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2015-03-26 | Quantitative analysis of magnetic spin and orbital moments from an oxidized iron (1 1 0) surface using electron magnetic circular dichroism | Understanding the ramifications of reduced crystalline symmetry on magnetic
behavior is a critical step in improving our understanding of nanoscale and
interfacial magnetism. However, investigations of such effects are often
controversial largely due to the challenges inherent in directly correlating
nanoscale stoichiometry and structure to magnetic behavior. Here, we describe
how to use Transmission Electron Microscope (TEM) to obtain Electron Magnetic
Circular Dichroism (EMCD) signals as a function of scattering angle to locally
probe the magnetic behavior of thin oxide layers grown on an Fe (1 1 0)
surface. Experiments and simulations both reveal a strong dependence of the
magnetic orbital to spin ratio on its scattering vector in reciprocal space. We
exploit this variation to extract the magnetic properties of the oxide cladding
layer, showing that it locally may exhibit an enhanced orbital to spin moment
ratio. This finding is supported here by both spatially and angularly resolved
EMCD measurements, opening up the way for compelling investigations into how
magnetic properties are affected by nanoscale features. | 1503.07681v2 |
2015-03-28 | Nearly-critical spin and charge fluctuations in KFe2As2 observed by high-pressure NMR | We report a high-pressure 75As NMR study on the heavily hole-doped iron
pnictide superconductor KFe2As2 (Tc~3.8 K). The low-energy spin fluctuations
are found to decrease with applied pressure up to 2 GPa, but then increase
again, changing in lockstep with the pressure-induced evolution of Tc. Their
diverging nature suggests close proximity to a magnetic quantum critical point
at a negative pressure of P~-0.6 GPa. Above 2.4 GPa, the 75As satellite spectra
split below 40 K, indicating a breaking of As site symmetry and an incipient
charge order. These pressure-controlled phenomena demonstrate the presence of
nearly-critical fluctuations in both spin and charge, providing essential input
for the origin of superconductivity. | 1503.08298v1 |
2015-03-31 | Theory of excitations and dielectric response at a spin-orbital quantum critical point | Despite possessing a local spin $2$ moment on the iron site and a Curie-Weiss
temperature of $45K$, the A site spinel FeSc$_2$S$_4$ does not magnetically
order down to 50mK. Previous theoretical work by Chen and Balents advanced an
explanation for this observation in the form of the "$J_2$-$\lambda$" model
which places FeSc$_2$S$_4$ close to a quantum critical point on the disordered
side of a quantum phase transition between a N\'{e}el ordered phase and a
"Spin-Orbital Liquid" in which spins and orbitals are entangled, quenching the
magnetization. We present new theoretical studies of the optical properties of
the $J_2$-$\lambda$ model, including a computation of the dispersion relation
for the quasiparticle excitations and the form of the collective response to
electric field. We argue that the latter directly probes a low energy
excitation continuum characteristic of quantum criticality, and that our
results reinforce the consistency of this model with experiment. | 1503.08887v2 |
2015-03-31 | Ultrafast near infrared photoinduced absorption in a multiferroic single crystal of bismuth ferrite | We studied the ultrafast third-order optical nonlinearity in a single crystal
of multiferroic bismuth ferrite (BiFeO3) in the near-infrared range of 0.5-1.0
eV, where the material is fundamentally transparent,at room temperature. With
pump pulses at 1.55 eV, which is off-resonant to the strong inter-band charge
transfer (CT) transition, we observed instantaneous transient absorption with
pencil-like temporal profile originating from the two-photon CT transition from
the oxygen 2p to the iron 3p levels. In contrast, under pumping with 3.10-eV
photons, the pencil-like absorption change was not observed but decay profiles
showed longer time constants. Although the two-photon absorption coefficient is
estimated to be 1.5 cm/GW, which is ten (hundred) times smaller than that of
two(one)-dimensional cuprates, it is larger than those of common semiconductors
such as ZnSe and GaAs at the optical communication wavelength. | 1503.08976v1 |
2015-03-31 | BeppoSAX observations of GRO J1744-28 | We present an analysis of BeppoSAX observations of the unique transient
bursting X-ray pulsar GRO J1744-28. The observations took place in March 1997
during the decay phase of the outburst. We find that the persistent broadband
X-ray continuum of the source is consistent with a cutoff power law typical for
the accreting pulsars. We also detect the fluorescence iron line at 6.7 keV and
an absorption feature at ~4.5 keV, which we interpret as a cyclotron line. The
corresponding magnetic field strength in the line forming region is ~3.7 x
10^11 G. Neither line is detected in the spectra of the bursts. However,
additional soft thermal component with kT ~2 keV was required to describe the
burst spectrum. We briefly discuss the nature of this component and argue that
among other possibilities it might be connected with thermonuclear flashes at
the neutron star surface which accompany the accretion-powered bursts in the
source. | 1503.09020v1 |
2015-04-01 | Possible Field-Temperature Phase Diagrams of Two-Band Superconductors with Paramagnetic Pair-Breaking | Possible field-temperature superconducting (SC) phase diagrams in two-band
quasi-two-dimensional materials with a strong paramagnetic pair-breaking (PPB)
are considered theoretically. Attention is paid to the case under a magnetic
field $perpendicular$ to the SC layers and to essential differences from the
counterpart in the ordinary single-band material. It is found by examining the
$H_{c2}(T)$ curve and the vortex lattices to be realized close to $H_{c2}$ that
a PPB-induced SC phase with a spatial modulation parallel to the field tends to
occur more easily than in the single-band case, and that a crisscrossing vortex
lattice proposed previously can occur in place of the conventional
Fulde-Ferrell-Larkin-Ovchinnikov state within a parameter range. The relevance
of the obtained results to FeSe and other iron-based superconductors is
discussed. | 1504.00112v2 |
2015-04-06 | Quantifying the complex permittivity and permeability of magnetic nanoparticles | The complex permittivity and permeability of superparamagnetic iron-oxide
nanoparticles has been quantified using a circular waveguide assembly with a
static magnetic field to align the nanoparticle's magnetization. The high
sensitivity of the measurement provides the precise resonant feature of
nanoparticles. The complex permeability in the vicinity of ferromagnetic
resonance (FMR) is in agreement with the nanoparticle's measured magnetization
via conventional magnetometry. A rigorous and self-consistent measure of
complex permittivities and permeabilities of nanoparticles is crucial to
ascertain accurately the dielectric behaviour as well as the frequency response
of nanoparticle magnetization, necessary ingredients when designing and
optimizing magnetic nanoparticles for biomedical applications. | 1504.01404v1 |
2015-04-09 | Plain s-wave superconductivity in single-layer FeSe on SrTiO3 probed by scanning tunneling microscopy | Single-layer FeSe film on SrTiO3(001) was recently found to be the champion
of interfacial superconducting systems, with a much enhanced superconductivity
than the bulk iron-based superconductors. Its superconducting mechanism is of
great interest. Although the film has a simple Fermi surface topology, its
pairing symmetry is unsettled. Here by using low-temperature scanning tunneling
microscopy (STM), we systematically investigated the superconductivity of
single-layer FeSe/SrTiO3(001) films. We observed fully gapped tunneling
spectrum and magnetic vortex lattice in the film. Quasi-particle interference
(QPI) patterns reveal scatterings between and within the electron pockets, and
put constraints on possible pairing symmetries. By introducing impurity atoms
onto the sample, we show that the magnetic impurities (Cr, Mn) can locally
suppress the superconductivity but the non-magnetic impurities (Zn, Ag and K)
cannot. Our results indicate that single-layer FeSe/SrTiO3 has a plain s-wave
paring symmetry whose order parameter has the same phase on all Fermi surface
sections. | 1504.02185v1 |
2015-04-09 | Spectropolarimetrically accurate magnetohydrostatic sunspot model for forward modelling in helioseismology | We present a technique to construct a spectropolarimetrically accurate
magneto-hydrostatic model of a large-scale solar magnetic field concentration,
mimicking a sunspot. Using the constructed model we perform a simulation of
acoustic wave propagation, conversion and absorption in the solar interior and
photosphere with the sunspot embedded into it. With the $6173\mathrm{\AA}$
magnetically sensitive photospheric absorption line of neutral iron, we
calculate observable quantities such as continuum intensities, Doppler
velocities, as well as full Stokes vector for the simulation at various
positions at the solar disk, and analyse the influence of non-locality of
radiative transport in the solar photosphere on helioseismic measurements.
Bisector shapes were used to perform multi-height observations. The differences
in acoustic power at different heights within the line formation region at
different positions at the solar disk were simulated and characterised. An
increase in acoustic power in the simulated observations of the sunspot umbra
away from the solar disk centre was confirmed as the slow magneto-acoustic
wave. | 1504.02189v1 |
2015-04-09 | The origin of nematic order in FeSe | The origin of the 90 K nematic transition in the chalcogenide FeSe, which
displays no magnetic order down to T=0, remains a major puzzle for a unifying
theory for the iron-based superconductors. We analyze this problem in light of
recent experimental data which reveal very small Fermi pockets in this
material. We show that the smallness of the Fermi energy leads to a
near-degeneracy between magnetic fluctuations and fluctuations in the
charge-current density-wave channel. While the two fluctuation modes cooperate
to promote the same preemptive Ising-nematic order, they compete for primary
order. We argue that this explains why in FeSe the nematic order emerges when
the magnetic correlation length is smaller than in other Fe-based materials,
and why no magnetism is observed. We discuss how pressure lifts this
near-degeneracy, resulting in a non-monotonic dependence of the nematic
transition with pressure, in agreement with experiments. | 1504.02315v1 |
2015-04-11 | Magnetic Nanorods Confined in a Lamellar Lyotropic Phase | The dilute lamellar phase of the nonionic surfactant C$_{12}$EO$_5$ was doped
with goethite (iron oxide) nanorods up to a fraction of 5 vol. %. The
interaction between the inclusions and the host phase was studied by polarized
optical microscopy (with or without an applied magnetic field) and by
small-angle X-ray scattering. We find that, when the orientation of the
nanorods is modified using the magnetic field, the texture of the lamellar
phase changes accordingly; one can thus induce a homeotropic-planar
reorientation transition. On the other hand, the lamellar phase induces an
attractive interaction between the nanorods. In more concentrated lamellar
phases (under stronger confinement) the particles form aggregates. This
behavior is not encountered for a similar system doped with spherical
particles, emphasizing the role of particle shape in the interaction between
doping particles and the host phase. | 1504.02879v1 |
2015-04-15 | Single Vortex Pinning and Penetration Depth in Superconducting NdFeAsO$_{1-x}$F$_x$ | We use a magnetic force microscope (MFM) to investigate single vortex pinning
and penetration depth in NdFeAsO$_{1-x}$F$_x$, one of the highest-$T_c$
iron-based superconductors. In fields up to 20 Gauss, we observe a disordered
vortex arrangement, implying that the pinning forces are stronger than the
vortex-vortex interactions. We measure the typical force to depin a single
vortex, $F_{\mathrm{depin}} \simeq 4.5$ pN, corresponding to a critical current
up to $J_c \simeq 7 \times 10^5$ A/cm$^2$. Furthermore, our MFM measurements
allow the first local and absolute determination of the superconducting
in-plane penetration depth in NdFeAsO$_{1-x}$F$_x$, $\lambda_{ab}=320 \pm 60$
nm, which is larger than previous bulk measurements. | 1504.04045v1 |
2015-04-16 | Origin of the Higher-$T_\rm{c}$ Phase in the K$_x$Fe$_{2-y}$Se$_2$ System | Single crystals of K$_x$Fe$_{2-y}$Se$_2$ are prepared by quenching at various
temperatures. The crystals obtained at higher quenching temperatures have a
surface morphology with mesh-like texture. They show a sharp superconducting
transition at $T_\rm{c}$ ~32 K with a large shielding volume fraction. On the
other hand, the crystals prepared without quenching show an onset
superconducting transition at ~44 K and a zero resistivity around ~33 K, and
they possess island-like regions on the surface with a larger amount of Fe
incorporation. In-situ high-temperature single crystal X-ray diffraction
measurements tell us the Fe-vacancy ordered phase is generated at a temperature
region around 270 {\deg}C via iron diffusion. The creation of this Fe-vacancy
ordered phase may become a driving force of the growth of the higher $T_\rm{c}$
phase. The superconductivity at ~44 K is attributed to a metallic phase with no
Fe-vacancy. | 1504.04197v2 |
2015-04-17 | Structural analysis of superconducting dipole prototype for HIAF | The High Intensity Heavy-Ion Accelerator Facility is a new project in the
Institute of Modern Physics. The dipole magnets of all rings are conceived as
fast cycled superconducting magnet with high magnetic field and large gap, the
warm iron and superconducting coil structure (superferric) is adopted. The
reasonable structure design of coil and cryostat is very important for reliable
operation. Based on the finite element software ANSYS, the mechanical analysis
of electromagnetic stress, the thermal stress in the cooling down and the
stress in the pumping are showed in detail. According to the analysis result,
the supporter structure is the key problem of coil system. With reasonable
support's structure design, the stress and the deformation of coil structure
can be reduced effectively, which ensure the stable operation of
superconducting coil system. | 1504.04468v1 |
2015-04-21 | Fast Luminous Blue Transients from Newborn Black Holes | Newborn black holes in collapsing massive stars can be accompanied by a
fallback disk. The accretion rate is typically super-Eddington and strong disk
outflows are expected. Such outflows could be directly observed in some failed
explosions of compact (blue supergiants or Wolf-Rayet stars) progenitors, and
may be more common than long-duration gamma-ray bursts. Using an analytical
model, we show that the fallback disk outflows produce blue UV-optical
transients with a peak bolometric luminosity of ~10^(42-43) erg s^-1 (peak
R-band absolute AB magnitudes of -16 to -18) and an emission duration of ~ a
few to ~ 10 days. The spectra are likely dominated intermediate mass elements,
but will lack much radioactive nuclei and iron-group elements. The above
properties are broadly consistent with some of the rapid blue transients
detected by Pan-STARRS and PTF. This scenario can be distinguished from
alternative models using radio observations within a few years after the
optical peak. | 1504.05582v1 |
2015-04-26 | Amplitude modes and dynamic coexistence of competing orders in multicomponent superconductors | We study the nonequilibrium dynamics of an electronic model with competing
spin-density-wave and unconventional superconductivity in the context of iron
pnictides. Focusing on the collisionless regime, we find that magnetic and
superconducting order parameters may coexist dynamically after a sudden quench,
even though the equilibrium thermodynamic state supports only one order
parameter. We consider various initial conditions concomitant with the phase
diagram and in a certain regime identify different oscillatory amplitude modes
with incommensurate frequencies for magnetic and superconducting responses. At
the technical level we solve the equations of motion for the electronic Green's
functions and self-consistency conditions by reducing the problem to a closed
set of Bloch equations in a pseudospin representation. For certain quench
scenarios the nonadiabatic dynamics of the pairing amplitude is completely
integrable and in principle can be found exactly. | 1504.06874v2 |
2015-04-28 | Single Photon Level Study of Microwave Properties of Lithium Niobate at milli-Kelvin Temperatures | Properties of doped and natural impurities in Lithium Niobate single crystals
are studied using the Whispering Gallery Mode method at low temperatures as a
function of magnetic field. The study reveals considerable coupling of
microwave photon modes to the Fe$^{3+}$ spin ensemble in iron-doped and
non-doped crystals. The $S=5/2$ structure of the Fe$^{3+}$ impurities
demonstrate Zero Field Splittings of $11.21$ and $20.96$ GHz, significant
asymmetry of the Zeeman lines and additional lines with anomalous
$\text{g}$-factors of $1.37$ and $3.95$. Also, interactions between different
transitions of the Fe$^{3+}$ ion is observed. An additional ion impurity
ensemble with a splitting of about $1.7$ GHz is shown to couple to the
dominating Fe$^{3+}$ spins and the effect on $Q$-factors of microwave photon
modes due to the Fe$^{3+}$ ion ensemble is also demonstrated. Measurements down
to less than one photon level are made with a loss tangent of order $10^{-5}$
determined. | 1504.07352v1 |
2015-04-28 | Phase diagram of the isovalent phosphorous-substituted 122-type iron pnictides | Recent experiments demonstrated that isovalent doping system gives the
similar phase diagram as the heterovalent doped cases. For example, with the
phosphorous (P)-doping, the magnetic order in
BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ compound is first suppressed, then the
superconductivity dome emerges to an extended doping region but eventually it
disappears at large $x$. With the help of a minimal two-orbital model for both
BaFe$_{2}$As$_{2}$ and BaFe$_{2}$P$_{2}$, together with the self-consistent
lattice Bogoliubov-de Gennes (BdG) equation, we calculate the phase diagram
against the P content $x$ in which the doped isovalent P-atoms are treated as
impurities. We show that our numerical results can qualitatively compare with
the experimental measurements. | 1504.07539v1 |
2015-04-30 | Ab initio calculation of Spin-Polarized Low-Energy Electron Diffraction Pattern for the systems Fe(001) and Fe(001)-p(1x1)-O | The construction of a multi-channel vector spin polarimeter requires the
development of a new detector type, which works as a spin polarizing mirror
with high reflectivity and asymmetry properties to guarantee for a high figure
of merit. Technical realizations are found by spin polarized electron
scattering from a surface at low energies. A very promising candidate for such
a detector suitable material consists of an oxygen passivated iron surface, as
for example a Fe(001)-p(1x1)-O surface. We investigate in detail the electronic
structure of this adsorbate system and calculate the corresponding
spin-polarized low-energy electron scattering. Our theoretical study is based
on the fully relativistic SPRKKR-method in the framework of density functional
theory. Furthermore, we use the local spin-density approximation in combination
with dynamical mean field theory to determine the electronic structure of
Fe(001)-p(1x1)-O and demonstrate that a significant impact of correlation
effects occurs in the calculated figure of merit. | 1504.08198v2 |
2015-04-30 | TITUS: An Intermediate Distance Detector for the Hyper-Kamiokande Neutrino Beam | The Tokai Intermediate Tank with Unoscillated Spectrum (TITUS) detector is a
proposed addition to the Hyper-Kamiokande (HK) experiment located approximately
2 km from the J-PARC neutrino beam. The design consists of a 2 kton Gadolinium
(Gd) doped water Cherenkov detector, surrounded by a magnetized iron detector
designed to range-out muons. The target material and location are chosen so
that the neutrino interactions and beam spectrum at TITUS will match those of
HK. Including a 0.1% Gd concentration allows for neutrino/antineutrino
discrimination via neutron tagging. The primary goal of TITUS is to directly
measure the neutrino flux and make cross-section measurements that reduce the
systematic uncertainty of the long-baseline oscillation physics program at HK
and enhance its sensitivity to CP violation. TITUS can also be used for physics
unrelated to the J-PARC beam, functioning as an independent detector for
supernova neutrino bursts and measuring the neutron rate to improve HK proton
decay searches. | 1504.08272v2 |
2015-05-05 | Formation depths of Fraunhofer lines | We have summed up our investigations performed in 1970--1993. The main task
of this paper is clearly to show processes of formation of spectral lines as
well as their distinction by validity and by location. For 503 photospheric
lines of various chemical elements in the wavelength range 300--1000 nm we list
in Table the average formation depths of the line depression and the line
emission for the line centre and on the half-width of the line, the average
formation depths of the continuum emission as well as the effective widths of
the layer of the line depression formation. Dependence of average depths of
line depression formation on excitation potential, equivalent widths, and
central line depth are demonstrated by iron lines. | 1505.00975v1 |
2015-05-06 | Tetragonal magnetic phase in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ from x-ray and neutron diffraction | Combined neutron and x-ray diffraction experiments demonstrate the formation
of a low-temperature minority tetragonal phase in
Ba$_{0.76}$K$_{0.24}$Fe$_2$As$_2$ in addition to the majority magnetic,
orthorhombic phase. A coincident enhancement in the magnetic ($\frac{1}{2}$
$\frac{1}{2}$ 1) peaks shows that this minority phase is of the same type that
was observed in Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ ($0.24 \leq x \leq 0.28$), in
which the magnetic moments reorient along the $c$-axis. This is evidence that
the tetragonal magnetic phase is a universal feature of the hole-doped
iron-based superconductors. | 1505.01433v1 |
2015-05-11 | Two-dimensional Cs-vacancy superstructure in iron-based superconductor $Cs_{0.8}Fe_{1.6}Se_2$ | Single crystal neutron diffraction is combined with synchrotron x-ray
scattering to identify the different superlattice phases present in
$Cs_{0.8}Fe_{1.6}Se_2$. A combination of single crystal refinements and first
principles modelling are used to provide structural solutions for the
$\sqrt{5}\times\sqrt{5}$ and $\sqrt{2}\times\sqrt{2}$ superlattice phases. The
$\sqrt{5}\times\sqrt{5}$ superlattice structure is predominantly composed of
ordered Fe vacancies and Fe distortions, whereas the $\sqrt{2}\times\sqrt{2}$
superlattice is composed of ordered Cs vacancies. The Cs vacancies only order
within the plane, causing Bragg rods in reciprocal space. By mapping x-ray
diffraction measurements with narrow spatial resolution over the surface of the
sample, the structural domain pattern was determined, consistent with the
notion of a majority antiferromagnetic $\sqrt{5}\times\sqrt{5}$ phase and a
superconducting $\sqrt{2}\times\sqrt{2}$ phase. | 1505.02527v1 |
2015-05-11 | Interplay of structure, magnetism, and superconductivity in Se substituted iron telluride with excess Fe | We investigated the evolution of the temperature-composition phase diagram of
Fe$_{1+y}$Te upon Se substitution. In particular, the effect of Se substitution
on the two-step, coupled magneto-structural transition in Fe$_{1+y}$Te single
crystals is investigated. To this end, the nominal Fe excess was kept at $y$ =
0.12. For low Se concentrations, the two magneto-structural transitions
displayed a tendency to merge. In spite of the high Fe-content,
superconductivity emerges for Se concentrations, $x \geq$ 0.1. We present a
temperature-composition phase diagram to demonstrate the interplay of
structure, magnetism, and superconductivity in these ternary Fe chalcogenides. | 1505.02542v1 |
2015-05-14 | Nucleosynthesis in a Primordial Supernova: Carbon and Oxygen Abundances in SMSS J031300.36-670839.31 | SMSS J031300.36-670839.3 (hereafter SM0313-6708) is a sub-giant halo star,
with no detectable Fe lines and large overabundances of C and Mg relative to
Ca. We obtained VLT-UVES spectra extending to 3060 Angstroms showing strong OH
A-X band lines enabling an oxygen abundance to be derived. The OH A-X band
lines in SM0313-6708 are much stronger than the CH C-X band lines. Spectrum
synthesis fits indicate an [O/C] ratio of 0.02 +- 0.175. Our high S/N UVES data
also enabled us to lower the Fe abundance limit to [Fe/H]{3D},NLTE < -7.52 (3
sigma). These data support our previous suggestion that the star formed from
the iron-poor ejecta of a single massive star Population III supernova. | 1505.03756v2 |
2015-05-16 | Glide reflection symmetry, Brillouin zone folding and superconducting pairing for the $P4/nmm$ space group | Motivated by the studies of the superconducting pairing states in the
iron-based superconductors, we analyze the effects of Brillouin zone folding
procedure from a space group symmetry perspective for a general class of
materials with the $P4/nmm$ space group. The Brillouin zone folding amounts to
working with an effective one-Fe unit cell, instead of the crystallographic
two-Fe unit cell. We show that the folding procedure can be justified by the
validity of a glide reflection symmetry throughout the crystallographic
Brillouin zone and by the existence of a minimal double degeneracy along the
edges of the latter. We also demonstrate how the folding procedure fails when a
local spin-orbit coupling is included although the latter does not break any of
the space group symmetries of the bare Hamiltonian. In light of these general
symmetry considerations, we further discuss the implications of the glide
reflection symmetry for the superconducting pairing in an effective
multi-orbital $t-J_{1}-J_{2}$ model. We find that the $P4/nmm$ space group
symmetry allows only pairing states with even parity under the glide reflection
and zero total momentum. | 1505.04227v2 |
2015-05-17 | Palladium and silver abundances in stars with [Fe/H] > -2.6 | Palladium (Pd) and silver (Ag) are the key elements for probing the weak
component in the rapid neutron-capture process (r-process) of stellar
nucleosynthesis. We performed a detailed analysis of the high-resolution and
high signal-to-noise ratio near-UV spectra from the archive of HIRES on the
Keck telescope, UVES on the VLT, and HDS on the Subaru Telescope, to determine
the Pd and Ag abundances of 95 stars. This sample covers a wide metallicity
range with -2.6 $\lesssim$ [Fe/H] $\lesssim$ +0.1, and most of them are dwarfs.
The plane-parallel LTE MAFAGS-OS model atmosphere was adopted, and the spectral
synthesis method was used to derive the Pd and Ag abundances from Pd I
{\lambda} 3404 {\AA} and Ag I {\lambda} 3280/3382 {\AA} lines. We found that
both elements are enhanced in metal-poor stars, and their ratios to iron show
flat trends at -0.6 < [Fe/H] < +0.1. The abundance ratios of [Ag/H] and [Pd/H]
are well correlated over the whole abundance range. This implies that Pd and Ag
have similar formation mechanisms during the Galactic evolution. | 1505.04356v1 |
2015-05-19 | Electronic nematic susceptibility of iron-based superconductors | We review our recent experimental results on the electronic nematic phase in
electron- and hole-doped BaFe$_2$As$_2$ and FeSe. The nematic susceptibility is
extracted from shear-modulus data (obtained using a three-point-bending method
in a capacitance dilatometer) using Landau theory and is compared to the
nematic susceptibility obtained from elastoresistivity and Raman data. FeSe is
particularly interesting in this context, because of a large nematic, i.e., a
structurally distorted but paramagnetic, region in its phase diagram. Scaling
of the nematic susceptibility with the spin lattice relaxation rate from NMR,
as predicted by the spin-nematic theory, is found in both electron- and
hole-doped BaFe$_2$As$_2$, but not in FeSe. The intricate relationship of the
nematic susceptibility to spin and orbital degrees of freedom is discussed. | 1505.05120v1 |
2015-05-22 | Probing Atomic Structure and Majorana Wavefunctions in Mono-Atomic Fe-chains on Superconducting Pb-Surface | Motivated by the striking promise of quantum computation, Majorana bound
states (MBSs) in solid-state systems have attracted wide attention in recent
years. In particular, the wavefunction localization of MBSs is a key feature
and crucial for their future implementation as qubits. Here, we investigate the
spatial and electronic characteristics of topological superconducting chains of
iron atoms on the surface of Pb(110) by combining scanning tunneling microscopy
(STM) and atomic force microscopy (AFM). We demonstrate that the Fe chains are
mono-atomic, structured in a linear fashion, and exhibit zero-bias conductance
peaks at their ends which we interprete as signature for a Majorana bound
state. Spatially resolved conductance maps of the atomic chains reveal that the
MBSs are well localized at the chain ends (below 25 nm), with two localization
lengths as predicted by theory. Our observation lends strong support to use
MBSs in Fe chains as qubits for quantum computing devices. | 1505.06078v2 |
2015-05-22 | Indications of Negative Evolution for the Sources of the Highest Energy Cosmic Rays | Using recent measurements of the spectrum and chemical composition of the
highest energy cosmic rays, we consider the sources of these particles. We find
that the data strongly prefers models in which the sources of the ultra-high
energy cosmic rays inject predominantly intermediate mass nuclei, with
comparatively few protons or heavy nuclei, such as iron or silicon. If the
number density of sources per comoving volume does not evolve with redshift,
the injected spectrum must be very hard ($\alpha\simeq 1$) in order to fit the
spectrum observed at Earth. Such a hard spectral index would be surprising and
difficult to accommodate theoretically. In contrast, much softer spectral
indices, consistent with the predictions of Fermi acceleration ($\alpha\simeq
2$), are favored in models with negative source evolution. With this
theoretical bias, these observations thus favor models in which the sources of
the highest energy cosmic rays are preferentially located within the
low-redshift universe. | 1505.06090v2 |
2015-05-25 | Bandwidth and Electron Correlation-Tuned Superconductivity in Rb$_{0.8}$Fe$_{2}$(Se$_{1-z}$S$_z$)$_2$ | We present a systematic angle-resolved photoemission spectroscopy study of
the substitution-dependence of the electronic structure of
Rb$_{0.8}$Fe$_{2}$(Se$_{1-z}$S$_z$)$_2$ (z = 0, 0.5, 1), where
superconductivity is continuously suppressed into a metallic phase. Going from
the non-superconducting Rb$_{0.8}$Fe$_{2}$(Se$_{1-z}$S$_z$)$_2$ to
superconducting Rb$_{0.8}$Fe$_{2}$Se$_2$, we observe little change of the Fermi
surface topology, but a reduction of the overall bandwidth by a factor of 2 as
well as an increase of the orbital-dependent renormalization in the $d_{xy}$
orbital. Hence for these heavily electron-doped iron chalcogenides, we have
identified electron correlation as explicitly manifested in the quasiparticle
bandwidth to be the important tuning parameter for superconductivity, and that
moderate correlation is essential to achieving high $T_C$. | 1505.06636v2 |
2015-05-25 | Magnetic Ground State of an Individual Fe2+ Ion in Strained Semiconductor Nanostructure | We investigate spin properties of a Fe2+ dopant, known for having single
nondegenerate ground state in bulk host semiconductor. Due to zero magnetic
moment such a ground state is of little use for spintronics and solotronics. We
show that this well-established picture of Fe2+ spin configuration can be
contradicted by subjecting the Fe2+ ion to sufficiently high strain, e.g.,
resulting from lattice mismatched epitaxial heterostructures. Our analysis
reveals that high strain induces qualitative change in the ion energy spectrum
and results in doubly degenerate ground state with spin projection Sz=+/-2. An
experimental proof of this concept is demonstrated using a new system: an
epitaxial quantum dot containing individual Fe2+ ion. Magnetic character of the
Fe2+ ground state in a CdSe/ZnSe dot is revealed in photoluminescence
experiments by exploiting a coupling between a confined exciton and the single
iron impurity. | 1505.06763v1 |
2015-05-27 | Energy and Direction Estimation of Neutrinos in muonless events at ICAL | In this paper, we study events without identifiable muon tracks in the Iron
Calorimeter detector at the India-based Neutrino Observatory. Such events are
dominated by high energy (E$_\nu>$1 GeV) $\nu_e$ charged current interactions,
which have been studied only in a few experiments so far. The charged
particles, produced in these neutrino interactions, give rise to a set of hits
in the detector. We attempt to reconstruct the energy and the direction of the
neutrino in such events. We study the energy distribution for a given pattern
of hits of these events and find that the Landau distribution provides a good
fit. % The parameters of the fit can be correlated to the energy of the
neutrino. We define two kinematic variables based on the hit distribution and
use them to determine the cosine of the polar angle of the neutrino direction
($\cos \theta$). There is a moderate correlation between these variables and
the $\cos \theta$. These provide us enough information to prepare calibration
charts for looking up the energy and direction of the incident neutrino. | 1505.07295v1 |
2015-05-28 | Properties of AGN coronae in the NuSTAR era | The focussing optics of NuSTAR have enabled high signal-to-noise spectra to
be obtained from many X-ray bright Active Galactic Nuclei (AGN) and Galactic
Black Hole Binaries (BHB). Spectral modelling then allows robust
characterization of the spectral index and upper energy cutoff of the coronal
power-law continuum, after accounting for reflection and absorption effects.
Spectral-timing studies, such as reverberation and broad iron line fitting, of
these sources yield coronal sizes, often showing them to be small and in the
range of 3 to 10 gravitational radii in size. Our results indicate that coronae
are hot and radiatively compact, lying close to the boundary of the region in
the compactness - temperature diagram which is forbidden due to runaway pair
production. The coincidence suggests that pair production and annihilation are
essential ingredients in the coronae of AGN and BHB and that they control the
shape of the observed spectra. | 1505.07603v1 |
2015-05-28 | Se content $x$ dependence of electron correlation strength in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ | The iron chalcogenide Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ on the Te-rich side is
known to exhibit the strongest electron correlations among the Fe-based
superconductors, and is non-superconducting for $x$ < 0.1. In order to
understand the origin of such behaviors, we have performed ARPES studies of
Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ ($x$ = 0, 0.1, 0.2, and 0.4). The obtained mass
renormalization factors for different energy bands are qualitatively consistent
with DFT + DMFT calculations. Our results provide evidence for strong orbital
dependence of mass renormalization, and systematic data which help us to
resolve inconsistencies with other experimental data. The unusually strong
orbital dependence of mass renormalization in Te-rich
Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ arises from the dominant contribution to the Fermi
surface of the $d_{xy}$ band, which is the most strongly correlated and may
contribute to the suppression of superconductivity. | 1505.07637v1 |
2015-05-30 | Large increase of the anisotropy factor in the overdoped region of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ as probed by fluctuation spectroscopy | We study the diamagnetism induced by thermal fluctuations above the
superconducting transition of the iron pnictide Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$
with different doping levels. The measurements are performed with magnetic
fields up to 7 T applied in the two main crystal directions. These data provide
double information: first, they confirm at a quantitative level the
applicability to these materials of a 3D-anisotropic Ginzburg-Landau approach
valid in the finite field regime. Then, they allow to determine the
doping-level dependence of the in-plane coherence length and of the
superconducting anisotropy factor, $\gamma$. Our results provide a stringent
confirmation of the large increase of $\gamma$ with the doping level, as
recently proposed from magnetoresistivity measurements. The implications of the
applicability of the model used to a multiband superconductor are discussed. | 1506.00170v1 |
2015-06-04 | Exploring the feasibility of Fe(Se,Te) conductors by ex-situ Powder-in-Tube method | In this work, the feasibility condition of Powder-In-Tube (PIT) processed
wires of Fe(Se,Te) superconductor has been investigated. We faced several
technical issues that are extensively described and discussed. In particular,
we tested different metals and alloys as external sheaths (Cu, Ag, Nb, Ta, Ni,
Fe, cupronickel, brass) concluding that the only sheath that does not affect
substantially the Fe(Se,Te) phase is Fe. On the other hand, Fe sheath
introduces excess iron in the Fe(Se,Te) phase, which affects the
superconducting properties; we investigated the effects of the thermal
treatments and of the powder composition in order to avoid it. The maximum Jc
value obtained in our samples is 4*10^2 A/cm2, comparable to other published
values of PIT conductors of the 11 family. We conclude that the fabrication of
Fe(Se,Te) wires by PIT method is quite challenging and other approaches should
be developed. | 1506.01630v1 |
2015-06-09 | Optical observations of a SN 2002cx-like peculiar supernova SN 2013en in UGC 11369 | We present optical observations of a SN 2002cx-like supernova SN 2013en in
UGC 11369, spanning from a phase near maximum light (t= +1 d) to t= +60 d with
respect to the R-band maximum. Adopting a distance modulus of mu=34.11 +/- 0.15
mag and a total extinction (host galaxy+Milky Way) of $A_V \sim1.5$ mag, we
found that SN 2013en peaked at $M(R)\sim -18.6$ mag, which is underluminous
compared to the normal SNe Ia. The near maximum spectra show lines of Si II, Fe
II, Fe III, Cr II, Ca II and other intermediate-mass and iron group elements
which all have lower expansion velocities (i.e., ~ 6000 km/s). The photometric
and spectroscopic evolution of SN 2013en is remarkably similar to those of SN
2002cx and SN 2005hk, suggesting that they are likely to be generated from a
similar progenitor scenario or explosion mechanism. | 1506.02845v1 |
2015-06-09 | Synthesis, Crystal Structure and Magnetism of Eu3Sc2O5Fe2As2 | The iron arsenide Eu3Fe2O5Fe2As2 was synthesized at 1173-1373 K in a
resistance furnace and characterized by X-ray powder diffraction with Rietveld
analysis: Sr3Fe2O5Cu2S2-type, I4/mmm, a = 406.40(1) pm, c = 2646.9(1) pm.
Layers of edge-sharing FeAs4/4 tetrahedra are separated by perovskite-like
oxide blocks. No structural transition occurs in the temperature range from 10
to 300 K. Magnetic measurements have revealed Curie-Weiss behavior with an
effective magnetic moment of 7.79 muB per europium atom in agreement with the
theoretical value of 7.94 muB for Eu2+. A drop in the magnetic susceptibility
at 5 K indicates possible antiferromagnetic ordering. 151Eu and 57Fe
M\"ossbauer spectroscopic measurements have confirmed a beginning cooperative
magnetic phenomenon by showing significantly broadened spectra at 4.8 K
compared to those at 78 K. | 1506.02925v1 |
2015-06-10 | Interaction of discrete breathers with primary lattice defects in bcc Fe | The interaction of discrete breathers with the primary lattice defects in
transition metals such as vacancy, dislocation, and surface is analyzed on the
example of bcc iron employing atomistic simulations. Scattering of discrete
breathers on the lattice defects induces localized atomic excitations, with
intensity and relaxation time depending on the defect structure and breather
kinetic energy. The dissipation of the intrinsic breather energy due to the
scattering is computed and analyzed. It is concluded that the
breather-to-defect energy transfer may stipulate the activation of the lattice
defects causing unexpected athermal effects such as enhanced mass transfer or
electroplasticity, already experimentally reported but so far not fully
understood at the atomic-scale level. | 1506.03507v1 |
2015-06-12 | Near-field microwave imaging of inhomogeneous K$_x$Fe$_y$Se$_2$: separation of topographic and electric features | It is important for modern scanning microwave microscopes to overcome the
effect of the surface roughness. Here, we report microwave conductivity imaging
of the phase-separated iron chalcogenide K$_x$Fe$_y$Se$_2$ ($x=0.8$,
$y=1.6$-$2$), in which electric conductivity-induced contrast is distinguished
from topography-induced contrast using a combination of a scanning tunneling
microscope and a scanning microwave microscope (STM-SMM). We observed the
characteristic modulation of the local electric property that originates from
the mesoscopic phase separation of the metallic and semiconducting phases in
two different scanning modes: constant current (CC) mode and constant $Q$ (CQ)
mode. In particular, CQ scanning is useful because we obtain a qualitative
image in which the topographic contrast is largely eliminated without
degradation of the spatial resolution. | 1506.03911v1 |
2015-06-17 | Giant enhancement in critical current density, up to a hundredfold, in superconducting NaFe0.97Co0.03As single crystals under hydrostatic pressure | Tremendous efforts towards improvement in the critical current density (Jc)
of iron based superconductors (FeSCs), especially at relatively low
temperatures and magnetic fields, have been made so far through different
methods, resulting in real progress. Jc at high temperatures in high fields
still needs to be further improved, however, in order to meet the requirements
of practical applications. Here, we demonstrate a simple approach to achieve
this. Hydrostatic pressure can significantly enhance Jc in NaFe0.97Co0.03As
single crystals by at least tenfold at low field and more than a hundredfold at
high fields. Significant enhancement in the in-field performance of
NaFe0.97Co0.03As single crystal in terms of pinning force density (Fp) is found
at high pressures. At high fields, the Fp is over 20 and 80 times higher than
under ambient pressure at12K and 14K, respectively, at P=1GPa. We believe that
the Co-doped NaFeAs compounds are very exciting and deserve to be more
intensively investigated. Finally, it is worthwhile to say that by using
hydrostatic pressure, we can achieve more milestones in terms of high Jc values
in different superconductors. | 1506.05183v1 |
2015-06-17 | Local spin-density-wave order inside vortex cores in multiband superconductors | Coexistence of antiferromagnetic order with superconductivity in many
families of newly discovered iron-based superconductors has renewed interest to
this old problem. Due to competition between the two types of order, one can
expect appearance of the antiferromagnetism inside the cores of the vortices
generated by the external magnetic field. The structure of a vortex in type II
superconductors holds significant importance from the theoretical and the
application points of view. Here we consider the internal vortex structure in a
two-band s$_\pm$ superconductor near a spin-density-wave instability. We treat
the problem in a completely self-consistent manner within the quasiclassical
Eilenberger formalism. We study the structure of the s$_\pm$ superconducting
order and magnetic field-induced spin-density-wave order near an isolated
vortex. We examine the effect of this spin-density-wave state inside the vortex
cores on the local density of states. | 1506.05440v2 |
2015-06-26 | Correlated electron behavior of metalorganic molecules: insights from density functional theory combined with many-body effects using exact diagonalization | A proper theoretical description of electronic structure of the 3d orbitals
in the metal centers of functional metalorganics is a challenging problem. In
this letter, we apply density functional theory and an exact diagonalization
method in a many body approach to study the ground state electronic
configuration of an iron porphyrin (FeP) molecule. Our study reveals that
dynamical correlation effects are important, and FeP is a potential candidate
for realizing a spin crossover due to a subtle balance of crystal field
effects, on-site Coulomb repulsion and hybridization between the Fe d-orbitals
and ligand N p-states. The mechanism of switching between two close lying
electronic configurations of Fe-d orbitals is shown. We discuss the generality
of the suggested approach and the possibility to properly describe the
electronic structure and related low energy physics of the whole class of
correlated metal centered organometallic molecules. | 1506.07973v1 |
2015-06-29 | Neutrino-driven explosions of ultra-stripped type Ic supernovae generating binary neutron stars | We study explosion characteristics of ultra-stripped supernovae (SNe), which
are candidates of SNe generating binary neutron stars (NSs). As a first step,
we perform stellar evolutionary simulations of bare carbon-oxygen cores of mass
from 1.45 to 2.0 $M_\odot$ until the iron cores become unstable and start
collapsing. We then perform axisymmetric hydrodynamics simulations with
spectral neutrino transport using these stellar evolution outcomes as initial
conditions. All models exhibit successful explosions driven by neutrino
heating. The diagnostic explosion energy, ejecta mass, Ni mass, and NS mass are
typically $\sim 10^{50}$ erg, $\sim 0.1 M_\odot$, $\sim 0.01M_\odot$, and
$\approx 1.3 M_\odot$, which are compatible with observations of
rapidly-evolving and luminous transient such as SN 2005ek. We also find that
the ultra-stripped SN is a candidate for producing the secondary low-mass NS in
the observed compact binary NSs like PSR J0737-3039. | 1506.08827v2 |
2015-07-01 | Onset of the Meissner effect at 65 K in FeSe thin film grown on Nb doped SrTiO3 substrate | We report the Meissner effect studies on an FeSe thin film grown on Nb doped
SrTiO3 substrate by molecular beam epitaxy. Two-coil mutual inductance
measurement clearly demonstrates the onset of diamagnetic screening at 65 K,
which is consistent with the gap opening temperature determined by previous
angle resolved photoemission spectroscopy results. The applied magnetic field
causes a broadening of the superconducting transition near the onset
temperature, which is the typical behavior for quasi-two-dimensional
superconductors. Our results provide direct evidence that FeSe thin film grown
on Nb doped SrTiO3 substrate has an onset TC ~ 65 K, which is the highest among
all iron based superconductors discovered so far. | 1507.00129v2 |
2015-07-06 | Isotope shifts and hyperfine structure of the laser cooling Fe I 358-nm line | We report on the measurement of the isotope shifts of the $3d^74s \,\, a \,
{}^5\!F_5 - 3d^74p \,\, z \, {}^5\!G^o_6$ Fe~I line at 358~nm between all four
stable isotopes ${}^{54}$Fe, ${}^{56}$Fe, ${}^{57}$Fe and ${}^{58}$Fe, as well
as the hyperfine structure of that line for ${}^{57}$Fe, the only stable
isotope having a nonzero nuclear spin. This line is of primary importance for
laser cooling applications. In addition, an experimental value of the field and
specific mass shift coefficients of the transition is reported as well as the
hyperfine structure magnetic dipole coupling constant $A$ of the transition
excited state in $^{57}$Fe, namely $A(3d^74p \,\, z \, {}^5\!G^o_6)=31.241(48)$
MHz. The measurements were carried out by means of laser-induced fluorescence
spectroscopy performed on an isotope-enriched iron atomic beam. All measured
frequency shifts are reported with uncertainties below the third percent level. | 1507.01498v1 |
2015-07-07 | New Superconductivity Dome in LaFeAsO$_{1-x}$F$_{x}$ Accompanied by Structural Transition | High temperature superconductivity is often found in the vicinity of
antiferromagnetism. This is also true in LaFeAsO$_{1-x}$F$_{x}$ ($x \leq$ 0.2)
and many other iron-based superconductors, which leads to proposals that
superconductivity is mediated by fluctuations associated with the nearby
magnetism. Here we report the discovery of a new superconductivity dome without
low-energy magnetic fluctuations in LaFeAsO$_{1-x}$F$_{x}$ with 0.25$\leq x
\leq$0.75, where the maximal critical temperature $T_c$ at $x_{opt}$ =
0.5$\sim$0.55 is even higher than that at $x \leq$ 0.2. By nuclear magnetic
resonance and Transmission Electron Microscopy, we show that a C4 rotation
symmetry-breaking structural transition takes place for $x>$ 0.5 above $T_c$.
Our results point to a new paradigm of high temperature superconductivity. | 1507.01750v2 |
2015-07-08 | Towards quantifying the role of exact exchange in predictions of transition metal complex properties | We estimate the prediction sensitivity with respect to Hartree-Fock exchange
in approximate density functionals for representative Fe(II) and Fe(III)
octahedral complexes. Based on the observation that the range of parameters
spanned by the most widely-employed functionals is relatively narrow, we
compute electronic structure property and spin-state orderings across a
relatively broad range of Hartree-Fock exchange (0-50%) ratios. For the entire
range considered, we consistently observe linear relationships between
spin-state ordering that differ only based on the element of the direct ligand
and thus may be broadly employed as measures of functional sensitivity in
predictions of organometallic compounds. The role Hartree-Fock exchange in
hybrid functionals is often assumed to play is to correct self-interaction
error-driven electron delocalization (e.g. from transition metal centers to
neighboring ligands). Surprisingly, we instead observe that increasing
Hartree-Fock exchange reduces charge on iron centers, corresponding to
effective delocalization of charge to ligands, thus challenging notions of the
role of Hartree-Fock exchange in shifting predictions of spin-state ordering. | 1507.02261v1 |
2015-07-11 | End states and subgap structure in proximity-coupled chains of magnetic adatoms | A recent experiment [Nadj-Perge et al., Science 346, 602 (2014)] provides
evidence for Majorana zero modes in iron (Fe) chains on the superconducting
Pb(110) surface. Here, we study this system by scanning tunneling microscopy
using superconducting tips. This high-resolution technique resolves a rich
subgap structure, including zero-energy excitations in some chains. We compare
the symmetry properties of the data under voltage reversal against theoretical
expectations and provide evidence that the putative Majorana signature overlaps
with a previously unresolved low-energy resonance. Interpreting the data within
a Majorana framework suggests that the topological gap is significantly smaller
than previously believed. Aided by model calculations, we also analyze
higher-energy features of the subgap spectrum and their relation to high-bias
peaks which we associate with the Fe d-bands. | 1507.03104v2 |
2015-07-13 | Localized High Frequency Electrodynamic Behavior of Optimally-doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ Single Crystal films | Localized high frequency (several GHz) electrodynamic properties of a high
quality epitaxial, single-crystal Iron-Pnicitde Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$
thin film near optimal doping (x=0.08) are measured under a localized and
strong RF magnetic field, created by a near-field microwave microscope. Two
reflection electrodynamic measurements, including linear and the third harmonic
responses, are performed to understand the electromagnetic properties of
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. Our measurement results show that
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ has have a wide superconducting transition width
and may have a multi-gap nature. In addition, based on the $1/T^2$ dependence
of the third harmonics signal at lower temperature,
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ shows the possibility of nodal behavior. | 1507.03318v1 |
2015-07-14 | Observational consequences of turbulent pressure in the envelopes of massive stars | The major mass fraction of the envelope of hot luminous stars is radiatively
stable. However, the partial ionisation of hydrogen, helium and iron gives rise
to extended sub-surface convection zones in all of them. In this work, we
investigate the effect of the pressure induced by the turbulent motion in these
zones based on the mixing length theory, and search for observable
consequences. We find that the turbulent pressure fraction can amount up to ~5%
in OB supergiants, and to ~30% in cooler supergiants. The resulting structural
changes are, however, not significantly affecting the evolutionary tracks
compared to previous calculations. Instead, a comparison of macroturbulent
velocities derived from high quality spectra of OB stars with the turbulent
pressure fraction obtained in corresponding stellar models reveals a strong
correlation of these two quantities. We discuss a possible physical connection,
and conclude that turbulent pressure fluctuations may drive high-order
oscillations, which - as conjectured earlier - manifest themselves as
macroturbulence in the photospheres of hot luminous stars. | 1507.03988v1 |
2015-07-15 | Neutron investigation of the magnetic scattering in an iron-based ferromagnetic superconductor | Neutron diffraction and small angle scattering experiments have been carried
out on the double-isotopic polycrystalline sample (7Li0.82Fe0.18OD)FeSe.
Profile refinements of the diffraction data establish the composition and
reveal an essentially single phase material with lattice parameters of a=
3.7827 {\AA} and c= 9.1277 {\AA} at 4 K, in the ferromagnetic-superconductor
regime, with a bulk superconducting transition of TC = 18 K. Small angle
neutron scattering (SANS) measurements in zero applied field reveal the onset
of ferromagnetic order below TF ~ 12.5 K, with a wave vector and temperature
dependence consistent with an inhomogeneous ferromagnet of spontaneous vortices
or domains in a mixed state. No oscillatory long range ordered magnetic state
is observed. Field dependent measurements establish a separate component of
magnetic scattering from the vortex lattice, which occurs at the expected wave
vector. The temperature dependence of the vortex scattering does not indicate
any contribution from the ferromagnetism, consistent with diffraction data that
indicate that the ordered ferromagnetic moment is quite small. | 1507.04311v1 |
2015-07-16 | Strong quantum effects in an almost classical antiferromagnet on a kagome lattice | Two ubiquitous features of frustrated spin systems stand out: massive
degeneracy of their ground states and flat, or dispersionless, excitation
branches. In real materials, the former is frequently lifted by secondary
interactions or quantum fluctuations, in favor of an ordered or spin-liquid
state, but the latter often survive. We demonstrate that flat modes may
precipitate remarkably strong quantum effects even in the systems that are
otherwise written off as almost entirely classical. The resultant spectral
features should be reminiscent of the quasiparticle breakdown in quantum
systems, only here the effect is strongly amplified by the flatness of
spin-excitation branches, leading to the damping that is not vanishingly small
even at $S\!\gg\!1$. We provide a theoretical analysis of excitation spectrum
of the $S=5/2$ iron-jarosite to illustrate our findings and to suggest further
studies of this and other frustrated spin systems. | 1507.04738v2 |
2015-07-21 | Edge states and local electronic structure around an adsorbed impurity in a topological superconductor | Recently topological superconducting states has attracted a lot of interest.
In this work, we consider a topo- logical superconductor with $Z_2$ topological
mirror order [1] and s$\pm$-wave superconducting pairing symmetry, within a
two-orbital model originally designed for iron-based superconductivity [2]. We
predict the existence of gapless edge states. We also study the local
electronic structure around an adsorbed interstitial magnetic impurity in the
system, and find the existence of low-energy in-gap bound states even with a
weak spin polar- ization on the impurity. We also discuss the relevance of our
results to the recent STM experiment on Fe(Te,Se) compound with adsorbed Fe
impurity [3], for which our density functional calculations show the Fe
impurity is spin polarized. | 1507.05904v1 |