text
stringlengths 89
2.49k
| category
stringclasses 19
values |
|---|---|
Oxygen vacancies in strained SrTiO$_{3}$ thin films: formation enthalpy
and manipulation: We report the enthalpy of oxygen vacancy formation in thin films of
electron-doped SrTiO$_{3}$, under different degrees of epitaxial stress. We
demonstrate that both compressive and tensile strain decrease this energy at a
very similar rate, and promote the formation of stable doubly ionized oxygen
vacancies. Moreover, we also show that unintentional cationic vacancies
introduced under typical growth conditions, produce a characteristic rotation
pattern of TiO$_6$ octahedra. The local concentration of oxygen vacancies can
be modulated by an electric field with an AFM tip, changing not only the local
electrical potential, but also producing a non-volatile mechanical response
whose sign (up/down) can be reversed by the electric field. | cond-mat_mtrl-sci |
Water adsorption on the P-rich GaP(100) surface: Optical spectroscopy
from first principles: The contact of water with semiconductors typically changes its surface
electronic structure by oxidation or corrosion processes. A detailed knowledge
- or even control of - the surface structure is highly desirable, as it impacts
the performance of opto-electronic devices from gas-sensing to energy
conversion applications. It is also a prerequisite for density functional
theory-based modelling of the electronic structure in contact with an
electrolyte. The P-rich GaP(100) surface is extraordinary with respect to its
contact with gas-phase water, as it undergoes a surface reordering, but does
not oxidise. We investigate the underlying changes of the surface in contact
with water by means of theoretically derived reflection anisotropy spectroscopy
(RAS). A comparison of our results with experiment reveals that a water-induced
hydrogen-rich phase on the surface is compatible with the boundary conditions
from experiment, reproducing the optical spectra. We discuss potential reaction
paths that comprise a water-enhanced hydrogen mobility on the surface. Our
results also show that computational RAS - required for the interpretation of
experimental signatures - is feasible for GaP in contact with water double
layers. Here, RAS is sensitive to surface electric fields, which are an
important ingredient of the Helmholtz-layer. This paves the way for future
investigations of RAS at the semiconductor-electrolyte interface. | cond-mat_mtrl-sci |
Chemical and nuclear catalysis driven by localized anharmonic vibrations: In many-body nonlinear systems with sufficient anharmonicity, a special kind
of lattice vibrations, namely, Localized Anharmonic Vibrations (LAV) can be
excited either thermally or by external triggering, in which the amplitude of
atomic oscillations greatly exceeds that of harmonic oscillations (phonons)
that determine the system temperature. Coherency and persistence of LAV may
have drastic effect on chemical and nuclear reaction rates due to time-periodic
modulation of reaction sites. One example is a strong acceleration of chemical
reaction rates driven by thermally-activated "jumps" over the reaction barrier
due to the time-periodic modulation of the barrier height in the LAV vicinity.
At sufficiently low temperatures, the reaction rate is controlled by quantum
tunneling through the barrier rather than by classical jumping over it. A giant
increase of sub-barrier transparency was demonstrated for a parabolic potential
well with the time-periodic eigenfrequency, when the modulation frequency
exceeds the eigenfrequency by a factor of ~2 (parametric regime). Such regime
can be realized for a hydrogen or deuterium atom in metal hydrides/deuterides,
such as NiH or PdD, in the vicinity of LAV. We present an analytical solution
of the Schrodinger equation for a nonstationary harmonic oscillator, analyze
the parametric regime in details and discuss its applications to the tunnel
effect and to D-D fusion in PdD lattice. We obtain simple analytical
expressions for the increase of amplitude and energy of zero-point oscillations
(ZPO) induced by the parametric modulation. Based on that, we demonstrate a
drastic increase of the D-D fusion rate with in-creasing number of modulation
periods evaluated in the framework of Schwinger model, which takes into account
suppression of the Coulomb barrier due to lattice vibrations. | cond-mat_mtrl-sci |
Particle-hole cumulant approach for inelastic losses in x-ray spectra: Inelastic losses in core level x-ray spectra arise from many-body
excitations, leading to broadening and damping as well as satellite peaks in
x-ray photoemission (XPS) and x-ray absorption (XAS) spectra. Here we present a
practical approach for calculating these losses based on a cumulant
representation of the particle-hole Green's function, a quasi-boson
approximation, and a partition of the cumulant into extrinsic, intrinsic and
interference terms. The intrinsic losses are calculated using real-time,
time-dependent density functional theory while the extrinsic losses are
obtained from the GW approximation of the photo-electron self-energy and the
interference terms are approximated. These effects are included in the spectra
using a convolution with an energy dependent particle-hole spectral function.
The approach elucidates the nature of the spectral functions in XPS and XAS and
explains the significant cancellation between extrinsic and intrinsic losses.
Edge-singularity effects in metals are also accounted for. Illustrative results
are presented for the XPS and XAS for both weakly and more correlated systems. | cond-mat_mtrl-sci |
The spontaneous exchange bias effect in La2-xCaxCoMnO6 series: Structural, electronic and magnetic properties of polycrystalline
La2-xCaxCoMnO6 (0 $\leq$ x $\leq$ 0.75) compounds are investigated by x-ray
diffraction and magnetometry. All the samples have an orthorhombic structure
and show a slight decrease in the unit cell with Ca-doping.
Temperature-dependent magnetization measurements reveal a complex magnetic
behavior with two ferromagnetic transitions. These transitions are ascribed to
Co2+--Mn4+ and Co3+--Mn3+ couplings and suggest the presence of additional
antiferromagnetic couplings in these disordered compounds. Field-dependent
magnetization curves, measured after cooling the samples in a zero external
magnetic field, reveal the spontaneous exchange bias (SEB) effect for the
Ca-doped samples. The strengthening of the uncompensated magnetic coupling at
the interfaces, caused by the increased antiferromagnetic phase, explains the
increase of SEB with increasing the Ca-content. | cond-mat_mtrl-sci |
Light and electric field control of ferromagnetism in magnetic quantum
structures: A strong influence of illumination and electric bias on the Curie temperature
and saturation value of the magnetization is demonstrated for semiconductor
structures containing a modulation-doped p-type Cd0.96Mn0.04Te quantum well
placed in various built-in electric fields. It is shown that both light beam
and bias voltage generate an isothermal and reversible cross-over between the
paramagnetic and ferromagnetic phases, in the way that is predetermined by the
structure design. The observed behavior is in quantitative agreement with the
expectations for systems, in which ferromagnetic interactions are mediated by
the weakly disordered two-dimensional hole liquid. | cond-mat_mtrl-sci |
Spin Hall effect emerging from a chiral magnetic lattice without
spin-orbit coupling: The spin Hall effect (SHE), which converts a charge current into a transverse
spin current, has long been believed to be a phenomenon induced by the
spin--orbit coupling. Here, we propose an alternative mechanism to realize the
intrinsic SHE through a chiral magnetic structure that breaks the spin rotation
symmetry. No spin--orbit coupling is needed even when the scalar spin chirality
vanishes, different from the case of the topological Hall effect. In known
chiral antiferromagnetic compounds Mn$_3X$ ($X=$ Ga, Ge, and Sn), for example,
we indeed obtain large spin Hall conductivities based on \textit{ab initio}
calculations. Apart further developing the conceptual understanding of the SHE,
our work suggests an alternative strategy to design spin Hall materials without
involving heavy elements, which may be advantageous for technological
applications. | cond-mat_mtrl-sci |
Large magnetocaloric effect in the kagome ferromagnet
Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$: Single-crystal growth, magnetic properties, and magnetocaloric effect of the
$S = 3/2$ kagome ferromagnet Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$
(trigonal, space group: $P\bar{3}c1$) are reported. Magnetization data suggest
dominant ferromagnetic intra-plane coupling with a weak anisotropy and the
onset of ferromagnetic ordering at $T_{\rm C} \simeq 2.6$ K. Microscopic
analysis reveals a very small ratio of interlayer to intralayer ferromagnetic
couplings ($J_{\perp}/J \simeq 0.02$). Electron spin resonance data suggest the
presence of short-range correlations above $T_{\rm C}$ and confirms
quasi-two-dimensional character of the spin system. A large magnetocaloric
effect characterized by isothermal entropy change of $-\Delta S_{\rm m}\simeq
31$ J kg$^{-1}$ K$^{-1}$ and adiabatic temperature change of $-\Delta T_{\rm
ad}\simeq 9$ K upon a field sweep of 7 T is observed around $T_{\rm C}$. This
leads to a large relative cooling power of $RCP \simeq 284$ J kg$^{-1}$. The
large magnetocaloric effect, together with negligible hysteresis render
Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$ a promising material for magnetic
refrigeration at low temperatures. The magnetocrystalline anisotropy constant
$K \simeq -7.42 \times 10^4$ erg cm$^{-3}$ implies that the compound is an
easy-plane type ferromagnet with the hard axis normal to the $ab$-plane,
consistent with the magnetization data. | cond-mat_mtrl-sci |
$(111)$ surface states of SnTe: The characterization and applications of topological insulators depend
critically on their protected surface states, which, however, can be obscured
by the presence of trivial dangling bond states. Our first principle
calculations show that this is the case for the pristine $(111)$ surface of
SnTe. Yet, the predicted surface states unfold when the dangling bond states
are passivated in proper chemisorption. We further extract the anisotropic
Fermi velocities, penetration lengths and anisotropic spin textures of the
unfolded $\bar\Gamma$- and $\bar M$-surface states, which are consistent with
the theory in http://dx.doi.org/10.1103/PhysRevB.86.081303 Phys. Rev. B 86,
081303 (R). More importantly, this chemisorption scheme provides an external
control of the relative energies of different Dirac nodes, which is
particularly desirable in multi-valley transport. | cond-mat_mtrl-sci |
Photo-induced persistent inversion of germanium in a 200-nm-deep surface
region: The controlled manipulation of the charge carrier concentration in nanometer
thin layers is the basis of current semiconductor technology and of fundamental
importance for device applications. Here we show that it is possible to induce
a persistent inversion from n- to p-type in a 200-nm-thick surface layer of a
germanium wafer by illumination with white and blue light. We induce the
inversion with a half-life of ~12 hours at a temperature of 220 K which
disappears above 280 K. The photo-induced inversion is absent for a sample with
a 20-nm-thick gold capping layer providing a Schottky barrier at the interface.
This indicates that charge accumulation at the surface is essential to explain
the observed inversion. The contactless change of carrier concentration is
potentially interesting for device applications in opto-electronics where the
gate electrode and gate oxide could be replaced by the semiconductor surface. | cond-mat_mtrl-sci |
Effect of uniaxial stress on low-frequency dispersion of dielectric
constant in high-resistivity GaSe crystals: Low-frequency dielectric spectra of high-resistivity GaSe layered crystals
have been studied on the samples clamped between two insulating parallel plates
at frequencies up to 100 kHz. The measurements have been carried out at
different uniaxial stresses up to $2.4\times10^5$ Pa applied along the c-axis
normal to crystal layer's plane. It is revealed that the dielectric spectra of
high-resistivity GaSe layered crystals with insulating plates obey a universal
power law ${\sim}\omega^{n-1}$, where ${\omega}$ is the angular frequency and
$n\approx 0.8$, earlier observed on high-resistivity GaSe crystals with
indium-soldered contacts. The same type of spectra on the crystals with
different types of contacts (insulating and ohmic) confirms the bulk character
of the observed polarization caused by hopping charge carriers. It is shown
that the frequency-dependent dielectric constant increases linearly with the
uniaxial stress characterized by the coefficient
${\Delta}{\epsilon}/({\epsilon}{\Delta}{p})=8{\times}10^{-7}$ Pa$^{-1}$. A
slight increase of power $1-n$ with the stress is observed, that leads to a
stronger dielectric dispersion. The strong stress dependence of the
low-frequency dielectric constant in high-resistivity GaSe crystals may be
referred to the presence of the formations of elementary dipoles, rotations of
which correspond to hops of localized charge carriers. | cond-mat_mtrl-sci |
On the Structure of ${\rm ZnI_2}$: A new structure for ${\rm ZnI_2}$ is proposed which it exists in tetragonal
state. In this structure the ${\rm ZnI_2}$ molecule exists in a nonlinear array
and forms the basis of the tetragonal unit cell with one basis per unit cell.
The structural analysis based on the reflections listed in ASTM 30-1479 shows
that the proposed structure is correct. | cond-mat_mtrl-sci |
Coherence properties of infrared thermal emission from heated metallic
nanowires: Coherence properties of the infrared thermal radiation from individual heated
nanowires are investigated as function of nanowire dimensions. Interfering the
thermally induced radiation from a heated nanowire with its image in a nearby
moveable mirror, well-defined fringes are observed. From the fringe visibility,
the coherence length of the thermal emission radiation from the narrowest
nanowires was estimated to be at least 20 um which is much larger than expected
from a classical blackbody radiator. A significant increase in coherence and
emission efficiency is observed for smaller nanowires. | cond-mat_mtrl-sci |
Structural and electronic properties of solid molecular hydrogen from
many-electron theories: We study the structural and electronic properties of phase III of solid
hydrogen using accurate many-electron theories and compare to state-of-the-art
experimental findings. The atomic structures of phase III modelled by C2/c-24
crystals are fully optimized on the level of second-order perturbation theory,
demonstrating that previously employed structures optimized on the level of
approximate density functionals exhibit errors in the H$_2$ bond lengths that
cause significant discrepancies in the computed quasi particle band gaps and
vibrational frequencies compared to experiment. Using the newly optimized
atomic structures, we study the band gap closure and change in vibrational
frequencies as a function of pressure. Our findings are in good agreement with
recent experimental observations and may prove useful in resolving
long-standing discrepancies between experimental estimates of metallization
pressures possibly caused by disagreeing pressure calibrations. | cond-mat_mtrl-sci |
Observation of Coexisting Dirac Bands and Moiré Flat Bands in
Magic-Angle Twisted Trilayer Graphene: Moir\'e superlattices that consist of two or more layers of two-dimensional
materials stacked together with a small twist angle have emerged as a tunable
platform to realize various correlated and topological phases, such as Mott
insulators, unconventional uperconductivity and quantum anomalous Hall effect.
Recently, the magic-angle twisted trilayer graphene (MATTG) has shown both
robust superconductivity similar to magic-angle twisted bilayer graphene
(MATBG) and other unique properties, including the Pauli-limit violating and
re-entrant superconductivity. These rich properties are deeply rooted in its
electronic structure under the influence of distinct moir\'e potential and
mirror symmetry. Here, combining nanometer-scale spatially resolved
angle-resolved photoemission spectroscopy (nano-ARPES) and scanning tunneling
microscopy/spectroscopy (STM/STS), we systematically measure the yet unexplored
band structure of MATTG near charge neutrality. Our measurements reveal the
coexistence of the distinct dispersive Dirac band with the emergent moir\'e
flat band, showing nice agreement with the theoretical calculations. These
results serve as a stepstone for further understanding of the unconventional
superconductivity in MATTG. | cond-mat_mtrl-sci |
Femtosecond Demagnetization and Hot Hole Relaxation in Ferromagnetic
GaMnAs: We have studied ultrafast photoinduced demagnetization in GaMnAs via
two-color time-resolved magneto-optical Kerr spectroscopy. Below-bandgap
midinfrared pump pulses strongly excite the valence band, while near-infrared
probe pulses reveal sub-picosecond demagnetization that is followed by an
ultrafast ($\sim$1 ps) partial recovery of the Kerr signal. Through comparison
with InMnAs, we attribute the signal recovery to an ultrafast energy relaxation
of holes. We propose that the dynamical polarization of holes through $p$-$d$
scattering is the source of the observed probe signal. These results support
the physical picture of femtosecond demagnetization proposed earlier for
InMnAs, identifying the critical roles of both energy and spin relaxation of
hot holes. | cond-mat_mtrl-sci |
Portable implementation of a quantum thermal bath for molecular dynamics
simulations: Recently, Dammak and coworkers (H. Dammak, Y. Chalopin, M. Laroche, M.
Hayoun, and J.J. Greffet. Quantumthermal bath for molecular dynamics
simulation. Phys. Rev. Lett., 103:190601, 2009.) proposed that the quantum
statistics of vibrations in condensed systems at low temperature could be
simulated by running molecular dynamics simulations in the presence of a
colored noise with an appropriate power spectral density. In the present
contribution, we show how this method can be implemented in a flexible manner
and at a low computational cost by synthesizing the corresponding noise 'on the
fly'. The proposed algorithm is tested for a simple harmonic chain as well as
for a more realistic model of aluminium crystal. The energy and Debye-Waller
factor are shown to be in good agreement with those obtained from harmonic
approximations based on the phonon spectrum of the systems. The limitations of
the method associated with anharmonic effects are also briefly discussed. Some
perspectives for disordered materials and heat transfer are considered. | cond-mat_mtrl-sci |
Effect of Pressure on Electrical and optical Properties of Metal Doped
TiO$_2$: A comparative study of the electrical and optical properties has been done on
3d-doped TiO$_2$. Ti$_{1-x}$M$_x$O$_2$ (M= Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn)
powder and its corresponding pellets, with doping concentration $x= 0.05$. The
samples were prepared using the solid-state route. Optical and electrical
measurements have been performed for all prepared samples and interestingly, it
is observed that due to external pressure (i.e. strain) both the properties
change significantly. A rigorous theoretical calculation has also been carried
out to verify the experimental band gap obtained from optical absorption
spectroscopy. In case of pellet sample band gap decreases as compared to the
powder sample due to variation of pressure inside the structures. Role of
doping has also been investigated both in pellet and powder forms and we found
that the band gap decreases as the atomic number of dopants increases. A
cross-over behavior is seen in pellet samples on doping with Ni, Cu and Zn
(i.e. band gap increases with an increase in the atomic number of dopant).
Electrical resistivity measurements have been carried out for both pellet and
powder samples and it is found that in the case of strained samples the value
of resistivity is smaller while in the case of strain-free samples it is quite
large. We believe that the present study suggests a novel approach for tuning
the electrical and optical properties of semiconducting oxides either from
doping or from applied pressure (or strain). | cond-mat_mtrl-sci |
Generalized interface models for transport phenomena: unusual scale
effects in composite nanomaterials: The effective transport properties of heterogeneous nanoscale materials and
structures are affected by several geometrical and physical factors. Among them
the presence of imperfect interfaces plays a central role being often at the
origin of the scale effects. To describe real contacts between different phases
some classical schemes have been introduced in literature, namely the low and
the high conducting interface models. Here, we introduce a generalized
formalism, which is able to take into account the properties of both previous
schemes and, at the same time, it implements more complex behaviors, already
observed in recent investigations. We apply our models to the calculation of
the effective conductivity in a paradigmatic structure composed of a dispersion
of particles. In particular we describe the conductivity dependence upon the
size of the inclusions finding an unusual non-monotone scale effect with a
pronounced peak at a given particle size. We introduce some intrinsic length
scales governing the universal scaling laws. | cond-mat_mtrl-sci |
Effects of macroscopic-polarization built-in electrostatic fields in
III-V nitrides multi-quantum-wells: Huge built-in electric fields have been predicted to exist in wurtzite III-V
nitrides thin films and multilayers. Such fields originate from heterointerface
discontinuities of the macroscopic bulk polarization of the nitrides. Here we
discuss the background theory, the role of spontaneous polarization in this
context, and the practical implications of built-in polarization fields in
nitride nanostructures. To support our arguments, we present detailed
self-consistent tight-binding simulations of typical nitride QW structures in
which polarization effects are dominant. | cond-mat_mtrl-sci |
Magnetic Skyrmionic Polarons: We study a two-dimensional electron gas exchanged-coupled to a system of
classical magnetic ions. For large Rashba spin-orbit coupling a single electron
can become self-trapped in a skyrmion spin texture self-induced in the magnetic
ions system. This new quasiparticle carries electrical and topological charge
as well as a large spin, and we named it as magnetic skyrmionic polaron. We
study the range of parameters; temperature, exchange coupling, Rashba coupling
and magnetic field, for which the magnetic skyrmionic polaron is the
fundamental state in the system. The dynamics of this quasiparticle is studied
using the collective coordinate approximation, and we obtain that in presence
of an electric field the new quasiparticle shows, because the chirality of the
skyrmion, a Hall effect. Finally we argue that the magnetic skyrmionic polarons
can be found in large Rashba spin-orbit coupling semiconductors as GeMnTe. | cond-mat_mtrl-sci |
Complex structures of dense lithium: electronic origin: Lithium - the lightest alkali metal - exhibits unexpected structures and
electronic behaviour at high pressures. As the heavier alkalis, Li is bcc at
ambient pressure and transforms first to fcc (at 7.5 GPa). The post-fcc
high-pressure form Li-cI16 (at 40-60 GPa) is similar to Na-cI16 and related to
more complex structures of heavy alkalis Rb-oC52 and Cs-oC84. The other high
pressure phases for Li (oC88, oC40, oC24) found at pressures up to 130 GPa are
specific the only to Li. The different route of Li high-pressure structures
correlates with its special electronic configuration containing the only 3
electrons (at 1s and 2s levels). Crystal structures for Li are analyzed within
the model of Fermi sphere - Brillouin zone interactions. Stability of post-fcc
structures for Li can be supported by Hume-Rothery arguments when new Brillouin
zone plains appear close to the Fermi level producing pseudogaps near the Fermi
level and decreasing the crystal energy. The filling of Brillouin-Jones zones
by electron states for a given structure defines the physical properties as
optical reflectivity, electrical resistivity and superconductivity. To
understand complexity of structural and physical properties of Li above 60 GPa
is necessary to assume the valence electrons band overlap with the upper core
electrons and increase the valence electron count under compression. | cond-mat_mtrl-sci |
Deciphering Cryptic Behavior in Bimetallic Transition Metal Complexes
with Machine Learning: The rational tailoring of transition metal complexes is necessary to address
outstanding challenges in energy utilization and storage. Heterobimetallic
transition metal complexes that exhibit metal-metal bonding in stacked "double
decker" ligand structures are an emerging, attractive platform for catalysis,
but their properties are challenging to predict prior to laborious synthetic
efforts. We demonstrate an alternative, data-driven approach to uncovering
structure-property relationships for rational bimetallic complex design. We
tailor graph-based representations of the metal-local environment for these
heterobimetallic complexes for use in training of multiple linear regression
and kernel ridge regression (KRR) models. Focusing on oxidation potentials, we
obtain a set of 28 experimentally characterized complexes to develop a multiple
linear regression model. On this training set, we achieve good accuracy (mean
absolute error, MAE, of 0.25 V) and preserve transferability to unseen
experimental data with a new ligand structure. We trained a KRR model on a
subset of 330 structurally characterized heterobimetallics to predict the
degree of metal-metal bonding. This KRR model predicts relative metal-metal
bond lengths in the test set to within 5%, and analysis of key features reveals
the fundamental atomic contributions (e.g., the valence electron configuration)
that most strongly influence the behavior of complexes. Our work provides
guidance for rational bimetallic design, suggesting that properties including
the formal shortness ratio should be transferable from one period to another. | cond-mat_mtrl-sci |
Band gap engineering by Bi intercalation of graphene on Ir(111): We report on the structural and electronic properties of a single bismuth
layer intercalated underneath a graphene layer grown on an Ir(111) single
crystal. Scanning tunneling microscopy (STM) reveals a hexagonal surface
structure and a dislocation network upon Bi intercalation, which we attribute
to a $\sqrt{3}\times\sqrt{3}R30{\deg}$ Bi structure on the underlying Ir(111)
surface. Ab-initio calculations show that this Bi structure is the most
energetically favorable, and also illustrate that STM measurements are most
sensitive to C atoms in close proximity to intercalated Bi atoms. Additionally,
Bi intercalation induces a band gap ($E_g=0.42\,$eV) at the Dirac point of
graphene and an overall n-doping ($\sim 0.39\,$eV), as seen in angular-resolved
photoemission spectroscopy. We attribute the emergence of the band gap to the
dislocation network which forms favorably along certain parts of the moir\'e
structure induced by the graphene/Ir(111) interface. | cond-mat_mtrl-sci |
Towards High-Performance Two-Dimensional Black Phosphorus Optoelectronic
Devices: the Role of Metal Contacts: The metal contacts on 2D black phosphorus field-effect transistor and
photodetectors are studied. The metal work functions can significantly impact
the Schottky barrier at the metal-semiconductor contact in black phosphorus
devices. Higher metal work functions lead to larger output hole currents in
p-type transistors, while ambipolar characteristics can be observed with lower
work function metals. Photodetectors with record high photoresponsivity (223
mA/W) are demonstrated on black phosphorus through contact-engineering. | cond-mat_mtrl-sci |
Phonon Dispersion Effects and the Thermal Conductivity Reduction in
GaAs/AlAs Superlattices: The experimentally observed order-of-magnitude reduction in the thermal
conductivity along the growth axis of (GaAs)_n/(AlAs)_n (or n x n)
superlattices is investigated theoretically for (2x2), (3x3) and (6x6)
structures using an accurate model of the lattice dynamics. The modification of
the phonon dispersion relation due to the superlattice geometry leads to
flattening of the phonon branches and hence to lower phonon velocities. This
effect is shown to account for a factor-of-three reduction in the thermal
conductivity with respect to bulk GaAs along the growth direction; the
remainder is attributable to a reduction in the phonon lifetime. The
dispersion-related reduction is relatively insensitive to temperature (100 < T
< 300K) and n. The phonon lifetime reduction is largest for the (2x2)
structures and consistent with greater interface scattering. The thermal
conductivity reduction is shown to be appreciably more sensitive to GaAs/AlAs
force constant differences than to those associated with molecular masses. | cond-mat_mtrl-sci |
Bayesian calibration of interatomic potentials for binary alloys: Developing reliable interatomic potential models with quantified predictive
accuracy is crucial for atomistic simulations. Commonly used potentials, such
as those constructed through the embedded atom method (EAM), are derived from
semi-empirical considerations and contain unknown parameters that must be
fitted based on training data. In the present work, we investigate Bayesian
calibration as a means of fitting EAM potentials for binary alloys. The
Bayesian setting naturally assimilates probabilistic assertions about uncertain
quantities. In this way, uncertainties about model parameters and model errors
can be updated by conditioning on the training data and then carried through to
prediction. We apply these techniques to investigate an EAM potential for a
family of gold-copper systems in which the training data correspond to
density-functional theory values for lattice parameters, mixing enthalpies, and
various elastic constants. Through the use of predictive distributions, we
demonstrate the limitations of the potential and highlight the importance of
statistical formulations for model error. | cond-mat_mtrl-sci |
A regression-based feature selection study of the Curie temperature of
transition-metal rare-earth compounds: prediction and understanding: The Curie temperature ($T_C$) of binary alloy compounds consisting of 3$d$
transition-metal and 4$f$ rare-earth elements is analyzed by a machine learning
technique. We first demonstrate that nonlinear regression can accurately
reproduce $T_C$ of the compounds. The prediction accuracy for $T_C$ is
maximized when five to ten descriptors are selected, with the rare-earth
concentration being the most relevant. We then discuss an attempt to utilize a
regression-based model selection technique to learn the relation between the
descriptors and the actuation mechanism of the corresponding physical
phenomenon, i.e., $T_C$ in the present case. | cond-mat_mtrl-sci |
Atomic defects and dopants in ternary Z-phase transition-metal nitrides
CrMN with M=V, Nb, Ta investigated with density functional theory: A density functional theory study of atomic defects and dopants in ternary
Z-phase transition-metal nitrides CrMN with M=V, Nb, or Ta is presented.
Various defect formation energies of native point defects and of substitutional
atoms of other metal elements which are abundant in the steel as well, are
evaluated. The dependence thereof on the thermodynamic environment, i.e. the
chemical conditions of a growing Z-phase precipitate, is studied and different
growth scenarios are compared. The results obtained may help to relate results
of experimental atomic-scale analysis, by atom probe tomography or transmission
electron microscopy, to the theoretical modeling of the formation process of
the Z phase from binary transition metal nitrides. | cond-mat_mtrl-sci |
Electrical transport and percolation in magnetoresistive manganite /
insulating oxide composites: case of La0.7Ca0.3MnO3 / Mn3O4: We report the results of electrical resistivity measurements carried out on
well-sintered La0.7Ca0.3MnO3 / Mn3O4 composite samples with almost constant
composition of the magnetoresistive manganite phase (La0.7Ca0.3MnO3). A
percolation threshold (fc) occurs when the La0.7Ca0.3MnO3 volume fraction is ~
0.19. The dependence of the electrical resistivity as a function of
La0.7Ca0.3MnO3 volume fraction (fLCMO) can be described by percolation-like
phenomenological equations. Fitting the conducting regime (fLCMO > fc) by the
percolation power law returns a critical exponent t value of 2.0 +/- 0.2 at
room temperature and 2.6 +/-0.2 at 5 K. The increase of t is ascribed to the
influence of the grain boundaries on the electrical conduction process at low
temperature. | cond-mat_mtrl-sci |
Instability of the rhodium magnetic moment as origin of the metamagnetic
phase transition in alpha-FeRh: Based on ab initio total energy calculations we show that two magnetic states
of rhodium atoms together with competing ferromagnetic and antiferromagnetic
exchange interactions are responsible for a temperature induced metamagnetic
phase transition, which experimentally is observed for stoichiometric
alpha-FeRh. A first-principle spin-based model allows to reproduce this
first-order metamagnetic transition by means of Monte Carlo simulations.
Further inclusion of spacial variation of exchange parameters leads to a
realistic description of the experimental magneto-volume effects in alpha-FeRh. | cond-mat_mtrl-sci |
Incipient triple point for adsorbed xenon monolayers: Pt(111) versus
graphite substrates: Simulation evidence of an incipient triple point is reported for xenon
submonolayers adsorbed on the (111) surface of platinum. This is in stark
contrast to the "normal" triple point found in simulations and experiments for
xenon on the basal plane surface of graphite. The motions of the atoms in the
surface plane are treated with standard 2D "NVE" molecular dynamics simulations
using modern interactions. The simulation evidence strongly suggests an
incipient triple point in the 120-150 K range for adsorption on the Pt (111)
surface while the adsorption on graphite shows a normal triple point at about
100 K. | cond-mat_mtrl-sci |
Magnetization dynamics and damping due to electron-phonon scattering in
a ferrimagnetic exchange model: We present a microscopic calculation of magnetization damping for a magnetic
"toy model." The magnetic system consists of itinerant carriers coupled
antiferromagnetically to a dispersionless band of localized spins, and the
magnetization damping is due to coupling of the itinerant carriers to a phonon
bath in the presence of spin-orbit coupling. Using a mean-field approximation
for the kinetic exchange model and assuming the spin-orbit coupling to be of
the Rashba form, we derive Boltzmann scattering integrals for the distributions
and spin coherences in the case of an antiferromagnetic exchange splitting,
including a careful analysis of the connection between lifetime broadening and
the magnetic gap. For the Elliott-Yafet type itinerant spin dynamics we extract
dephasing and magnetization times T_1 and T_2 from initial conditions
corresponding to a tilt of the magnetization vector, and draw a comparison to
phenomenological equations such as the Landau-Lifshitz or the Gilbert damping.
We also analyze magnetization precession and damping for this system including
an anisotropy field and find a carrier mediated dephasing of the localized spin
via the mean-field coupling. | cond-mat_mtrl-sci |
Theoretical estimates for flat voids coalescence by internal necking: Coalescence of voids by internal necking is in most cases the last
microscopic event related to ductile fracture and corresponds to a localized
plastic flow between adjacent voids. Macroscopic load associated to the onset
of coalescence is classically estimated based on limit analysis. However, a
rigorous upper-bound mathematical expression for the limitload required for
flat voids coalescence that remains finite for penny-shaped voids/cracks is
still unavailable. Therefore, based on limit analysis, theoretical upper-bound
estimates - both integral expression and closed-form formula - are obtained for
the limit-load of cylindrical flat voids in cylindrical unit-cell subjected to
boundary conditions allowing the assessment of coalescence, for axisymmetric
stress state. These estimates, leading to finite limit-loads for pennyshaped
cracks, are shown to be in very good agreement with numerical limit analysis,
for both cylindrical and spheroidal voids. Approximate formula is also given
for coalescence under combined tension and shear loading. These coalescence
criteria can thus be used to predict onset of coalescence of voids by internal
necking in ductile fracture modelling. | cond-mat_mtrl-sci |
Coherent generation of symmetry-forbidden phonons by light-induced
electron-phonon interactions in magnetite: Symmetry breaking across phase transitions often causes changes in selection
rules and emergence of optical modes which can be detected via spectroscopic
techniques or generated coherently in pump-probe experiments. In second-order
or weakly first-order transitions, fluctuations of the order parameter are
present above the ordering temperature, giving rise to intriguing precursor
phenomena, such as critical opalescence. Here, we demonstrate that in magnetite
(Fe$_3$O$_4$) light excitation couples to the critical fluctuations of the
charge order and coherently generates structural modes of the ordered phase
above the critical temperature of the Verwey transition. Our findings are
obtained by detecting coherent oscillations of the optical constants through
ultrafast broadband spectroscopy and analyzing their dependence on temperature.
To unveil the coupling between the structural modes and the electronic
excitations, at the origin of the Verwey transition, we combine our results
from pump-probe experiments with spontaneous Raman scattering data and
theoretical calculations of both the phonon dispersion curves and the optical
constants. Our methodology represents an effective tool to study the real-time
dynamics of critical fluctuations across phase transitions. | cond-mat_mtrl-sci |
Thermoelectric properties of semiconducting materials with parabolic and
pudding-mold band structures: We theoretically investigate the thermoelectric properties of semiconducting
(gapped) materials by varying the degrees of polynomials in their energy
dispersion relations, in which either the valence or conduction energy
dispersion depends on the wave vector raised to the power of two, four, and
six. The thermoelectric transport coefficients such as the Seebeck coefficient,
electrical conductivity, and thermal conductivity are calculated within the
linearized Boltzmann transport theory combined with the relaxation time
approximation. We consider various effects such as band gaps, dimensionalities,
and dispersion powers to understand the conditions that can give the optimal
thermoelectric efficiency or figure of merit ($ZT$). Our calculations show that
the so-called pudding-mold band structure produces larger electrical and
thermal conductivities than the parabolic band, but no significant difference
is found in the Seebeck coefficients of the pudding-mold and parabolic bands.
Furthermore, we find that a high $ZT$ can be obtained by tuning the band gap of
the material to an optimum value simultaneously with breaking the band
symmetry. The largest $ZT$ is found in a combination of two-contrasting
polynomial powers in the dispersion relations of valence and conduction bands.
This band asymmetry also shifts the charge neutrality away from the undoped
level and allows optimal $ZT$ to be located at a smaller chemical potential.
With some reasonable values of thermal conductivity parameters, the maximum
$ZT$ for the bulk systems can be larger than 1, while for one-dimensional
systems it can even reach almost 4. We expect this work to trigger
high-throughput calculations for screening of potential thermoelectric
materials combining various polynomial powers in the energy dispersion
relations of semiconductors. | cond-mat_mtrl-sci |
Spin transport in a magnetic insulator with zero effective damping: Applications based on spin currents strongly profit from the control and
reduction of their effective damping and their transport properties. We here
experimentally observe magnon mediated transport of spin (angular) momentum
through a 13.4 nm thin yttrium iron garnet film with full control of the
magnetic damping via spin-orbit torque. Above a critical spin-orbit torque, the
fully compensated damping manifests itself as an increase of magnon
conductivity by almost two orders of magnitude. We compare our results to
theoretical expectations based on recently predicted current induced magnon
condensates and discuss other possible origins of the observed critical
behaviour. | cond-mat_mtrl-sci |
Gate-Voltage Tunability of Plasmons in Single and Multi-layer Graphene
Structures: Analytical Description and Concepts for Terahertz Devices: The strong light-matter interaction in graphene over a broad frequency range
has opened up a plethora of photonics applications of graphene. The goal of
this paper is to present the voltage tunability of plasmons in gated single-
and multi-layer graphene structures. Device concepts for plasmonic
interconnects and antennas and their performance for THz communication are
presented. For the first time, the role of gate voltage and the thickness of
the gate dielectric on the characteristics of plasmon propagation in graphene
are quantified by accounting for both the interface trap capacitance and the
quantum capacitance. The gate voltage serves as a powerful knob to tweak the
carrier concentration and allows building electrically reconfigurable terahertz
devices. By optimizing the gate voltage to maximize the plasmon propagation
length in a gated multi-layer graphene geometry, we derive simple scaling
trends that give intuitive insight into device modeling and design. | cond-mat_mtrl-sci |
DFT Studies of 2D Materials Inspired by Lie Algebras: Inspired by the root systems of Lie algebras of rank 2, we propose a
mathematical method to engineer new 2D materials with double periodic
structures tessellating the plane. Concretely, we investigate two geometries
relaying on squares and hexagons exhibiting D4 D4 and D6 D6 dihedral group
invariances, respectively. Due to lack of empirical verifications of such
double configurations, we provide a numerical investigation by help of the open
source quantum espresso. Motivated by hybrid structures of graphene, silicene,
germanene, we investigate two models involving D4 D4 and D6 D6 dihedral
symmetries which we refer to as Si4Ge4 and Si6C6 compounds, respectively. For
simplicities, we study only the opto-electronic physical properties by applying
an electromagnetic source propagating in linear and isotropic mediums. We
believe that the Lie algebra inspiration of such 2D material studies, via
density functional theory techniques, could open new roads to think about
higher dimensional cases by implementing generalized Cartan matrices. | cond-mat_mtrl-sci |
Reconciling the ionic and covalent pictures in rare-earth nickelates: The properties of AMO3 perovskite oxides, where M is a 3d transition metal,
depend strongly on the level of covalency between the metal d and oxygen p
orbitals. With their complex spin orders and metal-insulator transition,
rare-earth nickelates verge between dominantly ionic and covalent characters.
Accordingly, the nature of their ground state is highly debated. Here, we
reconcile the ionic and covalent visions of the insulating state of nickelates.
Through first-principles calculations, we show that it is reminiscent of the
ionic charge disproportionation picture (with strictly low-spin 4+ and
high-spin 2+ Ni sites) while exhibiting strong covalence effects with oxygen
electrons shifted toward the depleted Ni cations, mimicking a configuration
with identical Ni sites. Our results further hint at strategies to control
electronic and magnetic phases of transition metal oxide perovskites. | cond-mat_mtrl-sci |
A systematic study of magnetodynamic properties at finite temperatures
in doped permalloy from first principles calculations: By means of first principles calculations, we have systematically
investigated how the magnetodynamic properties Gilbert damping, magnetization
and exchange stiffness are affected when permalloy (Py)
(Fe$_{0.19}$Ni$_{0.81}$) is doped with 4d or 5d transition metal impurities. We
find that the trends in the Gilbert damping can be understood from relatively
few basic parameters such as the density of states at the Fermi level, the
spin-orbit coupling and the impurity concentration. % The temperature
dependence of the Gilbert damping is found to be very weak which we relate to
the lack of intraband transitions in alloys. % Doping with $4d$ elements has no
major impact on the studied Gilbert damping, apart from diluting the host.
However, the $5d$ elements have a profound effect on the damping and allows it
to be tuned over a large interval while maintaining the magnetization and
exchange stiffness. % As regards spin stiffness, doping with early transition
metals results in considerable softening, whereas late transition metals have a
minor impact. % Our result agree well with earlier calculations where
available. In comparison to experiments, the computed Gilbert damping appears
slightly underestimated while the spin stiffness show good general agreement. | cond-mat_mtrl-sci |
Influence of different exchange-correlation potentials on twisted
structures of bilayer XS2 (X= Mo, Cr): In this work, we employ the LDA, GGA and GGA with four vdW corrections to
study crystal and electronic structures of bilayer transition metal
dichalcogenides (TMDs) with different twist angles. We find the GGA interlayer
distance of bilayer MoS2 has good agreement with experimental value while vdW
correction method still needs to be further improved. Our results indicate the
GGA interlayer distances of bilayer XS2 (X= Mo, Cr) with twist angles are
smaller than that of normal bilayer, which is the opposite in the LDA case. The
GGA results show that reduced bandgap is due to the reduction of interlayer
distance and, flat valley and conductivity bands appear owing to twist angle.
Our study not only supports valuable information for application possibility of
twisted two-dimensional (2D) materials but also stimulates more related
research. | cond-mat_mtrl-sci |
Atomic multiplet and charge-transfer screening effects in 1$s$ and 2$p$
core-level X-ray photoelectron spectra of early 3$d$ transition-metal oxides: We present a comparative analysis of 1$s$ and 2$p$ core-level hard X-ray
photoelectron spectroscopy (HAXPES) spectra in metallic VO$_2$ and CrO$_2$.
Even though the V 1$s$ and 2$p$ spectra in VO$_2$ display similar line shapes
except the absence or presence of a spin-orbit coupling splitting, the Cr 1$s$
and 2$p$ spectra exhibit distinct main-line shapes. The experimental HAXPES
spectra are analyzed by the Anderson impurity model based on the density
functional theory + dynamical mean-field theory and a conventional MO$_6$
cluster model. We elucidate the complex interplay between formation of the
intra-atomic multiplet and charge transfer effect on the chemical bonding
followed by the 1$s$ and 2$p$ core electron excitations. We demonstrate the
advantage of the 1$s$ excitations to the routinely-employed 2$p$ excitations
for distinguishing between metal-ligand and metal-metal charge transfer
contributions in early 3$d$ transition-metal oxides. | cond-mat_mtrl-sci |
Thermal expansion and pressure effect in MnWO4: MnWO4 has attracted attention because of its ferroelectric property induced
by frustrated helical spin order. Strong spin-lattice interaction is necessary
to explain ferroelectricity associated with this type of magnetic order.We have
conducted thermal expansion measurements along the a, b, c axes revealing the
existence of strong anisotropic lattice anomalies at T1=7.8 K, the temperature
of the magnetic lock-in transition into a commensurate low-temperature
(reentrant paraelectric) phase. The effect of hydrostatic pressure up to 1.8
GPa on the FE phase is investigated by measuring the dielectric constant and
the FE polarization. The low- temperature commensurate and paraelectric phase
is stabilized and the stability range of the ferroelectric phase is diminished
under pressure. | cond-mat_mtrl-sci |
Monte Carlo and kinetic Monte Carlo methods: This article reviews the basic computational techniques for carrying out
multi-scale simulations using statistical methods, with the focus on
simulations of epitaxial growth. First, the statistical-physics background
behind Monte Carlo simulations is briefly described. The kinetic Monte Carlo
(kMC) method is introduced as an extension of the more wide-spread
thermodynamic Monte Carlo methods, and algorithms for kMC simulations,
including parallel ones, are discussed in some detail. The step from the
atomistic picture to the more coarse-grained description of Monte Carlo
simulations is exemplified for the case of surface diffusion. Here, the aim is
the derivation of rate constants from knowledge about the underlying atomic
processes. Both the simple approach of Transition State Theory, as well as more
recent approaches using accelerated molecular dynamics are reviewed. Finally, I
address the point that simplifications often need to be introduced in practical
Monte Carlo simulations in order to reduce the complexity of 'real' atomic
processes. Different 'flavors' of kMC simulations and the potential pitfalls
related to the reduction of complexity are presented in the context of
simulations of epitaxial growth. | cond-mat_mtrl-sci |
Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide: Understanding defect effect on carrier dynamics is essential for both
fundamental physics and potential applications of transition metal
dichalcogenides. Here, the phenomenon of oxygen impurities trapping
photo-excited carriers has been studied with ultrafast pump-probe spectroscopy.
Oxygen impurities are intentionally created in exfoliated multilayer MoSe2 with
Ar+ plasma irradiation and air exposure. After plasma treatment, the signal of
transient absorption first increases and then decreases, which is a signature
of defect capturing carriers. With larger density of oxygen defects, the
trapping effect becomes more prominent. The trapping defect densities are
estimated from the transient absorption signal, and its increasing trend in the
longer-irradiated sample agrees with the results from X-ray photoelectron
spectroscopy. First principle calculations with density functional theory
reveal that oxygen atoms occupying Mo vacancies create mid-gap defect states,
which are responsible for the carrier trapping. Our findings shed light on the
important role of oxygen defects as carrier trappers in transition metal
dichalcogenides, and facilitates defect engineering in relevant material and
device applications. | cond-mat_mtrl-sci |
Ambient temperature pressure driven alkane dehydrogenation by palladium
metal: Dehydrogenation of alkanes is of increasing importance in fulfilling global
demand for olefins and offers a potential source of carbon-neutral hydrogen as
a co-product. Currently commercial dehydrogenation processes occur at
high-temperatures (500-900$^\circ$C) which is energy intensive and results in
side reactions and rapid coking of the catalysts. In addition the hydrogen
produced is often burned to maintain temperature and to inhibit the back
reaction. Here we demonstrate pressure as a parameter to enable novel chemical
catalytic processes and demonstrate ambient-temperature dehydrogenation of
alkanes by palladium at 50-100 MPa pressures, with both hydrogen gas and
olefins recovered on decompression. This reaction follows a fundamentally
different path to current commercial high-temperature low-pressure
dehydrogenation processes with the palladium catalyst reversibly forming a
hydride intermediate. | cond-mat_mtrl-sci |
A phase field model combined with genetic algorithm for polycrystalline
hafnium zirconium oxide ferroelectrics: Ferroelectric hafnium zirconium oxide (HZO) thin films show significant
promise for applications in ferroelectric random-access memory, ferroelectric
field-effect transistors, and ferroelectric tunneling junctions. However, there
are shortcomings in understanding ferroelectric switching, which is crucial in
the operation of these devices. Here a computational model based on phase field
method is developed to simulate the switching behavior of polycrystalline HZO
thin films. Furthermore, we introduce a novel approach to optimize the
effective Landau coefficients describing the free energy of HZO by combining
the phase field model with a genetic algorithm. We validate the model by
accurately simulating switching curves for HZO thin films with different
ferroelectric phase fractions. The simulated domain dynamics during switching
also shows amazing similarity to the available experimental observations. The
present work also provides fundamental insights into enhancing the
ferroelectricity in HZO thin films by controlling grain morphology and
crystalline texture. It can potentially be extended to improve the
ferroelectric properties of other hafnia based thin films. | cond-mat_mtrl-sci |
Molecular Motion on Semiconductor Surface via Tip-enhanced Multiple
Excitation: In a low-temperature study with a scanning tunneling microscope (STM), the
irreducible lateral motion of a CO molecule adsorbed on a Si(001) surface
showed a hyperlinear dependence on the tunneling current. This dependence
implies that the adsorbate displacement is caused by multiple excitations of
adsorbate vibration modes, a situation thus far observed only at metal
surfaces. The local vibronic temperature at the atomic scale on the surface
heated by ohmic inelastic scattering of tunneling electrons indicates that
there is an activation barrier of 0.11 eV for the irreversible motion of CO, in
agreement with the adiabatic potential obtained from first-principles
calculation. The highly efficient local heating is caused by a mid-gap state at
the surface induced by the electric field of the STM tip. | cond-mat_mtrl-sci |
First-principles prediction of oxygen octahedral rotations in
perovskite-structure EuTiO3: We present a systematic first-principles study of the structural and
vibrational properties of perovskite-structure EuTiO3. Our calculated phonon
spectrum of the high-symmetry cubic structural prototype shows strong M- and
R-point instabilities, indicating a tendency to symmetry-lowering structural
deformations composed of rotations and tilts of the oxygen octahedra.
Subsequent explicit study of 14 different octahedral tilt-patterns showed that
the I4/mcm, Imma, and R\bar{3}c structures, all with antiferrodistortive
rotations of the octahedra, have significantly lower total energy than the
prototype Pm\bar{3}m structure. We discuss the dynamical stability of these
structures, and the influence of the antiferrodistortive structural distortions
on the vibrational, optical, and magnetic properties of EuTiO3, in the context
of recent unexplained experimental observations. | cond-mat_mtrl-sci |
Band restructuring of ordered/disordered blue TiO2 for visible
photocatalyst: Black TiO2 with/without noble metal has been proposed for visible
photocatalyst, still leaving poor catalyst efficiency. Alternatively,
phase-mixed TiO2 such as anatase and rutile has been commonly used for visible
catalysts with the inevitable inclusion of noble metal. Here, we perform a
noble metal-free visible photocatalyst blue TiO2 with type-II band-aligned
ordered anatase/disordered rutile structure, via phase-selective reduction with
alkali metals. The changed band alignment in this heterostructure was
identified by absorption and ultraviolet photoemission spectroscopy, which was
further confirmed by transient charge separation. The band alignment of type-I
and type-II was clearly restructured by converting from ordered to disordered
phase with a prolonged reduction period and as followed light absorbance
enhancement also observed. Initiated type-I in a pristine sample, the type-II
was organized from disordered rutile phase in 3-day Li-reduction. The type-II
disordered rutile TiO2 heterostructure exhibits a remarkable photocatalytic
performance by 55 times higher than conventional P25 TiO2 in solar-light driven
hydrogen evolution reaction owing to an efficient electron and hole separation
of type-II heterojunction. Furthermore, this restructured heterojunction
type-II TiO2 demanded 10 times less Pt amount as a co-catalyst for the
comparable photocatalytic performance, compared to Pt decorated type-I pristine
anatase/rutile phase-mixed TiO2. | cond-mat_mtrl-sci |
Doping graphene with metal contacts: Making devices with graphene necessarily involves making contacts with
metals. We use density functional theory to study how graphene is doped by
adsorption on metal substrates and find that weak bonding on Al, Ag, Cu, Au and
Pt, while preserving its unique electronic structure, can still shift the Fermi
level with respect to the conical point by $\sim 0.5$ eV. At equilibrium
separations, the crossover from $p$-type to $n$-type doping occurs for a metal
work function of $\sim 5.4$ eV, a value much larger than the graphene work
function of 4.5 eV. The numerical results for the Fermi level shift in graphene
are described very well by a simple analytical model which characterizes the
metal solely in terms of its work function, greatly extending their
applicability. | cond-mat_mtrl-sci |
Giant magnetostriction in Tb-doped Fe83Ga17 melt-spun ribbons: Giant magnetostriction is achieved in the slightly Tb-doped Fe83Ga17
melt-spun ribbons. The tested average perpendicular magnetostriction is -886
ppm along the melt-spun ribbon direction in the Fe82.89Ga16.88Tb0.23 alloy. The
calculated parallel magnetostriction is 1772 ppm, more than 4 times as large as
that of binary Fe83Ga17 alloy. The enhanced magnetostriction should be
attributed to a small amount of Tb solution into the A2 matrix phase during
rapid solidification. The localized strong magnetocrystalline anisotropy of Tb
element is suggested to cause the giant magnetostriction. | cond-mat_mtrl-sci |
Magnetic phase diagram of Ce2Fe17: Rare-earth-based permanent-magnet materials rich in iron have relatively low
ferromagnetic ordering temperatures. This is believed to be due to the presence
of antiferromagnetic exchange interactions, besides the ferromagnetic
interactions responsible for the magnetic order. The magnetic properties of
Ce2Fe17 are anomalous. Instead of ferromagnetic, it is antiferromagnetic, and
instead of one ordering temperature, it shows two, at the Neel temperature TN ~
208 K and at TT ~ 124 K. Ce2Fe17, doped by 0.5% Ta, also shows two ordering
temperatures, one to an antiferromagnetic phase, at TN ~ 214 K, and one to a
ferromagnetic phase, at T0 ~ 75 K. In order to clarify this behavior,
single-crystalline samples were prepared by solution growth, and characterized
by electron microscopy, single crystal x-ray diffraction, temperature-dependent
specific heat, and magnetic field and temperature-dependent electrical
resistivity and magnetization. From these measurements, magnetic H-T phase
diagrams were determined for both Ta-doped Ce2Fe17 and undoped Ce2Fe17. These
phase diagrams can be very well described in terms of a theory that gives
magnetic phase diagrams of systems with competing antiferro- and
ferromagnetism. | cond-mat_mtrl-sci |
Impact of Sb degrees of freedom on the charge density wave phase diagram
of the kagome metal CsV$_3$Sb$_5$: Elucidating the microscopic mechanisms responsible for the charge density
wave (CDW) instability of the AV$_3$Sb$_5$ (A=Cs, K, Rb) family of kagome
metals is critical for understanding their unique properties, including
superconductivity. In these compounds, distinct CDW phases with wave-vectors at
the $M$ and $L$ points are energetically favorable, opening the possibility of
tuning the type of CDW order by appropriate external parameters. Here, we shed
light on the CDW landscape of CsV$_3$Sb$_5$ via a combination of
first-principles calculations and phenomenology, which consists of extracting
the coefficients of the CDW Landau free-energy expansion from density
functional theory. We find that while the main structural distortions of the
kagome lattice in the staggered tri-hexagonal CDW phase are along the
nearest-neighbor V-V bonds, distortions associated with the Sb ions play a
defining role in the energy gain in this and all other CDW states. Moreover,
the coupling between ionic displacements from different unit cells is small,
thus explaining the existence of multiple CDW instabilities with different
modulations along the c-axis. We also investigate how pressure and temperature
impact the CDW phase of CsV$_3$Sb$_5$. Increasing pressure does not change the
staggered tri-hexagonal CDW ground state, even though the $M$-point CDW
instability disappears before the $L$-point one, a behavior that we attribute
to the large nonlinear coupling between the order parameters. Upon changing the
temperature, we find a narrow regime in which another transition can take
place, toward a tri-hexagonal Star-of-David CDW phase. We discuss the
implications of our results by comparing them with experiments on this
compound. | cond-mat_mtrl-sci |
Shock and Release Temperatures in Molybdenum: Shock and release temperatures in Mo were calculated, taking account of
heating from plastic flow predicted using the Steinberg-Guinan model. Plastic
flow was calculated self-consistently with the shock jump conditions: this is
necessary for a rigorous estimate of the locus of shock states accessible. The
temperatures obtained were significantly higher than predicted assuming ideal
hydrodynamic loading. The temperatures were compared with surface emission
spectrometry measurements for Mo shocked to around 60GPa and then released into
vacuum or into a LiF window. Shock loading was induced by the impact of a
planar projectile, accelerated by high explosive or in a gas gun. Surface
velocimetry showed an elastic wave at the start of release from the shocked
state; the amplitude of the elastic wave matched the prediction to around 10%,
indicating that the predicted flow stress in the shocked state was reasonable.
The measured temperatures were consistent with the simulations, indicating that
the fraction of plastic work converted to heat was in the range 70-100% for
these loading conditions. | cond-mat_mtrl-sci |
Powder Diffraction Data and Mesomorphic Properties for 4-Butyloxyphenyl
4'-Decyloxybenzoate: Unit cell parameters obtained from X-ray powder diffraction data are
presented for the crystalline phase of a liquid crystal 4-butyloxyphenyl
4'-decyloxybenzoate: a = 23.098 (4) {\AA}, b = 5.974 (6) {\AA}, c = 12.357 (10)
{\AA}, \b{eta} = 121.53 (8){\deg}, unit-cell volume V = 1453.56 {\AA}3.
Temperature dependent X-ray diffraction data confirmed the existence of smectic
A and smectic C mesophases and a more ordered, tilted crystalline smectic
phase. Possibility of existence of previously reported smectic B phase as well
as another crystalline phase was refuted. | cond-mat_mtrl-sci |
Investigation of re-entrant relaxor behaviour in lead cobalt niobate
ceramic: The temperature dependent dielectric properties revealed re-entrant relaxor
behaviour (Tm ~130 K and 210 K for 1 kHz) below a high temperature diffused
phase transition, Tc ~270 K in lead cobalt niobate (PCN). Multiple
positive/negative magnetodielectric effect and deviation from straight line at
~130 K is observed in temperature dependence of inverse susceptibility, which
depicts origin of frustration. Microstructure examination depicts closely
packed grains with grain size ~8-10 microm and XRD pattern revealed single
phase pseudo cubic crystal structure having Pm3m symmetry with lattice constant
~4.0496(2) {\AA}. Rietveld Refinement on XRD data yields larger value of
thermal parameters, implying Pb and O are disordered along <111> and <110>
directions respectively. Observation of A1g (780 cm-1) mode in Raman
spectroscopy and F-spot in SAED pattern along <110> unit axis in TEM suggests
presence of nano scale 1:1 Co and Nb non-stoichiometric chemical ordering
(CORs), akin to lead magnesium niobate (PMN). K-edge XANES spectra reveals the
presence of cobalt in two oxidation states (Co2+ and Co3+); whereas, niobium
exists in Nb3+ state. Therefore, these local-average structural properties
suggest chemical, structural and spatial heterogeneities. Such multiple
heterogeneities are believed to play a crucial role in producing re-entrant
relaxor behaviour. | cond-mat_mtrl-sci |
The Effects of Vacancy and Oxidation on Black Phosphorus Nanoresonators: Black phosphorene is not stable at ambient conditions, so atomic defects and
oxidation effects are unavoidable in black phosphorus samples in the
experiment. The effects of these defects on the performance of the black
phosphorus nanoresonators are still unclear. Here, we perform classical
molecular dynamics to investigate the effects of the vacancy and oxidation on
single-layer black phosphorus nanoresonators at different temperatures. We find
that the vacancy causes strong reduction in the quality factor of the
nanoresonators, while the oxidation has weaker effect on the nanoresonators.
More specifically, a 2% concentration of randomly distributed single vacancies
is able to reduce the quality factor by about 80% and 40% at 4.2K and 50K,
respectively. We also find that the quality factor of the nanoresonator is not
sensitive to the distribution pattern of the vacancy defects. | cond-mat_mtrl-sci |
Hamiltonian Transformation for Band Structure Calculations: First-principles electronic band structure calculations are essential for
understanding periodic systems in condensed matter physics and materials
science. We propose an accurate and parameter-free method, called Hamiltonian
transformation (HT), to calculate band structures in both density functional
theory (DFT) and post-DFT calculations with plane-wave basis sets. The cost of
HT is independent of the choice of the density functional and scales as
$\mathcal{O}(N_e^3N_k\log N_k)$, where $N_e$ and $N_k$ are the number of
electrons and the number of $\mathbf{k}$-points. Compared to the widely used
Wannier interpolation (WI), HT adopts an eigenvalue transformation to construct
a spatial localized representation of the spectrally truncated Hamiltonian. HT
also uses a non-iterative algorithm to change the basis sets to circumvent the
construction of the maximally localized Wannier functions. As a result, HT can
significantly outperform WI in terms of the accuracy of the band structure
calculation. We also find that the eigenvalue transformation can be of
independent interest, and can be used to improve the accuracy of the WI for
systems with entangled bands. | cond-mat_mtrl-sci |
Multiferroic Properties of Nanocrystalline BaTiO3: Some of the Multiferroics [1] form a rare class of materials that exhibit
magnetoelectric coupling arising from the coexistence of ferromagnetism and
ferroelectricity, with potential for many technological applications.[2,3] Over
the last decade, an active research on multiferroics has resulted in the
identification of a few routes that lead to multiferroicity in bulk
materials.[4-6] While ferroelectricity in a classic ferroelectric such as
BaTiO3 is expected to diminish with the reducing particle size,[7,8]
ferromagnetism cannot occur in its bulk form.[9] Here, we use a combination of
experiment and first-principles simulations to demonstrate that multiferroic
nature emerges in intermediate size nanocrystalline BaTiO3, ferromagnetism
arising from the oxygen vacancies at the surface and ferroelectricity from the
core. A strong coupling between a surface polar phonon and spin is shown to
result in a magnetocapacitance effect observed at room temperature, which can
open up possibilities of new electro-magneto-mechanical devices at the
nano-scale. | cond-mat_mtrl-sci |
Resonant Raman of OH/OD vibrations and photoluminescence studies in
LiTaO3 thin film: Resonant Raman spectra (RRS) of O-H and O-D vibration and libration modes,
their combinations and higher harmonics have been observed in LiTaO3
polycrystalline thin films. RRS peaks are superimposed on photoluminescence
(PL) spectrum. Monochromatic light from a xenon lamp is used as excitation
source. PL spectrum shows two broad peaks, first near the band gap in UV
(4.4-4.8eV) and another in the sub band gap region (< 4.0 eV). Band gap PL
along with RRS peaks are reported for the first time. Photoluminescence
excitation spectrum (PLE) shows a peak at 4.8 eV. Peak positions and full width
at half maximum (FWHM) of RRS peaks depend upon the excitation energy.
Dispersions of the fundamental and the third harmonic of the stretching mode of
O-H with excitation energy are about 800 cm-1/eV and 2000 cm-1/eV respectively.
This dispersion is much higher than reported in any other material. | cond-mat_mtrl-sci |
Quantitative Temperature Dependence of Longitudinal Spin Seebeck Effect
at High Temperatures: This article reports temperature-dependent measurements of longitudinal spin
Seebeck effects (LSSEs) in Pt/Y$_3$Fe$_5$O$_{12}$ (YIG)/Pt systems in a high
temperature range from room temperature to above the Curie temperature of YIG.
The experimental results show that the magnitude of the LSSE voltage in the
Pt/YIG/Pt systems rapidly decreases with increasing the temperature and
disappears above the Curie temperature. The critical exponent of the LSSE
voltage in the Pt/YIG/Pt systems at the Curie temperature was estimated to be
3, which is much greater than that for the magnetization curve of YIG. This
difference highlights the fact that the mechanism of the LSSE cannot be
explained in terms of simple static magnetic properties in YIG. | cond-mat_mtrl-sci |
Antiferromagnetic Spin Orientation and Magnetic Domain Structure in
Epitaxially Grown MnN Studied using Optical Second Harmonic Generation: MnN is a centrosymmetric collinear antiferromagnet belonging to the
transition metal nitride family with a high Neel temperature, a low anisotropy
field, and a large magnetic moment per Mn atom. Despite several recent
experimental and theoretical studies, the spin symmetry (magnetic point group)
and magnetic domain structure of the material remain unknown. In this work, we
use optical second harmonic generation (SHG) to study the magnetic structure of
thin epitaxially-grown single-crystal (001) MnN films. Our work shows that spin
moments in MnN are tilted away from the [001] direction and the components of
the spin moments in the (001) plane are aligned along one of the two possible
in-plane symmetry axes ([100] or [110]) resulting in a magnetic point group
symmetry of 2/m1'. Our work rules out magnetic point group symmetries 4/mmm1'
and mmm1' that have been previously discussed in the literature. Four different
spin domains consistent with the 2/m1' magnetic point group symmetry are
possible in MnN. A statistical model based on the observed variations in the
polarization-dependent intensity of the second harmonic signal collected over
large sample areas puts an upper bound of 0.65 microns on the mean domain size.
Our results show that SHG can be used to probe the magnetic order in metallic
antiferromagnets. This work is expected to contribute to the recent efforts in
using antiferromagnets for spintronic applications. | cond-mat_mtrl-sci |
Preventing corona effects: multi-phosphonic acid poly(ethylene glycol)
copolymers for stable stealth iron oxide nanoparticles: When disperse in biological fluids, engineered nanoparticles are selectively
coated with proteins, resulting in the formation of a protein corona. It is
suggested that the protein corona is critical in regulating the conditions of
entry into the cytoplasm of living cells. Recent reports describe this
phenomenon as ubiquitous and independent of the nature of the particle. For
nanomedicine applications however, there is a need to design advanced and
cost-effective coatings that are resistant to protein adsorption and that
increase the biodistribution in vivo. In this study, phosphonic acid
poly(ethylene glycol) copolymers were synthesized and used to coat iron oxide
particles. The copolymer composition was optimized to provide simple and
scalable protocols as well as long-term stability in culture media. It is shown
that polymers with multiple phosphonic acid functionalities and PEG chains
outperform other types of coating, including ligands, polyelectrolytes and
carboxylic acid functionalized PEG. PEGylated particles exhibit moreover
exceptional low cellular uptake, of the order of 100 femtograms of iron per
cell. The present approach demonstrates that the surface chemistry of
engineered particles is a key parameter in the interactions with cells. It also
opens up new avenues for the efficient functionalization of inorganic surfaces. | cond-mat_mtrl-sci |
Symmetrical laws of structure of helicoidally-like biopolymers in the
framework of algebraic topology. II. α-helix and DNA structures: In the framework of algebraic topology the closed sequence of 4-dimensional
polyhedra (algebraic polytopes) was defined. This sequence is started by the
polytope {240}, discovered by Coxeter, and is determined by the second
coordination sphere of 8-dimensional lattice E8. The second polytope of
sequence allows to determine a topologically stable rod substructure that
appears during multiplication by a non-crystallographic axis 40/11 of the
starting union of 4 tetrahedra with common vertex. When positioning the
appropriate atoms tin positions of special symmetry of the staring 4
tetrahedra, such helicoid determines an {\alpha}-helix. The third polytope of
sequence allows to determine the helicoidally-like union of rods with 12-fold
axis, which can be compare with Z-DNA structures. This model is defined as a
local lattice rod packing, contained within a surface of helicoidally similar
type, which ensures its topological stability, as well as possibility for it to
be transformed into other forms of DNA structures. Formation of such structures
corresponds to lifting a configuration degeneracy, and the stability of a state
- to existence of a point of bifurcation. Furthermore, in the case of DNA
structures, a second "security check" possibly takes place in the form of local
lattice (periodic) property using the lattices other than the main ones. | cond-mat_mtrl-sci |
Limitations for the determination of piezoelectric constants with
piezoresponse force microscopy: At first sight piezoresponse force microscopy (PFM) seems an ideal technique
for the determination of piezoelectric coefficients (PCs), thus making use of
its ultra-high vertical resolution (<0.1 pm/V). Christman et al. \cite{Chr98}
first used PFM for this purpose. Their measurements, however, yielded only
reasonable results of unsatisfactory accuracy, amongst others caused by an
incorrect calibration of the setup. In this contribution a reliable calibration
procedure is given followed by a careful analysis of the encounted difficulties
determining PCs with PFM. We point out different approaches for their solution
and expose why, without an extensive effort, those difficulties can not be
circumvented. | cond-mat_mtrl-sci |
Study of the elastocaloric effect and mechanical behavior for the NiTi
shape memory alloys: The NiTi shape memory alloy exhibited excellent superelastic property and
elastocaloric effect. Large temperature changes of 30 K upon loading and -19 K
upon unloading were obtained at room temperature, which were higher than those
of the other NiTi-based materials and among the highest values reported in the
elastocaloric materials. The asymmetry of the measured temperature changes
between loading and unloading process was ascribed to the friction dissipation.
The large temperature changes originated from the large entropy change during
the stress-induced martensite transformation (MT) and the reverse MT. A large
coefficient-of-performance of the material (COPmater) of 11.7 was obtained,
which decreased with increasing the applied strain. These results are very
attractive in the present solid-state cooling which is potential to replace the
vapor compression refrigeration technologies. | cond-mat_mtrl-sci |
Highly anisotropic two-dimensional metal in monolayer MoOCl$_2$: Anisotropy is a general feature in materials. Strong anisotropy could lead to
interesting physical properties and useful applications. Here, based on
first-principles calculations and theoretical analysis, we predict a stable
two-dimensional (2D) material---the monolayer MoOCl$_2$, and show that it
possesses intriguing properties related to its high anisotropy. Monolayer
MoOCl$_2$ can be readily exfoliated from the van der Waals layered bulk, which
has already been synthesized. We show that a high in-plane anisotropy manifests
in the structural, phononic, mechanical, electronic, and optical properties of
monolayer MoOCl$_2$. The material is a metal with highly anisotropic Fermi
surfaces, giving rise to open orbits at the Fermi level, which can be probed in
magneto-transport. Remarkably, the combination of high anisotropy and metallic
character makes monolayer MoOCl$_2$ an almost ideal hyperbolic material. It has
two very wide hyperbolic frequency windows from 0.41 eV (99 THz) to 2.90 eV
(701 THz), and from 3.63 eV (878 THz) to 5.54 eV (1340 THz). The former window
has a large overlap with the visible spectrum, and the dissipation for most
part of this window is very small. The window can be further tuned by the
applied strain, such that at a chosen frequency, a transition between elliptic
and hyperbolic character can be induced by strain. Our work discovers a highly
anisotropic 2D metal with extraordinary properties, which holds great potential
for electronic and optical applications. | cond-mat_mtrl-sci |
Monte Carlo simulation of GaAs(001) homoepitaxy: By carrying out Monte Carlo simulations based on the two-species atomic-scale
kinetic growth model of GaAs(001) homoepitaxy and comparing the results with
scanning tunneling microscope images, we show that initial growing islands
undergo the structural transformation before adopting the proper beta2(2x4)
reconstruction. | cond-mat_mtrl-sci |
Giant interfacial perpendicular magnetic anisotropy in MgO/CoFe/capping
layer structures: Magnetic tunnel junction (MTJ) based on CoFeB/MgO/CoFeB structures is of
great interest due to its application in the spin-transfer-torque magnetic
random access memory (STT-MRAM). Large interfacial perpendicular magnetic
anisotropy (PMA) is required to achieve high thermal stability. Here we use
first-principles calculations to investigate the magnetic anisotropy energy
(MAE) of MgO/CoFe/capping layer structures, where the capping materials include
5d metals Hf, Ta, Re, Os, Ir, Pt, Au and 6p metals Tl, Pb, Bi. We demonstrate
that it is feasible to enhance PMA by using proper capping materials.
Relatively large PMA is found in the structures with capping materials of Hf,
Ta, Os, Ir and Pb. More importantly, the MgO/CoFe/Bi structure gives rise to
giant PMA (6.09 mJ/m2), which is about three times larger than that of the
MgO/CoFe/Ta structure. The origin of the MAE is elucidated by examining the
contributions to MAE from each atomic layer and orbital. These findings provide
a comprehensive understanding of the PMA and point towards the possibility to
achieve advanced-node STT-MRAM with high thermal stability. | cond-mat_mtrl-sci |
The local atomic quasicrystal structure of the icosahedral Mg25Y11Zn64
alloy: A local and medium range atomic structure model for the face centred
icosahedral (fci) Mg25Y11Zn64 alloy has been established in a sphere of r = 27
A. The model was refined by least squares techniques using the atomic pair
distribution (PDF) function obtained from synchrotron powder diffraction. Three
hierarchies of the atomic arrangement can be found: (i) five types of local
coordination polyhedra for the single atoms, four of which are of Frank-Kasper
type. In turn, they (ii) form a three-shell (Bergman) cluster containing 104
atoms, which is condensed sharing its outer shell with its neighbouring
clusters and (iii) a cluster connecting scheme corresponding to a
three-dimensional tiling leaving space for few glue atoms. Inside adjacent
clusters, Y8-cubes are tilted with respect to each other and thus allow for
overall icosahedral symmetry. It is shown that the title compound is
essentially isomorphic to its holmium analogue. Therefore fci-Mg-Y-Zn can be
seen as the representative structure type for the other rare earth analogues
fci-Mg-Zn-RE (RE = Dy, Er, Ho, Tb) reported in the literature. | cond-mat_mtrl-sci |
Microscale simulation of adhesive and cohesive failure in rough
interfaces: Multi-material lightweight designs, e.g. the combination of aluminum with
fiber-reinforced composites, are a key feature for the development of
innovative and resource-efficient products. The connection properties of such
bi-material interfaces are influenced by the geometric structure on different
length scales. In this article a modeling strategy is presented to study the
failure behavior of rough interfaces within a computational homogenization
scheme. We study different local phenomena and their effects on the overall
interface characteristics, e.g. the surface roughness and different local
failure types as cohesive failure of the bulk material and adhesive failure of
the local interface. Since there is a large separation in the length scales of
the surface roughness, which is in the micrometer range, and conventional
structural components, we employ a numerical homogenization approach to extract
effective traction-separation laws to derive effective interface parameters.
Adhesive interface failure is modeled by cohesive elements based on a
traction-separation law and cohesive failure of the bulk material is described
by an elastic-plastic model with progressive damage evolution. | cond-mat_mtrl-sci |
Encoding Complexity within Supramolecular Analogues of Frustrated
Magnets: At the heart of systems chemistry lies the idea that supramolecular
interactions can give rise to complex and unexpected collective states that
emerge on a fundamentally different lengthscale to that of the interactions
themselves. While in certain cases - e.g. the self-assembly of virus-like
polyhedral cages from coordination building blocks - it is possible to control
emergence in a systematic manner, the development of general approaches remains
a fundamental challenge in the field. In the conceptually-related domain of
frustrated magnetism - where collective states give rise to exotic physics of
relevance to data storage and spintronics - the task of predicting emergent
behaviour is simplified through control over the geometry and form of the
magnetic interactions from which complexity arises. Seeking to combine
approaches from these two fields, we study here the solid phases of inorganic
polymer chains assembled from non-magnetic gold(I)/silver(I) cations and
cyanide anions. We show the periodic inter-chain potential encodes a
supramolecular interaction that can be tuned to mimic different magnetic
interactions between XY spins ("spin rotors"). Because the chains pack on a
triangular lattice, the crystal structures of gold(I)/silver(I) cyanides can be
interpreted in terms of the phase behaviour of triangular XY magnets. Complex
magnetic states predicted for this family - including hidden quadrupolar order
and emergent spin-vortex quasiparticles - are realised for the first time in
the structural chemistry of these cyanide polymers. In this way we demonstrate
both how simple inorganic materials might behave as structural analogues of
otherwise-unrealisable "toy" spin models, and also how a theoretical
understanding of those models might be used to predict and control emergent
phenomena in chemical systems. | cond-mat_mtrl-sci |
Yield criterion and finite strain behavior of random porous isotropic
materials: The mechanical response of isotropic elastoplastic materials containing
random distributions of initially spherical voids is investigated
computationally based on Fast Fourier Transform simulations. Numerical
limit-analysis simulations at constant stress triaxiality allow to determine
the yield surfaces, leading in particular to the determination of a
Representative Volume Element size for the onset of coalescence / inhomogeneous
yielding. Moreover, two different coalescence regimes are observed that differ
by the presence of shearing. The yield surfaces are found to be consistent with
the combination of two models proposed in the literature, a GTN-type model
calibrated for homogeneous yielding of random porous materials and an
inhomogeneous yielding model accounting for both coalescence with or without
shear. Finite strain simulations performed for different hardening moduli and
stress triaxialities under axisymmetric loading conditions confirm the
existence of a RVE up to the onset of inhomogeneous yielding. Coalescence
strains are found to be significantly smaller for random porous materials than
for periodic distribution of voids. A homogenized model is finally proposed
that reproduces quantitatively the behavior of isotropic elastoplastic
materials containing random distributions of voids under finite strains. | cond-mat_mtrl-sci |
Weak antilocalization in a noncentrosymmetric CaAgBi single crystal: We report on the single crystal growth and transport properties of a
topological semimetal CaAgBi which crystallises in the hexagonal $ABC-$type
structure with the non-centrosymmetric space group $\mathit{P6_3mc}$ (No. 186).
The transverse magnetoresistance measurements with current in the basal plane
of the hexagonal crystal structure reveal a value of about 30 % for I //
[10-10] direction and about 50 % for I // [1-210] direction at 10 K in an
applied magnetic field of 14 T. The magnetoresistance shows a cusp-like
behavior in the low magnetic-field region, suggesting the presence of weak
antilocalization effect for temperatures less than 100 K. The Hall measurements
reveal that predominant charge carriers are $p$ type exhibiting a linear
behavior for fields up to 14 T and can be explained based on the single band
model. The magnetoconductance of CaAgBi is analysed based on the modified
Hikami-Larkin-Nagaoka (HLN) model. Our first-principles calculations within a
density-functional theory framework reveal that CaAgBi supports a topological
Dirac semimetal state with Dirac points located on the rotational axis slightly
above the Fermi level and are protected by $C_{6v}$ point-group symmetry. The
Fermi surface consists of both the electron and hole pockets. However, the size
of hole pockets is much larger than electron pockets suggesting the dominant
$p$ type carriers in accord with our experimental results. | cond-mat_mtrl-sci |
Systematical, experimental investigations on LiMgZ (Z= P, As, Sb) wide
band gap semiconductors: This work reports on the experimental investigation of the wide band gap
compounds LiMgZ (Z = P, As, Sb), which are promising candidates for
opto-electronics and anode materials for Lithium batteries. The compounds
crystallize in the cubic (C1_b) MgAgAs structure (space group F-43m). The
polycrystalline samples were synthesized by solid state reaction methods. X-ray
and neutron diffraction measurements show a homogeneous, single-phased samples.
The electronic properties were studied using the direct current (DC) method.
Additionally UV-VIS diffuse reflectance spectra were recorded in order to
investigate the band gap nature. The measurements show that all compounds
exhibit semiconducting behavior with direct band gaps of 1.0 eV to 2.3 eV
depending on the Z element. A decrease of the peak widths in the static 7Li
nuclear magnetic resonance (NMR) spectra with increasing temperature was
observed, which can directly be related to an increase of Li ion mobility. | cond-mat_mtrl-sci |
High entropy van der Waals materials (Review article): By breaking the restrictions on traditional alloying strategy, the high
entropy concept has promoted the exploration of the central area of phase
space, thus broadening the horizon of alloy exploitation. This review
highlights the marriage of the high entropy concept and van der Waals systems
to form a new family of materials category, namely the high entropy van der
Waals materials (HEX, HE = high entropy, X= anion clusters) and describe the
current issues and next challenges. The design strategy for HEX has integrated
the local feature (e.g., composition, spin, and valence states) of structural
units in high entropy materials and the holistic degrees of freedom (e.g.,
stacking, twisting, and intercalating species) in van der Waals materials, and
has been successfully employed for the discovery of high entropy
dichalcogenides, phosphorus tri-chalcogenides, halogens, and MXene. The rich
combination and random distribution of the multiple metallic constituents on
the nearly-regular 2D lattice give rise to a flexible platform to study the
correlation features behind a range of selected physical properties, e.g.,
superconductivity, magnetism, and metal-insulator transition. The deliberate
design of structural units and their stacking configuration can also create
novel catalysts to enhance their performance in a bunch of chemical reactions. | cond-mat_mtrl-sci |
Synthesis and Local Probe Gating of a Monolayer Metal-Organic Framework: Achieving large-area uniform two-dimensional (2D) metal-organic frameworks
(MOFs) and controlling their electronic properties on inert surfaces is a big
step towards future applications in electronic devices. Here we successfully
fabricated a 2D monolayer Cu-dicyanoanthracene (DCA) MOF with long-range order
on an epitaxial graphene surface. Its structural and electronic properties are
studied by low-temperature scanning tunneling microscopy (STM) and spectroscopy
(STS) complemented by density-functional theory (DFT) calculations. We
demonstrate access to multiple molecular charge states in the 2D MOF using
tip-induced local electric fields. We expect that a similar strategy could be
applied to fabricate and characterize 2D MOFs with exotic, engineered
electronic states. | cond-mat_mtrl-sci |
Giant Anisotropy of Spin-Orbit Splitting at the Bismuth Surface: We investigate the bismuth (111) surface by means of time and angle resolved
photoelectron spectroscopy. The parallel detection of the surface states below
and above the Fermi level reveals a giant anisotropy of the Spin-Orbit (SO)
spitting. These strong deviations from the Rashba-like coupling cannot be
treated in $\textbf{k}\cdot \textbf{p}$ perturbation theory. Instead, first
principle calculations could accurately reproduce the experimental dispersion
of the electronic states. Our analysis shows that the giant anisotropy of the
SO splitting is due to a large out-of plane buckling of the spin and orbital
texture. | cond-mat_mtrl-sci |
Conditions for the formation of pure birnessite during the oxidation of
Mn(II) cations in aqueous alkaline medium: Birnessite was synthetized through redox reaction by mixing MnO4-, Mn2+ and
OH- solutions. The Mn(VII): Mn(II) ratio of 0.33 was chosen and three methods
were used consisting in a quick mixing under vigorous stirring of two of the
three reagents and then on the dropwise addition of the third one. The obtained
solids were characterized by XRD, FTIR and XPS spectroscopies. Their average
oxidation states were determined from ICP and CEC measurements while their
surface properties were investigated by XPS. This study provides an increased
understanding of the importance of dissolved oxygen in the formation of
birnessite and hausmannite and shows the ways to obtain pure birnessite. The
role of counter-ion ie. Na+ or K+ was also examined. | cond-mat_mtrl-sci |
A General Framework for Liquid Marbles: Liquid marbles refer to liquid droplets that are covered with a layer of
non-wetting particles. They are observed in nature and have practical
significance. However, a generalized framework for analyzing liquid marbles as
they inflate or deflate is unavailable. The present study fills this gap by
developing an analytical framework based on liquid-particle and
particle-particle interactions. We demonstrate that the potential final states
of evaporating liquid marbles are characterized by one of the following: (I)
constant surface area, (II) particle ejection, or (III) multilayering. Based on
these insights, a single-parameter evaporation model for liquid marbles is
developed. Model predictions are in excellent agreement with experimental
evaporation data for water liquid marbles of particle sizes ranging from 7
nanometers to 300 micrometers (over four orders of magnitude) and chemical
compositions ranging from hydrophilic to superhydrophobic. These findings lay
the groundwork for the rational design of liquid marble applications. | cond-mat_mtrl-sci |
Interfacial-Redox-Induced Tuning of Superconductivity in
YBa$_{2}$Cu$_{3}$O$_{7-δ}$: Solid state ionic approaches for modifying ion distributions in getter/oxide
heterostructures offer exciting potentials to control material properties. Here
we report a simple, scalable approach allowing for total control of the
superconducting transition in optimally doped YBa$_{2}$Cu$_{3}$O$_{7-{\delta}}$
(YBCO) films via a chemically-driven ionic migration mechanism. Using a thin Gd
capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films,
oxygen is found to leach from deep within the YBCO. Progressive reduction of
the superconducting transition is observed, with complete suppression possible
for a sufficiently thick Gd layer. These effects arise from the combined impact
of redox-driven electron doping and modification of the YBCO microstructure due
to oxygen migration and depletion. This work demonstrates an effective ionic
control of superconductivity in oxides, an interface induced effect that goes
well into the quasi-bulk regime, opening up possibilities for electric field
manipulation. | cond-mat_mtrl-sci |
Correlation of microdistortions with misfit volumes in High Entropy
Alloys: The yield strengths of High Entropy Alloys have recently been correlated with
measured picometer-scale atomic distortions. Here, the root mean square
microdistortion in a multicomponent alloy is shown to be nearly proportional to
the misfit-volume parameter that enters into a predictive model of solute
strengthening. Analysis of two model ternary alloy families, face-centered
cubic Cr-Fe-Ni and body-centered cubic Nb-Mo-V, demonstrates the correlation
over a wide composition space. The reported correlation of yield strength with
microdistortion is thus a consequence of the correlation between
microdistortion and misfit parameter and the derived dependence of yield
strength on the misfit parameter. | cond-mat_mtrl-sci |
Stress transmission in planar disordered solid foams: Stress transmission in planar open-cell cellular solids is analysed using a
recent theory developed for marginally rigid granular assemblies. This is made
possible by constructing a one-to-one mapping between the two systems. General
trivalent networks are mapped onto assemblies of rough grains, while networks
where Plateau rules are observed, are mapped onto assemblies of smooth grains.
The constitutive part of the stress transmission equations couples the stress
directly to the local rotational disorder of the cellular structure via a new
fabric tensor. An intriguing consequence of the analysis is that the stress
field can be determined in terms of the microstructure alone independent of
stress-strain information. This redefines the problem of structure-property
relationship in these materials and poses questions on the relations between
this formalism and elasticity theory. The deviation of the stress transmission
equations from those of conventional solids has been interpreted in the context
of granular assemblies as a new state of solid matter and the relevance of this
interpretation to the state of matter of cellular solids is discussed. | cond-mat_mtrl-sci |
Induced Giant Piezoelectricity in Centrosymmetric Oxides: Piezoelectrics are materials that linearly deform in response to an applied
electric field. As a fundamental prerequisite, piezoelectric material must
possess a non centrosymmetric crystal structure. For more than a century, this
remains the major obstacle for finding new piezoelectric materials. We
circumvent this limitation by breaking the crystallographic symmetry, and
inducing large and sustainable piezoelectric effects in centrosymmetric
materials by electric field induced rearrangement of oxygen vacancies
Surprisingly, the results show the generation of extraordinarily large
piezoelectric responses d33 ~200,000 pm/V), in cubic fluorite Gd-doped CeO2-x
films, which is two orders of magnitude larger than in the presently best known
lead based piezoelectric relaxor ferroelectric oxide. These findings open
opportunities to design new piezoelectric materials from environmentally
friendly centrosymmetric ones. | cond-mat_mtrl-sci |
Interface collisions with diffusive mass transport: We report on a linear Langevin model that describes the evolution of the
roughness of two interfaces that move towards each other and are coupled by a
diffusion field. This model aims at describing the closing of the gap between
two two-dimensional material domains during growth, and the subsequent
formation of a rough grain boundary. We assume that deposition occurs in the
gap between the two domains and that the growth units diffuse and may attach to
the edges of the domains. These units can also detach from edges, diffuse, and
re-attach elsewhere. For slow growth, the edge roughness increases monotonously
and then saturates at some equilibrium value. For fast growth, the roughness
exhibits a maximum just before the collision between the two interfaces, which
is followed by a minimum. The peak of the roughness can be dominated by
statistical fluctuations or by edge instabilities. A phase diagram with three
regimes is obtained: slow growth without peak, peak dominated by statistical
fluctuations, and peak dominated by instabilities. These results reproduce the
main features observed in Kinetic Monte Carlo simulations. | cond-mat_mtrl-sci |
Scaling of the thermally induced sign inversion of longitudinal spin
Seebeck effect in a compensated ferrimagnet: Role of magnetic anisotropy: We report on a systematic investigation of the longitudinal spin Seebeck
effect (LSSE) in a GGG(Gd3Ga5O12)/GdIG(Gd3Fe5O12)/Pt film series exhibiting an
in-plane magnetic easy axis with a compensation temperature (T_Comp) that
decreases from 270 to 220 K when decreasing GdIG film thickness from 272 to 31
nm, respectively. For all the films, the LSSE signal flips its sign below
T_Comp. We demonstrate a universal scaling behavior of the temperature
dependence of LSSE signal for our GdIG films around their respective T_Comp.
Additionally, we demonstrate LSSE in a 31 nm GdIG film grown on a
lattice-mismatched GSGG (Gd3Sc2Ga3O12) substrate that exhibits an out-of-plane
magnetic easy axis at room temperature. However, this sample reveals a spin
reorientation transition where the magnetic easy axis changes its orientation
to in-plane at low temperatures. We observed a clear distinction in the LSSE
signal for the GSGG/GdIG(31 nm)/Pt heterostructure, relative to
GGG/GdIG(31nm)/Pt showing an in-plane magnetic easy axis. Our findings
underscore a strong correlation between the LSSE signal and the orientation of
magnetic easy axis in compensated ferrimagnets and opens the possibility to
tune LSSE through effective anisotropy. | cond-mat_mtrl-sci |
Polarization Morphology and Electrocaloric Response of Strained
Ferroelectric Core-Shell Nanorods and Nanowires: Using Landau-Ginzburg-Devonshire (LGD) approach we proposed the analytical
description of the Vegard strains influence on the spontaneous polarization and
electrocaloric response in ferroelectric core-shell nanorods. The nanorod core
presents a defect-free single-crystalline ferroelectric material, and the
Vegard strains are induced by elastic defects in the ultra-thin shell. The
finite element modeling (FEM) based on the LGD approach reveals transitions of
domain structure morphology induced by the Vegard strains in the BaTiO3
nanorods. Namely, tensile Vegard strains induce and support the single-domain
state in the central part of the nanorod, while the curled domain structures
appear near the unscreened or partially screened ends of the rod. The
vortex-like domains propagate toward the central part of the rod and fill it
entirely, when the rod is covered by a shell with compressive Vegard strains
above some critical value. The critical value depends on the nanorod sizes,
aspect ratio, and screening conditions at its ends. Both analytical theory and
FEM predict that the tensile Vegard strains in the shell increase the nanorod
polarization, lattice tetragonality, and electrocaloric response well-above the
values corresponding to the bulk material. The physical reason of the increase
is the strong electrostriction coupling between the mismatch-type elastic
strains induced in the core by the Vegard strains in the shell. Comparison with
the earlier XRD data confirmed an increase of tetragonality ratio in tensiled
BaTiO3 nanorods compared to the bulk material. Obtained analytical expressions,
which are suitable for the description of strain-induced changes in a wide
class of multiaxial ferroelectric core-shell nanorods and nanowires, can be
useful for strain engineering of advanced ferroelectric nanomaterials for
electrocaloric applications and negative capacitance elements. | cond-mat_mtrl-sci |
Ultrafast Epitaxial Growth of Metre-Sized Single-Crystal Graphene on
Industrial Cu Foil: A foundation of the modern technology that uses single-crystal silicon has
been the growth of high-quality single-crystal Si ingots with diameters up to
12 inches or larger. For many applications of graphene, large-area high-quality
(ideally of single-crystal) material will be enabling. Since the first growth
on copper foil a decade ago, inch-sized single-crystal graphene has been
achieved. We present here the growth, in 20 minutes, of a graphene film of 5 x
50 cm2 dimension with > 99% ultra-highly oriented grains. This growth was
achieved by: (i) synthesis of sub-metre-sized single-crystal Cu(111) foil as
substrate; (ii) epitaxial growth of graphene islands on the Cu(111) surface;
(iii) seamless merging of such graphene islands into a graphene film with high
single crystallinity and (iv) the ultrafast growth of graphene film. These
achievements were realized by a temperature-driven annealing technique to
produce single-crystal Cu(111) from industrial polycrystalline Cu foil and the
marvellous effects of a continuous oxygen supply from an adjacent oxide. The
as-synthesized graphene film, with very few misoriented grains (if any), has a
mobility up to ~ 23,000 cm2V-1s-1 at 4 K and room temperature sheet resistance
of ~ 230 ohm/square. It is very likely that this approach can be scaled up to
achieve exceptionally large and high-quality graphene films with single
crystallinity, and thus realize various industrial-level applications at a low
cost. | cond-mat_mtrl-sci |
First-principles methodology for studying magnetotransport in narrow-gap
semiconductors: an application to Zirconium Pentatelluride ZrTe5: The origin of anomalous resistivity peak and accompanied sign reversal of
Hall resistivity of ZrTe$_5$ has been under debate for a long time. Although
various theoretical models have been proposed to account for these intriguing
transport properties, a systematic study from first principles view is still
lacking. In this work, we present a first principles calculation combined with
Boltzmann transport theory to investigate the transport properties in
narrow-gap semiconductors at different temperatures and doping densities within
the relaxation time approximation. Regarding the sensitive
temperature-dependent chemical potential and relaxation time of semiconductors,
we take proper approximation to simulate these two variables, and then
comprehensively study the transport properties of ZrTe$_5$ both in the absence
and presence of an applied magnetic field. Without introducing topological
phases and correlation interactions, we qualitatively reproduced crucial
features observed in experiments, including zero-field resistivity anomaly,
nonlinear Hall resistivity with sign reversal, and non-saturating
magnetoresistance at high temperatures. Our calculation allows a systematic
interpretation of the observed properties in terms of multi-carrier and Fermi
surface geometry. Our method can be extended to other narrow-gap semiconductors
and further pave the way to explore interesting and novel transport properties
of this field. | cond-mat_mtrl-sci |
Lattice expansion and non-collinear to collinear ferrimagnetic order in
MnCr$_2$O$_4$ nanoparticle: We report magnetic behaviour of MnCr$_2$O$_4$, which belongs to a special
class of spinel, known as chromite. Bulk MnCr$_2$O$_4$ shows a sequence of
magnetic states, which follows paramagnetic (PM) to collinear ferrimagnetic
(FM) state below T$_C$ $\sim$ 45 K and collinear FM state to non-collinear FM
state below T$_S$ $\sim$ 18 K. The non-collinear spin structure has been
modified on decreasing the particle size, and magnetic transition at T$_S$
decreases in nanoparticle samples. However, ferrimagnetic order is still
dominating in nanoparticles, except the observation of superparamagnetic like
blocking and decrease of spontaneous magnetization for nanoparticle. This may,
according to the core-shell model of ferrimagnetic nanoparticle, be the surface
disorder effect of nanoparticle. The system also show the increase of T$_C$ in
nanoparticle samples, which is not consistent with the core-shell model. The
analysis of the M(T) data, applying spin wave theory, has shown an unusual
Bloch exponent value 3.35 for bulk MnCr$_2$O$_4$, which decreases and
approaches to 1.5, a typical value for any standard ferromagnet, with
decreasing the particle size. MnCr$_2$O$_4$ has shown a few more unusual
behaviour. For example, lattice expansion in nanoparticle samples. The present
work demonstrates the correlation between a systematic increase of lattice
parameter and the gradual decrease of B site non-collinear spin structure in
the light of magnetism of MnCr$_2$O$_4$ nanoparticles. | cond-mat_mtrl-sci |
Strongly Constrained and Appropriately Normed Semilocal Density
Functional: The ground-state energy, electron density, and related properties of ordinary
matter can be computed efficiently when the exchange-correlation energy as a
functional of the density is approximated semilocally. We propose the first
meta-GGA (meta-generalized gradient approximation) that is fully constrained,
obeying all 17 known exact constraints that a meta-GGA can. It is also exact or
nearly exact for a set of appropriate norms, including rare-gas atoms and
nonbonded interactions. This SCAN (strongly constrained and appropriately
normed) meta-GGA achieves remarkable accuracy for systems where the exact
exchange-correlation hole is localized near its electron, and especially for
lattice constants and weak interactions. | cond-mat_mtrl-sci |
New apparatus for DTA at 2000 bar: thermodynamic studies on Au, Ag, Al
and HTSC oxides: A new DTA (Differential Thermal Analysis) device was designed and installed
in a Hot Isostatic Pressure (HIP) furnace in order to perform high-pressure
thermodynamic investigations up to 2 kbar and 1200C. Thermal analysis can be
carried out in inert or oxidising atmosphere up to p(O2) = 400 bar. The
calibration of the DTA apparatus under pressure was successfully performed
using the melting temperature (Tm) of pure metals (Au, Ag and Al) as standard
calibration references. The thermal properties of these metals have been
studied under pressure. The values of DV (volume variation between liquid and
solid at Tm), ROsm (density of the solid at Tm) and ALPHAm (linear thermal
expansion coefficient at Tm) have been extracted. A very good agreement was
found with the existing literature and new data were added. This HP-DTA
apparatus is very useful for studying the thermodynamics of those systems where
one or more volatile elements are present, such as high TC superconducting
oxides. DTA measurements have been performed on Bi,Pb(2223) tapes up to 2 kbar
under reduced oxygen partial pressure (p(O2) = 0.07 bar). The reaction leading
to the formation of the 2223 phase was found to occur at higher temperatures
when applying pressure: the reaction DTA peak shifted by 49C at 2 kbar compared
to the reaction at 1 bar. This temperature shift is due to the higher stability
of the Pb-rich precursor phases under pressure, as the high isostatic pressure
prevents Pb from evaporating. | cond-mat_mtrl-sci |
A scalable parallel Monte Carlo algorithm for atomistic simulations of
precipitation in alloys: We present an extension of the semi-grandcanonical (SGC) ensemble that we
refer to as the variance-constrained semi-grandcanonical (VC-SGC) ensemble. It
allows for transmutation Monte Carlo simulations of multicomponent systems in
multiphase regions of the phase diagram and lends itself to scalable
simulations on massively parallel platforms. By combining transmutation moves
with molecular dynamics steps structural relaxations and thermal vibrations in
realistic alloys can be taken into account. In this way, we construct a robust
and efficient simulation technique that is ideally suited for large-scale
simulations of precipitation in multicomponent systems in the presence of
structural disorder. To illustrate the algorithm introduced in this work, we
study the precipitation of Cu in nanocrystalline Fe. | cond-mat_mtrl-sci |
Deep Learning and Crystal Plasticity: A Preconditioning Approach for
Accurate Orientation Evolution Prediction: Efficient and precise prediction of plasticity by data-driven models relies
on appropriate data preparation and a well-designed model. Here we introduce an
unsupervised machine learning-based data preparation method to maximize the
trainability of crystal orientation evolution data during deformation. For
Taylor model crystal plasticity data, the preconditioning procedure improves
the test score of an artificial neural network from 0.831 to 0.999, while
decreasing the training iterations by an order of magnitude. The efficacy of
the approach was further improved with a recurrent neural network. Electron
backscattered (EBSD) lab measurements of crystal rotation during rolling were
compared with the results of the surrogate model, and despite error introduced
by Taylor model simplifying assumptions, very reasonable agreement between the
surrogate model and experiment was observed. Our method is foundational for
further data-driven studies, enabling the efficient and precise prediction of
texture evolution from experimental and simulated crystal plasticity results. | cond-mat_mtrl-sci |
Fourier analysis of the IR response of van der Waals materials: In this letter, we report on an analytical technique for optical
investigations of semitransparent samples. By Fourier transforming optical
spectra with Fabry-Perot resonances we extract information about sample
thickness and its discrete variations. Moreover, this information is used to
recover optical spectra devoid of Fabry-Perot fringes, which simplifies optical
modelling, and can reveal previously concealed spectral features. To illustrate
its use, we apply our technique to a Si wafer as well as six different
cleavable layered materials, including topological insulators, thermoelectrics,
and magnetic insulators. In the layered materials, we find strong evidence of
large step edges and thickness inhomogeneity, and cannot conclusively exclude
the presence of voids in the bulk of cleaved samples. This could strongly
affect the interpretation of transport and optical data of crystals with
topologically protected surfaces states. | cond-mat_mtrl-sci |
Impact of electron solvation on ice structures at the molecular scale: We determine the impact of electron solvation on D$_2$O structures adsorbed
on Cu(111) with low temperature scanning tunneling microscopy, two-photon
photoemission, and ab initio theory. UV photons generating solvated electrons
lead not only to transient, but also to permanent structural changes through
the rearrangement of individual molecules. The persistent changes occur near
sites with a high density of dangling OH groups that facilitate electron
solvation. We conclude that energy dissipation during solvation triggers
permanent molecular rearrangement via vibrational excitation. | cond-mat_mtrl-sci |
Bandgap of two-dimensional materials: Thorough assessment of modern
exchange-correlation functionals: The density functional theory (DFT) approximations that are the most accurate
for the calculation of band gap of bulk materials are hybrid functionals like
HSE06, the MBJ potential, and the GLLB-SC potential. More recently, generalized
gradient approximations (GGA), like HLE16, or meta-GGAs, like (m)TASK, have
proven to be also quite accurate for the band gap. Here, the focus is on 2D
materials and the goal is to provide a broad overview of the performance of DFT
functionals by considering a large test set of 298 2D systems. The present work
is an extension of our recent studies [Rauch et al., Phys. Rev. B 101, 245163
(2020) and Patra et al., J. Phys. Chem. C 125, 11206 (2021)]. Due to the lack
of experimental results for the band gap of 2D systems, $G_{0}W_{0}$ results
were taken as reference. It is shown that the GLLB-SC potential and mTASK
functional provide the band gaps that are the closest to $G_{0}W_{0}$.
Following closely, the local MBJ potential has a pretty good accuracy that is
similar to the accuracy of the more expensive hybrid functional HSE06. | cond-mat_mtrl-sci |
Electronic structure of Ba(Zn0.875Mn0.125)2As2 studied by angle-resolved
photoemission spectroscopy: Electronic structure of single crystalline
Ba(Zn$_{0.875}$Mn$_{0.125}$)$_{2}$As$_{2}$, parent compound of the recently
founded high-temperature ferromagnetic semiconductor, was studied by
high-resolution photoemission spectroscopy (ARPES). Through systematically
photon energy and polarization dependent measurements, the energy bands along
the out-of-plane and in-plane directions were experimentally determined. Except
the localized states of Mn, the measured band dispersions agree very well with
the first-principle calculations of undoped BaZn$_{2}$As$_{2}$. A new feature
related to Mn 3d states was identified at the binding energies of about -1.6 eV
besides the previously observed feature at about -3.3 eV. We suggest that the
hybridization between Mn and As orbitals strongly enhanced the density of
states around -1.6 eV. Although our resolution is much better compared with
previous soft X-ray photoemission experiments, no clear hybridization gap
between Mn 3d states and the valence bands proposed by previous model
calculations was detected. | cond-mat_mtrl-sci |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.