publicationDate,title,abstract,id
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
2021-01-25,Raman Spectroscopy and Aging of the Low-Loss Ferrimagnet Vanadium Tetracyanoethylene,"Vanadium tetracyanoethylene (V[TCNE]$_{x}$, $x\approx 2$) is an organic-based
ferrimagnet with a high magnetic ordering temperature $\mathrm{T_C>600 ~K}$,
low magnetic damping, and growth compatibility with a wide variety of
substrates. However, similar to other organic-based materials, it is sensitive
to air. Although encapsulation of V[TCNE]$_{x}$ with glass and epoxy extends
the film lifetime from an hour to a few weeks, what is limiting its lifetime
remains poorly understood. Here we characterize encapsulated V[TCNE]$_{x}$
films using confocal microscopy, Raman spectroscopy, ferromagnetic resonance
and SQUID magnetometry. We identify the relevant features in the Raman spectra
in agreement with \textit{ab initio} theory, reproducing $\mathrm{C=C,C\equiv
N}$ vibrational modes. We correlate changes in the effective dynamic
magnetization with changes in Raman intensity and in photoluminescence. Based
on changes in Raman spectra, we hypothesize possible structural changes and
aging mechanisms in V[TCNE]$_x$. These findings enable a local optical probe of
V[TCNE]$_{x}$ film quality, which is invaluable in experiments where assessing
film quality with local magnetic characterization is not possible.",2101.10240v1
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
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
2009-07-02,Tuning Molecule-Mediated Spin Coupling in Bottom-Up Fabricated Vanadium-TCNE Nanostructures,"We have fabricated hybrid magnetic complexes from V atoms and
tetracyanoethylene (TCNE) ligands via atomic manipulation with a cryogenic
scanning tunneling microscope. Using tunneling spectroscopy we observe
spin-polarized molecular orbitals as well as Kondo behavior. For complexes
having two V atoms, the Kondo behavior can be quenched for different molecular
arrangements, even as the spin-polarized orbitals remain unchanged. This is
explained by variable spin-spin (i.e., V-V) ferromagnetic coupling through a
single TCNE molecule, as supported by density functional calculations.",0907.0490v1
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
2019-01-10,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
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
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
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
2023-08-07,$\textit{In situ}$ electric-field control of ferromagnetic resonance in the low-loss organic-based ferrimagnet V[TCNE]$_{x\sim 2}$,"We demonstrate indirect electric-field control of ferromagnetic resonance
(FMR) in devices that integrate the low-loss, molecule-based, room-temperature
ferrimagnet vanadium tetracyanoethylene (V[TCNE]$_{x \sim 2}$) mechanically
coupled to PMN-PT piezoelectric transducers. Upon straining the V[TCNE]$_x$
films, the FMR frequency is tuned by more than 6 times the resonant linewidth
with no change in Gilbert damping for samples with $\alpha = 6.5 \times
10^{-5}$. We show this tuning effect is due to a strain-dependent magnetic
anisotropy in the films and find the magnetoelastic coefficient $|\lambda_S|
\sim (1 - 4.4)$ ppm, backed by theoretical predictions from DFT calculations
and magnetoelastic theory. Noting the rapidly expanding application space for
strain-tuned FMR, we define a new metric for magnetostrictive materials,
$\textit{magnetostrictive agility}$, given by the ratio of the magnetoelastic
coefficient to the FMR linewidth. This agility allows for a direct comparison
between magnetostrictive materials in terms of their comparative efficacy for
magnetoelectric applications requiring ultra-low loss magnetic resonance
modulated by strain. With this metric, we show V[TCNE]$_x$ is competitive with
other magnetostrictive materials including YIG and Terfenol-D. This combination
of ultra-narrow linewidth and magnetostriction in a system that can be directly
integrated into functional devices without requiring heterogeneous integration
in a thin-film geometry promises unprecedented functionality for electric-field
tuned microwave devices ranging from low-power, compact filters and circulators
to emerging applications in quantum information science and technology.",2308.03353v1
2005-04-14,Ferromagnetism and Curie temperature of Vanadium-doped Nitrides,"Electronic structures, exchange interaction mechanism between magnetic ions
and Curie temperature of Vanadium - doped Nitrides (AlN, GaN, and InN) are
studied within KKR-LSDA-CPA. It is found that the ferromagnetic super-exchange
interaction mechanism is dominant at low concentrations of Vanadium, but the
anti-ferromagnetic super-exchange interaction appears and reduces the
stabilization of ferromagnetism at sufficiently high concentrations (x > 0.10),
especially for Vanadium-doped AlN and Vanadium- doped GaN. The estimation of
the Curie temperature within the mean field approximation shows the Curie
temperature of Vanadium-doped Nitrides exceeding the room temperature with a
few constituents of Vanadium.",0504355v1
2015-11-24,Comparative computational study of lithium and sodium insertion in van der Waals and covalent tetracyanoethylene (TCNE) -based crystals as promising materials for organic lithium and sodium ion batteries,"We present a comparative ab initio study of Li and Na insertion in molecular
(van der Waals) crystals of TCNE (tetracyanoethylene) as well as in covalent
Li/Na-TCNE crystals. We confirm the structure of previously synthesized
(covalent) Li-TCNE crystal as well as predict the existence of its Na-TCNE
analogue. In the molecular/covalent TCNE crystals, insertion sites are
identified with the binding energy of Li and Na up to 2.7/1.8 and 2.6/1.8 eV
stronger than Li and Na cohesive energy, respectively, in dilute
concentrations. Up to 5.5/2.5 and 3/2 Li and Na atoms per TCNE unit can be
inserted in the molecular/covalent crystals, respectively, while preserving the
structure, with maximum voltages, respectively, 3.5/2.2 and 3.3/2.7 V.
Significantly, up to capacity of 418 mAh g-1 for both Li and Na in the
molecular crystal and 198 mAh g-1 for Li and 177 mAh g-1 for Na in the covalent
crystal, the insertion of Li and Na would not lead to reactions with common
electrolytes. Tetracyanoethylene-based molecular and covalent crystals could
therefore become efficient organic cathode and anode materials for Li and Na
ion batteries.",1511.07649v2
2020-09-25,Current progress in vanadium oxide nanostructures and its composites as supercapacitor electrodes,"In recent years, vanadium oxides have gained immense attention in the field
of energy storage devices due to their low-cost, layered structure and
multi-valency despite their limited electrical conductivity and lower
structural stability. In this brief review, we have tried to focus on
electrochemical properties of the stoichiometric vanadium oxides along with
VO_x composites. The morphology engineering, doping with heteroatom and
formation of composites with carbon-based materials and/or conducting polymers
in enhancing the supercapacitive performances of the vanadium oxides are
discussed in details. Finally, the potentiality and challenges of vanadium
oxides nanocomposites for supercapacitor applications are discussed.",2009.12034v1
2006-06-13,A Mechanism for Photoinduced Effects In Tetracyanoethylene-Based Organic Magnets,"The photoinduced magnetism in manganese-tetracyanoethylene (Mn-TCNE)
molecule-based magnets is ascribed to charge-transfer excitations from
manganese to TCNE. Charge-transfer energies are calculated using Density
Functional Theory; photoinduced magnetization is described using a model
Hamiltonian based on a double-exchange mechanism. Photoexciting electrons from
the manganese core spin into the lowest unoccupied orbital of TCNE with photon
energies around 3 eV increases the magnetization through a reduction of the
canting angle of the manganese core spins for an average electron density on
TCNE less than one. When photoexciting with a smaller energy, divalent TCNE
molecules are formed. The delocalization of the excited electron causes a local
spin flip of a manganese core spin.",0606343v1
2008-10-27,Strongly reshaped organic-metal interfaces: Tetracyanoethylene on Cu(100),"The interaction of the strong electron-acceptor tetracyanoethylene (TCNE)
with the Cu(100) surface has been studied with scanning tunneling microscopy
experiments and first-principles density functional theory calculations. We
compare two different adsorption models with the experimental results and show
that the molecular self-assembly is caused by a strong structural modification
of the Cu(100) surface rather than the formation of a coordination network by
diffusing Cu adatoms. Surface atoms become highly buckled and the chemisorption
of TCNE is accompanied by a partial charge-transfer.",0810.4862v2
2015-02-12,Lithium and sodium storage on tetracyanoethylene (TCNE) and TCNE-(doped)-graphene complexes: a computational study,"Li and Na attachment to free tetracyanoethylene (TCNE) molecules and TCNE
adsorbed on doped graphene is studied using density functional theory. While
TCNE is adsorbed only weakly on ideal graphene, we identified a configuration
in which TCNE is chemisorbed on Al-doped graphene via molecule C atom and a
surface oxygen atom. Up to four (five) Li and Na atoms can be stored on both
free (adsorbed) TCNE with binding energies stronger than cohesive energies of
the Li and Na metals. When storing up to three atoms per molecule, it should be
possible to avoid reduction of common battery electrolytes. TCNE immobilized on
a conducting graphene-based substrate could therefore become an efficient anode
material for organic Li and Na ion batteries. Importantly, there is no
significant difference either in specific capacity (per unit mass of material
excluding Li/Na) nor in voltage between Li and Na storage, which makes this
kind of approach very promising for post-Li storage in general.",1502.03533v1
2016-06-21,Adsorption and STM imaging of tetracyanoethylene on Ag(001): An ab-initio study,"We investigate the adsorption of a single tetracyanoethylene (TCNE) molecule
on the silver (001) surface. Adsorption structures, electronic properties, and
scanning tunneling microscopy (STM) images are calculated within
density-functional theory. Adsorption occurs most favorably in on-top
configuration, with the C=C double bond directly above a silver atom and the
four N atoms bound to four neighboring Ag atoms. The lowest unoccupied
molecular orbital of TCNE becomes occupied due to electron transfer from the
substrate. This state dominates the electronic spectrum and the STM image at
moderately negative bias. We discuss and employ a spatial extrapolation
technique for the calculation of STM and scanning tunneling spectroscopy (STS)
images. Our calculated images are in good agreement with experimental data.",1606.06435v1
2023-07-27,A comprehensive guide for measuring total vanadium concentration and state of charge of vanadium electrolytes using UV-Visible spectroscopy,"This paper presents an exhaustive how-to guide on measuring the total
vanadium concentration and state of charge of vanadium electrolytes using
UV-Visible spectroscopy. The study is provided with an open-access database
(https://github.com/AngeAM/SOC_Vanadium_Spectra_2023.git) that supports the
methods and procedures and facilitates access to the calibration data. The
study covers the three types of electrolyte solutions relevant to vanadium
redox flow batteries, namely the anolyte $V^{II}/V^{III}$, the catholyte
$V^{IV}V^V$, and the $V^{III}/V^{IV}$ commercial electrolyte, meant to be
preconditioned to either $V^{III}$ or $V^{IV}$ before battery operation.
Analytical expressions to calculate the concentration of different vanadium
species in the electrolyte solutions are provided based on either empirical
correlations or spectral deconvolution methods. The paper also examines the
limitations of the measurement technique and provides insightful
recommendations for future research. The open-access database provided by the
authors is expected to serve as a valuable repository for scholars and
scientists working in the field of vanadium redox flow batteries.",2307.15009v1
2008-12-16,Electrical properties of vanadium oxide subject to hydrogen plasma treatment,"The effect of doping with hydrogen on the electrical properties of vanadium
oxide is studied. For vanadium oxide films, subject to cold hydrogen plasma
treatment, the temperature dependence of resistance with a maximum at T ~ 100 K
is observed. Also, the dependence of the a.c. resistance on frequency is
studied. A strategy for fabrication new superconducting materials is discussed.",0812.2973v1
2009-04-08,Organometallic Vanadium-Borazine Systems: Efficient One- Dimensional Half-Metallic Spin Filters,"Using density functional theory, we have investigated the electronic and
magnetic properties of finite-size as well as infinitely periodic
organometallic vanadium-borazine systems for their possible applications in
spintronics devices. From our calculations, we find the finite-size
vanadium-borazine systems to be structurally more stable in comparison to their
isoelectronic benzene counterparts. All the finite-size vanadium-borazine
systems are found to be ferromagnetically stabilized, with the infinite
one-dimensional wire exhibiting robust half-metallic behaviour. The finite-size
clusters are also found to exhibit efficient spin filter properties when
coupled to graphene electrodes.",0904.1342v1
2017-03-01,Infrared properties of micromachined vanadium oxide thin films,"This paper discusses questions of synthesizing and pressing vanadium oxides
to create film-forming materials that can be used in producing optical
coatings. Based on the film-forming materials thus created, technological
processes have been developed for fabricating coatings from vanadium dioxide by
two methods of producing thin films: vacuum evaporation and magnetron
sputtering. Questions of using films made from vanadium oxide in optical
instrumentation are considered.",1703.00127v1
2019-10-03,Vanadium trimers randomly aligned along the c-axis direction in layered LiVO2,"Herein, we discuss the identification of vanadium trimers in layered LiVO2
and its sulfide analog of LiVS2 with two-dimensional triangular lattices. Our
comprehensive structural studies using synchrotron X-ray diffraction
experiments clarified that vanadium trimers are randomly aligned along the
c-axis direction in LiVO2, while the long-range ordering of vanadium trimers
along the c-axis direction appears in LiVS2. Our results solve the longstanding
issue of cluster patterns in LiVO2 and provide an experimental basis for
identifying the mechanism of trimer formation.",1910.01337v1
2020-01-08,Electrical and optical properties of hydrated amorphous vanadium oxide,"Electrical and optical properties of amorphous vanadium oxide thin films
obtained by electrochemical anodic oxidation are studied. It is shown that
under cathodic polarization the hydrogen insertion into vanadium oxide from an
electrolyte occurs. Metal-insulator transition in amorphous HxVO2 is found to
be preserved up to high concentration (x ~ 1.5) of hydrogen. Memory switching
with the N-type negative differential resistance, associated with the H+ ionic
transfer, is observed in ""V/hydrated amorphous vanadium oxide/Au"" sandwich
structures.",2001.02418v1
2022-09-03,Hall Sensor Based Measurements of the Magnetic Phase Diagram of Superconducting Vanadium,"Vanadium is an elemental type-II superconductor which has a pure
superconducting phase, a vortex phase, and a non-superconducting phase. We
designed and carried out a low-budget experiment in our undergraduate physics
lab to measure the magnetic field a close distance from the surface of a pure
Vanadium disk sample isothermally with a Hall sensor across a range of
different temperatures and external magnetic fields. We then used these
measurements to compute the superconducting phase diagram of Vanadium. After
completing measurements at eight temperatures ranging from 2.6K to 5K and
magnetic fields ranging from -0.2T to 0.2T, we were able to detect the Meissner
phase (clearly), and a vortex phase (less clearly). Using the graphs of the
hall bar magnetic field versus external magnetic field, we were able to derive
a rough estimate for the upper critical field and the lower critical field of
Vanadium at the eight temperatures wherein we conducted measurements. The
results are in general accord with the phase diagram of Vanadium found in the
literature, except that the critical temperature appears to be less than 5.43K
and 5.13K (about 4.8K). Further, we observed critical fields lower than those
found in the literature. We hypothesize this is due mostly to impurities in our
Vanadium sample (purchased through Amazon.com), which would reduce the measured
critical temperature below that of pure Vanadium, consistent with our results.",2209.01324v1
2024-01-17,Effects of Vanadium Doping on the Optical Response and Electronic Structure of WS$_{2}$ Monolayers,"Two-dimensional dilute magnetic semiconductors has been recently reported in
semiconducting transition metal dichalcogenides by the introduction of
spin-polarized transition metal atoms as dopants. This is the case of
vanadium-doped WS$_2$ and WSe$_2$ monolayers, which exhibits a ferromagnetic
ordering even above room temperature. However, a broadband characterization of
their electronic band structure and its dependence on vanadium concentration is
still lacking. Therefore, here we perform power-dependent photoluminescence,
resonant four-wave mixing, and differential reflectance spectroscopy to study
the optical transitions close to the A exciton energy of vanadium-doped WS$_2$
monolayers with distinct concentrations. Instead of a single A exciton peak,
vanadium-doped samples exhibit two photoluminescence peaks associated with
transitions to occupied and unoccupied bands. Moreover, resonant Raman
spectroscopy and resonant second-harmonic generation measurements revealed a
blueshift in the B exciton but no energy change in the C exciton as vanadium is
introduced in the monolayers. Density functional theory calculations showed
that the band structure is sensitive to the Hubbard \(U\) correction for
vanadium and several scenarios are proposed to explain the two
photoluminescence peaks around the A exciton energy region. Our work provides
the first broadband optical characterization of these two-dimensional dilute
magnetic semiconductors, shedding light on the novel electronic features of
WS$_{2}$ monolayers which are tunable by the vanadium concentration.",2401.09402v1
2006-02-05,Magnetic properties of vanadium-oxide nanotubes probed by static magnetization and {51}V NMR,"Measurements of the static magnetic susceptibility and of the nuclear
magnetic resonance of multiwalled vanadium-oxide nanotubes are reported. In
this nanoscale magnet the structural low-dimensionality and mixed valency of
vanadium ions yield a complex temperature dependence of the static
magnetization and the nuclear relaxation rates. Analysis of the different
contributions to the magnetism allows to identify individual interlayer
magnetic sites as well as strongly antiferromagnetically coupled vanadium spins
(S = 1/2) in the double layers of the nanotube's wall. In particular, the data
give strong indications that in the structurally well-defined vanadium-spin
chains in the walls, owing to an inhomogeneous charge distribution,
antiferromagnetic dimers and trimers occur. Altogether, about 30% of the
vanadium ions are coupled in dimers, exhibiting a spin gap of the order of 700
K, the other ~ 30% comprise individual spins and trimers, whereas the remaining
\~ 40% are nonmagnetic.",0602114v2
2012-01-27,The effects of Vanadium on the strength of a bcc Fe Σ3(111)[1-10] grain boundary,"The effects of micro-alloying element, vanadium, on a bcc Fe \Sigma
3(111)[1-10] symmetric tilt grain boundary strength are studied using density
functional theory calculations. The lowest energy configuration of the grain
boundary structure are obtained from the first-principles calculations. The
substitutional and interstitial point defect formation energies of vanadium in
the grain boundary are compared. The substitutional defect is prefered to
interstitial one. The segregation energies of vanadium onto the grain boundary
and its fractured surfaces are computed. The cohesive energy calculation of the
grain boundary with and without vanadium show that vanadium strengthen the bcc
iron \Sigma 3(111)[1-10] grain boundary.",1201.5915v1
2016-07-10,Effect of isothermal holding temperature on the precipitation hardening in Vanadium-microalloyed steels with varying carbon and nitrogen levels,"Combined effect of Carbon and Nitrogen levels and isothermal holding
temperature on the microstructure, precipitation and the tensile properties of
Vanadium microalloyed steels with 0.05 weight percent Vanadium were studied.
Two different Vanadium steels, one having higher Carbon and lower Nitrogen
content, HCLN steel, and the other having lower Carbon and higher Nitrogen
content, LCHN steel, were prepared and subjected to isothermal holding
treatment over a temperature range of 500 to 750 degree Celsius, after
hot-deformation. Maximum precipitation strengthening from fine Vanadium
carbonitride precipitates has been found at intermediate isothermal holding
temperatures i.e. 600 to 650 degree Celsius in both the steels. In spite of the
significantly smaller fraction of pearlite and bainite, coarser average ferrite
grain size and lower interaction of precipitation and dislocation in the
ferrite matrix, the yield strength of LCHN steel was close to HCLN steel. This
can be attributed to the higher precipitation strengthening ranging from 20 to
50 MPa resulted from the finer Vanadium precipitates in LCHN steel than that of
HCLN steel.",1607.02721v1
2009-04-17,Novel magnetic properties of graphene: Presence of both ferromagnetic and antiferromagnetic features and other aspects,"Investigations of the magnetic properties of graphenes prepared by different
methods reveal that dominant ferromagnetic interactions coexist along with
antiferromagnetic interactions in all the samples. Thus, all the graphene
samples exhibit room-temperature magnetic hysteresis. The magnetic properties
depend on the number of layers and the sample area, small values of both
favoring larger magnetization. Molecular charge-transfer affects the magnetic
properties of graphene, interaction with a donor molecule such as
tetrathiafulvalene having greater effect than an electron-withdrawing molecule
such as tetracyanoethylene",0904.2739v1
2011-09-05,Charge-transfer excitations in molecular donor-acceptor complexes within the many-body Bethe-Salpeter approach,"We study within the perturbative many-body $GW$ and Bethe-Salpeter approach
the low lying singlet charge-transfer excitations in molecular donor-acceptor
complexes associating benzene, naphtalene and anthracene derivatives with the
tetracyanoethylene acceptor. Our calculations demonstrate that such techniques
can reproduce the experimental data with a mean average error of 0.1-0.15 eV
for the present set of dimers, in excellent agreement with the best
time-dependent density functional studies with optimized range-separated
functionals. The present results pave the way to the study of photoinduced
charge transfer processes in photovoltaic devices with a parameter-free
\textit{ab initio} approach showing equivalent accuracy for finite and extended
systems.",1109.0824v1
2014-11-28,Growth control of the oxidation state in vanadium oxide thin films,"Precise control of the chemical valence or oxidation state of vanadium in
vanadium oxide thin films is highly desirable for not only fundamental
research, but also technological applications that utilize the subtle change in
the physical properties originating from the metal- insulator transition (MIT)
near room temperature. However, due to the multivalent nature of vanadium and
the lack of a good understanding on growth control of the oxidation state,
stabilization of phase pure vanadium oxides with a single oxidation state is
extremely challenging. Here, we systematically varied the growth conditions to
clearly map out the growth window for preparing phase pure epitaxial vanadium
oxides by pulsed laser deposition for providing a guideline to grow high
quality thin films with well-defined oxidation states of V2(+3)O3, V(+4)O2, and
V2(+5)O5. A well pronounced MIT was only observed in VO2 films grown in a very
narrow range of oxygen partial pressure P(O2). The films grown either in lower
(< 10 mTorr) or higher P(O2) (> 25 mTorr) result in V2O3 and V2O5 phases,
respectively, thereby suppressing the MIT for both cases. We have also found
that the resistivity ratio before and after the MIT of VO2 thin films can be
further enhanced by one order of magnitude when the films are further oxidized
by post-annealing at a well-controlled oxidizing ambient. This result indicates
that stabilizing vanadium into a single valence state has to compromise with
insufficient oxidation of an as grown thin film and, thereby, a subsequent
oxidation is required for an improved MIT behavior.",1411.7922v1
2005-10-13,Theory of successive transitions in vanadium spinels and order of orbitals and spins,"We have theoretically studied successive transitions in vanadium spinel
oxides with (t_2g)^2 electron configuration. These compounds show a structural
transition at ~ 50K and an antiferromagnetic transition at ~ 40K. Since
threefold t_2g orbitals of vanadium cations are occupied partially and
vanadiums constitute a geometrically-frustrated pyrochlore lattice, the system
provides a particular example to investigate the interplay among spin, orbital
and lattice degrees of freedom on frustrated lattice. We examine the models
with the Jahn-Teller coupling and/or the spin-orbital superexchange
interaction, and conclude that keen competition between these two contributions
explains the thermodynamics of vanadium spinels. Effects of quantum
fluctuations as well as relativistic spin-orbit coupling are also discussed.",0510331v1
2008-03-19,Electrodynamics of the vanadium oxides VO2 and V2O3,"The optical/infrared properties of films of vanadium dioxide (VO2) and
vanadium sesquioxide (V2O3) have been investigated via ellipsometry and
near-normal incidence reflectance measurements from far infrared to ultraviolet
frequencies. Significant changes occur in the optical conductivity of both VO2
and V2O3 across the metal-insulator transitions at least up to (and possibly
beyond) 6 eV. We argue that such changes in optical conductivity and electronic
spectral weight over a broad frequency range is evidence of the important role
of electronic correlations to the metal-insulator transitions in both of these
vanadium oxides. We observe a sharp optical transition with possible final
state (exciton) effects in the insulating phase of VO2. This sharp optical
transition occurs between narrow a1g bands that arise from the
quasi-one-dimensional chains of vanadium dimers. Electronic correlations in the
metallic phases of both VO2 and V2O3 lead to reduction of the kinetic energy of
the charge carriers compared to band theory values, with paramagnetic metallic
V2O3 showing evidence of stronger correlations compared to rutile metallic VO2.",0803.2739v1
2014-10-23,The interplay between the structural and magnetic properties of vanadium dioxide from first principles,"The effects of the spin and lattice degrees of freedom on the electronic
structure of M1 vanadium dioxide are explored using a quasiparticle
description. Contraction of the inter-vanadium spacing of the Peierls pairings
stabilizes bonding electrons, reducing polarizability and thus widening the
band gap. Increases of this inter-vanadium spacing of as little as 1 \% reduce
this stabilization, resulting in a crossover to ferromagnetic behaviour
accomplished by half of the valence electrons inhabiting the leading edge of
the conduction band in localized atomic-like orbitals, as the antiferromagnetic
order becomes unstable with respect to rearrangement according to Hund's first
rule. The data indicates that the magnetic structure of M1 vanadium dioxide may
be finely balanced; the antiferromagnetic order is a consequence of the
overlapping nuclear potential of the Peierls pairs, and input which disrupts
this will have a significant effect on magnetic properties.",1410.6522v1
2015-12-30,Thermal radiative near field transport between vanadium dioxide and silicon oxide across the metal insulator transition,"The thermal radiative near field transport between vanadium dioxide and
silicon oxide at submicron distances is expected to exhibit a strong dependence
on the state of vanadium dioxide which undergoes a metal-insulator transition
near room temperature. We report the measurement of near field thermal
transport between a heated silicon oxide micro-sphere and a vanadium dioxide
thin film on a titanium oxide (rutile) substrate. The temperatures of the 15 nm
vanadium dioxide thin film varied to be below and above the
metal-insulator-transition, the sphere temperatures were varied in a range
between 100 and 200 Celsius. The measurements were performed using a
vacuum-based scanning thermal microscope with a cantilevered resistive thermal
sensor. We observe a thermal conductivity per unit area between the sphere and
the film with a distance dependence following a power law trend and a
conductance contrast larger than 2 for the two different phase states of the
film.",1512.09050v2
2016-02-25,Atomistic modeling of the anomalous helium behaviors in vanadium,"Ab initio calculations have been performed to clarify the primary behaviors
of He atoms in vanadium and to generate the database for the development of the
interatomic potential for V-He system within the framework of
the""s-band""model.The calculated formation energies of the
tetrahedral,octahedral and substitutional He defects,as well as those for
He2,He3 and He2V clusters are reasonable when compared with relevant
experimental results and ab initio calculations under the same conditions.The
applicability of the present V-He potential for atomistic simulations to
investigate the moving,clustering,and trapping of interstitial He atoms in
vanadium are demonstrated.Similar to the results in iron and tungsten,the
interstitial He atoms can migrate quickly in vanadium. However,He atoms
aggregate into clusters with low binding energies,and the trapping of He atoms
depend much on the pre-existing traps in vanadium.",1602.07826v1
2022-09-20,The actual pressure and temperature at the melt of elemental vanadium,"It is claimed that all of the pressure scales of the reported melting curves
derived by diamond anvil cell experiments require a correction which takes into
account the pressure thermal shift, where vanadium is an illustrative example.
The linear behavior of the thermal pressure (Pth) vs. the temperature, as
predicted by first principles theoretical assumptions is then experimentally
confirmed. This allows extrapolation to determine of the actual pressure and
thermal temperature at the melt. Accounting for the role of the pressure
transmitting media in diamond anvil cell experiments, the analysis of elemental
vanadium melting curve is presented. It is shown that the appropriate
correction of shock waves melting data which takes into account the radiation
absorbed by the LiF window, applies only to vanadium metal. The correct
pressure scale of vanadium metal as derived by diamond anvil cell is presented.",2209.09836v1
2003-02-24,Crystal structure and charge order below the metal-insulator transition in the vanadium bronze $β$-SrV$_6$O$_{15}$,"Single crystal X-ray diffraction measurements were performed on
one-dimensional mixed valence vanadium bronze $\beta$-SrV$_6$O$_{15}$ in which
a metal-insulator transition exists at 170 K. Above 170 K the P2$_1$/a
structure with zigzag order of Sr in the tunnels of vanadium is confirmed. In
the structure below 170 K, the P2$_1$/a space group is retained with a b-axis
threefold increase and a charge order appears. A Bond Valence Sum analysis
shows that the charge order may consist in clusters of V$^{4+}$ regularly
spaced along 1D direction.",0302476v1
2004-01-13,"Structural, orbital, and magnetic order in vanadium spinels","Vanadium spinels (ZnV_2O_4, MgV_2O_4, and CdV_2O_4) exhibit a sequence of
structural and magnetic phase transitions, reflecting the interplay of lattice,
orbital, and spin degrees of freedom. We offer a theoretical model taking into
account the relativistic spin-orbit interaction, collective Jahn-Teller effect,
and spin frustration. Below the structural transition, vanadium ions exhibit
ferroorbital order and the magnet is best viewed as two sets of
antiferromagnetic chains with a single-ion Ising anisotropy. Magnetic order,
parametrized by two Ising variables, appears at a tetracritical point.",0401203v3
2005-05-02,Orbital order in vanadium spinels,"Motivated by recent theoretical and experimental controversy, we present a
theoretical study to clarify the orbital symmetry of the ground state of
vanadium spinel oxides AV$_2$O$_4$ (A=Zn, Mg, Cd). The study is based on an
effective Hamiltonian with spin-orbital superexchange interaction and a local
spin-orbit coupling term. We construct a classical phase-diagram and prove the
complex orbital nature of the ground state. Remarkably, with our new analysis
we predict correctly also the coherent tetragonal flattening of oxygen
octahedra. Finally, through analytical considerations as well as numerical
ab-initio simulations, we propose how to detect the predicted complex orbital
ordering through vanadium K edge resonant x-ray scattering.",0505031v1
2007-04-29,Pinned Low Energy Electronic Excitation in Metal Exchanged Vanadium Oxide Nanoscrolls,"We measured the optical properties of mixed valent vanadium oxide nanoscrolls
and their metal exchanged derivatives in order to investigate the charge
dynamics in these compounds. In contrast to the prediction of a metallic state
for the metal exchanged derivatives within a rigid band model, we find that the
injected charges in Mn$^{2+}$ exchanged vanadium oxide nanoscrolls are pinned.
A low-energy electronic excitation associated with the pinned carriers appears
in the far infrared and persists at low temperature, suggesting that the
nanoscrolls are weak metals in their bulk form, dominated by inhomogeneous
charge disproportionation and Madelung energy effects.",0704.3861v1
2007-11-05,Magnetic excitations in vanadium spinels,"We study magnetic excitations in vanadium spinel oxides AV$_2$O$_4$ (A=Zn,
Mg, Cd) using two models: first one is a superexchange model for vanadium S=1
spins, second one includes in addition spin-orbit coupling, and crystal
anisotropy. We show that the experimentally observed magnetic ordering can be
obtained in both models, however the orbital ordering is different with and
without spin-orbit coupling and crystal anisotropy. We demonstrate that this
difference strongly affects the spin-wave excitation spectrum above the
magnetically ordered state, and argue that the neutron measurement of such
dispersion is a way to distinguish between the two possible orbital orderings
in AV$_2$O$_4$.",0711.0732v1
2009-07-08,Atomic scale characterization of deformation induced interfacial mixing in a Cu/V nanocomposite wire,"The microstructure of a Cu/V nanocomposite wire processed by cold drawing was
investigated by high resolution transmission electron microscopy and atom probe
tomography. The experimental data clearly reveal some deformation induced
interfacial mixing where the vanadium filaments are nanoscaled. The mixed layer
is a 2nm wide vanadium gradient in the fcc Cu phase. This mechanical mixing
leads to the local fragmentation and dissolution of the filaments and to the
formation of vanadium super saturated solid solutions in fcc Cu.",0907.1400v1
2014-11-26,Spectroscopy of vanadium (III) doped gallium lanthanum sulphide chalcogenide glass,"Vanadium doped gallium lanthanum sulphide glass (V:GLS) displays three
absorption bands at 580, 730 and 1155 nm identified by photoluminescence
excitation measurements. Broad photoluminescence, with a full width half
maximum (FWHM) of 500 nm, is observed peaking at 1500 nm when exciting at 514,
808 and 1064 nm. The fluorescence lifetime and quantum efficiency at 300 K were
measured to be 33.4 us and 4 % respectively. From the available spectroscopic
data we propose the vanadium ions valence to be 3+ and be in tetrahedral
coordination The results indicate potential for development of a laser or
optical amplifier based on V:GLS.",1411.7156v1
2018-12-12,The root cause of hydrogen induced changes in optical transmission of vanadium,"The changes in the optical transmission of thin vanadium layers upon hydrogen
absorption are found to be dominated by the volume changes of the layers and
not directly linked to concentration. This effect is demonstrated by utilising
the difference in the hydrogen induced expansion of V layers in Fe/V and Cr/V
superlattices. Hydrogen resides solely in the vanadium layers in these
superlattices, while occupying different sites, causing different lattice
expansion. Quantitative agreement is obtained between the experimental results
and first principle density functional calculations.",1812.04917v1
2020-08-24,Structural properties and Raman spectra of columbite-type NiNb$_{2-x}$V$_x$O$_6$ synthesized under high pressure,"The complete set of structural parameters of the new series of compounds
NiNb$_{2-x}$V$_x$O$_6$ ($0\leq x \leq 2$) with the unusual columbite-type
structure is presented here. In the samples containing vanadium, this
crystalline structure was stabilized by synthesis in conditions of high
pressure and high temperature. Here we report here the first Raman spectrum for
the NiV$_2$O$_6$-\textsl{Pbcn} polymorph and extend the list of the previously
observed modes for the NiNb$_2$O$_6$. The evolution of the vibrational Raman
spectrum produced when the vanadium is substituted for niobium along the series
is also presented and discussed. This evolution is interpreted by taking into
account the changes in the local structural environment of the niobium/vanadium
atoms and its influence over the nickel-oxygen bonds around them. The presence
of vanadium atoms favors an increase of the symmetry in the arrangement of
oxygen atoms around the nickel-ones; in counterpart, the vanadium is in an
octahedral environment which is more distorted than that of the niobium.
Because of these apparently subtle differences, the homogeneous distribution of
vanadium in the solid solution NiNb$_{2-x}$V$_x$O$_6$ is not possible.",2008.10629v1
2021-05-21,Vanadium Dioxide Thin Films Synthesized Using Low Thermal Budget Atmospheric Oxidation,"Vanadium dioxide is a complex oxide material, which shows large resistivity
and optical reflectance change while transitioning from the insulator to metal
phase at ~68 {\deg}C. In this work, we use a modified atmospheric thermal
oxidation method to oxidize RF-sputtered Vanadium films. Structural,
surface-morphology and phase-transition properties of the oxidized films as a
function of oxidation duration are presented. Phase-pure VO2 films are obtained
by oxidizing ~130 nm Vanadium films in short oxidation duration of ~30 seconds.
Compared to previous reports on VO2 synthesis using atmospheric oxidation of
Vanadium films of similar thickness, we obtain a reduction in oxidation
duration by more than one order. Synthesized VO2 thin film shows resistance
switching of ~3 orders of magnitude. We demonstrate optical reflectance
switching in long-wave infrared wavelengths in VO2 films synthesized using
atmospheric oxidation of Vanadium. The extracted refractive index of VO2 in the
insulating and in the metallic phase is in good agreement with VO2 synthesized
using other methods. The considerable reduction in oxidation time of VO2
synthesis while retaining good resistance and optical switching properties will
help in integration of VO2 in limited thermal budget processes, enabling
further applications of this phase-transition material.",2105.10264v1
2005-12-12,Abundances of Vanadium and Bromine in 3 Cen A: Additional Odd-Z Anomalies,"We report abundance excesses of 1.2 and 2.6 dex, respectively, for vanadium
and bromine in the hot, peculiar star 3 Cen A. Abundances for these two odd-Z
elements have not been previously reported for this star. Taken with previous
work, they strengthen the case of the origin of the abundance peculiarities by
diffusion.",0512294v1
2004-03-10,"Comment on ""Molecular dynamics study of the threshold displacement energy in vanadium""","The simulation study on Frenkel pair creation in vanadium by L.A. Zepeda-Ruiz
et al. (Phys. Rev. B 67, 134114, 2003) is criticized for its lack of reference
to existing experimental work and for generally inadequate treatment of
literature data. Hence, the validity of the ensuing discussion of various
results (e.g. the value of the minimum TDE and the Frenkel pair stability in V)
is questioned.",0403275v1
2009-07-11,Discovery of the Vanadium Isotopes,"Twenty-four vanadium isotopes have so far been observed; the discovery of
these isotopes is discussed. For each isotope a brief summary of the first
refereed publication, including the production and identification method, is
presented.",0907.1994v1
2012-10-12,Effect of ion and electron irradiation on microstructure of vanadium,"The evolution of microstructures in pure vanadium under electron irradiation
was investigated.",1210.3414v3
2014-02-09,Work function measurements of vanadium doped diamond-like carbon films by ultraviolet photoelectron spectroscopy,"Vanadium doped diamond-like carbon films prepared by unbalanced magnetron
sputtering have been investigated by X-ray and ultraviolet photoelectron
spectroscopy measurements for the purpose of revealing electronic structures
including values of work function on the surfaces. In addition to these
photoelectron measurements, X-ray diffraction measurements have been performed
to characterize the crystal structures.",1402.1911v2
2014-11-27,The critical relaxation of the model of iron-vanadium magnetic superlattice,"The critical relaxation of iron-vanadium magnetic superlattice in case of the
equality between interlayer and intralayer exchange interactions is
investigated. The dynamic and static critical exponents of the model are
calculated. A value of the critical temperature is evaluated.",1411.7505v1
2009-12-07,High temperature ferromagnetism of Li-doped vanadium oxide nanotubes,"The nature of a puzzling high temperature ferromagnetism of doped
mixed-valent vanadium oxide nanotubes reported earlier by Krusin-Elbaum et al.,
Nature 431 (2004) 672, has been addressed by static magnetization, muon spin
relaxation, nuclear magnetic and electron spin resonance spectroscopy
techniques. A precise control of the charge doping was achieved by
electrochemical Li intercalation. We find that it provides excess electrons,
thereby increasing the number of interacting magnetic vanadium sites, and, at a
certain doping level, yields a ferromagnetic-like response persisting up to
room temperature. Thus we confirm the surprising previous results on the
samples prepared by a completely different intercalation method. Moreover our
spectroscopic data provide first ample evidence for the bulk nature of the
effect. In particular, they enable a conclusion that the Li nucleates
superparamagnetic nanosize spin clusters around the intercalation site which
are responsible for the unusual high temperature ferromagnetism of vanadium
oxide nanotubes.",0912.1237v1
2010-02-23,Growth model investigation of Vanadium-Benzene Polymer,"Electronic and magnetic properties of VnBzn+1 sandwich clusters are
investigated using density functional theory. Growth model is applied to
investigate the property change of Vanadium-Benzene sandwich clusters. Our
results show that, for n<8, all V ions tend to put their spin in ferromagnetic
state, and that the magnetic moments of VnBzn+1 increase linearly with n.
Finite-size effects induce a nonmonotonous behavior of the VnBzn+1 magnetic
properties. In the case of n=8, the electronic properties of VnBzn+1 has the
same characteristics with the counterpart of Vanadium-Benzene infinite wire,
hence, the critical length of VnBzn+1 is defined eight. Furthermore, our
results demonstrate that Vanadium-Benzene infinite wire is a proper material
for spin polarized transport and has a high stability in the presence of
external electronic and magnetic fields.",1002.4323v1
2011-09-08,Current oscillations in Vanadium Dioxide: evidence for electrically triggered percolation avalanches,"In this work, we experimentally and theoretically explore voltage controlled
oscillations occurring in micro-beams of vanadium dioxide. These oscillations
are a result of the reversible insulator to metal phase transition in vanadium
dioxide. Examining the structure of the observed oscillations in detail, we
propose a modified percolative-avalanche model which allows for
voltage-triggering. This model captures the periodicity and waveshape of the
oscillations as well as several other key features. Importantly, our modeling
shows that while temperature plays a critical role in the vanadium dioxide
phase transition, electrically induced heating cannot act as the primary
instigator of the oscillations in this configuration. This realization leads us
to identify electric field as the most likely candidate for driving the phase
transition.",1109.1834v1
2013-08-29,Grain boundary ferromagnetism in vanadium-doped In$_2$O$_3$ thin films,"Room temperature ferromagnetism was observed in In$_2$O$_3 thin films doped
with 5 at.% vanadium, prepared by pulsed laser deposition at substrate
temperatures ranging from 300 to 600 $\,^{\circ}{\rm C}$. X-ray absorption fine
structure measurement indicated that vanadium was substitutionally dissolved in
the In$_2$O$_3$ host lattice, thus excluding the existence of secondary phases
of vanadium compounds. Magnetic measurements based on SQUID magnetometry and
magnetic circular dichroism confirm that the magnetism is at grain boundaries
and also in the grains. The overall magnetization originates from the competing
effects between grains and grain boundaries.",1308.6492v1
2015-03-10,Vanadium sacrificial layers as a novel approach for the fabrication of freestanding Heusler Shape Memory Alloys,"In this study we report a method for the preparation of freestanding
magnetocaloric thin films. Non-stoichiometric Heusler alloys Ni-Mn-Sn,
Ni-Co-Mn-Sn and Ni-Co-Mn-Al are prepared via sputter deposition. A sacrificial
vanadium layer is added between the substrate and the Heusler film. By means of
selective wet-chemical etching the vanadium layer can be removed. Conditions
for the crystallization of Vanadium layers and epitaxial growth of the Heusler
films are indicated. Magnetic and structural properties of freestanding and
as-prepared films are compared in detail. The main focus of this study is on
the influence of substrate constraints on the Martensitic transistion.",1503.02987v1
2017-09-14,An Ab Initio Description of the Mott Metal-Insulator Transition of M$_{2}$ Vanadium Dioxide,"Using an \textit{ab initio} approach based on the GW approximation which
includes strong local \textbf{k}-space correlations, the Metal-Insulator
Transition of M$_2$ vanadium dioxide is broken down into its component parts
and investigated. Similarly to the M$_{1}$ structure, the Peierls pairing of
the M$_{2}$ structure results in bonding-antibonding splitting which stabilizes
states in which the majority of the charge density resides on the Peierls
chain. This is insufficient to drop all of the bonding states into the lower
Hubbard band however. An antiferroelectric distortion on the neighboring
vanadium chain is required to reduce the repulsion felt by the Peierls bonding
states by increasing the distances between the vanadium and apical oxygen
atoms, lowering the potential overlap thus reducing the charge density
accumulation and thereby the electronic repulsion. The antibonding states are
simultaneously pushed into the upper Hubbard band. The data indicate that
sufficiently modified GW calculations are able to describe the interplay of the
atomic and electronic structures occurring in Mott metal-insulator transitions.",1709.04602v1
2019-08-26,Vanadium spin qubits as telecom quantum emitters in silicon carbide,"Solid state quantum emitters with spin registers are promising platforms for
quantum communication, yet few emit in the narrow telecom band necessary for
low-loss fiber networks. Here we create and isolate near-surface single
vanadium dopants in silicon carbide (SiC) with stable and narrow emission in
the O-band (1278-1388 nm), with brightness allowing cavity-free detection in a
wafer-scale CMOS-compatible material. In vanadium ensembles, we characterize
the complex d1 orbital physics in all five available sites in 4H-SiC and
6H-SiC. The optical transitions are sensitive to mass shifts from local silicon
and carbon isotopes, enabling optically resolved nuclear spin registers.
Optically detected magnetic resonance in the ground and excited orbital states
reveals a variety of hyperfine interactions with the vanadium nuclear spin and
clock transitions for quantum memories. Finally, we demonstrate coherent
quantum control of the spin state. These results provide a path for telecom
emitters in the solid-state for quantum applications.",1908.09817v1
2021-07-28,Controllable p-type Doping of 2D WSe2 via Vanadium Substitution,"Scalable substitutional doping of two-dimensional (2D) transition metal
dichalcogenides (TMDCs) is a prerequisite to developing next-generation logic
and memory devices based on 2D materials. To date, doping efforts are still
nascent. Here, we report scalable growth and vanadium (V) doping of 2D WSe2 at
front-end-of-line (FEOL) and back-end-of-line (BEOL) compatible temperatures of
800 {\deg}C and 400 {\deg}C, respectively. A combination of experimental and
theoretical studies confirm that vanadium atoms substitutionally replace
tungsten in WSe2, which results in p-type doping via the introduction of
discrete defect levels that lie close to the valence band maxima. The p-type
nature of the V dopants is further verified by constructed field-effect
transistors, where hole conduction becomes dominant with increasing vanadium
concentration. Hence, our study presents a method to precisely control the
density of intentionally introduced impurities, which is indispensable in the
production of electronic-grade wafer-scale extrinsic 2D semiconductors.",2107.13413v1
2022-06-13,Vanadium in Silicon Carbide: Telecom-ready spin centres with long relaxation lifetimes and hyperfine-resolved optical transitions,"Vanadium in silicon carbide (SiC) is emerging as an important candidate
system for quantum technology due to its optical transitions in the telecom
wavelength range. However, several key characteristics of this defect family
including their spin relaxation lifetime (T1), charge state dynamics, and level
structure are not fully understood. In this work, we determine the T1 of an
ensemble of vanadium defects, demonstrating that it can be greatly enhanced at
low temperature. We observe a large spin contrast exceeding 90% and long
spin-relaxation times of up to 25s at 100mK, and of order 1s at 1.3K. These
measurements are complemented by a characterization of the ensemble charge
state dynamics. The stable electron spin furthermore enables high-resolution
characterization of the systems' hyperfine level structure via two-photon
magneto-spectroscopy. The acquired insights point towards high-performance
spin-photon interfaces based on vanadium in SiC.",2206.06240v1
2023-07-05,Low temperature spin Seebeck effect in non-magnetic vanadium dioxide,"The spin Seebeck effect (SSE) is sensitive to thermally driven magnetic
excitations in magnetic insulators. Vanadium dioxide in its insulating low
temperature phase is expected to lack magnetic degrees of freedom, as vanadium
atoms are thought to form singlets upon dimerization of the vanadium chains.
Instead, we find a paramagnetic SSE response in VO2 films that grows as the
temperature decreases below 50 K. The field and temperature dependent SSE
voltage is qualitatively consistent with a general model of paramagnetic SSE
response and inconsistent with triplet spin transport. The microscopic nature
of the magnetic excitations in VO2 requires further examination.",2307.02594v1
2023-11-09,Strain-Hardening Stages and Structure Evolution in Pure Niobium and Vanadium upon High Pressure Torsion,"High pressure torsion (HPT) is one of the ways to form nanostructured
materials with high strength properties. However, HPT hardening mechanisms vary
from material to material and are poorly understood for some BCC metals,
particularly niobium and vanadium. This work aims to identify strain hardening
stages for Nb and V metals during HPT. Two approaches have been used to
identify the deformation stages during high pressure torsion. The approaches
are based on the application of a ""piecewise"" model, taking into account the
different deformation mechanisms that determine the type of the forming
structure, and on the analysis of the hardness vs. true strain dependence
according to the $H$$-$${e}^{0.5}$ law. We compared the identified stages with
the results of the electron microscopic study of the structure. Both models
describe well the structural changes observed microscopically in HPT-deformed
niobium. However, we have shown that only the piecewise model gives an adequate
description of the stages of structure development in vanadium. We have
provided an explanation for the observed difference in the behavior of niobium
and vanadium upon HPT.",2311.05549v2
2020-05-05,Monolayer Vanadium-doped Tungsten Disulfide: A Room-Temperature Dilute Magnetic Semiconductor,"Dilute magnetic semiconductors, achieved through substitutional doping of
spin-polarized transition metals into semiconducting systems, enable
experimental modulation of spin dynamics in ways that hold great promise for
novel magneto-electric or magneto-optical devices, especially for
two-dimensional systems such as transition metal dichalcogenides that
accentuate interactions and activate valley degrees of freedom. Practical
applications of 2D magnetism will likely require room-temperature operation,
air stability, and (for magnetic semiconductors) the ability to achieve optimal
doping levels without dopant aggregation. Here we describe room-temperature
ferromagnetic order obtained in semiconducting vanadium-doped tungsten
disulfide monolayers produced by a reliable single-step film sulfidation method
across an exceptionally wide range of vanadium concentrations, up to 12 at%
with minimal dopant aggregation. These monolayers develop p-type transport as a
function of vanadium incorporation and rapidly reach ambipolarity.
Ferromagnetism peaks at an intermediate vanadium concentration of a few atomic
percent and decreases for higher concentrations, which is consistent with
quenching due to orbital hybridization at closer vanadium-vanadium spacings, as
supported by transmission electron microscopy, magnetometry and
first-principles calculations. Room-temperature two-dimensional dilute magnetic
semiconductors provide a new component to expand the functional scope of van
der Waals heterostructures and bring semiconducting magnetic 2D
heterostructures them into the realm of practical application.",2005.01965v1
2000-12-18,"Structure determination, valence, and superexchange in the dimerized low temperature phase of alpha'-NaV2O5","We report results of a new analysis for the low-temperature structure of
alpha'-NaV2O5 from synchrotron x-ray diffraction experiments. We confirm the
existence of two inequivalent ladder structures in each vanadium layer. Based
on our structural data we perform a bond-valence calculation for the vanadium
sites in the low temperature state. Due to an asymmetric charge ordering we
obtain only two different vanadium valences despite the three inequivalent
sites. This explains the 51V-NMR observation of only two resonant peaks in the
charge ordered phase. By use of a Slater-Koster method to obtain hopping matrix
elements and cluster calculations we obtain effective vanadium-vanadium
hoppings which compare well to LDA results. Using these in a cluster
calculation we obtain a superexchange of 0.047 eV between electrons on
neighbouring rungs of the same ladder for the undistorted phase. For the
distorted phase we find a significant alternation in the shifts of the oxygen
atoms along the legs of one of the two ladder types which leads to a
significant exchange dimerisation \delta_J \approx 0.25.",0012327v3
2013-11-12,Influence of the C/O ratio on titanium and vanadium oxides in protoplanetary disks,"Context. The observation of carbon-rich disks have motivated several studies
questioning the influence of the C/O ratio on their gas phase composition in
order to establish the connection between the metallicity of hot-Jupiters and
that of their parent stars.
  Aims. We to propose a method that allows the characterization of the adopted
C/O ratio in protoplanetary disks independently from the determination of the
host star composition. Titanium and vanadium chemistries are investigated
because they are strong optical absorbers and also because their oxides are
known to be sensitive to the C/O ratio in some exoplanet atmospheres.
  Methods. We use a commercial package based on the Gibbs energy minimization
technique to compute the titanium and vanadium equilibrium chemistries in
protoplanetary disks for C/O ratios ranging from 0.05 to 10. Our calculations
are performed for pressures ranging from 1e-6 to 1e-2 bar, and for temperatures
ranging from 50 to 2000 K.
  Results. We find that the vanadium nitride/vanadium oxide and titanium
hydride/titanium oxide gas phase ratios strongly depend on the C/O ratio in the
hot parts of disks (T > 1000 K). Our calculations suggest that, in these
regions, these ratios can be used as tracers of the C/O value in protoplanetary
disks.",1311.2916v1
2014-11-20,Concentration dependence of the fluorescence decay profile in transition metal doped chalcogenide glass,"In this paper we present the fluorescence decay profiles of vanadium and
titanium doped gallium lanthanum sulphide (GLS) glass at various doping
concentrations between 0.01 and 1% (molar). We demonstrate that below a
critical doping concentration the fluorescence decay profile can be fitted with
the stretched exponential function: exp[-(t/{\tau})\b{eta}], where {\tau} is
the fluorescence lifetime and \b{eta} is the stretch factor. At low
concentrations the lifetime for vanadium and titanium doped GLS was 30 {\mu}s
and 67 {\mu}s respectively. We validate the use of the stretched exponential
model and discuss the possible microscopic phenomenon it arises from. We also
demonstrate that above a critical doping concentration of around 0.1% (molar)
the fluorescence decay profile can be fitted with the double exponential
function: a*exp-(t/{\tau}1)+ b*exp-(t/{\tau}2), where {\tau}1and {\tau}2 are
characteristic fast and slow components of the fluorescence decay profile, for
vanadium the fast and slow components are 5 {\mu}s and 30 {\mu}s respectively
and for titanium they are 15 {\mu}s and 67 {\mu}s respectively. We also show
that the fluorescence lifetime of vanadium and titanium at low concentrations
in the oxide rich host gallium lanthanum oxy-sulphide (GLSO) is 43 {\mu}s and
97 {\mu}s respectively, which is longer than that in GLS. From this we deduce
that vanadium and titanium fluorescing ions preferentially substitute into high
efficiency oxide sites until at a critical concentration they become saturated
and low efficiency sulphide sites start to be filled.",1411.7045v1
2016-11-04,Stability of rhombohedral phases in vanadium at high-pressure and high-temperature: first-principles investigations,"The pressure-induced transition of vanadium from BCC to rhombohedral
structures is unique and intriguing among transition metals. In this work, the
stability of these phases is revisited by using density functional theory. At
finite temperatures, a novel transition of rhombohedral phases back to BCC
phase induced by thermal electrons is discovered. This reentrant transition is
found not driven by phonons, instead it is the electronic entropy that
stabilizes the latter phase, which is totally out of expectation. Parallel to
this transition, we find a peculiar and strong increase of the shear modulus
C44 with increasing temperature. It is counter-intuitive in the sense that it
suggests an unusual harding mechanism of vanadium by temperature. With these
stability analyses, the high-pressure and finite-temperature phase diagram of
vanadium is proposed. Furthermore, the dependence of the stability of RH phases
on the Fermi energy and chemical environment is investigated. The results
demonstrate that the position of the Fermi level has a significant impact on
the phase stability, and follows the band-filling argument. Besides the Fermi
surface nesting, we find that the localization/delocalization of the d orbitals
also contributes to the instability of rhombohedral distortions in vanadium.",1611.01426v1
2018-07-25,$d-p$ model and spin-orbital order in the vanadium perovskites,"Using the multi-band $d-p$ model and unrestricted Hartree-Fock approximation
we investigate the electronic structure and spin-orbital order in
three-dimensional VO$_3$ lattice. The main aim of this investigation is testing
if simple $d-p$ model, with partly filled $3d$ orbitals (at vanadium ions) and
$2p$ orbitals (at oxygen ions), is capable of reproducing correctly nontrivial
coexisting spin-orbital order observed in the vanadium perovskites. We point
out that the multi-band $d-p$ model has to include partly filled $e_g$ orbitals
at vanadium ions. The results suggest weak self-doping as an important
correction beyond the ionic model and reproduce the possible ground states with
broken spin-orbital symmetry on vanadium ions: either $C$-type alternating
orbital order accompanied by $G$-type antiferromagnetic spin order, or $G$-type
alternating orbital order accompanied by $C$-type antiferromagnetic spin order.
Both states are experimentally observed and compete with each other in YVO$_3$
while only the latter was observed in LaVO$_3$. Orbital order is induced and
stabilized by particular patterns of oxygen distortions arising from the
Jahn-Teller effect. In contrast to time-consuming \textit{ab-initio}
calculations, the computations using $d-p$ model are very quick and should be
regarded as very useful in solid state physics, provided the parameters are
selected carefully.",1807.09879v1
2022-08-29,Multi Spectral Switchable Infra-Red Reflectance Resonances in Highly Subwavelength Partially Oxidized Vanadium Thin Films,"Phase transition materials are promising for realization of switchable
optics. In this work, we show reflectance resonances in the near-infrared and
long-wave infrared wavelengths in highly subwavelength partially oxidized
Vanadium thin films. These partially oxidized films consist of a multilayer of
Vanadium dioxide and Vanadium as shown using Raman spectroscopy and four-probe
measurements. As Vanadium dioxide is a phase transition material that shows
insulator to metal phase transition at 68 C, the observed infra-red resonances
can be switched with temperature into a high-reflectance state. The wavelength
of these resonances are passively tunable as a function of the oxidation
duration. The obtained reflectance resonance at near-infrared wavelength red
shifts from 1.78 um to 2.68 um with increasing oxidation duration while the
long-wavelength infrared resonance blue shifts from 12.68 um to 9.96 um. To
find the origin of the reflectance resonances, we model the reflectance spectra
as a function of the oxidation duration using the transfer matrix method. The
presented model captures the dual reflectance resonances reasonably well. These
passive wavelength-tunable and switchable resonances with easy to fabricate
lithography-free multilayer structure will be useful for multispectral
applications such as camouflage, spectral selective microbolometer, and thermal
management.",2208.13458v1
2005-10-06,A unified electrostatic and cavitation model for first-principles molecular dynamics in solution,"The electrostatic continuum solvent model developed by Fattebert and Gygi is
combined with a first-principles formulation of the cavitation energy based on
a natural quantum-mechanical definition for the surface of a solute. Despite
its simplicity, the cavitation contribution calculated by this approach is
found to be in remarkable agreement with that obtained by more complex
algorithms relying on a large set of parameters. Our model allows for very
efficient Car-Parrinello simulations of finite or extended systems in solution,
and demonstrates a level of accuracy as good as that of established
quantum-chemistry continuum solvent methods. We apply this approach to the
study of tetracyanoethylene dimers in dichloromethane, providing valuable
structural and dynamical insights on the dimerization phenomenon.",0510157v1
2008-08-23,Effects of interaction of electron donor and accepter molecules on the electronic structure of graphene,"Effects of interaction of graphene with electron donor and acceptor molecules
have been investigated by employing Raman spectroscopy. The G band softens
progressively with the increasing concentration of tetrathiafulvalene (TTF)
which is an electron donor while the band stiffens with increasing
concentration of tetracyanoethylene (TCNE) which is an electron-acceptor
molecule. Both TTF and TCNE broaden the G-band. The 2D band position is also
affected by interaction with TTF and TCNE. The intensity of the 2D-band
decreases markedly with the concentration of either. The ratio of intensities
of the 2D and G bands decreases with increase in TTF and TCNE concentrations.
The electrical resistivity of graphene varies in opposite directions on
interaction with TTF and TCNE. All these effects occur due to molecular
charge-transfer, as evidenced by the observation of charge-transfer bands in
the electronic absorption spectra.",0808.3165v1
2010-08-03,Electrical spin injection from an organic-based ferrimagnet in a hybrid organic/inorganic heterostructure,"We report the successful extraction of spin polarized current from the
organic-based room temperature ferrimagnetic semiconductor V[TCNE]x (x~2, TCNE:
tetracyanoethylene; TC ~ 400 K, EG ~ 0.5 eV, s ~ 10-2 S/cm) and its subsequent
injection into a GaAs/AlGaAs light-emitting diode (LED). The spin current
tracks the magnetization of V[TCNE]x~2, is weakly temperature dependent, and
exhibits heavy hole / light hole asymmetry. This result has implications for
room temperature spintronics and the use of inorganic materials to probe spin
physics in organic and molecular systems.",1008.0675v2
2013-06-03,Adsorption site determination of a molecular monolayer via inelastic tunneling,"We have combined scanning tunneling microscopy with inelastic electron
tunneling spectroscopy (IETS) and density functional theory (DFT) to study a
tetracyanoethylene monolayer on Ag(100). Images show that the molecules arrange
in locally ordered patterns with three non-equivalent, but undeterminable,
adsorption sites. While scanning tunneling spectroscopy only shows subtle
variations of the local electronic structure at the three different positions,
we find that vibrational modes are very sensitive to the local atomic
environment. IETS detects sizeable mode frequency shifts of the molecules
located at the three topographically detected sites, which permits us to
determine the molecular adsorption sites through identification with DFT
calculations.",1306.0463v1
2014-12-07,Simple Cubic Carbon Phase C21-sc: A Promising Superhard Carbon Conductor,"Traditionally, all superhard carbon phases including diamond are electric
insulators and all conductive carbon phases including graphite are mechanically
soft. Based on first-principles calculation results, we report a superhard but
conductive carbon phase C21-sc which can be obtained through increasing the sp3
bonds in the previously proposed soft and conductive phase C20-sc (Phys. Rev. B
74, 172101 2006). We also show that further increase of sp3 bonds in C21-sc
results in a superhard and insulating phase C22-sc with sp3 bonds only. With
C20-sc, C21-sc, C22-sc and graphite, the X-ray diffraction peaks from the
unidentified carbon material synthesized by compressing the mixture of
tetracyanoethylene and carbon black (Carbon, 41, 1309, 2003) can be understood.
In view of its positive stability, superhard and conductive features, and the
strong possibility of existence in previous experiments, C21-sc is a promising
multi-functional material with potential applications in extreme conditions.",1412.2301v2
2015-10-04,Thermoelectric properties of graphene/boron nitride heterostructures,"Using density functional theory combined with a Green's function scattering
approach, we examine the thermoelectric properties of hetero-nanoribbons formed
from alternating lengths of graphene and boron nitride. In such structures, the
boron nitride acts as a tunnel barrier, which weakly couples states in the
graphene, to form mini-bands . In un-doped nanoribbons, the mini bands are
symmetrically positioned relative to the Fermi energy and do not enhance
thermoelectric performance significantly. In contrast, when the ribbons are
doped by electron donating or electron accepting adsorbates, the thermopower S
and electronic figure of merit are enhanced and either positive or negative
thermopowers can be obtained. In the most favourable case, doping with the
electron donor tetrathiafulvalene (TTF) increases the room-temperature
thermopower to -284 {\mu}v/K and doping by the electron acceptor
tetracyanoethylene (TCNE) increases S to 210 {\mu}v/K. After including both
electron and phonon contributions to the thermal conductance, figures of merit
ZT up to of order 0.9 are obtained.",1510.00948v1
2016-01-02,Gating of single molecule junction conductance by charge transfer complex formation,"The solid-state structures of organic charge transfer (CT) salts are critical
in determining their mode of charge transport, and hence their unusual
electrical properties, which range from semiconducting through metallic to
superconducting. In contrast, using both theory and experiment, we show here
that the conductance of metal | single molecule | metal junctions involving
aromatic donor moieties (dialkylterthiophene, dialkylbenzene) increase by over
an order of magnitude upon formation of charge transfer (CT) complexes with
tetracyanoethylene (TCNE). This enhancement occurs because CT complex formation
creates a new resonance in the transmission function, close to the metal
contact Fermi energy, that is a signal of room-temperature quantum
interference.",1601.00222v1
2017-09-15,Leaving the Valley: Charting the Energy Landscape of Metal/Organic Interfaces via Machine Learning,"The rich polymorphism exhibited by inorganic/organic interfaces is a major
challenge for materials design. In this work we present a method to efficiently
explore the potential energy surface and predict the formation energies of
polymorphs and defects. This is achieved by training a machine learning model
on a list of only 100 candidate structures that are evaluated via
dispersion-corrected Density Functional Theory (DFT) calculations. We
demonstrate the power of this approach for tetracyanoethylene on Ag(100) and
explain the anisotropic ordering that is observed experimentally.",1709.05417v3
2018-09-24,Correlation of Kondo effect and molecular conformation of the acceptor molecule in the TTF-TCNE charge transfer complex,"A Kondo resonance has been observed on purely organic molecules in several
combinations of charge transfer complexes on a metal surface. It has been
regarded as a fingerprint of the transfer of one electron from the donor to the
extended $\pi$ orbital of the acceptor's LUMO. Here, we investigate the
stoichiometric checkerboard structure of tetrathiafulvalene (TTF) and
tetracyanoethylene (TCNE) on a Au(111) surface using scanning tunneling and
atomic force microscopy at 4.8 K. We find a bistable state of the TCNE
molecules with distinct structural and electronic properties. The two states
represent different conformations of the TCNE within the structure. One of them
exhibits a Kondo resonance, whereas the other one does not, despite of both
TCNE types being singly charged.",1809.09229v1
2021-06-30,Towards targeted kinetic trapping of organic-inorganic interfaces: A computational case study,"Properties of inorganic-organic interfaces, such as their interface dipole,
strongly depend on the structural arrangements of the organic molecules. A
prime example is tetracyanoethylene (TCNE) on Cu(111), which shows two
different phases with significantly different work functions. However, the
thermodynamically pre-ferred phase is not always the one that is best suited
for a given application. Rather, it may be desirable to selectively grow a
kinetically trapped structure. In this work, we employ density functional
theory and transi-tion state theory to discuss under which conditions such a
kinetic trapping might be possible for the model system of TCNE on Cu.
Specifically, we want to trap the molecules in the first layer in a flat-lying
orientation. This requires temperatures that are sufficiently low to suppress
the re-orientation of the molecules, which is thermodynamically more favorable
for high dosages, but still high enough to enable ordered growth through
diffusion of molecules. Based on the temperature-dependent diffusion and
re-orientation rates, we propose a temperature range at which the
re-orientation can be successfully suppressed.",2107.00477v2
2021-11-29,"Effect of salt concentration on the solubility, ion-dynamics, and transport properties of dissolved vanadium ions in lithium-ion battery electrolytes: Generalized solubility limit approach (Part II)","In this article, we study the transport properties of superconcentrated
electrolytes using Molecular Dynamics simulations, which have been shown
experimentally to retard elemental dissolution in vanadium containing cathode
materials. Five compositions between one and seven molar lithium
bis(trifluoromethanesulfonyl)imide in 1,3-Dioxolane and 1,2-Dimethoxyethane
solvent mixture are studied using non-polarizable Optimized Potentials for
Liquid Simulations - All Atom force field. The simulated physico-chemical
properties such as ionic conductivity, self-diffusion coefficients, and density
are observed to match well with the results obtained through experiments.
Radial Distribution Function analysis reveals a strong co-ordination between
salt anions and vanadium cations as the electrolyte transitions from a
salt-in-solvent type to solvent-in-salt type electrolyte. A high anion content
in the first solvation shell of vanadium cations is observed for
solvent-in-salt type electrolytes, through ion-clustering calculations.
Solvation free energy calculations using Free Energy Perturbation method
indicate that the active material dissolution should be retarded by using
superconcentrated electrolytes. Ion-dynamics of the clusters reveal that
vanadium cation transport occurs against its concentration gradient due to
strong coulombic interactions with the salt anions in superconcentrated
electrolytes. The improvement in the cycleability of several vanadium
containing cathode materials provides a robust proof for the theoretical
framework described in this manuscript.",2111.14591v1
2024-01-05,Vanadium oxide clusters in substellar atmospheres: A quantum chemical study,"We aim to understand the formation of cloud condensation nuclei in
oxygen-rich substellar atmospheres by calculating fundamental properties of the
energetically most favorable vanadium oxide molecules and clusters. A
hierarchical optimization approach is applied in order to find the most
favorable structures for clusters of (VO)$_{N}$ and (VO$_2$)$_{N}$ for N=1-10,
and (V$_2$O$_5$)$_{N}$ for N=1-4 and to calculate their thermodynamical
potentials. The candidate geometries are initially optimized applying classical
interatomic potentials and then refined at the B3LYP/cc-pVTZ level of theory to
obtain accurate zero-point energies and thermochemical quantities. We present
previously unreported vanadium oxide cluster structures as lowest-energy
isomers. We report revised cluster energies and their thermochemical
properties. Chemical equilibrium calculations are used to asses the impact of
the updated and newly derived thermodynamic potentials on the gas-phase
abundances of vanadium-bearing species. In chemical equilibrium, larger
clusters from different stoichiometric families are found to be the most
abundant vanadium-bearing species for temperatures below ~1000 K, while
molecular VO is the most abundant between ~1000 K and ~2000 K. We determine the
nucleation rates of each stoichiometric family for a given (T$_{gas}$,
p$_{gas}$) profile of a brown dwarf using classical and non-classical
nucleation theory. Small differences in the revised Gibbs free energies of the
clusters have a large impact on the abundances of vanadium bearing species in
chemical equilibrium at temperatures below ~1000 K, which subsequently has an
impact on the nucleation rates of each stoichiometric family. We find that with
the revised and more accurate cluster data non-classical nucleation rates are
up to 15 orders of magnitude higher than classical nucleation rates.",2401.02784v1
2001-03-21,Spin-Peierls transition with strong structural fluctuations in the vanadium oxide VOSb$_{2}$O$_{4}$,"We report on the magnetic susceptibility and electron spin resonance
measurements on polycrystalline samples of the vanadium oxide
VOSb$_{2}$O$_{4}$, a quasi-one dimensional S=1/2 Heisenberg system. We show
that the susceptibility vanishes at zero temperature, as in a gapped system,
and we argue that this is due to a spin-Peierls transition with strong
structural fluctuations.",0103441v1
2004-01-26,Strongly Non-Arrhenius Self-Interstitial Diffusion in Vanadium,"We study diffusion of self-interstitial atoms (SIAs) in vanadium via
molecular dynamics simulations. The <111>-split interstitials are observed to
diffuse one-dimensionally at low temperature, but rotate into other <111>
directions as the temperature is increased. The SIA diffusion is highly
non-Arrhenius. At T<600 K, this behavior arises from temperature-dependent
correlations. At T>600 K, the Arrhenius expression for thermally activated
diffusion breaks down when the migration barriers become small compared to the
thermal energy. This leads to Arrhenius diffusion kinetics at low T and
diffusivity proportional to temperature at high T.",0401525v1
2004-09-30,Dynamical singlets and correlation-assisted Peierls transition in VO2,"A theory of the metal-insulator transition in vanadium dioxide from the
high-temperature rutile to the low- temperature monoclinic phase is proposed on
the basis of cluster dynamical mean field theory, in conjunction with the
density functional scheme. The interplay of strong electronic Coulomb
interactions and structural distortions, in particular the dimerization of
vanadium atoms in the low temperature phase, plays a crucial role. We find that
VO2 is not a conventional Mott insulator, but that the formation of dynamical
V-V singlet pairs due to strong Coulomb correlations is necessary to trigger
the opening of a Peierls gap.",0410005v1
2005-05-08,Orbital Disordering and metal-insulator transition with hole-doping in perovskite-type vanadium oxides,"Filling-control metal-insulator transitions (MITs) and related electronic
phase diagrams have been investigated for hole-doped vanadium oxides,
Pr_{1-x}Ca_xVO_3, Nd_{1-x}Sr_xVO_3 and Y_{1-x}Ca_xVO_3, with perovskite
structure. The increase of the doping level x causes the melting of the G-type
(and C-type) orbital order, prior to or concomitantly with the MIT, due partly
to the doped-hole motion and partly to the ramdom potential arising from the
quenched disorder. In particular, the G-type spin- and C-type orbital-ordered
phase present in Y_{1-x}Ca_xVO_3 disappears immediately upon hole doping,
around x=0.02. On the other hand, the critical doping level x for MIT is
governed by the electron-correlation strength of the undoped parent compound.",0505190v1
2005-09-13,Electronic Structure and Exchange Interactions of Na$_{2}$V$_{3}$O$_{7}$,"We have performed first-principle calculations of the electronic structure
and exchange couplings for the nanotube compound Na$_{2}$V$_{3}$O$_{7}$ using
the LDA+U approach. Our results show that while the intra-ring exchange
interactions are mainly antiferromagnetic, the inter-ring couplings are {\it
ferromagnetic}. We argue that this is a consequence of the strong hybridization
between filled and vacant 3d vanadium orbitals due to the low symmetry of
Na$_{2}$V$_{3}$O$_{7}$, which results into strong - and often dominant -
ferromagnetic contributions to the total exchange interaction between vanadium
atoms. A comparison with results of previous works is included.",0509315v1
2005-11-30,Evidence for electronic phase separation between orbital orderings in SmVO3,"We report evidence for phase coexistence of orbital orderings of different
symmetry in SmVO$_3$ by high resolution X-Ray powder diffraction. The phase
coexistence is triggered by an antiferromagnetic ordering of the vanadium spins
near 130K, below an initial orbital ordering near 200K. The phase coexistence
is the result of the intermediate ionic size of samarium coupled to exchange
striction at the vanadium spin ordering.",0511744v1
2006-10-09,V2O3(0001) on Au(111) and W(110): Metal to Insulator Transition Induced by Surface Termination,"Thin films of V2O3 have been grown on Au(111) and W(110). It is possible to
prepare two different surface terminations: the first one is vanadium
terminated whereas the second one exhibits additional oxygen atoms, forming
vanadyl groups with the surface vanadium atoms. The electronic structure was
studied for both terminations by photoelectron spectroscopy. While the first
surface is metallic at room temperature like V2O3 bulk, the second surface with
the vanadyl groups shows a gap at the Fermi level.",0610232v2
2006-11-28,Correlated metallic state of vanadium dioxide,"The metal-insulator transition and unconventional metallic transport in
vanadium dioxide (VO$_2$) are investigated with a combination of spectroscopic
ellipsometry and reflectance measurements. The data indicates that electronic
correlations, not electron-phonon interactions, govern charge dynamics in the
metallic state of VO$_2$. This study focuses on the frequency and temperature
dependence of the conductivity in the regime of extremely short mean free path
violating the Ioffe-Regel-Mott limit of metallic transport. The standard
quasiparticle picture of charge conduction is found to be untenable in metallic
VO$_2$.",0611724v1
2006-12-13,Lattice dynamics and thermodynamics of bcc vanadium at high pressures,"We investigate the lattice dynamics and thermodynamics of nonmagnetic bcc
vanadium as a function of temperature and pressure, using the first principles
linear response linear-muffin-tin-orbital method. The calculated phonon density
of states (DOS) show strong temperature dependence, different from inelastic
neutron scattering measurements where the phonon DOS show little change from
room temperature up to 1273 K. We obtain the Helmholtz free energy including
both electronic and phonon contributions and calculate various equation of
state properties such as the bulk modulus and the thermal expansion
coefficient. A detailed comparison has been made with available experimental
measurements.",0612338v1
2007-02-08,Accurate estimations of circumstellar and interstellar lines of quadruply ionized vanadium using the coupled cluster approach,"Accurate {\it ab initio} calculations have been carried out to study the
valence electron removal energies and oscillator strengths of astrophysically
important electromagnetic transitions of quadruply ionized vanadium, $V^{4+}$.
Many important electron correlations are considered to all-orders using the
relativistic coupled-cluster theory. Calculated ionization potentials and fine
structure splittings are compared with the experimental values, wherever
available. To our knowledge, oscillator strengths of electric dipole
transitions are predicted for the first time for most of the transitions. The
transitions span in the range of ultraviolet, visible and near infrared regions
and are important for astrophysical observations.",0702066v1
2009-12-08,Analytical evidence for quantum states in aqueous vanadium pentoxide with positron lifetime spectroscopy,"The possibility of registration of quantum states, such as the coalescence of
droplets (tactoids) in the sol phase of aqueous vanadium pentoxide V$_2$O$_5$,
with positron annihilation lifetime spectroscopy is discussed. The decrease of
the long-living positronium (Ps) lifetime term in the result of the coalescence
of V$_2$O$_5$ tactoids is predicted.",0912.1570v3
2010-04-27,Magnetic Ordering in Blocking Layer and Highly Anisotropic Electronic Structure of High-Tc Iron-based Superconductor Sr2VFeAsO3: LDA+U Studies,"We calculate electronic structures of a high-Tc iron-based superconductor
Sr2VFeAsO3 by LDA+U method. We assume a checker-board antiferromagnetic order
on blocking layers including vanadium and strong correlation in d-orbits of
vanadium through the Hubbard U. While the standard LDA brings about metallic
blocking layers and complicated Fermi surface as in the previous literatures,
our calculation changes the blocking layer into insulating one and the Fermi
surface becomes quite similar to those of other iron-based superconductors.
Moreover, the appearance of the insulating blocking layers predicts high
anisotropy on quasi-particle transports and new types of intrinsic Josephson
effects.",1004.4741v1
2010-06-22,Hall carrier density and magnetoresistance measurements in thin film vanadium dioxide across the metal-insulator transition,"Temperature dependent magneto-transport measurements in magnetic fields of up
to 12 Tesla were performed on thin film vanadium dioxide (VO2) across the
metal-insulator transition (MIT). The Hall carrier density increases by 4
orders of magnitude at the MIT and accounts almost entirely for the resistance
change. The Hall mobility varies little across the MIT and remains low,
~0.1cm2/V sec. Electrons are found to be the major carriers on both sides of
the MIT. Small positive magnetoresistance in the semiconducting phase is
measured.",1006.4376v1
2010-10-13,Hybrid InAs nanowire-vanadium proximity SQUID,"We report the fabrication and characterization of superconducting quantum
interference devices (SQUIDs) based on InAs nanowires and vanadium
superconducting electrodes. These mesoscopic devices are found to be extremely
robust against thermal cycling and to operate up to temperatures of $\sim2.5$~K
with reduced power dissipation. We show that our geometry allows to obtain
nearly-symmetric devices with very large magnetic-field modulation of the
critical current. All these properties make these devices attractive for
on-chip quantum-circuit implementation.",1010.2545v1
2013-02-13,Theory of ferromagnetism in vanadium-oxide based perovskites,"The conditions under which ferromagnetism may occur in transition metal
oxides with partially filled $t_{2g}$ shells such as vanadium-based perovskites
are studied using a combination of density functional and single-site dynamical
mean field methods. For reasonable values of the correlation strength,
rotations of the VO$_6$ octahedra play an important role in enabling
ferromagnetism, with ferromagnetism typically occurring for rotations larger
than a nonzero critical value. Ferromagnetism is suppressed near the Mott
insulating phase but the phase boundary is otherwise only weakly dependent on
carrier concentration. Design rules are suggested for new oxide systems
exhibiting ferromagnetism.",1302.3062v2
2013-03-16,Designing ferromagnetism in vanadium-oxide based superlattices,"Motivated by recent reports (Phys. Rev. B\textbf{80}, 241102) of
room-temperature ferromagnetism in vanadium-oxide based superlattices, a
single-site dynamical mean field study of the dependence of the
paramagnetic-ferromagnetic phase boundary on superlattice geometry was
performed. An examination of variants of the experimentally determined crystal
structure indicate that ferromagnetism is found only in a small and probably
inaccessible region of the phase diagram. Design criteria for increasing the
range over which ferromagnetism might exist are proposed.",1303.4016v2
2013-09-27,Multiferroicity in V-doped PbTiO$_{3}$,"We report \emph{ab initio} predictions on the proper multiferroic
(ferromagnetic, insulating and ferroelectric) character of PbTiO$_{3}$ doped
with vanadium. V impurities coupled ferromagnetically carry a magnetization of
1 $\mu_{\rm B}$ each. The coupling is expected to be strong, since the
paramagnetic solution is higher by 150 meV/vanadium, and no stable
antiferromagnetic solution was found. The electronic gap in the doped system is
about 0.2-0.3 eV in GGA, hence the system is properly multiferroic. V doping
increases the spontaneous polarization in PbTiO$_{3}$, with an approximate
percentual rate of 0.7 $\mu$C/cm$^{2}$.",1309.7205v1
2014-04-07,A hybrid vanadium fluoride with structurally isolated S = 1 kagome layers,"A new organically-templated vanadium (III) fluoride, (NH4)2(C2H8N)[V3F12],
has been prepared using an ionothermal approach. This compound has a unique
layered structure featuring distorted S = 1 kagome planes separated by the
cationic species. The compound exhibits magnetic frustration, with a canted
antiferromagnetic ground state. On further cooling within the ground state a
pronounced change in magnetisation kinetics is observed.",1404.1755v1
2014-07-17,Effect of uniaxial strain on the site occupancy of hydrogen in vanadium from density-functional calculations,"We investigate the influence of uniaxial strain on site occupancy of hydrogen
vanadium, using density functional theory. The site occupancy is found to be
strongly influenced by the strain state of the lattice. The results provide the
conceptual framework of the atomistic description of the observed hysteresis in
the alpha to beta phase transition in bulk, as well as the preferred octahedral
occupancy of hydrogen in strained V layers.",1407.4652v1
2016-01-23,"Vanadium Dioxide: Metal-Insulator Transition, Electrical Switching and Oscillations. A Review of State of the Art and Recent Progress","Vanadium dioxide is currently considered as one of the most promising
metarials for oxide elcteronics. Both planar and sandwich thin-film MOM devices
based on VO2 exhibit electrical switching with an S-shaped I-V characteristic,
and this switching effect is associated with the metal-insulator transition
(MIT). In an electrical circuit containing such a switching device, relaxation
oscillations are observed if the load line intersects the I-V curve at a unique
point in NDR region. All these effects are potentially prospective for
designing various devices of oxide electronics, particularly, elements of
dynamical neural networks based on coupled oscillators.",1601.06246v1
2017-07-24,Magnetic properties of quasi one-dimensional vanadium-benzene nanowire affected by gas molecules: a first-principle study,"Magnetic properties of quasi one-dimensional vanadium-benzene nanowires
(VBNW) are investigated theoretically with the absorption of gas molecules-NO
and NO2. With the increase adsorption of NO on VBNW, the phase transition from
half metal to ferromagnetic metal and last to paramagnetic semiconductor can be
observed. With the increase of NO2 on VBNW, half metallic property can be
enhanced at first and decreased later. Thus, the electronic and magnetic
properties of VBNW can be sensitive and selective to NO and NO2, revealing the
potential applications in spintronic sensors of these two kinds of molecules.",1707.07423v1
2017-10-16,Vanadium substitution: a simple and efficient way to improve UV sensing in ZnO,"UV sensing in pure ZnO is due to oxygen adsorption/desorption process from
ZnO surface. Vanadium doping improves UV sensitivity of ZnO. Enhancement in UV
sensitivity in doped ZnO is attributed to trapping and de-trapping of electrons
at V4+ & V5+-related defect states. An extra electron in the V4+ state is
excited under UV illumination while in absence of the same a trapping happens
at the V5+ state. An insight to the mechanism is obtained by an analytic study
of the response phenomenon.",1710.05998v1
2019-01-16,Optical Properties of Vanadium in 4H Silicon Carbide for Quantum Technology,"We study the optical properties of tetravalent vanadium impurities in 4H
silicon carbide (4H SiC). Emission from two crystalline sites is observed at
wavelengths of 1.28 \mum and 1.33 \mum, with optical lifetimes of 163 ns and 43
ns. Group theory and ab initio density functional supercell calculations enable
unequivocal site assignment and shed light on the spectral features of the
defects. We conclude with a brief outlook on applications in quantum photonics.",1901.05371v2
2020-01-08,Electrical conductivity of vanadium dioxide switching channel,"The electrical conductivity of the switching channel of vanadium dioxide
thin-film sandwich structures is studied over a wide temperature range (15-300
K). It is shown that the electrical resistance of the channel varies with
temperature as R~exp(aT-b/T) in the high-temperature region (above 70 K). The
experimental results are discussed from the viewpoint of the small polaron
hopping conduction theory which takes into account the influence of thermal
lattice vibrations onto the resonance integral.",2001.03053v1
2020-01-08,UV-laser modification and selective ion-beam etching of amorphous vanadium pentoxide thin films,"We present the results on excimer laser modification and patterning of
amorphous vanadium pentoxide films. Wet positive resist-type and Ar ion-beam
negative resist-type etching techniques were employed to develop UV-modified
films. V2O5 films were found to possess sufficient resistivity compared to
standard electronic materials thus to be promising masks for sub-micron
lithog-raphy",2001.03054v1
2021-04-16,Ferromagnetism in 2D Vanadium Diselenide,"Two-dimensional (2D) Van der Waals ferromagnets carry the promise of
ultimately miniature spintronics and information storage devices. Among the
newly discovered 2D ferromagnets all inherit the magnetic ordering from their
bulk ancestors. Here we report a new 2D ferromagnetic semiconductor at room
temperature, 2H phase vanadium diselenide (VSe2) which show ferromagnetic at 2D
form only. This unique 2D ferromagnetic semiconductor manifests an enhanced
magnetic ordering owing to structural anisotropy at 2D limit.",2104.07937v1
2022-01-26,Terahertz Optical Properties and Birefringence in Single Crystal Vanadium doped [100] \b{eta}-Ga2O3,"We report the Terahertz optical properties of the Vanadium doped [100]
\b{eta}-Ga2O3 using Terahertz Time-Domain Spectroscopy (THz-TDS). The V-doped
\b{eta}-Ga2O3 crystal shows strong birefringence in the 0.2-2.4 THz range.
Further, phase retardation by the V-doped \b{eta}-Ga2O3 has been measured over
the whole THz range by Terahertz Time-Domain Polarimetry (THz-TDP). It is
observed that the V-doped \b{eta}-Ga2O3 crystal behaves both as a quarter
waveplate (QWP) at 0.38, 1.08, 1.71, 2.28 THz, and a half waveplate (HWP) at
0.74 and 1.94 THz, respectively.",2201.10740v1
2024-01-26,A Tunable Terahertz Metamaterial Perfect Absorber Based on Vanadium Dioxide,"In this paper, a tunable metamaterial perfect absorber based on vanadium
dioxide (VO2) is designed in the terahertz frequency range. The proposed
structure is simulated by the numerical method of three-dimensional Finite
Difference Time Domain (FDTD). Simulation results show that the absorption
spectrum of the device in the normal incidence of a plane wave light in the
range of 1 THz to 12 THz, has three polarization-insensitive resonance peaks
whose amplitude changes for the different conductivities of VO2 and shifts in a
certain frequency range. This device also having a tolerance of 6 degrees of
incidence angle can provide a relatively stable absorption spectrum.",2401.15218v1
2006-08-08,Multi-vanadium substituted polyoxometalates as efficient electrocatalysts for the oxidation of l-cysteine at low potential on glassy carbon electrodes,"The electrochemical behaviours of the sandwich-type complex
[As2W18(VO)3O66]11- were studied in a pH 7 medium and compared with those of
the three following Dawson-type vanadium-substituted complexes: [P2V2W16O62]8-
(P2V2W16), [P2MoV2W15O62]8- (P2MoV2W15) and [P2V3W15O62]9- (P2V3W15).
Electrochemistry shows that the sandwich-type POM contains 2 VIV centers and
one VV center and must be formulated As2V2IVVW18, in agreement with titration,
elemental analysis and magnetic measurements on this element.. All the POMs of
this work proved efficient for the oxidation of L-cysteine. Comparison of the
present results with those of mono-Vanadium substituted POMs indicates that
accumulation of vanadium atoms in the POM framework is beneficial in the
electrocatalytic process. In addition, the present work highlights the
important influence of the POM structure in the electrocatalytic oxidation of
L-cysteine. The remarkable outcome of this work is that the potential for the
oxidation of L-cysteine in the presence of the selected POMs has been
substantially driven in the negative direction compared to the case of glassy
carbon alone, a feature which is associated with faster kinetics. The stability
of the systems must also be pointed out.",0608085v1
2016-09-12,"Structure, hydrolysis and diffusion of aqueous vanadium ions from Car-Parrinello molecular dynamics","A molecular level understanding of the properties of electroactive vanadium
species in aqueous solution is crucial for enhancing the performance of
vanadium redox flow batteries (RFB). Here, we employ Car-Parrinello molecular
dynamics (CPMD) simulations based on density functional theory to investigate
the hydration structures, first hydrolysis reaction and diffusion of aqueous
V$^{2+}$, V$^{3+}$, VO$^{2+}$, and VO$_2^+$ ions at 300 K. The results indicate
that the first hydration shell of both V$^{2+}$ and V$^{3+}$ contains six water
molecules, while VO$^{2+}$ is coordinated to five and VO$_2^+$ to three water
ligands. The first acidity constants (p$K_\mathrm{a}$) estimated using
metadynamics simulations are 2.47, 3.06 and 5.38 for aqueous V$^{3+}$, VO$_2^+$
and VO$^{2+}$, respectively, while V$^{2+}$ is predicted to be a fairly weak
acid in aqueous solution with a p$K_\mathrm{a}$ value of 6.22. We also show
that the presence of chloride ions in the first coordination sphere of the
aqueous VO$_2^+$ ion has a significant impact on water hydrolysis leading to a
much higher p$K_\mathrm{a}$ value of 4.8. This should result in a lower
propensity of aqueous VO$_2^+$ for oxide precipitation reaction in agreement
with experimental observations for chloride-based electrolyte solutions. The
computed diffusion coefficients of vanadium species in water at room
temperature are found to increase as V$^{3+}$ $<$ VO$_2^+$ $<$ VO$^{2+}$ $<$
V$^{2+}$ and thus correlate with the simulated hydrolysis constants, namely,
the higher the p$K_\mathrm{a}$ value, the greater the diffusion coefficient.",1609.03463v1
2017-01-19,Ultrafast Electron-Lattice Coupling Dynamics in VO2 and V2O3 Thin Films,"Ultrafast optical pump - optical probe and optical pump - terahertz probe
spectroscopy were performed on vanadium dioxide (VO2) and vanadium sesquioxide
(V2O3) thin films over a wide temperature range. A comparison of the
experimental data from these two different techniques and two different
vanadium oxides, in particular a comparison of the electronic oscillations
generated by the photoinduced longitudinal acoustic modulation, reveals the
strong electron-phonon coupling that exists in the metallic state of both
materials. The low energy Drude response of V2O3 appears more susceptible than
VO2 to ultrafast strain control. Additionally, our results provide a
measurement of the temperature dependence of the sound velocity in both
systems, revealing a four- to fivefold increase in VO2 and a three- to fivefold
increase in V2O3 across the phase transition. Our data also confirm
observations of strong damping and phonon anharmonicity in the metallic phase
of VO2, and suggest that a similar phenomenon might be at play in the metallic
phase of V2O3. More generally, our simple table-top approach provides relevant
and detailed information about dynamical lattice properties of vanadium oxides,
opening the way to similar studies in other complex materials.",1701.05531v1
2018-03-21,3D-printed conductive static mixers enable all-vanadium redox flow battery using slurry electrodes,"State-of-the-art all-vanadium redox flow batteries employ porous carbonaceous
materials as electrodes. The battery cells possess non-scalable fixed
electrodes inserted into a cell stack. In contrast, a conductive particle
network dispersed in the electrolyte, known as slurry electrode, may be
beneficial for a scalable redox flow battery. In this work, slurry electrodes
are successfully introduced to an all-vanadium redox flow battery. Activated
carbon and graphite powder particles are dispersed up to 20 wt% in the vanadium
electrolyte and charge-discharge behavior is inspected via polarization
studies. Graphite powder slurry is superior over activated carbon with a
polarization behavior closer to the standard graphite felt electrodes.
3D-printed conductive static mixers introduced to the slurry channel improve
the charge transfer via intensified slurry mixing and increased surface area.
Consequently, a significant increase in the coulombic efficiency up to 95% and
energy efficiency up to 65% is obtained. Our results show that slurry
electrodes supported by conductive static mixers can be competitive to
state-of-the-art electrodes yielding an additional degree of freedom in battery
design. Research into carbon properties (particle size, internal surface area,
pore size distribution) tailored to the electrolyte system and optimization of
the mixer geometry may yield even better battery properties.",1804.05759v1
2019-03-06,Effective interaction for vanadium oxyhydrides Sr$_{n+1}$V$_n$O$_{2n+1}$H$_n$ ($n=1$ and $n\to\infty$): A constrained-RPA study,"Transition metal oxides have been one of the central objects in the studies
of electron correlation effects because of their rich variety of physical
properties mainly depending on the transition metal element. On the other hand,
exploiting the anion degrees of freedom is less popular but can be another
promising way to control properties of strongly correlated materials. In
particular, oxyhydrides offer a unique playground of strongly correlated
low-dimensional electronic structure, where the $s$ orbitals of hydrogen breaks
a chemical bond between the cation $t_{2g}$ orbitals. In this study, we
evaluate the effective interaction, i.e., the screened Coulomb interaction
parameters in low-energy effective models, for vanadium oxyhydrides
Sr$_{n+1}$V$_n$O$_{2n+1}$H$_n$ ($n=1,\infty$) using the constrained
random-phase approximation (cRPA). We find that the effective interaction in
the $t_{2g}$ model, where only the $t_{2g}$ orbitals are explicitly considered,
is strongly screened by the $e_g$ bands compared with that for oxides, because
the $e_g$ bands are much entangled with the $t_{2g}$ bands in the oxyhydrides.
On the other hand, the effective interaction is rather strong in the $d$ model,
where all the vanadium $d$ orbitals are explicitly considered, owing to a large
energy separation between the V-$d$ bands and the anion bands (O-$p$ and
H-$s$), because the O-$p$ states are stabilized by the existence of the
hydrogen atoms. These findings suggest that non-trivial and unique correlation
effects can take place in vanadium oxyhydrides.",1903.02222v2
2020-05-01,Tunable Ferromagnetism and Thermally Induced Spin Flip in Vanadium-doped Tungsten Diselenide Monolayers at Room Temperature,"The outstanding optoelectronic and valleytronic properties of transition
metal dichalcogenides (TMDs) have triggered intense research efforts by the
scientific community. An alternative to induce long-range ferromagnetism (FM)
in TMDs is by introducing magnetic dopants to form a dilute magnetic
semiconductor. Enhancing ferromagnetism in these semiconductors not only
represents a key step towards modern TMD-based spintronics, but also enables
exploration of new and exciting dimensionality-driven magnetic phenomena. To
this end, we show tunable ferromagnetism at room temperature and a thermally
induced spin flip (TISF) in monolayers of V-doped WSe2. As vanadium
concentrations increase within the WSe2 monolayers the saturation magnetization
increases, and it is optimal at ~4at.% vanadium; the highest doping/alloying
level ever achieved for V-doped WSe2 monolayers. The TISF occurs at ~175 K and
becomes more pronounced upon increasing the temperature towards room
temperature. We demonstrate that TISF can be manipulated by changing the
vanadium concentration within the WSe2 monolayers. We attribute TISF to the
magnetic field and temperature dependent flipping of the nearest W-site
magnetic moments that are antiferromagnetically coupled to the V magnetic
moments in the ground state. This is fully supported by a recent spin-polarized
density functional theory calculation. Our findings pave the way for the
development of novel spintronic and valleytronic nanodevices based on
atomically thin magnetic semiconductors and stimulate further studies in this
rapidly expanding research field of 2D magnetism.",2005.00493v1
2022-01-31,Evidence for mechanical softening-hardening dual anomaly in transition metals from shock compressed vanadium,"Solid usually becomes harder and tougher under compression, and turns softer
at elevated temperature. Recently, compression-induced softening and
heating-induced hardening (CISHIH) dual anomaly was predicted in group VB
elements such as vanadium. Here, the evidence for this counterintuitive
phenomenon is reported. By using accurate high-temperature high-pressure sound
velocities measured at Hugoniot states generated by shock-waves, together with
first-principles calculations, we observe not only the prominent
compression-induced sound velocity reduction, but also strong heating-induced
sound velocity enhancement, in shocked vanadium. The former corresponds to the
softening in shear modulus by compression, whereas the latter reflects the
reverse hardening by heat. These experiments also unveil another anomaly in
Young's modulus that wasn't reported before. Based on the experimental and
theoretical data, we infer that vanadium might transition from BCC into two
different rhombohedral (RH1 and RH2) phases at about 79GPa and 116GPa along the
Hugoniot, respectively, which implies a dramatic difference in static and
dynamic loading, as well as the significance of deviatoric stress and
rate-relevant effects in high-pressure phase transition dynamics.",2201.13085v1
2022-10-18,Coherent spin dynamics of hyperfine-coupled vanadium impurities in silicon carbide,"Progress with quantum technology has for a large part been realized with the
nitrogen-vacancy centre in diamond. Part of its properties, however, are
nonideal and this drives research into other spin-active crystal defects.
Several of these come with much stronger energy scales for spin-orbit and
hyperfine coupling, but how this affects their spin coherence is little
explored. Vanadium in silicon carbide is such a system, with technological
interest for its optical emission at a telecom wavelength and compatibility
with semiconductor industry. Here we show coherent spin dynamics of an ensemble
of vanadium defects around a clock-transition, studied while isolated from, or
coupled to neighbouring nuclear spins. We find spin dephasing times up to 7.2
$\mu$s, and via spin-echo studies coherence lifetimes that go well beyond tens
of microseconds. We demonstrate operation points where strong coupling to
neighbouring nuclear spins does not compromise the coherence of the central
vanadium spin, which identifies how these can be applied as a coherent spin
register. Our findings are relevant for understanding a wide class of defects
with similar energy scales and crystal symmetries, that are currently explored
in diamond, silicon carbide, and hexagonal boron nitride.",2210.09942v1
2023-06-29,"Novel 2D vanadium sulphides: synthesis, atomic structure engineering and charge density waves","Two new ultimately thin vanadium rich 2D materials based on VS2 are created
via molecular beam epitaxy and investigated using scanning tunneling
microscopy, X-ray photoemission spectroscopy and density-functional theory
calculations. The controlled synthesis of stoichiometric single-layer VS2 or
either of the two vanadium-rich materials is achieved by varying the sample
coverage and the sulphur pressure during annealing. Through annealing of small
stoichiometric single-layer VS2 islands without S pressure, S-vacancies
spontaneously order in 1D arrays, giving rise to patterned adsorption. Via the
comparison of density-functional theory calculations with scanning tunneling
microscopy data, the atomic structure of the S-depleted phase, with a
stoichiometry of V4S7, is determined. By depositing larger amounts of vanadium
and sulphur, which are subsequently annealed in a S-rich atmosphere,
self-intercalated ultimately thin V5S8-derived layers are obtained, which host
2x2 V-layers between sheets of VS2. We provide atomic models for the thinnest
V5S8-derived structures. Finally, we use scanning tunneling spectroscopy to
investigate the charge density wave observed in the 2D V5S8-derived islands.",2306.16792v2
2023-09-21,Analytical electron microscopy analysis of insulating and metallic phases in nanostructured vanadium dioxide,"Vanadium dioxide (VO$_2$) is a strongly-correlated material that exhibits
insulator-to-metal transition (IMT) near room temperature, which makes it a
promising candidate for applications in nanophotonics or optoelectronics.
However, creating VO$_2$ nanostructures with the desired functionality can be
challenging due to microscopic inhomogeneities that can significantly impact
the local optical and electronic properties. Thin lamellas, produced by focused
ion beam milling from a homogeneous layer, provide a useful prototype for
studying VO$_2$ at the truly microscopic level using a scanning transmission
electron microscope (STEM). High-resolution imaging is used to identify
structural inhomogeneities while electron energy-loss spectroscopy (EELS)
supported by statistical analysis helps to detect V$_x$O$_y$ stoichiometries
with a reduced oxidation number of vanadium at the areas of thickness below 70
nm. On the other hand, the thicker areas are dominated by vanadium dioxide,
where the signatures of IMT are detected in both core-loss and low-loss EELS
experiments with in-situ heating. The experimental results are interpreted with
ab-initio and semi-classical calculations. This work shows that structural
inhomogeneities such as pores and cracks present no harm to the desired optical
properties of VO$_2$ samples.",2309.11980v2
2024-01-30,Vanadium-Doped Molybdenum Disulfide Monolayers with Tunable Electronic and Magnetic Properties: Do Vanadium-Vacancy Pairs Matter?,"Monolayers of molybdenum disulfide (MoS2) are the most studied
two-dimensional (2D) transition-metal dichalcogenides (TMDs), due to its
exceptional optical, electronic, and opto-electronic properties. Recent studies
have shown the possibility of incorporating a small amount of magnetic
transition metals (e.g., Fe, Co, Mn, V) into MoS2 to form a 2D dilute magnetic
semiconductor (2D-DMS). However, the origin of the observed ferromagnetism has
remained elusive, due to the presence of randomly generated sulfur vacancies
during synthesis that can pair with magnetic dopants to form complex
dopant-vacancy configurations altering the magnetic order induced by the
dopants. By combining high-angle annular dark-field scanning transmission
electron microscopy (HAADF-STEM) imaging with first-principles density
functional theory (DFT) calculations and magnetometry data, we demonstrate the
critical effects of sulfur vacancies and their pairings with vanadium atoms on
the magnetic ordering in V-doped MoS2 (V-MoS2) monolayers. Additionally, we
fabricated a series of field effect transistors on these V-MoS2 monolayers and
observed the emergence of p-type behavior as the vanadium concentration
increased. Our study sheds light on the origin of ferromagnetism in V-MoS2
monolayers and provides a foundation for future research on defect engineering
to tune the electronic and magnetic properties of atomically thin TMD-based
DMSs.",2401.16806v1
2001-11-14,Interlayer exchange coupling and giant magnetoresistance in Fe/V (001) superlattices,"Magnetization and magnetoresistivity studies of Fe/V (001) superlattices are
reported. The first giant magnetoresistance peak with respect to the vanadium
and iron layer thicknesses is investigated. The interlayer antiferromagnetic
coupling strength is found to show a peak at a vanadium layer thickness of
  13 atomic monolayers ($\approx$ 20 \AA) with a full width at half maximum of
about 2 monolayers. The antiferromagnetic coupling shows a maximum at an iron
layer thickness of about 6 monolayers ($\approx$ 9 \AA) for series of
superlattices with vanadium thicknesses around 13 monolayers. The magnitude of
the giant magnetoresistance shows a similar variation as the antiferromagnetic
coupling strength.",0111255v1
2003-05-05,Molecular dynamics study of melting of a bcc metal-vanadium II : thermodynamic melting,"We present molecular dynamics simulations of the thermodynamic melting
transition of a bcc metal, vanadium using the Finnis-Sinclair potential. We
studied the structural, transport and energetic properties of slabs made of 27
atomic layers with a free surface. We investigated premelting phenomena at the
low-index surfaces of vanadium; V(111), V(001), and V(011), finding that as the
temperature increases, the V(111) surface disorders first, then the V(100)
surface, while the V(110) surface remains stable up to the melting temperature.
Also, as the temperature increases, the disorder spreads from the surface layer
into the bulk, establishing a thin quasiliquid film in the surface region. We
conclude that the hierarchy of premelting phenomena is inversely proportional
to the surface atomic density, being most pronounced for the V(111) surface
which has the lowest surface density.",0305082v1
2004-03-29,The vanadium Magneli phases VnO(2n-1),"To compare the metal-insulator transitions (MITs) of VO2 and V2O3 we analyze
the relations between the structural and electronic properties of the vanadium
Magneli phases. These materials set up the homologous series VnO(2n-1) (3 <= n
<= 9) and have crystal structures comprising typical dioxide-like and
sesquioxide-like regions. As the MITs of the vanadium Magneli phases are
accompanied by structural transformations, we are able to discuss the effects
of characteristic changes in the local atomic environments. The systematic
investigation of the transport properties is based on a new and unifying
description of the crystal structures of the Magneli phases including VO2 and
V2O3. Our results lead to a comprehensive understanding of the MITs in the
Magneli class and shed new light on the role of particular electronic states
for the MIT of V2O3.",0403689v1
2009-04-03,Metal-insulator transition in vanadium dioxide nanobeams: probing sub-domain properties of strongly correlated materials,"Many strongly correlated electronic materials, including high-temperature
superconductors, colossal magnetoresistance and metal-insulator-transition
(MIT) materials, are inhomogeneous on a microscopic scale as a result of domain
structure or compositional variations. An important potential advantage of
nanoscale samples is that they exhibit the homogeneous properties, which can
differ greatly from those of the bulk. We demonstrate this principle using
vanadium dioxide, which has domain structure associated with its dramatic MIT
at 68 degrees C. Our studies of single-domain vanadium dioxide nanobeams reveal
new aspects of this famous MIT, including supercooling of the metallic phase by
50 degrees C; an activation energy in the insulating phase consistent with the
optical gap; and a connection between the transition and the equilibrium
carrier density in the insulating phase. Our devices also provide a
nanomechanical method of determining the transition temperature, enable
measurements on individual metal-insulator interphase walls, and allow general
investigations of a phase transition in quasi-one-dimensional geometry.",0904.0596v1
2010-04-19,Evidence for strong Coulomb correlations in metallic phase of vanadium dioxide,"The influence of Coulomb correlation on magnetic and spectral properties in
metallic rutile phase of vanadium dioxide is studied by state of the art
LDA+DMFT method. Calculation results in strongly correlated metallic state with
an effective mass renormalization $m^*/m\approx2$. Uniform magnetic
susceptibility shows Curie-Weiss temperature dependence with effective magnetic
moment, $p_{eff}^{theor} = 1.54 \mu_B$, in a good agreement with the
experimental value $p_{eff}^{exp} = 1.53 \mu_B$ that is close to ideal value
for V$^{4+}$ ion with the spin $S=1/2$, $p_{eff} = 1.73 \mu_B$. Calculated
spectral function shows well developed Hubbard bands observabale in the recent
experimental photoemission spectra. We conclude that VO2 in rutile phase is
strongly correlated metal with local magnetic moments formed by vanadium
$d$-electrons.",1004.3214v1
2010-07-20,Quantum criticality of vanadium chains with strong relativistic spin-orbit interaction,"We study quantum phase transitions induced by the on-site spin-orbit
interaction lambda(L.S) in a toy model of vanadium chains. In the lambda->0
limit, the decoupled spin and orbital sectors are described by a Haldane and an
Ising chain, respectively. The gapped ground state is composed of a
ferro-orbital order and a spin liquid with finite correlation lengths. In the
opposite limit, strong spin-orbital entanglement results in a simultaneous spin
and orbital-moment ordering, which can be viewed as an orbital liquid. Using a
combination of analytical arguments and density-matrix renormalization group
calculation, we show that an intermediate phase, where the ferro-orbital state
is accompanied by a spin Neel order, is bounded on both sides by Ising
transition lines. Implications for vanadium compounds CaV2O4 and ZnV2O4 are
also discussed.",1007.3472v3
2010-07-22,Strong reduction of V4+ amount in vanadium oxide/hexadecylamine nanotubes by doping with Co2+ and Ni2+ ions: EPR and magnetic studies,"In this work we present a complete characterization and magnetic study of
vanadium oxide/hexadecylamine nanotubes (VOx/Hexa NT's) doped with Co2+ and
Ni2+ ions. The morphology of the NT's has been characterized by Transmission
Electron Microscopy (TEM) while the metallic elements have been quantified by
Instrumental Neutron Activation Analysis (INAA) technique. The static and
dynamic magnetic properties were studied collecting data of magnetization as a
function of magnetic field and temperature and by Electron Paramagnetic
Resonance (EPR). We observed that the incorporation of metallic ions (Co2+,
S=3/2 and Ni2+, S=1) decrease notably the amount of V4+ ions in the system,
from 14-16% (non-doped case) to 2-4%, with respect to the total vanadium atoms
into the tubular nanostructure, improving considerably their potential
technological applications as Li-ion batteries cathodes.",1007.3968v1
2011-04-15,The effect of hydrogen on the magnetic properties of FeV superlattice,"The electronic and magnetic structures of a hydrogenated and hydrogen free
superlattice of 3 iron monolayers and 9 vanadium monolayers are studied using
the first principle full-potential augmented-plane-wave method as implemented
in WIEN2k package. The volume, the total energy and the magnetic moments of the
system are studied versus the hydrogen positions at the octahedral sites within
the superlattice and also versus the filling of the vanadium octahedral
location by hydrogen atoms. It is found that the hydrogen locations at the
interior of vanadium layer are energetically more favourable. The local Fe
magnetic moment and the average magnetic moment per supercell are found to
increase as the H position moves towards the Fe-V interface. On the other hand,
the average magnetic moment per supercell is found to initially decrease up to
filling by 3 H atoms and then increases afterwards. To our knowledge, this is
the first reporting on the increase in the computed magnetic moment with
hydrogenation. These trends of magnetic moments are attributed to the volume
changes resulting from hydrogenation and not to electronic hydrogen-metal
interaction.",1104.2965v1
2011-12-30,Refractive index of vanadium determined by resonant diffraction of soft x-rays,"The dispersive part of the refractive index of vanadium is determined by
measuring the angular displacement of the first order diffraction peak of a
V/Fe superlattice. The measurements were made using elliptically polarized
synchrotron radiation which was scanned through the V L2,3 absorption edges for
different incident scattering angles. The x-ray scattering technique provides
access to direct determination of the dispersive part of the refractive index
through an absorption resonance. The influence of absorption at the resonances
is shown by comparing the absorption correction to the dispersion correction.
The results demonstrate that 1-{\delta} is larger than unity at the L2,3
resonances of vanadium and the optical consequences are discussed.",1201.0109v1
2013-01-20,Theory of resonant inelastic X-ray scattering in vanadium oxides: how to detect $d$-$d$ excitations?,"Generic low-energy spectral features related to $d$-$d$ excitations in nearly
cubic vanadium perovskites are predicted for the expected $L$-edge resonant
inelastic X-ray scattering (RIXS) measurements. Model Hamiltonian describing
local electronic properties, including crystal-field effects, of vanadium 3d
orbitals in the basic $t_{2g}^2$ configuration is formulated with the help of
complementary {\it ab initio} quantum-chemical cluster calculations. In the
presence of 2p-core hole, the local Hamiltonian includes strong 2p-3d electron
interactions. As a prerequisite for evaluating RIXS transition amplitudes
beyond the fast collision approximation, a symmetry-group approach is applied
to generate a basis set of many-electron wavefunctions of the intermediate
core-hole states accessible in RIXS processes. Although a comprehensive
description of the core-hole multiplets still remains a formidable task and
requires using specially designed numerical codes, for particular resonant
states the analysis is simplified and the calculation of RIXS amplitude can be
carried out analytically to the end.",1301.4683v1
2013-07-17,Magnetic Field Induced Transition in Vanadium Spinels,"We study vanadium spinels $A$V$_2$O$_4$ ($A$ = Cd, Mg) in pulsed magnetic
fields up to 65 T. A jump in magnetization at $\mu_0 H \approx$ 40 T is
observed in the single-crystal MgV$_2$O$_4$, indicating a field induced quantum
phase transition between two distinct magnetic orders. In the multiferroic
CdV$_2$O$_4$, the field-induced transition is accompanied by a suppression of
the electric polarization. By modeling the magnetic properties in the presence
of strong spin-orbit coupling characteristic of vanadium spinels, we show that
both features of the field-induced transition can be successfully explained by
including the effects of the local trigonal crystal field.",1307.4775v2
2013-10-03,Computation of the correlated metal-insulator transition in vanadium dioxide from first principles,"Vanadium dioxide(VO$_2$) is a paradigmatic example of a strongly correlated
system that undergoes a metal-insulator transition at a structural phase
transition. To date, this transition has necessitated significant post-hoc
adjustments to theory in order to be described properly. Here we report
standard state-of-the-art first principles quantum Monte Carlo (QMC)
calculations of the structural dependence of the properties of VO$_2$. Using
this technique, we simulate the interactions between electrons explicitly,
which allows for the metal-insulator transition to naturally emerge,
importantly without ad-hoc adjustments. The QMC calculations show that the
structural transition directly causes the metal-insulator transition and a
change in the coupling of vanadium spins. This change in the spin coupling
results in a prediction of a momentum-independent magnetic excitation in the
insulating state. While two-body correlations are important to set the stage
for this transition, they do not change significantly when VO$_2$ becomes an
insulator. These results show that it is now possible to account for electron
correlations in a quantitatively accurate way that is also specific to
materials.",1310.1066v2
2013-12-30,Ultracompact Vanadium Dioxide Dual-Mode Plasmonic Waveguide Electroabsorption Modulator,"Subwavelength modulators play an indispensable role in integrated
photonic-electronic circuits. Due to weak light-matter interactions, it is
always a challenge to develop a modulator with a nanometer scale footprint, low
switching energy, low insertion loss and large modulation depth. In this paper,
we propose the design of a vanadium dioxide dual-mode plasmonic waveguide
electroabsorption modulator using a metal-insulator-VO$_2$-insulator-metal
(MIVIM) waveguide platform. By varying the index of vanadium dioxide, the
modulator can route plasmonic waves through the low-loss dielectric insulator
layer during the ""on"" state and high-loss VO$_2$ layer during the ""off"" state,
thereby significantly reducing the insertion loss while maintaining a large
modulation depth. This ultracompact waveguide modulator, for example, can
achieve a large modulation depth of ~10dB with an active size of only
200x50x220nm$^3$ (or ~{\lambda}$^3$/1700), requiring a drive-voltage of ~4.6V.
This high performance plasmonic modulator could potentially be one of the keys
towards fully-integrated plasmonic nanocircuits in the next-generation chip
technology.",1312.7636v1
2014-01-16,Separating electric field and thermal effects across the metal-insulator transition in vanadium oxide nanobeams,"We present results from an experimental study of the equilibrium and
non-equilibrium transport properties of vanadium oxide nanobeams near the
metal-insulator transition (MIT). Application of a large electric field in the
insulating phase across the nanobeams produces an abrupt MIT and the individual
roles of thermal and non-thermal effects in driving the transition are studied.
Transport measurements at temperatures ($T$) far below the critical temperature
($T_c$) of MIT, in several nanoscale vanadium oxide devices, show that both $T$
and electric field play distinctly separate, but critical roles in inducing the
MIT. Specifically, at $T << T_c$ electric field dominates the MIT through an
avalanche-type process, whereas thermal effects become progressively critical
as $T$ approaches $T_c$.",1401.4129v1
2014-07-04,A photoinduced metallic phase of monoclinic vanadium dioxide,"The complex interplay between several active degrees of freedom (charge,
lattice, orbtial and spin order) is thought to determine the electronic
properties of many oxides, but the respective role of the various contributions
is often extremely difficult to determine. Vanadium dioxide (VO$_2$) is a
particularly notorious example. Here we report on combined ultrafast electron
diffraction (UED) and infrared transmissivity experiments in which we directly
watch and separate the lattice and charge density reorganizations that are
associated with the optically-induced semiconductor-metal transition (SMT) in
VO$_2$. These studies have uncovered a previously unreported photoinduced
transition to a metastable state with the periodic lattice distortion (PLD)
characteristic of the insulator intact, but differing by a 1D rearrangement of
charge density along the octahedrally coordinated vanadium dimer chains and a
transition to metal-like mid IR optical properties. The results demonstrate
that UED is capable of following details of both lattice and electronic
structural dynamics on the ultrafast timescale.",1407.1304v1
2014-11-27,The critical dynamics of the models of iron-vanadium magnetic superlattice,"We report the results of a numerical investigation of static and dynamic
critical behavior of the anisotropic easy-plane Heisenberg model which is used
as a model for iron-vanadium magnetic superlattices. Models of iron-vanadium
magnetic superlattice investigated with different values of intralayer and
interlayer exchange interactions ratio. Numerical experiment technique, based
on the dynamic finite-size scaling theory. Basic features of the time evolution
of dynamic parameters for the researched systems are studied. The effect of
interlayer exchange interaction value on a character of the critical dynamics
is determined. It is confirmed the possibility of application of the
Hamiltonian with strong easy-plane anisotropy for investigation both static and
dynamic critical behavior of complex planar magnetic models.",1411.7509v1
2016-03-03,Measurement of collective excitations in VO$_2$ by resonant inelastic X-ray scattering,"Vanadium dioxide is of broad interest as a spin-1/2 electron system that
realizes a metal-insulator transition near room temperature, due to a
combination of strongly correlated and itinerant electron physics. Here,
resonant inelastic X-ray scattering is used to measure the excitation spectrum
of charge, spin, and lattice degrees of freedom at the vanadium L-edge under
different polarization and temperature conditions. These spectra reveal the
evolution of energetics across the metal-insulator transition, including the
low temperature appearance of a strong candidate for the singlet-triplet
excitation of a vanadium dimer.",1603.01164v1
2017-03-07,Pressure-Temperature Phase Diagram of Vanadium Dioxide,"The complexity of strongly correlated electron physics in vanadium dioxide is
exemplified as its rich phase diagrams of all kinds, which in turn shed light
on the mechanisms behind its various phase transitions. In this work, we map
out the hydrostatic pressure - temperature phase diagram of vanadium dioxide
nanobeams by independently varying pressure and temperature with a diamond
anvil cell. In addition to the well-known insulating M1 (monoclinic) and
metallic R (tetragonal) phases, the diagram identifies the existence at high
pressures of the insulating M1' (monoclinic, more conductive than M1) phase,
and two metallic phases of X (monoclinic) and O (orthorhombic, at high
temperature only). Systematic optical and electrical measurements combined with
density functional calculations allow us to delineate their phase boundaries as
well as reveal some basic features of the transitions.",1703.02477v1
2017-12-19,Vanadium Transitions in the Spectrum of Arcturus,"We derive a new abundance for vanadium in the bright, mildly metal-poor red
giant Arcturus. This star has an excellent high-resolution spectral atlas and
well-understood atmospheric parameters, and it displays a rich set of neutral
vanadium lines that are available for abundance extraction. We employ a newly
recorded set of laboratory FTS spectra to investigate any potential
discrepancies in previously reported V I log(gf) values near 900 nm. These new
spectra support our earlier laboratory transition data and the calibration
method utilized in that study. We then perform a synthetic spectrum analysis of
weak V I features in Arcturus, deriving log {\epsilon}(V) = 3.54 $\pm$ 0.01
({\sigma} = 0.04) from 55 lines. There are no significant abundance trends with
wavelength, line strength, or lower excitation energy.",1712.06942v1
2018-09-18,Vertex corrections in self-consistent GW$Γ$ calculations: ground state properties of vanadium,"Self-consistent vertex corrected GW calculations have been performed to
evaluate the equilibrium volume and corresponding bulk modulus of 3d transition
metal vanadium. The study demonstrates the feasibility of this approach. The
accuracy of the results obtained for vanadium is considerably better as
compared to the accuracy of the results obtained with self consistent GW method
without vertex corrections. It is shown that vertex corrections are important
not only for the self energy, but also for the irreducible polarizability. The
contribution to the value of the $\Psi$-functional from the vertex corrections
is about 30 times smaller than the contribution from the GW part (correlation
only!), suggesting the fast convergence of the expansion in terms of the
screened interaction for this material. Consideration of the Ward-Takahashi
Identity shows that the first order vertex correction eliminates considerable
(about 80\%) part of the mismatch found in scGW calculations. The remaining
small mismatch in the Ward-Takahashi Identity leaves, however, a possibility of
the calculated low energy electronic structure being affected by higher order
diagrams (not included in this study).",1809.06654v2
2019-08-14,Exploring the behavior of vanadium under high-pressure and high-temperature conditions,"We report a combined experimental and theoretical study of the melting curve
and the structural behavior of vanadium under extreme pressure and temperature.
We performed powder x-ray diffraction experiments up to 120 GPa and 4000 K,
determining the phase boundary of the bcc-to-rhombohedral transition and
melting temperatures at different pressures. Melting temperatures have also
been established from the observation of temperature plateaus during laser
heating, and the results from the density-functional theory calculations.
Results obtained from our experiments and calculations are fully consistent and
lead to an accurate determination of the melting curve of vanadium. These
results are discussed in comparison with previous studies. The melting
temperatures determined in this study are higher than those previously obtained
using the speckle method, but also considerably lower than those obtained from
shock-wave experiments and linear muffin-tin orbital calculations. Finally, a
high-pressure high-temperature equation of state up to 120 GPa and 2800 K has
also been determined.",1908.05166v1
2020-01-08,Influence of doping on the properties of vanadium oxide gel films,"Effect of doping with H and W on the properties of V2O5 and VO2 derived from
V2O5 gel has been studied. It is shown that the treatment of V2O5 in
low-temperature RF hydrogen plasma for 1 to 10 min. leads to either hydration
of vanadium pentoxide or its reduction (depending on the treatment conditions)
to lower vanadium oxides. For some samples, which are subject to plasma
treatment in the discharge active zone, a non-ordinary temperature dependence
of resistance, with a maxi-mum at T ~ 100 K, is observed. For W-doped VO2
films, it is shown that substitution of V4+ with W6+ results in a decrease of
the temperature of metal-insulator transition. Also, it has been shown that the
doping of the initial films with ~3 at.% of W reduces the statistical scatter
in the thresh-old parameters of the switching devices with S-shaped I-V
characteristics on the basis of V2O5 gel films.",2001.02402v1
2020-01-08,Effect of memory electrical switching in metal/vanadium oxide/silicon structures with VO2 films obtained by the sol-gel method,"Electrical switching and rectifying properties of the metal-VO2-Si
structures, on both p-type and n-type silicon, with vanadium dioxide films
obtained by an acetylacetonate sol-gel method, are studied. The switching
effect is shown to be due to the semiconductor-to-metal phase transition (SMPT)
in vanadium dioxide. The shift of the switching threshold voltage, accompanied
by the memory effect, in forward bias of the p-Si-VO2 anisotype heterojunction
is observed. To explain this effect, a model is proposed which suggests the
existence of an additional series resistance associated with a channel at the
VO2/Si interface, where a SiOx layer forms during the VO2 deposition process.
This resistance is responsible for both threshold switching characteristics,
and the memory effect, and the oxygen ion electromigration process is shown to
underlie this effect. Potential applications of the observed phenomena,
combining the effects of ReRAM and SMPT, in oxide electronics are discussed.",2001.03055v1
2020-01-08,Vanadium oxide thin films and fibers obtained by acetylacetonate sol-gel method,"Vanadium oxide films and fibers have been fabricated by the acetylacetonate
sol-gel method followed by annealing in wet nitrogen. The samples are
characterized by X-ray diffraction and electrical conductivity measurements.
The effects of a sol aging, the precursor decomposition and the gas atmosphere
composition on the annealing process, structure and properties of the films are
discussed. The two-stage temperature regime of annealing of amorphous films in
wet nitrogen for formation of the well crystallized VO2 phase is chosen: 1)
25-550 C and 2) 550-600 C. The obtained films demonstrate the metal-insulator
transition and electrical switching. Also, the effect of the
polyvinylpyrrolidone additive concentration and electrospinning parameters on
qualitative (absence of defects and gel drops) and quantitative (length and
diameter) characteristics of vanadium oxide fibers is studied.",2001.11329v1
2020-09-02,Broadband operation of active terahertz quarter-wave plate achieved with vanadium-dioxide-based metasurface switchable by current injection,"We demonstrate the broadband operation of a switchable terahertz quarter-wave
plate achieved with an active metasurface employing vanadium dioxide. For this
purpose, we utilize anisotropically deformed checkerboard structures, which
present broadband characteristics compatible with deep modulation. Moreover,
the metasurface is integrated with a current injection circuit to achieve state
switching; this injection circuit can also be employed to monitor the electric
state of vanadium dioxide. We estimate the Stokes parameters derived from the
experimental transmission spectra of the fabricated metasurface and confirm the
helicity switching of circularly polarized waves near a designed frequency of
0.66THz. The relative bandwidth is evaluated as 0.52, which is 4.2 times
broader than that in a previous study.",2009.00832v1
2020-12-03,Design of on-chip plasmonic modulator with vanadium-dioxide in hybrid orthogonal junctions on Silicon-on-Insulator,"We present a plasmonic modulator based on hybrid orthogonal silver junctions
using vanadium dioxide as the modulating material on the silicon on insulator.
The modulator has an ultra-compact footprint of 1.8{\mu}m x 1{\mu}m with a
100nm x 100nm modulating section based on the orthogonal geometry. We take
advantage of large change in the refractive index of vanadium dioxide during
its phase transition to achieve a high modulation depth of 46.89dB/{\mu}m. We
also provide a fabrication strategy for the development of this device. The
device geometry has potential applications in the development of next
generation high frequency photonic modulators for optical communications which
require a nanometer scale footprint, large modulation depth and small insertion
losses.",2012.01695v1
2021-02-16,"Spin injection into vanadium dioxide films from a typical ferromagnetic metal, across the metal-insulator transition of the vanadium dioxide films","A vanadium dioxide VO2 film shows metal-insulator transition (MIT) induced by
changing environmental temperature. We report the temperature dependence of
electromotive force properties generated in VO2/Ni80Fe20 bilayer junctions
under the ferromagnetic resonance (FMR) of the Ni80Fe20 layer. An electromotive
force generated in a VO2/Ni80Fe20 bilayer junction under the FMR showed a small
change across the MIT temperature of the VO2 film, while the VO2 film
resistance drastically changed. This behavior was not only explained with the
temperature dependence of the electromotive force property generated in the
Ni80Fe20 film itself under the FMR, but also with the generated electromotive
forces due to the inverse spin-Hall effect (ISHE) in the VO2 film under the FMR
of the Ni80Fe20 film. That is, we successfully demonstrated the spin injection
from a Ni80Fe20 film into a VO2 film across the MIT temperature of the VO2
film.",2102.07998v1
2021-06-07,Modeling refractory high-entropy alloys with efficient machine-learned interatomic potentials: defects and segregation,"We develop a fast and accurate machine-learned interatomic potential for the
Mo-Nb-Ta-V-W quinary system and use it to study segregation and defects in the
body-centred cubic refractory high-entropy alloy MoNbTaVW. In the bulk alloy,
we observe clear ordering of mainly Mo-Ta and V-W binaries at low temperatures.
In damaged crystals, our simulations reveal clear segregation of vanadium, the
smallest atom in the alloy, to compressed interstitial-rich regions like
radiation-induced dislocation loops. Vanadium also dominates the population of
single self-interstitial atoms. In contrast, due to its larger size and low
surface energy, niobium segregates to spacious regions like the inner surfaces
of voids. When annealing samples with supersaturated concentrations of defects,
we find that in complete contrast to W, interstitial atoms in MoNbTaVW cluster
to create only small ($\sim 1$ nm) experimentally invisible dislocation loops
enriched by vanadium. By comparison to W, we explain this by the reduced but
three-dimensional migration of interstitials, the immobility of dislocation
loops, and the increased mobility of vacancies in the high-entropy alloy, which
together promote defect recombination over clustering.",2106.03369v2
2021-06-26,High Infrared Reflectance Modulation in VO2 Films Synthesized on Glass and ITO coated Glass substrates using Atmospheric Oxidation of Vanadium,"Vanadium Dioxide (VO2) is a strongly correlated material, which exhibits
insulator to metal transition at ~68 C along with large resistivity and
infrared optical reflectance modulation. In this work, we use atmospheric
pressure thermal oxidation of Vanadium to synthesize VO2 films on glass and ITO
coated glass substrates. With the optimized short oxidation durations of 2 min
and 4 min, the synthesized VO2 film shows high optical reflectance switching in
long-wavelength infrared on glass substrates and mid-wavelength infrared on ITO
coated glass substrates, respectively. Peak reflectance switching values of
~76% and ~79% are obtained on the respective substrates, which are among the
highest reported values. Using the reflectance data, we extract VO2 complex
refractive index in infrared wavelengths, in both the insulating and metallic
phases. The extracted refractive index shows good agreement with VO2
synthesized using other methods. This demonstration of high optical reflectance
switching in VO2 thin films, grown on low cost glass and ITO coated glass
substrates, using a simple low thermal budget process will aid in enhancing VO2
applications in the optical domain.",2106.14006v1
2022-01-20,Effect of vanadium catalysts on hydrogen evolution from NaBH4,"NaBH4 is a very cheap and hydrogen rich material and a potential hydrogen
store. However, high temperature of its thermal decomposition (above
530{\deg}C) renders it inapplicable in practical use. Here, we have studied the
effect of addition of diverse V containing catalysts on thermal hydrogen
desorption. It turns out that mechanochemical doping of NaBH4 with vanadium
metal, its oxides or nanoparticles lowers the temperature of pyrolysis
significantly. Notably, NaBH4 milled for 3 hrs with 25 wt.% of V2O5 or VO2
releases ca. 70 % of stored hydrogen in the temperature range from ca.
370{\deg}C to 450{\deg}C. On the other hand, precursors and solvents used to
prepare rather uniform vanadium nanoparticles (ca. 4nm) suspended in THF or
less uniform and larger ones (ca. 28nm) in ortho difluorobenzene, have adverse
effect on purity of hydrogen evolved.",2201.08121v1
2022-03-02,Electro-optic modulator based on Vanadium dioxide epsilon-near-zero vertical nanowire triple-cavity in Silicon Waveguide,"We present an electro-optic modulator exploiting an array of Vanadium dioxide
nanorods operating in the epsilon near regime as the active switching material
integrated in a silicon waveguide. The modulator takes advantage of the
insulator-to-metal transition of Vanadium dioxide to achieve a robust
modulation depth of 19.7 dB/micron with a broad wavelength of operation. Using
simulations, we demonstrate how the effective permittivity of the metamaterial
can be tuned to a near-zero value by varying the nanorod geometry. The paper
also proposes a novel hexagonal array design which achieves low insertion
losses while retaining a strong modulation depth. The results provide insight
into the design of ultra-compact modulators with high operation frequencies and
low insertion losses.",2203.01431v1
2022-08-25,Spin Seebeck effect at low temperatures in the nominally paramagnetic insulating state of vanadium dioxide,"The low temperature monoclinic, insulating phase of vanadium dioxide is
ordinarily considered nonmagnetic, with dimerized vanadium atoms forming spin
singlets, though paramagnetic response is seen at low temperatures. We find a
nonlocal spin Seebeck signal in VO2 films that appears below 30 K and which
increases with decreasing temperature. The spin Seebeck response has a
non-hysteretic dependence on in-plane external magnetic field. This
paramagnetic spin Seebeck response is discussed in terms of prior findings on
paramagnetic spin Seebeck effects and expected magnetic excitations of the
monoclinic ground state.",2208.12284v1
2022-08-29,Vanadium oxide metal-insulator phase transition in different types of one-dimensional photonic microcavities,"The optical properties of vanadium dioxide ($VO_2$) can be tuned via
metal-insulator transition. In this work different types of one-dimensional
photonic structure-based microcavities that embed vanadium dioxide have been
studied in the spectral range between 900 nm and 2000 nm. In particular, $VO_2$
has been sandwiched between: i) two photonic crystals made of $SiO_2$ and
$ZrO_2$; ii) two aperiodic structures made of $SiO_2$ and $ZrO_2$ that follow
the Thue-Morse sequence; iii) two disordered photonic structures, made of
$SiO_2$ and $ZrO_2$ in which the disorder is introduced either by a random
sequence of the two materials or by a random variation of the thicknesses of
the layers; iv) two four material-based photonic crystals made of $SiO_2$,
$Al_2O_3$, $Y_2O_3$, and $ZrO_2$. The ordered structures i and iv show,
respectively, one and two intense transmission valleys with defect modes, while
the aperiodic and disordered structures ii and iii show a manifold of
transmission valleys due to their complex layered configurations. The
metal-insulator transition of $VO_2$, controlled by temperature, results in a
modulation of the optical properties of the microcavities.",2208.13452v1
2023-03-06,Thermal hysteresis in amplification and attenuation of surface-plasmon-polariton waves,"The propagation of surface-plasmon-polariton (SPP) waves at the planar
interface of a metal and a dielectric material was investigated for a
dielectric material with strongly temperature-dependent constitutive
properties. The metal was silver and the dielectric material was vanadium
multioxide impregnated with a combination of active dyes. Depending upon the
volume fraction of vanadium multioxide, either attenuation or amplification of
the SPP waves may be achieved; the degree of attenuation or amplification is
strongly dependent on both the temperature and whether the temperature is
increasing or decreasing. At intermediate volume fractions of vanadium
multioxide, for a fixed temperature, a SPP wave may experience attenuation if
the temperature is increasing but experience amplification if the temperature
is decreasing.",2303.03087v1
2024-03-11,Active Control of Bound States in the Continuum in Toroidal Metasurfaces,"The remarkable properties of toroidal metasurfaces, featuring ultrahigh-Q
bound states in the continuum (BIC) resonances and nonradiating anapole modes,
have garnered significant attention. The active manipulation of toroidal
resonance characteristics offers substantial potential for advancing tunable
metasurfaces. Our study specifically explores the application of vanadium
dioxide, a widely used phase change material in active photonics and
room-temperature bolometric detectors, to control BIC resonances in toroidal
metasurfaces. The phase change transition of vanadium dioxide occurs in a
narrow temperature range providing a large variation in material resistivity.
Through heating thin film patches of vanadium dioxide integrated into a
metasurface comprising gold split-ring resonators on a sapphire substrate, we
achieve remarkable control over the amplitude and frequency of toroidal dipole
BIC resonances due to their high sensitivity to losses present in the system.
Breaking the symmetry of meta-atoms reveals enhanced tunability. The predicted
maximum change in the amplitude of toroidal dipole BIC resonances reaches 14 dB
with a temperature variation of approximately 10oC. The proposed tunable
metasurface holds promise for various applications, including active photonic
systems and room temperature bolometers.",2403.06345v1
2024-03-17,Modified Steinberg-Guinan elasticity model to describe softening-hardening dual anomaly in vanadium,"Constitutive models are essential for describing the mechanical behavior of
materials under high temperatures and pressures, among which the
Steinberg-Guinan (SG) model is widely adopted. Recent work has discovered a
peculiar dual anomaly of compression-induced softening and heating-induced
hardening in the elasticity of compressed vanadium [Phys. Rev. B 104, 134102
(2021)], which is beyond the capability of the SG model to describe. In this
work, a modified SG constitutive model is proposed to embody such an anomalous
behavior. Elemental vanadium is considered as an example to demonstrate the
effectiveness of this improved model in describing the dual anomalies of
mechanical elasticity. This new variant of the SG model can also be applied to
other materials that present an irregular variation in the mechanical
elasticity, and is important to faithfully model and simulate the mechanical
response of materials under extreme conditions.",2403.11179v1
2024-03-19,Superconducting transition temperatures of pure vanadium and vanadium-titanium alloys in the presence of dynamical electronic correlations,"Ordinary superconductors are widely assumed insensitive to small
concentrations of random nonmagnetic impurities, whereas strong disorder
suppresses superconductivity, ultimately leading to a superconductor-insulator
transition. In between these limiting cases, a most fascinating regime may
emerge where disorder enhances superconductivity. This effect is discussed here
for the $\beta$-phase of vanadium-titanium alloys. Disorder is modeled using
the coherent potential approximation while local electronic interactions are
treated using dynamical mean-field theory. The McMillan formula is employed to
estimate the superconducting transition temperature, showing a maximum at a Ti
concentration of around $0.33$ for a local Coulomb interaction $U$ in the range
of $2$ to $3$ eV. Our calculations quantitatively agree with the experimentally
observed concentration dependent increase of $T_c$, and its maximal value of
about $20\%$.",2403.13202v1
2009-01-28,Extension of the spin-1/2 frustrated square lattice model: the case of layered vanadium phosphates,"We study the influence of the spin lattice distortion on the properties of
frustrated magnetic systems and consider the applicability of the spin-1/2
frustrated square lattice model to materials lacking tetragonal symmetry. We
focus on the case of layered vanadium phosphates AA'VO(PO4)2 (AA' = Pb2, SrZn,
BaZn, and BaCd). To provide a proper microscopic description of these
compounds, we use extensive band structure calculations for real materials and
model structures and supplement this analysis with simulations of thermodynamic
properties, thus facilitating a direct comparison with the experimental data.
Due to the reduced symmetry, the realistic spin model of layered vanadium
phosphates AA'VO(PO4)2 includes four inequivalent exchange couplings: J1 and
J1' between nearest-neighbors and J2 and J2' between next-nearest-neighbors.
The estimates of individual exchange couplings suggest different regimes, from
J1'/J1 and J2'/J2 close to 1 in BaCdVO(PO4)2, a nearly regular frustrated
square lattice, to J1'/J1 ~ 0.7 and J2'/J2 ~ 0.4 in SrZnVO(PO4)2, a frustrated
square lattice with sizable distortion. The underlying structural differences
are analyzed, and the key factors causing the distortion of the spin lattice in
layered vanadium compounds are discussed. We propose possible routes for
finding new frustrated square lattice materials among complex vanadium oxides.
Full diagonalization simulations of thermodynamic properties indicate the
similarity of the extended model to the regular one with averaged couplings. In
case of moderate frustration and moderate distortion, valid for all the
AA'VO(PO4)2 compounds reported so far, the distorted spin lattice can be
considered as a regular square lattice with the couplings (J1+J1')/2 between
nearest-neighbors and (J2+J2')/2 between next-nearest-neighbors.",0901.4498v2
2018-08-25,Electron dynamics method using a locally projected group diabatic Fock matrix for molecules and aggregates,"We propose a method using reduced size of Hilbert space to describe an
electron dynamics in molecule and aggregate based on our previous theoretical
scheme [ T. Yonehara and T. Nakajima, J. Chem. Phys. \textbf{147}, 074110
(2017) ]. The real-time time-dependent density functional theory is combined
with newly introduced projected group diabatic Fock matrix. First, this
projection method is applied to a test donor--acceptor dimer, namely, a
naphthalene--tetracyanoethylene with and without initial local excitations and
light fields. Secondly, we calculate an absorption spectrum of
five-unit-polythiophene monomer. The importance of feedback of instantaneous
density to Fock matrix is also clarified. In all cases, half of the orbitals
were safely reduced without loss of accuracy in descriptions of properties. The
present scheme provides one possible way to investigate and analyze a complex
excited electron dynamics in molecular aggregates within a moderate
computational cost.",1808.08362v4
2021-07-01,How much does surface polymorphism influence the work function of organic/metal interfaces?,"Molecules adsorbing on metal surfaces form a variety of different surface
polymorphs. How strongly this polymorphism affects interface properties is a
priori unknown. In this work we investigate how strongly the surface
polymorphism influences the interface work functions for various metal/organic
interfaces. To evaluate the whole bandwidth of possible polymorphs, we perform
full theoretical structure search, probing millions of polymorph candidates.
All of these candidates might be observed in reality, either by kinetic
trapping or by thermodynamic occupation. Employing first-principles
calculations and machine learning we predict and analyze the work function
changes for those millions of candidates for three physically distinct model
systems: the weakly interacting naphthalene on Cu(111), the strongly
interacting anthraquinone on Ag(111), and tetracyanoethylene, which undergoes a
re-orientation from lying to standing polymorphs on the Cu(111) surface. These
thorough investigations indicate that kinetic trapping of flat lying molecules
can lead to work function differences of a few hundred meV. If the molecules
also reorientate, this can increase to a change of several eV. We further show
that the spread in work function decreases when working in thermodynamic
equilibrium, but thermally occupied phases still lead to an intrinsic
uncertainty at elevated temperatures.",2107.00560v3
2006-04-12,"Neutron diffraction study of YVO3, NdVO3, and TbVO3","The structural and magnetic properties of YVO3, NdVO3 and TbVO3 were
investigated by single-crystal and powder neutron diffraction. YVO3 shows a
structural phase transition at 200 K from an orthorhombic structure with the
space group Pbnm to a monoclinic one with the space group P21/b. But
supplementary high-resolution synchrotron diffraction experiments showed that
the monoclinic distortion is extremely small. A group theoretical analysis
shows that this magnetic state in the monoclinic phase is incompatible with the
lattice structure, unless terms of higher than bilinear order in the spin
operators are incorporated in the spin Hamiltonian. This observation is
discussed in the light of recent theories invoking unusual many-body
correlations between the vanadium t2g orbitals. A structural phase transition
back to the orthorhombic space group Pbnm is observed upon cooling below 77 K.
This transition is accompanied by a rearrangement of the magnetic structure
into a mode compatible with the lattice structure. The crystal structures of
NdVO3 and TbVO3 are closely similar to that of YVO3. However, only a single
magnetic phase transition was found in the vanadium sublattice down to 9.5 K.
Below 60 K the magnetic moments of the Nd- and Tb-ions are gradually polarized
by the ordered vanadium moments. Below 11 K, we found a noncollinear order of
the terbium moments.",0604305v1
2012-12-11,Insights into thermonuclear supernovae from the incomplete silicon burning process,"Type Ia supernova (SNIa) explosions synthesize a few tenths to several tenths
of a solar mass, whose composition is the result of incomplete silicon burning
that reaches peak temperatures of 4 GK to 5 GK. The elemental abundances are
sensitive to the physical conditions in the explosion, making their measurement
a promising clue to uncovering the properties of the progenitor star and of the
explosion itself. Using a parameterized description of the thermodynamic
history of matter undergoing incomplete silicon burning, we computed the final
composition for a range of parameters wide enough to encompass current models
of SNIa. Then, we searched for combinations of elemental abundances that trace
the parameters values and are potentially measurable. For this purpose, we
divide the present study into two epochs of SNIa, namely the optical epoch,
from a few weeks to several months after the explosion, and the X-ray epoch,
which refers to the time period in which the supernova remnant is young,
starting one or two hundred years age and ending a thousand years after the
event. During the optical epoch, the only SNIa property that can be extracted
from the detection of incomplete silicon burning elements is the neutron excess
of the progenitor white dwarf at thermal runaway, which can be determined
through measuring the ratio of the abundance of manganese to that of titanium,
chromium, or vanadium. Conversely, in the X-ray epoch, any abundance ratio
built using a couple of elements from titanium, vanadium, chromium, or
manganese may constrain the initial neutron excess. Furthermore, measuring the
ratio of the abundances of vanadium to manganese in the X-ray might shed light
on the timescale of the thermonuclear explosion.",1212.2410v1
2013-06-05,Measurements of ultracold neutron upscattering and absorption in polyethylene and vanadium,"The study of neutron cross sections for elements used as efficient
``absorbers'' of ultracold neutrons (UCN) is crucial for many precision
experiments in nuclear and particle physics, cosmology and gravity. In this
context, ``absorption'' includes both the capture and upscattering of neutrons
to the energies above the UCN energy region. The available data, especially for
hydrogen, do not agree between themselves or with the theory. In this report we
describe measurements performed at the Los Alamos National Laboratory UCN
facility of the UCN upscattering cross sections for vanadium and for hydrogen
in CH$_2$ using simultaneous measurements of the radiative capture cross
sections for these elements. We measured $\sigma_{up}=1972\pm130$ b for
hydrogen in CH$_2$, which is below theoretical expectations, and $\sigma_{up} <
25\pm9$ b for vanadium, in agreement with the expectation for the neutron
heating by thermal excitations in solids.",1306.1261v1
2016-02-15,Correlating the Energetics and Atomic Motions of the Metal-Insulator Transition of M1 Vanadium Dioxide,"Materials that undergo reversible metal-insulator transitions are obvious
candidates for new generations of devices. For such potential to be realised,
the underlying microscopic mechanisms of such transitions must be fully
determined. In this work we probe the correlation between the energy landscape
and electronic structure of the metal-insulator transition of vanadium dioxide
and the atomic motions occurring using first principles calculations and high
resolution X-ray diffraction. Calculations find an energy barrier between the
high and low temperature phases corresponding to contraction followed by
expansion of the distances between vanadium atoms on neighbouring sub-lattices.
X-ray diffraction reveals anisotropic strain broadening in the low temperature
structure's crystal planes, however only for those with spacings affected by
this compression/expansion. GW calculations reveal that traversing this barrier
destabilises the bonding/anti-bonding splitting of the low temperature phase.
This precise atomic description of the origin of the energy barrier separating
the two structures will facilitate more precise control over the transition
characteristics for new applications and devices.",1602.04563v2
2016-10-19,Mathematical Modeling of Electrolyte Flow Dynamic Patterns and Volumetric Flow Penetrations in the Flow Channel over Porous Electrode Layered System in Vanadium Flow Battery with Serpentine Flow Field Design,"In this work, a two-dimensional mathematical model is developed to study the
flow patterns and volumetric flow penetrations in the flow channel over the
porous electrode layered system in vanadium flow battery with serpentine flow
field design. The flow distributions at the interface between the flow channel
and porous electrode are examined. It is found that the non-linear pressure
distributions can distinguish the interface flow distributions under the ideal
plug flow and ideal parabolic flow inlet boundary conditions. However, the
volumetric flow penetration within the porous electrode beneath the flow
channel through the integration of interface flow velocity reveals that this
value is identical under both ideal plug flow and ideal parabolic flow inlet
boundary conditions. The volumetric flow penetrations under the advection
effects of flow channel and landing/rib are estimated. The maximum current
density achieved in the flow battery can be predicted based on the 100% amount
of electrolyte flow reactant consumption through the porous electrode beneath
both flow channel and landing channel. The corresponding theoretical maximum
current densities achieved in vanadium flow battery with one and three layers
of SGL 10AA carbon paper electrode have reasonable agreement with experimental
results under a proper permeability.",1610.06171v2
2017-06-02,Subcycle insulator-to-metal transition in vanadium dioxide by terahertz-field-driven tunneling,"In vanadium dioxide, the interplay between coherent lattice transformation
and electronic correlation drives an insulator-to-metal transition (IMT). This
phase commutation can be triggered by temperature, pressure, doping or
deposition of optical energy. Here we demonstrate that an atomically-strong
terahertz electric field initiates a metastable ultrafast IMT in vanadium
dioxide without a concomitant lattice transformation. The free-space terahertz
field acts as off-resonant excitation with photon energy below the lattice
phonons and the interband transitions. Differently from optical and infrared
excitation, terahertz interaction leads to a full IMT by interband Zener
tunneling with a negligible entropy deposition. In previous experiments the
temporal dynamics of IMT in VO2 could be only indirectly inferred. We
disentangle the electronic and lattice contributions to the IMT on a
sub-picosecond timescale. Near the critical temperature the IMT becomes
dissipative and the terahertz field concludes the lattice-assisted metallic
nucleation initiated by heating. The method of strong-field induced phase
transition presented here is applicable to a wide class of strongly correlated
systems and will enable the discovery of novel metastable phases.",1706.00616v2
2019-02-12,The role of defects in the metal-insulator transition in VO$_{2}$ and V$_2$O$_3$,"The vanadates VO$_2$ and V$_2$O$_3$ are prototypical examples of strongly
correlated materials that exhibit a metal-insulator transition. While the phase
transitions in these materials have been studied extensively, there is a
limited understanding of how the properties of these materials are affected by
the presence of defects and doping. In this study we investigate the impact of
native point defects in the form of Frenkel defects on the structural, magnetic
and electronic properties of VO$_2$ and V$_2$O$_3$, using first-principles
calculations. In VO$_2$ the vanadium Frenkel pairs lead to a non-trivial
insulating state. The unpaired vanadium interstitial bonds to a single dimer,
which leads to a trimer that has one singlet state and one localized
single-electron $S=1/2$ state. The unpaired broken dimer created by the
vanadium vacancy also has a localized $S=1/2$ state. Thus, the insulating state
is created by the singlet dimers, the trimer and the two localized $S=1/2$
states. Oxygen Frenkel pairs, on the other hand, lead to a metallic state in
VO$_2$, but are expected to be present in much lower concentrations. In
contrast, the Frenkel defects in V$_2$O$_3$ do not directly suppress the
insulating character of the material. However, the disorder created by defects
in V$_2$O$_3$ alters the local magnetic moments and in turn reduces the energy
cost of a transition between the insulating and conducting phases of the
material. We also find self-trapped small polarons in V$_2$O$_3$, which has
implications for transport properties in the insulating phase.",1902.04526v1
2019-12-26,Strong local moment antiferromagnetic spin fluctuations in V-doped LiFeAs,"We use neutron scattering to study vanadium (hole)-doped LiFe$_{1-x}$V$_x$As.
In the undoped state, LiFeAs exhibits superconductivity at $T_c=18$ K and
transverse incommensurate spin excitations similar to electron overdoped iron
pnictides. Upon vanadium-doping to form LiFe$_{0.955}$V$_{0.045}$, the
transverse incommensurate spin excitations in LiFeAs transform into
longitudinally elongated in a similar fashion as that of potassium (hole) doped
Ba$_{0.7}$K$_{0.3}$Fe$_2$As$_2$, but with dramatically enhanced magnetic
scattering and elimination of superconductivity. This is different from the
suppression of the overall magnetic excitations in hole doped BaFe$_2$As$_2$
and the enhancement of superconductivity near optimal hole doping. These
results are consistent with density function theory plus dynamic mean field
theory calculations, suggesting that vanadium-doping in LiFeAs may induce an
enlarged effective magnetic moment $S_{eff}$ with a spin crossover ground state
arising from the inter-orbital scattering of itinerant electrons.",1912.11860v1
2020-01-14,"Interplay among Work Function, electronic structure and stoichiometry in nanostructured vanadium oxides films","The work function is the parameter of greatest interest in many technological
applications involving charge exchange mechanisms at the interface. The
possibility to produce samples with a controlled work function is then
particularly interesting, albeit challenging. We synthetized nanostructured
vanadium oxides films by a room temperature Supersonic Cluster Beam Deposition
method, obtaining samples with tunable stoichiometry and work function (3.7-7
eV). We present an investigation of the electronic structure of several
vanadium oxides films as a function of the oxygen content via in-situ Auger,
valence-band photoemission spectroscopy and work function measurements. The
experiments probed the partial 3d density of states, highlighting the presence
of strong V3d-O2p and V3d-V4s hybridization which influence 3d occupation. We
show how controlling the stoichiometry of the sample implies a control over
work function, and that the access to nanoscale quantum confinement can be
exploited to increase the work function of the sample relative to the bulk
analogue. In general, the knowledge of the interplay among work function,
electronic structure, and stoichiometry is strategic to match nanostructured
oxides to their target applications.",2001.04695v1
2020-10-21,Substitutional VSn nanodispersed in MoS$_2$ film for Pt-scalable catalyst,"Among transition metal dichalcogenides (TMdCs) as alternatives for Pt-based
catalysts, metallic-TMdCs catalysts have highly reactive basal-plane but are
unstable. Meanwhile, chemically stable semiconducting-TMdCs show limiting
catalytic activity due to their inactive basal-plane. Here, we propose metallic
vanadium sulfide (VSn) nanodispersed in a semiconducting MoS2 film (V-MoS2) as
an efficient catalyst. During synthesis, vanadium atoms are substituted into
hexagonal monolayer MoS2 to form randomly distributed VSn units. The V-MoS2
film on a Cu electrode exhibits Pt-scalable catalytic performance; current
density of 1000 mA cm-2 at 0.6 V, overpotential of -0.06 V at a current density
of 10 mA cm-2 and exchange current density of 0.65 mA cm-2 at 0 V with
excellent cycle stability for hydrogen-evolution-reaction (HER). The high
intrinsic HER performance of V-MoS2 is explained by the efficient electron
transfer from the Cu electrode to chalcogen vacancies near vanadium sites with
optimal Gibbs free energy (-0.02 eV). This study adds insight into ways to
engineer TMdCs at the atomic-level to boost intrinsic catalytic activity for
hydrogen evolution.",2010.10908v1
2021-04-15,Fermi surface mapping and the nature of charge density wave order in the kagome superconductor CsV$_3$Sb$_5$,"The recently discovered family of AV$_3$Sb$_5$ (A: K, Rb Cs) kagome metals
possess a unique combination of nontrivial band topology, superconducting
ground states, and signatures of electron correlations manifest via competing
charge density wave order. Little is understood regarding the nature of the
charge density wave (CDW) instability inherent to these compounds and the
potential correlation with the accompanying onset of a large anomalous Hall
response. To understand the impact of the CDW order on the electronic structure
in these systems, we present quantum oscillation measurements on single
crystals of CsV$_3$Sb$_5$. Our data provides direct evidence that the CDW
invokes a substantial reconstruction of the Fermi surface pockets associated
with the vanadium orbitals and the kagome lattice framework. In conjunction
with density functional theory modeling, we are able to identify split
oscillation frequencies originating from reconstructed pockets built from
vanadium orbitals and Dirac-like bands. Complementary diffraction measurements
are further able to demonstrate that the CDW instability has a correlated
phasing between neighboring V$_3$Sb$_5$ planes. These results provide critical
insights into the underlying CDW instability in AV$_3$Sb$_5$ kagome metals and
support minimal models of CDW order arising from within the vanadium-based
kagome lattice.",2104.07230v3
2021-07-03,Effect of Vanadium Thickness and Deposition Temperature on VO2 Synthesis using Atmospheric Pressure Thermal Oxidation,"Vanadium dioxide (VO2) is a phase transition material that undergoes a
reversible insulator-metal phase transition at ~ 68 C. Atmospheric pressure
thermal oxidation (APTO) of vanadium (V) is a simple VO2 synthesis method in
which V thin film is oxidized in open air. For an optimum oxidation duration,
VO2 films are obtained with good phase transition properties. We recently
reported a modified APTO process using a step temperature profile for oxidation
(Thin Solid Films 706, 138003 (2020)). We demonstrated an ultra-low thermal
budget synthesis of VO2 thin films with good electrical and optical phase
transition properties. For a 130 nm room-temperature RF sputtered V thin film,
an optimum oxidation duration of ~ 30 s was obtained. In this work, we study
how the starting V film thickness and deposition temperature affects the
optimum oxidation duration. V thin films of varying thickness (15-212 nm) and
120 nm thick V films with varying deposition temperature (~27-450 C) are
prepared using RF magnetron sputtering. These films are oxidized for different
oxidation durations and characterized using Raman and four-probe measurements
to find the optimum oxidation duration for each deposition condition. We find
that the optimum oxidation duration increases with the increase in V film
thickness and V deposition temperature. We model the effect of V film thickness
and deposition temperature on the optimal oxidation time using a parabolic law
which can be used to obtain the optimal oxidation times for intermediate V
thicknesses/deposition temperatures.",2107.01413v1
2021-11-28,Investigations on the improved cycling stability of Kazakhstanite phase Fe-V-O layered oxide by using superconcentrated electrolytes: Generalized solubility limit approach (Part I),"In this article, we address the issue of vanadium dissolution pertinent in
the layered Fe5V15O39(OH)9.9H2O using the solubility limit approach. This
layered oxide is prepared via a low-cost solution phase synthesis route and
crystallizes in the Kazakhstanite phase (Space Group: C2/m), confirmed using
selected area electron diffraction and x-ray diffraction.The layered oxide
exhibits the 2 electron redox reaction of vanadium (V5+ to V3+) along with the
1 electron redox reaction of iron within the voltage window of 1.5-3.8V. This
results in a high specific capacity of ~350mAhg-1 which can be extracted from
this material. However, the transition from V4+ to V3+ is identified to
initiate a dissolution process at ~2.5V, resulting in a loss of active material
and poor cycling stability. The vanadium dissolution is found to be arrested by
switching to a superconcentrated electrolyte, wherein the amount of 'free'
solvent is low. An electrolyte, consisting of seven molar lithium
bis(trifluoromethanesulfonyl)imide in 1,3-Dioxolane: 1,2-Dimethoxyethane = 1:1
(v:v), is found to be suitable in providing the best cycling stability amongst
the other compositions tested. The electrochemical characteristics of the
passivation layers formed over lithium foil are mathematically modeled to
indicate the preference of superconcentrated electrolytes over relatively
dilute ones.",2111.14198v1
2022-03-18,Transient dynamics of the phase transition in VO2 revealed by mega electron-volt ultrafast electron diffraction,"Vanadium dioxide (VO2) exhibits an insulator-to-metal transition accompanied
by a structural transition near room temperature. This transition can be
triggered by an ultrafast laser pulse. Exotic transient states, such as a
metallic state without structural transition, were also proposed. These unique
characteristics let VO2 have great potential in thermal switchable devices and
photonic applications. Although great efforts have been made, the atomic
pathway during the photoinduced phase transition is still not clear. Here, we
synthesized freestanding quasi-single-crystal VO2 films and examined their
photoinduced structural phase transition with mega-electron-volt ultrafast
electron diffraction. Leveraging the high signal-to-noise ratio and high
temporal resolution, we observe that the disappearance of vanadium dimers and
zigzag chains does not coincide with the transformation of crystal symmetry.
After photoexcitation, the initial structure is strongly modified within 200
femtoseconds, resulting in a transient monoclinic structure without vanadium
dimers and zigzag chains. Then, it continues to evolve to the final tetragonal
structure in approximately 5 picoseconds. In addition, only one laser fluence
threshold instead of two thresholds suggested in polycrystalline samples was
observed in our quasi-single-crystal samples. Our findings provide new
essential information for a comprehensive understanding of the photoinduced
ultrafast phase transition in VO2.",2203.09776v2
2023-01-16,Polaronic and Mott insulating phase of layered magnetic vanadium trihalide VCl3,"Two-dimensional (2D) van der Waals (vdW) magnetic $3d$-transition metal
trihalides are a new class of functional materials showing exotic physical
properties useful for spintronic and memory storage applications. In this
article, we report the synthesis and electromagnetic characterization of
single-crystalline vanadium trichloride, VCl$_3$, a novel 2D layered vdW Mott
insulator, which has a rhombohedral structure (R$\overline{3}$, No. 148) at
room temperature. VCl$_3$ undergoes a structural phase transition at 103 K and
a subsequent antiferromagnetic transition at 21.8 K. Combining core levels and
valence bands x-ray photoemission spectroscopy (XPS) with first-principles
density functional theory (DFT) calculations, we demonstrate the Mott Hubbard
insulating nature of VCl$_3$ and the existence of electron small 2D magnetic
polarons localized on V atom sites by V-Cl bond relaxation. The polarons
strongly affect the electromagnetic properties of VCl$_3$ promoting the
occupation of dispersion-less spin-polarized V-3d $a_{1g}$ states and band
inversion with $e^{'}_{g}$ states. Within the polaronic scenario, it is
possible to reconcile different experimental evidences on vanadium trihalides,
suggesting that also VI$_3$ hosts polarons. Our results highlight the complex
physical behavior of this class of crystals determined by charge trapping,
lattice distortions, correlation effects, mixed valence states, and magnetic
states.",2301.06501v4
2023-02-23,"YbV$_3$Sb$_4$ and EuV$_3$Sb$_4$, vanadium-based kagome metals with Yb$^{2+}$ and Eu$^{2+}$ zig-zag chains","Here we present YbV$_3$Sb$_4$ and EuV$_3$Sb$_4$, two new compounds exhibiting
slightly distorted vanadium-based kagome nets interleaved with zig-zag chains
of divalent Yb$^{2+}$ and Eu$^{2+}$ ions. Single crystal growth methods are
reported alongside magnetic, electronic, and thermodynamic measurements.
YbV$_3$Sb$_4$ is a nonmagnetic metal with no collective phase transitions
observed between 60mK and 300K. Conversely, EuV$_3$Sb$_4$ is a magnetic kagome
metal exhibiting easy-plane ferromagnetic-like order below $T_\text{C}$=32K
with signatures of noncollinearity under low field. Our discovery of
YbV$_3$Sb$_4$ and EuV$_3$Sb$_4$ demonstrate another direction for the discovery
and development of vanadium-based kagome metals while incorporating the
chemical and magnetic degrees of freedom offered by a rare-earth sublattice.",2302.12354v3
2023-09-12,"Pentagon, Hexagon, or Bridge? Identifying the Location of a Single Vanadium Cation on Buckminsterfullerene Surface","Buckminsterfullerene C60 has received extensive research interest ever since
its discovery. In addition to its interesting intrinsic properties of
exceptional stability and electron-accepting ability, the broad chemical
tunability by decoration or substitution on the C60-fullerene surface makes it
a fascinating molecule. However, to date there is uncertainty about the binding
location of such decorations on the C60 surface, even for a single adsorbed
metal atom. In this work, we report the gas-phase synthesis of the C60V+
complex and its in-situ characterization by mass spectrometry and in-frared
spectroscopy with the help of quantum chemical calculations and molecular
dynamics simulations. We identify the most probable binding position of a
vanadium cation on C60 above a pentagon center in eta5-fashion, demonstrate a
high thermal stability for this complex, and explore the bonding nature between
C60 and the vanadium cation, reveal-ing that large orbital and electrostatic
interactions lie at the origin of the stability of the eta5-C60V+ complex.",2309.05890v1
2023-09-20,Effects of disorder on the magnetic properties of the Heusler alloy V$_{2}$FeAl,"Magnetic properties of multicomponent alloys depend sensitively on the degree
of atomic order on the different crystallographic sites. In this work we
demonstrate the magnetic contrast between bulk and thin-film samples of the
Heusler alloy V$_{2}$FeAl. Arc-melted bulk ingots show practically no site
preference of the elements (A2 structure), whereas magnetron-sputtered
thin-film samples display a higher degree of atomic ordering with a tendency
towards XA-type order. Electronic structure calculations favour ferrimagnetic
XA-type ordering, and the effect of different pairwise atomic disorder on the
element specific and net magnetic moments are evaluated to reproduce
experimental observations. XA-type thin-films with iron moment of 1.24
$\mu_{\mathrm{B}}$ determined by X-ray magnetic circular dichroism are in
agreement with calculation, but the measured net moment of 1.0
$\mu_{\mathrm{B}}$ per formula unit and average vanadium moment are smaller
than expected from calculations. The measured Curie temperature is
approximately 500 K. Films with a higher degree of disorder have a
T$_{\mathrm{C}}$ close to 300 K with a net moment of 0.1 $\mu_{\mathrm{B}}$ at
low temperature. The large calculated vanadium moments are destroyed by partial
disorder on $4d$ vanadium sites. By contrast, the arc-melted and annealed bulk
alloy with a fully-disordered A2 structure shows no spontaneous magnetization;
it is a Pauli paramagnet with dimensionless susceptibility
$\chi_{\mathrm{v}}=-2.95\times10^{-4}$.",2309.11480v1
2024-01-07,Dynamic Multi Color Switching using Ultrathin Vanadium Oxide on Aluminium based Asymmetric Fabry-Perot Resonant Structure,"Vanadium dioxide ($VO_{2}$) exhibits strong infrared optical switching due to
its insulator-metal phase-transition property. However, in the visible
wavelengths, it's intrinsic optical switching is quite low. Current research
explores solutions like multilayering, intricate structural patterning, high
thermal budget processes and costly metals for improved color switching.
Nonetheless, the color gamut coverage with these methodologies remains notably
limited. This work overcomes these limitations and demonstrates dynamic
multi-colour switching covering a large color gamut using a simple,
unpatterned, ultrathin ($\sim$ $\frac{\lambda}{14}$, where wavelength $\lambda$
is taken as 575 nm at the center of visible spectrum) asymmetric
Fabry-P\'{e}rot structure of $VO_{2}$ on Aluminium (Al). We use the transfer
matrix method to design the $VO_{2}/Aluminium\,(Al)/Sapphire$ structure for
maximum visible reflectance switching. $VO_{2}$ films are synthesized using a
simple, low thermal budget atmospheric oxidation of Vanadium (V). With varying
oxidation durations, different colors of the oxidized samples are observed.
Consistent and reversible color-switching is observed visibly and in
reflectance measurements with the change in temperature from low (RT $\sim$
30$^{\circ}$C) to high (HT $\sim$ 100$^{\circ}$C) or vice versa due to the
phase transition property of the $VO_{2}$ layer in the structure. Compared to
the existing studies, this work shows a significant change in chromaticities
and covers a large color gamut when plotted on the CIE chromaticity diagram.
This work has potential applications in the fields of display, thermochromic
structures, and visible camouflage.",2401.03543v1
2024-02-02,Decoupled few-femtosecond phase transitions in vanadium dioxide,"The nature of the insulator-to-metal phase transition in vanadium dioxide
(VO2) is one of the longest-standing problems in condensed-matter physics.
Ultrafast spectroscopy has long promised to determine whether the transition is
primarily driven by the electronic or structural degree of freedom, but
measurements to date have been stymied by their sensitivity to only one of
these components and/or their limited temporal resolution. Here we use
ultra-broadband few-femtosecond pump-probe spectroscopy to resolve the
electronic and structural phase transitions in VO2 at their fundamental time
scales. We find that the system transforms into a bad-metallic phase within 10
fs after photoexcitation, but requires another 100 fs to complete the
transition, during which we observe electronic oscillations and a partial
re-opening of the bandgap, signalling a transient semi-metallic state.
Comparisons with tensor-network simulations and density-functional theory
calculations show these features originate from oscillations around the
equilibrium high-symmetry atomic positions during an unprecedentedly fast
structural transition, in which the vanadium dimers separate and untwist with
two different timescales. Our results resolve the complete structural and
electronic nature of the light-induced phase transition in VO2 and establish
ultra-broadband few-femtosecond spectroscopy as a powerful new tool for
studying quantum materials out of equilibrium.",2402.01266v2
2024-03-05,Highly Reproducible and CMOS-compatible VO2-based Oscillators for Brain-inspired Computing,"With remarkable electrical and optical switching properties induced at low
power and near room temperature (68C), vanadium dioxide (VO2) has sparked
rising interest in unconventional computing among the phase-change materials
research community. The scalability and the potential to compute beyond the von
Neumann model make VO2 especially appealing for implementation in oscillating
neural networks for artificial intelligence (AI) applications, to solve
constraint satisfaction problems, and for pattern recognition. Its integration
into large networks of oscillators on a Silicon platform still poses challenges
associated with the stabilization in the correct oxidation state and the
ability to fabricate a structure with predictable electrical behavior showing
very low variability. In this work, the role played by the different annealing
parameters applied by three methods (slow thermal annealing, flash annealing,
and rapid thermal annealing), following the vanadium oxide atomic layer
deposition (ALD), on the formation of VO2 grains is studied and an optimal
substrate stack configuration that minimizes variability between devices is
proposed. Material and electrical characterizations are performed on the
different films and a step-by-step recipe to build reproducible VO2-based
oscillators is presented, which is argued to be made possible thanks to the
introduction of a hafnium oxide (HfO2) layer between the silicon substrate and
the vanadium oxide layer. Up to seven nearly identical VO2-based devices are
contacted simultaneously to create a network of oscillators, paving the way for
large-scale implementation of VO2 oscillating neural networks.",2403.02822v1
2009-05-17,Temperature dependence of hydrogen local mode frequencies in the ordering bcc metal hydrides,"A simple physical conception explaining the experimentally observed
temperature dependence of the local mode frequencies in the ordering metal
hydrides is proposed. Calculations are performed for bcc vanadium deuterides.
The model can be applied to other physical phenomena which are related to the
hydrogen motion in metal lattices.",0905.2733v1
2020-02-24,Vanadium-based neutron Beam Monitor,"A prototype quasi-parasitic thermal neutron beam monitor based on isotropic
neutron scattering from a thin natural vanadium foil and standard $^3$He
proportional counters is conceptualized, designed, simulated, calibrated, and
commissioned. The European Spallation Source designed to deliver the highest
integrated neutron flux originating from a pulsed source is currently under
construction in Lund, Sweden. The effort to investigate a vanadium-based
neutron beam monitor was triggered by a list of requirements for Beam Monitors
permanently placed in the ESS neutron beams in order to provide reliable
monitoring at complex beamlines: low attenuation, linear response over a wide
range of neutron fluxes, near to constant efficiency for neutron wavelengths in
a range of 0.6-10 \r{A}, calibration stability and the possibility to place the
system in vacuum are all desirable characteristics. The scattering-based
prototype, employing a natural vanadium foil and standard $^3$He proportional
counters, was investigated at the V17 and V20 neutron beamlines of the
Helmholtz-Zentrum in Berlin, Germany, in several different geometrical
configurations of the $^3$He proportional counters around the foil. Response
linearity is successfully demonstrated for foil thicknesses ranging from 0.04
mm to 3.15 mm. Attenuation lower than 1% for thermal neutrons is demonstrated
for the 0.04 mm and 0.125 mm foils. The geometries used for the experiment were
simulated allowing for absolute flux calibration and establishing the possible
range of efficiencies for various designs of the prototype. The operational
flux limits for the beam monitor prototype were established as a dependency of
the background radiation and prototype geometry. The herein demonstrated
prototype monitors can be employed for neutron fluxes ranging from
$10^3-10^{10}$ n/s/cm$^2$.",2002.10108v1
2024-02-06,Unveiling the charge density wave mechanism in vanadium-based Bi-layered kagome metals,"The charge density wave (CDW), as a hallmark of vanadium-based kagome
superconductor AV3Sb5 (A = K, Rb, Cs), has attracted intensive attention.
However, the fundamental controversy regarding the underlying mechanism of CDW
therein persists. Recently, the vanadium-based bi-layered kagome metal ScV6Sn6,
reported to exhibit a long-range charge order below 94 K, has emerged as a
promising candidate to further clarify this core issue. Here, employing
micro-focusing angle-resolved photoemission spectroscopy ({\mu}-ARPES) and
first-principles calculations, we systematically studied the unique CDW order
in vanadium-based bi-layered kagome metals by comparing ScV6Sn6 with its
isostructural counterpart YV6Sn6, which lacks a CDW ground state. Combining
ARPES data and the corresponding joint density of states (DOS), we suggest that
the VHS nesting mechanism might be invalid in these materials. Besides, in
ScV6Sn6, we identified multiple hybridization energy gaps resulting from
CDW-induced band folding, along with an anomalous band dispersion, implying a
potential electron-phonon coupling driven mechanism underlying the formation of
the CDW order. Our finding not only comprehensively maps the electronic
structure of V-based bi-layer kagome metals but also provide constructive
experimental evidence for the unique origin of CDW in this system.",2402.03765v1
2011-12-06,Exchange Parameters in the TCNE-based Magnets as Estimated with use of the Effective Hamiltonian Crystal Field Method,"Estimates of the exchange interaction parameter (J) between electronic
magnetic momenta residing in the d-shells of several divalent first row
transition metal ions, and those residing on the [TCNE]$^-$. anion-radicals
(TCNE = tetracyanoethylene) that are bonded to the metal ion are obtained by
the use of the EHCF method in the cluster approximation. The clusters geometry
was set according to that observed for Mn$^{II}$(TCNE)$_{1/2}$(I$_3$)$_{1/2}$
and M$^{II}$(TCNE)(C$_4$(CN)$_8$)$_{1/2}$ (M = Fe, Mn) with the [TCNE]$^{-.}$.
For the Mn$^{II}$(TCNE)$_{1/2}$(I$_3$)$_{1/2}$ two cluster types are
considered: one with the geometry corresponding to [TCNE]$^{-.}$ in the
two-dimensional layer, and another connecting the layers. For
M$^{II}$(TCNE)(C$_4$(CN)$_8$)$_{1/2}$ only the layer position of the
[TCNE]$^{-.}$ is considered. In all cases the LUMO (singly filled in the
material) is predominantly concentrated on the ethylenic C-C bond and is
antibonding. The J value for the Mn$^{II}$(TCNE)$_{1/2}$(I$_3$)$_{1/2}$ layer
is at least as twice as strong as one for the similar geometry for
M$^{II}$(TCNE)(C$_4$(CN)$_8$)$_{1/2}$. The correct order of magnitude of the
calculated exchange parameters, as compared with those extracted from
experimentsindicates the general validity of this approach. However, the direct
comparison of these J values with the critical temperatures is not possible
since these latter heavily depend on details of the interlayer coupling not
available through the EHCF procedure. In order to get more information on the
geometry dependence of the exchange parameters, the dependence on the tilt
angle of the TCNE with respect to the plane formed by the metal ions and four
equatorial NCMe groups was investigated. In all cases, a remarkably strong
dependence, having different trends depending on the number of electrons in the
d-shells of respective transition metal ions was observed.",1112.1362v1
2014-01-14,Charge transfer excitations from excited state Hartree-Fock subsequent minimization scheme,"Photoinduced charge-transfer processes play a key role for novel photovoltaic
phenomena and devices. Thus, the development of ab initio methods that allow
for an accurate and computationally inexpensive treatment of charge-transfer
excitations is a topic that nowadays attracts a lot of scientific attention. In
this paper we extend an approach recently introduced for the description of
single and double excitations (M. Tassi, I. Theophilou and S. Thanos, Int. J.
Quantum Chem., (113), 690 (2013), M. Tassi, I. Theophilou and S. Thanos, J.
Chem. Phys. (138), 124107 (2013)) to allow for the description of
intermolecular charge-transfer excitations. We describe an excitation where an
electron is transferred from a donor system to an acceptor one, keeping the
excited state orthogonal to the ground state and avoiding variational collapse.
These conditions are achieved by decomposing the space spanned by the
Hartree-Fock (HF) ground state orbitals into four subspaces: The subspace
spanned by the occupied orbitals that are localized in the region of the donor
molecule, the corresponding for the acceptor ones and two more subspaces
containing the virtual orbitals that are localized in the neighborhood of the
donor and the acceptor, respectively. Next, we create a Slater determinant with
a hole in the subspace of occupied orbitals of the donor and a particle in the
virtual subspace of the acceptor. Subsequently we optimize both the hole and
the particle by minimizing the HF energy functional in the corresponding
subspaces. Finally, we test our approach by calculating the lowest
charge-transfer excitation energies for a set of tetracyanoethylene-hydrocarbon
complexes, that have been used earlier as a test set for such kind of
excitations.",1401.3271v4
2018-04-14,"Effects of Graphene/BN Encapsulation, Surface Functionalization and Molecular Adsorption on the Electronic Properties of Layered InSe: A First-Principles Study","By using first-principles calculations, we investigated the effects of
graphene/boron nitride (BN) encapsulation, surface functionalization by
metallic elements (K, Al, Mg and typical transition metals) and molecules
(tetracyanoquinodimethane (TCNQ) and tetracyanoethylene (TCNE)) on the
electronic properties of layered indium selenide (InSe). It was found that an
opposite trend of charge transfer is possible for graphene (donor) and BN
(acceptor), which is dramatically different from phosphorene where both
graphene and BN play the same role (donor). For InSe/BN heterostructure, a
change of the interlayer distance due to an out-of-plane compression can
effectively modulate the band gap. Strong acceptor abilities to InSe were found
for the TCNE and TCNQ molecules. For K, Al and Mg-doped monolayer InSe, the
charge transfer from K and Al atoms to the InSe surface was observed, causing
an n-type conduction of InSe, while p-type conduction of InSe observed in case
of the Mg-doping. The atomically thin structure of InSe enables the possible
observation and utilization of the dopant-induced vertical electric field
across the interface. A proper adoption of the n- or p-type dopants allows for
the modulation of the work function, the Fermi level pinning, the band bending,
and the photo-adsorbing efficiency near the InSe surface/interface.
Investigation on the adsorption of transition metal atoms on InSe showed that
Ti-, V-, Cr-, Mn-, Co-adsorbed InSe are spin-polarized, while Ni-, Cu-, Pd-,
Ag- and Au-adsorbed InSe are non-spin-polarized. Our results shed lights on the
possible ways to protect InSe structure and modulate its electronic properties
for nanoelectronics and electrochemical device applications.",1804.05180v1
2001-10-18,Strong Quasiparticle Trapping In A 6x6 Array Of Vanadium-Aluminum Superconducting Tunnel Junctions,"A 6x6 array of symmetrical V/Al/AlOx/Al/V Superconducting Tunnel Junctions
(STJs) was fabricated. The base electrode is a high quality epitaxial film with
a residual resistance ratio (RRR) of ~30. The top film is polycrystalline with
an RRR of ~10. The leakage currents of the 25x25 mm^2 junctions are of the
order of 0.5 pA/mm^2 at a bias voltage of 100 mV, which corresponds to a
dynamical resistance of ~ 3 10^5 ohms. When the array was illuminated by 6 keV
X-ray photons from a 55Fe radioactive source the single photon charge output
was found to be low and strongly dependent on the temperature of the devices.
This temperature dependence at X-ray energies can be explained by the existence
of a very large number of quasiparticle (QP) traps in the Vanadium. QPs are
confined in these traps, having a lower energy gap than the surrounding
material, and are therefore not available for tunneling. The number of traps
can be derived from the energy dependence of the responsivity of the devices
(charge output per electron volt of photon input energy).",0110374v1
2001-12-02,"Evidence for short range orbital order in paramagnetic insulating (Al,V)_2O_3","The local structure of (Al_0.06V_0.94)_2O_3 in the paramagnetic insulating
(PI) and antiferromagnetically ordered insulating (AFI) phase has been
investigated using hard and soft x-ray absorption techniques. It is shown that:
1) on a local scale, the symmetry of the vanadium sites in both the PI and the
AFI phase is the same; and 2) the vanadium 3d - oxygen 2p hybridization, as
gauged by the oxygen 1s absorption edge, is the same for both phases, but
distinctly different from the paramagnetic metallic phase of pure V_2O_3. These
findings can be understood in the context of a recently proposed model which
relates the long range monoclinic distortion of the antiferromagnetically
ordered state to orbital ordering, if orbital short range order in the PI phase
is assumed. The measured anisotropy of the x-ray absorption spectra is
discussed in relation to spin-polarized density functional calculations.",0112016v1
2002-09-30,"Electronic structure of magnetic molecules V15: LDA+U calculations, X-ray emission and photoelectron spectra","Electronic structure of V$_{15}$ magnetic molecules (K$_6$ [V$_{15}$ As$_6$
O$_{42}$ (H$_2$O)] \cdot 8H$_2$O)$ has been studied using LSDA+U band structure
calculations, and measurements of X-ray photoelectron (valence band, core
levels) and X-ray fluorescence spectra (vanadium K$\beta_5$ and L$_{2,3}$, and
oxygen K$\alpha$). Experiments confirm that vanadium ions are tetravalent in
V$_{15}$, and their local atomic structure is close to that of CaV$_3$O$_7$.
Comparison of experimental data with the results of electronic structure
calculations show that the LSDA+U method provides a description of the
electronic structure of V$_{15}$ which agrees well with experiments.",0209665v1
2003-01-15,Growth and properties of strained VOx thin films with controlled stoichiometry,"We have succeeded in growing epitaxial films of rocksalt VOx on MgO(001)
substrates. The oxygen content as a function of oxygen flux was determined
using 18O2-RBS and the vanadium valence using XAS. The upper and lower
stoichiometry limits found are similar to the ones known for bulk material
(0.8<x<1.3). From the RHEED oscillation period a large number of vacancies for
both vanadium and oxygen were deduced, i.e. ~16% for stoichiometric VO. These
numbers are, surprisingly, very similar to those for bulk material and
consequently quite strain-insensitive. XAS measurements reveal that the
vacancies give rise to strong low symmetry ligand fields to be present. The
electrical conductivity of the films is much lower than the conductivity of
bulk samples which we attribute to a decrease in the direct overlap between t2g
orbitals in the coherently strained layers. The temperature dependence of the
conductivity is consistent with a variable range hopping mechanism.",0301263v3
2003-01-20,Variation of orbital symmetry of the localized 3d^1 electron of the V^{4+} ion upon the metal-insulator transition in VO_2,"The electronic structure of the vanadium dioxide VO_2 in the tetragonal R and
two monocinic M_1 and M_2 structural modifications was calculated in frames of
the local-density functional (LDA) approach and the LSDA + U formalism of
correction for correlation effects. Based on the results of calculation, we
argue in favor of the Mott-Hubbard mechanism of the metal-insulator transition.
It is shown that the transition is accompanied by the change of the orbital in
which the 3d^1 electron of the V^{4+} ion is localized. The symmetry of the
filled orbital is determined by the local oxygen environment of the vanadium
atoms in each structural phase. Depending on the dispersion of the band that
corresponds to the orbital occupied, the investigated compound can be either a
metal or an insulator.",0301347v1
2003-02-20,Correlation and surface effects in Vanadium Oxides,"Recent photoemission experiments have shown strong surface modifications in
the spectra from vanadium oxides as (V,Cr)_2O_3 or
  (Sr,Ca)VO_3. The effective mass is enhanced at the surface and the coherent
part of the surface spectrum is narrowed as compared to the bulk. The
quasiparticle weight is more sensitive at the surface than in the bulk against
bandwidth variations. We investigate these effects theoretically considering
the single-band Hubbard model for a film geometry. A simplified dynamical
mean-field scheme is used to calculate the main features of the interacting
layer-dependent spectral function. It turns out that the experimentally
confirmed effects are inherent properties of a system of strongly correlated
electrons. The reduction of the weight and the variance of the coherent part of
the surface spectrum can be traced back to the reduced surface coordination
number. Surface correlation effects can be strongly amplified by changes of the
hopping integrals at the surface.",0302427v1
2003-04-09,Molecular dynamics study of melting of a bcc metal-vanadium I: mechanical melting,"We present molecular dynamics simulations of the homogeneous (mechanical)
melting transition of a bcc metal, vanadium. We study both the nominally
perfect crystal as well as one that includes point defects. According to the
Born criterion, a solid cannot be expanded above a critical volume, at which a
'rigidity catastrophe' occurs. This catastrophe is caused by the vanishing of
the elastic shear modulus. We found that this critical volume is independent of
the route by which it is reached whether by heating the crystal, or by adding
interstitials at a constant temperature which expand the lattice. Overall,
these results are similar to what was found previously for an fcc metal,
copper. The simulations establish a phase diagram of the mechanical melting
temperature as a function of the concentration of interstitials. Our results
show that the Born model of melting applies to bcc metals in both the nominally
perfect state and in the case where point defects are present.",0304215v1
2004-06-21,"Orbital ordering and one-dimensional magnetic correlation in vanadium spinel oxides AV2O4 (A = Zn, Mg, or Cd)","We present our theoretical results on the mechanism of two transitions in
vanadium spinel oxides $A$V$_2$O$_4$ ($A$=Zn, Mg, or Cd) in which magnetic V
cations constitute a geometrically-frustrated pyrochlore structure. We have
derived an effective spin-orbital-lattice coupled model in the strong
correlation limit of the multiorbital Hubbard model, and applied Monte Carlo
simulation to the model. The results reveal that the higher-temperature
transition is a layered antiferro-type orbital ordering accompanied by
tetragonal Jahn-Teller distortion, and the lower-temperature transition is an
antiferromagnetic spin ordering. The orbital order lifts the magnetic
frustration partially, and induces spatial anisotropy in magnetic exchange
interactions. In the intermediate phase, the system can be considered to
consist of weakly-coupled antiferromagnetic chains lying in the perpendicular
planes to the tetragonal distortion.",0406462v1
2004-09-21,Spin Frustration and Orbital Order in Vanadium Spinels,"We present the results of our theoretical study on the effects of geometrical
frustration and the interplay between spin and orbital degrees of freedom in
vanadium spinel oxides $A$V$_2$O$_4$ ($A$ = Zn, Mg or Cd). Introducing an
effective spin-orbital-lattice coupled model in the strong correlation limit
and performing Monte Carlo simulation for the model, we propose a reduced spin
Hamiltonian in the orbital ordered phase to capture the stabilization mechanism
of the antiferromagnetic order. Orbital order drastically reduces spin
frustration by introducing spatial anisotropy in the spin exchange
interactions, and the reduced spin model can be regarded as weakly-coupled
one-dimensional antiferromagnetic chains. The critical exponent estimated by
finite-size scaling analysis shows that the magnetic transition belongs to the
three-dimensional Heisenberg universality class. Frustration remaining in the
mean-field level is reduced by thermal fluctuations to stabilize a collinear
ordering.",0409526v1
2005-10-06,Organometallic Benzene-Vanadium Wire: One-Dimensional Half-Metallic Ferromagnet,"Using density functional theory we have performed theoretical investigations
of the electronic properties of a free-standing one-dimensional organometallic
vanadium-benzene wire. This system represents the limiting case of multi-decker
V_n(C6H6)_{n+1} clusters which can be synthesized. We predict that the ground
state of the wire is a 100% spin-polarized ferromagnet (half-metal). Its
density of states is metallic at the Fermi energy for the minority electrons
and shows a semiconductor gap for the majority electrons. We found that the
half-metallic behavior is conserved up to 12%, longitudinal elongation of the
wire. However, under further stretching, the system exhibits a transition to a
high-spin ferromagnetic state that is accompanied by an abrupt jump of the
magnetic moment and a gain of exchange energy.",0510144v1
2007-04-15,Elastic properties of vanadium pentoxide aggregates and topological defects,"We study the aqueous solution of vanadium pentoxide by using topology
methods. The experiments by Zocher, Kaznacheev, and Dogic exhibited, that in
the sol phases of $V_2O_5-H_2O$, the tactoid droplets of $V_2O_5$ can coalesce.
In the magnetic field, this effect is associated with a gauge field action,
viz. we consider coalescence (in the topologically more convenient term,
""junction"") of droplets as annihilation of topological defects, concerning with
the tactoid geometry. We have shown, that in the magnetic field, the tactoid
junction is mainly caused by non-Abelian monopoles (vortons), whereas the
Abelian defects almost do not annihilate. Taking into account this annihilation
mechanism, the estimations of time-aging of the $V_2O_5-H_2O$ sols may be
specified",0704.1893v7
2007-07-06,Softening of the insulating phase near Tc for the photo-induced insulator-to-metal phase transition in vanadium dioxide,"We use optical-pump terahertz-probe spectroscopy to investigate the
near-threshold behavior of the photoinduced insulator-to-metal (IM) transition
in vanadium dioxide thin films. Upon approaching Tc a reduction in the fluence
required to drive the IM transition is observed, consistent with a softening of
the insulating state due to an increasing metallic volume fraction (below the
percolation limit). This phase coexistence facilitates the growth of a
homogeneous metallic conducting phase following superheating via
photoexcitation. A simple dynamic model using Bruggeman effective medium theory
describes the observed initial condition sensitivity.",0707.1054v2
2007-11-27,Elastic constants and volume changes associated with two high-pressure rhombohedral phase transformations in vanadium,"We present results from ab initio calculations of the mechanical properties
of the rhombohedral phase (beta) of vanadium metal reported in recent
experiments, and other predicted high-pressure phases (gamma and bcc), focusing
on properties relevant to dynamic experiments. We find that the volume change
associated with these transitions is small: no more than 0.15% (for beta -
gamma). Calculations of the single crystal and polycrystal elastic moduli
(stress-strain coefficients) reveal a remarkably small discontinuity in the
shear modulus and other elastic properties across the phase transitions even at
zero temperature where the transitions are first order.",0711.4383v2
2008-01-25,Ab initio study of spin-dependent transport in carbon nanotubes with iron and vanadium adatoms,"We present an ab initio study of spin dependent transport in armchair carbon
nanotubes with transition metal adsorbates, iron or vanadium. We neglect the
effect of tube curvature and model the nanotube by graphene with periodic
boundary conditions. A density functional theory based nonequilibrium Green's
function method is used to compute the electronic structure and zero-bias
conductance. The presence of the adsorbate causes a strong scattering of
electrons of one spin type only. The scattering is shown to be due to coupling
of the two armchair band states to the metal 3d orbitals with matching symmetry
causing Fano resonances appearing as dips in the transmission function. The
spin type (majority/minority) being scattered depends on the adsorbate and is
explained in terms of d-state filling. The results are qualitatively reproduced
using a simple tight-binding model, which is then used to investigate the
dependence of the transmission on the nanotube width. We find a decrease in the
width of the transmission dip as the tube-size increases.",0801.3997v1
2008-02-15,Magnetic properties of Ag(2)VOP(2)O(7): an unexpected spin dimer system,"Magnetic properties of the silver vanadium phosphate Ag(2)VOP(2)O(7) are
studied by means of magnetic susceptibility measurements and electronic
structure calculations. In spite of the layered crystal structure suggesting 1D
or 2D magnetic behavior, this compound can be understood as a spin dimer
system. The fit of the magnetic susceptibility indicates an intradimer
interaction of about 30 K in perfect agreement with the computational results.
Our study emphasizes the possible pitfalls in interpreting experimental data on
structural basis only and points out the importance of microscopic models for
the understanding of the magnetic properties of vanadium phosphates.",0802.2293v1
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
2008-07-21,First Principles Calculations of Spin-Dependent Conductance of Graphene Flakes,"Using ab initio density functional theory and quantum transport calculations
based on nonequilibrium Green's function formalism we study structural,
electronic, and transport properties of hydrogen-terminated short graphene
nanoribbons (graphene flakes) and their functionalization with vanadium atoms.
Rectangular graphene flakes are stable, having geometric and electronic
structures quite similar to that of extended graphene nanoribbons. We show that
a spin-polarized current can be produced by pure, hydrogenated rectangular
graphene flakes by exploiting the spatially-separated edge states of the flake
using asymmetric, non-magnetic contacts. Functionalization of the graphene
flake with magnetic adatoms such as vanadium also leads to spin-polarized
currents even with symmetric contacts. We observe and discuss sharp
discontinuities in the transmission spectra which arise from Fano resonances of
localized states in the flake.",0807.3174v2
2008-07-25,Materials Design using Correlated Oxides: Optical Properties of Vanadium Dioxide,"Materials with strong electronic Coulomb interactions play an increasing role
in modern materials applications. ""Thermochromic"" systems, which exhibit
thermally induced changes in their optical response, provide a particularly
interesting case. The optical switching associated with the metal-insulator
transition of vanadium dioxide (VO2), for example, has been proposed for use in
""intelligent"" windows, which selectively filter radiative heat in hot weather
conditions. In this work, we develop the theoretical tools for describing such
a behavior. Using a novel scheme for the calculation of the optical
conductivity of correlated materials, we obtain quantitative agreement with
experiments for both phases of VO2. On the example of an optimized
energy-saving window setup, we further demonstrate that theoretical materials
design has now come into reach, even for the particularly challenging class of
correlated electron systems.",0807.4044v1
2008-11-07,Multi-orbital Effects in Optical Properties of Vanadium Sesquioxide,"Vanadium sesquioxide, V2O3, boasts a rich phase diagram whose description
necessitates the accounting for many-body Coulomb correlations. Spectral
properties of this compound have been successfully addressed within dynamical
mean field theory to an extent that results of recent angle resolved
photoemission experiments have been correctly predicted. While photoemission
spectroscopy probes the occupied part of the one-particle spectrum, optical
experiments measure transitions into empty states and thus provide
complementary information. In this work, we focus on the optical properties of
V2O3 in its paramagnetic phases by employing our recently developed
""generalized Peierls approach"". Compared to experiments, we obtain results in
overall satisfactory agreement. Further, we rationalize that the experimentally
observed temperature dependence stems from the different coherence scales of
the involved charge carriers.",0811.1098v1
2009-06-18,Experimental evidence of anapolar moments in the antiferromagnetic insulating phase of V2O3 obtained from x-ray resonant Bragg diffraction,"We have investigated the antiferromagnetic insulating phase of the
Mott-Hubbard insulator V$_2$O$_3$ by resonant x-ray Bragg diffraction at the
vanadium K-edge. Combining the information obtained from azimuthal angle scans,
linear incoming polarization scans and by fitting collected data to the
scattering amplitude derived from the established chemical I2/a and magnetic
space groups we provide evidence of the ordering motif of anapolar moments
(which results from parity violation coupling to an electromagnetic field).
Experimental data (azimuthal dependence and polarization analysis) collected at
space-group forbidden Bragg reflections are successfully accounted within our
model in terms of vanadium magnetoelectric multipoles. We demonstrate that
resonant x-ray diffraction intensities in all space-group forbidden Bragg
reflections of the kind $(hkl)_m$ with odd $h$ are produced by an E1-E2 event.
The determined tensorial parameters offer a test for ab-initio calculations in
this material, that can lead to a deeper and more quantitative understanding of
the physical properties of V$_2$O$_3$.",0906.3541v1
2009-07-02,Exploring the spin-1/2 frustrated square lattice model with high-field magnetization measurements,"We report on high-field magnetization measurements for a number of layered
vanadium phosphates that were recently recognized as spin-1/2 frustrated square
lattice compounds with ferromagnetic nearest-neighbor couplings (J_1) and
antiferromagnetic next-nearest-neighbor couplings (J_2). The saturation fields
of the materials lie in the range from 4 to 24 T and show excellent agreement
with the previous estimates of the exchange couplings deduced from low-field
thermodynamic measurements. The consistency of the high-field data with the
regular frustrated square lattice model provides experimental evidence for a
weak impact of spatial anisotropy on the nearest-neighbor couplings in layered
vanadium phosphates. The variation of the J_2/J_1 ratio within the compound
family facilitates the experimental access to the evolution of the
magnetization curve upon the change of the frustration magnitude. Our results
support the recent theoretical prediction by Thalmeier et al. [Phys. Rev. B,
77, 104441 (2008)] and give evidence for the enhanced bending of the
magnetization curves due to the increasing frustration of the underlying spin
system.",0907.0391v1
2009-12-30,Quantum 120-degree model on pyrochlore lattice: orbital ordering in MnV2O4,"We present an analytical model of orbital ordering in vanadium spinel MnV2O4.
The model is based on recent first-principles calculation indicating a strong
trigonal distortion at the vanadium sites of this compound [Phys. Rev. Lett.
102, 216405 (2009)]. At the single-ion level, the trigonal crystal field leaves
a doubly degenerate atomic ground state and breaks the approximate rotational
symmetry of t2g orbitals. We find that the effective interaction between the
low-energy doublets is described by a quantum antiferromagnetic 120-degree
model on the pyrochlore lattice. We obtain the classical ground state and show
its stability against quantum fluctuations. The corresponding orbital order
consisting of two inequivalent orbital chains is consistent with the
experimentally observed tetragonal symmetry. A periodic modulation of electron
density function along orbital chains is shown to arise from the staggering of
local trigonal axes. In the presence of orbital order, single-ion spin
anisotropy arising from relativistic spin-orbit interaction stabilizes the
experimentally observed orthogonal magnetic structure.",0912.5171v2
2010-02-15,Electronic correlations in vanadium chalcogenides: BaVSe3 versus BaVS3,"Albeit structurally and electronically very similar, at low temperature the
quasi-one-dimensional vanadium sulfide BaVS3 shows a metal-to-insulator
transition via the appearance of a charge-density-wave state, while BaVSe3
apparently remains metallic down to zero temperature. This different behavior
upon cooling is studied by means of density functional theory and its
combination with the dynamical mean-field theory and the rotationally-invariant
slave-boson method. We reveal several subtle differences between these
chalcogenides that provide indications for the deviant behavior of BaVSe3 at
low temperature. In this regard, a smaller Hubbard U in line with an increased
relevance of the Hund's exchange J plays a vital role.",1002.2924v2
2010-07-18,Vanadium pentoxide (V2O5): a van der Waals density functional study,"The past few years has brought renewed focus on the physics behind the class
of materials characterized by long-range interactions and wide regions of low
electron density, sparse matter. There is now much work on developing the
appropriate algorithms and codes able to correctly describe this class of
materials within a parameter-free quantum physical description. In particular,
van der Waals (vdW) forces play a major role in building up material cohesion
in sparse matter. This work presents an application to the vanadium pentoxide
(V2O5) bulk structure of two versions of the vdW-DF method, a first-principles
procedure for the inclusion of vdW interactions in the context of density
functional theory (DFT). In addition to showing improvement compared to
traditional semilocal calculations of DFT, we discuss the choice of various
exchange functionals and point out issues that may arise when treating systems
with large amounts of vacuum.",1007.3045v1
2010-09-24,Control of Correlations in Sr4V2O6Fe2As2 by Chemical Stoichiometry,"We show using a combination of powder X-ray and neutron diffraction, first
principles calculations, temperature- and field-dependent magnetization, heat
capacity and resistivity data that the superconducting behavior of
`Sr$_4$V$_2$O$_6$Fe$_2$As$_2$' is dependent on synthesis conditions,
particularly, heating profiles result in unintentional chemical doping. This
compound can be tuned from a state in which the vanadium electrons are
itinerant with a high electronic density of states, to a state where the
vanadium-oxide layers are insulating and presumably antiferromagnetic.",1009.4911v1
2011-09-12,Nanoscale imaging of the electronic and structural transitions in vanadium dioxide,"We investigate the electronic and structural changes at the nanoscale in
vanadium dioxide (VO2) in the vicinity of its thermally driven phase
transition. Both electronic and structural changes exhibit phase coexistence
leading to percolation. In addition, we observe a dichotomy between the local
electronic and structural transitions. Nanoscale x-ray diffraction reveals
local, non-monotonic switching of the lattice structure, a phenomenon that is
not seen in the electronic insulator-to-metal transition mapped by near-field
infrared microscopy.",1109.3347v1
2012-02-07,Vanadium dioxide : A Peierls-Mott insulator stable against disorder,"Vanadium dioxide undergoes a first order metal-insulator transition at 340 K.
In this work, we develop and carry out state of the art linear scaling DFT
calculations refined with non-local dynamical mean-field theory. We identify a
complex mechanism, a Peierls-assisted orbital selection Mott instability, which
is responsible for the insulating M$_1$ phase, and furthermore survives a
moderate degree of disorder.",1202.1423v2
2012-02-24,Quantum Monte-Carlo study of magnetic ordering in ZnV2O4,"We study the magnetic ordering of Vanadium spinels by Quantum Monte Carlo
simulations of a three-band Hubbard model. Vanadium spinels, AV$_2$O$_4$,
exhibit a unique ""up-up-down-down"" spin ordering at low temperatures. While
this magnetic ordering was originally measured in 1973, its origin has remained
unclear for many years due to the lack of unbiased approaches for solving the
relevant model. A three-band Hubbard model on the spinel lattice (corner
sharing tetrahedra) is a minimal Hamiltonian for describing the $t_{2g}$
electrons of the V$^{2+}$ ions. One of the main difficulties is that this
family of compounds belongs to the elusive intermediate-coupling regime ($U
\gtrsim t$) for which there is no small parameter that can justify a
perturbative expansion. We present a controlled Quantum Monte-Carlo approach to
the three-band Hubbard model relevant for this materials that reproduces the
up-up-down-down spin ordering. The method is free of the sign problem that is
usually the main limiting factor for simulating fermionic systems in dimension
higher than one.",1202.5366v1
2012-04-12,Magnetic order and ice rules in the multiferroic spinel FeV2O4,"We present a neutron diffraction study of FeV2O4, which is rare in exhibiting
spin and orbital degrees of freedom on both cation sublattices of the spinel
structure. Our data confirm the existence of three structural phase transitions
previously identified with x-ray powder diffraction, and reveal that the lower
two transitions are associated with sequential collinear and canted
ferrimagnetic transitions involving both cation sites. Through consideration of
local crystal and spin symmetry, we further conclude that Fe2+ cations are
ferro-orbitally ordered below 135K and V3+ orbitals order at 60K in accordance
with predictions for vanadium spinels with large trigonal distortions and
strong spin-orbit coupling. Intriguingly, the direction of ordered vanadium
spins at low temperatures obey `ice rules' more commonly associated with the
frustrated rare-earth pyrochlore systems.",1204.2812v3
2012-05-04,Orbital disorder induced by charge fluctuations in vanadium spinels,"Motivated by recent experiments on vanadium spinels, $A$V$_2$O$_4$, that show
an increasing degree of electronic delocalization for smaller cation sizes, we
study the evolution of orbital ordering (OO) between the strong and
intermediate-coupling regimes of a multi-orbital Hubbard Hamiltonian. The
underlying magnetic ordering of the Mott insulating state leads to a rapid
suppression of OO due to enhanced charge fluctuations along ferromagnetic
bonds. Orbital double-occupancy is rather low at the transition point
indicating that the system is in the crossover region between strong and
intermediate-coupling regimes when the orbital degrees of freedom become
disordered.",1205.1081v1
2012-05-18,Controlling exchange coupling strength in NixCu100-x thin films,"Thickness (dF) and concentration (x) dependence of the Curie temperature of
NixCu100-x(dF) ferromagnetic alloy layers (x =0.55,0.65, dF =[3nm{\div}12nm])
being in contact with a vanadium layer was studied. The Curie temperature of
the ferromagnetic layers depends on the thickness when it is comparable with
the interface layer between the F and the vanadium layers, which is attributed
to the proximity coupling of the interface region with the rest of the F layer.
The present study provides valuable information for fabrication of samples with
controlled exchange coupling strength for studies of superconductor/ferromagnet
(S/F) proximity effects.",1205.4149v1
2012-10-02,Low-voltage broadband hybrid plasmonic-vanadium dioxide switches,"Surface plasmon polaritons can substantially reduce the sizes of optical
devices, since they can concentrate light to (sub)wavelength scales. However,
(sub)wavelength-scale electro-optic plasmonic switches or modulators with high
efficiency, low insertion loss, and high extinction ratios remain a challenge
due to their small active volumes. Here, we use the insulator-metal phase
transition of a correlated-electron material, vanadium dioxide, to overcome
this limitation and demonstrate compact, broadband, and efficient plasmonic
switches with integrated electrical control. The devices are micron-scale in
length and operate near a wavelength of 1550 nm. The switching bandwidths
exceed 100 nm and applied voltages of only 400 mV are sufficient to attain
extinction ratios in excess of 20 dB. Our results illustrate the potential of
using phase transition materials for highly efficient and ultra-compact
plasmonic switches and modulators.",1210.0785v2
2013-02-15,Orbital reformation with vanadium trimerization in $d^2$ triangular lattice LiVO$_2$ revealed by $^{51}$V NMR,"LiVO$_2$ is a model system of the valence bond solid (VBS) in $3d^2$
triangular lattice. The origin of the VBS formation has remained controversial.
We investigate the microscopic mechanism by elucidating the $d$ orbital
character via on-site $^{51}$V NMR measurements in a single crystal up to 550 K
across a structural transition temperature $T_c$. The Knight shift, $K$, and
nuclear quadrupole frequency, $\delta \nu$, show that the 3d orbital with local
trigonal symmetry are reconstructed into a $d_{yz}d_{zx}$ orbital order below
$T_c$. Together with the NMR spectra with three-fold rotational symmetry, we
confirm a vanadium trimerization with $d$-$d$ $\sigma$ bonds. The Knight shift
extracts the large Van-Vleck orbital susceptibility, $\chi^{\rm VV} = 3.6
\times 10^{-4}$, in a paramagnetic state above $T_c$, which is comparable to
the spin susceptibility. The results suggest that orbitally induced Peierls
transition in the proximity of the frustrated itinerant state is the dominant
driving force of the trimerization transition.",1302.3662v1
2013-04-30,Vanadium dioxide as a natural disordered metamaterial: perfect thermal emission and large broadband negative differential thermal emittance,"We experimentally demonstrate that a thin (~150 nm) film of vanadium dioxide
(VO2) deposited on sapphire has an anomalous thermal emittance profile when
heated, which arises due to the optical interaction between the film and the
substrate when the VO2 is at an intermediate state of its insulator-metal
transition (IMT). Within the IMT region, the VO2 film comprises nanoscale
islands of metal- and dielectric-phase, and can thus be viewed as a natural,
disordered metamaterial. This structure displays ""perfect"" blackbody-like
thermal emissivity over a narrow wavelength range (~40 cm-1), surpassing the
emissivity of our black soot reference. We observed large broadband negative
differential thermal emittance over a >10 {\deg}C range: upon heating, the
VO2/sapphire structure emitted less thermal radiation and appeared colder on an
infrared camera. We anticipate that emissivity engineering with thin film
geometries comprising VO2 will find applications in infrared camouflage,
thermal regulation, infrared tagging and labeling.",1305.0033v1
2013-05-16,MicroSQUIDs based on V/Cu/V Josephson nanojunctions,"We report on the fabrication and characterization of microSQUID devices based
on nanoscale vanadium/copper/vanadium Josephson weak links. Magnetically driven
quantum interference patterns have been measured for temperatures in the 0.24 -
2 K range. As magnetometers, these devices obtain flux-to-voltage transfer
function values as high as 450 microV/Phi_0 leading to promising magnetic flux
resolution Phi_N < 3 microPhi_0 / Hz^(1/2), being here limited by the room
temperature preamplification stage. Significant improvements in the flux noise
performance figures, which are already competitive with existing
state-of-the-art commercial SQUIDs systems, are expected either with cryogenic
preamplification or with the adoption of optimized electronic readout stages
based on active feedback schemes.",1305.3863v1
2013-06-18,Gapless spin liquid ground state in the S=1/2 vanadium oxyfluoride kagome antiferromagnet [NH4]2[C7H14N][V7O6F18],"The vanadium oxyfluoride [NH4]2[C7H14N][V7O6F18] (DQVOF) is a geometrically
frustrated magnetic bilayer material. The structure consists of S=1/2 kagome
planes of V4+ d1 ions with S=1 V3+ d2 ions located between the kagome layers.
Muon spin relaxation measurements demonstrate the absence of spin freezing down
to 40 mK despite an energy scale of 60 K for antiferromagnetic exchange
interactions. From magnetization and heat capacity measurements we conclude
that the S=1 spins of the interplane V3+ ions are weakly coupled to the kagome
layers, such that DQVOF can be viewed as an experimental model for S=1/2 kagome
physics, and that it displays a gapless spin liquid ground state.",1306.4269v1
2013-07-14,Voltage-triggered Ultra-fast Metal-insulator Transition in Vanadium Dioxide Switches,"Electrically driven metal-insulator transition in vanadium dioxide (VO2) is
of interest in emerging memory devices, neural computation, and high speed
electronics. We report on the fabrication of out-of-plane VO2
metal-insulator-metal (MIM) structures and reproducible high-speed switching
measurements in these two-terminal devices. We have observed a clear
correlation between electrically-driven ON/OFF current ratio and
thermally-induced resistance change during metal-insulator transition. It is
also found that sharp metal-insulator transition could be triggered by external
voltage pulses within 2 ns at room temperature and the achieved ON/OFF ratio is
greater than two orders of magnitude with good endurance.",1307.3774v1
2013-07-22,Transport behavior and electronic structure of phase pure VO2 thin films grown on c-plane sapphire under different O2 partial pressure,"We grew highly textured phase pure VO2 thin films on c-plane Al2O3 substrates
with different oxygen partial pressure. X-ray absorption and photoemission
spectroscopy confirm the identical valence state of vanadium ions despite the
different oxygen pressure during the deposition. As the O2 flow rate increases,
the [010] lattice parameter for monoclinic VO2 was reduced and coincidently
distinctive changes in the metal- semiconductor transition (MST) and transport
behaviors were observed despite the identical valence state of vanadium in
these samples. We discuss the effect of the oxygen partial pressure on the
monoclinic structure and electronic structure of VO2, and consequently the MST.",1307.5871v1
2013-07-23,Anisotropic Infrared Response of Vanadium Dioxide Microcrystals,"Vanadium dioxide (VO2) undergoes a phase transition at a temperature of 340 K
between an insulating monoclinic M1 phase and a conducting rutile phase.
Accurate measurements of possible anisotropy of the electronic properties and
phonon features of VO2 in the insulating monoclinic M1 and metallic rutile
phases are a prerequisite for understanding the phase transition in this
correlated system. Recently, it has become possible to grow single domain
untwinned VO2 microcrystals which makes it possible to investigate the true
anisotropy of VO2. We performed polarized transmission infrared
micro-spectroscopy on these untwinned microcrystals in the spectral range
between 200 cm-1 and 6000 cm-1 and have obtained the anisotropic phonon
parameters and low frequency electronic properties in the insulating monoclinic
M1 and metallic rutile phases. We have also performed ab initio GGA+U total
energy calculations of phonon frequencies for both phases. We find our
measurements and calculations to be in good agreement.",1307.6022v1
2013-10-18,Epitaxial Growth of VO$_{2}$ by Periodic Annealing,"We report the growth of ultrathin VO$_{2}$ films on rutile TiO$_{2}$ (001)
substrates via reactive molecular-beam epitaxy. The films were formed by the
cyclical deposition of amorphous vanadium and its subsequent oxidation and
transformation to VO$_{2}$ via solid-phase epitaxy. Significant metal-insulator
transitions were observed in films as thin as 2.3 nm, where a resistance change
{\Delta}R/R of 25 was measured. Low angle annular dark field scanning
transmission electron microscopy was used in conjunction with electron energy
loss spectroscopy to study the film/substrate interface and revealed the
vanadium to be tetravalent and the titanium interdiffusion to be limited to 1.6
nm.",1310.5021v4
2014-05-13,Hydrogen Ordering and New Polymorph of Layered Perovskite Oxyhydrides: Sr2VO4-xHx,"Compositionally tunable vanadium oxyhydrides Sr2VO4-xHx (x = 0 - 1) without
considerable anion vacancy were synthesized by high-pressure solid state
reaction. The crystal structures and their properties were characterized by
powder neutron diffraction, synchrotron X-ray diffraction, thermal desorption
spectroscopy, and first-principles density functional theory (DFT)
calculations. The hydrogen anions selectively replaced equatorial oxygen sites
in the VO6 layers via statistical substitution of hydrogen in the low x region
(x < 0.2). A new orthorhombic phase (Immm) with an almost entirely
hydrogen-ordered structure formed from the K2NiF4-type tetragonal phase with x
> 0.7. Based on the DFT calculations, the degree of oxygen/hydrogen anion
ordering is strongly correlated with the bonding interaction between vanadium
and the ligands.",1405.3011v1
2014-07-03,Fast ignition driven by quasi-monoenergetic ions: Optimal ion type and reduction of ignition energies with an ion beam array,"Fast ignition of inertial fusion targets driven by quasi-monoenergetic ion
beams is investigated by means of numerical simulations. Light and intermediate
ions such as lithium, carbon, aluminium and vanadium have been considered.
Simulations show that the minimum ignition energies of an ideal configuration
of compressed Deuterium-Tritium are almost independent on the ion atomic
number. However, they are obtained for increasing ion energies, which scale,
approximately, as Z^2, where Z is the ion atomic number. Assuming that the ion
beam can be focused into 10 {\mu}m spots, a new irradiation scheme is proposed
to reduce the ignition energies. The combination of intermediate Z ions, such
as 5.5 GeV vanadium, and the new irradiation scheme allows a reduction of the
number of ions required for ignition by, roughly, three orders of magnitude
when compared with the standard proton fast ignition scheme.",1407.1013v1
2014-09-16,Thermally Driven Analog of the Barkhausen Effect at the Metal-Insulator Transition in Vanadium Dioxide,"The physics of the metal-insulator transition (MIT) in vanadium dioxide
remains a subject of intense interest. Because of the complicating effects of
elastic strain on the phase transition, there is interest in comparatively
strain-free means of examining VO2 material properties. We report contact-free,
low-strain studies of the MIT through an inductive bridge approach sensitive to
the magnetic response of VO2 powder. Rather than observing the expected
step-like change in susceptibility at the transition, we argue that the
measured response is dominated by an analog of the Barkhausen effect, due to
the extremely sharp jump in the magnetic response of each grain as a function
of time as the material is cycled across the phase boundary. This effect
suggests that future measurements could access the dynamics of this and similar
phase transitions.",1409.4817v2
2014-11-27,Short-Time Dynamics of Fe2/V13 Magnetic Superlattice Models,"Critical relaxation from a low-temperature fully ordered state of Fe2/V13
iron-vanadium magnetic superlattice models has been studied using the method of
short-time dynamics. Systems with three variants of the ratio R of inter- to
intralayer exchange coupling have been considered. Particles with N = 262144
spins have been simulated with periodic boundary conditions. Calculations have
been performed using the standard Metropolis algorithm of the Monte Carlo
method. The static critical exponents of magnetization and correlation radius,
as well as the dynamic critical exponent, have been calculated for three R
values. It is established that a small decrease in the exchange ratio (from R =
1.0 to 0.8) does not significantly influence the character of the short-time
dynamics in the models studied. A further significant decrease in this ratio
(to R = 0.01), for which a transition from three-dimensional to
quasi-twodimensional magnetism is possible, leads to significant changes in the
dynamic behavior of iron-vanadium magnetic superlattice models.",1411.7546v1
2014-12-01,Experimental Verification of Van Vleck Nature of Long-Range Ferromagnetic Order in Vanadium-Doped Three-Dimensional Topological Insulator Sb$_{2}$Te$_{3}$,"We demonstrate by high resolution low temperature electron energy loss
spectroscopy (EELS) measurements that the long range ferromagnetic (FM) order
in vanadium (V)-doped topological insulator Sb$_2$Te$_3$ has the nature of van
Vleck-type ferromagnetism. The positions and the relative amplitudes of two
core-level peaks (L$_3$ and L$_2$) of the V EELS spectrum show unambiguous
change when the sample is cooled from room temperature to T=10K.
Magnetotransport and comparison of the measured and simulated EELS spectra
confirm that these changes originate from onset of FM order. Crystal field
analysis indicates that in V-doped Sb$_2$Te$_3$, partially filled core states
contribute to the FM order. Since van Vleck magnetism is a result of summing
over all states, this magnetization of core level verifies the van Vleck-type
ferromagnetism in a direct manner.",1412.0636v1
2015-07-10,Thermopower analysis of metal-insulator transition temperature modulations in vanadium dioxide thin films with lattice distortion,"Insulator-to-metal (MI) phase transition in vanadium dioxide (VO2) thin films
with controlled lattice distortion was investigated by thermopower
measurements. VO2 epitaxial films with different crystallographic orientations,
grown on (0001) alpha-Al2O3, (11-20) alpha-Al2O3, and (001) TiO2 substrates,
showed significant decrease of absolute value of Seebeck coefficient (S) from
~200 to 23 microV K-1, along with a sharp drop in electrical resistivity (rho),
due to the transition from an insulator to a metal. The MI transition
temperatures observed both in rho (Trho) and S (TS) for the VO2 films
systematically decrease with lattice shrinkage in the pseudo-rutile structure
along c-axis, accompanying a broadening of the MI transition temperature width.
Moreover, the onset TS, where the insulating phase starts to become metallic,
is much lower than onset Trho. This difference is attributed to the sensitivity
of S for the detection of hidden metallic domains in the majority insulating
phase, which cannot be detected in rho-measurements. Consequently,
S-measurements provide a straightforward and excellent approach for a deeper
understanding of the MI transition process in VO2.",1507.02752v1
2015-09-01,Novel single phase vanadium dioxide nanostructured films for methane sensing near room temperature,"Methane (CH_4) gas sensing properties of novel vanadium dioxide (VO_2)
nanostructured films is reported for the first time. The single phase
nanostructures are synthesized by pulsed dc-magnetron sputtering of V target
followed by oxidation in O_2 atmosphere at 550 ^oC. The partial pressure of O_2
is controlled to obtain stoichiometric VO_2 with the samples showing rutile
monoclinic crystalline symmetry and regions of rod shaped nano-architectures.
These nanostructured films exhibit a reversible semiconductor to metal
transition in the temperature range of 60-70 ^oC. Gas sensing experiments are
carried out in the temperature span from 25 ^oC to 200 ^oC in presence of CH_4.
These experiments reveal that the films respond very well at temperatures as
low as 50 ^oC, in the semiconducting state.",1509.00203v1
2016-01-27,Induced superconductivity and engineered Josephson tunneling devices in epitaxial (111)-oriented gold/vanadium heterostructures,"We report a unique experimental approach to create topological
superconductors by inducing superconductivity into epitaxial metallic thin film
with strong spin-orbit coupling. Utilizing molecular beam epitaxy technique
under ultra-high vacuum condition, we are able to achieve (111) oriented single
phase of gold (Au) thin film grown on a well-oriented vanadium (V) s-wave
superconductor film with clean interface. We obtained atomically smooth Au thin
films with thicknesses even down to below a nanometer showing near-ideal
surface quality. The as-grown V/Au bilayer heterostructure exhibits
superconducting transition at around 4 K. Clear Josephson tunneling and Andreev
reflection are observed in S-I-S tunnel junctions fabricated from the epitaxial
bi-layers. The barrier thickness dependent tunneling and the associated
subharmonic gap structures (SGS) confirmed the induced superconductivity in Au
(111), paving the way for engineering thin film heterostructure based p-wave
superconductors and nano devices for Majorana fermion.",1601.07591v1
2016-02-24,Dynamically Babinet-invertible metasurface: a capacitive-inductive reconfigurable filter for terahertz waves using vanadium-dioxide metal-insulator transition,"This paper proposes a reconfigurable planar metamaterial that can be switched
between capacitive and inductive responses using local changes in the
electrical conductivity of its constituent material. The proposed device is
based on Babinet's principle and exploits the singular electromagnetic
responses of metallic checkerboard structures, which are dependent on the local
electrical conductivity. Utilizing the heating-induced metal-insulator
transition of vanadium dioxide ($\mathrm{VO}_2$), the proposed metamaterial is
designed to compensate for the effect of the substrate and is experimentally
characterized in the terahertz regime. This reconfigurable metamaterial can be
utilized as a switchable filter and as a switchable phase shifter for terahertz
waves.",1602.07408v2
2016-06-10,Unified Band Theoretic Description of Electronic and Magnetic Properties of Vanadium Dioxide Phases,"The debate about whether the insulating phases of vanadium dioxide (VO2) can
be described by band theory or must be described by a theory of strong electron
correlations remains unresolved even after decades of research. Energy-band
calculations using hybrid exchange functionals or including self-energy
corrections account for the insulating or metallic nature of different phases,
but have not yet successfully accounted for the observed magnetic orderings.
Strongly-correlated theories have had limited quantitative success. Here we
report that, by using hard pseudopotentials and an optimized hybrid exchange
functional, the energy gaps and magnetic orderings of both monoclinic VO2
phases and the metallic nature of the high-temperature rutile phase are
consistent with available experimental data, obviating an explicit role for
strong correlations. We also report a potential candidate for the newly-found
metallic monoclinic phase and present a detailed magnetic structure of the M2
monoclinic phase.",1606.03162v1
2016-07-07,Distributed Authorization in Vanadium,"In this tutorial, we present an authorization model for distributed systems
that operate with limited internet connectivity. Reliable internet access
remains a luxury for a majority of the world's population. Even for those who
can afford it, a dependence on internet connectivity may lead to sub-optimal
user experiences. With a focus on decentralized deployment, we present an
authorization model that is suitable for scenarios where devices right next to
each other (such as a sensor or a friend's phone) should be able to communicate
securely in a peer-to-peer manner. The model has been deployed as part of an
open-source distributed application framework called Vanadium. As part of this
tutorial, we survey some of the key ideas and techniques used in distributed
authorization, and explain how they are combined in the design of our model.",1607.02192v2
2016-08-29,The insulating phases of vanadium dioxide are Mott-Hubbard insulators,"We present the first comprehensive broadband optical spectroscopy data on two
insulating phases of vanadium dioxide (VO2): monoclinic M2 and triclinic. The
main result of our work is that the energy gap and the electronic structure are
essentially unaltered by the first-order structural phase transition between
the M2 and triclinic phases. Moreover, the optical interband features in the M2
and triclinic phases are remarkably similar to those observed in the
well-studied monoclinic M1 insulating phase of VO2. As the energy gap is
insensitive to the different lattice structures of the three insulating phases,
we rule out Peierls effects as the dominant contributor to the opening of the
gap. Rather, the energy gap arises from intra-atomic Coulomb correlations.",1608.08227v1
2016-10-13,Activation cross-sections of proton induced reactions on vanadium in the 37-65 MeV energy range,"Experimental excitation functions for proton induced reactions on natural
vanadium in the 37-65 MeV energy range were measured with the activation method
using a stacked foil irradiation technique. By using high resolution gamma
spectrometry cross-section data for the production of $^{51,48}$Cr, $^{48}$V,
$^{48,47,46,44m,44g,43}$Sc and $^{43,42}$K were determined. Comparisons with
the earlier published data are presented and results predicted by different
theoretical codes (EMPIRE and TALYS) are included. Thick target yields were
calculated from a fit to our experimental excitation curves and compared with
the earlier experimental yield data. Depth distribution curves to be used for
thin layer activation (TLA) are also presented.",1610.03954v1
2016-11-16,Electronic Correlations in Vanadium Revealed by Electron-Positron Annihilation Measurements,"The electronic structure of vanadium measured by Angular Correlation of
electron-positron Annihilation Radiation (ACAR) is compared with the
predictions of the combined Density Functional and Dynamical Mean-Field Theory
(DMFT). Reconstructing the momentum density from five 2D projections we were
able to determine the full Fermi surface and found excellent agreement with the
DMFT calculations. In particular, we show that the local, dynamic self-energy
corrections contribute to the anisotropy of the momentum density and need to be
included to explain the experimental results.",1611.05361v1
2016-12-02,Highly repeatable nanoscale phase coexistence in vanadium dioxide films,"The metal-insulator transition (MIT) in vanadium dioxide (VO2) has the
potential to lead to a number of disruptive technologies, including ultra-fast
data storage, optical switches, and transistors which move beyond the
limitations of silicon. For applications, VO2 films are deposited on
crystalline substrates to prevent cracking observed in bulk VO2 crystals across
the thermally driven MIT. Near the MIT, VO2 films exhibit nanoscale coexistence
between metallic and insulating phases, which opens up further potential
applications such as memristors, tunable capacitors, and optically engineered
devices such as perfect absorbers. It is generally believed that the formation
of phase domains must be affected to some extent by random processes which lead
to unreliable performance in nanoscale MIT based devices. Here we show that
nanoscale randomness is suppressed in the thermally driven MIT in sputtered VO2
films; the observed domain patterns of metallic and insulating phases in the
vicinity of the MIT in these films behave in a strikingly reproducible way.
This result opens the door for realizing reliable nanoscale VO2 devices.",1612.00855v1
2016-12-14,Volume Contraction at a Grain Boundary in Vanadium,"It is a conventional wisdom that symmetry breakdown at grain boundaries in
crystals introduces volume expansion and there has been no confirmed evidence
of volume contraction at a grain boundary in any kind of crystals. We report
surprising volume contraction at the Sigma3(111) grain boundary in vanadium.
The lattice distortion near this grain boundary is found to drive the local
structure toward a hypothetical {\omega} phase, which is only slightly less
stable than the bcc phase. Compressing lattice constant a of the {\omega}-phase
down to the value of bcc, as is the case at the Sigma3(111) grain boundary,
results in smaller c than in bcc structure, and hence the volume contraction.
We also find such a volume contraction impedes the segregation of Ti and H,
while enhances that of Cr, and hence a significant influence on material
properties. The discovery adds fundamental new knowledge of condensed matter,
and may also point to new techniques in grain boundary engineering of novel
materials through volume control.",1612.04498v2
2016-12-23,Theoretical evaluation of [V$^{IV}$(α-C$_3$S$_5$)]${^2-}$ as nuclear-spin sensitive single-molecule spin transistor,"In a straightforward application of molecular nanospintronics to quantum
computing, single-molecule spin transistors can be used to measure and control
nuclear spin qubits. A jump in the conductance occurs when the electronic spin
inverts its polarization, and this happens at a so-called anticrossing between
energy levels, which in turn only takes place at a specific magnetic field
determined by the nuclear spin state. So far, this procedure has only been
implemented for the terbium(III) bis(phthalocyaninato) complex. Here we explore
theoretically whether a similar behavior is expected for a highly stable
molecular spin qubit, the vanadium tris-dithiolate complex
[V$^{IV}$({\alpha}-C$_3$S$_5$)]${^2-}$. We consider such molecule sandwiched
into a two-terminal device and determine the spin-dependent conductance. We
verify that the transport channel at minimal bias voltage does not overlap with
the occupied spin orbitals, indicating that the spin states may survive in the
conduction regime. We estimate some physical parameters to guide the
experiments, and verify the robustness of the theoretical methodology by
applying it to two chemically related vanadium complexes.",1612.08017v2
2017-04-17,\textit{Ab initio} molecular dynamics study of the structural and electronic transition in VO$_2$,"The temperature-induced structural and electronic transformation in VO$_2$
between the monoclinic M1 and tetragonal rutile phases was studied by means of
\textit{ab initio} molecular dynamics, based on density functional theory with
Hubbard correction (DFT+U). Analysis of the dynamical processes associated with
the structural transformation was carried out on the atomic scale by following
the time evolution of dimerization amplitudes of vanadium atom chains and the
twisting angle of vanadium dimers. The electronic transition was studied by
tracing the changes in projected densities of states and their correlation with
the evolution of the structural transformation. Our results reveal a strong
interconnection between the structural and electronic transformations and show
that they take place on the same time scale.",1704.04917v1
2017-05-10,Role of Vanadyl Oxygen in Understanding Metallic Behavior of V2O5(001) Nanorods,"Vanadium pentoxide (V2O5), the most stable member of vanadium oxide family,
exhibits interesting semiconductor to metal transition in the temperature range
of 530-560 K. The metallic behavior originates because of the reduction of V2O5
through oxygen vacancies. In the present report, V2O5 nanorods in the
orthorhombic phase with crystal orientation of (001) are grown using vapor
transport process. Among three nonequivalent oxygen atoms in a VO5 pyramidal
formula unit in V2O5 structure, the role of terminal vanadyl oxygen (OI) in the
formation of metallic phase above the transition temperature is established
from the temperature-dependent Raman spectroscopic studies. The origin of the
metallic behavior of V2O5 is also understood due to the breakdown of pdpi bond
between OI and nearest V atom instigated by the formation of vanadyl OI
vacancy, confirmed from the downward shift of the bottom most split-off
conduction bands in the material with increasing temperature.",1705.03620v2
2017-05-17,High Thermoelectric Figure of Merit by Resonant Dopant in Half-Heusler Alloys,"Half-Heusler alloys have been one of the benchmark high temperature
thermoelectric materials owing to their thermal stability and promising figure
of merit ZT. Simonson et al. early showed that small amounts of vanadium doped
in Hf0.75Zr0.25NiSn enhanced the Seebeck coefficient and correlated the change
with the increased density of states near the Fermi level. We herein report a
systematic study on the role of vanadium (V), niobium (Nb), and tantalum (Ta)
as prospective resonant dopants in enhancing the ZT of n-type half-Heusler
alloys based on Hf0.6Zr0.4NiSn0.995Sb0.005. The V doping was found to increase
the Seebeck coefficient in the temperature range 300-1000 K, consistent with a
resonant doping scheme. In contrast, Nb and Ta act as normal n-type dopants, as
evident by the systematic decrease in electrical resistivity and Seebeck
coefficient. The combination of enhanced Seebeck coefficient due to the
presence of V resonant states and the reduced thermal conductivity has led to a
state-of-the-art ZT of 1.3 near 850 K in n-type
(Hf0.6Zr0.4)0.99V0.01NiSn0.995Sb0.005 alloys.",1705.06100v1
2017-11-14,Imaging the nanoscale phase separation in vanadium dioxide thin films at terahertz frequencies,"We use apertureless scattering near-field optical microscopy (SNOM) to
investigate the nanoscale optical response of vanadium dioxide (VO2) thin films
through a temperature-induced insulator-to-metal transition (IMT). We compare
images of the transition at both mid-infrared (MIR) and terahertz (THz)
frequencies, using a custom-built broadband THz-SNOM compatible with both
cryogenic and elevated temperatures. We observe that the character of spatial
inhomogeneities in the VO2 film strongly depends on the probing frequency. In
addition, we find that individual insulating (or metallic) domains have a
temperature-dependent optical response, in contrast to the assumptions of a
classical first-order phase transition. We discuss these results in light of
dynamical mean-field theory calculations of the dimer Hubbard model recently
applied to VO2.",1711.05242v1
2018-01-20,Limiting optical diodes enabled by the phase transition of vanadium dioxide,"A limiting optical diode is an asymmetric nonlinear device that is
bidirectionally transparent at low power, but becomes opaque when illuminated
by sufficiently intense light incident from a particular direction. We explore
the use of a phase-transition material, vanadium dioxide (VO2), as an active
element of limiting optical diodes. The VO2 phase transition can be triggered
by optical absorption, resulting in a change in refractive index orders of
magnitude larger than what can be achieved with conventional nonlinearities. As
a result, a limiting optical diode based on incident-direction-dependent
absorption in a VO2 layer can be very thin, and can function at low powers
without field enhancement, resulting in broadband operation. We demonstrate a
simple thin-film limiting optical diode comprising a transparent substrate, a
VO2 film, and a semi-transparent metallic layer. For sufficiently high incident
intensity, our proof-of-concept device realizes broadband asymmetric
transmission across the near infrared, and is approximately ten times thinner
than the free-space wavelength.",1801.06728v1
2018-07-20,Novel flow field design for vanadium redox flow batteries via topology optimization,"This paper presents a three-dimensional topology optimization method for the
design of flow field in vanadium redox flow batteries (VRFBs). We focus on
generating a novel flow field configuration for VRFBs via topology
optimization, which has been attracted attention as a powerful design tool
based on numerical optimization. An attractive feature of topology optimization
is that a topology optimized configuration can be automatically generated
without presetting a promising design candidate. In this paper, we formulate
the topology optimization problem as a maximization problem of the electrode
surface concentration in the negative electrode during the charging process.
The aim of this optimization problem is to obtain a topology optimized flow
field that enables the improvement of mass transfer effect in a VRFB. We
demonstrate that a novel flow field configuration can be obtained through the
numerical investigation. To clarify the performance of the topology optimized
flow field, we investigate the mass transfer effect through the comparison with
reference flow fields---parallel and interdigitated flow fields---and the
topology optimized flow field. In addition, we discuss the power loss that
takes account of the polarization loss and pumping power, at various operating
conditions.",1807.07764v1
2018-12-31,Vanadium in yttrium aluminum garnet: charge states and localization in the lattice,"Vanadium ions charge states and their incorporation in the yttrium aluminum
garnet Y3Al5O12 (YAG) lattice were studied by the correlated optical absorption
and electron paramagnetic resonance (EPR) measurements. In as-grown crystals,
the occupation of the V3+ at both the octahedral and tetrahedral aluminum sites
was proven. The V3+ to V4+ charge transformation was observed after annealing
in air, whereas annealing in the hydrogen atmosphere resulted exclusively in a
slight weakening of the V3+ absorption bands due to partial recharge of these
ions. Spin Hamiltonian parameters of the V3+ and V4+ ions at the tetrahedral
sites including the zero field splitting and the 51V hyperfine constants have
been determined using the high-frequency, up to 300 GHz, EPR measurements. From
the analysis of the spin Hamiltonian parameters in the framework of the crystal
field theory, the ground state energy levels splitting of the V3+ and V4+ ions
were calculated. The charge distribution over the tetrahedral V3+ and its
nearest oxygen surroundings was found to be strongly inhomogeneous whereas the
tetrahedral V4+ ion concentrated the charge with very weak participation of
surrounding ligands. Furthermore, the correlation of the optical and EPR data
allowed the proper assignment of the optical absorption peaks in YAG:V
crystals.",1812.11877v1
2019-02-05,Towards Commercializing Vanadium Dioxide Films: Investigation of the Impact of Different Interface on the Deterioration Process for Largely Extended Service Life,"Long term stability is the most pressing issue that impedes commercialization
of Vanadium Dioxide (VO2) based functional films, which show a gradual loss of
relative phase transition performance, especially in humid conditions when
serving as smart windows. Here, we investigated the impact of different
interface on the deterioration process of VO2 films and proposed a novel
encapsulation structure for largely extended service life. Hydrophobic and
stable hafnium dioxide (HfO2) layers have been incorporated with VO2 films for
encapsulated surfaces and cross sections. With modified thickness and structure
of HfO2 layers, the degradation process of VO2 can be effectively suppressed.
The proposed films can retain stable phase transition performances under high
relative humidity (90%) and temperature (60 Celsius) over 100 days, which is
equal to about 16 years in the real environment. Improving the stability of VO2
materials is a necessary step towards commercializing production of high
performance films for long term use.",1902.01617v1
2019-03-18,Effects of Vanadium doping on BaFe2As2,"We report an investigation of the structural, magnetic and electronic
properties of Ba(Fe(1-x)V(x))2As2 using x-ray, transport, magnetic
susceptibility and neutron scattering measurements. The vanadium substitutions
in Fe sites are possible up to 40\%. Hall effect measurements indicate strong
hole-doping effect through V doping, while no superconductivity is observed in
all samples down to 2K. The antiferromagnetic and structural transition
temperature of BaFe2As2 is gradually suppressed to finite temperature then
vanishes at x=0.245 with the emergence of spin glass behavior, suggesting an
avoided quantum critical point (QCP). Our results demonstrate that the avoided
QCP and spin glass state which were previously reported in the superconducting
phase of Co/Ni-doped BaFe2As2 can also be realized in non-superconducting
Ba(Fe(1-x)V(x))2As2.",1903.07233v1
2019-04-09,First-principles investigation of spin-phonon coupling in vanadium-based molecular spin qubits,"Paramagnetic molecules can show long spin-coherence times, which make them
good candidates as quantum bits. Reducing the efficiency of the spin-phonon
interaction is the primary challenge towards achieving long coherence times
over a wide temperature range in soft molecular lattices. The lack of a
microscopic understanding about the role of vibrations in spin relaxation
strongly undermines the possibility to chemically design better performing
molecular qubits. Here we report a first-principles characterization of the
main mechanism contributing to the spin-phonon coupling for a class of
vanadium(IV) molecular qubits. Post Hartree Fock and Density Functional Theory
are used to determine the effect of both reticular and intra-molecular
vibrations on the modulation of the Zeeman energy for four molecules showing
different coordination geometries and ligands. This comparative study provides
the first insight into the role played by coordination geometry and ligand
field strength in determining the spin-lattice relaxation time of molecular
qubits, opening the avenue to a rational design of new compounds.",1904.04922v1
2019-05-25,Actively tunable terahertz electromagnetically induced transparency analogue based on vanadium-oxide-assisted metamaterials,"Recently, phase-change materials (PCMs) have drawn more attention due to the
dynamically tunable optical properties. Here, we investigate the active control
of electromagnetically induced transparency (EIT) analogue based on terahertz
(THz) metamaterials integrated with vanadium oxide (VO2). Utilizing the
insulator-to-metal transition of VO2, the amplitude of EIT peak can be actively
modulated with a significant modulation depth. Meanwhile the group delay within
the transparent window can also be dynamically tuned, achieving the active
control of slow light effect. Furthermore, we also introduce independently
tunable transparent peaks as well as group delay based on a double-peak EIT
with good tuning performance. Finally, based on broadband EIT, the active
tuning of quality factor of the EIT peak is also realized. This work introduces
active EIT control with more degree of freedom by employing VO2, and can find
potential applications in future wireless and ultrafast THz communication
systems as multi-channel filters, switches, spacers, logic gates and
modulators.",1905.10623v1
2019-09-26,A detailed electronic structure study of Vanadium metal by using different beyond-DFT methods,"We report a detailed electronic structure calculation for Vanadium (V) using
DFT, DFT+$U$, $G_0W_0$, $GW_0$ and DFT+DMFT methods. The calculated values of
$W$, $U$ and $J$ by cRPA method are $\sim$1.1, $\sim$3.4 and $\sim$0.52 eV,
respectively. The comparison between calculated spectra (CS) and experimental
spectra (ES) suggests that $W$ ($U$) is more accurate for DFT+$U$ (DFT+DMFT)
method. The CS, obtained by these methods, give fairly good agreement with ES
for peaks' positions except $GW_0$. The shallowness of the dips lying $\sim$
-1.5 eV and $\sim$1.0 eV in ES are properly explained by DFT+DMFT method only,
due to the presence of incoherent $t_{2g}$ states. This work suggests that for
the proper explanation of ES, sophisticated many-body theory is needed even for
the simple metal.",1909.12267v2
2019-10-19,Wafer-size VO2 film prepared by water-vapor oxidant,"The growth of wafer-scale and uniform monoclinic VO2 film was a challenge if
considering the multivalent vanadium atom and the various phase structures of
VO2 compound. Directly oxidizing metallic vanadium film in oxygen gas seemed to
be an easy way, while the oxidation parameters were extremely sensitive due to
the critical preparation window. Here we proposed a facile thermal oxidation by
water-vapor to produce wafer-scale VO2 films with high quality. Results
indicated that by using the water-vapor oxidant, the temperature window for VO2
growth was greatly broadened. In addition, the obtained wafer-size VO2 film
showed very uniform surface and sharp resistance change. The chemical reaction
routes with water-vapor were calculated, which favored the VO2 film growth. Our
results not only demonstrated that the water-vapor could be used as a modest
oxidizing agent, but also showed the unique advantage for large size VO2 film
preparation.",1910.08776v3
2019-11-27,"Controlled Preparation, Characterization, and Bandgap Modulation of RF Sputtered Antimony Vanadium Oxide (SbVO4) Thin Films","In this paper, RF sputtered antimony vanadium oxide (SbVO4) thin films and
its characterization are reported. High purity sputtering targets were
fabricated by sintering a mixture of Sb2O3 and V2O5 powders. Thin films were
deposited by reactive sputtering at various temperature and argon/oxygen
partial pressures. Several growth parameters and surface chemistry were studied
by applying numerous optical and electrochemical characterization technique. It
is found that SbVO4 exhibits an indirect band gap in the range of 1.89 eV to
2.36 eV and has desirable valence band position to drive water oxidation
reaction under illumination. The bandgap depends heavily on stoichiometry of
the film and can be modulated by incorporating controlled amount of oxygen gas
in plasma environment. Optimized SbVO4 photoanodes was designed and tested for
photoelectrochemical water oxidation catalysis. Preliminary studies show that
these electrodes possess n-type catalytic behavior in alkaline media. The
prepared SbVO4 thin films, with typical film thickness was around 400 nm,
contain nanoparticles having the sizes of 10-15 nm.",1911.12171v1
2019-11-27,Study of RF Sputtered Antimony Alloyed Bismuth Vanadium Oxide (Sb:BiVO4) Thin Films for Enhanced Photoelectrochemical (PEC) performance from Bandgap Modulation to Thickness Optimization,"Monoclinic scheelite bismuth vanadate (BiVO4) is a promising photoanode for
water splitting yet the PEC performance is limited due to its relatively higher
(2.4 eV) band gap. Here, we successfully decreased its the band gap to 1.72 eV
by controlled antimony alloying. Low bandgap antimony alloyed bismuth vanadium
oxide (Sb:BiVO4) thin film was prepared by RF sputtering of high purity
homemade target, fabricated by solid-state reaction using a mixture of Sb2O3,
Bi2O3, and V2O5 powders with desired stoichiometric ratios. Several growth
parameters, powder crystallography, post-deposition effects, and surface
treatments, thickness dependence, effect of electrolytes on photocorrosion were
studied along with its optical and electrochemical characterization. We
discovered that Sb:BiVO4 is a direct band gap material in the visible light
range (1.72 eV) and a valence band position suitable for driving water
oxidation reaction under illumination. Furthermore, hole diffusion length is
increased with antimony alloying and achieved optimum thickness of 400 nm for
higher photocurrent. The controllably prepared Sb:BiVO4 particles are having
the sizes of 10-15 nm in room temperature deposition and can be grown up to 0.5
microns under air annealing.",1911.12191v1
2019-12-03,Multimorphism and gap opening of charge-density-wave phases in monolayer VTe2,"Vanadium dichalcogenides have attracted increasing interests for the charge
density wave phenomena and possible ferromagnetism. Here, we report on the
multiphase behavior and gap opening in monolayer VTe2 grown by molecular beam
epitaxy. Scanning tunneling microscopy and spectroscopy study revealed the
(4*4) metallic and gapped (2root3*2root3)charge-density wave pahses with an
energy gap of ~40 meV. Through the in-plane condensation of vanadium atoms, the
typical star-of-David clusters and truncated triangle-shaped clusters are
formed in (4*4) and (2root3*2root3) phases respectively, resulting in different
surface morphologies and electronic structures as confirmed by density
functional theory calculations with on-site coulomb repulsion. The CDW-driven
reorganization of the atomic structure weakens the ferromagnetic superexchange
coupling and strengthens the antiferromagnetic exchange coupling on the
contrary, suppressing the long-range magnetic order in monolayer VTe2.",1912.01336v1
2019-12-10,Ligand-field contributions to spin-phonon coupling in a family of Vanadium molecular qubits from multi-reference electronic structure theory,"Molecular electronic spins represent one of the most promising building
blocks for the design of quantum computing architectures. However, the
advancement of this technology requires the increase of spin lifetime at
ambient temperature. Spin-phonon coupling has been recognized as the key
interaction dictating spin relaxation at high temperature in molecular crystals
and the search for chemical-design principles to control such interaction are a
fundamental challenge in the field. Here we present a multi-reference
first-principles analysis of the g-tensor and the spin-phonon coupling in a
series of four exa-coordinate Vanadium(IV) molecular complexes, where the
catecholate ligand donor atom is progressively changed from Oxygen to Sulphur,
Selenium and Tellurium. A ligand field interpretation of the multi-reference
electronic structure theory results made it possible to rationalize the
correlation between the molecular g-shifts and the average spin-phonon coupling
coefficients, revealing the role of spin-orbit coupling, chemical bond
covalency and energy splitting of d-like orbitals in spin relaxation. Our study
reveals the simultaneous increase of metal-ligand covalency and electronic
excited state energy separation as key elements of an optimal strategy towards
long spin-lattice lifetimes in molecular qubits.",1912.04545v1
2020-02-11,Bound hole states associated to individual vanadium atoms incorporated into monolayer WSe$_2$,"Doping a two-dimensional semiconductor with magnetic atoms is a possible
route to induce magnetism in the material. We report on the atomic structure
and electronic properties of monolayer WSe$_2$ intentionally doped with
vanadium atoms by means of scanning transmission electron microscopy and
scanning tunneling microscopy and spectroscopy. Most of the V atoms incorporate
at W sites. These V$_W$ dopants are negatively charged, which induces a
localized bound state located 140 meV above the valence band maximum. The
overlap of the electronic potential of two charged V$_W$ dopants generates
additional in-gap states. Eventually, the negative charge may suppress the
magnetic moment on the V$_W$ dopants.",2002.04371v1
2020-02-28,Experimental Demonstration of Dynamic Thermal Regulation using Vanadium Dioxide Thin Films,"We present an experimental demonstration of passive, dynamic thermal
regulation in a solid-state system with temperature-dependent thermal
emissivity switching. We achieve this effect using a multilayered device,
comprised of a vanadium dioxide (VO2) thin film on a silicon substrate with a
gold back reflector. We experimentally characterize the optical properties of
the VO2 film and use the results to optimize device design. Using a calibrated,
transient calorimetry experiment we directly measure the temperature
fluctuations arising from a time-varying heat load. Under laboratory
conditions, we find that the device regulates temperature better than a
constant emissivity sample. We use the experimental results to validate our
thermal model, which can be used to predict device performance under the
conditions of outer space. In this limit, thermal fluctuations are halved with
reference to a constant-emissivity sample.",2003.00031v1
2020-03-22,Thermoelectric probe of defect state induced by ionic liquid gating in vanadium dioxide,"Thermoelectric measurements detect the asymmetry between the density of
states above and below the chemical potential in a material. It provides
insights into small variations in the density of states near the chemical
potential, complementing electron transport measurements. Here, combined
resistance and thermoelectric power measurements are performed on vanadium
dioxide (VO2), a prototypical correlated electron material, under ionic-liquid
(IL) gating. With IL gating, charge transport below the
metal-to-insulator-transition (MIT) temperature remains in the thermally
activated regime, while the Seebeck coefficient exhibits an apparent transition
from semiconducting to metallic behavior. The contrasting behavior indicates
changes in electronic structure upon IL gating, due to the formation of oxygen
defect states. The experimental results are corroborated by numerical
simulations based on a model density of states incorporating a gating induced
defect band. This study reveals thermoelectric measurements to be a convenient
and sensitive probe for the role of defect states induced by IL gating in
suppressing the MIT in VO2, which remains benign in charge transport
measurements, and possibly for studying defect sates in other materials.",2003.09840v1
2020-05-12,Vanadium gate-controlled Josephson half-wave nanorectifier,"Recently, the possibility to tune the critical current of conventional
metallic superconductors via electrostatic gating was shown in wires, Josephson
weak-links and superconductor-normal metal-superconductor junctions. Here we
exploit such a technique to demonstrate a gate-controlled vanadium-based Dayem
nano-bridge operated as a \emph{half-wave} rectifier at $3$ K. Our devices
exploit the gate-driven modulation of the critical current of the Josephson
junction, and the resulting steep variation of its normal-state resistance, to
convert an AC signal applied to the gate electrode into a DC one across the
junction. All-metallic superconducting gated rectifiers could provide the
enabling technology to realize tunable photon detectors and diodes useful for
superconducting electronics circuitry.",2005.05671v1
2020-05-13,Spectrally-Selective Vanadium Dioxide Based Tunable Metafilm Emitter for Dynamic Radiative Cooling,"Dynamic radiative cooling with variable emissive power is experimentally
demonstrated in this study by a wavelength-selective tunable metafilm emitter,
which consists of an opaque aluminum film, a sputtered silicon spacer, and a
thermochromic vanadium dioxide (VO2) layer fabricated by a furnace oxidation
method. The temperature-dependent spectral emittance, experimentally obtained
from spectral reflectance measurements, clearly shows a pronounced emission
peak around 10 um wavelength when the VO2 experiences an insulator-to-metal
phase transition near 65C. The tunable metafilm emitter achieves a significant
total emittance increase from 0.14 at room temperature to 0.6 at 100C.
Theoretical modeling based on thin-film optics indicates that the emission
enhancement at high temperatures is realized by Fabry-Perot cavity resonance
with the metallic VO2 film. Moreover, a calorimetry-based thermal vacuum
experiment was conducted and the enhanced thermal emission of the fabricated
tunable metafilm sample was experimentally demonstrated at temperatures higher
than the phase transition temperature, compared to black, aluminum and doped
silicon samples, whose emittance changes little near room temperatures. The
developed tunable metafilm emitter with variable spectrally-selective emittance
in the mid-infrared holds great promise for both terrestrial and
extraterrestrial dynamic radiative cooling applications.",2005.06140v1
2020-06-06,Controlling Metal-Insulator Transitions in Vanadium Oxide Thin Films by Modifying Oxygen Stoichiometry,"Vanadium oxides are strongly correlated materials which display
metal-insulator transitions as well as various structural and magnetic
properties that depend heavily on oxygen stoichiometry. Therefore, it is
crucial to precisely control oxygen stoichiometry in these materials,
especially in thin films. This work demonstrates a high-vacuum gas evolution
technique which allows for the modification of oxygen concentration in VOX thin
films by carefully tuning thermodynamic conditions. We were able to control the
evolution between VO2, V3O5, and V2O3 phases on sapphire substrates, overcoming
the narrow phase stability of adjacent Magn\'eli phases. A variety of annealing
routes were found to achieve the desired phases and eventually to control the
metal-insulator transition (MIT). The pronounced MIT of the transformed films
along with the detailed structural investigations based on x-ray diffraction
measurements and reciprocal space mapping show that optimal stoichiometry is
obtained and stabilized. Using this technique, we find that the thin film V-O
phase diagram differs from that of the bulk material due to strain and finite
size effects. Our study demonstrates new pathways to strategically tune the
oxygen stoichiometry in complex oxides and provides a roadmap for understanding
the phase stability of VOX thin films.",2006.03998v1
2020-06-12,Occupation switching of $d$ orbitals probed via hyperfine interactions in vanadium dioxide,"Metal-insulator transition was microscopically investigated by
orbital-resolved nuclear magnetic resonance (OR-NMR) spectroscopy in a single
crystal of vanadium dioxide VO$_2$. Observations of the anisotropic $^{51}$V
Knight shift and the nuclear quadrupole frequency allow us to evaluate
orbital-dependent spin susceptibility and $d$ orbital occupations. The result
is consistent with the degenerated $t_{2g}$ orbitals in a correlated metallic
phase and the $d$ orbital ordering in a nonmagnetic insulating phase. The
predominant orbital pointing along the chain facilitates a spin-singlet
formation triggering metal-insulator transition. The asymmetry of magnetic and
electric hyperfine tensors suggests the $d$ orbital reformation favored by a
low-symmetry crystal field, forming a localized molecular orbital. The result
highlights the cooperative electron correlation and electron-phonon coupling in
Mott transition with orbital degrees of freedom.",2006.07030v1
2020-06-07,A Statistical Description of Nuclear Reaction Models for Medical Radionuclides: the Paradigmatic Case of $^{47}$Sc Production with Thick Vanadium Targets,"We have introduced a tool to describe in a simple and efficient way the
outcomes of known nuclear reaction codes. It differs from the customary use
where typically a specific single model is selected and the remaining
disregarded. The use of simple statistical procedures allows to introduce a
more general theoretical evaluation with quantitative uncertainty, constructed
on the variability of the built-in theoretical models. We apply the technique
to study the production of $^{47}$Sc (a radio-nuclide with potential
theranostic applications in nuclear medicine) with a proton beam impinging on a
thick natural Vanadium target. We find an energy range with significant
production of $^{47}$Sc, and a minimum co-production of $^{46}$Sc, the
radioactive contaminant that has to be avoided as much as possible because of
its much longer half life than $^{47}$Sc (83.79 d vs 3.3492 d).",2006.07146v1
2020-08-27,Mapping nanoscale charge states and phase domains with quantitative hyperspectral coherent diffractive imaging spectroscopy,"The critical properties of functional materials and nanoscale devices often
originate from the coexistence of different thermodynamic phases and / or
oxidization states, but sample makeup is seldom completely known a priori.
Coherent diffractive imaging (CDI) provides the spatial resolution needed to
observe nanoscale coexistence while returning the full amplitude and phase
information of an object, but to date lacks the spectral information necessary
for composition identification. Here we demonstrate CDI spectroscopy (CDIS),
acquiring images of the prototypical quantum material vanadium oxide across the
vanadium L2,3 and oxygen K X-ray absorption edges with nanometer scale
resolution. Using the hyperspectral X-ray image we show coexistence of multiple
oxidization states and phases in a single sample and extract the full complex
refractive index of V2O5 and the monoclinic insulating and rutile conducting
phases of VO2. These results constrain the role of hidden phases in the
insulator-to-metal transition in VO2.",2008.11992v1
2020-08-06,A kinetic description of Vanadium carbide coating formed by the plasma electrolytic method,"The old creation method of this coating is using salt bath for an approximate
time of 6 to 10 hours, which means wasting a lot of time and energy while the
formation of this coating with the plasma electrolytic method just takes about
15 minutes and it can increase the efficiency strongly on an industrial scale.
In this research, the process has been applied on the samples of 1.2436 steel
in different periods. Then, thickness of the coating layers has been measured
by using SEM images and thermodynamic data of Vanadium carbide formation. These
results have been expanded to access and present a confirmed model to predict
the thickness of this diffusion-based coating as a function of time and
reaction temperature. The proven model can be used to demonstrate and prove the
kinetic advantage of this method and also find the optimal value of applying
time and temperature.",2010.02706v1
2020-11-16,Topologically stable bimerons and skyrmions in vanadium dichalcogenide Janus monolayers,"We investigate the magnetic phase diagram of 1T-vanadium dichalcogenide
monolayers in Janus configuration (VSeTe, VSSe, and VSTe) from first
principles. The magnetic exchange, magnetocrystalline anisotropy and
Dzyaloshinskii-Moriya interaction (DMI) are computed using density functional
theory calculations, while the temperature- and field-dependent magnetic phase
diagram is simulated using large-scale atomistic spin modeling in the presence
of thermal fluctuations. The boundaries between magnetic ordered phases and
paramagnetic phases are determined by cross-analyzing the average topological
charge with the magnetic susceptibility and its derivatives. We find that in
such Janus monolayers, DMI is large enough to stabilize non-trivial chiral
textures. In VSeTe monolayer, an asymmetrical bimeron lattice state is
stabilized for in-plane field configuration whereas skyrmion lattice is formed
for out-of-plane field configuration. In VSSe monolayer, a skyrmion lattice is
stabilized for out-of-plane field configuration. This study demonstrates that
non-centrosymmetric van der Waals magnetic monolayers can support topological
textures close to room temperature.",2011.07813v1
2020-11-30,Spatially non-homogeneous metallization of VO2: a TDDFT+DMFT analysis,"We provide insights into the atomistic details of the ultrafast
spatially-resolved breakdown of the insulating M1 phase in bulk VO2 employing
an ab initio technique based on time-dependent density-functional theory and
dynamical mean-field theory (DMFT-TDDFT). We find that the system is initially
metallized preferentially along the vanadium-dimer chains (CR axis), with a
subsequent growth of CR-elongated metallic bubbles. Moreover, we trace the
breakdown of the insulating phase to two types of oxygen atoms, resulting from
vanadium dimerization, which produce an unusual charge-density modulation in
the oxygen-atom chains with significant charge transfer to the inter-dimer
distance. These results are in qualitative agreement with experimental data and
shed light on the interplay between valence charge and lattice structure and
its role in the ultrafast response of strongly correlated insulators.",2011.14864v1
2021-01-09,Interface-Driven Thermo-Electric Switching Performance of VO$^+$ Diffused Soda-Lime Glass,"Strongly confined NaVO$^+$ segregation and its thermo-responsive
functionality at the interface between simple sputter-deposited amorphous
vanadium oxide thin films and soda-lime glass was substantiated in the present
study by in-situ temperature-controlled Time of Flight Secondary Ion Mass
Spectrometry (ToF-SIMS). The obtained ToF-SIMS depth profiles provided
unambiguous evidence for a reversible transformation that caused systematic
switching of the NaVO$^+$/ Na$^+$ and Na$^+$/ VO$^+$ intensities upon cycling
the temperature between 25 $^\circ$C and 340 $^\circ$C. Subsequently, NaVO
complexes were found to be reversibly formed (at 300 $^\circ$C) in vanadium
oxide diffused glass, leading to thermo-responsive electrical behaviour of the
thin film glass system. This new segregation -- and diffusion-dependent
multifunctionality of NaVO$^+$ -- points towards applications as an advanced
material for thermo-optical switches, in smart windows or in thermal sensors.",2101.03330v1
2021-03-23,Doping Concentration Modulation in Vanadium Doped Monolayer Molybdenum Disulfide for Synaptic Transistors,"Doping is an effective way to modify the electronic property of
two-dimensional (2D) materials and endow them with new functionalities.
However, wide-range control of the substitutional doping concentration with
large scale uniformity remains challenging in 2D materials. Here we report
in-situ chemical vapor deposition growth of vanadium (V) doped monolayer
molybdenum disulfide (MoS2) with widely tunable doping concentrations ranging
from 0.3 to 13.1 at%. The key to regulate the doping concentration lies in the
use of appropriate V precursors with different doping abilities, which also
generate a large-scale uniform doping effect. Artificial synaptic transistors
were fabricated by using the heavily doped MoS2 as the channel material for the
first time. Synaptic potentiation, depression and repetitive learning processes
are mimicked by the gate-tunable channel conductance change in such transistors
with abundant V atoms to trap/detrap electrons. This work shows a feasible
method to dope monolayer 2D semiconductors and demonstrates their use in
artificial synaptic transistors.",2103.12262v1
2021-05-06,Balancing orbital effects and onsite Coulomb repulsion through Na modulations in NaxVO2,"Vanadium oxides have been highly attractive for over half a century since the
discovery of the metal insulator transition near room temperatures. Here NaxVO2
is studied through a systematic comparison with other layered sodium metal
oxides with early 3d transition metals, first disclosing a unified evolution
pattern of Na density waves through in situ XRD analysis. Combining ab-initio
simulations and theoretical modelings, a sodium-modulated Peierls-like
transition mechanism is then proposed for the bonding formation of metal ion
dimers. More importantly, the unique trimer structure in NaxVO2 is shown to be
very sensitive to the onsite Coulomb repulsion value, suggesting a delicate
balance between strong electronic correlations and orbital effects that can be
precisely modulated by both Na compositions and atomic stackings. This unveils
a unique opportunity to design strongly correlated materials with tailored
electronic transitions through electrochemical modulations and crystallographic
designs, to elegantly balance various competition effects. We think the
understanding will also help further elucidate complicated electronic behaviors
in other vanadium oxide systems.",2105.02416v1
2021-07-07,Optimization of Electrolyte Rebalancing in Vanadium Redox Flow Batteries,"This paper presents a novel algorithm to optimize energy capacity restoration
of vanadium redox flow batteries (VRFBs). VRFB technologies can have their
lives prolonged through a partially restoration of the lost capacity by
electrolyte rebalancing. Our algorithm finds the optimal number and time of
these rebalancing services to minimize the service cost, while maximizing the
revenues from energy arbitrage. We show that the linearized form of this
problem can be analytically solved, and that the objective function is convex.
To solve the complete problem, we develop a two-step mixed integer linear
programming (MILP) algorithm, which first finds the bounds for optimal number
of services and then optimizes the number, and time of the services. We then
present a theoretical analysis and optimization results for a case study of
energy arbitrage in New York ISO.",2107.03339v1
2021-11-04,Glass-like Electronics in Vanadium Dioxide,"Functional materials can offer new paradigms for miniaturized and
energy-efficient electronics, providing a complementary or even alternative
platform to metal-oxide-semiconductors. Here we report on electronically
accessible long-lived structural states in Vanadium Dioxide that can offer a
scheme for data storage and processing. We show that such states can be
electrically manipulated and tracked beyond 10,000 seconds after excitation,
exhibiting similar features of glasses, which are beyond the classic
metastability in Mott systems. Glass-like electronics can potentially overcome
some of the fundamental limitations in conventional metal-oxide-semiconductor
electronics, and open avenues for neuromorphic computation and multi-level
memories.",2111.03181v1
2022-01-20,Spin valve effect in two-dimensional VSe$_2$ system,"Vanadium based dichalcogenides, VSe$_2$, are two-dimensional materials in
which magnetic Vanadium atoms are arranged in a hexagonal lattice and are
coupled ferromagnetically within the plane. However, adjacent atomic planes are
coupled antiferromagnetically. This provides new and interesting opportunities
for application in spintronics and data storage and processing technologies. A
spin valve magnetoresistance may be achieved when magnetic moments of both
atomic planes are driven to parallel alignment by an external magnetic field.
The resistance change associated with the transition from antiparallel to the
parallel configuration is qualitatively similar to that observed in
artificially layered metallic magnetic structures. Detailed electronic
structure of VSe$_2$ was obtained from DFT calculations. Then, the ballistic
spin-valve magnetoresistance was determined within the Landauer formalism. In
addition, we also analyze thermal and thermoelectric properties. Both phases of
VSe$_2$, denoted as H and T, are considered.",2201.08420v1
2022-01-30,Electrolyte Flow Rate Control for Vanadium Redox Flow Batteries using the Linear Parameter Varying Framework,"In this article, an electrolyte flow rate control approach is developed for
an all-vanadium redox flow battery (VRB) system based on the linear parameter
varying (LPV) framework. The electrolyte flow rate is regulated to provide a
trade-off between stack voltage efficiency and pumping energy losses, so as to
achieve optimal battery energy efficiency. The nonlinear process model is
embedded in a linear parameter varying state-space description and a set of
state feedback controllers are designed to handle fluctuations in current
during both charging and discharging. Simulation studies have been conducted
under different operating conditions to demonstrate the performance of the
proposed approach. This control approach was further implemented on a
laboratory scale VRB system.",2201.12812v2
2022-02-03,Tuning carrier density and phase transitions in oxide semiconductors using focused ion beams,"We demonstrate spatial modification of the optical properties of thin-film
metal oxides, zinc oxide and vanadium dioxide as representatives, using a
commercial focused ion beam (FIB) system. Using a Ga+ FIB and thermal
annealing, we demonstrated variable doping of a band semiconductor, zinc oxide
(ZnO), achieving carrier concentrations from 10^18 cm-3 to 10^20 cm-3. Using
the same FIB without subsequent thermal annealing, we defect-engineered a
correlated semiconductor, vanadium dioxide (VO2), locally modifying its
insulator-to-metal transition (IMT) temperature by range of ~25 degrees C. Such
area-selective modification of metal oxides by direct writing using a FIB
provides a simple, mask-less route to the fabrication of optical structures,
especially when multiple or continuous levels of doping or defect density are
required.",2202.01777v3
2022-03-04,Pressure-induced structural phase transition of vanadium: A revisit from the perspective of ensemble theory,"For realistic crystals, the free energy strictly formulated in ensemble
theory can hardly be obtained because of the difficulty in solving the
high-dimension integral of the partition function, the dilemma of which makes
it even a doubt if the rigorous ensemble theory is applicable to phase
transitions of condensed matters. In the present work, the partition function
of crystal vanadium under compression up to $320$ GPa at room temperature is
solved by an approach developed very recently, and the derived equation of
state is in a good agreement with all the experimental measurements, especially
the latest one covering the widest pressure range up to $300$ GPa. Furthermore,
the derived Gibbs free energy proves the very argument to understand most of
the experiments reported in the past decade on the pressure-induced phase
transition, and, especially, a novel phase transition sequence concerning three
different phases observed very recently and the measured angles of two phases
agree with our theoretical results excellently.",2203.02125v1
2022-03-09,"Vanadium doped beta-Ga2O3 single crystals: Growth, Optical and Terahertz characterization","We report the growth of electrically-resistive vanadium-doped beta-Ga2O3
single crystals via the optical floating zone technique. By carefully
controlling the growth parameters V-doped crystals with very high electrical
resistivity compared to the usual n-type V-doped beta-Ga2O3 (ne~10^(18)/cm^3)
can be synthesized. The optical properties of such high resistive V-doped
b-Ga2O3 are significantly different compared to the undoped and n-doped
crystals. We study the polarization-dependent Raman spectra,
polarization-dependent transmission, temperature-dependent photoluminescence in
the optical wavelength range and the THz transmission properties in the 0.2 -
2.6 THz range. The V-doped insulating Ga2O3 crystals show strong birefringence
with refractive index contrast Dn of 0.3+-0.02 at 1 THz, suggesting it to be an
ideal material for optical applications in the THz region.",2203.04941v1
2022-05-31,Highly anisotropic magnetism in the vanadium-based kagome metal TbV6Sn6,"RV6Sn6 (R=rare earth) compounds are appealing materials platforms for
exploring the interplay between R-site magnetism and nontrivial band topology
associated with the nonmagnetic vanadium-based kagome network. Here we present
the synthesis and characterization of the kagome metal TbV6Sn6 via
single-crystal x-ray diffraction, magnetization, transport, and heat capacity
measurements. Magnetization measurements reveal strong, uniaxial magnetic
anisotropy rooted in the alignment of Tb3{\AA} moments in the interplane
direction below 4.3(2) K. TbV6Sn6 exhibits multiband transport behavior with
high mobilities of charge carriers, and our measurements suggest TbV6Sn6 is a
promising candidate for hosting Chern gaps driven via the interplay between
Tb-site magnetic order and the band topology of the V-site kagome network.",2205.15559v3
2022-06-12,Electronic and magnetic properties of intermetallic Kagome magnets $R$V$_6$Sn$_6$ ($R$ = Tb - Tm),"We present a systematic study of the structure, electronic, and magnetic
properties of a new branch of intermetalllic compounds, $R$V$_6$Sn$_6$ ($R$ =
Tb - Tm) by using X-ray diffraction, magnetic susceptibility, magnetization,
electrical transport, and heat-capacity measurements. These compounds feature a
combination of a non-magnetic vanadium kagome sublattice and a magnetic
rare-earth triangular sublattice that supports various spin anisotropies based
on different $R$ ions. We find magnetic orders for the $R$ = Tb, Dy, and Ho
compounds at 4.4, 3, 2.5 K, respectively, while no ordering is detected down to
0.4 K for the $R$ = Er and Tm compounds with easy-plane anisotropies.
Electronically, we found no superconductivity or charge ordering transition
down to 0.4 K for any member of this family, while all compounds exhibit
multi-band transport properties that originate from the band topology of the
vanadium kagome sublattice.",2206.05653v1
2022-06-13,Electrically driven reprogrammable vanadium dioxide metasurface using binary control for broadband beam-steering,"Resonant optical phased arrays are a promising way to reach fully
reconfigurable metasurfaces in the optical and NIR regimes with low energy
consumption, low footprint and high reliability. Continuously tunable resonant
structures suffer from inherent drawbacks such as low phase range,
amplitude-phase correlation or extreme sensitivity that makes precise control
at the individual element level very challenging. In order to bypass these
issues, we use 1-bit (binary) control for beam steering for an innovative
nano-resonator antenna and explore the theoretical capabilities of such phased
arrays. A thermally realistic structure based on vanadium dioxide sandwiched in
a metal-insulator-metal structure is proposed and optimized using inverse
design to enhance its performance at 1550 nm. Continuous beam steering over
90{\deg} range is successfully achieved using binary control, with excellent
agreement with predictions based on the theoretical first principles
description of phased arrays. Furthermore a broadband response from 1500 nm to
1700 nm is achieved. The robustness of the design manufacturing imperfections
is also demonstrated. This simplified approach can be implemented to optimize
tunable nanophotonic phased array metasurfaces based on other materials or
phased shifting mechanisms for various functionalities.",2206.06056v1
2022-07-02,Thermal hysteresis in scattering by vanadium-dioxide spheres,"Vanadium dioxide (VO2) transforms from purely monoclinic to purely tetragonal
on being heated from 58 deg C to 72 deg C, the transformation being reversible
but hysteretic. Electromagnetically, VO2 transforms from a dissipative
dielectric to another dissipative dielectric if the free-space wavelength is
less than 1100 nm, but from a dissipative dielectric to a plasmonic metal (or
vice versa) if the free-space wavelength exceeds 1100 nm. Calculating the
extinction, total scattering, absorption, radiation-pressure, back-scattering,
and forward-scattering efficiencies of a VO2 sphere, we found clear signatures
of thermal hysteresis in (i) the forward-scattering, back-scattering, and
absorption efficiencies for free-space wavelength less than 1100 nm, and (ii)
the forward-scattering, back-scattering, total scattering, and absorption
efficiencies for free-space wavelength more than 1100 nm. Vacuum and
null-permittivity quasistates occur between 58 deg C and 72 deg C, when
tetragonal VO2 is a plasmonic metal, once each on the heating branch and once
each on the cooling branch of thermal hysteresis. But none of the six
efficiencies show significant differences between the two quasistates.",2207.00895v1
2022-07-13,Influence of germanium substitution on the structural and electronic stability of the competing vanadium dioxide phases,"We present a density-functional theory (DFT) study of the structural,
electronic, and chemical bonding behaviour in germanium (Ge)-doped vanadium
dioxide (VO$_2$). Our motivation is to explain the reported increase of the
metal-insulator transition temperature under Ge doping and to understand how
much of the fundamental physics and chemistry behind it can be captured at the
conventional DFT level. We model doping using a supercell approach, with
various concentrations and different spatial distributions of Ge atoms in
VO$_2$. Our results suggest that the addition of Ge atoms strongly perturbs the
high-symmetry metallic rutile phase and induces structural distortions that
partially resemble the dimerization of the experimental insulating structure.
Our work, therefore, hints at a possible explanation of the observed increase
in transition temperature under Ge doping, motivating further studies into
understanding the interplay of structural and electronic transitions in VO$_2$.",2207.06153v2
2022-09-20,Characteristics of a Nickel Vanadium redox flow battery Based on COMSOL,"The overpotential, dissociation rate, electrode potential distributions and
current density are suggested in this study to analyze the Nickel Vanadium
Redox Flow Battery (NVRFB). Due to its large capacity and ecofriendly
properties, NVRFB may be a viable option in the present state of energy
constraint and environmental pollution. Due to their low cost and high energy
density, nickel-based flow batteries have gained popularity. This study
demonstrates that the Ni2+/Ni+ and V5+/V4+ ions have a higher rate of
dissociation at the membrane and a lower rate at the inlet, where the
electrolyte flow velocity is greater; Because the membrane undergoes more
oxidation-reduction reactions, the electrolyte flow rate is critical in the
redox flow cell; Additionally, we see that when electrode thickness is reduced,
current density and electrode potential increase while overpotential decreases;
the model's equations are solved using the finite-element method in the COMSOL
Multiphysics program. An electrolyte-electrode interface connection is used to
simulate the reaction. The dissociation rate indicates that the
oxidation-reduction process happens at a lower membrane potential. Improving
the electrolyte flow rate enhances battery performance. Compression of the
electrodes enhances conductivity and battery performance.",2209.09872v1
2022-10-03,The effect of vanadium substitution on the structural and magnetic properties of (Fe$_{1-x}$V$_{x}$)$_{3}$Ga$_{4}$,"Fe$_{3}$Ga$_{4}$ displays a complex magnetic phase diagram that is sensitive
and tunable with both electronic and crystallographic structure changes. In
order to explore this tunability, vanadium-doped
(Fe$_{1-x}$V$_{x}$)$_{3}$Ga$_{4}$ has been synthesized and characterized.
High-resolution synchrotron X-ray diffraction and Rietveld refinement show that
samples up to 20\% V-doping remain isostructural to Fe$_{3}$Ga$_{4}$ and
display a linear increase in unit cell volume as doping is increased. Magnetic
measurements reveal a suppression of the antiferromagnetic helical spin-density
wave (SDW) with V-doping, revealed by changes in both the low-temperature
ferromagnetic-antiferromagentic (FM-AFM) transition (T$_{1}$) and
high-temperature AFM-FM transition (T$_{2}$). At 7.5\% V-doping, the
metamagnetic behavior of the helical AFM SDW phase is no longer observed. These
results offer an avenue to effective tuning of the magnetic order in
Fe$_{3}$Ga$_{4}$ for devices, as well as increased understanding of the
magnetism in this system.",2210.01039v1
2022-12-28,Low-temperature magnetoresistance hysteresis in Vanadium-doped Bi$_{2}$Te$_{2.4}$Se$_{0.6}$ bulk topological insulators,"Bi$_{2}$Te$_{2.4}$Se$_{0.6}$ single crystals show gapless topological surface
states and doping ($x$) with Vanadium allows to shift the chemical potential in
the bulk band gap. Accordingly, the resistivity, carrier density, and mobility
are constant below 10 K and the magnetoresistance shows weak antilocalization
as expected for low-temperature transport properties dominated by gapless
surface states of so-called three-dimensional topological ""insulators"".
However, the magnetoresistance also shows a hysteresis depending on the sweep
rate and the magnetic field direction. Here, we provide evidence that such
magnetoresistance hysteresis is enhanced if both three-dimensional bulk states
and quasi-two-dimensional topological states contribute to the transport ($x$ =
0 and 0.03), and it is mostly suppressed if the topological states govern
transport ($x$ = 0.015). The results are discussed in terms of spin-dependent
scattering between the different available states",2212.14078v2
2023-01-10,Short-range order and increased transition temperature in LiVO2 with weakened trimer frustration,"Vanadium atoms in layered LiVO2 form in-plane periodic vanadium trimers at
low temperatures, but the trimers appear randomly in the stacking direction
because there are many trimer configurations with comparable lattice energy. We
detailed an original modeling scheme to represent glassy states with a
completely disordered trimer configuration in the stacking structure. Through
PDF analysis using this model, we show that the synthesis method can yield two
types of low-temperature stacking structures: a completely disordered stacking
structure and a short-range order in the stacking structure. The phase
transition temperature of the former sample is about 15 K lower than that of
the latter. We discuss that this is due to the strong trimer frustration that
appears in the sample without short-range order, which suppresses the phase
transition temperature, similar to the frustration effect in conventional spin
systems.",2301.03833v1
2023-05-30,VO2 Phase Change Electrodes in Li-ion Batteries,"Use of electrode materials that show phase change behavior and hence drastic
changes in electrochemical activity during operation, have not been explored
for Li-ion batteries. Here we demonstrate the vanadium oxide (VO2) cathode that
undergoes metal-insulator transition due to first-order structural phase
transition at accessible temperature of 68{\deg}C for battery operation. Using
a suitable electrolyte operable across the phase transition range and
compatible with vanadium oxide cathodes, we studied the effect of electrode
structure change on lithium insertion followed by the electrochemical
characteristics above and below the phase transition temperature. The
high-temperature VO2 phase shows significantly improved capacitance, enhanced
current rate capabilities, improved electrical conductivity and lithium-ion
diffusivity compared to the insulating low temperature phase. This opens up new
avenues for electrode designs, allowing manipulation of electrochemical
reactions around phase transition temperatures, and in particular enhancing
electrochemical properties at elevated temperatures contrary to existing
classes of battery chemistries that lead to performance deterioration at
elevated temperatures.",2305.19002v1
2023-06-29,Kagome surface states and weak electronic correlation in vanadium-kagome metals,"RV6Sn6 (R = Y and lanthanides) with two-dimensional vanadium-kagome surface
states is an ideal platform to investigate kagome physics and manipulate the
kagome features to realize novel phenomena. Utilizing the micron-scale
spatially resolved angle-resolved photoemission spectroscopy and
first-principles calculations, we report a systematical study of the electronic
structures of RV6Sn6 (R = Gd, Tb, and Lu) on the two cleaved surfaces, i.e.,
the V- and RSn1-terminated (001) surfaces. The calculated bands without any
renormalization match well with the main ARPES dispersive features, indicating
the weak electronic correlation in this system. We observe 'W'-like kagome
surface states around the Brillouin zone corners showing R-element-dependent
intensities, which is probably due to various coupling strengths between V and
RSn1 layers. Our finding suggests an avenue for tuning electronic states by
interlayer coupling based on two-dimensional kagome lattices.",2306.16724v1
2023-07-18,Room-temperature magnetism and controlled cation distribution in vanadium ferrite thin films,"Spinel oxides demonstrate significant technological promise due to the vast
array of interrelated physical properties that their unique structure supports.
Specifically, the Fe1+xV2-xO4 spinel system garners extensive interest due to
the presence of orbitally ordered states and multiferroism. This study focuses
on the elaboration of high-quality Fe2VO4 (x = 1) thin films on MgO substrates
via pulsed laser deposition. Structural analyses confirm the epitaxial growth
of the films, their high crystallinity and fully strained nature. The cationic
distribution and stoichiometry were investigated using Resonant Elastic X-ray
Scattering experiments, in conjunction with comprehensive characterization of
the films' physical and electrical properties. The films exhibit
room-temperature magnetism, with a magnetization consistent with the
(Fe3+)Td[Fe2+V3+2]OhO4 inverse spinel structure unveiled by anomalous
diffraction. This work represents the inaugural successful deposition of Fe2VO4
thin films, thereby expanding the family of spinel vanadium oxide thin films
with a new member that demonstrates room-temperature magnetic properties.",2307.09598v1
2023-09-13,Amorphous VO$_x$ films with high temperature coefficient of the resistivity grown by reactive e-beam evaporation of V metal,"Amorphous VO$_x$ films without a hysteretic phase transition are stable with
respect to thermal cycling and highly demanded as sensitive elements of the
resistive thermometers and microbolometers. In this paper we present simple and
low-temperature growth of amorphous vanadium oxide films by reactive electron
beam evaporation of vanadium metal in $\sim 10^{-4}$ mBar oxygen atmosphere.
The temperature coefficient of the resistivity (TCR) of the films is weakly
sensitive to substrate material and temperature and could be tuned by oxygen
pressure in the growth chamber up to -2.2\% /K. The resistivity value is stable
for months. It depends on the substrate material and substrate temperature
during the evaporation. Simplicity and controllability of the method should
lead to various laboratory and industrial applications.",2309.07036v1
2023-09-20,Radiative thermal switch via metamaterials made of vanadium dioxide-coated nanoparticles,"In this work, a thermal switch is proposed based on the phase-change material
vanadium dioxide (VO2) within the framework of near-field radiative heat
transfer (NFRHT). The radiative thermal switch consists of two metamaterials
filled with core-shell nanoparticles, with the shell made of VO2. Compared to
traditional VO2 slabs, the proposed switch exhibits a more than 2-times
increase in the switching ratio, reaching as high as 90.29% with a 100 nm
vacuum gap. The improved switching effect is attributed to the capability of
the VO2 shell to couple with the core, greatly enhancing heat transfer with the
insulating VO2, while blocking the motivation of the core in the metallic state
of VO2. As a result, this efficiently enlarges the difference in photonic
characteristics between the insulating and metallic states of the structure,
thereby improving the ability to rectify the NFRHT. The proposed switch opens
pathways for active control of NFRHT and holds practical significance for
developing thermal photon-based logic circuits.",2309.10983v1
2023-11-21,Iris Presentation Attack: Assessing the Impact of Combining Vanadium Dioxide Films with Artificial Eyes,"Iris recognition systems, operating in the near infrared spectrum (NIR), have
demonstrated vulnerability to presentation attacks, where an adversary uses
artifacts such as cosmetic contact lenses, artificial eyes or printed iris
images in order to circumvent the system. At the same time, a number of
effective presentation attack detection (PAD) methods have been developed.
These methods have demonstrated success in detecting artificial eyes (e.g.,
fake Van Dyke eyes) as presentation attacks. In this work, we seek to alter the
optical characteristics of artificial eyes by affixing Vanadium Dioxide (VO2)
films on their surface in various spatial configurations. VO2 films can be used
to selectively transmit NIR light and can, therefore, be used to regulate the
amount of NIR light from the object that is captured by the iris sensor. We
study the impact of such images produced by the sensor on two state-of-the-art
iris PA detection methods. We observe that the addition of VO2 films on the
surface of artificial eyes can cause the PA detection methods to misclassify
them as bonafide eyes in some cases. This represents a vulnerability that must
be systematically analyzed and effectively addressed.",2311.12773v1
2023-12-11,High-speed sensing of RF signals with phase change materials,"RF radiation spectrum is central to wireless and radar systems among numerous
high-frequency device technologies. Here, we demonstrate sensing of RF signals
in the technologically relevant 2.4 GHz range utilizing vanadium dioxide (VO2),
a quantum material that has garnered significant interest for its
insulator-to-metal transition. We find the electrical resistance of both
stoichiometric as well as off-stoichiometric vanadium oxide films can be
modulated with RF wave exposures from a distance. The response of the materials
to the RF waves can be enhanced by either increasing the power received by the
sample or reducing channel separation. We report a significant ~73% drop in
resistance with a 5 {\mu}m channel gap of the VO2 film at a characteristic
response time of 16 microseconds. The peak sensitivity is proximal to the phase
transition temperature boundary that can be engineered via doping and crystal
chemistry. Dynamic sensing measurements highlight the films' rapid response and
broad-spectrum sensitivity. Engineering electronic phase boundaries in
correlated electron systems could offer new capabilities in emerging
communication technologies.",2312.06459v1
2023-12-19,Study on electromagnetically induced transparency effects in Dirac and VO$_2$ hybrid material structure,"In this paper, we present a metamaterial structure of Dirac and vanadium
dioxide and investigate its optical properties using the finite-difference
time-domain (FDTD) technique. Using the phase transition feature of vanadium
dioxide, the design can realize active tuning of the PIT effect at terahertz
frequency, thereby converting from a single PIT to a double PIT. When VO$_2$ is
in the insulating state, the structure is symmetric to obtain a single-band PIT
effect; When VO$_2$ is in the metallic state, the structure turns asymmetric to
realize a dual-band PIT effect. This design provides a reference direction for
the design of actively tunable metamaterials. Additionally, it is discovered
that the transparent window's resonant frequency and the Dirac material's Fermi
level in this structure have a somewhat linear relationship. In addition, the
structure achieves superior refractive index sensitivity in the terahertz band,
surpassing 1 THz/RIU. Consequently, the concept exhibits encouraging potential
for application in refractive index sensors and optical switches.",2312.11814v1
2024-01-10,Origin of discrete electrical switching in chemically heterogeneous vanadium oxide crystals,"Electrically driven insulator-metal transitions in prototypical quantum
materials such as VO2 offer a foundational platform for designing novel
solid-state devices. Tuning the V: O stoichiometry offers a vast electronic
phase space with non-trivial collective properties. Here, we report the
discovery of discrete threshold switching voltages with constant threshold
voltage difference between cycles in vanadium oxide crystals. The observed
threshold fields over 10000 cycles are ~100X lower than that noted for
stoichiometric VO2 and show unique discrete behaviour. We correlate the
observed discrete memristor behaviour with the valence change mechanism and
fluctuations in the chemical composition of spatially distributed VO2-VnO2n-1
complex oxide phases. Design of chemical heterogeneity in Mott insulators,
therefore, offers an intriguing path to realizing low-energy neuromorphic
devices.",2401.05222v1
2024-02-20,Single-site DFT+DMFT for vanadium dioxide using bond-centered orbitals,"We present a combined density-functional theory and single-site dynamical
mean-field theory (DMFT) study of vanadium dioxide (VO$_2$) using an
unconventional set of bond-centered orbitals as the basis of the correlated
subspace. VO$_2$ is a prototypical material undergoing a metal-insulator
transition (MIT), hosting both intriguing physical phenomena and the potential
for industrial applications. With our choice of correlated subspace basis, we
investigate the interplay of structural dimerization and electronic
correlations in VO$_2$ in a computationally cheaper way compared to other
state-of-the-art methods such as cluster DMFT. Our approach allows us to treat
the rutile and M1 monoclinic VO$_2$ phases on an equal footing and to vary the
dimerizing distortion continuously, exploring the energetics of the transition
between the two phases. The choice of basis presented in this work hence offers
a complementary view on the long-standing discussion of the MIT in VO$_2$ and
suggests possible future extensions to other similar materials hosting
molecular-orbital-like states.",2402.12798v1
2024-03-02,Nonlinear dynamics and stability analysis of locally-active Mott memristors using a physics-based compact model,"Locally-active memristors are a class of emerging nonlinear dynamic circuit
elements that hold promise for scalable yet biomimetic neuromorphic circuits.
Starting from a physics-based compact model, we performed small-signal
linearization analyses and applied Chua's local activity theory to a
one-dimensional locally-active vanadium dioxide Mott memristor based on an
insulator-to-metal phase transition. This approach allows a connection between
the dynamical behaviors of a Mott memristor and its physical device parameters,
which could guide materials and device development for neuromorphic circuit
applications. We also examined the applicability of local analyses on a
second-order circuit consists of a vanadium dioxide memristor coupled to an
external reactive element, specifically a parallel capacitor. Finally, we show
that global nonlinear techniques including nullclines and phase portraits
provide insights on instabilities and persistent oscillations near
non-hyperbolic fixed points, such as the creation of a stable limit cycle in a
supercritical Hopf bifurcation, with some of the bifurcation characteristics
distinctive from the general predictions.",2403.01036v1
2024-03-15,Ultra-high endurance silicon photonic memory using vanadium dioxide,"Silicon photonics arises as a viable solution to address the stringent
resource demands of emergent technologies, such as neural networks. Within this
framework, photonic memories are fundamental building blocks of photonic
integrated circuits that have not yet found a standardized solution due to
several trade-off among different metrics such as energy consumption, speed,
footprint, or fabrication complexity, to name a few. In particular, a photonic
memory exhibiting ultra-high endurance performance (> 10^6 cycles) has been
elusive to date. Here, we report an ultra-high endurance silicon photonic
memory using vanadium dioxide (VO_2) exhibiting a record cyclability of up to
10^7 cycles without degradation. Moreover, our memory features an ultra-compact
footprint below 5 {\mu}m with potential for nanosecond and picojoule
programming performance. Our silicon photonic memory could find application in
emerging photonic applications demanding high number of memory updates such as
photonic neural networks with in-situ training.",2403.10162v1
2009-10-12,Frustrated square lattice with spatial anisotropy: crystal structure and magnetic properties of PbZnVO(PO4)2,"Crystal structure and magnetic properties of the layered vanadium phosphate
PbZnVO(PO4)2 are studied using x-ray powder diffraction, magnetization and
specific heat measurements, as well as band structure calculations. The
compound resembles AA'VO(PO4)2 vanadium phosphates and fits to the extended
frustrated square lattice model with the couplings J(1), J(1)' between
nearest-neighbors and J(2), J(2)' between next-nearest-neighbors. The
temperature dependence of the magnetization yields estimates of averaged
nearest-neighbor and next-nearest-neighbor couplings, J(1) ~ -5.2 K and J(2) ~
10.0 K, respectively. The effective frustration ratio alpha=J(2)/J(1) amounts
to -1.9 and suggests columnar antiferromagnetic ordering in PbZnVO(PO4)2.
Specific heat data support the estimates of J(1) and J(2) and indicate a likely
magnetic ordering transition at 3.9 K. However, the averaged couplings
underestimate the saturation field, thus pointing to the spatial anisotropy of
the nearest-neighbor interactions. Band structure calculations confirm the
identification of ferromagnetic J(1), J(1)' and antiferromagnetic J(2), J(2)'
in PbZnVO(PO4)2 and yield J(1)'-J(1) ~ 1.1 K in excellent agreement with the
experimental value of 1.1 K, deduced from the difference between the expected
and experimentally measured saturation fields. Based on the comparison of
layered vanadium phosphates with different metal cations, we show that a
moderate spatial anisotropy of the frustrated square lattice has minor
influence on the thermodynamic properties of the model. We discuss relevant
geometrical parameters, controlling the exchange interactions in these
compounds, and propose a new route towards strongly frustrated square lattice
materials.",0910.2258v1
2011-11-07,Atomic and itinerant effects at the transition metal x-ray absorption K-pre-edge exemplified in the case of V$_2$O$_3$,"X-ray absorption spectroscopy is a well established tool for obtaining
information about orbital and spin degrees of freedom in transition metal- and
rare earth-compounds. For this purpose usually the dipole transitions of the L-
(2p to 3d) and M- (3d to 4f) edges are employed, whereas higher order
transitions such as quadrupolar 1s to 3d in the K-edge are rarely studied in
that respect. This is due to the fact that usually such quadrupolar transitions
are overshadowed by dipole allowed 1s to 4p transitions and, hence, are visible
only as minor features in the pre-edge region. Nonetheless, these features
carry a lot of valuable information, similar to the dipole L-edge transition,
which is not accessible in experiments under pressure due to the absorption of
the diamond anvil pressurecell. We recently performed a theoretical and
experimental analysis of such a situation for the metal insulator transition of
(V(1-x)Crx)2O3. Since the importance of the orbital degrees of freedom in this
transition is widely accepted, a thorough understanding of quadrupole
transitions of the vanadium K-pre-edge provides crucial information about the
underlying physics. Moreover, the lack of inversion symetry at the vanadium
site leads to onsite mixing of vanadium 3d- and 4p- states and related quantum
mechanical interferences between dipole and quadrupole transitions. Here we
present a theoretical analysis of experimental high resolution x-ray absorption
spectroscopy at the V pre-K edge measured in partial fluorescence yield mode
for single crystals. We carried out density functional as well as configuration
interaction calculations in order to capture effects coming from both,
itinerant and atomic limits.",1111.1544v1
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
2015-12-08,Terahertz Coded Aperture Mask using a Vanadium Dioxide Bowtie Antenna Array,"Terahertz imaging systems have received substantial attention from the
scientific community for their use in astronomy, spectroscopy, plasma
diagnostics and security. One approach to designing such systems is to use
focal plane arrays. Although the principle of these systems is straightforward,
realizing practical architectures has proven deceptively difficult. A different
approach to imaging consists of spatially encoding the incoming flux of
electromagnetic energy prior to detection using a reconfigurable mask. This
technique is referred to as coded aperture or Hadamard imaging. This paper
details the design, fabrication and testing of a prototype coded aperture mask
operating at WR 1.5 (500 to 750 GHz) that uses the switching properties of
vanadium dioxide (VO2). The reconfigurable mask consists of bowtie antennas
with vanadium dioxide VO2 elements at the feed points. From the symmetry, a
unit cell of the array can be represented by an equivalent waveguide whose
dimensions limit the maximum operating frequency. In this design, the cutoff
frequency of the unit cell is 640 GHz. The VO2 devices are grown using
reactive-biased target ion beam deposition. A reflection coefficient (S11)
measurement of the mask in the WR 1.5 (500 to 750 GHz) band is conducted. The
results are compared with circuit models and found to be in good agreement. A
simulation of the transmission response of the mask is conducted and shows a
transmission modulation of up to 28 dB. This project is a first step towards
the development of a full coded aperture imaging system operating at WR 1.5
with VO2 as the mask switching element.",1512.02697v1
2018-02-12,"Superconducting transition temperatures in the electronic and magnetic phase diagrams of Sr2VFeAsO3-delta, a superconductor","We elucidate the magnetic phases and superconducting transition temperatures
(Tc) in Sr2VFeAsO3-delta (21113V), an iron-based superconductor with a
thick-blocking layer fabricated from a perovskite-related transition metal
oxide. At low temperatures (T < 37.1 K), 21113V exhibited a superconducting
phase in the range 0.031 =< delta =< 0.145 and an antiferromagnetic (AFM) iron
sublattice in the range 0.267 =< delta =< 0.664. Mixed-valent vanadium
exhibited a dominant AFM phase in 0.031 =< delta =< 0.088, and a partial
ferrimagnetic (Ferri.) phase in the range 0.124 =< delta =< 0.664. The Ferri.
phase was the most dominant at a delta value of 0.267, showing an AFM phase of
Fe at T < 20 K. Increasing the spontaneous magnetic moments reduced the
magnetic shielding volume fraction due to the superconducting phase. This
result was attributed to the magnetic phase of vanadium, which dominates the
superconductivity of Fe in 21113V. The Tc-delta curve showed two maxima. The
smaller and larger of Tc maxima occurred at delta = 0.073 and delta = 0.145,
respectively; the latter resides on the phase boundary between AFM and the
partial Ferri. phases of vanadium. 21113V is a useful platform for verifing new
mechanisms of Tc enhancement in iron-based superconductors.",1802.03907v2
2020-08-12,Vanadium Abundance Derivations in 255 Metal-poor Stars,"We present vanadium (V) abundances for 255 metal-poor stars, derived from
high-resolution optical spectra from the Magellan Inamori Kyocera Echelle
spectrograph on the Magellan Telescopes at Las Campanas Observatory, the Robert
G. Tull Coud\'{e} Spectrograph on the Harlan J. Smith Telescope at McDonald
Observatory, and the High Resolution Spectrograph on the Hobby-Eberly Telescope
at McDonald Observatory. We use updated V I and V II atomic transition data
from recent laboratory studies, and we increase the number of lines examined
(from 1 to 4 lines of V I, and from 2 to 7 lines of V II). As a result, we
reduce the V abundance uncertainties for most stars by more than 20% and expand
the number of stars with V detections from 204 to 255. In the metallicity range
$-$4.0 $<$ [Fe/H] $< -$1.0, we calculate the mean ratios [V I/Fe I]$ = -0.10
\pm 0.01 (\sigma = 0.16)$ from 128 stars with $\geq$ 2 V I lines detected, [V
II/Fe II] $= +0.13 \pm 0.01 (\sigma = 0.16)$ from 220 stars with $\geq$ 2 V II
lines detected, and [V II/V I] $= +0.25 \pm 0.01 (\sigma = 0.15)$ from 119
stars. We suspect this offset is due to non-LTE effects, and we recommend using
[V II/Fe II], which is enhanced relative to the solar ratio, as a better
representation of [V/Fe]. We provide more extensive evidence for abundance
correlations detected previously among scandium, titanium, and vanadium, and we
identify no systematic effects in the analysis that can explain these
correlations.",2008.05500v1
2021-10-19,Graphene-Based Electrodes in a Vanadium Redox Flow Battery Produced by Rapid Low-Pressure Combined Gas Plasma Treatments,"The development of high-power density vanadium redox flow batteries (VRFBs)
with high energy efficiencies (EEs) is crucial for the widespread dissemination
of this energy storage technology. In this work, we report the production of
novel hierarchical carbonaceous nanomaterials for VRFB electrodes with high
catalytic activity toward the vanadium redox reactions (VO2+/VO2+ and V2+/V3+).
The electrode materials are produced through a rapid (minute timescale)
low-pressure combined gas plasma treatment of graphite felts (GFs) in an
inductively coupled radio frequency reactor. By systematically studying the
effects of either pure gases (O2 and N2) or their combination at different gas
plasma pressures, the electrodes are optimized to reduce their kinetic
polarization for the VRFB redox reactions. To further enhance the catalytic
surface area of the electrodes, single-/few-layer graphene, produced by highly
scalable wet-jet milling exfoliation of graphite, is incorporated into the GFs
through an infiltration method in the presence of a polymeric binder. Depending
on the thickness of the proton-exchange membrane (Nafion 115 or Nafion XL), our
optimized VRFB configurations can efficiently operate within a wide range of
charge/discharge current densities, exhibiting energy efficiencies up to 93.9%,
90.8%, 88.3%, 85.6%, 77.6%, and 69.5% at 25, 50, 75, 100, 200, and 300 mA cm-2,
respectively. Our technology is cost-competitive when compared to commercial
ones (additional electrode costs < 100 euro m-2) and shows EEs rivalling the
record-high values reported for efficient systems to date. Our work remarks on
the importance to study modified plasma conditions or plasma methods
alternative to those reported previously (e.g., atmospheric plasmas) to improve
further the electrode performances of the current VRFB systems.",2110.10062v1
2022-11-22,"Mixed Integer Linear Program model for optimized scheduling of a vanadium redox flow battery with variable efficiencies, capacity fade, and electrolyte maintenance","Redox Flow Batteries are a promising option for large-scale stationary energy
storage. The vanadium redox flow battery is the most widely commercialized
system thanks to its chemical stability and performance. This work aims to
optimize the scheduling of a vanadium flow battery that stores energy produced
by a renewable power plant, keeping into account a thorough characterization of
the battery performance, with variable efficiencies and capacity fade effects.
A detailed characterization of the battery performance improves the calculation
of the optimal number of cycles and revenue associated with the battery use if
compared to the results obtained using simpler models, which take into account
constant efficiencies and no capacity fade effects. The presented problem is
nonlinear due to the functions of the battery efficiency, which depend upon
charging and discharging powers and state of charge with nonlinear, non-convex
correlations. The problem is linearized using convex hulls. The optimization
program also calculates the progressive battery capacity fade due to undesired
secondary electrochemical reactions and the economic impact of capacity
restoration through periodic maintenance. The final problem is solved as a
Mixed-Integer Linear Program (MILP) to guarantee the global optimality of the
linearized problem. The proposed optimization model has been applied to two
different case studies: a case of energy arbitrage and a case of load-shifting.
The optimization results have been compared to those obtained with constant
battery efficiency models, which do not consider the capacity fade effects.
Results show that simpler models overestimate the optimal number of cycles of
the battery and the revenue by up to 15% if they do not take into account the
degradation model of the battery, and respectively up to 32% and 42% if they
also assume constant efficiency for the battery.",2211.12333v2
2023-05-30,Fluid dynamics of mixing in the tanks of small vanadium redox flow batteries: Insights from order-of-magnitude estimates and transient two-dimensional simulations,"This paper investigates the fluid dynamics of mixing in the tanks of
small-scale vanadium redox flow batteries. These systems use two redox pairs
dissolved in separate electrolytes to convert electrical energy into chemical
energy, a process that can be reversed in an efficient way to restore the
initial electrical energy with negligible chemical losses. After flowing
through the electrochemical cell, the electrolytes are stored in separate
tanks, where they discharge as submerged jets with small temperature and
composition changes compared to the electrolyte already present in the tanks.
The subsequent mixing process is critical for battery performance, as imperfect
mixing tends to reduce the energy capacity and may lead to asymmetric battery
operation. The analysis starts using order-of-magnitude estimates to determine
the conditions under which the mixing process is dominated by momentum or
buoyancy. Transient two-dimensional simulations illustrate the different flow
regimes that emerge in the tanks under laminar flow conditions. The results
show that, contrary to the common assumption, the electrolytes do not mix well
in the tanks. In the presence of high-momentum -- and, specially, positively
buoyant -- jets, a significant fraction of the electrolyte remains unmixed and
unreacted for long periods, thus reducing the energy capacity. The results also
show that the availability of reliable electrolyte properties is crucial for
the accuracy of the numerical simulations, as, under the mixed convection
conditions that typically prevail in vanadium redox flow batteries, small
density variations can significantly impact the long-term mixing of the
electrolytes. In particular, in momentum-dominated flows the cumulative effect
of density changes over time eventually leads to flow instabilities that
significantly promote mixing; therefore, they should be taken into account in
future studies.",2305.19071v1
2023-09-21,Zigzag chain order of LiVSe$_2$ developing away from the vanadium trimer phase transition boundary,"The phenomenon of self-assembly of constituent elements to form molecules at
low temperatures appears ubiquitously in transition metal compounds with
orbital degrees of freedom. Recent progress in local structure studies using
synchrotron radiation x-rays is shifting the interest in structural studies in
such molecule-forming systems from the low-temperature ordered phase to the
short-range order that appears like a precursor at high temperatures. In this
study, we discuss both experimentally and theoretically the relationship
between the trimer structure that appears in the layered LiV$X_2$ ($X$ = O, S,
Se) system with a two-dimensional triangular lattice of vanadium and the zigzag
chain-like local structure that appears near the phase transition boundary
where molecular formation occurs. The vanadium trimerization that persistently
appears in both low-temperature phases of LiVO$_2$ and LiVS$_2$ disappears in
LiVSe$_2$, and a regular triangular lattice is thought to be realized in
LiVSe$_2$, but this study reveals that the zigzag chain local distortion
appears with a finite correlation length. This zigzag chain state local
distortions are similar to the motif of local distortions in the
high-temperature phase of LiVS$_2$, indicating that the local distortions are
persistent away from the trimer phase transition boundary. On the other hand,
it is concluded that the zigzag chain order appearing in LiVSe$_2$ is more
stable than that in LiVS$_2$ in terms of the temperature variation of atomic
displacement and correlation length. The zigzag chain order is considered to be
competitive with the trimer order appearing in the LiV$X_2$ system. In this
paper, we discuss the similarities and differences between the parameters that
stabilize these electronic phases and the local distortions that appear in
other molecular formation systems.",2309.11749v1
1995-10-29,Spin Gap of Two-Dimensional Antiferromagnet Representing CaV$_4$O$_9$,"We examined a two-dimensional Heisenberg model with two kinds of exchange
energies, $J_e$ and $J_c$. This model describes localized spins at vanadium
ions in a layer of CaV$_4$O$_9$, for which a spin gap is found by a recent
experiment. Comparing the high temperature expansion of the magnetic
susceptibility to experimental data, we determined the exchange energies as
$J_e \simeq$ 610 K and $J_c \simeq$ 150 K. By the numerical diagonalization we
estimated the spin gap as $\Delta \sim 0.2J_e \simeq$ 120 K, which consists
with the experimental value 107 K. Frustration by finite $J_c$ enhances the
spin gap.",9510160v1
1997-09-10,Observation of Spin Freezing in CaV4O9 and CaV2O5 by uSR,"We have performed muon spin relaxation and susceptibility measurements of
CaV_4O_9 and CaV_2O_5, systems in which spin gap-like behavior has been found.
We observe spin freezing in the two compounds below 12K and 50K, respectively.
Our results indicate the possibility that a substantial fraction of the of the
vanadium magnetic moment is not in a spin singlet sate at low temperatures in
either material.",9709123v1
1999-09-03,Effective Dimensionality and Band Structure of alpha-Nav_2o_5 Compound: 1D or 2D?,"The AV_nO_{2n+1} mixed valence compounds (n=1, A=Na) are classified as
dimerized layered system with strongly interacting d-electrons of vanadium
ions. The derived band gaps, energy dispersion relations and density of
electronic states are in a good agreement with available experimental and
theoretical data. The correlated band gap provides the insulating state of the
high-temperature alpha-NaV_2O_5 phase whereas the state, earlier misrepresented
as the spin-Peierls state, is governed, in fact, by opening of the Coulomb gap.",9909046v1
1999-09-24,Biquadratic interactions and spin-Peierls transition in the spin 1 chain LiVGe2O6,"The magnetic susceptibility of a new one-dimensional, S=1 system, the
vanadium oxide LiVGe2O6, has been measured. Contrary to previous S=1 chains, it
exhibits an abrupt drop at 22 K typical of a spin-Peierls transition, and it is
consistent with a gapless spectrum above this temperature. We propose that this
behaviour is due to the presence of a significant biquadratic exchange
interaction, a suggestion supported by quantum chemistry calculations that take
into account the quasi-degeneracy of the t2g levels.",9909357v1
1999-10-25,Electron-phonon and spin-phonon coupling in NaV$_{2}$O$_{5}$: charge fluctuations effect,"We show that the asymmetric crystal environment of the V site in the ladder
compound NaV$_{2}$O$_{5}$ leads to a strong coupling of vanadium 3d electrons
to phonons. This coupling causes fluctuations of the charge on the V ions, and
favors a transition to a charge-ordered state at low temperatures. In the low
temperature phase the charge fluctuations modulate the spin-spin superexchange
interaction, resulting in a strong spin-phonon coupling.",9910403v1
2000-12-01,Theory for Phase Transitions in Insulating Vanadium Oxide,"We show that the recently proposed S=2 bond model with orbital degrees of
freedom for insulating V$_{2}$O$_{3}$ not only explains the anomalous magnetic
ordering, but also other mysteries of the magnetic phase transition. The model
contains an additional orbital degree of freedom that exhibits a zero
temperature quantum phase transtion in the Ising universality class.",0012019v3
2000-12-15,Phase transition in a magnetic field in V2-yO3 (y=0 and 0.04),"The effect of a magnetic field on the two long-range-ordered magnetic phases,
the collinear insulating antiferromagnetic (AFI) and the incommensurate
metallic transverse spin density wave (SDW), is investigated for the vanadium
sesquioxide system. A field of 18 T has little effect on the AFI phase of a
nominal V2O3 sample. The transverse SDW phase of V1.96O3 can be suppressed by a
4.6(3)-T magnetic field applied in the plane of spiraling spins, while the same
magnetic field applied along the spiral axis has little effect on the SDW
phase.",0012297v1
2001-09-06,Transition between orbital orderings in YVO3,"Evidence has been found for a change in the ordered occupation of the
vanadium d-orbitals at the 77K phase transition in YVO3, manifested by a change
in the type of Jahn-Teller distortion. The orbital ordering above 77K is not
destroyed at the magnetic ordering temperature of 116K, but is present as far
as a second structural phase transition at 200K. The transition between orbital
orderings is caused by an increase in octahedral tilting with decreasing
temperature.",0109118v1
2001-10-05,LiV2O4: evidence for two-stage screening,"LiV2O4, a frustrated mixed valent metal (d^1 <-> d^2), is argued to undergo
two spin-screening processes. The first quenches the effective spin to produce
the spin 1/2 behavior seen below room temperature[1], while the second produces
the heavy fermi liquid character seen at low temperatures[2]. We present here a
preliminary discussion of a t-J model with strong Hund's coupling of the
strongly correlated d-electrons. Valence fluctuations of the Hubbard operators
(S = {1/2} <-> 1) combined with the frustration of the underlying corner-shared
tetrahedral vanadium lattice are the essential components of our model.",0110115v2
2002-11-08,Hall Effect in Nested Antiferromagnets Near the Quantum Critical Point,"We investigate the behavior of the Hall coefficient in the case of
antiferromagnetism driven by Fermi surface nesting, and find that the Hall
coefficient should abruptly increase with the onset of magnetism, as recently
observed in vanadium doped chromium. This effect is due to the sudden removal
of flat portions of the Fermi surface upon magnetic ordering. Within this
picture, the Hall coefficient should scale as the square of the residual
resistivity divided by the impurity concentration, which is consistent with
available data.",0211149v2
2002-11-13,Spin-fluctuations in the quarter-filled Hubbard ring : significances to LiV$_2$O$_4$,"Using the quantum Monte Carlo method, we investigate the spin dynamics of
itinerant electrons in the one-dimensional Hubbard system. Based on the model
calculation, we have studied the spin-fluctuations in the quarter-filled
metallic Hubbard ring, which is aimed at the vanadium ring or chain defined
along corner-sharing tetrahedra of LiV$_2$O$_4$, and found the dramatic changes
of magnetic responses and spin-fluctuation characteristics with the
temperature. Such results can explain the central findings in the recent
neutron scattering experiment for LiV$_2$O$_4$.",0211261v1
2003-09-08,Computer Modeling of Electronic Properties of Scroll-like V2O5-based Nanotubes,"Atomic models of quasi-one-dimensional 1D vanadium oxide nanostructures -
nanotubes of various morphology (cylinder or scroll-like) formed by rolling
(010) single layers of V2O5 are constructed and their electronic properties are
studied using the tight-binding band theory. Compared to the cylindrical zigzag
(n,0) and armchair (n,n)-like nanotubes, which are uniformly semi-conducting
with the bang gap of about 2.5-2.7 eV, the band gap of the scroll-like tubes
trends significantly to vanish (up to about 0.1 eV) depending on the atomic
configurations of the tubes and inter-wall distances.",0309179v1
2004-01-23,Optical properties and Raman scattering of vanadium ladder compounds,"We investigate electronic and optical properties of the V-based ladder
compounds NaV2O5, the iso-structural CaV2O5, as well as MgV2O5, which differs
from NaV2O5 and CaV2O5 in the c axis stacking. We calculate ab initio the A_g
phonon modes in these compounds as a basis for the investigation of the
electron-phonon and spin-phonon coupling. The phonon modes together with the
dielectric tensors as a function of the corresponding ion displacements are the
starting point for the calculation of the A_g Raman scattering.",0401458v1
2004-02-01,High Resolution Study of Magnetic Ordering at Absolute Zero,"High fidelity pressure measurements in the zero temperature limit provide a
unique opportunity to study the behavior of strongly interacting, itinerant
electrons with coupled spin and charge degrees of freedom. Approaching the
exactitude that has become the hallmark of experiments on classical critical
phenomena, we characterize the quantum critical behavior of the model,
elemental antiferromagnet chromium, lightly doped with vanadium. We resolve the
sharp doubling of the Hall coefficient at the quantum critical point and trace
the dominating effects of quantum fluctuations up to surprisingly high
temperatures.",0402016v1
2004-05-07,Soliton binding and low-lying singlets in frustrated odd-legged S=1/2 spin tubes,"Motivated by the intriguing properties of the vanadium spin tube Na2V3O7, we
show that an effective spin-chirality model similar to that of standard
Heisenberg odd-legged S=1/2 spin tubes can be derived for frustrated inter-ring
couplings, but with a spin-chirality coupling constant alpha that can be
arbitrarily small. Using density matrix renormalization group and analytical
arguments, we show that, while spontaneous dimerization is always present,
solitons become bound into low-lying singlets as alpha is reduced. Experimental
implications for strongly frustrated tubes are discussed.",0405131v1
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
2006-01-11,Coulombian disorder in Charge Density Waves,"When unscreened charged impurities are introduced in charge density wave
system the long wavelength component of the disorder is long ranged and
dominates static correlation functions. We calculate the x-ray intensity and
find that it is identical to the one produced by thermal fluctuations in a
disorder-free smectic-A. We suggest that this effect could be observed in the
blue bronze material K$_{0.3}$MoO$_3$ doped with charged impurities such as
Vanadium.",0601239v1
2006-07-16,Lattice and elastic constants of titanium-niobium monoborides containing aluminum and vanadium,"First-principles electronic-structure computes the lattice and elastic
constants of single-crystal TiB and NbB and changes with Nb, Ti, Al, and V
solutes. The data is built into an interpolation formula for lattice and
elastic constants of the quartenary (TiNbAlV)B with dilute Al and V
concentrations. The lattice and elastic constants of borides in two Ti alloys
containing Nb and Al are predicted from microprobe measurements.",0607398v1
2006-09-14,Strain induced pressure effect in pulsed laser deposited thin films of the strongly correlated oxide V2O3,"V2O3 thin films about 10 nm thick were grown on Al2O3 (0001) by pulsed laser
deposition. The XRD analysis is in agreement with R-3c space group. Some of
them exhibit the metal / insulator transition characteristic of V2O3 bulk
material and others samples exhibit a metallic behavior. For the latter, the
XPS analysis indicates an oxidation state of +III for vanadium. There is no
metal / insulator transition around 150 K in this sample and a strongly
correlated Fermi liquid rho = AT2 behavior of the resistivity at low
temperature is observed, with a value of A of 1.2 10-4 ohm cm, 3 times larger
than the bulk value at 25 kbar.",0609342v1
2006-12-01,Site-Selective NMR in the Quasi-1D Conductor beta-Sr0.33V2O5,"We report 51V NMR experiments in the metallic phase of the
quasi-one-dimensional (1D) conductor beta-Sr0.33V2O5. The Knight shift and the
quadrupole splitting of all six vanadium sites were determined as a function of
the direction of magnetic field perpendicular to the conducting b-axis.
Magnetic properties of the three 1D structural units are remarkably
heterogeneous. In particular, the V2 ladder unit shows pronounced charge
disproportionation among the two (V2a and V2b) sites. At one of these sites,
the absolute value of the Knight shift and 1/(T1T) increase steeply with
decreasing temperature, suggesting development of ferromagnetic correlation.",0612017v1
2006-12-30,Low relaxation rate in a low-Z alloy of iron,"The longest relaxation time and sharpest frequency content in ferromagnetic
precession is determined by the intrinsic (Gilbert) relaxation rate \emph{$G$}.
For many years, pure iron (Fe) has had the lowest known value of $G=\textrm{57
Mhz}$ for all pure ferromagnetic metals or binary alloys. We show that an
epitaxial iron alloy with vanadium (V) possesses values of $G$ which are
significantly reduced, to 35$\pm$5 Mhz at 27% V. The result can be understood
as the role of spin-orbit coupling in generating relaxation, reduced through
the atomic number $Z$.",0701004v1
1996-09-25,Strained tetragonal states and Bain paths in metals,"Paths of tetragonal states between two phases of a material, such as bcc and
fcc, are called Bain paths. Two simple Bain paths can be defined in terms of
special imposed stresses, one of which applies directly to strained epitaxial
films. Each path goes far into the range of nonlinear elasticity and reaches a
range of structural parameters in which the structure is inherently unstable.
In this paper we identify and analyze the general properties of these paths by
density functional theory. Special examples include vanadium, cobalt and
copper, and the epitaxial path is used to identify an epitaxial film as related
uniquely to a bulk phase.",9609008v1
2007-04-05,Phonon instability and structural phase transition in Vanadium under high pressure,"Results of the first-principles calculations are presented for the group-VB
metals V, Nb and Ta up to couple of megabar pressure. An unique structural
phase transition sequence BCC-->(at 60 GPa) rhombohedral (angle=110.5
degree)-->(at ~ 160 GPa)rhombohedral(angle=108.5 degree)--> (at ~ 430 GPa) BCC
is predicted in V. We also find that BCC-V becomes mechanically and
vibrationally unstable at around 112 GPa pressure. Similar transitions are
absent in Nb and Ta.",0704.0696v1
2007-07-05,Observation of a uniform temperature dependence in the electrical resistance across the structural phase transition in thin film vanadium oxide ($VO_{2}$),"An electrical study of thin $VO_{2}$ films in the vicinity of the structural
phase transition at $68^{0}C$ shows (a) that the electrical resistance $R$
follows $log (R)$ $\propto$ $-T$ over the $T$-range, $20 < T < 80 ^{0}C$
covering both sides of the structural transition, and (b) a history dependent
hysteresis loop in $R$ upon thermal cycling. These features are attributed here
to transport through a granular network.",0707.0885v1
2008-10-20,Modulation Doping of a Mott Quantum Well by a Proximate Polar Discontinuity,"We present evidence for hole injection into LaAlO3/LaVO3/LaAlO3 quantum wells
near a polar surface of LaAlO3 (001). As the surface is brought in proximity to
the LaVO3 layer, an exponential drop in resistance and a decreasing positive
Seebeck coefficient is observed below a characteristic coupling length of 10-15
unit cells. We attribute this behavior to a crossover from an atomic
reconstruction of the AlO2-terminated LaAlO3 surface to an electronic
reconstruction of the vanadium valence. These results suggest a general
approach to tunable hole-doping in oxide thin film heterostructures.",0810.3469v1
2009-01-07,Phase-transition driven memristive system,"Memristors are passive circuit elements which behave as resistors with
memory. The recent experimental realization of a memristor has triggered
interest in this concept and its possible applications. Here, we demonstrate
memristive response in a thin film of Vanadium Dioxide. This behavior is driven
by the insulator-to-metal phase transition typical of this oxide. We discuss
several potential applications of our device, including high density
information storage. Most importantly, our results demonstrate the potential
for a new realization of memristive systems based on phase transition
phenomena.",0901.0899v2
2009-03-24,Light dynamics in glass-vanadium dioxide nanocomposite waveguides with thermal nonlinearity,"We address the propagation of laser beams in Si02-VO2 nanocomposite
waveguides with thermo-optical nonlinearity. We show that the large
modifications of the absorption coefficient as well as notable changes of
refractive index of VO2 nanoparticles embedded into the SiO2 host media that
accompany the semiconductor-to-metal phase transition may lead to optical
limiting in the near-infrared wave range.",0903.4150v1
2009-04-17,Electronic properties of superconducting Sr4V2Fe2As2O6 versus Sr4Sc2Fe2As2O6,"First principle FLAPW-GGA calculations have been performed with the purpose
to understand the peculiarities of band structure and Fermi surface topology
for recently discovered superconductor: Sr4V2Fe2As2O - in comparison with
isostructural phase Sr4Sc2Fe2As2O6. Our main finding is that the replacement of
Sc on vanadium leads to drastic transformation of electronic, magnetic and
conductive properties of these materials: as against non-magnetic
Sr4Sc2Fe2As2O6 which is formed from non-magnetic conducting [Fe2As2] and
insulating [Sr4Sc2O6] blocks, Sr4V2Fe2As2O6 consists from non-magnetic
conducting [Fe2As2] blocks and [Sr4V2O6] blocks which exhibit magnetic
half-metallic properties.",0904.2671v1
2011-01-21,One-Dimensional Organometallic V-Anthracene Wire and Its B-N Analogue: Efficient Half-Metallic Spin Filters,"Using density functional theory, we have investigated the structural,
electronic and magnetic properties of infinitely periodic organometallic
vanadium-anthracene ($[V_2Ant]_\infinity)$ and $[V_4(BNAnt)_2]_\infinity$(where
BNAnt is B-N analogue of anthracene) for their possible application in
spintronics. From our calculations, we find that one-dimensional
$[V_2Ant]_\infinity$ and $[V_4(BNAnt)_2]_\infinity$ wires exhibit robust
ferromagnetic half-metallic and metallic behavior, respectively. The finite
sized $V_6Ant_2$ and $V_6(BNAnt)_2$ clusters are also found to exhibit
efficient spin filter properties when coupled to graphene electrodes on either
side.",1101.4115v1
2011-02-17,Ground State of the Quasi-1D \bvs\ resolved by Resonant Magnetic X-ray Scattering,"Resonant-magnetic x-ray scattering (RMXS) near the vanadium
$L_{2,3}$-absorption edges has been used to investigate the low temperature
magnetic structure of high quality \bvs\ single crystals. Below $T_N$ = 31 K,
the strong resonance revealed a triple-incommensurate magnetic ordering at wave
vector (0.226 0.226 $\xi$) in the hexagonal notation, with $\xi$ = 0.033. The
simulations of the experimental RMXS spectra with a time-dependent density
functional theory indicate an antiferromagnetic order with the spins polarized
along $a$ in the monoclinic structure.",1102.3589v1
2011-08-17,The hyperfine properties of a hydrogenated Fe/V superlattice,": We study the effect of hydrogen on the electronic, magnetic and hyperfine
structures of an iron-vanadium superlattice consisting of three Fe monolayers
and nine V monolayers. The contact charge density ({\rho}), the contact
hyperfine field (Bhf) and the electronic field gradient (EFG) at the Fe sites
for different H locations and H fillings are calculated using the first
principle full-potential linear-augmented-plane-wave (FP-LAPW) method . It is
found that sizeable changes in the hyperfine properties are obtained only when
H is in the interface region.",1108.3388v1
2011-11-02,Alternating spin-orbital order in tetragonal Sr2VO4,"Considering spin-orbit coupling, the tetragonal crystal-field, and all
relevant superexchange processes including quantum interference, we derive
expressions for the energy levels of the vanadium ions in tetragonal Sr2VO4.
The used parameters of the model Hamiltonian allow to describe well the
excitation spectra observed in neutron scattering and optical experiments at
low temperatures. The free energy exhibits a minimum which corresponds to a
novel alternating spin-orbital order with strong thermal fluctuation of the
orbital mixing parameter.",1111.0668v1
2011-12-07,VO2 nanosheets: controlling the THz properties through strain engineering,"We investigate far-infrared properties of strain engineered vanadium dioxide
nanosheets through epitaxial growth on a (100)R TiO2 substrate. The nanosheets
exhibit large uniaxial strain leading to highly uniform and oriented cracks
along the rutile c-axis. Dramatic anisotropy arises for both the
metal-insulator transition temperature, which is different from the structural
transition temperature along the cR axis, and the metallic state conductivity.
Detailed analysis reveals a Mott-Hubbard like behavior along the rutile cR
axis.",1112.1573v1
2012-06-13,Ferromagnetism and orbital order in a topological ferroelectric,"We explore via density functional calculations the magnetic doping of a
topological ferroelectric as an unconventional route to multiferroicity.
Vanadium doping of the layered perovskite La$_{2}$Ti$_{2}$O$_{7}$ largely
preserves electric polarization and produces robust ferromagnetic order, hence
proper multiferroicity. The marked tendency of dopants to cluster into chains
results in an insulating character at generic doping. Ferromagnetism stems from
the symmetry breaking of the multi-orbital V system via an unusual
""antiferro""-orbital order, and from the host's low-symmetry layered structure.",1206.2879v2
2013-03-21,Metal Insulator Transition of Cr doped V2O3 calculated by hybrid density functional,"The electronic structure of vanadium sesquioxide in its different phases has
been calculated using the screened exchange (sX) hybrid functional. The hybrid
functional reproduces the electronic properties of all three phases, the
paramagnetic metal (PM) phase, the anti-ferromagnetic insulating phase, and the
Cr-doped paramagnetic insulating (PI) phase. A fully relaxed supercell model of
Cr-doped V2O3 has a polaronic distortion around the substitutional Cr atoms and
this local strain drives the PI-PM transition. The PI phase has a calculated
band gap of 0.15eV in good agreement with experiment.",1303.5320v1
2013-09-05,Switchable thermal antenna by phase transition,"We introduce a thermal antenna which can be actively switched by phase
transition. The source makes use of periodically patterned vanadium dioxide, a
metal-insulator phase transition material which supports a surface
phonon-polariton (SPP) in the infrared range in its crystalline phase. Using
electrodes properly registred with respect to the pattern, the phase transition
of VO2 can be localy triggered within few microseconds and the SPP can be
diffracted making the thermal emission highly directionnal. This switchable
antenna could find broad applications in the domain of active thermal coatings
or in those of infrared spectroscopy and sensing.",1309.1263v1
2014-06-11,Self-organized vanadium and nitrogen co-doped titania nanotube arrays with enhanced photocatalytic reduction of CO2 into CH4,"Self-organized V-N co-doped TiO2 nanotube arrays (TNAs) with various doping
amount were synthesized by anodizing in association with hydrothermal
treatment. Impacts of V-N co-doping on the morphologies, phase structures, and
photoelectrochemical properties of the TNAs films were thoroughly investigated.
The co-doped TiO2 photocatalysts show remarkably enhanced photocatalytic
activity for the CO2 photoreduction to methane under ultraviolet illumination.
The mechanism of the enhanced photocatalytic activity is discussed in detail.",1406.2765v1
2014-06-11,Theoretical unification of hybrid-DFT and DFT+U methods for the treatment of localized orbitals,"We formulate the on-site occupation dependent exchange correlation energy and
effective potential of hybrid functionals for localized states and connect them
to the on-site correction term of the DFT+U method. Our derivation provides a
theoretical justification for adding a DFT+U-like onsite potential in hybrid
DFT calculations to resolve issues caused by overscreening of localized states.
The resulting scheme, hybrid- DFT+Vw, is tested for chromium impurity in
wurtzite AlN and vanadium impurity in 4H-SiC, which are paradigm examples of
systems with different degree of localization between host and impurity
orbitals.",1406.2944v1
2015-02-18,Random Field Driven Spatial Complexity at the Mott Transition in VO2,"We report the first application of critical cluster techniques to the Mott
metal-insulator transition in vanadium dioxide. We show that the geometric
properties of the metallic and insulating puddles observed by scanning
near-field infrared microscopy are consistent with the system passing near
criticality of the random field Ising model as temperature is varied. The
resulting large barriers to equilibrium may be the source of the unusually
robust hysteresis phenomena associated with the metal-insulator transition in
this system.",1502.05426v1
2016-01-07,First-principles derived complexion diagrams for phase boundaries in doped cemented carbides,"This article reviews a method for calculating an equilibrium interfacial
phase diagram depicting regions of stability for different interface structures
as function of temperature and chemical potentials. Density functional theory
(DFT) is used for interfacial energies, Monte Carlo simulations together with
cluster expansions based on DFT results for obtaining configurational free
energies, and CALPHAD-type modeling for describing the thermodynamic properties
of the adjoining bulk phases. An explicit case, vanadium doped cemented
carbides, is chosen to illustrate the progress in the research field and the
interfacial diagram, a complexion diagram, for the phase boundary WC(0001)/Co
is constructed as function of temperature and chemical potentials.",1601.01588v1
2017-08-10,Silicon waveguide modulator with embedded phase change material,"Phase-change materials (PCMs) have emerged as promising active elements in
silicon (Si) photonic systems. In this work, we design, fabricate, and
characterize a hybrid Si-PCM optical modulator. By integrating vanadium dioxide
(a PCM) within a Si photonic waveguide, in a non-resonant geometry, we
demonstrate ~ 10 dB broadband modulation with a PCM length of 500 nm.",1708.04297v1
2017-09-13,Electrochemical Impedance Imaging via the Distribution of Diffusion Times,"We develop a mathematical framework to analyze electrochemical impedance
spectra in terms of a distribution of diffusion times (DDT) for a parallel
array of random finite-length Warburg (diffusion) or Gerischer
(reaction-diffusion) circuit elements. A robust DDT inversion method is
presented based on Complex Nonlinear Least Squares (CNLS) regression with
Tikhonov regularization and illustrated for three cases of nanostructured
electrodes for energy conversion: (i) a carbon nanotube supercapacitor, (ii) a
silicon nanowire Li-ion battery, and (iii) a porous-carbon vanadium flow
battery. The results demonstrate the feasibility of non-destructive ""impedance
imaging"" to infer microstructural statistics of random, heterogeneous
materials.",1709.04147v1
2018-07-19,New approximation to compute the incoherent scattering function of harmonic lattices,"A new method to compute the incoherent scattering function of harmonic
lattices is introduced. It is based in a saddle point approximation for each
term of the phonon expansion, and is simple enough to be used in practice. The
method gives very accurate results even for the tails of the scattering
function, and is more accurate than the usual gaussian approximation, which can
be derived from this saddle point approximation in the limit in which the order
of the phonon expansion term becomes large. Numerical comparisons are provided
using vanadium as a test case.",1807.07330v1
2019-03-02,Reconfigurable Electromagnetic Structures: X-Band and Beyond,"Developing reconfigurable millimeter-wave (mmWave) antennas and devices is an
outstanding challenge, with switch technologies being a primary impediment.
Recently, it has been shown that vanadium dioxide (VO2), a thermochromic
material whose resistance changes with temperature, could provide a path
forward in developing reconfigurable mmWave devices. As an initial step towards
this vision, we investigate the integration of VO2 switches in reconfigurable
components at 15 GHz. In particular, a frequency reconfigurable antenna and a
reconfigurable phase shifter are shown. The low loss and minimal parasitics of
VO2 technology have the potential to enable devices at 15 GHz and beyond.",1906.03034v1
2019-07-24,Chemical complexity in high entropy alloys: A pair-interaction perspective,"The recently proposed pair-interaction model is applied to study a series of
refractory high entropy alloys. The results demonstrate the simplicity,
robustness, and high accuracy of this model in predicting the configuration
energies of NbMoTaW, NbMoTaWV and NbMoTaWTi. The element-element pair
interaction parameters obtained from the linear regression of first-principle
data provide a new perspective to understand the strengthening mechanism in
HEAs, as revealed by comparing the effects of adding vanadium and titanium.
Using the pair-interaction model, an expression for the intrinsic energy
fluctuation is derived, which provides guidance on both theoretical modeling
and first principles calculation.",1907.10223v1
2019-12-16,On the thermodynamic derivation of Nernst relation,"What is the maximum voltage of a cell with a given electrochemical reaction?
The answer to this question has been given more than a century ago by Walther
Nernst and bears his name. Unfortunately, the assumptions behind the answer
have been forgotten by many authors, which leads to wrong forms of the Nernst
relation. Such mistakes can be overcome by applying a correct thermodynamic
derivation independently of the form in which the reaction is written. The
correct form of Nernst relation is important for instance in modelling of
vanadium redox flow batteries or zinc-air batteries. In particular, the
presence of corrosion can impact the OCV in the case of zinc-air batteries.",1912.07724v1
2020-01-28,A C/V$_2$O$_5$ core-sheath nanofibrous cathode with mixed-ion intercalation for aluminium-ion batteries,"A new nanofibrous material, consisting of a conductive carbon core and an
external layer made of vanadium oxide, has been studied as a cathode for
aluminium-ion batteries. The material enables a mixed-ion intercalation
mechanism, resulting in the alternating insertion of $\text{Al}^{3+}$ and
$\text{AlCl}_4^-$ in the $\text{V}_2\text{O}_5$ and carbon layers,
respectively. This is a highly desirable feature for cathode materials which
may increase the energy density of future batteries by optimising the
utilisation of the electrolyte.",2001.10153v2
2020-03-19,Energy Resolution and Neutron Flux of the 4SEASONS Spectrometer Revisited,"The elastic energy resolution, integrated intensity, and peak intensity of
the direct-geometry neutron chopper spectrometer 4SEASONS at Japan Proton
Accelerator Research Complex (J-PARC) were re-investigated. This was done with
respect to the incident energy and the rotation speed of the Fermi chopper
using incoherent scattering of vanadium and simple analytical formulas. The
model calculations reproduced the observed values satisfactorily. The present
work should be useful for estimating in instrument performance in experiments.",2003.08785v1
2020-04-14,Can Fermi surface nesting alone drive the charge-density-wave transition in monolayer vanadium diselenide?,"We demonstrate that charge-density-wave formation is possible via a purely
electronic mechanism in monolayers of the transition metal dichalcogenide
1T-VSe$_2$. Via a renormalization group treatment of an extended Hubbard model
we examine the competition of superconducting and density-wave fluctuations as
sections of the Fermi surface are tuned to perfect nesting. We find regions of
charge-density-wave order when the Heisenberg exchange interaction is
comparable to the Coulomb repulsion, and $d$-wave superconductivity for purely
repulsive interactions. We discuss the possible role of lattice vibrations in
enhancing the effective Heisenberg exchange.",2004.06665v1
2020-06-09,"Evidence of liquid-liquid phase transition in compressed Ar probed by the thermal expansion of Mo, Ta and W at high pressures","The long standing controversy between the melting curves of the bcc Mo,Ta,W
and vanadium
  (V) metals measured by diamond anvil Cells (DAC) and the shock dynamic
experiments is explained by the behavior the liquid or solid pressure
transmitting mediums compressed by the thermal expansion of the these
transition metals. This explains the observed isobaric behavior of the laser
heated DAC experiments containing different transmitting mediums reported in
the literature, thus solving the standing enigma described in very many
publications.",2006.05472v1
2020-07-10,Thermodynamic dislocation theory: Application to bcc-crystals,"This paper presents the thermodynamic dislocation theory containing several
modifications over its first version which was originally proposed by Langer,
Bouchbinder, and Lookman (2010). Employing a small set of physics-based
material parameters identified by the large scale least squares analysis, we
show that the theory can fit the stress-strain curves of bcc crystals niobium,
tantalum, tungsten, and vanadium over a wide range of temperatures and strain
rates.",2007.05429v1
2021-01-18,Electron-hole asymmetry in electron systems with orbital degeneracy and correlated hopping,"Microscopic models of electronic subsystems with orbital degeneracy of energy
states and non-diagonal matrix elements of electron interactions (correlated
hopping) are considered within the configuration-operator approach. Equations
for arbitrary-temperature numerical calculation of the doublon concentration
for the integer band filling $n=1$ at different forms of the model density of
states are derived. The energy spectra obtained within the Green function
method are discussed with special emphasis on the role of correlated hopping in
transition from itinerant to localized behavior observed in vanadium Magneli
phases V$_n$O$_{2n-1}$, transition-metal dichalcogenides NiS$_{2-x}$Se$_x$,
fulleride A$_n$C$_{60}$ systems.",2101.07051v1
2021-06-13,Multi-spectral programmable absorbers,"We designed and demonstrated a multi-spectral programmable perfect absorber
that exploits two different phase-change materials. This programmability is
possible by resonantly coupling two phase change materials, a Ge2Sb2Te5 layer
to vanadium dioxide nanoparticles (VO2 NPs). The perfect absorption is
attributed to the coalescence of gap plasmon modes excited between the NPs and
waveguide cavity-like modes excited between the film and the NPs. The
absorptance peak (>90%) can be tuned to four different infrared (IR)
wavelengths from 1906 to 2960 nm by heating the structure to different
temperatures. The perfect absorber is reconfigurable, lithography-free,
large-scale, polarization-insensitive omnidirectional. Our strategy opens a new
path for programmable infrared photonics.",2106.06940v1
2023-01-19,Critical Slowing Down at the Abrupt Mott Transition: When the First-Order Phase Transition Becomes Zeroth-Order and Looks Like Second-Order,"We report that the thermally-induced Mott transition in vanadium sesquioxide
shows critical-slowing-down and enhanced variance ('critical opalescence') of
the order parameter fluctuations measured through low-frequency
resistance-noise spectroscopy. Coupled with the observed increase of also the
phase-ordering time, these features suggest that the strong abrupt transition
is controlled by a critical-like singularity in the hysteretic metastable
phase. The singularity is identified with the spinodal point and is a likely
consequence of the strain-induced long-range interaction.",2301.08254v1
2023-11-27,Symmetry breaking in vanadium trihalides,"In the light of new experimental evidence we study the insulating ground
state of the $3d^2$-transition metal trihalides VX$_3$ (X=Cl, I). Based on
Density Functional Theory with the Hubbard correction (DFT$+U$) we
systematically show how these systems host multiple metastable states
characterized by different orbital ordering and electronic behaviour. Our
calculations reveal the importance of imposing a precondition in the on site
$d$ density matrix and of considering a symmetry broken unit cell to correctly
take into account the correlation effects in a mean field framework.
Furthermore we ultimately found a ground state with the $a_{1g}$ orbital
occupied in a distorted VX$_6$ octahedra driven by an optical phonon mode.",2311.16068v1
2001-01-17,One-dimensional dynamics of the d-electrons in $α'$-NaV$_{2}$O$_{5}$,"We have studied the electronic properties of the ladder compound
$\alpha'$-NaV$_{2}$O$_{5}$, adopting a joint experimental and theoretical
approach. The momentum-dependent loss function was measured using electron
energy-loss spectroscopy in transmission. The optical conductivity derived from
the loss function by a Kramers-Kronig analysis agrees well with our results
from LSDA+U band-structure calculations upon application of an
antiferromagnetic alignment of the V~3$d_{xy}$ spins along the legs and an
on-site Coulomb interaction U of between 2 and 3 eV. The decomposition of the
calculated optical conductivity into contributions from transitions between
selected energy regions of the DOS reveals the origin of the observed
anisotropy of the optical conductivity. In addition, we have investigated the
plasmon excitations related to transitions between the vanadium states within
an effective 16 site vanadium cluster model. Good agreement between the
theoretical and experimental loss function was obtained using the hopping
parameters derived from the tight binding fit to the band-structure and
moderate Coulomb interactions between the electrons within the ab plane.",0101256v1
2004-06-02,Orbital and magnetic transitions in geometrically-frustrated vanadium spinels -- Monte Carlo study of an effective spin-orbital-lattice coupled model --,"We present our theoretical and numerical results on thermodynamic properties
and the microscopic mechanism of two successive transitions in vanadium spinel
oxides $A$V$_2$O$_4$ ($A$=Zn, Mg, or Cd) obtained by Monte Carlo calculations
of an effective spin-orbital-lattice model in the strong correlation limit.
Geometrical frustration in the pyrochlore lattice structure of V cations
suppresses development of spin and orbital correlations, however, we find that
the model exhibits two transitions at low temperatures. First, a discontinuous
transition occurs with an orbital ordering assisted by the tetragonal
Jahn-Teller distortion. The orbital order reduces the frustration in spin
exchange interactions, and induces antiferromagnetic correlations in
one-dimensional chains lying in the perpendicular planes to the tetragonal
distortion. Secondly, at a lower temperature, a three-dimensional
antiferromagnetic order sets in continuously, which is stabilized by the
third-neighbor interaction among the one-dimensional antiferromagnetic chains.
Thermal fluctuations are crucial to stabilize the collinear magnetic state by
the order-by-disorder mechanism. The results well reproduce the experimental
data such as transition temperatures, temperature dependence of the magnetic
susceptibility, changes of the entropy at the transitions, and the magnetic
ordering structure at low temperatures. Quantum fluctuation effect is also
examined by the linear spin wave theory at zero temperature. The staggered
moment in the ground state is found to be considerably reduced from saturated
value, and reasonably agrees with the experimental data.",0406039v1
2009-04-02,Infrared spectroscopy and nano-imaging of the insulator-to-metal transition in vanadium dioxide,"We present a detailed infrared study of the insulator-to-metal transition
(IMT) in vanadium dioxide (VO2) thin films. Conventional infrared spectroscopy
was employed to investigate the IMT in the far-field. Scanning near-field
infrared microscopy directly revealed the percolative IMT with increasing
temperature. We confirmed that the phase transition is also percolative with
cooling across the IMT. We present extensive near-field infrared images of
phase coexistence in the IMT regime in VO2. We find that the coexisting
insulating and metallic regions at a fixed temperature are static on the time
scale of our measurements. A novel approach for analyzing the far-field and
near-field infrared data within the Bruggeman effective medium theory was
employed to extract the optical constants of the incipient metallic puddles at
the onset of the IMT. We found divergent effective carrier mass in the metallic
puddles that demonstrates the importance of electronic correlations to the IMT
in VO2. We employ the extended dipole model for a quantitative analysis of the
observed near-field infrared amplitude contrast and compare the results with
those obtained with the basic dipole model.",0904.0294v1
2009-07-09,Optical Properties of Correlated Materials -- or Why Intelligent Windows may look Dirty,"Materials with strong electronic Coulomb correlations play an increasing role
in modern materials applications. ""Thermochromic"" systems, which exhibit
thermally induced changes in their optical response, provide a particularly
interesting case. The optical switching associated with the metal-insulator
transition of vanadium dioxide, for example, has been proposed for use in
numerous applications, ranging from anti-laser shields to ""intelligent""
windows, which selectively filter radiative heat in hot weather conditions. Are
present-day electronic structure techniques able to describe, or -- eventually
even predict -- such a kind of behavior ? How far are we from materials design
using correlated oxides ? These are the central questions we try to address in
this article. We review recent attempts of calculating optical properties of
correlated materials within dynamical mean field theory, and summarize results
for vanadium dioxide obtained within a novel scheme aiming at particularly
simple and efficient calculations of optical transition matrix elements within
localized basis sets. Finally, by optimizing the geometry of ""intelligent
windows"", we argue that this kind of technique can in principle be used to
provide guidance for experiments, thus giving a rather optimistic answer to the
above questions.",0907.1575v1
2010-06-22,Three-terminal field effect devices utilizing thin film vanadium oxide as the channel layer,"Electrostatic control of the metal-insulator transition (MIT) in an oxide
semiconductor could potentially impact the emerging field of oxide electronics.
Vanadium dioxide is of particular interest due to the fact that the MIT happens
in the vicinity of room temperature and it is considered to exhibit the Mott
transition. We present a detailed account of our experimental investigation
into three-terminal field effect transistor-like devices using thin film VO2 as
the channel layer. The gate is separated from the channel through an insulating
gate oxide layer, enabling true probing of the field effect with minimal or no
interference from large leakage currents flowing directly from the electrode.
The influence of the fabrication of multiple components of the device,
including the gate oxide deposition, on the VO2 film characteristics is
discussed. Further, we discuss the effect of the gate voltage on the device
response, point out some of the unusual characteristics including temporal
dependence. A reversible unipolar modulation of the channel resistance upon the
gate voltage is demonstrated for the first time in optimally engineered
devices. The results presented in this work are of relevance towards
interpreting gate voltage response in such oxides as well as addressing
challenges in advancing gate stack processing for oxide semiconductors.",1006.4373v1
2012-01-18,On the metal-insulator-transition in vanadium dioxide,"Vanadium dioxide (VO$_2$) undergoes a metal-insulator transition (MIT) at 340
K with the structural change between tetragonal and monoclinic crystals as the
temperature is lowered. The conductivity $\sigma$ drops at MIT by four orders
of magnitude. The low-temperature monoclinic phase is known to have a lower
ground-state energy. The existence of a $k$-vector ${\boldsymbol k}$ is
prerequisite for the conduction since the ${\boldsymbol k}$ appears in the
semiclassical equation of motion for the conduction electron (wave packet).
Each wave packet is, by assumption, composed of the plane waves proceeding in
the ${\boldsymbol k}$ direction perpendicular to the plane. The tetragonal
(VO$_2$)$_3$ unit cells are periodic along the crystal's $x$-, $y$-, and
z-axes, and hence there are three-dimensional $k$-vectors. The periodicity
using the non-orthogonal bases does not legitimize the electron dynamics in
solids. There are one-dimensional ${\boldsymbol k}$ along the c-axis for a
monoclinic crystal. We believe this decrease in the dimensionality of the
$k$-vectors is the cause of the conductivity drop. Triclinic and trigonal
(rhombohedral) crystals have no $k$-vectors, and hence they must be insulators.
The majority carriers in graphite are ""electrons"", which is shown by using an
orthogonal unit cell for the hexagonal lattice.",1201.3700v1
2012-10-29,Temperature driven Vanadium clusterization and band gap enlargement in the layered misfit compound (LaS)$_{1.196}$VS$_2$,"Intriguing properties of the misfit layered chalcogenide
(LaS)$_{1.196}$VS$_2$ crystals were investigated by transport, optical
measurements, angle-resolved photoemission (ARPES) and x-ray diffraction.
Although no clear anomaly is found in transport properties as a function of
temperature, a large spectral weight transfer, up to at least 1 eV, is observed
by both optical and photoemission spectroscopies. ARPES reveals that a nearly
filled band with negative curvature, close enough from the Fermi level at 300K
to produce metallic-like behaviour as observed in optical conductivity spectra.
At low temperature, the band structure is strongly modified, yielding to an
insulating state with a optical gap of 120 meV. An accurate (3+1)D analysis of
x-ray diffraction data shows that, although a phase transition does not occur,
structural distortions increase as temperature is decreased, and vanadium
clusterization is enhanced. We found that the changes of electronic properties
and structure are intimately related. This indicates that structural distorsion
play a major role in the insulating nature of (LaS)$_{1.196}$VS$_2$ and that
electronic correlation may not be important, contrary to previous belief. These
results shed a new light on the mechanism at the origin of non-linear electric
properties observed in (LaS)$_{1.196}$VS$_2$.",1210.8341v1
2014-05-07,A S=1/2 vanadium-based geometrically frustrated spinel system Li2ZnV3O8,"We report the synthesis and characterization of Li2ZnV3O8, which is a new
Zn-doped LiV2O4 system containing only tetravalent vanadium. A Curie-Weiss
susceptibility with a Curie-Weiss temperature of <theta>CW ~214 K suggests the
presence of strong antiferromagnetic correlations in this system. We have
observed a splitting between the zero-field cooled ZFC and field cooled FC
susceptibility curves below 6 K. A peak is present in the ZFC curve around 3.5
K suggestive of spin-freezing . Similarly, a broad hump is also seen in the
inferred magnetic heat capacity around 9 K. The consequent entropy change is
only about 8% of the value expected for an ordered S = 1=2 system. This
reduction indicates continued presence of large disorder in the system in spite
of the large <theta>CW, which might result from strong geometric frustration in
the system. We did not find any temperature T dependence in our 7Li nuclear
magnetic resonance NMR shift down to 6 K (an abrupt change in the shift takes
place below 6 K) though considerable T-dependence has been found in literature
for LiV2O4- undoped or with other Zn/Ti contents. Consistent with the above
observation, the 7Li nuclear spin-lattice relaxation rate 1/T1 is relatively
small and nearly T-independent except a small increase close to the freezing
temperature, once again, small compared to undoped or 10% Zn or 20% Ti-doped
LiV2O4.",1405.1697v2
2014-10-01,Evidence for magnetic clusters in Ni$_{1-x}$V$_{x}$ close to the quantum critical concentration,"The d-metal alloy Ni$_{1-x}$V$_{x}$ undergoes a quantum phase transition from
a ferromagnetic ground state to a paramagnetic ground state as the vanadium
concentration $x$ is increased. We present magnetization, ac-susceptibility and
muon-spin relaxation data at several vanadium concentrations near the critical
concentration $x_c \approx11.6%$ at which the onset of ferromagnetic order is
suppressed to zero temperature. Below $x_c$, the muon data reveal a broad
magnetic field distribution indicative of long-range ordered ferromagnetic
state with spatial disorder. We show evidence of magnetic clusters in the
ferromagnetic phase and close to the phase boundary in this disordered
itinerant system as an important generic ingredient of a disordered quantum
phase transition. In contrast, the temperature dependence of the magnetic
susceptibility above $x_c$ is best described in terms of a magnetic quantum
Griffiths phase with a power-law distribution of fluctuation rates of dynamic
magnetic clusters. At the lowest temperatures, the onset of a short-range
ordered cluster-glass phase is recognized by an increase in the muon
depolarization in transverse fields and maxima in ac-susceptibility.",1410.0094v1
2015-02-16,Low-temperature magnetic ordering and structural distortions in Vanadium Sesquioxide (V$_2$O$_3$),"Vanadium Sesquioxide (V$_2$O$_3$) is an antiferromagnetic insulator below
$T_{\mathrm N}\approx$ 155 K. The magnetic order is not of C- or G-type as one
would infer from the bipartite character of the hexagonal basal plane in the
high-temperature corundum structure. In fact, the N\'eel transition is
accompanied by a monoclinic distortion that makes one bond of the honeycomb
plane inequivalent from the other two, thus justifying a magnetic structure
with one ferromagnetic bond and two antiferromagnetic ones. We show here that
the magnetic ordering, the accompanying monoclinic structural distortion, the
magnetic anisotropy and also the recently discovered high-pressure monoclinic
phase, can all be accurately described by conventional electronic structure
calculations within GGA and GGA+U. Our results are in line with DMFT
calculations for the paramagnetic phase, which predict that the insulating
character is driven by a correlation-enhanced crystal field splitting between
$e^\pi_g$ and $a_{1g}$ orbitals that pushes the latter above the chemical
potential. We find that the $a_{1g}$ orbital, although almost empty in the
insulating phase, is actually responsible for the unusual magnetic order as it
leads to magnetic frustration whose effect is similar to a
next-nearest-neighbor exchange in a Heisenberg model on a honeycomb lattice.",1502.04555v1
2015-10-28,Systematics in the metal-insulator transition temperatures in vanadium oxides,"Nine of the known vanadium oxides, VO$_{2-1/n}$ (n - a positive or negative
integer) with n=2 - 6, 8, 9, $\infty$ and -6, undergo metal-insulator
transitions accompanied by structural transitions, at various temperatures
T$_{MIT}$ (V$_7$O$_{13}$ is metallic above T=0). Among the persistent efforts
to determine the driving force(s) of these transitions, electron-electron
(Mott-like) and electron-phonon (Peierls-like) interactions, there were several
attempts to find systematics in T$_{MIT}$ as function of n. Here we present an
unexpectedly simple and illuminating systematics that holds for positive n: if
T$_{MIT}$ is the absolute value of the difference between T$_M$(n) and
T$_P$(n), which represent the contributions of electron-electron and
electron-phonon interactions, respectively, all data points of T$_M$-T$_P$
versus 1/n lie on, or close to, two simple straight lines; one is T$_M$-T$_P$=
T$_{\infty}$(7/n-1) for V$_3$O$_5$, V$_4$O$_7$, V$_5$O$_9$, V$_7$O$_{13}$,
V$_8$O$_{15}$, V$_9$O$_{17}$ and VO$_2$ and the other is T$_M$-T$_P$=
T$_{\infty}$(3/n-1) for V$_2$O$_3$, V$_6$O$_{11}$ and VO$_2$.",1510.08286v1
2016-01-27,Ultrafast THz Field Control of Electronic and Structural Interactions in Vanadium Dioxide,"Vanadium dioxide, an archetypal correlated-electron material, undergoes an
insulator-metal transition near room temperature that exhibits
electron-correlation-driven and structurally-driven physics. Using ultrafast
optical spectroscopy and x-ray scattering we show that these processes can be
disentangled in the time domain. Specifically, following intense sub-picosecond
electric-field excitation, a partial collapse of the insulating gap occurs
within the first ps. Subsequently, this electronic reconfiguration initiates a
change in lattice symmetry taking place on a slower timescale. We identify the
kinetic energy increase of electrons tunneling in the strong electric field as
the driving force, illustrating a novel method to control electronic
interactions in correlated materials on an ultrafast timescale.",1601.07490v2
2016-02-19,Temperature and Electric Field Induced Metal-Insulator Transition in Atomic Layer Deposited Vanadium Dioxide Thin Films,"Amorphous vanadium dioxide (VO$_{2}$) films deposited by atomic layer
deposition (ALD) were crystallized with an ex situ anneal at 660-670 ${\deg}$C
for 1-2 hours under a low oxygen pressure (10$^{-4}$ to 10$^{-5}$ Torr). Under
these conditions the crystalline VO$_{2}$ phase was maintained, while formation
of the V$_{2}$O$_{5}$ phase was suppressed. Electrical transition from the
insulator to the metallic phase was observed in the 37-60 ${\deg}$C range, with
a R$_{ON}$/R$_{OFF}$ ratio of up to about 750 and critical transition
temperature of 7-10 ${\deg}$C. Electric field applied across two-terminal
device structures induced a reversible phase change, with a room temperature
transition field of about 25 kV/cm in the VO$_{2}$ sample processed with the 2
hr long anneal. Both the width and slope of the field induced MIT hysteresis
were dependent upon the VO$_{2}$ crystalline quality.",1602.06340v1
2016-04-08,Detection of H2O and evidence for TiO/VO in an ultra hot exoplanet atmosphere,"We present a primary transit observation for the ultra hot (Teq~2400K) gas
giant expolanet WASP-121b, made using the Hubble Space Telescope Wide Field
Camera 3 in spectroscopic mode across the 1.12-1.64 micron wavelength range.
The 1.4 micron water absorption band is detected at high confidence (5.4 sigma)
in the planetary atmosphere. We also reanalyze ground-based photometric
lightcurves taken in the B, r', and z' filters. Significantly deeper transits
are measured in these optical bandpasses relative to the near-infrared
wavelengths. We conclude that scattering by high-altitude haze alone is
unlikely to account for this difference, and instead interpret it as evidence
for titanium oxide and vanadium oxide absorption. Enhanced opacity is also
inferred across the 1.12-1.3 micron wavelength range, possibly due to iron
hydride absorption. If confirmed, WASP-121b will be the first exoplanet with
titanium oxide, vanadium oxide, and iron hydride detected in transmission. The
latter are important species in M/L dwarfs, and their presence is likely to
have a significant effect on the overall physics and chemistry of the
atmosphere, including the production of a strong thermal inversion.",1604.02310v1
2016-04-14,Ultrafast mid-infrared nanoscopy of strained vanadium dioxide nanobeams,"Long regarded as a model system for studying insulator-to-metal phase
transitions, the correlated electron material vanadium dioxide (VO$_2$) is now
finding novel uses in device applications. Two of its most appealing aspects
are its accessible transition temperature ($\sim$341 K) and its rich phase
diagram. Strain can be used to selectively stabilize different VO$_2$
insulating phases by tuning the competition between electron and lattice
degrees of freedom. It can even break the mesoscopic spatial symmetry of the
transition, leading to a quasi-periodic ordering of insulating and metallic
nanodomains. Nanostructuring of strained VO$_2$ could potentially yield unique
components for future devices. However, the most spectacular property of VO$_2$
- its ultrafast transition - has not yet been studied on the length scale of
its phase heterogeneity. Here, we use ultrafast near-field microscopy in the
mid-infrared to study individual, strained VO$_2$ nanobeams on the 10 nm scale.
We reveal a previously unseen correlation between the local steady-state
switching susceptibility and the local ultrafast response to below-threshold
photoexcitation. These results suggest that it may be possible to tailor the
local photo-response of VO$_2$ using strain and thereby realize new types of
ultrafast nano-optical devices.",1604.04306v1
2016-06-30,Quantum phase transitions and multicriticality in Ta(Fe1-xVx)2,"We present a comprehensive study of synthesis, structure analysis, transport
and thermodynamic properties of the C14 Laves phase Ta(Fe1-xVx)2. Our
measurements confirm the appearance of spin-density wave (SDW) order within a
dome-like region of the x - T phase diagram with vanadium content 0.02 < x <
0.3. Our results indicate that on approaching TaFe2 from the vanadium-rich
side, ferromagnetic (FM) correlations increase faster than the
antiferromagnetic (AFM) ones. This results in an exchange-enhanced
susceptibility and in the suppression of the SDW transition temperature for x <
0.13 forming the dome-like shape of the phase diagram. This effect is strictly
related to a significant lattice distortion of the crystal structure manifested
in the c/a ratio. At x = 0.02 both FM and AFM energy scales have similar
strength and the system remains paramagnetic down to 2 K with an extremely
large Stoner enhancement factor of about 400. Here, spin fluctuations dominate
the temperature dependence of the resistivity \rho ~ T ^ 3/2 and of the
specific heat C/T ~ - log(T) which deviate from their conventional Fermi liquid
forms, inferring the presence of a quantum critical point of dual nature.",1606.09416v1
2016-09-16,Ab initio calculations to support accurate modelling of the rovibronic spectroscopy calculations of vanadium monoxide (VO),"Accurate knowledge of the rovibronic near-infrared and visible spectra of
vanadium monoxide (VO) is very important for studies of cool stellar and hot
planetary atmospheres. Here, the required ab initio dipole moment and
spin-orbit coupling curves for VO are produced. This data forms the basis of a
new VO line list considering 13 different electronic states and containing over
277 million transitions. Open shell transition, metal diatomics are challenging
species to model through ab initio quantum mechanics due to the large number of
low-lying electronic states, significant spin-orbit coupling and strong static
and dynamic electron correlation. Multi-reference configuration interaction
methodologies using orbitals from a complete active space self-consistent-field
(CASSCF) calculation are the standard technique for these systems. We use
different state-specific or minimal-state CASSCF orbitals for each electronic
state to maximise the calculation accuracy. The off-diagonal dipole moment
controls the intensity of electronic transitions. We test finite-field
off-diagonal dipole moments, but found that (1) the accuracy of the excitation
energies were not sufficient to allow accurate dipole moments to be evaluated
and (2) computer time requirements for perpendicular transitions were
prohibitive. The best off-diagonal dipole moments are calculated using
wavefunctions with different CASSCF orbitals.",1609.05073v1
2017-03-21,"Anomalous Lattice Behavior of Vanadium Pentaoxide (V2O5): X- Ray Diffraction, Inelastic Neutron Scattering and ab-initio Lattice Dynamics","We present the structural and dynamical studies of layered vanadium
pentaoxide (V2O5). The temperature dependent X-ray diffraction measurements
reveal highly anisotropic and anomalous thermal expansion from 12 K to 853 K.
The results do not show any evidence of structural phase transition or
decomposition of {\alpha}-V2O5, contrary to the previous transmission electron
microscopy (TEM) and electron energy loss spectroscopy (EELS) experiments. The
inelastic neutron scattering measurements performed up to 673 K corroborate the
result of our X-ray diffraction measurements. The analysis of the experimental
data is carried out using ab-initio lattice dynamics calculation. The important
role of van der-Waals dispersion and Hubbard interactions on the structure and
dynamics is revealed through the ab-initio calculations. The calculated
anisotropic thermal expansion behavior agrees well with temperature dependent
X- ray diffraction. The mechanism of anisotropic thermal expansion and
anisotropic linear compressibility is discussed in terms of calculated
anisotropy in Gr\""uneisen parameters and elastic coefficients. The calculated
Gibbs free energy in various phases of V2O5 is used to understand the high
pressure and temperature phase diagram of the compound. Softening of elastic
constant (C66) with pressure suggests a possibility of shear mechanism for
{\alpha} to \b{eta} phase transformation under pressure.",1703.07132v1
2017-05-04,Early Solar System irradiation quantified by linked vanadium and beryllium isotope variations in meteorites,"X-ray emission in young stellar objects (YSOs) is orders of magnitude more
intense than in main sequence stars1,2, suggestive of cosmic ray irradiation of
surrounding accretion disks. Protoplanetary disk irradiation has been detected
around YSOs by HERSCHEL3. In our solar system, short-lived 10Be (half-life =
1.39 My4), which cannot be produced by stellar nucleosynthesis, was discovered
in the oldest solar system solids, the calcium-aluminium-rich inclusions
(CAIs)5. The high 10Be abundance, as well as detection of other irradiation
tracers6,7, suggest 10Be likely originates from cosmic ray irradiation caused
by solar flares8. Nevertheless, the nature of these flares (gradual or
impulsive), the target (gas or dust), and the duration and location of
irradiation remain unknown. Here we use the vanadium isotopic composition,
together with initial 10Be abundance to quantify irradiation conditions in the
early Solar System9. For the initial 10Be abundances recorded in CAIs, 50V
excesses of a few per mil relative to chondrites have been predicted10,11. We
report 50V excesses in CAIs up to 4.4 per mil that co-vary with 10Be abundance.
Their co-variation dictates that excess 50V and 10Be were synthesised through
irradiation of refractory dust. Modelling of the production rate of 50V and
10Be demonstrates that the dust was exposed to solar cosmic rays produced by
gradual flares for less than 300 years at about 0.1 au from the protoSun.",1705.01808v1
2017-05-25,Topological Weyl and Node-Line Semimetals in Ferromagnetic Vanadium-Phosphorous-Oxide $β$-V$_2$OPO$_4$ Compound,"We propose that the topological semimetal features can co-exist with
ferromagnetic ground state in vanadium-phosphorous-oxide $\beta$-V$_2$OPO$_4$
compound from first-principles calculations. In this magnetic system with
inversion symmetry, the direction of magnetization is able to manipulate the
symmetric protected band structures from a node-line type to a Weyl one in the
presence of spin-orbital-coupling. The node-line semimetal phase is protected
by the mirror symmetry with the reflection-invariant plane perpendicular to
magnetic order. Within mirror symmetry breaking due to the magnetization along
other directions, the gapless node-line loop will degenerate to only one pair
of Weyl points protected by the rotational symmetry along the magnetic axis,
which are largely separated in momentum space. Such Weyl semimetal phase
provides a nice candidate with the minimum number of Weyl points in a condensed
matter system. The results of surface band calculations confirm the non-trivial
topology of this proposed compound. This findings provide a realistic candidate
for the investigation of topological semimetals with time-reversal symmetry
breaking, particularly towards the realization of quantum anomalous Hall effect
in Weyl semimetals.",1705.09234v1
2017-07-25,Characteristics of superconducting state in vanadium: the Eliashberg equations and semi-analytical formulas,"The superconducting state in vanadium characterizes with the critical
temperature ($T_{c}$) equal to $5.3$~K. The Coulomb pseudopotential, calculated
with the help of the Eliashberg equations, possesses anomalously high value
$\mu^{\star}\left(3\Omega_{\rm max}\right)=0.259$ or
$\mu^{\star}\left(10\Omega_{\rm max}\right)=0.368$ ($\Omega_{\rm max}$ denotes
the maximum phonon frequency). Despite the relatively large electron-phonon
coupling constant ($\lambda=0.91$), the quantities such as: the order parameter
($\Delta$), the specific heat ($C$), and the thermodynamic critical field
($H_{c}$) determine the values of the dimensionless ratios not deviating much
from the predictions of the BCS theory: $R_{\Delta}=2\Delta\left(0\right)/
k_{B}T_{c}=3.68$, $R_{C}=\Delta C\left(T_{c}\right)/
C^{N}\left(T_{c}\right)=1.69$, and $R_{H}=T_{c}C^{N}\left(T_{c}\right)/
H^{2}_{c}\left(0\right)=0.171$. This result is associated with the reduction of
the strong-coupling and the retardation effects by the high value of the
Coulomb pseudopotential. It has been shown that the results of the Eliashberg
formalism can be relatively precisely reproduced with the help of the
semi-analytical formulas, if the value of $\mu^{\star}$ is determined on the
basis of the $T_{c}$-Allen-Dynes expression ($\mu^{\star}_{AD}=0.198$). The
attention should be paid to the fact that in the numerical and in the
semi-analytical approach the comparable values of the thermodynamic parameters
for the same $\mu^{\star}$ have been obtained only in the vicinity of the point
$\mu^{\star}=0.1$.",1707.08228v1
2018-03-05,Weak metal-metal transition in the vanadium oxytelluride Rb$_{1-δ}$V$_2$Te$_2$O,"We report the synthesis, crystal structure, physical properties, and
first-principles calculations of a vanadium-based oxytelluride
Rb$_{1-\delta}$V$_2$Te$_2$O ($\delta\approx0.2$). The crystal structure bears
two-dimensional V$_2$O square nets sandwiched with tellurium layers, mimicking
the structural motifs of cuprate and iron-based superconductors. The material
exhibits metallic conductivity with dominant hole-type charge carriers. A weak
metal-to-metal transition takes place at $\sim$100 K, which is conformably
characterized by a slight kink/hump in the electrical resistivity, jumps in the
Hall and Seebeck coefficients, a minute drop in the magnetic susceptibility,
and a small peak in the heat capacity. Neither Bragg-peak splitting nor
superlattice reflections can be detected within the resolution of conventional
x-ray diffractions. The band-structure calculations show that V-3$d$ orbitals
dominate the electronic states at around Fermi energy where a $d_{yz}/d_{xz}$
orbital polarization shows up. There are three Fermi-surface sheets that seem
unfavorable for nesting. Our results suggest an orbital or spin-density-wave
order for the low-temperature state and, upon suppression of the competing
order, emergence of superconductivity could be expected.",1803.01605v2
2018-04-11,Universal renormalization group flow toward perfect Fermi-surface nesting driven by enhanced electron-electron correlations in monolayer vanadium diselenide,"In the present study we examine nature of a charge ordering transition in
monolayer vanadium diselenide ($VSe_{2}$), which would be distinguished from
that of $VSe_{2}$ bulk samples, driven by more enhanced electron-electron
correlations. Recently, angle resolved photoemission spectroscopy measurements
uncovered that the Fermi surface nesting becomes perfect, where the dynamics of
hot electrons is dispersionless along the orthogonal direction of the nesting
wave-vector. In addition, scanning tunneling microscopy measurements confirmed
that the resulting CDW state shows essentially the same modulation pattern as
the three dimensional system of $VSe_{2}$. Here, we perform the renormalization
group analysis based on an effective field theory in terms of critical CDW
fluctuations and hot electrons of imperfect Fermi-surface nesting. As a result,
we reveal that the imperfect nesting universally flows into perfect nesting in
two dimensions, where the Fermi velocity along the orthogonal direction of the
nesting vector vanishes generically. We argue that this electronic
reconstruction is responsible for the observation that the CDW transition
temperature is much more enhanced to be around $T_{c} > 300$ $K$ than that of
the bulk sample.",1804.03766v3
2018-05-03,How optical excitation controls the structure and properties of vanadium dioxide,"We combine ultrafast electron diffraction and time-resolved terahertz
spectroscopy measurements to unravel the connection between structure and
electronic transport properties during the photoinduced insulator-metal
transitions in vanadium dioxide. We determine the structure of the metastable
monoclinic metal phase, which exhibits anti-ferroelectric charge order arising
from a thermally activated, orbital-selective phase transition in the electron
system. The relative contribution of this photoinduced monoclinic metal (which
has no equilibrium analog) and the photoinduced rutile metal (known from the
equilibrium phase diagram) to the time and pump-fluence dependent multi-phase
character of the film is established, as is the respective impact of these two
distinct phase transitions on the observed changes in terahertz conductivity.
Our results represent an important new example of how light can control the
properties of strongly correlated materials and elucidate that multi-modal
experiements are essential when seeking a detailed connection between ultrafast
changes in optical-electronic properties and lattice structure in complex
materials.",1805.01430v1
2018-11-19,"Elastic anisotropy and thermal properties of extended linear chain compounds MV$_2$Ga$_4$ (M = Sc, Zr, Hf) from ab-initio calculations","MV$_2$Ga$_4$ (M = Sc, Zr, Hf) compounds belong to an emerging class of
materials showing a unique combination of unusual superconducting behavior with
extended linear chains in the crystal structure. In order to gain insights
{into} its mechanical and thermal properties, we have performed
first-principles electronic-structure calculations in the framework of the
Density Functional Theory (DFT). From the calculated second-order elastic
constants, we have systematically shown that the extended linear vanadium chain
substructures indeed give rise to an anisotropic regime in the elastic and
mechanical moduli. The high density of valence and conduction electrons along
the linear vanadium chains leads to a directional dependence of the reciprocal
linear compressibility, Young's modulus and shear modulus. Poisson's ratio for
several elongation directions is also drastically affected by the presence of
extended V chains. If the elongation is along the V chains, all compounds
exhibit {practically} the same Poisson ratio in directions perpendicular to it,
further highlighting the importance of the V chains to the mechanical
properties. Moreover, based on our results, we have discussed the possible
consequences of the elastic anisotropy on the superconducting properties of the
compounds. Finally, using the Debye-Gr\""uneisen approximation, our calculations
of thermal properties show {a good agreement with the available experimental
low temperature heat capacity data above the superconducting critical
temperature.",1811.07814v1
2018-11-22,Defect-induced orbital polarization and collapse of orbital order in doped vanadium perovskites,"We explore mechanisms of orbital order decay in doped Mott insulators
$R_{1-x}$(Sr,Ca)$_x$VO$_3$ ($R=\,$Pr,Y,La) caused by charged (Sr,Ca) defects.
Our unrestricted Hartree-Fock analysis focuses on the combined effect of
random, charged impurities and associated doped holes up to $x=0.5$. The study
is based on a generalized multi-band Hubbard model for the relevant vanadium
$t_{2g}$ electrons, and includes the long-range (i) Coulomb potentials of
defects and (ii) electron-electron interactions. We show that the rotation of
occupied $t_{2g}$ orbitals, induced by the electric field of defects, is a very
efficient perturbation that largely controls the suppression of orbital order
in these compounds. We investigate the inverse participation number spectra and
find that electron states remain localized on few sites even in the regime
where orbital order is collapsed. From the change of kinetic and superexchange
energy we can conclude that the motion of doped holes, which is the dominant
effect for the reduction of magnetic order in high-$T_c$ compounds, is of
secondary importance here.",1811.09009v2
2019-01-08,Optical properties of thin-film vanadium dioxide from the visible to the far infrared,"The insulator-to-metal transition (IMT) in vanadium dioxide (VO2) can enable
a variety of optics applications, including switching and modulation, optical
limiting, and tuning of optical resonators. Despite the widespread interest in
optics, the optical properties of VO2 across its IMT are scattered throughout
the literature, and are not available in some wavelength regions. We
characterized the complex refractive index of VO2 thin films across the IMT for
free-space wavelengths from 300 nm to 30 {\mu}m, using broadband spectroscopic
ellipsometry, reflection spectroscopy, and the application of effective-medium
theory.
  We studied VO2 thin films of different thickness, on two different substrates
(silicon and sapphire), and grown using different synthesis methods (sputtering
and sol gel). While there are differences in the optical properties of VO2
synthesized under different conditions, they are relatively minor compared to
the change resulting from the IMT, most notably in the ~2 - 11 {\mu}m range
where the insulating phase of VO2 has relatively low optical loss. We found
that the macroscopic optical properties of VO2 are much more robust to
sample-to-sample variation compared to the electrical properties, making the
refractive-index datasets from this article broadly useful for modeling and
design of VO2-based optical and optoelectronic components.",1901.02517v1
2019-01-25,"Tailoring Vanadium Dioxide Film Orientation using Nanosheets: A Combined Microscopy, Diffraction, Transport and Soft X-ray in Transmission Study","VO2 is a much-discussed material for oxide electronics and neuromorphic
computing applications. Here, heteroepitaxy of vanadium dioxide (VO2) was
realized on top of oxide nanosheets that cover either the amorphous silicon
dioxide surfaces of Si substrates or X-ray transparent silicon nitride
membranes. The out-of-plane orientation of the VO2 thin films was controlled at
will between (011)M1/(110)R and (-402)M1/(002)R by coating the bulk substrates
with Ti0.87O2 and NbWO6 nanosheets, respectively, prior to VO2 growth.
Temperature dependent X-ray diffraction and automated crystal orientation
mapping in microprobe TEM mode (ACOM-TEM) characterized the high phase purity,
the crystallographic and orientational properties of the VO2 films. Transport
measurements and soft X-ray absorption in transmission are used to probe the
VO2 metal-insulator transition, showing results of a quality equal to those
from epitaxial films on bulk single-crystal substrates. Successful local
manipulation of two different VO2 orientations on a single substrate is
demonstrated using VO2 grown on lithographically-patterned lines of Ti0.87O2
and NbWO6 nanosheets investigated by electron backscatter diffraction. Finally,
the excellent suitability of these nanosheet-templated VO2 films for advanced
lensless imaging of the metal-insulator transition using coherent soft X-rays
is discussed.",1901.08823v1
2019-06-21,Quantum Oscillations of Robust Topological Surface States up to 50 K in Thick Bulk-insulating Topological Insulator,"As personal electronic devices increasingly rely on cloud computing for
energy-intensive calculations, the power consumption associated with the
information revolution is rapidly becoming an important environmental issue.
Several approaches have been proposed to construct electronic devices with low
energy consumption. Among these, the low-dissipation surface states of
topological insulators (TIs) are widely employed. To develop TI-based devices,
a key factor is the maximum temperature at which the Dirac surface states
dominate the transport behavior. Here, we employ Shubnikov-de Haas oscillations
(SdH) as a means to study the surface state survival temperature in a high
quality vanadium doped Bi1.08Sn0.02Sb0.9Te2S single crystal system. The
temperature and angle dependence of the SdH show that: 1) crystals with
different vanadium (V) doping levels are insulating in the 3-300 K region, 2)
the SdH oscillations show two-dimensional behavior, indicating that the
oscillations arise from the pure surface states; and 3) at 50 K, the V0.04
single crystals (Vx:Bi1.08-xSn0.02Sb0.9Te2S, where x = 0.04) still show clear
sign of SdH oscillations, which demonstrate that the surface dominant transport
behavior can survive above 50 K. The robust surface states in our V doped
single crystal systems provide an ideal platform to study the Dirac fermions
and their interaction with other materials above 50 K.",1906.09953v1
2019-06-25,Unraveling the Mott-Peierls intrigue in Vanadium dioxide,"Vanadium dioxide is one of the most studied strongly correlated materials.
Nonetheless, the intertwining between electronic correlation and lattice
effects has precluded a comprehensive description of the rutile metal to
monoclinic insulator transition, in turn triggering a longstanding ""the chicken
or the egg"" debate about which comes first, the Mott localisation or the
Peierls distortion. Here, we suggest that this problem is in fact ill-posed:
the electronic correlations and the lattice vibrations conspire to stabilise
the monoclinic insulator, and so they must be both considered not to miss
relevant pieces of the VO$_2$ physics. Specifically, we design a minimal model
for VO$_2$ that includes all the important physical ingredients: the electronic
correlations, the multi-orbital character, and the two components
antiferrodistortive mode that condenses in the monoclinic insulator. We solve
this model by dynamical mean-field theory within the adiabatic Born-Oppenheimer
approximation. Consistently with the first-order character of the
metal-insulator transition, the Born-Oppenheimer potential has a rich
landscape, with minima corresponding to the undistorted phase and to the four
equivalent distorted ones, and which translates into an equally rich
thermodynamics that we uncover by the Monte Carlo method. Remarkably, we find
that a distorted metal phase intrudes between the low-temperature distorted
insulator and high-temperature undistorted metal, which sheds new light on the
debated experimental evidence of a monoclinic metallic phase.",1906.10632v2
2019-08-13,Competing Phases in Epitaxial Vanadium Dioxide at Nanoscale,"Phase competition in correlated oxides offers tantalizing opportunities as
many intriguing physical phenomena occur near the phase transitions. Owing to a
sharp metal-insulator transition (MIT) near room temperature, correlated
vanadium dioxide (VO2) exhibits a strong competition between insulating and
metallic phases that is important for practical applications. However, the
phase boundary undergoes strong modification when strain is involved, yielding
complex phase transitions. Here, we report the emergence of the nanoscale M2
phase domains in VO2 epitaxial films under anisotropic strain relaxation. The
phase states of the films are imaged by multi-length-scale probes, detecting
the structural and electrical properties in individual local domains. Competing
evolution of the M1 and M2 phases indicates a critical role of lattice-strain
on both the stability of the M2 Mott phase and the energetics of the MIT in VO2
films. This study demonstrates how strain engineering can be utilized to design
phase states, which allow deliberate control of MIT behavior at the nanoscale
in epitaxial VO2 films.",1908.04774v1
2020-04-07,Robustness of superconducting properties to transition metal substitution and impurity phases in Fe1-xVxSe,"We have performed transverse- and zero-field muon spin rotation/relaxation
experiments, as well as magnetometry measurements, on samples of Fe1-xVxSe and
their Li+NH3 intercalates Li0.6(NH2)0.2(NH3)0.8 Fe1-x Vx Se. We examine the low
vanadium substitution regime: x = 0.005, 0.01, and 0.02. The intercalation
reaction significantly increases the critical temperature (Tc) and the
superfluid stiffness for all x. The nonintercalated samples all exhibit Tc =
8.5 K while the intercalated samples all show an enhanced Tc > 40 K. Vanadium
substitution has a negligible effect on Tc, but seems to suppress the
superfluid stiffness for the nonintercalated samples and weakly enhance it for
the intercalated materials. The optimal substitution level for the intercalated
samples is found to be x = 0.01, with Tc = 41 K and {\lambda}_{ab}(0) = 0.18
{\mu}m. The nonintercalated samples can be modeled with either a single d-wave
superconducting gap or with an anisotropic gap function based on recent
quasiparticle imaging experiments, whereas the intercalates display multigap
nodal behavior which can be fitted using s + d- or d + d-wave models.
Magnetism, likely from iron impurities, appears after the intercalation
reaction and coexists and competes with the superconductivity. However, it
appears that the superconductivity is remarkably robust to the impurity phase,
providing an avenue to stably improve the superconducting properties of
transition metal substituted FeSe.",2004.03625v1
2020-07-14,Temperature Dependence of Reconfigurable Bandstop Filters Using Vanadium Dioxide Switches,"In this letter we report and investigate the temperature dependency of
various radio frequency parameters (RF) for a fabricated reconfigurable
bandstop filter with vanadium dioxide (VO2) switches measured up to 55 GHz.
Here the insulator to metal (ITM) and metal to insulator transition (MIT)
hysteresis of the VO2 thin film influence on the RF characteristics of the
filters is analyzed from 25 {\deg}C and 120 {\deg}C in heating and cooling. The
resonance frequency and maximum insertion loss (IL) stability and sensitivity
with temperature variations are explored. It is noticed that increasing the
temperature with 50 {\deg}C from 25 {\deg}C (or decreasing it with 50 {\deg}C
from 120 {\deg}C) will result in a less than 1% fractional frequency shift in
respect to the off and on resonance frequencies. The sharp DC conductivity
levels variations of the VO2 thin film around the transition temperatures
translate into sharp effects on the resonance characteristics of the filters.
On the contrary, the maximum IL levels are less sensitive to the DC films sharp
conductivity changes around the VO2 transition temperature. Last, we see that
the RF parameters in heating and cooling at 80 {\deg}C, above (but close to)
the DC transition temperatures of VO2 exhibit completely different resonance
frequencies. The RF results reported close to the transition temperatures for
the VO2 thin films can diverge in heating and cooling, thus of a more
insightful understanding of VO2 reconfigurable RF devices has to include
temperature dependent measurements at various temperatures below MIT and ITM in
the RF ranges too",2007.07152v1
2020-09-02,Understanding modes of negative differential resistance in amorphous and polycrystalline vanadium oxides,"Metal-oxide-metal devices based on amorphous VOx are shown to exhibit one of
two distinct negative differential resistance (NDR) characteristics depending
on the maximum current employed for electroforming. For low compliance currents
they exhibit a smooth S-type characteristic and have a temperature-dependent
device resistance characterised by an activation energy of 0.25 eV, consistent
with conduction in polycrystalline VO2, while for high-compliance currents they
exhibit an abrupt snap-back characteristic and a resistance characterised by an
activation energy of 0.025 eV, consistent with conduction in oxygen deficient
VOx. In both cases, the temperature dependence of the switching voltage implies
that the conductivity change is due to the insulator-metal transition in VO2.
From this analysis it is concluded that electroforming at low currents creates
a conductive filament comprised largely of polycrystalline VO2, while
electroforming at high currents creates a composite structure comprised of VO2
and a conductive halo of oxygen deficient VOx. The effect of electroforming on
the NDR mode is then explained with reference to a lumped element model of
filamentary conduction that includes the effect of a parallel resistance
created by the halo. These results provide new insight into the NDR response of
vanadium-oxide-based devices and a basis for designing devices with specific
characteristics.",2009.00810v1
2021-01-29,Tunable Doping of Rhenium and Vanadium into Transition Metal Dichalcogenides for Two-Dimensional Electronics,"Two-dimensional (2D) transition metal dichalcogenides (TMDCs) with unique
electrical properties are fascinating materials used for future electronics.
However, the strong Fermi level pinning effect at the interface of TMDCs and
metal electrodes always leads to high contact resistance, which seriously
hinders their application in 2D electronics. One effective way to overcome this
is to use metallic TMDCs or transferred metal electrodes as van der Waals (vdW)
contacts. Alternatively, using highly conductive doped TMDCs will have a
profound impact on the contact engineering of 2D electronics. Here, a novel
chemical vapor deposition using mixed molten salts is established for
vapor-liquid-solid growth of high-quality rhenium (Re) and vanadium (V)-doped
TMDC monolayers with high controllability and reproducibility. A tunable
semiconductor to metal transition is observed in the Re and V-doped TMDCs.
Electrical conductivity increases up to a factor of 108 in the degenerate
V-doped WS2 and WSe2. Using V-doped WSe2 as vdW contact, the on-state current
and on/off ratio of WSe2-based field-effect transistors have been substantially
improved (from ~10-8 to 10-5 A; ~104 to 108), compared to metal contacts.
Future studies on lateral contacts and interconnects using doped TMDCs will
pave the way for 2D integrated circuits and flexible electronics.",2101.12423v1
2021-01-29,Doping isolated one-dimensional antiferro-magnetic semiconductor Vanadium tetrasulfide ($VS_4$) nanowires with carriers induces half-metallicity,"Quasi one-dimensional (1D) vanadium tetrasulfide ($VS_4$) nanowires (NWs) are
synthetic semiconductors which combine with each other through Van der Waals
interactions to form bulk phases. However, the properties of these individual
nanowires remain unknown. Nevertheless, our calculations of their stability
indicate that $VS_4$) NWs can be separated from their bulk structures.
Accordingly, we theoretically investigated the geometrical, electronic, and
magnetic properties of bulk phase and isolated $VS_4$ NWs. Our results indicate
that both bulk phase and isolated $VS_4$ NWs are semiconductors with band gaps
of 2.24 and 2.64 eV, respectively, and that they prefer the antiferromagnetic
(AFM) ground state based on DFT calculations. These calculations also suggested
that isolated $VS_4$ NWs show half-metallic antiferromagnetism upon electron
and hole doping because carrier doping splits the spin degeneracy to induce
local spin polarisation. As a result, spin polarisation currents in isolated
$VS_4$ NWs can be manipulated with locally applied gate voltage. Therefore,
these 1D AFM materials have a high potential for advancing both fundamental
research and spintronic applications because they are more resistant to
magnetic perturbation than their 1D ferromagnetic counterparts.",2101.12658v1
2021-02-16,Nodal superconductivity and superconducting domes in the topological Kagome metal CsV3Sb5,"Recently superconductivity was discovered in the Kagome metal AV3Sb5 (A = K,
Rb, and Cs), which has an ideal Kagome lattice of vanadium. These V-based
superconductors also host charge density wave (CDW) and topological nontrivial
band structure. Here we report the ultralow-temperature thermal conductivity
and high pressure resistance measurements on CsV3Sb5 with Tc = 2.5 K, the
highest among AV3Sb5. A finite residual linear term of thermal conductivity at
zero magnetic field and its rapid increase in fields suggest nodal
superconductivity. By applying pressure, the Tc of CsV3Sb5 increases first,
then decreases to lower than 0.3 K at 11.4 GPa, showing a clear first
superconducting dome peaked around 0.8 GPa. Above 11.4 GPa, superconductivity
re-emerges, suggesting a second superconducting dome. Both nodal
superconductivity and superconducting domes point to unconventional
superconductivity in this V-based superconductor. While our finding of nodal
superconductivity puts a strong constrain on the pairing state of the first
dome, which should be related to the CDW instability, the superconductivity of
the second dome may present another exotic pairing state in this ideal Kagome
lattice of vanadium.",2102.08356v2
2021-03-18,Plasmonically enhanced spectrally selective narrowband MWIR and LWIR light detection based on hybrid nanopatterned graphene and phase changing vanadium oxide heterostructure operating close to room temperature,"We present the model of an ultrasensitive mid-infrared (mid-IR) photodetector
operating in the mid-wavelength infrared (MWIR) and long-wavelength infrared
(LWIR) domains consisting of a hybrid heterostructure made of nanopatterned
graphene (NPG) and vanadium dioxide (VO$_2$) which exhibits a large
responsivity of $R\sim 10^4$ V/W, a detectivity exceeding $D^*\sim 10^{10}$ J,
and a sensitivity in terms of noise-equivalent power $\mathrm{NEP}\sim 100$
fW/$\sqrt{\rm Hz}$ close to room temperature by taking advantage of the phase
change of a thin VO$_2$ film. Our proposed photodetector can reach an
absorption of nearly 100\% in monolayer graphene due to localized surface
plasmons (LSPs) around the patterned circular holes. The geometry of the
nanopattern and an electrostatic gate potential can be used to tune the
absorption peak in the mid-IR regime between 3 and 12 $\mu$m. After the photon
absorption by the NPG sheet and the resulting phase change of VO$_2$ from
insulating to metallic phase the applied bias voltage $V_b$ triggers a current
through the VO$_2$ sheet, which can be detected electronically in about 1 ms,
shorter than the detection times of current VO$_2$ bolometers. Our envisioned
mid-IR photodetector reaches detectivities of cryogenically cooled HgCdTe
photodetectors and sensitivities larger than VO$_2$ microbolometers while
operating close to room temperature.",2103.10311v1
2021-05-04,Role of Sb in the superconducting kagome metal CsV$_3$Sb$_5$ revealed by its anisotropic compression,"Pressure evolution of the superconducting kagome metal CsV$_3$Sb$_5$ is
studied with single-crystal x-ray diffraction and density-functional
band-structure calculations. A highly anisotropic compression observed up to 5
GPa is ascribed to the fast shrinkage of the Cs-Sb distances and suppression of
Cs rattling motion. This prevents Sb displacements required to stabilize the
three-dimensional charge-density-wave (CDW) order and elucidates the
disappearance of the CDW already at 2 GPa despite only minor changes in the
electronic structure of the normal state. At higher pressures, vanadium bands
still change only marginally, whereas antimony bands undergo a major
reconstruction caused by the gradual formation of the interlayer Sb-Sb bonds.
Our results exclude pressure tuning of vanadium kagome bands as the main
mechanism for the non-trivial evolution of superconductivity in real-world
kagome metals. Concurrently, we establish the central role of Sb atoms in the
stabilization of a three-dimensional CDW and Fermi surface reconstruction.",2105.01397v4
2021-05-11,Emergence of new van Hove singularities in the charge density wave state of a topological kagome metal RbV3Sb5,"Quantum materials with layered kagome structures have drawn considerable
attention due to their unique lattice geometry, which gives rise to flat bands
co-existing with Dirac-like dispersions. The interplay between strong Coulomb
correlations and nontrivial band topology in these systems results in various
exotic phenomena. Recently, vanadium-based materials with layered kagome
structures are discovered to be topological metals, which exhibit charge
density wave (CDW) properties, significant anomalous Hall effect, and unusual
superconductivity at low temperatures. Here we exploit high-resolution
angle-resolved photoemission spectroscopy to investigate the electronic
structure evolution induced by the CDW transition in a vanadium-based kagome
material RbV3Sb5. A remarkable band renormalization in the CDW state is
observed, which is consistent with first principles calculations based on an
inverse star-of-David superstructure. The CDW phase transition gives rise to a
partial energy gap opening at the Fermi level, a shift in the band dispersion,
and most importantly, the emergence of new van Hove singularities associated
with large density of states, which are absent in the normal phase and may be
related to superconductivity observed at lower temperatures. Our work would
shed light on the microscopic mechanisms for the formation of the CDW and
superconducting states in these topological kagome metals.",2105.05117v1
2021-06-13,Switchable induced-transmission filters enabled by vanadium dioxide,"Abstract: An induced-transmission filter (ITF) uses an ultrathin layer of
metal positioned at an electric-field node within a dielectric thin-film
bandpass filter to select one transmission band while suppressing other
transmission bands that would have been present without the metal layer. Here,
we introduce a switchable mid-infrared ITF where the metal film can be
""switched on and off"", enabling the modulation of the filter response from
single-band to multiband. The switching is enabled by a deeply subwavelength
film of vanadium dioxide (VO2), which undergoes a reversible insulator-to-metal
phase transition. We designed and experimentally demonstrated an ITF that can
switch between two states: one broad passband across the long-wave infrared
(LWIR, 8 - 12 um) and one narrow passband at ~8.8 um. Our work generalizes the
ITF -- previously a niche type of bandpass filter -- into a new class of
tunable devices. Furthermore, our unique fabrication process -- which begins
with thin-film VO2 on a suspended membrane -- enables the integration of VO2
into any thin-film assembly that is compatible with physical vapor deposition
(PVD) processes, and is thus a new platform for realizing tunable thin-film
filters.",2106.07093v1
2021-07-01,"An oxygen-rich, tetrahedral surface phase on high-temperature rutile VO$_2$(110)$_\text{T}$ single crystals","Vanadium dioxide undergoes a metal-insulator transition (MIT) from an
insulating (monoclinic) to a metallic (tetragonal) phase close to room
temperature, which makes it a promising functional material for many
applications, e.g. as chemical sensors. Not much is known about its surface and
interface properties, although these are critical in many of its applications.
This work presents an atomic-scale investigation of the tetragonal rutile
VO$_2$(110)$_\text{T}$ single-crystal surface and reports results obtained with
scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and
X-ray photoelectron spectroscopy (XPS), supported by density-functional
theory-based (DFT) calculations. The surface reconstructs into an oxygen-rich
(2$\times$2) superstructure that coexists with small patches of the underlying,
unreconstructed (110)-(1$\times$1) surface. The best structural model for the
(2$\times$2) surface termination, conceptually derived from a vanadium
pentoxide (001) monolayer, consists of rings of corner-shared tetrahedra. Over
a wide range of oxygen chemical potentials this reconstruction is more stable
than the unreconstructed (110) surface as well as models proposed in the
literature.",2107.00350v1
2021-10-11,Absolute calibration of the latent heat of transition using differential thermal analysis,"We describe a simple and accurate differential thermal analysis set up to
measure the latent heat of solid state materials undergoing abrupt phase
transitions in the temperature range from 77 K to above room temperature. We
report a numerical technique for the absolute calibration of the latent heat of
the transition, without the need of a reference sample. The technique is
applied to three different samples -- vanadium sesquioxide undergoing the Mott
transition, bismuth barium ruthenate undergoing a magnetoelastic transition,
and an intermetallic Heusler compound. In each case, the inferred latent heat
value agrees with the literature value to within its error margins. To further
demonstrate the importance of absolute calibration, we show that the changes in
the latent heat of the Mott transition in vanadium sesquioxide (V$_2$O$_3$)
stays constant to within 2% even as the depth of supersaturation changes by
about 10 K, in non-equilibrium dynamic hysteresis measurements. We also apply
this technique for the measurement of the temperature-dependent specific heat.",2110.05046v1
2021-10-18,Superconductivity modulated by structural phase transitions in pressurized vanadium-based kagome metals,"The interplay of superconductivity with electronic and structural
instabilities on the kagome lattice provides a fertile ground for the emergence
of unusual phenomena. The vanadium-based kagome metals $A$V$_3$Sb$_5$ ($A=$ K,
Rb, Cs) exhibit superconductivity on an almost ideal kagome lattice, with the
superconducting transition temperature $T_{\rm c}$ forming two domes upon
pressure-tuning. The first dome arises from the competition between
superconductivity and a charge-density-wave, whereas the origin for the second
dome remains unclear. Herein, we show that the appearance of the second
superconducting dome in KV$_3$Sb$_5$ and RbV$_3$Sb$_5$ is associated with
transitions from hexagonal $P6$/$mmm$ to monoclinic $P2$/$m$ structures,
evidenced by splitting of structural peaks from synchrotron powder X-ray
diffraction experiments and imaginary phonon frequencies in first-principles
calculations. In KV$_3$Sb$_5$, transition to an orthorhombic $Pmmm$ structure
is further observed for pressure $p\gtrsim20$ GPa, and is correlated with the
strong suppression of $T_{\rm c}$ in the second superconducting dome. Our
findings indicate distortions of the crystal structure modulates
superconductivity in $A$V$_3$Sb$_5$ under pressure, providing a platform to
study the emergence of superconductivity in the presence of multiple structural
instabilities.",2110.09056v1
2022-01-11,Recent advance in phase transition of vanadium oxide based solar reflectors and the fabrication progress,"Vanadium dioxide (VO2) as a phase-change material controls the transferred
heat during phase transition process between metal and insulator states. At
temperature above 68C, the rutile structure VO2 keeps the heat out and
increases the IR radiation reflectivity, while at the lower temperature the
monoclinic structure VO2 acts as the transparent material and increase the
transmission radiation. In this paper, we first present the metal-insulator
phase transition (MIT) of the VO2 in high and low temperatures. Then we
simulate the meta-surface VO2 of metamaterial reflector by Ansys HFSS to show
the emittance tunability of the rutile and monoclinic phase of the VO2. In next
section, we will review the recent progress in the deposition of thermochromic
VO2 on glass and silicon substrate with modifying the pressure of sputtering
gases and temperature of the substrate. Finally, we present the results of the
in-situ sputtered VOx thin film on thick SiO2 substrate in different
combination of oxygen and argon environment by V2O5 target at temperature
higher than 300C and then, analyze it with x-ray diffraction (XRD) method. The
thermochromic VO2 based metamaterial structures open a new route to the passive
energy-efficient optical solar reflector in the past few years.",2201.03722v2
2022-03-03,Enhanced physics-constrained deep neural networks for modeling vanadium redox flow battery,"Numerical modeling and simulation have become indispensable tools for
advancing a comprehensive understanding of the underlying mechanisms and
cost-effective process optimization and control of flow batteries. In this
study, we propose an enhanced version of the physics-constrained deep neural
network (PCDNN) approach [1] to provide high-accuracy voltage predictions in
the vanadium redox flow batteries (VRFBs). The purpose of the PCDNN approach is
to enforce the physics-based zero-dimensional (0D) VRFB model in a neural
network to assure model generalization for various battery operation
conditions. Limited by the simplifications of the 0D model, the PCDNN cannot
capture sharp voltage changes in the extreme SOC regions. To improve the
accuracy of voltage prediction at extreme ranges, we introduce a second
(enhanced) DNN to mitigate the prediction errors carried from the 0D model
itself and call the resulting approach enhanced PCDNN (ePCDNN). By comparing
the model prediction with experimental data, we demonstrate that the ePCDNN
approach can accurately capture the voltage response throughout the
charge--discharge cycle, including the tail region of the voltage discharge
curve. Compared to the standard PCDNN, the prediction accuracy of the ePCDNN is
significantly improved. The loss function for training the ePCDNN is designed
to be flexible by adjusting the weights of the physics-constrained DNN and the
enhanced DNN. This allows the ePCDNN framework to be transferable to battery
systems with variable physical model fidelity.",2203.01985v1
2022-05-25,Hidden magnetic order on a kagome lattice for KV3Sb5,"KV3Sb5 has recently attracted a considerable attention, due to its low
temperature superconducting properties, which are heralded by a charge density
wave. The apparent presence of a very weak magnetism does not result in long
range ordering. We propose a model compatible with a detectable internal
magnetic field with no evidence of magnetic long-range order. It invokes higher
order terms in the vanadium magnetization density compatible with the presence
of a ""hidden order"" of Dirac (polar) multipoles. The Dirac dipole, known as an
anapole or toroidal dipole, is one of a family of electronic multipoles visible
in x-ray and magnetic neutron diffraction while undetectable with standard
laboratory-based techniques. Two plausible hidden orders are studied with a
view to testing in future x-ray and neutron Bragg diffraction experiments
whether they are trustworthy. One candidate is magneto-electric and restricted
to the linear type, while the other candidate cannot show a magneto-electric
effect of any type. The latter hosts a vanadium entity that is both a true
scalar and magnetic, and its presence epitomizes an absence of loop currents.
Diffraction patterns for the two proposed hidden magnetic orders are distinctly
different, fortunately. Corresponding scattering amplitudes for resonant x-ray
and neutron Bragg diffraction are calculated with standard chemical and
magnetic symmetry tools, and atomic physics.",2205.12657v1
2022-06-06,"Anisotropic magnetic property of single crystals $R$V$_6$Sn$_6$ ($R$ = Y, Gd - Tm, Lu)","$R$V${_6}$Sn${_6}$ ($R$ = Y, Gd - Tm, Lu) single crystals are synthesized by
Sn-flux method and their physical properties are characterized by
magnetization, resistivity, and specific heat measurements. Powder X-ray
diffraction patterns of all samples can be well indexed with the hexagonal
HfFe$_6$Ge$_6$-type structure, where rare-earth atoms form hexagonal layers and
vanadium atoms form Kagome layers. At high temperatures, magnetic
susceptibility measurements of moment bearing rare-earths ($R$ = Gd - Tm)
follow Curie-Weiss behavior. Effective moments estimated from the
polycrystalline average of magnetic susceptibility curves are consistent with
the values for free $R^{3+}$ ion. Strong magnetic anisotropy due to crystalline
electric field effects is observed for moment bearing rare-earths, except
GdV$_6$Sn$_6$. The easy magnetization direction is determined to be $c$-axis
for $R$ = Tb - Ho and $ab$-plane for $R$ = Er, and Tm. The vanadium ions in
$R$V${_6}$Sn${_6}$ possess no magnetic moment. The compounds for $R$ = Y and Lu
exhibit typical characteristics of paramagnetic metals. At low temperatures,
the magnetic ordering is confirmed from magnetization, specific heat, and
resistivity: the highest $T_{N} = 4.9$~K for GdV$_6$Sn$_6$ and the lowest
$T_{N} = 2.3$~K for HoV$_6$Sn$_6$. No magnetic ordering is observed down to
1.8~K for $R$ = Er and Tm. A slight deviation of the magnetic ordering
temperature from the de Gennes scaling suggests the dominant
Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between rare-earth
moments in metallic $R$V${_6}$Sn${_6}$ compounds.",2206.02924v1
2022-06-23,Complementary Vanadium Dioxide Metamaterials with Enhanced Modulation Amplitude at Terahertz Frequencies,"One route to create tunable metamaterials is through integration with
""on-demand"" dynamic quantum materials, such as vanadium dioxide (VO2). This
enables new modalities to create high performance devices for historically
challenging applications. Indeed, dynamic materials have often been integrated
with metamaterials to imbue artificial structures with some degree of
tunability. Conversely, metamaterials can be used to enhance and extend the
natural tuning range of dynamic materials. Utilizing a complementary split ring
resonator array deposited on a VO2 film, we demonstrate enhanced terahertz
transmission modulation upon traversing the insulator-to-metal transition (IMT)
at ~340 K. Our complementary metamaterial increases the modulation amplitude of
the original VO2 film from 42% to 68.3% at 0.47 THz upon crossing the IMT,
corresponding to an enhancement of 62.4%. Moreover, temperature dependent
transmission measurements reveal a redshift of the resonant frequency arising
from a giant increase of the permittivity of the VO2 film. Maxwell-Garnett
effective medium theory was employed to explain the permittivity change upon
transitioning through the IMT. Our results highlight that symbiotic integration
of metamaterial arrays with quantum materials provides a powerful approach to
engineer emergent functionality.",2206.11930v1
2022-06-24,Transition from Pauli paramagnetism to Curie-Weiss behaviour in vanadium,"We study electron correlations and their impact on magnetic properties of bcc
vanadium by a combination of density functional and dynamical mean-field
theory. The calculated uniform magnetic susceptibility {in bcc structure} is of
Pauli type at low temperatures, while it obeys the Curie-Weiss law at higher
temperatures. Thus, we qualitatively reproduce the experimental temperature
dependence of magnetic susceptibility without introducing the martensitic phase
transition. Our results for local spin-spin correlation function and local
susceptibility reveal that the Curie-Weiss behavior appears due to partial
formation of local magnetic moments, which originate from $t_{2g}$ states and
occur due to local spin correlations caused by Hund's rule coupling. At the
same time, the fermionic quasiparticles remain well-defined, while the
formation of local moments is accompanied by a deviation from the Fermi-liquid
behavior. In particular, the self-energy of the $t_{2g}$ states shows the
non-analytic frequency dependence, which is a characteristic of the
spin-freezing behavior, while the quasiparticle damping changes approximately
linearly with temperature in the intermediate temperature range $200$--$700$~K.
By analyzing the momentum dependence of static magnetic susceptibility, we find
incommensurate magnetic correlations, which may provide a mechanism for
unconventional superconductivity at low temperatures.",2206.12383v3
2022-07-28,Evolution of short-range magnetic correlations in ferromagnetic Ni-V alloys,"We experimentally study how the magnetic correlations develop in a binary
alloy close to the ferromagnetic quantum critical point with small-angle
neutron scattering (SANS). Upon alloying the itinerant ferromagnet nickel with
vanadium, the ferromagnetic order is continuously suppressed. The critical
temperature Tc vanishes when vanadium concentrations reach the critical value
of xc=0.116 indicating a quantum critical point separating the ferromagnetic
and paramagnetic phases. Earlier magnetization and $\mu$SR data have indicated
the presence of magnetic inhomogeneities in Ni(1-x)V(x) and, in particular,
recognize the magnetic clusters close to xc, on the paramagnetic and on the
ferromagnetic sides with nontrivial dynamical properties [R. Wang et al., Phys.
Rev. Lett. 118, 267202 (2017)]. We present the results of SANS study with full
polarization analysis of polycrystalline Ni(1-x)V(x) samples with x=0.10 and
x=0.11 with low critical temperatures Tc below 50 K. For both Ni-V samples
close to xc we find isotropic magnetic short-range correlations in the
nanometer-scale persisting at low temperatures. They are suppressed gradually
in higher magnetic fields. In addition, signatures of long-range ordered
magnetic domains are present below Tc. The fraction of these magnetic clusters
embedded in the ferromagnetic ordered phase grows towards xc and agrees well
with the cluster fraction estimate from the magnetization and $\mu$SR data. Our
SANS studies provide new insights into the nature of the inhomogeneities in a
ferromagnetic alloy close to a quantum critical point.",2207.14196v2
2022-09-01,Towards Morphologically Induced Anisotropy in Thermally Hysteretic Dielectric Properties of Vanadium Dioxide,"The Bruggeman homogenization formalism was used to numerically investigate
the dielectric properties of a columnar thin film (CTF) made from vanadium
dioxide. For visible and near-infrared wavelengths, the CTF is
electromagnetically equivalent to a homogeneous orthorhombic material. Over the
58 deg C -- 72 deg C temperature range, the eigenvalues of the CTF's relative
permittivity dyadic are highly sensitive to temperature, and vary according to
whether the CTF is being heated or cooled. The anisotropy revealed through the
eigenvalues, and the anisotropy of the associated hysteresis, were investigated
in relation to temperature for CTFs of different porosities and columnar cross
sections. When the free-space wavelength is 800 nm, the CTF is a dissipative
dielectric material that exhibits temperature-dependent anisotropy and
anisotropic hysteresis. In contrast, when the free-space wavelength is 1550 nm,
the CTF can be either a dissipative dielectric material, a hyperbolic material
or a metal-like material, depending on the temperature and the porosity of the
CTF. As the porosity of the CTF decreases from 0.55 to 0.3, the anisotropy of
the CTF becomes more pronounced, as does the anisotropy of the hysteresis. Only
relatively modest variations in anisotropy and hysteresis arise in response to
varying the columnar cross-sectional shape, as compared to the variations
induced by varying the porosity.",2209.00492v1
2022-09-20,Optical Characterization of a Single Quantum Emitter Based on Vanadium Phthalocyanine Molecules,"Single quantum emitters play a fundamental role in the development of quantum
technologies such as quantum repeaters, and quantum information processing.
Isolating individual molecules with stable optical emission is an essential
step for these applications, specially for those molecules that present large
coherence times at room temperature. Among them, vanadium-oxide phthalocyanine
(VOPc) molecules stand out as promising candidates due to their large coherence
times measured in ensemble. However, the optical properties of individual
molecules have not yet been reported. Here we show that single VOPc molecules
with stable optical properties at room temperature can be isolated. We find
that the optical response of the molecule under laser illumination of different
polarization agrees well with a system having pyramidal C$_{4v}$ symmetry.
Furthermore, the molecule reveals a non-radiative transition rate that depends
on the excitation wavelength when its lifetime is interrogated. We provide
theoretical calculations that support our experimental findings and provide
insight to the role of phonons and internal electronic structure of the
molecule. These results demonstrate that this single paramagnetic molecule can
function as a single quantum emitter while displaying optical stability under
ambient conditions to have their intrinsic properties investigated.",2209.09842v1
2022-09-21,Classical and Machine Learning Interatomic Potentials for BCC Vanadium,"BCC transition metals (TMs) exhibit complex temperature and strain-rate
dependent plastic deformation behaviour controlled by individual crystal
lattice defects. Classical empirical and semi-empirical interatomic potentials
have limited capability in modelling defect properties such as the screw
dislocation core structures and Peierls barriers in the BCC structure. Machine
learning (ML) potentials, trained on DFT-based datasets, have shown some
successes in reproducing dislocation core properties. However, in group VB TMs,
the most widely-used DFT functionals produce erroneous shear moduli C44 which
are undesirably transferred to machine-learning interatomic potentials, leaving
current ML approaches unsuitable for this important class of metals and alloys.
Here, we develop two interatomic potentials for BCC vanadium (V) based on (i)
an extension of the partial electron density and screening parameter in the
classical semi-empirical modified embedded-atom method (XMEAM-V) and (ii) a
recent hybrid descriptor in the ML Deep Potential framework (DP-HYB-V). We
describe distinct features in these two disparate approaches, including their
dataset generation, training procedure, weakness and strength in modelling
lattice and defect properties in BCC V. Both XMEAM-V and DP-HYB-V reproduce a
broad range of defect properties relevant to plastic deformation and fracture.
In particular, XMEAM-V reproduces nearly all mechanical and thermodynamic
properties at DFT accuracies and with C44 near experimental value. XMEAM-V also
naturally exhibits the anomalous slip at 77 K widely observed in group VB and
VIB TMs and outperforms all existing, publically available interatomic
potentials for V. The XMEAM thus provides a practical path to developing
accurate and efficient interatomic potentials for nonmagnetic BCC TMs and
possibly multi-principal element TM alloys.",2209.12322v1
2022-09-29,Site-specific atomic substitution in a giant magnetocaloric Fe$_2$P-type system,"Giant magnetocaloric (GMC) materials constitute a requirement for near room
temperature magnetic refrigeration. (Fe,Mn)$_2$(P,Si) is a GMC compound with
strong magnetoelastic coupling. The main hindrance towards application of this
material is a comparably large temperature hysteresis, which can be reduced by
metal site substitution with a nonmagnetic element. However, the
(Fe,Mn)$_2$(P,Si) compound has two equally populated metal sites, the
tetrahedrally coordinated $3f$ and the pyramidally coordinated $3g$ sites. The
magnetic and magnetocaloric properties of such compounds are highly sensitive
to the site specific occupancy of the magnetic atoms. Here we have attempted to
study separately the effect of $3f$ and $3g$ site substitution with equal
amounts of vanadium. Using formation energy calculations, the site preference
of vanadium and its influence on the magnetic phase formation are described. A
large difference in the isothermal entropy change (as high as 44\%) with
substitution in the $3f$ and $3g$ sites is observed. The role of the lattice
parameter change with temperature and the strength of the magnetoelastic
coupling on the magnetic properties are highlighted.",2209.14962v1
2022-11-26,Investigating the effect of electronic correlation on transport properties and phononic states of Vanadium,"In the present work, we have tried to investigate the importance of
electronic correlation on transport properties and phononic states of Vanadium
(V). The temperature-dependent electrical resistivity ($\rho$) and electronic
part of thermal conductivity ($\kappa_e$) due to electron-electron interactions
(EEIs) and electron-phonon interactions (EPIs) are computed. The values of
$\rho$ due to EEIs are found to be extremely small in comparison to $\rho$ due
to EPIs. For instance, at 300 K, the calculated value of $\rho$ due to EEIs
(EPIs) is $\sim$ 0.859$\times10^{-3}$ ($\sim$ 0.20) $\mu\Omega$m. The
magnitudes of $\kappa_e$ due to EPIs are found to be in good agreement with the
experimental results. These observations indicate the negligible importance of
EEIs to these quantities for V. However, at 300 K, the value of Seebeck
coefficient ($S$) at DFT+DMFT level ($\sim$ -0.547 $\mu$VK$^{-1}$) is found to
be entirely different than at DFT level ($\sim 7.401$ $\mu$VK$^{-1}$). Also,
the DFT+DMFT value of $S$ at 300 K is in good match with the available
experimental data (-1.06 $\mu$VK$^{-1}$, 1.0 $\mu$VK$^{-1}$). Apart from this,
the study of phononic states at DFT and DFT+DMFT level is performed. The
obtained phononic band structure and phonon DOS at DFT+DMFT differ to a good
extent from that at DFT. The maximum energy of phononic state obtained at DFT
(DFT+DMFT) is $\sim$ 33.83 ($\sim$ 35.15) meV, where the result of DFT+DMFT is
obtained more closer to the experimental data (35.15, 36.98 & 41.77 meV). These
results highlight the importance of electronic correlation on $S$ & phononic
states of simple correlated V metal.",2211.14504v2
2023-01-06,Economic and Energetic Assessment of a Hybrid Vanadium Redox Flow and Lithium-ion batteries considering Power Sharing Strategies Impact,"Hybrid energy storage systems (HESS) combine different energy storage
technologies aiming at overall system performance and lifetime improvement
compared to a single technology system. In this work, control combinations for
a vanadium redox flow battery (VRFB, 5/60 kW/kWh) and a lithium-ion battery
(LIB, 3.3/9.8 kW/kWh) are investigated for the design of a HESS. Four real-time
energy management/power allocation scenarios are considered for the operation
of the hybrid storage solution through a 15-year economic and energetic
analysis. The results obtained for each scenario are compared with a single
technology battery performance. In the definition of the scenarios, real
electricity generation is considered from two solar photovoltaic installations
(3.2 kWp and 6.7 kWp) and an estimated representative load of a services
building. HESS performance is evaluated through specific energy and economic
key performance indicators. The results obtained indicate that the use of
customized energy management strategies (EMSs) renders the VRFB and LIB
characteristics complementary, besides enhancing the competitiveness of VRFB,
as a single technology. Moreover, the HESS management impacts the seasonality
factor, contributing to the overall electric system smart management.",2301.02535v3
2023-01-07,Modulation-Doping a Correlated Electron Insulator,"Correlated electron materials (CEMs) host a rich variety of condensed matter
phases. Vanadium dioxide (VO2) is a prototypical CEM with a
temperature-dependent metal-to-insulator (MIT) transition with a concomitant
crystal symmetry change. External control of MIT in VO2 - especially without
inducing structural changes - has been a long-standing challenge. In this work,
we design and synthesize modulation-doped VO2-based thin film heterostructures
that closely emulate a textbook example of filling control in a correlated
electron insulator. Using a combination of charge transport, hard x-ray
photoelectron spectroscopy, and structural characterization, we show that the
insulating state can be doped to achieve carrier densities greater than 5x10^21
cm^(-3) without inducing any measurable structural changes. We find that the
MIT temperature (T_MIT) continuously decreases with increasing carrier
concentration. Remarkably, the insulating state is robust even at doping
concentrations as high as ~0.2 e-/vanadium. Finally, our work reveals
modulation-doping as a viable method for electronic control of phase
transitions in correlated electron oxides with the potential for use in future
devices based on electric-field controlled phase transitions.",2301.02798v1
2023-05-02,Ultra-narrow inhomogeneous spectral distribution of telecom-wavelength vanadium centres in isotopically-enriched silicon carbide,"Spin-active quantum emitters have emerged as a leading platform for quantum
technologies. However, one of their major limitations is the large spread in
optical emission frequencies, which typically extends over tens of GHz. Here,
we investigate single V4+ vanadium centres in 4H-SiC, which feature
telecom-wavelength emission and a coherent S=1/2 spin state. We perform
spectroscopy on single emitters and report the observation of spin-dependent
optical transitions, a key requirement for spin-photon interfaces. By
engineering the isotopic composition of the SiC matrix, we reduce the
inhomogeneous spectral distribution of different emitters down to 100 MHz,
significantly smaller than any other single quantum emitter. Additionally, we
tailor the dopant concentration to stabilise the telecom-wavelength V4+ charge
state, thereby extending its lifetime by at least two orders of magnitude.
These results bolster the prospects for single V emitters in SiC as material
nodes in scalable telecom quantum networks.",2305.01757v2
2023-08-23,"Plastic deformation mechanisms during nanoindentation of W, Mo, V body-centered cubic single crystals and their corresponding W-Mo, W-V equiatomic random solid solutions","Deformation plasticity mechanisms in alloys and compounds may unveil the
material capacity towards optimal mechanical properties. We conduct a series of
molecular dynamics (MD) simulations to investigate plasticity mechanisms due to
nanoindentation in pure tungsten, molybdenum and vanadium body-centered cubic
single crystals, as well as the also body-centered cubic, equiatomic, random
solid solutions (RSS) of tungsten--molybdenum and tungsten--vanadium alloys.
Our analysis focuses on a thorough, side-by-side comparison of dynamic
deformation processes, defect nucleation, and evolution, along with
corresponding stress--strain curves. We also check the surface morphology of
indented samples through atomic shear strain mapping. As expected, the presence
of Mo and V atoms in W matrices introduces lattice strain and distortion,
increasing material resistance to deformation and slowing down dislocation
mobility of dislocation loops with a Burgers vector of 1/2 $\langle 111
\rangle$. Our side-by-side comparison displays a remarkable suppression of the
plastic zone size in equiatomic W--V RSS, but not in equiatomic W--Mo RSS
alloys, displaying a clear prediction for optimal hardening response equiatomic
W--V RSS alloys. If the small-depth nanoindentation plastic response is
indicative of overall mechanical performance, it is possible to conceive a
novel MD-based pathway towards material design for mechanical applications in
complex, multi-component alloys.",2308.12206v1
2023-09-15,Ab initio investigation of the lattice dynamics and thermophysical properties of BCC Vanadium and Niobium,"In the present work, we have performed the phonon dispersion calculations of
body-centered cubic Vanadium (V) and Niobium (Nb) with the supercell approach
using different supercell size. Using DFT method, the calculated phonon spectra
of V and Nb are found to be in a good agreement with the available experimental
data. Our calculated results show a \enquote{dip}-like feature in the
longitudinal acoustic phonon mode along the $\Gamma$-H high symmetric path for
both transition metals in the case of supercell size 4$\times$4$\times$4.
However, in supercell size 2$\times$2$\times$2 and 3$\times$3$\times$3, the
\enquote{dip}-like feature is not clearly visible. In addition to this,
thermodynamical properties are also computed, which compare well with the
experimental data. Apart from this, the phonon lifetime due to electron-phonon
interactions ($\tau_{\rm{eph}}^{\rm{ph}}$) and phonon-phonon interactions
($\tau_{\rm{ph}}^{\rm{ph}}$) are calculated. The effect of phonon-phonon
interactions (PPI) is studied by computing the average phonon lifetime for all
acoustic branches. The value of $\tau_{\rm{eph}}^{\rm{ph}}$ of V (Nb) is found
to be 23.16 (24.70)$\times 10^{-15}$ s at $100$ K, which gets decreased to 1.51
(1.85)$\times 10^{-15}$ s at 1000 K. The $\tau_{\rm{ph}}^{\rm{ph}}$ of V (Nb)
is found to be 8.59 (18.09)$\times 10^{-12}$ and 0.83 (1.76)$\times 10^{-12}$ s
at 100 and 1000 K, respectively. Nextly, the lattice thermal conductivity is
computed using linearized phonon Boltzmann equation. The present work suggests
that studying the variation of phonon dispersion with supercell size is crucial
for understanding the phonon properties of solids accurately.",2309.08245v1
2023-10-13,Atomic scale imaging of the negative charge induced by a single vanadium dopant atom in monolayer WSe$_2$ using 4D-STEM,"There has been extensive activity exploring the doping of semiconducting
two-dimensional (2D) transition metal dichalcogenides in order to tune their
electronic and magnetic properties. The outcome of doping depends on various
factors, including the intrinsic properties of the host material, the nature of
the dopants used, their spatial distribution as well as their interactions with
other types of defects. A thorough atomic-level analysis is essential to fully
understand these mechanisms. In this work, vanadium doped WSe$_2$ monolayer
grown by molecular beam epitaxy is investigated using four-dimensional scanning
transmission electron microscopy (4D-STEM). Through center of mass-based
reconstruction, atomic scale maps are produced, allowing the visualization of
both the electric field and the electrostatic potential around individual V
atoms. To provide quantitative insights, these results are successfully
compared with multislice image simulations based on ab initio calculations,
accounting for lens aberrations. Finally, a negative charge around the V
dopants is detected as a drop in the electrostatic potential, unambiguously
demonstrating that 4D-STEM can be used to detect and to accurately analyze
single dopant charge states in semiconducting 2D materials.",2310.09246v1
2023-12-01,Active Loss Engineering in Vanadium Dioxide Based BIC Metasurfaces,"Metasurfaces have unlocked significant advancements across photonics, yet
their efficient active control remains challenging. The active materials
required often lack continuous tunability, exhibit inadequate refractive index
(RI) changes, or suffer from high losses. These aspects pose an inherent
limitation for resonance-shifting based switching: when RI changes are small,
the resulting shift is also minor. Conversely, high RI changes typically come
with high intrinsic losses necessitating broad modes because narrow ones cannot
tolerate such losses. Therefore, larger spectral shifts are required to
effectively detune the modes. This paper introduces a novel active metasurface
approach that converts the constraint of high intrinsic losses into a
beneficial feature. This is achieved by controlling the losses in a hybrid
vanadium dioxide (VO$_{2}$) - silicon metasurface, supporting
symmetry-protected bound states in the continuum (BICs) within the infrared
spectrum. By leveraging the temperature-controlled losses in VO$_{2}$ and
combining them with the inherent far-field-coupling tunability of BICs, we gain
unprecedented precision in independently controlling both the radiative and
nonradiative losses of the resonant system. Our dual-control mechanism allows
us to optimize our metasurfaces and we experimentally demonstrate quality
factors above 200, a maximum reflectance amplitude of 90%, a relative switching
contrast of 78%, and continuous tuning from under- to over-coupling within the
infrared spectral range. This study provides a foundation for experimentally
and technologically simple, fine-tunable, active metasurfaces for applications
ranging from molecular sensors to filters and optical modulators.",2312.00547v1
2024-02-14,Switchable optical trapping based on Mie-resonant phase-change nanoparticles,"Optical tweezers have revolutionized the manipulation of nanoscale objects,
offering versatile control over trapped particles. Typically, the manipulation
and trapping capabilities of optical tweezers rely on adjusting either the
trapping laser beams or the optical environment surrounding the trapped
nanoparticles. In our study, we present a novel approach to achieve tunable and
switchable trapping using optical tweezers. We utilize nanoparticles made of a
phase-change material (vanadium dioxide or VO$_2$), trapped by a focused
Gaussian beam. By varying the intensity of the trapping beam, we can optically
control the phase and optomechanical properties of the vanadium dioxide
nanoparticles. At lower intensities, the nanoparticle remains in its monoclinic
phase, and it is trapped by attractive optical forces. However, at higher
intensities, the optical beam induces a phase transition in the nanoparticle to
the rutile phase, which dramatically alters the optical forces, transforming
them from attractive to repulsive, thereby pushing the nanoparticle away from
the beam. Importantly, this effect is reversible, allowing the same particle to
be attracted and repelled repeatedly. The observed phenomenon is governed by
Mie resonances supported by the nanoparticle and their alterations during the
phase transition of the VO$_2$ material. Our findings introduce a versatile new
addition to the optical tweezers toolbox for optomechanical manipulation of
nanoparticles.",2402.08947v1
2016-06-14,On the multiferroic skyrmion-host GaV4S8,"The lacunar spinel GaV4S8 exhibits orbital ordering at 44 K and shows a
complex magnetic phase diagram below 12.7 K, which includes ferromagnetic and
cycloidal spin order. At low but finite external magnetic fields, N\'eel-type
skyrmions are formed in this material. Skyrmions are whirl-like spin vortices
that have received great theoretical interest because of their non-trivial spin
topology and that are also considered as basic entities for new data-storage
technologies. Interestingly, we found that the orbitally ordered phase shows
sizable ferroelectric polarization and that excess spin-driven polarizations
appear in all magnetic phases, including the skyrmion-lattice phase. Hence,
GaV4S8 shows simultaneous magnetic and polar order and belongs to the class of
multiferroics, materials that attracted enormous attention in recent years.
Here, we summarize the existing experimental information on the magnetic,
electronic, and dielectric properties of GaV4S8. By performing detailed
magnetic susceptibility, resistivity, specific heat, and dielectric
experiments, we complement the low-temperature phase diagram. Specifically, we
show that the low-temperature and low-field ground state of GaV4S8 seems to
have a more complex spin configuration than purely collinear ferromagnetic spin
order. In addition, at the structural Jahn-Teller transition the magnetic
exchange interaction changes from antiferromagnetic to ferromagnetic. We also
provide experimental evidence that the vanadium V4 clusters in GaV4S8 can be
regarded as molecular units with spin 1/2. However, at high temperatures
deviations in the susceptibility show up, indicating that either the magnetic
moments of the vanadium atoms fluctuate independently or excited states of the
V4 molecule become relevant.",1606.04511v1
2019-05-23,Reconfigurable radiofrequency electronic functions designed with 3D Smith Charts in Metal-Insulator-Transition Materials,"Recently, the field of Metal-Insulator-Transition (MIT) materials has emerged
as an unconventional solution for novel energy efficient electronic functions,
such as steep slope subthermionic switches, neuromorphic hardware,
reconfigurable radiofrequency functions, new types of sensors, teraherz and
optoelectronic devices. Designing radiofrequency (RF) electronic circuits with
a MIT material like vanadium dioxide, VO2, requires the understanding of its
physics and appropriate models and tools, with predictive capability over large
range of frequency (1-100GHz). Here, we develop 3D Smith charts for devices and
circuits having complex frequency dependences, like the ones resulting by the
use of MIT materials. The novel foundation of a 3D Smith chart involves here
the geometrical fundamental notions of oriented curvature and variable
homothety in order to clarify first theoretical inconsistencies in Foster and
Non Foster circuits, where the driving point impedances exhibit mixed clockwise
and counter-clockwise frequency dependent paths on the Smith chart as frequency
increases. We show here the unique visualization capability of a 3D Smith
chart, which allows to quantify orientation over variable frequency. The new 3D
Smith chart is applied as a 3D multi-parameter modelling and design environment
for the complex case of Metal-Insulator-Transition (MIT) materials where their
permittivity is dependent on the frequency. In this work, we apply 3D Smith
charts to on Vanadium Dioxide (VO2) reconfigurable Peano inductors. We report
fabricated inductors with record quality factors using VO2 phase transition to
program multiple tuning states, operating in the range 4 GHz to 10 GHz.
Finally, we fabricate new Peano curves filters used to extract the
frequency-dependent dielectric constant of VO2 within 1 GHz-50 GHz for the
accurate design of RF electronic applications with phase change materials",1905.09701v1
2020-01-07,Electrical switching and oscillations in vanadium dioxide,"We have studied electrical switching with S-shaped I-V characteristics in
two-terminal MOM devices based on vanadium dioxide thin films. The switching
effect is associated with the metal-insulator phase transition. Relaxation
oscillations are observed in circuits with VO2-based switches. Dependences of
the oscillator critical frequency Fmax, threshold power and voltage, as well as
the time of current rise, on the switching structure size are obtained by
numerical simulation. The empirical dependence of the threshold voltage on the
switching region dimensions and film thickness is found. It is shown that, for
the VO2 channel sizes of 10*10 nm, Fmax can reach the value of 300 MHz at a
film thickness of ~20 nm. Next, it is shown that oscillatory neural networks
can be implemented on the basis of coupled VO2 oscillators. For the weak
capacitive coupling, we revealed the dependence of the phase difference upon
synchronization on the coupling capacitance value. When the switches are scaled
down, the limiting time of synchronization is reduced to Ts ~13 {\mu}s, and the
number of oscillation periods for the entering to the synchronization mode
remains constant, Ns ~ 17. In the case of weak thermal coupling in the
synchronization mode, we observe in-phase behavior of oscillators, and there is
a certain range of parameters of the supply current, in which the
synchronization effect becomes possible. With a decrease in dimensions, a
decrease in the thermal coupling action radius is observed, which can vary in
the range from 0.5 to 50 {\mu}m for structures with characteristic dimensions
of 0.1 to 5 {\mu}m, respectively. Thermal coupling may have a promising effect
for realization of a 3D integrated oscillatory neural network.",2001.01913v1
2020-03-24,Canted antiferromagnetic order in the monoaxial chiral magnets V$_{1/3}$TaS$_2$ and V$_{1/3}$NbS$_{2}$,"The Dzyaloshinskii-Moriya (DM) interaction is present in the transition metal
dichalcogenides (TMDC) magnets of form $M_{1/3}T$S$_{2}$ ($M$ $=$ 3d transition
metal, $T$ $\in$ {Nb, Ta}), given that the intercalants $M$ form
$\sqrt{3}\times\sqrt{3}$ superlattices within the structure of the parent
materials $T$S$_2$ and break the centrosymmetry. Competition between the DM and
ferromagnetic exchange interactions in these systems has been shown to
stabilize a topological defect known as a chiral soliton in select intercalated
TMDCs, initiating interest both in terms of fundamental physics and the
potential for technological applications. In the current article, we report on
our study of the materials V$_{1/3}$TaS$_2$ and V$_{1/3}$NbS$_2$, using a
combination of x-ray powder diffraction, magnetization and single crystal
neutron diffraction. Historically identified as ferromagnets, our diffraction
results instead reveal that vanadium spins in these compounds are arranged into
an A-type antiferromagnetic configuration at low temperatures. Refined moments
are 1.37(6)$\mu_{B}$ and 1.50(9)$\mu_{B}$ for V$_{1/3}$TaS$_2$ and
V$_{1/3}$NbS$_2$, respectively. Transition temperatures $T_{c}$~$=$~32K for
V$_{1/3}$TaS$_{2}$ and 50K for V$_{1/3}$NbS$_{2}$ are obtained from the
magnetization and neutron diffraction results. We attribute the small net
magnetization observed in the low-temperature phases to a subtle
($\sim$2$^{\circ}$) canting of XY-spins in the out-of-plane direction. These
new results are indicative of dominant antiferromagnetic exchange interactions
between the vanadium moments in adjacent ab-planes, likely eliminating the
possibility of identifying stable chiral solitons in the current materials.",2003.10967v4
2021-03-16,Roton pair density wave and unconventional strong-coupling superconductivity in a topological kagome metal,"The transition-metal kagome lattice materials host frustrated, correlated,
and topological quantum states of matter. Recently, a new family of
vanadium-based kagome metals AV3Sb5 (A=K, Rb, and Cs) with topological band
structures has been discovered. These layered compounds are nonmagnetic and
undergo charge density wave transitions before developing superconductivity at
low temperatures. Here we report the observation of unconventional
superconductivity and pair density wave (PDW) in CsV3Sb5 using scanning
tunneling microscope/spectroscopy (STM/STS) and Josephson STS. We find that
CsV3Sb5 exhibits a V-shaped pairing gap {\Delta}~0.5 meV and is a
strong-coupling superconductor (2{\Delta}/kBTc~5) that coexists with 4a0
unidirectional and 2a0X2a0 charge order. Remarkably, we discover a 3Q PDW
accompanied by bidirectional 4a0/3 spatial modulations of the superconducting
gap, coherence peak and gap-depth in the tunneling conductance. We term this
novel quantum state a roton-PDW associated with an underlying vortex-antivortex
lattice that can account for the observed conductance modulations. Probing the
electronic states in the vortex halo in an applied magnetic field, in
strong-field that suppresses superconductivity, and in zero-field above Tc
reveals that the PDW is a primary state responsible for an emergent pseudogap
and intertwined electronic order. Our findings show striking analogies and
distinctions to the phenomenology of high-Tc cuprate superconductors, and
provide groundwork for understanding the microscopic origin of correlated
electronic states and superconductivity in vanadium-based kagome metals.",2103.09188v2
2021-03-25,Thermodynamic Analysis of Titanium Removal from Molten iron Smelted with Large Amounts of Sodium Additives,"High purity iron is obtained from vanadium-titanium magnetite (VTM) by
one-step coal-based direct reduction-smelting process with coal as reductant
and sodium carbonate (Na2CO3) as additives. Industrial experiments show that
the process of treating molten iron with a large amount of Na2CO3 is effective
in removing titanium from molten iron. However, the studies are rarely
conducted in thermodynamic relationship between titanium and other components
of molten iron, under the condition of a large amount of Na2CO3 additives. In
this study, through the thermodynamic database software Factsage8.0, the
effects of melting temperature, sodium content and oxygen content on the
removal of titanium from molten iron are studied. The results of thermodynamic
calculation show that the removal of titanium from molten iron needs to be
under the condition of oxidation, and the temperature should be below the
critical temperature of titanium removal (the highest temperature at which
titanium can be removed). Relatively low temperature and high oxygen content
contribute to the removal of titanium from molten iron. The high oxygen content
is conducive to the simultaneous removal of titanium and phosphorus from molten
iron. In addition, from a thermodynamic point of view, excessive sodium
addition inhibits the removal of titanium from molten iron.",2103.13572v1
2021-04-19,Orbital ordering and fluctuations in a kagome superconductor CsV3Sb5,"Recently, competing electronic instabilities, including superconductivity and
density-wave-like order, have been discovered in vanadium-based kagome metals
AV3Sb5 (A = K, Rb, Cs) with a nontrivial band topology. This finding stimulates
wide interests to study the interplay of these competing electronic orders and
possible exotic excitations in the superconducting state. Here, in order to
further clarify the nature of density-wave-like transition in these kagome
superconductors, we performed 51V and 133Cs nuclear magnetic resonance (NMR)
measurements on the CsV3Sb5 single crystal. A first-order phase transition
associated with orbital ordering is revealed by observing a sudden splitting of
orbital shift in 51V NMR spectrum at the structural transition temperature Ts ~
94 K. In contrast, the quadrupole splitting from a charge-density-wave (CDW)
order on 51V NMR spectrum only appears gradually below Ts with a typical
second-order transition behavior, suggesting that the CDW order is a secondary
electronic order. Moreover, combined with 133Cs NMR spectrum, the present
result also confirms a three-dimensional structural modulation with a 2ax2ax2c
period. Above Ts, the temperature-dependent Knight shift and nuclear
spin-lattice relaxation rate (1/T1) further indicate the existence of
remarkable magnetic fluctuations from vanadium 3d orbitals, which are
suppressed due to orbital ordering below Ts. The present results strongly
support that, besides CDW order, the previously claimed density-wave-like
transition also involves a dominant orbital order, suggesting a rich orbital
physics in these kagome superconductors.",2104.09173v1
2021-11-24,Titanium oxide and chemical inhomogeneity in the atmosphere of the exoplanet WASP-189b,"The temperature of an atmosphere decreases with increasing altitude, unless a
shortwave absorber exists that causes a temperature inversion. Ozone plays this
role in the Earth`s atmosphere. In the atmospheres of highly irradiated
exoplanets, shortwave absorbers are predicted to be titanium oxide (TiO) and
vanadium oxide (VO). Detections of TiO and VO have been claimed using both low
and high spectral resolution observations, but later observations have failed
to confirm these claims or overturned them. Here we report the unambiguous
detection of TiO in the ultra-hot Jupiter WASP-189b using high-resolution
transmission spectroscopy. This detection is based on applying the
cross-correlation technique to many spectral lines of TiO from 460 to 690 nm.
Moreover, we report detections of metals, including neutral and singly ionised
iron and titanium, as well as chromium, magnesium, vanadium and manganese (Fe,
Fe+, Ti, Ti+, Cr, Mg, V, Mn). The line positions of the detected species
differ, which we interpret as a consequence of spatial gradients in their
chemical abundances, such that they exist in different regions or dynamical
regimes. This is direct observational evidence for the three-dimensional
thermo-chemical stratification of an exoplanet atmosphere derived from
high-resolution ground-based spectroscopy.",2111.12732v2
2022-06-06,Evidence of Spin Frustration in Vanadium Diselenide Monolayer Magnet,"Monolayer VSe2, featuring both charge density wave and magnetism phenomena,
represents a unique van der Waals magnet in the family of metallic
two-dimensional transition-metal dichalcogenides (2D-TMDs). Herein, by means of
in-situ microscopic and spectroscopic techniques, including scanning tunneling
microscopy/spectroscopy, synchrotron X-ray and angle-resolved photoemission,
and X-ray absorption, direct spectroscopic signatures are established, that
identify the metallic 1T-phase and vanadium 3d1 electronic configuration in
monolayer VSe2 grown on graphite by molecular-beam epitaxy. Element-specific
X-ray magnetic circular dichroism, complemented with magnetic susceptibility
measurements, further reveals monolayer VSe2 as a frustrated magnet, with its
spins exhibiting subtle correlations, albeit in the absence of a long-range
magnetic order down to 2 K and up to a 7 T magnetic field. This observation is
attributed to the relative stability of the ferromagnetic and antiferromagnetic
ground states, arising from its atomic-scale structural features, such as
rotational disorders and edges. The results of this study extend the current
understanding of metallic 2D-TMDs in the search for exotic low-dimensional
quantum phenomena, and stimulate further theoretical and experimental studies
on van der Waals monolayer magnets.",2206.02427v1
2022-07-26,Loop currents in $A$V$_3$Sb$_5$ kagome metals: multipolar and toroidal magnetic orders,"Experiments in the recently discovered vanadium-based kagome metals have
suggested that their charge-ordered state displays not only bond distortions,
characteristic of a ``real"" charge density-wave (rCDW), but also time-reversal
symmetry-breaking, typical of loop currents described by an ``imaginary"" charge
density-wave (iCDW). Here, we combine density-functional theory, group-theory,
and phenomenological modeling to investigate the complex charge-ordered states
that arise from interactions between the low-energy van Hove singularities
present in the electronic structure of $A$V$_3$Sb$_5$. We find two broad
classes of mixed iCDW-rCDW configurations: triple-$\mathbf{Q}$ iCDW,
triple-$\mathbf{Q}$ rCDW order, dubbed $3\mathbf{Q}$-$3\mathbf{Q}$, and
double-$\mathbf{Q}$ iCDW, single-$\mathbf{Q}$ rCDW order, dubbed
$2\mathbf{Q}$-$1\mathbf{Q}$. Moreover, we identify seven different types of
iCDW order, stemming from the different vanadium-orbital and kagome-sublattice
structures of the two pairs of van Hove singularities present above and below
the Fermi level. While the $2\mathbf{Q}$-$1\mathbf{Q}$ states trigger an
orthorhombic distortion that breaks the threefold rotational symmetry of the
kagome lattice, the $3\mathbf{Q}$-$3\mathbf{Q}$ states induce various types of
subsidiary uniform magnetic orders, from conventional ferromagnetism to
magnetic octupolar, magnetic toroidal, and even magnetic monopolar order. We
show that these exotic orders display unique magneto-striction,
magneto-electric, and magneto-electric-striction properties that can be probed
experimentally to identify which iCDW state is realized in these compounds. We
briefly discuss the impact of an out-of-plane modulation of the charge order
and the interplay between these complex charge-ordered states and
superconductivity.",2207.12820v2
2022-11-29,"Structural evolution of the kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, and Cs) through charge density wave order","The kagome superconductors KV$_3$Sb$_5$, RbV$_3$Sb$_5$, and CsV$_3$Sb$_5$ are
known to display charge density wave (CDW) order which impacts the topological
characteristics of their electronic structure. Details of their structural
ground states and how they evolve with temperature are revealed here using
single crystal X-ray crystallographic refinements as a function of temperature,
carried out with synchrotron radiation. The compounds KV$_3$Sb$_5$ and
RbV$_3$Sb$_5$ present 2$\times$2$\times$2 superstructures in the $Fmmm$ space
group with a staggered tri-hexagonal deformation of vanadium layers.
CsV$_3$Sb$_5$ displays more complex structural evolution, whose details have
been unravelled by applying machine learning methods to the scattering data.
Upon cooling through the CDW transition, CsV$_3$Sb$_5$ displays a staged
progression of ordering from a 2$\times$2$\times$1 supercell and a
2$\times$2$\times$2 supercell into a final 2$\times$2$\times$4 supercell that
persists to $T$ = 11 K and exhibits an average structure where vanadium layers
display both tri-hexagonal and Star of David patterns of deformations.
Diffraction from CsV$_3$Sb$_5$ under pulsed magnetic fields up to $\mu_0H$ = 28
T suggest the real component of the CDW state is insensitive to external
magnetic fields.",2211.16602v3
2022-12-15,Non-trivial band topology and orbital-selective electronic nematicity in a new titanium-based kagome superconductor,"Electronic nematicity that spontaneously breaks rotational symmetry has been
shown as a generic phenomenon in correlated quantum systems including
high-temperature superconductors and the AV3Sb5 (A = K, Rb, Cs) family with a
kagome network. Identifying the driving force has been a central challenge for
understanding nematicity. In iron-based superconductors, the problem is
complicated because the spin, orbital and lattice degrees of freedom are
intimately coupled. In vanadium-based kagome superconductors AV3Sb5, the
electronic nematicity exhibits an intriguing entanglement with the charge
density wave order (CDW), making understanding its origin difficult. Recently,
a new family of titanium-based kagome superconductors ATi3Bi5 has been
synthesized. In sharp contrast to its vanadium-based counterpart, the
electronic nematicity occurs in the absence of CDW. ATi3Bi5 provides a new
window to explore the mechanism of electronic nematicity and its interplay with
the orbital degree of freedom. Here, we combine polarization-dependent
angle-resolved photoemission spectroscopy with density functional theory to
directly reveal the band topology and orbital characters of the multi-orbital
RbTi3Bi5. The promising coexistence of flat bands, type-II Dirac nodal line and
nontrivial Z2 topological states is identified in RbTi3Bi5. Remarkably, our
study clearly unveils the orbital character change along the G-M and G-K
directions, implying a strong intrinsic inter-orbital coupling in the Ti-based
kagome metals, reminiscent of iron-based superconductors. Furthermore,
doping-dependent measurements directly uncover the orbital-selective features
in the kagome bands, which can be well explained by the d-p hybridization. The
suggested d-p hybridization, in collaboration with the inter-orbital coupling,
could account for the electronic nematicity in ATi3Bi5.",2212.07958v1
2023-02-10,"Unidirectional electron-phonon coupling as a ""fingerprint'' of the nematic state in a kagome superconductor","Electronic nematicity has been commonly observed in juxtaposition with
unconventional superconductivity. Understanding the nature of the nematic
state, as well as its consequence on the electronic band structure and
superconductivity, has become a pivotal focus in condensed matter physics. Here
we use spectroscopic imaging-scanning tunneling microscopy to visualize how the
interacting quasiparticles organize themselves in the nematic state of kagome
superconductor CsV$_{3-x}$Ti$_x$Sb$_5$, in which twofold symmetric (C$_2$)
quasiparticle scattering interference of the vanadium kagome bands emerges
below the bulk nematic transition temperature (T$_{nem}$). Surprisingly, we
find that the coupling to collective modes, i.e., the phonon, dramatically
alters the electrons self-energy and renormalizes the Fermi velocity of the
in-plane vanadium d$_{xy/x^2-y^2}$ bands only along the C$_2$ direction, making
the low-energy dispersion and electron dynamics highly nonequivalent along the
three lattice directions. The anti-correlation between T$_{nem}$ and the
superconducting transition temperature upon Ti substitution further suggests a
possible competition between superconductivity and electron nematicity in this
series, with a principal superconducting gap opening on the same V bands once
the nematic state is totally suppressed. The organizing principle of these
quasiparticles provides essential information for understanding the interplay
between charge density wave and superconductivity in these kagome
superconductors, and also reveals a previously unexplored form that expands the
landscape for modelling electronic nematicity in systems where electron
correlations and lattice degree of freedom act in concert.",2302.05115v1
2023-04-13,Phonon promoted charge density wave in topological kagome metal ScV$_{6}$Sn$_{6}$,"Charge density wave (CDW) orders in vanadium-based kagome metals have
recently received tremendous attention due to their unique properties and
intricate interplay with exotic correlated phenomena, topological and
symmetry-breaking states. However, the origin of the CDW order remains a topic
of debate. The discovery of ScV$_{6}$Sn$_{6}$, a vanadium-based bilayer kagome
metal exhibiting an in-plane $\sqrt{3}$ x $\sqrt{3} $ $\textit{R}$30${\deg}$
CDW order with time-reversal symmetry breaking, provides a novel platform to
explore the underlying mechanism behind the unconventional CDW. Here, we
combine high-resolution angle-resolved photoemission spectroscopy, Raman
scattering measurements and density functional theory to investigate the
electronic structures and phonon modes of ScV$_{6}$Sn$_{6}$ and their evolution
with temperature. We identify topologically nontrivial Dirac surface states and
multiple van Hove singularities (VHSs) in the vicinity of the Fermi level, with
one VHS near the K point exhibiting nesting wave vectors in proximity to the
$\sqrt{3}$ x $\sqrt{3}$ $\textit{R}$30${\deg}$ CDW wave vector. Additionally,
Raman measurements indicate a strong intrinsic electron-phonon coupling in
ScV$_{6}$Sn$_{6}$, as evidenced by the presence of a two-phonon mode and a
large frequency amplitude mode. Our findings highlight the fundamental role of
lattice degrees of freedom in promoting the CDW in ScV$_{6}$Sn$_{6}$ and
provide important insights into the fascinating correlation phenomena observed
in kagome metals.",2304.06431v1
2023-05-03,Universal sublinear resistivity in vanadium kagome materials hosting charge density waves,"The recent discovery of a charge density (CDW) state in ScV$_6$Sn$_6$ at
$T_{\textrm{CDW}}$ = 91 K offers new opportunities to understand the origins of
electronic instabilities in topological kagome systems. By comparing to the
isostructural non-CDW compound LuV$_6$Sn$_6$, we unravel interesting electrical
transport properties in ScV$_6$Sn$_6$, above and below the charge ordering
temperature. We observed that by applying a magnetic field along the $a$ axis,
the temperature behavior of the longitudinal resistivity in ScV$_6$Sn$_6$
changes from metal-like to insulator-like above the CDW transition. We show
that in the charge ordered state ScV$_6$Sn$_6$ follows the Fermi liquid
behavior while above that, it transforms into a non-Fermi liquid phase in which
the resistivity varies sublinearly over a broad temperature range. The
sublinear resistivity, which scales by $T^{3/5}$ is a common feature among
other vanadium-containing kagome compounds exhibiting CDW states such as
KV$_3$Sb$_5$, RbV$_3$Sb$_5$, and CsV$_3$Sb$_5$. By contrast, the non-Fermi
liquid behavior does not occur in LuV$_6$Sn$_6$. We explain the $T^{3/5}$
universal scaling behavior from the Coulomb scattering between Dirac electrons
and Van Hove singularities; common features in the electronic structure of
kagome materials. Finally, we show anomalous Hall-like behavior in
ScV$_6$Sn$_6$ below $T_{\textrm{CDW}}$, which is absent in the Lu compound.
Comparing the transport properties of ScV$_6$Sn$_6$ and LuV$_6$Sn$_6$ is
valuable to highlight the impacts of the unusual CDW in the Sc compound.",2305.02393v2
2023-06-14,Vanadium oxide and a sharp onset of cold-trapping on a giant exoplanet,"The abundance of refractory elements in giant planets can provide key
insights into their formation histories. Due to the Solar System giants' low
temperatures, refractory elements condense below the cloud deck limiting
sensing capabilities to only highly volatile elements. Recently, ultra-hot
giant exoplanets have allowed for some refractory elements to be measured
showing abundances broadly consistent with the solar nebula with titanium
likely condensed out of the photosphere. Here we report precise abundance
constraints of 14 major refractory elements on the ultra-hot giant planet
WASP-76b that show distinct deviations from proto-solar, and a sharp onset in
condensation temperature. In particular, we find nickel to be enriched, a
possible sign of the accretion of a differentiated object's core during the
planet's evolution. Elements with condensation temperatures below 1,550 K
otherwise closely match those of the Sun before sharply transitioning to being
strongly depleted above 1,550 K, well explained by nightside cold-trapping. We
further unambiguously detect vanadium oxide on WASP-76b, a molecule long
hypothesized to drive atmospheric thermal inversions, and also observe a global
east-west asymmetry in its absorption signals. Overall, our findings indicate
that giant planets have a mostly stellar-like refractory elemental content and
suggest that temperature sequences of hot Jupiter spectra can show abrupt
transitions wherein a mineral species is either present, or completely absent
if a cold-trap exists below its condensation temperature.",2306.08739v1
2023-06-20,Unlocking the Inaccessible Energy Density of Sodium Vanadium Fluorophosphate Electrode Materials by Transition Metal Mixing,"Sodium (Na) vanadium (V) fluorophosphate $\mathrm{Na_xV_2(PO_4)_2F_3}$ (NVPF)
is a highly attractive intercalation electrode material due to its high
operation voltage, large capacity, and long cycle life. However, several issues
limit the full utilization of NVPF's energy density: 1) the high voltage
plateau associated with extracting the ""third"" Na ion in the reaction N$_1$VPF
$\rightarrow$ VPF (~4.9 V vs Na/Na$^+$) appears above the electrochemical
stability window of most practical electrolytes (~4.5 V); 2) a sudden drop in
Na-ion diffusivity is observed near composition $\mathrm{Na_1V_2(PO_4)_2F_3}$.
Therefore, it is important to investigate the potential substitution of V by
other transition metals in NVPF derivatives, which can access the extraction of
the third Na-ion. In this work, we investigate the partial substitution of V
with molybdenum (Mo), niobium (Nb), or tungsten (W) in NVPF to improve its
energy density. We examine the structural and electrochemical behaviors of
$\mathrm{Na_xV_{2-y}Mo_y(PO_4)_2F_3}$, $\mathrm{Na_xV_{2-y}Nb_y(PO_4)_2F_3}$,
and $\mathrm{Na_xW_{2}(PO_4)_2F_3}$ across the whole Na composition region of 0
$\leq$ x $\leq$ 4, and at various transition metal substitution levels, namely,
y=0.5, 1.0, 1.5, 2.0 for Mo, and y=1.0, 2.0 for Nb. We find that partial
substitution of 50% V by Mo in NVPF reduces the voltage plateau for extracting
the third Na ion by 0.6 Volts, which enables further Na extraction from
$\mathrm{Na_1Mo_{2}(PO_4)_2F_3}$ and increases the theoretical gravimetric
capacity from ~128 to ~174 mAh/g. Analysis of the migration barriers for
Na-ions in $\mathrm{Na_xVMo(PO_4)_2F_3}$ unveils improved kinetic properties
over NVPF. The proposed $\mathrm{Na_xVMo(PO_4)_2F_3}$ material provides an
optimal gravimetric energy density of ~577.3 Wh/kg versus ~507 Wh/kg for the
pristine NVPF, which amounts to an increase of ~13.9%.",2306.11275v1
2023-09-21,VO$_2$ under hydrostatic pressure: Isostructural phase transition close to a critical end-point,"The high-pressure behavior of monoclinic VO$_2$ is revisited by a combination
of Raman spectroscopy and X-ray diffraction on a single crystal under
hydrostatic conditions at room temperature. A soft mode is observed up to P$_c$
= 13.9(1) GPa. At this pressure, an isostructural phase transition between two
monoclinic phases M$_1$ and M$_1$' hinders this instability. The features of
this transformation (no apparent volume jump) indicate that the compression at
ambient temperature passes close to a critical point. An analysis based on the
Landau theory of phase transitions gives a complete description of the P-T
phase diagram. The M1' is characterized by spontaneous displacements of the
oxygen sub-lattice without any strong modification of the VV dimers distances
nor the twist angle of vanadium chains. The spontaneous displacements of oxygen
and the spontaneous deformations of the ($b_{M1}$, $c_{M1}$) plane follow the
same quadratic dependence with pressure and scales with spontaneous shifts of
the Raman phonons located at 225, 260 and 310 cm$^{-1}$. Pressure-induced
shifts of the Raman peaks allows for new assignment of several Raman modes. In
particular, the A$_g$(1)+B$_g$(1) modes at 145 cm$^{-1}$ are identified as the
vanadium displacive phonons. A second transformation in the metallic phase X,
which is found triclinic (P$\bar1$) is observed starting at 32 GPa, with a wide
coexistence region (up to 42 GPa). Upon decompression, phase X transforms,
between 20 GPa and 3 GPa, to another phase that is neither the M$_1$' nor M$_1$
phase. The structural transitions identified under pressure match with all the
previously reported electronic modifications confirming that lattice and
electronic degrees of freedom are closely coupled in this correlated material.",2309.12020v1
2023-12-29,Bilayer Vanadium Dioxide Thin Film with Elevated Transition Temperatures and High Resistance Switching,"Despite widespread interest in the phase-change applications of vanadium
dioxide (VO$_2$), the fabrication of high-quality VO$_2$ thin films with
elevated transition temperatures (TIMT) and high Insulator-Metal-Transition
resistance switching still remains a challenge. This study introduces a
two-step atmospheric oxidation approach to fabricate bilayer VO$_{2-x}$/VO$_2$
films on a c-plane sapphire substrate. To quantify the impact of the VO$_2$
buffer layer, a single-layer VO$_2$ film of the same thickness was also
fabricated. The bilayer VO$_{2-x}$/VO$_2$ films wherein the top VO$_{2-x}$ film
was under-oxidized demonstrated an elevation in TIMT reaching ~97 $^\circ$C,
one of the highest reported to date for VO$_2$ films and is achieved in a
doping-free manner. Our results also reveal a one-order increase in resistance
switching, with the optimum bilayer VO$_2$/VO$_2$ film exhibiting ~3.6 orders
of switching from 25 $^\circ$C to 110 $^\circ$C, compared to the optimum
single-layer VO$_2$ reference film. This is accompanied by a one-order decrease
in the on-state resistance in its metallic phase. The elevation in TIMT,
coupled with increased strain extracted from the XRD characterization of the
bilayer film, suggests the possibility of compressive strain along the c-axis.
These VO$_{2-x}$/VO$_2$ films also demonstrate a significant change in the
slope of their resistance vs temperature curves contrary to the conventional
smooth transition. This feature was ascribed to the rutile/monoclinic
quasi-heterostructure formed due to the top VO$_{2-x}$ film having a reduced
TIMT. Our findings carry significant implications for both the lucid
fabrication of VO$_2$ thin film devices as well as the study of phase
transitions in correlated oxides.",2312.17437v1
2024-02-02,Light-induced phase transitions in vanadium dioxide: a tensor network study,"Nonequilibrium phase transitions driven by light pulses represent a rapidly
developing field in condensed matter physics. As one of the archetypal strongly
correlated materials, vanadium dioxide (VO2) undergoes a structural phase
transition (SPT) from a monoclinic (M1) to rutile (R) structure and an
insulator-to-metal transition (IMT) either when heated above 340 K or when
excited by an ultrafast laser pulse. Here, we present a tensor network study of
the light-induced phase transitions in VO2 based on a quasi-one-dimensional
model with all the important ingredients -- multi-orbital character,
electron-lattice coupling, and electron-electron correlations -- being
included. We show that this model qualitatively captures the equilibrium
properties of VO2 by calculating the ground state phase diagram and
finite-temperature phase transitions. A hybrid quantum-classical tensor-network
method is used to simulate the dynamics following photoexcitation. We find that
the structure can transform faster than the harmonic phonon modes of M1 phase,
suggesting lattice nonlinearity is key in the SPT. We also find separate
timescales for the evolution of dimerization and tilt distortions in the
lattice dynamics, as well as the loss and subsequent partial restoration
behavior of the displacements, which can provide an explanation for the complex
dynamics observed in recent experiments [C. Brahms et al., arXiv:XXXX.XXXXX].
Moreover, decoupled SPT and IMT dynamics are observed in the numerical
simulations: while the initial M1 structure transforms to the R one in tens of
femtoseconds, the IMT occurs quasi-instantaneously, consistent with recent
experimental findings. Our theoretical studies provide insight into the
light-induced phase transitions of VO2, revealing unexpected non-monotonic
transformation pathways and paving the way for future studies of non-thermal
phase transformations.",2402.01247v2
1997-10-15,Near-infrared narrow-band photometry of M-giant and Mira stars: models meet observations,"From near-infrared, narrow-band photometry of 256 oxygen-rich Mira variables
we obtain evidence about the loops that these stars follow in colour-colour
diagrams. We also find a phase lag between indices related to molecular
band-strength of titanium oxide and vanadium oxide. We compute colours for
normal M-giants and Miras using hydrostatic and hydrodynamic model atmospheres
and very extensive up-to-date line lists. Normal M-giants colours are well
reproduced, reaching a high quantitative agreement with observations for
spectral types earlier than M7. The out-of-phase variations of the various
spectral features of Miras are also acceptably reproduced, despite limitations
in the modelling. This enables us to confirm that the phase lag phenomenon
results from the propagation of perturbations in the extended atmosphere. It
opens new perspectives in the spectral modelling of Miras.",9710157v1
1996-03-04,Convergent expansions for properties of the Heisenberg model for CaV$_4$O$_9$,"We have carried out a wide range of calculations for the $S=1/2$ Heisenberg
model with nearest- and second-neighbor interactions on a two-dimensional
lattice which describes the geometry of the vanadium ions in the spin-gap
system CaV$_4$O$_9$. The methods used were convergent high-order perturbation
expansions (``Ising'' and ``Plaquette'' expansions at $T=0$, as well as
high-temperature expansions) for quantities such as the uniform susceptibility,
sublattice magnetization, and triplet elementary excitation spectrum.
Comparison with the data for CaV$_4$O$_9$ indicates that its magnetic
properties are well described by nearest-neighbor exchange of about 200K in
conjunction with second-neighbor exchange of about 100K.",9603025v1
1997-11-06,"Raman, infrared and optical spectra of the spin-Peierls compound NaV_2O_5","We have measured polarized spectra of Raman scattering, infrared and optical
transmission of NaV_2O_5 single crystals above the temperature of the
spin-Peierls transition Tsp=35 K. Some of the far-infrared (FIR) phonon lines
are strongly asymmetric, due to the spin-phonon interaction. In addition to the
phonon lines, a broad band was observed in the c(aa)c Raman spectrum and in the
E||a FIR transmission spectrum. A possible origin of these bands is discussed.
The absorption band at 10000 cm-1 1.25 eV is attributed to vanadium d-d
electronic transitions while the absorption edge above 3 eV is supposed to
correspond to the onset of charge-transfer transitions.",9711048v1
1998-01-12,Band-width control in a perovskite-type 3d^1 correlated metal Ca_1-xSr_xVO_3. II. Optical spectroscopy investigation,"Optical conductivity spectra of single crystals of Ca_1-xSr_xVO_3 have been
studied to elucidate how the electronic behavior depends on the strength of the
electron correlation without changing the nominal number of electrons per
vanadium atom. The effective mass deduced by the analysis of the Drude-like
contribution do not show critical enhancement, even though the system is close
to the Mott transition. Besides the Drude-like contribution, two anomalous
features were observed in the optical conductivity spectra of the intraband
transition within the 3d band. These features can be assigned to transitions
involving the incoherent and coherent bands near the Fermi level. The large
spectral weight redistribution in this system, however, does not involve a
large mass enhancement.",9801086v2
1998-02-03,Magnetic properties of the coupled ladder system MgV$_2$O$_5$,"We present magnetic susceptibility measurements on MgV$_2$O$_5$, a compound
in which the vanadium oxide planes have the same topology as in CaV$_2$O$_5$.
The most striking property is that there is an energy gap of about 15 K, much
smaller than in CaV$_2$O$_5$, where the values reported are of the order of 500
K. We show that this may be understood naturally in terms of the phase diagram
of the frustrated, coupled ladder system. This analysis leads to the prediction
that MgV$_2$O$_5$ should have incommensurate dynamic spin fluctuations.",9802029v1
1998-06-05,Inversion symmetry in the spin-Peierls compound NaV2O5,"At room-temperature NaV2O5 was found to have the centrosymmetric space group
Pmmn. This space group implies the presence of only one kind of V site in
contrast with previous reports of the non-centrosymmetric counterpart P21mn.
This indicates a non-integer valence state of vanadium.
  Furthermore, this symmetry has consequences for the interpretation of the
transition at 34 K, which was ascribed to a spin-Peierls transition of one
dimensional chains of V4+.",9806081v2
1998-08-15,High frequency dielectric and magnetic anomaly at the phase transition in NaV2O5,"We found anomalies in the temperature dependence of the dielectric and the
magnetic susceptibiliy of NaV_2O_5 in the microwave and far infrared frequency
ranges. The anomalies occur at the phase transition temperature T_c, at which
the spin gap opens. The real parts of the dielectric constants epsilon_a and
epsilon_c decrease below T_c. The decrease of epsilon_a (except for the narrow
region close to T_c) is proportional to the intensity of the x-ray reflection
appearing at T_c. The dielectric constant anomaly can be explained by the
zigzag charge ordering in the ab-plane appearing below T_c. The anomaly of the
microwave magnetic losses is probably related to the coupling between the spin
and charge degrees of freedom in vanadium ladders.",9808165v2
1999-06-14,Electronic structure and magnetic interactions in LiV2O4,"We present results of all-electron electronic structure calculations for the
recently discovered d electron heavy fermion compound LiV_2O_4. The augmented
spherical wave calculations are based on density functional theory within the
local density approximation. The electronic properties near the Fermi energy
originate almost exclusively from V 3d t_{2g} states, which fall into two
equally occupied subbands: While sigma-type metal-metal bonding leads to rather
broad bands, small pi-type p-d overlap causes a narrow peak at E_F. Without the
geometric frustration inherent in the crystal structure, spin-polarized
calculations reveal an antiferromagnetic ground state and ferromagnetic order
at slightly higher energy. Since direct d-d exchange interaction plays only a
minor role, ordering of the localized vanadium moments can be attributed
exclusively to a rather weak superexchange interaction. With the magnetic order
suppressed by the geometric frustration, the remaining spin fluctuations
suggest an explanation of the low temperature behaviour of the specific heat.",9906194v1
1999-06-25,Direct exchange in the edge-sharing spin-1/2 compound MgVO_3,"Bandstructure calculations with different spin arrangement for the spin-chain
compound MgVO_3 have been performed, and paramagnetic as well as magnetic
solutions with ferro- and antiferromagnetically ordered chains are found, the
magnetic solutions being by 0.22 eV per formula unit lower than the
paramagnetic one. The orbital analysis of the narrow band crossing the Fermi
level in the paramagnetic solution reveals that the band has almost pure
vanadium 3d character, the lobes of the relevant d-orbitals at the neighboring
in-chain sites being directed towards each other, which suggests direct
exchange. The tight-binding analysis of the band confirms the strong exchange
transfer between neighboring in-chain V-ions. Besides, some additional
superexchange transfer terms are found, which give rise both to in-plane
coupling between the chains and to frustration, the dominant frustration
occuring due to the interchain interaction.",9906385v1
1999-07-15,First evidence for charge ordering in NaV$_2$O$_5$ from Raman spectroscopy,"We argue on the basis of symmetry selection rules and Raman scattering
spectra that NaV$_2$O$_5$ undergoes a charge ordering phase transition at
T$_c$=34 K. Such a transition is characterized by the redistribution of the
charges at the phase transition and corresponds to the change of the vanadium
ions, from uniform V$^{4.5+}$ to two different V$^{4+}$ and V$^{5+}$ states. In
the low temperature phase the V$^{4+}$ ions are forming a ""zig-zag"" ladder
structure along the {\bf b}-axis, consistent with the symmetry of the P2/b
space group.",9907218v1
1999-12-22,Valence band excitations in V_2O_5,"We present a joint theoretical and experimental investigation of the
electronic and optical properties of vanadium pentoxide. Electron energy-loss
spectroscopy in transmission was employed to measure the momentum-dependent
loss function. This in turn was used to derive the optical conductivity, which
is compared to the results of band structure calculations. A good qualitative
and quantitative agreement between the theoretical and the experimental optical
conductivity was observed. The experimentally observed anisotropy of the
optical properties of V_2O_5 could be understood in the light of an analysis of
the theoretical data involving the decomposition of the calculated optical
conductivity into contributions from transitions into selected energy regions
of the conduction band. In addition, based upon a tight binding fit to the band
structure, values are given for the effective V3d_xy-O2p hopping terms and are
compared to the corresponding values for alpha'-NaV_2O_5.",9912400v1
2000-01-13,Friedel Oscillations and Charge-density Waves Pinning in Quasi-one-dimensional Conductors: An X-ray Access,"We present an x-ray diffraction study of the Vanadium-doped blue bronze
K0.3(Mo0.972V0.028)O3. At low temperature, we have observed both an intensity
asymmetry of the +-2kF satellite reflections relative to the pure compound, and
a profile asymmetry of each satellite reflections. We show that the profile
asymmetry is due to Friedel oscillation around the V substituant and that the
intensity asymmetry is related to the charge density wave (CDW) pinning. These
two effects, intensity and profile asymmetries, gives for the first time access
to the local properties of CDW in disordered systems, including the pinning and
even the phase shift of FOs.",0001189v2
2000-03-08,X-ray Anomalous Scattering Study of a Charge-Ordered State in NaV2O5,"Charge ordering of V4+ and V5+ in NaV2O5 has been studied by an X-ray
diffraction technique using anomalous scattering near a vanadium K-absorption
edge to critically enhance a contrast between the two ions. A dramatic energy
dependence of the superlattice intensities is observed below Tc=35 K.
Consequently, the charge ordering pattern is the zigzag-type ladders with the
unit cell 2a*2b*4c, but not the chain-type originally proposed for the
spin-Peierls state. Charge disproportionation suggested in our model as the
average valence V^{4.5+-delta_c/2} is observed below T_C, showing continuous
variation of delta_c as a function of temperature.",0003129v3
2000-06-26,Charge-ordering phase transition and order-disorder effects in the Raman spectra of NaV2O5,"In the ac polarized Raman spectra of NaV2O5 we have found anomalous phonon
broadening, and an energy shift of the low-frequency mode as a function of the
temperature. These effects are related to the breaking of translational
symmetry, caused by electrical disorder that originates from the fluctuating
nature of the V {4.5+} valence state of vanadium. The structural correlation
length, obtained from comparisons between the measured and calculated Raman
scattering spectra, diverges at T< 5 K, indicating the existence of the
long-range charge order at very low temperatures, probably at T=0 K.",0006408v2
2000-09-29,Spin-Isospin Model of NaV_2O_5,"We argue that in the quarter-filled ladder compound NaV_2O_5 the
quasi-one-dimensional spin system is strongly coupled to a low-energy
antiferroelectric mode of the excitonic type. This mode results from the
interplay between the electron hopping along the rungs of the vanadium two-leg
ladders and the Coulomb repulsion between electrons. The charge ordering
observed in sodium vanadate at T_c = 34K corresponds to the softening of the
antiferroelectric mode. We consider the spin-isospin model, which describes the
spin and low-energy charge degrees of freedom in sodium vanadate. Within this
model we explain the observed anomalous temperature-dependence of the
dielectric susceptibility at T_c and the midinfrared absorption continuum. We
identify the broad structure in the low-energy optical absorption spectrum of
NaV_2O_5 with the continuum formed by two spinons and one low-energy charge
excitation.",0009464v1
2001-01-18,Modeling the electronic behavior of $γ$-LiV2O5: a microscopic study,"We determine the electronic structure of the one-dimensional spin-1/2
Heisenberg compound $\gamma$-LiV$_2$O$_5$, which has two inequivalent vanadium
ions, V(1) and V(2), via density-functional calculations. We find a relative
V(1)-V(2) charge ordering of roughly $70:30$. We discuss the influence of the
charge ordering on the electronic structure and the magnetic behavior. We give
estimates of the basic hopping matrix elements and compare with the most
studied $\alpha '$-NaV$_2$O$_5$.",0101282v3
2001-05-08,Charge excitations in NaV$_{2}$O$_{5}$,"We calculate the electron-energy loss spectrum and the optical conductivity
for NaV$_{2}$O$_{5}$ using the standard Lanczos algorithm. The vanadium ions in
NaV$_{2}$O$_{5}$ form a system of coupled ladders which can be described by a
quarter-filled extended Hubbard model. Since this system has a large unit cell,
one has to be very careful to avoid finite-size effects in the calculations. We
show this by performing exact diagonalization of different clusters with up to
16 sites. The calculated loss function for the extended Hubbard model shows
good agreement with experimental spectra. Furthermore, a qualitative
description of the optical conductivity is obtained with the same Hamiltonian,
and the same set of model parameters. The comparison with the experiment shows
that interladder hopping is of minor importance for a realistic description of
charge excitations in NaV$_{2}$O$_{5}$. We find that the character of the
excitations depends strongly on the direction of momentum transfer.",0105165v2
2001-06-12,Charge excitations in LiV$_{2}$O$_{5}$ and NaV$_{2}$O$_{5}$: similarities and differences,"We calculate the optical conductivity of LiV$_{2}$O$_{5}$ and
NaV$_{2}$O$_{5}$ using exact numerical diagonalization of a quarter-filled
extended Hubbard model on a system of coupled ladders. In particular,
electronic correlations are treated exactly, and a quantitative agreement
between calculated and experimentally observed optical conductivity of these
two vanadium oxides is presented. Furthermore, it is found that
LiV$_{2}$O$_{5}$ differs from NaV$_{2}$O$_{5}$ not only in the charge ordering
pattern but also in the nature of the inter-ladder coupling: In contrast to
LiV$_{2}$O$_{5}$, in NaV$_{2}$O$_{5}$ neighboring ladders are coupled by a
strong Coulomb repulsion, and not by inter-ladder hopping.",0106213v3
2001-06-21,Superconductivity mediated by charge fluctuations in layered molecular crystals,"In elemental metals the pairing of electrons leading to superconductivity is
mediated by the vibrations of the crystal lattice and the pairs have spherical
symmetry. For layered organic molecular crystals there is no consensus about
the pairing symmetry or the mechanism of the superconductivity. We show
theoretically that the mechanism of superconductivity for members of the
(BEDT-TTF)2X family with the theta and beta'' crystal structures is mediated by
charge fluctuations and the superconducting order parameter has dxy symmetry.
This is in contrast to the kappa-(BEDT-TTF)2X family, for which theoretical
calculations give superconductivity mediated by spin fluctuations and with
dx2-y2 symmetry. Our conclusions are based on systematic many-body calculations
using slave-boson theory of an extended Hubbard model. This is the simplest
model that can describe the competition between metallic, superconducting,
insulating, and charge ordered phases that occurs in the theta and beta''
materials. Based on the theory presented we predict several materials that
should become superconducting under pressure.",0106425v2
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
2001-09-05,X-ray anomalous scattering investigations on the charge order in $α^\prime$-NaV$_2$O$_5$,"Anomalous x-ray diffraction studies show that the charge ordering in
$\alpha^\prime$-NaV$_2$O$_5$ is of zig-zag type in all vanadium ladders. We
have found that there are two models of the stacking of layers along
\emph{c-}direction, each of them consisting of 2 degenerated patterns, and that
the experimental data is well reproduced if the 2 patterns appears
simultaneously. We believe that the low temperature structure contains stacking
faults separating regions corresponding to the four possible patterns.",0109091v1
2001-11-07,Atomic scale imaging and spectroscopy of the V_2O_3 (0001)-surface: bulk versus surface effects,"We present atomic scale images of a V_2O_3 (0001)-surface, which show that
the surface is susceptible to reconstruction by dimerization of vanadium ions.
The atomic order of the surface depends sensitively on the surface preparation.
Scanning tunneling spectroscopy proves a dimerized surface has a gap in the
electronic density of states at the Fermi energy, while a surface prepared by
sputtering and successive annealing shows no dimerization and no gap.
Photoemission spectra depend sensitively on the surface structure and are
consistent with scanning tunneling spectroscopy data. The measurements explain
inconsistencies in photoemission experiments performed on such oxides in the
past.",0111113v1
2002-01-27,Electron Correlation Effects in Resonant Inelastic X-ray Scattering of NaV2O5,"Element- and site-specific resonant inelastic x-ray scattering spectroscopy
(RIXS) is employed to investigate electron correlation effects in {$\rm
NaV_2O_5$}. In contrast to single photon techniques, RIXS at the vanadium $L_3$
edge is able to probe $d-d^*$ transitions between V d-bands. A sharp energy
loss feature is observed at -1.56 eV, which is well reproduced by a model
calculation including correlation effects. The calculation identifies the loss
feature as excitation between the lower and upper Hubbard bands and permits an
accurate determination of the Hubbard interaction term $U= 3.0 \pm 0.2$ eV.",0201497v1
2002-02-05,Holographic analysis of diffraction structure factors,"We combine the theory of inside-source/inside-detector x-ray fluorescence
holography and Kossel lines/x ray standing waves in kinematic approximation to
directly obtain the phases of the diffraction structure factors. The influence
of Kossel lines and standing waves on holography is also discussed. We obtain
partial phase determination from experimental data obtaining the sign of the
real part of the structure factor for several reciprocal lattice vectors of a
vanadium crystal.",0202061v2
2002-02-24,Orbital-resolved Soft X-Ray Spectroscopy in NaV2O5,"We demonstrate that angle-resolved soft x-ray spectroscopy can resolve
absorption by inequivalent oxygen sites and by different orbitals belonging to
the same site in NaV2O5. By rotating the polarization direction, we see a
dramatic change in the absorption spectra at the oxygen K edge. Our theory
identifies the detailed composition of the spectra and predicts a correct
energy-ordering of the orbitals of three inequivalent oxygen atoms. Because
different orbitals dominate absorption spectra at different energies and
angles, one can excite at a specific site and ``orbital''. In contrast,
absorption at the vanadium L edge does not show large changes when varying the
polarization direction. The reason for this is that different excitation
channels (involving different initial states for the excited electron) overlap
in energy and vary in compensating ways, obscuring each channel's sensitive
polarization dependence.",0202424v1
2002-03-11,Pressure dependence of phase transitions in the quasi one-dimensional metal-insulator transition system beta-Na1/3V2O5,"The pressure dependence of phase transitions in the quasi one-dimensional
vanadium oxide $\beta$-Na$_{1/3}$V$_2$O$_5$ has been studied by magnetic
susceptibility and electrical resistivity measurements. The pressure dependence
of the various transition temperatures is quite differently. The transition at
T=240 K, previously reported and attributed to ordering on Na sites, and a
second transition at $T \approx 222$ K, reported here for the first time and
attributed to a further increase of order on Na sites, are almost independent
of pressure. On the other hand, the metal-insulator (MI) transition at
$T_{MI}=130$ K shifts to lower temperatures, while the magnetic transition at
$T_N=24$ K shifts to higher temperatures with increasing pressure. We discuss
the different pressure dependencies of $T_{MI}$ and $T_N$ in terms of
increasing interchain coupling and the MI transition to be of Peierls type.",0203234v1
2002-03-14,Optical properties of NaxV2O5,"The optical properties of sodium-deficient NaxV2O5 (0.85 < x <1) single
crystals are analyzed in the wide energy range, from 0.012 to 4.5 eV, using
ellipsometry, infrared reflectivity, and Raman scattering techniques. The
material remains insulating up to the maximal achieved hole concentration of
about 15%. In sodium deficient samples the optical absorption peak associated
to the fundamental electronic gap develops at about 0.44 eV. It corresponds to
the transition between vanadium dxy and the impurity band, which forms in the
middle of the pure NaV2O5 gap. Raman spectra measured with incident photon
energy larger then 2 eV show strong resonant behavior, due to the presence of
the hole-doping activated optical transitions, peaked at 2.8 eV.",0203298v1
2002-06-25,Genuine Electronic States of Vanadium Perovskites Revealed by High-Energy Photoemission,"Bulk-sensitive high-resolution photoemission was carried out on a prototype
3d1 metallic Sr1-xCaxVO3. In a strong contrast to so far reported results, the
bulk spectral functions are revealed to be insensitive to x. The conservation
of the density of states at the Fermi level in spite of the electron
correlation is clarified by the successful suppression and deconvolution of the
surface contribution. Our study has demonstrated the importance of high-energy
and high-resolution photoemission spectroscopy for revealing detailed bulk
electronic states of transition metal oxides.",0206471v2
2002-07-24,A theoretical study of the structural phases of Group 5B - 6B metals and their transport properties,"In order to predict the stable and metastable phases of the bcc metals in the
block of the Periodic Table defined by groups 5B to 6B and periods 4 to 6, as
well as the structure dependence of their transport properties, we have
performed full potential computations of the total energies per unit cell as a
function of the c/a ratio at constant experimental volume. In all cases, a
metastable body centered tetragonal (bct) phase was predicted from the
calculations. The total energy differences between the calculated stable and
metastable phases ranged from 0.09 eV/cell (vanadium) to 0.39 eV/cell
(tungsten). The trends in resistivity as a function of structure and atomic
number are discussed in terms of a model of electron transport in metals.
Theoretical calculations of the electrical resistivity and other transport
properties show that bct phases derived from group 5B elements are more
conductive than the corresponding bcc phases, while bct phases formed from
group 6B elements are less conductive than the corresponding bcc phases.
Special attention is paid to the phases of tantalum where we show that the
frequently observed beta phase is not a simple tetragonal distortion of bcc
tantalum.",0207590v1
2002-09-17,Sr2V3O9 and Ba2V3O9: quasi one-dimensional spin-systems with an anomalous low temperature susceptibility,"The magnetic behaviour of the low-dimensional Vanadium-oxides Sr2V3O9 and
Ba2V3O9 was investigated by means of magnetic susceptibility and specific heat
measurements. In both compounds, the results can be very well described by an
S=1/2 Heisenberg antiferromagnetic chain with an intrachain exchange of J = 82
K and J = 94 K in Sr2V3O9 and Ba2V3O9, respectively. In Sr2V3O9,
antiferromagnetic ordering at T_N = 5.3 K indicate a weak interchain exchange
of the order of J_perp ~ 2 K. In contrast, no evidence for magnetic order was
found in Ba2V3O9 down to 0.5 K, pointing to an even smaller interchain
coupling. In both compounds, we observe a pronounced Curie-like increase of the
susceptibility below 30 K, which we tentatively attribute to a staggered field
effect induced by the applied magnetic field. Results of LDA calculations
support the quasi one-dimensional character and indicate that in Sr2V3O9, the
magnetic chain is perpendicular to the structural one with the magnetic
exchange being transferred through VO4 tetrahedra.",0209409v1
2002-10-09,Magnetic susceptibility in quasi one-dimensional Ba2V3O9: chain segmentation versus the staggered field effect,"A pronounced Curie-like upturn of the magnetic susceptibility chi(T) of the
quasi one-dimensional spin chain compound Ba2V3O9 has been found recently.
Frequently this is taken as a signature for a staggered field mechanism due to
the presence of g-factor anisotropy and Dzyaloshinskii-Moriya interaction. We
calculate this contribution within a realistic structure of vanadium 3d- and
oxygen 2p-orbitals and conclude that this mechanism is far too small to explain
experimental results. We propose that the Curie term is rather due to a
segmentation of spin chains caused by broken magnetic bonds which leads to
uncompensated S=1/2 spins of segments with odd numbers of spins. Using a
finite-temperature Lanczos method we calculate their effective moment and show
that ~1% of broken magnetic bonds is sufficient to reproduce the anomalous
low-T behavior of chi(T) in Ba2V3O9.",0210204v2
2002-11-25,Calculation of thermal expansion coefficient of Fe/sub 3/Al with the addition of transition metal elements,"The addition of transition metal elements can significantly modify physical
properties of intermetalic compounds. We studied the influence of Molybdenum
and Vanadium additives on thermal expansion coefficient (CTE) of Fe/sub 3/Al
and FeAl over the wide range of temperatures. The site preference of both
transition metals was determined by full-potential LMTO method within the
grandcanonical formalism. At low temperatures CTEs were found directly from the
FP-LMTO calculations by incorporating them into the Debye model of a solid. The
obtained thermal expansion for pure Fe/sub 3/Al and FeAl is within 10% of its
experimentally measured values. At high temperatures we performed molecular
dynamics simulations based on our many-body atomistic potentials. The
parameters were fitted to reproduce the total energy of a crystal under various
types of deformations obtained by FP-LMTO method and were tested with respect
to different structures and vacancy formation energies. Our calculations show
that addition of V decreases the CTEs of both iron-aluminides, while the
addition of Mo makes Fe/sub 3/Al DO3 structure unstable.",0211569v1
2003-01-24,Raman scattering study of charge ordering in beta-Ca0.33V2O5,"Polarized Raman spectra of the calcium vanadium oxide bronze beta-Ca0.33V2O5
are measured in the temperature range between 300 K and 20 K. The charge
ordering phase transition at about 150 K is characterized by the appearance of
the new Raman-active modes in the spectra, and by anomalies in the electronic
background scattering. The high temperature Raman scattering spectra of
beta-Ca0.33V2O5 are in apparent resemblance with those of alpha'-NaV2O5, which
suggests that there is a similar charge-phonon dynamics in both compounds. The
study of dynamical properties and the symmetry analysis of the Raman modes show
that in the mixed valence state of beta-Ca0.33V2O5 the electrons are
delocalized into V1-O5-V3 orbitals. We propose that in the charge ordered state
below 150 K the d-electrons localize within V1-V3 ladders, either in ""zig-zag""
fashion like in alpha'-NaV2O5 or in the forms of the double chains like in
gamma-LiV2O5.",0301477v1
2003-05-12,Magnetic transition and orbital degrees of freedom in vanadium spinels,"We propose a scenario for the two phase transitions in $A$V$_2$O$_4$ ($A$=Zn,
Mg, Cd), based on an effective spin-orbital model on the pyrochlore lattice. At
high temperatures, spin correlations are strongly frustrated due to the lattice
structure, and the transition at $\sim$50 [K] is an orbital order, supported by
Jahn-Teller lattice distortion. This orbital order introduces spatial
modulation of spin exchange couplings depending on the bond direction. This
partially releases the frustration, and leads to a spin order at $\sim$40 [K].
We also study the stable spin configuration by taking account of third-neighbor
exchange couplings and quantum fluctuations. The result is consistent with the
experimental results.",0305269v1
2003-07-10,New Sol-Gel Synthesis of Ordered Nanostructured Doped ZnO Films,"A novel sol-gel route to c-axis orientated undoped and Co, Fe, Mn and V doped
ZnO films is reported. Sols were prepared from a hydrated zinc acetate
precursor and dimethyl formamide (DMF) solvent. Films were spin-coated on to
hydrophilic sapphire substrates then dried, annealed and post-annealed,
producing almost purely uniaxial ZnO crystallites and a high degree of
long-range structural order. Specific orientation of hexagonal crystallites is
demonstrated both perpendicular and parallel to the substrate surface. Cobalt
doping resulted in the formation of columnar ZnO nanocrystals. Vanadium doped
films formed the spinel oxide ZnAl2O4, resulting from the reaction between ZnO
and the sapphire substrate. Structural, optical and morphological
characterisation demonstrated the high quality of the films.",0307254v1
2003-11-27,Fabrication and Electrical Properties of Pure VO2 Phase Films,"We have grown VO2 thin films by laser ablation for electronic device
applications. In obtaining the thin films of the pure VO2 phase, oxygen partial
pressure is a critical parameter because vanadium oxides have several phases
with the oxygen concentration. It is found that the pure VO2 films are
epitaxially grown on Al2O3 substrate in the narrow ranges of 55-60 mTorr in an
Ar+10% O2 ambient, and that the mixed phase films are synthesized when the
deposition pressure slightly deviates from the optimum pressure. The (100)
oriented VO2 films undergo an abrupt metal-insulator transition (MIT) with
resistance change of an order of 104 at 338K. In the films of mixed phases, the
small change of the resistance is observed at the same temperature. The
polycrystalline films grown on SiO2/Si substrate undergo a broaden MIT of the
resistance. Furthermore, the abrupt MIT and collective current motion appearing
in metal are observed when the electric field is applied to the film.",0311616v2
2003-12-05,Magnetotransport near a quantum critical point in a simple metal,"We use geometric considerations to study transport properties, such as the
conductivity and Hall coefficient, near the onset of a nesting-driven spin
density wave in a simple metal. In particular, motivated by recent experiments
on vanadium-doped chromium, we study the variation of transport coefficients
with the onset of magnetism within a mean-field treatment of a model that
contains nearly nested electron and hole Fermi surfaces. We show that most
transport coefficients display a leading dependence that is linear in the
energy gap. The coefficient of the linear term, though, can be small. In
particular, we find that the Hall conductivity $\sigma_{xy}$ is essentially
unchanged, due to electron-hole compensation, as the system goes through the
quantum critical point. This conclusion extends a similar observation we made
earlier for the case of completely flat Fermi surfaces to the immediate
vicinity of the quantum critical point where nesting is present but not
perfect.",0312168v1
2004-04-02,"Charge order, orbital order, and electron localization in the Magneli phase Ti4O7","The metal-insulator transition of the Magneli phase Ti4O7 is studied by means
of augmented spherical wave (ASW) electronic structure calculations as based on
density functional theory and the local density approximation. The results show
that the metal-insulator transition arises from a complex interplay of charge
order, orbital order, and singlet formation of those Ti 3d states which mediate
metal-metal bonding inside the four-atom chains characteristic of the material.
Ti4O7 thus combines important aspects of Fe3O4 and VO2. While the charge
ordering closely resembles that observed at the Verwey transition, the orbital
order and singlet formation appear to be identical to the mechanisms driving
the metal-insulator transition of vanadium dioxide.",0404059v1
2004-04-03,"Optical properties, electron-phonon coupling, and Raman scattering of vanadium ladder compounds","The electronic structure of two V-based ladder compounds, the quarter-filled
NaV$_2$O$_5$ in the symmetric phase and the iso-structural half-filled
CaV$_2$O$_5$ is investigated by ab initio calculations. Based on the
bandstructure we determine the dielectric tensor $\epsilon(\omega)$ of these
systems in a wide energy range. The frequencies and eigenvectors of the fully
symmetric A$_{g}$ phonon modes and the corresponding electron-phonon and
spin-phonon coupling parameters are also calculated from first-principles. We
determine the Raman scattering intensities of the A$_g$ phonon modes as a
function of polarization and frequency of the exciting light.
  All results, i.e. shape and magnitude of the dielectric function, phonon
frequencies and Raman intensities show very good agreement with available
experimental data.",0404076v2
2004-06-03,Proximity effect of vanadium on spin-density-wave magnetism in Cr films,"The spin-density wave (SDW) state in thin chromium films is well known to be
strongly affected by proximity effects from neighboring layers. To date the
main attention has been given to effects arising from exchange interactions at
interfaces. In the present work we report on combined neutron and synchrotron
scattering studies of proximity effects in Cr/V films where the boundary
condition is due to the hybridization of Cr with paramagnetic V at the
interface. We find that the V/Cr interface has a strong and long-range effect
on the polarization, period, and the N\'{e}el temperature of the SDW in rather
thick Cr films. This unusually strong effect is unexpected and not predicted by
theory.",0406071v1
2004-08-04,Rietveld refinement and electron density distribution in CuxV2O5,"Room temperature powder x-ray diffraction measurements have been carried out
on polycrystalline samples of CuxV2O5 with x = 0.33, 0.40, 0.50, 0.55 and 0.60,
prepared by solid state reaction in evacuated quartz tubes. All the samples
were found to be in single-phase monoclinic structure. Rietveld full profile
refinement of the x-ray diffraction data yielded the lattice parameters and
fractional atom coordinates. Maximum entropy inversion of the diffraction data
was done in order to obtain the variation in the spatial electron density
distribution as a function of copper composition. These density maps show
progressive positional and charge disorders in the V-O network induced by
addition of copper. It is shown that the electrons donated by copper atoms are
transferred to the specific vanadium atom sites.",0408084v1
2004-11-27,Point defect dynamics in bcc metals,"We present an analysis of the time evolution of self-interstitial atom and
vacancy (point defect) populations in pure bcc metals under constant
irradiation flux conditions. Mean-field rate equations are developed in
parallel to a kinetic Monte Carlo (kMC) model. When only considering the
elementary processes of defect production, defect migration, recombination and
absorption at sinks, the kMC model and rate equations are shown to be
equivalent and the time evolution of the point defect populations is analyzed
using simple scaling arguments. We show that the typically large mismatch of
the rates of interstitial and vacancy migration in bcc metals can lead to a
vacancy population that grows as the square root of time. The vacancy cluster
size distribution under both irreversible and reversible attachment can be
described by a simple exponential function. We also consider the effect of
highly mobile interstitial clusters and apply the model with parameters
appropriate for vanadium and $\alpha-$iron.",0411680v1
2005-05-12,Orbital selective insulator-metal transition in V2O3 under external pressure,"We present a detailed account of the physics of Vanadium sesquioxide (${\rm
V_2O_3}$), a benchmark system for studying correlation induced metal-insulator
transition(s). Based on a detailed perusal of a wide range of experimental
data, we stress the importance of multi-orbital Coulomb interactions in concert
with first-principles LDA bandstructure for a consistent understanding of the
PI-PM MIT under pressure. Using LDA+DMFT, we show how the MIT is of the orbital
selective type, driven by large changes in dynamical spectral weight in
response to small changes in trigonal field splitting under pressure. Very good
quantitative agreement with ($i$) the switch of orbital occupation and ($ii$)
S=1 at each $V^{3+}$ site across the MIT, and ($iii$) carrier effective mass in
the PM phase, is obtained. Finally, using the LDA+DMFT solution, we have
estimated screening induced renormalisation of the local, multi-orbital Coulomb
interactions. Computation of the one-particle spectral function using these
screened values is shown to be in excellent quantitative agreement with very
recent experimental (PES and XAS) results. These findings provide strong
support for an orbital-selective Mott transition in paramagnetic ${\rm
V_2O_3}$.",0505317v1
2005-06-17,On the nature of the magnetic ground-state wave function of V_2O_3,"After a brief historical introduction, we dwell on two recent experiments in
the low-temperature, monoclinic phase of V_2O_3: K-edge resonant x-ray
scattering and non-reciprocal linear dichroism, whose interpretations are in
conflict, as they require incompatible magnetic space groups. Such a conflict
is critically reviewed, in the light of the present literature, and new
experimental tests are suggested, in order to determine unambiguously the
magnetic group. We then focus on the correlated, non-local nature of the
ground-state wave function, that is at the basis of some drawbacks of the LDA+U
approach: we singled out the physical mechanism that makes LDA+U unreliable,
and indicate the way out for a possible remedy. Finally we explain, by means of
a symmetry argument related to the molecular wave function, why the magnetic
moment lies in the glide plane, even in the absence of any local symmetry at
vanadium sites.",0506431v1
2005-06-23,"Interplay among Spin, Orbital and Lattice Degrees of Freedom in $t_{2g}$ Electron Systems with Edge-Sharing Network of Octahedra","Transition metal oxides whose lattice structure has edge-sharing network of
octahedra constitute a diverse group of intriguing materials besides compounds
with corner-sharing octahedra such as perovskites. We present a theoretical
investigation of the interplay among spin, orbital and lattice degrees of
freedom in these materials. We focus on $t_{2g}$ electron systems where a keen
competition among those degrees of freedom is expected to emerge under a
relatively weak Jahn-Teller coupling. We study the interplay between spin and
orbital degrees of freedom in vanadium spinels and titanium pyroxenes. We
clarify the important role of the strong anisotropy in the orbital interactions
due to the edge-sharing geometry. We also discuss the interplay between spin
and lattice in chromium spinels focusing on the magnetization process under the
external magnetic field.",0506598v1
2005-08-09,Structural precursor to the metal-insulator transition in V_2O_3,"The temperature dependence of the local structure of V_2O_3 in the vicinity
of the metal to insulator transition (MIT) has been investigated using hard
X-ray absorption spectroscopy. It is shown that the vanadium pair distance
along the hexagonal c-axis changes abruptly at the MIT as expected. However, a
continuous increase of the tilt of these pairs sets in already at higher
temperatures and reaches its maximum value at the onset of the electronic and
magnetic transition. These findings confirm recent theoretical results which
claim that electron-lattice coupling is important for the MIT in V_2O_3. Our
results suggest that interactions in the basal plane play a decisive role for
the MIT and orbital degrees of freedom drive the MIT via changes in
hybridization.",0508221v2
2006-04-17,Quantum distortion caused by magneto-elastic coupling for antiferromagnetic tetrahedral cluster of spin-1/2,"We study the effects of magneto-elastic coupling on the degenerate ground
spin-state of the antiferromagnetic Heisenberg model on a regular tetrahedron
of spin-1/2.
  When displacement of spin is considered as a classical variable, the
degeneracy is lifted through the distance dependence of exchange coupling,
i.e., a kind of Jahn-Teller effect.
  On the other hand, when displacement is considered as a quantum variable, the
degeneracy of the ground spin-state is not lifted although the tetrahedron is
distorted by quantum fluctuation caused by magneto-elastic coupling for a
component of the normal coordinates of the degenerate mode. We propose a new
model for the structural phase transition of vanadium and nickel spinels: the
tetragonal distortion is caused by quantum fluctuation and there exists a kind
of hidden order with respect to the non-magnetic ground spin-states.",0604397v2
2006-04-21,Muon spin rotation measurements of the vortex state in vanadium: A comparative analysis using iterative and analytical solutions of the Ginzburg-Landau equations,"We report muon spin rotation measurements on a single crystal of the marginal
type-II superconductor V. The measured internal magnetic field distributions
are modeled assuming solutions of the Ginzburg-Landau (GL) equations for an
ideal vortex lattice obtained using (i) an iterative procedure developed by
E.H. Brandt, Phys. Rev. Lett. 78, 2208 (1997) and (ii) a variational method.
Both models yield qualitatively similar results. The magnetic penetration depth
(lambda) and the GL coherence length (xi) determined from the data analysis
exhibit strong field dependences, which are attributed to changes in the
electronic structure of the vortex lattice. The zero-field extrapolated values
of lambda and the GL parameter kappa agree well with values obtained by other
experimental techniques that probe the Meissner state.",0604501v1
2006-05-02,Spin Singlet State in Heptamers Emerging in Spinel Oxide AlV$_2$O$_4$,"We present our theoretical results on the electronic state in vanadium spinel
oxide AlV$_2$O$_4$. The material is a mixed-valent system with the average
valence V$^{2.5+}$, and V cations constitute a frustrated pyrochlore structure.
It shows a structural transition at $\sim 700$ K, leading to the formation of
seven V-sites clusters -- heptamers. We study the electronic state of the
heptamer by explicitly taking account of orbital degree of freedom as well as
electron correlations. We show that the ground state of the heptamer for
realistic parameters becomes spin-singlet because of strong $\sigma$-type
bonding states of $t_{2g}$ orbitals. The temperature dependence of the magnetic
susceptibility in experiments is naturally understood by this singlet formation
in heptamers. Our results indicate that in AlV$_2$O$_4$ orbital physics is
relevant to stabilize a cluster-type singlet state instead of a previously
proposed charge-ordered state with valence skipping.",0605046v2
2007-01-12,Enhanced Crystal Field Splitting and Orbital Selective Coherence by Strong Correlations in V_2O_3,"We present a study of the paramagnetic metallic and insulating phases of
vanadium sesquioxide by means of the $N$th order muffin-tin orbital
implementation of density functional theory combined with dynamical mean-field
theory. The transition is shown to be driven by a correlation-induced
enhancement of the crystal field splitting within the $t_{2g}$ manifold, which
results in a suppression of the hybridization between the $a_{1g}$ and
$e_g^{\pi}$ bands. We discuss the changes in the effective quasi-particle band
structure caused by the correlations and the corresponding self-energies. At
temperatures of about 400 K we find the $a_{1g}$ orbitals to display coherent
quasi-particle behavior, while a large imaginary part of the self-energy and
broad features in the spectral function indicate that the $e_g^{\pi}$ orbitals
are still far above their coherence temperature. The local spectral functions
are in excellent agreement with recent bulk sensitive photoemission data.
Finally, we also make a prediction for angle-resolved photoemission experiments
by calculating momentum-resolved spectral functions.",0701263v1
2007-03-21,DFT investigation of 3d transition metal NMR shielding tensors in diamagnetic systems using the gauge-including projector augmented-wave method,"We present a density functional theory based method for calculating NMR
shielding tensors for 3d transition metal nuclei using periodic boundary
conditions. Calculations employ the gauge-including projector augmented-wave
pseudopotentials method. The effects of ultrasoft pseudopotential and induced
approximations on the second-order magnetic response are intensively examined.
The reliability and the strength of the approach for 49Ti and 51V nuclei is
shown by comparison with traditional quantum chemical methods, using benchmarks
of finite organometallic systems. Application to infinite systems is validated
through comparison to experimental data for the 51V nucleus in various vanadium
oxide based compounds. The successful agreement obtained for isotropic chemical
shifts contrasts with full estimation of the shielding tensor eigenvalues,
revealing the limitation of pure exchange-correlation functionals compared to
their exact-exchange corrected analogues.",0703553v2
2003-10-15,Secondary Electron Yield Measurements of TiN Coating and TiZrV Getter Film,"In the beam pipe of the positron Main Damping Ring (MDR) of the Next Linear
Collider (NLC), ionization of residual gases and secondary electron emission
give rise to an electron cloud which can cause the loss of the circulating
beam. One path to avoid the electron cloud is to ensure that the vacuum wall
has low secondary emission yield and, therefore, we need to know the secondary
emission yield (SEY) for candidate wall coatings. We report on SEY measurements
at SLAC on titanium nitride (TiN) and titanium-zirconium-vanadium (TiZrV) thin
sputter-deposited films, as well as describe our experimental setup.",0310071v2
2007-02-06,{\it Ab initio} calculations of forbidden transition probabilities and lifetimes of low-lying states in V$^{4+}$,"Electric quadrupole (E2) and magnetic dipole (M1) transition amplitudes among
the low-lying states of quadruply ionized vanadium V$^{4+}$, important in
various field of experimental and astrophysics are presented very accurately.
Most of these results are reported for the first time in the literature.
Relativistic coupled-cluster theory with single, double and leading triple
excitations has been employed for these calculations. Estimation of different
correlation effects arising through the above formalism have been highlighted
by studying core and valence electrons excitations to the excited states. The
lifetime of the first excited $D$- state is found to be long.",0702052v2
2007-04-19,Competing itinerant and localized states in strongly correlated BaVS$_3$,"The electronic structure of the quasi-lowdimensional vanadium sulfide \bavs3
is investigated for the different phases above the magnetic ordering
temperature. By means of density functional theory and its combination with
dynamical-mean field theory, we follow the evolution of the relevant low-energy
electronic states on cooling. Hence we go in the metallic regime from the room
temperature hexagonal phase to the orthorhombic phase after the first
structural transition, and close with the monoclinic insulating phase below the
metal-insulator transition. Due to the low symmetry and expected intersite
correlations, the latter phase is treated within cellular dynamical mean-field
theory. It is generally discussed how the intriguing interplay between
band-structure and strong-correlation effects leads to the stabilization of the
various electronic phases with decreasing temperature.",0704.2501v1
2007-05-22,Magnetocaloric effect in the frustrated square lattice J1-J2 model,"We investigate the magnetocaloric properties of the two-dimensional
frustrated J1-J2 model on a square lattice. This model describes well the
magnetic behavior of two classes of quasi-two-dimensional S = 1/2 vanadates,
namely the Li2VOXO4 (X = Si, Ge) and AA'VO(PO4)2 (A, A' = Pb, Zn, Sr, Ba)
compounds. The magnetocaloric effect (MCE) consists in the adiabatic
temperature change upon changing the external magnetic field. In frustrated
systems, the MCE can be enhanced close to the saturation field because of
massive degeneracies among low lying excitations. We discuss results for the
MCE in the two distinct antiferromagnetic regimes of the phase diagram.
Numerical finite temperature Lanczos as well as analytical methods based on the
spin wave expansion are employed and results are compared. We give explicit
values for the saturation fields of the vanadium compounds. We predict that at
subcritical fields there is first a (positive) maximum followed by sign change
of the MCE, characteristic of all magnetically ordered phases.",0705.3094v2
2007-06-14,Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3,"The charge response in the barium vanadium sulfide (BaVS3) single crystals is
characterized by dc resistivity and low frequency dielectric spectroscopy. A
broad relaxation mode in MHz range with huge dielectric constant ~= 10^6
emerges at the metal-to-insulator phase transition TMI ~= 67 K, weakens with
lowering temperature and eventually levels off below the magnetic transition
Tchi ~= 30 K. The mean relaxation time is thermally activated in a manner
similar to the dc resistivity. These features are interpreted as signatures of
the collective charge excitations characteristic for the orbital ordering that
gradually develops below TMI and stabilizes at long-range scale below Tchi.",0706.2079v4
2007-08-15,Insulator to semiconductor transition and magnetic properties of the one-dimensional S = 1/2 system In_2VO_5,"We report structural, magnetization, electrical resistivity and nuclear- and
electron spin resonance data of the complex transition metal oxide In_2VO_5 in
which structurally well-defined V-O chains are realized. An itinerant character
of the vanadium d-electrons and ferromagnetic correlations, revealed at high
temperatures, are contrasted with the insulating behavior and predominantly
antiferromagnetic exchange between the localized V^{4+} S = 1/2-magnetic
moments which develop below a certain characteristic temperature T* ~ 120 K.
Eventually the compound exhibits short-range magnetic order at $T_SRO ~ 20 K.
We attribute this crossover occurring around T* to the unusual anisotropic
thermal contraction of the lattice which changes significantly the overlap
integrals and the character of magnetic intra- and interchain interactions.",0708.2088v1
2007-09-05,Thickness-dependence of the electronic properties in V2O3 thin films,"High quality vanadium sesquioxide V2O3 films (170-1100 {\AA}) were grown
using the pulsed laser deposition technique on (0001)-oriented sapphire
substrates, and the effects of film thickness on the lattice strain and
electronic properties were examined. X-ray diffraction indicates that there is
an in-plane compressive lattice parameter (a), close to -3.5% with respect to
the substrate and an out-of-plane tensile lattice parameter (c) . The thin film
samples display metallic character between 2-300 K, and no metal-to-insulator
transition is observed. At low temperature, the V2O3 films behave as a strongly
correlated metal, and the resistivity (\rho) follows the equation \rho =\rho_0
+ A T^2, where A is the transport coefficient in a Fermi liquid. Typical values
of A have been calculated to be 0.14 \mu\Omega cm K^{-2}, which is in agreement
with the coefficient reported for V2O3 single crystals under high pressure.
Moreover, a strong temperature-dependence of the Hall resistance confirms the
electronic correlations of these V2O3 thin films samples.",0709.0692v1
2007-10-09,Spin ladder compound Pb(0.55)Cd(0.45)V(2)O(5): synthesis and investigation,"The complex oxide Pb(0.55)Cd(0.45)V(2)O(5) was synthesized and investigated
by means of X-ray powder diffraction, electron diffraction, magnetic
susceptibility measurements and band structure calculations. Its structure is
similar to that of MV(2)O(5) compounds (M = Na, Ca) giving rise to a spin
system of coupled S=1/2 two-leg ladders. Magnetic susceptibility measurements
reveal a spin gap-like behavior with \Delta ~ 270 K and a spin singlet ground
state. Band structure calculations suggest Pb(0.55)Cd(0.45)V(2)O(5) to be a
system of weakly coupled dimers in perfect agreement with the experimental
data. Pb(0.55)Cd(0.45)V(2)O(5) provides an example of the modification of the
spin system in layered vanadium oxides by cation substitution. Simple
correlations between the cation size, geometrical parameters and exchange
integrals for the MV(2)O(5)-type oxides are established and discussed.",0710.1806v1
2008-01-08,Mott transition in VO2 revealed by infrared spectroscopy and nano-imaging,"Electrons in correlated insulators are prevented from conducting by Coulomb
repulsion between them. When an insulator-to-metal transition is induced in a
correlated insulator by doping or heating, the resulting conducting state can
be radically different from that characterized by free electrons in
conventional metals. We report on the electronic properties of a prototypical
correlated insulator vanadium dioxide (VO2) in which the metallic state can be
induced by increasing temperature. Scanning near-field infrared microscopy
allows us to directly image nano-scale metallic puddles that appear at the
onset of the insulator-to-metal transition. In combination with far-field
infrared spectroscopy, the data reveal the Mott transition with divergent
quasiparticle mass in the metallic puddles. The experimental approach employed
here sets the stage for investigations of charge dynamics on the nanoscale in
other inhomogeneous correlated electron systems.",0801.1171v1
2008-01-09,Frustrated spin-1/2 square lattice in the layered perovskite PbVO(3),"We report on the magnetic properties of the layered perovskite PbVO(3). The
results of magnetic susceptibility and specific heat measurements as well as
band structure calculations consistently suggest that the S=1/2 square lattice
of vanadium atoms in PbVO(3) is strongly frustrated due to
next-nearest-neighbor antiferromagnetic interactions. The ratio of
next-nearest-neighbor (J(2)) to nearest-neighbor (J(1)) exchange integrals is
estimated to be J(2)/J(1)\approx 0.2-0.4. Thus, PbVO(3) is within or close to
the critical region of the J(1)-J(2) frustrated square lattice. Supporting
this, no sign of long-range magnetic ordering was found down to 1.8 K.",0801.1434v1
2008-03-10,Multiple Avalanches Across the Metal-Insulator Transition of Vanadium Oxide Nano-scaled Junctions,"The metal insulator transition of nano-scaled $VO_2$ devices is drastically
different from the smooth transport curves generally reported. The temperature
driven transition occurs through a series of resistance jumps ranging over 2
decades in amplitude, indicating that the transition is caused by avalanches.
We find a power law distribution of the jump amplitudes, demonstrating an
inherent property of the $VO_2$ films. We report a surprising relation between
jump amplitude and device size. A percolation model captures the general
transport behavior, but cannot account for the statistical behavior.",0803.1190v3
2008-06-03,Site symmetry and crystal symmetry: a spherical tensor analysis,"The relation between the properties of a specific crystallographic site and
the properties of the full crystal is discussed by using spherical tensors. The
concept of spherical tensors is introduced and the way it transforms under the
symmetry operations of the site and from site to site is described in detail.
The law of spherical tensor coupling is given and illustrated with the example
of the electric dipole and quadrupole transitions in x-ray absorption
spectroscopy. The main application of the formalism is the reduction of
computation time in the calculation of the properties of crystals by band
structure methods. The general approach is illustrated by the examples of
substitutional chromium in spinel and substitutional vanadium in garnet.",0806.0438v1
2008-06-13,Remote hole-doping of Mott insulators on the nanometer scale,"At interfaces between polar and nonpolar perovskite oxides, an unusual
electron-doping has been previously observed, due to electronic
reconstructions. We report on remote hole-doping at an interface composed of
only polar layers, revealed by high-resolution hard x-ray core-level
photoemission spectroscopy. In LaAlO3/LaVO3/LaAlO3 trilayers, the vanadium
valence systematically evolves from the bulk value of V3+ to higher oxidation
states with decreasing LaAlO3 cap layer thickness. These results provide a
synthetic approach to hole-doping transition metal oxide heterointerfaces
without invoking a polar discontinuity.",0806.2191v1
2008-06-30,Electrostatic modification of infrared response in gated structures based on VO2,"We investigate the changes in the infrared response due to charge carriers
introduced by electrostatic doping of the correlated insulator vanadium dioxide
(VO2) integrated in the architecture of the field effect transistor.
Accumulation of holes at the VO2 interface with the gate dielectric leads to an
increase in infrared absorption. This phenomenon is observed only in the
insulator-to-metal transition regime of VO2 with coexisting metallic and
insulating regions. We postulate that doped holes lead to the growth of the
metallic islands thereby promoting percolation, an effect that persists upon
removal of the applied gate voltage.",0806.4826v1
2008-07-11,Magnetic interactions and high-field properties of Ag(2)VOP(2)O(7): frustrated alternating chain close to the dimer limit,"We report on high-field magnetic properties of the silver vanadium phosphate
Ag(2)VOP(2)O(7). This compound has a layered crystal structure, but the
specific topology of the V-P-O framework gives rise to a one-dimensional spin
system, a frustrated alternating chain. Low-field magnetization measurements
and band structure calculations show that Ag(2)VOP(2)O(7) is close to the dimer
limit with the largest nearest-neighbor interaction of about 30 K. High-field
magnetization data reveal the critical fields \mu_0H_{c1} of about 23 T
(closing of the spin gap) and \mu_0H_{c2} of about 30 T (saturation by full
alignment of the magnetic moments). From H_{c1} to H_{c2} the magnetization
increases sharply similar to the system of isolated dimers. Thus, the magnetic
frustration in Ag(2)VOP(2)O(7) bears little influence on the high-field
properties of this compound.",0807.1849v1
2008-08-09,Electronic transport and specific heat of 1T- VSe2,"The results of low temperature thermoelectric power and the specific heat of
1T-VSe2 (Vanadium diselenide) have been reported along with the electrical
resistivity, and Hall coefficient of the compound. The Charge Density Wave
(CDW) transition is observed near 110K temperature in all these properties. The
Thermoelectric power has been measured from 15K to 300K spanning the
incommensurate and commensurate CDW regions. We observed a weak anomaly at the
CDW transition for the first time in the specific heat of VSe2. The linear
temperature dependence of resistivity and thermoelectric power at higher
temperatures suggests a normal metallic behavior and electron-phonon scattering
above the CDW transition. The positive thermoelectric power and negative Hall
coefficient along with strongly temperature dependent behavior in the CDW phase
suggest a mixed conduction related to the strongly hybridized s-p-d bands in
this compound.",0808.1332v2
2008-08-24,Effective Hamiltonian of Three-orbital Hubbard Model on Pyrochlore Lattice: Application to LiV_2O_4,"We investigate heavy fermion behaviors in the vanadium spinel LiV_2O_4. We
start from a three-orbital Hubbard model on the pyrochlore lattice and derive
its low-energy effective Hamiltonian by an approach of real-space
renormalization group type. One important tetrahedron configuration in the
rochlore lattice has a three-fold orbital degeneracy and spin S=1, and
correspondingly, the effective Hamiltonian has spin and orbital exchange
interactions of Kugel-Khomskii type as well as correlated electron hoppings.
Analyzing the effective Hamiltonian, we find that ferromagnetic double exchange
processes compete with antiferromagnetic superexchange processes and various
spin and orbital exchange processes are competing to each other. These results
suggest the absence of phase transition in spin and orbital spaces down to very
low temperatures and their large fluctuations in the low-energy sector, which
are key issues for understanding the heavy fermion behavior in LiV_2O_4.",0808.3250v1
2008-12-09,Magnetic Structure and Interactions in the Quasi-1D Antiferromagnet CaV$_2$O$_4$,"CaV$_2$O$_4$ is a spin-1 antiferromagnet, where the magnetic vanadium ions
are arranged on quasi-one-dimensional (1D) zig-zag chains with potentially
frustrated antiferromagnetic exchange interactions. High temperature
susceptibility and single-crystal neutron diffraction measurements are used to
deduce the non-collinear magnetic structure, dominant exchange interactions and
orbital configurations. The results suggest that at high temperatures
CaV$_2$O$_4$ behaves as a Haldane chain, but at low temperatures, orbital
ordering lifts the frustration and it becomes a spin-1 ladder.",0812.1808v2
2009-02-10,Maximally localized Wannier function within linear combination of pseudo-atomic orbital method: Implementation and applications to transition-metal-benzene complex,"Construction of maximally localized Wannier functions (MLWFs) has been
implemented within the linear combination of pseudo-atomic orbital (LCPAO)
method. Detailed analysis using MLWFs is applied to three closely related
materials, single benzene (Bz) molecule, organometallic Vanadium-Bz infinite
chain, and V$_2$Bz$_{3}$ sandwich cluster. Two important results come out from
the present analysis: 1) for the infinite chain, the validity of the basic
assumption in the mechanism of Kanamori and Terakura for the ferromagnetic (FM)
state stability is confirmed; 2) for V$_2$Bz$_3$, an important role played by
the difference in the orbital energy between the edge Bzs and the middle Bz is
newly revealed: the on-site energy of p$\delta$ states of edge Bzs is higher
than that of middle Bz, which further reduces the FM stability of V$_2$Bz$_3$.",0902.1584v1
2009-04-10,Transition of stoichiometricSr2VO3FeAs to a superconducting state at 37.2 K,"The superconductor Sr4V2O6Fe2As2 with transition temperature at 37.2 K has
been fabricated. It has a layered structure with the space group of p4/nmm, and
with the lattice constants a = 3.9296Aand c = 15.6732A. The observed large
diamagnetization signal and zero-resistance demonstrated the bulk
superconductivity. The broadening of resistive transition was measured under
different magnetic fields leading to the discovery of a rather high upper
critical field. The results also suggest a large vortex liquid region which
reflects high anisotropy of the system. The Hall effect measurements revealed
dominantly electron-like charge carriers in this material. The
superconductivity in the present system may be induced by oxygen deficiency or
the multiple valence states of vanadium.",0904.1732v5
2009-04-17,Variational study of the antiferromagnetic insulating phase of V2O3 based on Nth order Muffin-Tin-Orbitals,"Motivated by recent results of $N$th order muffin-tin orbital (NMTO)
implementation of the density functional theory (DFT), we re-examine
low-temperature ground-state properties of the anti-ferromagnetic insulating
phase of vanadium sesquioxide V$_2$O$_3$. The hopping matrix elements obtained
by the NMTO-downfolding procedure differ significantly from those previously
obtained in electronic structure calculations and imply that the in-plane
hopping integrals are as important as the out-of-plane ones. We use the NMTO
hopping matrix elements as input and perform a variational study of the ground
state. We show that the formation of stable molecules throughout the crystal is
not favorable in this case and that the experimentally observed magnetic
structure can still be obtained in the atomic variational regime. However the
resulting ground state (two $t_{2g}$ electrons occupying the degenerate $e_g$
doublet) is in contrast with many well established experimental observations.
We discuss the implications of this finding in the light of the non-local
electronic correlations certainly present in this compound.",0904.2757v1
2009-04-22,Optical properties of correlated materials -- Generalized Peierls approach and its application to VO2,"The aim of the present paper is to present a versatile scheme for the
computation of optical properties of solids, with particular emphasis on
realistic many-body calculations for correlated materials. Geared at the use
with localized basis sets, we extend the commonly known lattice ""Peierls
substitution"" approach to the case of multi-atomic unit cells. We show in how
far this generalization can be deployed as an approximation to the full Fermi
velocity matrix elements that enter the continuum description of the response
of a solid to incident light. We further devise an upfolding scheme to
incorporate optical transitions, that involve high energy orbitals that had
been downfolded in the underlying many-body calculation of the electronic
structure. As an application of the scheme, we present results on a material of
longstanding interest, vanadium dioxide, VO2. Using dynamical mean-field data
of both, the metallic and the insulating phase, we calculate the corresponding
optical conductivities, elucidate optical transitions and find good agreement
with experimental results.",0904.3388v1
2009-05-28,Large-J approach to strongly coupled spin-orbital systems,"We present a novel approach to study the ground state and elementary
excitations in compounds where spins and orbitals are entangled by on-site
relativistic spin-orbit interaction. The appropriate degrees of freedom are
localized states with an effective angular momentum J. We generalize J to
arbitrary large values while maintaining the delicate spin-orbital
entanglement. After projecting the inter-site exchange interaction to the
manifold of effective spins, a systematic 1/J expansion of the effective
Hamiltonian is realized using the Holstein-Primakoff transformation.
Applications to representative compounds Sr2IrO4 and particularly vanadium
spinels AV2O4 are discussed.",0905.4728v3
2009-06-24,Detection of sub-lattice magnetism in sigma-phase Fe-V compounds by zero-field NMR,"The first successful measurements of a sub-lattice magnetism with 51V NMR
techniques in the sigma-phase Fe(100-x)Vx alloys with x = 34.4, 39.9 and 47.9
are reported. Vanadium atoms present on all five crystallographic sites are
magnetic. Their magnetic properties are characteristic of a given site, which
strongly depend on the composition. The strongest magnetism exhibit sites A and
the weakest one sites D. The estimated average magnetic moment per V atom
decreases from 0.36 muB for x = 34.4 to 0.20 muB for x = 47.9. The magnetism
revealed at V atoms is linearly correlated with the magnetic moment of Fe
atoms, which implies that the former is induced by the latter.",0906.4458v2
2009-07-31,Two inequivalent sublattices and orbital ordering in MnV2O4 studied by 51V NMR,"We report detailed 51V NMR spectra in a single crystal of MnV2O4. The
vanadium spectrum reveals two peaks in the orbitally ordered state, which arise
from different internal hyperfine fields at two different V sublattices. These
internal fields evolve smoothly with externally applied field, and show no
change in structure that would suggest a change of the orbital ordering. The
result is consistent with the orbital ordering model recently proposed by
Sarkar et al. [Phys. Rev. Lett. 102, 216405 (2009)] in which the same orbital
that is a mixture of t_2g orbitals rotates by about 45$^\circ$ alternately
within and between orbital chains in the I4_1/a tetragonal space group.",0907.5574v2
2009-08-24,The two-dimensional frustrated Heisenberg model on the orthorhombic lattice,"We discuss new high-field magnetization data recently obtained by Tsirlin et
al. for layered vanadium phosphates in the framework of the square-lattice
model. Our predictions for the saturation fields compare exceptionally well to
the experimental findings, and the strong bending of the curves below
saturation agrees very well with the experimental field dependence. Furthermore
we discuss the remarkably good agreement of the frustrated Heisenberg model on
the square lattice in spite of the fact that the compounds described with this
model actually have a lower crystallographic symmetry. We present results from
our calculations on the thermodynamics of the model on the orthorhombic (i.e.,
rectangular) lattice, in particular the temperature dependence of the magnetic
susceptibility. This analysis also sheds light on the discussion of magnetic
frustration and anisotropy of a class of iron pnictide parent compounds, where
several alternative suggestions for the magnetic exchange models were proposed.",0908.3442v1
2009-09-03,Spin correlations and exchange in square lattice frustrated ferromagnets,"The J1-J2 model on a square lattice exhibits a rich variety of different
forms of magnetic order that depend sensitively on the ratio of exchange
constants J2/J1. We use bulk magnetometry and polarized neutron scattering to
determine J1 and J2 unambiguously for two materials in a new family of vanadium
phosphates, Pb2VO(PO4)2 and SrZnVO(PO4)2, and we find that they have
ferromagnetic J1. The ordered moment in the collinear antiferromagnetic ground
state is reduced, and the diffuse magnetic scattering is enhanced, as the
predicted bond-nematic region of the phase diagram is approached.",0909.0702v1
2009-09-24,"Magnetic, Thermal, and Transport Properties of the Mixed Valent Vanadium Oxides LuV4O8 and YV4O8","LV4O8 (L = Yb, Y, Lu) compounds are reported to crystallize in a structure
similar to that of the orthorhombic CaFe2O4 structure-type, and contain four
inequivalent V sites arranged in zigzag chains. We confirm the structure and
report the magnetic, thermal, and transport properties of polycrystalline YV4O8
and LuV4O8. A first-order like phase transition is observed at 50 K in both
YV4O8 and LuV4O8. The symmetry remains the same with the lattice parameters
changing discontinously. The structural transition in YV4O8 leads to partial
dimerization of the V atoms resulting in a sudden sharp drop in the magnetic
susceptibility. The V spins that do not form dimers order in a canted
antiferromagnetic state. The magnetic susceptibility of LuV4O8 shows a sharp
peak at ~ 50 K. The magnetic entropies calculated from heat capacity versus
temperature measurements indicate bulk magnetic transitions below 90 K for both
YV4O8 and LuV4O8.",0909.4572v2
2009-10-08,Structural and transport properties of Sr2VO{3-delta}FeAs superconductors with different oxygen deficiencies,"Sr2VO{3-delta}FeAs superconductors with different oxygen deficiencies have
been successfully fabricated. It is found that the superconducting transition
temperature drops down monotonically with the increase of oxygen deficiency.
The diminishing of superconductivity is accompanied by the enhancement of
residual resistivity, indicating an unraveled scattering effect induced by the
oxygen deficiency. The highest superconducting transition temperature at about
40 K is achieved near the stoichiometrical sample Sr$_2$VO$_{3}$FeAs.
Surprisingly, the X-ray photoelectron spectroscopy (XPS) shows that the
vanadium has a ""5+"" valence state in the samples. The Hall effect measurements
reveal that the density of charge carriers (electron-like here) varies
qualitatively with the increase of oxygen deficiency. Magnetotransport
measurements show that the superconducting transition changes from
one-step-like shape at low fields to two-step-like one at high fields,
indicating a high anisotropy.",0910.1537v2
2009-12-06,Quantum Griffiths Phase in the weak itinerant ferromagnetic alloy Ni$_{1-x}$V$_x$,"We present magnetization ($M$) data of the $d$-metal alloy Ni$_{1-x}$V$_x$ at
vanadium concentrations close to $x_c \approx 11.4%$ where the onset of
long-range ferromagnetic (FM) order is suppressed to zero temperature. Above
$x_c$, the temperature ($T$) and magnetic field ($H$) dependencies of the
magnetization are best described by simple nonuniversal power laws. The
exponents of $M/H \sim T^{-\gamma}$ and $M \sim H^\alpha$ are related by
$1-\gamma=\alpha$ for wide temperature ($10K < T \leq 300K$) and field ($H \leq
5T$) ranges. $\gamma$ is strongly $x$ dependent, decreasing from 1 at $x\approx
x_c$ to $\gamma < 0.1$ for x=15%. This behavior is not compatible with either
classical or quantum critical behavior in a clean 3D FM. Instead it closely
follows the predictions for a quantum Griffiths phase associated with a quantum
phase transition in a disordered metal. Deviations at the lowest temperatures
hint at a freezing of large clusters and the onset of a cluster glass phase,
presumably due to RKKY interactions in this alloy.",0912.1146v2
2010-03-19,Non-congruence of thermally driven structural and electronic transition in VO2,"Coupled structural and electronic phase transitions underlie the
multifunctional properties of strongly-correlated materials. For example,
colossal magnetoresistance1,2 in manganites involves phase transition from
paramagnetic insulator to ferromagnetic metal linked to a structural
Jahn-Teller distortion3. Vanadium dioxide (VO2) likewise exhibits an
insulator-to-metal transition (IMT) at ~67oC with abrupt changes in transport
and optical properties and coupled to a structural phase transition (SPT) from
monoclinic to tetragonal4. The IMT and SPT hystereses are signatures of
first-order phase transition tracking the nucleation to stabilization of a new
phase. Here we have for the first time measured independently the IMT and SPT
hystereses in epitaxial VO2 films, and shown that the hystereses are not
congruent. From the measured volume fractions of the two phases in the region
of strong correlation, we have computed the evolving dielectric function under
an effective-medium approximation. But the computed dielectric functions could
not reproduce the measured IMT, implying that there is a strongly correlated
metallic phase that is not in the stable rutile structure, consistent with
Qazilbash et al5. Search for a corresponding macroscopic structural
intermediate also yielded negative result.",1003.3876v1
2010-05-06,Effects of strain on the electronic structure of VO_2,"We present cluster-DMFT (CTQMC) calculations based on a downfolded
tight-binding model in order to study the electronic structure of vanadium
dioxide (VO_2) both in the low-temperature (M_1) and high-temperature (rutile)
phases. Motivated by the recent efforts directed towards tuning the physical
properties of VO_2 by depositing films on different supporting surfaces of
different orientations we performed calculations for different geometries for
both phases. In order to investigate the effects of the different growing
geometries we applied both contraction and expansion for the lattice parameter
along the rutile c-axis in the 3-dimensional translationally invariant systems
miming the real situation. Our main focus is to identify the mechanisms
governing the formation of the gap characterizing the M_1 phase and its
dependence on strain. We found that the increase of the band-width with
compression along the axis corresponding to the rutile c-axis is more important
than the Peierls bonding-antibonding splitting.",1005.0886v1
2010-06-12,Role of van der Waals bonding in layered oxide: Bulk vanadium pentoxide,"Sparse matter is characterized by regions with low electron density and its
understanding calls for methods to accurately calculate both the van der Waals
(vdW) interactions and other bonding. Here we present a first-principles
density functional theory (DFT) study of a layered oxide (V2O5) bulk structure
which shows charge voids in between the layers and we highlight the role of the
vdW forces in building up material cohesion. The result of previous
first-principles studies involving semilocal approximations to the
exchange-correlation functional in DFT gave results in good agreement with
experiments for the two in-plane lattice parameters of the unit cell but
overestimated the parameter for the stacking direction. To recover the third
parameter we include the nonlocal (dispersive) vdW interactions through the
vdW-DF method [Dion et al., Phys. Rev. Lett. 92, 246401 (2004)] testing also
various choices of exchange flavors. We find that the transferable
first-principle vdW-DF calculations stabilizes the bulk structure. The vdW-DF
method gives results in fairly good agreement with experiments for all three
lattice parameters.",1006.2494v1
2010-06-19,High resolution spectroscopic study of red clump stars in the Galaxy: iron group elements,"The main atmospheric parameters and abundances of the iron group elements
(vanadium, chromium, iron, cobalt and nickel) are determined for 62 red giant
""clump"" stars revealed in the Galactic field by the Hipparcos orbiting
observatory. The stars form a homogeneous sample with the mean value of
temperature T=4750 +- 160K, of surface gravity log g = 2.41 +- 0.26 and the
mean value of metallicity [Fe/H] = -0.04 +- 0.15 dex. A Gaussian fit to the
[Fe/H] distribution produces the mean [Fe/H] = -0.01 dex and dispersion of
[Fe/H] = 0.08 dex. The near-solar metallicity and small dispersion of [Fe/H] of
clump stars of the Galaxy obtained in this work confirm the theoretical model
of the Hipparcos clump by Girardi & Salaris (2001). This suggests that nearby
clump stars are (in the mean) relatively young objects, reflecting mainly the
near-solar metallicities developed in the local disk during the last few Gyrs
of its history. We find iron group element to iron abundance ratios in clump
giants to be close to solar.",1006.3857v1
2010-06-21,Signatures of a quantum Griffiths phase in a d-metal alloy close to its ferromagnetic quantum critical point,"We report magnetization ($M$) measurements close to the ferromagnetic quantum
phase transition of the d-metal alloy Ni$_{1-x}$V$_x$ at a vanadium
concentration of $x_c \approx 11.4 %$. In the diluted regime ($x>x_c$), the
temperature ($T$) and magnetic field ($H$) dependencies of the magnetization
are characterized by nonuniversal power laws and display $H/T$ scaling in a
wide temperature and field range. The exponents vary strongly with $x$ and
follow the predictions of a quantum Griffiths phase. We also discuss the
deviations and limits of the quantum Griffiths phase as well as the phase
boundaries due to bulk and cluster physics.",1006.4094v2
2010-08-16,Suppression of superconductivity by V-doping and possible magnetic order in Sr2VO3FeAs,"Superconductivity at 33 K in Sr2VO3FeAs is completely suppressed by small
amounts of V-doping in Sr2VO3[Fe0.93(+/-0.01)V0.07(+/-0.01)]As. The crystal
structures and exact stoichiometries are determined by combined neutron- and
x-ray powder diffraction. Sr2VO3FeAs is shown to be very sensitive to Fe/V
mixing, which interferes with or even suppresses superconductivity. This
inhomogeneity may be intrinsic and explains scattered reports regarding Tc and
reduced superconducting phase fractions in Sr2VO3FeAs. Neutron diffraction data
collected at 4 K indicates incommensurate mag- netic ordering of the
V-sublattice with a propagation vector q = (0,0,0.306). This suggests strongly
correlated vanadium, which does not contribute significantly to the Fermi
surface of Sr2VO3FeAs.",1008.2687v2
2010-09-02,Spin and Orbital Order of the Vanadium Spinel MgV2O4,"We present a unique study of the frustrated spinel MgV2O4 which possesses
highly coupled spin, lattice and orbital degrees of freedom. Using large
single-crystal and powder samples, we find a distortion from spinel at room
temperature (space group F-43m) which allows for a greater trigonal distortion
of the VO6 octahedra and a low temperature space group (I-4m2) that maintains
the mirror plane symmetry. The magnetic structure that develops below 42 K
consists of antiferromagnetic chains with a strongly reduced moment while
inelastic neutron scattering reveals one-dimensional behavior and a single band
of excitations. The implications of these results are discussed in terms of
various orbital ordering scenarios. We conclude that although spin-orbit
coupling must be significant to maintain the mirror plane symmetry, the
trigonal distortion is large enough to mix the 3d levels leading to a wave
function of mixed real and complex orbitals.",1009.0429v2
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
2011-03-29,Reconfigurable Gradient Index using VO2 Memory Metamaterials,"We demonstrate tuning of a metamaterial device that incorporates a form of
spatial gradient control. Electrical tuning of the metamaterial is achieved
through a vanadium dioxide layer which interacts with an array of split ring
resonators. We achieved a spatial gradient in the magnitude of permittivity,
writeable using a single transient electrical pulse. This induced gradient in
our device is observed on spatial sc ales on the order of one wavelength at 1
THz. Thus, we show the viability of elements for use in future devices with
potential applications in beamforming and communications",1103.5729v2
2011-04-12,VO2: A Novel View from Band Theory,"New calculations for vanadium dioxide, one of the most controversely
discussed materials for decades, reveal that band theory as based on density
functional theory is well capable of correctly describing the electronic and
magnetic properties of the metallic as well as both the insulating M1 and M2
phases. Considerable progress in the understanding of the physics of VO2 is
achieved by the use of the recently developed hybrid functionals, which include
part of the electron-electron interaction exactly and thereby improve on the
weaknesses of semilocal exchange functionals as provided by the local density
and generalized gradient approximations. Much better agreement with
photoemission data as compared to previous calculations is found and a
consistent description of the rutile-type early transition-metal dioxides is
achieved.",1104.2214v1
2011-04-15,Ultrafast Insulator-Metal Phase Transition in VO2 Studied by Multiterahertz Spectroscopy,"The ultrafast photoinduced insulator-metal transition in VO2 is studied at
different temperatures and excitation fluences using multi-THz probe pulses.
The spectrally resolved mid-infrared response allows us to trace separately the
dynamics of lattice and electronic degrees of freedom with a time resolution of
40 fs. The critical fluence of the optical pump pulse which drives the system
into a long-lived metallic state is found to increase with decreasing
temperature. Under all measurement conditions we observe a modulation of the
eigenfrequencies of the optical phonon modes induced by their anharmonic
coupling to the coherent wave packet motion of V-V dimers at 6.1 THz.
Furthermore, we find a weak quadratic coupling of the electronic response to
the coherent dimer oscillation resulting in a modulation of the electronic
conductivity at twice the frequency of the wave packet motion. The findings are
discussed in the framework of a qualitative model based on an approximation of
local photoexcitation of the vanadium dimers from the insulating state.",1104.2984v1
2011-05-06,The interplay between dopants and oxygen vacancies in the magnetism of V-doped TiO2,"Density functional theory calculations indicate that the incorporation of V
into Ti lattice positions of rutile TiO2 leads to magnetic V4+ species, but the
extension and sign of the coupling between dopant moments confirm that
ferromagnetic order cannot be reached via low-concentration doping in the
non-defective oxide. Oxygen vacancies can introduce additional magnetic
centres, and we show here that one of the effects of vanadium doping is to
reduce the formation energies of these defects. In the presence of both V
dopants and O vacancies all the spins tend to align with the same orientation.
We conclude that V doping favours the ferromagnetic behaviour of TiO2 not only
by introducing spins associated with the dopant centres but also by increasing
the concentration of oxygen vacancies with respect to the pure oxide.",1105.1289v1
2011-05-22,Evidence for a capacitor network near the metal insulator transition in VO2 thin films probed by in-plane impedance spectroscopy,"Impedance spectroscopy measurements were performed in high quality Vanadium
dioxide (VO2) thin films. This technique allows us investigate the resistive
and capacitive contribution to the dielectric response near the metal-insulator
transition (MIT). A non ideal RC behavior was found in our films from room
temperature up to 334 K. A decrease of the total capacitance was found in this
region, possibly due to interface effects. Above the MIT, the system behaves
like a metal as expected, and a modified equivalent circuit is necessary to
describe the impedance data adequately. Around the MIT, an increase of the
total capacitance is observed.",1105.4308v1
2011-08-30,Orbital ordering in the geometrically frustrated MgV$_2$O$_4$: \emph{Ab initio} electronic structure calculations,"In the light of recent interesting experimental work on MgV$_2$O$_4$ we
employ the density functional theory to investigate the crucial role played by
different interaction parameters in deciding its electronic and magnetic
properties. The strong Coulomb correlation in presence of antiferromagnetic
(AFM) coupling is responsible for the insulating ground state. In the ground
state the $d_{xz}$ and $d_{yz}$ orbitals are ordered and intra-chain vanadium
ions are antiferromagnetically coupled. The calculation gives small spin-orbit
coupling (SOC), which provides a tilt of $\sim11.3^0$ to the magnetic moment
from the z-axis. In the presence of weak SOC and strong exchange coupling, the
experimentally observed small magnetic moment and low AFM transition
temperature appear to arise from spin fluctuation due to activeness of
geometrical frustration.",1108.5907v2
2011-09-30,Ab initio computation of d-d excitation energies in low-dimensional Ti and V oxychlorides,"Using a quantum chemical cluster-in-solid computational scheme, we calculate
the local d-d excitation energies for two strongly correlated Mott insulators,
the oxychlorides TiOCl and VOCl. TiOCl harbors quasi-one-dimensional spin
chains made out of S = 1/2 Ti3+ ions while the electronic structure of VOCl
displays a more two-dimensional character. We find in both cases that the
lowest-energy d-d excitations are within the t2g subshell, starting at 0.34 eV
and indicating that orbital degeneracies are significantly lifted. In the
vanadium oxychloride, spin triplet to singlet excitations are calculated to be
1 eV higher in energy. For TiOCl, the computed d-level electronic structure and
the symmetries of the wavefunctions are in very good agreement with resonant
inelastic x-ray scattering results and optical absorption data. For VOCl,
future resonant inelastic x-ray scattering experiments will constitute a direct
test of the symmetry and energy of about a dozen of different d-d excitations
that we predict here.",1109.6797v2
2011-10-09,Anisotropic compression in the high pressure regime of pure and Cr-doped vanadium dioxide,"We present structural studies of V$_{1-x}$Cr$_x$O$_2$ (pure, 0.7% and 2.5% Cr
doped) compounds at room temperature in a diamond anvil cell for pressures up
to 20 GPa using synchrotron x-ray powder diffraction. All the samples studied
show a persistence of the monoclinic $M_1$ symmetry between 4 and 12 GPa. Above
12 GPa, the monoclinic $M_1$ symmetry changes to isostructural $M_x$ phase
(space group $P2_1/c$) with a significant anisotropy in lattice compression of
the $b$-$c$ plane of the $M_{1}$ phase. This behavior can be reconciled
invoking the pressure induced charge-delocalization.",1110.1844v2
2011-11-15,BaVS$_3$ probed by V L edge X-ray absorption spectroscopy,"Polarization dependent vanadium L edge X-ray absorption spectra of BaVS$_3$
single crystals are measured in the four phases of the compound. The difference
between signals with the polarization \textbf{E}$\perp$\textbf{c} and
\textbf{E}$\parallel$\textbf{c} (linear dichroism) changes with temperature.
Besides increasing intensity of one of the maxima, a new structure appears in
the pre-edge region below the metal-insulator transition. More careful
examination brings to light that the changes start already with pretransitional
charge density wave fluctuations. Simple symmetry analysis suggests that the
effect is related to rearrangements in $E_{g}$ and $A_{1g}$ states, and is
compatible with the formation of four inequivalent V sites along the V-S chain.",1111.3476v1
2012-01-06,Electric-field-driven phase transition in vanadium dioxide,"We report on local probe measurements of current-voltage and electrostatic
force-voltage characteristics of electric-field-induced insulator to metal
transition in VO2 thin film. In conducting AFM mode, switching from the
insulating to metallic state occurs for electric-field threshold
E~6.5\times10^7 Vm-1 at 300K. Upon lifting the tip above the sample surface, we
find that the transition can also be observed through a change in electrostatic
force and in tunneling current. In this noncontact regime, the transition is
characterized by random telegraphic noise. These results show that electric
field alone is sufficient to induce the transition; however, the electronic
current provides a positive feedback effect that amplifies the phenomena.",1201.1459v1
2012-01-30,"Exchange constants and spin waves of the orbital ordered, non-collinear spinel MnV$_2$O$_4$","We study the exchange constants of MnV$_2$O$_4$ using magnetic force theorem
and local spin density approximation of density functional theory supplemented
with a correction due to on-site Hubbard interaction U. We obtain the exchanges
for three different orbital orderings of the Vanadium atoms of the spinel. We
then map the exchange constants to a Heisenberg model with single-ion
anisotropy and solve for the spin-wave excitations in the non-collinear, low
temperature phase of the spinel. The single-ion anisotropy parameters are
obtained from an atomic multiplet exact-diagonalization program, taking into
effect the crystal-field splitting and the spin-orbit coupling. We find good
agreement between the spin waves of one of our orbital ordered setups with
previously reported experimental spin waves as determined by neutron
scattering. We can therefore determine the correct orbital order from various
proposals that exist in the literature.",1201.6375v2
2012-02-09,Interlayer magnetic frustration driven quantum spin disorder in honeycomb compound In$_{3}$Cu$_{2}$VO$_{9}$,"We present electronic and magnetic properties of a honeycomb compound
In$_{3}$Cu$_{2}$VO$_{9}$ in this paper. We find that the parent phase is a
charge transfer insulator with an energy gap of about 1.5 eV. Singly occupied
d$_{3z^{2}-r^{2}}$ electrons of copper ions contribute an $S$ = 1/2 spin, while
vanadium ions show nonmagnetism. Oxygen 2$p$ orbitals hybridizing with a small
fraction of Cu 3$d$ orbitals dominate the density of states near $E_{F}$. The
planar nearest-neighbor, next-nearest-neighbor and interplane superexchange
couplings of Cu spins are $J_{1}$ $\approx$ 16.2 meV, $J_{2}$ $\approx$ 0.3 meV
and $J_{z}$ $\approx$ 1.2 meV, suggesting a low-dimensional antiferromagnet
\cite{Sondhi10}. We propose that the magnetic frustration along the c-axis
leads to a quantum spin disorder in In$_{3}$Cu$_{2}$VO$_{9}$, in accordance
with the recent experiments. {abstract}",1202.1861v3
2012-03-11,Calculations of the thermodynamic and kinetic properties of LiV3O8,"The phase behavior and kinetic pathways of Li1+xV3O8 are investigated by
means of density functional theory (DFT) and a cluster expansion (CE)
methodology that approximates the system Hamiltonian in order to identify the
lowest energy configurations. Although DFT calculations predict the correct
ground state for a given composition, both GGA and LDA fail to obtain phase
stability consistent with experiment due to strongly localized vanadium 3d
electrons. A DFT+U methodology recovers the correct phase stability for an
optimized U value of 3.0eV. GGA+U calculations with this value of U predict
electronic structures that qualitatively agree with experiment. The resulting
calculations indicate solid solution behavior from LiV3O8 to Li2.5V3O8 and
two-phase coexistence between Li2.5V3O8 and Li4V3O8. Analysis of the lithiation
sequence from LiV3O8 to Li2.5V3O8 reveals the mechanism by which lithium
intercalation proceeds in this material. Calculations of lithium migration
energies for different lithium concentrations and configurations provides
insight into the relevant diffusion pathways and their relationship to
structural properties.",1203.2314v1
2012-04-11,Frustrated magnets and quantum paramagnetic phases at finite temperature,"We develop a general framework, which combines exact diagonalization in small
clusters with a density matrix variational principle, to study frustrated
magnets at finite temperature. This thermodynamic hierarchical mean-field
technique is used to determine the phase diagram and magnetization process of
the three-dimensional spin-1/2 $J_1$-$J_2$ antiferromagnet on a stacked square
lattice. Its non-magnetic phase exhibits a thermal crossover from a quantum to
a classical paramagnet at a temperature $T=T_0$ which can be extracted from
thermodynamic measurements. At low temperature an applied magnetic field
stabilizes, through order-by-disorder, a variety of phases with non-trivial
spin textures and a magnetization plateau at half-saturation which continuously
disappears at $T\sim T_0$. Our results are relevant for frustrated vanadium
oxides.",1204.2559v2
2012-05-31,Effect of spin-orbit coupling on magnetic and orbital order in MgV$_2$O$_4$,"Recent measurements on MgV$_2$O$_4$ single crystal have reignited the debate
on the role of spin-orbit (SO) coupling in dictating the orbital order in
Vanadium spinel systems. Density functional theory calculations were performed
using the full-potential linearized augmented-plane-wave method within the
local spin density approximation (LSDA), Coulomb correlated LSDA+U, and with SO
interaction (LSDA+U+SO) to study the magnetic and orbital ordering in low
temperature phase of MgV$_2$O$_4$. It is observed that the spin-orbit coupling
in the experimentally observed antiferromagnetic phase, affects the orbital
order differently in alternate V-atom chains along c-axis. This observation is
found to be consistent with the experimental prediction that the effect of
spin-orbit coupling is intermediate between that in case of ZnV$_2$O$_4$ and
MnV$_2$O$_4$.",1205.6950v1
2012-08-07,An x-ray diffraction study of the temperature-induced structural phase transitions in SmVO3,"Through powder x-ray diffraction we have investigated the structural behavior
of SmVO3, in which orbital and magnetic degrees of freedom are believed to be
closely coupled to the crystal lattice. We have found, contrary to previous
reports, that SmVO3 exists in a single, monoclinic, phase below 200 K. The
associated crystallographic distortion is then stabilized through the
magnetostriction that occurs below 134 K. The crystal structure has been
refined using synchrotron x-ray powder diffraction data measured throughout the
structural phase diagram, showing a substantial Jahn-Teller distortion of the
VO6 octahedra in the monoclinic phase, compatible with the expected G-type
orbital order. Changes in the vanadium ion crystal field due to the structural
and magnetic transitions have then been probed by resonant x-ray diffraction.",1208.1485v1
2012-08-16,Terahertz nano antenna enabled early transition in VO2,"We study terahertz transmission through nano-patterned vanadium dioxide thin
film. It is found that the patterning allows the lowering of the apparent
transition temperature. For the case of the smallest width nano antennas, the
transition temperature is lower by as many as ten degrees relative to the bare
film, so that the nano patterned hysteresis curves completely separate
themselves from their bare film counterparts. This early transition comes from
the one order of magnitude enhanced effective dielectric constants by nano
antennas. This phenomenon opens up the possibility of transition temperature
engineering.",1208.3269v1
2012-09-27,Density functional theory study of rutile VO2 surfaces,"We present the results of a density functional theory (DFT) investigation of
the surfaces of rutile-like vanadium dioxide, VO2(R). We calculate the surface
energies of low Miller index planes, and find that the most stable surface
orientation is the (110). The equilibrium morphology of a VO2(R) particle has
an acicular shape, laterally confined by (110) planes and topped by (011)
planes. The redox properties of the (110) surface are investigated by
calculating the relative surface free energies of the non-stoichiometric
compositions as a function of oxygen chemical potential. It is found that the
VO2(110) surface is oxidized with respect to the stoichiometric composition,
not only at ambient conditions but also at the more reducing conditions under
which bulk VO2 is stable in comparison with bulk V2O5. The adsorbed oxygen
forms surface vanadyl species much more favorably than surface peroxo species.",1209.6177v1
2012-10-16,Thermodynamics of the two-dimensional frustrated J1-J2 Heisenberg ferromagnet in the collinear stripe regime: Susceptibility and correlation length,"We calculate the temperature dependence of the correlation length xi and the
uniform susceptibility chi_0 of the frustrated J1-J2 square-lattice Heisenberg
ferromagnet in the collinear stripe phase using Green-function technique. The
height chi_{max} and the position T(chi_{max}) of the maximum in the chi_0(T)
curve exhibit a characteristic dependence on the frustration parameter J2/|J1|,
which is well described by power laws, chi_{max}=a(J2-J2^c)^{-nu} and
T(chi_{max})=b(J_2-J_2^c), where J2^c = 0.4 and nu is of the order of unity.The
correlation length diverges at low temperatures as xi \propto e^{A/T}, where A
increases with growing J2/|J1|. We also compare our results with recent
measurements on layered vanadium phosphates and find reasonable agreement.",1210.4414v2
2012-10-29,Substrate Effect on Optical Properties of Insulator-Metal Transition in VO2 Thin Films,"In this paper we used Raman spectroscopy to investigate the optical
properties of vanadium dioxide (VO2) thin films during the thermally induced
insulating to metallic phase transition. We observed a significant difference
in transition temperature in similar VO2 films grown on quartz and sapphire
substrates: the film grown on quartz displayed the phase transition at a lower
temperature (Tc=50C) compared a film grown on sapphire (Tc=68C). We also
investigated differences in the detected Raman signal for different wavelengths
and polarizations of the excitation laser. We found that for either substrate,
a longer wavelength (in our case 785 nm) yielded the clearest VO2 Raman
spectra, with no polarization dependence.",1210.7746v2
2012-11-12,An orbital-selective spin liquid in a frustrated heavy fermion spinel LiV$_2$O$_4$,"The pronounced enhancement of the effective mass is the primary phenomenon
associated with strongly correlated electrons. In the presence of local
moments, the large effective mass is thought to arise from Kondo coupling, the
interaction between itinerant and localised electrons. However, in d electron
systems, the origin is not clear because of the competing Hund's rule coupling.
Here we experimentally address the microscopic origin for the heaviest d
fermion in a vanadium spinel LiV2O4 having geometrical frustration. Utilising
orbital-selective 51V NMR, we elucidate the orbital-dependent local moment that
exhibits no long-range magnetic order despite persistent antiferromagnetic
correlations. A frustrated spin liquid, Hund-coupled to itinerant electrons,
has a crucial role in forming heavy fermions with large residual entropy. Our
method is important for the microscopic observation of the orbital-selective
localisation in a wide range of materials including iron pnictides, cobaltates,
manganites, and ruthnates.",1211.2844v1
2013-01-17,Quantum anomalous Hall effect and tunable topological states in 3d transition metals doped silicene,"We engineer quantum anomalous Hall effect in silicene via doping 3d
transition metals. We show that there exists a stable quantum anomalous Hall
effect in Vanadium doped silicene using both analytical model and Wannier
interpolation. We also predict the quantum valley Hall effect and electrically
tunable topological states could be realized in certain transition metal doped
silicene where the energy band inversion occurs. Our finding provides new
scheme for the realization of quantum anomalous Hall effect and platform for
electrically controllable topological states.",1301.4081v4
2013-02-13,Nucleosynthesis in the gamma-ray burst accretion disks and associated outflows,"We investigate nucleosynthesis inside the gamma-ray burst (GRB) accretion
disks formed by the Type II collapsars and outflows launched from these disks.
We deal with accretion disks having relatively low accretion rates: 0.001 M_sun
s^{-1} <~ Mdot <~ 0.01 M_sun s^{-1} and hence they are predominantly advection
dominated. We report the synthesis of several unusual nuclei like 31P, 39K,
43Sc, 35Cl and various isotopes of titanium, vanadium, chromium, manganese and
copper in the disk. We also confirm that isotopes of iron, cobalt, nickel,
argon, calcium, sulphur and silicon get synthesized in the disk, as shown by
previous authors. Much of these heavy elements thus synthesized are ejected
from the disk and survive in the outflows. Indeed, emission lines of many of
these heavy elements have been observed in the X-ray afterglows of several
GRBs.",1302.3067v1
2013-02-20,Finite-temperature Gutzwiller approximation and the phase diagram of a toy-model for V2O3,"We exploit exact inequalities that refer to the entropy of a distribution to
derive a simple variational principle at finite temperature for trial density
matrices of Gutzwiller and Jastrow type. We use the result to extend at finite
temperature the Gutzwiller approximation, which we apply to study a two-orbital
model that we believe captures some essential features of V$_2$O$_3$. We indeed
find that the phase diagram of the model bears many similarities to that of
real vanadium sesquioxide. In addition, we show that in a Bethe lattice, where
the finite temperature Gutzwiller approximation provides a rigorous upper bound
of the actual free energy, the results compare well with the exact phase
diagram obtained by the dynamical mean field theory.",1302.4927v1
2013-04-09,Exchange randomness and spin dynamics in the frustrated magnetic Keplerate {W72V30},"The magnetic properties and spin dynamics of the spin frustrated
polyoxometalate {W72V30}, where 30 V^{4+} ions (s = 1/2) occupy the sites of an
icosidodecahedron, have been investigated by low temperature magnetization,
magnetic susceptibility, proton and vanadium nuclear magnetic resonance, and
theoretical studies. The field-dependent magnetization at 0.5 K increases
monotonically up to 50 T without any sign of staircase behavior. This
low-temperature behavior cannot be explained by a Heisenberg model based on a
single value of the nearest-neighbor exchange coupling. We analyze this
behavior upon assuming a rather broad distribution of nearest-neighbor exchange
interactions. Slow spin dynamics of {W72V30} at low temperatures is observed
from the magnetic field and temperature dependence of nuclear spin-lattice
relaxation rate 1/T_1 measurements.",1304.2603v1
2013-04-22,Structural and magnetic properties in the quantum S=1/2 dimer systems Ba3(Cr1-xVx)2O8 with site disorder,"We report a comprehensive study of dc susceptibility, specific heat, neutron
diffraction, and inelastic neutron scattering measurements on polycrystalline
Ba3(Cr1-xVx)2O8 samples, where x=0, 0.06, 0.15, and 0.53. A Jahn-Teller
structure transition occurs for x=0, 0.06, and 0.15 samples and the transition
temperature is reduced upon vanadium substitution from 70(2) K at x=0 to 60(2)
K at x=0.06 and 0.15. The structure becomes less distorted as x increases and
such transition disappears at x=0.53. The observed magnetic excitation spectrum
indicates that the singlet ground state remains unaltered and spin gap energy
\Delta=1.3(1) meV is identical within the instrument resolution for all x. In
addition, the dispersion bandwidth W decreases with increase of x. At x=0.53, W
is reduced to 1.4(1) meV from 2.0(1) meV at x=0.",1304.6069v2
2013-06-10,On the feasibility to study inverse proximity effect in a single S/F bilayer by Polarized Neutron Reflectometry,"Here we report on a feasibility study aiming to explore the potential of
Polarized Neutron Reflectometry (PNR) for detecting the inverse proximity
effect in a single superconducting/ferromagnetic bilayer. Experiments,
conducted on the V(40nm)/Fe(1nm) S/F bilayer, have shown that experimental spin
asymmetry measured at T = 0.5TC is shifted towards higher Q values compared to
the curve measured at T = 1.5TC. Such a shift can be described by the
appearance in superconducting vanadium of magnetic sub-layer with thickness of
7 nm and magnetization of +0.8 kG.",1306.2173v2
2013-06-17,Direct in-situ observation of structural transition driven actuation in VO2 utilizing electron transparent cantilevers,"Direct imaging and quantification of actuation in nanostructures that undergo
structural phase transitions could advance our understanding of collective
phenomena in the solid state. Here, we demonstrate visualization of structural
phase transition induced actuation in a model correlated insulator vanadium
dioxide by in-situ Fresnel contrast imaging of electron transparent
cantilevers. We quantify abrupt, reversible cantilever motion occurring due to
the stress relaxation across the structural transition from monoclinic to
tetragonal phase with increasing temperature. Deflections measured in such
nanoscale cantilevers can be directly correlated with macroscopic stress
measurements by wafer curvature studies as well as temperature dependent
electrical conduction allowing one to interrogate lattice dynamics across
length scales.",1306.3931v1
2013-07-08,Weak magnetism and the Mott-state of vanadium in superconducting Sr2VO3FeAs,"We report neutron scattering data and DFT calculations of the stoichiometric
iron-arsenide superconductor Sr2VO3FeAs. Rietveld refinements of neutron powder
patterns confirm the ideal composition without oxygen deficiencies. Experiments
with polarized neutrons prove weak magnetic ordering in the V-sublattice of
Sr2VO3FeAs at ~ 45 K with a probable propagation vector q = (1/8,1/8,0). The
ordered moment of ~ 0.1 muB is too small to remove the V-3d bands from the
Fermi level by magnetic exchange splitting, and much smaller than predicted
from a recent LDA+U study. By using DFT calculations with a GGA+EECE functional
we recover the typical quasi-nested Fermi-surface even without magnetic moment.
From this we suggest that the V-atoms are in a Mott-state where the electronic
correlations are dominated by on-site Coulomb-repulsion which shifts the V-3d
states away from the Fermi energy. Our results are consistent with
photoemission data and clearly reveal that Sr2VO3FeAs is a typical
iron-arsenide superconductor with quasi-nested hole- and electron-like Fermi
surface sheets, and constitutes no new paradigm. We suggest that intrinsic
electron-doping through V3+/V4+ mixed valence is responsible for the absence of
SDW ordering.",1307.2138v1
2013-07-15,Temperature Gated Thermal Rectifier,"Heat flow control is essential for widespread applications of heating,
cooling, energy conversion and utilization. Here we demonstrate the first
observation of temperature-gated thermal rectification in vanadium dioxide
beams, in which an environment temperature actively modulates asymmetric heat
flow. In this three terminal device, there are two switchable states, which can
be accessed by global heating: Rectifier state and Resistor state. In the
Rectifier state, up to 28% thermal rectification is observed. In the Resistor
state, the thermal rectification is significantly suppressed (below 4%). This
temperature-gated rectifier can have substantial implications ranging from
autonomous thermal management of micro/nanoscale devices to thermal energy
conversion and storage.",1307.4069v2
2013-09-06,Metal-insulator transition induced in SrTi_{1-x}V_xO_3 thin films,"Epitaxial SrTi1-xVxO3 thin films with thicknesses of ~16 nm were grown on
(001)-oriented LSAT substrates using the pulsed electron-beam deposition
technique. The transport study revealed a temperature driven metal-insulator
transition (MIT) at 95 K for the film with x = 0.67. The films with higher
vanadium concentration (x > 0.67) were metallic, and the electrical resistivity
followed the T^2 law corresponding to a Fermi liquid system. In the insulating
region of x < 0.67, the temperature dependence of electrical resistivity for
the x = 0.5 and 0.33 films can be scaled with the variable range hopping model.
The possible mechanisms behind the observed MIT were discussed, including the
effects of electron correlation, lattice distortion and Anderson localization.",1309.1745v1
2013-10-22,Nucleosynthesis inside accretion disks and outflows formed during core collapse of massive stars,"We investigate nucleosynthesis inside the gamma-ray burst (GRB) accretion
disks and in the outflows launched from these disks mainly in the context of
Type II collapsars. We report the synthesis of several unusual nuclei like 31P,
39K, 43Sc, 35Cl and various isotopes of titanium, vanadium, chromium, manganese
and copper in the disk. We also confirm the presence of iron-group and
alpha-elements in the disk, as shown by previous authors. Much of these heavy
elements thus synthesized are ejected from the disk and survive in the
outflows. While emission lines of several of these elements have been observed
in the X-ray afterglows of GRBs by BeppoSAX, Chandra, XMM-Newton etc., Swift
seems to have not found these lines yet.",1310.5911v1
2013-10-30,Novel Rattling of K Atoms in Aluminium-Doped Defect Pyrochlore Tungstate,"Rattling dynamics have been identified as fundamental to superconductivity in
defect pyrochlore osmates and aluminium vanadium intermetallics, as well as low
thermal conductivity in clathrates and filled skutterudites. Combining
inelastic neutron scattering (INS) measurements and ab initio molecular
dynamics (MD) simulations, we use a new approach to investigate rattling in the
Al-doped defect pyrochlore tungstates: AAl0.33W1.67O6 (A = K, Rb, Cs). We find
that although all the alkali metals rattle, the rattling of the K atoms is
unique, involving local diffusion within a cage as well as vibrations at a
range of frequencies. We show that the novel K rattling is driven by the strong
anharmonicity and anisotropy of the local potentials around the K atoms, both
of which are a consequence of the small size of the K rattler. Our findings may
open new possibilities for phonon engineering in thermoelectric materials.",1310.8137v2
2014-03-05,Design and demonstration of a quasi-monoenergetic neutron source,"The design of a neutron source capable of producing 24 and 70 keV neutron
beams with narrow energy spread is presented. The source exploits
near-threshold kinematics of the $^{7}$Li(p,n)$^{7}$Be reaction while taking
advantage of the interference `notches' found in the scattering cross-sections
of iron. The design was implemented and characterized at the Center for
Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory.
Alternative filters such as vanadium and manganese are also explored and the
possibility of studying the response of different materials to low-energy
nuclear recoils using the resultant neutron beams is discussed.",1403.1285v2
2014-03-21,Quantum Monte Carlo study of one-dimensional transition metal organometallic cluster systems and their suitability as spin filters,"We present calculations of electronic and magnetic structures of
vanadium-benzene multidecker clusters V$_{n}$Bz$_{n+1}$ ($n$ = 1 - 3) using
advanced quantum Monte Carlo methods. These and related systems have been
identified as prospective spin filters in spintronic applications, assuming
that their ground states are half-metallic ferromagnets. Although we find that
magnetic properties of these multideckers are consistent with ferromagnetic
coupling, their electronic structures do not appear to be half-metallic as
previously assumed. In fact, they are ferromagnetic insulators with large and
broadly similar $\uparrow$-/$\downarrow$-spin gaps. This makes the potential of
these and related materials as spin filtering devices very limited, unless they
are further modified or functionalized.",1403.5411v1
2014-04-14,Impurity induced enhancement of perpendicular magnetic anisotropy in Fe/MgO tunnel junctions,"Using first-principles calculations, we investigated the impact of chromium
(Cr) and vanadium (V) impurities on the magnetic anisotropy and spin
polarization in Fe/MgO magnetic tunnel junctions. It is demonstrated using
layer resolved anisotropy calculation technique, that while the impurity near
the interface has a drastic effect in decreasing the perpendicular magnetic
anisotropy (PMA), its position within the bulk allows maintaining high surface
PMA. Moreover, the effective magnetic anisotropy has a strong tendency to go
from in-plane to out-of-plane character as a function of Cr and V concentration
favoring out-of-plane magnetization direction for ~1.5 nm thick Fe layers at
impurity concentrations above 20 %. At the same time, spin polarization is not
affected and even enhanced in most situations favoring an increase of tunnel
magnetoresistance (TMR) values.",1404.3523v1
2014-05-23,Extreme Ultraviolet Transient Grating Spectroscopy of Vanadium Dioxide,"Nonlinear spectroscopy in the extreme ultraviolet (EUV) and soft x-ray
spectral range offers the opportunity for element selective probing of
ultrafast dynamics using core-valence transitions (Mukamel et al., Acc. Chem.
Res. 42, 553 (2009)). We demonstrate a step on this path showing core-valence
sensitivity in transient grating spectroscopy with EUV probing. We study the
optically induced insulator-to-metal transition (IMT) of a VO2 film with EUV
diffraction from the optically excited sample. The VO2 exhibits a change in the
3p-3d resonance of V accompanied by an acoustic response. Due to the broadband
probing we are able to separate the two features.",1405.5964v1
2014-05-29,Reconfigurable anisotropy and functional transformations with VO$_{2}$-based metamaterial electric circuits,"We demonstrate an innovative multifunctional artificial material that
combines exotic metamaterial properties and the environmentally responsive
nature of phase change media. The tunable metamaterial is designed with the aid
of two interwoven coordinate-transformation equations and implemented with a
network of thin film resistors and vanadium dioxide ($VO_{2}$). The strong
temperature dependence of $VO_{2}$ electrical conductivity results in a
relevant modification of the resistor network behavior, and we provide
experimental evidence for a reconfigurable metamaterial electric circuit (MMEC)
that not only mimics a continuous medium but is also capable of responding to
thermal stimulation through dynamic variation of its spatial anisotropy. Upon
external temperature change the overall effective functionality of the material
switches between a ""truncated-cloak"" and ""concentrator"" for electric currents.
Possible applications may include adaptive matching resistor networks,
multifunctional electronic devices, and equivalent artificial materials in the
magnetic domain. Additionally, the proposed technology could also be relevant
for thermal management of integrated circuits",1405.7743v1
2014-07-16,Density matrix approach to the orbital ordering in the spinel vanadates: A case study,"In this work we apply the density matrices approach to orbital ordering (OO)
in order to study the OO of the spinel vanadates AV$_{2}$O$_{4}$ (A $\equiv$
Zn, Cd and Mg), which is normally believed to be responsible for the structural
transition from cubic to tetragonal phase observed in these compounds. The
density matrices of vanadium atoms are obtained by using {\it state-of-the-art}
full-potential linearized augmented plane wave method based GGA+U calculations.
In the absence of spin-orbit coupling, the present study shows the existence of
anti-ferro OO in the global (local octahedral) coordinate system where $d_{xz}$
and $d_{yz}$ ($d_{xz}$+$d_{yz}$ and $d_{xz}$-$d_{yz}$) orbitals are mainly
occupied at the neighboring V sites for all the compounds.",1407.4217v2
2014-08-01,Charge transfer across transition metal oxide interfaces: emergent conductance and new electronic structure,"We perform density functional theory plus dynamical mean field theory
calculations to investi- gate internal charge transfer in an artificial
superlattice composed of alternating layers of vanadate and manganite
perovskite and Ruddlesden-Popper structure materials. We show that the elec-
tronegativity difference between vanadium and manganese causes moderate charge
transfer from VO2 to MnO2 layers in both perovskite and Ruddlesden-Popper based
superlattices, leading to hole doping of the VO2 layer and electron doping of
the MnO2 layer. Comparison of the perovskite and Ruddlesden-Popper based
heterostructures provides insights into the role of the apical oxy- gen. Our
first principles simulations demonstrate that the combination of internal
charge transfer and quantum confinement provided by heterostructuring is a
powerful approach to engineering electronic structure and tailoring correlation
effects in transition metal oxides.",1408.0217v2
2014-08-06,Wavelength-Tunable Infrared Metamaterial by Tailoring Magnetic Resonance Condition with VO2 Phase Transition,"In this work, we report the design of a wavelength-tunable infrared
metamaterial by exciting magnetic resonance with phase transition of vanadium
dioxide (VO2). Numerical simulation shows a broad absorption peak at the
wavelength of 10.9 um when VO2 is a metal, but it shifts to 15.1 um when VO2
changes to dielectric phase below its phase transition temperature of 68degC.
The large tunability of 38.5% in the resonance wavelength stems from the
different excitation conditions of magnetic resonance assisted by plasmon in
metallic VO2 but optical phonons in dielectric VO2. The physical mechanism is
elucidated with the aid of electromagnetic field distribution at the resonance
wavelengths. A hybrid magnetic resonance mode due to plasmon-phonon coupling is
also discussed. The results here would be beneficial for active control in
novel electronic, optical and thermal devices.",1408.1356v1
2014-09-29,Optoelectronic excitations and photovoltaic effect in strongly correlated materials,"Solar cells based on conventional semiconductors have low efficiency in
converting solar energy into electricity because the excess energy beyond the
gap of an incident solar photon is converted into heat by phonons. Here we show
by ab initio methods that the presence of strong Coulomb interactions in
strongly correlated insulators (SCI) causes the highly photo-excited
electron-hole pair to decay fast into multiple electron-hole pairs via impact
ionization (II). We show that the II rate in the insulating $M_1$ phase of
vanadium dioxide (chosen for this study as it is considered a prototypical SCI)
is two orders of magnitude higher than in Si and much higher than the rate of
hot electron/hole decay due to phonons. Our results indicate that a rather
broad class of materials may be harnessed for an efficient solar-to-electrical
energy conversion that has been not considered before.",1409.8261v1
2014-09-30,"Coupling and Electrical Control of Structural, Orbital and Magnetic Orders in Perovskites","Perovskite oxides are already widely used in industry and have huge potential
for novel device applications thanks to the rich physical behaviour displayed
in these materials. The key to the functional electronic properties exhibited
by perovskites is often the so-called Jahn-Teller distortion. For applications,
an electrical control of the Jahn-Teller distortions, which is so far out of
reach, would therefore be highly desirable. Based on universal symmetry
arguments, we determine new lattice mode couplings that can provide exactly
this paradigm, and exemplify the effect from first-principles calculations. The
proposed mechanism is completely general, however for illustrative purposes, we
demonstrate the concept on vanadium based perovskites where we reveal an
unprecedented orbital ordering and Jahn-Teller induced ferroelectricity. Thanks
to the intimate coupling between Jahn-Teller distortions and electronic degrees
of freedom, the electric field control of Jahn-Teller distortions is of general
relevance and may find broad interest in various functional devices.",1409.8422v1
2014-10-06,Vacuum Thermal Switch Made of Phase Transition Materials Considering Thin Film and Substrate Effects,"In the present study, we demonstrate a vacuum thermal switch based on
near-field thermal radiation between phase transition materials, i.e., vanadium
dioxide (VO2), whose phase changes from insulator to metal at 341 K. Similar
modulation effect has already been demonstrated and it will be extended to
thin-film structure with substrate in this paper. Strong coupling of surface
phonon polaritons between two insulating VO2 plates significantly enhances the
near-field heat flux, which on the other hand is greatly reduced when the VO2
emitter becomes metallic, resulting strong thermal switching effect.
Fluctuational electrodynamics predicts more than 80% heat transfer reduction at
sub-30-nm vacuum gaps and 50% at vacuum gap of 1 micron. By replacing the bulk
VO2 receiver with a thin film of several tens of nanometers, the switching
effect can be further improved over a broad range of vacuum gaps from 10 nm to
1 um. In addition, for the purpose of more practical setup in experiments and
applications, the SiO2 substrate effect is also considered for the structure
with thin-film emitter or receiver.",1410.1269v1
2014-10-24,Dynamic conductivity scaling in photoexcited V2O3 thin films,"Optical-pump terahertz-probe spectroscopy is used to investigate ultrafast
far-infrared conductivity dynamics during the insulator-to-metal transition
(IMT) in vanadium sesquioxide (V2O3). The resultant conductivity increase
occurs on a tens of ps timescale, exhibiting a strong dependence on the initial
temperature and fluence. We have identified a scaling of the conductivity
dynamics upon renormalizing the time axis with a simple power law (alpha = 1/2)
that depends solely on the initial, final, and conductivity onset temperatures.
Qualitative and quantitative considerations indicate that the dynamics arise
from nucleation and growth of the metallic phase which can be described by the
Avrami model. We show that the temporal scaling arises from spatial scaling of
the growth of the metallic volume fraction, highlighting the self-similar
nature of the dynamics. Our results illustrate the important role played by
mesoscopic effects in phase transition dynamics.",1410.6804v1
2014-11-17,The Mott criterion: So simple and yet so complex,"September 30, 2015 marks the 110th anniversary of the birth of the famous
English physicist N. F. Mott. This article is dedicated to his memory. Here we
consider the problem of metal-insulator transition. It is shown that the Mott
criterion $a_{B}(n_{c})^{1/3} \approx 0.25$ is applicable not only to heavily
doped semiconductors, but also to many other materials and systems, including
strongly correlated transition-metal and rare-earth-metal compounds, such as
vanadium oxides. A special emphasis is placed to the 'paramagnetic metal -
antiferromagnetic insulator' transition in $V_{2}O_{3}$ doped with chromium. In
Supplement we also briefly consider the history and state of the art of the
Mott transition problem.",1411.4372v2
2015-03-26,Correlation-driven insulator-metal transition in near-ideal vanadium dioxide films,"We use polarization- and temperature-dependent x-ray absorption spectroscopy,
in combination with photoelectron microscopy, x-ray diffraction and electronic
transport measurements, to study the driving force behind the insulator-metal
transition in VO2. We show that both the collapse of the insulating gap and the
concomitant change in crystal symmetry in homogeneously strained
single-crystalline VO2 films are preceded by the purely-electronic softening of
Coulomb correlations within V-V singlet dimers. This process starts 7 K (+/-
0.3 K) below the transition temperature, as conventionally defined by
electronic transport and x-ray diffraction measurements, and sets the energy
scale for driving the near-room-temperature insulator-metal transition in this
technologically-promising material.",1503.07892v1
2015-04-09,Control of plasmonic nanoantennas by reversible metal-insulator transition,"Nanophotonic (nanoplasmonic) structures confine, guide, and concentrate light
on the nanoscale. Advancement of nanophotonics critically depends on active
nanoscale control of these phenomena. Localized control of the insulator and
metallic phases of vanadium dioxide (VO2) would open up a universe of
applications in nanophotonics via modulation of the local dielectric
environment of nanophotonic structures allowing them to function as active
devices. Here we show dynamic reversible control of VO2 insulator-to-metal
transition (IMT) locally on the scale of 15 nm or less and control of
nanoantennas, observed in the near-field for the first time. Using
polarization-selective near-field imaging techniques, we monitor simultaneously
the IMT in VO2 and the change of plasmons on gold infrared nanoantennas.
Structured nanodomains of the metallic VO2 locally and reversibly transform
infrared plasmonic dipolar antennas to monopole antennas. Fundamentally, the
IMT in VO2 can be triggered on femtosecond timescale to allow ultrafast
nanoscale control of optical phenomena. These unique capabilities open up
exciting novel applications in active nanophotonics.",1504.02286v2
2015-04-21,Non-Contact Friction for Ion-Surface Interactions,"Non-contact friction forces are exerted on physical systems through
dissipative processes, when the two systems are not in physical contact with
each other, or, in quantum mechanical terms, when the overlap of their wave
functions is negligible. Non-contact friction is mediated by the exchange of
virtual quanta, with the additional requirement that the scattering process
needs to have an inelastic component. For finite-temperature ion-surface
interactions, the friction is essentially caused by Ohmic resistance due to the
motion of the image charge moving in a dielectric material. A conceivable
experiment is difficult because the friction force needs to be isolated from
the interaction with the image charge, which significantly distorts the ion's
flight path. We propose an experimental setup which is designed to minimize the
influence of the image charge interaction though a compensation mechanism, and
evaluate the energy loss due to non-contact friction for helium ions (He+)
interacting with gold, vanadium, titanium and graphite surfaces. Interactions
with the infinite series of mirror charges in the plates are summed in terms of
the logarithmic derivatives of the Gamma function, and of the Hurwitz zeta
function.",1504.05180v1
2015-04-27,Optical properties of V2O3 in its whole phase diagram,"Vanadium sesquioxide V2O3 is considered a textbook example of Mott-Hubbard
physics. In this paper we present an extended optical study of its whole
temperature/doping phase diagram as obtained by doping the pure material with
M=Cr or Ti atoms (V1-xMx)2O3. We reveal that its thermodynamically stable
metallic and insulating phases, although macroscopically equivalent, show very
different low-energy electrodynamics. The Cr and Ti doping drastically change
both the antiferromagnetic gap and the paramagnetic metallic properties. A
slight chromium content induces a mesoscopic electronic phase separation, while
the pure compound is characterized by short-lived quasiparticles at high
temperature. This study thus provides a new comprehensive scenario of the
Mott-Hubbard physics in the prototype compound V2O3.",1504.07279v1
2015-04-29,Hubbard Physics in the PAW GW Approximation,"It is demonstrated that the signatures of the Hubbard Model in the strongly
interacting regime can be simulated by modifying the screening in the limit of
zero wavevector in Projector-Augmented Wave GW calculations for systems without
significant nesting. This modification, when applied to the Mott insulator CuO,
results in the opening of the Mott gap by the splitting of states at the Fermi
level into upper and lower Hubbard bands, and exhibits a giant transfer of
spectral weight upon electron doping. The method is also employed to clearly
illustrate that the M$_{1}$ and M$_{2}$ forms of vanadium dioxide are
fundamentally different types of insulator. Standard GW calculations are
sufficient to open a gap in M$_{1}$ VO$_{2}$, which arise from the Peierls
pairings filling the valence band, creating homopolar bonds. The valence band
wavefunctions are stabilized with respect to the conduction band, reducing
polarizability and pushing the conduction band eigenvalues to higher energy.
The M$_{2}$ structure however opens a gap from strong on-site interactions; it
is a Mott insulator.",1504.07739v2
2015-06-03,Proximity-induced magnetism in transition-metal substituted graphene,"We investigate the interactions between two identical magnetic impurities
substituted into a graphene superlattice. Using a first-principles approach, we
calculate the electronic and magnetic properties for transition-metal
substituted graphene systems with varying spatial separation. These
calculations are compared for three different magnetic impurities, manganese,
chromium, and vanadium. We determine the electronic band structure, density of
states, and Millikan populations (magnetic moment) for each atom, as well as
calculate the exchange parameter between the two magnetic atoms as a function
of spatial separation. We find that the presence of magnetic impurities
establishes a distinct magnetic moment in the graphene lattice, where the
interactions are highly dependent on the spatial and magnetic characteristic
between the magnetic atoms and the carbon atoms, which leads to either
ferromagnetic or antiferromagnetic behavior. Furthermore, through an analysis
of the calculated exchange energies and partial density of states, it is
determined that interactions between the magnetic atoms can be classified as an
RKKY interaction.",1506.01319v1
2015-07-01,Electronic structure of tungsten-doped vanadium dioxide,"A common method of adjusting the metal-insulator transition temperature of
M$_{1}$ VO$_{2}$ is via disruption of the Peierls pairing by doping, or
inputting stress or strain. However, since adding even small amounts of dopants
will change the band structure, it is unclear how doped VO$_{2}$ retains its
insulating character observed in experiments. While strong correlations may be
responsible for maintaining a gap, theoretical evidence for this has been very
difficult to obtain due to the complexity of the many-body problem involved. In
this work we use GW calculations modified to include strong local
$\textbf{k}$-space interactions to investigate the changes in band structure
from tungsten doping. We find that the combination of carrier doping and the
experimentally observed structural defects introduced by inclusion of tungsten
are consistent with a change from band-like to Mott-insulating behavior.",1507.00105v4
2015-07-12,High-Temperature Quantum Anomalous Hall Effect in n-p Codoped Topological Insulators,"The quantum anomalous Hall effect (QAHE) is a fundamental quantum transport
phenomenon that manifests as a quantized transverse conductance in response to
a longitudinally applied electric field in the absence of an external magnetic
field, and promises to have immense application potentials in future
dissipation-less quantum electronics. Here we present a novel kinetic pathway
to realize the QAHE at high temperatures by $n$-$p$ codoping of
three-dimensional topological insulators. We provide proof-of-principle
numerical demonstration of this approach using vanadium-iodine (V-I) codoped
Sb$_2$Te$_3$ and demonstrate that, strikingly, even at low concentrations of
$\sim$2\% V and $\sim$1\% I, the system exhibits a quantized Hall conductance,
the tell-tale hallmark of QAHE, at temperatures of at least $\sim$ 50 Kelvin,
which is three orders of magnitude higher than the typical temperatures at
which it has been realized so far. The proposed approach is conceptually
general and may shed new light in experimental realization of high-temperature
QAHE.",1507.03218v1
2015-09-08,Kondo effect goes anisotropic in vanadate oxide superlattices,"We study the transport properties in SrVO3/LaVO3 (SVO/LVO) superlattices
deposited on SrTiO3 (STO) substrates. We show that the electronic conduction
occurs in the metallic LVO layers with a galvanomagnetism typical of a 2D Fermi
surface. In addition, a Kondo-like component appears in both the thermal
variation of resistivity and the magnetoresistance. Surprisingly, in this
system where the STO interface does not contribute to the measured conduction,
the Kondo correction is strongly anisotropic. We show that the growth
temperature allows a direct control of this contribution. Finally, the key role
of vanadium mixed valency stabilized by oxygen vacancies is enlightened.",1509.02265v1
2015-09-09,Metal-insulator Transition in VO2: a DFT+DMFT perspective,"We present a theoretical investigation of the electronic structure of rutile
(metallic) and M$_1$ and M$_2$ monoclinic (insulating) phases of VO$_2$
employing a fully self-consistent combination of density functional theory and
embedded dynamical mean field theory calculations. We describe the electronic
structure of the metallic and both insulating phases of VO$_2$, and propose a
distinct mechanism for the gap opening. We show that Mott physics plays an
essential role in all phases of VO$_2$: undimerized vanadium atoms undergo
classical Mott transition through local moment formation (in the M$_2$ phase),
while strong superexchange within V-dimers adds significant dynamic intersite
correlations, which remove the singularity of self-energy for dimerized
V-atoms. The resulting transition from rutile to dimerized M$_1$ phase is
adiabatically connected to Peierls-like transition, but is better characterized
as the Mott transition in the presence of strong intersite exchange. As a
consequence of Mott physics, the gap in the dimerized M$_1$ phase is
temperature dependent. The sole increase of electronic temperature collapses
the gap, reminiscent of recent experiments.",1509.02968v3
2015-11-27,Orbital Ordering in Fe$_{1-x}$Mn$_x$V$_2$O$_4$: A First Principles Study,"Long range orbital order has been investigated in Fe$_{1-x}$Mn$_x$V$_2$O$_4$
as a function of doping (x) using first principles density functional theory
calculations including the effects of Coulomb correlation and spin-orbit
interaction within GGA+U and GGA+U+SO approximations. Through a detailed
analysis of corresponding Wannier orbital projections of the Vanadium d bands,
we have clearly established that for x$\le$0.6, the orbital order at V sites
consists of a linear superposition of d$_{xz}$ and d$_{yz}$ orbitals of the
type d$_{xz}\pm$d$_{yz}$. Within each ab-plane a ferro-orbital ordering of
either d$_{xz}$+d$_{yz}$ or d$_{xz}$-d$_{yz}$ is observed which alternates in
successive ab-planes along the c-direction. On the contrary, for x$>$0.6, it is
the d$_{xz}$ or d$_{yz}$ orbital which orders at V sites in successive
ab-planes along c-direction (so called A-type ordering). At Fe sites, we
observe an orbital ordering of d$_{x^2-y^2}$ orbitals for x$\le$0.6 and
d$_{z^2}$ orbitals for x$>$0.6. Effect of spin-orbit interaction on orbital
ordering is found to be not significant in the entire range of doping studied.",1511.08673v1
2016-04-12,Ab initio determination of spin Hamiltonians with anisotropic exchange interactions: the case of the pyrochlore ferromagnet Lu$_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
2016-05-17,Quantum Spin Liquid in a Breathing Kagome Lattice,"Motivated by recent experiments on the vanadium oxyfluoride material DQVOF,
we examine possible spin liquid phases on a breathing kagome lattice of S=1/2
spins. By performing a projective symmetry group analysis, we determine the
possible phases for both fermionic and bosonic $\mathbb{Z}_2$ spin liquids on
this lattice, and establish the correspondence between the two. The nature of
the ground state of the Heisenberg model on the isotropic kagome lattice is a
hotly debated topic, with both $\mathbb{Z}_2$ and U(1) spin liquids argued to
be plausible ground states. Using variational Monte Carlo techniques, we show
that a gapped $\mathbb{Z}_2$ spin liquid emerges as the clear ground state in
the presence of this breathing anisotropy. Our results suggest that the
breathing anisotropy helps to stabilize this spin liquid ground state, which
may aid us in understanding the results of experiments and help to direct
future numerical studies on these systems.",1605.05322v1
2016-06-24,Charge-order-induced ferroelectricity in LaVO$_{3}$/SrVO$_{3}$ superlattices,"The structure and properties of the 1:1 superlattice of LaVO$_{3}$ and
SrVO$_{3}$ are investigated with a first-principles
density-functional-theory-plus-$U$ (DFT+$U$) method. The lowest energy states
are antiferromagnetic charge-ordered Mott-insulating phases. In one of these
insulating phases, layered charge ordering combines with the layered cation
ordering to produce a polar structure with nonzero spontaneous polarization
normal to the interfaces. This polarization is produced by electron transfer
between the V$^{3+}$ and V$^{4+}$ layers, and is comparable to that of
conventional ferroelectrics. The energy of this polar state relative to the
nonpolar ground state is only 3 meV per vanadium. Under tensile strain, this
energy difference can be further reduced, suggesting that the polar phase can
be induced by applied electric field, yielding an antiferroelectric
double-hysteresis loop. If the system does not switch back to the nonpolar
state on removal of the field, a ferroelectric-type hysteresis loop could be
observed.",1606.07835v1
2016-07-20,Studies on the Magnetic Ground State of a Spin Möbius Strip,"Here we report the synthesis, structure and detailed characterisation of
three n-membered oxovanadium rings,
Na$_n$[(V=O)$_n$Na$_n$(H$_2$O)$_n$($\alpha$, $\beta$, or
$\gamma$-CD)$_2$]$m$H$_2$O (n = 6, 7, or 8), prepared by the reactions of
(V=O)SO$_4$$\cdot$$x$H$_2$O with $\alpha$, $\beta$, or $\gamma$-cyclodextrins
(CDs) and NaOH in water. Their alternating heterometallic vanadium/sodium
cyclic core structures were sandwiched between two CD moieties such that O-Na-O
groups separated neighbouring vanadyl ions. Antiferromagnetic interactions
between the $S$ = 1/2 vanadyl ions led to $S$ = 0 ground states for the
even-membered rings, but to two quasi-degenerate $S$ = 1/2 states for the
spin-frustrated heptanuclear cluster.",1607.06130v1
2016-08-11,Hall measurement of ultra thin vanadium dioxide thin films,"In this work, we present the investigation of temperature dependent hall
measurement of the ultra thin VO$_2$ films grown on Si/SiO$_2$ substrate.
Experimental results suggest that electrons are the predominant carrier both in
the semiconducting and metallic phases. The decrease of the resistivity with
increasing temperature is mainly caused by the increase in the number density
of charge carriers. The temperature dependence of the carrier concentration
indicates the VO$_2$ films has a band gap of 0.40$\pm$0.09 $ev$ in the
semiconducting phase. Analysis of hall effect data based on a composite cube
model suggests that the sample has some untransitional phase with a length that
is 1/4 of the grains.",1608.03378v2
2016-08-12,Growth of ultra thin vanadium dioxide thin films using magnetron sputtering,"In this work, the results of fabricating ultra thin VO$_2$ films on the
technologically relevant amorphous SiO$_2$ surface using reactive DC magnetron
sputtering are presented. Results indicate that a post deposition anneal in low
partial pressures of oxygen is an effective way at stabilizing the
VO$_2$(M$_{1}$) phase on the SiO$_2$ surface. VO$_2$ films with a thickness of
42nm show a continuous microstructure, and undergo a resistivity change of more
than a factor of 200 as the temperature of the film increases above 72$^{o}C$.
The film shows hysteresis in the metal-insulator transition temperature upon
heating and cooling with a width of approximately 8$^o$C. The resistivity of
the low temperature semiconducting phase is found to be thermally activated
with an activation energy 0.16$\pm$0.03 $ev$. Stress measurements using X-ray
diffraction indicate that the ultra thin VO$_2$ film has a large tensile stress
of 2.0$\pm$0.2 $GPa$. This value agrees well with the calculated thermal stress
due to differential thermal expansion between the VO$_2$ thin film and silicon
substrate. The stress leads to a shift of the metal-insulator transition
temperature by approximately 4$^{o}C$.",1608.03911v1
2016-09-20,Levelized Cost of Energy for PV and Grid Scale Energy Storage Systems,"With the increasing penetration of renewable energy sources and energy
storage devices in the power system, it is important to evaluate the cost of
the system by using Levelized Cost of Energy (LCOE). In this paper a new
metric, Levelized Cost of Delivery (LCOD) is proposed to calculate the LCOE for
the energy storage. The recent definitions in LCOE for renewable energy system
has been reviewed. From fundamental principles, it is demonstrated that there
is a need to introduce a new method to evaluate LCOE of the system as the
conventional LCOE is not applicable for renewable energy storage systems. Three
years of solar irradiance data in Africa collected from Johannesburg and the
national load data from Kenya are obtained for case studies. The proposed cost
calculation methods are evaluated for two types of storage technologies
(Vanadium Redox Battery (VRB) and Lithium-ion) with real-life data. It shows
that the marginal LCOE and LCOD indices can be used to assist policymakers to
consider the discount rate and the type of storage technology for a cost
effective renewable storage energy system.",1609.06000v1
2016-10-16,A Simple Analytical Model of Coupled Single Flow Channel over Porous Electrode in Vanadium Redox Flow Battery with Serpentine Flow Channel,"A simple analytical model of a layered system comprised of a single passage
of a serpentine flow channel and a parallel underlying porous electrode (or
porous layer) is proposed. This analytical model is derived from Navier-Stokes
motion in the flow channel and Darcy-Brinkman model in the porous layer. The
continuities of flow velocity and normal stress are applied at the interface
between the flow channel and the porous layer. The effects of the inlet
volumetric flow rate, thickness of the flow channel and thickness of a typical
carbon fiber paper porous layer on the volumetric flow rate within this porous
layer are studied. The maximum current density based on the electrolyte
volumetric flow rate is predicted, and found to be consistent with reported
numerical simulation. It is found that, for a mean inlet flow velocity of 33.3
cm s-1, the analytical maximum current density is estimated to be 377 mA cm-2,
which compares favorably with experimental result reported by others of ~400 mA
cm-2.",1610.04923v1
2016-11-29,Direct observation of the M2 phase with its Mott transition in a VO$_2$ film,"In VO$_2$, the explicit origin of the insulator-to-metal transition is still
disputable between Peierls and Mott insulators. Along with the controversy, its
second monoclinic (M2) phase has received considerable attention due to the
presence of electron correlation in undimerized vanadium ions. However, the
origin of the M2 phase is still obscure. Here, we study a granular VO$_2$ film
using conductive atomic force microscopy and Raman scattering. Upon the
structural transition from monoclinic to rutile, we observe directly an
intermediate state showing the coexistence of monoclinic M1 and M2 phases. The
conductivity near the grain boundary in this regime is six times larger than
that of the grain core, producing a donut-like landscape. Our results reveal an
intra-grain percolation process, indicating that VO$_2$ with the M2 phase is a
Mott insulator.",1611.09508v1
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
2017-02-08,Dynamically controlled plasmonic nano-antenna phased array utilizing vanadium dioxide,"We propose and analyze theoretically an approach for realizing a tunable
optical phased-array antenna utilizing the properties of VO2 for electronic
beam steering applications in the near-IR spectral range. The device is based
on a 1D array of slot nano-antennas engraved in a thin Au film grown over VO2
layer. The tuning is obtained by inducing a temperature gradient over the
device, which changes the refractive index of the VO2, and hence modifies the
phase response of the elements comprising the array, by producing a thermal
gradient within the underlying PCM layer. Using a 10-element array, we show
that an incident beam can be steered up to with respect to the normal, by
applying a gradient of less than 10{\deg}C.",1702.02333v1
2017-02-16,Consecutive Insulator-Metal-Insulator Phase Transitions of Vanadium Dioxide by Hydrogen Doping,"We report modulation of a reversible phase transition in VO2 films by
hydrogen doping. A metallic phase and a new insulating phase are successively
observed at room temperature as the doping concentration increases. It is
suggested that the polarized charges from doped hydrogens play an important
role. These charges gradually occupy V3d-O2p hybridized orbitals and
consequently modulate the filling of the VO2 crystal conduction band-edge
states, which eventually evolve into new valence band-edge states. This
demonstrates the exceptional sensitivity of VO2 electronic properties to
electron concentration and orbital occupancy, providing key information for the
phase transition mechanism.",1702.05027v2
2017-03-09,Doping-induced superconductivity of ZrB$_2$ and HfB$_2$,"Unlike the widely studied $s$-type two-gap superconductor MgB$_2$, the
chemically similar compounds ZrB$_2$ and HfB$_2$ do not superconduct above 1 K.
Yet, it has been shown that small amounts of self- or extrinsic doping (in
particular with vanadium), can induce superconductivity in these materials.
Based on results of different macro- and microscopic measurements, including
magnetometry, nuclear magnetic resonance (NMR), resistivity, and muon-spin
rotation ($\mu$SR), we present a comparative study of
Zr$_{0.96}$V$_{0.04}$B$_2$ and Hf$_{0.97}$V$_{0.03}$B$_2$. Their key magnetic
and superconducting features are determined and the results are considered
within the theoretical framework of multiband superconductivity proposed for
MgB$_2$. Detailed Fermi surface (FS) and electronic structure calculations
reveal the difference between MgB$_2$ and transition-metal diborides.",1703.03253v1
2017-04-03,Rechargeable redox flow batteries: Maximum current density with electrolyte flow reactant penetration in a serpentine flow structure,"Rechargeable redox flow batteries with serpentine flow field designs have
been demonstrated to deliver higher current density and power density in medium
and large-scale stationary energy storage applications. Nevertheless, the
fundamental mechanisms involved with improved current density in flow batteries
with flow field designs have not been understood. Here we report a maximum
current density concept associated with stoichiometric availability of
electrolyte reactant flow penetration through the porous electrode that can be
achieved in a flow battery system with a ""zero-gap""serpentine flow field
architecture. This concept can explain a higher current density achieved within
allowing reactions of all species soluble in the electrolyte. Further
validations with experimental data are confirmed by an example of a vanadium
flow battery with a serpentine flow structure over carbon paper electrode.",1704.00744v2
2017-04-05,High operating temperature in V-based superconducting quantum interference proximity transistors,"Here we report the fabrication and characterization of fully superconducting
quantum interference proximity transistors (SQUIPTs) based on the
implementation of vanadium (V) in the superconducting loop. At low temperature,
the devices show high flux-to-voltage (up to 0.52$\ \textrm{mV}/\Phi_0$) and
flux-to-current (above 12$\ \textrm{nA}/\Phi_0$) transfer functions, with the
best estimated flux sensitivity $\sim$2.6$\ \mu\Phi_0/\sqrt{\textrm{Hz}}$
reached under fixed voltage bias, where $\Phi_0$ is the flux quantum. The
interferometers operate up to $T_\textrm{bath}\simeq$ 2 $ \textrm{K}$, with an
improvement of 70$\%$ of the maximal operating temperature with respect to
early SQUIPTs design. The main features of the V-based SQUIPT are described
within a simplified theoretical model. Our results open the way to the
realization of SQUIPTs that take advantage of the use of higher-gap
superconductors for ultra-sensitive nanoscale applications that operate at
temperatures well above 1 K.",1704.01284v2
2017-04-17,"Mesoscopic quantum effects in a bad metal, hydrogen-doped vanadium dioxide","The standard treatment of quantum corrections to semiclassical electronic
conduction assumes that charge carriers propagate many wavelengths between
scattering events, and succeeds in explaining multiple phenomena (weak
localization magnetoresistance (WLMR), universal conductance fluctuations,
Aharonov-Bohm oscillations) observed in polycrystalline metals and doped
semiconductors in various dimensionalities. We report apparent WLMR and
conductance fluctuations in H$_{x}$VO$_{2}$, a poor metal (in violation of the
Mott-Ioffe-Regel limit) stabilized by the suppression of the VO$_{2}$
metal-insulator transition through atomic hydrogen doping. Epitaxial thin
films, single-crystal nanobeams, and nanosheets show similar phenomenology,
though the details of the apparent WLMR seem to depend on the combined effects
of the strain environment and presumed doping level. Self-consistent
quantitative analysis of the WLMR is challenging given this and the high
resistivity of the material, since the quantitative expressions for WLMR are
derived assuming good metallicity. These observations raise the issue of how to
assess and analyze mesoscopic quantum effects in poor metals.",1704.05080v1
2017-04-29,Infrared-transmittance tunable metal-insulator conversion device with thin-film-transistor-type structure on a glass substrate,"Infrared (IR) transmittance tunable metal-insulator conversion was
demonstrated on glass substrate by using thermochromic vanadium dioxide (VO2)
as the active layer in three-terminal thin-film-transistor-type device with
water-infiltrated glass as the gate insulator. Alternative positive/negative
gate-voltage applications induce the reversible protonation/deprotonation of
VO2 channel, and two-orders of magnitude modulation of sheet-resistance and 49%
modulation of IR-transmittance were simultaneously demonstrated at room
temperature by the metal-insulator phase conversion of VO2 in a non-volatile
manner. The present device is operable by the room-temperature protonation in
all-solid-state structure, and thus it will provide a new gateway to future
energy-saving technology as advanced smart window.",1705.00130v1
2017-06-06,Structural properties of thin-film ferromagnetic topological insulators,"We present a comprehensive study of the crystal structure of the thin-film,
ferromagnetic topological insulator (Bi, Sb)$_{2-x}$V$_x$Te$_3$. The
dissipationless quantum anomalous Hall edge states it manifests are of
particular interest for spintronics, as a natural spin filter or pure spin
source, and as qubits for topological quantum computing. For ranges typically
used in experiments, we investigate the effect of doping, substrate choice and
film thickness on the (Bi, Sb)$_2$Te$_3$ unit cell using high-resolution X-ray
diffractometry. Scanning transmission electron microscopy and energy-dispersive
X-ray spectroscopy measurements provide local structural and interfacial
information. We find that the unit cell is unaffected in-plane by vanadium
doping changes, and remains unchanged over a thickness range of 4--10 quintuple
layers (1 QL $\approx$ 1 nm). The in-plane lattice parameter ($a$) also remains
the same in films grown on different substrate materials. However, out-of-plane
the $c$-axis is reduced in films grown on less closely lattice-matched
substrates, and increases with the doping level.",1706.01848v1
2017-06-26,Artifact Free Transient Near-Field Nanoscopy,"We report on the first implementation of ultrafast near field nanoscopy
carried out with the transient pseudoheterodyne detection method (Tr-pHD). This
method is well suited for efficient and artifact free pump-probe
scattering-type near-field optical microscopy with nanometer scale resolution.
The Tr-pHD technique is critically compared to other data acquisition methods
and found to offer significant advantages. Experimental evidence for the
advantages of Tr-pHD is provided in the Near-IR frequency range. Crucial
factors involved in achieving proper performance of the Tr-pHD method with
pulsed laser sources are analyzed and detailed in this work. We applied this
novel method to time-resolved and spatially resolved studies of the
photo-induced effects in the insulator-to-metal transition system vanadium
dioxide with nanometer scale resolution.",1706.08478v2
2017-06-30,Stability of the spin-$1/2$ kagome ground state with breathing anisotropy,"We numerically study the spin-$1/2$ breathing kagome lattice. In this
variation of the kagome Heisenberg antiferromagnet, the spins belonging to
upward and downward facing triangles have different coupling strengths. Using
the density matrix renormalization group (DMRG) method and exact
diagonalization, we show that the kagome antiferromagnet spin liquid is
extremely robust to this anisotropy. Materials featuring this anisotropy -- and
especially the recently studied vanadium compound
$[{\mathrm{NH}}_{4}{]}_{2}[{\mathbf{C}}_{7}{\mathbf{H}}_{14}\mathbf{N}][{\mathbf{V}}_{7}{\mathbf{O}}_{6}{\mathbf{F}}_{18}]$
(DQVOF) -- may thus be very good candidates to realize the much studied kagome
spin liquid. Further, we closely examine the limit of strong breathing
anisotropy and find indications of a transition to a nematic phase.",1706.10105v1
2017-07-03,"Ab initio study of Li, Mg and Al insertion into rutile VO2: Fast diffusion and enhanced voltages","Vanadium oxides are among the most promising materials that can be used as
electrodes in rechargeable metal-ion batteries. In this work, we systematically
investigate thermodynamic, electronic and kinetic properties associated with
the insertion of Li, Mg and Al atoms in rutile VO2. Using first-principles
calculations, we systematically study the structural evolution and voltage
curves of LixVO2, MgxVO2 and AlxVO2 (0<x<1) compounds. The calculated lithium
intercalation voltage starts at 3.50 V for single-atom insertion and decreases
to 2.23 V for full lithiation, to the LiVO2 compound, which agrees well with
experimental results. The Mg insertion features a plateau about 1.6 V up to
Mg0.5VO2 and then another plateau-like region at around 0.5 V up to Mg1VO2. The
predicted voltage curve for Al insertion starts at 1.98 V, followed by two
plateaus at 1.48 V and 1.17 V. The diffusion barrier of Li, Mg and Al in the
tunnel structure of VO2 is 0.06, 0.33 and 0.50 eV, respectively. The
demonstrated excellent Li, Mg and Al mobility, high structural stability and
high specific capacity suggest a promising potential of rutile VO2 electrodes
especially for multivalent batteries.",1707.00447v1
2017-07-15,Structural and magnetic properties of lightly doped M-type hexaferrites,"Vanadium substituted SrM hexaferrites (SrFe12-xVxO19 with x = 0.2, 0.4) and
Eu-substituted BaM hexaferrite (Ba0.8Eu0.2Fe12O19) were prepared by high energy
ball milling and sintering at 1200{\deg} C. X-ray diffraction measurements
revealed that the V-substituted SrM samples exhibited phase separation
resulting in the coexistence of the pure SrM magnetic phase with nonmagnetic
Sr3(VO4)2 vanadate and {\alpha}-Fe2O3 iron oxide phase. Also, the
Eu-substituted BaM hexaferrite revealed the formation of the pure BaM phase
coexisting with {\alpha}-Fe2O3 secondary phase, and Eu-garnet minor phase.
Although the magnetic properties of the samples deteriorated with respect to
pure hexaferrite properties, the magnetic parameters of the substituted samples
were found to be of potential importance for practical applications. Further,
the results of the study suggest methods for the preparation of high quality
SrM hexaferrites, and hexaferrite/garnet composites.",1707.04709v1
2017-10-24,A tilted pulse-front setup for femtosecond extreme ultraviolet transient grating spectroscopy in highly non-collinear geometries,"We demonstrate a tilted pulse-front transient grating technique that allows
to optimally utilize time-resolution as well as transient grating line density
while probing under grazing incidence as typically done in extreme ultraviolet
(EUV) or soft x-ray (SXR) experiments. Our optical setup adapts the pulse front
tilt of the two pulses that create the transient grating to the relative tilt
grazing incident pulse. We demonstrate the technique using all 800 nm
femtosecond laser pulses for transient grating generation on a vanadium dioxide
film. We probe that grating via diffraction of a third 800 nm pulse. The time
resolution of 100 fs is an improvement by a factor 30 compared to our previous
experiments on the same system (1,2). The scheme paves the way for EUV and SXR
probing of optically induced transient gratings on any material.",1710.08817v1
2017-11-08,Renormalized Lattice Dynamics and Thermal Transport in VO$_{2}$,"Vanadium dioxide (VO$_{2}$) undergoes a first-order metal-insulator
transition (MIT) upon cooling near room temperature, concomitant with
structural change from rutile to monoclinic. Accurate characterization of
lattice vibrations is vital for elucidating the transition mechanism. To
investigate the lattice dynamics and thermal transport properties of VO$_{2}$
across the MIT, we present a phonon renormalization scheme based on
self-consistent phonon theory through iteratively refining vibrational free
energy. Using this technique, we compute temperature-dependent phonon
dispersion and lifetimes, and point out the importance of both magnetic and
vibrational entropy in driving the MIT. The predicted phonon dispersion and
lifetimes show quantitative agreement with experimental measurements. We
demonstrate that lattice thermal conductivity of rutile VO$_{2}$ is nearly
temperature independent as a result of strong intrinsic anharmonicity, while
that of monoclinic VO$_{2}$ varies according to $1/T$. Due to phonon softening
and enhanced scattering rates, the lattice thermal conductivity is deduced to
be substantially lower in the rutile phase, suggesting that Wiedemann-Franz law
might not be strongly violated in rutile VO$_{2}$.",1711.02819v3
2017-11-17,Atom Probe Analysis of Ex Situ Gas-Charged Stable Hydrides,"In this work, we report on the atom probe tomography analysis of two metallic
hydrides formed by pressurized charging using an ex situ hydrogen charging
cell, in the pressure range of 200-500 kPa (2-5 bar). Specifically we report on
the deuterium charging of Pd/Rh and V systems. Using this ex situ system, we
demonstrate the successful loading and subsequent atom probe analysis of
deuterium within a Pd/Rh alloy, and demonstrate that deuterium is likely
present within the oxide-metal interface of a native oxide formed on vanadium.
Through these experiments, we demonstrate the feasibility of ex situ hydrogen
analysis for hydrides via atom probe tomography, and thus a practical route to
three-dimensional imaging of hydrogen in hydrides at the atomic scale.",1711.06667v1
2017-11-27,"Hysteresis features of the transition-metal dichalcogenides VX$_2$ (X=S, Se, and Te)","Very recently, it has been shown that vanadium dichalcogenides (VX$_2$, X=S,
Se and Te) monolayers show intrinsic ferromagnetism, and their critical
temperatures are nearly to or beyond room temperature. Hence, they would have
wide potential applications in next-generation nanoelectronic and spintronic
devices. In this work, being inspired by a recent study we systematically
perform Monte Carlo simulations based on single-site update Metropolis
algorithm to investigate the hysteresis features of VX$_2$ monolayers for a
wide range of temperatures up to 600 K. Our simulation results indicate that,
both remanence and coercivity values tend to decrease with increasing
temperature. Furthermore, it is found that hysteresis curves start to evolve
from rectangular at the lower temperature regions to nearly S-shaped with
increasing temperature.",1711.09622v1
2017-12-18,Proposal for a dual spin filter based on [VO(C$_3$S$_4$O)$_2$]$^{2-}$,"Polynuclear magnetic molecules often present dense transmission spectra with
many overlapping conduction spin channels. Single-metal complexes display a
sparser density of states, which in the presence of a fixed external magnetic
field makes them interesting candidates for spin filtering. Here we perform a
DFT study of a family of bis- and tris-dithiolate vanadium complexes sandwiched
between Au(111) electrodes and demonstrate that [VO(C$_3$S$_4$O)$_2$]$^{2-}$
can behave as a dual spin filter. This means that an external stimulus can
switch between the selective transmission of spin-up and spin-down carriers. By
using an electrostatic gate as external stimulus we show that the onset for the
spin-up conductance is at a voltage V$_g$ = -0.51 V but a small shift to V$_g$
= -0.63 V is capable of activating spin-down transport. For both cases, we
estimate a large low-bias conductance (approx. 2 {\mu}S at Vbias < 50 mV) with
excellent spin selectivity (> 99.5%). We conclude by commenting on the general
molecular requirements for the chemical design of further such spintronics
components.",1712.06331v1
2018-01-01,Radiative thermal runaway due to negative differential thermal emission across a solid-solid phase transition,"Thermal runaway occurs when a rise in system temperature results in heat
generation rates exceeding dissipation rates. Here we demonstrate that thermal
runaway occurs in thermal radiative systems, given a sufficient level of
negative differential thermal emission. By exploiting the insulator-to-metal
phase transition of vanadium dioxide, we show that a small increase in heat
generation (e.g., 10 nW/mm2) can result in a large change in surface
temperature (e.g., ~35 K), as the thermal emitter switches from high emissivity
to low emissivity. While thermal runaway is typically associated with
catastrophic failure mechanisms, detailed understanding and control of this
phenomenon may give rise to new opportunities in infrared sensing, camouflage,
and rectification.",1801.00376v1
2018-01-02,Spectroscopic study of native defects in the semiconductor to metal phase transition in V2O5 nanostructure,"Vanadium is a transition metal with multiple oxidation states and V2O5 is the
most stable form among them. Besides catalysis, chemical sensing and
photo-chromatic applications, V2O5 is also reported to exhibit a semiconductor
to metal transition (SMT) at a temperature range of 530-560K. Even though,
there are debates in using the term 'SMT' for V2O5, the metallic behavior above
transition temperature and its origin are of great interests in the scientific
community. In this study, V2O5 nanostructures were deposited on SiO2/Si
substrate by vapour transport method using Au as catalyst. Temperature
dependent electrical measurement confirms the SMT in V2O5 without any
structural change. Temperature dependent photoluminescence analysis proves the
appearance of oxygen vacancy related peaks due to reduction of V2O5 above the
transition temperature, as also inferred from temperature dependent Raman
spectroscopic studies. The newly evolved defect levels in the V2O5 electronic
structure with increasing temperature is also understood from the downward
shift of the bottom most split-off conduction bands due to breakdown of pd{\pi}
bonds leading to metallic behavior in V2O5 above the transition temperature.",1801.00546v1
2018-01-02,Vanadyl dithiolate single molecule transistors: the next spintronic frontier?,"The role of Chemistry in the road towards quantum devices is the design of
elementary pieces with a built-in function. A brilliant example is the use of
molecular transistors as nuclear spin detectors, which, up to now, has been
implemented only on [TbPc$_2$]$^-$. We argue that this is an artificial
constraint and critically discuss the limitations of current theoretical
approaches to assess the potential of molecules for their use in spintronics.
In connection with this, we review the recent progress in the preparation of
highly coherent spin qubits based on vanadium dithiolate complexes and argue
that the use of vanadyl dithiolates as single molecule transistors to read and
control a triple nuclear spin qubit could give rise to new phenomena, notably
including a low-current nuclear spin detection scheme by means of a spin valve
effect.",1801.00595v1
2018-01-11,An innovative technique for the investigation of the 4-fold forbidden beta-decay of $^{50}$V,"For the first time a Vanadium-based crystal was operated as cryogenic
particle detector. The scintillating low temperature calorimetric technique was
used for the characterization of a 22 g YVO$_4$ crystal aiming at the
investigation of the 4-fold forbidden non-unique $\beta^-$ decay of $^{50}$V.
The excellent bolometric performance of the compound together with high light
output of the crystal makes it an outstanding technique for the study of such
elusive rate process. The internal radioactive contaminations of the crystal
are also investigated showing that an improvement on the current status of
material selection and purification are needed, $^{235/238}$U and $^{232}$Th
are measured at the level of 28 mBq/kg, 1.3 Bq/kg and 28 mBq/kg, respectively.
In this work, we also discuss a future upgrade of the experimental set-up which
may pave the road for the detection of the rare $^{50}$V $\beta^-$ decay.",1801.03980v2
2018-03-05,Mott transition with Holographic Spectral function,"We show that the Mott transition can be realized in a holographic model of a
fermion with bulk mass, $m$, and a dipole interaction of coupling strength $p$.
The phase diagram contains gapless, pseudo-gap and gapped phases and the first
one can be further divided into four sub-classes. We compare the spectral
densities of our holographic model with the Dynamical Mean Field Theory (DMFT)
results for Hubbard model as well as the experimental data of Vanadium Oxide
materials. Interestingly, single-site and cluster DMFT results of Hubbard model
share some similarities with the holographic model of different parameters,
although the spectral functions are quite different due to the asymmetry in the
holography part. The theory can fit the X-ray absorption spectrum (XAS) data
quite well, but once the theory parameters are fixed with the former it can fit
the photoelectric emission spectrum (PES) data only if we symmetrize the
spectral function.",1803.01864v2
2018-03-12,"Kramers degeneracy and relaxation in vanadium, niobium and tantalum clusters","In this work we use magnetic deflection of V, Nb, and Ta atomic clusters to
measure their magnetic moments. While only a few of the clusters show weak
magnetism, all odd-numbered clusters deflect due to the presence of a single
unpaired electron. Surprisingly, for majority of V and Nb clusters an
atomic-like behavior is found, which is a direct indication of the absence of
spin-lattice interaction. This is in agreement with Kramers degeneracy theorem
for systems with a half-integer spin. This purely quantum phenomenon is
surprisingly observed for large systems of more than 20 atoms, and also
indicates various quantum relaxation processes, via Raman two-phonon and Orbach
high-spin mechanisms. In heavier, Ta clusters, the relaxation is always
present, probably due to larger masses and thus lower phonon energies, as well
as increased spin-orbit coupling.",1803.04320v1
2018-03-21,Novel single-layer vanadium sulphide phases,"VS2 is a challenging material to prepare stoichiometrically in the bulk, and
the single layer has not been successfully isolated before now. Here we report
the first realization of single-layer VS2, which we have prepared epitaxially
with high quality on Au(111) in the octahedral (1T) structure. We find that we
can deplete the VS2 lattice of S by annealing in vacuum so as to create an
entirely new two-dimensional compound that has no bulk analogue. The transition
is reversible upon annealing in an H2S gas atmosphere. We report the structural
properties of both the stoichiometric and S-depleted compounds on the basis of
low-energy electron diffraction, X-ray photoelectron spectroscopy and
diffraction, and scanning tunneling microscopy experiments.",1803.07999v1
2018-03-31,Work Function Characterization of the Directionally Solidified LaB6 VB2 Eutectic,"With its low work function and high mechanical strength, the LaB6/VB2
eutectic system is an interesting candidate for high performance thermionic
emitters. For the development of device applications, it is important to
understand the origin, value, and spatial distribution of the work function in
this system. Here we combine thermal emission electron microscopy and low
energy electron microscopy with Auger electron spectroscopy and physical vapour
deposition of the constituent elements to explore physical and chemical
conditions governing the work function of these surfaces. Our results include
the observation that work function is lower (and emission intensity is higher)
on VB2 inclusions than on the LaB6 matrix. We also observe that the deposition
of atomic monolayer doses of vanadium results in surprisingly significant
lowering of the work function with values as low as 1.1 eV.",1804.00166v1
2018-05-25,Reconfigurable terahertz quarter-wave plate for helicity switching based on Babinet inversion of anisotropic checkerboard metasurface,"Dynamic helicity switching by utilizing metasurfaces is challenging because
it requires deep modulation of polarization states. To realize such helicity
switching, this paper proposes a dynamic metasurface functioning as a
switchable quarter-wave plate, the fast axis of which can be dynamically
rotated by $90^\circ$. The device is based on the critical transition of an
anisotropic metallic checkerboard, which realizes the deep modulation and
simultaneous design of the switchable states. After verifying the functionality
of the ideally designed device in a simulation, we tune its structural
parameters to realize practical experiments in the terahertz frequency range.
By evaluating the fabricated sample with vanadium dioxide, the conductivity of
which can be controlled by temperature, its dynamic helicity switching function
is demonstrated.",1805.10034v1
2018-06-08,A Model of an Oscillatory Neural Network with Multilevel Neurons for Pattern Recognition and Computing,"The current study uses a novel method of multilevel neurons and high order
synchronization effects described by a family of special metrics, for pattern
recognition in an oscillatory neural network (ONN). The output oscillator
(neuron) of the network has multilevel variations in its synchronization value
with the reference oscillator, and allows classification of an input pattern
into a set of classes. The ONN model is implemented on thermally-coupled
vanadium dioxide oscillators. The ONN is trained by the simulated annealing
algorithm for selection of the network parameters. The results demonstrate that
ONN is capable of classifying 512 visual patterns (as a cell array 3 * 3,
distributed by symmetry into 102 classes) into a set of classes with a maximum
number of elements up to fourteen. The classification capability of the network
depends on the interior noise level and synchronization effectiveness
parameter. The model allows for designing multilevel output cascades of neural
networks with high net data throughput. The presented method can be applied in
ONNs with various coupling mechanisms and oscillator topology.",1806.03079v2
2018-06-11,Oscillations of the Critical Temperature in a (Fe/Cr/Fe)/V/Fe Heterostructure,"The superconducting and magnetic properties of the (Fe/Cr/Fe)/V/Fe layered
system with variable thickness of the chromium layer have been experimentally
and theoretically studied. The magnetic properties of the system have been
studied by the ferromagnetic resonance method, and the superconducting
transition temperature has been measured from the jump in the magnetic
susceptibility. A wide variety of magnetic states are observed in the system;
in particular, the structure of small domains can arise in the iron layer
placed between vanadium and chromium. It has been shown experimentally that the
critical temperature $T_c$ of the superconducting transition undergoes
nonmonotonic oscillations with a noticeable amplitude in the given system with
the change in the thickness of the Cr layer. The proposed model based on the
proximity effect theory makes it possible to relate these $T_c$ oscillations to
the features of the magnetic structure of the samples.",1806.04180v1
2018-07-02,Mapping of Yu-Shiba-Rusinov states from an extended scatterer,"We investigate the spatial evolution of Yu-Shiba-Rusinov (YSR) states
resulting from the interaction between Copper phthalocyanine molecules and a
superconducting Vanadium (100) surface with submolecular resolution. Each
molecule creates several YSR states at different energies showing distinctly
different spatial intensity patterns. Surprisingly, on the molecules the
largest YSR intensities are found not at the metal center, but close to one of
the pyrrolic N-atoms, demonstrating strong molecular substrate interactions via
the organic ligands. Energy resolved YSR maps reveal that the different YSR
states originate from different molecular orbitals. We also follow the YSR
states well-beyond the extent of the molecules and find clear oscillations of
the YSR intensities without strong particle hole scattering phase differences.
This is in contrast to expectations from a point scattering model and recent
experimental findings on atomic impurities on superconductors. Our results can
be explained by treating the molecular system as extended scatterer. Our
findings provide new insights that are crucial to interpret the effects of a
variety of magnetic systems on superconductors, in particular also those
discussed in the context of Majorana bound states, which because of their size
can not be considered point like as well.",1807.00646v1
2018-08-02,Kinetic Spinodal Instabilities in the Mott Transition in V2O3: Evidence from Hysteresis Scaling and Dissipative Phase Ordering,"We present the first systematic observation of scaling of thermal hysteresis
with the temperature scanning rate around an abrupt thermodynamic transition in
correlated electron systems. We show that the depth of supercooling and
superheating in vanadium sesquioxide (V2O3) shifts with the temperature quench
rates. The dynamic scaling exponent is close to the mean field prediction of
2/3. These observations, combined with the purely dissipative continuous
ordering seen in ""quench-and-hold"" experiments, indicate departures from
classical nucleation theory toward a barrier-free phase ordering associated
with critical dynamics. Observation of critical-like features and scaling in a
thermally induced abrupt phase transition suggests that the presence of a
spinodal-like instability is not just an artifact of the mean field theories
but can also exist in the transformation kinetics of real systems, surviving
fluctuations.",1808.00693v1
2018-08-27,Vanadium dioxide circuits emulate neurological disorders,"Information in the central nervous system (CNS) is conducted via electrical
signals known as action potentials and is encoded in time. Several neurological
disorders including depression, Attention Deficit Hyperactivity Disorder
(ADHD), originate in faulty brain signaling frequencies. Here, we present a
Hodgkin-Huxley model analog for a strongly correlated VO2 artificial neuron
system that undergoes an electrically-driven insulator-metal transition. We
demonstrate that tuning of the insulating phase resistance in VO2 threshold
switch circuits can enable direct mimicry of neuronal origins of disorders in
the central nervous system. The results introduce use of circuits based on
quantum materials as complementary to model animal studies for neuroscience,
especially when precise measurements of local electrical properties or
competing parallel paths for conduction in complex neural circuits can be a
challenge to identify onset of breakdown or diagnose early symptoms of disease.",1808.08820v1
2018-09-20,Biological plausibility and stochasticity in scalable VO2 active memristor neurons,"Neuromorphic networks of artificial neurons and synapses can solve
computational hard problems with energy efficiencies unattainable for von
Neumann architectures. For image processing, silicon neuromorphic processors
outperform graphic processing units (GPUs) in energy efficiency by a large
margin, but they deliver much lower chip-scale throughput. The
performance-efficiency dilemma for silicon processors may not be overcome by
Moore's law scaling of complementary metal-oxide-semiconductor (CMOS)
field-effect transistors. Scalable and biomimetic active memristor neurons and
passive memristor synapses form a self-sufficient basis for a transistorless
neural network. However, previous demonstrations of memristor neurons only
showed simple integrate-and-fire (I&F) behaviors and did not reveal the rich
dynamics and computational complexity of biological neurons. Here we show that
neurons built with nanoscale vanadium dioxide active memristors possess all
three classes of excitability and most of the known biological neuronal
dynamics, and are intrinsically stochastic. With the favorable size and power
scaling, there is a path toward an all-memristor neuromorphic cortical
computer.",1809.07867v1
2018-09-25,Phase change materials for nano-polaritonics: a case study of hBN/VO2 heterostructures,"Polaritonic excitation and control in van der Waals (vdW) materials exhibit
superior merits than conventional materials and thus hold new promise for
exploring light matter interactions. In this work, we created vdW
heterostructures combining hexagonal boron nitride (hBN) and a representative
phase change material - vanadium dioxide (VO2). Using infrared
nano-spectroscopy and nano-imaging, we demonstrated the dynamic tunability of
hyperbolic phonon polaritons in hBN/VO2 heterostructures by temperature control
in a precise and reversible fashion. The dynamic tuning of the polaritons stems
from the change of local dielectric properties of the VO2 sublayer through
insulator to metal transition by the temperature control. The high
susceptibility of polaritons to electronic phase transitions opens
possibilities for applications of vdW materials in combination with correlated
phase change materials.",1809.09652v1
2018-09-27,The origin of the vanadium dioxide transition entropy,"The reversible metal-insulator transition in VO$_2$ at $T_\text{C} \approx
340$ K has been closely scrutinized yet its thermodynamic origin remains
ambiguous. We discuss the origin of the transition entropy by calculating the
electron and phonon contributions at $T_\text{C}$ using density functional
theory. The vibration frequencies are obtained from harmonic phonon
calculations, with the soft modes that are imaginary at zero temperature
renormalized to real values at $T_\text{C}$ using experimental information from
diffuse x-ray scattering at high-symmetry wavevectors. Gaussian Process
Regression is used to infer the transformed frequencies for wavevectors across
the whole Brillouin zone, and in turn compute the finite temperature phonon
partition function to predict transition thermodynamics. Using this method, we
predict the phase transition in VO$_2$ is driven five to one by phonon entropy
over electronic entropy, and predict a total transition entropy that accounts
for $95$ % of the calorimetric value.",1809.10703v2
2018-09-27,Epitaxial Growth and Electrical Properties of VO2 on LSAT (111) substrate,"We report the epitaxial growth and the electrical properties, especially the
metal-to-insulator transition (MIT), of vanadium dioxide (VO2) thin films
synthesized on LSAT (111) ([LaAlO3]0.3[Sr2AlTaO6]0.7) substrates by pulsed
laser deposition. X-ray diffraction studies show that the epitaxial
relationship between the VO2 thin films and LSAT substrate is given as
VO2(020)||LSAT(111) and VO2[001]||LSAT[11-2]. We observed a sharp four orders
of magnitude change in the longitudinal resistance for the VO2 thin films
around the transition temperature. We also measured distinct Raman spectra
below and above the transition point indicating a concomitant structural
transition between the insulator and metallic phases, in agreement with past
investigations.",1809.10748v1
2018-12-11,The effect of crystal structure in sputtering of two-component single crystal targets,"In the present paper the forward sputtering of atoms of the components under
ion bombardment of the single crystal VSi2 (0001) face is studied with the use
of molecular dynamics computer simulations. Sputtering of atoms of the
components under ion bombardment of the virtual single crystals VV2 and SiSi2
consisting of atoms of the same mass but located in the lattice sites of V and
Si sublattices is calculated. The new effect in the selective sputtering of
two-component targets is studied: atoms from the vanadium lattice sites are
preferentially forward sputtered for equal masses and equal surface binding
energies of atoms of the components. This effect one could name the effect of
nonidentity of lattice sites of the components in the complex lattice of VSi2
(of the C40 type) to the propagation of momentum in collision cascades and thus
the effect of structure in the selective sputtering of two-component single
crystal targets.
  Key words: sputtering, transmission of ions, selective sputtering,
transmission sputtering, two-component targets, VSi2, thin single-crystal
films.",1812.04310v1
2018-12-14,VI3 - a new layered ferromagnetic semiconductor,"Two-dimensional (2D) materials are promising candidates for next-generation
electronic devices. In this regime, insulating 2D ferromagnets, which remain
rare, are of special importance due to their potential for enabling new device
architectures. Here we report the discovery of ferromagnetism in a layered van
der Waals semiconductor, VI3, which is based on honeycomb vanadium layers
separated by an iodine-iodine van der Waals gap. It has a BiI3-type structure
(R-3, No.148) at room temperature, and our experimental evidence suggests that
it may undergo a subtle structural phase transition at 78 K. VI3 becomes
ferromagnetic at 49 K, below which magneto-optical Kerr effect imaging clearly
shows ferromagnetic domains, which can be manipulated by the applied external
magnetic field. The optical band gap determined by reflectance measurements is
0.6 eV, and the material is highly resistive.",1812.05982v1
2019-02-04,"Passive radiative ""thermostat"" enabled by phase-change photonic nanostructures","A thermostat senses the temperature of a physical system and switches heating
or cooling devices on or off, regulating the flow of heat to maintain the
system's temperature near a desired setpoint. Taking advantage of recent
advances in radiative heat transfer technologies, here we propose a passive
radiative ""thermostat"" based on phase-change photonic nanostructures for
thermal regulation at room temperature. By self-adjusting their visible to
mid-IR absorptivity and emissivity responses depending on the ambient
temperature, the proposed devices use the sky to passively cool or heat during
day-time using the phase-change transition temperature as the setpoint, while
at night-time temperature is maintained at or below ambient. We simulate the
performance of a passive nanophotonic thermostat design based on vanadium
dioxide thin films, showing daytime passive cooling (heating) with respect to
ambient in hot (cold) days, maintaining an equilibrium temperature
approximately locked within the phase transition region. Passive radiative
thermostats can potentially enable novel thermal management technologies, e.g.
to moderate diurnal temperature in regions with extreme annual thermal swings.",1902.01354v1
2019-02-12,Development of high-sensitivity chip calorimeters for cellular metabolic heat sensing,"Cellular metabolic rate is a good indicator of the physiological state of
cells and its changes, which can be measured by total heat flux accompanying
metabolism. Chip calorimeters can provide label-free and high throughput
measurements of cellular metabolic rate, however, lack of high power resolution
and microfluidic sample handling capability has been preventing their wide
applications. We report high-resolution chip calorimeters integrated with
thin-film parylene microfluidics, which can reliably measure metabolic heat
from mammalian cells with controlled stimuli. The molding and bonding technique
allowed fast and reliable parylene microfluidic channel fabrications and highly
sensitive vanadium oxide thermistor enabled temperature resolution as small as
~ 15 {\mu}K, which led to a three-orders-of-magnitude improvement in volume
specific power resolutions. Measurements of metabolic heat were successfully
demonstrated with adherent and nonadherent cells. We expect the chip
calorimeter will provide a universal platform for fundamental cell-biology
studies and biomedical applications including cell-based assay for drug
discovery.",1902.04503v2
2019-02-25,Superconductivity in the surface state of noble metal gold and its Fermi level tuning by EuS dielectric,"The induced superconductivity (SC) in a robust and scalable quantum material
with strong Rashba spin-orbit coupling is particularly attractive for
generating topological superconductivity and Majorana bound states (MBS). Gold
(111) thin film has been proposed as a promising candidate because of the large
Rashba energy, the predicted topological nature and the possibility for
large-scale MBS device fabrications. We experimentally demonstrate two
important steps towards achieving such a goal. We successfully show induced SC
in the Shockley surface state (SS) of ultrathin Au(111) layers grown over
epitaxial vanadium films, which is easily achievable on a wafer scale. The
emergence of SC in the SS, which is physically separated from a bulk
superconductor, is attained by indirect quasiparticle scattering processes
instead of by conventional interfacial Andreev reflections. We further show the
ability to tune the SS Fermi level (E_F) by interfacing SS with a high-k
dielectric ferromagnetic insulator EuS. The shift of E_F from ~ 550 mV to ~34mV
in superconducting SS is an important step towards realizing MBS in this robust
system.",1902.09664v1
2019-03-10,An Adaptive Observer Design for Charge-State and Crossover Estimation in Disproportionation Redox Flow Batteries undergoing Self-Discharge,"This article considers a model formulation and an adaptive observer design
for the simultaneous estimation of the state of charge and crossover flux in
disproportionation redox flow batteries. This novel nonaqueous battery
chemistry allows a simple isothermal lumped parameter model to be formulated.
The transport of vanadium through the porous separator is a key unknown
function of battery variables and it is approximated in the space of continuous
functions. The state and parameter observer adaptation laws are derived using
Lyapunov analysis applied to the estimation error, the stability and
convergence of which are proved. Numerical values of observer gains are
calculated by solving a polytopic linear matrix inequality and equality problem
via convex optimization. The performance of this design is evaluated on a
laboratory flow battery prototype, and it is shown that the crossover flux can
be considered a linear function of state of charge for this battery
configuration during self-discharge.",1903.04073v1
2019-05-18,A Fixed-Flexible BESS Allocation Scheme for Transmission Networks Considering Uncertainties,"Battery energy storage systems (BESSs) can play a key role in mitigating the
intermittency and uncertainty associated with adding large amounts of renewable
energy to the bulk power system (BPS). Two BESS technologies that have gained
prominence in this regard are Lithium-ion (LI) BESS and Vanadium redox flow
(VRF) BESS. This paper proposes a fixed-flexible BESS allocation scheme that
exploits the complementary characteristics of LI and VRF BESSs to attain
optimal techno-economic benefits in a wind-integrated BPS. Studies carried out
on relatively large transmission networks demonstrate that benefits such as
reduction in system operation cost, wind spillage, voltage fluctuations, and
discounted payback period, can be realized by using the proposed scheme.",1905.07536v3
2019-05-27,A State Observer Design for Simultaneous Estimation of Charge State and Crossover in Self-Discharging Disproportionation Redox Flow Batteries,"This paper presents an augmented state observer design for the simultaneous
estimation of charge state and crossover flux in disproportionation redox flow
batteries, which exhibits exponential estimation error convergence to a bounded
residual set. The crossover flux of vanadium through the porous separator is
considered as an unknown function of the battery states, model-approximated as
the output of a persistently excited linear system. This parametric model and
the simple isothermal lumped parameter model of the battery are combined to
form an augmented space state representation suitable for the observer design,
which is carried out via Lyapunov stability theory including the
error-uncertainty involved in the approximation of the crossover flux. The
observer gain is calculated by solving a polytopic linear matrix inequality
problem via convex optimization. The performance of this design is evaluated
with a laboratory flow battery prototype undergoing self-discharge.",1905.11396v1
2019-06-08,Possible emergence of a skyrmion phase in ferroelectric GaMo$_4$S$_8$,"Polar lacunar spinels, such as GaV$_4$S$_8$ and GaV$_4$Se$_8$, were proposed
to host skyrmion phases under magnetic field. In this work, we put forward, as
a candidate for N\'eel-type skyrmion lattice, the isostructural GaMo$_4$S$_8$,
here systematically studied via both first-principles calculations and Monte
Carlo simulations of model Hamiltonian. Electric polarization, driven by
Jahn-Teller distortion, is predicted to arise in GaMo$_4$S$_8$, showing a
comparable size but an opposite sign with respect to that evaluated in V-based
counterparts and explained in terms of different electron counting arguments
and resulting distortions. Interestingly, a larger spin-orbit coupling of 4d
orbitals with respect to 3d orbitals in vanadium-spinels leads to stronger
Dzyaloshinskii-Moriya interactions, which are beneficial to stabilize a
cycloidal spin texture, as well as smaller-sized skyrmions (radius<10 nm).
Furthermore, the possibly large exchange anisotropy of GaMo4S8 may lead to a
ferroelectric-ferromagnetic ground state, as an alternative to the
ferroelectric-skyrmionic one, calling for further experimental verification.",1906.03399v1
2019-07-10,Dynamical mean-field study of Vanadium diselenide monolayer ferromagnetism,"To understand the magnetism of VSe$_2$, whose monolayer form has recently
been reported to be a room temperature ferromagnet, Here, the combined method
of conventional density functional theory with dynamical mean-field theory has
been adopted. This higher-level computation method enables us to resolve many
of existing controversies and contradictions in between theory and experiment.
First of all, this new approach is shown to give the correct magnetic
properties of both bulk and two-dimensional limit of VSe$_2$ which demonstrates
its superiority to the conventional methods. The results demonstrate that
monolayer VSe$_2$ without charge density waves is a ferromagnet with ordering
temperature of 250K. From the direct simulation of temperature-dependent
magnetic susceptibility and ordered moment, it is shown that its ferromagnetism
is clearly two-dimensional in nature. Further, it is shown that this
ferromagnetic order is vulnerable to extra charge dopings which provides the
important insight to elucidate recent experimental controversies.",1907.04790v3
2019-08-02,Real-Space Investigation of the Charge Density Wave in VTe2 Monolayer with Rotational and Mirror Symmetries Broken,"Recently the charge density wave (CDW) in vanadium dichalcogenides have
attracted increasing research interests, but a real-space investigation on the
symmetry breaking of the CDW state in VTe2 monolayer is still lacking. We have
investigated the CDW of VTe2 monolayer by low energy electron diffraction
(LEED) and scanning tunneling microscope (STM). While the LEED experiments
revealed a (4X4) CDW transition at 192+-2 K, our low-temperature STM
experiments resolved the (4X4) lattice distortions and charge-density
modulation in real space, and further unveiled a 1D modulation that breaks the
three-fold rotational and mirror symmetries in the CDW state. In accordance
with the CDW state at low temperature, a CDW gap of 12 meV was detected by
scanning tunneling spectroscopy (STS) at 4.9 K. Our work provides real-space
evidence on the symmetry breaking of the (4X4) CDW state in VTe2 monolayer, and
implies there is a certain mechanism, beyond the conventional Fermi surface
nesting or the q-dependent electron-phonon coupling, is responsible for the
formation of CDW state in VTe2 monolayer.",1908.00714v1
2019-08-19,Planar Hall Effect and Anisotropic Magnetoresistance in a polar-polar interface of LaVO$_3$-KTaO$_3$ with strong spin-orbit coupling,"Among the perovskite oxide family, KTaO$_3$ (KTO) has recently attracted
considerable interest as a possible system for the realization of the Rashba
effect. In this work, we improvise a novel conducting interface by juxtaposing
KTO with another insulator, namely LaVO$_3$ (LVO) and report planar Hall effect
(PHE) and anisotropic magnetoresistance (AMR) measurements. This interface
exhibits a signature of strong spin-orbit coupling. Our experimental
observation of two fold AMR at low magnetic fields can be intuitively
understood using a phenomenological theory for a Rashba spin-split system. At
high fields ($\sim$8 T), we see a two fold to four fold transition in the AMR
that could not be explained using only Rashba spin-split energy spectra. We
speculate that it might be generated through an intricate process arising from
the interplay between strong spin-orbit coupling, broken inversion symmetery,
relativistic conduction electron and possible uncompensated localized vanadium
spins.",1908.06636v1
2019-08-27,"A Systematic Study to Improve the Performance of SrCoO3 as anAnion-Intercalation-Type Electrode for Supercapacitors Through Interface, Oxygen Vacancies, and Doping","Supercapacitors have recently gained popularity as possible energy storage
systems due to their high cycling ability and increased power density. However,
one of the major drawbacks of supercapacitors is that they have a low energy
density, which makes them less effective than batteries. Herein, we explore
different methods of increasing the supercapacitor performance of the
perovskite SrCoO3. We carry out first-principles calculations to systematically
study how SrCoO3/graphene interface, oxygen vacancies, and doping improve the
performance of strontium cobaltite as an anion-intercalation-type
supercapacitor. The results show that the SrCoO3/graphene interface is
relatively stable with a formation energy of 1.3 eV and is highly conductive,
which makes it a promising material for supercapacitors. We also find that
inducing oxygen vacancies in SrCoO3 significantly increases the conductivity of
this material. Results of doping calculations reveal that doping with Mo, V, P,
and Nb all increase the stability and conductivity of SrCoO3. We find that
niobium is the most stable and most conductive of all four dopants. In
addition, we find that vanadium is a very promising novel dopant for SrCoO3 as
an anion-intercalation-type supercapacitor electrode material.",1908.10494v1
2019-08-31,Magnetocaloric properties of V6 molecular magnet,"The paper presents a theoretical study of magnetocaloric properties of
polyoxovanadate molecular magnet V6 containing 6 vanadium ions carrying quantum
spins $S = 1/2$. The characteristic property of such structure is the presence
of two weakly interacting spin triangles with all-antiferromagnetic couplings.
The properties of the system are described using the exact numerical
diagonalization approach applied to the quantum Heisenberg model and utilizing
a field ensemble formalism. The dependence of the magnetic entropy and magnetic
specific heat on the temperature and external magnetic field is calculated and
extensively discussed. The magnetocaloric properties are quantified by
isothermal entropy change and entropy derivative over the magnetic field. An
interesting behaviour of isothermal entropy change is found, with high degree
of tunability of the magnetocaloric effect with the initial and final magnetic
field values.",1909.00242v2
2019-09-18,"NMR, magnetization, and heat capacity studies of the uniform spin-1/2 chain compound Bi6V3O16","We report the local (NMR) and bulk (magnetization and heat capacity)
properties of the vanadium-based S=1/2 uniform spin chain compound
Bi6V3O16(Bi4V2O10.66). In the low-temperature {\alpha} phase, the magnetic ions
(V4+) are arranged in one-dimensional chains. The magnetic susceptibility shows
a broad maximum around 50 K signifying a short-range magnetic order. Heat
capacity measurements also reveal low-dimensional magnetism. The 51V magic
angle spinning nuclear magnetic resonance measurements clearly show that the
magnetic V4+ and nonmagnetic V5+ species are located on different
crystallographic sites with no mixed occupation. The spin susceptibility
calculated from the shift of the 51V NMR spectra reproduces the behavior
observed in magnetic susceptibility and agrees well with the S=1/2 uniform spin
chain model with J=113(5) K.",1909.08594v1
2019-09-25,{\textit{Ab initio} determination of pseudospin for paramagnetic defects in SiC,"Paramagnetic point defects in solids may exhibit a rich set of interesting
and not yet fully resolved physics. In particular, character of wavefunctions
and electron-phonon coupling in these defects may highly influence their
interaction with external magnetic fields. Complex interplay between the
electronic orbitals, phonons and electron spin determines the effective
pseudospin of the system that we demonstrate on vanadium and molybdenum defects
in hexagonal silicon carbide (SiC) by means of \textit{ab initio} calculations.
In this Rapid Communication, we find a giant anisotropy in the $g$-tensor of
these defects with Kramers doublet spin ground state, resulting in reduced and
vanishing interaction with the magnetic field in parallel and transverse
directions, respectively. The consequences of our finding in the application of
these defects for quantum information processing are briefly discussed.",1909.11587v3
2019-10-08,Tunable Multifocal THz Metalens Based on Metal-Insulator Transition of VO2 Film,"Recently, metalenses which consist of metasurface arrays, have attracted
attention due to their more condensed size in comparison with conventional
lenses. In this paper, we propose a reconfigurable coding metasurface
hybridized with vanadium dioxide (VO2) for wavefront manipulation at terahertz
(THz) frequencies. At room temperature, the unit-cell can reflect as a ""1"" bit
under linearly y polarized illuminated waves. Besides, when the temperature is
increased, VO2 would be in a fully metallic state; therefore, unit-cell can act
as a ""0"" reflection phase. Furthermore, by changing the unit-cells arrangements
on a metalens surface, the proposed device can focus the incident beam at any
position according to a particular design. Numerical simulations demonstrate
that the designed VO2-assisted metasurface can generate one and multi-focal
spot in reflection mode as expected. Also, theoretical results depict an
excellent agreement with obtained simulation results. The presented metalens
has notable potential in THz high-resolution imaging and optical coding.",1910.03149v1
2019-10-31,Ultrafast reprogrammable multifunctional vanadium-dioxide-assisted metasurface for dynamic THz wavefront engineering,"In this paper, for the first time, a new generation of ultrafast
reprogrammable multi-mission bias encoded metasurface is proposed for dynamic
THz wavefront engineering by employing VO2 reversible and fast monoclinic to
tetragonal phase transition. The multi-functionality of our designed VO2 based
coding metasurface (VBCM) was guaranteed by elaborately designed meta-atom
comprising three-patterned VO2 thin films whose operational statuses can be
dynamically tuned among four states of ""00""- ""11"" by merely changing the
biasing voltage controlled by an external FPGA platform. Capitalizing on such
meta-atom design and by driving VBCM with different spiral-like and
spiral-parabola-like coding sequences, single vortex beam and focused vortex
beam with interchangeable OAM modes were satisfactorily generated respectively.
Additionally, by adopting superposition theorem and convolution operation,
symmetric/asymmetric multiple beams and arbitrarily-oriented multiple vortex
beams in pre-demined directions with different topological charges are
realized. The versatility of our designed VBCM also has equipped a platform to
focus the incident THz wavefront into a pre-determined point which can be
dynamically altered. Several illustrative examples successfully have clarified
that proposed VBCM is a promising candidate for solving crucial THz challenges
such as high data rate wireless communication where ultrafast switching between
several missions is required.",1910.14272v1
2019-12-23,Spin-$\frac{1}{2}$ kagome Heisenberg antiferromagnet with strong breathing anisotropy,"We study the zero-temperature phase diagram of the spin-$\frac{1}{2}$
Heisenberg model with breathing anisotropy (i.e., with different coupling
strength on the upward and downward triangles) on the kagome lattice. Our study
relies on large scale tensor network simulations based on infinite projected
entangled-pair state and infinite projected entangled-simplex state methods
adapted to the kagome lattice. Our energy analysis suggests that the U(1)
algebraic quantum spin-liquid (QSL) ground-state of the isotropic Heisenberg
model is stable up to very large breathing anisotropy until it breaks down to a
critical lattice-nematic phase that breaks rotational symmetry in real space
through a first-order quantum phase transition. Our results also provide
further insight into the recent experiment on vanadium oxyfluoride compounds
which has been shown to be relevant platforms for realizing QSL in the presence
of breathing anisotropy.",1912.10756v2
2020-01-09,High-Temperature Ferromagnetic Semiconductors: Janus Monolayer Vanadium Trihalides,"Two-dimensional (2D) intrinsic ferromagnetic semiconductors are expected to
stand out in the spintronic field. Recently, the monolayer VI$_{3}$ has been
experimentally synthesized but the weak ferromagnetism and low Curie
temperature ($T_C$) limit its potential application. Here we report that the
Janus structure can elevate the $T_C$ to 240 K. And it is discussed that the
reason for high $T_C$ in Janus structure originates from the lower virtual
exchange gap between $t_{2g}$ and $e_{g}$ states of nearest-neighbor V atoms.
Besides, $T_C$ can be further substantially enhanced by tensile strain due to
the increasing ferromagnetism driven by rapidly quenched direct exchange
interaction. Our work supports a feasible approach to enhance Curie temperature
of monolayer VI$_{3}$ and unveils novel stable intrinsic FM semiconductors for
realistic applications in spintronics.",2001.02846v2
2020-01-10,Electrical and Spectroscopic Properties of SiC Detectors,"In this paper we compare suitability of three different semi-insulating bulk
silicon carbide (SiC) materials for fabrication of radiation detectors. We
prepared planar sensors with various metal contact combination and
characterized detector quality by the alpha spectroscopy and I-V characteristic
measurements. We observed that 4H-SiC material from the II-VI company is not
suitable for radiation detector fabrication due to high concentration of
vanadium doping. We also present a poor charge collection efficiency of the
4H-SiC Norstel material due to high concentration of residual impurities and we
evaluated low mobility-lifetime product (alpha)= 3.5E-7 cm2/V from the alpha
spectroscopy. We demonstrate that the 4H-SiC material from the Cree company is
the best candidate for the production of radiation detectors. We evaluated
mobility-lifetime product (alpha)= 5.4E-6 cm2/V using AuTi/SiC/TiAu contact
structure.",2001.03351v1
2020-01-20,Gate modulation of the long-range magnetic order in a vanadium-doped WSe2 semiconductor,"We demonstrate the gate-tunability of the long-range magnetic order in a
p-type V-doped WSe2 monolayer using ab initio calculations. We found that at a
low V-doping concentration limit, the long-range ferromagnetic order is
enhanced by increasing the hole density. In contrast, the short-range
antiferromagnetic order is manifested at a high electron density by full
compensation of the p-type V doping concentration. The hole-mediated long-range
magnetic exchange is ~70 meV, thus strongly suggesting the ferromagnetism in
V-doped WSe2 at room temperature. Our findings on strong coupling between
charge and spin order in V-doped WSe2 provide plenty of room for
multifunctional gate-tunable spintronics.",2001.06941v1
2020-01-23,Magnetic-field-induced insulator-metal transition in W-doped VO2 at 500 T,"Metal-insulator (MI) transitions in correlated electron systems have long
been a central and controversial issue in material science. Vanadium dioxide
(VO2) exhibits a first-order MI transition at 340 K. For more than half a
century, it has been debated whether electronic correlation or the structural
instability due to dimerised V ions is the more essential driving force behind
this MI transition. Here, we show that an ultrahigh magnetic field of 500 T
renders the insulator phase of tungsten (W)-doped VO2 metallic. The spin Zeeman
effect on the d electrons of the V ions dissociates the dimers in the
insulating phase, resulting in the delocalisation of electrons. Because the
Mott-Hubbard gap essentially does not depend on the spin degree of freedom, the
structural instability is likely to be the more essential driving force behind
the MI transition.",2001.08580v1
2020-02-10,Dynamic control of mode modulation and spatial multiplexing using hybrid metasurfaces,"Designing reconfigurable metasurfaces that can dynamically control scattered
electromagnetic waves and work in the near-infrared (NIR) and optical regimes
remains a challenging task, which is hindered by the static material property
and fixed structures. Phase change materials (PCMs) can provide high contrast
optical refractive indexes at high frequencies between amorphous and crystal
states, therefore are promising as feasible materials for reconfigurable
metasurfaces. Here, we propose a hybrid metasurface that can arbitrarily
modulate the complex amplitude of incident light with uniform amplitude and
full $2\pi$ phase coverage by utilizing composite concentric rings (CCRs) with
different ratios of gold and PCMs. Our designed metasurface possesses a
bi-functionality that is capable of splitting beams or generating vortex beams
by thermal switching between metal and semiconductor states of vanadium oxide
(VO2), respectively. It can be easily integrated into low loss photonic
circuits with an ultra-small footprint. Our metadevice serves as a novel
paradigm for active control of beams, which may open new opportunities for
signal processing, memory storage, holography, and anti-counterfeiting.",2002.04074v1
2020-02-17,"Structural, electronic, and magnetic properties of Vanadium-based Janus dichalcogenide monolayers: A first-principles study","The structural, electronic, and magnetic properties of VSSe, VSeTe, VSTe
monolayers in both 2H and 1T phases are investigated via first-principles
calculations. The 2H phase is energetically favorable in VSSe and VSeTe,
whereas the 1T phase is lower in energy in VSTe. For V-based Janus monolayers
in the 2H phase, calculations of the magnetic anisotropy show an easy-plane for
the magnetic moment. As such, they should not exhibit a ferromagnetic phase
transition, but instead, a Berezinskii-Kosterlitz-Thouless (BKT) transition. A
classical XY model with nearest-neighbor coupling estimates critical
temperatures (T$_{BKT}$) ranging from 106 K for VSSe to 46 K for VSTe.",2002.07104v3
2020-03-23,Tracking ultrafast solid-state dynamics using high harmonic spectroscopy,"We establish time-resolved high harmonic generation (tr-HHG) as a powerful
spectroscopy for photoinduced dynamics in strongly correlated materials through
a detailed investigation of the insulator-to-metal transitions in vanadium
dioxide. We benchmark our technique by comparing our measurements to
established momentum-resolved ultrafast electron diffraction, and theoretical
density functional calculations. Tr-HHG allows distinguishing of individual
dynamic channels, including a transition to a thermodynamically hidden phase.
In addition, the HHG yield is shown to be modulated at a frequency
characteristic of a coherent phonon in the equilibrium monoclinic phase over a
wide range of excitation fluences. These results demonstrate that tr-HHG is
capable of tracking complex dynamics in solids through its sensitivity to the
band structure.",2003.10430v1
2020-04-01,Spectral self-adaptive absorber/emitter for harvesting energy from the sun and outer space,"The sun (~6000 K) and outer space (~3 K) are the original heat source and
sink for human beings on Earth. The energy applications of absorbing solar
irradiation and harvesting the coldness of outer space for energy utilization
have attracted considerable interest from researchers. However, combining these
two functions in a static device for continuous energy harvesting is
unachievable due to the intrinsic infrared spectral conflict. In this study, we
developed spectral self-adaptive absorber/emitter (SSA/E) for daytime
photothermal and nighttime radiative sky cooling modes depending on the phase
transition of the vanadium dioxide coated layer. A 24-hour day-night test
showed that the fabricated SSA/E has continuous energy harvesting ability and
improved overall energy utilization performance, thus showing remarkable
potential in future energy applications.",2004.00459v1
2020-04-08,The limit of spin lifetime in solid-state electronic spins,"The development of spin qubits for quantum technologies requires their
protection from the main source of finite-temperature decoherence: atomic
vibrations. Here we eliminate one of the main barriers to the progress in this
field by providing a complete first-principles picture of spin relaxation that
includes up to two-phonon processes. Our method is based on machine learning
and electronic structure theory and makes the prediction of spin lifetime in
realistic systems feasible. We study a prototypical vanadium-based molecular
qubit and reveal that the spin lifetime at high temperature is limited by Raman
processes due to a small number of THz intra-molecular vibrations. These
findings effectively change the conventional understanding of spin relaxation
in this class of materials and open new avenues for the rational design of
long-living spin systems.",2004.04007v1
2020-04-11,Coherent Growth and Characterization of van der Waals 1T-VSe$_2$ Layers on GaAs(111)B Using Molecular Beam Epitaxy,"We report epitaxial growth of vanadium diselenide (VSe$_2$) thin films in the
octahedrally-coordinated (1T) structure on GaAs(111)B substrates by molecular
beam epitaxy. Film thickness from a single monolayer (ML) up to 30 ML is
demonstrated. Structural and chemical studies using by x-ray diffraction,
transmission electron microscopy, scanning tunneling microscopy and x-ray
photoelectron spectroscopy indicate high quality thin films. Further studies
show that monolayer VSe$_2$ films on GaAs are not air-stable and are
susceptible to oxidation within a matter of hours, which indicates that a
protective capping layer should be employed for device applications. This work
demonstrates that VSe$_2$, a candidate van der Waals material for possible
spintronic and electronic applications, can be integrated with III-V
semiconductors via epitaxial growth for 2D/3D hybrid devices.",2004.05506v1
2020-04-15,Tunable electronic structure and stoichiometry dependent disorder in Nanostructured VO$_x$ films,"We present and discuss an original method to synthesize disordered
Nanostructured (NS) VO$_x$ films with controlled stoichiometry and tunable
electronic structures. In these NS films, the original lattice symmetry of the
bulk vanadium oxides is broken and atoms are arranged in a highly disordered
structure . The stoichiometry-dependent disorder as a function of the oxygen
concentration has been characterized by in-situ X-ray Absorption Near-Edge
Structure (XANES) spectroscopy identifying the spectroscopic fingerprints.
Results show structural rearrangements that deviate from the octahedral
symmetry with different coexisting disordered phases. The modulation of the
electronic structure of the NS films based on the resulted stoichiometry and
the quantum confinement in the NS particles are also discussed. We demonstrate
the possibility to modulate the electronic structure of VO$_x$ NS films
accessing new disordered atomic configurations with a controlled stoichiometry
that provides an extraordinary opportunity to match a wide number of
technological applications.",2004.07042v1
2020-04-17,High-pressure characterization of multifunctional CrVO4,"The structural stability and physical properties of CrVO4 under compression
were studied by X-ray diffraction, Raman spectroscopy, optical absorption,
resistivity measurements, and ab initio calculations up to 10 GPa.
High-pressure X-ray diffraction and Raman measurements show that CrVO4
undergoes a phase transition from the ambient pressure orthorhombic CrVO4-type
structure (Cmcm space group, phase III) to the high-pressure monoclinic CrVO4-V
phase, which is isomorphic to the wolframite structure. Such a phase transition
(CrVO4-type - wolframite), driven by pressure, also was previously observed in
indium vanadate. The crystal structure of both phases and the pressure
dependence in unit-cell parameters, Raman-active modes, resistivity, and
electronic band gap, is reported. Vanadium atoms are sixth-fold coordinated in
the wolframite phase, which is related to the collapse in the volume at the
phase transition. Besides, we also observed drastic changes in the phonon
spectrum, a drop of the band-gap, and a sharp decrease of resistivity. All the
observed phenomena are explained with the help of first-principles
calculations.",2004.08072v1
2020-05-06,Correlation induced emergent charge order in metallic vanadium dioxide,"Recent progress in growth and characterization of thin-film VO$_2$ has shown
its electronic properties can be significantly modulated by epitaxial matching.
To throw new light on the concept of `Mott engineering', we develop a
symmetry-consistent approach to treat structural distortions and electronic
correlations in epitaxial VO$_2$ films under strain, and compare our design
with direct experimental probes. We find strong evidence for the emergence of
correlation-driven charge order deep in the metallic phase, and our results
indicate that exotic phases of VO$_2$ can be controlled with epitaxial
stabilization.",2005.02957v1
2020-05-20,Frequency mixing in a ferrimagnetic sphere resonator,"Frequency mixing in ferrimagnetic resonators based on yttrium and calcium
vanadium iron garnets (YIG and CVBIG) is employed for studying their nonlinear
interactions. The ferrimagnetic Kittel mode is driven by applying a pump tone
at a frequency close to resonance. We explore two nonlinear frequency mixing
configurations. In the first one, mixing between a transverse pump tone and an
added longitudinal weak signal is explored, and the experimental results are
compared with the predictions of the Landau-Zener-Stuckelberg model. In the
second one, intermodulation measurements are employed by mixing pump and signal
tones both in the transverse direction for studying a bifurcation between a
stable spiral and a stable node attractors. Our results are applicable for
developing sensitive signal receivers with high gain for both the radio
frequency and the microwave bands.",2005.09864v1
2020-06-18,Switchable bifunctional metasurfaces: nearly perfect retroreflection and absorption at THz regime,"Here we make use of vanadium dioxide (VO2) to design a bifunctional
metasurface working at the same targeted frequency. With the increase of
temperature, the functionality of the designed metasurface can switch from a
multi-channel retroreflector to a perfect absorber, caused by the phase
transition of VO2 from insulator to conductor. Different from traditional
bifunctional metasurfaces designed by simple composition of two
functionalities, our proposed bifunctional metasurface is based on the
interaction between two functionalities. The device shows good potential for
the combination of wavefront manipulation and optical absorption, therefore
providing a promising approach for switchable detection and anti-detection
devices.",2006.10229v1
2020-06-15,Influence of Ti/V Cation-Exchange in Na$_2$Ti$_3$O$_7$ on Na-Ion Negative Electrode Performance: an Insight from First-Principles Study,"Sodium-titanate \ce{Na2Ti3O7} (NTO) is regarded as a highly promising anode
material with a very low voltage for Na-ion batteries and capacitors, but
suffered from relatively low specific capacity and poor electron conductivity.
Here we report a first-principles study of electrochemical properties of NTO
and its vanadium-modified compounds, \ce{Na2Ti2VO7} and \ce{Na2TiV2O7} (NTVO),
offering an insight into their detailed working mechanism and an evidence of
enhancing anode performance by Ti/V cation exchange. Our calculations reveal
that the specific capacity can increase from 177 mAh g$^{-1}$ in NTO to over
280 mAh g$^{-1}$ in NTVO when using \ce{NaTi_{3-$x$}V_{$x$}O7} ($x$ = 1, 2) as
a starting material for Na insertion due to higher oxidation state of
\ce{V^{+5}}, together with lower voltages and small volume expansion rates
below 3\%. With Ti/V exchange, we obtain slightly higher activation energies
for Na ion migrations along the two different pathways, but find an obvious
improvement of electronic transport in NTVO.",2006.10669v1
2020-07-08,Non-contact Thermal Transistor Effects Modulated by Nanoscale Mechanical Deformation,"Thermal management has become a promising field in recent years due to the
limitation of energy resources and the global warming. An important topic in
improving the efficiency of thermal energy utilization is how to control the
flows of heat, and thermal rectifiers, such as the thermal transistor, have
been proposed as units for modulating the flow of heat. In this work, a
reconfigurable non-contact thermal transistor with two-dimensional grating is
introduced. The thermal transistor consists of three parts: source, gate, and
drain, with the gate working around the phase-transition temperature of
vanadium dioxide, a type of phase-transition material. Results show that the
unit has a clear transistor-like behavior. The surface phonon/plasmon
polaritons supported by the insulating/metallic states that contribute to the
radiative thermal transport can be modulated at a nanoscale separation. And the
dynamic amplification factor ranges from 15.4 to 30.6 when the stretchable
polydimethylsiloxane is subjected to tension or compression. This work sheds
light on studies about the controllable small-scale thermal transport due to
mechanical deformations.",2007.03889v1
2020-07-14,High pressure crystal structures of orthovanadates and their properties,"Pressure-induced phase transitions in orthovanadates have led to interesting
physical phenomena. The observed transitions usually involve large volume
collapses and drastic changes in the electronic and vibrational properties of
the materials. In some cases, the phase transitions implicate coordination
changes in vanadium, which has important consequences in the physical
properties of vanadates. In this Perspective, we explore the current knowledge
of the behavior of MVO4 vanadates under compression. In particular, we
summarize studies of the structural, vibrational, and electronic properties and
a few illustrative examples of high-pressure research in the compounds of
interest are discussed. A systematic understanding of the high-pressure
behavior of MVO4 compounds is presented, putting the emphasis on results that
could be relevant for practical applications. Recent advances and future
challenges in the study of orthovanadates under extreme pressure will be
reviewed, along with conclusions that could have consequences for the studies
of related oxides. Some ideas on topics that may lead to exciting breakthroughs
in the near future will be presented too.",2007.07146v1
2020-07-15,Atomic origin for hydrogenation promoted bulk oxygen vacancies removal in vanadium dioxide,"Oxygen vacancies (VO), a common type of point defects in metal oxides
materials, play important roles on the physical and chemical properties. To
obtain stoichiometric oxide crystal, the pre-existing VO is always removed via
careful post-annealing treatment at high temperature in air or oxygen
atmosphere. However, the annealing conditions is difficult to control and the
removal of VO in bulk phase is restrained due to high energy barrier of VO
migration. Here, we selected VO2 crystal film as the model system and developed
an alternative annealing treatment aided by controllable hydrogen doping, which
can realizes effective removal of VO defects in VO2-{\delta} crystal at lower
temperature. This finding is attributed to the hydrogenation accelerated oxygen
vacancies recovery in VO2-{\delta} crystal. Theoretical calculations revealed
that the H-doping induced electrons are prone to accumulate around the oxygen
defects in VO2-{\delta} film, which facilitates the diffusion of VO and thus
makes it easier to be removed. The methodology is expected to be applied to
other metal oxides for oxygen-related point defects control.",2007.07505v1
2020-07-15,Janus Monolayers of Magnetic Transition Metal Dichalcogenides as an All-in-One Platform for Spin-Orbit Torque,"We theoretically predict that vanadium-based Janus dichalcogenide monolayers
constitute an ideal platform for spin-orbit-torque memories. Using first
principles calculations, we demonstrate that magnetic exchange and magnetic
anisotropy energies are higher for heavier chalcogen atoms, while the broken
inversion symmetry in the Janus form leads to the emergence of Rashba-like
spin-orbit coupling. The spin-orbit torque efficiency is evaluated using
optimized quantum transport methodology and found to be comparable to heavy
nonmagnetic metals. The coexistence of magnetism and spin-orbit coupling in
such materials with tunable Fermi-level opens new possibilities for monitoring
magnetization dynamics in the perspective of non-volatile magnetic random
access memories.",2007.07579v1
2020-07-15,Nanoscale axial position and orientation measurement of hexagonal boron nitride quantum emitters using a tunable nanophotonic environment,"Color centers in hexagonal boron nitride (hBN) have emerged as promising
candidates for single-photon emitters (SPEs) due to their bright emission
characteristics at room temperature. In contrast to mono- and few-layered hBN,
color centers in multi-layered flakes show superior emission characteristics
such as higher saturation counts and spectral stability. Here, we report a
method for determining both the axial position and three-dimensional dipole
orientation of SPEs in thick hBN flakes by tuning the photonic local density of
states using vanadium dioxide (VO2), a phase change material. Emitters under
study exhibit a strong surface-normal dipole orientation, providing some
insight on the atomic structure of hBN SPEs, deeply embedded in thick crystals.
We have optimized a hot pickup technique to reproducibly transfer flakes of hBN
from VO2 onto SiO2/Si substrate and relocated the same emitters. Our approach
serves as a practical method to systematically characterize SPEs in hBN prior
to integration in quantum photonics systems.",2007.07811v2
2020-07-22,Temporal dynamics of all-optical switching in hybrid VO2/Si waveguides,"Vanadium dioxide (VO2) is one of the most promising materials for developing
hybrid photonic integrated devices (PICs). However, despite switching times as
low as a few femtoseconds have been reported, the all-optical temporal dynamics
of VO2 embedded in a waveguide using an in-plane optical signal remain still
hidden. Here, we experimentally investigate this behavior in hybrid VO2/Si
waveguides by using pump-probe measurements at telecom wavelengths. Our results
show switching times in the micro and nanosecond range, suggesting that the
phase transition is triggered thermally from the light absorbed by the VO2 and
the temporal response is governed by thermal conductive dynamics. By properly
engineering the optical pulse, we prospect switching times of nanoseconds with
an energy consumption of a few nanojoules. Our results unveil a new temporal
dynamic that would be useful for developing future all-optical VO2 photonic
integrated devices.",2007.11452v1
2020-08-06,Decay scheme of 50V,"Investigation of the $^{50}$V electron-capture to the $2^+$ 1553.8 keV level
of $^{50}$Ti and search for $\beta^-$ decay of $^{50}$V to the $2^+$ 783.3 keV
level of $^{50}$Cr (both those decays are fourfold forbidden with $\Delta
J^{\Delta \pi}=4^+$) have been performed using a vanadium sample of natural
isotopic abundance with mass of 955 g. The measurements were conducted with the
help of an ultra low-background HPGe-detector system located 225 m underground
in the laboratory HADES (Belgium). The measured value of the half-life of
$^{50}$V for electron capture was
$T^{\mathrm{EC}}_{1/2}=(2.77^{+0.20}_{-0.19})\times 10^{17}$ yr. The
$\beta^-$-decay branch was not detected and the corresponding lower bound of
the half-life was $T^{\beta}_{1/2}\geq 8.9\times 10^{18}$ yr at the 90\%
confidence level.",2008.02751v1
2020-09-16,Properties and influence of microstructure and crystal defects in Fe$_2$VAl modified by laser surface remelting,"Laser surface remelting can be used to manipulate the microstructure of cast
material. Here, we present a detailed analysis of the microstructure of
Fe$_2$VAl following laser surface remelting. Within the melt pool, elongated
grains grow nearly epitaxially from the heat-affected zone. These grains are
separated by low-angle grain boundaries with 1{\deg}-5{\deg} misorientations.
Segregation of vanadium, carbon, and nitrogen at grain boundaries and
dislocations is observed using atom probe tomography. The local electrical
resistivity was measured by an in-situ four-point-probe technique. A smaller
increase in electrical resistivity is observed at these low-angle grain
boundaries compared to high-angle grain boundaries in a cast sample. This
indicates that grain boundary engineering could potentially be used to
manipulate thermoelectric properties.",2009.07685v2
2020-09-29,Cascaded superconducting junction refrigerators: optimization and performance limits,"We demonstrate highly transparent silicon-vanadium and silicon-aluminum
tunnel junctions with relatively low sub-gap leakage current and discuss how a
trade-off typically encountered between transparency and leakage affects their
refrigeration performance. We theoretically investigate cascaded
superconducting tunnel junction refrigerators with two or more refrigeration
stages. In particular, we develop an approximate method that takes into account
self-heating effects but still allows us to optimize the cascade a single stage
at a time. We design a cascade consisting of energy-efficient refrigeration
stages, which makes cooling of, e.g., quantum devices from above 1 K to below
100 mK a realistic experimental target.",2009.14166v3
2020-10-12,Electron beam modification of vanadium dioxide oscillators,"The paper presents the results of a study of electron-beam modification (EBM)
of VO2-switch I-V curve threshold parameters and the self-oscillation frequency
of a circuit containing such a switching device. EBM in vacuum is reversible
and the parameters are restored when exposed to air at pressure of 150 Pa. At
EBM with a dose of 3 C/cm2, the voltages of switching-on (Vth) and off (Vh), as
well as the OFF-state resistance Roff, decrease down to 50% of the initial
values, and the oscillation frequency increases by 30% at a dose of 0.7 C/cm2.
Features of physics of EBM of an oscillator are outlined considering the
contribution of the metal and semiconductor phases of the switching channel.
Con-trolled modification allows EBM forming of switches with preset parameters.
Also, it might be used in artifi-cial oscillatory neural networks for pattern
recognition based on frequency shift keying.",2010.05460v1
2020-10-23,Observation of photoelectric nonvolatile memory and oscillations in VO2 at room temperature,"Vanadium dioxide (VO2) is a phase change material that can reversibly change
between high and low resistivity states through electronic and structural phase
transitions. Thus far, VO2 memory devices have essentially been volatile at
room temperature, and nonvolatile memory has required non-ambient surroundings
(e.g., elevated temperatures, electrolytes) and long write times. Here, we
report the first observation of optically addressable nonvolatile memory in VO2
at room temperature with a readout by voltage oscillations. The read and write
times had to be kept shorter than about 150 {\mu}s. The writing of the memory
and onset of the voltage oscillations had a minimum optical power threshold.
This discovery demonstrates the potential of VO2 for new computing devices and
architectures, such as artificial neurons and oscillatory neural networks.",2010.12531v1
2020-11-09,Multifocal Terahertz Lens with Adjustable Focal Points,"The conventional lens's tunability drawback always restricts their
application compared to the metasurface lens (metalens). On the other side,
reconfigurable metalenses offer the benefits of ultrathin thickness and capable
of tunability. Therefore achieving reconfigurable functionalities in a single
metasurface has attracted significant research interest for potential terahertz
(THz) applications. In this paper, an adjustable metasurface is presented using
Vanadium dioxide (VO2) to manipulate the electromagnetic waves and provide the
full reflection phase. The phase-change metasurface is composed of a VO2
nanofilm, a silicon spacer, and a gold layer embedded in the structure's
bottom. By employing the reconfigurable metasurface with the specific phase
distribution, the incident beam can converge to determined points in any
arbitrary manner, including the number of the focal points, focal points
location, and power intensity ratio. Numerical simulations demonstrate that the
proposed reconfigurable metasurface can concentrate power on one or more than
one focal point in reflection modes as expected. Additionally, the VO2-based
metasurface can control concentration width in a real-time manner using a novel
proposed method. The simulation and theoretical results are in good agreement
to verify the validity and feasibility of 2-bit metalens design, which has
considerable potential in wireless high-speed communication and
super-resolution imaging.",2011.04790v1
2020-12-11,A reflective mm-wave photonic limiter,"Millimeter wave (mm-wave) communications and radar receivers capable of
processing small signals must be protected from high-power signals, which can
damage sensitive receiver components. Many of these systems arguably can be
protected by using photonic limiting techniques, in addition to electronic
limiting circuits in receiver front-ends. Here we demonstrate, experimentally
and numerically, a free-space, reflective mm-wave limiter based on a multilayer
structure involving a nanolayer of vanadium dioxide (VO2), experiencing a
thermal insulator-to-metal transition. The multilayer acts as a variable
reflector, controlled by the input power. At low input power levels, VO2
remains dielectric, and the multilayer exhibits resonant transmittance. When
the input power exceeds a threshold level, the emerging metallic phase renders
the multilayer highly reflective while dissipating a small portion of the input
power without damage to the limiter. In the case of a Gaussian beam, the
limiter has a nearly constant output above the limiting threshold input.",2012.07631v1
2020-12-23,Direct observation of the electrically triggered Insulator-Metal transition in V3O5 far below the transition temperature,"Resistive switching is one of the key phenomena for applications such as
nonvolatile memories or neuromorphic computing. V3O5, a compound of the
vanadium oxide Magn\'eli series, is one of the rare materials to exhibit an
insulator-metal transition (IMT) above room temperature (Tc ~ 415 K). Here we
demonstrate both static dc resistive switching (RS) and fast oscillatory
spiking regimes in V3O5 devices at room temperature (120 K below the phase
transition temperature) by applying an electric field. We use operando optical
imaging to track a reflectivity change during the RS and find that a
percolating high temperature metallic phase filament is formed. This
demonstrates that the electrically induced RS triggers the phase transition.
Furthermore, we optically capture the spiking oscillations that we link to the
negative differential resistance regime and find the filament forms and
dissolves via a periodic spatio-temporal instability that we describe by
numerical simulations.",2012.13009v1
2021-01-12,Tuning the Magnetic and Electronic Properties of Monolayer VI3 by 3d Transition Metal Doping: A First-Principles Study,"Two-dimensional (2D) materials with robust magnetism have drawn immense
attention for their promising applications in spintronics. Recently, intrinsic
ferromagnetic vanadium triiodide (VI3) has been synthesized experimentally. To
enhance its spintronic property, we modified VI3 by interstitial doping with 3d
transition metals (TM) and used first-principles calculations to investigate
the geometric structure, formation energy, electronic property, and magnetism
of pristine VI3 and 3d TM-doped VI3 monolayer. Among eight transition metal
(Sc-, Ti-, V-, Cr-, Mn-, Fe-, Co-, and Ni-) doped VI3 materials, four of them
(Ti-, V-, Mn-, and Ni-doped VI3) show robust magnetism with full spin
polarization near the Fermi energy. Our research demonstrates that Ti-doped VI3
results in half-metallic semiconductor properties (HMS), while V-doped VI3 and
Ni-doped VI3 result in half-semiconductor properties (HSC). Surprisingly,
Mn-doped VI3 exhibits an unusual bipolar magnetic semiconductor property (BMS).
This unique combination of strong ferromagnetism and 100% spin polarization
with a half-metallic, half-semiconductor, or bipolar semiconductor property
renders 3d TM-doped VI3 as potential candidates for next generation
semiconductor spintronic applications. These spin-polarized materials will be
extremely useful for spin-current generation and other spintronic applications.",2101.05071v2
2021-01-17,Quantifying residual stress in Helium implanted surfaces and its implication for blistering,"Helium implantation in surfaces is of interest for plasma facing materials
and other nuclear applications. Vanadium as both a representative bcc material
and a material relevant for fusion applications is implanted using a Helium ion
beam microscope, and the resulting swelling and nanomechanical properties are
quantified. These values are put in correlation to data obtained from micro
residual stress measurements using a focused ion beam based ring-core
technique. We found that the swelling measured is similar to literature values.
Further we are able to measure the surface stress caused by the implantation
and find it approaches the yield strength of the material at blistering doses.
The simple calculations performed in the present work, along with several
geometrical considerations deduced from experimental results confirm the
driving force for blister formation comes from bulging resulting mainly from
gas pressure buildup, rather than solely stress induced buckling.",2101.06780v1
2021-01-18,Casimir-Polder Interaction of an Atom with a Cavity Wall Made of Phase-Change Material out of Thermal Equilibrium,"We consider the out-of-thermal-equilibrium Casimir-Polder interaction between
atoms of He$^*$, Na, Cs, and Rb and a cavity wall made of sapphire coated with
a vanadium dioxide film which undergoes the dielectric-to-metal phase
transition with increasing wall temperature. Numerical computations of the
Casimir-Polder force and its gradient as the functions of atom-wall separation
and wall temperature are made when the latter exceeds the temperature of the
environment. The obtained results are compared with those in experiment on
measuring the gradient of the Casimir-Polder force between $^{87}$Rb atoms and
a silica glass wall out of thermal equilibrium. It is shown that the use of
phase-change wall material increases significantly the force magnitude and
especially the force gradient, as opposed to the case of dielectric wall.",2101.06995v1
2021-01-28,Optically tunable Mie-resonance VO2 nanoantennas for metasurfaces in the visible,"Metasurfaces are ultrathin nanostructured surfaces that can allow arbitrary
manipulation of light. Implementing dynamic tunability into their design could
allow the optical functions of metasurfaces to be rapidly modified at will. The
most pronounced and robust tunability of optical properties is provided by
phase-change materials such as vanadium dioxide (VO2) and germanium antimony
telluride (GST), but their implementations have been limited only to
near-infrared wavelengths. Here, we demonstrate that VO2 nanoantennas with
widely tunable Mie resonances can be utilized for designing tunable
metasurfaces in the visible range. In contrast to the dielectric-metallic phase
transition-induced tunability in previous demonstrations, we show that
dielectric Mie resonances in VO2 nanoantennas offer remarkable scattering and
extinction modulation depths (5-8 dB and 1-3 dB, respectively) for tunability
in the visible. Moreover, these strong resonances are optically switchable
using a continuous-wave laser. Our results establish VO2 nanostructures as
low-loss building blocks of optically tunable metasurfaces.",2101.12283v1
2021-02-11,Universal phase dynamics in VO2 switches revealed by ultrafast operando diffraction,"Strongly correlated materials that exhibit an insulator-metal transition are
key candidates in the search for new computing platforms. Understanding the
pathways and timescales underlying the electrically-driven insulator-metal
transition is crucial for uncovering the fundamental limits of device
operation. Using stroboscopic electron diffraction, we perform synchronized
time-resolved measurements of atomic motions and electronic transport in
operating vanadium dioxide switches. We discover an electrically-triggered,
isostructural state that forms transiently on microsecond timescales,
stabilized by local heterogeneities and interfacial interactions between the
equilibrium phases. This metastable phase bears striking similarity to that
formed under photoexcitation within picoseconds, suggesting a universal
transformation pathway across eight orders of magnitude of timescale. Our
results establish a new route for uncovering non-equilibrium and metastable
phases in correlated materials, and open avenues for engineering novel
dynamical behavior in nanoelectronics.",2102.06013v1
2021-02-16,Materials genes of heterogeneous catalysis from clean experiments and artificial intelligence,"Heterogeneous catalysis is an example of a complex materials function,
governed by an intricate interplay of several processes, e.g., the different
surface chemical reactions, and the dynamic re-structuring of the catalyst
material at reaction conditions. Modelling the full catalytic progression via
first-principles statistical mechanics is impractical, if not impossible.
Instead, we show here how a tailored artificial-intelligence approach can be
applied, even to a small number of materials, to model catalysis and determine
the key descriptive parameters (""materials genes"") reflecting the processes
that trigger, facilitate, or hinder catalyst performance. We start from a
consistent experimental set of ""clean data"", containing nine vanadium-based
oxidation catalysts. These materials were synthesized, fully characterized, and
tested according to standardized protocols. By applying the symbolic-regression
SISSO approach, we identify correlations between the few most relevant
materials properties and their reactivity. This approach highlights the
underlying physicochemical processes, and accelerates catalyst design.",2102.08269v1
2021-02-19,Slow dynamics of disordered zigzag chain molecules in layered LiVS2 under electron irradiation,"Electronic instabilities in transition metal compounds often spontaneously
form orbital molecules, which consist of orbital-coupled metal ions at low
temperature. Recent local structural studies utilizing the pair distribution
function revealed that preformed orbital molecules appear disordered even in
the high-temperature paramagnetic phase. However, it is unclear whether
preformed orbital molecules are dynamic or static. Here, we provide clear
experimental evidence of the slow dynamics of disordered orbital molecules
realized in the high-temperature paramagnetic phase of LiVS2, which exhibits
vanadium trimerization upon cooling below 314 K. Unexpectedly, the preformed
orbital molecules appear as a disordered zigzag chain that fluctuate in both
time and space under electron irradiation. Our findings should advance studies
on soft matter physics realized in an inorganic material due to disordered
orbital molecules.",2102.09795v1
2021-03-06,Thin-film radiative thermal diode with large rectification,"We propose a mechanism to substantially rectify radiative heat flow by
matching thin films of metal-to-insulator transition materials and polar
dielectrics in the electromagnetic near field. By leveraging the distinct
scaling behaviors of the local density of states with film thickness for metals
and insulators, we theoretically achieve rectification ratios over 140-a
10-fold improvement over the state of the art-with nanofilms of vanadium
dioxide and cubic boron nitride in the parallel-plane geometry at
experimentally feasible gap sizes (~100 nm). Our rational design offers
relative ease of fabrication, flexible choice of materials, and robustness
against deviations from optimal film thicknesses. We expect this work to
facilitate the application of thermal diodes in solid-state thermal circuits
and energy conversion devices.",2103.04035v1
2021-03-29,"Stability, electronic structure, and magnetic moment of Vanadium phthalocyanine grafted to the Au(111) surface","The studies of electronic and magnetic properties of V-Pc molecule adsorbed
onto Au(111) surface are based on ab-initio calculations in the framework of
density functional theory. We compute adsorption energies, investigate
interaction mechanisms between constituents of the hybrid system consisting of
V-Pc molecule and Au surface, and determine geometry changes in the system,
particularly in the grafted molecule. We find out that the energetically most
stable configuration of the V-Pc/Au(111) occurs when V-Pc is grafted to the Au
surface's fcc site, which leads to the reduction of the point group symmetry of
the hybrid system in comparison to the free standing V-Pc molecule. Further,
our studies reveal that the electronic structure and magnetic properties of the
V-Pc change significantly after adsorption to the Au(111). Generally, these
studies shed light on physical mechanisms of the V-Pc adsorption to metallic
surfaces and open up new prospects for design of novel spintronic devices.",2103.15639v1
2021-04-06,Electronic structure and charge-density wave transition in monolayer VS_{2},"Vanadium disulfide (VS_{2}) attracts elevated interests for its
charge-density wave (CDW) phase transition, ferromagnetism, and catalytic
reactivity, but the electronic structure of monolayer has not been well
understood yet. Here we report synthesis of epitaxial 1T VS_{2} monolayer on
bilayer graphene grown by molecular-beam epitaxy (MBE). Angle-resolved
photoemission spectroscopy (ARPES) measurements reveal that Fermi surface with
six elliptical pockets centered at the M points shows gap opening at low
temperature. Temperature-dependence of the gap size suggests existence of CDW
phase transition above room temperature. Our observations provide important
evidence to understand the strongly correlated electron physics and the related
surface catalytic properties in two-dimensional transition-metal
dichalcogenides (TMDCs).",2104.02217v1
2021-04-08,A short-range structural insight into lithium substituted barium vanadate glasses using Raman and EPR spectroscopy as probes,"We present a corroborative study of structural characterization of lithium
substituted barium vanadate glasses using Raman and Electron Paramagnetic
Resonance (EPR) spectroscopy. Investigation of the thermal and physical
properties of these glasses showed a gradual increase in the concentration of
non-bridging oxygen. Raman and EPR analysis gave an insight into the changing
structure of the glasses. Both the spectroscopic techniques confirmed that
vanadium is present in the glasses as distorted VO6 octahedra. From the
analysis of both spectroscopic techniques, it is proposed that the lithium ion
prefers to occupy planar positions of the VO6 octahedra thus reducing the
tetragonal distortion and making the environment around the network forming
unit in the glass matrix more homogenous as we increase lithium content. The
concentration of V4+ showed a non-monotonic variation with an increase in Li2O
as indicated by Raman studies and confirmed by EPR which indicates a structural
change in the distorted VO6 octahedra.",2104.03592v2
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
2021-04-29,Reconfigurable Multistate Optical Systems Enabled by VO2 Phase Transitions,"Reconfigurable optical systems are the object of continuing, intensive
research activities, as they hold great promise for realizing a new generation
of compact, miniaturized, and flexible optical devices. However, current
reconfigurable systems often tune only a single state variable triggered by an
external stimulus, thus, leaving out many potential applications. Here we
demonstrate a reconfigurable multistate optical system enabled by phase
transitions in vanadium dioxide (VO2). By controlling the phase-transition
characteristics of VO2 with simultaneous stimuli, the responses of the optical
system can be reconfigured among multiple states. In particular, we show a
quadruple-state dynamic plasmonic display that responds to both temperature
tuning and hydrogen-doping. Furthermore, we introduce an electron-doping scheme
to locally control the phase-transition behavior of VO2, enabling an optical
encryption device encoded by multiple keys. Our work points the way toward
advanced multistate reconfigurable optical systems, which substantially
outperform current optical devices in both breadth of capabilities and
functionalities.",2104.14232v1
2021-05-06,Optical forces on an oscillating dipole near VO$_2$ phase transition,"We investigate optical forces on oscillating dipoles close to a phase-change
vanadium dioxide (VO$_2$) film, which exhibits a metal-insulator transition
around $340$ K and low thermal hysteresis. This configuration is related to one
composed of an excited two-level quantum emitter and we employ a classical
description to capture important aspects of the radiation-matter interaction.
We consider both electric and magnetic dipoles for two different
configurations, namely, with the dipole moments parallel and perpendicular to
the VO$_2$ film. By using Bruggeman theory to describe the effective optical
response of the material, we show that, in the near-field regime, the force on
the dipoles can change from attractive to repulsive just by heating the film
for a selected frequency range. We demonstrate that the thermal hysteresis
present in the VO$_2$ transition clearly shows up in the behavior of the
optical forces, setting the grounds for alternative approaches to control
light-matter interactions using phase-change materials.",2105.02493v1
2021-05-13,Enhanced passive thermal stealth properties of VO$_2$ thin films via gradient W doping,"Thermal stealth and camouflage have been intensively studied for blending
objects with their surroundings against remote thermal image detection.
Adaptive control of infrared emissivity has been explored extensively as a
promising way of thermal stealth, but it still requires an additional feedback
control. Passive modulation of emissivity, however, has been remained as a
great challenge which requires a precise engineering of emissivity over wide
temperature range. Here, we report a drastic improvement of passive camouflage
thin films capable of concealing thermal objects at near room temperature
without any feedback control, which consists of a vanadium dioxide (VO2) layer
with gradient tungsten (W) concentration. The gradient W-doping widens the
metal-insulator transition width, accomplishing self-adaptive thermal stealth
with a smooth change of emissivity. Our simple approach, applicable to other
similar thermal camouflage materials for improving their passive cloaking, will
find wide applications, such as passive thermal camouflage, urban energy-saving
smart windows, and improved infrared sensors.",2105.06063v1
2021-05-20,Coexistence of ferromagnetism and spin-orbit coupling by incorporation of platinum in two-dimensional VSe$_2$,"We report on a novel material, namely two-dimensional (2D)
V$_{1-x}$Pt$_x$Se$_2$ alloy, exhibiting simultaneously ferromagnetic order and
Rashba spin-orbit coupling. While ferromagnetism is absent in 1T-VSe$_2$ due to
the competition with the charge density wave phase, we demonstrate
theoretically and experimentally that the substitution of vanadium by platinum
in VSe$_2$ (10-50 %) to form an homogeneous 2D alloy restores ferromagnetic
order with Curie temperatures of 6 K for 5 monolayers and 25 K for one
monolayer of V$_{0.65}$Pt$_{0.35}$Se$_2$. Moreover, the presence of platinum
atoms gives rise to Rashba spin-orbit coupling in (V,Pt)Se$_2$ providing an
original platform to study the interplay between ferromagnetism and spin-orbit
coupling in the 2D limit.",2105.10022v1
2021-06-21,Physics-constrained deep neural network method for estimating parameters in a redox flow battery,"In this paper, we present a physics-constrained deep neural network (PCDNN)
method for parameter estimation in the zero-dimensional (0D) model of the
vanadium redox flow battery (VRFB). In this approach, we use deep neural
networks (DNNs) to approximate the model parameters as functions of the
operating conditions. This method allows the integration of the VRFB
computational models as the physical constraints in the parameter learning
process, leading to enhanced accuracy of parameter estimation and cell voltage
prediction. Using an experimental dataset, we demonstrate that the PCDNN method
can estimate model parameters for a range of operating conditions and improve
the 0D model prediction of voltage compared to the 0D model prediction with
constant operation-condition-independent parameters estimated with traditional
inverse methods. We also demonstrate that the PCDNN approach has an improved
generalization ability for estimating parameter values for operating conditions
not used in the DNN training.",2106.11451v2
2021-06-24,Dynamic quarter-wave metasurface for efficient helicity inversion of polarization beyond the single-layer conversion limit,"Terahertz chiral sensing and polarization-multiplexing communication demand
subwavelength devices that dynamically invert polarization helicity.
Metasurfaces can enhance anisotropy and fine tunability at subwavelength scales
for this purpose. Although metasurfaces enabling deep modulation between
orthogonal polarizations have been designed, they suffer from low conversion
efficiencies. In this study, it is shown that the efficiency of conversion from
linear to circular polarization by conventional single-layer transmissive
metasurfaces cannot exceed a fundamental limit. A dynamic reflective
metasurface free from this limitation is then proposed. The device includes
multilayer structures working as a terahertz quarter-wave plate with switchable
slow and fast axes. A phase transition of vanadium dioxide induces the
necessary structural transformation of the metallic patterns. A practical
fabrication method, based on the room-temperature bonding technique of
sapphires, is presented. Dynamic helicity inversion is demonstrated at 0.90
THz, with a conversion efficiency of over 80 % that is beyond the fundamental
limit of single-layer transmissive metasurfaces (65 %) and more than four times
greater than that of previously reported devices.",2106.12932v2
2021-07-16,First principles studies of the electronic and structural properties of the rutile VO$_{2}$(110) surface and its oxygen-rich terminations,"We present a Density Functional Theory (DFT) study of the structural and
electronic properties of bare rutile VO$_{2}$(110) surfaces and its oxygen-rich
terminations. We discuss the performance of various DFT functionals, including
PBE, PBE+U (U = 2 eV), SCAN and SCAN+rVV functionals with non-magnetic and
ferromagnetic spin ordering. We predict the presence of a ring-like termination
that is electronically and structurally related to a V$_{2}$O$_{5}$(001)
monolayer and shows a higher stability than pure oxygen adsorption phases.
Despite the fact that the calculated phase stabilities depend on the chosen
functional, our results show that employing the spin-polarized SCAN functional
offers a good compromise yielding both a reasonable description of the
structural and electronic properties of the rutile VO$_{2}$ bulk phase and the
enthalpy of formation for different stages of vanadium oxidation.",2107.07899v1
2021-07-29,New results on proton-induced reactions on Vanadium for $^{47}$Sc production and the impact of level densities on theoretical cross sections,"New data for the $^\mbox{nat}$V(p,x) reactions have been measured in the
range 26-70 MeV, with production of the nuclides $^{47}$Sc, $^{43}$Sc,
$^{44m}$Sc, $^{44g}$Sc, $^{46}$Sc, $^{48}$Sc, $^{42}$K, $^{43}$K, $^{48}$V,
$^{48}$Cr, $^{49}$Cr, and $^{51}$Cr. The focus is on the production of
$^{47}$Sc, a $\beta^-$-emitter suitable for innovative radiotheranostic
applications in nuclear medicine. The measured cross sections for this
radionuclide and its contaminants are compared with the theoretical excitation
functions calculated with the TALYS code. In view of novel radiopharmaceutical
applications, it is essential to accurately describe these cross-sections for
the evaluation of yields, purities, and dose releases. Hence, we optimize the
level-density parameters of the microscopic models in the TALYS code to obtain
the best possible descriptions of the new data. We consider different
irradiation conditions to estimate the production yields from the cross
sections determined in this work.",2107.13773v1
2021-08-13,An Optimized Self-Adaptive Thermal Radiation Turn-Down Coating with Vanadium Dioxide Nanowire Array,"High performance metasurfaces for thermal radiative cooling applications can
be identified using computational optimization methods. This work has
identified an easy-to-fabricate temperature phase transition VO2 nanowire array
laid atop dielectric BaF2 Fabry-Perot cavity-on-metal with total coating
thickness of 2 um. This optimized structure has ability to self-adaptively
switch between high reflectance at low temperature to high emissivity at high
temperature in the broad thermal infrared spectrum. This design demonstrates
exceptional turn-down figure-of-merit compared to previously realized
configurations utilizing VO2 metasurfaces and multilayers. The mechanism is
achieved with a sub-wavelength nanowire array effective medium that switches
between anti-reflecting gradient coating and Fabry-Perot interference. This
thin metasurface coating could impact self-cooling of the solar cells,
batteries, and electrical devices where risk presents at high temperatures.",2108.06013v1
2021-08-24,Higher-order topological superconductivity from repulsive interactions in kagome and honeycomb systems,"We discuss a pairing mechanism in interacting two-dimensional multipartite
lattices that intrinsically leads to a second order topological superconducting
state with a spatially modulated gap. When the chemical potential is close to
Dirac points, oppositely moving electrons on the Fermi surface undergo an
interference phenomenon in which the Berry phase converts a repulsive
electron-electron interaction into an effective attraction. The topology of the
superconducting phase manifests as gapped edge modes in the quasiparticle
spectrum and Majorana Kramers pairs at the corners. We present symmetry
arguments which constrain the possible form of the electron-electron
interactions in these systems and classify the possible superconducting phases
which result. Exact diagonalization of the Bogoliubov-de Gennes Hamiltonian
confirms the existence of gapped edge states and Majorana corner states, which
strongly depend on the spatial structure of the gap. Possible applications to
vanadium-based superconducting kagome metals AV$_3$Sb$_3$ (A=K,Rb,Cs) are
discussed.",2108.10897v2
2021-09-14,Phase Stability and Sodium-Vacancy Orderings in a NaSICON Electrode,"We elucidate the thermodynamics of sodium (Na) intercalation into the sodium
super-ionic conductor (NaSICON)-type electrode, Na$_x$V$_2$(PO$_4$)$_3$, for
promising Na-ion batteries with high-power density. This is the first report of
a computational temperature-composition phase diagram of the NaSICON-type
electrode Na$_x$V$_2$(PO$_4$)$_3$. We identify two thermodynamically stable
phases at the compositions Na$_2$V$_2$(PO$_4$)$_3$ and
Na$_{3.5}$V$_2$(PO$_4$)$_3$, and their structural features are described for
the first time based on our computational analysis. We unveil the
crystal-structure and the electronic-structure origins of the ground-state
compositions associated with specific Na/vacancy arrangements, which are driven
by charge orderings on the vanadium sites. These results are significant for
the optimization of high-energy and power densities electrodes for sustainable
Na-ion batteries",2109.06997v1
2021-10-14,TRACK: A python code for calculating the transport properties of correlated electron systems using Kubo formalism,"Exploring the transport properties of different materials brings new avenue
for basic understanding of emergent phenomena and practical applications in
many different fields. Here, we report a program named as TRACK (TRAnsport
properties for Correlated materials using Kubo formalism) which is written in
Python 3 for calculating temperature dependent electrical conductivity,
electronic part of thermal conductivity, Seebeck coefficient and Lorenz number.
In this code, Kubo linear-response formalism is utilized for computing these
parameters using both interacting and non-interacting electronic structure
methods. The formula for transport coefficients is accordingly modified to
obtain the transport parameters under relaxation time approximation using
band-theory. The basic inputs of this program are the structural information,
dense k-points sampling in the irreducible part of the Brillouin zone and the
information of velocity matrix elements, which can be calculated using
third-party ab-initio package. TRACK is expected to calculate the transport
properties of different class of materials. The code has been benchmarked by
performing calculation on three different types of materials namely Vanadium
(V), FeSi and LaCoO$_3$, which are metal, semiconductor and Mott insulator,
respectively. The temperature dependent behaviour of the transport coefficients
for these materials show fairly good agreement with the corresponding
experimental data.",2110.07243v1
2021-12-05,Uniting the order and disorder dynamics in photoexcited VO2,"Photoinduced phase transition (PIPT) is always treated as a coherent process,
but ultrafast disordering in PIPT is observed in recent experiments. Utilizing
the real-time time-dependent density functional theory (rt-TDDFT) method, here,
we track the motion of individual vanadium (V) ions during PIPT in VO2 and
uncover that their coherent or disordered dynamics can be manipulated by tuning
the laser fluence. We find that the photoexcited holes generate a force on each
V-V dimer to drive their collective coherent motion, in competing with the
thermal-induced vibrations. If the laser fluence is so weak that the
photoexcited hole density is too low to drive the phase transition alone, the
PIPT is a disordered process due to the interference of thermal phonons. We
also reveal that the photoexcited holes populated by the V-V dimerized bonding
states will become saturated if the laser fluence is too strong, limiting the
timescale of photoinduced phase transition.",2112.02463v1
2021-12-08,Precise design of VO2 thin films for smart windows by employing thickness dependent refractive index,"Vanadium dioxide (VO2) is an adjustable refractive index material and has
capability of behaving as semiconductor or conductor depending on its
temperature. Such condition makes it as a material which can be employed in
fabricating thermochromic smart windows. The transmission characteristics of
these type of windows strongly depend on the thickness of the film. Therefore,
some calculations are required to optimize the VO2 thickness. Unfortunately,
refractive index of VO2 thin film is thickness dependent, therefore, in
calculating the transmission of light spectrum from VO2 thin films, a unique
refractive index cannot be utilized. In the present paper using three
theoretical models (Lorentz-Drude oscillator, Lorentz oscillator, and
Tauc-Lorentz) and employing experimental results from previous reports, we
provide a collection of thickness dependent refractive index of VO2 films. More
precise transmission can be achieved by using this set of refractive index data
in the calculations which agree with those of experiments. These results also
fairly make us capable to determine optimized film thickness for any desired
transmission performance. This method is useful for design of VO2 based
thermochromic smart windows.",2112.04582v1
2021-12-10,Coupling of electronic and structural degrees of freedom in vanadate superlattices,"Heterostructuring provides different ways to manipulate the orbital degrees
of freedom and to tailor orbital occupations in transition metal oxides.
However, the reliable prediction of these modifications remains a challenge.
Here, we present a detailed investigation of the relationship between the
crystal and electronic structure in YVO$_3$-LaAlO$_3$ superlattices by
combining ab initio theory, scanning transmission electron microscopy, and
x-ray diffraction. Density functional theory simulations including an on-site
Coulomb repulsion term, accurately predict the crystal structure and in
conjunction with x-ray diffraction, provide an explanation for the lifting of
degeneracy of the vanadium $d_{xz}$ and $d_{yz}$ orbitals, that was recently
observed in this system. In addition, we unravel the combined effects of
electronic confinement and octahedral connectivity by disentangling their
impact from that of epitaxial strain. Our results demonstrate that the specific
orientation of the substrate and the thickness of the YVO$_3$ slabs in the
multilayer, can be utilized to reliably engineer orbital polarization.",2112.05444v1
2021-12-30,"Realizing Kagome Band Structure in Two-Dimensional Kagome Surface States of $RV_{6}Sn_{6}$ ($R$=Gd,Ho)","We report angle resolved photoemission experiments on a newly discovered
family of kagome metals $RV_{6}Sn_{6}$ ($R$=Gd, Ho). Intrinsic bulk states and
surface states of the vanadium kagome layer are differentiated from those of
other atomic sublattices by the real-space resolution of the measurements with
a small beam spot. Characteristic Dirac cone, saddle point and flat bands of
the kagome lattice are observed. Our results establish the two-dimensional (2D)
kagome surface states as a new platform to investigate the intrinsic kagome
physics.",2112.15007v1
2022-01-20,Crystal structure evolution in the van der Waals transition metal trihalides,"Most transition-metal trihalides are dimorphic. The representative
chromium-based triad, CrCl3, CrBr3, CrI3, is characterized by the
low-temperature phase adopting the trigonal BiI3 type while the structure of
the high-temperature phase is monoclinic of AlCl3 type (C2/m). The structural
transition between the two crystallographic phases is of the first-order type
with large thermal hysteresis in CrCl3 and CrI3. We studied crystal structures
and structural phase transitions of vanadium-based counterparts VCl3, VBr3, and
VI3 by measuring specific heat, magnetization, and X-ray diffraction as
functions of temperature and observed an inverse situation. In these cases, the
high-temperature phase has a higher symmetry while the low-temperature
structure reveals a lower symmetry. The structural phase transition between
them shows no measurable hysteresis in contrast to CrX3. Possible relations of
the evolution of the ratio c/a of the unit cell parameters, types of crystal
structures, and nature of the structural transitions in V and Cr trihalides are
discussed.",2201.08088v1
2022-02-28,Entanglement between a muon spin and $I>\frac{1}{2}$ nuclear spins,"We report on the first example of quantum coherence between the spins of
muons and quadrupolar nuclei. We observe this effect in vanadium intermetallic
compounds which adopt the A15 crystal structure, and whose members include all
technologically dominant superconductors. The entangled states are extremely
sensitive to the local structural and electronic environments through the
electric field gradient at the quadrupolar nuclei. This case-study demonstrates
that positive muons can be used as a quantum sensing tool to probe also
structural and charge related phenomena in materials, even in the absence of
magnetic order.",2202.13742v1
2022-03-03,Symmetry-protected Dirac nodal lines and large spin Hall effect in $\mathbf{V_6Sb_4}$ with kagome bilayer,"Recently, a family of nonmagnetic kagome metals \textit{A}$\mathrm{V_3Sb_5}$
(\textit{A}=K, Rb, and Cs) has attracted significant attention for realizing
the intertwining of quantum order and nontrivial topology. However, these
compounds have been identified to host complex band structures. Therefore, it
is desirable to design and synthesize novel kagome materials with a simple band
topology and good transport properties. In this study, using first-principles
calculations, we present the electronic properties and the intrinsic spin Hall
effect of V$_6$Sb$_4$, the latest experimentally synthesized vanadium-based
compounds with a kagome bilayer. In the absence of spin-orbital coupling (SOC),
this compound is a Dirac nodal line semimetal with symmetry-protected nodal
rings near the Fermi level. Within the SOC, the spin-rotation symmetry breaks
the gaps of the nodal rings with a small band gap. Furthermore, based on the
Wannier tight-binding approach and the Kubo formula, we propose a large spin
Hall effect in V$_6$Sb$_4$, which intrinsically originates from the spin Berry
curvature. Our work further expands nonmagnetic kagome compounds for
applications in spintronics accompanied by exotic quantum order.",2203.01558v1
2022-03-23,"Thermally switchable polarization manipulation and diodelike transmission in scalable, resonator-free, mid-infrared metasurfaces with vanadium-dioxide grids","We conceptualized three-array scalable metasurfaces comprising only three
thin strip grids and numerically demonstrated their characteristics in the
mid-infrared spectral regime for switchable polarization manipulation and
related asymmetric diodelike transmission (AT). A few or all of the grids were
taken to be made of VO$_2$, a phase-change material. For each proposed
metasurface, two effective structures and, therefore, two different
functionalities exist, corresponding to the metallic and insulating phases of
VO$_2$. The achieved scenarios of functionality switching that depend on the
VO$_2$ phase are shown to significantly depend in the way in which VO$_2$ is
incorporated to the metasurface. Switchable bands of polarization manipulation
are up to 40 THz wide. The AT band can be modulated when Fabry--Perot
(anti-)resonances come into play.",2203.12754v1
2022-04-10,V$_2$C-based lithium batteries: The influence of magnetic phase and Hubbard interaction,"MXenes are a family of two-dimensional materials that could be attractive for
use as electrodes in lithium batteries due to their high specific capacity. For
this purpose, it is necessary to evaluate magnitudes such as the lithium
adsorption energy and the magnitude of the open-circuit voltage for different
lithium concentrations. In this paper, we show through first principles
calculations that in a V$_2$C monolayer we must consider the high correlation
between the electrons belonging to vanadium to obtain correct results of these
quantities. We include this correlation employing the Hubbard coupling
parameter obtained by a linear response method. We found that the system is
antiferromagnetic and that the quantities studied depend on the magnetic phase
considered. Indirectly, experimental results could validate the theoretical
value of the theoretical Hubbard parameter.",2204.04764v1
2022-04-11,Chemical state analysis of reactively sputtered zinc vanadium nitride: The Auger parameter as a tool in materials design,"Photoelectron spectroscopy is an important tool for the development of new
materials. However, especially for nitride semiconductors, the formation of
surface oxides, surface band bending as well as the lack of a suitable charge
reference often prevent a robust analysis. Here, we perform a comprehensive
chemical state analysis of the Zn-V-N phase space using the Auger parameter
concept, which is less sensitive to such uncertainties. Phase-pure Zn2VN3, VN,
and Zn3N2 samples are analyzed using XPS/HAXPES after transfer in inert-gas
atmosphere. In addition, high-throughput chemical state analysis is performed
on combinatorial Zn1-xVxN thin film libraries. The evolution of the Zn Auger
parameter in Zn1-xVxN is consistent with previous mapping of the structural and
functional properties. Strikingly, the study reveals a narrower stability range
of wurtzite Zn1-xVxN than our previous high-throughput XRD screening,
highlighting the sensitivity of the measurement approach. The procedures
applied here are transferable to many other material systems and could be
particularly useful for the high-throughput development of materials with low
crystallinity where insights from XRD screenings are limited.",2204.05401v1
2022-04-12,On the Existence of Photoluminescence and Room-Temperature Spin Polarization in Ambipolar V doped MoS$_2$ Monolayers,"Opto-spintronics is an emerging field where ultra-thin
magnetic-semiconductors having high spin-valley coupling play an important
role. Here, we demonstrate substitutional vanadium (V) doping in MoS$_2$
lattice in different extent, leading to the coexistence of photoluminescence
(PL), valleypolarization (~32%), and valley splitting (~28 meV shift in PL with
helicity $\sigma^+$ and $\sigma^-$ of light excitation). A large V doping
causes semiconductor to metal transition in MoS$_2$ but with medium level
causing the existence of photoluminescence with high spin polarization. The
ambipolar nature of medium level V doped MoS$_2$ is shown here indicating its
potential as an opto-electronic material. The presence of V-dopants and their
different level of content are proven by both spectroscopic and microscopic
methods.A detailed temperature and power dependent photoluminescence studies
along with density functional theory-based calculations in support unravels the
emergence of the co-existence of spin-valley coupling and photoluminescence.
This study shows the potential of doping MoS$_2$ for deriving new materials for
next generation room temperature opto-spintronics.",2204.05887v1
2022-04-20,Nuclear Spin Quantum Memory in Silicon Carbide,"Transition metal (TM) defects in silicon carbide (SiC) are a promising
platform for applications in quantum technology. Some TM defects, e.g.
vanadium, emit in one of the telecom bands, but the large ground state
hyperfine manifold poses a problem for applications which require pure quantum
states. We develop a driven, dissipative protocol to polarize the nuclear spin,
based on a rigorous theoretical model of the defect. We further show that
nuclear-spin polarization enables the use of well-known methods for
initialization and long-time coherent storage of quantum states. The proposed
nuclear-spin preparation protocol thus marks the first step towards an
all-optically controlled integrated platform for quantum technology with TM
defects in SiC.",2204.09295v2
2022-04-27,Finite-temperature surface elasticity of crystalline solids,"Surface energies and surface elasticity largely affect the mechanical
response of nanostructures as well as the physical phenomenon associated with
surfaces such as evaporation and adsorption. Studying surface energies at
finite temperatures is therefore of immense interest for nanoscale
applications. However, calculating surface energies and derived quantities from
atomistic ensembles is usually limited to zero temperature or involve
cumbersome thermodynamic integration techniques at finite temperature. Here, we
illustrate a technique to identify the energy and elastic properties of
surfaces of solids at non-zero temperature based on a Gaussian phase packets
(GPP) approach (which in the isothermal limit coincides with a maximum-entropy
formulation). Using this setup, we investigate the effect of temperature on the
surface properties of different crystal faces for six pure metals -- copper,
nickel, alumimum, iron, tungsten and vanadium -- thus covering both FCC and BCC
lattice structures. While the obtained surface energies and stresses usually
show a decreasing trend with increasing temperature, the elastic constants do
not show such a consistent trend across the different materials and are quite
sensitive to temperature changes. Validation is performed by comparing the
obtained surface energy densities of selected BCC and FCC materials to those
calculated via molecular dynamics.",2204.13028v1
2022-05-18,Higgs-Leggett mechanism for the elusive 6e superconductivity observed in Kagome vanadium-based superconductors,"A recent Little-Parks experiment on Kagome-structured superconductor
CsV_3Sb_5 demonstrated remarkable resistance oscillations with period
\phi0/3=hc/6e. Here, we perform analysis based on a theory involving three 2e
superconductivity (SC) order parameters associated with the three reciprocal
lattice vectors which connect M points of the hexagonal Brillouin zone. In a
ring geometry we unveil that, as a series of metastable states, phase of one SC
order parameter winds 2\pi more or less than the other two ones around the
ring, which yields local free energy minima at integer multiples of \phi0/3.
Intriguingly, the ground-state degeneracy associated with a Z_2 chirality is
crucial, and the Higgs-Leggett mechanism stabilizes domain walls (DW) between
chiral domains. At low temperatures DW are expelled from the system resulting
in free energy minima only at integer multiples of \phi_0. Our theory explains
successfully the 6e SC observed in experiments, which opens a door for
approaching rich physics of Kagome superconductors.",2205.08732v2
2022-06-17,Theory of ultrafast magnetization of non-magnetic semiconductors with localized conduction bands,"The magnetization of a non-magnetic semiconductor by femtosecond light pulses
is crucial to achieve an all-optical control of the spin dynamics in materials
and to develop faster memory devices. However, the conditions for its detection
are largely unknown. In this work we identify the criteria for the observation
of ultrafast magnetization and critically discuss the difficulties hindering
its experimental detection. We show that ultrafast magnetization of a non
magnetic semiconductor can be observed in compounds with very localized
conduction band states and more delocalized valence bands, such as in the case
of a p-d charge transfer gap. By using constrained and time dependent density
functional theory simulations, we demonstrate that a transient ferrimagnetic
state can be induced in diamagnetic semiconductor V2O5 via ultrafast pulses at
realistic fluences. The ferrimagnetic state has opposite magnetic moments on
vanadium (conduction) and oxygen (valence) states. Our methodology outruns the
case of V2O5 as it identifies the key requirements for a computational
screening of ultrafast magnetism in non-magnetic semiconductors.",2206.08624v1
2022-09-06,Crossover between electron-electron and electron-phonon mediated pairing on the Kagome lattice,"We study electron-electron and electron-phonon mediated pairing in the
Holstein extended Hubbard model on the Kagome lattice near the van Hove
fillings, and investigate their combined effects on electron pairing states. We
find that their combination can promote exotic pairings in a crossover region,
where the filling is close to a van Hove singularity. In particular, at the
$p$-type van Hove filling the $E_{1u}$ ($p$-wave) and $B_{2u}$
($f_{y^3-3yx^2}$-wave) pairings become leading, and at the $m$-type van Hove
filling the $E_{1u}$ and $A_{2g}$ ($i$-wave) pairings get promoted. Moreover,
we show that the electron-phonon interaction acquires a significant momentum
dependence, due to the sublattice texture of the Fermi surfaces, which can
promote non $s$-wave pairing. We present a detailed analysis of these pairing
propensities and discuss implications for the vanadium-based kagome
superconductors AV$_3$Sb$_5$.",2209.02351v2
2022-09-08,Titanium-based kagome superconductor CsTi_3Bi_5 and topological states,"Since the discovery of a new family of vanadium-based kagome superconductor
AV3Sb5 (A=K, Rb, and Cs) with topological band structures, extensive effort has
been devoted to exploring the origin of superconducting states and the
intertwined orders. Meanwhile, searching for new types of superconductors with
novel physical properties and higher superconducting transition temperatures
has always been a major thread in the history of superconductor research. Here
we report a successful fabrication and the topological states of a
Titanium-based kagome metal CsTi3Bi5 (CT3B5) crystal. The as-grown CT3B5
crystal is of high quality and possesses a perfect two-dimensional kagome net
of Titanium. The superconductivity of the CT3B5 crystal shows that the critical
temperature Tc is of ~4.8 K. First-principle calculations predict that the
CT3B5 has robust topological surface states, implying that CT3B5 is a Z2
topological kagome superconductor. This finding provides a new type of
superconductors and the base for exploring the origin of superconductivity and
topological states in kagome superconductors.",2209.03840v1
2022-09-21,"Correction of the pressure scales reliant to the thermal pressures shifts in iron (3d) vanadium (4d), iridium and platinum (5d) transition metals","In many published articles isobaric behavior in DAC chambers has been
reported On the other hand, a clear isochoric behavior in the chambers were
reported. Thus, Isochoric or isobaric condition remain an open question. In
isochoric condition, upon increasing the temperature, at each pressure, the
examined sample and the PTM both are subject to increases of their volumes
which is suppressed by the chamber finite volume, provoking an increase in the
thermal pressure over the whole system. For this reason, the pressure scale
reported in DAC experiments do not represent the actual pressure experienced by
the sample in the cell. The different response of the PTMs to P,T changes is
the reason for the variety of melting curves reported in the literature. It is
concluded that the pressure scale in DAC experiments need a correction taking
in to account the thermal pressure shift. Trusting the first principals DFT- Z
methodology it is shown that the melting curve derived by this method should
serve as an anchor for the pressure scale correction.",2209.10188v2
2022-09-26,Tunable Directional Filter for Mid-Infrared Optical Transmission Switching,"Controlling the spectral and angular response of infrared (IR) radiation is a
challenging task of paramount importance to various emerging photonic
applications. Here, we overcome these problems by proposing and analyzing a new
design of a tunable narrowband directional optical transmission filter. The
presented thermally controlled multilayer filter leverages the temperature
dependent phase change properties of vanadium dioxide (VO2) to enable efficient
and reversible fast optical switching by using a pump-probe laser excitation
setup. More specifically, transmission is blocked for high intensity probe
lasers due to the VO2 metallic properties induced at elevated temperatures
while at low probe laser intensities high transmission through the filter
occurs only for a narrowband IR range confined to near normal incident angles.
The proposed multilayer composite dielectric filter is expected to have
applications in optical communications, where it can act as dual functional
infrared filter and optical switch.",2209.13035v1
2022-10-06,Kinetically Decoupled Electrical and Structural Phase Transitions in VO2,"Vanadium dioxide (VO2) has drawn significant attention for its near room
temperature insulator to metal transition and associated structural phase
transition. The underlying Physics behind the temperature induced insulator to
metal and concomitant structural phase transition in VO2 is yet to be fully
understood. We have investigated the kinetics of the above phase transition
behaviors of VO2 with the help of resistivity measurements and Raman
spectroscopy. Resistance thermal hysteresis scaling and relaxation measurements
across the temperature induced insulator to metal transition reveal the unusual
behaviour of this first-order phase transition, whereas Raman relaxation
measurements show that the temperature induced structural phase transition in
VO2 follows usual behaviour and is consistent with mean field prediction. At
higher temperature sweeping rates decoupling of insulator to metal transition
and structural phase transition have been confirmed. The observed anomalous
first order phase transition behavior in VO2 is attributed to the
unconventional quasi particle dynamics, i.e. significantly lowered electronic
thermal conductivity across insulator to metal transition, which is confirmed
by ultrafast optical pump-probe time domain thermoreflectance measurements.",2210.02691v1
2022-10-07,Enhanced Magnetism in Heterostructures with Transition-Metal Dichalcogenide Monolayers,"Two-dimensional materials and their heterostructures have opened up new
possibilities for magnetism at the nanoscale. In this study, we utilize
first-principles simulations to investigate the structural, electronic, and
magnetic properties of $\textrm{Fe}/\textrm{WSe}_2/\textrm{Pt}$ systems
containing pristine, defective, or doped $\textrm{WSe}_2$ monolayers. The
proximity effects of the ferromagnetic Fe layer are studied by considering
defective and vanadium-doped $\textrm{WSe}_2$ monolayers. All heterostructures
are found to be ferromagnetic, and the insertion of the transition-metal
dichalcogenide results in a redistribution of spin orientation and an increased
density of magnetic atoms due to the magnetized $\textrm{WSe}_2$. There is an
increase in the overall total density of states at the Fermi level due to
$\textrm{WSe}_2$; however, the transition-metal dichalcogenide may lose its
distinct semiconducting properties due to the stronger than van der Waals
coupling. Spin-resolved electronic structure properties are linked to larger
spin Seebeck coefficients found in heterostructures with $\textrm{WSe}_2$
monolayers.",2210.03817v1
2022-10-21,Triangular Kondo lattice in $\mathrm{YbV_6Sn_6}$ and its quantum critical behaviors in magnetic field,"We report magnetization, heat capacity and electrical resistivity for a newly
discovered heavy fermion (HF) compound $\mathrm{YbV_6Sn_6}$ which is
crystallized in a hexagonal $\mathrm{HfFe_6Ge_6}$-type structure, highlighted
by the stacking of triangular ytterbium sublattice and kagome vanadium
sublattice. Above 2 K, $\mathrm{YbV_6Sn_6}$ shows typical HF properties due to
the Kondo effect on the Kramers doublet of $\mathrm{Yb^{3+}}$ ions in crystal
electric field. A remarkable magnetic ordering occurs at $T_{\mathrm{N}}$ =
0.40 K in zero field while a weak external field suppresses the ordering and
induces non-Fermi liquid (NFL) behavior. In strong magnetic field the compound
shows a heavy Fermi liquid state. $\mathrm{YbV_6Sn_6}$ presents as one of the
few examples of Yb-based HF compounds hosting triangular Kondo lattice on which
a magnetic field induces quantum criticality near zero temperature.",2210.12117v1
2022-10-20,Nanoscale optical nonreciprocity with nonlinear metasurfaces,"Optical nonreciprocity is manifested as a difference in transmission of light
between a pair of modes for the opposite directions of excitation.
Nonreciprocal optics is traditionally realized with relatively bulky components
such as optical isolators based on the Faraday rotation [1,2]. This represents
a roadblock to the future miniaturization and integration of optical systems
requiring the development of subwavelength nonreciprocal components. Here we
demonstrate nonreciprocal transmission through a half-a-micron-thick silicon
metasurface hybridized with vanadium dioxide (VO2) layer. Reciprocity is broken
by VO2 undergoing a phase transition from its insulating to conductive phase.
The phase transition is induced all-optically by an incident light, and it is
designed to occur under different conditions for the opposite directions of
illumination. Resonant response of the metasurface enhances nonreciprocity.
Such hybrid metasurfaces can exhibit nonreciprocity at intensities as low as
150 W/cm2. Our demonstration brings optical nonreciprocity concepts to the
realm of nanoscale science.",2210.14952v4
2022-12-19,An empirical spectroscopic model for eleven electronic states of VO,"Previously-determined empirical energy levels are used to construct a
rovibronic model for the $\mathrm{X}\,^4\Sigma^-$, $\mathrm{A}'\,^4\Phi$,
$\mathrm{A}\,^4\Pi$, $\mathrm{B}\,^4\Pi$, $\mathrm{C}\,^4\Sigma^-$,
$\mathrm{D}\,^4\Delta$, $\mathrm{1}\,^2\Delta$, $\mathrm{1}\,^2\Sigma^+$,
$\mathrm{1}\,^2\Phi$, $\mathrm{1}\,^2\Pi$ and $\mathrm{2}\,^2\Pi$ electronic
states of vanadium mononoxide. The spectrum of VO is characterized by many
couplings and crossings between the states associated with these curves. The
model is based on the use of potential energy curves, represented as extended
Morse oscillators, and couplings (spin-orbit, spin-spin, angular momentum),
represented by polynomials, which are tuned to the data plus an empirical
allowance for spin-rotation couplings. The curves are used as input for the
variational nuclear motion code \duo. For the $\mathrm{X}\,^4\Sigma^-$,
$\mathrm{1}\,^2\Phi$ and $\mathrm{1}\,^2\Pi$ states the model reproduces the
observed energy to significantly better than 0.1 $\mathrm{cm}^{-1}$. For the
other states the standard deviations are between 0.25 and 1.5
$\mathrm{cm}^{-1}$. Further experimental data and consideration of hyperfine
effects would probably be needed to significantly improve this situation.",2212.09223v1
2022-12-20,Ultrafast electron dynamics in a topological surface state observed in two-dimensional momentum space,"We study ultrafast population dynamics in the topological surface state of
Sb$_2$Te$_3$ in two-dimensional momentum space with time- and angle-resolved
two-photon photoemission. Linear polarized mid-infrared pump pulses are used to
permit a direct optical excitation across the Dirac point. We show that this
resonant excitation is strongly enhanced within the Dirac cone along three of
the six $\bar{\Gamma}$-$\bar{M}$ directions and results in a macroscopic
photocurrent when the plane of incidence is aligned along a
$\bar{\Gamma}$-$\bar{K}$ direction. Our experimental approach makes it possible
to disentangle the decay of transiently excited population and photocurent by
elastic and inelastic electron scattering within the full Dirac cone in
unprecedented detail. This is utilized to show that doping of Sb$_2$Te$_3$ by
vanadium atoms strongly enhances inelastic electron scattering to lower
energies, but only scarcely affects elastic scattering around the Dirac cone.",2212.10153v1
2022-12-21,Rare collapse of fermionic quasiparticles upon coupling to local bosons,"We study the stability of the fermionic quasiparticle in a fermion-boson
model on a Bethe lattice, with fermions interacting with local bosons via a
polaronic-type coupling. We solve the problem by mapping it onto a
non-interacting chain with site-dependent potential. We show that, despite a
finite number of bosonic excitations costing zero energy, among the many
analyzed cases, the occurrence of a complete collapse of the quasiparticle is
rare. The quasiparticle disappearance becomes easier with an increase in: (i)
the total number of bosons with zero energy, and (ii) the relative strength of
the coupling between bosons and fermions. The postulated model can, among other
things, be applied to study systems in which fermions are introduced into
antiferromagnetic (or antiferro-orbital) domains surrounded by ferromagnetic
(or ferro-orbital) ordered states. This might take place in the overdoped
cuprates or upon doping manganese or vanadium oxides. Finally, we show how this
model leads to an in-depth understanding of the onset of quasiparticles in the
1D and 2D $t$-$J^z$ model.",2212.11065v2
2023-01-02,Non-Equilibrium Spark Plasma Reactive Doping Enables Highly Adjustable Metal to Insulator Transitions and Improved Mechanical Stability for VO2,"Although vanadium dioxide (VO2) exhibits the most abrupt metal to insulator
transition (MIT) properties near room-temperature, the present regulation of
their MIT functionalities is insufficient owing to the high complexity and
susception associated with V4+. Herein, we demonstrate a spark plasma assisted
reactive sintering (SPARS) approach to simultaneously achieve in situ doping
and sintering of VO2 within largely short period (~10 minutes). This enables
high convenience and flexibility in regulating the electronic structure of VO2
via dopant elements covering Ti, W, Nb, Mo, Cr and Fe, leading to a wide
adjustment in their metal to insulator transition temperature (TMIT) and basic
resistivity. Furthermore, the mechanical strengths of the doped-VO2 were
meanwhile largely improved via the compositing effect of high melting-point
dopant oxide. The high adjustability in MIT properties and improved mechanical
properties further paves the way towards practical applications of VO2 in power
electronics, thermochromism and infrared camouflage.",2301.00634v1
2023-01-16,Analyze the Effects of COVID-19 on Energy Storage Systems: A Techno-Economic Approach,"During the COVID-19 pandemic, the U.S. power sector witnessed remarkable
electricity demand changes in many geographical regions. these changes were
evident in population-dense cities. This paper incorporates a techno-economic
analysis of energy storage systems to investigate the pandemic's influence on
ESS development, In particular, we employ a linear program-based revenue
maximization model to capture the revenues of ESS from participating in the
electricity market, by performing arbitrage on energy trading, and regulation
market, by providing regulation services to stabilize the grid's frequency. We
consider five dominant energy storage technologies in the U.S., namely,
Lithium-ion, Advanced Lead Acid, Flywheel, Vanadium Redox Flow, and
Lithium-Iron Phosphate storage technologies. Extensive numerical results
conducted on the case of New York City allow us to highlight the negative
impact that COVID-19 had on the NYC power sector.",2301.06268v1
2023-01-26,Hydrogen atom/molecule adsorption on 2D metallic porphyrin: A first-principles study,"Hydrogen is a promising element for applications in new energy sources like
fuel cells. One key issue for such applications is storing hydrogen. And, to
improve storage capacity, understanding the interaction mechanism between
hydrogen and possible storage materials is critical. This work uses DFT
simulations to comprehensively investigate the adsorption mechanism of H/H$_2$
on the 2D metallic porphyrins with one transition metal in its center. Our
results suggest that the mechanism for adsorption of H (H$_2$) is chemisorption
(physisorption). The maximum adsorption energy for atomic hydrogen was $-3.7$
eV for 2D porphyrins embedded with vanadium or chromium atoms. Our results also
revealed charge transfer of up $-0.43$ e to chemisorbed H atoms. In contrast,
the maximum adsorption energy calculated for molecular hydrogen was $-122.5$
meV for 2D porphyrins embedded with scandium atoms. Furthermore, charge
transfer was minimal for physisorption. Finally, we also determined that
uniaxial strain has a minimal effect on the adsorption properties of 2D
metallic porphyrins.",2301.11466v1
2023-02-24,Impact of the orbital current order on the superconducting properties of the kagome superconductors,"Motivated by recent experimental evidences signalling the chiral charge order
in the vanadium-based kagome superconductors, we theoretically investigate the
impact of the chiral flux charge order over the experimental outcomes for the
normal and the SC properties. It is revealed that the spectral weight on the
Fermi surface (FS) is partially gaped by the chiral flux charge order with the
reservation of the spectral weight on the $M$ points and the midpoint between
the two adjacent $M$ points, resulting in the momentum-dependent energy gap
being consistent with the recent experimental observations. More importantly,
by considering the influence of the chiral flux charge order, we find that a
conventional fully gapped SC pairing state evolves into a nodal gap feature for
the spectral weight due to the spectral gap modulations on the FS. As a result,
the U-shaped density of states (DOS) deforms to the V-shaped one along with the
residual DOS near the Fermi energy. These results bear some resemblance to the
experimental observations, and may serve as a promising proposal to mediate the
divergent or seemingly contradictory experimental outcomes about the SC pairing
symmetry.",2302.12377v1
2023-03-12,Near-field imaging of domain switching in in-operando VO$_{2}$ devices,"Experimental insight in the nanoscale dynamics underlying switching in novel
memristive devices is limited owing to the scarcity of techniques that can
probe the electronic structure of these devices. Scattering scanning near-field
optical microscopy is a relatively novel approach to probe the optical response
of materials with a spatial resolution well below the diffraction limit. We use
this non-invasive tool to demonstrate that it provides detailed information on
the origin and memory behaviour of ultra-thin films of vanadium dioxide.
Simultaneously recorded $I(V)$ characteristics and near-field maps show that
discontinuities in the I(V) characteristics arise from the sudden switching of
insulating domains to metallic domains. At the threshold voltage, the domains
form a continuous current path. The metallic domains persist once the bias
voltage is removed, but narrow monoclinic regions appear around the domain
boundaries. The key advantage of our approach is that it provides detailed
information on the electronic structure at length scales raging from tens of
nanometers up to tens of microns and is easily applied under \textit{in
operando} conditions.",2303.06703v1
2023-04-28,Ab-initio study of short-range ordering in vanadium-based disordered rocksalt structures,"Disordered rocksalt Li-excess (DRX) compounds are attractive new cathode
materials for Li-ion batteries as they contain resource-abundant metals and do
not require the use of cobalt or nickel. Understanding the delithiation process
and cation short-range ordering (SRO) in DRX compounds is essential to
improving these promising cathode materials. Herein, we use first-principles
calculations along with the cluster-expansion approach to model the disorder in
DRX Li2-xVO3, 0 < x < 1. We discuss the SRO of Li in tetrahedral and octahedral
sites, and the order in which Li delithiates and V oxidizes with respect to
local environments. We reveal that the number of nearest-neighbor V dictates
the order of delithiation from octahedral sites and that V are oxidized in a
manner that minimizes the electrostatic interactions among V. Our results
provide valuable insight for tailoring the performance of V-based DRX cathode
materials in general by controlling the SRO features that reduce energy
density.",2304.14645v1
2023-05-05,Ultrafast all-optical manipulation of the charge-density-wave in VTe$_{2}$,"The charge-density wave (CDW) phase in the layered transition-metal
dichalcogenide VTe$_{2}$ is strongly coupled to the band inversion involving
vanadium and tellurium orbitals. In particular, this coupling leads to a
selective disappearance of the Dirac-type states that characterize the normal
phase, when the CDW phase sets in. Here, by means of broadband time-resolved
optical spectroscopy (TR-OS), we investigate the ultrafast reflectivity changes
caused by collective and single particle excitations in the CDW ground state of
VTe$_{2}$. Remarkably, our measurements show the presence of two collective
(amplitude) modes of the CDW ground state. By applying a double-pulse
excitation scheme, we show the possibility to manipulate these modes,
demonstrating a more efficient way to control and perturb the CDW phase in
VTe$_{2}$.",2305.03528v1
2023-05-09,Multibridge VO2-Based Resistive Switching Devices in a Two-Terminal Configuration,"Vanadium dioxide (VO2) exhibits a hysteretic insulator-to-metal transition
near room temperature, forming the foundation for various forms of resistive
switching devices. Usually, these are realized in the form of two-terminal
bridge-like structures. We show here that by incorporating multiple, parallel
VO2 bridges in a single two-terminal device, a wider range of possible
characteristics can be obtained, including a manifold of addressable resistance
states. Different device configurations are studied, in which the number of
bridges, the bridge dimensions and the interbridge distances are varied. The
switching characteristics of the multibridge devices are influenced by the
thermal crosstalk between the bridges. Scanning Thermal Microscopy has been
used to image the current distributions at various voltage/current bias
conditions. This work presents a route to realize devices exhibiting highly
non-linear, multistate current-voltage characteristics, with potential
applications in e.g., tunable electronic components and novel, neuromorphic
information processing circuitry.",2305.05259v1
2023-05-10,Quadratic heat capacity and high-field magnetic phases of V5S8,"We report the observation of an unexpected quadratic temperature dependence
of the heat capacity in the vanadium sulphide metal V5S8 at low temperatures
which is independent of applied magnetic field. We find that the behaviour of
the heat capacity is consistent with an unconventional phonon spectrum which is
linear in wavevector in the c direction but quadratic in the a-b plane,
indicating a form of geometrical elastic criticality. In the case of V5S8 we
also observe an unusual intermediate transition at high magnetic fields between
the expected spin-flop and spin-flip transitions. We demonstrate that the
intermediate field-induced transition is in agreement with a model of two
sublattices with frustrated inter- and intra-sublattice spin couplings.",2305.06175v1
2023-05-20,Strong substrate influence on atomic structure and properties of epitaxial VO2 thin films,"The metal-insulator transition (MIT) observed in vanadium dioxide (VO2) has
been a topic of great research interest for past decades, with the underlying
physics yet not fully understood due to the complex electron interactions and
structures involved. The ability to understand and tune the MIT behaviour is of
vital importance from the perspective of both underlying fundamental science as
well as potential applications. In this work, we use scanning transmission
electron microscopy (STEM) to investigate cross-section lamella of the VO2
films deposited using pulsed laser deposition (PLD) on three substrates: c-cut
sapphire, TiO2(101) and TiO2(001). Advanced STEM imaging is performed in which
also the oxygen atom columns are resolved. We link the overall film quality and
structures on atomic and nanoscale to the electrical transition
characteristics. We observe poor MIT characteristics on c-sapphire due to the
presence of very small domains with six orientation variants, and on TiO2 (001)
due to the presence of cracks induced by stress relaxation. However, the MIT on
TiO2 (101) behaves favourably, despite similar stress relaxation which,
however, only lead to domain boundaries but no cracks.",2305.12108v1
2023-05-31,Physics-informed machine learning of redox flow battery based on a two-dimensional unit cell model,"In this paper, we present a physics-informed neural network (PINN) approach
for predicting the performance of an all-vanadium redox flow battery, with its
physics constraints enforced by a two-dimensional (2D) mathematical model. The
2D model, which includes 6 governing equations and 24 boundary conditions,
provides a detailed representation of the electrochemical reactions, mass
transport and hydrodynamics occurring inside the redox flow battery. To solve
the 2D model with the PINN approach, a composite neural network is employed to
approximate species concentration and potentials; the input and output are
normalized according to prior knowledge of the battery system; the governing
equations and boundary conditions are first scaled to an order of magnitude
around 1, and then further balanced with a self-weighting method. Our numerical
results show that the PINN is able to predict cell voltage correctly, but the
prediction of potentials shows a constant-like shift. To fix the shift, the
PINN is enhanced by further constrains derived from the current collector
boundary. Finally, we show that the enhanced PINN can be even further improved
if a small number of labeled data is available.",2306.01010v2
2023-06-06,Vanadium-Based Superconductivity in a Breathing Kagome Compound Ta2V3.1Si0.9,"Superconductivity in V-based kagome metals has recently raised great interest
as they exhibit the competing ground states associated with the flat bands and
topological electronic structures. Here we report the discovery of
superconductivity in Ta2V3.1Si0.9 with a superconducting transition temperature
Tc of 7.5 K, much higher than those in previously reported kagome metals at
ambient pressure. While the V ions form a two-dimensional breathing kagome
structure, the length difference between two different V-V bonds is just 0.04,
making it very close to the perfect kagome structure. Our results show that
Ta2V3.1Si0.9 is a moderate-coupled superconductor with a large upper critical
field that is close to the Pauli limit. DFT calculations give a
van-Hove-singularity band located at Fermi energy, which may explain the
relatively high Tc observed in this material.",2306.03370v2
2023-06-16,Determination of the nearest-neighbor interaction in VO$_2$ via fractal dimension analysis,"The Ising model is one of the simplest and most well-established concepts to
simulate phase transformations in complex materials. However, its most central
constant, the interaction strength J between two nearest neighbors, is hard to
obtain. Here we show how this basic constant can be determined with a fractal
dimension analysis of measured domain structures. We apply this approach to
vanadium dioxide, a strongly correlated material with a first-order
insulator-to-metal phase-transition with enigmatic properties. We obtain a
nearest-neighbor interaction of 13.8 meV, a value close to the thermal energy
at room temperature. Consequently, even far below the transition temperature,
there are spontaneous local phase-flips from the insulating into the metallic
phase. These fluctuations explain several measured anomalies in VO$_2$, in
particular the low thermal carrier activation energy and the finite
conductivity of the insulating phase. As a method, our fractal dimension
analysis links the Ising model to macroscopic material constants for almost any
first-order phase transition.",2306.09722v2
2023-07-20,Reconfigurable cascaded thermal neuristors for neuromorphic computing,"While the complementary metal-oxide semiconductor (CMOS) technology is the
mainstream for the hardware implementation of neural networks, we explore an
alternative route based on a new class of spiking oscillators we call thermal
neuristors, which operate and interact solely via thermal processes. Utilizing
the insulator-to-metal transition in vanadium dioxide, we demonstrate a wide
variety of reconfigurable electrical dynamics mirroring biological neurons.
Notably, inhibitory functionality is achieved just in a single oxide device,
and cascaded information flow is realized exclusively through thermal
interactions. To elucidate the underlying mechanisms of the neuristors, a
detailed theoretical model is developed, which accurately reflects the
experimental results. This study establishes the foundation for scalable and
energy-efficient thermal neural networks, fostering progress in brain-inspired
computing.",2307.11256v2
2023-07-31,Structural and Magnetic Properties of V-Ti-Si Alloy Superconductors,"The structural and magnetic properties of the as-cast and annealed
V$_{0.6-x}$Si$_x$Ti$_{0.4}$ ($x$ = 0, 0.05, 0.10, 0.15) alloy superconductors
are reported here. It is found that addition of silicon to the V-Ti alloys
results in eutectic precipitation of Ti$_{5}$Si$_3$-phase in the body centred
cubic (bcc) $\beta$-V-Ti matrix. In the as-cast V$_{0.6-x}$Si$_x$Ti$_{0.4}$
alloys, the superconducting transition temperature (T$_{C}$) changes
non-monotonically with increasing silicon content whereas after annealing, it
is about 7.7 K for all the alloys. On the other hand, the upper critical field
decreases and the coherence length increases after annealing in the x = 0.10
alloy. The variations in the superconducting properties in the alloys are
related to the solubility of 6 at.% Si in the V$_{0.60}$Ti$_{0.40}$ alloy and
the vanadium enrichment in the $\beta$ matrix due to the precipitation of
Ti$_{5}$Si$_3$ phase.",2307.16623v2
2023-08-04,Kinetics of Li transport in vanadium-based disordered rocksalt structures,"Disordered rocksalt Li-excess (DRX) compounds have emerged as promising new
cathode materials for lithium-ion batteries, as they can consist solely of
resource-abundant metals and eliminate the need for cobalt or nickel. A deeper
understanding of the lithium-ion transport kinetics in DRX compounds is
essential for enhancing their rate performance. This study employs
first-principles calculations, cluster expansion techniques, and kinetic Monte
Carlo simulations to investigate the Li+ transport properties in DRX Li2-xVO3,
where 0 <= x <= 1. Our findings underscore (i) the necessity of accounting for
both tetrahedral and octahedral Li occupancy when predicting the transport
properties in DRX materials, (ii) the factors influencing the variation in the
diffusion coefficients with Li content in Li2-xVO3, and (iii) the impact of Li+
correlated motion on the kinetics of Li+ transport. We reveal that the relative
stability of tetrahedral and octahedral Li determines the number of active
sites within the percolation network, subsequently affecting the Li+ transport
properties. Furthermore, we demonstrate that the wide site-energy distribution
causes correlated motion in Li2-xVO3, which hinders Li+ transport. Although our
study focuses on Li2-xVO3 as a model system, the insights gained apply to all
DRX materials, given their inherently broader site-energy distributions.",2308.02725v1
2023-08-28,Formation of Domains within Lower-to-higher Symmetry Structural Transition in CrI$_3$,"CrI$_3$ represents one of the most important van der Waals systems on the
route to understanding two-dimensional magnetic phenomena. Being arranged in a
specific layered structure it also provides a unique opportunity to investigate
structural transformations in dimension-confined systems. CrI$_3$ is dimorphic
and possesses a higher symmetry low-temperature phase, which is quite uncommon.
It contrasts with vanadium trihalides which show a higher symmetry
high-temperature. An explanation of this distinct behavior together with a
large cycle-dependent transition hysteresis is still an open question. Our
low-temperature X-ray diffraction study conducted on CrI$_3$ single crystals
complemented by magnetization and specific heat measurements was focused mainly
on specific features of the structural transition during cooling. Our results
manifest that the structural transition during cooling relates to the formation
of structural domains despite the lower symmetry structure transforming to a
higher symmetry one. We propose that these domains could control the transition
temperature and also the size of thermal hysteresis.",2308.14548v1
2023-09-02,Linear Trimer Formation by Three-Center-Four-Electron Bonding in RuP,"In molecules like hydrogen, most chemical bonds are formed by sharing two
electrons from each atom in the bonding molecular orbital
(two-center-two-electron (2c2e) bonding). There are, however, different kinds
of chemical bonding. The I3- molecule, for example, is noteworthy because three
iodine atoms are linearly united by sharing four electrons
(three-center-four-electron (3c4e) bonding). Some inorganic solids undergo
phase transitions that result in the formation of ""molecules"" in their
crystalline frameworks, which are often accompanied by dramatic changes in
physical properties; the metal-to-insulator transition (MIT) in vanadium
dioxide, for example, occurs with the formation of dimer molecules with 2c2e
bonding. We repot the creation of a linear ruthenium trimer with 3c4e bonding
in ruthenium monopnictide at its MIT. Charge transfer from polymerized
phosphorous to ruthenium produces this unusual molecule, with all conduction
electrons trapped by the bonding molecular orbital. Our results demonstrate
that molecules are crucial even in solid crystals as they impact their
electronic properties.",2309.00838v1
2023-09-23,All-optical seeding of a light-induced phase transition with correlated disorder,"Ultrafast manipulation of vibrational coherence is an emergent route to
control the structure of solids. However, this strategy can only induce
long-range correlations and cannot modify atomic structure locally, which is
required in many technologically-relevant phase transitions. Here, we
demonstrate that ultrafast lasers can generate incoherent structural
fluctuations which are more efficient for material control than coherent
vibrations, extending optical control to a wider range of materials. We observe
that local, non-equilibrium lattice distortions generated by a weak laser pulse
reduce the energy barrier to switch between insulating and metallic states in
vanadium dioxide by 6%. Seeding inhomogeneous structural-fluctuations presents
an alternative, more energy efficient, route for controlling materials that may
be applicable to all solids, including those used in data and energy storage
devices.",2309.13275v1
2023-10-06,"Kagome KMn$_3$Sb$_5$ metal: Magnetism, lattice dynamics, and anomalous Hall conductivity","Kagome metals are reported to exhibit remarkable properties, including
superconductivity, charge density wave order, and a large anomalous Hall
conductivity, which facilitate the implementation of spintronic devices. In
this work, we study a novel kagome metal based on Mn magnetic sites in a
KMn$_3$Sb$_5$ stoichiometry. By means of first-principles density functional
theory calculations, we demonstrate that the studied compound is dynamically
stable, locking the ferromagnetic order as the ground state configuration, thus
preventing the charge-density-wave state as reported in its vanadium-based
counterpart KV$_3$Sb$_5$. Our calculations predict that KMn$_3$Sb$_5$ exhibits
an out-of-plane (001) ferromagnetic response as the ground state, allowing for
the emergence of topologically protected Weyl nodes near the Fermi level and
nonzero anomalous Hall conductivity ($\sigma_{ij}$) in this centrosymmetric
system. We obtain a tangible $\sigma_{xy} = 314$ S$\cdot$cm$^{-1}$ component,
which is comparable to that of other kagome metals. Finally, we explore the
effect of the on-site Coulomb repulsion ($+U$) on the structural and electronic
properties and find that, although the lattice parameters and $\sigma_{xy}$
moderately vary with increasing $+U$, KMn$_3$Sb$_5$ stands as an ideal stable
ferromagnetic kagome metal with a large anomalous Hall conductivity response.",2310.04339v1
2023-10-16,Molecular orbital formation and metastable short-range ordered structure in VO$_2$,"The metal-insulator transition (MIT) in vanadium dioxide VO$_2$ due to V-V
dimerization has been extensively discussed for decades. While it is widely
acknowledged that electron correlations, Peierls instabilities, and molecular
orbital formations are crucial for understanding the MIT of VO$_2$, the primary
origin of the MIT remains controversial. In this study, we delve into the
crystal structure and orbital state of VO$_2$ through synchrotron x-ray
diffraction experiments. The molecular orbital formation corresponding to the
V-V dimerization is directly observed in the low-temperature insulating
monoclinic phase, called the M1 phase, as indicated by the valence electron
density distribution. Moreover, diffuse scattering observed in the
high-temperature metal phase of rutile structure suggests the presence of
short-range correlation of V displacements, which is not directly attributed to
the structural fluctuation of the M1 phase. The short-range order in the rutile
phase will be the key to understanding the MIT in this system.",2310.10193v2
2023-10-22,Atomic arrangement of van der Waals heterostructures using X-ray scattering and crystal truncation rod analysis,"Vanadium diselenide (VSe2) has intriguing physical properties such as
unexpected ferromagnetism at the two-dimensional limit. However, the
experimental results for room temperature ferromagnetism are still
controversial and depend on the detailed crystal structure and stoichiometry.
Here we introduce crystal truncation rod (CTR) analysis to investigate the
atomic arrangement of bilayer VSe2 and bilayer graphene (BLG) hetero-structures
grown on a 6H-SiC(0001) substrate. Using non-destructive CTR analysis, we were
able to obtain electron density profiles and detailed crystal structure of the
VSe2/BLG heterostructures. Specifically, the out-of-plane lattice parameters of
each VSe2 layer were modulated by the interface compared to that of the bulk
VSe2 1T phase. The atomic arrangement of the VSe2/BLG heterostructure provides
deeper understanding and insight for elucidating the magnetic properties of the
van der Waals heterostructure.",2310.14207v1
2023-10-30,Strain Engineering for Transition Metal Defects in SiC,"Transition metal (TM) defects in silicon carbide (SiC) are a promising
platform for applications in quantum technology as some of these defects, e.g.
vanadium (V), allow for optical emission in one of the telecom bands. For other
defects it was shown that straining the crystal can lead to beneficial effects
regarding the emission properties. Motivated by this, we theoretically study
the main effects of strain on the electronic level structure and optical
electric-dipole transitions of the V defect in SiC. In particular we show how
strain can be used to engineer the g-tensor, electronic selection rules, and
the hyperfine interaction. Based on these insights we discuss optical Lambda
systems and a path forward to initializing the quantum state of strained TM
defects in SiC.",2310.19719v1
2023-11-15,Infrared thermochromic antenna composite for self-adaptive thermoregulation,"Self-adaptive thermoregulation, the mechanism living organisms use to balance
their temperature, holds great promise for decarbonizing cooling and heating
processes. The functionality can be effectively emulated by engineering the
thermal emissivity of materials to adapt to background temperature variations.
Yet, solutions that marry large emissivity switching (${\Delta}\epsilon$) with
scalability, cost-effectiveness and design freedom are still lacking. Here, we
fill this gap by introducing infrared dipole antennas made of tunable
thermochromic materials. We demonstrate that non-spherical antennas (rods,
stars and flakes) made of vanadium-dioxide can exhibit a massive (~200-fold)
increase in their absorption cross-section as temperature rises. Embedding
these antennas in polymer films, or simply spraying them directly, creates
free-form thermoregulation composites, featuring an outstanding
${\Delta}\epsilon\sim0.6$ in spectral ranges that can be tuned at will. Our
research paves the way for versatile self-adaptive heat management solutions
(coatings, fibers, membranes and films) that could find application in
radiative-cooling, heat-sensing, thermal-camouflage, and other.",2311.08633v1
2023-11-26,"Quantum oscillations in kagome metals (Ti, Zr, Hf)V6Sn6 at van Hove filling","Kagome materials have recently drawn great attention due to the interplay
between nontrivial band topology, electron correlations and van Hove
singularities related many-body orders. Here we report three new vanadium-based
kagome metals, TiV6Sn6, ZrV6Sn6 and HfV6Sn6, and conduct a comprehensive
investigation on their structural, magnetic and electrical transport
properties. All three compounds exhibit large unsaturated magnetoresistances
and multiband Hall effects at low temperatures, indicating the existence of
multiple highly mobile carriers. Both the diagonal and off-diagonal resistivity
show quantum oscillations with nontrivial Berry phases and high quantum
mobilities. First-principles calculations together with quantum oscillation
analyses suggest the van Hove singularities at the M point for the three
compounds all locate in close vicinity of the Fermi level, and there also exist
multiple topological nontrivial band crossings including a nodal ring and a
massive Dirac cone. Our work extends the kagome R166 family and paves the way
for searching possible van Hove physics in the V kagome lattice.",2311.15239v1
2023-12-07,Confinement-Induced Isosymmetric Metal-Insulator Transition in Ultrathin Epitaxial V2O3 Films,"Dimensional confinement has shown to be an effective strategy to tune
competing degrees of freedom in complex oxides. Here, we achieved atomic
layered growth of trigonal vanadium sesquioxide (V2O3) by means of
oxygen-assisted molecular beam epitaxy. This led to a series of high-quality
epitaxial ultrathin V2O3 films down to unit cell thickness, enabling the study
of the intrinsic electron correlations upon confinement. By electrical and
optical measurements, we demonstrate a dimensional confinement-induced
metal-insulator transition in these ultrathin films. We shed light on the
Mott-Hubbard nature of this transition, revealing an abrupt vanishing of the
quasiparticle weight as demonstrated by photoemission spectroscopy.
Furthermore, we prove that dimensional confinement acts as an effective
out-of-plane stress. This highlights the structural component of correlated
oxides in a confined architecture, while opening an avenue to control both
in-plane and out-of-plane lattice components by epitaxial strain and
confinement, respectively.",2312.04425v1
2023-12-01,Comparative study of Kondo effect in Vanadium dichalcogenides VX$_2$ (X=Se & Te),"We report on the electrical transport, magnetotransport, and magnetic
properties studies on the transition metal dichalcogenides VSe$_2$ and VTe$_2$
and draw a comprehensive comparison between them. We observe Kondo effect in
both systems induced by the exchange interaction between localized moments and
conduction electrons at low temperature, resulting into resistance upturn at 6
K for VSe$_2$ and 17 K for VTe$_2$. From the field dependent resistance
measurements we find that the data is fitted best with modified Hamann equation
corrected by the quantum Brillouin function for VSe$_2$, while the data is
fitted best with modified Hamann equation corrected by the classical Langevin
function for VTe$_2$. Interestingly, we observe a contrasting magnetoresistance
(MR) property between these systems across the Kondo temperature. That means,
negative MR is found in both systems in the Kondo state. In the normal state MR
is positive for VSe$_2$, while it is negligible for VTe$_2$. In addition, both
systems show weak ferromagnetism at low temperature due to intercalated V
atoms.",2312.09258v1
2023-12-15,Structural dimerization and charge-orbital ordering in a ferromagnetic semiconductor LiV2S4 monolayer,"With the rise of two-dimensional (2D) materials, unique properties that are
completely distinct from bulk counterparts continue to emerge at
low-dimensional scales, presenting numerous opportunities and challenges. It
also provides a new perspective for the study of transition metal system. Here,
based on density functional theory (DFT), the physical properties of 2D
monolayer LiV2S4 have been studied. Remarkable changes have been observed,
i.e., vanadium dimerization, ferromagnetism, charge distribution and
metal-insulator transition (MIT). It is argued that the electronic instability
leads to the V dimerization which further lifts the degeneracy of charge
distribution and stabilizes the charge and spin ordering state.",2312.09937v1
2023-12-20,Collective dynamics and long-range order in thermal neuristor networks,"In the pursuit of scalable and energy-efficient neuromorphic devices, recent
research has unveiled a novel category of spiking oscillators, termed ``thermal
neuristors."" These devices function via thermal interactions among neighboring
vanadium dioxide resistive memories, closely mimicking the behavior of
biological neurons. Here, we show that the collective dynamical behavior of
networks of these neurons showcases a rich phase structure, tunable by
adjusting the thermal coupling and input voltage. Notably, we have identified
phases exhibiting long-range order that, however, does not arise from
criticality, but rather from the time non-local response of the system. In
addition, we show that these thermal neuristor arrays achieve high accuracy in
image recognition tasks through reservoir computing, without taking advantage
of this long-range order. Our findings highlight a crucial aspect of
neuromorphic computing with possible implications on the functioning of the
brain: criticality may not be necessary for the efficient performance of
neuromorphic systems in certain computational tasks.",2312.12899v1
2023-12-23,"Reduction of Magnetic Interaction Due to Clustering in Doped Transition-Metal Dichalcogenides: A Case Study of Mn, V, Fe-Doped $\rm WSe_2$","Using Hubbard U corrected density functional theory calculations, lattice
Monte-Carlo, and spin-Monte-Carlo simulations, we investigate the impact of
dopant clustering on the magnetic properties of WSe2~doped with period four
transition metals. We use manganese (Mn) and iron (Fe) as candidate n-type
dopants and vanadium (V) as the candidate p-type dopants, substituting the
tungsten (W) atom in WSe2. Specifically, we determine the strength of the
exchange interaction in the Fe-, Mn-, and V-doped WSe2~ in the presence of
clustering. We show that the clusters of dopants are energetically more stable
than discretely doped systems. Further, we show that in the presence of dopant
clustering, the magnetic exchange interaction significantly reduces because the
magnetic order in clustered WSe2~becomes more itinerant. Finally, we show that
the clustering of the dopant atoms has a detrimental effect on the magnetic
interaction, and to obtain an optimal Curie temperature, it is important to
control the distribution of the dopant atoms.",2312.15171v1
2024-01-07,Double tunable metamaterial perfect absorber,"This paper focuses on the simulation of a tunable metamaterial absorber
designed for the infrared region. Adsorbents offer three different mechanisms
to adjust their absorption characteristics. The first method involves changes
in temperature. When the temperature changes, the electrical conductivity of
the active material, vanadium dioxide (VO2), also changes. This transition
between insulating and conducting phases at different temperatures provides the
possibility to adjust the absorption spectrum of the metamaterial. By
exploiting the thermochromic properties of VO2, the conductivity of the
material can be dynamically adjusted over a wide range. Through temperature
control, the conductivity of VO2 changes from 200 S/m to 2*105 S/m, resulting
in a continuous adjustment of the absorption peak intensity. The second method
relies on applying an electric field. By applying an electric field in the
transverse direction of the lead zirconate titanate )PZT( material, the size of
the piezoelectric material undergoes changes and bending. Consequently, the
entire structure bends, leading to a change in the shape of the absorber. This
mechanism allows for precise control of the absorber's shape by applying
different voltages, which in turn enables the modification of the absorption
peak.",2401.03421v1
2024-01-26,Quantum Oscillations Measurement of the Heavy Electron Mass near the van Hove Singularity in a Kagome Metal,"Kagome metals with the Fermi energy tuned near the van Hove singularities
(vHss) have shown to host exotic phases including unconventional
superconductivity and a chiral flux phase arising from a charge density wave.
However, most quantum oscillations studies of the electronic structure of
kagome metals focus on compounds which electronically or magnetically order,
obscuring the unperturbed vHs. Here we present quantum oscillation measurements
of YV$_6$Sn$_6$ which contains a pristine kagome lattice free from long range
order. We discovered quantum oscillations corresponding to a large orbit
($\approx$70% of the Brillouin Zone area) with the heaviest mass ever observed
in vanadium based kagome metals ($\approx3.3 m_e$), consistent with a Fermi
pocket whose Fermi level is near the vHs. Comparing with first principles
calculations suggests that the effective mass of this pocket is highly
sensitive to the position of Fermi level. Our study establishes the enhanced
density of states associated with a vHs in a kagome metal, allowing further
insight into a potential driving mechanism for the unconventional electronic
orderings in this class of materials.",2401.14699v1
2024-01-30,Coupling strongly correlated electron systems to a tunable electronic reservoir,"We study the effect of coupling an electronic reservoir to a Hubbard model
and to a Dimer Hubbard Model.
  This is motivated by recent experiments on the effect of illumination on the
insulator-metal transition in a vanadium oxides and photo-conductive cadmium
sulfide heterostructure. We model the system as an electronic reservoir
hybridized to the correlated system. We assume that the light intensity
controls the hybridization coupling strength. We find that the light intensity
acts similarly as the temperature in the weak interaction regime. This is
consistent with the role played by electronic reservoirs in out-of-equilibrium
systems. In contrast, qualitative differences appear at strong coupling. We
show that modeling the V$_2$O$_3$ compound with a Hubbard model, our results
describe qualitatively well the observed illumination-driven suppression of the
insulator-metal transition. In contrast, in the DHM results fail to capture the
mild suppression observed in the case of VO$_2$. This indicates that the
lattice may play an important role in this case.",2401.17016v1
2024-04-04,Competitive coexistence of ferromagnetism and metal--insulator transition of VO$_2$ nanoparticles,"We investigated the magnetic and electric properties of nanometer-sized
vanadium dioxide (VO$_2$) particles. VO$_2$ nanoparticles were formed by
milling VO$_2$ powder. We measured the magnetic field dependence of the
magnetization of the VO$_2$ powder and nanoparticles. The VO$_2$ powder did not
exhibit ferromagnetism, whereas the VO$_2$ nanoparticles exhibited
ferromagnetism. In addition, we fabricated samples by bridging between
electrodes with the VO$_2$ nanoparticles, and the temperature dependence of
their resistance was measured. Metal-insulator transitions (MITs) were
observed, and the temperature range where the MIT occurred was wider than that
in a typical bulk VO$_2$. The VO$_2$ nanoparticles exhibited these properties
of ferromagnetism and MIT possibly because of the surface and size effects of
the VO$_2$ nanoparticles. These results indicate the first observation of the
competitive coexistence of ferromagnetism and MIT of VO$_2$ nanoparticles.",2404.03207v1
2024-04-05,Resonant Raman signatures of exciton polarons in a transition metal oxide: BiVO$_4$,"In this work we investigate the delocalized excitons and excitons trapped by
a polaron formation in \BVO{} by means of resonant Raman spectroscopy. We
record Raman spectra with 16 laser lines between 1.9 and \SI{2.6}{\eV} and
analyze intensity variations of the Raman peaks for different vibrational
modes. The resonant Raman cross sections of the \Ag{} modes contain two types
of resonances. The first high-energy resonance near \SI{2.45}{\eV} belongs to a
transition between delocalized states; it is close to absorption edge measured
at \SI{2.3}{\eV} and exhibits a characteristic \SI{50}{\meV} anisotropy between
polarization parallel and perpendicular to the $c$ axis. The high energy Raman
resonance occurs inside the gap at \SI{1.94}{\eV} for all crystallographic
directions.
  The in-gap resonance can involve a localized transition. We attribute it to
an exciton-polaron, formed by a small localized electron polaron of Holstein
type and delocalized holes. It manifests in the vibrations of vanadium and
oxygen atoms where polaron localization occurs and the resonance energy matches
theoretical predictions. The vibrational modes couple to the polaron with
different efficiency determined from resonant Raman profiles.",2404.04112v1
2021-08-27,The CARMENES search for exoplanets around M dwarfs: Not-so-fine hyperfine-split vanadium lines in cool star spectra,"(Abridged) We characterize a series of neutral vanadium atomic absorption
lines in the 800--910nm wavelength region of high signal-to-noise,
high-resolution, telluric-corrected M-dwarf spectra from the CARMENES survey.
Many of these lines are prominent and exhibit a distinctive broad and
flat-bottom shape, which is a result of hyperfine structure (HFS). We
investigate the potential and implications of these HFS split lines for
abundance analysis of cool stars. With standard spectral synthesis routines, as
provided by the spectroscopy software iSpec and the latest atomic data
(including HFS) available from the VALD3 database, we modeled these striking
line profiles. We used them to measure V abundances of cool dwarfs. We
determined V abundances for 135 early M dwarfs (M0.0V to M3.5V) in the CARMENES
guaranteed time observations sample. They exhibit a [V/Fe]-[Fe/H] trend
consistent with that derived from nearby FG dwarfs. The tight ($\pm$ 0.1 dex)
correlation between [V/H] and [Fe/H] suggests the potential application of V as
an alternative metallicity indicator in M dwarfs. We also show hints that
neglecting to model HFS could partially explain the temperature correlation in
V abundance measurements observed in previous studies of samples involving
dwarf stars with $T_{\rm eff} \lesssim 5300$K. Our work suggests that HFS can
impact certain absorption lines in cool photospheres more severely than in
Sun-like ones. Therefore, we advocate that HFS should be carefully treated in
abundance studies in stars cooler than $\sim 5000$K. On the other hand, strong
HFS split lines in high-resolution spectra present an opportunity for precision
chemical analyses of large samples of cool stars. The V-to-Fe trends exhibited
by the local M dwarfs continue to challenge theoretical models of V production
in the Galaxy.",2108.12442v1
1998-04-29,"Magnetic correlations and quantum criticality in the insulating antiferromagnetic, insulating spin liquid, renormalized Fermi liquid, and metallic antiferromagnetic phases of the Mott system V_2O_3","Magnetic correlations in all four phases of pure and doped vanadium
sesquioxide V_2O_3 have been examined by magnetic thermal neutron scattering.
While the antiferromagnetic insulator can be accounted for by a Heisenberg
localized spin model, the long range order in the antiferromagnetic metal is an
incommensurate spin-density-wave, resulting from a Fermi surface nesting
instability. Spin dynamics in the strongly correlated metal are dominated by
spin fluctuations in the Stoner electron-hole continuum. Furthermore, our
results in metallic V_2O_3 represent an unprecedentedly complete
characterization of the spin fluctuations near a metallic quantum critical
point, and provide quantitative support for the SCR theory for itinerant
antiferromagnets in the small moment limit. Dynamic magnetic correlations for
energy smaller than k_BT in the paramagnetic insulator carry substantial
magnetic spectral weight. However, the correlation length extends only to the
nearest neighbor distance. The phase transition to the antiferromagnetic
insulator introduces a sudden switching of magnetic correlations to a different
spatial periodicity which indicates a sudden change in the underlying spin
Hamiltonian. To describe this phase transition and also the unusual short range
order in the paramagnetic state, it seems necessary to take into account the
orbital degrees of freedom associated with the degenerate d-orbitals at the
Fermi level in V_2O_3.",9804320v3
2001-02-01,Exact diagonalisation study of charge order in the quarter-filled two-leg ladder system NaV2O5,"The charge ordering transition in the layer compound NaV2O5 is studied by
means of exact diagonalization methods for finite systems. The 2-leg ladders of
the V-Trellis lattice are associated with one spin variable of the vanadium
3d-electron in the rung and a pseudospin variable that describes its positional
degree of freedom. The charge ordering (CO) due to intersite Coulomb
interactions is described by an effective Ising-like Hamiltonian for the
pseudo-spins that are coupled to the spin fluctuations along the ladder. We
employ a Lanczos algortihm on 2D lattice to compute charge (pseudo-spin) and
spin-correlation functions and the energies of the low lying excited states. A
CO-phase diagram is constructed and the effect of intra-ladder exchange on the
CO transition is studied. It is shown that a phase with no-longe range order
(no-LRO) exists between the in-line and zig-zag ordered structures. We provide
a finite-size scaling analysis for the spin excitation gap and also discuss the
type of excitations. In addition we studied the effect of bond-alternation of
spin exchange and derived a scaling form for the spin gap in terms of the
dimerization parameter.",0102007v1
2002-10-24,The metal-insulator transitions of VO2: A band theoretical approach,"The results of first principles electronic structure calculations for the
metallic rutile and the insulating monoclinic M1 phase of vanadium dioxide are
presented. In addition, the insulating M2 phase is investigated for the first
time. The density functional calculations allow for a consistent understanding
of all three phases. In the rutile phase metallic conductivity is carried by
metal t_2g orbitals, which fall into the one-dimensional d_parallel band, and
the isotropically dispersing e_g^pi bands. Hybridization of both types of bands
is weak. In the M1 phase splitting of the d_parallel band due to metal-metal
dimerization and upshift of the e_g^pi bands due to increased p-d overlap lead
to an effective separation of both types of bands. Despite incomplete opening
of the optical band gap due to the shortcomings of the local density
approximation, the metal-insulator transition can be understood as a
Peierls-like instability of the d_parallel band in an embedding background of
e_g^pi electrons. In the M2 phase, the metal-insulator transition arises as a
combined embedded Peierls-like and antiferromagnetic instability. The results
for VO2 fit into the general scenario of an instability of the rutile-type
transition-metal dioxides at the beginning of the d-series towards dimerization
or antiferromagnetic ordering within the characteristic metal chains. This
scenario was successfully applied before to MoO2 and NbO2. In the d^1
compounds, the d_parallel and e_g^pi bands can be completely separated, which
leads to the observed metal-insulator transitions.",0210558v1
2003-08-12,"Structural, electronic, and magneto-optical properties of YVO$_3$","Optical and magneto-optical properties of YVO$_3$ single crystal were studied
in FIR, visible, and UV regions. Two structural phase transitions at 75 K and
200 K were observed and established to be of the first and second order,
respectively. The lattice has an orthorhombic $Pbnm$ symmetry both above 200 K
as well as below 75 K, and is found to be dimerized monoclinic $Pb11$ in
between. We identify YVO$_3$ as a Mott-Hubbard insulator with the optical gap
of 1.6 eV. The electronic excitations in the visible spectrum are determined by
three $d$-bands at 1.8, 2.4, and 3.3 eV, followed by the charge-transfer
transitions at about 4 eV. The observed structure is in good agreement with
LSDA+$U$ band structure calculations. By using ligand field considerations, we
assigned these bands to the transitions to the $^4A_{2g}$, $^2E_{g} +
^2T_{1g}$, and $^2T_{2g}$ states. The strong temperature dependence of these
bands is in agreement with the formation of orbital order. Despite the small
net magnetic moment of 0.01 $\mu_B$ per vanadium, the Kerr effect of the order
of $0.01^\circ$ was observed for all three $d$-bands in the magnetically
ordered phase $T_{\text{N\'eel}}<116 K$. A surprisingly strong enhancement of
the Kerr effect was found below 75 K, reaching a maximum of $0.1^\circ$. The
effect is ascribed to the non-vanishing net orbital magnetic moment.",0308224v1
2005-03-31,"Local Electronic Structure of PbVO3, a New Member of PbTiO3 Family, studied by XANES/ELNES","Recently an interesting multi-ferroic system PbVO3 [Chem. Mater. 2004] was
successfully prepared using a high-pressure and high-temperature technique. The
crystallographic features were reported. In this note we concentrate on the
theoretical XANES spectra by considering the K-edge of Vanadium. The
tetragonality [c/a=1.229 at 300 K] of PbVO3 is the largest in the PbTiO3 family
of compounds. Thus one is led naturally to examine the effect of the change of
tetragonality and the axial oxygen position on the electronic structure [i.e.
XANES spectrum]. We study this effect in two ways. At a given temperature we
vary the tetragonality and the axial oxygen position and quantify it in terms
of XANES difference spectrum. Secondly, we compute the XANES spectra at three
different temperatures, 90 K, 300 K, and 530 K and quantify the change in terms
of the difference spectrum. We note that in this compound the tetragonality
increases almost monotonically with temperature from 12 K to 570 K without
transition to the cubic phase under ambient pressure. A key objective of the
current investigation is to gain an understanding of various absorption
features in the vicinity of K-edge of V, in terms of valence, local site
symmetry, local coordination geometry, local bond distances, charge transfer,
and local projected density of states. We consider both the polarized and the
unpolarized XANES spectra. In short we have performed a local electronic study,
which nicely complements the crystallographic features reported recently in
PbVO3.",0503737v1
2006-03-06,Metal-insulator transition in electric field: A viewpoint from the switching effect,"The proposed switching mechanism is based on an electronically-induced
metal-insulator transition occurring in conditions of the excess
non-equilibrium carrier density under the applied electric field. First, this
mechanism is developed on the basis of a phenomenological approach. This model
not only allows the qualitative description of the switching mechanism, but it
is in quantitative agreement with the experimental results, in particular, with
those concerning the critical concentration and threshold field. The mechanism
takes into account the dependence of the carrier density on electric field, as
well as the scaling of the critical field. Next, we show that such a
'macroscopic' approach can be supported by some microscopic model. The
quintessence of this approach consists in the fact that an electronically
induced metal-insulator transition may be described in terms of
Bardeen-Cooper-Schrieffer (BCS) formalism developed earlier for the charge
density wave concept. It is shown that for the combination of both the types of
interaction (electron-electron and electron-phonon), the formation of a
collective excitation - an electron crystal of charge density wave - in the
model of exciton insulator is possible, which, in a general case, can also be
accompanied by a structural transition. The results for vanadium dioxide are
examined within the frameworks of the developed approach.",0603132v1
2006-08-17,"Photoemission study of (V$_{1-x}$M$_x$)$_2$O$_3$ (M=Cr, Ti)","We present high-resolution bulk-sensitive photoemission spectra of
(V$_{1-x}$M$_x$)$_2$O$_3$ (M=Cr, Ti). The measurements were made for the
paramagnetic metal (PM), paramagnetic insulator (PI), and antiferromagnetic
insulator (AFI) phases of (V$_{1-x}$M$_x$)$_2$O$_3$ with the samples of $x$ =
0, 0.012, and 0.028 for Cr-doping and $x$ = 0.01 for Ti-doping. In the PM
phase, we observe a prominent quasiparticle peak in general agreement with
theory, which combines dynamical mean-field theory with the local density
approximation (LDA+DMFT). The quasiparticle peak shows a significantly larger
peak width and weight than in the theory. For both the PI and AFI phases, the
vanadium 3d parts of the valence spectra are not simple one peak structures.
For the PI phase, there is not yet a good theoretical understanding of these
structures. The size of the electron removal gap increases, and spectral weight
accumulates in the energy range closer to the chemical potential, when the PI
to AFI transition occurs. Spectra taken in the same phases with different
compositions show interesting monotonic changes as the dopant concentration
increases, regardless of the dopant species. With increased Cr-doping, the AFI
phase gap decreases and the PI phase gap increases.",0608380v1
2006-10-24,"Synthesis, Structure, and Ferromagnetism of a New Oxygen Defect Pyrochlore System Lu2V2O_{7-x} (x = 0.40-0.65)","A new fcc oxygen defect pyrochlore structure system Lu2V2O_{7-x} with x =
0.40 to 0.65 was synthesized from the known fcc ferromagnetic semiconductor
pyrochlore compound Lu2V2O7 which can be written as Lu2V2O6O' with two
inequivalent oxygen sites O and O'. Rietveld x-ray diffraction refinements
showed significant Lu-V antisite disorder for x >= 0.5. The lattice parameter
versus x (including x = 0) shows a distinct maximum at x ~ 0.4. We propose that
these observations can be explained if the oxygen defects are on the O'
sublattice of the structure. The magnetic susceptibility versus temperature
exhibits Curie-Weiss behavior above 150 K for all x, with a Curie constant C
that increases with x as expected in an ionic model. However, the magnetization
measurements also show that the (ferromagnetic) Weiss temperature theta and the
ferromagnetic ordering temperature T_C both strongly decrease with increasing x
instead of increasing as expected from C(x). The T_C decreases from 73 K for x
= 0 to 21 K for x = 0.65. Furthermore, the saturation moment at a field of 5.5
T at 5 K is nearly independent of x, with the value expected for a fixed spin
1/2 per V. The latter three observations suggest that Lu2V2O_{7-x} may contain
localized spin 1/2 vanadium moments in a metallic background that is induced by
oxygen defect doping, instead of being a semiconductor as suggested by the C(x)
dependence.",0610680v1
2007-11-26,Quantum fluctuations in high field magnetization of 2D square lattice J1-J2 antiferromagnets,"The J1-J2 square lattice Heisenberg model with spin S=1/2 has three phases
with long-range magnetic order and two unconventionally ordered phases
depending on the ratio of exchange constants. It describes a number of recently
found layered vanadium oxide compounds. A simple means of investigating the
ground state is the study of the magnetization curve and high-field
susceptibility. We discuss these quantities by using the spin-wave theory and
the exact diagonalization in the whole J1-J2 plane. We compare both results and
find good overall agreement in the sectors of the phase diagram with magnetic
order. Close to the disordered regions the magnetization curve shows strong
deviations from the classical linear behaviour caused by large quantum
fluctuations and spin-wave approximation breaks down. On the FM side (J1<0)
where one approaches the quantum gapless spin nematic ground state this region
is surprisingly large. We find that inclusion of second order spin-wave
corrections does not lead to fundamental improvement. Quantum corrections to
the tilting angle of the ordered moments are also calculated. They may have
both signs, contrary to the always negative first order quantum corrections to
the magnetization. Finally we investigate the effect of the interlayer coupling
and find that the quasi-2D picture remains valid up to |J_\perp/J1| ~ 0.3.",0711.4054v1
2008-01-08,Light Transition Metal Monatomic Chains,"In this paper we investigated structural, electronic and magnetic properties
of 3d (light) transition metal (TM) atomic chains using first-principles
pseudopotential plane wave calculations. Periodic linear, dimerized linear and
planar zigzag chain structures and their short segments consisting of finite
number of atoms have been considered. Like Cu, the periodic, linear chains of
Mn, Co and Ni correspond to a local shallow minimum. However, for most of the
infinite periodic chains, neither linear nor dimerized linear structures are
favored; to lower their energy the chains undergo a structural transformation
to form planar zigzag and dimerized zigzag geometry. Dimerization in both
infinite and finite chains are much stronger than the usual Peierls distortion
and appear to depend on the number of 3d-electrons. As a result of
dimerization, a significant energy lowering occurs which, in turn, influences
the stability and physical properties. Metallic linear chain of Vanadium
becomes half-metallic upon dimerization. Infinite linear chain of Scandium also
becomes half-metallic upon transformation to zigzag structure. An interplay
between the magnetic ground state and atomic as well as electronic structure of
the chain has been revealed. The end effects influence the geometry, energetics
and magnetic ground state of the finite chains. Structure optimization
performed using noncollinear approximation indicates significant differences
from the collinear approximation. Variation of the cohesive energy of infinite
and finite-size chains with respect to the number of 3d-electrons are found to
mimic the bulk behavior pointed out by Friedel. The spin-orbit coupling of
finite chains are found to be negligibly small.",0801.1178v1
2008-04-29,Strong frustration due to competing ferromagnetic and antiferromagnetic interactions: magnetic properties of M(VO)2(PO4)2 (M = Ca and Sr),"We present a detailed investigation of the magnetic properties of complex
vanadium phosphates M(VO)2(PO4)2 (M = Ca, Sr) by means of magnetization,
specific heat, 31P NMR measurements, and band structure calculations.
Experimental data evidence the presence of ferro- and antiferromagnetic
interactions in M(VO)2(PO4)2 resulting in a nearly vanishing Curie-Weiss
temperature \theta_{CW} < 1 K that contrasts with the maximum of magnetic
susceptibility at 3 K. Specific heat and NMR measurements also reveal weak
exchange couplings with the thermodynamic energy scale J_c = 10-15 K.
Additionally, the reduced maximum of the magnetic specific heat indicates
strong frustration of the spin system. Band structure calculations show that
the spin systems of the M(VO)2(PO4)2 compounds are essentially
three-dimensional with the frustration caused by competing ferro- and
antiferromagnetic interactions. Both calcium and strontium compounds undergo
antiferromagnetic long-range ordering at T_N = 1.5 K and 1.9 K, respectively.
The spin model reveals an unusual example of controllable frustration in
three-dimensional magnetic systems.",0804.4667v2
2008-05-22,Intrinsic and non-local Gilbert damping in polycrystalline nickel studied by Ti:Sapphire laser fs spectroscopy,"The use of femtosecond laser pulses generated by a Ti:Sapphire laser system
allows us to gain an insight into the magnetization dynamics on time scales
from sub-picosecond up to 1 ns directly in the time domain. This experimental
technique is used to excite a polycrystalline nickel (Ni) film optically and
probe the dynamics afterwards. Different spin wave modes (the Kittel mode,
perpendicular standing spin-wave modes (PSSW) and dipolar spin-wave modes
(Damon-Eshbach modes)) are identified as the Ni thickness is increased. The
Kittel mode allows determination of the Gilbert damping parameter alpha
extracted from the magnetization relaxation time tau_alpha. The non-local
damping by spin currents emitted into a non-magnetic metallic layer of vanadium
(V), palladium (Pd) and the rare earth dysprosium (Dy) are studied for
wedge-shaped Ni films 1 nm-30 nm. The damping parameter increases from
alpha=0.045 intrinsic for nickel to alpha>0.10 for the heavy materials, such as
Pd and Dy, for the thinnest Ni films below 10 nm thickness. Also, for the
thinnest reference Ni film thickness, an increased magnetic damping below 4 nm
is observed. The origin of this increase is discussed within the framework of
line broadening by locally different precessional frequencies within the laser
spot region.",0805.3495v1
2008-09-10,TiO and VO broad band absorption features in the optical spectrum of the atmosphere of the hot-Jupiter HD209458b,"The presence of titanium oxide (TiO) and vanadium oxide (VO) gas phase
species is searched for in the atmosphere of the hot Jupiter HD209458b. We
compared a model for the planets transmitted spectrum to multi-wavelength
eclipse-depth measurements (from 3000 to 10000 Angstrom), obtained by Sing et
al. (2008a) using archived HST-STIS time series spectra. We make use of these
observations to search for spectral signatures from extra absorbers in the
planet atmosphere between 6000 and 8000 Angstrom. Along with sodium depletion
and Rayleigh scattering recently published for this exoplanet atmosphere, an
extra absorber of uncertain origin, redward of the sodium lines, resides in the
atmosphere of the planet. Furthermore, this planet has a stratosphere
experiencing a thermal inversion caused by the capture of optical stellar flux
by absorbers that resides at altitude. Recent models have predicted that the
presence of TiO and VO in the atmosphere of HD209458b may be responsible for
this temperature inversion. Although no specific TiO and VO spectral band head
signatures have been identified unambiguously in the observed spectrum, we
suggest here that the opacities of those molecules are possible candidates to
explain the remaining continuous broad band absorption observed between 6200
and 8000 Angstrom. To match reasonably well the data, the abundances of TiO and
VO molecules are evaluated from ten to one thousand times below solar. This
upper limit result is in agreement with expected variations with altitude due
to depletion effects such as condensation.",0809.1865v1
2008-11-07,Effective Band Structure of Correlated Materials - The Case of VO2,"Vanadium dioxide, VO2, and its metal-insulator transition at T=340K continues
to receive considerable interest. The question whether the physics of the
insulating low-temperature phase is dominated by the Mott or the Peierls
scenario, i.e. by correlation or band effects, is still under debate. A recent
cluster dynamical mean field theory calculation [Biermann et al, Phys. Rev.
Lett., 94, 026404 (2005)] suggests a combination of both effects,
characterizing the transition as of a correlation assisted Peierls type. In
this paper we present a detailed analysis of the excitation spectrum of the
insulating M1 phase of VO2, based on this calculation. We implement a scheme to
analytically continue self-energies from Matsubara to real frequencies, and
study the physics of the strong interactions, as well as the corresponding
changes with respect to the density functional theory (LDA) band structure. We
find that in the M1 phase life-time effects are rather negligible, indeed
allowing for an effective band structure description. A frequency independent
but orbital dependent potential, constructed as an approximation to the full
cluster dynamical mean field self-energy, turns out to satisfactory reproduce
the fully interacting one-particle spectrum, acting as a scissors operator
which pushes the a1g bonding and egp bands apart and, thus, opening the gap.",0811.1104v1
2009-02-23,Can TiO Explain Thermal Inversions in the Upper Atmospheres of Irradiated Giant Planets?,"Spitzer Space Telescope infrared observations indicate that several
transiting extrasolar giant planets have thermal inversions in their upper
atmospheres. Above a relative minimum, the temperature appears to increase with
altitude. Such an inversion probably requires a species at high altitude that
absorbs a significant amount of incident optical/UV radiation. Some authors
have suggested that the strong optical absorbers titanium oxide (TiO) and
vanadium oxide (VO) could provide the needed additional opacity. However, if
regions of the atmosphere are cold enough for Ti and V to be sequestered into
solids they might rain out and be severely depleted. With a model of the
vertical distribution of a refractory species in gaseous and condensed form, we
address the question of whether enough TiO (or VO) could survive aloft in an
irradiated planet's atmosphere to produce a thermal inversion. We find that,
without significant macroscopic mixing, a heavy species such as TiO --
especially one that can condense in a cold-trap region -- cannot persist in a
planet's upper atmosphere. In our model, the amount of macroscopic mixing that
would be required to loft TiO to millibar pressures ranges from ~10^7 to
~10^{11} cm^2/s on HD 209458b, HD 149026b, TrES-4, OGLE-TR-56b, and WASP-12b,
depending on the planet and on assumed condensate sizes. Such large values may
be problematic for the TiO hypothesis. [Abridged]",0902.3995v2
2010-01-20,Theory of Room Temperature Ferromagnet V(TCNE)_x (1.5 < x < 2): Role of Hidden Flat Bands,"Theoretical studies on the possible origin of room temperature ferromagnetism
(ferromagnetic once crystallized) in the molecular transition metal complex,
V(TCNE)_x (1.5<x<2) have been carried out. For this family, there have been no
definite understanding of crystal structure so far because of sample quality,
though the effective valence of V is known to be close to +2. Proposing a new
crystal structure for the stoichiometric case of x=2, where the valence of each
TCNE molecule is -1 and resistivity shows insulating behavior, exchange
interaction among d-electrons on adjacent V atoms has been estimated based on
the cluster with 3 vanadium atoms and one TCNE molecule. It turns out that
Hund's coupling among d orbitals within the same V atoms and antiferromagnetic
coupling between d oribitals and LUMO of TCNE (bridging V atoms) due to
hybridization result in overall ferromagnetism (to be precise, ferrimagnetism).
This view based on localized electrons is supplemented by the band picture,
which indicates the existence of a flat band expected to lead to ferromagnetism
as well consistent with the localized view. The off-stoichiometric cases (x<2),
which still show ferromagnetism but semiconducting transport properties, have
been analyzed as due to Anderson localization.",1001.3512v2
2011-01-06,Quantum phase transitions in a strongly entangled spin-orbital chain: A field-theoretical approach,"Motivated by recent experiments on quasi-1D vanadium oxides, we study quantum
phase transitions in a one-dimensional spin-orbital model describing a Haldane
chain and a classical Ising chain locally coupled by the relativistic
spin-orbit interaction. By employing a field-theoretical approach, we analyze
the topology of the ground-state phase diagram and identify the nature of the
phase transitions. In the strong coupling limit, a long-range N\'eel order of
entangled spin and orbital angular momentum appears in the ground state. We
find that, depending on the relative scales of the spin and orbital gaps, the
linear chain follows two distinct routes to reach the N\'eel state. First, when
the orbital exchange is the dominating energy scale, a two-stage ordering takes
place in which the magnetic transition is followed by melting of the orbital
Ising order; both transitions belong to the two-dimensional Ising universality
class. In the opposite limit, the low-energy orbital modes undergo a continuous
reordering transition which represents a line of Gaussian critical points. On
this line the orbital degrees of freedom form a Tomonaga-Luttinger liquid. We
argue that the emergence of the Gaussian criticality results from merging of
the two Ising transitions in the strong hybridization region where the
characteristic spin and orbital energy scales become comparable. Finally, we
show that, due to the spin-orbit coupling, an external magnetic field acting on
the spins can induce an orbital Ising transition.",1101.1268v3
2012-01-30,High--field NMR of the quasi--1D antiferromagnet LiCuVO$_4$,"We report on NMR studies of the quasi one--dimensional (1D) antiferromagnetic
$S=1/2$ chain cuprate LiCuVO$_4$ in magnetic fields $H$ up to $\mu_0H$ = 30 T
($\approx 70$% of the saturation field $H_{\rm sat}$). NMR spectra in fields
higher than $H_{\rm c2}$ ($\mu_0H_{\rm c2} \approx 7.5$ T) and temperatures
$T<T_{\rm N}$ can be described within the model of a spin-modulated phase in
which the magnetic moments are aligned parallel to the applied field $H$ and
their values alternate sinusoidally along the magnetic chains. Based on
theoretical concepts about magnetically frustrated 1D chains, the field
dependence of the modulation strength of the magnetic structure is deduced from
our experiments. Relaxation time $T_2$ measurements of the $^{51}$V nuclei show
that $T_2$ depends on the particular position of the probing $^{51}$V nucleus
with respect to the magnetic copper moments within the 1D chains: the largest
$T_2$ value is observed for the vanadium nuclei which are very next to the
magnetic Cu$^{2+}$ ion with largest ordered magnetic moment. This observation
is in agreement with the expectation for the spin-modulated magnetic structure.
The $(H,T)$ magnetic phase diagram of LiCuVO$_4$ is discussed.",1201.6182v1
2012-07-15,"Synthesis and Characterization of TiFe0.7-xMn0.3Vx (x=0.05, and 0.1) and Ti1-yTayFe0.7Mn0.3 (y=0.2, and 0.4) Nanostructured Metal Hydrides for Low Temperature Applications","Metal hydrides (MH) are often preferred to absorb and desorb hydrogen at
ambient temperature and pressure with a high volumetric density. These hydrogen
storage alloys create promising prospects for hydrogen storage and can solve
the energetic and environmental issues. In the present research work, the goal
of our studies is to find the influence of partial substitution of small
amounts of vanadium and tantalum on the hydrogenation properties of
TiFe0.7-xMn0.3Vx (x=0.05, and 0.1) and Ti1-yTayFe0.7Mn0.3 (y=0.2, and 0.4)
alloys, respectively. The nominal compositions of these materials are
TiFe0.65Mn0.3V0.05, TiFe0.6Mn0.3V0.1, Ti0.8Ta0.2Fe0.7Mn0.3, and
Ti0.6Ta0.4Fe0.7Mn0.3. All samples were synthesized by arc-melting high purity
elements under argon atmosphere. The structural and microstructural properties
of the samples were studied by using XRD and SEM, respectively, while the
corresponding microchemistry was determined by obtaining EDS measurements at
specific regions of the samples. Mapping was obtained in order to investigate
atomic distribution in microstructure. Moreover, to ensure the associations
between the properties and structure, all samples were examined by an optical
microscope for accessional characterization. From all these microscopic
examinations a variety of photomicrographs were taken with different
magnifications. The hydrogenation properties were obtained by using a Magnetic
Suspension Balance (Rubotherm). In this equipment, the hydrogen desorption and
re-absorption, can be investigated at constant hydrogen pressures in the range
of 1 to 20 MPa (flow-through mode). At least 3.43 wt. % of absorbed hydrogen
amount was measured while the effect of substitutions was investigated at the
same temperature.",1207.3503v1
2012-09-04,Photoresponse of a strongly correlated material determined by scanning photocurrent microscopy,"The generation of a current by light is a key process in optoelectronic and
photovoltaic devices. In band semiconductors, depletion fields associated with
interfaces separate long-lived photo-induced carriers. However, in systems with
strong electron-electron and electron-phonon correlations it is unclear what
physics will dominate the photoresponse. Here we investigate photocurrent in a
vanadium dioxide, an exemplary strongly correlated material known for its
dramatic metal-insulator transition (MIT) at Tc = 68 C which could be useful
for optoelectronic detection and switching up to ultraviolet wavelengths. Using
scanning photocurrent microscopy (SPCM) on individual suspended VO2 nanobeams
we observe photoresponse peaked at the metal-insulator boundary but extended
throughout both insulating and metallic phases. We determine that the response
is photo-thermal, implying efficient carrier relaxation to a local equilibrium
in a manner consistent with strong correlations. Temperature dependent
measurements reveal subtle phase changes within the insulating state. We
further demonstrate switching of the photocurrent by optical control of the
metal-insulator boundary arrangement. Our work shows the value of SPCM applied
to nanoscale crystals for investigating strongly correlated materials, and the
results are relevant for designing and controlling optoelectronic devices
employing such materials.",1209.0677v2
2012-09-24,Small quantum dots of diluted magnetic III-V semiconductor compound,"In this chapter quantum many body theoretical methods have been used to study
properties of GaAs - and InAs - based, small semiconductor compound quantum
dots (QDs) containing manganese or vanadium atoms. Interest to such systems has
grown since experimental synthesis of nanoscale magnetic semiconductors, that
is, nanoscale semiconductor compounds with enhanced magnetic properties. This
enhancement is achieved by several methods, and in particular by doping common
semiconductor compounds with some atoms, such as Mn or V. Experimental studies
indicate that the electron spin density in the case of thin nanoscale magnetic
semiconductor films and QDs may be delocalized. As described in this chapter,
quantum many body theory-based, computational synthesis (i.e., virtual
synthesis) of tetrahedral symmetry GaAs and InAs small pyramidal QDs doped with
sabstitutional Mn or V atoms proves that such QDs are small magnetic molecules
that indeed, possess delocalized and polarized electron spin density. Such
delocalization provides a physical mechanism responsible for stabilization of
these nanoscale molecular magnets, and leads to the development of what can be
described as spin-polarized holes of the electron charge deficit. In some QDs,
numerical values of the electron spin density distribution are relatively
large, indicating that such semiconductor systems may be used as nanomaterials
for spintronic and magneto-optical sensor applications.",1209.5341v1
2012-10-04,Electron spin resonance and exchange paths in the orthorhombic dimer system Sr2VO4,"We report on magnetization and electron spin resonance (ESR) measurements of
Sr$_{2}$VO$_4$ with orthorhombic symmetry. In this dimer system the $V^{4+}$
ions are in tetrahedral environment and are coupled by an antiferromagnetic
intra-dimer exchange constant $J/k_B \approx$ 100 K to form a singlet ground
state without any phase transitions between room temperature and 2 K. Based on
an extended-H\""{u}ckel-Tight-Binding analysis we identify the strongest
exchange interaction to occur between two inequivalent vanadium sites via two
intermediate oxygen ions. The ESR absorption spectra can be well described by a
single Lorentzian line with an effective g-factor $g$ = 1.89. The temperature
dependence of the ESR intensity is well described by a dimer model in agreement
with the magnetization data. The temperature dependence of the ESR linewidth
can be modeled by a superposition of a linear increase with temperature with a
slope $\alpha$ = 1.35 Oe/K and a thermally activated behavior with an
activation energy $\Delta/k_B$ = 1418 K, both of which point to spin-phonon
coupling as the dominant relaxation mechanism in this compound.",1210.1349v1
2013-01-17,Defect states and spin-orbital physics in doped vanadates Y1-xCaxVO3,"We present a model for typical charged defects in weakly doped Y1-xCaxVO3
perovskites and study how they influence the magnetic and orbital order.
Starting from a multiband Hubbard model, we show that the charge carriers
introduced by doping are bound to the Ca defects with large binding energy of
about 1 eV at small doping, and give rise to the in-gap absorption band
observed in the optical spectroscopy. The central position of a generic Ca
defect with eight equidistant vanadium neighbors implies a partly filled defect
band and permits activated transport due to Coulomb disorder. We explore the
effect of bound charge carriers on the dynamics of the (yz,zx) orbital and spin
degrees of freedom in the context of a spin-orbital t-J model. After deriving
the superexchange interactions around the doped hole, we show that the
transition from G-type to C-type antiferromagnetic (AF) order is triggered by
the kinetic energy of doped holes via the double-exchange mechanism. The defect
states lead to local modification of orbital correlations within ferromagnetic
chains along the c axis; some of them contain hole defects, while the
charge-orbital coupling suppresses locally (yz,zx) orbital fluctuations in the
others. Thereby, Ca defects provide a physical mechanism for spin-orbital
dimerization along the ferromagnetic bonds, suggesting that, in the C-AF phase
of weakly doped Y1-xCaxVO3, dimerization increases with doping.",1301.4026v1
2013-06-03,Electrical switching dynamics and broadband microwave characteristics of VO2 RF devices,"Vanadium dioxide is a correlated electron system that features a
metal-insulator phase transition (MIT) above room temperature and is of
interest in high speed switching devices. Here, we integrate VO2 into
two-terminal coplanar waveguides and demonstrate a large resistance modulation
of the same magnitude (>10^3) in both electrically (i.e. by bias voltage,
referred to as E-MIT) and thermally (T-MIT) driven transitions. We examine
transient switching characteristics of the E-MIT and observe two
distinguishable time scales for switching. We find an abrupt jump in
conductivity with a rise time of the order of 10 ns followed by an oscillatory
damping to steady state on the order of several {\mu}s. We characterize the RF
power response in the On state and find that high RF input power drives VO2
further into the metallic phase, indicating that electromagnetic
radiation-switching of the phase transition may be possible. We measure
S-parameter RF properties up to 13.5 GHz. Insertion loss is markedly flat at
2.95 dB across the frequency range in the On state and sufficient isolation of
over 25 dB is observed in the Off state. We are able to simulate the RF
response accurately using both lumped element and 3D electromagnetic models.
Extrapolation of our results suggests that optimizing device geometry can
reduce insertion loss further and maintain broadband flatness up to 40 GHz.",1306.0292v1
2013-07-23,Orbital superexchange and crystal field simultaneously at play in YVO3: resonant inelastic x-ray scattering at the V L edge and the O K edge,"We report on the observation of orbital excitations in YVO3 by means of
resonant inelastic x-ray scattering (RIXS) at energies across the vanadium L3
and oxygen K absorption edges. Due to the excellent experimental resolution we
are able to resolve the intra-t2g excitations at 0.1-0.2 eV, 1.07 eV, and 1.28
eV, the lowest excitations from the t2g into the eg levels at 1.86 eV, and
further excitations above 2.2 eV. For the intra-t2g excitations at 0.1-0.2 eV,
the RIXS peaks show small shifts of the order of 10-40 meV as a function of
temperature and of about 13-20 meV as a function of the transferred momentum
q||a. We argue that the latter reflects a finite dispersion of the orbital
excitations. For incident energies tuned to the oxygen K edge, RIXS is more
sensitive to intersite excitations. We observe excitations across the
Mott-Hubbard gap and find an additional feature at 0.4 eV which we attribute to
two-orbiton scattering, i.e., an exchange of orbitals between adjacent sites.
Altogether, these results indicate that both superexchange interactions and the
coupling to the lattice are important for a quantitative understanding of the
orbital excitations in YVO3.",1307.6027v1
2013-08-21,Measurement of a solid-state triple point at the metal-insulator transition in VO2,"First-order phase transitions in solids are notoriously challenging to study.
The combination of change in unit cell shape, long range of elastic distortion,
and flow of latent heat leads to large energy barriers resulting in domain
structure, hysteresis, and cracking. The situation is still worse near a triple
point where more than two phases are involved. The famous metal-insulator
transition (MIT) in vanadium dioxide, a popular candidate for ultrafast optical
and electrical switching applications, is a case in point. Even though VO2 is
one of the simplest strongly correlated materials, experimental difficulties
posed by the first-order nature of the MIT as well as the involvement of at
least two competing insulating phases have led to persistent controversy about
its nature. Here, we show that studying single-crystal VO2 nanobeams in a
purpose-built nanomechanical strain apparatus allows investigation of this
prototypical phase transition with unprecedented control and precision. Our
results include the striking finding that the triple point of the metallic and
two insulating phases is at the transition temperature, T_tr = T_c, which we
determine to be 65.0 +- 0.1 C. The findings have profound implications for the
mechanism of the MIT in VO2, but in addition they demonstrate the importance of
such an approach for mastering phase transitions in many other strongly
correlated materials, such as manganites and iron-based superconductors.",1308.4741v1
2013-11-05,Suppression of ferromagnetism in the La(V$_x$Cr$_{1-x}$)Sb$_3$ system,"To explore the possibility of quantum phase transitions and even quantum
criticality in LaCrSb$_3$ based compounds, we performed measurements under
pressure as well as a vanadium substitution study. The Curie temperature of
LaCrSb$_3$ was found to be invariant under pressure. Although pressure was not
able to suppress the ferromagnetism, chemical substitution was used as another
parameter to tune the magnetism. We grew La(V$_x$Cr$_{1-x}$)Sb$_3$ (\textit{x}
= 0 -- 1.0) single crystals, and studied the series by measurements of
temperature and field dependent magnetic susceptibility, magnetization,
resistivity, and specific heat. Ferromagnetism has been observed for $x \leq
0.22$, and the system manifests a strong anisotropy in its ordered state. The
Curie temperature decreases monotonically as the V concentration increases. For
$0.42 \leq x \leq 0.73$, the system enters a new magnetic state at low
temperatures, and no magnetic ordering above 1.8 K can be observed for $x \geq
0.88$. The effective moment $\mu_{\rm eff}$/Cr varies only slightly as the V
concentration increases, from 3.9 $\mu_{\rm B}$ for $x$ = 0 to 2.9 $\mu_{\rm
B}$ for $x$ = 0.88. Features related to quantum criticality have not been
observed in the La(V$_x$Cr$_{1-x}$)Sb$_3$ system.",1311.1186v1
2013-12-26,A Novel High-Pressure Monoclinic Metallic Phase of V2O3,"Vanadium sesquioxide, V2O3, is a prototypical metal-to-insulator system
where, in temperature-dependent studies, the transition always coincides with a
corundum-to-monoclinic structural transition. As a function of pressure, V2O3
follows the expected behavior of increased metallicity due to a larger
bandwidth for pressures up to 12.5 GPa. Surprisingly, for higher pressures when
the structure becomes unstable, the resistance starts to increase. Around 32.5
GPa at 300 K, we observe a novel pressure-induced corundum-to-monoclinic
transition between two metallic phases, showing that the structural phase
transition can be decoupled from the metal-insulator transition. Using X-ray
Raman scattering, we find that screening effects, which are strong in the
corundum phase, become weakened at high pressures. Theoretical calculations
indicate that this can be related to a decrease in coherent quasiparticle
strength, suggesting that the high-pressure phase is likely a critical
correlated metal, on the verge of Mott-insulating behavior.",1312.7063v1
2014-03-20,Electronic Properties of Graphene Encapsulated with Different Two-Dimensional Atomic Crystals,"Hexagonal boron nitride is the only substrate that has so far allowed
graphene devices exhibiting micron-scale ballistic transport. Can other
atomically flat crystals be used as substrates for making quality graphene
heterostructures? Here we report on our search for alternative substrates. The
devices fabricated by encapsulating graphene with molybdenum or tungsten
disulphides and hBN are found to exhibit consistently high carrier mobilities
of about 60,000 cm$^{2}$V$^{-1}$s$^{-1}$. In contrast, encapsulation with
atomically flat layered oxides such as mica, bismuth strontium calcium copper
oxide and vanadium pentoxide results in exceptionally low quality of graphene
devices with mobilities of ~ 1,000 cm$^{2}$ V$^{-1}$s$^{-1}$. We attribute the
difference mainly to self-cleansing that takes place at interfaces between
graphene, hBN and transition metal dichalcogenides. Surface contamination
assembles into large pockets allowing the rest of the interface to become
atomically clean. The cleansing process does not occur for graphene on
atomically flat oxide substrates.",1403.5225v2
2014-06-10,In situ diffraction study of catalytic hydrogenation of VO2: Stable phases and origins of metallicity,"Controlling electronic population through chemical doping is one way to tip
the balance between competing phases in materials with strong electronic
correlations. Vanadium dioxide exhibits a first-order phase transition at
around 338 K between a high temperature, tetragonal, metallic state (T) and a
low temperature, monoclinic, insulating state (M1), driven by electron-electron
and electron-lattice interactions. Intercalation of VO2 with atomic hydrogen
has been demonstrated, with evidence that this doping suppresses the
transition. However, the detailed effects of intercalated H on the crystal and
electronic structure of the resulting hydride have not been previously
reported. Here we present synchrotron and neutron diffraction studies of this
material system, mapping out the structural phase diagram as a function of
temperature and hydrogen content. In addition to the original T and M1 phases,
we find two orthorhombic phases, O1 and O2, which are stabilized at higher
hydrogen content. We present density functional calculations that confirm the
metallicity of these states and discuss the physical basis by which hydrogen
stabilizes conducting phases, in the context of the metal-insulator transition.",1406.2704v1
2014-08-25,Cluster spin glass behavior in geometrically frustrated Zn3V3O8,"We report the bulk magnetic properties of a yet unexplored vanadium-based
multivalued spinel system, Zn3V3O8. A Curie-Weiss fit of our dc magnetic
susceptibility data in the temperature region from 140 to 300 K yields a Curie
constant C = 0.75cm3K/mole V, theta CW = -370 K. We have observed a splitting
between the zero field cooled ZFC and field cooled FC susceptibility curves
below a temperature Tirr of about 6.3 K. The value of the 'frustration
parameter' nearly equals to 100 suggests that the system is strongly
frustrated. From the ac susceptibility measurements we find a logarithmic
variation of freezing temperature (Tf ) with frequency attesting to the
formation of a spin glass below Tf . However, the value of the characteristic
frequency obtained from the Vogel-Fulcher fit suggests that the ground state is
closer to a cluster glass rather than a conventional spin glass. We explored
further consequences of the spin glass behavior and observed aging phenomena
and memory effect (both in ZFC and FC). We found that a positive temperature
cycle erases the memory, as predicted by the hierarchical model. From the heat
capacity CP data, a hump-like anomaly was observed in CP /T at about 3.75 K.
Below this temperature the magnetic heat capacity shows a nearly linear
dependence with T which is consistent with the formation of a spin glass state
below Tf in Zn3V3O8.",1408.5702v1
2014-12-17,Inhomogeneity in the ultrafast insulator-to-metal transition dynamics in VO$_2$,"The insulator-to-metal transition (IMT) of the simple binary compound of
vanadium dioxide VO$_2$ at $\sim 340$ K has been puzzling since its discovery
more than five decades ago. A wide variety of photon and electron probes have
been applied in search of a satisfactory microscopic mechanistic explanation.
However, many of the conclusions drawn have implicitly assumed a {\em
homogeneous} material response. Here, we reveal inherently {\em inhomogeneous}
behavior in the study of the dynamics of individual VO$_2$ micro-crystals using
a combination of femtosecond pump-probe microscopy with nano-IR imaging. The
time scales of the photoinduced bandgap reorganization in the ultrafast IMT
vary from $\simeq 40 \pm 8$ fs, i.e., shorter than a suggested phonon
bottleneck, to $\sim 200\pm20$ fs, with an average value of $80 \pm 25$ fs,
similar to results from previous studies on polycrystalline thin films. The
variation is uncorrelated with crystal size, orientation, transition
temperature, and initial insulating phase. This together with details of the
nano-domain behavior during the thermally-induced IMT suggests a significant
sensitivity to local variations in, e.g., doping, defects, and strain of the
microcrystals. The combination of results points to an electronic mechanism
dominating the photoinduced IMT in VO$_2$, but also highlights the difficulty
of deducing mechanistic information where the intrinsic response in correlated
matter may not yet have been reached.",1412.5495v1
2014-12-24,Probing the magnetic phases in the Ni-V alloy close to the disordered ferromagnetic quantum critical point with muSR,"Zero (ZF) and longitudinal field (LF) muon spin relaxation data of the {\it
d}-metal alloy Ni$_{1-x}$V$_{x}$ are presented at several vanadium
concentrations $x$ below and above the critical $x_c \approx 11$% where
long-range ferromagnetic (FM) order is suppressed. The clear single precession
frequency observed for Ni, as expected for a homogeneous FM, changes to rather
damped oscillation with small V substitution at $x=4$%, confirming magnetic
inhomogeneities caused by the less magnetic V environments in the magnetic Ni
matrix. Furthermore, local fields and spatial field distributions can be
estimated to characterize different inhomogeneous regimes developing with $x$
in the FM phase of Ni$_{1-x}$V$_{x}$. In the regime of $x=7-10$% a Kubo Toyabe
function well describes the low temperature ZF and LF asymmetry data supporting
a static Gaussian field distribution. Closer to the quantum critical
concentration a single scale static Kubo Toyabe function with one field
distribution is not sufficient to describe the muon relaxation. These data
indicate that further changes in spatial distributions and dynamics are
evolving as expected within the critical regime of a disordered quantum
critical point.",1412.7671v1
2015-03-27,HAADF-STEM Study of Mo/V Distributions in Mo-V-Te-Ta-O M1 Phases and Their Correlations with Surface Reactivity,"The MoVTeTaO M1 phases were prepared by conventional hydrothermal (HT) and
microwave-assisted HT synthesis methods (MW) employing two different Ta
precursors, Ta ethoxide and a custom-made Ta oxalate complex. The profile
intensity analysis of the HAADF-STEM image of M1 phases oriented along [hk0]
directions from the surface to bulk region of HAADF-STEM images indicated that
the chemical composition of surface ab planes is very similar to their
composition in the bulk. The HAADF-STEM image analysis showed that synthesis
methods have a significant impact on the Mo/V distribution in the MoVTeTaO M1
phases and their reactivity in propane ammoxidation. Enhanced acrylonitrile
(ACN) yield and 1st order irreversible reaction rate constants for propane
consumption, normalized to the estimated surface ab plane areas, correlated
with increased V content in the proposed catalytic center (S2-S4-S4-S7-S7).
These observations lend further support to the idea that multiple VOx sites
present in the surface ab planes may be responsible for the activity and
selectivity of the M1 phase in propane ammoxidation.",1503.08100v2
2015-06-15,Phase transition in bulk single crystals and thin films of VO2 by nano-infrared spectroscopy and imaging,"We have systematically studied a variety of vanadium dioxide (VO2)
crystalline forms, including bulk single crystals and oriented thin films,
using infrared (IR) near-field spectroscopic imaging techniques. By measuring
the IR spectroscopic responses of electrons and phonons in VO2 with
sub-grain-size spatial resolution (~20 nm), we show that epitaxial strain in
VO2 thin films not only triggers spontaneous local phase separations but also
leads to intermediate electronic and lattice states that are intrinsically
different from those found in bulk. Generalized rules of strain and symmetry
dependent mesoscopic phase inhomogeneity are also discussed. These results set
the stage for a comprehensive understanding of complex energy landscapes that
may not be readily determined by macroscopic approaches.",1506.04786v1
2015-07-15,Structural anomalies and short-range magnetic correlations in the orbitally degenerated system Sr$_2$VO$_4$,"We report on the electronic ground state of a layered perovskite vanadium
oxide Sr$_2$VO$_4$ studied by the combined use of synchrotron radiation x-ray
diffraction (SR-XRD) and muon spin rotation/relaxation ($\mu$SR) techniques,
where $\mu$SR measurements were extended down to 30 mK. We found an
intermediate orthorhombic phase between $T_{\rm c2} \sim$~130 K and $T_{\rm c1}
\sim$~100 K, whereas a tetragonal phase appears for $T > T_{\rm c2}$ and $T <
T_{\rm c1}$. The absence of long-range magnetic order was confirmed by $\mu$SR
at the reentrant tetragonal phase below $T_{\rm c1}$, where the relative
enhancement in the $c$-axis length versus that of the $a$-axis length was
observed. However, no clear indication of the lowering of the tetragonal
lattice symmetry with superlattice modulation, which is expected in the orbital
order state with superstructure of $d_{yz}$ and $d_{zx}$ orbitals, was observed
by SR-XRD below $T_{\rm c1}$. Instead, it was inferred from $\mu$SR that a
magnetic state developed below $T_{\rm c0} \sim$~10 K, which was characterized
by the highly inhomogeneous and fluctuating local magnetic fields down to 30
mK. We argue that the anomalous magnetic ground state below $T_{\rm c0}$
originates from the coexistence of ferromagnetic and antiferromagnetic
correlations.",1507.04139v1
2015-07-16,$\mathbb{Z}_2$ gauge theory for valence bond solids on the kagome lattice,"We present an effective $\mathbb{Z}_2$ gauge theory that captures various
competing phases in spin-1/2 kagome lattice antiferromagnets: the topological
$\mathbb{Z}_2$ spin liquid (SL) phase, and the 12-site and 36-site valence bond
solid (VBS) phases. Our effective theory is a generalization of the recent
$\mathbb{Z}_2$ gauge theory proposed for SL phases by Wan and Tchernyshyov. In
particular, we investigate possible VBS phases that arise from vison
condensations in the SL. In addition to the 12-site and 36-site VBS phases,
there exists 6-site VBS that is closely related to the symmetry-breaking
valence bond modulation patterns observed in the recent density matrix
renormalization group simulations. We find that our results have remarkable
consistency with a previous study using a differnt $\mathbb{Z}_2$ gauge theory.
Motivated by the lattice geometry in the recently reported vanadium oxyfluoride
kagome antiferromagnet, our gauge theory is extended to incorporate lowered
symmetry by inequivalent up- and down-triangles. We investigate effects of this
anisotropy on the 12-site, 36-site, and 6-site VBS phases. The 12-site VBS is
stable to anisotropy while the 36-site VBS undergoes severe dimer melting.
Interestingly, any analogue of the 6-site VBS is not found in this approach. We
discuss the implications of these findings and also compare the results with a
different type of $\mathbb{Z}_2$ gauge theory used in previous studies.",1507.04747v2
2015-11-18,Iron-Group Abundances in the Metal-Poor Main Sequence Turnoff Star HD~84937,"We have derived new very accurate abundances of the Fe-group elements Sc
through Zn (Z = 21-30) in the bright main-sequence turnoff star HD 84937, based
on high-resolution spectra covering the visible and ultraviolet spectral
regions. New or recent laboratory transition data for 14 species of seven
elements have been used. Abundances from more than 600 lines of non-Fe species
have been combined with about 550 Fe lines in HD 84937 to yield abundance
ratios of high precision. The abundances have been determined from both neutral
and ionized transitions, which generally are in agreement with each other. We
find no substantial departures from standard LTE Saha ionization balance in
this [Fe/H] = -2.32 star. Noteworthy among the abundances are: [Co/Fe] = 0.14
and [Cu/Fe] = -0.83, in agreement with past studies abundance trends in this
and other low metallicity stars; and <[Sc,Ti,V/Fe]> = 0.31, which has not been
noted previously. A detailed examination of scandium, titanium, and vanadium
abundances in large-sample spectroscopic surveys reveals that they are
positively correlated in stars with [Fe/H] < -2; HD 84937 lies at the high end
of this correlation. These trends constrain the synthesis mechanisms of
Fe-group elements. We also examine the GCE abundance trends of the Fe-group
elements, including a new nucleosynthesis model with jet-like explosion
effects.",1511.05985v1
2016-02-26,Electronic correlations in monolayer VS$_2$,"The layered transition metal dichalcogenide vanadium disulfide (VS$_2$),
which nominally has one electron in the $3d$ shell, is potent for strong
correlation physics and is possibly another realization of an effective
one-band model beyond the cuprates. Here monolayer VS$_2$ in both the trigonal
prismatic and octahedral phases is investigated using density functional theory
plus Hubbard $U$ (DFT+$U$) calculations. Trigonal prismatic VS$_2$ has an
isolated low-energy band that emerges from a confluence of crystal field
splitting and direct V--V hopping. Within spin density functional theory,
ferromagnetism splits the isolated band of the trigonal prismatic structure,
leading to a low-band-gap $S=1/2$ ferromagnetic Stoner insulator; the
octahedral phase is higher in energy. Including the on-site interaction $U$
increases the band gap, leads to Mott insulating behavior, and for sufficiently
high values stabilizes the ferromagnetic octahedral phase. The validity of DFT
and DFT+$U$ for these two-dimensional materials with potential for strong
electronic correlations is discussed. A clear benchmark is given by examining
the experimentally observed charge density wave (CDW) in octahedral VS$_2$, for
which DFT grossly overestimates the bond length differences compared to known
experiments; the presence of CDWs is also probed for the trigonal prismatic
phase. Finally, we investigate why only the octahedral phase has been observed
in experiments and discuss the possibility of realizing the trigonal prismatic
phase. Our work suggests trigonal prismatic VS$_2$ is a promising candidate for
strongly correlated electron physics that, if realized, could be experimentally
probed in an unprecedented fashion due to its monolayer nature.",1602.08483v1
2016-03-01,"High pressure structural, electronic, and optical properties of polymorphic InVO4 phases","In the present work, we report a detailed density functional theory
calculation on polymorphic InVO$_4$ phases by means of projector augmented wave
method. The computed first-order structural phase transformation from
orthorhombic \emph{(Cmcm)} to monoclinic \emph{(P2/c)} structure is found to
occur around 5.6 GPa along with a large volume collapse of 16.6$\%$, which is
consistent with previously reported experimental data. This transformation also
leads to an increase in the coordination number of vanadium atom from 4 to 6.
The computed equilibrium and high pressure structural properties of both
InVO$_4$ phases, including unit cell parameters, equation of state, and bulk
moduli, are in good agreement with the available experimental data. In
addition, compressibility is found to be highly anisotropic and the
\emph{b}-axis being more compressible than the other for both the structures.
Electronic band structures for both the phases were calculated, and the band
gap for orthorhombic and monoclinic InVO$_4$ are found to be 4.02 and 1.67 eV,
respectively, within the Tran-Blaha Modified Becke-Johnson potential as
implemented in linearized augmented planewave method. We further examined the
optical properties such as dielectric function, refractive index, and
absorption spectra for both the structures. From the implications of these
results, it can be proposed that the high pressure InVO$_4$ phase can be more
useful than orthorhombic phase for photo catalytic applications.",1603.00240v1
2016-09-07,Vertex coloring of graphs via phase dynamics of coupled oscillatory networks,"While Boolean logic has been the backbone of digital information processing,
there are classes of computationally hard problems wherein this conventional
paradigm is fundamentally inefficient. Vertex coloring of graphs, belonging to
the class of combinatorial optimization represents such a problem; and is well
studied for its wide spectrum of applications in data sciences, life sciences,
social sciences and engineering and technology. This motivates alternate, and
more efficient non-Boolean pathways to their solution. Here, we demonstrate a
coupled relaxation oscillator based dynamical system that exploits the
insulator-metal transition in vanadium dioxide (VO2), to efficiently solve the
vertex coloring of graphs. By harnessing the natural analogue between
optimization, pertinent to graph coloring solutions, and energy minimization
processes in highly parallel, interconnected dynamical systems, we harness the
physical manifestation of the latter process to approximate the optimal
coloring of k-partite graphs. We further indicate a fundamental connection
between the eigen properties of a linear dynamical system and the spectral
algorithms that can solve approximate graph coloring. Our work not only
elucidates a physics-based computing approach but also presents tantalizing
opportunities for building customized analog co-processors for solving hard
problems efficiently.",1609.02079v2
2016-10-15,An Alternative Strategy to Control the Electronic Properties of Bilayer Graphene: Semi-metal to Metal Transition and a 2D Material with Dirac Cone,"The AB-stacked bilayer graphene (BLG) is a pure semiconductor whose band gap
and properties can be tuned by various methods such as doping or applying gate
voltage. Here we show an alternative method to control the electronic
properties of BLG; intercalation of transition metal atoms between the two
monolayers graphene (MLG). A theoretical investigation has been performed on
two-dimensional MLG, BLG, BLG-intercalated nanostructured materials, all of
which are energetically stable. Our study reveals that only MLG and
BLG-intercalated with one Vanadium (V) atom (BLG-1V) materials have a Dirac
Cone at the K-point. This study reveals a new strategy to control the material
properties of BLG to exhibit various behaviors, including: metallic,
semi-metallic, and semiconducting by varying the concentration and spin
arrangement of the V atoms in BLG. In all the cases, the present DFT
calculations show that the 2p$_z$ sub-shells of C atoms in graphene and the
3d$_{yz}$ sub-shells of the V atoms provide the electron density near the Fermi
level controlling the material properties of the BLG-intercalated materials.
Thus we prove that out-of-plane atoms can influence in plane electronic
densities in BLG.",1610.04777v2
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
2016-10-22,Sensitivity of stellar electron-capture rates to parent neutron number: A case study on a continuous chain of twenty Vanadium isotopes,"Gamow-Teller (GT) strength distributions (B(GT)) in electron-capture (EC)
daughters stemming from the parent ground state are computed with the
shell-model in the full pf-shell space, with quasi-particle random-phase
approximation (QRPA) in the formalism of Krumlinde and M\""oller and with an
Approximate Method (AM) for assigning an effective B(GT). These are compared to
data available from decay and charge-exchange (CE) experiments across titanium
isotopes in the pf-shell from A=43 to A=62, the largest set available for any
chain of isotopes in the pf-shell. The present study is the first to examine
B(GT) and the associated EC rates across a particular chain of isotopes with
the purpose of examining rate sensitivities as neutron number increases. EC
rates are also computed for a wide variety of stellar electron densities and
temperatures providing concise estimates of the relative size of rate
sensitivities for particular astrophysical scenarios. This work underscores the
astrophysical motivation for CE experiments in inverse kinematics for nuclei
away from stability at the luminosities of future Radioactive Ion Beam
Facilities.",1610.06992v1
2016-11-07,Unique catalytic decomposition and reduction of hydrogen peroxide and organic peroxides on magnesia (001) promoted by oxide-metal hybrid nanostructure,"The detection, removal and reduction of hydrogen peroxide is of significant
importance for its increasing application in the areas of environment, food,
electrochemistry and clinical laboratory. Herein the dissociative adsorption
behavior of hydrogen peroxide on ultrathin magnesia (001) films deposited on
transition metal is uncovered for the first time by employing periodic
density-functional theory calculations with van der Waals corrections. The
hydrogen peroxide is dissociated smoothly and reduced to surface hydroxyls on
MgO(001)/TM, and the dissociative adsorption energies of all the considered
fragmentation configurations are substantially negative, demonstrating
dissociation and reduction of hydrogen peroxide on MgO(001)/TM is
thermodynamically favorable. The dissociative adsorption energy decreases
monotonously with increasing film thickness, demonstrating the lower reactivity
of thick oxide films. Moreover, we examined the suitability of several
transition metal slabs (molybdenum, silver, vanadium, tungsten and gold)
combined with magnesia for splitting hydrogen peroxide. The dissociative
adsorption energies enhance when the lattice constants of substrate slabs
increase, indicating the chemisorption strength can be tuned by category of
metal slabs as well as thickness of oxide film. The mechanism of reactivity
enhancement for energetically and dynamically favorable decomposition of
hydrogen peroxide on supported magnesia is elucidated by characterizing the
geometric structures and electronic properties. The fragmentation and reduction
of diethyl peroxide and peroxyacetone are also studied to reveal the catalytic
activity of ultrathin magnesia toward splitting organic peroxides. The results
are wished to provide useful clue for detecting and reducing hydrogen peroxide
and organic peroxides by employing oxide-metal hybrid nanostructure.",1611.01897v2
2017-03-09,Zero-field Edge Magnetoplasmons in a Magnetic Topological Insulator,"Incorporating ferromagnetic dopants, such as chromium or vanadium, into thin
films of the three-dimensional (3D) topological insulator (TI) (Bi,Sb)2Te3 has
recently led to the realisation of the quantum anomalous Hall effect (QAHE), a
unique phase of quantum matter. These materials are of great interest, since
they may support electrical currents that flow without resistance via edge
channels, even at zero magnetic field. To date, the QAHE has been investigated
using low-frequency transport measurements. However, transport requires
contacting the sample and results can be difficult to interpret due to the
presence of parallel conductive paths, via either the bulk or surface, or
because additional non-chiral edge channels may exist. Here, we move beyond
transport measurements by probing the microwave response of a magnetised disk
of Cr-(Bi,Sb)2Te3. We identify features associated with chiral edge
magnetoplasmons (EMPs), a signature that robust edge-channels are indeed
intrinsic to this material system. Our results provide a measure of the
velocity of edge excitations without contacting the sample, and pave the way
for a new, on-chip circuit element of practical importance: the TI, zero-field
microwave circulator.",1703.03122v1
2017-03-28,Metallic vanadium disulfide nanosheets as a platform material for multifunctional electrode applications,"Nano-thick metallic transition metal dichalcogenides such as VS$_{2}$ are
essential building blocks for constructing next-generation electronic and
energy-storage applications, as well as for exploring unique physical issues
associated with the dimensionality effect. However, such 2D layered materials
have yet to be achieved through either mechanical exfoliation or bottom-up
synthesis. Herein, we report a facile chemical vapor deposition route for
direct production of crystalline VS$_{2}$ nanosheets with sub-10 nm thicknesses
and domain sizes of tens of micrometers. The obtained nanosheets feature
spontaneous superlattice periodicities and excellent electrical conductivities
(~3$\times$10$^{3}$ S cm$^{-1}$), which has enabled a variety of applications
such as contact electrodes for monolayer MoS$_{2}$ with contact resistances of
~1/4 to that of Ni/Au metals, and as supercapacitor electrodes in aqueous
electrolytes showing specific capacitances as high as 8.6$\times$10$^{2}$ F
g$^{-1}$. This work provides fresh insights into the delicate
structure-property relationship and the broad application prospects of such
metallic 2D materials.",1703.09582v1
2017-04-20,Hydrogenating VO2 with protons in acid solution,"Hydrogenation is an effective way to tune material property1-5. Traditional
techniques for doping hydrogen atoms into solid materials are very costly due
to the need for noble metal catalysis and high-temperature/pressure annealing
treatment or even high energy proton implantation in vacuum condition5-8. Acid
solution contains plenty of freely-wandering protons, but it is difficult to
act as a proton source for doping, since the protons always cause corrosions by
destroying solid lattices before residing into them. Here we achieve a facile
way to hydrogenate monoclinic vanadium dioxide (VO2) with protons in acid
solution by attaching suitable metal to it. Considering the Schottky contact at
the metal/VO2 interface, electrons flow from metal to VO2 due to workfunction
difference and simultaneously attract free protons in acid solution to
penetrate, forming the hydrogens dopants inside VO2 lattice. This metal-acid
treatment constitutes an electron-proton co-doping strategy, which not only
protects the VO2 lattice from corrosion, but also causes pronounced
insulator-to-metal transitions. In addition, the metal-acid induced hydrogen
doping behavior shows a ripple effect, and it can spread contagiously up to
wafer-size area (>2 inch) even triggered by a tiny metal particle attachment
(~1.0mm). This will stimulate a new way of simple and cost-effective atomic
doping technique for some other oxide materials.",1704.06220v2
2017-04-28,"Dirac Cone in two dimensional bilayer graphene by intercalation with V, Nb, and Ta transition metals","Bilayer graphene (BLG) is semiconductor whose band gap and properties can be
tuned by various methods such as doping or applying gate voltage. Here, we show
how to tune electronic properties of BLG by intercalation of transition metal
(TM) atoms between two monolayer graphene (MLG) using a novel
dispersion-corrected first-principle density functional theory approach. We
intercalated V, Nb, and Ta atoms between two MLG. We found that the symmetry,
the spin, and the concentration of TM atoms in BLG-intercalated materials are
the important parameters to control and to obtain a Dirac Cone in their band
structures. Our study reveals that the BLG intercalated with one Vanadium (V)
atom, BLG-1V, has a Dirac Cone at the K-point. In all the cases, the present
DFT calculations show that the 2$p_z$ sub-shells of C atoms in graphene and the
3$d_{yz}$ sub-shells of the TM atoms provide the electron density near the
Fermi level E$\mathrm{_F}$ which controls the material properties. Thus, we
show that out-of-plane atoms can influence in-plane electronic densities in
BLG, and enumerate the conditions necessary to control the Dirac point. This
study presents a new strategy for controlling the material properties of BLG so
that they exhibit various behaviors, including: metal, semi-metal, and
semiconductor by varying the concentration and spin arrangement of the TM atoms
in BLG while offering insight into the physical properties of 2D
BLG-intercalated materials.",1704.08800v4
2017-05-11,Role of 1-D finite size Heisenberg chain in increasing metal to insulator transition temperature in hole rich VO2,"VO2 samples are grown with different oxygen concentrations leading to
different monoclinic, M1 and triclinic, T insulating phases which undergo a
first order metal to insulator transition (MIT) followed by a structural phase
transition (SPT) to rutile tetragonal phase. The metal insulator transition
temperature (Tc) was found to be increased with increasing native defects.
Vanadium vacancy (VV) is envisaged to create local strains in the lattice which
prevents twisting of the V-V dimers promoting metastable monoclinic, M2 and T
phases at intermediate temperatures. It is argued that MIT is driven by strong
electronic correlation. The low temperature insulating phase can be considered
as a collection of one-dimensional (1-D) half-filled band, which undergoes Mott
transition to 1-D infinitely long Heisenberg spin 1/2 chains leading to
structural distortion due to spin-phonon coupling. Presence of VV creates
localized holes (d0) in the nearest neighbor, thereby fragmenting the spin 1/2
chains at nanoscale, which in turn increase the Tc value more than that of an
infinitely long one. The Tc value scales inversely with the average size of
fragmented Heisenberg spin 1/2 chains following a critical exponent of 2/3,
which is exactly the same predicted theoretically for Heisenberg spin 1/2 chain
at nanoscale undergoing SPT (spin-Peierls transition). Thus, the observation of
MIT and SPT at the same time in VO2 can be explained from our phenomenological
model of reduced 1-D Heisenberg spin 1/2 chains. The reported increase
(decrease) in Tc value of VO2 by doping with metal having valency less (more)
than four, can also be understood easily with our unified model, for the first
time, considering finite size scaling of Heisenberg chains.",1705.04012v1
2017-05-11,Nature of the spin liquid ground state in a breathing kagome compound studied by NMR and series expansion,"In the vanadium oxyfluoride compound
(NH$_4$)$_2$[C$_7$H$_{14}$N][V$_7$O$_6$F$_{18}$] (DQVOF), the V$^{4+}$ (3d$^1$,
$S=1/2$) ions realize a unique, highly frustrated breathing kagome lattice
composed of alternately-sized, corner-sharing equilateral triangles. Here we
present an $^{17}$O NMR study of DQVOF, which isolates the local susceptibility
of the breathing kagome network. By a fit to series expansion we extract the
ratio of the interactions within the breathing kagome plane, $J_\triangledown /
J_\vartriangle = 0.55(4)$, and the mean antiferromagnetic interaction
$\bar{J}=60(7)$~K. Spin lattice, $T_1$, measurements reveal an essentially
gapless excitation spectrum with a maximum gap $\Delta / \bar{J}=0.007(7)$. Our
study provides new impetus for further theoretical investigations in order to
establish whether the gapless spin liquid behavior displayed by DQVOF is
intrinsic to its breathing kagome lattice or whether it is due to perturbations
to this model, such as a residual coupling of the V$^{4+}$ ions in the
breathing kagome planes to the interlayer V$^{3+}$ ($S=1$) spins.",1705.04177v1
2017-06-25,Frustration-driven C4 symmetric orders in a hetero-structured iron-based superconductor,"A subtle balance between competing interactions in strongly correlated
systems can be easily tipped by additional interfacial interactions in a
heterostructure. This often induces exotic phases with unprecedented
properties, as recently exemplified by high-Tc superconductivity in FeSe
monolayer on the nonmagnetic SrTiO3. When the proximity-coupled layer is
magnetically active, even richer phase diagrams are expected in iron-based
superconductors (FeSCs), which however has not been explored due to the lack of
a proper material system. One promising candidate is Sr2VO3FeAs, a
naturally-assembled heterostructure of a FeSC and a Mott-insulating vanadium
oxide. Here, using high-quality single crystals and high-accuracy 75As and 51V
nuclear magnetic resonance (NMR) measurements, we show that a novel electronic
phase is emerging in the FeAs layer below T0 ~ 155 K without either static
magnetism or a crystal symmetry change, which has never been observed in other
FeSCs. We find that frustration of the otherwise dominant Fe stripe and V Neel
fluctuations via interfacial coupling induces a charge/orbital order with
C4-symmetry in the FeAs layers, while suppressing the Neel antiferromagnetism
in the SrVO3 layers. These findings demonstrate that the magnetic proximity
coupling is effective to stabilize a hidden order in FeSCs and, more generally,
in strongly correlated heterostructures.",1706.08157v1
2017-08-14,High-Kinetic Inductance Additive Manufactured Superconducting Microwave Cavity,"Investigations into the microwave surface impedance of superconducting
resonators have led to the development of single photon counters that rely on
kinetic inductance for their operation. While concurrent progress in additive
manufacturing, `3D printing', opens up a previously inaccessible design space
for waveguide resonators. In this manuscript, we present results from the first
synthesis of these two technologies in a titanium, aluminum, vanadium
(Ti-6Al-4V) superconducting radio frequency resonator which exploits a design
unattainable through conventional fabrication means. We find that Ti-6Al-4V has
two distinct superconducting transition temperatures observable in heat
capacity measurements. The higher transition temperature is in agreement with
DC resistance measurements. While the lower transition temperature, not
previously known in literature, is consistent with the observed temperature
dependence of the superconducting microwave surface impedance. From the surface
reactance, we extract a London penetration depth of $8\pm3{\mu}$m - roughly an
order of magnitude larger than other titanium alloys and several orders of
magnitude larger than other conventional elemental superconductors. This large
London penetration depth suggests that Ti-6Al-4V may be a suitable material for
high kinetic inductance applications such as single photon counting or
parametric amplification used in quantum computing.",1708.04273v1
2017-07-29,Optimizing Thermochromism of Solution-Processed VO$_2$ Nanocomposite Films for Chromogenic Fenestration,"Vanadium (IV) oxide is one of the most promising materials for thermochromic
films due to its unique, reversible crystal phase transition from monoclinic
(M1) to rutile (R) at its critical temperature (T$_c$) which corresponds to a
change in optical properties: above T$_c$, VO$_2$ films exhibit a decreased
transmittance for wavelengths of light in the near-infrared region. However, a
high transmittance modulation often sacrifices luminous transmittance which is
necessary for commercial and residential applications of this technology. In
this study, we explore the potential for synthesis of VO$_2$ films in a matrix
of metal oxide nanocrystals, using In$_2$O$_3$, TiO$_2$, and ZnO as diluents.
We seek to optimize the annealing conditions to yield desirable optical
properties. Although the films diluted with TiO$_2$ and ZnO failed to show
transmittance modulation, those diluted with In$_2$O$_3$ exhibited strong
thermochromism. Our investigation introduces a novel window film consisting of
a 0.93 metal ionic molar ratio VO$_2$-In$_2$O$_3$ nanocrystalline matrix,
demonstrating a significant increase in luminous transmittance without any
measurable impact on thermochromic character. Furthermore, solution-processing
mitigates costs, allowing this film to be synthesized 4x-7x cheaper than
industry standards. This study represents a crucial development in film
chemistry and paves the way for further application of VO$_2$ nanocomposite
films in chromogenic fenestration.",1709.06944v1
2018-01-24,Ultrafast orbital manipulation and Mott physics in multi-band correlated materials,"Multiorbital correlated materials are often on the verge of multiple
electronic phases (metallic, insulating, super- conducting, charge and
orbitally ordered), which can be explored and controlled by small changes of
the external parameters. The use of ultrashort light pulses as a mean to
transiently modify the band population is leading to fundamentally new results.
In this paper we will review recent advances in the field and we will discuss
the pos- sibility of manipulating the orbital polarization in correlated
multi-band solid state systems. This technique can provide new understanding of
the ground state properties of many interesting classes of quantum materials
and offers a new tool to induce transient emergent properties with no
counterpart at equilibrium. We will address: the discovery of high-energy
Mottness in superconducting copper oxides and its impact on our understanding
of the cuprate phase diagram; the instability of the Mott insulating phase in
photoexcited vanadium oxides; the manipulation of orbital-selective
correlations in iron-based superconductors; the pumping of local electronic
excitons and the consequent transient effective quasiparticle cooling in
alkali-doped fullerides. Finally, we will discuss a novel route to manipulate
the orbital polarization in a a k-resolved fashion.",1801.07957v1
2018-05-21,Reconfigurable Mid-Infrared Hyperbolic Metasurfaces using Phase-Change Materials,"Metasurfaces offer the potential to control light propagation at the
nanoscale for applications in both free-space and surface-confined geometries.
Existing metasurfaces frequently utilize metallic polaritonic elements with
high absorption losses, and/or fixed geometrical designs that serve a single
function. Here we overcome these limitations by demonstrating a reconfigurable
hyperbolic metasurface comprising of a heterostructure of isotopically enriched
hexagonal boron nitride (hBN) in direct contact with the phase-change material
(PCM) vanadium dioxide (VO2). Spatially localized metallic and dielectric
domains in VO2 change the wavelength of the hyperbolic phonon polaritons
(HPhPs) supported in hBN by a factor 1.6 at 1450cm-1. This induces in-plane
launching, refraction and reflection of HPhPs in the hBN, proving
reconfigurable control of in-plane HPhP propagation at the nanoscale15. These
results exemplify a generalizable framework based on combining hyperbolic media
and PCMs in order to design optical functionalities such as resonant cavities,
beam steering, waveguiding and focusing with nanometric control.",1805.08292v1
2018-06-18,Room-temperature ferromagnetism in monolayer WSe2 semiconductor via vanadium dopant,"Diluted magnetic semiconductors including Mn-doped GaAs are attractive for
gate-controlled spintronics but Curie transition at room temperature with
long-range ferromagnetic order is still debatable to date. Here, we report the
room-temperature ferromagnetic domains with long-range order in semiconducting
V-doped WSe2 monolayer synthesized by chemical vapor deposition. Ferromagnetic
order is manifested using magnetic force microscopy up to 360K, while retaining
high on/off current ratio of ~105 at 0.1% V-doping concentration. The
V-substitution to W sites keep a V-V separation distance of 5 nm without V-V
aggregation, scrutinized by high-resolution scanning transmission-electron
microscopy, which implies the possibility of the Ruderman-Kittel-Kasuya-Yoshida
interaction (or Zener model) by establishing the long-range ferromagnetic order
in V-doped WSe2 monolayer through free hole carriers. More importantly, the
ferromagnetic order is clearly modulated by applying a back gate. Our findings
open new opportunities for using two-dimensional transition metal
dichalcogenides for future spintronics.",1806.06479v2
2018-08-22,Evidence against anomalous compositions for giants in the Galactic Nuclear Star Cluster,"Very strong Sc I lines have been found recently in cool M giants in the
Nuclear Star Cluster in the Galactic Center. Interpreting these as anomalously
high scandium abundances in the Galactic Center would imply a unique
enhancement signature and chemical evolution history for nuclear star clusters,
and a potential test for models of chemical enrichment in these objects. We
present high resolution K-band spectra (NIRSPEC/Keck II) of cool M giants
situated in the solar neighborhood and compare them with spectra of M giants in
the Nuclear Star Cluster. We clearly identify strong Sc I lines in our solar
neighborhood sample as well as in the Nuclear Star Cluster sample. The strong
Sc I lines in M giants are therefore not unique to stars in the Nuclear Star
Cluster and we argue that the strong lines are a property of the line formation
process that currently escapes accurate theoretical modeling. We further
conclude that for giant stars with effective temperatures below approximately
3800 K these Sc I lines should not be used for deriving the scandium abundances
in any astrophysical environment until we better understand how these lines are
formed. We also discuss the lines of vanadium, titanium, and yttrium identified
in the spectra, which demonstrate a similar striking increase in strength below
3500 K effective temperature.",1808.07489v1
2018-10-08,Metasurface reconfiguration through lithium ion intercalation in a transition metal oxide,"In the latest years the optical engineer's toolbox has welcomed a new
concept, the metasurface. In a metasurface, properly tailored material
inclusions are able to reshape the electromagnetic field of an incident beam.
Change of amplitude, phase and polarization can be addressed within a thickness
of only a fraction of a wavelength. By means of this concept, a radical gain in
compactness of optical components is foreseen, even of the most complex ones;
other unique features like that of analog computing have also been identified.
With this huge potential ready to be disclosed, lack of tunability is still a
main barrier to be broken. Metasurfaces must now be made reconfigurable, i.e.
able to modify and memorize their state, possibly with a small amount of
energy. In this Communication we report low-energy, self-holding metasurface
reconfiguration through lithium intercalation in a vanadium pentoxide layer
integrated within the photonic device. By a proper meta-atom design, operation
on amplitude and phase of linearly polarized light has been demonstrated. In
addition, manipulation of circularly polarized light in the form of tunable
chirality and tunable handedness-preserving reflection has been implemented.
These operations are accomplished using as low as 50 pJ/{\mu}m^{2}, raising
lithium intercalation in transition metal oxides as one of the most energy
efficient self-holding tuning mechanisms known so far for metasurfaces, with
significant perspectives in the whole field of nanophotonics.",1810.03351v1
2019-01-17,Electron-proton Co-doping Induced Metal-insulator Transition in VO2 Film via Surface Self-assembled Ascorbic Acid Molecules,"Charge doping is an effective way to induce metal-insulate transition (MIT)
in correlated materials for many important utilizations, which is however
practically limited by problem of low stability. In this study, we have
achieved pronounced phase modulation and stabilized the metallic state of
monoclinic vanadium dioxide (VO2) at room temperature, via a novel
electron-proton co-doping mechanism driven by surface absorption of
self-assembled L-ascorbic acid (AA) molecules. The ionized AA- species in
solution donate effective electrons to the adsorbed VO2 surface, which then
electrostatically attract surrounding protons to penetrate, and eventually
results in stable hydrogen-doped metallic VO2. The variations of phase and
electronic structures as well as the electron occupancy of V-3d/O-2p hybrid
orbitals were examined by synchrotron characterizations and first-principle
theoretical simulations, which explain the formation of stable metallic state.
Importantly, the adsorbed molecules protect hydrogen dopants from escaping out
of lattice and thereby stabilize the metallic phase for VO2. Such an
electron-proton co-doping mechanism driven by suitable molecules absorption
would open a new door for engineering properties of correlated oxide materials.",1901.05597v2
2019-01-22,Battery selection for optimal grid-outage resilient photovoltaic and battery systems,"The first and most important purpose of the current research work is to
investigate the effects that different battery types have on the optimal
configuration of photovoltaic (PV) and battery systems, from both economic and
resilience perspectives. Many industry reports, as well as research papers,
have already highlighted the crucial role that storage systems have in the
coming years in the electricity sector, especially when combined with renewable
energy systems (RES). Given the high cost of storage technologies, there is an
urgent need for optimizing such integrated energy systems. In this paper, a
simulation-based method is adopted and improved, in order to compare different
battery types based on their characteristics by considering projected trends in
the future. After the introduction of four different battery types, i.e.
lead-acid, sodium sulphur, vanadium redox and lithium-ion, the mathematical
model for the optimization problem is presented, along with the required
explanations. Subsequently, a case study is described and the numerical
assumptions are defined. Therein, our specific focus addresses the different
values that the four battery types possess in three critical parameters, i.e.
battery cost, efficiency and depth of discharge (DoD). Finally, results and
discussion are provided in an illustrative and informative way. This model
provides a useful guide for relevant future work in the area, and also serves
as a baseline for more comprehensive methodologies regarding optimal sizing of
photovoltaic and battery systems.",1901.11389v1
2019-03-08,Local Superconductivity in Vanadium Iron Arsenide,"We investigate the chemical substitution of group 5 into BaFe2As2 (122) iron
arsenide, in the effort to understand why Fe-site hole doping of this compound
(e.g., using group 5 or 6) does not yield bulk superconductivity. We find an
increase in c-lattice parameter of the BaFe2As2 with the substitution of V, Nb,
or Ta; the reduction in c predicts the lack of bulk superconductivity [1] that
is confirmed here through transport and magnetization results. However, our
spectroscopy measurements find a coexistence of antiferromagnetic and local
superconducting nanoscale regions in V-122, observed for the first time in a
transition-metal hole-doped iron arsenide. In BaFe2As2, there is a complex
connection between local parameters such as composition and lattice strain,
average lattice details, and the emergence of bulk quantum states such as
superconductivity and magnetism. [1] L. M. N. Konzen, and A. S. Sefat, J.
Phys.: Condens. Matter 29 (2017), 083001.",1903.03660v3
2019-03-26,Isostructural Mott Transition in 2D honeycomb antiferromagnet V$_{0.9}$PS$_3$,"We present the observation of an isostructural Mott insulator-metal
transition in van-der-Waals honeycomb antiferromagnet V$_{0.9}$PS$_3$ through
high-pressure x-ray diffraction and transport measurements. The MPX$_3$ family
of magnetic van-der-Waals materials (M denotes a first row transition metal and
X either S or Se) are currently the subject of broad and intense attention, but
the vanadium compounds have until this point not been studied beyond their
basic properties. We observe insulating variable-range-hopping type resistivity
in V$_{0.9}$PS$_3$, with a gradual increase in effective dimensionality with
increasing pressure, followed by a transition to a metallic resistivity
temperature dependence between 112 and 124 kbar. The metallic state
additionally shows a low-temperature upturn we tentatively attribute to the
Kondo Effect. A gradual structural distortion is seen between 26-80 kbar, but
no structural change at higher pressures corresponding to the insulator-metal
transition. We conclude that the insulator-metal transition occurs in the
absence of any distortions to the lattice - an isostructural Mott transition in
a new class of two-dimensional material, and in strong contrast to the behavior
of the other MPX$_3$ compounds.",1903.10971v1
2019-05-16,"Phase composition and transformations in magnetron-sputtered (Al,V)2O3 coatings","Coatings of (Al1-xVx)2O3, with x ranging from 0 to 1, were deposited by
pulsed DC reactive sputter deposition on Si(100) at a temperature of 550
{\deg}C. XRD showed three different crystal structures depending on V-metal
fraction in the coating: {\alpha}-V2O3 rhombohedral structure for 100 at.% V, a
defect spinel structure for the intermediate region, 63 - 42 at.% V. At lower
V-content, 18 and 7 at.%, a gamma-alumina-like solid solution was observed,
shifted to larger d-spacing compared to pure {\gamma}-Al2O3. The microstructure
changes from large columnar faceted grains for {\alpha}-V2O3 to smaller
equiaxed grains when lowering the vanadium content toward pure {\gamma}-Al2O3.
Annealing in air resulted in formation of V2O5 crystals on the surface of the
coating after annealing to 500 {\deg}C for 42 at.% V and 700 {\deg}C for 18
at.% V metal fraction respectively. The highest thermal stability was shown for
pure {\gamma}-Al2O3-coating, which transformed to {\alpha}-Al2O3 after
annealing to 1100{\deg} C. Highest hardness was observed for the Al-rich
oxides, ~24 GPa. The latter decreased with increasing V-content, larger than 7
at.% V metal fraction. The measured hardness after annealing in air decreased
in conjunction with the onset of further oxidation of the coatings.",1905.06714v2
2019-07-05,Localized Triggering of the Insulator-Metal Transition in VO2 using a Single Carbon Nanotube,"Vanadium dioxide (VO2) has been widely studied for its rich physics and
potential applications, undergoing a prominent insulator-metal transition (IMT)
near room temperature. The transition mechanism remains highly debated, and
little is known about the IMT at nanoscale dimensions. To shed light on this
problem, here we use ~1 nm wide carbon nanotube (CNT) heaters to trigger the
IMT in VO2. Single metallic CNTs switch the adjacent VO2 at less than half the
voltage and power required by control devices without a CNT, with switching
power as low as ~85 ${\mu}W$ at 300 nm device lengths. We also obtain potential
and temperature maps of devices during operation using Kelvin Probe Microscopy
(KPM) and Scanning Thermal Microscopy (SThM). Comparing these with
three-dimensional electrothermal simulations, we find that the local heating of
the VO2 by the CNT play a key role in the IMT. These results demonstrate the
ability to trigger IMT in VO2 using nanoscale heaters, and highlight the
significance of thermal engineering to improve device behaviour.",1907.02681v1
2019-07-30,Energy Conversion via Metal Nanolayers,"Current approaches for electric power generation from nanoscale conducting or
semi-conducting layers in contact with moving aqueous droplets are promising as
they show efficiencies of around 30 percent, yet, even the most successful ones
pose challenges regarding fabrication and scaling. Here, we report stable,
all-inorganic single-element structures synthesized in a single step that
generate electrical current when alternating salinity gradients flow along its
surface in a liquid flow cell. 10 nm to 30 nm thin nanolayers of iron,
vanadium, or nickel produce several tens of mV and several microA cm^-2 at
aqueous flow velocities of just a few cm s^-1. The principle of operation is
strongly sensitive to charge-carrier motion in the thermal oxide nano-overlayer
that forms spontaneously in air and then self terminates. Indeed, experiments
suggest a role for intra-oxide electron transfer for Fe, V, and Ni nanolayers,
as their thermal oxides contain several metal oxidation states, whereas
controls using Al or Cr nanolayers, which self-terminate with oxides that are
redox inactive under the experimental conditions, exhibit dramatically
diminished performance. The nanolayers are shown to generate electrical current
in various modes of application with moving liquids, including sliding liquid
droplets, salinity gradients in a flowing liquid, and in the oscillatory motion
of a liquid without a salinity gradient.",1907.13170v1
2019-07-20,Temperature Tunable Optical Transmission control of VO2 nanostructures by IR based 1-D Photonic crystals as hybrid Photonic absorbers,"Effect of 1-D photonic crystals on optical transmission of VO2 is studied by
depositing thin films of VO2 nanoparticles on SiO2/TiO2 distributed Bragg
reflectors (DBR) in the near infrared (IR) spectrum as per earlier theoretical
predictions of J. Phys. D: Appl. Phys. 51 375102 (2018). Monoclinic VO2
nanoparticles with tuned crystallinity were synthesized by a facile solution
processing method. Moderately crystalline (MC) and highly crystalline (HC) VO2
nanostructures were obtained by varying its synthesis temperature and post
growth annealing conditions. Both MC VO2 and HC VO2 films exhibit expected
reduction in optical transmission in the IR region due to its structural phase
transition from monoclinic (insulator) to rutile (metallic) around critical
temperature of 68 {\deg}C. By combining VO2 films on a 40% transmitting DBR
structure, the average optical transmission further went down to ~ 20%. Number
of stacks of DBR plays a key role in such effective reduction of optical
transmission in IR. When the number of stacks of DBR is further increased from
4 to 7, optical transmission of metallic VO2 films on DBR nearly vanishes in
NearIR spectrum in such vanadium dioxide/1D photonic crystal based composite
photonic structures. Such temperature controlled, enhanced, broad band optical
response can be a promising design for VO2 nanoparticle based hybrid photonic
absorbers for various smart window applications.",1908.02840v2
2019-08-20,Superconductivity in V$_{1-x}$Zr$_x$ alloys]{Evolution of high field superconductivity and high critical current density in the as-cast V$_{1-x}$Zr$_x$ alloys,"We report here the structural, electrical and magnetic properties of as-cast
V$_{1-x}$Zr$_x$ alloys ($x$ =0 - 0.4) at low temperatures. We observe that all
the alloys undergo successive peritectic and eutectic reactions during cooling
from the melt which leads to the formation of five phases, namely, a body
centred cubic $\beta$-V phase, two phases with slightly different compositions
having face centred cubic ZrV$_2$ structure, a hexagonal closed packed
$\alpha$-Zr phase, and the $\beta$-Zr precipitates. The amount of each phase is
found to be dependent on the concentration of zirconium in vanadium. The
$\beta$-V and ZrV$_2$ phases show superconductivity below 5.3~K and 8.5~K
respectively. We show that the critical current density is large for V-rich
V$_{1-x}$Zr$_x$ alloys with $x >$ 0.1. The grain boundaries generated from the
eutectic reaction, and the point defects formed due to the variation in the
composition are found to be responsible for the pinning of flux lines in low
and high magnetic fields respectively. Our studies reveal that the choice of
the composition and the heat treatment which leads to eutectic reaction are
important in improving the critical current density in this alloy system.",1908.07288v1
2019-09-23,Electronic and Magnetic Characterization of Epitaxial VSe$_2$ Monolayers on Superconducting NbSe$_2$,"Vertical integration of two-dimensional (2D) van der Waals (vdW) materials
with different quantum ground states is predicted to lead to novel electronic
properties that are not found in the constituent layers. Here, we present the
direct synthesis of superconductor-magnet hybrid heterostructures by combining
superconducting niobium diselenide (NbSe$_2$) with the monolayer (ML) vanadium
diselenide (VSe$_2$). More significantly, the in-situ growth in ultra-high
vacuum (UHV) allows to produce a clean and an atomically sharp interfaces.
Combining different characterization techniques and density-functional theory
(DFT) calculations, we investigate the electronic and magnetic properties of
VSe$_2$ on NbSe$_2$. Low temperature scanning tunneling microscopy (STM)
measurements demonstrate a reduction of the superconducting gap on VSe$_2$
layer. This together with the lack of charge density wave signatures indicates
magnetization of the sheet, but not of a conventional itinerant ferromagnet.",1909.10208v3
2019-09-27,Field-Effect Control of Metallic Superconducting Systems,"Despite metals are believed to be insensitive to field-effect and
conventional Bardeen-Cooper-Schrieffer (BCS) theories predict the electric
field to be ineffective on conventional superconductors, a number of gating
experiments showed the possibility of modulating the conductivity of metallic
thin films and the critical temperature of conventional superconductors. All
these experimental features have been explained by simple charge
accumulation/depletion. In 2018, electric field control of supercurrent in
conventional metallic superconductors has been demonstrated in a range of
electric fields where the induced variation of charge carrier concentration in
metals is negligibly small. In fact, no changes of normal state resistance and
superconducting critical temperature were reported. Here, we review the
experimental results obtained in the realization of field-effect metallic
superconducting devices exploiting this unexplained phenomenon. We will start
by presenting the seminal results on superconducting BCS wires and
nano-constriction Josephson junctions (Dayem bridges) made of different
materials, such as titanium, aluminum and vanadium. Then, we show the mastering
of the Josephson supercurrent in superconductor-normal metal-superconductor
proximity transistors suggesting that the presence of induced superconducting
correlations are enough to see this unconventional field-effect. Later, we
present the control of the interference pattern in a superconducting quantum
interference device indicating the coupling of the electric field with
thesuperconducting phase. Among the possible applications of the presented
phenomenology, we conclude this review by proposing some devices that may
represent a breakthrough in superconducting quantum and classical computation.",1909.12721v2
2019-11-01,Shadow epitaxy for in-situ growth of generic semiconductor/superconductor devices,"Uniform, defect-free crystal interfaces and surfaces are crucial ingredients
for realizing high-performance nanoscale devices. A pertinent example is that
advances in gate-tunable and topological superconductivity using
semiconductor/superconductor electronic devices are currently built on the hard
proximity-induced superconducting gap obtained from epitaxial indium
arsenide/aluminium heterostructures. Fabrication of devices requires selective
etch processes; these exist only for InAs/Al hybrids, precluding the use of
other, potentially superior material combinations. We present a crystal growth
platform -- based on three-dimensional structuring of growth substrates --
which enables synthesis of semiconductor nanowire hybrids with in-situ
patterned superconductor shells. This platform eliminates the need for etching,
thereby enabling full freedom in choice of hybrid constituents. We realise and
characterise all the most frequently used architectures in superconducting
hybrid devices, finding increased yield and electrostatic stability compared to
etched devices, along with evidence of ballistic superconductivity. In addition
to aluminium, we present hybrid devices based on tantalum, niobium and
vanadium.
  This is the submitted version of the manuscript. The accepted, peer reviewed
version is available from Advanced Materials:
http://doi.org/10.1002/adma.201908411
  Previous title: Shadow lithography for in-situ growth of generic
semiconductor/superconductor devices",1911.00460v2
2019-12-15,Orbital order and electron itinerancy in CoV$_{2}$O$_{4}$ and Mn$ _{0.5} $Co$ _{0.5} $V$_{2}$O$_{4}$ from first principles,"In view of the recent experimental predictions of a weak structural
transition in CoV$_{2}$O$_{4}$ we explore the possible orbital order states in
its low temperature tetragonal phases from first principles density functional
theory calculations. We observe that the tetragonal phase with I4$_1/amd$
symmetry is associated with an orbital order involving complex orbitals with a
reasonably large orbital moment at Vanadium sites while in the phase with
I4$_1/a$ symmetry, the real orbitals with quenched orbital moment constitute
the orbital order. Further, to study the competition between orbital order and
electron itinerancy we considered Mn$_{0.5}$Co$_{0.5}$V$_{2}$O$_{4}$ as one of
the parent compounds, CoV$_{2}$O$_{4}$, lies near itinerant limit while the
other, MnV$_{2}$O$_{4}$, lies deep inside the orbitally ordered insulating
regime. Orbital order and electron transport have been investigated using first
principles density functional theory and Boltzmann transport theory in
CoV$_{2}$O$_{4}$, MnV$_{2}$O$_{4}$ and Mn$_{0.5}$Co$_{0.5}$V$_{2}$O$_{4}$. Our
results show that as we go from MnV$_{2}$O$_{4}$ to CoV$_{2}$O$_{4}$ there is
enhancement in the electron's itinerancy while the nature of orbital order
remains unchanged.",1912.07051v1
2019-12-27,"Anomalous, anomalous Hall effect in the layered, Kagome, Dirac semimetal KV$_3$Sb$_5$","The electronic anomalous Hall effect (AHE), where charge carriers acquire a
velocity component orthogonal to an applied electric field, is one of the most
fundamental and widely studied phenomena in physics. There are several
different AHE mechanisms known, and material examples are highly sought after,
however in the highly conductive (skew scattering) regime the focus has
centered around ferromagnetic metals. Here we report the observation of a giant
extrinsic AHE in KV$_3$Sb$_5$, an exfoliable, Dirac semimetal with a Kagome
layer of Vanadium atoms. Although there has been no reports of magnetic
ordering down to 0.25 K, the anomalous Hall conductivity (AHC) reaches
$\approx$ 15,507 $\Omega^{-1}$cm$^{-1}$ with an anomalous Hall ratio (AHR) of
$\approx$ 1.8$ \%$; an order of magnitude larger than Fe. Defying expectations
from skew scattering theory, KV$_3$Sb$_5$ shows an enhanced skew scattering
effect that scales quadratically, not linearly, with the longitudinal
conductivity ($\sigma_{xx}$), opening the possibility of reaching an anomalous
Hall angle (AHA) of 90$^{\circ}$ in metals; an effect thought reserved for
quantum anomalous Hall insulators. This observation raises fundamental
questions about the AHE and opens a new frontier for AHE (and correspondingly
SHE) exploration, stimulating investigation in a new direction of materials,
including metallic geometrically frustrated magnets, spin-liquid candidates,
and cluster magnets.",1912.12288v1
2020-01-12,Spatial correlation of the thermally generated electromagnetic field in layered media,"A general formulation for the cross-spectral density tensor enabling
calculation of the spatial correlation of the thermally generated
electromagnetic field in layered media is derived. The formulation is based on
fluctuational electrodynamics, and is thus applicable in the near and far field
of heat sources. The resulting cross-spectral density tensor is written in
terms of a single integration over the parallel wavevector, as the angular
integrations leading to numerical instability are evaluated analytically. Using
this formulation, the spatial correlation length in the near field of a film
made of silicon carbide (SiC) supporting surface phonon-polaritons (SPhPs) in
the infrared is analyzed. It is shown that the spatial correlation length of a
SiC heat source suspended in vacuum decreases substantially by decreasing its
thickness owing to SPhP coupling. In the limit of a 10-nm-thick SiC film, the
spatial correlation length is similar to that of a blackbody. The results also
reveal that it is possible to control the spatial coherence of a thin SiC heat
source via dielectric and metallic substrates, respectively allowing and
preventing SPhP coupling. This suggests that active modulation of thermal
emission via thin films supporting surface polaritons in the infrared is
possible by using a phase change material substrate such as vanadium dioxide.",2001.03999v1
2020-01-28,Evolution of superconductivity and antiferromagnetic order in Ba(Fe$_{0.92-x}$Co$_{0.08}$V$_x$)$_2$As$_2$,"The vanadium doping effects on superconductivity and magnetism of iron
pnictides are investigated in Ba(Fe$_{0.92-x}$Co$_{0.08}$V$_x$)$_2$As$_2$ by
transport, susceptibility and neutron scattering measurements. The doping of
magnetic impurity V causes a fast suppression of superconductivity with T$_c$
reduced at a rate of 7.4~K/1\%V. On the other hand, the long-range commensurate
$C$-type antiferromagnetic order is recovered upon the V doping. The value of
ordered magnetic moments of Ba(Fe$_{0.92-x}$Co$_{0.08}$V$_x$)$_2$As$_2$ follows
a dome-like evolution versus doping concentration x. A possible Griffiths-type
antiferromagnetic region of multiple coexisting phases in the phase diagram of
Ba(Fe$_{0.92-x}$Co$_{0.08}$V$_x$)$_2$As$_2$ is identified, in accordance with
previous theoretical predictions based on a cooperative behavior of the
magnetic impurities and the conduction electrons mediating the
Ruderman-Kittel-Kasuya-Yosida interactions between them.",2001.10151v1
2020-02-18,Evidence of itinerant holes for long-range magnetic order in tungsten diselenide semiconductor with vanadium dopants,"One primary concern in diluted magnetic semiconductors (DMSs) is how to
establish a long-range magnetic order with a low magnetic doping concentration
to maintain the gate tunability of the host semiconductor, as well as to
increase Curie temperature. Two-dimensional van der Waals semiconductors have
been recently investigated to demonstrate the magnetic order in DMSs; however,
a comprehensive understanding of the mechanism responsible for the gate-tunable
long-range magnetic order in DMSs has not been achieved yet. Here, we introduce
a monolayer tungsten diselenide (WSe2) semiconductor with V dopants to
demonstrate the long-range magnetic order through itinerant spin-polarized
holes. The V atoms are sparsely located in the host lattice by substituting W
atoms, which is confirmed by scanning tunneling microscopy and high-resolution
transmission electron microscopy. The V impurity states and the valence band
edge states are overlapped, which is congruent with density functional theory
calculations. The field-effect transistor characteristics reveal the itinerant
holes within the hybridized band; this clearly resembles the Zener model. Our
study gives an insight into the mechanism of the long-range magnetic order in
V-doped WSe2, which can also be used for other magnetically doped
semiconducting transition metal dichalcogenides.",2002.07333v1
2020-03-05,Massively strained VO2 thin film growth on RuO2,"Strain engineering vanadium dioxide thin films is one way to alter this
material's characteristic first order transition from semiconductor to metal.
In this study we extend the exploitable strain regime by utilizing the very
large lattice mismatch of 8.78 % occurring in the VO$_2$/RuO$_2$ system along
the c axis of the rutile structure. We have grown VO$_2$ thin films on single
domain RuO$_2$ islands of two distinct surface orientations by atomic
oxygen-supported reactive MBE. These films were examined by spatially resolved
photoelectron and x-ray absorption spectroscopy, confirming the correct
stoichiometry. Low energy electron diffraction then reveals the VO$_2$ films to
grow indeed fully strained on RuO$_2$(110), exhibiting a previously unreported
($2\times2$) reconstruction. On TiO$_2$(110) substrates, we reproduce this
reconstruction and attribute it to an oxygen-rich termination caused by the
high oxygen chemical potential. On RuO$_2$(100) on the other hand, the films
grow fully relaxed. Hence, the presented growth method allows for simultaneous
access to a remarkable strain window ranging from bulk-like structures to
massively strained regions.",2003.02723v1
2020-01-06,Tunable Casimir equilibria with phase change materials: from quantum trapping to its release,"A stable suspension of nanoscale particles due to the Casimir force is of
great interest for many applications such as sensing, non-contract
nano-machines. However, the suspension properties are difficult to change once
the devices are fabricated. Vanadium dioxide (VO$_2$) is a phase change
material, which undergoes a transition from a low-temperature insulating phase
to a high-temperature metallic phase around a temperature of 340 K. In this
work, we study Casimir forces between a nanoplate (gold or Teflon) and a
layered structure containing a VO$_2$ film. It is found that stable Casimir
suspensions of nanoplates can be realized in a liquid environment, and the
equilibrium distances are determined, not only by the layer thicknesses but
also by the matter phases of VO$_2$. Under proper designs, a switch from
quantum trapping of the gold nanoplate (""on"" state) to its release (""off""
state) as a result of the metal-to-insulator transition of VO$_2$, is revealed.
On the other hand, the quantum trapping and release of a Teflon nanoplate is
found under the insulator-to-metal transition of VO$_2 $. Our findings offer
the possibility of designing switchable devices for applications in micro-and
nano-electromechanical systems.",2003.04100v1
2020-03-14,Orbital localization error of density functional theory in shear properties of vanadium and niobium,"It is believed that the density functional theory (DFT) describes most
elements with s, p and d orbitals very well, except some materials that having
strongly localized and correlated valence electrons. In this work, we find that
the widely employed exchange-correlation (xc) functionals, including LDA, GGA
and meta-GGA, underestimate the shear modulus and phase stability of V and Nb
greatly. The advanced hybrid functional that is usually better for correlated
system, on the other hand, completely fails in these two simple metals. This
striking failure is revealed due to the orbital localization error in GGA,
which is further deteriorated by hybrid functionals. This observation is
corroborated by a similar failure of DFT+U and van der Waals functionals when
applied to V and Nb. To remedy this problem, an semi-empirical approach of
DFT+J is proposed which can delocalize electrons by facilitating the on-site
exchange. Furthermore, it is observed that including density derivatives
slightly improves the performance of the semi-local functionals, with meta-GGA
outperforms GGA, and the latter is better than LDA. This discovery indicates
the possibility and necessity to include higher-order density derivatives
beyond the Laplacian level for the purpose to remove the orbital localization
error (mainly from d orbitals) and delocalization error (mainly from s and p
orbitals) completely in V and Nb, so that to achieve a better description of
their electronic structures. The same strategy can be applied to other d
electron system and f electron system.",2003.06626v1
2020-03-16,Dynamically Reconfigurable Microwave Circuits Leveraging Abrupt Phase-Change Material,"This article proposes a concept for dynamically reconfigurable distributed
microwave circuits by leveraging the abrupt conductivity transition in
phase-change materials (PCM). Metallic surface-inclusions (<<$\lambda$) are
embedded in the PCM film$-$vanadium dioxide (VO2)$-$to provide low-loss and
reconfigurability. To validate this concept, a variety of co-planar waveguide
transmission lines are designed and fabricated with metallic surface-inclusions
in VO2 films, and the lines' performance are characterized up to 50 GHz. The
measured results are used to develop a transmission-line based model and an
equivalent circuit model of the VO2 with surface-inclusions to aid in the rapid
design of new structures. Additionally, an electromagnetic model was developed
and indicates that loss can be close to that of conventional metallic
distributed circuits with 100 to 200 nm thick VO2 films. With these thicker
films, two practical realizations of this concept were designed and simulated:
a tunable dipole from 2.13 to 9.07 GHz and a tunable triple-stub matching
network from 5 to 40 GHz with high $|\Gamma|$. Therefore, the proposed method
appears viable for the realization of arbitrary programmable distributed
circuits and antennas in the microwave and low-millimeter-wave bands.",2003.07264v1
2020-04-09,Magnetic Damping in Epitaxial Fe Alloyed with Vanadium and Aluminum,"To develop low-moment, low-damping metallic ferromagnets for power-efficient
spintronic devices, it is crucial to understand how magnetic relaxation is
impacted by the addition of nonmagnetic elements. Here, we compare magnetic
relaxation in epitaxial Fe films alloyed with light nonmagnetic elements of V
and Al. FeV alloys exhibit lower intrinsic damping compared to pure Fe, reduced
by nearly a factor of 2, whereas damping in FeAl alloys increases with Al
content. Our experimental and computational results indicate that reducing the
density of states at the Fermi level, rather than the average atomic number,
has a more significant impact in lowering damping in Fe alloyed with light
elements. Moreover, FeV is confirmed to exhibit an intrinsic Gilbert damping
parameter of $\simeq$0.001, among the lowest ever reported for ferromagnetic
metals.",2004.04840v3
2020-05-18,Enhanced Infrared Emission by Thermally Switching the Excitation of Magnetic Polariton with Scalable Microstructured VO2 Metasurfaces,"Dynamic radiative cooling attracts fast-increasing interest due to its
adaptability to changing environment and promises for more energy-savings than
the static counterpart. Here we demonstrate enhanced infrared emission by
thermally switching the excitation of magnetic polariton with microstructured
vanadium dioxide (VO2) metasurfaces fabricated via scalable and etch-free
processes. Temperature-dependent infrared spectroscopy clearly shows that the
spectral emittance of fabricated tunable metasurfaces at wavelengths from 2 to
6 um is significantly enhanced when heated beyond its phase transition
temperature, where the magnetic polariton is excited with metallic VO2. The
tunable emittance spectra are also demonstrated to be insensitive to incidence
and polarization angles such that the VO2 metasurface can be treated as a
diffuse infrared emitter. Numerical optical simulation and analytical
inductance-capacitance model elucidate the suppression or excitation of
magnetic polariton with insulating or metallic VO2 upon phase transition. The
effect of enhanced thermal emission with the tunable VO2 metasurface is
experimentally demonstrated with a thermal vacuum test. For the same heating
power of 0.2 W, the steady-state temperature of the tunable VO2 metasurface
emitter after phase transition is found to be 20degC lower than that of a
reference V2O5 emitter whose static spectral emittance is almost the same as
that of the VO2 metasurface before phase transition. The radiative thermal
conductance for the tunable metasurface emitter is found to be 3.96 W/m2K with
metallic VO2 at higher temperatures and 0.68 W/m2K with insulating VO2 at lower
temperatures, clearly demonstrating almost six-fold enhancement in radiative
heat dissipation.",2005.08913v1
2020-06-05,Ultrathin broadband reflective optical limiter,"Optical limiters are nonlinear devices that feature decreasing transmittance
with increasing incident optical intensity, and thus can protect sensitive
components from high-intensity illumination. The ideal optical limiter reflects
rather than absorbs light in its active (""limiting"") state, minimizing risk of
damage to the limiter itself. Previous efforts to realize reflective limiters
were based on embedding nonlinear layers into relatively thick multilayer
photonic structures, resulting in substantial fabrication complexity, reduced
speed and, in some instances, limited working bandwidth. We overcome these
tradeoffs by using the insulator-to-metal transition in vanadium dioxide (VO2)
to achieve intensity-dependent modulation of resonant transmission through
aperture antennas. Due to the dramatic change of optical properties across the
insulator-to-metal transition, low-quality-factor resonators were sufficient to
achieve high on-off ratios in device transmittance. As a result, our ultra-thin
reflective limiter (thickness ~1/100 of the free-space wavelength) is broadband
in terms of operating wavelength (> 2 um at 10 um) and angle of incidence (up
to ~50$\deg$ away from the normal).",2006.03595v3
2020-06-23,3D Smith chart constant quality factor semi-circles contours for positive and negative resistance circuits,"The article proves first that the constant quality factor (Q) contours for
passive circuits, while represented on a 2D Smith chart, form circle arcs on a
coaxal circle family. Furthermore, these circle arcs represent semi-circles
families in the north hemisphere while represented on a 3D Smith chart. On the
contrary we show that the constant Q contours for active circuits with negative
resistance form complementary circle arcs on the same family of coaxal circles
in the exterior of the 2D Smith chart. Also, we find out that these constant Q
contours represent complementary semi-circles in the south hemisphere while
represented on the 3D Smith chart for negative resistance circuits. The
constant Q - computer aided design (CAD) implementation of the Q semi-circles
on the 3D Smith chart is then successfully used to evaluate the quality factor
variations of newly fabricated Vanadium dioxide inductors first, directly from
their reflection coefficient, as the temperature is increased from room
temperature to 50 degrees Celsius ({\deg}C). Thus, a direct multi-parameter
frequency dependent analysis is proposed including Q, inductance and reflection
coefficient for inductors. Then, quality factor direct analysis is used for two
tunnel diode small signal equivalent circuits analysis, allowing for the first
time the Q and input impedance direct analysis on Smith chart representation of
a circuit, including negative resistance",2006.13315v1
2020-07-03,Light-controlled room temperature ferromagnetism in vanadium-doped tungsten diselenide semiconducting monolayers,"Atomically thin transition metal dichalcogenide (TMD) semiconductors hold
enormous potential for modern optoelectronic devices and quantum computing
applications. By inducing long-range ferromagnetism (FM) in these
semiconductors through the introduction of small amounts of a magnetic dopant,
it is possible to extend their potential in emerging spintronic applications.
Here, we demonstrate light-mediated, room temperature (RT) FM, in V-doped WS2
(V-WS2) monolayers. We probe this effect using the principle of magnetic LC
resonance, which employs a soft ferromagnetic Co-based microwire coil driven
near its resonance in the radio frequency (RF) regime. The combination of LC
resonance with an extraordinary giant magneto-impedance effect, renders the
coil highly sensitive to changes in the magnetic flux through its core. We then
place the V-WS2 monolayer at the core of the coil where it is excited with a
laser while its change in magnetic permeability is measured. Notably, the
magnetic permeability of the monolayer is found to depend on the laser
intensity, thus confirming light control of RT magnetism in this
two-dimensional (2D) material. Guided by density functional calculations, we
attribute this phenomenon to the presence of excess holes in the conduction and
valence bands, as well as carriers trapped in the magnetic doping states, which
in turn mediates the magnetization of the V-WS2 monolayer. These findings
provide a unique route to exploit light-controlled ferromagnetism in low
powered 2D spintronic devices capable of operating at RT.",2007.01505v1
2020-07-06,Sub-picosecond all-optical switching in a hybrid VO2:silicon waveguide at 1550 nm,"Achieving ultrafast all-optical switching in a silicon waveguide geometry is
a key milestone on the way to an integrated platform capable of handling the
increasing demands for higher speed and higher capacity for information
transfer. Given the weak electro-optic and thermo-optic effects in silicon,
there has been intense interest in hybrid structures in which that switching
could be accomplished by integrating another material into the waveguide,
including the phase-changing material, vanadium dioxide (VO2). It has long been
known that the phase transition in VO2 can be triggered by ultrafast laser
pulses, and that pump-laser fluence is a critical parameter governing the
recovery time of thin films irradiated by femtosecond laser pulses near 800 nm.
However, thin-film experiments are not a priori reliable guides to using VO2
for all-optical switching in on-chip silicon photonics because of the large
changes in VO2 optical constants in the telecommunications band, the
requirement of low insertion loss, and the limits on switching energy
permissible in integrated photonic systems. Here we report the first
measurements to show that the reversible, ultrafast photo-induced phase
transition in VO2 can be harnessed to achieve sub-picosecond switching when
small VO2 volumes are integrated in a silicon waveguide as a modulating
element. Switching energies above threshold are of order 600 fJ/switch. These
results suggest that VO2 can now be pursued as a strong candidate for
all-optical switching with sub-picosecond on-off times.",2007.02812v1
2020-08-14,Laser Cooling of Transition Metal Atoms,"We propose the application of laser cooling to a number of transition-metal
atoms, allowing numerous bosonic and fermionic atomic gases to be cooled to
ultra-low temperatures. The non-zero electron orbital angular momentum of these
atoms implies that strongly atom-state-dependent light-atom interactions occur
even for light that is far-detuned from atomic transitions. At the same time,
many transition-metal atoms have small magnetic dipole moments in their
low-energy states, reducing the rate of dipolar-relaxation collisions.
Altogether, these features provide compelling opportunities for future
ultracold-atom research. Focusing on the case of atomic titanium, we identify
the metastable $a ^5F_5$ state as supporting a $J \rightarrow J+1$ optical
transition with properties similar to the D2 transition of alkali atoms, and
suited for laser cooling. The high total angular momentum and electron spin of
this state suppresses leakage out of the the nearly closed optical transition
to a branching ratio estimated below $\sim 10^{-5}$. Following the pattern
exemplified by titanium, we identify optical transitions that are suited for
laser cooling of elements in the scandium group (Sc, Y, La), the titanium group
(Ti, Zr), the vanadium group (V, Nb), the manganese group (Mn, Tc), and the
iron group (Fe, Ru).",2008.06147v4
2020-08-14,Metallic Diluted Dimerization in VO2 Tweeds,"The observation of electronic phase separation textures in vanadium dioxide
(VO2), a prototypical electron-correlated oxide, has recently added new
perspectives on the long standing debate about its metal-insulator transition
and its applications. Yet, the lack of atomically-resolved information on
phases accompanying such complex patterns still hinders a comprehensive
understanding of the transition and its implementation in practical devices. In
this work, atomic resolution imaging and spectroscopy unveils the existence of
ferroelastic tweed structures on ~5 nm length scales, well below the resolution
limit of currently employed spectroscopic imaging techniques. Moreover, density
functional theory calculations show that such fine tweeds are formed by a
metallic structure, formed by partially dimerized V-chains, that appears
tetragonal (rutile) on average, but is locally monoclinic. These observations
suggest that the metal/insulator coexistence among low-symmetry forms of VO2 is
driven by lattice degrees of freedom, and provide a multiscale perspective for
the interpretation of existing data, whereby phase coexistence and structural
intermixing can occur all the way down to the atomic scale.",2008.06548v2
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
2020-10-09,"An exploratory view of water quality and sediment characteristics in Musa Estuary, the Persian Gulf","The results of mentioned physicochemical and biological analysis show that in
more polluted sampling points, the number of resistance benthos (Bivalves) are
more than other points of the study area. Heavy metal concentration in sediment
is between 10 to 1000 times more than water samples especially for Arsenic,
Mercury, and Vanadium. Dissolved Oxygen is acceptable all over the study area
(more than 4 mg/L) based on environmental quality standards for dissolved
oxygen which present the range of 2-9 mg/l for sensitive aquatic lives to less
sensitive ones. However the presence of chemicals causes increase in the value
of Chemical Oxygen Demand (around 500 to 1000 mg/L) and consequent decrease in
water purification ability Nutrient uptake capacity which shown as the ratio of
Nitrogen to phosphorus is appropriate (around 6) in parts of Musa estuary with
more than 3 meters depth . The study area is a semi-closed tide-dominant water
body with shallow depth, in comparison to the oceans. A 2-D modelling is
applied to simulate current speed and direction. The results of this modelling
shows except in the creeks, we have relatively suitable circulation of water in
the estuary and current speed may exceed 0.8 m/s. It means Musa estuary is not
a dead zone and in some parts which have enough water circulation and nutrient
uptake for aquatic life, but huge amount of heavy metal (especially Mercury) in
the sediment and water samples, indicates that all aquatic bodies overwhelmed
by dangerous heavy metals.",2010.04669v1
2020-10-20,Pressure-induced huge increase of Curie temperature of the van der Waals ferromagnet VI3,"Evolution of magnetism in single crystals of the van der Waals compound VI3
in external pressure up to 7.3 GPa studied by measuring magnetization and ac
magnetic susceptibility is reported. Four magnetic phase transitions, at T1 =
54.5 K, T2 = 53 K, TC = 49.5 K, and TFM = 26 K, respectively have been observed
at ambient pressure. The first two have been attributed to the onset of
ferromagnetism in specific crystal-surface layers. The bulk ferromagnetism is
characterized by the magnetic ordering transition at Curie temperature TC and
the transition between two different ferromagnetic phases TFM, accompanied by a
structure transition from monoclinic to triclinic symmetry upon cooling. The
pressure effects on magnetic parameters were studied with three independent
techniques. TC was found to be almost unaffected by pressures up to 0.6 GPa
whereas TFM increases rapidly with increasing pressure and reaches TC at a
triple point at ~ 0.85 GPa. At higher pressures, only one magnetic phase
transition is observed moving to higher temperatures with increasing pressure
to reach 99 K at 7.3 GPa. In contrast, the low-temperature bulk magnetization
is dramatically reduced by applying pressure (by more than 50% at 2.5 GPa)
suggesting a possible pressure-induced reduction of vanadium magnetic moment.
We discussed these results in light of recent theoretical studies to analyze
exchange interactions and provide how to increase the Curie temperature of VI3.",2010.10319v1
2020-11-13,CsV$_3$Sb$_5$: a $\mathbb{Z}_2$ topological kagome metal with a superconducting ground state,"Recently discovered alongside its sister compounds KV$_3$Sb$_5$ and
RbV$_3$Sb$_5$, CsV$_3$Sb$_5$ crystallizes with an ideal kagome network of
vanadium and antimonene layers separated by alkali metal ions. This work
presents the electronic properties of CsV$_3$Sb$_5$, demonstrating bulk
superconductivity in single crystals with a T$_{c} = 2.5$K. The normal state
electronic structure is studied via angle-resolved photoemission spectroscopy
(ARPES) and density functional theory (DFT), which categorize CsV$_3$Sb$_5$ as
a $\mathbb{Z}_2$ topological metal. Multiple protected Dirac crossings are
predicted in close proximity to the Fermi level ($E_F$), and signatures of
normal state correlation effects are also suggested by a high temperature
charge density wave-like instability. The implications for the formation of
unconventional superconductivity in this material are discussed.",2011.06745v1
2020-12-10,"Anisotropic proximity-induced superconductivity and edge supercurrent in Kagome metal, K1-xV3Sb5","Materials with transition metals in triangular lattices are of great interest
for their potential combination of strong correlation, exotic magnetism and
electronic topology. Kagome nets are of particular importance since the
discovery of geometrically frustrated magnetism and topological band structures
in crystals like Herbertsmithite and Fe3Sn2, respectively. KV3Sb5 was
discovered to be a layered topological metal with a Kagome net of vanadium.
Here, we fabricated Josephson Junctions (JJ) of K1-xV3Sb5 and induced
superconductivity over long junction lengths. Through magnetoresistance and
current vs. phase measurements, we observed magnetic field sweeping direction
dependent magnetoresistance, and an anisotropic interference pattern with a
Fraunhofer pattern for in-plane magnetic field, but a suppression of critical
current for out-of-plane magnetic field. These results indicate an anisotropic
internal magnetic field in K1-xV3Sb5 which influences the superconducting
coupling in the junction, possibly giving rise to spin-triplet
superconductivity. In addition, the observation of long-lived fast oscillations
shows evidence of spatially localized conducting channels arising from edge
states. These observations pave the way for studying unconventional
superconductivity and Josephson device based on Kagome metals with electron
correlation and topology.",2012.05898v3
2020-12-19,Parallelising Electrocatalytic Nitrogen Fixation Beyond Heterointerfacial Boundary,"The nitrogen (N2) reduction reaction (NRR) is an eco-friendly alternative to
the Haber-Bosch process to produce ammonia (NH3) with high sustainability.
However, the significant magnitude of uphill energies in the multi-step NRR
pathways is a bottleneck of its serial reactions. Herein, the concept of a
parallelized reaction is proposed to actively promote NH3 production via the
NRR using a multi-phase vanadium oxide-nitride (V2O3/VN) hybrid system. Density
functional theory calculations revealed that the V2O3/VN junction parallelizes
NRR pathways by exchanging intermediate N2H4* and NH2* products to avoid
massive uphill energies towards final NH3 generation. Such an exchange is
driven by the difference in coverage of the two species. The impact of the
reaction parallelization strategy for multi-step nitrogen reduction is
demonstrated with the V2O3/VN junction by an increased ammonia yield rate of
18.36 micro mol h^-1 cm^-2 and a Faraday efficiency (FE) of 26.62% at -0.3 V
versus a reversible hydrogen electrode in a 0.1 M aqueous KOH electrolyte, of
which FE value exhibits 16.95 times and 6.22 times higher than that of
single-phase V2O3 and VN, respectively. The introduction of multi-phase
oxides/nitrides into a transition metal-based electrocatalyst can thus be a
promising approach to realizing an alternative method for N2 fixation.",2012.10625v2
2020-12-22,Orbital Rotations induced by Charges of Polarons and Defects in Doped Vanadates,"We explore the competiton of doped holes and defects that leads to the loss
of orbital order in vanadate perovskites. In compounds such as La$_{1-{\sf
x}}$Ca$_{\,\sf x}$VO$_3$ spin and orbital order result from super-exchange
interactions described by an extended three-orbital degenerate Hubbard-Hund
model for the vanadium $t_{2g}$ electrons. Long-range Coulomb potentials of
charged Ca$^{2+}$ defects and $e$-$e$ interactions control the emergence of
defect states inside the Mott gap. The quadrupolar components of the Coulomb
fields of doped holes induce anisotropic orbital rotations of degenerate
orbitals. These rotations modify the spin-orbital polaron clouds and compete
with orbital rotations induced by defects. Both mechanisms lead to a mixing of
orbitals, and cause the suppression of the asymmetry of kinetic energy in the
$C$-type magnetic phase. We find that the gradual decline of orbital order with
doping, a characteristic feature of the vanadates, however, has its origin not
predominantly in the charge carriers, but in the off-diagonal couplings of
orbital rotations induced by the charges of the doped ions.",2012.11919v1
2021-01-12,Room temperature Mott metal-insulator transition in V2O3 compounds induced via strain-engineering,"Vanadium sesquioxide (V2O3) is an archetypal Mott insulator in which the
atomic positions and electron correlations change as temperature, pressure or
doping are varied giving rise to different structural, magnetic or electronic
phase transitions. Remarkably, the isostructural Mott transition in Cr-doped
V2O3 between paramagnetic metallic and insulating phase observed in bulk has
been elusive in thin film compounds so far. Here, via continuous lattice
deformations induced by heteroepitaxy we demonstrate a room temperature Mott
metal-insulator transition in 1.5% Cr-doped and pure V2O3 thin films. By means
of a controlled epitaxial strain, not only the structure but also the intrinsic
electronic and optical properties of the thin films are stabilized at different
intermediate states between the metallic and insulating phases, inaccessible in
bulk materials. This leads to films with unique features such as a colossal
change in room temperature resistivity (DR/R up to 100,000 %) and a broad range
of optical constant values, as consequence of a strain-modulated bandgap. We
propose a new phase diagram for pure and Cr-doped V2O3 thin films with the
engineered in-plane lattice constant as a tuneable parameter. Our results
demonstrate that controlling phase transitions in correlated systems by
epitaxial strain offers a radical new approach to create the next generation of
Mott devices.",2101.04409v1
2021-01-30,Gate control of superconductivity in mesoscopic all-metallic devices,"It was recently demonstrated the possibility to tune, through the application
of a control gate voltage, the superconducting properties of mesoscopic devices
based on Bardeen-Cooper-Schrieffer metals. In spite of the several experimental
evidence obtained on different materials and geometries, a description of the
microscopic mechanism at the basis of such unconventional effect has not been
provided yet. This work discusses the technological potential of gate control
of superconductivity in metallic superconductors and revises the experimental
results which provide information regarding a possible thermal origin of the
effect: in the first place, we review experiments performed on high critical
temperature elemental superconductors (niobium and vanadium) and show how
devices based on these materials can be exploited to realize basic electronic
tools such as, e. g., a half-wave rectifier. In a second part, we discuss the
origin of the gating effect by showing the gate-driven suppression of the
supercurrent in a suspended titanium wire and by providing a comparison between
thermal and electric switching current probability distributions. Furthermore,
we discuss the cold field-emission of electrons from the gate by means of
finite element simulations and compare the results with experimental data.
Finally, the presented data provide a strong indication regarding the
unlikelihood of thermal origin of the gating effect.",2102.00215v2
2021-02-02,Pressure-induced 1T to 3R structural phase transition in metallic VSe2: X-ray diffraction and first-principles theory,"We study pressure-induced structural evolution of vanadium diselenide (VSe2),
a 1T polymorphic member of the transition metal di-chalcogenide (TMD) family
using synchrotron-based powder X-ray diffraction (PXRD) and first-principles
density functional theory (DFT). Our XRD results reveal anomalies at P ~4 GPa
in c/a ratio, V-Se bond length and Se-V-Se bond angle signalling an
isostructural transition. This is followed by a first order structural
transition from 1T (space group P-3m1) phase to a 3R (space group R-3m) phase
at P ~11 GPa due to sliding of adjacent Se-V-Se layers. We present various
scenarios to understand the experimental results within DFT and find that the
1T to 3R transition can be captured only after inclusion of enthalpic
correction associated with errors in cell volume with underestimated transition
pressure. The abrupt increase in the Debye-Waller factors of Se atoms by a
factor of ~4 and hence the anharmonic effects across the structural transition
pressure are hitherto not reported so far and hint a possible way to understand
the mismatch between the experimental and theoretical transition pressure
values.",2102.01560v2
2021-03-24,Effect of Electric Fields on the Director Field and Shape of Nematic Tactoids,"Tactoids are spindle shaped-droplets of a uniaxial nematic phase suspended in
the co-existing isotropic phase. They are found in dispersions of a wide
variety of elongated colloidal particles, including actin, fd virus, carbon
nanotubes, vanadium peroxide and chitin nanocrystals. Recent experiments on
tactoids of chitin nanocrystals in water show that electric fields can very
strongly elongate tactoids even though the dielectric properties of the
co-existing isotropic and nematic phases differ only subtly. We develop a model
for partially bipolar tactoids, where the degree of bipolarness of the director
field is free to adjust to optimize the sum of the elastic, surface and Coulomb
energies of the system. By means of a combination of a scaling analysis and a
numerical study, we investigate the elongation and director field's behavior of
the tactoids as a function of their size, the strength of the electric field,
the surface tension, anchoring strength, the elastic constants and the electric
susceptibility anisotropy. We find that tactoids cannot elongate significantly
due to an external electric field, unless the director field is bipolar or
quasi bipolar and is somehow frozen in the field-free configuration. Presuming
that this is the case, we find reasonable agreement with experimental data.",2103.13000v1
2021-04-29,Double-dome superconductivity under pressure in the V-based Kagome metals AV3Sb5 (A = Rb and K),"We present high-pressure electrical transport measurements on the newly
discovered V-based superconductors $A$V$_3$Sb$_5$ ($A$ = Rb and K), which have
an ideal Kagome lattice of vanadium. Two superconducting domes under pressure
are observed in both compounds, as previously observed in their sister compound
CsV$_3$Sb$_5$. For RbV$_3$Sb$_5$, the $T_c$ increases from 0.93 K at ambient
pressure to the maximum of 4.15 K at 0.38 GPa in the first dome. The second
superconducting dome has the highest $T_c$ of 1.57 K at 28.8 GPa. KV$_3$Sb$_5$
displays a similar double-dome phase diagram, however, its two maximum $T_c$s
are lower, and the $T_c$ drops faster in the second dome than RbV$_3$Sb$_5$. An
integrated temperature-pressure phase diagram of $A$V$_3$Sb$_5$ ($A$ = Cs, Rb
and K) is constructed, showing that the ionic radius of the intercalated
alkali-metal atoms has a significant effect. Our work demonstrates that
double-dome superconductivity under pressure is a common feature of these
V-based Kagome metals.",2104.14487v1
2021-05-05,Phenomenological model for the third-harmonic magnetic response of superconductors: application to Sr$_{2}$RuO$_{4}$,"We employ the phenomenological Lawrence-Doniach model to compute the
contributions of the superconducting fluctuations to the third-harmonic
magnetic response, denoted here by $\overline{M_{3}}$, which can be measured in
a precise way using ac magnetic fields and lock-in techniques. We show that, in
an intermediate temperature regime, this quantity behaves as the third-order
nonlinear susceptibility, which shows a power-law dependence with the reduced
temperature $\epsilon=\frac{T-T_{c}}{T_{c}}$ as $\epsilon^{-5/2}$. Very close
to $T_{c}$, however, $\overline{M_{3}}$ saturates due to the nonzero amplitude
of the ac field. We compare our theoretical results with experimental data for
three conventional superconductors -- lead, niobium, and vanadium -- and for
the unconventional superconductor Sr$_{2}$RuO$_{4}$ (SRO). We find good
agreement between theory and experiment for the elemental superconductors,
although the theoretical values for the critical field systematically deviate
from the experimental ones. In the case of SRO, however, the phenomenological
model completely fails to describe the data, as the third-harmonic response
remains sizable over a much wider reduced temperature range compared to Pb, Nb,
and V. We show that an inhomogeneous distribution of $T_{c}$ can partially
account for this discrepancy, since regions with a locally higher $T_{c}$
contribute to the fluctuation $\overline{M_{3}}$ significantly more than
regions with the ""nominal"" $T_{c}$ of the clean system. However, the
exponential temperature dependence of $\overline{M_{3}}$ first reported in Ref.
[3] is not captured by the model with inhomogeneity. We conclude that, while
inhomogeneity is an important ingredient to understand the superconducting
fluctuations of SRO and other perovskite superconductors, additional effects
may be at play, such as non-Gaussian fluctuations or rare-region effects.",2105.01813v1
2021-05-13,Interlayer coupling and ultrafast hot electron transfer dynamics in metallic VSe2/graphene van der Waals heterostructures,"Atomically thin vanadium diselenide (VSe2 ) is a two-dimensional transition
metal dichalcogenide exhibiting attractive properties due to its metallic
1T-phase. With the recent development of methods to manufacture high-quality
monolayer VSe 2 on van der Waals materials, the outstanding properties of VSe2
-based heterostructures have been widely studied for diverse applications.
Dimensional reduction and interlayer coupling with a van der Waals substrate
lead to its distinguishable characteristics from its bulk counterparts.
However, only a few fundamental studies have investigated the interlayer
coupling effects and hot electron transfer dynamics in VSe2 heterostructures.
In this work, we reveal ultrafast and efficient interlayer hot electron
transfer and interlayer coupling effects in VSe2 /graphene heterostructures.
Femtosecond time-resolved reflectivity measurements showed that hot electrons
in VSe 2 were transferred to graphene within a 100-fs timescale with high
efficiency. Besides, coherent acoustic phonon dynamics indicated interlayer
coupling in VSe2 /graphene heterostructures and efficient thermal energy
transfer to three-dimensional substrates. Our results provide valuable insights
into the intriguing properties of metallic transition metal dichalcogenide
heterostructures and motivate designing optoelectronic and photonic devices
with tailored properties.",2105.06034v1
2021-05-20,Competition of superconductivity and charge density wave in selective oxidized CsV3Sb5 thin flakes,"The recently discovered layered kagome metals AV$_3$Sb$_5$ (A = K, Rb, and
Cs) with vanadium kagome networks provide a novel platform to explore
correlated quantum states intertwined with topological band structures. Here we
report the prominent effect of hole doping on both superconductivity and charge
density wave (CDW) order, achieved by selective oxidation of exfoliated thin
flakes. A superconducting dome is revealed as a function of the effective
doping content. The superconducting transition temperature ($T_{\mathrm{c}}$)
and upper critical field in thin flakes are significantly enhanced compared
with the bulk, which is accompanied by the suppression of CDW. Our detailed
analyses establish the pivotal role of van Hove singularities (VHSs) in
promoting correlated quantum orders in these kagome metals. Our experiment not
only demonstrates the intriguing nature of superconducting and CDW orders, but
also provides a novel route to tune the carrier concentration through both
selective oxidation and electric gating. This establishes AV$_3$Sb$_5$ as a
tunable 2D platform for the further exploration of topology and correlation
among 3$d$ electrons in kagome lattices.",2105.09898v4
2021-06-03,Computational Design of Microarchitected Flow-Through Electrodes for Energy Storage,"Porous flow-through electrodes are used as the core reactive component across
electrochemical technologies. Controlling the fluid flow, species transport,
and reactive environment is critical to attaining high performance. However,
conventional electrode materials like felts and papers provide few
opportunities for precise engineering of the electrode and its microstructure.
To address these limitations, architected electrodes composed of unit cells
with spatially varying geometry determined via computational optimization are
proposed. Resolved simulation is employed to develop a homogenized description
of the constituent unit cells. These effective properties serve as inputs to a
continuum model for the electrode when used in the negative half cell of a
vanadium redox flow battery. Porosity distributions minimizing power loss are
then determined via computational design optimization to generate architected
porosity electrodes. The architected electrodes are compared to bulk, uniform
porosity electrodes and found to lead to increased power efficiency across
operating flow rates and currents. The design methodology is further used to
generate a scaled-up electrode with comparable power efficiency to the
bench-scale systems. The variable porosity architecture and computational
design methodology presented here thus offers a novel pathway for automatically
generating spatially engineered electrode structures with improved power
performance.",2106.01512v1
2021-07-23,No observation of chiral flux current in the topological kagome metal CsV$_{3}$Sb$_{5}$,"Compounds with kagome lattice usually host many exotic quantum states,
including the quantum spin liquid, non-trivial topological Dirac bands and a
strongly renormalized flat band, etc. Recently an interesting vanadium based
kagome family $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, or Cs) was discovered, and these
materials exhibit multiple interesting properties, including unconventional
saddle-point driving charge density wave (CDW) state, superconductivity, etc.
Furthermore, some experiments show anomalous Hall effect which inspires that
there might be some chiral flux current states. Here we report scanning
tunneling measurements by using spin-polarized tips. Although we have observed
clearly the $2a_0\times2a_0$ CDW and $4a_0$ stripe orders, the well-designed
experiments with refined spin-polarized tips do not reveal any trace of the
chiral flux current phase in CsV$_3$Sb$_5$ within the limits of experimental
accuracy. No observation of the local magnetic moment in our experiments may
put an upper bound constraint on the magnitude of magnetic moments induced by
the possible chiral loop current which has a time-reversal symmetry breaking
along $c$-axis in CsV$_{3}$Sb$_{5}$.",2107.11326v2
2021-08-10,Toward nonthermal control of excited quantum materials: framework and investigations by ultrafast electron scattering and imaging,"Quantum material systems upon applying ultrashort laser pulses provide a rich
platform to access excited material phases and their transformations that are
not entirely like their equilibrium counterparts. The addressability and
potential controls of metastable or long-trapped out-of-equilibrium phases have
motivated interests both for the purposes of understanding the nonequilibrium
physics and advancing the quantum technologies. Thus far, the dynamical
spectroscopic probes eminently focus on microscopic electronic and phonon
responses. For characterizing the long-range dynamics, such as order parameter
fields and fluctuation effects, the ultrafast scattering probes offer direct
sensitivity. Bridging the connections between the microscopic dynamics and
macroscopic responses is central toward establishing the nonequilibrium physics
behind the light-induced phases. Here, we present a path toward such
understanding by cross-examining the structure factors associated with
different dynamical states obtained from ultrafast electrons scattering,
imaging, and modeling. We give the basic theoretical framework on describing
the non-equilibrium scattering problems and briefly describe how such framework
relates to the out-of-equilibrium phenomena. We give effective models outlining
the emergences of nonthermal critical points, hidden phases, and
non-equilibrium relaxational responses from vacuum-suspended rare-earth
tritellurides, tantalum disulfides thin films, and vanadium dioxide
nanocrystalline materials upon light excitations.",2108.04860v1
2021-08-21,Evolution of superconductivity and charge order in pressurized RbV$_3$Sb$_5$,"The kagome metals $A$V$_3$Sb$_5$ ($A=$ K, Rb, Cs) under ambient pressure
exhibit an unusual charge order, from which superconductivity emerges. In this
work, by applying hydrostatic pressure using a liquid pressure medium and
carrying out electrical resistance measurements for RbV$_3$Sb$_5$, we find the
charge order becomes suppressed under a modest pressure $p_{\rm c}$
($1.4<p_{\rm c}<1.6$ GPa), while the superconducting transition temperature
$T_{\rm c}$ is maximized. $T_{\rm c}$ is then gradually weakened with further
increase of pressure and reaches a minimum around 14.3 GPa, before exhibiting
another maximum around 22.8 GPa, signifying the presence of a second
superconducting dome. Distinct normal state resistance anomalies are found to
be associated with the second superconducting dome, similar to KV$_3$Sb$_5$.
Our findings point to qualitatively similar temperature-pressure phase diagrams
in KV$_3$Sb$_5$ and RbV$_3$Sb$_5$, and suggest a close link between the second
superconducting dome and the high-pressure resistance anomalies.",2108.09434v2
2021-09-09,A deep learned nanowire segmentation model using synthetic data augmentation,"Automatized object identification and feature analysis of experimental image
data are indispensable for data-driven material science; deep-learning-based
segmentation algorithms have been shown to be a promising technique to achieve
this goal. However, acquiring high-resolution experimental images and assigning
labels in order to train such algorithms is challenging and costly in terms of
both time and labor. In the present work, we apply synthetic images, which
resemble the experimental image data in terms of geometrical and visual
features, to train state-of-art deep learning-based Mask R-CNN algorithms to
segment vanadium pentoxide (V2O5) nanowires, a canonical cathode material,
within optical intensity-based images from spectromicroscopy. The performance
evaluation demonstrates that even though the deep learning model is trained on
pure synthetically generated structures, it can segment real optical
intensity-based spectromicroscopy images of complex V2O5 nanowire structures in
overlapped particle networks, thus providing reliable statistical information.
The model can further be used to segment nanowires in scanning electron
microscopy (SEM) images, which are fundamentally different from the training
dataset known to the model. The proposed methodology of using a purely
synthetic dataset to train the deep learning model can be extended to any
optical intensity-based images of variable particle morphology, extent of
agglomeration, material class, and beyond.",2109.04429v2
2021-09-15,Study of the electronic properties of topological kagome metals YV$_6$Sn$_6$ and GdV$_6$Sn$_6$,"The synthesis and characterization of vanadium-based kagome metals
YV$_6$Sn$_6$ and GdV$_6$Sn$_6$ are presented. X-ray diffraction, magnetization,
magnetotransport, and heat capacity measurements reveal an ideal kagome network
of V-ions coordinated by Sn and separated by triangular lattice planes of
rare-earth ions. The onset of low-temperature, likely noncollinear, magnetic
order of Gd spins is detected in GdV$_6$Sn$_6$, while V-ions in both compounds
remain nonmagnetic. Density functional theory calculations are presented
modeling the band structures of both compounds, which can be classified as
$\mathbb{Z}_2$ topological metals in the paramagnetic state. Both compounds
exhibit high mobility, multiband transport and present an interesting platform
for controlling the interplay between magnetic order associated with the
$R$-site sublattice and nontrivial band topology associated with the V-based
kagome network. Our results invite future exploration of other $R$V$_6$Sn$_6$
($R$=rare earth) variants where this interplay can be tuned via $R$-site
substitution.",2109.07394v2
2021-10-11,Boosting room temperature tunnel magnetoresistance in hybrid magnetic tunnel junctions under electric bias,"Spin-resolved electron symmetry filtering is a key mechanism behind giant
tunneling magnetoresistance (TMR) in Fe/MgO/Fe and similar magnetic tunnel
junctions (MTJs), providing room temperature functionality in modern spin
electronics. However, the core process of the electron symmetry filtering
breaks down under applied bias, dramatically reducing the TMR above 0.5 V. This
strongly hampers the application range of MTJs. To circumvent the problem,
resonant tunneling between ferromagnetic electrodes through quantum well states
in thin layers has been used so far. This mechanism, however, is mainly
effective at low temperatures. Here, a fundamentally different approach is
demonstrated, providing a strong TMR boost under applied bias in
V/MgO/Fe/MgO/Fe/Co hybrids. This pathway uses spin orbit coupling (SOC)
controlled interfacial states in vanadium, which contrary to the V(001) bulk
states are allowed to tunnel to Fe(001) at low biases. The experimentally
observed strong increase of TMR with bias is modelled using two nonlinear
resistances in series, with the low bias conductance of the first (V/MgO/Fe)
element being boosted by the SOC-controlled interfacial states, while the
conductance of the second (Fe/MgO/Fe) junctions controlled by the relative
alignment of the two ferromagnetic layers. These results pave a way to
unexplored and fundamentally different spintronic device schemes, with
tunneling magnetoresistance uplifted under applied electric bias.",2110.05061v1
2021-10-19,A new class of bilayer kagome lattice compounds with Dirac nodal lines and pressure-induced superconductivity,"Kagome lattice composed of transition-metal ions provides a great opportunity
to explore the intertwining between geometry, electronic orders and band
topology. The discovery of multiple competing orders that connect intimately
with the underlying topological band structure in nonmagnetic kagome metals
$A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) further pushes this topic to the quantum
frontier. Here we report the discovery and characterization of a new class of
vanadium-based compounds with kagome bilayers, namely $A$V$_6$Sb$_6$ ($A$ = K,
Rb, Cs) and V$_6$Sb$_4$, which, together with $A$V$_3$Sb$_5$, compose a series
of kagome compounds with a generic chemical formula
($A_{m-1}$Sb$_{2m}$)(V$_3$Sb)$_n$ (m = 1, 2; n = 1, 2). Theoretical
calculations combined with angle-resolved photoemission measurements reveal
that these compounds feature Dirac nodal lines in close vicinity to the Fermi
level. Pressure-induced superconductivity in $A$V$_6$Sb$_6$ further suggests
promising emergent phenomena in these materials. The establishment of a new
family of layered kagome materials paves the way for designer of fascinating
kagome systems with diverse topological nontrivialities and collective ground
states.",2110.09782v2
2021-11-11,Charge Order Stabilized Quantum Spin Liquid in Hollandite K$_2$V$_8$O$_{16}$,"Quantum spin liquid is an elusive state that display strong many-body
entanglement with potential applications in future quantum computing. This
study reports muon spin relaxation ($\mu^+$SR) measurements on a novel
high-pressure synthesized material, the Hollandite K$_{2}$V$_8$O$_{16}$. In
this quasi-one-dimensional compound, charge ordering (CO) at $T_{\rm
MIT}\approx160$~K effectively isolates half of the vanadium chains and
model-like Heisenberg spin-1/2 chains are realized. Our zero field $\mu^+$SR
measurements show exponential like relaxation down to the lowest temperature
$T=100$~mK and the absence of long range ordering is confirmed. The relaxation
rate is found to be temperature independent below $T_{\rm QSL}\approx2$~K and
measurements in longitudinal field confirms a highly dynamic ground state.
These results represents the first confirmation of quantum spin liquid (QSL)
behavior within the Hollandite family, stabilized by the CO. Finally, the
presence of strong local electron correlation and one dimensional Fermi surface
suggest this QSL to be a gapless Tomonaga-Luttinger liquid (TLL), which here
uniquely presents itself in a stoichiometric compound under zero applied
magnetic field and at ambient pressure.",2111.06177v1
2021-12-06,First principles theory for the magnetic and charge instabilities in AV$_3$Sb$_5$ systems,"Vanadium-based materials AV$_3$Sb$_5$ (A=K, Rb, Cs) with layered kagome
lattice structures have drawn great attention recently due to the discoveries
of topologically nontrivial band structures, charge density wave states, giant
anomalous Hall effect, as well as unusual superconducting phase at low
temperatures. In this work, we theoretically study the magnetic and charge
instabilities for this class of materials based on first principles
calculations. We develop a method to calculate the generalized susceptibility
tensor defined in the sublattice-orbital-spin space, with the effects of
Coulomb interactions treated by generalized random phase approximation (RPA).
The RPA susceptibility calculations indicate that there are three leading
ferromagnetic instability modes at $\Gamma$ point, which are further verified
by unrestricted self-consistent Hartree-Fock calculations including both the
on-site and inter-site Coulomb interactions. The inclusion of inter-site
interactions tend to suppress the spin ferromagnetism due to charge transfer
from the V to Sb sites, leading to weak spin magnetic moments $\sim
0.1\mu_{\textrm{B}}$ per V atom with small intrinsic anomalous Hall
conductivity. Current loops can be generated in such weak spin ferromagnetic
states as a result of spin-orbit coupling effects. The electronic structures in
the ferromagnetic states are significantly reconstructed which have nearly
compensated electron and hole carriers from two bands. On the other hand, we do
not find any diverging instability mode at $M$ point driven by
electron-electron Coulomb interactions. First principles phonon calculations
indicate that there are unstable phonon modes which tend to drive the system
into an inverse star-of-David structure. Our results indicate that there may be
separate phase transitions in the magnetic and charge channels in the system.",2112.02808v1
2022-01-13,"Charge density wave order in kagome metal AV$_3$Sb$_5$ (A= Cs, Rb, K)","We employ polarization-resolved electronic Raman spectroscopy and density
functional theory to study the primary and secondary order parameters, as well
as their interplay, in the charge density wave (CDW) state of the kagome metal
AV3Sb5. Previous x-ray diffraction data at 15K established that the CDW order
in CsV3Sb5 comprises of a 2x2x4 structure: one layer of inverse-star-of-David
and three consecutive layers of star-of-David pattern. We analyze the lattice
distortions based the 2x2x4 structure at 15K, and find that U lattice
distortion is the primary order parameter while M and L distortions are
secondary order parameters for Vanadium displacements. This conclusion is
confirmed by the calculation of bare susceptibility that shows a broad peak at
around qz=0.25 along the hexagonal Brillouin zone face central line (U-line).
We also identify several phonon modes emerging in the CDW state, which are
lattice vibration modes related to V and Sb atoms as well as alkali atoms. The
detailed temperature evolution of these modes' frequencies, HWHM, and
integrated intensities support a phase diagram with two successive structural
phase transitions in CsV3Sb5: the first one with a primary order parameter
appearing at TS=94K and the second isostructural one appearing at around 70K.",2201.05188v2
2022-02-02,Sensing enhancement of a Fabry-Perot THz cavity using switchable VO2 mirrors,"We experimentally investigate the sensing properties of an open cavity
operating in the THz regime and realized by employing as mirrors two thin
vanadium dioxide (VO2) films grown on silicon parallel plates and separated by
a variable length. The phase transition of VO2 is used to control the behavior
of the system between two different responses: a high transmission mode to the
incident radiation (VO2 in the insulating state) and a high sensitivity to tiny
changes in the cavity refractive index (VO2 in the conducting state). In the
first state, the low loss regime enables to adjust the cavity length and easily
optimize the resonances due to the Fabry-Perot (FP) effect in the Si plates and
in the cavity volume. The activation of the metallic-like state instead, by
damping the FP oscillations in the plates, promotes the onset of a comb-like
spectrum that can be exploited as a versatile tool for accurate sensing
applications. Using both an analytical model and full-wave simulations, we
estimate the device response to variation in the refractive index of the cavity
volume, showing that the proposed structure can achieve sensitivity values
among the highest reported for THz sensors.",2202.00921v1
2022-02-09,"Be, V and Cu in the halo star CS 31082-001 from near-UV spectroscopy","The 'First Stars' programme revealed the metal-poor halo star CS 31082-001 to
be r-process and actinide rich, including the first measurement of a uranium
abundance for an old star. To better characterise and understand such rare
objects, we present the first abundance estimates of three elements (Be, V, Cu)
for CS 31082-001 from analysis of its near-ultraviolet spectrum. Beryllium is
rarely measured in giant stars, and we confirm that its abundance in this star
is low due to the rather cool effective temperature that causes destruction of
both Be and Li in its atmosphere. Vanadium and copper are iron-peak elements
that are starting to be used as chemical-tagging indicators to investigate the
origin of stellar populations. We find V and Cu abundances for CS 31082-001
that are comparable to other metal-poor stars, and present new chemical
evolution models to investigate our results. In the case of V, extra
nucleosynthesis due to interaction of neutrinos with matter is included in the
models to be able to reproduce the measured abundance. Given the availability
of high-quality spectroscopy of CS 31082-001, we also explore other atomic
lines in the near-ultraviolet as a template for future studies of metal-poor
stars with the planned CUBES instrument in development for the Very Large
Telescope.",2202.04450v1
2022-02-26,Topology optimization of 3D flow fields for flow batteries,"As power generated from renewables becomes more readily available, the need
for power-efficient energy storage devices, such as redox flow batteries,
becomes critical for successful integration of renewables into the electrical
grid. An important component in a redox flow battery is the planar flow field,
which is usually composed of two-dimensional channels etched into a backing
plate. As reactant-laden electrolyte flows into the flow battery, the channels
in the flow field distribute the fluid throughout the reactive porous
electrode. We utilize topology optimization to design flow fields with full
three-dimensional geometry variation, i.e., 3D flow fields. Specifically, we
focus on vanadium redox flow batteries and use the optimization algorithm to
generate 3D flow fields evolved from standard interdigitated flow fields by
minimizing the electrical and flow pressure power losses. To understand how
these designs improve performance, we analyze the polarization of the reactant
concentration and exchange current within the electrode to highlight how the
designed flow fields mitigate the presence of electrode dead zones. While
interdigitated flow fields can be heuristically engineered to yield high
performance by tuning channel and land dimensions, such a process can be
tedious; this work provides a framework for automating that design process.",2202.13032v1
2022-03-04,In-situ nitriding of Fe2VAl during laser surface remelting to manipulate microstructure and crystalline defects,"Tailoring the physical properties of complex materials for targeted
applications requires optimizing the microstructure and crystalline defects
that influence electrical and thermal transport, and mechanical properties.
Laser surface remelting can be used to modify the sub-surface microstructure of
bulk materials and hence manipulate their properties locally. Here, we
introduce an approach to perform remelting in a reactive nitrogen atmosphere,
in order to form nitrides and induce segregation of nitrogen to structural
defects. These defects arise from the fast solidification of the full-Heusler
Fe2VAl compound that is a promising thermoelectric material. Advanced scanning
electron microscopy, including electron channelling contrast imaging and
three-dimensional electron backscatter diffraction, is complemented by atom
probe tomography to study the distribution of crystalline defects and their
local chemical composition. We reveal a high density of dislocations, which are
stable due to their character as geometrically necessary dislocations. At these
dislocations and low-angle grain boundaries, we observe segregation of nitrogen
and vanadium, which can be enhanced by repeated remelting in nitrogen
atmosphere. We propose that this approach can be generalized to other additive
manufacturing processes to promote local segregation and precipitation states,
thereby manipulating physical properties.",2203.02209v1
2022-03-07,Tuning the competition between superconductivity and charge order in kagome superconductor Cs(V1-xNbx)3Sb5,"The recently discovered coexistence of superconductivity and charge density
wave order in the kagome systems AV3Sb5 (A = K, Rb, Cs) has stimulated enormous
interest. According to theory, a vanadium-based kagome system may host a flat
band, nontrivial linear dispersive Dirac surface states and electronic
correlation. Despite intensive investigations, it remains controversial about
the origin of the charge density wave (CDW) order, how does the
superconductivity relate to the CDW, and whether the anomalous Hall effect
(AHE) arises primarily from the kagome lattice or the CDW order. We report an
extensive investigation on Cs(V1-xNbx)3Sb5 samples with systematic Nb doping.
Our results show that the Nb doping induces apparent suppression of CDW order
and promotes superconductivity; meanwhile, the AHE and magnetoresistance (MR)
will be significantly weakened together with the CDW order. Combining with our
density functional calculations, we interpret these effects by an antiphase
shift of the Fermi energy with respect to the saddle points near M and the
Fermi surface centered around {\Gamma}. It is found that the former depletes
the filled states for the CDW instability and worsens the nesting condition for
CDW order; while the latter lifts the Fermi level upward and enlarges the Fermi
surface surrounding the {\Gamma} point, and thus promotes superconductivity.
Our results uncover a delicate but unusual competition between the CDW order
and superconductivity.",2203.03524v1
2022-04-26,"LAMOST J045019.27+394758.7, with peculiar abundances of N, Na, V, Zn, is possibly a Sculptor dwarf galaxy escapee","We present the results of the high-resolution (R$\sim$60,000) spectroscopic
analysis of the star LAMOSTJ045019.27+394758.7 (hereafter J045) from the list
of carbon stars of LAMOST DR2. From our analysis, we find that J045 does not
exhibit the spectral characteristics of carbon stars. It is found to be a
metal-poor ( [Fe/H] = $-$1.05) giant that shows very unusual elemental
abundances, particularly for N, Na, V, and Zn. J045 shows ${\alpha}$-elements
(Mg, Si, Ca) with near-solar values ($<$[$\alpha$/Fe]$>$ = 0.09) in contrast to
Galactic stars that show [$\alpha$/Fe] in the range 0.2 to 0.3 dex. In J045, Sc
and Ti are under abundant with [X/Fe] $\le$ $-$0.25. Vanadium gives [V/Fe] =
0.51 and zinc is under-abundant with [Zn/Fe] = $-$0.62. The object exhibits
near-solar abundances for Sr, Y, Ba, Pr, and Sm. The La is marginally enhanced,
and Ce and Nd are marginally under-abundant in J045. With [Ba/Eu] = $-$0.38,
the object falls into the category of neutron-capture rich r-I stars. The
estimated abundances of various elements show that the observed abundance
pattern is not compatible with the abundances characteristic of Galactic
metal-poor stars but matches quite closely with the abundance pattern of
Sculptor Dwarf galaxy stars of similar metallicity. Based on the above
observational evidences, we suggest that the object is a possible Sculptor
Dwarf Galaxy escapee.",2204.12099v1
2022-05-09,Charge density wave in kagome lattice intermetallic ScV6Sn6,"Materials hosting kagome lattices have drawn interest for the diverse
magnetic and electronic states generated by geometric frustration. In the
$A$V$_3$Sb$_5$ compounds ($A$ = K, Rb, Cs), stacked vanadium kagome layers give
rise to unusual charge density waves (CDW) and superconductivity. Here we
report single-crystal growth and characterization of ScV$_6$Sn$_6$, a hexagonal
HfFe$_6$Ge$_6$-type compound that shares this structural motif. We identify a
first-order phase transition at 92 K. Single crystal X-ray and neutron
diffraction reveal a charge density wave modulation of the atomic lattice below
this temperature. This is a distinctly different structural mode than that
observed in the $A$V$_3$Sb$_5$ compounds, but both modes have been anticipated
in kagome metals. The diverse HfFe$_6$Ge$_6$ family offers more opportunities
to tune ScV$_6$Sn$_6$ and explore density wave order in kagome lattice
materials.",2205.04582v2
2022-05-12,"Investigation of lattice dynamics, magnetism and electronic transport in $β$-Na$_{0.33}$V$_2$O$_{5}$","We investigate the electronic and magnetic properties as well as lattice
dynamics and spin-phonon coupling of $\beta$-Na$_{0.33}$V$_2$O$_5$ using
temperature-dependent Raman scattering, dc-magnetization and dc-resistivity,
x-ray photoemission, and absorption spectroscopy. The Rietveld refinement of
XRD pattern with space group C2/m confirms the monoclinic structure. The
analysis of temperature-dependent Raman spectra in a temperature range of
13--673~K reveals an anharmonic dependence of the phonon frequency and full
width at half maximum, which is accredited to the symmetric phonon decay.
However, below about 40 K, the hardening of the phonon frequency beyond
anharmonicity is attributed to the spin-phonon coupling. Interestingly, the
estimated effective magnetic moment $\mu_{\rm eff}=$ 0.63~$\mu_B$ from the
magnetization data manifests a mixed-valence state of V ions in 4+ (18$\pm$1\%)
and 5+ (82$\pm$1\%). A similar ratio of V$^{4+}$ to V$^{5+}$ is also observed
in the x-ray photoemission and x-ray absorption near-edge spectra and that is
found to be consistent with the sample stoichiometry. In addition, the V$^{4+}$
ions are distributed between different vanadium (V1 and V3) sites. The analysis
of extended x-ray absorption fine structure at different V-sites gives the
corresponding V--O bond lengths, which are utilized in the assignment of Raman
modes. Moreover, the temperature-dependent resistivity resembles a
semiconducting behavior where the charge carrier transport is facilitated by
the band conduction at higher temperatures and via hopping $\le$260~K.",2205.06019v2
2022-05-19,Ubiquitous Superconducting Diode Effect in Superconductor Thin Films,"The macroscopic coherence in superconductors supports dissipationless
supercurrents which could play a central role in emerging quantum technologies.
Accomplishing unequal supercurrents in the forward and backward directions
would enable unprecedented functionalities. This nonreciprocity of critical
supercurrents is called superconducting (SC) diode effect. We demonstrate
strong SC diode effect in conventional SC thin films, such as niobium and
vanadium, employing external magnetic fields as small as 1 Oe. Interfacing the
SC layer with a ferromagnetic semiconductor EuS, we further accomplish
non-volatile SC diode effect reaching a giant efficiency of 65%. By careful
control experiments and theoretical modeling, we demonstrate that the critical
supercurrent nonreciprocity in SC thin films could be easily accomplished with
asymmetrical vortex edge/surface barriers and the universal Meissner screening
current governing the critical currents. Our engineering of the SC diode effect
in simple systems opens door for novel technologies. Meanwhile, we reveal the
ubiquity of Meissner screening effect induced SC diode effect in
superconducting films, which should be eliminated with great care in the search
of exotic superconducting states harboring finite-momentum Cooper pairing.",2205.09276v5
2022-06-08,Macroscopic Quantum Tunneling of a Topological Ferromagnet,"The recent advent of topological states of matter spawned many significant
discoveries. The quantum anomalous Hall effect[1-3] is a prime example due to
its potential for applications in quantum metrology[4, 5] as well as its
influence on fundamental research into the underlying topological and magnetic
states[6-11] and axion electrodynamics[2, 12-14]. Here, we perform electronic
transport studies on a (V,Bi,Sb)2Te3 ferromagnetic topological insulator
nanostructure in the quantum anomalous Hall regime. This allows us access to
the dynamics of an individual ferromagnetic domain. The volume of the domain is
estimated to be about 85 000 nm3, containing some 50 000 vanadium atoms, spread
over a macroscopic distance of 115 nm. Telegraph noise resulting from the
magnetization fluctuations of this domain is observed in the Hall signal.
Careful analysis of the influence of temperature and external magnetic field on
the domain switching statistics provides evidence for quantum tunneling of
magnetization[15-22] in a macrospin state. This ferromagnetic macrospin is not
only the largest magnetic object in which quantum tunneling has been observed,
but also the first observation of the effect in a topological state of matter.",2206.03972v1
2022-08-29,Superconductivity from repulsive interactions on the kagome lattice,"The discovery of superconductivity in layered vanadium-based kagome metals
$A$V$_3$Sb$_5$ ($A$: K, Rb, Cs) has added a new family of materials to the
growing class of possible unconventional superconductors. However, the nature
of the superconducting pairing in these materials remains elusive. We present a
microscopic theoretical study of the leading superconducting instabilities on
the kagome lattice based on spin- and charge-fluctuation mediated Cooper
pairing. The applied methodology includes effects of both on-site and
nearest-neighbor repulsive Coulomb interactions. Near the upper van Hove
filling -- relevant for the $A$V$_3$Sb$_5$ materials -- we find a rich phase
diagram with several pairing symmetries being nearly degenerate. In particular,
while a substantial fraction of the phase diagram is occupied by a spin-singlet
order parameter transforming as a two-dimensional irreducible representation of
the point group, several nodal spin-triplet pairing states remain competitive.
We compute the band and interaction parameter-dependence of the hierarchy of
the leading superconducting instabilities, and determine the detailed momentum
dependence of the resulting preferred gap structures. Crucially, for moderate
values of the interaction parameters, the individual pairing states depend
strongly on momentum and exhibit multiple nodes on the Fermi surface. We
discuss the properties of these superconducting gap structures in light of
recent experimental developments of the $A$V$_3$Sb$_5$ materials.",2208.13521v1
2022-09-20,Characteristics of a Titanium Manganese redox flow battery based on Comsol,"A simulation model and design of Titanium Manganese Redox Flow Battery
(TMRFB) is proposed to study the distribution of dissociation rate,
overpotential, current density, and electrode potential. TMRFB is one of the
most promising new energy storages because of its high capacity and
eco-friendly characteristics in the current condition of energy scarcity and
environmental pollution. Moreover, Mn-based flow batteries are gaining
popularity due to their inexpensive cost and high energy density in lieu of all
vanadium redox flow batteries which are expensive. This research shows that the
surface dissociation rate of Ti4+/ Ti3+ and Mn3+/Mn2+ ions are higher at the
membrane and lower at the inlet where the velocity of the electrolyte flow is
higher; Furthermore, our work reveals that when the thickness of the electrode
is compressed from 4.5 mm to 3 mm, overpotential reduces whereas current
density and electrode potential increases. The COMSOL Multiphysics software is
used to solve the model's equations using the finite element approach. From the
dissociation rate it is concluded that less potential is required at the
membrane for the oxidation reduction reaction and with optimized electrolyte
flow rate battery performance can be improved. Thus, electrode compression
increases conductivity and battery performance.",2209.09904v1
2022-10-12,Microscopic nature of the charge-density wave in kagome superconductor RbV$_3$Sb$_5$,"The recently discovered vanadium-based kagome metals AV$_3$Sb$_5$ (A = K, Rb,
Cs) offer the possibility to study the interplay between competing electronic
orderings, such as charge density order and superconductivity. We focus on the
former and provide a comprehensive set of $^{51}$V, $^{87}$Rb, and $^{121}$Sb
magnetic resonance measurements on an RbV$_3$Sb$_5$ single crystal. Elucidating
the symmetries and properties of the CDW phase is essential to understanding
the unconventional electronic orderings occurring in this material. We
establish the structure of the $2\times 2 \times 2$ superlattice that describes
the system below the charge density wave transition by combining both
experimental and computational methods, with a methodology that can be readily
applied to the remaining compounds of the same family. Our results give
compelling evidence that the CDW structure occurring below 103 K for
RbV$_3$Sb$_5$ is the so-called Inverse Start of David pattern $\pi$-shifted
along the c axis (also known as staggered tri-hexagonal).",2210.06523v1
2022-10-26,Chemical tuning of a honeycomb magnet through a critical point,"BaCo2(AsO4)2 (BCAO) has seen extensive study since its initial identification
as a proximate Kitaev quantum spin liquid candidate. Thought to be described by
the highly anisotropic XXZ-J_1-J_3 model, the ease with which magnetic order is
suppressed in the system indicates proximity to a spin liquid phase. Upon
chemical tuning via partial arsenic substitution with vanadium, we show an
initial suppression of long-range incommensurate order in the BCAO system to T
= 3.0 K, followed by increased spin freezing at higher substitution levels.
Between these two regions, at around 10% substitution, the system is shown to
pass through a critical point where the competing J_1/J_3 exchange interactions
become more balanced, producing a more complex magnetic ground state, likely
stabilized by quantum fluctuations. This state shows how slight compositional
change in magnetically-frustrated systems may be leveraged to tune ground state
degeneracies and potentially realize a quantum spin liquid state.",2210.14439v2
2022-11-03,Mechanochemical synthesis of pseudobinary Ti-V hydrides and their conversion reaction with Li and Na,"Lithium-ion batteries (LiBs) based on insertion electrodes reach intrinsic
capacity limits. Performance improvements and cost reduction require
alternative reaction mechanisms and novel battery chemistries such as
conversion reactions and sodium-ion batteries (NaBs), respectively. We here
study the formation of Ti1-xVxH2 hydrides (0 < x < 1) and their electrochemical
properties as anodes in LiBs and NaBs half-cells. Hydrides were synthesized by
mechanochemistry of the metal powders under hydrogen atmosphere (PH2~ 8 MPa).
For V contents below 80 at.% (x < 0.8), single-phase pseudobinary dihydride
compounds Ti1-xVxH2 are formed. They crystallize in the fluorite-type structure
and are highly nanostructured (crystallite size < 10 nm). Their lattice
parameter decreases linearly with the V content leading to hydride
destabilization. Electrochemical studies were first carried out in Li-ion half
cells with full conversion between Ti1-xVxH2 hydrides and lithium. The
potential of the conversion reaction can be gradually tuned with the vanadium
content due to its destabilization effect. Furthermore, different paths for the
conversion reaction are observed for Ti-rich (x < 0.25) and V-rich (x > 0.7)
alloys. Na-ion half-cell measurements prove the reactivity between (V,Ti)H2
hydrides and sodium, albeit with significant kinetic limitations",2211.01550v1
2022-11-07,Optical spectroscopy and band structure calculations of structural phase transition in the Vanadium-based kagome metal ScV$_6$Sn$_6$,"In condensed matter physics, materials with kagome lattice display a range of
exotic quantum states, including charge density wave (CDW), superconductivity
and magnetism. Recently, the intermetallic kagome metal ScV6Sn6 was discovered
to undergo a first-order structural phase transition with the formation of a
root3xroot3x3 CDW at around 92 K. The bulk electronic band properties are
crucial to understanding the origin of the structural phase transition. Here,
we conducted an optical spectroscopy study in combination with band structure
calculations across the structural transition. Our findings showed abrupt
changes in the optical reflectivity/conductivity spectra as a result of the
structural transition, without any observable gap formation behavior. The
optical measurements and band calculations actually reveal a sudden change of
the band structure after transition. It is important to note that this phase
transition is of the first-order type, which distinguishes it from conventional
density-wave type condensations. Our results provide an insight into the origin
of the structural phase transition in this new and unique kagome lattice.",2211.03412v2
2022-11-29,Electronic nematicity without charge density waves in titanium-based kagome metal,"Layered crystalline materials that consist of transition metal atoms on a
kagome network have emerged as a versatile platform to study unusual electronic
phenomena. For example, in the vanadium-based kagome superconductors AV3Sb5
(where A can stand for K, Cs, or Rb) there is a parent charge density wave
phase that appears to simultaneously break both the translational and the
rotational symmetry of the lattice. Here, we show a contrasting situation where
electronic nematic order - the breaking of rotational symmetry without the
breaking of translational symmetry - can occur without a corresponding charge
density wave. We use spectroscopic-imaging scanning tunneling microscopy to
study the kagome metal CsTi3Bi5 that is isostructural to AV3Sb5 but with a
titanium atom kagome network. CsTi3Bi5 does not exhibit any detectable charge
density wave state, but comparison to density functional theory calculations
reveals substantial electronic correlation effects at low energies. Comparing
the amplitudes of scattering wave vectors along different directions, we
discover an electronic anisotropy that breaks the six-fold symmetry of the
lattice, arising from both in-plane and out-of-plane titanium-derived d
orbitals. Our work uncovers the role of electronic orbitals in CsTi3Bi5,
suggestive of a hexagonal analogue of the nematic bond order in Fe-based
superconductors.",2211.16477v2
2022-12-21,Universal scaling of tunable Yu-Shiba-Rusinov states across the quantum phase transition,"Quantum magnetic impurities give rise to a wealth of phenomena attracting
tremendous research interest in recent years. On a normal metal, magnetic
impurities generate the correlation-driven Kondo effect. On a superconductor,
bound states emerge inside the superconducting gap called the Yu-Shiba-Rusinov
(YSR) states. Theoretically, quantum impurity problems have been successfully
tackled by numerical renormalization group (NRG) theory, where the Kondo and
YSR physics are shown to be unified and the normalized YSR energy scales
universally with the Kondo temperature divided by the superconducting gap.
However, experimentally the Kondo temperature is usually extracted from
phenomenological approaches, which gives rise to significant uncertainties and
cannot account for magnetic fields properly. Using scanning tunneling
microscopy at 10mK, we apply a magnetic field to several YSR impurities on a
vanadium tip to reveal the Kondo effect and employ the microscopic single
impurity Anderson model with NRG to fit the Kondo spectra in magnetic fields
accurately and extract the corresponding Kondo temperature unambiguously. Some
YSR states move across the quantum phase transition (QPT) due to the changes in
atomic forces during tip approach, yielding a continuous universal scaling with
quantitative precision for quantum spin-1/2 impurities.",2212.11332v1
2023-01-17,"Enhancement of photocatalytic performance of V2O5 by rare-earth ions doping, synthesized by facile hydrothermal technique","The rare-earth (RE) elements [Holmium (Ho) and Ytterbium (Yb)] doped vanadium
pentoxide (V2O5) with a series of doping concentrations (1 mol.%, 3 mol.%, and
5 mol.%) have been successfully synthesized using environment-friendly facile
hydrothermal method. The effect of RE ions on the photocatalytic efficiency of
doped V2O5 has also been analyzed. The stable orthorhombic crystal structure of
doped V2O5 confirms by the X-ray diffraction with no secondary phase, and
high-stressed conditions are generated for the 3 mol.%. The crystallite size,
strain, and dislocation density are calculated to perceive the doping effect on
the bare V2O5. The optical characteristics have been measured using UV-vis
spectroscopy. The absorptions are found to be increased with increasing doping
concentrations; however, the bandgap remains in the visible range. The
photocatalytic properties are examined for the compounds with varying pH, and
it is observed that higher efficiency is exhibited for the pH 7 and catalyst
concentration 500 ppm. The highest degradation efficiency is found to be 93%
and 95% for the 3 mol.% of Ho and Yb-doped V2O5 samples within 2 hours,
respectively. It is elucidated that the RE ions significantly impact the
catalytic behavior of V2O5, and the mechanism behind these extraordinary
efficiencies has been explained thoroughly.",2301.06666v1
2023-01-31,Hybrid Cathode Lithium Battery Discharge Simulation for Implantable Cardioverter Defibrillators Using a Coupled Electro-Thermal Dynamic Model,"This paper investigates the impact of implantable cardioverter defibrillator
(ICD)'s load on its lithium battery power sources through a coupled
electro-thermal dynamic model simulation. ICDs are one of the effective
treatments available to significantly improve survival of patients with fatal
arrhythmia (abnormal heart rhythm) disorders. Using a lithium battery power
source, this life-saving device sends electrical shocks or pulses to regulate
the heartbeat. The service life and reliability of an ICD is primarily
expressed by its battery's lifespan and performance. In this paper we
investigate the terminal voltage, depth of discharge (DOD) and temperature
dynamics of the implantable lithium battery with a combined cathode material,
namely carbon-monofluoride and silver vanadium oxide (Li/CFx-SVO). Modeling the
implantable batteries characteristics is a well-established topic in
literature; however, to the best of the author's knowledge, the impact of the
high-energy shocks (defibrillation) and low-energy device power supply
(housekeeping) on the ICD's battery operation is relatively less-explored. Our
analysis reveals that the battery terminal voltage is primarily affected by
small but continuous housekeeping discharge current in the range of uA, rather
than intermittent high defibrillation current demand in the range of several
amps. The results can be used to improve the device design control and
operation, thus extending the service life in patients and reducing the need
for invasive replacement surgery.",2302.00069v1
2023-02-13,"Two elementary band representation model, Fermi surface nesting, and surface topological superconductivity in $A$V$_{3}$Sb$_ {5}$ ($A = \text{K, Rb, Cs}$)","The recently discovered vanadium-based Kagome metals $A$V$_{3}$Sb$_{5}$ ($A =
\text{K, Rb, Cs}$) are of great interest with the interplay of charge density
wave (CDW) order, band topology and superconductivity. In this paper, by
identifying elementary band representations (EBRs), we construct a two-EBR
graphene-Kagome model to capture the two low-energy van-Hove-singularity
dispersions and, more importantly, the nontrivial band topology in these Kagome
metals. This model consists of $A_g@3g$ (V-$d_{x^2-y^2/z^2}$, Kagome sites) and
$A_2''@2d$ EBRs (Sb1-$p_z$, honeycomb sites). We have investigated the Fermi
surface instability by calculating the electronic susceptibility
$\chi(\mathbf{q})$. Prominent Fermi-surface nesting peaks are obtained at three
L points, where the $z$ component of the nesting vector shows intimate
relationship with the anticrossing point along M--L. The nesting peaks at L are
consistent with the $2\times 2\times 2$ CDW reconstruction in these compounds.
In addition, the sublattice-resolved bare susceptibility is calculated and
similar sharp peaks are observed at the L points, indicating a strong
antiferromagnetic fluctuation. Assuming a bulk $s$-wave superconducting
pairing, helical surface states and nontrivial superconducting gap are obtained
on the (001) surface. In analogous to FeTe$_{1-x}$Se$_{x}$ superconductor, our
results establish another material realization of a stoichiometric
superconductor with nontrivial band topology, providing a promising platform
for studying exotic Majorana physics in condensed matter",2302.06211v2
2023-03-01,Anisotropic magnetism and electronic properties of the kagome metal SmV6Sn6,"Kagome magnets are expected to feature emergent properties due to the
interplays among geometry, magnetism, electronic correlation, and band
topology. The magnetism and topological electronic states can be tuned via the
rare earth engineering in RV6Sn6 kagome metals, where R is a rare earth
element. Herein, we present the synthesis and characterization of SmV6Sn6, a
metal with two-dimensional kagome nets of vanadium and frustrated triangular Sm
lattice. Partial of the Sm atoms are shifted from the normal R positions by c/2
along the c axis. Magnetic measurements reveal obvious anisotropy, where the
easy magnetic axis is within the ab plane. Electronic transports show multiband
behaviors below 200 K. Density functional theory calculations find that the
electronic structure of SmV6Sn6 hosts flat bands, Dirac cone, and saddle point
arising from the V-3d electrons near the Fermi level. No evidence for the
existence of charge density wave or magnetic order down to 2 K can be observed.
Thus, SmV6Sn6 can be viewed as a modest disordered derivative of the RV6Sn6
structure, in which the disordered rare earth ions can suppress the magnetic
order and charge density wave in the RV6Sn6 kagome family.",2303.00627v1
2023-03-20,Violation of Emergent Rotational Symmetry in the Hexagonal Kagome Superconductor CsV3Sb5,"Superconductivity is caused by electron pairs that are canonically isotropic,
whereas some exotic superconductors are known to exhibit non-trivial anisotropy
stemming from unconventional pairings. However, superconductors with hexagonal
symmetry, the highest rotational symmetry allowed in crystals, exceptionally
have strong constraint that is called emergent rotational symmetry (ERS):
anisotropic properties should be very weak especially near the critical
temperature Tc even for unconventional pairings such as d-wave states. Here, we
investigate superconducting anisotropy of the recently-found hexagonal Kagome
superconductor CsV3Sb5, which is known to exhibit various intriguing phenomena
originating from its undistorted Kagome lattice formed by vanadium atoms. Based
on calorimetry performed under accurate two-axis field-direction control, we
discover a combination of six- and two-fold anisotropies in the in-plane upper
critical field. Both anisotropies, robust up to very close to Tc, are beyond
predictions of standard theories. We infer that this clear ERS violation with
nematicity is best explained by multi-component nematic superconducting order
parameter in CsV3Sb5 intertwined with symmetry breakings caused by the
underlying charge-density-wave order.",2303.11072v1
2023-03-22,Open-tunneled oxides as intercalation host for multivalent ion (Ca and Al) batteries: A DFT study,"Lithium-ion batteries (LIBs) are ubiquitous in everyday applications.
However, Lithium (Li) is a limited resource on the planet and is therefore not
sustainable. As an alternative to lithium, earth-abundant and cheaper
multivalent metals such as aluminum (Al) and calcium (Ca) have been actively
researched in battery systems. However, finding suitable intercalation hosts
for multivalent-ion batteries is urgently needed. Open-tunneled oxides are a
particular category of microparticles distinguished by the presence of
integrated one-dimensional channels or nanopores. This work focuses on two
promising open-tunnel oxides, viz: Niobium Tungsten Oxide (NTO) and Molybdenum
Vanadium Oxide (MoVO). We find that the MoVO structure can adsorb greater
numbers of multivalent ions than NTO due to its larger surface area and
different shapes. The MoVO structure can adsorb Ca, Li, and Al ions with
adsorption potential at around 4 to 5 eV. However, the adsorption potential for
hexagonal channels of Al ion drops to 1.73 eV because of less channel area. NTO
structure has an insertion/adsorption potential of 4.4 eV, 3.4 eV, and 0.9 eV
for one Li, Ca, and Al, respectively. In general, Ca ion is more adsorbable
than Al ion in both MoVO and NTO structures. Bader charge analysis and charge
density plot reveals the role of charge transfer and ion size on the insertion
of multivalent ions such as Ca and Al into MoVO and NTO systems. Our results
provide general guidelines to explore other multivalent ions for battery
applications.",2303.12301v1
2023-05-09,Anisotropic hybridization probed by polarization dependent x-ray absorption spectroscopy in VI3 van der Waals Mott ferromagnet,"Polarization dependent x-ray absorption spectroscopy was used to study the
magnetic ground state and the orbital occupation in bulk-phase VI$_3$ van der
Waals crystals below and above the ferromagnetic and structural transitions.
X-ray natural linear dichroism and X-ray magnetic circular dichroism spectra
acquired at the V $L_{2,3}$ edges are compared against multiplet cluster
calculations within the frame of the ligand field theory to quantify the
intra-atomic electronic interactions at play and evaluate the effects of
symmetry reduction occurring in a trigonally distorted VI$_6$ unit. We observed
a non zero linear dichroism proving the presence of an anisotropic charge
density distribution around the V$^{3+}$ ion due to the unbalanced
hybridization between the Vanadium and the ligand states. Such hybridization
acts as an effective trigonal crystal field, slightly lifting the degeneracy of
the $t_{2g}^2$ ground state. However, the energy splitting associated to the
distortion underestimates the experimental band gap, suggesting that the
insulating ground state is stabilized by Mott correlation effects rather than
via a Jahn-Teller mechanism. Our results clarify the role of the distortion in
VI$_3$ and establish a benchmark for the study of the spectroscopic properties
of other van der Waals halides, including emerging 2D materials with mono and
few-layers thickness, whose fundamental properties might be altered by reduced
dimensions and interface proximity.",2305.05360v1
2023-05-24,Hybrid-MPET: an open-source simulation software for hybrid electrode batteries,"As the design of single-component battery electrodes has matured, the battery
industry has turned to hybrid electrodes with blends of two or more active
materials to enhance battery performance. Leveraging the best properties of
each material while mitigating their drawbacks, multi-component hybrid
electrodes open a vast new design space that could be most efficiently explored
through simulations. In this article, we introduce a mathematical modeling
framework and open-source battery simulation software package for Hybrid
Multiphase Porous Electrode Theory (Hybrid-MPET), capable of accounting for the
parallel reactions, phase transformations and multiscale heterogeneities in
hybrid porous electrodes. Hybrid-MPET models can simulate both solid solution
and multiphase active materials in hybrid electrodes at intra-particle and
inter-particle scales. Its modular design also allows the combination of
different active materials at any capacity fraction. To illustrate the novel
features of Hybrid-MPET, we present experimentally validated models of
silicon-graphite (Si-Gr) anodes used in electric vehicle batteries and carbon
monofluoride (CFx) - silver vanadium oxide (SVO) cathodes used in implantable
medical device batteries. The results demonstrate the potential of Hybrid-MPET
models to accelerate the development of hybrid electrode batteries by providing
fast predictions of their performance over a wide range of design parameters
and operating protocols.",2305.15599v1
2023-06-14,The unexpected optical and ultraviolet variability of the standard star $α$ Sex (HD 87887),"The analysis of the available TESS light curves of $\alpha$ Sex (HD 87887)
reveals low-frequency pulsations with a period of about 9.1 hours in this
spectroscopic A0 III standard star. The IUE observations in December 1992
reveal large flux variations both in the far UV and in the mid UV which are
accompanied by variations of the brightness in the V band recorded by the Fine
Error Sensor on board IUE. The ultraviolet variability could be due to an
eclipse by an hitherto undetected companion of smaller radius, possibly 2.5
R$_{\odot}$ but this needs confirmation by further monitoring possibly with
TESS. An abundance determination yields solar abundances for most elements.
Only carbon and strontium are underabundant and titanium, vanadium and barium
mildly overabundant. Identification is provided for most of the lines absorbing
more than 2% in the optical spectrum of $\alpha$ Sex. Stellar evolution
modeling shows that $\alpha$ Sex is near the terminal-age main sequence, and
its mass, radius and age are estimated to be $M = 2.57 \pm 0.32$ M$_{\odot}$,
$R = 3.07 \pm 0.90$ R$_{\odot}$, $A = 385 \pm 77$ Myr, respectively.",2306.08551v1
2023-06-23,Semi-Empirical Haken-Strobl Model for Molecular Spin Qubits,"Understanding the physical processes that determine the relaxation $T_{1}$
and dephasing $T_2$ times of molecular spin qubits is critical for envisioned
applications in quantum metrology and information processing. Recent spin-echo
$T_1$ measurements of solid-state molecular spin qubits have stimulated the
development of quantum mechanical models for predicting intrinsic spin qubit
timescales using first-principles electronic structure methods. We develop an
alternative semi-empirical approach to construct Redfield quantum master
equations for molecular spin qubits using a stochastic Haken-Strobl model for a
central spin with a fluctuating gyromagnetic tensor due to spin-lattice
interaction and a fluctuating local magnetic field due to interactions with
other lattice spins. Using a vanadium-based spin qubit as a case study, we
compute qubit population and decoherence timescales as a function of
temperature and magnetic field using a bath spectral density parametrized with
a small number of $T_{1}$ measurements. The theory quantitatively agrees with
experimental data over a range of conditions beyond those used to parametrize
the model, demonstrating the generalization potential of the method. The
ability of the model to describe the temperature dependence of the ratio
$T_2/T_1$ is discussed and possible applications for designing novel
molecule-based quantum magnetometers are suggested.",2306.13790v1
2023-07-31,Spin decoherence in VOPc@graphene nanoribbon complexes,"Carbon nanoribbon or nanographene qubit arrays can facilitate
quantum-to-quantum transduction between light, charge, and spin, making them an
excellent testbed for fundamental science in quantum coherent systems and for
the construction of higher-level qubit circuits. In this work, we study spin
decoherence due to coupling with a surrounding nuclear spin bath of an
electronic molecular spin of a vanadyl phthalocyanine (VOPc) molecule
integrated on an armchair-edged graphene nanoribbon (GNR). Density functional
theory (DFT) is used to obtain ground state atomic configurations. Decay of
spin coherence in Hahn echo experiments is then simulated using the cluster
correlation expansion method with a spin Hamiltonian involving hyperfine and
electric field gradient tensors calculated from DFT. We find that the
decoherence time $T_2$ is anisotropic with respect to magnetic field
orientation and determined only by the hydrogen nuclear spins both on VOPc and
GNR. Large electron spin echo envelope modulation (ESEEM) due to nitrogen and
vanadium nuclear spins is present at specific field ranges and can be
completely suppressed by tuning the magnetic field. The relation between these
field ranges and the hyperfine interactions is analyzed. The effects of
interactions with the nuclear quadrupole moments are also studied, validating
the applicability and limitations of the spin Hamiltonian when they are
disregarded.",2307.16403v1
2023-08-13,On the relative abundances of Cavansite and Pentagonite,"Cavansite is a visually stunning blue vanadosilicate mineral with limited
occurrences worldwide, whereas Pentagonite is a closely related dimorph with
similar physical and chemical properties, yet is extremely rare compared to
Cavansite. The reasons behind Pentagonite's exceptional rarity remain largely
unknown.
  In this study, (a) density functional theory (DFT) is utilized to investigate
the electronic structures of Cavansite and Pentagonite at ground state and
finite pressures; (b) a two-state Boltzmann probability model is then employed
to construct a comprehensive phase diagram that reveals the abundance of each
species across a wide range of pressure and temperature conditions; and (c)
dehydration characteristics of these two minerals are explored.
  The present analysis reveals the key factors that contribute to the relative
scarcity of Pentagonite, including differences in structural arrangement and
electronic configurations between the two minerals. Specifically, it shows that
(a) because of the peculiar arrangements of SiO4 polyhedra, Cavansite forms a
compact structure (about 2.7% less in volume) resulting in lower energy; (b) at
a temperature of about 650K only about 1% Pentagonite can form; (c) vanadium
induces a highly localized state in both of these otherwise large-band-gap
insulators resulting in an extremely weak magnetic phase that is unlikely to be
observed at any reasonable finite temperature; and (d) water molecules are
loosely bound inside the microporous crystals of Cavansite and Pentagonite,
suggesting potential applications of these minerals in various technological
fields.",2308.06825v2
2023-09-03,Observation of multiple flat bands and topological Dirac states in a new titanium based slightly distorted kagome metal YbTi3Bi4,"Kagome lattices have emerged as an ideal platform for exploring various
exotic quantum phenomena such as correlated topological phases, frustrated
lattice geometry, unconventional charge density wave orders, Chern quantum
phases, superconductivity, etc. In particular, the vanadium based nonmagnetic
kagome metals AV3Sb5 (A= K, Rb, and Cs) have seen a flurry of research interest
due to the discovery of multiple competing orders. Here, we report the
discovery of a new Ti based kagome metal YbTi3Bi4 and employ angle-resolved
photoemission spectroscopy (ARPES), magnetotransport in combination with
density functional theory calculations to investigate its electronic structure.
We reveal spectroscopic evidence of multiple flat bands arising from the kagome
lattice of Ti with predominant Ti 3d character. Through our calculations of the
Z2 indices, we have identified that the system exhibits topological
nontriviality with surface Dirac cones at the Gamma point and a quasi
two-dimensional Dirac state at the K point which is further confirmed by our
ARPES measured band dispersion. These results establish YbTi3Bi4 as a novel
platform for exploring the intersection of nontrivial topology, and electron
correlation effects in this newly discovered Ti based kagome lattice.",2309.01176v1
2023-09-26,Intelligent radiative thermostat induced by near-field radiative thermal diode,"A radiative thermostat system senses its own temperature and automatically
modulates heat transfer by turning on/off the cooling to maintain its
temperature near a desired set point. Taking advantage of far- and near-field
radiative thermal technologies, we propose an intelligent radiative thermostat
induced by the combination of passive radiative cooling and near-field
radiative thermal diode for thermal regulation at room temperature. The top
passive radiative cooler in thermostat system with static thermal emissivity
uses the cold outer space to passively cool itself all day, which can provide
the bottom structure with the sub-ambient cold source. Meanwhile, using the
phase-transition material vanadium dioxide, the bottom structure forms a
near-field radiative thermal diode with the top cooler, which can significantly
regulate the heat transfer between two terminals of the diode and then realize
a stable temperature of the bottom structure. Besides, the backsided heat input
of the thermostat has been taken into account according to real-world
applications. Thermal performance of the proposed radiative thermostat design
has been analyzed, showing that the coupling effect of static passive radiative
cooling and dynamic internal heat transfer modulation can maintain an
equilibrium temperature approximately locked within the phase transition
region. Besides, after considering empirical indoor-to-outdoor heat flux,
rendering its thermal performance closer to that of passive solar residential
building walls, the calculation result proves that the radiative thermostat
system can effectively modulate the temperature and stabilize it within a
controllable range. Passive radiative thermostats driven by near-field
radiative thermal diode can potentially enable intelligent temperature
regulation technologies, for example, to moderate diurnal temperature in
regions with extreme thermal swings.",2309.15303v1
2023-10-01,Electronic properties of kagome metal ScV6Sn6 using high field torque magnetometry,"This work presents electronic properties of the kagome metal ScV6Sn6 using de
Haas-van Alphen (dHvA) oscillations and density functional theory (DFT)
calculations. The torque signal with the applied fields up to 43 T shows clear
dHvA oscillations with six major frequencies, five of them are below 400 T (low
frequencies) and one is nearly 2800 T (high frequency). The Berry phase
calculated using the Landau level fan diagram near the quantum limit is
approximately {\pi}, which suggests the non-trivial band topology in ScV6Sn6.
To explain the experimental data, we computed the electronic band structure and
Fermi surface using DFT in both the pristine and charge density wave (CDW)
phases. Our results confirm that the CDW phase is energetically favorable, and
the Fermi surface undergoes a severe reconstruction in the CDW state.
Furthermore, the angular dependence of the dHvA frequencies are consistent with
the DFT calculations. The detailed electronic properties presented here are
invaluable for understanding the electronic structure and CDWorder in ScV6Sn6,
as well as in other vanadium-based kagome systems.",2310.00751v1
2023-10-10,Stochastic 3D modeling of nanostructured NVP/C active material particles for sodium-ion batteries,"A data-driven modeling approach is presented to quantify the influence of
morphology on effective properties in nanostructured sodium vanadium phosphate
$\mathrm{Na}_3\mathrm{V}_2(\mathrm{PO}_4)_3$/ carbon composites (NVP/C), which
are used as cathode material in sodium-ion batteries. This approach is based on
the combination of advanced imaging techniques, experimental nanostructure
characterization and stochastic modeling of the 3D nanostructure consisting of
NVP, carbon and pores. By 3D imaging and subsequent post-processing involving
image segmentation, the spatial distribution of NVP is resolved in 3D, and the
spatial distribution of carbon and pores is resolved in 2D. Based on this
information, a parametric stochastic model, specifically a Pluri-Gaussian
model, is calibrated to the 3D morphology of the nanostructured NVP/C
particles. Model validation is performed by comparing the nanostructure of
simulated NVP/C composites with image data in terms of morphological
descriptors which have not been used for model calibration. Finally, the
stochastic model is used for predictive simulation to quantify the effect of
varying the amount of carbon while keeping the amount of NVP constant. The
presented methodology opens new possibilities for a ressource-efficient
optimization of the morphology of NVP/C particles by modeling and simulation.",2310.07017v1
2023-10-14,"Chiral magnetism, lattice dynamics, and anomalous Hall conductivity in the novel V$_3$AuN antiferromagnetic antiperovskite","Antiferromagnetic antiperovskites, where magnetically active 3$d$ metal
cations are placed in the octahedral corners of a perovskite structure, are in
the spotlight due to their intertwined magnetic structure and topological
properties. Especially their anomalous Hall conductivity, which can be
controlled by applied strain and/or electric field, makes them highly
attractive in different electronic applications. Here, we present the study and
theoretical understanding of a new antiperovskite compound that can offer
enormous opportunities in a broad set of applications. Using first-principles
calculations, we investigated the structure, lattice dynamics, noncollinear
magnetic ordering, and electronic behavior in the Vanadium-based antiperovskite
V$_3$AuN. We found an antiperovskite structure centered on N similar to the
Mn$_3A$N family as the structural ground state. In such a phase, a
\emph{Pm$\bar{3}$m} ground state was found in contrast to the \emph{Cmcm}
post-antiperovskite layered structure, as in the V$_3A$N, $A$ = Ga, Ge, As, and
P. We studied the lattice dynamics and electronic properties, demonstrating its
vibrational stability in the cubic structure and a chiral antiferromagnetic
noncollinear ordering as a magnetic ground state. Finally, we found that the
anomalous Hall conductivity, associated with the topological features induced
by the magnetic symmetry, is $\sigma_{xy}$ = $-$291 S$\cdot$cm$^{-1}$
($\sigma_{111}$ = $-$504 S$\cdot$cm$^{-1}$). The latter is the largest reported
in the antiferromagnetic antiperovskite family of compounds.",2310.09616v1
2023-11-22,Reconfigurable Image Processing Metasurfaces with Phase-Change Materials,"Optical metasurfaces have been enabling reduced footprint and power
consumption, as well as faster speeds, in the context of analog computing and
image processing. While various image processing and optical computing
functionalities have been recently demonstrated using metasurfaces, most of the
considered devices are static and lack reconfigurability. Yet, the ability to
dynamically reconfigure processing operations is key for metasurfaces to be
able to compete with practical computing systems. Here, we demonstrate a
passive edge-detection metasurface operating in the near-infrared regime whose
image processing response can be drastically modified by temperature variations
smaller than 10{\deg} C around a CMOS-compatible temperature of 65{\deg} C.
Such reconfigurability is achieved by leveraging the insulator-to-metal phase
transition of a thin buried layer of vanadium dioxide which, in turn, strongly
alters the nonlocal response of the metasurface. Importantly, this
reconfigurability is accompanied by performance metrics - such as high
numerical aperture, high efficiency, isotropy, and polarization-independence -
close to optimal, and it is combined with a simple geometry compatible with
large-scale manufacturing. Our work paves the way to a new generation of
ultra-compact, tunable, passive devices for all-optical computation, with
potential applications in augmented reality, remote sensing and bio-medical
imaging.",2311.13109v1
2023-12-29,Optical detection of small polarons in vanadium dioxide and their critical role in mediating metal-insulator transition,"In the pursuit of advanced photoelectric devices, researchers have uncovered
near room-temperature metal-insulator transitions (MIT) in non-volatile VO2.
Although theoretical investigations propose that polaron dynamics mediate the
MIT, direct experimental evidence remains scarce. In this study, we present
direct evidence of the polaron state in insulating VO2 through high-resolution
spectroscopic ellipsometry measurements and first-principles calculations. We
demonstrate that polaron dynamics play a complementary role in facilitating
Peierls and Mott transitions to contribute to the MIT processes. Moreover, our
observations and characterizations of conventional metallic and correlated
plasmons in the respective phases of the VO2 film provide valuable insights
into their electron structures. This study provides an understanding of the MIT
mechanism in correlated systems and highlights how polarons, lattice
distortions and electron correlations facilitate the phase transition processes
in strongly-correlated systems, while further inspiring the development of new
device functionalities.",2312.17419v1
2024-01-31,Phase Transition of the single-layer vanadium diselenide on Au(111) with distinguished electronic structures,"Herein, we report the reversible structure transition of the single-layer
VSe2 grown on Au (111) with the alternating thermal annealing and Se
replenishment. Using scanning tunnelling microscopy (STM) and angle-resolved
photoemission spectroscopy (ARPES), we demonstrate the epitaxially growth of
high-quality VSe2 on Au (1 1 1) with the octahedral (1T) structure and the
Se-vacancy-induced structure transformation of VSe2 from the metallic Morrie
(1T) phase to the semiconducting (2H) phase with an increased in-plane lattice
constant. With the convincing agreement between the experimental results and
DFT calculations, the nanostructure near the gain boundary in the defective
intermediate phase is confirmed, as well as the reaction pathway with Se
gradually depleted at elevated temperatures. It is revealed that the Se density
of the line-shape defects plays an important role in the formation of the 2H
domain phase, due to the increment of the in-plane lattice parameter after the
Se desorption, and the better thermal stability of 2H phase compared to the 1T
phase. Importantly, the proper control of the density of Se could feasibly
manipulate the ratio between the 1T and 2H phase in the steak domain, which is
regard as a good platform for the 2D homojunctions in nanoelectronics.",2401.17562v1
2024-02-06,Physics-based Modeling of Pulse and Relaxation of High-rate Li/CF$_{x}$-SVO batteries in Implantable Medical Devices,"We present a physics-based model that accurately predicts the performance of
Medtronic's implantable medical device battery lithium/carbon monofluoride
(CF$_x$) - silver vanadium oxide (SVO) under both low-rate background
monitoring and high-rate pulsing currents. The distinct properties of multiple
active materials are reflected by parameterizing their thermodynamics,
kinetics, and mass transport properties separately. Diffusion limitations of
Li$^+$ in SVO are used to explain cell voltage transient behavior during pulse
and post-pulse relaxation. We also introduce change in cathode electronic
conductivity, Li metal anode surface morphology, and film resistance buildup to
capture evolution of cell internal resistance throughout multi-year electrical
tests. We share our insights on how the Li$^+$ redistribution process between
active materials can restore pulse capability of the hybrid electrode, allow
CF$_x$ to indirectly contribute to capacity release during pulsing, and affect
the operation protocols and design principles of batteries with other hybrid
electrodes. We also discuss additional complexities in porous electrode model
parameterization and electrochemical characterization techniques due to
parallel reactions and solid diffusion pathways across active materials. We
hope our models implemented in the Hybrid Multiphase Porous Electrode Theory
(Hybrid-MPET) framework can complement future experimental research and
accelerate development of multi-active material electrodes with targeted
performance.",2402.03677v1
2024-02-16,X-ray Linear Dichroic Tomography of Crystallographic and Topological Defects,"The functionality of materials is determined by their composition and
microstructure, that is, the distribution and orientation of crystalline
grains, grain boundaries and the defects within them. The characterisation of
the material's microstructure is therefore critical for materials applications
such as catalysis, energy storage and buildings. Until now, characterization
techniques that map the distribution of grains, their orientation, and the
presence of defects have either been limited to surface investigations, to
spatial resolutions of a few hundred nanometres, or to systems of thickness
around one hundred nanometres, thus requiring destructive sample preparation
for measurements and preventing the study of system-representative volumes or
the investigation of materials under operational conditions. Here, we present
X-ray linear dichroic orientation tomography, a quantitative, non-invasive
technique that allows for an intra- and inter-granular characterisation of
extended polycrystalline and amorphous materials in three dimensions (3D). We
present the detailed characterisation of a polycrystalline sample of vanadium
pentoxide (V2O5), a key catalyst in the production of sulfuric acid. In
addition to determining the nanoscale composition, we map the crystal
orientation throughout the polycrystalline sample with 73 nm spatial
resolution. We identify grains, as well as twist, tilt, and twin grain
boundaries. We further observe the creation and annihilation of topological
defects promoted by the presence of volume crystallographic defects in 3D. Our
method's non-destructive and spectroscopic nature opens the door to in-operando
combined chemical and microstructural investigations of functional materials,
including energy and mechanical materials in existing industries, as well as
quantum materials for future technologies.",2402.10647v1
2024-03-12,Metastable ordered states induced by low temperature annealing of δ-Ag2/3V2O5,"In {\delta}-Ag2/3V2O5 with charge degrees of freedom in V, it is known that
the charge ordering state and physical properties of V that appear at low
temperatures depend strongly on the ordering state of Ag. In this study, we
focused on the Ag ions in the interlayer and studied the structure using
synchrotron radiation powder diffraction in dependence on temperature. We found
that when the sample is slowly cooled from room temperature and ordering occurs
at the Ag sites, V4+/V5+ charge ordering of V and subsequent V4+-V4+ structural
dimers are produced. Although quenching the sample from room temperature
suppresses the ordering of Ag, annealing at around 160 K promotes partial
ordering of Ag and allows a metastable phase to be realized. This metastable
phase is maintained even when the temperature is lowered again, producing a
remarkable change in low-temperature properties. These results indicate that
the ordered state of Ag, which is the key to control the charge-ordered state
and physical properties, can be controlled by low-temperature annealing. The
results of this study may provide a methodology for the realization of
metastable states in a wide range of material groups of vanadium compounds,
where competition among various charge ordered states underlies the physical
properties.",2403.07276v1
2024-03-20,Picosecond Femtojoule Resistive Switching in Nanoscale VO$_{2}$ Memristors,"Beyond-Moore computing technologies are expected to provide a sustainable
alternative to the von Neumann approach not only due to their down-scaling
potential but also via exploiting device-level functional complexity at the
lowest possible energy consumption. The dynamics of the Mott transition in
correlated electron oxides, such as vanadium dioxide, has been identified as a
rich and reliable source of such functional complexity. However, its full
potential in high-speed and low-power operation has been largely unexplored. We
fabricated nanoscale VO$_{2}$ devices embedded in a broad-band test circuit to
study the speed and energy limitations of their resistive switching operation.
Our picosecond time-resolution, real-time resistive switching experiments and
numerical simulations demonstrate that tunable low-resistance states can be set
by the application of 20~ps long, $<$1.7~V amplitude voltage pulses at 15~ps
incubation times and switching energies starting from a few femtojoule.
Moreover, we demonstrate that at nanometer-scale device sizes not only the
electric field induced insulator-to-metal transition, but also the thermal
conduction limited metal-to-insulator transition can take place at timescales
of 100's of picoseconds. These orders of magnitude breakthroughs open the route
to the design of high-speed and low-power dynamical circuits for a plethora of
neuromorphic computing applications from pattern recognition to numerical
optimization.",2403.13530v1
2024-04-11,The photoinduced hidden metallic phase of monoclinic VO2 driven by local nucleation via a self-amplification process,"The insulator-to-metal transition (IMT) in vanadium dioxide (VO2) has
garnered extensive attention for its potential applications in ultrafast
switches, neuronal network architectures, and storage technologies. However, a
significant controversy persists regarding the formation of the IMT,
specifically concerning whether a complete structural phase transition from
monoclinic (M1) to rutile (R) phase is necessary. Here we employ the real-time
time-dependent density functional theory (rt-TDDFT) to track the dynamic
evolution of atomic and electronic structures in photoexcited VO2, revealing
the emergence of a long-lived monoclinic metal phase (MM) under low electronic
excitation. The emergence of the metal phase in the monoclinic structure
originates from the dissociation of the local V-V dimer, driven by the
self-trapped and self-amplified dynamics of photoexcited holes, rather than by
a pure electron-electron correction. On the other hand, the M1-to-R phase
transition does appear at higher electronic excitation. Our findings validate
the existence of MM phase and provide a comprehensive picture of the IMT in
photoexcited VO2.",2404.07693v1
2001-06-14,New Analyses of Star-to-Star Abundance Variations Among Bright Giants in the Mildly Metal-Poor Globular Cluster M5,"We present a chemical composition analysis of 36 giant stars in the mildly
metal-poor globular cluster M5 (NGC 5904). The analysis makes use of high
resolution data acquired at the Keck I telescope as well as a re-analysis of
high resolution spectra acquired for an earlier study at Lick Observatory. We
employed two analysis techniques: one, adopting standard spectroscopic
constraints, and two, adopting an analysis consistent with the non-LTE precepts
as recently described by Thevenin & Idiart. The abundance ratios we derive for
magnesium, silicon, calcium, scandium, titanium, vanadium, nickel, barium and
europium in M5 show no significant abundance variations and the ratios are
comparable to those of halo field stars. However, large variations are seen in
the abundances of oxygen, sodium and aluminum, the elements that are sensitive
to proton-capture nucleosynthesis. In comparing the abundances of M5 and M4
(NGC 6121), another mildly metal-poor globular cluster, we find that silicon,
aluminum, barium and lanthanum are overabundant in M4 with respect to what is
seen in M5, confirming and expanding the results of previous studies. In
comparing the abundances between these two clusters and others having
comparable metallicities, we find that the anti-correlations observed in M5 are
similar to those found in more metal-poor clusters, M3, M10 and M13, whereas
the behavior in M4 is more like that of the more metal-rich globular cluster
M71. We conclude that among stars in Galactic globular clusters, there is no
definitive ``single'' value of [el/Fe] at a given [Fe/H] for at least some
alpha-capture, odd-Z and slow neutron-capture process elements, in this case,
silicon, aluminum, barium and lanthanum.",0106249v1
2005-11-15,"Discovery of a Very Young Field L Dwarf, 2MASS J01415823-4633574","While following up L dwarf candidates selected photometrically from the Two
Micron All Sky Survey, we uncovered an unusual object designated 2MASS
J01415823-4633574. Its optical spectrum exhibits very strong bands of vanadium
oxide but abnormally weak absorptions by titanium oxide, potassium, and sodium.
Morphologically such spectroscopic characteristics fall intermediate between
old, field early-L dwarfs (log(g)~5) and very late M giants (log(g)~0), leading
us to favor low gravity as the explanation for the unique spectral signatures
of this L dwarf. Such a low gravity can be explained only if this L dwarf is
much lower in mass than a typical old field L dwarf of similar temperature and
is still contracting to its final radius. These conditions imply a very young
age. Further evidence of youth is found in the near-infrared spectrum,
including a triangular-shaped H-band continuum reminiscent of young brown dwarf
candidates discovered in the Orion Nebula Cluster. Using the above information
along with comparisons to brown dwarf atmospheric and interior models, our
current best estimate is that this L dwarf has an age of 1-50 Myr and a mass of
6-25 M_Jupiter. The location of 2MASS 0141-4633 on the sky coupled with a
distance estimate of ~35 pc and the above age estimate suggests that this
object may be a brown dwarf member of either the 30-Myr-old Tucana/Horologium
Association or the ~12-Myr-old beta Pic Moving Group.",0511462v1
2010-05-20,Metals in the Exosphere of the Highly Irradiated Planet WASP-12b,"We present near-UV transmission spectroscopy of the highly irradiated
transiting exoplanet WASP-12b, obtained with the Cosmic Origins Spectrograph on
the Hubble Space Telescope. The spectra cover three distinct wavelength ranges:
NUVA (2539-2580 {\AA}), NUVB (2655-2696 {\AA}), and NUVC (2770-2811 {\AA}).
Three independent methods all reveal enhanced transit depths attributable to
absorption by resonance lines of metals in the exosphere of WASP-12b. Light
curves of total counts in the NUVA and NUVC wavelength ranges show a detection
at a 2.5{\sigma} level. We detect extra absorption in the Mg II
{\lambda}{\lambda}2800 resonance line cores at the 2.8{\sigma} level. The NUVA,
NUVB, and NUVC light curves imply effective radii of 2.69+/-0.24 RJ,
2.18+/-0.18 RJ, and 2.66+/-0.22 RJ respectively, suggesting the planet is
surrounded by an absorbing cloud which overfills the Roche lobe. We detect
enhanced transit depths at the wavelengths of resonance lines of neutral
sodium, tin, and manganese, and at singly ionized ytterbium, scandium,
manganese, aluminum, vanadium, and magnesium. We also find the statistically
expected number of anomalous transit depths at wavelengths not associated with
any known resonance line. Our data are limited by photon noise, but taken as a
whole the results are strong evidence for an extended absorbing exosphere
surrounding the planet. The NUVA data exhibit an early ingress, contrary to
model expectations; we speculate this could be due to the presence of a disk of
previously stripped material.",1005.3656v1
2011-04-20,Phase separation and frustrated square lattice magnetism of Na1.5VOPO4F0.5,"Crystal structure, electronic structure, and magnetic behavior of the
spin-1/2 quantum magnet Na1.5VOPO4F0.5 are reported. The disorder of Na atoms
leads to a sequence of structural phase transitions revealed by synchrotron
x-ray powder diffraction and electron diffraction. The high-temperature
second-order alpha <--> beta transition at 500 K is of the order-disorder type,
whereas the low-temperature beta <--> gamma+gamma' transition around 250 K is
of the first order and leads to a phase separation toward the polymorphs with
long-range (gamma) and short-range (gamma') order of Na. Despite the complex
structural changes, the magnetic behavior of Na1.5VOPO4F0.5 probed by magnetic
susceptibility, heat capacity, and electron spin resonance measurements is well
described by the regular frustrated square lattice model of the
high-temperature alpha-polymorph. The averaged nearest-neighbor and
next-nearest-neighbor couplings are J1 ~ -3.7 K and J2 ~ 6.6 K, respectively.
Nuclear magnetic resonance further reveals the long-range ordering below TN =
2.6 K in low magnetic fields. Although the experimental data are consistent
with the simplified square-lattice description, band structure calculations
suggest that the ordering of Na atoms introduces a large number of inequivalent
exchange couplings that split the square lattice into plaquettes. Additionally,
the direct connection between the vanadium polyhedra induces an unusually
strong interlayer coupling having effect on the transition entropy and the
transition anomaly in the specific heat. Peculiar features of the
low-temperature crystal structure and the relation to isostructural materials
suggest Na1.5VOPO4F0.5 as a parent compound for the experimental study of
tetramerized square lattices as well as frustrated square lattices with
different values of spin.",1104.4017v1
2013-01-31,Defect states and excitations in a Mott insulator with orbital degrees of freedom: Mott-Hubbard gap versus optical and transport gaps in doped systems,"We address the role played by charged defects in doped Mott insulators with
active orbital degrees of freedom. It is observed that defects feature a rather
complex and rich physics, which is well captured by a degenerate Hubbard model
extended by terms that describe crystal-field splittings and orbital-lattice
coupling, as well as by terms generated by defects such as the Coulomb
potential terms that act both on doped holes and on electrons within occupied
orbitals at undoped sites. We show that the multiplet structure of the excited
states generated in such systems by strong electron interactions is well
described within the unrestricted Hartree-Fock approximation, once the symmetry
breaking caused by the onset of magnetic and orbital order is taken into
account. Furthermore, we uncover new spectral features that arise within the
Mott-Hubbard gap and in the multiplet spectrum at high energies due to the
presence of defect states and strong correlations. These features reflect the
action on electrons/holes of the generalized defect potential that affects
charge and orbital degrees of freedom, and indirectly also spin ones. The
present study elucidates the mechanism behind the Coulomb gap appearing in the
band of defect states and investigates the dependence on the electron-electron
interactions and the screening by the orbital polarization field. As an
illustrative example of our general approach, we present explicit calculations
for the model describing three t_2g orbital flavors in the perovskite vanadates
doped by divalent Sr or Ca ions, such as in La_(1-x)Sr_xVO_3 and
Y_(1-x)Ca_xVO_3 systems. We analyze the orbital densities at vanadium ions in
the vicinity of defects, and the excited defect states which determine the
optical and transport gaps in doped systems.",1302.0020v2
2013-05-08,Nucleosynthesis in the accretion disks of Type II collapsars,"We investigate nucleosynthesis inside the gamma-ray burst (GRB) accretion
disks formed by the Type II collapsars. In these collapsars, the core collapse
of massive stars first leads to the formation of a proto-neutron star and a
mild supernova explosion is driven. However, this supernova ejecta lack
momentum and falls back onto the neutron star which gets transformed to a
stellar mass black hole. In order to study the hydrodynamics and
nucleosynthesis of such an accretion disk formed from the fallback material of
the supernova ejecta, we use the well established hydrodynamic models. In such
a disk neutrino cooling becomes important in the inner disk where the
temperature and density are higher. Higher the accretion rate (dot{M}), higher
is the density and temperature in the disks. In this work we deal with
accretion disks with relatively low accretion rates: 0.001 M_sun s^{-1}
\lesssim dot{M} \lesssim 0.01 M_sun s^{-1} and hence these disks are
predominantly advection dominated. We use He-rich and Si-rich abundances as the
initial condition of nucleosynthesis at the outer disk, and being equipped with
the disk hydrodynamics and the nuclear network code, we study the abundance
evolution as matter inflows and falls into the central object. We investigate
the variation in the nucleosynthesis products in the disk with the change in
the initial abundance at the outer disk and also with the change in the mass
accretion rate. We report the synthesis of several unusual nuclei like {31}P,
{39}K, {43}Sc, {35}Cl, and various isotopes of titanium, vanadium, chromium,
manganese and copper. We also confirm that isotopes of iron, cobalt, nickel,
argon, calcium, sulphur and silicon get synthesized in the disk, as shown by
previous authors. Much of these heavy elements thus synthesized are ejected
from the disk via outflows and hence they should leave their signature in
observed data.",1305.1755v1
2013-07-17,BaV3O8: A possible Majumdar-Ghosh system with S=1/2,"BaV3O8 contains both magnetic V4+(S=1/2) ions and non-magnetic V5+(S=0) ions.
The V4+ ions are arranged in a coupled Majumdar-Ghosh chain like network. Our
magnetic susceptibility chi(T) data fit well with the Curie-Weiss formula in
the temperature range of 80-300K and it yields a Curie constant
C=0.39cm3K/mole-V4+ and an antiferromagnetic Weiss temperature theta=-26K. The
chi(T) curve shows a broad maximum at T~25K indicative of short-range order
(SRO) and an anomaly corresponding to long-range order (LRO) at TN~6K. The
value of the frustration index (f=mod[theta/TN]~5) suggests that the system is
moderately frustrated. Above the LRO temperature the experimental magnetic
susceptibility data match well with the coupled Majumdar-Ghosh chain model with
the ratio of the nnn (next-nearest neighbor) to nn (nearest neighbor) magnetic
coupling alpha=2 and Jnnn/kB=40K. In a mean-field approach when considering the
inter-chain interactions, we obtain the total inter-chain coupling to be about
16K. The LRO anomaly at TN is also observe in the specific heat Cp(T) data and
is not sensitive to an applied magnetic field up to 90kOe. A 51V NMR signal
corresponding to the non-magnetic vanadium was observed. Anomalies at 6K were
observed in the variation with temperature of the 51V NMR linewidth and in the
spin-lattice relaxation rate 1/T1, indicating that they are sensitive to the
LRO onset and fluctuations at the magnetic V sites. The existence of two
components (one short and another long) is observed in the spin-spin relaxation
rate 1/T2 data in the vicinity of TN. The shorter component seems to be
intimately connected with the magnetically ordered state. We suggest that both
magnetically ordered and non-long range ordered (non-LRO) regions coexist in
this compound below the long range ordering temperature.",1307.4730v1
2015-02-06,Transcriptomic and metabolomic analysis of copper stress acclimation in Ectocarpus siliculosus highlights signaling and tolerance mechanisms in brown algae,"Brown algae are sessile macro-organisms of great ecological relevance in
coastal ecosystems. They evolved independently from land plants and other
multicellular lineages, and therefore hold several original ontogenic and
metabolic features. Most brown algae grow along the coastal zone where they
face frequent environmental changes, including exposure to toxic levels of
heavy metals such as copper (Cu). We carried out large-scale transcriptomic and
metabolomic analyses to decipher the short-term acclimation of the brown algal
model E. siliculosus to Cu stress, and compared these data to results known for
other abiotic stressors. This comparison demonstrates that Cu induces oxidative
stress in E. siliculosus as illustrated by the transcriptomic overlap between
Cu and H2O2 treatments. The common response to Cu and H2O2 consisted in the
activation of the oxylipin and the repression of inositol signaling pathways,
together with the regulation of genes coding for several
transcription-associated proteins. Concomitantly, Cu stress specifically
activated a set of genes coding for orthologs of ABC transporters, a P1B-type
ATPase, ROS detoxification systems such as a vanadium-dependent
bromoperoxidase, and induced an increase of free fatty acid contents. Finally
we observed, as a common abiotic stress mechanism, the activation of autophagic
processes on one hand and the repression of genes involved in nitrogen
assimilation on the other hand. Comparisons with data from green plants
indicate that some processes involved in Cu and oxidative stress response are
conserved across these two distant lineages. At the same time the high number
of yet uncharacterized brown alga-specific genes induced in response to copper
stress underlines the potential to discover new components and molecular
interactions unique to these organisms. Of particular interest for future
research is the potential cross-talk between reactive oxygen species (ROS)-,
myo-inositol-, and oxylipin signaling.",1502.02001v1
2015-05-07,Role of orbital degrees of freedom in investigating the magnetic properties of geometrically frustrated vanadium spinels,"The inconsistency about the degree of geometrical frustration has been a long
issue in AV$_{2}$O$_{4}$ (A $\equiv$ Zn, Cd and Mg) compounds, which arises
from the two experimental results: (i) frustration indices and (ii) magnetic
moments. In the present study, we try to understand such inconsistency by using
{\it ab initio} electronic structure calculations. The orbital degrees of
freedom are found to play an important role in understanding the geometrically
frustrated magnetic behaviour of these compounds. The inclusion of the orbital
and spin angular momenta for calculating the frustration indices improves the
understanding about the degree of geometrical frustration in these compounds.
The calculated values of the frustration indices ($f$$_{\it J}$) are largest
for MgV$_{2}$O$_{4}$ and smallest for CdV$_{2}$O$_{4}$ for 3.3$\leq$ $U
\leq$5.3 eV. In this range of $U$, the calculated values of
$\Delta$M$_{2}$=M$_{\rm total}$-M$_{\rm exp}$ are largest for MgV$_{2}$O$_{4}$
and smallest for CdV$_{2}$O$_{4}$. Hence, the consistency about the degree of
geometrical frustration is achieved. The absolute values of the nearest
neighbour exchange coupling constant ({\it J$_{nn}$}) between V spins are found
to be largest for MgV$_{2}$O$_{4}$ and smallest for CdV$_{2}$O$_{4}$, which
indicate that the calculated absolute values of the Curie-Weiss temperature
($\varTheta$$_{CW}$)$_{\it J}$ are highest for MgV$_{2}$O$_{4}$ and smallest
for CdV$_{2}$O$_{4}$ for 3.3$\leq$ $U \leq$5.3 eV. In this range of $U$, the
magnetic transition temperature ($T$$_{N}$)$_{\it J}$ is found to be $\sim$150
K, $\sim$60 K and $\sim$22 K for MgV$_{2}$O$_{4}$, ZnV$_{2}$O$_{4}$ and
CdV$_{2}$O$_{4}$, respectively, which shows that the order of ($T$$_{N}$)$_{\it
J}$ is similar to that of ($T$$_{N}$)$_{\rm exp}$ for these compounds.",1505.01601v2
2015-12-30,"The phase transition in VO2 probed using x-ray, visible and infrared radiations","Vanadium dioxide (VO2) is a model system that has been used to understand
closely-occurring multiband electronic (Mott) and structural (Peierls)
transitions for over half a century due to continued scientific and
technological interests. Among the many techniques used to study VO2, the most
frequently used involve electromagnetic radiation as a probe. Understanding of
the distinct physical information provided by different probing radiations is
incomplete, mostly owing to the complicated nature of the phase transitions.
Here we use transmission of spatially averaged infrared ({\lambda}=1500 nm) and
visible ({\lambda}=500 nm) radiations followed by spectroscopy and nanoscale
imaging using x-rays ({\lambda}=2.25-2.38 nm) to probe the same VO2 sample
while controlling the ambient temperature across its hysteretic phase
transitions and monitoring its electrical resistance. We directly observed
nanoscale puddles of distinct electronic and structural compositions during the
transition. The two main results are that, during both heating and cooling, the
transition of infrared and visible transmission occur at significantly lower
temperatures than the Mott transition; and the electronic (Mott) transition
occurs before the structural (Peierls) transition in temperature. We use our
data to provide insights into possible microphysical origins of the different
transition characteristics. We highlight that it is important to understand
these effects because small changes in the nature of the probe can yield
quantitatively, and even qualitatively, different results when applied to a
non-trivial multiband phase transition. Our results guide more judicious use of
probe type and interpretation of the resulting data.",1512.08921v2
2016-06-01,An optical transmission spectrum of the transiting hot Jupiter in the metal-poor WASP-98 planetary system,"The WASP-98 planetary system represents a rare case of a hot Jupiter hosted
by a metal-poor main-sequence star. We present a follow-up study of this system
based on multi-band photometry and high-resolution spectroscopy. Two new
transit events of WASP-98b were simultaneously observed in four passbands
(g,r,i,z), using the telescope-defocussing technique, yielding eight
high-precision light curves with point-to-point scatters of less than 1 mmag.
We also collected three spectra of the parent star with a high-resolution
spectrograph, which we used to remeasure its spectral characteristics, in
particular its metallicity. We found this to be very low, Fe/H]=-0.49, but
larger than was previously reported, [Fe/H]=-0.60. We used these new
photometric and spectroscopic data to refine the orbital and physical
properties of this planetary system, finding that the stellar and planetary
mass measurements are significantly larger than those in the discovery paper.
In addition, the multi-band light curves were used to construct an optical
transmission spectrum of WASP-98b and probe the characteristics of its
atmosphere at the terminator. We measured a lower radius at z compared with the
other three passbands. The maximum variation is between the r and z bands, has
a confidence level of roughly 6 sigma and equates to 5.5 pressure scale
heights. We compared this spectrum to theoretical models, investigating several
possible types of atmospheres, including hazy, cloudy, cloud-free, and clear
atmospheres with titanium and vanadium oxide opacities. We could not find a
good fit to the observations, except in the extreme case of a clear atmosphere
with TiO and VO opacities, in which the condensation of Ti and V was
suppressed. As this case is unrealistic, our results suggest the presence of an
additional optical-absorbing species in the atmosphere of WASP-98b, of unknown
chemical nature.",1606.00432v1
2016-11-07,Self-consistent evaluation of effective Coulomb interaction of V atom and its importance to understand the comparative electronic behaviour of vanadium spinels,"In present work, we try to understand the importance of effective Coulomb
interaction ($U_{ef}$) between localized electrons of V atom to understand the
comparative electronic behaviour of AV$_{2}$O$_{4}$ (A=Zn, Cd and Mg)
compounds. The suitable values of $d$-linearization energy ($E_{d}$) of
impurity V atom for calculating the $U_{ef}$ for these compounds are found to
be $\geq$44.89 eV above the Fermi level. Corresponding to these values of
$E_{d}$, the self-consistently calculated values of effective $U_{LSDA}$
($U_{PBEsol}$) for ZnV$_{2}$O$_{4}$, MgV$_{2}$O$_{4}$ and CdV$_{2}$O$_{4}$ are
$\sim$5.73 ($\sim$5.92), $\sim$6.06 ($\sim$6.22) and $\sim$5.59 ($\sim$5.71)
eV, respectively. The calculated values of $\frac{t}{U_{ef}}$ ($t$ is the
transfer integral between neighbouring sites) increases with decreasing V-V
distance from CdV$_{2}$O$_{4}$ to MgV$_{2}$O$_{4}$ to ZnV$_{2}$O$_{4}$ and are
found to be consistent with experimentally reported band gap. The values of
$\frac{t}{U_{ef}}$ for ZnV$_{2}$O$_{4}$, MgV$_{2}$O$_{4}$ and CdV$_{2}$O$_{4}$
are found to be $\sim$0.023, $\sim$0.020 and $\sim$0.018, respectively. Hence,
CdV$_{2}$O$_{4}$ with small (large) $\frac{t}{U_{ef}}$ (experimental band gap)
as compared to ZnV$_{2}$O$_{4}$ and MgV$_{2}$O$_{4}$ is found to be in
localized-electron regime, while ZnV$_{2}$O$_{4}$ and MgV$_{2}$O$_{4}$ are
intermediate between localized and an itinerant-electron regime. The calculated
values of lattice parameters $a_{LSDS}$ ($a_{PBEsol}$) are found to be
$\sim$1.7\%, $\sim$2.0\% and $\sim$2.4\% ($\sim$0.6\%, $\sim$0.7\% and
$\sim$0.7\%) smaller than $a_{exp}$ for CdV$_{2}$O$_{4}$, MgV$_{2}$O$_{4}$ and
ZnV$_{2}$O$_{4}$, respectively, which indicates that the PBEsol functional
predicts the lattice parameters in good agreement with the experimental data.",1611.02028v1
2016-12-05,Electronic structure study of vanadium spinels by using density functional theory and dynamical mean field theory,"Theoretically, various physical properties of AV$_{2}$O$_{4}$ (A=Zn, Cd and
Mg) spinels have been extensively studied for last 15 years. Besides of this,
no systematic comparative study has been done for these compounds, where the
material specific parameters are used. Here, we report the comparative
electronic behaviour of these spinels by using a combination of density
functional theory and dynamical mean-field theory, where the self-consistent
calculated Coulomb interaction $U$ and Hund's coupling $J$ (determined by
Yukawa screening $\lambda$) are used. The main features, such as insulating
band gaps ($E_{g}$), degree of itinerancy of V 3$d$ electrons and position of
lower Hubbard band are observed for these parameters in these spinels. The
calculated values of $E_{g}$ for ZnV$_{2}$O$_{4}$, CdV$_{2}$O$_{4}$ and
MgV$_{2}$O$_{4}$ are found to be $\sim$0.9 eV, $\sim$0.95 eV and $\sim$1.15 eV,
respectively, where the values of $E_{g}$ are close to experiment for
ZnV$_{2}$O$_{4}$ and MgV$_{2}$O$_{4}$. The position of lower Hubbard band are
observed around $\sim$-1.05 eV, $\sim$-1.25 eV and $\sim$-1.15 eV for
ZnV$_{2}$O$_{4}$, CdV$_{2}$O$_{4}$ and MgV$_{2}$O$_{4}$, respectively, which
are also in good agreement with the experimental data for ZnV$_{2}$O$_{4}$. The
order of average impurity hybridization function of V site are found to be
ZnV$_{2}$O$_{4}$$>$MgV$_{2}$O$_{4}$$>$CdV$_{2}$O$_{4}$. Hence, the degree of
localization of V 3$d$ electrons is largest for CdV$_{2}$O$_{4}$ and smallest
for ZnV$_{2}$O$_{4}$, which is in accordance with our earlier results. Hence,
present work shows the importance of material specific parameters to understand
the comparative electronic behaviour of these compounds.",1612.01273v2
2017-08-03,An ultrahot gas-giant exoplanet with a stratosphere,"Infrared radiation emitted from a planet contains information about the
chemical composition and vertical temperature profile of its atmosphere. If
upper layers are cooler than lower layers, molecular gases will produce
absorption features in the planetary thermal spectrum. Conversely, if there is
a stratosphere - where temperature increases with altitude - these molecular
features will be observed in emission. It has been suggested that stratospheres
could form in highly irradiated exoplanets, but the extent to which this occurs
is unresolved both theoretically and observationally. A previous claim for the
presence of a stratosphere remains open to question, owing to the challenges
posed by the highly variable host star and the low spectral resolution of the
measurements. Here we report a near-infrared thermal spectrum for the ultrahot
gas giant WASP-121b, which has an equilibrium temperature of approximately
2,500 kelvin. Water is resolved in emission, providing a detection of an
exoplanet stratosphere at 5-sigma confidence. These observations imply that a
substantial fraction of incident stellar radiation is retained at high
altitudes in the atmosphere, possibly by absorbing chemical species such as
gaseous vanadium oxide and titanium oxide.",1708.01076v1
2017-08-16,Stochastic IMT (insulator-metal-transition) neurons: An interplay of thermal and threshold noise at bifurcation,"Artificial neural networks can harness stochasticity in multiple ways to
enable a vast class of computationally powerful models. Electronic
implementation of such stochastic networks is currently limited to addition of
algorithmic noise to digital machines which is inherently inefficient; albeit
recent efforts to harness physical noise in devices for stochasticity have
shown promise. To succeed in fabricating electronic neuromorphic networks we
need experimental evidence of devices with measurable and controllable
stochasticity which is complemented with the development of reliable
statistical models of such observed stochasticity. Current research literature
has sparse evidence of the former and a complete lack of the latter. This
motivates the current article where we demonstrate a stochastic neuron using an
insulator-metal-transition (IMT) device, based on electrically induced
phase-transition, in series with a tunable resistance. We show that an IMT
neuron has dynamics similar to a piecewise linear FitzHugh-Nagumo (FHN) neuron
and incorporates all characteristics of a spiking neuron in the device
phenomena. We experimentally demonstrate spontaneous stochastic spiking along
with electrically controllable firing probabilities using Vanadium Dioxide
(VO$_2$) based IMT neurons which show a sigmoid-like transfer function. The
stochastic spiking is explained by two noise sources - thermal noise and
threshold fluctuations, which act as precursors of bifurcation. As such, the
IMT neuron is modeled as an Ornstein-Uhlenbeck (OU) process with a fluctuating
boundary resulting in transfer curves that closely match experiments. As one of
the first comprehensive studies of a stochastic neuron hardware and its
statistical properties, this article would enable efficient implementation of a
large class of neuro-mimetic networks and algorithms.",1708.06238v4
2017-09-13,Detection of titanium oxide in the atmosphere of a hot Jupiter,"As an exoplanet transits its host star, some of the light from the star is
absorbed by the atoms and molecules in the planet's atmosphere, causing the
planet to seem bigger; plotting the planet's observed size as a function of the
wavelength of the light produces a transmission spectrum. Measuring the tiny
variations in the transmission spectrum, together with atmospheric modelling,
then gives clues to the properties of the exoplanet's atmosphere. Chemical
species composed of light elements$-$such as hydrogen, oxygen, carbon, sodium
and potassium$-$have in this way been detected in the atmospheres of several
hot giant exoplanets, but molecules composed of heavier elements have thus far
proved elusive. Nonetheless, it has been predicted that metal oxides such as
titanium oxide (TiO) and vanadium oxide occur in the observable regions of the
very hottest exoplanetary atmospheres, causing thermal inversions on the
dayside. Here we report the detection of TiO in the atmosphere of the
hot-Jupiter planet WASP-19b. Our combined spectrum, with its wide spectral
coverage, reveals the presence of TiO (to a confidence level of 7.7{\sigma}), a
strongly scattering haze (7.4{\sigma}) and sodium (3.4{\sigma}), and confirms
the presence of water (7.9{\sigma}) in the atmosphere.",1709.04118v1
2017-10-14,Fingerprints of spin-orbital polarons and of their disorder in the photoemission spectra of doped Mott insulators with orbital degeneracy,"We explore the effects of disordered charged defects on the electronic
excitations observed in the photoemission spectra of doped transition metal
oxides in the Mott insulating regime by the example of the
$R_{1-x}$Ca$_x$VO$_3$ perovskites, where $R=$La,$\dots$,Lu. A fundamental
characteristic of these vanadium $d^2$ compounds with partly filled $t_{2g}$
valence orbitals is the persistence of spin and orbital order up to high
doping, in contrast to the loss of magnetic order in high-$T_c$ cuprates at low
defect concentration. We demonstrate that the disordered electronic structure
of doped Mott-Hubbard insulators can be obtained with high precision within the
unrestricted Hartree-Fock approximation. In particular: (i) the atomic
multiplet excitations in the inverse photoemission spectra and the various
defect-related states and satellites are well reproduced, (ii) a robust Mott
gap survives up to large doping, and (iii) we show that the defect states
inside the Mott gap develop a soft gap at the Fermi energy. The soft defect
states gap can be characterized by a shape and a scale parameter extracted from
a Weibull statistical sampling of the density of states near the chemical
potential. We demonstrate that charge defects trigger small spin-orbital
polarons, with their internal kinetic energy responsible for the opening of the
soft defect states gap. The small size of spin-orbital polarons is inferred by
an analysis of the inverse participation ratio which explains the origin of the
robustness of spin and orbital order. Using realistic parameters for
La$_{1-x}$Ca$_x$VO$_3$, we show that its soft gap is well reproduced as well as
the marginal doping dependence of the position of the chemical potential
relative to the center of the lower Hubbard band.",1710.05171v1
2018-02-22,"Super-Solar Metallicity Stars in the Galactic Center Nuclear Star Cluster: Unusual Sc, V, and Y Abundances","We present adaptive-optics assisted near-infrared high-spectral resolution
observations of late-type giants in the nuclear star cluster of the Milky Way.
The metallicity and elemental abundance measurements of these stars offer us an
opportunity to understand the formation and evolution of the nuclear star
cluster. In addition, their proximity to the supermassive black hole ($\sim
0.5$ pc) offers a unique probe of the star formation and chemical enrichment in
this extreme environment. We observed two stars identified by medium
spectral-resolution observations as potentially having very high metallicities.
We use spectral-template fitting with the PHOENIX grid and Bayesian inference
to simultaneously constrain the overall metallicity, [M/H], alpha-element
abundance [$\alpha$/Fe], effective temperature, and surface gravity of these
stars. We find that one of the stars has very high metallicity ([M/H] $> 0.6$)
and the other is slightly above solar metallicity. Both Galactic center stars
have lines from scandium (Sc), vanadium (V), and yttrium (Y) that are much
stronger than allowed by the PHOENIX grid. We find, using the spectral
synthesis code Spectroscopy Made Easy, that [Sc/Fe] may be an order of
magnitude above solar. For comparison, we also observed an empirical calibrator
in NGC6791, the highest metallicity cluster known ([M/H] $\sim 0.4$). Most
lines are well matched between the calibrator and the Galactic center stars,
except for Sc, V, and Y, which confirms that their abundances must be
anomalously high in these stars. These unusual abundances, which may be a
unique signature of nuclear star clusters, offer an opportunity to test models
of chemical enrichment in this region.",1802.08270v2
2018-05-02,Performance Enhancement of Nanoscale ${\rm VO}_2$ Modulators Using Hybrid Plasmonics,"Vanadium dioxide (${\rm VO}_2$) is a phase change material (PCM) that
exhibits a large change in complex refractive index on the order of unity upon
switching from its dielectric to its metallic phase. Although this property is
key for the design of ultracompact optical modulators of only a few microns in
footprint, the high absorption of ${\rm VO}_2$ leads to appreciable insertion
loss (IL) that limits the modulator performance. In this paper, through theory
and numerical modeling, we report on a new paradigm, which demonstrates how the
use of a hybrid plasmonic waveguide to construct a ${\rm VO}_2$ based modulator
can improve the performance by minimizing its IL while achieving high
extinction ratio (ER) in comparison to a purely dielectric waveguide. The
hybrid plasmonic waveguide that contains an additional metal layer with even
higher loss than ${\rm VO}_2$ enables unique approaches to engineer the
electric field (E-field) intensity distribution within the cross section of the
modulator. The resulting figure-of-merit, FoM = ER/IL, is much higher than what
is possible by simply incorporating ${\rm VO}_2$ into a silicon wire waveguide.
A practical modulator design using this new approach, which also includes input
and output couplers yields ER = 3.8 dB/{\mu}m and IL = 1.4 dB/{\mu}m (FoM =
2.7), with a 3-dB optical bandwidth >500 nm, in a device length = 2 {\mu}m, and
cross-sectional dimensions = 200 nm $\times$ 450 nm. To the best of our
knowledge, this is one of the smallest modulator designs proposed to date that
also exhibits amongst the highest ER, FoM, and optical bandwidth, in comparison
to existing designs. In addition to ${\rm VO}_2$, we investigate two other PCMs
incorporated within the waveguide structure. The improvements obtained for
${\rm VO}_2$ modulators do not extend to other PCMs.",1805.01005v1
2018-07-15,LiZn$_{2}$V$_{3}$O$_{8}$: A new geometrically frustrated cluster spin-glass,"We have investigated the structural and magnetic properties of a new cubic
spinel LiZn$_{2}$V$_{3}$O$_{8}$ (LZVO) through x-ray diffraction, dc and ac
susceptibility, magnetic relaxation, aging, memory effect, heat capacity and
$^{7}$Li nuclear magnetic resonance (NMR) measurements. A Curie-Weiss fit of
the dc susceptibility $\chi_{\mathrm{dc}}$($\mathit{T}$) yields a Curie-Weiss
temperature $\mathrm{\theta}_{\mathrm{CW}}$ = -185 K. This suggests strong
antiferromagnetic (AFM) interactions among the magnetic vanadium ions. The dc
and ac susceptibility data indicate the spin-glass behavior below a freezing
temperature $T_{f}$ $\simeq$ 3 K. The frequency dependence of the $T_{f}$ is
characterized by the Vogel-Fulcher law and critical dynamic scaling behavior or
power law. From both fitting, we obtained the value of the characteristic
angular frequency $\omega_{0}$ $\approx$ 3.56$\times$10$^{6}$ Hz, the dynamic
exponent $\mathit{zv}$ $\approx$ 2.65, and the critical time constant
$\tau_{0}$ $\approx$ 1.82$\times$10$^{-6}$ s, which falls in the conventional
range for typical cluster spin-glass (CSG) systems. The value of relative shift
in freezing temperature $\delta T_{f}$ $\simeq$ 0.039 supports a CSG ground
states. We also found aging phenomena and memory effects in LZVO. The
asymmetric response of the magnetic relaxation below $T_{f}$ supports the
hierarchical model. Heat capacity data show no long-range or short-range
ordering down to 2 K. Only about 25% magnetic entropy change $(\Delta
S_{\mathrm{m}})$ signifies the presence of strong frustration in the system.
The $^{7}$Li NMR spectra show a shift and broadening with decreasing
temperature. The spin-lattice and spin-spin relaxation rates show anomalies due
to spin freezing around 3 K as the bulk magnetization.",1807.05612v3
2019-10-03,Control of spintronic and electronic properties of bimetallic and vacancy-ordered vanadium carbide MXenes via surface functionalization,"MXenes are 2D transition metal carbides with high potential for overcoming
limitations of conventional two-dimensional electronics. In this context,
various MXenes have shown magnetic properties suitable for applications in
spintronics, yet the number of MXenes reported so far is far smaller than their
parental MAX phases. Therefore, we have studied the structural, electronic and
magnetic properties of bimetallic and vacancy-ordered MXenes derived from a new
$(V_{2/3}Zr_{1/3})_2AlC $ MAX phase to assess whether MXene exfoliation would
return stable magnetic materials. In particular, we have investigated the
properties of pristine and surface-functionalized $(V_{2/3}Zr_{1/3})_2CX_2$
bimetallic and $(V_{2/3} \square_{1/3})_2CX2$ vacancy-ordered MXenes with X =
O, F and OH. Our density functional theory (DFT) calculations showed that
modifying the MXene stoichiometry and/or MXene surface functionalization
changes MXene properties. After testing all possible combinations of metallic
motifs and functionalization, we identified $V_{2/3}Zr_{1/3})_2CX_2$,
$(V_{2/3}\square_{1/3})_2CF_2$ and $(V_{2/3}\square_{1/3})_2C(OH)_2$ as stable
structures. Among them, $(V_{2/3}Zr_{1/3})_2CO_2$ MXene is predicted to be an
FM intrinsic half-semiconductor with a remarkably high Curie temperature
($T_C$)) of 270 K. The $(V_{2/3}Zr_{1/3})_2C(OH)_2$ MXene exhibits a rather low
work function (WF) (1.37 eV) and is thus a promising candidate for ultra-low
work function electron emitters. Conversely, the $(V_{2/3}\square_{1/3})_2CF_2$
MXene has a rather high WF and hence can be used as a hole injector for
Schottky-barrier-free contact applications. Overall, our proof-of-concept study
shows that theoretical predictions of MXene exfoliation and properties support
further experimental research towards developing spintronics devices.",1910.01509v1
2020-01-02,Detailed Iron-Peak Element Abundances in Three Very Metal-Poor Stars,"We have obtained new detailed abundances of the Fe-group elements Sc through
Zn (Z=21-30) in three very metal-poor ([Fe/H] $\approx -3$) stars: BD 03 740,
BD -13 3442 and CD -33 1173. High-resolution ultraviolet HST/STIS spectra in
the wavelength range 2300-3050\AA\ were gathered, and complemented by an
assortment of optical echelle spectra. The analysis featured recent laboratory
atomic data for number of neutral and ionized species for all Fe-group elements
except Cu and Zn. A detailed examination of scandium, titanium, and vanadium
abundances in large-sample spectroscopic surveys indicates that they are
positively correlated in stars with [Fe/H]<-$2. The abundances of these
elements in BD 03 740, BD -13 3442 and CD -33 1173 and HD 84937. (studied in a
previous paper of this series) are in accord with these trends and lie at the
high end of the correlations. Six elements have detectable neutral and ionized
features, and generally their abundances are in reasonable agreement. For Cr we
find only minimal abundance disagreement between the neutral (mean of
[Cri/Fe]=+0.01) and ionized species (mean of [Crii/Fe]=+0.08), unlike most
studies in the past. The prominent exception is Co, for which the neutral
species indicates a significant overabundance (mean of [Co/H]=-2.53), while no
such enhancement is seen for the ionized species (mean of [Coii/H]=-2.93).
These new stellar abundances, especially the correlations among Sc, Ti, and V,
suggest that models of element production in early high-mass metal-poor stars
should be revisited.",2001.00541v1
2020-01-06,Thermal coupling and effect of subharmonic synchronization in a system of two VO2 based oscillators,"We explore a prototype of an oscillatory neural network (ONN) based on
vanadium dioxide switching devices. The model system under study represents two
oscillators based on thermally coupled VO2 switches. Numerical simulation shows
that the effective action radius RTC of coupling depends both on the total
energy released during switching and on the average power. It is experimentally
and numerically proved that the temperature change dT commences almost
synchronously with the released power peak and T-coupling reveals itself up to
a frequency of about 10 kHz. For the studied switching structure configuration,
the RTC value varies over a wide range from 4 to 45 mkm, depending on the
external circuit capacitance C and resistance Ri, but the variation of Ri is
more promising from the practical viewpoint. In the case of a ""weak"" coupling,
synchronization is accompanied by attraction effect and decrease of the main
spectra harmonics width. In the case of a ""strong"" coupling, the number of
effects increases, synchronization can occur on subharmonics resulting in
multilevel stable synchronization of two oscillators. An advanced algorithm for
synchronization efficiency and subharmonic ratio calculation is proposed. It is
shown that of the two oscillators the leading one is that with a higher main
frequency, and, in addition, the frequency stabilization effect is observed.
Also, in the case of a strong thermal coupling, the limit of the supply current
parameters, for which the oscillations exist, expands by ~ 10 %. The obtained
results have a universal character and open up a new kind of coupling in ONNs,
namely, T-coupling, which allows for easy transition from 2D to 3D integration.
The effect of subharmonic synchronization hold promise for application in
classification and pattern recognition.",2001.01382v1
2020-03-18,Imaging domain reversal in an ultrathin van der Waals ferromagnet,"The recent isolation of two-dimensional van der Waals magnetic materials has
uncovered rich physics that often differs from the magnetic behaviour of their
bulk counterparts. However, the microscopic details of fundamental processes
such as the initial magnetization or domain reversal, which govern the magnetic
hysteresis, remain largely unknown in the ultrathin limit. Here we employ a
widefield nitrogen-vacancy (NV) microscope to directly image these processes in
few-layer flakes of magnetic semiconductor vanadium triiodide (VI$_3$). We
observe complete and abrupt switching of most flakes at fields
$H_c\approx0.5-1$ T (at 5 K) independent of thickness down to two atomic
layers, with no intermediate partially-reversed state. The coercive field
decreases as the temperature approaches the Curie temperature ($T_c\approx50$
K), however, the switching remains abrupt. We then image the initial
magnetization process, which reveals thickness-dependent domain wall depinning
fields well below $H_c$. These results point to ultrathin VI$_3$ being a
nucleation-type hard ferromagnet, where the coercive field is set by the
anisotropy-limited domain wall nucleation field. This work illustrates the
power of widefield NV microscopy to investigate magnetization processes in van
der Waals ferromagnets, which could be used to elucidate the origin of the hard
ferromagnetic properties of other materials and explore field- and
current-driven domain wall dynamics.",2003.08470v1
2020-04-01,"A simulational study of the indirect geometry neutron spectrometer, BIFROST at the European Spallation Source, from neutron source position to detector position","The European Spallation Source (ESS) is intended to become the most powerful
spallation neutron source in the world and the flagship of neutron science in
the upcoming decades. The exceptionally high neutron flux will provide unique
opportunities for scientific experiments, but also set high requirements for
the detectors. One of the most challenging aspects is the rate capability and
in particular the peak instantaneous rate capability, i.e. the number of
neutrons hitting the detector per channel or cm$^2$ at the peak of the neutron
pulse. The primary purpose of this paper is to estimate the incident rates that
are anticipated for the BIFROST instrument planned for ESS, and also to
demonstrate the use of powerful simulation tools for the correct interpretation
of neutron transport in crystalline materials. A full simulation model of the
instrument from source to detector position, implemented with the use of
multiple simulation software packages is presented. For a single detector tube
instantaneous incident rates with a maximum of 1.7 GHz for a Bragg peak from a
single crystal, and 0.3 MHz for a vanadium sample are found. This paper also
includes the first application of a new pyrolytic graphite model, and a
comparison of different simulation tools to highlight their strengths and
weaknesses.",2004.00335v2
2020-05-05,"ARES I: WASP-76 b, A Tale of Two HST Spectra","We analyse the transmission and emission spectra of the ultra-hot Jupiter
WASP-76b, observed with the G141 grism of the Hubble Space Telescope's Wide
Field Camera 3 (WFC3). We reduce and fit the raw data for each observation
using the open-source software Iraclis before performing a fully Bayesian
retrieval using the publicly available analysis suite TauRex 3. Previous
studies of the WFC3 transmission spectra of WASP-76 b found hints of titanium
oxide (TiO) and vanadium oxide (VO) or non-grey clouds. Accounting for a
fainter stellar companion to WASP-76, we reanalyse this data and show that
removing the effects of this background star changes the slope of the spectrum,
resulting in these visible absorbers no longer being detected, eliminating the
need for a non-grey cloud model to adequately fit the data but maintaining the
strong water feature previously seen. However, our analysis of the emission
spectrum suggests the presence of TiO and an atmospheric thermal inversion,
along with a significant amount of water. Given the brightness of the host star
and the size of the atmospheric features, WASP-76 b is an excellent target for
further characterisation with HST, or with future facilities, to better
understand the nature of its atmosphere, to confirm the presence of TiO and to
search for other optical absorbers.",2005.02374v2
2020-10-26,"Database, Features, and Machine Learning Model to Identify Thermally Driven Metal-Insulator Transition Compounds","Metal-insulator transition (MIT) compounds are materials that may exhibit
insulating or metallic behavior, depending on the physical conditions, and are
of immense fundamental interest owing to their potential applications in
emerging microelectronics. There is a dearth of thermally-driven MIT materials,
however, which makes delineating these compounds from those that are
exclusively insulating or metallic challenging. Here we report a material
database comprising temperature-controlled MITs (and metals and insulators with
similar chemical composition and stoichiometries to the MIT compounds) from
high quality experimental literature, built through a combination of
materials-domain knowledge and natural language processing. We featurize the
dataset using compositional, structural, and energetic descriptors, including
two MIT relevant energy scales, an estimated Hubbard interaction and the charge
transfer energy, as well as the structure-bond-stress metric referred to as the
global-instability index (GII). We then perform supervised classification,
constructing three electronic-state classifiers: metal vs non-metal (M),
insulator vs non-insulator (I), and MIT vs non-MIT (T). We identify two
important descriptors that separate metals, insulators, and MIT materials in a
2D feature space: the average deviation of the covalent radius and the range of
the Mendeleev number. We further elaborate on other important features (GII and
Ewald energy), and examine how they affect classification of binary vanadium
and titanium oxides. We discuss the relationship of these atomic features to
the physical interactions underlying MITs in the rare-earth nickelate family.
Last, we implement an online version of the classifiers, enabling quick
probabilistic class predictions by uploading a crystallographic structure file.",2010.13306v2
2020-12-09,The Breakdown of Mott Physics at VO$_2$ Surfaces,"Transition metal oxides such as vanadium dioxide (VO$_2$), niobium dioxide
(NbO$_2$), and titanium sesquioxide (Ti$_2$O$_3$) are known to undergo a
temperature-dependent metal-insulator transition (MIT) in conjunction with a
structural transition within their bulk. However, it is not typically discussed
how breaking crystal symmetry via surface termination affects the complicated
MIT physics. Using synchrotron-based x-ray spectroscopy, low energy electron
diffraction (LEED), low energy electron microscopy (LEEM), transmission
electron microscopy (TEM), and several other experimental techniques, we show
that suppression of the bulk structural transition is a common feature at
VO$_2$ surfaces. Our density functional theory (DFT) calculations further
suggest that this is due to inherent reconstructions necessary to stabilize the
surface, which deviate the electronic structure away from the bulk d$^1$
configuration. Our findings have broader ramifications not only for the
characterization of other ""Mott-like"" MITs, but also for any potential device
applications of such materials.",2012.05306v1
2020-12-22,An Approach to Implement Photovoltaic Self-Consumption and Ramp-Rate Control Algorithm with a Vanadium Redox Flow Battery Day-to-Day Forecast Charging,"The variability of the solar resource is mainly caused by cloud passing,
causing rapid power fluctuations on the output of photovoltaic (PV) systems.
The fluctuations can negatively impact the electric grid, and smoothing
techniques can be used as attempts to correct it. However, the integration of a
PV+VRFB to deal with the extreme power ramps at a building scale is
underexplored in the literature, as well as its effectiveness in combination
with other energy management strategies (EMSs). This work is focused on using a
VRFB to control the power output of the PV installation, maintaining the ramp
rate within a non-violation limit and within a battery state of charge (SOC)
range, appropriate to perform the ramp rate management. Based on the model
simulation, energy key-performance indicators (KPI) are studied, and validation
in real-time is carried. Three EMSs are simulated: a self-consumption
maximization (SCM), and SCM with ramp rate control (SCM+RR), and this last
strategy includes a night battery charging based on a day ahead weather
forecast (SCM+RR+WF). Results show a battery SOC management control is
essential to apply these EMSs on VRFB, and the online weather forecast proves
to be efficient in real-time application. SCM+RR+WF is a robust approach to
manage PV+VRFB systems in wintertime (studied application), and high PV
penetration building areas make it a feasible approach. Over the studied week,
the strategy successfully controlled 100% of the violating power ramps, also
obtaining a self-consumption ratio (SCR) of 59% and a grid-relief factor (GRF)
of 61%.",2012.11955v4
2021-02-26,Role of chemical disorder in tuning the Weyl points in vanadium doped Co$_2$TiSn,"The lack of time-reversal symmetry and Weyl fermions give exotic transport
properties to Co-based Heusler alloys. In the present study, we have
investigated the role of chemical disorder on the variation of Weyl points in
Co\textsubscript{2}Ti\textsubscript{1-x}V\textsubscript{x}Sn magnetic Weyl
semimetal candidate. We employ the first principle approach to track the
evolution of the nodal lines responsible for the appearance of Weyl node in
Co$_2$TiSn as a function of V substitution in place of Ti. By increasing the V
concentration in place of Ti, the nodal line moves toward Fermi level and
remains at Fermi level around the middle composition. Further increase of the V
content, leads shifting of nodal line away from Fermi level. Density of state
calculation shows half-metallic behavior for the entire range of composition.
The magnetic moment on each Co atom as a function of V concentration increases
linearly up to x=0.4, and after that, it starts decreasing. We also
investigated the evolution of the Weyl nodes and Fermi arcs with chemical
doping. The first-principles calculations reveal that via replacing almost half
of the Ti with V, the intrinsic anomalous Hall conductivity increased twice as
compared to the undoped composition. Our results indicate that the composition
close to the 50\% V doped Co$_2$TiSn, will be an ideal composition for the
experimental investigation of Weyl physics.",2102.13389v2
2021-05-04,Twofold van Hove singularity and origin of charge order in topological kagome superconductor CsV3Sb5,"The layered vanadium antimonides AV3Sb5 (A = K, Rb, Cs) are a recently
discovered family of topological kagome metals with a rich phenomenology of
strongly correlated electronic phases including charge order and
superconductivity. Understanding how the singularities inherent to the kagome
electronic structure are linked to the observed many-body phases is a topic of
great interest and relevance. Here, we combine angle-resolved photoemission
spectroscopy and density functional theory to reveal multiple kagome-derived
van Hove singularities (vHs) coexisting near the Fermi level of CsV3Sb5 and
analyze their contribution to electronic symmetry breaking. Intriguingly, the
vHs in CsV3Sb5 have two distinct flavors - p-type and m-type - which originate
from their pure and mixed sublattice characters, respectively. This twofold vHs
is unique property of the kagome lattice, and its flavor critically determines
the pairing symmetry and ground states emerging in AV3Sb5 series. We establish
that, among the multiple vHs in CsV3Sb5, the m-type vHs of the dxz/dyz kagome
band and the p-type vHs of the dxy/dx2-y2 kagome band cross the Fermi level to
set the stage for electronic symmetry breaking. The former band exhibits
pronounced Fermi surface nesting, while the latter contributes via higher-order
vHs. Our work reveals the essential role of kagome-derived vHs for the
collective phenomena realized in the AV3Sb5 family, paving the way to a deeper
understanding of strongly correlated topological kagome systems.",2105.01689v2
2021-05-19,Competing superconductivity and charge-density wave in Kagome metal CsV3Sb5: evidence from their evolutions with sample thickness,"Recently superconductivity and topological charge-density wave (CDW) were
discovered in the Kagome metals $A$V$_3$Sb$_5$ ($A$ = Cs, Rb, and K), which
have an ideal Kagome lattice of vanadium. Here we report resistance
measurements on thin flakes of CsV$_3$Sb$_5$ to investigate the evolution of
superconductivity and CDW with sample thickness. The CDW transition temperature
${\it T}_{\rm CDW}$ decreases from 94 K in bulk to a minimum of 82 K at
thickness of 60 nm, then increases to 120 K as the thickness is reduced further
to 4.8 nm (about five monolayers). Since the CDW order in CsV$_3$Sb$_5$ is
quite three-dimensional (3D) in the bulk sample, the non-monotonic evolution of
${\it T}_{\rm CDW}$ with reducing sample thickness can be explained by a 3D to
2D crossover around 60 nm. Strikingly, the superconducting transition
temperature ${\it T}_{\rm c}$ shows an exactly opposite evolution, increasing
from 3.64 K in the bulk to a maximum of 4.28 K at thickness of 60 nm, then
decreasing to 0.76 K at 4.8 nm. Such exactly opposite evolutions provide strong
evidence for competing superconductivity and CDW, which helps us to understand
these exotic phases in $A$V$_3$Sb$_5$ Kagome metals.",2105.09248v1
2021-08-03,A Comparative Study of Atmospheric Chemistry with VULCAN,"We present an update of the open-source photochemical kinetics code VULCAN
(Tsai et al. 2017; https://github.com/exoclime/VULCAN) to include C-H-N-O-S
networks and photochemistry. Additional new features are advection transport,
condensation, various boundary conditions, and temperature-dependent UV
cross-sections. First, we validate our photochemical model for hot Jupiter
atmospheres by performing an intercomparison of HD 189733b models between Moses
et al. (2011), Venot et al. (2012), and VULCAN, to diagnose possible sources of
discrepancy. Second, we set up a model of Jupiter extending from the deep
troposphere to upper stratosphere to verify the kinetics for low temperature.
Our model reproduces hydrocarbons consistent with observations, and the
condensation scheme successfully predicts the locations of water and ammonia
ice clouds. We show that vertical advection can regulate the local ammonia
distribution in the deep atmosphere. Third, we validate the model for oxidizing
atmospheres by simulating Earth and find agreement with observations. Last,
VULCAN is applied to four representative cases of extrasolar giant planets:
WASP-33b, HD 189733b, GJ 436b, and 51 Eridani b. We look into the effects of
the C/O ratio and chemistry of titanium/vanadium species for WASP-33b; we
revisit HD 189733b for the effects of sulfur and carbon condensation; the
effects of internal heating and vertical mixing ($K_{\textrm{zz}}$) are
explored for GJ 436b; we test updated planetary properties for 51 Eridani b
with S$_8$ condensates. We find sulfur can couple to carbon or nitrogen and
impact other species such as hydrogen, methane, and ammonia. The observable
features of the synthetic spectra and trends in the photochemical haze
precursors are discussed for each case.",2108.01790v2
2021-10-21,Titanium doped kagome superconductor CsV3-xTixSb5 and two distinct phases,"The vanadium-based kagome superconductor CsV3Sb5 has attracted tremendous
attention due to its unexcepted anomalous Hall effect (AHE), charge density
waves (CDWs), nematicity, and a pseudogap pair density wave (PDW) coexisting
with unconventional strong-coupling superconductivity (SC). The origins of
CDWs, unconventional SC, and their correlation with different electronic states
in this kagome system are of great significance, but so far, are still under
debate. Chemical doping in the kagome layer provides one of the most direct
ways to reveal the intrinsic physics, but remains unexplored. Here, we report,
for the first time, the synthesis of Ti-substituted CsV3Sb5 single crystals and
its rich phase diagram mapping the evolution of intertwining electronic states.
The Ti atoms directly substitute for V in the kagome layers. CsV3-xTixSb5 shows
two distinct SC phases upon substitution. The Ti slightly-substituted phase
displays an unconventional V-shaped SC gap, coexisting with weakening CDW, PDW,
AHE, and nematicity. The Ti highly-substituted phase has a U-shaped SC gap
concomitant with a short-range rotation symmetry breaking CDW, while long-range
CDW, twofold symmetry of in-plane resistivity, AHE, and PDW are absent.
Furthermore, we also demonstrate the chemical substitution of V atoms with
other elements such as Cr and Nb, showing a different modulation on the SC
phase and CDWs. These findings open up a way to synthesise a new family of
doped CsV3Sb5 materials, and further representing a new platform for tuning the
different correlated electronic states and superconducting pairing in kagome
superconductors.",2110.11228v2
2022-01-30,Type-II Dirac Nodal Lines in double-kagome-layered CsV$_8$Sb$_{12}$,"Lorentz-violating type-II Dirac nodal line semimetals (DNLSs), hosting curves
of band degeneracy formed by two dispersion branches with the same sign of
slope, represent a novel states of matter. While being studied extensively in
theory, convincing experimental evidences of type-II DNLSs remain elusive.
Recently, Vanadium-based kagome materials have emerged as a fertile ground to
study the interplay between lattice symmetry and band topology. In this work,
we study the low-energy band structure of double-kagome-layered
CsV$_8$Sb$_{12}$ and identify it as a scarce type-II DNLS protected by mirror
symmetry. We have observed multiple DNLs consisting of type-II Dirac cones
close to or almost at the Fermi level via angle-resolved photoemission
spectroscopy (ARPES). First-principle analyses show that spin-orbit coupling
only opens a small gap, resulting effectively gapless ARPES spectra, yet
generating large spin Berry curvature. These type-II DNLs, together with the
interaction between a low-energy van Hove singularity and quasi-1D band as we
observed in the same material, suggest CsV$_8$Sb$_{12}$ as an ideal platform
for exploring novel transport properties such as chiral anomaly, the Klein
tunneling and fractional quantum Hall effect.",2201.12851v2
2022-02-17,Ultrafast X-ray imaging of the light-induced phase transition in VO2,"Using light to control transient phases in quantum materials is an emerging
route to engineer new properties and functionality, with both thermal and
non-thermal phases observed out of equilibrium. Transient phases are expected
to be heterogeneous, either through photo-generated domain growth or by
generating topological defects, and this impacts the dynamics of the system.
However, this nanoscale heterogeneity has not been directly observed. Here we
use time- and spectrally resolved coherent X-ray imaging to track the
prototypical light induced insulator-to-metal phase transition in vanadium
dioxide on the nanoscale with femtosecond time resolution. We show that the
early-time dynamics are independent of the initial spatial heterogeneity and
observe a 200 fs switch to the metallic phase. A heterogeneous response emerges
only after hundreds of picoseconds. Through spectroscopic imaging, we reveal
that the transient metallic phase is a highly orthorhombically strained rutile
metallic phase, an interpretation that is in contrast to those based on
spatially averaged probes. Our results demonstrate the critical importance of
spatially and spectrally resolved measurements for understanding and
interpreting the transient phases of quantum materials.",2202.08585v2
2022-02-20,Diurnal variations in the stratosphere of the ultrahot giant exoplanet WASP-121b,"The temperature profile of a planetary atmosphere is a key diagnostic of
radiative and dynamical processes governing the absorption, redistribution, and
emission of energy. Observations have revealed dayside stratospheres that
either cool or warm with altitude for a small number of gas giant exoplanets,
while other dayside stratospheres are consistent with constant temperatures.
Here we report spectroscopic phase curve measurements for the gas giant
WASP-121b, which constrain stratospheric temperatures throughout the diurnal
cycle. Variations measured for a water vapour spectral feature reveal a
temperature profile that transitions from warming with altitude on the dayside
hemisphere to cooling with altitude on the nightside hemisphere. The data are
well explained by models assuming chemical equilibrium, with water molecules
thermally dissociating at low pressures on the dayside and recombining on the
nightside. Nightside temperatures are low enough for perovskite (CaTiO3) to
condense, which could deplete titanium from the gas phase and explain recent
non-detections at the day-night terminator. Nightside temperatures are also
consistent with the condensation of refractory species such as magnesium, iron,
and vanadium. Detections of these metals at the day-night terminator suggest,
however, that if they do form nightside clouds, cold trapping does not
efficiently remove them from the upper atmosphere. Horizontal winds and
vertical mixing could keep these refractory condensates aloft in the upper
atmosphere of the nightside hemisphere until they are recirculated to the
hotter dayside hemisphere and vaporised.",2202.09884v1
2022-02-26,Evolution of Dopant-Concentration-Induced Magnetic Exchange Interaction in Topological Insulator Thin Films,"Two essential ingredients for the quantum anomalous Hall (QAH) effect, i.e.
topological and magnetic orders, can be combined by doping magnetic ions into a
topological insulator (TI) film. Through this approach, the QAH effect has been
realized in chromium (Cr)- and/or vanadium (V)-doped TI (Bi,Sb)2Te3 thin films.
In this work, we synthesize both V- and Cr-doped Bi2Te3 thin films with
controlled dopant concentration using molecular beam epitaxy (MBE). By
performing magneto-transport measurements, we find that both systems show an
unusual but yet similar ferromagnetic response with respect to magnetic dopant
concentration, specifically the Curie temperature does not increase
monotonically but shows a local maximum at a critical dopant concentration. Our
angle-resolved photoemission spectroscopy (ARPES) measurements show that the
Cr/V doping introduces hole carriers into Bi2Te3, which consequently move the
chemical potential toward the charge neutral point. In addition, the Cr/V
doping also reduces the spin-orbit coupling of Bi2Te3 which drives it from a
nontrivial TI to a trivial semiconductor. The unusual ferromagnetic response
observed in Cr/V-doped Bi2Te3 thin films is attributed to the
dopant-concentration-induced magnetic exchange interaction, which displays the
evolution from the van Vleck-type ferromagnetism in a nontrivial magnetic TI to
the Ruderman-Kittel-Kasuya-Yosida (RKKY)-type ferromagnetism in a trivial
diluted magnetic semiconductor. Our work provides insights into the
ferromagnetic properties of magnetically doped TI thin films and facilitates
the pursuit of high-temperature QAH effect.",2202.13217v1
2022-03-23,Charge order breaks time-reversal symmetry in CsV$_3$Sb$_5$,"The recently discovered vanadium-based kagome metals $A$V$_{3}$Sb$_{5}$
($A$~=~K,~Rb,~Cs) exhibit superconductivity at low-temperatures and charge
density wave (CDW) order at high-temperatures. A prominent feature of the
charge ordered state in this family is that it breaks time-reversal symmetry
(TRSB), which is connected to the underlying topological nature of the band
structure. In this work, a powerful combination of zero-field and high-field
muon-spin rotation/relaxation is used to study the signatures of TRSB of the
charge order in CsV$_3$Sb$_5$, as well as its anisotropic character. By
tracking the temperature evolution of the in-plane and out-of-plane components
of the muon-spin polarization, an enhancement of the internal field width
sensed by the muon-spin ensemble was observed below $T_{\rm TRSB}=T_{\rm
CDW}\simeq95$~K. Additional increase of the internal field width, accompanied
by a change of the local field direction at the muon site from the $ab$-plane
to the $c$-axis, was detected below $T^\ast\simeq30$~K. Remarkably, this
two-step feature becomes well pronounced when a magnetic field of 8~T is
applied along the crystallographic $c-$axis, thus indicating a field-induced
enhancement of the electronic response at the CDW transition. These results
point to a TRSB in CsV$_3$Sb$_5$ by charge order with an onset of
${\simeq}~95$~K, followed by an enhanced electronic response below
${\simeq}~30$~K. The observed two-step transition is discussed within the
framework of different charge-order instabilities, which, in accordance with
density functional theory calculations, are nearly degenerate in energy.",2203.12317v1
2022-05-30,Uniaxial ferromagnetism in the kagome metal TbV${_6}$Sn${_6}$,"The synthesis and characterization of the vanadium-based kagome metal
TbV${_6}$Sn${_6}$ is presented. X-ray measurements confirm this material forms
with the same crystal structure type as the recently investigated kagome metals
GdV$_6$Sn$_6$ and YV$_6$Sn$_6$, with space group symmetry P6/mmm. A signature
of a phase transition at 4.1K is observed in heat capacity, resistivity, and
magnetic susceptibility measurements, and both resistivity and magnetization
measurements exhibit hysteresis in magnetic field. Furthermore, a strikingly
large anisotropy in the magnetic susceptibility was observed, with the c-axis
susceptibility nearly 100 times the ab plane susceptibility at 2K. This is
highly suggestive of uniaxial ferromagnetism, and the large size of
9.4$\mu_b$/f.u. indicates the Tb$^{3+}$ $4f$ electronic moments cooperatively
align perpendicular to the V kagome lattice plane. The entropy at the phase
transition is nearly Rln(2), indicating that the CEF ground state of the
Tb$^{3+}$ ion is a doublet, and therefore the sublattice of $4f$ electrons in
this material can be shown to map at low temperatures to the Ising model in a
D$_{6h}$ symmetry environment. Hall measurements at temperatures from 300K to
1.7K can be described by two-band carrier transport at temperatures below
around 150K, with a large increase in both hole and electron mobilities,
similar to YV$_6$Sn$_6$, and an anomalous Hall effect is seen below the
ordering temperature. Angle-resolved photoemission measurements above the
magnetic ordering temperature reveal typical kagome dispersions. Our study
presents TbV${_6}$Sn${_6}$ as an ideal system to study the interplay between
Ising ferromagnetism and non-trivial electronic states emerging from a kagome
lattice.",2205.14802v1
2022-09-22,Loop-current charge density wave driven by long-range Coulomb repulsion on the kagome lattice,"Recent experiments on vanadium-based nonmagnetic kagom\'e metals
$A$V$_3$Sb$_5$ ($A=$ K, Rb, Cs) revealed evidence for possible spontaneous
time-reversal symmetry (TRS) breaking in the charge density wave (CDW) ordered
state. The long-sought-after quantum order of loop currents has been suggested
as a candidate for the TRS breaking state. However, a microscopic model for the
emergence of the loop-current CDW due to electronic correlations is still
lacking. Here, we calculate the susceptibility of the real and imaginary bond
orders on the kagom\'e lattice near van Hove filling, and reveal the importance
of next-nearest-neighbor Coulomb repulsion $V_2$ in triggering the instability
toward imaginary bond ordered CDW. The concrete effective single-orbital
$t$-$V_1$-$V_2$ model on the kagom\'e lattice is then studied, where $t$ and
$V_1$ are the hopping and Coulomb repulsion on the nearest-neighbor bonds. We
obtain the mean-field ground states, analyze their properties, and determine
the phase diagram in the plane spanned by $V_1$ and $V_2$ at van Hove filling.
The region dominated by $V_1$ is occupied by a $2a_0 \times 2a_0$ real CDW
insulator with the inverse of Star-of-David (ISD) bond configuration.
Increasing $V_2$ indeed drives a first-order transition from ISD to stabilized
loop-current insulators that exhibit four possible current patterns of
different topological properties, leading to orbital Chern insulators. We then
extend these results away from van Hove filling and show that electron doping
helps the stabilization of loop currents, and gives rise to doped orbital Chern
insulators with emergent Chern Fermi pockets carrying large Berry curvature and
orbital magnetic moment. Our findings provide a concrete model realization of
the loop-current Chern metal at the mean-field level for the TRS breaking
normal state of the kagom\'e superconductors.",2209.10768v2
2022-10-13,High-strain-induced local modification of the electronic properties of VO$_2$ thin films,"Vanadium dioxide (VO2) is a popular candidate for electronic and optical
switching applications due to its well-known semiconductor-metal transition.
Its study is notoriously challenging due to the interplay of long and short
range elastic distortions, as well as the symmetry change, and the electronic
structure changes. The inherent coupling of lattice and electronic degrees of
freedom opens the avenue towards mechanical actuation of single domains. In
this work, we show that we can manipulate and monitor the reversible
semiconductor-to-metal transition of VO2 while applying a controlled amount of
mechanical pressure by a nanosized metallic probe using an atomic force
microscope. At a critical pressure, we can reversibly actuate the phase
transition with a large modulation of the conductivity. Direct tunneling
through the VO2-metal contact is observed as the main charge carrier injection
mechanism before and after the phase transition of VO2. The tunneling barrier
is formed by a very thin but persistently insulating surfacelayer of the VO2.
The necessary pressure to induce the transition decreases with temperature. In
addition, we measured the phase coexistence line in a hitherto unexplored
regime. Our study provides valuable information on pressure-induced electronic
modifications of the VO2 properties, as well as on nanoscale metal-oxide
contacts, which can help in the future design of oxide electronics.",2210.07408v1
2022-10-24,Anomalous Hall effect and two-dimensional Fermi surfaces in the charge-density-wave state of kagome metal RbV$_3$Sb$_5$,"AV$_3$Sb$_5$ (A=Cs, K, Rb) are recently discovered superconducting systems
($T_{\rm c}\sim0.9-2.5$ K) in which the vanadium atoms adopt the kagome
structure. Intriguingly, these systems enter a charge-density-wave (CDW) phase
($T_{\rm CDW}\sim80-100$ K), and further evidence shows that the time-reversal
symmetry is broken in the CDW phase. Concurrently, the anomalous Hall effect
has been observed in KV$_3$Sb$_5$ and CsV$_3$Sb$_5$ inside the novel CDW phase.
Here, we report a comprehensive study of a high-quality RbV$_3$Sb$_5$ single
crystal with magnetotransport measurements. Our data demonstrate the emergence
of anomalous Hall effect in RbV$_3$Sb$_5$ when the charge-density-wave state
develops. The magnitude of anomalous Hall resistivity at the low temperature
limit is comparable to the reported values in KV$_3$Sb$_5$ and CsV$_3$Sb$_5$.
The magnetoresistance channel further reveals a rich spectrum of quantum
oscillation frequencies, many of which have not been reported before. In
particular, a large quantum oscillation frequency (2235 T), which occupies
$\sim$56% of the Brillouin zone area, has been recorded. For the quantum
oscillation frequencies with sufficient signal-to-noise ratio, we further
perform field-angle dependent measurements and our data indicate
two-dimensional Fermi surfaces in RbV$_3$Sb$_5$. Our results provide
indispensable information for understanding the anomalous Hall effect and band
structure in kagome metals AV$_3$Sb$_5$.",2210.13250v2
2022-11-22,Superconductivity and orbital-selective nematic order in a new titanium-based kagome metal CsTi3Bi5,"Fabrication of new types of superconductors with novel physical properties
has always been a major thread in the research of superconducting materials. An
example is the enormous interests generated by the cascade of correlated
topological quantum states in the newly discovered vanadium-based kagome
superconductors AV3Sb5 (A=K, Rb, and Cs) with a Z2 topological band structure.
Here we report the successful fabrication of single-crystals of titanium-based
kagome metal CsTi3Bi5 and the observation of superconductivity and electronic
nematicity. The onset of the superconducting transition temperature Tc is
around 4.8 K. In sharp contrast to the charge density wave superconductor
AV3Sb5, we find that the kagome superconductor CsTi3Bi5 preserves translation
symmetry, but breaks rotational symmetry and exhibits an electronic nematicity.
The angular-dependent magnetoresistivity shows a remarkable two-fold rotational
symmetry as the magnetic field rotates in the kagome plane. The scanning
tunneling microscopy and spectroscopic imaging detect rotational-symmetry
breaking C2 quasiparticle interference patterns (QPI) at low energies,
providing further microscopic evidence for electronic nematicity. Combined with
first-principle calculations, we find that the nematic QPI is orbital-selective
and dominated by the Ti dxz and dyz orbitals, possibly originating from the
intriguing orbital bond nematic order. Our findings in the new ""135"" material
CsTi3Bi5 provide new directions for exploring the multi-orbital correlation
effect and the role of orbital or bond order in the electron liquid crystal
phases evidenced by the symmetry breaking states in kagome superconductors.",2211.12264v1
2023-02-27,Emergence of a new band and the Lifshitz transition in kagome metal ScV$_6$Sn$_6$ with charge density wave,"Topological kagome systems have been a topic of great interest in condensed
matter physics due totheir unique electronic properties. The vanadium-based
kagome materials are particularly intrigu-ing since they exhibit exotic
phenomena such as charge density wave (CDW) and
unconventionalsuperconductivity. The origin of these electronic instabilities
is not fully understood, and the re-cent discovery of a charge density wave in
ScV6Sn6provides a new avenue for investigation. In thiswork, we investigate the
electronic structure of the novel kagome metal ScV6Sn6using angle
resolvedphotoemission spectroscopy (ARPES), scanning tunneling microscopy
(STM), and first-principlesdensity functional theory calculations. Our analysis
reveals for the first time the temperature-dependent band changes of ScV6Sn6and
identifies a new band that exhibits a strong signatureof a structure with CDW
below the critical temperature. Further analysis revealed that this newband is
due to the surface kagome layer of the CDW structure. In addition, a Lifshitz
transition isidentified in the ARPES spectra that is related to the saddle
point moving across the Fermi levelat the critical temperature for the CDW
formation. This result shows the CDW behavior may alsobe related to nesting of
the saddle point, similar to related materials. However, no energy gap is
observed at the Fermi level and thus the CDW is not a typical Fermi surface
nesting scenario. These results provide new insights into the underlying
physics of the CDW in the kagome materials and could have implications for the
development of materials with new functionality.",2302.14041v1
2023-06-13,Tiny Sc allows the chains to rattle: Impact of Lu and Y doping on the charge density wave in ScV$_6$Sn$_6$,"The kagome metals display an intriguing variety of electronic and magnetic
phases arising from the connectivity of atoms on a kagome lattice. A growing
number of these materials with vanadium kagome nets host charge density waves
(CDWs) at low temperatures including ScV$_6$Sn$_6$, CsV$_3$Sb$_5$, and
V$_3$Sb$_2$. Curiously, only the Sc version of the $R$V$_6$Sn$_6$
HfFe$_6$Ge$_6$-type materials hosts a CDW ($R = $Gd-Lu, Y, Sc). In this study
we investigate the role of rare earth size in CDW formation in the
$R$V$_6$Sn$_6$ compounds. Magnetization measurements on our single crystals of
(Sc,Lu)V$_6$Sn$_6$ and (Sc,Y)V$_6$Sn$_6$ establish that the CDW is suppressed
by substitution of Sc by larger Lu or Y. Single crystal x-ray diffraction
reveals that compressible Sn-Sn bonds accommodate the larger rare earth atoms
within loosely packed $R$-Sn-Sn chains without significantly expanding the
lattice. We propose that Sc provides the extra room in these chains crucial to
CDW formation in ScV$_6$Sn$_6$. Our rattling chain model explains why both
physical pressure and substitution by larger rare earths hinder CDW formation
despite opposite impacts on lattice size. We emphasize the cooperative effect
of pressure and rare earth size by demonstrating that pressure further
suppresses the CDW in a Lu-doped ScV$_6$Sn$_6$ crystal. Our model not only
addresses why a CDW only forms in the $R$V$_6$Sn$_6$ materials with tiny Sc, it
also advances to our understanding of why unusual CDWs form in the kagome
metals.",2306.07868v1
2023-06-24,Performance improvement of three-body radiative diode driven by graphene surface plasmon polaritons,"As an analogue to electrical diode, a radiative thermal diode allows
radiation to transfer more efficiently in one direction than in the opposite
direction by operating in a contactless mode. In this study, we demonstrated
that, within the framework of three-body photon thermal tunneling, the
rectification performance of three-body radiative diode can be greatly improved
by bringing graphene into the system. The system is composed of three parallel
slabs, with the hot and cold terminals of the diode coated with graphene films,
and the intermediate body made of vanadium dioxide (VO2). The rectification
factor of the proposed radiative thermal diode reaches 300 % with a 350 nm
separation distance between the hot and cold terminals of the diode. With the
help of graphene, the rectification performance of the radiative thermal diode
can be improved by over 11 times. By analyzing the spectral heat flux and
energy transmission coefficients, it was found that the improved performance is
primarily attributed to the surface plasmon polaritons (SPPs) of graphene. They
excite the modes of insulating VO2 in the forward-biased scenario by forming
strongly coupled modes between graphene and VO2, and thus dramatically enhance
the heat flux. While, for the reverse-biased scenario, the VO2 is at its
metallic state and thus graphene SPPs cannot work by three-body photon thermal
tunneling. Furthermore, the improvement was also investigated for different
chemical potentials of graphene, and geometric parameters of the three-body
system. Our findings demonstrate the feasibility of using thermal-photon-based
logical circuits, creating radiation-based communication technology, and
implementing thermal management approaches at the nanoscale.",2306.13851v1
2023-07-13,Layer-dependent electronic structures and magnetic ground states of polar-polar $\rm{LaVO_3/KTaO_3}$ (001) interfaces,"Employing a first-principles and model Hamiltonian approach, we work out the
electronic properties of polar-polar LaVO$_3$/KTaO$_3$ (LVO/KTO, 001)
hetero-interfaces, considering configurations with up to six layers of KTO and
five layers of LVO. Our computational analyses indicate the existence of
multiple Lifshitz transitions (LT) within the $t_{2g}$ bands, potentially
manifesting in elevated thermal conductivity and Seebeck coefficient. The
Lifshitz transitions can be fine-tuned by adjusting the number of LVO layers or
applying gate voltage. Contrary to experimental report, the spin-orbit coupling
is found to be negligible, originating solely from the Ta $5d_{xy}$-derived
band of KTO, while the 5$d_{xz}$ and 5$d_{yz}$ bands are considerably away from
the Fermi level and LVO overlayers playing no role in it. The magnetic
properties of the interfaces, due to Vanadium ions, exhibit a pronounced
sensitivity to the number of LVO and KTO layers. Spin-polarized density of
states reveals that the interfaces featuring ferromagnetic (FM) ground states
are \textit{half-metallic}. The small energy differences between
antiferromagnetic (AFM) and ferromagnetic (FM) configurations suggest a
potential coexistence of competing AFM and FM ground states within these
interfaces. Moreover, we find that an insulator to metal transition at the
interface requires a distinct number of LVO layers: a half-metallic FM state
appears for two and more LVO layers and a metallic AFM state is found from four
LVO layers onwards. The experimentally observed critical thickness for
metallicity, therefore, requires consideration of the magnetic interactions.
Additionally, we observe that the oxygen-mediated superexchange mechanism
governs the behavior of AFM insulators, whereas direct exchange is responsible
for the metallic and \textit{half-metallic} traits.",2307.06904v2
2023-08-08,Delocalized polaron and Burstein-Moss shift induced by Li in $α$-$\textrm{V}_{2}\textrm{O}_{5}$: DFT+DMFT study,"We performed density functional theory (DFT)+$U$ and dynamical mean field
theory (DMFT) calculations with continuous time quantum Monte Carlo impurity
solver to investigate the electronic properties of V$_2$O$_5$ and
Li$_x$V$_2$O$_5$ ($x$ = 0.125 and 0.25). Pristine V$_2$O$_5$ is a
charge-transfer insulator with strong O $p$-V $d$ hybridization, and exhibits a
large band gap ($E_{\textrm{gap}}$) as well as non-zero conduction band (CB)
gap. We show that the band gap, the number of $d$ electrons of vanadium, $N_d$,
and conduction band (CB) gap for V$_2$O$_5$ obtained from our DMFT calculations
are in excellent agreement with the experimental values. While the DFT+$U$
approach replicates the experimental band gap, it overestimates the value of
$N_d$ and underestimates the CB gap. In the presence of low Li doping, the
electronic properties of V$_2$O$_5$ are mainly driven by a polaronic mechanism,
the electron spin resonance and electron nuclear double resonance
spectroscopies observed the coexistence of free and bound polarons. Notably,
our DMFT results identify both polaron types, with the bound polaron being
energetically preferred, while DFT+$U$ method predicts only the free polaron.
Our DMFT analysis also reveals that increased Li doping leads to electron
filling in the conduction band, shifting the Fermi level, this result
consistent with the observed Burstein-Moss shift upon enhanced Li doping and we
thus demonstrate that the DFT+DMFT approach can be used for accurate and
realistic description of strongly correlated materials.",2308.04043v3
2023-08-14,"Coupled electron-crystal lattice symmetry reduction in insulating materials with transition metal dopants: Cu-doped Pb$_{10}$(PO$_4$)$_6$O, and Vanadium doped SrTiO$_3$","We study two materials and clarify the mechanisms at play in stabilizing an
insulating state, and an impurity level in the bandgap using ab initio
calculations: Cu-doped Pb$_{10}$(PO$_4$)$_6$O ('LK-99') and V-doped SrTiO$_3$,
both transition metal-doped insulators. In both cases, the electron degeneracy
and crystal lattice symmetry are broken, leading to an insulating state, and a
magnetically and orbitally polarized impurity state within the bulk gap,
clearly separated from the doped material's valence and conduction bands.
Doping slightly lowers the band gap of the bulk materials, with possible
applications related to bandgap tuning (e.g. photocatalysis). We also resolve
previously noticed inconsistencies between density functional theory results
and experiment regarding doping site energetics, crystal structure, and
transparency in Cu-doped phosphate lead apatite 'LK-99'. Doping one of each
type of site in the same unit cell ($20 \%$ doping) leads to two spin-polarized
impurity bands in the material's gap. The local transition metal ion sites may
behave like color centers (or f-centers), possibly giving color at low
temperatures to what we predict to be a transparent, insulating material in the
recently synthesized LK-99 at low temperatures. The results shown here likely
exclude superconductivity by known mechanisms. We also note the potential of
this type of doping to lead to altermagnetism - though the Cu-doped lead
apatite does not seem particularly promising in this regard. Throughout this
paper, we also discuss tools that can be used to quickly understand the local
d-orbital symmetry of the impurity, and the coupling between the crystal
structure and the impurity electronic order on the d-shell.",2308.07295v2
2023-08-14,Anharmonic Strong-Coupling Effects at the Origin of the Charge Density Wave in CsV$_3$Sb$_5$,"The formation of charge density waves (CDW) is a long-standing open problem
particularly in dimensions higher than one. Various observations in the
vanadium antimonides discovered recently, such as the missing Kohn anomaly in
the acoustic phonons or the latent heat at the transition $T_{\rm CDW}$ = 95 K
, further underpin this notion. Here, we study the Kagome metal CsV$_3$Sb$_5$
using polarized inelastic light scattering. The electronic energy gap $2\Delta$
as derived from the redistribution of the continuum is much larger than
expected from mean-field theory and reaches values above 20 for $2\Delta/k_{\rm
B}T_{\rm CDW}$. The A$_{1g}$ phonon has a discontinuity at $T_{\rm CDW}$ and a
precursor starting 20 K above $T_{\rm CDW}$. Density functional theory
qualitatively reproduces the redistribution of the electronic continuum at the
CDW transition and the phonon energies of the pristine and distorted
structures. The linewidths of all A$_{1g}$ and E$_{2g}$ phonon lines including
those emerging below $T_{\rm CDW}$ were analyzed in terms of anharmonic
symmetric decay revealing strong phonon-phonon coupling. In addition, we
observe two CDW amplitude modes (AMs): one in A$_{1g}$ symmetry and one in
E$_{2g}$ symmetry. The temperature dependence of both modes deviates from the
prediction of mean-field theory. The A$_{1g}$ AM displays an asymmetric
Fano-type lineshape, suggestive of strong electron-phonon coupling. The
asymmetric A$_{1g}$ AM, along with the discontinuity of the A$_{1g}$ phonon,
the large phonon-phonon coupling parameters and the large gap ratio, indicate
the importance of anharmonic strong phonon-phonon and electron-phonon coupling
for the CDW formation in CsV$_3$Sb$_5$.",2308.07482v1
2023-09-24,Superconductivity emerging from density-wave-like order in a correlated kagome metal,"Unconventional superconductivity (USC) in a highly correlated kagome system
has been theoretically proposed for years, yet the experimental realization is
hard to achieve. The recently discovered vanadium-based kagome materials, which
exhibit both superconductivity and charge density wave (CDW) orders, are
nonmagnetic and weakly correlated, thus unlikely host USC as theories proposed.
Here we report the discovery of a chromium-based kagome metal, CsCr$_3$Sb$_5$,
which is contrastingly characterised by strong electron correlations,
frustrated magnetism, and characteristic flat bands close to the Fermi level.
Under ambient pressure, it undergoes a concurrent structural and magnetic phase
transition at 55 K, accompanying with a stripe-like $4a_0$ structural
modulation. At high pressure, the phase transition evolves into two
transitions, probably associated with CDW and antiferromagnetic
spin-density-wave orderings, respectively. These density-wave (DW)-like orders
are gradually suppressed with pressure and, remarkably, a superconducting dome
emerges at 3.65-8.0 GPa. The maximum of the superconducting transition
temperature, $T_\mathrm{c}^{\mathrm{max}}=$ 6.4 K, appears when the DW-like
orders are completely suppressed at 4.2 GPa, and the normal state exhibits a
non-Fermi-liquid behaviour, reminiscent of USC and quantum criticality in
iron-based superconductors. Our work offers an unprecedented platform for
investigating possible USC in a correlated kagome system.",2309.13514v2
2023-09-29,Temporal credit assignment for one-shot learning utilizing a phase transition material,"Design of hardware based on biological principles of neuronal computation and
plasticity in the brain is a leading approach to realizing energy- and
sample-efficient artificial intelligence and learning machines. An important
factor in selection of the hardware building blocks is the identification of
candidate materials with physical properties suitable to emulate the large
dynamic ranges and varied timescales of neuronal signaling. Previous work has
shown that the all-or-none spiking behavior of neurons can be mimicked by
threshold switches utilizing phase transitions. Here we demonstrate that
devices based on a prototypical metal-insulator-transition material, vanadium
dioxide (VO2), can be dynamically controlled to access a continuum of
intermediate resistance states. Furthermore, the timescale of their intrinsic
relaxation can be configured to match a range of biologically-relevant
timescales from milliseconds to seconds. We exploit these device properties to
emulate three aspects of neuronal analog computation: fast (~1 ms) spiking in a
neuronal soma compartment, slow (~100 ms) spiking in a dendritic compartment,
and ultraslow (~1 s) biochemical signaling involved in temporal credit
assignment for a recently discovered biological mechanism of one-shot learning.
Simulations show that an artificial neural network using properties of VO2
devices to control an agent navigating a spatial environment can learn an
efficient path to a reward in up to 4 fold fewer trials than standard methods.
The phase relaxations described in our study may be engineered in a variety of
materials, and can be controlled by thermal, electrical, or optical stimuli,
suggesting further opportunities to emulate biological learning.",2310.00066v1
2023-12-07,Colossal orbital Zeeman effect driven by tunable spin-Berry curvature in a kagome metal,"Berry phase and the related concept of Berry curvature can give rise to many
unconventional phenomena in solids. In this work, we discover colossal orbital
Zeeman effect of topological origin in a newly synthesized bilayer kagome metal
TbV6Sn6. We use spectroscopic-imaging scanning tunneling microscopy to study
the magnetic field induced renormalization of the electronic band structure.
The nonmagnetic vanadium d-orbitals form Dirac crossings at the K point with a
small mass gap and strong Berry curvature induced by the spin-orbit coupling.
We reveal that the magnetic field leads to the splitting of gapped Dirac
dispersion into two branches with giant momentum-dependent g factors, resulting
in the substantial renormalization of the Dirac band. These measurements
provide a direct observation of the magnetic field controlled orbital Zeeman
coupling to the enormous orbital magnetic moments of up to 200 Bohr magnetons
near the gapped Dirac points. Interestingly, the effect is increasingly
non-linear, and becomes gradually suppressed at higher magnetic fields.
Theoretical modeling further confirms the existence of orbital magnetic moments
in TbV6Sn6 produced by the non-trivial spin-Berry curvature of the Bloch wave
functions. Our work provides the first direct insight into the
momentum-dependent nature of topological orbital moments and their tunability
by magnetic field concomitant with the evolution of the spin-Berry curvature.
Significantly large orbital magnetic moments driven by the Berry curvature can
also be generated by other quantum numbers beyond spin, such as the valley in
certain graphene-based structures, which may be unveiled using the same tools
highlighted in our work.",2312.04445v1
2024-03-08,Quasiparticle band structure and excitonic optical response in V2O5 bulk and monolayer,"The electronic band structure of V$_2$O$_5$ is calculated using an
all-electron quasiparticle self-consistent (QS) $GW$ method, including
electron-hole ladder diagrams in the screening of $W$. The optical dielectric
function calculated with the Bethe-Salpeter equation exhibits excitons with
large binding energy, consistent with spectroscopic ellipsometry data and other
recent calculations. Sharp peaks in the direction perpendicular to the layers
at high energy are found to be an artifact of the truncation of the numbers of
bands included in the BSE calculation of the macroscopic dielectric function.
The $\varepsilon_1(\omega=0)$ gives indices of refraction in good agreement
with experiment. The excitons are charge transfer excitons with the hole
primarily on oxygen and electrons on vanadium, but depending on which exciton,
the distribution over different oxygens changes. The exciton wave functions
have a spread of about 5-15\AA, with asymmetric character for the electron
distribution around the hole depending on which oxygen the hole is fixed at.
The monolayer quasiparticle gap increases inversely proportional to interlayer
distance once the initial interlayer covalent couplings are removed which is
thanks to the long-range nature of the self-energy and the reduced screening in
a 2D system. The optical gap on the other hand is relatively independent of
interlayer spacing because of the compensation between the self-energy gap
shift and the exciton binding energy, both of which are proportional to the
screened Coulomb interaction $\hat{W}$. Recent experimental results on very
thin layer V$_2$O$_5$ obtained by chemical exfoliation provide experimental
support for an increase in gap.",2403.05473v2
2024-03-12,Strong asymptotic giant branch stars' spectral features in distant quiescent galaxies: Impact on galaxy evolution,"Age-dating and weighting stellar populations in galaxies at various cosmic
epochs are essential steps to study galaxy formation through cosmic times.
Evolutionary population synthesis models with different input physics are used
towards this aim. In particular, the contribution from the thermally pulsing
asymptotic-giant-branch (TP-AGB) stellar phase, which peaks for
intermediate-age 0.6-2 Gyr systems, has been debated upon for decades. Here we
report the detection of strong cool star signatures in the rest-frame
near-infrared spectra of three young (~1 Gyr), massive (~10^10 Msun) quiescent
galaxies at large look-back time, z=1-2, using JWST/NIRSpec. The co-existence
of oxygen- and carbon-type absorption features, spectral edges and features
from rare species such as Vanadium, and possibly Zirconium, reveal a strong
contribution from TP-AGB stars. Population synthesis models with significant
TP-AGB contribution reproduce the observations considerably better than those
with weak TP-AGB, which are those commonly used. These findings call for
revisions of published stellar population fitting results, pointing to lower
masses and younger ages, with additional implications on cosmic dust production
and chemical enrichment. These results will stimulate new generations of
improved models informed by these and future observations.",2403.07414v1