superconducting ground state can be achieved. The data reveal marked decrease
of the spin gap derived from Knight shift measurements upon Ca substitution and
also under pressure and confirm the onset of low lying spin excitations around
P_{c} as previously reported. The spin gap in Sr_{2}Ca_{12}Cu_{24}O_{41} is
strongly reduced above 20 kbar. However, the data of an experiment performed at
P=36 kbar where superconductivity has been detected at 6.7K by an inductive
technique have shown that a significant amount of spin excitations remains
gapped at 80K when superconductivity sets in. The standard relaxation model
with two and three-magnon modes explains fairly well the activated relaxation
data in the intermediate temperature regime corresponding to gapped spin
excitations using the spin gap data derived from Knight shift experiments.The
data of Gaussian relaxation rates of heavily doped samples support the
limitation of the coherence lenght at low temperature by the average distance
between doped holes. We discuss the interplay between superconductivity and the
spin gap and suggest that these new results support the exciting prospect of
superconductivity induced by the interladder tunnelling of preformed pairs as
long as the pressure remains lower than the pressure corresponding to the
maximum of the superconducting critical temperature.",0110559v1
2004-10-04,Dissipative dynamics of topological defects in frustrated Heisenberg spin systems,"We study the dynamics of topological defects of a frustrated spin system
displaying spiral order. As a starting point we consider the SO(3) nonlinear
sigma model to describe long-wavelength fluctuations around the noncollinear
spiral state. Besides the usual spin-wave magnetic excitations, the model
allows for topologically non-trivial static solutions of the equations of
motion, associated with the change of chirality (clockwise or counterclockwise)
of the spiral. We consider two types of these topological defects, single
vortices and vortex-antivortex pairs, and quantize the corresponding solutions
by generalizing the semiclassical approach to a non-Abelian field theory. The
use of the collective coordinates allows us to represent the defect as a
particle coupled to a bath of harmonic oscillators, which can be integrated out
employing the Feynman-Vernon path-integral formalism. The resulting effective
action for the defect indicates that its motion is damped due to the scattering
by the magnons. We derive a general expression for the damping coefficient of
the defect, and evaluate its temperature dependence in both cases, for a single
vortex and for a vortex-antivortex pair. Finally, we consider an application of
the model for cuprates, where a spiral state has been argued to be realized in
the spin-glass regime. By assuming that the defect motion contributes to the
dissipative dynamics of the charges, we can compare our results with the
measured inverse mobility in a wide range of temperature. The relatively good
agreement between our calculations and the experiments confirms the possible
relevance of an incommensurate spiral order for lightly doped cuprates.",0410082v3
2005-05-14,"Magnetic, electronic and Shubnikov-de Haas investigation of the dense Kondo system CeAgSb2","Of the dense Kondo materials in the class CeTSb2 (where T = Au, Ag, Ni, Cu,
or Pd), CeAgSb2 is special due to its complex magnetic ground state, which
exhibits both ferro- and anti-ferromagnetic character below an ordering
temperature TO ~ 9.8 K. To further elucidate a description this magnetic ground
state, we have carried out a systematic study of single crystalline CeAgSb2 by
magnetic, electrical magneto-transport, and Shubnikov-de Haas (SdH) studies
over a broad range of temperature and magnetic field. We have constructed the
magnetic phase diagram based solely on magnetoresistance data. Here, depending
on the orientation of the magnetic field H, either ferromagnetic or
antiferromagnetic ordering occurs below TO. The resistivity of this compound
below TO does not follow a simple Fermi liquid behavior, but requires an
additional contribution from conduction electron scattering from boson
excitations with an energy gap, D. At zero field the temperature dependent
resistivity below TO is most consistent with antiferromagnetic order, based on
the transport theory which includes magnon scattering. Crystal field effect
theory applied to the susceptibility data yields splitting energies from the
ground state to the first and second excited states of 53 K and 137 K,
respectively. Although there is some uncertainty in the Kondo temperature
determination, we estimate TK ~ 23 K from our analysis. In the Fermi surface
studies, the measurements show very small Fermi surface sections, not predicted
by band structure calculations, and the SdH amplitudes are very sensitive to
field direction. Only by considering lens orbits between the main Fermi surface
cylinders can the SdH results be reconciled with the Fermi surface topology
predicted from band structure.",0505362v1
1998-09-16,Slow Magnetic Monopole: Interaction with Matter and New Possibility of Their Detection,"The possibility of existence of a magnetic monopole has been surveyed by P.
Dirac even in 1931, and then from the point of view of the modern theory by
A.M. Polyakov and G.~ 'tHooft in 1974. Numerous and unsuccessful attempts of
experimental search for monopole in cosmic rays and on accelerators in high
energy particle collisions have been done. Also the searches have been carried
out in mica for monopole tracks as well as for relict monopoles, entrapped by
ferromagnetic inclusions in iron-ores, moon rock and meteorites. These
entrapped monopoles, when released, would have the lowest velocities
$\beta<10^{-6}$ and do not yield ionization at all, and are hard to detect.
Therefore it is necessary to examine thoroughly the mechanisms of slow heavy
monopole interaction with matter and their scale of energy loss.
  We discuss here the interaction of a massive slow magnetic monopole with
magnetically ordered matter, with conductors, superconductors and with
condensed matter in general. Our results indicate that the energy loss of a
slow supermassive monopole reach $10^{8} eV/cm$ and more if we take into
consideration the Cherenkov radiation of magnons or phonons and conductivity of
the media. A new method of search for cosmic and relict monopoles by
magnetically ordered film is considered too. This approach resembles the
traditional method of nuclear emulsion chamber. Apparently the proposed method
is particularly attractive for detection of relict monopoles, released from
melting iron ore.",9809420v3
2002-04-09,"On the Geometry and Mass of Static, Asymptotically AdS Spacetimes, and the Uniqueness of the AdS Soliton","We prove two theorems, announced in hep-th/0108170, for static spacetimes
that solve Einstein's equation with negative cosmological constant. The first
is a general structure theorem for spacetimes obeying a certain convexity
condition near infinity, analogous to the structure theorems of Cheeger and
Gromoll for manifolds of non-negative Ricci curvature. For spacetimes with
Ricci-flat conformal boundary, the convexity condition is associated with
negative mass. The second theorem is a uniqueness theorem for the negative mass
AdS soliton spacetime. This result lends support to the new positive mass
conjecture due to Horowitz and Myers which states that the unique lowest mass
solution which asymptotes to the AdS soliton is the soliton itself. This
conjecture was motivated by a nonsupersymmetric version of the AdS/CFT
correspondence. Our results add to the growing body of rigorous mathematical
results inspired by the AdS/CFT correspondence conjecture. Our techniques
exploit a special geometric feature which the universal cover of the soliton
spacetime shares with familiar ``ground state'' spacetimes such as Minkowski
spacetime, namely, the presence of a null line, or complete achronal null
geodesic, and the totally geodesic null hypersurface that it determines. En
route, we provide an analysis of the boundary data at conformal infinity for
the Lorentzian signature static Einstein equations, in the spirit of the
Fefferman-Graham analysis for the Riemannian signature case. This leads us to
generalize to arbitrary dimension a mass definition for static asymptotically
AdS spacetimes given by Chru\'sciel and Simon. We prove equivalence of this
mass definition with those of Ashtekar-Magnon and Hawking-Horowitz.",0204081v4
2007-11-20,Highly Frustrated Magnetic Clusters: The kagome on a sphere,"We present a detailed study of the low-energy excitations of two existing
finite-size realizations of the planar kagome Heisenberg antiferromagnet on the
sphere, the cuboctahedron and the icosidodecahedron. After highlighting a
number of special spectral features (such as the presence of low-lying singlets
below the first triplet and the existence of localized magnons) we focus on two
major issues. The first concerns the nature of the excitations above the
plateau phase at 1/3 of the saturation magnetization Ms. Our exact
diagonalizations for the s=1/2 icosidodecahedron reveal that the low-lying
plateau states are adiabatically connected to the degenerate collinear
``up-up-down'' ground states of the Ising point, at the same time being well
isolated from higher excitations. A complementary physical picture emerges from
the derivation of an effective quantum dimer model which reveals the central
role of the topology and the intrinsic spin s. We also give a prediction for
the low energy excitations and thermodynamic properties of the spin s=5/2
icosidodecahedron Mo72Fe30. In the second part we focus on the low-energy
spectra of the s>1/2 Heisenberg model in view of interpreting the broad
inelastic neutron scattering response reported for Mo72Fe30. To this end we
demonstrate the simultaneous presence of several broadened low-energy ``towers
of states'' or ``rotational bands'' which arise from the large discrete spatial
degeneracy of the classical ground states, a generic feature of highly
frustrated clusters. This semiclassical interpretation is further corroborated
by their striking symmetry pattern which is shown, by an independent group
theoretical analysis, to be a characteristic fingerprint of the classical
coplanar ground states.",0711.3231v2
2008-02-07,Spin-wave interaction in two-dimensional ferromagnets with dipolar forces,"We discuss the spin-wave interaction in two-dimensional (2D) Heisenberg
ferromagnet (FM) with dipolar forces at $T_C\gg T\ge0$ using 1/S expansion. A
comprehensive analysis is carried out of the first 1/S corrections to the
spin-wave spectrum. In particular, similar to 3D FM discussed in our previous
paper A.V. Syromyatnikov, PRB {\bf 74}, 014435 (2006), we obtain that the
spin-wave interaction leads to the {\it gap} in the spectrum $\epsilon_{\bf k}$
renormalizing greatly the bare gapless spectrum at small momenta $k$.
Expressions for the spin-wave damping $\Gamma_{\bf k}$ are derived
self-consistently and it is concluded that magnons are well-defined
quasi-particles in both quantum and classical 2D FMs at small $T$. We observe
thermal enhancement of both $\Gamma_{\bf k}$ and $\Gamma_{\bf k}/\epsilon_{\bf
k}$ at small momenta. In particular, a peak appears in $\Gamma_{\bf k}$ and
$\Gamma_{\bf k}/\epsilon_{\bf k}$ at small $k$ and at any given direction of
$\bf k$. If $S\sim1$ the height of the peak in $\Gamma_{\bf k}/\epsilon_{\bf
k}$ is not larger than a value proportional to $T/D\ll1$, where $D$ is the
spin-wave stiffness. In the case of large spins $S\gg1$ the peak in
$\Gamma_{\bf k}/\epsilon_{\bf k}$ cannot be greater than that of the classical
2D FM found at $k=0$ which height is small only {\it numerically}: $\Gamma_{\bf
0}/\epsilon_{\bf 0}\approx0.16$ for the simple square lattice. Frustrating
next-nearest-neighbor exchange coupling increases $\Gamma_{\bf 0}/\epsilon_{\bf
0}$ in classical 2D FM only slightly. We find expressions for spin Green's
functions and the magnetization. The latter differs from the well-known result
by S.V. Maleev, Sov. Phys. JETP {\bf 43}, 1240 (1976). The effect of the
exchange anisotropy is also discussed briefly.",0802.0958v1
2009-02-03,Pattern recognition and PID for COMPASS RICH-1,"A package for pattern recognition and PID by COMPASS RICH-1 has been
developed and used for the analysis of COMPASS data collected in the years 2002
to 2004, and 2006-2007 with the upgraded RICH-1 photon detectors. It has
allowed the full characterization of the detector in the starting version and
in the upgraded one, as well as the PID for physics results. We report about
the package structure and algorithms, and the detector characterization and PID
results.",0902.0444v1
2010-10-29,Revisiting the Phase Transition of Spin-1/2 Heisenberg Model with a Spatially Staggered Anisotropy on the Square Lattice,"Puzzled by the indication of a new critical theory for the spin-1/2
Heisenberg model with a spatially staggered anisotropy on the square lattice as
suggested in \cite{Wenzel08}, we re-investigate the phase transition of this
model induced by dimerization using first principle Monte Carlo simulations. We
focus on studying the finite-size scaling of $\rho_{s1} L$ and $\rho_{s2} L$,
where $L$ stands for the spatial box size used in the simulations and
$\rho_{si}$ with $i \in \{1,2\}$ is the spin-stiffness in $i$-direction. From
our Monte Carlo data, we find that $\rho_{s2} L$ suffers a much less severe
correction compared to that of $\rho_{s1} L$. Therefore $\rho_{s2} L$ is a
better quantity than $\rho_{s1} L$ for finite-size scaling analysis concerning
the limitation of the availability of large volumes data in our study. Further,
motivated by the so-called cubical regime in magnon chiral perturbation theory,
we additionally perform a finite-size scaling analysis on our Monte Carlo data
with the assumption that the ratio of spatial winding numbers squared is fixed
through all simulations. As a result, the physical shape of the system remains
fixed in our calculations. The validity of this new idea is confirmed by
studying the phase transition driven by spatial anisotropy for the ladder
anisotropic Heisenberg model. With this new strategy, even from $\rho_{s1} L$
which receives the most serious correction among the observables considered in
this study, we arrive at a value for the critical exponent $\nu$ which is
consistent with the expected $O(3)$ value by using only up to $L = 64$ data
points.",1010.6267v1
2011-06-14,Intense paramagnon excitations in a large family of high-temperature superconductors,"In the search for the mechanism of high-temperature superconductivity,
intense research has been focused on the evolution of the spin excitation
spectrum upon doping from the antiferromagnetic insulating to the
superconducting states of the cuprates. Because of technical limitations, the
experimental investigation of doped cuprates has been largely focused on
low-energy excitations in a small range of momentum space. Here we use resonant
inelastic x-ray scattering to show that a large family of superconductors,
encompassing underdoped YBa$_2$Cu$_4$O$_8$ and overdoped YBa$_2$Cu$_3$O$_{7}$,
exhibits damped spin excitations (paramagnons) with dispersions and spectral
weights closely similar to those of magnons in undoped cuprates. %The results
are in excellent agreement with the spin excitations obtained by exact
diagonalization of the $\bf t-J$ Hamiltonian on finite-sized clusters. The
comprehensive experimental description of this surprisingly simple spectrum
permits quantitative tests of magnetic Cooper pairing models. A numerical
solution of the Eliashberg equations for the magnetic spectrum of
YBa$_2$Cu$_3$O$_{7}$ reproduces its superconducting transition temperature
within a factor of two, a level of agreement comparable to Eliashberg theories
of conventional superconductors.",1106.2641v1
2011-09-25,Spiky strings and the AdS/CFT correspondence,"We first focus on the finite-gap formalism for type IIB strings in AdS_3 x
S^1, which allows to encode the semiclassical spectrum of a very large family
of string solutions in a Riemann surface, the spectral curve. Then, we show
that, in the large angular momentum limit, it separates into two distinct
surfaces, allowing the derivation of an explicit expression for the spectrum,
which is correspondingly characterised by two separate branches. The latter may
be interpreted in terms of two kinds of spikes appearing on the strings:
""large"" spikes, yielding an infinite contribution to the energy and angular
momentum of the string, and ""small"" spikes, representing finite excitations
over the background of the ""large"" spikes.
  On the other side of the AdS/CFT correspondence, we consider the sl(2) sector
of N=4 super Yang-Mills theory. The corresponding 1-loop spectrum, in the large
conformal spin limit, is also encoded in a spectral curve and characterised in
terms of the so-called holes. We show that, with the appropriate
identifications and with the usual extrapolation from weak to strong 't Hooft
coupling described by the cusp anomalous dimension, the large-S spectra of
gauge theory and of string theory coincide. Furthermore, we explain how ""small""
and ""large"" holes may be identified with ""small"" and ""large"" spikes.
  Finally, we discuss several explicit spiky string solutions in AdS_3 which
display the finite-gap spectrum. We compute their spectral curves in the large
S limit, finding that they correspond to specific regions of the moduli space
of the finite-gap curves. We also explain how ""large"" spikes may be used in
order to extract a discrete system of degrees of freedom from string theory,
which can then be matched with the degrees of freedom of the dual gauge theory
operators, and how ""small"" spikes are in fact very similar to the Giant Magnons
living in R x S^2.",1109.5401v1
2012-07-20,"Ferromagnetic Ordering in CeIr2B2: Transport, magnetization, specific heat and NMR studies","We present a complete characterization of ferromagnetic system CeIr2B2 using
powder x-ray diffraction XRD, magnetic susceptibility chi(T), isothermal
magnetization M(H), specific heat C(T), electrical resistivity rho(T,H), and
thermoelectric power S(T) measurements. Furthermore 11B NMR study was performed
to probe the magnetism on a microscopic scale. The chi(T), C(T) and rho(T) data
confirm bulk ferromagnetic ordering with Tc = 5.1 K. Ce ions in CeIr2B2 are in
stable trivalent state. Our low-temperature C(T) data measured down to 0.4 K
yield Sommerfeld coefficient gamma = 73(4) mJ/molK2 which is much smaller than
the previously reported value of gamma = 180 mJ/molK2 deduced from the specific
heat measurement down to 2.5 K. For LaIr2B2 gamma = 6(1) mJ/molK2 which implies
the density of states at the Fermi level D(EF) = 2.54 states/(eV f.u.) for both
spin directions. The renormalization factor for quasi-particle density of
states and hence for quasi-particle mass due to 4f correlations in CeIr2B2 is
12. The Kondo temperature TK ~ 4 K is estimated from the jump in specific heat
of CeIr2B2 at Tc. Both C(T) and rho(T) data exhibit gapped-magnon behavior in
magnetically ordered state with an energy gap Eg ~ 3.5 K. The rho data as a
function of magnetic field H indicate a large negative magnetoresistance (MR)
which is highest for T = 5 K.While at 5 K the negative MR keeps on increasing
up to 10 T, at 2 K an upturn is observed near H = 3.5 T. On the other hand, the
thermoelectric power data have small absolute values (S ~ 7 {\mu}V/K)
indicating a weak Kondo interaction. A shoulder in S(T) at about 30 K followed
by a minimum at ~ 10 K is attributed to crystal electric field (CEF) effects
and the onset of magnetic ordering. 11B NMR line broadening provides strong
evidence of ferromagnetic correlations below 40 K.",1207.5061v1
2013-02-26,Superconducting pairing and the pseudogap in nematic striped La2-xSrxCuO4,"The individual k\parallel and k\perp stripe excitations in fluctuating
spin-charge stripes have not been observed yet. In Raman scattering if we set,
for example, incident and scattered light polarizations to two possible stripe
directions, we can observe the fluctuating stripe as if it is static. Using the
different symmetry selection rule between the B1g two-magnon scattering and the
B1g and B2g isotropic electronic scattering, we succeeded to obtain the
k\parallel and k\perp strip magnetic excitations separately in La2-xSrxCuO4.
Only the k\perp stripe excitations appear in the wide-energy isotropic
electronic Raman scattering, indicating that the charge transfer is restricted
to the direction perpendicular to the stripe. This is the same as the Burgers
vector of an edge dislocation which easily slides perpendicularly to the
stripe. Hence charges at the edge dislocation move together with the
dislocation perpendicularly to the stripe, while other charges are localized. A
looped edge dislocation has lower energy than a single edge dislocation. The
superconducting coherence length is close to the inter-charge stripe distance
at x \le 0.2. Therefore we conclude that Cooper pairs are formed at looped edge
dislocations. The restricted charge transfer direction naturally explains the
opening of a pseudogap around (0, {\pi}) for the stripe parallel to the b axis
and the reconstruction of the Fermi surface to have a flat plane near (0,
{\pi}). They break the four-fold rotational symmetry. Furthermore the
systematic experiments revealed the carrier density dependence of the isotropic
and anisotropic electronic excitations, the spin density wave and/or charge
density wave gap near ({\pi}/2, {\pi}/2), and the strong coupling between the
electronic states near ({\pi}/2, {\pi}/2) and the zone boundary phonons at
({\pi}, {\pi}).",1302.6917v1
2014-10-01,Search for a spin-nematic phase in the quasi-one-dimensional frustrated magnet LiCuVO$_4$,"We have performed NMR experiments on the quasi one-dimensional frustrated
spin-1/2 system LiCuVO$_4$ in magnetic fields $H$ applied along the c-axis up
to field values near the saturation field $H_{\rm sat}$. For the field range
$H_{\rm c2}<H<H_{\rm c3}$ ($\mu_0H_{\rm c2}\approx 7.5$T and $\mu_0H_{\rm c3} =
[40.5 \pm 0.2]$T) the $^{51}$V NMR spectra at $T$ = 380mK exhibit a
characteristic double-horn pattern, as expected for a spin-modulated phase in
which the magnetic moments of Cu$^{2+}$ ions are aligned parallel to the
applied field $H$ and their magnitudes change sinusoidally along the magnetic
chains. For higher fields, the $^{51}$V NMR spectral shape changes from the
double-horn pattern into a single Lorentzian line. For this Lorentzian line,
the internal field at the $^{51}$V nuclei stays constant for $\mu_0 H > 41.4$T,
indicating that the majority of magnetic moments in LiCuVO$_4$ are already
saturated in this field range. This result is inconsistent with the previously
observed linear field dependence of the magnetization $M(H)$ for $H_{\rm
c3}<H<H_{\rm sat}$ with $\mu_0H_{\rm sat}=45$T [L. E. Svistov {\it et al}.,
JETP Letters {\bf 93}, 21 (2011)]. We argue that the discrepancy is due to
non-magnetic defects in the samples. The results of the spin-lattice relaxation
rate of $^7$Li nuclei indicate an energy gap which grows with field twice as
fast as the Zeeman energy of a single spin, therefore, suggesting that the
two-magnon bound state is the lowest energy excitation. The energy gap tends to
close at $\mu_0H \approx 41$T. Our results suggest that the theoretically
predicted spin-nematic phase, if it exists in LiCuVO$_4$, can be established
only within the narrow field range $40.5 < \mu_0 H < 41.4$T .",1410.0160v1
2014-10-23,The 2014 Magnetism Roadmap,"Magnetism is a very fascinating and dynamic field. Especially in the last 30
years it has experienced many major advances in the full range from novel
fundamental phenomena to new products. Applications such as hard disk drives
and magnetic sensors are part of our daily life, and new applications, such as
in non-volatile computer random access memory, are expected to surface shortly.
Thus it is timely for describing the current status, and current and future
challenges in the form of a Roadmap article. This 2014 Magnetism Roadmap
provides a view on several selected, currently very active innovative
developments. It consists of 12 sections, each written by an expert in the
field and addressing a specific subject, with strong emphasize on future
potential. This Roadmap cannot cover the entire field. We have selected several
highly relevant areas without attempting to provide a full review - a future
update will have room for more topics. The scope covers mostly nano-magnetic
phenomena and applications, where surfaces and interfaces provide additional
functionality. New developments in fundamental topics such as interacting
nano-elements, novel magnon-based spintronics concepts, spin-orbit torques and
spin-caloric phenomena are addressed. New materials, such as organic magnetic
materials and permanent magnets are covered. New applications are presented
such as nano-magnetic logic, non-local and domain-wall based devices,
heat-assisted magnetic recording, magnetic random access memory, and
applications in biotechnology. May the Roadmap serve as a guideline for future
emerging research directions in modern magnetism.",1410.6404v1
2015-04-07,Generation of coherent spin-wave modes in Yttrium Iron Garnet microdiscs by spin-orbit torque,"Spin-orbit effects [1-4] have the potential of radically changing the field
of spintronics by allowing transfer of spin angular momentum to a whole new
class of materials. In a seminal letter to Nature [5], Kajiwara et al. showed
that by depositing Platinum (Pt, a normal metal) on top of a 1.3 $\mu$m thick
Yttrium Iron Garnet (YIG, a magnetic insulator), one could effectively transfer
spin angular momentum through the interface between these two different
materials. The outstanding feature was the detection of auto-oscillation of the
YIG when enough dc current was passed in the Pt. This finding has created a
great excitement in the community for two reasons: first, one could control
electronically the damping of insulators, which can offer improved properties
compared to metals, and here YIG has the lowest damping known in nature;
second, the damping compensation could be achieved on very large objects, a
particularly relevant point for the field of magnonics [6,7] whose aim is to
use spin-waves as carriers of information. However, the degree of coherence of
the observed auto-oscillations has not been addressed in ref. [5]. In this
work, we emphasize the key role of quasi-degenerate spin-wave modes, which
increase the threshold current. This requires to reduce both the thickness and
lateral size in order to reach full damping compensation [8] , and we show
clear evidence of coherent spin-orbit torque induced auto-oscillation in
micron-sized YIG discs of thickness 20 nm.",1504.01512v1
2015-06-16,Evidence of quantum dimer excitations in Sr$_3$Ir$_2$O$_7$,"The magnetic excitation spectrum in the bilayer iridate Sr$_3$Ir$_2$O$_7$ has
been investigated using high-resolution resonant inelastic x-ray scattering
(RIXS) performed at the iridium L$_3$ edge and theoretical techniques. A study
of the systematic dependence of the RIXS spectrum on the orientation of the
wavevector transfer, $\mathbf{Q}$, with respect to the iridium-oxide bilayer
has revealed that the magnon dispersion is comprised of two branches well
separated in energy and gapped across the entire Brillouin zone. Our results
contrast with those of an earlier study which reported the existence of a
single dominant branch. While these earlier results were interpreted as two
overlapping modes within a spin-wave model of weakly coupled iridium-oxide
planes, our results are more reminiscent of those expected for a system of
weakly coupled dimers. In this latter approach the lower and higher energy
modes find a natural explanation as those corresponding to transverse and
longitudinal fluctuations, respectively. We have therefore developed a
bond-operator theory which describes the magnetic dispersion in
Sr$_3$Ir$_2$O$_7$ in terms of quantum dimer excitations. In our model
dimerisation is produced by the leading Heisenberg exchange, $J_c$, which
couples iridium ions in adjacent planes of the bilayer. The Hamiltonian also
includes in plane exchange, $J$, as well as further neighbour couplings and
relevant anisotropies. The bond-operator theory provides an excellent account
of the dispersion of both modes, while the measured $\mathbf{Q}$ dependence of
the RIXS intensities is in reasonable qualitative accord with the spin-spin
correlation function calculated from the theory. We discuss our results in the
context of the quantum criticality of bilayer dimer systems in the presence of
anisotropic interactions derived from strong spin-orbit coupling.",1506.04877v1
2015-06-23,Semi-phenomenological analysis of neutron scattering results for quasi-two dimensional quantum anti-ferromagnet,"The available results from the inelastic neutron scattering experiment
performed on the quasi-two dimensional spin $\frac{1}{2}$ anti-ferromagnetic
material $La_2 Cu O_4$ have been analysed theoretically. The formalism of ours
is based on a semi-classical like treatment involving a model of an ideal gas
of mobile vortices and anti-vortices built on the background of the
N$\acute{e}$el state, using the bipartite classical spin configuration
corresponding to an XY- anisotropic Heisenberg anti-ferromagnet on a square
lattice. The results for the integrated intensities for our spin $\frac{1}{2}$
model corresponding to different temperatures, show occurrence of vigorous
unphysical oscillations, when convoluted with a realistic spectral window
function. These results indicate failure of the conventional semi-classical
theoretical model of ideal vortex/anti-vortex gas arising in the
Berezinskii-Kosterlitz-Thouless theory for the low spin magnetic systems. A
full fledged quantum mechanical formalism and calculations seem crucial for the
understanding of topological excitations in such low spin systems. Furthermore,
a severe disagreement is found to occur at finite values of energy transfer
between the integrated intensities obtained theoretically from the conventional
formalism and those obtained experimentally. This further suggests strongly
that the full quantum treatment should also incorporate the interaction between
the fragile-magnons and the topological excitations. This is quite plausible in
view of the recent work establishing such a process in XXZ quantum ferromagnet
on 2D lattice. The high spin XXZ quasi-two dimensional antiferromagnet like
$MnPS_3$ however follows the conventional theory quite well",1506.06856v3
2016-02-28,"Effective field theory and the scattering process for magnons in the ferromagnet, antiferromagnet, and ferrimagnet","We discuss that a low-energy effective Lagrangian relying on SO(3)
$\rightarrow$ SO(2) is applicable for a ferrimagnet as well as a ferromagnet
and an antiferromagnet. The analysis of the particle states shows that there
exist not only massless modes with the dispersion relations $\omega \propto
|\bm{k}|,\, |\bm{k}|^2$, i.e., the so-called type-I and type-II Nambu-Goldstone
modes, respectively, but also gapped modes with $\omega \propto
m^2+|\bm{k}|^2$. We clarify how the coefficients of the terms with one time
derivative and those with two time derivatives in the effective Lagrangian
determine the order parameters specifying whether the system is in a
ferromagnetic, antiferromagnetic or ferrimagnetic state; we stress that the
gapped mode related to the spontaneous symmetry breaking appears only in the
ferrimagnetic system and not in the ferromagnetic and antiferromagnetic
systems. We also establish the power counting scheme and calculate the
scattering amplitudes and thereby the scattering lengths between the two
Nambu-Goldstone bosons. We show that the scattering length of the gapped mode
is finite and proportional to the gap. This characteristic property of the
gapped NG mode can be used to discriminate it from gapped excitations which
originate in other mechanisms. Finally, we study the effects of the explicit
symmetry breaking that are given by an external magnetic field and a single-ion
anisotropy, and show that the external magnetic fields do not have any effects
on the scattering amplitudes in all the spin systems as was known for the
ferromagnet system. In contrast, the anisotropy does affect the scattering
amplitudes, the phase shift, and the scattering length except for spin 1/2
systems. This result supports the possibility of the Efimov effect in spin
systems discussed in previous studies.",1602.08692v2
2018-11-26,Supersymmetric $t$-$J$ models with long-range interactions: partition function and spectrum,"We study the spectrum of the long-range supersymmetric su$(m)$ $t$-$J$ model
of Kuramoto and Yokoyama in the presence of an external magnetic field and a
charge chemical potential. To this end, we first establish the precise
equivalence of a large class of models of this type to a family of su$(1|m)$
spin chains with long-range exchange interactions and a suitable chemical
potential term. We exploit this equivalence to compute in closed form the
partition function of the long-range $t$-$J$ model, which we then relate to
that of an inhomogeneous vertex model with simple interactions. From the
structure of this partition function we are able to deduce an exact formula for
the restricted partition function of the long-range $t$-$J$ model in subspaces
with well-defined magnon content in terms of its analogue for the equivalent
vertex model. This yields a complete analytical description of the spectrum in
the latter subspaces, including the precise degeneracy of each level, by means
of the supersymmetric version of Haldane's motifs and their related skew Young
tableaux. As an application, we determine the structure of the motifs
associated with the ground state of the spin $1/2$ model in the thermodynamic
limit in terms of the magnetic field strength and the charge chemical
potential. This leads to a complete characterization of the distinct ground
state phases, determined by their spin content, in terms of the magnetic field
strength and the charge chemical potential.",1811.10297v2
2011-11-14,New pixelized Micromegas detector with low discharge rate for the COMPASS experiment,"New Micromegas (Micro-mesh gaseous detectors) are being developed in view of
the future physics projects planned by the COMPASS collaboration at CERN.
Several major upgrades compared to present detectors are being studied:
detectors standing five times higher luminosity with hadron beams, detection of
beam particles (flux up to a few hundred of kHz/mm^{2}, 10 times larger than
for the present Micromegas detectors) with pixelized read-out in the central
part, light and integrated electronics, and improved robustness. Two solutions
of reduction of discharge impact have been studied, with Micromegas detectors
using resistive layers and using an additional GEM foil. Performance of such
detectors has also been measured. A large size prototypes with nominal active
area and pixelized read-out has been produced and installed at COMPASS in 2010.
In 2011 prototypes featuring an additional GEM foil, as well as an resistive
prototype, are installed at COMPASS and preliminary results from those
detectors presented very good performance. We present here the project and
report on its status, in particular the performance of large size prototypes
with an additional GEM foil.",1111.3337v4
2014-05-28,Anomalous dimensions from rotating open strings in AdS/CFT,"We propose a new entry within the dictionary of the AdS/CFT duality at strong
coupling: in the limit of a large spin or a large R-charge, the anomalous
dimension of the gauge theory operator dual to a semiclassical rotating string
is proportional to the string proper length. This conjecture is motivated by a
generalization to strings of the rule for computing anomalous dimensions of
massive particles and supergravity fields in the anti-de Sitter space. We show
that this proportionality holds for a rotating closed string in global AdS
space, representing a high spin operator made of fields in the adjoint
representation. It is also valid for closed strings rotating in $S^5$
(representing operators with large R-charge), for closed strings with multiple
AdS spin, and for giant magnons. Based on this conjecture, we calculate the
anomalous dimension $\delta$ of operators made of fields in the fundamental
representation, associated with high spin mesons, and which are represented by
rotating open strings attached to probe D7-branes. The result is a logarithmic
dependence upon the spin, $\delta\sim \sqrt{\lambda}\ln S$, similar to the
closed string case. We show that the operator properties --- anomalous
dimension and spin --- are obtained from measurements made by a local observer
in the anti-de Sitter space. For the open string case, this ensures that these
quantities are independent of the mass scale introduced by the D7-branes (the
quark mass), as expected on physical grounds. In contrast, properties of the
gauge theory states, like the energy, correspond to measurements by a gauge
theory observer and depend upon the mass scale --- once again, as expected.",1405.7388v2
2014-06-17,Phonons and Hybrid Modes in the High and Low Temperature Far Infrared Dynamics of Hexagonal TmMnO3,"We report on TmMnO3 far infrared emissivity and reflectivity spectra from
1910 K to 4 K. At the highest temperature the number of infrared bands is lower
than that predicted for centrosymmetric P63/mmc (D6h4) (Z=2) space group due
high temperature anharmonicity and possible defect induced bitetrahedra
misalignments. On cooling, at ~1600 K TmMnO3 goes from non-polar to an
antiferroelectric-ferroelectric polar phase reaching the ferroelectric onset at
the ~700 K. The 300 K reflectivity is fitted using 19 oscillators and this
number of phonons is maintained down to 4 K. A weak phonon anomaly in the band
profile at 217 cm-1 (4 K) suggests subtle Rare Earth magnetoelectric couplings
at ~TN and below. A low energy collective excitation is identified as a THz
instability associated with room temperature eg electrons in a d-orbital
fluctuating environment. It condenses into two modes that emerge pinned to the
E-type antiferromagmetic order hardening simultaneously down to 4 K. They obey
power laws with TN as the critical temperature and match known zone center
magnons. The one peaking at 26 cm-1, with critical exponent \b{eta}=0.42 as for
antiferromagnetic order in a hexagonal lattice, is dependent on the Rare Earth.
The band at ~50 cm-1, with \b{eta}=0.25, splits at ~TN into two peaks. The
weaker band of the two is assimilated to the upper branch of gap opening in the
transverse acoustical (TA) phonon branch crossing the magnetic dispersion found
in YMnO3. (Petit et al, 2007 PRL 99, 266604). The stronger second at ~36 cm-1
corresponds to the lower branch of the TA gap. We assign both excitations as
zone center magnetoelectric hybrid quasiparticles concluding that in NdMnO3
perovskite the equivalent picture corresponds to an instability which may be
driven by an external field to transform NdMnO3 into a multiferroic compound by
perturbation enhancing the TA phonon-magnetic correlation.",1406.4382v1
2020-12-10,Magnetic order and magneto-transport in half-metallic ferrimagnetic Mn$_y$Ru$_x$Ga thin films,"The ruthenium content of half-metallic Mn$_2$Ru$_x$Ga thin films, with a
biaxially-strained inverse Heusler structure, controls the ferrimagnetism that
determines their magnetic and electronic properties. An extensive study of
Mn$_y$Ru$_x$Ga films on MgO (100) substrates with $1.8 \leq y \leq 2.6$ and $x
= 0.5$, 0.7 or 0.9, including crystallographic, magnetic order,
magneto-transport and spin polarisation is undertaken to map specific
composition-dependent properties in this versatile ternary system. A comparison
of experimental densities obtained from X-ray reflectivity with calculated
densities indicates full site occupancy for all compositions, which implies
chemical disorder. All moments lie on the Slater-Pauling plot with slope 1 and
all except $x = 0.5$, $y = 2.2$ exhibit magnetic compensation at \tcmp~below
500~K. The coercivity near \tcmp~exceeds 10~T. Increasing the Mn or Ru content
raises \tcmp, but increasing Ru also decreases the spin polarisation determined
by point contact Andreev reflection. Molecular field theory is used to model
the temperature dependence of the net ferrimagnetic moment and three principal
exchange coefficients are deduced. Marked differences in the shape of anomalous
Hall and net magnetisation hysteresis loops are explained by substantial
canting of the small net moment by up to \SI{40}{\degree} relative to the
$c$-axis in zero field, which is a result of slight non-collinearity of the
Mn$^{4c}$ sublattice moments due to competing intra-sublattice exchange
interactions arising from antisite disorder and excess Mn in the unit cell.
Consequences are reduced spin polarisation and an enhanced intrinsic
contribution to the anomalous Hall effect. The systematic investigation of the
physical properties as a function of $x$ and $y$ will guide the selection of
compositions to meet the requirements for magnonic and spintronic MRG-based
devices.",2012.05736v5
2020-12-11,Antiferromagnetic Kitaev interaction in $J_\rm{eff}=1/2$ cobalt honeycomb materials Na$_3$Co$_2$SbO$_6$ and Na$_2$Co$_2$TeO$_6$,"Finding new materials with antiferromagnetic (AFM) Kitaev interaction is an
urgent issue to broaden and enrich the quantum magnetism research
significantly. By carrying out inelastic neutron scattering experiments and
subsequent analysis, we conclude that Na$_3$Co$_2$SbO$_6$ and
Na$_2$Co$_2$TeO$_6$ are new honeycomb cobalt-based AFM Kitaev systems. The
spin-orbit excitons at 20-28~meV in both compounds strongly supports the idea
that Co$^{2+}$ ions of both compounds have a spin-orbital entangled
$J_\rm{eff}=1/2$ state. Furthermore, we found that a generalized
Kitaev-Heisenberg Hamiltonian can well describe the spin-wave excitations of
both compounds with additional 3rd nearest-neighbor interaction. Our best-fit
parameters show large AFM Kitaev terms and off-diagonal symmetric anisotropy
terms of a similar magnitude in both compounds. We should stress that our
parameters' optimized magnetic structures are consistent with the magnetic
structures reported from neutron diffraction studies. Moreover, there is also
the magnon-damping effect at the higher energy part of the spin waves, as
usually observed in other Kitaev magnets. We demonstrate that
Na$_3$Co$_2$SbO$_6$ and Na$_2$Co$_2$TeO$_6$ are the first experimental
realization of AFM Kitaev magnets based on the systematic studies of the spin
waves and analysis.",2012.06167v3
2020-12-22,Fragility of $\mathcal{Z}_2$ topological invariant characterizing triplet excitations in a bilayer kagome magnet,"The discovery by Kane and Mele of a model of spinful electrons characterized
by a $\mathcal{Z}_2$ topological invariant had a lasting effect on the study of
electronic band structures. Given this, it is natural to ask whether similar
topology can be found in the band-like excitations of magnetic insulators, and
recently models supporting $\mathcal{Z}_2$ topological invariants have been
proposed for both magnon [Kondo et al. Phys. Rev. B 99, 041110(R) (2019)] and
triplet [D. G. Joshi and A. P. Schnyder, Phys. Rev. B 100, 020407 (2019)]
excitations. In both cases, magnetic excitations form time--reversal (TR)
partners, which mimic the Kramers pairs of electrons in the Kane-Mele model but
do not enjoy the same type of symmetry protection. In this paper, we revisit
this problem in the context of the triplet excitations of a spin model on the
bilayer kagome lattice. Here the triplet excitations provide a faithful analog
of the Kane-Mele model as long as the Hamiltonian preserves the TR$\times$U(1)
symmetry. We find that exchange anisotropies, allowed by the point group and
typical in realistic models, break the required TR$\times$U(1) symmetry and
instantly destroy the $\mathcal{Z}_2$ band topology. We further consider the
effects of TR breaking by an applied magnetic field. In this case, the lifting
of spin-degeneracy leads to a triplet Chern insulator, which is stable against
the breaking of TR$\times$U(1) symmetry. Kagome bands realize both a quadratic
and a linear band touching, and we provide a thorough characterization of the
Berry curvature associated with both cases. We also calculate the
triplet-mediated spin Nernst and thermal Hall signals which could be measured
in experiments. These results suggest that the $\mathcal{Z}_2$ topology of
band-like excitations in magnets may be intrinsically fragile compared to their
electronic counterparts.",2012.11765v3
2018-07-11,Probing the Dzyaloshinskii-Moriya interaction via the propagation of spin waves in ferromagnetic thin films,"The Dzyaloshinskii-Moriya interaction (DMI) has attracted considerable recent
attention owing to the intriguing physics behind and the fundamental role it
played in stabilizing magnetic solitons, such as magnetic skyrmions and chiral
domain walls. A number of experimental efforts have been devoted to probe the
DMI, among which the most popular method is the Brillouin light scattering
spectroscopy (BLS) to measure the frequency difference of spin waves with
opposite wave vectors $\pm\mathbf{k}$ perpendicular to the in-plane
magnetization $\mathbf{m}$. Such a technique, however, is not applicable for
the cases of $\mathbf{k}\parallel\mathbf{m}$, since the spin-wave reciprocity
is recovered then. For a narrow magnetic strip, it is also difficult to measure
the DMI strength using BLS because of the spatial resolution limit of lights.
To fill these gaps, we propose to probe the DMI via the propagation of spin
waves in ferromagnetic films. We show that the DMI can cause the
non-collinearity of the group velocities of spin waves with
$\pm\mathbf{k}\parallel\mathbf{m}$. In heterogeneous magnetic thin films with
different DMIs, negative refractions of spin waves emerge at the interface
under proper conditions. These findings enable us to quantify the DMI strength
by measuring the angle between the two spin-wave beams with
$\pm\mathbf{k}\parallel\mathbf{m}$ in homogeneous film and by measuring the
incident and negative refraction angles in heterogeneous films. For a narrow
magnetic strip, we propose a nonlocal scheme to determine the DMI strength via
nonlinear three-magnon processes. We implement theoretical calculations and
micromagnetic simulations to verify our ideas. The results presented here are
helpful for future measurement of the DMI and for designing novel spin-wave
spintronic devices.",1807.04025v2
2018-12-03,Magnetoelastic hybrid excitations in CeAuAl$_3$,"The interactions between elementary excitations such as phonons, plasmons,
magnons, or particle-hole pairs, drive emergent functionalities and electronic
instabilities such as multiferroic behaviour, anomalous thermoelectric
properties, polar order, or superconductivity. Whereas various hybrid
excitations have been studied extensively, the feed-back of prototypical
elementary excitations on the crystal electric fields (CEF), defining the
environment in which the elementary excitations arise, has been explored for
very strong coupling only. We report high-resolution neutron spectroscopy and
ab-initio phonon calculations of {\ceaual}, an archetypal fluctuating valence
compound. The high resolution of our data allows us to quantify the energy
scales of three coupling mechanisms between phonons, CEF-split localized 4f
electron states, and conduction electrons. Although these interactions do not
appear to be atypically strong for this class of materials, we resolve, for the
first time, a profound renormalization of low-energy quasiparticle excitations
on all levels. The key anomalies of the spectrum we observe comprise (1) the
formation of a CEF-phonon bound state with a comparatively low density of
acoustic phonons reminiscent of vibronic modes observed in other materials,
where they require a pronounced abundance of optical phonons, (2) an
anti-crossing of CEF states and acoustic phonons, and (3) a strong broadening
of CEF states due to the hybridization with more itinerant excitations. The
fact that all of these features are well resolved in CeAuAl$_3$ suggests that
similar hybrid excitations should also be dominant in a large family of related
materials. This promises a predictive understanding towards the discovery of
new magneto-elastic functionalities and instabilities.",1812.01020v1
2018-12-23,Voltage control of the long-range p-d exchange coupling in a ferromagnet-semiconductor quantum well hybrid structure,"Voltage control of ferromagnetism on the nanometer scale is highly appealing
for the development of novel electronic devices. Here a key challenge is to
implement and combine low power consumption, high operation speed, reliable
reversibility and compatibility with semiconductor technology. Hybrid
structures based on the assembly of ferromagnetic and semiconducting building
blocks are attractive candidates in that respect as such systems bring together
the properties of the isolated constituents: They are expected to show magnetic
order as a ferromagnet and to be electrically tunable as a semiconductor. Here
we demonstrate the electrical control of the exchange coupling in a hybrid
consisting of a ferromagnetic Co layer and a semiconductor CdTe quantum well,
separated by a thin non-magnetic (Cd,Mg)Te barrier. The effective magnetic
field of the exchange interaction reaches up to 2.5 Tesla and can be turned on
and off by application of 1 V bias across the heterostructure. The mechanism of
this electric field control is essentially different from the conventional
concept, in which wavefunctions are spatially redistributed to vary the
exchange interaction, requiring high field strengths. Here we address instead
control of the novel exchange mechanism that is mediated by elliptically
polarized phonons emitted from the ferromagnet, i.e. the phononic ac Stark
effect. An essential parameter of this coupling is the splitting between heavy
and light hole states in the quantum well which can be varied by the electric
field induced band bending. Thereby the splitting can be tuned with respect to
the magnon-phonon resonance energy in the ferromagnet, leading to maximum
coupling for flat band conditions. Our results demonstrate the feasibility of
electrically controlled exchange coupling in hybrid semiconductor
nanostructures at quite moderate electric field strengths.",1812.09680v1
2019-02-01,Summary Report of Physics Beyond Colliders at CERN,"Physics Beyond Colliders is an exploratory study aimed at exploiting the full
scientific potential of CERN's accelerator complex and its scientific
infrastructure in the next two decades through projects complementary to the
LHC, HL-LHC and other possible future colliders. These projects should target
fundamental physics questions that are similar in spirit to those addressed by
high-energy colliders, but that require different types of beams and
experiments. A kick-off workshop held in September 2016 identified a number of
areas of interest and working groups have been set-up to study and develop
these directions. All projects currently under consideration are presented
including physics motivation, a brief outline of the experimental set-up and
the status of the corresponding beam and detector technological studies. The
proposals are also put in context of the worldwide landscape and their
implementation issues are discussed.",1902.00260v1
2019-02-12,Ultrafast X-Ray Induced Changes of the Electronic and Magnetic Response of Solids Due to Valence Electron Redistribution,"We report a novel mechanism, consisting of redistribution of valence
electrons near the Fermi level, during interactions of intense femtosecond
X-ray pulses with a Co/Pd multilayer. The changes in Co 3d valence shell
occupation were directly revealed by fluence-dependent changes of the Co L$_3$
X-ray absorption and magnetic circular dichroism spectra near the excitation
threshold. The valence shell redistribution arises from inelastic scattering of
high energy Auger electrons and photoelectrons that lead to transient holes
below and electrons above the Fermi level on the femtosecond time scale. The
valence electron reshuffling effect scales with the energy deposited by X-rays
and within 17 fs extends to valence states within 2 eV of the Fermi level. As a
consequence the sample demagnetizes by more than twenty percent due to magnon
generation.",1902.04611v1
2019-08-02,Giant magnetoelastic coupling in Love acoustic waveguide based on uniaxial multilayered TbCo2/FeCo nanostructured thin film on Quartz ST-cut,"Coupling between dynamic strain and magnetization in ferromagnetic thin films
has attracted special consideration as it presents both intriguing fundamental
physics problems and technological importance for potential multi-functional
devices and information handling. The dynamic strain can be generated by
acoustic waves including bulk, surface or guided waves. In this work, we
propose the theoretical and experimental investigation of the interaction of
pure shear horizontal (SH) wave with a uniaxial multilayered TbCo2/FeCo thin
film in a delay line configuration fabricated on Quartz ST-90X cut. We evaluate
theoretically the evolution of phase velocity as a function of magnetic field
and experimentally the variation of the transmission coefficient. A
piezomagnetic model was developed allowing us to calculate the elastic
stiffness constants of the multilayer as a function of the applied magnetic
field. The model was also implemented for acoustic waves dispersion curves
calculation. We show that the evolution of phase velocity is dominated by the
C66 elastic stiffness constant variation as expected for the case of shear
horizontal surface wave. The fabricated device let us exciting both fundamental
and third harmonic shear mode at 410 MHz and 1.2 GHz, respectively. For both
modes, the theoretical results corroborate very well the experimental ones. At
1.2 GHz the mode exhibits a maximum phase velocity shift close to 2.5% and an
attenuation reaching 500 dB/cm, for a sensitivity as high as 250 ppm/Oe. The
reported theoretical model and experimental results are of tremendous interest
for the development of advanced devices for magnetic field sensing applications
as well as investigating magnon-phonon interaction at a fundamental level.",1908.10443v2
2019-08-29,Pushing down the lateral dimension of single and coupled magnetic dots to the nanometric scale: characteristics and evolution of the spin-wave eigenmodes,"Planar magnetic nanoelements, either single- or multi-layered, are exploited
in a variety of current or forthcoming spintronic and/or ICT devices, such as
read heads, magnetic memory cells, spin-torque nano-oscillators, nanomagnetic
logic circuits, magnonic crystals and artificial spin-ices. The lateral
dimensions of the elemental magnetic components have been squeezed down during
the last decade to a few tens of nanometers, but they are still an order of
magnitude larger that the exchange correlation length of the constituent
materials. This means that the spectrum of spin-wave eigenmodes, occurring in
the GHz range, is relatively complex and cannot be described within a simple
macrospin approximation. On the other hand, a detailed knowledge of the
dynamical spectrum is needed to understand or to predict crucial
characteristics of the devices. With this focused review we aim at the analysis
and the rationalization of the characteristics of the eigenmodes spectrum of
magnetic nanodots, paying special attention to the following key points: (i)
Consider and compare the case of in-plane and out of-plane orientation of the
magnetization, as well as of single- and multi-layered dots, putting in
evidence similarities and diversities, and proposing a unifying nomenclature
and labelling scheme; (ii) Underline the evolution of the spectrum when the
lateral size of magnetic dots is squeezed down from hundreds to tens of
nanometers, as in current devices, with emphasis given to the occurrence of
soft modes and to the change of spatial localization of the fundamental mode
for in-plane magnetized dots; (iii) Extend the analysis from isolated elements
to twins of dots, as well as to dense arrays of dipolarly interacting dots,
showing how the discretized eigenmodes distinctive of the single element
transform in finite-width frequency bands of spin waves propagating through the
array.",1908.11098v1
2019-11-26,Free-standing and positionable microwave antenna device for magneto-optical spectroscopy experiments,"Modern spectroscopic techniques for the investigation of magnetization
dynamics in micro- and nano- structures or thin films use typically microwave
antennas which are directly fabricated on top of the sample by means of
electron-beam-lithography (EBL). Following this approach, every magnetic
structure on the sample needs its own antenna, resulting in additional EBL
steps and layer deposition processes. We demonstrate a new approach for
magnetization excitation that is suitable for optical and non-optical
spectroscopy techniques. By patterning the antenna on a separated flexible
glass cantilever and insulating it electrically, we solved the before mentioned
issues. Since we use flexible transparent glass as a substrate, optical
spectroscopy techniques like Brillouin-light-scattering microscopy ({\mu}BLS),
time resolved magneto-optical Kerr effect measurements (TRMOKE) or optical
detected magnetic resonance (ODMR) measurements can be carried out at visible
laser wavelengths. As the antenna is detached from the sample it can be freely
positioned in all three dimensions to adress only the desired magnetic sample
structures and to achieve effective excitation. We demonstrate the
functionality of these antennas using {\mu}BLS and compare coherently and
thermally excited magnon spectra to show the enhancement of the signal by a
factor of about 400 due to the excitation by the antenna. Moreover, we succeed
to characterize yttrium iron garnet thin films with spatial resolution using
optical ferromagnetic resonance (FMR) experiments. We analyse the spatial
excitation profile of the antenna by measuring the magnetization dynamics in
two dimensions. The technique is furthermore applied to investigate
injection-locking of spin Hall nano-oscillators.",1911.11517v1
2019-12-03,"Temperature dependence of the $(π,0)$ anomaly in the excitation spectrum of the 2D quantum Heisenberg antiferromagnet","It is well established that in the low-temperature limit, the two-dimensional
quantum Heisenberg antiferromagnet on a square lattice (2DQHAFSL) exhibits an
anomaly in its spectrum at short-wavelengths on the zone-boundary. In the
vicinity of the $(\pi,0)$ point the pole in the one-magnon response exhibits a
downward dispersion, is heavily damped and attenuated, giving way to an
isotropic continuum of excitations extending to high energies. The origin of
the anomaly and the presence of the continuum are of current theoretical
interest, with suggestions focused around the idea that the latter evidences
the existence of spinons in a two-dimensional system. Here we present the
results of neutron inelastic scattering experiments and Quantum Monte Carlo
calculations on the metallo-organic compound Cu(DCOO)$_2\cdot 4$D$_2$O (CFTD),
an excellent physical realisation of the 2DQHAFSL, designed to investigate how
the anomaly at $(\pi,0)$ evolves up to finite temperatures $T/J\sim2/3$. Our
data reveal that on warming the anomaly survives the loss of long-range,
three-dimensional order, and is thus a robust feature of the two-dimensional
system. With further increase of temperature the zone-boundary response
gradually softens and broadens, washing out the $(\pi,0)$ anomaly. This is
confirmed by a comparison of our data with the results of finite-temperature
Quantum Monte Carlo simulations where the two are found to be in good accord.
At lower energies, in the vicinity of the antiferromagnetic zone centre, there
was no significant softening of the magnetic excitations over the range of
temperatures investigated.",1912.01364v1
2019-12-28,Non-Equilibrium Heat Transport in Pt and Ru Probed by an Ultrathin Co Thermometer,"Non-equilibrium of electrons, phonons, and magnons in metals is a fundamental
phenomenon in condensed matter physics and serves as an important driver in the
field of ultrafast magnetism. In this work, we demonstrate that the
magnetization of a sub-nm-thick Co layer with perpendicular magnetic anisotropy
can effectively serve as a thermometer to monitor non-equilibrium dynamics in
adjacent metals, Pt and Ru, via time-resolved magneto-optic Kerr effect. The
temperature evolutions of the Co thermometer embedded in Pt layers of different
thicknesses, 6-46 nm, are adequately described by a phenomenological three
temperature model with a consistent set of materials parameters. We do not
observe any systematic deviations between the model and the data that can be
caused by a non-thermal distribution of electronic excitations. We attribute
the consistently good agreement between the model and the data to strong
electron-electron interaction in Pt. By using Pt/Co/Pt and Pt/Co/Pt/Ru
structures, we determine the electron-phonon coupling parameters of Pt and Ru,
g_ep(Pt)=(6+/-1)x10^17 W m-3 K-1 and g_ep(Ru)=(9+/-2)x10^17 W m-3 K-1. We also
find that the length scales of non-equilibrium between electrons and phonons
are l_ep=({\Lambda}_e/g_ep)^1/2 =9 nm for Pt and 7 nm for Ru, shorter than
their optical absorption depths, 11 and 13 nm, respectively. Therefore, the
optically thick Pt and Ru layers show two steps of temperature rise: The
initial jump of electron temperature that occurs within 1 ps is caused by
direct optical excitation and electronic heat transport within a distance l_ep
for the Co layer. The second temperature rise is caused by heat transport by
electrons and phonons that are near thermal equilibrium. We contrast
two-temperature modeling of heat transport in Pt an Ru films to calculations
for Cu, which has a much longer non-equilibrium length scale, l_ep=63 nm.",1912.12425v1
2016-10-27,Hybridization gap in the heavy-fermion compound UPd$_2$Al$_3$ via quasiparticle scattering spectroscopy,"We present results from point-contact spectroscopy of the antiferromagnetic
heavy-fermion superconductor UPd$_2$Al$_3$: conductance spectra are taken from
single crystals with two major surface orientations as a function of
temperature and magnetic field, and analyzed using a theory of co-tunneling
into an Anderson lattice. Spectroscopic signatures are clearly identified
including the distinct asymmetric double-peak structure arising from the
opening of a hybridization gap when a coherent heavy Fermi liquid is formed.
Both the hybridization gap, found to be 7.2 $\pm$ 0.3 meV at 4 K, and the
conductance enhancement above a flat background decrease upon increasing
temperature. While the hybridization gap is extrapolated to remain finite up to
$\sim$28 K, close to the temperature around which the magnetic susceptibility
displays a broad peak, the conductance enhancement vanishes at $\sim$18 K,
slightly above the antiferromagnetic transition temperature ($T_\textrm{N}$
$\approx$ 14 K). This rapid decrease of the conductance enhancement is
understood as a consequence of the junction drifting away from the ballistic
regime due to increased scattering off magnons associated with the localized U
5$f$ electrons. This shows that while the hybridization gap opening is not
directly associated with the antiferromagnetic ordering, its visibility in the
conductance is greatly affected by the temperature-dependent magnetic
excitations. Our findings are not only consistent with the 5$f$ dual-nature
picture in the literature but also shed new light on the interplay between the
itinerant and localized electrons in UPd$_2$Al$_3$.",1610.08601v3
2020-06-18,Spin Wave Normalization Towards all Magnonic Circuits,"The key enabling factor for Spin Wave (SW) technology utilization for
building ultra low power circuits is the ability to energy efficiently cascade
SW basic computation blocks. SW Majority gates, which constitute a universal
gate set for this paradigm, operating on phase encoded data are not input
output coherent in terms of SW amplitude, and as such, their cascading requires
information representation conversion from SW to voltage and back, which is by
no means energy effective. In this paper, a novel conversion free SW gate
cascading scheme is proposed that achieves SW amplitude normalization by means
of a directional coupler. After introducing the normalization concept, we
utilize it in the implementation of three simple circuits and, to demonstrate
its bigger scale potential, of a 2-bit inputs SW multiplier. The proposed
structures are validated by means of the Object Oriented Micromagnetic
Framework (OOMMF) and GPU-accelerated Micromagnetics (MuMax3). Furthermore, we
assess the normalization induced energy overhead and demonstrate that the
proposed approach consumes 20% to 33% less energy when compared with the
transducers based conventional counterpart. Finally, we introduce a
normalization based SW 2-bit inputs multiplier design and compare it with
functionally equivalent SW transducer based and 16nm CMOS designs. Our
evaluation indicate that the proposed approach provided 26% and 6.25x energy
reductions when compared with the conventional approach and 16nm CMOS
counterpart, respectively, which demonstrates that our proposal is energy
effective and opens the road towards the full utilization of the SW paradigm
potential and the development of SW only circuits.",2006.10432v3
2020-09-12,Anomalous thermodynamics of a quantum spin system with large residual entropy,"In contrast to strongly frustrated classical systems, their quantum
counterparts typically have a non-degenerate ground state. A counterexample is
the celebrated Heisenberg sawtooth spin chain with ferromagnetic zigzag bonds
$J_1$ and competing antiferromagnetic basal bonds $J_2$. At a quantum phase
transition point $|J_2/J_1|=1/2$, this model exhibits a flat one-magnon
excitation band leading to a massively degenerate ground-state manifold which
results in a large residual entropy. Thus, for the spin-half model, the
residual entropy amounts to exactly one half of its maximum value
$\lim_{T\to\infty} S(T)/N = \ln2$. In the present paper we study in detail the
role of the spin quantum number $s$ and the magnetic field $H$ in the parameter
region around the transition (flat-band) point. For that we use full exact
diagonalization up to $N=20$ lattice sites and the finite-temperature Lanczos
method up to $N=36$ sites to calculate the density of states as well as the
temperature dependence of the specific heat, the entropy and the
susceptibility. The study of chain lengths up to $N=36$ allows a careful
finite-size analysis. At the flat-band point we find extremely small
finite-size effects for spin $s=1/2$, i.e., the numerical data virtually
correspond to the thermodynamic limit. In all other cases the finite-size
effects are still small and become visible at very low temperatures. In a
sizeable parameter region around the flat-band point the former massively
degenerate ground-state manifold acts as a large manifold of low-lying
excitations leading to extraordinary thermodynamic properties at the transition
point as well as in its vicinity such as an additional low-temperature maximum
in the specific heat. Moreover, there is a very strong influence of the
magnetic field on the low-temperature thermodynamics including an enhanced
magnetocaloric effect.",2009.05809v2
2020-09-29,"Twisted Bilayer Graphene V: Exact Analytic Many-Body Excitations in Twisted Bilayer Graphene Coulomb Hamiltonians: Charge Gap, Goldstone Modes and Absence of Cooper Pairing","We find exact analytic expressions for the energies and wavefunctions of the
charged and neutral excitations above the exact ground states (at rational
filling per unit cell) of projected Coulomb Hamiltonians in twisted bilayer
graphene. Our exact expressions are valid for any form of the Coulomb
interaction and any form of $AA$ and $AB/BA$ tunneling. The single charge
excitation energy is a convolution of the Coulomb potential with a quantum
geometric tensor of the TBG bands. The neutral excitations are (high-symmetry
group) magnons, and their dispersion is analytically calculated in terms of the
form factors of the active bands in TBG. The two-charge excitation energy and
wavefunctions are also obtained, and a sufficient condition on the graphene
eigenstates for obtaining a Cooper-pair from Coulomb interactions is obtained.
For the actual TBG bands at the first magic angle, we can analytically show
that the Cooper pair binding energy is zero in all such projected Coulomb
models, implying that either phonons and/or non-zero kinetic energy are needed
for superconductivity. Since the [Phys. Rev. Lett. 122, 246401] showed that the
kinetic energy bounds on the superexchange energy are less $10^{-3}$ in Coulomb
units, the phonon mechanism becomes then very likely. If nonetheless the
superconductivity is due to kinetic terms which render the bands non-flat, one
prediction of our theory is that the highest $T_c$ would not occur at the
highest DOS.",2009.14200v4
2020-09-30,How coordinate Bethe ansatz works for Inozemtsev model,"Three decades ago, Inozemtsev found an isotropic long-range spin chain with
elliptic pair potential that interpolates between the Heisenberg and
Haldane-Shastry (HS) spin chains while admitting an exact solution throughout,
based on a connection with the elliptic quantum Calogero-Sutherland model.
Though Inozemtsev's spin chain is widely believed to be quantum integrable, the
underlying algebraic reason for its exact solvability is not yet well
understood. As a step in this direction we refine Inozemtsev's `extended
coordinate Bethe ansatz' and clarify various aspects of the model's exact
spectrum and its limits.
  We identify quasimomenta in terms of which the $M$-particle energy is close
to being (functionally) additive, as one would expect from the limiting models;
our expression is additive iff the energy of the elliptic Calogero-Sutherland
system is so. This enables us to rewrite the energy and Bethe-ansatz equations
on the elliptic curve, turning the spectral problem into a rational problem as
might be expected for an isotropic spin chain.
  We treat the $M=2$ particle sector and its limits in detail. We identify an
$S$-matrix that is independent of positions. We show that the Bethe-ansatz
equations reduce to those of Heisenberg in one limit and give rise to the
`motifs' of HS in the other limit. We show that, as the interpolation parameter
changes, the `scattering states' from Heisenberg become Yangian highest-weight
states for HS, while bound states become ($\mathfrak{sl}_2$-highest weight
versions of) affine descendants of the magnons from $M=1$. For bound states we
find a generalisation of the known equation for the `critical length' for the
Heisenberg spin chain. We discuss completeness for $M=2$ by passing to the
elliptic curve.
  Our review of the two-particle sectors of the Heisenberg and HS spin chains
may be of independent interest.",2009.14513v3
2015-07-23,Ultrafast Laser-Induced Melting of Long-Range Magnetic Order in Multiferroic TbMnO3,"We performed ultrafast time-resolved near-infrared pump, resonant soft X-ray
diffraction probe measurements to investigate the coupling between the
photoexcited electronic system and the spin cycloid magnetic order in
multiferroic TbMnO3 at low temperatures. We observe melting of the long range
antiferromagnetic order at low excitation fluences with a decay time constant
of 22.3 +- 1.1 ps, which is much slower than the ~1 ps melting times previously
observed in other systems. To explain the data we propose a simple model of the
melting process where the pump laser pulse directly excites the electronic
system, which then leads to an increase in the effective temperature of the
spin system via a slower relaxation mechanism. Despite this apparent increase
in the effective spin temperature, we do not observe changes in the wavevector
q of the antiferromagnetic spin order that would typically correlate with an
increase in temperature under equilibrium conditions. We suggest that this
behavior results from the extremely low magnon group velocity that hinders a
change in the spin-spiral wavevector on these time scales.",1507.06628v1
2015-12-09,"Standard model of the rare-earths, analyzed from the Hubbard I approximation","In this work we examine critically the electronic structure of the rare-earth
elements by use of the so-called Hubbard I approximation. From the theoretical
side all measured features of both occupied and unoccupied states are
reproduced, without significant deviations between observations and theory. We
also examine cohesive properties like the equilibrium volume and bulk modulus,
where we find, in general, a good agreement between theory and measurements. In
addition we have reproduced the spin and orbital moments of these elements, as
they are reflected from measurements of the saturation moment. We have also
employed the Hubbard I approximation to extract the interatomic exchange
parameters of an effective spin Hamiltonian for the heavy rare earths. We show
that the Hubbard I approximation gives results which are consistent with
calculations where $4f$ electrons are treated as core states for Gd. The latter
approach was also used to address the series of the heavy/late rare-earths. Via
Monte Carlo simulations we obtained ordering temperatures which reproduce
measurements within about $20\%$. We have further illustrated the accuracy of
these exchange parameters by comparing measured and calculated magnetic
configurations for the heavy rare earths and the magnon dispersion for Gd. The
Hubbard I approximation is compared to other theories of the electronic
structure, and we argue that it is superior. We discuss the relevance of our
results in general, and how this makes it possible to treat the electronic
structure of materials containing rare-earth elements, such as permanent
magnets, magnetostrictive compounds, photovoltaics, optical fibers, topological
insulators, and molecular magnets.",1512.02848v2
2016-11-17,Holographic Dynamics from Multiscale Entanglement Renormalization Ansatz,"The Multiscale Entanglement Renormalization Ansatz (MERA) is a tensor network
based variational ansatz that is capable of capturing many of the key physical
properties of strongly correlated ground states such as criticality and
topological order. MERA also shares many deep relationships with the AdS/CFT
(gauge-gravity) correspondence by realizing a UV complete holographic duality
within the tensor networks framework. Motivated by this, we have re-purposed
the MERA tensor network as an analysis tool to study the real-time evolution of
the 1D transverse Ising model in its low energy excited state sector. We
performed this analysis by allowing the ancilla qubits of the MERA tensor
network to acquire quantum fluctuations, which yields a unitary transform
between the physical (boundary) and ancilla qubit (bulk) Hilbert spaces. This
then defines a reversible quantum circuit which is used as a `holographic
transform' to study excited states and their real-time dynamics from the point
of the bulk ancillae. In the gapped paramagnetic phase of the transverse field
Ising model, we demonstrate the holographic duality between excited states
induced by single spin-flips (Ising `magnons') acting on the ground state and
single ancilla qubit spin-flips. The single ancillae qubit excitation is shown
to be stable in the bulk under real-time evolution and hence defines a stable
holographic quasiparticle which we have named the `hologron'. The `dictionary'
between the bulk and boundary is determined and realizes many features of the
holographic correspondence in a non-CFT limit of the boundary theory. As an
added spin-off, this dictionary together with the extension to multi-hologron
sectors gives us a systematic way to construct quantitatively accurate low
energy effective Hamiltonians.",1611.05877v1
2018-08-26,Resonating quantum three-coloring wavefunctions for the kagome quantum antiferromagnet,"Motivated by the recent discovery of a macroscopically degenerate exactly
solvable point of the spin-$1/2$ $XXZ$ model for $J_z/J=-1/2$ on the kagome
lattice [H. J. Changlani et al. Phys. Rev. Lett 120, 117202 (2018)] -- a result
that holds for arbitrary magnetization -- we develop an exact mapping between
its exact quantum three-coloring wavefunctions and the characteristic localized
and topological magnons. This map, involving resonating two-color loops, is
developed to represent exact many-body ground state wavefunctions for special
high magnetizations. Using this map we show that these exact ground state
solutions are valid for any $J_z/J \geq -1/2$. This demonstrates the
equivalence of the ground-state wavefunction of the Ising, Heisenberg and $XY$
regimes all the way to the $J_z/J=-1/2$ point for these high magnetization
sectors. In the hardcore bosonic language, this means that a certain class of
exact many-body solutions, previously argued to hold for purely repulsive
interactions ($J_z \geq 0$), actually hold for attractive interactions as well,
up to a critical interaction strength. For the case of zero magnetization,
where the ground state is not exactly known, we perform density matrix
renormalization group calculations. Based on the calculation of the ground
state energy and measurement of order parameters, we provide evidence for a
lack of any qualitative change in the ground state on finite clusters in the
Ising ($J_z \gg J$), Heisenberg ($J_z=J$) and $XY$ ($J_z=0$) regimes,
continuing adiabatically to the vicinity of the macroscopically degenerate
$J_z/J=-1/2$ point. These findings offer a framework for recent results in the
literature, and also suggest that the $J_z/J=-1/2$ point is an unconventional
quantum critical point whose vicinity may contain the key to resolving the
spin-$1/2$ kagome problem.",1808.08633v1
2019-07-17,Topical Review on Skyrmions and Hall Transport,"We review recent progresses towards an understanding of the Skyrmion Hall
transport in insulating as well as conducting materials. First, we consider a
theoretical breakthrough based on the quantum field theory Ward identity, a
first principle analysis, relying on symmetries and conservation laws. Broken
parity (inversion) symmetry plays a crucial role in Skyrmion Hall transport. In
addition to the well known thermal and electric Hall conductivities, our
analysis has led us to the discovery of a new and unforeseen physical quantity,
Hall viscosity - an anti-symmetric part of the viscosity tensor. We propose a
simple way to confirm the existence of Hall viscosity in the measurements of
Hall conductivity as a function of momentum. We provide various background
materials to assist the readers to understand the quantum field theory Ward
identity.
  In the second part, we review recent theoretical and experimental
advancements of the Skyrmion Hall effects and the topological (Magnon) Hall
effects for conducting (insulting) magnets. For this purpose, we consider two
enveloping themes: spin torque and thermo-electromagnetic effect. First, we
overview various spin torques, such as spin transfer torque, spin-orbit torque,
and spin Hall torque, and generalized Landau-Lifshitz-Gilbert equations and
Thiele equations using a phenomenological approach. Second, we consider
irreversible thermodynamics to survey possible thermo-electromagnetic effects,
such as Seebeck, Peltier and Thompson effects in the presence of the electric
currents, along with the Hall effects in the presence of a background magnetic
field. Recently developed spin Seebeck effects are also a significant part of
the survey. We also accommodate extensive background materials to make this
review self-contained. Finally, we revisit the Skyrmion Hall transport from the
Ward identity view point.",1907.07696v1
2019-10-11,"Low-Damping Ferromagnetic Resonance in Electron-Beam Patterned, High-$Q$ Vanadium Tetracyanoethylene Magnon Cavities","Integrating patterned, low-loss magnetic materials into microwave devices and
circuits presents many challenges due to the specific conditions that are
required to grow ferrite materials, driving the need for flip-chip and other
indirect fabrication techniques. The low-loss ($\alpha = 3.98 \pm 0.22 \times
10^{-5}$), room-temperature ferrimagnetic coordination compound vanadium
tetracyanoethylene ($\mathrm{V[TCNE]}_x$) is a promising new material for these
applications that is potentially compatible with semiconductor processing. Here
we present the deposition, patterning, and characterization of
$\mathrm{V[TCNE]}_x$ thin films with lateral dimensions ranging from 1 micron
to several millimeters. We employ electron-beam lithography and liftoff using
an aluminum encapsulated poly(methyl methacrylate), poly(methyl
methacrylate-methacrylic acid) copolymer bilayer (PMMA/P(MMA-MAA)) on sapphire
and silicon. This process can be trivially extended to other common
semiconductor substrates. Films patterned via this method maintain low-loss
characteristics down to 25 microns with only a factor of 2 increase down to 5
microns. A rich structure of thickness and radially confined spin-wave modes
reveals the quality of the patterned films. Further fitting, simulation, and
analytic analysis provides an exchange stiffness, $A_{ex} = 2.2 \pm 0.5 \times
10^{-10}$ erg/cm, as well as insights into the mode character and surface spin
pinning. Below a micron, the deposition is non-conformal, which leads to
interesting and potentially useful changes in morphology. This work establishes
the versatility of $\mathrm{V[TCNE]}_x$ for applications requiring highly
coherent magnetic excitations ranging from microwave communication to quantum
information.",1910.05325v1
2020-01-17,Temporal and spectral fingerprints of ultrafast all-coherent spin switching,"Future information technology demands ultimately fast, low-loss quantum
control. Intense light fields have facilitated important milestones, such as
inducing novel states of matter, accelerating electrons ballistically, or
coherently flipping the valley pseudospin. These dynamics leave unique
signatures, such as characteristic bandgaps or high-order harmonic radiation.
The fastest and least dissipative way of switching the technologically most
important quantum attribute - the spin - between two states separated by a
potential barrier is to trigger an all-coherent precession. Pioneering
experiments and theory with picosecond electric and magnetic fields have
suggested this possibility, yet observing the actual dynamics has remained out
of reach. Here, we show that terahertz (1 THz = 10$^{12}$ Hz) electromagnetic
pulses allow coherent navigation of spins over a potential barrier and we
reveal the corresponding temporal and spectral fingerprints. This goal is
achieved by coupling spins in antiferromagnetic TmFeO$_{3}$ with the locally
enhanced THz electric field of custom-tailored antennas. Within their duration
of 1 ps, the intense THz pulses abruptly change the magnetic anisotropy and
trigger a large-amplitude ballistic spin motion. A characteristic phase flip,
an asymmetric splitting of the magnon resonance, and a long-lived offset of the
Faraday signal are hallmarks of coherent spin switching into adjacent potential
minima, in agreement with a numerical simulation. The switchable spin states
can be selected by an external magnetic bias. The low dissipation and the
antenna's sub-wavelength spatial definition could facilitate scalable spin
devices operating at THz rates.",2001.06255v1
2020-01-29,Magnetic Fluctuations and the Spin-Orbit Interaction in Mott Insulating CoO,"Motivated by the presence of an unquenched orbital angular momentum in CoO, a
team at Chalk River, including a recently hired research officer Roger Cowley,
performed the first inelastic neutron scattering experiments on the classic
Mott insulator [Sakurai $\textit{et al.}$ 1968 Phys. Rev. $\mathbf{167}$ 510].
Despite identifying magnon modes at the zone boundary, the team was unable to
parameterise the low energy magnetic excitation spectrum below $T\rm{_{N}}$
using conventional pseudo-bosonic approaches. It would not be for another 40
years that Roger, now at Oxford and motivated by the discovery of the
high-$T_{c}$ cuprate superconductors [Bednorz & Muller 1986 Z. Phys. B
$\mathbf{64}$ 189], would make another attempt at the parameterisation of the
magnetic excitation spectrum that had previously alluded him. Upon his return
to CoO, Roger found a system embroiled in controversy, with some of its most
fundamental parameters still remaining undetermined. Faced with such a
formidable task, Roger performed a series of inelastic neutron scattering
experiments in the early 2010's on both CoO and a magnetically dilute
structural analogue MgO. These experiments would prove instrumental in the
determination of both single-ion [Cowley $\textit{et al.}$ 2013 Phys. Rev. B
$\mathbf{88}$ 205117] and cooperative magnetic parameters [Sarte $\textit{et
al.}$ 2018 Phys. Rev. B $\mathbf{98}$ 024415] for CoO. Both these sets of
parameters would eventually be used in a spin-orbit exciton model [Sarte
$\textit{et al.}$ 2019 Phys. Rev. B $\mathbf{100}$ 075143], developed by his
longtime friend and collaborator Bill Buyers, to successfully parameterise the
complex spectrum that both measured at Chalk River almost 50 years prior. The
story of CoO is of one that has come full circle, one filled with both
spectacular failures and intermittent, yet profound, little victories.",2001.10923v1
2020-04-24,Skyrmionic order and magnetically induced polarization change in lacunar spinel compounds GaV$_{4}$S$_{8}$ and GaMo$_{4}$S$_{8}$: comparative theoretical study,"We show how low-energy electronic models derived from the first-principles
electronic structure calculations can help to rationalize the magnetic
properties of two lacunar spinel compounds GaM4S8 with light (M=V) and heavy
(M=Mo) transition-metal elements, which are responsible for different
spin-orbit interaction strength. In the model, each magnetic lattice point was
associated with the M4S4 molecule, and the model itself was formulated in the
basis of molecular Wannier functions constructed for three magnetic t2 bands.
The effects of rhombohedral distortion, spin-orbit interaction, band filling,
and the screening of Coulomb interactions in the t2 bands are discussed in
details. The electronic model is further treated in the superexchange
approximation, which allows us to derive an effective spin model for the energy
and electric polarization ($P$) depending on the relative orientation of spins
in the bonds, and study the properties of this model by means of classical
Monte Carlo simulations with the emphasis on the possible formation of the
skyrmionic phase. While isotropic exchange interactions clearly dominate in
GaV4S8, all types of interactions -- isotropic, antisymmetric, and symmetric
anisotropic -- are comparable in the case of GaMo4S8. Particularly, large
uniaxial exchange anisotropy has a profound effect on the properties of
GaMo4S8. On the one hand, it raises the Curie temperature by opening a gap in
the spectrum of magnon excitations. On the other hand, it strongly affects the
skyrmionic phase by playing the role of a molecular field, which facilitates
the formation of skyrmions, but makes them relatively insensitive to the
external magnetic field in the large part of the phase diagram. We predict
reversal of the magnetic dependence of $P$ in the case of GaMo4S8 caused by the
reversal of direction of the rhombohedral distortion.",2004.11548v1
2020-05-28,"Dirac-type nodal spin liquid revealed by refined quantum many-body solver using neural-network wave function, correlation ratio, and level spectroscopy","Pursuing fractionalized particles that do not bear properties of conventional
measurable objects, exemplified by bare particles in the vacuum such as
electrons and elementary excitations such as magnons, is a challenge in
physics. Here we show that a machine-learning method for quantum many-body
systems that has achieved state-of-the-art accuracy reveals the existence of a
quantum spin liquid (QSL) phase in the region $0.49\lesssim
J_2/J_1\lesssim0.54$ convincingly in spin-1/2 frustrated Heisenberg model with
the nearest and next-nearest neighbor exchanges, $J_1$ and $J_2$, respectively,
on the square lattice. This is achieved by combining with the cutting-edge
computational schemes known as the correlation ratio and level spectroscopy
methods to mitigate the finite-size effects. The quantitative one-to-one
correspondence between the correlations in the ground state and the excitation
spectra enables the reliable identification and estimation of the QSL and its
nature. The spin excitation spectra containing both singlet and triplet gapless
Dirac-like dispersions signal the emergence of gapless fractionalized spin-1/2
Dirac-type spinons in the distinctive QSL phase. Unexplored critical behavior
with coexisting and dual power-law decays of N\'{e}el antiferromagnetic and
dimer correlations is revealed. The power-law decay exponents of the two
correlations differently vary with $J_2/J_1$ in the QSL phase and thus have
different values except for a single point satisfying the symmetry of the two
correlations. The isomorph of excitations with the cuprate $d$-wave
superconductors implies a tight connection between the present QSL and
superconductivity. This achievement demonstrates that the quantum-state
representation using machine learning techniques, which had mostly been limited
to benchmarks, is a promising tool for investigating grand challenges in
quantum many-body physics.",2005.14142v6
2020-07-22,An Electromagnetic Approach to Cavity Spintronics,"The fields of cavity quantum electrodynamics and magnetism have recently
merged into \textit{`cavity spintronics'}, investigating a quasiparticle that
emerges from the strong coupling between standing electromagnetic waves
confined in a microwave cavity resonator and the quanta of spin waves, magnons.
This phenomenon is now expected to be employed in a variety of devices for
applications ranging from quantum communication to dark matter detection. To be
successful, most of these applications require a vast control of the coupling
strength, resulting in intensive efforts to understanding coupling by a variety
of different approaches. Here, the electromagnetic properties of both resonator
and magnetic samples are investigated to provide a comprehensive understanding
of the coupling between these two systems. Because the coupling is a
consequence of the excitation vector fields, which directly interact with
magnetisation dynamics, a highly-accurate electromagnetic perturbation theory
is employed which allows for predicting the resonant hybrid mode frequencies
for any field configuration within the cavity resonator, without any fitting
parameters. The coupling is shown to be strongly dependent not only on the
excitation vector fields and sample's magnetic properties but also on the
sample's shape. These findings are illustrated by applying the theoretical
framework to two distinct experiments: a magnetic sphere placed in a
three-dimensional resonator, and a rectangular, magnetic prism placed on a
two-dimensional resonator. The theory provides comprehensive understanding of
the overall behaviour of strongly coupled systems and it can be easily modified
for a variety of other systems.",2007.11483v2
2020-08-21,"Macroscopic, layered onion shell like magnetic domain structure generated in YIG film using ultrashort, megagauss magnetic pulses","Study of the formation and evolution of large scale, ordered structures is an
enduring theme in science. The generation, evolution and control of large sized
magnetic domains are intriguing and challenging tasks, given the complex nature
of competing interactions present in any magnetic system. Here, we demonstrate
large scale non-coplanar ordering of spins, driven by picosecond, megagauss
magnetic pulses derived from a high intensity, femtosecond laser. Our studies
on a specially designed Yttrium Iron Garnet (YIG)/dielectric/metal film
sandwich target, show the creation of complex, large, concentric, elliptical
shaped magnetic domains which resemble the layered shell structure of an onion.
The largest shell has a major axis of over hundreds of micrometers, in stark
contrast to conventional sub micrometer scale polygonal, striped or bubble
shaped magnetic domains found in magnetic materials, or the large dumbbell
shaped domains produced in magnetic films irradiated with accelerator based
relativistic electron beams. Through micromagnetic simulations, we show that
the giant magnetic field pulses create ultrafast terahertz (THz) spin waves. A
snapshot of these fast propagating spin waves is stored as the layered onion
shell shaped domains in the YIG film. Typically, information transport via spin
waves in magnonic devices occurs in the gigahertz (GHz) regime, where the
devices are susceptible to thermal disturbances at room temperature. Our
intense laser light pulse - YIG sandwich target combination, paves the way for
room temperature table-top THz spin wave devices, which operate just above or
in the range of the thermal noise floor. This dissipation-less device offers
ultrafast control of spin information over distances of few hundreds of
microns.",2008.09473v1
2020-11-23,2-input 4-output Programmable Spin Wave Logic Gate,"To bring Spin Wave (SW) based computing paradigm into practice and develop
ultra low power Magnonic circuits and computation platforms, one needs basic
logic gates that operate and can be cascaded within the SW domain without
requiring back and forth conversion between the SW and voltage domains. To
achieve this, SW gates have to possess intrinsic fanout capabilities, be
input-output data representation coherent, and reconfigurable. In this paper,
we address the first and the last requirements and propose a novel 4-output
programmable SW logic. First, we introduce the gate structure and demonstrate
that, by adjusting the gate output detection method, it can parallelly evaluate
any 4-element subset of the 2-input Boolean function set AND, NAND, OR, NOR,
XOR, and XNOR. Furthermore, we adjust the structure such that all its 4 outputs
produce SWs with the same energy and demonstrate that it can evaluate Boolean
function sets while providing fanout capabilities ranging from 1 to 4. We
validate our approach by instantiating and simulating different gate
configurations such as 4-output AND/OR, 4-output XOR/XNOR, output energy
balanced 4-output AND/OR, and output energy balanced 4-output XOR/XNOR by means
of Object Oriented Micromagnetic Framework (OOMMF) simulations. Finally, we
evaluate the performance of our proposal in terms of delay and energy
consumption and compare it against existing state-of-the-art SW and 16nm CMOS
counterparts. The results indicate that for the same functionality, our
approach provides 3x and 16x energy reduction, when compared with conventional
SW and 16nm CMOS implementations, respectively.",2011.11316v2
2021-03-25,Twist engineering of the two-dimensional magnetism in double bilayer chromium triiodide homostructures,"Twist engineering, or the alignment of two-dimensional (2D) crystalline
layers with desired orientations, has led to tremendous success in modulating
the charge degree of freedom in hetero- and homo-structures, in particular, in
achieving novel correlated and topological electronic phases in moir\'e
electronic crystals. However, although pioneering theoretical efforts have
predicted nontrivial magnetism and magnons out of twisting 2D magnets,
experimental realization of twist engineering spin degree of freedom remains
elusive. Here, we leverage the archetypal 2D Ising magnet chromium triiodide
(CrI3) to fabricate twisted double bilayer homostructures with tunable twist
angles and demonstrate the successful twist engineering of 2D magnetism in
them. Using linear and circular polarization-resolved Raman spectroscopy, we
identify magneto-Raman signatures of a new magnetic ground state that is
sharply distinct from those in natural bilayer (2L) and four-layer (4L) CrI3.
With careful magnetic field and twist angle dependence, we reveal that, for a
very small twist angle (~ 0.5 degree), this emergent magnetism can be
well-approximated by a weighted linear superposition of those of 2L and 4L CI3
whereas, for a relatively large twist angle (~ 5 degree), it mostly resembles
that of isolated 2L CrI3. Remarkably, at an intermediate twist angle (~ 1.1
degree), its magnetism cannot be simply inferred from the 2L and 4L cases,
because it lacks sharp spin-flip transitions that are present in 2L and 4L CrI3
and features a dramatic Raman circular dichroism that is absent in natural 2L
and 4L ones. Our results demonstrate the possibility of designing and
controlling the spin degree of freedom in 2D magnets using twist engineering.",2103.13537v2
2021-03-28,Magnetic field-induced softening of spin waves and hard-axis order in Kondo-lattice ferromagnet CeAgSb$_{2}$,"A significant number of Kondo-lattice ferromagnets order perpendicular to the
easy magnetization axis dictated by the crystalline electric field. The nature
of this phenomenon has attracted considerable attention, but remains poorly
understood. In the present paper we use inelastic neutron scattering supported
by magnetization and specific heat measurements to study the spin dynamics in
the hard-axis ferromagnet CeAgSb2. In the zero field state we observed two
sharp magnon modes, which are associated with Ce ordering and extended up to
$\approx 3 meV with a considerable spin gap of 0.6 meV. Application of a
magnetic field perpendicular to the moment direction reduces the spectral
intensity and suppresses the gap and significantly enhances the low-temperature
specific heat at a critical field of Bc ~ 2.8 T via a mean-field-like
transition. Above the transition, in the field polarized state, the gap
eventually reopens due to the Zeeman effect. We modeled the observed dispersion
using linear spin-wave theory (LSWT) taking into account the ground state Gamma
6 doublet and exchange anisotropy. Our model correctly captures the essential
features of the spin dynamics including magnetic dispersion, distribution of
the spectral intensity as well as the field-induced behavior, although several
minor features remain obscure. The observed spectra do not show significant
broadening due to the finite lifetime of the quasiparticles. Along with a
moderate electronic specific heat coefficient gamma = 46 mJ/mol K2 this
indicates that the Kondo coupling is relatively weak and the Ce moments are
well localized. Altogether, our results provide profound insight into the spin
dynamics of the hard-axis ferromagnet CeAgSb2 and can be used as solid ground
for studying magnetic interactions in isostructural compounds including
CeAuSb2, which exhibits nematicity and unusual mesoscale magnetic textures.",2103.15137v2
2021-04-21,Atomic Layer Deposition of Yttrium Iron Garnet Thin Films for 3D Magnetic Structures,"A wide variety of new phenomena such as novel magnetization configurations
have been predicted to occur in three dimensional magnetic nanostructures.
However, the fabrication of such structures is often challenging due to the
specific shapes required, such as magnetic tubes and spirals. Furthermore, the
materials currently used to assemble these structures are predominantly
magnetic metals that do not allow to study the magnetic response of the system
separately from the electronic one. In the field of spintronics, the
prototypical material used for such experiments is the ferrimagnetic insulator
yttrium iron garnet (Y$_3$Fe$_5$O$_{12}$, YIG). YIG is one of the best
materials especially for magnonic studies due to its low Gilbert damping. Here,
we report the first successful fabrication of YIG thin films via atomic layer
deposition. To that end we utilize a supercycle approach based on the
combination of sub-nanometer thin layers of the binary systems Fe$_2$O$_3$ and
Y$_2$O$_3$ in the correct atomic ratio on Y$_3$Al$_5$O$_{12}$ substrates with a
subsequent annealing step. Our process is robust against typical growth-related
deviations, ensuring a good reproducibility. The ALD-YIG thin films exhibit a
good crystalline quality as well as magnetic properties comparable to other
deposition techniques. One of the outstanding characteristics of atomic layer
deposition is its ability to conformally coat arbitrarily-shaped substrates.
ALD hence is the ideal deposition technique to grant an extensive freedom in
choosing the shape of the magnetic system. The atomic layer deposition of YIG
enables the fabrication of novel three dimensional magnetic nanostructures,
which in turn can be utilized for experimentally investigating the phenomena
predicted in those structures.",2104.10293v2
2021-06-14,One-dimensional ghost imaging with an electron microscope: a route towards ghost imaging with inelastically scattered electrons,"In quantum mechanics, entanglement and correlations are not just a mere
sporadic curiosity, but rather common phenomena at the basis of an interacting
quantum system. In electron microscopy, such concepts have not been extensively
explored yet in all their implications; in particular, inelastic scattering can
be reanalyzed in terms of correlation between the electron beam and the sample.
While classical inelastic scattering simply implies loss of coherence in the
electron beam, performing a joint measurement on the electron beam and the
sample excitation could restore the coherence and the lost information. Here,
we propose to exploit joint measurement in electron microscopy for a surprising
and counter-intuitive application of the concept of ghost imaging. Ghost
imaging, first proposed in quantum photonics, can be applied partially in
electron microscopy by performing joint measurement between the portion of the
transmitted electron beam and a photon emitted from the sample reaching a
bucket detector. This would permit us to form a one-dimensional virtual image
of an object that even has not interacted with the electron beam directly. This
technique is extremely promising for low-dose imaging that requires the
minimization of radiation exposure for electron-sensitive materials, because
the object interacts with other form of waves, e.g., photons/surface plasmon
polaritons, and not the electron beam. We demonstrate this concept
theoretically for any inelastic electron-sample interaction in which the
electron excites a single quantum of a collective mode, such as a photon,
plasmon, phonon, magnon, or any optical polariton.",2106.08955v1
2021-06-22,"XY magnetism, Kitaev exchange, and long-range frustration in the $J_{\rm eff}=1/2$ honeycomb cobaltates","The quest for Kitaev quantum spin liquids has led to great interest in
honeycomb quantum magnets with strong spin-orbit coupling. It has been recently
proposed that even Mott insulators with $3d$ transition metal ions, having
nominally weak spin-orbit coupling, can realize such exotic physics. Motivated
by this, we study the rhombohedral honeycomb cobaltates CoTiO$_3$,
BaCo$_2$(PO$_4$)$_2$, and BaCo$_2$(AsO$_4$)$_2$, using $\textit{ab initio}$
density functional theory, which takes into account realistic crystal field
distortions and chemical information, in conjunction with exact diagonalization
numerics. We show that these Co$^{2+}$ magnets host $j_\text{eff}=1/2$ local
moments with highly anisotropic $g$-factors, and we extract their full spin
Hamiltonians including longer-range and anisotropic exchange couplings. For
CoTiO$_3$, we find a nearest-neighbor easy-plane ferromagnetic $XXZ$ model with
additional bond-dependent anisotropies and interlayer exchange, which supports
three-dimensional (3D) Dirac nodal line magnons. In contrast, for
BaCo$_2$(PO$_4$)$_2$ and BaCo$_2$(AsO$_4$)$_2$, we find a strongly suppressed
interlayer coupling, and significant frustration from additional third-neighbor
antiferromagnetic exchange mediated by P/As. Such bond-anisotropic $J_1$-$J_3$
spin models can support collinear zig-zag or coplanar spiral ground states; we
discuss their dynamical spin correlations which reveal a gapped Goldstone mode,
and argue that the effective parameters of these pseudospin-$1/2$ models may be
strongly renormalized by coupling to a low energy spin-exciton. Our results
call for re-examining proposals for realizing Kitaev spin liquids in the
honeycomb cobaltates.",2106.11982v3
2021-07-22,When can localized spins interacting with conduction electrons in ferro- or antiferromagnets be described classically via the Landau-Lifshitz equation: Transition from quantum many-body entangled to quantum-classical nonequilibrium states,"Experiments in spintronics and magnonics operate with macroscopically large
number of localized spins within ferromagnetic (F) or antiferromagnetic (AF)
materials, so that their nonequilibrium dynamics is standardly described by the
Landau-Lifshitz (LL) equation treating localized spins as classical vectors of
fixed length. However, spin is a genuine quantum degree of freedom, and even
though quantum effects become progressively less important for spin value
$S>1$, they exist for all $S < \infty$. While this has motivated exploration of
limitations/breakdown of the LL equation, by using examples of F insulators,
analogous comparison of fully quantum many-body vs. quantum (for
electrons)-classical (for localized spins) dynamics in systems where
nonequilibrium conduction electrons are present is lacking. Here we employ
quantum Heisenberg F or AF chains of $N=4$ sites, whose localized spins
interact with conduction electrons via $sd$ exchange interaction, to perform
such comparison by starting from unentangled pure (at zero temperature) or
mixed (at finite temperature) quantum state of localized spins as the initial
condition. This reveals that quantum-classical dynamics can faithfully
reproduce fully quantum dynamics in the F metallic case, but only when spin
$S$, Heisenberg exchange between localized spins and $sd$ exchange are
sufficiently small. Increasing any of these three parameters can lead to
substantial deviations, which are explained by the dynamical buildup of
entanglement between localized spins and/or between them and electrons. In the
AF metallic case, substantial deviations appear even at early times, despite
starting from unentangled N\'{e}el state, which therefore poses a challenge on
how to rigorously justify wide usage of the LL equation in phenomenological
modeling of antiferromagnetic spintronics experiments.",2107.10776v3
2021-07-22,Spin-orbit torque control of spin waves in a ferromagnetic waveguide,"Spin-orbit torque (SOT) created by a spin current injected into a ferromagnet
by an adjacent heavy metal represents an efficient tool for the excitation and
manipulation of spin waves. Here we report the micromagnetic simulations
describing the influence of SOT on the propagation of spin waves in the
$\mathrm{W}/\mathrm{CoFeB}/\mathrm{MgO}$ nanostructure having
voltage-controlled magnetic anisotropy (VCMA). The simulations show that two
spin waves travelling in the opposite directions can be generated in the center
of the $\mathrm{CoFeB}$ waveguide via the modulation of VCMA induced by a
microwave voltage locally applied to the $\mathrm{MgO}$ nanolayer. The
amplitudes of these waves exponentially decrease with the propagation distance
with similar decay lengths of about 2.5 $\mu$m. In the presence of a direct
electric current injected into the $\mathrm{W}$ film beneath the waveguide
center, the decay lengths of two spin waves change in the opposite way owing to
different directions of the electric currents flowing in the underlying halves
of the $\mathrm{W}$ layer. Remarkably, above the critical current density
$J_\mathrm{W} \approx 2 \times 10^{10}$ A m$^{-2}$, SOT provides the
amplification of the spin wave propagating in one half of the waveguide and
strongly accelerates the attenuation of the wave travelling in the other half.
As a result, a long-distance spin-wave propagation takes place in a half of the
$\mathrm{CoFeB}$ waveguide only. Furthermore, by reversing the polarity of the
dc voltage applied to the heavy-metal layer one can change the propagation area
and switch the travel direction of the spin wave in the ferromagnetic
waveguide. Thus, the $\mathrm{W}/\mathrm{CoFeB}/\mathrm{MgO}$ nanostructure can
be employed as an electrically controlled magnonic device converting the
electrical input signal into a spin signal, which can be transmitted to one of
two outputs of the device.",2107.10795v2
2021-07-27,The Keck Planet Imager and Characterizer: A dedicated single-mode fiber injection unit for high resolution exoplanet spectroscopy,"The Keck Planet Imager and Characterizer (KPIC) is a purpose-built instrument
to demonstrate new technological and instrumental concepts initially developed
for the exoplanet direct imaging field. Located downstream of the current Keck
II adaptive optic system, KPIC contains a fiber injection unit (FIU) capable of
combining the high-contrast imaging capability of the adaptive optics system
with the high dispersion spectroscopy capability of the current Keck high
resolution infrared spectrograph (NIRSPEC). Deployed at Keck in September 2018,
this instrument has already been used to acquire high resolution spectra ($R >
30,000$) of multiple targets of interest. In the near term, it will be used to
spectrally characterize known directly imaged exoplanets and low-mass brown
dwarf companions visible in the northern hemisphere with a spectral resolution
high enough to enable spin and planetary radial velocity measurements as well
as Doppler imaging of atmospheric weather phenomena. Here we present the design
of the FIU, the unique calibration procedures needed to operate a single-mode
fiber instrument and the system performance.",2107.12556v1
2021-08-16,Symmetry- and curvature effects on spin waves in vortex-state hexagonal nanotubes,"Analytic and numerical studies on curved magnetic nano-objects predict
numerous exciting effects that can be referred to as magneto-chiral effects,
which do not originate from intrinsic Dzyaloshinskii-Moriya interaction or
interface-induced anisotropies. In constrast, these chiral effects stem from
isotropic exchange or dipole-dipole interaction, present in all magnetic
materials, which acquire asymmetric contributions in case of curved geometry of
the specimen. As a result, for example, the spin-wave dispersion in round
magnetic nanotubes becomes asymmetric, namely spin waves of the same frequency
propagating in opposite directions along the nanotube exhibit different
wavelenghts. Here, using time-resolved scanning transmission X-ray microscopy
experiments, standard micromagntic simulations and a dynamic-matrix approach,
we show that the spin-wave spectrum undergoes additional drastic changes when
transitioning from a continuous to a discrete rotational symmetry, i.e. from
round to hexagonal nanotubes, which are much easier to fabricate. The polygonal
shape introduces localization of the modes both to the sharp, highly curved
corners and flat edges. Moreover, due to the discrete rotational symmetry, the
degenerate nature of the modes with azimuthal wave vectors known from round
tubes is partly lifted, resulting in singlet and duplet modes. For comparison
with our experiments, we calculate the microwave absorption from the
numerically obtained mode profiles which shows that a dedicated antenna design
is paramount for magnonic applications in 3D nano-structures. To our knowledge
these are the first experiments directly showing real space spin-wave
propagation in 3D nano objects.",2108.07103v1
2021-10-11,Transition of laser-induced terahertz spin currents from torque- to conduction-electron-mediated transport,"Spin transport is crucial for future spintronic devices operating at
bandwidths up to the terahertz (THz) range. In F|N thin-film stacks made of a
ferro/ferrimagnetic layer F and a normal-metal layer N, spin transport is
mediated by (1) spin-polarized conduction electrons and/or (2) torque between
electron spins. To identify a cross-over from (1) to (2), we study laser-driven
spin currents in F|Pt stacks where F consists of model materials with different
degrees of electrical conductivity. For the magnetic insulators YIG, GIG and
maghemite, identical dynamics is observed. It arises from the THz interfacial
spin Seebeck effect (SSE), is fully determined by the relaxation of the
electrons in the metal layer and provides an estimate of the spin-mixing
conductance of the GIG/Pt interface. Remarkably, in the half-metallic
ferrimagnet Fe3O4 (magnetite), our measurements reveal two spin-current
components with opposite direction. The slower, positive component exhibits SSE
dynamics and is assigned to torque-type magnon excitation of the A- and B-spin
sublattices of Fe3O4. The faster, negative component arises from the
pyro-spintronic effect and can consistently be assigned to ultrafast
demagnetization of e-sublattice minority-spin hopping electrons. This
observation supports the magneto-electronic model of Fe3O4. In general, our
results provide a new route to the contact-free separation of torque- and
conduction-electron-mediated spin currents.",2110.05462v2
2021-10-13,"A Combined First Principles Study of the Structural, Magnetic, and Phonon Properties of Monolayer CrI$_{3}$","The first magnetic 2D material discovered, monolayer (ML) CrI$_3$, is
particularly fascinating due to its ground state ferromagnetism. Yet, because
monolayer materials are difficult to probe experimentally, much remains
unresolved about ML CrI$_{3}$'s structural, electronic, and magnetic
properties. Here, we leverage Density Functional Theory (DFT) and high-accuracy
Diffusion Monte Carlo (DMC) simulations to predict lattice parameters, magnetic
moments, and spin-phonon and spin-lattice coupling of ML CrI$_{3}$. We exploit
a recently developed surrogate Hessian DMC line search technique to determine
CrI$_{3}$'s monolayer geometry with DMC accuracy, yielding lattice parameters
in good agreement with recently-published STM measurements - an accomplishment
given the $\sim 10$% variability in previous DFT-derived estimates depending
upon the functional. Strikingly, we find previous DFT predictions of ML
CrI$_3$'s magnetic spin moments are correct on average across a unit cell, but
miss critical local spatial fluctuations in the spin density revealed by more
accurate DMC. DMC predicts magnetic moments in ML CrI$_3$ are 3.62 $\mu_B$ per
chromium and -0.145 $\mu_B$ per iodine; both larger than previous DFT
predictions. The large disparate moments together with the large spin-orbit
coupling of CrI$_3$'s I-$\textit{p}$ orbital suggests a ligand
superexchange-dominated magnetic anisotropy in ML CrI$_3$, corroborating recent
observations of magnons in its 2D limit. We also find ML CrI$_3$ exhibits a
substantial spin-phonon coupling of $\sim$3.32 cm$^{-1}$. Our work thus
establishes many of ML CrI$_{3}$'s key properties, while also continuing to
demonstrate the pivotal role DMC can assume in the study of magnetic and other
2D materials.",2110.06731v1
2021-10-25,Germanium monosulfide as a natural platform for highly anisotropic THz polaritons,"Terahertz (THz) electromagnetic radiation is key to optically access
collective excitations such as magnons (spins), plasmons (electrons), or
phonons (atomic vibrations), thus bridging between optics and solid-state
physics. Confinement of THz light to the nanometer length scale is desirable
for local probing of such excitations in low dimensional systems, thereby
inherently circumventing the large footprint and low spectral density of
far-field THz optics. For that purpose, phonon polaritons (PhPs, i.e., light
coupled to lattice vibrations in polar crystals) in anisotropic van der Waals
(vdW) materials have recently emerged as a promising platform for THz
nanooptics; yet the amount of explored, viable materials is still exiguous.
Hence, there is a demand for the exploration of novel materials that feature
not only THz PhPs at different spectral regimes, but also exhibit unique
anisotropic (directional) electrical, thermoelectric, and vibronic properties.
To that end, we introduce here the semiconducting alpha-germanium(II) sulfide
(GeS) as an intriguing candidate. By employing THz nano-spectroscopy supported
by theoretical analysis, we provide a thorough characterization of the
different in-plane hyperbolic and elliptical PhP modes in GeS. We find not only
PhPs with long life times ($\tau$ > 2 ps) and excellent THz light confinement
($\lambda_0/\lambda$ > 45), but also an intrinsic, phonon-induced anomalous
dispersion as well as signatures of naturally occurring PhP canalization within
one single GeS slab.",2110.13295v2
2021-12-02,Structures of magnetic excitations in the spin-1/2 kagome-lattice antiferromagnets Cs$_2$Cu$_3$SnF$_{12}$ and Rb$_2$Cu$_3$SnF$_{12}$,"We show the structures of magnetic excitations in spin-1/2 kagome-lattice
antiferromagnets Cs$_2$Cu$_3$SnF$_{12}$ and Rb$_2$Cu$_3$SnF$_{12}$ investigated
by inelastic neutron scattering in wide energy and momentum ranges. For
Cs$_2$Cu$_3$SnF$_{12}$, four single-magnon excitation modes were observed.
Low-energy three modes are assigned to be transverse modes and the high-energy
fourth mode is suggested to be an amplitude mode. It was found that the broad
excitation continuum without a marked structure spreads in a wide energy range
from $0.15J$ to approximately $2.5J$ in contrast to the clearly structured
excitation continuum observed in the spin-1/2 triangular-lattice Heisenberg
antiferromagnet. These findings strongly suggest spinon excitations as
elementary excitations in Cs$_2$Cu$_3$SnF$_{12}$. In Rb$_2$Cu$_3$SnF$_{12}$,
singlet-triplet excitations from the pinwheel VBS state and their ghost modes
caused by the enlargement of the chemical unit cell were clearly confirmed. It
was found that the excitation continuum is structured in the low-energy region
approximately below $J_{\mathrm{avg}}$ and the almost structureless high-energy
excitation continuum extends to approximately $2.6J_{\mathrm{avg}}$. The
characteristics of the high-energy excitation continuum are common to both
Cs$_2$Cu$_3$SnF$_{12}$ and Rb$_2$Cu$_3$SnF$_{12}$, irrespective of their ground
states. The experimental results strongly suggest that the spin liquid
component remains in the ground state as quantum fluctuations in
Cs$_2$Cu$_3$SnF$_{12}$ and Rb$_2$Cu$_3$SnF$_{12}$.",2112.00934v2
2022-02-15,Thermodynamics of the spin-half square-kagome lattice antiferromagnet,"Over the last decade, the interest in the spin-$1/2$ Heisenberg
antiferromagnet (HAF) on the square-kagome (also called shuriken) lattice has
been growing as a model system of quantum magnetism with a quantum paramagnetic
ground state, flat-band physics near the saturation field, and quantum scars.
Here, we present large-scale numerical investigations of the specific heat
$C(T)$, the entropy $S(T)$ as well as the susceptibility $\chi(T)$ by means of
the finite-temperature Lanczos method for system sizes of
$N=18,24,30,36,42,48$, and $N=54$. We find that the specific heat exhibits a
low-temperature shoulder below the major maximum which can be attributed to
low-lying singlet excitations filling the singlet-triplet gap, which is
significantly larger than the singlet-singlet gap. This observation is further
supported by the behavior of the entropy $S(T)$, where a change in the
curvature is present just at about $T/J=0.2$, the same temperature where the
shoulder in $C$ sets in. For the susceptibility the low-lying singlet
excitations are irrelevant, and the singlet-triplet gap leads to an
exponentially activated low-temperature behavior. The maximum in $\chi(T)$ is
found at a pretty low temperature $T_{\rm max}/J=0.146$ (for $N=42$) compared
to $T_{\rm max}/J=0.935$ for the unfrustrated square-lattice HAF signaling the
crucial role of frustration also for the susceptibility. We find a striking
similarity of our square-kagome data with the corresponding ones for the kagome
HAF down to very low $T$. The magnetization process featuring plateaus and
jumps and the field dependence of the specific heat that exhibits
characteristic peculiarities attributed to the existence of a flat one-magnon
band are as well discussed.",2202.07357v2
2022-04-11,Curvilinear spin-wave dynamics beyond the thin-shell approximation: Magnetic nanotubes as a case study,"Surface curvature of magnetic systems can lead to many static and dynamic
effects which are not present in flat systems of the same material. These
emergent magnetochiral effects can lead to frequency nonreciprocity of spin
waves, which has been shown to be a bulk effect of dipolar origin and is
related to a curvature-induced symmetry breaking in the magnetic volume
charges. So far, such effects have been investigated theoretically mostly for
thin shells, where the spatial profiles of the spin waves can be assumed to be
homogeneous along the thickness. Here, using a finite-element dynamic-matrix
approach, we investigate the transition of the spin-wave spectrum from thin to
thick curvilinear shells, at the example of magnetic nanotubes in the vortex
state. With increasing thickness, we observe the appearance of higher-order
radial modes which are strongly hybridized and resemble the
perpendicular-standing-waves (PSSWs) in flat films. Along with an increasing
dispersion asymmetry, we uncover the curvature-induced non-reciprocity of the
mode profiles. This is explained in a very simple picture general for thick
curvilinear shells, considering the inhomogeneity of the emergent geometric
volume charges along the thickness of the shell. Such curvature-induced
mode-profile asymmetry also leads to non-reciprocal hybridization which can
facilitate unidirectional spin-wave propagation. With that, we also show how
curvature allows for nonlinear three-wave splitting of a higher-order radial
mode into secondary modes which can also propagate unidirectionally. We believe
that our study provides a significant contribution to the understanding of the
spin-wave dynamics in curvilinear magnetic systems, but also advertises these
for novel magnonic applications.",2204.05065v1
2022-04-22,Would Magnonic Circuits Outperform CMOS Counterparts?,"In the early stages of a novel technology development, it is difficult to
provide a comprehensive assessment of its potential capabilities and impact.
Nevertheless, some preliminary estimates can be drawn and are certainly of
great interest and in this paper we follow this line of reasoning within the
framework of the Spin Wave (SW) computing paradigm. In particular, we are
interested in assessing the technological development horizon that needs to be
reached in order to unleash the full SW paradigm potential such that SW
circuits can outperform CMOS counterparts in terms of energy consumption. In
view of the zero power SWs propagation through ferromagnetic waveguides, the
overall SW circuit power consumption is determined by the one associated to SWs
generation and sensing by means of transducers. While current antenna based
transducers are clearly power hungry recent developments indicate that
magneto-electric (ME) cells have a great potential for ultra-low power SW
generation and sensing. Given that MEs have been only proposed at the
conceptual level and no actual experimental demonstration has been reported we
cannot evaluate the impact of their utilization on the SW circuit energy
consumption. However, we can perform a reverse engineering alike analysis to
determine ME delay and power consumption upper bounds that can place SW
circuits in the leading position. To this end, we utilize a 32-bit Brent-Kung
Adder (BKA) as discussion vehicle and compute the maximum ME delay and power
consumption that could potentially enable a SW implementation able to
outperform its 7nm CMOS counterpart. We evaluate different BKA SW
implementations that rely on conversion or normalization gate cascading and
consider continuous or pulsed SW generation scenarios. 31nW is the maximum
transducer power consumption for which a 32-bit BKA SW implementation can
outperform its 7nm CMOS counterpart.",2204.10732v1
2022-05-23,"Highly tunable spin Hall magnetoresistance in room-temperature magnetoelectric multiferroic, $\text{Sr}_{3}\text{Co}_{2}\text{Fe}_{24}\text{O}_{41}|$Pt hybrids","We present spin transport studies on a low-field, room-temperature
magnetoelectric multiferroic polycrystalline
$\text{Sr}_{3}\text{Co}_{2}\text{Fe}_{24}\text{O}_{41}$ (SCFO)|Pt
heterostructure wherein a highly tunable transverse conical magnetic phase is
responsible for static and dynamic magnetoelectric coupling. We measured
angular dependence of spin Hall magnetoresistance (SMR) at constant magnetic
fields ($H$) in the range of 50 to 100 kOe. Application of field below the
critical value (2.5 kOe), yielded negative SMR and the $H$-evolution of
normalized SMR exhibited a negative gradient. Further, an increase in the $H$
resulted in the positive slope of normalized SMR Vs. $H$ and later at higher
$H$ around 14 kOe, a crossover from negative to positive SMR was observed. We
employed a simple model for estimating the equilibrium magnetic configuration
and computed the SMR modulation at various values of $H$. We argue that the
tilting of the cone is dominant and in turn responsible for the observed nature
of SMR below 2.5 kOe while, the closing of the cone-angle is pronounced at
higher fields causing a reversal in sign of the SMR from negative to positive.
Importantly, SMR experiments revealed that a change in the helicity with a
reversal of the magnetic field has no influence on the observed SMR.
Longitudinal spin Seebeck effect (LSSE) signal was measured to be 500 nV at 280
K, under application of thermal gradient, $\Delta T = 23$ K and field, 60 kOe.
The observed LSSE signal, originating from pure magnon spin current, showed a
similar $H$-dependent behavior as that of the magnetization of SCFO. Our
detailed spin transport studies on polycrystalline SCFO|Pt heterostructure
demonstrate high tunability of the amplitude and the sign of the SMR,
highlighting its potential for novel spintronic devices such as SMR-based spin
valves and voltage-controlled spin transport devices.",2205.11539v1
2022-08-16,Liger at Keck Observatory: Design of the Data Reduction System and Software Interfaces,"Liger is a second generation near-infrared imager and integral field
spectrograph (IFS) for the W. M. Keck Observatory that will utilize the
capabilities of the Keck All-sky Precision Adaptive-optics (KAPA) system. Liger
operates at a wavelength range of 0.81 {\mu}m - 2.45 {\mu}m and utilizes a
slicer and a lenslet array IFS with varying spatial plate scales and fields of
view resulting in hundreds of modes available to the astronomer. Because of the
high level of complexity in the raw data formats for the slicer and lenslet IFS
modes, Liger must be designed in conjunction with a Data Reduction System (DRS)
which will reduce data from the instrument in real-time and deliver
science-ready data products to the observer. The DRS will reduce raw imager and
IFS frames from the readout system and provide 2D and 3D data products via
custom quick-look visualization tools suited to the presentation of IFS data.
The DRS will provide the reduced data to the Keck Observatory Archive (KOA) and
will be available to astronomers for offline post-processing of observer data.
We present an initial design for the DRS and define the interfaces between
observatory and instrument software systems.",2208.07937v1
2022-08-16,Liger at Keck Observatory: Imager Detector and IFS Pick-off Mirror Assembly,"Liger is a next-generation near-infrared imager and integral field
spectrograph (IFS) planned for the W.M. Keck Observatory. Liger is designed to
take advantage of improved adaptive optics (AO) from the Keck All-Sky Precision
Adaptive Optics (KAPA) upgrade currently underway. Liger operates at 0.84-2.45
$\mu$m with spectral resolving powers of R$\sim$4,000-10,000. Liger makes use
of a sequential imager and spectrograph design allowing for simultaneous
observations. There are two spectrograph modes: a lenslet with high spatial
sampling of 14 and 31 mas, and a slicer with 75 and 150 mas sampling with an
expanded field of view. Two pick-off mirrors near the imager detector direct
light to these two IFS channels. We present the design and structural analysis
for the imager detector and IFS pick-off mirror mounting assembly that will be
used to align and maintain stability throughout its operation. A piezoelectric
actuator will be used to step through $\rm3\,mm$ of travel during alignment of
the instrument to determine the optimal focus for both the detector and
pick-off mirrors which will be locked in place during normal operation. We will
demonstrate that the design can withstand the required gravitational and
shipping loads and can be aligned within the positioning tolerances for the
optics.",2208.07938v1
2022-08-23,Design of SCALES: A 2-5 Micron Coronagraphic Integral Field Spectrograph for Keck Observatory,"We present the design of SCALES (Slicer Combined with Array of Lenslets for
Exoplanet Spectroscopy) a new 2-5 micron coronagraphic integral field
spectrograph under construction for Keck Observatory. SCALES enables
low-resolution (R~50) spectroscopy, as well as medium-resolution (R~4,000)
spectroscopy with the goal of discovering and characterizing cold exoplanets
that are brightest in the thermal infrared. Additionally, SCALES has a 12x12""
field-of-view imager that will be used for general adaptive optics science at
Keck. We present SCALES's specifications, its science case, its overall design,
and simulations of its expected performance. Additionally, we present progress
on procuring, fabricating and testing long lead-time components.",2208.10721v1
2022-09-06,Enhancement of spin-orbit torque efficiency by tailoring interfacial spin-orbit coupling in Pt-based magnetic multilayers,"We study inserting Co layer thickness-dependent spin transport and spin-orbit
torques (SOTs) in the Pt/Co/Py trilayers by spin-torque ferromagnetic
resonance. The interfacial perpendicular magnetic anisotropy energy density
($K_s = 2.7~erg/cm^2$), which is dominated by interfacial spin-orbit coupling
(ISOC) in the Pt/Co interface, total effective spin-mixing conductance
($G_{eff,tot} = 0.42 {\times} 10^{15}~{\Omega}^{-1} m^{-2}$) and two-magnon
scattering (${\beta}_{TMS} = 0.46~nm^2$) are first characterized, and the
damping-like torque (${\xi}_{DL}$ = 0.103) and field-like torque (${\xi}_{FL}$
= -0.017) efficiencies are also calculated quantitatively by varying the
thickness of the inserting Co layer. The significant enhancement of
${\xi}_{DL}$ and ${\xi}_{FL}$ in Pt/Co/Py than Pt/Py bilayer system originates
from the interfacial Rashba-Edelstein effect due to the strong ISOC between
Co-3d and Pt-5d orbitals at the Pt/Co interface. Additionally, we find a
considerable out-of-plane spin polarization SOT, which is ascribed to the spin
anomalous Hall effect and possible spin precession effect due to IPMA-induced
perpendicular magnetization at the Pt/Co interface. Our results demonstrate
that the ISOC of the Pt/Co interface plays a vital role in spin transport and
SOTs-generation. Our finds offer an alternative approach to improve the
conventional SOTs efficiencies and generate unconventional SOTs with
out-of-plane spin polarization to develop low power Pt-based spintronic via
tailoring the Pt/FM interface.",2209.02282v1
2022-10-20,Ultrafast behavior of induced and intrinsic magnetic moments in CoFeB/Pt bilayers probed by element-specific measurements in the extreme ultraviolet spectral range,"The ultrafast and element-specific response of magnetic systems containing
ferromagnetic 3d transition metals and 4d/5d heavy metals is of interest both
from a fundamental as well as an applied research perspective. However, to date
no consensus about the main microscopic processes describing the interplay
between intrinsic 3d and induced 4d/5d magnetic moments upon femtosecond laser
excitation exist. In this work, we study the ultrafast response of CoFeB/Pt
bilayers by probing element-specific, core-to-valence band transitions in the
extreme ultraviolet spectral range using high harmonic radiation. We show that
the combination of magnetic scattering simulations and analysis of the energy-
and time-dependent magnetic asymmetries allows to accurately disentangle the
element-specific response in spite of overlapping Co and Fe M$_{2,3}$ as well
as Pt O$_{2,3}$ and N$_7$ resonances. We find a considerably smaller
demagnetization time constant as well as much larger demagnetization amplitudes
of the induced moment of Pt compared to the intrinsic moment of CoFeB. Our
results are in agreement with enhanced spin-flip probabilities due to the high
spin-orbit coupling localized at the heavy metal Pt, as well as with the
recently formulated hypothesis that a laser generated, incoherent magnon
population within the ferromagnetic film leads to an overproportional reduction
of the induced magnetic moment of Pt.",2210.11390v3
2022-10-25,Nuclear and magnetic spin structure of the antiferromagnetic triangular lattice compound LiCrTe$_2$ investigated by $μ^+$SR as well as neutron and X-ray diffraction,"Two$-$dimensional (2D) triangular lattices antiferromagnets (2D$-$TLA) often
manifest intriguing physical and technological properties, due to the strong
interplay between lattice geometry and electronic properties. The recently
synthesized 2$-$dimensional transition metal dichalcogenide LiCrTe$_2$, being a
2D$-$TLA, enriched the range of materials which can present such properties. In
this work, muon spin rotation ($\mu^+$SR) and neutron powder diffraction (NPD)
have been utilized to reveal the true magnetic nature and ground state of
LiCrTe$_2$. From high$-$resolution NPD the magnetic spin order at
base$-$temperature is not, as previously suggested, helical, but rather
collinear antiferromagnetic (AFM) with ferromagnetic (FM) spin coupling within
the $ab-$plane and AFM coupling along the $c-$axis. The ordered magnetic Cr
moment is established as $\mu_{\rm Cr}=$ 2.36 $\mu_{\rm B}$. From detailed
$\mu^+$SR measurements we observe an AFM ordering temperature $T_{\rm
N}\approx$ 125 K. This value is remarkably higher than the one previously
reported by magnetic bulk measurements. From $\mu^+$SR we are able to extract
the magnetic order parameter, whose critical exponent allows us to categorize
LiCrTe$_2$ in the 3D Heisenberg AFM universality class. Finally, by combining
our magnetic studies with high$-$resolution synchrotron X$-$ray diffraction
(XRD), we find a clear coupling between the nuclear and magnetic spin lattices.
This suggests the possibility for a strong magnon$-$phonon coupling, similar to
what has been previously observed in the closely related compound LiCrO$_2$.",2210.14079v1
2022-11-03,Skyrmion Jellyfish in Driven Chiral Magnets,"Chiral magnets can host topological particles known as skyrmions, which carry
an exactly quantised topological charge $Q=-1$. In the presence of an
oscillating magnetic field ${\bf B}_1(t)$, a single skyrmion embedded in a
ferromagnetic background will start to move with constant velocity ${\bf
v}_{\text{trans}}$. The mechanism behind this motion is similar to the one used
by a jellyfish when it swims through water. We show that the skyrmion's motion
is a universal phenomenon, arising in any magnetic system with translational
modes. By projecting the equation of motion onto the skyrmion's translational
modes and going to quadratic order in ${\bf B}_1(t)$, we obtain an analytical
expression for ${\bf v}_{\text{trans}}$ as a function of the system's linear
response. The linear response and consequently ${\bf v}_{\text{trans}}$ are
influenced by the skyrmion's internal modes and scattering states, as well as
by the ferromagnetic background's Kittel mode. The direction and speed of ${\bf
v}_{\text{trans}}$ can be controlled by changing the polarisation, frequency
and phase of the driving field ${\bf B}_1(t)$. For systems with small Gilbert
damping parameter $\alpha$, we identify two distinct physical mechanisms used
by the skyrmion to move. At low driving frequencies, the skyrmion's motion is
driven by friction, and $v_{\text{trans}}\sim\alpha$, whereas at higher
frequencies above the ferromagnetic gap, the skyrmion moves by magnon emission,
and $v_{\text{trans}}$ becomes independent of $\alpha$.",2211.01714v5
2022-12-14,Method of difference-differential equations for some Bethe ansatz solvable models,"In studies of one-dimensional Bethe ansatz solvable models, a Fredholm
integral equation of the second kind with a difference kernel on a finite
interval often appears. This equation does not generally admit a closed-form
solution and hence its analysis is quite complicated. Here we study a family of
such equations concentrating on their moments. We find exact relations between
the moments in the form of difference-differential equations. The latter
results significantly advance the analysis, enabling one to practically
determine all the moments from the explicit knowledge of the lowest one. As
applications, first we study the moments of the quasimomentum distribution in
the Lieb-Liniger model and find explicit analytical results. The latter moments
determine several basic quantities, e.g., the $N$-body local correlation
functions. We prove the equivalence between different expressions found in the
literature for the three-body local correlation functions and find an exact
result for the four-body local correlation function in terms of the moments of
the quasimomentum distributions. We eventually find the analytical results for
the three- and four-body correlation functions in the form of asymptotic series
in the regimes of weak and strong interactions. Next, we study the exact form
of the low-energy spectrum of a magnon (a polaron) excitation in the
two-component Bose gas described by the Yang-Gaudin model. We find its explicit
form, which depends on the moments of the quasimomentum distributions of the
Lieb-Liniger model. Then, we address a seemingly unrelated problem of
capacitance of a circular capacitor and express the exact result for the
capacitance in the parametric form. In the most interesting case of short plate
separations, the parametric form has a single logarithmic term. This should be
contrasted with the explicit result that has a complicated structure of
logarithms.",2212.07480v1
2023-02-28,Quantifying interaction mechanism in infinite layer nickelate superconductors,"The relationship between the long-range antiferromagnetic order in cuprates
and the high-temperature superconductivity in these compounds represents
unresolved, nearly four-decades long scientific problem. Because recently
discovered nickelate superconductors are crystallographical counterparts of
cuprates, many properties and difficulties into describing these compounds are
common to both families. Recently, Fowlie et al (2022 Nature Physics 18 1043)
aimed to detect the antiferromagnetic order in $R_{1-x}Sr_{x}NiO_{2}$ (R = Nd,
Pr, La, x ~ 0.2) films by using the muon spin rotation (muSR) technique. This
research group reported on the existence of short-range antiferromagnetic order
in all studied nickelates. Here, we aimed to reveal the existence of this
interaction in the same nickelate films by analyzing the temperature dependent
resistivity, $\rho(T)$, reported by the same research group. Global $\rho(T)$
data fits to the advanced Bloch-Gr\""uneisen model showed that each of
R1-xSrxNiO2 compounds can be characterized by a unique power-law exponent, p
(where p=2 for the electron-electron scattering, p=3 for the electron-magnon
scattering, and p=5 for the electron-phonon scattering), and global
characteristic temperature, $T_{\omega}$ (which has the meaning of the Debye
temperature at p=5). We found that p=2.0 in Nd- and Pr-based compounds, and
p=1.3 for La-based compound. The latter value does not have any interpretation
within established theoretical models. We also analyzed $\rho(T)$ data for
$Nd_{1-x}Sr_{x}NiO_{2}$ (0.125 < x < 0.325) reported by Lee et al (2022
arXiv2203.02580). Because our analysis showed that p-values in nickelates are
remarkably different from p=3, we call for the developent of a new theoretical
model to describe $\rho(T)$ in materials exhibiting a short-range
antiferromagnetic order.",2302.14729v3
2023-03-13,Unusual magnetotransport and anomalous Hall effect in quasi-two-dimensional van der Waals ferromagnet Fe$_4$GeTe$_2$,"Fe$_4$GeTe$_2$, an itinerant vdW ferromagnet (FM) having Curie temperature
(T$_C$) close to room temperature ($\sim 270$ K), exhibits another transition
(T$_{SR}$ $\sim$ 120 K) where the easy axis of magnetization changes from
in-plane to the out-of-plane direction in addition to T$_C$. Here, we have
studied the magnetotransport in a multilayer Hall bar device fabricated on 300
nm Si/SiO$_2$ substrate. Interestingly, the zero field resistivity shows a
negligible change in resistivity near T$_C$ unlike the typical metallic FM,
whereas, it exhibits a dramatic fall below T$_{SR}$. Also, the resistivity
shows a weak anomaly at T $ \sim $ 38 K (T$_Q$), below which the resistivity
shows a quadratic temperature dependence according to the Fermi liquid
behavior. Temperature-dependent Hall data exhibits important consequences. The
ordinary Hall coefficient changes sign near T$_{SR}$ indicating the change in
majority carriers. In a similar manner, the magnetoresistance (MR) data shows
significantly large negative MR near T$_{SR}$ and becomes positive below T$_Q$.
The observations of anomaly in the resistivity, sign-change of the ordinary
Hall coefficient and maximum negative MR near T$_{SR}$, together suggest a
possible Fermi surface reconstruction associated with the spin reorientation
transition. Furthermore, analysis of the Hall data reveals a significant
anomalous Hall conductivity (AHC) from $\sim 123 \Omega^{-1}$ cm$^{-1}$ (at T
$\approx$ 5 K) to the maximum value of $\sim 366 \Omega^{-1}$ cm$^{-1}$ near
T$_{SR}$. While the low-temperature part may originate due to the intrinsic KL
mechanism, our analysis indicates that the temperature-dependent AHC is
primarily appearing due to the side-jump mechanism as a result of the spin-flip
electron-magnon scattering. Our study demonstrates an interplay between
magnetism and band topology and its consequence on electron transport in
Fe$_4$GeTe$_2$.",2303.07440v1
2023-05-05,Spin-phonon scattering-induced low thermal conductivity in a van der Waals layered ferromagnet Cr$_2$Si$_2$Te$_6$,"Layered van der Waals (vdW) magnets are prominent playgrounds for developing
magnetoelectric, magneto-optic and spintronic devices. In spintronics,
particularly in spincaloritronic applications, low thermal conductivity
($\kappa$) is highly desired. Here, by combining thermal transport measurements
with density functional theory calculations, we demonstrate low $\kappa$ down
to 1 W m$^{-1}$ K$^{-1}$ in a typical vdW ferromagnet Cr$_2$Si$_2$Te$_6$. In
the paramagnetic state, development of magnetic fluctuations way above
$T_\mathrm{c}=$ 33 K strongly reduces $\kappa$ via spin-phonon scattering,
leading to low $\kappa \sim$ 1 W m$^{-1}$ K$^{-1}$ over a wide temperature
range, in comparable to that of amorphous silica. In the magnetically ordered
state, emergence of resonant magnon-phonon scattering limits $\kappa$ below
$\sim$ 2 W m$^{-1}$ K$^{-1}$, which would be three times larger if magnetic
scatterings were absent. Application of magnetic fields strongly suppresses the
spin-phonon scattering, giving rise to large enhancements of $\kappa$. Our
calculations well capture these complex behaviours of $\kappa$ by taking the
temperature- and magnetic-field-dependent spin-phonon scattering into account.
Realization of low $\kappa$ which is easily tunable by magnetic fields in
Cr$_2$Si$_2$Te$_6$, may further promote spincaloritronic applications of vdW
magnets. Our theoretical approach may also provide a generic understanding of
spin-phonon scattering, which appears to play important roles in various
systems.",2305.13268v1
2023-06-01,Large Charge 't Hooft Limit of $\mathcal{N}=4$ Super-Yang-Mills,"The planar integrability of $\mathcal{N}=4$ super-Yang-Mills (SYM) is the
cornerstone for numerous exact observables. We show that the large charge
sector of the ${\rm SU}(2)$ $\mathcal{N}=4$ SYM provides another interesting
solvable corner which exhibits striking similarities despite being far from the
planar limit. We study non-BPS operators obtained by small deformations of
half-BPS operators with $R$-charge $J$ in the limit $J\to\infty$ with
$\lambda_{J}\equiv g_{\rm YM}^2 J/2$ fixed. The dynamics in this {\it large
charge 't Hooft limit} is constrained by a centrally-extended
$\mathfrak{psu}(2|2)^2$ symmetry that played a crucial role for the planar
integrability. To the leading order in $1/J$, the spectrum is fully fixed by
this symmetry, manifesting the magnon dispersion relation familiar from the
planar limit, while it is constrained up to a few constants at the next order.
We also determine the structure constant of two large charge operators and the
Konishi operator, revealing a rich structure interpolating between the
perturbative series at weak coupling and the worldline instantons at strong
coupling. In addition we compute heavy-heavy-light-light (HHLL) four-point
functions of half-BPS operators in terms of resummed conformal integrals and
recast them into an integral form reminiscent of the hexagon formalism in the
planar limit. For general ${\rm SU}(N)$ gauge groups, we study integrated HHLL
correlators by supersymmetric localization and identify a dual matrix model of
size $J/2$ that reproduces our large charge result at $N=2$. Finally we discuss
a relation to the physics on the Coulomb branch and explain how the dilaton
Ward identity emerges from a limit of the conformal block expansion. We comment
on generalizations including the large spin 't Hooft limit, the combined large
$N$-large $J$ limits, and applications to general $\mathcal{N}=2$
superconformal field theories.",2306.00929v1
2023-06-09,Coexistence of symmetry-protected topological order and Neel order in the spin-1/2 ladder antiferromagnet C9H18N2CuBr4,"Topological phases of matter are beyond the paradigm of Landau's symmetry
breaking and have challenged our understanding of condensed matter systems.
Here we report a new type of symmetry-protected topological phase of matter in
the spin-1/2 coupled two-leg ladder antiferromagnet C9H18N2CuBr4, DLCB for
short. In this two-sublattice antiferromagnet with a weak easy-axis anisotropy,
we find no evidence of a conventional spin-flop transition in the magnetization
with the magnetic field applied parallel to the easy axis at T=0.4 K, well
below TN=2.0 K. Moreover, the temperature dependence of the gapped transverse
excitations across TN indicates that they are not the conventional S=1 magnons
associated with explicit symmetry breaking. Instead, the thermal
renormalization of the gap energy shows a remarkable agreement with a
calculation for the three-dimensional O(3) nonlinear sigma model. Accordingly,
the spin gap in DLCB is not due to the spin anisotropy but to the separation
between a spin singlet state and a triplet excited state. Since an
antiferromagnetic spin-1/2 ladder systems can be mapped onto the spin-1 chain,
the notion of the Haldane gap is proposed to explain the opening of the spin
gap in DLCB. Therefore, the ground state of DLCB is best described as a quantum
superposition of a Haldane phase and a Neel-ordered phase, which resembles the
quantum state of a qubit in quantum computing. Our results indicate the
presence of a symmetry-protected topological order coexisting with an
antiferromagnetic order in this material.",2306.06021v3
2023-06-22,"Gilbert damping in metallic ferromagnets from Schwinger-Keldysh field theory: Intrinsically nonlocal and nonuniform, and made anisotropic by spin-orbit coupling","Understanding the origin of damping mechanisms in magnetization dynamics of
metallic ferromagnets is a fundamental problem for nonequilibrium many-body
physics of systems where quantum conduction electrons interact with localized
spins assumed to be governed by the classical Landau-Lifshitz-Gilbert (LLG)
equation. It is also of critical importance for applications, as damping
affects energy consumption and speed of spintronic and magnonic devices. Since
the 1970s, a variety of linear-response and scattering theory approaches have
been developed to produce widely used formulas for computation of
spatially-independent Gilbert scalar parameter as the magnitude of the Gilbert
damping term in the LLG equation. The largely unexploited for this purpose
Schwinger-Keldysh field theory (SKFT) offers additional possibilities, such as
to rigorously derive an extended LLG equation by integrating quantum electrons
out. Here we derive such equation whose Gilbert damping for metallic
ferromagnets is nonlocal, i.e., dependent on all localized spins at a given
time, and nonuniform, even if all localized spins are collinear and spin-orbit
coupling (SOC) is absent. This is in sharp contrast to standard lore, where
nonlocal damping is considered to emerge only if localized spins are
noncollinear; for such situations, direct comparison on the example of magnetic
domain wall shows that SKFT-derived nonlocal damping is an order of magnitude
larger than the previously considered one. Switching on SOC makes such nonlocal
damping anisotropic, in contrast to standard lore where SOC is usually
necessary to obtain nonzero Gilbert damping scalar parameter. Our analytical
formulas, with their nonlocality being more prominent in low spatial
dimensions, are fully corroborated by numerically exact quantum-classical
simulations.",2306.13013v4
2023-10-02,Quantum spin nematic phase in a square-lattice iridate,"Spin nematic (SN) is a magnetic analog of classical liquid crystals, a fourth
state of matter exhibiting characteristics of both liquid and solid.
Particularly intriguing is a valence-bond SN, in which spins are quantum
entangled to form a multi-polar order without breaking time-reversal symmetry,
but its unambiguous experimental realization remains elusive. Here, we
establish a SN phase in the square-lattice iridate Sr$_2$IrO$_4$, which
approximately realizes a pseudospin one-half Heisenberg antiferromagnet (AF) in
the strong spin-orbit coupling limit. Upon cooling, the transition into the SN
phase at T$_C$ $\approx$ 263 K is marked by a divergence in the static spin
quadrupole susceptibility extracted from our Raman spectra, and concomitant
emergence of a collective mode associated with the spontaneous breaking of
rotational symmetries. The quadrupolar order persists in the antiferromagnetic
(AF) phase below T$_N$ $\approx$ 230 K, and becomes directly observable through
its interference with the AF order in resonant x-ray diffraction, which allows
us to uniquely determine its spatial structure. Further, we find using resonant
inelastic x-ray scattering a complete breakdown of coherent magnon excitations
at short-wavelength scales, suggesting a resonating-valence-bond-like quantum
entanglement in the AF state. Taken together, our results reveal a quantum
order underlying the N\'eel AF that is widely believed to be intimately
connected to the mechanism of high temperature superconductivity (HTSC).",2310.00886v2
2023-11-20,Electrical control of magnetism by electric field and current-induced torques,"While early magnetic memory designs relied on magnetization switching by
locally generated magnetic fields, key insights in condensed matter physics
later suggested the possibility to do it electrically. In the 1990s,
Slonczewzki and Berger formulated the concept of current-induced spin torques
in magnetic multilayers through which a spin-polarized current may switch the
magnetization of a ferromagnet. This discovery drove the development of
spin-transfer-torque magnetic random-access memories (STT-MRAMs). More recent
research unveiled spin-orbit-torques (SOTs) and will lead to a new generation
of devices including SOT-MRAMs. Parallel to these advances, multiferroics and
their magnetoelectric coupling experienced a renaissance, leading to novel
device concepts for information and communication technology such as the MESO
transistor. The story of the electrical control of magnetization is that of a
dance between fundamental research (in spintronics, condensed matter physics,
and materials science) and technology (MRAMs, MESO, microwave emitters,
spin-diodes, skyrmion-based devices, components for neuromorphics, etc). This
pas de deux led to major breakthroughs over the last decades (pure spin
currents, magnetic skyrmions, spin-charge interconversion, etc). As a result,
this field has propelled MRAMs into consumer electronics products but also
fueled discoveries in adjacent research areas such as ferroelectrics or
magnonics. Here, we cover recent advances in the control of magnetism by
electric fields and by current-induced torques. We first review fundamental
concepts in these two directions, then discuss their combination, and finally
present various families of devices harnessing the electrical control of
magnetic properties for various application fields. We conclude by giving
perspectives in terms of both emerging fundamental physics concepts and new
directions in materials science.",2311.11724v1
2023-12-18,"Moiré Fractional Chern Insulators III: Hartree-Fock Phase Diagram, Magic Angle Regime for Chern Insulator States, the Role of the Moiré Potential and Goldstone Gaps in Rhombohedral Graphene Superlattices","We investigate in detail the $\nu=+1$ displacement-field-tuned interacting
phase diagram of $L=3,4,5,6,7$ layer rhombohedral graphene aligned to hBN
(R$L$G/hBN). Our calculations account for the 3D nature of the Coulomb
interaction, the inequivalent stacking orientations $\xi=0,1$, the effects of
the filled valence bands, and the choice of `interaction scheme' for specifying
the many-body Hamiltonian. We show that the latter has a dramatic impact on the
Hartree-Fock phase boundaries and the properties of the phases, including for
pentalayers (R5G/hBN) with large displacement field $D$ where recent
experiments observed a Chern insulator at $\nu=+1$ and fractional Chern
insulators for $\nu<1$. In this large $D$ regime, the low-energy conduction
bands are polarized away from the aligned hBN layer, and are hence
well-described by the folded bands of moir\'eless rhombohedral graphene at the
non-interacting level. Despite this, the filled valence bands develop
moir\'e-periodic charge density variations which can generate an effective
moir\'e potential, thereby explicitly breaking the approximate continuous
translation symmetry in the conduction bands, and leading to contrasting
electronic topology in the ground state for the two stacking arrangements.
Within time-dependent Hartree-Fock theory, we further characterize the strength
of the moir\'e pinning potential in the Chern insulator phase by computing the
low-energy $\mathbf{q}=0$ collective mode spectrum, where we identify competing
gapped pseudophonon and valley magnon excitations. Our results emphasize the
importance of careful examination of both the single-particle and interaction
model for a proper understanding of the correlated phases in R$L$G/hBN.",2312.11617v1
2023-12-18,"Twisted van der Waals Quantum Materials: Fundamentals, Tunability and Applications","Twisted vdW quantum materials have emerged as a rapidly developing field of
2D semiconductors. These materials establish a new central research area and
provide a promising platform for studying quantum phenomena and investigating
the engineering of novel optoelectronic properties such as single-photon
emission, non-linear optical response, magnon physics, and topological
superconductivity. These captivating electronic and optical properties result
from, and can be tailored by, the interlayer coupling using moir\'e patterns
formed by vertically stacking atomic layers with controlled angle
misorientation or lattice mismatch. Their outstanding properties and the high
degree of tunability position them as compelling building blocks for both
compact quantum-enabled devices and classical optoelectronics. This article
offers a comprehensive review of recent advancements in the understanding and
manipulation of twisted van der Waals structures and presents a survey of the
state-of-the-art research on moir\'e superlattices, encompassing
interdisciplinary interests. It delves into fundamental theories, synthesis and
fabrication, and visualization techniques, and the wide range of novel physical
phenomena exhibited by these structures, with a focus on their potential for
practical device integration in applications ranging from quantum information
to biosensors, and including classical optoelectronics such as modulators,
light emitting diodes (LEDs), lasers, and photodetectors. It highlights the
unique ability of moir\'e superlattices to connect multiple disciplines,
covering chemistry, electronics, optics, photonics, magnetism, topological and
quantum physics. This comprehensive review provides a valuable resource for
researchers interested in moir\'e superlattices, shedding light on their
fundamental characteristics and their potential for transformative applications
in various fields.",2312.11757v1
2024-03-01,Emergence of interfacial magnetism in strongly-correlated nickelate-titanate superlattices,"Strongly-correlated transition-metal oxides are widely known for their
various exotic phenomena. This is exemplified by rare-earth nickelates such as
LaNiO$_{3}$, which possess intimate interconnections between their electronic,
spin, and lattice degrees of freedom. Their properties can be further enhanced
by pairing them in hybrid heterostructures, which can lead to hidden phases and
emergent phenomena. An important example is the LaNiO$_{3}$/LaTiO$_{3}$
superlattice, where an interlayer electron transfer has been observed from
LaTiO$_{3}$ into LaNiO$_{3}$ and is predicted to result in a high-spin state.
However, macroscopic emergence of magnetic order has so far not been observed.
Here, by using muon spin rotation, x-ray absorption, and resonant inelastic
x-ray scattering, we present direct evidence of an emergent antiferromagnetic
order with high magnon energy and exchange interactions at the
LaNiO$_{3}$/LaTiO$_{3}$ interface. As the magnetism is purely interfacial, a
single LaNiO$_{3}$/LaTiO$_{3}$ interface can essentially behave as an
atomically thin quasi-two-dimensional antiferromagnet, potentially allowing its
technological utilisation in advanced spintronic devices. Furthermore, its
strong quasi-two-dimensional magnetic correlations and orbitally-polarized
planar ligand holes make its electronic and magnetic configurations resemble
the precursor states of superconducting cuprates and nickelates, but with an S
$\rightarrow$ 1 spin state instead.",2403.00728v1
2024-04-03,Unraveling the Mn $L_3$-edge RIXS spectrum of lightly manganese doped Sr$_{3}$Ru$_{2}$O$_{7}$,"Resonant inelastic x-ray scattering (RIXS) experiment was performed at the Mn
$L_3$ edge. A 10 $\%$ Mn-doped Sr$_{3}$Ru$_{2}$O$_{7}$ compound, where the
Mn$^{3+}$ ions are in the 3$d^4$ state, were probed for $dd$ excitations. The
dilute doping concentration allows one to treat the dopant Mn$^{3+}$ ions as
effectively free in the host ruthenium compound. The local nature of $dd$ RIXS
spectroscopy permits one to use a single-site model to simulate the
experimental spectra. The simulated spectra reproduces the in-plane [100]
experimental RIXS spectrum. We also predict the intensity for the in-plane
[110] direction and the out-of-plane spin orientation configuration [001].
Based on our single-ion model we were able to fit the experimental data to
obtain the crystal field parameters, the 10Dq value, and the intra-orbital
spin-flip energy 2$\mathcal{J}$(or $3J_{H}$, where $J_{H}$ is the Hund's
energy) of the Mn$^{3+}$ ion. Utilizing our computed RIXS quantum transition
amplitudes between the various $d$ orbitals of the Mn$^{3+}$ ion, the
expression for the Kramers-Heisenberg cross section, and a self-consistent
fitting procedure we also identify the energy boundaries of the non-spin-flip
and spin-flip $dd$ excitations present in the experimental data. From our
fitting procedure we obtain $2\mathcal{J} (3J_{H})=2.06$ eV, a value which is
in excellent agreement with that computed from the free ion Racah parameters.
We also identified the charge transfer boundary. In addition to predicting the
microscopic parameters, we find a quantum spin-flip transition in the non-cross
($\sigma_{in}-\sigma_{out}$, $\pi_{in}-\pi_{out}$) x-ray polarization channels
of the $dd$ RIXS spectra. A similar transition, was previously predicted to
occur in the $\pi-\pi$ channel of the magnon spectrum in the non-collinear
non-coplanar Kagome compound composed of Cu$^{2+}$ 3d$^{9}$ ion.",2404.02963v1
2014-07-09,The Integral Field Spectrograph for the Gemini Planet Imager,"The Gemini Planet Imager (GPI) is a complex optical system designed to
directly detect the self-emission of young planets within two arcseconds of
their host stars. After suppressing the starlight with an advanced AO system
and apodized coronagraph, the dominant residual contamination in the focal
plane are speckles from the atmosphere and optical surfaces. Since speckles are
diffractive in nature their positions in the field are strongly wavelength
dependent, while an actual companion planet will remain at fixed separation. By
comparing multiple images at different wavelengths taken simultaneously, we can
freeze the speckle pattern and extract the planet light adding an order of
magnitude of contrast. To achieve a bandpass of 20%, sufficient to perform
speckle suppression, and to observe the entire two arcsecond field of view at
diffraction limited sampling, we designed and built an integral field
spectrograph with extremely low wavefront error and almost no chromatic
aberration. The spectrograph is fully cryogenic and operates in the wavelength
range 1 to 2.4 microns with five selectable filters. A prism is used to produce
a spectral resolution of 45 in the primary detection band and maintain high
throughput. Based on the OSIRIS spectrograph at Keck, we selected to use a
lenslet-based spectrograph to achieve an rms wavefront error of approximately
25 nm. Over 36,000 spectra are taken simultaneously and reassembled into image
cubes that have roughly 192x192 spatial elements and contain between 11 and 20
spectral channels. The primary dispersion prism can be replaced with a
Wollaston prism for dual polarization measurements. The spectrograph also has a
pupil-viewing mode for alignment and calibration.",1407.2314v1
2023-09-20,The development of HISPEC for Keck and MODHIS for TMT: science cases and predicted sensitivities,"HISPEC is a new, high-resolution near-infrared spectrograph being designed
for the W.M. Keck II telescope. By offering single-shot, R=100,000 between 0.98
- 2.5 um, HISPEC will enable spectroscopy of transiting and non-transiting
exoplanets in close orbits, direct high-contrast detection and spectroscopy of
spatially separated substellar companions, and exoplanet dynamical mass and
orbit measurements using precision radial velocity monitoring calibrated with a
suite of state-of-the-art absolute and relative wavelength references. MODHIS
is the counterpart to HISPEC for the Thirty Meter Telescope and is being
developed in parallel with similar scientific goals. In this proceeding, we
provide a brief overview of the current design of both instruments, and the
requirements for the two spectrographs as guided by the scientific goals for
each. We then outline the current science case for HISPEC and MODHIS, with
focuses on the science enabled for exoplanet discovery and characterization. We
also provide updated sensitivity curves for both instruments, in terms of both
signal-to-noise ratio and predicted radial velocity precision.",2309.11050v1
2012-02-18,Leading order determination of the gluon polarisation from DIS events with high-p_T hadron pairs,"We present a determination of the gluon polarisation Delta g/g in the
nucleon, based on the longitudinal double-spin asymmetry of DIS events with a
pair of large transverse-momentum hadrons in the final state. The data were
obtained by the COMPASS experiment at CERN using a 160 GeV/c polarised muon
beam scattering off a polarised ^6LiD target. The gluon polarisation is
evaluated by a Neural Network approach for three intervals of the gluon
momentum fraction x_g covering the range 0.04 < x_g < 0.27. The values obtained
at leading order in QCD do not show any significant dependence on x_g. Their
average is Delta g/g = 0.125 +/- 0.060 (stat.) +/- 0.063 (syst.) at x_g=0.09
and a scale of mu^2 = 3 (GeV/c)^2.",1202.4064v1
2012-02-28,Transverse spin effects in hadron-pair production from semi-inclusive deep inelastic scattering,"First measurements of azimuthal asymmetries in hadron-pair production in
deep-inelastic scattering of muons on transversely polarised ^6LiD (deuteron)
and NH_3 (proton) targets are presented. The data were taken in the years
2002-2004 and 2007 with the COMPASS spectrometer using a muon beam of 160 GeV/c
at the CERN SPS. The asymmetries provide access to the transversity
distribution functions, without involving the Collins effect as in single
hadron production. The sizeable asymmetries measured on the NH_ target indicate
non-vanishing u-quark transversity and two-hadron interference fragmentation
functions. The small asymmetries measured on the ^6LiD target can be
interpreted as indication for a cancellation of u- and d-quark transversities.",1202.6150v2
2012-07-09,Measurement of the Cross Section for High-p_T Hadron Production in Scattering of 160 GeV/c Muons off Nucleons,"The differential cross section for production of charged hadrons with high
transverse momenta in scattering of 160\,GeV/$c$ muons off nucleons at low
photon virtualities has been measured at the COMPASS experiment at CERN. The
results, which cover transverse momenta from 1.1\,GeV/$c$ to 3.6\,GeV/$c$, are
compared to a perturbative Quantum Chromodynamics (pQCD) calculation, in order
to evaluate the applicability of pQCD to this process in the kinematic domain
of the experiment. The shape of the calculated differential cross section as a
function of transverse momentum is found to be in good agreement with the
experimental data, but the absolute scale is underestimated by next-to-leading
order (NLO) pQCD. The inclusion of all-order resummation of large logarithmic
threshold corrections reduces the discrepancy from a factor of three to four to
a factor of two. The dependence of the cross section on the pseudo-rapidity and
on virtual photon energy fraction is investigated. Finally the dependence on
the charge of the hadrons is discussed.",1207.2022v2
2012-07-18,Exclusive rho^0 muoproduction on transversely polarised protons and deuterons,"The transverse target spin azimuthal asymmetry A_UT in hard exclusive
production of rho^0 mesons was measured at COMPASS by scattering 160 GeV/c
muons off transversely polarised protons and deuterons. The measured asymmetry
is sensitive to the nucleon helicity-flip generalised parton distributions E^q,
which are related to the orbital angular momentum of quarks in the nucleon. The
Q^2, x_B and p_t^2 dependence of A_UT is presented in a wide kinematic range.
Results for deuterons are obtained for the first time. The measured asymmetry
is small in the whole kinematic range for both protons and deuterons, which is
consistent with the theoretical interpretation that contributions from GPDs E^u
and E^d approximately cancel.",1207.4301v2
2012-11-07,D* and D Meson Production in Muon Nucleon Interactions at 160 GeV/c,"The production of D* and D mesons in inelastic scattering of 160 GeV/c muons
off a ^6LiD target has been investigated with the COMPASS spectrometer at CERN
for 0.003 (GeV/c)^2 < Q^2 < 10 (GeV/c)^2 and 3x10^-5< x_Bj < 0.1. The study is
based on 8100 events where a D^0 or anti D^0 is detected subsequently to a D*+
or D*- decay, and on 34000 events, where only a D^0 or anti D^0 is detected.
Kinematic distributions of D*, D and K*_2 are given as a function of their
energy E, transverse momentum p_T, energy fraction z, and of the virtual photon
variables nu, Q^2 and x_Bj. Semi-inclusive differential D* production
cross-sections are compared with theoretical predictions for D* production via
photon-gluon fusion into open charm. The total observed production
cross-section for D*+/- mesons with laboratory energies between 22 and 86 GeV
is 1.9 nb. Significant cross-section asymmetries are observed between D*+ and
D*- production for nu<40 GeV and z>0.6.",1211.1575v2
2012-11-29,Leading and Next-to-Leading Order Gluon Polarisation in the Nucleon and Longitudinal Double Spin Asymmetries from Open Charm Muoproduction,"The gluon polarisation in the nucleon was measured using open charm
production by scattering 160 GeV/c polarised muons off longitudinally polarised
protons or deuterons. The data were taken by the COMPASS collaboration between
2002 and 2007. A detailed account is given of the analysis method that includes
the application of neural networks. Several decay channels of D^0 mesons are
investigated. Longitudinal spin asymmetries of the D meson production
cross-sections are extracted in bins of D^0 transverse momentum and energy. At
leading order QCD accuracy the average gluon polarisation is determined as
(Delta g/G)^LO=-0.06 +/- 0.21 (stat.) +/- 0.08 (syst.) at the scale <mu^2> ~13
(GeV/c)^2 and an average gluon momentum fraction <x>~ 0.11. For the first time,
the average gluon polarisation is also obtained at next-to-leading order QCD
accuracy as (Delta g/G)^NLO = -0.13 +/- 0.15 (stat.) +/- 0.15 (syst.) at the
scale <mu^2> ~ 13 (\GeV/c)^2 and <x> ~ 0.20.",1211.6849v2
2013-04-03,Study of Σ(1385) and Ξ(1321) hyperon and antihyperon production in deep inelastic muon scattering,"Large samples of \Lambda, \Sigma(1385) and \Xi(1321) hyperons produced in
deep-inelastic muon scattering off a ^6LiD target were collected with the
COMPASS experimental setup at CERN. The relative yields of \Sigma(1385)^+,
\Sigma(1385)^-, \bar{\Sigma}(1385)^-, \bar{\Sigma}(1385)^+, \Xi(1321)^-, and
\bar{\Xi}(1321)^+ hyperons decaying into \Lambda(\bar{\Lambda})\pi were
measured. The heavy hyperon to \Lambda and heavy antihyperon to \bar{\Lambda}
yield ratios were found to be in the range 3.8% to 5.6% with a relative
uncertainty of about 10%. They were used to tune the parameters relevant for
strange particle production of the LEPTO Monte Carlo generator.",1304.0952v2
2013-05-31,Hadron Transverse Momentum Distributions in Muon Deep Inelastic Scattering at 160 GeV/$c$,"Multiplicities of charged hadrons produced in deep inelastic muon scattering
off a $^6$LiD target have been measured as a function of the DIS variables
$x_{Bj}$, $Q^2$, $W^2$ and the final state hadron variables $p_T$ and $z$. The
$p_T^2$ distributions are fitted with a single exponential function at low
values of $p_T^2$ to determine the dependence of $\langle p_T^2 \rangle$ on
$x_{Bj}$, $Q^2$, $W^2$ and $z$. The $z$-dependence of $\langle p_T^2 \rangle$
is shown to be a potential tool to extract the average intrinsic transverse
momentum squared of partons, $\langle k_{\perp}^2 \rangle$, as a function of
$x_{Bj}$ and $Q^2$ in a leading order QCD parton model.",1305.7317v1
2013-10-05,Transverse target spin asymmetries in exclusive $ρ^0$ muoproduction,"Exclusive production of $\rho^0$ mesons was studied at the COMPASS experiment
by scattering 160 GeV/$c$ muons off transversely polarised protons. Five
single-spin and three double-spin azimuthal asymmetries were measured as a
function of $Q^2$, $x_{Bj}$, or $p_{T}^{2}$. The $\sin \phi_S$ asymmetry is
found to be $-0.019 \pm 0.008(stat.) \pm 0.003(syst.)$. All other asymmetries
are also found to be of small magnitude and consistent with zero within
experimental uncertainties. Very recent calculations using a GPD-based model
agree well with the present results. The data is interpreted as evidence for
the existence of chiral-odd, transverse generalized parton distributions.",1310.1454v2
2014-01-24,Measurement of azimuthal hadron asymmetries in semi-inclusive deep inelastic scattering off unpolarised nucleons,"Spin-averaged asymmetries in the azimuthal distributions of positive and
negative hadrons produced in deep inelastic scattering were measured using the
CERN SPS muon beam at $160$ GeV/c and a $^6$LiD target. The amplitudes of the
three azimuthal modulations $\cos\phi_h$, $\cos2\phi_h$ and $\sin\phi_h$ were
obtained binning the data separately in each of the relevant kinematic
variables $x$, $z$ or $p_T^{\,h}$ and binning in a three-dimensional grid of
these three variables. The amplitudes of the $\cos \phi_h$ and $\cos 2\phi_h$
modulations show strong kinematic dependencies both for positive and negative
hadrons.",1401.6284v1
2014-01-30,A high-statistics measurement of transverse spin effects in dihadron production from muon-proton semi-inclusive deep-inelastic scattering,"A measurement of the azimuthal asymmetry in dihadron production in
deep-inelastic scattering of muons on transversely polarised proton (NH$_{3}$)
targets are presented. They provide independent access to the transversity
distribution functions through the measurement of the Collins asymmetry in
single hadron production. The data were taken in the year $2010$ with the
COMPASS spectrometer using a $160\,\mbox{GeV}/c$ muon beam of the CERN SPS,
increasing by a factor of about three the available statistics of the
previously published data taken in the year $2007$. The measured sizeable
asymmetry is in good agreement with the published data. An approximate equality
of the Collins asymmetry and the dihadron asymmetry is observed, suggesting a
common physical mechanism in the underlying fragmentation.",1401.7873v1
2014-07-23,Search for exclusive photoproduction of Z$_c^{\pm}$(3900) at COMPASS,"A search for the exclusive production of the $Z_c^{\pm}(3900)$ hadron by
virtual photons has been performed in the channel $Z_c^{\pm}(3900)\rightarrow
J/\psi \pi^{\pm}$. The data cover the range from 7 GeV to 19 GeV in the
centre-of-mass energy of the photon-nucleon system. The full set of the COMPASS
data set collected with a muon beam between 2002 and 2011 has been used. An
upper limit for the ratio $BR(Z_c^{\pm}(3900)\rightarrow J/\psi \pi^{\pm}
)\times \sigma_{
  \gamma~N \rightarrow Z_c^{\pm}(3900)~ N} /\sigma_{\gamma~N \rightarrow
  J/\psi~ N}$ of $3.7\times10^{-3}$ has been established at the confidence
level of 90%.",1407.6186v1
2014-08-19,"Odd and Even Partial Waves of $ηπ^-$ and $η'π^-$ in $π^-p\toη^{(\prime)}π^-p$ at $191\,\textrm{GeV}/c$","Exclusive production of $\eta\pi^-$ and $\eta'\pi^-$ has been studied with a
$191\,\textrm{GeV}/c$ $\pi^-$ beam impinging on a hydrogen target at COMPASS
(CERN). Partial-wave analyses reveal different odd/even angular momentum ($L$)
characteristics in the inspected invariant mass range up to
$3\,\textrm{GeV}/c^2$. A striking similarity between the two systems is
observed for the $L=2,4,6$ intensities (scaled by kinematical factors) and the
relative phases. The known resonances $a_2(1320)$ and $a_4(2040)$ are in line
with this similarity. In contrast, a strong enhancement of $\eta'\pi^-$ over
$\eta\pi^-$ is found for the $L=1,3,5$ waves, which carry non-$q\bar q$ quantum
numbers. The $L=1$ intensity peaks at $1.7\,\textrm{GeV}/c^2$ in $\eta'\pi^-$
and at $1.4\,\textrm{GeV}/c^2$ in $\eta\pi^-$, the corresponding phase motions
with respect to $L=2$ are different.",1408.4286v1
2014-08-19,Collins and Sivers asymmetries in muonproduction of pions and kaons off transversely polarised proton,"Measurements of the Collins and Sivers asymmetries for charged pions and
charged and neutral kaons produced in semi-inclusive deep-inelastic scattering
of high energy muons off transversely polarised protons are presented. The
results were obtained using all the available COMPASS proton data, which were
taken in the years 2007 and 2010. The Collins asymmetries exhibit in the
valence region a non-zero signal for pions and there are hints of non-zero
signal also for kaons. The Sivers asymmetries are found to be positive for
positive pions and kaons and compatible with zero otherwise.",1408.4405v1
2014-10-07,The COMPASS Setup for Physics with Hadron Beams,"The main characteristics of the COMPASS experimental setup for physics with
hadron beams are described. This setup was designed to perform exclusive
measurements of processes with several charged and/or neutral particles in the
final state. Making use of a large part of the apparatus that was previously
built for spin structure studies with a muon beam, it also features a new
target system as well as new or upgraded detectors. The hadron setup is able to
operate at the high incident hadron flux available at CERN. It is characterised
by large angular and momentum coverages, large and nearly flat acceptances, and
good two and three-particle mass resolutions. In 2008 and 2009 it was
successfully used with positive and negative hadron beams and with liquid
hydrogen and solid nuclear targets. This article describes the new and upgraded
detectors and auxiliary equipment, outlines the reconstruction procedures used,
and summarises the general performance of the setup.",1410.1797v1
2015-01-23,Observation of a new narrow axial-vector meson $a_1(1420)$,"The COMPASS collaboration at CERN has measured diffractive dissociation of
190 GeV/$c$ pions into the $\pi^-\pi^-\pi^+$ final state using a stationary
hydrogen target. A partial-wave analysis (PWA) was performed in bins of $3\pi$
mass and four-momentum transfer using the isobar model and the so far largest
PWA model consisting of 88 waves. A narrow $J^{PC} = 1^{++}$ signal is observed
in the $f_0(980)\,\pi$ channel. We present a resonance-model study of a subset
of the spin-density matrix selecting $3\pi$ states with $J^{PC} = 2^{++}$ and
$4^{++}$ decaying into $\rho(770)\,\pi$ and with $J^{PC} = 1^{++}$ decaying
into $f_0(980)\,\pi$. We identify a new $a_1$ meson with mass
$(1414^{+15}_{-13})$ MeV/$c^2$ and width $(153^{+8}_{-23})$ MeV/$c^2$. Within
the final states investigated in our analysis, we observe the new $a_1(1420)$
decaying only into $f_0(980)\,\pi$, suggesting its exotic nature.",1501.05732v3
2015-03-31,The Spin Structure Function $g_1^{\rm p}$ of the Proton and a Test of the Bjorken Sum Rule,"New results for the double spin asymmetry $A_1^{\rm p}$ and the proton
longitudinal spin structure function $g_1^{\rm p}$ are presented. They were
obtained by the COMPASS collaboration using polarised 200 GeV muons scattered
off a longitudinally polarised NH$_3$ target. The data were collected in 2011
and complement those recorded in 2007 at 160\,GeV, in particular at lower
values of $x$. They improve the statistical precision of $g_1^{\rm p}(x)$ by
about a factor of two in the region $x\lesssim 0.02$. A next-to-leading order
QCD fit to the $g_1$ world data is performed. It leads to a new determination
of the quark spin contribution to the nucleon spin, $\Delta \Sigma$ ranging
from 0.26 to 0.36, and to a re-evaluation of the first moment of $g_1^{\rm p}$.
The uncertainty of $\Delta \Sigma$ is mostly due to the large uncertainty in
the present determinations of the gluon helicity distribution. A new evaluation
of the Bjorken sum rule based on the COMPASS results for the non-singlet
structure function $g_1^{\rm NS}(x,Q^2)$ yields as ratio of the axial and
vector coupling constants $|g_{\rm A}/g_{\rm V}| = 1.22 \pm 0.05~({\rm stat.})
\pm 0.10~({\rm syst.})$, which validates the sum rule to an accuracy of about
9\%.",1503.08935v1
2015-09-11,Longitudinal double spin asymmetries in single hadron quasi-real photoproduction at high $p_T$,"We measured the longitudinal double spin asymmetries $A_{LL}$ for single
hadron muo-production off protons and deuterons at photon virtuality $Q^2$ <
1(GeV/$\it c$)$^2$ for transverse hadron momenta $p_T$ in the range 0.7
GeV/$\it c$ to 4 GeV/$\it c$ . They were determined using COMPASS data taken
with a polarised muon beam of 160 GeV/$\it c$ or 200 GeV/$\it c$ impinging on
polarised $\mathrm{{}^6LiD}$ or $\mathrm{NH_3}$ targets. The experimental
asymmetries are compared to next-to-leading order pQCD calculations, and are
sensitive to the gluon polarisation $\Delta G$ inside the nucleon in the range
of the nucleon momentum fraction carried by gluons $0.05 < x_g < 0.2$.",1509.03526v2
2016-04-10,Multiplicities of charged pions and unidentified charged hadrons from deep-inelastic scattering of muons off an isoscalar target,"Multiplicities of charged pions and unidentified hadrons produced in
deep-inelastic scattering were measured in bins of the Bjorken scaling variable
$x$, the relative virtual-photon energy $y$ and the relative hadron energy $z$.
Data were obtained by the COMPASS Collaboration using a 160 GeV muon beam and
an isoscalar target ($^6$LiD). They cover the kinematic domain in the photon
virtuality $Q^2$ > 1(GeV/c$)^2$, $0.004 < x < 0.4$, $0.2 < z < 0.85$ and $0.1 <
y < 0.7$. In addition, a leading-order pQCD analysis was performed using the
pion multiplicity results to extract quark fragmentation functions.",1604.02695v2
2016-06-12,Exclusive $ω$ meson muoproduction on transversely polarised protons,"Exclusive production of $\omega$ mesons was studied at the COMPASS experiment
by scattering $160~\mathrm{GeV}/\mathit{c}$ muons off transversely polarised
protons. Five single-spin and three double-spin azimuthal asymmetries were
measured in the range of photon virtuality $1~(\mathrm{GeV}/\mathit{c})^2 < Q^2
< 10~(\mathrm{GeV}/\mathit{c})^2$, Bjorken scaling variable $0.003 <
x_{\mathit{Bj}} < 0.3$ and transverse momentum squared of the $\omega$ meson
$0.05~(\mathrm{GeV}/\mathit{c})^2 < p_{T}^{2} <
0.5~(\mathrm{GeV}/\mathit{c})^2$. The measured asymmetries are sensitive to the
nucleon helicity-flip Generalised Parton Distributions (GPD) $E$ that are
related to the orbital angular momentum of quarks, the chiral-odd GPDs $H_{T}$
that are related to the transversity Parton Distribution Functions, and the
sign of the $\pi\omega$ transition form factor. The results are compared to
recent calculations of a GPD-based model.",1606.03725v2
2014-03-11,Measurement of radiative widths of $a_2(1320)$ and $π_2(1670)$,"The COMPASS Collaboration at CERN has investigated the reaction $\pi^- \gamma
\rightarrow \pi^-\pi^-\pi^+$ embedded in the Primakoff reaction of
$190~\textrm{GeV}$ pions scattering in the Coulomb field of a lead target,
$\pi^- \text{Pb} \rightarrow \pi^-\pi^-\pi^+ \text{Pb}$. Exchange of quasi-real
photons is selected by isolating the sharp Coulomb peak observed at momentum
transfer below $0.001~(\text{GeV}/c)^2$. Using a partial-wave analysis the
amplitudes and relative phases of the $a_2(1320)$ and $\pi_2(1670)$ mesons have
been extracted, and the Coulomb and the diffractive contributions have been
disentangled. Measuring absolute production cross sections we have determined
the radiative width of the $a_2(1320)$ to be $\Gamma_0(a_2(1320) \rightarrow
\pi\gamma) = (358 \pm 6_{\textrm{stat}} \pm 42_{\textrm{syst}})~\textrm{keV}$.
As the first measurement, $\Gamma_0(\pi_2(1670) \rightarrow \pi\gamma) = (181
\pm 11_{\textrm{stat}} \pm 27_{\textrm{syst}})~\textrm{keV} \cdot
(\textrm{BR}^{\textrm{PDG}}_{f_2 \pi}/\textrm{BR}_{f_2 \pi})$ is obtained for
the radiative width of the $\pi_2(1670)$, where in this analysis the branching
ratio $\textrm{BR}^{\textrm{PDG}}_{f_2 \pi}=0.56$ has been used. We compare
these values to previous measurements and theoretical predictions.",1403.2644v1
2011-11-25,First Measurement of Chiral Dynamics in π^- γ-> π^- π^- π^+,"The COMPASS collaboration at CERN has investigated the \pi^- \gamma -> \pi^-
\pi^- \pi^+ reaction at center-of-momentum energy below five pion masses,
sqrt(s) < 5 m(\pi), embedded in the Primakoff reaction of 190 GeV pions
impinging on a lead target. Exchange of quasi-real photons is selected by
isolating the sharp Coulomb peak observed at smallest momentum transfers, t' <
0.001 (GeV/c)^2. Using partial-wave analysis techniques, the scattering
intensity of Coulomb production described in terms of chiral dynamics and its
dependence on the 3\pi-invariant mass m(3\pi) = sqrt(s) were extracted. The
absolute cross section was determined in seven bins of $\sqrt{s}$ with an
overall precision of 20%. At leading order, the result is found to be in good
agreement with the prediction of chiral perturbation theory over the whole
energy range investigated.",1111.5954v2
2014-05-25,Spin alignment and violation of the OZI rule in exclusive $ω$ and $φ$ production in pp collisions,"Exclusive production of the isoscalar vector mesons $\omega$ and $\phi$ is
measured with a 190 GeV$/c$ proton beam impinging on a liquid hydrogen target.
Cross section ratios are determined in three intervals of the Feynman variable
$x_{F}$ of the fast proton. A significant violation of the OZI rule is found,
confirming earlier findings. Its kinematic dependence on $x_{F}$ and on the
invariant mass $M_{p\mathrm{V}}$ of the system formed by fast proton
$p_\mathrm{fast}$ and vector meson $V$ is discussed in terms of diffractive
production of $p_\mathrm{fast}V$ resonances in competition with central
production. The measurement of the spin density matrix element $\rho_{00}$ of
the vector mesons in different selected reference frames provides another
handle to distinguish the contributions of these two major reaction types.
Again, dependences of the alignment on $x_{F}$ and on $M_{p\mathrm{V}}$ are
found. Most of the observations can be traced back to the existence of several
excited baryon states contributing to $\omega$ production which are absent in
the case of the $\phi$ meson. Removing the low-mass $M_{p\mathrm{V}}$ resonant
region, the OZI rule is found to be violated by a factor of eight,
independently of $x_\mathrm{F}$.",1405.6376v1
2014-05-25,Measurement of the charged-pion polarisability,"The COMPASS collaboration at CERN has investigated pion Compton scattering,
$\pi^-\gamma\rightarrow \pi^-\gamma$, at centre-of-mass energy below 3.5 pion
masses. The process is embedded in the reaction
$\pi^-\mathrm{Ni}\rightarrow\pi^-\gamma\;\mathrm{Ni}$, which is initiated by
190\,GeV pions impinging on a nickel target. The exchange of quasi-real photons
is selected by isolating the sharp Coulomb peak observed at smallest momentum
transfers, $Q^2<0.0015$\,(GeV/$c$)$^2$. From a sample of 63\,000 events the
pion electric polarisability is determined to be $\alpha_\pi\ =\ (\,2.0\ \pm\
0.6_{\mbox{\scriptsize stat}}\ \pm\ 0.7_{\mbox{\scriptsize syst}}\,) \times
10^{-4}\,\mbox{fm}^3$ under the assumption $\alpha_\pi=-\beta_\pi$, which
relates the electric and magnetic dipole polarisabilities. It is the most
precise measurement of this fundamental low-energy parameter of strong
interaction, that has been addressed since long by various methods with
conflicting outcomes. While this result is in tension with previous dedicated
measurements, it is found in agreement with the expectation from chiral
perturbation theory. An additional measurement replacing pions by muons, for
which the cross-section behavior is unambigiously known, was performed for an
independent estimate of the systematic uncertainty.",1405.6377v2
2016-08-24,Multiplicities of charged kaons from deep-inelastic muon scattering off an isoscalar target,"Precise measurements of charged-kaon multiplicities in deep inelastic
scattering were performed. The results are presented in three-dimensional bins
of the Bjorken scaling variable x, the relative virtual-photon energy y, and
the fraction z of the virtual-photon energy carried by the produced hadron. The
data were obtained by the COMPASS Collaboration by scattering 160 GeV muons off
an isoscalar 6 LiD target. They cover the kinematic domain 1 (GeV/c)2 < Q2 < 60
(GeV/c)^2 in the photon virtuality, 0.004 < x < 0.4, 0.1 < y < 0.7, 0.20 < z <
0.85, and W > 5 GeV/c^2 in the invariant mass of the hadronic system. The
results from the sum of the z-integrated K+ and K- multiplicities at high x
point to a value of the non-strange quark fragmentation function larger than
obtained by the earlier DSS fit.",1608.06760v1
2017-10-03,Longitudinal double-spin asymmetry $A_1^{\rm p}$ and spin-dependent structure function $g_1^{\rm p}$ of the proton at small values of $x$ and $Q^2$,"We present a precise measurement of the proton longitudinal double-spin
asymmetry $A_1^{\rm p}$ and the proton spin-dependent structure function
$g_1^{\rm p}$ at photon virtualities $0.006~({\rm GeV}/c)^2<Q^2 < 1~({\rm
GeV}/c)^2$ in the Bjorken $x$ range of $4 \times 10^{-5} < x < 4 \times
10^{-2}$. The results are based on data collected by the COMPASS Collaboration
at CERN using muon beam energies of $160~{\rm GeV}$ and $200~{\rm GeV}$. The
statistical precision is more than tenfold better than that of the previous
measurement in this region. In the whole range of $x$, the measured values of
$A_1^{\rm p}$ and $g_1^{\rm p}$ are found to be positive. It is for the first
time that spin effects are found at such low values of $x$.",1710.01014v1
2018-09-09,Measurement of $P_T$-weighted Sivers asymmetries in leptoproduction of hadrons,"The transverse spin asymmetries measured in semi-inclusive leptoproduction of
hadrons, when weighted with the hadron transverse momentum $P_T$, allow for the
extraction of important transverse-momentum-dependent distribution functions.
In particular, the weighted Sivers asymmetries provide direct information on
the Sivers function, which is a leading-twist distribution that arises from a
correlation between the transverse momentum of an unpolarised quark in a
transversely polarised nucleon and the spin of the nucleon. Using the
high-statistics data collected by the COMPASS Collaboration in 2010 with a
transversely polarised proton target, we have evaluated two types of
$P_T$-weighted Sivers asymmetries, which are both proportional to the product
of the firsttransverse moment of the Sivers function and of the fragmentation
function. The results are compared to the standard unweighted Sivers
asymmetries and used to extract the first transverse moments of the Sivers
distributions for $u$ and $d$ quarks.",1809.02936v1
2012-05-23,Experimental investigation of transverse spin asymmetries in muon-p SIDIS processes: Collins asymmetries,"The COMPASS Collaboration at CERN has measured the transverse spin azimuthal
asymmetry of charged hadrons produced in semi-inclusive deep inelastic
scattering using a 160 GeV positive muon beam and a transversely polarised NH_3
target. The Collins asymmetry of the proton was extracted in the Bjorken x
range 0.003<x<0.7. These new measurements confirm with higher accuracy previous
measurements from the COMPASS and HERMES collaborations, which exhibit a
definite effect in the valence quark region. The asymmetries for negative and
positive hadrons are similar in magnitude and opposite in sign. They are
compatible with model calculations in which the u-quark transversity is
opposite in sign and somewhat larger than the d-quark transversity distribution
function. The asymmetry is extracted as a function of Bjorken $x$, the relative
hadron energy $z$ and the hadron transverse momentum p_T^h. The high statistics
and quality of the data also allow for more detailed investigations of the
dependence on the kinematic variables. These studies confirm the leading-twist
nature of the Collins asymmetry.",1205.5121v1
2012-05-23,Experimental investigation of transverse spin asymmetries in muon-p SIDIS processes: Sivers asymmetries,"The COMPASS Collaboration at CERN has measured the transverse spin azimuthal
asymmetry of charged hadrons produced in semi-inclusive deep inelastic
scattering using a 160 GeV positive muon beam and a transversely polarised NH_3
target. The Sivers asymmetry of the proton has been extracted in the Bjorken x
range 0.003<x<0.7. The new measurements have small statistical and systematic
uncertainties of a few percent and confirm with considerably better accuracy
the previous COMPASS measurement. The Sivers asymmetry is found to be
compatible with zero for negative hadrons and positive for positive hadrons, a
clear indication of a spin-orbit coupling of quarks in a transversely polarised
proton. As compared to measurements at lower energy, a smaller Sivers asymmetry
for positive hadrons is found in the region x > 0.03. The asymmetry is
different from zero and positive also in the low x region, where sea-quarks
dominate. The kinematic dependence of the asymmetry has also been investigated
and results are given for various intervals of hadron and virtual photon
fractional energy. In contrast to the case of the Collins asymmetry, the
results on the Sivers asymmetry suggest a strong dependence on the
four-momentum transfer to the nucleon, in agreement with the most recent
calculations.",1205.5122v1
2019-03-28,Measurement of the cross section for hard exclusive $π^0$ leptoproduction,"We report on a measurement of hard exclusive $\pi^0$ muoproduction on the
proton by COMPASS using 160 GeV/$c$ polarised $\mu^+$ and $\mu^-$ beams of the
CERN SPS impinging on a liquid hydrogen target. From the average of the
measured $\mu^+$ and $\mu^-$ cross sections, the virtual-photon proton cross
section is determined as a function of the squared four-momentum transfer
between initial and final proton in the range $0.08\,(\text{GeV/}c)^2 < |t| <
0.64\,(\text{GeV/}c)^2$. The average kinematics of the measurement are $\langle
Q^2 \rangle =2.0\; {(\text{GeV}/c)^2}$, $\langle \nu \rangle = 12.8\;
{\text{GeV}}$, $\langle x_{Bj} \rangle = 0.093 $ and $\langle -t \rangle =
0.256\; {(\text{GeV}/c)^2} $. Fitting the azimuthal dependence reveals a
combined contribution by transversely and longitudinally polarised photons of
$(8.1 \ \pm \ 0.9_{\text{stat}}{}_{- \ 1.0}^{+ \
1.1}\big\rvert_{\text{sys}})\,{\text{nb}}/{(\text{GeV}/c)^{2}}$, as well as
transverse-transverse and longitudinal-transverse interference contributions of
$(-6.0 \pm 1.3_{\text{stat}}{}_{- \ 0.7}^{+ \
0.7}\big\rvert_{\text{sys}})\,{\text{nb}}/{(\text{GeV}/c)^{2}}$ and $(1.4 \pm
0.5_{\text{stat}}{}_{- \ 0.2}^{+ \
0.3}\big\rvert_{\text{sys}})\,{\text{nb}}/{(\text{GeV}/c)^{2}}$, respectively.
Our results provide important input for modelling Generalised Parton
Distributions. In the context of the phenomenological Goloskokov-Kroll model,
the statistically significant transverse-transverse interference contribution
constitutes clear experimental evidence for the chiral-odd GPD
$\overline{E}_T$.",1903.12030v1
2019-08-02,AEDGE: Atomic Experiment for Dark Matter and Gravity Exploration in Space,"We propose in this White Paper a concept for a space experiment using cold
atoms to search for ultra-light dark matter, and to detect gravitational waves
in the frequency range between the most sensitive ranges of LISA and the
terrestrial LIGO/Virgo/KAGRA/INDIGO experiments. This interdisciplinary
experiment, called Atomic Experiment for Dark Matter and Gravity Exploration
(AEDGE), will also complement other planned searches for dark matter, and
exploit synergies with other gravitational wave detectors. We give examples of
the extended range of sensitivity to ultra-light dark matter offered by AEDGE,
and how its gravitational-wave measurements could explore the assembly of
super-massive black holes, first-order phase transitions in the early universe
and cosmic strings. AEDGE will be based upon technologies now being developed
for terrestrial experiments using cold atoms, and will benefit from the space
experience obtained with, e.g., LISA and cold atom experiments in microgravity.
  This paper is based on a submission (v1) in response to the Call for White
Papers for the Voyage 2050 long-term plan in the ESA Science Programme. ESA
limited the number of White Paper authors to 30. However, in this version (v2)
we have welcomed as supporting authors participants in the Workshop on Atomic
Experiments for Dark Matter and Gravity Exploration held at CERN: ({\tt
https://indico.cern.ch/event/830432/}), as well as other interested scientists,
and have incorporated additional material.",1908.00802v2
2019-12-21,Contribution of exclusive diffractive processes to the measured azimuthal asymmetries in SIDIS,"Hadron leptoproduction in Semi-Inclusive measurements of Deep-Inelastic
Scattering (SIDIS) on unpolarised nucleons allows one to get information on the
intrinsic transverse momentum of quarks in a nucleon and on the Boer-Mulders
function through the measurement of azimuthal modulations in the cross section.
These modulations were recently measured by the HERMES experiment at DESY on
proton and deuteron targets, and by the COMPASS experiment using the CERN SPS
muon beam and a $^6$LiD target. In both cases, the amplitudes of the
$\cos\phi_h$ and $\cos 2\phi_h$ modulations show strong kinematic dependences
for both positive and negative hadrons. It has been known since some time that
the measured final-state hadrons in those SIDIS experiments receive a
contribution from exclusive diffractive production of vector mesons,
particularly important at large values of $z$, the fraction of the virtual
photon energy carried by the hadron. In previous measurements of azimuthal
asymmetries this contribution was not taken into account, because it was not
known that it could distort the azimuthal modulations. Presently, a method to
evaluate the contribution of the exclusive reactions to the azimuthal
asymmetries measured by COMPASS has been developed. The subtraction of this
contribution results in a better understanding of the kinematic effects, and
the remaining non-zero $\cos 2\phi_h$ modulation gives indication for a
non-zero Boer-Mulders effect.",1912.10322v1
2017-07-10,New analysis of $ηπ$ tensor resonances measured at the COMPASS experiment,"We present a new amplitude analysis of the $\eta\pi$ $D$-wave in $\pi^- p\to
\eta\pi^- p$ measured by COMPASS. Employing an analytical model based on the
principles of the relativistic $S$-matrix, we find two resonances that can be
identified with the $a_2(1320)$ and the excited $a_2^\prime(1700)$, and perform
a comprehensive analysis of their pole positions. For the mass and width of the
$a_2$ we find $M=(1307 \pm 1 \pm 6)$~MeV and $\Gamma=(112 \pm 1 \pm 8)$~MeV,
and for the excited state $a_2^\prime$ we obtain $M=(1720 \pm 10 \pm 60)$~MeV
and $\Gamma=(280\pm 10 \pm 70)$~MeV, respectively.",1707.02848v1
2020-06-09,Triangle Singularity as the Origin of the $a_1(1420)$,"The COMPASS experiment recently discovered a new isovector resonance-like
signal with axial-vector quantum numbers, the $a_1(1420)$, decaying to
$f_0(980)\pi$. With a mass too close to and a width smaller than the
axial-vector ground state $a_1(1260)$, it was immediately interpreted as a new
light exotic meson, similar to the $X$, $Y$, $Z$ states in the hidden-charm
sector. We show that a resonance-like signal fully matching the experimental
data is produced by the decay of the $a_1(1260)$ resonance into $K^\ast(\to
K\pi)\bar{K}$ and subsequent rescattering through a triangle singularity into
the coupled $f_0(980)\pi$ channel. The amplitude for this process is calculated
using a new approach based on dispersion relations. The triangle-singularity
model is fitted to the partial-wave data of the COMPASS experiment. Despite
having less parameters, this fit shows a slightly better quality than the one
using a resonance hypothesis and thus eliminates the need for an additional
resonance in order to describe the data. We thereby demonstrate for the first
time in the light-meson sector that a resonance-like structure in the
experimental data can be described by rescattering through a triangle
singularity, providing evidence for a genuine three-body effect.",2006.05342v3
2020-09-07,Spin Density Matrix Elements in Exclusive $ω$ Meson Muoproduction $^*$,"We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard
exclusive $\omega$ meson muoproduction on the proton at COMPASS using 160
GeV/$c$ polarised $ \mu ^{+}$ and $ \mu ^{-}$ beams impinging on a liquid
hydrogen target. The measurement covers the range 5.0 GeV/$c^2$ $< W <$ 17.0
GeV/$c^2$, with the average kinematics $\langle Q^{2} \rangle=$ 2.1
(GeV/$c$)$^2$, $\langle W \rangle= 7.6$ GeV/$c^2$, and $\langle p^{2}_{\rm T}
\rangle = 0.16$ (GeV/$c$)$^2$. Here, $Q^2$ denotes the virtuality of the
exchanged photon, $W$ the mass of the final hadronic system and $p_T$ the
transverse momentum of the $\omega$ meson with respect to the virtual-photon
direction. The measured non-zero SDMEs for the transitions of transversely
polarised virtual photons to longitudinally polarised vector mesons
($\gamma^*_T \to V_L$) indicate a violation of $s$-channel helicity
conservation. Additionally, we observe a sizeable contribution of
unnatural-parity-exchange (UPE) transitions that decreases with increasing $W$.
The results provide important input for modelling Generalised Parton
Distributions (GPDs). In particular, they may allow to evaluate in a
model-dependent way the contribution of UPE transitions and assess the role of
parton helicity-flip GPDs in exclusive $\omega$ production.",2009.03271v3
2015-07-27,Interplay among transversity induced asymmetries in hadron leptoproduction,"In the fragmentation of a transversely polarized quark several left-right
asymmetries are possible for the hadrons in the jet. When only one unpolarized
hadron is selected, it exhibits an azimuthal modulation known as Collins
effect. When a pair of oppositely charged hadrons is observed, three
asymmetries can be considered, a di-hadron asymmetry and two single hadron
asymmetries. In lepton deep inelastic scattering on transversely polarized
nucleons all these asymmetries are coupled with the transversity distribution.
From the high statistics COMPASS data on oppositely charged hadron-pair
production we have investigated for the first time the dependence of these
three asymmetries on the difference of the azimuthal angles of the two hadrons.
The similarity of transversity induced single and di-hadron asymmetries is
discussed. A new analysis of the data allows to establish quantitative
relationships among them, providing for the first time strong experimental
indication that the underlying fragmentation mechanisms are all driven by a
common physical process.",1507.07593v2
2016-09-23,Sivers asymmetry extracted in SIDIS at the hard scale of the Drell-Yan process at COMPASS,"Proton transverse-spin azimuthal asymmetries are extracted from the COMPASS
2010 semi-inclusive hadron measurements in deep inelastic muon-nucleon
scattering in those four regions of the photon virtuality $Q^2$, which
correspond to the four regions of the di-muon mass $\sqrt{Q^2}$ used in the
ongoing analysis of the COMPASS Drell-Yan measurements. This allows for a
future direct comparison of the nucleon transverse-momentum-dependent parton
distribution functions extracted from these two alternative measurements.
Various two-dimensional kinematic dependences are presented for the azimuthal
asymmetries induced by the Sivers transverse-momentum-dependent parton
distribution function. The integrated Sivers asymmetries are found to be
positive with an accuracy that appears to be sufficient to test the sign change
of the Sivers function predicted by Quantum Chromodynamics.",1609.07374v2
2017-04-03,First measurement of transverse-spin-dependent azimuthal asymmetries in the Drell-Yan process,"The first measurement of transverse-spin-dependent azimuthal asymmetries in
the pion-induced Drell-Yan (DY) process is reported. We use the CERN SPS 190
GeV/$c$, $\pi^{-}$ beam and a transversely polarized ammonia target. Three
azimuthal asymmetries giving access to different transverse-momentum-dependent
(TMD) parton distribution functions (PDFs) are extracted using dimuon events
with invariant mass between 4.3 GeV/$c^2$ and 8.5 GeV/$c^2$. The observed sign
of the Sivers asymmetry is found to be consistent with the fundamental
prediction of Quantum Chromodynamics (QCD) that the Sivers TMD PDFs extracted
from DY have a sign opposite to the one extracted from semi-inclusive
deep-inelastic scattering (SIDIS) data. We present two other asymmetries
originating from the pion Boer-Mulders TMD PDFs convoluted with either the
nucleon transversity or pretzelosity TMD PDFs. These DY results are obtained at
a hard scale comparable to that of a recent COMPASS SIDIS measurement and hence
allow unique tests of fundamental QCD universality predictions.",1704.00488v1
2020-03-26,Antiproton over proton and K$^-$ over K$^+$ multiplicity ratios at high $z$ in DIS,"The $\bar{\rm p} $ over p multiplicity ratio is measured in deep-inelastic
scattering for the first time using (anti-) protons carrying a large fraction
of the virtual-photon energy, $z>0.5$. The data were obtained by the COMPASS
Collaboration using a 160 GeV muon beam impinging on an isoscalar $^6$LiD
target. The regime of deep-inelastic scattering is ensured by requiring $Q^2$ >
1 (GeV/$c$)$^2$ for the photon virtuality and $W > 5$ GeV/$c^2$ for the
invariant mass of the produced hadronic system. The range in Bjorken-$x$ is
restricted to $0.01 < x < 0.40$. Protons and antiprotons are identified in the
momentum range $20 \div 60$ GeV/$c$. In the whole studied $z$-region, the
$\bar{\rm p}$ over p multiplicity ratio is found to be below the lower limit
expected from calculations based on leading-order perturbative Quantum
Chromodynamics (pQCD). Extending our earlier analysis of the K$^-$ over K$^+$
multiplicity ratio by including now events with larger virtual-photon energies,
this ratio becomes closer to the expectation of next-to-leading order pQCD. The
results of both analyses strengthen our earlier conclusion that the phase space
available for hadronisation should be taken into account in the pQCD formalism.",2003.11791v1
2021-04-28,Probing transversity by measuring $Λ$ polarisation in SIDIS,"Based on the observation of sizeable target-transverse-spin asymmetries in
single-hadron and hadron-pair production in Semi-Inclusive measurements of Deep
Inelastic Scattering (SIDIS), the chiral-odd transversity quark distribution
functions $h_1^q$ are nowadays well established. Several possible channels to
access these functions were originally proposed. One candidate is the
measurement of the polarisation of $\Lambda$ hyperons produced in SIDIS off
transversely polarised nucleons, where the transverse polarisation of the
struck quark might be transferred to the final-state hyperon. In this article,
we present the COMPASS results on the transversity-induced polarisation of
$\Lambda$ and $\bar{\Lambda}$ hyperons produced in SIDIS off transversely
polarised protons. Within the experimental uncertainties, no significant
deviation from zero was observed. The results are discussed in the context of
different models taking into account previous experimental results on $h_1^u$
and $h_1^d$.",2104.13585v2
2021-08-03,The exotic meson $π_1(1600)$ with $J^{PC} = 1^{-+}$ and its decay into $ρ(770)π$,"We study the spin-exotic $J^{PC} = 1^{-+}$ amplitude in single-diffractive
dissociation of 190 GeV$/c$ pions into $\pi^-\pi^-\pi^+$ using a hydrogen
target and confirm the $\pi_1(1600) \to \rho(770) \pi$ amplitude, which
interferes with a nonresonant $1^{-+}$ amplitude. We demonstrate that
conflicting conclusions from previous studies on these amplitudes can be
attributed to different analysis models and different treatment of the
dependence of the amplitudes on the squared four-momentum transfer and we thus
reconcile their experimental findings. We study the nonresonant contributions
to the $\pi^-\pi^-\pi^+$ final state using pseudo-data generated on the basis
of a Deck model. Subjecting pseudo-data and real data to the same partial-wave
analysis, we find good agreement concerning the spectral shape and its
dependence on the squared four-momentum transfer for the $J^{PC} = 1^{-+}$
amplitude and also for amplitudes with other $J^{PC}$ quantum numbers. We
investigate for the first time the amplitude of the $\pi^-\pi^+$ subsystem with
$J^{PC} = 1^{--}$ in the $3\pi$ amplitude with $J^{PC} = 1^{-+}$ employing the
novel freed-isobar analysis scheme. We reveal this $\pi^-\pi^+$ amplitude to be
dominated by the $\rho(770)$ for both the $\pi_1(1600)$ and the nonresonant
contribution. We determine the $\rho(770)$ resonance parameters within the
three-pion final state. These findings largely confirm the underlying
assumptions for the isobar model used in all previous partial-wave analyses
addressing the $J^{PC} = 1^{-+}$ amplitude.",2108.01744v2
2022-04-04,Double $J/ψ$ production in pion-nucleon scattering at COMPASS,"We present the study of the production of double $J/\psi$ mesons using
COMPASS data collected with a 190 GeV/$c$ $\pi^-$ beam scattering off NH$_{3}$,
Al and W targets. Kinematic distributions of the collected double $J/\psi$
events are analysed, and the double $J/\psi$ production cross section is
estimated for each of the COMPASS targets. The results are compared to
predictions from single- and double-parton scattering models as well as the
pion intrinsic charm and the tetraquark exotic resonance hypotheses. It is
demonstrated that the single parton scattering production mechanism gives the
dominant contribution that is sufficient to describe the data. An upper limit
on the double intrinsic charm content of pion is evaluated. No significant
signatures that could be associated with exotic tetraquarks are found in the
double $J/\psi$ mass spectrum.",2204.01817v1
2022-10-30,Spin Density Matrix Elements in Exclusive $ρ^0$ Meson Muoproduction,"We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard
exclusive $\rho ^0$ meson muoproduction at COMPASS using 160~GeV/$c$ polarised
$ \mu ^{+}$ and $ \mu ^{-}$ beams impinging on a liquid hydrogen target. The
measurement covers the kinematic range 5.0~GeV/$c^2$ $< W <$ 17.0~GeV/$c^2$,
1.0 (GeV/$c$)$^2$ $< Q^2 <$ 10.0 (GeV/$c$)$^2$ and 0.01 (GeV/$c$)$^2$ $<
p_{\rm{T}}^2 <$ 0.5 (GeV/$c$)$^2$. Here, $W$ denotes the mass of the final
hadronic system, $Q^2$ the virtuality of the exchanged photon, and $p_{\rm{T}}$
the transverse momentum of the $\rho ^0$ meson with respect to the
virtual-photon direction. The measured non-zero SDMEs for the transitions of
transversely polarised virtual photons to longitudinally polarised vector
mesons ($\gamma^*_T \to V^{ }_L$) indicate a violation of $s$-channel helicity
conservation. Additionally, we observe a dominant contribution of
natural-parity-exchange transitions and a very small contribution of
unnatural-parity-exchange transitions, which is compatible with zero within
experimental uncertainties. The results provide important input for modelling
Generalised Parton Distributions (GPDs). In particular, they may allow one to
evaluate in a model-dependent way the role of parton helicity-flip GPDs in
exclusive $\rho ^0$ production.",2210.16932v2
2022-10-31,Collins and Sivers transverse-spin asymmetries in inclusive muoproduction of $ρ^0$ mesons,"The production of vector mesons in deep inelastic scattering is an
interesting yet scarsely explored channel to study the transverse spin
structure of the nucleon and the related phenomena. The COMPASS collaboration
has performed the first measurement of the Collins and Sivers asymmetries for
inclusively produced $\rho^0$ mesons. The analysis is based on the data set
collected in deep inelastic scattering in $2010$ using a $160\,\,\rm{GeV}/c$
$\mu^+$ beam impinging on a transversely polarized $\rm{NH}_3$ target. The
$\rho^{0}$ mesons are selected from oppositely charged hadron pairs, and the
asymmetries are extracted as a function of the Bjorken-$x$ variable, the
transverse momentum of the pair and the fraction of the energy $z$ carried by
the pair. Indications for positive Collins and Sivers asymmetries are observed.",2211.00093v2
2023-12-28,Final COMPASS results on the transverse-spin-dependent azimuthal asymmetries in the pion-induced Drell-Yan process,"The COMPASS Collaboration performed measurements of the Drell-Yan process in
2015 and 2018 using a 190 GeV/c $\pi^{-}$ beam impinging on a transversely
polarised ammonia target. Combining the data of both years, we present final
results on the amplitudes of the five azimuthal modulations in the dimuon
production cross section. Three of these transverse-spin-dependent azimuthal
asymmetries (TSAs) probe the nucleon leading-twist Sivers, transversity, and
pretzelosity transverse-momentum dependent (TMD) parton distribution functions
(PDFs). The other two are induced by subleading effects. These TSAs provide
unique new inputs for the study of the nucleon TMD PDFs and their universality
properties. In particular, the Sivers TSA observed in this measurement is
consistent with the fundamental QCD prediction of a sign change of naive
time-reversal-odd TMD PDFs when comparing the Drell-Yan process with
semi-inclusive measurements of deep inelastic scattering. Also, within the
context of model predictions, the observed transversity TSA is consistent with
the expectation of a sign change for the Boer-Mulders function.",2312.17379v1
2023-12-30,High-statistics measurement of Collins and Sivers asymmetries for transversely polarised deuterons,"New results are presented on a high-statistics measurement of Collins and
Sivers asymmetries of charged hadrons produced in deep inelastic scattering of
muons on a transversely polarised $^6$LiD target. The data were taken in 2022
with the COMPASS spectrometer using the 160 \gevv\ muon beam at CERN, balancing
the existing data on transversely polarised proton targets. The first results
from about two-thirds of the new data have total uncertainties smaller by up to
a factor of three compared to the previous deuteron measurements. Using all the
COMPASS proton and deuteron results, both the transversity and the Sivers
distribution functions of the $u$ and $d$ quark, as well as the tensor charge
in the measured $x$-range are extracted. In particular, the accuracy of the $d$
quark results is significantly improved.",2401.00309v1
2008-02-15,Collins and Sivers asymmetries for pions and kaons in muon-deuteron DIS,"The measurements of the Collins and Sivers asymmetries of identified hadrons
produced in deep-inelastic scattering of 160 GeV/c muons on a transversely
polarised 6LiD target at COMPASS are presented. The results for charged pions
and charged and neutral kaons correspond to all data available, which were
collected from 2002 to 2004. For all final state particles both the Collins and
Sivers asymmetries turn out to be small, compatible with zero within the
statistical errors, in line with the previously published results for not
identified charged hadrons, and with the expected cancellation between the u-
and d-quark contributions.",0802.2160v2
2015-09-03,Resonance Production and $ππ$ S-wave in $π^- + p \rightarrow π^- π^- π^+ + p_{recoil}$ at 190 GeV/c,"The COMPASS collaboration has collected the currently largest data set on
diffractively produced $\pi^-\pi^-\pi^+$ final states using a negative pion
beam of 190 GeV/c momentum impinging on a stationary proton target. This data
set allows for a systematic partial-wave analysis in 100 bins of three-pion
mass, $0.5 < m_{3\pi} < 2.5$ GeV/c$^2$ , and in 11 bins of the reduced
four-momentum transfer squared, $0.1 < t < 1.0$ (GeV/c)$^2$ . This
two-dimensional analysis offers sensitivity to genuine one-step resonance
production, i.e. the production of a state followed by its decay, as well as to
more complex dynamical effects in nonresonant $3\pi$ production. In this paper,
we present detailed studies on selected $3\pi$ partial waves with $J^{PC} =
0^{-+}$, $1^{++}$, $2^{-+}$, $2^{++}$, and $4^{++}$. In these waves, we observe
the well-known ground-state mesons as well as a new narrow axial-vector meson
$a_1(1420)$ decaying into $f_0(980) \pi$. In addition, we present the results
of a novel method to extract the amplitude of the $\pi^-\pi^+$ subsystem with
$I^{G}J^{PC} = 0^+ 0^{++}$ in various partial waves from the $\pi^-\pi^-\pi^+$
data. Evidence is found for correlation of the $f_0(980)$ and $f_0(1500)$
appearing as intermediate $\pi^- \pi^+$ isobars in the decay of the known
$\pi(1800)$ and $\pi_2(1880)$.",1509.00992v2
2018-02-02,K$^{-}$ over K$^{+}$ multiplicity ratio for kaons produced in DIS with a large fraction of the virtual-photon energy,"The K$^{-}$ over K$^{+}$ multiplicity ratio is measured in deep-inelastic
scattering, for the first time for kaons carrying a large fraction $z$ of the
virtual-photon energy. The data were obtained by the COMPASS collaboration
using a 160 GeV muon beam and an isoscalar $^6$LiD target. The regime of
deep-inelastic scattering is ensured by requiring $Q^2>1$ (GeV/$c)^2$ for the
photon virtuality and $W>5$ GeV/$c^2$ for the invariant mass of the produced
hadronic system. Kaons are identified in the momentum range from 12 GeV/$c$ to
40 GeV/$c$, thereby restricting the range in Bjorken-$x$ to $0.01<x<0.40$. The
$z$-dependence of the multiplicity ratio is studied for $z>0.75$. For very
large values of $z$, $i.e.$ $z>0.8$, we observe the kaon multiplicity ratio to
fall below the lower limits expected from calculations based on leading and
next-to-leading order perturbative quantum chromodynamics. Also, the kaon
multiplicity ratio shows a strong dependence on the missing mass of the
single-kaon production process. This suggests that within the perturbative
quantum chromodynamics formalism an additional correction may be required,
which takes into account the phase space available for hadronisation.",1802.00584v2
2018-02-08,Transverse Extension of Partons in the Proton probed by Deeply Virtual Compton Scattering,"We report on the first measurement of exclusive single-photon muoproduction
on the proton by COMPASS using 160 GeV/$c$ polarized $\mu^+$ and $\mu^-$ beams
of the CERN SPS impinging on a liquid hydrogen target. We determine the
dependence of the average of the measured $\mu^+$ and $\mu^-$ cross sections
for deeply virtual Compton scattering on the squared four-momentum transfer $t$
from the initial to the final final proton. The slope $B$ of the $t$-dependence
is fitted with a single exponential function, which yields $B=(4.3 \ \pm \
0.6_{\text{stat}}\_{- \ 0.3}^{+ \ 0.1}\big\rvert_{\text{sys}})
(\text{GeV}/c)^{-2}$. This result can be converted into an average transverse
extension of partons in the proton, $\sqrt{\langle r_{\perp}^2 \rangle} = (0.58
\ \pm \ 0.04_{\text{stat}}\_{- \ 0.02}^{+ \
0.01}\big\rvert_{\text{sys}})\text{fm}$. For this measurement, the average
virtuality of the photon mediating the interaction is $\langle Q^2 \rangle =
1.8\,(\text{GeV/}c)^2$ and the average value of the Bjorken variable is
$\langle x_{\text{Bj}} \rangle = 0.056$.",1802.02739v2
2018-02-16,Light isovector resonances in $π^- p \to π^-π^-π^+ p$ at 190 GeV/${\it c}$,"We have performed the most comprehensive resonance-model fit of
$\pi^-\pi^-\pi^+$ states using the results of our previously published
partial-wave analysis (PWA) of a large data set of diffractive-dissociation
events from the reaction $\pi^- + p \to \pi^-\pi^-\pi^+ + p_\text{recoil}$ with
a 190 GeV/$c$ pion beam. The PWA results, which were obtained in 100 bins of
three-pion mass, $0.5 < m_{3\pi} < 2.5$ GeV/$c^2$, and simultaneously in 11
bins of the reduced four-momentum transfer squared, $0.1 < t' < 1.0$
$($GeV$/c)^2$, are subjected to a resonance-model fit using Breit-Wigner
amplitudes to simultaneously describe a subset of 14 selected waves using 11
isovector light-meson states with $J^{PC} = 0^{-+}$, $1^{++}$, $2^{++}$,
$2^{-+}$, $4^{++}$, and spin-exotic $1^{-+}$ quantum numbers. The model
contains the well-known resonances $\pi(1800)$, $a_1(1260)$, $a_2(1320)$,
$\pi_2(1670)$, $\pi_2(1880)$, and $a_4(2040)$. In addition, it includes the
disputed $\pi_1(1600)$, the excited states $a_1(1640)$, $a_2(1700)$, and
$\pi_2(2005)$, as well as the resonancelike $a_1(1420)$. We measure the
resonance parameters mass and width of these objects by combining the
information from the PWA results obtained in the 11 $t'$ bins. We extract the
relative branching fractions of the $\rho(770) \pi$ and $f_2(1270) \pi$ decays
of $a_2(1320)$ and $a_4(2040)$, where the former one is measured for the first
time. In a novel approach, we extract the $t'$ dependence of the intensity of
the resonances and of their phases. The $t'$ dependence of the intensities of
most resonances differs distinctly from the $t'$ dependence of the nonresonant
components. For the first time, we determine the $t'$ dependence of the phases
of the production amplitudes and confirm that the production mechanism of the
Pomeron exchange is common to all resonances.",1802.05913v2
2017-01-10,First measurement of the Sivers asymmetry for gluons from SIDIS data,"The Sivers function describes the correlation between the transverse spin of
a nucleon and the transverse motion of its partons. It was extracted from
measurements of the azimuthal asymmetry of hadrons produced in semi-inclusive
deep inelastic scattering of leptons off transversely polarised nucleon
targets, and it turned out to be non-zero for quarks. In this letter the
evaluation of the Sivers asymmetry for gluons in the same process is presented.
The analysis method is based on a Monte Carlo simulation that includes three
hard processes: photon-gluon fusion, QCD Compton scattering and leading-order
virtual-photon absorption process. The Sivers asymmetries of the three
processes are simultaneously extracted using the LEPTO event generator and a
neural network approach. The method is applied to samples of events containing
at least two hadrons with large transverse momentum from the COMPASS data taken
with a 160 GeV/$c$ muon beam scattered off transversely polarised deuterons and
protons. With a significance of more than two standard deviations a negative
value is obtained for the gluon Sivers asymmetry. The result of a similar
analysis for a Collins-like asymmetry for gluons is consistent with zero.",1701.02453v1
2017-09-21,Transverse-momentum-dependent Multiplicities of Charged Hadrons in Muon-Deuteron Deep Inelastic Scattering,"A semi-inclusive measurement of charged hadron multiplicities in deep
inelastic muon scattering off an isoscalar target was performed using data
collected by the COMPASS Collaboration at CERN. The following kinematic domain
is covered by the data: photon virtuality $Q^{2}>1$ (GeV/$c$)$^2$, invariant
mass of the hadronic system $W > 5$ GeV/$c^2$, Bjorken scaling variable in the
range $0.003 < x < 0.4$, fraction of the virtual photon energy carried by the
hadron in the range $0.2 < z < 0.8$, square of the hadron transverse momentum
with respect to the virtual photon direction in the range 0.02 (GeV/$c)^2 <
P_{\rm{hT}}^{2} < 3$ (GeV/$c$)$^2$. The multiplicities are presented as a
function of $P_{\rm{hT}}^{2}$ in three-dimensional bins of $x$, $Q^2$, $z$ and
compared to previous semi-inclusive measurements. We explore the
small-$P_{\rm{hT}}^{2}$ region, i.e. $P_{\rm{hT}}^{2} < 1$ (GeV/$c$)$^2$, where
hadron transverse momenta are expected to arise from non-perturbative effects,
and also the domain of larger $P_{\rm{hT}}^{2}$, where contributions from
higher-order perturbative QCD are expected to dominate. The multiplicities are
fitted using a single-exponential function at small $P_{\rm{hT}}^{2}$ to study
the dependence of the average transverse momentum $\langle
P_{\rm{hT}}^{2}\rangle$ on $x$, $Q^2$ and $z$. The power-law behaviour of the
multiplicities at large $P_{\rm{hT}}^{2}$ is investigated using various
functional forms. The fits describe the data reasonably well over the full
measured range.",1709.07374v1
2016-12-02,Final COMPASS results on the deuteron spin-dependent structure function $g_1^{\rm d}$ and the Bjorken sum rule,"Final results are presented from the inclusive measurement of deep-inelastic
polarised-muon scattering on longitudinally polarised deuterons using a $^6$LiD
target. The data were taken at $160~{\rm GeV}$ beam energy and the results are
shown for the kinematic range $1~({\rm GeV}/c)^2 < Q^2 < 100~({\rm GeV}/c)^2$
in photon virtuality, $0.004<x<0.7$ in the Bjorken scaling variable and $W >
4~{\rm GeV}/c^2$ in the mass of the hadronic final state. The deuteron
double-spin asymmetry $A_1^{\rm d}$ and the deuteron longitudinal-spin
structure function $g_1^{\rm d}$ are presented in bins of $x$ and $Q^2$.
Towards lowest accessible values of $x$, $g_1^{\rm d}$ decreases and becomes
consistent with zero within uncertainties. The presented final $g_1^{\rm d}$
values together with the recently published final $g_1^{\rm p}$ values of
COMPASS are used to again evaluate the Bjorken sum rule and perform the QCD fit
to the $g_1$ world data at next-to-leading order of the strong coupling
constant. In both cases, changes in central values of the resulting numbers are
well within statistical uncertainties. The flavour-singlet axial charge $a_0$,
{which is identified in the $\overline{\rm MS}$ renormalisation scheme with the
total contribution of quark helicities to the nucleon spin}, is extracted from
only the COMPASS deuteron data with negligible extrapolation uncertainty: $a_0
(Q^2 = 3~({\rm GeV}/c)^2) = 0.32 \pm 0.02_{\rm stat} \pm0.04_{\rm syst} \pm
0.05_{\rm evol}$. Together with the recent results on the proton spin structure
function $g_1^{\rm p}$, the results on $g_1^{\rm d}$ constitute the COMPASS
legacy on the measurements of $g_1$ through inclusive spin-dependent deep
inelastic scattering.",1612.00620v1
2017-07-06,Search for muoproduction of $X(3872)$ at COMPASS and indication of a new state $\widetilde{X}(3872)$,"We have searched for exclusive production of exotic charmonia in the reaction
$\mu^+~N \rightarrow \mu^+ (J\!/\!\psi\pi^+\pi^-)\pi^{\pm}~N'$ using COMPASS
data collected with incoming muons of 160 GeV/$c$ and 200 GeV/$c$ momentum. In
the $J\!/\!\psi\pi^+\pi^-$ mass distribution we observe a signal with a
statistical significance of 4.1 $\sigma$. Its mass and width are consistent
with those of the $X(3872)$. The shape of the $\pi^+\pi^-$ mass distribution
from the observed decay into $J\!/\!\psi\pi^+\pi^-$ shows disagreement with
previous observations for $X(3872)$. The observed signal may be interpreted as
a possible evidence of a new charmonium state. It could be associated with a
neutral partner of $X(3872)$ with $C = -1$ predicted by a tetraquark model. The
product of cross section and branching fraction of the decay of the observed
state into $J\!/\!\psi\pi^+\pi^-$ is determined to be
71$\pm$28(stat)$\pm$39(syst) pb.",1707.01796v2
2015-12-16,Leading-order determination of the gluon polarisation from semi-inclusive deep inelastic scattering data,"Using a novel analysis technique, the gluon polarisation in the nucleon is
re-evaluated using the longitudinal double-spin asymmetry measured in the cross
section of semi-inclusive single-hadron muoproduction with photon virtuality
$Q^2>1~({\rm GeV}/c)^2$. The data were obtained by the COMPASS experiment at
CERN using a 160 GeV/$c$ polarised muon beam impinging on a polarised $^6$LiD
target. By analysing the full range in hadron transverse momentum $p_{\rm T}$,
the different $p_{\rm T}$-dependences of the underlying processes are separated
using a neural-network approach. In the absence of pQCD calculations at
next-to-leading order in the selected kinematic domain, the gluon polarisation
$\Delta g/g$ is evaluated at leading order in pQCD at a hard scale of $\mu^2=
\langle Q^2 \rangle = 3 ({\rm GeV}/c)^2$. It is determined in three intervals
of the nucleon momentum fraction carried by gluons, $x_{\rm g}$, covering the
range $0.04 \!<\! x_{ \rm g}\! <\! 0.28$~ and does not exhibit a significant
dependence on $x_{\rm g}$. The average over the three intervals, $\langle
\Delta g/g \rangle = 0.113 \pm 0.038_{\rm (stat.)}\pm 0.036_{\rm (syst.)}$ at
$\langle x_{\rm g} \rangle \approx 0.10$, suggests that the gluon polarisation
is positive in the measured $x_{\rm g}$ range.",1512.05053v2
2016-09-20,Azimuthal asymmetries of charged hadrons produced in high-energy muon scattering off longitudinally polarised deuterons,"Single hadron azimuthal asymmetries of positive and negative hadrons produced
in muon semi-inclusive deep inelastic scattering off longitudinally polarised
deuterons are determined using the 2006 COMPASS data and also combined all
deuteron COMPASS data. For each hadron charge, the dependence of the azimuthal
asymmetry on the hadron azimuthal angle $\phi$ is obtained by means of a
five-parameter fitting function that besides a $\phi$-independent term includes
four modulations predicted by theory: $\sin\phi$, $\sin 2 \phi$, $\sin 3\phi$
and $\cos\phi$. The amplitudes of the five terms have been extracted, first,
for the hadrons in the whole available kinematic region. In further fits,
performed for hadrons from a restricted kinematic region, the $\phi$-dependence
is determined as a function of one of three variables (Bjorken-$x$, fractional
energy of virtual photon taken by the outgoing hadron and hadron transverse
momentum), while disregarding the others. Except the $\phi$-independent term,
all the modulation amplitudes are very small, and no clear kinematic dependence
could be observed within experimental uncertainties.",1609.06062v2
2018-08-02,Letter of Intent: A New QCD facility at the M2 beam line of the CERN SPS (COMPASS++/AMBER),"A New QCD facility at the M2 beam line of the CERN SPS
  COMPASS++/AMBER",1808.00848v6